Manual: g++-15
GCC(1) GNU GCC(1)
NAME
gcc - GNU project C and C++ compiler
SYNOPSIS
gcc [-c|-S|-E] [-std=standard]
[-g] [-pg] [-Olevel]
[-Wwarn...] [-Wpedantic]
[-Idir...] [-Ldir...]
[-Dmacro[=defn]...] [-Umacro]
[-foption...] [-mmachine-option...]
[-o outfile] [@file] infile...
Only the most useful options are listed here; see below for the remainder. g++ accepts mostly the same options as
gcc.
DESCRIPTION
When you invoke GCC, it normally does preprocessing, compilation, assembly and linking. The "overall options" allow
you to stop this process at an intermediate stage. For example, the -c option says not to run the linker. Then the
output consists of object files output by the assembler.
Other options are passed on to one or more stages of processing. Some options control the preprocessor and others
the compiler itself. Yet other options control the assembler and linker; most of these are not documented here,
since you rarely need to use any of them.
Most of the command-line options that you can use with GCC are useful for C programs; when an option is only useful
with another language (usually C++), the explanation says so explicitly. If the description for a particular option
does not mention a source language, you can use that option with all supported languages.
The usual way to run GCC is to run the executable called gcc, or machine-gcc when cross-compiling, or
machine-gcc-version to run a specific version of GCC. When you compile C++ programs, you should invoke GCC as g++
instead.
The gcc program accepts options and file names as operands. Many options have multi-letter names; therefore multiple
single-letter options may not be grouped: -dv is very different from -d -v.
You can mix options and other arguments. For the most part, the order you use doesn't matter. Order does matter
when you use several options of the same kind; for example, if you specify -L more than once, the directories are
searched in the order specified. Also, the placement of the -l option is significant.
Many options have long names starting with -f or with -W---for example, -fmove-loop-invariants, -Wformat and so on.
Most of these have both positive and negative forms; the negative form of -ffoo is -fno-foo. This manual documents
only one of these two forms, whichever one is not the default.
Some options take one or more arguments typically separated either by a space or by the equals sign (=) from the
option name. Unless documented otherwise, an argument can be either numeric or a string. Numeric arguments must
typically be small unsigned decimal or hexadecimal integers. Hexadecimal arguments must begin with the 0x prefix.
Arguments to options that specify a size threshold of some sort may be arbitrarily large decimal or hexadecimal
integers followed by a byte size suffix designating a multiple of bytes such as "kB" and "KiB" for kilobyte and
kibibyte, respectively, "MB" and "MiB" for megabyte and mebibyte, "GB" and "GiB" for gigabyte and gigibyte, and so
on. Such arguments are designated by byte-size in the following text. Refer to the NIST, IEC, and other relevant
national and international standards for the full listing and explanation of the binary and decimal byte size
prefixes.
OPTIONS
Option Summary
Here is a summary of all the options, grouped by type. Explanations are in the following sections.
Overall Options
-c -S -E -o file -dumpbase dumpbase -dumpbase-ext auxdropsuf -dumpdir dumppfx -x language -v -###
--help[=class[,...]] --target-help --version -pass-exit-codes -pipe -specs=file -wrapper @file
-ffile-prefix-map=old=new -fcanon-prefix-map -fplugin=file -fplugin-arg-name=arg -fdump-ada-spec[-slim]
-fada-spec-parent=unit -fdump-go-spec=file
C Language Options
-ansi -std=standard -aux-info filename -fno-asm -fno-builtin -fno-builtin-function -fcond-mismatch
-ffreestanding -fgimple -fgnu-tm -fgnu89-inline -fhosted -flax-vector-conversions -fms-extensions
-fpermitted-flt-eval-methods=standard -fplan9-extensions -fsigned-bitfields -funsigned-bitfields -fsigned-char
-funsigned-char -fstrict-flex-arrays[=n] -fsso-struct=endianness
C++ Language Options
-fabi-version=n -fno-access-control -faligned-new=n -fno-assume-sane-operators-new-delete -fchar8_t
-fcheck-new -fconcepts -fconstexpr-depth=n -fconstexpr-cache-depth=n -fconstexpr-loop-limit=n
-fconstexpr-ops-limit=n -fno-elide-constructors -fno-enforce-eh-specs -fno-gnu-keywords -fno-immediate-escalation
-fno-implicit-templates -fno-implicit-inline-templates -fno-implement-inlines -fmodule-header[=kind]
-fmodule-only -fmodules -fmodule-implicit-inline -fno-module-lazy -fmodule-mapper=specification
-fmodule-version-ignore -fms-extensions -fnew-inheriting-ctors -fnew-ttp-matching -fno-nonansi-builtins
-fnothrow-opt -fno-operator-names -fno-optional-diags -fno-pretty-templates -frange-for-ext-temps -fno-rtti
-fsized-deallocation -fstrong-eval-order[=kind] -ftemplate-backtrace-limit=n -ftemplate-depth=n
-fno-threadsafe-statics -fuse-cxa-atexit -fno-weak -nostdinc++ -fvisibility-inlines-hidden
-fvisibility-ms-compat -fext-numeric-literals -flang-info-include-translate[=header]
-flang-info-include-translate-not -flang-info-module-cmi[=module] -stdlib=libstdc++,libc++ -Wabi-tag
-Wcatch-value -Wcatch-value=n -Wno-class-conversion -Wclass-memaccess -Wcomma-subscript
-Wconditionally-supported -Wno-conversion-null -Wctad-maybe-unsupported -Wctor-dtor-privacy
-Wdangling-reference -Wno-defaulted-function-deleted -Wno-delete-incomplete -Wdelete-non-virtual-dtor
-Wno-deprecated-array-compare -Wdeprecated-copy -Wdeprecated-copy-dtor -Wno-deprecated-enum-enum-conversion
-Wno-deprecated-enum-float-conversion -Weffc++ -Wno-elaborated-enum-base -Wno-exceptions -Wextra-semi
-Wno-global-module -Wno-inaccessible-base -Wno-inherited-variadic-ctor -Wno-init-list-lifetime
-Winvalid-constexpr -Winvalid-imported-macros -Wno-invalid-offsetof -Wno-literal-suffix -Wmismatched-new-delete
-Wmismatched-tags -Wmultiple-inheritance -Wnamespaces -Wnarrowing -Wnoexcept -Wnoexcept-type
-Wnon-virtual-dtor -Wpessimizing-move -Wno-placement-new -Wplacement-new=n -Wrange-loop-construct
-Wredundant-move -Wredundant-tags -Wreorder -Wregister -Wstrict-null-sentinel -Wno-subobject-linkage
-Wtemplates -Wno-non-template-friend -Wold-style-cast -Woverloaded-virtual -Wno-pmf-conversions -Wself-move
-Wsign-promo -Wsized-deallocation -Wsuggest-final-methods -Wsuggest-final-types -Wsuggest-override
-Wno-template-body -Wno-template-id-cdtor -Wtemplate-names-tu-local -Wno-terminate -Wno-vexing-parse
-Wvirtual-inheritance -Wno-virtual-move-assign -Wvolatile
Objective-C and Objective-C++ Language Options
-fconstant-string-class=class-name -fgnu-runtime -fnext-runtime -fno-nil-receivers -fobjc-abi-version=n
-fobjc-call-cxx-cdtors -fobjc-direct-dispatch -fobjc-exceptions -fobjc-gc -fobjc-nilcheck -fobjc-std=objc1
-fno-local-ivars -fivar-visibility=[public|protected|private|package] -freplace-objc-classes -fzero-link
-gen-decls -Wassign-intercept -Wno-property-assign-default -Wno-protocol -Wobjc-root-class -Wselector
-Wstrict-selector-match -Wundeclared-selector
OpenMP and OpenACC Options
-foffload=arg -foffload-options=arg -fopenacc -fopenacc-dim=geom -fopenmp -fopenmp-simd
-fopenmp-target-simd-clone[=device-type]
Diagnostic Message Formatting Options
-fmessage-length=n -fdiagnostics-plain-output -fdiagnostics-show-location=[once|every-line]
-fdiagnostics-color=[auto|never|always] -fdiagnostics-urls=[auto|never|always] -fdiagnostics-format=[text|sarif-
stderr|sarif-file|json|json-stderr|json-file] -fdiagnostics-add-output=DIAGNOSTICS-OUTPUT-SPEC
-fdiagnostics-set-output=DIAGNOSTICS-OUTPUT-SPEC -fno-diagnostics-json-formatting -fno-diagnostics-show-option
-fno-diagnostics-show-caret -fno-diagnostics-show-event-links -fno-diagnostics-show-labels
-fno-diagnostics-show-line-numbers -fno-diagnostics-show-cwe -fno-diagnostics-show-rules
-fno-diagnostics-show-highlight-colors -fdiagnostics-minimum-margin-width=width -fdiagnostics-parseable-fixits
-fdiagnostics-generate-patch -fdiagnostics-show-template-tree -fno-elide-type
-fdiagnostics-path-format=[none|separate-events|inline-events] -fdiagnostics-show-path-depths -fno-show-column
-fdiagnostics-column-unit=[display|byte] -fdiagnostics-column-origin=origin
-fdiagnostics-escape-format=[unicode|bytes] -fdiagnostics-text-art-charset=[none|ascii|unicode|emoji]
Warning Options
-fsyntax-only -fmax-errors=n -Wpedantic -pedantic-errors -fpermissive -w -Wextra -Wall -Wabi=n -Waddress
-Wno-address-of-packed-member -Waggregate-return -Walloc-size -Walloc-size-larger-than=byte-size -Walloc-zero
-Walloca -Walloca-larger-than=byte-size -Wno-aggressive-loop-optimizations -Warith-conversion -Warray-bounds
-Warray-bounds=n -Warray-compare -Warray-parameter -Warray-parameter=n -Wno-attributes -Wattribute-alias=n
-Wno-attribute-alias -Wno-attribute-warning -Wbidi-chars=[none|unpaired|any|ucn] -Wbool-compare -Wbool-operation
-Wno-builtin-declaration-mismatch -Wno-builtin-macro-redefined -Wc90-c99-compat -Wc99-c11-compat
-Wc11-c23-compat -Wc23-c2y-compat -Wc++-compat -Wc++11-compat -Wc++14-compat -Wc++17-compat -Wc++20-compat
-Wno-c++11-extensions -Wno-c++14-extensions -Wno-c++17-extensions -Wno-c++20-extensions -Wno-c++23-extensions
-Wcalloc-transposed-args -Wcast-align -Wcast-align=strict -Wcast-function-type -Wcast-qual -Wchar-subscripts
-Wclobbered -Wcomment -Wcompare-distinct-pointer-types -Wno-complain-wrong-lang -Wconversion
-Wno-coverage-mismatch -Wno-cpp -Wdangling-else -Wdangling-pointer -Wdangling-pointer=n -Wdate-time
-Wno-deprecated -Wno-deprecated-declarations -Wno-designated-init -Wdisabled-optimization
-Wno-discarded-array-qualifiers -Wno-discarded-qualifiers -Wno-div-by-zero -Wdouble-promotion
-Wduplicated-branches -Wduplicated-cond -Wempty-body -Wno-endif-labels -Wenum-compare -Wenum-conversion
-Wenum-int-mismatch -Werror -Werror=* -Wexpansion-to-defined -Wfatal-errors -Wflex-array-member-not-at-end
-Wfloat-conversion -Wfloat-equal -Wformat -Wformat=2 -Wno-format-contains-nul -Wno-format-extra-args
-Wformat-nonliteral -Wformat-overflow=n -Wformat-security -Wformat-signedness -Wformat-truncation=n
-Wformat-y2k -Wframe-address -Wframe-larger-than=byte-size -Wno-free-nonheap-object -Wheader-guard
-Wno-if-not-aligned -Wno-ignored-attributes -Wignored-qualifiers -Wno-incompatible-pointer-types -Whardened
-Wimplicit -Wimplicit-fallthrough -Wimplicit-fallthrough=n -Wno-implicit-function-declaration
-Wno-implicit-int -Winfinite-recursion -Winit-self -Winline -Wno-int-conversion -Wint-in-bool-context
-Wno-int-to-pointer-cast -Wno-invalid-memory-model -Winvalid-pch -Winvalid-utf8 -Wno-unicode
-Wjump-misses-init -Wlarger-than=byte-size -Wleading-whitespace=kind -Wlogical-not-parentheses -Wlogical-op
-Wlong-long -Wno-lto-type-mismatch -Wmain -Wmaybe-uninitialized -Wmemset-elt-size -Wmemset-transposed-args
-Wmisleading-indentation -Wmissing-attributes -Wmissing-braces -Wmissing-field-initializers
-Wmissing-format-attribute -Wmissing-include-dirs -Wmissing-noreturn -Wmusttail-local-addr
-Wmaybe-musttail-local-addr -Wno-missing-profile -Wno-multichar -Wmultistatement-macros -Wnonnull
-Wnonnull-compare -Wnormalized=[none|id|nfc|nfkc] -Wnull-dereference -Wno-odr -Wopenacc-parallelism -Wopenmp
-Wopenmp-simd -Wno-overflow -Woverlength-strings -Wno-override-init-side-effects -Wpacked
-Wno-packed-bitfield-compat -Wpacked-not-aligned -Wpadded -Wparentheses -Wno-pedantic-ms-format
-Wpointer-arith -Wno-pointer-compare -Wno-pointer-to-int-cast -Wno-pragmas -Wno-pragma-once-outside-header
-Wno-prio-ctor-dtor -Wno-psabi -Wredundant-decls -Wrestrict -Wno-return-local-addr -Wreturn-type
-Wno-scalar-storage-order -Wsequence-point -Wshadow -Wshadow=global -Wshadow=local -Wshadow=compatible-local
-Wno-shadow-ivar -Wno-shift-count-negative -Wno-shift-count-overflow -Wshift-negative-value -Wno-shift-overflow
-Wshift-overflow=n -Wsign-compare -Wsign-conversion -Wno-sizeof-array-argument -Wsizeof-array-div
-Wsizeof-pointer-div -Wsizeof-pointer-memaccess -Wstack-protector -Wstack-usage=byte-size -Wstrict-aliasing
-Wstrict-aliasing=n -Wstrict-overflow -Wstrict-overflow=n -Wstring-compare -Wno-stringop-overflow
-Wno-stringop-overread -Wno-stringop-truncation -Wstrict-flex-arrays -Wsuggest-attribute=attribute-name -Wswitch
-Wno-switch-bool -Wswitch-default -Wswitch-enum -Wno-switch-outside-range -Wno-switch-unreachable -Wsync-nand
-Wsystem-headers -Wtautological-compare -Wtrailing-whitespace -Wtrailing-whitespace=kind -Wtrampolines
-Wtrigraphs -Wtrivial-auto-var-init -Wno-tsan -Wtype-limits -Wundef -Wuninitialized -Wunknown-pragmas
-Wunsuffixed-float-constants -Wunterminated-string-initialization -Wunused -Wunused-but-set-parameter
-Wunused-but-set-variable -Wunused-const-variable -Wunused-const-variable=n -Wunused-function -Wunused-label
-Wunused-local-typedefs -Wunused-macros -Wunused-parameter -Wno-unused-result -Wunused-value -Wunused-variable
-Wuse-after-free -Wuse-after-free=n -Wuseless-cast -Wno-varargs -Wvariadic-macros
-Wvector-operation-performance -Wvla -Wvla-larger-than=byte-size -Wno-vla-larger-than -Wvolatile-register-var
-Wwrite-strings -Wno-xor-used-as-pow -Wzero-as-null-pointer-constant -Wzero-length-bounds
Static Analyzer Options
-fanalyzer -fanalyzer-call-summaries -fanalyzer-checker=name -fno-analyzer-feasibility -fanalyzer-fine-grained
-fanalyzer-show-events-in-system-headers -fno-analyzer-state-merge -fno-analyzer-state-purge
-fno-analyzer-suppress-followups -fanalyzer-transitivity -fno-analyzer-undo-inlining -fanalyzer-verbose-edges
-fanalyzer-verbose-state-changes -fanalyzer-verbosity=level -fdump-analyzer -fdump-analyzer-callgraph
-fdump-analyzer-exploded-graph -fdump-analyzer-exploded-nodes -fdump-analyzer-exploded-nodes-2
-fdump-analyzer-exploded-nodes-3 -fdump-analyzer-exploded-paths -fdump-analyzer-feasibility
-fdump-analyzer-infinite-loop -fdump-analyzer-json -fdump-analyzer-state-purge -fdump-analyzer-stderr
-fdump-analyzer-supergraph -fdump-analyzer-untracked -Wno-analyzer-double-fclose -Wno-analyzer-double-free
-Wno-analyzer-exposure-through-output-file -Wno-analyzer-exposure-through-uninit-copy
-Wno-analyzer-fd-access-mode-mismatch -Wno-analyzer-fd-double-close -Wno-analyzer-fd-leak
-Wno-analyzer-fd-phase-mismatch -Wno-analyzer-fd-type-mismatch -Wno-analyzer-fd-use-after-close
-Wno-analyzer-fd-use-without-check -Wno-analyzer-file-leak -Wno-analyzer-free-of-non-heap
-Wno-analyzer-imprecise-fp-arithmetic -Wno-analyzer-infinite-loop -Wno-analyzer-infinite-recursion
-Wno-analyzer-jump-through-null -Wno-analyzer-malloc-leak -Wno-analyzer-mismatching-deallocation
-Wno-analyzer-null-argument -Wno-analyzer-null-dereference -Wno-analyzer-out-of-bounds
-Wno-analyzer-overlapping-buffers -Wno-analyzer-possible-null-argument -Wno-analyzer-possible-null-dereference
-Wno-analyzer-putenv-of-auto-var -Wno-analyzer-shift-count-negative -Wno-analyzer-shift-count-overflow
-Wno-analyzer-stale-setjmp-buffer -Wno-analyzer-tainted-allocation-size -Wno-analyzer-tainted-assertion
-Wno-analyzer-tainted-array-index -Wno-analyzer-tainted-divisor -Wno-analyzer-tainted-offset
-Wno-analyzer-tainted-size -Wanalyzer-symbol-too-complex -Wanalyzer-too-complex
-Wno-analyzer-undefined-behavior-ptrdiff -Wno-analyzer-undefined-behavior-strtok
-Wno-analyzer-unsafe-call-within-signal-handler -Wno-analyzer-use-after-free
-Wno-analyzer-use-of-pointer-in-stale-stack-frame -Wno-analyzer-use-of-uninitialized-value
-Wno-analyzer-va-arg-type-mismatch -Wno-analyzer-va-list-exhausted -Wno-analyzer-va-list-leak
-Wno-analyzer-va-list-use-after-va-end -Wno-analyzer-write-to-const -Wno-analyzer-write-to-string-literal
C and Objective-C-only Warning Options
-Wbad-function-cast -Wdeprecated-non-prototype -Wfree-labels -Wmissing-declarations -Wmissing-parameter-name
-Wmissing-parameter-type -Wdeclaration-missing-parameter-type -Wmissing-prototypes
-Wmissing-variable-declarations -Wnested-externs -Wold-style-declaration -Wold-style-definition
-Wstrict-prototypes -Wtraditional -Wtraditional-conversion -Wdeclaration-after-statement -Wpointer-sign
Debugging Options
-g -glevel -gdwarf -gdwarf-version -gbtf -gctf -gctflevel -gprune-btf -gno-prune-btf -ggdb
-grecord-gcc-switches -gno-record-gcc-switches -gstrict-dwarf -gno-strict-dwarf -gas-loc-support
-gno-as-loc-support -gas-locview-support -gno-as-locview-support -gcodeview -gcolumn-info -gno-column-info
-gdwarf32 -gdwarf64 -gstatement-frontiers -gno-statement-frontiers -gvariable-location-views
-gno-variable-location-views -ginternal-reset-location-views -gno-internal-reset-location-views -ginline-points
-gno-inline-points -gvms -gz[=type] -gsplit-dwarf -gdescribe-dies -gno-describe-dies -fdebug-prefix-map=old=new
-fdebug-types-section -fno-eliminate-unused-debug-types -femit-struct-debug-baseonly -femit-struct-debug-reduced
-femit-struct-debug-detailed[=spec-list] -fno-eliminate-unused-debug-symbols -femit-class-debug-always
-fno-merge-debug-strings -fno-dwarf2-cfi-asm -fvar-tracking -fvar-tracking-assignments
Optimization Options
-faggressive-loop-optimizations -falign-functions[=n[:m:[n2[:m2]]]] -falign-jumps[=n[:m:[n2[:m2]]]]
-falign-labels[=n[:m:[n2[:m2]]]] -falign-loops[=n[:m:[n2[:m2]]]] -fmin-function-alignment=[n] -fno-allocation-dce
-fallow-store-data-races -fassociative-math -fauto-profile -fauto-profile[=path] -fauto-inc-dec
-fbranch-probabilities -fcaller-saves -fcombine-stack-adjustments -fconserve-stack -ffold-mem-offsets
-fcompare-elim -fcprop-registers -fcrossjumping -fcse-follow-jumps -fcse-skip-blocks -fcx-fortran-rules
-fcx-limited-range -fdata-sections -fdce -fdelayed-branch -fdelete-null-pointer-checks -fdevirtualize
-fdevirtualize-speculatively -fdevirtualize-at-ltrans -fdse -fearly-inlining -fipa-sra
-fexpensive-optimizations -ffat-lto-objects -ffast-math -ffinite-math-only -ffloat-store
-fexcess-precision=style -ffinite-loops -fforward-propagate -ffp-contract=style -ffunction-sections -fgcse
-fgcse-after-reload -fgcse-las -fgcse-lm -fgraphite-identity -fgcse-sm -fhoist-adjacent-loads
-fif-conversion -fif-conversion2 -findirect-inlining -finline-stringops[=fn] -finline-functions
-finline-functions-called-once -finline-limit=n -finline-small-functions -fipa-modref -fipa-cp -fipa-cp-clone
-fipa-bit-cp -fipa-vrp -fipa-pta -fipa-profile -fipa-pure-const -fipa-reference -fipa-reference-addressable
-fipa-reorder-for-locality -fipa-stack-alignment -fipa-icf -fira-algorithm=algorithm
-flate-combine-instructions -flifetime-dse -flive-patching=level -fira-region=region -fira-hoist-pressure
-fira-loop-pressure -fno-ira-share-save-slots -fno-ira-share-spill-slots -fisolate-erroneous-paths-dereference
-fisolate-erroneous-paths-attribute -fivopts -fkeep-inline-functions -fkeep-static-functions
-fkeep-static-consts -flimit-function-alignment -flive-range-shrinkage -floop-block -floop-interchange
-floop-strip-mine -floop-unroll-and-jam -floop-nest-optimize -floop-parallelize-all -flra-remat -flto
-flto-compression-level -flto-partition=alg -flto-incremental=path -flto-incremental-cache-size=n -fmalloc-dce
-fmerge-all-constants -fmerge-constants -fmodulo-sched -fmodulo-sched-allow-regmoves -fmove-loop-invariants
-fmove-loop-stores -fno-branch-count-reg -fno-defer-pop -fno-fp-int-builtin-inexact -fno-function-cse
-fno-guess-branch-probability -fno-inline -fno-math-errno -fno-peephole -fno-peephole2
-fno-printf-return-value -fno-sched-interblock -fno-sched-spec -fno-signed-zeros -fno-toplevel-reorder
-fno-trapping-math -fno-zero-initialized-in-bss -fomit-frame-pointer -foptimize-crc -foptimize-sibling-calls
-fpartial-inlining -fpeel-loops -fpredictive-commoning -fprefetch-loop-arrays -fprofile-correction
-fprofile-use -fprofile-use=path -fprofile-partial-training -fprofile-values -fprofile-reorder-functions
-freciprocal-math -free -frename-registers -freorder-blocks -freorder-blocks-algorithm=algorithm
-freorder-blocks-and-partition -freorder-functions -frerun-cse-after-loop -freschedule-modulo-scheduled-loops
-frounding-math -fsave-optimization-record -fsched2-use-superblocks -fsched-pressure -fsched-spec-load
-fsched-spec-load-dangerous -fsched-stalled-insns-dep[=n] -fsched-stalled-insns[=n] -fsched-group-heuristic
-fsched-critical-path-heuristic -fsched-spec-insn-heuristic -fsched-rank-heuristic -fsched-last-insn-heuristic
-fsched-dep-count-heuristic -fschedule-fusion -fschedule-insns -fschedule-insns2 -fsection-anchors
-fselective-scheduling -fselective-scheduling2 -fsel-sched-pipelining -fsel-sched-pipelining-outer-loops
-fsemantic-interposition -fshrink-wrap -fshrink-wrap-separate -fsignaling-nans -fsingle-precision-constant
-fsplit-ivs-in-unroller -fsplit-loops -fsplit-paths -fsplit-wide-types -fsplit-wide-types-early -fssa-backprop
-fssa-phiopt -fstdarg-opt -fstore-merging -fstrict-aliasing -fipa-strict-aliasing -fthread-jumps -ftracer
-ftree-bit-ccp -ftree-builtin-call-dce -ftree-ccp -ftree-ch -ftree-coalesce-vars -ftree-copy-prop
-ftree-cselim -ftree-dce -ftree-dominator-opts -ftree-dse -ftree-forwprop -ftree-fre -fcode-hoisting
-ftree-loop-if-convert -ftree-loop-im -ftree-phiprop -ftree-loop-distribution -ftree-loop-distribute-patterns
-ftree-loop-ivcanon -ftree-loop-linear -ftree-loop-optimize -ftree-loop-vectorize -ftree-parallelize-loops=n
-ftree-pre -ftree-partial-pre -ftree-pta -ftree-reassoc -ftree-scev-cprop -ftree-sink -ftree-slsr
-ftree-sra -ftree-switch-conversion -ftree-tail-merge -ftree-ter -ftree-vectorize -ftree-vrp
-ftrivial-auto-var-init -funconstrained-commons -funit-at-a-time -funroll-all-loops -funroll-loops
-funsafe-math-optimizations -funswitch-loops -fipa-ra -fvariable-expansion-in-unroller -fvect-cost-model
-fvpt -fweb -fwhole-program -fwpa -fuse-linker-plugin -fzero-call-used-regs --param name=value -O -O0 -O1
-O2 -O3 -Os -Ofast -Og -Oz
Program Instrumentation Options
-p -pg -fprofile-arcs --coverage -ftest-coverage -fcondition-coverage -fpath-coverage -fprofile-abs-path
-fprofile-dir=path -fprofile-generate -fprofile-generate=path -fprofile-info-section
-fprofile-info-section=name -fprofile-note=path -fprofile-prefix-path=path -fprofile-update=method
-fprofile-filter-files=regex -fprofile-exclude-files=regex -fprofile-reproducible=[multithreaded|parallel-
runs|serial] -fsanitize=style -fsanitize-recover -fsanitize-recover=style -fsanitize-trap
-fsanitize-trap=style -fasan-shadow-offset=number -fsanitize-sections=s1,s2,...
-fsanitize-undefined-trap-on-error -fbounds-check -fcf-protection
-fcf-protection=[full|branch|return|none|check] -fharden-compares -fharden-conditional-branches -fhardened
-fharden-control-flow-redundancy -fhardcfr-skip-leaf -fhardcfr-check-exceptions -fhardcfr-check-returning-calls
-fhardcfr-check-noreturn-calls=[always|no-xthrow|nothrow|never] -fstack-protector -fstack-protector-all
-fstack-protector-strong -fstack-protector-explicit -fstack-check -fstack-limit-register=reg
-fstack-limit-symbol=sym -fno-stack-limit -fsplit-stack -fstrub=disable -fstrub=strict -fstrub=relaxed
-fstrub=all -fstrub=at-calls -fstrub=internal -fvtable-verify=[std|preinit|none] -fvtv-counts -fvtv-debug
-finstrument-functions -finstrument-functions-once -finstrument-functions-exclude-function-list=sym,sym,...
-finstrument-functions-exclude-file-list=file,file,... -fprofile-prefix-map=old=new
-fpatchable-function-entry=N[,M]
Preprocessor Options
-Aquestion=answer -A-question[=answer] -C -CC -Dmacro[=defn] -dD -dI -dM -dN -dU -fdebug-cpp
-fdirectives-only -fdollars-in-identifiers -fexec-charset=charset -fextended-identifiers
-finput-charset=charset -fmacro-prefix-map=old=new -fmax-include-depth=depth -fno-canonical-system-headers
-fpch-deps -fpch-preprocess -fpreprocessed -ftabstop=width -ftrack-macro-expansion -fwide-exec-charset=charset
-fworking-directory -H -imacros file -include file -M -MD -MF -MG -MM -MMD -MP -MQ -MT -Mno-modules
-no-integrated-cpp -P -pthread -remap -traditional -traditional-cpp -trigraphs -Umacro -undef -Wp,option
-Xpreprocessor option
Assembler Options
-Wa,option -Xassembler option
Linker Options
object-file-name -fuse-ld=linker -llibrary -nostartfiles -nodefaultlibs -nolibc -nostdlib -nostdlib++ -e
entry --entry=entry -pie -pthread -r -rdynamic -s -static -static-pie -static-libgcc -static-libstdc++
-static-libasan -static-libtsan -static-liblsan -static-libubsan -shared -shared-libgcc -symbolic -T script
-Wl,option -Xlinker option -u symbol -z keyword
Directory Options
-Bprefix -Idir -I- -idirafter dir -imacros file -imultilib dir -iplugindir=dir -iprefix file -iquote dir
-isysroot dir -isystem dir -iwithprefix dir -iwithprefixbefore dir --embed-dir=dir -Ldir
-no-canonical-prefixes --no-sysroot-suffix -nostdinc -nostdinc++ --sysroot=dir
Code Generation Options
-fcall-saved-reg -fcall-used-reg -ffixed-reg -fexceptions -fnon-call-exceptions -fdelete-dead-exceptions
-funwind-tables -fasynchronous-unwind-tables -fno-gnu-unique -finhibit-size-directive -fcommon -fno-ident
-fpcc-struct-return -fpic -fPIC -fpie -fPIE -fno-plt -fno-jump-tables -fno-bit-tests -frecord-gcc-switches
-freg-struct-return -fshort-enums -fshort-wchar -fverbose-asm -fpack-struct[=n] -fleading-underscore
-ftls-model=model -fstack-reuse=reuse_level -ftrampolines -ftrampoline-impl=[stack|heap] -ftrapv -fwrapv
-fvisibility=[default|internal|hidden|protected] -fstrict-volatile-bitfields -fsync-libcalls
-fzero-init-padding-bits=value
Developer Options
-dletters -dumpspecs -dumpmachine -dumpversion -dumpfullversion -fcallgraph-info[=su,da] -fchecking
-fchecking=n -fdbg-cnt-list -fdbg-cnt=counter-value-list -fdisable-ipa-pass_name -fdisable-rtl-pass_name
-fdisable-rtl-pass-name=range-list -fdisable-tree-pass_name -fdisable-tree-pass-name=range-list -fdump-debug
-fdump-earlydebug -fdump-noaddr -fdump-unnumbered -fdump-unnumbered-links -fdump-final-insns[=file]
-fdump-ipa-all -fdump-ipa-cgraph -fdump-ipa-inline -fdump-lang-all -fdump-lang-switch
-fdump-lang-switch-options -fdump-lang-switch-options=filename -fdump-passes -fdump-rtl-pass
-fdump-rtl-pass=filename -fdump-statistics -fdump-tree-all -fdump-tree-switch -fdump-tree-switch-options
-fdump-tree-switch-options=filename -fcompare-debug[=opts] -fcompare-debug-second -fenable-kind-pass
-fenable-kind-pass=range-list -fira-verbose=n -flto-report -flto-report-wpa -fmem-report-wpa -fmem-report
-fpre-ipa-mem-report -fpost-ipa-mem-report -fopt-info -fopt-info-options[=file] -fmultiflags -fprofile-report
-frandom-seed=string -fsched-verbose=n -fsel-sched-verbose -fsel-sched-dump-cfg -fsel-sched-pipelining-verbose
-fstats -fstack-usage -ftime-report -ftime-report-details -fvar-tracking-assignments-toggle -gtoggle
-print-file-name=library -print-libgcc-file-name -print-multi-directory -print-multi-lib
-print-multi-os-directory -print-prog-name=program -print-search-dirs -Q -print-sysroot
-print-sysroot-headers-suffix -save-temps -save-temps=cwd -save-temps=obj -time[=file]
Machine-Dependent Options
AArch64 Options (AArch64 Options) -mabi=name -mbig-endian -mlittle-endian -mgeneral-regs-only -mcmodel=tiny
-mcmodel=small -mcmodel=large -mstrict-align -mno-strict-align -momit-leaf-frame-pointer -mtls-dialect=desc
-mtls-dialect=traditional -mtls-size=size -mfix-cortex-a53-835769 -mfix-cortex-a53-843419
-mlow-precision-recip-sqrt -mlow-precision-sqrt -mlow-precision-div -mpc-relative-literal-loads
-msign-return-address=scope -mbranch-protection=none|standard|pac-ret[+leaf +b-key]|bti|gcs -mharden-sls=opts
-march=name -mcpu=name -mtune=name -moverride=string -mverbose-cost-dump -mstack-protector-guard=guard
-mstack-protector-guard-reg=sysreg -mstack-protector-guard-offset=offset -mtrack-speculation -moutline-atomics
-mearly-ldp-fusion -mlate-ldp-fusion -Wexperimental-fmv-target
Adapteva Epiphany Options (Adapteva Epiphany Options) -mhalf-reg-file -mprefer-short-insn-regs -mbranch-cost=num
-mcmove -mnops=num -msoft-cmpsf -msplit-lohi -mpost-inc -mpost-modify -mstack-offset=num -mround-nearest
-mlong-calls -mshort-calls -msmall16 -mfp-mode=mode -mvect-double -max-vect-align=num -msplit-vecmove-early
-m1reg-reg
AMD GCN Options (AMD GCN Options) -march=gpu -mtune=gpu -mstack-size=bytes
ARC Options (ARC Options) -mbarrel-shifter -mjli-always -mcpu=cpu -mA6 -mARC600 -mA7 -mARC700 -mdpfp
-mdpfp-compact -mdpfp-fast -mno-dpfp-lrsr -mea -mno-mpy -mmul32x16 -mmul64 -matomic -mnorm -mspfp
-mspfp-compact -mspfp-fast -msimd -msoft-float -mswap -mcrc -mdsp-packa -mdvbf -mlock -mmac-d16 -mmac-24
-mrtsc -mswape -mtelephony -mxy -misize -mannotate-align -marclinux -marclinux_prof -mlong-calls
-mmedium-calls -msdata -mirq-ctrl-saved -mrgf-banked-regs -mlpc-width=width -G num -mvolatile-cache
-mtp-regno=regno -malign-call -mauto-modify-reg -mbbit-peephole -mno-brcc -mcase-vector-pcrel
-mcompact-casesi -mno-cond-exec -mearly-cbranchsi -mexpand-adddi -mindexed-loads -mlra -mlra-priority-none
-mlra-priority-compact -mlra-priority-noncompact -mmillicode -mmixed-code -mq-class -mRcq -mRcw
-msize-level=level -mtune=cpu -mmultcost=num -mcode-density-frame -munalign-prob-threshold=probability
-mmpy-option=multo -mdiv-rem -mcode-density -mll64 -mfpu=fpu -mrf16 -mbranch-index
ARM Options (ARM Options) -mapcs-frame -mno-apcs-frame -mabi=name -mapcs-stack-check -mno-apcs-stack-check
-mapcs-reentrant -mno-apcs-reentrant -mgeneral-regs-only -msched-prolog -mno-sched-prolog -mlittle-endian
-mbig-endian -mbe8 -mbe32 -mfloat-abi=name -mfp16-format=name -mthumb-interwork -mno-thumb-interwork -mcpu=name
-march=name -mfpu=name -mtune=name -mprint-tune-info -mstructure-size-boundary=n -mabort-on-noreturn
-mlong-calls -mno-long-calls -msingle-pic-base -mno-single-pic-base -mpic-register=reg -mnop-fun-dllimport
-mpoke-function-name -mthumb -marm -mflip-thumb -mtpcs-frame -mtpcs-leaf-frame -mcaller-super-interworking
-mcallee-super-interworking -mtp=name -mtls-dialect=dialect -mword-relocations -mfix-cortex-m3-ldrd
-mfix-cortex-a57-aes-1742098 -mfix-cortex-a72-aes-1655431 -munaligned-access -mneon-for-64bits -mslow-flash-data
-masm-syntax-unified -mrestrict-it -mverbose-cost-dump -mpure-code -mcmse -mfix-cmse-cve-2021-35465
-mstack-protector-guard=guard -mstack-protector-guard-offset=offset -mfdpic
-mbranch-protection=none|standard|pac-ret[+leaf] [+bti]|bti[+pac-ret[+leaf]]
AVR Options (AVR Options) -mmcu=mcu -mabsdata -maccumulate-args -mcvt -mbranch-cost=cost -mfuse-add=level
-mfuse-move=level -mfuse-move2 -mcall-prologues -mgas-isr-prologues -mint8 -mflmap -mdouble=bits
-mlong-double=bits -mno-call-main -mn_flash=size -mfract-convert-truncate -mno-interrupts -mmain-is-OS_task
-mrelax -mrmw -mstrict-X -mtiny-stack -mrodata-in-ram -msplit-bit-shift -msplit-ldst -mshort-calls
-mskip-bug -muse-nonzero-bits -nodevicelib -nodevicespecs -Waddr-space-convert -Wmisspelled-isr
Blackfin Options (Blackfin Options) -mcpu=cpu[-sirevision] -msim -momit-leaf-frame-pointer
-mno-omit-leaf-frame-pointer -mspecld-anomaly -mno-specld-anomaly -mcsync-anomaly -mno-csync-anomaly -mlow-64k
-mno-low64k -mstack-check-l1 -mid-shared-library -mno-id-shared-library -mshared-library-id=n
-mleaf-id-shared-library -mno-leaf-id-shared-library -msep-data -mno-sep-data -mlong-calls -mno-long-calls
-mfast-fp -minline-plt -mmulticore -mcorea -mcoreb -msdram -micplb
C6X Options (C6X Options) -mbig-endian -mlittle-endian -march=cpu -msim -msdata=sdata-type
CRIS Options (CRIS Options) -mcpu=cpu -march=cpu -mtune=cpu -mmax-stack-frame=n -metrax4 -metrax100 -mpdebug
-mcc-init -mno-side-effects -mstack-align -mdata-align -mconst-align -m32-bit -m16-bit -m8-bit
-mno-prologue-epilogue -melf -maout -sim -sim2 -mmul-bug-workaround -mno-mul-bug-workaround
C-SKY Options (C-SKY Options) -march=arch -mcpu=cpu -mbig-endian -EB -mlittle-endian -EL -mhard-float
-msoft-float -mfpu=fpu -mdouble-float -mfdivdu -mfloat-abi=name -melrw -mistack -mmp -mcp -mcache
-msecurity -mtrust -mdsp -medsp -mvdsp -mdiv -msmart -mhigh-registers -manchor -mpushpop -mmultiple-stld
-mconstpool -mstack-size -mccrt -mbranch-cost=n -mcse-cc -msched-prolog -msim
Cygwin and MinGW Options (Cygwin and MinGW Options) -mconsole -mcrtdll=library -mdll -mnop-fun-dllimport
-mthread -municode -mwin32 -mwindows -fno-set-stack-executable
Darwin Options (Darwin Options) -all_load -allowable_client -arch -arch_errors_fatal -arch_only -bind_at_load
-bundle -bundle_loader -client_name -compatibility_version -current_version -dead_strip -dependency-file
-dylib_file -dylinker_install_name -dynamic -dynamiclib -exported_symbols_list -filelist -flat_namespace
-force_cpusubtype_ALL -force_flat_namespace -headerpad_max_install_names -iframework -image_base -init
-install_name -keep_private_externs -multi_module -multiply_defined -multiply_defined_unused -noall_load
-no_dead_strip_inits_and_terms -nodefaultrpaths -nofixprebinding -nomultidefs -noprebind -noseglinkedit
-pagezero_size -prebind -prebind_all_twolevel_modules -private_bundle -read_only_relocs -sectalign
-sectobjectsymbols -whyload -seg1addr -sectcreate -sectobjectsymbols -sectorder -segaddr
-segs_read_only_addr -segs_read_write_addr -seg_addr_table -seg_addr_table_filename -seglinkedit -segprot
-segs_read_only_addr -segs_read_write_addr -single_module -static -sub_library -sub_umbrella
-twolevel_namespace -umbrella -undefined -unexported_symbols_list -weak_reference_mismatches -whatsloaded -F
-gused -gfull -mmacosx-version-min=version -mkernel -mone-byte-bool
DEC Alpha Options (DEC Alpha Options) -mno-fp-regs -msoft-float -mieee -mieee-with-inexact -mieee-conformant
-mfp-trap-mode=mode -mfp-rounding-mode=mode -mtrap-precision=mode -mbuild-constants -mcpu=cpu-type -mtune=cpu-
type -mbwx -mmax -mfix -mcix -msafe-bwa -msafe-partial -mfloat-vax -mfloat-ieee -mexplicit-relocs
-msmall-data -mlarge-data -msmall-text -mlarge-text -mmemory-latency=time
eBPF Options (eBPF Options) -mbig-endian -mlittle-endian -mframe-limit=bytes -mxbpf -mco-re -mno-co-re -mjmpext
-mjmp32 -malu32 -mv3-atomics -mbswap -msdiv -msmov -mcpu=version -masm=dialect -minline-memops-threshold=bytes
FR30 Options (FR30 Options) -msmall-model -mno-lsim
FT32 Options (FT32 Options) -msim -mlra -mnodiv -mft32b -mcompress -mnopm
FRV Options (FRV Options) -mgpr-32 -mgpr-64 -mfpr-32 -mfpr-64 -mhard-float -msoft-float -malloc-cc
-mfixed-cc -mdword -mno-dword -mdouble -mno-double -mmedia -mno-media -mmuladd -mno-muladd -mfdpic
-minline-plt -mgprel-ro -multilib-library-pic -mlinked-fp -mlong-calls -malign-labels -mlibrary-pic -macc-4
-macc-8 -mpack -mno-pack -mno-eflags -mcond-move -mno-cond-move -moptimize-membar -mno-optimize-membar -mscc
-mno-scc -mcond-exec -mno-cond-exec -mvliw-branch -mno-vliw-branch -mmulti-cond-exec -mno-multi-cond-exec
-mnested-cond-exec -mno-nested-cond-exec -mtomcat-stats -mTLS -mtls -mcpu=cpu
GNU/Linux Options (GNU/Linux Options) -mglibc -muclibc -mmusl -mbionic -mandroid -tno-android-cc
-tno-android-ld
H8/300 Options (H8/300 Options) -mrelax -mh -ms -mn -mexr -mno-exr -mint32 -malign-300
HPPA Options (HPPA Options) -march=architecture-type -matomic-libcalls -mbig-switch -mcaller-copies
-mdisable-fpregs -mdisable-indexing -mordered -mfast-indirect-calls -mgas -mgnu-ld -mhp-ld
-mfixed-range=register-range -mcoherent-ldcw -mjump-in-delay -mlinker-opt -mlong-calls -mlong-load-store
-mno-atomic-libcalls -mno-disable-fpregs -mno-disable-indexing -mno-fast-indirect-calls -mno-gas
-mno-jump-in-delay -mno-long-load-store -mno-portable-runtime -mno-soft-float -mno-space-regs -msoft-float
-mpa-risc-1-0 -mpa-risc-1-1 -mpa-risc-2-0 -mportable-runtime -mschedule=cpu-type -mspace-regs -msoft-mult
-msio -mwsio -munix=unix-std -nolibdld -static -threads
IA-64 Options (IA-64 Options) -mbig-endian -mlittle-endian -mgnu-as -mgnu-ld -mno-pic -mvolatile-asm-stop
-mregister-names -msdata -mno-sdata -mconstant-gp -mauto-pic -mfused-madd -minline-float-divide-min-latency
-minline-float-divide-max-throughput -mno-inline-float-divide -minline-int-divide-min-latency
-minline-int-divide-max-throughput -mno-inline-int-divide -minline-sqrt-min-latency -minline-sqrt-max-throughput
-mno-inline-sqrt -mdwarf2-asm -mearly-stop-bits -mfixed-range=register-range -mtls-size=tls-size -mtune=cpu-
type -milp32 -mlp64 -msched-br-data-spec -msched-ar-data-spec -msched-control-spec -msched-br-in-data-spec
-msched-ar-in-data-spec -msched-in-control-spec -msched-spec-ldc -msched-spec-control-ldc
-msched-prefer-non-data-spec-insns -msched-prefer-non-control-spec-insns -msched-stop-bits-after-every-cycle
-msched-count-spec-in-critical-path -msel-sched-dont-check-control-spec -msched-fp-mem-deps-zero-cost
-msched-max-memory-insns-hard-limit -msched-max-memory-insns=max-insns
LM32 Options (LM32 Options) -mbarrel-shift-enabled -mdivide-enabled -mmultiply-enabled -msign-extend-enabled
-muser-enabled
LoongArch Options (LoongArch Options) -march=arch-type -mtune=tune-type -mabi=base-abi-type -mfpu=fpu-type
-msimd=simd-type -msoft-float -msingle-float -mdouble-float -mlsx -mno-lsx -mlasx -mno-lasx -mbranch-cost=n
-maddr-reg-reg-cost=n -mcheck-zero-division -mno-check-zero-division -mcond-move-int -mno-cond-move-int
-mcond-move-float -mno-cond-move-float -memcpy -mno-memcpy -mstrict-align -mno-strict-align -G num
-mmax-inline-memcpy-size=n -mexplicit-relocs=style -mexplicit-relocs -mno-explicit-relocs -mdirect-extern-access
-mno-direct-extern-access -mcmodel=code-model -mrelax -mpass-mrelax-to-as -mrecip -mrecip=opt -mfrecipe
-mno-frecipe -mdiv32 -mno-div32 -mlam-bh -mno-lam-bh -mlamcas -mno-lamcas -mld-seq-sa -mno-ld-seq-sa
-mtls-dialect=opt -mannotate-tablejump -mno-annotate-tablejump
M32C Options (M32C Options) -mcpu=cpu -msim -memregs=number
M32R/D Options (M32R/D Options) -m32r2 -m32rx -m32r -mdebug -malign-loops -mno-align-loops -missue-rate=number
-mbranch-cost=number -mmodel=code-size-model-type -msdata=sdata-type -mno-flush-func -mflush-func=name
-mno-flush-trap -mflush-trap=number -G num
M680x0 Options (M680x0 Options) -march=arch -mcpu=cpu -mtune=tune -m68000 -m68020 -m68020-40 -m68020-60
-m68030 -m68040 -m68060 -mcpu32 -m5200 -m5206e -m528x -m5307 -m5407 -mcfv4e -mbitfield -mno-bitfield
-mc68000 -mc68020 -mnobitfield -mrtd -mno-rtd -mdiv -mno-div -mshort -mno-short -mhard-float -m68881
-msoft-float -mpcrel -malign-int -mstrict-align -msep-data -mno-sep-data -mshared-library-id=n
-mid-shared-library -mno-id-shared-library -mxgot -mno-xgot -mlong-jump-table-offsets
MCore Options (MCore Options) -mhardlit -mno-hardlit -mdiv -mno-div -mrelax-immediates -mno-relax-immediates
-mwide-bitfields -mno-wide-bitfields -m4byte-functions -mno-4byte-functions -mcallgraph-data
-mno-callgraph-data -mslow-bytes -mno-slow-bytes -mno-lsim -mlittle-endian -mbig-endian -m210 -m340
-mstack-increment
MicroBlaze Options (MicroBlaze Options) -msoft-float -mhard-float -msmall-divides -mcpu=cpu -mmemcpy
-mxl-soft-mul -mxl-soft-div -mxl-barrel-shift -mxl-pattern-compare -mxl-stack-check -mxl-gp-opt
-mno-clearbss -mxl-multiply-high -mxl-float-convert -mxl-float-sqrt -mbig-endian -mlittle-endian -mxl-reorder
-mxl-mode-app-model -mpic-data-is-text-relative
MIPS Options (MIPS Options) -EL -EB -march=arch -mtune=arch -mips1 -mips2 -mips3 -mips4 -mips32 -mips32r2
-mips32r3 -mips32r5 -mips32r6 -mips64 -mips64r2 -mips64r3 -mips64r5 -mips64r6 -mips16 -mno-mips16
-mflip-mips16 -minterlink-compressed -mno-interlink-compressed -minterlink-mips16 -mno-interlink-mips16
-mabi=abi -mabicalls -mno-abicalls -mshared -mno-shared -mplt -mno-plt -mxgot -mno-xgot -mgp32 -mgp64
-mfp32 -mfpxx -mfp64 -mhard-float -msoft-float -mno-float -msingle-float -mdouble-float -modd-spreg
-mno-odd-spreg -mabs=mode -mnan=encoding -mdsp -mno-dsp -mdspr2 -mno-dspr2 -mmcu -mmno-mcu -meva -mno-eva
-mvirt -mno-virt -mxpa -mno-xpa -mcrc -mno-crc -mginv -mno-ginv -mmicromips -mno-micromips -mmsa -mno-msa
-mloongson-mmi -mno-loongson-mmi -mloongson-ext -mno-loongson-ext -mloongson-ext2 -mno-loongson-ext2
-mfpu=fpu-type -msmartmips -mno-smartmips -mpaired-single -mno-paired-single -mdmx -mno-mdmx -mips3d
-mno-mips3d -mmt -mno-mt -mllsc -mno-llsc -mlong64 -mlong32 -msym32 -mno-sym32 -Gnum -mlocal-sdata
-mno-local-sdata -mextern-sdata -mno-extern-sdata -mgpopt -mno-gopt -membedded-data -mno-embedded-data
-muninit-const-in-rodata -mno-uninit-const-in-rodata -mcode-readable=setting -msplit-addresses
-mno-split-addresses -mexplicit-relocs -mno-explicit-relocs -mexplicit-relocs=release -mcheck-zero-division
-mno-check-zero-division -mdivide-traps -mdivide-breaks -mload-store-pairs -mno-load-store-pairs -mstrict-align
-mno-strict-align -mno-unaligned-access -munaligned-access -mmemcpy -mno-memcpy -mlong-calls -mno-long-calls
-mmad -mno-mad -mimadd -mno-imadd -mfused-madd -mno-fused-madd -nocpp -mfix-24k -mno-fix-24k -mfix-r4000
-mno-fix-r4000 -mfix-r4400 -mno-fix-r4400 -mfix-r5900 -mno-fix-r5900 -mfix-r10000 -mno-fix-r10000
-mfix-rm7000 -mno-fix-rm7000 -mfix-vr4120 -mno-fix-vr4120 -mfix-vr4130 -mno-fix-vr4130 -mfix-sb1
-mno-fix-sb1 -mflush-func=func -mno-flush-func -mbranch-cost=num -mbranch-likely -mno-branch-likely
-mcompact-branches=policy -mfp-exceptions -mno-fp-exceptions -mvr4130-align -mno-vr4130-align -msynci
-mno-synci -mlxc1-sxc1 -mno-lxc1-sxc1 -mmadd4 -mno-madd4 -mrelax-pic-calls -mno-relax-pic-calls
-mmcount-ra-address -mframe-header-opt -mno-frame-header-opt
MMIX Options (MMIX Options) -mlibfuncs -mno-libfuncs -mepsilon -mno-epsilon -mabi=gnu -mabi=mmixware
-mzero-extend -mknuthdiv -mtoplevel-symbols -melf -mbranch-predict -mno-branch-predict -mbase-addresses
-mno-base-addresses -msingle-exit -mno-single-exit
MN10300 Options (MN10300 Options) -mmult-bug -mno-mult-bug -mno-am33 -mam33 -mam33-2 -mam34 -mtune=cpu-type
-mreturn-pointer-on-d0 -mno-crt0 -mrelax -mliw -msetlb
Moxie Options (Moxie Options) -meb -mel -mmul.x -mno-crt0
MSP430 Options (MSP430 Options) -msim -masm-hex -mmcu= -mcpu= -mlarge -msmall -mrelax -mwarn-mcu
-mcode-region= -mdata-region= -msilicon-errata= -msilicon-errata-warn= -mhwmult= -minrt -mtiny-printf
-mmax-inline-shift=
NDS32 Options (NDS32 Options) -mbig-endian -mlittle-endian -mreduced-regs -mfull-regs -mcmov -mno-cmov
-mext-perf -mno-ext-perf -mext-perf2 -mno-ext-perf2 -mext-string -mno-ext-string -mv3push -mno-v3push -m16bit
-mno-16bit -misr-vector-size=num -mcache-block-size=num -march=arch -mcmodel=code-model -mctor-dtor -mrelax
Nvidia PTX Options (Nvidia PTX Options) -m64 -mmainkernel -moptimize
OpenRISC Options (OpenRISC Options) -mboard=name -mnewlib -mhard-mul -mhard-div -msoft-mul -msoft-div
-msoft-float -mhard-float -mdouble-float -munordered-float -mcmov -mror -mrori -msext -msfimm -mshftimm
-mcmodel=code-model
PDP-11 Options (PDP-11 Options) -mfpu -msoft-float -mac0 -mno-ac0 -m40 -m45 -m10 -mint32 -mno-int16
-mint16 -mno-int32 -msplit -munix-asm -mdec-asm -mgnu-asm -mlra
PowerPC Options See RS/6000 and PowerPC Options.
PRU Options (PRU Options) -mmcu=mcu -minrt -mno-relax -mloop -mabi=variant
RISC-V Options (RISC-V Options) -mbranch-cost=N-instruction -mplt -mno-plt -mabi=ABI-string -mfdiv -mno-fdiv
-mfence-tso -mno-fence-tso -mdiv -mno-div -misa-spec=ISA-spec-string -march=ISA-string -mtune=processor-string
-mpreferred-stack-boundary=num -msmall-data-limit=N-bytes -msave-restore -mno-save-restore -mshorten-memrefs
-mno-shorten-memrefs -mstrict-align -mno-strict-align -mcmodel=medlow -mcmodel=medany -mcmodel=large
-mexplicit-relocs -mno-explicit-relocs -mrelax -mno-relax -mriscv-attribute -mno-riscv-attribute
-malign-data=type -mbig-endian -mlittle-endian -mstack-protector-guard=guard -mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset -mcsr-check -mno-csr-check -mmovcc -mno-movcc -minline-atomics
-mno-inline-atomics -minline-strlen -mno-inline-strlen -minline-strcmp -mno-inline-strcmp -minline-strncmp
-mno-inline-strncmp -mtls-dialect=desc -mtls-dialect=trad
RL78 Options (RL78 Options) -msim -mmul=none -mmul=g13 -mmul=g14 -mallregs -mcpu=g10 -mcpu=g13 -mcpu=g14
-mg10 -mg13 -mg14 -m64bit-doubles -m32bit-doubles -msave-mduc-in-interrupts
RS/6000 and PowerPC Options (RS/6000 and PowerPC Options) -mcpu=cpu-type -mtune=cpu-type -mcmodel=code-model
-mpowerpc64 -maltivec -mno-altivec -mpowerpc-gpopt -mno-powerpc-gpopt -mpowerpc-gfxopt -mno-powerpc-gfxopt
-mmfcrf -mno-mfcrf -mpopcntb -mno-popcntb -mpopcntd -mno-popcntd -mfprnd -mno-fprnd -mcmpb -mno-cmpb
-mhard-dfp -mno-hard-dfp -mfull-toc -mminimal-toc -mno-fp-in-toc -mno-sum-in-toc -m64 -m32 -mxl-compat
-mno-xl-compat -mpe -malign-power -malign-natural -msoft-float -mhard-float -mmultiple -mno-multiple
-mupdate -mno-update -mavoid-indexed-addresses -mno-avoid-indexed-addresses -mfused-madd -mno-fused-madd
-mbit-align -mno-bit-align -mstrict-align -mno-strict-align -mrelocatable -mno-relocatable -mrelocatable-lib
-mno-relocatable-lib -mtoc -mno-toc -mlittle -mlittle-endian -mbig -mbig-endian -mdynamic-no-pic -mswdiv
-msingle-pic-base -mprioritize-restricted-insns=priority -msched-costly-dep=dependence_type
-minsert-sched-nops=scheme -mcall-aixdesc -mcall-eabi -mcall-freebsd -mcall-linux -mcall-netbsd
-mcall-openbsd -mcall-sysv -mcall-sysv-eabi -mcall-sysv-noeabi -mtraceback=traceback_type -maix-struct-return
-msvr4-struct-return -mabi=abi-type -msecure-plt -mbss-plt -msplit-patch-nops -mlongcall -mno-longcall
-mpltseq -mno-pltseq -mblock-move-inline-limit=num -mblock-compare-inline-limit=num
-mblock-compare-inline-loop-limit=num -mno-block-ops-unaligned-vsx -mstring-compare-inline-limit=num -misel
-mno-isel -mvrsave -mno-vrsave -mmulhw -mno-mulhw -mdlmzb -mno-dlmzb -mprototype -mno-prototype -msim -mmvme
-mads -myellowknife -memb -msdata -msdata=opt -mreadonly-in-sdata -mvxworks -G num -mrecip -mrecip=opt
-mno-recip -mrecip-precision -mno-recip-precision -mveclibabi=type -mfriz -mno-friz
-mpointers-to-nested-functions -mno-pointers-to-nested-functions -msave-toc-indirect -mno-save-toc-indirect
-mpower8-fusion -mno-mpower8-fusion -mcrypto -mno-crypto -mhtm -mno-htm -mquad-memory -mno-quad-memory
-mquad-memory-atomic -mno-quad-memory-atomic -mcompat-align-parm -mno-compat-align-parm -mfloat128
-mno-float128 -mfloat128-hardware -mno-float128-hardware -mgnu-attribute -mno-gnu-attribute
-mstack-protector-guard=guard -mstack-protector-guard-reg=reg -mstack-protector-guard-offset=offset -mprefixed
-mno-prefixed -mpcrel -mno-pcrel -mmma -mno-mmma -mrop-protect -mno-rop-protect -mprivileged -mno-privileged
RX Options (RX Options) -m64bit-doubles -m32bit-doubles -fpu -nofpu -mcpu= -mbig-endian-data
-mlittle-endian-data -msmall-data -msim -mno-sim -mas100-syntax -mno-as100-syntax -mrelax -mmax-constant-size=
-mint-register= -mpid -mallow-string-insns -mno-allow-string-insns -mjsr -mno-warn-multiple-fast-interrupts
-msave-acc-in-interrupts
S/390 and zSeries Options (S/390 and zSeries Options) -mtune=cpu-type -march=cpu-type -mhard-float -msoft-float
-mhard-dfp -mno-hard-dfp -mlong-double-64 -mlong-double-128 -mbackchain -mno-backchain -mpacked-stack
-mno-packed-stack -msmall-exec -mno-small-exec -mmvcle -mno-mvcle -m64 -m31 -mdebug -mno-debug -mesa
-mzarch -mhtm -mvx -mzvector -mtpf-trace -mno-tpf-trace -mtpf-trace-skip -mno-tpf-trace-skip -mfused-madd
-mno-fused-madd -mwarn-framesize -mwarn-dynamicstack -mstack-size -mstack-guard -mhotpatch=halfwords,halfwords
SH Options (SH Options) -m1 -m2 -m2e -m2a-nofpu -m2a-single-only -m2a-single -m2a -m3 -m3e -m4-nofpu
-m4-single-only -m4-single -m4 -m4a-nofpu -m4a-single-only -m4a-single -m4a -m4al -mb -ml -mdalign
-mrelax -mbigtable -mfmovd -mrenesas -mno-renesas -mnomacsave -mieee -mno-ieee -mbitops -misize
-minline-ic_invalidate -mpadstruct -mprefergot -musermode -multcost=number -mdiv=strategy
-mdivsi3_libfunc=name -mfixed-range=register-range -maccumulate-outgoing-args -matomic-model=atomic-model
-mbranch-cost=num -mzdcbranch -mno-zdcbranch -mcbranch-force-delay-slot -mfused-madd -mno-fused-madd -mfsca
-mno-fsca -mfsrra -mno-fsrra -mpretend-cmove -mtas
Solaris 2 Options (Solaris 2 Options) -mclear-hwcap -mno-clear-hwcap -mimpure-text -mno-impure-text -pthreads
SPARC Options (SPARC Options) -mcpu=cpu-type -mtune=cpu-type -mcmodel=code-model -mmemory-model=mem-model -m32
-m64 -mapp-regs -mno-app-regs -mfaster-structs -mno-faster-structs -mflat -mno-flat -mfpu -mno-fpu
-mhard-float -msoft-float -mhard-quad-float -msoft-quad-float -mstack-bias -mno-stack-bias -mstd-struct-return
-mno-std-struct-return -munaligned-doubles -mno-unaligned-doubles -muser-mode -mno-user-mode -mv8plus
-mno-v8plus -mvis -mno-vis -mvis2 -mno-vis2 -mvis3 -mno-vis3 -mvis3b -mno-vis3b -mvis4 -mno-vis4 -mvis4b
-mno-vis4b -mcbcond -mno-cbcond -mfmaf -mno-fmaf -mfsmuld -mno-fsmuld -mpopc -mno-popc -msubxc -mno-subxc
-mfix-at697f -mfix-ut699 -mfix-ut700 -mfix-gr712rc
System V Options (System V Options) -Qy -Qn -YP,paths -Ym,dir
V850 Options (V850 Options) -mlong-calls -mno-long-calls -mep -mno-ep -mprolog-function -mno-prolog-function
-mspace -mtda=n -msda=n -mzda=n -mapp-regs -mno-app-regs -mdisable-callt -mno-disable-callt -mv850e2v3
-mv850e2 -mv850e1 -mv850es -mv850e -mv850 -mv850e3v5 -mloop -mrelax -mlong-jumps -msoft-float -mhard-float
-mgcc-abi -mrh850-abi -mbig-switch
VAX Options (VAX Options) -munix -mgnu -md -md-float -mg -mg-float -mlra
Visium Options (Visium Options) -mdebug -msim -mfpu -mno-fpu -mhard-float -msoft-float -mcpu=cpu-type
-mtune=cpu-type -msv-mode -muser-mode
VMS Options (VMS Options) -mvms-return-codes -mdebug-main=prefix -mmalloc64 -mpointer-size=size
VxWorks Options (VxWorks Options) -mrtp -msmp -non-static -Bstatic -Bdynamic -Xbind-lazy -Xbind-now
x86 Options (x86 Options) -mtune=cpu-type -march=cpu-type -mtune-ctrl=feature-list -mdump-tune-features
-mno-default -mfpmath=unit -masm=dialect -mno-fancy-math-387 -mno-fp-ret-in-387 -m80387 -mhard-float
-msoft-float -mno-wide-multiply -mrtd -malign-double -mpreferred-stack-boundary=num
-mincoming-stack-boundary=num -mcld -mcx16 -msahf -mmovbe -mcrc32 -mmwait -mrecip -mrecip=opt -mvzeroupper
-mprefer-avx128 -mprefer-vector-width=opt -mpartial-vector-fp-math -mmove-max=bits -mstore-max=bits
-mnoreturn-no-callee-saved-registers -mmmx -msse -msse2 -msse3 -mssse3 -msse4.1 -msse4.2 -msse4 -mavx
-mavx2 -mavx512f -mavx512cd -mavx512vl -mavx512bw -mavx512dq -mavx512ifma -mavx512vbmi -msha -maes
-mpclmul -mfsgsbase -mrdrnd -mf16c -mfma -mpconfig -mwbnoinvd -mptwrite -mclflushopt -mclwb -mxsavec
-mxsaves -msse4a -m3dnow -m3dnowa -mpopcnt -mabm -mbmi -mtbm -mfma4 -mxop -madx -mlzcnt -mbmi2 -mfxsr
-mxsave -mxsaveopt -mrtm -mhle -mlwp -mmwaitx -mclzero -mpku -mthreads -mgfni -mvaes -mwaitpkg -mshstk
-mmanual-endbr -mcet-switch -mforce-indirect-call -mavx512vbmi2 -mavx512bf16 -menqcmd -mvpclmulqdq
-mavx512bitalg -mmovdiri -mmovdir64b -mavx512vpopcntdq -mavx512vnni -mprfchw -mrdpid -mrdseed -msgx
-mavx512vp2intersect -mserialize -mtsxldtrk -mamx-tile -mamx-int8 -mamx-bf16 -muintr -mhreset -mavxvnni
-mamx-fp8 -mavx512fp16 -mavxifma -mavxvnniint8 -mavxneconvert -mcmpccxadd -mamx-fp16 -mprefetchi -mraoint
-mamx-complex -mavxvnniint16 -msm3 -msha512 -msm4 -mapxf -musermsr -mavx10.1 -mavx10.1-256 -mavx10.1-512
-mevex512 -mavx10.2 -mamx-avx512 -mamx-tf32 -mamx-transpose -mmovrs -mamx-movrs -mcldemote -mms-bitfields
-mno-align-stringops -minline-all-stringops -minline-stringops-dynamically -mstringop-strategy=alg -mkl
-mwidekl -mmemcpy-strategy=strategy -mmemset-strategy=strategy -mpush-args -maccumulate-outgoing-args
-m128bit-long-double -m96bit-long-double -mlong-double-64 -mlong-double-80 -mlong-double-128 -mregparm=num
-msseregparm -mveclibabi=type -mvect8-ret-in-mem -mpc32 -mpc64 -mpc80 -mdaz-ftz -mstackrealign
-momit-leaf-frame-pointer -mno-red-zone -mno-tls-direct-seg-refs -mcmodel=code-model -mabi=name
-maddress-mode=mode -m32 -m64 -mx32 -m16 -miamcu -mlarge-data-threshold=num -msse2avx -mfentry
-mrecord-mcount -mnop-mcount -m8bit-idiv -minstrument-return=type -mfentry-name=name -mfentry-section=name
-mavx256-split-unaligned-load -mavx256-split-unaligned-store -malign-data=type -mstack-protector-guard=guard
-mstack-protector-guard-reg=reg -mstack-protector-guard-offset=offset -mstack-protector-guard-symbol=symbol
-mgeneral-regs-only -mcall-ms2sysv-xlogues -mrelax-cmpxchg-loop -mindirect-branch=choice
-mfunction-return=choice -mindirect-branch-register -mharden-sls=choice -mindirect-branch-cs-prefix -mneeded
-mno-direct-extern-access -munroll-only-small-loops -mlam=choice
x86 Windows Options See Cygwin and MinGW Options.
Xstormy16 Options (Xstormy16 Options) -msim
Xtensa Options (Xtensa Options) -mconst16 -mno-const16 -mfused-madd -mno-fused-madd -mforce-no-pic
-mserialize-volatile -mno-serialize-volatile -mtext-section-literals -mno-text-section-literals -mauto-litpools
-mno-auto-litpools -mtarget-align -mno-target-align -mlongcalls -mno-longcalls -mabi=abi-type
-mextra-l32r-costs=cycles -mstrict-align -mno-strict-align
zSeries Options See S/390 and zSeries Options.
Options Controlling the Kind of Output
Compilation can involve up to four stages: preprocessing, compilation proper, assembly and linking, always in that
order. GCC is capable of preprocessing and compiling several files either into several assembler input files, or
into one assembler input file; then each assembler input file produces an object file, and linking combines all the
object files (those newly compiled, and those specified as input) into an executable file.
For any given input file, the file name suffix determines what kind of compilation is done:
file.c
C source code that must be preprocessed.
file.i
C source code that should not be preprocessed.
file.ii
C++ source code that should not be preprocessed.
file.m
Objective-C source code. Note that you must link with the libobjc library to make an Objective-C program work.
file.mi
Objective-C source code that should not be preprocessed.
file.mm
file.M
Objective-C++ source code. Note that you must link with the libobjc library to make an Objective-C++ program
work. Note that .M refers to a literal capital M.
file.mii
Objective-C++ source code that should not be preprocessed.
file.h
C, C++, Objective-C or Objective-C++ header file to be turned into a precompiled header (default), or C, C++
header file to be turned into an Ada spec (via the -fdump-ada-spec switch).
file.cc
file.cp
file.cxx
file.cpp
file.CPP
file.c++
file.C
C++ source code that must be preprocessed. Note that in .cxx, the last two letters must both be literally x.
Likewise, .C refers to a literal capital C.
file.mm
file.M
Objective-C++ source code that must be preprocessed.
file.mii
Objective-C++ source code that should not be preprocessed.
file.hh
file.H
file.hp
file.hxx
file.hpp
file.HPP
file.h++
file.tcc
C++ header file to be turned into a precompiled header or Ada spec.
file.f
file.for
file.ftn
file.fi
Fixed form Fortran source code that should not be preprocessed.
file.F
file.FOR
file.fpp
file.FPP
file.FTN
Fixed form Fortran source code that must be preprocessed (with the traditional preprocessor).
file.f90
file.f95
file.f03
file.f08
file.fii
Free form Fortran source code that should not be preprocessed.
file.F90
file.F95
file.F03
file.F08
Free form Fortran source code that must be preprocessed (with the traditional preprocessor).
file.cob
file.COB
file.cbl
file.CBL
COBOL source code.
file.go
Go source code.
file.d
D source code.
file.di
D interface file.
file.dd
D documentation code (Ddoc).
file.ads
Ada source code file that contains a library unit declaration (a declaration of a package, subprogram, or
generic, or a generic instantiation), or a library unit renaming declaration (a package, generic, or subprogram
renaming declaration). Such files are also called specs.
file.adb
Ada source code file containing a library unit body (a subprogram or package body). Such files are also called
bodies.
file.s
Assembler code.
file.S
file.sx
Assembler code that must be preprocessed.
other
An object file to be fed straight into linking. Any file name with no recognized suffix is treated this way.
You can specify the input language explicitly with the -x option:
-x language
Specify explicitly the language for the following input files (rather than letting the compiler choose a default
based on the file name suffix). This option applies to all following input files until the next -x option.
Possible values for language are:
c c-header cpp-output
c++ c++-header c++-system-header c++-user-header c++-cpp-output
objective-c objective-c-header objective-c-cpp-output objc-cpp-output
objective-c++ objective-c++-header objective-c++-cpp-output
objc++-cpp-output
assembler assembler-with-cpp
ada adascil adawhy
cobol
d
f77 f77-cpp-input f95 f95-cpp-input
go
modula-2 modula-2-cpp-output
rust
lto
Note that -x does not imply a particular language standard. For example -x f77 may also require -std=legacy for
some older source codes.
-x none
Turn off any specification of a language, so that subsequent files are handled according to their file name
suffixes (as if -x has not been used at all).
If you only want some of the stages of compilation, you can use -x (or filename suffixes) to tell gcc where to start,
and one of the options -c, -S, or -E to say where gcc is to stop. Note that some combinations (for example, -x cpp-
output -E) instruct gcc to do nothing at all.
-c Compile or assemble the source files, but do not link. The linking stage simply is not done. The ultimate
output is in the form of an object file for each source file.
By default, the object file name for a source file is made by replacing the suffix .c, .i, .s, etc., with .o.
Unrecognized input files, not requiring compilation or assembly, are ignored.
-S Stop after the stage of compilation proper; do not assemble. The output is in the form of an assembler code file
for each non-assembler input file specified.
By default, the assembler file name for a source file is made by replacing the suffix .c, .i, etc., with .s.
Input files that don't require compilation are ignored.
-E Stop after the preprocessing stage; do not run the compiler proper. The output is in the form of preprocessed
source code, which is sent to the standard output.
Input files that don't require preprocessing are ignored.
-o file
Place the primary output in file file. This applies to whatever sort of output is being produced, whether it be
an executable file, an object file, an assembler file or preprocessed C code.
If -o is not specified, the default is to put an executable file in a.out, the object file for source.suffix in
source.o, its assembler file in source.s, a precompiled header file in source.suffix.gch, and all preprocessed C
source on standard output.
Though -o names only the primary output, it also affects the naming of auxiliary and dump outputs. See the
examples below. Unless overridden, both auxiliary outputs and dump outputs are placed in the same directory as
the primary output. In auxiliary outputs, the suffix of the input file is replaced with that of the auxiliary
output file type; in dump outputs, the suffix of the dump file is appended to the input file suffix. In
compilation commands, the base name of both auxiliary and dump outputs is that of the primary output; in compile
and link commands, the primary output name, minus the executable suffix, is combined with the input file name.
If both share the same base name, disregarding the suffix, the result of the combination is that base name,
otherwise, they are concatenated, separated by a dash.
gcc -c foo.c ...
will use foo.o as the primary output, and place aux outputs and dumps next to it, e.g., aux file foo.dwo for
-gsplit-dwarf, and dump file foo.c.???r.final for -fdump-rtl-final.
If a non-linker output file is explicitly specified, aux and dump files by default take the same base name:
gcc -c foo.c -o dir/foobar.o ...
will name aux outputs dir/foobar.* and dump outputs dir/foobar.c.*.
A linker output will instead prefix aux and dump outputs:
gcc foo.c bar.c -o dir/foobar ...
will generally name aux outputs dir/foobar-foo.* and dir/foobar-bar.*, and dump outputs dir/foobar-foo.c.* and
dir/foobar-bar.c.*.
The one exception to the above is when the executable shares the base name with the single input:
gcc foo.c -o dir/foo ...
in which case aux outputs are named dir/foo.* and dump outputs named dir/foo.c.*.
The location and the names of auxiliary and dump outputs can be adjusted by the options -dumpbase, -dumpbase-ext,
-dumpdir, -save-temps=cwd, and -save-temps=obj.
-dumpbase dumpbase
This option sets the base name for auxiliary and dump output files. It does not affect the name of the primary
output file. Intermediate outputs, when preserved, are not regarded as primary outputs, but as auxiliary
outputs:
gcc -save-temps -S foo.c
saves the (no longer) temporary preprocessed file in foo.i, and then compiles to the (implied) output file foo.s,
whereas:
gcc -save-temps -dumpbase save-foo -c foo.c
preprocesses to in save-foo.i, compiles to save-foo.s (now an intermediate, thus auxiliary output), and then
assembles to the (implied) output file foo.o.
Absent this option, dump and aux files take their names from the input file, or from the (non-linker) output
file, if one is explicitly specified: dump output files (e.g. those requested by -fdump-* options) with the input
name suffix, and aux output files (those requested by other non-dump options, e.g. "-save-temps",
"-gsplit-dwarf", "-fcallgraph-info") without it.
Similar suffix differentiation of dump and aux outputs can be attained for explicitly-given -dumpbase
basename.suf by also specifying -dumpbase-ext .suf.
If dumpbase is explicitly specified with any directory component, any dumppfx specification (e.g. -dumpdir or
-save-temps=*) is ignored, and instead of appending to it, dumpbase fully overrides it:
gcc foo.c -c -o dir/foo.o -dumpbase alt/foo \
-dumpdir pfx- -save-temps=cwd ...
creates auxiliary and dump outputs named alt/foo.*, disregarding dir/ in -o, the ./ prefix implied by
-save-temps=cwd, and pfx- in -dumpdir.
When -dumpbase is specified in a command that compiles multiple inputs, or that compiles and then links, it may
be combined with dumppfx, as specified under -dumpdir. Then, each input file is compiled using the combined
dumppfx, and default values for dumpbase and auxdropsuf are computed for each input file:
gcc foo.c bar.c -c -dumpbase main ...
creates foo.o and bar.o as primary outputs, and avoids overwriting the auxiliary and dump outputs by using the
dumpbase as a prefix, creating auxiliary and dump outputs named main-foo.* and main-bar.*.
An empty string specified as dumpbase avoids the influence of the output basename in the naming of auxiliary and
dump outputs during compilation, computing default values :
gcc -c foo.c -o dir/foobar.o -dumpbase " ...
will name aux outputs dir/foo.* and dump outputs dir/foo.c.*. Note how their basenames are taken from the input
name, but the directory still defaults to that of the output.
The empty-string dumpbase does not prevent the use of the output basename for outputs during linking:
gcc foo.c bar.c -o dir/foobar -dumpbase " -flto ...
The compilation of the source files will name auxiliary outputs dir/foo.* and dir/bar.*, and dump outputs
dir/foo.c.* and dir/bar.c.*. LTO recompilation during linking will use dir/foobar. as the prefix for dumps and
auxiliary files.
-dumpbase-ext auxdropsuf
When forming the name of an auxiliary (but not a dump) output file, drop trailing auxdropsuf from dumpbase before
appending any suffixes. If not specified, this option defaults to the suffix of a default dumpbase, i.e., the
suffix of the input file when -dumpbase is not present in the command line, or dumpbase is combined with dumppfx.
gcc foo.c -c -o dir/foo.o -dumpbase x-foo.c -dumpbase-ext .c ...
creates dir/foo.o as the main output, and generates auxiliary outputs in dir/x-foo.*, taking the location of the
primary output, and dropping the .c suffix from the dumpbase. Dump outputs retain the suffix: dir/x-foo.c.*.
This option is disregarded if it does not match the suffix of a specified dumpbase, except as an alternative to
the executable suffix when appending the linker output base name to dumppfx, as specified below:
gcc foo.c bar.c -o main.out -dumpbase-ext .out ...
creates main.out as the primary output, and avoids overwriting the auxiliary and dump outputs by using the
executable name minus auxdropsuf as a prefix, creating auxiliary outputs named main-foo.* and main-bar.* and dump
outputs named main-foo.c.* and main-bar.c.*.
-dumpdir dumppfx
When forming the name of an auxiliary or dump output file, use dumppfx as a prefix:
gcc -dumpdir pfx- -c foo.c ...
creates foo.o as the primary output, and auxiliary outputs named pfx-foo.*, combining the given dumppfx with the
default dumpbase derived from the default primary output, derived in turn from the input name. Dump outputs also
take the input name suffix: pfx-foo.c.*.
If dumppfx is to be used as a directory name, it must end with a directory separator:
gcc -dumpdir dir/ -c foo.c -o obj/bar.o ...
creates obj/bar.o as the primary output, and auxiliary outputs named dir/bar.*, combining the given dumppfx with
the default dumpbase derived from the primary output name. Dump outputs also take the input name suffix:
dir/bar.c.*.
It defaults to the location of the output file, unless the output file is a special file like "/dev/null".
Options -save-temps=cwd and -save-temps=obj override this default, just like an explicit -dumpdir option. In
case multiple such options are given, the last one prevails:
gcc -dumpdir pfx- -c foo.c -save-temps=obj ...
outputs foo.o, with auxiliary outputs named foo.* because -save-temps=* overrides the dumppfx given by the
earlier -dumpdir option. It does not matter that =obj is the default for -save-temps, nor that the output
directory is implicitly the current directory. Dump outputs are named foo.c.*.
When compiling from multiple input files, if -dumpbase is specified, dumpbase, minus a auxdropsuf suffix, and a
dash are appended to (or override, if containing any directory components) an explicit or defaulted dumppfx, so
that each of the multiple compilations gets differently-named aux and dump outputs.
gcc foo.c bar.c -c -dumpdir dir/pfx- -dumpbase main ...
outputs auxiliary dumps to dir/pfx-main-foo.* and dir/pfx-main-bar.*, appending dumpbase- to dumppfx. Dump
outputs retain the input file suffix: dir/pfx-main-foo.c.* and dir/pfx-main-bar.c.*, respectively. Contrast
with the single-input compilation:
gcc foo.c -c -dumpdir dir/pfx- -dumpbase main ...
that, applying -dumpbase to a single source, does not compute and append a separate dumpbase per input file. Its
auxiliary and dump outputs go in dir/pfx-main.*.
When compiling and then linking from multiple input files, a defaulted or explicitly specified dumppfx also
undergoes the dumpbase- transformation above (e.g. the compilation of foo.c and bar.c above, but without -c). If
neither -dumpdir nor -dumpbase are given, the linker output base name, minus auxdropsuf, if specified, or the
executable suffix otherwise, plus a dash is appended to the default dumppfx instead. Note, however, that unlike
earlier cases of linking:
gcc foo.c bar.c -dumpdir dir/pfx- -o main ...
does not append the output name main to dumppfx, because -dumpdir is explicitly specified. The goal is that the
explicitly-specified dumppfx may contain the specified output name as part of the prefix, if desired; only an
explicitly-specified -dumpbase would be combined with it, in order to avoid simply discarding a meaningful
option.
When compiling and then linking from a single input file, the linker output base name will only be appended to
the default dumppfx as above if it does not share the base name with the single input file name. This has been
covered in single-input linking cases above, but not with an explicit -dumpdir that inhibits the combination,
even if overridden by -save-temps=*:
gcc foo.c -dumpdir alt/pfx- -o dir/main.exe -save-temps=cwd ...
Auxiliary outputs are named foo.*, and dump outputs foo.c.*, in the current working directory as ultimately
requested by -save-temps=cwd.
Summing it all up for an intuitive though slightly imprecise data flow: the primary output name is broken into a
directory part and a basename part; dumppfx is set to the former, unless overridden by -dumpdir or -save-temps=*,
and dumpbase is set to the latter, unless overriden by -dumpbase. If there are multiple inputs or linking, this
dumpbase may be combined with dumppfx and taken from each input file. Auxiliary output names for each input are
formed by combining dumppfx, dumpbase minus suffix, and the auxiliary output suffix; dump output names are only
different in that the suffix from dumpbase is retained.
When it comes to auxiliary and dump outputs created during LTO recompilation, a combination of dumppfx and
dumpbase, as given or as derived from the linker output name but not from inputs, even in cases in which this
combination would not otherwise be used as such, is passed down with a trailing period replacing the compiler-
added dash, if any, as a -dumpdir option to lto-wrapper; being involved in linking, this program does not
normally get any -dumpbase and -dumpbase-ext, and it ignores them.
When running sub-compilers, lto-wrapper appends LTO stage names to the received dumppfx, ensures it contains a
directory component so that it overrides any -dumpdir, and passes that as -dumpbase to sub-compilers.
-v Print (on standard error output) the commands executed to run the stages of compilation. Also print the version
number of the compiler driver program and of the preprocessor and the compiler proper.
-###
Like -v except the commands are not executed and arguments are quoted unless they contain only alphanumeric
characters or "./-_". This is useful for shell scripts to capture the driver-generated command lines.
--help
Print (on the standard output) a description of the command-line options understood by gcc. If the -v option is
also specified then --help is also passed on to the various processes invoked by gcc, so that they can display
the command-line options they accept. If the -Wextra option has also been specified (prior to the --help
option), then command-line options that have no documentation associated with them are also displayed.
--target-help
Print (on the standard output) a description of target-specific command-line options for each tool. For some
targets extra target-specific information may also be printed.
--help={class|[^]qualifier}[,...]
Print (on the standard output) a description of the command-line options understood by the compiler that fit into
all specified classes and qualifiers. These are the supported classes:
optimizers
Display all of the optimization options supported by the compiler.
warnings
Display all of the options controlling warning messages produced by the compiler.
target
Display target-specific options. Unlike the --target-help option however, target-specific options of the
linker and assembler are not displayed. This is because those tools do not currently support the extended
--help= syntax.
params
Display the values recognized by the --param option.
language
Display the options supported for language, where language is the name of one of the languages supported in
this version of GCC. If an option is supported by all languages, one needs to select common class.
common
Display the options that are common to all languages.
These are the supported qualifiers:
undocumented
Display only those options that are undocumented.
joined
Display options taking an argument that appears after an equal sign in the same continuous piece of text,
such as: --help=target.
separate
Display options taking an argument that appears as a separate word following the original option, such as: -o
output-file.
Thus for example to display all the undocumented target-specific switches supported by the compiler, use:
--help=target,undocumented
The sense of a qualifier can be inverted by prefixing it with the ^ character, so for example to display all
binary warning options (i.e., ones that are either on or off and that do not take an argument) that have a
description, use:
--help=warnings,^joined,^undocumented
The argument to --help= should not consist solely of inverted qualifiers.
Combining several classes is possible, although this usually restricts the output so much that there is nothing
to display. One case where it does work, however, is when one of the classes is target. For example, to display
all the target-specific optimization options, use:
--help=target,optimizers
The --help= option can be repeated on the command line. Each successive use displays its requested class of
options, skipping those that have already been displayed. If --help is also specified anywhere on the command
line then this takes precedence over any --help= option.
If the -Q option appears on the command line before the --help= option, then the descriptive text displayed by
--help= is changed. Instead of describing the displayed options, an indication is given as to whether the option
is enabled, disabled or set to a specific value (assuming that the compiler knows this at the point where the
--help= option is used).
Here is a truncated example from the ARM port of gcc:
% gcc -Q -mabi=2 --help=target -c
The following options are target specific:
-mabi= 2
-mabort-on-noreturn [disabled]
-mapcs [disabled]
The output is sensitive to the effects of previous command-line options, so for example it is possible to find
out which optimizations are enabled at -O2 by using:
-Q -O2 --help=optimizers
Alternatively you can discover which binary optimizations are enabled by -O3 by using:
gcc -c -Q -O3 --help=optimizers > /tmp/O3-opts
gcc -c -Q -O2 --help=optimizers > /tmp/O2-opts
diff /tmp/O2-opts /tmp/O3-opts | grep enabled
--version
Display the version number and copyrights of the invoked GCC.
-pass-exit-codes
Normally the gcc program exits with the code of 1 if any phase of the compiler returns a non-success return code.
If you specify -pass-exit-codes, the gcc program instead returns with the numerically highest error produced by
any phase returning an error indication. The C, C++, and Fortran front ends return 4 if an internal compiler
error is encountered.
-pipe
Use pipes rather than temporary files for communication between the various stages of compilation. This fails to
work on some systems where the assembler is unable to read from a pipe; but the GNU assembler has no trouble.
-specs=file
Process file after the compiler reads in the standard specs file, in order to override the defaults which the gcc
driver program uses when determining what switches to pass to cc1, cc1plus, as, ld, etc. More than one
-specs=file can be specified on the command line, and they are processed in order, from left to right.
-wrapper
Invoke all subcommands under a wrapper program. The name of the wrapper program and its parameters are passed as
a comma separated list.
gcc -c t.c -wrapper gdb,--args
This invokes all subprograms of gcc under gdb --args, thus the invocation of cc1 is gdb --args cc1 ....
-ffile-prefix-map=old=new
When compiling files residing in directory old, record any references to them in the result of the compilation as
if the files resided in directory new instead. Specifying this option is equivalent to specifying all the
individual -f*-prefix-map options. This can be used to make reproducible builds that are location independent.
Directories referenced by directives are not affected by these options. See also -fmacro-prefix-map,
-fdebug-prefix-map, -fprofile-prefix-map and -fcanon-prefix-map.
-fcanon-prefix-map
For the -f*-prefix-map options normally comparison of old prefix against the filename that would be normally
referenced in the result of the compilation is done using textual comparison of the prefixes, or ignoring
character case for case insensitive filesystems and considering slashes and backslashes as equal on DOS based
filesystems. The -fcanon-prefix-map causes such comparisons to be done on canonicalized paths of old and the
referenced filename.
-fplugin=name.so
Load the plugin code in file name.so, assumed to be a shared object to be dlopen'd by the compiler. The base
name of the shared object file is used to identify the plugin for the purposes of argument parsing (See
-fplugin-arg-name-key=value below). Each plugin should define the callback functions specified in the Plugins
API.
-fplugin-arg-name-key=value
Define an argument called key with a value of value for the plugin called name.
-fdump-ada-spec[-slim]
For C and C++ source and include files, generate corresponding Ada specs.
-fada-spec-parent=unit
In conjunction with -fdump-ada-spec[-slim] above, generate Ada specs as child units of parent unit.
-fdump-go-spec=file
For input files in any language, generate corresponding Go declarations in file. This generates Go "const",
"type", "var", and "func" declarations which may be a useful way to start writing a Go interface to code written
in some other language.
@file
Read command-line options from file. The options read are inserted in place of the original @file option. If
file does not exist, or cannot be read, then the option will be treated literally, and not removed.
Options in file are separated by whitespace. A whitespace character may be included in an option by surrounding
the entire option in either single or double quotes. Any character (including a backslash) may be included by
prefixing the character to be included with a backslash. The file may itself contain additional @file options;
any such options will be processed recursively.
Compiling C++ Programs
C++ source files conventionally use one of the suffixes .C, .cc, .cpp, .CPP, .c++, .cp, or .cxx; C++ header files
often use .hh, .hpp, .H, or (for shared template code) .tcc; preprocessed C++ files use the suffix .ii; and C++20
module interface units sometimes use .ixx, .cppm, .cxxm, .c++m, or .ccm.
GCC recognizes files with these names and compiles them as C++ programs even if you call the compiler the same way as
for compiling C programs (usually with the name gcc).
However, the use of gcc does not add the C++ library. g++ is a program that calls GCC and automatically specifies
linking against the C++ library. It treats .c, .h and .i files as C++ source files instead of C source files unless
-x is used. This program is also useful when precompiling a C header file with a .h extension for use in C++
compilations. On many systems, g++ is also installed with the name c++.
When you compile C++ programs, you may specify many of the same command-line options that you use for compiling
programs in any language; or command-line options meaningful for C and related languages; or options that are
meaningful only for C++ programs.
Options Controlling C Dialect
The following options control the dialect of C (or languages derived from C, such as C++, Objective-C and
Objective-C++) that the compiler accepts:
-ansi
In C mode, this is equivalent to -std=c90. In C++ mode, it is equivalent to -std=c++98.
-std=
Determine the language standard. This option is currently only supported when compiling C or C++.
The compiler can accept several base standards, such as c90 or c++98, and GNU dialects of those standards, such
as gnu90 or gnu++98. When a base standard is specified, the compiler accepts all programs following that
standard plus those using GNU extensions that do not contradict it. For example, -std=c90 turns off certain
features of GCC that are incompatible with ISO C90, such as the "asm" and "typeof" keywords, but not other GNU
extensions that do not have a meaning in ISO C90, such as omitting the middle term of a "?:" expression. On the
other hand, when a GNU dialect of a standard is specified, all features supported by the compiler are enabled,
even when those features change the meaning of the base standard. As a result, some strict-conforming programs
may be rejected. The particular standard is used by -Wpedantic to identify which features are GNU extensions
given that version of the standard. For example -std=gnu90 -Wpedantic warns about C++ style // comments, while
-std=gnu99 -Wpedantic does not.
A value for this option must be provided; possible values are
c90
c89
iso9899:1990
Support all ISO C90 programs (certain GNU extensions that conflict with ISO C90 are disabled). Same as -ansi
for C code.
iso9899:199409
ISO C90 as modified in amendment 1.
c99
c9x
iso9899:1999
iso9899:199x
ISO C99. This standard is substantially completely supported, modulo bugs and floating-point issues (mainly
but not entirely relating to optional C99 features from Annexes F and G). See
for more information. The names c9x and iso9899:199x are deprecated.
c11
c1x
iso9899:2011
ISO C11, the 2011 revision of the ISO C standard. This standard is substantially completely supported,
modulo bugs, floating-point issues (mainly but not entirely relating to optional C11 features from Annexes F
and G) and the optional Annexes K (Bounds-checking interfaces) and L (Analyzability). The name c1x is
deprecated.
c17
c18
iso9899:2017
iso9899:2018
ISO C17, the 2017 revision of the ISO C standard (published in 2018). This standard is same as C11 except
for corrections of defects (all of which are also applied with -std=c11) and a new value of
"__STDC_VERSION__", and so is supported to the same extent as C11.
c23
c2x
iso9899:2024
ISO C23, the 2023 revision of the ISO C standard (published in 2024). The name c2x is deprecated.
c2y The next version of the ISO C standard, still under development. The support for this version is
experimental and incomplete.
gnu90
gnu89
GNU dialect of ISO C90 (including some C99 features).
gnu99
gnu9x
GNU dialect of ISO C99. The name gnu9x is deprecated.
gnu11
gnu1x
GNU dialect of ISO C11. The name gnu1x is deprecated.
gnu17
gnu18
GNU dialect of ISO C17.
gnu23
gnu2x
GNU dialect of ISO C23. This is the default for C code. The name gnu2x is deprecated.
gnu2y
The next version of the ISO C standard, still under development, plus GNU extensions. The support for this
version is experimental and incomplete. The name gnu2x is deprecated.
c++98
c++03
The 1998 ISO C++ standard plus the 2003 technical corrigendum and some additional defect reports. Same as
-ansi for C++ code.
gnu++98
gnu++03
GNU dialect of -std=c++98.
c++11
c++0x
The 2011 ISO C++ standard plus amendments. The name c++0x is deprecated.
gnu++11
gnu++0x
GNU dialect of -std=c++11. The name gnu++0x is deprecated.
c++14
c++1y
The 2014 ISO C++ standard plus amendments. The name c++1y is deprecated.
gnu++14
gnu++1y
GNU dialect of -std=c++14. The name gnu++1y is deprecated.
c++17
c++1z
The 2017 ISO C++ standard plus amendments. The name c++1z is deprecated.
gnu++17
gnu++1z
GNU dialect of -std=c++17. This is the default for C++ code. The name gnu++1z is deprecated.
c++20
c++2a
The 2020 ISO C++ standard plus amendments. Support is experimental, and could change in incompatible ways in
future releases. The name c++2a is deprecated.
gnu++20
gnu++2a
GNU dialect of -std=c++20. Support is experimental, and could change in incompatible ways in future
releases. The name gnu++2a is deprecated.
c++23
c++2b
The 2023 ISO C++ standard plus amendments (published in 2024). Support is experimental, and could change in
incompatible ways in future releases. The name c++2b is deprecated.
gnu++23
gnu++2b
GNU dialect of -std=c++23. Support is experimental, and could change in incompatible ways in future
releases. The name gnu++2b is deprecated.
c++2c
c++26
The next revision of the ISO C++ standard, planned for 2026. Support is highly experimental, and will almost
certainly change in incompatible ways in future releases.
gnu++2c
gnu++26
GNU dialect of -std=c++2c. Support is highly experimental, and will almost certainly change in incompatible
ways in future releases.
-aux-info filename
Output to the given filename prototyped declarations for all functions declared and/or defined in a translation
unit, including those in header files. This option is silently ignored in any language other than C.
Besides declarations, the file indicates, in comments, the origin of each declaration (source file and line),
whether the declaration was implicit, prototyped or unprototyped (I, N for new or O for old, respectively, in the
first character after the line number and the colon), and whether it came from a declaration or a definition (C
or F, respectively, in the following character). In the case of function definitions, a K&R-style list of
arguments followed by their declarations is also provided, inside comments, after the declaration.
-fno-asm
Do not recognize "asm", "inline" or "typeof" as a keyword, so that code can use these words as identifiers. You
can use the keywords "__asm__", "__inline__" and "__typeof__" instead. In C, -ansi implies -fno-asm.
In C++, "inline" is a standard keyword and is not affected by this switch. You may want to use the
-fno-gnu-keywords flag instead, which disables "typeof" but not "asm" and "inline". In C99 mode (-std=c99 or
-std=gnu99), this switch only affects the "asm" and "typeof" keywords, since "inline" is a standard keyword in
ISO C99. In C23 mode (-std=c23 or -std=gnu23), this switch only affects the "asm" keyword, since "typeof" is a
standard keyword in ISO C23.
-fno-builtin
-fno-builtin-function
Don't recognize built-in functions that do not begin with __builtin_ as prefix.
GCC normally generates special code to handle certain built-in functions more efficiently; for instance, calls to
"alloca" may become single instructions which adjust the stack directly, and calls to "memcpy" may become inline
copy loops. The resulting code is often both smaller and faster, but since the function calls no longer appear
as such, you cannot set a breakpoint on those calls, nor can you change the behavior of the functions by linking
with a different library. In addition, when a function is recognized as a built-in function, GCC may use
information about that function to warn about problems with calls to that function, or to generate more efficient
code, even if the resulting code still contains calls to that function. For example, warnings are given with
-Wformat for bad calls to "printf" when "printf" is built in and "strlen" is known not to modify global memory.
With the -fno-builtin-function option only the built-in function function is disabled. function must not begin
with __builtin_. If a function is named that is not built-in in this version of GCC, this option is ignored.
There is no corresponding -fbuiltin-function option; if you wish to enable built-in functions selectively when
using -fno-builtin or -ffreestanding, you may define macros such as:
#define abs(n) __builtin_abs ((n))
#define strcpy(d, s) __builtin_strcpy ((d), (s))
-fcond-mismatch
Allow conditional expressions with mismatched types in the second and third arguments. The value of such an
expression is void. This option is not supported for C++.
-ffreestanding
Assert that compilation targets a freestanding environment. This implies -fno-builtin. A freestanding
environment is one in which the standard library may not exist, and program startup may not necessarily be at
"main". The most obvious example is an OS kernel. This is equivalent to -fno-hosted.
-fgimple
Enable parsing of function definitions marked with "__GIMPLE". This is an experimental feature that allows unit
testing of GIMPLE passes.
-fgnu-tm
When the option -fgnu-tm is specified, the compiler generates code for the Linux variant of Intel's current
Transactional Memory ABI specification document (Revision 1.1, May 6 2009). This is an experimental feature
whose interface may change in future versions of GCC, as the official specification changes. Please note that
not all architectures are supported for this feature.
For more information on GCC's support for transactional memory,
Note that the transactional memory feature is not supported with non-call exceptions (-fnon-call-exceptions).
-fgnu89-inline
The option -fgnu89-inline tells GCC to use the traditional GNU semantics for "inline" functions when in C99 mode.
Using this option is roughly equivalent to adding the "gnu_inline" function attribute to all inline functions.
The option -fno-gnu89-inline explicitly tells GCC to use the C99 semantics for "inline" when in C99 or gnu99 mode
(i.e., it specifies the default behavior). This option is not supported in -std=c90 or -std=gnu90 mode.
The preprocessor macros "__GNUC_GNU_INLINE__" and "__GNUC_STDC_INLINE__" may be used to check which semantics are
in effect for "inline" functions.
-fhosted
Assert that compilation targets a hosted environment. This implies -fbuiltin. A hosted environment is one in
which the entire standard library is available, and in which "main" has a return type of "int". Examples are
nearly everything except a kernel. This is equivalent to -fno-freestanding.
-flax-vector-conversions
Allow implicit conversions between vectors with differing numbers of elements and/or incompatible element types.
This option should not be used for new code.
-fms-extensions
Accept some non-standard constructs used in Microsoft header files.
In C++ code, this allows member names in structures to be similar to previous types declarations.
typedef int UOW;
struct ABC {
UOW UOW;
};
Some cases of unnamed fields in structures and unions are only accepted with this option.
Note that this option is off for all targets except for x86 targets using ms-abi.
-fpermitted-flt-eval-methods=style
ISO/IEC TS 18661-3 defines new permissible values for "FLT_EVAL_METHOD" that indicate that operations and
constants with a semantic type that is an interchange or extended format should be evaluated to the precision and
range of that type. These new values are a superset of those permitted under C99/C11, which does not specify the
meaning of other positive values of "FLT_EVAL_METHOD". As such, code conforming to C11 may not have been written
expecting the possibility of the new values.
-fpermitted-flt-eval-methods specifies whether the compiler should allow only the values of "FLT_EVAL_METHOD"
specified in C99/C11, or the extended set of values specified in ISO/IEC TS 18661-3.
style is either "c11" or "ts-18661-3" as appropriate.
The default when in a standards compliant mode (-std=c11 or similar) is -fpermitted-flt-eval-methods=c11. The
default when in a GNU dialect (-std=gnu11 or similar) is -fpermitted-flt-eval-methods=ts-18661-3.
The -fdeps-* options are used to extract structured dependency information for a source. This involves
determining what resources provided by other source files will be required to compile the source as well as what
resources are provided by the source. This information can be used to add required dependencies between
compilation rules of dependent sources based on their contents rather than requiring such information be
reflected within the build tools as well.
-fdeps-file=file
Where to write structured dependency information.
-fdeps-format=format
The format to use for structured dependency information. p1689r5 is the only supported format right now. Note
that when this argument is specified, the output of -MF is stripped of some information (namely C++ modules) so
that it does not use extended makefile syntax not understood by most tools.
-fdeps-target=file
Analogous to -MT but for structured dependency information. This indicates the target which will ultimately need
any required resources and provide any resources extracted from the source that may be required by other sources.
-fplan9-extensions
Accept some non-standard constructs used in Plan 9 code.
This enables -fms-extensions, permits passing pointers to structures with anonymous fields to functions that
expect pointers to elements of the type of the field, and permits referring to anonymous fields declared using a
typedef. This is only supported for C, not C++.
-fsigned-bitfields
-funsigned-bitfields
-fno-signed-bitfields
-fno-unsigned-bitfields
These options control whether a bit-field is signed or unsigned, when the declaration does not use either
"signed" or "unsigned". By default, such a bit-field is signed, because this is consistent: the basic integer
types such as "int" are signed types.
-fsigned-char
Let the type "char" be signed, like "signed char".
Note that this is equivalent to -fno-unsigned-char, which is the negative form of -funsigned-char. Likewise, the
option -fno-signed-char is equivalent to -funsigned-char.
-funsigned-char
Let the type "char" be unsigned, like "unsigned char".
Each kind of machine has a default for what "char" should be. It is either like "unsigned char" by default or
like "signed char" by default.
Ideally, a portable program should always use "signed char" or "unsigned char" when it depends on the signedness
of an object. But many programs have been written to use plain "char" and expect it to be signed, or expect it
to be unsigned, depending on the machines they were written for. This option, and its inverse, let you make such
a program work with the opposite default.
The type "char" is always a distinct type from each of "signed char" or "unsigned char", even though its behavior
is always just like one of those two.
-fstrict-flex-arrays (C and C++ only)
-fstrict-flex-arrays=level (C and C++ only)
Control when to treat the trailing array of a structure as a flexible array member for the purpose of accessing
the elements of such an array. The value of level controls the level of strictness.
-fstrict-flex-arrays is equivalent to -fstrict-flex-arrays=3, which is the strictest; a trailing array is treated
as a flexible array member only when it is declared as a flexible array member per C99 standard onwards.
The negative form -fno-strict-flex-arrays is equivalent to -fstrict-flex-arrays=0, which is the least strict. In
this case all trailing arrays of structures are treated as flexible array members.
There are two more levels in between 0 and 3, which are provided to support older code that uses the GCC zero-
length array extension ([0]) or one-element array as flexible array members ([1]). When level is 1, the trailing
array is treated as a flexible array member when it is declared as either [], [0], or [1]. When level is 2, the
trailing array is treated as a flexible array member when it is declared as either [], or [0].
You can control this behavior for a specific trailing array field of a structure by using the variable attribute
"strict_flex_array" attribute.
The -fstrict_flex_arrays option interacts with the -Wstrict-flex-arrays option.
-fsso-struct=endianness
Set the default scalar storage order of structures and unions to the specified endianness. The accepted values
are big-endian, little-endian and native for the native endianness of the target (the default). This option is
not supported for C++.
Warning: the -fsso-struct switch causes GCC to generate code that is not binary compatible with code generated
without it if the specified endianness is not the native endianness of the target.
Options Controlling C++ Dialect
This section describes the command-line options that are only meaningful for C++ programs. You can also use most of
the GNU compiler options regardless of what language your program is in. For example, you might compile a file
firstClass.C like this:
g++ -g -fstrict-enums -O -c firstClass.C
In this example, only -fstrict-enums is an option meant only for C++ programs; you can use the other options with any
language supported by GCC.
Some options for compiling C programs, such as -std, are also relevant for C++ programs.
Here is a list of options that are only for compiling C++ programs:
-fabi-version=n
Use version n of the C++ ABI. The default is version 0.
Version 0 refers to the version conforming most closely to the C++ ABI specification. Therefore, the ABI
obtained using version 0 will change in different versions of G++ as ABI bugs are fixed.
Version 1 is the version of the C++ ABI that first appeared in G++ 3.2.
Version 2 is the version of the C++ ABI that first appeared in G++ 3.4, and was the default through G++ 4.9.
Version 3 corrects an error in mangling a constant address as a template argument.
Version 4, which first appeared in G++ 4.5, implements a standard mangling for vector types.
Version 5, which first appeared in G++ 4.6, corrects the mangling of attribute const/volatile on function pointer
types, decltype of a plain decl, and use of a function parameter in the declaration of another parameter.
Version 6, which first appeared in G++ 4.7, corrects the promotion behavior of C++11 scoped enums and the
mangling of template argument packs, const/static_cast, prefix ++ and --, and a class scope function used as a
template argument.
Version 7, which first appeared in G++ 4.8, that treats nullptr_t as a builtin type and corrects the mangling of
lambdas in default argument scope.
Version 8, which first appeared in G++ 4.9, corrects the substitution behavior of function types with function-
cv-qualifiers.
Version 9, which first appeared in G++ 5.2, corrects the alignment of "nullptr_t".
Version 10, which first appeared in G++ 6.1, adds mangling of attributes that affect type identity, such as ia32
calling convention attributes (e.g. stdcall).
Version 11, which first appeared in G++ 7, corrects the mangling of sizeof... expressions and operator names.
For multiple entities with the same name within a function, that are declared in different scopes, the mangling
now changes starting with the twelfth occurrence. It also implies -fnew-inheriting-ctors.
Version 12, which first appeared in G++ 8, corrects the calling conventions for empty classes on the x86_64
target and for classes with only deleted copy/move constructors. It accidentally changes the calling convention
for classes with a deleted copy constructor and a trivial move constructor.
Version 13, which first appeared in G++ 8.2, fixes the accidental change in version 12.
Version 14, which first appeared in G++ 10, corrects the mangling of the nullptr expression.
Version 15, which first appeared in G++ 10.3, corrects G++ 10 ABI tag regression.
Version 16, which first appeared in G++ 11, changes the mangling of "__alignof__" to be distinct from that of
"alignof", and dependent operator names.
Version 17, which first appeared in G++ 12, fixes layout of classes that inherit from aggregate classes with
default member initializers in C++14 and up.
Version 18, which first appeared in G++ 13, fixes manglings of lambdas that have additional context.
Version 19, which first appeared in G++ 14, fixes manglings of structured bindings to include ABI tags.
Version 20, which first appeared in G++ 15, fixes manglings of lambdas in static data member initializers.
See also -Wabi.
-fabi-compat-version=n
On targets that support strong aliases, G++ works around mangling changes by creating an alias with the correct
mangled name when defining a symbol with an incorrect mangled name. This switch specifies which ABI version to
use for the alias.
With -fabi-version=0 (the default), this defaults to 13 (GCC 8.2 compatibility). If another ABI version is
explicitly selected, this defaults to 0. For compatibility with GCC versions 3.2 through 4.9, use
-fabi-compat-version=2.
If this option is not provided but -Wabi=n is, that version is used for compatibility aliases. If this option is
provided along with -Wabi (without the version), the version from this option is used for the warning.
-fno-access-control
Turn off all access checking. This switch is mainly useful for working around bugs in the access control code.
-faligned-new
Enable support for C++17 "new" of types that require more alignment than "void* ::operator new(std::size_t)"
provides. A numeric argument such as "-faligned-new=32" can be used to specify how much alignment (in bytes) is
provided by that function, but few users will need to override the default of alignof(std::max_align_t).
This flag is enabled by default for -std=c++17.
-fno-assume-sane-operators-new
The C++ standard allows replacing the global "new", "new[]", "delete" and "delete[]" operators, though a lot of
C++ programs don't replace them and just use the implementation provided version. Furthermore, the C++ standard
allows omitting those calls if they are made from new or delete expressions (and by extension the same is assumed
if "__builtin_operator_new" or "__builtin_operator_delete" functions are used). This option allows control over
some optimizations around calls to those operators. With "-fassume-sane-operators-new-delete" option GCC may
assume that calls to the replaceable global operators from new or delete expressions or from
"__builtin_operator_new" or "__builtin_operator_delete" calls don't read or modify any global variables or
variables whose address could escape to the operators (global state; except for "errno" for the "new" and "new[]"
operators). This allows most optimizations across those calls and is something that the implementation provided
operators satisfy unless "malloc" implementation details are observable in the code or unless "malloc" hooks are
used, but might not be satisfied if a program replaces those operators. This behavior is enabled by default.
With "-fno-assume-sane-operators-new-delete" option GCC must assume all these calls (whether from new or delete
expressions or called directly) may read and write global state unless proven otherwise (e.g. when GCC compiles
their implementation). Use this option if those operators are or may be replaced and code needs to expect such
behavior.
-fchar8_t
-fno-char8_t
Enable support for "char8_t" as adopted for C++20. This includes the addition of a new "char8_t" fundamental
type, changes to the types of UTF-8 string and character literals, new signatures for user-defined literals,
associated standard library updates, and new "__cpp_char8_t" and "__cpp_lib_char8_t" feature test macros.
This option enables functions to be overloaded for ordinary and UTF-8 strings:
int f(const char *); // #1
int f(const char8_t *); // #2
int v1 = f("text"); // Calls #1
int v2 = f(u8"text"); // Calls #2
and introduces new signatures for user-defined literals:
int operator""_udl1(char8_t);
int v3 = u8'x'_udl1;
int operator""_udl2(const char8_t*, std::size_t);
int v4 = u8"text"_udl2;
template int operator""_udl3();
int v5 = u8"text"_udl3;
The change to the types of UTF-8 string and character literals introduces incompatibilities with ISO C++11 and
later standards. For example, the following code is well-formed under ISO C++11, but is ill-formed when
-fchar8_t is specified.
const char *cp = u8"xx";// error: invalid conversion from
// `const char8_t*' to `const char*'
int f(const char*);
auto v = f(u8"xx"); // error: invalid conversion from
// `const char8_t*' to `const char*'
std::string s{u8"xx"}; // error: no matching function for call to
// `std::basic_string::basic_string()'
using namespace std::literals;
s = u8"xx"s; // error: conversion from
// `basic_string' to non-scalar
// type `basic_string' requested
-fcheck-new
Check that the pointer returned by "operator new" is non-null before attempting to modify the storage allocated.
This check is normally unnecessary because the C++ standard specifies that "operator new" only returns 0 if it is
declared throw(), in which case the compiler always checks the return value even without this option. In all
other cases, when "operator new" has a non-empty exception specification, memory exhaustion is signalled by
throwing "std::bad_alloc". See also new (nothrow).
-fconcepts
Enable support for the C++ Concepts feature for constraining template arguments. With -std=c++20 and above,
Concepts are part of the language standard, so -fconcepts defaults to on.
Some constructs that were allowed by the earlier C++ Extensions for Concepts Technical Specification, ISO 19217
(2015), but didn't make it into the standard, could additionally be enabled by -fconcepts-ts. The option
-fconcepts-ts was deprecated in GCC 14 and removed in GCC 15; users are expected to convert their code to C++20
concepts.
-fconstexpr-depth=n
Set the maximum nested evaluation depth for C++11 constexpr functions to n. A limit is needed to detect endless
recursion during constant expression evaluation. The minimum specified by the standard is 512.
-fconstexpr-cache-depth=n
Set the maximum level of nested evaluation depth for C++11 constexpr functions that will be cached to n. This is
a heuristic that trades off compilation speed (when the cache avoids repeated calculations) against memory
consumption (when the cache grows very large from highly recursive evaluations). The default is 8. Very few
users are likely to want to adjust it, but if your code does heavy constexpr calculations you might want to
experiment to find which value works best for you.
-fconstexpr-fp-except
Annex F of the C standard specifies that IEC559 floating point exceptions encountered at compile time should not
stop compilation. C++ compilers have historically not followed this guidance, instead treating floating point
division by zero as non-constant even though it has a well defined value. This flag tells the compiler to give
Annex F priority over other rules saying that a particular operation is undefined.
constexpr float inf = 1./0.; // OK with -fconstexpr-fp-except
-fconstexpr-loop-limit=n
Set the maximum number of iterations for a loop in C++14 constexpr functions to n. A limit is needed to detect
infinite loops during constant expression evaluation. The default is 262144 (1<<18).
-fconstexpr-ops-limit=n
Set the maximum number of operations during a single constexpr evaluation. Even when number of iterations of a
single loop is limited with the above limit, if there are several nested loops and each of them has many
iterations but still smaller than the above limit, or if in a body of some loop or even outside of a loop too
many expressions need to be evaluated, the resulting constexpr evaluation might take too long. The default is
33554432 (1<<25).
-fcontracts
Enable experimental support for the C++ Contracts feature, as briefly added to and then removed from the C++20
working paper (N4820). The implementation also includes proposed enhancements from papers P1290, P1332, and
P1429. This functionality is intended mostly for those interested in experimentation towards refining the
feature to get it into shape for a future C++ standard.
On violation of a checked contract, the violation handler is called. Users can replace the violation handler by
defining
void
handle_contract_violation (const std::experimental::contract_violation&);
There are different sets of additional flags that can be used together to specify which contracts will be checked
and how, for N4820 contracts, P1332 contracts, or P1429 contracts; these sets cannot be used together.
-fcontract-mode=[on|off]
Control whether any contracts have any semantics at all. Defaults to on.
-fcontract-assumption-mode=[on|off]
[N4820] Control whether contracts with level axiom should have the assume semantic. Defaults to on.
-fcontract-build-level=[off|default|audit]
[N4820] Specify which level of contracts to generate checks for. Defaults to default.
-fcontract-continuation-mode=[on|off]
[N4820] Control whether to allow the program to continue executing after a contract violation. That is, do
checked contracts have the maybe semantic described below rather than the never semantic. Defaults to off.
-fcontract-role=:,,
[P1332] Specify the concrete semantics for each contract level of a particular contract role.
-fcontract-semantic=[default|audit|axiom]:
[P1429] Specify the concrete semantic for a particular contract level.
-fcontract-strict-declarations=[on|off]
Control whether to reject adding contracts to a function after its first declaration. Defaults to off.
The possible concrete semantics for that can be specified with -fcontract-role or -fcontract-semantic are:
"ignore"
This contract has no effect.
"assume"
This contract is treated like C++23 "[[assume]]".
"check_never_continue"
"never"
"abort"
This contract is checked. If it fails, the violation handler is called. If the handler returns,
"std::terminate" is called.
"check_maybe_continue"
"maybe"
This contract is checked. If it fails, the violation handler is called. If the handler returns, execution
continues normally.
-fcoroutines
Enable support for the C++ coroutines extension (experimental).
-fdiagnostics-all-candidates
Permit the C++ front end to note all candidates during overload resolution failure, including when a deleted
function is selected.
-fno-elide-constructors
The C++ standard allows an implementation to omit creating a temporary that is only used to initialize another
object of the same type. Specifying this option disables that optimization, and forces G++ to call the copy
constructor in all cases. This option also causes G++ to call trivial member functions which otherwise would be
expanded inline.
In C++17, the compiler is required to omit these temporaries, but this option still affects trivial member
functions.
-fno-enforce-eh-specs
Don't generate code to check for violation of exception specifications at run time. This option violates the C++
standard, but may be useful for reducing code size in production builds, much like defining "NDEBUG". This does
not give user code permission to throw exceptions in violation of the exception specifications; the compiler
still optimizes based on the specifications, so throwing an unexpected exception results in undefined behavior at
run time.
-fextern-tls-init
-fno-extern-tls-init
The C++11 and OpenMP standards allow "thread_local" and "threadprivate" variables to have dynamic (runtime)
initialization. To support this, any use of such a variable goes through a wrapper function that performs any
necessary initialization. When the use and definition of the variable are in the same translation unit, this
overhead can be optimized away, but when the use is in a different translation unit there is significant overhead
even if the variable doesn't actually need dynamic initialization. If the programmer can be sure that no use of
the variable in a non-defining TU needs to trigger dynamic initialization (either because the variable is
statically initialized, or a use of the variable in the defining TU will be executed before any uses in another
TU), they can avoid this overhead with the -fno-extern-tls-init option.
On targets that support symbol aliases, the default is -fextern-tls-init. On targets that do not support symbol
aliases, the default is -fno-extern-tls-init.
-ffold-simple-inlines
-fno-fold-simple-inlines
Permit the C++ frontend to fold calls to "std::move", "std::forward", "std::addressof" and "std::as_const". In
contrast to inlining, this means no debug information will be generated for such calls. Since these functions
are rarely interesting to debug, this flag is enabled by default unless -fno-inline is active.
-fno-gnu-keywords
Do not recognize "typeof" as a keyword, so that code can use this word as an identifier. You can use the keyword
"__typeof__" instead. This option is implied by the strict ISO C++ dialects: -ansi, -std=c++98, -std=c++11, etc.
-fno-immediate-escalation
Do not enable immediate function escalation whereby certain functions can be promoted to consteval, as specified
in P2564R3. For example:
consteval int id(int i) { return i; }
constexpr int f(auto t)
{
return t + id(t); // id causes f to be promoted to consteval
}
void g(int i)
{
f (3);
}
compiles in C++20: "f" is an immediate-escalating function (due to the "auto" it is a function template and is
declared "constexpr") and id(t) is an immediate-escalating expression, so "f" is promoted to "consteval".
Consequently, the call to id(t) is in an immediate context, so doesn't have to produce a constant (that is the
mechanism allowing consteval function composition). However, with -fno-immediate-escalation, "f" is not promoted
to "consteval", and since the call to consteval function id(t) is not a constant expression, the compiler rejects
the code.
This option is turned on by default; it is only effective in C++20 mode or later.
-fimplicit-constexpr
Make inline functions implicitly constexpr, if they satisfy the requirements for a constexpr function. This
option can be used in C++14 mode or later. This can result in initialization changing from dynamic to static and
other optimizations.
-fno-implicit-templates
Never emit code for non-inline templates that are instantiated implicitly (i.e. by use); only emit code for
explicit instantiations. If you use this option, you must take care to structure your code to include all the
necessary explicit instantiations to avoid getting undefined symbols at link time.
-fno-implicit-inline-templates
Don't emit code for implicit instantiations of inline templates, either. The default is to handle inlines
differently so that compiles with and without optimization need the same set of explicit instantiations.
-fno-implement-inlines
To save space, do not emit out-of-line copies of inline functions controlled by "#pragma implementation". This
causes linker errors if these functions are not inlined everywhere they are called.
-fmodules
-fno-modules
Enable support for C++20 modules. The -fno-modules is usually not needed, as that is the default. Even though
this is a C++20 feature, it is not currently implicitly enabled by selecting that standard version.
-fmodule-header
-fmodule-header=user
-fmodule-header=system
Compile a header file to create an importable header unit.
-fmodule-implicit-inline
Member functions defined in their class definitions are not implicitly inline for modular code. This is
different to traditional C++ behavior, for good reasons. However, it may result in a difficulty during code
porting. This option makes such function definitions implicitly inline. It does however generate an ABI
incompatibility, so you must use it everywhere or nowhere. (Such definitions outside of a named module remain
implicitly inline, regardless.)
-fno-module-lazy
Disable lazy module importing and module mapper creation.
-fmodule-mapper=[hostname]:port[?ident]
-fmodule-mapper=|program[?ident] args...
-fmodule-mapper==socket[?ident]
-fmodule-mapper=<>[inout][?ident]
-fmodule-mapper=out[?ident]
-fmodule-mapper=file[?ident]
An oracle to query for module name to filename mappings. If unspecified the CXX_MODULE_MAPPER environment
variable is used, and if that is unset, an in-process default is provided.
-fmodule-only
Only emit the Compiled Module Interface, inhibiting any object file.
-fms-extensions
Disable Wpedantic warnings about constructs used in MFC, such as implicit int and getting a pointer to member
function via non-standard syntax.
-fnew-inheriting-ctors
Enable the P0136 adjustment to the semantics of C++11 constructor inheritance. This is part of C++17 but also
considered to be a Defect Report against C++11 and C++14. This flag is enabled by default unless
-fabi-version=10 or lower is specified.
-fnew-ttp-matching
Enable the P0522 resolution to Core issue 150, template template parameters and default arguments: this allows a
template with default template arguments as an argument for a template template parameter with fewer template
parameters. This flag is enabled by default for -std=c++17.
-fno-nonansi-builtins
Disable built-in declarations of functions that are not mandated by ANSI/ISO C. These include "ffs", "alloca",
"_exit", "index", "bzero", "conjf", and other related functions.
-fnothrow-opt
Treat a throw() exception specification as if it were a "noexcept" specification to reduce or eliminate the text
size overhead relative to a function with no exception specification. If the function has local variables of
types with non-trivial destructors, the exception specification actually makes the function smaller because the
EH cleanups for those variables can be optimized away. The semantic effect is that an exception thrown out of a
function with such an exception specification results in a call to "terminate" rather than "unexpected".
-fno-operator-names
Do not treat the operator name keywords "and", "bitand", "bitor", "compl", "not", "or" and "xor" as synonyms as
keywords.
-fno-optional-diags
Disable diagnostics that the standard says a compiler does not need to issue. Currently, the only such
diagnostic issued by G++ is the one for a name having multiple meanings within a class.
-fno-pretty-templates
When an error message refers to a specialization of a function template, the compiler normally prints the
signature of the template followed by the template arguments and any typedefs or typenames in the signature (e.g.
"void f(T) [with T = int]" rather than "void f(int)") so that it's clear which template is involved. When an
error message refers to a specialization of a class template, the compiler omits any template arguments that
match the default template arguments for that template. If either of these behaviors make it harder to
understand the error message rather than easier, you can use -fno-pretty-templates to disable them.
-frange-for-ext-temps
Enable lifetime extension of C++ range based for temporaries. With -std=c++23 and above this is part of the
language standard, so lifetime of the temporaries is extended until the end of the loop by default. This option
allows enabling that behavior also in earlier versions of the standard.
-fno-rtti
Disable generation of information about every class with virtual functions for use by the C++ run-time type
identification features ("dynamic_cast" and "typeid"). If you don't use those parts of the language, you can
save some space by using this flag. Note that exception handling uses the same information, but G++ generates it
as needed. The "dynamic_cast" operator can still be used for casts that do not require run-time type information,
i.e. casts to "void *" or to unambiguous base classes.
Mixing code compiled with -frtti with that compiled with -fno-rtti may not work. For example, programs may fail
to link if a class compiled with -fno-rtti is used as a base for a class compiled with -frtti.
-fsized-deallocation
Enable the built-in global declarations
void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;
as introduced in C++14. This is useful for user-defined replacement deallocation functions that, for example,
use the size of the object to make deallocation faster. Enabled by default under -std=c++14 and above. The flag
-Wsized-deallocation warns about places that might want to add a definition.
-fstrict-enums
Allow the compiler to optimize using the assumption that a value of enumerated type can only be one of the values
of the enumeration (as defined in the C++ standard; basically, a value that can be represented in the minimum
number of bits needed to represent all the enumerators). This assumption may not be valid if the program uses a
cast to convert an arbitrary integer value to the enumerated type. This option has no effect for an enumeration
type with a fixed underlying type.
-fstrong-eval-order
-fstrong-eval-order=kind
Evaluate member access, array subscripting, and shift expressions in left-to-right order, and evaluate assignment
in right-to-left order, as adopted for C++17. -fstrong-eval-order is equivalent to -fstrong-eval-order=all, and
is enabled by default with -std=c++17 or later.
-fstrong-eval-order=some enables just the ordering of member access and shift expressions, and is the default for
C++ dialects prior to C++17.
-fstrong-eval-order=none is equivalent to -fno-strong-eval-order.
-ftemplate-backtrace-limit=n
Set the maximum number of template instantiation notes for a single warning or error to n. The default value is
10.
-ftemplate-depth=n
Set the maximum instantiation depth for template classes to n. A limit on the template instantiation depth is
needed to detect endless recursions during template class instantiation. ANSI/ISO C++ conforming programs must
not rely on a maximum depth greater than 17 (changed to 1024 in C++11). The default value is 900, as the
compiler can run out of stack space before hitting 1024 in some situations.
-fno-threadsafe-statics
Do not emit the extra code to use the routines specified in the C++ ABI for thread-safe initialization of local
statics. You can use this option to reduce code size slightly in code that doesn't need to be thread-safe.
-fuse-cxa-atexit
Register destructors for objects with static storage duration with the "__cxa_atexit" function rather than the
"atexit" function. This option is required for fully standards-compliant handling of static destructors, but
only works if your C library supports "__cxa_atexit".
-fno-use-cxa-get-exception-ptr
Don't use the "__cxa_get_exception_ptr" runtime routine. This causes "std::uncaught_exception" to be incorrect,
but is necessary if the runtime routine is not available.
-fvisibility-inlines-hidden
This switch declares that the user does not attempt to compare pointers to inline functions or methods where the
addresses of the two functions are taken in different shared objects.
The effect of this is that GCC may, effectively, mark inline methods with "__attribute__ ((visibility
("hidden")))" so that they do not appear in the export table of a DSO and do not require a PLT indirection when
used within the DSO. Enabling this option can have a dramatic effect on load and link times of a DSO as it
massively reduces the size of the dynamic export table when the library makes heavy use of templates.
The behavior of this switch is not quite the same as marking the methods as hidden directly, because it does not
affect static variables local to the function or cause the compiler to deduce that the function is defined in
only one shared object.
You may mark a method as having a visibility explicitly to negate the effect of the switch for that method. For
example, if you do want to compare pointers to a particular inline method, you might mark it as having default
visibility. Marking the enclosing class with explicit visibility has no effect.
Explicitly instantiated inline methods are unaffected by this option as their linkage might otherwise cross a
shared library boundary.
-fvisibility-ms-compat
This flag attempts to use visibility settings to make GCC's C++ linkage model compatible with that of Microsoft
Visual Studio.
The flag makes these changes to GCC's linkage model:
1. It sets the default visibility to "hidden", like -fvisibility=hidden.
2. Types, but not their members, are not hidden by default.
3. The One Definition Rule is relaxed for types without explicit visibility specifications that are defined in
more than one shared object: those declarations are permitted if they are permitted when this option is not
used.
In new code it is better to use -fvisibility=hidden and export those classes that are intended to be externally
visible. Unfortunately it is possible for code to rely, perhaps accidentally, on the Visual Studio behavior.
Among the consequences of these changes are that static data members of the same type with the same name but
defined in different shared objects are different, so changing one does not change the other; and that pointers
to function members defined in different shared objects may not compare equal. When this flag is given, it is a
violation of the ODR to define types with the same name differently.
-fno-weak
Do not use weak symbol support, even if it is provided by the linker. By default, G++ uses weak symbols if they
are available. This option exists only for testing, and should not be used by end-users; it results in inferior
code and has no benefits. This option may be removed in a future release of G++.
-fext-numeric-literals (C++ and Objective-C++ only)
Accept imaginary, fixed-point, or machine-defined literal number suffixes as GNU extensions. When this option is
turned off these suffixes are treated as C++11 user-defined literal numeric suffixes. This is on by default for
all pre-C++11 dialects and all GNU dialects: -std=c++98, -std=gnu++98, -std=gnu++11, -std=gnu++14. This option
is off by default for ISO C++11 onwards (-std=c++11, ...).
-nostdinc++
Do not search for header files in the standard directories specific to C++, but do still search the other
standard directories. (This option is used when building the C++ library.)
-flang-info-include-translate
-flang-info-include-translate-not
-flang-info-include-translate=header
Inform of include translation events. The first will note accepted include translations, the second will note
declined include translations. The header form will inform of include translations relating to that specific
header. If header is of the form "user" or "" it will be resolved to a specific user or system header
using the include path.
-flang-info-module-cmi
-flang-info-module-cmi=module
Inform of Compiled Module Interface pathnames. The first will note all read CMI pathnames. The module form will
not reading a specific module's CMI. module may be a named module or a header-unit (the latter indicated by
either being a pathname containing directory separators or enclosed in "<>" or "").
-stdlib=libstdc++,libc++
When G++ is configured to support this option, it allows specification of alternate C++ runtime libraries. Two
options are available: libstdc++ (the default, native C++ runtime for G++) and libc++ which is the C++ runtime
installed on some operating systems (e.g. Darwin versions from Darwin11 onwards). The option switches G++ to use
the headers from the specified library and to emit "-lstdc++" or "-lc++" respectively, when a C++ runtime is
required for linking.
In addition, these warning options have meanings only for C++ programs:
-Wabi-tag (C++ and Objective-C++ only)
Warn when a type with an ABI tag is used in a context that does not have that ABI tag. See C++ Attributes for
more information about ABI tags.
-Wcomma-subscript (C++ and Objective-C++ only)
Warn about uses of a comma expression within a subscripting expression. This usage was deprecated in C++20 and
is going to be removed in C++23. However, a comma expression wrapped in "( )" is not deprecated. Example:
void f(int *a, int b, int c) {
a[b,c]; // deprecated in C++20, invalid in C++23
a[(b,c)]; // OK
}
In C++23 it is valid to have comma separated expressions in a subscript when an overloaded subscript operator is
found and supports the right number and types of arguments. G++ will accept the formerly valid syntax for code
that is not valid in C++23 but used to be valid but deprecated in C++20 with a pedantic warning that can be
disabled with -Wno-comma-subscript.
Enabled by default with -std=c++20 unless -Wno-deprecated, and after -std=c++23 regardless of -Wno-deprecated.
Before -std=c++20, enabled with explicit -Wdeprecated.
This warning is upgraded to an error by -pedantic-errors in C++23 mode or later.
-Wctad-maybe-unsupported (C++ and Objective-C++ only)
Warn when performing class template argument deduction (CTAD) on a type with no explicitly written deduction
guides. This warning will point out cases where CTAD succeeded only because the compiler synthesized the
implicit deduction guides, which might not be what the programmer intended. Certain style guides allow CTAD only
on types that specifically "opt-in"; i.e., on types that are designed to support CTAD. This warning can be
suppressed with the following pattern:
struct allow_ctad_t; // any name works
template struct S {
S(T) { }
};
// Guide with incomplete parameter type will never be considered.
S(allow_ctad_t) -> S;
-Wctor-dtor-privacy (C++ and Objective-C++ only)
Warn when a class seems unusable because all the constructors or destructors in that class are private, and it
has neither friends nor public static member functions. Also warn if there are no non-private methods, and
there's at least one private member function that isn't a constructor or destructor.
-Wdangling-reference (C++ and Objective-C++ only)
Warn when a reference is bound to a temporary whose lifetime has ended. For example:
int n = 1;
const int& r = std::max(n - 1, n + 1); // r is dangling
In the example above, two temporaries are created, one for each argument, and a reference to one of the
temporaries is returned. However, both temporaries are destroyed at the end of the full expression, so the
reference "r" is dangling. This warning also detects dangling references in member initializer lists:
const int& f(const int& i) { return i; }
struct S {
const int &r; // r is dangling
S() : r(f(10)) { }
};
Member functions are checked as well, but only their object argument:
struct S {
const S& self () { return *this; }
};
const S& s = S().self(); // s is dangling
Certain functions are safe in this respect, for example "std::use_facet": they take and return a reference, but
they don't return one of its arguments, which can fool the warning. Such functions can be excluded from the
warning by wrapping them in a "#pragma":
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wdangling-reference"
const T& foo (const T&) { ... }
#pragma GCC diagnostic pop
The "#pragma" can also surround the class; in that case, the warning will be disabled for all the member
functions.
-Wdangling-reference also warns about code like
auto p = std::minmax(1, 2);
where "std::minmax" returns "std::pair", and both references dangle after the end of the
full expression that contains the call to "std::minmax".
The warning does not warn for "std::span"-like classes. We consider classes of the form:
template
struct Span {
T* data_;
std::size len_;
};
as "std::span"-like; that is, the class is a non-union class that has a pointer data member and a trivial
destructor.
The warning can be disabled by using the "gnu::no_dangling" attribute.
This warning is enabled by -Wextra.
-Wdelete-non-virtual-dtor (C++ and Objective-C++ only)
Warn when "delete" is used to destroy an instance of a class that has virtual functions and non-virtual
destructor. It is unsafe to delete an instance of a derived class through a pointer to a base class if the base
class does not have a virtual destructor. This warning is enabled by -Wall.
-Wdeprecated-copy (C++ and Objective-C++ only)
Warn that the implicit declaration of a copy constructor or copy assignment operator is deprecated if the class
has a user-provided copy constructor or copy assignment operator, in C++11 and up. This warning is enabled by
-Wextra. With -Wdeprecated-copy-dtor, also deprecate if the class has a user-provided destructor.
-Wno-deprecated-enum-enum-conversion (C++ and Objective-C++ only)
Disable the warning about the case when the usual arithmetic conversions are applied on operands where one is of
enumeration type and the other is of a different enumeration type. This conversion was deprecated in C++20. For
example:
enum E1 { e };
enum E2 { f };
int k = f - e;
-Wdeprecated-enum-enum-conversion is enabled by default with -std=c++20. In pre-C++20 dialects, this warning can
be enabled by -Wenum-conversion or -Wdeprecated.
-Wno-deprecated-enum-float-conversion (C++ and Objective-C++ only)
Disable the warning about the case when the usual arithmetic conversions are applied on operands where one is of
enumeration type and the other is of a floating-point type. This conversion was deprecated in C++20. For
example:
enum E1 { e };
enum E2 { f };
bool b = e <= 3.7;
-Wdeprecated-enum-float-conversion is enabled by default with -std=c++20. In pre-C++20 dialects, this warning
can be enabled by -Wenum-conversion or -Wdeprecated.
-Wdeprecated-literal-operator (C++ and Objective-C++ only)
Warn that the declaration of a user-defined literal operator with a space before the suffix is deprecated. This
warning is enabled by default in C++23, or with explicit -Wdeprecated.
string operator "" _i18n(const char*, std::size_t); // deprecated
string operator ""_i18n(const char*, std::size_t); // preferred
-Wdeprecated-variadic-comma-omission (C++ and Objective-C++ only)
Warn that omitting a comma before the varargs "..." at the end of a function parameter list is deprecated. This
warning is enabled by default in C++26, or with explicit -Wdeprecated.
void f1(int...); // deprecated
void f1(int, ...); // preferred
template
void f2(T...); // ok
template
void f3(T......); // deprecated
-Wno-elaborated-enum-base
For C++11 and above, warn if an (invalid) additional enum-base is used in an elaborated-type-specifier. That is,
if an enum with given underlying type and no enumerator list is used in a declaration other than just a
standalone declaration of the enum. Enabled by default. This warning is upgraded to an error with
-pedantic-errors.
-Wno-init-list-lifetime (C++ and Objective-C++ only)
Do not warn about uses of "std::initializer_list" that are likely to result in dangling pointers. Since the
underlying array for an "initializer_list" is handled like a normal C++ temporary object, it is easy to
inadvertently keep a pointer to the array past the end of the array's lifetime. For example:
* If a function returns a temporary "initializer_list", or a local "initializer_list" variable, the array's
lifetime ends at the end of the return statement, so the value returned has a dangling pointer.
* If a new-expression creates an "initializer_list", the array only lives until the end of the enclosing full-
expression, so the "initializer_list" in the heap has a dangling pointer.
* When an "initializer_list" variable is assigned from a brace-enclosed initializer list, the temporary array
created for the right side of the assignment only lives until the end of the full-expression, so at the next
statement the "initializer_list" variable has a dangling pointer.
// li's initial underlying array lives as long as li
std::initializer_list li = { 1,2,3 };
// assignment changes li to point to a temporary array
li = { 4, 5 };
// now the temporary is gone and li has a dangling pointer
int i = li.begin()[0] // undefined behavior
* When a list constructor stores the "begin" pointer from the "initializer_list" argument, this doesn't extend
the lifetime of the array, so if a class variable is constructed from a temporary "initializer_list", the
pointer is left dangling by the end of the variable declaration statement.
-Winvalid-constexpr
Warn when a function never produces a constant expression. In C++20 and earlier, for every "constexpr" function
and function template, there must be at least one set of function arguments in at least one instantiation such
that an invocation of the function or constructor could be an evaluated subexpression of a core constant
expression. C++23 removed this restriction, so it's possible to have a function or a function template marked
"constexpr" for which no invocation satisfies the requirements of a core constant expression.
This warning is enabled as a pedantic warning by default in C++20 and earlier. In C++23, -Winvalid-constexpr can
be turned on, in which case it will be an ordinary warning. For example:
void f (int& i);
constexpr void
g (int& i)
{
// Warns by default in C++20, in C++23 only with -Winvalid-constexpr.
f(i);
}
-Winvalid-imported-macros
Verify all imported macro definitions are valid at the end of compilation. This is not enabled by default, as it
requires additional processing to determine. It may be useful when preparing sets of header-units to ensure
consistent macros.
-Wno-literal-suffix (C++ and Objective-C++ only)
Do not warn when a string or character literal is followed by a ud-suffix which does not begin with an
underscore. As a conforming extension, GCC treats such suffixes as separate preprocessing tokens in order to
maintain backwards compatibility with code that uses formatting macros from "". For example:
#define __STDC_FORMAT_MACROS
#include
#include
int main() {
int64_t i64 = 123;
printf("My int64: %" PRId64"\n", i64);
}
In this case, "PRId64" is treated as a separate preprocessing token.
This option also controls warnings when a user-defined literal operator is declared with a literal suffix
identifier that doesn't begin with an underscore. Literal suffix identifiers that don't begin with an underscore
are reserved for future standardization.
These warnings are enabled by default.
-Wno-narrowing (C++ and Objective-C++ only)
For C++11 and later standards, narrowing conversions are diagnosed by default, as required by the standard. A
narrowing conversion from a constant produces an error, and a narrowing conversion from a non-constant produces a
warning, but -Wno-narrowing suppresses the diagnostic. Note that this does not affect the meaning of well-formed
code; narrowing conversions are still considered ill-formed in SFINAE contexts.
With -Wnarrowing in C++98, warn when a narrowing conversion prohibited by C++11 occurs within { }, e.g.
int i = { 2.2 }; // error: narrowing from double to int
This flag is included in -Wall and -Wc++11-compat.
-Wnoexcept (C++ and Objective-C++ only)
Warn when a noexcept-expression evaluates to false because of a call to a function that does not have a non-
throwing exception specification (i.e. throw() or "noexcept") but is known by the compiler to never throw an
exception.
-Wnoexcept-type (C++ and Objective-C++ only)
Warn if the C++17 feature making "noexcept" part of a function type changes the mangled name of a symbol relative
to C++14. Enabled by -Wabi and -Wc++17-compat.
As an example:
template void f(T t) { t(); };
void g() noexcept;
void h() { f(g); }
In C++14, "f" calls "f", but in C++17 it calls "f".
-Wclass-memaccess (C++ and Objective-C++ only)
Warn when the destination of a call to a raw memory function such as "memset" or "memcpy" is an object of class
type, and when writing into such an object might bypass the class non-trivial or deleted constructor or copy
assignment, violate const-correctness or encapsulation, or corrupt virtual table pointers. Modifying the
representation of such objects may violate invariants maintained by member functions of the class. For example,
the call to "memset" below is undefined because it modifies a non-trivial class object and is, therefore,
diagnosed. The safe way to either initialize or clear the storage of objects of such types is by using the
appropriate constructor or assignment operator, if one is available.
std::string str = "abc";
memset (&str, 0, sizeof str);
The -Wclass-memaccess option is enabled by -Wall. Explicitly casting the pointer to the class object to "void *"
or to a type that can be safely accessed by the raw memory function suppresses the warning.
-Wnon-virtual-dtor (C++ and Objective-C++ only)
Warn when a class has virtual functions and an accessible non-virtual destructor itself or in an accessible
polymorphic base class, in which case it is possible but unsafe to delete an instance of a derived class through
a pointer to the class itself or base class. This warning is automatically enabled if -Weffc++ is specified.
The -Wdelete-non-virtual-dtor option (enabled by -Wall) should be preferred because it warns about the unsafe
cases without false positives.
-Wregister (C++ and Objective-C++ only)
Warn on uses of the "register" storage class specifier, except when it is part of the GNU Explicit Register
Variables extension. The use of the "register" keyword as storage class specifier has been deprecated in C++11
and removed in C++17. Enabled by default with -std=c++17.
-Wreorder (C++ and Objective-C++ only)
Warn when the order of member initializers given in the code does not match the order in which they must be
executed. For instance:
struct A {
int i;
int j;
A(): j (0), i (1) { }
};
The compiler rearranges the member initializers for "i" and "j" to match the declaration order of the members,
emitting a warning to that effect. This warning is enabled by -Wall.
-Wno-pessimizing-move (C++ and Objective-C++ only)
This warning warns when a call to "std::move" prevents copy elision. A typical scenario when copy elision can
occur is when returning in a function with a class return type, when the expression being returned is the name of
a non-volatile automatic object, and is not a function parameter, and has the same type as the function return
type.
struct T {
...
};
T fn()
{
T t;
...
return std::move (t);
}
But in this example, the "std::move" call prevents copy elision.
This warning is enabled by -Wall.
-Wno-redundant-move (C++ and Objective-C++ only)
This warning warns about redundant calls to "std::move"; that is, when a move operation would have been performed
even without the "std::move" call. This happens because the compiler is forced to treat the object as if it were
an rvalue in certain situations such as returning a local variable, where copy elision isn't applicable.
Consider:
struct T {
...
};
T fn(T t)
{
...
return std::move (t);
}
Here, the "std::move" call is redundant. Because G++ implements Core Issue 1579, another example is:
struct T { // convertible to U
...
};
struct U {
...
};
U fn()
{
T t;
...
return std::move (t);
}
In this example, copy elision isn't applicable because the type of the expression being returned and the function
return type differ, yet G++ treats the return value as if it were designated by an rvalue.
This warning is enabled by -Wextra.
-Wrange-loop-construct (C++ and Objective-C++ only)
This warning warns when a C++ range-based for-loop is creating an unnecessary copy. This can happen when the
range declaration is not a reference, but probably should be. For example:
struct S { char arr[128]; };
void fn () {
S arr[5];
for (const auto x : arr) { ... }
}
It does not warn when the type being copied is a trivially-copyable type whose size is less than 64 bytes.
This warning also warns when a loop variable in a range-based for-loop is initialized with a value of a different
type resulting in a copy. For example:
void fn() {
int arr[10];
for (const double &x : arr) { ... }
}
In the example above, in every iteration of the loop a temporary value of type "double" is created and destroyed,
to which the reference "const double &" is bound.
This warning is enabled by -Wall.
-Wredundant-tags (C++ and Objective-C++ only)
Warn about redundant class-key and enum-key in references to class types and enumerated types in contexts where
the key can be eliminated without causing an ambiguity. For example:
struct foo;
struct foo *p; // warn that keyword struct can be eliminated
On the other hand, in this example there is no warning:
struct foo;
void foo (); // "hides" struct foo
void bar (struct foo&); // no warning, keyword struct is necessary
-Wno-subobject-linkage (C++ and Objective-C++ only)
Do not warn if a class type has a base or a field whose type uses the anonymous namespace or depends on a type
with no linkage. If a type A depends on a type B with no or internal linkage, defining it in multiple
translation units would be an ODR violation because the meaning of B is different in each translation unit. If A
only appears in a single translation unit, the best way to silence the warning is to give it internal linkage by
putting it in an anonymous namespace as well. The compiler doesn't give this warning for types defined in the
main .C file, as those are unlikely to have multiple definitions. -Wsubobject-linkage is enabled by default.
-Weffc++ (C++ and Objective-C++ only)
Warn about violations of the following style guidelines from Scott Meyers' Effective C++ series of books:
* Define a copy constructor and an assignment operator for classes with dynamically-allocated memory.
* Prefer initialization to assignment in constructors.
* Have "operator=" return a reference to *this.
* Don't try to return a reference when you must return an object.
* Distinguish between prefix and postfix forms of increment and decrement operators.
* Never overload "&&", "||", or ",".
This option also enables -Wnon-virtual-dtor, which is also one of the effective C++ recommendations. However,
the check is extended to warn about the lack of virtual destructor in accessible non-polymorphic bases classes
too.
When selecting this option, be aware that the standard library headers do not obey all of these guidelines; use
grep -v to filter out those warnings.
-Wno-exceptions (C++ and Objective-C++ only)
Disable the warning about the case when an exception handler is shadowed by another handler, which can point out
a wrong ordering of exception handlers.
-Wstrict-null-sentinel (C++ and Objective-C++ only)
Warn about the use of an uncasted "NULL" as sentinel. When compiling only with GCC this is a valid sentinel, as
"NULL" is defined to "__null". Although it is a null pointer constant rather than a null pointer, it is
guaranteed to be of the same size as a pointer. But this use is not portable across different compilers.
-Wno-non-template-friend (C++ and Objective-C++ only)
Disable warnings when non-template friend functions are declared within a template. In very old versions of GCC
that predate implementation of the ISO standard, declarations such as friend int foo(int), where the name of the
friend is an unqualified-id, could be interpreted as a particular specialization of a template function; the
warning exists to diagnose compatibility problems, and is enabled by default.
-Wold-style-cast (C++ and Objective-C++ only)
Warn if an old-style (C-style) cast to a non-void type is used within a C++ program. The new-style casts
("dynamic_cast", "static_cast", "reinterpret_cast", and "const_cast") are less vulnerable to unintended effects
and much easier to search for.
-Woverloaded-virtual (C++ and Objective-C++ only)
-Woverloaded-virtual=n
Warn when a function declaration hides virtual functions from a base class. For example, in:
struct A {
virtual void f();
};
struct B: public A {
void f(int); // does not override
};
the "A" class version of "f" is hidden in "B", and code like:
B* b;
b->f();
fails to compile.
In cases where the different signatures are not an accident, the simplest solution is to add a using-declaration
to the derived class to un-hide the base function, e.g. add "using A::f;" to "B".
The optional level suffix controls the behavior when all the declarations in the derived class override virtual
functions in the base class, even if not all of the base functions are overridden:
struct C {
virtual void f();
virtual void f(int);
};
struct D: public C {
void f(int); // does override
}
This pattern is less likely to be a mistake; if D is only used virtually, the user might have decided that the
base class semantics for some of the overloads are fine.
At level 1, this case does not warn; at level 2, it does. -Woverloaded-virtual by itself selects level 2. Level
1 is included in -Wall.
-Wno-pmf-conversions (C++ and Objective-C++ only)
Disable the diagnostic for converting a bound pointer to member function to a plain pointer.
-Wsign-promo (C++ and Objective-C++ only)
Warn when overload resolution chooses a promotion from unsigned or enumerated type to a signed type, over a
conversion to an unsigned type of the same size. Previous versions of G++ tried to preserve unsignedness, but
the standard mandates the current behavior.
-Wtemplates (C++ and Objective-C++ only)
Warn when a primary template declaration is encountered. Some coding rules disallow templates, and this may be
used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can
still use the STL. One may also instantiate or specialize templates.
-Wmismatched-new-delete (C++ and Objective-C++ only)
Warn for mismatches between calls to "operator new" or "operator delete" and the corresponding call to the
allocation or deallocation function. This includes invocations of C++ "operator delete" with pointers returned
from either mismatched forms of "operator new", or from other functions that allocate objects for which the
"operator delete" isn't a suitable deallocator, as well as calls to other deallocation functions with pointers
returned from "operator new" for which the deallocation function isn't suitable.
For example, the "delete" expression in the function below is diagnosed because it doesn't match the array form
of the "new" expression the pointer argument was returned from. Similarly, the call to "free" is also diagnosed.
void f ()
{
int *a = new int[n];
delete a; // warning: mismatch in array forms of expressions
char *p = new char[n];
free (p); // warning: mismatch between new and free
}
The related option -Wmismatched-dealloc diagnoses mismatches involving allocation and deallocation functions
other than "operator new" and "operator delete".
-Wmismatched-new-delete is included in -Wall.
-Wmismatched-tags (C++ and Objective-C++ only)
Warn for declarations of structs, classes, and class templates and their specializations with a class-key that
does not match either the definition or the first declaration if no definition is provided.
For example, the declaration of "struct Object" in the argument list of "draw" triggers the warning. To avoid
it, either remove the redundant class-key "struct" or replace it with "class" to match its definition.
class Object {
public:
virtual ~Object () = 0;
};
void draw (struct Object*);
It is not wrong to declare a class with the class-key "struct" as the example above shows. The -Wmismatched-tags
option is intended to help achieve a consistent style of class declarations. In code that is intended to be
portable to Windows-based compilers the warning helps prevent unresolved references due to the difference in the
mangling of symbols declared with different class-keys. The option can be used either on its own or in
conjunction with -Wredundant-tags.
-Wmultiple-inheritance (C++ and Objective-C++ only)
Warn when a class is defined with multiple direct base classes. Some coding rules disallow multiple inheritance,
and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL,
so one can still use the STL. One may also define classes that indirectly use multiple inheritance.
-Wvirtual-inheritance
Warn when a class is defined with a virtual direct base class. Some coding rules disallow multiple inheritance,
and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL,
so one can still use the STL. One may also define classes that indirectly use virtual inheritance.
-Wno-virtual-move-assign
Suppress warnings about inheriting from a virtual base with a non-trivial C++11 move assignment operator. This
is dangerous because if the virtual base is reachable along more than one path, it is moved multiple times, which
can mean both objects end up in the moved-from state. If the move assignment operator is written to avoid moving
from a moved-from object, this warning can be disabled.
-Wnamespaces
Warn when a namespace definition is opened. Some coding rules disallow namespaces, and this may be used to
enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use
the STL. One may also use using directives and qualified names.
-Wno-template-body (C++ and Objective-C++ only)
Disable diagnosing errors when parsing a template, and instead issue an error only upon instantiation of the
template. This flag can also be used to downgrade such errors into warnings with Wno-error= or -fpermissive.
-Wno-template-id-cdtor (C++ and Objective-C++ only)
Disable the warning about the use of simple-template-id as the declarator-id of a constructor or destructor,
which became invalid in C++20 via DR 2237. For example:
template struct S {
S(); // should be S();
~S(); // should be ~S();
};
-Wtemplate-id-cdtor is enabled by default with -std=c++20; it is also enabled by -Wc++20-compat.
-Wtemplate-names-tu-local
Warn when a template body hides an exposure of a translation-unit-local entity. In most cases, referring to a
translation-unit-local entity (such as an internal linkage declaration) within an entity that is emitted into a
module's CMI is an error. However, within the initializer of a variable, or in the body of a non-inline
function, this is not an exposure and no error is emitted.
This can cause variable or function templates to accidentally become unusable if they reference such an entity,
because other translation units that import the template will never be able to instantiate it. This warning
attempts to detect cases where this might occur. The presence of an explicit instantiation silences the warning.
This flag is enabled by -Wextra.
-Wno-terminate (C++ and Objective-C++ only)
Disable the warning about a throw-expression that will immediately result in a call to "terminate".
-Wno-vexing-parse (C++ and Objective-C++ only)
Warn about the most vexing parse syntactic ambiguity. This warns about the cases when a declaration looks like a
variable definition, but the C++ language requires it to be interpreted as a function declaration. For instance:
void f(double a) {
int i(); // extern int i (void);
int n(int(a)); // extern int n (int);
}
Another example:
struct S { S(int); };
void f(double a) {
S x(int(a)); // extern struct S x (int);
S y(int()); // extern struct S y (int (*) (void));
S z(); // extern struct S z (void);
}
The warning will suggest options how to deal with such an ambiguity; e.g., it can suggest removing the
parentheses or using braces instead.
This warning is enabled by default.
-Wno-class-conversion (C++ and Objective-C++ only)
Do not warn when a conversion function converts an object to the same type, to a base class of that type, or to
void; such a conversion function will never be called.
-Wvolatile (C++ and Objective-C++ only)
Warn about deprecated uses of the "volatile" qualifier. This includes postfix and prefix "++" and "--"
expressions of "volatile"-qualified types, using simple assignments where the left operand is a
"volatile"-qualified non-class type for their value, compound assignments where the left operand is a
"volatile"-qualified non-class type, "volatile"-qualified function return type, "volatile"-qualified parameter
type, and structured bindings of a "volatile"-qualified type. This usage was deprecated in C++20.
Enabled by default with -std=c++20. Before -std=c++20, enabled with explicit -Wdeprecated.
-Waligned-new
Warn about a new-expression of a type that requires greater alignment than the alignof(std::max_align_t) but uses
an allocation function without an explicit alignment parameter. This option is enabled by -Wall.
Normally this only warns about global allocation functions, but -Waligned-new=all also warns about class member
allocation functions.
-Wno-placement-new
-Wplacement-new=n
Warn about placement new expressions with undefined behavior, such as constructing an object in a buffer that is
smaller than the type of the object. For example, the placement new expression below is diagnosed because it
attempts to construct an array of 64 integers in a buffer only 64 bytes large.
char buf [64];
new (buf) int[64];
This warning is enabled by default.
-Wplacement-new=1
This is the default warning level of -Wplacement-new. At this level the warning is not issued for some
strictly undefined constructs that GCC allows as extensions for compatibility with legacy code. For example,
the following "new" expression is not diagnosed at this level even though it has undefined behavior according
to the C++ standard because it writes past the end of the one-element array.
struct S { int n, a[1]; };
S *s = (S *)malloc (sizeof *s + 31 * sizeof s->a[0]);
new (s->a)int [32]();
-Wplacement-new=2
At this level, in addition to diagnosing all the same constructs as at level 1, a diagnostic is also issued
for placement new expressions that construct an object in the last member of structure whose type is an array
of a single element and whose size is less than the size of the object being constructed. While the previous
example would be diagnosed, the following construct makes use of the flexible member array extension to avoid
the warning at level 2.
struct S { int n, a[]; };
S *s = (S *)malloc (sizeof *s + 32 * sizeof s->a[0]);
new (s->a)int [32]();
-Wcatch-value
-Wcatch-value=n (C++ and Objective-C++ only)
Warn about catch handlers that do not catch via reference. With -Wcatch-value=1 (or -Wcatch-value for short)
warn about polymorphic class types that are caught by value. With -Wcatch-value=2 warn about all class types
that are caught by value. With -Wcatch-value=3 warn about all types that are not caught by reference.
-Wcatch-value is enabled by -Wall.
-Wconditionally-supported (C++ and Objective-C++ only)
Warn for conditionally-supported (C++11 [intro.defs]) constructs.
-Wno-defaulted-function-deleted (C++ and Objective-C++ only)
Warn when an explicitly defaulted function is deleted by the compiler. That can occur when the function's
declared type does not match the type of the function that would have been implicitly declared. This warning is
enabled by default.
-Wno-delete-incomplete (C++ and Objective-C++ only)
Do not warn when deleting a pointer to incomplete type, which may cause undefined behavior at runtime. This
warning is enabled by default.
-Wextra-semi (C++, Objective-C++ only)
Warn about redundant semicolons. There are various contexts in which an extra semicolon can occur. One is a
semicolon after in-class function definitions, which is valid in all C++ dialects (and is never a pedwarn):
struct S {
void foo () {};
};
Another is an extra semicolon at namespace scope, which has been allowed since C++11 (therefore is a pedwarn in
C++98):
struct S {
};
;
And yet another is an extra semicolon in class definitions, which has been allowed since C++11 (therefore is a
pedwarn in C++98):
struct S {
int a;
;
};
-Wno-global-module (C++ and Objective-C++ only)
Disable the diagnostic for when the global module fragment of a module unit does not consist only of preprocessor
directives.
-Wno-inaccessible-base (C++, Objective-C++ only)
This option controls warnings when a base class is inaccessible in a class derived from it due to ambiguity. The
warning is enabled by default. Note that the warning for ambiguous virtual bases is enabled by the -Wextra
option.
struct A { int a; };
struct B : A { };
struct C : B, A { };
-Wno-inherited-variadic-ctor
Suppress warnings about use of C++11 inheriting constructors when the base class inherited from has a C variadic
constructor; the warning is on by default because the ellipsis is not inherited.
-Wno-invalid-offsetof (C++ and Objective-C++ only)
Suppress warnings from applying the "offsetof" macro to a non-POD type. According to the 2014 ISO C++ standard,
applying "offsetof" to a non-standard-layout type is undefined. In existing C++ implementations, however,
"offsetof" typically gives meaningful results. This flag is for users who are aware that they are writing
nonportable code and who have deliberately chosen to ignore the warning about it.
The restrictions on "offsetof" may be relaxed in a future version of the C++ standard.
-Wsized-deallocation (C++ and Objective-C++ only)
Warn about a definition of an unsized deallocation function
void operator delete (void *) noexcept;
void operator delete[] (void *) noexcept;
without a definition of the corresponding sized deallocation function
void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;
or vice versa. Enabled by -Wextra along with -fsized-deallocation.
-Wsuggest-final-types
Warn about types with virtual methods where code quality would be improved if the type were declared with the
C++11 "final" specifier, or, if possible, declared in an anonymous namespace. This allows GCC to more
aggressively devirtualize the polymorphic calls. This warning is more effective with link-time optimization,
where the information about the class hierarchy graph is more complete.
-Wsuggest-final-methods
Warn about virtual methods where code quality would be improved if the method were declared with the C++11
"final" specifier, or, if possible, its type were declared in an anonymous namespace or with the "final"
specifier. This warning is more effective with link-time optimization, where the information about the class
hierarchy graph is more complete. It is recommended to first consider suggestions of -Wsuggest-final-types and
then rebuild with new annotations.
-Wsuggest-override
Warn about overriding virtual functions that are not marked with the "override" keyword.
-Wno-conversion-null (C++ and Objective-C++ only)
Do not warn for conversions between "NULL" and non-pointer types. -Wconversion-null is enabled by default.
Options Controlling Objective-C and Objective-C++ Dialects
(NOTE: This manual does not describe the Objective-C and Objective-C++ languages themselves.
This section describes the command-line options that are only meaningful for Objective-C and Objective-C++ programs.
You can also use most of the language-independent GNU compiler options. For example, you might compile a file
some_class.m like this:
gcc -g -fgnu-runtime -O -c some_class.m
In this example, -fgnu-runtime is an option meant only for Objective-C and Objective-C++ programs; you can use the
other options with any language supported by GCC.
Note that since Objective-C is an extension of the C language, Objective-C compilations may also use options specific
to the C front-end (e.g., -Wtraditional). Similarly, Objective-C++ compilations may use C++-specific options (e.g.,
-Wabi).
Here is a list of options that are only for compiling Objective-C and Objective-C++ programs:
-fconstant-string-class=class-name
Use class-name as the name of the class to instantiate for each literal string specified with the syntax
"@"..."". The default class name is "NXConstantString" if the GNU runtime is being used, and "NSConstantString"
if the NeXT runtime is being used (see below). On Darwin / macOS platforms, the -fconstant-cfstrings option, if
also present, overrides the -fconstant-string-class setting and cause "@"..."" literals to be laid out as
constant CoreFoundation strings. Note that -fconstant-cfstrings is an alias for the target-specific
-mconstant-cfstrings equivalent.
-fgnu-runtime
Generate object code compatible with the standard GNU Objective-C runtime. This is the default for most types of
systems.
-fnext-runtime
Generate output compatible with the NeXT runtime. This is the default for NeXT-based systems, including Darwin /
macOS. The macro "__NEXT_RUNTIME__" is predefined if (and only if) this option is used.
-fno-nil-receivers
Assume that all Objective-C message dispatches ("[receiver message:arg]") in this translation unit ensure that
the receiver is not "nil". This allows for more efficient entry points in the runtime to be used. This option
is only available in conjunction with the NeXT runtime and ABI version 0 or 1.
-fobjc-abi-version=n
Use version n of the Objective-C ABI for the selected runtime. This option is currently supported only for the
NeXT runtime. In that case, Version 0 is the traditional (32-bit) ABI without support for properties and other
Objective-C 2.0 additions. Version 1 is the traditional (32-bit) ABI with support for properties and other
Objective-C 2.0 additions. Version 2 is the modern (64-bit) ABI. If nothing is specified, the default is
Version 0 on 32-bit target machines, and Version 2 on 64-bit target machines.
-fobjc-call-cxx-cdtors
For each Objective-C class, check if any of its instance variables is a C++ object with a non-trivial default
constructor. If so, synthesize a special "- (id) .cxx_construct" instance method which runs non-trivial default
constructors on any such instance variables, in order, and then return "self". Similarly, check if any instance
variable is a C++ object with a non-trivial destructor, and if so, synthesize a special "- (void) .cxx_destruct"
method which runs all such default destructors, in reverse order.
The "- (id) .cxx_construct" and "- (void) .cxx_destruct" methods thusly generated only operate on instance
variables declared in the current Objective-C class, and not those inherited from superclasses. It is the
responsibility of the Objective-C runtime to invoke all such methods in an object's inheritance hierarchy. The
"- (id) .cxx_construct" methods are invoked by the runtime immediately after a new object instance is allocated;
the "- (void) .cxx_destruct" methods are invoked immediately before the runtime deallocates an object instance.
As of this writing, only the NeXT runtime on Mac OS X 10.4 and later has support for invoking the "- (id)
.cxx_construct" and "- (void) .cxx_destruct" methods.
-fobjc-direct-dispatch
Allow fast jumps to the message dispatcher. On Darwin this is accomplished via the comm page.
-fobjc-exceptions
Enable syntactic support for structured exception handling in Objective-C, similar to what is offered by C++.
This option is required to use the Objective-C keywords @try, @throw, @catch, @finally and @synchronized. This
option is available with both the GNU runtime and the NeXT runtime (but not available in conjunction with the
NeXT runtime on Mac OS X 10.2 and earlier).
-fobjc-gc
Enable garbage collection (GC) in Objective-C and Objective-C++ programs. This option is only available with the
NeXT runtime; the GNU runtime has a different garbage collection implementation that does not require special
compiler flags.
-fobjc-nilcheck
For the NeXT runtime with version 2 of the ABI, check for a nil receiver in method invocations before doing the
actual method call. This is the default and can be disabled using -fno-objc-nilcheck. Class methods and super
calls are never checked for nil in this way no matter what this flag is set to. Currently this flag does nothing
when the GNU runtime, or an older version of the NeXT runtime ABI, is used.
-fobjc-std=objc1
Conform to the language syntax of Objective-C 1.0, the language recognized by GCC 4.0. This only affects the
Objective-C additions to the C/C++ language; it does not affect conformance to C/C++ standards, which is
controlled by the separate C/C++ dialect option flags. When this option is used with the Objective-C or
Objective-C++ compiler, any Objective-C syntax that is not recognized by GCC 4.0 is rejected. This is useful if
you need to make sure that your Objective-C code can be compiled with older versions of GCC.
-freplace-objc-classes
Emit a special marker instructing ld(1) not to statically link in the resulting object file, and allow dyld(1) to
load it in at run time instead. This is used in conjunction with the Fix-and-Continue debugging mode, where the
object file in question may be recompiled and dynamically reloaded in the course of program execution, without
the need to restart the program itself. Currently, Fix-and-Continue functionality is only available in
conjunction with the NeXT runtime on Mac OS X 10.3 and later.
-fzero-link
When compiling for the NeXT runtime, the compiler ordinarily replaces calls to objc_getClass("...") (when the
name of the class is known at compile time) with static class references that get initialized at load time, which
improves run-time performance. Specifying the -fzero-link flag suppresses this behavior and causes calls to
objc_getClass("...") to be retained. This is useful in Zero-Link debugging mode, since it allows for individual
class implementations to be modified during program execution. The GNU runtime currently always retains calls to
objc_get_class("...") regardless of command-line options.
-fno-local-ivars
By default instance variables in Objective-C can be accessed as if they were local variables from within the
methods of the class they're declared in. This can lead to shadowing between instance variables and other
variables declared either locally inside a class method or globally with the same name. Specifying the
-fno-local-ivars flag disables this behavior thus avoiding variable shadowing issues.
-fivar-visibility=[public|protected|private|package]
Set the default instance variable visibility to the specified option so that instance variables declared outside
the scope of any access modifier directives default to the specified visibility.
-gen-decls
Dump interface declarations for all classes seen in the source file to a file named sourcename.decl.
-Wassign-intercept (Objective-C and Objective-C++ only)
Warn whenever an Objective-C assignment is being intercepted by the garbage collector.
-Wno-property-assign-default (Objective-C and Objective-C++ only)
Do not warn if a property for an Objective-C object has no assign semantics specified.
-Wno-protocol (Objective-C and Objective-C++ only)
If a class is declared to implement a protocol, a warning is issued for every method in the protocol that is not
implemented by the class. The default behavior is to issue a warning for every method not explicitly implemented
in the class, even if a method implementation is inherited from the superclass. If you use the -Wno-protocol
option, then methods inherited from the superclass are considered to be implemented, and no warning is issued for
them.
-Wobjc-root-class (Objective-C and Objective-C++ only)
Warn if a class interface lacks a superclass. Most classes will inherit from "NSObject" (or "Object") for
example. When declaring classes intended to be root classes, the warning can be suppressed by marking their
interfaces with "__attribute__((objc_root_class))".
-Wselector (Objective-C and Objective-C++ only)
Warn if multiple methods of different types for the same selector are found during compilation. The check is
performed on the list of methods in the final stage of compilation. Additionally, a check is performed for each
selector appearing in a @selector(...) expression, and a corresponding method for that selector has been found
during compilation. Because these checks scan the method table only at the end of compilation, these warnings
are not produced if the final stage of compilation is not reached, for example because an error is found during
compilation, or because the -fsyntax-only option is being used.
-Wstrict-selector-match (Objective-C and Objective-C++ only)
Warn if multiple methods with differing argument and/or return types are found for a given selector when
attempting to send a message using this selector to a receiver of type "id" or "Class". When this flag is off
(which is the default behavior), the compiler omits such warnings if any differences found are confined to types
that share the same size and alignment.
-Wundeclared-selector (Objective-C and Objective-C++ only)
Warn if a @selector(...) expression referring to an undeclared selector is found. A selector is considered
undeclared if no method with that name has been declared before the @selector(...) expression, either explicitly
in an @interface or @protocol declaration, or implicitly in an @implementation section. This option always
performs its checks as soon as a @selector(...) expression is found, while -Wselector only performs its checks in
the final stage of compilation. This also enforces the coding style convention that methods and selectors must
be declared before being used.
-print-objc-runtime-info
Generate C header describing the largest structure that is passed by value, if any.
Options Controlling OpenMP and OpenACC
GCC supports OpenMP extensions to the C, C++, and Fortran languages with the -fopenmp option. Similarly, OpenACC
extensions are supported in all three languages with -fopenacc.
-foffload=disable
-foffload=default
-foffload=target-list
Specify for which OpenMP and OpenACC offload targets code should be generated. The default behavior, equivalent
to -foffload=default, is to generate code for all supported offload targets. The -foffload=disable form
generates code only for the host fallback, while -foffload=target-list generates code only for the specified
comma-separated list of offload targets.
Offload targets are specified in GCC's internal target-triplet format. You can run the compiler with -v to show
the list of configured offload targets under "OFFLOAD_TARGET_NAMES".
-foffload-options=options
-foffload-options=target-triplet-list=options
With -foffload-options=options, GCC passes the specified options to the compilers for all enabled offloading
targets. You can specify options that apply only to a specific target or targets by using the
-foffload-options=target-list=options form. The target-list is a comma-separated list in the same format as for
the -foffload= option.
Typical command lines are
-foffload-options='-fno-math-errno -ffinite-math-only' \
-foffload-options=nvptx-none=-latomic
-foffload-options=amdgcn-amdhsa=-march=gfx906
-fopenacc
Enable handling of OpenACC directives #pragma acc in C/C++ and !$acc in free-form Fortran and !$acc, c$acc and
*$acc in fixed-form Fortran. This option implies -pthread, and thus is only supported on targets that have
support for -pthread.
-fopenacc-dim=geom
Specify default compute dimensions for parallel offload regions that do not explicitly specify them. The geom
value is a triple of :-separated sizes, in order gang, worker, and vector. A size can be omitted, to use a
target-specific default value.
-fopenmp
Enable handling of OpenMP directives #pragma omp, [[omp::directive(...)]], [[omp::decl(...)]], and
[[omp::sequence(...)]] in C/C++. In Fortran, it enables !$omp and the conditional compilation sentinel !$. In
fixed source form Fortran, the sentinels can also start with c or *.
This option implies -pthread, and thus is only supported on targets that have support for -pthread. -fopenmp
implies -fopenmp-simd.
-fopenmp-simd
Enable handling of OpenMP's "simd", "declare simd", "declare reduction", "assume", "ordered", "scan" and "loop"
directive, and of combined or composite directives with "simd" as constituent with "#pragma omp",
"[[omp::directive(...)]]", "[[omp::sequence(...)]]" and "[[omp::decl(...)]]" in C/C++ and "!$omp" in Fortran. It
additionally enables the conditional compilation sentinel !$ in Fortran. In fixed source form Fortran, the
sentinels can also start with c or *. Other OpenMP directives are ignored. Unless -fopenmp is additionally
specified, the "loop" region binds to the current task region, independent of the specified "bind" clause.
-fopenmp-target-simd-clone
-fopenmp-target-simd-clone=device-type
In addition to generating SIMD clones for functions marked with the "declare simd" directive, GCC also generates
clones for functions marked with the OpenMP "declare target" directive that are suitable for vectorization when
this option is in effect. The device-type may be one of "none", "host", "nohost", and "any", which correspond to
keywords for the "device_type" clause of the "declare target" directive; clones are generated for the
intersection of devices specified. -fopenmp-target-simd-clone is equivalent to -fopenmp-target-simd-clone=any
and -fno-openmp-target-simd-clone is equivalent to -fopenmp-target-simd-clone=none.
At -O2 and higher (but not -Os or -Og) this optimization defaults to -fopenmp-target-simd-clone=nohost; otherwise
it is disabled by default.
Options to Control Diagnostic Messages Formatting
Traditionally, diagnostic messages have been formatted irrespective of the output device's aspect (e.g. its width,
...). You can use the options described below to control the formatting algorithm for diagnostic messages, e.g. how
many characters per line, how often source location information should be reported. Note that some language front
ends may not honor these options.
-fmessage-length=n
Try to format error messages so that they fit on lines of about n characters. If n is zero, then no line-
wrapping is done; each error message appears on a single line. This is the default for all front ends.
Note - this option also affects the display of the #error and #warning pre-processor directives, and the
deprecated function/type/variable attribute. It does not however affect the pragma GCC warning and pragma GCC
error pragmas.
-fdiagnostics-plain-output
This option requests that diagnostic output look as plain as possible, which may be useful when running dejagnu
or other utilities that need to parse diagnostics output and prefer that it remain more stable over time.
-fdiagnostics-plain-output is currently equivalent to the following options: -fno-diagnostics-show-caret
-fno-diagnostics-show-line-numbers -fdiagnostics-color=never -fdiagnostics-urls=never
-fdiagnostics-path-format=separate-events -fdiagnostics-text-art-charset=none -fno-diagnostics-show-event-links
In the future, if GCC changes the default appearance of its diagnostics, the corresponding option to disable the
new behavior will be added to this list.
-fdiagnostics-show-location=once
Only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit source location
information once; that is, in case the message is too long to fit on a single physical line and has to be
wrapped, the source location won't be emitted (as prefix) again, over and over, in subsequent continuation lines.
This is the default behavior.
-fdiagnostics-show-location=every-line
Only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit the same source
location information (as prefix) for physical lines that result from the process of breaking a message which is
too long to fit on a single line.
-fdiagnostics-color[=WHEN]
-fno-diagnostics-color
Use color in diagnostics. WHEN is never, always, or auto. The default depends on how the compiler has been
configured, it can be any of the above WHEN options or also never if GCC_COLORS environment variable isn't
present in the environment, and auto otherwise. auto makes GCC use color only when the standard error is a
terminal, and when not executing in an emacs shell. The forms -fdiagnostics-color and -fno-diagnostics-color are
aliases for -fdiagnostics-color=always and -fdiagnostics-color=never, respectively.
The colors are defined by the environment variable GCC_COLORS. Its value is a colon-separated list of
capabilities and Select Graphic Rendition (SGR) substrings. SGR commands are interpreted by the terminal or
terminal emulator. (See the section in the documentation of your text terminal for permitted values and their
meanings as character attributes.) These substring values are integers in decimal representation and can be
concatenated with semicolons. Common values to concatenate include 1 for bold, 4 for underline, 5 for blink, 7
for inverse, 39 for default foreground color, 30 to 37 for foreground colors, 90 to 97 for 16-color mode
foreground colors, 38;5;0 to 38;5;255 for 88-color and 256-color modes foreground colors, 49 for default
background color, 40 to 47 for background colors, 100 to 107 for 16-color mode background colors, and 48;5;0 to
48;5;255 for 88-color and 256-color modes background colors.
The default GCC_COLORS is
error=01;31:warning=01;35:note=01;36:range1=32:range2=34:locus=01:\
quote=01:path=01;36:fixit-insert=32:fixit-delete=31:\
diff-filename=01:diff-hunk=32:diff-delete=31:diff-insert=32:\
type-diff=01;32:fnname=01;32:targs=35:valid=01;31:invalid=01;32\
highlight-a=01;32:highlight-b=01;34
where 01;31 is bold red, 01;35 is bold magenta, 01;36 is bold cyan, 32 is green, 34 is blue, 01 is bold, and 31
is red. Setting GCC_COLORS to the empty string disables colors. Supported capabilities are as follows.
"error="
SGR substring for error: markers.
"warning="
SGR substring for warning: markers.
"note="
SGR substring for note: markers.
"path="
SGR substring for colorizing paths of control-flow events as printed via -fdiagnostics-path-format=, such as
the identifiers of individual events and lines indicating interprocedural calls and returns.
"range1="
SGR substring for first additional range.
"range2="
SGR substring for second additional range.
"locus="
SGR substring for location information, file:line or file:line:column etc.
"quote="
SGR substring for information printed within quotes.
"fnname="
SGR substring for names of C++ functions.
"targs="
SGR substring for C++ function template parameter bindings.
"fixit-insert="
SGR substring for fix-it hints suggesting text to be inserted or replaced.
"fixit-delete="
SGR substring for fix-it hints suggesting text to be deleted.
"diff-filename="
SGR substring for filename headers within generated patches.
"diff-hunk="
SGR substring for the starts of hunks within generated patches.
"diff-delete="
SGR substring for deleted lines within generated patches.
"diff-insert="
SGR substring for inserted lines within generated patches.
"type-diff="
SGR substring for highlighting mismatching types within template arguments in the C++ frontend.
"valid="
SGR substring for highlighting valid elements within text art diagrams.
"invalid="
SGR substring for highlighting invalid elements within text art diagrams.
"highlight-a="
"highlight-b="
SGR substrings for contrasting two different things within diagnostics, such as a pair of mismatching types.
See -fdiagnostics-show-highlight-colors.
-fdiagnostics-urls[=WHEN]
Use escape sequences to embed URLs in diagnostics. For example, when -fdiagnostics-show-option emits text
showing the command-line option controlling a diagnostic, embed a URL for documentation of that option.
WHEN is never, always, or auto. auto makes GCC use URL escape sequences only when the standard error is a
terminal, and when not executing in an emacs shell or any graphical terminal which is known to be incompatible
with this feature, see below.
The default depends on how the compiler has been configured. It can be any of the above WHEN options.
GCC can also be configured (via the --with-diagnostics-urls=auto-if-env configure-time option) so that the
default is affected by environment variables. Under such a configuration, GCC defaults to using auto if either
GCC_URLS or TERM_URLS environment variables are present and non-empty in the environment of the compiler, or
never if neither are.
However, even with -fdiagnostics-urls=always the behavior is dependent on those environment variables: If
GCC_URLS is set to empty or no, do not embed URLs in diagnostics. If set to st, URLs use ST escape sequences.
If set to bel, the default, URLs use BEL escape sequences. Any other non-empty value enables the feature. If
GCC_URLS is not set, use TERM_URLS as a fallback. Note: ST is an ANSI escape sequence, string terminator ESC \,
BEL is an ASCII character, CTRL-G that usually sounds like a beep.
At this time GCC tries to detect also a few terminals that are known to not implement the URL feature, and have
bugs or at least had bugs in some versions that are still in use, where the URL escapes are likely to misbehave,
i.e. print garbage on the screen. That list is currently xfce4-terminal, certain known to be buggy gnome-
terminal versions, the linux console, and mingw. This check can be skipped with the -fdiagnostics-urls=always.
-fno-diagnostics-show-option
By default, each diagnostic emitted includes text indicating the command-line option that directly controls the
diagnostic (if such an option is known to the diagnostic machinery). Specifying the -fno-diagnostics-show-option
flag suppresses that behavior.
-fno-diagnostics-show-caret
By default, each diagnostic emitted includes the original source line and a caret ^ indicating the column. This
option suppresses this information. The source line is truncated to n characters, if the -fmessage-length=n
option is given. When the output is done to the terminal, the width is limited to the width given by the COLUMNS
environment variable or, if not set, to the terminal width.
-fno-diagnostics-show-labels
By default, when printing source code (via -fdiagnostics-show-caret), diagnostics can label ranges of source code
with pertinent information, such as the types of expressions:
printf ("foo %s bar", long_i + long_j);
~^ ~~~~~~~~~~~~~~~
| |
char * long int
This option suppresses the printing of these labels (in the example above, the vertical bars and the "char *" and
"long int" text).
-fno-diagnostics-show-event-links
By default, when printing execution paths (via -fdiagnostics-path-format=inline-events), GCC will print lines
connecting related events, such as the line connecting events 1 and 2 in:
3 | if (p)
| ^
| |
| (1) following `false' branch (when `p' is NULL)... ->-+
| |
| |
|+------------------------------------------------------------+
4 || return 0;
5 || return *p;
|| ~
|| |
|+-------->(2) ...to here
| (3) dereference of NULL `p'
This option suppresses the printing of such connector lines.
-fno-diagnostics-show-cwe
Diagnostic messages can optionally have an associated
CWE ("https://cwe.mitre.org/index.html") identifier. GCC itself only provides such metadata for some of the
-fanalyzer diagnostics. GCC plugins may also provide diagnostics with such metadata. By default, if this
information is present, it will be printed with the diagnostic. This option suppresses the printing of this
metadata.
-fno-diagnostics-show-rules
Diagnostic messages can optionally have rules associated with them, such as from a coding standard, or a
specification. GCC itself does not do this for any of its diagnostics, but plugins may do so. By default, if
this information is present, it will be printed with the diagnostic. This option suppresses the printing of this
metadata.
-fno-diagnostics-show-highlight-colors
GCC can use color for emphasis and contrast when printing diagnostic messages and quoting the user's source.
For example, in
demo.c: In function `test_bad_format_string_args':
../../src/demo.c:25:18: warning: format `%i' expects argument of type `int', but argument 2 has type `const char *' [-Wformat=]
25 | printf("hello %i", msg);
| ~^ ~~~
| | |
| int const char *
| %s
* the %i and "int" in the message and the "int" in the quoted source are colored using "highlight-a" (bold
green by default), and
* the "const char *" in the message and in the quoted source are both colored using "highlight-b" (bold blue by
default).
The intent is to draw the reader's eyes to the relationships between the various aspects of the diagnostic
message and the source, using color to group related elements and distinguish between mismatching ones.
This additional colorization is enabled by default if color printing is enabled (as per -fdiagnostics-color=),
but it can be separately disabled via -fno-diagnostics-show-highlight-colors.
-fno-diagnostics-show-line-numbers
By default, when printing source code (via -fdiagnostics-show-caret), a left margin is printed, showing line
numbers. This option suppresses this left margin.
-fdiagnostics-minimum-margin-width=width
This option controls the minimum width of the left margin printed by -fdiagnostics-show-line-numbers. It
defaults to 6.
-fdiagnostics-parseable-fixits
Emit fix-it hints in a machine-parseable format, suitable for consumption by IDEs. For each fix-it, a line will
be printed after the relevant diagnostic, starting with the string "fix-it:". For example:
fix-it:"test.c":{45:3-45:21}:"gtk_widget_show_all"
The location is expressed as a half-open range, expressed as a count of bytes, starting at byte 1 for the initial
column. In the above example, bytes 3 through 20 of line 45 of "test.c" are to be replaced with the given
string:
00000000011111111112222222222
12345678901234567890123456789
gtk_widget_showall (dlg);
^^^^^^^^^^^^^^^^^^
gtk_widget_show_all
The filename and replacement string escape backslash as "\\", tab as "\t", newline as "\n", double quotes as
"\"", non-printable characters as octal (e.g. vertical tab as "\013").
An empty replacement string indicates that the given range is to be removed. An empty range (e.g. "45:3-45:3")
indicates that the string is to be inserted at the given position.
-fdiagnostics-generate-patch
Print fix-it hints to stderr in unified diff format, after any diagnostics are printed. For example:
--- test.c
+++ test.c
@ -42,5 +42,5 @
void show_cb(GtkDialog *dlg)
{
- gtk_widget_showall(dlg);
+ gtk_widget_show_all(dlg);
}
The diff may or may not be colorized, following the same rules as for diagnostics (see -fdiagnostics-color).
-fdiagnostics-show-template-tree
In the C++ frontend, when printing diagnostics showing mismatching template types, such as:
could not convert 'std::map >()'
from 'map<[...],vector>' to 'map<[...],vector>
the -fdiagnostics-show-template-tree flag enables printing a tree-like structure showing the common and differing
parts of the types, such as:
map<
[...],
vector<
[double != float]>>
The parts that differ are highlighted with color ("double" and "float" in this case).
-fno-elide-type
By default when the C++ frontend prints diagnostics showing mismatching template types, common parts of the types
are printed as "[...]" to simplify the error message. For example:
could not convert 'std::map >()'
from 'map<[...],vector>' to 'map<[...],vector>
Specifying the -fno-elide-type flag suppresses that behavior. This flag also affects the output of the
-fdiagnostics-show-template-tree flag.
-fdiagnostics-path-format=KIND
Specify how to print paths of control-flow events for diagnostics that have such a path associated with them.
KIND is none, separate-events, or inline-events, the default.
none means to not print diagnostic paths.
separate-events means to print a separate "note" diagnostic for each event within the diagnostic. For example:
test.c:29:5: error: passing NULL as argument 1 to 'PyList_Append' which requires a non-NULL parameter
test.c:25:10: note: (1) when 'PyList_New' fails, returning NULL
test.c:27:3: note: (2) when 'i < count'
test.c:29:5: note: (3) when calling 'PyList_Append', passing NULL from (1) as argument 1
inline-events means to print the events "inline" within the source code. This view attempts to consolidate the
events into runs of sufficiently-close events, printing them as labelled ranges within the source.
For example, the same events as above might be printed as:
'test': events 1-3
25 | list = PyList_New(0);
| ^~~~~~~~~~~~~
| |
| (1) when 'PyList_New' fails, returning NULL
26 |
27 | for (i = 0; i < count; i++) {
| ~~~
| |
| (2) when 'i < count'
28 | item = PyLong_FromLong(random());
29 | PyList_Append(list, item);
| ~~~~~~~~~~~~~~~~~~~~~~~~~
| |
| (3) when calling 'PyList_Append', passing NULL from (1) as argument 1
Interprocedural control flow is shown by grouping the events by stack frame, and using indentation to show how
stack frames are nested, pushed, and popped.
For example:
'test': events 1-2
|
| 133 | {
| | ^
| | |
| | (1) entering 'test'
| 134 | boxed_int *obj = make_boxed_int (i);
| | ~~~~~~~~~~~~~~~~~~
| | |
| | (2) calling 'make_boxed_int'
|
+--> 'make_boxed_int': events 3-4
|
| 120 | {
| | ^
| | |
| | (3) entering 'make_boxed_int'
| 121 | boxed_int *result = (boxed_int *)wrapped_malloc (sizeof (boxed_int));
| | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| | |
| | (4) calling 'wrapped_malloc'
|
+--> 'wrapped_malloc': events 5-6
|
| 7 | {
| | ^
| | |
| | (5) entering 'wrapped_malloc'
| 8 | return malloc (size);
| | ~~~~~~~~~~~~~
| | |
| | (6) calling 'malloc'
|
<-------------+
|
'test': event 7
|
| 138 | free_boxed_int (obj);
| | ^~~~~~~~~~~~~~~~~~~~
| | |
| | (7) calling 'free_boxed_int'
|
(etc)
-fdiagnostics-show-path-depths
This option provides additional information when printing control-flow paths associated with a diagnostic.
If this is option is provided then the stack depth will be printed for each run of events within
-fdiagnostics-path-format=inline-events. If provided with -fdiagnostics-path-format=separate-events, then the
stack depth and function declaration will be appended when printing each event.
This is intended for use by GCC developers and plugin developers when debugging diagnostics that report
interprocedural control flow.
-fno-show-column
Do not print column numbers in diagnostics. This may be necessary if diagnostics are being scanned by a program
that does not understand the column numbers, such as dejagnu.
-fdiagnostics-column-unit=UNIT
Select the units for the column number. This affects traditional diagnostics (in the absence of
-fno-show-column), as well as JSON format diagnostics if requested.
The default UNIT, display, considers the number of display columns occupied by each character. This may be
larger than the number of bytes required to encode the character, in the case of tab characters, or it may be
smaller, in the case of multibyte characters. For example, the character "GREEK SMALL LETTER PI (U+03C0)"
occupies one display column, and its UTF-8 encoding requires two bytes; the character "SLIGHTLY SMILING FACE
(U+1F642)" occupies two display columns, and its UTF-8 encoding requires four bytes.
Setting UNIT to byte changes the column number to the raw byte count in all cases, as was traditionally output by
GCC prior to version 11.1.0.
-fdiagnostics-column-origin=ORIGIN
Select the origin for column numbers, i.e. the column number assigned to the first column. The default value of
1 corresponds to traditional GCC behavior and to the GNU style guide. Some utilities may perform better with an
origin of 0; any non-negative value may be specified.
-fdiagnostics-escape-format=FORMAT
When GCC prints pertinent source lines for a diagnostic it normally attempts to print the source bytes directly.
However, some diagnostics relate to encoding issues in the source file, such as malformed UTF-8, or issues with
Unicode normalization. These diagnostics are flagged so that GCC will escape bytes that are not printable ASCII
when printing their pertinent source lines.
This option controls how such bytes should be escaped.
The default FORMAT, unicode displays Unicode characters that are not printable ASCII in the form , and
bytes that do not correspond to a Unicode character validly-encoded in UTF-8-encoded will be displayed as
hexadecimal in the form .
For example, a source line containing the string before followed by the Unicode character U+03C0 ("GREEK SMALL
LETTER PI", with UTF-8 encoding 0xCF 0x80) followed by the byte 0xBF (a stray UTF-8 trailing byte), followed by
the string after will be printed for such a diagnostic as:
beforeafter
Setting FORMAT to bytes will display all non-printable-ASCII bytes in the form , thus showing the underlying
encoding of non-ASCII Unicode characters. For the example above, the following will be printed:
before<80>after
-fdiagnostics-text-art-charset=CHARSET
Some diagnostics can contain "text art" diagrams: visualizations created from text, intended to be viewed in a
monospaced font.
This option selects which characters should be used for printing such diagrams, if any. CHARSET is none, ascii,
unicode, or emoji.
The none value suppresses the printing of such diagrams. The ascii value will ensure that such diagrams are pure
ASCII ("ASCII art"). The unicode value will allow for conservative use of unicode drawing characters (such as
box-drawing characters). The emoji value further adds the possibility of emoji in the output (such as emitting
U+26A0 WARNING SIGN followed by U+FE0F VARIATION SELECTOR-16 to select the emoji variant of the character).
The default is emoji, except when the environment variable LANG is set to C, in which case the default is ascii.
-fdiagnostics-format=FORMAT
Select a different format for printing diagnostics. FORMAT is text, sarif-stderr, sarif-file, json, json-stderr,
or json-file.
Using this option replaces any additional "output sinks" added by -fdiagnostics-add-output=, or that set by
-fdiagnostics-set-output=.
The default is text.
The sarif-stderr and sarif-file formats both emit diagnostics in SARIF Version 2.1.0 format, either to stderr, or
to a file named source.sarif, respectively.
The various json, json-stderr, and json-file values are deprecated and refer to a legacy JSON-based output
format. The json format is a synonym for json-stderr. The json-stderr and json-file formats are identical,
apart from where the JSON is emitted to. With json-stderr, the JSON is emitted to stderr, whereas with json-file
it is written to source.gcc.json.
-fdiagnostics-add-output=DIAGNOSTICS-OUTPUT-SPEC
Add an additional "output sink" for emitting diagnostics.
DIAGNOSTICS-OUTPUT-SPEC should specify a scheme, optionally followed by ":" and one or more KEY=VALUE pairs, in
this form:
:=
:=,=
etc.
Schemes, keys, or values with a name prefixed "experimental" may change or be removed without notice.
SCHEME can be
text
Emit diagnostics to stderr using GCC's classic text output format.
Supported keys are:
color=[yes|no]
Override colorization settings from -fdiagnostics-color for this text output.
experimental-nesting=[yes|no]
Enable an experimental mode that emphasizes hierarchical relationships within diagnostics messages,
displaying location information on separate lines.
experimental-nesting-show-locations=[yes|no]
If "experimental-nesting=yes", then by default locations are shown; set this key to "no" to disable
printing such locations. This exists for use by GCC developers, for writing DejaGnu test cases.
experimental-nesting-show-levels=[yes|no]
This is a debugging option for use with "experimental-nesting=yes". Set this key to "yes" to print
explicit nesting levels in the output. This exists for use by GCC developers.
sarif
Emit diagnostics to a file in SARIF format.
Supported keys are:
file=FILENAME
Specify the filename to write the SARIF output to, potentially with a leading absolute or relative path.
If not specified, it defaults to source.sarif.
version=[2.1|2.2-prerelease]
Specify the version of SARIF to use for the output. If not specified, defaults to 2.1. "2.2-prerelease"
uses an unofficial draft of the future SARIF 2.2 specification and should only be used for
experimentation in this release.
For example,
-fdiagnostics-add-output=sarif:version=2.1,file=foo.2.1.sarif
-fdiagnostics-add-output=sarif:version=2.2-prerelease,file=foo.2.2.sarif
would add a pair of outputs, each writing to a different file, using versions 2.1 and 2.2 of the SARIF standard
respectively.
In EBNF:
=
| , ":", ;
= "text" | "sarif";
=
| "," ;
= "=" ;
= ? string without a '=' ? ;
= ? string without a ',' ? ;
-fdiagnostics-set-output=DIAGNOSTICS-OUTPUT-SPEC
This works in a similar way to -fdiagnostics-add-output= except that instead of adding an additional "output
sink" for diagnostics, it replaces all existing output sinks, such as from -fdiagnostics-format=,
-fdiagnostics-add-output=, or a prior call to -fdiagnostics-set-output=.
-fno-diagnostics-json-formatting
By default, when JSON is emitted for diagnostics (via -fdiagnostics-format=sarif-stderr,
-fdiagnostics-format=sarif-file, -fdiagnostics-format=json, -fdiagnostics-format=json-stderr,
-fdiagnostics-format=json-file), GCC will add newlines and indentation to visually emphasize the hierarchical
structure of the JSON.
Use -fno-diagnostics-json-formatting to suppress this whitespace. It must be passed before the option it is to
affect.
This is intended for compatibility with tools that do not expect the output to contain newlines, such as that
emitted by older GCC releases.
Options to Request or Suppress Warnings
Warnings are diagnostic messages that report constructions that are not inherently erroneous but that are risky or
suggest there may have been an error.
The following language-independent options do not enable specific warnings but control the kinds of diagnostics
produced by GCC.
-fsyntax-only
Check the code for syntax errors, but don't do anything beyond that.
-fmax-errors=n
Limits the maximum number of error messages to n, at which point GCC bails out rather than attempting to continue
processing the source code. If n is 0 (the default), there is no limit on the number of error messages produced.
If -Wfatal-errors is also specified, then -Wfatal-errors takes precedence over this option.
-w Inhibit all warning messages.
-Werror
Turn all warnings into errors.
-Werror=
Turn the specified warning into an error. The specifier for a warning is appended; for example -Werror=switch
turns the warnings controlled by -Wswitch into errors. This switch takes a negative form, to be used to negate
-Werror for specific warnings; for example -Wno-error=switch makes -Wswitch warnings not be errors, even when
-Werror is in effect.
The warning message for each controllable warning includes the option that controls the warning. That option can
then be used with -Werror= and -Wno-error= as described above. (Printing of the option in the warning message
can be disabled using the -fno-diagnostics-show-option flag.)
Note that specifying -Werror=foo automatically implies -Wfoo. However, -Wno-error=foo does not imply anything.
-Wfatal-errors
This option causes the compiler to abort compilation on the first error occurred rather than trying to keep going
and printing further error messages.
You can request many specific warnings with options beginning with -W, for example -Wunused-variable to request
warnings on declarations of variables that are never used. Each of these specific warning options also has a
negative form beginning with -Wno- to turn off warnings; for example, -Wno-unused-variable. This manual lists only
one of the two forms, whichever is not the default. For further language-specific options also refer to C++ Dialect
Options and Objective-C and Objective-C++ Dialect Options. Additional warnings can be produced by enabling the
static analyzer;
Some options, such as -Wall and -Wextra, turn on other options, such as -Wunused, which may turn on further options,
such as -Wunused-variable. The combined effect of positive and negative forms is that more specific options have
priority over less specific ones, independently of their position in the command line. For options of the same
specificity, the last one takes effect. Options enabled or disabled via pragmas take effect as if they appeared at
the end of the command line.
When an unrecognized warning option is requested (e.g., -Wunknown-warning), GCC gives an error stating that the
option is not recognized. However, if the -Wno- form is used, the behavior is slightly different: no diagnostic is
produced for -Wno-unknown-warning unless other diagnostics are being produced. This allows the use of new -Wno-
options with old compilers, but if something goes wrong, the compiler warns that an unrecognized option is present.
The effectiveness of some warnings depends on optimizations also being enabled. For example, -Wsuggest-final-types is
more effective with link-time optimization. Some other warnings may not be issued at all unless optimization is
enabled. While optimization in general improves the efficacy of warnings about control and data-flow problems, in
some cases it may also cause false positives.
-Wpedantic
-pedantic
Issue all the warnings demanded by strict ISO C and ISO C++; diagnose all programs that use forbidden extensions,
and some other programs that do not follow ISO C and ISO C++. This follows the version of the ISO C or C++
standard specified by any -std option used.
Valid ISO C and ISO C++ programs should compile properly with or without this option (though a rare few require
-ansi or a -std option specifying the version of the standard). However, without this option, certain GNU
extensions and traditional C and C++ features are supported as well. With this option, they are diagnosed (or
rejected with -pedantic-errors).
-Wpedantic does not cause warning messages for use of the alternate keywords whose names begin and end with __.
This alternate format can also be used to disable warnings for non-ISO __intN types, i.e. __intN__. Pedantic
warnings are also disabled in the expression that follows "__extension__". However, only system header files
should use these escape routes; application programs should avoid them.
Some warnings about non-conforming programs are controlled by options other than -Wpedantic; in many cases they
are implied by -Wpedantic but can be disabled separately by their specific option, e.g. -Wpedantic
-Wno-pointer-sign.
Where the standard specified with -std represents a GNU extended dialect of C, such as gnu90 or gnu99, there is a
corresponding base standard, the version of ISO C on which the GNU extended dialect is based. Warnings from
-Wpedantic are given where they are required by the base standard. (It does not make sense for such warnings to
be given only for features not in the specified GNU C dialect, since by definition the GNU dialects of C include
all features the compiler supports with the given option, and there would be nothing to warn about.)
-pedantic-errors
Give an error whenever the base standard (see -Wpedantic) requires a diagnostic, in some cases where there is
undefined behavior at compile-time and in some other cases that do not prevent compilation of programs that are
valid according to the standard. This is not equivalent to -Werror=pedantic: the latter option is unlikely to be
useful, as it only makes errors of the diagnostics that are controlled by -Wpedantic, whereas this option also
affects required diagnostics that are always enabled or controlled by options other than -Wpedantic.
If you want the required diagnostics that are warnings by default to be errors instead, but don't also want to
enable the -Wpedantic diagnostics, you can specify -pedantic-errors -Wno-pedantic (or -pedantic-errors
-Wno-error=pedantic to enable them but only as warnings).
Some required diagnostics are errors by default, but can be reduced to warnings using -fpermissive or their
specific warning option, e.g. -Wno-error=narrowing.
Some diagnostics for non-ISO practices are controlled by specific warning options other than -Wpedantic, but are
also made errors by -pedantic-errors. For instance:
-Wattributes (for standard attributes) -Wchanges-meaning (C++) -Wcomma-subscript (C++23 or later)
-Wdeclaration-after-statement (C90 or earlier) -Welaborated-enum-base (C++11 or later) -Wimplicit-int (C99 or
later) -Wimplicit-function-declaration (C99 or later) -Wincompatible-pointer-types -Wint-conversion -Wlong-long
(C90 or earlier) -Wmain -Wnarrowing (C++11 or later) -Wpointer-arith -Wpointer-sign -Wincompatible-pointer-types
-Wregister (C++17 or later) -Wvla (C90 or earlier) -Wwrite-strings (C++11 or later)
-fpermissive
Downgrade some required diagnostics about nonconformant code from errors to warnings. Thus, using -fpermissive
allows some nonconforming code to compile. Some C++ diagnostics are controlled only by this flag, but it also
downgrades some C and C++ diagnostics that have their own flag:
-Wdeclaration-missing-parameter-type (C and Objective-C only) -Wimplicit-function-declaration (C and Objective-C
only) -Wimplicit-int (C and Objective-C only) -Wincompatible-pointer-types (C and Objective-C only)
-Wint-conversion (C and Objective-C only) -Wnarrowing (C++ and Objective-C++ only) -Wreturn-mismatch (C and
Objective-C only) -Wtemplate-body (C++ and Objective-C++ only)
The -fpermissive option is the default for historic C language modes (-std=c89, -std=gnu89, -std=c90,
-std=gnu90).
-Wall
This enables all the warnings about constructions that some users consider questionable, and that are easy to
avoid (or modify to prevent the warning), even in conjunction with macros. This also enables some language-
specific warnings described in C++ Dialect Options and Objective-C and Objective-C++ Dialect Options.
-Wall turns on the following warning flags:
-Waddress -Waligned-new (C++ and Objective-C++ only) -Warray-bounds=1 (only with -O2) -Warray-compare
-Warray-parameter=2 -Wbool-compare -Wbool-operation -Wc++11-compat -Wc++14-compat -Wc++17compat -Wc++20compat
-Wcatch-value (C++ and Objective-C++ only) -Wchar-subscripts -Wclass-memaccess (C++ and Objective-C++ only)
-Wcomment -Wdangling-else -Wdangling-pointer=2 -Wdelete-non-virtual-dtor (C++ and Objective-C++ only)
-Wduplicate-decl-specifier (C and Objective-C only) -Wenum-compare (in C/ObjC; this is on by default in C++)
-Wenum-int-mismatch (C and Objective-C only) -Wformat=1 -Wformat-contains-nul -Wformat-diag -Wformat-extra-args
-Wformat-overflow=1 -Wformat-truncation=1 -Wformat-zero-length -Wframe-address -Wimplicit (C and Objective-C
only) -Wimplicit-function-declaration (C and Objective-C only) -Wimplicit-int (C and Objective-C only)
-Winfinite-recursion -Winit-self (C++ and Objective-C++ only) -Wint-in-bool-context -Wlogical-not-parentheses
-Wmain (only for C/ObjC and unless -ffreestanding) -Wmaybe-uninitialized -Wmemset-elt-size
-Wmemset-transposed-args -Wmisleading-indentation (only for C/C++) -Wmismatched-dealloc -Wmismatched-new-delete
(C++ and Objective-C++ only) -Wmissing-attributes -Wmissing-braces (only for C/ObjC) -Wmultistatement-macros
-Wnarrowing (C++ and Objective-C++ only) -Wnonnull -Wnonnull-compare -Wopenmp-simd (C and C++ only)
-Woverloaded-virtual=1 (C++ and Objective-C++ only) -Wpacked-not-aligned -Wparentheses -Wpessimizing-move (C++
and Objective-C++ only) -Wpointer-sign (only for C/ObjC) -Wrange-loop-construct (C++ and Objective-C++ only)
-Wreorder (C++ and Objective-C++ only) -Wrestrict -Wreturn-type -Wself-move (C++ and Objective-C++ only)
-Wsequence-point -Wsign-compare (C++ and Objective-C++ only) -Wsizeof-array-div -Wsizeof-pointer-div
-Wsizeof-pointer-memaccess -Wstrict-aliasing -Wstrict-overflow=1 -Wswitch -Wtautological-compare -Wtrigraphs
-Wuninitialized -Wunknown-pragmas -Wunused -Wunused-but-set-variable -Wunused-const-variable=1 (only for C/ObjC)
-Wunused-function -Wunused-label -Wunused-local-typedefs -Wunused-value -Wunused-variable -Wuse-after-free=2
-Wvla-parameter -Wvolatile-register-var -Wzero-length-bounds
Note that some warning flags are not implied by -Wall. Some of them warn about constructions that users
generally do not consider questionable, but which occasionally you might wish to check for; others warn about
constructions that are necessary or hard to avoid in some cases, and there is no simple way to modify the code to
suppress the warning. Some of them are enabled by -Wextra but many of them must be enabled individually.
-Wextra
This enables some extra warning flags that are not enabled by -Wall. (This option used to be called -W. The
older name is still supported, but the newer name is more descriptive.)
-Wabsolute-value (only for C/ObjC) -Walloc-size -Wcalloc-transposed-args -Wcast-function-type -Wclobbered
-Wdangling-reference (C++ only) -Wdeprecated-copy (C++ and Objective-C++ only) -Wempty-body -Wenum-conversion
(only for C/ObjC) -Wexpansion-to-defined -Wignored-qualifiers (only for C/C++) -Wimplicit-fallthrough=3
-Wmaybe-uninitialized -Wmissing-field-initializers -Wmissing-parameter-name (C/ObjC only)
-Wmissing-parameter-type (C/ObjC only) -Wold-style-declaration (C/ObjC only) -Woverride-init (C/ObjC only)
-Wredundant-move (C++ and Objective-C++ only) -Wshift-negative-value (in C++11 to C++17 and in C99 and newer)
-Wsign-compare (C++ and Objective-C++ only) -Wsized-deallocation (C++ and Objective-C++ only) -Wstring-compare
-Wtype-limits -Wuninitialized -Wunterminated-string-initialization -Wunused-parameter (only with -Wunused or
-Wall) -Wunused-but-set-parameter (only with -Wunused or -Wall)
The option -Wextra also prints warning messages for the following cases:
* A pointer is compared against integer zero with "<", "<=", ">", or ">=".
* (C++ only) An enumerator and a non-enumerator both appear in a conditional expression.
* (C++ only) Ambiguous virtual bases.
* (C++ only) Subscripting an array that has been declared "register".
* (C++ only) Taking the address of a variable that has been declared "register".
* (C++ only) A base class is not initialized in the copy constructor of a derived class.
-Wabi (C, Objective-C, C++ and Objective-C++ only)
Warn about code affected by ABI changes. This includes code that may not be compatible with the vendor-neutral
C++ ABI as well as the psABI for the particular target. The latter warnings are also controlled separately by
-Wpsabi, which is implied by -Wabi.
Since G++ now defaults to updating the ABI with each major release, normally -Wabi warns only about C++ ABI
compatibility problems if there is a check added later in a release series for an ABI issue discovered since the
initial release. -Wabi warns about more things if an older ABI version is selected (with -fabi-version=n).
-Wabi can also be used with an explicit version number to warn about C++ ABI compatibility with a particular
-fabi-version level, e.g. -Wabi=2 to warn about changes relative to -fabi-version=2.
If an explicit version number is provided and -fabi-compat-version is not specified, the version number from this
option is used for compatibility aliases. If no explicit version number is provided with this option, but
-fabi-compat-version is specified, that version number is used for C++ ABI warnings.
Although an effort has been made to warn about all such cases, there are probably some cases that are not warned
about, even though G++ is generating incompatible code. There may also be cases where warnings are emitted even
though the code that is generated is compatible.
You should rewrite your code to avoid these warnings if you are concerned about the fact that code generated by
G++ may not be binary compatible with code generated by other compilers.
Known incompatibilities in -fabi-version=2 (which was the default from GCC 3.4 to 4.9) include:
* A template with a non-type template parameter of reference type was mangled incorrectly:
extern int N;
template struct S {};
void n (S) {2}
This was fixed in -fabi-version=3.
* SIMD vector types declared using "__attribute ((vector_size))" were mangled in a non-standard way that does
not allow for overloading of functions taking vectors of different sizes.
The mangling was changed in -fabi-version=4.
* "__attribute ((const))" and "noreturn" were mangled as type qualifiers, and "decltype" of a plain declaration
was folded away.
These mangling issues were fixed in -fabi-version=5.
* Scoped enumerators passed as arguments to a variadic function are promoted like unscoped enumerators, causing
"va_arg" to complain. On most targets this does not actually affect the parameter passing ABI, as there is
no way to pass an argument smaller than "int".
Also, the ABI changed the mangling of template argument packs, "const_cast", "static_cast", prefix
increment/decrement, and a class scope function used as a template argument.
These issues were corrected in -fabi-version=6.
* Lambdas in default argument scope were mangled incorrectly, and the ABI changed the mangling of "nullptr_t".
These issues were corrected in -fabi-version=7.
* When mangling a function type with function-cv-qualifiers, the un-qualified function type was incorrectly
treated as a substitution candidate.
This was fixed in -fabi-version=8, the default for GCC 5.1.
* decltype(nullptr) incorrectly had an alignment of 1, leading to unaligned accesses. Note that this did not
affect the ABI of a function with a "nullptr_t" parameter, as parameters have a minimum alignment.
This was fixed in -fabi-version=9, the default for GCC 5.2.
* Target-specific attributes that affect the identity of a type, such as ia32 calling conventions on a function
type (stdcall, regparm, etc.), did not affect the mangled name, leading to name collisions when function
pointers were used as template arguments.
This was fixed in -fabi-version=10, the default for GCC 6.1.
-Wpsabi (C, Objective-C, C++ and Objective-C++ only)
-Wpsabi enables warnings about processor-specific ABI changes, such as changes in alignment requirements or how
function arguments are passed. On several targets, including AArch64, ARM, x86, MIPS, RS6000/PowerPC, and S/390,
these details have changed between different versions of GCC and/or different versions of the C or C++ language
standards in ways that affect binary compatibility of compiled code. With -Wpsabi, GCC can detect potentially
incompatible usages and warn you about them.
-Wpsabi is enabled by default, and is also implied by -Wabi.
-Wno-changes-meaning (C++ and Objective-C++ only)
C++ requires that unqualified uses of a name within a class have the same meaning in the complete scope of the
class, so declaring the name after using it is ill-formed:
struct A;
struct B1 { A a; typedef A A; }; // warning, 'A' changes meaning
struct B2 { A a; struct A { }; }; // error, 'A' changes meaning
By default, the B1 case is only a warning because the two declarations have the same type, while the B2 case is
an error. Both diagnostics can be disabled with -Wno-changes-meaning. Alternately, the error case can be
reduced to a warning with -Wno-error=changes-meaning or -fpermissive.
Both diagnostics are also suppressed by -fms-extensions.
-Wchar-subscripts
Warn if an array subscript has type "char". This is a common cause of error, as programmers often forget that
this type is signed on some machines. This warning is enabled by -Wall.
-Wno-coverage-mismatch
Warn if feedback profiles do not match when using the -fprofile-use option. If a source file is changed between
compiling with -fprofile-generate and with -fprofile-use, the files with the profile feedback can fail to match
the source file and GCC cannot use the profile feedback information. By default, this warning is enabled and is
treated as an error. -Wno-coverage-mismatch can be used to disable the warning or -Wno-error=coverage-mismatch
can be used to disable the error. Disabling the error for this warning can result in poorly optimized code and
is useful only in the case of very minor changes such as bug fixes to an existing code-base. Completely
disabling the warning is not recommended.
-Wno-coverage-too-many-conditions
Warn if -fcondition-coverage is used and an expression have too many terms and GCC gives up coverage. Coverage
is given up when there are more terms in the conditional than there are bits in a "gcov_type_unsigned". This
warning is enabled by default.
-Wno-coverage-too-many-paths
Warn if -fpath-coverage is used and a function has too many paths and GCC gives up coverage. Giving up is
controlled by -fpath-coverage-limit. This warning is enabled by default.
-Wno-coverage-invalid-line-number
Warn in case a function ends earlier than it begins due to an invalid linenum macros. The warning is emitted
only with --coverage enabled.
By default, this warning is enabled and is treated as an error. -Wno-coverage-invalid-line-number can be used to
disable the warning or -Wno-error=coverage-invalid-line-number can be used to disable the error.
-Wno-cpp (C, Objective-C, C++, Objective-C++ and Fortran only)
Suppress warning messages emitted by "#warning" directives.
-Wdouble-promotion (C, C++, Objective-C and Objective-C++ only)
Give a warning when a value of type "float" is implicitly promoted to "double". CPUs with a 32-bit "single-
precision" floating-point unit implement "float" in hardware, but emulate "double" in software. On such a
machine, doing computations using "double" values is much more expensive because of the overhead required for
software emulation.
It is easy to accidentally do computations with "double" because floating-point literals are implicitly of type
"double". For example, in:
float area(float radius)
{
return 3.14159 * radius * radius;
}
the compiler performs the entire computation with "double" because the floating-point literal is a "double".
-Wduplicate-decl-specifier (C and Objective-C only)
Warn if a declaration has duplicate "const", "volatile", "restrict" or "_Atomic" specifier. This warning is
enabled by -Wall.
-Wformat
-Wformat=n
Check calls to "printf" and "scanf", etc., to make sure that the arguments supplied have types appropriate to the
format string specified, and that the conversions specified in the format string make sense. This includes
standard functions, and others specified by format attributes, in the "printf", "scanf", "strftime" and "strfmon"
(an X/Open extension, not in the C standard) families (or other target-specific families). Which functions are
checked without format attributes having been specified depends on the standard version selected, and such checks
of functions without the attribute specified are disabled by -ffreestanding or -fno-builtin.
The formats are checked against the format features supported by GNU libc version 2.2. These include all ISO C90
and C99 features, as well as features from the Single Unix Specification and some BSD and GNU extensions. Other
library implementations may not support all these features; GCC does not support warning about features that go
beyond a particular library's limitations. However, if -Wpedantic is used with -Wformat, warnings are given
about format features not in the selected standard version (but not for "strfmon" formats, since those are not in
any version of the C standard).
-Wformat=1
-Wformat
Option -Wformat is equivalent to -Wformat=1, and -Wno-format is equivalent to -Wformat=0. Since -Wformat
also checks for null format arguments for several functions, -Wformat also implies -Wnonnull. Some aspects
of this level of format checking can be disabled by the options: -Wno-format-contains-nul,
-Wno-format-extra-args, and -Wno-format-zero-length. -Wformat is enabled by -Wall.
-Wformat=2
Enable -Wformat plus additional format checks. Currently equivalent to -Wformat -Wformat-nonliteral
-Wformat-security -Wformat-y2k.
-Wno-format-contains-nul
If -Wformat is specified, do not warn about format strings that contain NUL bytes.
-Wno-format-extra-args
If -Wformat is specified, do not warn about excess arguments to a "printf" or "scanf" format function. The C
standard specifies that such arguments are ignored.
Where the unused arguments lie between used arguments that are specified with $ operand number specifications,
normally warnings are still given, since the implementation could not know what type to pass to "va_arg" to skip
the unused arguments. However, in the case of "scanf" formats, this option suppresses the warning if the unused
arguments are all pointers, since the Single Unix Specification says that such unused arguments are allowed.
-Wformat-overflow
-Wformat-overflow=level
Warn about calls to formatted input/output functions such as "sprintf" and "vsprintf" that might overflow the
destination buffer. When the exact number of bytes written by a format directive cannot be determined at
compile-time it is estimated based on heuristics that depend on the level argument and on optimization. While
enabling optimization will in most cases improve the accuracy of the warning, it may also result in false
positives.
-Wformat-overflow
-Wformat-overflow=1
Level 1 of -Wformat-overflow enabled by -Wformat employs a conservative approach that warns only about calls
that most likely overflow the buffer. At this level, numeric arguments to format directives with unknown
values are assumed to have the value of one, and strings of unknown length to be empty. Numeric arguments
that are known to be bounded to a subrange of their type, or string arguments whose output is bounded either
by their directive's precision or by a finite set of string literals, are assumed to take on the value within
the range that results in the most bytes on output. For example, the call to "sprintf" below is diagnosed
because even with both a and b equal to zero, the terminating NUL character ('\0') appended by the function
to the destination buffer will be written past its end. Increasing the size of the buffer by a single byte
is sufficient to avoid the warning, though it may not be sufficient to avoid the overflow.
void f (int a, int b)
{
char buf [13];
sprintf (buf, "a = %i, b = %i\n", a, b);
}
-Wformat-overflow=2
Level 2 warns also about calls that might overflow the destination buffer given an argument of sufficient
length or magnitude. At level 2, unknown numeric arguments are assumed to have the minimum representable
value for signed types with a precision greater than 1, and the maximum representable value otherwise.
Unknown string arguments whose length cannot be assumed to be bounded either by the directive's precision, or
by a finite set of string literals they may evaluate to, or the character array they may point to, are
assumed to be 1 character long.
At level 2, the call in the example above is again diagnosed, but this time because with a equal to a 32-bit
"INT_MIN" the first %i directive will write some of its digits beyond the end of the destination buffer. To
make the call safe regardless of the values of the two variables, the size of the destination buffer must be
increased to at least 34 bytes. GCC includes the minimum size of the buffer in an informational note
following the warning.
An alternative to increasing the size of the destination buffer is to constrain the range of formatted
values. The maximum length of string arguments can be bounded by specifying the precision in the format
directive. When numeric arguments of format directives can be assumed to be bounded by less than the
precision of their type, choosing an appropriate length modifier to the format specifier will reduce the
required buffer size. For example, if a and b in the example above can be assumed to be within the precision
of the "short int" type then using either the %hi format directive or casting the argument to "short" reduces
the maximum required size of the buffer to 24 bytes.
void f (int a, int b)
{
char buf [23];
sprintf (buf, "a = %hi, b = %i\n", a, (short)b);
}
-Wno-format-zero-length
If -Wformat is specified, do not warn about zero-length formats. The C standard specifies that zero-length
formats are allowed.
-Wformat-nonliteral
If -Wformat is specified, also warn if the format string is not a string literal and so cannot be checked, unless
the format function takes its format arguments as a "va_list".
-Wformat-security
If -Wformat is specified, also warn about uses of format functions that represent possible security problems. At
present, this warns about calls to "printf" and "scanf" functions where the format string is not a string literal
and there are no format arguments, as in "printf (foo);". This may be a security hole if the format string came
from untrusted input and contains %n. (This is currently a subset of what -Wformat-nonliteral warns about, but
in future warnings may be added to -Wformat-security that are not included in -Wformat-nonliteral.)
-Wformat-signedness
If -Wformat is specified, also warn if the format string requires an unsigned argument and the argument is signed
and vice versa.
-Wformat-truncation
-Wformat-truncation=level
Warn about calls to formatted input/output functions such as "snprintf" and "vsnprintf" that might result in
output truncation. When the exact number of bytes written by a format directive cannot be determined at compile-
time it is estimated based on heuristics that depend on the level argument and on optimization. While enabling
optimization will in most cases improve the accuracy of the warning, it may also result in false positives.
Except as noted otherwise, the option uses the same logic -Wformat-overflow.
-Wformat-truncation
-Wformat-truncation=1
Level 1 of -Wformat-truncation enabled by -Wformat employs a conservative approach that warns only about
calls to bounded functions whose return value is unused and that will most likely result in output
truncation.
-Wformat-truncation=2
Level 2 warns also about calls to bounded functions whose return value is used and that might result in
truncation given an argument of sufficient length or magnitude.
-Wformat-y2k
If -Wformat is specified, also warn about "strftime" formats that may yield only a two-digit year.
-Wnonnull
Warn about passing a null pointer for arguments marked as requiring a non-null value by the "nonnull" function
attribute.
-Wnonnull is included in -Wall and -Wformat. It can be disabled with the -Wno-nonnull option.
-Wnonnull-compare
Warn when comparing an argument marked with the "nonnull" function attribute against null inside the function.
-Wnonnull-compare is included in -Wall. It can be disabled with the -Wno-nonnull-compare option.
-Wnull-dereference
Warn if the compiler detects paths that trigger erroneous or undefined behavior due to dereferencing a null
pointer. This option is only active when -fdelete-null-pointer-checks is active, which is enabled by
optimizations in most targets. The precision of the warnings depends on the optimization options used.
-Wno-musttail-local-addr
Do not warn about passing a pointer (or in C++, a reference) to a local variable or label to argument of a
"musttail" call. Those variables go out of scope before the tail call instruction.
-Wmaybe-musttail-local-addr
Warn when address of a local variable can escape to a "musttail" call, unless it goes out of scope already before
the "musttail" call.
int foo (int *);
int
bar (int *x)
{
if (x[0] == 1)
{
int a = 42;
foo (&a);
/* Without the C attribute this call would not
be tail called, because address of the C variable escapes
and the second foo call could dereference it. With the attribute
the local variables are assumed to go out of scope immediately
before the tail call instruction and the compiler warns about
this. */
[[gnu::musttail]] return foo (nullptr);
}
else
{
{
int a = 42;
foo (&a);
}
/* The C variable isn't already in scope, so even when it
escaped, even without C attribute it would be
undefined behavior to dereference it and the compiler could
turn this into a tail call. No warning is diagnosed here. */
[[gnu::musttail]] return foo (nullptr);
}
}
This warning is enabled by -Wextra.
-Wnrvo (C++ and Objective-C++ only)
Warn if the compiler does not elide the copy from a local variable to the return value of a function in a context
where it is allowed by [class.copy.elision]. This elision is commonly known as the Named Return Value
Optimization. For instance, in the example below the compiler cannot elide copies from both v1 and v2, so it
elides neither.
std::vector f()
{
std::vector v1, v2;
// ...
if (cond) return v1;
else return v2; // warning: not eliding copy
}
-Winfinite-recursion
Warn about infinitely recursive calls. The warning is effective at all optimization levels but requires
optimization in order to detect infinite recursion in calls between two or more functions. -Winfinite-recursion
is included in -Wall.
Compare with -Wanalyzer-infinite-recursion which provides a similar diagnostic, but is implemented in a different
way (as part of -fanalyzer).
-Winit-self (C, C++, Objective-C and Objective-C++ only)
Warn about uninitialized variables that are initialized with themselves. Note this option can only be used with
the -Wuninitialized option.
For example, GCC warns about "i" being uninitialized in the following snippet only when -Winit-self has been
specified:
int f()
{
int i = i;
return i;
}
This warning is enabled by -Wall in C++.
-Wno-implicit-int (C and Objective-C only)
This option controls warnings when a declaration does not specify a type. This warning is enabled by default, as
an error, in C99 and later dialects of C, and also by -Wall. The error can be downgraded to a warning using
-fpermissive (along with certain other errors), or for this error alone, with -Wno-error=implicit-int.
This warning is upgraded to an error by -pedantic-errors.
-Wno-implicit-function-declaration (C and Objective-C only)
This option controls warnings when a function is used before being declared. This warning is enabled by default,
as an error, in C99 and later dialects of C, and also by -Wall. The error can be downgraded to a warning using
-fpermissive (along with certain other errors), or for this error alone, with
-Wno-error=implicit-function-declaration.
This warning is upgraded to an error by -pedantic-errors.
-Wimplicit (C and Objective-C only)
Same as -Wimplicit-int and -Wimplicit-function-declaration. This warning is enabled by -Wall.
-Whardened
Warn when -fhardened did not enable an option from its set (for which see -fhardened). For instance, using
-fhardened and -fstack-protector at the same time on the command line causes -Whardened to warn because
-fstack-protector-strong will not be enabled by -fhardened.
This warning is enabled by default and has effect only when -fhardened is enabled.
-Wimplicit-fallthrough
-Wimplicit-fallthrough is the same as -Wimplicit-fallthrough=3 and -Wno-implicit-fallthrough is the same as
-Wimplicit-fallthrough=0.
-Wimplicit-fallthrough=n
Warn when a switch case falls through. For example:
switch (cond)
{
case 1:
a = 1;
break;
case 2:
a = 2;
case 3:
a = 3;
break;
}
This warning does not warn when the last statement of a case cannot fall through, e.g. when there is a return
statement or a call to function declared with the noreturn attribute. -Wimplicit-fallthrough= also takes into
account control flow statements, such as ifs, and only warns when appropriate. E.g.
switch (cond)
{
case 1:
if (i > 3) {
bar (5);
break;
} else if (i < 1) {
bar (0);
} else
return;
default:
...
}
Since there are occasions where a switch case fall through is desirable, GCC provides an attribute,
"__attribute__ ((fallthrough))", that is to be used along with a null statement to suppress this warning that
would normally occur:
switch (cond)
{
case 1:
bar (0);
__attribute__ ((fallthrough));
default:
...
}
C++17 and C23 provide a standard way to suppress the -Wimplicit-fallthrough warning using "[[fallthrough]];"
instead of the GNU attribute. In C++11 or C++14 users can use "[[gnu::fallthrough]];", which is a GNU extension.
Instead of these attributes, it is also possible to add a fallthrough comment to silence the warning. The whole
body of the C or C++ style comment should match the given regular expressions listed below. The option argument
n specifies what kind of comments are accepted:
*<-Wimplicit-fallthrough=0 disables the warning altogether.>
*<-Wimplicit-fallthrough=1 matches ".*" regular>
expression, any comment is used as fallthrough comment.
*<-Wimplicit-fallthrough=2 case insensitively matches>
".*falls?[ \t-]*thr(ough|u).*" regular expression.
*<-Wimplicit-fallthrough=3 case sensitively matches one of the>
following regular expressions:
*<"-fallthrough">
*<"@fallthrough@">
*<"lint -fallthrough[ \t]*">
*<"[ \t.!]*(ELSE,? |INTENTIONAL(LY)? )?FALL(S | |-)?THR(OUGH|U)[ \t.!]*(-[^\n\r]*)?">
*<"[ \t.!]*(Else,? |Intentional(ly)? )?Fall((s | |-)[Tt]|t)hr(ough|u)[ \t.!]*(-[^\n\r]*)?">
*<"[ \t.!]*([Ee]lse,? |[Ii]ntentional(ly)? )?fall(s | |-)?thr(ough|u)[ \t.!]*(-[^\n\r]*)?">
*<-Wimplicit-fallthrough=4 case sensitively matches one of the>
following regular expressions:
*<"-fallthrough">
*<"@fallthrough@">
*<"lint -fallthrough[ \t]*">
*<"[ \t]*FALLTHR(OUGH|U)[ \t]*">
*<-Wimplicit-fallthrough=5 doesn't recognize any comments as>
fallthrough comments, only attributes disable the warning.
The comment needs to be followed after optional whitespace and other comments by "case" or "default" keywords or
by a user label that precedes some "case" or "default" label.
switch (cond)
{
case 1:
bar (0);
/* FALLTHRU */
default:
...
}
The -Wimplicit-fallthrough=3 warning is enabled by -Wextra.
-Wno-if-not-aligned (C, C++, Objective-C and Objective-C++ only)
Control if warnings triggered by the "warn_if_not_aligned" attribute should be issued. These warnings are
enabled by default.
-Wignored-qualifiers (C and C++ only)
Warn if the return type of a function has a type qualifier such as "const". For ISO C such a type qualifier has
no effect, since the value returned by a function is not an lvalue. For C++, the warning is only emitted for
scalar types or "void". ISO C prohibits qualified "void" return types on function definitions, so such return
types always receive a warning even without this option.
This warning is also enabled by -Wextra.
-Wno-ignored-attributes (C and C++ only)
This option controls warnings when an attribute is ignored. This is different from the -Wattributes option in
that it warns whenever the compiler decides to drop an attribute, not that the attribute is either unknown, used
in a wrong place, etc. This warning is enabled by default.
-Wmain
Warn if the type of "main" is suspicious. "main" should be a function with external linkage, returning int,
taking either zero arguments, two, or three arguments of appropriate types. This warning is enabled by default
in C++ and is enabled by either -Wall or -Wpedantic.
This warning is upgraded to an error by -pedantic-errors.
-Wmisleading-indentation (C and C++ only)
Warn when the indentation of the code does not reflect the block structure. Specifically, a warning is issued
for "if", "else", "while", and "for" clauses with a guarded statement that does not use braces, followed by an
unguarded statement with the same indentation.
In the following example, the call to "bar" is misleadingly indented as if it were guarded by the "if"
conditional.
if (some_condition ())
foo ();
bar (); /* Gotcha: this is not guarded by the "if". */
In the case of mixed tabs and spaces, the warning uses the -ftabstop= option to determine if the statements line
up (defaulting to 8).
The warning is not issued for code involving multiline preprocessor logic such as the following example.
if (flagA)
foo (0);
#if SOME_CONDITION_THAT_DOES_NOT_HOLD
if (flagB)
#endif
foo (1);
The warning is not issued after a "#line" directive, since this typically indicates autogenerated code, and no
assumptions can be made about the layout of the file that the directive references.
This warning is enabled by -Wall in C and C++.
-Wmissing-attributes
Warn when a declaration of a function is missing one or more attributes that a related function is declared with
and whose absence may adversely affect the correctness or efficiency of generated code. For example, the warning
is issued for declarations of aliases that use attributes to specify less restrictive requirements than those of
their targets. This typically represents a potential optimization opportunity. By contrast, the
-Wattribute-alias=2 option controls warnings issued when the alias is more restrictive than the target, which
could lead to incorrect code generation. Attributes considered include "alloc_align", "alloc_size", "cold",
"const", "hot", "leaf", "malloc", "nonnull", "noreturn", "nothrow", "pure", "returns_nonnull", and
"returns_twice".
In C++, the warning is issued when an explicit specialization of a primary template declared with attribute
"alloc_align", "alloc_size", "assume_aligned", "format", "format_arg", "malloc", or "nonnull" is declared without
it. Attributes "deprecated", "error", and "warning" suppress the warning..
You can use the "copy" attribute to apply the same set of attributes to a declaration as that on another
declaration without explicitly enumerating the attributes. This attribute can be applied to declarations of
functions, variables, or types.
-Wmissing-attributes is enabled by -Wall.
For example, since the declaration of the primary function template below makes use of both attribute "malloc"
and "alloc_size" the declaration of the explicit specialization of the template is diagnosed because it is
missing one of the attributes.
template
T* __attribute__ ((malloc, alloc_size (1)))
allocate (size_t);
template <>
void* __attribute__ ((malloc)) // missing alloc_size
allocate (size_t);
-Wmissing-braces
Warn if an aggregate or union initializer is not fully bracketed. In the following example, the initializer for
"a" is not fully bracketed, but that for "b" is fully bracketed.
int a[2][2] = { 0, 1, 2, 3 };
int b[2][2] = { { 0, 1 }, { 2, 3 } };
This warning is enabled by -Wall.
-Wmissing-include-dirs (C, C++, Objective-C, Objective-C++ and Fortran only)
Warn if a user-supplied include directory does not exist. This option is disabled by default for C, C++,
Objective-C and Objective-C++. For Fortran, it is partially enabled by default by warning for -I and -J, only.
-Wno-missing-profile
This option controls warnings if feedback profiles are missing when using the -fprofile-use option. This option
diagnoses those cases where a new function or a new file is added between compiling with -fprofile-generate and
with -fprofile-use, without regenerating the profiles. In these cases, the profile feedback data files do not
contain any profile feedback information for the newly added function or file respectively. Also, in the case
when profile count data (.gcda) files are removed, GCC cannot use any profile feedback information. In all these
cases, warnings are issued to inform you that a profile generation step is due. Ignoring the warning can result
in poorly optimized code. -Wno-missing-profile can be used to disable the warning, but this is not recommended
and should be done only when non-existent profile data is justified.
-Wmismatched-dealloc
Warn for calls to deallocation functions with pointer arguments returned from allocation functions for which the
former isn't a suitable deallocator. A pair of functions can be associated as matching allocators and
deallocators by use of attribute "malloc". Unless disabled by the -fno-builtin option the standard functions
"calloc", "malloc", "realloc", and "free", as well as the corresponding forms of C++ "operator new" and "operator
delete" are implicitly associated as matching allocators and deallocators. In the following example "mydealloc"
is the deallocator for pointers returned from "myalloc".
void mydealloc (void*);
__attribute__ ((malloc (mydealloc, 1))) void*
myalloc (size_t);
void f (void)
{
void *p = myalloc (32);
// ...use p...
free (p); // warning: not a matching deallocator for myalloc
mydealloc (p); // ok
}
In C++, the related option -Wmismatched-new-delete diagnoses mismatches involving either "operator new" or
"operator delete".
Option -Wmismatched-dealloc is included in -Wall.
-Wmultistatement-macros
Warn about unsafe multiple statement macros that appear to be guarded by a clause such as "if", "else", "for",
"switch", or "while", in which only the first statement is actually guarded after the macro is expanded.
For example:
#define DOIT x++; y++
if (c)
DOIT;
will increment "y" unconditionally, not just when "c" holds. The can usually be fixed by wrapping the macro in a
do-while loop:
#define DOIT do { x++; y++; } while (0)
if (c)
DOIT;
This warning is enabled by -Wall in C and C++.
-Wparentheses
Warn if parentheses are omitted in certain contexts, such as when there is an assignment in a context where a
truth value is expected, or when operators are nested whose precedence people often get confused about.
Also warn if a comparison like "x<=y<=z" appears; this is equivalent to "(x<=y ? 1 : 0) <= z", which is a
different interpretation from that of ordinary mathematical notation.
Also warn for dangerous uses of the GNU extension to "?:" with omitted middle operand. When the condition in the
"?": operator is a boolean expression, the omitted value is always 1. Often programmers expect it to be a value
computed inside the conditional expression instead.
For C++ this also warns for some cases of unnecessary parentheses in declarations, which can indicate an attempt
at a function call instead of a declaration:
{
// Declares a local variable called mymutex.
std::unique_lock (mymutex);
// User meant std::unique_lock lock (mymutex);
}
This warning is enabled by -Wall.
-Wno-self-move (C++ and Objective-C++ only)
This warning warns when a value is moved to itself with "std::move". Such a "std::move" typically has no effect.
struct T {
...
};
void fn()
{
T t;
...
t = std::move (t);
}
This warning is enabled by -Wall.
-Wsequence-point
Warn about code that may have undefined semantics because of violations of sequence point rules in the C and C++
standards.
The C and C++ standards define the order in which expressions in a C/C++ program are evaluated in terms of
sequence points, which represent a partial ordering between the execution of parts of the program: those executed
before the sequence point, and those executed after it. These occur after the evaluation of a full expression
(one which is not part of a larger expression), after the evaluation of the first operand of a "&&", "||", "? :"
or "," (comma) operator, before a function is called (but after the evaluation of its arguments and the
expression denoting the called function), and in certain other places. Other than as expressed by the sequence
point rules, the order of evaluation of subexpressions of an expression is not specified. All these rules
describe only a partial order rather than a total order, since, for example, if two functions are called within
one expression with no sequence point between them, the order in which the functions are called is not specified.
However, the standards committee have ruled that function calls do not overlap.
It is not specified when between sequence points modifications to the values of objects take effect. Programs
whose behavior depends on this have undefined behavior; the C and C++ standards specify that "Between the
previous and next sequence point an object shall have its stored value modified at most once by the evaluation of
an expression. Furthermore, the prior value shall be read only to determine the value to be stored.". If a
program breaks these rules, the results on any particular implementation are entirely unpredictable.
Examples of code with undefined behavior are "a = a++;", "a[n] = b[n++]" and "a[i++] = i;". Some more
complicated cases are not diagnosed by this option, and it may give an occasional false positive result, but in
general it has been found fairly effective at detecting this sort of problem in programs.
The C++17 standard will define the order of evaluation of operands in more cases: in particular it requires that
the right-hand side of an assignment be evaluated before the left-hand side, so the above examples are no longer
undefined. But this option will still warn about them, to help people avoid writing code that is undefined in C
and earlier revisions of C++.
The standard is worded confusingly, therefore there is some debate over the precise meaning of the sequence point
rules in subtle cases. Links to discussions of the problem, including proposed formal definitions, may be found
on the GCC readings page, at .
This warning is enabled by -Wall for C and C++.
-Wno-return-local-addr
Do not warn about returning a pointer (or in C++, a reference) to a variable that goes out of scope after the
function returns.
-Wreturn-mismatch
Warn about return statements without an expressions in functions which do not return "void". Also warn about a
"return" statement with an expression in a function whose return type is "void", unless the expression type is
also "void". As a GNU extension, the latter case is accepted without a warning unless -Wpedantic is used.
Attempting to use the return value of a non-"void" function other than "main" that flows off the end by reaching
the closing curly brace that terminates the function is undefined.
This warning is specific to C and enabled by default. In C99 and later language dialects, it is treated as an
error. It can be downgraded to a warning using -fpermissive (along with other warnings), or for just this
warning, with -Wno-error=return-mismatch.
-Wreturn-type
Warn whenever a function is defined with a return type that defaults to "int" (unless -Wimplicit-int is active,
which takes precedence). Also warn if execution may reach the end of the function body, or if the function does
not contain any return statement at all.
Attempting to use the return value of a non-"void" function other than "main" that flows off the end by reaching
the closing curly brace that terminates the function is undefined.
Unlike in C, in C++, flowing off the end of a non-"void" function other than "main" results in undefined behavior
even when the value of the function is not used.
This warning is enabled by default in C++ and by -Wall otherwise.
-Wno-shift-count-negative
Controls warnings if a shift count is negative. This warning is enabled by default.
-Wno-shift-count-overflow
Controls warnings if a shift count is greater than or equal to the bit width of the type. This warning is
enabled by default.
-Wshift-negative-value
Warn if left shifting a negative value. This warning is enabled by -Wextra in C99 (and newer) and C++11 to C++17
modes.
-Wno-shift-overflow
-Wshift-overflow=n
These options control warnings about left shift overflows.
-Wshift-overflow=1
This is the warning level of -Wshift-overflow and is enabled by default in C99 and C++11 modes (and newer).
This warning level does not warn about left-shifting 1 into the sign bit. (However, in C, such an overflow
is still rejected in contexts where an integer constant expression is required.) No warning is emitted in
C++20 mode (and newer), as signed left shifts always wrap.
-Wshift-overflow=2
This warning level also warns about left-shifting 1 into the sign bit, unless C++14 mode (or newer) is
active.
-Wswitch
Warn whenever a "switch" statement has an index of enumerated type and lacks a "case" for one or more of the
named codes of that enumeration. (The presence of a "default" label prevents this warning.) "case" labels that
do not correspond to enumerators also provoke warnings when this option is used, unless the enumeration is marked
with the "flag_enum" attribute. This warning is enabled by -Wall.
-Wswitch-default
Warn whenever a "switch" statement does not have a "default" case.
-Wswitch-enum
Warn whenever a "switch" statement has an index of enumerated type and lacks a "case" for one or more of the
named codes of that enumeration. "case" labels that do not correspond to enumerators also provoke warnings when
this option is used, unless the enumeration is marked with the "flag_enum" attribute. The only difference
between -Wswitch and this option is that this option gives a warning about an omitted enumeration code even if
there is a "default" label.
-Wno-switch-bool
Do not warn when a "switch" statement has an index of boolean type and the case values are outside the range of a
boolean type. It is possible to suppress this warning by casting the controlling expression to a type other than
"bool". For example:
switch ((int) (a == 4))
{
...
}
This warning is enabled by default for C and C++ programs.
-Wno-switch-outside-range
This option controls warnings when a "switch" case has a value that is outside of its respective type range.
This warning is enabled by default for C and C++ programs.
-Wno-switch-unreachable
Do not warn when a "switch" statement contains statements between the controlling expression and the first case
label, which will never be executed. For example:
switch (cond)
{
i = 15;
...
case 5:
...
}
-Wswitch-unreachable does not warn if the statement between the controlling expression and the first case label
is just a declaration:
switch (cond)
{
int i;
...
case 5:
i = 5;
...
}
This warning is enabled by default for C and C++ programs.
-Wsync-nand (C and C++ only)
Warn when "__sync_fetch_and_nand" and "__sync_nand_and_fetch" built-in functions are used. These functions
changed semantics in GCC 4.4.
-Wtrivial-auto-var-init
Warn when "-ftrivial-auto-var-init" cannot initialize the automatic variable. A common situation is an automatic
variable that is declared between the controlling expression and the first case label of a "switch" statement.
-Wunused-but-set-parameter
Warn whenever a function parameter is assigned to, but otherwise unused (aside from its declaration).
To suppress this warning use the "unused" attribute.
This warning is also enabled by -Wunused together with -Wextra.
-Wunused-but-set-variable
Warn whenever a local variable is assigned to, but otherwise unused (aside from its declaration). This warning
is enabled by -Wall.
To suppress this warning use the "unused" attribute.
This warning is also enabled by -Wunused, which is enabled by -Wall.
-Wunused-function
Warn whenever a static function is declared but not defined or a non-inline static function is unused. This
warning is enabled by -Wall.
-Wunused-label
Warn whenever a label is declared but not used. This warning is enabled by -Wall.
To suppress this warning use the "unused" attribute.
-Wunused-local-typedefs (C, Objective-C, C++ and Objective-C++ only)
Warn when a typedef locally defined in a function is not used. This warning is enabled by -Wall.
-Wunused-parameter
Warn whenever a function parameter is unused aside from its declaration. This option is not enabled by
"-Wunused" unless "-Wextra" is also specified.
To suppress this warning use the "unused" attribute.
-Wno-unused-result
Do not warn if a caller of a function marked with attribute "warn_unused_result" does not use its return value.
The default is -Wunused-result.
-Wunused-variable
Warn whenever a local or static variable is unused aside from its declaration. This option implies
-Wunused-const-variable=1 for C, but not for C++. This warning is enabled by -Wall.
To suppress this warning use the "unused" attribute.
-Wunused-const-variable
-Wunused-const-variable=n
Warn whenever a constant static variable is unused aside from its declaration.
To suppress this warning use the "unused" attribute.
-Wunused-const-variable=1
Warn about unused static const variables defined in the main compilation unit, but not about static const
variables declared in any header included.
-Wunused-const-variable=1 is enabled by either -Wunused-variable or -Wunused for C, but not for C++. In C
this declares variable storage, but in C++ this is not an error since const variables take the place of
"#define"s.
-Wunused-const-variable=2
This warning level also warns for unused constant static variables in headers (excluding system headers). It
is equivalent to the short form -Wunused-const-variable. This level must be explicitly requested in both C
and C++ because it might be hard to clean up all headers included.
-Wunused-value
Warn whenever a statement computes a result that is explicitly not used. To suppress this warning cast the unused
expression to "void". This includes an expression-statement or the left-hand side of a comma expression that
contains no side effects. For example, an expression such as "x[i,j]" causes a warning, while "x[(void)i,j]" does
not.
This warning is enabled by -Wall.
-Wunused
All the above -Wunused options combined, except those documented as needing to be specified explicitly.
In order to get a warning about an unused function parameter, you must either specify -Wextra -Wunused (note that
-Wall implies -Wunused), or separately specify -Wunused-parameter and/or -Wunused-but-set-parameter.
-Wunused enables only -Wunused-const-variable=1 rather than -Wunused-const-variable, and only for C, not C++.
-Wuse-after-free (C, Objective-C, C++ and Objective-C++ only)
-Wuse-after-free=n
Warn about uses of pointers to dynamically allocated objects that have been rendered indeterminate by a call to a
deallocation function. The warning is enabled at all optimization levels but may yield different results with
optimization than without.
-Wuse-after-free=1
At level 1 the warning attempts to diagnose only unconditional uses of pointers made indeterminate by a
deallocation call or a successful call to "realloc", regardless of whether or not the call resulted in an
actual reallocation of memory. This includes double-"free" calls as well as uses in arithmetic and
relational expressions. Although undefined, uses of indeterminate pointers in equality (or inequality)
expressions are not diagnosed at this level.
-Wuse-after-free=2
At level 2, in addition to unconditional uses, the warning also diagnoses conditional uses of pointers made
indeterminate by a deallocation call. As at level 2, uses in equality (or inequality) expressions are not
diagnosed. For example, the second call to "free" in the following function is diagnosed at this level:
struct A { int refcount; void *data; };
void release (struct A *p)
{
int refcount = --p->refcount;
free (p);
if (refcount == 0)
free (p->data); // warning: p may be used after free
}
-Wuse-after-free=3
At level 3, the warning also diagnoses uses of indeterminate pointers in equality expressions. All uses of
indeterminate pointers are undefined but equality tests sometimes appear after calls to "realloc" as an
attempt to determine whether the call resulted in relocating the object to a different address. They are
diagnosed at a separate level to aid gradually transitioning legacy code to safe alternatives. For example,
the equality test in the function below is diagnosed at this level:
void adjust_pointers (int**, int);
void grow (int **p, int n)
{
int **q = (int**)realloc (p, n *= 2);
if (q == p)
return;
adjust_pointers ((int**)q, n);
}
To avoid the warning at this level, store offsets into allocated memory instead of pointers. This approach
obviates needing to adjust the stored pointers after reallocation.
-Wuse-after-free=2 is included in -Wall.
-Wuseless-cast (C, Objective-C, C++ and Objective-C++ only)
Warn when an expression is cast to its own type. This warning does not occur when a class object is converted to
a non-reference type as that is a way to create a temporary:
struct S { };
void g (S&&);
void f (S&& arg)
{
g (S(arg)); // make arg prvalue so that it can bind to S&&
}
-Wuninitialized
Warn if an object with automatic or allocated storage duration is used without having been initialized. In C++,
also warn if a non-static reference or non-static "const" member appears in a class without constructors.
In addition, passing a pointer (or in C++, a reference) to an uninitialized object to a "const"-qualified
argument of a built-in function known to read the object is also diagnosed by this warning.
(-Wmaybe-uninitialized is issued for ordinary functions.)
If you want to warn about code that uses the uninitialized value of the variable in its own initializer, use the
-Winit-self option.
These warnings occur for individual uninitialized elements of structure, union or array variables as well as for
variables that are uninitialized as a whole. They do not occur for variables or elements declared "volatile".
Because these warnings depend on optimization, the exact variables or elements for which there are warnings
depend on the precise optimization options and version of GCC used.
Note that there may be no warning about a variable that is used only to compute a value that itself is never
used, because such computations may be deleted by data flow analysis before the warnings are printed.
In C++, this warning also warns about using uninitialized objects in member-initializer-lists. For example, GCC
warns about "b" being uninitialized in the following snippet:
struct A {
int a;
int b;
A() : a(b) { }
};
-Wno-invalid-memory-model
This option controls warnings for invocations of __atomic Builtins, __sync Builtins, and the C11 atomic generic
functions with a memory consistency argument that is either invalid for the operation or outside the range of
values of the "memory_order" enumeration. For example, since the "__atomic_store" and "__atomic_store_n" built-
ins are only defined for the relaxed, release, and sequentially consistent memory orders the following code is
diagnosed:
void store (int *i)
{
__atomic_store_n (i, 0, memory_order_consume);
}
-Winvalid-memory-model is enabled by default.
-Wmaybe-uninitialized
For an object with automatic or allocated storage duration, if there exists a path from the function entry to a
use of the object that is initialized, but there exist some other paths for which the object is not initialized,
the compiler emits a warning if it cannot prove the uninitialized paths are not executed at run time.
In addition, passing a pointer (or in C++, a reference) to an uninitialized object to a "const"-qualified
function argument is also diagnosed by this warning. (-Wuninitialized is issued for built-in functions known to
read the object.) Annotating the function with attribute "access (none)" indicates that the argument isn't used
to access the object and avoids the warning.
These warnings are only possible in optimizing compilation, because otherwise GCC does not keep track of the
state of variables.
These warnings are made optional because GCC may not be able to determine when the code is correct in spite of
appearing to have an error. Here is one example of how this can happen:
{
int x;
switch (y)
{
case 1: x = 1;
break;
case 2: x = 4;
break;
case 3: x = 5;
}
foo (x);
}
If the value of "y" is always 1, 2 or 3, then "x" is always initialized, but GCC doesn't know this. To suppress
the warning, you need to provide a default case with assert(0) or similar code.
This option also warns when a non-volatile automatic variable might be changed by a call to "longjmp". The
compiler sees only the calls to "setjmp". It cannot know where "longjmp" will be called; in fact, a signal
handler could call it at any point in the code. As a result, you may get a warning even when there is in fact no
problem because "longjmp" cannot in fact be called at the place that would cause a problem.
Some spurious warnings can be avoided if you declare all the functions you use that never return as "noreturn".
This warning is enabled by -Wall or -Wextra.
-Wunknown-pragmas
Warn when a "#pragma" directive is encountered that is not understood by GCC. If this command-line option is
used, warnings are even issued for unknown pragmas in system header files. This is not the case if the warnings
are only enabled by the -Wall command-line option.
-Wno-pragmas
Do not warn about misuses of pragmas, such as incorrect parameters, invalid syntax, or conflicts between pragmas.
See also -Wunknown-pragmas.
-Wno-pragma-once-outside-header
Do not warn when "#pragma once" is used in a file that is not a header file, such as a main file.
-Wno-prio-ctor-dtor
Do not warn if a priority from 0 to 100 is used for constructor or destructor. The use of constructor and
destructor attributes allow you to assign a priority to the constructor/destructor to control its order of
execution before "main" is called or after it returns. The priority values must be greater than 100 as the
compiler reserves priority values between 0--100 for the implementation.
-Wstrict-aliasing
This option is only active when -fstrict-aliasing is active. It warns about code that might break the strict
aliasing rules that the compiler is using for optimization. The warning does not catch all cases, but does
attempt to catch the more common pitfalls. It is included in -Wall. It is equivalent to -Wstrict-aliasing=3
-Wstrict-aliasing=n
This option is only active when -fstrict-aliasing is active. It warns about code that might break the strict
aliasing rules that the compiler is using for optimization. Higher levels correspond to higher accuracy (fewer
false positives). Higher levels also correspond to more effort, similar to the way -O works. -Wstrict-aliasing
is equivalent to -Wstrict-aliasing=3.
Level 1: Most aggressive, quick, least accurate. Possibly useful when higher levels do not warn but
-fstrict-aliasing still breaks the code, as it has very few false negatives. However, it has many false
positives. Warns for all pointer conversions between possibly incompatible types, even if never dereferenced.
Runs in the front end only.
Level 2: Aggressive, quick, not too precise. May still have many false positives (not as many as level 1
though), and few false negatives (but possibly more than level 1). Unlike level 1, it only warns when an address
is taken. Warns about incomplete types. Runs in the front end only.
Level 3 (default for -Wstrict-aliasing): Should have very few false positives and few false negatives. Slightly
slower than levels 1 or 2 when optimization is enabled. Takes care of the common pun+dereference pattern in the
front end: "*(int*)&some_float". If optimization is enabled, it also runs in the back end, where it deals with
multiple statement cases using flow-sensitive points-to information. Only warns when the converted pointer is
dereferenced. Does not warn about incomplete types.
-Wstrict-overflow
-Wstrict-overflow=n
This option is only active when signed overflow is undefined. It warns about cases where the compiler optimizes
based on the assumption that signed overflow does not occur. Note that it does not warn about all cases where
the code might overflow: it only warns about cases where the compiler implements some optimization. Thus this
warning depends on the optimization level.
An optimization that assumes that signed overflow does not occur is perfectly safe if the values of the variables
involved are such that overflow never does, in fact, occur. Therefore this warning can easily give a false
positive: a warning about code that is not actually a problem. To help focus on important issues, several
warning levels are defined. No warnings are issued for the use of undefined signed overflow when estimating how
many iterations a loop requires, in particular when determining whether a loop will be executed at all.
-Wstrict-overflow=1
Warn about cases that are both questionable and easy to avoid. For example the compiler simplifies "x + 1 >
x" to 1. This level of -Wstrict-overflow is enabled by -Wall; higher levels are not, and must be explicitly
requested.
-Wstrict-overflow=2
Also warn about other cases where a comparison is simplified to a constant. For example: "abs (x) >= 0".
This can only be simplified when signed integer overflow is undefined, because "abs (INT_MIN)" overflows to
"INT_MIN", which is less than zero. -Wstrict-overflow (with no level) is the same as -Wstrict-overflow=2.
-Wstrict-overflow=3
Also warn about other cases where a comparison is simplified. For example: "x + 1 > 1" is simplified to "x >
0".
-Wstrict-overflow=4
Also warn about other simplifications not covered by the above cases. For example: "(x * 10) / 5" is
simplified to "x * 2".
-Wstrict-overflow=5
Also warn about cases where the compiler reduces the magnitude of a constant involved in a comparison. For
example: "x + 2 > y" is simplified to "x + 1 >= y". This is reported only at the highest warning level
because this simplification applies to many comparisons, so this warning level gives a very large number of
false positives.
-Wstring-compare
Warn for calls to "strcmp" and "strncmp" whose result is determined to be either zero or non-zero in tests for
such equality owing to the length of one argument being greater than the size of the array the other argument is
stored in (or the bound in the case of "strncmp"). Such calls could be mistakes. For example, the call to
"strcmp" below is diagnosed because its result is necessarily non-zero irrespective of the contents of the array
"a".
extern char a[4];
void f (char *d)
{
strcpy (d, "string");
...
if (0 == strcmp (a, d)) // cannot be true
puts ("a and d are the same");
}
-Wstring-compare is enabled by -Wextra.
-Wno-stringop-overflow
-Wstringop-overflow
-Wstringop-overflow=type
Warn for code that can be statically determined to cause buffer overflows or memory overruns, such as calls to
"memcpy" and "strcpy" that overflow the destination buffer. The optional argument is one greater than the type
of Object Size Checking to perform to determine the size of the destination. The argument is meaningful only for
string functions that operate on character arrays; raw memory functions like "memcpy" always use type-zero Object
Size Checking.
The option also warns for calls that specify a size in excess of the largest possible object or at most "SIZE_MAX
/ 2" bytes.
The option produces the best results with optimization enabled but can detect a small subset of simple buffer
overflows even without optimization in calls to the GCC built-in functions like "__builtin_memcpy" that
correspond to the standard functions. In any case, the option warns about just a subset of buffer overflows
detected by the corresponding overflow checking built-ins, such as "__builtin___memcpy_chk", which can perform
run-time checking if the access cannot be identified as safe at compile time.
For example, the option issues a warning for the "strcpy" call below because it copies at least 5 characters (the
string "blue" including the terminating NUL) into the buffer of size 4.
enum Color { blue, purple, yellow };
const char* f (enum Color clr)
{
static char buf [4];
const char *str;
switch (clr)
{
case blue: str = "blue"; break;
case purple: str = "purple"; break;
case yellow: str = "yellow"; break;
}
return strcpy (buf, str); // warning here
}
The effect of this option is not limited to string or memory manipulation functions. In this example, a warning
is diagnosed because a 1-element array is passed to a function requiring at least a 4-element array argument:
void f (int[static 4]);
void g (void)
{
int *p = (int *) malloc (1 * sizeof(int));
f (p); // warning here
}
Option -Wstringop-overflow=2 is enabled by default.
-Wstringop-overflow
-Wstringop-overflow=1
The -Wstringop-overflow=1 option uses type-zero Object Size Checking to determine the sizes of destination
objects. At this setting the option does not warn for writes past the end of subobjects of larger objects
accessed by pointers unless the size of the largest surrounding object is known. When the destination may be
one of several objects it is assumed to be the largest one of them. On Linux systems, when optimization is
enabled at this setting the option warns for the same code as when the "_FORTIFY_SOURCE" macro is defined to
a non-zero value.
-Wstringop-overflow=2
The -Wstringop-overflow=2 option uses type-one Object Size Checking to determine the sizes of destination
objects. At this setting the option warns about overflows when writing to members of the largest complete
objects whose exact size is known. However, it does not warn for excessive writes to the same members of
unknown objects referenced by pointers since they may point to arrays containing unknown numbers of elements.
This is the default setting of the option.
-Wstringop-overflow=3
The -Wstringop-overflow=3 option uses type-two Object Size Checking to determine the sizes of destination
objects. At this setting the option warns about overflowing the smallest object or data member. This is the
most restrictive setting of the option that may result in warnings for safe code.
-Wstringop-overflow=4
The -Wstringop-overflow=4 option uses type-three Object Size Checking to determine the sizes of destination
objects. At this setting the option warns about overflowing any data members, and when the destination is
one of several objects it uses the size of the largest of them to decide whether to issue a warning.
Similarly to -Wstringop-overflow=3 this setting of the option may result in warnings for benign code.
-Wno-stringop-overread
Warn for calls to string manipulation functions such as "memchr", or "strcpy" that are determined to read past
the end of the source sequence.
Option -Wstringop-overread is enabled by default.
-Wno-stringop-truncation
Do not warn for calls to bounded string manipulation functions such as "strncat", "strncpy", and "stpncpy" that
may either truncate the copied string or leave the destination unchanged.
In the following example, the call to "strncat" specifies a bound that is less than the length of the source
string. As a result, the copy of the source will be truncated and so the call is diagnosed. To avoid the
warning use "bufsize - strlen (buf) - 1)" as the bound.
void append (char *buf, size_t bufsize)
{
strncat (buf, ".txt", 3);
}
As another example, the following call to "strncpy" results in copying to "d" just the characters preceding the
terminating NUL, without appending the NUL to the end. Assuming the result of "strncpy" is necessarily a NUL-
terminated string is a common mistake, and so the call is diagnosed. To avoid the warning when the result is not
expected to be NUL-terminated, call "memcpy" instead.
void copy (char *d, const char *s)
{
strncpy (d, s, strlen (s));
}
In the following example, the call to "strncpy" specifies the size of the destination buffer as the bound. If
the length of the source string is equal to or greater than this size the result of the copy will not be NUL-
terminated. Therefore, the call is also diagnosed. To avoid the warning, specify "sizeof buf - 1" as the bound
and set the last element of the buffer to "NUL".
void copy (const char *s)
{
char buf[80];
strncpy (buf, s, sizeof buf);
...
}
In situations where a character array is intended to store a sequence of bytes with no terminating "NUL" such an
array may be annotated with attribute "nonstring" to avoid this warning. Such arrays, however, are not suitable
arguments to functions that expect "NUL"-terminated strings. To help detect accidental misuses of such arrays
GCC issues warnings unless it can prove that the use is safe.
-Wstrict-flex-arrays (C and C++ only)
Warn about improper usages of flexible array members according to the level of the "strict_flex_array (level)"
attribute attached to the trailing array field of a structure if it's available, otherwise according to the level
of the option -fstrict-flex-arrays=level. "-Wstrict-flex-arrays" is effective only when level is greater than
0.
When level=1, warnings are issued for a trailing array reference of a structure that have 2 or more elements if
the trailing array is referenced as a flexible array member.
When level=2, in addition to level=1, additional warnings are issued for a trailing one-element array reference
of a structure if the array is referenced as a flexible array member.
When level=3, in addition to level=2, additional warnings are issued for a trailing zero-length array reference
of a structure if the array is referenced as a flexible array member.
This option is more effective when -ftree-vrp is active (the default for -O2 and above) but some warnings may be
diagnosed even without optimization.
-Wsuggest-attribute=attribute-name
Warn for cases where adding an attribute may be beneficial. The attribute-names currently supported are listed
below.
-Wsuggest-attribute=pure
-Wsuggest-attribute=const
-Wsuggest-attribute=noreturn
-Wmissing-noreturn
-Wsuggest-attribute=malloc
-Wsuggest-attribute=returns_nonnull
Warn about functions that might be candidates for attributes "pure", "const", "noreturn", "malloc" or
"returns_nonnull". The compiler only warns for functions visible in other compilation units or (in the case
of "pure" and "const") if it cannot prove that the function returns normally. A function returns normally if
it doesn't contain an infinite loop or return abnormally by throwing, calling "abort" or trapping. This
analysis requires option -fipa-pure-const, which is enabled by default at -O and higher. Higher optimization
levels improve the accuracy of the analysis.
-Wsuggest-attribute=format
-Wmissing-format-attribute
Warn about function pointers that might be candidates for "format" attributes. Note these are only possible
candidates, not absolute ones. GCC guesses that function pointers with "format" attributes that are used in
assignment, initialization, parameter passing or return statements should have a corresponding "format"
attribute in the resulting type. I.e. the left-hand side of the assignment or initialization, the type of
the parameter variable, or the return type of the containing function respectively should also have a
"format" attribute to avoid the warning.
GCC also warns about function definitions that might be candidates for "format" attributes. Again, these are
only possible candidates. GCC guesses that "format" attributes might be appropriate for any function that
calls a function like "vprintf" or "vscanf", but this might not always be the case, and some functions for
which "format" attributes are appropriate may not be detected.
-Wsuggest-attribute=cold
Warn about functions that might be candidates for "cold" attribute. This is based on static detection and
generally only warns about functions which always leads to a call to another "cold" function such as wrappers
of C++ "throw" or fatal error reporting functions leading to "abort".
-Walloc-size
Warn about calls to allocation functions decorated with attribute "alloc_size" that specify insufficient size for
the target type of the pointer the result is assigned to, including those to the built-in forms of the functions
"aligned_alloc", "alloca", "calloc", "malloc", and "realloc".
-Walloc-zero
Warn about calls to allocation functions decorated with attribute "alloc_size" that specify zero bytes, including
those to the built-in forms of the functions "aligned_alloc", "alloca", "calloc", "malloc", and "realloc".
Because the behavior of these functions when called with a zero size differs among implementations (and in the
case of "realloc" has been deprecated) relying on it may result in subtle portability bugs and should be avoided.
-Wcalloc-transposed-args
Warn about calls to allocation functions decorated with attribute "alloc_size" with two arguments, which use
"sizeof" operator as the earlier size argument and don't use it as the later size argument. This is a coding
style warning. The first argument to "calloc" is documented to be number of elements in array, while the second
argument is size of each element, so "calloc (n, sizeof (int))" is preferred over "calloc (sizeof (int), n)". If
"sizeof" in the earlier argument and not the latter is intentional, the warning can be suppressed by using
"calloc (sizeof (struct S) + 0, n)" or "calloc (1 * sizeof (struct S), 4)" or using "sizeof" in the later
argument as well.
-Walloc-size-larger-than=byte-size
Warn about calls to functions decorated with attribute "alloc_size" that attempt to allocate objects larger than
the specified number of bytes, or where the result of the size computation in an integer type with infinite
precision would exceed the value of PTRDIFF_MAX on the target. -Walloc-size-larger-than=PTRDIFF_MAX is enabled
by default. Warnings controlled by the option can be disabled either by specifying byte-size of SIZE_MAX or more
or by -Wno-alloc-size-larger-than.
-Wno-alloc-size-larger-than
Disable -Walloc-size-larger-than= warnings. The option is equivalent to -Walloc-size-larger-than=SIZE_MAX or
larger.
-Walloca
This option warns on all uses of "alloca" in the source.
-Walloca-larger-than=byte-size
This option warns on calls to "alloca" with an integer argument whose value is either zero, or that is not
bounded by a controlling predicate that limits its value to at most byte-size. It also warns for calls to
"alloca" where the bound value is unknown. Arguments of non-integer types are considered unbounded even if they
appear to be constrained to the expected range.
For example, a bounded case of "alloca" could be:
void func (size_t n)
{
void *p;
if (n <= 1000)
p = alloca (n);
else
p = malloc (n);
f (p);
}
In the above example, passing "-Walloca-larger-than=1000" would not issue a warning because the call to "alloca"
is known to be at most 1000 bytes. However, if "-Walloca-larger-than=500" were passed, the compiler would emit a
warning.
Unbounded uses, on the other hand, are uses of "alloca" with no controlling predicate constraining its integer
argument. For example:
void func ()
{
void *p = alloca (n);
f (p);
}
If "-Walloca-larger-than=500" were passed, the above would trigger a warning, but this time because of the lack
of bounds checking.
Note, that even seemingly correct code involving signed integers could cause a warning:
void func (signed int n)
{
if (n < 500)
{
p = alloca (n);
f (p);
}
}
In the above example, n could be negative, causing a larger than expected argument to be implicitly cast into the
"alloca" call.
This option also warns when "alloca" is used in a loop.
-Walloca-larger-than=PTRDIFF_MAX is enabled by default but is usually only effective when -ftree-vrp is active
(default for -O2 and above).
See also -Wvla-larger-than=byte-size.
-Wno-alloca-larger-than
Disable -Walloca-larger-than= warnings. The option is equivalent to -Walloca-larger-than=SIZE_MAX or larger.
-Warith-conversion
Do warn about implicit conversions from arithmetic operations even when conversion of the operands to the same
type cannot change their values. This affects warnings from -Wconversion, -Wfloat-conversion, and
-Wsign-conversion.
void f (char c, int i)
{
c = c + i; // warns with B<-Wconversion>
c = c + 1; // only warns with B<-Warith-conversion>
}
-Warray-bounds
-Warray-bounds=n
Warn about out of bounds subscripts or offsets into arrays. This warning is enabled by -Wall. It is more
effective when -ftree-vrp is active (the default for -O2 and above) but a subset of instances are issued even
without optimization.
By default, the trailing array of a structure will be treated as a flexible array member by -Warray-bounds or
-Warray-bounds=n if it is declared as either a flexible array member per C99 standard onwards ([]), a GCC zero-
length array extension ([0]), or an one-element array ([1]). As a result, out of bounds subscripts or offsets
into zero-length arrays or one-element arrays are not warned by default.
You can add the option -fstrict-flex-arrays or -fstrict-flex-arrays=level to control how this option treat
trailing array of a structure as a flexible array member:
when level<=1, no change to the default behavior.
when level=2, additional warnings will be issued for out of bounds subscripts or offsets into one-element arrays;
when level=3, in addition to level=2, additional warnings will be issued for out of bounds subscripts or offsets
into zero-length arrays.
-Warray-bounds=1
This is the default warning level of -Warray-bounds and is enabled by -Wall; higher levels are not, and must
be explicitly requested.
-Warray-bounds=2
This warning level also warns about the intermediate results of pointer arithmetic that may yield out of
bounds values. This warning level may give a larger number of false positives and is deactivated by default.
-Wunterminated-string-initialization (C and Objective-C only)
Warn about character arrays initialized as unterminated character sequences with a string literal, unless the
declaration being initialized has the "nonstring" attribute. For example:
char arr[3] = "foo"; /* Warning. */
char arr2[3] __attribute__((nonstring)) = "bar"; /* No warning. */
This warning is enabled by -Wextra. If -Wc++-compat is enabled, the warning has slightly different wording and
warns even if the declaration being initialized has the "nonstring" warning, as in C++ such initializations are
an error.
-Warray-compare
Warn about equality and relational comparisons between two operands of array type. This comparison was
deprecated in C++20. For example:
int arr1[5];
int arr2[5];
bool same = arr1 == arr2;
-Warray-compare is enabled by -Wall.
-Warray-parameter
-Warray-parameter=n
Warn about redeclarations of functions involving parameters of array or pointer types of inconsistent kinds or
forms, and enable the detection of out-of-bounds accesses to such parameters by warnings such as -Warray-bounds.
If the first function declaration uses the array form for a parameter declaration, the bound specified in the
array is assumed to be the minimum number of elements expected to be provided in calls to the function and the
maximum number of elements accessed by it. Failing to provide arguments of sufficient size or accessing more
than the maximum number of elements may be diagnosed by warnings such as -Warray-bounds or -Wstringop-overflow.
At level 1, the warning diagnoses inconsistencies involving array parameters declared using the "T[static N]"
form.
For example, the warning triggers for the second declaration of "f" because the first one with the keyword
"static" specifies that the array argument must have at least four elements, while the second allows an array of
any size to be passed to "f".
void f (int[static 4]);
void f (int[]); // warning (inconsistent array form)
void g (void)
{
int *p = (int *)malloc (1 * sizeof (int));
f (p); // warning (array too small)
...
}
At level 2 the warning also triggers for redeclarations involving any other inconsistency in array or pointer
argument forms denoting array sizes. Pointers and arrays of unspecified bound are considered equivalent and do
not trigger a warning.
void g (int*);
void g (int[]); // no warning
void g (int[8]); // warning (inconsistent array bound)
-Warray-parameter=2 is included in -Wall. The -Wvla-parameter option triggers warnings for similar
inconsistencies involving Variable Length Array arguments.
The short form of the option -Warray-parameter is equivalent to -Warray-parameter=2. The negative form
-Wno-array-parameter is equivalent to -Warray-parameter=0.
-Wattribute-alias=n
-Wno-attribute-alias
Warn about declarations using the "alias" and similar attributes whose target is incompatible with the type of
the alias.
-Wattribute-alias=1
The default warning level of the -Wattribute-alias option diagnoses incompatibilities between the type of the
alias declaration and that of its target. Such incompatibilities are typically indicative of bugs.
-Wattribute-alias=2
At this level -Wattribute-alias also diagnoses cases where the attributes of the alias declaration are more
restrictive than the attributes applied to its target. These mismatches can potentially result in incorrect
code generation. In other cases they may be benign and could be resolved simply by adding the missing
attribute to the target. For comparison, see the -Wmissing-attributes option, which controls diagnostics
when the alias declaration is less restrictive than the target, rather than more restrictive.
Attributes considered include "alloc_align", "alloc_size", "cold", "const", "hot", "leaf", "malloc",
"nonnull", "noreturn", "nothrow", "pure", "returns_nonnull", and "returns_twice".
-Wattribute-alias is equivalent to -Wattribute-alias=1. This is the default. You can disable these warnings
with either -Wno-attribute-alias or -Wattribute-alias=0.
-Wbidi-chars=[none|unpaired|any|ucn]
Warn about possibly misleading UTF-8 bidirectional control characters in comments, string literals, character
constants, and identifiers. Such characters can change left-to-right writing direction into right-to-left (and
vice versa), which can cause confusion between the logical order and visual order. This may be dangerous; for
instance, it may seem that a piece of code is not commented out, whereas it in fact is.
There are three levels of warning supported by GCC. The default is -Wbidi-chars=unpaired, which warns about
improperly terminated bidi contexts. -Wbidi-chars=none turns the warning off. -Wbidi-chars=any warns about any
use of bidirectional control characters.
By default, this warning does not warn about UCNs. It is, however, possible to turn on such checking by using
-Wbidi-chars=unpaired,ucn or -Wbidi-chars=any,ucn. Using -Wbidi-chars=ucn is valid, and is equivalent to
-Wbidi-chars=unpaired,ucn, if no previous -Wbidi-chars=any was specified.
NOTE: In Ubuntu 25.10 and later versions, -Wbidi-chars is enabled by default for C, C++, ObjC, ObjC++, unless
-Wno-bidi-chars or -Wbidi-chars=none is found.
-Wbool-compare
Warn about boolean expression compared with an integer value different from "true"/"false". For instance, the
following comparison is always false:
int n = 5;
...
if ((n > 1) == 2) { ... }
This warning is enabled by -Wall.
-Wbool-operation
Warn about suspicious operations on expressions of a boolean type. For instance, bitwise negation of a boolean
is very likely a bug in the program. For C, this warning also warns about incrementing or decrementing a
boolean, which rarely makes sense. (In C++, decrementing a boolean is always invalid. Incrementing a boolean is
invalid in C++17, and deprecated otherwise.)
This warning is enabled by -Wall.
-Wduplicated-branches
Warn when an if-else has identical branches. This warning detects cases like
if (p != NULL)
return 0;
else
return 0;
It doesn't warn when both branches contain just a null statement. This warning also warn for conditional
operators:
int i = x ? *p : *p;
-Wduplicated-cond
Warn about duplicated conditions in an if-else-if chain. For instance, warn for the following code:
if (p->q != NULL) { ... }
else if (p->q != NULL) { ... }
-Wframe-address
Warn when the __builtin_frame_address or __builtin_return_address is called with an argument greater than 0.
Such calls may return indeterminate values or crash the program. The warning is included in -Wall.
-Wno-discarded-qualifiers (C and Objective-C only)
Do not warn if type qualifiers on pointers are being discarded. Typically, the compiler warns if a "const char
*" variable is passed to a function that takes a "char *" parameter. This option can be used to suppress such a
warning.
-Wno-discarded-array-qualifiers (C and Objective-C only)
Do not warn if type qualifiers on arrays which are pointer targets are being discarded. Typically, the compiler
warns if a "const int (*)[]" variable is passed to a function that takes a "int (*)[]" parameter. This option
can be used to suppress such a warning.
-Wno-incompatible-pointer-types (C and Objective-C only)
Do not warn when there is a conversion between pointers that have incompatible types. This warning is for cases
not covered by -Wno-pointer-sign, which warns for pointer argument passing or assignment with different
signedness.
By default, in C99 and later dialects of C, GCC treats this issue as an error. The error can be downgraded to a
warning using -fpermissive (along with certain other errors), or for this error alone, with
-Wno-error=incompatible-pointer-types.
This warning is upgraded to an error by -pedantic-errors.
-Wno-int-conversion (C and Objective-C only)
Do not warn about incompatible integer to pointer and pointer to integer conversions. This warning is about
implicit conversions; for explicit conversions the warnings -Wno-int-to-pointer-cast and -Wno-pointer-to-int-cast
may be used.
By default, in C99 and later dialects of C, GCC treats this issue as an error. The error can be downgraded to a
warning using -fpermissive (along with certain other errors), or for this error alone, with
-Wno-error=int-conversion.
This warning is upgraded to an error by -pedantic-errors.
-Wzero-as-null-pointer-constant
Warn when a literal 0 is used as null pointer constant.
-Wzero-length-bounds
Warn about accesses to elements of zero-length array members that might overlap other members of the same object.
Declaring interior zero-length arrays is discouraged because accesses to them are undefined.
For example, the first two stores in function "bad" are diagnosed because the array elements overlap the
subsequent members "b" and "c". The third store is diagnosed by -Warray-bounds because it is beyond the bounds
of the enclosing object.
struct X { int a[0]; int b, c; };
struct X x;
void bad (void)
{
x.a[0] = 0; // -Wzero-length-bounds
x.a[1] = 1; // -Wzero-length-bounds
x.a[2] = 2; // -Warray-bounds
}
Option -Wzero-length-bounds is enabled by -Warray-bounds.
-Wno-div-by-zero
Do not warn about compile-time integer division by zero. Floating-point division by zero is not warned about, as
it can be a legitimate way of obtaining infinities and NaNs.
-Wsystem-headers
Print warning messages for constructs found in system header files. Warnings from system headers are normally
suppressed, on the assumption that they usually do not indicate real problems and would only make the compiler
output harder to read. Using this command-line option tells GCC to emit warnings from system headers as if they
occurred in user code. However, note that using -Wall in conjunction with this option does not warn about
unknown pragmas in system headers---for that, -Wunknown-pragmas must also be used.
-Wtautological-compare
Warn if a self-comparison always evaluates to true or false. This warning detects various mistakes such as:
int i = 1;
...
if (i > i) { ... }
This warning also warns about bitwise comparisons that always evaluate to true or false, for instance:
if ((a & 16) == 10) { ... }
will always be false.
This warning is enabled by -Wall.
-Wtrailing-whitespace
-Wtrailing-whitespace=kind
Warn about trailing whitespace at the end of lines, including inside of comments, but excluding trailing
whitespace in raw string literals. "-Wtrailing-whitespace" is equivalent to "-Wtrailing-whitespace=blanks" and
warns just about trailing space and horizontal tab characters. "-Wtrailing-whitespace=any" warns about those or
trailing form feed or vertical tab characters. "-Wno-trailing-whitespace" or "-Wtrailing-whitespace=none"
disables the warning, which is the default. This is a coding style warning.
-Wleading-whitespace=kind
Warn about style issues in leading whitespace, but not about the amount of indentation. Some projects use coding
styles where only spaces are used for indentation, others use only tabs, others use zero or more tabs (for
multiples of "-ftabstop=n") followed by zero or fewer than n spaces. No warning is emitted on lines which
contain solely whitespace (although "-Wtrailing-whitespace=" warning might be emitted), no warnings are emitted
inside of raw string literals. Warnings are also emitted for leading whitespace inside of multi-line comments.
"-Wleading-whitespace=spaces" warns about leading whitespace other than spaces for projects which want to indent
just by spaces. "-Wleading-whitespace=tabs" warns about leading whitespace other than horizontal tabs for
projects which want to indent just by horizontal tabs. "-Wleading-whitespace=blanks" warns about leading
whitespace other than spaces and horizontal tabs, or about horizontal tab after a space in the leading
whitespace, or about n or more consecutive spaces in leading whitespace (where n is argument of "-ftabstop=n", 8
by default). "-Wleading-whitespace=none" disables the warning, which is the default. This is a coding style
warning.
-Wtrampolines
Warn about trampolines generated for pointers to nested functions. A trampoline is a small piece of data or code
that is created at run time on the stack when the address of a nested function is taken, and is used to call the
nested function indirectly. For some targets, it is made up of data only and thus requires no special treatment.
But, for most targets, it is made up of code and thus requires the stack to be made executable in order for the
program to work properly.
-Wfloat-equal
Warn if floating-point values are used in equality comparisons.
The idea behind this is that sometimes it is convenient (for the programmer) to consider floating-point values as
approximations to infinitely precise real numbers. If you are doing this, then you need to compute (by analyzing
the code, or in some other way) the maximum or likely maximum error that the computation introduces, and allow
for it when performing comparisons (and when producing output, but that's a different problem). In particular,
instead of testing for equality, you should check to see whether the two values have ranges that overlap; and
this is done with the relational operators, so equality comparisons are probably mistaken.
-Wtraditional (C and Objective-C only)
Warn about certain constructs that behave differently in traditional and ISO C. Also warn about ISO C constructs
that have no traditional C equivalent, and/or problematic constructs that should be avoided.
* Macro parameters that appear within string literals in the macro body. In traditional C macro replacement
takes place within string literals, but in ISO C it does not.
* In traditional C, some preprocessor directives did not exist. Traditional preprocessors only considered a
line to be a directive if the # appeared in column 1 on the line. Therefore -Wtraditional warns about
directives that traditional C understands but ignores because the # does not appear as the first character on
the line. It also suggests you hide directives like "#pragma" not understood by traditional C by indenting
them. Some traditional implementations do not recognize "#elif", so this option suggests avoiding it
altogether.
* A function-like macro that appears without arguments.
* The unary plus operator.
* The U integer constant suffix, or the F or L floating-point constant suffixes. (Traditional C does support
the L suffix on integer constants.) Note, these suffixes appear in macros defined in the system headers of
most modern systems, e.g. the _MIN/_MAX macros in "". Use of these macros in user code might
normally lead to spurious warnings, however GCC's integrated preprocessor has enough context to avoid warning
in these cases.
* A function declared external in one block and then used after the end of the block.
* A "switch" statement has an operand of type "long".
* A non-"static" function declaration follows a "static" one. This construct is not accepted by some
traditional C compilers.
* The ISO type of an integer constant has a different width or signedness from its traditional type. This
warning is only issued if the base of the constant is ten. I.e. hexadecimal or octal values, which typically
represent bit patterns, are not warned about.
* Usage of ISO string concatenation is detected.
* Initialization of automatic aggregates.
* Identifier conflicts with labels. Traditional C lacks a separate namespace for labels.
* Initialization of unions. If the initializer is zero, the warning is omitted. This is done under the
assumption that the zero initializer in user code appears conditioned on e.g. "__STDC__" to avoid missing
initializer warnings and relies on default initialization to zero in the traditional C case.
* Conversions by prototypes between fixed/floating-point values and vice versa. The absence of these
prototypes when compiling with traditional C causes serious problems. This is a subset of the possible
conversion warnings; for the full set use -Wtraditional-conversion.
* Use of ISO C style function definitions. This warning intentionally is not issued for prototype declarations
or variadic functions because these ISO C features appear in your code when using libiberty's traditional C
compatibility macros, "PARAMS" and "VPARAMS". This warning is also bypassed for nested functions because
that feature is already a GCC extension and thus not relevant to traditional C compatibility.
-Wtraditional-conversion (C and Objective-C only)
Warn if a prototype causes a type conversion that is different from what would happen to the same argument in the
absence of a prototype. This includes conversions of fixed point to floating and vice versa, and conversions
changing the width or signedness of a fixed-point argument except when the same as the default promotion.
-Wdeclaration-after-statement (C and Objective-C only)
Warn when a declaration is found after a statement in a block. This construct, known from C++, was introduced
with ISO C99 and is by default allowed in GCC. It is not supported by ISO C90.
This warning is upgraded to an error by -pedantic-errors.
-Wshadow
Warn whenever a local variable or type declaration shadows another variable, parameter, type, class member (in
C++), or instance variable (in Objective-C) or whenever a built-in function is shadowed. Note that in C++, the
compiler warns if a local variable shadows an explicit typedef, but not if it shadows a struct/class/enum. If
this warning is enabled, it includes also all instances of local shadowing. This means that -Wno-shadow=local
and -Wno-shadow=compatible-local are ignored when -Wshadow is used. Same as -Wshadow=global.
-Wno-shadow-ivar (Objective-C only)
Do not warn whenever a local variable shadows an instance variable in an Objective-C method.
-Wshadow=global
Warn for any shadowing. Same as -Wshadow.
-Wshadow=local
Warn when a local variable shadows another local variable or parameter.
-Wshadow=compatible-local
Warn when a local variable shadows another local variable or parameter whose type is compatible with that of the
shadowing variable. In C++, type compatibility here means the type of the shadowing variable can be converted to
that of the shadowed variable. The creation of this flag (in addition to -Wshadow=local) is based on the idea
that when a local variable shadows another one of incompatible type, it is most likely intentional, not a bug or
typo, as shown in the following example:
for (SomeIterator i = SomeObj.begin(); i != SomeObj.end(); ++i)
{
for (int i = 0; i < N; ++i)
{
...
}
...
}
Since the two variable "i" in the example above have incompatible types, enabling only -Wshadow=compatible-local
does not emit a warning. Because their types are incompatible, if a programmer accidentally uses one in place of
the other, type checking is expected to catch that and emit an error or warning. Use of this flag instead of
-Wshadow=local can possibly reduce the number of warnings triggered by intentional shadowing. Note that this
also means that shadowing "const char *i" by "char *i" does not emit a warning.
This warning is also enabled by -Wshadow=local.
-Wlarger-than=byte-size
Warn whenever an object is defined whose size exceeds byte-size. -Wlarger-than=PTRDIFF_MAX is enabled by
default. Warnings controlled by the option can be disabled either by specifying byte-size of SIZE_MAX or more or
by -Wno-larger-than.
Also warn for calls to bounded functions such as "memchr" or "strnlen" that specify a bound greater than the
largest possible object, which is PTRDIFF_MAX bytes by default. These warnings can only be disabled by
-Wno-larger-than.
-Wno-larger-than
Disable -Wlarger-than= warnings. The option is equivalent to -Wlarger-than=SIZE_MAX or larger.
-Wframe-larger-than=byte-size
Warn if the size of a function frame exceeds byte-size. The computation done to determine the stack frame size
is approximate and not conservative. The actual requirements may be somewhat greater than byte-size even if you
do not get a warning. In addition, any space allocated via "alloca", variable-length arrays, or related
constructs is not included by the compiler when determining whether or not to issue a warning.
-Wframe-larger-than=PTRDIFF_MAX is enabled by default. Warnings controlled by the option can be disabled either
by specifying byte-size of SIZE_MAX or more or by -Wno-frame-larger-than.
-Wno-frame-larger-than
Disable -Wframe-larger-than= warnings. The option is equivalent to -Wframe-larger-than=SIZE_MAX or larger.
-Wfree-nonheap-object
Warn when attempting to deallocate an object that was either not allocated on the heap, or by using a pointer
that was not returned from a prior call to the corresponding allocation function. For example, because the call
to "stpcpy" returns a pointer to the terminating nul character and not to the beginning of the object, the call
to "free" below is diagnosed.
void f (char *p)
{
p = stpcpy (p, "abc");
// ...
free (p); // warning
}
-Wfree-nonheap-object is included in -Wall.
-Wstack-usage=byte-size
Warn if the stack usage of a function might exceed byte-size. The computation done to determine the stack usage
is conservative. Any space allocated via "alloca", variable-length arrays, or related constructs is included by
the compiler when determining whether or not to issue a warning.
The message is in keeping with the output of -fstack-usage.
* If the stack usage is fully static but exceeds the specified amount, it's:
warning: stack usage is 1120 bytes
* If the stack usage is (partly) dynamic but bounded, it's:
warning: stack usage might be 1648 bytes
* If the stack usage is (partly) dynamic and not bounded, it's:
warning: stack usage might be unbounded
-Wstack-usage=PTRDIFF_MAX is enabled by default. Warnings controlled by the option can be disabled either by
specifying byte-size of SIZE_MAX or more or by -Wno-stack-usage.
-Wno-stack-usage
Disable -Wstack-usage= warnings. The option is equivalent to -Wstack-usage=SIZE_MAX or larger.
-Wunsafe-loop-optimizations
Warn if the loop cannot be optimized because the compiler cannot assume anything on the bounds of the loop
indices. With -funsafe-loop-optimizations warn if the compiler makes such assumptions.
-Wno-pedantic-ms-format (MinGW targets only)
When used in combination with -Wformat and -pedantic without GNU extensions, this option disables the warnings
about non-ISO "printf" / "scanf" format width specifiers "I32", "I64", and "I" used on Windows targets, which
depend on the MS runtime.
-Wpointer-arith
Warn about anything that depends on the "size of" a function type or of "void". GNU C assigns these types a size
of 1, for convenience in calculations with "void *" pointers and pointers to functions. In C++, warn also when
an arithmetic operation involves "NULL". This warning is also enabled by -Wpedantic.
This warning is upgraded to an error by -pedantic-errors.
-Wno-pointer-compare
Do not warn if a pointer is compared with a zero character constant. This usually means that the pointer was
meant to be dereferenced. For example:
const char *p = foo ();
if (p == '\0')
return 42;
Note that the code above is invalid in C++11.
This warning is enabled by default.
-Wno-tsan
Disable warnings about unsupported features in ThreadSanitizer.
ThreadSanitizer does not support "std::atomic_thread_fence" and can report false positives.
-Wtype-limits
Warn if a comparison is always true or always false due to the limited range of the data type, but do not warn
for constant expressions. For example, warn if an unsigned variable is compared against zero with "<" or ">=".
This warning is also enabled by -Wextra.
-Wabsolute-value (C and Objective-C only)
Warn for calls to standard functions that compute the absolute value of an argument when a more appropriate
standard function is available. For example, calling abs(3.14) triggers the warning because the appropriate
function to call to compute the absolute value of a double argument is "fabs". The option also triggers warnings
when the argument in a call to such a function has an unsigned type. This warning can be suppressed with an
explicit type cast and it is also enabled by -Wextra.
-Wcomment
-Wcomments
Warn whenever a comment-start sequence /* appears in a /* comment, or whenever a backslash-newline appears in a
// comment. This warning is enabled by -Wall.
-Wtrigraphs
Warn if any trigraphs are encountered that might change the meaning of the program. Trigraphs within comments
are not warned about, except those that would form escaped newlines.
This option is implied by -Wall. If -Wall is not given, this option is still enabled unless trigraphs are
enabled. To get trigraph conversion without warnings, but get the other -Wall warnings, use -trigraphs -Wall
-Wno-trigraphs.
-Wundef
Warn if an undefined identifier is evaluated in an "#if" directive. Such identifiers are replaced with zero.
-Wexpansion-to-defined
Warn whenever defined is encountered in the expansion of a macro (including the case where the macro is expanded
by an #if directive). Such usage is not portable. This warning is also enabled by -Wpedantic and -Wextra.
-Wunused-macros
Warn about macros defined in the main file that are unused. A macro is used if it is expanded or tested for
existence at least once. The preprocessor also warns if the macro has not been used at the time it is redefined
or undefined.
Built-in macros, macros defined on the command line, and macros defined in include files are not warned about.
Note: If a macro is actually used, but only used in skipped conditional blocks, then the preprocessor reports it
as unused. To avoid the warning in such a case, you might improve the scope of the macro's definition by, for
example, moving it into the first skipped block. Alternatively, you could provide a dummy use with something
like:
#if defined the_macro_causing_the_warning
#endif
-Wno-endif-labels
Do not warn whenever an "#else" or an "#endif" are followed by text. This sometimes happens in older programs
with code of the form
#if FOO
...
#else FOO
...
#endif FOO
The second and third "FOO" should be in comments. This warning is on by default.
-Wbad-function-cast (C and Objective-C only)
Warn when a function call is cast to a non-matching type. For example, warn if a call to a function returning an
integer type is cast to a pointer type.
-Wc90-c99-compat (C and Objective-C only)
Warn about features not present in ISO C90, but present in ISO C99. For instance, warn about use of variable
length arrays, "long long" type, "bool" type, compound literals, designated initializers, and so on. This option
is independent of the standards mode. Warnings are disabled in the expression that follows "__extension__".
-Wc99-c11-compat (C and Objective-C only)
Warn about features not present in ISO C99, but present in ISO C11. For instance, warn about use of anonymous
structures and unions, "_Atomic" type qualifier, "_Thread_local" storage-class specifier, "_Alignas" specifier,
"Alignof" operator, "_Generic" keyword, and so on. This option is independent of the standards mode. Warnings
are disabled in the expression that follows "__extension__".
-Wc11-c23-compat (C and Objective-C only)
-Wc11-c2x-compat (C and Objective-C only)
Warn about features not present in ISO C11, but present in ISO C23. For instance, warn about omitting the string
in "_Static_assert", use of [[]] syntax for attributes, use of decimal floating-point types, and so on. This
option is independent of the standards mode. Warnings are disabled in the expression that follows
"__extension__". The name -Wc11-c2x-compat is deprecated.
When not compiling in C23 mode, these warnings are upgraded to errors by -pedantic-errors.
-Wc23-c2y-compat (C and Objective-C only)
-Wc23-c2y-compat (C and Objective-C only)
Warn about features not present in ISO C23, but present in ISO C2Y. For instance, warn about "_Generic"
selecting with a type name instead of an expression. This option is independent of the standards mode. Warnings
are disabled in the expression that follows "__extension__".
When not compiling in C2Y mode, these warnings are upgraded to errors by -pedantic-errors.
-Wc++-compat (C and Objective-C only)
Warn about ISO C constructs that are outside of the common subset of ISO C and ISO C++, e.g. request for implicit
conversion from "void *" to a pointer to non-"void" type.
-Wc++11-compat (C++ and Objective-C++ only)
Warn about C++ constructs whose meaning differs between ISO C++ 1998 and ISO C++ 2011, e.g., identifiers in ISO
C++ 1998 that are keywords in ISO C++ 2011. This warning turns on -Wnarrowing and is enabled by -Wall.
-Wc++14-compat (C++ and Objective-C++ only)
Warn about C++ constructs whose meaning differs between ISO C++ 2011 and ISO C++ 2014. This warning is enabled
by -Wall.
-Wc++17-compat (C++ and Objective-C++ only)
Warn about C++ constructs whose meaning differs between ISO C++ 2014 and ISO C++ 2017. This warning is enabled
by -Wall.
-Wc++20-compat (C++ and Objective-C++ only)
Warn about C++ constructs whose meaning differs between ISO C++ 2017 and ISO C++ 2020. This warning is enabled
by -Wall.
-Wno-c++11-extensions (C++ and Objective-C++ only)
Do not warn about C++11 constructs in code being compiled using an older C++ standard. Even without this option,
some C++11 constructs will only be diagnosed if -Wpedantic is used.
-Wno-c++14-extensions (C++ and Objective-C++ only)
Do not warn about C++14 constructs in code being compiled using an older C++ standard. Even without this option,
some C++14 constructs will only be diagnosed if -Wpedantic is used.
-Wno-c++17-extensions (C++ and Objective-C++ only)
Do not warn about C++17 constructs in code being compiled using an older C++ standard. Even without this option,
some C++17 constructs will only be diagnosed if -Wpedantic is used.
-Wno-c++20-extensions (C++ and Objective-C++ only)
Do not warn about C++20 constructs in code being compiled using an older C++ standard. Even without this option,
some C++20 constructs will only be diagnosed if -Wpedantic is used.
-Wno-c++23-extensions (C++ and Objective-C++ only)
Do not warn about C++23 constructs in code being compiled using an older C++ standard. Even without this option,
some C++23 constructs will only be diagnosed if -Wpedantic is used.
-Wno-c++26-extensions (C++ and Objective-C++ only)
Do not warn about C++26 constructs in code being compiled using an older C++ standard. Even without this option,
some C++26 constructs will only be diagnosed if -Wpedantic is used.
-Wcast-qual
Warn whenever a pointer is cast so as to remove a type qualifier from the target type. For example, warn if a
"const char *" is cast to an ordinary "char *".
Also warn when making a cast that introduces a type qualifier in an unsafe way. For example, casting "char **"
to "const char **" is unsafe, as in this example:
/* p is char ** value. */
const char **q = (const char **) p;
/* Assignment of readonly string to const char * is OK. */
*q = "string";
/* Now char** pointer points to read-only memory. */
**p = 'b';
-Wcast-align
Warn whenever a pointer is cast such that the required alignment of the target is increased. For example, warn
if a "char *" is cast to an "int *" on machines where integers can only be accessed at two- or four-byte
boundaries.
-Wcast-align=strict
Warn whenever a pointer is cast such that the required alignment of the target is increased. For example, warn
if a "char *" is cast to an "int *" regardless of the target machine.
-Wcast-function-type
Warn when a function pointer is cast to an incompatible function pointer. In a cast involving function types
with a variable argument list only the types of initial arguments that are provided are considered. Any
parameter of pointer-type matches any other pointer-type. Any benign differences in integral types are ignored,
like "int" vs. "long" on ILP32 targets. Likewise type qualifiers are ignored. The function type "void (*)
(void)" is special and matches everything, which can be used to suppress this warning. In a cast involving
pointer to member types this warning warns whenever the type cast is changing the pointer to member type. This
warning is enabled by -Wextra.
-Wcast-user-defined
Warn when a cast to reference type does not involve a user-defined conversion that the programmer might expect to
be called.
struct A { operator const int&(); } a;
auto r = (int&)a; // warning
This warning is enabled by default.
-Wwrite-strings
When compiling C, give string constants the type "const char[length]" so that copying the address of one into a
non-"const" "char *" pointer produces a warning. These warnings help you find at compile time code that can try
to write into a string constant, but only if you have been very careful about using "const" in declarations and
prototypes. Otherwise, it is just a nuisance. This is why we did not make -Wall request these warnings.
When compiling C++, warn about the deprecated conversion from string literals to "char *". This warning is
enabled by default for C++ programs.
This warning is upgraded to an error by -pedantic-errors in C++11 mode or later.
-Wclobbered
Warn for variables that might be changed by "longjmp" or "vfork". This warning is also enabled by -Wextra.
-Wno-complain-wrong-lang
By default, language front ends complain when a command-line option is valid, but not applicable to that front
end. This may be disabled with -Wno-complain-wrong-lang, which is mostly useful when invoking a single compiler
driver for multiple source files written in different languages, for example:
$ g++ -fno-rtti a.cc b.f90
The driver g++ invokes the C++ front end to compile a.cc and the Fortran front end to compile b.f90. The latter
front end diagnoses f951: Warning: command-line option '-fno-rtti' is valid for C++/D/ObjC++ but not for Fortran,
which may be disabled with -Wno-complain-wrong-lang.
-Wcompare-distinct-pointer-types (C and Objective-C only)
Warn if pointers of distinct types are compared without a cast. This warning is enabled by default.
-Wconversion
Warn for implicit conversions that may alter a value. This includes conversions between real and integer, like
"abs (x)" when "x" is "double"; conversions between signed and unsigned, like "unsigned ui = -1"; and conversions
to smaller types, like "sqrtf (M_PI)". Do not warn for explicit casts like "abs ((int) x)" and "ui = (unsigned)
-1", or if the value is not changed by the conversion like in "abs (2.0)". Warnings about conversions between
signed and unsigned integers can be disabled by using -Wno-sign-conversion.
For C++, also warn for confusing overload resolution for user-defined conversions; and conversions that never use
a type conversion operator: conversions to "void", the same type, a base class or a reference to them. Warnings
about conversions between signed and unsigned integers are disabled by default in C++ unless -Wsign-conversion is
explicitly enabled.
Warnings about conversion from arithmetic on a small type back to that type are only given with
-Warith-conversion.
-Wdangling-else
Warn about constructions where there may be confusion to which "if" statement an "else" branch belongs. Here is
an example of such a case:
{
if (a)
if (b)
foo ();
else
bar ();
}
In C/C++, every "else" branch belongs to the innermost possible "if" statement, which in this example is "if
(b)". This is often not what the programmer expected, as illustrated in the above example by indentation the
programmer chose. When there is the potential for this confusion, GCC issues a warning when this flag is
specified. To eliminate the warning, add explicit braces around the innermost "if" statement so there is no way
the "else" can belong to the enclosing "if". The resulting code looks like this:
{
if (a)
{
if (b)
foo ();
else
bar ();
}
}
This warning is enabled by -Wparentheses.
-Wdangling-pointer
-Wdangling-pointer=n
Warn about uses of pointers (or C++ references) to objects with automatic storage duration after their lifetime
has ended. This includes local variables declared in nested blocks, compound literals and other unnamed
temporary objects. In addition, warn about storing the address of such objects in escaped pointers. The warning
is enabled at all optimization levels but may yield different results with optimization than without.
-Wdangling-pointer=1
At level 1, the warning diagnoses only unconditional uses of dangling pointers.
-Wdangling-pointer=2
At level 2, in addition to unconditional uses the warning also diagnoses conditional uses of dangling
pointers.
The short form -Wdangling-pointer is equivalent to -Wdangling-pointer=2, while -Wno-dangling-pointer and
-Wdangling-pointer=0 have the same effect of disabling the warnings. -Wdangling-pointer=2 is included in -Wall.
This example triggers the warning at level 1; the address of the unnamed temporary is unconditionally referenced
outside of its scope.
char f (char c1, char c2, char c3)
{
char *p;
{
p = (char[]) { c1, c2, c3 };
}
// warning: using dangling pointer 'p' to an unnamed temporary
return *p;
}
In the following function the store of the address of the local variable "x" in the escaped pointer *p triggers
the warning at level 1.
void g (int **p)
{
int x = 7;
// warning: storing the address of local variable 'x' in '*p'
*p = &x;
}
In this example, the array a is out of scope when the pointer s is used. Since the code that sets "s" is
conditional, the warning triggers at level 2.
extern void frob (const char *);
void h (char *s)
{
if (!s)
{
char a[12] = "tmpname";
s = a;
}
// warning: dangling pointer 's' to 'a' may be used
frob (s);
}
-Wdate-time
Warn when macros "__TIME__", "__DATE__" or "__TIMESTAMP__" are encountered as they might prevent bit-wise-
identical reproducible compilations.
-Wempty-body
Warn if an empty body occurs in an "if", "else" or "do while" statement. This warning is also enabled by
-Wextra.
-Wno-endif-labels
Do not warn about stray tokens after "#else" and "#endif".
-Wenum-compare
Warn about a comparison between values of different enumerated types. In C++ enumerated type mismatches in
conditional expressions are also diagnosed and the warning is enabled by default. In C this warning is enabled
by -Wall.
-Wenum-conversion
Warn when a value of enumerated type is implicitly converted to a different enumerated type. This warning is
enabled by -Wextra in C.
-Wenum-int-mismatch (C and Objective-C only)
Warn about mismatches between an enumerated type and an integer type in declarations. For example:
enum E { l = -1, z = 0, g = 1 };
int foo(void);
enum E foo(void);
In C, an enumerated type is compatible with "char", a signed integer type, or an unsigned integer type. However,
since the choice of the underlying type of an enumerated type is implementation-defined, such mismatches may
cause portability issues. In C++, such mismatches are an error. In C, this warning is enabled by -Wall and
-Wc++-compat.
-Wjump-misses-init (C, Objective-C only)
Warn if a "goto" statement or a "switch" statement jumps forward across the initialization of a variable, or
jumps backward to a label after the variable has been initialized. This only warns about variables that are
initialized when they are declared. This warning is only supported for C and Objective-C; in C++ this sort of
branch is an error in any case.
-Wjump-misses-init is included in -Wc++-compat. It can be disabled with the -Wno-jump-misses-init option.
-Wsign-compare
Warn when a comparison between signed and unsigned values could produce an incorrect result when the signed value
is converted to unsigned. In C++, this warning is also enabled by -Wall. In C, it is also enabled by -Wextra.
-Wsign-conversion
Warn for implicit conversions that may change the sign of an integer value, like assigning a signed integer
expression to an unsigned integer variable. An explicit cast silences the warning. In C, this option is enabled
also by -Wconversion.
-Wflex-array-member-not-at-end (C and C++ only)
Warn when a structure containing a C99 flexible array member as the last field is not at the end of another
structure. This warning warns e.g. about
struct flex { int length; char data[]; };
struct mid_flex { int m; struct flex flex_data; int n; };
-Wfloat-conversion
Warn for implicit conversions that reduce the precision of a real value. This includes conversions from real to
integer, and from higher precision real to lower precision real values. This option is also enabled by
-Wconversion.
-Wno-scalar-storage-order
Do not warn on suspicious constructs involving reverse scalar storage order.
-Wsizeof-array-div
Warn about divisions of two sizeof operators when the first one is applied to an array and the divisor does not
equal the size of the array element. In such a case, the computation will not yield the number of elements in
the array, which is likely what the user intended. This warning warns e.g. about
int fn ()
{
int arr[10];
return sizeof (arr) / sizeof (short);
}
This warning is enabled by -Wall.
-Wsizeof-pointer-div
Warn for suspicious divisions of two sizeof expressions that divide the pointer size by the element size, which
is the usual way to compute the array size but won't work out correctly with pointers. This warning warns e.g.
about "sizeof (ptr) / sizeof (ptr[0])" if "ptr" is not an array, but a pointer. This warning is enabled by
-Wall.
-Wsizeof-pointer-memaccess
Warn for suspicious length parameters to certain string and memory built-in functions if the argument uses
"sizeof". This warning triggers for example for "memset (ptr, 0, sizeof (ptr));" if "ptr" is not an array, but a
pointer, and suggests a possible fix, or about "memcpy (&foo, ptr, sizeof (&foo));". -Wsizeof-pointer-memaccess
also warns about calls to bounded string copy functions like "strncat" or "strncpy" that specify as the bound a
"sizeof" expression of the source array. For example, in the following function the call to "strncat" specifies
the size of the source string as the bound. That is almost certainly a mistake and so the call is diagnosed.
void make_file (const char *name)
{
char path[PATH_MAX];
strncpy (path, name, sizeof path - 1);
strncat (path, ".text", sizeof ".text");
...
}
The -Wsizeof-pointer-memaccess option is enabled by -Wall.
-Wno-sizeof-array-argument
Do not warn when the "sizeof" operator is applied to a parameter that is declared as an array in a function
definition. This warning is enabled by default for C and C++ programs.
-Wmemset-elt-size
Warn for suspicious calls to the "memset" built-in function, if the first argument references an array, and the
third argument is a number equal to the number of elements, but not equal to the size of the array in memory.
This indicates that the user has omitted a multiplication by the element size. This warning is enabled by -Wall.
-Wmemset-transposed-args
Warn for suspicious calls to the "memset" built-in function where the second argument is not zero and the third
argument is zero. For example, the call "memset (buf, sizeof buf, 0)" is diagnosed because "memset (buf, 0,
sizeof buf)" was meant instead. The diagnostic is only emitted if the third argument is a literal zero.
Otherwise, if it is an expression that is folded to zero, or a cast of zero to some type, it is far less likely
that the arguments have been mistakenly transposed and no warning is emitted. This warning is enabled by -Wall.
-Waddress
Warn about suspicious uses of address expressions. These include comparing the address of a function or a
declared object to the null pointer constant such as in
void f (void);
void g (void)
{
if (!f) // warning: expression evaluates to false
abort ();
}
comparisons of a pointer to a string literal, such as in
void f (const char *x)
{
if (x == "abc") // warning: expression evaluates to false
puts ("equal");
}
and tests of the results of pointer addition or subtraction for equality to null, such as in
void f (const int *p, int i)
{
return p + i == NULL;
}
Such uses typically indicate a programmer error: the address of most functions and objects necessarily evaluates
to true (the exception are weak symbols), so their use in a conditional might indicate missing parentheses in a
function call or a missing dereference in an array expression. The subset of the warning for object pointers can
be suppressed by casting the pointer operand to an integer type such as "intptr_t" or "uintptr_t". Comparisons
against string literals result in unspecified behavior and are not portable, and suggest the intent was to call
"strcmp". The warning is suppressed if the suspicious expression is the result of macro expansion. -Waddress
warning is enabled by -Wall.
-Wno-address-of-packed-member
Do not warn when the address of packed member of struct or union is taken, which usually results in an unaligned
pointer value. This is enabled by default.
-Wlogical-op
Warn about suspicious uses of logical operators in expressions. This includes using logical operators in
contexts where a bit-wise operator is likely to be expected. Also warns when the operands of a logical operator
are the same:
extern int a;
if (a < 0 && a < 0) { ... }
-Wlogical-not-parentheses
Warn about logical not used on the left hand side operand of a comparison. This option does not warn if the
right operand is considered to be a boolean expression. Its purpose is to detect suspicious code like the
following:
int a;
...
if (!a > 1) { ... }
It is possible to suppress the warning by wrapping the LHS into parentheses:
if ((!a) > 1) { ... }
This warning is enabled by -Wall.
-Waggregate-return
Warn if any functions that return structures or unions are defined or called. (In languages where you can return
an array, this also elicits a warning.)
-Wno-aggressive-loop-optimizations
Do not warn if the compiler detects undefined behavior in a loop with a constant number of iterations.
-Waggressive-loop-optimizations is enabled by default.
-Wno-attributes
Do not warn if an unexpected "__attribute__" is used, such as unrecognized attributes, function attributes
applied to variables, etc. This does not stop errors for incorrect use of supported attributes.
Warnings about ill-formed uses of standard attributes are upgraded to errors by -pedantic-errors.
Additionally, using -Wno-attributes=, it is possible to suppress warnings about unknown scoped attributes (in
C++11 and C23). For example, -Wno-attributes=vendor::attr disables warning about the following declaration:
[[vendor::attr]] void f();
It is also possible to disable warning about all attributes in a namespace using -Wno-attributes=vendor:: which
prevents warning about both of these declarations:
[[vendor::safe]] void f();
[[vendor::unsafe]] void f2();
Note that -Wno-attributes= does not imply -Wno-attributes.
-Wno-builtin-declaration-mismatch
Warn if a built-in function is declared with an incompatible signature or as a non-function, or when a built-in
function declared with a type that does not include a prototype is called with arguments whose promoted types do
not match those expected by the function. When -Wextra is specified, also warn when a built-in function that
takes arguments is declared without a prototype. The -Wbuiltin-declaration-mismatch warning is enabled by
default. To avoid the warning include the appropriate header to bring the prototypes of built-in functions into
scope.
For example, the call to "memset" below is diagnosed by the warning because the function expects a value of type
"size_t" as its argument but the type of 32 is "int". With -Wextra, the declaration of the function is diagnosed
as well.
extern void* memset ();
void f (void *d)
{
memset (d, '\0', 32);
}
-Wno-builtin-macro-redefined
Do not warn if certain built-in macros are redefined. This suppresses warnings for redefinition of
"__TIMESTAMP__", "__TIME__", "__DATE__", "__FILE__", and "__BASE_FILE__".
-Wfree-labels (C and Objective-C only)
Warn if a label is applied to a non-statement, or occurs at the end of a compound statement. Such labels are
allowed by C23 and later dialects of C, and are available as a GCC extension in all other dialects.
This warning is also enabled by -Wc11-c23-compat. It is turned into an error if building for a C version before
C23 by -pedantic-errors.
-Wheader-guard
Warn if a valid preprocessor header multiple inclusion guard has a "#define" directive right after "#ifndef" or
"#if !defined" directive for the multiple inclusion guard, which defines a different macro from the guard macro
with a similar name, the actual multiple inclusion guard macro isn't defined at the corresponding "#ifndef"
directive at the end of the header, and the "#define" directive defines an object-like macro with empty
definition. In such case, it often is just a misspelled guard name, either in the "#ifndef" or "#if !defined"
directive or in the subsequent "#define" directive. This warning is enabled by -Wall.
-Wstrict-prototypes (C and Objective-C only)
Warn if a function is declared or defined without specifying the argument types. (An old-style function
definition is permitted without a warning if preceded by a declaration that specifies the argument types.)
-Wold-style-declaration (C and Objective-C only)
Warn for obsolescent usages, according to the C Standard, in a declaration. For example, warn if storage-class
specifiers like "static" are not the first things in a declaration. This warning is also enabled by -Wextra.
-Wold-style-definition (C and Objective-C only)
Warn if an old-style function definition is used. A warning is given even if there is a previous prototype. A
definition using () is not considered an old-style definition in C23 mode, because it is equivalent to (void) in
that case, but is considered an old-style definition for older standards.
-Wdeprecated-non-prototype (C and Objective-C only)
Warn if a function declared with an empty parameter list () is called with one or more arguments, or if a
function definition with one or more parameters is encountered after such a declaration. Both cases are errors
in C23 and later dialects of C.
This warning is also enabled by -Wc11-c23-compat.
-Wmissing-parameter-name (C and Objective-C only)
Warn if a function definition omits a parameter name, specifying only its type. This can be used to document
that a parameter is unused in the definition. It is part of C23 and later dialects of C, and available as a GCC
extension in all other dialects.
This warning is also enabled by -Wc11-c23-compat. It is turned into an error if building for a C version before
C23 by -pedantic-errors.
-Wmissing-parameter-type (C and Objective-C only)
A function parameter is declared without a type specifier in K&R-style functions:
void foo(bar) { }
This warning is also enabled by -Wextra.
-Wno-declaration-missing-parameter-type (C and Objective-C only)
Do not warn if a function declaration contains a parameter name without a type. Such function declarations do
not provide a function prototype and prevent most type checking in function calls.
This warning is enabled by default. In C99 and later dialects of C, it is treated as an error. The error can be
downgraded to a warning using -fpermissive (along with certain other errors), or for this error alone, with
-Wno-error=declaration-missing-parameter-type.
This warning is upgraded to an error by -pedantic-errors.
-Wmissing-prototypes (C and Objective-C only)
Warn if a global function is defined without a previous prototype declaration. This warning is issued even if
the definition itself provides a prototype. Use this option to detect global functions that do not have a
matching prototype declaration in a header file. This option is not valid for C++ because all function
declarations provide prototypes and a non-matching declaration declares an overload rather than conflict with an
earlier declaration. Use -Wmissing-declarations to detect missing declarations in C++.
-Wmissing-variable-declarations (C and Objective-C only)
Warn if a global variable is defined without a previous declaration. Use this option to detect global variables
that do not have a matching extern declaration in a header file.
-Wmissing-declarations
Warn if a global function is defined without a previous declaration. Do so even if the definition itself
provides a prototype. Use this option to detect global functions that are not declared in header files. In C,
no warnings are issued for functions with previous non-prototype declarations; use -Wmissing-prototypes to detect
missing prototypes. In C++, no warnings are issued for function templates, or for inline functions, or for
functions in anonymous namespaces.
-Wmissing-field-initializers
Warn if a structure's initializer has some fields missing. For example, the following code causes such a
warning, because "x.h" is implicitly zero:
struct s { int f, g, h; };
struct s x = { 3, 4 };
In C this option does not warn about designated initializers, so the following modification does not trigger a
warning:
struct s { int f, g, h; };
struct s x = { .f = 3, .g = 4 };
In C this option does not warn about the universal zero initializer { 0 }:
struct s { int f, g, h; };
struct s x = { 0 };
Likewise, in C++ this option does not warn about the empty { } initializer, for example:
struct s { int f, g, h; };
s x = { };
This warning is included in -Wextra. To get other -Wextra warnings without this one, use -Wextra
-Wno-missing-field-initializers.
-Wno-missing-requires
By default, the compiler warns about a concept-id appearing as a C++20 simple-requirement:
bool satisfied = requires { C };
Here satisfied will be true if C is a valid expression, which it is for all T. Presumably the user meant to
write
bool satisfied = requires { requires C };
so satisfied is only true if concept C is satisfied for type T.
This warning can be disabled with -Wno-missing-requires.
-Wno-missing-template-keyword
The member access tokens ., -> and :: must be followed by the "template" keyword if the parent object is
dependent and the member being named is a template.
template
void DoStuff (X x)
{
x.template DoSomeOtherStuff(); // Good.
x.DoMoreStuff(); // Warning, x is dependent.
}
In rare cases it is possible to get false positives. To silence this, wrap the expression in parentheses. For
example, the following is treated as a template, even where m and N are integers:
void NotATemplate (my_class t)
{
int N = 5;
bool test = t.m < N > (0); // Treated as a template.
test = (t.m < N) > (0); // Same meaning, but not treated as a template.
}
This warning can be disabled with -Wno-missing-template-keyword.
-Wno-multichar
Do not warn if a multicharacter constant ('FOOF') is used. Usually they indicate a typo in the user's code, as
they have implementation-defined values, and should not be used in portable code.
-Wnormalized=[none|id|nfc|nfkc]
In ISO C and ISO C++, two identifiers are different if they are different sequences of characters. However,
sometimes when characters outside the basic ASCII character set are used, you can have two different character
sequences that look the same. To avoid confusion, the ISO 10646 standard sets out some normalization rules which
when applied ensure that two sequences that look the same are turned into the same sequence. GCC can warn you if
you are using identifiers that have not been normalized; this option controls that warning.
There are four levels of warning supported by GCC. The default is -Wnormalized=nfc, which warns about any
identifier that is not in the ISO 10646 "C" normalized form, NFC. NFC is the recommended form for most uses. It
is equivalent to -Wnormalized.
Unfortunately, there are some characters allowed in identifiers by ISO C and ISO C++ that, when turned into NFC,
are not allowed in identifiers. That is, there's no way to use these symbols in portable ISO C or C++ and have
all your identifiers in NFC. -Wnormalized=id suppresses the warning for these characters. It is hoped that
future versions of the standards involved will correct this, which is why this option is not the default.
You can switch the warning off for all characters by writing -Wnormalized=none or -Wno-normalized. You should
only do this if you are using some other normalization scheme (like "D"), because otherwise you can easily create
bugs that are literally impossible to see.
Some characters in ISO 10646 have distinct meanings but look identical in some fonts or display methodologies,
especially once formatting has been applied. For instance "\u207F", "SUPERSCRIPT LATIN SMALL LETTER N", displays
just like a regular "n" that has been placed in a superscript. ISO 10646 defines the NFKC normalization scheme
to convert all these into a standard form as well, and GCC warns if your code is not in NFKC if you use
-Wnormalized=nfkc. This warning is comparable to warning about every identifier that contains the letter O
because it might be confused with the digit 0, and so is not the default, but may be useful as a local coding
convention if the programming environment cannot be fixed to display these characters distinctly.
-Wno-attribute-warning
Do not warn about usage of functions declared with "warning" attribute. By default, this warning is enabled.
-Wno-attribute-warning can be used to disable the warning or -Wno-error=attribute-warning can be used to disable
the error when compiled with -Werror flag.
-Wno-deprecated
Do not warn about usage of deprecated features.
In C++, explicitly specifying -Wdeprecated also enables warnings about some features that are deprecated in later
language standards, specifically -Wcomma-subscript, -Wvolatile, -Wdeprecated-enum-float-conversion,
-Wdeprecated-enum-enum-conversion, -Wdeprecated-literal-operator, and -Wdeprecated-variadic-comma-omission.
-Wno-deprecated-declarations
Do not warn about uses of functions, variables, and types marked as deprecated by using the "deprecated"
attribute.
-Wno-overflow
Do not warn about compile-time overflow in constant expressions.
-Wno-odr
Warn about One Definition Rule violations during link-time optimization. Enabled by default.
-Wopenacc-parallelism
Warn about potentially suboptimal choices related to OpenACC parallelism.
-Wno-openmp
Warn about suspicious OpenMP code.
-Wopenmp-simd
Warn if the vectorizer cost model overrides the OpenMP simd directive set by user. The
-fsimd-cost-model=unlimited option can be used to relax the cost model.
-Woverride-init (C and Objective-C only)
Warn if an initialized field without side effects is overridden when using designated initializers.
This warning is included in -Wextra. To get other -Wextra warnings without this one, use -Wextra
-Wno-override-init.
-Wno-override-init-side-effects (C and Objective-C only)
Do not warn if an initialized field with side effects is overridden when using designated initializers. This
warning is enabled by default.
-Wpacked
Warn if a structure is given the packed attribute, but the packed attribute has no effect on the layout or size
of the structure. Such structures may be mis-aligned for little benefit. For instance, in this code, the
variable "f.x" in "struct bar" is misaligned even though "struct bar" does not itself have the packed attribute:
struct foo {
int x;
char a, b, c, d;
} __attribute__((packed));
struct bar {
char z;
struct foo f;
};
-Wnopacked-bitfield-compat
The 4.1, 4.2 and 4.3 series of GCC ignore the "packed" attribute on bit-fields of type "char". This was fixed in
GCC 4.4 but the change can lead to differences in the structure layout. GCC informs you when the offset of such
a field has changed in GCC 4.4. For example there is no longer a 4-bit padding between field "a" and "b" in this
structure:
struct foo
{
char a:4;
char b:8;
} __attribute__ ((packed));
This warning is enabled by default. Use -Wno-packed-bitfield-compat to disable this warning.
-Wpacked-not-aligned (C, C++, Objective-C and Objective-C++ only)
Warn if a structure field with explicitly specified alignment in a packed struct or union is misaligned. For
example, a warning will be issued on "struct S", like, "warning: alignment 1 of 'struct S' is less than 8", in
this code:
struct __attribute__ ((aligned (8))) S8 { char a[8]; };
struct __attribute__ ((packed)) S {
struct S8 s8;
};
This warning is enabled by -Wall.
-Wpadded
Warn if padding is included in a structure, either to align an element of the structure or to align the whole
structure. Sometimes when this happens it is possible to rearrange the fields of the structure to reduce the
padding and so make the structure smaller.
-Wredundant-decls
Warn if anything is declared more than once in the same scope, even in cases where multiple declaration is valid
and changes nothing.
-Wrestrict
Warn when an object referenced by a "restrict"-qualified parameter (or, in C++, a "__restrict"-qualified
parameter) is aliased by another argument, or when copies between such objects overlap. For example, the call to
the "strcpy" function below attempts to truncate the string by replacing its initial characters with the last
four. However, because the call writes the terminating NUL into "a[4]", the copies overlap and the call is
diagnosed.
void foo (void)
{
char a[] = "abcd1234";
strcpy (a, a + 4);
...
}
The -Wrestrict option detects some instances of simple overlap even without optimization but works best at -O2
and above. It is included in -Wall.
-Wnested-externs (C and Objective-C only)
Warn if an "extern" declaration is encountered within a function.
-Winline
Warn if a function that is declared as inline cannot be inlined. Even with this option, the compiler does not
warn about failures to inline functions declared in system headers.
The compiler uses a variety of heuristics to determine whether or not to inline a function. For example, the
compiler takes into account the size of the function being inlined and the amount of inlining that has already
been done in the current function. Therefore, seemingly insignificant changes in the source program can cause
the warnings produced by -Winline to appear or disappear.
-Winterference-size
Warn about use of C++17 "std::hardware_destructive_interference_size" without specifying its value with --param
destructive-interference-size. Also warn about questionable values for that option.
This variable is intended to be used for controlling class layout, to avoid false sharing in concurrent code:
struct independent_fields {
alignas(std::hardware_destructive_interference_size)
std::atomic one;
alignas(std::hardware_destructive_interference_size)
std::atomic two;
};
Here one and two are intended to be far enough apart that stores to one won't require accesses to the other to
reload the cache line.
By default, --param destructive-interference-size and --param constructive-interference-size are set based on the
current -mtune option, typically to the L1 cache line size for the particular target CPU, sometimes to a range if
tuning for a generic target. So all translation units that depend on ABI compatibility for the use of these
variables must be compiled with the same -mtune (or -mcpu).
If ABI stability is important, such as if the use is in a header for a library, you should probably not use the
hardware interference size variables at all. Alternatively, you can force a particular value with --param.
If you are confident that your use of the variable does not affect ABI outside a single build of your project,
you can turn off the warning with -Wno-interference-size.
-Wint-in-bool-context
Warn for suspicious use of integer values where boolean values are expected, such as conditional expressions (?:)
using non-boolean integer constants in boolean context, like "if (a <= b ? 2 : 3)". Or left shifting of signed
integers in boolean context, like "for (a = 0; 1 << a; a++);". Likewise for all kinds of multiplications
regardless of the data type. This warning is enabled by -Wall.
-Wno-int-to-pointer-cast
Suppress warnings from casts to pointer type of an integer of a different size. In C++, casting to a pointer type
of smaller size is an error. Wint-to-pointer-cast is enabled by default.
-Wno-pointer-to-int-cast (C and Objective-C only)
Suppress warnings from casts from a pointer to an integer type of a different size.
-Winvalid-pch
Warn if a precompiled header is found in the search path but cannot be used.
-Winvalid-utf8
Warn if an invalid UTF-8 character is found. This warning is on by default for C++23 if -finput-charset=UTF-8 is
used and turned into error with -pedantic-errors.
-Wno-unicode
Don't diagnose invalid forms of delimited or named escape sequences which are treated as separate tokens.
Wunicode is enabled by default.
-Wlong-long
Warn if "long long" type is used. This is enabled by either -Wpedantic or -Wtraditional in ISO C90 and C++98
modes. To inhibit the warning messages, use -Wno-long-long.
This warning is upgraded to an error by -pedantic-errors.
-Wvariadic-macros
Warn if variadic macros are used in ISO C90 mode, or if the GNU alternate syntax is used in ISO C99 mode. This
is enabled by either -Wpedantic or -Wtraditional. To inhibit the warning messages, use -Wno-variadic-macros.
-Wno-varargs
Do not warn upon questionable usage of the macros used to handle variable arguments like "va_start". These
warnings are enabled by default.
-Wvector-operation-performance
Warn if vector operation is not implemented via SIMD capabilities of the architecture. Mainly useful for the
performance tuning. Vector operation can be implemented "piecewise", which means that the scalar operation is
performed on every vector element; "in parallel", which means that the vector operation is implemented using
scalars of wider type, which normally is more performance efficient; and "as a single scalar", which means that
vector fits into a scalar type.
-Wvla
Warn if a variable-length array is used in the code. -Wno-vla prevents the -Wpedantic warning of the variable-
length array.
This warning is upgraded to an error by -pedantic-errors.
-Wvla-larger-than=byte-size
If this option is used, the compiler warns for declarations of variable-length arrays whose size is either
unbounded, or bounded by an argument that allows the array size to exceed byte-size bytes. This is similar to
how -Walloca-larger-than=byte-size works, but with variable-length arrays.
Note that GCC may optimize small variable-length arrays of a known value into plain arrays, so this warning may
not get triggered for such arrays.
-Wvla-larger-than=PTRDIFF_MAX is enabled by default but is typically only effective when -ftree-vrp is active
(default for -O2 and above).
See also -Walloca-larger-than=byte-size.
-Wno-vla-larger-than
Disable -Wvla-larger-than= warnings. The option is equivalent to -Wvla-larger-than=SIZE_MAX or larger.
-Wvla-parameter
Warn about redeclarations of functions involving arguments of Variable Length Array types of inconsistent kinds
or forms, and enable the detection of out-of-bounds accesses to such parameters by warnings such as
-Warray-bounds.
If the first function declaration uses the VLA form the bound specified in the array is assumed to be the minimum
number of elements expected to be provided in calls to the function and the maximum number of elements accessed
by it. Failing to provide arguments of sufficient size or accessing more than the maximum number of elements may
be diagnosed.
For example, the warning triggers for the following redeclarations because the first one allows an array of any
size to be passed to "f" while the second one specifies that the array argument must have at least "n" elements.
In addition, calling "f" with the associated VLA bound parameter in excess of the actual VLA bound triggers a
warning as well.
void f (int n, int[n]);
// warning: argument 2 previously declared as a VLA
void f (int, int[]);
void g (int n)
{
if (n > 4)
return;
int a[n];
// warning: access to a by f may be out of bounds
f (sizeof a, a);
...
}
-Wvla-parameter is included in -Wall. The -Warray-parameter option triggers warnings for similar problems
involving ordinary array arguments.
-Wvolatile-register-var
Warn if a register variable is declared volatile. The volatile modifier does not inhibit all optimizations that
may eliminate reads and/or writes to register variables. This warning is enabled by -Wall.
-Wno-xor-used-as-pow (C, C++, Objective-C and Objective-C++ only)
Disable warnings about uses of "^", the exclusive or operator, where it appears the code meant exponentiation.
Specifically, the warning occurs when the left-hand side is the decimal constant 2 or 10 and the right-hand side
is also a decimal constant.
In C and C++, "^" means exclusive or, whereas in some other languages (e.g. TeX and some versions of BASIC) it
means exponentiation.
This warning can be silenced by converting one of the operands to hexadecimal as well as by compiling with
-Wno-xor-used-as-pow.
-Wdisabled-optimization
Warn if a requested optimization pass is disabled. This warning does not generally indicate that there is
anything wrong with your code; it merely indicates that GCC's optimizers are unable to handle the code
effectively. Often, the problem is that your code is too big or too complex; GCC refuses to optimize programs
when the optimization itself is likely to take inordinate amounts of time.
-Wpointer-sign (C and Objective-C only)
Warn for pointer argument passing or assignment with different signedness. This option is only supported for C
and Objective-C. It is implied by -Wall and by -Wpedantic, which can be disabled with -Wno-pointer-sign.
This warning is upgraded to an error by -pedantic-errors.
-Wstack-protector
This option is only active when -fstack-protector is active. It warns about functions that are not protected
against stack smashing.
-Woverlength-strings
Warn about string constants that are longer than the "minimum maximum" length specified in the C standard.
Modern compilers generally allow string constants that are much longer than the standard's minimum limit, but
very portable programs should avoid using longer strings.
The limit applies after string constant concatenation, and does not count the trailing NUL. In C90, the limit
was 509 characters; in C99, it was raised to 4095. C++98 does not specify a normative minimum maximum, so we do
not diagnose overlength strings in C++.
This option is implied by -Wpedantic, and can be disabled with -Wno-overlength-strings.
-Wunsuffixed-float-constants (C and Objective-C only)
Issue a warning for any floating constant that does not have a suffix. When used together with -Wsystem-headers
it warns about such constants in system header files. This can be useful when preparing code to use with the
"FLOAT_CONST_DECIMAL64" pragma from the decimal floating-point extension to C99.
-Wno-lto-type-mismatch
During the link-time optimization, do not warn about type mismatches in global declarations from different
compilation units. Requires -flto to be enabled. Enabled by default.
-Wno-designated-init (C and Objective-C only)
Suppress warnings when a positional initializer is used to initialize a structure that has been marked with the
"designated_init" attribute.
Options That Control Static Analysis
-fanalyzer
This option enables an static analysis of program flow which looks for "interesting" interprocedural paths
through the code, and issues warnings for problems found on them.
This analysis is much more expensive than other GCC warnings.
In technical terms, it performs coverage-guided symbolic execution of the code being compiled. It is neither
sound nor complete: it can have false positives and false negatives. It is a bug-finding tool, rather than a
tool for proving program correctness.
The analyzer is only suitable for use on C code in this release.
Enabling this option effectively enables the following warnings:
-Wanalyzer-allocation-size -Wanalyzer-deref-before-check -Wanalyzer-double-fclose -Wanalyzer-double-free
-Wanalyzer-exposure-through-output-file -Wanalyzer-exposure-through-uninit-copy
-Wanalyzer-fd-access-mode-mismatch -Wanalyzer-fd-double-close -Wanalyzer-fd-leak -Wanalyzer-fd-phase-mismatch
-Wanalyzer-fd-type-mismatch -Wanalyzer-fd-use-after-close -Wanalyzer-fd-use-without-check -Wanalyzer-file-leak
-Wanalyzer-free-of-non-heap -Wanalyzer-imprecise-fp-arithmetic -Wanalyzer-infinite-loop
-Wanalyzer-infinite-recursion -Wanalyzer-jump-through-null -Wanalyzer-malloc-leak
-Wanalyzer-mismatching-deallocation -Wanalyzer-null-argument -Wanalyzer-null-dereference -Wanalyzer-out-of-bounds
-Wanalyzer-overlapping-buffers -Wanalyzer-possible-null-argument -Wanalyzer-possible-null-dereference
-Wanalyzer-putenv-of-auto-var -Wanalyzer-shift-count-negative -Wanalyzer-shift-count-overflow
-Wanalyzer-stale-setjmp-buffer -Wanalyzer-tainted-allocation-size -Wanalyzer-tainted-array-index
-Wanalyzer-tainted-assertion -Wanalyzer-tainted-divisor -Wanalyzer-tainted-offset -Wanalyzer-tainted-size
-Wanalyzer-undefined-behavior-ptrdiff -Wanalyzer-undefined-behavior-strtok
-Wanalyzer-unsafe-call-within-signal-handler -Wanalyzer-use-after-free
-Wanalyzer-use-of-pointer-in-stale-stack-frame -Wanalyzer-use-of-uninitialized-value
-Wanalyzer-va-arg-type-mismatch -Wanalyzer-va-list-exhausted -Wanalyzer-va-list-leak
-Wanalyzer-va-list-use-after-va-end -Wanalyzer-write-to-const -Wanalyzer-write-to-string-literal
This option is only available if GCC was configured with analyzer support enabled.
-Wanalyzer-symbol-too-complex
If -fanalyzer is enabled, the analyzer uses various heuristics to attempt to track the state of memory, but these
can be defeated by sufficiently complicated code.
By default, the analysis silently stops tracking values of expressions if they exceed the threshold defined by
--param analyzer-max-svalue-depth=value, and falls back to an imprecise representation for such expressions. The
-Wanalyzer-symbol-too-complex option warns if this occurs.
-Wanalyzer-too-complex
If -fanalyzer is enabled, the analyzer uses various heuristics to attempt to explore the control flow and data
flow in the program, but these can be defeated by sufficiently complicated code.
By default, the analysis silently stops if the code is too complicated for the analyzer to fully explore and it
reaches an internal limit. The -Wanalyzer-too-complex option warns if this occurs.
-Wno-analyzer-allocation-size
This warning requires -fanalyzer, which enables it; to disable it, use -Wno-analyzer-allocation-size.
This diagnostic warns for paths through the code in which a pointer to a buffer is assigned to point at a buffer
with a size that is not a multiple of "sizeof (*pointer)".
See CWE-131: Incorrect Calculation of Buffer Size ("https://cwe.mitre.org/data/definitions/131.html").
-Wno-analyzer-deref-before-check
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-deref-before-check to disable it.
This diagnostic warns for paths through the code in which a pointer is checked for "NULL" *after* it has already
been dereferenced, suggesting that the pointer could have been NULL. Such cases suggest that the check for NULL
is either redundant, or that it needs to be moved to before the pointer is dereferenced.
This diagnostic also considers values passed to a function argument marked with "__attribute__((nonnull))" as
requiring a non-NULL value, and thus will complain if such values are checked for "NULL" after returning from
such a function call.
This diagnostic is unlikely to be reported when any level of optimization is enabled, as GCC's optimization logic
will typically consider such checks for NULL as being redundant, and optimize them away before the analyzer
"sees" them. Hence optimization should be disabled when attempting to trigger this diagnostic.
-Wno-analyzer-double-fclose
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-double-fclose to disable it.
This diagnostic warns for paths through the code in which a "FILE *" can have "fclose" called on it more than
once.
See CWE-1341: Multiple Releases of Same Resource or Handle ("https://cwe.mitre.org/data/definitions/1341.html").
-Wno-analyzer-double-free
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-double-free to disable it.
This diagnostic warns for paths through the code in which a pointer can have a deallocator called on it more than
once, either "free", or a deallocator referenced by attribute "malloc".
See CWE-415: Double Free ("https://cwe.mitre.org/data/definitions/415.html").
-Wno-analyzer-exposure-through-output-file
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-exposure-through-output-file to disable it.
This diagnostic warns for paths through the code in which a security-sensitive value is written to an output file
(such as writing a password to a log file).
See CWE-532: Information Exposure Through Log Files ("https://cwe.mitre.org/data/definitions/532.html").
-Wanalyzer-exposure-through-uninit-copy
This warning requires both -fanalyzer and the use of a plugin to specify a function that copies across a "trust
boundary". Use -Wno-analyzer-exposure-through-uninit-copy to disable it.
This diagnostic warns for "infoleaks" - paths through the code in which uninitialized values are copied across a
security boundary (such as code within an OS kernel that copies a partially-initialized struct on the stack to
user space).
See CWE-200: Exposure of Sensitive Information to an Unauthorized Actor
("https://cwe.mitre.org/data/definitions/200.html").
-Wno-analyzer-fd-access-mode-mismatch
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-fd-access-mode-mismatch to disable it.
This diagnostic warns for paths through code in which a "read" on a write-only file descriptor is attempted, or
vice versa.
This diagnostic also warns for code paths in a which a function with attribute "fd_arg_read (N)" is called with a
file descriptor opened with "O_WRONLY" at referenced argument "N" or a function with attribute "fd_arg_write (N)"
is called with a file descriptor opened with "O_RDONLY" at referenced argument N.
-Wno-analyzer-fd-double-close
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-fd-double-close to disable it.
This diagnostic warns for paths through code in which a file descriptor can be closed more than once.
See CWE-1341: Multiple Releases of Same Resource or Handle ("https://cwe.mitre.org/data/definitions/1341.html").
-Wno-analyzer-fd-leak
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-fd-leak to disable it.
This diagnostic warns for paths through code in which an open file descriptor is leaked.
See CWE-775: Missing Release of File Descriptor or Handle after Effective Lifetime
("https://cwe.mitre.org/data/definitions/775.html").
-Wno-analyzer-fd-phase-mismatch
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-fd-phase-mismatch to disable it.
This diagnostic warns for paths through code in which an operation is attempted in the wrong phase of a file
descriptor's lifetime. For example, it will warn on attempts to call "accept" on a stream socket that has not
yet had "listen" successfully called on it.
See CWE-666: Operation on Resource in Wrong Phase of Lifetime
("https://cwe.mitre.org/data/definitions/666.html").
-Wno-analyzer-fd-type-mismatch
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-fd-type-mismatch to disable it.
This diagnostic warns for paths through code in which an operation is attempted on the wrong type of file
descriptor. For example, it will warn on attempts to use socket operations on a file descriptor obtained via
"open", or when attempting to use a stream socket operation on a datagram socket.
-Wno-analyzer-fd-use-after-close
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-fd-use-after-close to disable it.
This diagnostic warns for paths through code in which a read or write is called on a closed file descriptor.
This diagnostic also warns for paths through code in which a function with attribute "fd_arg (N)" or "fd_arg_read
(N)" or "fd_arg_write (N)" is called with a closed file descriptor at referenced argument "N".
-Wno-analyzer-fd-use-without-check
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-fd-use-without-check to disable it.
This diagnostic warns for paths through code in which a file descriptor is used without being checked for
validity.
This diagnostic also warns for paths through code in which a function with attribute "fd_arg (N)" or "fd_arg_read
(N)" or "fd_arg_write (N)" is called with a file descriptor, at referenced argument "N", without being checked
for validity.
-Wno-analyzer-file-leak
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-file-leak to disable it.
This diagnostic warns for paths through the code in which a "" "FILE *" stream object is leaked.
See CWE-775: Missing Release of File Descriptor or Handle after Effective Lifetime
("https://cwe.mitre.org/data/definitions/775.html").
-Wno-analyzer-free-of-non-heap
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-free-of-non-heap to disable it.
This diagnostic warns for paths through the code in which "free" is called on a non-heap pointer (e.g. an on-
stack buffer, or a global).
See CWE-590: Free of Memory not on the Heap ("https://cwe.mitre.org/data/definitions/590.html").
-Wno-analyzer-imprecise-fp-arithmetic
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-imprecise-fp-arithmetic to disable it.
This diagnostic warns for paths through the code in which floating-point arithmetic is used in locations where
precise computation is needed. This diagnostic only warns on use of floating-point operands inside the
calculation of an allocation size at the moment.
-Wno-analyzer-infinite-loop
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-infinite-loop to disable it.
This diagnostics warns for paths through the code which appear to lead to an infinite loop.
Specifically, the analyzer will issue this warning when it "sees" a loop in which:
* no externally-visible work could be being done within the loop
* there is no way to escape from the loop
* the analyzer is sufficiently confident about the program state throughout the loop to know that the above are
true
One way for this warning to be emitted is when there is an execution path through a loop for which taking the
path on one iteration implies that the same path will be taken on all subsequent iterations.
For example, consider:
while (1)
{
char opcode = *cpu_state.pc;
switch (opcode)
{
case OPCODE_FOO:
handle_opcode_foo (&cpu_state);
break;
case OPCODE_BAR:
handle_opcode_bar (&cpu_state);
break;
}
}
The analyzer will complain for the above case because if "opcode" ever matches none of the cases, the "switch"
will follow the implicit "default" case, making the body of the loop be a "no-op" with "cpu_state.pc" unchanged,
and thus using the same value of "opcode" on all subseqent iterations, leading to an infinite loop.
See CWE-835: Loop with Unreachable Exit Condition ('Infinite Loop')
("https://cwe.mitre.org/data/definitions/835.html").
-Wno-analyzer-infinite-recursion
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-infinite-recursion to disable it.
This diagnostics warns for paths through the code which appear to lead to infinite recursion.
Specifically, when the analyzer "sees" a recursive call, it will compare the state of memory at the entry to the
new frame with that at the entry to the previous frame of that function on the stack. The warning is issued if
nothing in memory appears to be changing; any changes observed to parameters or globals are assumed to lead to
termination of the recursion and thus suppress the warning.
This diagnostic is likely to miss cases of infinite recursion that are convered to iteration by the optimizer
before the analyzer "sees" them. Hence optimization should be disabled when attempting to trigger this
diagnostic.
Compare with -Winfinite-recursion, which provides a similar diagnostic, but is implemented in a different way.
See CWE-674: Uncontrolled Recursion ("https://cwe.mitre.org/data/definitions/674.html").
-Wno-analyzer-jump-through-null
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-jump-through-null to disable it.
This diagnostic warns for paths through the code in which a "NULL" function pointer is called.
-Wno-analyzer-malloc-leak
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-malloc-leak to disable it.
This diagnostic warns for paths through the code in which a pointer allocated via an allocator is leaked: either
"malloc", or a function marked with attribute "malloc".
See CWE-401: Missing Release of Memory after Effective Lifetime
("https://cwe.mitre.org/data/definitions/401.html").
-Wno-analyzer-mismatching-deallocation
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-mismatching-deallocation to disable it.
This diagnostic warns for paths through the code in which the wrong deallocation function is called on a pointer
value, based on which function was used to allocate the pointer value. The diagnostic will warn about mismatches
between "free", scalar "delete" and vector "delete[]", and those marked as allocator/deallocator pairs using
attribute "malloc".
See CWE-762: Mismatched Memory Management Routines ("https://cwe.mitre.org/data/definitions/762.html").
-Wno-analyzer-out-of-bounds
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-out-of-bounds to disable it.
This diagnostic warns for paths through the code in which a buffer is definitely read or written out-of-bounds.
The diagnostic applies for cases where the analyzer is able to determine a constant offset and for accesses past
the end of a buffer, also a constant capacity. Further, the diagnostic does limited checking for accesses past
the end when the offset as well as the capacity is symbolic.
See CWE-119: Improper Restriction of Operations within the Bounds of a Memory Buffer
("https://cwe.mitre.org/data/definitions/119.html").
For cases where the analyzer is able, it will emit a text art diagram visualizing the spatial relationship
between the memory region that the analyzer predicts would be accessed, versus the range of memory that is valid
to access: whether they overlap, are touching, are close or far apart; which one is before or after in memory,
the relative sizes involved, the direction of the access (read vs write), and, in some cases, the values of data
involved. This diagram can be suppressed using -fdiagnostics-text-art-charset=none.
-Wno-analyzer-overlapping-buffers
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-overlapping-buffers to disable it.
This diagnostic warns for paths through the code in which overlapping buffers are passed to an API for which the
behavior on such buffers is undefined.
Specifically, the diagnostic occurs on calls to the following functions
*<"memcpy">
*<"strcat">
*<"strcpy">
for cases where the buffers are known to overlap.
-Wno-analyzer-possible-null-argument
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-possible-null-argument to disable it.
This diagnostic warns for paths through the code in which a possibly-NULL value is passed to a function argument
marked with "__attribute__((nonnull))" as requiring a non-NULL value.
See CWE-690: Unchecked Return Value to NULL Pointer Dereference
("https://cwe.mitre.org/data/definitions/690.html").
-Wno-analyzer-possible-null-dereference
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-possible-null-dereference to disable it.
This diagnostic warns for paths through the code in which a possibly-NULL value is dereferenced.
See CWE-690: Unchecked Return Value to NULL Pointer Dereference
("https://cwe.mitre.org/data/definitions/690.html").
-Wno-analyzer-null-argument
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-null-argument to disable it.
This diagnostic warns for paths through the code in which a value known to be NULL is passed to a function
argument marked with "__attribute__((nonnull))" as requiring a non-NULL value.
See CWE-476: NULL Pointer Dereference ("https://cwe.mitre.org/data/definitions/476.html").
-Wno-analyzer-null-dereference
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-null-dereference to disable it.
This diagnostic warns for paths through the code in which a value known to be NULL is dereferenced.
See CWE-476: NULL Pointer Dereference ("https://cwe.mitre.org/data/definitions/476.html").
-Wno-analyzer-putenv-of-auto-var
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-putenv-of-auto-var to disable it.
This diagnostic warns for paths through the code in which a call to "putenv" is passed a pointer to an automatic
variable or an on-stack buffer.
See POS34-C. Do not call putenv() with a pointer to an automatic variable as the argument
("https://wiki.sei.cmu.edu/confluence/x/6NYxBQ").
-Wno-analyzer-shift-count-negative
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-shift-count-negative to disable it.
This diagnostic warns for paths through the code in which a shift is attempted with a negative count. It is
analogous to the -Wshift-count-negative diagnostic implemented in the C/C++ front ends, but is implemented based
on analyzing interprocedural paths, rather than merely parsing the syntax tree. However, the analyzer does not
prioritize detection of such paths, so false negatives are more likely relative to other warnings.
-Wno-analyzer-shift-count-overflow
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-shift-count-overflow to disable it.
This diagnostic warns for paths through the code in which a shift is attempted with a count greater than or equal
to the precision of the operand's type. It is analogous to the -Wshift-count-overflow diagnostic implemented in
the C/C++ front ends, but is implemented based on analyzing interprocedural paths, rather than merely parsing the
syntax tree. However, the analyzer does not prioritize detection of such paths, so false negatives are more
likely relative to other warnings.
-Wno-analyzer-stale-setjmp-buffer
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-stale-setjmp-buffer to disable it.
This diagnostic warns for paths through the code in which "longjmp" is called to rewind to a "jmp_buf" relating
to a "setjmp" call in a function that has returned.
When "setjmp" is called on a "jmp_buf" to record a rewind location, it records the stack frame. The stack frame
becomes invalid when the function containing the "setjmp" call returns. Attempting to rewind to it via "longjmp"
would reference a stack frame that no longer exists, and likely lead to a crash (or worse).
-Wno-analyzer-tainted-allocation-size
This warning requires -fanalyzer which enables it; use -Wno-analyzer-tainted-allocation-size to disable it.
This diagnostic warns for paths through the code in which a value that could be under an attacker's control is
used as the size of an allocation without being sanitized, so that an attacker could inject an excessively large
allocation and potentially cause a denial of service attack.
See CWE-789: Memory Allocation with Excessive Size Value ("https://cwe.mitre.org/data/definitions/789.html").
-Wno-analyzer-tainted-assertion
This warning requires -fanalyzer which enables it; use -Wno-analyzer-tainted-assertion to disable it.
This diagnostic warns for paths through the code in which a value that could be under an attacker's control is
used as part of a condition without being first sanitized, and that condition guards a call to a function marked
with attribute "noreturn" (such as the function "__builtin_unreachable"). Such functions typically indicate
abnormal termination of the program, such as for assertion failure handlers. For example:
assert (some_tainted_value < SOME_LIMIT);
In such cases:
* when assertion-checking is enabled: an attacker could trigger a denial of service by injecting an assertion
failure
* when assertion-checking is disabled, such as by defining "NDEBUG", an attacker could inject data that
subverts the process, since it presumably violates a precondition that is being assumed by the code.
Note that when assertion-checking is disabled, the assertions are typically removed by the preprocessor before
the analyzer has a chance to "see" them, so this diagnostic can only generate warnings on builds in which
assertion-checking is enabled.
For the purpose of this warning, any function marked with attribute "noreturn" is considered as a possible
assertion failure handler, including "__builtin_unreachable". Note that these functions are sometimes removed by
the optimizer before the analyzer "sees" them. Hence optimization should be disabled when attempting to trigger
this diagnostic.
See CWE-617: Reachable Assertion ("https://cwe.mitre.org/data/definitions/617.html").
The warning can also report problematic constructions such as
switch (some_tainted_value) {
case 0:
/* [...etc; various valid cases omitted...] */
break;
default:
__builtin_unreachable (); /* BUG: attacker can trigger this */
}
despite the above not being an assertion failure, strictly speaking.
-Wno-analyzer-tainted-array-index
This warning requires -fanalyzer which enables it; use -Wno-analyzer-tainted-array-index to disable it.
This diagnostic warns for paths through the code in which a value that could be under an attacker's control is
used as the index of an array access without being sanitized, so that an attacker could inject an out-of-bounds
access.
See CWE-129: Improper Validation of Array Index ("https://cwe.mitre.org/data/definitions/129.html").
-Wno-analyzer-tainted-divisor
This warning requires -fanalyzer which enables it; use -Wno-analyzer-tainted-divisor to disable it.
This diagnostic warns for paths through the code in which a value that could be under an attacker's control is
used as the divisor in a division or modulus operation without being sanitized, so that an attacker could inject
a division-by-zero.
See CWE-369: Divide By Zero ("https://cwe.mitre.org/data/definitions/369.html").
-Wno-analyzer-tainted-offset
This warning requires -fanalyzer which enables it; use -Wno-analyzer-tainted-offset to disable it.
This diagnostic warns for paths through the code in which a value that could be under an attacker's control is
used as a pointer offset without being sanitized, so that an attacker could inject an out-of-bounds access.
See CWE-823: Use of Out-of-range Pointer Offset ("https://cwe.mitre.org/data/definitions/823.html").
-Wno-analyzer-tainted-size
This warning requires -fanalyzer which enables it; use -Wno-analyzer-tainted-size to disable it.
This diagnostic warns for paths through the code in which a value that could be under an attacker's control is
used as the size of an operation such as "memset" without being sanitized, so that an attacker could inject an
out-of-bounds access.
See CWE-129: Improper Validation of Array Index ("https://cwe.mitre.org/data/definitions/129.html").
-Wno-analyzer-undefined-behavior-ptrdiff
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-undefined-behavior-ptrdiff to disable it.
This diagnostic warns for paths through the code in which a pointer subtraction occurs where the pointers refer
to different chunks of memory. Such code relies on undefined behavior, as pointer subtraction is only defined
for cases where both pointers point to within (or just after) the same array.
See CWE-469: Use of Pointer Subtraction to Determine Size ("https://cwe.mitre.org/data/definitions/469.html").
-Wno-analyzer-undefined-behavior-strtok
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-undefined-behavior-strtok to disable it.
This diagnostic warns for paths through the code in which a call is made to "strtok" with undefined behavior.
Specifically, passing NULL as the first parameter for the initial call to "strtok" within a process has undefined
behavior.
-Wno-analyzer-unsafe-call-within-signal-handler
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-unsafe-call-within-signal-handler to
disable it.
This diagnostic warns for paths through the code in which a function known to be async-signal-unsafe (such as
"fprintf") is called from a signal handler.
See CWE-479: Signal Handler Use of a Non-reentrant Function ("https://cwe.mitre.org/data/definitions/479.html").
-Wno-analyzer-use-after-free
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-use-after-free to disable it.
This diagnostic warns for paths through the code in which a pointer is used after a deallocator is called on it:
either "free", or a deallocator referenced by attribute "malloc".
See CWE-416: Use After Free ("https://cwe.mitre.org/data/definitions/416.html").
-Wno-analyzer-use-of-pointer-in-stale-stack-frame
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-use-of-pointer-in-stale-stack-frame to
disable it.
This diagnostic warns for paths through the code in which a pointer is dereferenced that points to a variable in
a stale stack frame.
-Wno-analyzer-va-arg-type-mismatch
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-va-arg-type-mismatch to disable it.
This diagnostic warns for interprocedural paths through the code for which the analyzer detects an attempt to use
"va_arg" to extract a value passed to a variadic call, but uses a type that does not match that of the expression
passed to the call.
See CWE-686: Function Call With Incorrect Argument Type ("https://cwe.mitre.org/data/definitions/686.html").
-Wno-analyzer-va-list-exhausted
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-va-list-exhausted to disable it.
This diagnostic warns for interprocedural paths through the code for which the analyzer detects an attempt to use
"va_arg" to access the next value passed to a variadic call, but all of the values in the "va_list" have already
been consumed.
See CWE-685: Function Call With Incorrect Number of Arguments
("https://cwe.mitre.org/data/definitions/685.html").
-Wno-analyzer-va-list-leak
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-va-list-leak to disable it.
This diagnostic warns for interprocedural paths through the code for which the analyzer detects that "va_start"
or "va_copy" has been called on a "va_list" without a corresponding call to "va_end".
-Wno-analyzer-va-list-use-after-va-end
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-va-list-use-after-va-end to disable it.
This diagnostic warns for interprocedural paths through the code for which the analyzer detects an attempt to use
a "va_list" after "va_end" has been called on it. "va_list".
-Wno-analyzer-write-to-const
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-write-to-const to disable it.
This diagnostic warns for paths through the code in which the analyzer detects an attempt to write through a
pointer to a "const" object. However, the analyzer does not prioritize detection of such paths, so false
negatives are more likely relative to other warnings.
-Wno-analyzer-write-to-string-literal
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-write-to-string-literal to disable it.
This diagnostic warns for paths through the code in which the analyzer detects an attempt to write through a
pointer to a string literal. However, the analyzer does not prioritize detection of such paths, so false
negatives are more likely relative to other warnings.
-Wno-analyzer-use-of-uninitialized-value
This warning requires -fanalyzer, which enables it; use -Wno-analyzer-use-of-uninitialized-value to disable it.
This diagnostic warns for paths through the code in which an uninitialized value is used.
See CWE-457: Use of Uninitialized Variable ("https://cwe.mitre.org/data/definitions/457.html").
The analyzer has hardcoded knowledge about the behavior of the following memory-management functions:
*<"alloca">
*
"__builtin_alloc_with_align", @item "__builtin_calloc", "__builtin_free", "__builtin_malloc", "__builtin_memcpy",
"__builtin_memcpy_chk", "__builtin_memset", "__builtin_memset_chk", "__builtin_realloc",
"__builtin_stack_restore", and "__builtin_stack_save"
*<"calloc">
*<"free">
*<"malloc">
*<"memset">
*<"operator delete">
*<"operator delete []">
*<"operator new">
*<"operator new []">
*<"realloc">
*<"strdup">
*<"strndup">
of the following functions for working with file descriptors:
*<"open">
*<"close">
*<"creat">
*<"dup", "dup2" and "dup3">
*<"isatty">
*<"pipe", and "pipe2">
*<"read">
*<"write">
*<"socket", "bind", "listen", "accept", and "connect">
of the following functions for working with "" streams:
*
"__builtin_fprintf_unlocked", "__builtin_fputc", "__builtin_fputc_unlocked", "__builtin_fputs",
"__builtin_fputs_unlocked", "__builtin_fwrite", "__builtin_fwrite_unlocked", "__builtin_printf",
"__builtin_printf_unlocked", "__builtin_putc", "__builtin_putchar", "__builtin_putchar_unlocked",
"__builtin_putc_unlocked", "__builtin_puts", "__builtin_puts_unlocked", "__builtin_vfprintf", and
"__builtin_vprintf"
*<"fopen">
*<"fclose">
*<"ferror">
*<"fgets">
*<"fgets_unlocked">
*<"fileno">
*<"fread">
*<"getc">
*<"getchar">
*<"fprintf">
*<"printf">
*<"fwrite">
and of the following functions:
*
"__builtin_expect_with_probability", "__builtin_strchr", "__builtin_strcpy", "__builtin_strcpy_chk",
"__builtin_strlen", "__builtin_va_copy", and "__builtin_va_start"
*
*<"getpass">
*<"longjmp">
*<"putenv">
*<"setjmp">
*<"siglongjmp">
*<"signal">
*<"sigsetjmp">
*<"strcat">
*<"strchr">
*<"strlen">
In addition, various functions with an "__analyzer_" prefix have special meaning to the analyzer, described in the
GCC Internals manual.
Pertinent parameters for controlling the exploration are:
*<--param analyzer-bb-explosion-factor=value>
*<--param analyzer-max-enodes-per-program-point=value>
*<--param analyzer-max-recursion-depth=value>
*<--param analyzer-min-snodes-for-call-summary=value>
The following options control the analyzer.
-fanalyzer-call-summaries
Simplify interprocedural analysis by computing the effect of certain calls, rather than exploring all paths
through the function from callsite to each possible return.
If enabled, call summaries are only used for functions with more than one call site, and that are sufficiently
complicated (as per --param analyzer-min-snodes-for-call-summary=value).
-fanalyzer-checker=name
Restrict the analyzer to run just the named checker, and enable it.
-fanalyzer-debug-text-art-headings
This option is intended for analyzer developers. If enabled, the analyzer will add extra annotations to any
diagrams it generates.
-fno-analyzer-feasibility
This option is intended for analyzer developers.
By default the analyzer verifies that there is a feasible control flow path for each diagnostic it emits: that
the conditions that hold are not mutually exclusive. Diagnostics for which no feasible path can be found are
rejected. This filtering can be suppressed with -fno-analyzer-feasibility, for debugging issues in this code.
-fanalyzer-fine-grained
This option is intended for analyzer developers.
Internally the analyzer builds an "exploded graph" that combines control flow graphs with data flow information.
By default, an edge in this graph can contain the effects of a run of multiple statements within a basic block.
With -fanalyzer-fine-grained, each statement gets its own edge.
-fanalyzer-show-duplicate-count
This option is intended for analyzer developers: if multiple diagnostics have been detected as being duplicates
of each other, it emits a note when reporting the best diagnostic, giving the number of additional diagnostics
that were suppressed by the deduplication logic.
-fanalyzer-show-events-in-system-headers
By default the analyzer emits simplified diagnostics paths by hiding events fully located within a system header.
With -fanalyzer-show-events-in-system-headers such events are no longer suppressed.
-fno-analyzer-state-merge
This option is intended for analyzer developers.
By default the analyzer attempts to simplify analysis by merging sufficiently similar states at each program
point as it builds its "exploded graph". With -fno-analyzer-state-merge this merging can be suppressed, for
debugging state-handling issues.
-fno-analyzer-state-purge
This option is intended for analyzer developers.
By default the analyzer attempts to simplify analysis by purging aspects of state at a program point that appear
to no longer be relevant e.g. the values of locals that aren't accessed later in the function and which aren't
relevant to leak analysis.
With -fno-analyzer-state-purge this purging of state can be suppressed, for debugging state-handling issues.
-fno-analyzer-suppress-followups
This option is intended for analyzer developers.
By default the analyzer will stop exploring an execution path after encountering certain diagnostics, in order to
avoid potentially issuing a cascade of follow-up diagnostics.
The diagnostics that terminate analysis along a path are:
*<-Wanalyzer-null-argument>
*<-Wanalyzer-null-dereference>
*<-Wanalyzer-use-after-free>
*<-Wanalyzer-use-of-pointer-in-stale-stack-frame>
*<-Wanalyzer-use-of-uninitialized-value>
With -fno-analyzer-suppress-followups the analyzer will continue to explore such paths even after such
diagnostics, which may be helpful for debugging issues in the analyzer, or for microbenchmarks for detecting
undefined behavior.
-fanalyzer-transitivity
This option enables transitivity of constraints within the analyzer.
-fno-analyzer-undo-inlining
This option is intended for analyzer developers.
-fanalyzer runs relatively late compared to other code analysis tools, and some optimizations have already been
applied to the code. In particular function inlining may have occurred, leading to the interprocedural execution
paths emitted by the analyzer containing function frames that don't correspond to those in the original source
code.
By default the analyzer attempts to reconstruct the original function frames, and to emit events showing the
inlined calls.
With -fno-analyzer-undo-inlining this attempt to reconstruct the original frame information can be disabled,
which may be of help when debugging issues in the analyzer.
-fanalyzer-verbose-edges
This option is intended for analyzer developers. It enables more verbose, lower-level detail in the descriptions
of control flow within diagnostic paths.
-fanalyzer-verbose-state-changes
This option is intended for analyzer developers. It enables more verbose, lower-level detail in the descriptions
of events relating to state machines within diagnostic paths.
-fanalyzer-verbosity=level
This option controls the complexity of the control flow paths that are emitted for analyzer diagnostics.
The level can be one of:
0 At this level, interprocedural call and return events are displayed, along with the most pertinent state-
change events relating to a diagnostic. For example, for a double-"free" diagnostic, both calls to "free"
will be shown.
1 As per the previous level, but also show events for the entry to each function.
2 As per the previous level, but also show events relating to control flow that are significant to triggering
the issue (e.g. "true path taken" at a conditional).
This level is the default.
3 As per the previous level, but show all control flow events, not just significant ones.
4 This level is intended for analyzer developers; it adds various other events intended for debugging the
analyzer.
-fdump-analyzer
Dump internal details about what the analyzer is doing to file.analyzer.txt. -fdump-analyzer-stderr overrides
this option.
-fdump-analyzer-stderr
Dump internal details about what the analyzer is doing to stderr. This option overrides -fdump-analyzer.
-fdump-analyzer-callgraph
Dump a representation of the call graph suitable for viewing with GraphViz to file.callgraph.dot.
-fdump-analyzer-exploded-graph
Dump a representation of the "exploded graph" suitable for viewing with GraphViz to file.eg.dot. Nodes are
color-coded based on state-machine states to emphasize state changes.
-fdump-analyzer-exploded-nodes
Emit diagnostics showing where nodes in the "exploded graph" are in relation to the program source.
-fdump-analyzer-exploded-nodes-2
Dump a textual representation of the "exploded graph" to file.eg.txt.
-fdump-analyzer-exploded-nodes-3
Dump a textual representation of the "exploded graph" to one dump file per node, to file.eg-id.txt. This is
typically a large number of dump files.
-fdump-analyzer-exploded-paths
Dump a textual representation of the "exploded path" for each diagnostic to file.idx.kind.epath.txt.
-fdump-analyzer-feasibility
Dump internal details about the analyzer's search for feasible paths. The details are written in a form suitable
for viewing with GraphViz to filenames of the form file.*.fg.dot, file.*.tg.dot, and file.*.fpath.txt.
-fdump-analyzer-infinite-loop
Dump internal details about the analyzer's search for infinite loops. The details are written in a form suitable
for viewing with GraphViz to filenames of the form file.*.infinite-loop.dot.
-fdump-analyzer-json
Dump a compressed JSON representation of analyzer internals to file.analyzer.json.gz. The precise format is
subject to change.
-fdump-analyzer-state-purge
As per -fdump-analyzer-supergraph, dump a representation of the "supergraph" suitable for viewing with GraphViz,
but annotate the graph with information on what state will be purged at each node. The graph is written to
file.state-purge.dot.
-fdump-analyzer-supergraph
Dump representations of the "supergraph" suitable for viewing with GraphViz to file.supergraph.dot and to
file.supergraph-eg.dot. These show all of the control flow graphs in the program, with interprocedural edges for
calls and returns. The second dump contains annotations showing nodes in the "exploded graph" and diagnostics
associated with them.
-fdump-analyzer-untracked
Emit custom warnings with internal details intended for analyzer developers.
Options for Debugging Your Program
To tell GCC to emit extra information for use by a debugger, in almost all cases you need only to add -g to your
other options. Some debug formats can co-exist (like DWARF with CTF) when each of them is enabled explicitly by
adding the respective command line option to your other options.
GCC allows you to use -g with -O. The shortcuts taken by optimized code may occasionally be surprising: some
variables you declared may not exist at all; flow of control may briefly move where you did not expect it; some
statements may not be executed because they compute constant results or their values are already at hand; some
statements may execute in different places because they have been moved out of loops. Nevertheless it is possible to
debug optimized output. This makes it reasonable to use the optimizer for programs that might have bugs.
If you are not using some other optimization option, consider using -Og with -g. With no -O option at all, some
compiler passes that collect information useful for debugging do not run at all, so that -Og may result in a better
debugging experience.
-g Produce debugging information in the operating system's native format (stabs, COFF, XCOFF, or DWARF). GDB can
work with this debugging information.
On most systems that use stabs format, -g enables use of extra debugging information that only GDB can use; this
extra information makes debugging work better in GDB but probably makes other debuggers crash or refuse to read
the program. If you want to control for certain whether to generate the extra information, use -gvms (see
below).
-ggdb
Produce debugging information for use by GDB. This means to use the most expressive format available (DWARF,
stabs, or the native format if neither of those are supported), including GDB extensions if at all possible.
-gdwarf
-gdwarf-version
Produce debugging information in DWARF format (if that is supported). The value of version may be either 2, 3, 4
or 5; the default version for most targets is 5 (with the exception of VxWorks, TPF and Darwin / macOS, which
default to version 2, and AIX, which defaults to version 4).
Note that with DWARF Version 2, some ports require and always use some non-conflicting DWARF 3 extensions in the
unwind tables.
Version 4 may require GDB 7.0 and -fvar-tracking-assignments for maximum benefit. Version 5 requires GDB 8.0 or
higher.
GCC no longer supports DWARF Version 1, which is substantially different than Version 2 and later. For
historical reasons, some other DWARF-related options such as -fno-dwarf2-cfi-asm) retain a reference to DWARF
Version 2 in their names, but apply to all currently-supported versions of DWARF.
-gbtf
Request BTF debug information. BTF is the default debugging format for the eBPF target. On other targets, like
x86, BTF debug information can be generated along with DWARF debug information when both of the debug formats are
enabled explicitly via their respective command line options.
-gprune-btf
-gno-prune-btf
Prune BTF information before emission. When pruning, only type information for types used by global variables
and file-scope functions will be emitted. If compiling for the BPF target with BPF CO-RE enabled, type
information will also be emitted for types used in BPF CO-RE relocations. In addition, struct and union types
which are only referred to via pointers from members of other struct or union types shall be pruned and replaced
with BTF_KIND_FWD, as though those types were only present in the input as forward declarations.
This option substantially reduces the size of produced BTF information, but at significant loss in the amount of
detailed type information. It is primarily useful when compiling for the BPF target, to minimize the size of the
resulting object, and to eliminate BTF information which is not immediately relevant to the BPF program loading
process.
This option is enabled by default for the BPF target when generating BTF information.
-gctf
-gctflevel
Request CTF debug information and use level to specify how much CTF debug information should be produced. If
-gctf is specified without a value for level, the default level of CTF debug information is 2.
CTF debug information can be generated along with DWARF debug information when both of the debug formats are
enabled explicitly via their respective command line options.
Level 0 produces no CTF debug information at all. Thus, -gctf0 negates -gctf.
Level 1 produces CTF information for tracebacks only. This includes callsite information, but does not include
type information.
Level 2 produces type information for entities (functions, data objects etc.) at file-scope or global-scope
only.
-gvms
Produce debugging information in Alpha/VMS debug format (if that is supported). This is the format used by DEBUG
on Alpha/VMS systems.
-gcodeview
Produce debugging information in CodeView debug format (if that is supported). This is the format used by
Microsoft Visual C++ on Windows.
-glevel
-ggdblevel
-gvmslevel
Request debugging information and also use level to specify how much information. The default level is 2.
Level 0 produces no debug information at all. Thus, -g0 negates -g.
Level 1 produces minimal information, enough for making backtraces in parts of the program that you don't plan to
debug. This includes descriptions of functions and external variables, and line number tables, but no
information about local variables.
Level 3 includes extra information, such as all the macro definitions present in the program. Some debuggers
support macro expansion when you use -g3.
If you use multiple -g options, with or without level numbers, the last such option is the one that is effective.
-gdwarf does not accept a concatenated debug level, to avoid confusion with -gdwarf-level. Instead use an
additional -glevel option to change the debug level for DWARF.
-fno-eliminate-unused-debug-symbols
By default, no debug information is produced for symbols that are not actually used. Use this option if you want
debug information for all symbols.
-femit-class-debug-always
Instead of emitting debugging information for a C++ class in only one object file, emit it in all object files
using the class. This option should be used only with debuggers that are unable to handle the way GCC normally
emits debugging information for classes because using this option increases the size of debugging information by
as much as a factor of two.
-fno-merge-debug-strings
Direct the linker to not merge together strings in the debugging information that are identical in different
object files. Merging is not supported by all assemblers or linkers. Merging decreases the size of the debug
information in the output file at the cost of increasing link processing time. Merging is enabled by default.
-fdebug-prefix-map=old=new
When compiling files residing in directory old, record debugging information describing them as if the files
resided in directory new instead. This can be used to replace a build-time path with an install-time path in the
debug info. It can also be used to change an absolute path to a relative path by using . for new. This can give
more reproducible builds, which are location independent, but may require an extra command to tell GDB where to
find the source files. See also -ffile-prefix-map and -fcanon-prefix-map.
-fvar-tracking
Run variable tracking pass. It computes where variables are stored at each position in code. Better debugging
information is then generated (if the debugging information format supports this information).
It is enabled by default when compiling with optimization (-Os, -O, -O2, ...), debugging information (-g) and the
debug info format supports it.
-fvar-tracking-assignments
Annotate assignments to user variables early in the compilation and attempt to carry the annotations over
throughout the compilation all the way to the end, in an attempt to improve debug information while optimizing.
Use of -gdwarf-4 is recommended along with it.
It can be enabled even if var-tracking is disabled, in which case annotations are created and maintained, but
discarded at the end. By default, this flag is enabled together with -fvar-tracking, except when selective
scheduling is enabled.
-gsplit-dwarf
If DWARF debugging information is enabled, separate as much debugging information as possible into a separate
output file with the extension .dwo. This option allows the build system to avoid linking files with debug
information. To be useful, this option requires a debugger capable of reading .dwo files.
-gdwarf32
-gdwarf64
If DWARF debugging information is enabled, the -gdwarf32 selects the 32-bit DWARF format and the -gdwarf64
selects the 64-bit DWARF format. The default is target specific, on most targets it is -gdwarf32 though. The
32-bit DWARF format is smaller, but can't support more than 2GiB of debug information in any of the DWARF debug
information sections. The 64-bit DWARF format allows larger debug information and might not be well supported by
all consumers yet.
-gdescribe-dies
Add description attributes to some DWARF DIEs that have no name attribute, such as artificial variables, external
references and call site parameter DIEs.
-gpubnames
Generate DWARF ".debug_pubnames" and ".debug_pubtypes" sections.
-ggnu-pubnames
Generate ".debug_pubnames" and ".debug_pubtypes" sections in a format suitable for conversion into a GDB index.
This option is only useful with a linker that can produce GDB index version 7.
-fdebug-types-section
When using DWARF Version 4 or higher, type DIEs can be put into their own ".debug_types" section instead of
making them part of the ".debug_info" section. It is more efficient to put them in a separate comdat section
since the linker can then remove duplicates. But not all DWARF consumers support ".debug_types" sections yet and
on some objects ".debug_types" produces larger instead of smaller debugging information.
-grecord-gcc-switches
-gno-record-gcc-switches
This switch causes the command-line options used to invoke the compiler that may affect code generation to be
appended to the DW_AT_producer attribute in DWARF debugging information. The options are concatenated with
spaces separating them from each other and from the compiler version. It is enabled by default. See also
-frecord-gcc-switches for another way of storing compiler options into the object file.
-gstrict-dwarf
Disallow using extensions of later DWARF standard version than selected with -gdwarf-version. On most targets
using non-conflicting DWARF extensions from later standard versions is allowed.
-gno-strict-dwarf
Allow using extensions of later DWARF standard version than selected with -gdwarf-version.
-gas-loc-support
Inform the compiler that the assembler supports ".loc" directives. It may then use them for the assembler to
generate DWARF2+ line number tables.
This is generally desirable, because assembler-generated line-number tables are a lot more compact than those the
compiler can generate itself.
This option will be enabled by default if, at GCC configure time, the assembler was found to support such
directives.
-gno-as-loc-support
Force GCC to generate DWARF2+ line number tables internally, if DWARF2+ line number tables are to be generated.
-gas-locview-support
Inform the compiler that the assembler supports "view" assignment and reset assertion checking in ".loc"
directives.
This option will be enabled by default if, at GCC configure time, the assembler was found to support them.
-gno-as-locview-support
Force GCC to assign view numbers internally, if -gvariable-location-views are explicitly requested.
-gcolumn-info
-gno-column-info
Emit location column information into DWARF debugging information, rather than just file and line. This option
is enabled by default.
-gstatement-frontiers
-gno-statement-frontiers
This option causes GCC to create markers in the internal representation at the beginning of statements, and to
keep them roughly in place throughout compilation, using them to guide the output of "is_stmt" markers in the
line number table. This is enabled by default when compiling with optimization (-Os, -O1, -O2, ...), and
outputting DWARF 2 debug information at the normal level.
-gvariable-location-views
-gvariable-location-views=incompat5
-gno-variable-location-views
Augment variable location lists with progressive view numbers implied from the line number table. This enables
debug information consumers to inspect state at certain points of the program, even if no instructions associated
with the corresponding source locations are present at that point. If the assembler lacks support for view
numbers in line number tables, this will cause the compiler to emit the line number table, which generally makes
them somewhat less compact. The augmented line number tables and location lists are fully backward-compatible,
so they can be consumed by debug information consumers that are not aware of these augmentations, but they won't
derive any benefit from them either.
This is enabled by default when outputting DWARF 2 debug information at the normal level, as long as there is
assembler support, -fvar-tracking-assignments is enabled and -gstrict-dwarf is not. When assembler support is
not available, this may still be enabled, but it will force GCC to output internal line number tables, and if
-ginternal-reset-location-views is not enabled, that will most certainly lead to silently mismatching location
views.
There is a proposed representation for view numbers that is not backward compatible with the location list format
introduced in DWARF 5, that can be enabled with -gvariable-location-views=incompat5. This option may be removed
in the future, is only provided as a reference implementation of the proposed representation. Debug information
consumers are not expected to support this extended format, and they would be rendered unable to decode location
lists using it.
-ginternal-reset-location-views
-gno-internal-reset-location-views
Attempt to determine location views that can be omitted from location view lists. This requires the compiler to
have very accurate insn length estimates, which isn't always the case, and it may cause incorrect view lists to
be generated silently when using an assembler that does not support location view lists. The GNU assembler will
flag any such error as a "view number mismatch". This is only enabled on ports that define a reliable estimation
function.
-ginline-points
-gno-inline-points
Generate extended debug information for inlined functions. Location view tracking markers are inserted at
inlined entry points, so that address and view numbers can be computed and output in debug information. This can
be enabled independently of location views, in which case the view numbers won't be output, but it can only be
enabled along with statement frontiers, and it is only enabled by default if location views are enabled.
-gz[=type]
Produce compressed debug sections in DWARF format, if that is supported. If type is not given, the default type
depends on the capabilities of the assembler and linker used. type may be one of none (don't compress debug
sections), or zlib (use zlib compression in ELF gABI format). If the linker doesn't support writing compressed
debug sections, the option is rejected. Otherwise, if the assembler does not support them, -gz is silently
ignored when producing object files.
-femit-struct-debug-baseonly
Emit debug information for struct-like types only when the base name of the compilation source file matches the
base name of file in which the struct is defined.
This option substantially reduces the size of debugging information, but at significant potential loss in type
information to the debugger. See -femit-struct-debug-reduced for a less aggressive option. See
-femit-struct-debug-detailed for more detailed control.
This option works only with DWARF debug output.
-femit-struct-debug-reduced
Emit debug information for struct-like types only when the base name of the compilation source file matches the
base name of file in which the type is defined, unless the struct is a template or defined in a system header.
This option significantly reduces the size of debugging information, with some potential loss in type information
to the debugger. See -femit-struct-debug-baseonly for a more aggressive option. See
-femit-struct-debug-detailed for more detailed control.
This option works only with DWARF debug output.
-femit-struct-debug-detailed[=spec-list]
Specify the struct-like types for which the compiler generates debug information. The intent is to reduce
duplicate struct debug information between different object files within the same program.
This option is a detailed version of -femit-struct-debug-reduced and -femit-struct-debug-baseonly, which serves
for most needs.
A specification has the syntax[dir:|ind:][ord:|gen:](any|sys|base|none)
The optional first word limits the specification to structs that are used directly (dir:) or used indirectly
(ind:). A struct type is used directly when it is the type of a variable, member. Indirect uses arise through
pointers to structs. That is, when use of an incomplete struct is valid, the use is indirect. An example is
struct one direct; struct two * indirect;.
The optional second word limits the specification to ordinary structs (ord:) or generic structs (gen:). Generic
structs are a bit complicated to explain. For C++, these are non-explicit specializations of template classes,
or non-template classes within the above. Other programming languages have generics, but
-femit-struct-debug-detailed does not yet implement them.
The third word specifies the source files for those structs for which the compiler should emit debug information.
The values none and any have the normal meaning. The value base means that the base of name of the file in which
the type declaration appears must match the base of the name of the main compilation file. In practice, this
means that when compiling foo.c, debug information is generated for types declared in that file and foo.h, but
not other header files. The value sys means those types satisfying base or declared in system or compiler
headers.
You may need to experiment to determine the best settings for your application.
The default is -femit-struct-debug-detailed=all.
This option works only with DWARF debug output.
-fno-dwarf2-cfi-asm
Emit DWARF unwind info as compiler generated ".eh_frame" section instead of using GAS ".cfi_*" directives.
-fno-eliminate-unused-debug-types
Normally, when producing DWARF output, GCC avoids producing debug symbol output for types that are nowhere used
in the source file being compiled. Sometimes it is useful to have GCC emit debugging information for all types
declared in a compilation unit, regardless of whether or not they are actually used in that compilation unit, for
example if, in the debugger, you want to cast a value to a type that is not actually used in your program (but is
declared). More often, however, this results in a significant amount of wasted space.
Options That Control Optimization
These options control various sorts of optimizations.
Without any optimization option, the compiler's goal is to reduce the cost of compilation and to make debugging
produce the expected results. Statements are independent: if you stop the program with a breakpoint between
statements, you can then assign a new value to any variable or change the program counter to any other statement in
the function and get exactly the results you expect from the source code.
Turning on optimization flags makes the compiler attempt to improve the performance and/or code size at the expense
of compilation time and possibly the ability to debug the program.
The compiler performs optimization based on the knowledge it has of the program. Compiling multiple files at once to
a single output file mode allows the compiler to use information gained from all of the files when compiling each of
them.
Not all optimizations are controlled directly by a flag. Only optimizations that have a flag are listed in this
section.
Most optimizations are completely disabled at -O0 or if an -O level is not set on the command line, even if
individual optimization flags are specified. Similarly, -Og suppresses many optimization passes.
Depending on the target and how GCC was configured, a slightly different set of optimizations may be enabled at each
-O level than those listed here. You can invoke GCC with -Q --help=optimizers to find out the exact set of
optimizations that are enabled at each level.
-O
-O1 Optimize. Optimizing compilation takes somewhat more time, and a lot more memory for a large function.
With -O, the compiler tries to reduce code size and execution time, without performing any optimizations that
take a great deal of compilation time.
-O is the recommended optimization level for large machine-generated code as a sensible balance between time
taken to compile and memory use: higher optimization levels perform optimizations with greater algorithmic
complexity than at -O.
-O turns on the following optimization flags:
-fauto-inc-dec -fbranch-count-reg -fcombine-stack-adjustments -fcompare-elim -fcprop-registers -fdce -fdefer-pop
-fdelayed-branch -fdse -fforward-propagate -fguess-branch-probability -fif-conversion -fif-conversion2
-finline-functions-called-once -fipa-modref -fipa-profile -fipa-pure-const -fipa-reference
-fipa-reference-addressable -fivopts -fmerge-constants -fmove-loop-invariants -fmove-loop-stores
-fomit-frame-pointer -freorder-blocks -fshrink-wrap -fshrink-wrap-separate -fsplit-wide-types -fssa-backprop
-fssa-phiopt -ftree-bit-ccp -ftree-ccp -ftree-ch -ftree-coalesce-vars -ftree-copy-prop -ftree-dce
-ftree-dominator-opts -ftree-dse -ftree-forwprop -ftree-fre -ftree-phiprop -ftree-pta -ftree-scev-cprop
-ftree-sink -ftree-slsr -ftree-sra -ftree-ter -funit-at-a-time
-O2 Optimize even more. GCC performs nearly all supported optimizations that do not involve a space-speed tradeoff.
As compared to -O, this option increases both compilation time and the performance of the generated code.
-O2 turns on all optimization flags specified by -O1. It also turns on the following optimization flags:
-falign-functions -falign-jumps -falign-labels -falign-loops -fcaller-saves -fcode-hoisting -fcrossjumping
-fcse-follow-jumps -fcse-skip-blocks -fdelete-null-pointer-checks -fdevirtualize -fdevirtualize-speculatively
-fexpensive-optimizations -ffinite-loops -fgcse -fgcse-lm -fhoist-adjacent-loads -finline-functions
-finline-small-functions -findirect-inlining -fipa-bit-cp -fipa-cp -fipa-icf -fipa-ra -fipa-sra -fipa-vrp
-fisolate-erroneous-paths-dereference -flra-remat -foptimize-crc -foptimize-sibling-calls -foptimize-strlen
-fpartial-inlining -fpeephole2 -freorder-blocks-algorithm=stc -freorder-blocks-and-partition -freorder-functions
-frerun-cse-after-loop -fschedule-insns -fschedule-insns2 -fsched-interblock -fsched-spec -fstore-merging
-fstrict-aliasing -fthread-jumps -ftree-builtin-call-dce -ftree-loop-vectorize -ftree-pre -ftree-slp-vectorize
-ftree-switch-conversion -ftree-tail-merge -ftree-vrp -fvect-cost-model=very-cheap
Please note the warning under -fgcse about invoking -O2 on programs that use computed gotos.
NOTE: In Ubuntu 8.10 and later versions, -D_FORTIFY_SOURCE=2, in Ubuntu 24.04 and later versions,
-D_FORTIFY_SOURCE=3, is set by default, and is activated when -O is set to 2 or higher. This enables additional
compile-time and run-time checks for several libc functions. To disable, specify either -U_FORTIFY_SOURCE or
-D_FORTIFY_SOURCE=0.
NOTE: In Debian 13 and Ubuntu 24.04 and later versions, -D_TIME_BITS=64 together with -D_FILE_OFFSET_BITS=64 is
set by default on the 32bit architectures armel, armhf, hppa, m68k, mips, mipsel, powerpc and sh4.
-O3 Optimize yet more. -O3 turns on all optimizations specified by -O2 and also turns on the following optimization
flags:
-fgcse-after-reload -fipa-cp-clone -floop-interchange -floop-unroll-and-jam -fpeel-loops -fpredictive-commoning
-fsplit-loops -fsplit-paths -ftree-loop-distribution -ftree-partial-pre -funswitch-loops
-fvect-cost-model=dynamic -fversion-loops-for-strides
-O0 Reduce compilation time and make debugging produce the expected results. This is the default.
At -O0, GCC completely disables most optimization passes; they are not run even if you explicitly enable them on
the command line, or are listed by -Q --help=optimizers as being enabled by default. Many optimizations
performed by GCC depend on code analysis or canonicalization passes that are enabled by -O, and it would not be
useful to run individual optimization passes in isolation.
-Os Optimize for size. -Os enables all -O2 optimizations except those that often increase code size:
-falign-functions -falign-jumps -falign-labels -falign-loops -fprefetch-loop-arrays
-freorder-blocks-algorithm=stc
It also enables -finline-functions, causes the compiler to tune for code size rather than execution speed, and
performs further optimizations designed to reduce code size.
-Ofast
Disregard strict standards compliance. -Ofast enables all -O3 optimizations. It also enables optimizations that
are not valid for all standard-compliant programs. It turns on -ffast-math, -fallow-store-data-races and the
Fortran-specific -fstack-arrays, unless -fmax-stack-var-size is specified, and -fno-protect-parens. It turns off
-fsemantic-interposition.
-Og Optimize debugging experience. -Og should be the optimization level of choice for the standard edit-compile-
debug cycle, offering a reasonable level of optimization while maintaining fast compilation and a good debugging
experience. It is a better choice than -O0 for producing debuggable code because some compiler passes that
collect debug information are disabled at -O0.
Like -O0, -Og completely disables a number of optimization passes so that individual options controlling them
have no effect. Otherwise -Og enables all -O1 optimization flags except for those that may interfere with
debugging:
-fbranch-count-reg -fdelayed-branch -fdse -fif-conversion -fif-conversion2 -finline-functions-called-once
-fmove-loop-invariants -fmove-loop-stores -fssa-phiopt -ftree-bit-ccp -ftree-dse -ftree-pta -ftree-sra
-Oz Optimize aggressively for size rather than speed. This may increase the number of instructions executed if those
instructions require fewer bytes to encode. -Oz behaves similarly to -Os including enabling most -O2
optimizations.
If you use multiple -O options, with or without level numbers, the last such option is the one that is effective.
Options of the form -fflag specify machine-independent flags. Most flags have both positive and negative forms; the
negative form of -ffoo is -fno-foo. In the table below, only one of the forms is listed---the one you typically use.
You can figure out the other form by either removing no- or adding it.
The following options control specific optimizations. They are either activated by -O options or are related to ones
that are. You can use the following flags in the rare cases when "fine-tuning" of optimizations to be performed is
desired.
-fno-defer-pop
For machines that must pop arguments after a function call, always pop the arguments as soon as each function
returns. At levels -O1 and higher, -fdefer-pop is the default; this allows the compiler to let arguments
accumulate on the stack for several function calls and pop them all at once.
-fforward-propagate
Perform a forward propagation pass on RTL. The pass tries to combine two instructions and checks if the result
can be simplified. If loop unrolling is active, two passes are performed and the second is scheduled after loop
unrolling.
This option is enabled by default at optimization levels -O1, -O2, -O3, -Os.
-favoid-store-forwarding
-fno-avoid-store-forwarding
Many CPUs will stall for many cycles when a load partially depends on previous smaller stores. This pass tries
to detect such cases and avoid the penalty by changing the order of the load and store and then fixing up the
loaded value.
Disabled by default.
-ffp-contract=style
-ffp-contract=off disables floating-point expression contraction. -ffp-contract=fast enables floating-point
expression contraction such as forming of fused multiply-add operations if the target has native support for
them. -ffp-contract=on enables floating-point expression contraction if allowed by the language standard. This
is implemented for C and C++, where it enables contraction within one expression, but not across different
statements.
The default is -ffp-contract=off for C in a standards compliant mode (-std=c11 or similar), -ffp-contract=fast
otherwise.
-fomit-frame-pointer
Omit the frame pointer in functions that don't need one. This avoids the instructions to save, set up and
restore the frame pointer; on many targets it also makes an extra register available.
On some targets this flag has no effect because the standard calling sequence always uses a frame pointer, so it
cannot be omitted.
Note that -fno-omit-frame-pointer doesn't guarantee the frame pointer is used in all functions. Several targets
always omit the frame pointer in leaf functions.
Enabled by default at -O1 and higher.
-foptimize-crc
Detect loops calculating CRC (performing polynomial long division) and replace them with a faster implementation.
Detect 8, 16, 32, and 64 bit CRC, with a constant polynomial without the leading 1 bit, for both bit-forward and
bit-reversed cases. If the target supports a CRC instruction and the polynomial used in the source code matches
the polynomial used in the CRC instruction, generate that CRC instruction. Otherwise, if the target supports a
carry-less-multiplication instruction, generate CRC using it; otherwise generate table-based CRC.
Enabled by default at -O2 and higher.
-foptimize-sibling-calls
Optimize sibling and tail recursive calls.
Enabled at levels -O2, -O3, -Os.
-foptimize-strlen
Optimize various standard C string functions (e.g. "strlen", "strchr" or "strcpy") and their "_FORTIFY_SOURCE"
counterparts into faster alternatives.
Enabled at levels -O2, -O3.
-finline-stringops[=fn]
Expand memory and string operations (for now, only "memset") inline, even when the length is variable or big
enough as to require looping. This is most useful along with -ffreestanding and -fno-builtin.
In some circumstances, it enables the compiler to generate code that takes advantage of known alignment and
length multipliers, but even then it may be less efficient than optimized runtime implementations, and grow code
size so much that even a less performant but shared implementation runs faster due to better use of code caches.
This option is disabled by default.
-fno-inline
Do not expand any functions inline apart from those marked with the "always_inline" attribute. This is the
default when not optimizing.
Single functions can be exempted from inlining by marking them with the "noinline" attribute.
-finline-small-functions
Integrate functions into their callers when their body is smaller than expected function call code (so overall
size of program gets smaller). The compiler heuristically decides which functions are simple enough to be worth
integrating in this way. This inlining applies to all functions, even those not declared inline.
Enabled at levels -O2, -O3, -Os.
-findirect-inlining
Inline also indirect calls that are discovered to be known at compile time thanks to previous inlining. This
option has any effect only when inlining itself is turned on by the -finline-functions or
-finline-small-functions options.
Enabled at levels -O2, -O3, -Os.
-finline-functions
Consider all functions for inlining, even if they are not declared inline. The compiler heuristically decides
which functions are worth integrating in this way.
If all calls to a given function are integrated, and the function is declared "static", then the function is
normally not output as assembler code in its own right.
Enabled at levels -O2, -O3, -Os. Also enabled by -fprofile-use and -fauto-profile.
-finline-functions-called-once
Consider all "static" functions called once for inlining into their caller even if they are not marked "inline".
If a call to a given function is integrated, then the function is not output as assembler code in its own right.
Enabled at levels -O1, -O2, -O3 and -Os, but not -Og.
-fearly-inlining
Inline functions marked by "always_inline" and functions whose body seems smaller than the function call overhead
early before doing -fprofile-generate instrumentation and real inlining pass. Doing so makes profiling
significantly cheaper and usually inlining faster on programs having large chains of nested wrapper functions.
Enabled by default.
-fipa-sra
Perform interprocedural scalar replacement of aggregates, removal of unused parameters and replacement of
parameters passed by reference by parameters passed by value.
Enabled at levels -O2, -O3 and -Os.
-finline-limit=n
By default, GCC limits the size of functions that can be inlined. This flag allows coarse control of this limit.
n is the size of functions that can be inlined in number of pseudo instructions.
Inlining is actually controlled by a number of parameters, which may be specified individually by using --param
name=value. The -finline-limit=n option sets some of these parameters as follows:
max-inline-insns-single
is set to n/2.
max-inline-insns-auto
is set to n/2.
See below for a documentation of the individual parameters controlling inlining and for the defaults of these
parameters.
Note: there may be no value to -finline-limit that results in default behavior.
Note: pseudo instruction represents, in this particular context, an abstract measurement of function's size. In
no way does it represent a count of assembly instructions and as such its exact meaning might change from one
release to an another.
-fno-keep-inline-dllexport
This is a more fine-grained version of -fkeep-inline-functions, which applies only to functions that are declared
using the "dllexport" attribute or declspec.
-fkeep-inline-functions
In C, emit "static" functions that are declared "inline" into the object file, even if the function has been
inlined into all of its callers. This switch does not affect functions using the "extern inline" extension in
GNU C90. In C++, emit any and all inline functions into the object file.
-fkeep-static-functions
Emit "static" functions into the object file, even if the function is never used.
-fkeep-static-consts
Emit variables declared "static const" when optimization isn't turned on, even if the variables aren't
referenced.
GCC enables this option by default. If you want to force the compiler to check if a variable is referenced,
regardless of whether or not optimization is turned on, use the -fno-keep-static-consts option.
-fmerge-constants
Attempt to merge identical constants (string constants and floating-point constants) across compilation units.
This option is the default for optimized compilation if the assembler and linker support it. Use
-fno-merge-constants to inhibit this behavior.
Enabled at levels -O1, -O2, -O3, -Os.
-fmerge-all-constants
Attempt to merge identical constants and identical variables.
This option implies -fmerge-constants. In addition to -fmerge-constants this considers e.g. even constant
initialized arrays or initialized constant variables with integral or floating-point types. Languages like C or
C++ require each variable, including multiple instances of the same variable in recursive calls, to have distinct
locations, so using this option results in non-conforming behavior.
-fmodulo-sched
Perform swing modulo scheduling immediately before the first scheduling pass. This pass looks at innermost loops
and reorders their instructions by overlapping different iterations.
-fmodulo-sched-allow-regmoves
Perform more aggressive SMS-based modulo scheduling with register moves allowed. By setting this flag certain
anti-dependences edges are deleted, which triggers the generation of reg-moves based on the life-range analysis.
This option is effective only with -fmodulo-sched enabled.
-fno-branch-count-reg
Disable the optimization pass that scans for opportunities to use "decrement and branch" instructions on a count
register instead of instruction sequences that decrement a register, compare it against zero, and then branch
based upon the result. This option is only meaningful on architectures that support such instructions, which
include x86, PowerPC, IA-64 and S/390. Note that the -fno-branch-count-reg option doesn't remove the decrement
and branch instructions from the generated instruction stream introduced by other optimization passes.
The default is -fbranch-count-reg at -O1 and higher, except for -Og.
-fno-function-cse
Do not put function addresses in registers; make each instruction that calls a constant function contain the
function's address explicitly.
This option results in less efficient code, but some strange hacks that alter the assembler output may be
confused by the optimizations performed when this option is not used.
The default is -ffunction-cse
-fno-zero-initialized-in-bss
If the target supports a BSS section, GCC by default puts variables that are initialized to zero into BSS. This
can save space in the resulting code.
This option turns off this behavior because some programs explicitly rely on variables going to the data
section---e.g., so that the resulting executable can find the beginning of that section and/or make assumptions
based on that.
The default is -fzero-initialized-in-bss except in Ada.
-fthread-jumps
Perform optimizations that check to see if a jump branches to a location where another comparison subsumed by the
first is found. If so, the first branch is redirected to either the destination of the second branch or a point
immediately following it, depending on whether the condition is known to be true or false.
Enabled at levels -O1, -O2, -O3, -Os.
-fsplit-wide-types
When using a type that occupies multiple registers, such as "long long" on a 32-bit system, split the registers
apart and allocate them independently. This normally generates better code for those types, but may make
debugging more difficult.
Enabled at levels -O1, -O2, -O3, -Os.
-fsplit-wide-types-early
Fully split wide types early, instead of very late. This option has no effect unless -fsplit-wide-types is
turned on.
This is the default on some targets.
-fcse-follow-jumps
In common subexpression elimination (CSE), scan through jump instructions when the target of the jump is not
reached by any other path. For example, when CSE encounters an "if" statement with an "else" clause, CSE follows
the jump when the condition tested is false.
Enabled at levels -O2, -O3, -Os.
-fcse-skip-blocks
This is similar to -fcse-follow-jumps, but causes CSE to follow jumps that conditionally skip over blocks. When
CSE encounters a simple "if" statement with no else clause, -fcse-skip-blocks causes CSE to follow the jump
around the body of the "if".
Enabled at levels -O2, -O3, -Os.
-frerun-cse-after-loop
Re-run common subexpression elimination after loop optimizations are performed.
Enabled at levels -O2, -O3, -Os.
-fgcse
Perform a global common subexpression elimination pass. This pass also performs global constant and copy
propagation.
Note: When compiling a program using computed gotos, a GCC extension, you may get better run-time performance if
you disable the global common subexpression elimination pass by adding -fno-gcse to the command line.
Enabled at levels -O2, -O3, -Os.
-fgcse-lm
When -fgcse-lm is enabled, global common subexpression elimination attempts to move loads that are only killed by
stores into themselves. This allows a loop containing a load/store sequence to be changed to a load outside the
loop, and a copy/store within the loop.
Enabled by default when -fgcse is enabled.
-fgcse-sm
When -fgcse-sm is enabled, a store motion pass is run after global common subexpression elimination. This pass
attempts to move stores out of loops. When used in conjunction with -fgcse-lm, loops containing a load/store
sequence can be changed to a load before the loop and a store after the loop.
Not enabled at any optimization level.
-fgcse-las
When -fgcse-las is enabled, the global common subexpression elimination pass eliminates redundant loads that come
after stores to the same memory location (both partial and full redundancies).
Not enabled at any optimization level.
-fgcse-after-reload
When -fgcse-after-reload is enabled, a redundant load elimination pass is performed after reload. The purpose of
this pass is to clean up redundant spilling.
Enabled by -O3, -fprofile-use and -fauto-profile.
-faggressive-loop-optimizations
This option tells the loop optimizer to use language constraints to derive bounds for the number of iterations of
a loop. This assumes that loop code does not invoke undefined behavior by for example causing signed integer
overflows or out-of-bound array accesses. The bounds for the number of iterations of a loop are used to guide
loop unrolling and peeling and loop exit test optimizations. This option is enabled by default.
-funconstrained-commons
This option tells the compiler that variables declared in common blocks (e.g. Fortran) may later be overridden
with longer trailing arrays. This prevents certain optimizations that depend on knowing the array bounds.
-fcrossjumping
Perform cross-jumping transformation. This transformation unifies equivalent code and saves code size. The
resulting code may or may not perform better than without cross-jumping.
Enabled at levels -O2, -O3, -Os.
-fauto-inc-dec
Combine increments or decrements of addresses with memory accesses. This pass is always skipped on architectures
that do not have instructions to support this. Enabled by default at -O1 and higher on architectures that
support this.
-fdce
Perform dead code elimination (DCE) on RTL. Enabled by default at -O1 and higher.
-fdse
Perform dead store elimination (DSE) on RTL. Enabled by default at -O1 and higher.
-fif-conversion
Attempt to transform conditional jumps into branch-less equivalents. This includes use of conditional moves,
min, max, set flags and abs instructions, and some tricks doable by standard arithmetics. The use of conditional
execution on chips where it is available is controlled by -fif-conversion2.
Enabled at levels -O1, -O2, -O3, -Os, but not with -Og.
-fif-conversion2
Use conditional execution (where available) to transform conditional jumps into branch-less equivalents.
Enabled at levels -O1, -O2, -O3, -Os, but not with -Og.
-fdeclone-ctor-dtor
The C++ ABI requires multiple entry points for constructors and destructors: one for a base subobject, one for a
complete object, and one for a virtual destructor that calls operator delete afterwards. For a hierarchy with
virtual bases, the base and complete variants are clones, which means two copies of the function. With this
option, the base and complete variants are changed to be thunks that call a common implementation.
Enabled by -Os.
-fdelete-null-pointer-checks
Assume that programs cannot safely dereference null pointers, and that no code or data element resides at address
zero. This option enables simple constant folding optimizations at all optimization levels. In addition, other
optimization passes in GCC use this flag to control global dataflow analyses that eliminate useless checks for
null pointers; these assume that a memory access to address zero always results in a trap, so that if a pointer
is checked after it has already been dereferenced, it cannot be null.
Note however that in some environments this assumption is not true. Use -fno-delete-null-pointer-checks to
disable this optimization for programs that depend on that behavior.
This option is enabled by default on most targets. On AVR and MSP430, this option is completely disabled.
Passes that use the dataflow information are enabled independently at different optimization levels.
-fdevirtualize
Attempt to convert calls to virtual functions to direct calls. This is done both within a procedure and
interprocedurally as part of indirect inlining (-findirect-inlining) and interprocedural constant propagation
(-fipa-cp). Enabled at levels -O2, -O3, -Os.
-fdevirtualize-speculatively
Attempt to convert calls to virtual functions to speculative direct calls. Based on the analysis of the type
inheritance graph, determine for a given call the set of likely targets. If the set is small, preferably of size
1, change the call into a conditional deciding between direct and indirect calls. The speculative calls enable
more optimizations, such as inlining. When they seem useless after further optimization, they are converted back
into original form.
-fdevirtualize-at-ltrans
Stream extra information needed for aggressive devirtualization when running the link-time optimizer in local
transformation mode. This option enables more devirtualization but significantly increases the size of streamed
data. For this reason it is disabled by default.
-fexpensive-optimizations
Perform a number of minor optimizations that are relatively expensive.
Enabled at levels -O2, -O3, -Os.
-free
Attempt to remove redundant extension instructions. This is especially helpful for the x86-64 architecture,
which implicitly zero-extends in 64-bit registers after writing to their lower 32-bit half.
Enabled for Alpha, AArch64, LoongArch, PowerPC, RISC-V, SPARC, h83000 and x86 at levels -O2, -O3, -Os.
-fno-lifetime-dse
In C++ the value of an object is only affected by changes within its lifetime: when the constructor begins, the
object has an indeterminate value, and any changes during the lifetime of the object are dead when the object is
destroyed. Normally dead store elimination will take advantage of this; if your code relies on the value of the
object storage persisting beyond the lifetime of the object, you can use this flag to disable this optimization.
To preserve stores before the constructor starts (e.g. because your operator new clears the object storage) but
still treat the object as dead after the destructor, you can use -flifetime-dse=1. The default behavior can be
explicitly selected with -flifetime-dse=2. -flifetime-dse=0 is equivalent to -fno-lifetime-dse.
-flive-range-shrinkage
Attempt to decrease register pressure through register live range shrinkage. This is helpful for fast processors
with small or moderate size register sets.
-fira-algorithm=algorithm
Use the specified coloring algorithm for the integrated register allocator. The algorithm argument can be
priority, which specifies Chow's priority coloring, or CB, which specifies Chaitin-Briggs coloring. Chaitin-
Briggs coloring is not implemented for all architectures, but for those targets that do support it, it is the
default because it generates better code.
-fira-region=region
Use specified regions for the integrated register allocator. The region argument should be one of the following:
all Use all loops as register allocation regions. This can give the best results for machines with a small
and/or irregular register set.
mixed
Use all loops except for loops with small register pressure as the regions. This value usually gives the
best results in most cases and for most architectures, and is enabled by default when compiling with
optimization for speed (-O, -O2, ...).
one Use all functions as a single region. This typically results in the smallest code size, and is enabled by
default for -Os or -O0.
-fira-hoist-pressure
Use IRA to evaluate register pressure in the code hoisting pass for decisions to hoist expressions. This option
usually results in smaller code, but it can slow the compiler down.
This option is enabled at level -Os for all targets.
-fira-loop-pressure
Use IRA to evaluate register pressure in loops for decisions to move loop invariants. This option usually
results in generation of faster and smaller code on machines with large register files (>= 32 registers), but it
can slow the compiler down.
This option is enabled at level -O3 for some targets.
-fno-ira-share-save-slots
Disable sharing of stack slots used for saving call-used hard registers living through a call. Each hard
register gets a separate stack slot, and as a result function stack frames are larger.
-fno-ira-share-spill-slots
Disable sharing of stack slots allocated for pseudo-registers. Each pseudo-register that does not get a hard
register gets a separate stack slot, and as a result function stack frames are larger.
-flra-remat
Enable CFG-sensitive rematerialization in LRA. Instead of loading values of spilled pseudos, LRA tries to
rematerialize (recalculate) values if it is profitable.
Enabled at levels -O2, -O3, -Os.
-fdelayed-branch
If supported for the target machine, attempt to reorder instructions to exploit instruction slots available after
delayed branch instructions.
Enabled at levels -O1, -O2, -O3, -Os, but not at -Og.
-fschedule-insns
If supported for the target machine, attempt to reorder instructions to eliminate execution stalls due to
required data being unavailable. This helps machines that have slow floating point or memory load instructions
by allowing other instructions to be issued until the result of the load or floating-point instruction is
required.
Conventionally enabled at optimization levels -O2 and -O3. However, many targets override this behavior. For
example, on x86, it is disabled at all levels, while on AArch64, it is enabled only at -O3.
-fschedule-insns2
Similar to -fschedule-insns, but requests an additional pass of instruction scheduling after register allocation
has been done. This is especially useful on machines with a relatively small number of registers and where
memory load instructions take more than one cycle.
Enabled at levels -O2, -O3, -Os.
-fno-sched-interblock
Disable instruction scheduling across basic blocks, which is normally enabled when scheduling before register
allocation, i.e. with -fschedule-insns or at -O2 or higher.
-fno-sched-spec
Disable speculative motion of non-load instructions, which is normally enabled when scheduling before register
allocation, i.e. with -fschedule-insns or at -O2 or higher.
-fsched-pressure
Enable register pressure sensitive insn scheduling before register allocation. This only makes sense when
scheduling before register allocation is enabled, i.e. with -fschedule-insns or at -O2 or higher. Usage of this
option can improve the generated code and decrease its size by preventing register pressure increase above the
number of available hard registers and subsequent spills in register allocation.
-fsched-spec-load
Allow speculative motion of some load instructions. This only makes sense when scheduling before register
allocation, i.e. with -fschedule-insns or at -O2 or higher.
-fsched-spec-load-dangerous
Allow speculative motion of more load instructions. This only makes sense when scheduling before register
allocation, i.e. with -fschedule-insns or at -O2 or higher.
-fsched-stalled-insns
-fsched-stalled-insns=n
Define how many insns (if any) can be moved prematurely from the queue of stalled insns into the ready list
during the second scheduling pass. -fno-sched-stalled-insns means that no insns are moved prematurely,
-fsched-stalled-insns=0 means there is no limit on how many queued insns can be moved prematurely.
-fsched-stalled-insns without a value is equivalent to -fsched-stalled-insns=1.
-fsched-stalled-insns-dep
-fsched-stalled-insns-dep=n
Define how many insn groups (cycles) are examined for a dependency on a stalled insn that is a candidate for
premature removal from the queue of stalled insns. This has an effect only during the second scheduling pass,
and only if -fsched-stalled-insns is used. -fno-sched-stalled-insns-dep is equivalent to
-fsched-stalled-insns-dep=0. -fsched-stalled-insns-dep without a value is equivalent to
-fsched-stalled-insns-dep=1.
-fsched2-use-superblocks
When scheduling after register allocation, use superblock scheduling. This allows motion across basic block
boundaries, resulting in faster schedules. This option is experimental, as not all machine descriptions used by
GCC model the CPU closely enough to avoid unreliable results from the algorithm.
This only makes sense when scheduling after register allocation, i.e. with -fschedule-insns2 or at -O2 or higher.
-fsched-group-heuristic
Enable the group heuristic in the scheduler. This heuristic favors the instruction that belongs to a schedule
group. This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or
at -O2 or higher.
-fsched-critical-path-heuristic
Enable the critical-path heuristic in the scheduler. This heuristic favors instructions on the critical path.
This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2
or higher.
-fsched-spec-insn-heuristic
Enable the speculative instruction heuristic in the scheduler. This heuristic favors speculative instructions
with greater dependency weakness. This is enabled by default when scheduling is enabled, i.e. with
-fschedule-insns or -fschedule-insns2 or at -O2 or higher.
-fsched-rank-heuristic
Enable the rank heuristic in the scheduler. This heuristic favors the instruction belonging to a basic block
with greater size or frequency. This is enabled by default when scheduling is enabled, i.e. with
-fschedule-insns or -fschedule-insns2 or at -O2 or higher.
-fsched-last-insn-heuristic
Enable the last-instruction heuristic in the scheduler. This heuristic favors the instruction that is less
dependent on the last instruction scheduled. This is enabled by default when scheduling is enabled, i.e. with
-fschedule-insns or -fschedule-insns2 or at -O2 or higher.
-fsched-dep-count-heuristic
Enable the dependent-count heuristic in the scheduler. This heuristic favors the instruction that has more
instructions depending on it. This is enabled by default when scheduling is enabled, i.e. with -fschedule-insns
or -fschedule-insns2 or at -O2 or higher.
-freschedule-modulo-scheduled-loops
Modulo scheduling is performed before traditional scheduling. If a loop is modulo scheduled, later scheduling
passes may change its schedule. Use this option to control that behavior.
-fselective-scheduling
Schedule instructions using selective scheduling algorithm. Selective scheduling runs instead of the first
scheduler pass.
-fselective-scheduling2
Schedule instructions using selective scheduling algorithm. Selective scheduling runs instead of the second
scheduler pass.
-fsel-sched-pipelining
Enable software pipelining of innermost loops during selective scheduling. This option has no effect unless one
of -fselective-scheduling or -fselective-scheduling2 is turned on.
-fsel-sched-pipelining-outer-loops
When pipelining loops during selective scheduling, also pipeline outer loops. This option has no effect unless
-fsel-sched-pipelining is turned on.
-fsemantic-interposition
Some object formats, like ELF, allow interposing of symbols by the dynamic linker. This means that for symbols
exported from the DSO, the compiler cannot perform interprocedural propagation, inlining and other optimizations
in anticipation that the function or variable in question may change. While this feature is useful, for example,
to rewrite memory allocation functions by a debugging implementation, it is expensive in the terms of code
quality. With -fno-semantic-interposition the compiler assumes that if interposition happens for functions the
overwriting function will have precisely the same semantics (and side effects). Similarly if interposition
happens for variables, the constructor of the variable will be the same. The flag has no effect for functions
explicitly declared inline (where it is never allowed for interposition to change semantics) and for symbols
explicitly declared weak.
-fshrink-wrap
Emit function prologues only before parts of the function that need it, rather than at the top of the function.
This flag is enabled by default at -O and higher.
-fshrink-wrap-separate
Shrink-wrap separate parts of the prologue and epilogue separately, so that those parts are only executed when
needed. This option is on by default, but has no effect unless -fshrink-wrap is also turned on and the target
supports this.
-fcaller-saves
Enable allocation of values to registers that are clobbered by function calls, by emitting extra instructions to
save and restore the registers around such calls. Such allocation is done only when it seems to result in better
code.
This option is always enabled by default on certain machines, usually those which have no call-preserved
registers to use instead.
Enabled at levels -O2, -O3, -Os.
-fcombine-stack-adjustments
Tracks stack adjustments (pushes and pops) and stack memory references and then tries to find ways to combine
them.
Enabled by default at -O1 and higher.
-fipa-ra
Use caller save registers for allocation if those registers are not used by any called function. In that case it
is not necessary to save and restore them around calls. This is only possible if called functions are part of
same compilation unit as current function and they are compiled before it.
Enabled at levels -O2, -O3, -Os, however the option is disabled if generated code will be instrumented for
profiling (-p, or -pg) or if callee's register usage cannot be known exactly (this happens on targets that do not
expose prologues and epilogues in RTL).
-fconserve-stack
Attempt to minimize stack usage. The compiler attempts to use less stack space, even if that makes the program
slower. This option implies setting the large-stack-frame parameter to 100 and the large-stack-frame-growth
parameter to 400.
-ftree-reassoc
Perform reassociation on trees. This flag is enabled by default at -O1 and higher.
-fcode-hoisting
Perform code hoisting. Code hoisting tries to move the evaluation of expressions executed on all paths to the
function exit as early as possible. This is especially useful as a code size optimization, but it often helps
for code speed as well. This flag is enabled by default at -O2 and higher.
-ftree-pre
Perform partial redundancy elimination (PRE) on trees. This flag is enabled by default at -O2 and -O3.
-ftree-partial-pre
Make partial redundancy elimination (PRE) more aggressive. This flag is enabled by default at -O3.
-ftree-forwprop
Perform forward propagation on trees. This flag is enabled by default at -O1 and higher.
-ftree-fre
Perform full redundancy elimination (FRE) on trees. The difference between FRE and PRE is that FRE only
considers expressions that are computed on all paths leading to the redundant computation. This analysis is
faster than PRE, though it exposes fewer redundancies. This flag is enabled by default at -O1 and higher.
-ftree-phiprop
Perform hoisting of loads from conditional pointers on trees. This pass is enabled by default at -O1 and higher.
-fhoist-adjacent-loads
Speculatively hoist loads from both branches of an if-then-else if the loads are from adjacent locations in the
same structure and the target architecture has a conditional move instruction. This flag is enabled by default
at -O2 and higher.
-ftree-copy-prop
Perform copy propagation on trees. This pass eliminates unnecessary copy operations. This flag is enabled by
default at -O1 and higher.
-fipa-pure-const
Discover which functions are pure or constant. Enabled by default at -O1 and higher.
-fipa-reference
Discover which static variables do not escape the compilation unit. Enabled by default at -O1 and higher.
-fipa-reference-addressable
Discover read-only, write-only and non-addressable static variables. Enabled by default at -O1 and higher.
-fipa-reorder-for-locality
Group call chains close together in the binary layout to improve code locality and minimize jump distances
between frequently called functions. Unlike -freorder-functions this pass considers the call chains between
functions and groups them together, rather than grouping all hot/normal/cold/never-executed functions into
separate sections. Unlike -fprofile-reorder-functions it aims to improve code locality throughout the runtime of
the program rather than focusing on program startup. This option is incompatible with an explicit
-flto-partition= option since it enforces a custom partitioning scheme. If using this option it is recommended
to also use profile feedback, but this option is not enabled by default otherwise.
-fipa-stack-alignment
Reduce stack alignment on call sites if possible. Enabled by default.
-fipa-pta
Perform interprocedural pointer analysis and interprocedural modification and reference analysis. This option
can cause excessive memory and compile-time usage on large compilation units. It is not enabled by default at
any optimization level.
-fipa-profile
Perform interprocedural profile propagation. The functions called only from cold functions are marked as cold.
Also functions executed once (such as "cold", "noreturn", static constructors or destructors) are identified.
Cold functions and loop less parts of functions executed once are then optimized for size. Enabled by default at
-O1 and higher.
-fipa-modref
Perform interprocedural mod/ref analysis. This optimization analyzes the side effects of functions (memory
locations that are modified or referenced) and enables better optimization across the function call boundary.
This flag is enabled by default at -O1 and higher.
-fipa-cp
Perform interprocedural constant propagation. This optimization analyzes the program to determine when values
passed to functions are constants and then optimizes accordingly. This optimization can substantially increase
performance if the application has constants passed to functions. This flag is enabled by default at -O2, -Os
and -O3. It is also enabled by -fprofile-use and -fauto-profile.
-fipa-cp-clone
Perform function cloning to make interprocedural constant propagation stronger. When enabled, interprocedural
constant propagation performs function cloning when externally visible function can be called with constant
arguments. Because this optimization can create multiple copies of functions, it may significantly increase code
size (see --param ipa-cp-unit-growth=value). This flag is enabled by default at -O3. It is also enabled by
-fprofile-use and -fauto-profile.
-fipa-bit-cp
When enabled, perform interprocedural bitwise constant propagation. This flag is enabled by default at -O2 and by
-fprofile-use and -fauto-profile. It requires that -fipa-cp is enabled.
-fipa-vrp
When enabled, perform interprocedural propagation of value ranges. This flag is enabled by default at -O2. It
requires that -fipa-cp is enabled.
-fipa-icf
Perform Identical Code Folding for functions and read-only variables. The optimization reduces code size and may
disturb unwind stacks by replacing a function by equivalent one with a different name. The optimization works
more effectively with link-time optimization enabled.
Although the behavior is similar to the Gold Linker's ICF optimization, GCC ICF works on different levels and
thus the optimizations are not same - there are equivalences that are found only by GCC and equivalences found
only by Gold.
This flag is enabled by default at -O2 and -Os.
-flate-combine-instructions
Enable two instruction combination passes that run relatively late in the compilation process. One of the passes
runs before register allocation and the other after register allocation. The main aim of the passes is to
substitute definitions into all uses.
Most targets enable this flag by default at -O2 and -Os.
-flive-patching=level
Control GCC's optimizations to produce output suitable for live-patching.
If the compiler's optimization uses a function's body or information extracted from its body to optimize/change
another function, the latter is called an impacted function of the former. If a function is patched, its
impacted functions should be patched too.
The impacted functions are determined by the compiler's interprocedural optimizations. For example, a caller is
impacted when inlining a function into its caller, cloning a function and changing its caller to call this new
clone, or extracting a function's pureness/constness information to optimize its direct or indirect callers, etc.
Usually, the more IPA optimizations enabled, the larger the number of impacted functions for each function. In
order to control the number of impacted functions and more easily compute the list of impacted function, IPA
optimizations can be partially enabled at two different levels.
The level argument should be one of the following:
inline-clone
Only enable inlining and cloning optimizations, which includes inlining, cloning, interprocedural scalar
replacement of aggregates and partial inlining. As a result, when patching a function, all its callers and
its clones' callers are impacted, therefore need to be patched as well.
-flive-patching=inline-clone disables the following optimization flags: -fwhole-program -fipa-pta
-fipa-reference -fipa-ra -fipa-icf -fipa-icf-functions -fipa-icf-variables -fipa-bit-cp -fipa-vrp
-fipa-pure-const -fipa-reference-addressable -fipa-stack-alignment -fipa-modref
inline-only-static
Only enable inlining of static functions. As a result, when patching a static function, all its callers are
impacted and so need to be patched as well.
In addition to all the flags that -flive-patching=inline-clone disables, -flive-patching=inline-only-static
disables the following additional optimization flags: -fipa-cp-clone -fipa-sra -fpartial-inlining -fipa-cp
When -flive-patching is specified without any value, the default value is inline-clone.
This flag is disabled by default.
Note that -flive-patching is not supported with link-time optimization (-flto).
-fisolate-erroneous-paths-dereference
Detect paths that trigger erroneous or undefined behavior due to dereferencing a null pointer. Isolate those
paths from the main control flow and turn the statement with erroneous or undefined behavior into a trap. This
flag is enabled by default at -O2 and higher and depends on -fdelete-null-pointer-checks also being enabled.
-fisolate-erroneous-paths-attribute
Detect paths that trigger erroneous or undefined behavior due to a null value being used in a way forbidden by a
"returns_nonnull" or "nonnull" attribute. Isolate those paths from the main control flow and turn the statement
with erroneous or undefined behavior into a trap. This is not currently enabled, but may be enabled by -O2 in
the future.
-ftree-sink
Perform forward store motion on trees. This flag is enabled by default at -O1 and higher.
-ftree-bit-ccp
Perform sparse conditional bit constant propagation on trees and propagate pointer alignment information. This
pass only operates on local scalar variables and is enabled by default at -O1 and higher, except for -Og. It
requires that -ftree-ccp is enabled.
-ftree-ccp
Perform sparse conditional constant propagation (CCP) on trees. This pass only operates on local scalar
variables and is enabled by default at -O1 and higher.
-fssa-backprop
Propagate information about uses of a value up the definition chain in order to simplify the definitions. For
example, this pass strips sign operations if the sign of a value never matters. The flag is enabled by default
at -O1 and higher.
-fssa-phiopt
Perform pattern matching on SSA PHI nodes to optimize conditional code. This pass is enabled by default at -O1
and higher, except for -Og.
-ftree-switch-conversion
Perform conversion of simple initializations in a switch to initializations from a scalar array. This flag is
enabled by default at -O2 and higher.
-ftree-tail-merge
Look for identical code sequences. When found, replace one with a jump to the other. This optimization is known
as tail merging or cross jumping. This flag is enabled by default at -O2 and higher. The compilation time in
this pass can be limited using max-tail-merge-comparisons parameter and max-tail-merge-iterations parameter.
-ftree-cselim
Perform conditional store elimination on trees. This flag is enabled by default at -O1 and higher on targets
that have conditional move instructions.
-ftree-dce
Perform dead code elimination (DCE) on trees. This flag is enabled by default at -O1 and higher.
-ftree-builtin-call-dce
Perform conditional dead code elimination (DCE) for calls to built-in functions that may set "errno" but are
otherwise free of side effects. This flag is enabled by default at -O2 and higher if -Os is not also specified.
-ffinite-loops
Assume that a loop with an exit will eventually take the exit and not loop indefinitely. This allows the
compiler to remove loops that otherwise have no side-effects, not considering eventual endless looping as such.
This option is enabled by default at -O2 for C++ with -std=c++11 or higher.
-ftree-dominator-opts
Perform a variety of simple scalar cleanups (constant/copy propagation, redundancy elimination, range propagation
and expression simplification) based on a dominator tree traversal. This also performs jump threading (to reduce
jumps to jumps). This flag is enabled by default at -O1 and higher.
-ftree-dse
Perform dead store elimination (DSE) on trees. A dead store is a store into a memory location that is later
overwritten by another store without any intervening loads. In this case the earlier store can be deleted. This
flag is enabled by default at -O1 and higher.
-ftree-ch
Perform loop header copying on trees. This is beneficial since it increases effectiveness of code motion
optimizations. It also saves one jump. This flag is enabled by default at -O1 and higher. It is not enabled
for -Os, since it usually increases code size.
-ftree-loop-optimize
Perform loop optimizations on trees. This flag is enabled by default at -O1 and higher.
-ftree-loop-linear
-floop-strip-mine
-floop-block
Perform loop nest optimizations. Same as -floop-nest-optimize. To use this code transformation, GCC has to be
configured with --with-isl to enable the Graphite loop transformation infrastructure.
-fgraphite-identity
Enable the identity transformation for graphite. For every SCoP we generate the polyhedral representation and
transform it back to gimple. Using -fgraphite-identity we can check the costs or benefits of the GIMPLE ->
GRAPHITE -> GIMPLE transformation. Some minimal optimizations are also performed by the code generator isl, like
index splitting and dead code elimination in loops.
-floop-nest-optimize
Enable the isl based loop nest optimizer. This is a generic loop nest optimizer based on the Pluto optimization
algorithms. It calculates a loop structure optimized for data-locality and parallelism. This option is
experimental.
-floop-parallelize-all
Use the Graphite data dependence analysis to identify loops that can be parallelized. Parallelize all the loops
that can be analyzed to not contain loop carried dependences without checking that it is profitable to
parallelize the loops.
-ftree-coalesce-vars
While transforming the program out of the SSA representation, attempt to reduce copying by coalescing versions of
different user-defined variables, instead of just compiler temporaries. This may severely limit the ability to
debug an optimized program compiled with -fno-var-tracking-assignments. In the negated form, this flag prevents
SSA coalescing of user variables. This option is enabled by default if optimization is enabled, and it does very
little otherwise.
-ftree-loop-if-convert
Attempt to transform conditional jumps in the innermost loops to branch-less equivalents. The intent is to
remove control-flow from the innermost loops in order to improve the ability of the vectorization pass to handle
these loops. This is enabled by default if vectorization is enabled.
-ftree-loop-distribution
Perform loop distribution. This flag can improve cache performance on big loop bodies and allow further loop
optimizations, like parallelization or vectorization, to take place. For example, the loop
DO I = 1, N
A(I) = B(I) + C
D(I) = E(I) * F
ENDDO
is transformed to
DO I = 1, N
A(I) = B(I) + C
ENDDO
DO I = 1, N
D(I) = E(I) * F
ENDDO
This flag is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile.
-ftree-loop-distribute-patterns
Perform loop distribution of patterns that can be code generated with calls to a library. This flag is enabled
by default at -O2 and higher, and by -fprofile-use and -fauto-profile.
This pass distributes the initialization loops and generates a call to memset zero. For example, the loop
DO I = 1, N
A(I) = 0
B(I) = A(I) + I
ENDDO
is transformed to
DO I = 1, N
A(I) = 0
ENDDO
DO I = 1, N
B(I) = A(I) + I
ENDDO
and the initialization loop is transformed into a call to memset zero.
-floop-interchange
Perform loop interchange outside of graphite. This flag can improve cache performance on loop nest and allow
further loop optimizations, like vectorization, to take place. For example, the loop
for (int i = 0; i < N; i++)
for (int j = 0; j < N; j++)
for (int k = 0; k < N; k++)
c[i][j] = c[i][j] + a[i][k]*b[k][j];
is transformed to
for (int i = 0; i < N; i++)
for (int k = 0; k < N; k++)
for (int j = 0; j < N; j++)
c[i][j] = c[i][j] + a[i][k]*b[k][j];
This flag is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile.
-floop-unroll-and-jam
Apply unroll and jam transformations on feasible loops. In a loop nest this unrolls the outer loop by some
factor and fuses the resulting multiple inner loops. This flag is enabled by default at -O3. It is also enabled
by -fprofile-use and -fauto-profile.
-ftree-loop-im
Perform loop invariant motion on trees. This pass moves only invariants that are hard to handle at RTL level
(function calls, operations that expand to nontrivial sequences of insns). With -funswitch-loops it also moves
operands of conditions that are invariant out of the loop, so that we can use just trivial invariantness analysis
in loop unswitching. The pass also includes store motion.
-ftree-loop-ivcanon
Create a canonical counter for number of iterations in loops for which determining number of iterations requires
complicated analysis. Later optimizations then may determine the number easily. Useful especially in connection
with unrolling.
-ftree-scev-cprop
Perform final value replacement. If a variable is modified in a loop in such a way that its value when exiting
the loop can be determined using only its initial value and the number of loop iterations, replace uses of the
final value by such a computation, provided it is sufficiently cheap. This reduces data dependencies and may
allow further simplifications. Enabled by default at -O1 and higher.
-fivopts
Perform induction variable optimizations (strength reduction, induction variable merging and induction variable
elimination) on trees. Enabled by default at -O1 and higher.
-ftree-parallelize-loops=n
Parallelize loops, i.e., split their iteration space to run in n threads. This is only possible for loops whose
iterations are independent and can be arbitrarily reordered. The optimization is only profitable on
multiprocessor machines, for loops that are CPU-intensive, rather than constrained e.g. by memory bandwidth.
This option implies -pthread, and thus is only supported on targets that have support for -pthread.
-ftree-pta
Perform function-local points-to analysis on trees. This flag is enabled by default at -O1 and higher, except
for -Og.
-ftree-sra
Perform scalar replacement of aggregates. This pass replaces structure references with scalars to prevent
committing structures to memory too early. This flag is enabled by default at -O1 and higher, except for -Og.
-fstore-merging
Perform merging of narrow stores to consecutive memory addresses. This pass merges contiguous stores of
immediate values narrower than a word into fewer wider stores to reduce the number of instructions. This is
enabled by default at -O2 and higher as well as -Os.
-ftree-ter
Perform temporary expression replacement during the SSA->normal phase. Single use/single def temporaries are
replaced at their use location with their defining expression. This results in non-GIMPLE code, but gives the
expanders much more complex trees to work on resulting in better RTL generation. This is enabled by default at
-O1 and higher.
-ftree-slsr
Perform straight-line strength reduction on trees. This recognizes related expressions involving multiplications
and replaces them by less expensive calculations when possible. This is enabled by default at -O1 and higher.
-ftree-vectorize
Perform vectorization on trees. This flag enables -ftree-loop-vectorize and -ftree-slp-vectorize if not
explicitly specified.
-ftree-loop-vectorize
Perform loop vectorization on trees. This flag is enabled by default at -O2 and by -ftree-vectorize,
-fprofile-use, and -fauto-profile.
-ftree-slp-vectorize
Perform basic block vectorization on trees. This flag is enabled by default at -O2 and by -ftree-vectorize,
-fprofile-use, and -fauto-profile.
-ftrivial-auto-var-init=choice
Initialize automatic variables with either a pattern or with zeroes to increase the security and predictability
of a program by preventing uninitialized memory disclosure and use. GCC still considers an automatic variable
that doesn't have an explicit initializer as uninitialized, -Wuninitialized and
-Wanalyzer-use-of-uninitialized-value will still report warning messages on such automatic variables and the
compiler will perform optimization as if the variable were uninitialized. With this option, GCC will also
initialize any padding of automatic variables that have structure or union types to zeroes. However, the current
implementation cannot initialize automatic variables that are declared between the controlling expression and the
first case of a "switch" statement. Using -Wtrivial-auto-var-init to report all such cases.
The three values of choice are:
* uninitialized doesn't initialize any automatic variables. This is C and C++'s default.
* pattern Initialize automatic variables with values which will likely transform logic bugs into crashes down
the line, are easily recognized in a crash dump and without being values that programmers can rely on for
useful program semantics. The current value is byte-repeatable pattern with byte "0xFE". The values used
for pattern initialization might be changed in the future.
* zero Initialize automatic variables with zeroes.
The default is uninitialized.
Note that the initializer values, whether zero or pattern, refer to data representation (in memory or machine
registers), rather than to their interpretation as numerical values. This distinction may be important in
languages that support types with biases or implicit multipliers, and with such extensions as hardbool. For
example, a variable that uses 8 bits to represent (biased) quantities in the "range 160..400" will be initialized
with the bit patterns 0x00 or 0xFE, depending on choice, whether or not these representations stand for values in
that range, and even if they do, the interpretation of the value held by the variable will depend on the bias. A
hardbool variable that uses say "0X5A" and 0xA5 for "false" and "true", respectively, will trap with either
choice of trivial initializer, i.e., zero initialization will not convert to the representation for "false", even
if it would for a "static" variable of the same type. This means the initializer pattern doesn't generally
depend on the type of the initialized variable. One notable exception is that (non-hardened) boolean variables
that fit in registers are initialized with "false" (zero), even when pattern is requested.
You can control this behavior for a specific variable by using the variable attribute "uninitialized".
-fvect-cost-model=model
Alter the cost model used for vectorization. The model argument should be one of unlimited, dynamic, cheap or
very-cheap. With the unlimited model the vectorized code-path is assumed to be profitable while with the dynamic
model a runtime check guards the vectorized code-path to enable it only for iteration counts that will likely
execute faster than when executing the original scalar loop. The cheap model disables vectorization of loops
where doing so would be cost prohibitive for example due to required runtime checks for data dependence or
alignment but otherwise is equal to the dynamic model. The very-cheap model disables vectorization of loops when
any runtime check for data dependence or alignment is required, it also disables vectorization of epilogue loops
but otherwise is equal to the cheap model.
The default cost model depends on other optimization flags and is either dynamic or cheap.
-fsimd-cost-model=model
Alter the cost model used for vectorization of loops marked with the OpenMP simd directive. The model argument
should be one of unlimited, dynamic, cheap. All values of model have the same meaning as described in
-fvect-cost-model and by default a cost model defined with -fvect-cost-model is used.
-ftree-vrp
Perform Value Range Propagation on trees. This is similar to the constant propagation pass, but instead of
values, ranges of values are propagated. This allows the optimizers to remove unnecessary range checks like
array bound checks and null pointer checks. This is enabled by default at -O2 and higher. Null pointer check
elimination is only done if -fdelete-null-pointer-checks is enabled.
-fsplit-paths
Split paths leading to loop backedges. This can improve dead code elimination and common subexpression
elimination. This is enabled by default at -O3 and above.
-fsplit-ivs-in-unroller
Enables expression of values of induction variables in later iterations of the unrolled loop using the value in
the first iteration. This breaks long dependency chains, thus improving efficiency of the scheduling passes.
A combination of -fweb and CSE is often sufficient to obtain the same effect. However, that is not reliable in
cases where the loop body is more complicated than a single basic block. It also does not work at all on some
architectures due to restrictions in the CSE pass.
This optimization is enabled by default.
-fvariable-expansion-in-unroller
With this option, the compiler creates multiple copies of some local variables when unrolling a loop, which can
result in superior code.
This optimization is enabled by default for PowerPC targets, but disabled by default otherwise.
-fpartial-inlining
Inline parts of functions. This option has any effect only when inlining itself is turned on by the
-finline-functions or -finline-small-functions options.
Enabled at levels -O2, -O3, -Os.
-fpredictive-commoning
Perform predictive commoning optimization, i.e., reusing computations (especially memory loads and stores)
performed in previous iterations of loops.
This option is enabled at level -O3. It is also enabled by -fprofile-use and -fauto-profile.
-fprefetch-loop-arrays
If supported by the target machine, generate instructions to prefetch memory to improve the performance of loops
that access large arrays.
This option may generate better or worse code; results are highly dependent on the structure of loops within the
source code.
Disabled at level -Os.
-fno-printf-return-value
Do not substitute constants for known return value of formatted output functions such as "sprintf", "snprintf",
"vsprintf", and "vsnprintf" (but not "printf" of "fprintf"). This transformation allows GCC to optimize or even
eliminate branches based on the known return value of these functions called with arguments that are either
constant, or whose values are known to be in a range that makes determining the exact return value possible. For
example, when -fprintf-return-value is in effect, both the branch and the body of the "if" statement (but not the
call to "snprint") can be optimized away when "i" is a 32-bit or smaller integer because the return value is
guaranteed to be at most 8.
char buf[9];
if (snprintf (buf, "%08x", i) >= sizeof buf)
...
The -fprintf-return-value option relies on other optimizations and yields best results with -O2 and above. It
works in tandem with the -Wformat-overflow and -Wformat-truncation options. The -fprintf-return-value option is
enabled by default.
-fno-peephole
-fno-peephole2
Disable any machine-specific peephole optimizations. The difference between -fno-peephole and -fno-peephole2 is
in how they are implemented in the compiler; some targets use one, some use the other, a few use both.
-fpeephole is enabled by default. -fpeephole2 enabled at levels -O2, -O3, -Os.
-fno-guess-branch-probability
Do not guess branch probabilities using heuristics.
GCC uses heuristics to guess branch probabilities if they are not provided by profiling feedback
(-fprofile-arcs). These heuristics are based on the control flow graph. If some branch probabilities are
specified by "__builtin_expect", then the heuristics are used to guess branch probabilities for the rest of the
control flow graph, taking the "__builtin_expect" info into account. The interactions between the heuristics and
"__builtin_expect" can be complex, and in some cases, it may be useful to disable the heuristics so that the
effects of "__builtin_expect" are easier to understand.
It is also possible to specify expected probability of the expression with "__builtin_expect_with_probability"
built-in function.
The default is -fguess-branch-probability at levels -O, -O2, -O3, -Os.
-freorder-blocks
Reorder basic blocks in the compiled function in order to reduce number of taken branches and improve code
locality.
Enabled at levels -O1, -O2, -O3, -Os.
-freorder-blocks-algorithm=algorithm
Use the specified algorithm for basic block reordering. The algorithm argument can be simple, which does not
increase code size (except sometimes due to secondary effects like alignment), or stc, the "software trace cache"
algorithm, which tries to put all often executed code together, minimizing the number of branches executed by
making extra copies of code.
The default is simple at levels -O1, -Os, and stc at levels -O2, -O3.
-freorder-blocks-and-partition
In addition to reordering basic blocks in the compiled function, in order to reduce number of taken branches,
partitions hot and cold basic blocks into separate sections of the assembly and .o files, to improve paging and
cache locality performance.
This optimization is automatically turned off in the presence of exception handling or unwind tables (on targets
using setjump/longjump or target specific scheme), for linkonce sections, for functions with a user-defined
section attribute and on any architecture that does not support named sections. When -fsplit-stack is used this
option is not enabled by default (to avoid linker errors), but may be enabled explicitly (if using a working
linker).
Enabled for x86 at levels -O2, -O3, -Os.
-freorder-functions
Reorder functions in the object file in order to improve code locality. Unlike -fipa-reorder-for-locality this
option prioritises grouping all functions within a category (hot/normal/cold/never-executed) together. This is
implemented by using special subsections ".text.hot" for most frequently executed functions and ".text.unlikely"
for unlikely executed functions. Reordering is done by the linker so object file format must support named
sections and linker must place them in a reasonable way.
This option isn't effective unless you either provide profile feedback (see -fprofile-arcs for details) or
manually annotate functions with "hot" or "cold" attributes.
Enabled at levels -O2, -O3, -Os.
-fstrict-aliasing
Allow the compiler to assume the strictest aliasing rules applicable to the language being compiled. For C (and
C++), this activates optimizations based on the type of expressions. In particular, an object of one type is
assumed never to reside at the same address as an object of a different type, unless the types are almost the
same. For example, an "unsigned int" can alias an "int", but not a "void*" or a "double". A character type may
alias any other type.
Pay special attention to code like this:
union a_union {
int i;
double d;
};
int f() {
union a_union t;
t.d = 3.0;
return t.i;
}
The practice of reading from a different union member than the one most recently written to (called "type-
punning") is common. Even with -fstrict-aliasing, type-punning is allowed, provided the memory is accessed
through the union type. So, the code above works as expected. However, this code might not:
int f() {
union a_union t;
int* ip;
t.d = 3.0;
ip = &t.i;
return *ip;
}
Similarly, access by taking the address, casting the resulting pointer and dereferencing the result has undefined
behavior, even if the cast uses a union type, e.g.:
int f() {
double d = 3.0;
return ((union a_union *) &d)->i;
}
The -fstrict-aliasing option is enabled at levels -O2, -O3, -Os.
-fipa-strict-aliasing
Controls whether rules of -fstrict-aliasing are applied across function boundaries. Note that if multiple
functions gets inlined into a single function the memory accesses are no longer considered to be crossing a
function boundary.
The -fipa-strict-aliasing option is enabled by default and is effective only in combination with
-fstrict-aliasing.
-falign-functions
-falign-functions=n
-falign-functions=n:m
-falign-functions=n:m:n2
-falign-functions=n:m:n2:m2
Align the start of functions to the next power-of-two greater than or equal to n, skipping up to m-1 bytes. This
ensures that at least the first m bytes of the function can be fetched by the CPU without crossing an n-byte
alignment boundary. This is an optimization of code performance and alignment is ignored for functions
considered cold. If alignment is required for all functions, use -fmin-function-alignment.
If m is not specified, it defaults to n.
Examples: -falign-functions=32 aligns functions to the next 32-byte boundary, -falign-functions=24 aligns to the
next 32-byte boundary only if this can be done by skipping 23 bytes or less, -falign-functions=32:7 aligns to the
next 32-byte boundary only if this can be done by skipping 6 bytes or less.
The second pair of n2:m2 values allows you to specify a secondary alignment: -falign-functions=64:7:32:3 aligns
to the next 64-byte boundary if this can be done by skipping 6 bytes or less, otherwise aligns to the next
32-byte boundary if this can be done by skipping 2 bytes or less. If m2 is not specified, it defaults to n2.
Some assemblers only support this flag when n is a power of two; in that case, it is rounded up.
-fno-align-functions and -falign-functions=1 are equivalent and mean that functions are not aligned.
If n is not specified or is zero, use a machine-dependent default. The maximum allowed n option value is 65536.
Enabled at levels -O2, -O3.
-flimit-function-alignment
If this option is enabled, the compiler tries to avoid unnecessarily overaligning functions. It attempts to
instruct the assembler to align by the amount specified by -falign-functions, but not to skip more bytes than the
size of the function.
-falign-labels
-falign-labels=n
-falign-labels=n:m
-falign-labels=n:m:n2
-falign-labels=n:m:n2:m2
Align all branch targets to a power-of-two boundary.
Parameters of this option are analogous to the -falign-functions option. -fno-align-labels and -falign-labels=1
are equivalent and mean that labels are not aligned.
If -falign-loops or -falign-jumps are applicable and are greater than this value, then their values are used
instead.
If n is not specified or is zero, use a machine-dependent default which is very likely to be 1, meaning no
alignment. The maximum allowed n option value is 65536.
Enabled at levels -O2, -O3.
-falign-loops
-falign-loops=n
-falign-loops=n:m
-falign-loops=n:m:n2
-falign-loops=n:m:n2:m2
Align loops to a power-of-two boundary. If the loops are executed many times, this makes up for any execution of
the dummy padding instructions. This is an optimization of code performance and alignment is ignored for loops
considered cold.
If -falign-labels is greater than this value, then its value is used instead.
Parameters of this option are analogous to the -falign-functions option. -fno-align-loops and -falign-loops=1
are equivalent and mean that loops are not aligned. The maximum allowed n option value is 65536.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3.
-falign-jumps
-falign-jumps=n
-falign-jumps=n:m
-falign-jumps=n:m:n2
-falign-jumps=n:m:n2:m2
Align branch targets to a power-of-two boundary, for branch targets where the targets can only be reached by
jumping. In this case, no dummy operations need be executed. This is an optimization of code performance and
alignment is ignored for jumps considered cold.
If -falign-labels is greater than this value, then its value is used instead.
Parameters of this option are analogous to the -falign-functions option. -fno-align-jumps and -falign-jumps=1
are equivalent and mean that loops are not aligned.
If n is not specified or is zero, use a machine-dependent default. The maximum allowed n option value is 65536.
Enabled at levels -O2, -O3.
-fmin-function-alignment
Specify minimal alignment of functions to the next power-of-two greater than or equal to n. Unlike
-falign-functions this alignment is applied also to all functions (even those considered cold). The alignment is
also not affected by -flimit-function-alignment
-fno-allocation-dce
Do not remove unused C++ allocations (using operator "new" and operator "delete") in dead code elimination.
See also -fmalloc-dce.
-fallow-store-data-races
Allow the compiler to perform optimizations that may introduce new data races on stores, without proving that the
variable cannot be concurrently accessed by other threads. Does not affect optimization of local data. It is
safe to use this option if it is known that global data will not be accessed by multiple threads.
Examples of optimizations enabled by -fallow-store-data-races include hoisting or if-conversions that may cause a
value that was already in memory to be re-written with that same value. Such re-writing is safe in a single
threaded context but may be unsafe in a multi-threaded context. Note that on some processors, if-conversions may
be required in order to enable vectorization.
Enabled at level -Ofast.
-funit-at-a-time
This option is left for compatibility reasons. -funit-at-a-time has no effect, while -fno-unit-at-a-time implies
-fno-toplevel-reorder and -fno-section-anchors.
Enabled by default.
-fno-toplevel-reorder
Do not reorder top-level functions, variables, and "asm" statements. Output them in the same order that they
appear in the input file. When this option is used, unreferenced static variables are not removed. This option
is intended to support existing code that relies on a particular ordering. For new code, it is better to use
attributes when possible.
-ftoplevel-reorder is the default at -O1 and higher, and also at -O0 if -fsection-anchors is explicitly
requested. Additionally -fno-toplevel-reorder implies -fno-section-anchors.
-funreachable-traps
With this option, the compiler turns calls to "__builtin_unreachable" into traps, instead of using them for
optimization. This also affects any such calls implicitly generated by the compiler.
This option has the same effect as -fsanitize=unreachable -fsanitize-trap=unreachable, but does not affect the
values of those options. If -fsanitize=unreachable is enabled, that option takes priority over this one.
This option is enabled by default at -O0 and -Og.
-fweb
Constructs webs as commonly used for register allocation purposes and assign each web individual pseudo register.
This allows the register allocation pass to operate on pseudos directly, but also strengthens several other
optimization passes, such as CSE, loop optimizer and trivial dead code remover. It can, however, make debugging
impossible, since variables no longer stay in a "home register".
Enabled by default with -funroll-loops.
-fwhole-program
Assume that the current compilation unit represents the whole program being compiled. All public functions and
variables with the exception of "main" and those merged by attribute "externally_visible" become static functions
and in effect are optimized more aggressively by interprocedural optimizers.
With -flto this option has a limited use. In most cases the precise list of symbols used or exported from the
binary is known the resolution info passed to the link-time optimizer by the linker plugin. It is still useful
if no linker plugin is used or during incremental link step when final code is produced (with -flto
-flinker-output=nolto-rel).
-flto[=n]
This option runs the standard link-time optimizer. When invoked with source code, it generates GIMPLE (one of
GCC's internal representations) and writes it to special ELF sections in the object file. When the object files
are linked together, all the function bodies are read from these ELF sections and instantiated as if they had
been part of the same translation unit.
To use the link-time optimizer, -flto and optimization options should be specified at compile time and during the
final link. It is recommended that you compile all the files participating in the same link with the same
options and also specify those options at link time. For example:
gcc -c -O2 -flto foo.c
gcc -c -O2 -flto bar.c
gcc -o myprog -flto -O2 foo.o bar.o
The first two invocations to GCC save a bytecode representation of GIMPLE into special ELF sections inside foo.o
and bar.o. The final invocation reads the GIMPLE bytecode from foo.o and bar.o, merges the two files into a
single internal image, and compiles the result as usual. Since both foo.o and bar.o are merged into a single
image, this causes all the interprocedural analyses and optimizations in GCC to work across the two files as if
they were a single one. This means, for example, that the inliner is able to inline functions in bar.o into
functions in foo.o and vice-versa.
Another (simpler) way to enable link-time optimization is:
gcc -o myprog -flto -O2 foo.c bar.c
The above generates bytecode for foo.c and bar.c, merges them together into a single GIMPLE representation and
optimizes them as usual to produce myprog.
The important thing to keep in mind is that to enable link-time optimizations you need to use the GCC driver to
perform the link step. GCC automatically performs link-time optimization if any of the objects involved were
compiled with the -flto command-line option. You can always override the automatic decision to do link-time
optimization by passing -fno-lto to the link command.
To make whole program optimization effective, it is necessary to make certain whole program assumptions. The
compiler needs to know what functions and variables can be accessed by libraries and runtime outside of the link-
time optimized unit. When supported by the linker, the linker plugin (see -fuse-linker-plugin) passes
information to the compiler about used and externally visible symbols. When the linker plugin is not available,
-fwhole-program should be used to allow the compiler to make these assumptions, which leads to more aggressive
optimization decisions.
When a file is compiled with -flto without -fuse-linker-plugin, the generated object file is larger than a
regular object file because it contains GIMPLE bytecodes and the usual final code (see -ffat-lto-objects). This
means that object files with LTO information can be linked as normal object files; if -fno-lto is passed to the
linker, no interprocedural optimizations are applied. Note that when -fno-fat-lto-objects is enabled the compile
stage is faster but you cannot perform a regular, non-LTO link on them.
When producing the final binary, GCC only applies link-time optimizations to those files that contain bytecode.
Therefore, you can mix and match object files and libraries with GIMPLE bytecodes and final object code. GCC
automatically selects which files to optimize in LTO mode and which files to link without further processing.
Generally, options specified at link time override those specified at compile time, although in some cases GCC
attempts to infer link-time options from the settings used to compile the input files.
If you do not specify an optimization level option -O at link time, then GCC uses the highest optimization level
used when compiling the object files. Note that it is generally ineffective to specify an optimization level
option only at link time and not at compile time, for two reasons. First, compiling without optimization
suppresses compiler passes that gather information needed for effective optimization at link time. Second, some
early optimization passes can be performed only at compile time and not at link time.
There are some code generation flags preserved by GCC when generating bytecodes, as they need to be used during
the final link. Currently, the following options and their settings are taken from the first object file that
explicitly specifies them: -fcommon, -fexceptions, -fnon-call-exceptions, -fgnu-tm and all the -m target flags.
The following options -fPIC, -fpic, -fpie and -fPIE are combined based on the following scheme:
B<-fPIC> + B<-fpic> = B<-fpic>
B<-fPIC> + B<-fno-pic> = B<-fno-pic>
B<-fpic/-fPIC> + (no option) = (no option)
B<-fPIC> + B<-fPIE> = B<-fPIE>
B<-fpic> + B<-fPIE> = B<-fpie>
B<-fPIC/-fpic> + B<-fpie> = B<-fpie>
Certain ABI-changing flags are required to match in all compilation units, and trying to override this at link
time with a conflicting value is ignored. This includes options such as -freg-struct-return and
-fpcc-struct-return.
Other options such as -ffp-contract, -fno-strict-overflow, -fwrapv, -fno-trapv or -fno-strict-aliasing are passed
through to the link stage and merged conservatively for conflicting translation units. Specifically
-fno-strict-overflow, -fwrapv and -fno-trapv take precedence; and for example -ffp-contract=off takes precedence
over -ffp-contract=fast. You can override them at link time.
Diagnostic options such as -Wstringop-overflow are passed through to the link stage and their setting matches
that of the compile-step at function granularity. Note that this matters only for diagnostics emitted during
optimization. Note that code transforms such as inlining can lead to warnings being enabled or disabled for
regions if code not consistent with the setting at compile time.
When you need to pass options to the assembler via -Wa or -Xassembler make sure to either compile such
translation units with -fno-lto or consistently use the same assembler options on all translation units. You can
alternatively also specify assembler options at LTO link time.
To enable debug info generation you need to supply -g at compile time. If any of the input files at link time
were built with debug info generation enabled the link will enable debug info generation as well. Any elaborate
debug info settings like the dwarf level -gdwarf-5 need to be explicitly repeated at the linker command line and
mixing different settings in different translation units is discouraged.
If LTO encounters objects with C linkage declared with incompatible types in separate translation units to be
linked together (undefined behavior according to ISO C99 6.2.7), a non-fatal diagnostic may be issued. The
behavior is still undefined at run time. Similar diagnostics may be raised for other languages.
Another feature of LTO is that it is possible to apply interprocedural optimizations on files written in
different languages:
gcc -c -flto foo.c
g++ -c -flto bar.cc
gfortran -c -flto baz.f90
g++ -o myprog -flto -O3 foo.o bar.o baz.o -lgfortran
Notice that the final link is done with g++ to get the C++ runtime libraries and -lgfortran is added to get the
Fortran runtime libraries. In general, when mixing languages in LTO mode, you should use the same link command
options as when mixing languages in a regular (non-LTO) compilation.
If object files containing GIMPLE bytecode are stored in a library archive, say libfoo.a, it is possible to
extract and use them in an LTO link if you are using a linker with plugin support. To create static libraries
suitable for LTO, use gcc-ar and gcc-ranlib instead of ar and ranlib; to show the symbols of object files with
GIMPLE bytecode, use gcc-nm. Those commands require that ar, ranlib and nm have been compiled with plugin
support. At link time, use the flag -fuse-linker-plugin to ensure that the library participates in the LTO
optimization process:
gcc -o myprog -O2 -flto -fuse-linker-plugin a.o b.o -lfoo
With the linker plugin enabled, the linker extracts the needed GIMPLE files from libfoo.a and passes them on to
the running GCC to make them part of the aggregated GIMPLE image to be optimized.
If you are not using a linker with plugin support and/or do not enable the linker plugin, then the objects inside
libfoo.a are extracted and linked as usual, but they do not participate in the LTO optimization process. In
order to make a static library suitable for both LTO optimization and usual linkage, compile its object files
with -flto -ffat-lto-objects.
Link-time optimizations do not require the presence of the whole program to operate. If the program does not
require any symbols to be exported, it is possible to combine -flto and -fwhole-program to allow the
interprocedural optimizers to use more aggressive assumptions which may lead to improved optimization
opportunities. Use of -fwhole-program is not needed when linker plugin is active (see -fuse-linker-plugin).
The current implementation of LTO makes no attempt to generate bytecode that is portable between different types
of hosts. The bytecode files are versioned and there is a strict version check, so bytecode files generated in
one version of GCC do not work with an older or newer version of GCC.
Link-time optimization does not work well with generation of debugging information on systems other than those
using a combination of ELF and DWARF.
If you specify the optional n, the optimization and code generation done at link time is executed in parallel
using n parallel jobs by utilizing an installed make program. The environment variable MAKE may be used to
override the program used.
You can also specify -flto=jobserver to use GNU make's job server mode to determine the number of parallel jobs.
This is useful when the Makefile calling GCC is already executing in parallel. You must prepend a + to the
command recipe in the parent Makefile for this to work. This option likely only works if MAKE is GNU make. Even
without the option value, GCC tries to automatically detect a running GNU make's job server.
Use -flto=auto to use GNU make's job server, if available, or otherwise fall back to autodetection of the number
of CPU threads present in your system.
-flto-partition=alg
Specify the partitioning algorithm used by the link-time optimizer. The value is either 1to1 to specify a
partitioning mirroring the original source files or balanced to specify partitioning into equally sized chunks
(whenever possible) or max to create new partition for every symbol where possible or cache to balance chunk
sizes while keeping related symbols together for better caching in incremental LTO. Specifying none as an
algorithm disables partitioning and streaming completely. The default value is balanced. While 1to1 can be used
as an workaround for various code ordering issues, the max partitioning is intended for internal testing only.
The value one specifies that exactly one partition should be used while the value none bypasses partitioning and
executes the link-time optimization step directly from the WPA phase.
-flto-incremental=path
Enable incremental LTO, with its cache in given existing directory. Can significantly shorten edit-compile
cycles with LTO.
When used with LTO (-flto), the output of translation units inside LTO is cached. Cached translation units are
likely to be encountered again when recompiling with small code changes, leading to recompile time reduction.
Multiple GCC instances can use the same cache in parallel.
-flto-incremental-cache-size=n
Specifies number of cache entries in incremental LTO after which to prune old entries. This is a soft limit,
temporarily there may be more entries.
-flto-compression-level=n
This option specifies the level of compression used for intermediate language written to LTO object files, and is
only meaningful in conjunction with LTO mode (-flto). GCC currently supports two LTO compression algorithms. For
zstd, valid values are 0 (no compression) to 19 (maximum compression), while zlib supports values from 0 to 9.
Values outside this range are clamped to either minimum or maximum of the supported values. If the option is not
given, a default balanced compression setting is used.
-fuse-linker-plugin
Enables the use of a linker plugin during link-time optimization. This option relies on plugin support in the
linker, which is available in gold or in GNU ld 2.21 or newer.
This option enables the extraction of object files with GIMPLE bytecode out of library archives. This improves
the quality of optimization by exposing more code to the link-time optimizer. This information specifies what
symbols can be accessed externally (by non-LTO object or during dynamic linking). Resulting code quality
improvements on binaries (and shared libraries that use hidden visibility) are similar to -fwhole-program. See
-flto for a description of the effect of this flag and how to use it.
This option is enabled by default when LTO support in GCC is enabled and GCC was configured for use with a linker
supporting plugins (GNU ld 2.21 or newer or gold).
-ffat-lto-objects
Fat LTO objects are object files that contain both the intermediate language and the object code. This makes them
usable for both LTO linking and normal linking. This option is effective only when compiling with -flto and is
ignored at link time.
-fno-fat-lto-objects improves compilation time over plain LTO, but requires the complete toolchain to be aware of
LTO. It requires a linker with linker plugin support for basic functionality. Additionally, nm, ar and ranlib
need to support linker plugins to allow a full-featured build environment (capable of building static libraries
etc). GCC provides the gcc-ar, gcc-nm, gcc-ranlib wrappers to pass the right options to these tools. With non
fat LTO makefiles need to be modified to use them.
Note that modern binutils provide plugin auto-load mechanism. Installing the linker plugin into
$libdir/bfd-plugins has the same effect as usage of the command wrappers (gcc-ar, gcc-nm and gcc-ranlib).
The default is -fno-fat-lto-objects on targets with linker plugin support.
-fcompare-elim
After register allocation and post-register allocation instruction splitting, identify arithmetic instructions
that compute processor flags similar to a comparison operation based on that arithmetic. If possible, eliminate
the explicit comparison operation.
This pass only applies to certain targets that cannot explicitly represent the comparison operation before
register allocation is complete.
Enabled at levels -O1, -O2, -O3, -Os.
-ffold-mem-offsets
-fno-fold-mem-offsets
Try to eliminate add instructions by folding them in memory loads/stores.
Enabled at levels -O2, -O3.
NOTE: On Debian/Ubuntu this is disabled by default on m68k-linux-gnu.
-fcprop-registers
After register allocation and post-register allocation instruction splitting, perform a copy-propagation pass to
try to reduce scheduling dependencies and occasionally eliminate the copy.
Enabled at levels -O1, -O2, -O3, -Os.
-fprofile-correction
Profiles collected using an instrumented binary for multi-threaded programs may be inconsistent due to missed
counter updates. When this option is specified, GCC uses heuristics to correct or smooth out such
inconsistencies. By default, GCC emits an error message when an inconsistent profile is detected.
This option is enabled by -fauto-profile.
-fprofile-partial-training
With "-fprofile-use" all portions of programs not executed during train run are optimized aggressively for size
rather than speed. In some cases it is not practical to train all possible hot paths in the program. (For
example, program may contain functions specific for a given hardware and training may not cover all hardware
configurations program is run on.) With "-fprofile-partial-training" profile feedback is ignored for all
functions not executed during the train run, leading them to be optimized as if they were compiled without
profile feedback. This leads to better performance when train run is not representative but also leads to
significantly bigger code.
-fprofile-use
-fprofile-use=path
Enable profile feedback-directed optimizations, and the following optimizations, many of which are generally
profitable only with profile feedback available:
-fbranch-probabilities -fprofile-values -funroll-loops -fpeel-loops -ftracer -fvpt -finline-functions
-fipa-cp -fipa-cp-clone -fipa-bit-cp -fpredictive-commoning -fsplit-loops -funswitch-loops
-fgcse-after-reload -ftree-loop-vectorize -ftree-slp-vectorize -fvect-cost-model=dynamic
-ftree-loop-distribute-patterns -fprofile-reorder-functions
Before you can use this option, you must first generate profiling information.
By default, GCC emits an error message if the feedback profiles do not match the source code. This error can be
turned into a warning by using -Wno-error=coverage-mismatch. Note this may result in poorly optimized code.
Additionally, by default, GCC also emits a warning message if the feedback profiles do not exist (see
-Wmissing-profile).
If path is specified, GCC looks at the path to find the profile feedback data files. See -fprofile-dir.
-fauto-profile
-fauto-profile=path
Enable sampling-based feedback-directed optimizations, and the following optimizations, many of which are
generally profitable only with profile feedback available:
-fbranch-probabilities -fprofile-values -funroll-loops -fpeel-loops -ftracer -fvpt -finline-functions
-fipa-cp -fipa-cp-clone -fipa-bit-cp -fpredictive-commoning -fsplit-loops -funswitch-loops
-fgcse-after-reload -ftree-loop-vectorize -ftree-slp-vectorize -fvect-cost-model=dynamic
-ftree-loop-distribute-patterns -fprofile-correction
path is the name of a file containing AutoFDO profile information. If omitted, it defaults to fbdata.afdo in the
current directory.
Producing an AutoFDO profile data file requires running your program with the perf utility on a supported
GNU/Linux target system. For more information, see .
E.g.
perf record -e br_inst_retired:near_taken -b -o perf.data \
-- your_program
Then use the create_gcov tool to convert the raw profile data to a format that can be used by GCC. You must also
supply the unstripped binary for your program to this tool. See .
E.g.
create_gcov --binary=your_program.unstripped --profile=perf.data \
--gcov=profile.afdo
The following options control compiler behavior regarding floating-point arithmetic. These options trade off between
speed and correctness. All must be specifically enabled.
-fexcess-precision=style
This option allows control over excess precision on machines where floating-point operations occur in a format
with more precision or range than the IEEE standard and interchange floating-point types. An example of such a
target is x87 floating point on x86 processors, which uses an 80-bit representation internally instead of the
64-bit IEEE format. For most programs, the excess precision is harmless, but some programs may rely on the
requirements of the C or C++ language standards for handling IEEE values.
By default, -fexcess-precision=fast is in effect; this means that operations may be carried out in a wider
precision than the types specified in the source if that would result in faster code, and it is unpredictable
when rounding to the types specified in the source code takes place. When compiling C or C++, if
-fexcess-precision=standard is specified then excess precision follows the rules specified in ISO C99 or C++; in
particular, both casts and assignments cause values to be rounded to their semantic types (whereas -ffloat-store
only affects assignments). This option is enabled by default for C or C++ if a strict conformance option such as
-std=c99 or -std=c++17 is used. -ffast-math enables -fexcess-precision=fast by default regardless of whether a
strict conformance option is used. If -fexcess-precision=16 is specified, constants and the results of
expressions with types "_Float16" and "__bf16" are computed without excess precision.
-fexcess-precision=standard is not implemented for languages other than C or C++. On the x86, it has no effect
if -mfpmath=sse or -mfpmath=sse+387 is specified; in the former case, IEEE semantics apply without excess
precision, and in the latter, rounding is unpredictable.
-ffloat-store
Do not store floating-point variables in registers, and inhibit other options that might change whether a
floating-point value is taken from a register or memory. This option has generally been subsumed by
-fexcess-precision=standard, which is more general. If you do use -ffloat-store, you may need to modify your
program to explicitly store intermediate computations in temporary variables since -ffloat-store handles rounding
to IEEE format only on assignments and not casts as -fexcess-precision=standard does.
-ffast-math
Sets the options -fno-math-errno, -funsafe-math-optimizations, -ffinite-math-only, -fno-rounding-math,
-fno-signaling-nans, -fcx-limited-range and -fexcess-precision=fast.
This option causes the preprocessor macro "__FAST_MATH__" to be defined.
This option is not turned on by any -O option besides -Ofast since it can result in incorrect output for programs
that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however,
yield faster code for programs that do not require the guarantees of these specifications.
-fno-math-errno
Do not set "errno" after calling math functions that are executed with a single instruction, e.g., "sqrt". A
program that relies on IEEE exceptions for math error handling may want to use this flag for speed while
maintaining IEEE arithmetic compatibility.
This option is not turned on by any -O option besides -Ofast since it can result in incorrect output for
programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may,
however, yield faster code for programs that do not require the guarantees of these specifications.
The default is -fmath-errno.
On Darwin systems, the math library never sets "errno". There is therefore no reason for the compiler to
consider the possibility that it might, and -fno-math-errno is the default.
-funsafe-math-optimizations
Allow optimizations for floating-point arithmetic that (a) assume that arguments and results are valid and (b)
may violate IEEE or ANSI standards. When used at link time, it may include libraries or startup files that
change the default FPU control word or other similar optimizations.
This option is not turned on by any -O option besides -Ofast since it can result in incorrect output for programs
that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however,
yield faster code for programs that do not require the guarantees of these specifications. Enables
-fno-signed-zeros, -fno-trapping-math, -fassociative-math and -freciprocal-math.
The default is -fno-unsafe-math-optimizations.
-fassociative-math
Allow re-association of operands in series of floating-point operations. This violates the ISO C and C++
language standard by possibly changing computation result. NOTE: re-ordering may change the sign of zero as well
as ignore NaNs and inhibit or create underflow or overflow (and thus cannot be used on code that relies on
rounding behavior like "(x + 2**52) - 2**52". May also reorder floating-point comparisons and thus may not be
used when ordered comparisons are required. This option requires that both -fno-signed-zeros and
-fno-trapping-math be in effect. Moreover, it doesn't make much sense with -frounding-math. For Fortran the
option is automatically enabled when both -fno-signed-zeros and -fno-trapping-math are in effect.
The default is -fno-associative-math.
-freciprocal-math
Allow the reciprocal of a value to be used instead of dividing by the value if this enables optimizations. For
example "x / y" can be replaced with "x * (1/y)", which is useful if "(1/y)" is subject to common subexpression
elimination. Note that this loses precision and increases the number of flops operating on the value.
The default is -fno-reciprocal-math.
-ffinite-math-only
Allow optimizations for floating-point arithmetic that assume that arguments and results are not NaNs or +-Infs.
This option is not turned on by any -O option besides -Ofast since it can result in incorrect output for programs
that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however,
yield faster code for programs that do not require the guarantees of these specifications.
The default is -fno-finite-math-only.
-fno-signed-zeros
Allow optimizations for floating-point arithmetic that ignore the signedness of zero. IEEE arithmetic specifies
the behavior of distinct +0.0 and -0.0 values, which then prohibits simplification of expressions such as x+0.0
or 0.0*x (even with -ffinite-math-only). This option implies that the sign of a zero result isn't significant.
The default is -fsigned-zeros.
-fno-trapping-math
Compile code assuming that floating-point operations cannot generate user-visible traps. These traps include
division by zero, overflow, underflow, inexact result and invalid operation. This option requires that
-fno-signaling-nans be in effect. Setting this option may allow faster code if one relies on "non-stop" IEEE
arithmetic, for example.
This option is not turned on by any -O option besides -Ofast since it can result in incorrect output for programs
that depend on an exact implementation of IEEE or ISO rules/specifications for math functions.
The default is -ftrapping-math.
Future versions of GCC may provide finer control of this setting using C99's "FENV_ACCESS" pragma. This command-
line option will be used along with -frounding-math to specify the default state for "FENV_ACCESS".
-frounding-math
Disable transformations and optimizations that assume default floating-point rounding behavior (round-to-
nearest). This option should be specified for programs that change the FP rounding mode dynamically, or that may
be executed with a non-default rounding mode. This option disables constant folding of floating-point
expressions at compile time (which may be affected by rounding mode) and arithmetic transformations that are
unsafe in the presence of sign-dependent rounding modes.
The default is -fno-rounding-math.
This option is experimental and does not currently guarantee to disable all GCC optimizations that are affected
by rounding mode. Future versions of GCC may provide finer control of this setting using C99's "FENV_ACCESS"
pragma. This command-line option will be used along with -ftrapping-math to specify the default state for
"FENV_ACCESS".
-fsignaling-nans
Compile code assuming that IEEE signaling NaNs may generate user-visible traps during floating-point operations.
Setting this option disables optimizations that may change the number of exceptions visible with signaling NaNs.
This option implies -ftrapping-math.
This option causes the preprocessor macro "__SUPPORT_SNAN__" to be defined.
The default is -fno-signaling-nans.
This option is experimental and does not currently guarantee to disable all GCC optimizations that affect
signaling NaN behavior.
-fno-fp-int-builtin-inexact
Do not allow the built-in functions "ceil", "floor", "round" and "trunc", and their "float" and "long double"
variants, to generate code that raises the "inexact" floating-point exception for noninteger arguments. ISO C99
and C11 allow these functions to raise the "inexact" exception, but ISO/IEC TS 18661-1:2014, the C bindings to
IEEE 754-2008, as integrated into ISO C23, does not allow these functions to do so.
The default is -ffp-int-builtin-inexact, allowing the exception to be raised, unless C23 or a later C standard is
selected. This option does nothing unless -ftrapping-math is in effect.
Even if -fno-fp-int-builtin-inexact is used, if the functions generate a call to a library function then the
"inexact" exception may be raised if the library implementation does not follow TS 18661.
-fsingle-precision-constant
Treat floating-point constants as single precision instead of implicitly converting them to double-precision
constants.
-fcx-limited-range
When enabled, this option states that a range reduction step is not needed when performing complex division.
Also, there is no checking whether the result of a complex multiplication or division is "NaN + I*NaN", with an
attempt to rescue the situation in that case. The default is -fno-cx-limited-range, but is enabled by
-ffast-math.
This option controls the default setting of the ISO C99 "CX_LIMITED_RANGE" pragma. Nevertheless, the option
applies to all languages.
-fcx-fortran-rules
Complex multiplication and division follow Fortran rules. Range reduction is done as part of complex division,
but there is no checking whether the result of a complex multiplication or division is "NaN + I*NaN", with an
attempt to rescue the situation in that case.
The default is -fno-cx-fortran-rules.
The following options control optimizations that may improve performance, but are not enabled by any -O options.
This section includes experimental options that may produce broken code.
-fbranch-probabilities
After running a program compiled with -fprofile-arcs, you can compile it a second time using
-fbranch-probabilities, to improve optimizations based on the number of times each branch was taken. When a
program compiled with -fprofile-arcs exits, it saves arc execution counts to a file called sourcename.gcda for
each source file. The information in this data file is very dependent on the structure of the generated code, so
you must use the same source code and the same optimization options for both compilations. See details about the
file naming in -fprofile-arcs.
With -fbranch-probabilities, GCC puts a REG_BR_PROB note on each JUMP_INSN and CALL_INSN. These can be used to
improve optimization. Currently, they are only used in one place: in reorg.cc, instead of guessing which path a
branch is most likely to take, the REG_BR_PROB values are used to exactly determine which path is taken more
often.
Enabled by -fprofile-use and -fauto-profile.
-fprofile-values
If combined with -fprofile-arcs, it adds code so that some data about values of expressions in the program is
gathered.
With -fbranch-probabilities, it reads back the data gathered from profiling values of expressions for usage in
optimizations.
Enabled by -fprofile-generate, -fprofile-use, and -fauto-profile.
-fprofile-reorder-functions
Function reordering based on profile instrumentation collects first time of execution of a function and orders
these functions in ascending order, aiming to optimize program startup through more efficient loading of text
segments.
Enabled with -fprofile-use.
-fvpt
If combined with -fprofile-arcs, this option instructs the compiler to add code to gather information about
values of expressions.
With -fbranch-probabilities, it reads back the data gathered and actually performs the optimizations based on
them. Currently the optimizations include specialization of division operations using the knowledge about the
value of the denominator.
Enabled with -fprofile-use and -fauto-profile.
-frename-registers
Attempt to avoid false dependencies in scheduled code by making use of registers left over after register
allocation. This optimization most benefits processors with lots of registers. Depending on the debug
information format adopted by the target, however, it can make debugging impossible, since variables no longer
stay in a "home register".
Enabled by default with -funroll-loops.
-fschedule-fusion
Performs a target dependent pass over the instruction stream to schedule instructions of same type together
because target machine can execute them more efficiently if they are adjacent to each other in the instruction
flow.
Enabled at levels -O2, -O3, -Os.
-ftracer
Perform tail duplication to enlarge superblock size. This transformation simplifies the control flow of the
function allowing other optimizations to do a better job.
Enabled by -fprofile-use and -fauto-profile.
-funroll-loops
Unroll loops whose number of iterations can be determined at compile time or upon entry to the loop.
-funroll-loops implies -frerun-cse-after-loop, -fweb and -frename-registers. It also turns on complete loop
peeling (i.e. complete removal of loops with a small constant number of iterations). This option makes code
larger, and may or may not make it run faster.
Enabled by -fprofile-use and -fauto-profile.
-funroll-all-loops
Unroll all loops, even if their number of iterations is uncertain when the loop is entered. This usually makes
programs run more slowly. -funroll-all-loops implies the same options as -funroll-loops.
-fpeel-loops
Peels loops for which there is enough information that they do not roll much (from profile feedback or static
analysis). It also turns on complete loop peeling (i.e. complete removal of loops with small constant number of
iterations).
Enabled by -O3, -fprofile-use, and -fauto-profile.
-fmalloc-dce
Control whether "malloc" (and its variants such as "calloc" or "strdup"), can be optimized away provided its
return value is only used as a parameter of "free" call or compared with "NULL". If -fmalloc-dce=1 is used, only
calls to "free" are allowed while with -fmalloc-dce=2 also comparisons with "NULL" pointer are considered safe to
remove.
The default is -fmalloc-dce=2. See also -fallocation-dce.
-fmove-loop-invariants
Enables the loop invariant motion pass in the RTL loop optimizer. Enabled at level -O1 and higher, except for
-Og.
-fmove-loop-stores
Enables the loop store motion pass in the GIMPLE loop optimizer. This moves invariant stores to after the end of
the loop in exchange for carrying the stored value in a register across the iteration. Note for this option to
have an effect -ftree-loop-im has to be enabled as well. Enabled at level -O1 and higher, except for -Og.
-fsplit-loops
Split a loop into two if it contains a condition that's always true for one side of the iteration space and false
for the other.
Enabled by -fprofile-use and -fauto-profile.
-funswitch-loops
Move branches with loop invariant conditions out of the loop, with duplicates of the loop on both branches
(modified according to result of the condition).
Enabled by -fprofile-use and -fauto-profile.
-fversion-loops-for-strides
If a loop iterates over an array with a variable stride, create another version of the loop that assumes the
stride is always one. For example:
for (int i = 0; i < n; ++i)
x[i * stride] = ...;
becomes:
if (stride == 1)
for (int i = 0; i < n; ++i)
x[i] = ...;
else
for (int i = 0; i < n; ++i)
x[i * stride] = ...;
This is particularly useful for assumed-shape arrays in Fortran where (for example) it allows better
vectorization assuming contiguous accesses. This flag is enabled by default at -O3. It is also enabled by
-fprofile-use and -fauto-profile.
-ffunction-sections
-fdata-sections
Place each function or data item into its own section in the output file if the target supports arbitrary
sections. The name of the function or the name of the data item determines the section's name in the output
file.
Use these options on systems where the linker can perform optimizations to improve locality of reference in the
instruction space. Most systems using the ELF object format have linkers with such optimizations. On AIX, the
linker rearranges sections (CSECTs) based on the call graph. The performance impact varies.
Together with a linker garbage collection (linker --gc-sections option) these options may lead to smaller
statically-linked executables (after stripping).
On ELF/DWARF systems these options do not degenerate the quality of the debug information. There could be issues
with other object files/debug info formats.
Only use these options when there are significant benefits from doing so. When you specify these options, the
assembler and linker create larger object and executable files and are also slower. These options affect code
generation. They prevent optimizations by the compiler and assembler using relative locations inside a
translation unit since the locations are unknown until link time. An example of such an optimization is relaxing
calls to short call instructions.
-fstdarg-opt
Optimize the prologue of variadic argument functions with respect to usage of those arguments.
-fsection-anchors
Try to reduce the number of symbolic address calculations by using shared "anchor" symbols to address nearby
objects. This transformation can help to reduce the number of GOT entries and GOT accesses on some targets.
For example, the implementation of the following function "foo":
static int a, b, c;
int foo (void) { return a + b + c; }
usually calculates the addresses of all three variables, but if you compile it with -fsection-anchors, it
accesses the variables from a common anchor point instead. The effect is similar to the following pseudocode
(which isn't valid C):
int foo (void)
{
register int *xr = &x;
return xr[&a - &x] + xr[&b - &x] + xr[&c - &x];
}
Not all targets support this option.
-fzero-call-used-regs=choice
Zero call-used registers at function return to increase program security by either mitigating Return-Oriented
Programming (ROP) attacks or preventing information leakage through registers.
The possible values of choice are the same as for the "zero_call_used_regs" attribute. The default is skip.
You can control this behavior for a specific function by using the function attribute "zero_call_used_regs".
--param name=value
In some places, GCC uses various constants to control the amount of optimization that is done. For example, GCC
does not inline functions that contain more than a certain number of instructions. You can control some of these
constants on the command line using the --param option.
The names of specific parameters, and the meaning of the values, are tied to the internals of the compiler, and
are subject to change without notice in future releases.
In order to get the minimal, maximal and default values of a parameter, use the --help=param -Q options.
In each case, the value is an integer. The following choices of name are recognized for all targets:
phiopt-factor-max-stmts-live
When factoring statements out of if/then/else, this is the max # of statements after the defining statement
to be allow to extend the lifetime of a name
predictable-branch-outcome
When branch is predicted to be taken with probability lower than this threshold (in percent), then it is
considered well predictable.
max-rtl-if-conversion-insns
RTL if-conversion tries to remove conditional branches around a block and replace them with conditionally
executed instructions. This parameter gives the maximum number of instructions in a block which should be
considered for if-conversion. The compiler will also use other heuristics to decide whether if-conversion is
likely to be profitable.
file-cache-files
Max number of files in the file cache. The file cache is used to print source lines in diagnostics and do
some source checks like -Wmisleading-indentation.
file-cache-lines
Max number of lines to index into file cache. When 0 this is automatically sized. The file cache is used to
print source lines in diagnostics and do some source checks like -Wmisleading-indentation.
max-rtl-if-conversion-predictable-cost
RTL if-conversion will try to remove conditional branches around a block and replace them with conditionally
executed instructions. These parameters give the maximum permissible cost for the sequence that would be
generated by if-conversion depending on whether the branch is statically determined to be predictable or not.
The units for this parameter are the same as those for the GCC internal seq_cost metric. The compiler will
try to provide a reasonable default for this parameter using the BRANCH_COST target macro.
max-crossjump-edges
The maximum number of incoming edges to consider for cross-jumping. The algorithm used by -fcrossjumping is
O(N^2) in the number of edges incoming to each block. Increasing values mean more aggressive optimization,
making the compilation time increase with probably small improvement in executable size.
min-crossjump-insns
The minimum number of instructions that must be matched at the end of two blocks before cross-jumping is
performed on them. This value is ignored in the case where all instructions in the block being cross-jumped
from are matched.
max-grow-copy-bb-insns
The maximum code size expansion factor when copying basic blocks instead of jumping. The expansion is
relative to a jump instruction.
max-goto-duplication-insns
The maximum number of instructions to duplicate to a block that jumps to a computed goto. To avoid O(N^2)
behavior in a number of passes, GCC factors computed gotos early in the compilation process, and unfactors
them as late as possible. Only computed jumps at the end of a basic blocks with no more than max-goto-
duplication-insns are unfactored.
max-delay-slot-insn-search
The maximum number of instructions to consider when looking for an instruction to fill a delay slot. If more
than this arbitrary number of instructions are searched, the time savings from filling the delay slot are
minimal, so stop searching. Increasing values mean more aggressive optimization, making the compilation time
increase with probably small improvement in execution time.
max-delay-slot-live-search
When trying to fill delay slots, the maximum number of instructions to consider when searching for a block
with valid live register information. Increasing this arbitrarily chosen value means more aggressive
optimization, increasing the compilation time. This parameter should be removed when the delay slot code is
rewritten to maintain the control-flow graph.
max-gcse-memory
The approximate maximum amount of memory in "kB" that can be allocated in order to perform the global common
subexpression elimination optimization. If more memory than specified is required, the optimization is not
done.
max-gcse-insertion-ratio
If the ratio of expression insertions to deletions is larger than this value for any expression, then RTL PRE
inserts or removes the expression and thus leaves partially redundant computations in the instruction stream.
max-pending-list-length
The maximum number of pending dependencies scheduling allows before flushing the current state and starting
over. Large functions with few branches or calls can create excessively large lists which needlessly consume
memory and resources.
max-modulo-backtrack-attempts
The maximum number of backtrack attempts the scheduler should make when modulo scheduling a loop. Larger
values can exponentially increase compilation time.
max-inline-functions-called-once-loop-depth
Maximal loop depth of a call considered by inline heuristics that tries to inline all functions called once.
max-inline-functions-called-once-insns
Maximal estimated size of functions produced while inlining functions called once.
max-inline-insns-single
Several parameters control the tree inliner used in GCC. This number sets the maximum number of instructions
(counted in GCC's internal representation) in a single function that the tree inliner considers for inlining.
This only affects functions declared inline and methods implemented in a class declaration (C++).
max-inline-insns-auto
When you use -finline-functions (included in -O3), a lot of functions that would otherwise not be considered
for inlining by the compiler are investigated. To those functions, a different (more restrictive) limit
compared to functions declared inline can be applied (--param max-inline-insns-auto).
max-inline-insns-small
This is the bound applied to calls that are considered relevant with -finline-small-functions.
max-inline-insns-size
This is the bound applied to calls that are optimized for size. Small growth may be desirable to anticipate
optimization opportunities exposed by inlining.
uninlined-function-insns
Number of instructions accounted by inliner for function overhead such as function prologue and epilogue.
uninlined-function-time
Extra time accounted by inliner for function overhead such as time needed to execute function prologue and
epilogue.
inline-heuristics-hint-percent
The scale (in percents) applied to inline-insns-single, inline-insns-single-O2, inline-insns-auto when inline
heuristics hints that inlining is very profitable (will enable later optimizations).
uninlined-thunk-insns
uninlined-thunk-time
Same as --param uninlined-function-insns and --param uninlined-function-time but applied to function thunks.
inline-min-speedup
When estimated performance improvement of caller + callee runtime exceeds this threshold (in percent), the
function can be inlined regardless of the limit on --param max-inline-insns-single and --param max-inline-
insns-auto.
large-function-insns
The limit specifying really large functions. For functions larger than this limit after inlining, inlining
is constrained by --param large-function-growth. This parameter is useful primarily to avoid extreme
compilation time caused by non-linear algorithms used by the back end.
large-function-growth
Specifies maximal growth of large functions caused by inlining in percents. For example, parameter value 100
limits large function growth to 2.0 times the original size.
large-unit-insns
The limit specifying large translation unit. Growth caused by inlining of units larger than this limit is
limited by --param inline-unit-growth. For small units this might be too tight. For example, consider a
unit consisting of function A that is inline and B that just calls A three times. If B is small relative to
A, the growth of unit is 300\% and yet such inlining is very sane. For very large units consisting of small
inlineable functions, however, the overall unit growth limit is needed to avoid exponential explosion of code
size. Thus for smaller units, the size is increased to --param large-unit-insns before applying --param
inline-unit-growth.
lazy-modules
Maximum number of concurrently open C++ module files when lazy loading.
inline-unit-growth
Specifies maximal overall growth of the compilation unit caused by inlining. For example, parameter value 20
limits unit growth to 1.2 times the original size. Cold functions (either marked cold via an attribute or by
profile feedback) are not accounted into the unit size.
ipa-cp-unit-growth
Specifies maximal overall growth of the compilation unit caused by interprocedural constant propagation. For
example, parameter value 10 limits unit growth to 1.1 times the original size.
ipa-cp-large-unit-insns
The size of translation unit that IPA-CP pass considers large.
large-stack-frame
The limit specifying large stack frames. While inlining the algorithm is trying to not grow past this limit
too much.
large-stack-frame-growth
Specifies maximal growth of large stack frames caused by inlining in percents. For example, parameter value
1000 limits large stack frame growth to 11 times the original size.
max-inline-insns-recursive
max-inline-insns-recursive-auto
Specifies the maximum number of instructions an out-of-line copy of a self-recursive inline function can grow
into by performing recursive inlining.
--param max-inline-insns-recursive applies to functions declared inline. For functions not declared inline,
recursive inlining happens only when -finline-functions (included in -O3) is enabled; --param max-inline-
insns-recursive-auto applies instead.
max-inline-recursive-depth
max-inline-recursive-depth-auto
Specifies the maximum recursion depth used for recursive inlining.
--param max-inline-recursive-depth applies to functions declared inline. For functions not declared inline,
recursive inlining happens only when -finline-functions (included in -O3) is enabled; --param max-inline-
recursive-depth-auto applies instead.
min-inline-recursive-probability
Recursive inlining is profitable only for function having deep recursion in average and can hurt for function
having little recursion depth by increasing the prologue size or complexity of function body to other
optimizers.
When profile feedback is available (see -fprofile-generate) the actual recursion depth can be guessed from
the probability that function recurses via a given call expression. This parameter limits inlining only to
call expressions whose probability exceeds the given threshold (in percents).
early-inlining-insns
Specify growth that the early inliner can make. In effect it increases the amount of inlining for code
having a large abstraction penalty.
max-early-inliner-iterations
Limit of iterations of the early inliner. This basically bounds the number of nested indirect calls the
early inliner can resolve. Deeper chains are still handled by late inlining.
comdat-sharing-probability
Probability (in percent) that C++ inline function with comdat visibility are shared across multiple
compilation units.
modref-max-bases
modref-max-refs
modref-max-accesses
Specifies the maximal number of base pointers, references and accesses stored for a single function by
mod/ref analysis.
modref-max-tests
Specifies the maxmal number of tests alias oracle can perform to disambiguate memory locations using the
mod/ref information. This parameter ought to be bigger than --param modref-max-bases and --param modref-max-
refs.
modref-max-depth
Specifies the maximum depth of DFS walk used by modref escape analysis. Setting to 0 disables the analysis
completely.
modref-max-escape-points
Specifies the maximum number of escape points tracked by modref per SSA-name.
modref-max-adjustments
Specifies the maximum number the access range is enlarged during modref dataflow analysis.
profile-func-internal-id
A parameter to control whether to use function internal id in profile database lookup. If the value is 0, the
compiler uses an id that is based on function assembler name and filename, which makes old profile data more
tolerant to source changes such as function reordering etc.
min-vect-loop-bound
The minimum number of iterations under which loops are not vectorized when -ftree-vectorize is used. The
number of iterations after vectorization needs to be greater than the value specified by this option to allow
vectorization.
gcse-cost-distance-ratio
Scaling factor in calculation of maximum distance an expression can be moved by GCSE optimizations. This is
currently supported only in the code hoisting pass. The bigger the ratio, the more aggressive code hoisting
is with simple expressions, i.e., the expressions that have cost less than gcse-unrestricted-cost.
Specifying 0 disables hoisting of simple expressions.
gcse-unrestricted-cost
Cost, roughly measured as the cost of a single typical machine instruction, at which GCSE optimizations do
not constrain the distance an expression can travel. This is currently supported only in the code hoisting
pass. The lesser the cost, the more aggressive code hoisting is. Specifying 0 allows all expressions to
travel unrestricted distances.
max-hoist-depth
The depth of search in the dominator tree for expressions to hoist. This is used to avoid quadratic behavior
in hoisting algorithm. The value of 0 does not limit on the search, but may slow down compilation of huge
functions.
max-tail-merge-comparisons
The maximum amount of similar bbs to compare a bb with. This is used to avoid quadratic behavior in tree
tail merging.
max-tail-merge-iterations
The maximum amount of iterations of the pass over the function. This is used to limit compilation time in
tree tail merging.
store-merging-allow-unaligned
Allow the store merging pass to introduce unaligned stores if it is legal to do so.
max-stores-to-merge
The maximum number of stores to attempt to merge into wider stores in the store merging pass.
max-store-chains-to-track
The maximum number of store chains to track at the same time in the attempt to merge them into wider stores
in the store merging pass.
max-stores-to-track
The maximum number of stores to track at the same time in the attemt to to merge them into wider stores in
the store merging pass.
max-unrolled-insns
The maximum number of instructions that a loop may have to be unrolled. If a loop is unrolled, this
parameter also determines how many times the loop code is unrolled.
max-average-unrolled-insns
The maximum number of instructions biased by probabilities of their execution that a loop may have to be
unrolled. If a loop is unrolled, this parameter also determines how many times the loop code is unrolled.
max-unroll-times
The maximum number of unrollings of a single loop.
max-peeled-insns
The maximum number of instructions that a loop may have to be peeled. If a loop is peeled, this parameter
also determines how many times the loop code is peeled.
max-peel-times
The maximum number of peelings of a single loop.
max-peel-branches
The maximum number of branches on the hot path through the peeled sequence.
max-completely-peeled-insns
The maximum number of insns of a completely peeled loop.
max-completely-peel-times
The maximum number of iterations of a loop to be suitable for complete peeling.
max-completely-peel-loop-nest-depth
The maximum depth of a loop nest suitable for complete peeling.
max-unswitch-insns
The maximum number of insns of an unswitched loop.
max-unswitch-depth
The maximum depth of a loop nest to be unswitched.
lim-expensive
The minimum cost of an expensive expression in the loop invariant motion.
min-loop-cond-split-prob
When FDO profile information is available, min-loop-cond-split-prob specifies minimum threshold for
probability of semi-invariant condition statement to trigger loop split.
iv-consider-all-candidates-bound
Bound on number of candidates for induction variables, below which all candidates are considered for each use
in induction variable optimizations. If there are more candidates than this, only the most relevant ones are
considered to avoid quadratic time complexity.
iv-max-considered-uses
The induction variable optimizations give up on loops that contain more induction variable uses.
iv-always-prune-cand-set-bound
If the number of candidates in the set is smaller than this value, always try to remove unnecessary ivs from
the set when adding a new one.
avg-loop-niter
Average number of iterations of a loop.
dse-max-object-size
Maximum size (in bytes) of objects tracked bytewise by dead store elimination. Larger values may result in
larger compilation times.
dse-max-alias-queries-per-store
Maximum number of queries into the alias oracle per store. Larger values result in larger compilation times
and may result in more removed dead stores.
scev-max-expr-size
Bound on size of expressions used in the scalar evolutions analyzer. Large expressions slow the analyzer.
scev-max-expr-complexity
Bound on the complexity of the expressions in the scalar evolutions analyzer. Complex expressions slow the
analyzer.
max-tree-if-conversion-phi-args
Maximum number of arguments in a PHI supported by TREE if conversion unless the loop is marked with simd
pragma.
vect-max-layout-candidates
The maximum number of possible vector layouts (such as permutations) to consider when optimizing to-be-
vectorized code.
vect-max-version-for-alignment-checks
The maximum number of run-time checks that can be performed when doing loop versioning for alignment in the
vectorizer.
vect-max-version-for-alias-checks
The maximum number of run-time checks that can be performed when doing loop versioning for alias in the
vectorizer.
vect-max-peeling-for-alignment
The maximum number of loop peels to enhance access alignment for vectorizer. Value -1 means no limit.
max-iterations-to-track
The maximum number of iterations of a loop the brute-force algorithm for analysis of the number of iterations
of the loop tries to evaluate.
hot-bb-count-fraction
The denominator n of fraction 1/n of the maximal execution count of a basic block in the entire program that
a basic block needs to at least have in order to be considered hot. The default is 10000, which means that a
basic block is considered hot if its execution count is greater than 1/10000 of the maximal execution count.
0 means that it is never considered hot. Used in non-LTO mode.
hot-bb-count-ws-permille
The number of most executed permilles, ranging from 0 to 1000, of the profiled execution of the entire
program to which the execution count of a basic block must be part of in order to be considered hot. The
default is 990, which means that a basic block is considered hot if its execution count contributes to the
upper 990 permilles, or 99.0%, of the profiled execution of the entire program. 0 means that it is never
considered hot. Used in LTO mode.
hot-bb-frequency-fraction
The denominator n of fraction 1/n of the execution frequency of the entry block of a function that a basic
block of this function needs to at least have in order to be considered hot. The default is 1000, which
means that a basic block is considered hot in a function if it is executed more frequently than 1/1000 of the
frequency of the entry block of the function. 0 means that it is never considered hot.
unlikely-bb-count-fraction
The denominator n of fraction 1/n of the number of profiled runs of the entire program below which the
execution count of a basic block must be in order for the basic block to be considered unlikely executed.
The default is 20, which means that a basic block is considered unlikely executed if it is executed in fewer
than 1/20, or 5%, of the runs of the program. 0 means that it is always considered unlikely executed.
max-predicted-iterations
The maximum number of loop iterations we predict statically. This is useful in cases where a function
contains a single loop with known bound and another loop with unknown bound. The known number of iterations
is predicted correctly, while the unknown number of iterations average to roughly 10. This means that the
loop without bounds appears artificially cold relative to the other one.
builtin-expect-probability
Control the probability of the expression having the specified value. This parameter takes a percentage (i.e.
0 ... 100) as input.
builtin-string-cmp-inline-length
The maximum length of a constant string for a builtin string cmp call eligible for inlining.
align-threshold
Select fraction of the maximal frequency of executions of a basic block in a function to align the basic
block.
align-loop-iterations
A loop expected to iterate at least the selected number of iterations is aligned.
tracer-dynamic-coverage
tracer-dynamic-coverage-feedback
This value is used to limit superblock formation once the given percentage of executed instructions is
covered. This limits unnecessary code size expansion.
The tracer-dynamic-coverage-feedback parameter is used only when profile feedback is available. The real
profiles (as opposed to statically estimated ones) are much less balanced allowing the threshold to be larger
value.
tracer-max-code-growth
Stop tail duplication once code growth has reached given percentage. This is a rather artificial limit, as
most of the duplicates are eliminated later in cross jumping, so it may be set to much higher values than is
the desired code growth.
tracer-min-branch-ratio
Stop reverse growth when the reverse probability of best edge is less than this threshold (in percent).
tracer-min-branch-probability
tracer-min-branch-probability-feedback
Stop forward growth if the best edge has probability lower than this threshold.
Similarly to tracer-dynamic-coverage two parameters are provided. tracer-min-branch-probability-feedback is
used for compilation with profile feedback and tracer-min-branch-probability compilation without. The value
for compilation with profile feedback needs to be more conservative (higher) in order to make tracer
effective.
stack-clash-protection-guard-size
Specify the size of the operating system provided stack guard as 2 raised to num bytes. Higher values may
reduce the number of explicit probes, but a value larger than the operating system provided guard will leave
code vulnerable to stack clash style attacks.
stack-clash-protection-probe-interval
Stack clash protection involves probing stack space as it is allocated. This param controls the maximum
distance between probes into the stack as 2 raised to num bytes. Higher values may reduce the number of
explicit probes, but a value larger than the operating system provided guard will leave code vulnerable to
stack clash style attacks.
max-cse-path-length
The maximum number of basic blocks on path that CSE considers.
max-cse-insns
The maximum number of instructions CSE processes before flushing.
ggc-min-expand
GCC uses a garbage collector to manage its own memory allocation. This parameter specifies the minimum
percentage by which the garbage collector's heap should be allowed to expand between collections. Tuning
this may improve compilation speed; it has no effect on code generation.
The default is 30% + 70% * (RAM/1GB) with an upper bound of 100% when RAM >= 1GB. If "getrlimit" is
available, the notion of "RAM" is the smallest of actual RAM and "RLIMIT_DATA" or "RLIMIT_AS". If GCC is not
able to calculate RAM on a particular platform, the lower bound of 30% is used. Setting this parameter and
ggc-min-heapsize to zero causes a full collection to occur at every opportunity. This is extremely slow, but
can be useful for debugging.
ggc-min-heapsize
Minimum size of the garbage collector's heap before it begins bothering to collect garbage. The first
collection occurs after the heap expands by ggc-min-expand% beyond ggc-min-heapsize. Again, tuning this may
improve compilation speed, and has no effect on code generation.
The default is the smaller of RAM/8, RLIMIT_RSS, or a limit that tries to ensure that RLIMIT_DATA or
RLIMIT_AS are not exceeded, but with a lower bound of 4096 (four megabytes) and an upper bound of 131072 (128
megabytes). If GCC is not able to calculate RAM on a particular platform, the lower bound is used. Setting
this parameter very large effectively disables garbage collection. Setting this parameter and ggc-min-expand
to zero causes a full collection to occur at every opportunity.
max-reload-search-insns
The maximum number of instruction reload should look backward for equivalent register. Increasing values
mean more aggressive optimization, making the compilation time increase with probably slightly better
performance.
max-cselib-memory-locations
The maximum number of memory locations cselib should take into account. Increasing values mean more
aggressive optimization, making the compilation time increase with probably slightly better performance.
max-sched-ready-insns
The maximum number of instructions ready to be issued the scheduler should consider at any given time during
the first scheduling pass. Increasing values mean more thorough searches, making the compilation time
increase with probably little benefit.
max-sched-region-blocks
The maximum number of blocks in a region to be considered for interblock scheduling.
max-pipeline-region-blocks
The maximum number of blocks in a region to be considered for pipelining in the selective scheduler.
max-sched-region-insns
The maximum number of insns in a region to be considered for interblock scheduling.
max-pipeline-region-insns
The maximum number of insns in a region to be considered for pipelining in the selective scheduler.
min-spec-prob
The minimum probability (in percents) of reaching a source block for interblock speculative scheduling.
max-sched-extend-regions-iters
The maximum number of iterations through CFG to extend regions. A value of 0 disables region extensions.
max-sched-insn-conflict-delay
The maximum conflict delay for an insn to be considered for speculative motion.
sched-spec-prob-cutoff
The minimal probability of speculation success (in percents), so that speculative insns are scheduled.
sched-state-edge-prob-cutoff
The minimum probability an edge must have for the scheduler to save its state across it.
sched-mem-true-dep-cost
Minimal distance (in CPU cycles) between store and load targeting same memory locations.
selsched-max-lookahead
The maximum size of the lookahead window of selective scheduling. It is a depth of search for available
instructions.
selsched-max-sched-times
The maximum number of times that an instruction is scheduled during selective scheduling. This is the limit
on the number of iterations through which the instruction may be pipelined.
selsched-insns-to-rename
The maximum number of best instructions in the ready list that are considered for renaming in the selective
scheduler.
sms-min-sc
The minimum value of stage count that swing modulo scheduler generates.
max-last-value-rtl
The maximum size measured as number of RTLs that can be recorded in an expression in combiner for a pseudo
register as last known value of that register.
max-combine-insns
The maximum number of instructions the RTL combiner tries to combine.
max-combine-search-insns
The maximum number of instructions that the RTL combiner searches in order to find the next use of a given
register definition. If this limit is reached without finding such a use, the combiner will stop trying to
optimize the definition.
Currently this limit only applies after certain successful combination attempts, but it could be extended to
other cases in future.
integer-share-limit
Small integer constants can use a shared data structure, reducing the compiler's memory usage and increasing
its speed. This sets the maximum value of a shared integer constant.
ssp-buffer-size
The minimum size of buffers (i.e. arrays) that receive stack smashing protection when -fstack-protector is
used.
This default before Ubuntu 10.10 was "8". Currently it is "4", to increase the number of functions protected
by the stack protector.
min-size-for-stack-sharing
The minimum size of variables taking part in stack slot sharing when not optimizing.
max-jump-thread-duplication-stmts
Maximum number of statements allowed in a block that needs to be duplicated when threading jumps.
max-jump-thread-paths
The maximum number of paths to consider when searching for jump threading opportunities. When arriving at a
block, incoming edges are only considered if the number of paths to be searched so far multiplied by the
number of incoming edges does not exhaust the specified maximum number of paths to consider.
max-fields-for-field-sensitive
Maximum number of fields in a structure treated in a field sensitive manner during pointer analysis.
prefetch-latency
Estimate on average number of instructions that are executed before prefetch finishes. The distance
prefetched ahead is proportional to this constant. Increasing this number may also lead to less streams
being prefetched (see simultaneous-prefetches).
simultaneous-prefetches
Maximum number of prefetches that can run at the same time.
l1-cache-line-size
The size of cache line in L1 data cache, in bytes.
l1-cache-size
The size of L1 data cache, in kilobytes.
l2-cache-size
The size of L2 data cache, in kilobytes.
prefetch-dynamic-strides
Whether the loop array prefetch pass should issue software prefetch hints for strides that are non-constant.
In some cases this may be beneficial, though the fact the stride is non-constant may make it hard to predict
when there is clear benefit to issuing these hints.
Set to 1 if the prefetch hints should be issued for non-constant strides. Set to 0 if prefetch hints should
be issued only for strides that are known to be constant and below prefetch-minimum-stride.
prefetch-minimum-stride
Minimum constant stride, in bytes, to start using prefetch hints for. If the stride is less than this
threshold, prefetch hints will not be issued.
This setting is useful for processors that have hardware prefetchers, in which case there may be conflicts
between the hardware prefetchers and the software prefetchers. If the hardware prefetchers have a maximum
stride they can handle, it should be used here to improve the use of software prefetchers.
A value of -1 means we don't have a threshold and therefore prefetch hints can be issued for any constant
stride.
This setting is only useful for strides that are known and constant.
destructive-interference-size
constructive-interference-size
The values for the C++17 variables "std::hardware_destructive_interference_size" and
"std::hardware_constructive_interference_size". The destructive interference size is the minimum recommended
offset between two independent concurrently-accessed objects; the constructive interference size is the
maximum recommended size of contiguous memory accessed together. Typically both will be the size of an L1
cache line for the target, in bytes. For a generic target covering a range of L1 cache line sizes, typically
the constructive interference size will be the small end of the range and the destructive size will be the
large end.
The destructive interference size is intended to be used for layout, and thus has ABI impact. The default
value is not expected to be stable, and on some targets varies with -mtune, so use of this variable in a
context where ABI stability is important, such as the public interface of a library, is strongly discouraged;
if it is used in that context, users can stabilize the value using this option.
The constructive interference size is less sensitive, as it is typically only used in a static_assert to make
sure that a type fits within a cache line.
See also -Winterference-size.
loop-interchange-max-num-stmts
The maximum number of stmts in a loop to be interchanged.
loop-interchange-stride-ratio
The minimum ratio between stride of two loops for interchange to be profitable.
min-insn-to-prefetch-ratio
The minimum ratio between the number of instructions and the number of prefetches to enable prefetching in a
loop.
prefetch-min-insn-to-mem-ratio
The minimum ratio between the number of instructions and the number of memory references to enable
prefetching in a loop.
use-canonical-types
Whether the compiler should use the "canonical" type system. Should always be 1, which uses a more efficient
internal mechanism for comparing types in C++ and Objective-C++. However, if bugs in the canonical type
system are causing compilation failures, set this value to 0 to disable canonical types.
switch-conversion-max-branch-ratio
Switch initialization conversion refuses to create arrays that are bigger than switch-conversion-max-branch-
ratio times the number of branches in the switch.
switch-lower-slow-alg-max-cases
Maximum number of cases for slow switch lowering algorithms to be used.
max-partial-antic-length
Maximum length of the partial antic set computed during the tree partial redundancy elimination optimization
(-ftree-pre) when optimizing at -O3 and above. For some sorts of source code the enhanced partial redundancy
elimination optimization can run away, consuming all of the memory available on the host machine. This
parameter sets a limit on the length of the sets that are computed, which prevents the runaway behavior.
Setting a value of 0 for this parameter allows an unlimited set length.
rpo-vn-max-loop-depth
Maximum loop depth that is value-numbered optimistically. When the limit hits the innermost rpo-vn-max-loop-
depth loops and the outermost loop in the loop nest are value-numbered optimistically and the remaining ones
not.
sccvn-max-alias-queries-per-access
Maximum number of alias-oracle queries we perform when looking for redundancies for loads and stores. If
this limit is hit the search is aborted and the load or store is not considered redundant. The number of
queries is algorithmically limited to the number of stores on all paths from the load to the function entry.
ira-max-loops-num
IRA uses regional register allocation by default. If a function contains more loops than the number given by
this parameter, only at most the given number of the most frequently-executed loops form regions for regional
register allocation.
ira-max-conflict-table-size
Although IRA uses a sophisticated algorithm to compress the conflict table, the table can still require
excessive amounts of memory for huge functions. If the conflict table for a function could be more than the
size in MB given by this parameter, the register allocator instead uses a faster, simpler, and lower-quality
algorithm that does not require building a pseudo-register conflict table.
ira-loop-reserved-regs
IRA can be used to evaluate more accurate register pressure in loops for decisions to move loop invariants
(see -O3). The number of available registers reserved for some other purposes is given by this parameter.
Default of the parameter is the best found from numerous experiments.
ira-consider-dup-in-all-alts
Make IRA to consider matching constraint (duplicated operand number) heavily in all available alternatives
for preferred register class. If it is set as zero, it means IRA only respects the matching constraint when
it's in the only available alternative with an appropriate register class. Otherwise, it means IRA will
check all available alternatives for preferred register class even if it has found some choice with an
appropriate register class and respect the found qualified matching constraint.
ira-simple-lra-insn-threshold
Approximate function insn number in 1K units triggering simple local RA.
lra-inheritance-ebb-probability-cutoff
LRA tries to reuse values reloaded in registers in subsequent insns. This optimization is called
inheritance. EBB is used as a region to do this optimization. The parameter defines a minimal fall-through
edge probability in percentage used to add BB to inheritance EBB in LRA. The default value was chosen from
numerous runs of SPEC2000 on x86-64.
loop-invariant-max-bbs-in-loop
Loop invariant motion can be very expensive, both in compilation time and in amount of needed compile-time
memory, with very large loops. Loops with more basic blocks than this parameter won't have loop invariant
motion optimization performed on them.
loop-max-datarefs-for-datadeps
Building data dependencies is expensive for very large loops. This parameter limits the number of data
references in loops that are considered for data dependence analysis. These large loops are no handled by
the optimizations using loop data dependencies.
max-vartrack-size
Sets a maximum number of hash table slots to use during variable tracking dataflow analysis of any function.
If this limit is exceeded with variable tracking at assignments enabled, analysis for that function is
retried without it, after removing all debug insns from the function. If the limit is exceeded even without
debug insns, var tracking analysis is completely disabled for the function. Setting the parameter to zero
makes it unlimited.
max-vartrack-expr-depth
Sets a maximum number of recursion levels when attempting to map variable names or debug temporaries to value
expressions. This trades compilation time for more complete debug information. If this is set too low,
value expressions that are available and could be represented in debug information may end up not being used;
setting this higher may enable the compiler to find more complex debug expressions, but compile time and
memory use may grow.
max-debug-marker-count
Sets a threshold on the number of debug markers (e.g. begin stmt markers) to avoid complexity explosion at
inlining or expanding to RTL. If a function has more such gimple stmts than the set limit, such stmts will
be dropped from the inlined copy of a function, and from its RTL expansion.
min-nondebug-insn-uid
Use uids starting at this parameter for nondebug insns. The range below the parameter is reserved
exclusively for debug insns created by -fvar-tracking-assignments, but debug insns may get (non-overlapping)
uids above it if the reserved range is exhausted.
ipa-sra-deref-prob-threshold
IPA-SRA replaces a pointer which is known not be NULL with one or more new parameters only when the
probability (in percent, relative to function entry) of it being dereferenced is higher than this parameter.
ipa-sra-ptr-growth-factor
IPA-SRA replaces a pointer to an aggregate with one or more new parameters only when their cumulative size is
less or equal to ipa-sra-ptr-growth-factor times the size of the original pointer parameter.
ipa-sra-ptrwrap-growth-factor
Additional maximum allowed growth of total size of new parameters that ipa-sra replaces a pointer to an
aggregate with, if it points to a local variable that the caller only writes to and passes it as an argument
to other functions.
ipa-sra-max-replacements
Maximum pieces of an aggregate that IPA-SRA tracks. As a consequence, it is also the maximum number of
replacements of a formal parameter.
sra-max-scalarization-size-Ospeed
sra-max-scalarization-size-Osize
The two Scalar Reduction of Aggregates passes (SRA and IPA-SRA) aim to replace scalar parts of aggregates
with uses of independent scalar variables. These parameters control the maximum size, in storage units, of
aggregate which is considered for replacement when compiling for speed (sra-max-scalarization-size-Ospeed) or
size (sra-max-scalarization-size-Osize) respectively.
sra-max-propagations
The maximum number of artificial accesses that Scalar Replacement of Aggregates (SRA) will track, per one
local variable, in order to facilitate copy propagation.
tm-max-aggregate-size
When making copies of thread-local variables in a transaction, this parameter specifies the size in bytes
after which variables are saved with the logging functions as opposed to save/restore code sequence pairs.
This option only applies when using -fgnu-tm.
graphite-max-nb-scop-params
To avoid exponential effects in the Graphite loop transforms, the number of parameters in a Static Control
Part (SCoP) is bounded. A value of zero can be used to lift the bound. A variable whose value is unknown at
compilation time and defined outside a SCoP is a parameter of the SCoP.
hardcfr-max-blocks
Disable -fharden-control-flow-redundancy for functions with a larger number of blocks than the specified
value. Zero removes any limit.
hardcfr-max-inline-blocks
Force -fharden-control-flow-redundancy to use out-of-line checking for functions with a larger number of
basic blocks than the specified value.
loop-block-tile-size
Loop blocking or strip mining transforms, enabled with -floop-block or -floop-strip-mine, strip mine each
loop in the loop nest by a given number of iterations. The strip length can be changed using the loop-block-
tile-size parameter.
ipa-jump-function-lookups
Specifies number of statements visited during jump function offset discovery.
ipa-cp-value-list-size
IPA-CP attempts to track all possible values and types passed to a function's parameter in order to propagate
them and perform devirtualization. ipa-cp-value-list-size is the maximum number of values and types it
stores per one formal parameter of a function.
ipa-cp-eval-threshold
IPA-CP calculates its own score of cloning profitability heuristics and performs those cloning opportunities
with scores that exceed ipa-cp-eval-threshold.
ipa-cp-max-recursive-depth
Maximum depth of recursive cloning for self-recursive function.
ipa-cp-min-recursive-probability
Recursive cloning only when the probability of call being executed exceeds the parameter.
ipa-cp-profile-count-base
When using -fprofile-use option, IPA-CP will consider the measured execution count of a call graph edge at
this percentage position in their histogram as the basis for its heuristics calculation.
ipa-cp-recursive-freq-factor
The number of times interprocedural copy propagation expects recursive functions to call themselves.
ipa-cp-recursion-penalty
Percentage penalty the recursive functions will receive when they are evaluated for cloning.
ipa-cp-single-call-penalty
Percentage penalty functions containing a single call to another function will receive when they are
evaluated for cloning.
ipa-max-agg-items
IPA-CP is also capable to propagate a number of scalar values passed in an aggregate. ipa-max-agg-items
controls the maximum number of such values per one parameter.
ipa-cp-loop-hint-bonus
When IPA-CP determines that a cloning candidate would make the number of iterations of a loop known, it adds
a bonus of ipa-cp-loop-hint-bonus to the profitability score of the candidate.
ipa-max-loop-predicates
The maximum number of different predicates IPA will use to describe when loops in a function have known
properties.
ipa-max-aa-steps
During its analysis of function bodies, IPA-CP employs alias analysis in order to track values pointed to by
function parameters. In order not spend too much time analyzing huge functions, it gives up and consider all
memory clobbered after examining ipa-max-aa-steps statements modifying memory.
ipa-max-switch-predicate-bounds
Maximal number of boundary endpoints of case ranges of switch statement. For switch exceeding this limit,
IPA-CP will not construct cloning cost predicate, which is used to estimate cloning benefit, for default case
of the switch statement.
ipa-max-param-expr-ops
IPA-CP will analyze conditional statement that references some function parameter to estimate benefit for
cloning upon certain constant value. But if number of operations in a parameter expression exceeds ipa-max-
param-expr-ops, the expression is treated as complicated one, and is not handled by IPA analysis.
lto-partitions
Specify desired number of partitions produced during WHOPR compilation. The number of partitions should
exceed the number of CPUs used for compilation.
lto-min-partition
Size of minimal partition for WHOPR (in estimated instructions). This prevents expenses of splitting very
small programs into too many partitions.
lto-max-partition
Size of max partition for WHOPR (in estimated instructions). to provide an upper bound for individual size
of partition. Meant to be used only with balanced partitioning.
lto-partition-locality-frequency-cutoff
The denominator n of fraction 1/n of the execution frequency of callee to be cloned for a particular caller.
Special value of 0 dictates to always clone without a cut-off.
lto-partition-locality-size-cutoff
Size cut-off for callee including inlined calls to be cloned for a particular caller.
lto-max-locality-partition
Maximal size of a locality partition for LTO (in estimated instructions). Value of 0 results in default
value being used.
lto-max-streaming-parallelism
Maximal number of parallel processes used for LTO streaming.
cxx-max-namespaces-for-diagnostic-help
The maximum number of namespaces to consult for suggestions when C++ name lookup fails for an identifier.
sink-frequency-threshold
The maximum relative execution frequency (in percents) of the target block relative to a statement's original
block to allow statement sinking of a statement. Larger numbers result in more aggressive statement sinking.
A small positive adjustment is applied for statements with memory operands as those are even more profitable
so sink.
max-stores-to-sink
The maximum number of conditional store pairs that can be sunk. Set to 0 if either vectorization
(-ftree-vectorize) or if-conversion (-ftree-loop-if-convert) is disabled.
case-values-threshold
The smallest number of different values for which it is best to use a jump-table instead of a tree of
conditional branches. If the value is 0, use the default for the machine.
jump-table-max-growth-ratio-for-size
The maximum code size growth ratio when expanding into a jump table (in percent). The parameter is used when
optimizing for size.
jump-table-max-growth-ratio-for-speed
The maximum code size growth ratio when expanding into a jump table (in percent). The parameter is used when
optimizing for speed.
tree-reassoc-width
Set the maximum number of instructions executed in parallel in reassociated tree. This parameter overrides
target dependent heuristics used by default if has non zero value.
sched-pressure-algorithm
Choose between the two available implementations of -fsched-pressure. Algorithm 1 is the original
implementation and is the more likely to prevent instructions from being reordered. Algorithm 2 was designed
to be a compromise between the relatively conservative approach taken by algorithm 1 and the rather
aggressive approach taken by the default scheduler. It relies more heavily on having a regular register file
and accurate register pressure classes. See haifa-sched.cc in the GCC sources for more details.
The default choice depends on the target.
max-slsr-cand-scan
Set the maximum number of existing candidates that are considered when seeking a basis for a new straight-
line strength reduction candidate.
asan-globals
Enable buffer overflow detection for global objects. This kind of protection is enabled by default if you
are using -fsanitize=address option. To disable global objects protection use --param asan-globals=0.
asan-stack
Enable buffer overflow detection for stack objects. This kind of protection is enabled by default when using
-fsanitize=address. To disable stack protection use --param asan-stack=0 option.
asan-instrument-reads
Enable buffer overflow detection for memory reads. This kind of protection is enabled by default when using
-fsanitize=address. To disable memory reads protection use --param asan-instrument-reads=0.
asan-instrument-writes
Enable buffer overflow detection for memory writes. This kind of protection is enabled by default when using
-fsanitize=address. To disable memory writes protection use --param asan-instrument-writes=0 option.
asan-memintrin
Enable detection for built-in functions. This kind of protection is enabled by default when using
-fsanitize=address. To disable built-in functions protection use --param asan-memintrin=0.
asan-use-after-return
Enable detection of use-after-return. This kind of protection is enabled by default when using the
-fsanitize=address option. To disable it use --param asan-use-after-return=0.
Note: By default the check is disabled at run time. To enable it, add "detect_stack_use_after_return=1" to
the environment variable ASAN_OPTIONS.
asan-instrumentation-with-call-threshold
If number of memory accesses in function being instrumented is greater or equal to this number, use callbacks
instead of inline checks. E.g. to disable inline code use --param
asan-instrumentation-with-call-threshold=0.
asan-kernel-mem-intrinsic-prefix
If nonzero, prefix calls to "memcpy", "memset" and "memmove" with __asan_ or __hwasan_ for
-fsanitize=kernel-address or -fsanitize=kernel-hwaddress, respectively.
hwasan-instrument-stack
Enable hwasan instrumentation of statically sized stack-allocated variables. This kind of instrumentation is
enabled by default when using -fsanitize=hwaddress and disabled by default when using
-fsanitize=kernel-hwaddress. To disable stack instrumentation use --param hwasan-instrument-stack=0, and to
enable it use --param hwasan-instrument-stack=1.
hwasan-random-frame-tag
When using stack instrumentation, decide tags for stack variables using a deterministic sequence beginning at
a random tag for each frame. With this parameter unset tags are chosen using the same sequence but beginning
from 1. This is enabled by default for -fsanitize=hwaddress and unavailable for -fsanitize=kernel-hwaddress.
To disable it use --param hwasan-random-frame-tag=0.
hwasan-instrument-allocas
Enable hwasan instrumentation of dynamically sized stack-allocated variables. This kind of instrumentation
is enabled by default when using -fsanitize=hwaddress and disabled by default when using
-fsanitize=kernel-hwaddress. To disable instrumentation of such variables use --param
hwasan-instrument-allocas=0, and to enable it use --param hwasan-instrument-allocas=1.
hwasan-instrument-reads
Enable hwasan checks on memory reads. Instrumentation of reads is enabled by default for both
-fsanitize=hwaddress and -fsanitize=kernel-hwaddress. To disable checking memory reads use --param
hwasan-instrument-reads=0.
hwasan-instrument-writes
Enable hwasan checks on memory writes. Instrumentation of writes is enabled by default for both
-fsanitize=hwaddress and -fsanitize=kernel-hwaddress. To disable checking memory writes use --param
hwasan-instrument-writes=0.
hwasan-instrument-mem-intrinsics
Enable hwasan instrumentation of builtin functions. Instrumentation of these builtin functions is enabled by
default for both -fsanitize=hwaddress and -fsanitize=kernel-hwaddress. To disable instrumentation of builtin
functions use --param hwasan-instrument-mem-intrinsics=0.
use-after-scope-direct-emission-threshold
If the size of a local variable in bytes is smaller or equal to this number, directly poison (or unpoison)
shadow memory instead of using run-time callbacks.
tsan-distinguish-volatile
Emit special instrumentation for accesses to volatiles.
tsan-instrument-func-entry-exit
Emit instrumentation calls to __tsan_func_entry() and __tsan_func_exit().
max-fsm-thread-path-insns
Maximum number of instructions to copy when duplicating blocks on a finite state automaton jump thread path.
threader-debug
threader-debug=[none|all] Enables verbose dumping of the threader solver.
parloops-chunk-size
Chunk size of omp schedule for loops parallelized by parloops.
parloops-schedule
Schedule type of omp schedule for loops parallelized by parloops (static, dynamic, guided, auto, runtime).
parloops-min-per-thread
The minimum number of iterations per thread of an innermost parallelized loop for which the parallelized
variant is preferred over the single threaded one. Note that for a parallelized loop nest the minimum number
of iterations of the outermost loop per thread is two.
max-ssa-name-query-depth
Maximum depth of recursion when querying properties of SSA names in things like fold routines. One level of
recursion corresponds to following a use-def chain.
max-speculative-devirt-maydefs
The maximum number of may-defs we analyze when looking for a must-def specifying the dynamic type of an
object that invokes a virtual call we may be able to devirtualize speculatively.
ranger-debug
Specifies the type of debug output to be issued for ranges.
unroll-jam-min-percent
The minimum percentage of memory references that must be optimized away for the unroll-and-jam transformation
to be considered profitable.
unroll-jam-max-unroll
The maximum number of times the outer loop should be unrolled by the unroll-and-jam transformation.
max-rtl-if-conversion-unpredictable-cost
Maximum permissible cost for the sequence that would be generated by the RTL if-conversion pass for a branch
that is considered unpredictable.
max-variable-expansions-in-unroller
If -fvariable-expansion-in-unroller is used, the maximum number of times that an individual variable will be
expanded during loop unrolling.
partial-inlining-entry-probability
Maximum probability of the entry BB of split region (in percent relative to entry BB of the function) to make
partial inlining happen.
max-tracked-strlens
Maximum number of strings for which strlen optimization pass will track string lengths.
gcse-after-reload-partial-fraction
The threshold ratio for performing partial redundancy elimination after reload.
gcse-after-reload-critical-fraction
The threshold ratio of critical edges execution count that permit performing redundancy elimination after
reload.
max-loop-header-insns
The maximum number of insns in loop header duplicated by the copy loop headers pass.
vect-epilogues-nomask
Enable loop epilogue vectorization using smaller vector size.
vect-partial-vector-usage
Controls when the loop vectorizer considers using partial vector loads and stores as an alternative to
falling back to scalar code. 0 stops the vectorizer from ever using partial vector loads and stores. 1
allows partial vector loads and stores if vectorization removes the need for the code to iterate. 2 allows
partial vector loads and stores in all loops. The parameter only has an effect on targets that support
partial vector loads and stores.
vect-inner-loop-cost-factor
The maximum factor which the loop vectorizer applies to the cost of statements in an inner loop relative to
the loop being vectorized. The factor applied is the maximum of the estimated number of iterations of the
inner loop and this parameter. The default value of this parameter is 50.
vect-induction-float
Enable loop vectorization of floating point inductions.
vect-force-slp
Force the use of SLP when vectorizing, fail if not possible.
vrp-block-limit
Maximum number of basic blocks before VRP switches to a lower memory algorithm.
vrp-sparse-threshold
Maximum number of basic blocks before VRP uses a sparse bitmap cache.
vrp-switch-limit
Maximum number of outgoing edges in a switch before VRP will not process it.
vrp-vector-threshold
Maximum number of basic blocks for VRP to use a basic cache vector.
avoid-fma-max-bits
Maximum number of bits for which we avoid creating FMAs.
fully-pipelined-fma
Whether the target fully pipelines FMA instructions. If non-zero, reassociation considers the benefit of
parallelizing FMA's multiplication part and addition part, assuming FMUL and FMA use the same units that can
also do FADD.
sms-loop-average-count-threshold
A threshold on the average loop count considered by the swing modulo scheduler.
sms-dfa-history
The number of cycles the swing modulo scheduler considers when checking conflicts using DFA.
graphite-allow-codegen-errors
Whether codegen errors should be ICEs when -fchecking.
sms-max-ii-factor
A factor for tuning the upper bound that swing modulo scheduler uses for scheduling a loop.
lra-max-considered-reload-pseudos
The max number of reload pseudos which are considered during spilling a non-reload pseudo.
max-pow-sqrt-depth
Maximum depth of sqrt chains to use when synthesizing exponentiation by a real constant.
max-dse-active-local-stores
Maximum number of active local stores in RTL dead store elimination.
asan-instrument-allocas
Enable asan allocas/VLAs protection.
max-iterations-computation-cost
Bound on the cost of an expression to compute the number of iterations.
max-isl-operations
Maximum number of isl operations, 0 means unlimited.
graphite-max-arrays-per-scop
Maximum number of arrays per scop.
max-vartrack-reverse-op-size
Max. size of loc list for which reverse ops should be added.
fsm-scale-path-stmts
Scale factor to apply to the number of statements in a threading path crossing a loop backedge when comparing
to --param=max-jump-thread-duplication-stmts.
uninit-control-dep-attempts
Maximum number of nested calls to search for control dependencies during uninitialized variable analysis.
uninit-max-chain-len
Maximum number of predicates anded for each predicate ored in the normalized predicate chain.
uninit-max-num-chains
Maximum number of predicates ored in the normalized predicate chain.
uninit-max-prune-work
Maximum amount of work done to prune paths where the variable is always initialized.
sched-autopref-queue-depth
Hardware autoprefetcher scheduler model control flag. Number of lookahead cycles the model looks into; at '
' only enable instruction sorting heuristic.
loop-versioning-max-inner-insns
The maximum number of instructions that an inner loop can have before the loop versioning pass considers it
too big to copy.
loop-versioning-max-outer-insns
The maximum number of instructions that an outer loop can have before the loop versioning pass considers it
too big to copy, discounting any instructions in inner loops that directly benefit from versioning.
ssa-name-def-chain-limit
The maximum number of SSA_NAME assignments to follow in determining a property of a variable such as its
value. This limits the number of iterations or recursive calls GCC performs when optimizing certain
statements or when determining their validity prior to issuing diagnostics.
store-merging-max-size
Maximum size of a single store merging region in bytes.
store-forwarding-max-distance
Maximum number of instruction distance that a small store forwarded to a larger load may stall. Value '0'
disables the cost checks for the avoid-store-forwarding pass.
hash-table-verification-limit
The number of elements for which hash table verification is done for each searched element.
max-find-base-term-values
Maximum number of VALUEs handled during a single find_base_term call.
analyzer-max-enodes-per-program-point
The maximum number of exploded nodes per program point within the analyzer, before terminating analysis of
that point.
analyzer-max-constraints
The maximum number of constraints per state.
analyzer-min-snodes-for-call-summary
The minimum number of supernodes within a function for the analyzer to consider summarizing its effects at
call sites.
analyzer-max-enodes-for-full-dump
The maximum depth of exploded nodes that should appear in a dot dump before switching to a less verbose
format.
analyzer-max-recursion-depth
The maximum number of times a callsite can appear in a call stack within the analyzer, before terminating
analysis of a call that would recurse deeper.
analyzer-max-svalue-depth
The maximum depth of a symbolic value, before approximating the value as unknown.
analyzer-max-infeasible-edges
The maximum number of infeasible edges to reject before declaring a diagnostic as infeasible.
gimple-fe-computed-hot-bb-threshold
The number of executions of a basic block which is considered hot. The parameter is used only in GIMPLE FE.
analyzer-bb-explosion-factor
The maximum number of 'after supernode' exploded nodes within the analyzer per supernode, before terminating
analysis.
analyzer-text-art-string-ellipsis-threshold
The number of bytes at which to ellipsize string literals in analyzer text art diagrams.
analyzer-text-art-ideal-canvas-width
The ideal width in characters of text art diagrams generated by the analyzer.
analyzer-text-art-string-ellipsis-head-len
The number of literal bytes to show at the head of a string literal in text art when ellipsizing it.
analyzer-text-art-string-ellipsis-tail-len
The number of literal bytes to show at the tail of a string literal in text art when ellipsizing it.
ranger-logical-depth
Maximum depth of logical expression evaluation ranger will look through when evaluating outgoing edge ranges.
ranger-recompute-depth
Maximum depth of instruction chains to consider for recomputation in the outgoing range calculator.
relation-block-limit
Maximum number of relations the oracle will register in a basic block.
transitive-relations-work-bound
Work bound when discovering transitive relations from existing relations.
min-pagesize
Minimum page size for warning and early break vectorization purposes.
openacc-kernels
Specify mode of OpenACC `kernels' constructs handling. With --param=openacc-kernels=decompose, OpenACC
`kernels' constructs are decomposed into parts, a sequence of compute constructs, each then handled
individually. This is work in progress. With --param=openacc-kernels=parloops, OpenACC `kernels' constructs
are handled by the parloops pass, en bloc. This is the current default.
openacc-privatization
Control whether the -fopt-info-omp-note and applicable -fdump-tree-*-details options emit OpenACC
privatization diagnostics. With --param=openacc-privatization=quiet, don't diagnose. This is the current
default. With --param=openacc-privatization=noisy, do diagnose.
cycle-accurate-model
Specifies whether GCC should assume that the scheduling description is mostly a cycle-accurate model of the
target processor the code is intended to run on, in the absence of cache misses. Nonzero means that the
selected scheduling model is accurate and likely describes an in-order processor, and that scheduling should
aggressively spill to try and fill any pipeline bubbles. This is the current default. Zero means the
scheduling description might not be available/accurate or perhaps not applicable at all, such as for modern
out-of-order processors.
The following choices of name are available on AArch64 targets:
aarch64-vect-compare-costs
When vectorizing, consider using multiple different approaches and use the cost model to choose the cheapest
one. This includes:
* Trying both SVE and Advanced SIMD, when SVE is available.
* Trying to use 64-bit Advanced SIMD vectors for the smallest data elements, rather than using 128-bit
vectors for everything.
* Trying to use "unpacked" SVE vectors for smaller elements. This includes storing smaller elements in
larger containers and accessing elements with extending loads and truncating stores.
aarch64-float-recp-precision
The number of Newton iterations for calculating the reciprocal for float type. The precision of division is
proportional to this param when division approximation is enabled. The default value is 1.
aarch64-double-recp-precision
The number of Newton iterations for calculating the reciprocal for double type. The precision of division is
proportional to this param when division approximation is enabled. The default value is 2.
aarch64-autovec-preference
Force an ISA selection strategy for auto-vectorization.
default
Use the default heuristics.
asimd-only
Use only Advanced SIMD for auto-vectorization.
sve-only
Use only SVE for auto-vectorization.
prefer-asimd
Use both Advanced SIMD and SVE. Prefer Advanced SIMD when the costs are deemed equal.
prefer-sve
Use both Advanced SIMD and SVE. Prefer SVE when the costs are deemed equal.
aarch64-ldp-policy
Fine-grained policy for load pairs. With --param=aarch64-ldp-policy=default, use the policy of the tuning
structure. This is the current default. With --param=aarch64-ldp-policy=always, emit ldp regardless of
alignment. With --param=aarch64-ldp-policy=never, do not emit ldp. With --param=aarch64-ldp-policy=aligned,
emit ldp only if the source pointer is aligned to at least double the alignment of the type.
aarch64-stp-policy
Fine-grained policy for store pairs. With --param=aarch64-stp-policy=default, use the policy of the tuning
structure. This is the current default. With --param=aarch64-stp-policy=always, emit stp regardless of
alignment. With --param=aarch64-stp-policy=never, do not emit stp. With --param=aarch64-stp-policy=aligned,
emit stp only if the source pointer is aligned to at least double the alignment of the type.
aarch64-ldp-alias-check-limit
Limit on the number of alias checks performed by the AArch64 load/store pair fusion pass when attempting to
form an ldp/stp. Higher values make the pass more aggressive at re-ordering loads over stores, at the
expense of increased compile time.
aarch64-ldp-writeback
Param to control which writeback opportunities we try to handle in the AArch64 load/store pair fusion pass.
A value of zero disables writeback handling. One means we try to form pairs involving one or more existing
individual writeback accesses where possible. A value of two means we also try to opportunistically form
writeback opportunities by folding in trailing destructive updates of the base register used by a pair.
aarch64-loop-vect-issue-rate-niters
The tuning for some AArch64 CPUs tries to take both latencies and issue rates into account when deciding
whether a loop should be vectorized using SVE, vectorized using Advanced SIMD, or not vectorized at all. If
this parameter is set to n, GCC will not use this heuristic for loops that are known to execute in fewer than
n Advanced SIMD iterations.
aarch64-vect-unroll-limit
The vectorizer will use available tuning information to determine whether it would be beneficial to unroll
the main vectorized loop and by how much. This parameter set's the upper bound of how much the vectorizer
will unroll the main loop. The default value is four.
The following choices of name are available on GCN targets:
gcn-preferred-vectorization-factor
Preferred vectorization factor: default, 32, 64.
The following choices of name are available on i386 and x86_64 targets:
x86-stlf-window-ninsns
Instructions number above which STFL stall penalty can be compensated.
x86-stv-max-visits
The maximum number of use and def visits when discovering a STV chain before the discovery is aborted.
Program Instrumentation Options
GCC supports a number of command-line options that control adding run-time instrumentation to the code it normally
generates. For example, one purpose of instrumentation is collect profiling statistics for use in finding program
hot spots, code coverage analysis, or profile-guided optimizations. Another class of program instrumentation is
adding run-time checking to detect programming errors like invalid pointer dereferences or out-of-bounds array
accesses, as well as deliberately hostile attacks such as stack smashing or C++ vtable hijacking. There is also a
general hook which can be used to implement other forms of tracing or function-level instrumentation for debug or
program analysis purposes.
-p
-pg Generate extra code to write profile information suitable for the analysis program prof (for -p) or gprof (for
-pg). You must use this option when compiling the source files you want data about, and you must also use it
when linking.
You can use the function attribute "no_instrument_function" to suppress profiling of individual functions when
compiling with these options.
-fprofile-arcs
Add code so that program flow arcs are instrumented. During execution the program records how many times each
branch and call is executed and how many times it is taken or returns. On targets that support constructors with
priority support, profiling properly handles constructors, destructors and C++ constructors (and destructors) of
classes which are used as a type of a global variable.
When the compiled program exits it saves this data to a file called auxname.gcda for each source file. The data
may be used for profile-directed optimizations (-fbranch-probabilities), or for test coverage analysis
(-ftest-coverage). Each object file's auxname is generated from the name of the output file, if explicitly
specified and it is not the final executable, otherwise it is the basename of the source file. In both cases any
suffix is removed (e.g. foo.gcda for input file dir/foo.c, or dir/foo.gcda for output file specified as -o
dir/foo.o).
Note that if a command line directly links source files, the corresponding .gcda files will be prefixed with the
unsuffixed name of the output file. E.g. "gcc a.c b.c -o binary" would generate binary-a.gcda and binary-b.gcda
files.
-fcondition-coverage
Add code so that program conditions are instrumented. During execution the program records what terms in a
conditional contributes to a decision, which can be used to verify that all terms in a Boolean function are
tested and have an independent effect on the outcome of a decision. The result can be read with "gcov
--conditions".
-fpath-coverage
Add code so that the paths taken are tracked. During execution the program records the prime paths taken. The
number of paths grows very fast with complexity, and to avoid exploding compile times GCC will give up
instrumentation if the approximate number of paths exceeds the limit controlled by -fpath-coverage-limit. The
result can be read with "gcov --prime-paths --prime-paths-lines --prime-paths-source",
-fpath-coverage-limit=limit
The threshold at which point -fpath-coverage gives up on instrumenting a function. This limit is approximate and
conservative, as GCC uses a pessimistic heuristic which slightly overcounts the running number of paths, and
gives up if the threshold is reached before finding all the paths. This option is not for fine grained control
over which functions to instrument - rather it is intended to limit the effect of path explosion and keep compile
times reasonable. The default is 250000.
--coverage
This option is used to compile and link code instrumented for coverage analysis. The option is a synonym for
-fprofile-arcs -ftest-coverage (when compiling) and -lgcov (when linking). See the documentation for those
options for more details.
* Compile the source files with -fprofile-arcs plus optimization and code generation options. For test
coverage analysis, use the additional -ftest-coverage option. You do not need to profile every source file
in a program.
* Compile the source files additionally with -fprofile-abs-path to create absolute path names in the .gcno
files. This allows gcov to find the correct sources in projects where compilations occur with different
working directories.
* Link your object files with -lgcov or -fprofile-arcs (the latter implies the former).
* Run the program on a representative workload to generate the arc profile information. This may be repeated
any number of times. You can run concurrent instances of your program, and provided that the file system
supports locking, the data files will be correctly updated. Unless a strict ISO C dialect option is in
effect, "fork" calls are detected and correctly handled without double counting.
Moreover, an object file can be recompiled multiple times and the corresponding .gcda file merges as long as
the source file and the compiler options are unchanged.
* For profile-directed optimizations, compile the source files again with the same optimization and code
generation options plus -fbranch-probabilities.
* For test coverage analysis, use gcov to produce human readable information from the .gcno and .gcda files.
Refer to the gcov documentation for further information.
With -fprofile-arcs, for each function of your program GCC creates a program flow graph, then finds a spanning
tree for the graph. Only arcs that are not on the spanning tree have to be instrumented: the compiler adds code
to count the number of times that these arcs are executed. When an arc is the only exit or only entrance to a
block, the instrumentation code can be added to the block; otherwise, a new basic block must be created to hold
the instrumentation code.
With -fcondition-coverage, for each conditional in your program GCC creates a bitset and records the exercised
boolean values that have an independent effect on the outcome of that expression.
With -fpath-coverage, GCC finds and enumerates and records the taken prime paths of each function, unless the
number of paths would exceed the limit controlled by -fpath-coverage-limit. If the limit is exceeded the
function is not instrumented as if -fpath-coverage was not used. A prime path is the longest sequence of unique
blocks, except possibly the first and last, which is not a subpath of any other path.
-ftest-coverage
Produce a notes file that the gcov code-coverage utility can use to show program coverage. Each source file's
note file is called auxname.gcno. Refer to the -fprofile-arcs option above for a description of auxname and
instructions on how to generate test coverage data. Coverage data matches the source files more closely if you
do not optimize.
-fprofile-abs-path
Automatically convert relative source file names to absolute path names in the .gcno files. This allows gcov to
find the correct sources in projects where compilations occur with different working directories.
-fprofile-dir=path
Set the directory to search for the profile data files in to path. This option affects only the profile data
generated by -fprofile-generate, -ftest-coverage, -fprofile-arcs and used by -fprofile-use and
-fbranch-probabilities and its related options. Both absolute and relative paths can be used. By default, GCC
uses the current directory as path, thus the profile data file appears in the same directory as the object file.
In order to prevent the file name clashing, if the object file name is not an absolute path, we mangle the
absolute path of the sourcename.gcda file and use it as the file name of a .gcda file. See details about the
file naming in -fprofile-arcs. See similar option -fprofile-note.
When an executable is run in a massive parallel environment, it is recommended to save profile to different
folders. That can be done with variables in path that are exported during run-time:
%p process ID.
%q{VAR}
value of environment variable VAR
-fprofile-generate
-fprofile-generate=path
Enable options usually used for instrumenting application to produce profile useful for later recompilation with
profile feedback based optimization. You must use -fprofile-generate both when compiling and when linking your
program.
The following options are enabled: -fprofile-arcs, -fprofile-values, -finline-functions, and -fipa-bit-cp.
If path is specified, GCC looks at the path to find the profile feedback data files. See -fprofile-dir.
To optimize the program based on the collected profile information, use -fprofile-use.
-fprofile-info-section
-fprofile-info-section=name
Register the profile information in the specified section instead of using a constructor/destructor. The section
name is name if it is specified, otherwise the section name defaults to ".gcov_info". A pointer to the profile
information generated by -fprofile-arcs is placed in the specified section for each translation unit. This
option disables the profile information registration through a constructor and it disables the profile
information processing through a destructor. This option is not intended to be used in hosted environments such
as GNU/Linux. It targets freestanding environments (for example embedded systems) with limited resources which
do not support constructors/destructors or the C library file I/O.
The linker could collect the input sections in a continuous memory block and define start and end symbols. A GNU
linker script example which defines a linker output section follows:
.gcov_info :
{
PROVIDE (__gcov_info_start = .);
KEEP (*(.gcov_info))
PROVIDE (__gcov_info_end = .);
}
The program could dump the profiling information registered in this linker set for example like this:
#include
#include
#include
extern const struct gcov_info *const __gcov_info_start[];
extern const struct gcov_info *const __gcov_info_end[];
static void
dump (const void *d, unsigned n, void *arg)
{
const unsigned char *c = d;
for (unsigned i = 0; i < n; ++i)
printf ("%02x", c[i]);
}
static void
filename (const char *f, void *arg)
{
__gcov_filename_to_gcfn (f, dump, arg );
}
static void *
allocate (unsigned length, void *arg)
{
return malloc (length);
}
static void
dump_gcov_info (void)
{
const struct gcov_info *const *info = __gcov_info_start;
const struct gcov_info *const *end = __gcov_info_end;
/* Obfuscate variable to prevent compiler optimizations. */
__asm__ ("" : "+r" (info));
while (info != end)
{
void *arg = NULL;
__gcov_info_to_gcda (*info, filename, dump, allocate, arg);
putchar ('\n');
++info;
}
}
int
main (void)
{
dump_gcov_info ();
return 0;
}
The merge-stream subcommand of gcov-tool may be used to deserialize the data stream generated by the
"__gcov_filename_to_gcfn" and "__gcov_info_to_gcda" functions and merge the profile information into .gcda files
on the host filesystem.
-fprofile-note=path
If path is specified, GCC saves .gcno file into path location. If you combine the option with multiple source
files, the .gcno file will be overwritten.
-fprofile-prefix-path=path
This option can be used in combination with profile-generate=profile_dir and profile-use=profile_dir to inform
GCC where is the base directory of built source tree. By default profile_dir will contain files with mangled
absolute paths of all object files in the built project. This is not desirable when directory used to build the
instrumented binary differs from the directory used to build the binary optimized with profile feedback because
the profile data will not be found during the optimized build. In such setups -fprofile-prefix-path=path with
path pointing to the base directory of the build can be used to strip the irrelevant part of the path and keep
all file names relative to the main build directory.
-fprofile-prefix-map=old=new
When compiling files residing in directory old, record profiling information (with --coverage) describing them as
if the files resided in directory new instead. See also -ffile-prefix-map and -fcanon-prefix-map.
-fprofile-update=method
Alter the update method for an application instrumented for profile feedback based optimization. The method
argument should be one of single, atomic or prefer-atomic. The first one is useful for single-threaded
applications, while the second one prevents profile corruption by emitting thread-safe code.
Warning: When an application does not properly join all threads (or creates an detached thread), a profile file
can be still corrupted.
Using prefer-atomic would be transformed either to atomic, when supported by a target, or to single otherwise.
The GCC driver automatically selects prefer-atomic when -pthread is present in the command line, otherwise the
default method is single.
If atomic is selected, then the profile information is updated using atomic operations on a best-effort basis.
Ideally, the profile information is updated through atomic operations in hardware. If the target platform does
not support the required atomic operations in hardware, however, libatomic is available, then the profile
information is updated through calls to libatomic. If the target platform neither supports the required atomic
operations in hardware nor libatomic, then the profile information is not atomically updated and a warning is
issued. In this case, the obtained profiling information may be corrupt for multi-threaded applications.
For performance reasons, if 64-bit counters are used for the profiling information and the target platform only
supports 32-bit atomic operations in hardware, then the performance critical profiling updates are done using two
32-bit atomic operations for each counter update. If a signal interrupts these two operations updating a
counter, then the profiling information may be in an inconsistent state.
-fprofile-filter-files=regex
Instrument only functions from files whose name matches any of the regular expressions (separated by semi-
colons).
For example, -fprofile-filter-files=main\.c;module.*\.c will instrument only main.c and all C files starting with
'module'.
-fprofile-exclude-files=regex
Instrument only functions from files whose name does not match any of the regular expressions (separated by semi-
colons).
For example, -fprofile-exclude-files=/usr/.* will prevent instrumentation of all files that are located in the
/usr/ folder.
-fprofile-reproducible=[multithreaded|parallel-runs|serial]
Control level of reproducibility of profile gathered by "-fprofile-generate". This makes it possible to rebuild
program with same outcome which is useful, for example, for distribution packages.
With -fprofile-reproducible=serial the profile gathered by -fprofile-generate is reproducible provided the
trained program behaves the same at each invocation of the train run, it is not multi-threaded and profile data
streaming is always done in the same order. Note that profile streaming happens at the end of program run but
also before "fork" function is invoked.
Note that it is quite common that execution counts of some part of programs depends, for example, on length of
temporary file names or memory space randomization (that may affect hash-table collision rate). Such non-
reproducible part of programs may be annotated by "no_instrument_function" function attribute. gcov-dump with -l
can be used to dump gathered data and verify that they are indeed reproducible.
With -fprofile-reproducible=parallel-runs collected profile stays reproducible regardless the order of streaming
of the data into gcda files. This setting makes it possible to run multiple instances of instrumented program in
parallel (such as with "make -j"). This reduces quality of gathered data, in particular of indirect call
profiling.
-fsanitize=address
Enable AddressSanitizer, a fast memory error detector. Memory access instructions are instrumented to detect
out-of-bounds and use-after-free bugs. The option enables -fsanitize-address-use-after-scope. See
for more details. The run-time behavior can be
influenced using the ASAN_OPTIONS environment variable. When set to "help=1", the available options are shown at
startup of the instrumented program. See
for a list of supported options.
The option cannot be combined with -fsanitize=thread or -fsanitize=hwaddress. Note that the only targets
-fsanitize=hwaddress is currently supported on are x86-64 (only with "-mlam=u48" or "-mlam=u57" options) and
AArch64, in both cases only in ABIs with 64-bit pointers.
When compiling with -fsanitize=address, you should also use -g to produce more meaningful output. To get more
accurate stack traces, it is possible to use options such as -O0, -O1, or -Og (which, for instance, prevent most
function inlining), -fno-optimize-sibling-calls (which prevents optimizing sibling and tail recursive calls; this
option is implicit for -O0, -O1, or -Og), or -fno-ipa-icf (which disables Identical Code Folding for functions).
Using -fno-omit-frame-pointer also improves stack traces. Since multiple runs of the program may yield
backtraces with different addresses due to ASLR (Address Space Layout Randomization), it may be desirable to turn
ASLR off. On Linux, this can be achieved with setarch `uname -m` -R ./prog.
-fsanitize=kernel-address
Enable AddressSanitizer for Linux kernel. See for more details.
-fsanitize=hwaddress
Enable Hardware-assisted AddressSanitizer, which uses a hardware ability to ignore the top byte of a pointer to
allow the detection of memory errors with a low memory overhead. Memory access instructions are instrumented to
detect out-of-bounds and use-after-free bugs. The option enables -fsanitize-address-use-after-scope. See
for more details. The run-time
behavior can be influenced using the HWASAN_OPTIONS environment variable. When set to "help=1", the available
options are shown at startup of the instrumented program. The option cannot be combined with -fsanitize=thread
or -fsanitize=address, and is currently only available on AArch64.
-fsanitize=kernel-hwaddress
Enable Hardware-assisted AddressSanitizer for compilation of the Linux kernel. Similar to
-fsanitize=kernel-address but using an alternate instrumentation method, and similar to -fsanitize=hwaddress but
with instrumentation differences necessary for compiling the Linux kernel. These differences are to avoid hwasan
library initialization calls and to account for the stack pointer having a different value in its top byte.
Note: This option has different defaults to the -fsanitize=hwaddress. Instrumenting the stack and alloca calls
are not on by default but are still possible by specifying the command-line options --param
hwasan-instrument-stack=1 and --param hwasan-instrument-allocas=1 respectively. Using a random frame tag is not
implemented for kernel instrumentation.
-fsanitize=pointer-compare
Instrument comparison operation (<, <=, >, >=) with pointer operands. The option must be combined with either
-fsanitize=kernel-address or -fsanitize=address The option cannot be combined with -fsanitize=thread. Note: By
default the check is disabled at run time. To enable it, add "detect_invalid_pointer_pairs=2" to the environment
variable ASAN_OPTIONS. Using "detect_invalid_pointer_pairs=1" detects invalid operation only when both pointers
are non-null.
-fsanitize=pointer-subtract
Instrument subtraction with pointer operands. The option must be combined with either -fsanitize=kernel-address
or -fsanitize=address The option cannot be combined with -fsanitize=thread. Note: By default the check is
disabled at run time. To enable it, add "detect_invalid_pointer_pairs=2" to the environment variable
ASAN_OPTIONS. Using "detect_invalid_pointer_pairs=1" detects invalid operation only when both pointers are non-
null.
-fsanitize=shadow-call-stack
Enable ShadowCallStack, a security enhancement mechanism used to protect programs against return address
overwrites (e.g. stack buffer overflows.) It works by saving a function's return address to a separately
allocated shadow call stack in the function prologue and restoring the return address from the shadow call stack
in the function epilogue. Instrumentation only occurs in functions that need to save the return address to the
stack.
Currently it only supports the aarch64 platform. It is specifically designed for linux kernels that enable the
CONFIG_SHADOW_CALL_STACK option. For the user space programs, runtime support is not currently provided in libc
and libgcc. Users who want to use this feature in user space need to provide their own support for the runtime.
It should be noted that this may cause the ABI rules to be broken.
On aarch64, the instrumentation makes use of the platform register "x18". This generally means that any code
that may run on the same thread as code compiled with ShadowCallStack must be compiled with the flag -ffixed-x18,
otherwise functions compiled without -ffixed-x18 might clobber "x18" and so corrupt the shadow stack pointer.
Also, because there is no userspace runtime support, code compiled with ShadowCallStack cannot use exception
handling. Use -fno-exceptions to turn off exceptions.
See for more details.
-fsanitize=thread
Enable ThreadSanitizer, a fast data race detector. Memory access instructions are instrumented to detect data
race bugs. See for more details. The run-time
behavior can be influenced using the TSAN_OPTIONS environment variable; see
for a list of supported options. The option
cannot be combined with -fsanitize=address, -fsanitize=leak.
When compiling with -fsanitize=thread, you should also use -g to produce more meaningful output.
Note that sanitized atomic builtins cannot throw exceptions when operating on invalid memory addresses with non-
call exceptions (-fnon-call-exceptions).
-fsanitize=leak
Enable LeakSanitizer, a memory leak detector. This option only matters for linking of executables. The
executable is linked against a library that overrides "malloc" and other allocator functions. See
for more details. The run-time
behavior can be influenced using the LSAN_OPTIONS environment variable. The option cannot be combined with
-fsanitize=thread.
-fsanitize=undefined
Enable UndefinedBehaviorSanitizer, a fast undefined behavior detector. Various computations are instrumented to
detect undefined behavior at runtime. See for more
details. The run-time behavior can be influenced using the UBSAN_OPTIONS environment variable. Current
suboptions are:
-fsanitize=shift
This option enables checking that the result of a shift operation is not undefined. Note that what exactly
is considered undefined differs slightly between C and C++, as well as between ISO C90 and C99, etc. This
option has two suboptions, -fsanitize=shift-base and -fsanitize=shift-exponent.
-fsanitize=shift-exponent
This option enables checking that the second argument of a shift operation is not negative and is smaller
than the precision of the promoted first argument.
-fsanitize=shift-base
If the second argument of a shift operation is within range, check that the result of a shift operation is
not undefined. Note that what exactly is considered undefined differs slightly between C and C++, as well as
between ISO C90 and C99, etc.
-fsanitize=integer-divide-by-zero
Detect integer division by zero.
-fsanitize=unreachable
With this option, the compiler turns the "__builtin_unreachable" call into a diagnostics message call
instead. When reaching the "__builtin_unreachable" call, the behavior is undefined.
-fsanitize=vla-bound
This option instructs the compiler to check that the size of a variable length array is positive.
-fsanitize=null
This option enables pointer checking. Particularly, the application built with this option turned on will
issue an error message when it tries to dereference a NULL pointer, or if a reference (possibly an rvalue
reference) is bound to a NULL pointer, or if a method is invoked on an object pointed by a NULL pointer.
-fsanitize=return
This option enables return statement checking. Programs built with this option turned on will issue an error
message when the end of a non-void function is reached without actually returning a value. This option works
in C++ only.
-fsanitize=signed-integer-overflow
This option enables signed integer overflow checking. We check that the result of "+", "*", and both unary
and binary "-" does not overflow in the signed arithmetics. This also detects "INT_MIN / -1" signed
division. Note, integer promotion rules must be taken into account. That is, the following is not an
overflow:
signed char a = SCHAR_MAX;
a++;
-fsanitize=bounds
This option enables instrumentation of array bounds. Various out of bounds accesses are detected. Flexible
array members, flexible array member-like arrays, and initializers of variables with static storage are not
instrumented, with the exception of flexible array member-like arrays for which "-fstrict-flex-arrays" or
"-fstrict-flex-arrays=" options or "strict_flex_array" attributes say they shouldn't be treated like flexible
array member-like arrays.
-fsanitize=bounds-strict
This option enables strict instrumentation of array bounds. Most out of bounds accesses are detected,
including flexible array member-like arrays. Initializers of variables with static storage are not
instrumented.
-fsanitize=alignment
This option enables checking of alignment of pointers when they are dereferenced, or when a reference is
bound to insufficiently aligned target, or when a method or constructor is invoked on insufficiently aligned
object.
-fsanitize=object-size
This option enables instrumentation of memory references using the "__builtin_dynamic_object_size" function.
Various out of bounds pointer accesses are detected.
-fsanitize=float-divide-by-zero
Detect floating-point division by zero. Unlike other similar options, -fsanitize=float-divide-by-zero is not
enabled by -fsanitize=undefined, since floating-point division by zero can be a legitimate way of obtaining
infinities and NaNs.
-fsanitize=float-cast-overflow
This option enables floating-point type to integer conversion checking. We check that the result of the
conversion does not overflow. Unlike other similar options, -fsanitize=float-cast-overflow is not enabled by
-fsanitize=undefined. This option does not work well with "FE_INVALID" exceptions enabled.
-fsanitize=nonnull-attribute
This option enables instrumentation of calls, checking whether null values are not passed to arguments marked
as requiring a non-null value by the "nonnull" function attribute.
-fsanitize=returns-nonnull-attribute
This option enables instrumentation of return statements in functions marked with "returns_nonnull" function
attribute, to detect returning of null values from such functions.
-fsanitize=bool
This option enables instrumentation of loads from bool. If a value other than 0/1 is loaded, a run-time
error is issued.
-fsanitize=enum
This option enables instrumentation of loads from an enum type. If a value outside the range of values for
the enum type is loaded, a run-time error is issued.
-fsanitize=vptr
This option enables instrumentation of C++ member function calls, member accesses and some conversions
between pointers to base and derived classes, to verify the referenced object has the correct dynamic type.
-fsanitize=pointer-overflow
This option enables instrumentation of pointer arithmetics. If the pointer arithmetics overflows, a run-time
error is issued.
-fsanitize=builtin
This option enables instrumentation of arguments to selected builtin functions. If an invalid value is
passed to such arguments, a run-time error is issued. E.g. passing 0 as the argument to "__builtin_ctz" or
"__builtin_clz" invokes undefined behavior and is diagnosed by this option.
Note that sanitizers tend to increase the rate of false positive warnings, most notably those around
-Wmaybe-uninitialized. We recommend against combining -Werror and [the use of] sanitizers.
While -ftrapv causes traps for signed overflows to be emitted, -fsanitize=undefined gives a diagnostic message.
This currently works only for the C family of languages.
-fno-sanitize=all
This option disables all previously enabled sanitizers. -fsanitize=all is not allowed, as some sanitizers cannot
be used together.
-fasan-shadow-offset=number
This option forces GCC to use custom shadow offset in AddressSanitizer checks. It is useful for experimenting
with different shadow memory layouts in Kernel AddressSanitizer.
-fsanitize-sections=s1,s2,...
Sanitize global variables in selected user-defined sections. si may contain wildcards.
-fsanitize-recover[=opts]
-fsanitize-recover= controls error recovery mode for sanitizers mentioned in comma-separated list of opts.
Enabling this option for a sanitizer component causes it to attempt to continue running the program as if no
error happened. This means multiple runtime errors can be reported in a single program run, and the exit code of
the program may indicate success even when errors have been reported. The -fno-sanitize-recover= option can be
used to alter this behavior: only the first detected error is reported and program then exits with a non-zero
exit code.
Currently this feature only works for -fsanitize=undefined (and its suboptions except for -fsanitize=unreachable
and -fsanitize=return), -fsanitize=float-cast-overflow, -fsanitize=float-divide-by-zero,
-fsanitize=bounds-strict, -fsanitize=kernel-address and -fsanitize=address. For these sanitizers error recovery
is turned on by default, except -fsanitize=address, for which this feature is experimental.
-fsanitize-recover=all and -fno-sanitize-recover=all is also accepted, the former enables recovery for all
sanitizers that support it, the latter disables recovery for all sanitizers that support it.
Even if a recovery mode is turned on the compiler side, it needs to be also enabled on the runtime library side,
otherwise the failures are still fatal. The runtime library defaults to "halt_on_error=0" for ThreadSanitizer
and UndefinedBehaviorSanitizer, while default value for AddressSanitizer is "halt_on_error=1". This can be
overridden through setting the "halt_on_error" flag in the corresponding environment variable.
Syntax without an explicit opts parameter is deprecated. It is equivalent to specifying an opts list of:
undefined,float-cast-overflow,float-divide-by-zero,bounds-strict
-fsanitize-address-use-after-scope
Enable sanitization of local variables to detect use-after-scope bugs. The option sets -fstack-reuse to none.
-fsanitize-trap[=opts]
The -fsanitize-trap= option instructs the compiler to report for sanitizers mentioned in comma-separated list of
opts undefined behavior using "__builtin_trap" rather than a "libubsan" library routine. If this option is
enabled for certain sanitizer, it takes precedence over the -fsanitizer-recover= for that sanitizer,
"__builtin_trap" will be emitted and be fatal regardless of whether recovery is enabled or disabled using
-fsanitize-recover=.
The advantage of this is that the "libubsan" library is not needed and is not linked in, so this is usable even
in freestanding environments.
Currently this feature works with -fsanitize=undefined (and its suboptions except for -fsanitize=vptr),
-fsanitize=float-cast-overflow, -fsanitize=float-divide-by-zero and -fsanitize=bounds-strict.
"-fsanitize-trap=all" can be also specified, which enables it for "undefined" suboptions,
-fsanitize=float-cast-overflow, -fsanitize=float-divide-by-zero and -fsanitize=bounds-strict. If
"-fsanitize-trap=undefined" or "-fsanitize-trap=all" is used and "-fsanitize=vptr" is enabled on the command
line, the instrumentation is silently ignored as the instrumentation always needs "libubsan" support,
-fsanitize-trap=vptr is not allowed.
-fsanitize-undefined-trap-on-error
The -fsanitize-undefined-trap-on-error option is deprecated equivalent of -fsanitize-trap=all.
-fsanitize-coverage=trace-pc
Enable coverage-guided fuzzing code instrumentation. Inserts a call to "__sanitizer_cov_trace_pc" into every
basic block.
-fsanitize-coverage=trace-cmp
Enable dataflow guided fuzzing code instrumentation. Inserts a call to "__sanitizer_cov_trace_cmp1",
"__sanitizer_cov_trace_cmp2", "__sanitizer_cov_trace_cmp4" or "__sanitizer_cov_trace_cmp8" for integral
comparison with both operands variable or "__sanitizer_cov_trace_const_cmp1", "__sanitizer_cov_trace_const_cmp2",
"__sanitizer_cov_trace_const_cmp4" or "__sanitizer_cov_trace_const_cmp8" for integral comparison with one operand
constant, "__sanitizer_cov_trace_cmpf" or "__sanitizer_cov_trace_cmpd" for float or double comparisons and
"__sanitizer_cov_trace_switch" for switch statements.
-fcf-protection=[full|branch|return|none|check]
-fcf-protection
Enable code instrumentation to increase program security by checking that target addresses of control-flow
transfer instructions (such as indirect function call, function return, indirect jump) are valid. This prevents
diverting the flow of control to an unexpected target. This is intended to protect against such threats as
Return-oriented Programming (ROP), and similarly call/jmp-oriented programming (COP/JOP).
The -fcf-protection= keywords are interpreted as follows.
The value "branch" tells the compiler to implement checking of validity of control-flow transfer at the point of
indirect branch instructions, i.e. call/jmp instructions.
The value "return" implements checking of validity at the point of returning from a function.
The value "full" is an alias for specifying both "branch" and "return".
The value "check" is used for the final link with link-time optimization (LTO). An error is issued if LTO object
files are compiled with different -fcf-protection values. The value "check" is ignored at the compile time.
The value "none" turns off instrumentation.
-fcf-protection is an alias for -fcf-protection=full. To override a previous -fcf-protection option on the
command line, add -fcf-protection=none and then -fcf-protection=kind.
The macro "__CET__" is defined when -fcf-protection is used. The first bit of "__CET__" is set to 1 for the
value "branch" and the second bit of "__CET__" is set to 1 for the "return".
You can also use the "nocf_check" attribute to identify which functions and calls should be skipped from
instrumentation.
Currently the x86 GNU/Linux target provides an implementation based on Intel Control-flow Enforcement Technology
(CET) which works for i686 processor or newer.
NOTE: In Ubuntu 19.10 and later versions, -fcf-protection is enabled by default for C, C++, ObjC, ObjC++, if none
of -fno-cf-protection nor -fcf-protection=* are found.
-fharden-compares
For every logical test that survives gimple optimizations and is not the condition in a conditional branch (for
example, conditions tested for conditional moves, or to store in boolean variables), emit extra code to compute
and verify the reversed condition, and to call "__builtin_trap" if the results do not match. Use with
-fharden-conditional-branches to cover all conditionals.
-fharden-conditional-branches
For every non-vectorized conditional branch that survives gimple optimizations, emit extra code to compute and
verify the reversed condition, and to call "__builtin_trap" if the result is unexpected. Use with
-fharden-compares to cover all conditionals.
-fharden-control-flow-redundancy
Emit extra code to set booleans when entering basic blocks, and to verify and trap, at function exits, when the
booleans do not form an execution path that is compatible with the control flow graph.
Verification takes place before returns, before mandatory tail calls (see below) and, optionally, before escaping
exceptions with -fhardcfr-check-exceptions, before returning calls with -fhardcfr-check-returning-calls, and
before noreturn calls with -fhardcfr-check-noreturn-calls). Tuning options --param hardcfr-max-blocks and
--param hardcfr-max-inline-blocks are available.
Tail call optimization takes place too late to affect control flow redundancy, but calls annotated as mandatory
tail calls by language front-ends, and any calls marked early enough as potential tail calls would also have
verification issued before the call, but these possibilities are merely theoretical, as these conditions can only
be met when using custom compiler plugins.
-fhardcfr-skip-leaf
Disable -fharden-control-flow-redundancy in leaf functions.
-fhardcfr-check-exceptions
When -fharden-control-flow-redundancy is active, check the recorded execution path against the control flow graph
at exception escape points, as if the function body was wrapped with a cleanup handler that performed the check
and reraised. This option is enabled by default; use -fno-hardcfr-check-exceptions to disable it.
-fhardcfr-check-returning-calls
When -fharden-control-flow-redundancy is active, check the recorded execution path against the control flow graph
before any function call immediately followed by a return of its result, if any, so as to not prevent tail-call
optimization, whether or not it is ultimately optimized to a tail call.
This option is enabled by default whenever sibling call optimizations are enabled (see -foptimize-sibling-calls),
but it can be enabled (or disabled, using its negated form) explicitly, regardless of the optimizations.
-fhardcfr-check-noreturn-calls=[always|no-xthrow|nothrow|never]
When -fharden-control-flow-redundancy is active, check the recorded execution path against the control flow graph
before "noreturn" calls, either all of them (always), those that aren't expected to return control to the caller
through an exception (no-xthrow, the default), those that may not return control to the caller through an
exception either (nothrow), or none of them (never).
Checking before a "noreturn" function that may return control to the caller through an exception may cause
checking to be performed more than once, if the exception is caught in the caller, whether by a handler or a
cleanup. When -fhardcfr-check-exceptions is also enabled, the compiler will avoid associating a "noreturn" call
with the implicitly-added cleanup handler, since it would be redundant with the check performed before the call,
but other handlers or cleanups in the function, if activated, will modify the recorded execution path and check
it again when another checkpoint is hit. The checkpoint may even be another "noreturn" call, so checking may end
up performed multiple times.
Various optimizers may cause calls to be marked as "noreturn" and/or "nothrow", even in the absence of the
corresponding attributes, which may affect the placement of checks before calls, as well as the addition of
implicit cleanup handlers for them. This unpredictability, and the fact that raising and reraising exceptions
frequently amounts to implicitly calling "noreturn" functions, have made no-xthrow the default setting for this
option: it excludes from the "noreturn" treatment only internal functions used to (re)raise exceptions, that are
not affected by these optimizations.
-fhardened
Enable a set of flags for C and C++ that improve the security of the generated code without affecting its ABI.
The precise flags enabled may change between major releases of GCC, but are currently:
-D_FORTIFY_SOURCE=3 -D_GLIBCXX_ASSERTIONS -ftrivial-auto-var-init=zero -fPIE -pie -Wl,-z,relro,-z,now
-fstack-protector-strong -fstack-clash-protection -fcf-protection=full (x86 GNU/Linux only)
The list of options enabled by -fhardened can be generated using the --help=hardened option.
When the system glibc is older than 2.35, -D_FORTIFY_SOURCE=2 is used instead.
This option is intended to be used in production builds, not merely in debug builds.
Currently, -fhardened is only supported on GNU/Linux targets.
-fhardened only enables a particular option if it wasn't already specified anywhere on the command line. For
instance, -fhardened -fstack-protector will only enable -fstack-protector, but not -fstack-protector-strong.
-fstack-protector
Emit extra code to check for buffer overflows, such as stack smashing attacks. This is done by adding a guard
variable to functions with vulnerable objects. This includes functions that call "alloca", and functions with
buffers larger than or equal to 8 bytes. The guards are initialized when a function is entered and then checked
when the function exits. If a guard check fails, an error message is printed and the program exits. Only
variables that are actually allocated on the stack are considered, optimized away variables or variables
allocated in registers don't count.
-fstack-protector-all
Like -fstack-protector except that all functions are protected.
-fstack-protector-strong
Like -fstack-protector but includes additional functions to be protected --- those that have local array
definitions, or have references to local frame addresses. Only variables that are actually allocated on the
stack are considered, optimized away variables or variables allocated in registers don't count.
NOTE: In Ubuntu 14.10 and later versions, -fstack-protector-strong is enabled by default for C, C++, ObjC,
ObjC++, if none of -fno-stack-protector, -nostdlib, nor -ffreestanding are found.
-fstack-protector-explicit
Like -fstack-protector but only protects those functions which have the "stack_protect" attribute.
-fstack-check
Generate code to verify that you do not go beyond the boundary of the stack. You should specify this flag if you
are running in an environment with multiple threads, but you only rarely need to specify it in a single-threaded
environment since stack overflow is automatically detected on nearly all systems if there is only one stack.
Note that this switch does not actually cause checking to be done; the operating system or the language runtime
must do that. The switch causes generation of code to ensure that they see the stack being extended.
You can additionally specify a string parameter: no means no checking, generic means force the use of old-style
checking, specific means use the best checking method and is equivalent to bare -fstack-check.
Old-style checking is a generic mechanism that requires no specific target support in the compiler but comes with
the following drawbacks:
1. Modified allocation strategy for large objects: they are always allocated dynamically if their size exceeds a
fixed threshold. Note this may change the semantics of some code.
2. Fixed limit on the size of the static frame of functions: when it is topped by a particular function, stack
checking is not reliable and a warning is issued by the compiler.
3. Inefficiency: because of both the modified allocation strategy and the generic implementation, code
performance is hampered.
Note that old-style stack checking is also the fallback method for specific if no target support has been added
in the compiler.
-fstack-check= is designed for Ada's needs to detect infinite recursion and stack overflows. specific is an
excellent choice when compiling Ada code. It is not generally sufficient to protect against stack-clash attacks.
To protect against those you want -fstack-clash-protection.
-fstack-clash-protection
Generate code to prevent stack clash style attacks. When this option is enabled, the compiler will only allocate
one page of stack space at a time and each page is accessed immediately after allocation. Thus, it prevents
allocations from jumping over any stack guard page provided by the operating system.
Most targets do not fully support stack clash protection. However, on those targets -fstack-clash-protection
will protect dynamic stack allocations. -fstack-clash-protection may also provide limited protection for static
stack allocations if the target supports -fstack-check=specific.
NOTE: In Ubuntu 19.10 and later versions, -fstack-clash-protection is enabled by default for C, C++, ObjC,
ObjC++, unless -fno-stack-clash-protection is found.
-fstack-limit-register=reg
-fstack-limit-symbol=sym
-fno-stack-limit
Generate code to ensure that the stack does not grow beyond a certain value, either the value of a register or
the address of a symbol. If a larger stack is required, a signal is raised at run time. For most targets, the
signal is raised before the stack overruns the boundary, so it is possible to catch the signal without taking
special precautions.
For instance, if the stack starts at absolute address 0x80000000 and grows downwards, you can use the flags
-fstack-limit-symbol=__stack_limit and -Wl,--defsym,__stack_limit=0x7ffe0000 to enforce a stack limit of 128KB.
Note that this may only work with the GNU linker.
You can locally override stack limit checking by using the "no_stack_limit" function attribute.
-fsplit-stack
Generate code to automatically split the stack before it overflows. The resulting program has a discontiguous
stack which can only overflow if the program is unable to allocate any more memory. This is most useful when
running threaded programs, as it is no longer necessary to calculate a good stack size to use for each thread.
This is currently only implemented for the x86 targets running GNU/Linux.
When code compiled with -fsplit-stack calls code compiled without -fsplit-stack, there may not be much stack
space available for the latter code to run. If compiling all code, including library code, with -fsplit-stack is
not an option, then the linker can fix up these calls so that the code compiled without -fsplit-stack always has
a large stack. Support for this is implemented in the gold linker in GNU binutils release 2.21 and later.
-fstrub=disable
Disable stack scrubbing entirely, ignoring any "strub" attributes. See
-fstrub=strict
Functions default to "strub" mode "disabled", and apply strictly the restriction that only functions associated
with "strub"-"callable" modes ("at-calls", "callable" and "always_inline" "internal") are "callable" by functions
with "strub"-enabled modes ("at-calls" and "internal").
-fstrub=relaxed
Restore the default stack scrub ("strub") setting, namely, "strub" is only enabled as required by "strub"
attributes associated with function and data types. "Relaxed" means that strub contexts are only prevented from
calling functions explicitly associated with "strub" mode "disabled". This option is only useful to override
other -fstrub=* options that precede it in the command line.
-fstrub=at-calls
Enable "at-calls" "strub" mode where viable. The primary use of this option is for testing. It exercises the
"strub" machinery in scenarios strictly local to a translation unit. This "strub" mode modifies function
interfaces, so any function that is visible to other translation units, or that has its address taken, will not
be affected by this option. Optimization options may also affect viability. See the "strub" attribute
documentation for details on viability and eligibility requirements.
-fstrub=internal
Enable "internal" "strub" mode where viable. The primary use of this option is for testing. This option is
intended to exercise thoroughly parts of the "strub" machinery that implement the less efficient, but interface-
preserving "strub" mode. Functions that would not be affected by this option are quite uncommon.
-fstrub=all
Enable some "strub" mode where viable. When both strub modes are viable, "at-calls" is preferred.
-fdump-ipa-strubm adds function attributes that tell which mode was selected for each function. The primary use
of this option is for testing, to exercise thoroughly the "strub" machinery.
-fvtable-verify=[std|preinit|none]
This option is only available when compiling C++ code. It turns on (or off, if using -fvtable-verify=none) the
security feature that verifies at run time, for every virtual call, that the vtable pointer through which the
call is made is valid for the type of the object, and has not been corrupted or overwritten. If an invalid
vtable pointer is detected at run time, an error is reported and execution of the program is immediately halted.
This option causes run-time data structures to be built at program startup, which are used for verifying the
vtable pointers. The options std and preinit control the timing of when these data structures are built. In
both cases the data structures are built before execution reaches "main". Using -fvtable-verify=std causes the
data structures to be built after shared libraries have been loaded and initialized. -fvtable-verify=preinit
causes them to be built before shared libraries have been loaded and initialized.
If this option appears multiple times in the command line with different values specified, none takes highest
priority over both std and preinit; preinit takes priority over std.
-fvtv-debug
When used in conjunction with -fvtable-verify=std or -fvtable-verify=preinit, causes debug versions of the
runtime functions for the vtable verification feature to be called. This flag also causes the compiler to log
information about which vtable pointers it finds for each class. This information is written to a file named
vtv_set_ptr_data.log in the directory named by the environment variable VTV_LOGS_DIR if that is defined or the
current working directory otherwise.
Note: This feature appends data to the log file. If you want a fresh log file, be sure to delete any existing
one.
-fvtv-counts
This is a debugging flag. When used in conjunction with -fvtable-verify=std or -fvtable-verify=preinit, this
causes the compiler to keep track of the total number of virtual calls it encounters and the number of
verifications it inserts. It also counts the number of calls to certain run-time library functions that it
inserts and logs this information for each compilation unit. The compiler writes this information to a file
named vtv_count_data.log in the directory named by the environment variable VTV_LOGS_DIR if that is defined or
the current working directory otherwise. It also counts the size of the vtable pointer sets for each class, and
writes this information to vtv_class_set_sizes.log in the same directory.
Note: This feature appends data to the log files. To get fresh log files, be sure to delete any existing ones.
-finstrument-functions
Generate instrumentation calls for entry and exit to functions. Just after function entry and just before
function exit, the following profiling functions are called with the address of the current function and its call
site. (On some platforms, "__builtin_return_address" does not work beyond the current function, so the call site
information may not be available to the profiling functions otherwise.)
void __cyg_profile_func_enter (void *this_fn,
void *call_site);
void __cyg_profile_func_exit (void *this_fn,
void *call_site);
The first argument is the address of the start of the current function, which may be looked up exactly in the
symbol table.
This instrumentation is also done for functions expanded inline in other functions. The profiling calls indicate
where, conceptually, the inline function is entered and exited. This means that addressable versions of such
functions must be available. If all your uses of a function are expanded inline, this may mean an additional
expansion of code size. If you use "extern inline" in your C code, an addressable version of such functions must
be provided. (This is normally the case anyway, but if you get lucky and the optimizer always expands the
functions inline, you might have gotten away without providing static copies.)
A function may be given the attribute "no_instrument_function", in which case this instrumentation is not done.
This can be used, for example, for the profiling functions listed above, high-priority interrupt routines, and
any functions from which the profiling functions cannot safely be called (perhaps signal handlers, if the
profiling routines generate output or allocate memory).
-finstrument-functions-once
This is similar to -finstrument-functions, but the profiling functions are called only once per instrumented
function, i.e. the first profiling function is called after the first entry into the instrumented function and
the second profiling function is called before the exit corresponding to this first entry.
The definition of "once" for the purpose of this option is a little vague because the implementation is not
protected against data races. As a result, the implementation only guarantees that the profiling functions are
called at least once per process and at most once per thread, but the calls are always paired, that is to say, if
a thread calls the first function, then it will call the second function, unless it never reaches the exit of the
instrumented function.
-finstrument-functions-exclude-file-list=file,file,...
Set the list of functions that are excluded from instrumentation (see the description of -finstrument-functions).
If the file that contains a function definition matches with one of file, then that function is not instrumented.
The match is done on substrings: if the file parameter is a substring of the file name, it is considered to be a
match.
For example:
-finstrument-functions-exclude-file-list=/bits/stl,include/sys
excludes any inline function defined in files whose pathnames contain /bits/stl or include/sys.
If, for some reason, you want to include letter , in one of sym, write ,. For example,
-finstrument-functions-exclude-file-list=',,tmp' (note the single quote surrounding the option).
-finstrument-functions-exclude-function-list=sym,sym,...
This is similar to -finstrument-functions-exclude-file-list, but this option sets the list of function names to
be excluded from instrumentation. The function name to be matched is its user-visible name, such as "vector
blah(const vector &)", not the internal mangled name (e.g., "_Z4blahRSt6vectorIiSaIiEE"). The match is done
on substrings: if the sym parameter is a substring of the function name, it is considered to be a match. For C99
and C++ extended identifiers, the function name must be given in UTF-8, not using universal character names.
-fpatchable-function-entry=N[,M]
Generate N NOPs right at the beginning of each function, with the function entry point before the Mth NOP. If M
is omitted, it defaults to 0 so the function entry points to the address just at the first NOP. The NOP
instructions reserve extra space which can be used to patch in any desired instrumentation at run time, provided
that the code segment is writable. The amount of space is controllable indirectly via the number of NOPs; the
NOP instruction used corresponds to the instruction emitted by the internal GCC back-end interface "gen_nop".
This behavior is target-specific and may also depend on the architecture variant and/or other compilation
options.
For run-time identification, the starting addresses of these areas, which correspond to their respective function
entries minus M, are additionally collected in the "__patchable_function_entries" section of the resulting
binary.
Note that the value of "__attribute__ ((patchable_function_entry (N,M)))" takes precedence over command-line
option -fpatchable-function-entry=N,M. This can be used to increase the area size or to remove it completely on
a single function. If "N=0", no pad location is recorded.
The NOP instructions are inserted at---and maybe before, depending on M---the function entry address, even before
the prologue. On PowerPC with the ELFv2 ABI, for a function with dual entry points, the local entry point is
this function entry address by default. See the -msplit-patch-nops option to change this.
The maximum value of N and M is 65535. On PowerPC with the ELFv2 ABI, for a function with dual entry points, the
supported values for M are 0, 2, 6 and 14 when not using -msplit-patch-nops.
Options Controlling the Preprocessor
These options control the C preprocessor, which is run on each C source file before actual compilation.
If you use the -E option, nothing is done except preprocessing. Some of these options make sense only together with
-E because they cause the preprocessor output to be unsuitable for actual compilation.
In addition to the options listed here, there are a number of options to control search paths for include files
documented in Directory Options. Options to control preprocessor diagnostics are listed in Warning Options.
-D name
Predefine name as a macro, with definition 1.
-D name=definition
The contents of definition are tokenized and processed as if they appeared during translation phase three in a
#define directive. In particular, the definition is truncated by embedded newline characters.
If you are invoking the preprocessor from a shell or shell-like program you may need to use the shell's quoting
syntax to protect characters such as spaces that have a meaning in the shell syntax.
If you wish to define a function-like macro on the command line, write its argument list with surrounding
parentheses before the equals sign (if any). Parentheses are meaningful to most shells, so you should quote the
option. With sh and csh, -D'name(args...)=definition' works.
-D and -U options are processed in the order they are given on the command line. All -imacros file and -include
file options are processed after all -D and -U options.
-U name
Cancel any previous definition of name, either built in or provided with a -D option.
-include file
Process file as if "#include "file"" appeared as the first line of the primary source file. However, the first
directory searched for file is the preprocessor's working directory instead of the directory containing the main
source file. If not found there, it is searched for in the remainder of the "#include "..."" search chain as
normal.
If multiple -include options are given, the files are included in the order they appear on the command line.
-imacros file
Exactly like -include, except that any output produced by scanning file is thrown away. Macros it defines remain
defined. This allows you to acquire all the macros from a header without also processing its declarations.
All files specified by -imacros are processed before all files specified by -include.
-undef
Do not predefine any system-specific or GCC-specific macros. The standard predefined macros remain defined.
-pthread
Define additional macros required for using the POSIX threads library. You should use this option consistently
for both compilation and linking. This option is supported on GNU/Linux targets, most other Unix derivatives,
and also on x86 Cygwin and MinGW targets.
-M Instead of outputting the result of preprocessing, output a rule suitable for make describing the dependencies of
the main source file. The preprocessor outputs one make rule containing the object file name for that source
file, a colon, and the names of all the included files, including those coming from -include or -imacros command-
line options.
Unless specified explicitly (with -MT or -MQ), the object file name consists of the name of the source file with
any suffix replaced with object file suffix and with any leading directory parts removed. If there are many
included files then the rule is split into several lines using \-newline. The rule has no commands.
This option does not suppress the preprocessor's debug output, such as -dM. To avoid mixing such debug output
with the dependency rules you should explicitly specify the dependency output file with -MF, or use an
environment variable like DEPENDENCIES_OUTPUT. Debug output is still sent to the regular output stream as
normal.
Passing -M to the driver implies -E, and suppresses warnings with an implicit -w.
-MM Like -M but do not mention header files that are found in system header directories, nor header files that are
included, directly or indirectly, from such a header.
This implies that the choice of angle brackets or double quotes in an #include directive does not in itself
determine whether that header appears in -MM dependency output.
-MF file
When used with -M or -MM, specifies a file to write the dependencies to. If no -MF switch is given the
preprocessor sends the rules to the same place it would send preprocessed output.
When used with the driver options -MD or -MMD, -MF overrides the default dependency output file.
If file is -, then the dependencies are written to stdout.
-MG In conjunction with an option such as -M requesting dependency generation, -MG assumes missing header files are
generated files and adds them to the dependency list without raising an error. The dependency filename is taken
directly from the "#include" directive without prepending any path. -MG also suppresses preprocessed output, as
a missing header file renders this useless.
This feature is used in automatic updating of makefiles.
-Mno-modules
Disable dependency generation for compiled module interfaces.
-MP This option instructs CPP to add a phony target for each dependency other than the main file, causing each to
depend on nothing. These dummy rules work around errors make gives if you remove header files without updating
the Makefile to match.
This is typical output:
test.o: test.c test.h
test.h:
-MT target
Change the target of the rule emitted by dependency generation. By default CPP takes the name of the main input
file, deletes any directory components and any file suffix such as .c, and appends the platform's usual object
suffix. The result is the target.
An -MT option sets the target to be exactly the string you specify. If you want multiple targets, you can
specify them as a single argument to -MT, or use multiple -MT options.
For example, -MT '$(objpfx)foo.o' might give
$(objpfx)foo.o: foo.c
-MQ target
Same as -MT, but it quotes any characters which are special to Make. -MQ '$(objpfx)foo.o' gives
$$(objpfx)foo.o: foo.c
The default target is automatically quoted, as if it were given with -MQ.
-MD -MD is equivalent to -M -MF file, except that -E is not implied. The driver determines file based on whether an
-o option is given. If it is, the driver uses its argument but with a suffix of .d, otherwise it takes the name
of the input file, removes any directory components and suffix, and applies a .d suffix.
If -MD is used in conjunction with -E, any -o switch is understood to specify the dependency output file, but if
used without -E, each -o is understood to specify a target object file.
Since -E is not implied, -MD can be used to generate a dependency output file as a side effect of the compilation
process.
-MMD
Like -MD except mention only user header files, not system header files.
-fpreprocessed
Indicate to the preprocessor that the input file has already been preprocessed. This suppresses things like
macro expansion, trigraph conversion, escaped newline splicing, and processing of most directives. The
preprocessor still recognizes and removes comments, so that you can pass a file preprocessed with -C to the
compiler without problems. In this mode the integrated preprocessor is little more than a tokenizer for the
front ends.
-fpreprocessed is implicit if the input file has one of the extensions .i, .ii or .mi. These are the extensions
that GCC uses for preprocessed files created by -save-temps.
-fdirectives-only
When preprocessing, handle directives, but do not expand macros.
The option's behavior depends on the -E and -fpreprocessed options.
With -E, preprocessing is limited to the handling of directives such as "#define", "#ifdef", and "#error". Other
preprocessor operations, such as macro expansion and trigraph conversion are not performed. In addition, the -dD
option is implicitly enabled.
With -fpreprocessed, predefinition of command line and most builtin macros is disabled. Macros such as
"__LINE__", which are contextually dependent, are handled normally. This enables compilation of files previously
preprocessed with "-E -fdirectives-only".
With both -E and -fpreprocessed, the rules for -fpreprocessed take precedence. This enables full preprocessing
of files previously preprocessed with "-E -fdirectives-only".
-fdollars-in-identifiers
Accept $ in identifiers.
-fextended-identifiers
Accept universal character names and extended characters in identifiers. This option is enabled by default for
C99 (and later C standard versions) and C++.
-fno-canonical-system-headers
When preprocessing, do not shorten system header paths with canonicalization.
-fmax-include-depth=depth
Set the maximum depth of the nested #include. The default is 200.
-fsearch-include-path[=kind]
Look for input files on the #include path, not just the current directory. This is particularly useful with
C++20 modules, for which both header units and module interface units need to be compiled directly:
g++ -c -std=c++20 -fmodules -fsearch-include-path bits/stdc++.h bits/std.cc
kind defaults to user, which looks on the #include "..." search path; you can also explicitly specify system for
the "#include <...>" search path.
-ftabstop=width
Set the distance between tab stops. This helps the preprocessor report correct column numbers in warnings or
errors, even if tabs appear on the line. If the value is less than 1 or greater than 100, the option is ignored.
The default is 8.
-ftrack-macro-expansion[=level]
Track locations of tokens across macro expansions. This allows the compiler to emit diagnostic about the current
macro expansion stack when a compilation error occurs in a macro expansion. Using this option makes the
preprocessor and the compiler consume more memory. The level parameter can be used to choose the level of
precision of token location tracking thus decreasing the memory consumption if necessary. Value 0 of level de-
activates this option. Value 1 tracks tokens locations in a degraded mode for the sake of minimal memory
overhead. In this mode all tokens resulting from the expansion of an argument of a function-like macro have the
same location. Value 2 tracks tokens locations completely. This value is the most memory hungry. When this
option is given no argument, the default parameter value is 2.
Note that "-ftrack-macro-expansion=2" is activated by default.
-fmacro-prefix-map=old=new
When preprocessing files residing in directory old, expand the "__FILE__" and "__BASE_FILE__" macros as if the
files resided in directory new instead. This can be used to change an absolute path to a relative path by using
. for new which can result in more reproducible builds that are location independent. This option also affects
"__builtin_FILE()" during compilation. See also -ffile-prefix-map and -fcanon-prefix-map.
-fexec-charset=charset
Set the execution character set, used for string and character constants. The default is UTF-8. charset can be
any encoding supported by the system's "iconv" library routine.
-fwide-exec-charset=charset
Set the wide execution character set, used for wide string and character constants. The default is one of
UTF-32BE, UTF-32LE, UTF-16BE, or UTF-16LE, whichever corresponds to the width of "wchar_t" and the big-endian or
little-endian byte order being used for code generation. As with -fexec-charset, charset can be any encoding
supported by the system's "iconv" library routine; however, you will have problems with encodings that do not fit
exactly in "wchar_t".
-finput-charset=charset
Set the input character set, used for translation from the character set of the input file to the source
character set used by GCC. If the locale does not specify, or GCC cannot get this information from the locale,
the default is UTF-8. This can be overridden by either the locale or this command-line option. Currently the
command-line option takes precedence if there's a conflict. charset can be any encoding supported by the
system's "iconv" library routine.
-fpch-deps
When using precompiled headers, this flag causes the dependency-output flags to also list the files from the
precompiled header's dependencies. If not specified, only the precompiled header are listed and not the files
that were used to create it, because those files are not consulted when a precompiled header is used.
-fpch-preprocess
This option allows use of a precompiled header together with -E. It inserts a special "#pragma", "#pragma GCC
pch_preprocess "filename"" in the output to mark the place where the precompiled header was found, and its
filename. When -fpreprocessed is in use, GCC recognizes this "#pragma" and loads the PCH.
This option is off by default, because the resulting preprocessed output is only really suitable as input to GCC.
It is switched on by -save-temps.
You should not write this "#pragma" in your own code, but it is safe to edit the filename if the PCH file is
available in a different location. The filename may be absolute or it may be relative to GCC's current
directory.
-fworking-directory
Enable generation of linemarkers in the preprocessor output that let the compiler know the current working
directory at the time of preprocessing. When this option is enabled, the preprocessor emits, after the initial
linemarker, a second linemarker with the current working directory followed by two slashes. GCC uses this
directory, when it's present in the preprocessed input, as the directory emitted as the current working directory
in some debugging information formats. This option is implicitly enabled if debugging information is enabled,
but this can be inhibited with the negated form -fno-working-directory. If the -P flag is present in the command
line, this option has no effect, since no "#line" directives are emitted whatsoever.
-A predicate=answer
Make an assertion with the predicate predicate and answer answer. This form is preferred to the older form -A
predicate(answer), which is still supported, because it does not use shell special characters.
-A -predicate=answer
Cancel an assertion with the predicate predicate and answer answer.
-C Do not discard comments. All comments are passed through to the output file, except for comments in processed
directives, which are deleted along with the directive.
You should be prepared for side effects when using -C; it causes the preprocessor to treat comments as tokens in
their own right. For example, comments appearing at the start of what would be a directive line have the effect
of turning that line into an ordinary source line, since the first token on the line is no longer a #.
-CC Do not discard comments, including during macro expansion. This is like -C, except that comments contained
within macros are also passed through to the output file where the macro is expanded.
In addition to the side effects of the -C option, the -CC option causes all C++-style comments inside a macro to
be converted to C-style comments. This is to prevent later use of that macro from inadvertently commenting out
the remainder of the source line.
The -CC option is generally used to support lint comments.
-P Inhibit generation of linemarkers in the output from the preprocessor. This might be useful when running the
preprocessor on something that is not C code, and will be sent to a program which might be confused by the
linemarkers.
-traditional
-traditional-cpp
Try to imitate the behavior of pre-standard C preprocessors, as opposed to ISO C preprocessors. See the GNU CPP
manual for details.
Note that GCC does not otherwise attempt to emulate a pre-standard C compiler, and these options are only
supported with the -E switch, or when invoking CPP explicitly.
-trigraphs
Support ISO C trigraphs. These are three-character sequences, all starting with ??, that are defined by ISO C to
stand for single characters. For example, ??/ stands for \, so '??/n' is a character constant for a newline.
The nine trigraphs and their replacements are
Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??-
Replacement: [ ] { } # \ ^ | ~
By default, GCC ignores trigraphs, but in standard-conforming modes it converts them. See the -std and -ansi
options.
-remap
Enable special code to work around file systems which only permit very short file names, such as MS-DOS.
-H Print the name of each header file used, in addition to other normal activities. Each name is indented to show
how deep in the #include stack it is. Precompiled header files are also printed, even if they are found to be
invalid; an invalid precompiled header file is printed with ...x and a valid one with ...! .
-dletters
Says to make debugging dumps during compilation as specified by letters. The flags documented here are those
relevant to the preprocessor. Other letters are interpreted by the compiler proper, or reserved for future
versions of GCC, and so are silently ignored. If you specify letters whose behavior conflicts, the result is
undefined.
-dM Instead of the normal output, generate a list of #define directives for all the macros defined during the
execution of the preprocessor, including predefined macros. This gives you a way of finding out what is
predefined in your version of the preprocessor. Assuming you have no file foo.h, the command
touch foo.h; cpp -dM foo.h
shows all the predefined macros.
If you use -dM without the -E option, -dM is interpreted as a synonym for -fdump-rtl-mach.
-dD Like -dM except that it outputs both the #define directives and the result of preprocessing. Both kinds of
output go to the standard output file.
-dN Like -dD, but emit only the macro names, not their expansions.
-dI Output #include directives in addition to the result of preprocessing.
-dU Like -dD except that only macros that are expanded, or whose definedness is tested in preprocessor
directives, are output; the output is delayed until the use or test of the macro; and #undef directives are
also output for macros tested but undefined at the time.
-fdebug-cpp
This option is only useful for debugging GCC. When used from CPP or with -E, it dumps debugging information
about location maps. Every token in the output is preceded by the dump of the map its location belongs to.
When used from GCC without -E, this option has no effect.
-Wp,option
You can use -Wp,option to bypass the compiler driver and pass option directly through to the preprocessor. If
option contains commas, it is split into multiple options at the commas. However, many options are modified,
translated or interpreted by the compiler driver before being passed to the preprocessor, and -Wp forcibly
bypasses this phase. The preprocessor's direct interface is undocumented and subject to change, so whenever
possible you should avoid using -Wp and let the driver handle the options instead.
-Xpreprocessor option
Pass option as an option to the preprocessor. You can use this to supply system-specific preprocessor options
that GCC does not recognize.
If you want to pass an option that takes an argument, you must use -Xpreprocessor twice, once for the option and
once for the argument.
-no-integrated-cpp
Perform preprocessing as a separate pass before compilation. By default, GCC performs preprocessing as an
integrated part of input tokenization and parsing. If this option is provided, the appropriate language front
end (cc1, cc1plus, or cc1obj for C, C++, and Objective-C, respectively) is instead invoked twice, once for
preprocessing only and once for actual compilation of the preprocessed input. This option may be useful in
conjunction with the -B or -wrapper options to specify an alternate preprocessor or perform additional processing
of the program source between normal preprocessing and compilation.
Passing Options to the Assembler
You can pass options to the assembler.
-Wa,option
Pass option as an option to the assembler. If option contains commas, it is split into multiple options at the
commas.
-Xassembler option
Pass option as an option to the assembler. You can use this to supply system-specific assembler options that GCC
does not recognize.
If you want to pass an option that takes an argument, you must use -Xassembler twice, once for the option and
once for the argument.
Options for Linking
These options come into play when the compiler links object files into an executable output file. They are
meaningless if the compiler is not doing a link step.
object-file-name
A file name that does not end in a special recognized suffix is considered to name an object file or library.
(Object files are distinguished from libraries by the linker according to the file contents.) If linking is
done, these object files are used as input to the linker.
-c
-S
-E If any of these options is used, then the linker is not run, and object file names should not be used as
arguments.
-flinker-output=type
This option controls code generation of the link-time optimizer. By default the linker output is automatically
determined by the linker plugin. For debugging the compiler and if incremental linking with a non-LTO object
file is desired, it may be useful to control the type manually.
If type is exec, code generation produces a static binary. In this case -fpic and -fpie are both disabled.
If type is dyn, code generation produces a shared library. In this case -fpic or -fPIC is preserved, but not
enabled automatically. This allows to build shared libraries without position-independent code on architectures
where this is possible, i.e. on x86.
If type is pie, code generation produces an -fpie executable. This results in similar optimizations as exec
except that -fpie is not disabled if specified at compilation time.
If type is rel, the compiler assumes that incremental linking is done. The sections containing intermediate code
for link-time optimization are merged, pre-optimized, and output to the resulting object file. In addition, if
-ffat-lto-objects is specified, binary code is produced for future non-LTO linking. The object file produced by
incremental linking is smaller than a static library produced from the same object files. At link time the
result of incremental linking also loads faster than a static library assuming that the majority of objects in
the library are used.
Finally nolto-rel configures the compiler for incremental linking where code generation is forced, a final binary
is produced, and the intermediate code for later link-time optimization is stripped. When multiple object files
are linked together the resulting code is better optimized than with link-time optimizations disabled (for
example, cross-module inlining happens), but most of benefits of whole program optimizations are lost.
During the incremental link (by -r) the linker plugin defaults to rel. With current interfaces to GNU Binutils it
is however not possible to incrementally link LTO objects and non-LTO objects into a single mixed object file.
If any of object files in incremental link cannot be used for link-time optimization, the linker plugin issues a
warning and uses nolto-rel. To maintain whole program optimization, it is recommended to link such objects into
static library instead. Alternatively it is possible to use H.J. Lu's binutils with support for mixed objects.
-fuse-ld=bfd
Use the bfd linker instead of the default linker.
-fuse-ld=gold
Use the gold linker instead of the default linker.
-fuse-ld=lld
Use the LLVM lld linker instead of the default linker.
-fuse-ld=mold
Use the Modern Linker (mold) instead of the default linker.
-llibrary
-l library
Search the library named library when linking. (The second alternative with the library as a separate argument
is only for POSIX compliance and is not recommended.)
The -l option is passed directly to the linker by GCC. Refer to your linker documentation for exact details.
The general description below applies to the GNU linker.
The linker searches a standard list of directories for the library. The directories searched include several
standard system directories plus any that you specify with -L.
Static libraries are archives of object files, and have file names like liblibrary.a. Some targets also support
shared libraries, which typically have names like liblibrary.so. If both static and shared libraries are found,
the linker gives preference to linking with the shared library unless the -static option is used.
It makes a difference where in the command you write this option; the linker searches and processes libraries and
object files in the order they are specified. Thus, foo.o -lz bar.o searches library z after file foo.o but
before bar.o. If bar.o refers to functions in z, those functions may not be loaded.
-lobjc
You need this special case of the -l option in order to link an Objective-C or Objective-C++ program.
-nostartfiles
Do not use the standard system startup files when linking. The standard system libraries are used normally,
unless -nostdlib, -nolibc, or -nodefaultlibs is used.
-nodefaultlibs
Do not use the standard system libraries when linking. Only the libraries you specify are passed to the linker,
and options specifying linkage of the system libraries, such as -static-libgcc or -shared-libgcc, are ignored.
The standard startup files are used normally, unless -nostartfiles is used.
The compiler may generate calls to "memcmp", "memset", "memcpy" and "memmove". These entries are usually
resolved by entries in libc. These entry points should be supplied through some other mechanism when this option
is specified.
-nolibc
Do not use the C library or system libraries tightly coupled with it when linking. Still link with the startup
files, libgcc or toolchain provided language support libraries such as libgnat, libgfortran or libstdc++ unless
options preventing their inclusion are used as well. This typically removes -lc from the link command line, as
well as system libraries that normally go with it and become meaningless when absence of a C library is assumed,
for example -lpthread or -lm in some configurations. This is intended for bare-board targets when there is
indeed no C library available.
-nostdlib
Do not use the standard system startup files or libraries when linking. No startup files and only the libraries
you specify are passed to the linker, and options specifying linkage of the system libraries, such as
-static-libgcc or -shared-libgcc, are ignored.
The compiler may generate calls to "memcmp", "memset", "memcpy" and "memmove". These entries are usually
resolved by entries in libc. These entry points should be supplied through some other mechanism when this option
is specified.
One of the standard libraries bypassed by -nostdlib and -nodefaultlibs is libgcc.a, a library of internal
subroutines which GCC uses to overcome shortcomings of particular machines, or special needs for some languages.
In most cases, you need libgcc.a even when you want to avoid other standard libraries. In other words, when you
specify -nostdlib or -nodefaultlibs you should usually specify -lgcc as well. This ensures that you have no
unresolved references to internal GCC library subroutines. (An example of such an internal subroutine is
"__main", used to ensure C++ constructors are called.)
-nostdlib++
Do not implicitly link with standard C++ libraries.
-e entry
--entry=entry
Specify that the program entry point is entry. The argument is interpreted by the linker; the GNU linker accepts
either a symbol name or an address.
-pie
Produce a dynamically linked position independent executable on targets that support it. For predictable
results, you must also specify the same set of options used for compilation (-fpie, -fPIE, or model suboptions)
when you specify this linker option.
-no-pie
Don't produce a dynamically linked position independent executable.
-static-pie
Produce a static position independent executable on targets that support it. A static position independent
executable is similar to a static executable, but can be loaded at any address without a dynamic linker. For
predictable results, you must also specify the same set of options used for compilation (-fpie, -fPIE, or model
suboptions) when you specify this linker option.
-pthread
Link with the POSIX threads library. This option is supported on GNU/Linux targets, most other Unix derivatives,
and also on x86 Cygwin and MinGW targets. On some targets this option also sets flags for the preprocessor, so
it should be used consistently for both compilation and linking.
-r Produce a relocatable object as output. This is also known as partial linking.
-rdynamic
Pass the flag -export-dynamic to the ELF linker, on targets that support it. This instructs the linker to add all
symbols, not only used ones, to the dynamic symbol table. This option is needed for some uses of "dlopen" or to
allow obtaining backtraces from within a program.
-s Remove all symbol table and relocation information from the executable.
-static
On systems that support dynamic linking, this overrides -pie and prevents linking with the shared libraries. On
other systems, this option has no effect.
-shared
Produce a shared object which can then be linked with other objects to form an executable. Not all systems
support this option. For predictable results, you must also specify the same set of options used for compilation
(-fpic, -fPIC, or model suboptions) when you specify this linker option.[1]
-shared-libgcc
-static-libgcc
On systems that provide libgcc as a shared library, these options force the use of either the shared or static
version, respectively. If no shared version of libgcc was built when the compiler was configured, these options
have no effect.
There are several situations in which an application should use the shared libgcc instead of the static version.
The most common of these is when the application wishes to throw and catch exceptions across different shared
libraries. In that case, each of the libraries as well as the application itself should use the shared libgcc.
Therefore, the G++ driver automatically adds -shared-libgcc whenever you build a shared library or a main
executable, because C++ programs typically use exceptions, so this is the right thing to do.
If, instead, you use the GCC driver to create shared libraries, you may find that they are not always linked with
the shared libgcc. If GCC finds, at its configuration time, that you have a non-GNU linker or a GNU linker that
does not support option --eh-frame-hdr, it links the shared version of libgcc into shared libraries by default.
Otherwise, it takes advantage of the linker and optimizes away the linking with the shared version of libgcc,
linking with the static version of libgcc by default. This allows exceptions to propagate through such shared
libraries, without incurring relocation costs at library load time.
However, if a library or main executable is supposed to throw or catch exceptions, you must link it using the G++
driver, or using the option -shared-libgcc, such that it is linked with the shared libgcc.
-static-libasan
When the -fsanitize=address option is used to link a program, the GCC driver automatically links against libasan.
If libasan is available as a shared library, and the -static option is not used, then this links against the
shared version of libasan. The -static-libasan option directs the GCC driver to link libasan statically, without
necessarily linking other libraries statically.
-static-libtsan
When the -fsanitize=thread option is used to link a program, the GCC driver automatically links against libtsan.
If libtsan is available as a shared library, and the -static option is not used, then this links against the
shared version of libtsan. The -static-libtsan option directs the GCC driver to link libtsan statically, without
necessarily linking other libraries statically.
-static-liblsan
When the -fsanitize=leak option is used to link a program, the GCC driver automatically links against liblsan.
If liblsan is available as a shared library, and the -static option is not used, then this links against the
shared version of liblsan. The -static-liblsan option directs the GCC driver to link liblsan statically, without
necessarily linking other libraries statically.
-static-libubsan
When the -fsanitize=undefined option is used to link a program, the GCC driver automatically links against
libubsan. If libubsan is available as a shared library, and the -static option is not used, then this links
against the shared version of libubsan. The -static-libubsan option directs the GCC driver to link libubsan
statically, without necessarily linking other libraries statically.
-static-libstdc++
When the g++ program is used to link a C++ program, it normally automatically links against libstdc++. If
libstdc++ is available as a shared library, and the -static option is not used, then this links against the
shared version of libstdc++. That is normally fine. However, it is sometimes useful to freeze the version of
libstdc++ used by the program without going all the way to a fully static link. The -static-libstdc++ option
directs the g++ driver to link libstdc++ statically, without necessarily linking other libraries statically.
-symbolic
Bind references to global symbols when building a shared object. Warn about any unresolved references (unless
overridden by the link editor option -Xlinker -z -Xlinker defs). Only a few systems support this option.
-T script
Use script as the linker script. This option is supported by most systems using the GNU linker. On some
targets, such as bare-board targets without an operating system, the -T option may be required when linking to
avoid references to undefined symbols.
-Xlinker option
Pass option as an option to the linker. You can use this to supply system-specific linker options that GCC does
not recognize.
If you want to pass an option that takes a separate argument, you must use -Xlinker twice, once for the option
and once for the argument. For example, to pass -assert definitions, you must write -Xlinker -assert -Xlinker
definitions. It does not work to write -Xlinker "-assert definitions", because this passes the entire string as
a single argument, which is not what the linker expects.
When using the GNU linker, it is usually more convenient to pass arguments to linker options using the
option=value syntax than as separate arguments. For example, you can specify -Xlinker -Map=output.map rather
than -Xlinker -Map -Xlinker output.map. Other linkers may not support this syntax for command-line options.
-Wl,option
Pass option as an option to the linker. If option contains commas, it is split into multiple options at the
commas. You can use this syntax to pass an argument to the option. For example, -Wl,-Map,output.map passes -Map
output.map to the linker. When using the GNU linker, you can also get the same effect with -Wl,-Map=output.map.
NOTE: In Ubuntu 8.10 and later versions, for LDFLAGS, the option -Wl,-z,relro is used. To disable, use
-Wl,-z,norelro.
-u symbol
Pretend the symbol symbol is undefined, to force linking of library modules to define it. You can use -u
multiple times with different symbols to force loading of additional library modules.
-z keyword
-z is passed directly on to the linker along with the keyword keyword. See the section in the documentation of
your linker for permitted values and their meanings.
Options for Directory Search
These options specify directories to search for header files, for libraries and for parts of the compiler:
-I dir
-iquote dir
-isystem dir
-idirafter dir
Add the directory dir to the list of directories to be searched for header files during preprocessing. If dir
begins with = or $SYSROOT, then the = or $SYSROOT is replaced by the sysroot prefix; see --sysroot and -isysroot.
Directories specified with -iquote apply only to the quote form of the directive, "#include "file"". Directories
specified with -I, -isystem, or -idirafter apply to lookup for both the "#include "file"" and "#include "
directives.
You can specify any number or combination of these options on the command line to search for header files in
several directories. The lookup order is as follows:
1. For the quote form of the include directive, the directory of the current file is searched first.
2. For the quote form of the include directive, the directories specified by -iquote options are searched in
left-to-right order, as they appear on the command line.
3. Directories specified with -I options are scanned in left-to-right order.
4. Directories specified with -isystem options are scanned in left-to-right order.
5. Standard system directories are scanned.
6. Directories specified with -idirafter options are scanned in left-to-right order.
You can use -I to override a system header file, substituting your own version, since these directories are
searched before the standard system header file directories. However, you should not use this option to add
directories that contain vendor-supplied system header files; use -isystem for that.
The -isystem and -idirafter options also mark the directory as a system directory, so that it gets the same
special treatment that is applied to the standard system directories.
If a standard system include directory, or a directory specified with -isystem, is also specified with -I, the -I
option is ignored. The directory is still searched but as a system directory at its normal position in the
system include chain. This is to ensure that GCC's procedure to fix buggy system headers and the ordering for
the "#include_next" directive are not inadvertently changed. If you really need to change the search order for
system directories, use the -nostdinc and/or -isystem options.
-I- Split the include path. This option has been deprecated. Please use -iquote instead for -I directories before
the -I- and remove the -I- option.
Any directories specified with -I options before -I- are searched only for headers requested with
"#include "file""; they are not searched for "#include ". If additional directories are specified with -I
options after the -I-, those directories are searched for all #include directives.
In addition, -I- inhibits the use of the directory of the current file directory as the first search directory
for "#include "file"". There is no way to override this effect of -I-.
-iprefix prefix
Specify prefix as the prefix for subsequent -iwithprefix options. If the prefix represents a directory, you
should include the final /.
-iwithprefix dir
-iwithprefixbefore dir
Append dir to the prefix specified previously with -iprefix, and add the resulting directory to the include
search path. -iwithprefixbefore puts it in the same place -I would; -iwithprefix puts it where -idirafter would.
-isysroot dir
This option is like the --sysroot option, but applies only to header files (except for Darwin targets, where it
applies to both header files and libraries). See the --sysroot option for more information.
-imultilib dir
Use dir as a subdirectory of the directory containing target-specific C++ headers.
-nostdinc
Do not search the standard system directories for header files. Only the directories explicitly specified with
-I, -iquote, -isystem, and/or -idirafter options (and the directory of the current file, if appropriate) are
searched.
-nostdinc++
Do not search for header files in the C++-specific standard directories, but do still search the other standard
directories. (This option is used when building the C++ library.)
--embed-dir=dir
Append dir directory to the list of searched directories for "#embed" preprocessing directive or "__has_embed"
macro. There are no default directories for "#embed".
If dir begins with = or $SYSROOT, then the = or $SYSROOT is replaced by the sysroot prefix; see --sysroot and
-isysroot.
-iplugindir=dir
Set the directory to search for plugins that are passed by -fplugin=name instead of -fplugin=path/name.so. This
option is not meant to be used by the user, but only passed by the driver.
-Ldir
Add directory dir to the list of directories to be searched for -l.
-Bprefix
This option specifies where to find the executables, libraries, include files, and data files of the compiler
itself.
The compiler driver program runs one or more of the subprograms cpp, cc1, as and ld. It tries prefix as a prefix
for each program it tries to run, both with and without machine/version/ for the corresponding target machine and
compiler version.
For each subprogram to be run, the compiler driver first tries the -B prefix, if any. If that name is not found,
or if -B is not specified, the driver tries two standard prefixes, /usr/lib/gcc/ and /usr/local/lib/gcc/. If
neither of those results in a file name that is found, the unmodified program name is searched for using the
directories specified in your PATH environment variable.
The compiler checks to see if the path provided by -B refers to a directory, and if necessary it adds a directory
separator character at the end of the path.
-B prefixes that effectively specify directory names also apply to libraries in the linker, because the compiler
translates these options into -L options for the linker. They also apply to include files in the preprocessor,
because the compiler translates these options into -isystem options for the preprocessor. In this case, the
compiler appends include to the prefix.
The runtime support file libgcc.a can also be searched for using the -B prefix, if needed. If it is not found
there, the two standard prefixes above are tried, and that is all. The file is left out of the link if it is not
found by those means.
Another way to specify a prefix much like the -B prefix is to use the environment variable GCC_EXEC_PREFIX.
As a special kludge, if the path provided by -B is [dir/]stageN/, where N is a number in the range 0 to 9, then
it is replaced by [dir/]include. This is to help with boot-strapping the compiler.
-no-canonical-prefixes
Do not expand any symbolic links, resolve references to /../ or /./, or make the path absolute when generating a
relative prefix.
--sysroot=dir
Use dir as the logical root directory for headers and libraries. For example, if the compiler normally searches
for headers in /usr/include and libraries in /usr/lib, it instead searches dir/usr/include and dir/usr/lib.
If you use both this option and the -isysroot option, then the --sysroot option applies to libraries, but the
-isysroot option applies to header files.
The GNU linker (beginning with version 2.16) has the necessary support for this option. If your linker does not
support this option, the header file aspect of --sysroot still works, but the library aspect does not.
--no-sysroot-suffix
For some targets, a suffix is added to the root directory specified with --sysroot, depending on the other
options used, so that headers may for example be found in dir/suffix/usr/include instead of dir/usr/include.
This option disables the addition of such a suffix.
Options for Code Generation Conventions
These machine-independent options control the interface conventions used in code generation.
Most of them have both positive and negative forms; the negative form of -ffoo is -fno-foo. In the table below, only
one of the forms is listed---the one that is not the default. You can figure out the other form by either removing
no- or adding it.
-fstack-reuse=reuse-level
This option controls stack space reuse for user declared local/auto variables and compiler generated temporaries.
reuse_level can be all, named_vars, or none. all enables stack reuse for all local variables and temporaries,
named_vars enables the reuse only for user defined local variables with names, and none disables stack reuse
completely. The default value is all. The option is needed when the program extends the lifetime of a scoped
local variable or a compiler generated temporary beyond the end point defined by the language. When a lifetime
of a variable ends, and if the variable lives in memory, the optimizing compiler has the freedom to reuse its
stack space with other temporaries or scoped local variables whose live range does not overlap with it. Legacy
code extending local lifetime is likely to break with the stack reuse optimization.
For example,
int *p;
{
int local1;
p = &local1;
local1 = 10;
....
}
{
int local2;
local2 = 20;
...
}
if (*p == 10) // out of scope use of local1
{
}
Another example:
struct A
{
A(int k) : i(k), j(k) { }
int i;
int j;
};
A *ap;
void foo(const A& ar)
{
ap = &ar;
}
void bar()
{
foo(A(10)); // temp object's lifetime ends when foo returns
{
A a(20);
....
}
ap->i+= 10; // ap references out of scope temp whose space
// is reused with a. What is the value of ap->i?
}
The lifetime of a compiler generated temporary is well defined by the C++ standard. When a lifetime of a
temporary ends, and if the temporary lives in memory, the optimizing compiler has the freedom to reuse its stack
space with other temporaries or scoped local variables whose live range does not overlap with it. However some of
the legacy code relies on the behavior of older compilers in which temporaries' stack space is not reused, the
aggressive stack reuse can lead to runtime errors. This option is used to control the temporary stack reuse
optimization.
-ftrapv
This option generates traps for signed overflow on addition, subtraction, multiplication operations. The options
-ftrapv and -fwrapv override each other, so using -ftrapv -fwrapv on the command-line results in -fwrapv being
effective. Note that only active options override, so using -ftrapv -fwrapv -fno-wrapv on the command-line
results in -ftrapv being effective.
-fwrapv
This option instructs the compiler to assume that signed arithmetic overflow of addition, subtraction and
multiplication wraps around using twos-complement representation. This flag enables some optimizations and
disables others. The options -ftrapv and -fwrapv override each other, so using -ftrapv -fwrapv on the command-
line results in -fwrapv being effective. Note that only active options override, so using -ftrapv -fwrapv
-fno-wrapv on the command-line results in -ftrapv being effective.
-fwrapv-pointer
This option instructs the compiler to assume that pointer arithmetic overflow on addition and subtraction wraps
around using twos-complement representation. This flag disables some optimizations which assume pointer overflow
is invalid.
-fstrict-overflow
This option implies -fno-wrapv -fno-wrapv-pointer and when negated implies -fwrapv -fwrapv-pointer.
-fexceptions
Enable exception handling. Generates extra code needed to propagate exceptions. For some targets, this implies
GCC generates frame unwind information for all functions, which can produce significant data size overhead,
although it does not affect execution. If you do not specify this option, GCC enables it by default for
languages like C++ that normally require exception handling, and disables it for languages like C that do not
normally require it. However, you may need to enable this option when compiling C code that needs to
interoperate properly with exception handlers written in C++. You may also wish to disable this option if you
are compiling older C++ programs that don't use exception handling.
-fnon-call-exceptions
Generate code that allows trapping instructions to throw exceptions. Note that this requires platform-specific
runtime support that does not exist everywhere. Moreover, it only allows trapping instructions to throw
exceptions, i.e. memory references or floating-point instructions. It does not allow exceptions to be thrown
from arbitrary signal handlers such as "SIGALRM". This enables -fexceptions.
-fdelete-dead-exceptions
Consider that instructions that may throw exceptions but don't otherwise contribute to the execution of the
program can be optimized away. This does not affect calls to functions except those with the "pure" or "const"
attributes. This option is enabled by default for the Ada and C++ compilers, as permitted by the language
specifications. Optimization passes that cause dead exceptions to be removed are enabled independently at
different optimization levels.
-funwind-tables
Similar to -fexceptions, except that it just generates any needed static data, but does not affect the generated
code in any other way. You normally do not need to enable this option; instead, a language processor that needs
this handling enables it on your behalf.
-fasynchronous-unwind-tables
Generate unwind table in DWARF format, if supported by target machine. The table is exact at each instruction
boundary, so it can be used for stack unwinding from asynchronous events (such as debugger or garbage collector).
-fno-gnu-unique
On systems with recent GNU assembler and C library, the C++ compiler uses the "STB_GNU_UNIQUE" binding to make
sure that definitions of template static data members and static local variables in inline functions are unique
even in the presence of "RTLD_LOCAL"; this is necessary to avoid problems with a library used by two different
"RTLD_LOCAL" plugins depending on a definition in one of them and therefore disagreeing with the other one about
the binding of the symbol. But this causes "dlclose" to be ignored for affected DSOs; if your program relies on
reinitialization of a DSO via "dlclose" and "dlopen", you can use -fno-gnu-unique.
-fpcc-struct-return
Return "short" "struct" and "union" values in memory like longer ones, rather than in registers. This convention
is less efficient, but it has the advantage of allowing intercallability between GCC-compiled files and files
compiled with other compilers, particularly the Portable C Compiler (pcc).
The precise convention for returning structures in memory depends on the target configuration macros.
Short structures and unions are those whose size and alignment match that of some integer type.
Warning: code compiled with the -fpcc-struct-return switch is not binary compatible with code compiled with the
-freg-struct-return switch. Use it to conform to a non-default application binary interface.
-freg-struct-return
Return "struct" and "union" values in registers when possible. This is more efficient for small structures than
-fpcc-struct-return.
If you specify neither -fpcc-struct-return nor -freg-struct-return, GCC defaults to whichever convention is
standard for the target. If there is no standard convention, GCC defaults to -fpcc-struct-return, except on
targets where GCC is the principal compiler. In those cases, we can choose the standard, and we chose the more
efficient register return alternative.
Warning: code compiled with the -freg-struct-return switch is not binary compatible with code compiled with the
-fpcc-struct-return switch. Use it to conform to a non-default application binary interface.
-fshort-enums
Allocate to an "enum" type only as many bytes as it needs for the declared range of possible values.
Specifically, the "enum" type is equivalent to the smallest integer type that has enough room. This option has
no effect for an enumeration type with a fixed underlying type.
Warning: the -fshort-enums switch causes GCC to generate code that is not binary compatible with code generated
without that switch. Use it to conform to a non-default application binary interface.
-fshort-wchar
Override the underlying type for "wchar_t" to be "short unsigned int" instead of the default for the target.
This option is useful for building programs to run under WINE.
Warning: the -fshort-wchar switch causes GCC to generate code that is not binary compatible with code generated
without that switch. Use it to conform to a non-default application binary interface.
-fcommon
In C code, this option controls the placement of global variables defined without an initializer, known as
tentative definitions in the C standard. Tentative definitions are distinct from declarations of a variable with
the "extern" keyword, which do not allocate storage.
The default is -fno-common, which specifies that the compiler places uninitialized global variables in the BSS
section of the object file. This inhibits the merging of tentative definitions by the linker so you get a
multiple-definition error if the same variable is accidentally defined in more than one compilation unit.
The -fcommon places uninitialized global variables in a common block. This allows the linker to resolve all
tentative definitions of the same variable in different compilation units to the same object, or to a non-
tentative definition. This behavior is inconsistent with C++, and on many targets implies a speed and code size
penalty on global variable references. It is mainly useful to enable legacy code to link without errors.
-fno-ident
Ignore the "#ident" directive.
-finhibit-size-directive
Don't output a ".size" assembler directive, or anything else that would cause trouble if the function is split in
the middle, and the two halves are placed at locations far apart in memory. This option is used when compiling
crtstuff.c; you should not need to use it for anything else.
-fverbose-asm
Put extra commentary information in the generated assembly code to make it more readable. This option is
generally only of use to those who actually need to read the generated assembly code (perhaps while debugging the
compiler itself).
-fno-verbose-asm, the default, causes the extra information to be omitted and is useful when comparing two
assembler files.
The added comments include:
* information on the compiler version and command-line options,
* the source code lines associated with the assembly instructions, in the form FILENAME:LINENUMBER:CONTENT OF
LINE,
* hints on which high-level expressions correspond to the various assembly instruction operands.
For example, given this C source file:
int test (int n)
{
int i;
int total = 0;
for (i = 0; i < n; i++)
total += i * i;
return total;
}
compiling to (x86_64) assembly via -S and emitting the result direct to stdout via -o -
gcc -S test.c -fverbose-asm -Os -o -
gives output similar to this:
.file "test.c"
# GNU C11 (GCC) version 7.0.0 20160809 (experimental) (x86_64-pc-linux-gnu)
[...snip...]
# options passed:
[...snip...]
.text
.globl test
.type test, @function
test:
.LFB0:
.cfi_startproc
# test.c:4: int total = 0;
xorl %eax, %eax #
# test.c:6: for (i = 0; i < n; i++)
xorl %edx, %edx # i
.L2:
# test.c:6: for (i = 0; i < n; i++)
cmpl %edi, %edx # n, i
jge .L5 #,
# test.c:7: total += i * i;
movl %edx, %ecx # i, tmp92
imull %edx, %ecx # i, tmp92
# test.c:6: for (i = 0; i < n; i++)
incl %edx # i
# test.c:7: total += i * i;
addl %ecx, %eax # tmp92,
jmp .L2 #
.L5:
# test.c:10: }
ret
.cfi_endproc
.LFE0:
.size test, .-test
.ident "GCC: (GNU) 7.0.0 20160809 (experimental)"
.section .note.GNU-stack,"",@progbits
The comments are intended for humans rather than machines and hence the precise format of the comments is subject
to change.
-frecord-gcc-switches
This switch causes the command line used to invoke the compiler to be recorded into the object file that is being
created. This switch is only implemented on some targets and the exact format of the recording is target and
binary file format dependent, but it usually takes the form of a section containing ASCII text. This switch is
related to the -fverbose-asm switch, but that switch only records information in the assembler output file as
comments, so it never reaches the object file. See also -grecord-gcc-switches for another way of storing
compiler options into the object file.
-fpic
Generate position-independent code (PIC) suitable for use in a shared library, if supported for the target
machine. Such code accesses all constant addresses through a global offset table (GOT). The dynamic loader
resolves the GOT entries when the program starts (the dynamic loader is not part of GCC; it is part of the
operating system). If the GOT size for the linked executable exceeds a machine-specific maximum size, you get an
error message from the linker indicating that -fpic does not work; in that case, recompile with -fPIC instead.
(These maximums are 8k on the SPARC, 28k on AArch64 and 32k on the m68k and RS/6000. The x86 has no such limit.)
Position-independent code requires special support, and therefore works only on certain machines. For the x86,
GCC supports PIC for System V but not for the Sun 386i. Code generated for the IBM RS/6000 is always position-
independent.
When this flag is set, the macros "__pic__" and "__PIC__" are defined to 1.
-fPIC
If supported for the target machine, emit position-independent code, suitable for dynamic linking and avoiding
any limit on the size of the global offset table. This option makes a difference on AArch64, m68k, PowerPC and
SPARC.
Position-independent code requires special support, and therefore works only on certain machines.
When this flag is set, the macros "__pic__" and "__PIC__" are defined to 2.
-fpie
-fPIE
These options are similar to -fpic and -fPIC, but the generated position-independent code can be only linked into
executables. Usually these options are used to compile code that will be linked using the -pie GCC option.
-fpie and -fPIE both define the macros "__pie__" and "__PIE__". The macros have the value 1 for -fpie and 2 for
-fPIE.
-fno-plt
Do not use the PLT for external function calls in position-independent code. Instead, load the callee address at
call sites from the GOT and branch to it. This leads to more efficient code by eliminating PLT stubs and
exposing GOT loads to optimizations. On architectures such as 32-bit x86 where PLT stubs expect the GOT pointer
in a specific register, this gives more register allocation freedom to the compiler. Lazy binding requires use
of the PLT; with -fno-plt all external symbols are resolved at load time.
Alternatively, the function attribute "noplt" can be used to avoid calls through the PLT for specific external
functions.
In position-dependent code, a few targets also convert calls to functions that are marked to not use the PLT to
use the GOT instead.
-fno-jump-tables
Do not use jump tables for switch statements even where it would be more efficient than other code generation
strategies. This option is of use in conjunction with -fpic or -fPIC for building code that forms part of a
dynamic linker and cannot reference the address of a jump table. On some targets, jump tables do not require a
GOT and this option is not needed.
-fno-bit-tests
Do not use bit tests for switch statements even where it would be more efficient than other code generation
strategies.
-ffixed-reg
Treat the register named reg as a fixed register; generated code should never refer to it (except perhaps as a
stack pointer, frame pointer or in some other fixed role).
reg must be the name of a register. The register names accepted are machine-specific and are defined in the
"REGISTER_NAMES" macro in the machine description macro file.
This flag does not have a negative form, because it specifies a three-way choice.
-fcall-used-reg
Treat the register named reg as an allocable register that is clobbered by function calls. It may be allocated
for temporaries or variables that do not live across a call. Functions compiled this way do not save and restore
the register reg.
It is an error to use this flag with the frame pointer or stack pointer. Use of this flag for other registers
that have fixed pervasive roles in the machine's execution model produces disastrous results.
This flag does not have a negative form, because it specifies a three-way choice.
-fcall-saved-reg
Treat the register named reg as an allocable register saved by functions. It may be allocated even for
temporaries or variables that live across a call. Functions compiled this way save and restore the register reg
if they use it.
It is an error to use this flag with the frame pointer or stack pointer. Use of this flag for other registers
that have fixed pervasive roles in the machine's execution model produces disastrous results.
A different sort of disaster results from the use of this flag for a register in which function values may be
returned.
This flag does not have a negative form, because it specifies a three-way choice.
-fpack-struct[=n]
Without a value specified, pack all structure members together without holes. When a value is specified (which
must be a small power of two), pack structure members according to this value, representing the maximum alignment
(that is, objects with default alignment requirements larger than this are output potentially unaligned at the
next fitting location.
Warning: the -fpack-struct switch causes GCC to generate code that is not binary compatible with code generated
without that switch. Additionally, it makes the code suboptimal. Use it to conform to a non-default application
binary interface.
-fleading-underscore
This option and its counterpart, -fno-leading-underscore, forcibly change the way C symbols are represented in
the object file. One use is to help link with legacy assembly code.
Warning: the -fleading-underscore switch causes GCC to generate code that is not binary compatible with code
generated without that switch. Use it to conform to a non-default application binary interface. Not all targets
provide complete support for this switch.
-ftls-model=model
Alter the thread-local storage model to be used. The model argument should be one of global-dynamic, local-
dynamic, initial-exec or local-exec. Note that the choice is subject to optimization: the compiler may use a
more efficient model for symbols not visible outside of the translation unit, or if -fpic is not given on the
command line.
The default without -fpic is initial-exec; with -fpic the default is global-dynamic.
-ftrampolines
For targets that normally need trampolines for nested functions, always generate them instead of using
descriptors. Otherwise, for targets that do not need them, like for example HP-PA or IA-64, do nothing.
A trampoline is a small piece of code that is created at run time on the stack when the address of a nested
function is taken, and is used to call the nested function indirectly. Therefore, it requires the stack to be
made executable in order for the program to work properly.
-fno-trampolines is enabled by default on a language by language basis to let the compiler avoid generating them,
if it computes that this is safe, and replace them with descriptors. Descriptors are made up of data only, but
the generated code must be prepared to deal with them. As of this writing, -fno-trampolines is enabled by
default only for Ada.
Moreover, code compiled with -ftrampolines and code compiled with -fno-trampolines are not binary compatible if
nested functions are present. This option must therefore be used on a program-wide basis and be manipulated with
extreme care.
For languages other than Ada, the "-ftrampolines" and "-fno-trampolines" options currently have no effect, and
trampolines are always generated on platforms that need them for nested functions.
-ftrampoline-impl=[stack|heap]
By default, trampolines are generated on stack. However, certain platforms (such as the Apple M1) do not permit
an executable stack. Compiling with -ftrampoline-impl=heap generate calls to "__gcc_nested_func_ptr_created" and
"__gcc_nested_func_ptr_deleted" in order to allocate and deallocate trampoline space on the executable heap.
These functions are implemented in libgcc, and will only be provided on specific targets: x86_64 Darwin, x86_64
and aarch64 Linux. PLEASE NOTE: Heap trampolines are not guaranteed to be correctly deallocated if you "setjmp",
instantiate nested functions, and then "longjmp" back to a state prior to having allocated those nested
functions.
-fvisibility=[default|internal|hidden|protected]
Set the default ELF image symbol visibility to the specified option---all symbols are marked with this unless
overridden within the code. Using this feature can very substantially improve linking and load times of shared
object libraries, produce more optimized code, provide near-perfect API export and prevent symbol clashes. It is
strongly recommended that you use this in any shared objects you distribute.
Despite the nomenclature, default always means public; i.e., available to be linked against from outside the
shared object. protected and internal are pretty useless in real-world usage so the only other commonly used
option is hidden. The default if -fvisibility isn't specified is default, i.e., make every symbol public.
A good explanation of the benefits offered by ensuring ELF symbols have the correct visibility is given by "How
To Write Shared Libraries" by Ulrich Drepper (which can be found at )---however
a superior solution made possible by this option to marking things hidden when the default is public is to make
the default hidden and mark things public. This is the norm with DLLs on Windows and with -fvisibility=hidden
and "__attribute__ ((visibility("default")))" instead of "__declspec(dllexport)" you get almost identical
semantics with identical syntax. This is a great boon to those working with cross-platform projects.
For those adding visibility support to existing code, you may find "#pragma GCC visibility" of use. This works
by you enclosing the declarations you wish to set visibility for with (for example) "#pragma GCC visibility
push(hidden)" and "#pragma GCC visibility pop". Bear in mind that symbol visibility should be viewed as part of
the API interface contract and thus all new code should always specify visibility when it is not the default;
i.e., declarations only for use within the local DSO should always be marked explicitly as hidden as so to avoid
PLT indirection overheads---making this abundantly clear also aids readability and self-documentation of the
code. Note that due to ISO C++ specification requirements, "operator new" and "operator delete" must always be
of default visibility.
Be aware that headers from outside your project, in particular system headers and headers from any other library
you use, may not be expecting to be compiled with visibility other than the default. You may need to explicitly
say "#pragma GCC visibility push(default)" before including any such headers.
"extern" declarations are not affected by -fvisibility, so a lot of code can be recompiled with
-fvisibility=hidden with no modifications. However, this means that calls to "extern" functions with no explicit
visibility use the PLT, so it is more effective to use "__attribute ((visibility))" and/or "#pragma GCC
visibility" to tell the compiler which "extern" declarations should be treated as hidden.
Note that -fvisibility does affect C++ vague linkage entities. This means that, for instance, an exception class
that is be thrown between DSOs must be explicitly marked with default visibility so that the type_info nodes are
unified between the DSOs.
An overview of these techniques, their benefits and how to use them is at .
-fstrict-volatile-bitfields
This option should be used if accesses to volatile bit-fields (or other structure fields, although the compiler
usually honors those types anyway) should use a single access of the width of the field's type, aligned to a
natural alignment if possible. For example, targets with memory-mapped peripheral registers might require all
such accesses to be 16 bits wide; with this flag you can declare all peripheral bit-fields as "unsigned short"
(assuming short is 16 bits on these targets) to force GCC to use 16-bit accesses instead of, perhaps, a more
efficient 32-bit access.
If this option is disabled, the compiler uses the most efficient instruction. In the previous example, that
might be a 32-bit load instruction, even though that accesses bytes that do not contain any portion of the bit-
field, or memory-mapped registers unrelated to the one being updated.
In some cases, such as when the "packed" attribute is applied to a structure field, it may not be possible to
access the field with a single read or write that is correctly aligned for the target machine. In this case GCC
falls back to generating multiple accesses rather than code that will fault or truncate the result at run time.
Note: Due to restrictions of the C/C++11 memory model, write accesses are not allowed to touch non bit-field
members. It is therefore recommended to define all bits of the field's type as bit-field members.
The default value of this option is determined by the application binary interface for the target processor.
-fsync-libcalls
This option controls whether any out-of-line instance of the "__sync" family of functions may be used to
implement the C++11 "__atomic" family of functions.
The default value of this option is enabled, thus the only useful form of the option is -fno-sync-libcalls. This
option is used in the implementation of the libatomic runtime library.
-fzero-init-padding-bits=value
Guarantee zero initialization of padding bits in automatic variable initializers. Certain languages guarantee
zero initialization of padding bits in certain cases, e.g. C23 when using empty initializers ("{}"), or C++ when
using zero-initialization or C guarantees that fields not specified in an initializer have their padding bits
zero initialized. This option allows to change when padding bits in initializers are guaranteed to be zero
initialized. The default is "-fzero-init-padding-bits=standard", which makes no further guarantees than the
corresponding standard. E.g.
struct A { char a; unsigned long long b; char c; };
union B { char a; unsigned long long b; };
struct A a = {}; // C23 guarantees padding bits are zero.
struct A b = { 1, 2, 3 }; // No guarantees.
union B c = {}; // C23 guarantees padding bits are zero.
union B d = { 1 }; // No guarantees.
"-fzero-init-padding-bits=unions" guarantees zero initialization of padding bits in unions on top of what the
standards guarantee, if the initializer of an union is empty (then all bits of the union are zero initialized) or
if the initialized member of the union is smaller than the size of the union (in that case guarantees padding
bits outside of the initialized member to be zero initialized). This was the GCC behavior before GCC 15 and in
the above example guarantees zero initialization of last "sizeof (unsigned long long) - 1" bytes in the union.
"-fzero-init-padding-bits=all" guarantees additionally zero initialization of padding bits of other aggregates,
so the padding in between "b.a" and "b.b" (if any) and tail padding in the structure (if any).
NOTE: In Ubuntu 25.10 and later versions, -fzero-init-padding-bits=all is enabled by default for C, C++, ObjC,
ObjC++, unless -fzero-init-padding-bits=standard is found.
GCC Developer Options
This section describes command-line options that are primarily of interest to GCC developers, including options to
support compiler testing and investigation of compiler bugs and compile-time performance problems. This includes
options that produce debug dumps at various points in the compilation; that print statistics such as memory use and
execution time; and that print information about GCC's configuration, such as where it searches for libraries. You
should rarely need to use any of these options for ordinary compilation and linking tasks.
Many developer options that cause GCC to dump output to a file take an optional =filename suffix. You can specify
stdout or - to dump to standard output, and stderr for standard error.
If =filename is omitted, a default dump file name is constructed by concatenating the base dump file name, a pass
number, phase letter, and pass name. The base dump file name is the name of output file produced by the compiler if
explicitly specified and not an executable; otherwise it is the source file name. The pass number is determined by
the order passes are registered with the compiler's pass manager. This is generally the same as the order of
execution, but passes registered by plugins, target-specific passes, or passes that are otherwise registered late are
numbered higher than the pass named final, even if they are executed earlier. The phase letter is one of i (inter-
procedural analysis), l (language-specific), r (RTL), or t (tree). The files are created in the directory of the
output file.
-fcallgraph-info
-fcallgraph-info=MARKERS
Makes the compiler output callgraph information for the program, on a per-object-file basis. The information is
generated in the common VCG format. It can be decorated with additional, per-node and/or per-edge information,
if a list of comma-separated markers is additionally specified. When the "su" marker is specified, the callgraph
is decorated with stack usage information; it is equivalent to -fstack-usage. When the "da" marker is specified,
the callgraph is decorated with information about dynamically allocated objects.
When compiling with -flto, no callgraph information is output along with the object file. At LTO link time,
-fcallgraph-info may generate multiple callgraph information files next to intermediate LTO output files.
-dletters
-fdump-rtl-pass
-fdump-rtl-pass=filename
Says to make debugging dumps during compilation at times specified by letters. This is used for debugging the
RTL-based passes of the compiler.
Some -dletters switches have different meaning when -E is used for preprocessing.
Debug dumps can be enabled with a -fdump-rtl switch or some -d option letters. Here are the possible letters for
use in pass and letters, and their meanings:
-fdump-rtl-alignments
Dump after branch alignments have been computed.
-fdump-rtl-asmcons
Dump after fixing rtl statements that have unsatisfied in/out constraints.
-fdump-rtl-auto_inc_dec
Dump after auto-inc-dec discovery. This pass is only run on architectures that have auto inc or auto dec
instructions.
-fdump-rtl-barriers
Dump after cleaning up the barrier instructions.
-fdump-rtl-bbpart
Dump after partitioning hot and cold basic blocks.
-fdump-rtl-bbro
Dump after block reordering.
-fdump-rtl-btl1
-fdump-rtl-btl2
-fdump-rtl-btl1 and -fdump-rtl-btl2 enable dumping after the two branch target load optimization passes.
-fdump-rtl-bypass
Dump after jump bypassing and control flow optimizations.
-fdump-rtl-combine
Dump after the RTL instruction combination pass.
-fdump-rtl-compgotos
Dump after duplicating the computed gotos.
-fdump-rtl-ce1
-fdump-rtl-ce2
-fdump-rtl-ce3
-fdump-rtl-ce1, -fdump-rtl-ce2, and -fdump-rtl-ce3 enable dumping after the three if conversion passes.
-fdump-rtl-cprop_hardreg
Dump after hard register copy propagation.
-fdump-rtl-csa
Dump after combining stack adjustments.
-fdump-rtl-cse1
-fdump-rtl-cse2
-fdump-rtl-cse1 and -fdump-rtl-cse2 enable dumping after the two common subexpression elimination passes.
-fdump-rtl-dce
Dump after the standalone dead code elimination passes.
-fdump-rtl-dbr
Dump after delayed branch scheduling.
-fdump-rtl-dce1
-fdump-rtl-dce2
-fdump-rtl-dce1 and -fdump-rtl-dce2 enable dumping after the two dead store elimination passes.
-fdump-rtl-eh
Dump after finalization of EH handling code.
-fdump-rtl-eh_ranges
Dump after conversion of EH handling range regions.
-fdump-rtl-expand
Dump after RTL generation.
-fdump-rtl-fwprop1
-fdump-rtl-fwprop2
-fdump-rtl-fwprop1 and -fdump-rtl-fwprop2 enable dumping after the two forward propagation passes.
-fdump-rtl-gcse1
-fdump-rtl-gcse2
-fdump-rtl-gcse1 and -fdump-rtl-gcse2 enable dumping after global common subexpression elimination.
-fdump-rtl-init-regs
Dump after the initialization of the registers.
-fdump-rtl-initvals
Dump after the computation of the initial value sets.
-fdump-rtl-into_cfglayout
Dump after converting to cfglayout mode.
-fdump-rtl-ira
Dump after iterated register allocation.
-fdump-rtl-jump
Dump after the second jump optimization.
-fdump-rtl-loop2
-fdump-rtl-loop2 enables dumping after the rtl loop optimization passes.
-fdump-rtl-mach
Dump after performing the machine dependent reorganization pass, if that pass exists.
-fdump-rtl-mode_sw
Dump after removing redundant mode switches.
-fdump-rtl-rnreg
Dump after register renumbering.
-fdump-rtl-outof_cfglayout
Dump after converting from cfglayout mode.
-fdump-rtl-peephole2
Dump after the peephole pass.
-fdump-rtl-postreload
Dump after post-reload optimizations.
-fdump-rtl-pro_and_epilogue
Dump after generating the function prologues and epilogues.
-fdump-rtl-sched1
-fdump-rtl-sched2
-fdump-rtl-sched1 and -fdump-rtl-sched2 enable dumping after the basic block scheduling passes.
-fdump-rtl-ree
Dump after sign/zero extension elimination.
-fdump-rtl-seqabstr
Dump after common sequence discovery.
-fdump-rtl-shorten
Dump after shortening branches.
-fdump-rtl-split1
-fdump-rtl-split2
-fdump-rtl-split3
-fdump-rtl-split4
-fdump-rtl-split5
These options enable dumping after five rounds of instruction splitting.
-fdump-rtl-sms
Dump after modulo scheduling. This pass is only run on some architectures.
-fdump-rtl-stack
Dump after conversion from GCC's "flat register file" registers to the x87's stack-like registers. This pass
is only run on x86 variants.
-fdump-rtl-subreg1
-fdump-rtl-subreg2
-fdump-rtl-subreg1 and -fdump-rtl-subreg2 enable dumping after the two subreg expansion passes.
-fdump-rtl-vartrack
Dump after variable tracking.
-fdump-rtl-vregs
Dump after converting virtual registers to hard registers.
-fdump-rtl-web
Dump after live range splitting.
-fdump-rtl-regclass
-fdump-rtl-subregs_of_mode_init
-fdump-rtl-subregs_of_mode_finish
-fdump-rtl-dfinit
-fdump-rtl-dfinish
These dumps are defined but always produce empty files.
-da
-fdump-rtl-all
Produce all the dumps listed above.
-dA Annotate the assembler output with miscellaneous debugging information.
-dD Dump all macro definitions, at the end of preprocessing, in addition to normal output.
-dH Produce a core dump whenever an error occurs.
-dp Annotate the assembler output with a comment indicating which pattern and alternative is used. The length
and cost of each instruction are also printed.
-dP Dump the RTL in the assembler output as a comment before each instruction. Also turns on -dp annotation.
-dx Just generate RTL for a function instead of compiling it. Usually used with -fdump-rtl-expand.
-fdump-debug
Dump debugging information generated during the debug generation phase.
-fdump-earlydebug
Dump debugging information generated during the early debug generation phase.
-fdump-noaddr
When doing debugging dumps, suppress address output. This makes it more feasible to use diff on debugging dumps
for compiler invocations with different compiler binaries and/or different text / bss / data / heap / stack / dso
start locations.
-freport-bug
Collect and dump debug information into a temporary file if an internal compiler error (ICE) occurs.
-fdump-unnumbered
When doing debugging dumps, suppress instruction numbers and address output. This makes it more feasible to use
diff on debugging dumps for compiler invocations with different options, in particular with and without -g.
-fdump-unnumbered-links
When doing debugging dumps (see -d option above), suppress instruction numbers for the links to the previous and
next instructions in a sequence.
-fdump-ipa-switch
-fdump-ipa-switch-options
Control the dumping at various stages of inter-procedural analysis language tree to a file. The file name is
generated by appending a switch specific suffix to the source file name, and the file is created in the same
directory as the output file. The following dumps are possible:
all Enables all inter-procedural analysis dumps.
cgraph
Dumps information about call-graph optimization, unused function removal, and inlining decisions.
inline
Dump after function inlining.
strubm
Dump after selecting "strub" modes, and recording the selections as function attributes.
strub
Dump "strub" transformations: interface changes, function wrapping, and insertion of builtin calls for stack
scrubbing and watermarking.
Additionally, the options -optimized, -missed, -note, and -all can be provided, with the same meaning as for
-fopt-info, defaulting to -optimized.
For example, -fdump-ipa-inline-optimized-missed will emit information on callsites that were inlined, along with
callsites that were not inlined.
By default, the dump will contain messages about successful optimizations (equivalent to -optimized) together
with low-level details about the analysis.
-fdump-ipa-clones
Create a dump file containing information about creation of call graph node clones and removals of call graph
nodes during inter-procedural optimizations and transformations. Its main intended use is that tools that create
live-patches can determine the set of functions that need to be live-patched to completely replace a particular
function (see -flive-patching). The file name is generated by appending suffix "ipa-clones" to the source file
name, and the file is created in the same directory as the output file. Each entry in the file is on a separate
line containing semicolon separated fields.
In the case of call graph clone creation, the individual fields are:
1. String "Callgraph clone".
2. Name of the function being cloned as it is presented to the assembler.
3. A number that uniquely represents the function being cloned in the call graph. Note that the number is
unique only within a compilation unit or within whole-program analysis but is likely to be different in the
two phases.
4. The file name of the source file where the function is defined.
5. The line on which the function definition is located.
6. The column where the function definition is located.
7. Name of the new function clone as it is presented to the assembler.
8. A number that uniquely represents the new function clone in the call graph. Note that the number is unique
only within a compilation unit or within whole-program analysis but is likely to be different in the two
phases.
9. The file name of the source file where the source code location of the new clone points to.
10. The line to which the source code location of the new clone points to.
11. The column to which the source code location of the new clone points to.
12. A string that determines the reason for cloning.
In the case of call graph clone removal, the individual fields are:
1. String "Callgraph removal".
2. Name of the function being removed as it would be presented to the assembler.
3. A number that uniquely represents the function being cloned in the call graph. Note that the number is
unique only within a compilation unit or within whole-program analysis but is likely to be different in the
two phases.
4. The file name of the source file where the function is defined.
5. The line on which the function definition is located.
6. The column where the function definition is located.
-fdump-lang
Dump language-specific information. The file name is made by appending .lang to the source file name.
-fdump-lang-all
-fdump-lang-switch
-fdump-lang-switch-options
-fdump-lang-switch-options=filename
Control the dumping of language-specific information. The options and filename portions behave as described in
the -fdump-tree option. The following switch values are accepted:
all Enable all language-specific dumps.
class
Dump class hierarchy information. Virtual table information is emitted unless 'slim' is specified. This
option is applicable to C++ only.
module
Dump module information. Options lineno (locations), graph (reachability), blocks (clusters), uid
(serialization), alias (mergeable), asmname (Elrond), eh (mapper) & vops (macros) may provide additional
information. This option is applicable to C++ only.
raw Dump the raw internal tree data. This option is applicable to C++ only.
-fdump-passes
Print on stderr the list of optimization passes that are turned on and off by the current command-line options.
-fdump-statistics-option
Enable and control dumping of pass statistics in a separate file. The file name is generated by appending a
suffix ending in .statistics to the source file name, and the file is created in the same directory as the output
file. If the -option form is used, -stats causes counters to be summed over the whole compilation unit while
-details dumps every event as the passes generate them. The default with no option is to sum counters for each
function compiled.
-fdump-tree-all
-fdump-tree-switch
-fdump-tree-switch-options
-fdump-tree-switch-options=filename
Control the dumping at various stages of processing the intermediate language tree to a file. If the -options
form is used, options is a list of - separated options which control the details of the dump. Not all options
are applicable to all dumps; those that are not meaningful are ignored. The following options are available
address
Print the address of each node. Usually this is not meaningful as it changes according to the environment
and source file. Its primary use is for tying up a dump file with a debug environment.
asmname
If "DECL_ASSEMBLER_NAME" has been set for a given decl, use that in the dump instead of "DECL_NAME". Its
primary use is ease of use working backward from mangled names in the assembly file.
slim
When dumping front-end intermediate representations, inhibit dumping of members of a scope or body of a
function merely because that scope has been reached. Only dump such items when they are directly reachable
by some other path.
When dumping pretty-printed trees, this option inhibits dumping the bodies of control structures.
When dumping RTL, print the RTL in slim (condensed) form instead of the default LISP-like representation.
raw Print a raw representation of the tree. By default, trees are pretty-printed into a C-like representation.
details
Enable more detailed dumps (not honored by every dump option). Also include information from the optimization
passes.
stats
Enable dumping various statistics about the pass (not honored by every dump option).
blocks
Enable showing basic block boundaries (disabled in raw dumps).
graph
For each of the other indicated dump files (-fdump-rtl-pass), dump a representation of the control flow graph
suitable for viewing with GraphViz to file.passid.pass.dot. Each function in the file is pretty-printed as a
subgraph, so that GraphViz can render them all in a single plot.
This option currently only works for RTL dumps, and the RTL is always dumped in slim form.
vops
Enable showing virtual operands for every statement.
lineno
Enable showing line numbers for statements.
uid Enable showing the unique ID ("DECL_UID") for each variable.
verbose
Enable showing the tree dump for each statement.
eh Enable showing the EH region number holding each statement.
scev
Enable showing scalar evolution analysis details.
optimized
Enable showing optimization information (only available in certain passes).
missed
Enable showing missed optimization information (only available in certain passes).
note
Enable other detailed optimization information (only available in certain passes).
all Turn on all options, except raw, slim, verbose and lineno.
optall
Turn on all optimization options, i.e., optimized, missed, and note.
To determine what tree dumps are available or find the dump for a pass of interest follow the steps below.
1. Invoke GCC with -fdump-passes and in the stderr output look for a code that corresponds to the pass you are
interested in. For example, the codes "tree-evrp", "tree-vrp1", and "tree-vrp2" correspond to the three
Value Range Propagation passes. The number at the end distinguishes distinct invocations of the same pass.
2. To enable the creation of the dump file, append the pass code to the -fdump- option prefix and invoke GCC
with it. For example, to enable the dump from the Early Value Range Propagation pass, invoke GCC with the
-fdump-tree-evrp option. Optionally, you may specify the name of the dump file. If you don't specify one,
GCC creates as described below.
3. Find the pass dump in a file whose name is composed of three components separated by a period: the name of
the source file GCC was invoked to compile, a numeric suffix indicating the pass number followed by the
letter t for tree passes (and the letter r for RTL passes), and finally the pass code. For example, the
Early VRP pass dump might be in a file named myfile.c.038t.evrp in the current working directory. Note that
the numeric codes are not stable and may change from one version of GCC to another.
-fopt-info
-fopt-info-options
-fopt-info-options=filename
Controls optimization dumps from various optimization passes. If the -options form is used, options is a list of
- separated option keywords to select the dump details and optimizations.
The options can be divided into three groups:
1. options describing what kinds of messages should be emitted,
2. options describing the verbosity of the dump, and
3. options describing which optimizations should be included.
The options from each group can be freely mixed as they are non-overlapping. However, in case of any conflicts,
the later options override the earlier options on the command line.
The following options control which kinds of messages should be emitted:
optimized
Print information when an optimization is successfully applied. It is up to a pass to decide which
information is relevant. For example, the vectorizer passes print the source location of loops which are
successfully vectorized.
missed
Print information about missed optimizations. Individual passes control which information to include in the
output.
note
Print verbose information about optimizations, such as certain transformations, more detailed messages about
decisions etc.
all Print detailed optimization information. This includes optimized, missed, and note.
The following option controls the dump verbosity:
internals
By default, only "high-level" messages are emitted. This option enables additional, more detailed, messages,
which are likely to only be of interest to GCC developers.
One or more of the following option keywords can be used to describe a group of optimizations:
ipa Enable dumps from all interprocedural optimizations.
loop
Enable dumps from all loop optimizations.
inline
Enable dumps from all inlining optimizations.
omp Enable dumps from all OMP (Offloading and Multi Processing) optimizations.
vec Enable dumps from all vectorization optimizations.
optall
Enable dumps from all optimizations. This is a superset of the optimization groups listed above.
If options is omitted, it defaults to optimized-optall, which means to dump messages about successful
optimizations from all the passes, omitting messages that are treated as "internals".
If the filename is provided, then the dumps from all the applicable optimizations are concatenated into the
filename. Otherwise the dump is output onto stderr. Though multiple -fopt-info options are accepted, only one of
them can include a filename. If other filenames are provided then all but the first such option are ignored.
Note that the output filename is overwritten in case of multiple translation units. If a combined output from
multiple translation units is desired, stderr should be used instead.
In the following example, the optimization info is output to stderr:
gcc -O3 -fopt-info
This example:
gcc -O3 -fopt-info-missed=missed.all
outputs missed optimization report from all the passes into missed.all, and this one:
gcc -O2 -ftree-vectorize -fopt-info-vec-missed
prints information about missed optimization opportunities from vectorization passes on stderr. Note that
-fopt-info-vec-missed is equivalent to -fopt-info-missed-vec. The order of the optimization group names and
message types listed after -fopt-info does not matter.
As another example,
gcc -O3 -fopt-info-inline-optimized-missed=inline.txt
outputs information about missed optimizations as well as optimized locations from all the inlining passes into
inline.txt.
Finally, consider:
gcc -fopt-info-vec-missed=vec.miss -fopt-info-loop-optimized=loop.opt
Here the two output filenames vec.miss and loop.opt are in conflict since only one output file is allowed. In
this case, only the first option takes effect and the subsequent options are ignored. Thus only vec.miss is
produced which contains dumps from the vectorizer about missed opportunities.
-fsave-optimization-record
Write a SRCFILE.opt-record.json.gz file detailing what optimizations were performed, for those optimizations that
support -fopt-info.
This option is experimental and the format of the data within the compressed JSON file is subject to change.
It is roughly equivalent to a machine-readable version of -fopt-info-all, as a collection of messages with source
file, line number and column number, with the following additional data for each message:
* the execution count of the code being optimized, along with metadata about whether this was from actual
profile data, or just an estimate, allowing consumers to prioritize messages by code hotness,
* the function name of the code being optimized, where applicable,
* the "inlining chain" for the code being optimized, so that when a function is inlined into several different
places (which might themselves be inlined), the reader can distinguish between the copies,
* objects identifying those parts of the message that refer to expressions, statements or symbol-table nodes,
which of these categories they are, and, when available, their source code location,
* the GCC pass that emitted the message, and
* the location in GCC's own code from which the message was emitted
Additionally, some messages are logically nested within other messages, reflecting implementation details of the
optimization passes.
-fsched-verbose=n
On targets that use instruction scheduling, this option controls the amount of debugging output the scheduler
prints to the dump files.
For n greater than zero, -fsched-verbose outputs the same information as -fdump-rtl-sched1 and -fdump-rtl-sched2.
For n greater than one, it also output basic block probabilities, detailed ready list information and unit/insn
info. For n greater than two, it includes RTL at abort point, control-flow and regions info. And for n over
four, -fsched-verbose also includes dependence info.
-fenable-kind-pass
-fdisable-kind-pass=range-list
This is a set of options that are used to explicitly disable/enable optimization passes. These options are
intended for use for debugging GCC. Compiler users should use regular options for enabling/disabling passes
instead.
-fdisable-ipa-pass
Disable IPA pass pass. pass is the pass name. If the same pass is statically invoked in the compiler
multiple times, the pass name should be appended with a sequential number starting from 1.
-fdisable-rtl-pass
-fdisable-rtl-pass=range-list
Disable RTL pass pass. pass is the pass name. If the same pass is statically invoked in the compiler
multiple times, the pass name should be appended with a sequential number starting from 1. range-list is a
comma-separated list of function ranges or assembler names. Each range is a number pair separated by a
colon. The range is inclusive in both ends. If the range is trivial, the number pair can be simplified as a
single number. If the function's call graph node's uid falls within one of the specified ranges, the pass is
disabled for that function. The uid is shown in the function header of a dump file, and the pass names can
be dumped by using option -fdump-passes.
-fdisable-tree-pass
-fdisable-tree-pass=range-list
Disable tree pass pass. See -fdisable-rtl for the description of option arguments.
-fenable-ipa-pass
Enable IPA pass pass. pass is the pass name. If the same pass is statically invoked in the compiler
multiple times, the pass name should be appended with a sequential number starting from 1.
-fenable-rtl-pass
-fenable-rtl-pass=range-list
Enable RTL pass pass. See -fdisable-rtl for option argument description and examples.
-fenable-tree-pass
-fenable-tree-pass=range-list
Enable tree pass pass. See -fdisable-rtl for the description of option arguments.
Here are some examples showing uses of these options.
# disable ccp1 for all functions
-fdisable-tree-ccp1
# disable complete unroll for function whose cgraph node uid is 1
-fenable-tree-cunroll=1
# disable gcse2 for functions at the following ranges [1,1],
# [300,400], and [400,1000]
# disable gcse2 for functions foo and foo2
-fdisable-rtl-gcse2=foo,foo2
# disable early inlining
-fdisable-tree-einline
# disable ipa inlining
-fdisable-ipa-inline
# enable tree full unroll
-fenable-tree-unroll
-fchecking
-fchecking=n
Enable internal consistency checking. The default depends on the compiler configuration. -fchecking=2 enables
further internal consistency checking that might affect code generation.
-frandom-seed=string
This option provides a seed that GCC uses in place of random numbers in generating certain symbol names that have
to be different in every compiled file. It is also used to place unique stamps in coverage data files and the
object files that produce them. You can use the -frandom-seed option to produce reproducibly identical object
files.
The string can either be a number (decimal, octal or hex) or an arbitrary string (in which case it's converted to
a number by computing CRC32).
The string should be different for every file you compile.
-save-temps
Store the usual "temporary" intermediate files permanently; name them as auxiliary output files, as specified
described under -dumpbase and -dumpdir.
When used in combination with the -x command-line option, -save-temps is sensible enough to avoid overwriting an
input source file with the same extension as an intermediate file. The corresponding intermediate file may be
obtained by renaming the source file before using -save-temps.
-save-temps=cwd
Equivalent to -save-temps -dumpdir ./.
-save-temps=obj
Equivalent to -save-temps -dumpdir outdir/, where outdir/ is the directory of the output file specified after the
-o option, including any directory separators. If the -o option is not used, the -save-temps=obj switch behaves
like -save-temps=cwd.
-time[=file]
Report the CPU time taken by each subprocess in the compilation sequence. For C source files, this is the
compiler proper and assembler (plus the linker if linking is done).
Without the specification of an output file, the output looks like this:
# cc1 0.12 0.01
# as 0.00 0.01
The first number on each line is the "user time", that is time spent executing the program itself. The second
number is "system time", time spent executing operating system routines on behalf of the program. Both numbers
are in seconds.
With the specification of an output file, the output is appended to the named file, and it looks like this:
0.12 0.01 cc1
0.00 0.01 as
The "user time" and the "system time" are moved before the program name, and the options passed to the program
are displayed, so that one can later tell what file was being compiled, and with which options.
-fdump-final-insns[=file]
Dump the final internal representation (RTL) to file. If the optional argument is omitted (or if file is "."),
the name of the dump file is determined by appending ".gkd" to the dump base name, see -dumpbase.
-fcompare-debug[=opts]
If no error occurs during compilation, run the compiler a second time, adding opts and -fcompare-debug-second to
the arguments passed to the second compilation. Dump the final internal representation in both compilations, and
print an error if they differ.
If the equal sign is omitted, the default -gtoggle is used.
The environment variable GCC_COMPARE_DEBUG, if defined, non-empty and nonzero, implicitly enables
-fcompare-debug. If GCC_COMPARE_DEBUG is defined to a string starting with a dash, then it is used for opts,
otherwise the default -gtoggle is used.
-fcompare-debug=, with the equal sign but without opts, is equivalent to -fno-compare-debug, which disables the
dumping of the final representation and the second compilation, preventing even GCC_COMPARE_DEBUG from taking
effect.
To verify full coverage during -fcompare-debug testing, set GCC_COMPARE_DEBUG to say
-fcompare-debug-not-overridden, which GCC rejects as an invalid option in any actual compilation (rather than
preprocessing, assembly or linking). To get just a warning, setting GCC_COMPARE_DEBUG to -w%n-fcompare-debug not
overridden will do.
-fcompare-debug-second
This option is implicitly passed to the compiler for the second compilation requested by -fcompare-debug, along
with options to silence warnings, and omitting other options that would cause the compiler to produce output to
files or to standard output as a side effect. Dump files and preserved temporary files are renamed so as to
contain the ".gk" additional extension during the second compilation, to avoid overwriting those generated by the
first.
When this option is passed to the compiler driver, it causes the first compilation to be skipped, which makes it
useful for little other than debugging the compiler proper.
-gtoggle
Turn off generation of debug info, if leaving out this option generates it, or turn it on at level 2 otherwise.
The position of this argument in the command line does not matter; it takes effect after all other options are
processed, and it does so only once, no matter how many times it is given. This is mainly intended to be used
with -fcompare-debug.
-fvar-tracking-assignments-toggle
Toggle -fvar-tracking-assignments, in the same way that -gtoggle toggles -g.
-Q When used on the command line prior to --help=, -Q acts as a modifier to the help output.
Otherwise, this option makes the compiler print out each function name as it is compiled, and print some
statistics about each pass when it finishes.
-ftime-report
Makes the compiler print some statistics to stderr about the time consumed by each pass when it finishes.
If SARIF output of diagnostics was requested via -fdiagnostics-format=sarif-file or
-fdiagnostics-format=sarif-stderr then the -ftime-report information is instead emitted in JSON form as part of
SARIF output. The precise format of this JSON data is subject to change, and the values may not exactly match
those emitted to stderr due to being written out at a slightly different place within the compiler.
-ftime-report-details
Record the time consumed by infrastructure parts separately for each pass.
-fira-verbose=n
Control the verbosity of the dump file for the integrated register allocator. The default value is 5. If the
value n is greater or equal to 10, the dump output is sent to stderr using the same format as n minus 10.
-flto-report
Prints a report with internal details on the workings of the link-time optimizer. The contents of this report
vary from version to version. It is meant to be useful to GCC developers when processing object files in LTO
mode (via -flto).
Disabled by default.
-flto-report-wpa
Like -flto-report, but only print for the WPA phase of link-time optimization.
-fmem-report
Makes the compiler print some statistics about permanent memory allocation when it finishes.
-fmem-report-wpa
Makes the compiler print some statistics about permanent memory allocation for the WPA phase only.
-fpre-ipa-mem-report
-fpost-ipa-mem-report
Makes the compiler print some statistics about permanent memory allocation before or after interprocedural
optimization.
-fmultiflags
This option enables multilib-aware "TFLAGS" to be used to build target libraries with options different from
those the compiler is configured to use by default, through the use of specs set up by compiler internals, by the
target, or by builders at configure time.
Like "TFLAGS", this allows the target libraries to be built for portable baseline environments, while the
compiler defaults to more demanding ones. That's useful because users can easily override the defaults the
compiler is configured to use to build their own programs, if the defaults are not ideal for their target
environment, whereas rebuilding the runtime libraries is usually not as easy or desirable.
Unlike "TFLAGS", the use of specs enables different flags to be selected for different multilibs. The way to
accomplish that is to build with make TFLAGS=-fmultiflags, after configuring --with-specs=%{fmultiflags:...}.
This option is discarded by the driver once it's done processing driver self spec.
It is also useful to check that "TFLAGS" are being used to build all target libraries, by configuring a non-
bootstrap compiler --with-specs='%{!fmultiflags:%emissing TFLAGS}' and building the compiler and target
libraries.
-fprofile-report
Makes the compiler print some statistics about consistency of the (estimated) profile and effect of individual
passes.
-fstack-usage
Makes the compiler output stack usage information for the program, on a per-function basis. The filename for the
dump is made by appending .su to the auxname. auxname is generated from the name of the output file, if
explicitly specified and it is not an executable, otherwise it is the basename of the source file. An entry is
made up of three fields:
* The name of the function.
* A number of bytes.
* One or more qualifiers: "static", "dynamic", "bounded".
The qualifier "static" means that the function manipulates the stack statically: a fixed number of bytes are
allocated for the frame on function entry and released on function exit; no stack adjustments are otherwise made
in the function. The second field is this fixed number of bytes.
The qualifier "dynamic" means that the function manipulates the stack dynamically: in addition to the static
allocation described above, stack adjustments are made in the body of the function, for example to push/pop
arguments around function calls. If the qualifier "bounded" is also present, the amount of these adjustments is
bounded at compile time and the second field is an upper bound of the total amount of stack used by the function.
If it is not present, the amount of these adjustments is not bounded at compile time and the second field only
represents the bounded part.
-fstats
Emit statistics about front-end processing at the end of the compilation. This option is supported only by the
C++ front end, and the information is generally only useful to the G++ development team.
-fdbg-cnt-list
Print the name and the counter upper bound for all debug counters.
-fdbg-cnt=counter-value-list
Set the internal debug counter lower and upper bound. counter-value-list is a comma-separated list of
name:lower_bound1-upper_bound1 [:lower_bound2-upper_bound2...] tuples which sets the name of the counter and list
of closed intervals. The lower_bound is optional and is zero initialized if not set. For example, with
-fdbg-cnt=dce:2-4:10-11,tail_call:10, dbg_cnt(dce) returns true only for second, third, fourth, tenth and
eleventh invocation. For dbg_cnt(tail_call) true is returned for first 10 invocations.
-print-file-name=library
Print the full absolute name of the library file library that would be used when linking---and don't do anything
else. With this option, GCC does not compile or link anything; it just prints the file name.
-print-multi-directory
Print the directory name corresponding to the multilib selected by any other switches present in the command
line. This directory is supposed to exist in GCC_EXEC_PREFIX.
-print-multi-lib
Print the mapping from multilib directory names to compiler switches that enable them. The directory name is
separated from the switches by ;, and each switch starts with an @ instead of the -, without spaces between
multiple switches. This is supposed to ease shell processing.
-print-multi-os-directory
Print the path to OS libraries for the selected multilib, relative to some lib subdirectory. If OS libraries are
present in the lib subdirectory and no multilibs are used, this is usually just ., if OS libraries are present in
libsuffix sibling directories this prints e.g. ../lib64, ../lib or ../lib32, or if OS libraries are present in
lib/subdir subdirectories it prints e.g. amd64, sparcv9 or ev6.
-print-multiarch
Print the path to OS libraries for the selected multiarch, relative to some lib subdirectory.
-print-prog-name=program
Like -print-file-name, but searches for a program such as cpp.
-print-libgcc-file-name
Same as -print-file-name=libgcc.a.
This is useful when you use -nostdlib or -nodefaultlibs but you do want to link with libgcc.a. You can do:
gcc -nostdlib ... `gcc -print-libgcc-file-name`
-print-search-dirs
Print the name of the configured installation directory and a list of program and library directories gcc
searches---and don't do anything else.
This is useful when gcc prints the error message installation problem, cannot exec cpp0: No such file or
directory. To resolve this you either need to put cpp0 and the other compiler components where gcc expects to
find them, or you can set the environment variable GCC_EXEC_PREFIX to the directory where you installed them.
Don't forget the trailing /.
-print-sysroot
Print the target sysroot directory that is used during compilation. This is the target sysroot specified either
at configure time or using the --sysroot option, possibly with an extra suffix that depends on compilation
options. If no target sysroot is specified, the option prints nothing.
-print-sysroot-headers-suffix
Print the suffix added to the target sysroot when searching for headers, or give an error if the compiler is not
configured with such a suffix---and don't do anything else.
-dumpmachine
Print the compiler's target machine (for example, i686-pc-linux-gnu)---and don't do anything else.
-dumpversion
Print the compiler version (for example, 3.0, 6.3.0 or 7)---and don't do anything else. This is the compiler
version used in filesystem paths and specs. Depending on how the compiler has been configured it can be just a
single number (major version), two numbers separated by a dot (major and minor version) or three numbers
separated by dots (major, minor and patchlevel version).
-dumpfullversion
Print the full compiler version---and don't do anything else. The output is always three numbers separated by
dots, major, minor and patchlevel version.
-dumpspecs
Print the compiler's built-in specs---and don't do anything else. (This is used when GCC itself is being built.)
Machine-Dependent Options
Each target machine supported by GCC can have its own options---for example, to allow you to compile for a particular
processor variant or ABI, or to control optimizations specific to that machine. By convention, the names of machine-
specific options start with -m.
Some configurations of the compiler also support additional target-specific options, usually for compatibility with
other compilers on the same platform.
AArch64 Options
These options are defined for AArch64 implementations:
-mabi=name
Generate code for the specified data model. Permissible values are ilp32 for SysV-like data model where int,
long int and pointers are 32 bits, and lp64 for SysV-like data model where int is 32 bits, but long int and
pointers are 64 bits.
The default depends on the specific target configuration. Note that the LP64 and ILP32 ABIs are not link-
compatible; you must compile your entire program with the same ABI, and link with a compatible set of libraries.
The ilp32 model is deprecated.
-mbig-endian
Generate big-endian code. This is the default when GCC is configured for an aarch64_be-*-* target.
-mgeneral-regs-only
Generate code which uses only the general-purpose registers. This will prevent the compiler from using floating-
point and Advanced SIMD registers but will not impose any restrictions on the assembler.
-mlittle-endian
Generate little-endian code. This is the default when GCC is configured for an aarch64-*-* but not an
aarch64_be-*-* target.
-mcmodel=tiny
Generate code for the tiny code model. The program and its statically defined symbols must be within 1MB of each
other. Programs can be statically or dynamically linked.
-mcmodel=small
Generate code for the small code model. The program and its statically defined symbols must be within 4GB of
each other. Programs can be statically or dynamically linked. This is the default code model.
-mcmodel=large
Generate code for the large code model. This makes no assumptions about addresses and sizes of sections.
Programs can be statically linked only. The -mcmodel=large option is incompatible with -mabi=ilp32, -fpic and
-fPIC.
-mtp=name
Specify the system register to use as a thread pointer. The valid values are tpidr_el0, tpidrro_el0, tpidr_el1,
tpidr_el2, tpidr_el3. For backwards compatibility the aliases el0, el1, el2, el3 are also accepted. The default
setting is tpidr_el0. It is recommended to compile all code intended to interoperate with the same value of this
option to avoid accessing a different thread pointer from the wrong exception level.
-mstrict-align
-mno-strict-align
Avoid or allow generating memory accesses that may not be aligned on a natural object boundary as described in
the architecture specification.
-momit-leaf-frame-pointer
-mno-omit-leaf-frame-pointer
Omit or keep the frame pointer in leaf functions. The former behavior is the default.
-mstack-protector-guard=guard
-mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset
Generate stack protection code using canary at guard. Supported locations are global for a global canary or
sysreg for a canary in an appropriate system register.
With the latter choice the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset
furthermore specify which system register to use as base register for reading the canary, and from what offset
from that base register. There is no default register or offset as this is entirely for use within the Linux
kernel.
-mtls-dialect=desc
Use TLS descriptors as the thread-local storage mechanism for dynamic accesses of TLS variables. This is the
default.
-mtls-dialect=traditional
Use traditional TLS as the thread-local storage mechanism for dynamic accesses of TLS variables.
-mtls-size=size
Specify bit size of immediate TLS offsets. Valid values are 12, 24, 32, 48. This option requires binutils 2.26
or newer.
-mfix-cortex-a53-835769
-mno-fix-cortex-a53-835769
Enable or disable the workaround for the ARM Cortex-A53 erratum number 835769. This involves inserting a NOP
instruction between memory instructions and 64-bit integer multiply-accumulate instructions. This flag will be
ignored if an architecture or cpu is specified on the command line which does not need the workaround.
-mfix-cortex-a53-843419
-mno-fix-cortex-a53-843419
Enable or disable the workaround for the ARM Cortex-A53 erratum number 843419. This erratum workaround is made
at link time and this will only pass the corresponding flag to the linker. This flag will be ignored if an
architecture or cpu is specified on the command line which does not need the workaround.
-mlow-precision-recip-sqrt
-mno-low-precision-recip-sqrt
Enable or disable the reciprocal square root approximation. This option only has an effect if -ffast-math or
-funsafe-math-optimizations is used as well. Enabling this reduces precision of reciprocal square root results
to about 16 bits for single precision and to 32 bits for double precision.
-mlow-precision-sqrt
-mno-low-precision-sqrt
Enable or disable the square root approximation. This option only has an effect if -ffast-math or
-funsafe-math-optimizations is used as well. Enabling this reduces precision of square root results to about 16
bits for single precision and to 32 bits for double precision. If enabled, it implies
-mlow-precision-recip-sqrt.
-mlow-precision-div
-mno-low-precision-div
Enable or disable the division approximation. This option only has an effect if -ffast-math or
-funsafe-math-optimizations is used as well. Enabling this reduces precision of division results to about 16
bits for single precision and to 32 bits for double precision.
-mtrack-speculation
-mno-track-speculation
Enable or disable generation of additional code to track speculative execution through conditional branches. The
tracking state can then be used by the compiler when expanding calls to "__builtin_speculation_safe_copy" to
permit a more efficient code sequence to be generated.
-moutline-atomics
-mno-outline-atomics
Enable or disable calls to out-of-line helpers to implement atomic operations. These helpers will, at runtime,
determine if the LSE instructions from ARMv8.1-A can be used; if not, they will use the load/store-exclusive
instructions that are present in the base ARMv8.0 ISA.
This option is only applicable when compiling for the base ARMv8.0 instruction set. If using a later revision,
e.g. -march=armv8.1-a or -march=armv8-a+lse, the ARMv8.1-Atomics instructions will be used directly. The same
applies when using -mcpu= when the selected cpu supports the lse feature. This option is on by default.
-march=name
Specify the name of the target architecture and, optionally, one or more feature modifiers. This option has the
form -march=arch{+[no]feature}*.
The table below summarizes the permissible values for arch and the features that they enable by default:
arch value : Architecture : Includes by default
armv8-a : Armv8-A : +fp, +simd
armv8.1-a : Armv8.1-A : armv8-a, +crc, +lse, +rdma
armv8.2-a : Armv8.2-A : armv8.1-a
armv8.3-a : Armv8.3-A : armv8.2-a, +pauth, +fcma, +jscvt
armv8.4-a : Armv8.4-A : armv8.3-a, +flagm, +fp16fml, +dotprod, +rcpc2
armv8.5-a : Armv8.5-A : armv8.4-a, +sb, +ssbs, +predres, +frintts, +flagm2
armv8.6-a : Armv8.6-A : armv8.5-a, +bf16, +i8mm
armv8.7-a : Armv8.7-A : armv8.6-a, +wfxt, +xs
armv8.8-a : Armv8.8-a : armv8.7-a, +mops
armv8.9-a : Armv8.9-a : armv8.8-a
armv9-a : Armv9-A : armv8.5-a, +sve, +sve2
armv9.1-a : Armv9.1-A : armv9-a, +bf16, +i8mm
armv9.2-a : Armv9.2-A : armv9.1-a, +wfxt, +xs
armv9.3-a : Armv9.3-A : armv9.2-a, +mops
armv9.4-a : Armv9.4-A : armv9.3-a, +sve2p1
armv9.5-a : Armv9.5-A : armv9.4-a, cpa, +faminmax, +lut
armv8-r : Armv8-R : armv8-r
The value native is available on native AArch64 GNU/Linux and causes the compiler to pick the architecture of the
host system. This option has no effect if the compiler is unable to recognize the architecture of the host
system. When -march=native is given and no other -mcpu or -mtune is given then GCC will pick the host CPU as the
CPU to tune for as well as select the architecture features from. That is, -march=native is treated as
-mcpu=native.
The permissible values for feature are listed in the sub-section on aarch64-feature-modifiers,,-march and -mcpu
Feature Modifiers. Where conflicting feature modifiers are specified, the right-most feature is used.
GCC uses name to determine what kind of instructions it can emit when generating assembly code. If -march is
specified without either of -mtune or -mcpu also being specified, the code is tuned to perform well across a
range of target processors implementing the target architecture.
-mtune=name
Specify the name of the target processor for which GCC should tune the performance of the code. Permissible
values for this option are: generic, cortex-a35, cortex-a53, cortex-a55, cortex-a57, cortex-a72, cortex-a73,
cortex-a75, cortex-a76, cortex-a76ae, cortex-a77, cortex-a65, cortex-a65ae, cortex-a34, cortex-a78, cortex-a78ae,
cortex-a78c, ares, exynos-m1, emag, falkor, oryon-1, neoverse-512tvb, neoverse-e1, neoverse-n1, neoverse-n2,
neoverse-v1, neoverse-v2, grace, neoverse-v3, neoverse-v3ae, neoverse-n3, olympus, cortex-a725, cortex-x925,
qdf24xx, saphira, phecda, xgene1, vulcan, octeontx, octeontx81, octeontx83, octeontx2, octeontx2t98,
octeontx2t96 octeontx2t93, octeontx2f95, octeontx2f95n, octeontx2f95mm, a64fx, fujitsu-monaka, thunderx,
thunderxt88, thunderxt88p1, thunderxt81, tsv110, hip12, thunderxt83, thunderx2t99, thunderx3t110, zeus,
cortex-a57.cortex-a53, cortex-a72.cortex-a53, cortex-a73.cortex-a35, cortex-a73.cortex-a53,
cortex-a75.cortex-a55, cortex-a76.cortex-a55, cortex-r82, cortex-r82ae, cortex-x1, cortex-x1c, cortex-x2,
cortex-x3, cortex-x4, cortex-a510, cortex-a520, cortex-a520ae, cortex-a710, cortex-a715, cortex-a720,
cortex-a720ae, ampere1, ampere1a, ampere1b, cobalt-100, apple-m1, apple-m2, apple-m3 and native.
The values cortex-a57.cortex-a53, cortex-a72.cortex-a53, cortex-a73.cortex-a35, cortex-a73.cortex-a53,
cortex-a75.cortex-a55, cortex-a76.cortex-a55, apple-m1, apple-m2, apple-m3, gb10 specify that GCC should tune for
a big.LITTLE system.
The value neoverse-512tvb specifies that GCC should tune for Neoverse cores that (a) implement SVE and (b) have a
total vector bandwidth of 512 bits per cycle. In other words, the option tells GCC to tune for Neoverse cores
that can execute 4 128-bit Advanced SIMD arithmetic instructions a cycle and that can execute an equivalent
number of SVE arithmetic instructions per cycle (2 for 256-bit SVE, 4 for 128-bit SVE). This is more general
than tuning for a specific core like Neoverse V1 but is more specific than the default tuning described below.
Additionally on native AArch64 GNU/Linux systems the value native tunes performance to the host system. This
option has no effect if the compiler is unable to recognize the processor of the host system.
Where none of -mtune=, -mcpu= or -march= are specified, the code is tuned to perform well across a range of
target processors.
This option cannot be suffixed by feature modifiers.
-mcpu=name
Specify the name of the target processor, optionally suffixed by one or more feature modifiers. This option has
the form -mcpu=cpu{+[no]feature}*, where the permissible values for cpu are the same as those available for
-mtune. The permissible values for feature are documented in the sub-section on
aarch64-feature-modifiers,,-march and -mcpu Feature Modifiers. Where conflicting feature modifiers are
specified, the right-most feature is used.
GCC uses name to determine what kind of instructions it can emit when generating assembly code (as if by -march)
and to determine the target processor for which to tune for performance (as if by -mtune). Where this option is
used in conjunction with -march or -mtune, those options take precedence over the appropriate part of this
option.
-mcpu=neoverse-512tvb is special in that it does not refer to a specific core, but instead refers to all Neoverse
cores that (a) implement SVE and (b) have a total vector bandwidth of 512 bits a cycle. Unless overridden by
-march, -mcpu=neoverse-512tvb generates code that can run on a Neoverse V1 core, since Neoverse V1 is the first
Neoverse core with these properties. Unless overridden by -mtune, -mcpu=neoverse-512tvb tunes code in the same
way as for -mtune=neoverse-512tvb.
-moverride=string
Override tuning decisions made by the back-end in response to a -mtune= switch. The syntax, semantics, and
accepted values for string in this option are not guaranteed to be consistent across releases.
This option is only intended to be useful when developing GCC.
-mverbose-cost-dump
Enable verbose cost model dumping in the debug dump files. This option is provided for use in debugging the
compiler.
-mpc-relative-literal-loads
-mno-pc-relative-literal-loads
Enable or disable PC-relative literal loads. With this option literal pools are accessed using a single
instruction and emitted after each function. This limits the maximum size of functions to 1MB. This is enabled
by default for -mcmodel=tiny.
-msign-return-address=scope
Select the function scope on which return address signing will be applied. Permissible values are none, which
disables return address signing, non-leaf, which enables pointer signing for functions which are not leaf
functions, and all, which enables pointer signing for all functions. The default value is none. This option has
been deprecated by -mbranch-protection.
-mbranch-protection=none|standard|pac-ret[+leaf+b-key]|bti|gcs
Select the branch protection features to use. none is the default and turns off all types of branch protection.
standard turns on all types of branch protection features. If a feature has additional tuning options, then
standard sets it to its standard level. pac-ret[+leaf] turns on return address signing to its standard level:
signing functions that save the return address to memory (non-leaf functions will practically always do this)
using the a-key. The optional argument leaf can be used to extend the signing to include leaf functions. The
optional argument b-key can be used to sign the functions with the B-key instead of the A-key. bti turns on
branch target identification mechanism. gcs turns on guarded control stack compatible code generation.
-mharden-sls=opts
Enable compiler hardening against straight line speculation (SLS). opts is a comma-separated list of the
following options:
retbr
blr
In addition, -mharden-sls=all enables all SLS hardening while -mharden-sls=none disables all SLS hardening.
-mearly-ra=scope
Determine when to enable an early register allocation pass. This pass runs before instruction scheduling and
tries to find a spill-free allocation of floating-point and vector code. It also tries to make use of strided
multi-register instructions, such as SME2's strided LD1 and ST1.
The possible values of scope are: all, which runs the pass on all functions; strided, which runs the pass on
functions that have access to strided multi-register instructions; and none, which disables the pass.
-mearly-ra=all is the default for -O2 and above, and for -Os. -mearly-ra=none is the default otherwise.
-mearly-ldp-fusion
Enable the copy of the AArch64 load/store pair fusion pass that runs before register allocation. Enabled by
default at -O and above.
-mlate-ldp-fusion
Enable the copy of the AArch64 load/store pair fusion pass that runs after register allocation. Enabled by
default at -O and above.
-msve-vector-bits=bits
Specify the number of bits in an SVE vector register. This option only has an effect when SVE is enabled.
GCC supports two forms of SVE code generation: "vector-length agnostic" output that works with any size of vector
register and "vector-length specific" output that allows GCC to make assumptions about the vector length when it
is useful for optimization reasons. The possible values of bits are: scalable, 128, 256, 512, 1024 and 2048.
Specifying scalable selects vector-length agnostic output. At present -msve-vector-bits=128 also generates
vector-length agnostic output for big-endian targets. All other values generate vector-length specific code.
The behavior of these values may change in future releases and no value except scalable should be relied on for
producing code that is portable across different hardware SVE vector lengths.
The default is -msve-vector-bits=scalable, which produces vector-length agnostic code.
-Wexperimental-fmv-target
Warn about use of experimental Function Multi Versioning. The Arm C Language Extension specification for
Function Multi Versioning is beta and subject to change. Any usage of FMV is caveated that future behavior change
and incompatibility is likely.
-march and -mcpu Feature Modifiers
Feature modifiers used with -march and -mcpu can be any of the following and their inverses nofeature:
crc Enable CRC extension. This is on by default for -march=armv8.1-a.
crypto
Enable Crypto extension. This also enables Advanced SIMD and floating-point instructions.
fp Enable floating-point instructions. This is on by default for all possible values for options -march and -mcpu.
simd
Enable Advanced SIMD instructions. This also enables floating-point instructions. This is on by default for all
possible values for options -march and -mcpu.
sve Enable Scalable Vector Extension instructions. This also enables Advanced SIMD and floating-point instructions.
lse Enable Large System Extension instructions. This is on by default for -march=armv8.1-a.
rdma
Enable Round Double Multiply Accumulate instructions. This is on by default for -march=armv8.1-a.
fp16
Enable FP16 extension. This also enables floating-point instructions.
fp16fml
Enable FP16 fmla extension. This also enables FP16 extensions and floating-point instructions. This option is
enabled by default for -march=armv8.4-a. Use of this option with architectures prior to Armv8.2-A is not
supported.
rcpc
Enable the RCpc extension. This enables the use of the LDAPR instructions for load-acquire atomic semantics, and
passes it on to the assembler, enabling inline asm statements to use instructions from the RCpc extension.
dotprod
Enable the Dot Product extension. This also enables Advanced SIMD instructions.
aes Enable the Armv8-a aes and pmull crypto extension. This also enables Advanced SIMD instructions.
sha2
Enable the Armv8-a sha2 crypto extension. This also enables Advanced SIMD instructions.
sha3
Enable the sha512 and sha3 crypto extension. This also enables Advanced SIMD instructions. Use of this option
with architectures prior to Armv8.2-A is not supported.
sm4 Enable the sm3 and sm4 crypto extension. This also enables Advanced SIMD instructions. Use of this option with
architectures prior to Armv8.2-A is not supported.
profile
Enable the Statistical Profiling extension. This option is only to enable the extension at the assembler level
and does not affect code generation.
rng Enable the Armv8.5-a Random Number instructions. This option is only to enable the extension at the assembler
level and does not affect code generation.
memtag
Enable the Armv8.5-a Memory Tagging Extensions. Use of this option with architectures prior to Armv8.5-A is not
supported.
sb Enable the Armv8-a Speculation Barrier instruction. This option is only to enable the extension at the assembler
level and does not affect code generation. This option is enabled by default for -march=armv8.5-a.
ssbs
Enable the Armv8-a Speculative Store Bypass Safe instruction. This option is only to enable the extension at the
assembler level and does not affect code generation. This option is enabled by default for -march=armv8.5-a.
predres
Enable the Armv8-a Execution and Data Prediction Restriction instructions. This option is only to enable the
extension at the assembler level and does not affect code generation. This option is enabled by default for
-march=armv8.5-a.
sve2
Enable the Armv8-a Scalable Vector Extension 2. This also enables SVE instructions.
sve2-bitperm
Enable SVE2 bitperm instructions. This also enables SVE2 instructions.
sve2-sm4
Enable SVE2 sm4 instructions. This also enables SVE2 instructions.
sve2-aes
Enable SVE2 aes instructions. This also enables SVE2 instructions.
sve2-sha3
Enable SVE2 sha3 instructions. This also enables SVE2 instructions.
sve2p1
Enable SVE2.1 instructions. This also enables SVE2 instructions.
tme Enable the Transactional Memory Extension.
i8mm
Enable 8-bit Integer Matrix Multiply instructions. This also enables Advanced SIMD and floating-point
instructions. This option is enabled by default for -march=armv8.6-a. Use of this option with architectures
prior to Armv8.2-A is not supported.
f32mm
Enable 32-bit Floating point Matrix Multiply instructions. This also enables SVE instructions. Use of this
option with architectures prior to Armv8.2-A is not supported.
f64mm
Enable 64-bit Floating point Matrix Multiply instructions. This also enables SVE instructions. Use of this
option with architectures prior to Armv8.2-A is not supported.
bf16
Enable brain half-precision floating-point instructions. This also enables Advanced SIMD and floating-point
instructions. This option is enabled by default for -march=armv8.6-a. Use of this option with architectures
prior to Armv8.2-A is not supported.
ls64
Enable the 64-byte atomic load and store instructions for accelerators.
mops
Enable the instructions to accelerate memory operations like "memcpy", "memmove", "memset". This option is
enabled by default for -march=armv8.8-a
flagm
Enable the Flag Manipulation instructions Extension.
flagm2
Enable the FlagM2 flag conversion instructions.
pauth
Enable the Pointer Authentication Extension.
cssc
Enable the Common Short Sequence Compression instructions.
sme Enable the Scalable Matrix Extension. This is only supported when SVE2 is also enabled.
sme-i16i64
Enable the FEAT_SME_I16I64 extension to SME. This also enables SME instructions.
sme-f64f64
Enable the FEAT_SME_F64F64 extension to SME. This also enables SME instructions.
sme2
Enable the Scalable Matrix Extension 2. This also enables SME instructions.
sme-b16b16
Enable the FEAT_SME_B16B16 extension to SME. This also enables SME2 and SVE_B16B16 instructions.
sme-f16f16
Enable the FEAT_SME_F16F16 extension to SME. This also enables SME2 instructions.
sme2p1
Enable the Scalable Matrix Extension version 2.1. This also enables SME2 instructions.
fcma
Enable the complex number SIMD extensions.
jscvt
Enable the "fjcvtzs" JavaScript conversion instruction.
frintts
Enable floating-point round to integral value instructions.
wfxt
Enable "wfet" and "wfit" instructions.
xs Enable the XS memory attribute extension.
lse128
Enable the LSE128 128-bit atomic instructions extension. This also enables LSE instructions.
d128
Enable support for 128-bit system register read/write instructions. This also enables the LSE128 extension.
gcs Enable support for Armv9.4-a Guarded Control Stack extension.
the Enable support for Armv8.9-a/9.4-a translation hardening extension.
rcpc2
Enable the RCpc2 extension.
rcpc3
Enable the RCpc3 (Release Consistency) extension.
fp8 Enable the fp8 (8-bit floating point) extension.
fp8fma
Enable the fp8 (8-bit floating point) multiply accumulate extension.
ssve-fp8fma
Enable the fp8 (8-bit floating point) multiply accumulate extension in streaming mode.
fp8dot4
Enable the fp8 (8-bit floating point) to single-precision 4-way dot product extension.
ssve-fp8dot4
Enable the fp8 (8-bit floating point) to single-precision 4-way dot product extension in streaming mode.
fp8dot2
Enable the fp8 (8-bit floating point) to half-precision 2-way dot product extension.
ssve-fp8dot2
Enable the fp8 (8-bit floating point) to half-precision 2-way dot product extension in streaming mode.
faminmax
Enable the Floating Point Absolute Maximum/Minimum extension.
lut Enable the Lookup Table extension.
cpa Enable the Checked Pointer Arithmetic instructions.
sve-b16b16
Enable the SVE non-widening brain floating-point ("bf16") extension. This only has an effect when "sve2" or
"sme2" are also enabled.
Feature crypto implies aes, sha2, and simd, which implies fp. Conversely, nofp implies nosimd, which implies
nocrypto, noaes and nosha2.
Adapteva Epiphany Options
These -m options are defined for Adapteva Epiphany:
-mhalf-reg-file
Don't allocate any register in the range "r32"..."r63". That allows code to run on hardware variants that lack
these registers.
-mprefer-short-insn-regs
Preferentially allocate registers that allow short instruction generation. This can result in increased
instruction count, so this may either reduce or increase overall code size.
-mbranch-cost=num
Set the cost of branches to roughly num "simple" instructions. This cost is only a heuristic and is not
guaranteed to produce consistent results across releases.
-mcmove
Enable the generation of conditional moves.
-mnops=num
Emit num NOPs before every other generated instruction.
-mno-soft-cmpsf
For single-precision floating-point comparisons, emit an "fsub" instruction and test the flags. This is faster
than a software comparison, but can get incorrect results in the presence of NaNs, or when two different small
numbers are compared such that their difference is calculated as zero. The default is -msoft-cmpsf, which uses
slower, but IEEE-compliant, software comparisons.
-mstack-offset=num
Set the offset between the top of the stack and the stack pointer. E.g., a value of 8 means that the eight bytes
in the range "sp+0...sp+7" can be used by leaf functions without stack allocation. Values other than 8 or 16 are
untested and unlikely to work. Note also that this option changes the ABI; compiling a program with a different
stack offset than the libraries have been compiled with generally does not work. This option can be useful if
you want to evaluate if a different stack offset would give you better code, but to actually use a different
stack offset to build working programs, it is recommended to configure the toolchain with the appropriate
--with-stack-offset=num option.
-mno-round-nearest
Make the scheduler assume that the rounding mode has been set to truncating. The default is -mround-nearest.
-mlong-calls
If not otherwise specified by an attribute, assume all calls might be beyond the offset range of the "b" / "bl"
instructions, and therefore load the function address into a register before performing a (otherwise direct)
call. This is the default.
-mshort-calls
If not otherwise specified by an attribute, assume all direct calls are in the range of the "b" / "bl"
instructions, so use these instructions for direct calls. The default is -mlong-calls.
-msmall16
Assume addresses can be loaded as 16-bit unsigned values. This does not apply to function addresses for which
-mlong-calls semantics are in effect.
-mfp-mode=mode
Set the prevailing mode of the floating-point unit. This determines the floating-point mode that is provided and
expected at function call and return time. Making this mode match the mode you predominantly need at function
start can make your programs smaller and faster by avoiding unnecessary mode switches.
mode can be set to one the following values:
caller
Any mode at function entry is valid, and retained or restored when the function returns, and when it calls
other functions. This mode is useful for compiling libraries or other compilation units you might want to
incorporate into different programs with different prevailing FPU modes, and the convenience of being able to
use a single object file outweighs the size and speed overhead for any extra mode switching that might be
needed, compared with what would be needed with a more specific choice of prevailing FPU mode.
truncate
This is the mode used for floating-point calculations with truncating (i.e. round towards zero) rounding
mode. That includes conversion from floating point to integer.
round-nearest
This is the mode used for floating-point calculations with round-to-nearest-or-even rounding mode.
int This is the mode used to perform integer calculations in the FPU, e.g. integer multiply, or integer
multiply-and-accumulate.
The default is -mfp-mode=caller
-mno-split-lohi
-mno-postinc
-mno-postmodify
Code generation tweaks that disable, respectively, splitting of 32-bit loads, generation of post-increment
addresses, and generation of post-modify addresses. The defaults are msplit-lohi, -mpost-inc, and -mpost-modify.
-mnovect-double
Change the preferred SIMD mode to SImode. The default is -mvect-double, which uses DImode as preferred SIMD
mode.
-max-vect-align=num
The maximum alignment for SIMD vector mode types. num may be 4 or 8. The default is 8. Note that this is an
ABI change, even though many library function interfaces are unaffected if they don't use SIMD vector modes in
places that affect size and/or alignment of relevant types.
-msplit-vecmove-early
Split vector moves into single word moves before reload. In theory this can give better register allocation, but
so far the reverse seems to be generally the case.
-m1reg-reg
Specify a register to hold the constant -1, which makes loading small negative constants and certain bitmasks
faster. Allowable values for reg are r43 and r63, which specify use of that register as a fixed register, and
none, which means that no register is used for this purpose. The default is -m1reg-none.
AMD GCN Options
These options are defined specifically for the AMD GCN port.
-march=gpu
-mtune=gpu
Set architecture type or tuning for gpu. Supported values for gpu are
gfx900
Compile for GCN5 Vega 10 devices (gfx900).
gfx902
Compile for GCN5 Vega gfx902 devices. (Experimental)
gfx904
Compile for GCN5 Vega gfx904 devices. (Experimental)
gfx906
Compile for GCN5 Vega 20 devices (gfx906).
gfx908
Compile for CDNA1 Instinct MI100 series devices (gfx908).
gfx909
Compile for GCN5 Vega gfx909 devices. (Experimental)
gfx90a
Compile for CDNA2 Instinct MI200 series devices (gfx90a).
gfx90c
Compile for GCN5 Vega 7 devices (gfx90c).
gfx9-generic
Compile generic code for Vega devices, executable on the following subset of GFX9 devices: gfx900, gfx902,
gfx904, gfx906, gfx909 and gfx90c. (Experimental)
gfx1030
Compile for RDNA2 gfx1030 devices (GFX10 series).
gfx1031
Compile for RDNA2 gfx1031 devices (GFX10 series). (Experimental)
gfx1032
Compile for RDNA2 gfx1032 devices (GFX10 series). (Experimental)
gfx1033
Compile for RDNA2 gfx1033 devices (GFX10 series). (Experimental)
gfx1034
Compile for RDNA2 gfx1034 devices (GFX10 series). (Experimental)
gfx1035
Compile for RDNA2 gfx1035 devices (GFX10 series). (Experimental)
gfx1036
Compile for RDNA2 gfx1036 devices (GFX10 series).
gfx10-3-generic
Compile generic code for GFX10-3 devices, executable on gfx1030, gfx1031, gfx1032, gfx1033, gfx1034, gfx1035,
and gfx1036. (Experimental)
gfx1100
Compile for RDNA3 gfx1100 devices (GFX11 series).
gfx1101
Compile for RDNA3 gfx1101 devices (GFX11 series). (Experimental)
gfx1102
Compile for RDNA3 gfx1102 devices (GFX11 series). (Experimental)
gfx1103
Compile for RDNA3 gfx1103 devices (GFX11 series).
gfx1150
Compile for RDNA3 gfx1150 devices (GFX11 series). (Experimental)
gfx1151
Compile for RDNA3 gfx1151 devices (GFX11 series). (Experimental)
gfx1152
Compile for RDNA3 gfx1152 devices (GFX11 series). (Experimental)
gfx1153
Compile for RDNA3 gfx1153 devices (GFX11 series). (Experimental)
gfx11-generic
Compile generic code for GFX11 devices, executable on gfx1100, gfx1101, gfx1102, gfx1103, gfx1150, gfx1151,
gfx1152, and gfx1153. (Experimental)
-msram-ecc=on
-msram-ecc=off
-msram-ecc=any
Compile binaries suitable for devices with the SRAM-ECC feature enabled, disabled, or either mode. This feature
can be enabled per-process on some devices. The compiled code must match the device mode. The default is any,
for devices that support it.
-mstack-size=bytes
Specify how many bytes of stack space will be requested for each GPU thread (wave-front). Beware that there may
be many threads and limited memory available. The size of the stack allocation may also have an impact on run-
time performance. The default is 32KB when using OpenACC or OpenMP, and 1MB otherwise.
-mxnack=on
-mxnack=off
-mxnack=any
Compile binaries suitable for devices with the XNACK feature enabled, disabled, or either mode. Some devices
always require XNACK and some allow the user to configure XNACK. The compiled code must match the device mode.
The default is -mxnack=any on devices that support Unified Shared Memory, and -mxnack=no otherwise.
ARC Options
The following options control the architecture variant for which code is being compiled:
-mbarrel-shifter
Generate instructions supported by barrel shifter. This is the default unless -mcpu=ARC601 or -mcpu=ARCEM is in
effect.
-mjli-always
Force to call a function using jli_s instruction. This option is valid only for ARCv2 architecture.
-mcpu=cpu
Set architecture type, register usage, and instruction scheduling parameters for cpu. There are also shortcut
alias options available for backward compatibility and convenience. Supported values for cpu are
arc600
Compile for ARC600. Aliases: -mA6, -mARC600.
arc601
Compile for ARC601. Alias: -mARC601.
arc700
Compile for ARC700. Aliases: -mA7, -mARC700. This is the default when configured with --with-cpu=arc700.
arcem
Compile for ARC EM.
archs
Compile for ARC HS.
em Compile for ARC EM CPU with no hardware extensions.
em4 Compile for ARC EM4 CPU.
em4_dmips
Compile for ARC EM4 DMIPS CPU.
em4_fpus
Compile for ARC EM4 DMIPS CPU with the single-precision floating-point extension.
em4_fpuda
Compile for ARC EM4 DMIPS CPU with single-precision floating-point and double assist instructions.
hs Compile for ARC HS CPU with no hardware extensions except the atomic instructions.
hs34
Compile for ARC HS34 CPU.
hs38
Compile for ARC HS38 CPU.
hs38_linux
Compile for ARC HS38 CPU with all hardware extensions on.
hs4x
Compile for ARC HS4x CPU.
hs4xd
Compile for ARC HS4xD CPU.
hs4x_rel31
Compile for ARC HS4x CPU release 3.10a.
arc600_norm
Compile for ARC 600 CPU with "norm" instructions enabled.
arc600_mul32x16
Compile for ARC 600 CPU with "norm" and 32x16-bit multiply instructions enabled.
arc600_mul64
Compile for ARC 600 CPU with "norm" and "mul64"-family instructions enabled.
arc601_norm
Compile for ARC 601 CPU with "norm" instructions enabled.
arc601_mul32x16
Compile for ARC 601 CPU with "norm" and 32x16-bit multiply instructions enabled.
arc601_mul64
Compile for ARC 601 CPU with "norm" and "mul64"-family instructions enabled.
nps400
Compile for ARC 700 on NPS400 chip.
em_mini
Compile for ARC EM minimalist configuration featuring reduced register set.
-mdpfp
-mdpfp-compact
Generate double-precision FPX instructions, tuned for the compact implementation.
-mdpfp-fast
Generate double-precision FPX instructions, tuned for the fast implementation.
-mno-dpfp-lrsr
Disable "lr" and "sr" instructions from using FPX extension aux registers.
-mea
Generate extended arithmetic instructions. Currently only "divaw", "adds", "subs", and "sat16" are supported.
Only valid for -mcpu=ARC700.
-mno-mpy
Do not generate "mpy"-family instructions for ARC700. This option is deprecated.
-mmul32x16
Generate 32x16-bit multiply and multiply-accumulate instructions.
-mmul64
Generate "mul64" and "mulu64" instructions. Only valid for -mcpu=ARC600.
-mnorm
Generate "norm" instructions. This is the default if -mcpu=ARC700 is in effect.
-mspfp
-mspfp-compact
Generate single-precision FPX instructions, tuned for the compact implementation.
-mspfp-fast
Generate single-precision FPX instructions, tuned for the fast implementation.
-msimd
Enable generation of ARC SIMD instructions via target-specific builtins. Only valid for -mcpu=ARC700.
-msoft-float
This option ignored; it is provided for compatibility purposes only. Software floating-point code is emitted by
default, and this default can overridden by FPX options; -mspfp, -mspfp-compact, or -mspfp-fast for single
precision, and -mdpfp, -mdpfp-compact, or -mdpfp-fast for double precision.
-mswap
Generate "swap" instructions.
-matomic
This enables use of the locked load/store conditional extension to implement atomic memory built-in functions.
Not available for ARC 6xx or ARC EM cores.
-mdiv-rem
Enable "div" and "rem" instructions for ARCv2 cores.
-mcode-density
Enable code density instructions for ARC EM. This option is on by default for ARC HS.
-mll64
Enable double load/store operations for ARC HS cores.
-mtp-regno=regno
Specify thread pointer register number.
-mmpy-option=multo
Compile ARCv2 code with a multiplier design option. You can specify the option using either a string or numeric
value for multo. wlh1 is the default value. The recognized values are:
0
none
No multiplier available.
1
w 16x16 multiplier, fully pipelined. The following instructions are enabled: "mpyw" and "mpyuw".
2
wlh1
32x32 multiplier, fully pipelined (1 stage). The following instructions are additionally enabled: "mpy",
"mpyu", "mpym", "mpymu", and "mpy_s".
3
wlh2
32x32 multiplier, fully pipelined (2 stages). The following instructions are additionally enabled: "mpy",
"mpyu", "mpym", "mpymu", and "mpy_s".
4
wlh3
Two 16x16 multipliers, blocking, sequential. The following instructions are additionally enabled: "mpy",
"mpyu", "mpym", "mpymu", and "mpy_s".
5
wlh4
One 16x16 multiplier, blocking, sequential. The following instructions are additionally enabled: "mpy",
"mpyu", "mpym", "mpymu", and "mpy_s".
6
wlh5
One 32x4 multiplier, blocking, sequential. The following instructions are additionally enabled: "mpy",
"mpyu", "mpym", "mpymu", and "mpy_s".
7
plus_dmpy
ARC HS SIMD support.
8
plus_macd
ARC HS SIMD support.
9
plus_qmacw
ARC HS SIMD support.
This option is only available for ARCv2 cores.
-mfpu=fpu
Enables support for specific floating-point hardware extensions for ARCv2 cores. Supported values for fpu are:
fpus
Enables support for single-precision floating-point hardware extensions.
fpud
Enables support for double-precision floating-point hardware extensions. The single-precision floating-point
extension is also enabled. Not available for ARC EM.
fpuda
Enables support for double-precision floating-point hardware extensions using double-precision assist
instructions. The single-precision floating-point extension is also enabled. This option is only available
for ARC EM.
fpuda_div
Enables support for double-precision floating-point hardware extensions using double-precision assist
instructions. The single-precision floating-point, square-root, and divide extensions are also enabled.
This option is only available for ARC EM.
fpuda_fma
Enables support for double-precision floating-point hardware extensions using double-precision assist
instructions. The single-precision floating-point and fused multiply and add hardware extensions are also
enabled. This option is only available for ARC EM.
fpuda_all
Enables support for double-precision floating-point hardware extensions using double-precision assist
instructions. All single-precision floating-point hardware extensions are also enabled. This option is only
available for ARC EM.
fpus_div
Enables support for single-precision floating-point, square-root and divide hardware extensions.
fpud_div
Enables support for double-precision floating-point, square-root and divide hardware extensions. This option
includes option fpus_div. Not available for ARC EM.
fpus_fma
Enables support for single-precision floating-point and fused multiply and add hardware extensions.
fpud_fma
Enables support for double-precision floating-point and fused multiply and add hardware extensions. This
option includes option fpus_fma. Not available for ARC EM.
fpus_all
Enables support for all single-precision floating-point hardware extensions.
fpud_all
Enables support for all single- and double-precision floating-point hardware extensions. Not available for
ARC EM.
-mirq-ctrl-saved=register-range, blink, lp_count
Specifies general-purposes registers that the processor automatically saves/restores on interrupt entry and exit.
register-range is specified as two registers separated by a dash. The register range always starts with "r0",
the upper limit is "fp" register. blink and lp_count are optional. This option is only valid for ARC EM and ARC
HS cores.
-mrgf-banked-regs=number
Specifies the number of registers replicated in second register bank on entry to fast interrupt. Fast interrupts
are interrupts with the highest priority level P0. These interrupts save only PC and STATUS32 registers to avoid
memory transactions during interrupt entry and exit sequences. Use this option when you are using fast
interrupts in an ARC V2 family processor. Permitted values are 4, 8, 16, and 32.
-mlpc-width=width
Specify the width of the "lp_count" register. Valid values for width are 8, 16, 20, 24, 28 and 32 bits. The
default width is fixed to 32 bits. If the width is less than 32, the compiler does not attempt to transform
loops in your program to use the zero-delay loop mechanism unless it is known that the "lp_count" register can
hold the required loop-counter value. Depending on the width specified, the compiler and run-time library might
continue to use the loop mechanism for various needs. This option defines macro "__ARC_LPC_WIDTH__" with the
value of width.
-mrf16
This option instructs the compiler to generate code for a 16-entry register file. This option defines the
"__ARC_RF16__" preprocessor macro.
-mbranch-index
Enable use of "bi" or "bih" instructions to implement jump tables.
The following options are passed through to the assembler, and also define preprocessor macro symbols.
-mdsp-packa
Passed down to the assembler to enable the DSP Pack A extensions. Also sets the preprocessor symbol
"__Xdsp_packa". This option is deprecated.
-mdvbf
Passed down to the assembler to enable the dual Viterbi butterfly extension. Also sets the preprocessor symbol
"__Xdvbf". This option is deprecated.
-mlock
Passed down to the assembler to enable the locked load/store conditional extension. Also sets the preprocessor
symbol "__Xlock".
-mmac-d16
Passed down to the assembler. Also sets the preprocessor symbol "__Xxmac_d16". This option is deprecated.
-mmac-24
Passed down to the assembler. Also sets the preprocessor symbol "__Xxmac_24". This option is deprecated.
-mrtsc
Passed down to the assembler to enable the 64-bit time-stamp counter extension instruction. Also sets the
preprocessor symbol "__Xrtsc". This option is deprecated.
-mswape
Passed down to the assembler to enable the swap byte ordering extension instruction. Also sets the preprocessor
symbol "__Xswape".
-mtelephony
Passed down to the assembler to enable dual- and single-operand instructions for telephony. Also sets the
preprocessor symbol "__Xtelephony". This option is deprecated.
-mxy
Passed down to the assembler to enable the XY memory extension. Also sets the preprocessor symbol "__Xxy".
The following options control how the assembly code is annotated:
-misize
Annotate assembler instructions with estimated addresses.
-mannotate-align
Does nothing. Preserved for backward compatibility.
The following options are passed through to the linker:
-marclinux
Passed through to the linker, to specify use of the "arclinux" emulation. This option is enabled by default in
tool chains built for "arc-linux-uclibc" and "arceb-linux-uclibc" targets when profiling is not requested.
-marclinux_prof
Passed through to the linker, to specify use of the "arclinux_prof" emulation. This option is enabled by default
in tool chains built for "arc-linux-uclibc" and "arceb-linux-uclibc" targets when profiling is requested.
The following options control the semantics of generated code:
-mlong-calls
Generate calls as register indirect calls, thus providing access to the full 32-bit address range.
-mmedium-calls
Don't use less than 25-bit addressing range for calls, which is the offset available for an unconditional branch-
and-link instruction. Conditional execution of function calls is suppressed, to allow use of the 25-bit range,
rather than the 21-bit range with conditional branch-and-link. This is the default for tool chains built for
"arc-linux-uclibc" and "arceb-linux-uclibc" targets.
-G num
Put definitions of externally-visible data in a small data section if that data is no bigger than num bytes. The
default value of num is 4 for any ARC configuration, or 8 when we have double load/store operations.
-mno-sdata
Do not generate sdata references. This is the default for tool chains built for "arc-linux-uclibc" and
"arceb-linux-uclibc" targets.
-mvolatile-cache
Use ordinarily cached memory accesses for volatile references. This is the default.
-mno-volatile-cache
Enable cache bypass for volatile references.
The following options fine tune code generation:
-malign-call
Does nothing. Preserved for backward compatibility.
-mauto-modify-reg
Enable the use of pre/post modify with register displacement.
-mbbit-peephole
Does nothing. Preserved for backward compatibility.
-mno-brcc
This option disables a target-specific pass in arc_reorg to generate compare-and-branch ("brcc") instructions.
It has no effect on generation of these instructions driven by the combiner pass.
-mcase-vector-pcrel
Use PC-relative switch case tables to enable case table shortening. This is the default for -Os.
-mcompact-casesi
Enable compact "casesi" pattern. This is the default for -Os, and only available for ARCv1 cores. This option
is deprecated.
-mno-cond-exec
Disable the ARCompact-specific pass to generate conditional execution instructions.
Due to delay slot scheduling and interactions between operand numbers, literal sizes, instruction lengths, and
the support for conditional execution, the target-independent pass to generate conditional execution is often
lacking, so the ARC port has kept a special pass around that tries to find more conditional execution generation
opportunities after register allocation, branch shortening, and delay slot scheduling have been done. This pass
generally, but not always, improves performance and code size, at the cost of extra compilation time, which is
why there is an option to switch it off. If you have a problem with call instructions exceeding their allowable
offset range because they are conditionalized, you should consider using -mmedium-calls instead.
-mearly-cbranchsi
Enable pre-reload use of the "cbranchsi" pattern.
-mexpand-adddi
Expand "adddi3" and "subdi3" at RTL generation time into "add.f", "adc" etc. This option is deprecated.
-mindexed-loads
Enable the use of indexed loads. This can be problematic because some optimizers then assume that indexed stores
exist, which is not the case.
-mlra
Does nothing. Preserved for backward compatibility.
-mlra-priority-none
Don't indicate any priority for target registers.
-mlra-priority-compact
Indicate target register priority for r0..r3 / r12..r15.
-mlra-priority-noncompact
Reduce target register priority for r0..r3 / r12..r15.
-mmillicode
When optimizing for size (using -Os), prologues and epilogues that have to save or restore a large number of
registers are often shortened by using call to a special function in libgcc; this is referred to as a millicode
call. As these calls can pose performance issues, and/or cause linking issues when linking in a nonstandard way,
this option is provided to turn on or off millicode call generation.
-mcode-density-frame
This option enable the compiler to emit "enter" and "leave" instructions. These instructions are only valid for
CPUs with code-density feature.
-mmixed-code
Does nothing. Preserved for backward compatibility.
-mq-class
Ths option is deprecated. Enable q instruction alternatives. This is the default for -Os.
-mRcq
Does nothing. Preserved for backward compatibility.
-mRcw
Does nothing. Preserved for backward compatibility.
-msize-level=level
Fine-tune size optimization with regards to instruction lengths and alignment. The recognized values for level
are:
0 No size optimization. This level is deprecated and treated like 1.
1 Short instructions are used opportunistically.
2 In addition, alignment of loops and of code after barriers are dropped.
3 In addition, optional data alignment is dropped, and the option Os is enabled.
This defaults to 3 when -Os is in effect. Otherwise, the behavior when this is not set is equivalent to level 1.
-mtune=cpu
Set instruction scheduling parameters for cpu, overriding any implied by -mcpu=.
Supported values for cpu are
ARC600
Tune for ARC600 CPU.
ARC601
Tune for ARC601 CPU.
ARC700
Tune for ARC700 CPU with standard multiplier block.
ARC700-xmac
Tune for ARC700 CPU with XMAC block.
ARC725D
Tune for ARC725D CPU.
ARC750D
Tune for ARC750D CPU.
core3
Tune for ARCv2 core3 type CPU. This option enable usage of "dbnz" instruction.
release31a
Tune for ARC4x release 3.10a.
-mmultcost=num
Cost to assume for a multiply instruction, with 4 being equal to a normal instruction.
-munalign-prob-threshold=probability
Does nothing. Preserved for backward compatibility.
The following options are maintained for backward compatibility, but are now deprecated and will be removed in a
future release:
-margonaut
Obsolete FPX.
-mbig-endian
-EB Compile code for big-endian targets. Use of these options is now deprecated. Big-endian code is supported by
configuring GCC to build "arceb-elf32" and "arceb-linux-uclibc" targets, for which big endian is the default.
-mlittle-endian
-EL Compile code for little-endian targets. Use of these options is now deprecated. Little-endian code is supported
by configuring GCC to build "arc-elf32" and "arc-linux-uclibc" targets, for which little endian is the default.
-mbarrel_shifter
Replaced by -mbarrel-shifter.
-mdpfp_compact
Replaced by -mdpfp-compact.
-mdpfp_fast
Replaced by -mdpfp-fast.
-mdsp_packa
Replaced by -mdsp-packa.
-mEA
Replaced by -mea.
-mmac_24
Replaced by -mmac-24.
-mmac_d16
Replaced by -mmac-d16.
-mspfp_compact
Replaced by -mspfp-compact.
-mspfp_fast
Replaced by -mspfp-fast.
-mtune=cpu
Values arc600, arc601, arc700 and arc700-xmac for cpu are replaced by ARC600, ARC601, ARC700 and ARC700-xmac
respectively.
-multcost=num
Replaced by -mmultcost.
ARM Options
These -m options are defined for the ARM port:
-mabi=name
Generate code for the specified ABI. Permissible values are: apcs-gnu, atpcs, aapcs, aapcs-linux and iwmmxt.
-mapcs-frame
Generate a stack frame that is compliant with the ARM Procedure Call Standard for all functions, even if this is
not strictly necessary for correct execution of the code. Specifying -fomit-frame-pointer with this option
causes the stack frames not to be generated for leaf functions. The default is -mno-apcs-frame. This option is
deprecated.
-mapcs
This is a synonym for -mapcs-frame and is deprecated.
-mthumb-interwork
Generate code that supports calling between the ARM and Thumb instruction sets. Without this option, on pre-v5
architectures, the two instruction sets cannot be reliably used inside one program. The default is
-mno-thumb-interwork, since slightly larger code is generated when -mthumb-interwork is specified. In AAPCS
configurations this option is meaningless.
-mno-sched-prolog
Prevent the reordering of instructions in the function prologue, or the merging of those instruction with the
instructions in the function's body. This means that all functions start with a recognizable set of instructions
(or in fact one of a choice from a small set of different function prologues), and this information can be used
to locate the start of functions inside an executable piece of code. The default is -msched-prolog.
-mfloat-abi=name
Specifies which floating-point ABI to use. Permissible values are: soft, softfp and hard.
Specifying soft causes GCC to generate output containing library calls for floating-point operations. softfp
allows the generation of code using hardware floating-point instructions, but still uses the soft-float calling
conventions. hard allows generation of floating-point instructions and uses FPU-specific calling conventions.
The default depends on the specific target configuration. Note that the hard-float and soft-float ABIs are not
link-compatible; you must compile your entire program with the same ABI, and link with a compatible set of
libraries.
-mgeneral-regs-only
Generate code which uses only the general-purpose registers. This will prevent the compiler from using floating-
point and Advanced SIMD registers but will not impose any restrictions on the assembler.
-mlittle-endian
Generate code for a processor running in little-endian mode. This is the default for all standard
configurations.
-mbig-endian
Generate code for a processor running in big-endian mode; the default is to compile code for a little-endian
processor.
-mbe8
-mbe32
When linking a big-endian image select between BE8 and BE32 formats. The option has no effect for little-endian
images and is ignored. The default is dependent on the selected target architecture. For ARMv6 and later
architectures the default is BE8, for older architectures the default is BE32. BE32 format has been deprecated
by ARM.
-march=name[+extension...]
This specifies the name of the target ARM architecture. GCC uses this name to determine what kind of
instructions it can emit when generating assembly code. This option can be used in conjunction with or instead
of the -mcpu= option.
Permissible names are: armv4t, armv5t, armv5te, armv6, armv6j, armv6k, armv6kz, armv6t2, armv6z, armv6zk, armv7,
armv7-a, armv7ve, armv8-a, armv8.1-a, armv8.2-a, armv8.3-a, armv8.4-a, armv8.5-a, armv8.6-a, armv9-a, armv7-r,
armv8-r, armv6-m, armv6s-m, armv7-m, armv7e-m, armv8-m.base, armv8-m.main, armv8.1-m.main, iwmmxt and iwmmxt2.
Additionally, the following architectures, which lack support for the Thumb execution state, are recognized but
support is deprecated: armv4.
Many of the architectures support extensions. These can be added by appending +extension to the architecture
name. Extension options are processed in order and capabilities accumulate. An extension will also enable any
necessary base extensions upon which it depends. For example, the +crypto extension will always enable the +simd
extension. The exception to the additive construction is for extensions that are prefixed with +no...: these
extensions disable the specified option and any other extensions that may depend on the presence of that
extension.
For example, -march=armv7-a+simd+nofp+vfpv4 is equivalent to writing -march=armv7-a+vfpv4 since the +simd option
is entirely disabled by the +nofp option that follows it.
Most extension names are generically named, but have an effect that is dependent upon the architecture to which
it is applied. For example, the +simd option can be applied to both armv7-a and armv8-a architectures, but will
enable the original ARMv7-A Advanced SIMD (Neon) extensions for armv7-a and the ARMv8-A variant for armv8-a.
The table below lists the supported extensions for each architecture. Architectures not mentioned do not support
any extensions.
armv5te
armv6
armv6j
armv6k
armv6kz
armv6t2
armv6z
armv6zk
+fp The VFPv2 floating-point instructions. The extension +vfpv2 can be used as an alias for this extension.
+nofp
Disable the floating-point instructions.
armv7
The common subset of the ARMv7-A, ARMv7-R and ARMv7-M architectures.
+fp The VFPv3 floating-point instructions, with 16 double-precision registers. The extension +vfpv3-d16 can
be used as an alias for this extension. Note that floating-point is not supported by the base ARMv7-M
architecture, but is compatible with both the ARMv7-A and ARMv7-R architectures.
+nofp
Disable the floating-point instructions.
armv7-a
+mp The multiprocessing extension.
+sec
The security extension.
+fp The VFPv3 floating-point instructions, with 16 double-precision registers. The extension +vfpv3-d16 can
be used as an alias for this extension.
+simd
The Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions. The extensions +neon and
+neon-vfpv3 can be used as aliases for this extension.
+vfpv3
The VFPv3 floating-point instructions, with 32 double-precision registers.
+vfpv3-d16-fp16
The VFPv3 floating-point instructions, with 16 double-precision registers and the half-precision
floating-point conversion operations.
+vfpv3-fp16
The VFPv3 floating-point instructions, with 32 double-precision registers and the half-precision
floating-point conversion operations.
+vfpv4-d16
The VFPv4 floating-point instructions, with 16 double-precision registers.
+vfpv4
The VFPv4 floating-point instructions, with 32 double-precision registers.
+neon-fp16
The Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions, with the half-precision floating-
point conversion operations.
+neon-vfpv4
The Advanced SIMD (Neon) v2 and the VFPv4 floating-point instructions.
+nosimd
Disable the Advanced SIMD instructions (does not disable floating point).
+nofp
Disable the floating-point and Advanced SIMD instructions.
armv7ve
The extended version of the ARMv7-A architecture with support for virtualization.
+fp The VFPv4 floating-point instructions, with 16 double-precision registers. The extension +vfpv4-d16 can
be used as an alias for this extension.
+simd
The Advanced SIMD (Neon) v2 and the VFPv4 floating-point instructions. The extension +neon-vfpv4 can be
used as an alias for this extension.
+vfpv3-d16
The VFPv3 floating-point instructions, with 16 double-precision registers.
+vfpv3
The VFPv3 floating-point instructions, with 32 double-precision registers.
+vfpv3-d16-fp16
The VFPv3 floating-point instructions, with 16 double-precision registers and the half-precision
floating-point conversion operations.
+vfpv3-fp16
The VFPv3 floating-point instructions, with 32 double-precision registers and the half-precision
floating-point conversion operations.
+vfpv4-d16
The VFPv4 floating-point instructions, with 16 double-precision registers.
+vfpv4
The VFPv4 floating-point instructions, with 32 double-precision registers.
+neon
The Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions. The extension +neon-vfpv3 can be
used as an alias for this extension.
+neon-fp16
The Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions, with the half-precision floating-
point conversion operations.
+nosimd
Disable the Advanced SIMD instructions (does not disable floating point).
+nofp
Disable the floating-point and Advanced SIMD instructions.
armv8-a
+crc
The Cyclic Redundancy Check (CRC) instructions.
+simd
The ARMv8-A Advanced SIMD and floating-point instructions.
+crypto
The cryptographic instructions.
+nocrypto
Disable the cryptographic instructions.
+nofp
Disable the floating-point, Advanced SIMD and cryptographic instructions.
+sb Speculation Barrier Instruction.
+predres
Execution and Data Prediction Restriction Instructions.
armv8.1-a
+simd
The ARMv8.1-A Advanced SIMD and floating-point instructions.
+crypto
The cryptographic instructions. This also enables the Advanced SIMD and floating-point instructions.
+nocrypto
Disable the cryptographic instructions.
+nofp
Disable the floating-point, Advanced SIMD and cryptographic instructions.
+sb Speculation Barrier Instruction.
+predres
Execution and Data Prediction Restriction Instructions.
armv8.2-a
armv8.3-a
+fp16
The half-precision floating-point data processing instructions. This also enables the Advanced SIMD and
floating-point instructions.
+fp16fml
The half-precision floating-point fmla extension. This also enables the half-precision floating-point
extension and Advanced SIMD and floating-point instructions.
+simd
The ARMv8.1-A Advanced SIMD and floating-point instructions.
+crypto
The cryptographic instructions. This also enables the Advanced SIMD and floating-point instructions.
+dotprod
Enable the Dot Product extension. This also enables Advanced SIMD instructions.
+nocrypto
Disable the cryptographic extension.
+nofp
Disable the floating-point, Advanced SIMD and cryptographic instructions.
+sb Speculation Barrier Instruction.
+predres
Execution and Data Prediction Restriction Instructions.
+i8mm
8-bit Integer Matrix Multiply instructions. This also enables Advanced SIMD and floating-point
instructions.
+bf16
Brain half-precision floating-point instructions. This also enables Advanced SIMD and floating-point
instructions.
armv8.4-a
+fp16
The half-precision floating-point data processing instructions. This also enables the Advanced SIMD and
floating-point instructions as well as the Dot Product extension and the half-precision floating-point
fmla extension.
+simd
The ARMv8.3-A Advanced SIMD and floating-point instructions as well as the Dot Product extension.
+crypto
The cryptographic instructions. This also enables the Advanced SIMD and floating-point instructions as
well as the Dot Product extension.
+nocrypto
Disable the cryptographic extension.
+nofp
Disable the floating-point, Advanced SIMD and cryptographic instructions.
+sb Speculation Barrier Instruction.
+predres
Execution and Data Prediction Restriction Instructions.
+i8mm
8-bit Integer Matrix Multiply instructions. This also enables Advanced SIMD and floating-point
instructions.
+bf16
Brain half-precision floating-point instructions. This also enables Advanced SIMD and floating-point
instructions.
armv8.5-a
+fp16
The half-precision floating-point data processing instructions. This also enables the Advanced SIMD and
floating-point instructions as well as the Dot Product extension and the half-precision floating-point
fmla extension.
+simd
The ARMv8.3-A Advanced SIMD and floating-point instructions as well as the Dot Product extension.
+crypto
The cryptographic instructions. This also enables the Advanced SIMD and floating-point instructions as
well as the Dot Product extension.
+nocrypto
Disable the cryptographic extension.
+nofp
Disable the floating-point, Advanced SIMD and cryptographic instructions.
+i8mm
8-bit Integer Matrix Multiply instructions. This also enables Advanced SIMD and floating-point
instructions.
+bf16
Brain half-precision floating-point instructions. This also enables Advanced SIMD and floating-point
instructions.
armv8.6-a
+fp16
The half-precision floating-point data processing instructions. This also enables the Advanced SIMD and
floating-point instructions as well as the Dot Product extension and the half-precision floating-point
fmla extension.
+simd
The ARMv8.3-A Advanced SIMD and floating-point instructions as well as the Dot Product extension.
+crypto
The cryptographic instructions. This also enables the Advanced SIMD and floating-point instructions as
well as the Dot Product extension.
+nocrypto
Disable the cryptographic extension.
+nofp
Disable the floating-point, Advanced SIMD and cryptographic instructions.
+i8mm
8-bit Integer Matrix Multiply instructions. This also enables Advanced SIMD and floating-point
instructions.
+bf16
Brain half-precision floating-point instructions. This also enables Advanced SIMD and floating-point
instructions.
armv7-r
+fp.sp
The single-precision VFPv3 floating-point instructions. The extension +vfpv3xd can be used as an alias
for this extension.
+fp The VFPv3 floating-point instructions with 16 double-precision registers. The extension +vfpv3-d16 can
be used as an alias for this extension.
+vfpv3xd-d16-fp16
The single-precision VFPv3 floating-point instructions with 16 double-precision registers and the half-
precision floating-point conversion operations.
+vfpv3-d16-fp16
The VFPv3 floating-point instructions with 16 double-precision registers and the half-precision floating-
point conversion operations.
+nofp
Disable the floating-point extension.
+idiv
The ARM-state integer division instructions.
+noidiv
Disable the ARM-state integer division extension.
armv7e-m
+fp The single-precision VFPv4 floating-point instructions.
+fpv5
The single-precision FPv5 floating-point instructions.
+fp.dp
The single- and double-precision FPv5 floating-point instructions.
+nofp
Disable the floating-point extensions.
armv8.1-m.main
+dsp
The DSP instructions.
+mve
The M-Profile Vector Extension (MVE) integer instructions.
+mve.fp
The M-Profile Vector Extension (MVE) integer and single precision floating-point instructions.
+fp The single-precision floating-point instructions.
+fp.dp
The single- and double-precision floating-point instructions.
+nofp
Disable the floating-point extension.
+cdecp0, +cdecp1, ... , +cdecp7
Enable the Custom Datapath Extension (CDE) on selected coprocessors according to the numbers given in the
options in the range 0 to 7.
+pacbti
Enable the Pointer Authentication and Branch Target Identification Extension.
armv8-m.main
+dsp
The DSP instructions.
+nodsp
Disable the DSP extension.
+fp The single-precision floating-point instructions.
+fp.dp
The single- and double-precision floating-point instructions.
+nofp
Disable the floating-point extension.
+cdecp0, +cdecp1, ... , +cdecp7
Enable the Custom Datapath Extension (CDE) on selected coprocessors according to the numbers given in the
options in the range 0 to 7.
armv8-r
+crc
The Cyclic Redundancy Check (CRC) instructions.
+fp.sp
The single-precision FPv5 floating-point instructions.
+simd
The ARMv8-A Advanced SIMD and floating-point instructions.
+crypto
The cryptographic instructions.
+nocrypto
Disable the cryptographic instructions.
+nofp
Disable the floating-point, Advanced SIMD and cryptographic instructions.
-march=native causes the compiler to auto-detect the architecture of the build computer. At present, this
feature is only supported on GNU/Linux, and not all architectures are recognized. If the auto-detect is
unsuccessful the option has no effect.
-march=unset causes the compiler to ignore any -march=... options that appear earlier on the command line and
behave as if the option was never passed. This is useful to avoid warnings about conflicting CPU and
architecture options when the two produce different architecture specifications.
-mtune=name
This option specifies the name of the target ARM processor for which GCC should tune the performance of the code.
For some ARM implementations better performance can be obtained by using this option. Permissible names are:
arm7tdmi, arm7tdmi-s, arm710t, arm720t, arm740t, strongarm, strongarm110, strongarm1100, strongarm1110, arm8,
arm810, arm9, arm9e, arm920, arm920t, arm922t, arm946e-s, arm966e-s, arm968e-s, arm926ej-s, arm940t, arm9tdmi,
arm10tdmi, arm1020t, arm1026ej-s, arm10e, arm1020e, arm1022e, arm1136j-s, arm1136jf-s, mpcore, mpcorenovfp,
arm1156t2-s, arm1156t2f-s, arm1176jz-s, arm1176jzf-s, generic-armv7-a, cortex-a5, cortex-a7, cortex-a8,
cortex-a9, cortex-a12, cortex-a15, cortex-a17, cortex-a32, cortex-a35, cortex-a53, cortex-a55, cortex-a57,
cortex-a72, cortex-a73, cortex-a75, cortex-a76, cortex-a76ae, cortex-a77, cortex-a78, cortex-a78ae, cortex-a78c,
cortex-a710, ares, cortex-r4, cortex-r4f, cortex-r5, cortex-r7, cortex-r8, cortex-r52, cortex-r52plus, cortex-m0,
cortex-m0plus, cortex-m1, cortex-m3, cortex-m4, cortex-m7, cortex-m23, cortex-m33, cortex-m35p, cortex-m52,
cortex-m55, cortex-m85, cortex-x1, cortex-x1c, cortex-m1.small-multiply, cortex-m0.small-multiply,
cortex-m0plus.small-multiply, exynos-m1, marvell-pj4, neoverse-n1, neoverse-n2, neoverse-v1, xscale, iwmmxt,
iwmmxt2, ep9312, fa526, fa626, fa606te, fa626te, fmp626, fa726te, star-mc1, xgene1.
Additionally, this option can specify that GCC should tune the performance of the code for a big.LITTLE system.
Permissible names are: cortex-a15.cortex-a7, cortex-a17.cortex-a7, cortex-a57.cortex-a53, cortex-a72.cortex-a53,
cortex-a72.cortex-a35, cortex-a73.cortex-a53, cortex-a75.cortex-a55, cortex-a76.cortex-a55.
-mtune=generic-arch specifies that GCC should tune the performance for a blend of processors within architecture
arch. The aim is to generate code that run well on the current most popular processors, balancing between
optimizations that benefit some CPUs in the range, and avoiding performance pitfalls of other CPUs. The effects
of this option may change in future GCC versions as CPU models come and go.
-mtune permits the same extension options as -mcpu, but the extension options do not affect the tuning of the
generated code.
-mtune=native causes the compiler to auto-detect the CPU of the build computer. At present, this feature is only
supported on GNU/Linux, and not all architectures are recognized. If the auto-detect is unsuccessful the option
has no effect.
-mcpu=name[+extension...]
This specifies the name of the target ARM processor. GCC uses this name to derive the name of the target ARM
architecture (as if specified by -march) and the ARM processor type for which to tune for performance (as if
specified by -mtune). Where this option is used in conjunction with -march or -mtune, those options take
precedence over the appropriate part of this option.
Many of the supported CPUs implement optional architectural extensions. Where this is so the architectural
extensions are normally enabled by default. If implementations that lack the extension exist, then the extension
syntax can be used to disable those extensions that have been omitted. For floating-point and Advanced SIMD
(Neon) instructions, the settings of the options -mfloat-abi and -mfpu must also be considered: floating-point
and Advanced SIMD instructions will only be used if -mfloat-abi is not set to soft; and any setting of -mfpu
other than auto will override the available floating-point and SIMD extension instructions.
For example, cortex-a9 can be found in three major configurations: integer only, with just a floating-point unit
or with floating-point and Advanced SIMD. The default is to enable all the instructions, but the extensions
+nosimd and +nofp can be used to disable just the SIMD or both the SIMD and floating-point instructions
respectively.
Permissible names for this option are the same as those for -mtune.
The following extension options are common to the listed CPUs:
+nodsp
Disable the DSP instructions on cortex-m33, cortex-m35p, cortex-m52, cortex-m55 and cortex-m85. Also disable
the M-Profile Vector Extension (MVE) integer and single precision floating-point instructions on cortex-m52,
cortex-m55 and cortex-m85.
+nopacbti
Disable the Pointer Authentication and Branch Target Identification Extension on cortex-m52 and cortex-m85.
+nomve
Disable the M-Profile Vector Extension (MVE) integer and single precision floating-point instructions on
cortex-m52, cortex-m55 and cortex-m85.
+nomve.fp
Disable the M-Profile Vector Extension (MVE) single precision floating-point instructions on cortex-m52,
cortex-m55 and cortex-m85.
+cdecp0, +cdecp1, ... , +cdecp7
Enable the Custom Datapath Extension (CDE) on selected coprocessors according to the numbers given in the
options in the range 0 to 7 on cortex-m52, cortex-m55 and star-mc1.
+nofp
Disables the floating-point instructions on arm9e, arm946e-s, arm966e-s, arm968e-s, arm10e, arm1020e,
arm1022e, arm926ej-s, arm1026ej-s, cortex-r5, cortex-r7, cortex-r8, cortex-m4, cortex-m7, cortex-m33,
cortex-m35p, cortex-m52, cortex-m55 and cortex-m85. Disables the floating-point and SIMD instructions on
generic-armv7-a, cortex-a5, cortex-a7, cortex-a8, cortex-a9, cortex-a12, cortex-a15, cortex-a17,
cortex-a15.cortex-a7, cortex-a17.cortex-a7, cortex-a32, cortex-a35, cortex-a53 and cortex-a55.
+nofp.dp
Disables the double-precision component of the floating-point instructions on cortex-r5, cortex-r7,
cortex-r8, cortex-r52, cortex-r52plus and cortex-m7.
+nosimd
Disables the SIMD (but not floating-point) instructions on generic-armv7-a, cortex-a5, cortex-a7 and
cortex-a9.
+crypto
Enables the cryptographic instructions on cortex-a32, cortex-a35, cortex-a53, cortex-a55, cortex-a57,
cortex-a72, cortex-a73, cortex-a75, exynos-m1, xgene1, cortex-a57.cortex-a53, cortex-a72.cortex-a53,
cortex-a73.cortex-a35, cortex-a73.cortex-a53 and cortex-a75.cortex-a55.
Additionally the generic-armv7-a pseudo target defaults to VFPv3 with 16 double-precision registers. It supports
the following extension options: mp, sec, vfpv3-d16, vfpv3, vfpv3-d16-fp16, vfpv3-fp16, vfpv4-d16, vfpv4, neon,
neon-vfpv3, neon-fp16, neon-vfpv4. The meanings are the same as for the extensions to -march=armv7-a.
-mcpu=generic-arch is also permissible, and is equivalent to -march=arch -mtune=generic-arch. See -mtune for
more information.
-mcpu=native causes the compiler to auto-detect the CPU of the build computer. At present, this feature is only
supported on GNU/Linux, and not all architectures are recognized. If the auto-detect is unsuccessful the option
has no effect.
-mcpu=unset causes the compiler to ignore any -mcpu=... options that appear earlier on the command line and
behave as if the option was never passed. This is useful to avoid warnings about conflicting CPU and
architecture options when the two produce different architecture specifications.
-mfpu=name
This specifies what floating-point hardware (or hardware emulation) is available on the target. Permissible
names are: auto, vfpv2, vfpv3, vfpv3-fp16, vfpv3-d16, vfpv3-d16-fp16, vfpv3xd, vfpv3xd-fp16, neon-vfpv3,
neon-fp16, vfpv4, vfpv4-d16, fpv4-sp-d16, neon-vfpv4, fpv5-d16, fpv5-sp-d16, fp-armv8, neon-fp-armv8 and
crypto-neon-fp-armv8. Note that neon is an alias for neon-vfpv3 and vfp is an alias for vfpv2.
The setting auto is the default and is special. It causes the compiler to select the floating-point and Advanced
SIMD instructions based on the settings of -mcpu and -march.
If the selected floating-point hardware includes the NEON extension (e.g. -mfpu=neon), note that floating-point
operations are not generated by GCC's auto-vectorization pass unless -funsafe-math-optimizations is also
specified. This is because NEON hardware does not fully implement the IEEE 754 standard for floating-point
arithmetic (in particular denormal values are treated as zero), so the use of NEON instructions may lead to a
loss of precision.
You can also set the fpu name at function level by using the target("fpu=") function attributes or pragmas.
-mfp16-format=name
Specify the format of the "__fp16" half-precision floating-point type. Permissible names are none, ieee, and
alternative; the default is none, in which case the "__fp16" type is not defined.
-mstructure-size-boundary=n
The sizes of all structures and unions are rounded up to a multiple of the number of bits set by this option.
Permissible values are 8, 32 and 64. The default value varies for different toolchains. For the COFF targeted
toolchain the default value is 8. A value of 64 is only allowed if the underlying ABI supports it.
Specifying a larger number can produce faster, more efficient code, but can also increase the size of the
program. Different values are potentially incompatible. Code compiled with one value cannot necessarily expect
to work with code or libraries compiled with another value, if they exchange information using structures or
unions.
This option is deprecated.
-mabort-on-noreturn
Generate a call to the function "abort" at the end of a "noreturn" function. It is executed if the function
tries to return.
-mlong-calls
-mno-long-calls
Tells the compiler to perform function calls by first loading the address of the function into a register and
then performing a subroutine call on this register. This switch is needed if the target function lies outside of
the 64-megabyte addressing range of the offset-based version of subroutine call instruction.
Even if this switch is enabled, not all function calls are turned into long calls. The heuristic is that static
functions, functions that have the "short_call" attribute, functions that are inside the scope of a "#pragma
no_long_calls" directive, and functions whose definitions have already been compiled within the current
compilation unit are not turned into long calls. The exceptions to this rule are that weak function definitions,
functions with the "long_call" attribute or the "section" attribute, and functions that are within the scope of a
"#pragma long_calls" directive are always turned into long calls.
This feature is not enabled by default. Specifying -mno-long-calls restores the default behavior, as does
placing the function calls within the scope of a "#pragma long_calls_off" directive. Note these switches have no
effect on how the compiler generates code to handle function calls via function pointers.
-msingle-pic-base
Treat the register used for PIC addressing as read-only, rather than loading it in the prologue for each
function. The runtime system is responsible for initializing this register with an appropriate value before
execution begins.
-mpic-register=reg
Specify the register to be used for PIC addressing. For standard PIC base case, the default is any suitable
register determined by compiler. For single PIC base case, the default is R9 if target is EABI based or stack-
checking is enabled, otherwise the default is R10.
-mpic-data-is-text-relative
Assume that the displacement between the text and data segments is fixed at static link time. This permits using
PC-relative addressing operations to access data known to be in the data segment. For non-VxWorks RTP targets,
this option is enabled by default. When disabled on such targets, it will enable -msingle-pic-base by default.
-mpoke-function-name
Write the name of each function into the text section, directly preceding the function prologue. The generated
code is similar to this:
t0
.ascii "arm_poke_function_name", 0
.align
t1
.word 0xff000000 + (t1 - t0)
arm_poke_function_name
mov ip, sp
stmfd sp!, {fp, ip, lr, pc}
sub fp, ip, #4
When performing a stack backtrace, code can inspect the value of "pc" stored at "fp + 0". If the trace function
then looks at location "pc - 12" and the top 8 bits are set, then we know that there is a function name embedded
immediately preceding this location and has length "((pc[-3]) & 0xff000000)".
-mthumb
-marm
Select between generating code that executes in ARM and Thumb states. The default for most configurations is to
generate code that executes in ARM state, but the default can be changed by configuring GCC with the
--with-mode=state configure option.
You can also override the ARM and Thumb mode for each function by using the target("thumb") and target("arm")
function attributes or pragmas.
-mflip-thumb
Switch ARM/Thumb modes on alternating functions. This option is provided for regression testing of mixed
Thumb/ARM code generation, and is not intended for ordinary use in compiling code.
-mtpcs-frame
Generate a stack frame that is compliant with the Thumb Procedure Call Standard for all non-leaf functions. (A
leaf function is one that does not call any other functions.) The default is -mno-tpcs-frame.
-mtpcs-leaf-frame
Generate a stack frame that is compliant with the Thumb Procedure Call Standard for all leaf functions. (A leaf
function is one that does not call any other functions.) The default is -mno-apcs-leaf-frame.
-mcallee-super-interworking
Gives all externally visible functions in the file being compiled an ARM instruction set header which switches to
Thumb mode before executing the rest of the function. This allows these functions to be called from non-
interworking code. This option is not valid in AAPCS configurations because interworking is enabled by default.
-mcaller-super-interworking
Allows calls via function pointers (including virtual functions) to execute correctly regardless of whether the
target code has been compiled for interworking or not. There is a small overhead in the cost of executing a
function pointer if this option is enabled. This option is not valid in AAPCS configurations because
interworking is enabled by default.
-mtp=name
Specify the access model for the thread local storage pointer. The model soft generates calls to
"__aeabi_read_tp". Other accepted models are tpidrurw, tpidruro and tpidrprw which fetch the thread pointer from
the corresponding system register directly (supported from the arm6k architecture and later). These system
registers are accessed through the CP15 co-processor interface and the argument cp15 is also accepted as a
convenience alias of tpidruro. The argument auto uses the best available method for the selected processor. The
default setting is auto.
-mtls-dialect=dialect
Specify the dialect to use for accessing thread local storage. Two dialects are supported---gnu and gnu2. The
gnu dialect selects the original GNU scheme for supporting local and global dynamic TLS models. The gnu2 dialect
selects the GNU descriptor scheme, which provides better performance for shared libraries. The GNU descriptor
scheme is compatible with the original scheme, but does require new assembler, linker and library support.
Initial and local exec TLS models are unaffected by this option and always use the original scheme.
-mword-relocations
Only generate absolute relocations on word-sized values (i.e. R_ARM_ABS32). This is enabled by default on
targets (uClinux, SymbianOS) where the runtime loader imposes this restriction, and when -fpic or -fPIC is
specified. This option conflicts with -mslow-flash-data.
-mfix-cortex-m3-ldrd
Some Cortex-M3 cores can cause data corruption when "ldrd" instructions with overlapping destination and base
registers are used. This option avoids generating these instructions. This option is enabled by default when
-mcpu=cortex-m3 is specified.
-mfix-cortex-a57-aes-1742098
-mno-fix-cortex-a57-aes-1742098
-mfix-cortex-a72-aes-1655431
-mno-fix-cortex-a72-aes-1655431
Enable (disable) mitigation for an erratum on Cortex-A57 and Cortex-A72 that affects the AES cryptographic
instructions. This option is enabled by default when either -mcpu=cortex-a57 or -mcpu=cortex-a72 is specified.
-munaligned-access
-mno-unaligned-access
Enables (or disables) reading and writing of 16- and 32- bit values from addresses that are not 16- or 32- bit
aligned. By default unaligned access is disabled for all pre-ARMv6, all ARMv6-M and for ARMv8-M Baseline
architectures, and enabled for all other architectures. If unaligned access is not enabled then words in packed
data structures are accessed a byte at a time.
The ARM attribute "Tag_CPU_unaligned_access" is set in the generated object file to either true or false,
depending upon the setting of this option. If unaligned access is enabled then the preprocessor symbol
"__ARM_FEATURE_UNALIGNED" is also defined.
-mneon-for-64bits
This option is deprecated and has no effect.
-mslow-flash-data
Assume loading data from flash is slower than fetching instruction. Therefore literal load is minimized for
better performance. This option is only supported when compiling for ARMv7 M-profile and off by default. It
conflicts with -mword-relocations.
-masm-syntax-unified
Assume inline assembler is using unified asm syntax. The default is currently off which implies divided syntax.
This option has no impact on Thumb2. However, this may change in future releases of GCC. Divided syntax should
be considered deprecated.
-mrestrict-it
Restricts generation of IT blocks to conform to the rules of ARMv8-A. IT blocks can only contain a single 16-bit
instruction from a select set of instructions. This option is on by default for ARMv8-A Thumb mode.
-mprint-tune-info
Print CPU tuning information as comment in assembler file. This is an option used only for regression testing of
the compiler and not intended for ordinary use in compiling code. This option is disabled by default.
-mverbose-cost-dump
Enable verbose cost model dumping in the debug dump files. This option is provided for use in debugging the
compiler.
-mpure-code
Do not allow constant data to be placed in code sections. Additionally, when compiling for ELF object format
give all text sections the ELF processor-specific section attribute "SHF_ARM_PURECODE". This option is only
available when generating non-pic code for M-profile targets.
-mcmse
Generate secure code as per the "ARMv8-M Security Extensions: Requirements on Development Tools Engineering
Specification", which can be found on .
-mfix-cmse-cve-2021-35465
Mitigate against a potential security issue with the "VLLDM" instruction in some M-profile devices when using
CMSE (CVE-2021-365465). This option is enabled by default when the option -mcpu= is used with "cortex-m33",
"cortex-m35p", "cortex-m52", "cortex-m55", "cortex-m85" or "star-mc1". The option -mno-fix-cmse-cve-2021-35465
can be used to disable the mitigation.
-mstack-protector-guard=guard
-mstack-protector-guard-offset=offset
Generate stack protection code using canary at guard. Supported locations are global for a global canary or tls
for a canary accessible via the TLS register. The option -mstack-protector-guard-offset= is for use with
-fstack-protector-guard=tls and not for use in user-land code.
-mfdpic
-mno-fdpic
Select the FDPIC ABI, which uses 64-bit function descriptors to represent pointers to functions. When the
compiler is configured for "arm-*-uclinuxfdpiceabi" targets, this option is on by default and implies -fPIE if
none of the PIC/PIE-related options is provided. On other targets, it only enables the FDPIC-specific code
generation features, and the user should explicitly provide the PIC/PIE-related options as needed.
Note that static linking is not supported because it would still involve the dynamic linker when the program
self-relocates. If such behavior is acceptable, use -static and -Wl,-dynamic-linker options.
The opposite -mno-fdpic option is useful (and required) to build the Linux kernel using the same
("arm-*-uclinuxfdpiceabi") toolchain as the one used to build the userland programs.
-mbranch-protection=none|standard|pac-ret[+leaf][+bti]|bti[+pac-ret[+leaf]]
Enable branch protection features (armv8.1-m.main only). none generate code without branch protection or return
address signing. standard[+leaf] generate code with all branch protection features enabled at their standard
level. pac-ret[+leaf] generate code with return address signing set to its standard level, which is to sign all
functions that save the return address to memory. leaf When return address signing is enabled, also sign leaf
functions even if they do not write the return address to memory. +bti Add landing-pad instructions at the
permitted targets of indirect branch instructions.
If the +pacbti architecture extension is not enabled, then all branch protection and return address signing
operations are constrained to use only the instructions defined in the architectural-NOP space. The generated
code will remain backwards-compatible with earlier versions of the architecture, but the additional security can
be enabled at run time on processors that support the PACBTI extension.
Branch target enforcement using BTI can only be enabled at runtime if all code in the application has been
compiled with at least -mbranch-protection=bti.
Any setting other than none is supported only on armv8-m.main or later.
The default is to generate code without branch protection or return address signing.
AVR Options
These options are defined for AVR implementations:
-mmcu=mcu
Specify the AVR instruction set architecture (ISA) or device type. The default for this option is "avr2".
The following AVR devices and ISAs are supported. Note: A complete device support consists of startup code
"crtmcu.o", a device header "avr/io*.h", a device library "libmcu.a" and a device-specs
("https://gcc.gnu.org/wiki/avr-gcc#spec-files") file "specs-mcu". Only the latter is provided by the compiler
according the supported "mcu"s below. The rest is supported by AVR-LibC
("https://github.com/avrdudes/avr-libc/"), or by means of "atpack" ("https://gcc.gnu.org/wiki/avr-gcc#atpack")
files from the hardware manufacturer.
"avr2"
"Classic" devices with up to 8 KiB of program memory. mcu = "attiny22", "attiny26", "at90s2313",
"at90s2323", "at90s2333", "at90s2343", "at90s4414", "at90s4433", "at90s4434", "at90c8534", "at90s8515",
"at90s8535".
"avr25"
"Classic" devices with up to 8 KiB of program memory and with the "MOVW" instruction. mcu = "attiny13",
"attiny13a", "attiny24", "attiny24a", "attiny25", "attiny261", "attiny261a", "attiny2313", "attiny2313a",
"attiny43u", "attiny44", "attiny44a", "attiny45", "attiny48", "attiny441", "attiny461", "attiny461a",
"attiny4313", "attiny84", "attiny84a", "attiny85", "attiny87", "attiny88", "attiny828", "attiny841",
"attiny861", "attiny861a", "ata5272", "ata6616c", "at86rf401".
"avr3"
"Classic" devices with 16 KiB up to 64 KiB of program memory. mcu = "at76c711", "at43usb355".
"avr31"
"Classic" devices with 128 KiB of program memory. mcu = "atmega103", "at43usb320".
"avr35"
"Classic" devices with 16 KiB up to 64 KiB of program memory and with the "MOVW" instruction. mcu =
"attiny167", "attiny1634", "atmega8u2", "atmega16u2", "atmega32u2", "ata5505", "ata6617c", "ata664251",
"at90usb82", "at90usb162".
"avr4"
"Enhanced" devices with up to 8 KiB of program memory. mcu = "atmega48", "atmega48a", "atmega48p",
"atmega48pa", "atmega48pb", "atmega8", "atmega8a", "atmega8hva", "atmega88", "atmega88a", "atmega88p",
"atmega88pa", "atmega88pb", "atmega8515", "atmega8535", "ata5795", "ata6285", "ata6286", "ata6289",
"ata6612c", "at90pwm1", "at90pwm2", "at90pwm2b", "at90pwm3", "at90pwm3b", "at90pwm81".
"avr5"
"Enhanced" devices with 16 KiB up to 64 KiB of program memory. mcu = "atmega16", "atmega16a", "atmega16hva",
"atmega16hva2", "atmega16hvb", "atmega16hvbrevb", "atmega16m1", "atmega16u4", "atmega161", "atmega162",
"atmega163", "atmega164a", "atmega164p", "atmega164pa", "atmega165", "atmega165a", "atmega165p",
"atmega165pa", "atmega168", "atmega168a", "atmega168p", "atmega168pa", "atmega168pb", "atmega169",
"atmega169a", "atmega169p", "atmega169pa", "atmega32", "atmega32a", "atmega32c1", "atmega32hvb",
"atmega32hvbrevb", "atmega32m1", "atmega32u4", "atmega32u6", "atmega323", "atmega324a", "atmega324p",
"atmega324pa", "atmega324pb", "atmega325", "atmega325a", "atmega325p", "atmega325pa", "atmega328",
"atmega328p", "atmega328pb", "atmega329", "atmega329a", "atmega329p", "atmega329pa", "atmega3250",
"atmega3250a", "atmega3250p", "atmega3250pa", "atmega3290", "atmega3290a", "atmega3290p", "atmega3290pa",
"atmega406", "atmega64", "atmega64a", "atmega64c1", "atmega64hve", "atmega64hve2", "atmega64m1",
"atmega64rfr2", "atmega640", "atmega644", "atmega644a", "atmega644p", "atmega644pa", "atmega644rfr2",
"atmega645", "atmega645a", "atmega645p", "atmega649", "atmega649a", "atmega649p", "atmega6450",
"atmega6450a", "atmega6450p", "atmega6490", "atmega6490a", "atmega6490p", "ata5790", "ata5790n", "ata5791",
"ata6613c", "ata6614q", "ata5782", "ata5831", "ata8210", "ata8510", "ata5787", "ata5835", "ata5700m322",
"ata5702m322", "at90pwm161", "at90pwm216", "at90pwm316", "at90can32", "at90can64", "at90scr100",
"at90usb646", "at90usb647", "at94k", "m3000".
"avr51"
"Enhanced" devices with 128 KiB of program memory. mcu = "atmega128", "atmega128a", "atmega128rfa1",
"atmega128rfr2", "atmega1280", "atmega1281", "atmega1284", "atmega1284p", "atmega1284rfr2", "at90can128",
"at90usb1286", "at90usb1287".
"avr6"
"Enhanced" devices with 3-byte PC, i.e. with more than 128 KiB of program memory. mcu = "atmega256rfr2",
"atmega2560", "atmega2561", "atmega2564rfr2".
"avrxmega2"
"XMEGA" devices with more than 8 KiB and up to 64 KiB of program memory. mcu = "atxmega8e5", "atxmega16a4",
"atxmega16a4u", "atxmega16c4", "atxmega16d4", "atxmega16e5", "atxmega32a4", "atxmega32a4u", "atxmega32c3",
"atxmega32c4", "atxmega32d3", "atxmega32d4", "atxmega32e5", "avr64da28", "avr64da28s", "avr64da32",
"avr64da32s", "avr64da48", "avr64da48s", "avr64da64", "avr64da64s", "avr64db28", "avr64db32", "avr64db48",
"avr64db64", "avr64dd14", "avr64dd20", "avr64dd28", "avr64dd32", "avr64du28", "avr64du32", "avr64ea28",
"avr64ea32", "avr64ea48", "avr64sd28", "avr64sd32", "avr64sd48".
"avrxmega3"
"XMEGA" devices with up to 64 KiB of combined program memory and RAM, and with program memory visible in the
RAM address space. mcu = "attiny202", "attiny204", "attiny212", "attiny214", "attiny402", "attiny404",
"attiny406", "attiny412", "attiny414", "attiny416", "attiny416auto", "attiny417", "attiny424", "attiny426",
"attiny427", "attiny804", "attiny806", "attiny807", "attiny814", "attiny816", "attiny817", "attiny824",
"attiny826", "attiny827", "attiny1604", "attiny1606", "attiny1607", "attiny1614", "attiny1616", "attiny1617",
"attiny1624", "attiny1626", "attiny1627", "attiny3214", "attiny3216", "attiny3217", "attiny3224",
"attiny3226", "attiny3227", "atmega808", "atmega809", "atmega1608", "atmega1609", "atmega3208", "atmega3209",
"atmega4808", "atmega4809", "avr16dd14", "avr16dd20", "avr16dd28", "avr16dd32", "avr16du14", "avr16du20",
"avr16du28", "avr16du32", "avr16ea28", "avr16ea32", "avr16ea48", "avr16eb14", "avr16eb20", "avr16eb28",
"avr16eb32", "avr16la14", "avr16la20", "avr16la28", "avr16la32", "avr32da28", "avr32da28s", "avr32da32",
"avr32da32s", "avr32da48", "avr32da48s", "avr32db28", "avr32db32", "avr32db48", "avr32dd14", "avr32dd20",
"avr32dd28", "avr32dd32", "avr32du14", "avr32du20", "avr32du28", "avr32du32", "avr32ea28", "avr32ea32",
"avr32ea48", "avr32eb14", "avr32eb20", "avr32eb28", "avr32eb32", "avr32la14", "avr32la20", "avr32la28",
"avr32la32", "avr32sd20", "avr32sd28", "avr32sd32".
"avrxmega4"
"XMEGA" devices with more than 64 KiB and up to 128 KiB of program memory. mcu = "atxmega64a3",
"atxmega64a3u", "atxmega64a4u", "atxmega64b1", "atxmega64b3", "atxmega64c3", "atxmega64d3", "atxmega64d4",
"avr128da28", "avr128da28s", "avr128da32", "avr128da32s", "avr128da48", "avr128da48s", "avr128da64",
"avr128da64s", "avr128db28", "avr128db32", "avr128db48", "avr128db64".
"avrxmega5"
"XMEGA" devices with more than 64 KiB and up to 128 KiB of program memory and more than 64 KiB of RAM. mcu =
"atxmega64a1", "atxmega64a1u".
"avrxmega6"
"XMEGA" devices with more than 128 KiB of program memory. mcu = "atxmega128a3", "atxmega128a3u",
"atxmega128b1", "atxmega128b3", "atxmega128c3", "atxmega128d3", "atxmega128d4", "atxmega192a3",
"atxmega192a3u", "atxmega192c3", "atxmega192d3", "atxmega256a3", "atxmega256a3b", "atxmega256a3bu",
"atxmega256a3u", "atxmega256c3", "atxmega256d3", "atxmega384c3", "atxmega384d3".
"avrxmega7"
"XMEGA" devices with more than 128 KiB of program memory and more than 64 KiB of RAM. mcu = "atxmega128a1",
"atxmega128a1u", "atxmega128a4u".
"avrtiny"
"Reduced Tiny" Tiny core devices with only 16 general purpose registers and 512 B up to 4 KiB of program
memory. mcu = "attiny4", "attiny5", "attiny9", "attiny10", "attiny102", "attiny104", "attiny20", "attiny40".
"avr1"
This ISA is implemented by the minimal AVR core and supported for assembler only. mcu = "attiny11",
"attiny12", "attiny15", "attiny28", "at90s1200".
-mabsdata
Assume that all data in static storage can be accessed by LDS / STS instructions. This option has only an effect
on reduced Tiny devices like ATtiny40. See also the "absdata" AVR Variable Attributes,variable attribute.
-mcvt
Use a compact vector table. Some devices support a CVT with only four entries: 0=Reset, 1=NMI, 2=Prio1 IRQ,
3=Prio0 IRQs. This option will link startup code from "crtmcu-cvt.o" instead of the usual "crtmcu.o". Apart
from providing a compact vector table, the startup code will set bit "CPUINT_CTRLA.CPUINT_CVT" which enables the
CVT on the device.
When you do not want the startup code to set "CPUINT_CTRLA.CPUINT_CVT", then you can satisfy symbol "__init_cvt"
so that the respective code is no more pulled in from "libmcu.a". For example, you can link with
"-Wl,--defsym,__init_cvt=0".
The CVT startup code is available since AVR-LibC v2.3 ("https://github.com/avrdudes/avr-libc/issues/1010").
-mdouble=bits
-mlong-double=bits
Set the size (in bits) of the "double" or "long double" type, respectively. Possible values for bits are 32 and
64. Whether or not a specific value for bits is allowed depends on the "--with-double=" and
"--with-long-double=" configure options ("https://gcc.gnu.org/install/configure.html#avr"), and the same applies
for the default values of the options.
-mgas-isr-prologues
Interrupt service routines (ISRs) may use the "__gcc_isr" pseudo instruction supported by GNU Binutils. If this
option is on, the feature can still be disabled for individual ISRs by means of the AVR Function
Attributes,,"no_gccisr" function attribute. This feature is activated per default if optimization is on (but not
with -Og, @pxref{Optimize Options}), and if GNU Binutils support PR21683 ("https://sourceware.org/PR21683").
-mint8
Assume "int" to be 8-bit integer. This affects the sizes of all types: a "char" is 1 byte, an "int" is 1 byte, a
"long" is 2 bytes, and "long long" is 4 bytes. Please note that this option does not conform to the C standards,
but it results in smaller code size.
-mmain-is-OS_task
Do not save registers in "main". The effect is the same like attaching attribute AVR Function
Attributes,,"OS_task" to "main". It is activated per default if optimization is on.
-mno-call-main
Don't run "main" by means of
XCALL main
XJMP exit
Instead, put "main" in section ".init9"
("https://avrdudes.github.io/avr-libc/avr-libc-user-manual/mem_sections.html#sec_dot_init") so that no call is
required. By setting this option the user asserts that "main" will not return.
This option can be used for devices with very limited resources in order to save a few bytes of code and stack
space. It will work as expected since AVR-LibC v2.3 ("https://github.com/avrdudes/avr-libc/issues/1012"). With
older versions, there will be no performance gain.
-mno-interrupts
Generated code is not compatible with hardware interrupts. Code size is smaller.
-mrelax
Try to replace "CALL" resp. "JMP" instruction by the shorter "RCALL" resp. "RJMP" instruction if applicable.
Setting -mrelax just adds the --mlink-relax option to the assembler's command line and the --relax option to the
linker's command line.
Jump relaxing is performed by the linker because jump offsets are not known before code is located. Therefore,
the assembler code generated by the compiler is the same, but the instructions in the executable may differ from
instructions in the assembler code.
Relaxing must be turned on if linker stubs are needed, see the section on "EIND" and linker stubs below.
-mrodata-in-ram
-mno-rodata-in-ram
Locate the ".rodata" sections for read-only data in RAM resp. in program memory. For most devices, there is no
choice and this option acts rather like an assertion.
Since v14 and for the AVR64* and AVR128* devices, ".rodata" is located in flash memory per default, provided the
required GNU Binutils support (PR31124 ("https://sourceware.org/PR31124")) is available. In that case,
-mrodata-in-ram can be used to return to the old layout with ".rodata" in RAM.
-mtiny-stack
Only change the lower 8 bits of the stack pointer.
-mfract-convert-truncate
Allow to use truncation instead of rounding towards zero for fractional fixed-point types.
-nodevicelib
Don't link against AVR-LibC's device specific library "libmcu.a".
Notice that since AVR-LibC v2.3, that library contains code that is essential for the correct functioning of a
program. In particular, it contains parts of the startup code like: "__init_sp"
("https://github.com/avrdudes/avr-libc/issues/1011") to initialize the stack pointer with symbol "__stack",
"__init_cvt" ("https://github.com/avrdudes/avr-libc/issues/1010") to set up the hardware to use a compact vector
table with -mcvt, "__call_main" ("https://github.com/avrdudes/avr-libc/issues/1012") to call "main" and "exit",
and "__do_flmap_init" ("https://github.com/avrdudes/avr-libc/issues/931") to set up FLMAP according to symbol
"__flmap".
-nodevicespecs
Don't add -specs=device-specs/specs-mcu to the compiler driver's command line. The user takes responsibility for
supplying the sub-processes like compiler proper, assembler and linker with appropriate command line options.
This means that the user has to supply her private device specs file by means of -specs=path-to-specs-file.
There is no more need for option -mmcu=mcu.
This option can also serve as a replacement for the older way of specifying custom device-specs files that needed
-B some-path to point to a directory which contains a folder named "device-specs" which contains a specs file
named "specs-mcu", where mcu was specified by -mmcu=mcu.
-Waddr-space-convert
Warn about conversions between address spaces in the case where the resulting address space is not contained in
the incoming address space.
-Wmisspelled-isr
Warn if the ISR is misspelled, i.e. without __vector prefix. Enabled by default.
AVR Optimization Options
The following options are pure optimization options. Options -mgas-isr-prologues, -mmain-is-OS_task, -mno-call-main
and -mrelax from above are only almost optimization options, since there are rare occasions where their different
code generation matters.
-maccumulate-args
Accumulate outgoing function arguments and acquire/release the needed stack space for outgoing function arguments
once in function prologue/epilogue. Without this option, outgoing arguments are pushed before calling a function
and popped afterwards. See also the -fdefer-pop Optimize Options,,optimization option.
Popping the arguments after the function call can be expensive on AVR so that accumulating the stack space might
lead to smaller executables because arguments need not be removed from the stack after such a function call.
This option can lead to reduced code size for functions that perform several calls to functions that get their
arguments on the stack like calls to printf-like functions.
-mbranch-cost=cost
Set the branch costs for conditional branch instructions to cost. Reasonable values for cost are small, non-
negative integers. The default branch cost is 0.
-mcall-prologues
Functions prologues/epilogues are expanded as calls to appropriate subroutines. Code size is smaller.
-mfuse-add
-mno-fuse-add
-mfuse-add=level
Optimize indirect memory accesses on reduced Tiny devices. The default uses "level=1" for optimizations -Og and
-O1, and "level=2" for higher optimizations. Valid values for level are 0, 1 and 2.
-mfuse-move
-mno-fuse-move
-mfuse-move=level
Run a post reload optimization pass that tries to fuse move instructions and to split multi-byte instructions
into 8-bit operations. The default uses "level=3" for optimization -O1, and "level=23" for higher optimizations.
Valid values for level are in the range 0 ... 23 which is a 3:2:2:2 mixed radix value. Each digit controls some
aspect of the optimization.
-mfuse-move2
Run a post combine optimization pass that tries to fuse move instructions.
-mstrict-X
Use address register "X" in a way proposed by the hardware. This means that "X" is only used in indirect, post-
increment or pre-decrement addressing.
Without this option, the "X" register may be used in the same way as "Y" or "Z" which then is emulated by
additional instructions. For example, loading a value with "X+const" addressing with a small non-negative "const
< 64" to a register Rn is performed as
adiw r26, const ; X += const
ld , X ; = *X
sbiw r26, const ; X -= const
-msplit-bit-shift
Split multi-byte shifts with a constant offset into a shift with a byte offset and a residual shift with a non-
byte offset. This optimization is turned on per default for -O2 and higher, including -Os but excluding -Oz.
Splitting of shifts with a constant offset that is a multiple of 8 is controlled by -mfuse-move.
-msplit-ldst
Split multi-byte loads and stores into several byte loads and stores. This optimization is turned on per default
for -O2 and higher.
-muse-nonzero-bits
Enable optimizations that are only possible when some bits in a register are always zero. This optimization is
turned on per default for -O2 and higher.
"EIND" and Devices with More Than 128 Ki Bytes of Flash
Pointers in the implementation are 16 bits wide. The address of a function or label is represented as word address
so that indirect jumps and calls can target any code address in the range of 64 Ki words.
In order to facilitate indirect jump on devices with more than 128 Ki bytes of program memory space, there is a
special function register called "EIND" that serves as most significant part of the target address when "EICALL" or
"EIJMP" instructions are used.
Indirect jumps and calls on these devices are handled as follows by the compiler and are subject to some limitations:
* The compiler never sets "EIND".
* The compiler uses "EIND" implicitly in "EICALL"/"EIJMP" instructions or might read "EIND" directly in order to
emulate an indirect call/jump by means of a "RET" instruction.
* The compiler assumes that "EIND" never changes during the startup code or during the application. In particular,
"EIND" is not saved/restored in function or interrupt service routine prologue/epilogue.
* For indirect calls to functions and computed goto, the linker generates stubs. Stubs are jump pads sometimes also
called trampolines. Thus, the indirect call/jump jumps to such a stub. The stub contains a direct jump to the
desired address.
* Linker relaxation must be turned on so that the linker generates the stubs correctly in all situations. See the
compiler option -mrelax and the linker option --relax. There are corner cases where the linker is supposed to
generate stubs but aborts without relaxation and without a helpful error message.
* The default linker script is arranged for code with "EIND = 0". If code is supposed to work for a setup with
"EIND != 0", a custom linker script has to be used in order to place the sections whose name start with
".trampolines" into the segment where "EIND" points to.
* The startup code from libgcc never sets "EIND". Notice that startup code is a blend of code from libgcc and AVR-
LibC. For the impact of AVR-LibC on "EIND", see the AVR-LibC user manual
("https://avrdudes.github.io/avr-libc/avr-libc-user-manual/").
* It is legitimate for user-specific startup code to set up "EIND" early, for example by means of initialization
code located in section ".init3". Such code runs prior to general startup code that initializes RAM and calls
constructors, but after the bit of startup code from AVR-LibC that sets "EIND" to the segment where the vector
table is located.
#include
static void
__attribute__((section(".init3"),naked,used,no_instrument_function))
init3_set_eind (void)
{
__asm volatile ("ldi r24,pm_hh8(__trampolines_start)\n\t"
"out %i0,r24" :: "n" (&EIND) : "r24","memory");
}
The "__trampolines_start" symbol is defined in the linker script.
* Stubs are generated automatically by the linker if the following two conditions are met:
-
(short for generate stubs) like so:
LDI r24, lo8(gs())
LDI r25, hi8(gs())
-
outside the segment where the stubs are located.
* The compiler emits such "gs" modifiers for code labels in the following situations:
-
-
-
command-line option.
-
tables you can specify the -fno-jump-tables command-line option.
-
-
* Jumping to non-symbolic addresses like so is not supported:
int main (void)
{
/* Call function at word address 0x2 */
return ((int(*)(void)) 0x2)();
}
Instead, a stub has to be set up, i.e. the function has to be called through a symbol ("func_4" in the example):
int main (void)
{
extern int func_4 (void);
/* Call function at byte address 0x4 */
return func_4();
}
and the application be linked with -Wl,--defsym,func_4=0x4. Alternatively, "func_4" can be defined in the linker
script.
Handling of the "RAMPD", "RAMPX", "RAMPY" and "RAMPZ" Special Function Registers
Some AVR devices support memories larger than the 64 KiB range that can be accessed with 16-bit pointers. To access
memory locations outside this 64 KiB range, the content of a "RAMP" register is used as high part of the address: The
"X", "Y", "Z" address register is concatenated with the "RAMPX", "RAMPY", "RAMPZ" special function register,
respectively, to get a wide address. Similarly, "RAMPD" is used together with direct addressing.
* The startup code initializes the "RAMP" special function registers with zero.
* If a AVR Named Address Spaces,named address space other than generic or "__flash" is used, then "RAMPZ" is set as
needed before the operation.
* If the device supports RAM larger than 64 KiB and the compiler needs to change "RAMPZ" to accomplish an
operation, "RAMPZ" is reset to zero after the operation.
* If the device comes with a specific "RAMP" register, the ISR prologue/epilogue saves/restores that SFR and
initializes it with zero in case the ISR code might (implicitly) use it.
* RAM larger than 64 KiB is not supported by GCC for AVR targets. If you use inline assembler to read from
locations outside the 16-bit address range and change one of the "RAMP" registers, you must reset it to zero
after the access.
AVR Built-in Macros
GCC defines several built-in macros so that the user code can test for the presence or absence of features. Almost
any of the following built-in macros are deduced from device capabilities and thus triggered by the -mmcu= command-
line option.
For even more AVR-specific built-in macros see AVR Named Address Spaces and AVR Built-in Functions.
"__AVR_ARCH__"
Build-in macro that resolves to a decimal number that identifies the architecture and depends on the -mmcu=mcu
option. Possible values are:
2, 25, 3, 31, 35, 4, 5, 51, 6
for mcu="avr2", "avr25", "avr3", "avr31", "avr35", "avr4", "avr5", "avr51", "avr6",
respectively and
100, 102, 103, 104, 105, 106, 107
for mcu="avrtiny", "avrxmega2", "avrxmega3", "avrxmega4", "avrxmega5", "avrxmega6", "avrxmega7", respectively.
If mcu specifies a device, this built-in macro is set accordingly. For example, with -mmcu=atmega8 the macro is
defined to 4.
"__AVR_Device__"
Setting -mmcu=device defines this built-in macro which reflects the device's name. For example, -mmcu=atmega8
defines the built-in macro "__AVR_ATmega8__", -mmcu=attiny261a defines "__AVR_ATtiny261A__", etc.
The built-in macros' names follow the scheme "__AVR_Device__" where Device is the device name as from the AVR
user manual. The difference between Device in the built-in macro and device in -mmcu=device is that the latter is
always lowercase.
If device is not a device but only a core architecture like avr51, this macro is not defined.
"__AVR_DEVICE_NAME__"
Setting -mmcu=device defines this built-in macro to the device's name. For example, with -mmcu=atmega8 the macro
is defined to "atmega8".
If device is not a device but only a core architecture like avr51, this macro is not defined.
"__AVR_CVT__"
The code is being compiled with option "-mcvt" to use a compact vector table.
"__AVR_XMEGA__"
The device / architecture belongs to the XMEGA family of devices.
"__AVR_HAVE_ADIW__"
The device has the "ADIW" and "SBIW" instructions.
"__AVR_HAVE_ELPM__"
The device has the "ELPM" instruction.
"__AVR_HAVE_ELPMX__"
The device has the "ELPM Rn,Z" and "ELPM Rn,Z+" instructions.
"__AVR_HAVE_LPMX__"
The device has the "LPM Rn,Z" and "LPM Rn,Z+" instructions.
"__AVR_HAVE_MOVW__"
The device has the "MOVW" instruction to perform 16-bit register-register moves.
"__AVR_HAVE_MUL__"
The device has a hardware multiplier.
"__AVR_HAVE_JMP_CALL__"
The device has the "JMP" and "CALL" instructions. This is the case for devices with more than 8 KiB of program
memory.
"__AVR_HAVE_EIJMP_EICALL__"
"__AVR_3_BYTE_PC__"
The device has the "EIJMP" and "EICALL" instructions. This is the case for devices with more than 128 KiB of
program memory. This also means that the program counter (PC) is 3 bytes wide.
"__AVR_2_BYTE_PC__"
The program counter (PC) is 2 bytes wide. This is the case for devices with up to 128 KiB of program memory.
"__AVR_HAVE_8BIT_SP__"
"__AVR_HAVE_16BIT_SP__"
The stack pointer (SP) register is treated as 8-bit respectively 16-bit register by the compiler. The definition
of these macros is affected by -mtiny-stack.
"__AVR_HAVE_SPH__"
"__AVR_SP8__"
The device has the SPH (high part of stack pointer) special function register or has an 8-bit stack pointer,
respectively. The definition of these macros is affected by -mmcu= and in the cases of -mmcu=avr2 and
-mmcu=avr25 also by -msp8.
"__AVR_HAVE_RAMPD__"
"__AVR_HAVE_RAMPX__"
"__AVR_HAVE_RAMPY__"
"__AVR_HAVE_RAMPZ__"
The device has the "RAMPD", "RAMPX", "RAMPY", "RAMPZ" special function register, respectively.
"__NO_INTERRUPTS__"
This macro reflects the -mno-interrupts command-line option.
"__AVR_ERRATA_SKIP__"
"__AVR_ERRATA_SKIP_JMP_CALL__"
Some AVR devices (AT90S8515, ATmega103) must not skip 32-bit instructions because of a hardware erratum. Skip
instructions are "SBRS", "SBRC", "SBIS", "SBIC" and "CPSE". The second macro is only defined if
"__AVR_HAVE_JMP_CALL__" is also set.
"__AVR_ISA_RMW__"
The device has Read-Modify-Write instructions (XCH, LAC, LAS and LAT).
"__AVR_SFR_OFFSET__=offset"
Instructions that can address I/O special function registers directly like "IN", "OUT", "SBI", etc. may use a
different address as if addressed by an instruction to access RAM like "LD" or "STS". This offset depends on the
device architecture and has to be subtracted from the RAM address in order to get the respective I/O address.
"__AVR_SHORT_CALLS__"
The -mshort-calls command line option is set.
"__AVR_PM_BASE_ADDRESS__=addr"
Some devices support reading from flash memory by means of "LD*" instructions. The flash memory is seen in the
data address space at an offset of "__AVR_PM_BASE_ADDRESS__". If this macro is not defined, this feature is not
available. If defined, the address space is linear and there is no need to put ".rodata" into RAM. This is
handled by the default linker description file, and is currently available for "avrtiny" and "avrxmega3". Even
more convenient, there is no need to use address spaces like "__flash" or features like attribute "progmem" and
"pgm_read_*".
"__AVR_HAVE_FLMAP__"
This macro is defined provided the following conditions are met:
*
This applies to the AVR64* and AVR128* devices.
*
This implies the compiler was configured with GNU Binutils that implement PR31124
("https://sourceware.org/PR31124").
"__AVR_RODATA_IN_RAM__"
This macro is undefined when the code is compiled for a core architecture.
When the code is compiled for a device, the macro is defined to 1 when the ".rodata" sections for read-only data
is located in RAM; and defined to 0, otherwise.
"__WITH_AVRLIBC__"
The compiler is configured to be used together with AVR-LibC. See the --with-avrlibc configure option.
"__HAVE_SIGNAL_N__"
The compiler supports the signal(num) and interrupt(num) AVR Function Attributes,,function attributes with an
argument num that specifies the number of the interrupt service routine.
"__HAVE_DOUBLE_MULTILIB__"
Defined if -mdouble= acts as a multilib option.
"__HAVE_DOUBLE32__"
"__HAVE_DOUBLE64__"
Defined if the compiler supports 32-bit double resp. 64-bit double. The actual layout is specified by option
-mdouble=.
"__DEFAULT_DOUBLE__"
The size in bits of "double" if -mdouble= is not set. To test the layout of "double" in a program, use the
built-in macro "__SIZEOF_DOUBLE__".
"__HAVE_LONG_DOUBLE32__"
"__HAVE_LONG_DOUBLE64__"
"__HAVE_LONG_DOUBLE_MULTILIB__"
"__DEFAULT_LONG_DOUBLE__"
Same as above, but for "long double" instead of "double".
"__WITH_DOUBLE_COMPARISON__"
Reflects the "--with-double-comparison={tristate|bool|libf7}" configure option
("https://gcc.gnu.org/install/configure.html#avr") and is defined to 2 or 3.
"__WITH_LIBF7_LIBGCC__"
"__WITH_LIBF7_MATH__"
"__WITH_LIBF7_MATH_SYMBOLS__"
Reflects the "--with-libf7={libgcc|math|math-symbols}" configure option
("https://gcc.gnu.org/install/configure.html#avr").
AVR Internal Options
The following options are used internally by the compiler and to communicate between device specs files and the
compiler proper. You don't need to set these options by hand, in particular they are not optimization options. Using
these options in the wrong way may lead to sub-optimal or wrong code. They are documented for completeness, and in
order to get a better understanding of device specs ("https://gcc.gnu.org/wiki/avr-gcc#spec-files") files.
-mn-flash=num
Assume that the flash memory has a size of num times 64 KiB. This determines which "__flashN" address spaces are
available.
-mflmap
The device has the "FLMAP" bit field located in special function register "NVMCTRL_CTRLB".
-mrmw
Assume that the device supports the Read-Modify-Write instructions "XCH", "LAC", "LAS" and "LAT".
-mshort-calls
Assume that "RJMP" and "RCALL" can target the whole program memory. This option is used for multilib generation
and selection for the devices from architecture "avrxmega3".
-mskip-bug
Generate code without skips ("CPSE", "SBRS", "SBRC", "SBIS", "SBIC") over 32-bit instructions.
-msp8
Treat the stack pointer register as an 8-bit register, i.e. assume the high byte of the stack pointer is zero.
This option is used by the compiler to select and build multilibs for architectures "avr2" and "avr25". These
architectures mix devices with and without "SPH".
Blackfin Options
-mcpu=cpu[-sirevision]
Specifies the name of the target Blackfin processor. Currently, cpu can be one of bf512, bf514, bf516, bf518,
bf522, bf523, bf524, bf525, bf526, bf527, bf531, bf532, bf533, bf534, bf536, bf537, bf538, bf539, bf542, bf544,
bf547, bf548, bf549, bf542m, bf544m, bf547m, bf548m, bf549m, bf561, bf592.
The optional sirevision specifies the silicon revision of the target Blackfin processor. Any workarounds
available for the targeted silicon revision are enabled. If sirevision is none, no workarounds are enabled. If
sirevision is any, all workarounds for the targeted processor are enabled. The "__SILICON_REVISION__" macro is
defined to two hexadecimal digits representing the major and minor numbers in the silicon revision. If
sirevision is none, the "__SILICON_REVISION__" is not defined. If sirevision is any, the "__SILICON_REVISION__"
is defined to be 0xffff. If this optional sirevision is not used, GCC assumes the latest known silicon revision
of the targeted Blackfin processor.
GCC defines a preprocessor macro for the specified cpu. For the bfin-elf toolchain, this option causes the
hardware BSP provided by libgloss to be linked in if -msim is not given.
Without this option, bf532 is used as the processor by default.
Note that support for bf561 is incomplete. For bf561, only the preprocessor macro is defined.
-msim
Specifies that the program will be run on the simulator. This causes the simulator BSP provided by libgloss to
be linked in. This option has effect only for bfin-elf toolchain. Certain other options, such as
-mid-shared-library and -mfdpic, imply -msim.
-momit-leaf-frame-pointer
Don't keep the frame pointer in a register for leaf functions. This avoids the instructions to save, set up and
restore frame pointers and makes an extra register available in leaf functions.
-mspecld-anomaly
When enabled, the compiler ensures that the generated code does not contain speculative loads after jump
instructions. If this option is used, "__WORKAROUND_SPECULATIVE_LOADS" is defined.
-mno-specld-anomaly
Don't generate extra code to prevent speculative loads from occurring.
-mcsync-anomaly
When enabled, the compiler ensures that the generated code does not contain CSYNC or SSYNC instructions too soon
after conditional branches. If this option is used, "__WORKAROUND_SPECULATIVE_SYNCS" is defined.
-mno-csync-anomaly
Don't generate extra code to prevent CSYNC or SSYNC instructions from occurring too soon after a conditional
branch.
-mlow64k
When enabled, the compiler is free to take advantage of the knowledge that the entire program fits into the low
64k of memory.
-mno-low64k
Assume that the program is arbitrarily large. This is the default.
-mstack-check-l1
Do stack checking using information placed into L1 scratchpad memory by the uClinux kernel.
-mid-shared-library
Generate code that supports shared libraries via the library ID method. This allows for execute in place and
shared libraries in an environment without virtual memory management. This option implies -fPIC. With a bfin-
elf target, this option implies -msim.
-mno-id-shared-library
Generate code that doesn't assume ID-based shared libraries are being used. This is the default.
-mleaf-id-shared-library
Generate code that supports shared libraries via the library ID method, but assumes that this library or
executable won't link against any other ID shared libraries. That allows the compiler to use faster code for
jumps and calls.
-mno-leaf-id-shared-library
Do not assume that the code being compiled won't link against any ID shared libraries. Slower code is generated
for jump and call insns.
-mshared-library-id=n
Specifies the identification number of the ID-based shared library being compiled. Specifying a value of 0
generates more compact code; specifying other values forces the allocation of that number to the current library
but is no more space- or time-efficient than omitting this option.
-msep-data
Generate code that allows the data segment to be located in a different area of memory from the text segment.
This allows for execute in place in an environment without virtual memory management by eliminating relocations
against the text section.
-mno-sep-data
Generate code that assumes that the data segment follows the text segment. This is the default.
-mlong-calls
-mno-long-calls
Tells the compiler to perform function calls by first loading the address of the function into a register and
then performing a subroutine call on this register. This switch is needed if the target function lies outside of
the 24-bit addressing range of the offset-based version of subroutine call instruction.
This feature is not enabled by default. Specifying -mno-long-calls restores the default behavior. Note these
switches have no effect on how the compiler generates code to handle function calls via function pointers.
-mfast-fp
Link with the fast floating-point library. This library relaxes some of the IEEE floating-point standard's rules
for checking inputs against Not-a-Number (NAN), in the interest of performance.
-minline-plt
Enable inlining of PLT entries in function calls to functions that are not known to bind locally. It has no
effect without -mfdpic.
-mmulticore
Build a standalone application for multicore Blackfin processors. This option causes proper start files and link
scripts supporting multicore to be used, and defines the macro "__BFIN_MULTICORE". It can only be used with
-mcpu=bf561[-sirevision].
This option can be used with -mcorea or -mcoreb, which selects the one-application-per-core programming model.
Without -mcorea or -mcoreb, the single-application/dual-core programming model is used. In this model, the main
function of Core B should be named as "coreb_main".
If this option is not used, the single-core application programming model is used.
-mcorea
Build a standalone application for Core A of BF561 when using the one-application-per-core programming model.
Proper start files and link scripts are used to support Core A, and the macro "__BFIN_COREA" is defined. This
option can only be used in conjunction with -mmulticore.
-mcoreb
Build a standalone application for Core B of BF561 when using the one-application-per-core programming model.
Proper start files and link scripts are used to support Core B, and the macro "__BFIN_COREB" is defined. When
this option is used, "coreb_main" should be used instead of "main". This option can only be used in conjunction
with -mmulticore.
-msdram
Build a standalone application for SDRAM. Proper start files and link scripts are used to put the application
into SDRAM, and the macro "__BFIN_SDRAM" is defined. The loader should initialize SDRAM before loading the
application.
-micplb
Assume that ICPLBs are enabled at run time. This has an effect on certain anomaly workarounds. For Linux
targets, the default is to assume ICPLBs are enabled; for standalone applications the default is off.
C6X Options
-march=name
This specifies the name of the target architecture. GCC uses this name to determine what kind of instructions it
can emit when generating assembly code. Permissible names are: c62x, c64x, c64x+, c67x, c67x+, c674x.
-mbig-endian
Generate code for a big-endian target.
-mlittle-endian
Generate code for a little-endian target. This is the default.
-msim
Choose startup files and linker script suitable for the simulator.
-msdata=default
Put small global and static data in the ".neardata" section, which is pointed to by register "B14". Put small
uninitialized global and static data in the ".bss" section, which is adjacent to the ".neardata" section. Put
small read-only data into the ".rodata" section. The corresponding sections used for large pieces of data are
".fardata", ".far" and ".const".
-msdata=all
Put all data, not just small objects, into the sections reserved for small data, and use addressing relative to
the "B14" register to access them.
-msdata=none
Make no use of the sections reserved for small data, and use absolute addresses to access all data. Put all
initialized global and static data in the ".fardata" section, and all uninitialized data in the ".far" section.
Put all constant data into the ".const" section.
CRIS Options
These options are defined specifically for the CRIS ports.
-march=architecture-type
-mcpu=architecture-type
Generate code for the specified architecture. The choices for architecture-type are v3, v8 and v10 for
respectively ETRAX 4, ETRAX 100, and ETRAX 100 LX. Default is v0.
-mtune=architecture-type
Tune to architecture-type everything applicable about the generated code, except for the ABI and the set of
available instructions. The choices for architecture-type are the same as for -march=architecture-type.
-mmax-stack-frame=n
Warn when the stack frame of a function exceeds n bytes.
-metrax4
-metrax100
The options -metrax4 and -metrax100 are synonyms for -march=v3 and -march=v8 respectively.
-mmul-bug-workaround
-mno-mul-bug-workaround
Work around a bug in the "muls" and "mulu" instructions for CPU models where it applies. This option is disabled
by default.
-mpdebug
Enable CRIS-specific verbose debug-related information in the assembly code. This option also has the effect of
turning off the #NO_APP formatted-code indicator to the assembler at the beginning of the assembly file.
-mcc-init
Do not use condition-code results from previous instruction; always emit compare and test instructions before use
of condition codes.
-mno-side-effects
Do not emit instructions with side effects in addressing modes other than post-increment.
-mstack-align
-mno-stack-align
-mdata-align
-mno-data-align
-mconst-align
-mno-const-align
These options (no- options) arrange (eliminate arrangements) for the stack frame, individual data and constants
to be aligned for the maximum single data access size for the chosen CPU model. The default is to arrange for
32-bit alignment. ABI details such as structure layout are not affected by these options.
-m32-bit
-m16-bit
-m8-bit
Similar to the stack- data- and const-align options above, these options arrange for stack frame, writable data
and constants to all be 32-bit, 16-bit or 8-bit aligned. The default is 32-bit alignment.
-mno-prologue-epilogue
-mprologue-epilogue
With -mno-prologue-epilogue, the normal function prologue and epilogue which set up the stack frame are omitted
and no return instructions or return sequences are generated in the code. Use this option only together with
visual inspection of the compiled code: no warnings or errors are generated when call-saved registers must be
saved, or storage for local variables needs to be allocated.
-melf
Legacy no-op option.
-sim
This option arranges to link with input-output functions from a simulator library. Code, initialized data and
zero-initialized data are allocated consecutively.
-sim2
Like -sim, but pass linker options to locate initialized data at 0x40000000 and zero-initialized data at
0x80000000.
C-SKY Options
GCC supports these options when compiling for C-SKY V2 processors.
-march=arch
Specify the C-SKY target architecture. Valid values for arch are: ck801, ck802, ck803, ck807, and ck810. The
default is ck810.
-mcpu=cpu
Specify the C-SKY target processor. Valid values for cpu are: ck801, ck801t, ck802, ck802t, ck802j, ck803,
ck803h, ck803t, ck803ht, ck803f, ck803fh, ck803e, ck803eh, ck803et, ck803eht, ck803ef, ck803efh, ck803ft,
ck803eft, ck803efht, ck803r1, ck803hr1, ck803tr1, ck803htr1, ck803fr1, ck803fhr1, ck803er1, ck803ehr1, ck803etr1,
ck803ehtr1, ck803efr1, ck803efhr1, ck803ftr1, ck803eftr1, ck803efhtr1, ck803s, ck803st, ck803se, ck803sf,
ck803sef, ck803seft, ck807e, ck807ef, ck807, ck807f, ck810e, ck810et, ck810ef, ck810eft, ck810, ck810v, ck810f,
ck810t, ck810fv, ck810tv, ck810ft, and ck810ftv.
-mbig-endian
-EB
-mlittle-endian
-EL Select big- or little-endian code. The default is little-endian.
-mfloat-abi=name
Specifies which floating-point ABI to use. Permissible values are: soft, softfp and hard.
Specifying soft causes GCC to generate output containing library calls for floating-point operations. softfp
allows the generation of code using hardware floating-point instructions, but still uses the soft-float calling
conventions. hard allows generation of floating-point instructions and uses FPU-specific calling conventions.
The default depends on the specific target configuration. Note that the hard-float and soft-float ABIs are not
link-compatible; you must compile your entire program with the same ABI, and link with a compatible set of
libraries.
-mhard-float
-msoft-float
Select hardware or software floating-point implementations. The default is soft float.
-mdouble-float
-mno-double-float
When -mhard-float is in effect, enable generation of double-precision float instructions. This is the default
except when compiling for CK803.
-mfdivdu
-mno-fdivdu
When -mhard-float is in effect, enable generation of "frecipd", "fsqrtd", and "fdivd" instructions. This is the
default except when compiling for CK803.
-mfpu=fpu
Select the floating-point processor. This option can only be used with -mhard-float. Values for fpu are fpv2_sf
(equivalent to -mno-double-float -mno-fdivdu), fpv2 (-mdouble-float -mno-divdu), and fpv2_divd (-mdouble-float
-mdivdu).
-melrw
-mno-elrw
Enable the extended "lrw" instruction. This option defaults to on for CK801 and off otherwise.
-mistack
-mno-istack
Enable interrupt stack instructions; the default is off.
The -mistack option is required to handle the "interrupt" and "isr" function attributes.
-mmp
Enable multiprocessor instructions; the default is off.
-mcp
Enable coprocessor instructions; the default is off.
-mcache
Enable coprocessor instructions; the default is off.
-msecurity
Enable C-SKY security instructions; the default is off.
-mtrust
Enable C-SKY trust instructions; the default is off.
-mdsp
-medsp
-mvdsp
Enable C-SKY DSP, Enhanced DSP, or Vector DSP instructions, respectively. All of these options default to off.
-mdiv
-mno-div
Generate divide instructions. Default is off.
-msmart
-mno-smart
Generate code for Smart Mode, using only registers numbered 0-7 to allow use of 16-bit instructions. This option
is ignored for CK801 where this is the required behavior, and it defaults to on for CK802. For other targets,
the default is off.
-mhigh-registers
-mno-high-registers
Generate code using the high registers numbered 16-31. This option is not supported on CK801, CK802, or CK803,
and is enabled by default for other processors.
-manchor
-mno-anchor
Generate code using global anchor symbol addresses.
-mpushpop
-mno-pushpop
Generate code using "push" and "pop" instructions. This option defaults to on.
-mmultiple-stld
-mstm
-mno-multiple-stld
-mno-stm
Generate code using "stm" and "ldm" instructions. This option isn't supported on CK801 but is enabled by default
on other processors.
-mconstpool
-mno-constpool
Create constant pools in the compiler instead of deferring it to the assembler. This option is the default and
required for correct code generation on CK801 and CK802, and is optional on other processors.
-mstack-size
-mno-stack-size
Emit ".stack_size" directives for each function in the assembly output. This option defaults to off.
-mccrt
-mno-ccrt
Generate code for the C-SKY compiler runtime instead of libgcc. This option defaults to off.
-mbranch-cost=n
Set the branch costs to roughly "n" instructions. The default is 1.
-msched-prolog
-mno-sched-prolog
Permit scheduling of function prologue and epilogue sequences. Using this option can result in code that is not
compliant with the C-SKY V2 ABI prologue requirements and that cannot be debugged or backtraced. It is disabled
by default.
-msim
Links the library libsemi.a which is in compatible with simulator. Applicable to ELF compiler only.
Cygwin and MinGW Options
These additional options are available for Microsoft Windows targets:
-mconsole
This option specifies that a console application is to be generated, by instructing the linker to set the PE
header subsystem type required for console applications. This option is available for Cygwin and MinGW targets
and is enabled by default on those targets.
-mcrtdll=library
Preprocess, compile or link with specified C RunTime DLL library. This option adjust predefined macros
"__CRTDLL__", "__MSVCRT__", "_UCRT" and "__MSVCRT_VERSION__" for specified CRT library, choose start file for CRT
library and link with CRT library. Recognized CRT library names for proprocessor are: "crtdll*", "msvcrt10*",
"msvcrt20*", "msvcrt40*", "msvcr40*", "msvcrtd*", "msvcrt-os*", "msvcr70*", "msvcr71*", "msvcr80*", "msvcr90*",
"msvcr100*", "msvcr110*", "msvcr120*" and "ucrt*". If this options is not specified then the default MinGW
import library "msvcrt" is used for linking and no other adjustment for preprocessor is done. MinGW import
library "msvcrt" is just a symlink to (or a copy of) another MinGW CRT import library chosen during MinGW
compilation. MinGW import library "msvcrt-os" is for Windows system CRT DLL library "msvcrt.dll" and in most
cases is the default MinGW import library. Generally speaking, changing the CRT DLL requires recompiling the
entire MinGW CRT. This option is for experimental and testing purposes only. This option is available for MinGW
targets.
-mdll
This option is available for Cygwin and MinGW targets. It specifies that a DLL---a dynamic link library---is to
be generated, enabling the selection of the required runtime startup object and entry point.
-mnop-fun-dllimport
This option is available for Cygwin and MinGW targets. It specifies that the "dllimport" attribute should be
ignored.
-mthreads
This option is available for MinGW targets. It specifies that MinGW-specific thread support is to be used.
-municode
This option is available for MinGW-w64 targets. It causes the "UNICODE" preprocessor macro to be predefined, and
chooses Unicode-capable runtime startup code.
-mwin32
This option is available for Cygwin and MinGW targets. It specifies that the typical Microsoft Windows
predefined macros are to be set in the pre-processor, but does not influence the choice of runtime
library/startup code.
-mwindows
This option is available for Cygwin and MinGW targets. It specifies that a GUI application is to be generated by
instructing the linker to set the PE header subsystem type appropriately.
-fno-set-stack-executable
This option is available for MinGW targets. It specifies that the executable flag for the stack used by nested
functions isn't set. This is necessary for binaries running in kernel mode of Microsoft Windows, as there the
User32 API, which is used to set executable privileges, isn't available.
-fwritable-relocated-rdata
This option is available for MinGW and Cygwin targets. It specifies that relocated-data in read-only section is
put into the ".data" section. This is a necessary for older runtimes not supporting modification of ".rdata"
sections for pseudo-relocation.
-mpe-aligned-commons
This option is available for Cygwin and MinGW targets. It specifies that the GNU extension to the PE file format
that permits the correct alignment of COMMON variables should be used when generating code. It is enabled by
default if GCC detects that the target assembler found during configuration supports the feature.
See also under x86 Options for standard options.
Darwin Options
These options are defined for all architectures running the Darwin operating system.
FSF GCC on Darwin does not create "fat" object files; it creates an object file for the single architecture that GCC
was built to target. Apple's GCC on Darwin does create "fat" files if multiple -arch options are used; it does so by
running the compiler or linker multiple times and joining the results together with lipo.
The subtype of the file created (like ppc7400 or ppc970 or i686) is determined by the flags that specify the ISA that
GCC is targeting, like -mcpu or -march. The -force_cpusubtype_ALL option can be used to override this.
The Darwin tools vary in their behavior when presented with an ISA mismatch. The assembler, as, only permits
instructions to be used that are valid for the subtype of the file it is generating, so you cannot put 64-bit
instructions in a ppc750 object file. The linker for shared libraries, /usr/bin/libtool, fails and prints an error
if asked to create a shared library with a less restrictive subtype than its input files (for instance, trying to put
a ppc970 object file in a ppc7400 library). The linker for executables, ld, quietly gives the executable the most
restrictive subtype of any of its input files.
-Fdir
Add the framework directory dir to the head of the list of directories to be searched for header files. These
directories are interleaved with those specified by -I options and are scanned in a left-to-right order.
A framework directory is a directory with frameworks in it. A framework is a directory with a Headers and/or
PrivateHeaders directory contained directly in it that ends in .framework. The name of a framework is the name
of this directory excluding the .framework. Headers associated with the framework are found in one of those two
directories, with Headers being searched first. A subframework is a framework directory that is in a framework's
Frameworks directory. Includes of subframework headers can only appear in a header of a framework that contains
the subframework, or in a sibling subframework header. Two subframeworks are siblings if they occur in the same
framework. A subframework should not have the same name as a framework; a warning is issued if this is violated.
Currently a subframework cannot have subframeworks; in the future, the mechanism may be extended to support this.
The standard frameworks can be found in /System/Library/Frameworks and /Library/Frameworks. An example include
looks like "#include ", where Framework denotes the name of the framework and header.h is
found in the PrivateHeaders or Headers directory.
-iframeworkdir
Like -F except the directory is a treated as a system directory. The main difference between this -iframework
and -F is that with -iframework the compiler does not warn about constructs contained within header files found
via dir. This option is valid only for the C family of languages.
-gused
Emit debugging information for symbols that are used. For stabs debugging format, this enables
-feliminate-unused-debug-symbols. This is by default ON.
-gfull
Emit debugging information for all symbols and types.
-fconstant-cfstrings
The -fconstant-cfstrings is an alias for -mconstant-cfstrings.
-mconstant-cfstrings
When the NeXT runtime is being used (the default on these systems), override any -fconstant-string-class setting
and cause "@"..."" literals to be laid out as constant CoreFoundation strings.
-mmacosx-version-min=version
The earliest version of MacOS X that this executable will run on is version. Typical values supported for
version include 12, 10.12, and 10.5.8.
If the compiler was built to use the system's headers by default, then the default for this option is the system
version on which the compiler is running, otherwise the default is to make choices that are compatible with as
many systems and code bases as possible.
-mkernel
Enable kernel development mode. The -mkernel option sets -static, -fno-common, -fno-use-cxa-atexit,
-fno-exceptions, -fno-non-call-exceptions, -fapple-kext, -fno-weak and -fno-rtti where applicable. This mode
also sets -mno-altivec, -msoft-float, -fno-builtin and -mlong-branch for PowerPC targets.
-mone-byte-bool
Override the defaults for "bool" so that "sizeof(bool)==1". By default sizeof(bool) is 4 when compiling for
Darwin/PowerPC and 1 when compiling for Darwin/x86, so this option has no effect on x86.
Warning: The -mone-byte-bool switch causes GCC to generate code that is not binary compatible with code generated
without that switch. Using this switch may require recompiling all other modules in a program, including system
libraries. Use this switch to conform to a non-default data model.
-mfix-and-continue
-ffix-and-continue
-findirect-data
Generate code suitable for fast turnaround development, such as to allow GDB to dynamically load .o files into
already-running programs. -findirect-data and -ffix-and-continue are provided for backwards compatibility.
-all_load
Loads all members of static archive libraries. See man ld(1) for more information.
-arch_errors_fatal
Cause the errors having to do with files that have the wrong architecture to be fatal.
-bind_at_load
Causes the output file to be marked such that the dynamic linker will bind all undefined references when the file
is loaded or launched.
-bundle
Produce a Mach-o bundle format file. See man ld(1) for more information.
-bundle_loader executable
This option specifies the executable that will load the build output file being linked. See man ld(1) for more
information.
-dynamiclib
When passed this option, GCC produces a dynamic library instead of an executable when linking, using the Darwin
libtool command.
-force_cpusubtype_ALL
This causes GCC's output file to have the ALL subtype, instead of one controlled by the -mcpu or -march option.
-nodefaultrpaths
Do not add default run paths for the compiler library directories to executables, modules or dynamic libraries.
On macOS 10.5 and later, the embedded runpath is added by default unless the user adds -nodefaultrpaths to the
link line. Run paths are needed (and therefore enforced) to build on macOS version 10.11 or later.
-allowable_client client_name
-client_name
-compatibility_version
-current_version
-dead_strip
-dependency-file
-dylib_file
-dylinker_install_name
-dynamic
-exported_symbols_list
-filelist
-flat_namespace
-force_flat_namespace
-headerpad_max_install_names
-image_base
-init
-install_name
-keep_private_externs
-multi_module
-multiply_defined
-multiply_defined_unused
-noall_load
-no_dead_strip_inits_and_terms
-nofixprebinding
-nomultidefs
-noprebind
-noseglinkedit
-pagezero_size
-prebind
-prebind_all_twolevel_modules
-private_bundle
-read_only_relocs
-sectalign
-sectobjectsymbols
-whyload
-seg1addr
-sectcreate
-sectobjectsymbols
-sectorder
-segaddr
-segs_read_only_addr
-segs_read_write_addr
-seg_addr_table
-seg_addr_table_filename
-seglinkedit
-segprot
-segs_read_only_addr
-segs_read_write_addr
-single_module
-static
-sub_library
-sub_umbrella
-twolevel_namespace
-umbrella
-undefined
-unexported_symbols_list
-weak_reference_mismatches
-whatsloaded
These options are passed to the Darwin linker. The Darwin linker man page describes them in detail.
DEC Alpha Options
These -m options are defined for the DEC Alpha implementations:
-mno-soft-float
-msoft-float
Use (do not use) the hardware floating-point instructions for floating-point operations. When -msoft-float is
specified, functions in libgcc.a are used to perform floating-point operations. Unless they are replaced by
routines that emulate the floating-point operations, or compiled in such a way as to call such emulations
routines, these routines issue floating-point operations. If you are compiling for an Alpha without floating-
point operations, you must ensure that the library is built so as not to call them.
Note that Alpha implementations without floating-point operations are required to have floating-point registers.
-mfp-reg
-mno-fp-regs
Generate code that uses (does not use) the floating-point register set. -mno-fp-regs implies -msoft-float. If
the floating-point register set is not used, floating-point operands are passed in integer registers as if they
were integers and floating-point results are passed in $0 instead of $f0. This is a non-standard calling
sequence, so any function with a floating-point argument or return value called by code compiled with
-mno-fp-regs must also be compiled with that option.
A typical use of this option is building a kernel that does not use, and hence need not save and restore, any
floating-point registers.
-mieee
The Alpha architecture implements floating-point hardware optimized for maximum performance. It is mostly
compliant with the IEEE floating-point standard. However, for full compliance, software assistance is required.
This option generates code fully IEEE-compliant code except that the inexact-flag is not maintained (see below).
If this option is turned on, the preprocessor macro "_IEEE_FP" is defined during compilation. The resulting code
is less efficient but is able to correctly support denormalized numbers and exceptional IEEE values such as not-
a-number and plus/minus infinity. Other Alpha compilers call this option -ieee_with_no_inexact.
DEBIAN SPECIFIC: This option is on by default for alpha-linux-gnu, unless -ffinite-math-only (which is part of
the -ffast-math set) is specified, because the software functions in the GNU libc math libraries generate
denormalized numbers, NaNs, and infs (all of which will cause a programs to SIGFPE when it attempts to use the
results without -mieee).
-mieee-with-inexact
This is like -mieee except the generated code also maintains the IEEE inexact-flag. Turning on this option
causes the generated code to implement fully-compliant IEEE math. In addition to "_IEEE_FP", "_IEEE_FP_EXACT" is
defined as a preprocessor macro. On some Alpha implementations the resulting code may execute significantly
slower than the code generated by default. Since there is very little code that depends on the inexact-flag, you
should normally not specify this option. Other Alpha compilers call this option -ieee_with_inexact.
-mfp-trap-mode=trap-mode
This option controls what floating-point related traps are enabled. Other Alpha compilers call this option -fptm
trap-mode. The trap mode can be set to one of four values:
n This is the default (normal) setting. The only traps that are enabled are the ones that cannot be disabled
in software (e.g., division by zero trap).
u In addition to the traps enabled by n, underflow traps are enabled as well.
su Like u, but the instructions are marked to be safe for software completion (see Alpha architecture manual for
details).
sui Like su, but inexact traps are enabled as well.
-mfp-rounding-mode=rounding-mode
Selects the IEEE rounding mode. Other Alpha compilers call this option -fprm rounding-mode. The rounding-mode
can be one of:
n Normal IEEE rounding mode. Floating-point numbers are rounded towards the nearest machine number or towards
the even machine number in case of a tie.
m Round towards minus infinity.
c Chopped rounding mode. Floating-point numbers are rounded towards zero.
d Dynamic rounding mode. A field in the floating-point control register (fpcr, see Alpha architecture
reference manual) controls the rounding mode in effect. The C library initializes this register for rounding
towards plus infinity. Thus, unless your program modifies the fpcr, d corresponds to round towards plus
infinity.
-mtrap-precision=trap-precision
In the Alpha architecture, floating-point traps are imprecise. This means without software assistance it is
impossible to recover from a floating trap and program execution normally needs to be terminated. GCC can
generate code that can assist operating system trap handlers in determining the exact location that caused a
floating-point trap. Depending on the requirements of an application, different levels of precisions can be
selected:
p Program precision. This option is the default and means a trap handler can only identify which program
caused a floating-point exception.
f Function precision. The trap handler can determine the function that caused a floating-point exception.
i Instruction precision. The trap handler can determine the exact instruction that caused a floating-point
exception.
Other Alpha compilers provide the equivalent options called -scope_safe and -resumption_safe.
-mieee-conformant
This option marks the generated code as IEEE conformant. You must not use this option unless you also specify
-mtrap-precision=i and either -mfp-trap-mode=su or -mfp-trap-mode=sui. Its only effect is to emit the line
.eflag 48 in the function prologue of the generated assembly file.
-mbuild-constants
Normally GCC examines a 32- or 64-bit integer constant to see if it can construct it from smaller constants in
two or three instructions. If it cannot, it outputs the constant as a literal and generates code to load it from
the data segment at run time.
Use this option to require GCC to construct all integer constants using code, even if it takes more instructions
(the maximum is six).
You typically use this option to build a shared library dynamic loader. Itself a shared library, it must
relocate itself in memory before it can find the variables and constants in its own data segment.
-mbwx
-mno-bwx
-mcix
-mno-cix
-mfix
-mno-fix
-mmax
-mno-max
Indicate whether GCC should generate code to use the optional BWX, CIX, FIX and MAX instruction sets. The
default is to use the instruction sets supported by the CPU type specified via -mcpu= option or that of the CPU
on which GCC was built if none is specified.
-msafe-bwa
-mno-safe-bwa
Indicate whether in the absence of the optional BWX instruction set GCC should generate multi-thread and async-
signal safe code for byte and aligned word memory accesses.
-msafe-partial
-mno-safe-partial
Indicate whether GCC should generate multi-thread and async-signal safe code for partial memory accesses,
including piecemeal accesses to unaligned data as well as block accesses to leading and trailing parts of
aggregate types or other objects in memory that do not respectively start and end on an aligned 64-bit data
boundary.
-mfloat-vax
-mfloat-ieee
Generate code that uses (does not use) VAX F and G floating-point arithmetic instead of IEEE single and double
precision.
-mexplicit-relocs
-mno-explicit-relocs
Older Alpha assemblers provided no way to generate symbol relocations except via assembler macros. Use of these
macros does not allow optimal instruction scheduling. GNU binutils as of version 2.12 supports a new syntax that
allows the compiler to explicitly mark which relocations should apply to which instructions. This option is
mostly useful for debugging, as GCC detects the capabilities of the assembler when it is built and sets the
default accordingly.
-msmall-data
-mlarge-data
When -mexplicit-relocs is in effect, static data is accessed via gp-relative relocations. When -msmall-data is
used, objects 8 bytes long or smaller are placed in a small data area (the ".sdata" and ".sbss" sections) and are
accessed via 16-bit relocations off of the $gp register. This limits the size of the small data area to 64KB,
but allows the variables to be directly accessed via a single instruction.
The default is -mlarge-data. With this option the data area is limited to just below 2GB. Programs that require
more than 2GB of data must use "malloc" or "mmap" to allocate the data in the heap instead of in the program's
data segment.
When generating code for shared libraries, -fpic implies -msmall-data and -fPIC implies -mlarge-data.
-msmall-text
-mlarge-text
When -msmall-text is used, the compiler assumes that the code of the entire program (or shared library) fits in
4MB, and is thus reachable with a branch instruction. When -msmall-data is used, the compiler can assume that
all local symbols share the same $gp value, and thus reduce the number of instructions required for a function
call from 4 to 1.
The default is -mlarge-text.
-mcpu=cpu_type
Set the instruction set and instruction scheduling parameters for machine type cpu_type. You can specify either
the EV style name or the corresponding chip number. GCC supports scheduling parameters for the EV4, EV5 and EV6
family of processors and chooses the default values for the instruction set from the processor you specify. If
you do not specify a processor type, GCC defaults to the processor on which the compiler was built.
Supported values for cpu_type are
ev4
ev45
21064
Schedules as an EV4 and has no instruction set extensions.
ev5
21164
Schedules as an EV5 and has no instruction set extensions.
ev56
21164a
Schedules as an EV5 and supports the BWX extension.
pca56
21164pc
21164PC
Schedules as an EV5 and supports the BWX and MAX extensions.
ev6
21264
Schedules as an EV6 and supports the BWX, FIX, and MAX extensions.
ev67
21264a
Schedules as an EV6 and supports the BWX, CIX, FIX, and MAX extensions.
Native toolchains also support the value native, which selects the best architecture option for the host
processor. -mcpu=native has no effect if GCC does not recognize the processor.
-mtune=cpu_type
Set only the instruction scheduling parameters for machine type cpu_type. The instruction set is not changed.
Native toolchains also support the value native, which selects the best architecture option for the host
processor. -mtune=native has no effect if GCC does not recognize the processor.
-mmemory-latency=time
Sets the latency the scheduler should assume for typical memory references as seen by the application. This
number is highly dependent on the memory access patterns used by the application and the size of the external
cache on the machine.
Valid options for time are
number
A decimal number representing clock cycles.
L1
L2
L3
main
The compiler contains estimates of the number of clock cycles for "typical" EV4 & EV5 hardware for the Level
1, 2 & 3 caches (also called Dcache, Scache, and Bcache), as well as to main memory. Note that L3 is only
valid for EV5.
eBPF Options
-mframe-limit=bytes
This specifies the hard limit for frame sizes, in bytes. Currently, the value that can be specified should be
less than or equal to 32767. Defaults to whatever limit is imposed by the version of the Linux kernel targeted.
-mbig-endian
Generate code for a big-endian target.
-mlittle-endian
Generate code for a little-endian target. This is the default.
-mjmpext
-mno-jmpext
Enable or disable generation of extra conditional-branch instructions. Enabled for CPU v2 and above.
-mjmp32
-mno-jmp32
Enable or disable generation of 32-bit jump instructions. Enabled for CPU v3 and above.
-malu32
-mno-alu32
Enable or disable generation of 32-bit ALU instructions. Enabled for CPU v3 and above.
-mv3-atomics
-mno-v3-atomics
Enable or disable instructions for general atomic operations introduced in CPU v3. Enabled for CPU v3 and above.
-mbswap
-mno-bswap
Enable or disable byte swap instructions. Enabled for CPU v4 and above.
-msdiv
-mno-sdiv
Enable or disable signed division and modulus instructions. Enabled for CPU v4 and above.
-msmov
-mno-smov
Enable or disable sign-extending move and memory load instructions. Enabled for CPU v4 and above.
-mcpu=version
This specifies which version of the eBPF ISA to target. Newer versions may not be supported by all kernels. The
default is v4.
Supported values for version are:
v1 The first stable eBPF ISA with no special features or extensions.
v2 Supports the jump extensions, as in -mjmpext.
v3 All features of v2, plus:
-<32-bit jump operations, as in -mjmp32>
-<32-bit ALU operations, as in -malu32>
-
v4 All features of v3, plus:
-
-
-
-mco-re
Enable BPF Compile Once - Run Everywhere (CO-RE) support. Requires and is implied by -gbtf.
-mno-co-re
Disable BPF Compile Once - Run Everywhere (CO-RE) support. BPF CO-RE support is enabled by default when
generating BTF debug information for the BPF target.
-mxbpf
Generate code for an expanded version of BPF, which relaxes some of the restrictions imposed by the BPF
architecture:
-
exit, respectively.
-masm=dialect
Outputs assembly instructions using eBPF selected dialect. The default is pseudoc.
Supported values for dialect are:
normal
Outputs normal assembly dialect.
pseudoc
Outputs pseudo-c assembly dialect.
-minline-memops-threshold=bytes
Specifies a size threshold in bytes at or below which memmove, memcpy and memset shall always be expanded inline.
Operations dealing with sizes larger than this threshold would have to be implemented using a library call
instead of being expanded inline, but since BPF doesn't allow libcalls, exceeding this threshold results in a
compile-time error. The default is 1024 bytes.
FR30 Options
These options are defined specifically for the FR30 port.
-msmall-model
Use the small address space model. This can produce smaller code, but it does assume that all symbolic values
and addresses fit into a 20-bit range.
-mno-lsim
Assume that runtime support has been provided and so there is no need to include the simulator library (libsim.a)
on the linker command line.
FT32 Options
These options are defined specifically for the FT32 port.
-msim
Specifies that the program will be run on the simulator. This causes an alternate runtime startup and library to
be linked. You must not use this option when generating programs that will run on real hardware; you must
provide your own runtime library for whatever I/O functions are needed.
-mlra
Does nothing. Preserved for backward compatibility.
-mnodiv
Do not use div and mod instructions.
-mft32b
Enable use of the extended instructions of the FT32B processor.
-mcompress
Compress all code using the Ft32B code compression scheme.
-mnopm
Do not generate code that reads program memory.
FRV Options
-mgpr-32
Only use the first 32 general-purpose registers.
-mgpr-64
Use all 64 general-purpose registers.
-mfpr-32
Use only the first 32 floating-point registers.
-mfpr-64
Use all 64 floating-point registers.
-mhard-float
Use hardware instructions for floating-point operations.
-msoft-float
Use library routines for floating-point operations.
-malloc-cc
Dynamically allocate condition code registers.
-mfixed-cc
Do not try to dynamically allocate condition code registers, only use "icc0" and "fcc0".
-mdword
Change ABI to use double word insns.
-mno-dword
Do not use double word instructions.
-mdouble
Use floating-point double instructions.
-mno-double
Do not use floating-point double instructions.
-mmedia
Use media instructions.
-mno-media
Do not use media instructions.
-mmuladd
Use multiply and add/subtract instructions.
-mno-muladd
Do not use multiply and add/subtract instructions.
-mfdpic
Select the FDPIC ABI, which uses function descriptors to represent pointers to functions. Without any
PIC/PIE-related options, it implies -fPIE. With -fpic or -fpie, it assumes GOT entries and small data are within
a 12-bit range from the GOT base address; with -fPIC or -fPIE, GOT offsets are computed with 32 bits. With a
bfin-elf target, this option implies -msim.
-minline-plt
Enable inlining of PLT entries in function calls to functions that are not known to bind locally. It has no
effect without -mfdpic. It's enabled by default if optimizing for speed and compiling for shared libraries
(i.e., -fPIC or -fpic), or when an optimization option such as -O3 or above is present in the command line.
-mTLS
Assume a large TLS segment when generating thread-local code.
-mtls
Do not assume a large TLS segment when generating thread-local code.
-mgprel-ro
Enable the use of "GPREL" relocations in the FDPIC ABI for data that is known to be in read-only sections. It's
enabled by default, except for -fpic or -fpie: even though it may help make the global offset table smaller, it
trades 1 instruction for 4. With -fPIC or -fPIE, it trades 3 instructions for 4, one of which may be shared by
multiple symbols, and it avoids the need for a GOT entry for the referenced symbol, so it's more likely to be a
win. If it is not, -mno-gprel-ro can be used to disable it.
-multilib-library-pic
Link with the (library, not FD) pic libraries. It's implied by -mlibrary-pic, as well as by -fPIC and -fpic
without -mfdpic. You should never have to use it explicitly.
-mlinked-fp
Follow the EABI requirement of always creating a frame pointer whenever a stack frame is allocated. This option
is enabled by default and can be disabled with -mno-linked-fp.
-mlong-calls
Use indirect addressing to call functions outside the current compilation unit. This allows the functions to be
placed anywhere within the 32-bit address space.
-malign-labels
Try to align labels to an 8-byte boundary by inserting NOPs into the previous packet. This option only has an
effect when VLIW packing is enabled. It doesn't create new packets; it merely adds NOPs to existing ones.
-mlibrary-pic
Generate position-independent EABI code.
-macc-4
Use only the first four media accumulator registers.
-macc-8
Use all eight media accumulator registers.
-mpack
Pack VLIW instructions.
-mno-pack
Do not pack VLIW instructions.
-mno-eflags
Do not mark ABI switches in e_flags.
-mcond-move
Enable the use of conditional-move instructions (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-cond-move
Disable the use of conditional-move instructions.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mscc
Enable the use of conditional set instructions (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-scc
Disable the use of conditional set instructions.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mcond-exec
Enable the use of conditional execution (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-cond-exec
Disable the use of conditional execution.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mvliw-branch
Run a pass to pack branches into VLIW instructions (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-vliw-branch
Do not run a pass to pack branches into VLIW instructions.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mmulti-cond-exec
Enable optimization of "&&" and "||" in conditional execution (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-multi-cond-exec
Disable optimization of "&&" and "||" in conditional execution.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mnested-cond-exec
Enable nested conditional execution optimizations (default).
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-mno-nested-cond-exec
Disable nested conditional execution optimizations.
This switch is mainly for debugging the compiler and will likely be removed in a future version.
-moptimize-membar
This switch removes redundant "membar" instructions from the compiler-generated code. It is enabled by default.
-mno-optimize-membar
This switch disables the automatic removal of redundant "membar" instructions from the generated code.
-mtomcat-stats
Cause gas to print out tomcat statistics.
-mcpu=cpu
Select the processor type for which to generate code. Possible values are frv, fr550, tomcat, fr500, fr450,
fr405, fr400, fr300 and simple.
GNU/Linux Options
These -m options are defined for GNU/Linux targets:
-mglibc
Use the GNU C library. This is the default except on *-*-linux-*uclibc*, *-*-linux-*musl* and
*-*-linux-*android* targets.
-muclibc
Use uClibc C library. This is the default on *-*-linux-*uclibc* targets.
-mmusl
Use the musl C library. This is the default on *-*-linux-*musl* targets.
-mbionic
Use Bionic C library. This is the default on *-*-linux-*android* targets.
-mandroid
Compile code compatible with Android platform. This is the default on *-*-linux-*android* targets.
When compiling, this option enables -mbionic, -fPIC, -fno-exceptions and -fno-rtti by default. When linking,
this option makes the GCC driver pass Android-specific options to the linker. Finally, this option causes the
preprocessor macro "__ANDROID__" to be defined.
-tno-android-cc
Disable compilation effects of -mandroid, i.e., do not enable -mbionic, -fPIC, -fno-exceptions and -fno-rtti by
default.
-tno-android-ld
Disable linking effects of -mandroid, i.e., pass standard Linux linking options to the linker.
H8/300 Options
These -m options are defined for the H8/300 implementations:
-mrelax
Shorten some address references at link time, when possible; uses the linker option -relax.
-mh Generate code for the H8/300H.
-ms Generate code for the H8S.
-mn Generate code for the H8S and H8/300H in the normal mode. This switch must be used either with -mh or -ms.
-ms2600
Generate code for the H8S/2600. This switch must be used with -ms.
-mexr
Extended registers are stored on stack before execution of function with monitor attribute. Default option is
-mexr. This option is valid only for H8S targets.
-mno-exr
Extended registers are not stored on stack before execution of function with monitor attribute. Default option is
-mno-exr. This option is valid only for H8S targets.
-mint32
Make "int" data 32 bits by default.
-malign-300
On the H8/300H and H8S, use the same alignment rules as for the H8/300. The default for the H8/300H and H8S is
to align longs and floats on 4-byte boundaries. -malign-300 causes them to be aligned on 2-byte boundaries.
This option has no effect on the H8/300.
HPPA Options
These -m options are defined for the HPPA family of computers:
-march=architecture-type
Generate code for the specified architecture. The choices for architecture-type are 1.0 for PA 1.0, 1.1 for PA
1.1, and 2.0 for PA 2.0 processors. Refer to /usr/lib/sched.models on an HP-UX system to determine the proper
architecture option for your machine. Code compiled for lower numbered architectures runs on higher numbered
architectures, but not the other way around.
-mpa-risc-1-0
-mpa-risc-1-1
-mpa-risc-2-0
Synonyms for -march=1.0, -march=1.1, and -march=2.0 respectively.
-matomic-libcalls
Generate libcalls for atomic loads and stores when sync libcalls are disabled. This option is enabled by
default. It only affects the generation of atomic libcalls by the HPPA backend.
Both the sync and libatomic libcall implementations use locking. As a result, processor stores are not atomic
with respect to other atomic operations. Processor loads up to DImode are atomic with respect to other atomic
operations provided they are implemented as a single access.
The PA-RISC architecture does not support any atomic operations in hardware except for the "ldcw" instruction.
Thus, all atomic support is implemented using sync and atomic libcalls. Sync libcall support is in libgcc.a.
Atomic libcall support is in libatomic.
This option generates "__atomic_exchange" calls for atomic stores. It also provides special handling for atomic
DImode accesses on 32-bit targets.
-mbig-switch
Does nothing. Preserved for backward compatibility.
-mcaller-copies
The caller copies function arguments passed by hidden reference. This option should be used with care as it is
not compatible with the default 32-bit runtime. However, only aggregates larger than eight bytes are passed by
hidden reference and the option provides better compatibility with OpenMP.
-mcoherent-ldcw
Use ldcw/ldcd coherent cache-control hint.
-mdisable-fpregs
Disable floating-point registers. Equivalent to "-msoft-float".
-mdisable-indexing
Prevent the compiler from using indexing address modes. This avoids some rather obscure problems when compiling
MIG generated code under MACH.
-mfast-indirect-calls
Generate code that assumes calls never cross space boundaries. This allows GCC to emit code that performs faster
indirect calls.
This option does not work in the presence of shared libraries or nested functions.
-mfixed-range=register-range
Generate code treating the given register range as fixed registers. A fixed register is one that the register
allocator cannot use. This is useful when compiling kernel code. A register range is specified as two registers
separated by a dash. Multiple register ranges can be specified separated by a comma.
-mgas
Enable the use of assembler directives only GAS understands.
-mgnu-ld
Use options specific to GNU ld. This passes -shared to ld when building a shared library. It is the default
when GCC is configured, explicitly or implicitly, with the GNU linker. This option does not affect which ld is
called; it only changes what parameters are passed to that ld. The ld that is called is determined by the
--with-ld configure option, GCC's program search path, and finally by the user's PATH. The linker used by GCC
can be printed using which `gcc -print-prog-name=ld`. This option is only available on the 64-bit HP-UX GCC,
i.e. configured with hppa*64*-*-hpux*.
-mhp-ld
Use options specific to HP ld. This passes -b to ld when building a shared library and passes +Accept
TypeMismatch to ld on all links. It is the default when GCC is configured, explicitly or implicitly, with the HP
linker. This option does not affect which ld is called; it only changes what parameters are passed to that ld.
The ld that is called is determined by the --with-ld configure option, GCC's program search path, and finally by
the user's PATH. The linker used by GCC can be printed using which `gcc -print-prog-name=ld`. This option is
only available on the 64-bit HP-UX GCC, i.e. configured with hppa*64*-*-hpux*.
-mlinker-opt
Enable the optimization pass in the HP-UX linker. Note this makes symbolic debugging impossible.
-mlong-calls
Generate code that uses long call sequences. This ensures that a call is always able to reach linker generated
stubs. The default is to generate long calls only when the distance from the call site to the beginning of the
function or translation unit, as the case may be, exceeds a predefined limit set by the branch type being used.
The limits for normal calls are 7,600,000 and 240,000 bytes, respectively for the PA 2.0 and PA 1.X
architectures. Sibcalls are always limited at 240,000 bytes.
Distances are measured from the beginning of functions when using the -ffunction-sections option, or when using
the -mgas and -mno-portable-runtime options together under HP-UX with the SOM linker.
It is normally not desirable to use this option as it degrades performance. However, it may be useful in large
applications, particularly when partial linking is used to build the application.
The types of long calls used depends on the capabilities of the assembler and linker, and the type of code being
generated. The impact on systems that support long absolute calls, and long pic symbol-difference or pc-relative
calls should be relatively small. However, an indirect call is used on 32-bit ELF systems in pic code and it is
quite long.
-mlong-load-store
Generate 3-instruction load and store sequences as sometimes required by the HP-UX 10 linker. This is equivalent
to the +k option to the HP compilers.
-mjump-in-delay
This option is ignored and provided for compatibility purposes only.
-mno-space-regs
Generate code that assumes the target has no space registers. This allows GCC to generate faster indirect calls
and use unscaled index address modes.
Such code is suitable for level 0 PA systems and kernels.
-mordered
Assume memory references are ordered and barriers are not needed.
-mportable-runtime
Use the portable calling conventions proposed by HP for ELF systems.
-mschedule=cpu-type
Schedule code according to the constraints for the machine type cpu-type. The choices for cpu-type are 700 7100,
7100LC, 7200, 7300 and 8000. Refer to /usr/lib/sched.models on an HP-UX system to determine the proper
scheduling option for your machine. The default scheduling is 8000.
-msio
Generate the predefine, "_SIO", for server IO. The default is -mwsio. This generates the predefines,
"__hp9000s700", "__hp9000s700__" and "_WSIO", for workstation IO. These options are available under HP-UX and
HI-UX.
-msoft-float
Generate output containing library calls for floating point. Warning: the requisite libraries are not available
for all HPPA targets. Normally the facilities of the machine's usual C compiler are used, but this cannot be
done directly in cross-compilation. You must make your own arrangements to provide suitable library functions
for cross-compilation.
-msoft-float changes the calling convention in the output file; therefore, it is only useful if you compile all
of a program with this option. In particular, you need to compile libgcc.a, the library that comes with GCC,
with -msoft-float in order for this to work.
-msoft-mult
Use software integer multiplication.
This disables the use of the "xmpyu" instruction.
-munix=unix-std
Generate compiler predefines and select a startfile for the specified UNIX standard. The choices for unix-std
are 93, 95 and 98. 93 is supported on all HP-UX versions. 95 is available on HP-UX 10.10 and later. 98 is
available on HP-UX 11.11 and later. The default values are 93 for HP-UX 10.00, 95 for HP-UX 10.10 though to
11.00, and 98 for HP-UX 11.11 and later.
-munix=93 provides the same predefines as GCC 3.3 and 3.4. -munix=95 provides additional predefines for
"XOPEN_UNIX" and "_XOPEN_SOURCE_EXTENDED", and the startfile unix95.o. -munix=98 provides additional predefines
for "_XOPEN_UNIX", "_XOPEN_SOURCE_EXTENDED", "_INCLUDE__STDC_A1_SOURCE" and "_INCLUDE_XOPEN_SOURCE_500", and the
startfile unix98.o.
It is important to note that this option changes the interfaces for various library routines. It also affects
the operational behavior of the C library. Thus, extreme care is needed in using this option.
Library code that is intended to operate with more than one UNIX standard must test, set and restore the variable
"__xpg4_extended_mask" as appropriate. Most GNU software doesn't provide this capability.
-nolibdld
Suppress the generation of link options to search libdld.sl when the -static option is specified on HP-UX 10 and
later.
-static
The HP-UX implementation of setlocale in libc has a dependency on libdld.sl. There isn't an archive version of
libdld.sl. Thus, when the -static option is specified, special link options are needed to resolve this
dependency.
On HP-UX 10 and later, the GCC driver adds the necessary options to link with libdld.sl when the -static option
is specified. This causes the resulting binary to be dynamic. On the 64-bit port, the linkers generate dynamic
binaries by default in any case. The -nolibdld option can be used to prevent the GCC driver from adding these
link options.
-threads
Add support for multithreading with the dce thread library under HP-UX. This option sets flags for both the
preprocessor and linker.
IA-64 Options
These are the -m options defined for the Intel IA-64 architecture.
-mbig-endian
Generate code for a big-endian target. This is the default for HP-UX.
-mlittle-endian
Generate code for a little-endian target. This is the default for AIX5 and GNU/Linux.
-mgnu-as
-mno-gnu-as
Generate (or don't) code for the GNU assembler. This is the default.
-mgnu-ld
-mno-gnu-ld
Generate (or don't) code for the GNU linker. This is the default.
-mno-pic
Generate code that does not use a global pointer register. The result is not position independent code, and
violates the IA-64 ABI.
-mvolatile-asm-stop
-mno-volatile-asm-stop
Generate (or don't) a stop bit immediately before and after volatile asm statements.
-mregister-names
-mno-register-names
Generate (or don't) in, loc, and out register names for the stacked registers. This may make assembler output
more readable.
-mno-sdata
-msdata
Disable (or enable) optimizations that use the small data section. This may be useful for working around
optimizer bugs.
-mconstant-gp
Generate code that uses a single constant global pointer value. This is useful when compiling kernel code.
-mauto-pic
Generate code that is self-relocatable. This implies -mconstant-gp. This is useful when compiling firmware
code.
-minline-float-divide-min-latency
Generate code for inline divides of floating-point values using the minimum latency algorithm.
-minline-float-divide-max-throughput
Generate code for inline divides of floating-point values using the maximum throughput algorithm.
-mno-inline-float-divide
Do not generate inline code for divides of floating-point values.
-minline-int-divide-min-latency
Generate code for inline divides of integer values using the minimum latency algorithm.
-minline-int-divide-max-throughput
Generate code for inline divides of integer values using the maximum throughput algorithm.
-mno-inline-int-divide
Do not generate inline code for divides of integer values.
-minline-sqrt-min-latency
Generate code for inline square roots using the minimum latency algorithm.
-minline-sqrt-max-throughput
Generate code for inline square roots using the maximum throughput algorithm.
-mno-inline-sqrt
Do not generate inline code for "sqrt".
-mfused-madd
-mno-fused-madd
Do (don't) generate code that uses the fused multiply/add or multiply/subtract instructions. The default is to
use these instructions.
-mno-dwarf2-asm
-mdwarf2-asm
Don't (or do) generate assembler code for the DWARF line number debugging info. This may be useful when not
using the GNU assembler.
-mearly-stop-bits
-mno-early-stop-bits
Allow stop bits to be placed earlier than immediately preceding the instruction that triggered the stop bit.
This can improve instruction scheduling, but does not always do so.
-mfixed-range=register-range
Generate code treating the given register range as fixed registers. A fixed register is one that the register
allocator cannot use. This is useful when compiling kernel code. A register range is specified as two registers
separated by a dash. Multiple register ranges can be specified separated by a comma.
-mtls-size=tls-size
Specify bit size of immediate TLS offsets. Valid values are 14, 22, and 64.
-mtune=cpu-type
Tune the instruction scheduling for a particular CPU, Valid values are itanium, itanium1, merced, itanium2, and
mckinley.
-milp32
-mlp64
Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer to 32 bits.
The 64-bit environment sets int to 32 bits and long and pointer to 64 bits. These are HP-UX specific flags.
-mno-sched-br-data-spec
-msched-br-data-spec
(Dis/En)able data speculative scheduling before reload. This results in generation of "ld.a" instructions and
the corresponding check instructions ("ld.c" / "chk.a"). The default setting is disabled.
-msched-ar-data-spec
-mno-sched-ar-data-spec
(En/Dis)able data speculative scheduling after reload. This results in generation of "ld.a" instructions and the
corresponding check instructions ("ld.c" / "chk.a"). The default setting is enabled.
-mno-sched-control-spec
-msched-control-spec
(Dis/En)able control speculative scheduling. This feature is available only during region scheduling (i.e.
before reload). This results in generation of the "ld.s" instructions and the corresponding check instructions
"chk.s". The default setting is disabled.
-msched-br-in-data-spec
-mno-sched-br-in-data-spec
(En/Dis)able speculative scheduling of the instructions that are dependent on the data speculative loads before
reload. This is effective only with -msched-br-data-spec enabled. The default setting is enabled.
-msched-ar-in-data-spec
-mno-sched-ar-in-data-spec
(En/Dis)able speculative scheduling of the instructions that are dependent on the data speculative loads after
reload. This is effective only with -msched-ar-data-spec enabled. The default setting is enabled.
-msched-in-control-spec
-mno-sched-in-control-spec
(En/Dis)able speculative scheduling of the instructions that are dependent on the control speculative loads.
This is effective only with -msched-control-spec enabled. The default setting is enabled.
-mno-sched-prefer-non-data-spec-insns
-msched-prefer-non-data-spec-insns
If enabled, data-speculative instructions are chosen for schedule only if there are no other choices at the
moment. This makes the use of the data speculation much more conservative. The default setting is disabled.
-mno-sched-prefer-non-control-spec-insns
-msched-prefer-non-control-spec-insns
If enabled, control-speculative instructions are chosen for schedule only if there are no other choices at the
moment. This makes the use of the control speculation much more conservative. The default setting is disabled.
-mno-sched-count-spec-in-critical-path
-msched-count-spec-in-critical-path
If enabled, speculative dependencies are considered during computation of the instructions priorities. This
makes the use of the speculation a bit more conservative. The default setting is disabled.
-msched-spec-ldc
Use a simple data speculation check. This option is on by default.
-msched-control-spec-ldc
Use a simple check for control speculation. This option is on by default.
-msched-stop-bits-after-every-cycle
Place a stop bit after every cycle when scheduling. This option is on by default.
-msched-fp-mem-deps-zero-cost
Assume that floating-point stores and loads are not likely to cause a conflict when placed into the same
instruction group. This option is disabled by default.
-msel-sched-dont-check-control-spec
Generate checks for control speculation in selective scheduling. This flag is disabled by default.
-msched-max-memory-insns=max-insns
Limit on the number of memory insns per instruction group, giving lower priority to subsequent memory insns
attempting to schedule in the same instruction group. Frequently useful to prevent cache bank conflicts. The
default value is 1.
-msched-max-memory-insns-hard-limit
Makes the limit specified by msched-max-memory-insns a hard limit, disallowing more than that number in an
instruction group. Otherwise, the limit is "soft", meaning that non-memory operations are preferred when the
limit is reached, but memory operations may still be scheduled.
LM32 Options
These -m options are defined for the LatticeMico32 architecture:
-mbarrel-shift-enabled
Enable barrel-shift instructions.
-mdivide-enabled
Enable divide and modulus instructions.
-mmultiply-enabled
Enable multiply instructions.
-msign-extend-enabled
Enable sign extend instructions.
-muser-enabled
Enable user-defined instructions.
LoongArch Options
These command-line options are defined for LoongArch targets:
-march=arch-type
Generate instructions for the machine type arch-type. -march=arch-type allows GCC to generate code that may not
run at all on processors other than the one indicated.
The choices for arch-type are:
native
Local processor type detected by the native compiler.
loongarch64
Generic LoongArch 64-bit processor.
la464
LoongArch LA464-based processor with LSX, LASX.
la664
LoongArch LA664-based processor with LSX, LASX and all LoongArch v1.1 instructions.
la64v1.0
LoongArch64 ISA version 1.0.
la64v1.1
LoongArch64 ISA version 1.1.
More information about LoongArch ISA versions can be found at
.
-mtune=tune-type
Optimize the generated code for the given processor target.
The choices for tune-type are:
native
Local processor type detected by the native compiler.
generic
Generic LoongArch processor.
loongarch64
Generic LoongArch 64-bit processor.
la464
LoongArch LA464 core.
la664
LoongArch LA664 core.
-mabi=base-abi-type
Generate code for the specified calling convention. base-abi-type can be one of:
lp64d
Uses 64-bit general purpose registers and 32/64-bit floating-point registers for parameter passing. Data
model is LP64, where int is 32 bits, while long int and pointers are 64 bits.
lp64f
Uses 64-bit general purpose registers and 32-bit floating-point registers for parameter passing. Data model
is LP64, where int is 32 bits, while long int and pointers are 64 bits.
lp64s
Uses 64-bit general purpose registers and no floating-point registers for parameter passing. Data model is
LP64, where int is 32 bits, while long int and pointers are 64 bits.
-mfpu=fpu-type
Generate code for the specified FPU type, which can be one of:
64 Allow the use of hardware floating-point instructions for 32-bit and 64-bit operations.
32 Allow the use of hardware floating-point instructions for 32-bit operations.
none
0 Prevent the use of hardware floating-point instructions.
-msimd=simd-type
Enable generation of LoongArch SIMD instructions for vectorization and via builtin functions. The value can be
one of:
lasx
Enable generating instructions from the 256-bit LoongArch Advanced SIMD Extension (LASX) and the 128-bit
LoongArch SIMD Extension (LSX).
lsx Enable generating instructions from the 128-bit LoongArch SIMD Extension (LSX).
none
No LoongArch SIMD instruction may be generated.
-msoft-float
Force -mfpu=none and prevents the use of floating-point registers for parameter passing. This option may change
the target ABI.
-msingle-float
Force -mfpu=32 and allow the use of 32-bit floating-point registers for parameter passing. This option may
change the target ABI.
-mdouble-float
Force -mfpu=64 and allow the use of 32/64-bit floating-point registers for parameter passing. This option may
change the target ABI.
-mlasx
-mno-lasx
-mlsx
-mno-lsx
Incrementally adjust the scope of the SIMD extensions (none / LSX / LASX) that can be used by the compiler for
code generation. Enabling LASX with mlasx automatically enables LSX, and diabling LSX with mno-lsx automatically
disables LASX. These driver-only options act upon the final msimd configuration state and make incremental
changes in the order they appear on the GCC driver's command line, deriving the final / canonicalized msimd
option that is passed to the compiler proper.
-mbranch-cost=n
Set the cost of branches to roughly n instructions.
-maddr-reg-reg-cost=n
Set the cost of ADDRESS_REG_REG to the value calculated by n.
-mcheck-zero-division
-mno-check-zero-divison
Trap (do not trap) on integer division by zero. The default is -mcheck-zero-division for -O0 or -Og, and
-mno-check-zero-division for other optimization levels.
-mcond-move-int
-mno-cond-move-int
Conditional moves for integral data in general-purpose registers are enabled (disabled). The default is
-mcond-move-int.
-mcond-move-float
-mno-cond-move-float
Conditional moves for floating-point registers are enabled (disabled). The default is -mcond-move-float.
-mmemcpy
-mno-memcpy
Force (do not force) the use of "memcpy" for non-trivial block moves. The default is -mno-memcpy, which allows
GCC to inline most constant-sized copies. Setting optimization level to -Os also forces the use of "memcpy", but
-mno-memcpy may override this behavior if explicitly specified, regardless of the order these options on the
command line.
-mstrict-align
-mno-strict-align
Avoid or allow generating memory accesses that may not be aligned on a natural object boundary as described in
the architecture specification. The default is -mno-strict-align.
-G num
Put global and static data smaller than num bytes into a small data section. The default value is 0.
-mmax-inline-memcpy-size=n
Inline all block moves (such as calls to "memcpy" or structure copies) less than or equal to n bytes. The
default value of n is 1024.
-mcmodel=code-model
Set the code model to one of:
tiny-static (Not implemented yet)
tiny (Not implemented yet)
normal
The text segment must be within 128MB addressing space. The data segment must be within 2GB addressing
space.
medium
The text segment and data segment must be within 2GB addressing space.
large (Not implemented yet)
extreme
This mode does not limit the size of the code segment and data segment. The -mcmodel=extreme option is
incompatible with -fplt and/or -mexplicit-relocs=none.
The default code model is "normal".
-mexplicit-relocs=style
Set when to use assembler relocation operators when dealing with symbolic addresses. The alternative is to use
assembler macros instead, which may limit instruction scheduling but allow linker relaxation. with
-mexplicit-relocs=none the assembler macros are always used, with -mexplicit-relocs=always the assembler
relocation operators are always used, with -mexplicit-relocs=auto the compiler will use the relocation operators
where the linker relaxation is impossible to improve the code quality, and macros elsewhere. The default value
for the option is determined with the assembler capability detected during GCC build-time and the setting of
-mrelax: -mexplicit-relocs=none if the assembler does not support relocation operators at all,
-mexplicit-relocs=always if the assembler supports relocation operators but -mrelax is not enabled,
-mexplicit-relocs=auto if the assembler supports relocation operators and -mrelax is enabled.
-mexplicit-relocs
An alias of -mexplicit-relocs=always for backward compatibility.
-mno-explicit-relocs
An alias of -mexplicit-relocs=none for backward compatibility.
-mdirect-extern-access
-mno-direct-extern-access
Do not use or use GOT to access external symbols. The default is -mno-direct-extern-access: GOT is used for
external symbols with default visibility, but not used for other external symbols.
With -mdirect-extern-access, GOT is not used and all external symbols are PC-relatively addressed. It is only
suitable for environments where no dynamic link is performed, like firmwares, OS kernels, executables linked with
-static or -static-pie. -mdirect-extern-access is not compatible with -fPIC or -fpic.
-mrelax
-mno-relax
Take (do not take) advantage of linker relaxations. If -mpass-mrelax-to-as is enabled, this option is also
passed to the assembler. The default is determined during GCC build-time by detecting corresponding assembler
support: -mrelax if the assembler supports both the -mrelax option and the conditional branch relaxation (it's
required or the ".align" directives and conditional branch instructions in the assembly code outputted by GCC may
be rejected by the assembler because of a relocation overflow), -mno-relax otherwise.
-mpass-mrelax-to-as
-mno-pass-mrelax-to-as
Pass (do not pass) the -mrelax or -mno-relax option to the assembler. The default is determined during GCC
build-time by detecting corresponding assembler support: -mpass-mrelax-to-as if the assembler supports the
-mrelax option, -mno-pass-mrelax-to-as otherwise. This option is mostly useful for debugging, or interoperation
with assemblers different from the build-time one.
-mrecip
This option enables use of the reciprocal estimate and reciprocal square root estimate instructions with
additional Newton-Raphson steps to increase precision instead of doing a divide or square root and divide for
floating-point arguments. These instructions are generated only when -funsafe-math-optimizations is enabled
together with -ffinite-math-only and -fno-trapping-math. This option is off by default. Before you can use this
option, you must sure the target CPU supports frecipe and frsqrte instructions. Note that while the throughput
of the sequence is higher than the throughput of the non-reciprocal instruction, the precision of the sequence
can be decreased by up to 2 ulp (i.e. the inverse of 1.0 equals 0.99999994).
-mrecip=opt
This option controls which reciprocal estimate instructions may be used. opt is a comma-separated list of
options, which may be preceded by a ! to invert the option:
all Enable all estimate instructions.
default
Enable the default instructions, equivalent to -mrecip.
none
Disable all estimate instructions, equivalent to -mno-recip.
div Enable the approximation for scalar division.
vec-div
Enable the approximation for vectorized division.
sqrt
Enable the approximation for scalar square root.
vec-sqrt
Enable the approximation for vectorized square root.
rsqrt
Enable the approximation for scalar reciprocal square root.
vec-rsqrt
Enable the approximation for vectorized reciprocal square root.
So, for example, -mrecip=all,!sqrt enables all of the reciprocal approximations, except for scalar square root.
-mfrecipe
-mno-frecipe
Use (do not use) "frecipe.{s/d}" and "frsqrte.{s/d}" instructions. When build with -march=la664, it is enabled
by default. The default is -mno-frecipe.
-mdiv32
-mno-div32
Use (do not use) "div.w[u]" and "mod.w[u]" instructions with input not sign-extended. When build with
-march=la664, it is enabled by default. The default is -mno-div32.
-mlam-bh
-mno-lam-bh
Use (do not use) "am{swap/add}[_db].{b/h}" instructions. When build with -march=la664, it is enabled by default.
The default is -mno-lam-bh.
-mlamcas
-mno-lamcas
Use (do not use) "amcas[_db].{b/h/w/d}" instructions. When build with -march=la664, it is enabled by default.
The default is -mno-lamcas.
-mld-seq-sa
-mno-ld-seq-sa
Whether a same-address load-load barrier ("dbar 0x700") is needed. When build with -march=la664, it is enabled
by default. The default is -mno-ld-seq-sa, the load-load barrier is needed.
-mtls-dialect=opt
This option controls which tls dialect may be used for general dynamic and local dynamic TLS models.
trad
Use traditional TLS. This is the default.
desc
Use TLS descriptors.
-mannotate-tablejump
-mno-annotate-tablejump
Create an annotation section ".discard.tablejump_annotate" to correlate the "jirl" instruction and the jump table
when a jump table is used to optimize the "switch" statement. Some external tools, for example objtool of the
Linux kernel building system, need the annotation to analysis the control flow. The default is
-mno-annotate-tablejump.
--param loongarch-vect-unroll-limit=n
The vectorizer will use available tuning information to determine whether it would be beneficial to unroll the
main vectorized loop and by how much. This parameter set's the upper bound of how much the vectorizer will
unroll the main loop. The default value is six.
M32C Options
-mcpu=name
Select the CPU for which code is generated. name may be one of r8c for the R8C/Tiny series, m16c for the M16C
(up to /60) series, m32cm for the M16C/80 series, or m32c for the M32C/80 series.
-msim
Specifies that the program will be run on the simulator. This causes an alternate runtime library to be linked
in which supports, for example, file I/O. You must not use this option when generating programs that will run on
real hardware; you must provide your own runtime library for whatever I/O functions are needed.
-memregs=number
Specifies the number of memory-based pseudo-registers GCC uses during code generation. These pseudo-registers
are used like real registers, so there is a tradeoff between GCC's ability to fit the code into available
registers, and the performance penalty of using memory instead of registers. Note that all modules in a program
must be compiled with the same value for this option. Because of that, you must not use this option with GCC's
default runtime libraries.
M32R/D Options
These -m options are defined for Renesas M32R/D architectures:
-m32r2
Generate code for the M32R/2.
-m32rx
Generate code for the M32R/X.
-m32r
Generate code for the M32R. This is the default.
-mmodel=small
Assume all objects live in the lower 16MB of memory (so that their addresses can be loaded with the "ld24"
instruction), and assume all subroutines are reachable with the "bl" instruction. This is the default.
The addressability of a particular object can be set with the "model" attribute.
-mmodel=medium
Assume objects may be anywhere in the 32-bit address space (the compiler generates "seth/add3" instructions to
load their addresses), and assume all subroutines are reachable with the "bl" instruction.
-mmodel=large
Assume objects may be anywhere in the 32-bit address space (the compiler generates "seth/add3" instructions to
load their addresses), and assume subroutines may not be reachable with the "bl" instruction (the compiler
generates the much slower "seth/add3/jl" instruction sequence).
-msdata=none
Disable use of the small data area. Variables are put into one of ".data", ".bss", or ".rodata" (unless the
"section" attribute has been specified). This is the default.
The small data area consists of sections ".sdata" and ".sbss". Objects may be explicitly put in the small data
area with the "section" attribute using one of these sections.
-msdata=sdata
Put small global and static data in the small data area, but do not generate special code to reference them.
-msdata=use
Put small global and static data in the small data area, and generate special instructions to reference them.
-G num
Put global and static objects less than or equal to num bytes into the small data or BSS sections instead of the
normal data or BSS sections. The default value of num is 8. The -msdata option must be set to one of sdata or
use for this option to have any effect.
All modules should be compiled with the same -G num value. Compiling with different values of num may or may not
work; if it doesn't the linker gives an error message---incorrect code is not generated.
-mdebug
Makes the M32R-specific code in the compiler display some statistics that might help in debugging programs.
-malign-loops
Align all loops to a 32-byte boundary.
-mno-align-loops
Do not enforce a 32-byte alignment for loops. This is the default.
-missue-rate=number
Issue number instructions per cycle. number can only be 1 or 2.
-mbranch-cost=number
number can only be 1 or 2. If it is 1 then branches are preferred over conditional code, if it is 2, then the
opposite applies.
-mflush-trap=number
Specifies the trap number to use to flush the cache. The default is 12. Valid numbers are between 0 and 15
inclusive.
-mno-flush-trap
Specifies that the cache cannot be flushed by using a trap.
-mflush-func=name
Specifies the name of the operating system function to call to flush the cache. The default is _flush_cache, but
a function call is only used if a trap is not available.
-mno-flush-func
Indicates that there is no OS function for flushing the cache.
M680x0 Options
These are the -m options defined for M680x0 and ColdFire processors. The default settings depend on which
architecture was selected when the compiler was configured; the defaults for the most common choices are given below.
-march=arch
Generate code for a specific M680x0 or ColdFire instruction set architecture. Permissible values of arch for
M680x0 architectures are: 68000, 68010, 68020, 68030, 68040, 68060 and cpu32. ColdFire architectures are
selected according to Freescale's ISA classification and the permissible values are: isaa, isaaplus, isab and
isac.
GCC defines a macro "__mcfarch__" whenever it is generating code for a ColdFire target. The arch in this macro
is one of the -march arguments given above.
When used together, -march and -mtune select code that runs on a family of similar processors but that is
optimized for a particular microarchitecture.
-mcpu=cpu
Generate code for a specific M680x0 or ColdFire processor. The M680x0 cpus are: 68000, 68010, 68020, 68030,
68040, 68060, 68302, 68332 and cpu32. The ColdFire cpus are given by the table below, which also classifies the
CPUs into families:
Family : -mcpu arguments
51 : 51 51ac 51ag 51cn 51em 51je 51jf 51jg 51jm 51mm 51qe 51qm
5206 : 5202 5204 5206
5206e : 5206e
5208 : 5207 5208
5211a : 5210a 5211a
5213 : 5211 5212 5213
5216 : 5214 5216
52235 : 52230 52231 52232 52233 52234 52235
5225 : 5224 5225
52259 : 52252 52254 52255 52256 52258 52259
5235 : 5232 5233 5234 5235 523x
5249 : 5249
5250 : 5250
5271 : 5270 5271
5272 : 5272
5275 : 5274 5275
5282 : 5280 5281 5282 528x
53017 : 53011 53012 53013 53014 53015 53016 53017
5307 : 5307
5329 : 5327 5328 5329 532x
5373 : 5372 5373 537x
5407 : 5407
5475 : 5470 5471 5472 5473 5474 5475 547x 5480 5481 5482 5483 5484 5485
-mcpu=cpu overrides -march=arch if arch is compatible with cpu. Other combinations of -mcpu and -march are
rejected.
GCC defines the macro "__mcf_cpu_cpu" when ColdFire target cpu is selected. It also defines
"__mcf_family_family", where the value of family is given by the table above.
-mtune=tune
Tune the code for a particular microarchitecture within the constraints set by -march and -mcpu. The M680x0
microarchitectures are: 68000, 68010, 68020, 68030, 68040, 68060 and cpu32. The ColdFire microarchitectures are:
cfv1, cfv2, cfv3, cfv4 and cfv4e.
You can also use -mtune=68020-40 for code that needs to run relatively well on 68020, 68030 and 68040 targets.
-mtune=68020-60 is similar but includes 68060 targets as well. These two options select the same tuning
decisions as -m68020-40 and -m68020-60 respectively.
GCC defines the macros "__mcarch" and "__mcarch__" when tuning for 680x0 architecture arch. It also defines
"mcarch" unless either -ansi or a non-GNU -std option is used. If GCC is tuning for a range of architectures, as
selected by -mtune=68020-40 or -mtune=68020-60, it defines the macros for every architecture in the range.
GCC also defines the macro "__muarch__" when tuning for ColdFire microarchitecture uarch, where uarch is one of
the arguments given above.
-m68000
-mc68000
Generate output for a 68000. This is the default when the compiler is configured for 68000-based systems. It is
equivalent to -march=68000.
Use this option for microcontrollers with a 68000 or EC000 core, including the 68008, 68302, 68306, 68307, 68322,
68328 and 68356.
-m68010
Generate output for a 68010. This is the default when the compiler is configured for 68010-based systems. It is
equivalent to -march=68010.
-m68020
-mc68020
Generate output for a 68020. This is the default when the compiler is configured for 68020-based systems. It is
equivalent to -march=68020.
-m68030
Generate output for a 68030. This is the default when the compiler is configured for 68030-based systems. It is
equivalent to -march=68030.
-m68040
Generate output for a 68040. This is the default when the compiler is configured for 68040-based systems. It is
equivalent to -march=68040.
This option inhibits the use of 68881/68882 instructions that have to be emulated by software on the 68040. Use
this option if your 68040 does not have code to emulate those instructions.
-m68060
Generate output for a 68060. This is the default when the compiler is configured for 68060-based systems. It is
equivalent to -march=68060.
This option inhibits the use of 68020 and 68881/68882 instructions that have to be emulated by software on the
68060. Use this option if your 68060 does not have code to emulate those instructions.
-mcpu32
Generate output for a CPU32. This is the default when the compiler is configured for CPU32-based systems. It is
equivalent to -march=cpu32.
Use this option for microcontrollers with a CPU32 or CPU32+ core, including the 68330, 68331, 68332, 68333,
68334, 68336, 68340, 68341, 68349 and 68360.
-m5200
Generate output for a 520X ColdFire CPU. This is the default when the compiler is configured for 520X-based
systems. It is equivalent to -mcpu=5206, and is now deprecated in favor of that option.
Use this option for microcontroller with a 5200 core, including the MCF5202, MCF5203, MCF5204 and MCF5206.
-m5206e
Generate output for a 5206e ColdFire CPU. The option is now deprecated in favor of the equivalent -mcpu=5206e.
-m528x
Generate output for a member of the ColdFire 528X family. The option is now deprecated in favor of the
equivalent -mcpu=528x.
-m5307
Generate output for a ColdFire 5307 CPU. The option is now deprecated in favor of the equivalent -mcpu=5307.
-m5407
Generate output for a ColdFire 5407 CPU. The option is now deprecated in favor of the equivalent -mcpu=5407.
-mcfv4e
Generate output for a ColdFire V4e family CPU (e.g. 547x/548x). This includes use of hardware floating-point
instructions. The option is equivalent to -mcpu=547x, and is now deprecated in favor of that option.
-m68020-40
Generate output for a 68040, without using any of the new instructions. This results in code that can run
relatively efficiently on either a 68020/68881 or a 68030 or a 68040. The generated code does use the 68881
instructions that are emulated on the 68040.
The option is equivalent to -march=68020 -mtune=68020-40.
-m68020-60
Generate output for a 68060, without using any of the new instructions. This results in code that can run
relatively efficiently on either a 68020/68881 or a 68030 or a 68040. The generated code does use the 68881
instructions that are emulated on the 68060.
The option is equivalent to -march=68020 -mtune=68020-60.
-mhard-float
-m68881
Generate floating-point instructions. This is the default for 68020 and above, and for ColdFire devices that
have an FPU. It defines the macro "__HAVE_68881__" on M680x0 targets and "__mcffpu__" on ColdFire targets.
-msoft-float
Do not generate floating-point instructions; use library calls instead. This is the default for 68000, 68010,
and 68832 targets. It is also the default for ColdFire devices that have no FPU.
-mdiv
-mno-div
Generate (do not generate) ColdFire hardware divide and remainder instructions. If -march is used without -mcpu,
the default is "on" for ColdFire architectures and "off" for M680x0 architectures. Otherwise, the default is
taken from the target CPU (either the default CPU, or the one specified by -mcpu). For example, the default is
"off" for -mcpu=5206 and "on" for -mcpu=5206e.
GCC defines the macro "__mcfhwdiv__" when this option is enabled.
-mshort
Consider type "int" to be 16 bits wide, like "short int". Additionally, parameters passed on the stack are also
aligned to a 16-bit boundary even on targets whose API mandates promotion to 32-bit.
-mno-short
Do not consider type "int" to be 16 bits wide. This is the default.
-mnobitfield
-mno-bitfield
Do not use the bit-field instructions. The -m68000, -mcpu32 and -m5200 options imply -mnobitfield.
-mbitfield
Do use the bit-field instructions. The -m68020 option implies -mbitfield. This is the default if you use a
configuration designed for a 68020.
-mrtd
Use a different function-calling convention, in which functions that take a fixed number of arguments return with
the "rtd" instruction, which pops their arguments while returning. This saves one instruction in the caller
since there is no need to pop the arguments there.
This calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to
call libraries compiled with the Unix compiler.
Also, you must provide function prototypes for all functions that take variable numbers of arguments (including
"printf"); otherwise incorrect code is generated for calls to those functions.
In addition, seriously incorrect code results if you call a function with too many arguments. (Normally, extra
arguments are harmlessly ignored.)
The "rtd" instruction is supported by the 68010, 68020, 68030, 68040, 68060 and CPU32 processors, but not by the
68000 or 5200.
The default is -mno-rtd.
-malign-int
-mno-align-int
Control whether GCC aligns "int", "long", "long long", "float", "double", and "long double" variables on a 32-bit
boundary (-malign-int) or a 16-bit boundary (-mno-align-int). Aligning variables on 32-bit boundaries produces
code that runs somewhat faster on processors with 32-bit busses at the expense of more memory.
Warning: if you use the -malign-int switch, GCC aligns structures containing the above types differently than
most published application binary interface specifications for the m68k.
Use the pc-relative addressing mode of the 68000 directly, instead of using a global offset table. At present,
this option implies -fpic, allowing at most a 16-bit offset for pc-relative addressing. -fPIC is not presently
supported with -mpcrel, though this could be supported for 68020 and higher processors.
-mno-strict-align
-mstrict-align
Do not (do) assume that unaligned memory references are handled by the system.
-msep-data
Generate code that allows the data segment to be located in a different area of memory from the text segment.
This allows for execute-in-place in an environment without virtual memory management. This option implies -fPIC.
-mno-sep-data
Generate code that assumes that the data segment follows the text segment. This is the default.
-mid-shared-library
Generate code that supports shared libraries via the library ID method. This allows for execute-in-place and
shared libraries in an environment without virtual memory management. This option implies -fPIC.
-mno-id-shared-library
Generate code that doesn't assume ID-based shared libraries are being used. This is the default.
-mshared-library-id=n
Specifies the identification number of the ID-based shared library being compiled. Specifying a value of 0
generates more compact code; specifying other values forces the allocation of that number to the current library,
but is no more space- or time-efficient than omitting this option.
-mxgot
-mno-xgot
When generating position-independent code for ColdFire, generate code that works if the GOT has more than 8192
entries. This code is larger and slower than code generated without this option. On M680x0 processors, this
option is not needed; -fPIC suffices.
GCC normally uses a single instruction to load values from the GOT. While this is relatively efficient, it only
works if the GOT is smaller than about 64k. Anything larger causes the linker to report an error such as:
relocation truncated to fit: R_68K_GOT16O foobar
If this happens, you should recompile your code with -mxgot. It should then work with very large GOTs. However,
code generated with -mxgot is less efficient, since it takes 4 instructions to fetch the value of a global
symbol.
Note that some linkers, including newer versions of the GNU linker, can create multiple GOTs and sort GOT
entries. If you have such a linker, you should only need to use -mxgot when compiling a single object file that
accesses more than 8192 GOT entries. Very few do.
These options have no effect unless GCC is generating position-independent code.
-mlong-jump-table-offsets
Use 32-bit offsets in "switch" tables. The default is to use 16-bit offsets.
MCore Options
These are the -m options defined for the Motorola M*Core processors.
-mhardlit
-mno-hardlit
Inline constants into the code stream if it can be done in two instructions or less.
-mdiv
-mno-div
Use the divide instruction. (Enabled by default).
-mrelax-immediate
-mno-relax-immediate
Allow arbitrary-sized immediates in bit operations.
-mwide-bitfields
-mno-wide-bitfields
Always treat bit-fields as "int"-sized.
-m4byte-functions
-mno-4byte-functions
Force all functions to be aligned to a 4-byte boundary.
-mcallgraph-data
-mno-callgraph-data
Emit callgraph information.
-mslow-bytes
-mno-slow-bytes
Prefer word access when reading byte quantities.
-mlittle-endian
-mbig-endian
Generate code for a little-endian target.
-m210
-m340
Generate code for the 210 processor.
-mno-lsim
Assume that runtime support has been provided and so omit the simulator library (libsim.a) from the linker
command line.
-mstack-increment=size
Set the maximum amount for a single stack increment operation. Large values can increase the speed of programs
that contain functions that need a large amount of stack space, but they can also trigger a segmentation fault if
the stack is extended too much. The default value is 0x1000.
MicroBlaze Options
-msoft-float
Use software emulation for floating point (default).
-mhard-float
Use hardware floating-point instructions.
-mmemcpy
Do not optimize block moves, use "memcpy".
-mno-clearbss
This option is deprecated. Use -fno-zero-initialized-in-bss instead.
-mcpu=cpu-type
Use features of, and schedule code for, the given CPU. Supported values are in the format vX.YY.Z, where X is a
major version, YY is the minor version, and Z is compatibility code. Example values are v3.00.a, v4.00.b,
v5.00.a, v5.00.b, v6.00.a.
-mxl-soft-mul
Use software multiply emulation (default).
-mxl-soft-div
Use software emulation for divides (default).
-mxl-barrel-shift
Use the hardware barrel shifter.
-mxl-pattern-compare
Use pattern compare instructions.
-msmall-divides
Use table lookup optimization for small signed integer divisions.
-mxl-stack-check
This option is deprecated. Use -fstack-check instead.
-mxl-gp-opt
Use GP-relative ".sdata"/".sbss" sections.
-mxl-multiply-high
Use multiply high instructions for high part of 32x32 multiply.
-mxl-float-convert
Use hardware floating-point conversion instructions.
-mxl-float-sqrt
Use hardware floating-point square root instruction.
-mbig-endian
Generate code for a big-endian target.
-mlittle-endian
Generate code for a little-endian target.
-mxl-reorder
Use reorder instructions (swap and byte reversed load/store).
-mxl-mode-app-model
Select application model app-model. Valid models are
executable
normal executable (default), uses startup code crt0.o.
xmdstub
for use with Xilinx Microprocessor Debugger (XMD) based software intrusive debug agent called xmdstub. This
uses startup file crt1.o and sets the start address of the program to 0x800.
bootstrap
for applications that are loaded using a bootloader. This model uses startup file crt2.o which does not
contain a processor reset vector handler. This is suitable for transferring control on a processor reset to
the bootloader rather than the application.
novectors
for applications that do not require any of the MicroBlaze vectors. This option may be useful for
applications running within a monitoring application. This model uses crt3.o as a startup file.
Option -xl-mode-app-model is a deprecated alias for -mxl-mode-app-model.
-mpic-data-is-text-relative
Assume that the displacement between the text and data segments is fixed at static link time. This allows data
to be referenced by offset from start of text address instead of GOT since PC-relative addressing is not
supported.
MIPS Options
-EB Generate big-endian code.
-EL Generate little-endian code. This is the default for mips*el-*-* configurations.
-march=arch
Generate code that runs on arch, which can be the name of a generic MIPS ISA, or the name of a particular
processor. The ISA names are: mips1, mips2, mips3, mips4, mips32, mips32r2, mips32r3, mips32r5, mips32r6,
mips64, mips64r2, mips64r3, mips64r5 and mips64r6. The processor names are: 4kc, 4km, 4kp, 4ksc, 4kec, 4kem,
4kep, 4ksd, 5kc, 5kf, 20kc, 24kc, 24kf2_1, 24kf1_1, 24kec, 24kef2_1, 24kef1_1, 34kc, 34kf2_1, 34kf1_1, 34kn,
74kc, 74kf2_1, 74kf1_1, 74kf3_2, 1004kc, 1004kf2_1, 1004kf1_1, i6400, i6500, interaptiv, loongson2e, loongson2f,
loongson3a, gs464, gs464e, gs264e, m4k, m14k, m14kc, m14ke, m14kec, m5100, m5101, octeon, octeon+, octeon2,
octeon3, orion, p5600, p6600, r2000, r3000, r3900, r4000, r4400, r4600, r4650, r4700, r5900, r6000, r8000,
rm7000, rm9000, r10000, r12000, r14000, r16000, sb1, sr71000, vr4100, vr4111, vr4120, vr4130, vr4300, vr5000,
vr5400, vr5500, xlr and xlp. The special value from-abi selects the most compatible architecture for the
selected ABI (that is, mips1 for 32-bit ABIs and mips3 for 64-bit ABIs).
The native Linux/GNU toolchain also supports the value native, which selects the best architecture option for the
host processor. -march=native has no effect if GCC does not recognize the processor.
In processor names, a final 000 can be abbreviated as k (for example, -march=r2k). Prefixes are optional, and vr
may be written r.
Names of the form nf2_1 refer to processors with FPUs clocked at half the rate of the core, names of the form
nf1_1 refer to processors with FPUs clocked at the same rate as the core, and names of the form nf3_2 refer to
processors with FPUs clocked a ratio of 3:2 with respect to the core. For compatibility reasons, nf is accepted
as a synonym for nf2_1 while nx and bfx are accepted as synonyms for nf1_1.
GCC defines two macros based on the value of this option. The first is "_MIPS_ARCH", which gives the name of
target architecture, as a string. The second has the form "_MIPS_ARCH_foo", where foo is the capitalized value
of "_MIPS_ARCH". For example, -march=r2000 sets "_MIPS_ARCH" to "r2000" and defines the macro
"_MIPS_ARCH_R2000".
Note that the "_MIPS_ARCH" macro uses the processor names given above. In other words, it has the full prefix
and does not abbreviate 000 as k. In the case of from-abi, the macro names the resolved architecture (either
"mips1" or "mips3"). It names the default architecture when no -march option is given.
-mtune=arch
Optimize for arch. Among other things, this option controls the way instructions are scheduled, and the
perceived cost of arithmetic operations. The list of arch values is the same as for -march.
When this option is not used, GCC optimizes for the processor specified by -march. By using -march and -mtune
together, it is possible to generate code that runs on a family of processors, but optimize the code for one
particular member of that family.
-mtune defines the macros "_MIPS_TUNE" and "_MIPS_TUNE_foo", which work in the same way as the -march ones
described above.
-mips1
Equivalent to -march=mips1.
-mips2
Equivalent to -march=mips2.
-mips3
Equivalent to -march=mips3.
-mips4
Equivalent to -march=mips4.
-mips32
Equivalent to -march=mips32.
-mips32r3
Equivalent to -march=mips32r3.
-mips32r5
Equivalent to -march=mips32r5.
-mips32r6
Equivalent to -march=mips32r6.
-mips64
Equivalent to -march=mips64.
-mips64r2
Equivalent to -march=mips64r2.
-mips64r3
Equivalent to -march=mips64r3.
-mips64r5
Equivalent to -march=mips64r5.
-mips64r6
Equivalent to -march=mips64r6.
-mips16
-mno-mips16
Generate (do not generate) MIPS16 code. If GCC is targeting a MIPS32 or MIPS64 architecture, it makes use of the
MIPS16e ASE.
MIPS16 code generation can also be controlled on a per-function basis by means of "mips16" and "nomips16"
attributes.
-mmips16e2
-mno-mips16e2
Use (do not use) the MIPS16e2 ASE. This option modifies the behavior of the -mips16 option such that it targets
the MIPS16e2 ASE.
-mflip-mips16
Generate MIPS16 code on alternating functions. This option is provided for regression testing of mixed
MIPS16/non-MIPS16 code generation, and is not intended for ordinary use in compiling user code.
-minterlink-compressed
-mno-interlink-compressed
Require (do not require) that code using the standard (uncompressed) MIPS ISA be link-compatible with MIPS16 and
microMIPS code, and vice versa.
For example, code using the standard ISA encoding cannot jump directly to MIPS16 or microMIPS code; it must
either use a call or an indirect jump. -minterlink-compressed therefore disables direct jumps unless GCC knows
that the target of the jump is not compressed.
-minterlink-mips16
-mno-interlink-mips16
Aliases of -minterlink-compressed and -mno-interlink-compressed. These options predate the microMIPS ASE and are
retained for backwards compatibility.
-mabi=32
-mabi=o64
-mabi=n32
-mabi=64
-mabi=eabi
Generate code for the given ABI.
Note that the EABI has a 32-bit and a 64-bit variant. GCC normally generates 64-bit code when you select a
64-bit architecture, but you can use -mgp32 to get 32-bit code instead.
For information about the O64 ABI, see .
GCC supports a variant of the o32 ABI in which floating-point registers are 64 rather than 32 bits wide. You can
select this combination with -mabi=32 -mfp64. This ABI relies on the "mthc1" and "mfhc1" instructions and is
therefore only supported for MIPS32R2, MIPS32R3 and MIPS32R5 processors.
The register assignments for arguments and return values remain the same, but each scalar value is passed in a
single 64-bit register rather than a pair of 32-bit registers. For example, scalar floating-point values are
returned in $f0 only, not a $f0/$f1 pair. The set of call-saved registers also remains the same in that the
even-numbered double-precision registers are saved.
Two additional variants of the o32 ABI are supported to enable a transition from 32-bit to 64-bit registers.
These are FPXX (-mfpxx) and FP64A (-mfp64 -mno-odd-spreg). The FPXX extension mandates that all code must
execute correctly when run using 32-bit or 64-bit registers. The code can be interlinked with either FP32 or
FP64, but not both. The FP64A extension is similar to the FP64 extension but forbids the use of odd-numbered
single-precision registers. This can be used in conjunction with the "FRE" mode of FPUs in MIPS32R5 processors
and allows both FP32 and FP64A code to interlink and run in the same process without changing FPU modes.
-mabicalls
-mno-abicalls
Generate (do not generate) code that is suitable for SVR4-style dynamic objects. -mabicalls is the default for
SVR4-based systems.
-mshared
-mno-shared
Generate (do not generate) code that is fully position-independent, and that can therefore be linked into shared
libraries. This option only affects -mabicalls.
All -mabicalls code has traditionally been position-independent, regardless of options like -fPIC and -fpic.
However, as an extension, the GNU toolchain allows executables to use absolute accesses for locally-binding
symbols. It can also use shorter GP initialization sequences and generate direct calls to locally-defined
functions. This mode is selected by -mno-shared.
-mno-shared depends on binutils 2.16 or higher and generates objects that can only be linked by the GNU linker.
However, the option does not affect the ABI of the final executable; it only affects the ABI of relocatable
objects. Using -mno-shared generally makes executables both smaller and quicker.
-mshared is the default.
-mplt
-mno-plt
Assume (do not assume) that the static and dynamic linkers support PLTs and copy relocations. This option only
affects -mno-shared -mabicalls. For the n64 ABI, this option has no effect without -msym32.
You can make -mplt the default by configuring GCC with --with-mips-plt. The default is -mno-plt otherwise.
-mxgot
-mno-xgot
Lift (do not lift) the usual restrictions on the size of the global offset table.
GCC normally uses a single instruction to load values from the GOT. While this is relatively efficient, it only
works if the GOT is smaller than about 64k. Anything larger causes the linker to report an error such as:
relocation truncated to fit: R_MIPS_GOT16 foobar
If this happens, you should recompile your code with -mxgot. This works with very large GOTs, although the code
is also less efficient, since it takes three instructions to fetch the value of a global symbol.
Note that some linkers can create multiple GOTs. If you have such a linker, you should only need to use -mxgot
when a single object file accesses more than 64k's worth of GOT entries. Very few do.
These options have no effect unless GCC is generating position independent code.
-mgp32
Assume that general-purpose registers are 32 bits wide.
-mgp64
Assume that general-purpose registers are 64 bits wide.
-mfp32
Assume that floating-point registers are 32 bits wide.
-mfp64
Assume that floating-point registers are 64 bits wide.
-mfpxx
Do not assume the width of floating-point registers.
-mhard-float
Use floating-point coprocessor instructions.
-msoft-float
Do not use floating-point coprocessor instructions. Implement floating-point calculations using library calls
instead.
-mno-float
Equivalent to -msoft-float, but additionally asserts that the program being compiled does not perform any
floating-point operations. This option is presently supported only by some bare-metal MIPS configurations, where
it may select a special set of libraries that lack all floating-point support (including, for example, the
floating-point "printf" formats). If code compiled with -mno-float accidentally contains floating-point
operations, it is likely to suffer a link-time or run-time failure.
-msingle-float
Assume that the floating-point coprocessor only supports single-precision operations.
-mdouble-float
Assume that the floating-point coprocessor supports double-precision operations. This is the default.
-modd-spreg
-mno-odd-spreg
Enable the use of odd-numbered single-precision floating-point registers for the o32 ABI. This is the default
for processors that are known to support these registers. When using the o32 FPXX ABI, -mno-odd-spreg is set by
default.
-mabs=2008
-mabs=legacy
These options control the treatment of the special not-a-number (NaN) IEEE 754 floating-point data with the
"abs.fmt" and "neg.fmt" machine instructions.
By default or when -mabs=legacy is used the legacy treatment is selected. In this case these instructions are
considered arithmetic and avoided where correct operation is required and the input operand might be a NaN. A
longer sequence of instructions that manipulate the sign bit of floating-point datum manually is used instead
unless the -ffinite-math-only option has also been specified.
The -mabs=2008 option selects the IEEE 754-2008 treatment. In this case these instructions are considered non-
arithmetic and therefore operating correctly in all cases, including in particular where the input operand is a
NaN. These instructions are therefore always used for the respective operations.
-mnan=2008
-mnan=legacy
These options control the encoding of the special not-a-number (NaN) IEEE 754 floating-point data.
The -mnan=legacy option selects the legacy encoding. In this case quiet NaNs (qNaNs) are denoted by the first
bit of their trailing significand field being 0, whereas signaling NaNs (sNaNs) are denoted by the first bit of
their trailing significand field being 1.
The -mnan=2008 option selects the IEEE 754-2008 encoding. In this case qNaNs are denoted by the first bit of
their trailing significand field being 1, whereas sNaNs are denoted by the first bit of their trailing
significand field being 0.
The default is -mnan=legacy unless GCC has been configured with --with-nan=2008.
-mllsc
-mno-llsc
Use (do not use) ll, sc, and sync instructions to implement atomic memory built-in functions. When neither
option is specified, GCC uses the instructions if the target architecture supports them.
-mllsc is useful if the runtime environment can emulate the instructions and -mno-llsc can be useful when
compiling for nonstandard ISAs. You can make either option the default by configuring GCC with --with-llsc and
--without-llsc respectively. --with-llsc is the default for some configurations; see the installation
documentation for details.
-mdsp
-mno-dsp
Use (do not use) revision 1 of the MIPS DSP ASE.
This option defines the preprocessor macro "__mips_dsp". It also defines "__mips_dsp_rev" to 1.
-mdspr2
-mno-dspr2
Use (do not use) revision 2 of the MIPS DSP ASE.
This option defines the preprocessor macros "__mips_dsp" and "__mips_dspr2". It also defines "__mips_dsp_rev"
to 2.
-msmartmips
-mno-smartmips
Use (do not use) the MIPS SmartMIPS ASE.
-mpaired-single
-mno-paired-single
Use (do not use) paired-single floating-point instructions.
This option requires hardware floating-point support to be enabled.
-mdmx
-mno-mdmx
Use (do not use) MIPS Digital Media Extension instructions. This option can only be used when generating 64-bit
code and requires hardware floating-point support to be enabled.
-mips3d
-mno-mips3d
Use (do not use) the MIPS-3D ASE. The option -mips3d implies -mpaired-single.
-mmicromips
-mno-micromips
Generate (do not generate) microMIPS code.
MicroMIPS code generation can also be controlled on a per-function basis by means of "micromips" and
"nomicromips" attributes.
-mmt
-mno-mt
Use (do not use) MT Multithreading instructions.
-mmcu
-mno-mcu
Use (do not use) the MIPS MCU ASE instructions.
-meva
-mno-eva
Use (do not use) the MIPS Enhanced Virtual Addressing instructions.
-mvirt
-mno-virt
Use (do not use) the MIPS Virtualization (VZ) instructions.
-mxpa
-mno-xpa
Use (do not use) the MIPS eXtended Physical Address (XPA) instructions.
-mcrc
-mno-crc
Use (do not use) the MIPS Cyclic Redundancy Check (CRC) instructions.
-mginv
-mno-ginv
Use (do not use) the MIPS Global INValidate (GINV) instructions.
-mloongson-mmi
-mno-loongson-mmi
Use (do not use) the MIPS Loongson MultiMedia extensions Instructions (MMI).
-mloongson-ext
-mno-loongson-ext
Use (do not use) the MIPS Loongson EXTensions (EXT) instructions.
-mloongson-ext2
-mno-loongson-ext2
Use (do not use) the MIPS Loongson EXTensions r2 (EXT2) instructions.
-mlong64
Force "long" types to be 64 bits wide. See -mlong32 for an explanation of the default and the way that the
pointer size is determined.
-mlong32
Force "long", "int", and pointer types to be 32 bits wide.
The default size of "int"s, "long"s and pointers depends on the ABI. All the supported ABIs use 32-bit "int"s.
The n64 ABI uses 64-bit "long"s, as does the 64-bit EABI; the others use 32-bit "long"s. Pointers are the same
size as "long"s, or the same size as integer registers, whichever is smaller.
-msym32
-mno-sym32
Assume (do not assume) that all symbols have 32-bit values, regardless of the selected ABI. This option is
useful in combination with -mabi=64 and -mno-abicalls because it allows GCC to generate shorter and faster
references to symbolic addresses.
-G num
Put definitions of externally-visible data in a small data section if that data is no bigger than num bytes. GCC
can then generate more efficient accesses to the data; see -mgpopt for details.
The default -G option depends on the configuration.
-mlocal-sdata
-mno-local-sdata
Extend (do not extend) the -G behavior to local data too, such as to static variables in C. -mlocal-sdata is the
default for all configurations.
If the linker complains that an application is using too much small data, you might want to try rebuilding the
less performance-critical parts with -mno-local-sdata. You might also want to build large libraries with
-mno-local-sdata, so that the libraries leave more room for the main program.
-mextern-sdata
-mno-extern-sdata
Assume (do not assume) that externally-defined data is in a small data section if the size of that data is within
the -G limit. -mextern-sdata is the default for all configurations.
If you compile a module Mod with -mextern-sdata -G num -mgpopt, and Mod references a variable Var that is no
bigger than num bytes, you must make sure that Var is placed in a small data section. If Var is defined by
another module, you must either compile that module with a high-enough -G setting or attach a "section" attribute
to Var's definition. If Var is common, you must link the application with a high-enough -G setting.
The easiest way of satisfying these restrictions is to compile and link every module with the same -G option.
However, you may wish to build a library that supports several different small data limits. You can do this by
compiling the library with the highest supported -G setting and additionally using -mno-extern-sdata to stop the
library from making assumptions about externally-defined data.
-mgpopt
-mno-gpopt
Use (do not use) GP-relative accesses for symbols that are known to be in a small data section; see -G,
-mlocal-sdata and -mextern-sdata. -mgpopt is the default for all configurations.
-mno-gpopt is useful for cases where the $gp register might not hold the value of "_gp". For example, if the
code is part of a library that might be used in a boot monitor, programs that call boot monitor routines pass an
unknown value in $gp. (In such situations, the boot monitor itself is usually compiled with -G0.)
-mno-gpopt implies -mno-local-sdata and -mno-extern-sdata.
-membedded-data
-mno-embedded-data
Allocate variables to the read-only data section first if possible, then next in the small data section if
possible, otherwise in data. This gives slightly slower code than the default, but reduces the amount of RAM
required when executing, and thus may be preferred for some embedded systems.
-muninit-const-in-rodata
-mno-uninit-const-in-rodata
Put uninitialized "const" variables in the read-only data section. This option is only meaningful in conjunction
with -membedded-data.
-mcode-readable=setting
Specify whether GCC may generate code that reads from executable sections. There are three possible settings:
-mcode-readable=yes
Instructions may freely access executable sections. This is the default setting.
-mcode-readable=pcrel
MIPS16 PC-relative load instructions can access executable sections, but other instructions must not do so.
This option is useful on 4KSc and 4KSd processors when the code TLBs have the Read Inhibit bit set. It is
also useful on processors that can be configured to have a dual instruction/data SRAM interface and that,
like the M4K, automatically redirect PC-relative loads to the instruction RAM.
-mcode-readable=no
Instructions must not access executable sections. This option can be useful on targets that are configured
to have a dual instruction/data SRAM interface but that (unlike the M4K) do not automatically redirect PC-
relative loads to the instruction RAM.
-msplit-addresses
-mno-split-addresses
Enable (disable) use of the %hi() and %lo() assembler relocation operators. This option has been superseded by
-mexplicit-relocs but is retained for backwards compatibility.
-mexplicit-relocs=none
-mexplicit-relocs=base
-mexplicit-relocs=pcrel
-mexplicit-relocs
-mno-explicit-relocs
These options control whether explicit relocs (such as %gp_rel) are used. The default value depends on the
version of GAS when GCC itself was built.
The "base" explicit-relocs support was introduced into GAS in 2001. The "pcrel" explicit-relocs support was
introduced into GAS in 2014, which supports %pcrel_hi and %pcrel_lo.
-mcheck-zero-division
-mno-check-zero-division
Trap (do not trap) on integer division by zero.
The default is -mcheck-zero-division.
-mdivide-traps
-mdivide-breaks
MIPS systems check for division by zero by generating either a conditional trap or a break instruction. Using
traps results in smaller code, but is only supported on MIPS II and later. Also, some versions of the Linux
kernel have a bug that prevents trap from generating the proper signal ("SIGFPE"). Use -mdivide-traps to allow
conditional traps on architectures that support them and -mdivide-breaks to force the use of breaks.
The default is usually -mdivide-traps, but this can be overridden at configure time using --with-divide=breaks.
Divide-by-zero checks can be completely disabled using -mno-check-zero-division.
-mload-store-pairs
-mno-load-store-pairs
Enable (disable) an optimization that pairs consecutive load or store instructions to enable load/store bonding.
This option is enabled by default but only takes effect when the selected architecture is known to support
bonding.
-mstrict-align
-mno-strict-align
-munaligned-access
-mno-unaligned-access
Disable (enable) direct unaligned access for MIPS Release 6. MIPSr6 requires load/store unaligned-access
support, by hardware or trap&emulate. So -mstrict-align may be needed by kernel. The options -munaligned-access
and -mno-unaligned-access are obsoleted, and only for backward-compatible.
-mmemcpy
-mno-memcpy
Force (do not force) the use of "memcpy" for non-trivial block moves. The default is -mno-memcpy, which allows
GCC to inline most constant-sized copies.
-mlong-calls
-mno-long-calls
Disable (do not disable) use of the "jal" instruction. Calling functions using "jal" is more efficient but
requires the caller and callee to be in the same 256 megabyte segment.
This option has no effect on abicalls code. The default is -mno-long-calls.
-mmad
-mno-mad
Enable (disable) use of the "mad", "madu" and "mul" instructions, as provided by the R4650 ISA.
-mimadd
-mno-imadd
Enable (disable) use of the "madd" and "msub" integer instructions. The default is -mimadd on architectures that
support "madd" and "msub" except for the 74k architecture where it was found to generate slower code.
-mfused-madd
-mno-fused-madd
Enable (disable) use of the floating-point multiply-accumulate instructions, when they are available. The
default is -mfused-madd.
On the R8000 CPU when multiply-accumulate instructions are used, the intermediate product is calculated to
infinite precision and is not subject to the FCSR Flush to Zero bit. This may be undesirable in some
circumstances. On other processors the result is numerically identical to the equivalent computation using
separate multiply, add, subtract and negate instructions.
-nocpp
Tell the MIPS assembler to not run its preprocessor over user assembler files (with a .s suffix) when assembling
them.
-mfix-24k
-mno-fix-24k
Work around the 24K E48 (lost data on stores during refill) errata. The workarounds are implemented by the
assembler rather than by GCC.
-mfix-r4000
-mno-fix-r4000
Work around certain R4000 CPU errata:
- A double-word or a variable shift may give an incorrect result if executed immediately after starting an
integer division.
- A double-word or a variable shift may give an incorrect result if executed while an integer multiplication is
in progress.
- An integer division may give an incorrect result if started in a delay slot of a taken branch or a jump.
-mfix-r4400
-mno-fix-r4400
Work around certain R4400 CPU errata:
- A double-word or a variable shift may give an incorrect result if executed immediately after starting an
integer division.
-mfix-r10000
-mno-fix-r10000
Work around certain R10000 errata:
- "ll"/"sc" sequences may not behave atomically on revisions prior to 3.0. They may deadlock on revisions 2.6
and earlier.
This option can only be used if the target architecture supports branch-likely instructions. -mfix-r10000 is the
default when -march=r10000 is used; -mno-fix-r10000 is the default otherwise.
-mfix-r5900
-mno-fix-r5900
Do not attempt to schedule the preceding instruction into the delay slot of a branch instruction placed at the
end of a short loop of six instructions or fewer and always schedule a "nop" instruction there instead. The
short loop bug under certain conditions causes loops to execute only once or twice, due to a hardware bug in the
R5900 chip. The workaround is implemented by the assembler rather than by GCC.
-mfix-rm7000
-mno-fix-rm7000
Work around the RM7000 "dmult"/"dmultu" errata. The workarounds are implemented by the assembler rather than by
GCC.
-mfix-vr4120
-mno-fix-vr4120
Work around certain VR4120 errata:
- "dmultu" does not always produce the correct result.
- "div" and "ddiv" do not always produce the correct result if one of the operands is negative.
The workarounds for the division errata rely on special functions in libgcc.a. At present, these functions are
only provided by the "mips64vr*-elf" configurations.
Other VR4120 errata require a NOP to be inserted between certain pairs of instructions. These errata are handled
by the assembler, not by GCC itself.
-mfix-vr4130
Work around the VR4130 "mflo"/"mfhi" errata. The workarounds are implemented by the assembler rather than by
GCC, although GCC avoids using "mflo" and "mfhi" if the VR4130 "macc", "macchi", "dmacc" and "dmacchi"
instructions are available instead.
-mfix-sb1
-mno-fix-sb1
Work around certain SB-1 CPU core errata. (This flag currently works around the SB-1 revision 2 "F1" and "F2"
floating-point errata.)
-mr10k-cache-barrier=setting
Specify whether GCC should insert cache barriers to avoid the side effects of speculation on R10K processors.
In common with many processors, the R10K tries to predict the outcome of a conditional branch and speculatively
executes instructions from the "taken" branch. It later aborts these instructions if the predicted outcome is
wrong. However, on the R10K, even aborted instructions can have side effects.
This problem only affects kernel stores and, depending on the system, kernel loads. As an example, a
speculatively-executed store may load the target memory into cache and mark the cache line as dirty, even if the
store itself is later aborted. If a DMA operation writes to the same area of memory before the "dirty" line is
flushed, the cached data overwrites the DMA-ed data. See the R10K processor manual for a full description,
including other potential problems.
One workaround is to insert cache barrier instructions before every memory access that might be speculatively
executed and that might have side effects even if aborted. -mr10k-cache-barrier=setting controls GCC's
implementation of this workaround. It assumes that aborted accesses to any byte in the following regions does
not have side effects:
1. the memory occupied by the current function's stack frame;
2. the memory occupied by an incoming stack argument;
3. the memory occupied by an object with a link-time-constant address.
It is the kernel's responsibility to ensure that speculative accesses to these regions are indeed safe.
If the input program contains a function declaration such as:
void foo (void);
then the implementation of "foo" must allow "j foo" and "jal foo" to be executed speculatively. GCC honors this
restriction for functions it compiles itself. It expects non-GCC functions (such as hand-written assembly code)
to do the same.
The option has three forms:
-mr10k-cache-barrier=load-store
Insert a cache barrier before a load or store that might be speculatively executed and that might have side
effects even if aborted.
-mr10k-cache-barrier=store
Insert a cache barrier before a store that might be speculatively executed and that might have side effects
even if aborted.
-mr10k-cache-barrier=none
Disable the insertion of cache barriers. This is the default setting.
-mflush-func=func
-mno-flush-func
Specifies the function to call to flush the I and D caches, or to not call any such function. If called, the
function must take the same arguments as the common "_flush_func", that is, the address of the memory range for
which the cache is being flushed, the size of the memory range, and the number 3 (to flush both caches). The
default depends on the target GCC was configured for, but commonly is either "_flush_func" or "__cpu_flush".
-mbranch-cost=num
Set the cost of branches to roughly num "simple" instructions. This cost is only a heuristic and is not
guaranteed to produce consistent results across releases. A zero cost redundantly selects the default, which is
based on the -mtune setting.
-mbranch-likely
-mno-branch-likely
Enable or disable use of Branch Likely instructions, regardless of the default for the selected architecture. By
default, Branch Likely instructions may be generated if they are supported by the selected architecture. An
exception is for the MIPS32 and MIPS64 architectures and processors that implement those architectures; for
those, Branch Likely instructions are not be generated by default because the MIPS32 and MIPS64 architectures
specifically deprecate their use.
-mcompact-branches=never
-mcompact-branches=optimal
-mcompact-branches=always
These options control which form of branches will be generated. The default is -mcompact-branches=optimal.
The -mcompact-branches=never option ensures that compact branch instructions will never be generated.
The -mcompact-branches=always option ensures that a compact branch instruction will be generated if available for
MIPS Release 6 onwards. If a compact branch instruction is not available (or pre-R6), a delay slot form of the
branch will be used instead.
If it is used for MIPS16/microMIPS targets, it will be just ignored now. The behavior for MIPS16/microMIPS may
change in future, since they do have some compact branch instructions.
The -mcompact-branches=optimal option will cause a delay slot branch to be used if one is available in the
current ISA and the delay slot is successfully filled. If the delay slot is not filled, a compact branch will be
chosen if one is available.
-mfp-exceptions
-mno-fp-exceptions
Specifies whether FP exceptions are enabled. This affects how FP instructions are scheduled for some processors.
The default is that FP exceptions are enabled.
For instance, on the SB-1, if FP exceptions are disabled, and we are emitting 64-bit code, then we can use both
FP pipes. Otherwise, we can only use one FP pipe.
-mvr4130-align
-mno-vr4130-align
The VR4130 pipeline is two-way superscalar, but can only issue two instructions together if the first one is
8-byte aligned. When this option is enabled, GCC aligns pairs of instructions that it thinks should execute in
parallel.
This option only has an effect when optimizing for the VR4130. It normally makes code faster, but at the expense
of making it bigger. It is enabled by default at optimization level -O3.
-msynci
-mno-synci
Enable (disable) generation of "synci" instructions on architectures that support it. The "synci" instructions
(if enabled) are generated when "__builtin___clear_cache" is compiled.
This option defaults to -mno-synci, but the default can be overridden by configuring GCC with --with-synci.
When compiling code for single processor systems, it is generally safe to use "synci". However, on many multi-
core (SMP) systems, it does not invalidate the instruction caches on all cores and may lead to undefined
behavior.
-mrelax-pic-calls
-mno-relax-pic-calls
Try to turn PIC calls that are normally dispatched via register $25 into direct calls. This is only possible if
the linker can resolve the destination at link time and if the destination is within range for a direct call.
-mrelax-pic-calls is the default if GCC was configured to use an assembler and a linker that support the ".reloc"
assembly directive and -mexplicit-relocs is in effect. With -mno-explicit-relocs, this optimization can be
performed by the assembler and the linker alone without help from the compiler.
-mmcount-ra-address
-mno-mcount-ra-address
Emit (do not emit) code that allows "_mcount" to modify the calling function's return address. When enabled,
this option extends the usual "_mcount" interface with a new ra-address parameter, which has type "intptr_t *"
and is passed in register $12. "_mcount" can then modify the return address by doing both of the following:
* Returning the new address in register $31.
* Storing the new address in "*ra-address", if ra-address is nonnull.
The default is -mno-mcount-ra-address.
-mframe-header-opt
-mno-frame-header-opt
Enable (disable) frame header optimization in the o32 ABI. When using the o32 ABI, calling functions will
allocate 16 bytes on the stack for the called function to write out register arguments. When enabled, this
optimization will suppress the allocation of the frame header if it can be determined that it is unused.
This optimization is off by default at all optimization levels.
-mlxc1-sxc1
-mno-lxc1-sxc1
When applicable, enable (disable) the generation of "lwxc1", "swxc1", "ldxc1", "sdxc1" instructions. Enabled by
default.
-mmadd4
-mno-madd4
When applicable, enable (disable) the generation of 4-operand "madd.s", "madd.d" and related instructions.
Enabled by default.
MMIX Options
These options are defined for the MMIX:
-mlibfuncs
-mno-libfuncs
Specify that intrinsic library functions are being compiled, passing all values in registers, no matter the size.
-mepsilon
-mno-epsilon
Generate floating-point comparison instructions that compare with respect to the "rE" epsilon register.
-mabi=mmixware
-mabi=gnu
Generate code that passes function parameters and return values that (in the called function) are seen as
registers $0 and up, as opposed to the GNU ABI which uses global registers $231 and up.
-mzero-extend
-mno-zero-extend
When reading data from memory in sizes shorter than 64 bits, use (do not use) zero-extending load instructions by
default, rather than sign-extending ones.
-mknuthdiv
-mno-knuthdiv
Make the result of a division yielding a remainder have the same sign as the divisor. With the default,
-mno-knuthdiv, the sign of the remainder follows the sign of the dividend. Both methods are arithmetically
valid, the latter being almost exclusively used.
-mtoplevel-symbols
-mno-toplevel-symbols
Prepend (do not prepend) a : to all global symbols, so the assembly code can be used with the "PREFIX" assembly
directive.
-melf
Generate an executable in the ELF format, rather than the default mmo format used by the mmix simulator.
-mbranch-predict
-mno-branch-predict
Use (do not use) the probable-branch instructions, when static branch prediction indicates a probable branch.
-mbase-addresses
-mno-base-addresses
Generate (do not generate) code that uses base addresses. Using a base address automatically generates a request
(handled by the assembler and the linker) for a constant to be set up in a global register. The register is used
for one or more base address requests within the range 0 to 255 from the value held in the register. The
generally leads to short and fast code, but the number of different data items that can be addressed is limited.
This means that a program that uses lots of static data may require -mno-base-addresses.
-msingle-exit
-mno-single-exit
Force (do not force) generated code to have a single exit point in each function.
MN10300 Options
These -m options are defined for Matsushita MN10300 architectures:
-mmult-bug
Generate code to avoid bugs in the multiply instructions for the MN10300 processors. This is the default.
-mno-mult-bug
Do not generate code to avoid bugs in the multiply instructions for the MN10300 processors.
-mam33
Generate code using features specific to the AM33 processor.
-mno-am33
Do not generate code using features specific to the AM33 processor. This is the default.
-mam33-2
Generate code using features specific to the AM33/2.0 processor.
-mam34
Generate code using features specific to the AM34 processor.
-mtune=cpu-type
Use the timing characteristics of the indicated CPU type when scheduling instructions. This does not change the
targeted processor type. The CPU type must be one of mn10300, am33, am33-2 or am34.
-mreturn-pointer-on-d0
When generating a function that returns a pointer, return the pointer in both "a0" and "d0". Otherwise, the
pointer is returned only in "a0", and attempts to call such functions without a prototype result in errors. Note
that this option is on by default; use -mno-return-pointer-on-d0 to disable it.
-mno-crt0
Do not link in the C run-time initialization object file.
-mrelax
Indicate to the linker that it should perform a relaxation optimization pass to shorten branches, calls and
absolute memory addresses. This option only has an effect when used on the command line for the final link step.
This option makes symbolic debugging impossible.
-mliw
Allow the compiler to generate Long Instruction Word instructions if the target is the AM33 or later. This is
the default. This option defines the preprocessor macro "__LIW__".
-mno-liw
Do not allow the compiler to generate Long Instruction Word instructions. This option defines the preprocessor
macro "__NO_LIW__".
-msetlb
Allow the compiler to generate the SETLB and Lcc instructions if the target is the AM33 or later. This is the
default. This option defines the preprocessor macro "__SETLB__".
-mno-setlb
Do not allow the compiler to generate SETLB or Lcc instructions. This option defines the preprocessor macro
"__NO_SETLB__".
Moxie Options
-meb
Generate big-endian code. This is the default for moxie-*-* configurations.
-mel
Generate little-endian code.
-mmul.x
Generate mul.x and umul.x instructions. This is the default for moxiebox-*-* configurations.
-mno-crt0
Do not link in the C run-time initialization object file.
MSP430 Options
These options are defined for the MSP430:
-masm-hex
Force assembly output to always use hex constants. Normally such constants are signed decimals, but this option
is available for testsuite and/or aesthetic purposes.
-mmcu=
Select the MCU to target. This is used to create a C preprocessor symbol based upon the MCU name, converted to
upper case and pre- and post-fixed with __. This in turn is used by the msp430.h header file to select an MCU-
specific supplementary header file.
The option also sets the ISA to use. If the MCU name is one that is known to only support the 430 ISA then that
is selected, otherwise the 430X ISA is selected. A generic MCU name of msp430 can also be used to select the 430
ISA. Similarly the generic msp430x MCU name selects the 430X ISA.
In addition an MCU-specific linker script is added to the linker command line. The script's name is the name of
the MCU with .ld appended. Thus specifying -mmcu=xxx on the gcc command line defines the C preprocessor symbol
"__XXX__" and cause the linker to search for a script called xxx.ld.
The ISA and hardware multiply supported for the different MCUs is hard-coded into GCC. However, an external
devices.csv file can be used to extend device support beyond those that have been hard-coded.
GCC searches for the devices.csv file using the following methods in the given precedence order, where the first
method takes precedence over the second which takes precedence over the third.
Include path specified with "-I" and "-L"
devices.csv is searched for in each of the directories specified by include paths and linker library search
paths.
Path specified by the environment variable MSP430_GCC_INCLUDE_DIR
Define the value of the global environment variable MSP430_GCC_INCLUDE_DIR to the full path to the directory
containing devices.csv, and GCC will search this directory for devices.csv. If devices.csv is found, this
directory is also registered as an include path and linker library path. Header files and linker scripts in
this directory can therefore be used without manually specifying "-I" and "-L" on the command line.
The msp430-elf{,bare}/include/devices directory
Finally, GCC examines msp430-elf{,bare}/include/devices from the toolchain root directory. This directory
does not exist in a default installation, but if the user has created it and copied devices.csv there, then
the MCU data is read. As above, this directory is also registered as an include path and linker library
path.
If none of the above search methods find devices.csv, then the hard-coded MCU data is used.
-mwarn-mcu
-mno-warn-mcu
This option enables or disables warnings about conflicts between the MCU name specified by the -mmcu option and
the ISA set by the -mcpu option and/or the hardware multiply support set by the -mhwmult option. It also toggles
warnings about unrecognized MCU names. This option is on by default.
-mcpu=
Specifies the ISA to use. Accepted values are msp430, msp430x and msp430xv2. This option is deprecated. The
-mmcu= option should be used to select the ISA.
-msim
Link to the simulator runtime libraries and linker script. Overrides any scripts that would be selected by the
-mmcu= option.
-mlarge
Use large-model addressing (20-bit pointers, 20-bit "size_t").
-msmall
Use small-model addressing (16-bit pointers, 16-bit "size_t").
-mrelax
This option is passed to the assembler and linker, and allows the linker to perform certain optimizations that
cannot be done until the final link.
mhwmult=
Describes the type of hardware multiply supported by the target. Accepted values are none for no hardware
multiply, 16bit for the original 16-bit-only multiply supported by early MCUs. 32bit for the 16/32-bit multiply
supported by later MCUs and f5series for the 16/32-bit multiply supported by F5-series MCUs. A value of auto can
also be given. This tells GCC to deduce the hardware multiply support based upon the MCU name provided by the
-mmcu option. If no -mmcu option is specified or if the MCU name is not recognized then no hardware multiply
support is assumed. "auto" is the default setting.
Hardware multiplies are normally performed by calling a library routine. This saves space in the generated code.
When compiling at -O3 or higher however the hardware multiplier is invoked inline. This makes for bigger, but
faster code.
The hardware multiply routines disable interrupts whilst running and restore the previous interrupt state when
they finish. This makes them safe to use inside interrupt handlers as well as in normal code.
-minrt
Enable the use of a minimum runtime environment - no static initializers or constructors. This is intended for
memory-constrained devices. The compiler includes special symbols in some objects that tell the linker and
runtime which code fragments are required.
-mtiny-printf
Enable reduced code size "printf" and "puts" library functions. The tiny implementations of these functions are
not reentrant, so must be used with caution in multi-threaded applications.
Support for streams has been removed and the string to be printed will always be sent to stdout via the "write"
syscall. The string is not buffered before it is sent to write.
This option requires Newlib Nano IO, so GCC must be configured with --enable-newlib-nano-formatted-io.
-mmax-inline-shift=
This option takes an integer between 0 and 64 inclusive, and sets the maximum number of inline shift instructions
which should be emitted to perform a shift operation by a constant amount. When this value needs to be exceeded,
an mspabi helper function is used instead. The default value is 4.
This only affects cases where a shift by multiple positions cannot be completed with a single instruction (e.g.
all shifts >1 on the 430 ISA).
Shifts of a 32-bit value are at least twice as costly, so the value passed for this option is divided by 2 and
the resulting value used instead.
-mcode-region=
-mdata-region=
These options tell the compiler where to place functions and data that do not have one of the "lower", "upper",
"either" or "section" attributes. Possible values are "lower", "upper", "either" or "any". The first three
behave like the corresponding attribute. The fourth possible value - "any" - is the default. It leaves
placement entirely up to the linker script and how it assigns the standard sections (".text", ".data", etc) to
the memory regions.
-msilicon-errata=
This option passes on a request to assembler to enable the fixes for the named silicon errata.
-msilicon-errata-warn=
This option passes on a request to the assembler to enable warning messages when a silicon errata might need to
be applied.
-mwarn-devices-csv
-mno-warn-devices-csv
Warn if devices.csv is not found or there are problem parsing it (default: on).
NDS32 Options
These options are defined for NDS32 implementations:
-mbig-endian
Generate code in big-endian mode.
-mlittle-endian
Generate code in little-endian mode.
-mreduced-regs
Use reduced-set registers for register allocation.
-mfull-regs
Use full-set registers for register allocation.
-mcmov
Generate conditional move instructions.
-mno-cmov
Do not generate conditional move instructions.
-mext-perf
Generate performance extension instructions.
-mno-ext-perf
Do not generate performance extension instructions.
-mext-perf2
Generate performance extension 2 instructions.
-mno-ext-perf2
Do not generate performance extension 2 instructions.
-mext-string
Generate string extension instructions.
-mno-ext-string
Do not generate string extension instructions.
-mv3push
Generate v3 push25/pop25 instructions.
-mno-v3push
Do not generate v3 push25/pop25 instructions.
-m16-bit
Generate 16-bit instructions.
-mno-16-bit
Do not generate 16-bit instructions.
-misr-vector-size=num
Specify the size of each interrupt vector, which must be 4 or 16.
-mcache-block-size=num
Specify the size of each cache block, which must be a power of 2 between 4 and 512.
-march=arch
Specify the name of the target architecture.
-mcmodel=code-model
Set the code model to one of
small
All the data and read-only data segments must be within 512KB addressing space. The text segment must be
within 16MB addressing space.
medium
The data segment must be within 512KB while the read-only data segment can be within 4GB addressing space.
The text segment should be still within 16MB addressing space.
large
All the text and data segments can be within 4GB addressing space.
-mctor-dtor
Enable constructor/destructor feature.
-mrelax
Guide linker to relax instructions.
Nvidia PTX Options
These options are defined for Nvidia PTX:
-m64
Ignored, but preserved for backward compatibility. Only 64-bit ABI is supported.
-march=architecture-string
Generate code for the specified PTX ISA target architecture. Valid architecture strings are sm_30, sm_35, sm_37,
sm_52, sm_53, sm_61, sm_70, sm_75, sm_80, and sm_89. The default depends on how the compiler has been
configured, see --with-arch.
This option sets the value of the preprocessor macro "__PTX_SM__"; for instance, for sm_35, it has the value 350.
-misa=architecture-string
Alias of -march=.
-march-map=architecture-string
Select the closest available -march= value that is not more capable. For instance, for -march-map=sm_50 select
-march=sm_37, and for -march-map=sm_53 select -march=sm_53.
-mptx=version-string
Generate code for the specified PTX ISA version. Valid version strings are 3.1, 4.1, 4.2, 5.0, 6.0, 6.3, 7.0,
7.3, and 7.8. The default PTX ISA version is the one that added support for the selected PTX ISA target
architecture, see -march=, but at least 6.3, or 7.3 for -march=sm_52 and higher.
This option sets the values of the preprocessor macros "__PTX_ISA_VERSION_MAJOR__" and
"__PTX_ISA_VERSION_MINOR__"; for instance, for 3.1 the macros have the values 3 and 1, respectively.
-mmainkernel
Link in code for a __main kernel. This is for stand-alone instead of offloading execution.
-moptimize
Apply partitioned execution optimizations. This is the default when any level of optimization is selected.
-msoft-stack
-mno-soft-stack
For -mno-soft-stack (the default, unless -mgomp has been specified), use PTX "native" stacks, that is, generate
code that uses ".local" memory or PTX "alloca" directly for stack storage. Unless -mptx=7.3 or higher and
-march=sm_52 or higher are active, variable-length arrays and dynamically allocating memory on the stack with
"alloca" are not supported.
For -msoft-stack (implied by -mgomp), generate code that does not use ".local" memory or PTX "alloca" directly
for stack storage. Instead, a per-warp stack pointer is maintained explicitly. This enables variable-length stack
allocation (with variable-length arrays or "alloca"), and when global memory is used for underlying storage,
makes it possible to access automatic variables from other threads, or with atomic instructions. This code
generation variant is used for OpenMP offloading, but the option is exposed on its own for the purpose of testing
the compiler; to generate code suitable for linking into programs using OpenMP offloading, use option -mgomp.
-muniform-simt
Switch to code generation variant that allows to execute all threads in each warp, while maintaining memory state
and side effects as if only one thread in each warp was active outside of OpenMP SIMD regions. All atomic
operations and calls to runtime (malloc, free, vprintf) are conditionally executed (iff current lane index equals
the master lane index), and the register being assigned is copied via a shuffle instruction from the master lane.
Outside of SIMD regions lane 0 is the master; inside, each thread sees itself as the master. Shared memory array
"int __nvptx_uni[]" stores all-zeros or all-ones bitmasks for each warp, indicating current mode (0 outside of
SIMD regions). Each thread can bitwise-and the bitmask at position "tid.y" with current lane index to compute
the master lane index.
-mgomp
Generate code for use in OpenMP offloading: enables -msoft-stack and -muniform-simt options, and selects
corresponding multilib variant.
OpenRISC Options
These options are defined for OpenRISC:
-mboard=name
Configure a board specific runtime. This will be passed to the linker for newlib board library linking. The
default is "or1ksim".
-mnewlib
This option is ignored; it is for compatibility purposes only. This used to select linker and preprocessor
options for use with newlib.
-msoft-div
-mhard-div
Select software or hardware divide ("l.div", "l.divu") instructions. This default is hardware divide.
-msoft-mul
-mhard-mul
Select software or hardware multiply ("l.mul", "l.muli") instructions. This default is hardware multiply.
-msoft-float
-mhard-float
Select software or hardware for floating point operations. The default is software.
-mdouble-float
When -mhard-float is selected, enables generation of double-precision floating point instructions. By default
functions from libgcc are used to perform double-precision floating point operations.
-munordered-float
When -mhard-float is selected, enables generation of unordered floating point compare and set flag ("lf.sfun*")
instructions. By default functions from libgcc are used to perform unordered floating point compare and set flag
operations.
-mcmov
Enable generation of conditional move ("l.cmov") instructions. By default the equivalent will be generated using
set and branch.
-mror
Enable generation of rotate right ("l.ror") instructions. By default functions from libgcc are used to perform
rotate right operations.
-mrori
Enable generation of rotate right with immediate ("l.rori") instructions. By default functions from libgcc are
used to perform rotate right with immediate operations.
-msext
Enable generation of sign extension ("l.ext*") instructions. By default memory loads are used to perform sign
extension.
-msfimm
Enable generation of compare and set flag with immediate ("l.sf*i") instructions. By default extra instructions
will be generated to store the immediate to a register first.
-mshftimm
Enable generation of shift with immediate ("l.srai", "l.srli", "l.slli") instructions. By default extra
instructions will be generated to store the immediate to a register first.
-mcmodel=small
Generate OpenRISC code for the small model: The GOT is limited to 64k. This is the default model.
-mcmodel=large
Generate OpenRISC code for the large model: The GOT may grow up to 4G in size.
PDP-11 Options
These options are defined for the PDP-11:
-mfpu
Use hardware FPP floating point. This is the default. (FIS floating point on the PDP-11/40 is not supported.)
Implies -m45.
-msoft-float
Do not use hardware floating point.
-mac0
Return floating-point results in ac0 (fr0 in Unix assembler syntax).
-mno-ac0
Return floating-point results in memory. This is the default.
-m40
Generate code for a PDP-11/40. Implies -msoft-float -mno-split.
-m45
Generate code for a PDP-11/45. This is the default.
-m10
Generate code for a PDP-11/10. Implies -msoft-float -mno-split.
-mint16
-mno-int32
Use 16-bit "int". This is the default.
-mint32
-mno-int16
Use 32-bit "int".
-msplit
Target has split instruction and data space. Implies -m45.
-munix-asm
Use Unix assembler syntax.
-mdec-asm
Use DEC assembler syntax.
-mgnu-asm
Use GNU assembler syntax. This is the default.
-mlra
Use the new LRA register allocator. By default, the old "reload" allocator is used.
PowerPC Options
These are listed under
PRU Options
These command-line options are defined for PRU target:
-minrt
Link with a minimum runtime environment. This can significantly reduce the size of the final ELF binary, but
some standard C runtime features are removed.
This option disables support for static initializers and constructors. Beware that the compiler could still
generate code with static initializers and constructors. It is up to the programmer to ensure that the source
program will not use those features.
The minimal startup code would not pass "argc" and "argv" arguments to "main", so the latter must be declared as
"int main (void)". This is already the norm for most firmware projects.
-mmcu=mcu
Specify the PRU hardware variant to use. A correspondingly named spec file would be loaded, passing the memory
region sizes to the linker and defining hardware-specific C macros.
Newlib provides only the "sim" spec, intended for running regression tests using a simulator. Specs for real
hardware can be obtained by installing the GnuPruMcu ("https://github.com/dinuxbg/gnuprumcu/") package.
-mno-relax
Make GCC pass the --no-relax command-line option to the linker instead of the --relax option.
-mloop
Allow (or do not allow) GCC to use the LOOP instruction.
-mabi=variant
Specify the ABI variant to output code for. -mabi=ti selects the unmodified TI ABI while -mabi=gnu selects a GNU
variant that copes more naturally with certain GCC assumptions. These are the differences:
Function Pointer Size
TI ABI specifies that function (code) pointers are 16-bit, whereas GNU supports only 32-bit data and code
pointers.
Optional Return Value Pointer
Function return values larger than 64 bits are passed by using a hidden pointer as the first argument of the
function. TI ABI, though, mandates that the pointer can be NULL in case the caller is not using the returned
value. GNU always passes and expects a valid return value pointer.
The current -mabi=ti implementation simply raises a compile error when any of the above code constructs is
detected. As a consequence the standard C library cannot be built and it is omitted when linking with -mabi=ti.
Relaxation is a GNU feature and for safety reasons is disabled when using -mabi=ti. The TI toolchain does not
emit relocations for QBBx instructions, so the GNU linker cannot adjust them when shortening adjacent LDI32
pseudo instructions.
RISC-V Options
These command-line options are defined for RISC-V targets:
-mbranch-cost=n
Set the cost of branches to roughly n instructions.
-mplt
-mno-plt
When generating PIC code, do or don't allow the use of PLTs. Ignored for non-PIC. The default is -mplt.
-mabi=ABI-string
Specify integer and floating-point calling convention. ABI-string contains two parts: the size of integer types
and the registers used for floating-point types. For example -march=rv64ifd -mabi=lp64d means that long and
pointers are 64-bit (implicitly defining int to be 32-bit), and that floating-point values up to 64 bits wide are
passed in F registers. Contrast this with -march=rv64ifd -mabi=lp64f, which still allows the compiler to
generate code that uses the F and D extensions but only allows floating-point values up to 32 bits long to be
passed in registers; or -march=rv64ifd -mabi=lp64, in which no floating-point arguments will be passed in
registers.
The default for this argument is system dependent, users who want a specific calling convention should specify
one explicitly. The valid calling conventions are: ilp32, ilp32f, ilp32d, lp64, lp64f, and lp64d. Some calling
conventions are impossible to implement on some ISAs: for example, -march=rv32if -mabi=ilp32d is invalid because
the ABI requires 64-bit values be passed in F registers, but F registers are only 32 bits wide. There are also
the ilp32e ABI that can only be used with the rv32e architecture and the lp64e ABI that can only be used with the
rv64e. Those ABIs are not well specified at present, and are subject to change.
-mfdiv
-mno-fdiv
Do or don't use hardware floating-point divide and square root instructions. This requires the F or D extensions
for floating-point registers. The default is to use them if the specified architecture has these instructions.
-mfence-tso
-mno-fence-tso
Do or don't use the fence.tso instruction, which is unimplemented on some processors (including those from
T-Head). If the fence.tso instruction is not available then a stronger fence is used instead.
-mdiv
-mno-div
Do or don't use hardware instructions for integer division. This requires the M extension. The default is to
use them if the specified architecture has these instructions.
-misa-spec=ISA-spec-string
Specify the version of the RISC-V Unprivileged (formerly User-Level) ISA specification to produce code conforming
to. The possibilities for ISA-spec-string are:
2.2 Produce code conforming to version 2.2.
20190608
Produce code conforming to version 20190608.
20191213
Produce code conforming to version 20191213.
The default is -misa-spec=20191213 unless GCC has been configured with --with-isa-spec= specifying a different
default version.
-march=ISA-string
Generate code for given RISC-V ISA (e.g. rv64im). ISA strings must be lower-case. Examples include rv64i,
rv32g, rv32e, and rv32imaf. Additionally, a special value help (-march=help) is accepted to list all supported
extensions.
The syntax of the ISA string is defined as follows:
"The string must start with rv32 or rv64, followed by"
i, e, or g, referred to as the base ISA.
"The subsequent part of the string is a list of extension names. Extension"
names can be categorized as multi-letter (e.g. zba) and single-letter (e.g. v). Single-letter extensions can
appear consecutively, but multi-letter extensions must be separated by underscores.
"An underscore can appear anywhere after the base ISA. It has no specific"
effect but is used to improve readability and can act as a separator.
"Extension names may include an optional version number, following the"
syntax p or , (e.g. m2p1 or m2).
Supported extension are listed below:
Extension Name : Supported Version : Description
i @tab 2.0, 2.1 @tab Base integer extension.
e @tab 2.0 @tab Reduced base integer extension.
g @tab - @tab General-purpose computing base extension, g will expand to i, m, a, f, d, zicsr and zifencei.
m @tab 2.0 @tab Integer multiplication and division extension.
a @tab 2.0, 2.1 @tab Atomic extension.
f @tab 2.0, 2.2 @tab Single-precision floating-point extension.
d @tab 2.0, 2.2 @tab Double-precision floating-point extension.
c @tab 2.0 @tab Compressed extension.
h @tab 1.0 @tab Hypervisor extension.
v @tab 1.0 @tab Vector extension.
zicsr
@tab 2.0 @tab Control and status register access extension.
zifencei
@tab 2.0 @tab Instruction-fetch fence extension.
zicond
@tab 1.0 @tab Integer conditional operations extension.
za64rs
@tab 1.0 @tab Reservation set size of 64 bytes.
za128rs
@tab 1.0 @tab Reservation set size of 128 bytes.
zawrs
@tab 1.0 @tab Wait-on-reservation-set extension.
zba @tab 1.0 @tab Address calculation extension.
zbb @tab 1.0 @tab Basic bit manipulation extension.
zbc @tab 1.0 @tab Carry-less multiplication extension.
zbs @tab 1.0 @tab Single-bit operation extension.
zfinx
@tab 1.0 @tab Single-precision floating-point in integer registers extension.
zdinx
@tab 1.0 @tab Double-precision floating-point in integer registers extension.
zhinx
@tab 1.0 @tab Half-precision floating-point in integer registers extension.
zhinxmin
@tab 1.0 @tab Minimal half-precision floating-point in integer registers extension.
zbkb
@tab 1.0 @tab Cryptography bit-manipulation extension.
zbkc
@tab 1.0 @tab Cryptography carry-less multiply extension.
zbkx
@tab 1.0 @tab Cryptography crossbar permutation extension.
zkne
@tab 1.0 @tab AES Encryption extension.
zknd
@tab 1.0 @tab AES Decryption extension.
zknh
@tab 1.0 @tab Hash function extension.
zkr @tab 1.0 @tab Entropy source extension.
zksed
@tab 1.0 @tab SM4 block cipher extension.
zksh
@tab 1.0 @tab SM3 hash function extension.
zkt @tab 1.0 @tab Data independent execution latency extension.
zk @tab 1.0 @tab Standard scalar cryptography extension.
zkn @tab 1.0 @tab NIST algorithm suite extension.
zks @tab 1.0 @tab ShangMi algorithm suite extension.
zihintntl
@tab 1.0 @tab Non-temporal locality hints extension.
zihintpause
@tab 1.0 @tab Pause hint extension.
zicboz
@tab 1.0 @tab Cache-block zero extension.
zicbom
@tab 1.0 @tab Cache-block management extension.
zicbop
@tab 1.0 @tab Cache-block prefetch extension.
zic64b
@tab 1.0 @tab Cache block size isf 64 bytes.
ziccamoa
@tab 1.0 @tab Main memory supports all atomics in A.
ziccif
@tab 1.0 @tab Main memory supports instruction fetch with atomicity requirement.
zicclsm
@tab 1.0 @tab Main memory supports misaligned loads/stores.
ziccrse
@tab 1.0 @tab Main memory supports forward progress on LR/SC sequences.
zicntr
@tab 2.0 @tab Standard extension for base counters and timers.
zihpm
@tab 2.0 @tab Standard extension for hardware performance counters.
ztso
@tab 1.0 @tab Total store ordering extension.
zve32x
@tab 1.0 @tab Vector extensions for embedded processors.
zve32f
@tab 1.0 @tab Vector extensions for embedded processors.
zve64x
@tab 1.0 @tab Vector extensions for embedded processors.
zve64f
@tab 1.0 @tab Vector extensions for embedded processors.
zve64d
@tab 1.0 @tab Vector extensions for embedded processors.
zvl32b
@tab 1.0 @tab Minimum vector length standard extensions
zvl64b
@tab 1.0 @tab Minimum vector length standard extensions
zvl128b
@tab 1.0 @tab Minimum vector length standard extensions
zvl256b
@tab 1.0 @tab Minimum vector length standard extensions
zvl512b
@tab 1.0 @tab Minimum vector length standard extensions
zvl1024b
@tab 1.0 @tab Minimum vector length standard extensions
zvl2048b
@tab 1.0 @tab Minimum vector length standard extensions
zvl4096b
@tab 1.0 @tab Minimum vector length standard extensions
zvbb
@tab 1.0 @tab Vector basic bit-manipulation extension.
zvbc
@tab 1.0 @tab Vector carryless multiplication extension.
zvkb
@tab 1.0 @tab Vector cryptography bit-manipulation extension.
zvkg
@tab 1.0 @tab Vector GCM/GMAC extension.
zvkned
@tab 1.0 @tab Vector AES block cipher extension.
zvknha
@tab 1.0 @tab Vector SHA-2 secure hash extension.
zvknhb
@tab 1.0 @tab Vector SHA-2 secure hash extension.
zvksed
@tab 1.0 @tab Vector SM4 Block Cipher extension.
zvksh
@tab 1.0 @tab Vector SM3 Secure Hash extension.
zvkn
@tab 1.0 @tab Vector NIST Algorithm Suite extension, zvkn will expand to zvkned, zvknhb, zvkb and zvkt.
zvknc
@tab 1.0 @tab Vector NIST Algorithm Suite with carryless multiply extension, zvknc will expand to zvkn and
zvbc.
zvkng
@tab 1.0 @tab Vector NIST Algorithm Suite with GCM extension, zvkng will expand to zvkn and zvkg.
zvks
@tab 1.0 @tab Vector ShangMi algorithm suite extension, zvks will expand to zvksed, zvksh, zvkb and zvkt.
zvksc
@tab 1.0 @tab Vector ShangMi algorithm suite with carryless multiplication extension, zvksc will expand to
zvks and zvbc.
zvksg
@tab 1.0 @tab Vector ShangMi algorithm suite with GCM extension, zvksg will expand to zvks and zvkg.
zvkt
@tab 1.0 @tab Vector data independent execution latency extension.
zfh @tab 1.0 @tab Half-precision floating-point extension.
zfhmin
@tab 1.0 @tab Minimal half-precision floating-point extension.
zvfh
@tab 1.0 @tab Vector half-precision floating-point extension.
zvfhmin
@tab 1.0 @tab Vector minimal half-precision floating-point extension.
zvfbfmin
@tab 1.0 @tab Vector BF16 converts extension.
zfa @tab 1.0 @tab Additional floating-point extension.
zmmul
@tab 1.0 @tab Integer multiplication extension.
zca @tab 1.0 @tab Integer compressed instruction extension.
zcf @tab 1.0 @tab Compressed single-precision floating point loads and stores extension.
zcd @tab 1.0 @tab Compressed double-precision floating point loads and stores extension.
zcb @tab 1.0 @tab Simple compressed instruction extension.
zce @tab 1.0 @tab Compressed instruction extensions for embedded processors.
zcmp
@tab 1.0 @tab Compressed push pop extension.
zcmt
@tab 1.0 @tab Table jump instruction extension.
smaia
@tab 1.0 @tab Advanced interrupt architecture extension.
smepmp
@tab 1.0 @tab PMP Enhancements for memory access and execution prevention on Machine mode.
smstateen
@tab 1.0 @tab State enable extension.
ssaia
@tab 1.0 @tab Advanced interrupt architecture extension for supervisor-mode.
sscofpmf
@tab 1.0 @tab Count overflow & filtering extension.
ssstateen
@tab 1.0 @tab State-enable extension for supervisor-mode.
sstc
@tab 1.0 @tab Supervisor-mode timer interrupts extension.
svinval
@tab 1.0 @tab Fine-grained address-translation cache invalidation extension.
svnapot
@tab 1.0 @tab NAPOT translation contiguity extension.
svpbmt
@tab 1.0 @tab Page-based memory types extension.
xcvmac
@tab 1.0 @tab Core-V multiply-accumulate extension.
xcvalu
@tab 1.0 @tab Core-V miscellaneous ALU extension.
xcvelw
@tab 1.0 @tab Core-V event load word extension.
xtheadba
@tab 1.0 @tab T-head address calculation extension.
xtheadbb
@tab 1.0 @tab T-head basic bit-manipulation extension.
xtheadbs
@tab 1.0 @tab T-head single-bit instructions extension.
xtheadcmo
@tab 1.0 @tab T-head cache management operations extension.
xtheadcondmov
@tab 1.0 @tab T-head conditional move extension.
xtheadfmemidx
@tab 1.0 @tab T-head indexed memory operations for floating-point registers extension.
xtheadfmv
@tab 1.0 @tab T-head double floating-point high-bit data transmission extension.
xtheadint
@tab 1.0 @tab T-head acceleration interruption extension.
xtheadmac
@tab 1.0 @tab T-head multiply-accumulate extension.
xtheadmemidx
@tab 1.0 @tab T-head indexed memory operation extension.
xtheadmempair
@tab 1.0 @tab T-head two-GPR memory operation extension.
xtheadsync
@tab 1.0 @tab T-head multi-core synchronization extension.
xventanacondops
@tab 1.0 @tab Ventana integer conditional operations extension.
When -march= is not specified, use the setting from -mcpu.
If both -march and -mcpu= are not specified, the default for this argument is system dependent, users who want a
specific architecture extensions should specify one explicitly.
When the RISC-V specifications define an extension as depending on other extensions, GCC will implicitly add the
dependent extensions to the enabled extension set if they weren't added explicitly.
-mcpu=processor-string
Use architecture of and optimize the output for the given processor, specified by particular CPU name.
Permissible values for this option are: sifive-e20, sifive-e21, sifive-e24, sifive-e31, sifive-e34, sifive-e76,
sifive-s21, sifive-s51, sifive-s54, sifive-s76, sifive-u54, sifive-u74, sifive-x280, sifive-xp450, sifive-x670,
thead-c906, tt-ascalon-d8, xiangshan-nanhu.
Note that -mcpu does not override -march or -mtune.
-mtune=processor-string
Optimize the output for the given processor, specified by microarchitecture or particular CPU name. Permissible
values for this option are: generic-ooo, rocket, sifive-3-series, sifive-5-series, sifive-7-series, size,
sifive-p400-series, sifive-p600-series, and all valid options for -mcpu=.
When -mtune= is not specified, use the setting from -mcpu, the default is rocket if both are not specified.
The size choice is not intended for use by end-users. This is used when -Os is specified. It overrides the
instruction cost info provided by -mtune=, but does not override the pipeline info. This helps reduce code size
while still giving good performance.
-mpreferred-stack-boundary=num
Attempt to keep the stack boundary aligned to a 2 raised to num byte boundary. If -mpreferred-stack-boundary is
not specified, the default is 4 (16 bytes or 128-bits).
Warning: If you use this switch, then you must build all modules with the same value, including any libraries.
This includes the system libraries and startup modules.
-msmall-data-limit=n
Put global and static data smaller than n bytes into a special section (on some targets).
-msave-restore
-mno-save-restore
Do or don't use smaller but slower prologue and epilogue code that uses library function calls. The default is
to use fast inline prologues and epilogues.
-mmovcc
-mno-movcc
Do or don't produce branchless conditional-move code sequences even with targets that do not have specific
instructions for conditional operations. If enabled, sequences of ALU operations are produced using base integer
ISA instructions where profitable.
-minline-atomics
-mno-inline-atomics
Do or don't use smaller but slower subword atomic emulation code that uses libatomic function calls. The default
is to use fast inline subword atomics that do not require libatomic.
-minline-strlen
-mno-inline-strlen
Do or do not attempt to inline strlen calls if possible. Inlining will only be done if the string is properly
aligned and instructions for accelerated processing are available. The default is to not inline strlen calls.
-minline-strcmp
-mno-inline-strcmp
Do or do not attempt to inline strcmp calls if possible. Inlining will only be done if the strings are properly
aligned and instructions for accelerated processing are available. The default is to not inline strcmp calls.
The --param riscv-strcmp-inline-limit=n parameter controls the maximum number of bytes compared by the inlined
code. The default value is 64.
-minline-strncmp
-mno-inline-strncmp
Do or do not attempt to inline strncmp calls if possible. Inlining will only be done if the strings are properly
aligned and instructions for accelerated processing are available. The default is to not inline strncmp calls.
The --param riscv-strcmp-inline-limit=n parameter controls the maximum number of bytes compared by the inlined
code. The default value is 64.
-mshorten-memrefs
-mno-shorten-memrefs
Do or do not attempt to make more use of compressed load/store instructions by replacing a load/store of 'base
register + large offset' with a new load/store of 'new base + small offset'. If the new base gets stored in a
compressed register, then the new load/store can be compressed. Currently targets 32-bit integer load/stores
only.
-mstrict-align
-mno-strict-align
Do not or do generate unaligned memory accesses. The default is set depending on whether the processor we are
optimizing for supports fast unaligned access or not.
-mscalar-strict-align
-mno-scalar-strict-align
Do not or do generate unaligned memory accesses. The default is set depending on whether the processor we are
optimizing for supports fast unaligned access or not. This is an alias for -mstrict-align.
-mvector-strict-align
-mno-vector-strict-align
Do not or do generate unaligned vector memory accesses. The default is set to off unless the processor we are
optimizing for explicitly supports element-misaligned vector memory access.
-mcmodel=medlow
Generate code for the medium-low code model. The program and its statically defined symbols must lie within a
single 2 GiB address range and must lie between absolute addresses -2 GiB and +2 GiB. Programs can be statically
or dynamically linked. This is the default code model unless GCC has been configured with --with-cmodel=
specifying a different default code model.
-mcmodel=medany
Generate code for the medium-any code model. The program and its statically defined symbols must be within any
single 2 GiB address range. Programs can be statically or dynamically linked.
The code generated by the medium-any code model is position-independent, but is not guaranteed to function
correctly when linked into position-independent executables or libraries.
-mcmodel=large
Generate code for a large code model, which has no restrictions on size or placement of symbols.
-mexplicit-relocs
-mno-exlicit-relocs
Use or do not use assembler relocation operators when dealing with symbolic addresses. The alternative is to use
assembler macros instead, which may limit optimization.
-mrelax
-mno-relax
Take advantage of linker relaxations to reduce the number of instructions required to materialize symbol
addresses. The default is to take advantage of linker relaxations.
-mriscv-attribute
-mno-riscv-attribute
Emit (do not emit) RISC-V attribute to record extra information into ELF objects. This feature requires at least
binutils 2.32.
-mcsr-check
-mno-csr-check
Enables or disables the CSR checking.
-malign-data=type
Control how GCC aligns variables and constants of array, structure, or union types. Supported values for type
are xlen which uses x register width as the alignment value, and natural which uses natural alignment. xlen is
the default.
-mbig-endian
Generate big-endian code. This is the default when GCC is configured for a riscv64be-*-* or riscv32be-*-*
target.
-mlittle-endian
Generate little-endian code. This is the default when GCC is configured for a riscv64-*-* or riscv32-*-* but not
a riscv64be-*-* or riscv32be-*-* target.
-mstack-protector-guard=guard
-mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset
Generate stack protection code using canary at guard. Supported locations are global for a global canary or tls
for per-thread canary in the TLS block.
With the latter choice the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset
furthermore specify which register to use as base register for reading the canary, and from what offset from that
base register. There is no default register or offset as this is entirely for use within the Linux kernel.
-mtls-dialect=desc
Use TLS descriptors as the thread-local storage mechanism for dynamic accesses of TLS variables.
-mtls-dialect=trad
Use traditional TLS as the thread-local storage mechanism for dynamic accesses of TLS variables. This is the
default.
RL78 Options
-msim
Links in additional target libraries to support operation within a simulator.
-mmul=none
-mmul=g10
-mmul=g13
-mmul=g14
-mmul=rl78
Specifies the type of hardware multiplication and division support to be used. The simplest is "none", which
uses software for both multiplication and division. This is the default. The "g13" value is for the hardware
multiply/divide peripheral found on the RL78/G13 (S2 core) targets. The "g14" value selects the use of the
multiplication and division instructions supported by the RL78/G14 (S3 core) parts. The value "rl78" is an alias
for "g14" and the value "mg10" is an alias for "none".
In addition a C preprocessor macro is defined, based upon the setting of this option. Possible values are:
"__RL78_MUL_NONE__", "__RL78_MUL_G13__" or "__RL78_MUL_G14__".
-mcpu=g10
-mcpu=g13
-mcpu=g14
-mcpu=rl78
Specifies the RL78 core to target. The default is the G14 core, also known as an S3 core or just RL78. The G13
or S2 core does not have multiply or divide instructions, instead it uses a hardware peripheral for these
operations. The G10 or S1 core does not have register banks, so it uses a different calling convention.
If this option is set it also selects the type of hardware multiply support to use, unless this is overridden by
an explicit -mmul=none option on the command line. Thus specifying -mcpu=g13 enables the use of the G13 hardware
multiply peripheral and specifying -mcpu=g10 disables the use of hardware multiplications altogether.
Note, although the RL78/G14 core is the default target, specifying -mcpu=g14 or -mcpu=rl78 on the command line
does change the behavior of the toolchain since it also enables G14 hardware multiply support. If these options
are not specified on the command line then software multiplication routines will be used even though the code
targets the RL78 core. This is for backwards compatibility with older toolchains which did not have hardware
multiply and divide support.
In addition a C preprocessor macro is defined, based upon the setting of this option. Possible values are:
"__RL78_G10__", "__RL78_G13__" or "__RL78_G14__".
-mg10
-mg13
-mg14
-mrl78
These are aliases for the corresponding -mcpu= option. They are provided for backwards compatibility.
-mallregs
Allow the compiler to use all of the available registers. By default registers "r24..r31" are reserved for use
in interrupt handlers. With this option enabled these registers can be used in ordinary functions as well.
-m64bit-doubles
-m32bit-doubles
Make the "double" data type be 64 bits (-m64bit-doubles) or 32 bits (-m32bit-doubles) in size. The default is
-m32bit-doubles.
-msave-mduc-in-interrupts
-mno-save-mduc-in-interrupts
Specifies that interrupt handler functions should preserve the MDUC registers. This is only necessary if normal
code might use the MDUC registers, for example because it performs multiplication and division operations. The
default is to ignore the MDUC registers as this makes the interrupt handlers faster. The target option -mg13
needs to be passed for this to work as this feature is only available on the G13 target (S2 core). The MDUC
registers will only be saved if the interrupt handler performs a multiplication or division operation or it calls
another function.
IBM RS/6000 and PowerPC Options
These -m options are defined for the IBM RS/6000 and PowerPC:
-mpowerpc-gpopt
-mno-powerpc-gpopt
-mpowerpc-gfxopt
-mno-powerpc-gfxopt
-mpowerpc64
-mno-powerpc64
-mmfcrf
-mno-mfcrf
-mpopcntb
-mno-popcntb
-mpopcntd
-mno-popcntd
-mfprnd
-mno-fprnd
-mcmpb
-mno-cmpb
-mhard-dfp
-mno-hard-dfp
You use these options to specify which instructions are available on the processor you are using. The default
value of these options is determined when configuring GCC. Specifying the -mcpu=cpu_type overrides the
specification of these options. We recommend you use the -mcpu=cpu_type option rather than the options listed
above.
Specifying -mpowerpc-gpopt allows GCC to use the optional PowerPC architecture instructions in the General
Purpose group, including floating-point square root. Specifying -mpowerpc-gfxopt allows GCC to use the optional
PowerPC architecture instructions in the Graphics group, including floating-point select.
The -mmfcrf option allows GCC to generate the move from condition register field instruction implemented on the
POWER4 processor and other processors that support the PowerPC V2.01 architecture. The -mpopcntb option allows
GCC to generate the popcount and double-precision FP reciprocal estimate instruction implemented on the POWER5
processor and other processors that support the PowerPC V2.02 architecture. The -mpopcntd option allows GCC to
generate the popcount instruction implemented on the POWER7 processor and other processors that support the
PowerPC V2.06 architecture. The -mfprnd option allows GCC to generate the FP round to integer instructions
implemented on the POWER5+ processor and other processors that support the PowerPC V2.03 architecture. The
-mcmpb option allows GCC to generate the compare bytes instruction implemented on the POWER6 processor and other
processors that support the PowerPC V2.05 architecture. The -mhard-dfp option allows GCC to generate the decimal
floating-point instructions implemented on some POWER processors.
The -mpowerpc64 option allows GCC to generate the additional 64-bit instructions that are found in the full
PowerPC64 architecture and to treat GPRs as 64-bit, doubleword quantities. GCC defaults to -mno-powerpc64.
-mcpu=cpu_type
Set architecture type, register usage, and instruction scheduling parameters for machine type cpu_type.
Supported values for cpu_type are 401, 403, 405, 405fp, 440, 440fp, 464, 464fp, 476, 476fp, 505, 601, 602, 603,
603e, 604, 604e, 620, 630, 740, 7400, 7450, 750, 801, 821, 823, 860, 970, 8540, a2, e300c2, e300c3, e500mc,
e500mc64, e5500, e6500, ec603e, G3, G4, G5, titan, power3, power4, power5, power5+, power6, power6x, power7,
power8, power9, power10, power11, powerpc, powerpc64, powerpc64le, rs64, and native.
-mcpu=powerpc, -mcpu=powerpc64, and -mcpu=powerpc64le specify pure 32-bit PowerPC (either endian), 64-bit big
endian PowerPC and 64-bit little endian PowerPC architecture machine types, with an appropriate, generic
processor model assumed for scheduling purposes.
Specifying native as cpu type detects and selects the architecture option that corresponds to the host processor
of the system performing the compilation. -mcpu=native has no effect if GCC does not recognize the processor.
The other options specify a specific processor. Code generated under those options runs best on that processor,
and may not run at all on others.
The -mcpu options automatically enable or disable the following options:
-maltivec -mfprnd -mhard-float -mmfcrf -mmultiple -mpopcntb -mpopcntd -mpowerpc64 -mpowerpc-gpopt
-mpowerpc-gfxopt -mmulhw -mdlmzb -mmfpgpr -mvsx -mcrypto -mhtm -mpower8-fusion -mquad-memory
-mquad-memory-atomic -mfloat128 -mfloat128-hardware -mprefixed -mpcrel -mmma -mrop-protect
The particular options set for any particular CPU varies between compiler versions, depending on what setting
seems to produce optimal code for that CPU; it doesn't necessarily reflect the actual hardware's capabilities.
If you wish to set an individual option to a particular value, you may specify it after the -mcpu option, like
-mcpu=970 -mno-altivec.
On AIX, the -maltivec and -mpowerpc64 options are not enabled or disabled by the -mcpu option at present because
AIX does not have full support for these options. You may still enable or disable them individually if you're
sure it'll work in your environment.
-mtune=cpu_type
Set the instruction scheduling parameters for machine type cpu_type, but do not set the architecture type or
register usage, as -mcpu=cpu_type does. The same values for cpu_type are used for -mtune as for -mcpu. If both
are specified, the code generated uses the architecture and registers set by -mcpu, but the scheduling parameters
set by -mtune.
-mcmodel=small
Generate PowerPC64 code for the small model: The TOC is limited to 64k.
-mcmodel=medium
Generate PowerPC64 code for the medium model: The TOC and other static data may be up to a total of 4G in size.
This is the default for 64-bit Linux.
-mcmodel=large
Generate PowerPC64 code for the large model: The TOC may be up to 4G in size. Other data and code is only
limited by the 64-bit address space.
-maltivec
-mno-altivec
Generate code that uses (does not use) AltiVec instructions, and also enable the use of built-in functions that
allow more direct access to the AltiVec instruction set. You may also need to set -mabi=altivec to adjust the
current ABI with AltiVec ABI enhancements.
When -maltivec is used, the element order for AltiVec intrinsics such as "vec_splat", "vec_extract", and
"vec_insert" match array element order corresponding to the endianness of the target. That is, element zero
identifies the leftmost element in a vector register when targeting a big-endian platform, and identifies the
rightmost element in a vector register when targeting a little-endian platform.
-mvrsave
-mno-vrsave
Generate VRSAVE instructions when generating AltiVec code.
-msecure-plt
Generate code that allows ld and ld.so to build executables and shared libraries with non-executable ".plt" and
".got" sections. This is a PowerPC 32-bit SYSV ABI option.
-mbss-plt
Generate code that uses a BSS ".plt" section that ld.so fills in, and requires ".plt" and ".got" sections that
are both writable and executable. This is a PowerPC 32-bit SYSV ABI option.
-msplit-patch-nops
When adding NOPs for a patchable area via the -fpatchable-function-entry option emit the "before" NOPs in front
of the global entry point and the "after" NOPs after the local entry point. This makes the sequence of NOPs not
consecutive when a global entry point is generated. Without this option the NOPs are emitted directly before and
after the local entry point, making them consecutive but moving global and local entry point further apart. If
only a single entry point is generated this option has no effect.
-misel
-mno-isel
This switch enables or disables the generation of ISEL instructions.
-mvsx
-mno-vsx
Generate code that uses (does not use) vector/scalar (VSX) instructions, and also enable the use of built-in
functions that allow more direct access to the VSX instruction set.
-mcrypto
-mno-crypto
Enable the use (disable) of the built-in functions that allow direct access to the cryptographic instructions
that were added in version 2.07 of the PowerPC ISA.
-mhtm
-mno-htm
Enable (disable) the use of the built-in functions that allow direct access to the Hardware Transactional Memory
(HTM) instructions that were added in version 2.07 of the PowerPC ISA.
-mpower8-fusion
-mno-power8-fusion
Generate code that keeps (does not keeps) some integer operations adjacent so that the instructions can be fused
together on power8 and later processors.
-mquad-memory
-mno-quad-memory
Generate code that uses (does not use) the non-atomic quad word memory instructions. The -mquad-memory option
requires use of 64-bit mode.
-mquad-memory-atomic
-mno-quad-memory-atomic
Generate code that uses (does not use) the atomic quad word memory instructions. The -mquad-memory-atomic option
requires use of 64-bit mode.
-mfloat128
-mno-float128
Enable/disable the __float128 keyword for IEEE 128-bit floating point and use either software emulation for IEEE
128-bit floating point or hardware instructions.
The VSX instruction set (-mvsx) must be enabled to use the IEEE 128-bit floating point support. The IEEE 128-bit
floating point is only supported on Linux.
The default for -mfloat128 is enabled on PowerPC Linux systems using the VSX instruction set, and disabled on
other systems.
If you use the ISA 3.0 instruction set (-mcpu=power9) on a 64-bit system, the IEEE 128-bit floating point support
will also enable the generation of ISA 3.0 IEEE 128-bit floating point instructions. Otherwise, if you do not
specify to generate ISA 3.0 instructions or you are targeting a 32-bit big endian system, IEEE 128-bit floating
point will be done with software emulation.
-mfloat128-hardware
-mno-float128-hardware
Enable/disable using ISA 3.0 hardware instructions to support the __float128 data type.
The default for -mfloat128-hardware is enabled on PowerPC Linux systems using the ISA 3.0 instruction set, and
disabled on other systems.
-m32
-m64
Generate code for 32-bit or 64-bit environments of Darwin and SVR4 targets (including GNU/Linux). The 32-bit
environment sets int, long and pointer to 32 bits and generates code that runs on any PowerPC variant. The
64-bit environment sets int to 32 bits and long and pointer to 64 bits, and generates code for PowerPC64, as for
-mpowerpc64.
-mfull-toc
-mno-fp-in-toc
-mno-sum-in-toc
-mminimal-toc
Modify generation of the TOC (Table Of Contents), which is created for every executable file. The -mfull-toc
option is selected by default. In that case, GCC allocates at least one TOC entry for each unique non-automatic
variable reference in your program. GCC also places floating-point constants in the TOC. However, only 16,384
entries are available in the TOC.
If you receive a linker error message that saying you have overflowed the available TOC space, you can reduce the
amount of TOC space used with the -mno-fp-in-toc and -mno-sum-in-toc options. -mno-fp-in-toc prevents GCC from
putting floating-point constants in the TOC and -mno-sum-in-toc forces GCC to generate code to calculate the sum
of an address and a constant at run time instead of putting that sum into the TOC. You may specify one or both
of these options. Each causes GCC to produce very slightly slower and larger code at the expense of conserving
TOC space.
If you still run out of space in the TOC even when you specify both of these options, specify -mminimal-toc
instead. This option causes GCC to make only one TOC entry for every file. When you specify this option, GCC
produces code that is slower and larger but which uses extremely little TOC space. You may wish to use this
option only on files that contain less frequently-executed code.
-maix64
-maix32
Enable 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bit "long" type, and the infrastructure needed
to support them. Specifying -maix64 implies -mpowerpc64, while -maix32 disables the 64-bit ABI and implies
-mno-powerpc64. GCC defaults to -maix32.
-mxl-compat
-mno-xl-compat
Produce code that conforms more closely to IBM XL compiler semantics when using AIX-compatible ABI. Pass
floating-point arguments to prototyped functions beyond the register save area (RSA) on the stack in addition to
argument FPRs. Do not assume that most significant double in 128-bit long double value is properly rounded when
comparing values and converting to double. Use XL symbol names for long double support routines.
The AIX calling convention was extended but not initially documented to handle an obscure K&R C case of calling a
function that takes the address of its arguments with fewer arguments than declared. IBM XL compilers access
floating-point arguments that do not fit in the RSA from the stack when a subroutine is compiled without
optimization. Because always storing floating-point arguments on the stack is inefficient and rarely needed,
this option is not enabled by default and only is necessary when calling subroutines compiled by IBM XL compilers
without optimization.
-mpe
Support IBM RS/6000 SP Parallel Environment (PE). Link an application written to use message passing with
special startup code to enable the application to run. The system must have PE installed in the standard
location (/usr/lpp/ppe.poe/), or the specs file must be overridden with the -specs= option to specify the
appropriate directory location. The Parallel Environment does not support threads, so the -mpe option and the
-pthread option are incompatible.
-malign-natural
-malign-power
On AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux, the option -malign-natural overrides the ABI-defined
alignment of larger types, such as floating-point doubles, on their natural size-based boundary. The option
-malign-power instructs GCC to follow the ABI-specified alignment rules. GCC defaults to the standard alignment
defined in the ABI.
On 64-bit Darwin, natural alignment is the default, and -malign-power is not supported.
-msoft-float
-mhard-float
Generate code that does not use (uses) the floating-point register set. Software floating-point emulation is
provided if you use the -msoft-float option, and pass the option to GCC when linking.
-mmultiple
-mno-multiple
Generate code that uses (does not use) the load multiple word instructions and the store multiple word
instructions. These instructions are generated by default on POWER systems, and not generated on PowerPC
systems. Do not use -mmultiple on little-endian PowerPC systems, since those instructions do not work when the
processor is in little-endian mode. The exceptions are PPC740 and PPC750 which permit these instructions in
little-endian mode.
-mupdate
-mno-update
Generate code that uses (does not use) the load or store instructions that update the base register to the
address of the calculated memory location. These instructions are generated by default. If you use -mno-update,
there is a small window between the time that the stack pointer is updated and the address of the previous frame
is stored, which means code that walks the stack frame across interrupts or signals may get corrupted data.
-mavoid-indexed-addresses
-mno-avoid-indexed-addresses
Generate code that tries to avoid (not avoid) the use of indexed load or store instructions. These instructions
can incur a performance penalty on Power6 processors in certain situations, such as when stepping through large
arrays that cross a 16M boundary. This option is enabled by default when targeting Power6 and disabled
otherwise.
-mfused-madd
-mno-fused-madd
Generate code that uses (does not use) the floating-point multiply and accumulate instructions. These
instructions are generated by default if hardware floating point is used. The machine-dependent -mfused-madd
option is now mapped to the machine-independent -ffp-contract=fast option, and -mno-fused-madd is mapped to
-ffp-contract=off.
-mmulhw
-mno-mulhw
Generate code that uses (does not use) the half-word multiply and multiply-accumulate instructions on the IBM
405, 440, 464 and 476 processors. These instructions are generated by default when targeting those processors.
-mdlmzb
-mno-dlmzb
Generate code that uses (does not use) the string-search dlmzb instruction on the IBM 405, 440, 464 and 476
processors. This instruction is generated by default when targeting those processors.
-mno-bit-align
-mbit-align
On System V.4 and embedded PowerPC systems do not (do) force structures and unions that contain bit-fields to be
aligned to the base type of the bit-field.
For example, by default a structure containing nothing but 8 "unsigned" bit-fields of length 1 is aligned to a
4-byte boundary and has a size of 4 bytes. By using -mno-bit-align, the structure is aligned to a 1-byte
boundary and is 1 byte in size.
-mno-strict-align
-mstrict-align
On System V.4 and embedded PowerPC systems do not (do) assume that unaligned memory references are handled by the
system.
-mrelocatable
-mno-relocatable
Generate code that allows (does not allow) a static executable to be relocated to a different address at run
time. A simple embedded PowerPC system loader should relocate the entire contents of ".got2" and 4-byte
locations listed in the ".fixup" section, a table of 32-bit addresses generated by this option. For this to
work, all objects linked together must be compiled with -mrelocatable or -mrelocatable-lib. -mrelocatable code
aligns the stack to an 8-byte boundary.
-mrelocatable-lib
-mno-relocatable-lib
Like -mrelocatable, -mrelocatable-lib generates a ".fixup" section to allow static executables to be relocated at
run time, but -mrelocatable-lib does not use the smaller stack alignment of -mrelocatable. Objects compiled with
-mrelocatable-lib may be linked with objects compiled with any combination of the -mrelocatable options.
-mno-toc
-mtoc
On System V.4 and embedded PowerPC systems do not (do) assume that register 2 contains a pointer to a global area
pointing to the addresses used in the program.
-mlittle
-mlittle-endian
On System V.4 and embedded PowerPC systems compile code for the processor in little-endian mode. The
-mlittle-endian option is the same as -mlittle.
-mbig
-mbig-endian
On System V.4 and embedded PowerPC systems compile code for the processor in big-endian mode. The -mbig-endian
option is the same as -mbig.
-mdynamic-no-pic
On Darwin / macOS systems, compile code so that it is not relocatable, but that its external references are
relocatable. The resulting code is suitable for applications, but not shared libraries.
-msingle-pic-base
Treat the register used for PIC addressing as read-only, rather than loading it in the prologue for each
function. The runtime system is responsible for initializing this register with an appropriate value before
execution begins.
-mprioritize-restricted-insns=priority
This option controls the priority that is assigned to dispatch-slot restricted instructions during the second
scheduling pass. The argument priority takes the value 0, 1, or 2 to assign no, highest, or second-highest
(respectively) priority to dispatch-slot restricted instructions.
-msched-costly-dep=dependence_type
This option controls which dependences are considered costly by the target during instruction scheduling. The
argument dependence_type takes one of the following values:
no No dependence is costly.
all All dependences are costly.
true_store_to_load
A true dependence from store to load is costly.
store_to_load
Any dependence from store to load is costly.
number
Any dependence for which the latency is greater than or equal to number is costly.
-minsert-sched-nops=scheme
This option controls which NOP insertion scheme is used during the second scheduling pass. The argument scheme
takes one of the following values:
no Don't insert NOPs.
pad Pad with NOPs any dispatch group that has vacant issue slots, according to the scheduler's grouping.
regroup_exact
Insert NOPs to force costly dependent insns into separate groups. Insert exactly as many NOPs as needed to
force an insn to a new group, according to the estimated processor grouping.
number
Insert NOPs to force costly dependent insns into separate groups. Insert number NOPs to force an insn to a
new group.
-mcall-sysv
On System V.4 and embedded PowerPC systems compile code using calling conventions that adhere to the March 1995
draft of the System V Application Binary Interface, PowerPC processor supplement. This is the default unless you
configured GCC using powerpc-*-eabiaix.
-mcall-sysv-eabi
-mcall-eabi
Specify both -mcall-sysv and -meabi options.
-mcall-sysv-noeabi
Specify both -mcall-sysv and -mno-eabi options.
-mcall-aixdesc
On System V.4 and embedded PowerPC systems compile code for the AIX operating system.
-mcall-linux
On System V.4 and embedded PowerPC systems compile code for the Linux-based GNU system.
-mcall-freebsd
On System V.4 and embedded PowerPC systems compile code for the FreeBSD operating system.
-mcall-netbsd
On System V.4 and embedded PowerPC systems compile code for the NetBSD operating system.
-mcall-openbsd
On System V.4 and embedded PowerPC systems compile code for the OpenBSD operating system.
-mtraceback=traceback_type
Select the type of traceback table. Valid values for traceback_type are full, part, and no.
-maix-struct-return
Return all structures in memory (as specified by the AIX ABI).
-msvr4-struct-return
Return structures smaller than 8 bytes in registers (as specified by the SVR4 ABI).
-mabi=abi-type
Extend the current ABI with a particular extension, or remove such extension. Valid values are: altivec, no-
altivec, ibmlongdouble, ieeelongdouble, elfv1, elfv2, and for AIX: vec-extabi, vec-default.
-mabi=ibmlongdouble
Change the current ABI to use IBM extended-precision long double. This is not likely to work if your system
defaults to using IEEE extended-precision long double. If you change the long double type from IEEE extended-
precision, the compiler issues a warning unless you use the -Wno-psabi option. Requires -mlong-double-128 to be
enabled.
-mabi=ieeelongdouble
Change the current ABI to use IEEE extended-precision long double. This is not likely to work if your system
defaults to using IBM extended-precision long double. If you change the long double type from IBM extended-
precision, the compiler issues a warning unless you use the -Wno-psabi option. Requires -mlong-double-128 to be
enabled.
-mabi=elfv1
Change the current ABI to use the ELFv1 ABI. This is the default ABI for big-endian PowerPC 64-bit Linux.
Overriding the default ABI requires special system support and is likely to fail in spectacular ways.
-mabi=elfv2
Change the current ABI to use the ELFv2 ABI. This is the default ABI for little-endian PowerPC 64-bit Linux.
Overriding the default ABI requires special system support and is likely to fail in spectacular ways.
-mgnu-attribute
-mno-gnu-attribute
Emit .gnu_attribute assembly directives to set tag/value pairs in a .gnu.attributes section that specify ABI
variations in function parameters or return values.
-mprototype
-mno-prototype
On System V.4 and embedded PowerPC systems assume that all calls to variable argument functions are properly
prototyped. Otherwise, the compiler must insert an instruction before every non-prototyped call to set or clear
bit 6 of the condition code register ("CR") to indicate whether floating-point values are passed in the floating-
point registers in case the function takes variable arguments. With -mprototype, only calls to prototyped
variable argument functions set or clear the bit.
-msim
On embedded PowerPC systems, assume that the startup module is called sim-crt0.o and that the standard C
libraries are libsim.a and libc.a. This is the default for powerpc-*-eabisim configurations.
-mmvme
On embedded PowerPC systems, assume that the startup module is called crt0.o and the standard C libraries are
libmvme.a and libc.a.
-mads
On embedded PowerPC systems, assume that the startup module is called crt0.o and the standard C libraries are
libads.a and libc.a.
-myellowknife
On embedded PowerPC systems, assume that the startup module is called crt0.o and the standard C libraries are
libyk.a and libc.a.
-mvxworks
On System V.4 and embedded PowerPC systems, specify that you are compiling for a VxWorks system.
-memb
On embedded PowerPC systems, set the "PPC_EMB" bit in the ELF flags header to indicate that eabi extended
relocations are used.
-meabi
-mno-eabi
On System V.4 and embedded PowerPC systems do (do not) adhere to the Embedded Applications Binary Interface
(EABI), which is a set of modifications to the System V.4 specifications. Selecting -meabi means that the stack
is aligned to an 8-byte boundary, a function "__eabi" is called from "main" to set up the EABI environment, and
the -msdata option can use both "r2" and "r13" to point to two separate small data areas. Selecting -mno-eabi
means that the stack is aligned to a 16-byte boundary, no EABI initialization function is called from "main", and
the -msdata option only uses "r13" to point to a single small data area. The -meabi option is on by default if
you configured GCC using one of the powerpc*-*-eabi* options.
-msdata=eabi
On System V.4 and embedded PowerPC systems, put small initialized "const" global and static data in the ".sdata2"
section, which is pointed to by register "r2". Put small initialized non-"const" global and static data in the
".sdata" section, which is pointed to by register "r13". Put small uninitialized global and static data in the
".sbss" section, which is adjacent to the ".sdata" section. The -msdata=eabi option is incompatible with the
-mrelocatable option. The -msdata=eabi option also sets the -memb option.
-msdata=sysv
On System V.4 and embedded PowerPC systems, put small global and static data in the ".sdata" section, which is
pointed to by register "r13". Put small uninitialized global and static data in the ".sbss" section, which is
adjacent to the ".sdata" section. The -msdata=sysv option is incompatible with the -mrelocatable option.
-msdata=default
-msdata
On System V.4 and embedded PowerPC systems, if -meabi is used, compile code the same as -msdata=eabi, otherwise
compile code the same as -msdata=sysv.
-msdata=data
On System V.4 and embedded PowerPC systems, put small global data in the ".sdata" section. Put small
uninitialized global data in the ".sbss" section. Do not use register "r13" to address small data however. This
is the default behavior unless other -msdata options are used.
-msdata=none
-mno-sdata
On embedded PowerPC systems, put all initialized global and static data in the ".data" section, and all
uninitialized data in the ".bss" section.
-mreadonly-in-sdata
Put read-only objects in the ".sdata" section as well. This is the default.
-mblock-move-inline-limit=num
Inline all block moves (such as calls to "memcpy" or structure copies) less than or equal to num bytes. The
minimum value for num is 32 bytes on 32-bit targets and 64 bytes on 64-bit targets. The default value is target-
specific.
-mblock-compare-inline-limit=num
Generate non-looping inline code for all block compares (such as calls to "memcmp" or structure compares) less
than or equal to num bytes. If num is 0, all inline expansion (non-loop and loop) of block compare is disabled.
The default value is target-specific.
-mblock-compare-inline-loop-limit=num
Generate an inline expansion using loop code for all block compares that are less than or equal to num bytes, but
greater than the limit for non-loop inline block compare expansion. If the block length is not constant, at most
num bytes will be compared before "memcmp" is called to compare the remainder of the block. The default value is
target-specific.
-mstring-compare-inline-limit=num
Compare at most num string bytes with inline code. If the difference or end of string is not found at the end of
the inline compare a call to "strcmp" or "strncmp" will take care of the rest of the comparison. The default is
64 bytes.
-G num
On embedded PowerPC systems, put global and static items less than or equal to num bytes into the small data or
BSS sections instead of the normal data or BSS section. By default, num is 8. The -G num switch is also passed
to the linker. All modules should be compiled with the same -G num value.
-mregnames
-mno-regnames
On System V.4 and embedded PowerPC systems do (do not) emit register names in the assembly language output using
symbolic forms.
-mlongcall
-mno-longcall
By default assume that all calls are far away so that a longer and more expensive calling sequence is required.
This is required for calls farther than 32 megabytes (33,554,432 bytes) from the current location. A short call
is generated if the compiler knows the call cannot be that far away. This setting can be overridden by the
"shortcall" function attribute, or by #pragma longcall(0).
Some linkers are capable of detecting out-of-range calls and generating glue code on the fly. On these systems,
long calls are unnecessary and generate slower code. As of this writing, the AIX linker can do this, as can the
GNU linker for PowerPC/64. It is planned to add this feature to the GNU linker for 32-bit PowerPC systems as
well.
On PowerPC64 ELFv2 and 32-bit PowerPC systems with newer GNU linkers, GCC can generate long calls using an inline
PLT call sequence (see -mpltseq). PowerPC with -mbss-plt and PowerPC64 ELFv1 (big-endian) do not support inline
PLT calls.
On Darwin/PPC systems, "#pragma longcall" generates "jbsr callee, L42", plus a branch island (glue code). The
two target addresses represent the callee and the branch island. The Darwin/PPC linker prefers the first address
and generates a "bl callee" if the PPC "bl" instruction reaches the callee directly; otherwise, the linker
generates "bl L42" to call the branch island. The branch island is appended to the body of the calling function;
it computes the full 32-bit address of the callee and jumps to it.
On Mach-O (Darwin) systems, this option directs the compiler emit to the glue for every direct call, and the
Darwin linker decides whether to use or discard it.
In the future, GCC may ignore all longcall specifications when the linker is known to generate glue.
-mpltseq
-mno-pltseq
Implement (do not implement) -fno-plt and long calls using an inline PLT call sequence that supports lazy linking
and long calls to functions in dlopen'd shared libraries. Inline PLT calls are only supported on PowerPC64 ELFv2
and 32-bit PowerPC systems with newer GNU linkers, and are enabled by default if the support is detected when
configuring GCC, and, in the case of 32-bit PowerPC, if GCC is configured with --enable-secureplt. -mpltseq code
and -mbss-plt 32-bit PowerPC relocatable objects may not be linked together.
-mtls-markers
-mno-tls-markers
Mark (do not mark) calls to "__tls_get_addr" with a relocation specifying the function argument. The relocation
allows the linker to reliably associate function call with argument setup instructions for TLS optimization,
which in turn allows GCC to better schedule the sequence.
-mrecip
-mno-recip
This option enables use of the reciprocal estimate and reciprocal square root estimate instructions with
additional Newton-Raphson steps to increase precision instead of doing a divide or square root and divide for
floating-point arguments. You should use the -ffast-math option when using -mrecip (or at least
-funsafe-math-optimizations, -ffinite-math-only, -freciprocal-math and -fno-trapping-math). Note that while the
throughput of the sequence is generally higher than the throughput of the non-reciprocal instruction, the
precision of the sequence can be decreased by up to 2 ulp (i.e. the inverse of 1.0 equals 0.99999994) for
reciprocal square roots.
-mrecip=opt
This option controls which reciprocal estimate instructions may be used. opt is a comma-separated list of
options, which may be preceded by a "!" to invert the option:
all Enable all estimate instructions.
default
Enable the default instructions, equivalent to -mrecip.
none
Disable all estimate instructions, equivalent to -mno-recip.
div Enable the reciprocal approximation instructions for both single and double precision.
divf
Enable the single-precision reciprocal approximation instructions.
divd
Enable the double-precision reciprocal approximation instructions.
rsqrt
Enable the reciprocal square root approximation instructions for both single and double precision.
rsqrtf
Enable the single-precision reciprocal square root approximation instructions.
rsqrtd
Enable the double-precision reciprocal square root approximation instructions.
So, for example, -mrecip=all,!rsqrtd enables all of the reciprocal estimate instructions, except for the
"FRSQRTE", "XSRSQRTEDP", and "XVRSQRTEDP" instructions which handle the double-precision reciprocal square root
calculations.
-mrecip-precision
-mno-recip-precision
Assume (do not assume) that the reciprocal estimate instructions provide higher-precision estimates than is
mandated by the PowerPC ABI. Selecting -mcpu=power6, -mcpu=power7 or -mcpu=power8 automatically selects
-mrecip-precision. The double-precision square root estimate instructions are not generated by default on low-
precision machines, since they do not provide an estimate that converges after three steps.
-mveclibabi=type
Specifies the ABI type to use for vectorizing intrinsics using an external library. The only type supported at
present is mass, which specifies to use IBM's Mathematical Acceleration Subsystem (MASS) libraries for
vectorizing intrinsics using external libraries. GCC currently emits calls to "acosd2", "acosf4", "acoshd2",
"acoshf4", "asind2", "asinf4", "asinhd2", "asinhf4", "atan2d2", "atan2f4", "atand2", "atanf4", "atanhd2",
"atanhf4", "cbrtd2", "cbrtf4", "cosd2", "cosf4", "coshd2", "coshf4", "erfcd2", "erfcf4", "erfd2", "erff4",
"exp2d2", "exp2f4", "expd2", "expf4", "expm1d2", "expm1f4", "hypotd2", "hypotf4", "lgammad2", "lgammaf4",
"log10d2", "log10f4", "log1pd2", "log1pf4", "log2d2", "log2f4", "logd2", "logf4", "powd2", "powf4", "sind2",
"sinf4", "sinhd2", "sinhf4", "sqrtd2", "sqrtf4", "tand2", "tanf4", "tanhd2", and "tanhf4" when generating code
for power7. Both -ftree-vectorize and -funsafe-math-optimizations must also be enabled. The MASS libraries must
be specified at link time.
-mfriz
-mno-friz
Generate (do not generate) the "friz" instruction when the -funsafe-math-optimizations option is used to optimize
rounding of floating-point values to 64-bit integer and back to floating point. The "friz" instruction does not
return the same value if the floating-point number is too large to fit in an integer.
-mpointers-to-nested-functions
-mno-pointers-to-nested-functions
Generate (do not generate) code to load up the static chain register ("r11") when calling through a pointer on
AIX and 64-bit Linux systems where a function pointer points to a 3-word descriptor giving the function address,
TOC value to be loaded in register "r2", and static chain value to be loaded in register "r11". The
-mpointers-to-nested-functions is on by default. You cannot call through pointers to nested functions or
pointers to functions compiled in other languages that use the static chain if you use
-mno-pointers-to-nested-functions.
-msave-toc-indirect
-mno-save-toc-indirect
Generate (do not generate) code to save the TOC value in the reserved stack location in the function prologue if
the function calls through a pointer on AIX and 64-bit Linux systems. If the TOC value is not saved in the
prologue, it is saved just before the call through the pointer. The -mno-save-toc-indirect option is the
default.
-mcompat-align-parm
-mno-compat-align-parm
Generate (do not generate) code to pass structure parameters with a maximum alignment of 64 bits, for
compatibility with older versions of GCC.
Older versions of GCC (prior to 4.9.0) incorrectly did not align a structure parameter on a 128-bit boundary when
that structure contained a member requiring 128-bit alignment. This is corrected in more recent versions of GCC.
This option may be used to generate code that is compatible with functions compiled with older versions of GCC.
The -mno-compat-align-parm option is the default.
-mstack-protector-guard=guard
-mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset
Generate stack protection code using canary at guard. Supported locations are global for global canary or tls
for per-thread canary in the TLS block (the default with GNU libc version 2.4 or later).
With the latter choice the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset
furthermore specify which register to use as base register for reading the canary, and from what offset from that
base register. The default for those is as specified in the relevant ABI.
-mpcrel
-mno-pcrel
Generate (do not generate) pc-relative addressing. The -mpcrel option requires that the medium code model
(-mcmodel=medium) and prefixed addressing (-mprefixed) options are enabled.
-mprefixed
-mno-prefixed
Generate (do not generate) addressing modes using prefixed load and store instructions. The -mprefixed option
requires that the option -mcpu=power10 (or later) is enabled.
-mmma
-mno-mma
Generate (do not generate) the MMA instructions. The -mma option requires that the option -mcpu=power10 (or
later) is enabled.
-mrop-protect
-mno-rop-protect
Generate (do not generate) ROP protection instructions when the target processor supports them. Currently this
option disables the shrink-wrap optimization (-fshrink-wrap).
-mprivileged
-mno-privileged
Generate (do not generate) code that will run in privileged state.
-mblock-ops-unaligned-vsx
-mno-block-ops-unaligned-vsx
Generate (do not generate) unaligned vsx loads and stores for inline expansion of "memcpy" and "memmove".
--param rs6000-vect-unroll-limit=
The vectorizer will check with target information to determine whether it would be beneficial to unroll the main
vectorized loop and by how much. This parameter sets the upper bound of how much the vectorizer will unroll the
main loop. The default value is four.
RX Options
These command-line options are defined for RX targets:
-m64bit-doubles
-m32bit-doubles
Make the "double" data type be 64 bits (-m64bit-doubles) or 32 bits (-m32bit-doubles) in size. The default is
-m32bit-doubles. Note RX floating-point hardware only works on 32-bit values, which is why the default is
-m32bit-doubles.
-fpu
-nofpu
Enables (-fpu) or disables (-nofpu) the use of RX floating-point hardware. The default is enabled for the RX600
series and disabled for the RX200 series.
Floating-point instructions are only generated for 32-bit floating-point values, however, so the FPU hardware is
not used for doubles if the -m64bit-doubles option is used.
Note If the -fpu option is enabled then -funsafe-math-optimizations is also enabled automatically. This is
because the RX FPU instructions are themselves unsafe.
-mcpu=name
Selects the type of RX CPU to be targeted. Currently three types are supported, the generic RX600 and RX200
series hardware and the specific RX610 CPU. The default is RX600.
The only difference between RX600 and RX610 is that the RX610 does not support the "MVTIPL" instruction.
The RX200 series does not have a hardware floating-point unit and so -nofpu is enabled by default when this type
is selected.
-mbig-endian-data
-mlittle-endian-data
Store data (but not code) in the big-endian format. The default is -mlittle-endian-data, i.e. to store data in
the little-endian format.
-msmall-data-limit=N
Specifies the maximum size in bytes of global and static variables which can be placed into the small data area.
Using the small data area can lead to smaller and faster code, but the size of area is limited and it is up to
the programmer to ensure that the area does not overflow. Also when the small data area is used one of the RX's
registers (usually "r13") is reserved for use pointing to this area, so it is no longer available for use by the
compiler. This could result in slower and/or larger code if variables are pushed onto the stack instead of being
held in this register.
Note, common variables (variables that have not been initialized) and constants are not placed into the small
data area as they are assigned to other sections in the output executable.
The default value is zero, which disables this feature. Note, this feature is not enabled by default with higher
optimization levels (-O2 etc) because of the potentially detrimental effects of reserving a register. It is up
to the programmer to experiment and discover whether this feature is of benefit to their program. See the
description of the -mpid option for a description of how the actual register to hold the small data area pointer
is chosen.
-msim
-mno-sim
Use the simulator runtime. The default is to use the libgloss board-specific runtime.
-mas100-syntax
-mno-as100-syntax
When generating assembler output use a syntax that is compatible with Renesas's AS100 assembler. This syntax can
also be handled by the GAS assembler, but it has some restrictions so it is not generated by default.
-mmax-constant-size=N
Specifies the maximum size, in bytes, of a constant that can be used as an operand in a RX instruction. Although
the RX instruction set does allow constants of up to 4 bytes in length to be used in instructions, a longer value
equates to a longer instruction. Thus in some circumstances it can be beneficial to restrict the size of
constants that are used in instructions. Constants that are too big are instead placed into a constant pool and
referenced via register indirection.
The value N can be between 0 and 4. A value of 0 (the default) or 4 means that constants of any size are
allowed.
-mrelax
Enable linker relaxation. Linker relaxation is a process whereby the linker attempts to reduce the size of a
program by finding shorter versions of various instructions. Disabled by default.
-mint-register=N
Specify the number of registers to reserve for fast interrupt handler functions. The value N can be between 0
and 4. A value of 1 means that register "r13" is reserved for the exclusive use of fast interrupt handlers. A
value of 2 reserves "r13" and "r12". A value of 3 reserves "r13", "r12" and "r11", and a value of 4 reserves
"r13" through "r10". A value of 0, the default, does not reserve any registers.
-msave-acc-in-interrupts
Specifies that interrupt handler functions should preserve the accumulator register. This is only necessary if
normal code might use the accumulator register, for example because it performs 64-bit multiplications. The
default is to ignore the accumulator as this makes the interrupt handlers faster.
-mpid
-mno-pid
Enables the generation of position independent data. When enabled any access to constant data is done via an
offset from a base address held in a register. This allows the location of constant data to be determined at run
time without requiring the executable to be relocated, which is a benefit to embedded applications with tight
memory constraints. Data that can be modified is not affected by this option.
Note, using this feature reserves a register, usually "r13", for the constant data base address. This can result
in slower and/or larger code, especially in complicated functions.
The actual register chosen to hold the constant data base address depends upon whether the -msmall-data-limit
and/or the -mint-register command-line options are enabled. Starting with register "r13" and proceeding
downwards, registers are allocated first to satisfy the requirements of -mint-register, then -mpid and finally
-msmall-data-limit. Thus it is possible for the small data area register to be "r8" if both -mint-register=4 and
-mpid are specified on the command line.
By default this feature is not enabled. The default can be restored via the -mno-pid command-line option.
-mno-warn-multiple-fast-interrupts
-mwarn-multiple-fast-interrupts
Prevents GCC from issuing a warning message if it finds more than one fast interrupt handler when it is compiling
a file. The default is to issue a warning for each extra fast interrupt handler found, as the RX only supports
one such interrupt.
-mallow-string-insns
-mno-allow-string-insns
Enables or disables the use of the string manipulation instructions "SMOVF", "SCMPU", "SMOVB", "SMOVU", "SUNTIL"
"SWHILE" and also the "RMPA" instruction. These instructions may prefetch data, which is not safe to do if
accessing an I/O register. (See section 12.2.7 of the RX62N Group User's Manual for more information).
The default is to allow these instructions, but it is not possible for GCC to reliably detect all circumstances
where a string instruction might be used to access an I/O register, so their use cannot be disabled
automatically. Instead it is reliant upon the programmer to use the -mno-allow-string-insns option if their
program accesses I/O space.
When the instructions are enabled GCC defines the C preprocessor symbol "__RX_ALLOW_STRING_INSNS__", otherwise it
defines the symbol "__RX_DISALLOW_STRING_INSNS__".
-mjsr
-mno-jsr
Use only (or not only) "JSR" instructions to access functions. This option can be used when code size exceeds
the range of "BSR" instructions. Note that -mno-jsr does not mean to not use "JSR" but instead means that any
type of branch may be used.
Note: The generic GCC command-line option -ffixed-reg has special significance to the RX port when used with the
"interrupt" function attribute. This attribute indicates a function intended to process fast interrupts. GCC
ensures that it only uses the registers "r10", "r11", "r12" and/or "r13" and only provided that the normal use of the
corresponding registers have been restricted via the -ffixed-reg or -mint-register command-line options.
S/390 and zSeries Options
These are the -m options defined for the S/390 and zSeries architecture.
-mhard-float
-msoft-float
Use (do not use) the hardware floating-point instructions and registers for floating-point operations. When
-msoft-float is specified, functions in libgcc.a are used to perform floating-point operations. When
-mhard-float is specified, the compiler generates IEEE floating-point instructions. This is the default.
-mhard-dfp
-mno-hard-dfp
Use (do not use) the hardware decimal-floating-point instructions for decimal-floating-point operations. When
-mno-hard-dfp is specified, functions in libgcc.a are used to perform decimal-floating-point operations. When
-mhard-dfp is specified, the compiler generates decimal-floating-point hardware instructions. This is the
default for -march=z9-ec or higher.
-mlong-double-64
-mlong-double-128
These switches control the size of "long double" type. A size of 64 bits makes the "long double" type equivalent
to the "double" type. This is the default.
-mbackchain
-mno-backchain
Store (do not store) the address of the caller's frame as backchain pointer into the callee's stack frame. A
backchain may be needed to allow debugging using tools that do not understand DWARF call frame information. When
-mno-packed-stack is in effect, the backchain pointer is stored at the bottom of the stack frame; when
-mpacked-stack is in effect, the backchain is placed into the topmost word of the 96/160 byte register save area.
In general, code compiled with -mbackchain is call-compatible with code compiled with -mno-backchain; however,
use of the backchain for debugging purposes usually requires that the whole binary is built with -mbackchain.
Note that the combination of -mbackchain, -mpacked-stack and -mhard-float is not supported. In order to build a
linux kernel use -msoft-float.
The default is to not maintain the backchain.
-mpacked-stack
-mno-packed-stack
Use (do not use) the packed stack layout. When -mno-packed-stack is specified, the compiler uses the all fields
of the 96/160 byte register save area only for their default purpose; unused fields still take up stack space.
When -mpacked-stack is specified, register save slots are densely packed at the top of the register save area;
unused space is reused for other purposes, allowing for more efficient use of the available stack space.
However, when -mbackchain is also in effect, the topmost word of the save area is always used to store the
backchain, and the return address register is always saved two words below the backchain.
As long as the stack frame backchain is not used, code generated with -mpacked-stack is call-compatible with code
generated with -mno-packed-stack. Note that some non-FSF releases of GCC 2.95 for S/390 or zSeries generated
code that uses the stack frame backchain at run time, not just for debugging purposes. Such code is not call-
compatible with code compiled with -mpacked-stack. Also, note that the combination of -mbackchain,
-mpacked-stack and -mhard-float is not supported. In order to build a linux kernel use -msoft-float.
The default is to not use the packed stack layout.
-msmall-exec
-mno-small-exec
Generate (or do not generate) code using the "bras" instruction to do subroutine calls. This only works reliably
if the total executable size does not exceed 64k. The default is to use the "basr" instruction instead, which
does not have this limitation.
-m64
-m31
When -m31 is specified, generate code compliant to the GNU/Linux for S/390 ABI. When -m64 is specified, generate
code compliant to the GNU/Linux for zSeries ABI. This allows GCC in particular to generate 64-bit instructions.
For the s390 targets, the default is -m31, while the s390x targets default to -m64.
-mzarch
-mesa
When -mzarch is specified, generate code using the instructions available on z/Architecture. When -mesa is
specified, generate code using the instructions available on ESA/390. Note that -mesa is not possible with -m64.
When generating code compliant to the GNU/Linux for S/390 ABI, the default is -mesa. When generating code
compliant to the GNU/Linux for zSeries ABI, the default is -mzarch.
-mhtm
-mno-htm
The -mhtm option enables a set of builtins making use of instructions available with the transactional execution
facility introduced with the IBM zEnterprise EC12 machine generation S/390 System z Built-in Functions. -mhtm is
enabled by default when using -march=zEC12.
-mvx
-mno-vx
When -mvx is specified, generate code using the instructions available with the vector extension facility
introduced with the IBM z13 machine generation. This option changes the ABI for some vector type values with
regard to alignment and calling conventions. In case vector type values are being used in an ABI-relevant
context a GAS .gnu_attribute command will be added to mark the resulting binary with the ABI used. -mvx is
enabled by default when using -march=z13.
-mzvector
-mno-zvector
The -mzvector option enables vector language extensions and builtins using instructions available with the vector
extension facility introduced with the IBM z13 machine generation. This option adds support for vector to be
used as a keyword to define vector type variables and arguments. vector is only available when GNU extensions
are enabled. It will not be expanded when requesting strict standard compliance e.g. with -std=c99. In addition
to the GCC low-level builtins -mzvector enables a set of builtins added for compatibility with AltiVec-style
implementations like Power and Cell. In order to make use of these builtins the header file vecintrin.h needs to
be included. -mzvector is disabled by default.
-mmvcle
-mno-mvcle
Generate (or do not generate) code using the "mvcle" instruction to perform block moves. When -mno-mvcle is
specified, use a "mvc" loop instead. This is the default unless optimizing for size.
-mdebug
-mno-debug
Print (or do not print) additional debug information when compiling. The default is to not print debug
information.
-march=cpu-type
Generate code that runs on cpu-type, which is the name of a system representing a certain processor type.
Possible values for cpu-type are z900/arch5, z990/arch6, z9-109, z9-ec/arch7, z10/arch8, z196/arch9, zEC12,
z13/arch11, z14/arch12, z15/arch13, z16/arch14, z17/arch15, and native.
The default is -march=z900.
Specifying native as cpu type can be used to select the best architecture option for the host processor.
-march=native has no effect if GCC does not recognize the processor.
-mtune=cpu-type
Tune to cpu-type everything applicable about the generated code, except for the ABI and the set of available
instructions. The list of cpu-type values is the same as for -march. The default is the value used for -march.
-mtpf-trace
-mno-tpf-trace
Generate code that adds (does not add) in TPF OS specific branches to trace routines in the operating system.
This option is off by default, even when compiling for the TPF OS.
-mtpf-trace-skip
-mno-tpf-trace-skip
Generate code that changes (does not change) the default branch targets enabled by -mtpf-trace to point to
specialized trace routines providing the ability of selectively skipping function trace entries for the TPF OS.
This option is off by default, even when compiling for the TPF OS and specifying -mtpf-trace.
-mfused-madd
-mno-fused-madd
Generate code that uses (does not use) the floating-point multiply and accumulate instructions. These
instructions are generated by default if hardware floating point is used.
-mwarn-framesize=framesize
Emit a warning if the current function exceeds the given frame size. Because this is a compile-time check it
doesn't need to be a real problem when the program runs. It is intended to identify functions that most probably
cause a stack overflow. It is useful to be used in an environment with limited stack size e.g. the linux kernel.
-mwarn-dynamicstack
Emit a warning if the function calls "alloca" or uses dynamically-sized arrays. This is generally a bad idea
with a limited stack size.
-mstack-guard=stack-guard
-mstack-size=stack-size
If these options are provided the S/390 back end emits additional instructions in the function prologue that
trigger a trap if the stack size is stack-guard bytes above the stack-size (remember that the stack on S/390
grows downward). If the stack-guard option is omitted the smallest power of 2 larger than the frame size of the
compiled function is chosen. These options are intended to be used to help debugging stack overflow problems.
The additionally emitted code causes only little overhead and hence can also be used in production-like systems
without greater performance degradation. The given values have to be exact powers of 2 and stack-size has to be
greater than stack-guard without exceeding 64k. In order to be efficient the extra code makes the assumption
that the stack starts at an address aligned to the value given by stack-size. The stack-guard option can only be
used in conjunction with stack-size.
-mhotpatch=pre-halfwords,post-halfwords
If the hotpatch option is enabled, a "hot-patching" function prologue is generated for all functions in the
compilation unit. The funtion label is prepended with the given number of two-byte NOP instructions (pre-
halfwords, maximum 1000000). After the label, 2 * post-halfwords bytes are appended, using the largest NOP like
instructions the architecture allows (maximum 1000000).
If both arguments are zero, hotpatching is disabled.
This option can be overridden for individual functions with the "hotpatch" attribute.
SH Options
These -m options are defined for the SH implementations:
-m1 Generate code for the SH1.
-m2 Generate code for the SH2.
-m2e
Generate code for the SH2e.
-m2a-nofpu
Generate code for the SH2a without FPU, or for a SH2a-FPU in such a way that the floating-point unit is not used.
-m2a-single-only
Generate code for the SH2a-FPU, in such a way that no double-precision floating-point operations are used.
-m2a-single
Generate code for the SH2a-FPU assuming the floating-point unit is in single-precision mode by default.
-m2a
Generate code for the SH2a-FPU assuming the floating-point unit is in double-precision mode by default.
-m3 Generate code for the SH3.
-m3e
Generate code for the SH3e.
-m4-nofpu
Generate code for the SH4 without a floating-point unit.
-m4-single-only
Generate code for the SH4 with a floating-point unit that only supports single-precision arithmetic.
-m4-single
Generate code for the SH4 assuming the floating-point unit is in single-precision mode by default.
-m4 Generate code for the SH4.
-m4-100
Generate code for SH4-100.
-m4-100-nofpu
Generate code for SH4-100 in such a way that the floating-point unit is not used.
-m4-100-single
Generate code for SH4-100 assuming the floating-point unit is in single-precision mode by default.
-m4-100-single-only
Generate code for SH4-100 in such a way that no double-precision floating-point operations are used.
-m4-200
Generate code for SH4-200.
-m4-200-nofpu
Generate code for SH4-200 without in such a way that the floating-point unit is not used.
-m4-200-single
Generate code for SH4-200 assuming the floating-point unit is in single-precision mode by default.
-m4-200-single-only
Generate code for SH4-200 in such a way that no double-precision floating-point operations are used.
-m4-300
Generate code for SH4-300.
-m4-300-nofpu
Generate code for SH4-300 without in such a way that the floating-point unit is not used.
-m4-300-single
Generate code for SH4-300 in such a way that no double-precision floating-point operations are used.
-m4-300-single-only
Generate code for SH4-300 in such a way that no double-precision floating-point operations are used.
-m4-340
Generate code for SH4-340 (no MMU, no FPU).
-m4-500
Generate code for SH4-500 (no FPU). Passes -isa=sh4-nofpu to the assembler.
-m4a-nofpu
Generate code for the SH4al-dsp, or for a SH4a in such a way that the floating-point unit is not used.
-m4a-single-only
Generate code for the SH4a, in such a way that no double-precision floating-point operations are used.
-m4a-single
Generate code for the SH4a assuming the floating-point unit is in single-precision mode by default.
-m4a
Generate code for the SH4a.
-m4al
Same as -m4a-nofpu, except that it implicitly passes -dsp to the assembler. GCC doesn't generate any DSP
instructions at the moment.
-mb Compile code for the processor in big-endian mode.
-ml Compile code for the processor in little-endian mode.
-mdalign
Align doubles at 64-bit boundaries. Note that this changes the calling conventions, and thus some functions from
the standard C library do not work unless you recompile it first with -mdalign.
-mrelax
Shorten some address references at link time, when possible; uses the linker option -relax.
-mbigtable
Use 32-bit offsets in "switch" tables. The default is to use 16-bit offsets.
-mbitops
Enable the use of bit manipulation instructions on SH2A.
-mfmovd
Enable the use of the instruction "fmovd". Check -mdalign for alignment constraints.
-mrenesas
Comply with the calling conventions defined by Renesas.
-mno-renesas
Comply with the calling conventions defined for GCC before the Renesas conventions were available. This option
is the default for all targets of the SH toolchain.
-mnomacsave
Mark the "MAC" register as call-clobbered, even if -mrenesas is given.
-mieee
-mno-ieee
Control the IEEE compliance of floating-point comparisons, which affects the handling of cases where the result
of a comparison is unordered. By default -mieee is implicitly enabled. If -ffinite-math-only is enabled
-mno-ieee is implicitly set, which results in faster floating-point greater-equal and less-equal comparisons.
The implicit settings can be overridden by specifying either -mieee or -mno-ieee.
-minline-ic_invalidate
Inline code to invalidate instruction cache entries after setting up nested function trampolines. This option
has no effect if -musermode is in effect and the selected code generation option (e.g. -m4) does not allow the
use of the "icbi" instruction. If the selected code generation option does not allow the use of the "icbi"
instruction, and -musermode is not in effect, the inlined code manipulates the instruction cache address array
directly with an associative write. This not only requires privileged mode at run time, but it also fails if the
cache line had been mapped via the TLB and has become unmapped.
-misize
Dump instruction size and location in the assembly code.
-mpadstruct
This option is deprecated. It pads structures to multiple of 4 bytes, which is incompatible with the SH ABI.
-matomic-model=model
Sets the model of atomic operations and additional parameters as a comma separated list. For details on the
atomic built-in functions see __atomic Builtins. The following models and parameters are supported:
none
Disable compiler generated atomic sequences and emit library calls for atomic operations. This is the
default if the target is not "sh*-*-linux*".
soft-gusa
Generate GNU/Linux compatible gUSA software atomic sequences for the atomic built-in functions. The
generated atomic sequences require additional support from the interrupt/exception handling code of the
system and are only suitable for SH3* and SH4* single-core systems. This option is enabled by default when
the target is "sh*-*-linux*" and SH3* or SH4*. When the target is SH4A, this option also partially utilizes
the hardware atomic instructions "movli.l" and "movco.l" to create more efficient code, unless strict is
specified.
soft-tcb
Generate software atomic sequences that use a variable in the thread control block. This is a variation of
the gUSA sequences which can also be used on SH1* and SH2* targets. The generated atomic sequences require
additional support from the interrupt/exception handling code of the system and are only suitable for single-
core systems. When using this model, the gbr-offset= parameter has to be specified as well.
soft-imask
Generate software atomic sequences that temporarily disable interrupts by setting "SR.IMASK = 1111". This
model works only when the program runs in privileged mode and is only suitable for single-core systems.
Additional support from the interrupt/exception handling code of the system is not required. This model is
enabled by default when the target is "sh*-*-linux*" and SH1* or SH2*.
hard-llcs
Generate hardware atomic sequences using the "movli.l" and "movco.l" instructions only. This is only
available on SH4A and is suitable for multi-core systems. Since the hardware instructions support only 32
bit atomic variables access to 8 or 16 bit variables is emulated with 32 bit accesses. Code compiled with
this option is also compatible with other software atomic model interrupt/exception handling systems if
executed on an SH4A system. Additional support from the interrupt/exception handling code of the system is
not required for this model.
gbr-offset=
This parameter specifies the offset in bytes of the variable in the thread control block structure that
should be used by the generated atomic sequences when the soft-tcb model has been selected. For other models
this parameter is ignored. The specified value must be an integer multiple of four and in the range 0-1020.
strict
This parameter prevents mixed usage of multiple atomic models, even if they are compatible, and makes the
compiler generate atomic sequences of the specified model only.
-mtas
Generate the "tas.b" opcode for "__atomic_test_and_set". Notice that depending on the particular hardware and
software configuration this can degrade overall performance due to the operand cache line flushes that are
implied by the "tas.b" instruction. On multi-core SH4A processors the "tas.b" instruction must be used with
caution since it can result in data corruption for certain cache configurations.
-mprefergot
When generating position-independent code, emit function calls using the Global Offset Table instead of the
Procedure Linkage Table.
-musermode
-mno-usermode
Don't allow (allow) the compiler generating privileged mode code. Specifying -musermode also implies
-mno-inline-ic_invalidate if the inlined code would not work in user mode. -musermode is the default when the
target is "sh*-*-linux*". If the target is SH1* or SH2* -musermode has no effect, since there is no user mode.
-multcost=number
Set the cost to assume for a multiply insn.
-mdiv=strategy
Set the division strategy to be used for integer division operations. strategy can be one of:
call-div1
Calls a library function that uses the single-step division instruction "div1" to perform the operation.
Division by zero calculates an unspecified result and does not trap. This is the default except for SH4,
SH2A and SHcompact.
call-fp
Calls a library function that performs the operation in double precision floating point. Division by zero
causes a floating-point exception. This is the default for SHcompact with FPU. Specifying this for targets
that do not have a double precision FPU defaults to "call-div1".
call-table
Calls a library function that uses a lookup table for small divisors and the "div1" instruction with case
distinction for larger divisors. Division by zero calculates an unspecified result and does not trap. This
is the default for SH4. Specifying this for targets that do not have dynamic shift instructions defaults to
"call-div1".
When a division strategy has not been specified the default strategy is selected based on the current target.
For SH2A the default strategy is to use the "divs" and "divu" instructions instead of library function calls.
-maccumulate-outgoing-args
Reserve space once for outgoing arguments in the function prologue rather than around each call. Generally
beneficial for performance and size. Also needed for unwinding to avoid changing the stack frame around
conditional code.
-mdivsi3_libfunc=name
Set the name of the library function used for 32-bit signed division to name. This only affects the name used in
the call division strategies, and the compiler still expects the same sets of input/output/clobbered registers as
if this option were not present.
-mfixed-range=register-range
Generate code treating the given register range as fixed registers. A fixed register is one that the register
allocator cannot use. This is useful when compiling kernel code. A register range is specified as two registers
separated by a dash. Multiple register ranges can be specified separated by a comma.
-mbranch-cost=num
Assume num to be the cost for a branch instruction. Higher numbers make the compiler try to generate more
branch-free code if possible. If not specified the value is selected depending on the processor type that is
being compiled for.
-mzdcbranch
-mno-zdcbranch
Assume (do not assume) that zero displacement conditional branch instructions "bt" and "bf" are fast. If
-mzdcbranch is specified, the compiler prefers zero displacement branch code sequences. This is enabled by
default when generating code for SH4 and SH4A. It can be explicitly disabled by specifying -mno-zdcbranch.
-mcbranch-force-delay-slot
Force the usage of delay slots for conditional branches, which stuffs the delay slot with a "nop" if a suitable
instruction cannot be found. By default this option is disabled. It can be enabled to work around hardware bugs
as found in the original SH7055.
-mfused-madd
-mno-fused-madd
Generate code that uses (does not use) the floating-point multiply and accumulate instructions. These
instructions are generated by default if hardware floating point is used. The machine-dependent -mfused-madd
option is now mapped to the machine-independent -ffp-contract=fast option, and -mno-fused-madd is mapped to
-ffp-contract=off.
-mfsca
-mno-fsca
Allow or disallow the compiler to emit the "fsca" instruction for sine and cosine approximations. The option
-mfsca must be used in combination with -funsafe-math-optimizations. It is enabled by default when generating
code for SH4A. Using -mno-fsca disables sine and cosine approximations even if -funsafe-math-optimizations is in
effect.
-mfsrra
-mno-fsrra
Allow or disallow the compiler to emit the "fsrra" instruction for reciprocal square root approximations. The
option -mfsrra must be used in combination with -funsafe-math-optimizations and -ffinite-math-only. It is
enabled by default when generating code for SH4A. Using -mno-fsrra disables reciprocal square root
approximations even if -funsafe-math-optimizations and -ffinite-math-only are in effect.
-mpretend-cmove
Prefer zero-displacement conditional branches for conditional move instruction patterns. This can result in
faster code on the SH4 processor.
-mfdpic
Generate code using the FDPIC ABI.
Solaris 2 Options
These -m options are supported on Solaris 2:
-mclear-hwcap
-mclear-hwcap tells the compiler to remove the hardware capabilities generated by the Solaris assembler. This is
only necessary when object files use ISA extensions not supported by the current machine, but check at runtime
whether or not to use them.
-mimpure-text
-mimpure-text, used in addition to -shared, tells the compiler to not pass -z text to the linker when linking a
shared object. Using this option, you can link position-dependent code into a shared object.
-mimpure-text suppresses the "relocations remain against allocatable but non-writable sections" linker error
message. However, the necessary relocations trigger copy-on-write, and the shared object is not actually shared
across processes. Instead of using -mimpure-text, you should compile all source code with -fpic or -fPIC.
These switches are supported in addition to the above on Solaris 2:
-pthreads
This is a synonym for -pthread.
SPARC Options
These -m options are supported on the SPARC:
-mno-app-regs
-mapp-regs
Specify -mapp-regs to generate output using the global registers 2 through 4, which the SPARC SVR4 ABI reserves
for applications. Like the global register 1, each global register 2 through 4 is then treated as an allocable
register that is clobbered by function calls. This is the default.
To be fully SVR4 ABI-compliant at the cost of some performance loss, specify -mno-app-regs. You should compile
libraries and system software with this option.
-mflat
-mno-flat
With -mflat, the compiler does not generate save/restore instructions and uses a "flat" or single register window
model. This model is compatible with the regular register window model. The local registers and the input
registers (0--5) are still treated as "call-saved" registers and are saved on the stack as needed.
With -mno-flat (the default), the compiler generates save/restore instructions (except for leaf functions). This
is the normal operating mode.
-mfpu
-mhard-float
Generate output containing floating-point instructions. This is the default.
-mno-fpu
-msoft-float
Generate output containing library calls for floating point. Warning: the requisite libraries are not available
for all SPARC targets. Normally the facilities of the machine's usual C compiler are used, but this cannot be
done directly in cross-compilation. You must make your own arrangements to provide suitable library functions
for cross-compilation. The embedded targets sparc-*-aout and sparclite-*-* do provide software floating-point
support.
-msoft-float changes the calling convention in the output file; therefore, it is only useful if you compile all
of a program with this option. In particular, you need to compile libgcc.a, the library that comes with GCC,
with -msoft-float in order for this to work.
-mhard-quad-float
Generate output containing quad-word (long double) floating-point instructions.
-msoft-quad-float
Generate output containing library calls for quad-word (long double) floating-point instructions. The functions
called are those specified in the SPARC ABI. This is the default.
As of this writing, there are no SPARC implementations that have hardware support for the quad-word floating-
point instructions. They all invoke a trap handler for one of these instructions, and then the trap handler
emulates the effect of the instruction. Because of the trap handler overhead, this is much slower than calling
the ABI library routines. Thus the -msoft-quad-float option is the default.
-mno-unaligned-doubles
-munaligned-doubles
Assume that doubles have 8-byte alignment. This is the default.
With -munaligned-doubles, GCC assumes that doubles have 8-byte alignment only if they are contained in another
type, or if they have an absolute address. Otherwise, it assumes they have 4-byte alignment. Specifying this
option avoids some rare compatibility problems with code generated by other compilers. It is not the default
because it results in a performance loss, especially for floating-point code.
-muser-mode
-mno-user-mode
Do not generate code that can only run in supervisor mode. This is relevant only for the "casa" instruction
emitted for the LEON3 processor. This is the default.
-mfaster-structs
-mno-faster-structs
With -mfaster-structs, the compiler assumes that structures should have 8-byte alignment. This enables the use
of pairs of "ldd" and "std" instructions for copies in structure assignment, in place of twice as many "ld" and
"st" pairs. However, the use of this changed alignment directly violates the SPARC ABI. Thus, it's intended
only for use on targets where the developer acknowledges that their resulting code is not directly in line with
the rules of the ABI.
-mstd-struct-return
-mno-std-struct-return
With -mstd-struct-return, the compiler generates checking code in functions returning structures or unions to
detect size mismatches between the two sides of function calls, as per the 32-bit ABI.
The default is -mno-std-struct-return. This option has no effect in 64-bit mode.
-mcpu=cpu_type
Set the instruction set, register set, and instruction scheduling parameters for machine type cpu_type.
Supported values for cpu_type are v7, cypress, v8, supersparc, hypersparc, leon, leon3, leon3v7, leon5,
sparclite, f930, f934, sparclite86x, sparclet, tsc701, v9, ultrasparc, ultrasparc3, niagara, niagara2, niagara3,
niagara4, niagara7 and m8.
Native Solaris and GNU/Linux toolchains also support the value native, which selects the best architecture option
for the host processor. -mcpu=native has no effect if GCC does not recognize the processor.
Default instruction scheduling parameters are used for values that select an architecture and not an
implementation. These are v7, v8, sparclite, sparclet, v9.
Here is a list of each supported architecture and their supported implementations.
v7 cypress, leon3v7
v8 supersparc, hypersparc, leon, leon3, leon5
sparclite
f930, f934, sparclite86x
sparclet
tsc701
v9 ultrasparc, ultrasparc3, niagara, niagara2, niagara3, niagara4, niagara7, m8
By default (unless configured otherwise), GCC generates code for the V7 variant of the SPARC architecture. With
-mcpu=cypress, the compiler additionally optimizes it for the Cypress CY7C602 chip, as used in the
SPARCStation/SPARCServer 3xx series. This is also appropriate for the older SPARCStation 1, 2, IPX etc.
With -mcpu=v8, GCC generates code for the V8 variant of the SPARC architecture. The only difference from V7 code
is that the compiler emits the integer multiply and integer divide instructions which exist in SPARC-V8 but not
in SPARC-V7. With -mcpu=supersparc, the compiler additionally optimizes it for the SuperSPARC chip, as used in
the SPARCStation 10, 1000 and 2000 series.
With -mcpu=sparclite, GCC generates code for the SPARClite variant of the SPARC architecture. This adds the
integer multiply, integer divide step and scan ("ffs") instructions which exist in SPARClite but not in SPARC-V7.
With -mcpu=f930, the compiler additionally optimizes it for the Fujitsu MB86930 chip, which is the original
SPARClite, with no FPU. With -mcpu=f934, the compiler additionally optimizes it for the Fujitsu MB86934 chip,
which is the more recent SPARClite with FPU.
With -mcpu=sparclet, GCC generates code for the SPARClet variant of the SPARC architecture. This adds the
integer multiply, multiply/accumulate, integer divide step and scan ("ffs") instructions which exist in SPARClet
but not in SPARC-V7. With -mcpu=tsc701, the compiler additionally optimizes it for the TEMIC SPARClet chip.
With -mcpu=v9, GCC generates code for the V9 variant of the SPARC architecture. This adds 64-bit integer and
floating-point move instructions, 3 additional floating-point condition code registers and conditional move
instructions. With -mcpu=ultrasparc, the compiler additionally optimizes it for the Sun UltraSPARC I/II/IIi
chips. With -mcpu=ultrasparc3, the compiler additionally optimizes it for the Sun UltraSPARC
III/III+/IIIi/IIIi+/IV/IV+ chips. With -mcpu=niagara, the compiler additionally optimizes it for Sun UltraSPARC
T1 chips. With -mcpu=niagara2, the compiler additionally optimizes it for Sun UltraSPARC T2 chips. With
-mcpu=niagara3, the compiler additionally optimizes it for Sun UltraSPARC T3 chips. With -mcpu=niagara4, the
compiler additionally optimizes it for Sun UltraSPARC T4 chips. With -mcpu=niagara7, the compiler additionally
optimizes it for Oracle SPARC M7 chips. With -mcpu=m8, the compiler additionally optimizes it for Oracle M8
chips.
-mtune=cpu_type
Set the instruction scheduling parameters for machine type cpu_type, but do not set the instruction set or
register set that the option -mcpu=cpu_type does.
The same values for -mcpu=cpu_type can be used for -mtune=cpu_type, but the only useful values are those that
select a particular CPU implementation. Those are cypress, supersparc, hypersparc, leon, leon3, leon3v7, leon5,
f930, f934, sparclite86x, tsc701, ultrasparc, ultrasparc3, niagara, niagara2, niagara3, niagara4, niagara7 and
m8. With native Solaris and GNU/Linux toolchains, native can also be used.
-mv8plus
-mno-v8plus
With -mv8plus, GCC generates code for the SPARC-V8+ ABI. The difference from the V8 ABI is that the global and
out registers are considered 64 bits wide. This is enabled by default on Solaris in 32-bit mode for all SPARC-V9
processors.
-mvis
-mno-vis
With -mvis, GCC generates code that takes advantage of the UltraSPARC Visual Instruction Set extensions. The
default is -mno-vis.
-mvis2
-mno-vis2
With -mvis2, GCC generates code that takes advantage of version 2.0 of the UltraSPARC Visual Instruction Set
extensions. The default is -mvis2 when targeting a cpu that supports such instructions, such as UltraSPARC-III
and later. Setting -mvis2 also sets -mvis.
-mvis3
-mno-vis3
With -mvis3, GCC generates code that takes advantage of version 3.0 of the UltraSPARC Visual Instruction Set
extensions. The default is -mvis3 when targeting a cpu that supports such instructions, such as niagara-3 and
later. Setting -mvis3 also sets -mvis2 and -mvis.
-mvis3b
-mno-vis3b
With -mvis3b, GCC generates code that takes advantage of version 3.0 of the UltraSPARC Visual Instruction Set
extensions, plus the additional VIS instructions introduced in the Oracle SPARC Architecture 2011. The default is
-mvis3b when targeting a cpu that supports such instructions, such as niagara-7 and later. Setting -mvis3b also
sets -mvis3, -mvis2 and -mvis.
-mvis4
-mno-vis4
With -mvis4, GCC generates code that takes advantage of version 4.0 of the UltraSPARC Visual Instruction Set
extensions. The default is -mvis4 when targeting a cpu that supports such instructions, such as niagara-7 and
later. Setting -mvis4 also sets -mvis3b, -mvis3, -mvis2 and -mvis.
-mvis4b
-mno-vis4b
With -mvis4b, GCC generates code that takes advantage of version 4.0 of the UltraSPARC Visual Instruction Set
extensions, plus the additional VIS instructions introduced in the Oracle SPARC Architecture 2017. The default
is -mvis4b when targeting a cpu that supports such instructions, such as m8 and later. Setting -mvis4b also sets
-mvis4, -mvis3b, -mvis3, -mvis2 and -mvis.
-mcbcond
-mno-cbcond
With -mcbcond, GCC generates code that takes advantage of the UltraSPARC Compare-and-Branch-on-Condition
instructions. The default is -mcbcond when targeting a CPU that supports such instructions, such as Niagara-4
and later.
-mfmaf
-mno-fmaf
With -mfmaf, GCC generates code that takes advantage of the UltraSPARC Fused Multiply-Add Floating-point
instructions. The default is -mfmaf when targeting a CPU that supports such instructions, such as Niagara-3 and
later.
-mfsmuld
-mno-fsmuld
With -mfsmuld, GCC generates code that takes advantage of the Floating-point Multiply Single to Double (FsMULd)
instruction. The default is -mfsmuld when targeting a CPU supporting the architecture versions V8 or V9 with FPU
except -mcpu=leon.
-mpopc
-mno-popc
With -mpopc, GCC generates code that takes advantage of the UltraSPARC Population Count instruction. The default
is -mpopc when targeting a CPU that supports such an instruction, such as Niagara-2 and later.
-msubxc
-mno-subxc
With -msubxc, GCC generates code that takes advantage of the UltraSPARC Subtract-Extended-with-Carry instruction.
The default is -msubxc when targeting a CPU that supports such an instruction, such as Niagara-7 and later.
-mfix-at697f
Enable the documented workaround for the single erratum of the Atmel AT697F processor (which corresponds to
erratum #13 of the AT697E processor).
-mfix-ut699
Enable the documented workarounds for the floating-point errata and the data cache nullify errata of the UT699
processor.
-mfix-ut700
Enable the documented workaround for the back-to-back store errata of the UT699E/UT700 processor.
-mfix-gr712rc
Enable the documented workaround for the back-to-back store errata of the GR712RC processor.
These -m options are supported in addition to the above on SPARC-V9 processors in 64-bit environments:
-m32
-m64
Generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer to 32 bits.
The 64-bit environment sets int to 32 bits and long and pointer to 64 bits.
-mcmodel=which
Set the code model to one of
medlow
The Medium/Low code model: 64-bit addresses, programs must be linked in the low 32 bits of memory. Programs
can be statically or dynamically linked.
medmid
The Medium/Middle code model: 64-bit addresses, programs must be linked in the low 44 bits of memory, the
text and data segments must be less than 2GB in size and the data segment must be located within 2GB of the
text segment.
medany
The Medium/Anywhere code model: 64-bit addresses, programs may be linked anywhere in memory, the text and
data segments must be less than 2GB in size and the data segment must be located within 2GB of the text
segment.
embmedany
The Medium/Anywhere code model for embedded systems: 64-bit addresses, the text and data segments must be
less than 2GB in size, both starting anywhere in memory (determined at link time). The global register %g4
points to the base of the data segment. Programs are statically linked and PIC is not supported.
-mmemory-model=mem-model
Set the memory model in force on the processor to one of
default
The default memory model for the processor and operating system.
rmo Relaxed Memory Order
pso Partial Store Order
tso Total Store Order
sc Sequential Consistency
These memory models are formally defined in Appendix D of the SPARC-V9 architecture manual, as set in the
processor's "PSTATE.MM" field.
-mstack-bias
-mno-stack-bias
With -mstack-bias, GCC assumes that the stack pointer, and frame pointer if present, are offset by -2047 which
must be added back when making stack frame references. This is the default in 64-bit mode. Otherwise, assume no
such offset is present.
Options for System V
These additional options are available on System V Release 4 for compatibility with other compilers on those systems:
-G Create a shared object. It is recommended that -symbolic or -shared be used instead.
-Qy Identify the versions of each tool used by the compiler, in a ".ident" assembler directive in the output.
-Qn Refrain from adding ".ident" directives to the output file (this is the default).
-YP,dirs
Search the directories dirs, and no others, for libraries specified with -l.
-Ym,dir
Look in the directory dir to find the M4 preprocessor. The assembler uses this option.
V850 Options
These -m options are defined for V850 implementations:
-mlong-calls
-mno-long-calls
Treat all calls as being far away (near). If calls are assumed to be far away, the compiler always loads the
function's address into a register, and calls indirect through the pointer.
-mno-ep
-mep
Do not optimize (do optimize) basic blocks that use the same index pointer 4 or more times to copy pointer into
the "ep" register, and use the shorter "sld" and "sst" instructions. The -mep option is on by default if you
optimize.
-mno-prolog-function
-mprolog-function
Do not use (do use) external functions to save and restore registers at the prologue and epilogue of a function.
The external functions are slower, but use less code space if more than one function saves the same number of
registers. The -mprolog-function option is on by default if you optimize.
-mspace
Try to make the code as small as possible. At present, this just turns on the -mep and -mprolog-function
options.
-mtda=n
Put static or global variables whose size is n bytes or less into the tiny data area that register "ep" points
to. The tiny data area can hold up to 256 bytes in total (128 bytes for byte references).
-msda=n
Put static or global variables whose size is n bytes or less into the small data area that register "gp" points
to. The small data area can hold up to 64 kilobytes.
-mzda=n
Put static or global variables whose size is n bytes or less into the first 32 kilobytes of memory.
-mv850
Specify that the target processor is the V850.
-mv850e3v5
Specify that the target processor is the V850E3V5. The preprocessor constant "__v850e3v5__" is defined if this
option is used.
-mv850e2v4
Specify that the target processor is the V850E3V5. This is an alias for the -mv850e3v5 option.
-mv850e2v3
Specify that the target processor is the V850E2V3. The preprocessor constant "__v850e2v3__" is defined if this
option is used.
-mv850e2
Specify that the target processor is the V850E2. The preprocessor constant "__v850e2__" is defined if this
option is used.
-mv850e1
Specify that the target processor is the V850E1. The preprocessor constants "__v850e1__" and "__v850e__" are
defined if this option is used.
-mv850es
Specify that the target processor is the V850ES. This is an alias for the -mv850e1 option.
-mv850e
Specify that the target processor is the V850E. The preprocessor constant "__v850e__" is defined if this option
is used.
If neither -mv850 nor -mv850e nor -mv850e1 nor -mv850e2 nor -mv850e2v3 nor -mv850e3v5 are defined then a default
target processor is chosen and the relevant __v850*__ preprocessor constant is defined.
The preprocessor constants "__v850" and "__v851__" are always defined, regardless of which processor variant is
the target.
-mdisable-callt
-mno-disable-callt
This option suppresses generation of the "CALLT" instruction for the v850e, v850e1, v850e2, v850e2v3 and v850e3v5
flavors of the v850 architecture.
This option is enabled by default when the RH850 ABI is in use (see -mrh850-abi), and disabled by default when
the GCC ABI is in use. If "CALLT" instructions are being generated then the C preprocessor symbol
"__V850_CALLT__" is defined.
-mrelax
-mno-relax
Pass on (or do not pass on) the -mrelax command-line option to the assembler.
-mlong-jumps
-mno-long-jumps
Disable (or re-enable) the generation of PC-relative jump instructions.
-msoft-float
-mhard-float
Disable (or re-enable) the generation of hardware floating point instructions. This option is only significant
when the target architecture is V850E2V3 or higher. If hardware floating point instructions are being generated
then the C preprocessor symbol "__FPU_OK__" is defined, otherwise the symbol "__NO_FPU__" is defined.
-mloop
Enables the use of the e3v5 LOOP instruction. The use of this instruction is not enabled by default when the
e3v5 architecture is selected because its use is still experimental.
-mrh850-abi
-mghs
Enables support for the RH850 version of the V850 ABI. This is the default. With this version of the ABI the
following rules apply:
* Integer sized structures and unions are returned via a memory pointer rather than a register.
* Large structures and unions (more than 8 bytes in size) are passed by value.
* Functions are aligned to 16-bit boundaries.
* The -m8byte-align command-line option is supported.
* The -mdisable-callt command-line option is enabled by default. The -mno-disable-callt command-line option is
not supported.
When this version of the ABI is enabled the C preprocessor symbol "__V850_RH850_ABI__" is defined.
-mgcc-abi
Enables support for the old GCC version of the V850 ABI. With this version of the ABI the following rules apply:
* Integer sized structures and unions are returned in register "r10".
* Large structures and unions (more than 8 bytes in size) are passed by reference.
* Functions are aligned to 32-bit boundaries, unless optimizing for size.
* The -m8byte-align command-line option is not supported.
* The -mdisable-callt command-line option is supported but not enabled by default.
When this version of the ABI is enabled the C preprocessor symbol "__V850_GCC_ABI__" is defined.
-m8byte-align
-mno-8byte-align
Enables support for "double" and "long long" types to be aligned on 8-byte boundaries. The default is to
restrict the alignment of all objects to at most 4-bytes. When -m8byte-align is in effect the C preprocessor
symbol "__V850_8BYTE_ALIGN__" is defined.
-mbig-switch
Generate code suitable for big switch tables. Use this option only if the assembler/linker complain about out of
range branches within a switch table.
-mapp-regs
This option causes r2 and r5 to be used in the code generated by the compiler. This setting is the default.
-mno-app-regs
This option causes r2 and r5 to be treated as fixed registers.
VAX Options
These -m options are defined for the VAX:
-munix
Do not output certain jump instructions ("aobleq" and so on) that the Unix assembler for the VAX cannot handle
across long ranges.
-mgnu
Do output those jump instructions, on the assumption that the GNU assembler is being used.
-md
-md-float
Use the D_floating data format for double-precision floating-point numbers instead of G_floating.
-mg
-mg-float
Use the G_floating data format for double-precision floating-point numbers instead of D_floating.
-mlra
-mno-lra
Enable Local Register Allocation. This is still experimental for the VAX, so by default the compiler uses
standard reload.
Visium Options
-mdebug
A program which performs file I/O and is destined to run on an MCM target should be linked with this option. It
causes the libraries libc.a and libdebug.a to be linked. The program should be run on the target under the
control of the GDB remote debugging stub.
-msim
A program which performs file I/O and is destined to run on the simulator should be linked with option. This
causes libraries libc.a and libsim.a to be linked.
-mfpu
-mhard-float
Generate code containing floating-point instructions. This is the default.
-mno-fpu
-msoft-float
Generate code containing library calls for floating-point.
-msoft-float changes the calling convention in the output file; therefore, it is only useful if you compile all
of a program with this option. In particular, you need to compile libgcc.a, the library that comes with GCC,
with -msoft-float in order for this to work.
-mcpu=cpu_type
Set the instruction set, register set, and instruction scheduling parameters for machine type cpu_type.
Supported values for cpu_type are mcm, gr5 and gr6.
mcm is a synonym of gr5 present for backward compatibility.
By default (unless configured otherwise), GCC generates code for the GR5 variant of the Visium architecture.
With -mcpu=gr6, GCC generates code for the GR6 variant of the Visium architecture. The only difference from GR5
code is that the compiler will generate block move instructions.
-mtune=cpu_type
Set the instruction scheduling parameters for machine type cpu_type, but do not set the instruction set or
register set that the option -mcpu=cpu_type would.
-msv-mode
Generate code for the supervisor mode, where there are no restrictions on the access to general registers. This
is the default.
-muser-mode
Generate code for the user mode, where the access to some general registers is forbidden: on the GR5, registers
r24 to r31 cannot be accessed in this mode; on the GR6, only registers r29 to r31 are affected.
VMS Options
These -m options are defined for the VMS implementations:
-mvms-return-codes
Return VMS condition codes from "main". The default is to return POSIX-style condition (e.g. error) codes.
-mdebug-main=prefix
Flag the first routine whose name starts with prefix as the main routine for the debugger.
-mmalloc64
Default to 64-bit memory allocation routines.
-mpointer-size=size
Set the default size of pointers. Possible options for size are 32 or short for 32 bit pointers, 64 or long for
64 bit pointers, and no for supporting only 32 bit pointers. The later option disables "pragma pointer_size".
VxWorks Options
The options in this section are defined for all VxWorks targets. Options specific to the target hardware are listed
with the other options for that target.
-mrtp
GCC can generate code for both VxWorks kernels and real time processes (RTPs). This option switches from the
former to the latter. It also defines the preprocessor macro "__RTP__".
-msmp
Select SMP runtimes for linking. Not available on architectures other than PowerPC, nor on VxWorks version 7 or
later, in which the selection is part of the VxWorks build configuration and the library paths are the same for
either choice.
-non-static
Link an RTP executable against shared libraries rather than static libraries. The options -static and -shared
can also be used for RTPs; -static is the default.
-Bstatic
-Bdynamic
These options are passed down to the linker. They are defined for compatibility with Diab.
-Xbind-lazy
Enable lazy binding of function calls. This option is equivalent to -Wl,-z,now and is defined for compatibility
with Diab.
-Xbind-now
Disable lazy binding of function calls. This option is the default and is defined for compatibility with Diab.
x86 Options
These -m options are defined for the x86 family of computers.
-march=cpu-type
Generate instructions for the machine type cpu-type. In contrast to -mtune=cpu-type, which merely tunes the
generated code for the specified cpu-type, -march=cpu-type allows GCC to generate code that may not run at all on
processors other than the one indicated. Specifying -march=cpu-type implies -mtune=cpu-type, except where noted
otherwise.
The choices for cpu-type are:
native
This selects the CPU to generate code for at compilation time by determining the processor type of the
compiling machine. Using -march=native enables all instruction subsets supported by the local machine (hence
the result might not run on different machines). Using -mtune=native produces code optimized for the local
machine under the constraints of the selected instruction set.
x86-64
A generic CPU with 64-bit extensions, MMX, SSE, SSE2, and FXSR instruction set support.
x86-64-v2
x86-64-v3
x86-64-v4
These choices for cpu-type select the corresponding micro-architecture level from the x86-64 psABI. On ABIs
other than the x86-64 psABI they select the same CPU features as the x86-64 psABI documents for the
particular micro-architecture level.
Since these cpu-type values do not have a corresponding -mtune setting, using -march with these values
enables generic tuning. Specific tuning can be enabled using the -mtune=other-cpu-type option with an
appropriate other-cpu-type value.
i386
Original Intel i386 CPU.
i486
Intel i486 CPU. (No scheduling is implemented for this chip.)
i586
pentium
Intel Pentium CPU with no MMX support.
lakemont
Intel Lakemont MCU, based on Intel Pentium CPU.
pentium-mmx
Intel Pentium MMX CPU, based on Pentium core with MMX instruction set support.
pentiumpro
Intel Pentium Pro CPU with no MMX support.
i686
When used with -march, the Pentium Pro instruction set is used, so the code runs on all i686 family chips.
When used with -mtune, it has the same meaning as generic.
pentium2
Intel Pentium II CPU, based on Pentium Pro core with MMX and FXSR instruction set support.
pentium3
pentium3m
Intel Pentium III CPU, based on Pentium Pro core with MMX, FXSR and SSE instruction set support.
pentium-m
Intel Pentium M; low-power version of Intel Pentium III CPU with MMX, SSE, SSE2 and FXSR instruction set
support. Used by Centrino notebooks.
pentium4
pentium4m
Intel Pentium 4 CPU with MMX, SSE, SSE2 and FXSR instruction set support.
prescott
Improved version of Intel Pentium 4 CPU with MMX, SSE, SSE2, SSE3 and FXSR instruction set support.
nocona
Improved version of Intel Pentium 4 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3 and FXSR instruction set
support.
core2
Intel Core 2 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, CX16, SAHF and FXSR instruction set
support.
nehalem
corei7
Intel Nehalem CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF and
FXSR instruction set support.
westmere
Intel Westmere CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF,
FXSR and PCLMUL instruction set support.
sandybridge
corei7-avx
Intel Sandy Bridge CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16,
SAHF, FXSR, AVX, XSAVE and PCLMUL instruction set support.
ivybridge
core-avx-i
Intel Ivy Bridge CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF,
FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND and F16C instruction set support.
haswell
core-avx2
Intel Haswell CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF,
FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE and HLE instruction set
support.
broadwell
Intel Broadwell CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF,
FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX and
PREFETCHW instruction set support.
skylake
Intel Skylake CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF,
FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX,
PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES and SGX instruction set support.
skylake-avx512
Intel Skylake Server CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16,
SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX,
PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ and AVX512CD
instruction set support.
cascadelake
Intel Cascade Lake CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16,
SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX,
PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ, AVX512CD and
AVX512VNNI instruction set support.
cannonlake
Intel Cannon Lake Server CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT,
CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED,
ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU,
AVX512VBMI, AVX512IFMA and SHA instruction set support.
cooperlake
Intel Cooper Lake CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16,
SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX,
PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ, AVX512CD,
AVX512VNNI and AVX512BF16 instruction set support.
icelake-client
Intel Ice Lake Client CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16,
SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX,
PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU,
AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2 , VPCLMULQDQ, AVX512BITALG, RDPID and
AVX512VPOPCNTDQ instruction set support.
icelake-server
Intel Ice Lake Server CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16,
SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX,
PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU,
AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2 , VPCLMULQDQ, AVX512BITALG, RDPID,
AVX512VPOPCNTDQ, PCONFIG, WBNOINVD and CLWB instruction set support.
tigerlake
Intel Tiger Lake CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF,
FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX,
PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD PKU,
AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID,
AVX512VPOPCNTDQ, MOVDIRI, MOVDIR64B, CLWB, AVX512VP2INTERSECT and KEYLOCKER instruction set support.
rocketlake
Intel Rocket Lake CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16,
SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX,
PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD PKU, AVX512VBMI,
AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID and AVX512VPOPCNTDQ
instruction set support.
alderlake
raptorlake
meteorlake
gracemont
Intel Alder Lake/Raptor Lake/Meteor Lake/Gracemont CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3,
SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE,
PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA,
LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL and AVX-VNNI instruction set support.
arrowlake
Intel Arrow Lake CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES,
PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB,
MOVDIRI, MOVDIR64B, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ,
SERIALIZE, HRESET, KL, WIDEKL, AVX-VNNI, UINTR, AVXIFMA, AVXVNNIINT8, AVXNECONVERT and CMPCCXADD instruction
set support.
arrowlake-s
lunarlake
Intel Arrow Lake S/Lunar Lake CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2,
POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-
SSE, CLWB, MOVDIRI, MOVDIR64B, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES,
VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL, AVX-VNNI, UINTR, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD,
AVXVNNIINT16, SHA512, SM3 and SM4 instruction set support.
pantherlake
Intel Panther Lake CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT,
AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE,
CLWB, MOVDIRI, MOVDIR64B, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES,
VPCLMULQDQ, SERIALIZE, HRESET, AVX-VNNI, UINTR, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, AVXVNNIINT16,
SHA512, SM3 and SM4 instruction set support.
sapphirerapids
emeraldrapids
Intel Sapphire Rapids/Emerald Rapids CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2,
POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE,
RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ,
AVX512CD, PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG,
RDPID, AVX512VPOPCNTDQ, PCONFIG, WBNOINVD, CLWB, MOVDIRI, MOVDIR64B, ENQCMD, CLDEMOTE, PTWRITE, WAITPKG,
SERIALIZE, TSXLDTRK, UINTR, AMX-BF16, AMX-TILE, AMX-INT8, AVX-VNNI, AVX512-FP16 and AVX512BF16 instruction
set support.
graniterapids
Intel Granite Rapids CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16,
SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX,
PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU,
AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID,
AVX512VPOPCNTDQ, PCONFIG, WBNOINVD, CLWB, MOVDIRI, MOVDIR64B, ENQCMD, CLDEMOTE, PTWRITE, WAITPKG, SERIALIZE,
TSXLDTRK, UINTR, AMX-BF16, AMX-TILE, AMX-INT8, AVX-VNNI, AVX512-FP16, AVX512BF16, AMX-FP16 and PREFETCHI
instruction set support.
graniterapids-d
Intel Granite Rapids D CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16,
SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX,
PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU,
AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID,
AVX512VPOPCNTDQ, PCONFIG, WBNOINVD, CLWB, MOVDIRI, MOVDIR64B, ENQCMD, CLDEMOTE, PTWRITE, WAITPKG, SERIALIZE,
TSXLDTRK, UINTR, AMX-BF16, AMX-TILE, AMX-INT8, AVX-VNNI, AVX512FP16, AVX512BF16, AMX-FP16, PREFETCHI and AMX-
COMPLEX instruction set support.
diamondrapids
Intel Diamond Rapids CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT,
CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED,
ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU,
AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID,
AVX512VPOPCNTDQ, PCONFIG, WBNOINVD, CLWB, MOVDIRI, MOVDIR64B, ENQCMD, CLDEMOTE, PTWRITE, WAITPKG, SERIALIZE,
TSXLDTRK, UINTR, AMX-BF16, AMX-TILE, AMX-INT8, AVX-VNNI, AVX512FP16, AVX512BF16, AMX-FP16, PREFETCHI, AMX-
COMPLEX, AVX10.1-512, AVX-IFMA, AVX-NE-CONVERT, AVX-VNNI-INT16, AVX-VNNI-INT8, CMPccXADD, SHA512, SM3, SM4,
AVX10.2-512, APX_F, AMX-AVX512, AMX-FP8, AMX-TF32, MOVRS and AMX-MOVRS instruction set support.
bonnell
atom
Intel Bonnell CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support.
silvermont
slm Intel Silvermont CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT,
CX16, SAHF, FXSR, PCLMUL, PREFETCHW and RDRND instruction set support.
goldmont
Intel Goldmont CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16,
SAHF, FXSR, PCLMUL, PREFETCHW, RDRND, AES, SHA, RDSEED, XSAVE, XSAVEC, XSAVES, XSAVEOPT, CLFLUSHOPT and
FSGSBASE instruction set support.
goldmont-plus
Intel Goldmont Plus CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT,
CX16, SAHF, FXSR, PCLMUL, PREFETCHW, RDRND, AES, SHA, RDSEED, XSAVE, XSAVEC, XSAVES, XSAVEOPT, CLFLUSHOPT,
FSGSBASE, PTWRITE, RDPID and SGX instruction set support.
tremont
Intel Tremont CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16,
SAHF, FXSR, PCLMUL, PREFETCHW, RDRND, AES, SHA, RDSEED, XSAVE, XSAVEC, XSAVES, XSAVEOPT, CLFLUSHOPT,
FSGSBASE, PTWRITE, RDPID, SGX, CLWB, GFNI-SSE, MOVDIRI, MOVDIR64B, CLDEMOTE and WAITPKG instruction set
support.
sierraforest
Intel Sierra Forest CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT,
AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE,
CLWB, MOVDIRI, MOVDIR64B, CLDEMOTE, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU,
VAES, VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL, AVX-VNNI, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD,
ENQCMD and UINTR instruction set support.
grandridge
Intel Grand Ridge CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT,
AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE,
CLWB, MOVDIRI, MOVDIR64B, CLDEMOTE, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU,
VAES, VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL, AVX-VNNI, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD,
ENQCMD and UINTR instruction set support.
clearwaterforest
Intel Clearwater Forest CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2,
POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-
SSE, CLWB, MOVDIRI, MOVDIR64B, CLDEMOTE, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU,
VAES, VPCLMULQDQ, SERIALIZE, HRESET, AVX-VNNI, ENQCMD, UINTR, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD,
AVXVNNIINT16, SHA512, SM3, SM4, USER_MSR and PREFETCHI instruction set support.
k6 AMD K6 CPU with MMX instruction set support.
k6-2
k6-3
Improved versions of AMD K6 CPU with MMX and 3DNow! instruction set support.
athlon
athlon-tbird
AMD Athlon CPU with MMX, 3dNOW!, enhanced 3DNow! and SSE prefetch instructions support.
athlon-4
athlon-xp
athlon-mp
Improved AMD Athlon CPU with MMX, 3DNow!, enhanced 3DNow! and full SSE instruction set support.
k8
opteron
athlon64
athlon-fx
Processors based on the AMD K8 core with x86-64 instruction set support, including the AMD Opteron, Athlon
64, and Athlon 64 FX processors. (This supersets MMX, SSE, SSE2, 3DNow!, enhanced 3DNow! and 64-bit
instruction set extensions.)
k8-sse3
opteron-sse3
athlon64-sse3
Improved versions of AMD K8 cores with SSE3 instruction set support.
amdfam10
barcelona
CPUs based on AMD Family 10h cores with x86-64 instruction set support. (This supersets MMX, SSE, SSE2,
SSE3, SSE4A, 3DNow!, enhanced 3DNow!, ABM and 64-bit instruction set extensions.)
bdver1
CPUs based on AMD Family 15h cores with x86-64 instruction set support. (This supersets FMA4, AVX, XOP, LWP,
AES, PCLMUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set
extensions.)
bdver2
AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets BMI, TBM, F16C, FMA,
FMA4, AVX, XOP, LWP, AES, PCLMUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit
instruction set extensions.)
bdver3
AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets BMI, TBM, F16C, FMA,
FMA4, FSGSBASE, AVX, XOP, LWP, AES, PCLMUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and
64-bit instruction set extensions.)
bdver4
AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, TBM, F16C,
FMA, FMA4, FSGSBASE, AVX, AVX2, XOP, LWP, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3,
SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.)
znver1
AMD Family 17h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, F16C, FMA,
FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3,
SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, and 64-bit instruction set
extensions.)
znver2
AMD Family 17h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, CLWB, F16C,
FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3,
SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, RDPID, WBNOINVD, and 64-bit
instruction set extensions.)
znver3
AMD Family 19h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, CLWB, F16C,
FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3,
SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, RDPID, WBNOINVD, PKU, VPCLMULQDQ,
VAES, and 64-bit instruction set extensions.)
znver4
AMD Family 19h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, CLWB, F16C,
FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3,
SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, RDPID, WBNOINVD, PKU, VPCLMULQDQ,
VAES, AVX512F, AVX512DQ, AVX512IFMA, AVX512CD, AVX512BW, AVX512VL, AVX512BF16, AVX512VBMI, AVX512VBMI2,
AVX512VNNI, AVX512BITALG, AVX512VPOPCNTDQ, GFNI and 64-bit instruction set extensions.)
znver5
AMD Family 1ah core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, CLWB, F16C,
FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3,
SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, RDPID, WBNOINVD, PKU, VPCLMULQDQ,
VAES, AVX512F, AVX512DQ, AVX512IFMA, AVX512CD, AVX512BW, AVX512VL, AVX512BF16, AVX512VBMI, AVX512VBMI2,
AVX512VNNI, AVX512BITALG, AVX512VPOPCNTDQ, GFNI, AVXVNNI, MOVDIRI, MOVDIR64B, AVX512VP2INTERSECT, PREFETCHI
and 64-bit instruction set extensions.)
btver1
CPUs based on AMD Family 14h cores with x86-64 instruction set support. (This supersets MMX, SSE, SSE2,
SSE3, SSSE3, SSE4A, CX16, ABM and 64-bit instruction set extensions.)
btver2
CPUs based on AMD Family 16h cores with x86-64 instruction set support. This includes MOVBE, F16C, BMI, AVX,
PCLMUL, AES, SSE4.2, SSE4.1, CX16, ABM, SSE4A, SSSE3, SSE3, SSE2, SSE, MMX and 64-bit instruction set
extensions.
winchip-c6
IDT WinChip C6 CPU, dealt in same way as i486 with additional MMX instruction set support.
winchip2
IDT WinChip 2 CPU, dealt in same way as i486 with additional MMX and 3DNow! instruction set support.
c3 VIA C3 CPU with MMX and 3DNow! instruction set support. (No scheduling is implemented for this chip.)
c3-2
VIA C3-2 (Nehemiah/C5XL) CPU with MMX and SSE instruction set support. (No scheduling is implemented for
this chip.)
c7 VIA C7 (Esther) CPU with MMX, SSE, SSE2 and SSE3 instruction set support. (No scheduling is implemented for
this chip.)
samuel-2
VIA Eden Samuel 2 CPU with MMX and 3DNow! instruction set support. (No scheduling is implemented for this
chip.)
nehemiah
VIA Eden Nehemiah CPU with MMX and SSE instruction set support. (No scheduling is implemented for this
chip.)
esther
VIA Eden Esther CPU with MMX, SSE, SSE2 and SSE3 instruction set support. (No scheduling is implemented for
this chip.)
eden-x2
VIA Eden X2 CPU with x86-64, MMX, SSE, SSE2 and SSE3 instruction set support. (No scheduling is implemented
for this chip.)
eden-x4
VIA Eden X4 CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX and AVX2 instruction set
support. (No scheduling is implemented for this chip.)
nano
Generic VIA Nano CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. (No scheduling is
implemented for this chip.)
nano-1000
VIA Nano 1xxx CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. (No scheduling is
implemented for this chip.)
nano-2000
VIA Nano 2xxx CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. (No scheduling is
implemented for this chip.)
nano-3000
VIA Nano 3xxx CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 instruction set support. (No
scheduling is implemented for this chip.)
nano-x2
VIA Nano Dual Core CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 instruction set support. (No
scheduling is implemented for this chip.)
nano-x4
VIA Nano Quad Core CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 instruction set support. (No
scheduling is implemented for this chip.)
lujiazui
ZHAOXIN lujiazui CPU with x86-64, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PCLMUL,
RDRND, XSAVE, XSAVEOPT, FSGSBASE, CX16, ABM, BMI, BMI2, FXSR, RDSEED instruction set support. While the CPUs
do support AVX and F16C, these aren't enabled by "-march=lujiazui" for performance reasons.
yongfeng
ZHAOXIN yongfeng CPU with x86-64, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, POPCNT, AES,
PCLMUL, RDRND, XSAVE, XSAVEOPT, FSGSBASE, CX16, ABM, BMI, BMI2, F16C, FXSR, RDSEED, AVX2, FMA, SHA, LZCNT
instruction set support.
shijidadao
ZHAOXIN shijidadao CPU with x86-64, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, POPCNT, AES,
PCLMUL, RDRND, XSAVE, XSAVEOPT, FSGSBASE, CX16, ABM, BMI, BMI2, F16C, FXSR, RDSEED, AVX2, FMA, SHA, LZCNT
instruction set support.
geode
AMD Geode embedded processor with MMX and 3DNow! instruction set support.
-mtune=cpu-type
Tune to cpu-type everything applicable about the generated code, except for the ABI and the set of available
instructions. While picking a specific cpu-type schedules things appropriately for that particular chip, the
compiler does not generate any code that cannot run on the default machine type unless you use a -march=cpu-type
option. For example, if GCC is configured for i686-pc-linux-gnu then -mtune=pentium4 generates code that is
tuned for Pentium 4 but still runs on i686 machines.
The choices for cpu-type are the same as for -march. In addition, -mtune supports 2 extra choices for cpu-type:
generic
Produce code optimized for the most common IA32/AMD64/EM64T processors. If you know the CPU on which your
code will run, then you should use the corresponding -mtune or -march option instead of -mtune=generic. But,
if you do not know exactly what CPU users of your application will have, then you should use this option.
As new processors are deployed in the marketplace, the behavior of this option will change. Therefore, if
you upgrade to a newer version of GCC, code generation controlled by this option will change to reflect the
processors that are most common at the time that version of GCC is released.
There is no -march=generic option because -march indicates the instruction set the compiler can use, and
there is no generic instruction set applicable to all processors. In contrast, -mtune indicates the
processor (or, in this case, collection of processors) for which the code is optimized.
intel
Produce code optimized for the most current Intel processors, which are Haswell and Silvermont for this
version of GCC. If you know the CPU on which your code will run, then you should use the corresponding
-mtune or -march option instead of -mtune=intel. But, if you want your application performs better on both
Haswell and Silvermont, then you should use this option.
As new Intel processors are deployed in the marketplace, the behavior of this option will change. Therefore,
if you upgrade to a newer version of GCC, code generation controlled by this option will change to reflect
the most current Intel processors at the time that version of GCC is released.
There is no -march=intel option because -march indicates the instruction set the compiler can use, and there
is no common instruction set applicable to all processors. In contrast, -mtune indicates the processor (or,
in this case, collection of processors) for which the code is optimized.
-mcpu=cpu-type
A deprecated synonym for -mtune.
-mfpmath=unit
Generate floating-point arithmetic for selected unit unit. The choices for unit are:
387 Use the standard 387 floating-point coprocessor present on the majority of chips and emulated otherwise.
Code compiled with this option runs almost everywhere. The temporary results are computed in 80-bit
precision instead of the precision specified by the type, resulting in slightly different results compared to
most of other chips. See -ffloat-store for more detailed description.
This is the default choice for non-Darwin x86-32 targets.
sse Use scalar floating-point instructions present in the SSE instruction set. This instruction set is supported
by Pentium III and newer chips, and in the AMD line by Athlon-4, Athlon XP and Athlon MP chips. The earlier
version of the SSE instruction set supports only single-precision arithmetic, thus the double and extended-
precision arithmetic are still done using 387. A later version, present only in Pentium 4 and AMD x86-64
chips, supports double-precision arithmetic too.
For the x86-32 compiler, you must use -march=cpu-type, -msse or -msse2 switches to enable SSE extensions and
make this option effective. For the x86-64 compiler, these extensions are enabled by default.
The resulting code should be considerably faster in the majority of cases and avoid the numerical instability
problems of 387 code, but may break some existing code that expects temporaries to be 80 bits.
This is the default choice for the x86-64 compiler, Darwin x86-32 targets, and the default choice for x86-32
targets with the SSE2 instruction set when -ffast-math is enabled.
sse,387
sse+387
both
Attempt to utilize both instruction sets at once. This effectively doubles the amount of available
registers, and on chips with separate execution units for 387 and SSE the execution resources too. Use this
option with care, as it is still experimental, because the GCC register allocator does not model separate
functional units well, resulting in unstable performance.
-masm=dialect
Output assembly instructions using selected dialect. Also affects which dialect is used for basic "asm" and
extended "asm". Supported choices (in dialect order) are att or intel. The default is att. Darwin does not
support intel.
-mieee-fp
-mno-ieee-fp
Control whether or not the compiler uses IEEE floating-point comparisons. These correctly handle the case where
the result of a comparison is unordered.
-m80387
-mhard-float
Generate output containing 80387 instructions for floating point.
-mno-80387
-msoft-float
Generate output containing library calls for floating point.
Warning: the requisite libraries are not part of GCC. Normally the facilities of the machine's usual C compiler
are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide
suitable library functions for cross-compilation.
On machines where a function returns floating-point results in the 80387 register stack, some floating-point
opcodes may be emitted even if -msoft-float is used.
-mno-fp-ret-in-387
Do not use the FPU registers for return values of functions.
The usual calling convention has functions return values of types "float" and "double" in an FPU register, even
if there is no FPU. The idea is that the operating system should emulate an FPU.
The option -mno-fp-ret-in-387 causes such values to be returned in ordinary CPU registers instead.
-mno-fancy-math-387
Some 387 emulators do not support the "sin", "cos" and "sqrt" instructions for the 387. Specify this option to
avoid generating those instructions. This option is overridden when -march indicates that the target CPU always
has an FPU and so the instruction does not need emulation. These instructions are not generated unless you also
use the -funsafe-math-optimizations switch.
-malign-double
-mno-align-double
Control whether GCC aligns "double", "long double", and "long long" variables on a two-word boundary or a one-
word boundary. Aligning "double" variables on a two-word boundary produces code that runs somewhat faster on a
Pentium at the expense of more memory.
On x86-64, -malign-double is enabled by default.
Warning: if you use the -malign-double switch, structures containing the above types are aligned differently than
the published application binary interface specifications for the x86-32 and are not binary compatible with
structures in code compiled without that switch.
-m96bit-long-double
-m128bit-long-double
These switches control the size of "long double" type. The x86-32 application binary interface specifies the
size to be 96 bits, so -m96bit-long-double is the default in 32-bit mode.
Modern architectures (Pentium and newer) prefer "long double" to be aligned to an 8- or 16-byte boundary. In
arrays or structures conforming to the ABI, this is not possible. So specifying -m128bit-long-double aligns
"long double" to a 16-byte boundary by padding the "long double" with an additional 32-bit zero.
In the x86-64 compiler, -m128bit-long-double is the default choice as its ABI specifies that "long double" is
aligned on 16-byte boundary.
Notice that neither of these options enable any extra precision over the x87 standard of 80 bits for a "long
double".
Warning: if you override the default value for your target ABI, this changes the size of structures and arrays
containing "long double" variables, as well as modifying the function calling convention for functions taking
"long double". Hence they are not binary-compatible with code compiled without that switch.
-mlong-double-64
-mlong-double-80
-mlong-double-128
These switches control the size of "long double" type. A size of 64 bits makes the "long double" type equivalent
to the "double" type. This is the default for 32-bit Bionic C library. A size of 128 bits makes the "long
double" type equivalent to the "__float128" type. This is the default for 64-bit Bionic C library.
Warning: if you override the default value for your target ABI, this changes the size of structures and arrays
containing "long double" variables, as well as modifying the function calling convention for functions taking
"long double". Hence they are not binary-compatible with code compiled without that switch.
-malign-data=type
Control how GCC aligns variables. Supported values for type are compat uses increased alignment value compatible
uses GCC 4.8 and earlier, abi uses alignment value as specified by the psABI, and cacheline uses increased
alignment value to match the cache line size. compat is the default.
-mlarge-data-threshold=threshold
When -mcmodel=medium or -mcmodel=large is specified, data objects larger than threshold are placed in large data
sections. The default is 65535.
-mrtd
Use a different function-calling convention, in which functions that take a fixed number of arguments return with
the "ret num" instruction, which pops their arguments while returning. This saves one instruction in the caller
since there is no need to pop the arguments there.
You can specify that an individual function is called with this calling sequence with the function attribute
"stdcall". You can also override the -mrtd option by using the function attribute "cdecl".
Warning: this calling convention is incompatible with the one normally used on Unix, so you cannot use it if you
need to call libraries compiled with the Unix compiler.
Also, you must provide function prototypes for all functions that take variable numbers of arguments (including
"printf"); otherwise incorrect code is generated for calls to those functions.
In addition, seriously incorrect code results if you call a function with too many arguments. (Normally, extra
arguments are harmlessly ignored.)
-mregparm=num
Control how many registers are used to pass integer arguments. By default, no registers are used to pass
arguments, and at most 3 registers can be used. You can control this behavior for a specific function by using
the function attribute "regparm".
Warning: if you use this switch, and num is nonzero, then you must build all modules with the same value,
including any libraries. This includes the system libraries and startup modules.
-msseregparm
Use SSE register passing conventions for float and double arguments and return values. You can control this
behavior for a specific function by using the function attribute "sseregparm".
Warning: if you use this switch then you must build all modules with the same value, including any libraries.
This includes the system libraries and startup modules.
-mvect8-ret-in-mem
Return 8-byte vectors in memory instead of MMX registers. This is the default on VxWorks to match the ABI of the
Sun Studio compilers until version 12. Only use this option if you need to remain compatible with existing code
produced by those previous compiler versions or older versions of GCC.
-mpc32
-mpc64
-mpc80
Set 80387 floating-point precision to 32, 64 or 80 bits. When -mpc32 is specified, the significands of results
of floating-point operations are rounded to 24 bits (single precision); -mpc64 rounds the significands of results
of floating-point operations to 53 bits (double precision) and -mpc80 rounds the significands of results of
floating-point operations to 64 bits (extended double precision), which is the default. When this option is
used, floating-point operations in higher precisions are not available to the programmer without setting the FPU
control word explicitly.
Setting the rounding of floating-point operations to less than the default 80 bits can speed some programs by 2%
or more. Note that some mathematical libraries assume that extended-precision (80-bit) floating-point operations
are enabled by default; routines in such libraries could suffer significant loss of accuracy, typically through
so-called "catastrophic cancellation", when this option is used to set the precision to less than extended
precision.
-mdaz-ftz
The flush-to-zero (FTZ) and denormals-are-zero (DAZ) flags in the MXCSR register are used to control floating-
point calculations.SSE and AVX instructions including scalar and vector instructions could benefit from enabling
the FTZ and DAZ flags when -mdaz-ftz is specified. Don't set FTZ/DAZ flags when -mno-daz-ftz or -shared is
specified, -mdaz-ftz will set FTZ/DAZ flags even with -shared.
-mstackrealign
Realign the stack at entry. On the x86, the -mstackrealign option generates an alternate prologue and epilogue
that realigns the run-time stack if necessary. This supports mixing legacy codes that keep 4-byte stack
alignment with modern codes that keep 16-byte stack alignment for SSE compatibility. See also the attribute
"force_align_arg_pointer", applicable to individual functions.
-mpreferred-stack-boundary=num
Attempt to keep the stack boundary aligned to a 2 raised to num byte boundary. If -mpreferred-stack-boundary is
not specified, the default is 4 (16 bytes or 128 bits).
Warning: When generating code for the x86-64 architecture with SSE extensions disabled,
-mpreferred-stack-boundary=3 can be used to keep the stack boundary aligned to 8 byte boundary. Since x86-64 ABI
require 16 byte stack alignment, this is ABI incompatible and intended to be used in controlled environment where
stack space is important limitation. This option leads to wrong code when functions compiled with 16 byte stack
alignment (such as functions from a standard library) are called with misaligned stack. In this case, SSE
instructions may lead to misaligned memory access traps. In addition, variable arguments are handled incorrectly
for 16 byte aligned objects (including x87 long double and __int128), leading to wrong results. You must build
all modules with -mpreferred-stack-boundary=3, including any libraries. This includes the system libraries and
startup modules.
-mincoming-stack-boundary=num
Assume the incoming stack is aligned to a 2 raised to num byte boundary. If -mincoming-stack-boundary is not
specified, the one specified by -mpreferred-stack-boundary is used.
On Pentium and Pentium Pro, "double" and "long double" values should be aligned to an 8-byte boundary (see
-malign-double) or suffer significant run time performance penalties. On Pentium III, the Streaming SIMD
Extension (SSE) data type "__m128" may not work properly if it is not 16-byte aligned.
To ensure proper alignment of this values on the stack, the stack boundary must be as aligned as that required by
any value stored on the stack. Further, every function must be generated such that it keeps the stack aligned.
Thus calling a function compiled with a higher preferred stack boundary from a function compiled with a lower
preferred stack boundary most likely misaligns the stack. It is recommended that libraries that use callbacks
always use the default setting.
This extra alignment does consume extra stack space, and generally increases code size. Code that is sensitive
to stack space usage, such as embedded systems and operating system kernels, may want to reduce the preferred
alignment to -mpreferred-stack-boundary=2.
-mmmx
-msse
-msse2
-msse3
-mssse3
-msse4
-msse4a
-msse4.1
-msse4.2
-mavx
-mavx2
-mavx512f
-mavx512cd
-mavx512vl
-mavx512bw
-mavx512dq
-mavx512ifma
-mavx512vbmi
-msha
-maes
-mpclmul
-mclflushopt
-mclwb
-mfsgsbase
-mptwrite
-mrdrnd
-mf16c
-mfma
-mpconfig
-mwbnoinvd
-mfma4
-mprfchw
-mrdpid
-mrdseed
-msgx
-mxop
-mlwp
-m3dnow
-m3dnowa
-mpopcnt
-mabm
-madx
-mbmi
-mbmi2
-mlzcnt
-mfxsr
-mxsave
-mxsaveopt
-mxsavec
-mxsaves
-mrtm
-mhle
-mtbm
-mmwaitx
-mclzero
-mpku
-mavx512vbmi2
-mavx512bf16
-mavx512fp16
-mgfni
-mvaes
-mwaitpkg
-mvpclmulqdq
-mavx512bitalg
-mmovdiri
-mmovdir64b
-menqcmd
-muintr
-mtsxldtrk
-mavx512vpopcntdq
-mavx512vp2intersect
-mavx512vnni
-mavxvnni
-mcldemote
-mserialize
-mamx-tile
-mamx-int8
-mamx-bf16
-mhreset
-mkl
-mwidekl
-mavxifma
-mavxvnniint8
-mavxneconvert
-mcmpccxadd
-mamx-fp16
-mprefetchi
-mraoint
-mamx-complex
-mavxvnniint16
-msm3
-msha512
-msm4
-mapxf
-musermsr
-mavx10.1
-mavx10.1-256
-mavx10.1-512
-mavx10.2
-mamx-avx512
-mamx-tf32
-mamx-transpose
-mamx-fp8
-mmovrs
-mamx-movrs
These switches enable the use of instructions in the MMX, SSE, AVX512CD, AVX512VL, AVX512BW, AVX512DQ,
AVX512IFMA, AVX512VBMI, SHA, AES, PCLMUL, CLFLUSHOPT, CLWB, FSGSBASE, PTWRITE, RDRND, F16C, FMA, PCONFIG,
WBNOINVD, FMA4, PREFETCHW, RDPID, RDSEED, SGX, XOP, LWP, 3DNow!, enhanced 3DNow!, POPCNT, ABM, ADX, BMI, BMI2,
LZCNT, FXSR, XSAVE, XSAVEOPT, XSAVEC, XSAVES, RTM, HLE, TBM, MWAITX, CLZERO, PKU, AVX512VBMI2, GFNI, VAES,
WAITPKG, VPCLMULQDQ, AVX512BITALG, MOVDIRI, MOVDIR64B, AVX512BF16, ENQCMD, AVX512VPOPCNTDQ, AVX512VNNI,
SERIALIZE, UINTR, HRESET, AMXTILE, AMXINT8, AMXBF16, KL, WIDEKL, AVXVNNI, AVX512-FP16, AVXIFMA, AVXVNNIINT8,
AVXNECONVERT, CMPCCXADD, AMX-FP16, PREFETCHI, RAOINT, AMX-COMPLEX, AVXVNNIINT16, SM3, SHA512, SM4, APX_F,
USER_MSR, AVX10.1, AVX10.2, AMX-AVX512, AMX-TF32, AMX-TRANSPOSE, AMX-FP8, MOVRS, AMX-MOVRS or CLDEMOTE extended
instruction sets. Each has a corresponding -mno- option to disable use of these instructions.
These extensions are also available as built-in functions: see x86 Built-in Functions, for details of the
functions enabled and disabled by these switches.
Note that -msse4 enables both SSE4.1 and SSE4.2 support, while -mno-sse4 turns off those features; neither form
of the option affects SSE4A support, controlled separately by -msse4a.
To generate SSE/SSE2 instructions automatically from floating-point code (as opposed to 387 instructions), see
-mfpmath=sse.
GCC depresses SSEx instructions when -mavx is used. Instead, it generates new AVX instructions or AVX equivalence
for all SSEx instructions when needed.
These options enable GCC to use these extended instructions in generated code, even without -mfpmath=sse.
Applications that perform run-time CPU detection must compile separate files for each supported architecture,
using the appropriate flags. In particular, the file containing the CPU detection code should be compiled
without these options.
-mdump-tune-features
This option instructs GCC to dump the names of the x86 performance tuning features and default settings. The
names can be used in -mtune-ctrl=feature-list.
-mtune-ctrl=feature-list
This option is used to do fine grain control of x86 code generation features. feature-list is a comma separated
list of feature names. See also -mdump-tune-features. When specified, the feature is turned on if it is not
preceded with ^, otherwise, it is turned off. -mtune-ctrl=feature-list is intended to be used by GCC developers.
Using it may lead to code paths not covered by testing and can potentially result in compiler ICEs or runtime
errors.
-mno-default
This option instructs GCC to turn off all tunable features. See also -mtune-ctrl=feature-list and
-mdump-tune-features.
-mcld
This option instructs GCC to emit a "cld" instruction in the prologue of functions that use string instructions.
String instructions depend on the DF flag to select between autoincrement or autodecrement mode. While the ABI
specifies the DF flag to be cleared on function entry, some operating systems violate this specification by not
clearing the DF flag in their exception dispatchers. The exception handler can be invoked with the DF flag set,
which leads to wrong direction mode when string instructions are used. This option can be enabled by default on
32-bit x86 targets by configuring GCC with the --enable-cld configure option. Generation of "cld" instructions
can be suppressed with the -mno-cld compiler option in this case.
-mvzeroupper
This option instructs GCC to emit a "vzeroupper" instruction before a transfer of control flow out of the
function to minimize the AVX to SSE transition penalty as well as remove unnecessary "zeroupper" intrinsics.
-mprefer-avx128
This option instructs GCC to use 128-bit AVX instructions instead of 256-bit AVX instructions in the auto-
vectorizer.
-mprefer-vector-width=opt
This option instructs GCC to use opt-bit vector width in instructions instead of default on the selected
platform.
-mpartial-vector-fp-math
This option enables GCC to generate floating-point operations that might affect the set of floating-point status
flags on partial vectors, where vector elements reside in the low part of the 128-bit SSE register. Unless
-fno-trapping-math is specified, the compiler guarantees correct behavior by sanitizing all input operands to
have zeroes in the unused upper part of the vector register. Note that by using built-in functions or inline
assembly with partial vector arguments, NaNs, denormal or invalid values can leak into the upper part of the
vector, causing possible performance issues when -fno-trapping-math is in effect. These issues can be mitigated
by manually sanitizing the upper part of the partial vector argument register or by using -mdaz-ftz to set
denormals-are-zero (DAZ) flag in the MXCSR register.
This option is enabled by default.
-mmove-max=bits
This option instructs GCC to set the maximum number of bits can be moved from memory to memory efficiently to
bits. The valid bits are 128, 256 and 512.
-mstore-max=bits
This option instructs GCC to set the maximum number of bits can be stored to memory efficiently to bits. The
valid bits are 128, 256 and 512.
none
No extra limitations applied to GCC other than defined by the selected platform.
128 Prefer 128-bit vector width for instructions.
256 Prefer 256-bit vector width for instructions.
512 Prefer 512-bit vector width for instructions.
-mnoreturn-no-callee-saved-registers
This option optimizes functions with "noreturn" attribute or "_Noreturn" specifier by not saving in the function
prologue callee-saved registers which are used in the function (except for the "BP" register). This option can
interfere with debugging of the caller of the "noreturn" function or any function further up in the call stack,
so it is not enabled by default.
-mcx16
This option enables GCC to generate "CMPXCHG16B" instructions in 64-bit code to implement compare-and-exchange
operations on 16-byte aligned 128-bit objects. This is useful for atomic updates of data structures exceeding
one machine word in size. The compiler uses this instruction to implement __sync Builtins. However, for
__atomic Builtins operating on 128-bit integers, a library call is always used.
-msahf
This option enables generation of "SAHF" instructions in 64-bit code. Early Intel Pentium 4 CPUs with Intel 64
support, prior to the introduction of Pentium 4 G1 step in December 2005, lacked the "LAHF" and "SAHF"
instructions which are supported by AMD64. These are load and store instructions, respectively, for certain
status flags. In 64-bit mode, the "SAHF" instruction is used to optimize "fmod", "drem", and "remainder" built-
in functions; see Other Builtins for details.
-mmovbe
This option enables use of the "movbe" instruction to optimize byte swapping of four and eight byte entities.
-mshstk
The -mshstk option enables shadow stack built-in functions from x86 Control-flow Enforcement Technology (CET).
-mcrc32
This option enables built-in functions "__builtin_ia32_crc32qi", "__builtin_ia32_crc32hi",
"__builtin_ia32_crc32si" and "__builtin_ia32_crc32di" to generate the "crc32" machine instruction.
-mmwait
This option enables built-in functions "__builtin_ia32_monitor", and "__builtin_ia32_mwait" to generate the
"monitor" and "mwait" machine instructions.
-mrecip
This option enables use of "RCPSS" and "RSQRTSS" instructions (and their vectorized variants "RCPPS" and
"RSQRTPS") with an additional Newton-Raphson step to increase precision instead of "DIVSS" and "SQRTSS" (and
their vectorized variants) for single-precision floating-point arguments. These instructions are generated only
when -funsafe-math-optimizations is enabled together with -ffinite-math-only and -fno-trapping-math. Note that
while the throughput of the sequence is higher than the throughput of the non-reciprocal instruction, the
precision of the sequence can be decreased by up to 2 ulp (i.e. the inverse of 1.0 equals 0.99999994).
Note that GCC implements "1.0f/sqrtf(x)" in terms of "RSQRTSS" (or "RSQRTPS") already with -ffast-math (or the
above option combination), and doesn't need -mrecip.
Also note that GCC emits the above sequence with additional Newton-Raphson step for vectorized single-float
division and vectorized sqrtf(x) already with -ffast-math (or the above option combination), and doesn't need
-mrecip.
-mrecip=opt
This option controls which reciprocal estimate instructions may be used. opt is a comma-separated list of
options, which may be preceded by a ! to invert the option:
all Enable all estimate instructions.
default
Enable the default instructions, equivalent to -mrecip.
none
Disable all estimate instructions, equivalent to -mno-recip.
div Enable the approximation for scalar division.
vec-div
Enable the approximation for vectorized division.
sqrt
Enable the approximation for scalar square root.
vec-sqrt
Enable the approximation for vectorized square root.
So, for example, -mrecip=all,!sqrt enables all of the reciprocal approximations, except for square root.
-mveclibabi=type
Specifies the ABI type to use for vectorizing intrinsics using an external library. Supported values for type
are svml for the Intel short vector math library, aocl for the math library (LibM) from AMD Optimizing CPU
Libraries (AOCL) and acml for the end-of-life AMD core math library (to which AOCL-LibM is the successor). To
use this option, both -ftree-vectorize and -funsafe-math-optimizations have to be enabled, and an SVML or ACML
ABI-compatible library must be specified at link time.
GCC currently emits calls to "vmldExp2", "vmldLn2", "vmldLog102", "vmldPow2", "vmldTanh2", "vmldTan2",
"vmldAtan2", "vmldAtanh2", "vmldCbrt2", "vmldSinh2", "vmldSin2", "vmldAsinh2", "vmldAsin2", "vmldCosh2",
"vmldCos2", "vmldAcosh2", "vmldAcos2", "vmlsExp4", "vmlsLn4", "vmlsLog104", "vmlsPow4", "vmlsTanh4", "vmlsTan4",
"vmlsAtan4", "vmlsAtanh4", "vmlsCbrt4", "vmlsSinh4", "vmlsSin4", "vmlsAsinh4", "vmlsAsin4", "vmlsCosh4",
"vmlsCos4", "vmlsAcosh4" and "vmlsAcos4" for corresponding function type when -mveclibabi=svml is used,
"amd_vrs4_acosf", "amd_vrs16_acosf", "amd_vrd8_asin", "amd_vrs4_asinf", "amd_vrs8_asinf", "amd_vrs16_asinf",
"amd_vrd2_atan", "amd_vrd8_atan", "amd_vrs4_atanf", "amd_vrs8_atanf", "amd_vrs16_atanf", "amd_vrd2_cos",
"amd_vrd4_cos", "amd_vrd8_cos", "amd_vrs4_cosf", "amd_vrs8_cosf", "amd_vrs16_cosf", "amd_vrs4_coshf",
"amd_vrs8_coshf", "amd_vrd2_erf", "amd_vrd4_erf", "amd_vrd8_erf", "amd_vrs4_erff", "amd_vrs8_erff",
"amd_vrs16_erff", "amd_vrd2_exp", "amd_vrd4_exp", "amd_vrd8_exp", "amd_vrs4_expf", "amd_vrs8_expf",
"amd_vrs16_expf", "amd_vrd2_exp10", "amd_vrs4_exp10f", "amd_vrd2_exp2", "amd_vrd4_exp2", "amd_vrd8_exp2",
"amd_vrs4_exp2f", "amd_vrs8_exp2f", "amd_vrs16_exp2f", "amd_vrs4_expm1f", "amd_vrd2_log", "amd_vrd4_log",
"amd_vrd8_log", "amd_vrs4_logf", "amd_vrs8_logf", "amd_vrs16_logf", "amd_vrd2_log10", "amd_vrs4_log10f",
"amd_vrs8_log10f", "amd_vrs16_log10f", "amd_vrd2_log1p", "amd_vrs4_log1pf", "amd_vrd2_log2", "amd_vrd4_log2",
"amd_vrd8_log2", "amd_vrs4_log2f", "amd_vrs8_log2f", "amd_vrs16_log2f", "amd_vrd2_pow", "amd_vrd4_pow",
"amd_vrd8_pow", "amd_vrs4_powf", "amd_vrs8_powf", "amd_vrs16_powf", "amd_vrd2_sin", "amd_vrd4_sin",
"amd_vrd8_sin", "amd_vrs4_sinf", "amd_vrs8_sinf", "amd_vrs16_sinf", "amd_vrd2_tan", "amd_vrd4_tan",
"amd_vrd8_tan", "amd_vrs16_tanf", "amd_vrs4_tanhf", "amd_vrs8_tanhf", "amd_vrs16_tanhf" for the corresponding
function type when -mveclibabi=aocl is used, and "__vrd2_sin", "__vrd2_cos", "__vrd2_exp", "__vrd2_log",
"__vrd2_log2", "__vrd2_log10", "__vrs4_sinf", "__vrs4_cosf", "__vrs4_expf", "__vrs4_logf", "__vrs4_log2f",
"__vrs4_log10f" and "__vrs4_powf" for the corresponding function type when -mveclibabi=acml is used.
-mabi=name
Generate code for the specified calling convention. Permissible values are sysv for the ABI used on GNU/Linux
and other systems, and ms for the Microsoft ABI. The default is to use the Microsoft ABI when targeting
Microsoft Windows and the SysV ABI on all other systems. You can control this behavior for specific functions by
using the function attributes "ms_abi" and "sysv_abi".
-mforce-indirect-call
Force all calls to functions to be indirect. This is useful when using Intel Processor Trace where it generates
more precise timing information for function calls.
-mmanual-endbr
Insert ENDBR instruction at function entry only via the "cf_check" function attribute. This is useful when used
with the option -fcf-protection=branch to control ENDBR insertion at the function entry.
-mcet-switch
By default, CET instrumentation is turned off on switch statements that use a jump table and indirect branch
track is disabled. Since jump tables are stored in read-only memory, this does not result in a direct loss of
hardening. But if the jump table index is attacker-controlled, the indirect jump may not be constrained by CET.
This option turns on CET instrumentation to enable indirect branch track for switch statements with jump tables
which leads to the jump targets reachable via any indirect jumps.
-mcall-ms2sysv-xlogues
Due to differences in 64-bit ABIs, any Microsoft ABI function that calls a System V ABI function must consider
RSI, RDI and XMM6-15 as clobbered. By default, the code for saving and restoring these registers is emitted
inline, resulting in fairly lengthy prologues and epilogues. Using -mcall-ms2sysv-xlogues emits prologues and
epilogues that use stubs in the static portion of libgcc to perform these saves and restores, thus reducing
function size at the cost of a few extra instructions.
-mtls-dialect=type
Generate code to access thread-local storage using the gnu or gnu2 conventions. gnu is the conservative default;
gnu2 is more efficient, but it may add compile- and run-time requirements that cannot be satisfied on all
systems.
-mpush-args
-mno-push-args
Use PUSH operations to store outgoing parameters. This method is shorter and usually equally fast as method
using SUB/MOV operations and is enabled by default. In some cases disabling it may improve performance because
of improved scheduling and reduced dependencies.
-maccumulate-outgoing-args
If enabled, the maximum amount of space required for outgoing arguments is computed in the function prologue.
This is faster on most modern CPUs because of reduced dependencies, improved scheduling and reduced stack usage
when the preferred stack boundary is not equal to 2. The drawback is a notable increase in code size. This
switch implies -mno-push-args.
-mthreads
Support thread-safe exception handling on MinGW. Programs that rely on thread-safe exception handling must
compile and link all code with the -mthreads option. When compiling, -mthreads defines -D_MT; when linking, it
links in a special thread helper library -lmingwthrd which cleans up per-thread exception-handling data.
-mms-bitfields
-mno-ms-bitfields
Enable/disable bit-field layout compatible with the native Microsoft Windows compiler.
If "packed" is used on a structure, or if bit-fields are used, it may be that the Microsoft ABI lays out the
structure differently than the way GCC normally does. Particularly when moving packed data between functions
compiled with GCC and the native Microsoft compiler (either via function call or as data in a file), it may be
necessary to access either format.
This option is enabled by default for Microsoft Windows targets. This behavior can also be controlled locally by
use of variable or type attributes. For more information, see x86 Variable Attributes and x86 Type Attributes.
The Microsoft structure layout algorithm is fairly simple with the exception of the bit-field packing. The
padding and alignment of members of structures and whether a bit-field can straddle a storage-unit boundary are
determine by these rules:
1. Structure members are stored sequentially in the order in which they are
declared: the first member has the lowest memory address and the last member the highest.
2. Every data object has an alignment requirement. The alignment requirement
for all data except structures, unions, and arrays is either the size of the object or the current packing
size (specified with either the "aligned" attribute or the "pack" pragma), whichever is less. For
structures, unions, and arrays, the alignment requirement is the largest alignment requirement of its
members. Every object is allocated an offset so that:
offset % alignment_requirement == 0
3. Adjacent bit-fields are packed into the same 1-, 2-, or 4-byte allocation
unit if the integral types are the same size and if the next bit-field fits into the current allocation unit
without crossing the boundary imposed by the common alignment requirements of the bit-fields.
MSVC interprets zero-length bit-fields in the following ways:
1. If a zero-length bit-field is inserted between two bit-fields that
are normally coalesced, the bit-fields are not coalesced.
For example:
struct
{
unsigned long bf_1 : 12;
unsigned long : 0;
unsigned long bf_2 : 12;
} t1;
The size of "t1" is 8 bytes with the zero-length bit-field. If the zero-length bit-field were removed,
"t1"'s size would be 4 bytes.
2. If a zero-length bit-field is inserted after a bit-field, "foo", and the
alignment of the zero-length bit-field is greater than the member that follows it, "bar", "bar" is aligned as
the type of the zero-length bit-field.
For example:
struct
{
char foo : 4;
short : 0;
char bar;
} t2;
struct
{
char foo : 4;
short : 0;
double bar;
} t3;
For "t2", "bar" is placed at offset 2, rather than offset 1. Accordingly, the size of "t2" is 4. For "t3",
the zero-length bit-field does not affect the alignment of "bar" or, as a result, the size of the structure.
Taking this into account, it is important to note the following:
1. If a zero-length bit-field follows a normal bit-field, the type of the
zero-length bit-field may affect the alignment of the structure as whole. For example, "t2" has a size of
4 bytes, since the zero-length bit-field follows a normal bit-field, and is of type short.
2. Even if a zero-length bit-field is not followed by a normal bit-field, it may
still affect the alignment of the structure:
struct
{
char foo : 6;
long : 0;
} t4;
Here, "t4" takes up 4 bytes.
3. Zero-length bit-fields following non-bit-field members are ignored:
struct
{
char foo;
long : 0;
char bar;
} t5;
Here, "t5" takes up 2 bytes.
-mno-align-stringops
Do not align the destination of inlined string operations. This switch reduces code size and improves
performance in case the destination is already aligned, but GCC doesn't know about it.
-minline-all-stringops
By default GCC inlines string operations only when the destination is known to be aligned to least a 4-byte
boundary. This enables more inlining and increases code size, but may improve performance of code that depends
on fast "memcpy" and "memset" for short lengths. The option enables inline expansion of "strlen" for all pointer
alignments.
-minline-stringops-dynamically
For string operations of unknown size, use run-time checks with inline code for small blocks and a library call
for large blocks.
-mstringop-strategy=alg
Override the internal decision heuristic for the particular algorithm to use for inlining string operations. The
allowed values for alg are:
rep_byte
rep_4byte
rep_8byte
Expand using i386 "rep" prefix of the specified size.
byte_loop
loop
unrolled_loop
Expand into an inline loop.
libcall
Always use a library call.
-mmemcpy-strategy=strategy
Override the internal decision heuristic to decide if "__builtin_memcpy" should be inlined and what inline
algorithm to use when the expected size of the copy operation is known. strategy is a comma-separated list of
alg:max_size:dest_align triplets. alg is specified in -mstringop-strategy, max_size specifies the max byte size
with which inline algorithm alg is allowed. For the last triplet, the max_size must be -1. The max_size of the
triplets in the list must be specified in increasing order. The minimal byte size for alg is 0 for the first
triplet and "max_size + 1" of the preceding range.
-mmemset-strategy=strategy
The option is similar to -mmemcpy-strategy= except that it is to control "__builtin_memset" expansion.
-momit-leaf-frame-pointer
Don't keep the frame pointer in a register for leaf functions. This avoids the instructions to save, set up, and
restore frame pointers and makes an extra register available in leaf functions. The option
-fomit-leaf-frame-pointer removes the frame pointer for leaf functions, which might make debugging harder.
-mtls-direct-seg-refs
-mno-tls-direct-seg-refs
Controls whether TLS variables may be accessed with offsets from the TLS segment register (%gs for 32-bit, %fs
for 64-bit), or whether the thread base pointer must be added. Whether or not this is valid depends on the
operating system, and whether it maps the segment to cover the entire TLS area.
For systems that use the GNU C Library, the default is on.
-msse2avx
-mno-sse2avx
Specify that the assembler should encode SSE instructions with VEX prefix. The option -mavx turns this on by
default.
-mfentry
-mno-fentry
If profiling is active (-pg), put the profiling counter call before the prologue. Note: On x86 architectures the
attribute "ms_hook_prologue" isn't possible at the moment for -mfentry and -pg.
-mrecord-mcount
-mno-record-mcount
If profiling is active (-pg), generate a __mcount_loc section that contains pointers to each profiling call. This
is useful for automatically patching and out calls.
-mnop-mcount
-mno-nop-mcount
If profiling is active (-pg), generate the calls to the profiling functions as NOPs. This is useful when they
should be patched in later dynamically. This is likely only useful together with -mrecord-mcount.
-minstrument-return=type
Instrument function exit in -pg -mfentry instrumented functions with call to specified function. This only
instruments true returns ending with ret, but not sibling calls ending with jump. Valid types are none to not
instrument, call to generate a call to __return__, or nop5 to generate a 5 byte nop.
-mrecord-return
-mno-record-return
Generate a __return_loc section pointing to all return instrumentation code.
-mfentry-name=name
Set name of __fentry__ symbol called at function entry for -pg -mfentry functions.
-mfentry-section=name
Set name of section to record -mrecord-mcount calls (default __mcount_loc).
-mskip-rax-setup
-mno-skip-rax-setup
When generating code for the x86-64 architecture with SSE extensions disabled, -mskip-rax-setup can be used to
skip setting up RAX register when there are no variable arguments passed in vector registers.
Warning: Since RAX register is used to avoid unnecessarily saving vector registers on stack when passing variable
arguments, the impacts of this option are callees may waste some stack space, misbehave or jump to a random
location. GCC 4.4 or newer don't have those issues, regardless the RAX register value.
-m8bit-idiv
-mno-8bit-idiv
On some processors, like Intel Atom, 8-bit unsigned integer divide is much faster than 32-bit/64-bit integer
divide. This option generates a run-time check. If both dividend and divisor are within range of 0 to 255,
8-bit unsigned integer divide is used instead of 32-bit/64-bit integer divide.
-mavx256-split-unaligned-load
-mavx256-split-unaligned-store
Split 32-byte AVX unaligned load and store.
-mstack-protector-guard=guard
-mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset
-mstack-protector-guard-symbol=symbol
Generate stack protection code using canary at guard. Supported locations are global for global canary or tls
for per-thread canary in the TLS block (the default). This option has effect only when -fstack-protector or
-fstack-protector-all is specified.
With the latter choice the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset
furthermore specify which segment register (%fs or %gs) to use as base register for reading the canary, and from
what offset from that base register. The default for those is as specified in the relevant ABI.
-mstack-protector-guard-symbol=symbol overrides the offset with a symbol reference to a canary in the TLS block.
-mgeneral-regs-only
Generate code that uses only the general-purpose registers. This prevents the compiler from using floating-
point, vector, mask and bound registers.
-mrelax-cmpxchg-loop
When emitting a compare-and-swap loop for __sync Builtins and __atomic Builtins lacking a native instruction,
optimize for the highly contended case by issuing an atomic load before the "CMPXCHG" instruction, and using the
"PAUSE" instruction to save CPU power when restarting the loop.
-mindirect-branch=choice
Convert indirect call and jump with choice. The default is keep, which keeps indirect call and jump unmodified.
thunk converts indirect call and jump to call and return thunk. thunk-inline converts indirect call and jump to
inlined call and return thunk. thunk-extern converts indirect call and jump to external call and return thunk
provided in a separate object file. You can control this behavior for a specific function by using the function
attribute "indirect_branch".
Note that -mcmodel=large is incompatible with -mindirect-branch=thunk and -mindirect-branch=thunk-extern since
the thunk function may not be reachable in the large code model.
Note that -mindirect-branch=thunk-extern is compatible with -fcf-protection=branch since the external thunk can
be made to enable control-flow check.
-mfunction-return=choice
Convert function return with choice. The default is keep, which keeps function return unmodified. thunk
converts function return to call and return thunk. thunk-inline converts function return to inlined call and
return thunk. thunk-extern converts function return to external call and return thunk provided in a separate
object file. You can control this behavior for a specific function by using the function attribute
"function_return".
Note that -mindirect-return=thunk-extern is compatible with -fcf-protection=branch since the external thunk can
be made to enable control-flow check.
Note that -mcmodel=large is incompatible with -mfunction-return=thunk and -mfunction-return=thunk-extern since
the thunk function may not be reachable in the large code model.
-mindirect-branch-register
Force indirect call and jump via register.
-mharden-sls=choice
Generate code to mitigate against straight line speculation (SLS) with choice. The default is none which
disables all SLS hardening. return enables SLS hardening for function returns. indirect-jmp enables SLS
hardening for indirect jumps. all enables all SLS hardening.
-mindirect-branch-cs-prefix
Add CS prefix to call and jmp to indirect thunk with branch target in r8-r15 registers so that the call and jmp
instruction length is 6 bytes to allow them to be replaced with lfence; call *%r8-r15 or lfence; jmp *%r8-r15 at
run-time.
-mapx-inline-asm-use-gpr32
For inline asm support with APX, by default the EGPR feature was disabled to prevent potential illegal
instruction with EGPR occurs. To invoke egpr usage in inline asm, use new compiler option
-mapx-inline-asm-use-gpr32 and user should ensure the instruction supports EGPR.
-mevex512
-mno-evex512
Enables/disables 512-bit vector. It will be default on if AVX512F is enabled.
These -m switches are supported in addition to the above on x86-64 processors in 64-bit environments.
-m32
-m64
-mx32
-m16
-miamcu
Generate code for a 16-bit, 32-bit or 64-bit environment. The -m32 option sets "int", "long", and pointer types
to 32 bits, and generates code that runs in 32-bit mode.
The -m64 option sets "int" to 32 bits and "long" and pointer types to 64 bits, and generates code for the x86-64
architecture. For Darwin only the -m64 option also turns off the -fno-pic and -mdynamic-no-pic options.
The -mx32 option sets "int", "long", and pointer types to 32 bits, and generates code for the x86-64
architecture.
The -m16 option is the same as -m32, except for that it outputs the ".code16gcc" assembly directive at the
beginning of the assembly output so that the binary can run in 16-bit mode.
The -miamcu option generates code which conforms to Intel MCU psABI. It requires the -m32 option to be turned
on.
-mno-red-zone
Do not use a so-called "red zone" for x86-64 code. The red zone is mandated by the x86-64 ABI; it is a 128-byte
area beyond the location of the stack pointer that is not modified by signal or interrupt handlers and therefore
can be used for temporary data without adjusting the stack pointer. The flag -mno-red-zone disables this red
zone.
-mcmodel=small
Generate code for the small code model: the program and its symbols must be linked in the lower 2 GB of the
address space. Pointers are 64 bits. Programs can be statically or dynamically linked. This is the default
code model.
-mcmodel=kernel
Generate code for the kernel code model. The kernel runs in the negative 2 GB of the address space. This model
has to be used for Linux kernel code.
-mcmodel=medium
Generate code for the medium model: the program is linked in the lower 2 GB of the address space. Small symbols
are also placed there. Symbols with sizes larger than -mlarge-data-threshold are put into large data or BSS
sections and can be located above 2GB. Programs can be statically or dynamically linked.
-mcmodel=large
Generate code for the large model. This model makes no assumptions about addresses and sizes of sections.
-maddress-mode=long
Generate code for long address mode. This is only supported for 64-bit and x32 environments. It is the default
address mode for 64-bit environments.
-maddress-mode=short
Generate code for short address mode. This is only supported for 32-bit and x32 environments. It is the default
address mode for 32-bit and x32 environments.
-mneeded
-mno-needed
Emit GNU_PROPERTY_X86_ISA_1_NEEDED GNU property for Linux target to indicate the micro-architecture ISA level
required to execute the binary.
-mno-direct-extern-access
Without -fpic nor -fPIC, always use the GOT pointer to access external symbols. With -fpic or -fPIC, treat
access to protected symbols as local symbols. The default is -mdirect-extern-access.
Warning: shared libraries compiled with -mno-direct-extern-access and executable compiled with
-mdirect-extern-access may not be binary compatible if protected symbols are used in shared libraries and
executable.
-munroll-only-small-loops
Controls conservative small loop unrolling. It is default enabled by O2, and unrolls loop with less than 4 insns
by 1 time. Explicit -f[no-]unroll-[all-]loops would disable this flag to avoid any unintended unrolling behavior
that user does not want.
-mlam=choice
LAM(linear-address masking) allows special bits in the pointer to be used for metadata. The default is none. With
u48, pointer bits in positions 62:48 can be used for metadata; With u57, pointer bits in positions 62:57 can be
used for metadata.
x86 Windows Options
Xstormy16 Options
These options are defined for Xstormy16:
-msim
Choose startup files and linker script suitable for the simulator.
Xtensa Options
These options are supported for Xtensa targets:
-mconst16
-mno-const16
Enable or disable use of "CONST16" instructions for loading constant values. The "CONST16" instruction is
currently not a standard option from Tensilica. When enabled, "CONST16" instructions are always used in place of
the standard "L32R" instructions. The use of "CONST16" is enabled by default only if the "L32R" instruction is
not available.
-mfused-madd
-mno-fused-madd
Enable or disable use of fused multiply/add and multiply/subtract instructions in the floating-point option.
This has no effect if the floating-point option is not also enabled. Disabling fused multiply/add and
multiply/subtract instructions forces the compiler to use separate instructions for the multiply and add/subtract
operations. This may be desirable in some cases where strict IEEE 754-compliant results are required: the fused
multiply add/subtract instructions do not round the intermediate result, thereby producing results with more bits
of precision than specified by the IEEE standard. Disabling fused multiply add/subtract instructions also
ensures that the program output is not sensitive to the compiler's ability to combine multiply and add/subtract
operations.
-mserialize-volatile
-mno-serialize-volatile
When this option is enabled, GCC inserts "MEMW" instructions before "volatile" memory references to guarantee
sequential consistency. The default is -mserialize-volatile. Use -mno-serialize-volatile to omit the "MEMW"
instructions.
-mforce-no-pic
For targets, like GNU/Linux, where all user-mode Xtensa code must be position-independent code (PIC), this option
disables PIC for compiling kernel code.
-mtext-section-literals
-mno-text-section-literals
These options control the treatment of literal pools. The default is -mno-text-section-literals, which places
literals in a separate section in the output file. This allows the literal pool to be placed in a data RAM/ROM,
and it also allows the linker to combine literal pools from separate object files to remove redundant literals
and improve code size. With -mtext-section-literals, the literals are interspersed in the text section in order
to keep them as close as possible to their references. This may be necessary for large assembly files. Literals
for each function are placed right before that function.
-mauto-litpools
-mno-auto-litpools
These options control the treatment of literal pools. The default is -mno-auto-litpools, which places literals
in a separate section in the output file unless -mtext-section-literals is used. With -mauto-litpools the
literals are interspersed in the text section by the assembler. Compiler does not produce explicit ".literal"
directives and loads literals into registers with "MOVI" instructions instead of "L32R" to let the assembler do
relaxation and place literals as necessary. This option allows assembler to create several literal pools per
function and assemble very big functions, which may not be possible with -mtext-section-literals.
-mtarget-align
-mno-target-align
When this option is enabled, GCC instructs the assembler to automatically align instructions to reduce branch
penalties at the expense of some code density. The assembler attempts to widen density instructions to align
branch targets and the instructions following call instructions. If there are not enough preceding safe density
instructions to align a target, no widening is performed. The default is -mtarget-align. These options do not
affect the treatment of auto-aligned instructions like "LOOP", which the assembler always aligns, either by
widening density instructions or by inserting NOP instructions.
-mlongcalls
-mno-longcalls
When this option is enabled, GCC instructs the assembler to translate direct calls to indirect calls unless it
can determine that the target of a direct call is in the range allowed by the call instruction. This translation
typically occurs for calls to functions in other source files. Specifically, the assembler translates a direct
"CALL" instruction into an "L32R" followed by a "CALLX" instruction. The default is -mno-longcalls. This option
should be used in programs where the call target can potentially be out of range. This option is implemented in
the assembler, not the compiler, so the assembly code generated by GCC still shows direct call
instructions---look at the disassembled object code to see the actual instructions. Note that the assembler uses
an indirect call for every cross-file call, not just those that really are out of range.
-mabi=name
Generate code for the specified ABI. Permissible values are: call0, windowed. Default ABI is chosen by the
Xtensa core configuration.
-mabi=call0
When this option is enabled function parameters are passed in registers "a2" through "a7", registers "a12"
through "a15" are caller-saved, and register "a15" may be used as a frame pointer. When this version of the ABI
is enabled the C preprocessor symbol "__XTENSA_CALL0_ABI__" is defined.
-mabi=windowed
When this option is enabled function parameters are passed in registers "a10" through "a15", and called function
rotates register window by 8 registers on entry so that its arguments are found in registers "a2" through "a7".
Register "a7" may be used as a frame pointer. Register window is rotated 8 registers back upon return. When
this version of the ABI is enabled the C preprocessor symbol "__XTENSA_WINDOWED_ABI__" is defined.
-mextra-l32r-costs=n
Specify an extra cost of instruction RAM/ROM access for "L32R" instructions, in clock cycles. This affects, when
optimizing for speed, whether loading a constant from literal pool using "L32R" or synthesizing the constant from
a small one with a couple of arithmetic instructions. The default value is 0.
-mstrict-align
-mno-strict-align
Avoid or allow generating memory accesses that may not be aligned on a natural object boundary as described in
the architecture specification. The default is -mno-strict-align for cores that support both unaligned loads and
stores in hardware and -mstrict-align for all other cores.
zSeries Options
These are listed under
ENVIRONMENT
This section describes several environment variables that affect how GCC operates. Some of them work by specifying
directories or prefixes to use when searching for various kinds of files. Some are used to specify other aspects of
the compilation environment.
Note that you can also specify places to search using options such as -B, -I and -L. These take precedence over
places specified using environment variables, which in turn take precedence over those specified by the configuration
of GCC.
LANG
LC_CTYPE
LC_MESSAGES
LC_ALL
These environment variables control the way that GCC uses localization information which allows GCC to work with
different national conventions. GCC inspects the locale categories LC_CTYPE and LC_MESSAGES if it has been
configured to do so. These locale categories can be set to any value supported by your installation. A typical
value is en_GB.UTF-8 for English in the United Kingdom encoded in UTF-8.
The LC_CTYPE environment variable specifies character classification. GCC uses it to determine the character
boundaries in a string; this is needed for some multibyte encodings that contain quote and escape characters that
are otherwise interpreted as a string end or escape.
The LC_MESSAGES environment variable specifies the language to use in diagnostic messages.
If the LC_ALL environment variable is set, it overrides the value of LC_CTYPE and LC_MESSAGES; otherwise,
LC_CTYPE and LC_MESSAGES default to the value of the LANG environment variable. If none of these variables are
set, GCC defaults to traditional C English behavior.
TMPDIR
If TMPDIR is set, it specifies the directory to use for temporary files. GCC uses temporary files to hold the
output of one stage of compilation which is to be used as input to the next stage: for example, the output of the
preprocessor, which is the input to the compiler proper.
GCC_COMPARE_DEBUG
Setting GCC_COMPARE_DEBUG is nearly equivalent to passing -fcompare-debug to the compiler driver. See the
documentation of this option for more details.
GCC_EXEC_PREFIX
If GCC_EXEC_PREFIX is set, it specifies a prefix to use in the names of the subprograms executed by the compiler.
No slash is added when this prefix is combined with the name of a subprogram, but you can specify a prefix that
ends with a slash if you wish.
If GCC_EXEC_PREFIX is not set, GCC attempts to figure out an appropriate prefix to use based on the pathname it
is invoked with.
If GCC cannot find the subprogram using the specified prefix, it tries looking in the usual places for the
subprogram.
The default value of GCC_EXEC_PREFIX is prefix/lib/gcc/ where prefix is the prefix to the installed compiler. In
many cases prefix is the value of "prefix" when you ran the configure script.
Other prefixes specified with -B take precedence over this prefix.
This prefix is also used for finding files such as crt0.o that are used for linking.
In addition, the prefix is used in an unusual way in finding the directories to search for header files. For
each of the standard directories whose name normally begins with /usr/local/lib/gcc (more precisely, with the
value of GCC_INCLUDE_DIR), GCC tries replacing that beginning with the specified prefix to produce an alternate
directory name. Thus, with -Bfoo/, GCC searches foo/bar just before it searches the standard directory
/usr/local/lib/bar. If a standard directory begins with the configured prefix then the value of prefix is
replaced by GCC_EXEC_PREFIX when looking for header files.
COMPILER_PATH
The value of COMPILER_PATH is a colon-separated list of directories, much like PATH. GCC tries the directories
thus specified when searching for subprograms, if it cannot find the subprograms using GCC_EXEC_PREFIX.
LIBRARY_PATH
The value of LIBRARY_PATH is a colon-separated list of directories, much like PATH. When configured as a native
compiler, GCC tries the directories thus specified when searching for special linker files, if it cannot find
them using GCC_EXEC_PREFIX. Linking using GCC also uses these directories when searching for ordinary libraries
for the -l option (but directories specified with -L come first).
LANG
This variable is used to pass locale information to the compiler. One way in which this information is used is
to determine the character set to be used when character literals, string literals and comments are parsed in C
and C++. When the compiler is configured to allow multibyte characters, the following values for LANG are
recognized:
C-JIS
Recognize JIS characters.
C-SJIS
Recognize SJIS characters.
C-EUCJP
Recognize EUCJP characters.
If LANG is not defined, or if it has some other value, then the compiler uses "mblen" and "mbtowc" as defined by
the default locale to recognize and translate multibyte characters.
GCC_EXTRA_DIAGNOSTIC_OUTPUT
If GCC_EXTRA_DIAGNOSTIC_OUTPUT is set to one of the following values, then additional text will be emitted to
stderr when fix-it hints are emitted. -fdiagnostics-parseable-fixits and -fno-diagnostics-parseable-fixits take
precedence over this environment variable.
fixits-v1
Emit parseable fix-it hints, equivalent to -fdiagnostics-parseable-fixits. In particular, columns are
expressed as a count of bytes, starting at byte 1 for the initial column.
fixits-v2
As "fixits-v1", but columns are expressed as display columns, as per -fdiagnostics-column-unit=display.
Some additional environment variables affect the behavior of the preprocessor.
CPATH
C_INCLUDE_PATH
CPLUS_INCLUDE_PATH
OBJC_INCLUDE_PATH
Each variable's value is a list of directories separated by a special character, much like PATH, in which to look
for header files. The special character, "PATH_SEPARATOR", is target-dependent and determined at GCC build time.
For Microsoft Windows-based targets it is a semicolon, and for almost all other targets it is a colon.
CPATH specifies a list of directories to be searched as if specified with -I, but after any paths given with -I
options on the command line. This environment variable is used regardless of which language is being
preprocessed.
The remaining environment variables apply only when preprocessing the particular language indicated. Each
specifies a list of directories to be searched as if specified with -isystem, but after any paths given with
-isystem options on the command line.
In all these variables, an empty element instructs the compiler to search its current working directory. Empty
elements can appear at the beginning or end of a path. For instance, if the value of CPATH is
":/special/include", that has the same effect as -I. -I/special/include.
DEPENDENCIES_OUTPUT
If this variable is set, its value specifies how to output dependencies for Make based on the non-system header
files processed by the compiler. System header files are ignored in the dependency output.
The value of DEPENDENCIES_OUTPUT can be just a file name, in which case the Make rules are written to that file,
guessing the target name from the source file name. Or the value can have the form file target, in which case
the rules are written to file file using target as the target name.
In other words, this environment variable is equivalent to combining the options -MM and -MF, with an optional
-MT switch too.
SUNPRO_DEPENDENCIES
This variable is the same as DEPENDENCIES_OUTPUT (see above), except that system header files are not ignored, so
it implies -M rather than -MM. However, the dependence on the main input file is omitted.
SOURCE_DATE_EPOCH
If this variable is set, its value specifies a UNIX timestamp to be used in replacement of the current date and
time in the "__DATE__" and "__TIME__" macros, so that the embedded timestamps become reproducible.
The value of SOURCE_DATE_EPOCH must be a UNIX timestamp, defined as the number of seconds (excluding leap
seconds) since 01 Jan 1970 00:00:00 represented in ASCII; identical to the output of "date +%s" on GNU/Linux and
other systems that support the %s extension in the "date" command.
The value should be a known timestamp such as the last modification time of the source or package and it should
be set by the build process.
BUGS
For instructions on reporting bugs, see .
FOOTNOTES
1. On some systems, gcc -shared needs to build supplementary stub code for constructors to work. On multi-libbed
systems, gcc -shared must select the correct support libraries to link against. Failing to supply the correct
flags may lead to subtle defects. Supplying them in cases where they are not necessary is innocuous. -shared
suppresses the addition of startup code to alter the floating-point environment as done with -ffast-math, -Ofast
or -funsafe-math-optimizations on some targets.
SEE ALSO
gpl(7), gfdl(7), fsf-funding(7), cpp(1), gcov(1), as(1), ld(1), gdb(1) and the Info entries for gcc, cpp, as, ld,
binutils and gdb.
AUTHOR
See the Info entry for gcc, or , for contributors to GCC.
COPYRIGHT
Copyright (c) 1988-2025 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation
License, Version 1.3 or any later version published by the Free Software Foundation; with the Invariant Sections
being "GNU General Public License" and "Funding Free Software", the Front-Cover texts being (a) (see below), and with
the Back-Cover Texts being (b) (see below). A copy of the license is included in the gfdl(7) man page.
(a) The FSF's Front-Cover Text is:
A GNU Manual
(b) The FSF's Back-Cover Text is:
You have freedom to copy and modify this GNU Manual, like GNU
software. Copies published by the Free Software Foundation raise
funds for GNU development.
gcc-15 2026-03-21 GCC(1)