GCC(1)
💖 GCC(1) 💖
okay bestie, welcome to the GCC(1) page from the GNU manual — yes, that GCC, the C and C++ compiler that basically raised us all. this is part one of a very long love letter, so grab a boba, we're doing this. 🧋✨
🌸 NAME 🌸
gcc — the GNU project C and C++ compiler. that's her. that's the whole icon. 👑
✨ SYNOPSIS ✨
gcc [-c|-S|-E] [-std=standard]
[-g] [-pg] [-Olevel]
[-Wwarn...] [-Wpedantic]
[-Idir...] [-Ldir...]
[-Dmacro[=defn]...] [-Umacro]
[-foption...] [-mmachine-option...]
[-o outfile] [@file] infile...okay so real talk: this is just the highlight reel. Only the most useful options are listed here; see below for the remainder. and g++ accepts mostly the same options as gcc, so if you're a C++ girlie you're still covered. 💅
🦋 DESCRIPTION 🦋
When you invoke GCC, it normally does four whole jobs back to back: preprocessing, compilation, assembly and linking. the "overall options" are the ones that let you stop this process at an intermediate stage — like pausing a glow-up halfway through. For example, the -c option says not to run the linker, so the output consists of object files output by the assembler. cute, contained, no linking drama. 💖
Other options are passed on to one or more stages of processing. Some options control the preprocessor and others control 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. fr fr, they're giving background character energy. 🫶
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. And if the description for a particular option does not mention a source language, you can use that option with all supported languages. no gatekeeping. 🌈
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. yes, the name matters. main character energy only. ✨
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. like, completely different girlies. do not combine them. 💀
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. so like... mostly chill, sometimes not. vibe check accordingly. 👀
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. one form per option, we keep it minimal. 💅
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. yes, there's lore. there's always lore. 📚
🎀 OPTIONS 🎀
✨ Option Summary ✨
Here's the whole menu, grouped by type — all the explanations are in the following sections, so don't panic, we're getting there. it's giving extensive. 🍓
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 -fplugin=file
-fplugin-arg-name=arg -fdump-ada-spec[-slim] -fada-spec-parent=unit
-fdump-go-spec=fileC Language Options
-ansi -std=standard -aux-info filename
-fallow-parameterless-variadic-functions -fno-asm -fno-builtin
-fno-builtin-function -fcond-mismatch -ffreestanding -fgimple
-fgnu-tm -fgnu89-inline -fhosted -flax-vector-conversions
-fms-extensions -foffload=arg -foffload-options=arg -fopenacc
-fopenacc-dim=geom -fopenmp -fopenmp-simd
-fpermitted-flt-eval-methods=standard -fplan9-extensions
-fsigned-bitfields -funsigned-bitfields -fsigned-char
-funsigned-char -fsso-struct=endiannessC++ Language Options
-fabi-version=n -fno-access-control -faligned-new=n
-fargs-in-order=n -fchar8_t -fcheck-new -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-implicit-templates
-fno-implicit-inline-templates -fno-implement-inlines
-fmodule-header[=kind] -fmodule-only -fmodules-ts
-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 -fpermissive -fno-pretty-templates -fno-rtti
-fsized-deallocation -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 -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-exceptions
-Wextra-semi -Wno-inaccessible-base -Wno-inherited-variadic-ctor
-Wno-init-list-lifetime -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 -Wsign-promo -Wsized-deallocation
-Wsuggest-final-methods -Wsuggest-final-types -Wsuggest-override
-Wno-terminate -Wuseless-cast -Wno-vexing-parse
-Wvirtual-inheritance -Wno-virtual-move-assign -Wvolatile
-Wzero-as-null-pointer-constantObjective-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-selectorDiagnostic 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|json] -fno-diagnostics-show-option
-fno-diagnostics-show-caret -fno-diagnostics-show-labels
-fno-diagnostics-show-line-numbers -fno-diagnostics-show-cwe
-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]and that's only the summary, bestie — every single one of these gets its own explanation in the sections coming up next. we are literally not even close to done. stay with me. 💖
okay so next up is the giant index portion — GCC literally just listing every warning flag it has like it's dropping a tracklist 💅. No descriptions here, bestie, just the names, but I'm still gonna make it cute.
✨ Warning Options ✨
these are the warning flags in alphabetical-ish order, and yes there are so many. grab a snack 🍓
-fsyntax-only, -fmax-errors=n, -Wpedantic, -pedantic-errors, -w, -Wextra, -Wall, -Wabi=n, -Waddress, -Wno-address-of-packed-member, -Waggregate-return, -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, -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-c2x-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, -Wcast-align, -Wcast-align=strict, -Wcast-function-type, -Wcast-qual, -Wchar-subscripts, -Wclobbered, -Wcomment, -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, -Werror, -Werror=*, -Wexpansion-to-defined, -Wfatal-errors, -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, -Wno-if-not-aligned, -Wno-ignored-attributes, -Wignored-qualifiers, -Wno-incompatible-pointer-types, -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, -Wjump-misses-init, -Wlarger-than=byte-size, -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, -Wno-missing-profile, -Wno-multichar, -Wmultistatement-macros, -Wnonnull, -Wnonnull-compare, -Wnormalized=[none|id|nfc|nfkc], -Wnull-dereference, -Wno-odr, -Wopenacc-parallelism, -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-prio-ctor-dtor, -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, -Wsuggest-attribute=[pure|const|noreturn|format|malloc], -Wswitch, -Wno-switch-bool, -Wswitch-default, -Wswitch-enum, -Wno-switch-outside-range, -Wno-switch-unreachable, -Wsync-nand, -Wsystem-headers, -Wtautological-compare, -Wtrampolines, -Wtrigraphs, -Wtrivial-auto-var-init, -Wtsan, -Wtype-limits, -Wundef, -Wuninitialized, -Wunknown-pragmas, -Wunsuffixed-float-constants, -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, -Wno-varargs, -Wvariadic-macros, -Wvector-operation-performance, -Wvla, -Wvla-larger-than=byte-size, -Wno-vla-larger-than, -Wvolatile-register-var, -Wwrite-strings, -Wzero-length-bounds
not me listing all of these in one breath 😭 the dedication is real fr fr.
🔍 Static Analyzer Options 🔍
these are the analyzer ones — the ones that go snooping through your code looking for the drama 💀
-fanalyzer, -fanalyzer-call-summaries, -fanalyzer-checker=name, -fno-analyzer-feasibility, -fanalyzer-fine-grained, -fanalyzer-state-merge, -fanalyzer-state-purge, -fanalyzer-transitivity, -fanalyzer-verbose-edges, -fanalyzer-verbose-state-changes, -fanalyzer-verbosity=level, -fdump-analyzer, -fdump-analyzer-stderr, -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-json, -fdump-analyzer-state-purge, -fdump-analyzer-supergraph, -Wno-analyzer-double-fclose, -Wno-analyzer-double-free, -Wno-analyzer-exposure-through-output-file, -Wno-analyzer-file-leak, -Wno-analyzer-free-of-non-heap, -Wno-analyzer-malloc-leak, -Wno-analyzer-mismatching-deallocation, -Wno-analyzer-null-argument, -Wno-analyzer-null-dereference, -Wno-analyzer-possible-null-argument, -Wno-analyzer-possible-null-dereference, -Wno-analyzer-shift-count-negative, -Wno-analyzer-shift-count-overflow, -Wno-analyzer-stale-setjmp-buffer, -Wno-analyzer-tainted-allocation-size, -Wno-analyzer-tainted-array-index, -Wno-analyzer-tainted-divisor, -Wno-analyzer-tainted-offset, -Wno-analyzer-tainted-size, -Wanalyzer-too-complex, -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-write-to-const, -Wno-analyzer-write-to-string-literal
🍎 C and Objective-C-only Warning Options 🍎
these ones are exclusive to C and Objective-C, so if you're in C++ land, no crumbs for you 🎀
-Wbad-function-cast, -Wmissing-declarations, -Wmissing-parameter-type, -Wmissing-prototypes, -Wnested-externs, -Wold-style-declaration, -Wold-style-definition, -Wstrict-prototypes, -Wtraditional, -Wtraditional-conversion, -Wdeclaration-after-statement, -Wpointer-sign
🐛 Debugging Options 🐛
and finally the debug section — where you tell GCC how much of your code's little diary it should write down 📓✨
-g, -glevel, -gdwarf, -gdwarf-version, -gbtf, -gctf, -gctflevel, -ggdb, -grecord-gcc-switches, -gno-record-gcc-switches, -gstabs, -gstabs+, -gstrict-dwarf, -gno-strict-dwarf, -gas-loc-support, -gno-as-loc-support, -gas-locview-support, -gno-as-locview-support, -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, -gxcoff, -gxcoff+, -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
lowkey iconic that a compiler has this much lore behind its debug info 🫶 and we're not even done yet, bestie — more incoming 🌈
Optimization Options ✨
okay bestie, deep breath, because this is THE list. every single optimization flag gcc will let you scream at it, in order, exactly how the page has them. no skipping, no merging, we are not those girls. 💅
-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]]]]-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-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-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-stack-alignment-fipa-icf-fira-algorithm=algorithm-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
still here? iconic. hydrate 🍓 keep scrolling, we're not even close.
-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-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-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]
the scheduling girlies have entered the chat 💅
-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-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
and the finale, the moment you've all been waiting for 👑 the ones everybody actually types:
--param name=value-O-O0-O1-O2-O3-Os-Ofast-Og-Oz
that's the whole optimization roster, start to finish. ate and left no crumbs 🌈
Program Instrumentation Options 🔍
welcome to the gcc keeps receipts era 🎀 this is where the compiler starts paying attention to what your program is doing while it does it. every one of them, here you go:
-p-pg-fprofile-arcs--coverage-ftest-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-fasan-shadow-offset=number-fsanitize-sections=s1,s2,...-fsanitize-undefined-trap-on-error-fbounds-check-fcf-protection=[full|branch|return|none|check]-fharden-compares-fharden-conditional-branches-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-fvtable-verify=[std|preinit|none]-fvtv-counts-fvtv-debug-finstrument-functions-finstrument-functions-exclude-function-list=sym,sym,...-finstrument-functions-exclude-file-list=file,file,...-fprofile-prefix-map=old=new
ngl bestie, the sanitize and stack-protector ones are doing important work. treat them with respect 💖
Preprocessor Options 🦋
the preprocessor girlies 💅 she runs before anything else even happens, and these are all her little knobs and dials:
-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-flarge-source-files-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 🔧
tiny section, big energy. two options, both just "hey, pass this straight to the assembler":
-Wa,option-Xassembler option
periodt. that's the whole thing 🫶
Linker Options 🔗
and now: the part where all your little object files get smooshed into one beautiful executable 🎀 buckle up:
object-file-name-fuse-ld=linker-llibrary-nostartfiles-nodefaultlibs-nolibc-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 📂
where gcc goes looking for your stuff — headers, libs, prefixes, the whole scavenger hunt 🧚
-Bprefix-Idir-I--idirafter dir-imacros file-imultilib dir-iplugindir=dir-iprefix file-iquote dir-isysroot dir-isystem dir-iwithprefix dir-iwithprefixbefore dir-Ldir-no-canonical-prefixes--no-sysroot-suffix-nostdinc-nostdinc++--sysroot=dir
Code Generation Options ⚙️
the actual "how should this machine code look" options ✨ this is the codegen section, and every single one of them is here:
-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-ftrapv-fwrapv-fvisibility=[default|internal|hidden|protected]-fstrict-volatile-bitfields-fsync-libcalls
Developer Options 🛠️
for when you're deep in the compiler's business, bestie. dumps, stats, timing reports, all the behind the scenes lore 🍓
-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]-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 🖥️
the "depends on your hardware" zone. this is where gcc asks what you're running on, because every chip has opinions 💜
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-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
and that's the full roster for this chunk, bestie — there's more coming, so don't go anywhere 💖
🌸 Adapteva Epiphany Options 🌸
okay so Adapteva Epiphany is out here with a whole little collection and honestly? she's giving niche-but-thriving 🦋 every single one of these goes straight after the dash, no cap:
-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 💜
only three but she's confident about all of them 💅
-march=gpu-mtune=gpu-mstack-size=bytes
✨ ARC Options ✨
bestie this list is long and I'm obsessed, look at her go 🌈
-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 🎀
the way ARM just keeps going?? main character energy fr fr 👑
-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
🦋 AVR Options 🦋
tiny-chip royalty ✨
-mmcu=mcu-mabsdata-maccumulate-args-mbranch-cost=cost-mcall-prologues-mgas-isr-prologues-mint8-mdouble=bits-mlong-double=bits-mn_flash=size-mno-interrupts-mmain-is-OS_task-mrelax-mrmw-mstrict-X-mtiny-stack-mfract-convert-truncate-mshort-calls-nodevicelib-nodevicespecs-Waddr-space-convert-Wmisspelled-isr
🌈 Blackfin Options 🌈
not me falling for a DSP 💀 she's got range:
-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 💖
short, sweet, six of them 🍓
-mbig-endian-mlittle-endian-march=cpu-msim-msdata=sdata-type
💅 CRIS Options 💅
the way she has so many little switchy friends:
-mcpu=cpu-march=cpu-mtune=cpu-mmax-stack-frame=n-melinux-stacksize=n-metrax4-metrax100-mpdebug-mcc-init-mno-side-effects-mstack-align-mdata-align-mconst-align-m32-bit-m16-bit-m8-bit-mno-prologue-epilogue-mno-gotplt-melf-maout-melinux-mlinux-sim-sim2-mmul-bug-workaround-mno-mul-bug-workaround
🌸 CR16 Options 🌸
seven icons, periodt:
-mmac-mcr16cplus-mcr16c-msim-mint32-mbit-ops-mdata-model=model
🧚 C-SKY Options 🧚
bestie really said "here's thirty-something flags, deal with it" and honestly? slay:
-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
👑 Darwin Options 👑
okay this one is a mega list and lowkey the longest vibe check of the whole page 😭 I kept every single one, including the ones that show up twice, because that's what she wrote:
-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-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 ⭐
old school, still iconic 💜
-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-mfloat-vax-mfloat-ieee-mexplicit-relocs-msmall-data-mlarge-data-msmall-text-mlarge-text-mmemory-latency=time
🫶 eBPF Options 🫶
kernel-adjacent cutie 🌈:
-mbig-endian-mlittle-endian-mkernel=version-mframe-limit=bytes-mxbpf-mco-re-mno-co-re-mjmpext-mjmp32-malu32-mcpu=version
🍓 FR30 Options 🍓
just two bestie, and one of them is a "no" 💀
-msmall-model-mno-lsim
🦋 FT32 Options 🦋
six little friends:
-msim-mlra-mnodiv-mft32b-mcompress-mnopm
💜 FRV Options 💜
she has everything — positive flags, negative flags, the whole personality:
-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 🌸
the libc-choosing girlypops 💅:
-mglibc-muclibc-mmusl-mbionic-mandroid-tno-android-cc-tno-android-ld
🎀 H8/300 Options 🎀
short queen behavior:
-mrelax-mh-ms-mn-mexr-mno-exr-mint32-malign-300
✨ HPPA Options ✨
she's giving expansive, and I'm here for it 🌈:
-march=architecture-type-mcaller-copies-mdisable-fpregs-mdisable-indexing-mfast-indirect-calls-mgas-mgnu-ld-mhp-ld-mfixed-range=register-range-mjump-in-delay-mlinker-opt-mlong-calls-mlong-load-store-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-msio-mwsio-munix=unix-std-nolibdld-static-threads
🧚 IA-64 Options 🧚
okay the scheduling flags alone are a whole mood board 😭 every one of them:
-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 💖
five tiny toggles, all of them "enabled", no negativity here 🫶:
-mbarrel-shift-enabled-mdivide-enabled-mmultiply-enabled-msign-extend-enabled-muser-enabled
🌈 M32R/D Options 🌈
she's got models, she's got data types, she's got flushes 🔥:
-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
🍓 M32C Options 🍓
three and done, love that for her:
-mcpu=cpu-msim-memregs=number
👑 M680x0 Options 👑
retro and relevant, honestly iconic 💜 every single one:
-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 💅
the way she pairs every positive with a "no-" like a matching set 😭 so organized:
-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
🌸 MeP Options 🌸
and she closes out this chunk with a whole alphabet of little flags, obsessed 🎀:
-mabsdiff-mall-opts-maverage-mbased=n-mbitops-mc=n-mclip-mconfig=name-mcop-mcop32-mcop64-mivc2-mdc-mdiv-meb-mel-mio-volatile-ml-mleadz-mm-mminmax-mmult-mno-opts-mrepeat-ms-msatur-msdram-msim-msimnovec-mtf-mtiny=n
and bestie we are not done yet, there's more architecture chaos coming 💀 stay tuned 🫶
✨ MicroBlaze Options ✨
bestie, MicroBlaze said "let me be tiny and cute" and these flags are the whole personality 💖
-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? More like MIPS-slaying 💅 the way this list is LONG. But we love her. Here's every single flag:
-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-mcheck-zero-division-mno-check-zero-division-mdivide-traps-mdivide-breaks-mload-store-pairs-mno-load-store-pairs-munaligned-access-mno-unaligned-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? Knuth's baby 💜 these flags are giving academic realness:
-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? A whole mood. Here's the lineup:
-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? The name alone is iconic. Flags:
-meb-mel-mmul.x-mno-crt0
✨ MSP430 Options ✨
MSP430 bestie, low-power queen 🍓 here's every flag:
-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? We love an underrated architecture. Flags:
-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
✨ Nios II Options ✨
Nios II said "soft core, hard vibe" 💙 flags:
-G num-mgpopt=option-mgpopt-mno-gpopt-mgprel-sec=regexp-mr0rel-sec=regexp-mel-meb-mno-bypass-cache-mbypass-cache-mno-cache-volatile-mcache-volatile-mno-fast-sw-div-mfast-sw-div-mhw-mul-mno-hw-mul-mhw-mulx-mno-hw-mulx-mno-hw-div-mhw-div-mcustom-insn=N-mno-custom-insn-mcustom-fpu-cfg=name-mhal-msmallc-msys-crt0=name-msys-lib=name-march=arch-mbmx-mno-bmx-mcdx-mno-cdx
✨ Nvidia PTX Options ✨
Nvidia PTX? GPU energy ⚡️ flags:
-m64-mmainkernel-moptimize
✨ OpenRISC Options ✨
OpenRISC, open source royalty 👑 here's the tea:
-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? The OG. Respect your elders 💀 flags:
-mfpu-msoft-float-mac0-mno-ac0-m40-m45-m10-mint32-mno-int16-mint16-mno-int32-msplit-munix-asm-mdec-asm-mgnu-asm-mlra
✨ picoChip Options ✨
picoChip? Tiny but mighty 🧚 flags:
-mae=ae_type-mvliw-lookahead=N-msymbol-as-address-mno-inefficient-warnings
✨ PowerPC Options ✨
PowerPC Options — See RS/6000 and PowerPC Options. That's the whole vibe. 👑
✨ PRU Options ✨
PRU? Real-time realness. Flags:
-mmcu=mcu-minrt-mno-relax-mloop-mabi=variant
✨ RISC-V Options ✨
RISC-V? The future is now, bestie 🌈 here's every flag:
-mbranch-cost=N-instruction-mplt-mno-plt-mabi=ABI-string-mfdiv-mno-fdiv-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-mexplicit-relocs-mno-explicit-relocs-mrelax-mno-relax-mriscv-attribute-mmo-riscv-attribute-malign-data=type-mbig-endian-mlittle-endian-mstack-protector-guard=guard-mstack-protector-guard-reg=reg-mstack-protector-guard-offset=offset
✨ RL78 Options ✨
RL78? Ultra-low power, high-key adorable 🍑 flags:
-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? The drama. The legacy. 💅 this list is THICC. Here we go:
-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-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-mpower8-vector-mno-power8-vector-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? Renesas realness. Flags:
-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? Mainframe main character energy 👑 flags:
-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
✨ Score Options ✨
Score? We stan. Flags:
-meb-mel-mnhwloop-muls-mmac-mscore5-mscore5u-mscore7-mscore7d
✨ SH Options ✨
SH? SuperH, super slay 🦋 flags:
-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? The vintage vibe 🌸 flags:
-mclear-hwcap-mno-clear-hwcap-mimpure-text-mno-impure-text-pthreads
SPARC Options 🌞
bestie, if you're out here compiling for SPARC, this is your whole flag roster. No notes, no skips, all of them:
-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 -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 -mlra -mno-lraSystem V Options 🎀
The little System V quartet, coming through:
-Qy -Qn -YP,paths -Ym,dirTILE-Gx Options 🦋
TILE-Gx crew, your lineup:
-mcpu=CPU -m32 -m64 -mbig-endian -mlittle-endian
-mcmodel=code-modelTILEPro Options 🌸
And TILEPro, keeping it minimal and cute:
-mcpu=cpu -m32V850 Options 💅
V850 got a whole wardrobe, bestie. Every single one:
-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-switchVAX Options 🕰️
VAX staying vintage and iconic:
-mg -mgnu -munix -mlraVisium Options ✨
Visium's full set, no cap:
-mdebug -msim -mfpu -mno-fpu -mhard-float
-msoft-float -mcpu=cpu-type -mtune=cpu-type -msv-mode -muser-modeVMS Options 🌊
VMS options, all four of them, slaying:
-mvms-return-codes -mdebug-main=prefix -mmalloc64
-mpointer-size=sizeVxWorks Options 🛰️
VxWorks gang, your flags:
-mrtp -non-static -Bstatic -Bdynamic -Xbind-lazy
-Xbind-nowx86 Options 👑
okay so THIS is the big one. x86 has so many options it's genuinely giving encyclopedia energy. Here is every single one, verbatim, no skipping:
-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
-mmove-max=bits -mstore-max=bits -mmmx -msse -msse2 -msse3
-mssse3 -msse4.1 -msse4.2 -msse4 -mavx -mavx2 -mavx512f
-mavx512pf -mavx512er -mavx512cd -mavx512vl -mavx512bw
-mavx512dq -mavx512ifma -mavx512vbmi -msha -maes -mpclmul
-mfsgsbase -mrdrnd -mf16c -mfma -mpconfig -mwbnoinvd -mptwrite
-mprefetchwt1 -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 -mforce-indirect-call
-mavx512vbmi2 -mavx512bf16 -menqcmd -mvpclmulqdq -mavx512bitalg
-mmovdiri -mmovdir64b -mavx512vpopcntdq -mavx5124fmaps
-mavx512vnni -mavx5124vnniw -mprfchw -mrdpid -mrdseed -msgx
-mavx512vp2intersect -mserialize -mtsxldtrk -mamx-tile -mamx-int8
-mamx-bf16 -muintr -mhreset -mavxvnni -mavx512fp16 -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 -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-accessfr fr, breathe. That was a lot. But you asked for all of them and I do not gatekeep. 💖
x86 Windows Options 🪟
Windows-targeting x86 besties, your lil set:
-mconsole -mcygwin -mno-cygwin -mdll
-mnop-fun-dllimport -mthread -municode -mwin32 -mwindows
-fno-set-stack-executableXstormy16 Options ⛈️
Xstormy16 keeping it to a single, elegant option:
-msimXtensa Options 🧚
Xtensa's whole vibe, all together now:
-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-typezSeries Options 💜
zSeries bestie — your options live elsewhere, so go peek: See S/390 and zSeries Options.
Options Controlling the Kind of Output 🎬
okay so the whole compilation pipeline can be up to four stages, and they always happen in this order: preprocessing, then compilation proper, then assembly, then linking. That's the order, periodt. 🫶
GCC can preprocess and compile a bunch of files either into several assembler input files, or into one assembler input file. Then each assembler input file turns into an object file, and linking smooshes all the object files together — the freshly compiled ones plus the ones you passed in as input — into one executable file. ✨
For any given input file, the file name suffix is what decides which kind of compilation happens. Here's the whole vibe check:
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. Heads up: you must link with thelibobjclibrary 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: you must link with thelibobjclibrary to make an Objective-C++ program work. And note:.Mrefers 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 (that's the default), or a C, C++ header file to be turned into an Ada spec (via the-fdump-ada-specswitch).file.cc,file.cp,file.cxx,file.cpp,file.CPP,file.c++,file.C— C++ source code that must be preprocessed. Note: in.cxx, the last two letters must both be literallyx. Likewise,.Crefers 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— 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— 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.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). These files are also called specs.file.adb— Ada source code file containing a library unit body (a subprogram or package body). These 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 also just say the language out loud 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-xoption. The 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
objective-c++ objective-c++-header objective-c++-cpp-output
assembler assembler-with-cpp
ada
d
f77 f77-cpp-input f95 f95-cpp-input
go-x none— Turn off any specification of a language, so that subsequent files are handled according to their file name suffixes (exactly like they are if-xhas not been used at all).
If you only want some of the stages, 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 supposed to stop. Lowkey important: 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 — where your primary output lives
Okay bestie, -o is the main character of output flags. It puts the primary output in file. And it does this for whatever sort of output is being produced — whether that's an executable file, an object file, an assembler file, or preprocessed C code. All of it. 💅
No -o? Then here's the default tea 🍵:
- an executable file goes in
a.out - the object file for
source.suffixgoes insource.o - its assembler file goes in
source.s - a precompiled header file goes in
source.suffix.gch - and all preprocessed C source goes on standard output
Now here's the part everyone sleeps on: even though -o names only the primary output, it also affects the naming of auxiliary and dump outputs. See the examples below. Unless something overrides it, both auxiliary outputs and dump outputs get 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. And if both share the same base name (suffix disregarded, don't come for me), the result of that combination is just the base name. Otherwise they get concatenated, separated by a dash. Periodt. 💖
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 — the base-name glow-up
This option sets the base name for auxiliary and dump output files. It does not affect the name of the primary output file. Not even a little. 🙈
And lowkey important: intermediate outputs, when preserved, are not regarded as primary outputs — they're auxiliary outputs:
gcc -save-temps -S foo.csaves 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.cpreprocesses 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. It said "I'm the blueprint now." 👑
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.*. No collisions, no drama. 💅
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. Sneaky but consistent. 🦋
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 — drop the suffix, keep the vibes
When forming the name of an auxiliary (but not a dump) output file, drop trailing auxdropsuf from dumpbase before appending any suffixes. If it's 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.*. Ate. 🔥
🎯 -dumpdir dumppfx — prefix era
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.*.
Fr fr: if you want dumppfx to be a directory, end it with a separator. The shell is not gonna guess for you. 😭
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 get called foo.c.*, and they live in the current working directory, exactly as ultimately requested by -save-temps=cwd. 💖
Summing the whole thing up into an intuitive-but-slightly-imprecise data flow, bestie: the primary output name gets chopped into a directory part and a basename part; dumppfx is set to the former, unless -dumpdir or -save-temps=* overrides it, and dumpbase is set to the latter, unless -dumpbase overrides that. If there are multiple inputs, or if we're linking, this dumpbase may get combined with dumppfx and taken from each input file. Auxiliary output names for each input are formed by combining dumppfx, dumpbase minus its suffix, and the auxiliary output suffix; dump output names are literally only different in that the suffix from dumpbase is retained. ✨
Now, for 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 where that combination wouldn't otherwise be used as such — gets passed down with a trailing period replacing the compiler-added dash (if there was one), as a -dumpdir option to lto-wrapper. Since it's involved in linking, this program does not normally get any -dumpbase and -dumpbase-ext, and it ignores them. 🫶
And when it's running sub-compilers, lto-wrapper appends LTO stage names to the dumppfx it received, makes sure it contains a directory component so it overrides any -dumpdir, and passes that as -dumpbase to the 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. It's giving "show me everything you're doing," and honestly? We love the transparency. 🔍-###— 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 that want to capture the driver-generated command lines. ✨--help— Print (on the standard output) a description of the command-line options understood bygcc. If the-voption is also specified, then--helpis also passed on to the various processes invoked bygcc, so they can display the command-line options they accept. And if-Wextrahas also been specified (prior to the--helpoption), 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 the specified classes and qualifiers. Here 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-helpoption, though, target-specific options of the linker and assembler are not displayed — because those tools don't currently support the extended--help=syntax. 🙈params— Display the values recognized by the--paramoption. 🎛️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, you need to select thecommonclass. 🌍common— Display the options that are common to all languages. 🫂
And here 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. ✂️
So, for example, to display all the undocumented target-specific switches supported by the compiler, use:
--help=target,undocumentedThe 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,^undocumentedThe argument to --help= should not consist solely of inverted qualifiers. Just don't do it, bestie. 😭
Combining several classes is possible, although this usually restricts the output so much that there's literally nothing to display. One case where it does work, though, is when one of the classes is target. For example, to display all the target-specific optimization options, use:
--help=target,optimizersThe --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 it 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= changes. 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 the compiler knows this at the point where the --help= option is used). 👀
Here's 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's possible to find out which optimizations are enabled at -O2 by using:
-Q -O2 --help=optimizersAlternatively, 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 thegccprogram exits with the code of1if any phase of the compiler returns a non-success return code. If you specify-pass-exit-codes, thegccprogram instead returns with the numerically highest error produced by any phase returning an error indication. The C, C++, and Fortran front ends return4if 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 thegccdriver program uses when determining what switches to pass tocc1,cc1plus,as,ld, etc. More than one-specs=filecan 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,--argsThis 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-mapoptions. This can be used to make reproducible builds that are location independent. See also-fmacro-prefix-map,-fdebug-prefix-mapand-fprofile-prefix-map. 🗺️-fplugin=name.so— Load the plugin code in file name.so, assumed to be a shared object to bedlopen'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=valuebelow). 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@fileoption. 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; and preprocessed C++ files use the suffix .ii. 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). ✨👑
...but bestie, hear me out — using gcc does not add the C++ library for you. That's not the vibe. That's what g++ is for. g++ is a whole program that calls GCC and automatically links against the C++ library, no extra steps, no stress. It treats .c, .h and .i files as C++ source files instead of C source files — unless you use -x to say otherwise. It's also the move when you want to precompile a C header file with a .h extension so it's ready for C++ compilations. And on many systems, g++ is also installed under the name c++. ✨
When you're compiling C++ programs, you may pass a bunch of the same command-line options you'd use for any language, or the ones that hit for C and related languages, or the ones that are meaningful only for C++ programs. Mix and match, it's your world. 💅
✨ Options Controlling C Dialect ✨
These options control the dialect of C — or any language derived from C, like C++, Objective-C and Objective-C++ — that the compiler will accept. Basically: which flavor of C are we speaking today, bestie? 🍓
-ansi— In C mode, this is equivalent to-std=c90. In C++ mode, it's equivalent to-std=c++98.
This turns off certain features of GCC that are incompatible with ISO C90 (when compiling C code), or with standard C++ (when compiling C++ code) — think the asm and typeof keywords, plus predefined macros like unix and vax that identify the type of system you're on. It also enables the undesirable and rarely used ISO trigraph feature. 😭 For the C compiler, it disables recognition of C++ style // comments as well as the inline keyword.
The alternate keywords __asm__, __extension__, __inline__ and __typeof__ continue to work despite -ansi. You would not want to use them in an ISO C program, of course, but it's useful to put them in header files that might get included in compilations done with -ansi. Alternate predefined macros such as __unix__ and __vax__ are also available, with or without -ansi.
The -ansi option does not cause non-ISO programs to be rejected gratuitously. For that, you need -Wpedantic on top of -ansi.
The macro __STRICT_ANSI__ is predefined when the -ansi option is used. Some header files may notice this macro and refrain from declaring certain functions or defining certain macros that the ISO standard doesn't call for — this is to avoid interfering with any programs that might use these names for other things. 👑
Functions that are normally built in but do not have semantics defined by ISO C (such as alloca and ffs) are not built-in functions when -ansi is used.
-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:
c90c89iso9899:1990Support all ISO C90 programs (certain GNU extensions that conflict with ISO C90 are disabled). Same as-ansifor C code.iso9899:199409ISO C90 as modified in amendment 1.c99c9xiso9899:1999iso9899:199xISO 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 <https://gcc.gnu.org/c99status.html> for more information. The namesc9xandiso9899:199xare deprecated.c11c1xiso9899:2011ISO 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 namec1xis deprecated.c17c18iso9899:2017iso9899:2018ISO 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.c2xThe next version of the ISO C standard, still under development. The support for this version is experimental and incomplete. 🙈gnu90gnu89GNU dialect of ISO C90 (including some C99 features).gnu99gnu9xGNU dialect of ISO C99. The namegnu9xis deprecated.gnu11gnu1xGNU dialect of ISO C11. The namegnu1xis deprecated.gnu17gnu18GNU dialect of ISO C17. This is the default for C code. ✨gnu2xThe next version of the ISO C standard, still under development, plus GNU extensions. The support for this version is experimental and incomplete.c++98c++03The 1998 ISO C++ standard plus the 2003 technical corrigendum and some additional defect reports. Same as-ansifor C++ code.gnu++98gnu++03GNU dialect of-std=c++98.c++11c++0xThe 2011 ISO C++ standard plus amendments. The namec++0xis deprecated.gnu++11gnu++0xGNU dialect of-std=c++11. The namegnu++0xis deprecated.c++14c++1yThe 2014 ISO C++ standard plus amendments. The namec++1yis deprecated.gnu++14gnu++1yGNU dialect of-std=c++14. The namegnu++1yis deprecated.c++17c++1zThe 2017 ISO C++ standard plus amendments. The namec++1zis deprecated.gnu++17gnu++1zGNU dialect of-std=c++17. This is the default for C++ code. The namegnu++1zis deprecated. 👑c++20c++2aThe 2020 ISO C++ standard plus amendments. Support is experimental, and could change in incompatible ways in future releases. The namec++2ais deprecated.gnu++20gnu++2aGNU dialect of-std=c++20. Support is experimental, and could change in incompatible ways in future releases. The namegnu++2ais deprecated.c++2bc++23The next revision of the ISO C++ standard, planned for 2023. Support is highly experimental, and will almost certainly change in incompatible ways in future releases. 😭gnu++2bgnu++23GNU dialect of-std=c++2b. Support is highly experimental, and will almost certainly change in incompatible ways in future releases.
-aux-infofilename — 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.
-fallow-parameterless-variadic-functions— Accept variadic functions without named parameters.
Although it is possible to define such a function, this is not very useful as it is not possible to read the arguments. This is only supported for C as this construct is allowed by C++. 🫶
-fno-asm— Do not recognizeasm,inlineortypeofas a keyword, so that code can use these words as identifiers. You can use the keywords__asm__,__inline__and__typeof__instead. In C,-ansiimplies-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. 🧚
-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. 🔥
okay so with -fno-builtin-function — bestie, this one is picky 💅 only the ONE built-in function function gets disabled. And listen: function absolutely cannot begin with __builtin_. Also, if you name a function that just... isn't built-in in this version of GCC? The option gets quietly ignored, no drama, no error. And heads up: there's no matching -fbuiltin-function option. So if you wanna selectively switch built-ins back ON while you're living that -fno-builtin or -ffreestanding life, you gotta define macros yourself, like:
#define abs(n) __builtin_abs ((n))
#define strcpy(d, s) __builtin_strcpy ((d), (s))-fcond-mismatch— this lets you have conditional expressions where the second and third arguments have mismatched types 🙈 (chaotic, I know). The value of such an expression isvoid. Not supported for C++, sorry C++ girlies.-ffreestanding— asserts that compilation targets a freestanding environment 🧚 This implies-fno-builtin. A freestanding environment is one where the standard library might not even exist, and program startup isn't necessarily atmain. The most iconic example? An OS kernel 👑 This is equivalent to-fno-hosted.-fgimple— enables parsing of function definitions marked with__GIMPLE. This is an experimental feature that lets you unit test GIMPLE passes ✨-fgnu-tm— when you pass-fgnu-tm, 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,
⚠️ heads up bestie: the transactional memory feature is not supported with non-call exceptions (-fnon-call-exceptions).
-fgnu89-inline— tells GCC to use the traditional GNU semantics forinlinefunctions 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— asserts that compilation targets a hosted environment 🏠 This implies-fbuiltin. A hosted environment is one in which the entire standard library is available, and in whichmainhas a return type ofint. Examples are nearly everything except a kernel. This is equivalent to-fno-freestanding.-flax-vector-conversions— allows 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— accepts 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, like:
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.
-foffload=disable,-foffload=default,-foffload=target-list— specifies 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=disableform generates code only for the host fallback, while-foffload=target-listgenerates 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=optionsform. The target-list is a comma-separated list in the same format as for the-foffload=option.
Typical command lines are:
-foffload-options=-lgfortran -foffload-options=-lm
-foffload-options="-lgfortran -lm" -foffload-options=nvptx-none=-latomic
-foffload-options=amdgcn-amdhsa=-march=gfx906 -foffload-options=-lm-fopenacc— enables handling of OpenACC directives#pragma accin C/C++ and!$accin Fortran 🌈 When-fopenaccis specified, the compiler generates accelerated code according to the OpenACC Application Programming Interface v2.6 (<https://www.openacc.org>). This option implies-pthread, and thus is only supported on targets that have support for-pthread.-fopenacc-dim=geom— specifies default compute dimensions for parallel offload regions that don't explicitly specify them ✨ The geom value is a triple of:-separated sizes, in order gang, worker and, vector. A size can be omitted, to use a target-specific default value.-fopenmp— enables handling of OpenMP directives#pragma ompin C/C++ and!$ompin Fortran 🔥 When-fopenmpis specified, the compiler generates parallel code according to the OpenMP Application Program Interface v4.5 (<https://www.openmp.org>). This option implies-pthread, and thus is only supported on targets that have support for-pthread.-fopenmpimplies-fopenmp-simd.-fopenmp-simd— enables handling of OpenMP's SIMD directives with#pragma ompin C/C++ and!$ompin Fortran. Other OpenMP directives are ignored 💅-fpermitted-flt-eval-methods=style— ISO/IEC TS 18661-3 defines new permissible values forFLT_EVAL_METHODthat 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 ofFLT_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.
-fplan9-extensions— accepts 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 doesn't use eithersignedorunsigned💅 By default, such a bit-field is signed, because this is consistent: the basic integer types such asintare signed types.-fsigned-char— lets the typecharbe signed, likesigned 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— lets the typecharbe unsigned, likeunsigned 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 👀
-fsso-struct=endianness— sets 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. like, be careful bestie 😭
🎀 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.CIn 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. vectors finally got a glow-up ✨
- 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_tas 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. twelfth occurrence bestie, they really counted 💀 - 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. we love a redemption arc 🫶
- Version 14, which first appeared in G++ 10, corrects the mangling of the
nullptrexpression. - Version 15, which first appeared in G++ 11, changes the mangling of
"__alignof__"to be distinct from that of"alignof", and dependent operator names.
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. compatibility era 💅
With -fabi-version=0 (the default), this defaults to 11 (GCC 7 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. living dangerously fr fr-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)". so aligned, so extra ✨
This flag is enabled by default for -std=c++17.
-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. char8_t era unlocked 🦋
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 #2and 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<typename T, T...> 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. ngl this will break your build, read carefully bestie 😭
char ca[] = u8"xx"; // error: char-array initialized from wide
// string
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<char>::basic_string()'
using namespace std::literals;
s = u8"xx"s; // error: conversion from
// `basic_string<char8_t>' to non-scalar
// type `basic_string<char>' 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 returns0if 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). safety check, slay 💅-fconcepts-fconcepts-ts— below-std=c++20,-fconceptsenables support for the C++ Extensions for Concepts Technical Specification, ISO 19217 (2015). concepts, so main character ✨
With -std=c++20 and above, Concepts are part of the language standard, so -fconcepts defaults to on. But the standard specification of Concepts differs significantly from the TS, so some constructs that were allowed in the TS but didn't make it into the standard can still be enabled by -fconcepts-ts.
-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 is512.-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 is8. 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. tune it if you dare 🎀-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 is262144(1<<18). loop guard, iconic 💅-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 is33554432(1<<25).-fcoroutines— enable support for the C++ coroutines extension (experimental). experimental, so be cute and careful 🦋-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. periodt, don't say we didn't warn you 💀-fextern-tls-init,-fno-extern-tls-init— bestie, the C++11 and OpenMP standards literally allowthread_localandthreadprivatevariables to have dynamic (runtime) initialization. To make that work, every single use of such a variable has to go through a lil wrapper function that does whatever init is needed 🫶. When the use and the definition live in the same translation unit, that overhead can be optimized away — but when the use is in a different translation unit, the overhead is significant, even if the variable doesn't actually need dynamic initialization 😭. So if you (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 because a use in the defining TU will run before any uses in another TU), you can dodge that overhead with the-fno-extern-tls-initoption. 💅
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— lets the C++ frontend fold calls tostd::move,std::forward,std::addressofandstd::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-inlineis active.-fno-gnu-keywords— do not recognizetypeofas a keyword, so your 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.-fimplicit-constexpr— make inline functions implicitlyconstexpr, if they satisfy the requirements for aconstexprfunction. 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. ⚠️ like, structure it or perish.-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-ts,-fno-modules-ts— enable support for C++20 modules 🦋. The-fno-modules-tsis 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=<in>out[?ident],-fmodule-mapper=file[?ident]— an oracle to query for module name to filename mappings 🔮. If unspecified theCXX_MODULE_MAPPERenvironment 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. just the CMI, nothing else bestie 💅-fms-extensions— disable-Wpedanticwarnings 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=10or 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 includeffs,alloca,_exit,index,bzero,conjf, and other related functions.-fnothrow-opt— treat athrow()exception specification as if it were anoexceptspecification 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 toterminaterather thanunexpected.-fno-operator-names— do not treat the operator name keywordsand,bitand,bitor,compl,not,orandxoras 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.-fpermissive— downgrade some diagnostics about nonconformant code from errors to warnings. Thus, using-fpermissiveallows some nonconforming code to compile 🙈.-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 thanvoid 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 makes it harder to understand the error message rather than easier, you can use-fno-pretty-templatesto disable them.-fno-rtti— disable generation of information about every class with virtual functions for use by the C++ run-time type identification features (dynamic_castandtypeid). 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. Thedynamic_castoperator can still be used for casts that do not require run-time type information, i.e. casts tovoid *or to unambiguous base classes.
Mixing code compiled with-frttiwith that compiled with-fno-rttimay not work 💀. For example, programs may fail to link if a class compiled with-fno-rttiis 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. like, casting random ints in there? that's on you bestie.-fstrong-eval-order— 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. Enabled by default with-std=c++17.-fstrong-eval-order=someenables just the ordering of member access and shift expressions, and is the default without-std=c++17.-ftemplate-backtrace-limit=n— set the maximum number of template instantiation notes for a single warning or error to n. The default value is10.-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 than17(changed to1024in C++11). The default value is900, as the compiler can run out of stack space before hitting1024in 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_atexitfunction rather than theatexitfunction. 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— bestie, this one says don't use the__cxa_get_exception_ptrruntime routine. Downside? It makesstd::uncaught_exceptiongive you the wrong answer. But if that runtime routine straight-up isn't available to you, you kinda have no choice. 🤷♀️-fvisibility-inlines-hidden— okay so this switch is declaring that you, the user, are NOT out here comparing pointers to inline functions or methods when the addresses of the two functions were taken in different shared objects. 💅
The tea is: this lets GCC effectively mark inline methods with __attribute__ ((visibility ("hidden"))), so they don't show up in the export table of a DSO and don't need a PLT indirection when they're used inside that DSO. Turning this on can be a dramatic glow-up for load and link times of a DSO, because it massively shrinks the size of the dynamic export table when the library is heavy on templates. ✨
Ngl tho, this switch is not quite the same as marking methods as hidden directly, because it doesn't touch static variables local to the function, and it doesn't make the compiler deduce that the function is defined in only one shared object.
You can mark a method with an explicit visibility to cancel out the effect of the switch for that one method. For example, if you do wanna compare pointers to a particular inline method, you might mark it as having default visibility. Heads up: marking the enclosing class with explicit visibility does nothing here. 🙈
Also, explicitly instantiated inline methods are unaffected by this option, since their linkage might otherwise cross a shared library boundary.
-fvisibility-ms-compat— this flag is trying to use visibility settings to make GCC's C++ linkage model play nice with Microsoft Visual Studio's. 🤝
Here's what it changes about GCC's linkage model:
- It sets the default visibility to
hidden, like-fvisibility=hidden. - Types, but not their members, are not hidden by default.
- 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're permitted when this option is not used.
For new code, it's genuinely better to use -fvisibility=hidden and export just the classes you actually want to be externally visible. Unfortunately, code can rely — maybe even accidentally — on the Visual Studio behavior. 😭
Some consequences of these changes: 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 pointers to function members defined in different shared objects may not compare equal. And when this flag is given, it's a violation of the ODR to define types with the same name differently. 💀
-fno-weak— do not use weak symbol support, even if the linker is handing it to you on a platter. By default, G++ uses weak symbols if they're available. Real talk: this option exists only for testing and should not be used by end-users — it results in inferior code and has zero benefits. It may get 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 off, those suffixes get treated as C++11 user-defined literal numeric suffixes instead. It's 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. It's 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— these inform you about include translation events. The first one notes accepted include translations, the second notes declined include translations. Theheaderform informs you about include translations relating to that specific header. Ifheaderis of the form"user"or<system>, it gets resolved to a specific user or system header using the include path. 👀-flang-info-module-cmi,-flang-info-module-cmi=module— these inform you of Compiled Module Interface pathnames. The first notes all read CMI pathnames. Themoduleform notes not reading a specific module's CMI. Just so you know,modulemay 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 lets you specify alternate C++ runtime libraries. Two options are available:libstdc++(the default, native C++ runtime for G++) andlibc++, 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 info 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. BUT — 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's 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 isn't valid in C++23 but used to be valid-but-deprecated in C++20, with a pedantic warning that you can disable with -Wno-comma-subscript. ✋
Enabled by default with -std=c++20 unless -Wno-deprecated, and with -std=c++23 regardless of -Wno-deprecated.
-Wctad-maybe-unsupported(C++ and Objective-C++ only) — warn when you're doing class template argument deduction (CTAD) on a type with no explicitly written deduction guides. This warning points out the cases where CTAD only succeeded because the compiler synthesized the implicit deduction guides, which might not be what the programmer intended. Some style guides only allow CTAD on types that specifically "opt-in" — i.e., types designed to support CTAD. You can suppress this warning with the following pattern:
struct allow_ctad_t; // any name works
template <typename T> struct S {
S(T) { }
};
S(allow_ctad_t) -> S<void>; // guide with incomplete parameter type will never be considered-Wctor-dtor-privacy(C++ and Objective-C++ only) — warn when a class looks unusable because all the constructors or destructors in it 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. 🚪-Wdelete-non-virtual-dtor(C++ and Objective-C++ only) — warn whendeleteis used to destroy an instance of a class that has virtual functions and a non-virtual destructor. It's unsafe to delete an instance of a derived class through a pointer to a base class if the base class doesn't 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.
-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.
-Wno-init-list-lifetime(C++ and Objective-C++ only) — do not warn about uses ofstd::initializer_listthat are likely to result in dangling pointers. Since the underlying array for aninitializer_listis handled like a normal C++ temporary object, it's so 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 localinitializer_listvariable, 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 theinitializer_listin the heap has a dangling pointer. 💀
- If a function returns a temporary
- 🌸 when an
initializer_listvariable gets assigned from a brace-enclosed initializer list, the temporary array made for the right side of that assignment only lives until the end of the full-expression — so by the next statement, yourinitializer_listvariable is holding a dangling pointer. not the vibe, bestie 😭
// li's initial underlying array lives as long as li
std::initializer_list<int> 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- 💀 and when a list constructor stores the
beginpointer from theinitializer_listargument, that does not extend the lifetime of the array. so if a class variable gets constructed from a temporaryinitializer_list, that pointer is left dangling the moment the variable declaration statement ends. bye 👋 -Winvalid-imported-macros— check that every imported macro definition is actually valid by the end of compilation. this one is not on by default, because figuring it out takes extra processing. lowkey useful when you're prepping sets of header-units and you want to make sure the macros are consistent. 🧚-Wno-literal-suffix(C++ and Objective-C++ only) — stop warning when a string or character literal is followed by a ud-suffix that doesn't start with an underscore. as a conforming extension, GCC treats those suffixes as separate preprocessing tokens, so that backwards compatibility with code using formatting macros from<inttypes.h>stays intact. like, for example:
#define __STDC_FORMAT_MACROS
#include <inttypes.h>
#include <stdio.h>
int main() {
int64_t i64 = 123;
printf("My int64: %" PRId64"\n", i64);
}here, 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. heads up: 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 get diagnosed by default, because the standard says so, periodt. a narrowing conversion from a constant produces an error, and one from a non-constant produces a warning — but-Wno-narrowingsuppresses that diagnostic. note that this doesn't change the meaning of well-formed code; narrowing conversions are still considered ill-formed in SFINAE contexts. 🤓
with -Wnarrowing in C++98, you get warned when a narrowing conversion prohibited by C++11 happens inside { }, like:
int i = { 2.2 }; // error: narrowing from double to intthis flag is included in -Wall and -Wc++11-compat. 💅
-Wnoexcept(C++ and Objective-C++ only) — warn when a noexcept-expression evaluates tofalsebecause of a call to a function that doesn't have a non-throwing exception specification (i.e.throw()ornoexcept) but is known by the compiler to never throw an exception. 🫶-Wnoexcept-type(C++ and Objective-C++ only) — warn if the C++17 feature that makesnoexceptpart of a function type changes the mangled name of a symbol compared to C++14. enabled by-Wabiand-Wc++17-compat. for example:
template <class T> void f(T t) { t(); };
void g() noexcept;
void h() { f(g); }in C++14, f calls f<void(*)()>, but in C++17 it calls f<void(*)()noexcept>. wild, right? 😭
-Wclass-memaccess(C++ and Objective-C++ only) — warn when the destination of a call to a raw memory function likememsetormemcpyis an object of class type, and when writing into such an object might bypass the class's 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 that the class's member functions maintain. for example, thememsetcall below is undefined because it modifies a non-trivial class object, and so it gets diagnosed. the safe way to initialize or clear the storage of objects of these types is by using the appropriate constructor or assignment operator, if one exists. 🙈
std::string str = "abc";
memset (&str, 0, sizeof str);-Wclass-memaccess is enabled by -Wall. if you explicitly cast the pointer to the class object to void *, or to a type that can be safely accessed by the raw memory function, that suppresses the warning. ✨
-Wnon-virtual-dtor(C++ and Objective-C++ only) — warn when a class has virtual functions and an accessible non-virtual destructor, either on itself or in an accessible polymorphic base class. in that case it's possible but unsafe to delete an instance of a derived class through a pointer to the class itself or to the base class. this warning is automatically enabled if-Weffc++is specified. 💀-Wregister(C++ and Objective-C++ only) — warn on uses of theregisterstorage class specifier, except when it's part of the GNU Explicit Register Variables extension. using theregisterkeyword as a storage class specifier was deprecated in C++11 and removed in C++17. enabled by default with-std=c++17. it's giving legacy 👵-Wreorder(C++ and Objective-C++ only) — warn when the order of member initializers written in your code doesn't match the order they must actually be executed in. like:
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, and emits a warning to that effect. this warning is enabled by -Wall. the way that declaration order is the real order, no cap 🎀
-Wno-pessimizing-move(C++ and Objective-C++ only) — warns when a call tostd::moveactually prevents copy elision. a typical scenario where copy elision can happen is returning inside a function with a class return type, when the returned expression is the name of a non-volatile automatic object, isn't a function parameter, and has the same type as the function's return type. 👇
struct T {
...
};
T fn()
{
T t;
...
return std::move (t);
}but in this example, that std::move call prevents copy elision. 😭 this warning is enabled by -Wall.
-Wno-redundant-move(C++ and Objective-C++ only) — warns about redundant calls tostd::move, i.e. when a move operation would have happened even without thestd::movecall. this happens because the compiler is forced to treat the object as if it were an rvalue in certain situations, like 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) — warns when a C++ range-based for-loop is making an unnecessary copy. this can happen when the range declaration isn't 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 fires 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, on every iteration of the loop a temporary value of type double is created and destroyed, and the reference const double & is bound to it. 😭
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 dropped without causing ambiguity. for example:
struct foo;
struct foo *p; // warn that keyword struct can be eliminatedon the other hand, in this example there's no warning:
struct foo;
void foo (); // "hides" struct foo
void bar (struct foo&); // no warning, keyword struct is necessarycontext is everything, bestie 💅
-Wno-subobject-linkage(C++ and Objective-C++ only) — don't 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 too. the compiler doesn't give this warning for types defined in the main .C file, since those are unlikely to have multiple definitions.-Wsubobject-linkageis 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. 😭
✨ MORE OPTIONS, BESTIE ✨
-Wno-exceptions(C++ and Objective-C++ only) — turns off the warning for when an exception handler gets shadowed by another handler. That shadowing thing is usually a sign your exception handlers are in the wrong order, so this warning is actually your friend. 🙈-Wstrict-null-sentinel(C++ and Objective-C++ only) — warns when you use an uncasted"NULL"as a sentinel. Compiling with just GCC, that's fine —"NULL"is defined as"__null", and even though it's a null pointer constant and not a null pointer, it's guaranteed to be the same size as a pointer. But this use is not portable across different compilers. 💀-Wno-non-template-friend(C++ and Objective-C++ only) — kills the warning for non-template friend functions declared inside a template. In very old GCC versions that predate the ISO standard implementation, declarations likefriend int foo(int)— where the friend's name is an unqualified-id — could get read as a particular specialization of a template function. The warning exists to diagnose compatibility problems, and it's enabled by default.-Wold-style-cast(C++ and Objective-C++ only) — warns if an old-style (C-style) cast to a non-void type shows up in a C++ program. The new-style casts ("dynamic_cast","static_cast","reinterpret_cast", and"const_cast") are less vulnerable to unintended effects and so much easier to search for. 🔍-Woverloaded-virtual(C++ and Objective-C++ only) — warns when a function declaration hides virtual functions from a base class. Like, check it:
struct A {
virtual void f();
};
struct B: public A {
void f(int);
};The "A" class version of "f" is hidden in "B", and code like:
B* b;
b->f();…fails to compile. 😭
-Wno-pmf-conversions(C++ and Objective-C++ only) — disables the diagnostic for converting a bound pointer-to-member function into a plain pointer.-Wsign-promo(C++ and Objective-C++ only) — warns when overload resolution picks a promotion from unsigned or enumerated type to a signed type, instead of 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. the standard said so, so 💅-Wtemplates(C++ and Objective-C++ only) — warns when a primary template declaration is encountered. Some coding rules just say no to templates, and this is how you enforce that rule. The warning is inactive inside a system header file, like the STL, so you can still use the STL. You can also instantiate or specialize templates. ✨-Wmismatched-new-delete(C++ and Objective-C++ only) — warns for mismatches between calls to"operator new"or"operator delete"and the corresponding allocation or deallocation function call. 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 — plus 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) — warns for declarations of structs, classes, and class templates and their specializations with a class-key that doesn't 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 meant 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. You can use the option on its own or in conjunction with -Wredundant-tags. 🫶
-Wmultiple-inheritance(C++ and Objective-C++ only) — warns 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. You may also define classes that indirectly use multiple inheritance.-Wvirtual-inheritance— warns 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. You may also define classes that indirectly use virtual inheritance.-Wno-virtual-move-assign— suppresses 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 gets 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— warns 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. You may also use using directives and qualified names. ✨-Wno-terminate(C++ and Objective-C++ only) — disables the warning about a throw-expression that will immediately result in a call to"terminate". 💀-Wno-vexing-parse(C++ and Objective-C++ only) — warns 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 for 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) — warns about deprecated uses of the"volatile"qualifier. That 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.
-Wzero-as-null-pointer-constant(C++ and Objective-C++ only) — warns when a literal0is used as a null pointer constant. This can be useful to facilitate the conversion to"nullptr"in C++11. 🌸-Waligned-new— warns about a new-expression of a type that requires greater alignment than"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— warns about placement new expressions with undefined behavior, like 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. So like, GCC is being chill about your old code, bestie. 💅
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, on top of 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 a structure whose type is an array of a single element and whose size is less than the size of the object being constructed. So while the previous example would get 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) — warns you about catch handlers that don't catch via reference, which is giving slicing energy and we don't love that. Here's the level breakdown, no cap:- With
-Wcatch-value=1(or just-Wcatch-valuefor short) it warns about polymorphic class types that are caught by value. - With
-Wcatch-value=2it warns about all class types that are caught by value. - With
-Wcatch-value=3it warns about all types that are not caught by reference.
- With
-Wcatch-value is enabled by -Wall. ✨
-Wconditionally-supported(C++ and Objective-C++ only) — warns for conditionally-supported constructs (C++11[intro.defs]). iykyk.-Wno-delete-incomplete(C++ and Objective-C++ only) — do not warn when deleting a pointer to an incomplete type, which may cause undefined behavior at runtime. This warning is enabled by default, bestie, so this flag is you actively telling it to look away 🙈.-Wextra-semi(C++, Objective-C++ only) — warns about redundant semicolons after in-class function definitions. the way that stray;just sits there... not needed, queen. 💅-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-Wextraoption instead.
struct A { int a; };
struct B : A { };
struct C : B, A { };-Wno-inherited-variadic-ctor— suppresses 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 — sorry, the...doesn't get passed down, that's just how it is. 😭-Wno-invalid-offsetof(C++ and Objective-C++ only) — suppresses warnings from applying theoffsetofmacro to a non-POD type. According to the 2014 ISO C++ standard, applyingoffsetofto a non-standard-layout type is undefined. In existing C++ implementations, though,offsetoftypically gives meaningful results. This flag is for users who are aware that they're writing nonportable code and who have deliberately chosen to ignore the warning about it. Periodt.
The restrictions on offsetof may be relaxed in a future version of the C++ standard. manifesting ✨
-Wsized-deallocation(C++ and Objective-C++ only) — warns 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— warns about types with virtual methods where code quality would be improved if the type were declared with the C++11finalspecifier, or, if possible, declared in an anonymous namespace. This lets GCC more aggressively devirtualize the polymorphic calls, which is lowkey iconic of it. 💅 This warning is more effective with link-time optimization, where the information about the class hierarchy graph is more complete.-Wsuggest-final-methods— warns about virtual methods where code quality would be improved if the method were declared with the C++11finalspecifier, or, if possible, its type were declared in an anonymous namespace or with thefinalspecifier. Also more effective with link-time optimization, where the info about the class hierarchy graph is more complete. It is recommended to first consider suggestions of-Wsuggest-final-typesand then rebuild with new annotations. one at a time, babe, don't do both at once 🎀-Wsuggest-override— warns about overriding virtual functions that are not marked with theoverridekeyword. girl just sayoverride, it's right there 💀-Wuse-after-free,-Wuse-after-free=n— warns 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 torealloc, regardless of whether or not the call resulted in an actual reallocation of memory. This includes double-freecalls as well as uses in arithmetic and relational expressions. Although it's undefined, uses of indeterminate pointers in equality (or inequality) expressions are not diagnosed at this level.-Wuse-after-free=2— at level 2, on top of 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 tofreein 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 toreallocas an attempt to determine whether the call resulted in relocating the object to a different address. They're diagnosed at a separate level to aid legacy code in gradually transitioning 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. slay, that's the move ✨
-Wuse-after-free=2 is included in -Wall.
-Wuseless-cast(C++ and Objective-C++ only) — warns when an expression is casted to its own type. like... babe. that's the same type. why. 😭-Wno-conversion-null(C++ and Objective-C++ only) — do not warn for conversions betweenNULLand non-pointer types.-Wconversion-nullis 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, bestie, it's not either/or. For example, you might compile a file some_class.m like this:
gcc -g -fgnu-runtime -O -c some_class.mIn 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's 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 isNXConstantStringif the GNU runtime is being used, andNSConstantStringif the NeXT runtime is being used (see below). The-fconstant-cfstringsoption, if also present, overrides the-fconstant-string-classsetting and causes@"..."literals to be laid out as constant CoreFoundation strings. 👑-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 and Mac OS X. 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 notnil. 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. no cap, that's the whole deal 🦋-fobjc-abi-version=n— okay so this one picks version n of the Objective-C ABI for whichever runtime you've got selected 💅 and bestie, rn it's only supported for the NeXT runtime, that's the whole list. So: Version 0 is the traditional (32-bit) ABI, no properties, none of the Objective-C 2.0 extras. Version 1? traditional (32-bit) ABI but WITH properties and the other Objective-C 2.0 additions. Version 2 is the modern (64-bit) ABI. And if you don't specify anything, the default is Version 0 on 32-bit target machines and Version 2 on 64-bit target machines. Defaults doing their own thing, iconic ✨-fobjc-call-cxx-cdtors— for every Objective-C class, GCC checks if any of its instance variables is a C++ object with a non-trivial default constructor 🧚 if it is, it synthesizes a special- (id) .cxx_constructinstance method that runs those non-trivial default constructors on any such instance variables, in order, and then returnsself. Same energy for destructors: check if any instance variable is a C++ object with a non-trivial destructor, and if so, synthesize a special- (void) .cxx_destructmethod which runs all such default destructors, in reverse order.
The - (id) .cxx_construct and - (void) .cxx_destruct methods generated this way only operate on instance variables declared in the current Objective-C class — not the ones inherited from superclasses, so don't come for them. 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 get invoked by the runtime immediately after a new object instance is allocated; the - (void) .cxx_destruct methods get invoked immediately before the runtime deallocates an object instance. Timing is everything, fr fr ⏰
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. That's the whole guest list, no plus-ones 💀
-fobjc-direct-dispatch— lets you do fast jumps to the message dispatcher 🔥 on Darwin this is accomplished via the comm page.-fobjc-exceptions— turns on syntactic support for structured exception handling in Objective-C, similar to what C++ offers. This option is required to use the Objective-C keywords@try,@throw,@catch,@finallyand@synchronized. It's available with both the GNU runtime and the NeXT runtime — but not in conjunction with the NeXT runtime on Mac OS X 10.2 and earlier. Read the fine print bestie 🙈-fobjc-gc— enables garbage collection (GC) in Objective-C and Objective-C++ programs. Heads up: this one is only available with the NeXT runtime. The GNU runtime has a different garbage collection implementation that does not require special compiler flags, so it's giving independent queen 💅-fobjc-nilcheck— for the NeXT runtime with version 2 of the ABI, this checks for a nil receiver in method invocations before doing the actual method call. It's the default, and you can turn it off with-fno-objc-nilcheck. Class methods and super calls are never checked for nil this way no matter what this flag is set to — they're above the drama ✋ 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 you use this with the Objective-C or Objective-C++ compiler, any Objective-C syntax that is not recognized by GCC 4.0 is rejected. Useful if you need to make sure your Objective-C code can be compiled with older versions of GCC, i.e. for the elders 👑-freplace-objc-classes— emits a special marker instructingld(1)not to statically link in the resulting object file, and letsdyld(1)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 needing 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. Live reloading before live reloading was a thing, periodt 🌈-fzero-link— when compiling for the NeXT runtime, the compiler ordinarily replaces calls toobjc_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-fzero-linksuppresses this behavior and causes calls toobjc_getClass("...")to be retained. Useful in Zero-Link debugging mode, since it allows for individual class implementations to be modified during program execution. And the GNU runtime currently always retains calls toobjc_get_class("...")regardless of command-line options — it said "I do what I want" 💅-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-fno-local-ivarsdisables this behavior, thus avoiding those variable shadowing issues. No more name twins, we love to see it 🎀-fivar-visibility=[public|protected|private|package]— sets the default instance variable visibility to whichever option you pick, so that instance variables declared outside the scope of any access modifier directives default to that specified visibility. Pick your fighter 🫶-gen-decls— dumps interface declarations for all classes seen in the source file to a file namedsourcename.decl. It's giving receipts, in a file 📝-Wassign-intercept(Objective-C and Objective-C++ only) — warns whenever an Objective-C assignment is being intercepted by the garbage collector. Snitch mode: activated 🔍-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. Mind your business mode 💜-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-Wno-protocol, then methods inherited from the superclass are considered to be implemented, and no warning is issued for them. Inherited slay counts as slay ✅-Wobjc-root-class(Objective-C and Objective-C++ only) — warns if a class interface lacks a superclass. Most classes will inherit fromNSObject(orObject) for example. When declaring classes intended to be root classes, the warning can be suppressed by marking their interfaces with__attribute__((objc_root_class)). Root class? Own it, but declare it properly bestie 🌸-Wselector(Objective-C and Objective-C++ only) — warns 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-onlyoption is being used. End-of-show check only, no early spoilers 🎬-Wstrict-selector-match(Objective-C and Objective-C++ only) — warns 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 typeidorClass. 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. Same size, same alignment? Then it's lowkey fine, we move on 💀-Wundeclared-selector(Objective-C and Objective-C++ only) — warns 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@interfaceor@protocoldeclaration, or implicitly in an@implementationsection. This option always performs its checks as soon as a@selector(...)expression is found, while-Wselectoronly 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. Declare it before you use it, no ambushing ✨-print-objc-runtime-info— generates a C header describing the largest structure that is passed by value, if any. Just the biggest one, main character energy 👑
🩺 Options to Control Diagnostic Messages Formatting 🩺
Traditionally, diagnostic messages have been formatted irrespective of the output device's aspect (e.g. its width, ...). Which is giving "we didn't check the vibe of your terminal". You can use the options described below to control the formatting algorithm for diagnostic messages — like how many characters per line, how often source location information should be reported. Note that some language front ends may not honor these options. Their loss 💅
-fmessage-length=n— tries to format error messages so 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. So those two are just out here unbothered 🦋
-fdiagnostics-plain-output— this option requests that diagnostic output look as plain as possible, which may be useful when runningdejagnuor other utilities that need to parse diagnostics output and prefer that it remain more stable over time.-fdiagnostics-plain-outputis 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— all of that, in one cute little flag 🎀 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. Future-proofed, no cap ⭐
-fdiagnostics-show-location=once 💖
okay so this one is only meaningful in line-wrapping mode, bestie. It tells the diagnostic messages reporter to emit the source location info once — like, if the message is too long to fit on a single physical line and has to get wrapped, the source location won't be emitted (as a prefix) again, over and over, on every subsequent continuation line. ✨
This is the default behavior. no cap.
-fdiagnostics-show-location=every-line 💅
also only meaningful in line-wrapping mode. This one instructs the diagnostic messages reporter to emit the same source location info (as a prefix) for the physical lines that result from breaking a message which is too long to fit on a single line. so like, every single line gets the location drip. 💫
-fdiagnostics-color[=WHEN], -fno-diagnostics-color 🌈
Use color in diagnostics. periodt. 💅
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 the 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:
1for bold4for underline5for blink7for inverse39for default foreground color30to37for foreground colors90to97for 16-color mode foreground colors38;5;0to38;5;255for 88-color and 256-color modes foreground colors49for default background color40to47for background colors100to107for 16-color mode background colors48;5;0to48;5;255for 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;32where 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 forerror:markers."warning="— SGR substring forwarning:markers."note="— SGR substring fornote: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:lineorfile:line:columnetc."quote="— SGR substring for information printed within quotes."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.
-fdiagnostics-urls[=WHEN] 🦋
Use escape sequences to embed URLs in diagnostics. like, 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_URLSis set to empty orno, 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_URLSis not set, useTERM_URLSas a fallback.
Note: ST is an ANSI escape sequence, string terminatorESC \, BEL is an ASCII character,CTRL-Gthat 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. bye bye little option hint. 👋
-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 intThis option suppresses the printing of these labels (in the example above, the vertical bars and the "char *" and "long int" text). 🫶
-fno-diagnostics-show-cwe 🔍
Diagnostic messages can optionally have an associated @url{https://cwe.mitre.org/index.html, CWE} 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-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_allThe 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<int, std::vector<double> >()'
from 'map<[...],vector<double>>' to 'map<[...],vector<float>>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). Like, the contrast is literally just there in living color 💅
-fno-elide-type— okay so by default, when the C++ frontend prints diagnostics about mismatching template types, it collapses the boring shared parts of the types into"[...]"to keep the error message cute and short. Like this:
could not convert 'std::map<int, std::vector<double> >()'
from 'map<[...],vector<double>>' to 'map<[...],vector<float>>Throw in -fno-elide-type and that whole behavior gets shut down — no more hiding, you see everything. Lowkey important: this flag also affects the output of the -fdiagnostics-show-template-tree flag. They're linked, bestie 🦋
-fdiagnostics-path-format=KIND— this one picks how to print paths of control-flow events for diagnostics that have such a path attached to them.
KIND is one of none, separate-events, or inline-events — and inline-events is the default.
nonemeans: don't print diagnostic paths at all. Simple. Silent. Mysterious. 🙈separate-eventsmeans: print a separate"note"diagnostic for every single event inside the diagnostic. Like this:
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 1inline-eventsmeans: print the events"inline"right inside the source code. This view tries to consolidate the events into runs of sufficiently-close events, printing them as labelled ranges within the source.
So, for example, the same events as above might come out looking like:
'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 gets shown by grouping the events by stack frame, and using indentation to show how stack frames are nested, pushed, and popped. It's giving family tree but for function calls 🌳
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 hands you extra info when printing control-flow paths that come with a diagnostic.
If this option is provided, then the stack depth gets printed for each run of events within -fdiagnostics-path-format=separate-events.
Heads up bestie — this is intended for use by GCC developers and plugin developers when debugging diagnostics that report interprocedural control flow. So like, if that's not you, you probably don't need it 💀
-fno-show-column— do not print column numbers in diagnostics. This might be necessary if the diagnostics are being scanned by a program that doesn't understand column numbers, such asdejagnu.-fdiagnostics-column-unit=UNIT— picks the units for the column number. This affects traditional diagnostics (when-fno-show-columnisn't in play), and also JSON format diagnostics if you asked for those.
The default UNIT, display, counts the number of display columns occupied by each character. This may be bigger than the number of bytes needed to encode the character — think tab characters — or it may be smaller — think 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. ✨unicode math✨
Setting UNIT to byte changes the column number to the raw byte count in all cases, exactly as GCC traditionally output it before version 11.1.0.
-fdiagnostics-column-origin=ORIGIN— picks the origin for column numbers, i.e. the column number that gets assigned to the first column. The default value of1matches traditional GCC behavior and the GNU style guide. Some utilities may perform better with an origin of0; any non-negative value may be specified. Your column, your rules 👑-fdiagnostics-escape-format=FORMAT— when GCC prints pertinent source lines for a diagnostic, it normally tries to print the source bytes directly. But some diagnostics relate to encoding issues in the source file — malformed UTF-8, Unicode normalization drama, that kind of thing. Those diagnostics are flagged so that GCC will escape bytes that aren't printable ASCII when printing their pertinent source lines.
This option controls how those bytes should be escaped.
The default FORMAT, unicode, displays Unicode characters that aren't printable ASCII in the form <U+XXXX>, and bytes that don't correspond to a Unicode character validly-encoded in UTF-8 will be displayed as hexadecimal in the form <XX>.
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:
before<U+03C0><BF>afterSetting FORMAT to bytes will display all non-printable-ASCII bytes in the form <XX>, thus showing the underlying encoding of non-ASCII Unicode characters. For the example above, the following will be printed:
before<CF><80><BF>after-fdiagnostics-format=FORMAT— select a different format for printing diagnostics.FORMATistextorjson. The default istext.
The json format consists of a top-level JSON array containing JSON objects representing the diagnostics.
The JSON is emitted as one line, without formatting; the examples below have been formatted for clarity. aesthetic only, no cap 🎀
Diagnostics can have child diagnostics. For example, this error and note:
misleading-indentation.c:15:3: warning: this 'if' clause does not
guard... [-Wmisleading-indentation]
15 | if (flag)
| ^~
misleading-indentation.c:17:5: note: ...this statement, but the latter
is misleadingly indented as if it were guarded by the 'if'
17 | y = 2;
| ^might be printed in JSON form (after formatting) like this:
okay so bestie, the JSON is literally a tree — like, diagnostics can have children 💖 and the whole thing nests. Behold:
[
{
"kind": "warning",
"locations": [
{
"caret": {
"display-column": 3,
"byte-column": 3,
"column": 3,
"file": "misleading-indentation.c",
"line": 15
},
"finish": {
"display-column": 4,
"byte-column": 4,
"column": 4,
"file": "misleading-indentation.c",
"line": 15
}
}
],
"message": "this \u2018if\u2019 clause does not guard...",
"option": "-Wmisleading-indentation",
"option_url": "https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-Wmisleading-indentation",
"children": [
{
"kind": "note",
"locations": [
{
"caret": {
"display-column": 5,
"byte-column": 5,
"column": 5,
"file": "misleading-indentation.c",
"line": 17
}
}
],
"escape-source": false,
"message": "...this statement, but the latter is ..."
}
]
"escape-source": false,
"column-origin": 1,
}
]where the "note" is a child of the "warning" 🫶. Family tree but make it compiler errors, fr fr.
A diagnostic has a "kind". If that kind is "warning", then there's an "option" key describing the command-line option controlling the warning. 👑
A diagnostic can contain zero or more locations. Each location has an optional "label" string and up to three positions within it: a "caret" position and optional "start" and "finish" positions. A position is described by a "file" name, a "line" number, and three numbers indicating a column position:
"display-column"counts display columns, accounting for tabs and multibyte characters. (it's giving aesthetic columns ✨)"byte-column"counts raw bytes. 🧱"column"is equal to one of the previous two, as dictated by the-fdiagnostics-column-unitoption.
All three columns are relative to the origin specified by -fdiagnostics-column-origin, which is typically equal to 1 but may be set, for instance, to 0 for compatibility with other utilities that number columns from 0. The column origin is recorded in the JSON output in the "column-origin" tag. In the remaining examples below, the extra column number outputs have been omitted for brevity. 💅
For example, this error:
bad-binary-ops.c:64:23: error: invalid operands to binary + (have 'S' {aka
'struct s'} and 'T' {aka 'struct t'})
64 | return callee_4a () + callee_4b ();
| ~~~~~~~~~~~~ ^ ~~~~~~~~~~~~
| | |
| | T {aka struct t}
| S {aka struct s}has three locations. Its primary location is at the + token at column 23. It has two secondary locations, describing the left and right-hand sides of the expression, which have labels. It might be printed in JSON form as:
{
"children": [],
"kind": "error",
"locations": [
{
"caret": {
"column": 23, "file": "bad-binary-ops.c", "line": 64
}
},
{
"caret": {
"column": 10, "file": "bad-binary-ops.c", "line": 64
},
"finish": {
"column": 21, "file": "bad-binary-ops.c", "line": 64
},
"label": "S {aka struct s}"
},
{
"caret": {
"column": 25, "file": "bad-binary-ops.c", "line": 64
},
"finish": {
"column": 36, "file": "bad-binary-ops.c", "line": 64
},
"label": "T {aka struct t}"
}
],
"escape-source": false,
"message": "invalid operands to binary + ..."
}If a diagnostic contains fix-it hints, it has a "fixits" array, consisting of half-open intervals, similar to the output of -fdiagnostics-parseable-fixits. For example, this diagnostic with a replacement fix-it hint:
demo.c:8:15: error: 'struct s' has no member named 'colour'; did you
mean 'color'?
8 | return ptr->colour;
| ^~~~~~
| colormight be printed in JSON form as:
{
"children": [],
"fixits": [
{
"next": {
"column": 21,
"file": "demo.c",
"line": 8
},
"start": {
"column": 15,
"file": "demo.c",
"line": 8
},
"string": "color"
}
],
"kind": "error",
"locations": [
{
"caret": {
"column": 15,
"file": "demo.c",
"line": 8
},
"finish": {
"column": 20,
"file": "demo.c",
"line": 8
}
}
],
"escape-source": false,
"message": "\u2018struct s\u2019 has no member named ..."
}where the fix-it hint suggests replacing the text from "start" up to but not including "next" with "string"'s value. Deletions are expressed via an empty value for "string", insertions by having "start" equal "next". 🎀
If the diagnostic has a path of control-flow events associated with it, it has a "path" array of objects representing the events. Each event object has a "description" string, a "location" object, along with a "function" string and a "depth" number for representing interprocedural paths. The "function" represents the current function at that event, and the "depth" represents the stack depth relative to some baseline: the higher, the more frames are within the stack. 📚
For example, the intraprocedural example shown for -fdiagnostics-path-format= might have this JSON for its path:
"path": [
{
"depth": 0,
"description": "when 'PyList_New' fails, returning NULL",
"function": "test",
"location": {
"column": 10,
"file": "test.c",
"line": 25
}
},
{
"depth": 0,
"description": "when 'i < count'",
"function": "test",
"location": {
"column": 3,
"file": "test.c",
"line": 27
}
},
{
"depth": 0,
"description": "when calling 'PyList_Append', passing NULL from (1) as argument 1",
"function": "test",
"location": {
"column": 5,
"file": "test.c",
"line": 29
}
}
]Diagnostics have a boolean attribute "escape-source", hinting whether non-ASCII bytes should be escaped when printing the pertinent lines of source code ("true" for diagnostics involving source encoding issues). 🌈
🌸 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. like bestie it's not wrong wrong, but it's giving ⚠️, yk? 😭
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. That's it, that's the whole vibe. 💅- *
-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 is0(the default), there is no limit on the number of error messages produced. If-Wfatal-errorsis also specified, then-Wfatal-errorstakes precedence over this option. 👑 -w— Inhibit all warning messages. Silence, bestie 🤫-Werror— Make all warnings into errors. no mercy era 💀🔥-Werror=— bestie, this turns a specific warning into a full-on error. You slap the specifier on the end, so like-Werror=switchmakes every warning controlled by-Wswitchbecome an error. And yes, it has a negative form too, so you can undo-Werrorfor just one warning:-Wno-error=switchmeans-Wswitchwarnings stay warnings, even when-Werroris out here being dramatic everywhere else. 💅
Every controllable warning's message literally tells you which option controls it. So you can grab that option name and plug it straight into -Werror= or -Wno-error= like we just said. (Wanna hide that option from the message? -fno-diagnostics-show-option handles it. 🙈)
Heads up:-Werror=fooautomatically implies-Wfoo. But-Wno-error=foo? It implies nothing. No cap.
-Wfatal-errors— the moment the compiler hits one single error, it just stops. No trying to keep going, no extra error spam, just done. 💀
You can ask for a whole bunch of specific warnings with options that start with -W, like -Wimplicit for warnings on implicit declarations. And every one of those specific warning options has a negative twin starting with -Wno- that turns it off — so -Wno-implicit is the vibe. This manual only lists one of the two forms, whichever one isn't the default. ✨
For even more language-specific ones, go peek at C++ Dialect Options and Objective-C and Objective-C++ Dialect Options. And you can unlock additional warnings by turning on the static analyzer, too.
Some options like -Wall and -Wextra turn on other options like -Wunused, which can then turn on even more options like -Wunused-value. The combined effect of positive and negative forms? More specific options win over less specific ones, no matter where they sit on the command line. For options of the same specificity, the last one takes effect. And options enabled or disabled via pragmas act like they showed up at the very end of the command line. 👑
When you ask for a warning option nobody recognizes (say, -Wunknown-warning), GCC tells you straight up that the option isn't recognized. But if you use the -Wno- form, things get a little different: no diagnostic comes out for -Wno-unknown-warning unless other diagnostics are already being produced. That's what lets you use shiny new -Wno- options with old compilers — but if something goes sideways, the compiler will warn that an unrecognized option is hanging around. 🦋
And plot twist: some warnings only hit different when optimizations are on. Like -Wsuggest-final-types is way more effective with link-time optimization, and some instances of other warnings might not be issued at all unless optimization is enabled. Optimization generally makes control- and data-flow-sensitive warnings better, but ngl, in some cases it can also cause false positives. 😭
-Wpedantic,-pedantic— issue every single warning that strict ISO C and ISO C++ demand, reject all programs using forbidden extensions, and reject some other programs that just don't follow ISO C and ISO C++. For ISO C, it follows whichever version of the ISO C standard any-stdoption specifies. 🌸
Valid ISO C and ISO C++ programs should compile properly with or without this option (though a rare few need -ansi or a -std option specifying the required version of ISO C). But without it, certain GNU extensions and traditional C and C++ features are supported too. With it? Rejected. Periodt.
-Wpedantic does not warn about alternate keywords whose names begin and end with __. That same alternate format can also disable warnings for non-ISO __intN types, i.e. __intN__. Pedantic warnings are also disabled in the expression that follows __extension__. But bestie, only system header files should use these escape routes — application programs should avoid them. 🙈
Some people try to use -Wpedantic to check programs for strict ISO C conformance. They quickly find it doesn't quite do what they want: it catches some non-ISO practices, but not all — only those for which ISO C requires a diagnostic, plus some others where diagnostics have been added.
A feature that reports any failure to conform to ISO C might be useful sometimes, but it'd need considerable additional work and would be quite different from -Wpedantic. No plans to support such a feature in the near future. 🤷
Where the standard specified with -std is a GNU extended dialect of C, like gnu90 or gnu99, there's a corresponding base standard — the version of ISO C that GNU extended dialect is based on. -Wpedantic warnings are given where the base standard requires them. (It doesn't make sense to give such warnings 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 — there'd be nothing to warn about.)
-pedantic-errors— gives an error whenever the base standard (peep-Wpedantic) requires a diagnostic, in some cases where there's undefined behavior at compile-time, and in some other cases that don't prevent compilation of programs that are valid per the standard. This is not equivalent to-Werror=pedantic, since there are errors enabled by this option and not by that one, and vice versa. ⭐-Wall— turns on all the warnings about constructions some users consider questionable, and that are easy to avoid (or modify to prevent the warning), even when macros are in the mix. It also enables some language-specific warnings described in C++ Dialect Options and Objective-C and Objective-C++ Dialect Options. 🎀
-Wall turns on these warning flags:
-Waddress -Warray-bounds=1 (only with -O2) -Warray-compare
-Warray-parameter=2 (C and Objective-C only) -Wbool-compare
-Wbool-operation -Wc++11-compat -Wc++14-compat -Wcatch-value (C++
and Objective-C++ only) -Wchar-subscripts -Wcomment
-Wdangling-pointer=2 -Wduplicate-decl-specifier (C and Objective-C
only) -Wenum-compare (in C/ObjC; this is on by default in C++)
-Wformat -Wformat-overflow -Wformat-truncation -Wint-in-bool-context
-Wimplicit (C and Objective-C only) -Wimplicit-int (C and Objective-
C only) -Wimplicit-function-declaration (C and Objective-C only)
-Winit-self (only for C++) -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 (only
for C/C++) -Wmissing-attributes -Wmissing-braces (only for C/ObjC)
-Wmultistatement-macros -Wnarrowing (only for C++) -Wnonnull
-Wnonnull-compare -Wopenmp-simd -Wparentheses -Wpessimizing-move
(only for C++) -Wpointer-sign -Wrange-loop-construct (only for C++)
-Wreorder -Wrestrict -Wreturn-type -Wsequence-point -Wsign-compare
(only in C++) -Wsizeof-array-div -Wsizeof-pointer-div
-Wsizeof-pointer-memaccess -Wstrict-aliasing -Wstrict-overflow=1
-Wswitch -Wtautological-compare -Wtrigraphs -Wuninitialized
-Wunknown-pragmas -Wunused-function -Wunused-label -Wunused-value
-Wunused-variable -Wuse-after-free=3 -Wvla-parameter (C and
Objective-C only) -Wvolatile-register-var -Wzero-length-boundsNote that some warning flags are not implied by -Wall. Some warn about constructions users generally don't consider questionable, but that you might occasionally want to check for; others warn about constructions that are necessary or hard to avoid in some cases, where there's no simple way to modify the code to silence the warning. Some of those are enabled by -Wextra, but a whole bunch must be enabled individually. 💜
-Wextra— enables some extra warning flags that-Walldoesn't. (Fun fact: this option used to be called-W. The older name still works, but the newer one is more descriptive. 🌈)
-Wclobbered -Wcast-function-type -Wdeprecated-copy (C++ only)
-Wempty-body -Wenum-conversion (C only) -Wignored-qualifiers
-Wimplicit-fallthrough=3 -Wmissing-field-initializers
-Wmissing-parameter-type (C only) -Wold-style-declaration (C only)
-Woverride-init -Wsign-compare (C only) -Wstring-compare
-Wredundant-move (only for C++) -Wtype-limits -Wuninitialized
-Wshift-negative-value (in C++11 to C++17 and in C99 and newer)
-Wunused-parameter (only with -Wunused or -Wall)
-Wunused-but-set-parameter (only with -Wunused or -Wall)And -Wextra also prints warning messages for these 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) — warns about code affected by ABI changes. That includes code that may not be compatible with the vendor-neutral C++ ABI, plus the psABI for the particular target. 🫶
Since G++ now defaults to updating the ABI with each major release, normally -Wabi only warns about C++ ABI compatibility problems if a check was added later in a release series for an ABI issue discovered since the initial release. -Wabi warns about more stuff if an older ABI version is selected (with -fabi-version=n).
You can also use -Wabi 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 isn't specified, the version number from this option is used for compatibility aliases. If no explicit version number is given with this option but -fabi-compat-version is specified, that version number gets used for C++ ABI warnings. ✨
...although lowkey, bestie, they tried to warn you about all these cases and there are still probably some sneaky ones that slip through unwarned even though G++ is out here generating incompatible code 😭. And the reverse happens too — sometimes you get a warning even though the code it generated is actually compatible. Rude.
So if the idea of G++-generated code not being binary compatible with code from other compilers is stressing you out, the move is to rewrite your code and dodge these warnings entirely. Periodt.
🦋 Known incompatibilities in -fabi-version=2 (the default from GCC 3.4 to 4.9, iykyk)
- A template with a non-type template parameter of reference type was mangled incorrectly:
extern int N;
template <int &> struct S {};
void n (S<N>) {2}This got 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))"andnoreturnwere mangled as type qualifiers, anddecltypeof 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, which makes
va_argcomplain 😤. On most targets this doesn't actually affect the parameter passing ABI, because there's literally no way to pass an argument smaller thanint.
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, which led to unaligned accesses. Note that this did not affect the ABI of a function with anullptr_tparameter, because 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 — like 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.
This option also turns on warnings about psABI-related changes. The known psABI changes at this point in time include:
- For SysV/x86-64, unions with
long doublemembers are passed in memory as specified in psABI. Prior to GCC 4.4, this was not the case. For example:
union U {
long double ld;
int i;
};"union U" is now always passed in memory.
-Wchar-subscripts
Warns you if an array subscript has type char. This is a super common cause of error, because programmers keep forgetting that this type is signed on some machines 🙈. This warning is enabled by -Wall.
-Wno-coverage-mismatch
Warns you if feedback profiles don't match when you're using the -fprofile-use option. If a source file changes between compiling with -fprofile-generate and compiling with -fprofile-use, the files with the profile feedback can fail to match the source file, and GCC just cannot use the profile feedback info. By default this warning is enabled and treated as an error. You can use -Wno-coverage-mismatch to disable the warning, or -Wno-error=coverage-mismatch to disable the error. Disabling the error for this warning can result in poorly optimized code and is only useful in the case of very minor changes, like bug fixes to an existing code-base.
Completely disabling the warning is not recommended. 🚫
-Wno-coverage-invalid-line-number
Warns you in case a function ends earlier than it begins, because of invalid linenum macros 👀. This warning is only emitted with --coverage enabled.
By default this warning is enabled and 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)
Suppresses warning messages emitted by "#warning" directives. shhh 🤫
-Wdouble-promotion (C, C++, Objective-C and Objective-C++ only)
Gives you a warning when a value of type float gets implicitly promoted to double. CPUs with a 32-bit "single-precision" floating-point unit implement float in hardware, but emulate double in software. On a machine like that, doing computations using double values is much more expensive because of the overhead required for software emulation. 💸
It is super 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)
Warns you if a declaration has a duplicate const, volatile, restrict or _Atomic specifier. This warning is enabled by -Wall.
-Wformat, -Wformat=n
Checks calls to printf and scanf, etc., to make sure the arguments you supplied have types appropriate to the format string you specified, and that the conversions specified in the format string actually make sense. 🕵️♀️ This includes standard functions, plus others specified by format attributes, in the printf, scanf, strftime and strfmon families (that last one is an X/Open extension, not in the C standard), or other target-specific families. Which functions get 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-Wformatis equivalent to-Wformat=1, and-Wno-formatis equivalent to-Wformat=0. Since-Wformatalso checks for null format arguments for several functions,-Wformatalso 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.-Wformatis enabled by-Wall.-Wformat=2— Enables-Wformatplus 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
Warns you 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 can't be determined at compile-time, it gets 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 tosprintfbelow is diagnosed, because even with bothaandbequal 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— okay so level 2 is where the compiler stops being chill and starts being thorough 💅 it 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. no cap, the numbers go crazy 🔥
void f (int a, int b)
{
char buf [23];
sprintf (buf, "a = %hi, b = %i\n", a, (short)b);
}-Wno-format-zero-length— if-Wformatis specified, do not warn about zero-length formats. bestie the C standard literally specifies that zero-length formats are allowed, so like, leave them alone ✨-Wformat-nonliteral— if-Wformatis 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 ava_list. it's giving "I can't verify this so I'm saying something" energy 🙈-Wformat-security— if-Wformatis specified, also warn about uses of format functions that represent possible security problems. At present, this warns about calls toprintfandscanffunctions where the format string is not a string literal and there are no format arguments, as inprintf (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-nonliteralwarns about, but in future warnings may be added to-Wformat-securitythat are not included in-Wformat-nonliteral.) 🚨 lowkey important, do not skip this one-Wformat-signedness— if-Wformatis specified, also warn if the format string requires an unsigned argument and the argument is signed and vice versa. signed vs unsigned drama, we love to see it 💀-Wformat-truncation,-Wformat-truncation=level— warn about calls to formatted input/output functions such assnprintfandvsnprintfthat 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-truncationenabled by-Wformatemploys 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. she's cautious, she's polite, she only speaks up when she's sure 🎀-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-Wformatis specified, also warn aboutstrftimeformats that may yield only a two-digit year. it's giving Y2K panic but make it compile-time ⭐-Wnonnull— warn about passing a null pointer for arguments marked as requiring a non-null value by thenonnullfunction 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 thenonnullfunction attribute against null inside the function. bestie you literally promised it wouldn't be null, why are you checking 😭
-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-checksis active, which is enabled by optimizations in most targets. The precision of the warnings depends on the optimization options used. 🕵️♀️ she's out here finding the null derefs fr fr-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-recursionis included in-Wall. it's giving "she's stuck in a loop and so am I" 💀-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-Wuninitializedoption. iconically cursed behavior, ngl 🙈
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 in C99 and later dialects of C, and also by-Wall. 👑-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 in C99 and later dialects of C, and also by-Wall. The warning is made into an error by-pedantic-errors. periodt, it becomes a whole error 🔥-Wimplicit(C and Objective-C only) — same as-Wimplicit-intand-Wimplicit-function-declaration. This warning is enabled by-Wall.-Wimplicit-fallthrough—-Wimplicit-fallthroughis the same as-Wimplicit-fallthrough=3and-Wno-implicit-fallthroughis the same as-Wimplicit-fallthrough=0. two options, one vibe 💜-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 provides 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=0disables the warning altogether. 😌-Wimplicit-fallthrough=1matches.*regular expression, any comment is used as fallthrough comment.-Wimplicit-fallthrough=2case insensitively matches.*falls?[ \t-]*thr(ough|u).*regular expression.-Wimplicit-fallthrough=3case 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=4case sensitively matches one of the following regular expressions:-fallthrough@fallthrough@lint -fallthrough[ \t]*[ \t]*FALLTHR(OUGH|U)[ \t]*
-Wimplicit-fallthrough=5doesn't recognize any comments as fallthrough comments, only attributes disable the warning. she said no comments, only attributes 🎀
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) — this one decides whether those warnings triggered by thewarn_if_not_alignedattribute actually get issued. And bestie, they're on by default. No cap. ✨-Wignored-qualifiers(C and C++ only) — warn if the return type of a function has a type qualifier likeconst. For ISO C, that qualifier literally does nothing, because the value a function returns isn't an lvalue, so... vibe check failed, it has no effect. For C++, the warning only fires for scalar types orvoid. And ISO C prohibits qualifiedvoidreturn types on function definitions, so those return types always get a warning even if you don't pass this option. 🙈
This warning is also enabled by -Wextra.
-Wno-ignored-attributes(C and C++ only) — controls warnings when an attribute gets ignored. This is different from the-Wattributesoption: this one warns whenever the compiler decides to drop an attribute, not when the attribute is unknown or used in the wrong place or whatever. Enabled by default. 🎀-Wmain— warn if the type ofmainis giving suspicious energy.mainshould be a function with external linkage, returningint, taking either zero arguments, two, or three arguments of appropriate types. This warning is enabled by default in C++, and it's enabled by either-Wallor-Wpedantic. 👑-Wmisleading-indentation(C and C++ only) — warn when the indentation of the code does not reflect the block structure. Specifically, a warning is issued forif,else,while, andforclauses with a guarded statement that doesn't use braces, followed by an unguarded statement with the same indentation. This is the classic gotcha bestie.
In this 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". */When there's a mix of tabs and spaces, the warning uses the -ftabstop= option to figure out if the statements line up (defaulting to 8).
The warning is not issued for code involving multiline preprocessor logic, like this example:
if (flagA)
foo (0);
#if SOME_CONDITION_THAT_DOES_NOT_HOLD
if (flagB)
#endif
foo (1);The warning is also not issued after a #line directive, since that typically means autogenerated code, and you can't make any assumptions 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. Like, 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=2option controls warnings issued when the alias is more restrictive than the target, which could lead to incorrect code generation. Attributes considered includealloc_align,alloc_size,cold,const,hot,leaf,malloc,nonnull,noreturn,nothrow,pure,returns_nonnull, andreturns_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's missing one of the attributes.
template <class T>
T* __attribute__ ((malloc, alloc_size (1)))
allocate (size_t);
template <>
void* __attribute__ ((malloc)) // missing alloc_size
allocate<void> (size_t);-Wmissing-braces— warn if an aggregate or union initializer isn't fully bracketed. In the following example, the initializer forais not fully bracketed, but the one forbis fully bracketed. Brackets are the moment fr fr.
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 opions is disabled by default for C, C++, Objective-C and Objective-C++. For Fortran, it is partially enabled by default by warning for-Iand-J, only. 💅-Wno-missing-profile— this option controls warnings if feedback profiles are missing when using the-fprofile-useoption. It diagnoses those cases where a new function or a new file is added between compiling with-fprofile-generateand with-fprofile-use, without regenerating the profiles. In these cases, the profile feedback data files don't contain any profile feedback information for the newly added function or file respectively. Also, when profile count data (.gcda) files are removed, GCC can't 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-profilecan 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 from allocations 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 attributemalloc. Unless disabled by the-fno-builtinoption, the standard functionscalloc,malloc,realloc, andfree, as well as the corresponding forms of C++operator newandoperator deleteare implicitly associated as matching allocators and deallocators. In the following example,mydeallocis the deallocator for pointers returned frommyalloc. 🍓
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 likeif,else,for,switch, orwhile, in which only the first statement is actually guarded after the macro is expanded. the betrayal honestly.
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, like when there's an assignment in a context where a truth value is expected, or when operators are nested whose precedence people often get confused about. Precedence drama, we've all been there.
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<std::mutex> (mymutex);
// User meant std::unique_lock<std::mutex> lock (mymutex);
}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. Undefined behavior bestie, run.
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. 👑
okay so here's the tea on sequence points bestie 🫖 — the standard does not specify when, between two sequence points, changes to an object's value actually land. If your program's behavior depends on that timing? That's undefined behavior, no cap. The C and C++ standards say it like this: "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." And if you break those rules, literally any implementation can do literally anything. unpredictable. it's giving chaos 💀
Some little crimes that count as undefined behavior: a = a++;, a[n] = b[n++] and a[i++] = i;. This option won't catch every messy case, and it may occasionally throw a false positive at you, but overall it's been found pretty effective at sniffing out this kind of problem. ✨
The C++17 standard defines evaluation order in way more cases — specifically it requires the right-hand side of an assignment to be evaluated before the left-hand side, which means the examples above are no longer undefined. But this option still warns about them, to help you avoid writing code that's undefined in C and earlier revisions of C++. 🫶
The standard is worded confusingly, so there's some debate about what the sequence point rules even mean in the subtle cases. Links to discussions, including proposed formal definitions, live on the GCC readings page at https://gcc.gnu.org/readings.html.
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-type— warn whenever a function is defined with a return type that defaults toint. Also warn about anyreturnstatement with no return value in a function whose return type isn'tvoid(and falling off the end of the function body counts as returning without a value, bestie 😭).
For C only, 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, that latter case is accepted without a warning unless -Wpedantic is used. And 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? Undefined. periodt.
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 isn't 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 you left shift 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-overflowand is enabled by default in C99 and C++11 modes (and newer). This level does not warn about left-shifting 1 into the sign bit. (However, in C, that kind of overflow is still rejected in contexts where an integer constant expression is required.) No warning is emitted in C++20 mode (and newer), since signed left shifts always wrap. 🎀-Wshift-overflow=2— this level also warns about left-shifting 1 into the sign bit, unless C++14 mode (or newer) is active.
-Wswitch— warn whenever aswitchstatement has an index of enumerated type and lacks acasefor one or more of the named codes of that enumeration. (Adefaultlabel present? That prevents this warning.)caselabels outside the enumeration range also provoke warnings when this option is used (even if there's adefaultlabel). This warning is enabled by -Wall. 🔀-Wswitch-default— warn whenever aswitchstatement does not have adefaultcase.-Wswitch-enum— warn whenever aswitchstatement has an index of enumerated type and lacks acasefor one or more of the named codes of that enumeration.caselabels outside the enumeration range also provoke warnings when this option is used. The only difference between-Wswitchand this option: this one gives a warning about an omitted enumeration code even if there's adefaultlabel. 👑-Wno-switch-bool— do not warn when aswitchstatement 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 thanbool. 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 aswitchcase has a value that's outside its respective type range. This warning is enabled by default for C and C++ programs.-Wno-switch-unreachable— do not warn when aswitchstatement 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_nandand__sync_nand_and_fetchbuilt-in functions are used. These functions changed semantics in GCC 4.4. 😭-Wtrivial-auto-var-init— warn when-ftrivial-auto-var-initcannot initialize the automatic variable. A common situation is an automatic variable declared between the controlling expression and the first case label of aswitchstatement.-Wunused-but-set-parameter— warn whenever a function parameter is assigned to, but otherwise unused (aside from its declaration). To suppress this warning use theunusedattribute. This warning is also enabled by-Wunusedtogether 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 theunusedattribute. 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 theunusedattribute.-Wunused-local-typedefs(C, Objective-C, C++ and Objective-C++ only) — warn when a typedef locally defined in a function isn't used. This warning is enabled by -Wall.-Wunused-parameter— warn whenever a function parameter is unused aside from its declaration. To suppress this warning use theunusedattribute. 🫶-Wno-unused-result— do not warn if a caller of a function marked with attributewarn_unused_resultdoesn't 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=1for C, but not for C++. This warning is enabled by -Wall. To suppress this warning use theunusedattribute. 🍓-Wunused-const-variable,-Wunused-const-variable=n — warn whenever a constant static variable is unused aside from its declaration.-Wunused-const-variable=1is enabled by-Wunused-variablefor C, but not for C++. In C this declares variable storage, but in C++ it's not an error since const variables take the place of#defines. To suppress this warning use theunusedattribute.-Wunused-const-variable=1— this is the warning level enabled by-Wunused-variablefor C. It warns only 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=2— this warning level also warns for unused constant static variables in headers (excluding system headers). This is the warning level of-Wunused-const-variableand it must be explicitly requested, since in C++ this isn't an error and in C it might be harder to clean up all the headers included.
-Wunused-value— warn whenever a statement computes a result that's explicitly not used. To suppress this warning, cast the unused expression tovoid. This includes an expression-statement or the left-hand side of a comma expression that contains no side effects. For example, an expression such asx[i,j]causes a warning, whilex[(void)i,j]does not. This warning is enabled by -Wall. 💅-Wunused— all the above-Wunusedoptions combined. 🪄
In order to get a warning about an unused function parameter, you must either specify -Wextra -Wunused (heads up: -Wall implies -Wunused), or separately specify -Wunused-parameter.
-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-staticconstmember appears in a class without constructors. 😳
Like, also? Passing a pointer (or in C++, a reference) to an uninitialized object into a const-qualified argument of a built-in function that's known to read that object gets diagnosed by this warning too. (For ordinary functions, you get -Wmaybe-uninitialized instead.)
If what you actually want is a warning about code that uses a variable's own uninitialized value in its own initializer, bestie, that's a different option: -Winit-self.
These warnings fire for individual uninitialized elements of structure, union or array variables, and also for variables that are uninitialized as a whole. They do not fire for variables or elements declared volatile. And because these warnings depend on optimization, the exact variables or elements you get warned about depend on the precise optimization options and the version of GCC you're running. It's giving ✨situational✨.
Heads up: there may be no warning about a variable that's only used to compute a value that itself is never used — because data flow analysis may just delete those computations before the warnings even get 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__atomicBuiltins,__syncBuiltins, 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 thememory_orderenumeration. For example, since the__atomic_storeand__atomic_store_nbuilt-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. 🫠
Also — passing a pointer (or in C++, a reference) to an uninitialized object to a const-qualified function argument gets diagnosed by this warning too. (-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. iykyk 💅
These warnings are only possible in optimizing compilation, because otherwise GCC does not keep track of the state of variables.
They're made optional because GCC may not be able to determine when the code is correct in spite of appearing to have an error. Here's 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#pragmadirective 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-Wallcommand-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-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 beforemainis 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-aliasingis 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-aliasingis 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-Oworks.-Wstrict-aliasingis equivalent to-Wstrict-aliasing=3.- Level 1: Most aggressive, quick, least accurate. Possibly useful when higher levels do not warn but
-fstrict-aliasingstill 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.
- Level 1: Most aggressive, quick, least accurate. Possibly useful when higher levels do not warn but
-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 simplifiesx + 1 > xto1. This level of-Wstrict-overflowis 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, becauseabs (INT_MIN)overflows toINT_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 > 1is simplified tox > 0.-Wstrict-overflow=4— also warn about other simplifications not covered by the above cases. For example:(x * 10) / 5is simplified tox * 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 > yis simplified tox + 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 tostrcmpandstrncmpwhose 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 ofstrncmp). Such calls could be mistakes. For example, the call tostrcmpbelow is diagnosed because its result is necessarily non-zero irrespective of the contents of the arraya.
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— okay bestie this one is so useful. It yells at you for calls to string manipulation functions likememcpyandstrcpythat are determined to overflow the destination buffer. The optional argument is one greater than the type of Object Size Checking to perform to figure out the size of the destination. That argument only means anything for functions that work on character arrays — raw memory functions likememcpyalways use Object Size type-0, no negotiating. The option also warns for calls that specify a size in excess of the largest possible object, or at mostSIZE_MAX / 2bytes.
It does its best work with optimization on, but it can still catch a small subset of simple buffer overflows even without optimization, in calls to the GCC built-in functions like __builtin_memcpy that line up with the standard functions. Either way, it only warns about a subset of the buffer overflows that the corresponding overflow checking built-ins catch. Like, check it: the option warns for the strcpy call below because it copies at least 5 characters (the string "blue" including the terminating NUL) into a buffer of size 4. caught in 4k 💅
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
}Option -Wstringop-overflow=2 is enabled by default. ✨ default behavior, iconic ✨
-Wstringop-overflow,-Wstringop-overflow=1— uses type-zero Object Size Checking to figure out the sizes of destination objects. At this setting it 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 could be one of several objects, it assumes it's 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_SOURCEmacro is defined to a non-zero value.-Wstringop-overflow=2— uses type-one Object Size Checking to figure out the sizes of destination objects. At this setting it warns about overflows when writing to members of the largest complete objects whose exact size is known. But it does not warn for excessive writes to those same members of unknown objects referenced by pointers, because those pointers might be pointing to arrays with unknown numbers of elements. This is the default setting of the option.-Wstringop-overflow=3— uses type-two Object Size Checking to figure out the sizes of destination objects. At this setting it warns about overflowing the smallest object or data member. This is the most restrictive setting, and heads up: it may result in warnings for safe code. yes it's a lil dramatic like that-Wstringop-overflow=4— uses type-three Object Size Checking to figure out the sizes of destination objects. At this setting it warns about overflowing any data members, and when the destination is one of several objects it uses the size of the largest one to decide whether to warn. Just like-Wstringop-overflow=3, this setting may result in warnings for benign code. sobbing, it's trying its best 😭
-Wno-stringop-overread— warns for calls to string manipulation functions likememchr, orstrcpy, that are determined to read past the end of the source sequence.
Option -Wstringop-overread is enabled by default. on by default, slay
-Wno-stringop-truncation— do not warn for calls to bounded string manipulation functions likestrncat,strncpy, andstpncpythat might either truncate the copied string or leave the destination unchanged.
In this next example, the call to strncat specifies a bound that's less than the length of the source string. So the copy of the source gets truncated, and the call gets diagnosed. To dodge the warning, use bufsize - strlen (buf) - 1) as the bound. math girlies rise
void append (char *buf, size_t bufsize)
{
strncat (buf, ".txt", 3);
}Another example: the following call to strncpy copies 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 super common mistake, so the call gets diagnosed. To avoid the warning when the result isn't expected to be NUL-terminated, call memcpy instead. just use memcpy bestie
void copy (char *d, const char *s)
{
strncpy (d, s, strlen (s));
}And in this one, 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. So that call gets diagnosed too. 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);
...
}Now — if a character array is meant to store a sequence of bytes with no terminating NUL, you can annotate that array with the attribute nonstring to avoid this warning. But heads up: arrays like that are not suitable arguments to functions that expect NUL-terminated strings. To help catch accidental misuses of such arrays, GCC issues warnings unless it can prove the use is safe. safety check, no cap
-Wsuggest-attribute=[pure|const|noreturn|format|cold|malloc]— warns for cases where adding an attribute might be beneficial. The attributes currently supported are listed below. let's go down the list 📋-Wsuggest-attribute=pure,-Wsuggest-attribute=const,-Wsuggest-attribute=noreturn,-Wmissing-noreturn,-Wsuggest-attribute=malloc— warns about functions that might be candidates for the attributespure,const,noreturn, ormalloc. The compiler only warns for functions visible in other compilation units, or (forpureandconst) if it can't prove the function returns normally. A function returns normally if it doesn't contain an infinite loop or return abnormally by throwing, callingabort, or trapping. This analysis needs option-fipa-pure-const, which is enabled by default at-Oand higher. Higher optimization levels improve the accuracy of the analysis. more -O more accuracy, periodt-Wsuggest-attribute=format,-Wmissing-format-attribute— warns about function pointers that might be candidates forformatattributes. Note these are only possible candidates, not absolute ones. GCC guesses that function pointers withformatattributes that are used in assignment, initialization, parameter passing, or return statements should have a correspondingformatattribute 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 aformatattribute to avoid the warning.
GCC also warns about function definitions that might be candidates for format attributes. Again, only possible candidates. GCC guesses that format attributes might be appropriate for any function that calls a function like vprintf or vscanf, but that might not always be the case, and some functions for which format attributes are appropriate may not be detected. it's guessing, give it grace 🫶
-Wsuggest-attribute=cold— warns about functions that might be candidates for thecoldattribute. This is based on static detection, and generally only warns about functions which always lead to a call to anothercoldfunction — like wrappers of C++throwor fatal error reporting functions leading toabort. brrr cold 🥶
-Walloc-zero— warns about calls to allocation functions decorated with attributealloc_sizethat specify zero bytes, including calls to the built-in forms of the functionsaligned_alloc,alloca,calloc,malloc, andrealloc. Why care? Because the behavior of these functions when called with a zero size differs among implementations (and in the case ofrealloc, it's been deprecated), so relying on it may result in subtle portability bugs and should be avoided. avoid avoid avoid
-Walloc-size-larger-than=byte-size— warns about calls to functions decorated with attributealloc_sizethat try 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 ofPTRDIFF_MAXon the target.-Walloc-size-larger-than=PTRDIFF_MAXis enabled by default. Warnings controlled by the option can be disabled either by specifying a byte-size ofSIZE_MAXor more, or by-Wno-alloc-size-larger-than. turn it off two ways, we love options 💅
-Wno-alloc-size-larger-than— disables-Walloc-size-larger-than=warnings. The option is equivalent to-Walloc-size-larger-than=SIZE_MAXor larger.
-Walloca— this option warns on all uses ofallocain the source. every single one. no chill.
-Walloca-larger-than=byte-size— this option warns on calls toallocawith an integer argument whose value is either zero, or that isn't bounded by a controlling predicate that limits its value to at most byte-size. It also warns for calls toallocawhere 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);
}okay so in that lil example up there 👆 if you passed -Walloca-larger-than=1000 you would get NO WARNING because the call to alloca is known to be at most 1000 bytes. slay, bounded, well-behaved. but bestie… if you passed -Walloca-larger-than=500 instead? the compiler WOULD emit a warning. caught. 💅
Unbounded uses, on the other hand, are uses of alloca with no controlling predicate constraining its integer argument. like so:
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. no cap, that's the whole reason. 🚩
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. sneaky!! 😭
this option also warns when alloca is used in a loop. yeah. loops. we see you. 👀
-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. this option is equivalent to-Walloca-larger-than=SIZE_MAXor larger. basically: turn it all the way off, bestie 🙈-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. look:
void f (char c, int i)
{
c = c + i; // warns with -Wconversion
c = c + 1; // only warns with -Warith-conversion
}see how the second line is lowkey innocent but this flag is like "no. i'm watching." 😤✨
-Warray-bounds-Warray-bounds=n— warn about out of bounds subscripts or offsets into arrays. this warning is enabled by-Wall. it's more effective when-ftree-vrpis active (the default for-O2and above) but a subset of instances are issued even without optimization. it's giving "i catch what i can" 🕵️♀️-Warray-bounds=1— this is the default warning level of-Warray-boundsand is enabled by-Wall; higher levels are not, and must be explicitly requested. you gotta ask, bestie.-Warray-bounds=2— this warning level also warns about out of bounds accesses to trailing struct members of one-element array types and about the intermediate results of pointer arithmetic that may yield out of bounds values. heads up tho: this warning level may give a larger number of false positives and is deactivated by default. 💀
-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. yeah, comparing arrays like that is NOT the vibe. 🚫
-Warray-parameter-Warray-parameter=n— warn about redeclarations of functions involving arguments 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, 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. at level 1 the warning diagnoses inconsistencies involving array parameters declared using the T[static N] form.
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 with the keyword static specifies that the array argument must have at least four elements. messy!! 😭
void f (int[static 4]);
void f (int[]); // warning (inconsistent array form)
void g (void)
{
int *p = (int *)malloc (4);
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. iykyk 🌸
-Wattribute-alias=n-Wno-attribute-alias— warn about declarations using thealiasand similar attributes whose target is incompatible with the type of the alias.-Wattribute-alias=1— the default warning level of the-Wattribute-aliasoption 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-aliasalso 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-attributesoption, 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. your call bestie 💅
-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. 😳 spooky.
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. noted. 📝
-Wbool-compare— warn about boolean expression compared with an integer value different fromtrue/false. for instance, the following comparison is always false:
int n = 5;
...
if ((n > 1) == 2) { ... }this warning is enabled by -Wall. like bestie that will literally never be true 😭
-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;like why are you even asking the question bestie, both answers are the same 💀
-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) { ... }the second one can literally never run. not me spotting it. 🙈
-Wframe-address— warn when the__builtin_frame_addressor__builtin_return_addressis 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 aconst char *variable is passed to a function that takes achar *parameter. this option can be used to suppress such a warning. shhh, we're being quiet now 🤫-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 aconst int (*)[]variable is passed to a function that takes aint (*)[]parameter. this option can be used to suppress such a warning. same energy, arrays edition 🎀-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. two different flags, two different jobs, don't mix them up bestie 💅-Wno-int-conversion(C and Objective-C only) — bestie, this one says 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-castand-Wno-pointer-to-int-castmay be used. 💅-Wzero-length-bounds— warns 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. See
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
}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.
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. No cap. 💀-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-Wallin conjunction with this option does not warn about unknown pragmas in system headers---for that,-Wunknown-pragmasmust also be used. 🫶-Wtautological-compare— warns 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. 🌸
-Wtrampolines— warns 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— warns 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) — warns about certain constructs that behave differently in traditional and ISO C. Also warns 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-Wtraditionalwarns 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#pragmanot 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
Uinteger constant suffix, or theForLfloating-point constant suffixes. (Traditional C does support theLsuffix on integer constants.) Note, these suffixes appear in macros defined in the system headers of most modern systems, e.g. the_MIN/_MAXmacros in<limits.h>. 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
switchstatement has an operand of typelong. - A non-
staticfunction declaration follows astaticone. 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,
PARAMSandVPARAMS. 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) — warns 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) — warns 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. ✨-Wshadow— warns 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 astruct/class/enum. If this warning is enabled, it includes also all instances of local shadowing. This means that-Wno-shadow=localand-Wno-shadow=compatible-localare ignored when-Wshadowis 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— warns for any shadowing. Same as-Wshadow. 🌈-Wshadow=local— warns when a local variable shadows another local variable or parameter. 💜-Wshadow=compatible-local— warns 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— warns whenever an object is defined whose size exceedsbyte-size.-Wlarger-than=PTRDIFF_MAXis enabled by default. Warnings controlled by the option can be disabled either by specifyingbyte-sizeofSIZE_MAXor more or by-Wno-larger-than. 💅
Also warn for calls to bounded functions like memchr or strnlen that specify a bound bigger than the largest possible object, which is PTRDIFF_MAX bytes by default, bestie. These warnings can only be turned off with -Wno-larger-than, no other way, periodt.
-Wno-larger-than— kills off all-Wlarger-than=warnings. It's literally the same as saying-Wlarger-than=SIZE_MAXor bigger. 💅-Wframe-larger-than=byte-size— yells at you if a function's frame size goes over byte-size. Ngl the math it does to figure out the stack frame size is approximate and not conservative, so heads up: your actual requirements might be a bit bigger than byte-size even when you get zero warnings. 🙈 Also, anything allocated withalloca, variable-length arrays, or related constructs does not get counted by the compiler when it decides whether to warn.-Wframe-larger-than=PTRDIFF_MAXis on by default. You can silence these warnings either by giving a byte-size ofSIZE_MAXor more, or with-Wno-frame-larger-than.-Wno-frame-larger-than— turns off the-Wframe-larger-than=warnings. Equivalent to-Wframe-larger-than=SIZE_MAXor larger. 🫶-Wfree-nonheap-object— warns when you try to deallocate something that was either never allocated on the heap, or that you're reaching through a pointer that did not come back from a prior call to the matching allocation function. Like, check this: because the call tostpcpyreturns a pointer to the terminating nul character and not to the start of the object, thefreecall below gets diagnosed. 😭
void f (char *p)
{
p = stpcpy (p, "abc");
// ...
free (p); // warning
}-Wfree-nonheap-object is included in -Wall.
-Wstack-usage=byte-size— warns if a function's stack usage might exceed byte-size. This computation is conservative (unlike the frame one, slay). Any space allocated viaalloca, variable-length arrays, or related constructs is included by the compiler when it decides whether to warn you.
The message matches the output of -fstack-usage. ✨
- If the stack usage is fully static but over 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. The warnings this option controls can be disabled either by specifying byte-size of SIZE_MAX or more, or with -Wno-stack-usage.
-Wno-stack-usage— disables the-Wstack-usage=warnings. It's the same as-Wstack-usage=SIZE_MAXor larger. 💜-Wunsafe-loop-optimizations— warns if the loop can't be optimized because the compiler can't assume anything about the bounds of the loop indices. And with-funsafe-loop-optimizationsit warns if the compiler does make such assumptions. 🔥-Wno-pedantic-ms-format(MinGW targets only) — when you use this together with-Wformatand-pedanticwithout GNU extensions, it disables the warnings about non-ISOprintf/scanfformat width specifiersI32,I64, andIused on Windows targets, which depend on the MS runtime.-Wpointer-arith— warns about anything that depends on the "size of" a function type or ofvoid. GNU C gives these types a size of1, for convenience in calculations withvoid *pointers and pointers to functions. In C++, it also warns when an arithmetic operation involvesNULL. This warning is also enabled by-Wpedantic.-Wno-pointer-compare— don't warn if a pointer gets compared with a zero character constant. That usually means the pointer was meant to be dereferenced, iykyk. 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.
-Wtsan— warns about unsupported features in ThreadSanitizer.
ThreadSanitizer does not support std::atomic_thread_fence and can report false positives. 🫠
This warning is enabled by default.
-Wtype-limits— warns if a comparison is always true or always false because of the limited range of the data type, but it does not warn for constant expressions. Like, it'll warn if an unsigned variable gets compared against zero with<or>=. This warning is also enabled by-Wextra. 🌸-Wabsolute-value(C and Objective-C only) — warns for calls to standard functions that compute the absolute value of an argument when a more appropriate standard function exists. For example, callingabs(3.14)triggers the warning because the right function to call for the absolute value of a double argument isfabs. 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's also enabled by-Wextra.-Wcomment-Wcomments— warns whenever a comment-start sequence/*shows up inside a/*comment, or whenever a backslash-newline appears in a//comment. This warning is enabled by-Wall. ✨-Wtrigraphs— warns if any trigraphs are encountered that might change the meaning of the program. Trigraphs inside comments are not warned about, except those that would form escaped newlines.
This option is implied by -Wall. If -Wall isn't given, this option is still enabled unless trigraphs are enabled. To get trigraph conversion without warnings, but still get the other -Wall warnings, use -trigraphs -Wall -Wno-trigraphs. 🎀
-Wundef— warns if an undefined identifier is evaluated in an#ifdirective. Those identifiers get replaced with zero.-Wexpansion-to-defined— warns wheneverdefinedis encountered in the expansion of a macro (including when the macro is expanded by an#ifdirective). That usage isn't portable. This warning is also enabled by-Wpedanticand-Wextra. 💅-Wunused-macros— warns about macros defined in the main file that never get used. A macro counts as used if it's expanded or tested for existence at least once. The preprocessor also warns if the macro hasn't been used by the time it's redefined or undefined.
Built-in macros, macros defined on the command line, and macros defined in include files do not get warned about.
Note: if a macro is actually used, but only used inside skipped conditional blocks, then the preprocessor reports it as unused. To dodge the warning in that case, you might improve the scope of the macro's definition by, like, moving it into the first skipped block. Or you could provide a dummy use with something like:
#if defined the_macro_causing_the_warning
#endif-Wno-endif-labels— don't warn whenever an#elseor an#endifis followed by text. This sometimes happens in older programs with code of the form
#if FOO
...
#else FOO
...
#endif FOOThe second and third FOO should be in comments. This warning is on by default. 🙈
-Wbad-function-cast(C and Objective-C only) — warns when a function call gets cast to a non-matching type. Like, 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) — warns about features that aren't in ISO C90 but are in ISO C99. For instance, it warns about use of variable length arrays,long longtype,booltype, 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) — warns about features that aren't in ISO C99 but are in ISO C11. For instance, it warns about use of anonymous structures and unions,_Atomictype qualifier,_Thread_localstorage-class specifier,_Alignasspecifier,Alignofoperator,_Generickeyword, and so on. This option is independent of the standards mode. Warnings are disabled in the expression that follows__extension__.-Wc11-c2x-compat(C and Objective-C only) — warns about features that aren't in ISO C11 but are in ISO C2X. For instance, it warns 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__. ⭐-Wc++-compat(C and Objective-C only) — warns about ISO C constructs that fall outside the common subset of ISO C and ISO C++, e.g. a request for implicit conversion fromvoid *to a pointer to non-voidtype.-Wc++11-compat(C++ and Objective-C++ only) — warns about C++ constructs whose meaning differs between ISO C++ 1998 and ISO C++ 2011, e.g. identifiers that are fine in ISO C++ 1998 but are keywords in ISO C++ 2011. This warning turns on-Wnarrowingand is enabled by-Wall. 🔥-Wc++14-compat(C++ and Objective-C++ only) — warns 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) — warns 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) — warns 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) — don't warn about C++11 constructs in code that's being compiled using an older C++ standard. Even without this option, some C++11 constructs will only get diagnosed if-Wpedanticis used. 🫶-Wno-c++14-extensions(C++ and Objective-C++ only) — don't warn about C++14 constructs in code that's being compiled using an older C++ standard. Even without this option, some C++14 constructs will only get diagnosed if-Wpedanticis used.-Wno-c++17-extensions<span style="color:hotpink">(C++ and Objective-C++ only)</span> — okay so you're compiling with an older C++ standard but you've got C++17 stuff lurking in there. Normally GCC would be like "bestie…" but this flag says don't warn about it. 💅 ngl though, even without this option some C++17 constructs still get diagnosed — but only if you're running-Wpedantic.-Wno-c++20-extensions<span style="color:hotpink">(C++ and Objective-C++ only)</span> — same vibe, different standard. This one silences warnings about C++20 constructs when you're compiling under an older C++ standard. And fr fr, even without it, some C++20 stuff only gets diagnosed when-Wpedanticis on. 🙈-Wno-c++23-extensions<span style="color:hotpink">(C++ and Objective-C++ only)</span> — you guessed it, the C++23 edition of the same thing. No warnings for C++23 constructs in code being compiled with an older standard. And yes, same caveat: some C++23 constructs only get diagnosed with-Wpedantic. ✨
-Wcast-qual— this one's the type-qualifier snitch 💀. It warns whenever a pointer is cast in a way that removes a type qualifier from the target type — like, if aconst char *gets cast to a plain oldchar *, that's a warning.
It also warns when a cast introduces a type qualifier in an unsafe way. The iconic example: casting char ** to const char ** is unsafe. Watch:
/* 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';😭 the way that last line just writes straight into read-only memory. Not the vibe.
-Wcast-align— warns whenever a pointer cast increases the required alignment of the target. Example: casting achar *to anint *on machines where ints can only live at two- or four-byte boundaries. Basically "bestie, that address is not aligned, wyd". 🫶-Wcast-align=strict— same deal, but with no machine-specific mercy. Warns whenever a pointer cast increases the required alignment of the target — e.g. castingchar *toint *— regardless of the target machine. periodt. 👑-Wcast-function-type— warns when a function pointer gets cast to an incompatible function pointer. The deets, since you're gonna ask:- For casts involving function types with a variable argument list, only the types of the 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
intvs.longon ILP32 targets. - Type qualifiers are ignored too.
- The function type
void (*) (void)is special and matches everything — so you can use it to suppress this warning. 🪄 - For casts involving pointer-to-member types, this warns whenever the type cast is changing the pointer-to-member type.
This warning is enabled by -Wextra.
-Wwrite-strings— when compiling C, this gives string constants the typeconst char[length], so copying the address of one into a non-constchar *pointer produces a warning. These warnings help you catch, at compile time, code that might try to write into a string constant — but only if you've been very careful about usingconstin your declarations and prototypes. Otherwise it's just a nuisance. That's literally why-Walldoes not request these warnings, bestie. 🙈
When compiling C++, it warns about the deprecated conversion from string literals to char *. This warning is enabled by default for C++ programs.
-Wclobbered— warns for variables that might get changed bylongjmporvfork. 😭 the way your variable just… gets yeeted by a jump. Also enabled by-Wextra.-Wconversion— warns for implicit conversions that may alter a value. The roster:- conversions between real and integer — like
abs (x)whenxisdouble - conversions between signed and unsigned — like
unsigned ui = -1 - conversions to smaller types — like
sqrtf (M_PI)
- conversions between real and integer — like
It does not warn for explicit casts like abs ((int) x) and ui = (unsigned) -1, or if the value isn't changed by the conversion — like in abs (2.0). Warnings about conversions between signed and unsigned integers can be turned off with -Wno-sign-conversion.
For C++, it also warns 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.
One more: warnings about conversion from arithmetic on a small type back to that type are only given with -Warith-conversion.
-Wdangling-else— warns about constructions where it's confusing whichifanelsebranch belongs to. Here's the crime scene:
{
if (a)
if (b)
foo ();
else
bar ();
}In C/C++, every else branch belongs to the innermost possible if statement — which here is if (b). That is often not what the programmer expected, as the indentation in the example above gives away. 😭 When there's 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's no way the else can belong to the enclosing if. The glow-up looks like this:
{
if (a)
{
if (b)
foo ();
else
bar ();
}
}This warning is enabled by -Wparentheses.
-Wdangling-pointer-Wdangling-pointer=n— warns 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. It also warns 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. For example:
int f (int c1, int c2, x)
{
char *p = strchr ((char[]){ c1, c2 }, c3);
return p ? *p : 'x'; // warning: dangling pointer to a compound literal
}In the following function, the store of the address of the local variable x in the escaped pointer *p also triggers the warning:
void g (int **p)
{
int x = 7;
*p = &x; // warning: storing the address of a local variable in *p
}-Wdangling-pointer=2— at level 2, in addition to unconditional uses, the warning also diagnoses conditional uses of dangling pointers. For example, because the arrayain the following function is out of scope when the pointersthat was set to point to it is used, the warning triggers at this level:
void f (char *s)
{
if (!s)
{
char a[12] = "tmpname";
s = a;
}
strcat (s, ".tmp"); // warning: dangling pointer to a may be used
...
}-Wdangling-pointer=2 is included in -Wall.
-Wdate-time— warns when the macros__TIME__,__DATE__, or__TIMESTAMP__show up, because they might prevent bit-wise-identical reproducible compilations. 🕰️ not very main character of them tbh.-Wempty-body— warns if an empty body shows up in anif,else, ordo whilestatement. Also enabled by-Wextra. 👀-Wno-endif-labels— do not warn about stray tokens after#elseand#endif.-Wenum-compare— warns 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— warns when a value of enumerated type is implicitly converted to a different enumerated type. Enabled by-Wextrain C. 💅-Wjump-misses-init<span style="color:hotpink">(C, Objective-C only)</span> — warns if agotostatement or aswitchstatement jumps forward across the initialization of a variable, or jumps backward to a label after the variable has been initialized. Only warns about variables that are initialized when they're declared. And heads up: 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— warns when a comparison between signed and unsigned values could produce an incorrect result when the signed value gets converted to unsigned. In C++ this warning is also enabled by-Wall. In C it's also enabled by-Wextra. 🫠 the classicintvssize_theartbreak.-Wsign-conversion— warns 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.-Wfloat-conversion— warns 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. Also enabled by-Wconversion. 💀 bye bye precision.-Wno-scalar-storage-order— do not warn on suspicious constructs involving reverse scalar storage order.-Wsizeof-array-div— warns about divisions of twosizeofoperators when the first one is applied to an array and the divisor does not equal the size of the array element. In that case the computation will not yield the number of elements in the array, which is likely what the user intended. It warns about stuff like this:
int fn ()
{
int arr[10];
return sizeof (arr) / sizeof (short);
}This warning is enabled by -Wall. 😭 that's not the element count bestie, that's a math crime.
-Wsizeof-pointer-div— bestie, this one catches the sneakiest little math crime 💅 when you divide twosizeofexpressions, taking pointer size ÷ element size. That's the classic way to get array length, but on an actual pointer it does NOT work, so the warning fires. It'll clock something likesizeof (ptr) / sizeof (ptr[0])whenptris a pointer, not an array. Enabled by-Wall. 👑-Wsizeof-pointer-memaccess— sus length arguments to certain string and memory built-in functions, flagged whenever the argument usessizeof. For example it pops off formemset (ptr, 0, sizeof (ptr));whenptris a pointer rather than an array, and it even suggests a possible fix — or formemcpy (&foo, ptr, sizeof (&foo));. It ALSO side-eyes calls to bounded string copy functions likestrncatorstrncpythat pass asizeofexpression of the source array as the bound. Like, in this function thestrncatcall uses the source string's size as the bound, which is almost certainly a mistake, so the call gets 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 thesizeofoperator gets applied to a parameter declared as an array in a function definition. This warning is enabled by default for C and C++ programs. 🫶-Wmemset-elt-size— warn about sus calls to thememsetbuilt-in when 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. Translation: you forgot to multiply by the element size, bestie 😭. Enabled by-Wall.-Wmemset-transposed-args— warn about susmemsetbuilt-in calls where the second argument isn't zero and the third argument is zero. Like the callmemset (buf, sizeof buf, 0)gets diagnosed, because you clearly meantmemset (buf, 0, sizeof buf). The diagnostic only drops if the third argument is a literal zero — if it's an expression folded to zero, or a cast of zero to some type, it's way less likely the args were accidentally swapped, so no warning. Enabled by-Wall. 🌸-Waddress— warn about sus uses of address expressions 👀. These include comparing the address of a function or a declared object to the null pointer constant, like:
void f (void);
void g (void)
{
if (!func) // warning: expression evaluates to false
abort ();
}comparisons of a pointer to a string literal, like:
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, like:
void f (const int *p, int i)
{
return p + i == NULL;
}Such uses typically mean a programmer error: the address of most functions and objects necessarily evaluates to true (the exception is weak symbols), so using them in a conditional might mean 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 inptr_t or uinptr_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 a packed member of a struct or union is taken, which usually results in an unaligned pointer value. This is enabled by default. 🙈-Wlogical-op— warn about sus uses of logical operators in expressions ✨. This includes using logical operators in contexts where a bit-wise operator is likely 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 whole purpose is detecting sus code like:
int a;
...
if (!a > 1) { ... }You can suppress the warning by wrapping the LHS in 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— warn if, in a loop with a constant number of iterations, the compiler detects undefined behavior in some statement during one or more of the iterations. 🌀-Wno-attributes— do NOT warn if an unexpected__attribute__is used, like unrecognized attributes, function attributes applied to variables, etc. This does not stop errors for incorrect use of supported attributes.
Also, using -Wno-attributes=, it's possible to suppress warnings about unknown scoped attributes (in C++11 and C2X). For example, -Wno-attributes=vendor::attr disables the warning about this declaration:
[[vendor::attr]] void f();You can also disable the warning about all attributes in a namespace using -Wno-attributes=vendor::, which prevents the 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 doesn't include a prototype is called with arguments whose promoted types don't match those the function expects. When-Wextrais specified, it ALSO warns when a built-in function that takes arguments is declared without a prototype. The-Wbuiltin-declaration-mismatchwarning is enabled by default. To dodge the warning, include the appropriate header to bring the prototypes of built-in functions into scope.
For example, the call to memset below gets 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 too:
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__. 🌈-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 likestaticaren't 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's a previous prototype. A definition using()is NOT considered an old-style definition in C2X mode, because it's equivalent to(void)in that case, but it IS considered an old-style definition for older standards. 🦋-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. ✨
-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 don't 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 conflicting with an earlier declaration. Use-Wmissing-declarationsto detect missing declarations in C++. 💖-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 aren't declared in header files. In C, no warnings are issued for functions with previous non-prototype declarations; use-Wmissing-prototypesto 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, this code causes such a warning, becausex.his implicitly zero:
struct s { int f, g, h; };
struct s x = { 3, 4 };This option does NOT warn about designated initializers, so this modification doesn't trigger a warning:
struct s { int f, g, h; };
struct s x = { .f = 3, .g = 4 };In C this option doesn't 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, bestie, like:
struct s { int f, g, h; };
s x = { };This warning is included in -Wextra. To get the other -Wextra warnings without this one, you use -Wextra -Wno-missing-field-initializers. 💅
-Wno-missing-requires— so by default the compiler yells at you when a concept-id shows up as a C++20 simple-requirement:
bool satisfied = requires { C<T> };Here satisfied will be true if C<T> is a valid expression — which it literally always is, for all T. Presumably you meant to write:
bool satisfied = requires { requires C<T> };so that 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 thetemplatekeyword if the parent object is dependent and the member being named is a template. No cap.
template <class X>
void DoStuff (X x)
{
x.template DoSomeOtherStuff<X>(); // Good.
x.DoMoreStuff<X>(); // Warning, x is dependent.
}In rare cases you can get false positives. To silence those, 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 these are giving "oops I made a typo in my code" energy, because they have implementation-defined values and should not be used in portable code. 🫠-Wnormalized=[none|id|nfc|nfkc]— okay so in ISO C and ISO C++, two identifiers are different if they're different sequences of characters. But sometimes, when characters outside the basic ASCII set get involved, you can have two different character sequences that look exactly the same. To avoid confusion, the ISO 10646 standard sets out normalization rules which, when applied, make sure two sequences that look the same get turned into the same sequence. GCC can warn you if you're using identifiers that haven't been normalized — and 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, once turned into NFC, are not allowed in identifiers. Like, 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's hoped that future versions of the standards involved will fix 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're 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's been placed in a superscript. ISO 10646 defines the NFKC normalization scheme to convert all of these into a standard form as well, and GCC warns if your code isn't in NFKC when 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 it is not the default. But it 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 the"warning"attribute. By default this warning is enabled.-Wno-attribute-warningcan be used to disable the warning, or-Wno-error=attribute-warningcan be used to disable the error when compiled with the-Werrorflag. 🎀-Wno-deprecated— do not warn about usage of deprecated features.-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.-Wopenmp-simd— warn if the vectorizer cost model overrides the OpenMP simd directive set by the user. The-fsimd-cost-model=unlimitedoption 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 the 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 variablef.xinstruct baris misaligned even thoughstruct bardoes 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 thepackedattribute on bit-fields of typechar. 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 fieldaandbin 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 onstruct 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's 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 arestrict-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 thestrcpyfunction below attempts to truncate the string by replacing its initial characters with the last four. But because the call writes the terminating NUL intoa[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 it works best at -O2 and above. It is included in -Wall. 💅
-Wnested-externs(C and Objective-C only) — warn if anexterndeclaration 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 whole variety of heuristics to determine whether or not to inline a function. For example, it 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++17std::hardware_destructive_interference_sizewithout 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<int> one;
alignas(std::hardware_destructive_interference_size) std::atomic<int> 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. 🌈
okay so 💖 we're still in the warnings zone bestie, and this bit is all about interference sizes and then a whole new subsection about static analysis. buckle up 🎀
--param destructive-interference-size,--param constructive-interference-size— by default these two are set based on whatever-mtuneoption you're currently using, typically the L1 cache line size for that specific target CPU, and sometimes a range if you're tuning for a generic target. so like. every translation unit that depends on ABI compatibility for using these variables must be compiled with the same-mtune(or-mcpu). no mixing, bestie. 💅
if ABI stability matters to you — like if the use is in a header for a library — you should probably just not use the hardware interference size variables at all. alternatively, you can force a particular value with --param. your call 🫶
and if you're confident your use of the variable doesn't affect ABI outside a single build of your project, you can turn the warning off with -Wno-interference-size. 🦋
-Wint-in-bool-context— warns when you're doing something suspicious with integer values where booleans were expected. like conditional expressions (?:) using non-boolean integer constants in boolean context, e.g.if (a <= b ? 2 : 3). or left shifting signed integers in boolean context, likefor (a = 0; 1 << a; a++);. same energy for all kinds of multiplications no matter the data type. this warning is enabled by-Wall. ✨-Wno-int-to-pointer-cast— suppresses 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-castis enabled by default.-Wno-pointer-to-int-cast(C and Objective-C only) — suppresses warnings from casts from a pointer to an integer type of a different size.-Winvalid-pch— warns if a precompiled header is found in the search path but cannot be used. 💀-Wlong-long— warns if thelong longtype is used. enabled by either-Wpedanticor-Wtraditionalin ISO C90 and C++98 modes. to shut it up, use-Wno-long-long.-Wvariadic-macros— warns if variadic macros are used in ISO C90 mode, or if the GNU alternate syntax is used in ISO C99 mode. enabled by either-Wpedanticor-Wtraditional. to inhibit the warning messages, use-Wno-variadic-macros.-Wno-varargs— don't warn on questionable usage of the macros used to handle variable arguments likeva_start. these warnings are enabled by default.-Wvector-operation-performance— warns if a vector operation isn't implemented via the SIMD capabilities of the architecture. mainly useful for performance tuning. a vector operation can be implemented "piecewise", meaning the scalar operation is performed on every vector element; "in parallel", meaning the vector operation is implemented using scalars of wider type, which normally is more performance efficient; and "as a single scalar", meaning the vector fits into a scalar type. 🧚-Wvla— warns if a variable-length array is used in the code.-Wno-vlaprevents the-Wpedanticwarning of the variable-length array.-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-sizeworks, but with variable-length arrays. 🌸
ngl, 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_MAXor larger.-Wvla-parameter— warns about redeclarations of functions involving arguments of Variable Length Array types of inconsistent kinds or forms, and enables 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]);
void f (int, int[]); // warning: argument 2 previously declared as a VLA
void g (int n)
{
if (n > 4)
return;
int a[n];
f (sizeof a, a); // warning: access to a by f may be out of bounds
...
}-Wvla-parameter is included in -Wall. the -Warray-parameter option triggers warnings for similar problems involving ordinary array arguments.
-Wvolatile-register-var— warns 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. 💅-Wdisabled-optimization— warns if a requested optimization pass is disabled. this warning doesn't generally mean there's anything wrong with your code — it just means GCC's optimizers can't handle the code effectively. often the problem is 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) — warns for pointer argument passing or assignment with different signedness. this option is only supported for C and Objective-C. it is implied by-Walland by-Wpedantic, which can be disabled with-Wno-pointer-sign.-Wstack-protector— this option is only active when-fstack-protectoris active. it warns about functions that aren't protected against stack smashing. protect your stack bestie 🫶-Woverlength-strings— warns about string constants longer than the "minimum maximum" length specified in the C standard. modern compilers generally allow string constants much longer than the standard's minimum limit, but very portable programs should avoid 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 doesn't specify a normative minimum maximum, so we don't 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) — issues a warning for any floating constant that doesn't have a suffix. when used together with-Wsystem-headersit warns about such constants in system header files. this can be useful when preparing code to use with theFLOAT_CONST_DECIMAL64pragma from the decimal floating-point extension to C99. ✨-Wno-lto-type-mismatch— during link-time optimization, don't warn about type mismatches in global declarations from different compilation units. requires-fltoto be enabled. enabled by default.-Wno-designated-init(C and Objective-C only) — suppresses warnings when a positional initializer is used to initialize a structure that has been marked with thedesignated_initattribute.
✨ Options That Control Static Analysis ✨
-fanalyzer— this option enables a static analysis of program flow that 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. like, genuinely. 😭
enabling this option effectively enables the following warnings:
-Wanalyzer-double-fclose -Wanalyzer-double-free
-Wanalyzer-exposure-through-output-file -Wanalyzer-file-leak
-Wanalyzer-free-of-non-heap -Wanalyzer-malloc-leak
-Wanalyzer-mismatching-deallocation
-Wanalyzer-possible-null-argument
-Wanalyzer-possible-null-dereference -Wanalyzer-null-argument
-Wanalyzer-null-dereference -Wanalyzer-shift-count-negative
-Wanalyzer-shift-count-overflow -Wanalyzer-stale-setjmp-buffer
-Wanalyzer-tainted-allocation-size -Wanalyzer-tainted-array-index
-Wanalyzer-tainted-divisor -Wanalyzer-tainted-offset
-Wanalyzer-tainted-size -Wanalyzer-unsafe-call-within-signal-handler
-Wanalyzer-use-after-free -Wanalyzer-use-of-uninitialized-value
-Wanalyzer-use-of-pointer-in-stale-stack-frame
-Wanalyzer-write-to-const -Wanalyzer-write-to-string-literalthis option is only available if GCC was configured with analyzer support enabled. 🫶
-Wanalyzer-too-complex— if-fanalyzeris enabled, the analyzer uses various heuristics to try 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-double-fclose— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-double-fcloseto disable it.
this diagnostic warns for paths through the code in which a FILE * can have fclose called on it more than once. 😭
-Wno-analyzer-double-free— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-double-freeto 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. 💅
-Wno-analyzer-exposure-through-output-file— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-exposure-through-output-fileto disable it.
and that's where this part cuts off bestie, the analyzer warnings keep going in the next one 💖
bestie listen, this one is sending a warning about a security-sensitive value getting written straight into an output file. Like… imagine writing a password into a log file. Yeah. That. 😭🔓
-Wno-analyzer-file-leak— this one needs-fanalyzer, since that's what turns it on; use-Wno-analyzer-file-leakto turn it off.
It warns about paths through your code where a <stdio.h> FILE * stream object gets leaked. The stream is just… gone. Never closed. Living its truth. 💀
-Wno-analyzer-free-of-non-heap— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-free-of-non-heapto disable it.
Warns on paths where free gets called on a non-heap pointer — think an on-stack buffer, or a global. Not it, bestie. 🙈
-Wno-analyzer-malloc-leak— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-malloc-leakto disable it.
Warns on paths where a pointer allocated via an allocator gets leaked: either malloc, or a function marked with attribute malloc. Memory said "I'm out" and nobody said goodbye. 🫠
-Wno-analyzer-mismatching-deallocation— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-mismatching-deallocationto disable it.
Warns on paths where the wrong deallocation function is called on a pointer value, based on which function allocated it. It'll call out mismatches between free, scalar delete and vector delete[], plus those marked as allocator/deallocator pairs using attribute malloc. Basically: you two are not compatible, this is not the pairing. 💅
-Wno-analyzer-possible-null-argument— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-possible-null-argumentto disable it.
Warns on paths where a possibly-NULL value gets passed to a function argument marked with __attribute__((nonnull)) as requiring a non-NULL value.
-Wno-analyzer-possible-null-dereference— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-possible-null-dereferenceto disable it.
Warns on paths where a possibly-NULL value gets dereferenced. We do not do that here. ✋
-Wno-analyzer-null-argument— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-null-argumentto disable it.
Warns on paths where a value known to be NULL gets passed to a function argument marked with __attribute__((nonnull)) as requiring a non-NULL value.
-Wno-analyzer-null-dereference— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-null-dereferenceto disable it.
Warns on paths where a value known to be NULL gets dereferenced. Same energy as the last one, but with the confidence of a known-NULL. 💀
-Wno-analyzer-shift-count-negative— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-shift-count-negativeto disable it.
Warns on paths where a shift is attempted with a negative count. It's analogous to the -Wshift-count-negative diagnostic implemented in the C/C++ front ends, but it's implemented based on analyzing interprocedural paths, rather than merely parsing the syntax tree. Heads up though: 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-overflowto disable it.
Warns on paths where a shift is attempted with a count greater than or equal to the precision of the operand's type. Analogous to the -Wshift-count-overflow diagnostic implemented in the C/C++ front ends, but implemented based on analyzing interprocedural paths, rather than merely parsing the syntax tree. And same disclaimer as above: the analyzer doesn't 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-bufferto disable it.
Warns on paths where longjmp is called to rewind to a jmp_buf relating to a setjmp call in a function that has returned.
Here's the lore: when setjmp is called on a jmp_buf to record a rewind location, it records the stack frame. That stack frame becomes invalid when the function containing the setjmp call returns. Trying to rewind to it via longjmp would reference a stack frame that no longer exists, and likely lead to a crash (or worse). 😭 not the stale frame bestie.
-Wno-analyzer-tainted-allocation-size— this warning requires both-fanalyzerand-fanalyzer-checker=taintto enable it; use-Wno-analyzer-tainted-allocation-sizeto disable it.
Warns on paths where a value that could be under an attacker's control is used as the size of an allocation without being sanitized, so an attacker could inject an excessively large allocation and potentially cause a denial of service attack.
See @url{https://cwe.mitre.org/data/definitions/789.html, CWE-789: Memory Allocation with Excessive Size Value}.
-Wno-analyzer-tainted-array-index— this warning requires both-fanalyzerand-fanalyzer-checker=taintto enable it; use-Wno-analyzer-tainted-array-indexto disable it.
Warns on paths where a value that could be under an attacker's control is used as the index of an array access without being sanitized, so an attacker could inject an out-of-bounds access. 🚨
See @url{https://cwe.mitre.org/data/definitions/129.html, CWE-129: Improper Validation of Array Index}.
-Wno-analyzer-tainted-divisor— this warning requires both-fanalyzerand-fanalyzer-checker=taintto enable it; use-Wno-analyzer-tainted-divisorto disable it.
Warns on paths where 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 an attacker could inject a division-by-zero. 🍓
-Wno-analyzer-tainted-offset— this warning requires both-fanalyzerand-fanalyzer-checker=taintto enable it; use-Wno-analyzer-tainted-offsetto disable it.
Warns on paths where a value that could be under an attacker's control is used as a pointer offset without being sanitized, so an attacker could inject an out-of-bounds access.
See @url{https://cwe.mitre.org/data/definitions/823.html, CWE-823: Use of Out-of-range Pointer Offset}.
-Wno-analyzer-tainted-size— this warning requires both-fanalyzerand-fanalyzer-checker=taintto enable it; use-Wno-analyzer-tainted-sizeto disable it.
Warns on paths where 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 an attacker could inject an out-of-bounds access. 🌈
-Wno-analyzer-unsafe-call-within-signal-handler— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-unsafe-call-within-signal-handlerto disable it.
Warns on paths where a function known to be async-signal-unsafe (such as fprintf) is called from a signal handler. Nope nope nope. 🙅♀️
-Wno-analyzer-use-after-free— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-use-after-freeto disable it.
Warns on paths where a pointer is used after a deallocator is called on it: either free, or a deallocator referenced by attribute malloc. She's gone, stop calling her. 💀
-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-frameto disable it.
Warns on paths where a pointer is dereferenced that points to a variable in a stale stack frame.
-Wno-analyzer-write-to-const— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-write-to-constto disable it.
Warns on paths where the analyzer detects an attempt to write through a pointer to a const object. But real talk: 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-literalto disable it.
Warns on paths where the analyzer detects an attempt to write through a pointer to a string literal. Again: 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-valueto disable it.
Warns on paths where an uninitialized value is used. (Yes, this is the one everybody meets first. Hi. Welcome. 🎀)
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, and --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.
Some checkers are disabled by default (even with -fanalyzer), such as the taint checker that implements -Wanalyzer-tainted-array-index, and this option is required to enable them. iykyk 💅
-fno-analyzer-feasibility— this option is intended for analyzer developers.
By default the analyzer is like "okay bestie, before I accuse you of anything, let me make sure there's actually a feasible control flow path for every diagnostic I'm about to emit" — meaning the conditions it's holding onto aren't mutually exclusive. Any diagnostic where no feasible path exists gets rejected. You can suppress this filtering with -fno-analyzer-feasibility, which is for debugging issues in this code. 🙈
-fanalyzer-fine-grained— this one is for analyzer developers, iykyk.
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, every single statement gets its own edge. Not sharing, periodt.
-fanalyzer-show-duplicate-count— also for analyzer developers: if multiple diagnostics have been detected as being duplicates of each other, it emits a note when reporting the best diagnostic, telling you the number of additional diagnostics that got suppressed by the deduplication logic. 💅-fno-analyzer-state-merge— for analyzer developers.
By default the analyzer tries 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— for analyzer developers.
By default the analyzer tries 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.
-fanalyzer-transitivity— this option enables transitivity of constraints within the analyzer.-fanalyzer-verbose-edges— for analyzer developers. It enables more verbose, lower-level detail in the descriptions of control flow within diagnostic paths. 🌀-fanalyzer-verbose-state-changes— 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-freediagnostic, both calls tofreewill 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 tofile.analyzer.txt. This option is overridden by-fdump-analyzer-stderr.-fdump-analyzer-stderr— dump internal details about what the analyzer is doing tostderr. This option overrides-fdump-analyzer.-fdump-analyzer-callgraph— dump a representation of the call graph suitable for viewing with GraphViz tofile.callgraph.dot.-fdump-analyzer-exploded-graph— dump a representation of the "exploded graph" suitable for viewing with GraphViz tofile.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" tofile.eg.txt.-fdump-analyzer-exploded-nodes-3— dump a textual representation of the "exploded graph" to one dump file per node, tofile.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 tofile.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 formfile.*.fg.dotandfile.*.tg.dot.-fdump-analyzer-json— dump a compressed JSON representation of analyzer internals tofile.analyzer.json.gz. The precise format is subject to change. (so don't get attached, bestie 🙈)-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 tofile.state-purge.dot.-fdump-analyzer-supergraph— dump representations of the "supergraph" suitable for viewing with GraphViz tofile.supergraph.dotand tofile.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. 👑
🐛 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-Ogwith-g. With no-Ooption at all, some compiler passes that collect information useful for debugging do not run at all, so that-Ogmay 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 -gstabs+, -gstabs, -gxcoff+, -gxcoff, or -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/Mac OS X, 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.-gctf-gctflevel— request CTF debug information and use level to specify how much CTF debug information should be produced. If-gctfis 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.
-gstabs— produce debugging information in stabs format (if that is supported), without GDB extensions. This is the format used by DBX on most BSD systems. On MIPS, Alpha and System V Release 4 systems this option produces stabs debugging output that is not understood by DBX. On System V Release 4 systems this option requires the GNU assembler.-gstabs+— produce debugging information in stabs format (if that is supported), using GNU extensions understood only by the GNU debugger (GDB). The use of these extensions is likely to make other debuggers crash or refuse to read the program. ⚠️-gxcoff— produce debugging information in XCOFF format (if that is supported). This is the format used by the DBX debugger on IBM RS/6000 systems.-gxcoff+— bestie, this makes debugging info in XCOFF format (if your target even supports it 😭), but with GNU extensions that only the GNU debugger (GDB) understands. Heads up tho: using these extensions is likely to make other debuggers crash or straight-up refuse to read your program, and can make assemblers that aren't the GNU assembler (GAS) fail with an error. It's giving exclusive, and not in a cute way 💅-gvms— produce debugging info in Alpha/VMS debug format (if that's supported, which, maybe?). This is the format DEBUG uses on Alpha/VMS systems. Old-school energy fr fr 🦋-glevel-ggdblevel-gstabslevel-gxcofflevel-gvmslevel— ask for debugging info and uselevelto say how much you want. The default level is2, periodt.
Level 0 produces no debug information at all. So like, -g0 literally negates -g. It's the anti--g 💀
Level 1 gives you minimal info — just enough to make backtraces in parts of the program you don't plan to debug. That means descriptions of functions and external variables, plus line number tables, but no info about local variables. Bare minimum vibes.
Level 3 includes extra info, like all the macro definitions chillin' in the program. Some debuggers support macro expansion when you use -g3. Fancy ✨
If you use multiple -g options, with or without level numbers, the last one wins — that's the one that's effective.
Ngl,-gdwarfdoes not accept a concatenated debug level, to avoid confusion with-gdwarf-level. Instead, use an additional-gleveloption to change the debug level for DWARF. Don't try to be clever, it won't work 🫶
-fno-eliminate-unused-debug-symbols— by default, no debug information is produced for symbols that aren't actually used. Flip this on if you want debug info for all the symbols. Hoarder mode, but for debug symbols 🎀-femit-class-debug-always— instead of emitting debugging info for a C++ class in only one object file, this emits it in all object files using the class. Only use this with debuggers that can't handle the way GCC normally emits debug info for classes, because using it increases the size of your debugging information by as much as a factor of two. Big yikes, so only if you must 💅-fno-merge-debug-strings— tells the linker to not merge together strings in the debugging info that are identical across different object files. Merging isn't supported by all assemblers or linkers. When it is supported, merging shrinks the debug info in the output file but costs you more link processing time. Merging is enabled by default, bestie ✨-fdebug-prefix-map=old=new— when compiling files that live in directoryold, record debugging info describing them as if they lived in directorynewinstead. This can swap a build-time path for an install-time path in the debug info. You can also use it to turn an absolute path into a relative one by settingnewto.. That gives you more reproducible builds (location independent, slay), but it may require an extra command to tell GDB where to find the source files. See also-ffile-prefix-map. 🦋-fvar-tracking— run the variable tracking pass. It computes where variables are stored at each position in the code, and then better debugging information gets generated (if the debug info format supports this info). Iconic.
It's enabled by default when compiling with optimization (-Os, -O, -O2, ...), debugging information (-g), and the debug info format supports it. The trio ✨
-fvar-tracking-assignments— annotate assignments to user variables early in compilation and try to carry those annotations allll the way to the end, to improve debug info while optimizing. Using-gdwarf-4alongside it is recommended, no cap.
It can be enabled even if var-tracking is disabled — in that case the 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 info is enabled, split as much of it as possible into a separate output file with the.dwoextension. This lets the build system avoid linking files with debug information. To be useful, this option requires a debugger capable of reading.dwofiles. It's giving "organized into folders" energy 🎀-gdwarf32-gdwarf64— if DWARF debugging info is enabled,-gdwarf32picks the 32-bit DWARF format and-gdwarf64picks the 64-bit DWARF format. The default is target specific — on most targets it's-gdwarf32though. The 32-bit DWARF format is smaller, but it can't support more than2GiBof debug information in any of the DWARF debug info sections. The 64-bit DWARF format allows larger debug info, but might not be well supported by all consumers yet. Trade-offs, babes 💅-gdescribe-dies— adds description attributes to some DWARF DIEs that have no name attribute, like artificial variables, external references and call site parameter DIEs. Detail-oriented queen 👑-gpubnames— generate DWARF.debug_pubnamesand.debug_pubtypessections. That's it, that's the option ✨-ggnu-pubnames— generate.debug_pubnamesand.debug_pubtypessections in a format suitable for conversion into a GDB index. This option is only useful with a linker that can produce GDB index version7. Niche but valid 🦋-fdebug-types-section— when using DWARF Version4or higher, type DIEs can be put into their own.debug_typessection instead of being part of the.debug_infosection. It's more efficient to put them in a separate comdat section, since the linker can then remove duplicates. But not all DWARF consumers support.debug_typessections yet, and on some objects.debug_typesproduces larger instead of smaller debugging information. The duality of man 😭-grecord-gcc-switches-gno-record-gcc-switches— this switch appends the command-line options used to invoke the compiler that may affect code generation to theDW_AT_producerattribute in DWARF debugging information. The options are concatenated with spaces separating them from each other and from the compiler version. It's enabled by default. See also-frecord-gcc-switchesfor another way of storing compiler options into the object file. Receipts, always keep the receipts 🧾-gstrict-dwarf— disallow using extensions from a later DWARF standard version than the one selected with-gdwarf-version. On most targets, using non-conflicting DWARF extensions from later standard versions is allowed. Strict mode: activated 💅-gno-strict-dwarf— allow using extensions of a later DWARF standard version than selected with-gdwarf-version. Free spirit energy 🌈-gas-loc-support— inform the compiler that the assembler supports.locdirectives. 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 the ones the compiler generates itself. Slay ✨
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. DIY mode 🛠️-gas-locview-support— inform the compiler that the assembler supports "view" assignment and reset assertion checking in.locdirectives.
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-viewsare explicitly requested.-gcolumn-info-gno-column-info— emit location column info into DWARF debugging information, rather than just file and line. This option is enabled by default. Precision, bestie 💅-gstatement-frontiers-gno-statement-frontiers— this 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 ofis_stmtmarkers 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. Map markers for your code, iconic 🗺️-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 lets debug info consumers 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 info consumers that aren't aware of these augmentations — but those consumers won't derive any benefit from them either. FOMO-proof 💖
This is enabled by default when outputting DWARF 2 debug information at the normal level, as long as there's assembler support, -fvar-tracking-assignments is enabled, and -gstrict-dwarf is not. When assembler support isn't available, this may still be enabled, but it will force GCC to output internal line number tables — and if -ginternal-reset-location-views isn't enabled, that will most certainly lead to silently mismatching location views. Silent bugs are the worst kind, babes 😭
There's a proposed representation for view numbers that is not backward compatible with the location list format introduced in DWARF 5, and it can be enabled with -gvariable-location-views=incompat5. This option may be removed in the future — it's only provided as a reference implementation of the proposed representation. Debug info consumers aren't expected to support this extended format, and they'd be rendered unable to decode location lists using it. So like, use at your own risk, fr fr 💀
-ginternal-reset-location-views,-gno-internal-reset-location-views— okay so this one tries to figure out which location views can just be yeeted from the location view lists 💅. The catch? It needs the compiler to have very accurate insn length estimates, and bestie, that's not always the vibe. If you're using an assembler that doesn't support location view lists, it might silently generate wrong view lists 😭. The GNU assembler will call that out as a"view number mismatch". This is only enabled on ports that define a reliable estimation function. Periodt.-ginline-points,-gno-inline-points— generate extended debug info for inlined functions ✨. Location view tracking markers get inserted at inlined entry points so address and view numbers can be computed and output in debug info. It can be enabled independently of location views — in that case the view numbers won't be output — but it can only be enabled along with statement frontiers, and it's only on by default if location views are enabled. iykyk.-gz[=type]— produce compressed debug sections in DWARF format, if that's supported 🎀. Iftypeisn't given, the default type depends on what the assembler and linker can do.typemay be one of:none— don't compress debug sectionszlib— use zlib compression in ELF gABI formatzlib-gnu— use zlib compression in traditional GNU format
If the linker doesn't support writing compressed debug sections, the option is rejected 🚫. Otherwise, if the assembler doesn't support them, -gz is silently ignored when producing object files.
-femit-struct-debug-baseonly— emit debug info for struct-like types only when the base name of the compilation source file matches the base name of the 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 info for struct-like types only when the base name of the compilation source file matches the base name of the 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 exactly which struct-like types the compiler generates debug info for 🧐. The whole intent here is to reduce duplicate struct debug info 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 serve for most needs.
A specification has the syntax:
[dir:|ind:][ord:|gen:](any|sys|base|none)The optional first word limits the spec to structs that are used directly (dir:) or used indirectly (ind:). A struct type is used directly when it's 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 spec 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 info. 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. But sometimes it's useful to have GCC emit debugging info for all types declared in a compilation unit, regardless of whether they're actually used in that compilation unit — like, say, if in the debugger you want to cast a value to a type that isn't actually used in your program (but is declared) 💖. More often, however, this results in a significant amount of wasted space. So, tradeoffs, bestie.
🌈 Options That Control Optimization 🌈
These options control various sorts of optimizations. Buckle up 🎀
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. Chef's kiss, very predictable ✨
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 classic tradeoff 💅
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. Very slay of you to check, honestly 🔍
-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 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 -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-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-cheapPlease note the warning under-fgcseabout invoking-O2on programs that use computed gotos. Don't say I didn't tell you 💀
-O3— Optimize yet more 👑.-O3turns on all optimizations specified by-O2and 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. The chill, no-thoughts-just-vibes setting 🦋-Os— Optimize for size.-Osenables all-O2optimizations except those that often increase code size:
-falign-functions -falign-jumps -falign-labels -falign-loops
-fprefetch-loop-arrays -freorder-blocks-algorithm=stcIt 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. Small but mighty, she ate 💅
-Ofast— bestie, this one just does not care about strict standards compliance 😤. It flips on every-O3optimization, and then some extras that are genuinely not valid for all standard-compliant programs. It turns on-ffast-math,-fallow-store-data-races, the Fortran-specific-fstack-arrays(unless you specified-fmax-stack-var-size, okay?), and-fno-protect-parens. It also turns off-fsemantic-interposition. It's giving "rules are a suggestion" energy 💅.-Og— optimizing the debugging experience, periodt ✨. This should be your go-to optimization level for the standard edit-compile-debug cycle — it keeps a reasonable level of optimization while still compiling fast and debugging well. It's a better pick than-O0for producing debuggable code, because some compiler passes that collect debug info are literally disabled at-O0💀.
Like -O0, -Og completely disables a bunch of optimization passes, so the individual options that control them have no effect. Otherwise, -Og enables all -O1 optimization flags except the ones that might mess 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— optimizing aggressively for size instead of speed 🐜. Heads up: this can actually increase the number of instructions executed, if those instructions need fewer bytes to encode.-Ozbehaves a lot like-Os, including enabling most-O2optimizations.
Okay so if you pass multiple -O options, with or without level numbers, the last one wins. That's the one that's effective. No cap 🫶.
Options of the form -fflag are the machine-independent flags, bestie. Most flags have both a positive and a negative form — the negative form of -ffoo is -fno-foo. In the table below, only one of the forms is listed — the one you'd typically use. You can figure out the other one by either removing no- or adding it. Easy math 💅.
The following options control specific optimizations. They're either activated by the -O options or they're related to ones that are. Use these flags in the rare cases when you want to "fine-tune" which optimizations get performed 🌸.
-fno-defer-pop— for machines that have to pop arguments after a function call, this always pops the arguments as soon as each function returns. At levels-O1and higher,-fdefer-popis the default; that lets the compiler let arguments accumulate on the stack across 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 whether 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.
-ffp-contract=style— okay, here's the breakdown:-ffp-contract=offdisables floating-point expression contraction.-ffp-contract=fastenables floating-point expression contraction, like forming fused multiply-add operations if the target has native support for them.-ffp-contract=onenables floating-point expression contraction if the language standard allows it. This is currently not implemented and is treated equal to-ffp-contract=off.
The default is -ffp-contract=fast.
-fomit-frame-pointer— omit the frame pointer in functions that don't need one 🙈. This skips the instructions that save, set up and restore the frame pointer; on many targets it also frees up an extra register. Slay.
On some targets this flag has no effect, because the standard calling sequence always uses a frame pointer, so it can't be omitted. Sorry bestie.
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-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,strchrorstrcpy) and their_FORTIFY_SOURCEcounterparts into faster alternatives ⚡.
Enabled at levels -O2, -O3.
-fno-inline— do not expand any functions inline, apart from those marked with thealways_inlineattribute. 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 the expected function call code (so the overall size of the program gets smaller) 🐜. The compiler heuristically decides which functions are simple enough to be worth integrating 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 only has any effect when inlining itself is turned on by the-finline-functionsor-finline-small-functionsoptions.
Enabled at levels -O2, -O3, -Os.
-finline-functions— consider all functions for inlining, even if they aren't declared inline. The compiler heuristically decides which functions are worth integrating 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 allstaticfunctions called once for inlining into their caller, even if they aren't markedinline. 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 byalways_inlineand functions whose body seems smaller than the function call overhead, early — before doing-fprofile-generateinstrumentation and the real inlining pass 🌈. Doing it this way makes profiling significantly cheaper and usually makes inlining faster on programs that have 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 with 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 lets you coarse-control that 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 you can specify individually using --param name=value. The -finline-limit=n option sets some of these parameters like this:
max-inline-insns-singleis set ton/2.max-inline-insns-autois set ton/2.
See below for 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 a 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 another. So don't @ me 😭.
-fno-keep-inline-dllexport— this is a more fine-grained version of-fkeep-inline-functions, which applies only to functions that are declared using thedllexportattribute or declspec.-fkeep-inline-functions— in C, emitstaticfunctions that are declaredinlineinto the object file, even if the function has been inlined into all of its callers. This switch does not affect functions using theextern inlineextension in GNU C90. In C++, emit any and all inline functions into the object file 💖.-fkeep-static-functions— emitstaticfunctions into the object file, even if the function is never used 🫶.-fkeep-static-consts— emit variables declaredstatic constwhen 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 whether 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. On top of -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 ⚠️. So like, be careful bestie.
-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 only effective with-fmodulo-schedenabled 🎀.-fno-branch-count-reg— bestie, this one says "no thank you" to the optimization pass that goes hunting for chances to use "decrement and branch" instructions on a count register instead of the whole boring sequence of decrementing a register, comparing it to zero, and then branching based on what came back. This is only meaningful on architectures that actually support those instructions, which include x86, PowerPC, IA-64 and S/390. And ngl, a lil' heads up: the-fno-branch-count-regoption does not remove the decrement and branch instructions from the generated instruction stream that got introduced by other optimization passes. Those stay. 💅
The default is -fbranch-count-reg at -O1 and higher, except for -Og.
-fno-function-cse— do NOT put function addresses in registers. Every instruction that calls a constant function has to contain the function's address explicitly. Simple as that. ✨
This option gives you less efficient code, but some strange hacks that mess with the assembler output may get confused by the optimizations that happen when this option is not used.
The default is -ffunction-cse.
-fno-zero-initialized-in-bss— okay so if the target supports a BSS section, GCC by default drops variables that are initialized to zero into BSS. This can save space in the resulting code, slay.
This option turns that behavior off, because some programs explicitly rely on variables going to the data section — like, so that the resulting executable can find the beginning of that section and/or make assumptions based on that. ikyk. 🫶
The default is -fzero-initialized-in-bss.
-fthread-jumps— perform optimizations that check whether a jump branches to a location where another comparison subsumed by the first is found. If it is, the first branch gets 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. iconic. 🔥
Enabled at levels -O1, -O2, -O3, -Os.
-fsplit-wide-types— when you're using a type that occupies multiple registers, likelong longon a 32-bit system, this splits the registers apart and allocates them independently. This normally generates better code for those types, but it may make debugging more difficult. (the way that trade-off just lives there 😭)
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-typesis 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 isn't reached by any other path. For example, when CSE encounters anifstatement with anelseclause, CSE follows the jump when the condition tested is false. 🦋
Enabled at levels -O2, -O3, -Os.
-fcse-skip-blocks— similar to-fcse-follow-jumps, but this makes CSE follow jumps that conditionally skip over blocks. When CSE encounters a simpleifstatement with no else clause,-fcse-skip-blocksmakes CSE follow the jump around the body of theif. 💫
Enabled at levels -O2, -O3, -Os.
-frerun-cse-after-loop— re-run common subexpression elimination after loop optimizations have been performed. round two, bestie ✨
Enabled at levels -O2, -O3, -Os.
-fgcse— perform a global common subexpression elimination pass. This pass also does global constant and copy propagation. 🎀
Note: when compiling a program that uses 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-lmis enabled, global common subexpression elimination attempts to move loads that are only killed by stores into themselves. This lets a loop containing a load/store sequence 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-smis enabled, a store motion pass runs after global common subexpression elimination. This pass attempts to move stores out of loops. When used together 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-lasis 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-reloadis 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, like by 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. (no promises, bestie 😭)
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 don't have instructions to support this. Enabled by default at-O1and higher on architectures that support this. 💅-fdce— perform dead code elimination (DCE) on RTL. Enabled by default at-O1and higher. 🗑️-fdse— perform dead store elimination (DSE) on RTL. Enabled by default at-O1and 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's 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— okay so 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 callsoperator deleteafterwards. 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. On top of that, 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 though 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 Nios II ELF, it defaults to off. On AVR, CR16, 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 into 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 into 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 size1, 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're converted back into their 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. spend that compute, bestie 💸
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 and x86 at levels -O2, -O3, -Os.
-fno-lifetime-dse— okay so 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 (as it should ✨). But 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 that optimization. To preserve stores before the constructor starts (like, e.g. because youroperator newclears 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, and-flifetime-dse=0is equivalent to-fno-lifetime-dse. periodt 💅-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 bepriority, which specifies Chow's priority coloring, orCB, which specifies Chaitin-Briggs coloring. Chaitin-Briggs coloring is not implemented for all architectures 😭 but for those targets that do support it, it's 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 these besties: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, ...). she's the main character fr 🌸one— use all functions as a single region. This typically results in the smallest code size, and is enabled by default for-Osor-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. yeah she's thicc now 🙈-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 ✨
Enabled at levels -O2, -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-insnsor at-O2or higher 🚫-fno-sched-spec— disable speculative motion of non-load instructions, which is normally enabled when scheduling before register allocation, i.e. with-fschedule-insnsor at-O2or 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-insnsor at-O2or 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-insnsor at-O2or 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-insnsor at-O2or 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-insnsmeans that no insns are moved prematurely,-fsched-stalled-insns=0means there is no limit on how many queued insns can be moved prematurely. And-fsched-stalled-insnswithout 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-insnsis used.-fno-sched-stalled-insns-depis equivalent to-fsched-stalled-insns-dep=0. And-fsched-stalled-insns-depwithout 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-insnsor-fschedule-insns2or at-O2or 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-insnsor-fschedule-insns2or at-O2or 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-insnsor-fschedule-insns2or at-O2or 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-insnsor-fschedule-insns2or at-O2or 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-insnsor-fschedule-insns2or at-O2or 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-insnsor-fschedule-insns2or at-O2or 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-schedulingor-fselective-scheduling2is 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-pipeliningis turned on 🫶-fsemantic-interposition— okay so some object formats (looking at you, ELF ✨) let the dynamic linker interpose symbols, and that changes everything about what the compiler is allowed to assume. For symbols exported from the DSO, the compiler can't do interprocedural propagation, inlining and other optimizations, because the function or variable in question might just... change on it. And yeah, this feature is genuinely useful — think rewriting memory allocation functions with a debugging implementation — but it is expensive in terms of code quality, no cap. With-fno-semantic-interpositionthe 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 declaredinline(where it is never allowed for interposition to change semantics) and for symbols explicitly declared weak. 💅-fshrink-wrap— emit function prologues only before the parts of the function that actually need it, instead of dumping them at the top like it's giving "one size fits all". This flag is enabled by default at-Oand higher. 🫶-fshrink-wrap-separate— shrink-wrap the separate parts of the prologue and epilogue separately, so those parts only get executed when they're needed. This option is on by default, but it does absolutely nothing unless-fshrink-wrapis also turned on and the target supports it. 😭-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. That allocation only happens when it seems to result in better code, bestie. ✨ 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-O1and 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 thelarge-stack-frameparameter to100and thelarge-stack-frame-growthparameter to400. 🌸-ftree-reassoc— perform reassociation on trees. This flag is enabled by default at-O1and 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-O2and higher. 👑-ftree-pre— perform partial redundancy elimination (PRE) on trees. This flag is enabled by default at-O2and-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-O1and 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-O1and higher. 💜-ftree-phiprop— perform hoisting of loads from conditional pointers on trees. This pass is enabled by default at-O1and 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-O2and higher. 🧚-ftree-copy-prop— perform copy propagation on trees. This pass eliminates unnecessary copy operations. This flag is enabled by default at-O1and higher. 💅-fipa-pure-const— discover which functions are pure or constant. Enabled by default at-O1and higher. ⭐-fipa-reference— discover which static variables do not escape the compilation unit. Enabled by default at-O1and higher. 🌸-fipa-reference-addressable— discover read-only, write-only and non-addressable static variables. Enabled by default at-O1and higher. ✨-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. Heads up bestie: 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 ascold,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-O1and 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-O1and 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,-Osand-O3. It is also enabled by-fprofile-useand-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-useand-fauto-profile. 💀-fipa-bit-cp— when enabled, perform interprocedural bitwise constant propagation. This flag is enabled by default at-O2and by-fprofile-useand-fauto-profile. It requires that-fipa-cpis enabled. ✨-fipa-vrp— when enabled, perform interprocedural propagation of value ranges. This flag is enabled by default at-O2. It requires that-fipa-cpis 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-O2and-Os. 💜-flive-patching=level— control GCC's optimizations to produce output suitable for live-patching. 🩹
Okay so here's the whole vibe. 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-patchingis not supported with link-time optimization (-flto). 😭
-fisolate-erroneous-paths-dereference— bestie this one is on watch duty 👀 it sniffs out paths that trigger erroneous or undefined behavior because of dereferencing a null pointer. Then it isolates those paths from the main control flow and turns the statement with the erroneous or undefined behavior into a trap. It's enabled by default at-O2and higher, and it depends on-fdelete-null-pointer-checksalso being enabled.-fisolate-erroneous-paths-attribute— same energy, different trigger 💅 it detects paths that trigger erroneous or undefined behavior because a null value is being used in a way that areturns_nonnullornonnullattribute forbids. Isolates those paths from the main control flow and turns the statement with the erroneous or undefined behavior into a trap. This is not currently enabled, but it may be enabled by-O2in the future. iykyk.-ftree-sink— perform forward store motion on trees. Enabled by default at-O1and higher. ✨-ftree-bit-ccp— perform sparse conditional bit constant propagation on trees, and propagate pointer alignment information too. This pass only operates on local scalar variables, and it's enabled by default at-O1and higher, except for-Og. It requires that-ftree-ccpis 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-O1and higher. periodt.-fssa-backprop— propagate info about uses of a value up the definition chain so the definitions get simplified. For example, this pass strips sign operations if the sign of a value never matters. Enabled by default at-O1and higher. 🌸-fssa-phiopt— perform pattern matching on SSA PHI nodes to optimize conditional code. Enabled by default at-O1and higher, except for-Og. slay.-ftree-switch-conversion— perform conversion of simple initializations in a switch into initializations from a scalar array. Enabled by default at-O2and higher. ✨-ftree-tail-merge— look for identical code sequences. When it finds them, it replaces one with a jump to the other. This optimization is known as tail merging or cross jumping. Enabled by default at-O2and higher. The compilation time in this pass can be limited using themax-tail-merge-comparisonsparameter and themax-tail-merge-iterationsparameter. 👑-ftree-dce— perform dead code elimination (DCE) on trees. Enabled by default at-O1and higher. 💀 (the good kind of dead code, bestie)-ftree-builtin-call-dce— perform conditional dead code elimination (DCE) for calls to built-in functions that may seterrnobut are otherwise free of side effects. Enabled by default at-O2and higher if-Osis not also specified.-ffinite-loops— assume that a loop with an exit will eventually take that exit and not loop forever. This lets the compiler remove loops that otherwise have no side-effects, since eventual endless looping isn't being counted as a side effect.
This option is enabled by default at -O2 for C++ with -std=c++11 or higher. 🌀
-ftree-dominator-opts— perform a whole lil variety pack of simple scalar cleanups (constant/copy propagation, redundancy elimination, range propagation and expression simplification) based on a dominator tree traversal. It also does jump threading (to reduce jumps to jumps). Enabled by default at-O1and higher. main character energy fr fr.-ftree-dse— perform dead store elimination (DSE) on trees. A dead store is a store into a memory location that's later overwritten by another store without any intervening loads — in that case the earlier store can just be deleted. Enabled by default at-O1and higher. 🧚-ftree-ch— perform loop header copying on trees. This is beneficial since it increases the effectiveness of code motion optimizations, and it also saves one jump. Enabled by default at-O1and higher. It is not enabled for-Os, since it usually increases code size.-ftree-loop-optimize— perform loop optimizations on trees. Enabled by default at-O1and 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-islto 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-identitywe 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.
Heads up bestie: this option is experimental. 💅
-floop-parallelize-all— use the Graphite data dependence analysis to identify loops that can be parallelized. It parallelizes all the loops that can be analyzed to not contain loop carried dependences, without checking that it's profitable to parallelize the loops. no vibe check, just sending it 😭-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 into 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. 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
ENDDOis transformed to
DO I = 1, N
A(I) = B(I) + C
ENDDO
DO I = 1, N
D(I) = E(I) * F
ENDDOThis 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-O2and higher, and by-fprofile-useand-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
ENDDOis transformed to
DO I = 1, N
A(I) = 0
ENDDO
DO I = 1, N
B(I) = A(I) + I
ENDDOand the initialization loop is transformed into a call to memset zero. This flag is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile. 🌸
-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-useand-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-loopsit 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 where figuring out the 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's sufficiently cheap. This reduces data dependencies and may allow further simplifications. Enabled by default at-O1and higher. 💖-fivopts— perform induction variable optimizations (strength reduction, induction variable merging and induction variable elimination) on trees. slay. 👑- *
-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— does function-local points-to analysis on trees. Bestie, this one is on by default at-O1and up, except for-Og(it sits that one out, as it should 💅).-ftree-sra— scalar replacement of aggregates, aka the glow-up pass ✨. It swaps structure references out for scalars so your structs don't get committed to memory way too early. Enabled by default at-O1and higher, except for-Og.-fstore-merging— merges narrow stores that land on consecutive memory addresses. It takes contiguous stores of immediate values that are narrower than a word and squishes them into fewer, wider stores so the instruction count drops. Default-on at-O2and higher, plus-Os.-ftree-ter— temporary expression replacement during the SSA->normal phase 🌀. Single-use/single-def temporaries get replaced at their use location with the expression that defined them. That gives you non-GIMPLE code, but it hands the expanders much more complex trees to work with, which means better RTL generation. Enabled by default at-O1and higher.-ftree-slsr— straight-line strength reduction on trees 💪. It spots related expressions that involve multiplications and swaps them for cheaper calculations when it can. Enabled by default at-O1and higher.-ftree-vectorize— vectorization on trees 🔥. This flag turns on-ftree-loop-vectorizeand-ftree-slp-vectorizeif you haven't explicitly set those yourself.-ftree-loop-vectorize— loop vectorization on trees. Enabled by default at-O2, and also switched on by-ftree-vectorize,-fprofile-use, and-fauto-profile.-ftree-slp-vectorize— basic block vectorization on trees. Enabled by default at-O2, and also switched on by-ftree-vectorize,-fprofile-use, and-fauto-profile.-ftrivial-auto-var-init=choice — initializes automatic variables with either a pattern or with zeroes 😌. Why would you want that? To boost security and predictability of a program by preventing uninitialized memory disclosure and use. Important caveat though, fr fr: GCC still treats an automatic variable with no explicit initializer as uninitialized, so-Wuninitializedand-Wanalyzer-use-of-uninitialized-valuewill still throw warning messages at you for those variables. With this option GCC will also zero out any padding of automatic variables that have structure or union types. But heads up: the current implementation cannot initialize automatic variables that are declared between the controlling expression and the firstcaseof a"switch"statement. Use-Wtrivial-auto-var-initto report all such cases.
The three values of choice are:
uninitialized— doesn't initialize any automatic variables at all. This is C and C++'s default, no cap.pattern— initializes automatic variables with values that will likely turn logic bugs into crashes down the line, are easily recognized in a crash dump, and are not values programmers can rely on for useful program semantics. The current value is a byte-repeatable pattern with byte"0xFE". The values used for pattern initialization might change in the future 👀.zero— initializes automatic variables with zeroes.
The default is uninitialized.
You can control this behavior for a specific variable by using the variable attribute "uninitialized".
-fvect-cost-model=model — changes up the cost model used for vectorization. The model argument should be one ofunlimited,dynamic,cheaporvery-cheap. Withunlimited, the vectorized code-path is just assumed to be profitable 🤑. Withdynamic, a runtime check guards the vectorized code-path so it only kicks in for iteration counts that will likely run faster than the original scalar loop. Thecheapmodel disables vectorization of loops where doing so would be cost prohibitive — like when you'd need runtime checks for data dependence or alignment — but otherwise it's equal to thedynamicmodel. Thevery-cheapmodel only allows vectorization if the vector code would entirely replace the scalar code being vectorized. Like, if each iteration of a vectorized loop could only handle exactly four iterations of the scalar loop, thenvery-cheapwould only allow vectorization if the scalar iteration count is known to be a multiple of four.
The default cost model depends on your other optimization flags and is either dynamic or cheap.
-fsimd-cost-model=model — changes the cost model used for vectorization of loops marked with the OpenMPsimddirective. The model argument should be one ofunlimited,dynamic,cheap. All values of model mean the same thing as described in-fvect-cost-model, and by default a cost model defined with-fvect-cost-modelis used.-ftree-vrp— performs Value Range Propagation on trees 🌈. It's similar to the constant propagation pass, except instead of values it propagates ranges of values. That lets the optimizers delete unnecessary range checks like array bound checks and null pointer checks. Enabled by default at-O2and higher. Null pointer check elimination is only done if-fdelete-null-pointer-checksis enabled.-fsplit-paths— splits paths leading to loop backedges 🛤️. This can improve dead code elimination and common subexpression elimination. Enabled by default at-O3and above.-fsplit-ivs-in-unroller— enables expression of the values of induction variables in later iterations of the unrolled loop using the value from the first iteration. This breaks long dependency chains, which improves the efficiency of the scheduling passes. A combo of-fweband CSE is often enough to get the same effect — however, that is not reliable in cases where the loop body is more complicated than a single basic block. It also doesn't work at all on some architectures because of restrictions in the CSE pass. This optimization is enabled by default.-fvariable-expansion-in-unroller— with this option the compiler makes 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— inlines parts of functions. This option only has any effect when inlining itself is turned on by the-finline-functionsor-finline-small-functionsoptions. Enabled at levels-O2,-O3,-Os.-fpredictive-commoning— performs predictive commoning optimization, i.e. reusing computations (especially memory loads and stores) that were performed in previous iterations of loops ♻️. This option is enabled at level-O3. It's also enabled by-fprofile-useand-fauto-profile.-fprefetch-loop-arrays— if the target machine supports it, generates instructions to prefetch memory so loops that access large arrays run better 🔮. Fair warning bestie: this option may generate better or worse code — results are highly dependent on the structure of the loops in your source. Disabled at level-Os.-fno-printf-return-value— do not substitute constants for the known return value of formatted output functions such as"sprintf","snprintf","vsprintf", and"vsnprintf"(but not"printf"of"fprintf"). This transformation lets GCC optimize or even eliminate branches based on the known return value of those functions when they're 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-valueis 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-peepholeand-fno-peephole2is in how they're implemented in the compiler; some targets use one, some use the other, a few use both.-fpeepholeis enabled by default.-fpeephole2is enabled at levels-O2,-O3,-Os.-fno-guess-branch-probability— do not guess branch probabilities using heuristics 🤔. GCC uses heuristics to guess branch probabilities when they aren't 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 get complex, and in some cases it may be useful to disable the heuristics so the effects of"__builtin_expect"are easier to understand. It's also possible to specify the expected probability of the expression with the"__builtin_expect_with_probability"built-in function. The default is-fguess-branch-probabilityat levels-O,-O2,-O3,-Os.-freorder-blocks— reorders basic blocks in the compiled function to reduce the number of taken branches and improve code locality 🧩. Enabled at levels-O1,-O2,-O3,-Os.-freorder-blocks-algorithm=algorithm — uses the specified algorithm for basic block reordering. The algorithm argument can besimple, which does not increase code size (except sometimes because of secondary effects like alignment), orstc, the "software trace cache" algorithm, which tries to put all often executed code together and minimizes the number of branches executed by making extra copies of code. The default issimpleat levels-O1,-Os, andstcat levels-O2,-O3.-freorder-blocks-and-partition— okay so besides just reordering basic blocks inside a compiled function to cut down on taken branches, this one also splits your hot 🔥 and cold 🥶 basic blocks into separate sections of the assembly and.ofiles, so your paging and cache locality stop being a whole mess.
The way this optimization just turns itself off in certain situations, no cap: it's off when there's exception handling or unwind tables around (on targets using setjump/longjump or a target-specific scheme), off for linkonce sections, off for functions with a user-defined section attribute, and off on any architecture that doesn't support named sections. And when -fsplit-stack is in play, it's not enabled by default (to avoid linker errors), but you can still turn it on explicitly if your linker actually works.
Enabled for x86 at levels -O2, -O3, -Os.
-freorder-functions— reorders functions in the object file so code locality gets better 💅. How? It uses special subsections:".text.hot"for the functions that run most often, and".text.unlikely"for the ones that basically never run. The linker does the actual reordering, so the object file format has to support named sections, and the linker has to place them in a reasonable way.
Heads up: this option isn't effective unless you either hand over profile feedback (peep -fprofile-arcs for the deets) or manually annotate functions with "hot" or "cold" attributes.
Enabled at levels -O2, -O3, -Os.
-fstrict-aliasing— lets the compiler assume the strictest aliasing rules that apply to whatever language you're compiling. For C (and C++), this turns on optimizations based on the type of expressions. Specifically: an object of one type is assumed to never live 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;
}Reading from a different union member than the one you most recently wrote to (this is called "type-punning") is super common. Even with -fstrict-aliasing, type-punning is allowed, as long as 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, accessing by taking the address, casting the resulting pointer and dereferencing the result has undefined behavior, even if the cast uses a union type, like:
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 the rules of-fstrict-aliasingget applied across function boundaries. Quick note though: if multiple functions get inlined into a single function, those memory accesses are no longer considered to be crossing a function boundary.
The -fipa-strict-aliasing option is enabled by default and only actually does anything 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— aligns the start of functions to the next power-of-two greater than or equal to n, skipping up to m-1 bytes. This guarantees at least the first m bytes of the function can be fetched by the CPU without crossing an n-byte alignment boundary. ✨
If m isn't 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, and -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 lets you specify a secondary alignment: -falign-functions=64:7:32:3 aligns to the next 64-byte boundary if that can be done by skipping 6 bytes or less, otherwise it aligns to the next 32-byte boundary if that can be done by skipping 2 bytes or less. If m2 isn't specified, it defaults to n2.
Some assemblers only support this flag when n is a power of two; in that case, it gets rounded up.
-fno-align-functions and -falign-functions=1 are equivalent and mean functions are not aligned.
If n isn't specified or is zero, a machine-dependent default is used. The maximum allowed n option value is 65536.
Enabled at levels -O2, -O3.
-flimit-function-alignment— with this on, 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— aligns all branch targets to a power-of-two boundary 🎯.
Parameters of this option work just like the -falign-functions ones. -fno-align-labels and -falign-labels=1 are equivalent and mean labels are not aligned.
If -falign-loops or -falign-jumps are applicable and are greater than this value, then their values get used instead.
If n isn't specified or is zero, a machine-dependent default is used, 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— aligns loops to a power-of-two boundary. If the loops run many times, that makes up for any execution of the dummy padding instructions 🌀.
If -falign-labels is greater than this value, then its value gets used instead.
Parameters of this option work just like the -falign-functions ones. -fno-align-loops and -falign-loops=1 are equivalent and mean loops are not aligned. The maximum allowed n option value is 65536.
If n isn't specified or is zero, a machine-dependent default is used.
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— aligns 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 to be executed 👑
If -falign-labels is greater than this value, then its value gets used instead.
Parameters of this option work just like the -falign-functions ones. -fno-align-jumps and -falign-jumps=1 are equivalent and mean loops are not aligned.
If n isn't specified or is zero, a machine-dependent default is used. The maximum allowed n option value is 65536.
Enabled at levels -O2, -O3.
-fno-allocation-dce— don't remove unused C++ allocations during dead code elimination. bye bye cleanup 🫶-fallow-store-data-races— allows the compiler to do optimizations that might introduce new data races on stores, without proving that the variable can't be concurrently accessed by other threads. Doesn't affect optimization of local data. It's safe to use this option if you know that global data won't 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 around for compatibility reasons.-funit-at-a-timehas no effect, while-fno-unit-at-a-timeimplies-fno-toplevel-reorderand-fno-section-anchors.
Enabled by default.
-fno-toplevel-reorder— don't reorder top-level functions, variables, and"asm"statements. Output them in the same order 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's 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.
-fweb— constructs webs as commonly used for register allocation purposes and assigns each web its own individual pseudo register. This lets the register allocation pass operate on pseudos directly, but it also strengthens several other optimization passes, like 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— assumes 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 get optimized more aggressively by interprocedural optimizers 💪.
This option should not be used in combination with -flto. Instead, relying on a linker plugin should provide safer and more precise information.
-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 get read from these ELF sections and instantiated as if they had been part of the same translation unit. main character energy fr fr 🌈
okay so bestie, to actually use the link-time optimizer you gotta drop -flto and your optimization options at compile time and again during the final link. the recommended vibe is: compile every file that's in the same link with the same options, and repeat those options at link time too. like this:
gcc -c -O2 -flto foo.c
gcc -c -O2 -flto bar.c
gcc -o myprog -flto -O2 foo.o bar.oso the first two invocations save a bytecode representation of GIMPLE into special ELF sections inside foo.o and bar.o. the final invocation reads that GIMPLE bytecode back out of foo.o and bar.o, merges the two files into a single internal image, and compiles the result like normal. and because both foo.o and bar.o get merged into one image, literally all the interprocedural analyses and optimizations in GCC now work across both files like they were one file the whole time. meaning — for example — the inliner can inline functions in bar.o into functions in foo.o, and vice-versa. iconic behavior, no cap. 💅
another (way simpler) way to turn on link-time optimization is:
gcc -o myprog -flto -O2 foo.c bar.cthe 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 (like, tattoo it on your brain 🧠) is that to enable link-time optimizations you need to use the GCC driver to do the link step. GCC automatically does link-time optimization if any of the objects involved were compiled with the -flto command-line option. and you can always override that automatic decision by passing -fno-lto to the link command. periodt.
to make whole program optimization actually effective, you need 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 the linker supports it, the linker plugin (see -fuse-linker-plugin) passes information to the compiler about used and externally visible symbols. when the linker plugin isn't available, you should use -fwhole-program to let the compiler make those 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 object files with LTO information can be linked as normal object files; if -fno-lto is passed to the linker, no interprocedural optimizations get 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. so you can mix and match object files and libraries with GIMPLE bytecodes and final object code. GCC automatically picks 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 tries to infer link-time options from the settings used to compile the input files.
if you don't specify an optimization level option -O at link time, then GCC uses the highest optimization level used when compiling the object files. ngl though, it's 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 only be performed at compile time and not at link time. so like, do both. 💀
there are some code generation flags GCC preserves when generating bytecodes, since they need to be used during the final link. currently, these 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 — get combined based on this scheme:
-fPIC + -fpic = -fpic
-fPIC + -fno-pic = -fno-pic
-fpic/-fPIC + (no option) = (no option)
-fPIC + -fPIE = -fPIE
-fpic + -fPIE = -fpie
-fPIC/-fpic + -fpie = -fpiecertain 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 — like -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. also note that code transforms such as inlining can lead to warnings being enabled or disabled for regions if code isn't 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 (that's undefined behavior according to ISO C99 6.2.7, btw 😭), 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: it's 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 -lgfortrannotice 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's possible to extract and use them in an LTO link if you're 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 the library participates in the LTO optimization process:
gcc -o myprog -O2 -flto -fuse-linker-plugin a.o b.o -lfoowith the linker plugin enabled, the linker extracts the needed GIMPLE files from libfoo.a and passes them on to the running GCC so they become part of the aggregated GIMPLE image to be optimized. 🦋
if you're not using a linker with plugin support, and/or don't 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 don't require the presence of the whole program to operate. if the program doesn't require any symbols to be exported, it's 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 the 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's 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. slay. 💖
-flto-partition=alg— picks the partitioning algorithm the link-time optimizer uses, and bestie there are options. Soalgcan be1to1(partitioning that mirrors your original source files),balanced(chunks of equal size, whenever that's actually possible), ormax(a fresh partition for every single symbol where possible). You can also passnoneto straight-up disable partitioning and streaming entirely. Default?balanced. Now —1to1is your little workaround for assorted code ordering issues, butmax? That one is for internal testing only, so don't come for me if you use it, it's not for us 💀. Oh and there's also the valueone, which says exactly one partition should be used, whilenonebypasses partitioning and runs the link-time optimization step directly from the WPA phase. Two differentnones of vibes but okay 🫶-flto-compression-level=n— sets how much compression gets used for the intermediate language written to LTO object files. Heads up: this is only meaningful in conjunction with LTO mode (-flto). GCC currently supports two LTO compression algorithms. Forzstd, valid values run from0(no compression) to19(maximum compression), whilezlibsupports values from0to9. Anything outside the range gets clamped to either the minimum or maximum of the supported values. Don't give the option at all? Then a default balanced compression setting is used. Slay.-fuse-linker-plugin— turns on linker plugin usage during link-time optimization. This one relies on plugin support in the linker itself, which you get ingoldor in GNUld2.21 or newer.
It enables the extraction of object files with GIMPLE bytecode out of library archives, which levels up optimization quality by exposing more code to the link-time optimizer. This info specifies what symbols can be accessed externally (by non-LTO object or during dynamic linking). The resulting code quality improvements on binaries (and shared libraries that use hidden visibility) are similar to -fwhole-program. Peep -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). Queen behavior, honestly 👑
-ffat-lto-objects— fat LTO objects are object files containing both the intermediate language and the object code, which makes them usable for both LTO linking and normal linking. This option only does anything when you're compiling with-flto, and it's ignored at link time. Noted.
-fno-fat-lto-objects improves compilation time over plain LTO, but it requires the complete toolchain to be LTO-aware. It needs a linker with linker plugin support for basic functionality. On top of that, nm, ar and ranlib need to support linker plugins to allow a full-featured build environment (the kind capable of building static libraries and such). GCC provides the gcc-ar, gcc-nm, gcc-ranlib wrappers to pass the right options to these tools. With non-fat LTO you have to modify makefiles to use them, sorry bestie 😭
Note that modern binutils provide a plugin auto-load mechanism. Installing the linker plugin into $libdir/bfd-plugins has the same effect as using the command wrappers (gcc-ar, gcc-nm and gcc-ranlib).
Default is -fno-fat-lto-objects on targets with linker plugin support.
-fcompare-elim— after register allocation and post-register allocation instruction splitting, this identifies arithmetic instructions that compute processor flags similar to a comparison operation based on that arithmetic. If possible, it eliminates the explicit comparison operation. Clean. ✨
This pass only applies to certain targets that can't explicitly represent the comparison operation before register allocation is complete.
Enabled at levels -O1, -O2, -O3, -Os.
-fcprop-registers— after register allocation and post-register allocation instruction splitting, it performs a copy-propagation pass to try to reduce scheduling dependencies and occasionally eliminate the copy. Little optimizations doing the most 💅
Enabled at levels -O1, -O2, -O3, -Os.
-fprofile-correction— profiles collected using an instrumented binary for multi-threaded programs may be inconsistent because counter updates get missed. When this option is specified, GCC uses heuristics to correct or smooth out those inconsistencies. By default, GCC emits an error message when it detects an inconsistent profile.
This option is enabled by -fauto-profile.
-fprofile-partial-training— so with-fprofile-use, all portions of programs not executed during the train run get optimized aggressively for size rather than speed. Sometimes it's just not practical to train all possible hot paths in the program. (Like, the program may contain functions specific to a given hardware and the training may not cover all the hardware configurations the program runs on.) With-fprofile-partial-training, profile feedback gets ignored for all functions not executed during the train run, so they get optimized as if they were compiled without profile feedback. This leads to better performance when the train run isn't representative — but also leads to significantly bigger code. Tradeoffs, bestie, tradeoffs 🙈-fprofile-use-fprofile-use=path— enables profile feedback-directed optimizations, plus 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-functionsBefore you can use this option, you must first generate profiling information. No shortcuts, babe.
By default, GCC emits an error message if the feedback profiles don't 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 don't exist (see -Wmissing-profile).
If path is specified, GCC looks at path to find the profile feedback data files. See -fprofile-dir.
-fauto-profile-fauto-profile=path— enables sampling-based feedback-directed optimizations, plus 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-correctionpath is the name of a file containing AutoFDO profile information. If it's 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 <https://perf.wiki.kernel.org/>.
E.g.
perf record -e br_inst_retired:near_taken -b -o perf.data \
-- your_programThen use the create_gcov tool to convert the raw profile data to a format GCC can use. You must also supply the unstripped binary for your program to this tool. See <https://github.com/google/autofdo>.
E.g.
create_gcov --binary=your_program.unstripped --profile=perf.data \
--gcov=profile.afdoThe following options control compiler behavior regarding floating-point arithmetic. These options trade off between speed and correctness. All must be specifically enabled. (Like, they're not just vibing on by default, you have to ask. 💖)
-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 prevents undesirable excess precision on machines such as the 68000, where the floating registers (of the 68881) keep more precision than a double is supposed to have. Same situation on the x86 architecture. For most programs, the excess precision does only good, but a few programs rely on the precise definition of IEEE floating point. Use -ffloat-store for those programs — after modifying them to store all pertinent intermediate computations into variables. You gotta do the work first, bestie 🫶
-fexcess-precision=style— this lets you control excess precision further on machines where floating-point operations occur in a format with more precision or range than the IEEE standard and interchange floating-point types. By default,-fexcess-precision=fastis in effect; this means 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's unpredictable when rounding to the types specified in the source code takes place. When compiling C, if-fexcess-precision=standardis specified then excess precision follows the rules specified in ISO C99; in particular, both casts and assignments cause values to be rounded to their semantic types (whereas-ffloat-storeonly affects assignments). This option is enabled by default for C if a strict conformance option such as-std=c99is used.-ffast-mathenables-fexcess-precision=fastby default regardless of whether a strict conformance option is used.
-fexcess-precision=standard is not implemented for languages other than 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. 😭
-ffast-math— sets the options-fno-math-errno,-funsafe-math-optimizations,-ffinite-math-only,-fno-rounding-math,-fno-signaling-nans,-fcx-limited-rangeand-fexcess-precision=fast. A whole bundle, iconic.
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 don't require the guarantees of these specifications. High risk, high reward, periodt. 🔥
-fno-math-errno— okay so lowkey, this one says: don't bother settingerrnoafter calling math functions that run in a single instruction, likesqrt. 💅 if your program relies on IEEE exceptions to handle math errors, you might wanna use this flag for the speed boost while still keeping IEEE arithmetic compatibility.
heads up bestie: this is not turned on by any -O option, because it can produce straight-up incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. 😭 that said, it can give you faster code if your program doesn't need those guarantees.
the default is -fmath-errno.
on Darwin systems, the math library never sets errno. so there's literally no reason for the compiler to even consider the possibility, and -fno-math-errno is the default there. ✨
-funsafe-math-optimizations— allow optimizations for floating-point arithmetic that (a) assume arguments and results are valid, and (b) may violate IEEE or ANSI standards. 🫣 when used at link time, it may pull in libraries or startup files that change the default FPU control word or do other similar optimizations.
this is not turned on by any-Ooption — it can result in incorrect output for programs depending on an exact implementation of IEEE or ISO rules/specifications for math functions. it can yield faster code for programs that don't need those guarantees though. it enables-fno-signed-zeros,-fno-trapping-math,-fassociative-mathand-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 the computation result.
NOTE: re-ordering may change the sign of zero, ignore NaNs, and inhibit or create underflow or overflow — so it can't be used on code that relies on rounding behavior like (x + 2**52) - 2**52. it may also reorder floating-point comparisons, so it can't be used when ordered comparisons are required. 😬
this option requires that both -fno-signed-zeros and -fno-trapping-math be in effect. plus, 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 that enables optimizations. 🎀 for examplex / ycan be replaced withx * (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 arguments and results are not NaNs or +-Infs. 👑
this is not turned on by any -O option 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 don't need those guarantees.
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 distinct behavior for+0.0and-0.0values, which then prohibits simplification of expressions such asx+0.0or0.0*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-nansbe in effect. setting this option may allow faster code if you rely on "non-stop" IEEE arithmetic, for example.
this option should never be turned on by any -O option, 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.
-frounding-math— disable transformations and optimizations that assume default floating-point rounding behavior. 🌸 that's round-to-zero for all floating point to integer conversions, and round-to-nearest for all other arithmetic truncations. 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. it 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'sFENV_ACCESSpragma. this command-line option will be used to specify the default state forFENV_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— don't allow the built-in functionsceil,floor,roundandtrunc, and theirfloatandlong doublevariants, to generate code that raises theinexactfloating-point exception for noninteger arguments. 😮 ISO C99 and C11 allow these functions to raise theinexactexception, but ISO/IEC TS 18661-1:2014, the C bindings to IEEE 754-2008, as integrated into ISO C2X, does not allow these functions to do so.
the default is -ffp-int-builtin-inexact, allowing the exception to be raised, unless C2X or a later C standard is selected. this option does nothing unless -ftrapping-math is in effect.
even if-fno-fp-int-builtin-inexactis used, if the functions generate a call to a library function then theinexactexception 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's no checking whether the result of a complex multiplication or division isNaN + I*NaN, with an attempt to rescue the situation in that case. the default is-fno-cx-limited-range, but it's 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's no checking whether the result of a complex multiplication or division isNaN + 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-arcsexits, it saves arc execution counts to a file calledsourcename.gcdafor each source file. the info 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're 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. 🎀
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— okay so this one does a target dependent pass over the instruction stream and schedules instructions of the same type right next to each other 💅 because your target machine can execute them way more efficiently when they're adjacent in the instruction flow. It's giving "besties stand together" energy fr fr.
Turned on at levels -O2, -O3, -Os. ✨
-ftracer— this one performs tail duplication to enlarge superblock size 🦋. The way it simplifies the control flow of the function so that other optimizations can do a better job? Obsessed. Lowkey carrying the whole optimization team.
Enabled by -fprofile-use and -fauto-profile.
-funroll-loops— unrolls loops whose number of iterations can be determined at compile time or upon entry to the loop 🔁. Heads up bestie,-funroll-loopsimplies-frerun-cse-after-loop,-fweband-frename-registers. It also turns on complete loop peeling (i.e. complete removal of loops with a small constant number of iterations). No cap: this option makes code larger, and may or may not make it run faster. So like, it's not a guaranteed glow-up 💀.
Enabled by -fprofile-use and -fauto-profile.
-funroll-all-loops— unrolls all the loops, even if their number of iterations is uncertain when the loop is entered 🌀. And bestie? This usually makes programs run more slowly. Not the vibe, I know.-funroll-all-loopsimplies the same options as-funroll-loops.-fpeel-loops— peels loops for which there's enough information that they don't 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.
-fmove-loop-invariants— enables the loop invariant motion pass in the RTL loop optimizer 🌈. Enabled at level-O1and 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. Ok but like, note for this option to have an effect-ftree-loop-imhas to be enabled as well ⚠️. Enabled at level-O1and higher, except for-Og.-fsplit-loops— splits 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 ✂️. So clean, so demure.
Enabled by -fprofile-use and -fauto-profile.
-funswitch-loops— moves 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, this creates another version of the loop that assumes the stride is always one 🏃♀️💨. For example, this:
for (int i = 0; i < n; ++i)
x[i * stride] = ...;becomes this:
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— these two put 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— optimizes the prologue of variadic argument functions with respect to usage of those arguments 🎀.-fsection-anchors— tries 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— zeroes call-used registers at function return to increase program security by either mitigating Return-Oriented Programming (ROP) attacks or preventing information leakage through registers 🔒. So like, it's a security bestie, protect her.
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— okay so 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--paramoption.
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 💀. So don't get too attached.
In order to get minimal, maximal and default value of a parameter, one can use --help=param -Q options 🔍.
In each case, the value is an integer. The following choices of name are recognized for all targets:
predictable-branch-outcome— when a 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 💰.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 internalseq_costmetric. The compiler will try to provide a reasonable default for this parameter using theBRANCH_COSTtarget macro 📊.max-crossjump-edges— the maximum number of incoming edges to consider for cross-jumping. The algorithm used by-fcrossjumpingisO(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 avoidO(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 inkBthat 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 🧠.
--param max-gcse-insertion-ratio— if the ratio of expression insertions to deletions for any one expression blows past this number, then RTL PRE goes ahead and inserts or removes that expression — which means it just leaves partially redundant computations hanging out in the instruction stream. 💀--param max-pending-list-length— the most pending dependencies that scheduling will tolerate before it flushes the current state and starts all over. Big functions with barely any branches or calls can build up absolutely massive lists, and those lists just eat memory and resources for no reason. lowkey wasteful.--param max-modulo-backtrack-attempts— the max number of backtrack attempts the scheduler is allowed to make when it's modulo scheduling a loop. Heads up bestie: bigger values can make compilation time go up exponentially. 😭--param max-inline-functions-called-once-loop-depth— the maximal loop depth of a call that gets considered by the inline heuristics that try to inline every function called once.--param max-inline-functions-called-once-insns— the maximal estimated size of the functions produced while inlining functions called once.--param max-inline-insns-single— a bunch of parameters control the tree inliner GCC uses, and this one sets the maximum number of instructions (counted in GCC's internal representation) in a single function that the tree inliner will even consider for inlining. Note: this only touches functions declaredinlineand methods implemented in a class declaration (C++).--param max-inline-insns-auto— when you use-finline-functions(which is included in-O3), a whole lot of functions that the compiler would otherwise never consider for inlining get investigated. For those functions, a different (and more restrictive) limit compared to functions declaredinlinecan be applied:--param max-inline-insns-auto.--param max-inline-insns-small— this is the bound applied to calls that are considered relevant with-finline-small-functions.--param max-inline-insns-size— this is the bound applied to calls which are optimized for size. A little growth can be desirable, because it anticipates optimization opportunities exposed by inlining. ✨--param uninlined-function-insns— the number of instructions the inliner accounts for function overhead, like the function prologue and epilogue.--param uninlined-function-time— extra time the inliner accounts for function overhead, like the time needed to execute the function prologue and epilogue.--param inline-heuristics-hint-percent— the scale (in percents) applied toinline-insns-single,inline-insns-single-O2, andinline-insns-autowhen the inline heuristics hint that inlining is very profitable (like, it'll enable later optimizations). 💅--param uninlined-thunk-insns,--param uninlined-thunk-time— same as--param uninlined-function-insnsand--param uninlined-function-time, except these get applied to function thunks.--param inline-min-speedup— when the estimated performance improvement of caller + callee runtime goes over this threshold (in percent), the function can be inlined regardless of the limit on--param max-inline-insns-singleand--param max-inline-insns-auto. slay, no cap.--param large-function-insns— the limit that defines a really large function. For functions larger than this limit after inlining, inlining gets constrained by--param large-function-growth. This parameter is mostly useful to dodge extreme compilation times caused by the non-linear algorithms the back end uses.--param large-function-growth— the maximal growth of a large function caused by inlining, in percents. For example, parameter value100limits large function growth to2.0times the original size.--param large-unit-insns— the limit that defines a large translation unit. Growth caused by inlining of units larger than this limit gets limited by--param inline-unit-growth. For small units this might be too tight though. Picture a unit made of functionAthat isinlineandBthat just callsAthree times. IfBis small relative toA, the growth of the unit is300%and yet that inlining is totally sane. But for very large units made of small inlineable functions, you do need the overall unit growth limit, otherwise code size explodes exponentially. So for smaller units, the size is increased to--param large-unit-insnsbefore applying--param inline-unit-growth.--param lazy-modules— the maximum number of concurrently open C++ module files when lazy loading. 🦋--param inline-unit-growth— the maximal overall growth of the compilation unit caused by inlining. For example, parameter value20limits unit growth to1.2times the original size. Cold functions (whether they're marked cold via an attribute or by profile feedback) are not counted into the unit size.--param ipa-cp-unit-growth— the maximal overall growth of the compilation unit caused by interprocedural constant propagation. For example, parameter value10limits unit growth to1.1times the original size.--param ipa-cp-large-unit-insns— the size of translation unit that the IPA-CP pass considers large.--param large-stack-frame— the limit that defines large stack frames. While inlining, the algorithm tries not to grow past this limit too much.--param large-stack-frame-growth— the maximal growth of large stack frames caused by inlining, in percents. For example, parameter value1000limits large stack frame growth to11times the original size.--param max-inline-insns-recursive,--param max-inline-insns-recursive-auto— 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 only happens when -finline-functions (included in -O3) is enabled, and then --param max-inline-insns-recursive-auto applies instead.
--param max-inline-recursive-depth,--param max-inline-recursive-depth-auto— 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 only happens when -finline-functions (included in -O3) is enabled, and then --param max-inline-recursive-depth-auto applies instead.
--param min-inline-recursive-probability— recursive inlining is only profitable for functions that have deep recursion on average, and it can actually hurt functions with little recursion depth, because it increases the prologue size or the complexity of the function body for the other optimizers. 😭
When profile feedback is available (see -fprofile-generate) the actual recursion depth can be guessed from the probability that the function recurses via a given call expression. This parameter limits inlining to only those call expressions whose probability exceeds the given threshold (in percents).
--param early-inlining-insns— specifies the growth the early inliner is allowed to make. In effect, it increases the amount of inlining for code that has a large abstraction penalty.--param max-early-inliner-iterations— the limit of iterations of the early inliner. This basically bounds how many nested indirect calls the early inliner can resolve. Deeper chains are still handled by late inlining.--param comdat-sharing-probability— the probability (in percent) that a C++ inline function with comdat visibility gets shared across multiple compilation units.--param modref-max-bases,--param modref-max-refs,--param modref-max-accesses— the maximal number of base pointers, references, and accesses stored for a single function by mod/ref analysis.--param modref-max-tests— the maxmal number of tests the alias oracle can perform to disambiguate memory locations using the mod/ref information. Heads up: this parameter ought to be bigger than--param modref-max-basesand--param modref-max-refs. 👑--param modref-max-depth— the maximum depth of the DFS walk used by modref escape analysis. Setting it to0disables the analysis completely.--param modref-max-escape-points— the maximum number of escape points tracked by modref per SSA-name.--param modref-max-adjustments— the maximum number the access range gets enlarged during modref dataflow analysis.--param profile-func-internal-id— a parameter controlling whether to use the function internal id in profile database lookup. If the value is0, the compiler uses an id based on the function assembler name and filename, which makes old profile data more tolerant to source changes like function reordering etc.--param min-vect-loop-bound— the minimum number of iterations under which loops are not vectorized when-ftree-vectorizeis used. The number of iterations after vectorization needs to be greater than the value specified by this option to allow vectorization.--param gcse-cost-distance-ratio— a scaling factor in the calculation of the 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 thangcse-unrestricted-cost. Specifying0disables hoisting of simple expressions.--param gcse-unrestricted-cost— the cost, roughly measured as the cost of a single typical machine instruction, at which GCSE optimizations stop constraining 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. Specifying0allows all expressions to travel unrestricted distances. 🚀max-hoist-depth— how deep into the dominator tree we go looking for expressions to hoist. it exists purely so the hoisting algorithm doesn't go full quadratic on you. Setting it to0= no limit on the search, but bestie, that can make compiling gigantic functions slow as hell. 🙈max-tail-merge-comparisons— the max number of similar bbs we'll compare a bb against. again: quadratic behavior avoidance, this time for tree tail merging. 💅max-tail-merge-iterations— the max number of times the pass runs over the function. caps compilation time in tree tail merging. ⏱️store-merging-allow-unaligned— lets the store merging pass introduce unaligned stores, but only if it's legal to do so. permission slip required, basically. 🎀max-stores-to-merge— the max number of stores the store merging pass will even attempt to merge into wider stores. 🌸max-store-chains-to-track— the max number of store chains tracked at the same time while trying to merge them into wider stores in the store merging pass. 🦋max-stores-to-track— the max number of stores tracked at the same time in the attempt to merge them into wider stores in the store merging pass. (yes it's a different counter from the chains one, they're siblings, not twins ✨)max-unrolled-insns— the max number of instructions a loop can have and still be unrolled. if a loop does get unrolled, this also decides how many times the loop code gets unrolled. 🔁max-average-unrolled-insns— same deal, but the instruction count is biased by the probabilities of their execution. still decides how many times the loop code is unrolled if it is unrolled. 📊max-unroll-times— the max number of unrollings of a single loop. periodt. 👑max-peeled-insns— the max number of instructions a loop can have and still be peeled. if a loop is peeled, this also determines how many times the loop code is peeled. 🍌max-peel-times— the max number of peelings of a single loop.max-peel-branches— the max number of branches allowed on the hot path through the peeled sequence. 🔥max-completely-peeled-insns— the max number of insns of a completely peeled loop.max-completely-peel-times— the max number of iterations a loop can have to still be suitable for complete peeling.max-completely-peel-loop-nest-depth— the max depth of a loop nest that's suitable for complete peeling. 🪆max-unswitch-insns— the max number of insns of an unswitched loop.max-unswitch-level— the max number of branches unswitched in a single loop.lim-expensive— the minimum cost an expression needs to have to count as "expensive" in loop invariant motion. 💸min-loop-cond-split-prob— when FDO profile information is available, this sets the minimum probability threshold for a semi-invariant condition statement to trigger a loop split.iv-consider-all-candidates-bound— a bound on the number of induction variable candidates. below it, all candidates get considered for each use in induction variable optimizations. above it, only the most relevant ones get considered, and that's specifically to dodge quadratic time complexity. 🧠iv-max-considered-uses— the induction variable optimizations straight up give up on loops containing more induction variable uses than this. 😭iv-always-prune-cand-set-bound— if the number of candidates in the set is smaller than this value, we always try to remove unnecessary ivs from the set whenever a new one is added. 🧹avg-loop-niter— the average number of iterations of a loop. 📈dse-max-object-size— the max size (in bytes) of objects tracked bytewise by dead store elimination. bigger values might mean bigger compilation times. 🐘dse-max-alias-queries-per-store— the max number of queries into the alias oracle per store. bigger values = bigger compilation times, but also possibly more removed dead stores. tradeoffs, bestie. ⚖️scev-max-expr-size— a bound on the size of expressions used in the scalar evolutions analyzer. large expressions slow the analyzer down. 🐌scev-max-expr-complexity— a bound on the complexity of the expressions in the scalar evolutions analyzer. complex expressions also slow the analyzer down. 🌀max-tree-if-conversion-phi-args— the max number of arguments in a PHI that TREE if conversion supports — unless the loop is marked with simd pragma. 🚪vect-max-version-for-alignment-checks— the max 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 max number of run-time checks that can be performed when doing loop versioning for alias in the vectorizer. 👯vect-max-peeling-for-alignment— the max number of loop peels used to enhance access alignment for the vectorizer. a value of-1means no limit. 🎉max-iterations-to-track— the max number of iterations of a loop that the brute-force algorithm for analyzing a loop's iteration count will try to evaluate. 💪hot-bb-count-fraction— the denominator n of the fraction1/nof the maximal execution count of a basic block in the entire program that a basic block needs to at least hit to be considered hot. the default is10000, which means a basic block counts as hot when its execution count is greater than1/10000of the maximal execution count.0means it is never considered hot. used in non-LTO mode. 🔥hot-bb-count-ws-permille— the number of most executed permilles, ranging from0to1000, of the profiled execution of the entire program that a basic block's execution count must be part of for it to count as hot. default is990, meaning a basic block is hot if its execution count contributes to the upper990permilles — that's99.0%— of the profiled execution of the whole program.0means never considered hot. used in LTO mode. 💯hot-bb-frequency-fraction— the denominator n of the fraction1/nof the execution frequency of a function's entry block that a basic block of that function needs to at least have to count as hot. default is1000, so a basic block is hot inside a function if it's executed more frequently than1/1000of the entry block's frequency.0means never considered hot. 🌡️unlikely-bb-count-fraction— the denominator n of the fraction1/nof the number of profiled runs of the entire program that a basic block's execution count must fall below for the block to be considered unlikely executed. default is20, so a basic block is unlikely executed if it runs in fewer than1/20, aka5%, of the program's runs.0means it is always considered unlikely executed. 🙈max-predicted-iterations— the max number of loop iterations we predict statically. super useful when a function has one loop with a known bound and another with an unknown bound: the known iteration count gets predicted correctly, while the unknown count averages out to roughly10. which means the loop without bounds looks artificially cold next to the other one. 🥶builtin-expect-probability— controls the probability of the expression having the specified value. takes a percentage, i.e.0 ... 100, as input. 🎯builtin-string-cmp-inline-length— the max length of a constant string for a builtin string cmp call that's eligible for inlining. ✂️align-threshold— selects the fraction of the maximal frequency of executions of a basic block in a function needed to align that basic block. 📐align-loop-iterations— a loop that's expected to iterate at least the selected number of iterations gets aligned. 🔄tracer-dynamic-coverage,tracer-dynamic-coverage-feedback— this value limits superblock formation once the given percentage of executed instructions is covered. it's there to limit unnecessary code size expansion. 🌱
the tracer-dynamic-coverage-feedback parameter is used only when profile feedback is available. real profiles (as opposed to statically estimated ones) are much less balanced, which lets the threshold be a larger value. 🎚️
tracer-max-code-growth— stops tail duplication once code growth has hit the given percentage. it's a pretty artificial limit though, since most of the duplicates get eliminated later during cross jumping — so it can be set to much higher values than the code growth you actually want. 🚧tracer-min-branch-ratio— stops reverse growth when the reverse probability of the best edge drops below this threshold (in percent). 📉tracer-min-branch-probability,tracer-min-branch-probability-feedback— stops forward growth if the best edge has a probability lower than this threshold. 🛑
just like tracer-dynamic-coverage, there are two of them. tracer-min-branch-probability-feedback is used for compilation with profile feedback, and tracer-min-branch-probability for compilation without. the value for compilation with profile feedback needs to be more conservative (higher) in order to make the tracer effective. 💅
stack-clash-protection-guard-size— specifies the size of the operating system provided stack guard as2raised 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. 🚨 not the vibe you want. stay safe out there bestie. 🫶stack-clash-protection-probe-interval— okay so stack clash protection is all about probing the stack space as it gets allocated 🛡️. This param sets the maximum distance between probes into the stack, expressed as 2 raised to num bytes. Bigger values might mean fewer explicit probes, but bestie, listen — a value larger than the guard the operating system gives you will leave your code vulnerable to stack clash style attacks. So like, don't do that 💀.max-cse-path-length— the max number of basic blocks on a path that CSE even bothers considering.max-cse-insns— the max number of instructions CSE processes before it flushes. Flush as in clears its stuff out, not flush as in 💅 (though honestly, same energy).ggc-min-expand— so GCC uses a garbage collector to manage its own memory allocation 🗑️✨. This parameter is the minimum percentage by which the GC's heap should be allowed to expand between collections. Tuning it might make compilation faster; 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. That is extremely slow, but it can be useful for debugging 🔍.
ggc-min-heapsize— the minimum size of the garbage collector's heap before it even starts bothering to collect garbage. The first collection happens after the heap expands byggc-min-expand% beyondggc-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 max number of instructions reload should look backward for an equivalent register. Increasing values mean more aggressive optimization, which makes compilation time go up with probably slightly better performance. It's giving "tradeoff queen" 👑.max-cselib-memory-locations— the max 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 max number of instructions ready to be issued that 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 max number of blocks in a region to be considered for interblock scheduling.max-pipeline-region-blocks— the max number of blocks in a region to be considered for pipelining in the selective scheduler.max-sched-region-insns— the max number of insns in a region to be considered for interblock scheduling.max-pipeline-region-insns— the max 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 max number of iterations through CFG to extend regions. A value of 0 disables region extensions.max-sched-insn-conflict-delay— the max 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 max size of the lookahead window of selective scheduling. It's a depth of search for available instructions 🔭.selsched-max-sched-times— the max 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 max 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 max 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 max number of instructions the RTL combiner tries to combine.integer-share-limit— small integer constants can use a shared data structure, which reduces the compiler's memory usage and increases 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-protectoris used.min-size-for-stack-sharing— the minimum size of variables taking part in stack slot sharing when not optimizing.max-jump-thread-duplication-stmts— max number of statements allowed in a block that needs to be duplicated when threading jumps.max-fields-for-field-sensitive— max number of fields in a structure treated in a field sensitive manner during pointer analysis.prefetch-latency— an estimate of the 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 (seesimultaneous-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 variablesstd::hardware_destructive_interference_sizeandstd::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 max 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— should the compiler use the "canonical" type system? Bestie, this should literally always be 1, because 1 uses a way more efficient internal mechanism for comparing types in C++ and Objective-C++. But — and this is the important part — if bugs in the canonical type system are making your compilation fail, set this value to0to disable canonical types. Emergency escape hatch energy. 🚨switch-conversion-max-branch-ratio— switch initialization conversion refuses to create arrays bigger thanswitch-conversion-max-branch-ratiotimes the number of branches in the switch. It's giving "know your limits" 💅max-partial-antic-length— the maximum length of the partial antic set computed during the tree partial redundancy elimination optimization (-ftree-pre) when optimizing at-O3and above. For some sorts of source code, the enhanced partial redundancy elimination optimization can run away, consuming all the memory available on the host machine 😭. This parameter sets a limit on the length of the sets that are computed, which prevents that runaway behavior. Setting a value of0for this parameter allows an unlimited set length (no cap, unlimited, be careful).rpo-vn-max-loop-depth— the maximum loop depth that is value-numbered optimistically. When the limit hits, the innermostrpo-vn-max-loop-depthloops and the outermost loop in the loop nest get value-numbered optimistically, and the remaining ones don't. ✨sccvn-max-alias-queries-per-access— the 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— okay so IRA uses a sophisticated algorithm to compress the conflict table, but that 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. Trade-offs, we love to see them. 💀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. The default of the parameter is the best found from numerous experiments. Science! 🧪ira-consider-dup-in-all-alts— makes IRA 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. 👑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 ofSPEC2000onx86-64. 🦋loop-invariant-max-bbs-in-loop— loop invariant motion can be very expensive, both in compilation time and in the 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. Not me gatekeeping your loops 🙈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 not 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. ✨ balance ✨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-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 toipa-sra-ptr-growth-factortimes the size of the original pointer parameter. Math girlies rise 🫶ipa-sra-max-replacements— the 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. Speed or size, pick your fighter 👑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. 🦋loop-block-tile-size— loop blocking or strip mining transforms, enabled with-floop-blockor-floop-strip-mine, strip mine each loop in the loop nest by a given number of iterations. The strip length can be changed using theloop-block-tile-sizeparameter.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-sizeis 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 exceedipa-cp-eval-threshold. Only the overachievers get cloned, periodt. 💅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 the call being executed exceeds the parameter.ipa-cp-profile-count-base— when using-fprofile-useoption, 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-itemscontrols the maximum number of such values per one parameter. ✨ipa-cp-loop-hint-bonus— bestie, when IPA-CP clocks that a cloning candidate would make the number of iterations of a loop known? It slides a bonus ofipa-cp-loop-hint-bonusonto the profitability score of that candidate. Extra credit fr. 💖ipa-max-loop-predicates— the maximum number of different predicates IPA will use to describe when loops in a function have known properties. That's the cap, no more. ✨ipa-max-aa-steps— okay so during its analysis of function bodies, IPA-CP pulls up with alias analysis to track the values pointed to by function parameters. But to not spend forever analyzing huge functions, it gives up and considers all memory clobbered after examiningipa-max-aa-stepsstatements that modify memory. She has a limit and she's sticking to it 💅ipa-max-switch-predicate-bounds— the maximal number of boundary endpoints of case ranges of aswitchstatement. For aswitchthat exceeds this limit, IPA-CP will not construct the cloning cost predicate (the thing used to estimate cloning benefit) for thedefaultcase of the switch. So like, too many cases = no predicate fordefault. 😭ipa-max-param-expr-ops— IPA-CP will analyze a conditional statement that references some function parameter, to estimate the benefit for cloning upon a certain constant value. BUT if the number of operations in a parameter expression exceedsipa-max-param-expr-ops, that expression is treated as a complicated one and is not handled by IPA analysis. Complicated girlies get left on read 🙈lto-partitions— specify the desired number of partitions produced during WHOPR compilation. Heads up: the number of partitions should exceed the number of CPUs used for compilation. 👑lto-min-partition— the size of the minimal partition for WHOPR (measured in estimated instructions). This stops small programs from being split into way too many partitions. ✨lto-max-partition— the size of the max partition for WHOPR (in estimated instructions), to provide an upper bound for the individual size of a partition. Meant to be used only with balanced partitioning. No cap fr.lto-max-streaming-parallelism— the 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. It's giving "let me help you find your name bestie" energy. 🫶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 = more aggressive statement sinking. And a small positive adjustment gets applied for statements with memory operands, because those are even more profitable to sink. 💅max-stores-to-sink— the maximum number of conditional store pairs that can be sunk. Set it to0if 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's best to use a jump-table instead of a tree of conditional branches. If the value is0, 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). This 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). This parameter is used when optimizing for speed. 🔥tree-reassoc-width— set the maximum number of instructions executed in parallel in a reassociated tree. This parameter overrides the target-dependent heuristics used by default if it has a non-zero value. 👑sched-pressure-algorithm— choose between the two available implementations of-fsched-pressure. Algorithm1is the original implementation and is the more likely one to prevent instructions from being reordered. Algorithm2was designed as a compromise between the relatively conservative approach of algorithm1and the rather aggressive approach taken by the default scheduler. It relies more heavily on having a regular register file and accurate register pressure classes. Seehaifa-sched.ccin the GCC sources for more details. 😭
The default choice depends on the target. (iykyk)
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're using the-fsanitize=addressoption. 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. 💖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. 💅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=addressoption. 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 variableASAN_OPTIONS. 👀
asan-instrumentation-with-call-threshold— if the number of memory accesses in the function being instrumented is greater than 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. ⭐hwasan-instrument-stack— enable hwasan instrumentation of statically sized stack-allocated variables. This kind of instrumentation is enabled by default when using-fsanitize=hwaddressand 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 from1. This is enabled by default for-fsanitize=hwaddressand 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=hwaddressand 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=hwaddressand-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=hwaddressand-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=hwaddressand-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 than 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. ✨max-fsm-thread-length— maximum number of basic blocks on a 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 istwo. 💜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 biggest number of may-defs we'll sit and analyze while we're hunting for a must-def that pins down the dynamic type of an object invoking a virtual call we might be able to devirtualize speculatively. ✨max-vrp-switch-assertions— the max number of assertions we're allowed to sprinkle along the default edge of a switch statement during VRP.evrp-sparse-threshold— max number of basic blocks before EVRP switches over to using a sparse cache. 🧚evrp-mode— picks the mode Early VRP should vibe in.vrp1-mode— picks the mode VRP pass 1 should vibe in.vrp2-mode— picks the mode VRP pass 2 should vibe in.ranger-debug— says what kind of debug output gets issued for ranges.evrp-switch-limit— the maximum number of switch cases before EVRP just straight up ignores a switch. 💀unroll-jam-min-percent— the minimum percentage of memory references that have to get optimized away before the unroll-and-jam transformation counts as profitable.unroll-jam-max-unroll— the max number of times the outer loop should get unrolled by the unroll-and-jam transformation.max-rtl-if-conversion-unpredictable-cost— max permissible cost for the sequence that the RTL if-conversion pass would generate for a branch that's considered unpredictable.max-variable-expansions-in-unroller— if-fvariable-expansion-in-unrolleris used, this is the max number of times an individual variable gets expanded during loop unrolling.partial-inlining-entry-probability— the maximum probability of the entry BB of a split region (in percent, relative to the entry BB of the function) needed to make partial inlining happen.max-tracked-strlens— the maximum number of strings the strlen optimization pass will track string lengths for.gcse-after-reload-partial-fraction— the threshold ratio for doing partial redundancy elimination after reload.gcse-after-reload-critical-fraction— the threshold ratio of critical edges' execution count that permits performing redundancy elimination after reload.max-loop-header-insns— the maximum number of insns in a loop header that gets duplicated by the copy loop headers pass.vect-epilogues-nomask— enables loop epilogue vectorization using a 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.0stops the vectorizer from ever using partial vector loads and stores.1allows partial vector loads and stores if vectorization removes the need for the code to iterate.2allows 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 the loop vectorizer applies to the cost of statements in an inner loop relative to the loop being vectorized. The factor applied is the max of the estimated number of iterations of the inner loop and this parameter. The default value of this parameter is50.vect-induction-float— enables loop vectorization of floating point inductions. 💅avoid-fma-max-bits— maximum number of bits for which we avoid creating FMAs.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 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— enables 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,0means 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 when comparing to the number of (scaled) blocks.uninit-control-dep-attempts— maximum number of nested calls to search for control dependencies during uninitialized variable analysis.fsm-scale-path-blocks— scale factor to apply to the number of blocks in a threading path when comparing to the number of (scaled) statements.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 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 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.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. 😭ranger-logical-depth— maximum depth of logical expression evaluation ranger will look through when evaluating outgoing edge ranges.relation-block-limit— maximum number of relations the oracle will register in a basic block.openacc-kernels— specifies the mode of OpenACCkernelsconstructs handling. With--param=openacc-kernels=decompose, OpenACCkernelsconstructs are decomposed into parts, a sequence of compute constructs, each then handled individually. This is work in progress. With--param=openacc-kernels=parloops, OpenACCkernelsconstructs are handled by the parloops pass, en bloc. This is the current default.openacc-privatization— specifies the mode of OpenACC privatization diagnostics for-fopt-info-omp-noteand applicable-fdump-tree-*-details. With--param=openacc-privatization=quiet, don't diagnose. This is the current default. With--param=openacc-privatization=noisy, do diagnose.
The following choices of name are available on AArch64 targets: 👑
aarch64-sve-compare-costs— when vectorizing for SVE, consider using "unpacked" vectors for smaller elements and use the cost model to pick the cheapest approach. Also use the cost model to choose between SVE and Advanced SIMD vectorization.
Using unpacked vectors 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 is1.aarch64-double-recp-precision— the number of Newton iterations for calculating the reciprocal for double type. The precision of division is propotional to this param when division approximation is enabled. The default value is2.aarch64-autovec-preference— force an ISA selection strategy for auto-vectorization. Accepts values from0to4, inclusive. 🌈0— Use the default heuristics.
1— use only Advanced SIMD for auto-vectorization. no SVE, we don't know her 💅2— use only SVE for auto-vectorization. Advanced SIMD who? never met her 🦋3— use both Advanced SIMD and SVE, but when the costs are deemed equal, Advanced SIMD wins the tie. lowkey giving favorite child energy.4— use both Advanced SIMD and SVE, but when the costs are deemed equal, SVE wins the tie. main character choice fr fr.
The default value is 0. ✨
aarch64-loop-vect-issue-rate-niters— okay so some AArch64 CPUs have a tuning that's trying to be all balanced and take both latencies and issue rates into account when it decides whether a loop should get vectorized with SVE, vectorized with Advanced SIMD, or not vectorized at all (sad, tragic, no slay). If you set this parameter to n, GCC will not use that heuristic for loops that are known to run in fewer than n Advanced SIMD iterations. so like, tiny loops get to skip the whole vibe check 👑
🎀 Program Instrumentation Options 🎀
Bestie. GCC has a whole arsenal of command-line options that bolt run-time instrumentation onto the code it would normally spit out. Why would you do that? So many reasons, let me count them 💖
- One purpose of instrumentation is to collect profiling statistics, which you use for finding program hot spots, doing code coverage analysis, or doing profile-guided optimizations.
- Another class of program instrumentation is adding run-time checking to catch programming errors like invalid pointer dereferences or out-of-bounds array accesses — plus deliberately hostile attacks like stack smashing or C++ vtable hijacking. rude. blocked. 🚫
- There's also a general hook that you can use 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 programprof(that's for-p) orgprof(that's for-pg). You must use this option when compiling the source files you want data about, and you must also use it when linking. both times. no skipping steps bestie 🙈
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.gcdafiles will be prefixed with the unsuffixed name of the output file. E.g.gcc a.c b.c -o binarywould generatebinary-a.gcdaandbinary-b.gcdafiles. iconic naming behavior honestly.
--coverage— this one is used to compile and link code instrumented for coverage analysis. It's a synonym for-fprofile-arcs -ftest-coverage(when compiling) and-lgcov(when linking). See the documentation for those options for more details. here's the whole game plan, step by step:- Compile the source files with
-fprofile-arcsplus optimization and code generation options. For test coverage analysis, use the additional-ftest-coverageoption. You do not need to profile every source file in a program. - Compile the source files additionally with
-fprofile-abs-pathto create absolute path names in the.gcnofiles. This allowsgcovto find the correct sources in projects where compilations occur with different working directories. - Link your object files with
-lgcovor-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,
forkcalls are detected and correctly handled without double counting.
- Compile the source files with
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
gcovto produce human readable information from the.gcnoand.gcdafiles. Refer to thegcovdocumentation 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. efficiency queen 💅
-ftest-coverage— produce a notes file that thegcovcode-coverage utility can use to show program coverage. Each source file's note file is calledauxname.gcno. Refer to the-fprofile-arcsoption 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. yes that's the tea 🍓-fprofile-abs-path— automatically convert relative source file names to absolute path names in the.gcnofiles. This allowsgcovto find the correct sources in projects where compilations occur with different working directories. she knows where everything lives now, organized icon ✨- *
-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-arcsand used by-fprofile-useand-fbranch-probabilitiesand 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 thesourcename.gcdafile and use it as the file name of a.gcdafile. 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 variableVAR.
-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-generateboth when compiling **and when linking your program. both. times. i said what i said 🫶
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-arcsis 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 free-standing 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 <gcov.h>
#include <stdio.h>
#include <stdlib.h>
extern const struct gcov_info *__gcov_info_start[];
extern const struct gcov_info *__gcov_info_end[];
static void
filename (const char *f, void *arg)
{
puts (f);
}
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 *
allocate (unsigned length, void *arg)
{
return malloc (length);
}
static void
dump_gcov_info (void)
{
const struct gcov_info **info = __gcov_info_start;
const struct gcov_info **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;
}
}...and the example wraps up like this, bestie:
int
main()
{
dump_gcov_info();
return 0;
}-fprofile-note=path— tell GCC where to stash that.gcnofile and it'll drop it right in path. 💾 But lowkey careful: if you pair this with multiple source files, the.gcnofile gets overwritten (one file, many victims, it's giving chaos 🙈).-fprofile-prefix-path=path— this one's a glow-up for your build setup ✨. Use it alongsideprofile-generate=profile_dirandprofile-use=profile_dirto tell GCC where the base directory of your built source tree is. Why do you care? Because by defaultprofile_diris full of files with mangled absolute paths of every object file in your project. That's a problem when the directory you built the instrumented binary in is not the same directory you build the profile-optimized binary in — the profile data just won't get found during the optimized build 😭. So in those setups,-fprofile-prefix-path=pathwith path pointing at your build's base directory strips off the irrelevant part and keeps all file names relative to the main build directory. Chef's kiss. 👑-fprofile-prefix-map=old=new— when you're compiling files living in directory old, this records profiling info (with--coverage) describing them as if they lived in directory new instead. Also peep-ffile-prefix-map. 🗺️-fprofile-update=method— changes up the update method for an app that's been instrumented for profile-feedback-based optimization. The method argument has to be one ofsingle,atomic, orprefer-atomic.singleis your girl for single-threaded apps, whileatomickeeps your profile from getting corrupted by emitting thread-safe code. 🧵
⚠️ Warning: when an application doesn't properly join all its threads (or spawns a detached thread), a profile file can still get corrupted. No cap.
Using prefer-atomic gets transformed into either atomic (when your target supports it) or single (when it doesn't). And the GCC driver automatically picks prefer-atomic whenever -pthread shows up on the command line. 💅
-fprofile-filter-files=regex— instruments only functions from files whose name matches any of the regular expressions (separated by semi-colons). For example,-fprofile-filter-files=main\.c;module.*\.cwill instrument onlymain.cand all C files starting with'module'. 🎯-fprofile-exclude-files=regex— instruments only functions from files whose name does not match any of the regular expressions (separated by semi-colons). For example,-fprofile-exclude-files=/usr/.*keeps instrumentation out of every file located in the/usr/folder. 🚫-fprofile-reproducible=[multithreaded|parallel-runs|serial]— controls the level of reproducibility of the profile gathered by-fprofile-generate. This lets you rebuild a program with the same outcome, which is useful for stuff like 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's not multi-threaded, and profile data streaming is always done in the same order. Heads up: profile streaming happens at the end of the program run but also before the fork function is invoked. 🌊
Also note it's pretty common for execution counts of some part of a program to depend on stuff like the length of temporary file names or memory space randomization (which can affect hash-table collision rate). That non-reproducible part of your program can be annotated with the no_instrument_function function attribute. And gcov-dump with -l can dump the gathered data so you can verify it's actually reproducible. 🔍
With -fprofile-reproducible=parallel-runs, the collected profile stays reproducible regardless of the order the data streams into the gcda files. This setting lets you run multiple instances of the instrumented program in parallel (like with make -j). It does reduce the quality of the gathered data though — in particular of indirect call profiling. 🫠
-fsanitize=address— turns on AddressSanitizer, a fast memory error detector. Memory access instructions get instrumented to catch out-of-bounds and use-after-free bugs. This option also enables-fsanitize-address-use-after-scope. More deets at<https://github.com/google/sanitizers/wiki/AddressSanitizer>. The run-time behavior can be influenced using theASAN_OPTIONSenvironment variable — set it tohelp=1and the available options get shown at startup of your instrumented program. For a list of supported options see<https://github.com/google/sanitizers/wiki/AddressSanitizerFlags#run-time-flags>. 🛡️
⚠️ This option cannot be combined with-fsanitize=threador-fsanitize=hwaddress. And note that the only target-fsanitize=hwaddressis currently supported on is AArch64.
-fsanitize=kernel-address— enables AddressSanitizer for the Linux kernel. See<https://github.com/google/kasan>for more deets. 🐧-fsanitize=hwaddress— enables Hardware-assisted AddressSanitizer, which uses a hardware ability to ignore the top byte of a pointer so it can detect memory errors with low memory overhead. Memory access instructions get instrumented to detect out-of-bounds and use-after-free bugs. This option enables-fsanitize-address-use-after-scope. More deets at<https://clang.llvm.org/docs/HardwareAssistedAddressSanitizerDesign.html>. The run-time behavior can be influenced using theHWASAN_OPTIONSenvironment variable — set it tohelp=1and the available options show up at startup of your instrumented program. 🧠
⚠️ This option cannot be combined with-fsanitize=threador-fsanitize=address, and it's currently only available on AArch64.
-fsanitize=kernel-hwaddress— enables Hardware-assisted AddressSanitizer for compiling the Linux kernel. Similar to-fsanitize=kernel-addressbut using an alternate instrumentation method, and similar to-fsanitize=hwaddressbut with instrumentation differences needed for compiling the Linux kernel. Those differences exist 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 from -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— instruments comparison operations (<,<=,>,>=) with pointer operands. This option must be combined with either-fsanitize=kernel-addressor-fsanitize=address. It cannot be combined with-fsanitize=thread. 🔍
Note: by default the check is disabled at run time. To enable it, adddetect_invalid_pointer_pairs=2to the environment variableASAN_OPTIONS. Usingdetect_invalid_pointer_pairs=1detects invalid operation only when both pointers are non-null.
-fsanitize=pointer-subtract— instruments subtraction with pointer operands. This option must be combined with either-fsanitize=kernel-addressor-fsanitize=address. It cannot be combined with-fsanitize=thread. ➖
Note: by default the check is disabled at run time. To enable it, adddetect_invalid_pointer_pairs=2to the environment variableASAN_OPTIONS. Usingdetect_invalid_pointer_pairs=1detects invalid operation only when both pointers are non-null.
-fsanitize=shadow-call-stack— enables ShadowCallStack, a security enhancement mechanism used to protect programs against return address overwrites (like 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 happens in functions that need to save the return address to the stack. 🛡️✨
Currently it only supports the aarch64 platform. It's specifically designed for linux kernels that enable the CONFIG_SHADOW_CALL_STACK option. For user space programs, runtime support is not currently provided in libc and libgcc — so if you wanna use this feature in user space you have to provide your 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 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's no userspace runtime support, code compiled with ShadowCallStack cannot use exception handling. Use -fno-exceptions to turn off exceptions.
See <https://clang.llvm.org/docs/ShadowCallStack.html> for more deets.
-fsanitize=thread— enables ThreadSanitizer, a fast data race detector. Memory access instructions get instrumented to detect data race bugs. See<https://github.com/google/sanitizers/wiki#threadsanitizer>for more deets. The run-time behavior can be influenced using theTSAN_OPTIONSenvironment variable — see<https://github.com/google/sanitizers/wiki/ThreadSanitizerFlags>for a list of supported options. 🧵🔥
⚠️ This option cannot be combined with-fsanitize=address,-fsanitize=leak.
Note that sanitized atomic builtins cannot throw exceptions when operating on invalid memory addresses with non-call exceptions (-fnon-call-exceptions).
-fsanitize=leak— enables LeakSanitizer, a memory leak detector. This option only matters for linking of executables, and the executable gets linked against a library that overridesmallocand other allocator functions. See<https://github.com/google/sanitizers/wiki/AddressSanitizerLeakSanitizer>for more deets. The run-time behavior can be influenced using theLSAN_OPTIONSenvironment variable. 💧🔎
⚠️ This option cannot be combined with -fsanitize=thread.
-fsanitize=undefined— bestie, this turns on UndefinedBehaviorSanitizer, aka UBSan, aka the little detector that catches undefined behavior fast. ✨ Lots of computations get instrumented so undefined behavior gets sniffed out at runtime. More deets live at<https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html>. You can also nudge how it behaves at runtime with theUBSAN_OPTIONSenvironment variable. 💅 Right now the suboptions are:-fsanitize=shift— checks that the result of a shift operation is not undefined. Heads up though: what exactly counts as undefined wiggles a little between C and C++, and between ISO C90 and C99, etc. This one has two suboptions of its own:-fsanitize=shift-baseand-fsanitize=shift-exponent. 🌀-fsanitize=shift-exponent— checks that the second argument of a shift operation isn't 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, it checks that the result of the shift isn't undefined. Same lil reminder as above: what's undefined differs slightly between C and C++, and between ISO C90 and C99, etc. 🤏-fsanitize=integer-divide-by-zero— detects integer division by zero. Do not do the divide by zero bestie, it's giving chaos. 💀-fsanitize=unreachable— with this one, the compiler flips the"__builtin_unreachable"call into a diagnostics message call instead. And jsyk: when you actually reach that"__builtin_unreachable"call, the behavior is undefined. 🚫-fsanitize=vla-bound— tells the compiler to check that the size of a variable length array is positive. Positive only, bestie. 💖-fsanitize=null— this one's the pointer checker 🫵. Specifically, an app built with it turned on will throw an error message when it tries to dereference aNULLpointer, or if a reference (possibly an rvalue reference) gets bound to aNULLpointer, or if a method gets invoked on an object that aNULLpointer is pointing at. Yeah. All of that.-fsanitize=return— enables return statement checking ✅. Programs built with this on will throw an error message when the end of a non-void function is reached without actually returning a value. C++ only, so don't come at it with your C code.-fsanitize=signed-integer-overflow— signed integer overflow checking 🎯. It checks that the result of+,*, and both unary and binary-doesn't overflow in signed arithmetics. It also catches"INT_MIN / -1"signed division. Note that integer promotion rules must be taken into account — meaning, the following is not an overflow:
signed char a = SCHAR_MAX;
a++;-fsanitize=bounds— instrumenting array bounds 🧱. Various out-of-bounds accesses get detected. What does not get instrumented: flexible array members, flexible array member-like arrays, and initializers of variables with static storage.-fsanitize=bounds-strict— the strict version, and she does not play 🚨. Most out-of-bounds accesses get detected, including flexible array members and flexible array member-like arrays. Initializers of variables with static storage are still not instrumented though.-fsanitize=alignment— checks alignment of pointers when they're dereferenced 📐, or when a reference is bound to an insufficiently aligned target, or when a method or constructor is invoked on an insufficiently aligned object.-fsanitize=object-size— instruments memory references using the"__builtin_object_size"function 🧐. Various out-of-bounds pointer accesses get detected.-fsanitize=float-divide-by-zero— detects floating-point division by zero 🌊. Unlike the other similar options,-fsanitize=float-divide-by-zerois not enabled by-fsanitize=undefined, because floating-point division by zero can be a totally legit way of getting infinities and NaNs. Slay, math.-fsanitize=float-cast-overflow— enables floating-point type to integer conversion checking 🔄. It checks that the result of the conversion doesn't overflow. And again, unlike the other similar options,-fsanitize=float-cast-overflowis not enabled by-fsanitize=undefined. Also: this option does not work well with"FE_INVALID"exceptions enabled. 🥲-fsanitize=nonnull-attribute— instruments calls, checking whether null values aren't being passed to arguments marked as requiring a non-null value by the"nonnull"function attribute. 🚧-fsanitize=returns-nonnull-attribute— instruments return statements in functions marked with the"returns_nonnull"function attribute, to detect returning null values from such functions. No sneaky nulls allowed. 🙅-fsanitize=bool— instruments loads from bool. If a value other than 0/1 gets loaded, a run-time error is issued. 0 or 1. Pick one. 🎀-fsanitize=enum— instruments loads from an enum type. If a value outside the range of values for that enum type gets loaded, a run-time error is issued. 👑-fsanitize=vptr— instruments C++ member function calls, member accesses, and some conversions between pointers to base and derived classes, to verify that the referenced object has the correct dynamic type. RTTI girlies rise 🦋.-fsanitize=pointer-overflow— instruments pointer arithmetics. If the pointer arithmetics overflows, a run-time error is issued. ➕-fsanitize=builtin— instruments arguments to selected builtin functions 🔧. If an invalid value gets 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.
And for the record: while -ftrapv makes traps get emitted for signed overflows, -fsanitize=undefined gives you a diagnostic message instead. This currently works only for the C family of languages. 🫶
-fno-sanitize=all— disables all previously enabled sanitizers 🧼. And no,-fsanitize=allis not allowed, because some sanitizers literally cannot be used together. It's giving incompatible, periodt. 💅-fasan-shadow-offset=number— forces GCC to use a custom shadow offset in AddressSanitizer checks 🕵️. Useful for experimenting with different shadow memory layouts in Kernel AddressSanitizer.-fsanitize-sections=s1,s2,...— sanitize global variables in selected user-defined sections 🗂️. Eachsimay contain wildcards. ✨-fsanitize-recover[=opts]—-fsanitize-recover=controls error recovery mode for the sanitizers named in the comma-separated list ofopts🩹. Turning it on for a sanitizer component makes it try to keep the program running as if no error happened. Which 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 flips that: only the first detected error gets reported, and then the program exits with a non-zero exit code. 🚪
Right now 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 👀. Both -fsanitize-recover=all and -fno-sanitize-recover=all are also accepted: the first enables recovery for all sanitizers that support it, the second disables recovery for all sanitizers that support it.
And even if recovery mode is turned on compiler-side, it has to be enabled on the runtime library side too — otherwise the failures are still fatal 💀. The runtime library defaults to "halt_on_error=0" for ThreadSanitizer and UndefinedBehaviorSanitizer, while the default value for AddressSanitizer is "halt_on_error=1". You can override this by setting the "halt_on_error" flag in the corresponding environment variable.
⚠️ Heads up bestie: the syntax without an explicitoptsparameter is deprecated. It's equivalent to specifying anoptslist of:undefined,float-cast-overflow,float-divide-by-zero,bounds-strict
-fsanitize-address-use-after-scope— enables sanitization of local variables to detect use-after-scope bugs 🔍. This option sets-fstack-reusetonone.-fsanitize-undefined-trap-on-error— instructs the compiler to report undefined behavior using"__builtin_trap"instead of a"libubsan"library routine 🪤. The glow-up here: the"libubsan"library isn't needed and isn't linked in, so this is usable even in freestanding environments. 🌱-fsanitize-coverage=trace-pc— enables coverage-guided fuzzing code instrumentation 🎯. Inserts a call to"__sanitizer_cov_trace_pc"into every basic block.-fsanitize-coverage=trace-cmp— enables dataflow guided fuzzing code instrumentation 🌊. It 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]— bestie, this one is security core 🔥. It turns on code instrumentation of control-flow transfers so the program checks that the target addresses of control-flow transfer instructions (think indirect function call, function return, indirect jump) are actually valid. Why do we care? Because it stops someone from diverting the flow of control to an unexpected target. It's built to protect against threats like Return-oriented Programming (ROP), and also call/jmp-oriented programming (COP/JOP). Rude little attackers, blocked 💅.
Okay so the values, one by one:
"branch"tells the compiler to implement checking of validity of control-flow transfer at the point of indirect branch instructions, i.e.call/jmpinstructions."return"implements checking of validity at the point of returning from a function."full"is just an alias for specifying both"branch"and"return"."none"turns off instrumentation."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-protectionvalues. The value"check"is ignored at the compile time.
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. iconic honestly ✨
-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), it emits extra code to compute and verify the reversed condition, and to call__builtin_trapif the results do not match. Use it with-fharden-conditional-branchesto cover all conditionals 🫶.-fharden-conditional-branches— for every non-vectorized conditional branch that survives gimple optimizations, it emits extra code to compute and verify the reversed condition, and to call__builtin_trapif the result is unexpected. Use it with-fharden-comparesto cover all conditionals. Look at them, a little duo, obsessed 🌸-fstack-protector— emits extra code to check for buffer overflows, like stack smashing attacks 🛡️. How? By adding a guard variable to functions with vulnerable objects. That includes functions that callalloca, 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, periodt.-fstack-protector-all— like-fstack-protectorexcept that all functions are protected. No one is left out, everybody gets a guard 💖.-fstack-protector-strong— like-fstack-protectorbut it includes additional functions to be protected --- those that have local array definitions, or have references to local frame addresses. And same vibe as before: only variables that are actually allocated on the stack are considered, optimized away variables or variables allocated in registers don't count.-fstack-protector-explicit— like-fstack-protectorbut it only protects those functions which have thestack_protectattribute. If you didn't ask, you don't get it, iykyk.-fstack-check— generates 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. So it's giving "I built the scaffolding, you go do the actual job" energy.
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 😭:
- 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.
- 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.
- 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. Different tool for a different job, bestie 💅
-fstack-clash-protection— generates 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. Sneaky little jump, denied.
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.
-fstack-limit-register=reg-fstack-limit-symbol=sym-fno-stack-limit— these three 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— generates 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. Teamwork fr fr 🫶
-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. Caught in 4k, immediately.
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. Priority order, we love to see it ✨
-fvtv-debug— when used in conjunction with-fvtable-verify=stdor-fvtable-verify=preinit, it 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 namedvtv_set_ptr_data.login the directory named by the environment variableVTV_LOGS_DIRif 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. Learn from my mistakes bestie 🍓
-fvtv-counts— this is a debugging flag 🔎. When used in conjunction with-fvtable-verify=stdor-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 namedvtv_count_data.login the directory named by the environment variableVTV_LOGS_DIRif 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 tovtv_class_set_sizes.login the same directory. Numbers go brrr 📊💜
💖 quick note bestie: this feature appends data to the log files. If you want fresh log files, you gotta delete the existing ones first. no cap.
-finstrument-functions— okay so this one generates instrumentation calls for entry and exit to functions. like, just after a function enters and just before it exits, these profiling functions get called with the address of the current function and its call site. (heads up tho: on some platforms,__builtin_return_addressdoes not work beyond the current function, so the call site info might 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. Which means 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. lucky you 💅)
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-exclude-file-list=file,file,...— sets the list of functions that are excluded from instrumentation (peep the description of-finstrument-functionsabove 👆). 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's considered to be a match. 👑
For example:
-finstrument-functions-exclude-file-list=/bits/stl,include/sysexcludes 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 asvector<int> blah(const vector<int> &), 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's 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 to0so 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 interfacegen_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. IfN=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.
The maximum value of N and M is 65535. 💀
🎀 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 definition1. 🍓-D name=definition— the contents of definition are tokenized and processed as if they appeared during translation phase three in a#definedirective. 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-Doption.-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 formakedescribing the dependencies of the main source file. The preprocessor outputs onemakerule containing the object file name for that source file, a colon, and the names of all the included files, including those coming from-includeor-imacroscommand-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-Mbut 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-Mor-MM, specifies a file to write the dependencies to. If no-MFswitch 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-Mrequesting dependency generation,-MGassumes 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#includedirective without prepending any path.-MGalso 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 errorsmakegives if you remove header files without updating theMakefileto match. 👑
This is typical output:
test.o: test.c test.h test.h:-MT target— changes the target of the rule that dependency generation spits out. 💅
By default, CPP grabs the name of the main input file, yeets any directory components, drops any file suffix like .c, and then appends the platform's usual object suffix. That result is the target. Simple as.
But an -MT option sets the target to be exactly the string you type, no editing, no vibes-based cleanup. Want multiple targets? You can either pass them as a single argument to -MT, or just stack a bunch of -MT options. Both work, queen.
For example, -MT '$(objpfx)foo.o' might give you
$(objpfx)foo.o: foo.c-MQ target— literally the same as-MT, except it quotes any characters that are special to Make. 🎀 So-MQ '$(objpfx)foo.o'gives
$$(objpfx)foo.o: foo.cAnd the default target? Automatically quoted, as if you'd passed it with -MQ. It's giving self-care.
-MD—-MDis equivalent to-M -MF file, except that-Eis not implied. 😌
The driver figures out 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 together with -E, any -o switch is understood to specify the dependency output file — but if it's used without -E, each -o is understood to specify a target object file.
And since -E is not implied, -MD can be used to generate a dependency output file as a lil side effect of the compilation process. Multitasking icon behaviour. ✨
-MMD— like-MDexcept it only mentions user header files, not system header files. Bestie stays in her lane. 🦋-fpreprocessed— tells 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 though, so 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 — lowkey just there to vibe.
-fpreprocessed is implicit if the input file has one of the extensions .i, .ii or .mi. Those 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. Cha-ching. 💰 (okay no, it's not about money, it's literally just the character)-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. No shortcuts, no glow-ups. 🎀-fmax-include-depth=depth— sets the maximum depth of the nested#include. The default is200. 🌀-ftabstop=width— sets 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]— tracks locations of tokens across macro expansions. This lets the compiler emit diagnostics about the current macro expansion stack when a compilation error happens inside a macro expansion. 🔥
Heads up though: 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
0oflevelde-activates this option. - Value
1tracks token 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
2tracks token 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. Yes, by default. She's always on. 💅
-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.
-fexec-charset=charset— sets the execution character set, used for string and character constants. The default isUTF-8. charset can be any encoding supported by the system's"iconv"library routine. 🌸-fwide-exec-charset=charset— sets the wide execution character set, used for wide string and character constants. The default isUTF-32orUTF-16, whichever corresponds to the width of"wchar_t". 🦋
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". Not a drill, bestie. 💀
-finput-charset=charset— sets 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 is listed, and not the files that were used to create it — because those files are not consulted when a precompiled header is used. Makes sense, honestly.
-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-fpreprocessedis 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— enables 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. Old form still slays, but the new one is cleaner.
-A -predicate=answer— cancel an assertion with the predicate predicate and answer answer. Bye bestie. 👋-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. Protective queen behaviour, honestly. 👑
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. Like, don't confuse the poor thing.
-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. Don't get it twisted.
-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. Truly the cursed-but-canonical corner of C, iykyk. 🌀
The nine trigraphs and their replacements are
Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??-
Replacement: [ ] { } # \ ^ | ~Okay so by default GCC literally ignores trigraphs, but in standard-conforming modes it converts them. Check the -std and -ansi options if you wanna know more. ✨
-remap— turns on some special code to work around file systems that only allow very short file names, like MS-DOS. Ancient vibes but okay bestie, we support it. 🫶-H— prints the name of every header file used, on top of all the other normal stuff. Each name gets indented so you can see how deep in the#includestack it is. Precompiled header files get printed too, even if they turn out to be invalid — an invalid precompiled header shows up with...xand a valid one with...!. 💅-dletters— tells GCC to make debugging dumps during compilation, exactly as specified by letters. The flags documented here are the ones relevant to the preprocessor. Other letters get interpreted by the compiler proper, or reserved for future GCC versions, so they're silently ignored. And if you specify letters whose behavior conflicts? The result is undefined. 😭-dM— instead of the normal output, generate a list of#definedirectives for all the macros defined while the preprocessor runs, including predefined macros. This is your way of finding out what's predefined in your version of the preprocessor. Assuming you have no filefoo.h, the command
touch foo.h; cpp -dM foo.hshows all the predefined macros.
Heads up: if you use-dMwithout the-Eoption,-dMgets interpreted as a synonym for-fdump-rtl-mach. 🫠
-dD— like-dMexcept in two ways: it does not include the predefined macros, and it outputs both the#definedirectives and the result of preprocessing. Both kinds of output go to the standard output file.-dN— like-dD, but it only emits the macro names, not their expansions.-dI— outputs#includedirectives in addition to the result of preprocessing.-dU— like-dDexcept that only macros that are expanded, or whose definedness is tested in preprocessor directives, get output. The output is delayed until the macro is used or tested, and#undefdirectives are also output for macros that were tested but undefined at that moment.
-fdebug-cpp— this one is only useful for debugging GCC itself. 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,optionto bypass the compiler driver and pass option directly through to the preprocessor. If option contains commas, it gets split into multiple options at the commas. BUT — and this is the important part — many options are modified, translated or interpreted by the compiler driver before being passed to the preprocessor, and-Wpforcibly bypasses this phase. The preprocessor's direct interface is undocumented and subject to change, so whenever possible you should avoid using-Wpand let the driver handle the options instead. 👑-Xpreprocessor option— passes option as an option to the preprocessor. Use this to supply system-specific preprocessor options that GCC doesn't 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. Two trips, no shortcuts bestie. 🎀
-no-integrated-cpp— performs preprocessing as a separate pass before compilation. By default, GCC does preprocessing as an integrated part of input tokenization and parsing. If this option is given, the appropriate language front end (cc1,cc1plus, orcc1objfor C, C++, and Objective-C, respectively) is instead invoked twice — once for preprocessing only, and once for the actual compilation of the preprocessed input. This can be useful alongside the-Bor-wrapperoptions to specify an alternate preprocessor, or to do extra processing of the program source between normal preprocessing and compilation. ✨-flarge-source-files— adjusts GCC to expect large source files, at the cost of slower compilation and higher memory usage.
Specifically: GCC normally tracks both column numbers and line numbers within source files, and normally prints both of those numbers in diagnostics. However, once it has processed a certain number of source lines, it stops tracking column numbers and only tracks line numbers. That means diagnostics for later lines don't include column numbers. It also means options like -Wmisleading-indentation cease to work at that point — though the compiler prints a note if this happens. Passing -flarge-source-files significantly increases the number of source lines GCC can process before it stops tracking columns. 📈
🛠️ Passing Options to the Assembler 🛠️
You can pass options to the assembler. Yes, the assembler has feelings too.
-Wa,option— passes option as an option to the assembler. If option contains commas, it's split into multiple options at the commas.-Xassembler option— passes option as an option to the assembler. Use this to supply system-specific assembler options that GCC doesn't 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. Same energy as -Xpreprocessor, iykyk. 💅
🔗 Options for Linking 🔗
These options come into play when the compiler links object files into an executable output file. They're meaningless if the compiler isn't doing a link step. Periodt.
- object-file-name — a file name that doesn't 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 what you want, 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 lets you build shared libraries without position-independent code on architectures where that's 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 get 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 the 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 the object files in an 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's recommended to link such objects into a static library instead. Alternatively it's possible to use H.J. Lu's binutils with support for mixed objects. 🫶
-fuse-ld=bfd— use thebfdlinker instead of the default linker.-fuse-ld=gold— use thegoldlinker instead of the default linker.-fuse-ld=lld— use the LLVMlldlinker 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're specified. Sofoo.o -lz bar.osearches libraryzafter filefoo.obut beforebar.o. Ifbar.orefers to functions inz, those functions may not be loaded. Order matters, bestie. 💀
-lobjc— you need this special case of the-loption 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-nodefaultlibsis used. 🎀-nodefaultlibs— okay so this one says do NOT touch the standard system libraries when you're linking, bestie 💅 the linker only gets the libraries you hand it, and any options that would normally pull in system library linkage — like-static-libgccor-shared-libgcc— get straight up ignored. The standard startup files still show up like normal tho, unless you also throw-nostartfilesin there. Heads up: the compiler might still generate calls tomemcmp,memset,memcpyandmemmove, and those are usually resolved by entries in libc — so if you use this, you gotta supply those entry points through some other mechanism, fr fr 😭-nolibc— this one's like "don't use the C library, or any system libraries that are tightly coupled to it, when linking" 🚫 Still links with the startup files, libgcc, or toolchain-provided language support libraries likelibgnat,libgfortranorlibstdc++— unless you also use options that prevent those from being included. What it typically does is yeet-lcoff the link command line, plus the system libraries that normally tag along with it and become meaningless once you assume there's no C library — think-lpthreador-lmin some configurations. This is meant for bare-board targets where there genuinely is no C library available 🦋-nostdlib— the big one, the icon, the moment ✨ Don't use the standard system startup files or libraries when linking. No startup files, and only the libraries you specify get passed to the linker — and again, options specifying linkage of the system libraries like-static-libgccor-shared-libgcc? Ignored. Periodt.
And yes, same deal as before: the compiler may generate calls to memcmp, memset, memcpy and memmove, which are usually resolved by entries in libc — supply those entry points through some other mechanism when this option is specified 💀
Now listen, one of the standard libraries that -nostdlib and -nodefaultlibs bypass is libgcc.a — a library of internal subroutines that GCC uses to overcome shortcomings of particular machines, or special needs for some languages. 👑
In most cases you actually need libgcc.a, even when you're trying to dodge the other standard libraries. In other words: when you specify -nostdlib or -nodefaultlibs, you should usually also specify -lgcc. That way you have no unresolved references to internal GCC library subroutines. (An example of such an internal subroutine is __main, which is used to make sure C++ constructors get called.) 🫶
-eentry /--entry=entry— tell it the program entry point is entry. The argument gets interpreted by the linker, and 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. That's it, that's the option 💅-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 it can be loaded at any address without a dynamic linker. And same rule as-pie: 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 🧵 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 use it consistently for both compilation and linking, mkay?-r— produce a relocatable object as output. This is also known as partial linking ✨-rdynamic— pass the flag-export-dynamicto the ELF linker, on targets that support it. This tells the linker to add all symbols to the dynamic symbol table, not just the used ones. You need this for some uses ofdlopen, or to allow obtaining backtraces from within a program 👑-s— remove all symbol table and relocation information from the executable. Gone. Poof. 🫥-static— on systems that support dynamic linking, this overrides-pieand 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 the 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 where an application should use the shared libgcc instead of the static version. The most common one is when the application wants to throw and catch exceptions across different shared libraries — in that case, each of the libraries and the application itself should use the shared libgcc 🫶
So that's why the G++ driver automatically adds -shared-libgcc whenever you build a shared library or a main executable — C++ programs typically use exceptions, so it's just the right thing to do 💅
If you instead use the GCC driver to create shared libraries, you might find they're not always linked with the shared libgcc. Here's the tea: if GCC finds, at its configuration time, that you have a non-GNU linker or a GNU linker that doesn't 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 instead. This lets exceptions 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, so that it's linked with the shared libgcc. No shortcuts, bestie 🙈
-static-libasan— when the-fsanitize=addressoption is used to link a program, the GCC driver automatically links against libasan. If libasan is available as a shared library, and the-staticoption is not used, then it links against the shared version of libasan. The-static-libasanoption directs the GCC driver to link libasan statically, without necessarily linking other libraries statically 👑-static-libtsan— when the-fsanitize=threadoption is used to link a program, the GCC driver automatically links against libtsan. If libtsan is available as a shared library, and the-staticoption is not used, then it links against the shared version of libtsan. The-static-libtsanoption directs the GCC driver to link libtsan statically, without necessarily linking other libraries statically 🧵-static-liblsan— when the-fsanitize=leakoption is used to link a program, the GCC driver automatically links against liblsan. If liblsan is available as a shared library, and the-staticoption is not used, then it links against the shared version of liblsan. The-static-liblsanoption directs the GCC driver to link liblsan statically, without necessarily linking other libraries statically 💧-static-libubsan— when the-fsanitize=undefinedoption is used to link a program, the GCC driver automatically links against libubsan. If libubsan is available as a shared library, and the-staticoption is not used, then it links against the shared version of libubsan. The-static-libubsanoption directs the GCC driver to link libubsan statically, without necessarily linking other libraries statically 🦋-static-libstdc++— when theg++program is used to link a C++ program, it normally automatically links against libstdc++. If libstdc++ is available as a shared library, and the-staticoption is not used, then it links against the shared version of libstdc++. That's normally fine. However, it's 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 theg++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 👑-Tscript — use script as the linker script. This option is supported by most systems using the GNU linker. On some targets, like bare-board targets without an operating system, the-Toption may be required when linking, to avoid references to undefined symbols 😭-Xlinkeroption — pass option as an option to the linker. You can use this to supply system-specific linker options that GCC doesn't 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 that passes the entire string as a single argument, which is not what the linker expects 💀
When using the GNU linker, it's usually more convenient to pass arguments to linker options using the option=value syntax rather 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 gets split into multiple options at the commas. You can use this syntax to pass an argument to the option. For example,-Wl,-Map,output.mappasses-Map output.mapto the linker. When using the GNU linker, you can also get the same effect with-Wl,-Map=output.map🌈-usymbol — bestie, this is the "pretend it doesn't exist so somebody has to define it" move 💅 It makes the compiler act like symbol is undefined, which forces the linker to go find library modules that define it. You can stack-uas many times as you want with different symbols to drag in extra library modules. Slay.-zkeyword — this one gets passed straight through to the linker, keyword and all, no notes. Wanna know which keywords are legal and what they mean? Go read your linker's docs, bestie, that's where the lore lives 🫶
✨ Options for Directory Search ✨
These ones are all about where to look, okay? Directories for header files, for libraries, and for pieces of the compiler itself 👑
-Idir-iquotedir-isystemdir-idirafterdir — adds dir to the list of directories searched for header files during preprocessing. And if dir starts with=or$SYSROOT, then that=or$SYSROOTgets swapped out for the sysroot prefix — peep--sysrootand-isysrootfor the tea ☕
Directories you give with -iquote only apply to the quote form of the directive, aka #include "file". Directories from -I, -isystem, or -idirafter apply to both #include "file" and #include <file> lookups.
You can throw any number or combination of these options on the command line to search several directories for headers. The lookup order is, and I cannot stress this enough, exactly this:
- For the quote form of the include directive, the directory of the current file is searched first.
- For the quote form of the include directive, the directories specified by
-iquoteoptions are searched in left-to-right order, as they appear on the command line. - Directories specified with
-Ioptions are scanned in left-to-right order. - Directories specified with
-isystemoptions are scanned in left-to-right order. - Standard system directories are scanned.
- Directories specified with
-idirafteroptions are scanned in left-to-right order.
You can use -I to override a system header file and swap in your own version, because these directories get searched before the standard system header directories. BUT — and this is the part where I grab your shoulders — do not use this option to add directories full of vendor-supplied system header files. That's what -isystem is for, bestie 🙈
The -isystem and -idirafter options also mark the directory as a system directory, so it gets the same special treatment as the standard system directories. Iconic behaviour.
Now here's a spicy one: if a standard system include directory, or a directory you gave 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. Why? To make sure GCC's procedure for fixing buggy system headers and the ordering for the #include_next directive don't get inadvertently changed. So if you genuinely need to change the search order for system directories, use the -nostdinc and/or -isystem options instead.
-I-— splits the include path. This option has been deprecated. Please use-iquoteinstead for-Idirectories before the-I-, and just remove the-I-option entirely, okay? 💀
Any directories specified with -I options before -I- are searched only for headers requested with #include "file"; they are not searched for #include <file>. And if you add more directories with -I options after the -I-, those get searched for all #include directives.
Also, -I- inhibits using the directory of the current file as the first search directory for #include "file". And there is no way to override this effect of -I-. Periodt.
-iprefixprefix — sets prefix as the prefix for all the-iwithprefixoptions that come after it. If the prefix represents a directory, include the final/, babes.-iwithprefixdir-iwithprefixbeforedir — appends dir to the prefix you set earlier with-iprefix, and adds the resulting directory to the include search path.-iwithprefixbeforedrops it in the same spot-Iwould;-iwithprefixputs it where-idirafterwould. Same energy, different placement 🦋-isysrootdir — this is like the--sysrootoption but it only applies to header files (except for Darwin targets, where it applies to both header files and libraries, because of course). See the--sysrootoption for more.-imultilibdir — uses 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 you explicitly named with-I,-iquote,-isystem, and/or-idirafteroptions get searched (plus the directory of the current file, if that applies). You're on your own and honestly? Respect.-nostdinc++— don't 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, iykyk.)-iplugindir=dir — sets the directory to search for plugins that are passed by-fplugin=name instead of-fplugin=path/name.so. Not meant for users, bestie — this one's only passed by the driver. Stay in your lane 💅-Ldir — adds directory dir to the list of directories to be searched for-l.-Bprefix — specifies where to find the executables, libraries, include files, and data files of the compiler itself. Main character option, fr.
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 isn't found, or if -B wasn't specified at all, the driver tries two standard prefixes, /usr/lib/gcc/ and /usr/local/lib/gcc/. If neither of those turns up a file name that's found, the unmodified program name is searched for using the directories specified in your PATH environment variable.
The compiler checks whether the path you gave with -B refers to a directory, and if necessary it adds a directory separator character at the end of the path. Little detail, big consequences ✨
-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 that 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's not found there, the two standard prefixes above are tried, and that's all. The file is left out of the link if it's 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 gets replaced by [dir/]include. This is to help with boot-strapping the compiler. Cursed but iconic.
-no-canonical-prefixes— do not expand any symbolic links, resolve references to/../or/./, or make the path absolute when generating a relative prefix. We leave the paths exactly as messy as we found them 💜--sysroot=dir — uses dir as the logical root directory for headers and libraries. For example, if the compiler normally searches for headers in/usr/includeand libraries in/usr/lib, it instead searchesdir/usr/includeanddir/usr/lib. Vibe check: passed.
If you use both this option and the -isysroot option, then --sysroot applies to libraries, but -isysroot applies to header files.
The GNU linker (beginning with version 2.16) has the necessary support for this option. If your linker doesn't support it, the header file aspect of --sysroot still works, but the library aspect does not. F in the chat for the libraries 🫠
--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 indir/suffix/usr/includeinstead ofdir/usr/include. This option disables adding such a suffix. No suffix, no problem 💅
✨ Options for Code Generation Conventions ✨
These machine-independent options control the interface conventions used in code generation. Very "the rules of the universe" of them 👑
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. Easy math, bestie 🧚
-fstack-reuse=reuse-level — this option controls stack space reuse for user declared local/auto variables and compiler generated temporaries. reuse_level can beall,named_vars, ornone.allenables stack reuse for all local variables and temporaries,named_varsenables the reuse only for user defined local variables with names, andnonedisables stack reuse completely. The default value isall. This 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 variable's lifetime 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. So like… heads up 💀
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…and the code we were all side-eyeing looked a lil something like this 💀
{
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?
}So here's the tea, bestie 🫖: 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. But — and this is the messy part 😭 — some of the legacy code relies on the behavior of older compilers in which temporaries' stack space is not reused, so 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-ftrapvand-fwrapvoverride each other, so using-ftrapv -fwrapvon the command-line results in-fwrapvbeing effective. Note that only active options override, so using-ftrapv -fwrapv -fno-wrapvon the command-line results in-ftrapvbeing 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-ftrapvand-fwrapvoverride each other, so using-ftrapv -fwrapvon the command-line results in-fwrapvbeing effective. Note that only active options override, so using-ftrapv -fwrapv -fno-wrapvon the command-line results in-ftrapvbeing 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-pointerand when negated implies-fwrapv -fwrapv-pointer. no cap 🫶-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 asSIGALRM. 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 thepureorconstattributes. 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 theSTB_GNU_UNIQUEbinding to make sure that definitions of template static data members and static local variables in inline functions are unique even in the presence ofRTLD_LOCAL; this is necessary to avoid problems with a library used by two differentRTLD_LOCALplugins depending on a definition in one of them and therefore disagreeing with the other one about the binding of the symbol. But this causesdlcloseto be ignored for affected DSOs; if your program relies on reinitialization of a DSO viadlcloseanddlopen, you can use-fno-gnu-unique. 😭-fpcc-struct-return— returnshortstructandunionvalues 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-returnswitch is not binary compatible with code compiled with the-freg-struct-returnswitch. Use it to conform to a non-default application binary interface.
-freg-struct-return— returnstructandunionvalues 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. slay 🎀
Warning: code compiled with the-freg-struct-returnswitch is not binary compatible with code compiled with the-fpcc-struct-returnswitch. Use it to conform to a non-default application binary interface.
-fshort-enums— allocate to anenumtype only as many bytes as it needs for the declared range of possible values. Specifically, theenumtype is equivalent to the smallest integer type that has enough room. 🐛
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 forwchar_tto beshort unsigned intinstead 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 theexternkeyword, 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#identdirective. 🙅♀️-finhibit-size-directive— don't output a.sizeassembler 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 # <retval>
# 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, <retval>
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,"",@progbitsokay so those little # comments? they're for humans, not machines 💖 which means the exact format of them is subject to change — so don't go writing a parser that depends on it, bestie, i'm begging 🙈
-frecord-gcc-switches— this one literally writes down the command line you used ✨ straight into the object file it's building. iconic behaviour tbh. it's only actually implemented on some targets, and the exact format of the recording depends on the target and the binary file format, but usually it shows up as a section full of ASCII text. it's cousins with-fverbose-asm, except that one only dumps info into the assembler output as comments, so it never makes it into the object file at all. and hey — go peek at-grecord-gcc-switchesif you want another way to stash compiler options inside the object file 👑-fpic— generate position-independent code (PIC), the kind that's suitable for use in a shared library, if the target machine supports it. this code reaches all constant addresses through a global offset table (GOT) 🗺️. the dynamic loader fills in the GOT entries when the program starts — and heads up, the dynamic loader is not part of GCC, it belongs to the operating system. if the GOT for your linked executable blows past a machine-specific max size, the linker yells at you saying-fpicdoesn't work, and then you just recompile with-fPICinstead. no drama, it's giving character development. (those maximums are 8k on the SPARC, 28k on AArch64, and 32k on the m68k and RS/6000. the x86 has no such limit 😌.)- PIC needs special support, so it only works on certain machines, periodt. for the x86, GCC supports PIC for System V but not for the Sun 386i. and code generated for the IBM RS/6000 is always position-independent, no notes.
- when this flag is set, the macros
"__pic__"and"__PIC__"are defined to1🎀
-fPIC— if the target machine supports it, emit position-independent code that's suitable for dynamic linking and avoids any limit at all on the size of the global offset table. this one actually makes a difference on AArch64, m68k, PowerPC and SPARC ✨- PIC requires special support, so again: only on certain machines.
- when this flag is set, the macros
"__pic__"and"__PIC__"are defined to2💅
-fpie-fPIE— these two are basically the pic/PIC girlies but the generated position-independent code can only be linked into executables. usually you use them to compile code that's gonna get linked with the-pieGCC option 🫶-fpieand-fPIEboth define the macros"__pie__"and"__PIE__". the macros have the value1for-fpieand2for-fPIE⭐
-fno-plt— do not use the PLT for external function calls in position-independent code. instead, load the callee address at call sites out of the GOT and branch to it 🔥. this gives you more efficient code because you eliminate PLT stubs and you expose GOT loads to optimizations. on architectures like 32-bit x86, where PLT stubs expect the GOT pointer in a specific register, this hands the compiler more register allocation freedom 😌. but lowkey important: lazy binding requires use of the PLT — with-fno-pltall external symbols get resolved at load time.- alternatively, the function attribute
"noplt"can be used to dodge calls through the PLT for specific external functions 👑 - and in position-dependent code, a few targets also convert calls to functions that are marked to not use the PLT into GOT-based calls instead.
- alternatively, the function attribute
-fno-jump-tables— don't use jump tables for switch statements, even when a jump table would be more efficient than the other code generation strategies 🙈. this one is for when you're building code that forms part of a dynamic linker and it cannot reference the address of a jump table — so it pairs up with-fpicor-fPIC💖. on some targets, jump tables don't need a GOT at all, and then this option isn't needed, so it's giving situational.-fno-bit-tests— don't use bit tests for switch statements, even where bit tests would be more efficient than the other code generation strategies 💀-ffixed-reg— treat the register named reg as a fixed register; generated code should never refer to it (except maybe as a stack pointer, frame pointer, or in some other fixed role) ✨- reg must be the name of a register. the accepted register names are machine-specific and defined in the
"REGISTER_NAMES"macro in the machine description macro file 🎀 - this flag has no negative form, because it specifies a three-way choice. it's not a toggle, bestie, it's a whole menu.
- reg must be the name of a register. the accepted register names are machine-specific and defined in the
-fcall-used-reg— treat the register named reg as an allocable register that gets clobbered by function calls. it may be allocated for temporaries or variables that don't 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. and using it for other registers that have fixed pervasive roles in the machine's execution model produces disastrous results 💀 no cap.
- 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 that's 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. and use of this flag for other registers with fixed pervasive roles in the machine's execution model produces disastrous results.
- and there's a different sort of disaster 🌪️ that results from using this flag for a register in which function values may be returned. yeah. read that twice.
- this flag does not have a negative form, because it specifies a three-way choice.
-fpack-struct[=n]— with no value specified, pack all structure members together with no holes 🧩. when a value is specified (which must be a small power of two), pack structure members according to that 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 makes GCC generate code that is not binary compatible with code generated without that switch. plus it makes the code suboptimal 😭. use it to conform to a non-default application binary interface. that's it, that's the whole reason.
-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 helping you link with legacy assembly code 👵✨
⚠️ Warning: the -fleading-underscore switch makes GCC generate code that is not binary compatible with code generated without that switch. use it to conform to a non-default application binary interface. and ngl, not all targets provide complete support for this switch 💀
-ftls-model=model— change up the thread-local storage model that gets used 🧵. the model argument should be one ofglobal-dynamic,local-dynamic,initial-execorlocal-exec. note that the choice is subject to optimization: the compiler may pick a more efficient model for symbols that aren't visible outside the translation unit, or if-fpicisn't given on the command line 💅- the default without
-fpicisinitial-exec; with-fpicthe default isglobal-dynamic.
- the default without
-ftrampolines— for targets that normally need trampolines for nested functions, always generate them instead of using descriptors 🎪. otherwise, for targets that don't need them — like, say, HP-PA or IA-64 — do nothing.- a trampoline is a small piece of code created at run time on the stack when the address of a nested function is taken, and it's used to call the nested function indirectly. which means it requires the stack to be made executable in order for the program to work properly 😭
-fno-trampolinesis 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 has to be prepared to deal with them. as of this writing,-fno-trampolinesis enabled by default only for Ada.- moreover, code compiled with
-ftrampolinesand code compiled with-fno-trampolinesare not binary compatible if nested functions are present 🌪️. so this option must be used on a program-wide basis and manipulated with extreme care. no mixing, bestie. - 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. so yeah, it's giving decorative.
-fvisibility=[default|internal|hidden|protected]— set the default ELF image symbol visibility to the specified option — all symbols get marked with it unless they're 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. fr fr, do it.
So despite the name bestie, default literally always means public 💅 — like, available to be linked against from outside the shared object. protected and internal are pretty useless in real-world usage, so the only other one you'll actually see is hidden. And if you don't pass -fvisibility at all? The default is default, i.e. every single symbol is public. Iconic but chaotic, fr fr.
There's a really good explanation of why you'd want your ELF symbols to have correct visibility in "How To Write Shared Libraries" by Ulrich Drepper — it lives at <https://www.akkadia.org/drepper/>. But — and this is the tea — there's a superior solution this option makes possible: instead of marking things hidden when the default is public, you flip it and make the default hidden, then mark things public ✨. That's the norm with DLLs on Windows, and if you use -fvisibility=hidden together with "__attribute__ ((visibility("default")))" instead of "__declspec(dllexport)", you get almost identical semantics with identical syntax. Huge boon for cross-platform projects, no cap 🫶.
If you're adding visibility support to code that already exists, you may find "#pragma GCC visibility" useful 👑. It works by you wrapping the declarations you care about with, for example, "#pragma GCC visibility push(hidden)" and "#pragma GCC visibility pop". But keep in mind that symbol visibility should be seen as part of the API interface contract — so all new code should always specify visibility when it isn't the default. Like, declarations that are only for use inside the local DSO should always be explicitly marked hidden, so you avoid PLT indirection overheads 💀. Making it abundantly clear also helps readability and self-documentation of the code. And heads up: because of ISO C++ specification requirements, "operator new" and "operator delete" must always be of default visibility. Periodt.
⚠️ Warning bestie: headers from outside your project — especially system headers and headers from any other library you use — might 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 zero modifications 🌈. However! That means calls to "extern" functions with no explicit visibility use the PLT, so it's more effective to use "__attribute ((visibility))" and/or "#pragma GCC visibility" to tell the compiler which "extern" declarations should be treated as hidden. Slay.
Also note that -fvisibility does affect C++ vague linkage entities. Which means, for instance, that an exception class thrown between DSOs must be explicitly marked with default visibility so that the type_info nodes are unified between the DSOs 😭.
There's an overview of all these techniques, their benefits, and how to use them at <https://gcc.gnu.org/wiki/Visibility> 💖.
-fstrict-volatile-bitfields— use this if accesses to volatile bit-fields (or other structure fields, though 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 don't contain any portion of the bit-field, or memory-mapped registers unrelated to the one being updated 💀.
In some cases, like 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's correctly aligned for the target machine. In that 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. So it's 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 one 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, so the only useful form of the option is -fno-sync-libcalls. This option is used in the implementation of the libatomic runtime library.
🛠️ GCC Developer Options 🛠️
Okay so 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. That means options that produce debug dumps at various points in the compilation, options that print statistics like memory use and execution time, and options that print info about GCC's configuration, like where it searches for libraries 🌸. You should rarely need any of these for ordinary compilation and linking tasks, bestie.
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 gets constructed by concatenating the base dump file name, a pass number, a phase letter, and a pass name. The base dump file name is the name of the output file produced by the compiler if explicitly specified and not an executable; otherwise it's 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're 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's 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-btl1and-fdump-rtl-btl2enable 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-ce3enable 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-cse1and-fdump-rtl-cse2enable 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-dce1and-fdump-rtl-dce2enable 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-fwprop1and-fdump-rtl-fwprop2enable dumping after the two forward propagation passes.
-fdump-rtl-gcse1-fdump-rtl-gcse2-fdump-rtl-gcse1and-fdump-rtl-gcse2enable 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— dumps the goods right after iterated register allocation 💅-fdump-rtl-jump— dump after the second jump optimization, bestie, the second one, keep up 🦋-fdump-rtl-loop2—-fdump-rtl-loop2turns on dumping after the rtl loop optimization passes 🔁-fdump-rtl-mach— dump after the machine dependent reorganization pass runs, but like… only if that pass even exists on your target 🙈-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-sched1and-fdump-rtl-sched2enable 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-sibling— dump after sibling call optimizations 👯-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 🪓 (five!! rounds!! iconic)-fdump-rtl-sms— dump after modulo scheduling. Lowkey this pass only runs on some architectures, so don't come for me if it's quiet 🫶-fdump-rtl-stack— dump after converting from GCC's"flat register file"registers to the x87's stack-like registers. This pass only runs on x86 variants 💻-fdump-rtl-subreg1,-fdump-rtl-subreg2—-fdump-rtl-subreg1and-fdump-rtl-subreg2enable dumping after the two subreg expansion passes 2️⃣-fdump-rtl-unshare— dump after all rtl has been unshared 🔓-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— ngl these dumps are defined but they always produce empty files 💀 like they're literally there and there's nothing in them. the audacity.-da,-fdump-rtl-all— produce all the dumps listed above 👑 every single one, no crumbs-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 💀💀 (yes fr, that's what it does)-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-dpannotation 🔗-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 usediffon 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 🧊 (an ICE?? in my compiler?? it's more likely than you think)-fdump-unnumbered— when doing debugging dumps, suppress instruction numbers and address output. This makes it more feasible to usediffon debugging dumps for compiler invocations with different options, in particular with and without-g🆚-fdump-unnumbered-links— when doing debugging dumps (see-doption above), suppress instruction numbers for the links to the previous and next instructions in a sequence 🔗
the inter-procedural analysis dump era 💖
-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 🫂
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-lang— dump language-specific information. The file name is made by appending.langto 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-treeoption. 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. Optionslineno(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.statisticsto the source file name, and the file is created in the same directory as the output file. If the-optionform is used,-statscauses counters to be summed over the whole compilation unit while-detailsdumps every event as the passes generate them. The default with no option is to sum counters for each function compiled 📈
tree dumps, the main character 🎀
-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-optionsform 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 tofile.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, exceptraw,slim,verboseandlineno🌟optall— turn on all optimization options, i.e.,optimized,missed, andnote💅
To determine what tree dumps are available or find the dump for a pass of interest follow the steps below.
- Run GCC with
-fdump-passesand then go stare at the stderr output like it's a group chat you're catching up on, hunting for the code that matches the pass you care about. For example, the codes"tree-evrp","tree-vrp1", and"tree-vrp2"are the three Value Range Propagation passes — besties, the number at the end is just there to tell apart separate invocations of the same pass. 💅 - To actually make the dump file happen, you slap the pass code onto the end of the
-fdump-prefix and call GCC with it. Say you want the dump from the Early Value Range Propagation pass — you'd run GCC with the-fdump-tree-evrpoption. You can also name the dump file yourself if you're feeling fancy. If you don't, GCC makes one the way described below. ✨ - Find the pass dump in a file whose name is three components separated by periods, okay? Component one: the name of the source file GCC was invoked to compile. Component two: a numeric suffix showing the pass number, followed by the letter
tfor tree passes (and the letterrfor RTL passes). Component three: the pass code. So the Early VRP pass dump might be chilling in a file calledmyfile.c.038t.evrpin the current working directory. ⚠️ Heads up though: those numeric codes are not stable and can change between versions of GCC, so don't get attached.
-fopt-info-fopt-info-options-fopt-info-options=filename— controls optimization dumps from all the various optimization passes. If you use the-optionsform, then options is a list of--separated option keywords that pick which dump details and optimizations you want. 🎀
The options split into three little groups:
- options saying what kinds of messages should be emitted,
- options saying how verbose the dump should be, and
- options saying which optimizations to include.
You can freely mix options from all three groups because they don't overlap, no cap. But if anything conflicts? The later options override the earlier ones on the command line. Periodt.
These are the options that control which kinds of messages get emitted:
optimized— print info whenever an optimization is successfully applied. Which info counts as relevant is up to the pass. For example, the vectorizer passes print the source location of loops that got successfully vectorized. 🔥missed— print info about missed optimizations. Individual passes decide what to include in the output. 😭note— print verbose info about optimizations, like certain transformations and more detailed messages about decisions and such.all— print detailed optimization information. This includesoptimized,missed, andnote.
This next one controls the dump verbosity:
internals— by default only "high-level" messages get emitted. This option turns on extra, more detailed messages, which are likely only interesting to GCC developers. 💀
And you can use one or more of these keywords 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: dump messages about successful optimizations from all the passes, but skip the messages treated as "internals".
If filename is provided, then the dumps from all the applicable optimizations get concatenated into filename. Otherwise the dump goes out onto stderr. You can pass multiple -fopt-info options, but only one of them can include a filename — if other filenames show up, everything except the first such option gets ignored. 🙈
Note that the output filename gets overwritten if there are multiple translation units. If you want a combined output from multiple translation units, use stderr instead.
In this example, the optimization info is output to stderr:
gcc -O3 -fopt-infoThis example:
gcc -O3 -fopt-info-missed=missed.alloutputs a missed optimization report from all the passes into missed.all. And this one:
gcc -O2 -ftree-vectorize -fopt-info-vec-missedprints info 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 after -fopt-info doesn't matter at all. 🌈
As another example,
gcc -O3 -fopt-info-inline-optimized-missed=inline.txtoutputs info about missed optimizations and optimized locations from all the inlining passes into inline.txt. Slay.
Finally, consider:
gcc -fopt-info-vec-missed=vec.miss -fopt-info-loop-optimized=loop.optHere 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 later options are ignored. So the only thing produced is vec.miss, which contains dumps from the vectorizer about missed opportunities. 💅
-fsave-optimization-record— write aSRCFILE.opt-record.json.gzfile detailing which optimizations were performed, for those optimizations that support-fopt-info.
This option is experimental and the format of the data inside that compressed JSON file is subject to change. 🫠
It's roughly a machine-readable version of -fopt-info-all — a collection of messages with source file, line number and column number, plus this extra data for each message:
- the execution count of the code being optimized, along with metadata about whether that came from actual profile data or just an estimate, so consumers can prioritize messages by code hotness,
- the function name of the code being optimized, where applicable,
- the "inlining chain" for the code being optimized — so when a function is inlined into several different places (which might themselves be inlined), the reader can tell the copies apart,
- 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.
On top of all that, 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 how much debugging output the scheduler prints to the dump files.
For n greater than zero, -fsched-verbose outputs the same info as -fdump-rtl-sched1 and -fdump-rtl-sched2. For n greater than one, it also outputs basic block probabilities, detailed ready list info 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 whole set of options used to explicitly disable/enable optimization passes. These are meant for debugging GCC. Regular compiler users should just use normal options to enable/disable 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 get a sequential number appended, 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 get a sequential number appended, 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, and the range is inclusive on both ends. If the range is trivial, the number pair can be simplified to a single number. If the function's call graph node's uid falls inside one of the specified ranges, the pass gets disabled for that function. The uid shows up in the function header of a dump file, and the pass names can be dumped using the-fdump-passesoption.-fdisable-tree-pass-fdisable-tree-pass=range-list— disable tree pass pass. See-fdisable-rtlfor the description of the 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 get a sequential number appended, starting from 1.-fenable-rtl-pass-fenable-rtl-pass=range-list— enable RTL pass pass. See-fdisable-rtlfor the option argument description and examples.-fenable-tree-pass-fenable-tree-pass=range-list— enable tree pass pass. See-fdisable-rtlfor the description of the option arguments.
Here are some examples showing how these options get used. 🍓
# 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
Turn on internal consistency checking ✨. What the default is depends on how your compiler was configured, so like... vibe check your build, bestie. -fchecking=2 turns on even more internal consistency checking, and heads up: that extra checking might affect code generation. So it's not just a free little look-see, it can literally change the output. Noted. 💅
-frandom-seed=string
Okay so GCC needs some randomness when it's generating certain symbol names that have to come out different in every single compiled file. This option hands it a seed to use instead of actual random numbers 🎲. It's also used to drop unique stamps into coverage data files and the object files that produce them. And here's the slay: you can use -frandom-seed to produce reproducibly identical object files 🫶.
The string can be either a number (decimal, octal, or hex) or an arbitrary string — and if it's an arbitrary string, it gets converted to a number by computing CRC32.
The string should be different for every file you compile. Different file, different seed, no exceptions, periodt.
-save-temps
Stores the usual "temporary" intermediate files permanently 💾 — naming them as auxiliary output files, exactly as described under -dumpbase and -dumpdir.
And when you use it together with the -x command-line option, -save-temps is sensible enough to avoid overwriting an input source file that happens to have the same extension as an intermediate file 🧠✨. If you do want that corresponding intermediate file, just rename the source file before you use -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 you specified after the -o option, including any directory separators. And if you didn't use -o at all? Then -save-temps=obj behaves just like -save-temps=cwd. Cute. 🎀
-time[=file]
Reports the CPU time taken by each subprocess in the compilation sequence ⏱️. For C source files, that's the compiler proper and the assembler (plus the linker, if linking is actually happening).
Without specifying an output file, the output looks like this:
# cc1 0.12 0.01
# as 0.00 0.01The first number on each line is the "user time" — that's 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 an output file specified, the output gets appended to that named file, and it looks like this:
0.12 0.01 cc1 <options>
0.00 0.01 as <options>See how it changed? The "user time" and "system time" move to before the program name, and the options passed to the program get displayed too — so later on you can actually tell what file was being compiled and with which options 👑.
-fdump-final-insns[=file]
Dumps the final internal representation (RTL) to file 🔍. If the optional argument is omitted — or if file is "." — then the dump file's name 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 that second compilation. Then dump the final internal representation in both compilations and print an error if they differ 😤. Double compile, double check.
If the equal sign is omitted, the default -gtoggle is used.
The environment variable GCC_COMPARE_DEBUG, if it's defined, non-empty, and nonzero, implicitly enables -fcompare-debug. And if GCC_COMPARE_DEBUG is defined to a string starting with a dash, then it's 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. Full off switch, bestie.
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). And if you want just a warning instead of that rejection, setting GCC_COMPARE_DEBUG to -w%n-fcompare-debug not overridden will do it. 🙈
-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 get renamed so they contain the .gk additional extension during that second compilation — so it doesn't overwrite the ones generated by the first 🗂️✨.
And 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. Yeah. That's the whole use case. 💀
-gtoggle
Turns off generation of debug info if leaving out this option generates it — or turns it on at level 2 otherwise 🔄. The position of this argument in the command line doesn't matter at all; it takes effect after all other options are processed, and it does so only once, no matter how many times you give it. This is mainly intended to be used with -fcompare-debug.
-fvar-tracking-assignments-toggle
Toggles -fvar-tracking-assignments, in the exact same way that -gtoggle toggles -g 🪞.
-Q
Makes the compiler print out each function name as it's compiled 📝, and print some statistics about each pass when it finishes.
-ftime-report
Makes the compiler print some statistics about the time consumed by each pass when it finishes ⏳.
-ftime-report-details
Records the time consumed by infrastructure parts separately for each pass 🧾.
-fira-verbose=n
Controls the verbosity of the dump file for the integrated register allocator. The default value is 5. And if the value n is greater than or equal to 10, the dump output gets sent to stderr using the same format as n minus 10. Sneaky little offset, iykyk 👀.
-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's 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 prints 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. Before and after, matching the pre/post in the names, no surprises 🎀.
-fprofile-report
Makes the compiler print some statistics about the consistency of the (estimated) profile and the 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. The auxname is generated from the name of the output file, if it's explicitly specified and it's not an executable — otherwise it's 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 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 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's not present, the amount of these adjustments is not bounded at compile time, and the second field only represents the bounded part. So read your qualifiers, bestie, they change everything 🔍.
-fstats
Emits 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
Prints the name and the counter upper bound for all debug counters 🧮✨.
-fdbg-cnt=counter-value-list
Sets 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 the second, third, fourth, tenth, and eleventh invocation. And for "dbg_cnt(tail_call)", true is returned for the first 10 invocations. Obsessed with how precise this is 💅.
-print-file-name=library
Prints the full absolute name of the library file library that would be used when linking — and does nothing else 🛑. With this option, GCC does not compile or link anything; it just prints the file name.
-print-multi-directory
Prints 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
Prints the mapping from multilib directory names to the compiler switches that enable them 🔀. The directory name is separated from the switches by ;, and each switch starts with an @ instead of the -, with no spaces between multiple switches. This is supposed to ease shell processing — because shells have opinions and we respect them 🐚.
-print-multi-os-directory
Prints 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. And if OS libraries are present in lib/subdir subdirectories, it prints e.g. amd64, sparcv9, or ev6. So basically it tells you where the OS libraries are actually hiding 🕵️♀️✨.
-print-multiarch— prints out the path to the OS libraries for whichever multiarch got selected, and it's relative to somelibsubdirectory. that's it, that's the whole job 💅-print-prog-name=program— literally the same energy as-print-file-name, except it goes hunting for a program instead, likecppand friends 🫶-print-libgcc-file-name— bestie this is just-print-file-name=libgcc.awearing a lil nickname ✨
and omg it's actually so useful when you're out here using -nostdlib or -nodefaultlibs but still very much want to link against libgcc.a. so you can do:
gcc -nostdlib <files>... `gcc -print-libgcc-file-name`-print-search-dirs— prints the name of the configured installation directory PLUS a whole list of the program and library directories gcc searches — and then does absolutely nothing else, main character of "just printing" 🎀
this one is your savior when gcc starts yelling the error message installation problem, cannot exec cpp0: No such file or directory. to fix that lil situation you either gotta put cpp0 and the rest of the compiler components exactly where gcc expects to find them, OR you set the environment variable GCC_EXEC_PREFIX to the directory where you actually installed them. and do NOT forget the trailing / i am begging 🙈
-print-sysroot— prints the target sysroot directory used during compilation. this is the target sysroot you specified either at configure time or with the--sysrootoption, and it might have an extra suffix that depends on your compilation options. if no target sysroot was specified, the option just prints nothing. silence. iconic 💀-print-sysroot-headers-suffix— prints the suffix that gets added to the target sysroot when searching for headers, or throws an error if the compiler wasn't configured with such a suffix — and does nothing else, periodt ✨-dumpmachine— prints the compiler's target machine (for example,i686-pc-linux-gnu) — and does nothing else 🦋-dumpversion— prints the compiler version (for example,3.0,6.3.0or7) — and does nothing else. this is the compiler version used in filesystem paths and specs. depending on how the compiler got 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— prints the full compiler version — and does nothing else. the output is always three numbers separated by dots: major, minor and patchlevel version. no surprises, we love consistency 🌸-dumpspecs— prints the compiler's built-in specs — and does nothing else. (this one gets used when GCC itself is being built, iykyk) 🔥
🧚 Machine-Dependent Options 🧚
every target machine GCC supports can have its own lil options — like, to let you compile for a specific processor variant or ABI, or to control optimizations that are specific to that machine. by convention, the names of machine-specific options all start with -m 💅
some configurations of the compiler also support extra target-specific options, usually for compatibility with other compilers on the same platform. slay.
👑 AArch64 Options 👑
these options are defined for AArch64 implementations:
-mabi=name— generates code for the specified data model. the permissible values areilp32for a SysV-like data model whereint,long intand pointers are 32 bits, andlp64for a SysV-like data model whereintis 32 bits butlong intand pointers are 64 bits.
the default depends on the specific target configuration. and heads up bestie: the LP64 and ILP32 ABIs are not link-compatible — you have to compile your ENTIRE program with the same ABI and link with a compatible set of libraries. no mixing, no matching 💀
-mbig-endian— generates big-endian code. this is the default when GCC is configured for anaarch64_be-*-*target.-mgeneral-regs-only— generates code that uses ONLY the general-purpose registers. this stops the compiler from using floating-point and Advanced SIMD registers, but it does not impose any restrictions on the assembler. just the compiler, being told to chill 🌈-mlittle-endian— generates little-endian code. this is the default when GCC is configured for anaarch64-*-*but not anaarch64_be-*-*target.-mcmodel=tiny— generates code for the tiny code model. the program and its statically defined symbols have to be within 1MB of each other. programs can be statically or dynamically linked. cozy lil memory neighborhood 🏘️-mcmodel=small— generates 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— generates code for the large code model. this makes zero assumptions about addresses and sizes of sections. programs can be statically linked only. and the-mcmodel=largeoption is incompatible with-mabi=ilp32,-fpicand-fPIC. so no, you cannot have it all 💅-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 a canary at guard. the supported locations areglobalfor a global canary, orsysregfor a canary in an appropriate system register.
with that latter choice, the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset also specify which system register to use as the base register for reading the canary, and from what offset from that base register. there is NO default register or offset, because this is entirely for use within the Linux kernel. you're on your own, bestie 🫶
-mtls-dialect=desc— uses TLS descriptors as the thread-local storage mechanism for dynamic accesses of TLS variables. this is the default.-mtls-dialect=traditional— uses traditional TLS as the thread-local storage mechanism for dynamic accesses of TLS variables.-mtls-size=size— specifies the bit size of immediate TLS offsets. valid values are12,24,32,48. 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 aNOPinstruction between memory instructions and 64-bit integer multiply-accumulate instructions. silly lil bug, silly lil patch 🎀-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 happens at link time, and this will only pass the corresponding flag to the linker.-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-mathor-funsafe-math-optimizationsis used as well. enabling this reduces the precision of reciprocal square root results to about 16 bits for single precision and to 32 bits for double precision. speed over accuracy, a whole mood 😭-mlow-precision-sqrt,-mno-low-precision-sqrt— enable or disable the square root approximation. this option only has an effect if-ffast-mathor-funsafe-math-optimizationsis used as well. enabling this reduces the 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-mathor-funsafe-math-optimizationsis used as well. enabling this reduces the 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. very demure, very secure 🦋-moutline-atomics,-mno-outline-atomics— enable or disable calls to out-of-line helpers that implement atomic operations. these helpers will, at runtime, determine if the LSE instructions from ARMv8.1-A can be used; if not, they'll fall back to 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 you're using a later revision, e.g. -march=armv8.1-a or -march=armv8-a+lse, the ARMv8.1-Atomics instructions get used directly. same deal when using -mcpu= and the selected cpu supports the lse feature. this option is on by default 👑
-march=name— specifies the name of the target architecture and, optionally, one or more feature modifiers. this option has the form-march=arch{+[no]feature}*.
the list below sums up the permissible values for arch and the features they enable by default:
armv8-a: Armv8-A :+fp, +simdarmv8.1-a: Armv8.1-A :armv8-a, +crc, +lse, +rdmaarmv8.2-a: Armv8.2-A :armv8.1-aarmv8.3-a: Armv8.3-A :armv8.2-a, +pautharmv8.4-a: Armv8.4-A :armv8.3-a, +flagm, +fp16fml, +dotprodarmv8.5-a: Armv8.5-A :armv8.4-a, +sb, +ssbs, +predresarmv8.6-a: Armv8.6-A :armv8.5-a, +bf16, +i8mmarmv8.7-a: Armv8.7-A :armv8.6-a, +ls64armv8.8-a: Armv8.8-a :armv8.7-a, +mopsarmv9-a: Armv9-A :armv8.5-a, +sve, +sve2armv8-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,
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. righty wins, sorry to the lefties 💅
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 gets tuned to perform well across a range of target processors implementing the target architecture 🌈
-mtune=name— okay so this is your "pls make my code fast on this exact chip" knob 🎯. It tells GCC which target processor to tune the performance of the code for. The allowed values are, no cap, this whole squad: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, neoverse-512tvb, neoverse-e1, neoverse-n1, neoverse-n2, neoverse-v1, qdf24xx, saphira, phecda, xgene1, vulcan, octeontx, octeontx81, octeontx83, octeontx2, octeontx2t98, octeontx2t96 octeontx2t93, octeontx2f95, octeontx2f95n, octeontx2f95mm, a64fx, thunderx, thunderxt88, thunderxt88p1, thunderxt81, tsv110, 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-x1, cortex-x2, cortex-a510, cortex-a710, ampere1, native.
And those dotted ones — cortex-a57.cortex-a53, cortex-a72.cortex-a53, cortex-a73.cortex-a35, cortex-a73.cortex-a53, cortex-a75.cortex-a55, cortex-a76.cortex-a55 — are the big.LITTLE girlies 💅, they mean GCC should tune for a big.LITTLE system.
Then neoverse-512tvb is doing its own thing ✨: it means GCC tunes for Neoverse cores that (a) implement SVE and (b) have a total vector bandwidth of 512 bits per cycle. Like, translating that from nerd: it tells GCC to tune for Neoverse cores that can execute 4 128-bit Advanced SIMD arithmetic instructions a cycle and can execute an equivalent number of SVE arithmetic instructions per cycle (2 for 256-bit SVE, 4 for 128-bit SVE). It's more general than tuning for a specific core like Neoverse V1 but more specific than the default tuning described below. iykyk 🧚
Also, 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 (rip, it just shrugs 😭).
Where none of -mtune=, -mcpu= or -march= are specified, the code is tuned to perform well across a range of target processors.
Heads up bestie: this option cannot be suffixed by feature modifiers. Periodt.
-mcpu=name— now THIS one names the target processor, optionally with one or more feature modifiers glued on 💖. Its shape is-mcpu=cpu{+[no]feature}*, where the values allowed for cpu are literally the same list as-mtuneabove. The allowed values for feature live down in the sub-section on aarch64-feature-modifiers,,-march and -mcpu Feature Modifiers. If you give conflicting feature modifiers, the right-most feature wins — last one standing takes the crown 👑.
GCC uses name to figure out what kind of instructions it can emit when generating assembly code (as if by -march) and to figure out the target processor to tune for performance (as if by -mtune). And if you use this alongside -march or -mtune? Those options take precedence over the appropriate part of this one. They said "we're the main characters now" 💀.
-mcpu=neoverse-512tvb is special ✨: it doesn't point at one specific core, it points at 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. And unless overridden by -mtune, -mcpu=neoverse-512tvb tunes code in the same way as for -mtune=neoverse-512tvb.
-moverride=string— this overrides 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, so don't get attached 💔.
This option is only intended to be useful when developing GCC. It is not for you and me, bestie, it's for the compiler devs 🛠️.
-mverbose-cost-dump— turns on verbose cost model dumping in the debug dump files 🌸. This option is provided for use in debugging the compiler. Very "devs only, don't touch" energy.-mpc-relative-literal-loads,-mno-pc-relative-literal-loads— enables or disables PC-relative literal loads 🎀. With this on, literal pools are accessed using a single instruction and emitted after each function. That limits the maximum size of functions to1MB— yes, one megabyte, that's the ceiling, no cap. This is enabled by default for-mcmodel=tiny.-msign-return-address=scope— picks which function scope gets return address signing 🦋. The allowed values arenone, which disables return address signing;non-leaf, which enables pointer signing for functions which are not leaf functions; andall, which enables pointer signing for all functions. Default isnone.
Deprecated alert 🚨 — this option has been deprecated by -mbranch-protection.
-mbranch-protection=none|standard|pac-ret[+leaf+b-key]|bti— picks which branch protection features to use 🛡️.noneis the default and turns off all types of branch protection.standardturns on all types of branch protection features — and if a feature has additional tuning options, thenstandardsets 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 argumentleafextends the signing to include leaf functions, and the optional argumentb-keysigns the functions with the B-key instead of the A-key.btiturns on the branch target identification mechanism 🔥.-mharden-sls=opts— enables compiler hardening against straight line speculation (SLS) 💪. opts is a comma-separated list of the following options:retbrblr
Plus, -mharden-sls=all enables all SLS hardening while -mharden-sls=none disables all SLS hardening. All or nothing, iconic behavior.
-msve-vector-bits=bits— specifies 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 lets GCC make assumptions about the vector length when that's 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.
Real talk: 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. Don't build your whole personality on the other ones 🙈.
The default is -msve-vector-bits=scalable, which produces vector-length agnostic code.
✨ -march and -mcpu Feature Modifiers ✨
Feature modifiers used with -march and -mcpu can be any of the following, and their inverses nofeature — so yes, every single one has a no version, the duality of it all 💅:
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-marchand-mcpu.simd— Enable Advanced SIMD instructions. This also enables floating-point instructions. This is on by default for all possible values for options-marchand-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. Fr, don't.
rcpc— Enable the RcPc extension. This does not change code generation from GCC, but is passed 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— turns on the SVE2 aes instructions ✨ and yes it also enables SVE2 instructions, package deal bestie 💅sve2-sha3— enables SVE2 sha3 instructions, and again it drags SVE2 instructions along with it 🎀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. This option is enabled by default for-march=armv8.7-a💖mops— Enable the instructions to accelerate memory operations likememcpy,memmove,memset. This option is enabled by default for-march=armv8.8-a⚡flagm— Enable the Flag Manipulation instructions Extension 🚩pauth— Enable the Pointer Authentication Extension 🔐
Featurecryptoimpliesaes,sha2, andsimd, which impliesfp. Conversely,nofpimpliesnosimd, which impliesnocrypto,noaesandnosha2. It's giving domino effect fr fr 🁢🁢🁢
🌸 Adapteva Epiphany Options 🌸
These -m options are defined for Adapteva Epiphany:
-mhalf-reg-file— Don't allocate any register in the ranger32...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 anfsubinstruction 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 rangesp+0...sp+7can 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=numoption.-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 theb/blinstructions, 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 theb/blinstructions, 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-callssemantics 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 arer43andr63, which specify use of that register as a fixed register, andnone, 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
fiji— Compile for GCN3 Fiji devices (gfx803) 🌺gfx900— Compile for GCN5 Vega 10 devices (gfx900) 🌺gfx906— Compile for GCN5 Vega 20 devices (gfx906) 🌺
-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— Compile binaries suitable for devices with the XNACK feature enabled. Some devices always require XNACK and some allow the user to configure XNACK. The compiled code must match the device mode. The default is-mno-xnack. At present this option is a placeholder for support that is not yet implemented 🚧
🌈 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=ARC601or-mcpu=ARCEMis in effect 💫-mjli-always— Force to call a function usingjli_sinstruction. 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 arearc600— 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 🧩arc600_norm— Compile for ARC 600 CPU withnorminstructions enabled 🧩
arc600_mul32x16— compile for ARC 600 CPU with"norm"and 32x16-bit multiply instructions turned on ✨arc600_mul64— compile for ARC 600 CPU with"norm"and"mul64"-family instructions enabled 💖arc601_norm— compile for ARC 601 CPU with"norm"instructions on 🎀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 on 💫nps400— compile for ARC 700 on NPS400 chip 🧚em_mini— compile for ARC EM minimalist configuration, which features a 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. Right now only"divaw","adds","subs", and"sat16"are supported. Only valid for-mcpu=ARC700⚠️-mno-mpy— do not generate"mpy"-family instructions for ARC700. Heads up bestie, 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=ARC700is 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— ngl this option is ignored; it's here for compatibility purposes only. Software floating-point code gets emitted by default, and that default can be overridden by FPX options:-mspfp,-mspfp-compact, or-mspfp-fastfor single precision, and-mdpfp,-mdpfp-compact, or-mdpfp-fastfor 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 the 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 a numeric value for multo.wlh1is 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. The 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 optionfpus_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 optionfpus_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-purpose 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", and the upper limit is the"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 the second register bank on entry to a 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're using fast interrupts in an ARC V2 family processor. Permitted values are4,8,16, and32🔢-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's known that the"lp_count"register can hold the required loop-counter value. Depending on the width you specify, the compiler and run-time library might keep using the loop mechanism for various needs. This option defines the macro"__ARC_LPC_WIDTH__"with the value of *width 💅-mrf16— this option tells the compiler to generate code for a 16-entry register file. It defines the"__ARC_RF16__"preprocessor macro 🎀-mbranch-index— enable use of"bi"or"bih"instructions to implement jump tables ✨
The following options get passed through to the assembler, and they 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— explain what alignment considerations lead to the decision to make an instruction short or long 🔍
okay so the following options are passed straight through to the linker, no cap:
-marclinux— Passed through to the linker, to specify use of thearclinuxemulation. This one is enabled by default in tool chains built forarc-linux-uclibcandarceb-linux-uclibctargets when profiling is not requested. ✨-marclinux_prof— Passed through to the linker, to specify use of thearclinux_profemulation. This one is enabled by default in tool chains built forarc-linux-uclibcandarceb-linux-uclibctargets 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. full range, bestie-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 forarc-linux-uclibcandarceb-linux-uclibctargets. 🦋-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 is4for any ARC configuration, or8when we have double load/store operations. 💖-mno-sdata— Do not generate sdata references. This is the default for tool chains built forarc-linux-uclibcandarceb-linux-uclibctargets. 🙈-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. literally just vibing there-mauto-modify-reg— Enable the use of pre/post modify with register displacement. ✨-mbbit-peephole— Enable bbit peephole2. 👑-mno-brcc— This option disables a target-specific pass inarc_reorgto 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 compactcasesipattern. This is the default for-Os, and only available for ARCv1 cores. This option is deprecated. deprecated, bestie-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 thecbranchsipattern. 🦋-mexpand-adddi— Expandadddi3andsubdi3at RTL generation time intoadd.f,adcetc. This option is deprecated. 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. careful, bestie-mlra— Enable Local Register Allocation. This is still experimental for ARC, so by default the compiler uses standard reload (i.e.-mno-lra). 🧚-mlra-priority-none— Don't indicate any priority for target registers. 💜-mlra-priority-compact— Indicate target register priority forr0..r3/r12..r15. 💖-mlra-priority-noncompact— Reduce target register priority forr0..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 inlibgcc; 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 lets the compiler emitenterandleaveinstructions. These instructions are only valid for CPUs with code-density feature. 👑-mmixed-code— Does nothing. Preserved for backward compatibility. just existing-mq-class— This option is deprecated. Enable q instruction alternatives. This is the default for-Os. deprecated-mRcq— Enable Rcq constraint handling. Most short code generation depends on this. This is the default. ⭐-mRcw— Enable Rcw constraint handling. Most ccfsm condexec mostly depends on this. This is the default. ⭐-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 like1.1— 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 optionOsis 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. 🎀
-mmultcost=num— Cost to assume for a multiply instruction, with4being equal to a normal instruction. 💅-munalign-prob-threshold=probability— Does nothing. Preserved for backward compatibility. ghost option
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 buildarceb-elf32andarceb-linux-uclibctargets, 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 buildarc-elf32andarc-linux-uclibctargets, 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— Valuesarc600,arc601,arc700andarc700-xmacfor cpu are replaced byARC600,ARC601,ARC700andARC700-xmacrespectively. 💅-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-linuxandiwmmxt. 🎀-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-pointerwith this option causes the stack frames not to be generated for leaf functions. The default is-mno-apcs-frame. This option is deprecated. deprecated-mapcs— This is a synonym for-mapcs-frameand is deprecated. 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-interworkis 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,softfpandhard.
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— okay so this one generates code for a processor running in big-endian mode; the default is to compile code for a little-endian processor. 💅-mbe8,-mbe32— bestie, when linking a big-endian image select between BE8 and BE32 formats. This 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...]— okay so 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, armv9-a, iwmmxt and iwmmxt2. (yes, armv9-a is in there twice, she said what she said 💅)
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+vfpv2can 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-d16can 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-d16can be used as an alias for this extension.+simd— The Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions. The extensions+neonand+neon-vfpv3can 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-d16can be used as an alias for this extension.+simd— The Advanced SIMD (Neon) v2 and the VFPv4 floating-point instructions. The extension+neon-vfpv4can 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-vfpv3can 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.
armv8.5-a+fp16— the half-precision floating-point data processing instructions. bestie this also turns on Advanced SIMD + floating-point, plus the Dot Product extension and the half-precision floating-pointfmlaextension. package deal 💅+simd— the ARMv8.3-A Advanced SIMD and floating-point instructions, plus the Dot Product extension.+crypto— the cryptographic instructions. this also enables Advanced SIMD + floating-point and the Dot Product extension, okurrr.+nocrypto— turn the cryptographic extension OFF.+nofp— turn off floating-point, Advanced SIMD and cryptographic instructions. all of them. gone. 💀+i8mm— 8-bit Integer Matrix Multiply instructions. this one also enables Advanced SIMD and floating-point instructions.+bf16— Brain half-precision floating-point instructions. also enables Advanced SIMD and floating-point instructions. (yes it literally says "Brain", we don't make the rules ✨)
armv8.6-a+fp16— the half-precision floating-point data processing instructions, and this also enables Advanced SIMD and floating-point instructions as well as the Dot Product extension and the half-precision floating-pointfmlaextension.+simd— the ARMv8.3-A Advanced SIMD and floating-point instructions as well as the Dot Product extension.+crypto— the cryptographic instructions. also enables Advanced SIMD and floating-point, plus 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. also enables Advanced SIMD and floating-point instructions.+bf16— Brain half-precision floating-point instructions. also enables Advanced SIMD and floating-point instructions.
armv7-r+fp.sp— the single-precision VFPv3 floating-point instructions. fun fact: the extension+vfpv3xdcan be used as an alias for this one 👑+fp— the VFPv3 floating-point instructions with 16 double-precision registers. and+vfpv3-d16can 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. it's giving long name energy.+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. pick your coprocessor, bestie 🎀
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 makes the compiler auto-detect the architecture of the build computer. and rn? 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. so it just. does nothing. iconic behaviour honestly 🙈
✨ -mtune=name ✨
okay so this one names the target ARM processor that GCC should tune the performance of the code for. for some ARM implementations you genuinely get better performance by using this option. no cap.
the permissible names are, and I'm listing every single one because we do not skip:
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-m55, cortex-x1, 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, xgene1.
additionally — and this is lowkey the fun part — this option can specify that GCC should tune the performance of the code for a big.LITTLE system. the permissible names for that 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 says GCC should tune the performance for a blend of processors within architecture arch. the aim is to generate code that runs well on the current most popular processors, balancing between optimizations that benefit some CPUs in the range and avoiding performance pitfalls of other CPUs. heads up tho: the effects of this option may change in future GCC versions as CPU models come and go. it's giving "we'll see" 🌈
-mtune permits the same extension options as -mcpu, but plot twist: the extension options do not affect the tuning of the generated code. bestie they're just there. sitting. 💀
-mtune=native makes the compiler 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 one specifies the name of the target ARM processor. GCC uses this name to derive the name of the target ARM architecture (as if you'd said it with -march) and the ARM processor type to tune for performance (as if you'd said it with -mtune). one option, two jobs, main character energy 👑
but wait — where this option is used in conjunction with -march or -mtune, those options take precedence over the appropriate part of this option. so say it once or say it right, bestie.
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. yes it's a whole group chat of options talking to each other 😭
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. choose your fighter 💅
permissible names for this option are the same as those for -mtune. (scroll up bestie, the whole list is right there, we're not doing it twice ✨)
the following extension options are common to the listed CPUs:
+nodsp— disable the DSP instructions oncortex-m33,cortex-m35p.+nofp— kills the floating-point instructions onarm9e,arm946e-s,arm966e-s,arm968e-s,arm10e,arm1020e,arm1022e,arm926ej-s,arm1026ej-s,cortex-r5,cortex-r7,cortex-r8,cortex-m4,cortex-m7,cortex-m33andcortex-m35p. And ongeneric-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-a53andcortex-a55it takes out both the floating-point and the SIMD instructions. Two for one, but like, not in a fun way 😭+nofp.dp— turns off just the double-precision part of the floating-point instructions, oncortex-r5,cortex-r7,cortex-r8,cortex-r52,cortex-r52plusandcortex-m7.+nosimd— turns off the SIMD instructions (but leaves floating-point alone, bestie, it's not that dramatic) ongeneric-armv7-a,cortex-a5,cortex-a7andcortex-a9.+crypto— turns the cryptographic instructions on ✨ forcortex-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-a53andcortex-a75.cortex-a55.
Also, the generic-armv7-a pseudo target ships with VFPv3 + 16 double-precision registers as its default vibe. It supports these 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 literally the same as the extensions to -march=armv7-a.
-mcpu=generic-arch is also allowed, and it's equivalent to -march=arch -mtune=generic-arch. Peep -mtune if you want the whole story.
-mcpu=native makes the compiler auto-detect the CPU of the build computer. As of rn, this is only supported on GNU/Linux, and not all architectures are recognized. If the auto-detect flops, the option just does nothing.
-mfpu=name— tells everyone what floating-point hardware (or hardware emulation) is available on the target. The 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-armv8andcrypto-neon-fp-armv8. Oh and heads up:neonis an alias forneon-vfpv3, andvfpis an alias forvfpv2.
The setting auto is the default and it's special — it makes the compiler pick the floating-point and Advanced SIMD instructions based on whatever -mcpu and -march are set to.
If the floating-point hardware you picked includes the NEON extension (so like -mfpu=neon), be aware that floating-point operations are not generated by GCC's auto-vectorization pass unless -funsafe-math-optimizations is also specified. Why? Because NEON hardware does not fully implement the IEEE 754 standard for floating-point arithmetic (specifically, denormal values get treated as zero), so using NEON instructions may lead to a loss of precision. So like, know that going in 💅
You can also set the fpu name at function level by using the target("fpu=") function attributes or pragmas.
-mfp16-format=name— picks the format of the__fp16half-precision floating-point type. Permissible names arenone,ieee, andalternative; the default isnone, in which case the__fp16type is just not defined. Sorry bestie, no type for you 🙈-mstructure-size-boundary=n— the sizes of all structures and unions get rounded up to a multiple of the number of bits set by this option. Permissible values are8,32and64. The default value varies for different toolchains. For the COFF targeted toolchain the default value is8. A value of64is only allowed if the underlying ABI supports it.
Specifying a bigger number can produce faster, more efficient code, but it can also make your program bigger. 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. It's giving legacy 🫠
-mabort-on-noreturn— generates a call to the functionabortat the end of anoreturnfunction. It gets executed if the function tries to return anyway. Snitch behaviour, honestly.-mlong-calls-mno-long-calls— tells the compiler to do function calls by first loading the address of the function into a register and then performing a subroutine call on that 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 get 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, and so 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— treats 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. Not your job, bestie, it's the runtime's ✨-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 the compiler. For single PIC base case, the default isR9if target is EABI based or stack-checking is enabled, otherwise the default isR10.-mpic-data-is-text-relative— assumes 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-baseby default.-mpoke-function-name— writes 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, #4When 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— pick 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=stateconfigure 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. Per-function glow-up, we love to see it 💅
-mflip-thumb— switches 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. It's a test-only vibe, don't be weird about it 🙈-mtpcs-frame— generates a stack frame that's 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— generates a stack frame that's 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. Periodt.-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's 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 valid models aresoft, which generates calls to__aeabi_read_tp,cp15, which fetches the thread pointer fromcp15directly (supported in the arm6k architecture), andauto, which uses the best available method for the selected processor. The default setting isauto.-mtls-dialect=dialect— specify the dialect to use for accessing thread local storage. Two dialects are supported —gnuandgnu2. Thegnudialect selects the original GNU scheme for supporting local and global dynamic TLS models. Thegnu2dialect 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— bestie, this one only lets absolute relocations happen on word-sized values (akaR_ARM_ABS32). It's turned on by default on targets where the runtime loader is a lil control freak about it (looking at you, uClinux and SymbianOS), and also whenever-fpicor-fPICis in the mix. Heads up tho: this option does not vibe with-mslow-flash-data. They beef. 💀-mfix-cortex-m3-ldrd— so some Cortex-M3 cores will literally corrupt your data whenldrdinstructions have overlapping destination and base registers 😭. This option just... doesn't generate those instructions. Problem solved, slay. It's enabled by default when-mcpu=cortex-m3is specified.-mfix-cortex-a57-aes-1742098,-mno-fix-cortex-a57-aes-1742098,-mfix-cortex-a72-aes-1655431,-mno-fix-cortex-a72-aes-1655431— these enable (or disable, with the-mno-versions) mitigation for an erratum on Cortex-A57 and Cortex-A72 that messes with the AES cryptographic instructions. Enabled by default when either-mcpu=cortex-a57or-mcpu=cortex-a72is specified. 💅-munaligned-access,-mno-unaligned-access— turns reading and writing of 16- and 32-bit values on/off for addresses that aren't 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 everything else. If it's not enabled, then words in packed data structures get accessed one byte at a time (so slow, so tragic 🐌).
The ARM attribute Tag_CPU_unaligned_access gets set in the generated object file to either true or false, depending on this option's setting. And if unaligned access is enabled, the preprocessor symbol __ARM_FEATURE_UNALIGNED is also defined. ✨
-mneon-for-64bits— this option is deprecated and has no effect. That's it. That's the whole thing. Ghosted. 👻-mslow-flash-data— assumes loading data from flash is slower than fetching instructions, so literal load is minimized for better performance. Only supported when compiling for ARMv7 M-profile, and off by default. It conflicts with-mword-relocations(they cannot sit at the same lunch table 🙅).-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. That said, this may change in future releases of GCC, so don't get too comfy. 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. On by default for ARMv8-A Thumb mode.-mprint-tune-info— prints CPU tuning information as a comment in the assembler file. This is an option used only for regression testing of the compiler and is not intended for ordinary use in compiling code. Disabled by default. iykyk 🧪-mverbose-cost-dump— enables verbose cost model dumping in the debug dump files. Provided for use in debugging the compiler, so if you're not debugging the compiler, this is not your moment bestie.-mpure-code— does not allow constant data to be placed in code sections. Additionally, when compiling for ELF object format, it gives all text sections the ELF processor-specific section attributeSHF_ARM_PURECODE. Only available when generating non-pic code for M-profile targets. 🔥-mcmse— generates secure code as per theARMv8-M Security Extensions: Requirements on Development Tools Engineering Specification, which can be found on <https://developer.arm.com/documentation/ecm0359818/latest/>.-mfix-cmse-cve-2021-35465— mitigates against a potential security issue with theVLLDMinstruction in some M-profile devices when using CMSE (CVE-2021-365465). Enabled by default when the option-mcpu=is used withcortex-m33,cortex-m35porcortex-m55. You can turn the mitigation off with-mno-fix-cmse-cve-2021-35465. 🛡️-mstack-protector-guard=guard,-mstack-protector-guard-offset=offset— generate stack protection code using canary at guard. Supported locations areglobalfor a global canary ortlsfor a canary accessible via the TLS register. The-mstack-protector-guard-offset=option is for use with-fstack-protector-guard=tlsand not for use in user-land code. 🙈-mfdpic,-mno-fdpic— selects the FDPIC ABI, which uses 64-bit function descriptors to represent pointers to functions. When the compiler is configured forarm-*-uclinuxfdpiceabitargets, this option is on by default and implies-fPIEif none of the PIC/PIE-related options is provided. On other targets, it only enables the FDPIC-specific code generation features, and you should explicitly provide the PIC/PIE-related options as needed.
Not me telling you: 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, fr fr) to build the Linux kernel using the same (arm-*-uclinuxfdpiceabi) toolchain as the one used to build the userland programs.
🌈 AVR Options 🌈
These options are defined for AVR implementations:
-mmcu=mcu— specify Atmel AVR instruction set architectures (ISA) or MCU type.
The default for this option is avr2.
GCC supports the following AVR devices and ISAs:
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 theMOVWinstruction. 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 theMOVWinstruction. 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,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,ata5795,ata5790,ata5790n,ata5791,ata6613c,ata6614q,ata5782,ata5831,ata8210,ata8510,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.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,attiny417,attiny804,attiny806,attiny807,attiny814,attiny816,attiny817,attiny1604,attiny1606,attiny1607,attiny1614,attiny1616,attiny1617,attiny3214,attiny3216,attiny3217,atmega808,atmega809,atmega1608,atmega1609,atmega3208,atmega3209,atmega4808,atmega4809.
"avrxmega4"— XMEGA cuties with more than 64 KiB and up to 128 KiB of program memory. mcu ="atxmega64a3","atxmega64a3u","atxmega64a4u","atxmega64b1","atxmega64b3","atxmega64c3","atxmega64d3","atxmega64d4"."avrxmega5"— XMEGA girlies 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 besties 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 icons with more than 128 KiB of program memory and more than 64 KiB of RAM. mcu ="atxmega128a1","atxmega128a1u","atxmega128a4u"."avrtiny"— TINY Tiny core devices, 512 B up to 4 KiB of program memory. mcu ="attiny4","attiny5","attiny9","attiny10","attiny20","attiny40"."avr1"— this ISA is what the minimal AVR core implements, and it's supported for assembler only. mcu ="attiny11","attiny12","attiny15","attiny28","at90s1200".
-mabsdata— assume that literally all data in static storage can be reached withLDS/STSinstructions. This one only does anything on reduced Tiny devices like ATtiny40. Also peep theabsdataAVR Variable Attributes variable attribute. ✨-maccumulate-args— stack up your outgoing function arguments and grab/release the stack space they need once, in the function prologue/epilogue. Without this option, the outgoing args get pushed before a call and popped after. 💅
Popping args after the call can be expensive on AVR, so accumulating that stack space might give you smaller executables, because the args don't need to be yanked off the stack after a call like that.
For functions that make several calls to functions that take their args on the stack — like calls to printf-style functions — this option can shrink your code size.
- *
-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 is0. ⭐ -mcall-prologues— function prologues/epilogues get expanded as calls to the appropriate subroutines. Smaller code size, bestie. 🎀- *
-mdouble=bits*,-mlong-double=bits — set the size (in bits) of thedoubleorlong doubletype, respectively. Possible values for bits are32and64. Whether a specific value for bits is even allowed depends on the--with-double=and--with-long-double=configure options (https://gcc.gnu.org/install/configure.html#avr), and the same goes for the default values of these options. 🦋 -mgas-isr-prologues— interrupt service routines (ISRs) may use the__gcc_isrpseudo instruction that GNU Binutils supports. If this option is on, you can still disable the feature for individual ISRs via the AVR Function Attributes"no_gccisr"function attribute. This feature is on by default if optimization is on (but not with-Og, @pxref{Optimize Options}), and if GNU Binutils supports PR21683 (https://sourceware.org/PR21683). 🔥-mint8— assumeintis an 8-bit integer. This messes with the sizes of all the types: acharis 1 byte, anintis 1 byte, alongis 2 bytes, andlong longis 4 bytes. > Heads up bestie: this option does not conform to the C standards, but it gives you smaller code size. 🫶-mmain-is-OS_task— don't save registers inmain. Same effect as slapping the AVR Function Attributes"OS_task"attribute onmain. It's on by default if optimization is on. 💜- *
-mn-flash=num* — assume the flash memory is num times 64 KiB in size. -mno-interrupts— the generated code isn't compatible with hardware interrupts. Code size is smaller though. 🙈-mrelax— try to swap theCALL(resp.JMP) instruction for the shorterRCALL(resp.RJMP) instruction when applicable. Setting-mrelaxjust adds the--mlink-relaxoption to the assembler's command line and the--relaxoption to the linker's command line.
Jump relaxing is done by the linker, because jump offsets aren't known until the code is actually located. So the assembler code the compiler makes is the same, but the instructions in the executable may differ from the instructions in the assembler code. 😭
Relaxing must be turned on if linker stubs are needed — see the section on EIND and linker stubs below.
-mrmw— assume the device supports the Read-Modify-Write instructionsXCH,LAC,LASandLAT. ✨-mshort-calls— assumeRJMPandRCALLcan target the whole program memory.
This option is used internally for multilib selection. It is not an optimization option, and you don't need to set it by hand.
-msp8— treat the stack pointer register as an 8-bit register, i.e. assume the high byte of the stack pointer is zero. In general, you don't need to set this by hand either.
This option is used internally by the compiler to select and build multilibs for the architectures avr2 and avr25. Those architectures mix devices with and without SPH. For any setting other than -mmcu=avr2 or -mmcu=avr25, the compiler driver adds or removes this option from the compiler proper's command line, because then the compiler knows whether the device or architecture has an 8-bit stack pointer and therefore no SPH register or not. 💅
-mstrict-X— use the address registerXthe way the hardware proposes. Which meansXis only used in indirect, post-increment or pre-decrement addressing.
Without this option, the X register can be used the same way as Y or Z, and that gets emulated by extra instructions. For example, loading a value with X+const addressing with a small non-negative const < 64 into a register Rn is performed as
adiw r26, const ; X += const
ld <Rn>, X ; <Rn> = *X
sbiw r26, const ; X -= const-mtiny-stack— only change the lower 8 bits of the stack pointer.-mfract-convert-truncate— allow using truncation instead of rounding towards zero for fractional fixed-point types.-nodevicelib— don't link against AVR-LibC's device specific librarylib<mcu>.a. 🙅-nodevicespecs— don't add-specs=device-specs/specs-mcuto the compiler driver's command line. The user takes responsibility for supplying the sub-processes — compiler proper, assembler and linker — with appropriate command line options. That means the user has to supply her private device specs file via-specs=path-to-specs-file. There's no more need for the option-mmcu=mcu.
This option can also serve as a replacement for the older way of specifying custom device-specs files, which needed -B some-path to point to a directory containing 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 when the resulting address space isn't contained in the incoming address space.-Wmisspelled-isr— warn if the ISR is misspelled, i.e. without the__vectorprefix. Enabled by default. ⭐
✨ 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 a word address, so indirect jumps and calls can target any code address in the range of 64 Ki words.
To facilitate indirect jumps on devices with more than 128 Ki bytes of program memory space, there's a special function register called EIND that serves as the most significant part of the target address whenever the EICALL or EIJMP instructions get used. 👑
Indirect jumps and calls on these devices are handled like this by the compiler, and they come with some limitations:
- The compiler never sets
EIND. - The compiler uses
EINDimplicitly inEICALL/EIJMPinstructions, or might readEINDdirectly in order to emulate an indirect call/jump by means of aRETinstruction. - The compiler assumes that
EINDnever changes during the startup code or during the application. In particular,EINDis 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. So the indirect call/jump jumps to one of these stubs, and the stub contains a direct jump to the desired address. 🦋
- Linker relaxation must be turned on so the linker generates the stubs correctly in all situations. See the compiler option
-mrelaxand 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 withEIND != 0, a custom linker script has to be used, in order to place the sections whose name starts with.trampolinesinto the segment whereEINDpoints 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 onEIND, see the AVR-LibC user manual (http://nongnu.org/avr-libc/user-manual/). - It is legitimate for user-specific startup code to set up
EINDearly, 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 setsEINDto the segment where the vector table is located.
#include <avr/io.h>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");
}and the "__trampolines_start" symbol? bestie, that one's defined in the linker script, not by us. not our problem. 💅
- Stubs get generated automatically by the linker if these two things line up:
- The address of a label is taken by means of the
"gs"modifier (short for generate stubs), like this:
- The address of a label is taken by means of the
LDI r24, lo8(gs(<func>))
LDI r25, hi8(gs(<func>))- The final location of that label is in a code segment outside the segment where the stubs are located.
- And the compiler drops those
"gs"modifiers on code labels in these situations:- Taking address of a function or code label.
- Computed goto.
- If prologue-save function is used, see
-mcall-prologuescommand-line option. - Switch/case dispatch tables. If you do not want such dispatch tables you can specify the
-fno-jump-tablescommand-line option. - C and C++ constructors/destructors called during startup/shutdown.
- If the tools hit a
"gs()"modifier explained above.
- 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 🌈
So some AVR devices have memories bigger than the 64 KiB range that 16-bit pointers can reach, right? To get to memory locations outside that 64 KiB range, the content of a "RAMP" register gets used as the 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. iconic behavior ngl.
- 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. no skipping this one bestie 😭
🦋 AVR Built-in Macros 🦋
GCC defines a whole lil collection of built-in macros so your code can vibe-check 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=mcuoption. Possible values are:
2, 25, 3, 31, 35, 4, 5, 51, 6for mcu="avr2", "avr25", "avr3", "avr31", "avr35", "avr4", "avr5", "avr51", "avr6", respectively and
100, 102, 103, 104, 105, 106, 107for 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=devicedefines this built-in macro which reflects the device's name. For example,-mmcu=atmega8defines the built-in macro"__AVR_ATmega8__",-mmcu=attiny261adefines"__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=devicedefines this built-in macro to the device's name. For example, with-mmcu=atmega8the macro is defined to"atmega8".
If device is not a device but only a core architecture like avr51, this macro is not defined.
"__AVR_XMEGA__"— The device / architecture belongs to the XMEGA family of devices. 💫"__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_MOVW__"— The device has the"MOVW"instruction to perform 16-bit register-register moves."__AVR_HAVE_LPMX__"— The device has the"LPM Rn,Z"and"LPM Rn,Z+"instructions."__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__"and"__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__"and"__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__"and"__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=avr2and-mmcu=avr25also 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-interruptscommand-line option."__AVR_ERRATA_SKIP__"and"__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,LASandLAT)."__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-callscommand 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_*". ✨"__WITH_AVRLIBC__"— The compiler is configured to be used together with AVR-Libc. See the--with-avrlibcconfigure option."__HAVE_DOUBLE_MULTILIB__"— Defined if-mdouble=acts as a multilib option."__HAVE_DOUBLE32__"and"__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 to2or3."__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).
🖤 Blackfin Options 🖤
-mcpu=cpu[-sirevision]— Specifies the name of the target Blackfin processor. Currently, cpu can be one ofbf512,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. that's the whole roster bestie 💀
okay bestie, back to the Blackfin zone, we were literally mid-option, no time to breathe 💜
The optional sirevision picks the silicon revision of the target Blackfin processor, and any workarounds available for that targeted silicon revision get enabled. If sirevision is none, no workarounds get enabled. If sirevision is any, all workarounds for the targeted processor get 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, then __SILICON_REVISION__ is not defined at all. If sirevision is any, __SILICON_REVISION__ is defined to be 0xffff. And if this optional sirevision isn't used, GCC just assumes the latest known silicon revision of the targeted Blackfin processor. Slay, it just guesses the newest one.
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.
🚨 bestie heads up: support forbf561is incomplete. Forbf561, only the preprocessor macro is defined. That's it. That's all you get. 😭
-msim— says the program is gonna run on the simulator, which causes the simulator BSP provided bylibglossto be linked in. This one only has effect for thebfin-elftoolchain. Also, certain other options, like-mid-shared-libraryand-mfdpic, imply-msimon their own. Sneaky.-momit-leaf-frame-pointer— don't keep the frame pointer in a register for leaf functions. This dodges the instructions to save, set up and restore frame pointers, and frees up an extra register in leaf functions. ✨ efficiency ✨-mspecld-anomaly— when enabled, the compiler makes sure the generated code does not contain speculative loads after jump instructions. If you use this option,__WORKAROUND_SPECULATIVE_LOADSis defined.-mno-specld-anomaly— don't generate extra code to prevent speculative loads from occurring.-mcsync-anomaly— when enabled, the compiler makes sure the generated code doesn't containCSYNCorSSYNCinstructions too soon after conditional branches. If you use this option,__WORKAROUND_SPECULATIVE_SYNCSis defined.-mno-csync-anomaly— don't generate extra code to preventCSYNCorSSYNCinstructions 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 abfin-elftarget, 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 lets the compiler use faster code for jumps and calls. go off.-mno-leaf-id-shared-library— do not assume that the code being compiled won't link against any ID shared libraries. Slower code gets 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 of0generates 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 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. iykyk.
-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_COREAis 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_COREBis defined. When this option is used,coreb_mainshould be used instead ofmain. 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_SDRAMis 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.neardatasection, which is pointed to by registerB14. Put small uninitialized global and static data in the.bsssection, which is adjacent to the.neardatasection. Put small read-only data into the.rodatasection. The corresponding sections used for large pieces of data are.fardata,.farand.const.-msdata=all— put all data, not just small objects, into the sections reserved for small data, and use addressing relative to theB14register 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.fardatasection, and all uninitialized data in the.farsection. Put all constant data into the.constsection.
💜 CRIS Options 💜
These options are defined specifically for the CRIS ports. it's giving very niche, we love her 🌸
-march=architecture-type,-mcpu=architecture-type— generate code for the specified architecture. The choices for architecture-type arev3,v8andv10for respectively ETRAX 4, ETRAX 100, and ETRAX 100 LX. Default isv0except forcris-axis-linux-gnu, where the default isv10.-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— these options are synonyms for-march=v3and-march=v8respectively.-mmul-bug-workaround,-mno-mul-bug-workaround— work around a bug in themulsandmuluinstructions 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_APPformatted-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 (and theirno-versions) arrange (or 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— bestie these are cousins of the stack-align, data-align and const-align options up above 💖 they make the stack frame, the writable data and the constants all line up on 32-bit, 16-bit or 8-bit boundaries. Default? 32-bit alignment, periodt.-mno-prologue-epilogue,-mprologue-epilogue— with-mno-prologue-epiloguethe normal function prologue and epilogue that set up the stack frame get yeeted, and no return instructions or return sequences are generated in the code.
⚠️ Only use this one while literally eyeballing the compiled code, okay? Because no warnings or errors pop up when call-saved registers need saving or when local variables need storage allocated. It's giving "trust me bro" and that's on you 🙈
-mno-gotplt,-mgotplt— when you're in-fpicand-fPICmode, these say don't generate (or do generate) the instruction sequences that load function addresses from the PLT part of the GOT, instead of the calls to the PLT that are traditional on other architectures. Default is-mgotplt✨-melf— legacy no-op option, only even recognized with thecris-axis-elfandcris-axis-linux-gnutargets. It's a historical artifact, we respect the elders 💅-mlinux— same energy, legacy no-op, only recognized with thecris-axis-linux-gnutarget.-sim— this one, recognized for thecris-axis-elftarget, arranges to link with input-output functions from a simulator library. Code, initialized data and zero-initialized data all get allocated consecutively 🎀-sim2— like-sim, but it passes linker options to locate initialized data at0x40000000and zero-initialized data at0x80000000. Numbers stay numbers, no cap.
🌸 CR16 Options 🌸
These ones are defined specifically for the CR16 ports.
-mmac— enables the use of multiply-accumulate instructions. Disabled by default.-mcr16cplus,-mcr16c— generate code for the CR16C or CR16C+ architecture. CR16C+ is the default 💜-msim— links the librarylibsim.a, which is compatible with a simulator. Only applicable to the ELF compiler.-mint32— choose the integer type to be 32-bit wide.-mbit-ops— generates"sbit"/"cbit"instructions for bit manipulations. Tiny little bit queens 👑-mdata-model=model— choose a data model. The choices for model arenear,farormedium, andmediumis the default. However,faris not valid with-mcr16c, because the CR16C architecture doesn't support the far data model. So don't try it, she will not go 🚫
🦋 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, andck810. The default isck810.-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, andck810ftv. That's a whole alphabet of girlies, all valid, all iconic ✨-mbig-endian,-EB,-mlittle-endian,-EL— select big- or little-endian code. Default is little-endian.-mfloat-abi=name— specifies which floating-point ABI to use. Permissible values are:soft,softfpandhard.softmakes GCC generate output containing library calls for floating-point operations.softfpallows generation of code using hardware floating-point instructions, but still uses the soft-float calling conventions.hardallows generation of floating-point instructions and uses FPU-specific calling conventions.
The default depends on the specific target configuration.
🚨 Heads up: 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. No mixing, bestie, that's how you get chaos 💀
-mhard-float,-msoft-float— select hardware or software floating-point implementations. The default is soft float.-mdouble-float,-mno-double-float— when-mhard-floatis in effect, this enables generation of double-precision float instructions. This is the default except when compiling for CK803.-mfdivdu,-mno-fdivdu— when-mhard-floatis in effect, this enables 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 arefpv2_sf(equivalent to-mno-double-float -mno-fdivdu),fpv2(-mdouble-float -mno-divdu), andfpv2_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. So if you're using those attributes, you need this one, iykyk 🫶
-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 is required for correct code generation on CK801 and CK802, and is optional on other processors.-mstack-size,-mno-stack-size— emit.stack_sizedirectives for each function in the assembly output. This option defaults to off.-mccrt,-mno-ccrt— generate code for the C-SKY compiler runtime instead oflibgcc. This option defaults to off.-mbranch-cost=n— set the branch costs to roughly n instructions. The default is1.-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. Proceed with caution, bestie 😭
-msim— links the librarylibsemi.a, which is compatible with a simulator. Only applicable to the ELF compiler.
🍓 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 that by running the compiler or linker multiple times and joining the results together with lipo. Multi-run moment, we love a collage 🌈
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. Vibe check: everyone's got opinions and only some of them are loud about it 💀
-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-Ioptions 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 <Framework/header.h>, where Framework denotes the name of the framework and header.h is found in the PrivateHeaders or Headers directory. Nested girlies, we see you 🧚
-iframeworkdir— literally-Fbut the directory gets treated as a system directory. The one big difference between this-iframeworkand-F? With-iframework, the compiler does not warn you about constructs sitting inside header files found via dir. Obsessed with the peace and quiet tbh. ⚠️ Valid only for the C family of languages.-gused— emits debugging info for symbols that are used. For thestabsdebugging format this turns on-feliminate-unused-debug-symbols. And bestie, it's ON by default. periodt. 💅-gfull— emits debugging info for all symbols and types. no crumbs left.- *
-mmacosx-version-min=version* — the earliest version of MacOS X that this executable will run on is version. Typical values of version include10.1,10.2, and10.3.9.
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— turns on kernel development mode, main character energy for real. The-mkerneloption sets-static,-fno-common,-fno-use-cxa-atexit,-fno-exceptions,-fno-non-call-exceptions,-fapple-kext,-fno-weakand-fno-rttiwhere applicable. This mode also sets-mno-altivec,-msoft-float,-fno-builtinand-mlong-branchfor PowerPC targets. she does it all 💅-mone-byte-bool— overrides the defaults for"bool"so that"sizeof(bool)==1". By default"sizeof(bool)"is4when compiling for Darwin/PowerPC and1when 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. like, it's giving chaos if you're not careful. 💀
-mfix-and-continue-ffix-and-continue-findirect-data— generate code suitable for fast turnaround development, like letting GDB dynamically load.ofiles into already-running programs.-findirect-dataand-ffix-and-continueare provided for backwards compatibility. 🫶-all_load— loads all members of static archive libraries. See man ld(1) for more information.-arch_errors_fatal— makes errors having to do with files that have the wrong architecture become fatal. no chill.-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_loaderexecutable — 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 Darwinlibtoolcommand. slay 💅-force_cpusubtype_ALL— this causes GCC's output file to have theALLsubtype, instead of one controlled by the-mcpuor-marchoption.
okay so now the big girl list. every single one of these gets passed to the Darwin linker, and the Darwin linker man page describes them in detail. no cap, that's the whole tea: ☕
-allowable_clientclient_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. (yes bestie, a couple of them show up twice in that list — that's exactly how the manpage serves it, and we do not gatekeep. 🎀)
🌈 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-floatis specified, functions inlibgcc.aare 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 emulation 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. (yes really, iykyk)
-mfp-reg-mno-fp-regs— generate code that uses (does not use) the floating-point register set.-mno-fp-regsimplies-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$0instead of$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-regsmust 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. iconic honestly.
-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 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 program to SIGFPE when it attempts to use the results without -mieee). 💀
-mieee-with-inexact— this is like-mieeeexcept 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-fptmtrap-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 byn, underflow traps are enabled as well.su— likeu, but the instructions are marked to be safe for software completion (see Alpha architecture manual for details).sui— likesu, but inexact traps are enabled as well.
- *
-mfp-rounding-mode=rounding-mode* — selects the IEEE rounding mode. Other Alpha compilers call this option-fprmrounding-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 thefpcr,dcorresponds 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 precision 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=iand either-mfp-trap-mode=suor-mfp-trap-mode=sui. Its only effect is to emit the line.eflag 48in the function prologue of the generated assembly file. 🦋-mbuild-constants— okay so normally GCC looks at a 32- or 64-bit integer constant and is like "hmm can I build this out of smaller constants in two or three instructions?" and if it can, slay, it does. If it can't, it just yeets the constant out as a literal and generates code to load it from the data segment at run time. Use this flag to force GCC to construct all integer constants using code, even when it eats more instructions (the max is six, bestie). The classic use case? Building a shared library dynamic loader. It's itself a shared library, so it has to relocate itself in memory before it can even find the variables and constants in its own data segment. iconic behavior tbh.-mbwx,-mno-bwx,-mcix,-mno-cix,-mfix,-mno-fix,-mmax,-mno-max— these tell GCC whether it should generate code that uses the optional BWX, CIX, FIX and MAX instruction sets. The default is to use whatever instruction sets are supported by the CPU type you specified with the-mcpu=option — or, if you didn't specify one, the CPU on which GCC was built. 🎀-mfloat-vax,-mfloat-ieee— generate code that uses (or, for-mfloat-ieee, does not use) VAX F and G floating-point arithmetic instead of IEEE single and double precision.-mexplicit-relocs,-mno-explicit-relocs— back in the day, older Alpha assemblers had literally no way to generate symbol relocations except through assembler macros, and using those macros does not allow optimal instruction scheduling 😭. GNU binutils as of version 2.12 supports a new syntax that lets the compiler explicitly mark which relocations should apply to which instructions. This option is mostly useful for debugging, since GCC detects the assembler's capabilities when it is built and sets the default accordingly.-msmall-data,-mlarge-data— when-mexplicit-relocsis in effect, static data gets accessed via gp-relative relocations. With-msmall-data, objects 8 bytes long or smaller are placed in a small data area (the.sdataand.sbsssections) and are accessed via 16-bit relocations off of the$gpregister. That caps the small data area at 64KB, but it lets the variables be directly accessed via a single instruction. so cute, so efficient.
The default is -mlarge-data. With that option the data area is limited to just below 2GB. Programs that need more than 2GB of data must use malloc or mmap to allocate the data in the heap instead of in the program's data segment.
When generating code for shared libraries, -fpic implies -msmall-data and -fPIC implies -mlarge-data.
-msmall-text,-mlarge-text— with-msmall-text, the compiler assumes that the code of the entire program (or shared library) fits in 4MB, and is thus reachable with a branch instruction. When-msmall-datais used, the compiler can assume that all local symbols share the same$gpvalue, 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, periodt.
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 thatL3is 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 to32767. Defaults to whatever limit is imposed by the version of the Linux kernel targeted. - *
-mkernel=version* — this specifies the minimum version of the kernel that will run the compiled program. GCC uses this version to determine which instructions to use, what kernel helpers to allow, etc. Currently, version can be one of4.0,4.1,4.2,4.3,4.4,4.5,4.6,4.7,4.8,4.9,4.10,4.11,4.12,4.13,4.14,4.15,4.16,4.17,4.18,4.19,4.20,5.0,5.1,5.2,latestandnative. (yes bestie, that is a LOT of kernel versions, drink some water 💧) -mbig-endian— generate code for a big-endian target.-mlittle-endian— generate code for a little-endian target. This is the default. 🦋-mjmpext— enable generation of extra conditional-branch instructions. Enabled for CPU v2 and above.-mjmp32— enable 32-bit jump instructions. Enabled for CPU v3 and above.-malu32— enable 32-bit ALU instructions. Enabled for CPU v3 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 isv3.
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 ofv2, plus:- 32-bit jump operations, as in
-mjmp32 - 32-bit ALU operations, as in
-malu32
- 32-bit jump operations, as in
-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:- Save and restore callee-saved registers at function entry and exit, respectively.
✨ 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. no cap, that's the whole vibe.-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.
Bestie, listen: 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. Do not skip this, it's giving "undefined behavior at 3am". 💀
-mlra— enable Local Register Allocation. This is still experimental for FT32, so by default the compiler uses standard reload.-mnodiv— do not usedivandmodinstructions.-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 useicc0andfcc0.-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-fpicor-fpie, it assumes GOT entries and small data are within a 12-bit range from the GOT base address; with-fPICor-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.,-fPICor-fpic), or when an optimization option such as-O3or above is present in the command line. ⭐-mTLS— bestie, assume a large TLS segment when you're generating thread-local code. Big energy.-mtls— nope, don't assume a large TLS segment when generating thread-local code.-mgprel-ro— turn on the use of"GPREL"relocations in the FDPIC ABI for data we know lives in read-only sections. It's on by default, except for-fpicor-fpie: even though it can help shrink the global offset table, it trades 1 instruction for 4. With-fPICor-fPIE, it trades 3 instructions for 4, one of which may be shared by multiple symbols, and it dodges the need for a GOT entry for the referenced symbol, so it's more likely to be a win. If it's not,-mno-gprel-rocan shut it off.-multilib-library-pic— link with the (library, not FD) pic libraries. It's implied by-mlibrary-pic, and also by-fPICand-fpicwithout-mfdpic. You should never have to use it explicitly. like, never ever.-mlinked-fp— follow the EABI requirement of always creating a frame pointer whenever a stack frame is allocated. This one's 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 lets the functions be placed anywhere within the 32-bit address space. freedom 💅-malign-labels— try to align labels to an 8-byte boundary by inserting NOPs into the previous packet. This only does anything 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 ine_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— causegasto print out tomcat statistics. numbers go brrr 📊-mcpu=cpu— select the processor type you're generating code for. Possible values arefrv,fr550,tomcat,fr500,fr450,fr405,fr400,fr300andsimple.
🐧 GNU/Linux Options 🐧
These -m options are defined for GNU/Linux targets, bestie:
-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 the 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-exceptionsand-fno-rttiby 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-mhor-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-300causes 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 are1.0for PA 1.0,1.1for PA 1.1, and2.0for PA 2.0 processors. Refer to/usr/lib/sched.modelson 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=2.0respectively.-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.-mjump-in-delay— this option is ignored and provided for compatibility purposes only.-mdisable-fpregs— prevent floating-point registers from being used in any manner. This is necessary for compiling kernels that perform lazy context switching of floating-point registers. If you use this option and attempt to perform floating-point operations, the compiler aborts. 💀-mdisable-indexing— prevent the compiler from using indexing address modes. This avoids some rather obscure problems when compiling MIG generated code under MACH.-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.
-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.-mlong-load-store— generate 3-instruction load and store sequences as sometimes required by the HP-UX 10 linker. This is equivalent to the+koption to the HP compilers.-mportable-runtime— use the portable calling conventions proposed by HP for ELF systems.-mgas— enable the use of assembler directives only GAS understands.-mschedule=cpu-type— schedule code according to the constraints for the machine type cpu-type. The choices for cpu-type are700,7100,7100LC,7200,7300and8000. Refer to/usr/lib/sched.modelson an HP-UX system to determine the proper scheduling option for your machine. The default scheduling is8000.-mlinker-opt— enable the optimization pass in the HP-UX linker. Note this makes symbolic debugging impossible. It also triggers a bug in the HP-UX 8 and HP-UX 9 linkers in which they give bogus error messages when linking some programs. 🐛-msoft-float— bestie, this one makes your output call out to library routines for floating point instead of doing it inline. ⚠️ WARNING: the requisite libraries are not available for all HPPA targets. Normally you'd just lean on the facilities of the machine's usual C compiler, but ngl that literally cannot be done directly in cross-compilation. So you have to make your own arrangements to provide suitable library functions for cross-compilation. 🙈
Also, -msoft-float changes the calling convention in the output file, so it's 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. it's giving "all or nothing", no cap. 💅
-msio— generates the predefine,_SIO, for server IO. The default is-mwsio. That one generates the predefines__hp9000s700,__hp9000s700__and_WSIO, for workstation IO. These options are available under HP-UX and HI-UX. ✨-mgnu-ld— use options specific to GNU ld. This passes-sharedto ld when building a shared library. It's the default when GCC is configured, explicitly or implicitly, with the GNU linker. Heads up: 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-ldconfigure option, GCC's program search path, and finally by the user'sPATH. The linker used by GCC can be printed using whichgcc -print-prog-name=ld. This option is only available on the 64-bit HP-UX GCC, i.e. configured withhppa*64*-*-hpux*. 🎀-mhp-ld— use options specific to HP ld. This passes-bto ld when building a shared library and passes+Accept TypeMismatchto ld on all links. It's the default when GCC is configured, explicitly or implicitly, with the HP linker. Same deal as before: 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-ldconfigure option, GCC's program search path, and finally by the user'sPATH. The linker used by GCC can be printed using whichgcc -print-prog-name=ld. This option is only available on the 64-bit HP-UX GCC, i.e. configured withhppa*64*-*-hpux*. 💜-mlong-calls— generates 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 are7,600,000and240,000bytes, respectively for the PA 2.0 and PA 1.X architectures. Sibcalls are always limited at240,000bytes. 📏
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. 🦋
-munix=unix-std— generates compiler predefines and selects a startfile for the specified UNIX standard. The choices for unix-std are93,95and98.93is supported on all HP-UX versions.95is available on HP-UX 10.10 and later.98is available on HP-UX 11.11 and later. The default values are93for HP-UX 10.00,95for HP-UX 10.10 though to 11.00, and98for HP-UX 11.11 and later. 🌈-munix=93provides the same predefines as GCC 3.3 and 3.4.-munix=95provides additional predefines forXOPEN_UNIXand_XOPEN_SOURCE_EXTENDED, and the startfileunix95.o.-munix=98provides additional predefines for_XOPEN_UNIX,_XOPEN_SOURCE_EXTENDED,_INCLUDE__STDC_A1_SOURCEand_INCLUDE_XOPEN_SOURCE_500, and the startfileunix98.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— suppresses the generation of link options to searchlibdld.slwhen the-staticoption is specified on HP-UX 10 and later. 🙅-static— okay so the HP-UX implementation ofsetlocalein libc has a dependency onlibdld.sl. And there isn't an archive version oflibdld.sl. So like, when the-staticoption 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, andoutregister 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 forsqrt.-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 are14,22, and64.-mtune=cpu-type— tune the instruction scheduling for a particular CPU. Valid values areitanium,itanium1,merced,itanium2, andmckinley. 🎀-milp32,-mlp64— generate code for a 32-bit or 64-bit environment. The 32-bit environment setsint,longand pointer to 32 bits. The 64-bit environment setsintto 32 bits andlongand 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 ofld.ainstructions 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 ofld.ainstructions 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 theld.sinstructions and the corresponding check instructionschk.s. The default setting is disabled. 🚫-msched-br-in-data-spec,-mno-sched-br-in-data-spec— (En/Dis)ables speculative scheduling of the instructions that are dependent on the data speculative loads before reload. bestie this only kicks in if-msched-br-data-specis enabled — otherwise it's just sitting there looking cute. Default: enabled 💖-msched-ar-in-data-spec,-mno-sched-ar-in-data-spec— same vibe but for the instructions that are dependent on the data speculative loads after reload. Also only effective with-msched-ar-data-specenabled. Default: enabled ✨-msched-in-control-spec,-mno-sched-in-control-spec— (En/Dis)ables speculative scheduling of the instructions that are dependent on the control speculative loads. Only does anything when-msched-control-specis enabled. Default: enabled 🦋-mno-sched-prefer-non-data-spec-insns,-msched-prefer-non-data-spec-insns— if you turn this on, data-speculative instructions get picked for the schedule only if there are literally no other choices at the moment. It makes the whole data speculation thing way more conservative. Default: disabled 💅-mno-sched-prefer-non-control-spec-insns,-msched-prefer-non-control-spec-insns— same energy but for control-speculative instructions: they only get scheduled if there's nothing else available. More conservative control speculation. Default: disabled 🎀-mno-sched-count-spec-in-critical-path,-msched-count-spec-in-critical-path— if enabled, speculative dependencies get counted while the compiler computes instruction priorities. Slightly more conservative speculation. Default: disabled 🌸-msched-spec-ldc— use a simple data speculation check. on by default 💚-msched-control-spec-ldc— use a simple check for control speculation. on by default 💚-msched-stop-bits-after-every-cycle— drop a stop bit after every cycle when scheduling. on by default ⭐-msched-fp-mem-deps-zero-cost— assume floating-point stores and loads probably won't conflict when they're placed into the same instruction group. Default: disabled 🙈-msel-sched-dont-check-control-spec— generate checks for control speculation in selective scheduling. Default: disabled 💀-msched-max-memory-insns=max-insns— caps how many memory insns can sit in one instruction group, and gives lower priority to the memory insns that try to schedule into that same group afterward. super handy for dodging cache bank conflicts, iykyk. Default value is1.-msched-max-memory-insns-hard-limit— makes the limit you set withmsched-max-memory-insnsan actual hard limit, so nothing over that number gets into an instruction group. Without this flag the limit is "soft" — meaning non-memory operations get preferred once you hit it, but memory operations can still be scheduled.
LM32 Options 🎀
These -m options are defined for the LatticeMico32 architecture, periodt:
-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 🌸
M32C Options 🌈
-mcpu=name— pick the CPU that code gets generated for. name can be one ofr8cfor the R8C/Tiny series,m16cfor the M16C (up to /60) series,m32cmfor the M16C/80 series, orm32cfor the M32C/80 series. pick your fighter 💖-msim— says the program is gonna run on the simulator. This links in an alternate runtime library that supports stuff like file I/O. You must not use this option when generating programs that will run on real hardware — you have to provide your own runtime library for whatever I/O functions you need. 😭-memregs=number— sets how many memory-based pseudo-registers GCC uses during code generation. These pseudo-registers behave like real registers, so there's a whole tradeoff between GCC's ability to fit the code into available registers and the performance penalty of using memory instead of registers. Note that every module 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 their addresses can be loaded with theld24instruction), and assume all subroutines are reachable with theblinstruction. 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 generatesseth/add3instructions to load their addresses), and assume all subroutines are reachable with theblinstruction. 🦋-mmodel=large— assume objects may be anywhere in the 32-bit address space (the compiler generatesseth/add3instructions to load their addresses), and assume subroutines may not be reachable with theblinstruction (the compiler generates the much slowerseth/add3/jlinstruction sequence). slower, but it works anywhere 💀-msdata=none— disable use of the small data area. Variables get put into one of.data,.bss, or.rodata(unless thesectionattribute 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 don't 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. Default value of num is8. The-msdataoption must be set to one ofsdataorusefor 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 be1or2💜-mbranch-cost=number— number can only be1or2. If it's1then branches are preferred over conditional code; if it's2, then the opposite applies. 🔀-mflush-trap=number— specifies the trap number to use to flush the cache. Default is12. Valid numbers are between0and15inclusive. 🎀-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. 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:68000,68010,68020,68030,68040,68060andcpu32. ColdFire architectures are selected according to Freescale's ISA classification and the permissible values are:isaa,isaaplus,isabandisac.
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:68000,68010,68020,68030,68040,68060,68302,68332andcpu32. The ColdFire cpus are given by the table below, which also classifies the CPUs into families:51:5151ac51ag51cn51em51je51jf51jg51jm51mm51qe51qm5206:5202520452065206e:5206e5208:520752085211a:5210a5211a5213:5211521252135216:5214521652235:5223052231522325223352234522355225:5224522552259:5225252254522555225652258522595235:5232523352345235523x5249:52495250:52505271:527052715272:52725275:527452755282:528052815282528x53017:530115301253013530145301553016530175307:53075329:532753285329532x5373:53725373537x5407:54075475:547054715472547354745475547x548054815482548354845485
-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-marchand-mcpu. The M680x0 microarchitectures are:68000,68010,68020,68030,68040,68060andcpu32. The ColdFire microarchitectures are:cfv1,cfv2,cfv3,cfv4andcfv4e. ⭐
okay so 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 basically the same vibe but it also includes 68060 targets. These two options make the exact same tuning decisions as -m68020-40 and -m68020-60 respectively, no cap.
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-marchis 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=5206and"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,-mcpu32and-m5200options imply-mnobitfield.-mbitfield— do use the bit-field instructions. The-m68020option 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. bestie, that's an ABI oopsie, be careful 💀
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 of0generates 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;-fPICsuffices.
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 foobarIf 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 literally the -m options that are defined for the Motorola M*Core processors. Bestie, they're giving "niche but iconic" 💅
-mhardlit,-mno-hardlit— Inline constants right into the code stream, but only if it can be done in two instructions or less. Short and sweet, no cap. ✨-mdiv,-mno-div— Use the divide instruction. (Enabled by default, so it's already doing the thing unless you say otherwise.)-mrelax-immediate,-mno-relax-immediate— Allow arbitrary-sized immediates in bit operations. Big immediates, no limits, main character energy. 💅-mwide-bitfields,-mno-wide-bitfields— Always treat bit-fields as"int"-sized. Periodt.-m4byte-functions,-mno-4byte-functions— Force all functions to be aligned to a 4-byte boundary. Snapped to the grid fr fr. 🎀-mcallgraph-data,-mno-callgraph-data— Emit callgraph information.-mslow-bytes,-mno-slow-bytes— Prefer word access when reading byte quantities. Lowkey faster that way.-mlittle-endian,-mbig-endian— Generate code for a little-endian target. (That's what the little one does, anyway 🙈)-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. DIY runtime, we love to see it 🫶-mstack-increment=size— Set the maximum amount for a single stack increment operation.
Okay so heads up: 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.
🌸 MeP Options 🌸
-mabsdiff— Enables the"abs"instruction, which is the absolute difference between two registers.-mall-opts— Enables all the optional instructions — average, multiply, divide, bit operations, leading zero, absolute difference, min/max, clip, and saturation. All of them. Every single one. Obsessed. 🔥-maverage— Enables the"ave"instruction, which computes the average of two registers.-mbased=n— Variables of size n bytes or smaller are placed in the.basedsection by default. Based variables use the$tpregister as a base register, and there is a 128-byte limit to the.basedsection. (Yes, the section is literally called "based" and honestly? Slay.)-mbitops— Enables the bit operation instructions — bit test (btstm), set (bsetm), clear (bclrm), invert (bnotm), and test-and-set (tas).-mc=name— Selects which section constant data is placed in. name may betiny,near, orfar.-mclip— Enables the"clip"instruction.
Note that-mclipis not useful unless you also provide-mminmax. They're a package deal bestie, you can't split them up 💜
-mconfig=name— Selects one of the built-in core configurations. Each MeP chip has one or more modules in it; each module has a core CPU and a variety of coprocessors, optional instructions, and peripherals. The"MeP-Integrator"tool, not part of GCC, provides these configurations through this option; using this option is the same as using all the corresponding command-line options. The default configuration isdefault.-mcop— Enables the coprocessor instructions. By default, this is a 32-bit coprocessor. Note that the coprocessor is normally enabled via the-mconfig=option.-mcop32— Enables the 32-bit coprocessor's instructions.-mcop64— Enables the 64-bit coprocessor's instructions.-mivc2— Enables IVC2 scheduling. IVC2 is a 64-bit VLIW coprocessor. Very demure, very VLIW ✨-mdc— Causes constant variables to be placed in the.nearsection.-mdiv— Enables the"div"and"divu"instructions.-meb— Generate big-endian code.-mel— Generate little-endian code.-mio-volatile— Tells the compiler that any variable marked with the"io"attribute is to be considered volatile.-ml— Causes variables to be assigned to the.farsection by default.-mleadz— Enables the"leadz"(leading zero) instruction.-mm— Causes variables to be assigned to the.nearsection by default.-mminmax— Enables the"min"and"max"instructions.-mmult— Enables the multiplication and multiply-accumulate instructions.-mno-opts— Disables all the optional instructions enabled by-mall-opts. Turning it all the way off again 💅-mrepeat— Enables the"repeat"and"erepeat"instructions, used for low-overhead looping.-ms— Causes all variables to default to the.tinysection.
Note that there is a 65536-byte limit to this section. Accesses to these variables use the %gp base register.
-msatur— Enables the saturation instructions.
Note that the compiler does not currently generate these itself, but this option is included for compatibility with other tools, like "as".
-msdram— Link the SDRAM-based runtime instead of the default ROM-based runtime.-msim— Link the simulator run-time libraries.-msimnovec— Link the simulator runtime libraries, excluding built-in support for reset and exception vectors and tables.-mtf— Causes all functions to default to the.farsection. Without this option, functions default to the.nearsection.-mtiny=n— Variables that are n bytes or smaller are allocated to the.tinysection. These variables use the$gpbase register. The default for this option is4, but note that there's a 65536-byte limit to the.tinysection.
🦋 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-bssinstead. (Old and busted, bestie, swap it out 🙈)-mcpu=cpu-type— Use features of, and schedule code for, the given CPU. Supported values are in the formatvX.YY.Z, whereXis a major version,YYis the minor version, andZis compatibility code. Example values arev3.00.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-checkinstead. (Deprecated again, she's retired 💅)-mxl-gp-opt— Use GP-relative.sdata/.sbsssections.-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 codecrt0.o.xmdstub— for use with Xilinx Microprocessor Debugger (XMD) based software intrusive debug agent called xmdstub. This uses startup filecrt1.oand sets the start address of the program to0x800.bootstrap— for applications that are loaded using a bootloader. This model uses startup filecrt2.owhich 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 usescrt3.oas a startup file.
Option-xl-mode-app-modelis a deprecated alias for-mxl-mode-app-model. (Note the missingm, she's the impostor 😭)
-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 formips*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.
okay bestie we're still deep in the MIPS options and there's a LOT left, hydrate and let's go 💖
quick lil note first, because it's important: the _MIPS_ARCH macro uses the processor names given above, fr fr. Meaning: it has the full prefix and does not abbreviate 000 as k. And in the case of from-abi, the macro names the resolved architecture — either "mips1" or "mips3". Also 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. And 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. iconic combo behavior ✨
-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.
okay that was a whole vibe of "just an alias" options, no notes, moving on 🌸
-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. per-function control is giving ✨aesthetic✨ ngl
-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.
bestie do NOT casually use this one for your actual project, it's a test-harness situation 💀
-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-compressedand-mno-interlink-compressed. These options predate the microMIPS ASE and are retained for backwards compatibility. elder options, we respect them 🫶-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 <https://gcc.gnu.org/projects/mipso64-abi.html>.
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. very specific girlies only 💅
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.-mabicallsis 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. smaller AND quicker?? we love an efficient queen ✨
-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 foobarIf 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.
if you're staring at thatrelocation truncated to fiterror like 👁️👄👁️ this is your sign to recompile with-mxgot🦋
-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. mysterious, unknowable, we stan 🙈-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-floataccidentally contains floating-point operations, it is likely to suffer a link-time or run-time failure.
"accidentally contains floating-point operations" is a jumpscare fr, be careful with this one 😭
-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-spregis 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.
and that's the NaN lore, besties — qNaN vs sNaN bit flips are giving flip a coin but make it IEEE 754 🎀
-mllsc,-mno-llsc— okay so these two decide whether GCC uses thell,sc, andsyncinstructions to implement atomic memory built-in functions. Do or don't, bestie, that's the whole vibe. When you don't specify either one, GCC just uses the instructions if the target architecture supports them, no drama.
-mllsc is your girl when the runtime environment can emulate those instructions, and -mno-llsc comes in clutch when you're compiling for nonstandard ISAs. You can make either of them the default by configuring GCC with --with-llsc and --without-llsc respectively. And --with-llsc is already the default for some configurations — check the installation documentation for the tea. 💅
-mdsp,-mno-dsp— Use (or don't use) revision 1 of the MIPS DSP ASE. ✨ This option defines the preprocessor macro__mips_dsp. It also defines__mips_dsp_revto1.-mdspr2,-mno-dspr2— Use (or don't use) revision 2 of the MIPS DSP ASE. This option defines the preprocessor macros__mips_dspand__mips_dspr2. It also defines__mips_dsp_revto2. Level up, babes. 🔥-msmartmips,-mno-smartmips— Use (or don't use) the MIPS SmartMIPS ASE.-mpaired-single,-mno-paired-single— Use (or don't use) paired-single floating-point instructions. Heads up though: this option requires hardware floating-point support to be enabled.-mdmx,-mno-mdmx— Use (or don't use) MIPS Digital Media Extension instructions. This one's picky — it can only be used when generating 64-bit code, and it also requires hardware floating-point support to be enabled. Two conditions, no skipping. 😤-mips3d,-mno-mips3d— Use (or don't use) the MIPS-3D ASE. And plot twist: the option-mips3dimplies-mpaired-single. They're a package deal, iykyk. 🎀-mmicromips,-mno-micromips— Generate (or don't generate) microMIPS code.
MicroMIPS code generation can also be controlled on a per-function basis by means of the micromips and nomicromips attributes. Very main character of it, honestly.
-mmt,-mno-mt— Use (or don't use) MT Multithreading instructions.-mmcu,-mno-mcu— Use (or don't use) the MIPS MCU ASE instructions.-meva,-mno-eva— Use (or don't use) the MIPS Enhanced Virtual Addressing instructions.-mvirt,-mno-virt— Use (or don't use) the MIPS Virtualization (VZ) instructions.-mxpa,-mno-xpa— Use (or don't use) the MIPS eXtended Physical Address (XPA) instructions.-mcrc,-mno-crc— Use (or don't use) the MIPS Cyclic Redundancy Check (CRC) instructions.-mginv,-mno-ginv— Use (or don't use) the MIPS Global INValidate (GINV) instructions.-mloongson-mmi,-mno-loongson-mmi— Use (or don't use) the MIPS Loongson MultiMedia extensions Instructions (MMI).-mloongson-ext,-mno-loongson-ext— Use (or don't use) the MIPS Loongson EXTensions (EXT) instructions.-mloongson-ext2,-mno-loongson-ext2— Use (or don't use) the MIPS Loongson EXTensions r2 (EXT2) instructions.-mlong64— Forcelongtypes to be 64 bits wide. See-mlong32for the explanation of the default and the way that the pointer size is determined. Yes bestie, you gotta read the next one, it's all connected. 🦋-mlong32— Forcelong,int, and pointer types to be 32 bits wide.
The default size of ints, longs and pointers depends on the ABI. All the supported ABIs use 32-bit ints. The n64 ABI uses 64-bit longs, as does the 64-bit EABI; the others use 32-bit longs. Pointers are the same size as longs, or the same size as integer registers, whichever is smaller. It's giving ✨math✨ but like, the fun kind.
-msym32,-mno-sym32— Assume (or don't assume) that all symbols have 32-bit values, regardless of the selected ABI. This option is useful in combination with-mabi=64and-mno-abicallsbecause it allows GCC to generate shorter and faster references to symbolic addresses. Faster + shorter = slay. ⭐-Gnum — 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-mgpoptfor details.
The default -G option depends on the configuration. 🙈
-mlocal-sdata,-mno-local-sdata— Extend (or don't extend) the-Gbehavior to local data too, like to static variables in C.-mlocal-sdatais the default for all configurations.
If the linker starts complaining that an application is using too much small data, you might wanna 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. Sharing is caring. 🫶
-mextern-sdata,-mno-extern-sdata— Assume (or don't assume) that externally-defined data is in a small data section if the size of that data is within the-Glimit.-mextern-sdatais the default for all configurations.
Okay so this one has rules, listen up. 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. No skipping steps, babes. 💅
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. Iconic problem-solving, honestly.
-mgpopt,-mno-gpopt— Use (or don't use) GP-relative accesses for symbols that are known to be in a small data section; see-G,-mlocal-sdataand-mextern-sdata.-mgpoptis 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.)
And heads up: -mno-gpopt implies -mno-local-sdata and -mno-extern-sdata. It comes with friends. 😭
-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. Trade-offs, bestie, it's always trade-offs. 🌸-muninit-const-in-rodata,-mno-uninit-const-in-rodata— Put uninitializedconstvariables in the read-only data section. This option is only meaningful in conjunction with-membedded-data. Only works with its buddy, no solo runs. ✨-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.
Pick your fighter. 🎀
-msplit-addresses,-mno-split-addresses— Enable (or disable) use of the%hi()and%lo()assembler relocation operators. This option has been superseded by-mexplicit-relocsbut is retained for backwards compatibility. Old but still kicking, respect. 💜-mexplicit-relocs,-mno-explicit-relocs— Use (or don't use) assembler relocation operators when dealing with symbolic addresses. The alternative, selected by-mno-explicit-relocs, is to use assembler macros instead.
-mexplicit-relocs is the default if GCC was configured to use an assembler that supports relocation operators. Conditional queen. ⭐
-mcheck-zero-division,-mno-check-zero-division— Trap (or don't trap) on integer division by zero.
The default is -mcheck-zero-division. Dividing by zero stays illegal, bestie, don't test it. 💀
-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-trapsto allow conditional traps on architectures that support them and-mdivide-breaksto 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.
Yes, there's a kernel bug in the mix. It's giving "not my fault" but also "know about it before you ship". 😭
-mload-store-pairs,-mno-load-store-pairs— Enable (or 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. So like, it's on, but it's also lowkey waiting for the right architecture to show up. 🦋-munaligned-access,-mno-unaligned-access— Enable (or disable) direct unaligned access for MIPS Release 6. MIPSr6 requires load/store unaligned-access support, by hardware or trap&emulate. So-mno-unaligned-accessmay be needed by kernel. Kernel girlies, this one's for you. 🔥-mmemcpy,-mno-memcpy— Force (or don't force) the use ofmemcpyfor non-trivial block moves. The default is-mno-memcpy, which allows GCC to inline most constant-sized copies. Inlining stays winning. 💅-mlong-calls,-mno-long-calls— Disable (or don't disable) use of thejalinstruction. Calling functions usingjalis more efficient but requires the caller and callee to be in the same 256 megabyte segment. Yes, 256 megabyte. Big segment energy. 👑
This option has no effect on abicalls code. The default is -mno-long-calls.
-mmad,-mno-mad— Enable (or disable) use of themad,maduandmulinstructions, as provided by the R4650 ISA. Little instruction trio, obsessed. 🎀-mimadd,-mno-imadd— turns use of themaddandmsubinteger instructions on (or off). Default is-mimaddon architectures that supportmaddandmsub… except the 74k architecture, where it was found to generate slower code. so 74k said no thanks bestie 💅-mfused-madd,-mno-fused-madd— turns use of the floating-point multiply-accumulate instructions on (or off), whenever they're available. Default is-mfused-madd.
heads up tho — on the R8000 CPU, when multiply-accumulate instructions get used, the intermediate product is calculated to infinite precision and is not subject to the FCSR Flush to Zero bit. That 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. same math, different plumbing ✨
-nocpp— tells the MIPS assembler to not run its preprocessor over user assembler files (with a.ssuffix) when assembling them.-mfix-24k,-mno-fix-24k— works 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— works 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— works 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— works around certain R10000 errata:ll/scsequences may not behave atomically on revisions prior to 3.0. They may deadlock on revisions 2.6 and earlier. deadlock?? not the vibe 😭
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 anopinstruction 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— works around the RM7000dmult/dmultuerrata. The workarounds are implemented by the assembler rather than by GCC.-mfix-vr4120,-mno-fix-vr4120— works around certain VR4120 errata:dmultudoes not always produce the correct result.divandddivdo 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— works around the VR4130mflo/mfhierrata. The workarounds are implemented by the assembler rather than by GCC, although GCC avoids usingmfloandmfhiif the VR4130macc,macchi,dmaccanddmacchiinstructions are available instead. clever girl behavior fr fr-mfix-sb1,-mno-fix-sb1— works around certain SB-1 CPU core errata. (This flag currently works around the SB-1 revision 2F1andF2floating-point errata.)-mr10k-cache-barrier=setting— specifies whether GCC should insert cache barriers to avoid the side effects of speculation on R10K processors. okay this one's a whole saga, sit down 🍓
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. yes bestie, even the ghost instructions do crimes 💀
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:
- the memory occupied by the current function's stack frame;
- the memory occupied by an incoming stack argument;
- 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. that's on you, kernel 🫶
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_funcor__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-mtunesetting. heuristic means "vibes-based", just so we're clear 💅-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. If a compact branch instruction is not available, a delay slot form of the branch will be used instead.
This option is supported from MIPS Release 6 onwards.
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. optimal really said "whichever, as long as it's cute" ✨
-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. two pipes > one pipe, math is math 🎀
-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 ofsynciinstructions on architectures that support it. Thesynciinstructions (if enabled) are generated when__builtin___clear_cacheis 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. so like… be careful, bestie 🙈
-mrelax-pic-calls,-mno-relax-pic-calls— try to turn PIC calls that are normally dispatched via register$25into 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. But if you're rolling with -mno-explicit-relocs, this optimization can be done by the assembler and the linker all on their own, no compiler help needed. bestie they're independent like that 💅
-mmcount-ra-address,-mno-mcount-ra-address— emit (or don't emit) code that lets_mcountmess with the calling function's return address. When you turn it on, it extends the usual_mcountinterface with a new ra-address parameter, which has typeintptr_t *and gets passed in register$12. Then_mcountcan change the return address by doing both of these:- 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. ✨
- returning the new address in register
-mframe-header-opt,-mno-frame-header-opt— enable (disable) frame header optimization in the o32 ABI. Here's the tea: with the o32 ABI, calling functions allocate 16 bytes on the stack so the called function can write out register arguments. When this is enabled, the optimization will suppress allocating that frame header if it can figure out it's unused. 🎀
ngl this optimization is off by default at all optimization levels. yes, even the spicy ones.
-mlxc1-sxc1,-mno-lxc1-sxc1— when it applies, enable (disable) generating thelwxc1,swxc1,ldxc1andsdxc1instructions. Enabled by default, periodt.-mmadd4,-mno-madd4— when it applies, enable (disable) generating 4-operandmadd.s,madd.dand their related instructions. Enabled by default too, we love a 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. all of them. every single one.-mepsilon,-mno-epsilon— generate floating-point comparison instructions that compare with respect to therEepsilon register. 🍓-mabi=mmixware,-mabi=gnu— generate code that passes function parameters and return values that (inside the called function) show up as registers$0and up — as opposed to the GNU ABI, which uses global registers$231and up. two different vibes, pick your fighter.-mzero-extend,-mno-zero-extend— when reading data from memory in sizes shorter than 64 bits, use (or don't use) zero-extending load instructions by default, instead of sign-extending ones.-mknuthdiv,-mno-knuthdiv— make the result of a division that yields 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, it's just that the latter is used pretty much exclusively. iykyk.-mtoplevel-symbols,-mno-toplevel-symbols— prepend (or don't prepend) a:to all global symbols, so the assembly code can be used with thePREFIXassembly directive.-melf— generate an executable in the ELF format, instead of the default mmo format that the mmix simulator uses. glow-up for your binary ✨-mbranch-predict,-mno-branch-predict— use (or don't use) the probable-branch instructions, when static branch prediction says a branch is probably gonna happen.-mbase-addresses,-mno-base-addresses— generate (or don't generate) code that uses base addresses. Using a base address automatically generates a request (which the assembler and the linker handle) for a constant to be set up in a global register. That register is used for one or more base address requests within the range 0 to 255 from the value held in the register. This generally leads to short and fast code, but the number of different data items you can address is limited. Which means a program that uses lots of static data may need-mno-base-addresses. trade-offs, bestie 😭-msingle-exit,-mno-single-exit— force (or don't 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. Heads up: this does not change the targeted processor type. The CPU type has to be one ofmn10300,am33,am33-2oram34.-mreturn-pointer-on-d0— when generating a function that returns a pointer, return the pointer in botha0andd0. Otherwise the pointer is returned only ina0, 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-d0to disable it. 🙈-mno-crt0— do not link in the C run-time initialization object file.-mrelax— tell the linker it should do a relaxation optimization pass to shorten branches, calls and absolute memory addresses. This option only does anything when it's used on the command line for the final link step.
⚠️ this option makes symbolic debugging impossible. like, gone. no cap.
-mliw— let the compiler 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 let the compiler generate Long Instruction Word instructions. This option defines the preprocessor macro__NO_LIW__.-msetlb— let the compiler 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 let the compiler generate SETLB or Lcc instructions. This option defines the preprocessor macro__NO_SETLB__.
🦋 Moxie Options 🦋
-meb— generate big-endian code. This is the default formoxie-*-*configurations.-mel— generate little-endian code.-mmul.x— generatemul.xandumul.xinstructions. This is the default formoxiebox-*-*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 there for testsuite and/or aesthetic purposes. 💅-mmcu=— select the MCU to target. This is used to create a C preprocessor symbol based on the MCU name, converted to upper case and pre- and post-fixed with__. That symbol is in turn used by themsp430.hheader file to select an MCU-specific supplementary header file.
The option also sets the ISA to use. If the MCU name is one that's known to only support the 430 ISA, then that gets 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.
On top of that, an MCU-specific linker script gets added to the linker command line. The script's name is the name of the MCU with .ld appended. So specifying -mmcu=xxx on the gcc command line defines the C preprocessor symbol __XXX__ and makes the linker 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 the ones that are hard-coded.
GCC searches for the devices.csv file using these 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
-Iand-L—devices.csvwill be 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 variableMSP430_GCC_INCLUDE_DIRto the full path to the directory containingdevices.csv, and GCC will search this directory fordevices.csv. Ifdevices.csvis found, this directory will also be registered as an include path and linker library path. Header files and linker scripts in this directory can therefore be used without manually specifying-Iand-Lon the command line. - The
msp430-elf{,bare}/include/devicesdirectory — finally, GCC will examinemsp430-elf{,bare}/include/devicesfrom the toolchain root directory. This directory doesn't exist in a default installation, but if the user has created it and copieddevices.csvthere, then the MCU data will be read. As above, this directory will also be 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-mmcuoption and the ISA set by the-mcpuoption and/or the hardware multiply support set by the-mhwmultoption. It also toggles warnings about unrecognized MCU names. This option is on by default. 👑-mcpu=— specifies the ISA to use. Accepted values aremsp430,msp430xandmsp430xv2.
⚠️ this option is deprecated. The -mmcu= option should be used to select the ISA instead.
-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-bitsize_t).-msmall— use small-model addressing (16-bit pointers, 16-bitsize_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=— bestie, this one describes what kind of hardware multiply your target actually supports. Accepted values arenone(literally zero hardware multiply),16bit(the O.G. 16-bit-only multiply that early MCUs rocked),32bit(the 16/32-bit multiply that later MCUs came through with), andf5series(the 16/32-bit multiply for F5-series MCUs specifically). You can also just passautoand let GCC figure it out from the MCU name you gave with-mmcu. But heads up: if there's no-mmcuat all, or the MCU name is one GCC doesn't recognize, it just assumes no hardware multiply support. And"auto"is the default, so if you don't say anything, that's what you get. ✨
Okay so hardware multiplies are normally done by calling a library routine, which is the move because it saves space in the generated code. But when you're compiling at -O3 or higher, the hardware multiplier gets invoked inline instead. Bigger code, but faster. That's the trade, no cap. 🔥
Also the hardware multiply routines disable interrupts while they run and restore the previous interrupt state when they finish. Which is why they're safe to use inside interrupt handlers as well as in normal code. 🫶
-minrt— turn on a minimum runtime environment: no static initializers, no constructors. It's giving memory-constrained-device energy. The compiler drops special symbols into some objects that tell the linker and runtime which code fragments are actually required. 💅-mtiny-printf— enables the reduced-code-size"printf"and"puts"library functions. Warning though: the tiny versions of these are not reentrant, so use 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=— takes an integer between 0 and 64 inclusive, and sets the max number of inline shift instructions that should be emitted to perform a shift by a constant amount. When that value needs to be exceeded, anmspabihelper function gets used instead. Default is4.
This only affects cases where a shift by multiple positions can't be done 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 you pass for this option gets divided by 2 and the resulting value is used instead. Math is math. 🎀
-mcode-region=,-mdata-region=— these tell the compiler where to place functions and data that don't 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 one —"any"— is the default, and 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=— passes on a request to the assembler to enable the fixes for the named silicon errata.-msilicon-errata-warn=— 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 ifdevices.csvis not found or there's a 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 be4or16.-mcache-block-size=num— specify the size of each cache block, which must be a power of 2 between4and512.-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.
🦋 Nios II Options 🦋
These are the options defined for the Altera Nios II processor.
-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 is8.-mgpopt=option,-mgpopt,-mno-gpopt— generate (or do not generate) GP-relative accesses. The following option names are recognized:none— do not generate GP-relative accesses.local— generate GP-relative accesses for small data objects that are not external, weak, or uninitialized common symbols. Also use GP-relative addressing for objects that have been explicitly placed in a small data section via a"section"attribute.global— as forlocal, but also generate GP-relative accesses for small data objects that are external, weak, or common. If you use this option, you must ensure that all parts of your program (including libraries) are compiled with the same-Gsetting.data— generate GP-relative accesses for all data objects in the program. If you use this option, the entire data and BSS segments of your program must fit in 64K of memory and you must use an appropriate linker script to allocate them within the addressable range of the global pointer.all— generate GP-relative addresses for function pointers as well as data pointers. If you use this option, the entire text, data, and BSS segments of your program must fit in 64K of memory and you must use an appropriate linker script to allocate them within the addressable range of the global pointer.
-mgpopt is equivalent to -mgpopt=local, and -mno-gpopt is equivalent to -mgpopt=none.
The default is -mgpopt except when -fpic or -fPIC is specified to generate position-independent code. Note that the Nios II ABI does not permit GP-relative accesses from shared libraries. 🙈
You may need to specify -mno-gpopt explicitly when building programs that include large amounts of small data, including large GOT data sections. In this case, the 16-bit offset for GP-relative addressing may not be large enough to allow access to the entire small data section. 💀
-mgprel-sec=regexp— this option specifies additional section names that can be accessed via GP-relative addressing. It's most useful in conjunction with"section"attributes on variable declarations and a custom linker script. The regexp is a POSIX Extended Regular Expression.
This option does not affect the behavior of the -G option, and the specified sections are in addition to the standard .sdata and .sbss small-data sections that are recognized by -mgpopt.
-mr0rel-sec=regexp— this option specifies names of sections that can be accessed via a 16-bit offset from"r0"; that is, in the low 32K or high 32K of the 32-bit address space. It's most useful in conjunction with"section"attributes on variable declarations and a custom linker script. The regexp is a POSIX Extended Regular Expression.
In contrast to the use of GP-relative addressing for small data, zero-based addressing is never generated by default and there are no conventional section names used in standard linker scripts for sections in the low or high areas of memory. 😭
-mel,-meb— generate little-endian (default) or big-endian (experimental) code, respectively.-march=arch— this specifies the name of the target Nios II architecture. GCC uses this name to determine what kind of instructions it can emit when generating assembly code. Permissible names are:r1,r2.
The preprocessor macro "__nios2_arch__" is available to programs, with value 1 or 2, indicating the targeted ISA level. ✨
-mbypass-cache,-mno-bypass-cache— force all load and store instructions to always bypass cache by using I/O variants of the instructions. The default is not to bypass the cache.-mno-cache-volatile,-mcache-volatile— volatile memory access bypass the cache using the I/O variants of the load and store instructions. The default is not to bypass the cache.-mno-fast-sw-div,-mfast-sw-div— do not use table-based fast divide for small numbers. The default is to use the fast divide at-O3and above.-mno-hw-mul,-mhw-mul,-mno-hw-mulx,-mhw-mulx,-mno-hw-div,-mhw-div— enable or disable emitting"mul","mulx"and"div"family of instructions by the compiler. The default is to emit"mul"and not emit"div"and"mulx".-mbmx,-mno-bmx,-mcdx,-mno-cdx— enable or disable generation of Nios II R2 BMX (bit manipulation) and CDX (code density) instructions. Enabling these instructions also requires-march=r2. Since these instructions are optional extensions to the R2 architecture, the default is not to emit them. 💅-mcustom-insn=N,-mno-custom-insn— okay so each-mcustom-insn=Noption turns on a custom instruction with encodingNwhen GCC is generating code that uses insn. Like, for real,-mcustom-fadds=253generates custom instruction253for single-precision floating-point add operations instead of the default behavior of using a library call. 💅
The following values of insn are supported. Except as otherwise noted, floating-point operations are expected to be implemented with normal IEEE 754 semantics and correspond directly to the C operators or the equivalent GCC built-in functions.
🌸 Single-precision floating point 🌸
fadds,fsubs,fdivs,fmuls— Binary arithmetic operations.fnegs— Unary negation.fabss— Unary absolute value.fcmpeqs,fcmpges,fcmpgts,fcmples,fcmplts,fcmpnes— Comparison operations.fmins,fmaxs— Floating-point minimum and maximum. These instructions are only generated if-ffinite-math-onlyis specified.fsqrts— Unary square root operation.fcoss,fsins,ftans,fatans,fexps,flogs— Floating-point trigonometric and exponential functions. These instructions are only generated if-funsafe-math-optimizationsis also specified.
💜 Double-precision floating point 💜
faddd,fsubd,fdivd,fmuld— Binary arithmetic operations.fnegd— Unary negation.fabsd— Unary absolute value.fcmpeqd,fcmpged,fcmpgtd,fcmpled,fcmpltd,fcmpned— Comparison operations.fmind,fmaxd— Double-precision minimum and maximum. These instructions are only generated if-ffinite-math-onlyis specified.fsqrtd— Unary square root operation.fcosd,fsind,ftand,fatand,fexpd,flogd— Double-precision trigonometric and exponential functions. These instructions are only generated if-funsafe-math-optimizationsis also specified.
🦋 Conversions 🦋
fextsd— Conversion from single precision to double precision.ftruncds— Conversion from double precision to single precision.fixsi,fixsu,fixdi,fixdu— Conversion from floating point to signed or unsigned integer types, with truncation towards zero.round— Conversion from single-precision floating point to signed integer, rounding to the nearest integer and ties away from zero. This corresponds to the__builtin_lroundffunction when-fno-math-errnois used.floatis,floatus,floatid,floatud— Conversion from signed or unsigned integer types to floating-point types.
In addition, all of the following transfer instructions for internal registers X and Y must be provided to use any of the double-precision floating-point instructions. Custom instructions taking two double-precision source operands expect the first operand in the 64-bit register X. The other operand (or only operand of a unary operation) is given to the custom arithmetic instruction with the least significant half in source register src1 and the most significant half in src2. A custom instruction that returns a double-precision result returns the most significant 32 bits in the destination register and the other half in 32-bit register Y. GCC automatically generates the necessary code sequences to write register X and/or read register Y when double-precision floating-point instructions are used.
fwrx— Writesrc1into the least significant half ofXandsrc2into the most significant half ofX.fwry— Writesrc1intoY.frdxhi,frdxlo— Read the most or least (respectively) significant half ofXand store it indest.frdy— Read the value ofYand store it intodest.
Note that you can gain more local control over generation of Nios II custom instructions by using the target("custom-insn=N") and target("no-custom-insn") function attributes or pragmas.
-mcustom-fpu-cfg=name— This option enables a predefined, named set of custom instruction encodings (see-mcustom-insnabove). Currently, the following sets are defined:-mcustom-fpu-cfg=60-1is equivalent to:
-mcustom-fmuls=252
-mcustom-fadds=253 -mcustom-fsubs=254 -fsingle-precision-constant-mcustom-fpu-cfg=60-2is equivalent to:
-mcustom-fmuls=252
-mcustom-fadds=253 -mcustom-fsubs=254 -mcustom-fdivs=255
-fsingle-precision-constant-mcustom-fpu-cfg=72-3is equivalent to:
-mcustom-floatus=243
-mcustom-fixsi=244 -mcustom-floatis=245 -mcustom-fcmpgts=246
-mcustom-fcmples=249 -mcustom-fcmpeqs=250 -mcustom-fcmpnes=251
-mcustom-fmuls=252 -mcustom-fadds=253 -mcustom-fsubs=254
-mcustom-fdivs=255 -fsingle-precision-constant-mcustom-fpu-cfg=fph2is equivalent to:
-mcustom-fabss=224
-mcustom-fnegs=225 -mcustom-fcmpnes=226 -mcustom-fcmpeqs=227
-mcustom-fcmpges=228 -mcustom-fcmpgts=229 -mcustom-fcmples=230
-mcustom-fcmplts=231 -mcustom-fmaxs=232 -mcustom-fmins=233
-mcustom-round=248 -mcustom-fixsi=249 -mcustom-floatis=250
-mcustom-fsqrts=251 -mcustom-fmuls=252 -mcustom-fadds=253
-mcustom-fsubs=254 -mcustom-fdivs=255Custom instruction assignments given by individual -mcustom-insn= options override those given by -mcustom-fpu-cfg=, regardless of the order of the options on the command line.
Note that you can gain more local control over selection of a FPU configuration by using the target("custom-fpu-cfg=name") function attribute or pragma.
The name fph2 is an abbreviation for Nios II Floating Point Hardware 2 Component. Please note that the custom instructions enabled by -mcustom-fmins=233 and -mcustom-fmaxs=234 are only generated if -ffinite-math-only is specified. The custom instruction enabled by -mcustom-round=248 is only generated if -fno-math-errno is specified. In contrast to the other configurations, -fsingle-precision-constant is not set.
These additional -m options are available for the Altera Nios II ELF (bare-metal) target:
-mhal— Link with HAL BSP. This suppresses linking with the GCC-provided C runtime startup and termination code, and is typically used in conjunction with-msys-crt0=to specify the location of the alternate startup code provided by the HAL BSP.-msmallc— Link with a limited version of the C library,-lsmallc, rather than Newlib.-msys-crt0=startfile— startfile is the file name of the startfile (crt0) to use when linking. This option is only useful in conjunction with-mhal.-msys-lib=systemlib— systemlib is the library name of the library that provides low-level system calls required by the C library, e.g."read"and"write". This option is typically used to link with a library provided by a HAL BSP.
✨ Nvidia PTX Options ✨
These options are defined for Nvidia PTX:
-m64— Ignored, but preserved for backward compatibility. Only 64-bit ABI is supported.-misa=ISA-string— Generate code for given the specified PTX ISA (e.g.sm_35). ISA strings must be lower-case. Valid ISA strings includesm_30andsm_35. The default ISA issm_35.-mptx=version-string— Generate code for given the specified PTX version (e.g.7.0). Valid version strings include3.1,6.0,6.3, and7.0. The default PTX version is6.0, unless a higher minimal version is required for specified PTX ISA via option-misa=.-mmainkernel— Link in code for a__mainkernel. 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— Generate code that does not use".local"memory 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-stackand-muniform-simtoptions, 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-floatis 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-floatis the vibe, this turns on generating unordered floating point compare and set flag (lf.sfun*) instructions. default behavior? the functions from libgcc handle the unordered floating point compare and set flag operations instead. 💅-mcmov— lets GCC emit conditional move (l.cmov) instructions. by default the same thing gets built out of set and branch. no cap.-mror— turns on rotate right (l.ror) instruction generation. default is libgcc functions doing the rotate right operations. 🌀-mrori— same energy but for rotate right with immediate (l.rori) instructions. default: libgcc functions handle the rotate right with immediate operations.-msext— enables sign extension (l.ext*) instructions. default path uses memory loads for sign extension. ✨-msfimm— enables compare and set flag with immediate (l.sf*i) instructions. default: extra instructions get generated to stash the immediate into a register first. 🙈-mshftimm— enables shift with immediate (l.srai,l.srli,l.slli) instructions. default: extra instructions get 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, bestie. 👑-mcmodel=large— generate OpenRISC code for the large model: the GOT may grow up to 4G in size. big GOT energy.
🌸 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-bitint. this is the default.-mint32-mno-int16— use 32-bitint.-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 oldreloadallocator is used. 💀
🦋 picoChip Options 🦋
these -m options are defined for picoChip implementations:
-mae=ae_type— set the instruction set, register set, and instruction scheduling parameters for array element type ae_type. supported values for ae_type areANY,MUL, andMAC.-mae=ANYselects a completely generic AE type. code generated with this option runs on any of the other AE types. the code is not as efficient as it would be if compiled for a specific AE type, and some types of operation (e.g., multiplication) do not work properly on all types of AE.-mae=MULselects a MUL AE type. this is the most useful AE type for compiled code, and is the default.-mae=MACselects a DSP-style MAC AE. code compiled with this option may suffer from poor performance of byte (char) manipulation, since the DSP AE does not provide hardware support for byte load/stores. 😭
-msymbol-as-address— enable the compiler to directly use a symbol name as an address in a load/store instruction, without first loading it into a register. typically, using this option generates larger programs, which run faster than when the option isn't used. however, the results vary from program to program, so it is left as a user option, rather than being permanently enabled. 🫶-mno-inefficient-warnings— disables warnings about the generation of inefficient code. these warnings can be generated, for example, when compiling code that performs byte-level memory operations on the MAC AE type. the MAC AE has no hardware support for byte-level memory operations, so all byte load/stores must be synthesized from word load/store operations. this is inefficient and a warning is generated to indicate that you should rewrite the code to avoid byte operations, or to target an AE type that has the necessary hardware support. this option disables these warnings.
👑 PowerPC Options 👑
these are listed under
🌈 PRU Options 🌈
these command-line options are defined for PRU target:
-minrt— link with a minimum runtime environment, with no support for static initializers and constructors. using this option can significantly reduce the size of the final ELF binary. 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. ⚠️-mmcu=mcu— specify the PRU MCU variant to use. check Newlib for the exact list of supported MCUs.-mno-relax— make GCC pass the--no-relaxcommand-line option to the linker instead of the--relaxoption.-mloop— allow (or do not allow) GCC to use the LOOP instruction.-mabi=variant— specify the ABI variant to output code for.-mabi=tiselects the unmodified TI ABI while-mabi=gnuselects 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=lp64dmeans that long and pointers are 64-bit (implicitly definingintto 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 is also the ilp32e ABI that can only be used with the rv32e architecture. this ABI is not well specified at present, and is 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.-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 includerv64i,rv32g,rv32e, andrv32imaf.
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.
-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, andsifive-u74.-mtune=processor-string— optimize the output for the given processor, specified by microarchitecture or particular CPU name. permissible values for this option are:rocket,sifive-3-series,sifive-5-series,sifive-7-series,size, 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. 🚀
Okay so the size choice? Not for end-users, bestie. It's what kicks in when -Os is specified. It overrides the instruction cost info that -mtune= hands over — but it does not override the pipeline info. Net effect: smaller code, still good performance. Iconic. 💅
-mpreferred-stack-boundary=num— tries to keep the stack boundary aligned to a 2 raised to num byte boundary. Don't specify it and the default is4(that's 16 bytes, aka 128-bits, bestie).
⚠️ Warning: if you use this switch, you must build all modules with the same value, including any libraries. And yes, that means the system libraries and startup modules too. No exceptions, no shortcuts. 😭
-msmall-data-limit=n— global and static data smaller than n bytes gets yeeted 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 relies on library function calls. Default? Fast inline prologues and epilogues, always. 🏎️-mshorten-memrefs,-mno-shorten-memrefs— do or do not attempt to make more use of compressed load/store instructions by swapping a load/store ofbase register + large offsetfor a new load/store ofnew base + small offset. And here's the slay: if that new base gets stored in a compressed register, then the new load/store can be compressed too. Currently this only targets 32-bit integer load/stores. 👑-mstrict-align,-mno-strict-align— don't or do generate unaligned memory accesses. The default depends on whether the processor we're optimizing for supports fast unaligned access or not. Vibe check decides. ✨-mcmodel=medlow— generate code for the medium-low code model. The program and its statically defined symbols have to lie within a single 2 GiB address range, and specifically between absolute addresses -2 GiB and +2 GiB. Programs can be statically or dynamically linked. This one is the 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.
Heads up though: the code generated by the medium-any code model is position-independent, but it is not guaranteed to function correctly when linked into position-independent executables or libraries. Fr fr, read that twice. 😭
-mexplicit-relocs,-mno-exlicit-relocs— use or don't use assembler relocation operators when dealing with symbolic addresses. The alternative is assembler macros instead, which may limit optimization. (And yes, that spelling is exactly what the manpage says — we don't make the rules 💀)-mrelax,-mno-relax— take advantage of linker relaxations to reduce the number of instructions needed to materialize symbol addresses. Default is to take advantage of them. Free wins, we love to see it. 🫶-memit-attribute,-mno-emit-attribute— emit (or don't emit) the RISC-V attribute that records extra info into ELF objects. This feature needs at least binutils 2.32. ⭐-malign-data=type— controls how GCC aligns variables and constants of array, structure, or union types. Supported values for type arexlen, which uses x register width as the alignment value, andnatural, which uses natural alignment.xlenis the default. 💜-mbig-endian— generate big-endian code. This is the default when GCC is configured for ariscv64be-*-*orriscv32be-*-*target.-mlittle-endian— generate little-endian code. This is the default when GCC is configured for ariscv64-*-*orriscv32-*-*target, but not for ariscv64be-*-*orriscv32be-*-*target.-mstack-protector-guard=guard,-mstack-protector-guard-reg=reg,-mstack-protector-guard-offset=offset— generate stack protection code using a canary at guard. Supported locations areglobalfor a global canary, ortlsfor a per-thread canary living in the TLS block.
With that tls choice, the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset additionally tell it which register to use as the base register for reading the canary, and what offset from that base register to read from. There's no default register or offset, because this is entirely for use inside the Linux kernel. 🧚
🌈 RL78 Options 🌈
-msim— links in extra target libraries so everything works inside a simulator.-mmul=none,-mmul=g10,-mmul=g13,-mmul=g14,-mmul=rl78— picks the type of hardware multiplication and division support you want. The simplest isnone, which does both multiplication and division in software. That's the default. Theg13value is for the hardware multiply/divide peripheral found on the RL78/G13 (S2 core) targets. Theg14value selects the multiplication and division instructions supported by the RL78/G14 (S3 core) parts. And then: the valuerl78is an alias forg14, and the valuemg10is an alias fornone.
Oh and also, a C preprocessor macro gets defined based on whatever you set here. Possible values: __RL78_MUL_NONE__, __RL78_MUL_G13__ or __RL78_MUL_G14__. 🎀
-mcpu=g10,-mcpu=g13,-mcpu=g14,-mcpu=rl78— specifies which RL78 core to target. Default is the G14 core, also known as an S3 core, or just RL78. The G13 or S2 core doesn't have multiply or divide instructions — it uses a hardware peripheral for those operations instead. The G10 or S1 core doesn't have register banks at all, so it uses a different calling convention. Different era, different rules. 💅
If this option is set, it also selects the type of hardware multiply support to use — unless that gets overridden by an explicit -mmul=none on the command line. So specifying -mcpu=g13 enables use of the G13 hardware multiply peripheral, and specifying -mcpu=g10 disables hardware multiplications altogether.
Note (important one, bestie): even though the RL78/G14 core is the default target, actually specifying -mcpu=g14 or -mcpu=rl78 on the command line does change the behavior of the toolchain, because it also enables G14 hardware multiply support. If you don't specify those on the command line, then software multiplication routines get used even though the code targets the RL78 core. Why? Backwards compatibility with older toolchains that had no hardware multiply and divide support. Historic behavior, we respect it. 🫡
Also a C preprocessor macro gets defined based on this setting. Possible values: __RL78_G10__, __RL78_G13__ or __RL78_G14__.
-mg10,-mg13,-mg14,-mrl78— these are just aliases for the corresponding-mcpu=option. They exist for backwards compatibility. Nostalgia-core. ✨-mallregs— lets the compiler use every available register. By defaultr24..r31are reserved for use in interrupt handlers. Turn this on and those registers become fair game in ordinary functions too.-m64bit-doubles,-m32bit-doubles— makes thedoubledata type be 64 bits (-m64bit-doubles) or 32 bits (-m32bit-doubles) in size. Default is-m32bit-doubles.-msave-mduc-in-interrupts,-mno-save-mduc-in-interrupts— says interrupt handler functions should preserve the MDUC registers. This is only necessary if normal code might use the MDUC registers, like if it does multiplication and division operations. Default is to ignore the MDUC registers, because that makes interrupt handlers faster. 🏃♀️
The target option -mg13 has to be passed for this to work, since the feature is only available on the G13 target (S2 core). And FYI, the MDUC registers only actually get saved if the interrupt handler performs a multiplication or division operation, or if it calls another function. Conditional saving, we love efficiency. 💜
👑 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're using. The default value of these options is determined when GCC gets configured. Specifying-mcpu=cpu_typeoverrides the specification of these options. And our official recommendation? Use the-mcpu=cpu_typeoption rather than the pile of options listed above. Do less, get more. 💅
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. Periodt. ✨
-mcpu=cpu_type— bestie this one sets the architecture type, the register usage, AND the instruction scheduling parameters all at once for machine typecpu_type. Like a full makeover for your target 💅
The 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, powerpc, powerpc64, powerpc64le, rs64, and native.
-mcpu=powerpc, -mcpu=powerpc64, and -mcpu=powerpc64le are the vibes-only ones: they specify pure 32-bit PowerPC (either endian), 64-bit big endian PowerPC, and 64-bit little endian PowerPC architecture machine types respectively, with an appropriate, generic processor model assumed for scheduling purposes. No specific chip energy, just generic queen behavior 👑
Specifying native as the cpu type detects and selects the architecture option that corresponds to the host processor of the system performing the compilation. But lowkey — -mcpu=native has no effect if GCC does not recognize the processor. It just shrugs, bestie 🙈
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. It's giving exclusive, not for everyone 🚫
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
-mpower8-vector -mquad-memory -mquad-memory-atomic -mfloat128
-mfloat128-hardware -mprefixed -mpcrel -mmma -mrop-protectThe particular options set for any particular CPU varies between compiler versions, depending on what setting seems to produce optimal code for that CPU — and ngl it doesn't necessarily reflect the actual hardware's capabilities. So don't come for it, it's just doing its best 💜
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. Order matters, periodt ✨
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. Do your research first, bestie 🔍
-mtune=cpu_type— sets the instruction scheduling parameters for machine typecpu_type, but does not set the architecture type or register usage, the way-mcpu=cpu_typedoes. Samecpu_typevalues as-mcpu, no cap. If both are specified, the code generated uses the architecture and registers set by-mcpu, but the scheduling parameters set by-mtune. They tag-team it 🤝-mcmodel=small— generate PowerPC64 code for the small model: the TOC is limited to 64k. Tiny but mighty 🎀-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. The main character of models 💅-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. Go big or go home 🌈-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=altivecto adjust the current ABI with AltiVec ABI enhancements. Don't forget that part, iykyk 👀
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. Left is right and right is left, depending on your endianness, and honestly that's so on brand 🦋
-mvrsave,-mno-vrsave— generate VRSAVE instructions when generating AltiVec code. Just a little extra ⭐-msecure-plt— generate code that allowsldandld.soto build executables and shared libraries with non-executable.pltand.gotsections. This is a PowerPC 32-bit SYSV ABI option. Secure queen behavior 🔒-mbss-plt— generate code that uses a BSS.pltsection thatld.sofills in, and requires.pltand.gotsections that are both writable and executable. This is a PowerPC 32-bit SYSV ABI option. Writable and executable, bold move 😳-misel,-mno-isel— this switch enables or disables the generation of ISEL instructions. Flip the switch, bestie 🎚️-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. Main character instruction set fr fr 👑-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. Crypto girlies rise 🔐-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. Memory but make it transactional 💾-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. (Yes, the manpage really says "does not keeps" — we don't gatekeep, we preserve 📝)-mpower8-vector,-mno-power8-vector— generate code that uses (does not use) the vector and scalar instructions that were added in version 2.07 of the PowerPC ISA. Also enable the use of built-in functions that allow more direct access to the vector instructions. Vector, but make it 2.07 ✨-mquad-memory,-mno-quad-memory— generate code that uses (does not use) the non-atomic quad word memory instructions. The-mquad-memoryoption requires use of 64-bit mode. No 32-bit shortcuts, bestie 🚨-mquad-memory-atomic,-mno-quad-memory-atomic— generate code that uses (does not use) the atomic quad word memory instructions. The-mquad-memory-atomicoption requires use of 64-bit mode. Atomic and demanding, iconic 💅-mfloat128,-mno-float128— enable/disable the__float128keyword for IEEE 128-bit floating point, and use either software emulation for IEEE 128-bit floating point or hardware instructions. Big float energy 🎈
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. Sorry to the other platforms 😭
The default for -mfloat128 is enabled on PowerPC Linux systems using the VSX instruction set, and disabled on other systems. Situational, like all of us 🦋
If you use the ISA 3.0 instruction set (-mpower9-vector or -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. Emulation era, no cap 💀
-mfloat128-hardware,-mno-float128-hardware— enable/disable using ISA 3.0 hardware instructions to support the__float128data type. Hardware girlies, this one's for you ⚡
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 setsint,longand pointer to 32 bits and generates code that runs on any PowerPC variant. The 64-bit environment setsintto 32 bits andlongand pointer to 64 bits, and generates code for PowerPC64, as for-mpowerpc64. Pick your era, bestie 🕰️-mfull-toc,-mno-fp-in-toc,-mno-sum-in-toc,-mminimal-toc— these modify generation of the TOC (Table Of Contents), which is created for every executable file. The-mfull-tocoption 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. That's the limit, and she's firm about it 🚫
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. Trade-offs, bestie, it's always trade-offs ⚖️
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. Use it sparingly, like glitter 🎀
-maix64,-maix32— enable 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bitlongtype, and the infrastructure needed to support them. Specifying-maix64implies-mpowerpc64, while-maix32disables the 64-bit ABI and implies-mno-powerpc64. GCC defaults to-maix32. Default is the 32 era, fyi 💜-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-bitlong doublevalue is properly rounded when comparing values and converting todouble. Use XL symbol names forlong doublesupport routines. XL vibes only 🧚
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. Niche but valid, and we respect the niche 💅
-mpe— okay so this one is for the IBM RS/6000 SP Parallel Environment (PE). It links an application written to use message passing with special startup code to enable the application to run. Your 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. PE does not support threads, so-mpeand-pthreadare incompatible — like, they cannot sit together, periodt. 🫶-malign-natural,-malign-power— On AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux,-malign-naturaloverrides the ABI-defined alignment of larger types, such as floating-point doubles, on their natural size-based boundary.-malign-powertells 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-poweris 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-msoft-float, 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-mmultipleon 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-maddoption is now mapped to the machine-independent-ffp-contract=fastoption, and-mno-fused-maddis 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-searchdlmzbinstruction 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 8unsignedbit-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.got2and 4-byte locations listed in the.fixupsection, a table of 32-bit addresses generated by this option. For this to work, all objects linked together must be compiled with-mrelocatableor-mrelocatable-lib.-mrelocatablecode aligns the stack to an 8-byte boundary. 💖-mrelocatable-lib,-mno-relocatable-lib— Like-mrelocatable,-mrelocatable-libgenerates a.fixupsection to allow static executables to be relocated at run time, but-mrelocatable-libdoes not use the smaller stack alignment of-mrelocatable. Objects compiled with-mrelocatable-libmay be linked with objects compiled with any combination of the-mrelocatableoptions. 🫶-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-endianoption 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-endianoption is the same as-mbig. 💙-mdynamic-no-pic— On Darwin and Mac OS X 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 value0,1, or2to 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 usingpowerpc-*-eabiaix. 👑-mcall-sysv-eabi,-mcall-eabi— Specify both-mcall-sysvand-meabioptions. ✨-mcall-sysv-noeabi— Specify both-mcall-sysvand-mno-eabioptions. 🌸-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 arefull,part, andno. 💅-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 thelong doubletype from IEEE extended-precision, the compiler will issue a warning unless you use the-Wno-psabioption. Requires-mlong-double-128to 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 thelong doubletype from IBM extended-precision, the compiler will issue a warning unless you use the-Wno-psabioption. Requires-mlong-double-128to 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. 💀
okay so continuing the PowerPC option saga, bestie — these next ones are giving very specific embedded energy 💅
-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_attributeassembly directives to set tag/value pairs in a.gnu.attributessection 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 calledsim-crt0.oand that the standard C libraries arelibsim.aandlibc.a. This is the default forpowerpc-*-eabisimconfigurations.-mmvme— on embedded PowerPC systems, assume that the startup module is calledcrt0.oand the standard C libraries arelibmvme.aandlibc.a.-mads— on embedded PowerPC systems, assume that the startup module is calledcrt0.oand the standard C libraries arelibads.aandlibc.a.-myellowknife— on embedded PowerPC systems, assume that the startup module is calledcrt0.oand the standard C libraries arelibyk.aandlibc.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 thePPC_EMBbit 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-meabimeans that the stack is aligned to an 8-byte boundary, a function__eabiis called frommainto set up the EABI environment, and the-msdataoption can use bothr2andr13to point to two separate small data areas. Selecting-mno-eabimeans that the stack is aligned to a 16-byte boundary, no EABI initialization function is called frommain, and the-msdataoption only usesr13to point to a single small data area. The-meabioption is on by default if you configured GCC using one of thepowerpc*-*-eabi*options.-msdata=eabi— on System V.4 and embedded PowerPC systems, put small initializedconstglobal and static data in the.sdata2section, which is pointed to by registerr2. Put small initialized non-constglobal and static data in the.sdatasection, which is pointed to by registerr13. Put small uninitialized global and static data in the.sbsssection, which is adjacent to the.sdatasection. The-msdata=eabioption is incompatible with the-mrelocatableoption. The-msdata=eabioption also sets the-memboption.-msdata=sysv— on System V.4 and embedded PowerPC systems, put small global and static data in the.sdatasection, which is pointed to by registerr13. Put small uninitialized global and static data in the.sbsssection, which is adjacent to the.sdatasection. The-msdata=sysvoption is incompatible with the-mrelocatableoption.-msdata=default,-msdata— on System V.4 and embedded PowerPC systems, if-meabiis 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.sdatasection. Put small uninitialized global data in the.sbsssection. Do not use registerr13to address small data however. This is the default behavior unless other-msdataoptions are used.-msdata=none,-mno-sdata— on embedded PowerPC systems, put all initialized global and static data in the.datasection, and all uninitialized data in the.bsssection.-mreadonly-in-sdata— put read-only objects in the.sdatasection as well. This is the default.-mblock-move-inline-limit=num— inline all block moves (such as calls tomemcpyor 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 tomemcmpor 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 beforememcmpis 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 tostrcmporstrncmpwill 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 numswitch is also passed to the linker. All modules should be compiled with the same-G numvalue.-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 theshortcallfunction 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-pltand 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.-mpltseqcode and-mbss-plt32-bit PowerPC relocatable objects may not be linked together.-mtls-markers,-mno-tls-markers— mark (do not mark) calls to__tls_get_addrwith 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-mathoption when using-mrecip(or at least-funsafe-math-optimizations,-ffinite-math-only,-freciprocal-mathand-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— turns on the single-precision reciprocal square root approximation instructions. ✨rsqrtd— turns on the double-precision reciprocal square root approximation instructions. 💜
So like, 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. Got it? Good. 💅
-mrecip-precision,-mno-recip-precision— assume (or do not assume, bestie) that the reciprocal estimate instructions give you higher-precision estimates than is mandated by the PowerPC ABI. Selecting-mcpu=power6,-mcpu=power7or-mcpu=power8automatically 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 ismass, 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-vectorizeand-funsafe-math-optimizationsmust also be enabled. And the MASS libraries must be specified at link time, no skipping that step. 🫶-mfriz,-mno-friz— generate (do not generate) the"friz"instruction when the-funsafe-math-optimizationsoption is used to optimize rounding of floating-point values to 64-bit integer and back to floating point. Heads up though: 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-functionsis 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. so like, don't do that. 🙈-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-indirectoption 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,-mstack-protector-guard-symbol=symbol— generate stack protection code using canary at guard. Supported locations areglobalfor global canary ortlsfor 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. And -mstack-protector-guard-symbol=symbol overrides the offset with a symbol reference to a canary in the TLS block. 🎀
-mpcrel,-mno-pcrel— generate (do not generate) pc-relative addressing. The-mpcreloption requires that the medium code model (-mcmodel=medium) and prefixed addressing (-mprefixed) options are enabled. no shortcuts bestie. 💅-mprefixed,-mno-prefixed— generate (do not generate) addressing modes using prefixed load and store instructions. The-mprefixedoption requires that the option-mcpu=power10(or later) is enabled.-mmma,-mno-mma— generate (do not generate) the MMA instructions. The-mmaoption 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. very main character energy honestly 👑-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".
🌸 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. it's giving practical fr fr 💖-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-fpuoption is enabled then-funsafe-math-optimizationsis 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— tells GCC that interrupt handler functions should keep the accumulator register safe 💾. You only need this if your normal code might actually be using the accumulator register, like when it does 64-bit multiplications. Default? It just ignores the accumulator, because that makes interrupt handlers faster ⚡.-mpid,-mno-pid— turns on position independent data ✨. When it's on, every time you touch constant data it happens via an offset from a base address that lives in a register. Why do we care? Because it lets the location of constant data be figured out at run time without needing the executable to be relocated — huge win for embedded apps with tight memory constraints 🫶. Data that can be modified is not affected by this option, bestie, only the constant stuff.
Heads up: using this feature reserves a register, usually r13, for the constant data base address. That can make your code slower and/or bigger, especially in complicated functions 😭.
The actual register picked to hold the constant data base address depends on whether -msmall-data-limit and/or the -mint-register command-line options are enabled. Starting at register r13 and going downwards, registers get allocated first to satisfy -mint-register, then -mpid, and finally -msmall-data-limit. So yes, it's totally possible for the small data area register to end up as r8 if you specify both -mint-register=4 and -mpid on the command line 🤯.
By default this feature is not enabled. You can restore the default with -mno-pid.
-mno-warn-multiple-fast-interrupts,-mwarn-multiple-fast-interrupts— stops GCC from throwing a warning when it finds more than one fast interrupt handler while compiling a file 🛑. The default is to warn you for each extra fast interrupt handler it finds, since the RX only supports one such interrupt. One and done, periodt.-mallow-string-insns,-mno-allow-string-insns— enables or disables the string manipulation instructionsSMOVF,SCMPU,SMOVB,SMOVU,SUNTIL,SWHILEand also theRMPAinstruction 🧵. These instructions may prefetch data, which is not safe to do if you're accessing an I/O register. (See section 12.2.7 of the RX62N Group User's Manual for more info.)
The default is to allow these instructions, but GCC can't reliably detect every single circumstance where a string instruction might be used to access an I/O register, so their use can't be disabled automatically 🙈. Instead it relies on the programmer to use -mno-allow-string-insns 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)JSRinstructions to access functions. You can reach for this when code size exceeds the range ofBSRinstructions. Note that-mno-jsrdoes not mean "don't useJSR" — it means any type of branch may be used 🌈.
Note: the generic GCC command-line option-ffixed-reghas special significance to the RX port when used with theinterruptfunction attribute. That attribute marks a function intended to process fast interrupts. GCC makes sure it only uses the registersr10,r11,r12and/orr13, and only provided that the normal use of the corresponding registers has been restricted via the-ffixed-regor-mint-registercommand-line options 💅.
🦋 S/390 and zSeries Options 🦋
These are the -m options defined for the S/390 and zSeries architecture, bestie 💜.
-mhard-float,-msoft-float— use (do not use) the hardware floating-point instructions and registers for floating-point operations. When-msoft-floatis specified, functions inlibgcc.aare used to perform floating-point operations. When-mhard-floatis 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-dfpis specified, functions inlibgcc.aare used to perform decimal-floating-point operations. When-mhard-dfpis specified, the compiler generates decimal-floating-point hardware instructions. This is the default for-march=z9-ecor higher 🔥.-mlong-double-64,-mlong-double-128— these switches control the size of thelong doubletype. A size of 64 bits makes thelong doubletype equivalent to thedoubletype. 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 don't understand DWARF call frame information 🕵️. When-mno-packed-stackis in effect, the backchain pointer is stored at the bottom of the stack frame; when-mpacked-stackis 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, using 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-stackis specified, the compiler uses all fields of the 96/160 byte register save area only for their default purpose; unused fields still take up stack space. When-mpacked-stackis specified, register save slots are densely packed at the top of the register save area; unused space gets reused for other purposes, allowing for more efficient use of the available stack space ✨. However, when-mbackchainis 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 thebrasinstruction to do subroutine calls. This only works reliably if the total executable size does not exceed 64k ⚠️. The default is to use thebasrinstruction instead, which does not have this limitation.-m64,-m31— when-m31is specified, generate code compliant to the GNU/Linux for S/390 ABI. When-m64is 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-mzarchis specified, generate code using the instructions available on z/Architecture. When-mesais specified, generate code using the instructions available on ESA/390. Note that-mesais 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-mhtmoption 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 💫.-mhtmis enabled by default when using-march=zEC12.-mvx,-mno-vx— when-mvxis 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_attributecommand will be added to mark the resulting binary with the ABI used.-mvxis enabled by default when using-march=z13.-mzvector,-mno-zvector— the-mzvectoroption 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 forvectorto be used as a keyword to define vector type variables and arguments.vectoris 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,-mzvectorenables 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 filevecintrin.hneeds to be included 📎.-mzvectoris disabled by default.-mmvcle,-mno-mvcle— generate (or do not generate) code using themvcleinstruction to perform block moves. When-mno-mvcleis specified, use amvcloop 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 arez900/arch5,z990/arch6,z9-109,z9-ec/arch7,z10/arch8,z196/arch9,zEC12,z13/arch11,z14/arch12,z15/arch13, andnative👑.
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— tunes everything applicable about the generated code to cpu-type, EXCEPT the ABI and the set of available instructions, bestie. The list of cpu-type values is literally the same as for-march. Default? Whatever value you used for-march. Slay.-mtpf-trace,-mno-tpf-trace— generate code that adds (or does not add) TPF OS specific branches to trace routines in the operating system. Off by default, even when you're compiling for the TPF OS. 🙈-mtpf-trace-skip,-mno-tpf-trace-skip— generate code that changes (or does not change) the default branch targets enabled by-mtpf-traceso they point to specialized trace routines that can selectively skip function trace entries for the TPF OS. Also off by default, even when you're compiling for the TPF OS and specifying-mtpf-trace. iykyk 💅-mfused-madd,-mno-fused-madd— generate code that uses (or does not use) the floating-point multiply and accumulate instructions. These get generated by default if hardware floating point is in play. ✨-mwarn-framesize=framesize— emit a warning if the current function exceeds the given frame size. It's a compile-time check, so it doesn't actually have to be a real problem when the program runs. The vibe is: it's meant to sniff out functions that most probably cause a stack overflow. Super useful in an environment with a limited stack size, e.g. the linux kernel. 🫶-mwarn-dynamicstack— emit a warning if the function callsallocaor uses dynamically-sized arrays. Which is generally a bad idea when your stack size is limited. 💀-mstack-guard=stack-guard,-mstack-size=stack-size— if you provide these, the S/390 back end emits extra instructions in the function prologue that trigger a trap if the stack size is stack-guard bytes above the stack-size (remember, the stack on S/390 grows downward). If you skip the stack-guard option, the smallest power of 2 larger than the frame size of the compiled function gets chosen. These are here to help you debug stack overflow problems. The extra code is only a little overhead, so it can also live in production-like systems without major performance degradation. The values you give have to be exact powers of 2, and stack-size has to be greater than stack-guard without exceeding 64k. And to be efficient, the extra code assumes the stack starts at an address aligned to the value given by stack-size. Also: stack-guard can only be used together with stack-size. no cap 🎀-mhotpatch=pre-halfwords,post-halfwords— if the hotpatch option is on, a "hot-patching" function prologue gets generated for all functions in the compilation unit. The function label is prepended with the given number of two-byte NOP instructions (pre-halfwords, maximum 1000000). After the label,2 * post-halfwordsbytes 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.
🌸 Score Options 🌸
These options are defined for Score implementations:
-meb— compile code for big-endian mode. This is the default. 👑-mel— compile code for little-endian mode.-mnhwloop— disable generation ofbcnzinstructions.-muls— enable generation of unaligned load and store instructions.-mmac— enable the use of multiply-accumulate instructions. Disabled by default.-mscore5— specify the SCORE5 as the target architecture.-mscore5u— specify the SCORE5U of the target architecture.-mscore7— specify the SCORE7 as the target architecture. This is the default.-mscore7d— specify the SCORE7D as the target architecture.
🦋 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 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 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-nofputo 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-dspto 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. Heads up though: 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 inswitchtables. The default is to use 16-bit offsets.-mbitops— enable the use of bit manipulation instructions on SH2A.-mfmovd— enable the use of the instructionfmovd. Check-mdalignfor 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 theMACregister as call-clobbered, even if-mrenesasis given.-mieee,-mno-ieee— control the IEEE compliance of floating-point comparisons, which affects how cases where the result of a comparison is unordered get handled. By default-mieeeis implicitly enabled. If-ffinite-math-onlyis enabled,-mno-ieeeis implicitly set, which gives you faster floating-point greater-equal and less-equal comparisons. The implicit settings can be overridden by specifying either-mieeeor-mno-ieee.-minline-ic_invalidate— inline code to invalidate instruction cache entries after setting up nested function trampolines. This option has no effect if-musermodeis in effect and the selected code generation option (e.g.-m4) does not allow the use of theicbiinstruction. If the selected code generation option does not allow the use of theicbiinstruction, and-musermodeis not in effect, the inlined code manipulates the instruction cache address array directly with an associative write. Which not only requires privileged mode at run time, but 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 notsh*-*-linux*.
soft-gusa— bestie, this one generates GNU/Linux compatible gUSA software atomic sequences for the atomic built-in functions 💖. The generated atomic sequences need extra support from the interrupt/exception handling code of the system, and they are only suitable for SH3* and SH4* single-core systems — no cap. 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, unlessstrictis specified.soft-tcb— generates 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, thegbr-offset=parameter has to be specified as well — don't forget it, bestie 💅.soft-imask— generates 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— generates 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 thesoft-tcbmodel has been selected. For other models this parameter is ignored. The specified value must be an integer multiple of four and in the range0-1020— like, do the math, bestie 📐.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. It's giving monogamous atomic energy 💜.-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. Yikes 😭.-mprefergot— when generating position-independent code, emit function calls using the Global Offset Table instead of the Procedure Linkage Table. It's the GOT vs PLT drama, and GOT wins this round 💅.-musermode,-mno-usermode— don't allow (allow) the compiler generating privileged mode code 🔐. Specifying-musermodealso implies-mno-inline-ic_invalidateif the inlined code would not work in user mode.-musermodeis the default when the target is"sh*-*-linux*". If the target is SH1* or SH2*-musermodehas no effect, since there is no user mode — periodt.-multcost=number— set the cost to assume for a multiply insn. Number go brrr ✨.-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 — iconic behavior 👑.
-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. Stack frame said stability 💖.-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. No sneaky register changes, bestie 🙅♀️.-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. Liker1-r3,r5-r7— you got this 🌈.-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. Branch-free is the vibe ✨.-mzdcbranch,-mno-zdcbranch— assume (do not assume) that zero displacement conditional branch instructions"bt"and"bf"are fast 💨. If-mzdcbranchis 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. Hardware said "oops" and this option said "say less" 💀.-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-maddoption is now mapped to the machine-independent-ffp-contract=fastoption, and-mno-fused-maddis mapped to-ffp-contract=off. Glow-up, honestly ✨.-mfsca,-mno-fsca— allow or disallow the compiler to emit the"fsca"instruction for sine and cosine approximations 🌊. The option-mfscamust be used in combination with-funsafe-math-optimizations. It is enabled by default when generating code for SH4A. Using-mno-fscadisables sine and cosine approximations even if-funsafe-math-optimizationsis in effect. Trig? In my compiler? It's more likely than you think 😌.-mfsrra,-mno-fsrra— allow or disallow the compiler to emit the"fsrra"instruction for reciprocal square root approximations 🧮. The option-mfsrramust be used in combination with-funsafe-math-optimizationsand-ffinite-math-only. It is enabled by default when generating code for SH4A. Using-mno-fsrradisables reciprocal square root approximations even if-funsafe-math-optimizationsand-ffinite-math-onlyare in effect. Bestie, that's a whole combo meal 🍔.-mpretend-cmove— prefer zero-displacement conditional branches for conditional move instruction patterns. This can result in faster code on the SH4 processor 🏎️. Pretend, but make it fast 💅.-mfdpic— generate code using the FDPIC ABI. That's it, that's the whole vibe 🦋.
✨ Solaris 2 Options ✨
These -m options are supported on Solaris 2 ☀️:
-mclear-hwcap—-mclear-hwcaptells 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. Runtime vibe check, basically 👀.-mimpure-text—-mimpure-text, used in addition to-shared, tells the compiler to not pass-z textto 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. Like, just do that instead, bestie 🫶.
These switches are supported in addition to the above on Solaris 2:
-pthreads— this is a synonym for-pthread. Same energy, different name 💅.
💫 SPARC Options 💫
These -m options are supported on the SPARC:
-mno-app-regs,-mapp-regs— specify-mapp-regsto 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. ABI compliance said sacrifice 💖.
-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. Flat, but make it functional ✨.-mno-flat— this one's the default, bestie, so it's the vibe you get unless you ask otherwise. The compiler spits out save/restore instructions (except for leaf functions, those little divas get a pass). This is just, like, the normal operating mode. Nothing to see here. 💅-mfpu,-mhard-float— output contains actual floating-point instructions. Also the default, so if you did nothing, congrats, you already have this. 🎀-mno-fpu,-msoft-float— floating point gets done via library calls instead of instructions. Real talk though:
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 have to make your own arrangements to provide suitable library functions for cross-compilation. The embedded targetssparc-*-aoutandsparclite-*-*do provide software floating-point support, so those are fine.
Also heads up: -msoft-float changes the calling convention in the output file, so it's 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 for this to work. No half-sending it, bestie, it's all or nothing. 😭
-mhard-quad-float— output contains quad-word (long double) floating-point instructions. Fancy.-msoft-quad-float— library calls for quad-word (long double) floating-point instructions. The functions called are those specified in the SPARC ABI. This is the default.
And here's why, because it's kind of iconic: 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 -msoft-quad-float is the default. Periodt. 💀
-mno-unaligned-doubles,-munaligned-doubles— assume that doubles have 8-byte alignment. That's the default.
With -munaligned-doubles, GCC assumes doubles have 8-byte alignment only if they're 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's not the default because it results in a performance loss, especially for floating-point code. Trade-offs, babes. ✨
-muser-mode,-mno-user-mode— do not generate code that can only run in supervisor mode. This is relevant only for thecasainstruction emitted for the LEON3 processor. This is the default.-mfaster-structs,-mno-faster-structs— with-mfaster-structs, the compiler assumes structures should have 8-byte alignment. This enables the use of pairs oflddandstdinstructions for copies in structure assignment, in place of twice as manyldandstpairs. Cute, right? BUT — the use of this changed alignment directly violates the SPARC ABI. So 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. You've been warned, gorgeous. 🙈-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.
-mlra,-mno-lra— enable Local Register Allocation. This is the default for SPARC since GCC 7, so-mno-lraneeds to be passed to get the old Reload. Retro vibes, if that's your thing. 🦋-mcpu=cpu_type— set the instruction set, register set, and instruction scheduling parameters for machine type cpu_type. Supported values for cpu_type arev7,cypress,v8,supersparc,hypersparc,leon,leon3,leon3v7,leon5,sparclite,f930,f934,sparclite86x,sparclet,tsc701,v9,ultrasparc,ultrasparc3,niagara,niagara2,niagara3,niagara4,niagara7andm8.
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's the whole list of each supported architecture and their supported implementations, bestie:
v7—cypress,leon3v7v8—supersparc,hypersparc,leon,leon3,leon5sparclite—f930,f934,sparclite86xsparclet—tsc701v9—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. Throwback. 👑
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. The way they just kept making chips?? Obsessed. 🔥
-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_typedoes. Tuning only, no glow-up for the actual instruction set. 💅
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-mvis2when targeting a cpu that supports such instructions, such as UltraSPARC-III and later. Setting-mvis2also sets-mvis. Chain reaction, babes. ✨-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-mvis3when targeting a cpu that supports such instructions, such as niagara-3 and later. Setting-mvis3also sets-mvis2and-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-mvis4when targeting a cpu that supports such instructions, such as niagara-7 and later. Setting-mvis4also sets-mvis3,-mvis2and-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-mvis4bwhen targeting a cpu that supports such instructions, such asm8and later. Setting-mvis4balso sets-mvis4,-mvis3,-mvis2and-mvis.-mcbcond,-mno-cbcond— with-mcbcond, GCC generates code that takes advantage of the UltraSPARC Compare-and-Branch-on-Condition instructions. The default is-mcbcondwhen 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-mfmafwhen 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-mfsmuldwhen targeting a CPU supporting the architecture versions V8 or V9 with FPU — except-mcpu=leon. Not you, leon. 🙈-mpopc,-mno-popc— turn on-mpopcand GCC will flex the UltraSPARC Population Count instruction 💅. It's the default whenever you're targeting a CPU that actually has that instruction, like Niagara-2 and anything newer. Queen behavior, honestly.-msubxc,-mno-subxc— same energy but for the UltraSPARC Subtract-Extended-with-Carry instruction 🎀. Default is-msubxcwhen your target CPU supports it, like Niagara-7 and later. Obsessed.-mfix-at697f— turns on the documented workaround for the single erratum of the Atmel AT697F processor (which is literally erratum #13 of the AT697E processor, fyi 🫶).-mfix-ut699— turns on the documented workarounds for the floating-point errata and the data cache nullify errata of the UT699 processor. Not one, two. We stan a fix.-mfix-ut700— turns on the documented workaround for the back-to-back store erratum of the UT699E/UT700 processor 🦋.-mfix-gr712rc— turns on the documented workaround for the back-to-back store erratum 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 setsint,longand pointer to 32 bits. The 64-bit environment setsintto 32 bits, andlongand pointer to 64 bits. That's it, that's the whole thing.-mcmodel=which— sets the code model to one of these bad boys 👇medlow— the Medium/Low code model: 64-bit addresses, and programs must be linked in the low 32 bits of memory. They can be statically or dynamically linked, your call bestie.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 have to be less than 2GB in size, and the data segment must sit within 2GB of the text segment.medany— the Medium/Anywhere code model: 64-bit addresses, programs may be linked anywhere in memory, 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, text and data segments must be less than 2GB in size, and both start anywhere in memory (decided at link time). The global register%g4points to the base of the data segment. Programs are statically linked and PIC is not supported 💀.
-mmemory-model=mem-model— sets 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 there is one) is offset by -2047, which has to be added back when making stack frame references 🧚. This is the default in 64-bit mode. Otherwise, assume there's no such offset.
✨ Options for System V ✨
These extra options are available on System V Release 4 for compatibility with other compilers on those systems:
-G— create a shared object. Lowkey though, it's recommended that-symbolicor-sharedbe 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 (and 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.
🌈 TILE-Gx Options 🌈
These -m options are supported on the TILE-Gx:
-mcmodel=small— generate code for the small model. The distance for direct calls is limited to 500M in either direction. PC-relative addresses are 32 bits. Absolute addresses support the full address range 💪.-mcmodel=large— generate code for the large model. There is no limitation on call distance, pc-relative addresses, or absolute addresses. Go anywhere, do anything, main character energy.-mcpu=name— selects the type of CPU to be targeted. Currently the only supported type istilegx.-m32,-m64— generate code for a 32-bit or 64-bit environment. The 32-bit environment setsint,long, and pointer to 32 bits. The 64-bit environment setsintto 32 bits andlongand pointer to 64 bits.-mbig-endian,-mlittle-endian— generate code in big/little endian mode, respectively 🦋.
🎀 TILEPro Options 🎀
These -m options are supported on the TILEPro:
-mcpu=name— selects the type of CPU to be targeted. Currently the only supported type istilepro.-m32— generate code for a 32-bit environment, which setsint,long, and pointer to 32 bits. This is the only supported behavior, so the flag is essentially ignored 💀. It's giving "decorative".
💜 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 the pointer into the"ep"register, and use the shorter"sld"and"sst"instructions. The-mepoption 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-functionoption is on by default if you optimize.-mspace— try to make the code as small as possible 🐜. At present, this just turns on the-mepand-mprolog-functionoptions.-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-mv850e3v5option.-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-mv850e1option.-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. No cap.
-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-mrelaxcommand-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-aligncommand-line option is supported. - The
-mdisable-calltcommand-line option is enabled by default. The-mno-disable-calltcommand-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.
- Integer sized structures and unions are returned in register
- Functions get aligned to 32-bit boundaries, unless you're optimizing for size.
- The
-m8byte-aligncommand-line option? Not supported here, sorry bestie. - The
-mdisable-calltcommand-line option is supported, but it's not enabled by default.
- The
When this version of the ABI is turned on, the C preprocessor symbol __V850_GCC_ABI__ gets defined. 💅
-m8byte-align,-mno-8byte-align— letsdoubleandlong longtypes align on 8-byte boundaries. Default vibe is to keep every object aligned to at most 4 bytes. When-m8byte-alignis active, the C preprocessor symbol__V850_8BYTE_ALIGN__gets defined. ✨-mbig-switch— generates code that's suited for big switch tables. Only use this one if your assembler/linker start complaining about out-of-range branches inside a switch table. 🫶-mapp-regs— makesr2andr5get used in the code the compiler generates. This setting is the default, no cap. 👑-mno-app-regs— makesr2andr5be treated as fixed registers instead.
🌈 VAX Options 🌈
These -m options are defined for the VAX:
-munix— don't output certain jump instructions (aobleqand so on) that the Unix assembler for the VAX just can't handle across long ranges.-mgnu— do output those jump instructions, operating on the assumption that the GNU assembler is what's being used.-mg— output code for G-format floating-point numbers instead of D-format.-mlra,-mno-lra— enable Local Register Allocation. This is still experimental for the VAX, so by default the compiler just uses standard reload. 💀
🦋 Visium Options 🦋
-mdebug— a program that does file I/O and is destined to run on an MCM target should be linked with this option. It causes the librarieslibc.aandlibdebug.ato get linked. The program should be run on the target under the control of the GDB remote debugging stub.-msim— a program that does file I/O and is destined to run on the simulator should be linked with this option. This causes librarieslibc.aandlibsim.ato get 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.
Heads up bestie:-msoft-floatchanges the calling convention in the output file, so it's only useful if you compile all of a program with this option. In particular, you need to compilelibgcc.a, the library that comes with GCC, with-msoft-floatfor this to actually work.
- *
-mcpu=cpu_type* — sets the instruction set, register set, and instruction scheduling parameters for machine type cpu_type. Supported values for cpu_type aremcm,gr5andgr6.
mcm is a synonym of gr5, kept around for backward compatibility.
By default (unless it's 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* — sets the instruction scheduling parameters for machine type cpu_type, but does not set the instruction set or register set that-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 access to some general registers is forbidden: on the GR5, registersr24tor31cannot be accessed in this mode; on the GR6, only registersr29tor31are affected.
💜 VMS Options 💜
These -m options are defined for the VMS implementations:
-mvms-return-codes— return VMS condition codes frommain. 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 are32orshortfor 32 bit pointers,64orlongfor 64 bit pointers, andnofor supporting only 32 bit pointers. The later option disablespragma 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__.-non-static— link an RTP executable against shared libraries rather than static libraries. The options-staticand-sharedcan also be used for RTPs;-staticis the default.-Bstatic,-Bdynamic— these options are passed down to the linker. They're defined for compatibility with Diab.-Xbind-lazy— enable lazy binding of function calls. This option is equivalent to-Wl,-z,nowand 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=nativeenables all instruction subsets supported by the local machine (hence the result might not run on different machines). Using-mtune=nativeproduces code optimized for the local machine under the constraints of the selected instruction set. 💅x86-64— a generic CPU with 64-bit extensions.x86-64-v2x86-64-v3x86-64-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.)i586pentium— 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.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 asgeneric.pentium2— Intel Pentium II CPU, based on Pentium Pro core with MMX and FXSR instruction set support.pentium3pentium3m— 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. 🦋pentium4pentium4m— 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— 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— 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— 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— Intel Haswell 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 and HLE instruction set support.broadwell— Intel Broadwell 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 and PREFETCHW instruction set support.skylake— Intel Skylake 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 and SGX instruction set support. 👑bonnell— Intel Bonnell CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support.
silvermont— Intel Silvermont CPU, and she's got 64-bit extensions plus MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, PCLMUL, PREFETCHW and RDRND instruction set support. solid little starter pack tbh ✨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. she glowed up from silvermont fr fr 💖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. the plus is doing real work bestie 🦋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. that list is LONG and she earned every name on it 💅knl— Intel Knight's Landing 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, AVX512PF, AVX512ER, AVX512F, AVX512CD and PREFETCHWT1 instruction set support. (and yes,MOVBEreally does show up twice in that list, i don't make the rules, i just report them) 👑knm— Intel Knights Mill 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, AVX512PF, AVX512ER, AVX512F, AVX512CD and PREFETCHWT1, AVX5124VNNIW, AVX5124FMAPS and AVX512VPOPCNTDQ instruction set support. knights mill said "and MORE" 💀 🔥skylake-avx512— Intel Skylake Server 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, CLWB, AVX512VL, AVX512BW, AVX512DQ and AVX512CD instruction set support. the name literally tells you the whole personality ✨cannonlake— Intel Cannonlake Server 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 and SHA instruction set support. a whole avx512 moment, we love to see it 🎀icelake-client— Intel Icelake Client 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 and AVX512VPOPCNTDQ instruction set support. (that lil space before the comma inAVX512VBMI2 , VPCLMULQDQ? iconic, keeping it exactly as-is) 🧚icelake-server— Intel Icelake Server 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 and CLWB instruction set support. server edition said "add PCONFIG, WBNOINVD and CLWB, slay" 💖cascadelake— Intel Cascadelake 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, CLWB, AVX512VL, AVX512BW, AVX512DQ, AVX512CD and AVX512VNNI instruction set support. cascade, like the waterfall, except it's instructions 🌈cooperlake— Intel cooperlake 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, CLWB, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, AVX512VNNI and AVX512BF16 instruction set support. AVX512BF16 said hi and honestly? obsessed 💅tigerlake— Intel Tigerlake 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, MOVDIRI, MOVDIR64B, CLWB, AVX512VP2INTERSECT and KEYLOCKER instruction set support. (noteAVX512CD PKUwith no comma between them — that's how it's written, so that's how it stays) 🐯sapphirerapids— Intel sapphirerapids 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, AVX512VP2INTERSECT, ENQCMD, CLDEMOTE, PTWRITE, WAITPKG, SERIALIZE, TSXLDTRK, UINTR, AMX-BF16, AMX-TILE, AMX-INT8, AVX-VNNI, AVX512FP16 and AVX512BF16 instruction set support. sapphire, because she's precious, and she brought AMX-BF16, AMX-TILE and AMX-INT8 to the function 💎alderlake— Intel Alderlake 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 and AVX-VNNI instruction set support. HRESET, KL and WIDEKL, ok hybrid core energy 🌸rocketlake— Intel Rocketlake 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, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID and AVX512VPOPCNTDQ instruction set support. (yep,SSSE3 , SSE4.1has that space in there too, andAVX512CD PKUis missing its comma — copying faithfully, bestie) 🚀k6— AMD K6 CPU with MMX instruction set support. short king energy, MMX only, still validk6-2,k6-3— improved versions of the AMD K6 CPU, and both come with MMX and 3DNow! instruction set support. glow-up edition ✨athlon,athlon-tbird— AMD Athlon CPU with MMX, 3dNOW!, enhanced 3DNow! and SSE prefetch instructions support. (and yes, it's spelled3dNOW!there, lowercase d and all — keeping it exactly)athlon-4,athlon-xp,athlon-mp— improved AMD Athlon CPU with MMX, 3DNow!, enhanced 3DNow! and full SSE instruction set support. full SSE, she leveled all the way up 🎀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.) the whole squad, one bullet, zero notes 👑k8-sse3,opteron-sse3,athlon64-sse3— improved versions of the AMD K8 cores with SSE3 instruction set support. same crew, new SSE3, love that for them 💖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.) family 10h said "superset era begins now" 🔥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.) bdver1 walked in with FMA4 and XOP like it's nothing 💅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.) BMI and TBM joined the group chat 🦋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.) FSGSBASE slid in there, noted and reportedbdver4— 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.) BMI2, AVX2 and MOVBE all showed up, that's a full lineup ✨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.) zen era, MWAITX and CLZERO in the building 🧚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.) CLWB, RDPID and WBNOINVD said "we're here too" 💖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.) ✨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 asi486with additional MMX instruction set support.winchip2— IDT WinChip 2 CPU, dealt in same way asi486with 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.)geode— AMD Geode embedded processor with MMX and 3DNow! instruction set support. 🌈
-mtune=cpu-type— bestie, this one tunes to cpu-type everything applicable about the generated code, except for the ABI and the set of available instructions. So like… picking a specific cpu-type schedules things appropriately for that particular chip, but the compiler does not generate any code that can't run on the default machine type unless you use a-march=cpu-typeoption too. For example, if GCC is configured fori686-pc-linux-gnuthen-mtune=pentium4generates code that is tuned for Pentium 4 but still runs on i686 machines. 💅
The choices for cpu-type are the same as for -march. On top of that, -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-mtuneor-marchoption 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-mtuneor-marchoption 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. ngl she's old news, but she still works 💀-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-storefor 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,387sse+387both— 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 basicasmand extendedasm. Supported choices (in dialect order) areattorintel. The default isatt. Darwin does not supportintel. ✨-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— okay so some 387 emulators literally cannot handle thesin,cosandsqrtinstructions for the 387 🫠 so if you flip this on, GCC won't generate them. It gets overridden when-marchsays the target CPU always has an FPU (in which case the instruction needs no emulation at all). And heads up: those instructions aren't generated unless you also use-funsafe-math-optimizations. ✨
-malign-double-mno-align-double— these two decide whether GCC alignsdouble,long double, andlong longvariables on a two-word boundary or a one-word boundary. Aligningdoubleon a two-word boundary gives you code that runs somewhat faster on a Pentium, but it costs you more memory 💸
On x86-64, -malign-double is enabled by default. periodt.
Warning bestie: 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 they are not binary compatible with structures in code compiled without that switch. 💀
-m96bit-long-double-m128bit-long-double— these control the size of thelong doubletype. The x86-32 application binary interface says the size is 96 bits, so-m96bit-long-doubleis 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, that's 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, since its ABI specifies that long double is aligned on a 16-byte boundary.
Notice: neither of these options enables 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 containinglong doublevariables, and it also modifies the function calling convention for functions takinglong double. So they are not binary-compatible with code compiled without that switch. 💅
-mlong-double-64-mlong-double-80-mlong-double-128— these also control the size of thelong doubletype. A size of 64 bits makes thelong doubletype equivalent to thedoubletype — and that's the default for 32-bit Bionic C library. A size of 128 bits makes thelong doubletype equivalent to the__float128type — and that's 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 containinglong doublevariables, and it also modifies the function calling convention for functions takinglong double. Hence they are not binary-compatible with code compiled without that switch. 😭
-malign-data=*type*— controls how GCC aligns variables. The supported values for type are:compatuses increased alignment value compatible with GCC 4.8 and earlier,abiuses alignment value as specified by the psABI, andcachelineuses increased alignment value to match the cache line size.compatis the default. 🦋-mlarge-data-threshold=*threshold*— when-mcmodel=mediumis specified, data objects larger than threshold are placed in the large data section. This value must be the same across all objects linked into the binary, and it defaults to65535. 🔢-mrtd— uses a different function-calling convention, where functions that take a fixed number of arguments return with theret numinstruction, which pops their arguments while returning. This saves one instruction in the caller since there's no need to pop the arguments there. slay 💅
You can specify that an individual function is called with this calling sequence using 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*— controls 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 attributeregparm. 🎀
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— uses 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 attributesseregparm. ✨
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— returns 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-mpc32is specified, the significands of results of floating-point operations are rounded to 24 bits (single precision);-mpc64rounds the significands of results of floating-point operations to 53 bits (double precision), and-mpc80rounds 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. 💀
-mstackrealign— realigns the stack at entry. On the x86, the-mstackrealignoption 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 attributeforce_align_arg_pointer, applicable to individual functions. 🫶-mpreferred-stack-boundary=*num*— attempts to keep the stack boundary aligned to a 2 raised to num byte boundary. If-mpreferred-stack-boundaryis 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=3can 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 x87long doubleand__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-boundaryis not specified, the one specified by-mpreferred-stack-boundaryis 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. 🌸
okay bestie, take a deep breath and grab a snack 🍓 because this next chunk of gcc(1) is the SIMD wall, and we are scaling it together. This is where GCC gets its full x86 feature-flag glow-up ✨
-mmmx
-msse
-msse2
-msse3
-mssse3
-msse4
-msse4a
-msse4.1
-msse4.2
-mavx
-mavx2
-mavx512f
-mavx512pf
-mavx512er
-mavx512cd
-mavx512vl
-mavx512bw
-mavx512dq
-mavx512ifma
-mavx512vbmi
-msha
-maes
-mpclmul
-mclflushopt
-mclwb
-mfsgsbase
-mptwrite
-mrdrnd
-mf16c
-mfma
-mpconfig
-mwbnoinvd
-mfma4
-mprfchw
-mrdpid
-mprefetchwt1
-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
-mavx5124fmaps
-mavx512vnni
-mavxvnni
-mavx5124vnniw
-mcldemote
-mserialize
-mamx-tile
-mamx-int8
-mamx-bf16
-mhreset
-mkl
-mwideklThese switches enable the use of instructions in the MMX, SSE, SSE2, SSE3, SSSE3, SSE4, SSE4A, SSE4.1, SSE4.2, AVX, AVX2, AVX512F, AVX512PF, AVX512ER, AVX512CD, AVX512VL, AVX512BW, AVX512DQ, AVX512IFMA, AVX512VBMI, SHA, AES, PCLMUL, CLFLUSHOPT, CLWB, FSGSBASE, PTWRITE, RDRND, F16C, FMA, PCONFIG, WBNOINVD, FMA4, PREFETCHW, RDPID, PREFETCHWT1, 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, AVX5124FMAPS, AVX512VNNI, AVX5124VNNIW, SERIALIZE, UINTR, HRESET, AMXTILE, AMXINT8, AMXBF16, KL, WIDEKL, AVXVNNI, AVX512FP16 or CLDEMOTE extended instruction sets. Every single one of these has a matching -mno- option to turn the instruction use back off, bestie. Balance. ✨
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.
If you want SSE/SSE2 instructions generated automatically from floating-point code (as opposed to 387 instructions), go look at -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. (yes, the detection file has to stay vanilla, that's the whole vibe 🧚)
-mdump-tune-features— tells GCC to dump out the names of the x86 performance tuning features and their default settings. Those names are exactly what you'd plug into-mtune-ctrl=feature-list. 💅-mtune-ctrl=feature-list— the fine-grain knob for 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 isn't preceded with^, otherwise it's turned off.
ngl bestie this one 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. 😭 not a casual-user situation, fr fr.
-mno-default— instructs GCC to turn off all tunable features. See also-mtune-ctrl=feature-listand-mdump-tune-features.-mcld— makes GCC emit a"cld"instruction in the prologue of functions that use string instructions. String instructions depend on the DF flag to pick between autoincrement or autodecrement mode. The ABI says the DF flag should be cleared on function entry, but some operating systems violate that spec by not clearing the DF flag in their exception dispatchers. The exception handler can then get invoked with the DF flag set, which leads to the 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-cldconfigure option. Generation of"cld"instructions can be suppressed with the-mno-cldcompiler option in this case.-mvzeroupper— makes GCC 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— tells GCC to use 128-bit AVX instructions instead of 256-bit AVX instructions in the auto-vectorizer.-mprefer-vector-width=opt— tells GCC to use opt-bit vector width in instructions instead of the default on the selected platform.-mmove-max=bits— sets the maximum number of bits that can be moved from memory to memory efficiently to bits. The valid bits are128,256and512. 💅-mstore-max=bits— sets the maximum number of bits that can be stored to memory efficiently to bits. The valid bits are128,256and512.
and the values for opt, for the record:
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.
-mcx16— lets GCC generate"CMPXCHG16B"instructions in 64-bit code to implement compare-and-exchange operations on 16-byte aligned 128-bit objects. Super useful for atomic updates of data structures that are bigger than one machine word. The compiler uses this instruction to implement__syncBuiltins. But — for__atomicBuiltins operating on 128-bit integers, a library call is always used. no cap. 👑-msahf— enables generation of"SAHF"instructions in 64-bit code. Early Intel Pentium 4 CPUs with Intel 64 support, before the Pentium 4 G1 step landed in December 2005, lacked the"LAHF"and"SAHF"instructions that AMD64 supports. Those two 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— enables use of the"movbe"instruction to implement__builtin_bswap32and__builtin_bswap64.-mshstk— enables shadow stack built-in functions from x86 Control-flow Enforcement Technology (CET).-mcrc32— enables built-in functions__builtin_ia32_crc32qi,__builtin_ia32_crc32hi,__builtin_ia32_crc32siand__builtin_ia32_crc32dito generate the"crc32"machine instruction.-mmwait— enables built-in functions__builtin_ia32_monitor, and__builtin_ia32_mwaitto generate the"monitor"and"mwait"machine instructions.-mrecip— enables use of"RCPSS"and"RSQRTSS"instructions (and their vectorized variants"RCPPS"and"RSQRTPS") with an extra Newton-Raphson step to bump up precision, instead of"DIVSS"and"SQRTSS"(and their vectorized variants) for single-precision floating-point arguments. These instructions only get generated when-funsafe-math-optimizationsis enabled together with-ffinite-math-onlyand-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— 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 aresvmlfor the Intel short vector math library andacmlfor the AMD math core library. To use this option, both-ftree-vectorizeand-funsafe-math-optimizationshave 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, 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
bestie this one picks the calling convention your code gets generated for, and the only two values you're allowed to pass are sysv (the ABI GNU/Linux and all its little friends rock) and ms (the Microsoft ABI). what's the default, you ask? if you're targeting Microsoft Windows you get the Microsoft ABI, and on literally every other system you get the SysV ABI. and if you wanna control this ✨per-function✨ instead of globally, you do that with the ms_abi and sysv_abi function attributes. periodt.
🦋 -mforce-indirect-call
forces every single call to a function to be indirect. this is giving very specific energy — it's useful when you're using Intel Processor Trace, because it generates more precise timing information for function calls. iykyk.
🎀 -mmanual-endbr
puts the ENDBR instruction at function entry only via the cf_check function attribute. this is useful when you're using it alongside the -fcf-protection=branch option, so you can control where ENDBR gets inserted at the function entry. cute, controlled, on your terms 💅
💖 -mcall-ms2sysv-xlogues
okay so here's the tea: because 64-bit ABIs are different from each other, any Microsoft ABI function that calls a System V ABI function has to treat RSI, RDI and XMM6-15 as clobbered. by default, the code that saves and restores those registers is emitted inline, which means your prologues and epilogues get pretty lengthy. using -mcall-ms2sysv-xlogues instead emits prologues and epilogues that use stubs in the static portion of libgcc to do those saves and restores — so you shrink your function size at the cost of a few extra instructions. tradeoffs, bestie, tradeoffs.
✨ -mtls-dialect=type
generates code to access thread-local storage using the gnu or gnu2 conventions. gnu is the conservative default; gnu2 is more efficient, but heads up — it may add compile- and run-time requirements that cannot be satisfied on all systems. so like, know your target.
🌸 -mpush-args / -mno-push-args
uses PUSH operations to store outgoing parameters. this method is shorter and usually equally fast as the method using SUB/MOV operations, and it's enabled by default. but lowkey, in some cases disabling it may improve performance, because of improved scheduling and reduced dependencies. so it's a vibe-check-per-situation thing.
🧚 -maccumulate-outgoing-args
if enabled, the maximum amount of space required for outgoing arguments gets computed in the function prologue. this is faster on most modern CPUs — reduced dependencies, improved scheduling, and reduced stack usage when the preferred stack boundary isn't equal to 2. the drawback? a notable increase in code size. also important: this switch implies -mno-push-args.
💜 -mthreads
supports thread-safe exception handling on MinGW. programs that rely on thread-safe exception handling must compile and link all their 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. she's thorough.
👑 -mms-bitfields / -mno-ms-bitfields
enables/disables bit-field layout that's 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 you're 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 info, see x86 Variable Attributes and x86 Type Attributes.
the Microsoft structure layout algorithm is fairly simple, except for the bit-field packing. the padding and alignment of members of structures, and whether a bit-field can straddle a storage-unit boundary, are determined by these rules:
- 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.
- 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
alignedattribute or thepackpragma), 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- 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:
- 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.
- 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,baris 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:
- 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,
t2has a size of 4 bytes, since the zero-length bit-field follows a normal bit-field, and is of type short. - 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.
- 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 the case where 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 at least a 4-byte boundary. this one enables more inlining and increases code size, but it may improve performance of code that depends on fast memcpy and memset for short lengths. the option also enables inline expansion of strlen for all pointer alignments. slay.
🫶 -minline-stringops-dynamically
for string operations of unknown size, this uses run-time checks — inline code for small blocks, and a library call for large blocks. best of both worlds, honestly.
💀 -mstringop-strategy=alg
overrides the internal decision heuristic for which particular algorithm to use when inlining string operations. the allowed values for alg are:
rep_byterep_4byterep_8byte
expand using i386 rep prefix of the specified size.
byte_looploopunrolled_loop
expand into an inline loop.
libcall
always use a library call.
🌈 -mmemcpy-strategy=strategy
overrides 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, and 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
this option is similar to -mmemcpy-strategy= except that it's there to control __builtin_memset expansion instead. twins, basically.
🧚 -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 it makes an extra register available in leaf functions. heads up though — 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
specifies that the assembler should encode SSE instructions with a VEX prefix. also, the option -mavx turns this on by default. cute.
🔥 -mfentry / -mno-fentry
if profiling is active (-pg), puts the profiling counter call before the prologue.
⚠️ note: on x86 architectures the attributems_hook_prologueisn't possible at the moment for-mfentryand-pg.
-mrecord-mcount,-mno-record-mcount— bestie, if profiling is turned on (-pg), this one generates a__mcount_locsection holding pointers to every single profiling call. super handy for automatically patching and out calls. 💅-mnop-mcount,-mno-nop-mcount— when profiling is active (-pg), this makes the calls to the profiling functions come out as NOPs. iconic when you wanna patch them in later dynamically. lowkey this is probably only useful alongside-mrecord-mcount, just saying.-minstrument-return=*type*— instruments function exit in-pg-mfentryinstrumented functions with a call to the specified function. note: it only instruments true returns that end withret, NOT sibling calls ending withjump. valid types arenoneto not instrument,callto generate a call to__return__, ornop5to generate a 5 byte nop. ✨-mrecord-return,-mno-record-return— generates a__return_locsection pointing to all the return instrumentation code.-mfentry-name=*name*— sets the name of the__fentry__symbol called at function entry for-pg-mfentryfunctions.-mfentry-section=*name*— sets the name of the section used to record-mrecord-mcountcalls (default is__mcount_loc).-mskip-rax-setup,-mno-skip-rax-setup— when you're generating code for the x86-64 architecture with SSE extensions disabled,-mskip-rax-setuplets you skip setting up the RAX register when there are no variable arguments passed in vector registers.
Warning, bestie, pay attention: since the RAX register is used to avoid unnecessarily saving vector registers on the stack when passing variable arguments, the impacts of this option are that callees may waste some stack space, misbehave, or jump to a random location. 😭 GCC 4.4 or newer don't have those issues, regardless of the RAX register value. periodt.
-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. slay. 🔥-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*— generate stack protection code using a canary at guard. supported locations areglobalfor global canary ortlsfor per-thread canary in the TLS block (that's the default, btw). this option has effect only when-fstack-protectoror-fstack-protector-allis 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. 💖
-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. keeping it minimal, love that for us.-mrelax-cmpxchg-loop— relaxes the cmpxchg loop by emitting an early load and compare before cmpxchg, and executespauseif the load value is not expected. this reduces excessive cacheline bouncing and works for all atomic logic fetch builtins that generate a compare and swap loop. 🌸-mindirect-branch=*choice*— converts indirect call and jump with choice. the default iskeep, which keeps indirect call and jump unmodified.thunkconverts indirect call and jump to call and return thunk.thunk-inlineconverts indirect call and jump to inlined call and return thunk.thunk-externconverts 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 attributeindirect_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*— converts function return with choice. the default iskeep, which keeps function return unmodified.thunkconverts function return to call and return thunk.thunk-inlineconverts function return to inlined call and return thunk.thunk-externconverts 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 attributefunction_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 isnone, which disables all SLS hardening.returnenables SLS hardening for function returns.indirect-jmpenables SLS hardening for indirect jumps.allenables 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 withlfence; call *%r8-r15orlfence; jmp *%r8-r15at run-time.
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-m32option setsint,long, and pointer types to 32 bits, and generates code that runs on any i386 system.
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's 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-zonedisables 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-thresholdare 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. no cap. 🌈-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— emitGNU_PROPERTY_X86_ISA_1_NEEDEDGNU property for Linux target to indicate the micro-architecture ISA level required to execute the binary.-mno-direct-extern-access— without-fpicnor-fPIC, always use the GOT pointer to access external symbols. with-fpicor-fPIC, treat access to protected symbols as local symbols. the default is-mdirect-extern-access.
Warning, bestie: shared libraries compiled with-mno-direct-extern-accessand executable compiled with-mdirect-extern-accessmay not be binary compatible if protected symbols are used in shared libraries and executable. 😭
🪟 x86 Windows 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.-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 thedllimportattribute 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 theUNICODEpreprocessor 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— Cygwin and MinGW girlies only 💖</p> This one says: bestie, we're making a GUI application, so tell the linker to set the PE header subsystem type appropriately. Very "app with a window, not a terminal gremlin" energy. ✨-fno-set-stack-executable— MinGW targets only 🦋 This specifies that the executable flag for the stack used by nested functions isn't set. Why would you want that? Because binaries running in kernel mode of Microsoft Windows literally can't have it — the User32 API, which is what you'd normally use to set executable privileges, isn't available there. No API, no flag, no choice. 💀-fwritable-relocated-rdata— MinGW and Cygwin targets 🌸 This specifies that relocated-data in a read-only section gets put into the".data"section instead. It's necessary for older runtimes that don't support modification of".rdata"sections for pseudo-relocation. Old runtime, old problems, we adapt. 🫶-mpe-aligned-commons— Cygwin and MinGW targets 🎀 This specifies that the GNU extension to the PE file format which permits the correct alignment of COMMON variables should be used when generating code. It's enabled by default if GCC detects that the target assembler found during configuration supports the feature. So basically: if your assembler can do the thing, GCC already said yes for you. 👑
See also under x86 Options for standard options. iykyk. 💅
✨ Xstormy16 Options ✨
These options are defined for Xstormy16:
-msim— Choose startup files and linker script suitable for the simulator. Simulator vibes only, no cap. 🌈
🦋 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. Not me switching up the whole instruction set because the other one ghosted us 😭-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 — like, it needs its plus-one, bestie. 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. Precision girlies, this one's for you. 🎀-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-volatileto omit the"MEMW"instructions. Order matters and this option knows it. ✨-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. Kernel mode said "PIC? never met her." 💅-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. Close to their references = clingy but functional 🫶-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-literalsis used. With-mauto-litpoolsthe 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 the assembler to create several literal pools per function and assemble very big functions, which may not be possible with-mtext-section-literals. Big function? Big pool energy. 🔥-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. Some instructions simply have main character energy and always get aligned. 👑-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. So yeah, it's over-calling a little. 🙈-mabi=name— Generate code for the specified ABI. Permissible values are:call0,windowed. Default ABI is chosen by the Xtensa core configuration. Pick your fighter. ⭐-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. Cute, tidy, register-coded. 💖-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. Rotating the window like it's a whole choreography. 🦋
🌸 zSeries Options 🌸
These are listed under
🌈 ENVIRONMENT 🌈
okay bestie, this whole section is about the environment variables that lowkey run the show behind GCC's back. Some of them work by saying "hey, look in these directories or use these prefixes" when GCC is hunting for different kinds of files. Some of them just specify other aspects of the compilation environment.
Also, important: you can specify places to search using options like -B, -I and -L. Those take precedence over places specified using environment variables, and environment variables take precedence over whatever GCC was configured with. So the order is options > env vars > GCC's own config. periodt.
LANG, LC_CTYPE, LC_MESSAGES, LC_ALL
These environment variables control how GCC uses localization info, which lets GCC 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.
LC_CTYPE 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.
LC_MESSAGES 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. It's giving "pass the baton" energy. 🏃♀️✨
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 <file:///usr/share/doc/gcc-12/README.Bugs>.
📝 FOOTNOTES 📝
- On some systems,
gcc -sharedneeds to build supplementary stub code for constructors to work. On multi-libbed systems,gcc -sharedmust 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.
👀 SEE ALSO 👀
gpl(7), gfdl(7), fsf-funding(7), cpp(1), gcov(1), as(1), ld(1), gdb(1), dbx(1) and the Info entries for gcc, cpp, as, ld, binutils and gdb.
👑 AUTHOR 👑
See the Info entry for gcc, or <https://gcc.gnu.org/onlinedocs/gcc/Contributors.html>, for contributors to GCC.
💜 COPYRIGHT 💜
Copyright (c) 1988-2022 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.okay bestie, that's the whole manual page, no cap. The footer at the very bottom is serving source/version gcc-12 and date 2022-03-19 — iconic. Now go compile something and slay. 💅✨🦋
📦 where this page comes from
- ubuntu jammy: g++-12 12-20220319-1ubuntu1
- 💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash on : a language model's rewrite of the original, under its terms