GCC(1)
💖 NAME 💖
gcc — the GNU project C and C++ compiler. Yeah bestie, this is the big one, from the GNU manual, section 1. Buckle up, it's giving legendary.
✨ SYNOPSIS ✨
gcc [-c|-S|-E] [-std=standard]
[-g] [-pg] [-Olevel]
[-Wwarn...] [-Wpedantic]
[-Idir...] [-Ldir...]
[-Dmacro[=defn]...] [-Umacro]
[-foption...] [-mmachine-option...]
[-o outfile] [@file] infile...Real talk: that's only the most useful options up there, okay? The rest are all further down, and there are so many. Also g++ accepts mostly the same options as gcc, so don't panic, the girlies are basically twins. 👯
🌈 DESCRIPTION 🌈
Okay so when you invoke GCC, it normally does four whole jobs in a row: preprocessing, compilation, assembly, and linking. That's the full pipeline, the whole main character arc. But the "overall options" let you stop that process at an intermediate stage whenever you want, like hitting pause mid-binge. For example, the -c option says do not run the linker, and then the output is just object files output by the assembler. Cute, efficient, we love her. 💅
Other options get passed on to one or more stages of processing. Some options control the preprocessor, others control the compiler itself. Yet other options control the assembler and the linker — and honestly most of those aren't documented here, because you rarely need to use any of them. (iykyk.)
Most of the command-line options you can use with GCC are useful for C programs. When an option is only useful with another language (usually C++, ofc), the explanation says so explicitly. And if the description for a particular option doesn't 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. And when you compile C++ programs, you should invoke GCC as g++ instead. That's the move. 🌟
The gcc program accepts options and file names as operands. Now listen — many options have multi-letter names, so therefore multiple single-letter options may not be grouped: -dv is very different from -d -v. Not the same thing, bestie, not even close. 😭
You can mix options and other arguments. For the most part, the order you use doesn't matter. But 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 yeah, position matters sometimes, it's giving ✨situationship✨.
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. Only one, don't come for me.
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, and 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. And if you want the full listing and explanation of the binary and decimal byte size prefixes, refer to the NIST, IEC, and other relevant national and international standards. Slay, standards bodies. 📏
👑 OPTIONS 👑
🎀 Option Summary 🎀
Here's a summary of all the options, grouped by type. The explanations are in the following sections — this part is just the lineup, the ✨cast reveal✨.
💫 Overall Options 💫
-c-S-E-o file-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=file
🌸 C Language Options 🌸
-ansi-std=standard-fgnu89-inline-fpermitted-flt-eval-methods=standard-aux-info filename-fallow-parameterless-variadic-functions-fno-asm-fno-builtin-fno-builtin-function-fgimple-fhosted-ffreestanding-fopenacc-fopenacc-dim=geom-fopenmp-fopenmp-simd-fms-extensions-fplan9-extensions-fsso-struct=endianness-fallow-single-precision-fcond-mismatch-flax-vector-conversions-fsigned-bitfields-fsigned-char-funsigned-bitfields-funsigned-char
🦋 C++ Language Options 🦋
-fabi-version=n-fno-access-control-faligned-new=n-fargs-in-order=n-fchar8_t-fcheck-new-fconstexpr-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-fms-extensions-fnew-inheriting-ctors-fnew-ttp-matching-fno-nonansi-builtins-fnothrow-opt-fno-operator-names-fno-optional-diags-fpermissive-fno-pretty-templates-frepo-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-Wabi=n-Wabi-tag-Wconversion-null-Wctor-dtor-privacy-Wdelete-non-virtual-dtor-Wdeprecated-copy-Wdeprecated-copy-dtor-Wliteral-suffix-Wmultiple-inheritance-Wno-init-list-lifetime-Wnamespaces-Wnarrowing-Wpessimizing-move-Wredundant-move-Wnoexcept-Wnoexcept-type-Wclass-memaccess-Wnon-virtual-dtor-Wreorder-Wregister-Weffc++-Wstrict-null-sentinel-Wtemplates-Wno-non-template-friend-Wold-style-cast-Woverloaded-virtual-Wno-pmf-conversions-Wno-class-conversion-Wno-terminate-Wsign-promo-Wvirtual-inheritance
🍓 Objective-C and Objective-C++ Language Options 🍓
-fconstant-string-class=class-name-fgnu-runtime-fnext-runtime-fno-nil-receivers-fobjc-abi-version=n-fobjc-call-cxx-cdtors-fobjc-direct-dispatch-fobjc-exceptions-fobjc-gc-fobjc-nilcheck-fobjc-std=objc1-fno-local-ivars-fivar-visibility=[public|protected|private|package]-freplace-objc-classes-fzero-link-gen-decls-Wassign-intercept-Wno-protocol-Wselector-Wstrict-selector-match-Wundeclared-selector
🧚 Diagnostic Message Formatting Options 🧚
-fmessage-length=n-fdiagnostics-show-location=[once|every-line]-fdiagnostics-color=[auto|never|always]-fdiagnostics-format=[text|json]-fno-diagnostics-show-option-fno-diagnostics-show-caret-fno-diagnostics-show-labels-fno-diagnostics-show-line-numbers-fdiagnostics-minimum-margin-width=width-fdiagnostics-parseable-fixits-fdiagnostics-generate-patch-fdiagnostics-show-template-tree-fno-elide-type-fno-show-column
And that's just the summary, bestie — every single one of these gets its own explanation in the sections coming up, so don't go anywhere. 💜
🌸 Warning Options 🌸
okay bestie, hydrate, stretch, sit down. this is the biggest wall of flags on the whole page and it is giving encyclopedia core. no descriptions in this stretch, just the full roster — and every single one is reproduced here exactly as the page has it. nothing renamed, nothing tidied, nothing merged. if it has a =n or a =byte-size or a bracket situation, that's because the page says so. no cap.
- the control room 👑 —
-fsyntax-only,-fmax-errors=n,-Wpedantic,-pedantic-errors,-w
yes, four of these five are not even -W flags. the compiler does not care about your little categories, and honestly? respect.
- the classics ✨ —
-Wextra,-Wall,-Waddress,-Waddress-of-packed-member - aggregate + alloc energy 🫶 —
-Waggregate-return,-Waligned-new,-Walloc-zero,-Walloc-size-larger-than=byte-size,-Walloca,-Walloca-larger-than=byte-size - arrays, attributes, and one aggressive loop situation 💀 —
-Wno-aggressive-loop-optimizations,-Warray-bounds,-Warray-bounds=n,-Wno-attributes,-Wattribute-alias=n - bools + every dialect-compat era at once 🌈 —
-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
the whole compatibility timeline is just. sitting there. slay.
- casting call 💅 —
-Wcast-align,-Wcast-align=strict,-Wcast-function-type,-Wcast-qual,-Wchar-subscripts,-Wcatch-value,-Wcatch-value=n,-Wclobbered,-Wcomment - the c-crowd: conversion, coverage, cpp, dangling, date, delete 🎀 —
-Wconditionally-supported,-Wconversion,-Wcoverage-mismatch,-Wno-cpp,-Wdangling-else,-Wdate-time,-Wdelete-incomplete - the d-block, heavy on the
-Wno-🍓 —-Wno-attribute-warning,-Wno-deprecated,-Wno-deprecated-declarations,-Wno-designated-init,-Wdisabled-optimization,-Wno-discarded-qualifiers,-Wno-discarded-array-qualifiers,-Wno-div-by-zero,-Wdouble-promotion,-Wduplicated-branches,-Wduplicated-cond - the e/f era, featuring the error girlies 😭 —
-Wempty-body,-Wenum-compare,-Wno-endif-labels,-Wexpansion-to-defined,-Werror,-Werror=*,-Wextra-semi,-Wfatal-errors,-Wfloat-equal - the format family reunion 🎀 (plus frames and free) —
-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 - j through i, part one 🦋 —
-Wjump-misses-init,-Whsa,-Wif-not-aligned,-Wignored-qualifiers,-Wignored-attributes,-Wincompatible-pointer-types,-Wimplicit,-Wimplicit-fallthrough,-Wimplicit-fallthrough=n,-Wimplicit-function-declaration,-Wimplicit-int - the rest of i + the lonely l 💜 —
-Winit-self,-Winline,-Wno-int-conversion,-Wint-in-bool-context,-Wno-int-to-pointer-cast,-Winvalid-memory-model,-Wno-invalid-offsetof,-Winvalid-pch,-Wlarger-than=byte-size - the l/m crew 💜 —
-Wlogical-op,-Wlogical-not-parentheses,-Wlong-long,-Wmain,-Wmaybe-uninitialized - memset + misleading indentation 👀 —
-Wmemset-elt-size,-Wmemset-transposed-args,-Wmisleading-indentation - the
-Wmissingsquad (seven of them, iconic) 💖 —-Wmissing-attributes,-Wmissing-braces,-Wmissing-field-initializers,-Wmissing-format-attribute,-Wmissing-include-dirs,-Wmissing-noreturn,-Wmissing-profile - multichar + multistatement 🎀 —
-Wno-multichar,-Wmultistatement-macros - n-block, and yes
-Wnormalizedreally takes[none|id|nfc|nfkc]—-Wnonnull,-Wnonnull-compare,-Wnormalized=[none|id|nfc|nfkc],-Wnull-dereference - o-block 🍓 —
-Wodr,-Wno-overflow,-Wopenmp-simd,-Woverride-init-side-effects,-Woverlength-strings - the p-block, part one 💅 —
-Wpacked,-Wpacked-bitfield-compat,-Wpacked-not-aligned,-Wpadded,-Wparentheses,-Wno-pedantic-ms-format,-Wplacement-new,-Wplacement-new=n - pointer + pragma + prio 🫶 —
-Wpointer-arith,-Wpointer-compare,-Wno-pointer-to-int-cast,-Wno-pragmas,-Wno-prio-ctor-dtor - r-block 🍓 —
-Wredundant-decls,-Wrestrict,-Wno-return-local-addr,-Wreturn-type,-Wsequence-point - the shadow crew 👥 —
-Wshadow,-Wno-shadow-ivar,-Wshadow=global,,-Wshadow=local,,-Wshadow=compatible-local
and yes bestie, -Wshadow=global, and -Wshadow=local, genuinely have a trailing comma in the page. we keep it. we do not ask questions. periodt. 💅
- shift + sign + sizeof 🌈 —
-Wshift-overflow,-Wshift-overflow=n,-Wshift-count-negative,-Wshift-count-overflow,-Wshift-negative-value,-Wsign-compare,-Wsign-conversion,-Wfloat-conversion,-Wno-scalar-storage-order,-Wsizeof-pointer-div,-Wsizeof-pointer-memaccess,-Wsizeof-array-argument - stack, strict, stringop, subobject 💜 —
-Wstack-protector,-Wstack-usage=byte-size,-Wstrict-aliasing,-Wstrict-aliasing=n,-Wstrict-overflow,-Wstrict-overflow=n,-Wstringop-overflow=n,-Wstringop-truncation,-Wsubobject-linkage - suggest + switch + sync + system ✨ —
-Wsuggest-attribute=[pure|const|noreturn|format|malloc],-Wsuggest-final-types,-Wsuggest-final-methods,-Wsuggest-override,-Wswitch,-Wswitch-bool,-Wswitch-default,-Wswitch-enum,-Wswitch-unreachable,-Wsync-nand,-Wsystem-headers - t-block 💖 —
-Wtautological-compare,-Wtrampolines,-Wtrigraphs,-Wtype-limits - u-block: undef, uninitialized, unknown pragmas, unsuffixed floats 🦋 —
-Wundef,-Wuninitialized,-Wunknown-pragmas,-Wunsuffixed-float-constants - the
-Wunusedcinematic universe 👑 —-Wunused,-Wunused-function,-Wunused-label,-Wunused-local-typedefs,-Wunused-macros,-Wunused-parameter,-Wno-unused-result,-Wunused-value,-Wunused-variable,-Wunused-const-variable,-Wunused-const-variable=n,-Wunused-but-set-parameter,-Wunused-but-set-variable - the grand finale 🎀 —
-Wuseless-cast,-Wvariadic-macros,-Wvector-operation-performance,-Wvla,-Wvla-larger-than=byte-size,-Wvolatile-register-var,-Wwrite-strings,-Wzero-as-null-pointer-constant
🎀 C and Objective-C-only Warning Options 🎀
okay so these ones are exclusive — the heading literally says C and Objective-C-only, so don't go expecting them everywhere, bestie. tiny list, elite list.
- the whole squad 💖 —
-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
twelve flags, zero fluff, all of them reproduced character for character. 🫶
🐛 Debugging Options 🐛
debugging time ✨ and wow there are a lot of -g variants. this is the part of the page that is basically "what flavour of debug info do you want, and how spicy".
- the
-ggirlies 👑 —-g,-glevel,-gdwarf,-gdwarf-version,-ggdb - recording + the format menu 💅 —
-grecord-gcc-switches,-gno-record-gcc-switches,-gstabs,-gstabs+,-gstrict-dwarf,-gno-strict-dwarf,-gvms,-gxcoff,-gxcoff+
yes there's a -gstabs+ and a -gxcoff+. plus signs are load-bearing. respect the plus signs. 💅
- the loc support pair 🦋 —
-gas-loc-support,-gno-as-loc-support,-gas-locview-support,-gno-as-locview-support - columns, statements, variables, inline points 🌸 —
-gcolumn-info,-gno-column-info,-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
notice the pattern? every single one has a -gno- twin. iconic symmetry, honestly.
- compression + splitting + describing dies 🎀 —
-gz[=type],-gsplit-dwarf,-gdescribe-dies,-gno-describe-dies - the
-fdebug crew 💜 —-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],-feliminate-unused-debug-symbols,-femit-class-debug-always,-fno-merge-debug-strings,-fno-dwarf2-cfi-asm,-fvar-tracking,-fvar-tracking-assignments
yep, the debugging section sneaks in a bunch of -f flags at the end. -fdebug-prefix-map=old=new takes old and new, keep them exactly as written, don't get creative. 🙈
🚀 Optimization Options 🚀
and now, the moment you've been waiting for: the OPTIMIZATION WALL. 🔥 this is the longest flag list on the entire page, it's basically a full alphabet of -f options, and it ends with the -O girlies everyone actually knows. every flag, every =n, every nested bracket situation — all here. 🫶
- the align quad ✨ —
-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]]]]
that nested bracket thing is [=n[:m:[n2[:m2]]]] and it appears four times in a row. copy it carefully, bestie, do not improvise the colons. 😭
- a + b block 🍓 —
-fassociative-math,-fauto-profile,-fauto-profile[=path],-fauto-inc-dec,-fbranch-probabilities,-fbranch-target-load-optimize,-fbranch-target-load-optimize2,-fbtr-bb-exclusive - the c-block, part one 💅 —
-fcaller-saves,-fcombine-stack-adjustments,-fconserve-stack,-fcompare-elim,-fcprop-registers,-fcrossjumping,-fcse-follow-jumps,-fcse-skip-blocks,-fcx-fortran-rules,-fcx-limited-range - d-block 🌈 —
-fdata-sections,-fdce,-fdelayed-branch,-fdelete-null-pointer-checks,-fdevirtualize,-fdevirtualize-speculatively,-fdevirtualize-at-ltrans,-fdse - e + the start of f 🌸 —
-fearly-inlining,-fipa-sra,-fexpensive-optimizations,-ffat-lto-objects - the math era 🔥 —
-ffast-math,-ffinite-math-only,-ffloat-store,-fexcess-precision=style,-fforward-propagate,-ffp-contract=style,-ffunction-sections
two of those take a style: -fexcess-precision=style and -ffp-contract=style. keep the word style right there, it's a placeholder situation.
- gcse × everything 💀 —
-fgcse,-fgcse-after-reload,-fgcse-las,-fgcse-lm,-fgraphite-identity,-fgcse-sm,-fhoist-adjacent-loads
-fgraphite-identity is wedged right in between -fgcse-lm and -fgcse-sm, which is a chaos move, but it's the page's chaos and we honour it. 🫡
- the inline era 👑 —
-fif-conversion,-fif-conversion2,-findirect-inlining,-finline-functions,-finline-functions-called-once,-finline-limit=n,-finline-small-functions - the
-fipacinematic universe 🎀 —-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 - ira + live patching 💅 —
-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
-flive-patching=level is sitting right in the middle of the ira girlies. that's just where the page put it. we move on. 😌
- isolate + ivopts 🦋 —
-fisolate-erroneous-paths-dereference,-fisolate-erroneous-paths-attribute,-fivopts - keep + limit + live range 🌸 —
-fkeep-inline-functions,-fkeep-static-functions,-fkeep-static-consts,-flimit-function-alignment,-flive-range-shrinkage - the
-floopsix 🌀 —-floop-block,-floop-interchange,-floop-strip-mine,-floop-unroll-and-jam,-floop-nest-optimize,-floop-parallelize-all - lra, lto, merge, modulo, move 💜 —
-flra-remat,-flto,-flto-compression-level,-flto-partition=alg,-fmerge-all-constants,-fmerge-constants,-fmodulo-sched,-fmodulo-sched-allow-regmoves,-fmove-loop-invariants - the
-fno-block 🙈 —-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
sixteen of them in one unbroken run. it's giving the villain arc of the option list. 💀
- omit, optimize, partial, peel, predictive, prefetch, profile 🍓 —
-fomit-frame-pointer,-foptimize-sibling-calls,-fpartial-inlining,-fpeel-loops,-fpredictive-commoning,-fprefetch-loop-arrays,-fprofile-correction,-fprofile-use,-fprofile-use=path,-fprofile-values,-fprofile-reorder-functions
yes, -fprofile-use and -fprofile-use=path are both listed, separately, right next to each other. both stay. 💅
- r-block: reciprocal,
-free, rename registers, reorder, rerun, reschedule, rounding 🍓 —-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
okay but -free really is a flag. it's literally just -free. not a typo, not a vibe, it's on the page. 💀
- the scheduler obsession 🎀 —
-fsave-optimization-record,-fsched2-use-superblocks,-fsched-pressure,-fsched-spec-load,-fsched-spec-load-dangerous,-fsched-stalled-insns-dep[=n],-fsched-stalled-insns[=n],-fsched-group-heuristic,-fsched-critical-path-heuristic,-fsched-spec-insn-heuristic,-fsched-rank-heuristic,-fsched-last-insn-heuristic,-fsched-dep-count-heuristic,-fschedule-fusion,-fschedule-insns,-fschedule-insns2
five separate -fsched-*-heuristic flags plus two more -fschedule-* ones. the scheduler said "I contain multitudes". 😭
- section, selective, sel-sched, semantic, shrink, signaling, single precision 🌈 —
-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 - split, ssa, stdarg, store, strict aliasing, thread jumps, tracer 💜 —
-fsplit-ivs-in-unroller,-fsplit-loops,-fsplit-paths,-fsplit-wide-types,-fssa-backprop,-fssa-phiopt,-fstdarg-opt,-fstore-merging,-fstrict-aliasing,-fthread-jumps,-ftracer - the
-ftreepass parade, part one 🌳 —-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
and -fcode-hoisting is chilling in the middle of the ftree girlies. once again: page's order, we don't argue. 🫡
- part two: the loops + everything after 💫 —
-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 - the final
-fstretch 💜 —-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
-fipa-ra shows up way after the rest of the -fipa family, sandwiched between -funswitch-loops and -fvariable-expansion-in-unroller. that's the page's doing. we keep it. 💅
- the big
-Ofinale 👑🔥 —--param name=value,-O,-O0,-O1,-O2,-O3,-Os,-Ofast,-Og
and there it is — the nine everyone actually types, with --param name=value leading the parade. note that -O is bare, no number, right there next to -O0 through -O3, plus -Os, -Ofast and -Og. all nine. exact. 🎀
🩺 Program Instrumentation Options 🩺
okay so this whole crew is the ✨watch-me-while-I-run✨ squad — they poke at your program while it's actually executing, so you can see where it goes, what it touches, and how long it lingers. Very main character. Very nosy. We love them. 💅
-p -pg -fprofile-arcs --coverage -ftest-coverage
-fprofile-abs-path -fprofile-dir=path -fprofile-generate
-fprofile-generate=path -fprofile-update=method
-fprofile-filter-files=regex -fprofile-exclude-files=regex
-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] -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,...no cap, the stack-protector variants and the sanitizer stuff are the ones people actually reach for when they want their code to tell on itself the second something goes sideways. 😭
📎 Preprocessor Options 📎
the preprocessor is literally the before — it runs before the compiler even gets a look in, doing its little text-swap magic. These flags are how you talk to that stage specifically. 🎀
-Aquestion=answer -A-question[=answer] -C -CC -Dmacro[=defn] -dD
-dI -dM -dN -dU -fdebug-cpp -fdirectives-only
-fdollars-in-identifiers -fexec-charset=charset
-fextended-identifiers -finput-charset=charset
-fmacro-prefix-map=old=new -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 -no-integrated-cpp -P -pthread
-remap -traditional -traditional-cpp -trigraphs -Umacro -undef
-Wp,option -Xpreprocessor optionthe whole -M family is giving dependency-list energy — perfect when you're wiring up a build system and need to know who depends on whom. iykyk. 🦋
🔧 Assembler Options 🔧
only two here and honestly? iconic minimalism. 👑
-Wa,option -Xassembler option🔗 Linker Options 🔗
the linker is the moment everything gets stitched together into one actual executable, and these are the knobs for that grand finale. it's giving final boss. 💖
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 keywordheads up bestie —-staticand-sharedare like, fundamentally different vibes. don't just throw them around, they change how your whole binary gets put together. 🫶
📁 Directory Options 📁
where do the headers live? where do the libraries live? this is the address book for your compiler. 💜
-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 ⚙️
this is the deep-magic zone. 🪄 calling conventions, struct returns, PIC/PIE, visibility, all of it. lowkey the most powerful section in the whole page.
-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 -fno-common -fno-ident
-fpcc-struct-return -fpic -fPIC -fpie -fPIE -fno-plt
-fno-jump-tables -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 🧪
the debugging-the-compiler-itself section. yes, bestie, GCC has options for inspecting GCC. the recursion?? the audacity?? we stan. 🌈
-dletters -dumpspecs -dumpmachine -dumpversion -dumpfullversion
-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]the -fdump-* family will literally spit out intermediate representations for you to stare at. obsessed. and -save-temps? keeps all the in-between files so you can go snooping later. 🍓
🖥️ Machine-Dependent Options 🖥️
and now for the part where GCC shows off that it speaks, like, every architecture. this is a whole tour, bestie — buckle up. ✨
🐧 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]|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🦋 Adapteva Epiphany Options 🦋
-mhalf-reg-file -mprefer-short-insn-regs
-mbranch-cost=num -mcmove -mnops=num -msoft-cmpsf -msplit-lohi
-mpost-inc -mpost-modify -mstack-offset=num -mround-nearest
-mlong-calls -mshort-calls -msmall16 -mfp-mode=mode -mvect-double
-max-vect-align=num -msplit-vecmove-early -m1reg-reg🌊 AMD GCN Options 🌊
three flags, maximum chill. 😌
-march=gpu -mtune=gpu -mstack-size=bytes🏹 ARC Options 🏹
this list is LONG long and honestly it ate. every single one of these is a real thing, we do not skip. 💅
-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-indexngl,-mRcqand-mRcware case-sensitive little gremlins. copy them exactly or the compiler will simply not know what you mean. 💀
🍓 ARM Options 🍓
the ARM list is the moment. it's giving encyclopedia. every bit of it stays. 💖
-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 -munaligned-access
-mneon-for-64bits -mslow-flash-data -masm-syntax-unified
-mrestrict-it -mverbose-cost-dump -mpure-code -mcmse🌸 AVR Options 🌸
microcontroller bestie era. 💜
-mmcu=mcu -mabsdata -maccumulate-args
-mbranch-cost=cost -mcall-prologues -mgas-isr-prologues -mint8
-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 🌑
-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, slays. 👑
-mbig-endian -mlittle-endian -march=cpu -msim
-msdata=sdata-type🧚 CRIS Options 🧚
-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 🎀
-mmac -mcr16cplus -mcr16c -msim -mint32 -mbit-ops
-mdata-model=model🦄 C-SKY Options 🦄
last one of this part, and it's a whole vibe. 🌈
-march=arch -mcpu=cpu -mbig-endian -EB
-mlittle-endian -EL -mhard-float -msoft-float -mfpu=fpu
-mdouble-float -mfdivdu -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-prologand that's where this part ends bestie — the architecture tour continues, so don't go anywhere. 🫶
🍎 Darwin Options 🍎
okay so if you're on Darwin, this is your era. literally the whole squad of Apple-flavored flags, no notes:
-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
not me noticing-sectobjectsymbols,-segs_read_only_addrand-segs_read_write_addreach show up twice in the list — that's not a typo of mine bestie, that's literally how the page prints it, I'm just the messenger 💅
-whyload is iconic though. no further comment.
🐉 DEC Alpha Options 🐉
-mno-fp-regs-msoft-float-mieee-mieee-with-inexact-mieee-conformant-mfp-trap-mode=mode-mfp-rounding-mode=mode-mtrap-precision=mode-mbuild-constants-mcpu=cpu-type-mtune=cpu-type-mbwx-mmax-mfix-mcix-mfloat-vax-mfloat-ieee-mexplicit-relocs-msmall-data-mlarge-data-msmall-text-mlarge-text-mmemory-latency=time
🌱 FR30 Options 🌱
short and sweet, tiny king energy:
-msmall-model-mno-lsim
✨ FT32 Options ✨
-msim-mlra-mnodiv-mft32b-mcompress-mnopm
🦋 FRV Options 🦋
okay this one ate, it's a whole wardrobe:
-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
-mtomcat-stats bestie?? tomcat?? in my compiler options?? obsessed, no idea, love her 🐱🐧 GNU/Linux Options 🐧
-mglibc-muclibc-mmusl-mbionic-mandroid-tno-android-cc-tno-android-ld
🎀 H8/300 Options 🎀
-mrelax-mh-ms-mn-mexr-mno-exr-mint32-malign-300
💜 HPPA Options 💜
-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 ⭐
this one is giving "my options list has a whole paragraph of options" and honestly? respect.
-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
-msched-stop-bits-after-every-cycle sounds like a self-care routine and honestly good for her 💅🧚 LM32 Options 🧚
-mbarrel-shift-enabled-mdivide-enabled-mmultiply-enabled-msign-extend-enabled-muser-enabled
🌸 M32R/D Options 🌸
-m32r2-m32rx-m32r-mdebug-malign-loops-mno-align-loops-missue-rate=number-mbranch-cost=number-mmodel=code-size-model-type-msdata=sdata-type-mno-flush-func-mflush-func=name-mno-flush-trap-mflush-trap=number-G num
🫶 M32C Options 🫶
-mcpu=cpu-msim-memregs=number
🍓 M680x0 Options 🍓
a whole lineage, she contains multitudes:
-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 🌈
-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 💖
-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
🦄 MicroBlaze Options 🦄
-msoft-float-mhard-float-msmall-divides-mcpu=cpu-mmemcpy-mxl-soft-mul-mxl-soft-div-mxl-barrel-shift-mxl-pattern-compare-mxl-stack-check-mxl-gp-opt-mno-clearbss-mxl-multiply-high-mxl-float-convert-mxl-float-sqrt-mbig-endian-mlittle-endian-mxl-reorder-mxl-mode-app-model-mpic-data-is-text-relative
👑 MIPS Options 👑
bestie. BESTIE. sit down. this is the main character of option lists. hydrate first, we're going in:
-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-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
-mmno-mcuand-mno-goptare just sitting there exactly like that in the source, and I am NOT touching them. fidelity besties, we don't "fix" the manpage 🙈
💀 MMIX Options 💀
-mlibfuncs-mno-libfuncs-mepsilon-mno-epsilon-mabi=gnu-mabi=mmixware-mzero-extend-mknuthdiv-mtoplevel-symbols-melf-mbranch-predict-mno-branch-predict-mbase-addresses-mno-base-addresses-msingle-exit-mno-single-exit
🎀 MN10300 Options 🎀
-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 🧚
four options, tiny, adorable, unstoppable:
-meb-mel-mmul.x-mno-crt0
💜 MSP430 Options 💜
-msim-masm-hex-mmcu=-mcpu=-mlarge-msmall-mrelax-mwarn-mcu-mcode-region=-mdata-region=-msilicon-errata=-msilicon-errata-warn=-mhwmult=-minrt
🦋 NDS32 Options 🦋
-mbig-endian-mlittle-endian-mreduced-regs-mfull-regs-mcmov-mno-cmov-mext-perf-mno-ext-perf-mext-perf2-mno-ext-perf2-mext-string-mno-ext-string-mv3push-mno-v3push-m16bit-mno-16bit-misr-vector-size=num-mcache-block-size=num-march=arch-mcmodel=code-model-mctor-dtor-mrelax
🌸 Nios II Options 🌸
-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 🌈
-m32-m64-mmainkernel-moptimize
✨ OpenRISC Options ✨
-mboard=name-mnewlib-mhard-mul-mhard-div-msoft-mul-msoft-div-mcmov-mror-msext-msfimm-mshftimm
💖 PDP-11 Options 💖
-mfpu-msoft-float-mac0-mno-ac0-m40-m45-m10-mint32-mno-int16-mint16-mno-int32-msplit-munix-asm-mdec-asm-mgnu-asm-mlra
🫶 picoChip Options 🫶
-mae=ae_type-mvliw-lookahead=N-msymbol-as-address-mno-inefficient-warnings
⭐ PowerPC Options ⭐
and for this one? the page just points you elsewhere, no list, no drama:
See RS/6000 and PowerPC Options.
periodt. that's the whole entry. go look over there bestie 💅
🦋 RISC-V Options
okay RISC-V bestie, come get your flags 💅 this one's got alignment, division, floating div, PLT behaviour, and a couple of mcmodels to pick between — all of it, no skips:
-mbranch-cost=N-instruction -mplt -mno-plt
-mabi=ABI-string -mfdiv -mno-fdiv -mdiv -mno-div -march=ISA-string
-mtune=processor-string -mpreferred-stack-boundary=num
-msmall-data-limit=N-bytes -msave-restore -mno-save-restore
-mstrict-align -mno-strict-align -mcmodel=medlow -mcmodel=medany
-mexplicit-relocs -mno-explicit-relocs -mrelax -mno-relax
-mriscv-attribute -mmo-riscv-attribute🌸 RL78 Options
small chip, big flag energy. sim, multiply modes, the whole g10/g13/g14 family, plus double sizes and that extremely specific -msave-mduc-in-interrupts:
-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
this list is long long, bestie — like, a whole semester of flags 😭 cpu/tune/cmodel at the top, then altivec, popcount, dfp, toc sizes, call ABIs, struct returns, secure PLT, longcalls, the power8 fusion/vector/crypto/htm/quad-memory batch, float128, gnu attributes and the stack-protector guard trio. every single one, in order:
-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
-mmfpgpr -mno-mfpgpr -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
-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🎀 RX Options
double sizes, fpu or no fpu, endianness data, small data, sim, AS100 syntax, relax, constant sizes, register counts, pid, string insns, jsr, interrupts — the whole menu fr:
-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
mainframe girlies rise 🌈 tune/arch, float hardness, dfp, long-double sizes, backchain, packed stack, small exec, mvcle, -m64/-m31, debug, esa, zarch, htm, vx, zvector, tpf-trace, fused-madd, frame warnings, stack size/guard and the -mhotpatch pair:
-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 -mfused-madd
-mno-fused-madd -mwarn-framesize -mwarn-dynamicstack -mstack-size
-mstack-guard -mhotpatch=halfwords,halfwords✨ Score Options
tiny list, big vibes. endianness, hardware loop, -muls, -mmac, and the score5/5u/7/7d crew:
-meb -mel -mnhwloop -muls -mmac -mscore5 -mscore5u
-mscore7 -mscore7d🫶 SH Options
the SuperH buffet — every m1 through m4al variant, endianness, dalign, relax, bigtable, fmovd, renesas-ness, nomacsave, ieee, bitops, isize, inline ic invalidation, padstruct, prefergot, usermode, multcost, div strategies and libfunc names, fixed ranges, outgoing args, atomic model, branch cost, zdcbranch, delay slots, fused-madd, fsca, fsrra, pretend-cmove and tas. iconic honestly:
-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
short and sweet, five flags, hwcap clearing, impure-text and pthreads:
-mclear-hwcap -mno-clear-hwcap -mimpure-text
-mno-impure-text -pthreads⭐ SPARC Options
cpu/tune/cmodel/memory-model, 32 vs 64, app-regs, faster structs, flat, fpu, float hardness, quad float, stack bias, std struct returns, unaligned doubles, user mode, v8plus, the whole mvis → mvis4b ladder, cbcond, fmaf, fsmuld, popc, subxc, the errata fixes (-mfix-at697f, -mfix-ut699, -mfix-ut700, -mfix-gr712rc) and lra:
-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-lra🧚 SPU Options
reloc warnings, safe vs unsafe DMA, branch hints, small/large mem, stdmain, fixed ranges, ea32/ea64, address space conversion, cache size and atomic updates:
-mwarn-reloc -merror-reloc -msafe-dma -munsafe-dma
-mbranch-hints -msmall-mem -mlarge-mem -mstdmain
-mfixed-range=register-range -mea32 -mea64
-maddress-space-conversion -mno-address-space-conversion
-mcache-size=cache-size -matomic-updates -mno-atomic-updates💖 System V Options
four of them, and they are giving very specific energy:
-Qy -Qn -YP,paths -Ym,dir🌸 TILE-Gx Options
cpu, 32/64, endianness, cmodel — that's the whole thing, slay:
-mcpu=CPU -m32 -m64 -mbig-endian -mlittle-endian
-mcmodel=code-model🎀 TILEPro Options
two flags. two. adorable:
-mcpu=cpu -m32🦋 V850 Options
long calls, ep, prolog function, space, the tda/sda/zda offsets, app-regs, callt disabling, every v850 variant, loop, relax, long jumps, float hardness, gcc/rh850 ABI and big switch:
-mlong-calls -mno-long-calls -mep -mno-ep
-mprolog-function -mno-prolog-function -mspace -mtda=n -msda=n
-mzda=n -mapp-regs -mno-app-regs -mdisable-callt
-mno-disable-callt -mv850e2v3 -mv850e2 -mv850e1 -mv850es -mv850e
-mv850 -mv850e3v5 -mloop -mrelax -mlong-jumps -msoft-float
-mhard-float -mgcc-abi -mrh850-abi -mbig-switch✨ VAX Options
three flags, old-school cool:
-mg -mgnu -munix🫶 Visium Options
debug, sim, fpu, float hardness, cpu/tune, sv-mode and user-mode:
-mdebug -msim -mfpu -mno-fpu -mhard-float
-msoft-float -mcpu=cpu-type -mtune=cpu-type -msv-mode -muser-mode👑 VMS Options
return codes, debug main prefix, malloc64 and pointer size:
-mvms-return-codes -mdebug-main=prefix -mmalloc64
-mpointer-size=size🌈 VxWorks Options
rtp, non-static, static/dynamic binding and lazy vs now binding:
-mrtp -non-static -Bstatic -Bdynamic -Xbind-lazy
-Xbind-now💜 x86 Options
okay this is THE one. the big one. the main character 😭 tune/arch, tune-ctrl, dump-tune-features, no-default, fpmath, asm dialect, the 387 stuff, stack boundaries, and then every single ISA extension you've ever heard of — mmx through avx512 and beyond, sha, aes, pclmul, fsgsbase, rdrnd, f16c, fma, pconfig, wbnoinvd, ptwrite, prefetchwt1, clflushopt, clwb, xsavec, xsaves, sse4a, 3dnow, 3dnowa, popcnt, abm, bmi, tbm, fma4, xop, adx, lzcnt, bmi2, fxsr, xsave, xsaveopt, rtm, hle, lwp, mwaitx, clzero, pku, threads, gfni, vaes, waitpkg, shstk, manual-endbr, force-indirect-call, avx512vbmi2, vpclmulqdq, avx512bitalg, movdiri, movdir64b, avx512vpopcntdq, avx5124fmaps, avx512vnni, avx5124vnniw, prfchw, rdpid, rdseed, sgx, cldemote, ms-bitfields — plus the stringop controls, push-args, outgoing args, long-double widths, regparm, sseregparm, veclibabi, vect8-ret-in-mem, pc32/64/80, stackrealign, omit-leaf-frame-pointer, red-zone, tls-direct-seg-refs, cmodel, abi, address-mode, -m32/-m64/-mx32/-m16/-miamcu, large-data-threshold, sse2avx, fentry, record-mcount, nop-mcount, 8bit-idiv, instrument-return, fentry-name, fentry-section, avx256 splits, align-data, the stack-protector guard quad, general-regs-only, call-ms2sysv-xlogues, indirect-branch, function-return and indirect-branch-register. breathe. here it is:
-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 -mrecip -mrecip=opt -mvzeroupper
-mprefer-avx128 -mprefer-vector-width=opt -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 -mvpclmulqdq -mavx512bitalg -mmovdiri -mmovdir64b
-mavx512vpopcntdq -mavx5124fmaps -mavx512vnni -mavx5124vnniw
-mprfchw -mrdpid -mrdseed -msgx -mcldemote -mms-bitfields
-mno-align-stringops -minline-all-stringops
-minline-stringops-dynamically -mstringop-strategy=alg
-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 -mindirect-branch=choice
-mfunction-return=choice -mindirect-branch-registerngl if you read that whole block in one go you deserve a little treat 🍓
🎀 x86 Windows Options
console, cygwin (and not-cygwin), dll, no fun dllimport, threads, unicode, win32, windows and no set-stack-executable:
-mconsole -mcygwin -mno-cygwin -mdll
-mnop-fun-dllimport -mthread -municode -mwin32 -mwindows
-fno-set-stack-executable✨ Xstormy16 Options
one flag. just the one. we love a minimalist:
-msim🦋 Xtensa Options
const16, fused-madd, force-no-pic, serialize-volatile, text-section-literals, auto-litpools, target-align and longcalls — each with its mno- twin where it's got one:
-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💜 zSeries Options
literally a pointer, not even a list:
See S/390 and zSeries Options.🎀 Options Controlling the Kind of Output
okay so compiling is a whole journey, and it has up to four stages, always in this exact order bestie, no skipping ahead: preprocessing, compilation proper, assembly and linking. 💅
GCC can preprocess and compile several files either into several assembler input files, or into one assembler input file; then each assembler input file produces an object file, and linking combines all the object files (those newly compiled, and those specified as input) into an executable file.
So like, for any given input file, the file name suffix is what decides what kind of compilation is done. It's giving "your file extension is your destiny" ✨
file.c— C source code that must be preprocessed.file.i— C source code that should not be preprocessed.file.ii— C++ source code that should not be preprocessed.file.m— Objective-C source code. Note that you must link with thelibobjclibrary to make an Objective-C program work.file.mi— Objective-C source code that should not be preprocessed.
Alright bestie, the file-suffix saga continues, and honestly? It's giving ✨encyclopedia✨ but we're gonna slay it 💅
file.mm,file.M— Objective-C++ source code! 🦋 Heads up though: you must link with thelibobjclibrary to make an Objective-C++ program actually work. And.Mmeans a literal capital M, don't play with me on that one, case is real. 😭file.mii— Objective-C++ source code that should not be preprocessed. hands off, leave it alone, it's thriving 🫶file.h— a C, C++, Objective-C or Objective-C++ header file, getting turned into a precompiled header (that's the default, iykyk), or a C / C++ header file getting turned into an Ada spec (that happens 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. ⚠️ tiny trap: in.cxx, those last two letters have to both be literallyx. Same energy with.C— that's a literal capitalC. periodt.file.mm,file.M— Objective-C++ source code that must be preprocessed. (yes bestie it's listed again, we're being thorough and I respect the commitment 💜)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 getting turned into a precompiled header or an 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. free form, free spirit 🦋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.brig— BRIG files, aka the binary representation of HSAIL. fancy ✨file.d— D source code.file.di— D interface file.file.dd— D documentation code (Ddoc).file.ads— an Ada source code file containing a library unit declaration (that means a declaration of a package, subprogram, or generic, or a generic instantiation), or a library unit renaming declaration (that's a package, generic, or subprogram renaming declaration). These files are also called specs, bestie. 💅file.adb— an Ada source code file containing a library unit body (a subprogram or package body). These are also called bodies. 🫶file.s— Assembler code.file.S,file.sx— Assembler code that must be preprocessed. 👀other— literally any object file you want fed straight into linking. Any file name with no recognized suffix gets treated this way. no notes. 🙈
Okay so if you wanna be extra and pick the language yourself instead of letting the compiler vibe-check your filename, there's the -x option for that:
-xlanguage — specifies explicitly the language for the following input files (instead of letting the compiler guess a default from the file name suffix). This option applies to all following input files until the next-xoption shows up. The possible values for language are:
c c-header cpp-output
c++ c++-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
brig-x none— turns off any specification of a language, so the files after it get handled according to their file name suffixes (exactly like they would if you'd never used-xat all). reset era 💅
Now, if you only want some of the compilation stages (main character energy, selective), 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 should stop. ⚠️ Fair warning: some combinations (for example, -x cpp-output -E) instruct gcc to do nothing at all. literally nothing. iconic but also 😭
-c— Compile or assemble the source files, but do not link. The linking stage simply is not done. The ultimate output is 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 that don't require compilation or assembly are ignored. 🎀-S— Stop after the stage of compilation proper; do not assemble. The output is an assembler code file for each non-assembler input file you 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 preprocessed source code, and it gets sent to the standard output. Input files that don't require preprocessing are ignored. 💖-ofile — Place output in file. This applies to whatever sort of output is being produced — whether that's an executable file, an object file, an assembler file or preprocessed C code. If-ois not specified, the default is to put an executable file ina.out, the object file forsource.suffixinsource.o, its assembler file insource.s, a precompiled header file insource.suffix.gch, and all preprocessed C source on standard output. 🫶-v— Print (on standard error output) the commands executed to run the stages of compilation. Also prints the version number of the compiler driver program, plus the version of the preprocessor and the compiler proper. nosy but make it useful 👑-###— Like-v, except the commands are not executed and arguments get quoted unless they contain only alphanumeric characters or"./-_". This one's 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 the-Wextraoption has also been specified (before the--helpoption), then command-line options that have no documentation associated with them are displayed too. deep lore unlocked 🔥--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. These are the supported classes:optimizers— Display all of the optimization options supported by the compiler.warnings— Display all of the options controlling warning messages produced by the compiler.target— Display target-specific options. ⚠️ Unlike the--target-helpoption though, target-specific options of the linker and assembler are not displayed. That's 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.common— Display the options that are common to all languages.
And these are the supported qualifiers:
undocumented— Display only those options that are undocumented.joined— Display options taking an argument that appears after an equal sign in the same continuous piece of text, such as:--help=target.separate— Display options taking an argument that appears as a separate word following the original option, such as:-o output-file.
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,^undocumented⚠️ The argument to --help= should not consist solely of inverted qualifiers.
Combining several classes is possible, although this usually restricts the output so much that there's nothing left 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 the ones that have already been displayed. And if --help is also specified anywhere on the command line, then that takes precedence over any --help= option. 💅
If the -Q option appears on the command line before the --help= option, then the descriptive text displayed by --help= is changed. Instead of describing the displayed options, you get an indication of 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. the receipts, bestie 🎀-pass-exit-codes— okay so normallygccbails with exit code1if any phase of the compiler returns a non-success return code. But if you pass this flag? Thegccprogram instead returns the numerically highest error produced by any phase returning an error indication. And bestie, the C, C++, and Fortran front ends return4if an internal compiler error happens. 💅-pipe— use pipes instead of temporary files for communication between the various stages of compilation. It fails to work on some systems where the assembler can’t read from a pipe, but the GNU assembler has no trouble. ✨-specs=file— process file after the compiler reads in the standard specs file, so you can override the defaults thegccdriver program uses when determining what switches to pass tocc1,cc1plus,as,ld, etc. You can specify more than one-specs=fileon 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. For example:
gcc -c t.c -wrapper gdb,--argsThis invokes all subprograms of gcc under gdb --args, so 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-mapand-fdebug-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 Goconst,type,var, andfuncdeclarations 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). 👑
However, the use of gcc does not add the C++ library. g++ is a program that calls GCC and automatically specifies linking against the C++ library. It treats .c, .h and .i files as C++ source files instead of C source files unless -x is used. This program is also useful when precompiling a C header file with a .h extension for use in C++ compilations. On many systems, g++ is also installed with the name c++. 💅
When you compile C++ programs, you may specify many of the same command-line options that you use for compiling programs in any language; or command-line options meaningful for C and related languages; or options that are meaningful only for C++ programs. ✨
🌈 Options Controlling C Dialect 🌈
The following options control the dialect of C (or languages derived from C, such as C++, Objective-C and Objective-C++) that the compiler accepts:
-ansi— In C mode, this is equivalent to-std=c90. In C++ mode, it is equivalent to-std=c++98.
This turns off certain features of GCC that are incompatible with ISO C90 (when compiling C code), or of standard C++ (when compiling C++ code), such as the asm and typeof keywords, and predefined macros such as unix and vax that identify the type of system you are using. 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 is useful to put them in header files that might be 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, -Wpedantic is required in addition to -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:
c90,c89,iso9899:1990— Support all ISO C90 programs (certain GNU extensions that conflict with ISO C90 are disabled). Same as-ansifor C code. 💖iso9899:199409— ISO C90 as modified in amendment 1. 🎀c99,c9x,iso9899:1999,iso9899:199x— ISO C99. This standard is substantially completely supported, modulo bugs and floating-point issues (mainly but not entirely relating to optional C99 features from Annexes F and G). See <http://gcc.gnu.org/c99status.html> for more information. The namesc9xandiso9899:199xare deprecated. 🦋c11,c1x,iso9899:2011— ISO C11, the 2011 revision of the ISO C standard. This standard is substantially completely supported, modulo bugs, floating-point issues (mainly but not entirely relating to optional C11 features from Annexes F and G) and the optional Annexes K (Bounds-checking interfaces) and L (Analyzability). The namec1xis deprecated. 🌈c17,c18,iso9899:2017,iso9899:2018— ISO C17, the 2017 revision of the ISO C standard (published in 2018). This standard is same as C11 except for corrections of defects (all of which are also applied with-std=c11) and a new value of__STDC_VERSION__, and so is supported to the same extent as C11. ✨c2x— The next version of the ISO C standard, still under development. The support for this version is experimental and incomplete. 😭gnu90,gnu89— GNU dialect of ISO C90 (including some C99 features). 💅gnu99,gnu9x— GNU dialect of ISO C99. The namegnu9xis deprecated. 🎀gnu11,gnu1x— GNU dialect of ISO C11. The namegnu1xis deprecated. 🦋gnu17,gnu18— GNU dialect of ISO C17. This is the default for C code. 👑gnu2x— The next version of the ISO C standard, still under development, plus GNU extensions. The support for this version is experimental and incomplete. 🌸c++98,c++03— The 1998 ISO C++ standard plus the 2003 technical corrigendum and some additional defect reports. Same as-ansifor C++ code. 💖gnu++98,gnu++03— GNU dialect of-std=c++98. ✨c++11,c++0x— The 2011 ISO C++ standard plus amendments. The namec++0xis deprecated. 🔥gnu++11,gnu++0x— GNU dialect of-std=c++11. The namegnu++0xis deprecated. 💅c++14,c++1y— The 2014 ISO C++ standard plus amendments. The namec++1yis deprecated. 🎀gnu++14,gnu++1y— GNU dialect of-std=c++14. This is the default for C++ code. The namegnu++1yis deprecated. 👑c++17,c++1z— The 2017 ISO C++ standard plus amendments. The namec++1zis deprecated. 🦋
gnu++17,gnu++1z— the GNU dialect of-std=c++17. Heads up bestie: the namegnu++1zis deprecated 💀c++2a— the next revision of the ISO C++ standard, tentatively planned for 2020. Support is highly experimental, and it will almost certainly change in incompatible ways in future releases 😭 so like, don't build your whole life on itgnu++2a— GNU dialect of-std=c++2a. Support is highly experimental, and will almost certainly change in incompatible ways in future releases 🙈 same warning, same energy
-fgnu89-inline— tells GCC to use the traditional GNU semantics forinlinefunctions when it's in C99 mode ✨
Using this option is roughly equivalent to adding the gnu_inline function attribute to all inline functions. Yes, all of them. Periodt.
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 👑
- *
-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.
-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.-ansiimplies-fno-asm.
In C++, this switch only affects the typeof keyword, since asm and inline are standard keywords. You may want to use the -fno-gnu-keywords flag instead, which has the same effect. 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 ✨
With the -fno-builtin-function option only the built-in function function is disabled. function must not begin with __builtin_. If a function is named that is not built-in in this version of GCC, this option is ignored. There is no corresponding -fbuiltin-function option; if you wish to enable built-in functions selectively when using -fno-builtin or -ffreestanding, you may define macros such as:
#define abs(n) __builtin_abs ((n))
#define strcpy(d, s) __builtin_strcpy ((d), (s))-fgimple— enable parsing of function definitions marked with__GIMPLE🧪 This is an experimental feature that allows unit testing of GIMPLE passes. Lowkey a lab-coat moment, ngl.-fhosted— assert that compilation targets a hosted environment. This implies-fbuiltin. A hosted environment is one in which the entire standard library is available, and in whichmainhas a return type ofint👑 Examples are nearly everything except a kernel. This is equivalent to-fno-freestanding.-ffreestanding— assert that compilation targets a freestanding environment. This implies-fno-builtin. A freestanding environment is one in which the standard library may not exist, and program startup may not necessarily be atmain. The most obvious example is an OS kernel 🌈 This is equivalent to-fno-hosted.-fopenacc— enable handling of OpenACC directives#pragma accin C/C++ and!$accin Fortran 🔥 When-fopenaccis specified, the compiler generates accelerated code according to the OpenACC Application Programming Interface v2.0https://www.openacc.org. This option implies-pthread, and thus is only supported on targets that have support for-pthread💅- *
-fopenacc-dim=geom* — specify default compute dimensions for parallel offload regions that do not explicitly specify. The geom value is a triple of:-separated sizes, in ordergang,workerand,vector. A size can be omitted, to use a target-specific default value 🎀 -fopenmp— enable handling of OpenMP directives#pragma ompin C/C++ and!$ompin Fortran 🚀 When-fopenmpis specified, the compiler generates parallel code according to the OpenMP Application Program Interface v4.5https://www.openmp.org. This option implies-pthread, and thus is only supported on targets that have support for-pthread.-fopenmpimplies-fopenmp-simd✨-fopenmp-simd— enable handling of OpenMP's SIMD directives with#pragma ompin C/C++ and!$ompin Fortran. Other OpenMP directives are ignored 🙈-fgnu-tm— when the option-fgnu-tmis specified, the compiler generates code for the Linux variant of Intel's current Transactional Memory ABI specification document (Revision 1.1, May 6 2009) 🧚 This is an experimental feature whose interface may change in future versions of GCC, as the official specification changes. Please note that not all architectures are supported for this feature 💀
For more information on GCC's support for transactional memory,
Note that the transactional memory feature is not supported with non-call exceptions (-fnon-call-exceptions) 🚫
-fms-extensions— accept some non-standard constructs used in Microsoft header files 🪟
In C++ code, this allows member names in structures to be similar to previous types declarations:
typedef int UOW;
struct ABC {
UOW UOW;
};Some cases of unnamed fields in structures and unions are only accepted with this option.
Note that this option is off for all targets but x86 targets using ms-abi 👀
-fplan9-extensions— accept some non-standard constructs used in Plan 9 code 🐇
This enables -fms-extensions, permits passing pointers to structures with anonymous fields to functions that expect pointers to elements of the type of the field, and permits referring to anonymous fields declared using a typedef. This is only supported for C, not C++ 💅
-fcond-mismatch— allow conditional expressions with mismatched types in the second and third arguments. The value of such an expression isvoid💀 This option is not supported for C++.-flax-vector-conversions— allow implicit conversions between vectors with differing numbers of elements and/or incompatible element types. This option should not be used for new code 🚫 like bestie, we're begging you, it's giving legacy.-funsigned-char— let 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 👑
-fsigned-char— let 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 🌸
-fsigned-bitfields,-funsigned-bitfields,-fno-signed-bitfields,-fno-unsigned-bitfields— bestie, these four are the vibe check for your bit-fields 🎀 when the declaration itself is out here being ambiguous and uses neithersignednorunsigned. So what's the default? Signed, periodt. Because it's consistent — the basic integer types likeintare signed types, and GCC is a girl who likes her rules to match 💅.-fsso-struct=endianness— sets the default scalar storage order of structures and unions to the endianness you pick. Accepted values arebig-endian,little-endian, andnativefor the native endianness of the target (which is, btw, the default). Not supported for C++, so don't even try it there 🙈.
⚠️ Warning, and this one's serious bestie: 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. That's giving "chaos on purpose" energy and you need to know it 💀.
✨ Options Controlling C++ Dialect ✨
This section is all about the command-line options that are only meaningful for C++ programs. But lowkey, 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. No gatekeeping here 🫶.
Here is a list of options that are only for compiling C++ programs:
-fabi-version=n— use version n of the C++ ABI. The default is version 0.
Version 0 refers to the version conforming most closely to the C++ ABI specification. Therefore, the ABI obtained using version 0 will change in different versions of G++ as ABI bugs are fixed 👀.
Version 1 is the version of the C++ ABI that first appeared in G++ 3.2.
Version 2 is the version of the C++ ABI that first appeared in G++ 3.4, and was the default through G++ 4.9.
Version 3 corrects an error in mangling a constant address as a template argument.
Version 4, which first appeared in G++ 4.5, implements a standard mangling for vector types.
Version 5, which first appeared in G++ 4.6, corrects the mangling of attribute const/volatile on function pointer types, decltype of a plain decl, and use of a function parameter in the declaration of another parameter.
Version 6, which first appeared in G++ 4.7, corrects the promotion behavior of C++11 scoped enums and the mangling of template argument packs, const/static_cast, prefix ++ and --, and a class scope function used as a template argument.
Version 7, which first appeared in G++ 4.8, that treats nullptr_t as a builtin type and corrects the mangling of lambdas in default argument scope.
Version 8, which first appeared in G++ 4.9, corrects the substitution behavior of function types with function-cv-qualifiers.
Version 9, which first appeared in G++ 5.2, corrects the alignment of nullptr_t.
Version 10, which first appeared in G++ 6.1, adds mangling of attributes that affect type identity, such as ia32 calling convention attributes (e.g. stdcall).
Version 11, which first appeared in G++ 7, corrects the mangling of sizeof... expressions and operator names. For multiple entities with the same name within a function, that are declared in different scopes, the mangling now changes starting with the twelfth occurrence. It also implies -fnew-inheriting-ctors.
Version 12, which first appeared in G++ 8, corrects the calling conventions for empty classes on the x86_64 target and for classes with only deleted copy/move constructors. It accidentally changes the calling convention for classes with a deleted copy constructor and a trivial move constructor. Oopsie 😭.
Version 13, which first appeared in G++ 8.2, fixes the accidental change in version 12. She said "let me fix that real quick" 💅.
See also -Wabi.
-fabi-compat-version=n— on targets that support strong aliases, G++ works around mangling changes by creating an alias with the correct mangled name when defining a symbol with an incorrect mangled name. This switch specifies which ABI version to use for the alias ✨.
With -fabi-version=0 (the default), this defaults to 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. Yes bestie, it's the "who needs private anyway" button 💀.-faligned-new— enable support for C++17newof types that require more alignment thanvoid* ::operator new(std::size_t)provides. A numeric argument such as-faligned-new=32can be used to specify how much alignment (in bytes) is provided by that function, but few users will need to override the default ofalignof(std::max_align_t)🎀.
This flag is enabled by default for -std=c++17. Slay.
-fchar8_t,-fno-char8_t— enable support forchar8_tas adopted for C++2a. This includes the addition of a newchar8_tfundamental 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_tand__cpp_lib_char8_tfeature test macros 🌈.
This option enables functions to be overloaded for ordinary and UTF-8 strings:
int f(const char *); // #1
int f(const char8_t *); // #2
int v1 = f("text"); // Calls #1
int v2 = f(u8"text"); // Calls #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. Yeah. It's giving "your old code is not safe" energy 😭:
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 byoperator newis non-null before attempting to modify the storage allocated 👑. This check is normally unnecessary because the C++ standard specifies thatoperator newonly returns 0 if it is declaredthrow(), in which case the compiler always checks the return value even without this option. In all other cases, whenoperator newhas a non-empty exception specification, memory exhaustion is signalled by throwingstd::bad_alloc. See also new (nothrow).-fconcepts— enable support for the C++ Extensions for Concepts Technical Specification, ISO 19217 (2015), which allows code like this absolute icon ✨:
template <class T> concept bool Addable = requires (T t) { t + t; };
template <Addable T> T add (T a, T b) { return a + b; }-fconstexpr-depth=n— set the maximum nested evaluation depth for C++11 constexpr functions to n. A limit is needed to detect endless recursion during constant expression evaluation 🔥. The minimum specified by the standard is 512.-fconstexpr-loop-limit=n— set the maximum number of iterations for a loop in C++14 constexpr functions to n. A limit is needed to detect infinite loops during constant expression evaluation 🙈. The default is 262144 (1<<18).-fconstexpr-ops-limit=n— set the maximum number of operations during a single constexpr evaluation. Even when number of iterations of a single loop is limited with the above limit, if there are several nested loops and each of them has many iterations but still smaller than the above limit, or if in a body of some loop or even outside of a loop too many expressions need to be evaluated, the resulting constexpr evaluation might take too long 😭. The default is 33554432 (1<<25).-fdeduce-init-list— enable deduction of a template type parameter asstd::initializer_listfrom a brace-enclosed initializer list, i.e.
template <class T> auto forward(T t) -> decltype (realfn (t))
{
return realfn (t);
}
void f()
{
forward({1,2}); // call forward<std::initializer_list<int>>
}This deduction was implemented as a possible extension to the originally proposed semantics for the C++11 standard, but was not part of the final standard, so it is disabled by default. This option is deprecated, and may be removed in a future version of G++. Bestie, don't get attached 💀.
-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. So it's still doing something, just not that thing 👑.
-fno-enforce-eh-specs— bestie, this one tells the compiler NOT to generate any code that checks whether you're violating your exception specifications at runtime. It literally violates the C++ standard 😭, but it can be a vibe for shrinking code size in production builds, kinda like definingNDEBUG. Lowkey important though: this does not give your code permission to throw exceptions that break the specs — the compiler still optimizes based on those specs, so throwing an unexpected exception = undefined behavior at runtime. No cap. 💀-fextern-tls-init/-fno-extern-tls-init— okay so the C++11 and OpenMP standards letthread_localandthreadprivatevariables have dynamic (runtime) initialization. To make that work, every use of such a variable goes through a wrapper function that does whatever init is needed. When the use and the definition live in the same translation unit, that overhead can just be optimized away ✨ — but when the use is in a different TU, there's significant overhead even if the variable doesn't actually need dynamic init. 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 — then you can dodge all that overhead with-fno-extern-tls-init. 💅
On targets that support symbol aliases, the default is -fextern-tls-init. On targets that don't support symbol aliases, the default is -fno-extern-tls-init.
-fno-gnu-keywords— do not recognizetypeofas a keyword, so your code can use that word as an identifier. You can use the keyword__typeof__instead. This option is implied by the strict ISO C++ dialects:-ansi,-std=c++98,-std=c++11, etc. 🎀-fno-implicit-templates— never emit code for non-inline templates that get instantiated implicitly (i.e. by use). Only emit code for explicit instantiations. ⚠️ Heads up bestie: if you use this, you must take care to structure your code so it includes all the necessary explicit instantiations, or you'll get undefined symbols at link time. The linker will be giving 😭.-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 those functions aren't inlined everywhere they're called. 💀-fms-extensions— disableWpedanticwarnings about constructs used in MFC, like 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. It's part of C++17 but also considered a Defect Report against C++11 and C++14. 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. Enabled by default for-std=c++17. 🌈-fno-nonansi-builtins— disable built-in declarations of functions that aren't 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. Fun fact bestie: 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. periodt. 💅-fno-optional-diags— disable diagnostics that the standard says a compiler doesn't 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. So using-fpermissivelets some nonconforming code actually compile. 🫶-fno-pretty-templates— okay so 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 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 the error message harder to understand instead of easier, you can use-fno-pretty-templatesto disable them. ✨-frepo— enable automatic template instantiation at link time. This option also implies-fno-implicit-templates. 🔥-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 don't 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. No cap, be careful bestie.
-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. So like, be honest with yourself about whether you're doing that.-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, because 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— don't use the__cxa_get_exception_ptrruntime routine. This causesstd::uncaught_exceptionto be incorrect, but is necessary if the runtime routine isn't available. 😭-fvisibility-inlines-hidden— this switch declares that the user does not attempt to compare pointers to inline functions or methods where the addresses of the two functions are taken in different shared objects. 👑
The effect of this is that GCC may, effectively, mark inline methods with __attribute__ ((visibility ("hidden"))) so that they don't appear in the export table of a DSO and don't require a PLT indirection when used within the DSO. Enabling this option can have a dramatic effect on load and link times of a DSO, because it massively reduces the size of the dynamic export table when the library makes heavy use of templates. 🔥
The behavior of this switch is not quite the same as marking the methods as hidden directly, because it doesn't affect static variables local to the function or cause the compiler to deduce that the function is defined in only one shared object. 🎀
You may mark a method as having a visibility explicitly to negate the effect of the switch for that method. For example, if you do want to compare pointers to a particular inline method, you might mark it as having default visibility. Marking the enclosing class with explicit visibility has no effect. 💅
Explicitly instantiated inline methods are unaffected by this option, as their linkage might otherwise cross a shared library boundary. ✨
-fvisibility-ms-compat — okay bestie, this flag is trying to make GCC's C++ linkage model match Microsoft Visual Studio's. It's giving "we can be compatible" energy. ✨
Here's what it changes:
- 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 are permitted when this option is not used.
In new code, it's better to use -fvisibility=hidden and export those classes that are intended to be externally visible. Unfortunately, it's possible for code to rely, perhaps accidentally, on the Visual Studio behavior.
Among the consequences of these changes are that static data members of the same type with the same name but defined in different shared objects are different, so changing one does not change the other; and that pointers to function members defined in different shared objects may not compare equal. When this flag is given, it is a violation of the ODR to define types with the same name differently. 💅
-fno-weak — do not use weak symbol support, even if the linker provides it. By default, G++ uses weak symbols if they are available. This option exists only for testing, and should not be used by end-users; it results in inferior code and has no benefits. This option may be removed in a future release of G++. 🚫
-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.) 📚
In addition, these optimization, warning, and code generation options have meanings only for C++ programs:
-Wabi (C, Objective-C, C++ and Objective-C++ only) — warn when G++ generates code that is probably not compatible with the vendor-neutral C++ ABI. Since G++ now defaults to updating the ABI with each major release, normally -Wabi will warn only if there is a check added later in a release series for an ABI issue discovered since the initial release. -Wabi will warn about more things if an older ABI version is selected (with -fabi-version=n).
-Wabi can also be used with an explicit version number to warn about compatibility with a particular -fabi-version level, e.g. -Wabi=2 to warn about changes relative to -fabi-version=2.
If an explicit version number is provided and -fabi-compat-version is not specified, the version number from this option is used for compatibility aliases. If no explicit version number is provided with this option, but -fabi-compat-version is specified, that version number is used for ABI warnings.
Although an effort has been made to warn about all such cases, there are probably some cases that are not warned about, even though G++ is generating incompatible code. There may also be cases where warnings are emitted even though the code that is generated is compatible.
You should rewrite your code to avoid these warnings if you are concerned about the fact that code generated by G++ may not be binary compatible with code generated by other compilers.
Known incompatibilities in -fabi-version=2 (which was the default from GCC 3.4 to 4.9) include:
- A template with a non-type template parameter of reference type was mangled incorrectly:
extern int N;
template <int &> struct S {};
void n (S<N>) {2}This was fixed in -fabi-version=3.
- SIMD vector types declared using
"__attribute ((vector_size))"were mangled in a non-standard way that does not allow for overloading of functions taking vectors of different sizes.
The mangling was changed in -fabi-version=4.
"__attribute ((const))"and"noreturn"were mangled as type qualifiers, and"decltype"of a plain declaration was folded away.
These mangling issues were fixed in -fabi-version=5.
- Scoped enumerators passed as arguments to a variadic function are promoted like unscoped enumerators, causing
"va_arg"to complain. On most targets this does not actually affect the parameter passing ABI, as there is no way to pass an argument smaller than"int".
Also, the ABI changed the mangling of template argument packs, "const_cast", "static_cast", prefix increment/decrement, and a class scope function used as a template argument.
These issues were corrected in -fabi-version=6.
- Lambdas in default argument scope were mangled incorrectly, and the ABI changed the mangling of
"nullptr_t".
These issues were corrected in -fabi-version=7.
- When mangling a function type with function-cv-qualifiers, the un-qualified function type was incorrectly treated as a substitution candidate.
This was fixed in -fabi-version=8, the default for GCC 5.1.
"decltype(nullptr)"incorrectly had an alignment of 1, leading to unaligned accesses. Note that this did not affect the ABI of a function with a"nullptr_t"parameter, as parameters have a minimum alignment.
This was fixed in -fabi-version=9, the default for GCC 5.2.
- Target-specific attributes that affect the identity of a type, such as ia32 calling conventions on a function type (stdcall, regparm, etc.), did not affect the mangled name, leading to name collisions when function pointers were used as template arguments.
This was fixed in -fabi-version=10, the default for GCC 6.1.
It also warns about psABI-related changes. The known psABI changes at this point include:
- For SysV/x86-64, unions with
"long double"members are passed in memory as specified in psABI. For example:
union U {
long double ld;
int i;
};"union U" is always passed in memory.
-Wabi-tag (C++ and Objective-C++ only) — warn when a type with an ABI tag is used in a context that does not have that ABI tag. See C++ Attributes for more information about ABI tags. 🏷️
-Wctor-dtor-privacy (C++ and Objective-C++ only) — warn when a class seems unusable because all the constructors or destructors in that class are private, and it has neither friends nor public static member functions. Also warn if there are no non-private methods, and there's at least one private member function that isn't a constructor or destructor. 🔒
-Wdelete-non-virtual-dtor (C++ and Objective-C++ only) — warn when "delete" is used to destroy an instance of a class that has virtual functions and non-virtual destructor. It is unsafe to delete an instance of a derived class through a pointer to a base class if the base class does not have a virtual destructor. This warning is enabled by -Wall. ⚠️
-Wdeprecated-copy (C++ and Objective-C++ only) — warn that the implicit declaration of a copy constructor or copy assignment operator is deprecated if the class has a user-provided copy constructor or copy assignment operator, in C++11 and up. This warning is enabled by -Wextra. With -Wdeprecated-copy-dtor, also deprecate if the class has a user-provided destructor. 📜
-Wno-init-list-lifetime (C++ and Objective-C++ only) — do not warn about uses of "std::initializer_list" that are likely to result in dangling pointers. Since the underlying array for an "initializer_list" is handled like a normal C++ temporary object, it is easy to inadvertently keep a pointer to the array past the end of the array's lifetime. For example:
- If a function returns a temporary
"initializer_list", or a local"initializer_list"variable, the array's lifetime ends at the end of the return statement, so the value returned has a dangling pointer. - If a new-expression creates an
"initializer_list", the array only lives until the end of the enclosing full-expression, so the"initializer_list"in the heap has a dangling pointer. - When an
"initializer_list"variable is assigned from a brace-enclosed initializer list, the temporary array created for the right side of the assignment only lives until the end of the full-expression, so at the next statement the"initializer_list"variable has a dangling pointer.
// li's initial underlying array lives as long as li
std::initializer_list<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- When a list constructor stores the
"begin"pointer from the"initializer_list"argument, this doesn't extend the lifetime of the array, so if a class variable is constructed from a temporary"initializer_list", the pointer is left dangling by the end of the variable declaration statement.
-Wliteral-suffix (C++ and Objective-C++ only) — warn when a string or character literal is followed by a ud-suffix which does not begin with an underscore. As a conforming extension, GCC treats such suffixes as separate preprocessing tokens in order to maintain backwards compatibility with code that uses formatting macros from "<inttypes.h>". For example:
#define __STDC_FORMAT_MACROS
#include <inttypes.h>
#include <stdio.h>
int main() {
int64_t i64 = 123;
printf("My int64: %" PRId64"\n", i64);
}In this case, "PRId64" is treated as a separate preprocessing token.
Additionally, warn when a user-defined literal operator is declared with a literal suffix identifier that doesn't begin with an underscore. Literal suffix identifiers that don't begin with an underscore are reserved for future standardization.
This warning is enabled by default. 📝
-Wlto-type-mismatch — during the link-time optimization warn about type mismatches in global declarations from different compilation units. Requires -flto to be enabled. Enabled by default. 🔗
-Wno-narrowing(C++ and Objective-C++ only) — bestie, for C++11 and later standards, narrowing conversions get diagnosed by default, because the standard said so, periodt. A narrowing conversion from a constant produces an error, and one from a non-constant produces a warning, but-Wno-narrowingsuppresses the whole diagnostic. 💅 Important tho: this does not change the meaning of well-formed code — narrowing conversions are still considered ill-formed in SFINAE contexts. No cap.
With -Wnarrowing in C++98, warn 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) — warns when anoexcept-expression evaluates tofalsebecause of a call to a function that doesn't have a non-throwing exception specification (i.e."throw()"or"noexcept") but is known by the compiler to never throw an exception. 👑-Wnoexcept-type(C++ and Objective-C++ only) — warns if the C++17 feature making"noexcept"part of a function type changes the mangled name of a symbol relative to C++14. Enabled by-Wabiand-Wc++17-compat. ✨
Example moment:
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) — warns when the destination of a call to a raw memory function such as"memset"or"memcpy"is an object of class type, and when writing into such an object might bypass the class non-trivial or deleted constructor or copy assignment, violate const-correctness or encapsulation, or corrupt virtual table pointers. Modifying the representation of such objects may violate invariants maintained by member functions of the class. For example, the call to"memset"below is undefined because it modifies a non-trivial class object and is, therefore, diagnosed. The safe way to either initialize or clear the storage of objects of such types is by using the appropriate constructor or assignment operator, if one is available. 🚨
std::string str = "abc";
memset (&str, 0, sizeof str);The -Wclass-memaccess option is enabled by -Wall. Explicitly casting the pointer to the class object to "void *" or to a type that can be safely accessed by the raw memory function suppresses the warning. 💖
-Wnon-virtual-dtor(C++ and Objective-C++ only) — warns when a class has virtual functions and an accessible non-virtual destructor itself or in an accessible polymorphic base class, in which case it's possible but unsafe to delete an instance of a derived class through a pointer to the class itself or base class. This warning is automatically enabled if-Weffc++is specified. 💀-Wregister(C++ and Objective-C++ only) — warns on uses of the"register"storage class specifier, except when it's part of the GNU Explicit Register Variables extension. The use of the"register"keyword as storage class specifier has been deprecated in C++11 and removed in C++17. Enabled by default with-std=c++17. 🙈-Wreorder(C++ and Objective-C++ only) — warns when the order of member initializers given in the code doesn't match the order in which they must be executed. 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, emitting a warning to that effect. This warning is enabled by -Wall. 🌸
-Wno-pessimizing-move(C++ and Objective-C++ only) — this warning warns when a call to"std::move"prevents copy elision. A typical scenario when copy elision can occur is when returning in a function with a class return type, when the expression being returned is the name of a non-volatile automatic object, and is not a function parameter, and has the same type as the function return type.
struct T {
...
};
T fn()
{
T t;
...
return std::move (t);
}But in this example, the "std::move" call prevents copy elision. 😤
This warning is enabled by -Wall.
-Wno-redundant-move(C++ and Objective-C++ only) — this warning warns about redundant calls to"std::move"; that is, when a move operation would have been performed even without the"std::move"call. This happens because the compiler is forced to treat the object as if it were an rvalue in certain situations such as returning a local variable, where copy elision isn't applicable. Consider:
struct T {
...
};
T fn(T t)
{
...
return std::move (t);
}Here, the "std::move" call is redundant. Because G++ implements Core Issue 1579, another example is:
struct T { // convertible to U
...
};
struct U {
...
};
U fn()
{
T t;
...
return std::move (t);
}In this example, copy elision isn't applicable because the type of the expression being returned and the function return type differ, yet G++ treats the return value as if it were designated by an rvalue. 🦋
This warning is enabled by -Wextra.
-fext-numeric-literals(C++ and Objective-C++ only) — accepts imaginary, fixed-point, or machine-defined literal number suffixes as GNU extensions. When this option is turned off these suffixes are treated as C++11 user-defined literal numeric suffixes. This is on by default for all pre-C++11 dialects and all GNU dialects:-std=c++98,-std=gnu++98,-std=gnu++11,-std=gnu++14. This option is off by default for ISO C++11 onwards (-std=c++11, ...). ⭐
The following -W... options are not affected by -Wall. (yes, really, these ones march to the beat of their own drum 🥁)
-Weffc++(C++ and Objective-C++ only) — warns 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. 😭
-Wstrict-null-sentinel(C++ and Objective-C++ only) — warns about the use of an uncasted"NULL"as sentinel. When compiling only with GCC this is a valid sentinel, as"NULL"is defined to"__null". Although it is a null pointer constant rather than a null pointer, it is guaranteed to be of the same size as a pointer. But this use is not portable across different compilers. 🚩-Wno-non-template-friend(C++ and Objective-C++ only) — disables warnings when non-template friend functions are declared within a template. In very old versions of GCC that predate implementation of the ISO standard, declarations such asfriend int foo(int), where the name of the friend is an unqualified-id, could be interpreted as a particular specialization of a template function; the warning exists to diagnose compatibility problems, and is enabled by default. 💜-Wold-style-cast(C++ and Objective-C++ only) — warns if an old-style (C-style) cast to a non-void type is used within a C++ program. The new-style casts ("dynamic_cast","static_cast","reinterpret_cast", and"const_cast") are less vulnerable to unintended effects and much easier to search for. ✨-Woverloaded-virtual(C++ and Objective-C++ only) — warns when a function declaration hides virtual functions from a base class. For example, in:
struct A {
virtual void f();
};
struct B: public A {
void f(int);
};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 to a plain pointer. 🙈-Wsign-promo(C++ and Objective-C++ only) — warns when overload resolution chooses a promotion from unsigned or enumerated type to a signed type, over a conversion to an unsigned type of the same size. Previous versions of G++ tried to preserve unsignedness, but the standard mandates the current behavior. 😤-Wtemplates(C++ and Objective-C++ only) — warns when a primary template declaration is encountered. Some coding rules disallow templates, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL. One may also instantiate or specialize templates. 🎀-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. One 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. One may also define classes that indirectly use virtual inheritance. 💅-Wnamespaces— bestie, this one yells at you whenever a namespace definition gets opened. Some coding standards are like "no namespaces ever" and this is the tool to enforce that. The warning stays quiet inside system header files (hi, STL 💖) so you can still use the STL without getting dragged. You can also still useusingdirectives and qualified names.-Wno-terminate(C++ and Objective-C++ only) — turns off the warning about athrow-expression that's gonna immediately result in a call toterminate. Like, it was always gonna end in tears, but now GCC won't narrate it. 💀-Wno-class-conversion(C++ and Objective-C++ only) — disables the warning for when a conversion function converts an object to the same type, to a base class of that type, or tovoid. Such a conversion function will never be called, so it's giving very much "ghost at the party nobody invited." 👻
✨ Options Controlling Objective-C and Objective-C++ Dialects ✨
NOTE: This manual does not describe the Objective-C and Objective-C++ languages themselves. That's a whole other vibe, we're just here for the flags. 🎀
Okay so this section is all about the command-line options that are only meaningful for Objective-C and Objective-C++ programs. You can still use most of the language-independent GNU compiler options though — they're universal, they slay everywhere. 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; the other options work with any language GCC supports. 🌈
Also, quick note: 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). Cross-pollination, we love to see it. 🦋
Here's the list of options that are only for compiling Objective-C and Objective-C++ programs:
-fconstant-string-class=class-name— useclass-nameas 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. Default queen behavior. 👑-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. Conditional, bestie. 💅-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.- *
-fobjc-abi-version=n* — use versionnof the Objective-C ABI for the selected runtime. This option is currently supported only for the NeXT runtime. In that case, Version 0 is the traditional (32-bit) ABI without support for properties and other Objective-C 2.0 additions. Version 1 is the traditional (32-bit) ABI with support for properties and other Objective-C 2.0 additions. Version 2 is the modern (64-bit) ABI. If nothing is specified, the default is Version 0 on 32-bit target machines, and Version 2 on 64-bit target machines. -fobjc-call-cxx-cdtors— for each Objective-C class, check if any of its instance variables is a C++ object with a non-trivial default constructor. If so, synthesize a special- (id) .cxx_constructinstance method which runs non-trivial default constructors on any such instance variables, in order, and then returnsself. Similarly, 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. Backwards, iconic. 🔄
The - (id) .cxx_construct and - (void) .cxx_destruct methods thusly generated only operate on instance variables declared in the current Objective-C class, and not those inherited from superclasses. It is the responsibility of the Objective-C runtime to invoke all such methods in an object's inheritance hierarchy. The - (id) .cxx_construct methods are invoked by the runtime immediately after a new object instance is allocated; the - (void) .cxx_destruct methods are invoked immediately before the runtime deallocates an object instance.
As of this writing, only the NeXT runtime on Mac OS X 10.4 and later has support for invoking the - (id) .cxx_construct and - (void) .cxx_destruct methods.
-fobjc-direct-dispatch— allow fast jumps to the message dispatcher. On Darwin this is accomplished via the comm page. Zoom zoom. 🏎️-fobjc-exceptions— enable syntactic support for structured exception handling in Objective-C, similar to what's offered by C++. This option is required to use the Objective-C keywords@try,@throw,@catch,@finallyand@synchronized. This option is available with both the GNU runtime and the NeXT runtime (but not available in conjunction with the NeXT runtime on Mac OS X 10.2 and earlier).-fobjc-gc— enable garbage collection (GC) in Objective-C and Objective-C++ programs. This option is only available with the NeXT runtime; the GNU runtime has a different garbage collection implementation that does not require special compiler flags. It just handles itself, no notes. 💅-fobjc-nilcheck— for the NeXT runtime with version 2 of the ABI, check for anilreceiver in method invocations before doing the actual method call. This is the default and can be disabled using-fno-objc-nilcheck. Class methods and super calls are never checked fornilin this way no matter what this flag is set to. Currently this flag does nothing when the GNU runtime, or an older version of the NeXT runtime ABI, is used.-fobjc-std=objc1— conform to the language syntax of Objective-C 1.0, the language recognized by GCC 4.0. This only affects the Objective-C additions to the C/C++ language; it does not affect conformance to C/C++ standards, which is controlled by the separate C/C++ dialect option flags. When this option is used with the Objective-C or Objective-C++ compiler, any Objective-C syntax that is not recognized by GCC 4.0 is rejected. This is useful if you need to make sure your Objective-C code can be compiled with older versions of GCC. Throwback compatibility, we respect it. 🫶-freplace-objc-classes— emit a special marker instructingld(1)not to statically link in the resulting object file, and allowdyld(1)to load it in at run time instead. This is used in conjunction with the Fix-and-Continue debugging mode, where the object file in question may be recompiled and dynamically reloaded in the course of program execution, without the need to restart the program itself. Currently, Fix-and-Continue functionality is only available in conjunction with the NeXT runtime on Mac OS X 10.3 and later. Hot-swapping your own code mid-run? Main character energy, fr. 🔥-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 the-fzero-linkflag suppresses this behavior and causes calls toobjc_getClass("...")to be retained. This is useful in Zero-Link debugging mode, since it allows for individual class implementations to be modified during program execution. The GNU runtime currently always retains calls toobjc_get_class("...")regardless of command-line options.-fno-local-ivars— by default instance variables in Objective-C can be accessed as if they were local variables from within the methods of the class they're declared in. This can lead to shadowing between instance variables and other variables declared either locally inside a class method or globally with the same name. Specifying the-fno-local-ivarsflag disables this behavior thus avoiding variable shadowing issues. No more name collisions, no more confusion. ✨-fivar-visibility=[public|protected|private|package]— set the default instance variable visibility to the specified option so that instance variables declared outside the scope of any access modifier directives default to the specified visibility.-gen-decls— dump interface declarations for all classes seen in the source file to a file namedsourcename.decl. Just spilling the tea, in writing. 📝-Wassign-intercept(Objective-C and Objective-C++ only) — warn whenever an Objective-C assignment is being intercepted by the garbage collector.-Wno-protocol(Objective-C and Objective-C++ only) — if a class is declared to implement a protocol, a warning is issued for every method in the protocol that is not implemented by the class. The default behavior is to issue a warning for every method not explicitly implemented in the class, even if a method implementation is inherited from the superclass. If you use the-Wno-protocoloption, then methods inherited from the superclass are considered to be implemented, and no warning is issued for them. Inherited counts as done, periodt. 💖-Wselector(Objective-C and Objective-C++ only) — warn if multiple methods of different types for the same selector are found during compilation. The check is performed on the list of methods in the final stage of compilation. Additionally, a check is performed for each selector appearing in a@selector(...)expression, and a corresponding method for that selector has been found during compilation. Because these checks scan the method table only at the end of compilation, these warnings are not produced if the final stage of compilation is not reached, for example because an error is found during compilation, or because the-fsyntax-onlyoption is being used. So if you bail early, no receipts. 🙈-Wstrict-selector-match(Objective-C and Objective-C++ only) — warn if multiple methods with differing argument and/or return types are found for a given selector when attempting to send a message using this selector to a receiver of 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, same vibes — we let it slide. 💜-Wundeclared-selector(Objective-C and Objective-C++ only, bestie) — warns you if there's a"@selector(...)"expression pointing at a selector that was never declared. And when do we call a selector "undeclared"? When no method with that name got declared before the"@selector(...)"shows up — whether that's explicitly in an@interfaceor@protocoldeclaration, or implicitly in an@implementationsection. ✨
Also lowkey important: this option does its checks the instant a "@selector(...)" expression is found, whereas -Wselector waits until the final stage of compilation to do its thing. It also enforces that coding-style convention that methods and selectors gotta be declared before you use them. no skipping ahead 💅
-print-objc-runtime-info— generates a C header describing the largest structure that gets passed by value, if there is one. slay.
🌈 Options to Control Diagnostic Messages Formatting 🌈
okay so historically, diagnostic messages were formatted with zero regard for the output device's aspect (like, its width, and so on 😭). But you can use the options below to control the formatting algorithm for diagnostics — like how many characters per line, how often source location info gets reported, all that. 🎀
heads up bestie: some language front ends may simply not honor these options. rude but true.
-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 happens at all — every error message shows up on a single line. This is the default for all front ends. 💖
ngl, note this: the option also affects the display of the #error and #warning pre-processor directives, plus the deprecated function/type/variable attribute. It does not, however, affect the pragma GCC warning and pragma GCC error pragmas.
-fdiagnostics-show-location=once— only meaningful in line-wrapping mode. Tells the diagnostic messages reporter to emit source location info once; like, if the message is too long to fit on a single physical line and has to wrap, the source location won't get emitted (as a prefix) again and again and again on the continuation lines. This is the default behavior. ✨-fdiagnostics-show-location=every-line— only meaningful in line-wrapping mode. Tells the diagnostic messages reporter to emit the same source location info (as a prefix) for the physical lines that result from breaking up a message that's too long for one line.-fdiagnostics-color[=WHEN]-fno-diagnostics-color— use color in diagnostics, bestie! 🌈 WHEN isnever,always, orauto. The default depends on how the compiler was configured — it can be any of those WHEN options, or alsoneverif theGCC_COLORSenvironment variable isn't present in the environment, andautootherwise.automeans use color only when standard error is a terminal. The forms-fdiagnostics-colorand-fno-diagnostics-colorare aliases for-fdiagnostics-color=alwaysand-fdiagnostics-color=never, respectively.
The colors are defined by the environment variable GCC_COLORS. Its value is a colon-separated list of capabilities and Select Graphic Rendition (SGR) substrings. SGR commands are interpreted by the terminal or terminal emulator. (See the section in the documentation of your text terminal for permitted values and their meanings as character attributes.) These substring values are integers in decimal representation and can be concatenated with semicolons. Common values to concatenate include 1 for bold, 4 for underline, 5 for blink, 7 for inverse, 39 for default foreground color, 30 to 37 for foreground colors, 90 to 97 for 16-color mode foreground colors, 38;5;0 to 38;5;255 for 88-color and 256-color modes foreground colors, 49 for default background color, 40 to 47 for background colors, 100 to 107 for 16-color mode background colors, and 48;5;0 to 48;5;255 for 88-color and 256-color modes background colors.
The default GCC_COLORS is
error=01;31:warning=01;35:note=01;36:range1=32:range2=34:locus=01:\
quote=01: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."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.
-fno-diagnostics-show-option— by default, every diagnostic emitted includes text telling you which command-line option directly controls that diagnostic (if such an option is known to the diagnostic machinery). Specifying the-fno-diagnostics-show-optionflag suppresses that behavior. 🙈-fno-diagnostics-show-caret— by default, each diagnostic emitted includes the original source line and a caret^pointing at the column. This option suppresses that info. The source line gets truncated to n characters if the-fmessage-length=noption is given. When output goes to the terminal, the width is limited to the width given by theCOLUMNSenvironment variable, or, if that's 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 info, like 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-line-numbers— by default, when printing source code (via-fdiagnostics-show-caret), a left margin gets printed showing line numbers. This option suppresses that left margin.-fdiagnostics-minimum-margin-width=width— controls the minimum width of the left margin printed by-fdiagnostics-show-line-numbers. It defaults to6.-fdiagnostics-parseable-fixits— emits fix-it hints in a machine-parseable format, suitable for consumption by IDEs. For each fix-it, a line gets 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— prints 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, like:
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, like:
map<
[...],
vector<
[double != float]>>The parts that differ are highlighted with color ("double" and "float" in this case). 🌸
-fno-elide-type— by default, when the C++ frontend prints diagnostics showing mismatching template types, common parts of the types get printed as"[...]"to simplify the error message. For example:
could not convert 'std::map<int, std::vector<double> >()'
from 'map<[...],vector<double>>' to 'map<[...],vector<float>>Specifying the -fno-elide-type flag suppresses that behavior. This flag also affects the output of the -fdiagnostics-show-template-tree flag. 👑
-fno-show-column— do not print column numbers in diagnostics. This may be necessary if diagnostics are being scanned by a program that doesn't understand the column numbers, such asdejagnu. fr fr.-fdiagnostics-format=FORMAT— select a different format for printing diagnostics. FORMAT istextorjson. The default istext. 💜
okay so the <code>json</code> format is literally just a top-level JSON array stuffed with JSON objects, and each object is one lil diagnostic. that's the whole vibe.
it comes out as one single line, no pretty formatting — the examples below got the glow-up manually just so you can actually read them, bestie.
diagnostics can have child diagnostics 🫶. like, check this error plus its 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;
| ^which, after it gets formatted (remember: real output is one line, no cap), might show up in JSON like this:
[
{
"kind": "warning",
"locations": [
{
"caret": {
"column": 3,
"file": "misleading-indentation.c",
"line": 15
},
"finish": {
"column": 4,
"file": "misleading-indentation.c",
"line": 15
}
}
],
"message": "this \u2018if\u2019 clause does not guard...",
"option": "-Wmisleading-indentation",
"children": [
{
"kind": "note",
"locations": [
{
"caret": {
"column": 5,
"file": "misleading-indentation.c",
"line": 17
}
}
],
"message": "...this statement, but the latter is ..."
}
]
},
...
]and see how that "note" is a child of the "warning"? that's the nepo baby dynamic, iconic.
every diagnostic has a "kind". if that kind is "warning", then boom, there's an "option" key telling you which command-line option is running the show for that warning 👑
a diagnostic can hold zero or more locations. each location has up to three positions inside it: a "caret" position plus optional "start" and "finish" positions. a location can also carry an optional "label" string. like, this error right here:
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 sits on the <code>"+"</code> token at column 23. then it's got two secondary locations describing the left and right-hand sides of the expression, and those ones are labeled. it might get printed in JSON 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}"
}
],
"message": "invalid operands to binary + ..."
}if a diagnostic has fix-it hints, it gets a "fixits" array — a bunch of half-open intervals, same energy as what <code>-fdiagnostics-parseable-fixits</code> spits out. for real, watch 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
}
}
],
"message": "\u2018struct s\u2019 has no member named ..."
}so the fix-it hint is basically saying swap the text from <code>"start"</code> up to but not including <code>"next"</code> with whatever <code>"string"</code> says ✨. and get this: deletions happen when <code>"string"</code> is empty, and insertions happen when <code>"start"</code> equals <code>"next"</code>. that's the whole trick, no notes 💅
✨ Options to Request or Suppress Warnings ✨
okay so warnings are diagnostic messages about constructions that aren't like, inherently broken, but are risky or lowkey hint that something went wrong somewhere 😭. not errors, just... suspicious behavior. the vibe check failed but nobody's hurt yet.
these next language-independent options don't switch on any specific warnings — they just control the kinds of diagnostics GCC hands you. so like, they're the directors, not the actors.
-fsyntax-only— check the code for syntax errors, and that's IT. nothing beyond that, bestie. no codegen, no extra credit. 🎀-fmax-errors=n— caps the max number of error messages at n, and once it hits that, GCC just bails instead of trying to keep grinding through the source code. if n is0(which is the default), there's literally no limit on how many error messages you'll get flooded with 🙈. and if-Wfatal-errorsis also specified? then-Wfatal-errorstakes precedence over this option. it said what it said.-w— inhibit ALL warning messages. every single one. silenced. 💀-Werror— make every warning into an error. brutal. iconic. no mercy 👑-Werror=— make the specified warning into an error. you append the specifier for the warning; for example-Werror=switchturns the warnings controlled by-Wswitchinto errors. this switch has a negative form too, so you can negate-Werrorfor specific warnings; for example-Wno-error=switchmakes-Wswitchwarnings NOT errors, even when-Werroris in effect. so you can have the strict vibe without fully committing.
the warning message for each controllable warning includes the option that controls it — that option can then be used with -Werror= and -Wno-error= exactly as described above. (and if seeing the option in the warning message annoys you, you can turn that off with the -fno-diagnostics-show-option flag.)
heads up bestie: specifying-Werror=fooautomatically implies-Wfoo. but-Wno-error=foodoes not imply anything. those are not the same situation, do not conflate them.
-Wfatal-errors— makes the compiler abort compilation on the first error that occurs, instead of trying to keep going and printing more error messages. one strike and it's out 🫶
you can request a ton of specific warnings with options starting with -W, like -Wimplicit to request warnings on implicit declarations. and every one of these specific warning options also has a negative form starting with -Wno- to switch warnings off, like -Wno-implicit. this manual only lists one of the two forms — whichever one isn't the default. for more language-specific options you should also go peek at C++ Dialect Options and Objective-C and Objective-C++ Dialect Options.
some options, like -Wall and -Wextra, turn on other options, like -Wunused, which can turn on further options, like -Wunused-value. it's giving chain reaction ✨. the combined effect of positive and negative forms is that more specific options have priority over less specific ones, no matter where they sit in the command-line. for options of the same specificity, the last one takes effect. and options enabled or disabled via pragmas take effect as if they appeared at the end of the command-line.
when an unrecognized warning option gets requested (like, say, -Wunknown-warning), GCC emits a diagnostic saying it doesn't recognize that option. BUT — and this is the plot twist — if the -Wno- form is used, the behavior is slightly different: no diagnostic is produced for -Wno-unknown-warning unless other diagnostics are being produced anyway. this is so you can use new -Wno- options with old compilers, but if something goes wrong, the compiler will warn you that an unrecognized option is present. considerate tbh 🌸
the effectiveness of some warnings depends on optimizations also being enabled. for example -Wsuggest-final-types is more effective with link-time optimization, and -Wmaybe-uninitialized will not warn at all unless optimization is enabled. so like... turn on your optimizations or those warnings are just gonna stand there silently 🙈
-Wpedantic, -pedantic
issue all the warnings demanded by strict ISO C and ISO C++ 🎀. reject all programs that use forbidden extensions, plus some other programs that don't follow ISO C and ISO C++ either. for ISO C, it follows the version of the ISO C standard specified by whatever -std option you used. strict is the moment, apparently.
Okay so valid ISO C and ISO C++ programs should compile properly with or without this option — though ngl a rare few of them do require -ansi or a -std option that spells out the version of ISO C they need. But here's the tea bestie: without this option, certain GNU extensions and traditional C and C++ features are supported as well. With this option? They get rejected. periodt. 💅
-Wpedantic does not cause warning messages for use of the alternate keywords whose names begin and end with __. Pedantic warnings are also disabled in the expression that follows __extension__. But okay, only system header files should use these escape routes — application programs should avoid them. fr fr. 🫶
Some users try to use -Wpedantic to check programs for strict ISO C conformance. They soon find that it does not do quite what they want 😭 it finds some non-ISO practices, but not all — only those for which ISO C requires a diagnostic, and some others for which diagnostics have been added. it's giving "close but not quite".
A feature to report any failure to conform to ISO C might be useful in some instances, but it would require considerable additional work and would be quite different from -Wpedantic. We don't have plans to support such a feature in the near future. 🙈
Where the standard specified with -std represents a GNU extended dialect of C, such as gnu90 or gnu99, there is a corresponding base standard, the version of ISO C on which the GNU extended dialect is based. Warnings from -Wpedantic are given where they are required by the base standard. (It does not make sense for such warnings to be given only for features not in the specified GNU C dialect, since by definition the GNU dialects of C include all features the compiler supports with the given option, and there would be nothing to warn about.)
-pedantic-errors— Give an error whenever the base standard (see-Wpedantic) requires a diagnostic, in some cases where there is undefined behavior at compile-time, and in some other cases that do not prevent compilation of programs that are valid according to the standard. Heads up: this is not equivalent to-Werror=pedantic, since there are errors enabled by this option and not enabled by the latter, and vice versa. 💀-Wall— This enables all the warnings about constructions that some users consider questionable, and that are easy to avoid (or modify to prevent the warning), even in conjunction with macros. This also enables some language-specific warnings described in C++ Dialect Options and Objective-C and Objective-C++ Dialect Options. the classic starter pack, iykyk ✨
-Wall turns on the following warning flags:
-Waddress-Warray-bounds=1(only with-O2)-Wbool-compare-Wbool-operation-Wc++11-compat-Wc++14-compat-Wcatch-value(C++ and Objective-C++ only)-Wchar-subscripts-Wcomment-Wduplicate-decl-specifier(C and Objective-C only)-Wenum-compare(in C/ObjC; this is on by default in C++)-Wformat-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++)-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-Wreorder-Wrestrict-Wreturn-type-Wsequence-point-Wsign-compare(only in C++)-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-Wvolatile-register-var
Note that some warning flags are not implied by -Wall. Some of them warn about constructions that users generally do not consider questionable, but which occasionally you might wish to check for; others warn about constructions that are necessary or hard to avoid in some cases, and there is no simple way to modify the code to suppress the warning. Some of them are enabled by -Wextra but many of them must be enabled individually. 🎀
-Wextra— This enables some extra warning flags that are not enabled by-Wall. (This option used to be called-W. The older name is still supported, but the newer name is more descriptive. glow-up fr 🌈)-Wclobbered-Wcast-function-type-Wdeprecated-copy(C++ only)-Wempty-body-Wignored-qualifiers-Wimplicit-fallthrough=3-Wmissing-field-initializers-Wmissing-parameter-type(C only)-Wold-style-declaration(C only)-Woverride-init-Wsign-compare(C only)-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-Wunusedor-Wall)-Wunused-but-set-parameter(only with-Wunusedor-Wall)
The option -Wextra also prints warning messages for the following cases:
- A pointer is compared against integer zero with
"<","<=",">", or">=". - (C++ only) An enumerator and a non-enumerator both appear in a conditional expression.
- (C++ only) Ambiguous virtual bases.
- (C++ only) Subscripting an array that has been declared
"register". - (C++ only) Taking the address of a variable that has been declared
"register". - (C++ only) A base class is not initialized in the copy constructor of a derived class.
-Wchar-subscripts— Warn if an array subscript has type"char". This is a common cause of error, as programmers often forget that this type is signed on some machines. This warning is enabled by-Wall. 🙈-Wno-coverage-mismatch— Warn if feedback profiles do not match when using the-fprofile-useoption. If a source file is changed between compiling with-fprofile-generateand with-fprofile-use, the files with the profile feedback can fail to match the source file and GCC cannot use the profile feedback information. By default, this warning is enabled and is treated as an error.-Wno-coverage-mismatchcan be used to disable the warning, or-Wno-error=coverage-mismatchcan be used to disable the error. Disabling the error for this warning can result in poorly optimized code and is useful only in the case of very minor changes such as bug fixes to an existing code-base. Completely disabling the warning is not recommended. > bestie do NOT just silence this one, it's there for a reason 😤-Wno-cpp— (C, Objective-C, C++, Objective-C++ and Fortran only)
Suppress warning messages emitted by "#warning" directives.
-Wdouble-promotion(C, C++, Objective-C and Objective-C++ only) — Give a warning when a value of type"float"is implicitly promoted to"double". CPUs with a 32-bit "single-precision" floating-point unit implement"float"in hardware, but emulate"double"in software. On such a machine, doing computations using"double"values is much more expensive because of the overhead required for software emulation. 💸
It is easy to accidentally do computations with "double" because floating-point literals are implicitly of type "double". For example, in:
float area(float radius)
{
return 3.14159 * radius * radius;
}the compiler performs the entire computation with "double" because the floating-point literal is a "double". not me accidentally making it slow 😭
-Wduplicate-decl-specifier(C and Objective-C only) — Warn if a declaration has duplicate"const","volatile","restrict"or"_Atomic"specifier. This warning is enabled by-Wall. ✨-Wformat,-Wformat=n— Check calls to"printf"and"scanf", etc., to make sure that the arguments supplied have types appropriate to the format string specified, and that the conversions specified in the format string make sense. This includes standard functions, and others specified by format attributes, in the"printf","scanf","strftime"and"strfmon"(an X/Open extension, not in the C standard) families (or other target-specific families). Which functions are checked without format attributes having been specified depends on the standard version selected, and such checks of functions without the attribute specified are disabled by-ffreestandingor-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. 👑-Wno-format-contains-nul— If-Wformatis specified, do not warn about format strings that contain NUL bytes.-Wno-format-extra-args— If-Wformatis 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— bestie, this one's here to catch calls to formatted input/output functions likesprintfandvsprintfthat might overflow the destination buffer 💀. When the exact number of bytes a format directive writes can't be figured out at compile-time, GCC estimates it using heuristics that depend on thelevelargument and on optimization. And like — turning on optimization will in most cases make the warning more accurate, but it can also hit you with false positives. No cap.-Wformat-overflow-Wformat-overflow=1— Level 1 of-Wformat-overflow, which is what-Wformatenables, is the chill conservative approach: it only warns 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 are assumed 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. So for example, the call tosprintfbelow gets diagnosed because even with bothaandbequal to zero, the terminating NUL character ('\0') that the function appends to the destination buffer will be written past its end 😭. Bumping the buffer size up by a single byte is enough to dodge the warning, though ngl it may not be enough to actually avoid the overflow.
void f (int a, int b)
{
char buf [13];
sprintf (buf, "a = %i, b = %i\n", a, b);
}-Wformat-overflow=2— Level 2 is the main-character version: it also warns 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 can't 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 diagnosed again, 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, you'd have to grow the destination buffer to at least 34 bytes. GCC includes the minimum size of the buffer in an informational note following the warning, which is genuinely so helpful of it 💅.
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. iconic behavior honestly:
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, don't warn about zero-length formats. The C standard specifies that zero-length formats are allowed, so like, it's fine 🫶.-Wformat=2— turns on-Wformatplus some additional format checks. Currently equivalent to-Wformat -Wformat-nonliteral -Wformat-security -Wformat-y2k. The whole bundle, periodt.-Wformat-nonliteral— if-Wformatis specified, also warn if the format string is not a string literal and so can't be checked, unless the format function takes its format arguments as ava_list.-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.) Keep an eye on that one fr.-Wformat-signedness— if-Wformatis specified, also warn if the format string requires an unsigned argument and the argument is signed, and vice versa. Mismatched vibes detected 💅.-Wformat-truncation-Wformat-truncation=level— warns 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 can't be determined at compile-time, it's estimated based on heuristics that depend on thelevelargument 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, this option uses the same logic as-Wformat-overflow.-Wformat-truncation-Wformat-truncation=1— Level 1 of-Wformat-truncation, enabled by-Wformat, employs a conservative approach that warns only about calls to bounded functions whose return value is unused and that will most likely result in output truncation.-Wformat-truncation=2— Level 2 also warns 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. the way that 1999 really said "we're not doing four digits" 😭
MORE WARNINGS, WE're still going 💖</p>
-Wnonnull— warns 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— warns when comparing an argument marked with thenonnullfunction attribute against null inside the function.
-Wnonnull-compare is included in -Wall. It can be disabled with the -Wno-nonnull-compare option.
-Wnull-dereference— warns 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.-Winit-self(C, C++, Objective-C and Objective-C++ only) — warns about uninitialized variables that are initialized with themselves. Note that this option can only be used with the-Wuninitializedoption.
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++. truly "i am my own worst enemy" energy 🙈
-Wimplicit-int(C and Objective-C only) — warns when a declaration doesn't specify a type. This warning is enabled by-Wall.-Wimplicit-function-declaration(C and Objective-C only) — gives a warning whenever a function is used before being declared. In C99 mode (-std=c99or-std=gnu99), this warning is enabled by default and it's made into an error by-pedantic-errors. This warning is also enabled by-Wall.-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=3, and-Wno-implicit-fallthroughis the same as-Wimplicit-fallthrough=0.-Wimplicit-fallthrough=n— warns 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 can't fall through, e.g. when there's a return statement or a call to a 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 falling through is genuinely desirable, GCC provides an attribute, __attribute__ ((fallthrough)), that's meant 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:
...
}okay so C++17 hands you a standard way to shush the -Wimplicit-fallthrough warning: just write [[fallthrough]]; instead of the GNU attribute. ✨ And if you're stuck on C++11 or C++14, you can use [[gnu::fallthrough]];, which is a GNU extension. But honestly? Instead of either attribute, you can also just drop a fallthrough comment in there to silence the warning. The whole body of the C or C++ style comment has to match one of the regular expressions listed below, bestie. The option argument n is what tells you which kind of comments get accepted:
-Wimplicit-fallthrough=0— disables the warning altogether, bye 👋-Wimplicit-fallthrough=1— matches the".*"regular expression, so literally any comment counts as a fallthrough comment.-Wimplicit-fallthrough=2— case insensitively matches the".*falls?[ \t-]*thr(ough|u).*"regular expression.-Wimplicit-fallthrough=3— case sensitively matches one of the following regular expressions:"-fallthrough""@fallthrough@""lint -fallthrough[ \t]*""[ \t.!]*(ELSE,? |INTENTIONAL(LY)? )?FALL(S | |-)?THR(OUGH|U)[ \t.!]*(-[^\n\r]*)?""[ \t.!]*(Else,? |Intentional(ly)? )?Fall((s | |-)[Tt]|t)hr(ough|u)[ \t.!]*(-[^\n\r]*)?""[ \t.!]*([Ee]lse,? |[Ii]ntentional(ly)? )?fall(s | |-)?thr(ough|u)[ \t.!]*(-[^\n\r]*)?"
-Wimplicit-fallthrough=4— case sensitively matches one of the following regular expressions:"-fallthrough""@fallthrough@""lint -fallthrough[ \t]*""[ \t]*FALLTHR(OUGH|U)[ \t]*"
-Wimplicit-fallthrough=5— doesn't recognize any comments as fallthrough comments. Only attributes get to disable the warning. periodt 💅
And the comment has 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. Like so:
switch (cond)
{
case 1:
bar (0);
/* FALLTHRU */
default:
...
}The -Wimplicit-fallthrough=3 warning is enabled by -Wextra. 👑
-Wif-not-aligned(C, C++, Objective-C and Objective-C++ only) — controls whether the warning triggered by thewarn_if_not_alignedattribute gets issued at all. This one is on by default, so if you want it gone, hit it with-Wno-if-not-aligned. 🫶-Wignored-qualifiers(C and C++ only) — warns you if the return type of a function has a type qualifier likeconst. For ISO C, that kind of type qualifier does nothing, because the value a function returns isn't an lvalue. For C++, the warning only fires for scalar types orvoid. And heads up: ISO C prohibits qualifiedvoidreturn types on function definitions, so those return types always get a warning, even if you never pass this option. 😭
This warning is also enabled by -Wextra.
-Wignored-attributes(C and C++ only) — warns when an attribute is ignored. Now, this is different from the-Wattributesoption, because it warns whenever the compiler decides to drop an attribute — not because the attribute is unknown, or used in a wrong place, etc. This warning is enabled by default.-Wmain— warns if the type ofmainis suspicious.mainshould be a function with external linkage, returning int, and 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) — warns when the indentation of your code does not reflect the block structure. Specifically, it fires forif,else,while, andforclauses with a guarded statement that doesn't use braces, followed by an unguarded statement sitting at the same indentation. In this example, the call tobaris misleadingly indented as if theifconditional were guarding it:
if (some_condition ())
foo ();
bar (); /* Gotcha: this is not guarded by the "if". */When you've got mixed 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:
if (flagA)
foo (0);
#if SOME_CONDITION_THAT_DOES_NOT_HOLD
if (flagB)
#endif
foo (1);And it's not issued after a #line directive either, since that usually means autogenerated code, and you can't make any assumptions about the layout of the file the directive points at. 🧚
This warning is enabled by -Wall in C and C++.
-Wmissing-attributes— warns when a declaration of a function is missing one or more attributes that a related function is declared with, where that absence may adversely affect the correctness or efficiency of the generated code. For example, the warning fires for declarations of aliases that use attributes to specify less restrictive requirements than their targets do. That 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 here 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 gets 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— warns if an aggregate or union initializer isn't fully bracketed. In the example below, the initializer foraisn't fully bracketed, but the one forbis. This warning is enabled by-Wallin C. 🎀
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 and Objective-C++ only) — warns if a user-supplied include directory does not exist. simple as that 💀-Wmissing-profile— warns if feedback profiles are missing when you're using the-fprofile-useoption. It diagnoses those cases where a new function or a new file gets added to the user code between compiling with-fprofile-generateand compiling with-fprofile-use, without regenerating the profiles. In those cases, the profile feedback data files contain no profile feedback information for the newly added function or file, respectively. Also, when profile count data (.gcda) files get removed, GCC can't use any profile feedback information. In all these cases, warnings are issued to let you know a profile generation step is due. You can use-Wno-missing-profileto disable the warning. And listen — ignoring the warning can result in poorly optimized code. Completely disabling the warning is not recommended, and you should only do it when non-existent profile data is justified. 🚨-Wmultistatement-macros— warns about unsafe multiple statement macros that look like they're guarded by a clause such asif,else,for,switch, orwhile, when actually only the first statement is guarded after the macro expands. For example:
#define DOIT x++; y++
if (c)
DOIT;will increment y unconditionally, not just when c holds. This 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— warns 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 in ways whose precedence people often get confused about.
It also warns if a comparison like x<=y<=z shows up; that's equivalent to (x<=y ? 1 : 0) <= z, which is a whole different interpretation from what ordinary mathematical notation would have you believe. 🙈
It also warns for dangerous uses of the GNU extension to ?: with the middle operand omitted. When the condition in the ? : operator is a boolean expression, the omitted value is always 1 — but often programmers expect it to be a value computed inside the conditional expression instead.
For C++ it also warns about 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— warns about code that may have undefined semantics because of violations of sequence point rules in the C and C++ standards. undefined behaviour is not a vibe, bestie 💅
Okay so real talk bestie, the C and C++ standards have this whole sequence points thing goin' on, and it's basically the ✨official timeline✨ of when stuff in your program actually happens. Like, a sequence point is a partial ordering between the parts of your program: the bits that run before the sequence point, and the bits that run after it. It's giving very much "we have a schedule and we're sticking to it" 💅
Where do these little checkpoint moments show up? After evaluating a full expression (meaning one that's not part of a bigger expression), after evaluating the first operand of a "&&", "||", "? :" or "," (comma) operator, before a function gets called (but after its arguments and the expression naming the called function are evaluated), and in certain other places. Iconic behavior honestly 🎀
But here's the tea: other than what the sequence point rules say, the order of evaluation of subexpressions inside an expression is not specified. All these rules only describe a partial order, not a total order — like, if two functions get called inside one expression with no sequence point between them, the order they get called in is just... vibes. Not specified. However, the standards committee has ruled that function calls do not overlap. Periodt. 👑
So when do modifications to objects actually take effect between sequence points? Not specified either, bestie. Programs that depend on this have undefined behavior. The C and C++ standards say, and I quote:
"Between the previous and next sequence point an object shall have its stored value modified at most once by the evaluation of an expression. Furthermore, the prior value shall be read only to determine the value to be stored."
If your program breaks these rules, the results on any particular implementation are entirely unpredictable 😭 like genuinely, good luck, thoughts and prayers.
Examples of code with undefined behavior? a = a++;, a[n] = b[n++] and a[i++] = i;. Some more complicated cases are not diagnosed by this option, and it may give an occasional false positive result, but in general it's been found fairly effective at detecting this sort of problem in programs. So like... it's trying its best, okay? 🫶
The C++17 standard will define the order of evaluation of operands in more cases: specifically it requires that the right-hand side of an assignment be evaluated before the left-hand side, so the above examples are no longer undefined. But this warning will still warn about them, to help people avoid writing code that's undefined in C and earlier revisions of C++. Thoughtful king behavior honestly ✨
The standard is worded confusingly, so there's some debate over the precise meaning of the sequence point rules in subtle cases. Links to discussions of the problem (including proposed formal definitions) may be found on the GCC readings page, at <http://gcc.gnu.org/readings.html>. iykyk 📖
This warning is enabled by -Wall for C and C++. 💖
-Wno-return-local-addr— bestie, do not warn about returning a pointer (or in C++, a reference) to a variable that goes out of scope after the function returns. Like okay we're just not gonna talk about it, live your truth 🙈-Wreturn-type— Warn whenever a function is defined with a return type that defaults to"int". Also warn about any"return"statement with no return value in a function whose return type is not"void"(falling off the end of the function body counts as returning without a value, btw).
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, the latter case is accepted without a warning unless -Wpedantic is used. Attempting to use the return value of a non-"void" function other than "main" that flows off the end by reaching the closing curly brace that terminates the function is undefined. 😬
Unlike in C, in C++, flowing off the end of a non-"void" function other than "main" results in undefined behavior even when the value of the function is not used. Rude.
This warning is enabled by default in C++ and by -Wall otherwise. 💜
-Wshift-count-negative— Warn if shift count is negative. This warning is enabled by default. Simple. Clean. We love a default. ✨-Wshift-count-overflow— Warn if shift count >= width of type. This warning is enabled by default. 💅-Wshift-negative-value— Warn if left shifting a negative value. This warning is enabled by-Wextrain C99 (and newer) and C++11 to C++17 modes. 🌀-Wshift-overflow-Wshift-overflow=n— Warn about left shift overflows. This warning is enabled by default in C99 and C++11 modes (and newer). Multiple warning levels, we're fancy.-Wshift-overflow=1— This is the warning level of-Wshift-overflowand is enabled by default in C99 and C++11 modes (and newer). This warning level does not warn about left-shifting 1 into the sign bit. (However, in C, such an overflow is still rejected in contexts where an integer constant expression is required.) No warning is emitted in C++2A mode (and newer), as signed left shifts always wrap. 🔄-Wshift-overflow=2— This warning level also warns about left-shifting 1 into the sign bit, unless C++14 mode (or newer) is active. 🔥
-Wswitch— Warn whenever a"switch"statement has an index of enumerated type and lacks a"case"for one or more of the named codes of that enumeration. (The presence of a"default"label prevents this warning, bestie — it's like a "we're good" pass.)"case"labels outside the enumeration range also provoke warnings when this option is used (even if there's a"default"label). This warning is enabled by-Wall. 🎀-Wswitch-default— Warn whenever a"switch"statement does not have a"default"case. Like, cover your bases babe. 💅-Wswitch-enum— Warn whenever a"switch"statement has an index of enumerated type and lacks a"case"for one or more of the named codes of that enumeration."case"labels outside the enumeration range also provoke warnings when this option is used. The only difference between-Wswitchand this option is that this option gives a warning about an omitted enumeration code even if there is a"default"label. Stricter queen energy. 👑-Wswitch-bool— Warn whenever a"switch"statement has an index of boolean type and the case values are outside the range of a boolean type. It's possible to suppress this warning by casting the controlling expression to a type other than"bool". For example:
switch ((int) (a == 4))
{
...
}This warning is enabled by default for C and C++ programs. 🌈
-Wswitch-unreachable— Warn whenever a"switch"statement contains statements between the controlling expression and the first case label, which will never be executed. Dead code alert!! For example:
switch (cond)
{
i = 15;
...
case 5:
...
}-Wswitch-unreachable does not warn if the statement between the controlling expression and the first case label is just a declaration:
switch (cond)
{
int i;
...
case 5:
i = 5;
...
}This warning is enabled by default for C and C++ programs. ✨
-Wsync-nand(C and C++ only) — Warn when"__sync_fetch_and_nand"and"__sync_nand_and_fetch"built-in functions are used. These functions changed semantics in GCC 4.4. Semantics glow-up gone wrong. 😭-Wunused-but-set-parameter— Warn whenever a function parameter is assigned to, but otherwise unused (aside from its declaration). Like bestie you set it but never used it, what was the point 😤
To suppress this warning use the "unused" attribute.
This warning is also enabled by -Wunused together with -Wextra. 💜
-Wunused-but-set-variable— Warn whenever a local variable is assigned to, but otherwise unused (aside from its declaration). This warning is enabled by-Wall.
To suppress this warning use the "unused" attribute.
This warning is also enabled by -Wunused, which is enabled by -Wall. 🦋
-Wunused-function— Warn whenever a static function is declared but not defined or a non-inline static function is unused. This warning is enabled by-Wall. 💅-Wunused-label— Warn whenever a label is declared but not used. This warning is enabled by-Wall.
To suppress this warning use the "unused" attribute. 🎀
-Wunused-local-typedefs(C, Objective-C, C++ and Objective-C++ only) — Warn when a typedef locally defined in a function is not used. This warning is enabled by-Wall. ✨-Wunused-parameter— Warn whenever a function parameter is unused aside from its declaration. We see that parameter just sitting there, doing nothing.
To suppress this warning use the "unused" attribute. 💖
-Wno-unused-result— Do not warn if a caller of a function marked with attribute"warn_unused_result"does not use its return value. The default is-Wunused-result. 🙈-Wunused-variable— Warn whenever a local or static variable is unused aside from its declaration. This option implies-Wunused-const-variable=1for C, but not for C++. This warning is enabled by-Wall.
To suppress this warning use the "unused" attribute. 💜
-Wunused-const-variable-Wunused-const-variable=n— Warn whenever a constant static variable is unused aside from its declaration.-Wunused-const-variable=1is enabled by-Wunused-variablefor C, but not for C++. In C this declares variable storage, but in C++ this is not an error since const variables take the place of"#define"s. Language differences, smh.
To suppress this warning use the "unused" attribute.
-Wunused-const-variable=1— This is the warning level that is 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 must be explicitly requested since in C++ this isn't an error and in C it might be harder to clean up all headers included. Deep cleaning mode activated. 🧹
-Wunused-value— Warn whenever a statement computes a result that is explicitly not used. To suppress this warning cast the unused expression to"void". This includes an expression-statement or the left-hand side of a comma expression that contains no side effects. For example, an expression such as"x[i,j]"causes a warning, while"x[(void)i,j]"does not. Clever girl workaround. 💅
This warning is enabled by -Wall. 🔥
-Wunused— All the above-Wunusedoptions combined. The whole collection, bestie. 👑
Like, if you are begging for a warning about a function parameter that nobody ever touches? Bestie, you have to pick a lane: either roll up with -Wextra -Wunused together (and lowkey note that -Wall already implies -Wunused, so don't double up thinking you're clever), or just specify -Wunused-parameter all on its own. ✨
-Wuninitialized— the "bestie you forgot to give this variable a value" alarm. 🔔 It yells if an automatic variable gets used before it's ever initialized, or if a variable might get clobbered by asetjmpcall. And in C++, it also warns when a non-static reference or a non-staticconstmember shows up in a class that has no constructors. Wild, right?
Wanna also catch code that uses the uninitialized value of a variable in its own initializer? That's a different vibe, so go grab -Winit-self. 🫶
These warnings pop off for individual uninitialized or clobbered elements of structure, union or array variables, and for variables that are uninitialized or clobbered as a whole. They do not happen for variables or elements declared volatile. And because these warnings depend on optimization, which exact variables or elements get flagged depends on the precise optimization options and the version of GCC you're running. 😭
Also heads up: there may be no warning at all about a variable that's only used to compute a value that itself is never used, because those computations may get deleted by data flow analysis before the warnings even get printed. Sneaky. 💀
-Winvalid-memory-model— warns when you invoke__atomicBuiltins,__syncBuiltins, or the C11 atomic generic functions with a memory consistency argument that's either invalid for the operation or straight-up outside the range of values of thememory_orderenumeration. 🚨
Example time, because this one's so much clearer with an example: since __atomic_store and __atomic_store_n built-ins are only defined for the relaxed, release, and sequentially consistent memory orders, this little guy below gets diagnosed:
void store (int *i)
{
__atomic_store_n (i, 0, memory_order_consume);
}-Winvalid-memory-model is enabled by default. 👑
-Wmaybe-uninitialized— okay so for an automatic (i.e. local) variable: if there's a path from the function entry to a use of the variable where it is initialized, but there also exist some other paths where it's not initialized, the compiler emits a warning if it can't prove the uninitialized paths never get executed at run time. It's giving "probably fine, but I'm not signing off on it." 🙈
These warnings are only possible in optimizing compilation, because otherwise GCC isn't keeping track of the state of variables at all. No notes, that's just how it is.
They're made optional because GCC may not be able to figure out when the code is correct even though it looks like it has an error. Here's one example of how that 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 that. 🤷 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 only sees 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. So you may get a warning even when there's genuinely no problem, because longjmp cannot in fact be called at the place that would cause one. 😮💨
Some spurious warnings can be avoided if you declare all the functions you use that never return as noreturn. Pro tip of the day. 💅
This warning is enabled by -Wall or -Wextra.
-Wunknown-pragmas— warns when a#pragmadirective shows up that GCC does not understand. And if you use this command-line option, warnings are even issued for unknown pragmas inside system header files. That is not the case if the warnings are only enabled by the-Wallcommand-line option — so like, that's the difference, iykyk. 👀-Wno-pragmas— do not warn about misuses of pragmas, like 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 a constructor or destructor. The use of constructor and destructor attributes lets you 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, because the compiler reserves priority values between 0--100 for the implementation. Like, that range is taken, bestie, don't touch it. 🚫-Wstrict-aliasing— this one's only active when-fstrict-aliasingis active. It warns about code that might break the strict aliasing rules the compiler is using for optimization. It doesn't catch all cases, but it does attempt to catch the more common pitfalls. It's included in-Wall, and it is equivalent to-Wstrict-aliasing=3. 🎀-Wstrict-aliasing=n— also only active when-fstrict-aliasingis active. Warns about code that might break the strict aliasing rules 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. And-Wstrict-aliasingis equivalent to-Wstrict-aliasing=3.
Let's break the levels down, because the way that this scales is honestly kind of iconic:
- Level 1: Most aggressive, quick, least accurate. Possibly useful when higher levels don't 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. the moment. 💖
-Wstrict-overflow-Wstrict-overflow=n— this one's 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. So this warning depends on the optimization level. 🌀
An optimization that assumes signed overflow doesn't occur is perfectly safe if the values of the variables involved are such that overflow never does, in fact, occur. So this warning can easily give a false positive: a warning about code that isn't actually a problem. To help you focus on the 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 > xto 1. 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. And-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. 😵💫
-Wstringop-overflow-Wstringop-overflow=type— warns 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 determine the size of the destination. The argument is meaningful only for functions that operate on character arrays, but not for raw memory functions likememcpy, which always make use of Object Size type-0. 👑
The option also warns for calls that specify a size in excess of the largest possible object, or at most SIZE_MAX / 2 bytes. It produces the best results with optimization enabled, but it can detect a small subset of simple buffer overflows even without optimization in calls to the GCC built-in functions like __builtin_memcpy that correspond to the standard functions. In any case, the option warns about just a subset of buffer overflows detected by the corresponding overflow checking built-ins. 🫠
For example, the option will issue a warning for the strcpy call below because it copies at least 5 characters (the string "blue", including the terminating NUL) into the buffer of size 4.
enum Color { blue, purple, yellow };
const char* f (enum Color clr)
{
static char buf [4];
const char *str;
switch (clr)
{
case blue: str = "blue"; break;
case purple: str = "purple"; break;
case yellow: str = "yellow"; break;
}
return strcpy (buf, str); // warning here
}see that little // warning here comment? yeah. that's the whole point. this is the code that gets flagged, bestie, and the compiler is literally just doing its job 💅
Option -Wstringop-overflow=2 is enabled by default.
-Wstringop-overflow,-Wstringop-overflow=1— okay so level one uses type-zero Object Size Checking to figure out how big the destination objects are. and like, this is the default setting of the option, no cap. at this level it will not warn you about writes that go past the end of subobjects of larger objects when they're accessed through pointers — unless the size of the largest surrounding object is actually known. and if the destination might be one of several objects? it just assumes it's the biggest one of them 🫶. on Linux systems, when optimization is turned on, at this setting the option warns for the exact same code as when the_FORTIFY_SOURCEmacro is defined to a non-zero value. iconic.-Wstringop-overflow=2— level two steps it up and uses type-one Object Size Checking for the destination sizes. at this setting it warns about overflows when you're writing to members of the largest complete objects whose exact size is known ✨. but — and this is the catch — it will not warn for excessive writes to those same members of unknown objects referenced by pointers, because like, those pointers might be pointing at arrays with unknown numbers of elements. it's giving "we can't prove it so we're not gonna say anything" 😭-Wstringop-overflow=3— level three goes full detective mode with type-two Object Size Checking. at this setting it warns about overflowing the smallest object or data member. and heads up: this is the most restrictive setting of the option, which means it may result in warnings for code that's actually safe 🙈-Wstringop-overflow=4— and level four? type-three Object Size Checking, periodt. 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 of them to decide whether to warn. same energy as-Wstringop-overflow=3though: this setting may also result in warnings for benign code 💀
-Wstringop-truncation— this one warns you about calls to bounded string manipulation functions likestrncat,strncpy, andstpncpythat might either truncate the copied string or leave the destination unchanged. messy.
okay so in the example below, the call to strncat specifies a bound that's less than the length of the source string. so as a result the copy of the source gets truncated, and the call gets diagnosed. to dodge the warning, use bufsize - strlen (buf) - 1) as the bound:
void append (char *buf, size_t bufsize)
{
strncat (buf, ".txt", 3);
}as another example, this next call to strncpy copies to d just the characters before 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 too. to avoid the warning when you don't actually expect a NUL-terminated result, call memcpy instead:
void copy (char *d, const char *s)
{
strncpy (d, s, strlen (s));
}and here's a third one 🍓: this call to strncpy specifies the size of the destination buffer as the bound. if the length of the source string is equal to or greater than this size, the result of the copy will not be NUL-terminated. therefore the call is also diagnosed. to avoid the warning, specify sizeof buf - 1 as the bound and set the last element of the buffer to NUL:
void copy (const char *s)
{
char buf[80];
strncpy (buf, s, sizeof buf);
...
}and in situations where a character array is meant to store a sequence of bytes with no terminating NUL, you can annotate such an array with the attribute nonstring to avoid this warning. but — important — such arrays are not suitable arguments to functions that expect NUL-terminated strings. to help catch accidental misuses of those arrays, GCC issues warnings unless it can prove the use is safe 👑
-Wsuggest-attribute=[pure|const|noreturn|format|cold|malloc]— this warns you about cases where adding an attribute might be beneficial. the attributes currently supported are listed below 🌈-Wsuggest-attribute=pure-Wsuggest-attribute=const-Wsuggest-attribute=noreturn-Wmissing-noreturn-Wsuggest-attribute=malloc— these warn about functions that might be candidates for the attributespure,const,noreturnormalloc. 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 the option-fipa-pure-const, which is enabled by default at-Oand higher ✨. and higher optimization levels improve how accurate the analysis is.-Wsuggest-attribute=format-Wmissing-format-attribute— these warn about function pointers that might be candidates forformatattributes. note: these are only possible candidates, not absolute ones, bestie. GCC guesses that function pointers withformatattributes 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 this might not always be the case, and some functions for which format attributes are appropriate may not be detected. it's giving "we tried our best" 😭
-Wsuggest-attribute=cold— warns about functions that might be candidates for thecoldattribute. this is based on static detection and generally will only warn about functions which always lead to a call to anothercoldfunction — like wrappers of C++throw, or fatal error reporting functions leading toabort💀
-Wsuggest-final-types— warns about types with virtual methods where code quality would improve 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. this warning is more effective with link time optimization, where the info about the class hierarchy graph is more complete. slay.-Wsuggest-final-methods— warns about virtual methods where code quality would improve if the method were declared with the C++11finalspecifier, or, if possible, its type were declared in an anonymous namespace or with thefinalspecifier. this warning is more effective with link-time optimization, where the information about the class hierarchy graph is more complete 🦋. and the recommendation? first consider the suggestions of-Wsuggest-final-types, and then rebuild with the new annotations. order matters, iykyk.-Wsuggest-override— warns about overriding virtual functions that aren't marked with theoverridekeyword 👀-Walloc-zero— warns about calls to allocation functions decorated with the attributealloc_sizethat specify zero bytes, including calls to the built-in forms ofaligned_alloc,alloca,calloc,malloc, andrealloc💅. and why do we care? because the behavior of these functions when called with a zero size differs between implementations — and in the case ofreallocit's been deprecated — so relying on it may result in subtle portability bugs and should be avoided. periodt.-Walloc-size-larger-than=byte-size— warns about calls to functions decorated with the 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 this option can be disabled either by specifying byte-size ofSIZE_MAXor more, or by-Wno-alloc-size-larger-than.-Wno-alloc-size-larger-than— disables-Walloc-size-larger-than=warnings. this option is equivalent to-Walloc-size-larger-than=SIZE_MAXor larger.-Walloca— warns on all uses ofallocain the source. every single one. no exceptions 👑-Walloca-larger-than=byte-size— warns on calls toallocawith an integer argument whose value is either zero, or that isn't bounded by a controlling predicate limiting its value to at most byte-size. it also warns for calls toallocawhere the bound value is unknown 🙈. and arguments of non-integer types are considered unbounded even if they look like they're constrained to the expected range.
for example, a bounded case of alloca could be:
void func (size_t n)
{
void *p;
if (n <= 1000)
p = alloca (n);
else
p = malloc (n);
f (p);
}in the above example, passing -Walloca-larger-than=1000 would not issue a warning, because the call to alloca is known to be at most 1000 bytes 🎀. but if -Walloca-larger-than=500 were passed, the compiler would emit a warning. math is math, bestie.
Unbounded uses, on the other hand, are uses of alloca with no controlling predicate constraining its integer argument. Like, the vibe check just isn't there. For example:
void func ()
{
void *p = alloca (n);
f (p);
}If -Walloca-larger-than=500 were passed, the above would trigger a warning, but this time because of the lack of bounds checking. Not me getting called out by the compiler 💀.
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. It's giving "math is hard" energy 😭.
This option also warns when alloca is used in a loop. Periodt.
-Walloca-larger-than=PTRDIFF_MAX is enabled by default but is usually only effective when -ftree-vrp is active (default for -O2 and above).
See also -Wvla-larger-than=byte-size.
-Wno-alloca-larger-than— Disable-Walloca-larger-than=warnings. The option is equivalent to-Walloca-larger-than=SIZE_MAXor larger. Slay, no more alloca drama.-Warray-bounds,-Warray-bounds=n— This option is only active when-ftree-vrpis active (default for-O2and above). It warns about subscripts to arrays that are always out of bounds. This warning is enabled by-Wall. It's giving "you're not him" to out-of-bounds indices.-Warray-bounds=1— This is the warning level of-Warray-boundsand is enabled by-Wall; higher levels are not, and must be explicitly requested. Lowkey the default queen.-Warray-bounds=2— This warning level also warns about out of bounds access for arrays at the end of a struct and for arrays accessed through pointers. This warning level may give a larger number of false positives and is deactivated by default. Highkey dramatic, but off by default.
-Wattribute-alias=n,-Wno-attribute-alias— Warn about declarations using thealiasand similar attributes whose target is incompatible with the type of the alias. Type mismatch? Not on my watch.-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. Basically the "something's off" radar.-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. It's giving extra strict bestie energy. Attributes considered includealloc_align,alloc_size,cold,const,hot,leaf,malloc,nonnull,noreturn,nothrow,pure,returns_nonnull, andreturns_twice. ✨ The whole squad of attributes ✨-Wattribute-aliasis equivalent to-Wattribute-alias=1. This is the default. You can disable these warnings with either-Wno-attribute-aliasor-Wattribute-alias=0. Default vibes, but you can mute it.
-Wbool-compare— Warn about boolean expression compared with an integer value different fromtrue/false. For instance, the following comparison is always false: Caught in 4K lying about booleans.
int n = 5;
...
if ((n > 1) == 2) { ... }This warning is enabled by -Wall. Wall says hi.
-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.) Boolean manipulation? Sus. 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 warns for conditional operators: Why are you like this? 😭
int i = x ? *p : *p;-Wduplicated-cond— Warn about duplicated conditions in an if-else-if chain. For instance, warn for the following code: Copy-paste who? Never heard of her.
if (p->q != NULL) { ... }
else if (p->q != NULL) { ... }-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. Crash risk? Not the vibe.-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. Const-correctness? She's sensitive.-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. Array qualifiers, but make it muted.-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. Pointer types not matching? Not today.-Wno-int-conversion(C and Objective-C only) — Do not warn about incompatible integer to pointer and pointer to integer conversions. This warning is about implicit conversions; for explicit conversions the warnings-Wno-int-to-pointer-castand-Wno-pointer-to-int-castmay be used. Implicit tea? Spilled.-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. Float division by zero is valid, periodt.-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. System headers stay loud? Only if you ask.-Wtautological-compare— Warn if a self-comparison always evaluates to true or false. This warning detects various mistakes such as:
int i = 1;
...
if (i > i) { ... }This warning also warns about bitwise comparisons that always evaluate to true or false, for instance:
if ((a & 16) == 10) { ... }will always be false. Girl, that comparison is a whole lie. This warning is enabled by -Wall.
-Wtrampolines— Warn about trampolines generated for pointers to nested functions. A trampoline is a small piece of data or code that is created at run time on the stack when the address of a nested function is taken, and is used to call the nested function indirectly. For some targets, it is made up of data only and thus requires no special treatment. But, for most targets, it is made up of code and thus requires the stack to be made executable in order for the program to work properly. Trampolines? In my stack? It's more likely than you think.-Wfloat-equal— Warn if floating-point values are used in equality comparisons. Floating point equality? Bestie, no. 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. Range overlap > exact equality. Iykyk.-Wtraditional(C and Objective-C only) — Warn about certain constructs that behave differently in traditional and ISO C. Also warn about ISO C constructs that have no traditional C equivalent, and/or problematic constructs that should be avoided. Old school C? The lore is deep.- 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. String literal tea.
- 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. Column 1 or it didn't happen. - A function-like macro that appears without arguments. No args? Rude.
- The unary plus operator. Unary plus, lowkey unnecessary.
- 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. Suffix drama, but GCC gets it. - A function declared external in one block and then used after the end of the block. Block scope? She's leaving.
- A
switchstatement has an operand of typelong. Long switch? Interesting choice.
- A non-
staticfunction declaration that comes after astaticone. Some old-school C compilers straight up refuse this construct, bestie. 💀 - The ISO type of an integer constant has a different width or signedness than its traditional type. Heads up tho: this warning only pops off if the base of the constant is ten. So hex or octal values — which are usually just bit patterns in a trench coat — get no warning at all.
- Usage of ISO string concatenation is detected. ✨
- Initialization of automatic aggregates. 🫶
- Identifier conflicts with labels. Traditional C didn't have a separate namespace for labels, so yeah, collision vibes.
- Initialization of unions. If the initializer is zero, the warning is skipped. This is done under the assumption that the zero initializer in user code shows up conditioned on stuff like
__STDC__, to dodge missing-initializer warnings, and it 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 only a subset of the possible conversion warnings — for the full set use
-Wtraditional-conversion. - Use of ISO C style function definitions. This warning is intentionally not issued for prototype declarations or variadic functions, because those ISO C features show up in your code when you use libiberty's traditional C compatibility macros,
PARAMSandVPARAMS. It's also bypassed for nested functions, since that feature is already a GCC extension and thus not relevant to traditional C compatibility. iykyk 💅
-Wtraditional-conversion(C and Objective-C only) — Warns if a prototype causes a type conversion that's different from what would happen to the same argument with no prototype around. This includes conversions of fixed point to floating and vice versa, plus conversions that change the width or signedness of a fixed-point argument — except when it's the same as the default promotion.-Wdeclaration-after-statement(C and Objective-C only) — Warns when a declaration shows up after a statement in a block. This construct, which is very C++ of it, was introduced with ISO C99 and is by default allowed in GCC. ISO C90 does not support it.-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 gets shadowed. Note that in C++, the compiler warns if a local variable shadows an explicit typedef, but not if it shadows astruct/class/enum. 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— The default for-Wshadow. Warns for any (global) shadowing.-Wshadow=local— Warns when a local variable shadows another local variable or parameter. This warning is enabled by-Wshadow=global.-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 whole idea behind this flag (on top of-Wshadow=local) is that when a local variable shadows another one of an incompatible type, it's most likely intentional — not a bug, not a typo — like in this example:
for (SomeIterator i = SomeObj.begin(); i != SomeObj.end(); ++i)
{
for (int i = 0; i < N; ++i)
{
...
}
...
}Since the two variables i in the example above have incompatible types, enabling only -Wshadow=compatible-local won't emit a warning. And because their types are incompatible, if a programmer accidentally uses one in place of the other, type checking will catch it and emit an error or warning. So not warning (about shadowing) here won't lead to undetected bugs. Using this flag instead of -Wshadow=local can possibly cut down the number of warnings triggered by intentional shadowing. 💅
This warning is enabled by -Wshadow=local.
-Wlarger-than=byte-size— Warns whenever an object is defined whose size exceeds byte-size.-Wlarger-than=PTRDIFF_MAXis enabled by default. Warnings controlled by this option can be disabled either by specifying a byte-size ofSIZE_MAXor more, or by-Wno-larger-than.-Wno-larger-than— Disables-Wlarger-than=warnings. This option is equivalent to-Wlarger-than=SIZE_MAXor larger.-Wframe-larger-than=byte-size— Warns if the size of a function frame exceeds byte-size. The computation done to figure out the stack frame size is approximate and not conservative. The actual requirements may be somewhat greater than byte-size even if you don't get a warning. On top of that, any space allocated viaalloca, variable-length arrays, or related constructs is not included by the compiler when it decides whether or not to issue a warning.-Wframe-larger-than=PTRDIFF_MAXis enabled by default. Warnings controlled by this option can be disabled either by specifying a byte-size ofSIZE_MAXor more, or by-Wno-frame-larger-than.-Wno-frame-larger-than— Disables-Wframe-larger-than=warnings. This option is equivalent to-Wframe-larger-than=SIZE_MAXor larger.-Wno-free-nonheap-object— Do not warn when you attempt to free an object that wasn't allocated on the heap. 🚫-Wstack-usage=byte-size— Warns if the stack usage of a function might exceed byte-size. The computation done to figure out stack usage is conservative. Any space allocated viaalloca, variable-length arrays, or related constructs is included by the compiler when it decides whether or not to issue a warning. 🌈
The message matches the output of -fstack-usage.
- If the stack usage is fully static but exceeds the specified amount, it's:
warning: stack usage is 1120 bytes- If the stack usage is (partly) dynamic but bounded, it's:
warning: stack usage might be 1648 bytes- If the stack usage is (partly) dynamic and not bounded, it's:
warning: stack usage might be unbounded-Wstack-usage=PTRDIFF_MAX is enabled by default. Warnings controlled by this option can be disabled either by specifying a byte-size of SIZE_MAX or more, or by -Wno-stack-usage.
-Wno-stack-usage— Disables-Wstack-usage=warnings. This option is equivalent to-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. With-funsafe-loop-optimizations, warns if the compiler makes such assumptions. 🙈-Wno-pedantic-ms-format(MinGW targets only) — When used together with-Wformatand-pedanticwithout GNU extensions, this option disables the warnings about non-ISOprintf/scanfformat width specifiersI32,I64, andIused on Windows targets, which depend on the MS runtime.-Waligned-new— Warns about a new-expression of a type that requires greater alignment thanalignof(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. 👑
-Wplacement-new-Wplacement-new=n— Warns about placement new expressions with undefined behavior, like constructing an object in a buffer that's smaller than the type of the object. For example, the placement new expression below gets diagnosed because it tries to construct an array of 64 integers in a buffer that's only 64 bytes large. 💀
char buf [64];
new (buf) int[64];This warning is enabled by default.
-Wplacement-new=1— This is the default warning level of-Wplacement-new. At this level the warning is not issued for some strictly undefined constructs that GCC allows as extensions for compatibility with legacy code. For example, the followingnewexpression 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 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. While the previous example would get diagnosed, the following construct uses the flexible member array extension to dodge 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]();-Wpointer-arith— Warns about anything that depends on the "size of" a function type or ofvoid. GNU C assigns these types a size of 1, 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.-Wpointer-compare— Warns if a pointer is compared with a zero character constant. This usually means the pointer was meant to be dereferenced. Like this:
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.
-Wtype-limits— bestie, this one's out here catching comparisons that are always true or always false because of the limited range of the data type. It does not warn for constant expressions tho, so don't @ it. Like, if an unsigned variable gets compared against zero with<or>=? Yeah, warning. And guess what —-Wextraalso turns this one on. 💅-Wabsolute-value(C and Objective-C only) — this one yells at you when you call a standard function that computes the absolute value of an argument and there's a more appropriate standard function sitting right there. For example, callingabs(3.14)triggers the warning because the appropriate function to call to compute the absolute value of a double argument isfabs. She also triggers warnings when the argument in a call to such a function has an unsigned type. You can shush her with an explicit type cast, and she's also enabled by-Wextra. 🫶-Wcomment-Wcomments— warns whenever a comment-start sequence/*appears in a/*comment, or whenever a backslash-newline appears in a//comment. Enabled by-Wall. Nested comment chaos? Caught. 😭-Wtrigraphs— warns if any trigraphs are encountered that might change the meaning of the program. Trigraphs within comments are not warned about, except those that would form escaped newlines.
This option is implied by -Wall. If -Wall is not given, this option is still enabled unless trigraphs are enabled. To get trigraph conversion without warnings, but get the other -Wall warnings, use -trigraphs -Wall -Wno-trigraphs. ✨
-Wundef— warns if an undefined identifier is evaluated in an#ifdirective. Such identifiers are replaced with zero. Simple as that, no cap.-Wexpansion-to-defined— warns whenever defined is encountered in the expansion of a macro (including the case where the macro is expanded by an#ifdirective). Such usage is not portable. This warning is also enabled by-Wpedanticand-Wextra.-Wunused-macros— warns about macros defined in the main file that are unused. A macro is used if it is expanded or tested for existence at least once. The preprocessor also warns if the macro has not been used at the time it is redefined or undefined.
Built-in macros, macros defined on the command line, and macros defined in include files are not warned about.
Note: If a macro is actually used, but only used in skipped conditional blocks, then the preprocessor reports it as unused. To avoid the warning in such a case, you might improve the scope of the macro's definition by, for example, moving it into the first skipped block. Alternatively, you could provide a dummy use with something like:
#if defined the_macro_causing_the_warning
#endif(iykyk, that little dummy block is the sneakiest slay 💀)
-Wno-endif-labels— do not warn whenever an#elseor an#endifare 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 is cast to a non-matching type. For example, warn if a call to a function returning an integer type is cast to a pointer type. That's giving "ma'am what are you doing" energy fr fr. 🦋-Wc90-c99-compat(C and Objective-C only) — warns about features not present in ISO C90, but present in ISO C99. For instance, warn 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 not present in ISO C99, but present in ISO C11. For instance, warn 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 not present in ISO C11, but present in ISO C2X. For instance, warn about omitting the string in_Static_assert. 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 are outside of the common subset of ISO C and ISO C++, e.g. 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 in ISO C++ 1998 that 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.-Wcast-qual— warns whenever a pointer is cast so as to remove a type qualifier from the target type. For example, warn if aconst char *is cast to an ordinarychar *.
Also warns when making a cast that introduces a type qualifier in an unsafe way. For example, casting char ** to const char ** is unsafe, as in this example:
/* p is char ** value. */
const char **q = (const char **) p;
/* Assignment of readonly string to const char * is OK. */
*q = "string";
/* Now char** pointer points to read-only memory. */
**p = 'b';like. the audacity of that last line. 💀
-Wcast-align— warns whenever a pointer is cast such that the required alignment of the target is increased. For example, warn if achar *is cast to anint *on machines where integers can only be accessed at two- or four-byte boundaries.-Wcast-align=strict— warns whenever a pointer is cast such that the required alignment of the target is increased. For example, warn if achar *is cast to anint *regardless of the target machine. Same energy, zero chill.-Wcast-function-type— warns when a function pointer is cast to an incompatible function pointer. In a cast involving function types with a variable argument list only the types of initial arguments that are provided are considered. Any parameter of pointer-type matches any other pointer-type. Any benign differences in integral types are ignored, likeintvs.longon ILP32 targets. Likewise type qualifiers are ignored. The function typevoid (*) (void)is special and matches everything, which can be used to suppress this warning. In a cast involving pointer to member types this warning warns whenever the type cast is changing the pointer to member type. This warning is enabled by-Wextra.-Wwrite-strings— when compiling C, give string constants the typeconst char[length]so that copying the address of one into a non-constchar *pointer produces a warning. These warnings help you find at compile time code that can try to write into a string constant, but only if you have been very careful about usingconstin declarations and prototypes. Otherwise, it is just a nuisance. This is why we did not make-Wallrequest these warnings.
When compiling C++, warn about the deprecated conversion from string literals to char *. This warning is enabled by default for C++ programs.
-Wcatch-value-Wcatch-value=n(C++ and Objective-C++ only) — warns about catch handlers that do not catch via reference. With-Wcatch-value=1(or-Wcatch-valuefor short) warn about polymorphic class types that are caught by value. With-Wcatch-value=2warn about all class types that are caught by value. With-Wcatch-value=3warn about all types that are not caught by reference.-Wcatch-valueis enabled by-Wall. Levels? We love a tiered system. 👑-Wclobbered— warns for variables that might be changed bylongjmporvfork. This warning is also enabled by-Wextra.-Wconditionally-supported(C++ and Objective-C++ only) — warns for conditionally-supported (C++11 [intro.defs]) constructs.-Wconversion— warns for implicit conversions that may alter a value. This includes conversions between real and integer, likeabs (x)whenxisdouble; conversions between signed and unsigned, likeunsigned ui = -1; and conversions to smaller types, likesqrtf (M_PI). Do not warn for explicit casts likeabs ((int) x)andui = (unsigned) -1, or if the value is not changed by the conversion like inabs (2.0). Warnings about conversions between signed and unsigned integers can be disabled by using-Wno-sign-conversion.
For C++, also warn for confusing overload resolution for user-defined conversions; and conversions that never use a type conversion operator: conversions to void, the same type, a base class or a reference to them. Warnings about conversions between signed and unsigned integers are disabled by default in C++ unless -Wsign-conversion is explicitly enabled.
-Wno-conversion-null(C++ and Objective-C++ only) — do not warn for conversions betweenNULLand non-pointer types.-Wconversion-nullis enabled by default.-Wzero-as-null-pointer-constant(C++ and Objective-C++ only) — warns when a literal 0 is used as null pointer constant. This can be useful to facilitate the conversion tonullptrin C++11. glow-up arc fr. ✨-Wsubobject-linkage(C++ and Objective-C++ only) — warns if a class type has a base or a field whose type uses the anonymous namespace or depends on a type with no linkage. If a type A depends on a type B with no or internal linkage, defining it in multiple translation units would be an ODR violation because the meaning of B is different in each translation unit. If A only appears in a single translation unit, the best way to silence the warning is to give it internal linkage by putting it in an anonymous namespace as well. The compiler doesn't give this warning for types defined in the main .C file, as those are unlikely to have multiple definitions.-Wsubobject-linkageis enabled by default.
-Wdangling-else— bestie, this one is here to save you from the classic "wait, whichifdoes thatelsebelong to??" chaos. It warns about constructions where there may be confusion about whichifstatement anelsebranch belongs to. Here's an example of such a case:
{
if (a)
if (b)
foo ();
else
bar ();
}In C/C++, every else branch belongs to the innermost possible if statement, which in this example is if (b). That is often not what the programmer expected — as illustrated in the above example by the indentation the programmer chose (the indentation is lying to you, bestie 😭). 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 resulting code looks like this:
{
if (a)
{
if (b)
foo ();
else
bar ();
}
}This warning is enabled by -Wparentheses. 💅
-Wdate-time— warns when the macros__TIME__,__DATE__or__TIMESTAMP__show up, because they might prevent bit-wise-identical reproducible compilations. Reproducible builds girlies, this one's for you 🫶-Wdelete-incomplete(C++ and Objective-C++ only) — warns when you delete a pointer to an incomplete type, which may cause undefined behavior at runtime. 🚨 This warning is enabled by default.-Wuseless-cast(C++ and Objective-C++ only) — warns when an expression is casted to its own type. Like… why would you do that bestie, that's giving nothing ✨-Wempty-body— warns if an empty body occurs in anif,elseordo whilestatement. This warning is also enabled by-Wextra. A body with zero body? not the vibe 💀-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.-Wextra-semi(C++, Objective-C++ only) — warns about a redundant semicolon after an in-class function definition. extra semicolons are NOT extra in a cute way 😭-Wjump-misses-init(C, Objective-C only) — 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. This only warns about variables that are initialized when they are declared. This warning is only supported for C and Objective-C; in C++ this sort of branch is an error in any case.
-Wjump-misses-init is included in -Wc++-compat. It can be disabled with the -Wno-jump-misses-init option.
-Wsign-compare— warns when a comparison between signed and unsigned values could produce an incorrect result when the signed value is converted to unsigned. signed vs unsigned drama, iconic honestly In C++, this warning is also enabled by-Wall. In C, it is also enabled by-Wextra.-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. This option is also enabled by-Wconversion.-Wno-scalar-storage-order— do not warn on suspicious constructs involving reverse scalar storage order. 🙈-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. match your deletes, bestie, it's giving symmetry 💅
-Wsizeof-pointer-div— warns for suspicious divisions of two sizeof expressions that divide the pointer size by the element size, which is the usual way to compute the array size but won't work out correctly with pointers. This warning warns e.g. aboutsizeof (ptr) / sizeof (ptr[0])ifptris not an array, but a pointer. This warning is enabled by-Wall. not the array-size math on a pointer 😭-Wsizeof-pointer-memaccess— warns for suspicious length parameters to certain string and memory built-in functions if the argument usessizeof. This warning triggers for example formemset (ptr, 0, sizeof (ptr));ifptris not an array, but a pointer, and suggests a possible fix, or aboutmemcpy (&foo, ptr, sizeof (&foo));.
-Wsizeof-pointer-memaccess also warns about calls to bounded string copy functions like strncat or strncpy that specify as the bound a sizeof expression of the source array. For example, in the following function the call to strncat specifies the size of the source string as the bound. That is almost certainly a mistake and so the call is diagnosed.
void make_file (const char *name)
{
char path[PATH_MAX];
strncpy (path, name, sizeof path - 1);
strncat (path, ".text", sizeof ".text");
...
}The -Wsizeof-pointer-memaccess option is enabled by -Wall.
-Wsizeof-array-argument— warns when thesizeofoperator is applied to a parameter that is declared as an array in a function definition. This warning is enabled by default for C and C++ programs. the array parameter decay betrayal 🦋-Wmemset-elt-size— warns for suspicious calls to thememsetbuilt-in function, if the first argument references an array, and the third argument is a number equal to the number of elements, but not equal to the size of the array in memory. This indicates that the user has omitted a multiplication by the element size. This warning is enabled by-Wall. forgot to multiply? we've all been there ngl-Wmemset-transposed-args— warns for suspicious calls to thememsetbuilt-in function where the second argument is not zero and the third argument is zero. For example, the callmemset (buf, sizeof buf, 0)is diagnosed becausememset (buf, 0, sizeof buf)was meant instead. The diagnostic is only emitted if the third argument is a literal zero. Otherwise, if it is an expression that is folded to zero, or a cast of zero to some type, it is far less likely that the arguments have been mistakenly transposed and no warning is emitted. This warning is enabled by-Wall. 💀-Waddress— warns about suspicious uses of memory addresses. These include using the address of a function in a conditional expression, such asvoid func(void); if (func), and comparisons against the memory address of a string literal, such asif (x == "abc"). Such uses typically indicate a programmer error: the address of a function always evaluates to true, so their use in a conditional usually indicate that the programmer forgot the parentheses in a function call; and comparisons against string literals result in unspecified behavior and are not portable in C, so they usually indicate that the programmer intended to usestrcmp. This warning is enabled by-Wall.-Waddress-of-packed-member— warns when the address of packed member of struct or union is taken, which usually results in an unaligned pointer value. This is enabled by default. packed structs stay humbling everybody-Wlogical-op— warns about suspicious uses of logical operators in expressions. This includes using logical operators in contexts where a bit-wise operator is likely to be expected. Also warns when the operands of a logical operator are the same:
extern int a;
if (a < 0 && a < 0) { ... }bestie you said the same thing twice 💅
-Wlogical-not-parentheses— warns about logical not used on the left hand side operand of a comparison. This option does not warn if the right operand is considered to be a boolean expression. Its purpose is to detect suspicious code like the following:
int a;
...
if (!a > 1) { ... }It is possible to suppress the warning by wrapping the LHS into parentheses:
if ((!a) > 1) { ... }This warning is enabled by -Wall. 🫶
-Waggregate-return— warns 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.) returning a whole struct? bold of you ✨-Wno-aggressive-loop-optimizations— warns if in a loop with 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, such as unrecognized attributes, function attributes applied to variables, etc. This does not stop errors for incorrect use of supported attributes.-Wno-builtin-declaration-mismatch— warns if a built-in function is declared with an incompatible signature or as a non-function, or when a built-in function declared with a type that does not include a prototype is called with arguments whose promoted types do not match those expected by the function. When-Wextrais specified, also warn when a built-in function that takes arguments is declared without a prototype. The-Wno-builtin-declaration-mismatchwarning is enabled by default. To avoid the warning include the appropriate header to bring the prototypes of built-in functions into scope.
For example, the call to memset below is diagnosed by the warning because the function expects a value of type size_t as its argument but the type of 32 is int. With -Wextra, the declaration of the function is diagnosed as well.
extern void* memset ();
void f (void *d)
{
memset (d, '\0', 32);
}-Wno-builtin-macro-redefined— do not warn if certain built-in macros are redefined. This suppresses warnings for redefinition of__TIMESTAMP__,__TIME__,__DATE__,__FILE__, and__BASE_FILE__. hands off the builtin macros bestie 🙈-Wstrict-prototypes(C and Objective-C only) — warns 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.) prototypes: use them, they're cute 💜-Wold-style-declaration(C and Objective-C only) — bestie, this one calls out obsolescent stuff in a declaration, per the C Standard. Like if storage-class specifiers such asstaticaren't the first thing in a declaration? Warning. This one also gets switched on by-Wextra. 💅-Wold-style-definition(C and Objective-C only) — warns you if an old-style function definition shows up. And it'll warn even if there's a previous prototype sitting right there. No cap.-Wmissing-parameter-type(C and Objective-C only) — this fires when a function parameter is declared with no type specifier in K&R-style functions, like:
void foo(bar) { }Also enabled by -Wextra. 🫶
-Wmissing-prototypes(C and Objective-C only) — warns if a global function gets defined without a previous prototype declaration. And it still warns even when the definition itself provides a prototype, because that's just how it is. Use this to hunt down global functions with no matching prototype declaration in a header file. Heads up though: this option is not valid for C++, because in C++ all function declarations provide prototypes and a non-matching declaration declares an overload rather than conflicting with an earlier declaration. For C++, you want-Wmissing-declarationsinstead.-Wmissing-declarations— warns if a global function is defined without a previous declaration, and it does this even if the definition itself provides a prototype. Use it to catch global functions that aren't declared in header files. In C, no warnings get issued for functions with previous non-prototype declarations — for that, use-Wmissing-prototypes. In C++, no warnings get issued for function templates, or for inline functions, or for functions in anonymous namespaces. ✨-Wmissing-field-initializers— warns when a structure's initializer is missing some fields. For example, this code triggers it, becausex.his implicitly zero:
struct s { int f, g, h; };
struct s x = { 3, 4 };It does not warn about designated initializers, so this little tweak is safe:
struct s { int f, g, h; };
struct s x = { .f = 3, .g = 4 };In C it also doesn't warn about the universal zero initializer { 0 }:
struct s { int f, g, h; };
struct s x = { 0 };And in C++ it doesn't warn about the empty { } initializer either, like:
struct s { int f, g, h; };
s x = { };This warning is included in -Wextra. Want the other -Wextra warnings without this one? Just do -Wextra -Wno-missing-field-initializers. 💖
-Wno-multichar— turns off the warning for multicharacter constants like'FOOF'. Usually those mean someone typo'd their code, since they have implementation-defined values and honestly shouldn't be in portable code at all. 🍓-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 you use characters outside the basic ASCII set, you can end up with two different character sequences that look identical. To avoid that chaos, the ISO 10646 standard sets out normalization rules that turn look-alike sequences into the same sequence. GCC can warn you when you're using identifiers that aren't 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 not in the ISO 10646 "C" normalized form, NFC. NFC is the recommended form for most uses, and it's equivalent to -Wnormalized.
Now the tragic part: some characters allowed in identifiers by ISO C and ISO C++ are, once turned into NFC, not allowed in identifiers. So there's literally 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. The hope is that future versions of the standards involved will fix this, which is why this option isn't the default. 🙈
You can switch the warning off for all characters by writing -Wnormalized=none or -Wno-normalized. Only do that 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" placed in a superscript. ISO 10646 defines the NFKC normalization scheme to convert all these into a standard form too, and GCC warns if your code isn't in NFKC when you use -Wnormalized=nfkc. This warning is comparable to warning about every identifier containing the letter O because it might be confused with the digit 0 — so yeah, it's not the default, but it can be useful as a local coding convention if the programming environment can't be fixed to display these characters distinctly.
-Wno-attribute-warning— don't warn about usage of functions declared with the"warning"attribute. By default this warning is enabled. You can use-Wno-attribute-warningto disable the warning, or-Wno-error=attribute-warningto disable the error when compiled with the-Werrorflag. 🎀-Wno-deprecated— don't warn about usage of deprecated features. Periodt.-Wno-deprecated-declarations— don't warn about uses of functions, variables, and types marked as deprecated via the"deprecated"attribute.-Wno-overflow— don't warn about compile-time overflow in constant expressions.-Wno-odr— warn about One Definition Rule violations during link-time optimization. Requires-flto-odr-type-mergingto be enabled. Enabled by default. 👑-Wopenmp-simd— warns if the vectorizer cost model overrides the OpenMP simd directive that the user set. The-fsimd-cost-model=unlimitedoption can be used to relax the cost model. 🔥-Woverride-init(C and Objective-C only) — warns if an initialized field without side effects gets overridden when using designated initializers. Included in-Wextra. Want the other-Wextrawarnings without this one?-Wextra -Wno-override-init. ✨-Woverride-init-side-effects(C and Objective-C only) — warns if an initialized field with side effects gets overridden when using designated initializers. Enabled by default. 😭-Wpacked— warns if a structure is given the packed attribute but that packed attribute has no effect on the layout or size of the structure. Such structures may end up mis-aligned for basically no benefit. For instance, in this code,f.xinstruct baris misaligned even thoughstruct baritself doesn't have the packed attribute:
struct foo {
int x;
char a, b, c, d;
} __attribute__((packed));
struct bar {
char z;
struct foo f;
};-Wpacked-bitfield-compat— the 4.1, 4.2 and 4.3 series of GCC ignore thepackedattribute on bit-fields of typechar. This got fixed in GCC 4.4, but the change can lead to differences in structure layout. GCC informs you when the offset of such a field changed in GCC 4.4. For example, there's 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 it. 🦋
-Wpacked-not-aligned(C, C++, Objective-C and Objective-C++ only) — warns if a structure field with explicitly specified alignment in a packed struct or union ends up misaligned. For example, a warning gets 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;
};Enabled by -Wall. 💜
-Wpadded— warns if padding is included in a structure, whether to align an element of the structure or to align the whole structure. Sometimes when this happens you can rearrange the fields to reduce the padding and make the structure smaller. Main character optimization energy. 👑-Wredundant-decls— warns if anything is declared more than once in the same scope, even in cases where the multiple declaration is valid and changes literally nothing. 🌈-Wno-restrict— warns 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, this call tostrcpybelow tries 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 gets diagnosed:
void foo (void)
{
char a[] = "abcd1234";
strcpy (a, a + 4);
...
}The -Wrestrict option catches some instances of simple overlap even without optimization, but it works best at -O2 and above. It's included in -Wall. 💅
-Wnested-externs(C and Objective-C only) — warns if anexterndeclaration shows up inside a function. 🧚-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.-Winline— warns if a function declared as inline can't be inlined. Even with this option, the compiler does not warn about failures to inline functions declared in system headers. 🙈
okay so bestie, the compiler is literally using a whole variety of heuristics to figure out whether or not to inline a function 💅. Like, it's taking into account the size of the function being inlined and the amount of inlining that has already been done in the current function. So the way that seemingly insignificant changes in the source program can cause the warnings produced by -Winline to appear or disappear? It's giving unpredictable, fr fr. 😭
-Wno-invalid-offsetof(C++ and Objective-C++ only) — bestie, this one suppresses warnings from applying the"offsetof"macro to a non-POD type. According to the 2014 ISO C++ standard, applying"offsetof"to a non-standard-layout type is literally undefined behavior 😱. In existing C++ implementations, though,"offsetof"typically gives meaningful results. This flag is for users who are aware that they are writing nonportable code and who have deliberately chosen to ignore the warning about it. Periodt.
The restrictions on "offsetof" may be relaxed in a future version of the C++ standard. iykyk
-Wint-in-bool-context— Warn for suspicious use of integer values where boolean values are expected, such as conditional expressions (?:) using non-boolean integer constants in boolean context, like"if (a <= b ? 2 : 3)". Or left shifting of signed integers in boolean context, like"for (a = 0; 1 << a; a++);". Likewise for all kinds of multiplications regardless of the data type. This warning is enabled by-Wall. No cap, this one is highkey useful.-Wno-int-to-pointer-cast— Suppress warnings from casts to pointer type of an integer of a different size. In C++, casting to a pointer type of smaller size is an error.Wint-to-pointer-castis enabled by default. Not me ignoring these warnings tho 💀-Wno-pointer-to-int-cast(C and Objective-C only) — Suppress warnings from casts from a pointer to an integer type of a different size. Only for C and Objective-C, bestie.-Winvalid-pch— Warn if a precompiled header is found in the search path but cannot be used. It's giving "why can't I use this PCH?" energy.-Wlong-long— Warn if"long long"type is used. This is enabled by either-Wpedanticor-Wtraditionalin ISO C90 and C++98 modes. To inhibit the warning messages, use-Wno-long-long. Old school vibes, fr.-Wvariadic-macros— Warn if variadic macros are used in ISO C90 mode, or if the GNU alternate syntax is used in ISO C99 mode. This is enabled by either-Wpedanticor-Wtraditional. To inhibit the warning messages, use-Wno-variadic-macros. Variadic macros are iconic, but C90 said no 💅-Wvarargs— Warn upon questionable usage of the macros used to handle variable arguments like"va_start". This is default. To inhibit the warning messages, use-Wno-varargs. Default behavior, bestie. Pay attention!-Wvector-operation-performance— Warn if vector operation is not implemented via SIMD capabilities of the architecture. Mainly useful for the performance tuning. Vector operation can be implemented"piecewise", which means that the scalar operation is performed on every vector element;"in parallel", which means that the vector operation is implemented using scalars of wider type, which normally is more performance efficient; and"as a single scalar", which means that vector fits into a scalar type. Performance queen 👑-Wno-virtual-move-assign— Suppress warnings about inheriting from a virtual base with a non-trivial C++11 move assignment operator. This is dangerous because if the virtual base is reachable along more than one path, it is moved multiple times, which can mean both objects end up in the moved-from state. If the move assignment operator is written to avoid moving from a moved-from object, this warning can be disabled. ⚠️ Danger, bestie! ⚠️-Wvla— Warn if a variable-length array is used in the code.-Wno-vlaprevents the-Wpedanticwarning of the variable-length array. VLAs are a vibe, but the warning is real.-Wvla-larger-than=byte-size— If this option is used, the compiler will warn 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.
Note that GCC may optimize small variable-length arrays of a known value into plain arrays, so this warning may not get triggered for such arrays.
-Wvla-larger-than=PTRDIFF_MAX is enabled by default but is typically only effective when -ftree-vrp is active (default for -O2 and above).
See also -Walloca-larger-than=byte-size. So many details, but we love that for us.
-Wno-vla-larger-than— Disable-Wvla-larger-than=warnings. The option is equivalent to-Wvla-larger-than=SIZE_MAXor larger. Turning it off like a light switch.-Wvolatile-register-var— Warn if a register variable is declared volatile. The volatile modifier does not inhibit all optimizations that may eliminate reads and/or writes to register variables. This warning is enabled by-Wall. Volatile register variables? In this economy?-Wdisabled-optimization— Warn if a requested optimization pass is disabled. This warning does not generally indicate that there is anything wrong with your code; it merely indicates that GCC's optimizers are unable to handle the code effectively. Often, the problem is that your code is too big or too complex; GCC refuses to optimize programs when the optimization itself is likely to take inordinate amounts of time. It's not you, it's your code being too extra. 💅-Wpointer-sign(C and Objective-C only) — Warn for pointer argument passing or assignment with different signedness. This option is only supported for C and Objective-C. It is implied by-Walland by-Wpedantic, which can be disabled with-Wno-pointer-sign. Signedness drama, fr fr.-Wstack-protector— This option is only active when-fstack-protectoris active. It warns about functions that are not protected against stack smashing. Stack smashing? Not on my watch. 🛡️-Woverlength-strings— Warn about string constants that are longer than the"minimum maximum"length specified in the C standard. Modern compilers generally allow string constants that are much longer than the standard's minimum limit, but very portable programs should avoid using longer strings.
The limit applies after string constant concatenation, and does not count the trailing NUL. In C90, the limit was 509 characters; in C99, it was raised to 4095. C++98 does not specify a normative minimum maximum, so we do not diagnose overlength strings in C++.
This option is implied by -Wpedantic, and can be disabled with -Wno-overlength-strings. 509 characters in C90? That's like, so short. 😭
-Wunsuffixed-float-constants(C and Objective-C only) — Issue a warning for any floating constant that does not have a suffix. When used together with-Wsystem-headersit warns about such constants in system header files. This can be useful when preparing code to use with the"FLOAT_CONST_DECIMAL64"pragma from the decimal floating-point extension to C99. Suffixes are important, bestie.-Wno-designated-init(C and Objective-C only) — Suppress warnings when a positional initializer is used to initialize a structure that has been marked with the"designated_init"attribute. Positional initializers? So last season.-Whsa— Issue a warning when HSAIL cannot be emitted for the compiled function or OpenMP construct. HSAIL? More like HSAIL no. 😂
✨ 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. Simple, right?
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. Optimized code is giving chaotic energy, but you can still debug it.
If you are not using some other optimization option, consider using -Og with -g. With no -O option at all, some compiler passes that collect information useful for debugging do not run at all, so that -Og may result in a better debugging experience. Pro tip: -Og is the debugging bestie.
-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). GDB girlies, this one's for you.
-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. Maximum GDB energy. 💅-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 4. DWARF Version 5 is only experimental. Experimental? We love a risk-taker.
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.
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. DWARF 1 is cancelled, bestie.
-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. Old school DBX vibes.-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. GDB-only, so other debuggers are gonna be like "what is this?" 💀-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. IBM RS/6000? Vintage, but iconic.-gxcoff+— bestie, this one makes debugging info in XCOFF format (if your setup even supports it), with GNU extensions that only GDB understands 💅. And like... heads up: those extensions will probably make other debuggers crash or flat-out refuse to read your program, and may make assemblers that aren't GNU assembler (GAS) throw an error. It's giving ✨ exclusive ✨, iykyk.-gvms— produces debugging info in Alpha/VMS debug format (if that's supported). It's literally the format DEBUG uses on Alpha/VMS systems. Niche queen behavior 🎀.-glevel-ggdblevel-gstabslevel-gxcofflevel-gvmslevel— okay so these ask for debugging info and letleveldecide how much info you get. Default level is 2, no cap.- Level 0 — produces no debug info at all. So
-g0straight-up negates-g. Nothing. Zero. Void 💀. - Level 1 — minimal info, just enough for 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 🙈.
- Level 3 — includes extra stuff, like all the macro definitions present in the program. Some debuggers support macro expansion when you use
-g3🔥.
- Level 0 — produces no debug info at all. So
If you use multiple -g options, with or without level numbers, the last one is the one that's effective. Last one wins, periodt.
Also:-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 👑
-feliminate-unused-debug-symbols— produces debugging info in stabs format (if that's supported), but only for symbols that are actually used. Declutter era 🧚.-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 debugging info for classes, because using it increases the size of debugging info by as much as a factor of two 😭. Big yikes, use sparingly.-fno-merge-debug-strings— tells the linker to not merge together strings in the debugging info that are identical across different object files. Not all assemblers or linkers support merging, fyi. Merging decreases the size of the debug info in the output file, at the cost of increasing link processing time. And bestie — merging is enabled by default ✨.-fdebug-prefix-map=old=new— when compiling files living in directoryold, this records debugging info describing them as if the files lived in directorynewinstead. You can use it to 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.. This gives you more reproducible builds, which are location independent, but it may require an extra command to tell GDB where to find the source files. See also-ffile-prefix-map👀.-fvar-tracking— runs the variable tracking pass. It computes where variables are stored at each position in code, so better debugging info gets generated (if the debugging info format supports that info).
It's enabled by default when compiling with optimization (-Os, -O, -O2, ...), debugging information (-g) and the debug info format supports it. Main character defaults 🌟.
-fvar-tracking-assignments— annotates assignments to user variables early in the compilation and tries to carry those annotations over throughout the whole compilation, all the way to the end, to improve debug info while optimizing. Using-gdwarf-4alongside it is recommended 💜.
It can be enabled even if var-tracking is disabled — in that case the annotations get created and maintained, but discarded at the end. By default this flag is enabled together with -fvar-tracking, except when selective scheduling is enabled. Sneaky exception, noted 📝.
-gsplit-dwarf— separates as much DWARF debugging info as possible into a separate output file with the extension.dwo. This lets the build system avoid linking files with debug info. To actually be useful, this option requires a debugger capable of reading.dwofiles. No debugger, no vibes 🦋.-gdescribe-dies— adds description attributes to some DWARF DIEs that have no name attribute, like artificial variables, external references and call site parameter DIEs. Giving the unnamed ones a little name tag 🏷️.-gpubnames— generates DWARF".debug_pubnames"and".debug_pubtypes"sections. Slay ✨.-ggnu-pubnames— generates".debug_pubnames"and".debug_pubtypes"sections in a format suitable for conversion into a GDB index. Only useful with a linker that can produce GDB index version 7, so check your linker first bestie 🔍.-fdebug-types-section— when using DWARF Version 4 or higher, type DIEs can be put into their own".debug_types"section instead of being part of the".debug_info"section. 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_types"sections yet, and on some objects".debug_types"produces larger instead of smaller debugging info. So it's giving... depends 😭.-grecord-gcc-switches-gno-record-gcc-switches— this switch causes the command-line options used to invoke the compiler that may affect code generation to be appended to theDW_AT_producerattribute in DWARF debugging info. 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.-gstrict-dwarf— disallows using extensions of 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 — but not here. Strict queen behavior 👑.-gno-strict-dwarf— allows using extensions of a later DWARF standard version than selected with-gdwarf-version. Live a little 🌈.-gas-loc-support— informs the compiler that the assembler supports".loc"directives. It may then use them so the assembler generates 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 🎀.
This option is enabled by default if, at GCC configure time, the assembler was found to support such directives.
-gno-as-loc-support— forces GCC to generate DWARF2+ line number tables internally, if DWARF2+ line number tables are to be generated. DIY era 💅.gas-locview-support— informs the compiler that the assembler supports"view"assignment and reset assertion checking in".loc"directives.
This option is enabled by default if, at GCC configure time, the assembler was found to support them.
gno-as-locview-support— forces GCC to assign view numbers internally, if-gvariable-location-viewsis explicitly requested.-gcolumn-info-gno-column-info— emits location column information into DWARF debugging info, rather than just file and line. This option is enabled by default ✨. Columns girlies rise.-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 of"is_stmt"markers in the line number table. It's enabled by default when compiling with optimization (-Os,-O,-O2, ...), and outputting DWARF 2 debug information at the normal level 🌸.-gvariable-location-views-gvariable-location-views=incompat5-gno-variable-location-views— augments 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 causes 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 debug info consumers that aren't aware of these augmentations can still consume them — but they won't derive any benefit from them either. It's giving "works, but why" 😭.
This is enabled by default when outputting DWARF 2 debug info 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'll 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 mismatch = the worst kind 💀.
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, and is only provided as a reference implementation of the proposed representation. Debug info consumers are not expected to support this extended format, and they'd be rendered unable to decode location lists using it. So like... be aware 🚨.
-ginternal-reset-location-views-gno-internal-reset-location-views— attempts to determine location views that can be omitted from location view lists. This requires the compiler to have very accurate insn length estimates, which isn't always the case, and it may cause incorrect view lists to be generated silently when using an assembler that doesn't support location view lists 😬. The GNU assembler will flag any such error as a"view number mismatch". This is only enabled on ports that define a reliable estimation function. Trust issues, but make it compiler 🔥.-ginline-points-gno-inline-points— generates extended debug info for inlined functions. Location view tracking markers are inserted at inlined entry points, so that address and view numbers can be computed and output in debug info ✨. This can be enabled independently of location views — in which case the view numbers won't be output — but it can only be enabled along with statement frontiers, and it's only enabled by default if location views are enabled. Chain of dependencies, bestie, it's giving ✨ecosystem✨.-gz[=type]— bestie, this one produces compressed debug sections in DWARF format, if that's even supported. If you don't hand it a type, the default type depends on what your assembler and linker can actually do. type can be one ofnone(no compression, we're keeping it raw),zlib(zlib compression in ELF gABI format), orzlib-gnu(zlib compression in the traditional GNU format). If your linker can't write compressed debug sections, the option gets rejected. Otherwise, if the assembler doesn't support them,-gzis just silently ignored when you're producing object files. 💅-femit-struct-debug-baseonly— emits debug info for struct-like types only when the base name of the compilation source file matches the base name of the file where that struct is defined.
This one shrinks your debugging info size by a lot, but the potential loss in type info handed to the debugger is significant. Peep -femit-struct-debug-reduced for a less aggressive vibe, and -femit-struct-debug-detailed if you want the full granular control.
This option only works with DWARF debug output, no cap. 🦋
-femit-struct-debug-reduced— emits debug info for struct-like types only when the base name of the compilation source file matches the base name of the file where the type is defined — unless the struct is a template or it's defined in a system header.
This shrinks debug info size a significant amount, with only some potential loss of type info to the debugger. See -femit-struct-debug-baseonly for a more aggressive option, and -femit-struct-debug-detailed for the fine-grained control.
Again — DWARF debug output only, bestie. ✨
-femit-struct-debug-detailed[=spec-list]— tells the compiler exactly which struct-like types it should generate debug info for. The whole intent here is to cut down on duplicate struct debug info between different object files in the same program.
This is the detailed version of -femit-struct-debug-reduced and -femit-struct-debug-baseonly, and honestly it serves most needs.
A specification has the syntax:
[dir:|ind:][ord:|gen:](any|sys|base|none)The optional first word limits the spec to structs used directly (dir:) or used indirectly (ind:). A struct type is used directly when it's the type of a variable, member. Indirect uses come through pointers to structs — that is, when using 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:). Okay so 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 languages have generics too, but -femit-struct-debug-detailed does not yet implement them. 🫠
The third word specifies the source files for those structs where the compiler should emit debug info. The values none and any have their normal meaning. The value base means the base of the name of the file where the type declaration appears must match the base of the name of the main compilation file. In practice, that means when you're compiling foo.c, debug info gets generated for types declared in that file and foo.h, but not other headers. The value sys means those types satisfying base or declared in system or compiler headers.
You may need to experiment to figure out the best settings for your application. fr fr, just poke at it.
The default is -femit-struct-debug-detailed=all.
DWARF debug output only. Periodt. 💜
-fno-dwarf2-cfi-asm— emits DWARF unwind info as a compiler generated.eh_framesection instead of using GAS.cfi_*directives. 🌸-fno-eliminate-unused-debug-types— okay so normally, when producing DWARF output, GCC avoids producing debug symbol output for types that are nowhere used in the source file it's compiling. But sometimes it's genuinely useful to have GCC emit debug info for all types declared in a compilation unit, regardless of whether they're actually used in that unit — like, say, you're in the debugger and you want to cast a value to a type that isn't actually used in your program (but is declared). More often though? This results in a significant amount of wasted space. So like, pick your poison. 💀
✨ Options That Control Optimization ✨
These options control various sorts of optimizations. We love a glow-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. Predictable queen behavior.
Turning on optimization flags makes the compiler attempt to improve the performance and/or code size at the expense of compilation time and possibly the ability to debug the program.
The compiler performs optimization based on the knowledge it has of the program. Compiling multiple files at once to a single output file mode allows the compiler to use information gained from all of the files when compiling each of them. Teamwork makes the dream work.
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. Receipts, always.
-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-profile
-fipa-pure-const -fipa-reference -fipa-reference-addressable
-fmerge-constants -fmove-loop-invariants -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 don't involve a space-speed tradeoff. Compared to-O, this bumps up both compilation time and the performance of the generated code. 🔥
-O2 turns on all optimization flags specified by -O. 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 -fgcse
-fgcse-lm -fhoist-adjacent-loads -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-pre
-ftree-switch-conversion -ftree-tail-merge -ftree-vrpPlease note the warning under -fgcse about invoking -O2 on programs that use computed gotos. ⚠️
NOTE: In Ubuntu 8.10 and later versions,-D_FORTIFY_SOURCE=2is set by default, and is activated when-Ois set to 2 or higher. This enables additional compile-time and run-time checks for several libc functions. To disable, specify either-U_FORTIFY_SOURCEor-D_FORTIFY_SOURCE=0.
-O3— Optimize yet more.-O3turns on all optimizations specified by-O2and also turns on the following optimization flags:
-fgcse-after-reload -finline-functions -fipa-cp-clone
-floop-interchange -floop-unroll-and-jam -fpeel-loops
-fpredictive-commoning -fsplit-paths -ftree-loop-distribute-patterns
-ftree-loop-distribution -ftree-loop-vectorize -ftree-partial-pre
-ftree-slp-vectorize -funswitch-loops -fvect-cost-model
-fversion-loops-for-strides-O0— Reduce compilation time and make debugging produce the expected results. This is the default. chill mode, no notes.-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. 🎀
-Ofast— Disregard strict standards compliance. living on the edge, bestie-Ofastenables all-O3optimizations. It also enables optimizations that are not valid for all standard-compliant programs. It turns on-ffast-mathand the Fortran-specific-fstack-arrays, unless-fmax-stack-var-sizeis specified, and-fno-protect-parens.-Og— Optimize debugging experience.-Ogshould be the optimization level of choice for the standard edit-compile-debug cycle, offering a reasonable level of optimization while maintaining fast compilation and a good debugging experience. It's a better choice than-O0for producing debuggable code because some compiler passes that collect debug information are disabled at-O0. 💖
ok so like its sibling -O0, -Og literally just turns off a whole bunch of optimization passes, which means any individual option controlling those passes is giving absolutely nothing — no effect, bestie, none. 💀 But otherwise? -Og turns on all the -O1 optimization flags except the ones that would mess with your debugging session:
-fbranch-count-reg -fdelayed-branch -fif-conversion
-fif-conversion2 -finline-functions-called-once
-fmove-loop-invariants -fssa-phiopt -ftree-bit-ccp -ftree-pta
-ftree-sraAnd the rule for stacking them up: if you use multiple -O options, with or without level numbers, the last such option is the one that's effective. Last one wins, periodt. 👑
Options of the form -fflag are your machine-independent flags, iykyk. Most flags come in 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 form by either removing no- or adding it. easy math. ✨
The following options control specific optimizations. They're either activated by -O options or they're related to ones that are. Reach for 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 makes them always pop the arguments the second each function returns. At levels-O1and higher,-fdefer-popis the default, which lets the compiler let arguments pile up on the stack across several function calls and pop them all at once. 🫶-fforward-propagate— runs 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 one is scheduled after loop unrolling. This option is enabled by default at optimization levels-O,-O2,-O3,-Os.-ffp-contract=style— okay so three vibes here:-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 it's allowed by the language standard. This is currently not implemented and treated equal to-ffp-contract=off.
The default is -ffp-contract=fast.
-fomit-frame-pointer— omits the frame pointer in functions that don't need one. This dodges the instructions to save, set up and restore the frame pointer, and on many targets it also frees up an extra register. 🎀 On some targets this flag has no effect because the standard calling sequence always uses a frame pointer, so it literally cannot be omitted. And note:-fno-omit-frame-pointerdoes not guarantee the frame pointer is used in all functions — several targets always omit it in leaf functions. Enabled by default at-Oand higher.-foptimize-sibling-calls— optimizes sibling and tail recursive calls. Enabled at levels-O2,-O3,-Os.-foptimize-strlen— optimizes 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 thenoinlineattribute.-finline-small-functions— integrates 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— inlines indirect calls too, when they're discovered to be known at compile time thanks to previous inlining. This option has any effect only when inlining itself is turned on by the-finline-functionsor-finline-small-functionsoptions. Enabled at levels-O2,-O3,-Os.-finline-functions— considers all functions for inlining, even if they're not 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 declaredstatic, then the function is normally not output as assembler code in its own right. Enabled at levels-O3,-Os. Also enabled by-fprofile-useand-fauto-profile.-finline-functions-called-once— considers allstaticfunctions called once for inlining into their caller, even if they're not 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,-O3and-Os, but not-Og. 🙈-fearly-inlining— inlines 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 so makes profiling significantly cheaper and usually makes inlining faster on programs that have large chains of nested wrapper functions. Enabled by default.-fipa-sra— performs interprocedural scalar replacement of aggregates, removal of unused parameters and replacement of parameters passed by reference by parameters passed by value. Enabled at levels-O2,-O3and-Os.-finline-limit=n— by default, GCC limits the size of functions that can be inlined. This flag gives you coarse control of that limit.nis the size of functions that can be inlined, measured in number of pseudo instructions. Inlining is actually controlled by a number of parameters, which you can specify individually by using--param name=value. The-finline-limit=noption sets some of these parameters like so:max-inline-insns-singleis set ton/2.max-inline-insns-autois set ton/2.
See below for a documentation of the individual parameters controlling inlining and for the defaults of these parameters.
Note: there may be no value of -finline-limit that results in default behavior.
Note: "pseudo instruction" here means, in this particular context, an abstract measurement of a function's size. It is in no way a count of assembly instructions, and as such its exact meaning might change from one release to another. 💀
-fno-keep-inline-dllexport— this is a more fine-grained version of-fkeep-inline-functions, which applies only to functions that are declared using thedllexportattribute or declspec.-fkeep-inline-functions— in C, emitsstaticfunctions 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++, it emits any and all inline functions into the object file.-fkeep-static-functions— emitsstaticfunctions into the object file, even if the function is never used.-fkeep-static-consts— emits 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-constsoption.-fmerge-constants— attempts 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-constantsto inhibit this behavior. Enabled at levels-O,-O2,-O3,-Os.-fmerge-all-constants— attempts to merge identical constants and identical variables. This option implies-fmerge-constants. In addition to-fmerge-constants, it considers e.g. even constant initialized arrays or initialized constant variables with integral or floating-point types. Languages like C or C++ require each variable — including multiple instances of the same variable in recursive calls — to have distinct locations, so using this option results in non-conforming behavior. ⚠️-fmodulo-sched— performs 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— performs more aggressive SMS-based modulo scheduling with register moves allowed. By setting this flag, certain anti-dependences edges are deleted, which triggers the generation of reg-moves based on the life-range analysis. This option is effective only with-fmodulo-schedenabled.-fno-branch-count-reg— disables the optimization pass that scans for opportunities to use "decrement and branch" instructions on a count register instead of instruction sequences that decrement a register, compare it against zero, and then branch based upon the result. This option is only meaningful on architectures that support such instructions, which include x86, PowerPC, IA-64 and S/390. Note that the-fno-branch-count-regoption doesn't remove the decrement and branch instructions from the generated instruction stream introduced by other optimization passes. The default is-fbranch-count-regat-O1and higher, except for-Og.-fno-function-cse— do not put function addresses in registers; make each instruction that calls a constant function contain the function's address explicitly. This option results in less efficient code, but some strange hacks that alter the assembler output may get confused by the optimizations performed when this option is not used. The default is-ffunction-cse.-fno-zero-initialized-in-bss— if the target supports a BSS section, GCC by default puts variables that are initialized to zero into BSS. This can save space in the resulting code. This option turns that behavior off, because some programs explicitly rely on variables going to the data section — e.g., so that the resulting executable can find the beginning of that section and/or make assumptions based on that. The default is-fzero-initialized-in-bss.-fthread-jumps— bestie this one is so clever 💅 it hunts for jumps that land somewhere where there's another comparison already subsumed by the first one. Found it? Then the first branch gets redirected — either to the destination of the second branch, or to the spot immediately after it, depending on whether the condition is known true or false. Basically it's playing matchmaker between branches.
Enabled at levels -O2, -O3, -Os.
-fsplit-wide-types— when a type takes up multiple registers (thinklong longon a 32-bit system), this splits those registers apart and allocates them independently. Normally it makes better code for those types ✨ but heads up, it can make debugging harder 😭.
Enabled at levels -O, -O2, -O3, -Os.
-fcse-follow-jumps— in common subexpression elimination (CSE), this scans through jump instructions when the jump's target isn't reached by any other path. Example: when CSE runs into anifstatement with anelseclause, CSE follows the jump when the condition tested is false. She's nosy and we love that 🫶
Enabled at levels -O2, -O3, -Os.
-fcse-skip-blocks— same energy as-fcse-follow-jumps, but it makes CSE follow jumps that conditionally skip over blocks. So when CSE meets a simpleifstatement with no else clause,-fcse-skip-blocksmakes CSE follow the jump around the body of theif. Iconic detour behavior 🌈
Enabled at levels -O2, -O3, -Os.
-frerun-cse-after-loop— literally what it says, no cap: re-run common subexpression elimination after loop optimizations have been performed. Round two, she's back 💖
Enabled at levels -O2, -O3, -Os.
-fgcse— performs a global common subexpression elimination pass. And it doesn't stop there, it also does global constant and copy propagation. Overachiever energy 👑
Note: when you're compiling a program that uses computed gotos (a GCC extension), you might get better run-time performance if you disable the global common subexpression elimination pass by adding -fno-gcse to the command line. Just saying 🙈
Enabled at levels -O2, -O3, -Os.
-fgcse-lm— when-fgcse-lmis on, global common subexpression elimination tries to move loads that are only killed by stores into themselves. Which means a loop with a load/store sequence can be changed into a load outside the loop plus a copy/store within the loop. Chef's kiss transformation 🍓
Enabled by default when -fgcse is enabled.
-fgcse-sm— when-fgcse-smis on, a store motion pass runs after global common subexpression elimination. This pass attempts to move stores out of loops. And when you pair it with-fgcse-lm, loops containing a load/store sequence can become a load before the loop and a store after the loop. Duo goals 💜
Not enabled at any optimization level.
-fgcse-las— when-fgcse-lasis on, the global common subexpression elimination pass eliminates redundant loads that come after stores to the same memory location (both partial and full redundancies). Redundancy found, redundancy deleted 💀
Not enabled at any optimization level.
-fgcse-after-reload— when-fgcse-after-reloadis on, a redundant load elimination pass is performed after reload. The whole point of this pass is to clean up redundant spilling ✨
Enabled by -fprofile-use and -fauto-profile.
-faggressive-loop-optimizations— this one tells the loop optimizer to use language constraints to derive bounds for the number of iterations of a loop. It assumes that loop code does not invoke undefined behavior — like, for example, causing signed integer overflows or out-of-bound array accesses. Those iteration bounds then guide loop unrolling and peeling and loop exit test optimizations.
Okay so this assumes no UB, and that assumption is doing a LOT of heavy lifting here — if your code does overflow signed ints or read out of bounds, that's on you and the results are not your friend 😭
This option is enabled by default.
-funconstrained-commons— this tells the compiler that variables declared in common blocks (e.g. Fortran) may later be overridden with longer trailing arrays. Which prevents certain optimizations that depend on knowing the array bounds. So it's like, we can't assume the size, so we back off 💅-fcrossjumping— performs the 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— combines increments or decrements of addresses with memory accesses. This pass is always skipped on architectures that don't have instructions to support it. Enabled by default at-Oand higher on architectures that do support this 🌸-fdce— performs dead code elimination (DCE) on RTL. Enabled by default at-Oand higher. Dead code, you are not the main character, goodbye 💅-fdse— performs dead store elimination (DSE) on RTL. Enabled by default at-Oand higher. Same vibe, different victim 🎀-fif-conversion— attempts 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 -O, -O2, -O3, -Os, but not with -Og.
-fif-conversion2— uses conditional execution (where available) to transform conditional jumps into branch-less equivalents.
Enabled at levels -O, -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 calls operator delete afterwards. For a hierarchy with virtual bases, the base and complete variants are clones — meaning two copies of the function. With this option, the base and complete variants are changed to be thunks that call a common implementation. Deduplication but make it C++ 👑
Enabled by -Os.
-fdelete-null-pointer-checks— assumes 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. Those assume that a memory access to address zero always results in a trap — so if a pointer is checked after it's 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— attempts 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. Virtual call, we know exactly who you are now 🔥-fdevirtualize-speculatively— attempts to convert calls to virtual functions into speculative direct calls. Based on the analysis of the type inheritance graph, it figures out for a given call the set of likely targets. If that set is small — preferably of size 1 — it changes the call into a conditional deciding between direct and indirect calls. These speculative calls enable more optimizations, such as inlining. And when they seem useless after further optimization, they get converted back into their original form. Manifesting, then un-manifesting 🌈-fdevirtualize-at-ltrans— streams 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. Trade-offs, bestie, always trade-offs 💅-fexpensive-optimizations— performs a number of minor optimizations that are relatively expensive. Yes she's expensive, she's worth it 💖
Enabled at levels -O2, -O3, -Os.
-free— attempts 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— in C++ the value of an object is only affected by changes within its lifetime: when the constructor begins, the object has an indeterminate value, and any changes during the lifetime of the object are dead when the object is destroyed. Normally dead store elimination will take advantage of this. If your code relies on the value of the object storage persisting beyond the lifetime of the object, you can use this flag to disable this optimization.
To preserve stores before the constructor starts (e.g. because your operator new clears the object storage) but still treat the object as dead after the destructor, you can use -flifetime-dse=1. The default behavior can be explicitly selected with -flifetime-dse=2. And -flifetime-dse=0 is equivalent to -fno-lifetime-dse.
-flive-range-shrinkage— attempts to decrease register pressure through register live range shrinkage. This is helpful for fast processors with small or moderate size register sets. Squish the ranges, free the registers 🎀-fira-algorithm=algorithm— uses 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 is the default because it generates better code. The upgrade was worth it ✨-fira-region=region— uses specified regions for the integrated register allocator. The region argument should be one of the following:all— use all loops as register allocation regions. This can give the best results for machines with a small and/or irregular register set.mixed— use all loops except for loops with small register pressure as the regions. This value usually gives the best results in most cases and for most architectures, and is enabled by default when compiling with optimization for speed (-O,-O2, ...).one— use all functions as a single region. This typically results in the smallest code size, and is enabled by default for-Osor-O0.
-fira-hoist-pressure— makes IRA do a lil vibe check on register pressure inside the code hoisting pass, so it can decide whether hoisting an expression is actually worth it. Usually this gives you smaller code, but bestie it can also slow the compiler tf down. ✨ Enabled at level-Osfor all targets.-fira-loop-pressure— IRA evaluates register pressure inside loops to decide whether to move loop invariants. On machines with big register files (>= 32registers) this usually generates faster and smaller code, but yeah it can slow the compiler down. Enabled at level-O3for some targets. 💅-fno-ira-share-save-slots— no more sharing stack slots for saving call-used hard registers that live through a call. Every hard register gets its own stack slot, and the consequence? Function stack frames get bigger. Not a vibe, but it's the tradeoff. 😭-fno-ira-share-spill-slots— same energy but for pseudo-registers. No sharing of stack slots allocated for pseudos. Every pseudo that doesn't land a hard register gets its own separate stack slot, and once again: bigger function stack frames. 💀-flra-remat— turns on CFG-sensitive rematerialization in LRA. Instead of loading up the values of spilled pseudos, LRA tries to rematerialize (recalculate) those values if it's profitable. Enabled at levels-O2,-O3,-Os. 🎀-fdelayed-branch— if the target machine supports it, try to reorder instructions so we can exploit the instruction slots sitting there after delayed branch instructions. Enabled at levels-O,-O2,-O3,-Os— but not at-Og. 🙈-fschedule-insns— if the target machine supports it, try reordering instructions to kill execution stalls caused by required data not being ready yet. This is a cheat code for machines with slow floating point or memory load instructions, because it lets other instructions issue while you wait for the load or FP result to actually be needed. Enabled at levels-O2,-O3. 🔥-fschedule-insns2— basically-fschedule-insns's sequel, but it asks for an additional pass of instruction scheduling after register allocation is done. 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— disables instruction scheduling across basic blocks, which is normally on when scheduling happens before register allocation, i.e. with-fschedule-insnsor at-O2or higher. ✨-fno-sched-spec— disables speculative motion of non-load instructions, which is normally on when scheduling happens before register allocation, i.e. with-fschedule-insnsor at-O2or higher. 💜-fsched-pressure— enables 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. Using it can improve the generated code and shrink it, by preventing register pressure from climbing above the number of available hard registers and the spills that would follow in register allocation. 🫶-fsched-spec-load— allows speculative motion of some load instructions. Again, only makes sense when scheduling before register allocation is enabled, i.e. with-fschedule-insnsor at-O2or higher. 🌸-fsched-spec-load-dangerous— allows speculative motion of more load instructions. Only makes sense when scheduling before register allocation is enabled, i.e. with-fschedule-insnsor at-O2or higher. It's giving dangerous, bestie, that's literally the name. 💀-fsched-stalled-insns,-fsched-stalled-insns=n— defines 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 no insns get moved prematurely.-fsched-stalled-insns=0means there's no limit on how many queued insns can be moved prematurely. And-fsched-stalled-insnswith no value is equivalent to-fsched-stalled-insns=1. 👑-fsched-stalled-insns-dep,-fsched-stalled-insns-dep=n— defines how many insn groups (cycles) get examined for a dependency on a stalled insn that's a candidate for premature removal from the queue of stalled insns. This only does anything during the second scheduling pass, and only if-fsched-stalled-insnsis used.-fno-sched-stalled-insns-depis equivalent to-fsched-stalled-insns-dep=0.-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, which results in faster schedules. This option is experimental, because not all machine descriptions used by GCC model the CPU closely enough to avoid unreliable results from the algorithm. Only makes sense when scheduling after register allocation, i.e. with-fschedule-insns2or at-O2or higher. 🧚-fsched-group-heuristic— enables the group heuristic in the scheduler. It favors the instruction that belongs to a schedule group. Enabled by default when scheduling is enabled, i.e. with-fschedule-insnsor-fschedule-insns2or at-O2or higher. 💖-fsched-critical-path-heuristic— enables the critical-path heuristic in the scheduler. It favors instructions sitting on the critical path. Enabled by default when scheduling is enabled, i.e. with-fschedule-insnsor-fschedule-insns2or at-O2or higher. ⭐-fsched-spec-insn-heuristic— enables the speculative instruction heuristic in the scheduler. It favors speculative instructions with greater dependency weakness. Enabled by default when scheduling is enabled, i.e. with-fschedule-insnsor-fschedule-insns2or at-O2or higher. 🦋-fsched-rank-heuristic— enables the rank heuristic in the scheduler. It favors the instruction belonging to a basic block with greater size or frequency. Enabled by default when scheduling is enabled, i.e. with-fschedule-insnsor-fschedule-insns2or at-O2or higher. 🍓-fsched-last-insn-heuristic— enables the last-instruction heuristic in the scheduler. It favors the instruction that's less dependent on the last instruction scheduled. Enabled by default when scheduling is enabled, i.e. with-fschedule-insnsor-fschedule-insns2or at-O2or higher. 💅-fsched-dep-count-heuristic— enables the dependent-count heuristic in the scheduler. It favors the instruction that has more instructions depending on it. Enabled by default when scheduling is enabled, i.e. with-fschedule-insnsor-fschedule-insns2or at-O2or higher. 👑-freschedule-modulo-scheduled-loops— modulo scheduling happens before traditional scheduling. So if a loop got modulo scheduled, later scheduling passes might change its schedule. Use this option to control that behavior. 🌸-fselective-scheduling— schedule instructions using the selective scheduling algorithm. Selective scheduling runs instead of the first scheduler pass. ✨-fselective-scheduling2— schedule instructions using the selective scheduling algorithm. Selective scheduling runs instead of the second scheduler pass. 💜-fsel-sched-pipelining— enables 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— some object formats, like ELF, let the dynamic linker interpose symbols. Which means: for symbols exported from the DSO, the compiler cannot do interprocedural propagation, inlining and other optimizations, because the function or variable in question might change. That feature is useful — say, for rewriting memory allocation functions with a debugging implementation — but it's expensive in terms of code quality. With-fno-semantic-interposition, the compiler assumes that if interposition happens for functions, the overwriting function will have precisely the same semantics (and side effects). Similarly, if interposition happens for variables, the constructor of the variable will be the same. The flag has no effect for functions explicitly declared inline (where interposition changing semantics is never allowed) and for symbols explicitly declared weak. 🌈-fshrink-wrap— emit function prologues only before the parts of the function that actually need them, instead of dumping them at the top of the function. Enabled by default at-Oand higher. 🎀-fshrink-wrap-separate— shrink-wrap separate parts of the prologue and epilogue separately, so those parts only get executed when needed. On by default, but has no effect unless-fshrink-wrapis also turned on and the target supports it. 🦋-fcaller-saves— enables allocating values to registers that get clobbered by function calls, by emitting extra instructions to save and restore those registers around such calls. That allocation only happens when it seems like it results in better code. On certain machines this option is always enabled by default — usually the ones with 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, 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 aren't used by any called function. In that case there's no need to save and restore them around calls. This is only possible if the called functions are part of the same compilation unit as the current function and they're compiled before it. Enabled at levels-O2,-O3,-Os— however, the option is disabled if the generated code will be instrumented for profiling (-p, or-pg) or if the callee's register usage can't be known exactly (this happens on targets that don't expose prologues and epilogues in RTL). 🔥-fconserve-stack— bestie, this one is literally trying to make your stack usage smaller. The compiler will attempt to use less stack space, even if that makes the program slower. It implies setting thelarge-stack-frameparameter to100and thelarge-stack-frame-growthparameter to400. 🫶-ftree-reassoc— does reassociation on trees. Enabled by default at-Oand higher. ✨-fcode-hoisting— performs code hoisting, which is the move where it tries to shove the evaluation of expressions that run on all paths to the function exit as early as possible. It's especially iconic as a code size optimization, but ngl it often helps code speed too. Enabled by default at-O2and higher. 🔥-ftree-pre— performs partial redundancy elimination (PRE) on trees. Enabled by default at-O2and-O3. 💅-ftree-partial-pre— makes partial redundancy elimination (PRE) more aggressive. Enabled by default at-O3. 😤-ftree-forwprop— performs forward propagation on trees. Enabled by default at-Oand higher. 🌸-ftree-fre— performs full redundancy elimination (FRE) on trees. So the tea on FRE vs PRE: 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. Enabled by default at-Oand higher. ✨-ftree-phiprop— performs hoisting of loads from conditional pointers on trees. This pass is enabled by default at-Oand higher. 🦋-fhoist-adjacent-loads— speculatively hoists 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. Enabled by default at-O2and higher. 🎀-ftree-copy-prop— performs copy propagation on trees. This pass eliminates unnecessary copy operations. Enabled by default at-Oand higher. 💜-fipa-pure-const— discovers which functions are pure or constant. Enabled by default at-Oand higher. 👑-fipa-reference— discovers which static variables do not escape the compilation unit. Enabled by default at-Oand higher. 🌈-fipa-reference-addressable— discovers read-only, write-only and non-addressable static variables. Enabled by default at-Oand higher. ⭐-fipa-stack-alignment— reduces stack alignment on call sites if possible. Enabled by default. 🧚-fipa-pta— performs interprocedural pointer analysis and interprocedural modification and reference analysis. Heads up though: 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— performs interprocedural profile propagation. The functions called only from cold functions are marked as cold. Also functions executed once (such as"cold","noreturn", static constructors or destructors) are identified. Cold functions and loop less parts of functions executed once are then optimized for size. Enabled by default at-Oand higher. 🫶-fipa-cp— performs interprocedural constant propagation. This optimization analyzes the program to determine when values passed to functions are constants and then optimizes accordingly. It can substantially increase performance if the application has constants passed to functions. Enabled by default at-O2,-Osand-O3. It is also enabled by-fprofile-useand-fauto-profile. 🔥-fipa-cp-clone— performs function cloning to make interprocedural constant propagation stronger. When enabled, interprocedural constant propagation performs function cloning when an 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 ipcp-unit-growth=value). Enabled by default at-O3. It is also enabled by-fprofile-useand-fauto-profile. 💀-fipa-bit-cp— when enabled, performs interprocedural bitwise constant propagation. Enabled by default at-O2and by-fprofile-useand-fauto-profile. It requires that-fipa-cpis enabled. ✨-fipa-vrp— when enabled, performs interprocedural propagation of value ranges. Enabled by default at-O2. It requires that-fipa-cpis enabled. 🌸-fipa-icf— performs Identical Code Folding for functions and read-only variables. This optimization reduces code size and may disturb unwind stacks by replacing a function by an equivalent one with a different name. The optimization works more effectively with link-time optimization enabled.
So like, even though the behavior is similar to the Gold Linker's ICF optimization, GCC ICF works on different levels and thus the optimizations are not same — there are equivalences that are found only by GCC and equivalences found only by Gold.
This flag is enabled by default at -O2 and -Os. 🦋
-flive-patching=level— controls 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, that latter one is called an impacted function of the former. And 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 functions, 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
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— detects paths that trigger erroneous or undefined behavior due to dereferencing a null pointer. It isolates those paths from the main control flow and turns the statement with erroneous or undefined behavior into a trap. Enabled by default at-O2and higher and depends on-fdelete-null-pointer-checksalso being enabled. 💅-fisolate-erroneous-paths-attribute— detects paths that trigger erroneous or undefined behavior due to a null value being used in a way forbidden by a"returns_nonnull"or"nonnull"attribute. It isolates those paths from the main control flow and turns the statement with erroneous or undefined behavior into a trap. This is not currently enabled, but may be enabled by-O2in the future. 🙈-ftree-sink— performs forward store motion on trees. Enabled by default at-Oand higher. 🍓-ftree-bit-ccp— performs sparse conditional bit constant propagation on trees and propagates pointer alignment information. This pass only operates on local scalar variables and is enabled by default at-O1and higher, except for-Og. It requires that-ftree-ccpis enabled. ✨-ftree-ccp— performs sparse conditional constant propagation (CCP) on trees. This pass only operates on local scalar variables and is enabled by default at-Oand higher. 💜-fssa-backprop— propagates information about uses of a value up the definition chain in order to simplify the definitions. For example, this pass strips sign operations if the sign of a value never matters. Enabled by default at-Oand higher. 🧚-fssa-phiopt— performs pattern matching on SSA PHI nodes to optimize conditional code. This pass is enabled by default at-O1and higher, except for-Og. 🌈-ftree-switch-conversion— performs conversion of simple initializations in a switch to initializations from a scalar array. Enabled by default at-O2and higher. 🔥-ftree-tail-merge— looks for identical code sequences. When found, 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 usingmax-tail-merge-comparisonsparameter andmax-tail-merge-iterationsparameter. 🎀-ftree-dce— performs dead code elimination (DCE) on trees. Enabled by default at-Oand higher. 💀-ftree-builtin-call-dce— performs conditional dead code elimination (DCE) for calls to built-in functions that may set"errno"but are otherwise free of side effects. Enabled by default at-O2and higher if-Osis not also specified. 🌸-ftree-dominator-opts— bestie this one does a whole lil spa day of simple scalar cleanups — constant/copy propagation, redundancy elimination, range propagation and expression simplification — all based on a dominator tree traversal. It also does jump threading (aka: jumps to jumps get reduced, iconic). Enabled by default at-Oand higher. 💅-ftree-dse— dead store elimination (DSE) on trees, periodt. A dead store is when you write into a memory location and then it gets overwritten by another store with no loads in between — so that earlier store is just… useless and can be deleted. Enabled by default at-Oand higher. 🗑️✨-ftree-ch— loop header copying on trees. Why do we love her? Because she makes code motion optimizations way more effective, and also saves one jump. Enabled by default at-Oand higher. BUT she's not enabled for-Os, since she usually makes your code bigger. 🙈-ftree-loop-optimize— loop optimizations on trees, no cap. Enabled by default at-Oand higher. 🔁-ftree-loop-linear-floop-strip-mine-floop-block— these three do loop nest optimizations. Same as-floop-nest-optimize. For this code transformation to work, GCC has to be configured with--with-islso the Graphite loop transformation infrastructure is enabled. 🏗️-fgraphite-identity— enables the identity transformation for graphite. For every SCoP we generate the polyhedral representation and transform it right back to gimple. With-fgraphite-identityyou can check the costs or benefits of theGIMPLE -> GRAPHITE -> GIMPLEtransformation. Some minimal optimizations also get done by the code generatorisl, like index splitting and dead code elimination in loops. 🔄💖-floop-nest-optimize— enables theislbased loop nest optimizer. It's 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— uses the Graphite data dependence analysis to spot loops that can be parallelized, then parallelizes all the loops it can analyze as not containing loop carried dependences — without even checking whether parallelizing them is profitable. It's giving chaos but make it optimized. 🌈-ftree-coalesce-vars— while transforming the program out of SSA representation, this tries to reduce copying by coalescing versions of different user-defined variables, not just compiler temporaries. Warning tho: this may severely limit your ability to debug an optimized program compiled with-fno-var-tracking-assignments. In negated form, this flag stops SSA coalescing of user variables. Enabled by default if optimization is enabled, and it does very little otherwise. 🫶-ftree-loop-if-convert— attempts to transform conditional jumps in the innermost loops into branch-less equivalents. The whole point is to remove control-flow from the innermost loops so the vectorization pass can handle them better. Enabled by default if vectorization is enabled. 🎀-ftree-loop-distribution— performs loop distribution. This can improve cache performance on big loop bodies and lets further loop optimizations happen, like parallelization or vectorization. For example, this loop:
DO I = 1, N
A(I) = B(I) + C
D(I) = E(I) * F
ENDDOgets transformed to:
DO I = 1, N
A(I) = B(I) + C
ENDDO
DO I = 1, N
D(I) = E(I) * F
ENDDOEnabled by default at -O3. It's also enabled by -fprofile-use and -fauto-profile. 🔥
-ftree-loop-distribute-patterns— performs loop distribution of patterns that can be code generated with calls to a library. Enabled by default at-O3, and by-fprofile-useand-fauto-profile.
This pass distributes the initialization loops and generates a call to memset zero. For example, this loop:
DO I = 1, N
A(I) = 0
B(I) = A(I) + I
ENDDOgets transformed to:
DO I = 1, N
A(I) = 0
ENDDO
DO I = 1, N
B(I) = A(I) + I
ENDDOand the initialization loop gets turned into a call to memset zero. Enabled by default at -O3. It's also enabled by -fprofile-use and -fauto-profile. ✨
-floop-interchange— performs loop interchange outside of graphite. This can improve cache performance on loop nests and lets further loop optimizations, like vectorization, take place. For example, this 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];gets 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];Enabled by default at -O3. It's also enabled by -fprofile-use and -fauto-profile. 💜
-floop-unroll-and-jam— applies 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. Enabled by default at-O3. It's also enabled by-fprofile-useand-fauto-profile. 🍓-ftree-loop-im— performs 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— creates a canonical counter for the number of iterations in loops where figuring out the iteration count requires complicated analysis. Later optimizations can then determine the number easily. Extra useful in connection with unrolling. 🔢-ftree-scev-cprop— performs final value replacement. If a variable is modified in a loop such that its value when exiting the loop can be determined using only its initial value and the number of loop iterations, then uses of the final value get replaced by such a computation — provided it's sufficiently cheap. This reduces data dependencies and may allow further simplifications. Enabled by default at-Oand higher. 👑-fivopts— performs induction variable optimizations (strength reduction, induction variable merging and induction variable elimination) on trees. 💫-ftree-parallelize-loops=n— parallelizes loops, i.e., splits 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 by e.g. memory bandwidth. This option implies-pthread, and thus is only supported on targets that have support for-pthread. 🧵🧚-ftree-pta— performs function-local points-to analysis on trees. Enabled by default at-O1and higher, except for-Og. 📍-ftree-sra— performs scalar replacement of aggregates. This pass replaces structure references with scalars so structures don't get committed to memory too early. Enabled by default at-O1and higher, except for-Og. 🧩-fstore-merging— performs merging of narrow stores to consecutive memory addresses. This pass merges contiguous stores of immediate values narrower than a word into fewer wider stores, so the instruction count goes down. Enabled by default at-O2and higher as well as-Os. 🧷-ftree-ter— performs temporary expression replacement during the SSA->normal phase. Single use/single def temporaries get replaced at their use location with their defining expression. The result is non-GIMPLE code, but it gives the expanders much more complex trees to work on, which results in better RTL generation. Enabled by default at-Oand higher. 🔧-ftree-slsr— performs straight-line strength reduction on trees. It recognizes related expressions involving multiplications and replaces them with less expensive calculations when possible. Enabled by default at-Oand higher. 🪄-ftree-vectorize— performs vectorization on trees. This flag enables-ftree-loop-vectorizeand-ftree-slp-vectorizeif they aren't explicitly specified. ⚡-ftree-loop-vectorize— performs loop vectorization on trees. Enabled by default at-O3and by-ftree-vectorize,-fprofile-use, and-fauto-profile. 🔁⚡-ftree-slp-vectorize— performs basic block vectorization on trees. Enabled by default at-O3and by-ftree-vectorize,-fprofile-use, and-fauto-profile. 🧱⚡-fvect-cost-model=model— changes up the cost model used for vectorization. The model argument should be one ofunlimited,dynamicorcheap. With theunlimitedmodel the vectorized code-path is assumed to be profitable; with thedynamicmodel 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 — for example due to required runtime checks for data dependence or alignment — but otherwise it's equal to thedynamicmodel. The default cost model depends on other optimization flags and is eitherdynamicorcheap. 💸-fsimd-cost-model=model— changes up 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
Perform Value Range Propagation on trees. It's giving constant propagation's cooler cousin — except instead of yeeting values around, we're propagating ranges of values. Which means the optimizers get to delete range checks they don't need anymore, like array bound checks and null pointer checks 💅. Enabled by default at -O2 and higher. Oh and heads up — null pointer check elimination only happens if -fdelete-null-pointer-checks is enabled. No cap.
🦋 -fsplit-paths
Split paths leading to loop backedges. This can improve dead code elimination and common subexpression elimination. Enabled by default at -O3 and above. Slay.
🎀 -fsplit-ivs-in-unroller
Enables expression of values of induction variables in later iterations of the unrolled loop using the value in the first iteration. Basically it breaks those long dependency chains, which makes the scheduling passes way more efficient. Iconic behavior honestly.
A combination of -fweb and CSE is often sufficient to obtain the same effect. However, that is not reliable in cases where the loop body is more complicated than a single basic block. It also does not work at all on some architectures due to restrictions in the CSE pass. 😭
This optimization is enabled by default.
✨ -fvariable-expansion-in-unroller
With this option, the compiler creates multiple copies of some local variables when unrolling a loop, which can result in superior code. Obsessed.
💖 -fpartial-inlining
Inline parts of functions. Lowkey a picky one tho — this option has any effect only when inlining itself is turned on by the -finline-functions or -finline-small-functions options.
Enabled at levels -O2, -O3, -Os.
🧚 -fpredictive-commoning
Perform predictive commoning optimization, i.e., reusing computations (especially memory loads and stores) performed in previous iterations of loops. Main character energy fr fr.
This option is enabled at level -O3. It is also enabled by -fprofile-use and -fauto-profile.
🌸 -fprefetch-loop-arrays
If supported by the target machine, generate instructions to prefetch memory to improve the performance of loops that access large arrays.
Real talk bestie: this option may generate better or worse code; results are highly dependent on the structure of loops within the source code. So vibe check your benchmarks before you commit.
Disabled at level -Os.
💀 -fno-printf-return-value
Do not substitute constants for known return value of formatted output functions such as sprintf, snprintf, vsprintf, and vsnprintf (but not printf of fprintf). This transformation allows GCC to optimize or even eliminate branches based on the known return value of these functions called with arguments that are either constant, or whose values are known to be in a range that makes determining the exact return value possible. For example, when -fprintf-return-value is in effect, both the branch and the body of the if statement (but not the call to snprint) can be optimized away when i is a 32-bit or smaller integer because the return value is guaranteed to be at most 8.
char buf[9];
if (snprintf (buf, "%08x", i) >= sizeof buf)
...The -fprintf-return-value option relies on other optimizations and yields best results with -O2 and above. It works in tandem with the -Wformat-overflow and -Wformat-truncation options. The -fprintf-return-value option is enabled by default. 🫶
🔥 -fno-peephole
🔥 -fno-peephole2
Disable any machine-specific peephole optimizations. The difference between -fno-peephole and -fno-peephole2 is in how they are implemented in the compiler; some targets use one, some use the other, a few use both. It's giving ✨implementation details✨.
-fpeephole is enabled by default. -fpeephole2 enabled at levels -O2, -O3, -Os.
👑 -fno-guess-branch-probability
Do not guess branch probabilities using heuristics. Okay so story time:
GCC uses heuristics to guess branch probabilities if they are not provided by profiling feedback (-fprofile-arcs). These heuristics are based on the control flow graph. If some branch probabilities are specified by __builtin_expect, then the heuristics are used to guess branch probabilities for the rest of the control flow graph, taking the __builtin_expect info into account. The interactions between the heuristics and __builtin_expect can be complex, and in some cases, it may be useful to disable the heuristics so that the effects of __builtin_expect are easier to understand. Not me needing to debug my own compiler vibes 😭
It is also possible to specify expected probability of the expression with __builtin_expect_with_probability built-in function.
The default is -fguess-branch-probability at levels -O, -O2, -O3, -Os.
⭐ -freorder-blocks
Reorder basic blocks in the compiled function in order to reduce number of taken branches and improve code locality.
Enabled at levels -O, -O2, -O3, -Os.
💜 -freorder-blocks-algorithm=algorithm
Use the specified algorithm for basic block reordering. The algorithm argument can be simple, which does not increase code size (except sometimes due to secondary effects like alignment), or stc, the "software trace cache" algorithm, which tries to put all often executed code together, minimizing the number of branches executed by making extra copies of code.
The default is simple at levels -O, -Os, and stc at levels -O2, -O3.
🌈 -freorder-blocks-and-partition
In addition to reordering basic blocks in the compiled function, in order to reduce number of taken branches, partitions hot and cold basic blocks into separate sections of the assembly and .o files, to improve paging and cache locality performance. Hot code and cold code are literally in different buildings now, iconic separation.
This optimization is automatically turned off in the presence of exception handling or unwind tables (on targets using setjump/longjump or target specific scheme), for linkonce sections, for functions with a user-defined section attribute and on any architecture that does not support named sections. When -fsplit-stack is used this option is not enabled by default (to avoid linker errors), but may be enabled explicitly (if using a working linker). 🙈
Enabled for x86 at levels -O2, -O3, -Os.
🦋 -freorder-functions
Reorder functions in the object file in order to improve code locality. This is implemented by using special subsections .text.hot for most frequently executed functions and .text.unlikely for unlikely executed functions. Reordering is done by the linker so object file format must support named sections and linker must place them in a reasonable way. So like, the linker is doing the final glow-up 💅
This option isn't effective unless you either provide profile feedback (see-fprofile-arcsfor details) or manually annotate functions withhotorcoldattributes. Otherwise it's just sitting there with nothing to work with 😭
Enabled at levels -O2, -O3, -Os.
🎀 -fstrict-aliasing
Allow the compiler to assume the strictest aliasing rules applicable to the language being compiled. For C (and C++), this activates optimizations based on the type of expressions. In particular, an object of one type is assumed never to reside at the same address as an object of a different type, unless the types are almost the same. For example, an unsigned int can alias an int, but not a void* or a double. A character type may alias any other type. No cap.
Pay special attention to code like this:
union a_union {
int i;
double d;
};
int f() {
union a_union t;
t.d = 3.0;
return t.i;
}The practice of reading from a different union member than the one most recently written to (called "type-punning") is common. Even with -fstrict-aliasing, type-punning is allowed, provided the memory is accessed through the union type. So, the code above works as expected. However, this code might not:
int f() {
union a_union t;
int* ip;
t.d = 3.0;
ip = &t.i;
return *ip;
}Similarly, access by taking the address, casting the resulting pointer and dereferencing the result has undefined behavior, even if the cast uses a union type, e.g.:
int f() {
double d = 3.0;
return ((union a_union *) &d)->i;
}⚠️ Undefined behavior means the compiler is allowed to do literally whatever, bestie. Not a vibe. Do not do this.
The -fstrict-aliasing option is enabled at levels -O2, -O3, -Os.
✨ -falign-functions
✨ -falign-functions=n
✨ -falign-functions=n:m
✨ -falign-functions=n:m:n2
✨ -falign-functions=n:m:n2:m2
Align the start of functions to the next power-of-two greater than n, skipping up to m-1 bytes. This ensures that at least the first m bytes of the function can be fetched by the CPU without crossing an n-byte alignment boundary.
If m is not specified, it defaults to n.
Examples: -falign-functions=32 aligns functions to the next 32-byte boundary, -falign-functions=24 aligns to the next 32-byte boundary only if this can be done by skipping 23 bytes or less, -falign-functions=32:7 aligns to the next 32-byte boundary only if this can be done by skipping 6 bytes or less.
The second pair of n2:m2 values allows you to specify a secondary alignment: -falign-functions=64:7:32:3 aligns to the next 64-byte boundary if this can be done by skipping 6 bytes or less, otherwise aligns to the next 32-byte boundary if this can be done by skipping 2 bytes or less. If m2 is not specified, it defaults to n2.
Some assemblers only support this flag when n is a power of two; in that case, it is rounded up.
-fno-align-functions and -falign-functions=1 are equivalent and mean that functions are not aligned.
If n is not specified or is zero, use a machine-dependent default. The maximum allowed n option value is 65536.
Enabled at levels -O2, -O3.
💖 -flimit-function-alignment
If this option is enabled, the compiler tries to avoid unnecessarily overaligning functions. It attempts to instruct the assembler to align by the amount specified by -falign-functions, but not to skip more bytes than the size of the function. Respectful, mindful, unproblematic 💅
🌸 -falign-labels
🌸 -falign-labels=n
🌸 -falign-labels=n:m
🌸 -falign-labels=n:m:n2
🌸 -falign-labels=n:m:n2:m2
Align all branch targets to a power-of-two boundary.
Parameters of this option are analogous to the -falign-functions option. -fno-align-labels and -falign-labels=1 are equivalent and mean that labels are not aligned.
If -falign-loops or -falign-jumps are in the mix and their values are bigger than this one? Then their values win, periodt. No contest. ✨
If n isn't given, or it's zero, you get a machine-dependent default that is very likely to be 1 — which means no alignment at all, bestie, we're just rawdogging the layout. And the max allowed n value is 65536. That's the ceiling. Do not test it. 💀
Turned on at levels -O2, -O3.
✨ the -falign-loops family ✨
-falign-loops-falign-loops=n-falign-loops=n:m-falign-loops=n:m:n2-falign-loops=n:m:n2:m2
Okay so this one aligns loops to a power-of-two boundary. And like… why would you do that? Because if the loops run a lot, the payoff totally makes up for all the dummy padding instructions you had to insert. It's giving investment. It's giving "spend a little to win a lot". 💅
The parameters here work exactly the same way as the -falign-functions option — same deal, same structure, just for loops. And heads up: -fno-align-loops and -falign-loops=1 are equivalent, and both of them mean loops are simply not aligned. The maximum allowed n value is 65536.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3. 👑
✨ the -falign-jumps family ✨
-falign-jumps-falign-jumps=n-falign-jumps=n:m-falign-jumps=n:m:n2-falign-jumps=n:m:n2:m2
This one aligns branch targets to a power-of-two boundary — specifically branch targets that can only be reached by jumping. And the cute part? In this case, no dummy operations need to be executed. Free real estate, bestie. 🎀
Parameters are analogous to the -falign-functions option, same as above. -fno-align-jumps and -falign-jumps=1 are equivalent and mean that loops are not aligned.
If n is not specified or is zero, use a machine-dependent default. The maximum allowed n option value is 65536.
Enabled at levels -O2, -O3.
✨ -funit-at-a-time ✨
-funit-at-a-time— honestly? This one is just vibing here for compatibility reasons. It has no effect whatsoever. Meanwhile-fno-unit-at-a-timeimplies-fno-toplevel-reorderand-fno-section-anchors.
Enabled by default. (The no-effect one, not the other one. iykyk.)
✨ -fno-toplevel-reorder ✨
Do not reorder top-level functions, variables, and asm statements. Output them in the exact same order that they appear in the input file. Like, respect the queue, please. 🙈
When you use this option, unreferenced static variables are not removed. So yeah, that's the tradeoff.
This option is intended to support existing code that relies on a particular ordering. For new code, it is better to use attributes when possible — like, attributes are the glow-up, this flag is the nostalgia. 🦋
-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, the loop optimizer, and the trivial dead code remover. Multi-purpose slay. 💖
But it can, however, make debugging impossible, since variables no longer stay in a "home register". So like… pick your fighter, bestie.
Enabled by default with -funroll-loops.
✨ -fwhole-program ✨
Assume that the current compilation unit represents the whole program being compiled. All public functions and variables — except main and those merged by attribute externally_visible — become static functions, and in effect get optimized more aggressively by interprocedural optimizers. Main character energy for exactly one translation unit. 👑
This option should not be used in combination with -flto. Instead, relying on a linker plugin should provide safer and more precise information. Don't do it. I'm serious. 😭
✨ -flto[=n] ✨
Okay this is the big one, the moment we've all been waiting for. 💅
This option runs the standard link-time optimizer. When invoked with source code, it generates GIMPLE (one of GCC's internal representations) and writes it to special ELF sections in the object file. When the object files are linked together, all the function bodies are read from these ELF sections and instantiated as if they had been part of the same translation unit. It's giving "surprise, we were one file all along". ✨
To use the link-time optimizer, -flto and optimization options should be specified at compile time and during the final link. It is recommended that you compile all the files participating in the same link with the same options and also specify those options at link time. For example:
gcc -c -O2 -flto foo.c
gcc -c -O2 -flto bar.c
gcc -o myprog -flto -O2 foo.o bar.oThe first two invocations to GCC save a bytecode representation of GIMPLE into special ELF sections inside foo.o and bar.o. The final invocation reads the GIMPLE bytecode from foo.o and bar.o, merges the two files into a single internal image, and compiles the result as usual. Since both foo.o and bar.o are merged into a single image, this causes all the interprocedural analyses and optimizations in GCC to work across the two files as if they were a single one. This means, for example, that the inliner is able to inline functions in bar.o into functions in foo.o and vice-versa. Bestie crossover episode. 🫶
Another (simpler) way to enable 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 is that to enable link-time optimizations you need to use the GCC driver to perform the link step. GCC automatically performs link-time optimization if any of the objects involved were compiled with the -flto command-line option. You can always override the automatic decision to do link-time optimization by passing -fno-lto to the link command. You're the one in charge, always. 💅
To make whole program optimization effective, it is necessary to make certain whole program assumptions. The compiler needs to know what functions and variables can be accessed by libraries and runtime outside of the link-time optimized unit. When supported by the linker, the linker plugin (see -fuse-linker-plugin) passes information to the compiler about used and externally visible symbols. When the linker plugin is not available, -fwhole-program should be used to allow the compiler to make these assumptions, which leads to more aggressive optimization decisions.
When a file is compiled with -flto without -fuse-linker-plugin, the generated object file is larger than a regular object file because it contains GIMPLE bytecodes and the usual final code (see -ffat-lto-objects). This means that object files with LTO information can be linked as normal object files; if -fno-lto is passed to the linker, no interprocedural optimizations are applied. Note that when -fno-fat-lto-objects is enabled the compile stage is faster but you cannot perform a regular, non-LTO link on them.
When producing the final binary, GCC only applies link-time optimizations to those files that contain bytecode. Therefore, you can mix and match object files and libraries with GIMPLE bytecodes and final object code. GCC automatically selects which files to optimize in LTO mode and which files to link without further processing. It just knows. ✨
Generally, options specified at link time override those specified at compile time, although in some cases GCC attempts to infer link-time options from the settings used to compile the input files.
If you do not specify an optimization level option -O at link time, then GCC uses the highest optimization level used when compiling the object files. Note that it is generally ineffective to specify an optimization level option only at link time and not at compile time, for two reasons:
- Compiling without optimization suppresses compiler passes that gather information needed for effective optimization at link time.
- Some early optimization passes can be performed only at compile time and not at link time.
So yeah. Do it at compile time. Fr fr. 😭
There are some code generation flags preserved by GCC when generating bytecodes, as they need to be used during the final link. Currently, the following options and their settings are taken from the first object file that explicitly specifies them: -fPIC, -fpic, -fpie, -fcommon, -fexceptions, -fnon-call-exceptions, -fgnu-tm and all the -m target flags.
Certain ABI-changing flags are required to match in all compilation units, and trying to override this at link time with a conflicting value is ignored. This includes options such as -freg-struct-return and -fpcc-struct-return.
Other options such as -ffp-contract, -fno-strict-overflow, -fwrapv, -fno-trapv or -fno-strict-aliasing are passed through to the link stage and merged conservatively for conflicting translation units. Specifically -fno-strict-overflow, -fwrapv and -fno-trapv take precedence; and for example -ffp-contract=off takes precedence over -ffp-contract=fast. You can override them at link time.
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. Consistency is the vibe check here. 🎀
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 explicitely repeated at the linker command line and mixing different settings in different translation units is discouraged.
If LTO encounters objects with C linkage declared with incompatible types in separate translation units to be linked together (undefined behavior according to ISO C99 6.2.7), a non-fatal diagnostic may be issued. The behavior is still undefined at run time. Similar diagnostics may be raised for other languages.
Another feature of LTO is that it is possible to apply interprocedural optimizations on files written in different languages:
gcc -c -flto foo.c
g++ -c -flto bar.cc
gfortran -c -flto baz.f90
g++ -o myprog -flto -O3 foo.o bar.o baz.o -lgfortranNotice that the final link is done with g++ to get the C++ runtime libraries and -lgfortran is added to get the Fortran runtime libraries. In general, when mixing languages in LTO mode, you should use the same link command options as when mixing languages in a regular (non-LTO) compilation. Multilingual queen behavior. 👑
okay so picture this 💭 — you've got object files holding GIMPLE bytecode just vibing inside a library archive, let's say libfoo.a. Bestie, you can pull those out and use them in an LTO link, but only if your linker has plugin support. To build static libraries that are LTO-ready, use gcc-ar and gcc-ranlib instead of plain old ar and ranlib. And to peek at the symbols inside object files with GIMPLE bytecode, use gcc-nm. Those commands only work if ar, ranlib and nm were themselves compiled with plugin support, iykyk. Then at link time, hit it with -fuse-linker-plugin so the library actually joins the LTO optimization party:
gcc -o myprog -O2 -flto -fuse-linker-plugin a.o b.o -lfooWith the linker plugin on, the linker yanks the GIMPLE files it needs out of libfoo.a and hands them to the running GCC, so they become part of the aggregated GIMPLE image getting optimized. iconic ✨
But listen 👀 — if you're not using a linker with plugin support and/or you don't enable the linker plugin, then the objects inside libfoo.a still get extracted and linked like normal, they just don't get to sit at the LTO table. No optimization invite for them, sorry babe. So if you want a static library that works for both LTO optimization and usual linkage, compile its object files with -flto -ffat-lto-objects. Dual citizenship, we love that for her 🫶
Link-time optimizations do not need the whole program present to do their thing. If your program doesn't need any symbols exported, you can combine -flto and -fwhole-program to let the interprocedural optimizers make more aggressive assumptions — which may open up better optimization opportunities. But heads up: you don't need -fwhole-program when the linker plugin is active (see -fuse-linker-plugin). Periodt 💅
⚠️ The current implementation of LTO does not even try to generate bytecode that's portable between different types of hosts. The bytecode files are versioned and there's a strict version check, so bytecode files made by one version of GCC will not work with an older or newer version of GCC. Fr fr, no cap.
😭 Also: 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 runs in parallel using n parallel jobs, by utilizing an installed make program. The environment variable MAKE may be used to override the program used. Slay, parallelism 💖
You can also specify -flto=jobserver to use GNU make's job server mode to figure out 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.
Use -flto=auto to use GNU make's job server, if it's available, or otherwise fall back to autodetection of the number of CPU threads present in your system. Adaptive queen 🌸
-flto-partition=alg— tell the link-time optimizer which partitioning algorithm to use. The value is either1to1to mirror the original source files, orbalancedto split into equally sized chunks (whenever possible), ormaxto create a new partition for every symbol where possible. Specifyingnoneas an algorithm disables partitioning and streaming completely. The default value isbalanced. While1to1can be used as a workaround for various code ordering issues, themaxpartitioning is intended for internal testing only 🙈. The valueonespecifies that exactly one partition should be used, while the valuenonebypasses partitioning and executes the link-time optimization step directly from the WPA phase.-flto-odr-type-merging— enable streaming of mangled type names of C++ types and their unification at link time. This increases the size of LTO object files, but enables diagnostics about One Definition Rule violations. Bigger files, better receipts 🔍-flto-compression-level=n— sets the level of compression used for the intermediate language written to LTO object files, and is only meaningful in conjunction with LTO mode (-flto). Valid values are0(no compression) to9(maximum compression). Values outside this range are clamped to either0or9. If the option isn't given, a default balanced compression setting is used. Squish responsibly 🎀-fuse-linker-plugin— enables the use of a linker plugin during link-time optimization. This option relies on plugin support in the linker, which is available in gold or in GNU ld 2.21 or newer.
This option enables the extraction of object files with GIMPLE bytecode out of library archives. This improves the quality of optimization by exposing more code to the link-time optimizer. This information specifies what symbols can be accessed externally (by non-LTO object or during dynamic linking). Resulting code quality improvements on binaries (and shared libraries that use hidden visibility) are similar to -fwhole-program. See -flto for a description of the effect of this flag and how to use it.
This option is enabled by default when LTO support in GCC is enabled and GCC was configured for use with a linker supporting plugins (GNU ld 2.21 or newer or gold). Default-on, main character energy 👑
-ffat-lto-objects— fat LTO objects are object files that contain both the intermediate language and the object code. This makes them usable for both LTO linking and normal linking. This option is effective only when compiling with-fltoand is ignored at link time.
-fno-fat-lto-objects improves compilation time over plain LTO, but requires the complete toolchain to be aware of LTO. It requires a linker with linker plugin support for basic functionality. Additionally, nm, ar and ranlib need to support linker plugins to allow a full-featured build environment (capable of building static libraries etc). GCC provides the gcc-ar, gcc-nm, gcc-ranlib wrappers to pass the right options to these tools. With non fat LTO, makefiles need to be modified to use them. 😮💨
Note that modern binutils provide plugin auto-load mechanism. Installing the linker plugin into $libdir/bfd-plugins has the same effect as usage of the command wrappers (gcc-ar, gcc-nm and gcc-ranlib).
The default is -fno-fat-lto-objects on targets with linker plugin support.
-fcompare-elim— after register allocation and post-register allocation instruction splitting, identify arithmetic instructions that compute processor flags similar to a comparison operation based on that arithmetic. If possible, eliminate the explicit comparison operation. Clean ✂️
This pass only applies to certain targets that cannot explicitly represent the comparison operation before register allocation is complete.
Enabled at levels -O, -O2, -O3, -Os.
-fcprop-registers— after register allocation and post-register allocation instruction splitting, perform a copy-propagation pass to try to reduce scheduling dependencies and occasionally eliminate the copy. Enabled at levels-O,-O2,-O3,-Os. Efficiency bestie 💜-fprofile-correction— profiles collected using an instrumented binary for multi-threaded programs may be inconsistent because of missed counter updates. When this option is specified, GCC uses heuristics to correct or smooth out such inconsistencies. By default, GCC emits an error message when an inconsistent profile is detected. 😭
This option is enabled by -fauto-profile.
-fprofile-use/-fprofile-use=path— enable 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 profile, no vibes 🚫
By default, GCC emits an error message if the feedback profiles do not match the source code. This error can be turned into a warning by using -Wno-error=coverage-mismatch. Note this may result in poorly optimized code. Additionally, by default, GCC also emits a warning message if the feedback profiles do not exist (see -Wmissing-profile).
If path is specified, GCC looks at the path to find the profile feedback data files. See -fprofile-dir.
-fauto-profile/-fauto-profile=path— enable sampling-based feedback-directed optimizations, 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 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 that can be used by GCC. 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 — none of them are on by default, so you have to opt in every single time 🧚
-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. Keeping it in memory, keeping it honest 💅
Okay so this is the tail end of -ffloat-store — it stops machines like the 68000 from doing that thing where the floating registers (the 68881 ones, bestie) hoard more precision than a double is ever supposed to have. Same deal on x86. And like, for most programs that extra precision is only doing good, but a few programs are out here relying on the exact IEEE floating point definition, and for those you use -ffloat-store — after you've gone in and edited them so all the pertinent intermediate computations get stored into variables. 😌
-fexcess-precision=style— this one gives you further control over excess precision on machines where floating-point ops happen in a format with more precision or range than the IEEE standard and the interchange floating-point types. By default,-fexcess-precision=fastis in effect — meaning ops may be carried out in a wider precision than the types specified in the source if that makes the code faster, and it is unpredictable when rounding to the source-specified types actually happens. When compiling C, if you say-fexcess-precision=standardthen excess precision follows the rules from ISO C99 — and specifically, 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 like-std=c99is used. And-ffast-mathenables-fexcess-precision=fastby default, no matter whether a strict conformance option is used or not.-fexcess-precision=standardis not implemented for languages other than C. On x86 it has no effect if-mfpmath=sseor-mfpmath=sse+387is specified — in the former case, IEEE semantics apply with no excess precision at all, 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. This option causes the preprocessor macro__FAST_MATH__to be defined. ✨ It is not turned on by any-Ooption 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 need those guarantees. 🏎️-fno-math-errno— don't seterrnoafter calling math functions that get executed with a single instruction, e.g.sqrt. A program that relies on IEEE exceptions for math error handling might want this flag for speed while still keeping IEEE arithmetic compatibility. This option is not turned on by any-Ooption 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. The default is-fmath-errno. On Darwin systems, the math library never setserrno. So there's genuinely no reason for the compiler to even consider the possibility, and-fno-math-errnois 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 include libraries or startup files that change the default FPU control word or other similar optimizations. 😬 This option is not turned on by any-Ooption 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. 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 as well as ignore NaNs and inhibit or create underflow or overflow — and thus cannot be used on code that relies on rounding behavior like(x + 2**52) - 2**52. It may also reorder floating-point comparisons, and thus may not be used when ordered comparisons are required. This option requires that both-fno-signed-zerosand-fno-trapping-mathbe in effect. And moreover, it doesn't make much sense with-frounding-math. For Fortran the option is automatically enabled when both-fno-signed-zerosand-fno-trapping-mathare in effect. The default is-fno-associative-math. 🦋-freciprocal-math— allow the reciprocal of a value to be used instead of dividing by the value, if this enables optimizations. For 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 option is not turned on by any-Ooption 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. The default is-fno-finite-math-only.-fno-signed-zeros— allow optimizations for floating-point arithmetic that ignore the signedness of zero. IEEE arithmetic specifies the behavior of distinct+0.0and-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're relying on "non-stop" IEEE arithmetic, for example. This option should never be turned on by any-Ooption, 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. You should specify this option for programs that change the FP rounding mode dynamically, or that may be executed with a non-default rounding mode. This option disables constant folding of floating-point expressions at compile time (which may be affected by rounding mode) and arithmetic transformations that are unsafe in the presence of sign-dependent rounding modes. The default is-fno-rounding-math.
Heads up bestie: 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.
Also experimental, babes: it 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 the"inexact"floating-point exception for noninteger arguments. ISO C99 and C11 allow these functions to raise the"inexact"exception, but ISO/IEC TS 18661-1:2014, the C bindings to IEEE 754-2008, does not allow these functions to do so. The default is-ffp-int-builtin-inexact, allowing the exception to be raised. This option does nothing unless-ftrapping-mathis in effect. Even if-fno-fp-int-builtin-inexactis used, if the functions generate a call to a library function then the"inexact"exception may be raised if the library implementation does not follow TS 18661. 😭-fsingle-precision-constant— treat floating-point constants as single precision instead of implicitly converting them to double-precision constants. 🎀-fcx-limited-range— when enabled, this option states that a range reduction step is not needed when performing complex division. Also, there'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 C99CX_LIMITED_RANGEpragma. 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. 🧚
Okay so 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. You have been warned, bestie. 💅
-fbranch-probabilities— bestie, this is the glow-up round 💅. So you ran your program once compiled with-fprofile-arcs, right? Now compile it a second time with-fbranch-probabilitiesand GCC will optimize based on how many times each branch actually got taken. When a program built with-fprofile-arcsexits, it saves arc execution counts into a file calledsourcename.gcdafor each source file. Heads up tho: that data file is extremely dependent on the structure of the generated code, so you must use the same source code and the same optimization options for both compilations. No switching up on it. 🙈
With -fbranch-probabilities, GCC puts a REG_BR_PROB note on each JUMP_INSN and CALL_INSN. These can be used to improve optimization. Currently, they are only used in one place: in reorg.c, 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. Iconic.
Enabled by -fprofile-use and -fauto-profile.
-fprofile-values— if you pair this 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. It collects the first time of execution of a function and orders these functions in ascending order. Little lineup, very organized, we love to see it 🎀
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. Periodt.
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. But fair warning bestie: depending on the debug information format adopted by the target, it can make debugging impossible, since variables no longer stay in a "home register". 💀
Enabled by default with -funroll-loops.
-fschedule-fusion— performs a target dependent pass over the instruction stream to schedule instructions of same type together because the target machine can execute them more efficiently if they are adjacent to each other in the instruction flow. Besties standing next to each other, always more powerful 🫶
Enabled at levels -O2, -O3, -Os.
-ftracer— perform tail duplication to enlarge superblock size. This transformation simplifies the control flow of the function allowing other optimizations to do a better job. Setting the stage for the other passes to eat 🔥
Enabled by -fprofile-use and -fauto-profile.
-funroll-loops— unroll loops whose number of iterations can be determined at compile time or upon entry to the loop.-funroll-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). This option makes code larger, and may or may not make it run faster. So like… it might slay, it might not, no cap. 😭
Enabled by -fprofile-use and -fauto-profile.
-funroll-all-loops— unroll all loops, even if their number of iterations is uncertain when the loop is entered. This usually makes programs run more slowly. Not the vibe, genuinely.-funroll-all-loopsimplies the same options as-funroll-loops.-fpeel-loops— peels loops for which there is enough information that they do not roll much (from profile feedback or static analysis). It also turns on complete loop peeling (i.e. complete removal of loops with small constant number of iterations).
Enabled by -O3, -fprofile-use, and -fauto-profile.
-fmove-loop-invariants— enables the loop invariant motion pass in the RTL loop optimizer. Enabled at level-O1and higher, except for-Og.-fsplit-loops— split a loop into two if it contains a condition that's always true for one side of the iteration space and false for the other. The way it just divides them 👑
Enabled by -fprofile-use and -fauto-profile.
-funswitch-loops— move branches with loop invariant conditions out of the loop, with duplicates of the loop on both branches (modified according to result of the condition).
Enabled by -fprofile-use and -fauto-profile.
-fversion-loops-for-strides— if a loop iterates over an array with a variable stride, create another version of the loop that assumes the stride is always one. Like, for example:
for (int i = 0; i < n; ++i)
x[i * stride] = ...;becomes:
if (stride == 1)
for (int i = 0; i < n; ++i)
x[i] = ...;
else
for (int i = 0; i < n; ++i)
x[i * stride] = ...;This is particularly useful for assumed-shape arrays in Fortran where (for example) it allows better vectorization assuming contiguous accesses. This flag is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile.
-ffunction-sections-fdata-sections— place each function or data item into its own section in the output file if the target supports arbitrary sections. The name of the function or the name of the data item determines the section's name in the output file. Everyone gets their own lil room ✨
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.
-fbranch-target-load-optimize— perform branch target register load optimization before prologue / epilogue threading. The use of target registers can typically be exposed only during reload, thus hoisting loads out of loops and doing inter-block scheduling needs a separate optimization pass.-fbranch-target-load-optimize2— perform branch target register load optimization after prologue / epilogue threading. (Yes bestie, the sequel 🎬)-fbtr-bb-exclusive— when performing branch target register load optimization, don't reuse branch target registers within any basic block.-fstdarg-opt— optimize the prologue of variadic argument functions with respect to usage of those arguments.
NOTE: In Ubuntu 14.10 and later versions,-fstack-protector-strongis enabled by default for C, C++, ObjC, ObjC++, if none of-fno-stack-protector,-nostdlib, nor-ffreestandingare found.
-fsection-anchors— try to reduce the number of symbolic address calculations by using shared "anchor" symbols to address nearby objects. This transformation can help to reduce the number of GOT entries and GOT accesses on some targets. 🌈
For example, the implementation of the following function "foo":
static int a, b, c;
int foo (void) { return a + b + c; }usually calculates the addresses of all three variables, but if you compile it with -fsection-anchors, it accesses the variables from a common anchor point instead. The effect is similar to the following pseudocode (which isn't valid C):
int foo (void)
{
register int *xr = &x;
return xr[&a - &x] + xr[&b - &x] + xr[&c - &x];
}Not all targets support this option. Sad but true. 💔
--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.
Real talk: 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 allowable choices for name are:
predictable-branch-outcome— when branch is predicted to be taken with probability lower than this threshold (in percent), then it is considered well predictable.max-rtl-if-conversion-insns— RTL if-conversion tries to remove conditional branches around a block and replace them with conditionally executed instructions. This parameter gives the maximum number of instructions in a block which should be considered for if-conversion. The compiler will also use other heuristics to decide whether if-conversion is likely to be profitable.max-rtl-if-conversion-predictable-costmax-rtl-if-conversion-unpredictable-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 max number of incoming edges we're even willing to look at for cross-jumping. The algorithm inside-fcrossjumpingis O(N^2) in the number of incoming edges per block, so like… crank it up and you get more aggressive optimization, compilation time goes up, and the executable probably shrinks by a lil bit. That's the trade bestie 💅min-crossjump-insns— the minimum number of instructions that have to match at the end of two blocks before cross-jumping even happens on them. Exception era: this value gets ignored if all the instructions in the block being cross-jumped from are matched.max-grow-copy-bb-insns— the max code size expansion factor when you copy basic blocks instead of jumping. The expansion is measured relative to a jump instruction, iykyk.max-goto-duplication-insns— the max number of instructions to duplicate into a block that jumps to a computed goto. To dodge O(N^2) behavior in a bunch of passes, GCC factors computed gotos early in compilation and unfactors them as late as possible. Only computed jumps sitting at the end of a basic block with no more thanmax-goto-duplication-insnsget unfactored. iconic little dance nglmax-delay-slot-insn-search— the max number of instructions to consider while hunting for an instruction to fill a delay slot. Search past this arbitrary number and the time savings from filling the delay slot are basically minimal, so we stop looking. Higher values = more aggressive optimization = more compilation time, with probably only a tiny improvement in execution time. 🫶max-delay-slot-live-search— when trying to fill delay slots, this is the max number of instructions to consider while searching for a block with valid live register information. Higher values here mean more aggressive optimization and more compilation time. Lowkey a known issue: this parameter should be removed when the delay slot code is rewritten to maintain the control-flow graph.max-gcse-memory— the approximate max amount of memory that can be allocated to do the global common subexpression elimination optimization. Need more memory than you specified? Then the optimization simply doesn't happen. 😭max-gcse-insertion-ratio— if the ratio of expression insertions to deletions is bigger than this value for any expression, then RTL PRE inserts or removes the expression — which leaves partially redundant computations sitting in the instruction stream.max-pending-list-length— the max number of pending dependencies scheduling allows before it flushes the current state and starts over from scratch. Big functions with few branches or calls can build excessively large lists that needlessly eat memory and resources. 💀max-modulo-backtrack-attempts— the max number of backtrack attempts the scheduler should make when modulo scheduling a loop. Larger values can make compilation time grow exponentially. So like, be careful babe.max-inline-insns-single— several parameters control the tree inliner used in GCC, and this number sets the max instructions (counted in GCC's internal representation) in a single function that the tree inliner will consider for inlining. This only affects functions declaredinlineand methods implemented in a class declaration (C++). ✨max-inline-insns-auto— when you use-finline-functions(which is included in-O3), a lot of functions that the compiler would otherwise never consider get investigated for inlining. For those functions, a different (more restrictive) limit compared to functions declaredinlinecan be applied.max-inline-insns-small— the bound applied to calls considered relevant with-finline-small-functions.max-inline-insns-size— the bound applied to calls that are optimized for size. A little growth may be desirable so you can anticipate the optimization opportunities that inlining exposes. 🦋uninlined-function-insns— the number of instructions the inliner accounts for function overhead, like function prologue and epilogue.uninlined-function-time— extra time the inliner accounts for function overhead, like the time needed to execute the function prologue and epilogue.uninlined-thunk-insns,uninlined-thunk-time— same as--param uninlined-function-insnsand--param uninlined-function-timebut applied to function thunks. twins fr frinline-min-speedup— when the estimated performance improvement of caller + callee runtime exceeds this threshold (in percent), the function can be inlined regardless of the limit on--param max-inline-insns-singleand--param max-inline-insns-auto. Periodt. 👑large-function-insns— the limit that defines "really large" functions. For functions bigger than this limit after inlining, inlining is constrained by--param large-function-growth. This parameter is mainly useful to avoid extreme compilation time caused by the non-linear algorithms the back end uses.large-function-growth— the maximal growth of a large function caused by inlining, in percents. Example: parameter value100limits large function growth to 2.0 times the original size.large-unit-insns— the limit that defines a large translation unit. Growth caused by inlining of units bigger than this limit is limited by--param inline-unit-growth. For small units this might be way too tight. Picture a unit made of function A that isinlineand B that just calls A three times. If B is small relative to A, the growth of the unit is 300%, and yet that inlining is very sane. For very large units made of small inlineable functions though, the overall unit growth limit is needed to avoid exponential explosion of code size. So for smaller units, the size is increased to--param large-unit-insnsbefore--param inline-unit-growthgets applied.inline-unit-growth— the maximal overall growth of the compilation unit caused by inlining. Example: parameter value20limits unit growth to 1.2 times the original size. Cold functions (either marked cold via an attribute or by profile feedback) are not counted into the unit size. 🙈ipcp-unit-growth— the maximal overall growth of the compilation unit caused by interprocedural constant propagation. Example: parameter value10limits unit growth to 1.1 times the original size.large-stack-frame— the limit that defines large stack frames. While inlining, the algorithm tries not to grow past this limit too much.large-stack-frame-growth— the maximal growth of large stack frames caused by inlining, in percents. Example: parameter value1000limits large stack frame growth to 11 times the original size.max-inline-insns-recursive,max-inline-insns-recursive-auto— the max 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.
max-inline-recursive-depth,max-inline-recursive-depth-auto— the max 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.
min-inline-recursive-probability— recursive inlining is only profitable for functions that have deep recursion on average, and it can hurt functions with little recursion depth by increasing the prologue size or the complexity of the function body for 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 only to call expressions whose probability exceeds the given threshold (in percents).
early-inlining-insns— 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.max-early-inliner-iterations— the iteration limit of the early inliner. Basically this bounds the number of nested indirect calls the early inliner can resolve. Deeper chains are still handled by late inlining. ✨comdat-sharing-probability— the probability (in percent) that a C++ inline function with comdat visibility gets shared across multiple compilation units.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 function assembler name and filename, which makes old profile data more tolerant to source changes such as function reordering and so on.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. 🌈gcse-cost-distance-ratio— okay so this is the scaling factor in calculation of the maximum distance an expression can be moved by GCSE optimizations. This is currently supported only in the code hoisting pass, ngl. The bigger the ratio, the more aggressive code hoisting is with simple expressions, bestie — i.e., the expressions that have cost less thangcse-unrestricted-cost. Specifying0disables hoisting of simple expressions. 💅gcse-unrestricted-cost— Cost, roughly measured as the cost of a single typical machine instruction, at which GCSE optimizations do not constrain the distance an expression can travel. This is currently supported only in the code hoisting pass. The lesser the cost, the more aggressive code hoisting is. Specifying0allows all expressions to travel unrestricted distances. 🦋max-hoist-depth— The depth of search in the dominator tree for expressions to hoist. This is used to avoid quadratic behavior in hoisting algorithm. The value of0does not limit on the search, but may slow down compilation of huge functions, bestie. 😭max-tail-merge-comparisons— The maximum amount of similar bbs to compare a bb with. This is used to avoid quadratic behavior in tree tail merging. 💀max-tail-merge-iterations— The maximum amount of iterations of the pass over the function. This is used to limit compilation time in tree tail merging, lowkey. ⏳store-merging-allow-unaligned— Allow the store merging pass to introduce unaligned stores if it is legal to do so. ✨max-stores-to-merge— The maximum number of stores to attempt to merge into wider stores in the store merging pass. 🫶max-unrolled-insns— The maximum number of instructions that a loop may have to be unrolled. If a loop is unrolled, this parameter also determines how many times the loop code is unrolled. 🔁max-average-unrolled-insns— The maximum number of instructions biased by probabilities of their execution that a loop may have to be unrolled. If a loop is unrolled, this parameter also determines how many times the loop code is unrolled. 📊max-unroll-times— The maximum number of unrollings of a single loop. 🔂max-peeled-insns— The maximum number of instructions that a loop may have to be peeled. If a loop is peeled, this parameter also determines how many times the loop code is peeled. 🍌max-peel-times— The maximum number of peelings of a single loop. 🍊max-peel-branches— The maximum number of branches on the hot path through the peeled sequence. 🔥max-completely-peeled-insns— The maximum number of insns of a completely peeled loop. 💯max-completely-peel-times— The maximum number of iterations of a loop to be suitable for complete peeling. 🌀max-completely-peel-loop-nest-depth— The maximum depth of a loop nest suitable for complete peeling. 🪆max-unswitch-insns— The maximum number of insns of an unswitched loop. 🔀max-unswitch-level— The maximum number of branches unswitched in a single loop. 🌿lim-expensive— The minimum cost of an expensive expression in the loop invariant motion. 💸iv-consider-all-candidates-bound— Bound on number of candidates for induction variables, below which all candidates are considered for each use in induction variable optimizations. If there are more candidates than this, only the most relevant ones are considered to avoid quadratic time complexity. 🧠iv-max-considered-uses— The induction variable optimizations give up on loops that contain more induction variable uses. 🛑iv-always-prune-cand-set-bound— If the number of candidates in the set is smaller than this value, always try to remove unnecessary ivs from the set when adding a new one. ✂️avg-loop-niter— Average number of iterations of a loop. 📈dse-max-object-size— Maximum size (in bytes) of objects tracked bytewise by dead store elimination. Larger values may result in larger compilation times. 🐘dse-max-alias-queries-per-store— Maximum number of queries into the alias oracle per store. Larger values result in larger compilation times and may result in more removed dead stores. 🗑️scev-max-expr-size— Bound on size of expressions used in the scalar evolutions analyzer. Large expressions slow the analyzer. 🐌scev-max-expr-complexity— Bound on the complexity of the expressions in the scalar evolutions analyzer. Complex expressions slow the analyzer. 🧩max-tree-if-conversion-phi-args— Maximum number of arguments in a PHI supported by TREE if conversion unless the loop is marked with simd pragma. 🔀vect-max-version-for-alignment-checks— The maximum number of run-time checks that can be performed when doing loop versioning for alignment in the vectorizer. 📐vect-max-version-for-alias-checks— The maximum number of run-time checks that can be performed when doing loop versioning for alias in the vectorizer. 👯vect-max-peeling-for-alignment— The maximum number of loop peels to enhance access alignment for vectorizer. Value-1means no limit. ♾️max-iterations-to-track— The maximum number of iterations of a loop the brute-force algorithm for analysis of the number of iterations of the loop tries to evaluate. 🔍hot-bb-count-ws-permille— A basic block profile count is considered hot if it contributes to the given permillage (i.e.0...1000) of the entire profiled execution. 🔥hot-bb-frequency-fraction— Select fraction of the entry block frequency of executions of basic block in function given basic block needs to have to be considered hot. 🌡️max-predicted-iterations— The maximum number of loop iterations we predict statically. This is useful in cases where a function contains a single loop with known bound and another loop with unknown bound. The known number of iterations is predicted correctly, while the unknown number of iterations average to roughly10. This means that the loop without bounds appears artificially cold relative to the other one. ❄️builtin-expect-probability— Control the probability of the expression having the specified value. This parameter takes a percentage (i.e.0 ... 100) as input. 🎲builtin-string-cmp-inline-length— The maximum length of a constant string for a builtin string cmp call eligible for inlining. 📏align-threshold— Select fraction of the maximal frequency of executions of a basic block in a function to align the basic block. 📐align-loop-iterations— A loop expected to iterate at least the selected number of iterations is aligned. 🔄tracer-dynamic-coverage,tracer-dynamic-coverage-feedback— This value is used to limit superblock formation once the given percentage of executed instructions is covered. This limits unnecessary code size expansion. Thetracer-dynamic-coverage-feedbackparameter is used only when profile feedback is available. The real profiles (as opposed to statically estimated ones) are much less balanced allowing the threshold to be larger value. 📊tracer-max-code-growth— Stop tail duplication once code growth has reached given percentage. This is a rather artificial limit, as most of the duplicates are eliminated later in cross jumping, so it may be set to much higher values than is the desired code growth. 🌱tracer-min-branch-ratio— Stop reverse growth when the reverse probability of best edge is less than this threshold (in percent). 📉tracer-min-branch-probability,tracer-min-branch-probability-feedback— Stop forward growth if the best edge has probability lower than this threshold. Similarly totracer-dynamic-coveragetwo parameters are provided.tracer-min-branch-probability-feedbackis used for compilation with profile feedback andtracer-min-branch-probabilitycompilation without. The value for compilation with profile feedback needs to be more conservative (higher) in order to make tracer effective. 🎯stack-clash-protection-guard-size— Specify the size of the operating system provided stack guard as 2 raised to num bytes. Higher values may reduce the number of explicit probes, but a value larger than the operating system provided guard will leave code vulnerable to stack clash style attacks. 🛡️stack-clash-protection-probe-interval— Stack clash protection involves probing stack space as it is allocated. This param controls the maximum distance between probes into the stack as 2 raised to num bytes. Higher values may reduce the number of explicit probes, but a value larger than the operating system provided guard will leave code vulnerable to stack clash style attacks. 🛡️max-cse-path-length— The maximum number of basic blocks on path that CSE considers. 🛣️max-cse-insns— The maximum number of instructions CSE processes before flushing. 🚽ggc-min-expand— GCC uses a garbage collector to manage its own memory allocation. This parameter specifies the minimum percentage by which the garbage collector's heap should be allowed to expand between collections. Tuning this may improve compilation speed; it has no effect on code generation. The default is30% + 70% * (RAM/1GB)with an upper bound of100%whenRAM >= 1GB. Ifgetrlimitis available, the notion ofRAMis the smallest of actual RAM andRLIMIT_DATAorRLIMIT_AS. If GCC is not able to calculate RAM on a particular platform, the lower bound of30%is used. Setting this parameter andggc-min-heapsizeto zero causes a full collection to occur at every opportunity. This is extremely slow, but can be useful for debugging. 🐢ggc-min-heapsize— the minimum size the garbage collector's heap can be before it even bothers to go collect garbage 💅 the very first collection happens after the heap grows byggc-min-expand% pastggc-min-heapsize. and again, same deal as before: tuning this might speed up compilation, and it has zero effect on the code that actually gets generated, no cap.
the default is the smaller of RAM/8, RLIMIT_RSS, or a limit that tries to make sure RLIMIT_DATA or RLIMIT_AS don't get exceeded — but with a lower bound of 4096 (four megabytes) and an upper bound of 131072 (128 megabytes). if GCC can't figure out your RAM on some platform, the lower bound is what gets used. setting this parameter very large effectively disables garbage collection entirely 😳 and setting this parameter and ggc-min-expand to zero makes a full collection happen at every single opportunity.
max-reload-search-insns— the max number of instructions reload should look backward for an equivalent register. bigger values = more aggressive optimization, which means compilation time goes up but you probably get slightly better performance ✨max-cselib-memory-locations— the max number of memory locations cselib should take into account. bigger values = more aggressive optimization, longer compile, 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 moment during the first scheduling pass. bigger values = more thorough searches, compile time goes up, and you probably get very 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 of0disables region extensions entirely.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 get 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 a store and a load targeting the same memory locations.selsched-max-lookahead— the max size of the lookahead window of selective scheduling. it's basically a depth of search for available instructions 🔍selsched-max-sched-times— the max number of times an instruction gets scheduled during selective scheduling. it's the limit on how many iterations the instruction may be pipelined through.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 share a data structure, which cuts the compiler's memory usage and boosts its speed 🏎️ this sets the max value of a shared integer constant.ssp-buffer-size— the minimum size of buffers (i.e. arrays) that receive stack smashing protection when-fstack-protectionis used.
fun lore drop: this default was"8"before Ubuntu 10.10, and currently it's"4"— the change was to increase the number of functions protected by the stack protector 🛡️
min-size-for-stack-sharing— the minimum size of variables taking part in stack slot sharing when you're 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 that get treated in a field sensitive manner during pointer analysis.prefetch-latency— an estimate of the average number of instructions executed before a prefetch finishes. the distance prefetched ahead is proportional to this constant. bumping this number may also mean fewer streams get prefetched (seesimultaneous-prefetches).simultaneous-prefetches— max number of prefetches that can run at the same time.l1-cache-line-size— the size of a 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 that the stride is non-constant may make it hard to predict when there's a clear benefit to issuing those hints 🤔
set to 1 if prefetch hints should be issued for non-constant strides. set to 0 if prefetch hints should only be issued for strides known to be constant and below prefetch-minimum-stride.
prefetch-minimum-stride— the 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 that case there may be conflicts between the hardware prefetchers and the software prefetchers. if the hardware prefetchers have a maximum stride they can handle, that value should be used here to improve the use of software prefetchers.
a value of -1 means there's no threshold, and therefore prefetch hints can be issued for any constant stride.
this setting is only useful for strides that are known and constant.
loop-interchange-max-num-stmts— the max number of stmts in a loop to be interchanged.loop-interchange-stride-ratio— the minimum ratio between the stride of two loops for interchange to be profitable.min-insn-to-prefetch-ratio— the minimum ratio between the number of instructions and the number of prefetches to enable prefetching in a loop.prefetch-min-insn-to-mem-ratio— the minimum ratio between the number of instructions and the number of memory references to enable prefetching in a loop.use-canonical-types— whether the compiler should use the"canonical"type system. this should always be1, which uses a more efficient internal mechanism for comparing types in C++ and Objective-C++ 👑 but if bugs in the canonical type system are causing compilation failures, set this value to0to disable canonical types.switch-conversion-max-branch-ratio— switch initialization conversion refuses to create arrays that are bigger thanswitch-conversion-max-branch-ratiotimes the number of branches in the switch.max-partial-antic-length— max 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 just run away, eating all the memory available on the host machine 😭 this parameter sets a limit on the length of the sets that get computed, which prevents that runaway behavior. setting a value of0for this parameter allows an unlimited set length.rpo-vn-max-loop-depth— max 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 are value-numbered optimistically, and the remaining ones are not.sccvn-max-alias-queries-per-access— max number of alias-oracle queries performed 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— even though IRA uses a sophisticated algorithm to compress the conflict table, that table can still need 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 doesn't require building a pseudo-register conflict table.
ira-loop-reserved-regs— okay so IRA can be used to figure out way more accurate register pressure inside loops, which feeds into the decision of whether to move loop invariants (peep-O3for that whole saga). This param is literally just "how many registers do we keep on reserve for other stuff". Default? The best value they found from numerous experiments 💅 — they did the science so we don't have to.lra-inheritance-ebb-probability-cutoff— LRA tries to reuse values that got reloaded into registers in the insns that come after. That lil optimization is called inheritance 🫶, and EBB is the region where it happens. This parameter sets the minimal fall-through edge probability, in percentage, needed to add a BB to the inheritance EBB in LRA. The default came from numerous runs of SPEC2000 on x86-64 — iconique, they benchmarked it into existence.loop-invariant-max-bbs-in-loop— loop invariant motion can get expensive bestie, like, both in compilation time and in how much compile-time memory it eats, especially with super large loops 😭. So: any loop packing more basic blocks than this parameter? It just doesn't get loop invariant motion optimization done on it. Sorry not sorry.loop-max-datarefs-for-datadeps— building data dependencies is lowkey pricey for very large loops. This parameter caps the number of data references in loops that get considered for data dependence analysis. Those big chonky loops? Not handled by the optimizations that rely on loop data dependencies. the way it just... opts out 💀max-vartrack-size— sets a maximum number of hash table slots to use during variable tracking dataflow analysis of any function. If you blow past this limit while variable tracking at assignments is enabled, analysis for that function gets retried without it, after removing all debug insns from the function. And if you still exceed the limit even with no debug insns? Var tracking analysis is completely disabled for that function 😳. Set the parameter to zero and it becomes unlimited.max-vartrack-expr-depth— the max number of recursion levels when trying to map variable names or debug temporaries to value expressions. It's a trade: compilation time ⚖️ more complete debug info. Set it too low and value expressions that are available and could be represented in debug info might just... not get used 🙈. Set it higher and the compiler may find more complex debug expressions — but compile time and memory use may grow. Pick your fighter.max-debug-marker-count— a threshold on the number of debug markers (e.g. begin stmt markers) so we don't get a complexity explosion at inlining or at expanding to RTL 💥. If a function has more such gimple stmts than the set limit, those stmts get dropped from the inlined copy of the function, and from its RTL expansion. gone. bye.min-nondebug-insn-uid— use uids starting at this parameter for nondebug insns. Everything below the parameter is reserved exclusively for debug insns created by-fvar-tracking-assignments, but debug insns may still get (non-overlapping) uids above it if that reserved range runs out. no cap fr fr.ipa-sra-ptr-growth-factor— IPA-SRA will only replace a pointer to an aggregate with one or more new parameters when their cumulative size is less than or equal toipa-sra-ptr-growth-factortimes the size of the original pointer parameter. it's giving ✨budget✨sra-max-scalarization-size-Ospeed/sra-max-scalarization-size-Osize— the two Scalar Reduction of Aggregates passes (SRA and IPA-SRA) want to swap the scalar parts of aggregates out for uses of independent scalar variables. These two params control the maximum size, in storage units, of aggregate that's considered for replacement when compiling for speed (sra-max-scalarization-size-Ospeed) or for size (sra-max-scalarization-size-Osize). two moods, one mission 🦋sra-max-propagations— the maximum number of artificial accesses that Scalar Replacement of Aggregates (SRA) will track, per one local variable, in order to make copy propagation easier. tracking, but make it efficient.tm-max-aggregate-size— when making copies of thread-local variables in a transaction, this parameter specifies the size in bytes after which variables get saved with the logging functions, as opposed to save/restore code sequence pairs. Heads up: this option only applies when you're 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 🔓. And btw, a variable whose value is unknown at compilation time and defined outside a SCoP counts as a parameter of the SCoP. noted.loop-block-tile-size— the loop blocking / strip mining transforms, enabled with-floop-blockor-floop-strip-mine, strip mine each loop in the loop nest by a given number of iterations. Wanna change that strip length? That's what theloop-block-tile-sizeparameter is for 💅ipa-cp-value-list-size— IPA-CP tries to track all possible values and types passed to a function's parameter, so it can propagate them and do devirtualization.ipa-cp-value-list-sizeis the maximum number of values and types it stores per one formal parameter of a function. a whole lil database per parameter, obsessed.ipa-cp-eval-threshold— IPA-CP calculates its own score of cloning profitability heuristics, and it only performs those cloning opportunities whose scores exceedipa-cp-eval-threshold. it has standards ✨ipa-cp-recursion-penalty— the percentage penalty that recursive functions receive when they're being evaluated for cloning. recursion tax fr.ipa-cp-single-call-penalty— the percentage penalty that functions containing a single call to another function receive when they're evaluated for cloning. one call? slight penalty. rules are rules.ipa-max-agg-items— IPA-CP can also propagate a number of scalar values passed in an aggregate.ipa-max-agg-itemscontrols the maximum number of such values per one parameter. cap it.ipa-cp-loop-hint-bonus— when IPA-CP figures out that a cloning candidate would make the number of iterations of a loop known, it adds a bonus ofipa-cp-loop-hint-bonusto that candidate's profitability score. bonus points bestie 🎀ipa-cp-array-index-hint-bonus— when IPA-CP figures out that a cloning candidate would make the index of an array access known, it adds a bonus ofipa-cp-array-index-hint-bonusto the candidate's profitability score. another W.ipa-max-aa-steps— during its analysis of function bodies, IPA-CP employs alias analysis to track values pointed to by function parameters. But it does not wanna spend forever analyzing huge functions, so it gives up and considers all memory clobbered after examiningipa-max-aa-stepsstatements that modify memory. tapping out respectfully 😭lto-partitions— specify the desired number of partitions produced during WHOPR compilation. And pro tip from the docs themselves: 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 exists to prevent the expense of splitting very small programs into way too many partitions. don't over-split, it's giving chaos.lto-max-partition— the size of the max partition for WHOPR (also in estimated instructions), so there's an upper bound for the individual size of a partition. Meant to be used only with balanced partitioning ⚠️lto-max-streaming-parallelism— the maximal number of parallel processes used for LTO streaming. parallelism, but make it bounded 🌈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 the compiler trying to be helpful, respectfully, within limits 💜sink-frequency-threshold— the maximum relative execution frequency (in percents) of the target block relative to a statement's original block, which is what allows statement sinking of a statement. Bigger 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. special treatment, deservedly.max-stores-to-sink— the maximum number of conditional store pairs that can be sunk. Set it to 0 if either vectorization (-ftree-vectorize) or if-conversion (-ftree-loop-if-convert) is disabled. zero means none, no cap.allow-store-data-races— allow optimizers to introduce new data races on stores 😳. Set to1to allow, otherwise to0. > bestie this one is literally in the name — turning it on lets the compiler create brand new data races on stores. be so serious before you touch it.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, just use the default for the machine. let the machine decide ✨tree-reassoc-width— set the maximum number of instructions executed in parallel in a reassociated tree. This parameter overrides the target-dependent heuristics that are used by default, if it has a non-zero value. your rules now, bestie.sched-pressure-algorithm— choose between the two available implementations of-fsched-pressure:- Algorithm 1 is the original implementation, and it's the more likely one to prevent instructions from being reordered.
- Algorithm 2 was designed as a compromise between the relatively conservative approach taken by algorithm 1 and the rather aggressive approach taken by the default scheduler. It relies more heavily on having a regular register file and accurate register pressure classes.
Peep haifa-sched.c in the GCC sources for more details 👀 The default choice depends on the target.
max-slsr-cand-scan— set the maximum number of existing candidates that get considered when seeking a basis for a new straight-line strength reduction candidate. shopping for a base, with a limit 🛍️asan-globals— enable buffer overflow detection for global objects. This kind of protection is enabled by default if you are using the-fsanitize=addressoption. To disable global objects protection, use--param asan-globals=0. off switch located ✅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 the--param asan-stack=0option. same energy, different stack 🫶asan-instrument-reads— turns on buffer overflow detection for memory reads. It's already on by default whenever you're using-fsanitize=address, so this is really a "don't touch unless you're turning it off" situation. Wanna disable read protection?--param asan-instrument-reads=0. periodt. 💅asan-instrument-writes— same energy but for memory writes: buffer overflow detection. Also enabled by default with-fsanitize=address. To turn write protection off, hit it with the--param asan-instrument-writes=0option. 🦋asan-memintrin— enables detection for built-in functions. Enabled by default when you use-fsanitize=address, bestie. To disable built-in function protection, use--param asan-memintrin=0. ✨asan-use-after-return— enables detection of use-after-return. Enabled by default when you use 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. iykyk 🙈
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. 🔥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. 🧚max-fsm-thread-path-insns— the maximum number of instructions to copy when duplicating blocks on a finite state automaton jump thread path.max-fsm-thread-length— the maximum number of basic blocks on a finite state automaton jump thread path.max-fsm-thread-paths— the maximum number of new jump thread paths to create for a finite state automaton.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). pick your fighter 💀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. Heads up though: for a parallelized loop nest, the minimum number of iterations of the outermost loop per thread is two. 🌈max-ssa-name-query-depth— maximum depth of recursion when querying properties of SSA names in things like fold routines. One level of recursion corresponds to following a use-def chain. literally a chain, bestie ⛓️hsa-gen-debug-stores— enable emission of special debug stores within HSA kernels which are then read and reported by libgomp plugin. Generation of these stores is disabled by default; use--param hsa-gen-debug-stores=1to enable it.max-speculative-devirt-maydefs— the maximum number of may-defs we analyze when looking for a must-def specifying the dynamic type of an object that invokes a virtual call we may be able to devirtualize speculatively. (say that five times fast 😭)max-vrp-switch-assertions— the maximum number of assertions to add along the default edge of a switch statement during VRP.unroll-jam-min-percent— the minimum percentage of memory references that must be optimized away for the unroll-and-jam transformation to be considered profitable.unroll-jam-max-unroll— the maximum number of times the outer loop should be unrolled by the unroll-and-jam transformation.max-rtl-if-conversion-unpredictable-cost— maximum permissible cost for the sequence that would be generated by the RTL if-conversion pass for a branch that is considered unpredictable.max-variable-expansions-in-unroller— if-fvariable-expansion-in-unrolleris used, the maximum number of times that an individual variable will be expanded during loop unrolling.tracer-min-branch-probability-feedback— stop forward growth if the probability of best edge is less than this threshold (in percent). Used when profile feedback is available. 📊partial-inlining-entry-probability— maximum probability of the entry BB of split region (in percent relative to entry BB of the function) to make partial inlining happen.max-tracked-strlens— maximum number of strings for which strlen optimization pass will track string lengths.gcse-after-reload-partial-fraction— the threshold ratio for performing partial redundancy elimination after reload.gcse-after-reload-critical-fraction— the threshold ratio of critical edges execution count that permit performing redundancy elimination after reload.max-loop-header-insns— the maximum number of insns in loop header duplicated by the copy loop headers pass.vect-epilogues-nomask— enable loop epilogue vectorization using smaller vector size. slay ✨slp-max-insns-in-bb— maximum number of instructions in basic block to be considered for SLP vectorization.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.hot-bb-count-fraction— select fraction of the maximal count of repetitions of basic block in program given basic block needs to have to be considered hot (used in non-LTO mode). 🔥max-inline-insns-recursive-auto— the maximum number of instructions non-inline function can grow to via recursive inlining.graphite-allow-codegen-errors— whether codegen errors should be ICEs when-fchecking.sms-max-ii-factor— a factor for tuning the upper bound that swing modulo scheduler uses for scheduling a loop.lra-max-considered-reload-pseudos— the max number of reload pseudos which are considered during spilling a non-reload pseudo.max-pow-sqrt-depth— maximum depth of sqrt chains to use when synthesizing exponentiation by a real constant.max-dse-active-local-stores— maximum number of active local stores in RTL dead store elimination.asan-instrument-allocas— enable asan allocas/VLAs protection. 🫶max-iterations-computation-cost— bound on the cost of an expression to compute the number of iterations.max-isl-operations— maximum number of isl operations,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.unlikely-bb-count-fraction— the minimum fraction of profile runs a given basic block execution count must be not to be considered unlikely.tracer-dynamic-coverage-feedback— the percentage of function, weighted by execution frequency, that must be covered by trace formation. Used when profile feedback is available.max-inline-recursive-depth-auto— the maximum depth of recursive inlining for non-inline functions.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.fsm-maximum-phi-arguments— maximum number of arguments a PHI may have before the FSM threader will not try to thread through its block.uninit-control-dep-attempts— maximum number of nested calls to search for control dependencies during uninitialized variable analysis.indir-call-topn-profile— track top N target addresses in indirect-call profile.max-once-peeled-insns— the maximum number of insns of a peeled loop that rolls only once.sra-max-scalarization-size-Osize— maximum size, in storage units, of an aggregate which should be considered for scalarization when compiling for size.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. (yes that is literally a single space in quotes, no cap 💀)loop-versioning-max-inner-insns— the maximum number of instructions that an inner loop can have before the loop versioning pass considers it too big to copy.loop-versioning-max-outer-insns— the maximum number of instructions that an outer loop can have before the loop versioning pass considers it too big to copy, discounting any instructions in inner loops that directly benefit from versioning.ssa-name-def-chain-limit— the maximum number ofSSA_NAMEassignments 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.
✨ Program Instrumentation Options ✨
okay so GCC supports a whole bunch of command-line options that control adding run-time instrumentation to the code it normally generates. 🎀
For example, one purpose of instrumentation is to collect profiling statistics for use in finding program hot spots, code coverage analysis, or profile-guided optimizations. 📊
Another class of program instrumentation is adding run-time checking to detect programming errors like invalid pointer dereferences or out-of-bounds array accesses, as well as deliberately hostile attacks such as stack smashing or C++ vtable hijacking. yikes but also iconic that GCC just catches that for you 💅
There is also a general hook which can be used to implement other forms of tracing or function-level instrumentation for debug or program analysis purposes. 🦋
-p,-pg— bestie, this one makes GCC generate extra code to write out profile information that's suitable for the analysis programprof(that's the-pgirlie) orgprof(that's-pg). ✨ And listen: you must use this option when you're compiling the source files you want data about, and you also must use it when linking. Both. No skipping steps. 💅
You can use the function attribute no_instrument_function to suppress profiling of individual functions when compiling with these options. So if one function is being extra and you don't want it in your profile, that's your escape hatch. 🎀
-fprofile-arcs— this one adds code so that program flow arcs get instrumented. 🦋 During execution, the program records how many times each branch and call is executed, and how many times it is taken or returns. And 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. classy behavior fr.
When the compiled program exits it saves this data to a file called auxname.gcda for each source file. 📁 That data can 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 it's explicitly specified and it is not the final executable, otherwise it's 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). 🍓
--coverage— this option 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. 👑- 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. 👯 - 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. 📖
- Compile the source files with
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. graph theory jumpscare but make it slay 💅
-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. And heads up: coverage data matches the source files more closely if you do not optimize. (yes, really, leave the optimizer out of this one bestie)-fprofile-abs-path— automatically convert relative source file names to absolute path names in the.gcnofiles. This letsgcovfind the correct sources in projects where compilations occur with different working directories. 🧚-fprofile-dir=path— set the directory to search for the profile data files in to path. This option affects only the profile data generated by-fprofile-generate,-ftest-coverage,-fprofile-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, so the profile data file appears in the same directory as the object file. And 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. name mangling, but for paths, iconic ✨
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 an application to produce a profile that's useful for later recompilation with profile feedback based optimization. And you must use-fprofile-generateboth when compiling and when linking your program. 👑
The following options are enabled: -fprofile-arcs, -fprofile-values, -finline-functions, and -fipa-bit-cp.
If path is specified, GCC looks at the path to find the profile feedback data files. See -fprofile-dir.
To optimize the program based on the collected profile information, use -fprofile-use. 🫶
-fprofile-update=method— alter the update method for an application instrumented for profile feedback based optimization. The method argument should be one ofsingle,atomicorprefer-atomic. The first one is useful for single-threaded applications, while the second one prevents profile corruption by emitting thread-safe code. 🧵
Warning: When an application does not properly join all threads (or creates a detached thread), a profile file can be still corrupted. 😭
Using prefer-atomic would be transformed either to atomic, when supported by a target, or to single otherwise. The GCC driver automatically selects prefer-atomic when -pthread is present in the command line. 💅
-fprofile-filter-files=regex— instrument only functions from files where names match any regular expression (separated by a semi-colon). 🎯
For example, -fprofile-filter-files=main.c;module.*.c will instrument only main.c and all C files starting with 'module'.
-fprofile-exclude-files=regex— instrument only functions from files where names do not match all the regular expressions (separated by a semi-colon). 🚫
For example, -fprofile-exclude-files=/usr/* will prevent instrumentation of all files that are located in /usr/ folder.
-fsanitize=address— enable AddressSanitizer, a fast memory error detector. 💀 Memory access instructions are instrumented to detect out-of-bounds and use-after-free bugs. The option enables-fsanitize-address-use-after-scope. See <https://github.com/google/sanitizers/wiki/AddressSanitizer> for more details. The run-time behavior can be influenced using theASAN_OPTIONSenvironment variable. When set tohelp=1, the available options are shown at startup of the instrumented program. See <https://github.com/google/sanitizers/wiki/AddressSanitizerFlags#run-time-flags> for a list of supported options. The option cannot be combined with-fsanitize=thread. 🚨-fsanitize=kernel-address— enable AddressSanitizer for Linux kernel. See <https://github.com/google/kasan/wiki> for more details. 🐧-fsanitize=pointer-compare— instrument comparison operation (<,<=,>,>=) with pointer operands. The option must be combined with either-fsanitize=kernel-addressor-fsanitize=address. The option 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— instrument subtraction with pointer operands. The option must be combined with either-fsanitize=kernel-addressor-fsanitize=address. The option 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=thread— enable ThreadSanitizer, a fast data race detector. 🏃♀️ Memory access instructions are instrumented to detect data race bugs. See <https://github.com/google/sanitizers/wiki#threadsanitizer> for more details. 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. The 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— enable LeakSanitizer, a memory leak detector. 💧 This option only matters for linking of executables and the executable is linked against a library that overridesmallocand other allocator functions. See <https://github.com/google/sanitizers/wiki/AddressSanitizerLeakSanitizer> for more details. The run-time behavior can be influenced using theLSAN_OPTIONSenvironment variable. The option cannot be combined with-fsanitize=thread. 🚫-fsanitize=undefined— enable UndefinedBehaviorSanitizer, a fast undefined behavior detector. 🙈 Various computations are instrumented to detect undefined behavior at runtime. See <https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html> for more details. The run-time behavior can be influenced using theUBSAN_OPTIONSenvironment variable. Current suboptions are:-fsanitize=shift— okay bestie, this turns on checking that a shift operation’s result isn’t undefined. ✨ Heads up: what exactly counts as undefined is a little different between C and C++, and also between ISO C90 and C99, etc. This option has two suboptions:-fsanitize=shift-baseand-fsanitize=shift-exponent. 💅-fsanitize=shift-exponent— this one checks that the second argument of a shift operation is not negative and is smaller than the precision of the promoted first argument. no cap, it’s giving “stay in your lane” energy. 🎀-fsanitize=shift-base— if the second argument of a shift operation is within range, this checks that the result of the shift operation is not undefined. Note that what exactly is considered undefined differs slightly between C and C++, as well as between ISO C90 and C99, etc. 🦋-fsanitize=integer-divide-by-zero— detect integer division by zero as well asINT_MIN / -1division. literally catching the math crimes. 💀-fsanitize=unreachable— with this option, the compiler turns the__builtin_unreachablecall into a diagnostics message call instead. When reaching the__builtin_unreachablecall, the behavior is undefined. so like, don’t actually go there, bestie. 😭-fsanitize=vla-bound— this option instructs the compiler to check that the size of a variable length array is positive. positive vibes only, fr fr. 🌸-fsanitize=null— this option enables pointer checking. Particularly, the application built with this option turned on will issue an error message when it tries to dereference a NULL pointer, or if a reference (possibly an rvalue reference) is bound to a NULL pointer, or if a method is invoked on an object pointed by a NULL pointer. it’s giving “bestie, that pointer is not okay.” 🫶-fsanitize=return— this option enables return statement checking. Programs built with this option turned on will issue an error message when the end of a non-void function is reached without actually returning a value. This option works in C++ only. C++ girlies, this one’s for you. 💜-fsanitize=signed-integer-overflow— this option enables signed integer overflow checking. We check that the result of+,*, and both unary and binary-does not overflow in the signed arithmetics. Note, integer promotion rules must be taken into account. That is, the following is not an overflow:
signed char a = SCHAR_MAX;
a++;-fsanitize=bounds— this option enables instrumentation of array bounds. Various out of bounds accesses are detected. Flexible array members, flexible array member-like arrays, and initializers of variables with static storage are not instrumented. so it’s a vibe check for your array indexes, but not for every special case. 👑-fsanitize=bounds-strict— this option enables strict instrumentation of array bounds. Most out of bounds accesses are detected, including flexible array members and flexible array member-like arrays. Initializers of variables with static storage are not instrumented. stricter than your group chat’s admin, honestly. 🌈-fsanitize=alignment— this option enables checking of alignment of pointers when they are dereferenced, or when a reference is bound to insufficiently aligned target, or when a method or constructor is invoked on insufficiently aligned object. alignment is the new self-care. ✨-fsanitize=object-size— this option enables instrumentation of memory references using the__builtin_object_sizefunction. Various out of bounds pointer accesses are detected. pointer goes out of bounds? not on this option’s watch. 💅-fsanitize=float-divide-by-zero— detect floating-point division by zero. Unlike other similar options,-fsanitize=float-divide-by-zerois not enabled by-fsanitize=undefined, since floating-point division by zero can be a legitimate way of obtaining infinities and NaNs. so it’s lowkey valid sometimes. 🦋-fsanitize=float-cast-overflow— this option enables floating-point type to integer conversion checking. We check that the result of the conversion does not overflow. Unlike other similar options,-fsanitize=float-cast-overflowis not enabled by-fsanitize=undefined. This option does not work well withFE_INVALIDexceptions enabled. messy behavior, avoid. 😭-fsanitize=nonnull-attribute— this option enables instrumentation of calls, checking whether null values are not passed to arguments marked as requiring a non-null value by thenonnullfunction attribute. if the function said “no nulls,” respect it. 🎀-fsanitize=returns-nonnull-attribute— this option enables instrumentation of return statements in functions marked withreturns_nonnullfunction attribute, to detect returning of null values from such functions. don’t return null when you promised non-null. periodt. 💅-fsanitize=bool— this option enables instrumentation of loads from bool. If a value other than 0/1 is loaded, a run-time error is issued. bool is not a vibe spectrum, bestie. it’s 0 or 1. 💀-fsanitize=enum— this option enables instrumentation of loads from an enum type. If a value outside the range of values for the enum type is loaded, a run-time error is issued. enum values stay in their lane or catch this error. 🔥-fsanitize=vptr— this option enables instrumentation of C++ member function calls, member accesses and some conversions between pointers to base and derived classes, to verify the referenced object has the correct dynamic type. it’s the “are you really who you say you are” check. 👑-fsanitize=pointer-overflow— this option enables instrumentation of pointer arithmetics. If the pointer arithmetics overflows, a run-time error is issued. pointer math said “I’m overflowing” and the sanitizer said “not today.” 🌈-fsanitize=builtin— this option enables instrumentation of arguments to selected builtin functions. If an invalid value is passed to such arguments, a run-time error is issued. E.g. passing 0 as the argument to__builtin_ctzor__builtin_clzinvokes undefined behavior and is diagnosed by this option. builtins have rules too. ✨
While -ftrapv causes traps for signed overflows to be emitted, -fsanitize=undefined gives a diagnostic message. This currently works only for the C family of languages. 🫶
-fno-sanitize=all— this option disables all previously enabled sanitizers.-fsanitize=allis not allowed, as some sanitizers cannot be used together. you can’t just turn everything on like it’s a party, some of them don’t get along. 💅-fasan-shadow-offset=number— this option forces GCC to use custom shadow offset in AddressSanitizer checks. It is useful for experimenting with different shadow memory layouts in Kernel AddressSanitizer. shadow memory customization, we love to see it. 🦋-fsanitize-sections=s1,s2,...— sanitize global variables in selected user-defined sections.simay contain wildcards. wildcards said “I contain multitudes.” ✨-fsanitize-recover[=opts]—-fsanitize-recover=controls error recovery mode for sanitizers mentioned in comma-separated list of opts. Enabling this option for a sanitizer component causes it to attempt to continue running the program as if no error happened. This means multiple runtime errors can be reported in a single program run, and the exit code of the program may indicate success even when errors have been reported. The-fno-sanitize-recover=option can be used to alter this behavior: only the first detected error is reported and program then exits with a non-zero exit code. 😭
Currently this feature only works for -fsanitize=undefined (and its suboptions except for -fsanitize=unreachable and -fsanitize=return), -fsanitize=float-cast-overflow, -fsanitize=float-divide-by-zero, -fsanitize=bounds-strict, -fsanitize=kernel-address and -fsanitize=address. For these sanitizers error recovery is turned on by default, except -fsanitize=address, for which this feature is experimental. -fsanitize-recover=all and -fno-sanitize-recover=all is also accepted, the former enables recovery for all sanitizers that support it, the latter disables recovery for all sanitizers that support it.
Even if a recovery mode is turned on the compiler side, it needs to be also enabled on the runtime library side, otherwise the failures are still fatal. The runtime library defaults to halt_on_error=0 for ThreadSanitizer and UndefinedBehaviorSanitizer, while default value for AddressSanitizer is halt_on_error=1. This can be overridden through setting the halt_on_error flag in the corresponding environment variable.
Syntax without an explicit opts parameter is deprecated. It is equivalent to specifying an opts list of:
undefined,float-cast-overflow,float-divide-by-zero,bounds-strict-fsanitize-address-use-after-scope— enable sanitization of local variables to detect use-after-scope bugs. The option sets-fstack-reuseto none. no sneaky reuse, we’re checking that scope. 🎀-fsanitize-undefined-trap-on-error— the-fsanitize-undefined-trap-on-erroroption instructs the compiler to report undefined behavior using__builtin_traprather than alibubsanlibrary routine. The advantage of this is that thelibubsanlibrary is not needed and is not linked in, so this is usable even in freestanding environments. no extra library, just traps. iconic. 💜-fsanitize-coverage=trace-pc— enable coverage-guided fuzzing code instrumentation. Inserts a call to__sanitizer_cov_trace_pcinto every basic block. every basic block gets a little callback. 🌸-fsanitize-coverage=trace-cmp— enable dataflow guided fuzzing code instrumentation. Inserts a call to__sanitizer_cov_trace_cmp1,__sanitizer_cov_trace_cmp2,__sanitizer_cov_trace_cmp4or__sanitizer_cov_trace_cmp8for integral comparison with both operands variable or__sanitizer_cov_trace_const_cmp1,__sanitizer_cov_trace_const_cmp2,__sanitizer_cov_trace_const_cmp4or__sanitizer_cov_trace_const_cmp8for integral comparison with one operand constant,__sanitizer_cov_trace_cmpfor__sanitizer_cov_trace_cmpdfor float or double comparisons and__sanitizer_cov_trace_switchfor switch statements. it’s giving “we track every comparison, bestie.” 🔥-fcf-protection=[full|branch|return|none|check]— enable code instrumentation of control-flow transfers to increase program security by checking that target addresses of control-flow transfer instructions (such as indirect function call, function return, indirect jump) are valid. This prevents diverting the flow of control to an unexpected target. This is intended to protect against such threats as Return-oriented Programming (ROP), and similarly call/jmp-oriented programming (COP/JOP). control-flow said “you shall not pass” to unexpected targets. 👑
branch is the "bestie, watch every jump" setting — it tells the compiler to check that control-flow transfers at indirect branch instructions (aka call/jmp) are actually valid. return does the same glow-up but at the moment a function returns. full is just a cute alias for saying both branch and return at once. And none? That's the off switch — no instrumentation, no vibes. 💅
check is the special one for the final link with link-time optimization (LTO). If your LTO object files were compiled with different -fcf-protection values, it throws an error, like "girl these don't match?? 😭". Important: check is ignored at compile time — it only matters at link. fr fr.
The macro __CET__ gets defined whenever -fcf-protection is in play. The first bit of __CET__ is set to 1 for the value branch, and the second bit of __CET__ is set to 1 for return. 🎀
You can also use the nocf_check attribute to call out which functions and calls should be skipped from instrumentation — like a VIP list but for "don't check me, I'm fine". 🙈
Right now the x86 GNU/Linux target is the one serving this, with an implementation based on Intel Control-flow Enforcement Technology (CET), and it works for i686 processor or newer. iconic.
NOTE: In Ubuntu 19.10 and later versions,-fcf-protectionis enabled by default for C, C++, ObjC, ObjC++, if none of-fno-cf-protectionnor-fcf-protection=*are found.
🛡️ -fstack-protector and friends 🛡️
-fstack-protector— emits extra code to catch buffer overflows, like those stack smashing attacks. How? By adding a guard variable to functions that have vulnerable objects. That includes functions that callalloca, plus functions with buffers larger than 8 bytes. The guards get initialized when a function is entered and then checked when the function exits. If a guard check fails, an error message prints and the program exits. periodt. 💀-fstack-protector-all— same as-fstack-protectorexcept that all functions are protected. no exceptions, everyone gets a guard. 👑-fstack-protector-strong— like-fstack-protectorbut it includes additional functions to protect --- those that have local array definitions, or have references to local frame addresses.-fstack-protector-explicit— like-fstack-protectorbut only protects those functions which have thestack_protectattribute.
📚 -fstack-check 📚
-fstack-check— generates code to verify that you don't go beyond the boundary of the stack. You should specify this flag if you're running in an environment with multiple threads, but you only rarely need it in a single-threaded environment, because stack overflow is automatically detected on nearly all systems if there's only one stack.
Note that this switch does not actually cause checking to be done — the operating system or the language runtime has to do that. The switch just causes generation of code to ensure that they see the stack being extended.
You can additionally specify a string parameter: no means no checking, generic means force the use of old-style checking, specific means use the best checking method and is equivalent to bare -fstack-check.
Old-style checking is a generic mechanism that requires no specific target support in the compiler, but bestie, it comes with these 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. iykyk.
💥 -fstack-clash-protection 💥
-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. So it prevents allocations from jumping over any stack guard page provided by the operating system. slay.
Most targets do not fully support stack clash protection. However, on those targets -fstack-clash-protection will protect dynamic stack allocations. -fstack-clash-protection may also provide limited protection for static stack allocations if the target supports -fstack-check=specific.
NOTE: In Ubuntu 19.10 and later versions,-fstack-clash-protectionis enabled by default for C, C++, ObjC, ObjC++, unless-fno-stack-clash-protectionis found.
🚧 Stack limits 🚧
-fstack-limit-register=reg-fstack-limit-symbol=sym-fno-stack-limit
These 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's 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
-Wl,--defsym,__stack_limit=0x7ffe0000to 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 🌈
-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, because 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.
🔮 -fvtable-verify=[std|preinit|none] 🔮
This option is only available when compiling C++ code. It turns on (or off, if using -fvtable-verify=none) the security feature that verifies at run time, for every virtual call, that the vtable pointer through which the call is made is valid for the type of the object, and has not been corrupted or overwritten. If an invalid vtable pointer is detected at run time, an error is reported and execution of the program is immediately halted. 💀
This option causes run-time data structures to be built at program startup, which are used for verifying the vtable pointers. The options std and preinit control the timing of when these data structures are built. In both cases the data structures are built before execution reaches main. Using -fvtable-verify=std causes the data structures to be built after shared libraries have been loaded and initialized. -fvtable-verify=preinit causes them to be built before shared libraries have been loaded and initialized.
If this option appears multiple times in the command line with different values specified, none takes highest priority over both std and preinit; preinit takes priority over std. ranking unlocked. 👑
🐛 -fvtv-debug 🐛
When used in conjunction with -fvtable-verify=std or -fvtable-verify=preinit, this causes debug versions of the runtime functions for the vtable verification feature to be called. This flag also causes the compiler to log information about which vtable pointers it finds for each class. This information is written to a file named vtv_set_ptr_data.log in the directory named by the environment variable VTV_LOGS_DIR if that is defined, or the current working directory otherwise.
Note: This feature appends data to the log file. If you want a fresh log file, be sure to delete any existing one.
🧮 -fvtv-counts 🧮
This is a debugging flag. When used in conjunction with -fvtable-verify=std or -fvtable-verify=preinit, this causes the compiler to keep track of the total number of virtual calls it encounters and the number of verifications it inserts. It also counts the number of calls to certain run-time library functions that it inserts and logs this information for each compilation unit. The compiler writes this information to a file named vtv_count_data.log in the directory named by the environment variable VTV_LOGS_DIR if that is defined, or the current working directory otherwise. It also counts the size of the vtable pointer sets for each class, and writes this information to vtv_class_set_sizes.log in the same directory.
Note: This feature appends data to the log files. To get fresh log files, be sure to delete any existing ones.
🎀 -finstrument-functions 🎀
-finstrument-functions— generates instrumentation calls for entry and exit to functions. Just after function entry and just before function exit, the following profiling functions are called with the address of the current function and its call site. (On some platforms,__builtin_return_addressdoes not work beyond the current function, so the call site information may not be available to the profiling functions otherwise.)
void __cyg_profile_func_enter (void *this_fn,
void *call_site);
void __cyg_profile_func_exit (void *this_fn,
void *call_site);The first argument is the address of the start of the current function, which may be looked up exactly in the symbol table. 💖
Okay so bestie, this instrumentation thing? It's also done for functions that get expanded inline in other functions. The profiling calls show where the inline function is conceptually entered and exited. That means addressable versions of those functions have to be available. If all your uses of a function are expanded inline, you might end up with extra code size. And if you use extern inline in your C code, an addressable version must be provided. (Usually that's the case anyway, but if you get lucky and the optimizer always expands them inline, you might have gotten away without providing static copies.) No cap.
A function can be given the attribute no_instrument_function, and then this instrumentation is skipped. This is useful for, like, the profiling functions listed above, high-priority interrupt routines, and any functions from which the profiling functions cannot safely be called (maybe signal handlers, if the profiling routines generate output or allocate memory). So don't @ me.
-finstrument-functions-exclude-file-list=file,file,...— Set the list of functions that are excluded from instrumentation (see the description of-finstrument-functions). If the file that contains a function definition matches with one of file, then that function is not instrumented. The match is done on substrings: if the file parameter is a substring of the file name, it's considered 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. So slay.
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). Okay so that's a thing.
-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 a match. For C99 and C++ extended identifiers, the function name must be given in UTF-8, not using universal character names. So keep that in mind. 🦋-fpatchable-function-entry=N[,M]— Generate N NOPs right at the beginning of each function, with the function entry point before the Mth NOP. If M is omitted, it defaults to 0 so the function entry points to the address just at the first NOP. The NOP instructions reserve extra space which can be used to patch in any desired instrumentation at run time, provided that the code segment is writable. The amount of space is controllable indirectly via the number of NOPs; the NOP instruction used corresponds to the instruction emitted by the internal GCC back-end 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. If N=0, no pad location is recorded.
The NOP instructions are inserted at---and maybe before, depending on M---the function entry address, even before the prologue. Periodt.
✨ 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. So yeah, they're a whole vibe.
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. So check those out if you need them.
-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're 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. So order matters, bestie.
-U name— Cancel any previous definition of name, either built in or provided with a-Doption. Bye Felicia. 👋-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. So you get the good stuff without the extra. 🌸
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. So keep it minimal. ✨-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. Threads, bestie! 🧵-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. So it's a whole package. 💅
-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. So don't @ me.
-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.
-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— Change the target of the rule emitted by dependency generation. By default CPP takes the name of the main input file, deletes any directory components and any file suffix such as.c, and appends the platform's usual object suffix. The result is the target. 🎯
An -MT option sets the target to be exactly the string you specify. If you want multiple targets, you can specify them as a single argument to -MT, or use multiple -MT options.
For example, -MT '$(objpfx)foo.o' might give
$(objpfx)foo.o: foo.c-MQ target— Same as-MT, but it quotes any characters which are special to Make.-MQ '$(objpfx)foo.o'gives
$$(objpfx)foo.o: foo.cThe default target is automatically quoted, as if it were given with -MQ. So it's the extra-safe version. 💖
-MD— lowkey the same as-M -MF file, except it does not drag-Ealong for the ride. So how does it pickfile? The driver vibes it out based on whether you gave an-ooption. If you did, it takes that argument and slaps a.dsuffix on it. If you didn't, it grabs the input file's name, yeets any directory components and suffix, and appends.d. 💅
Now if -MD is hanging out with -E, every -o switch is understood to mean the dependency output file. But if it's used without -E, each -o is understood to mean a target object file instead. And since -E is not implied, -MD can be used to generate a dependency output file as a side effect of the compilation process. Iconic multi-tasking behavior, honestly. ✨
-MMD— like-MDexcept it only mentions user header files, not system header files. gatekeeping the system headers, we love to see it 🎀-fpreprocessed— tells the preprocessor "bestie, this input file has already been preprocessed, chill." This suppresses stuff like macro expansion, trigraph conversion, escaped newline splicing, and processing of most directives. It still recognizes and removes comments though, so you can pass a file preprocessed with-Cto the compiler without any problems. In this mode the integrated preprocessor is little more than a tokenizer for the front ends. 💀
-fpreprocessed is implicit if the input file has one of the extensions .i, .ii or .mi. Those are the extensions that GCC uses for preprocessed files created by -save-temps. iykyk 🦋
-fdirectives-only— when preprocessing, handle directives but do not expand macros. The way it behaves depends on-Eand-fpreprocessed.
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. it's giving rules-lawyer energy and I respect it ⭐
-fdollars-in-identifiers— accept$in identifiers. money moves only 💸✨-fextended-identifiers— accept universal character names 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.-ftabstop=width— set the distance between tab stops. This helps the preprocessor report correct column numbers in warnings or errors, even if tabs appear on the line. If the value is less than 1 or greater than 100, the option is ignored. The default is 8. the 8 is sacred, don't touch it 🌸-ftrack-macro-expansion[=level]— tracks the locations of tokens across macro expansions. This lets the compiler emit a diagnostic about the current macro expansion stack when a compilation error occurs inside a macro expansion. Using this option makes the preprocessor and the compiler consume more memory, so be a little mindful. 🙈
The level parameter chooses how precise the token location tracking is, which decreases memory consumption if you need that. Value 0 of level de-activates this option. Value 1 tracks 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 2 tracks token locations completely, and this value is the most memory hungry. When this option is given no argument, the default parameter value is 2. 👑
Note that -ftrack-macro-expansion=2 is activated by default.
-fmacro-prefix-map=old=new— when preprocessing files residing in directory old, expand the__FILE__and__BASE_FILE__macros as if the files resided in directory new instead. This can be used to change an absolute path to a relative path by using.for new, which can result in more reproducible builds that are location independent. This option also affects__builtin_FILE()during compilation. See also-ffile-prefix-map. 🔥-fexec-charset=charset— set the execution character set, used for string and character constants. The default isUTF-8. charset can be any encoding supported by the system'siconvlibrary routine.-fwide-exec-charset=charset— set the wide execution character set, used for wide string and character constants. The default isUTF-32orUTF-16, whichever corresponds to the width ofwchar_t. Just like-fexec-charset, charset can be any encoding supported by the system'siconvlibrary routine; however, you will have problems with encodings that do not fit exactly inwchar_t. no cap, this one will bite you 😭-finput-charset=charset— set the input character set, used for translation from the character set of the input file to the source character set used by GCC. If the locale does not specify, or GCC cannot get this information from the locale, the default isUTF-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'siconvlibrary 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.-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, plus its filename. When-fpreprocessedis in use, GCC recognizes this#pragmaand loads the PCH. 🫶
This option is off by default, because the resulting preprocessed output is only really suitable as input to GCC. It is switched on by -save-temps.
You should not write this #pragma in your own code, but it is safe to edit the filename if the PCH file is available in a different location. The filename may be absolute or it may be relative to GCC's current directory.
-fworking-directory— enable generation of linemarkers in the preprocessor output that let the compiler know the current working directory at the time of preprocessing. When this option is enabled, the preprocessor emits, after the initial linemarker, a second linemarker with the current working directory followed by two slashes. GCC uses this directory, when it's present in the preprocessed input, as the directory emitted as the current working directory in some debugging information formats. This option is implicitly enabled if debugging information is enabled, but this can be inhibited with the negated form-fno-working-directory. If the-Pflag is present in the command line, this option has no effect, since no#linedirectives 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. shell-friendly, we stan 🍓-A -predicate=answer— cancel an assertion with the predicate predicate and answer answer.-C— do not discard comments. All comments are passed through to the output file, except for comments in processed directives, which are deleted along with the directive.
You should be prepared for side effects when using -C; it causes the preprocessor to treat comments as tokens in their own right. For example, comments appearing at the start of what would be a directive line have the effect of turning that line into an ordinary source line, since the first token on the line is no longer a #. ⚠️
-CC— do not discard comments, including during macro expansion. This is like-C, except that comments contained within macros are also passed through to the output file where the macro is expanded.
In addition to the side effects of the -C option, the -CC option causes all C++-style comments inside a macro to be converted to C-style comments. This is to prevent later use of that macro from inadvertently commenting out the remainder of the source line.
The -CC option is generally used to support lint comments. ✨
-P— inhibit generation of linemarkers in the output from the preprocessor. This might be useful when running the preprocessor on something that is not C code, and will be sent to a program which might be confused by the linemarkers.-traditional-traditional-cpp— try to imitate the behavior of pre-standard C preprocessors, as opposed to ISO C preprocessors. See the GNU CPP manual for details. throwback era fr 💖
Note that GCC does not otherwise attempt to emulate a pre-standard C compiler, and these options are only supported with the -E switch, or when invoking CPP explicitly.
-trigraphs— support ISO C trigraphs. These are three-character sequences, all starting with??, that are defined by ISO C to stand for single characters. For example,??/stands for\, so'??/n'is a character constant for a newline.
The nine trigraphs and their replacements are
Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??-
Replacement: [ ] { } # \ ^ | ~By default, GCC ignores trigraphs, but in standard-conforming modes it converts them. See the -std and -ansi options. these are cursed and also kind of iconic 💀
-remap— enable special code to work around file systems which only permit very short file names, such as MS-DOS.-H— print the name of each header file used, in addition to other normal activities. Each name is indented to show how deep in the#includestack it is. Precompiled header files are also printed, even if they are found to be invalid; an invalid precompiled header file is printed with...xand a valid one with...!. 🎀-dletters— okay bestie, this one says to make debugging dumps during compilation as specified by letters. The flags documented here are those relevant to the preprocessor. Other letters? They get interpreted by the compiler proper, or they're reserved for future versions of GCC, so they're silently ignored. And if you specify letters whose behavior conflicts, the result is undefined. 💀 no cap.-dM— Instead of the normal output, generate a list of#definedirectives for all the macros defined during the execution of the preprocessor, including predefined macros. This is literally your way of finding out what is 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. ✨
If you use -dM without the -E option, -dM is interpreted as a synonym for -fdump-rtl-mach. iykyk.
-dD— Like-dMexcept in two respects: 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 emit only the macro names, not their expansions. periodt.-dI— Output#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, are output; the output is delayed until the use or test of the macro; and#undefdirectives are also output for macros tested but undefined at the time. highkey specific but we love that.
-fdebug-cpp— This option is only useful for debugging GCC. When used from CPP or with-E, it dumps debugging information about location maps. Every token in the output is preceded by the dump of the map its location belongs to. When used from GCC without-E, this option has no effect. 🙈-Wp,option— You can use-Wp,optionto bypass the compiler driver and pass option directly through to the preprocessor. If option contains commas, it is split into multiple options at the commas. However, many options are modified, translated or interpreted by the compiler driver before being passed to the preprocessor, and-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. not me bypassing the driver 😭-Xpreprocessoroption — Pass option as an option to the preprocessor. You can use this to supply system-specific preprocessor options that GCC does not recognize. If you want to pass an option that takes an argument, you must use-Xpreprocessortwice, once for the option and once for the argument. iconic behavior.-no-integrated-cpp— Perform preprocessing as a separate pass before compilation. By default, GCC performs preprocessing as an integrated part of input tokenization and parsing. If this option is provided, the appropriate language front end (cc1,cc1plus, orcc1objfor C, C++, and Objective-C, respectively) is instead invoked twice, once for preprocessing only and once for actual compilation of the preprocessed input. This option may be useful in conjunction with the-Bor-wrapperoptions to specify an alternate preprocessor or perform additional processing of the program source between normal preprocessing and compilation. slay.
✨ Passing Options to the Assembler ✨
You can pass options to the assembler. 💅
-Wa,option— Pass option as an option to the assembler. If option contains commas, it is split into multiple options at the commas.-Xassembleroption — Pass option as an option to the assembler. You can use this to supply system-specific assembler options that GCC does not recognize. If you want to pass an option that takes an argument, you must use-Xassemblertwice, once for the option and once for the argument.
💖 Options for Linking 💖
These options come into play when the compiler links object files into an executable output file. They are meaningless if the compiler is not doing a link step. 🫶
object-file-name— A file name that does not end in a special recognized suffix is considered to name an object file or library. (Object files are distinguished from libraries by the linker according to the file contents.) If linking is done, these object files are used as input to the linker.-c,-S,-E— If any of these options is used, then the linker is not run, and object file names should not be used as arguments.-flinker-output=type— This option controls the code generation of the link time optimizer. By default the linker output is determined by the linker plugin automatically. For debugging the compiler and in the case of incremental linking to non-lto object file is desired, it may be useful to control the type manually.- If type is
execthe code generation is configured to produce static binary. In this case-fpicand-fpieare both disabled. - If type is
dynthe code generation is configured to produce shared library. In this case-fpicor-fPICis preserved, but not enabled automatically. This makes it possible to build shared libraries without position independent code on architectures this is possible, i.e. on x86. - If type is
piethe code generation is configured to produce-fpieexecutable. This gives similar optimizations asexecexcept that-fpieis not disabled if specified at compilation time. - If type is
relthe compiler assumes that incremental linking is done. The sections containing intermediate code for link-time optimization are merged, pre-optimized, and output to the resulting object file. In addition, if-ffat-lto-objectsis specified the binary code is produced for future non-lto linking. The object file produced by incremental linking will be smaller than a static library produced from the same object files. At link-time the result of incremental linking will also load faster to compiler than a static library assuming that majority of objects in the library are used. - Finally
nolto-relconfigures the compiler for incremental linking where code generation is forced, 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 will be optimized better than with link time optimizations disabled (for example, the cross-module inlining will happen), most of benefits of whole program optimizations are however lost. - During the incremental link (by
-r) the linker plugin will default torel. With current interfaces to GNU Binutils it is however not possible to link incrementally LTO objects and non-LTO objects into a single mixed object file. In the case any of object files in incremental link cannot be used for link-time optimization the linker plugin will output a warning and usenolto-rel. To maintain the whole program optimization it is recommended to link such objects into static library instead. Alternatively it is possible to use H.J. Lu's binutils with support for mixed objects.
- If type is
-fuse-ld=bfd— Use the bfd linker instead of the default linker.-fuse-ld=gold— Use the gold linker instead of the default linker.-fuse-ld=lld— Use the LLVM lld linker instead of the default linker.-llibrary,-llibrary — Search the library named library when linking. (The second alternative with the library as a separate argument is only for POSIX compliance and is not recommended.)
The -l option is passed directly to the linker by GCC. Refer to your linker documentation for exact details. The general description below applies to the GNU linker.
The linker searches a standard list of directories for the library. The directories searched include several standard system directories plus any that you specify with -L.
Static libraries are archives of object files, and have file names like liblibrary.a. Some targets also support shared libraries, which typically have names like liblibrary.so. If both static and shared libraries are found, the linker gives preference to linking with the shared library unless the -static option is used.
It makes a difference where in the command you write this option; the linker searches and processes libraries and object files in the order they are specified. Thus, foo.o -lz bar.o searches library z after file foo.o but before bar.o. If bar.o refers to functions in z, those functions may not be loaded.
-lobjc— You need this special case of the-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— Do not use the standard system libraries when linking. Only the libraries you specify are passed to the linker, and options specifying linkage of the system libraries, such as-static-libgccor-shared-libgcc, are ignored. The standard startup files are used normally, unless-nostartfilesis used.
The compiler may generate calls to "memcmp", "memset", "memcpy" and "memmove". These entries are usually resolved by entries in libc. These entry points should be supplied through some other mechanism when this option is specified.
-nolibc— Do not use the C library or system libraries tightly coupled with it when linking. Still link with the startup files, libgcc or toolchain provided language support libraries such as libgnat, libgfortran or libstdc++ unless options preventing their inclusion are used as well. This typically removes-lcfrom the link command line, as well as system libraries that normally go with it and become meaningless when absence of a C library is assumed, for example-lpthreador-lmin some configurations. This is intended for bare-board targets when there is indeed no C library available.-nostdlib— Do not use the standard system startup files or libraries when linking. No startup files and only the libraries you specify are passed to the linker, and options specifying linkage of the system libraries, such as-static-libgccor-shared-libgcc, are ignored.
okay so bestie, real talk before we get to the linker flags: the compiler may generate calls to memcmp, memset, memcpy and memmove. 💅 Those entries are usually resolved by entries in libc. But when this option is specified, those entry points should be supplied through some other mechanism, okay? No cap.
One of the standard libraries bypassed by -nostdlib and -nodefaultlibs is libgcc.a — a library of internal subroutines which GCC uses to overcome shortcomings of particular machines, or special needs for some languages. It's giving "secret sauce." 🫶
In most cases, you need libgcc.a even when you want to avoid other standard libraries. In other words, when you specify -nostdlib or -nodefaultlibs you should usually specify -lgcc as well. This ensures that you have no unresolved references to internal GCC library subroutines. (An example of such an internal subroutine is __main, used to ensure C++ constructors are called.) ✨
-eentry,--entry=entry — specify that the program entry point is entry. The argument is interpreted by the linker; the GNU linker accepts either a symbol name or an address. 🎯-pie— produce a dynamically linked position independent executable on targets that support it. For predictable results, you must also specify the same set of options used for compilation (-fpie,-fPIE, or model suboptions) when you specify this linker option. Consistency is the vibe check. ✅-no-pie— don't produce a dynamically linked position independent executable. That's it, that's the flag. 🙅♀️-static-pie— produce a static position independent executable on targets that support it. A static position independent executable is similar to a static executable, but can be loaded at any address without a dynamic linker. For predictable results, you must also specify the same set of options used for compilation (-fpie,-fPIE, or model suboptions) when you specify this linker option. 🌈-pthread— link with the POSIX threads library. This option is supported on GNU/Linux targets, most other Unix derivatives, and also on x86 Cygwin and MinGW targets. On some targets this option also sets flags for the preprocessor, so it should be used consistently for both compilation and linking. Don't half-do it bestie. 🧵-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 instructs the linker to add all symbols, not only used ones, to the dynamic symbol table. This option is needed for some uses ofdlopenor to allow obtaining backtraces from within a program. 🕵️♀️-s— remove all symbol table and relocation information from the executable. Stripped, minimal, sleek. 💅-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 static version, respectively. If no shared version of libgcc was built when the compiler was configured, these options have no effect.
There are several situations in which an application should use the shared libgcc instead of the static version. The most common of these is when the application wishes to throw and catch exceptions across different shared libraries. In that case, each of the libraries as well as the application itself should use the shared libgcc. 😭
Therefore, the G++ driver automatically adds -shared-libgcc whenever you build a shared library or a main executable, because C++ programs typically use exceptions, so this is the right thing to do. G++ said "I got you." 🫶
If, instead, you use the GCC driver to create shared libraries, you may find that they are not always linked with the shared libgcc. If GCC finds, at its configuration time, that you have a non-GNU linker or a GNU linker that does not support option --eh-frame-hdr, it links the shared version of libgcc into shared libraries by default. Otherwise, it takes advantage of the linker and optimizes away the linking with the shared version of libgcc, linking with the static version of libgcc by default. This allows exceptions to propagate through such shared libraries, without incurring relocation costs at library load time.
However, if a library or main executable is supposed to throw or catch exceptions, you must link it using the G++ driver, or using the option -shared-libgcc, such that it is linked with the shared libgcc. ⚠️
-static-libasan— when the-fsanitize=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 this 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 this 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 this 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 this 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 this links against the shared version of libstdc++. That is normally fine. However, it is sometimes useful to freeze the version of libstdc++ used by the program without going all the way to a fully static link. The-static-libstdc++option directs 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. Rare flag, limited edition. ✨-Tscript — use script as the linker script. This option is supported by most systems using the GNU linker. On some targets, such as bare-board targets without an operating system, the-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 does not recognize.
If you want to pass an option that takes a separate argument, you must use -Xlinker twice, once for the option and once for the argument. For example, to pass -assert definitions, you must write -Xlinker -assert -Xlinker definitions. It does not work to write -Xlinker "-assert definitions", because this passes the entire string as a single argument, which is not what the linker expects. 💀
When using the GNU linker, it is usually more convenient to pass arguments to linker options using the option=value syntax than as separate arguments. For example, you can specify -Xlinker -Map=output.map rather than -Xlinker -Map -Xlinker output.map. Other linkers may not support this syntax for command-line options.
-Wl,option — pass option as an option to the linker. If option contains commas, it is split into multiple options at the commas. You can use this syntax to pass an argument to the option. For example,-Wl,-Map,output.mappasses-Map output.mapto the linker. When using the GNU linker, you can also get the same effect with-Wl,-Map=output.map.
NOTE: In Ubuntu 8.10 and later versions, for LDFLAGS, the option-Wl,-z,relrois used. To disable, use-Wl,-z,norelro. 🐧
-usymbol — pretend the symbol symbol is undefined, to force linking of library modules to define it. You can use-umultiple times with different symbols to force loading of additional library modules. Stack 'em up. 🎀-zkeyword —-zis passed directly on to the linker along with the keyword keyword. See the section in the documentation of your linker for permitted values and their meanings. 👀
🎀 Options for Directory Search 🎀
These options specify directories to search for header files, for libraries and for parts of the compiler. It's giving "where my stuff at." 🔍
-Idir,-iquotedir,-isystemdir,-idirafterdir — add the directory dir to the list of directories to be searched for header files during preprocessing. If dir begins with=or$SYSROOT, then the=or$SYSROOTis replaced by the sysroot prefix; see--sysrootand-isysroot.
Directories specified with -iquote apply only to the quote form of the directive, #include "file". Directories specified with -I, -isystem, or -idirafter apply to lookup for both the #include "file" and #include <file> directives. 💖
You can specify any number or combination of these options on the command line to search for header files in several directories. The lookup order is as follows:
- 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 get searched left-to-right, in the order they show up 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, swapping in your own version, because these directories get searched before the standard system header file directories. But bestie, do not use this option to add directories that contain vendor-supplied system header files — use -isystem for that, okay? 🫶
The -isystem and -idirafter options also mark the directory as a system directory, so it gets the same special treatment that the standard system directories get. ✨
If a standard system include directory, or a directory specified with -isystem, is also specified with -I, then 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 to fix buggy system headers and the ordering for the #include_next directive don't get inadvertently changed. If you really need to change the search order for system directories, use the -nostdinc and/or -isystem options instead.
-I-— Split the include path. This option has been deprecated. Please use-iquoteinstead for-Idirectories before the-I-, and remove the-I-option. 😭
Any directories specified with -I options before -I- are searched only for headers requested with #include "file"; they are not searched for #include <file>. If additional directories are specified with -I options after the -I-, those directories are searched for all #include directives.
On top of that, -I- inhibits the use of the directory of the current file as the first search directory for #include "file". And ngl, there is no way to override this effect of -I-. It's giving locked-in. 🔒
-iprefix prefix— Specify prefix as the prefix for subsequent-iwithprefixoptions. If the prefix represents a directory, you should include the final/.-iwithprefix dir/-iwithprefixbefore dir— Append dir to the prefix specified previously with-iprefix, and add the resulting directory to the include search path.-iwithprefixbeforeputs it in the same place-Iwould;-iwithprefixputs it where-idirafterwould. Same energy, different placement. 💅-isysroot dir— This option is like the--sysrootoption, but it applies only to header files (except for Darwin targets, where it applies to both header files and libraries). See the--sysrootoption for more info. 🍓-imultilib dir— Use dir as a subdirectory of the directory containing target-specific C++ headers.-nostdinc— Do not search the standard system directories for header files. Only the directories explicitly specified with-I,-iquote,-isystem, and/or-idirafteroptions (and the directory of the current file, if appropriate) are searched. You're on your own, main character energy. 👑-nostdinc++— Do not search for header files in the C++-specific standard directories, but do still search the other standard directories. (This option is used when building the C++ library.) 🌈-iplugindir=dir— Set the directory to search for plugins that are passed by-fplugin=nameinstead of-fplugin=path/name.so. This option is not meant to be used by the user — it's only passed by the driver. Not for you, bestie, it's a driver thing. 🙈-Ldir— Add directory dir to the list of directories to be searched for-l.-Bprefix— This option specifies where to find the executables, libraries, include files, and data files of the compiler itself. ✨
The compiler driver program runs one or more of the subprograms cpp, cc1, as and ld. It tries prefix as a prefix for each program it tries to run, both with and without machine/version/ for the corresponding target machine and compiler version.
For each subprogram to be run, the compiler driver first tries the -B prefix, if any. If that name isn't found, or if -B isn't specified, the driver tries two standard prefixes, /usr/lib/gcc/ and /usr/local/lib/gcc/. If neither of those results in a file name that's found, the unmodified program name is searched for using the directories specified in your PATH environment variable. 🛣️
The compiler checks to see if the path provided by -B refers to a directory, and if necessary it adds a directory separator character at the end of the path.
-B prefixes that effectively specify directory names also apply to libraries in the linker, because the compiler translates these options into -L options for the linker. They also apply to include files in the preprocessor, because the compiler translates these options into -isystem options for the preprocessor. In this case, the compiler appends include to the prefix.
The runtime support file libgcc.a can also be searched for using the -B prefix, if needed. If it's not found there, the two standard prefixes above are tried, and that is all. The file is left out of the link if it isn't 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's replaced by [dir/]include. This is to help with boot-strapping the compiler.
-no-canonical-prefixes— Do not expand any symbolic links, resolve references to/../or/./, or make the path absolute when generating a relative prefix.--sysroot=dir— Use dir as the logical root directory for headers and libraries. For example, if the compiler normally searches for headers in/usr/includeand libraries in/usr/lib, it instead searchesdir/usr/includeanddir/usr/lib. 🦋
If you use both this option and the -isysroot option, then the --sysroot option applies to libraries, but the -isysroot option applies to header files.
The GNU linker (beginning with version 2.16) has the necessary support for this option. If your linker doesn't support this option, the header file aspect of --sysroot still works, but the library aspect does not. 😬
--no-sysroot-suffix— For some targets, a suffix is added to the root directory specified with--sysroot, depending on the other options used, so that headers may for example be found indir/suffix/usr/includeinstead ofdir/usr/include. This option disables the addition of such a suffix.
✨ Options for Code Generation Conventions ✨
These machine-independent options control the interface conventions used in code generation. 💅
Most of them have both positive and negative forms; the negative form of -ffoo is -fno-foo. In the table below, only one of the forms is listed — the one that is not the default. You can figure out the other form by either removing no- or adding it. iykyk. 🎀
-fstack-reuse=reuse-level— This option controls stack space reuse for user declared local/auto variables and compiler generated temporaries.reuse_levelcan 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. The option is needed when the program extends the lifetime of a scoped local variable or a compiler generated temporary beyond the end point defined by the language. When a lifetime of a variable ends, and if the variable lives in memory, the optimizing compiler has the freedom to reuse its stack space with other temporaries or scoped local variables whose live range does not overlap with it. Legacy code extending local lifetime is likely to break with the stack reuse optimization. 😭
For example,
int *p;
{
int local1;
p = &local1;
local1 = 10;
....
}
{
int local2;
local2 = 20;
...
}
if (*p == 10) // out of scope use of local1
{
}Another example:
struct A
{
A(int k) : i(k), j(k) { }
int i;
int j;
};
A *ap;
void foo(const A& ar)
{
ap = &ar;
}
void bar()
{
foo(A(10)); // temp object's lifetime ends when foo returns
{
A a(20);
....
}
ap->i+= 10; // ap references out of scope temp whose space
// is reused with a. What is the value of ap->i?
}The lifetime of a compiler generated temporary is well defined by the C++ standard. When a lifetime of a temporary ends, and if the temporary lives in memory, the optimizing compiler has the freedom to reuse its stack space with other temporaries or scoped local variables whose live range does not overlap with it. However some of the legacy code relies on the behavior of older compilers in which temporaries' stack space is not reused, 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— bestie, this one tells the compiler: assume that signed arithmetic overflow of addition, subtraction and multiplication wraps around using twos-complement representation. 🔄✨
This flag enables some optimizations and disables others. The options -ftrapv and -fwrapv override each other, so using -ftrapv -fwrapv on the command-line results in -fwrapv being effective. Note that only active options override, so using -ftrapv -fwrapv -fno-wrapv on the command-line results in -ftrapv being effective. 👑
-fwrapv-pointer— same energy but for pointers, fr. 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 one implies-fno-wrapv -fno-wrapv-pointer, and when negated it implies-fwrapv -fwrapv-pointer. it's giving "package deal". 🎀-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. 💀-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 option is enabled by default for the Ada front end, as permitted by the Ada language specification. 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. 💅
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. so tiny, so efficient.
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. ⚠️
-fno-common— 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. 🧠
Unix C compilers have traditionally allocated storage for 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 is the behavior specified by -fcommon, and is the default for GCC on most targets. On the other hand, this behavior is not required by ISO C, and on some targets may carry a speed or code size penalty on variable references. 🐌
The -fno-common option specifies that the compiler should instead place 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 defined in more than one compilation unit. Compiling with -fno-common is useful on targets for which it provides better performance, or if you wish to verify that the program will work on other systems that always treat uninitialized variable definitions this way. ✨
-fno-ident— Ignore the#identdirective. 🙈 simple as that bestie.-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 compilingcrtstuff.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 comments? they're for humans, not for machines, so the precise format of the comments is subject to change. don't build a parser around them bestie, that's not the vibe 💅
-frecord-gcc-switches— this one literally makes the command line you used to invoke the compiler get recorded into the object file that's being created. so cute, so archival. but: it's only implemented on some targets, and the exact format of the recording depends on the target and the binary file format. usually it takes the form of a section containing ASCII text. it's related to-fverbose-asm, except that one only records info in the assembler output file as comments</>, so it never reaches the object file. also peep-grecord-gcc-switchesfor another way of storing compiler options into the object file 🎀-fpic— generate position-independent code (PIC) suitable for use in a shared library, if supported for the target machine. such code accesses all constant addresses through a global offset table (GOT). the dynamic loader resolves the GOT entries when the program starts (and no, the dynamic loader is not part of GCC — it's part of the operating system, okurrr). if the GOT size for the linked executable exceeds a machine-specific maximum size, you get an error message from the linker saying that-fpicdoes not work; in that case, recompile with-fPICinstead. (those maximums are8kon the SPARC,28kon AArch64 and32kon the m68k and RS/6000. the x86 has no such limit, main character energy fr fr.)
position-independent code requires special support, and therefore works only on certain machines. for the x86, GCC supports PIC for System V but not for the Sun 386i. code generated for the IBM RS/6000 is always position-independent.
when this flag is set, the macros __pic__ and __PIC__ are defined to 1 ✨
-fPIC— if supported for the target machine, emit position-independent code, suitable for dynamic linking and avoiding any limit on the size of the global offset table. this option makes a difference on AArch64, m68k, PowerPC and SPARC. 👑
position-independent code requires special support, and therefore works only on certain machines.
when this flag is set, the macros __pic__ and __PIC__ are defined to 2.
-fpie,-fPIE— these are like-fpicand-fPICbut the generated position-independent code can only be linked into executables. usually you use these to compile code that's gonna be linked using the-pieGCC option. 🧚
-fpie and -fPIE both define the macros __pie__ and __PIE__. the macros have the value 1 for -fpie and 2 for -fPIE.
-fno-plt— do not use the PLT for external function calls in position-independent code. instead, load the callee address at call sites from the GOT and branch to it. this leads to more efficient code by eliminating PLT stubs and exposing GOT loads to optimizations. on architectures such as 32-bit x86 where PLT stubs expect the GOT pointer in a specific register, this gives more register allocation freedom to the compiler. lazy binding requires use of the PLT; with-fno-pltall external symbols are resolved at load time 💀
alternatively, the function attribute noplt can be used to avoid calls through the PLT for specific external functions.
in position-dependent code, a few targets also convert calls to functions that are marked to not use the PLT to use the GOT instead.
-fno-jump-tables— do not use jump tables forswitchstatements, even where it would be more efficient than other code generation strategies. this one is of use in conjunction with-fpicor-fPICfor building code that forms part of a dynamic linker and cannot reference the address of a jump table. on some targets, jump tables don't require a GOT and this option isn't needed at all. 🌈-ffixed-reg— treat the register named reg as a fixed register; generated code should never refer to it (except perhaps as a stack pointer, frame pointer or in some other fixed role). 🙈
reg must be the name of a register. the register names accepted are machine-specific and are defined in the REGISTER_NAMES macro in the machine description macro file.
heads up bestie: this flag does not have a negative form, because it specifies a three-way choice. no -fno- version, periodt.
-fcall-used-reg— treat the register named reg as an allocable register that is clobbered by function calls. it may be allocated for temporaries or variables that do not live across a call. functions compiled this way do not save and restore the register reg.
it is an error to use this flag with the frame pointer or stack pointer. use of this flag for other registers that have fixed pervasive roles in the machine's execution model produces disastrous results. 💀
same deal: this flag does not have a negative form, because it specifies a three-way choice.
-fcall-saved-reg— treat the register named reg as an allocable register saved by functions. it may be allocated even for temporaries or variables that live across a call. functions compiled this way save and restore the register reg if they use it.
it is an error to use this flag with the frame pointer or stack pointer. use of this flag for other registers that have fixed pervasive roles in the machine's execution model produces disastrous results.
and a different sort of disaster results from the use of this flag for a register in which function values may be returned. 😭
yes, again: no negative form, three-way choice, you know the drill.
-fpack-struct[=n]— without a value specified, pack all structure members together without holes. when a value is specified (which must be a small power of two), pack structure members according to this value, representing the maximum alignment (that is, objects with default alignment requirements larger than this are output potentially unaligned at the next fitting location).
warning: the -fpack-struct switch causes GCC to generate code that is not binary compatible with code generated without that switch. additionally, it makes the code suboptimal. use it to conform to a non-default application binary interface.
-fleading-underscore— this option and its counterpart,-fno-leading-underscore, forcibly change the way C symbols are represented in the object file. one use is to help link with legacy assembly code.
warning: the -fleading-underscore switch causes GCC to generate code that is not binary compatible with code generated without that switch. use it to conform to a non-default application binary interface. not all targets provide complete support for this switch.
-ftls-model=model— alter the thread-local storage model to be used. the model argument should be one ofglobal-dynamic,local-dynamic,initial-execorlocal-exec. note that the choice is subject to optimization: the compiler may use a more efficient model for symbols not visible outside of the translation unit, or if-fpicis not given on the command line. iykyk 💅
the default without -fpic is initial-exec; with -fpic the default is global-dynamic.
-ftrampolines— for targets that normally need trampolines for nested functions, always generate them instead of using descriptors. otherwise, for targets that do not need them, like for example HP-PA or IA-64, do nothing.
a trampoline is a small piece of code that is created at run time on the stack when the address of a nested function is taken, and is used to call the nested function indirectly. therefore, it requires the stack to be made executable in order for the program to work properly. 💀
-fno-trampolines is enabled by default on a language by language basis to let the compiler avoid generating them, if it computes that this is safe, and replace them with descriptors. descriptors are made up of data only, but the generated code must be prepared to deal with them. as of this writing, -fno-trampolines is enabled by default only for Ada.
moreover, code compiled with -ftrampolines and code compiled with -fno-trampolines are not binary compatible if nested functions are present. this option must therefore be used on a program-wide basis and be manipulated with extreme care. like, bestie, be careful. 🫶
-fvisibility=[default|internal|hidden|protected]— set the default ELF image symbol visibility to the specified option — all symbols are marked with this unless overridden within the code. using this feature can very substantially improve linking and load times of shared object libraries, produce more optimized code, provide near-perfect API export and prevent symbol clashes. it is strongly recommended that you use this in any shared objects you distribute. 👑
despite the nomenclature, default always means public; i.e., available to be linked against from outside the shared object. protected and internal are pretty useless in real-world usage so the only other commonly used option is hidden. the default if -fvisibility isn't specified is default, i.e., make every symbol public.
...okay bestie, here's the actual glow-up: instead of marking stuff hidden when the default is public, you flip the whole vibe — make the default hidden and then explicitly mark the things you want public. That's literally the norm with DLLs on Windows, and if you pair -fvisibility=hidden with "__attribute__ ((visibility("default")))" instead of "__declspec(dllexport)" you get almost identical semantics with identical syntax. Cross-platform girlies, this one is for you. 💖
And if you're retrofitting visibility onto code that already exists, "#pragma GCC visibility" is gonna be your new obsession. How it works: you wrap the declarations you care about in, like, "#pragma GCC visibility push(hidden)" and then "#pragma GCC visibility pop". But listen — symbol visibility is part of your API interface contract, okay? So all new code should always say what it means when it's not the default. Declarations that are only for use inside the local DSO should be marked explicitly hidden so you dodge PLT indirection overheads — and being loud about it also makes the code readable and self-documenting, which is a whole slay. 🎀 Heads up tho: because of ISO C++ specification requirements, "operator new" and "operator delete" must always be default visibility. No exceptions, bestie.
Also!! Headers from outside your project — especially system headers and headers from any other library you use — may not be expecting to be compiled with visibility other than the default. So you may need to explicitly say "#pragma GCC visibility push(default)" before including any of those. 🫶
"extern" declarations are not affected by -fvisibility, so a lot of code can be recompiled with -fvisibility=hidden with zero modifications. But (and this is the catch) that means calls to "extern" functions with no explicit visibility go through 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. 👑
Oh, and note that -fvisibility does affect C++ vague linkage entities. Meaning: an exception class that gets thrown between DSOs must be explicitly marked with default visibility so the type_info nodes are unified between the DSOs. No cap.
Wanna see the whole overview of these techniques, their benefits, and how to use them? It's at <http://gcc.gnu.org/wiki/Visibility>. ✨
-fstrict-volatile-bitfields— use this one when accesses to volatile bit-fields (or other structure fields, tho the compiler usually honors those types anyway) should be a single access of the width of the field's type, aligned to a natural alignment if possible. Example time: targets with memory-mapped peripheral registers might need 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 those targets) to force GCC into 16-bit accesses instead of, maybe, 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 touches bytes that don't contain any portion of the bit-field, or memory-mapped registers unrelated to the one being updated. messy, but efficient.
In some cases, like when the "packed" attribute is applied to a structure field, it may just not be possible to access the field with a single read or write that's correctly aligned for the target machine. When that happens, 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. fr fr.
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 it is -fno-sync-libcalls. This option is used in the implementation of the libatomic runtime library. 🧚
🛠️ GCC Developer Options 🛠️
Okay so this section is for the GCC devs themselves — the compiler-testing girlies, the bug-investigating girlies, the "why is my compile-time so slow" girlies. It covers options that produce debug dumps at various points in the compilation; that print statistics like memory use and execution time; and that print info about GCC's configuration, like where it searches for libraries. For ordinary compilation and linking tasks, you should rarely need any of these. 💅
Many developer options that cause GCC to dump output to a file take an optional =filename suffix. You can specify stdout or - to dump to standard output, and stderr for standard error. 🌈
If =filename is omitted, a default dump file name is constructed by concatenating the base dump file name, a pass number, 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.
-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— dump after iterated register allocation.-fdump-rtl-jump— dump after the second jump optimization.-fdump-rtl-loop2—-fdump-rtl-loop2enables dumping after the rtl loop optimization passes.-fdump-rtl-mach— dump after performing the machine dependent reorganization pass, if that pass exists.-fdump-rtl-mode_sw— dump after removing redundant mode switches.-fdump-rtl-rnreg— dump after register renumbering.-fdump-rtl-outof_cfglayout— dump after converting from cfglayout mode.-fdump-rtl-peephole2— dump after the peephole pass.-fdump-rtl-postreload— dump after post-reload optimizations.-fdump-rtl-pro_and_epilogue— dump after generating the function prologues and epilogues.-fdump-rtl-sched1,-fdump-rtl-sched2—-fdump-rtl-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— dumps everything right after branch shortening bestie ✨-fdump-rtl-sibling— dumps after sibling call optimizations 💅-fdump-rtl-split1,-fdump-rtl-split2,-fdump-rtl-split3,-fdump-rtl-split4,-fdump-rtl-split5— these bad boys enable dumping after five rounds of instruction splitting. five!! 🔥-fdump-rtl-sms— dumps after modulo scheduling. heads up tho: this pass is only run on some architectures 🙈-fdump-rtl-stack— dumps after the conversion from GCC's "flat register file" registers over to the x87's stack-like registers. also architecture-locked: this pass is only run on x86 variants 💀-fdump-rtl-subreg1,-fdump-rtl-subreg2—-fdump-rtl-subreg1and-fdump-rtl-subreg2enable dumping after the two subreg expansion passes 🫶-fdump-rtl-unshare— dumps after all rtl has been unshared ✨-fdump-rtl-vartrack— dumps after variable tracking 🌸-fdump-rtl-vregs— dumps after converting virtual registers to hard registers 👑-fdump-rtl-web— dumps 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— okay so these dumps are *defined*… but they always produce empty files. not a single byte of lore. iconic in a cursed way tbh 💀-da,-fdump-rtl-all— produce all the dumps listed above 🎀-dA— annotate the assembler output with miscellaneous debugging information.-dD— dump all macro definitions at the end of preprocessing, on top of the normal output.-dH— produce a core dump whenever an error occurs.-dp— annotate the assembler output with a comment indicating which pattern and alternative is used. the length and cost of each instruction also get 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 together with-fdump-rtl-expand✨
-fdump-debug— dump the debugging information generated during the debug generation phase 🐛-fdump-earlydebug— dump the debugging information generated during the early debug generation phase 🐛✨-fdump-noaddr— when doing debugging dumps, suppress address output. this makes it way 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. so if it crashes, at least it leaves receipts 😭-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 the-doption above), suppress instruction numbers for the links to the previous and next instructions in a sequence.
-fdump-ipa-switch,-fdump-ipa-switch-options— control the dumping at various stages of inter-procedural analysis language tree to a file. the file name is generated by appending a switch specific suffix to the source file name, and the file is created in the same directory as the output file. the following dumps are possible:all— enables all inter-procedural analysis dumps ✨cgraph— dumps information about call-graph optimization, unused function removal, and inlining decisions.inline— dump after function inlining.
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-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 💅raw— dump the raw internal tree data. this option is applicable to C++ only.
-fdump-passes— print onstderrthe 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 📊
-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.
- invoke GCC with
-fdump-passesand in the stderr output look for a code that corresponds to the pass you are interested in. for example, the codestree-evrp,tree-vrp1, andtree-vrp2correspond to the three Value Range Propagation passes. the number at the end distinguishes distinct invocations of the same pass 👀 - to enable the creation of the dump file, append the pass code to the
-fdump-option prefix and invoke GCC with it. for example, to enable the dump from the Early Value Range Propagation pass, invoke GCC with the-fdump-tree-evrpoption. optionally, you may specify the name of the dump file. if you don't specify one, GCC creates as described below. - find the pass dump in a file whose name is composed of three components separated by a period: the name of the source file GCC was invoked to compile, a numeric suffix indicating the pass number followed by the letter
tfor tree passes (and the letterrfor RTL passes), and finally the pass code. for example, the Early VRP pass dump might be in a file namedmyfile.c.038t.evrpin the current working directory. note that the numeric codes are not stable and may change from one version of GCC to another 💀
-fopt-info,-fopt-info-options,-fopt-info-options=filename— controls optimization dumps from various optimization passes. if the-optionsform is used, options is a list of-separated option keywords to select the dump details and optimizations.
the options can be divided into three groups:
- options describing what kinds of messages should be emitted,
- options describing the verbosity of the dump, and
- options describing which optimizations should be included.
The options from each group can be freely mixed as they are non-overlapping. However, in case of any conflicts, the later options override the earlier options on the command line. Periodt. Last one wins, bestie.
Which kinds of messages get emitted 💬
optimized— prints the tea ☕ when an optimization is successfully applied. It's up to each pass to decide which info is even relevant. Like, the vectorizer passes print the source location of loops that got successfully vectorized. Slay.missed— prints info about the optimizations that didn't happen 😭. Individual passes control what actually goes into the output.note— verbose info about optimizations, like certain transformations, more detailed messages about decisions, etc. The commentary track, basically.all— the whole buffet: detailed optimization information, which includesoptimized,missed, andnote. All of it. Iconic.
The one and only verbosity knob 🔍
internals— by default only "high-level" messages are emitted. This option unlocks the additional, more detailed messages that are likely to only be of interest to GCC developers. So like, dev-only content, iykyk.
Pick your optimization group 🎯
One or more of these keywords can be used:
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. The mother of all groups 👑
If options is omitted, it defaults to optimized-optall, which means: dump messages about successful optimizations from all the passes, while omitting messages that are treated as "internals". Simple as that.
If the filename is provided, then the dumps from all the applicable optimizations are concatenated into the filename. Otherwise the dump goes straight onto stderr. And here's the catch, bestie — though multiple -fopt-info options are accepted, only one of them can include a filename. If other filenames are provided then all but the first such option are ignored. Tragic but true 💀.
Note that the output filename is overwritten in case of multiple translation units. If you want a combined output from multiple translation units, use stderr instead. No, you can't have both, that's the whole point.
In the following example, the optimization info is output to stderr:
gcc -O3 -fopt-infoThis example:
gcc -O3 -fopt-info-missed=missed.alloutputs missed optimization report from all the passes into missed.all, and this one:
gcc -O2 -ftree-vectorize -fopt-info-vec-missedprints information about missed optimization opportunities from vectorization passes on stderr. Note that -fopt-info-vec-missed is equivalent to -fopt-info-missed-vec. The order of the optimization group names and message types listed after -fopt-info does not matter ✨. Mix and match, it's giving flexible.
As another example,
gcc -O3 -fopt-info-inline-optimized-missed=inline.txtoutputs information about missed optimizations as well as optimized locations from all the inlining passes into inline.txt.
Finally, consider:
gcc -fopt-info-vec-missed=vec.miss -fopt-info-loop-optimized=loop.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 subsequent options are ignored. Thus only vec.miss is produced, which contains dumps from the vectorizer about missed opportunities. First come, first served, no cap 🫶.
-fsave-optimization-record 📝
Write a SRCFILE.opt-record.json.gz file detailing what optimizations were performed, for those optimizations that support -fopt-info.
This option is experimental and the format of the data within the compressed JSON file is subject to change. Don't build your whole personality around it.
It is roughly equivalent to a machine-readable version of -fopt-info-all, as a collection of messages with source file, line number and column number, with the following additional data for each message:
- the execution count of the code being optimized, along with metadata about whether this was from actual profile data, or just an estimate, allowing consumers to prioritize messages by code hotness 🔥,
- the function name of the code being optimized, where applicable,
- the "inlining chain" for the code being optimized, so that when a function is inlined into several different places (which might themselves be inlined), the reader can distinguish between the copies,
- objects identifying those parts of the message that refer to expressions, statements or symbol-table nodes, which of these categories they are, and, when available, their source code location,
- the GCC pass that emitted the message, and
- the location in GCC's own code from which the message was emitted.
Additionally, some messages are logically nested within other messages, reflecting implementation details of the optimization passes. Nested, like a lil matryoshka 🪆.
-fsched-verbose=n 🗓️
On targets that use instruction scheduling, this option controls the amount of debugging output the scheduler prints to the dump files.
For n greater than zero, -fsched-verbose outputs the same information as -fdump-rtl-sched1 and -fdump-rtl-sched2. For n greater than one, it also outputs basic block probabilities, detailed ready list information and unit/insn info. For n greater than two, it includes RTL at abort point, control-flow and regions info. And for n over four, -fsched-verbose also includes dependence info. Crank the dial, get more lore 📈.
-fenable-kind-pass / -fdisable-kind-pass=range-list 🎛️
This is a set of options used to explicitly disable/enable optimization passes. These options are intended for use for debugging GCC. Compiler users should use regular options for enabling/disabling passes instead — this is the backstage door, not the front entrance 🔐.
-fdisable-ipa-pass— disable IPA pass pass. pass is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from1.-fdisable-rtl-pass-fdisable-rtl-pass=range-list— disable RTL pass pass. pass is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from1. range-list is a comma-separated list of function ranges or assembler names. Each range is a number pair separated by a colon. The range is inclusive in both ends. If the range is trivial, the number pair can be simplified as a single number. If the function's call graph node's uid falls within one of the specified ranges, the pass is disabled for that function. The uid is shown in the function header of a dump file, and the pass names can be dumped by using option-fdump-passes.-fdisable-tree-pass-fdisable-tree-pass=range-list— disable tree pass pass. See-fdisable-rtlfor the description of option arguments.-fenable-ipa-pass— enable IPA pass pass. pass is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from1.-fenable-rtl-pass-fenable-rtl-pass=range-list— enable RTL pass pass. See-fdisable-rtlfor option argument description and examples.-fenable-tree-pass-fenable-tree-pass=range-list— enable tree pass pass. See-fdisable-rtlfor the description of option arguments.
Here are some examples showing uses of these options.
# disable ccp1 for all functions
-fdisable-tree-ccp1
# disable complete unroll for function whose cgraph node uid is 1
-fenable-tree-cunroll=1
# disable gcse2 for functions at the following ranges [1,1],
# [300,400], and [400,1000]
# disable gcse2 for functions foo and foo2
-fdisable-rtl-gcse2=foo,foo2
# disable early inlining
-fdisable-tree-einline
# disable ipa inlining
-fdisable-ipa-inline
# enable tree full unroll
-fenable-tree-unroll-fchecking / -fchecking=n ✅
Enable internal consistency checking. The default depends on the compiler configuration. -fchecking=2 enables further internal consistency checking that might affect code generation. Double-checking the double-check, respectfully 💅.
-frandom-seed=string 🎲
This option provides a seed that GCC uses in place of random numbers in generating certain symbol names that have to be different in every compiled file. It is also used to place unique stamps in coverage data files and the object files that produce them. You can use the -frandom-seed option to produce reproducibly identical object files. Determinism girlies, this one's for you ✨.
The string can either be a number (decimal, octal or hex) or an arbitrary string (in which case it's converted to a number by computing CRC32).
The string should be different for every file you compile. Every. Single. One.
-save-temps / -save-temps=cwd 🧊
Store the usual "temporary" intermediate files permanently; place them in the current directory and name them based on the source file. Thus, compiling foo.c with -c -save-temps produces files foo.i and foo.s, as well as foo.o. This creates a preprocessed foo.i output file even though the compiler now normally uses an integrated preprocessor. No secrets here, everything's out in the open 👀.
When used in combination with the -x command-line option, -save-temps is sensible enough to avoid over writing an input source file with the same extension as an intermediate file. The corresponding intermediate file may be obtained by renaming the source file before using -save-temps. Thoughtful of it, honestly 🎀.
okay bestie so real talk — if you run GCC in parallel (like, compiling a bunch of different source files that share a common base name in different subdirectories, or the same source file built for multiple output destinations), it's lowkey likely that the different parallel compilers will interfere with each other and overwrite the temporary files. not the vibe. for instance:
gcc -save-temps -o outdir1/foo.o indir1/foo.c&
gcc -save-temps -o outdir2/foo.o indir2/foo.c&may result in foo.i and foo.o being written to simultaneously by both compilers. 💀 two compilers, one file name, chaos.
💾 -save-temps=obj
Stores the usual "temporary" intermediate files permanently ✨. If the -o option is used, the temporary files are based on the object file. If -o is not used, the -save-temps=obj switch behaves like -save-temps.
For example:
gcc -save-temps=obj -c foo.c
gcc -save-temps=obj -c bar.c -o dir/xbar.o
gcc -save-temps=obj foobar.c -o dir2/yfoobarcreates foo.i, foo.s, dir/xbar.i, dir/xbar.s, dir2/yfoobar.i, dir2/yfoobar.s, and dir2/yfoobar.o. look at all those little files bestie, they're thriving 🎀
⏱️ -time[=file]
Reports the CPU time taken by each subprocess in the compilation sequence. For C source files, this is the compiler proper and assembler (plus the linker if linking is done).
Without the specification of an output file, the output looks like this:
# cc1 0.12 0.01
# as 0.00 0.01The first number on each line is the "user time", that is time spent executing the program itself. The second number is "system time", time spent executing operating system routines on behalf of the program. Both numbers are in seconds.
With the specification of an output file, the output is appended to the named file, and it looks like this:
0.12 0.01 cc1 <options>
0.00 0.01 as <options>The "user time" and the "system time" are moved before the program name, and the options passed to the program are displayed, so that one can later tell what file was being compiled, and with which options. receipts, basically 🧾
🗂️ -fdump-final-insns[=file]
Dumps the final internal representation (RTL) to file. If the optional argument is omitted (or if file is .), the name of the dump file is determined by appending .gkd to the compilation output file name.
🐛 -fcompare-debug[=opts]
If no error occurs during compilation, run the compiler a second time, adding opts and -fcompare-debug-second to the arguments passed to the second compilation. Dump the final internal representation in both compilations, and print an error if they differ.
If the equal sign is omitted, the default -gtoggle is used.
The environment variable GCC_COMPARE_DEBUG, if defined, non-empty and nonzero, implicitly enables -fcompare-debug. If GCC_COMPARE_DEBUG is defined to a string starting with a dash, then it is used for opts, otherwise the default -gtoggle is used.
-fcompare-debug=, with the equal sign but without opts, is equivalent to -fno-compare-debug, which disables the dumping of the final representation and the second compilation, preventing even GCC_COMPARE_DEBUG from taking effect.
To verify full coverage during -fcompare-debug testing, set GCC_COMPARE_DEBUG to say -fcompare-debug-not-overridden, which GCC rejects as an invalid option in any actual compilation (rather than preprocessing, assembly or linking). To get just a warning, setting GCC_COMPARE_DEBUG to -w%n-fcompare-debug not overridden will do. 👑
🥈 -fcompare-debug-second
This option is implicitly passed to the compiler for the second compilation requested by -fcompare-debug, along with options to silence warnings, and omitting other options that would cause the compiler to produce output to files or to standard output as a side effect. Dump files and preserved temporary files are renamed so as to contain the .gk additional extension during the second compilation, to avoid overwriting those generated by the first.
When this option is passed to the compiler driver, it causes the first compilation to be skipped, which makes it useful for little other than debugging the compiler proper. ngl that's a very niche little job description 💅
🔁 -gtoggle
Turn off generation of debug info, if leaving out this option generates it, or turn it on at level 2 otherwise. The position of this argument in the command line does not matter; it takes effect after all other options are processed, and it does so only once, no matter how many times it is given. This is mainly intended to be used with -fcompare-debug.
🔀 -fvar-tracking-assignments-toggle
Toggles -fvar-tracking-assignments, in the same way that -gtoggle toggles -g. mirror behaviour, iconic ✨
📢 -Q
Makes the compiler print out each function name as it is compiled, and print some statistics about each pass when it finishes.
⏳ -ftime-report
Makes the compiler print some statistics about the time consumed by each pass when it finishes.
🔬 -ftime-report-details
Record the time consumed by infrastructure parts separately for each pass.
🎛️ -fira-verbose=n
Control the verbosity of the dump file for the integrated register allocator. The default value is 5. If the value n is greater or equal to 10, the dump output is sent to stderr using the same format as n minus 10.
📊 -flto-report
Prints a report with internal details on the workings of the link-time optimizer. The contents of this report vary from version to version. It is meant to be useful to GCC developers when processing object files in LTO mode (via -flto).
Disabled by default.
📈 -flto-report-wpa
Like -flto-report, but only print for the WPA phase of Link Time Optimization.
🧠 -fmem-report
Makes the compiler print some statistics about permanent memory allocation when it finishes.
🧩 -fmem-report-wpa
Makes the compiler print some statistics about permanent memory allocation for the WPA phase only.
📉 -fpre-ipa-mem-report, -fpost-ipa-mem-report
Makes the compiler print some statistics about permanent memory allocation before or after interprocedural optimization.
🧾 -fprofile-report
Makes the compiler print some statistics about consistency of the (estimated) profile and effect of individual passes.
📚 -fstack-usage
Makes the compiler output stack usage information for the program, on a per-function basis. The filename for the dump is made by appending .su to the auxname. auxname is generated from the name of the output file, if explicitly specified and it is not an executable, otherwise it is the basename of the source file. An entry is made up of three fields:
- The name of the function.
- A number of bytes.
- One or more qualifiers:
static,dynamic,bounded.
The qualifier static means that the function manipulates the stack statically: a fixed number of bytes are allocated for the frame on function entry and released on function exit; no stack adjustments are otherwise made in the function. The second field is this fixed number of bytes.
The qualifier dynamic means that the function manipulates the stack dynamically: in addition to the static allocation described above, stack adjustments are made in the body of the function, for example to push/pop arguments around function calls. If the qualifier bounded is also present, the amount of these adjustments is bounded at compile time and the second field is an upper bound of the total amount of stack used by the function. If it is not present, the amount of these adjustments is not bounded at compile time and the second field only represents the bounded part.
📐 -fstats
Emit statistics about front-end processing at the end of the compilation. This option is supported only by the C++ front end, and the information is generally only useful to the G++ development team. iykyk 🫶
🔢 -fdbg-cnt-list
Print the name and the counter upper bound for all debug counters.
🧮 -fdbg-cnt=counter-value-list
Set the internal debug counter lower and upper bound. counter-value-list is a comma-separated list of name:lower_bound:upper_bound tuples which sets the lower and the upper bound of each debug counter name. The lower_bound is optional and is zero initialized if not set. All debug counters have the initial upper bound of UINT_MAX; thus dbg_cnt returns true always unless the upper bound is set by this option. For example, with -fdbg-cnt=dce:2:4,tail_call:10, dbg_cnt(dce) returns true only for third and fourth invocation. For dbg_cnt(tail_call) true is returned for first 10 invocations.
🏷️ -print-file-name=library
Print the full absolute name of the library file library that would be used when linking---and don't do anything else. With this option, GCC does not compile or link anything; it just prints the file name.
📁 -print-multi-directory
Print the directory name corresponding to the multilib selected by any other switches present in the command line. This directory is supposed to exist in GCC_EXEC_PREFIX.
🗺️ -print-multi-lib
Print the mapping from multilib directory names to compiler switches that enable them. The directory name is separated from the switches by ;, and each switch starts with an @ instead of the -, without spaces between multiple switches. This is supposed to ease shell processing. cute, tidy, slay ✨
🐧 -print-multi-os-directory
Print the path to OS libraries for the selected multilib, relative to some lib subdirectory. If OS libraries are present in the lib subdirectory and no multilibs are used, this is usually just ., if OS libraries are present in libsuffix sibling directories this prints e.g. ../lib64, ../lib or ../lib32, or if OS libraries are present in lib/subdir subdirectories it prints e.g. amd64, sparcv9 or ev6.
🏗️ -print-multiarch
Print the path to OS libraries for the selected multiarch, relative to some lib subdirectory.
🔍 -print-prog-name=program
Like -print-file-name, but searches for a program such as cpp.
🧱 -print-libgcc-file-name
Same as -print-file-name=libgcc.a.
This is useful when you use -nostdlib or -nodefaultlibs but you do want to link with libgcc.a. You can do:
gcc -nostdlib <files>... `gcc -print-libgcc-file-name`bestie the backticks inside the backticks are giving inception but they work, trust 💜
-print-search-dirs— spits out the name of the configured installation directory plus the whole list of program and library directoriesgccgoes looking in, and then it does absolutely nothing else, bestie. it's a one-job icon. 💅
ok so this one is your little detective when gcc starts screaming the error message installation problem, cannot exec cpp0: No such file or directory. to fix that vibe you either gotta put cpp0 and all the other 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 /. fr fr the slash matters. 😭
-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 riding along depending on your compilation options. if no target sysroot was specified at all? it prints nothing. silence. the void. 🫶-print-sysroot-headers-suffix— prints the suffix that gets added to the target sysroot when it's searching for headers, or it throws an error if the compiler wasn't configured with such a suffix — and again, it does not do anything else. one task, no notes. ✨-dumpmachine— prints the compiler's target machine (like, for example,i686-pc-linux-gnu) — and don't do anything else. that's it. that's the whole personality. 🎀-dumpversion— prints the compiler version (for example3.0,6.3.0or7) — and don't do anything else. this is the compiler version that gets used in filesystem paths and specs. depending on how the compiler was 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). she contains multitudes, it depends. 🌸-dumpfullversion— prints the full compiler version — and don't do anything else. this one's output is always three numbers separated by dots: major, minor and patchlevel version. no ambiguity, no games. ⭐-dumpspecs— prints the compiler's built-in specs — and don't do anything else. (this one gets used when GCC itself is being built, iykyk.) 🔥
🦋 Machine-Dependent Options 🦋
okay so every target machine GCC supports can have its own lil set of 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, machine-specific option names all start with -m. that's the dress code. 💅
some configurations of the compiler also support extra target-specific options, usually for compatibility with other compilers living on the same platform. sharing is caring. 🫶
👑 AArch64 Options 👑
these options are defined for AArch64 implementations:
-mabi=name— generate code for the specified data model. permissible values areilp32for the SysV-like data model whereint,long intand pointers are 32 bits, andlp64for the SysV-like data model whereintis 32 bits butlong intand pointers are 64 bits.
the default depends on the specific target configuration. and heads up: the LP64 and ILP32 ABIs are not link-compatible. you must compile your entire program with the same ABI, and link with a compatible set of libraries. mixing them is giving chaos, not giving slay. 😭
-mbig-endian— generate big-endian code. this is the default when GCC is configured for anaarch64_be-*-*target.-mgeneral-regs-only— generate code that uses only the general-purpose registers. this stops the compiler from using floating-point and Advanced SIMD registers, but it puts no restrictions at all on the assembler. the assembler is free, we don't control her. 🦋-mlittle-endian— generate little-endian code. this is the default when GCC is configured for anaarch64-*-*but not anaarch64_be-*-*target.-mcmodel=tiny— generate code for the tiny code model. the program and its statically defined symbols must be within 1MB of each other. programs can be statically or dynamically linked. tiny but mighty. 🎀-mcmodel=small— generate code for the small code model. the program and its statically defined symbols must be within 4GB of each other. programs can be statically or dynamically linked. this is the default code model. the main character. 👑-mcmodel=large— generate code for the large code model. this makes no assumptions about addresses and sizes of sections. programs can be statically linked only. no dynamic linking for you, bestie.-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. pick your fighter. 💅-momit-leaf-frame-pointer,-mno-omit-leaf-frame-pointer— omit or keep the frame pointer in leaf functions. the former behavior (omitting) is the default.-mstack-protector-guard=guard,-mstack-protector-guard-reg=reg,-mstack-protector-guard-offset=offset— generate stack protection code using canary at guard. supported locations areglobalfor a global canary, orsysregfor a canary in an appropriate system register.
with the latter choice, the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset furthermore specify which system register to use as base register for reading the canary, and from what offset from that base register. there is no default register or offset, because this is entirely for use within the Linux kernel. yep, kernel girlies only. 🧚
-mstack-protector-guard=guard,-mstack-protector-guard-reg=reg,-mstack-protector-guard-offset=offset— generate stack protection code using canary at guard. supported locations areglobalfor a global canary, orsysregfor a canary in an appropriate system register.
with the latter choice, the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset furthermore specify which system register to use as base register for reading the canary, and from what offset from that base register. there is no default register or offset, as this is entirely for use within the Linux kernel. (yes bestie, she shows up twice in the source. we don't question the manpage, we just vibe. 🙈)
-mtls-dialect=desc— use TLS descriptors as the thread-local storage mechanism for dynamic accesses of TLS variables. this is the default. ✨-mtls-dialect=traditional— use traditional TLS as the thread-local storage mechanism for dynamic accesses of TLS variables. vintage aesthetic. 🎀-mtls-size=size— specify bit size of immediate TLS offsets. valid values are12,24,32,48. this option requires binutils 2.26 or newer, so check your binutils before you commit. 💅-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. she's patching the bug bestie, show some respect. 🫶-mfix-cortex-a53-843419,-mno-fix-cortex-a53-843419— enable or disable the workaround for the ARM Cortex-A53 erratum number 843419. this erratum workaround is made at link time, and this will only pass the corresponding flag to the linker. so it's not doing the work, it's delegating. queen behavior honestly. 👑-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 it reduces precision of reciprocal square root results to about 16 bits for single precision and to 32 bits for double precision. fast and loose, on purpose. 🔥-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 it reduces precision of square root results to about 16 bits for single precision and to 32 bits for double precision. and if it's enabled, it implies-mlow-precision-recip-sqrt. package deal. 💖-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 it reduces precision of division results to about 16 bits for single precision and to 32 bits for double precision. same energy, different math. ⭐-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_copyto permit a more efficient code sequence to be generated. fancy. 🌈-moutline-atomics,-mno-outline-atomics— enable or disable calls to out-of-line helpers to implement atomic operations. these helpers will, at runtime, determine if the LSE instructions from ARMv8.1-A can be used; if not, they'll use the load/store-exclusive instructions that are present in the base ARMv8.0 ISA. she checks at runtime, she's adaptable. 💅
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 will be used directly. the same applies when using -mcpu= when the selected cpu supports the lse feature.
-march=name— specify the name of the target architecture and, optionally, one or more feature modifiers. this option has the form-march=arch{+[no]feature}*.
the permissible values for arch are armv8-a, armv8.1-a, armv8.2-a, armv8.3-a, armv8.4-a, armv8.5-a or native.
the value armv8.5-a implies armv8.4-a and enables compiler support for the ARMv8.5-A architecture extensions.
the value armv8.4-a implies armv8.3-a and enables compiler support for the ARMv8.4-A architecture extensions.
the value armv8.3-a implies armv8.2-a and enables compiler support for the ARMv8.3-A architecture extensions.
the value armv8.2-a implies armv8.1-a and enables compiler support for the ARMv8.2-A architecture extensions.
the value armv8.1-a implies armv8-a and enables compiler support for the ARMv8.1-A architecture extension. in particular, it enables the +crc, +lse, and +rdma features. ok chain reaction, we love a lineage. 👑
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. last one wins, no cap.
GCC uses name to determine what kind of instructions it can emit when generating assembly code. if -march is specified without either of -mtune or -mcpu also being specified, the code is tuned to perform well across a range of target processors implementing the target architecture. versatile queen. ✨
-mtune=name— okay so this one tells GCC the name of the target processor it should tune the performance of the code for. The permissible values are:generic,cortex-a35,cortex-a53,cortex-a55,cortex-a57,cortex-a72,cortex-a73,cortex-a75,cortex-a76,ares,exynos-m1,emag,falkor,neoverse-e1,neoverse-n1,neoverse-n2,neoverse-v1,neoverse-512tvb,qdf24xx,saphira,phecda,xgene1,vulcan,octeontx,octeontx81,octeontx83,a64fx,thunderx,thunderxt88,thunderxt88p1,thunderxt81,tsv110,thunderxt83,thunderx2t99,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,native. ✨
The values cortex-a57.cortex-a53, cortex-a72.cortex-a53, cortex-a73.cortex-a35, cortex-a73.cortex-a53, cortex-a75.cortex-a55, cortex-a76.cortex-a55 specify that GCC should tune for a big.LITTLE system. big.LITTLE energy, periodt.
The value neoverse-512tvb specifies that GCC should tune for Neoverse cores that (a) implement SVE and (b) have a total vector bandwidth of 512 bits per cycle. In other words, the option tells GCC to tune for Neoverse cores that can execute 4 128-bit Advanced SIMD arithmetic instructions a cycle and that can execute an equivalent number of SVE arithmetic instructions per cycle (2 for 256-bit SVE, 4 for 128-bit SVE). This is more general than tuning for a specific core like Neoverse V1 but is more specific than the default tuning described below. so like, not just one core, but a whole squad.
Additionally on native AArch64 GNU/Linux systems the value native tunes performance to the host system. This option has no effect if the compiler is unable to recognize the processor of the host system.
Where none of -mtune=, -mcpu= or -march= are specified, the code is tuned to perform well across a range of target processors.
Heads up: this option cannot be suffixed by feature modifiers. No add-ons, bestie.
-mcpu=name— Specify the name of the target processor, optionally suffixed by one or more feature modifiers. This option has the form-mcpu=cpu{+[no]feature}*, where the permissible values forcpuare the same as those available for-mtune. The permissible values forfeatureare documented in the sub-section onaarch64-feature-modifiers,,-march and -mcpu Feature Modifiers. Where conflicting feature modifiers are specified, the right-most feature is used. right-most wins, no cap.
GCC uses name to determine what kind of instructions it can emit when generating assembly code (as if by -march) and to determine the target processor for which to tune for performance (as if by -mtune). Where this option is used in conjunction with -march or -mtune, those options take precedence over the appropriate part of this option. so -march and -mtune are the main characters here.
-mcpu=neoverse-512tvb is special in that it does not refer to a specific core, but instead refers to all Neoverse cores that (a) implement SVE and (b) have a total vector bandwidth of 512 bits a cycle. Unless overridden by -march, -mcpu=neoverse-512tvb generates code that can run on a Neoverse V1 core, since Neoverse V1 is the first Neoverse core with these properties. Unless overridden by -mtune, -mcpu=neoverse-512tvb tunes code in the same way as for -mtune=neoverse-512tvb. it’s giving “we’re all Neoverse V1 if you squint” but make it configurable.
-moverride=string— Override tuning decisions made by the back-end in response to a-mtune=switch. The syntax, semantics, and accepted values forstringin this option are not guaranteed to be consistent across releases. so like, don’t expect stability, bestie.
This option is only intended to be useful when developing GCC. compiler devs only, iykyk.
-mverbose-cost-dump— Enable verbose cost model dumping in the debug dump files. This option is provided for use in debugging the compiler. debugging vibes only.-mpc-relative-literal-loads,-mno-pc-relative-literal-loads— Enable or disable PC-relative literal loads. With this option literal pools are accessed using a single instruction and emitted after each function. This limits the maximum size of functions to 1MB. This is enabled by default for-mcmodel=tiny. 1MB max, so keep your functions cute and compact.-msign-return-address=scope— Select the function scope on which return address signing will be applied. Permissible 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. The default value isnone. This option has been deprecated by-mbranch-protection. deprecated, so like, it’s giving old news.-mbranch-protection=none|standard|pac-ret[+leaf]|bti— Select the branch protection features to use.noneis the default and turns off all types of branch protection.standardturns on all types of branch protection features. 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 argumentleafcan be used to extend the signing to include leaf functions.btiturns on branch target identification mechanism. protection, but make it fashion.-mharden-sls=opts— Enable compiler hardening against straight line speculation (SLS).optsis a comma-separated list of the following options:retbrblr
In addition, -mharden-sls=all enables all SLS hardening while -mharden-sls=none disables all SLS hardening. all or nothing, bestie.
-msve-vector-bits=bits— Specify the number of bits in an SVE vector register. This option only has an effect when SVE is enabled.
GCC supports two forms of SVE code generation: "vector-length agnostic" output that works with any size of vector register and "vector-length specific" output that allows GCC to make assumptions about the vector length when it is useful for optimization reasons. The possible values of bits are: scalable, 128, 256, 512, 1024 and 2048. Specifying scalable selects vector-length agnostic output. At present -msve-vector-bits=128 also generates vector-length agnostic output. All other values generate vector-length specific code. The behavior of these values may change in future releases and no value except scalable should be relied on for producing code that is portable across different hardware SVE vector lengths. so like, only scalable is the safe bet for portability.
The default is -msve-vector-bits=scalable, which produces vector-length agnostic code. default slays.
✨ -march and -mcpu Feature Modifiers ✨
Feature modifiers used with -march and -mcpu can be any of the following and their inverses nofeature:
crc— Enable CRC extension. This is on by default for-march=armv8.1-a.crypto— Enable Crypto extension. This also enables Advanced SIMD and floating-point instructions.fp— Enable floating-point instructions. This is on by default for all possible values for options-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.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. This option is only to enable the extension at the assembler level and does not affect code generation.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.
Feature crypto implies aes, sha2, and simd, which implies fp. Conversely, nofp implies nosimd, which implies nocrypto, noaes and nosha2. the implication chain is giving family tree.
🌸 Adapteva Epiphany Options 🌸
These -m options are defined for Adapteva Epiphany:
-mhalf-reg-file— Don't allocate any register in the range"r32"..."r63". That allows code to run on hardware variants that lack these registers. save those registers for later.-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. could go either way, bestie.-mbranch-cost=num— tells the compiler to treat branches as costing roughly num "simple" instructions. It's just a heuristic though, so ngl, it's not guaranteed to give you the same results across releases 💅.-mcmove— turn on generation of conditional moves. slay.-mnops=num— emit num NOPs before every other generated instruction. that's a lot of vibes.-mno-soft-cmpsf— for single-precision floating-point comparisons, emit anfsubinstruction and test the flags. faster than a software comparison, but 😭 it can give you incorrect results when NaNs are around, or when two different small numbers get compared and their difference comes out as zero. the default is-msoft-cmpsf, which is slower but IEEE-compliant (software comparisons).-mstack-offset=num— sets the offset between the top of the stack and the stack pointer. like, a value of 8 means 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. 🚨 also heads up: this option changes the ABI — compiling a program with a different stack offset than the libraries were compiled with generally does not work. it can be useful if you want to evaluate whether a different stack offset gives you better code, but to actually use a different stack offset to build working programs, you're recommended to configure the toolchain with the appropriate--with-stack-offset=numoption instead.-mno-round-nearest— makes the scheduler assume the rounding mode is set to truncating. the default is-mround-nearest.-mlong-calls— unless an attribute says otherwise, assume all calls might be beyond the offset range of theb/blinstructions, so load the function address into a register before performing an (otherwise direct) call. this is the default ✨.-mshort-calls— unless an attribute says otherwise, assume all direct calls are in range of theb/blinstructions, so just use those 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 where-mlong-callssemantics are in effect.-mfp-mode=mode— sets the prevailing mode of the floating-point unit. this decides the floating-point mode that's provided and expected at function call and return time. making this mode match the one you predominantly need at function start can make your programs smaller and faster by avoiding unnecessary mode switches.modecan be one of these:caller— any mode at function entry is valid, and it's 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 drop 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 a more specific choice of prevailing FPU mode would need.truncate— 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— the mode used for floating-point calculations with round-to-nearest-or-even rounding mode.int— 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 aremsplit-lohi,-mpost-inc, and-mpost-modify.-mnovect-double— changes the preferred SIMD mode toSImode. the default is-mvect-double, which usesDImodeas the preferred SIMD mode.-max-vect-align=num— the maximum alignment for SIMD vector mode types. num may be4or8. the default is8. 💀 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 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).
-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 is32KBwhen using OpenACC or OpenMP, and1MBotherwise.
🌈 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 calling 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 are:arc600— compile for ARC600. aliases:-mA6,-mARC600.arc601— compile for ARC601. alias:-mARC601.arc700— compile for ARC700. aliases:-mA7,-mARC700. this is the default when configured with--with-cpu=arc700.arcem— compile for ARC EM.archs— compile for ARC HS.em— compile for ARC EM CPU with no hardware extensions.em4— compile for ARC EM4 CPU.em4_dmips— compile for ARC EM4 DMIPS CPU.em4_fpus— compile for ARC EM4 DMIPS CPU with the single-precision floating-point extension.em4_fpuda— compile for ARC EM4 DMIPS CPU with single-precision floating-point and double assist instructions.hs— compile for ARC HS CPU with no hardware extensions except the atomic instructions.hs34— compile for ARC HS34 CPU.hs38— compile for ARC HS38 CPU.hs38_linux— compile for ARC HS38 CPU with all hardware extensions on.arc600_norm— compile for ARC 600 CPU withnorminstructions enabled.arc600_mul32x16— compile for ARC 600 CPU withnormand 32x16-bit multiply instructions enabled.arc600_mul64— compile for ARC 600 CPU withnormandmul64-family instructions enabled.arc601_norm— compile for ARC 601 CPU withnorminstructions enabled.arc601_mul32x16— compile for ARC 601 CPU withnormand 32x16-bit multiply instructions enabled.arc601_mul64— compile for ARC 601 CPU withnormandmul64-family instructions enabled.nps400— compile for ARC 700 on NPS400 chip.em_mini— compile for ARC EM minimalist configuration featuring reduced register set.
-mdpfp-mdpfp-compact— generate double-precision FPX instructions, tuned for the compact implementation.-mdpfp-fast— generate double-precision FPX instructions, tuned for the fast implementation.-mno-dpfp-lrsr— disablelrandsrinstructions from using FPX extension aux registers.-mea— generate extended arithmetic instructions. currently onlydivaw,adds,subs, andsat16are supported. this is always enabled for-mcpu=ARC700.-mno-mpy— do not generatempy-family instructions for ARC700. this option is deprecated 😬.-mmul32x16— generate 32x16-bit multiply and multiply-accumulate instructions.-mmul64— generatemul64andmulu64instructions. only valid for-mcpu=ARC600.-mnorm— generatenorminstructions. 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— this option is ignored; it's provided for compatibility purposes only 🙃. software floating-point code is emitted by default, and this 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— generateswapinstructions.-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— enabledivandreminstructions for ARCv2 cores.-mcode-density— enable code density instructions for ARC EM. this option is on by default for ARC HS.-mll64— enable double load/store operations for ARC HS cores.-mtp-regno=regno— specify thread pointer register number.-mmpy-option=multo— okay so this one is for compiling ARCv2 code with a specific multiplier design option 💅 and you can hand it either a string or a numeric value for multo. The default iswlh1, and here's the whole lineup of recognized values, bestie:0,none— literally no multiplier available. tragic but valid 💀1,w— 16x16 multiplier, fully pipelined. This turns on these instructions:mpywandmpyuw.2,wlh1— 32x32 multiplier, fully pipelined (1 stage). Additionally enables:mpy,mpyu,mpym,mpymu, andmpy_s.3,wlh2— 32x32 multiplier, fully pipelined (2 stages). Additionally enables:mpy,mpyu,mpym,mpymu, andmpy_s.4,wlh3— two 16x16 multipliers, blocking, sequential. Additionally enables:mpy,mpyu,mpym,mpymu, andmpy_s.5,wlh4— one 16x16 multiplier, blocking, sequential. Additionally enables:mpy,mpyu,mpym,mpymu, andmpy_s.6,wlh5— one 32x4 multiplier, blocking, sequential. Additionally enables:mpy,mpyu,mpym,mpymu, andmpy_s.7,plus_dmpy— ARC HS SIMD support.8,plus_macd— ARC HS SIMD support.9,plus_qmacw— ARC HS SIMD support.
Heads up bestie: this option is only available for ARCv2 cores. No ARCv2, no multiplier glow-up ✨
-mfpu=fpu— this one enables support for specific floating-point hardware extensions for ARCv2 cores 🧚 and 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— this specifies general-purposes registers that the processor automatically saves/restores on interrupt entry and exit 😌 register-range is written as two registers separated by a dash. The register range always starts with"r0", and the upper limit is the"fp"register.blinkandlp_countare optional. This option is only valid for ARC EM and ARC HS cores.-mrgf-banked-regs=number— specifies the number of registers replicated in second register bank on entry to fast interrupt. Fast interrupts are interrupts with the highest priority level P0 🔥 These interrupts save only PC and STATUS32 registers to avoid memory transactions during interrupt entry and exit sequences. Use this option when you'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 are8,16,20,24,28and32bits. The default width is fixed to32bits. 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 specified, the compiler and run-time library might continue to use the loop mechanism for various needs. This option defines macro"__ARC_LPC_WIDTH__"with the value of width.-mrf16— this option instructs the compiler to generate code for a 16-entry register file. This option defines the"__ARC_RF16__"preprocessor macro 🌸-mbranch-index— enable use of"bi"or"bih"instructions to implement jump tables. slay ✨
The following options are passed through to the assembler, and also define preprocessor macro symbols. 👑
-mdsp-packa— passed down to the assembler to enable the DSP Pack A extensions. Also sets the preprocessor symbol"__Xdsp_packa". This option is deprecated.-mdvbf— passed down to the assembler to enable the dual Viterbi butterfly extension. Also sets the preprocessor symbol"__Xdvbf". This option is deprecated.-mlock— passed down to the assembler to enable the locked load/store conditional extension. Also sets the preprocessor symbol"__Xlock".-mmac-d16— passed down to the assembler. Also sets the preprocessor symbol"__Xxmac_d16". This option is deprecated.-mmac-24— passed down to the assembler. Also sets the preprocessor symbol"__Xxmac_24". This option is deprecated.-mrtsc— passed down to the assembler to enable the 64-bit time-stamp counter extension instruction. Also sets the preprocessor symbol"__Xrtsc". This option is deprecated.-mswape— passed down to the assembler to enable the swap byte ordering extension instruction. Also sets the preprocessor symbol"__Xswape".-mtelephony— passed down to the assembler to enable dual- and single-operand instructions for telephony. Also sets the preprocessor symbol"__Xtelephony". This option is deprecated.-mxy— passed down to the assembler to enable the XY memory extension. Also sets the preprocessor symbol"__Xxy".
The following options control how the assembly code is annotated: 💖
-misize— annotate assembler instructions with estimated addresses.-mannotate-align— explain what alignment considerations lead to the decision to make an instruction short or long. (she's nosy but she's helpful 💅)
The following options are passed through to the linker: 🔗
-marclinux— passed through to the linker, to specify use of the"arclinux"emulation. This option is enabled by default in tool chains built for"arc-linux-uclibc"and"arceb-linux-uclibc"targets when profiling is not requested.-marclinux_prof— passed through to the linker, to specify use of the"arclinux_prof"emulation. This option is enabled by default in tool chains built for"arc-linux-uclibc"and"arceb-linux-uclibc"targets when profiling is requested.
The following options control the semantics of generated code: 🦋
-mlong-calls— generate calls as register indirect calls, thus providing access to the full 32-bit address range. big reach, iconic 🌈-mmedium-calls— don't use less than 25-bit addressing range for calls, which is the offset available for an unconditional branch-and-link instruction. Conditional execution of function calls is suppressed, to allow use of the 25-bit range, rather than the 21-bit range with conditional branch-and-link. This is the default for tool chains built for"arc-linux-uclibc"and"arceb-linux-uclibc"targets.-G num— put definitions of externally-visible data in a small data section if that data is no bigger than num bytes. The default value of num 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 for"arc-linux-uclibc"and"arceb-linux-uclibc"targets.-mvolatile-cache— use ordinarily cached memory accesses for volatile references. This is the default.-mno-volatile-cache— enable cache bypass for volatile references.
The following options fine tune code generation: 🎀
-malign-call— do alignment optimizations for call instructions.-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 in arc_reorg to generate compare-and-branch ("brcc") instructions. It has no effect on generation of these instructions driven by the combiner pass. (not me disabling a whole pass 😭)-mcase-vector-pcrel— use PC-relative switch case tables to enable case table shortening. This is the default for-Os.-mcompact-casesi— enable compact"casesi"pattern. This is the default for-Os, and only available for ARCv1 cores. This option is deprecated.-mno-cond-exec— disable the ARCompact-specific pass to generate conditional execution instructions.
okay so because of delay slot scheduling and the way operand numbers, literal sizes, instruction lengths, and conditional-execution support all vibe together, the target-independent pass that generates conditional execution is often kinda lacking. That's why the ARC port keeps a special pass around that hunts for more conditional-execution generation opportunities after register allocation, branch shortening, and delay slot scheduling have already happened. This pass generally—but not always—improves performance and code size, at the cost of extra compilation time, which is why there's an option to switch it off. If you have a problem with call instructions exceeding their allowable offset range because they are conditionalized, you should consider using -mmedium-calls instead. 🫶
-mearly-cbranchsi— Enable pre-reload use of the"cbranchsi"pattern. ✨-mexpand-adddi— Expand"adddi3"and"subdi3"at RTL generation time into"add.f","adc"etc. This option is deprecated. 💀-mindexed-loads— Enable the use of indexed loads. This can be problematic because some optimizers then assume that indexed stores exist, which is not the case. 😭-mlra— 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 enable the compiler to emit"enter"and"leave"instructions. These instructions are only valid for CPUs with code-density feature. 🎀-mmixed-code— Tweak register allocation to help 16-bit instruction generation. This generally has the effect of decreasing the average instruction size while increasing the instruction count. 🦋-mq-class— Enable q instruction alternatives. This is the default for-Os. 👑-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— Set probability threshold for unaligning branches. When tuning for ARC700 and optimizing for speed, branches without filled delay slot are preferably emitted unaligned and long, unless profiling indicates that the probability for the branch to be taken is below probability. The default is(REG_BR_PROB_BASE/2), i.e.5000. 🌸
Heads up, bestie: the following options are maintained for backward compatibility, but are now deprecated and will be removed in a future release: 🫣
-margonaut— Obsolete FPX.-mbig-endian,-EB— Compile code for big-endian targets. Use of these options is now deprecated. Big-endian code is supported by configuring GCC to build"arceb-elf32"and"arceb-linux-uclibc"targets, for which big endian is the default.-mlittle-endian,-EL— Compile code for little-endian targets. Use of these options is now deprecated. Little-endian code is supported by configuring GCC to build"arc-elf32"and"arc-linux-uclibc"targets, for which little endian is the default.-mbarrel_shifter— Replaced by-mbarrel-shifter.-mdpfp_compact— Replaced by-mdpfp-compact.-mdpfp_fast— Replaced by-mdpfp-fast.-mdsp_packa— Replaced by-mdsp-packa.-mEA— Replaced by-mea.-mmac_24— Replaced by-mmac-24.-mmac_d16— Replaced by-mmac-d16.-mspfp_compact— Replaced by-mspfp-compact.-mspfp_fast— Replaced by-mspfp-fast.-mtune=cpu— 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. 🫠-mapcs— This is a synonym for-mapcs-frameand is deprecated. 💀-mthumb-interwork— Generate code that supports calling between the ARM and Thumb instruction sets. Without this option, on pre-v5 architectures, the two instruction sets cannot be reliably used inside one program. The default is-mno-thumb-interwork, since slightly larger code is generated when-mthumb-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— Generate code for a processor running in big-endian mode; the default is to compile code for a little-endian processor. 🐘-mbe8,-mbe32— When linking a big-endian image select between BE8 and BE32 formats. The option has no effect for little-endian images and is ignored. The default is dependent on the selected target architecture. For ARMv6 and later architectures the default is BE8, for older architectures the default is BE32. BE32 format has been deprecated by ARM. ⚠️-march=name[+extension...]— This specifies the name of the target ARM architecture. GCC uses this name to determine what kind of instructions it can emit when generating assembly code. This option can be used in conjunction with or instead of the-mcpu=option.
Permissible names are: armv4t, armv5t, armv5te, armv6, armv6j, armv6k, armv6kz, armv6t2, armv6z, armv6zk, armv7, armv7-a, armv7ve, armv8-a, armv8.1-a, armv8.2-a, armv8.3-a, armv8.4-a, armv8.5-a, armv7-r, armv8-r, armv6-m, armv6s-m, armv7-m, armv7e-m, armv8-m.base, armv8-m.main, iwmmxt and iwmmxt2.
Additionally, the following architectures, which lack support for the Thumb execution state, are recognized but support is deprecated: armv4.
Many of the architectures support extensions. These can be added by appending +extension to the architecture name. Extension options are processed in order and capabilities accumulate. An extension will also enable any necessary base extensions upon which it depends. For example, the +crypto extension will always enable the +simd extension. The exception to the additive construction is for extensions that are prefixed with +no...: these extensions disable the specified option and any other extensions that may depend on the presence of that extension.
For example, -march=armv7-a+simd+nofp+vfpv4 is equivalent to writing -march=armv7-a+vfpv4 since the +simd option is entirely disabled by the +nofp option that follows it. 🌟
okay so most of these extension names are like, generically named?? but they hit completely differently depending on which architecture you apply them to. ✨ 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. same name, different era, we love a chameleon 💅
The table below lists the supported extensions for each architecture. Architectures not mentioned do not support any extensions. fr fr, if it's not on this list it's a no.
🌸 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. (taking it away bestie 😭)
🌸 armv7
The common subset of the ARMv7-A, ARMv7-R and ARMv7-M architectures. like the friend group they all share 👑
+fp— the VFPv3 floating-point instructions, with 16 double-precision registers. the extension+vfpv3-d16can be used as an alias for this extension. ngl, 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). 🙈 note: it does NOT take the float away, just the SIMD.+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.
🌸 armv8.4-a
+fp16— the half-precision floating-point data processing instructions. this also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension and the half-precision floating-point fmla extension.+simd— the ARMv8.3-A Advanced SIMD and floating-point instructions as well as the Dot Product extension.+crypto— the cryptographic instructions. this also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension.+nocrypto— disable the cryptographic extension.+nofp— disable the floating-point, Advanced SIMD and cryptographic instructions.+sb— Speculation Barrier Instruction.+predres— Execution and Data Prediction Restriction Instructions.
🌸 armv8.5-a
+fp16— the half-precision floating-point data processing instructions. this also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension and the half-precision floating-point fmla extension.+simd— the ARMv8.3-A Advanced SIMD and floating-point instructions as well as the Dot Product extension.+crypto— the cryptographic instructions. this also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension.+nocrypto— disable the cryptographic extension.+nofp— disable the floating-point, Advanced SIMD and cryptographic instructions.
🌸 armv7-r
+fp.sp— the single-precision VFPv3 floating-point instructions. the extension+vfpv3xdcan be used as an alias for this extension.+fp— the VFPv3 floating-point instructions with 16 double-precision registers. the extension+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.+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.+dsp— the DSP instructions. that's it, that's the whole vibe 💅+nodsp— turns the DSP extension off. bye bestie 🫶+fp— the single-precision floating-point instructions.+fp.dp— the single- and double-precision floating-point instructions. both. iconic.+nofp— disables the floating-point extension entirely. it's giving "we don't do math here" 😭
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— disables the cryptographic instructions.+nofp— disables the floating-point, Advanced SIMD and cryptographic instructions. all three, gone, no cap 💀
okay so -march=native makes the compiler auto-detect the architecture of the build computer, main character energy fr. but lowkey: 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. so if nothing happens… that's why 🙈
-mtune=name
this one says: hey GCC, tune the performance of the code for this ARM processor specifically ✨ for some ARM implementations you can get better performance by using this option. the permissible names are, in order:
arm7tdmi, arm7tdmi-s, arm710t, arm720t, arm740t, strongarm, strongarm110,
strongarm1100, 0strongarm1110, 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, ares,
cortex-r4, cortex-r4f, cortex-r5, cortex-r7, cortex-r8, cortex-r52, cortex-m0,
cortex-m0plus, cortex-m1, cortex-m3, cortex-m4, cortex-m7, cortex-m23,
cortex-m33, 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.and additionally 💫 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 though: 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 those extension options do not affect the tuning of the generated code. noted. 🫶
-mtune=native makes the compiler auto-detect the CPU of the build computer. at present, again, this feature is only supported on GNU/Linux, and not all architectures are recognized. if the auto-detect is unsuccessful, the option has no effect.
-mcpu=name[+extension...]
this specifies the name of the target ARM processor. GCC uses this name to derive:
- the name of the target ARM architecture (as if you'd specified it with
-march), and - the ARM processor type to tune for performance (as if you'd specified it with
-mtune).
where this option is used together with -march or -mtune, those options take precedence over the appropriate part of this one. periodt 👑
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 -mfloat-abi and -mfpu must also be considered: floating-point and Advanced SIMD instructions will only be used if -mfloat-abi is not set to soft; and any setting of -mfpu other than auto will override the available floating-point and SIMD extension instructions.
for example, cortex-a9 can be found in three major configurations: integer only, with just a floating-point unit, or with floating-point and Advanced SIMD. the default is to enable all the instructions, but the extensions +nosimd and +nofp can be used to disable just the SIMD, or both the SIMD and floating-point instructions, respectively ✨
the permissible names for this option are the same as those for -mtune. iykyk 💅
the following extension options are common to the listed CPUs:
+nodsp— disables the DSP instructions oncortex-m33.+nofp— disables 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-m7andcortex-m33. and it disables the floating-point and SIMD instructions 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-a55.+nofp.dp— disables the double-precision component of the floating-point instructions oncortex-r5,cortex-r7,cortex-r8,cortex-r52andcortex-m7.+nosimd— disables the SIMD (but not floating-point) instructions ongeneric-armv7-a,cortex-a5,cortex-a7andcortex-a9.+crypto— enables the cryptographic instructions oncortex-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. 🔐
additionally, the generic-armv7-a pseudo target defaults to VFPv3 with 16 double-precision registers 🎀 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 the same as for the extensions to -march=armv7-a.
-mcpu=generic-arch is also permissible, and is equivalent to -march=arch -mtune=generic-arch. see -mtune for more information 👑
-mcpu=native 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.
-mfpu=name
this specifies what floating-point hardware (or hardware emulation) is available on the target. permissible names: auto, vfpv2, vfpv3, vfpv3-fp16, vfpv3-d16, vfpv3-d16-fp16, vfpv3xd, vfpv3xd-fp16, neon-vfpv3, neon-fp16, vfpv4, vfpv4-d16, fpv4-sp-d16, neon-vfpv4, fpv5-d16, fpv5-sp-d16, fp-armv8, neon-fp-armv8 and crypto-neon-fp-armv8. note that neon is an alias for neon-vfpv3 and vfp is an alias for vfpv2. two aliases, no extra charge 💖
the setting auto is the default and is special ✨ it causes the compiler to select the floating-point and Advanced SIMD instructions based on the settings of -mcpu and -march.
if the selected floating-point hardware includes the NEON extension (e.g.-mfpu=neon), note that floating-point operations are not generated by GCC's auto-vectorization pass unless-funsafe-math-optimizationsis also specified. this is because NEON hardware does not fully implement the IEEE 754 standard for floating-point arithmetic (in particular denormal values are treated as zero), so the use of NEON instructions may lead to a loss of precision. bestie. read that twice. 😭
you can also set the fpu name at function level by using the target("fpu=") function attributes or pragmas.
-mfp16-format=name
specify the format of the __fp16 half-precision floating-point type. permissible names are none, ieee, and alternative; the default is none, in which case the __fp16 type is not defined. 🙈
-mstructure-size-boundary=n
the sizes of all structures and unions are rounded up to a multiple of the number of bits set by this option. permissible values are 8, 32 and 64. the default value varies for different toolchains. for the COFF targeted toolchain the default value is 8. a value of 64 is only allowed if the underlying ABI supports it.
specifying a larger number can produce faster, more efficient code, but can also increase the size of the program. different values are potentially incompatible 😬 code compiled with one value cannot necessarily expect to work with code or libraries compiled with another value, if they exchange information using structures or unions.
this option is deprecated. 👑 it's giving legacy energy.
-mabort-on-noreturn
generate a call to the function abort at the end of a noreturn function. it is executed if the function tries to return. dramatic exit, honestly 💀
-mlong-calls
-mno-long-calls
tells the compiler to perform function calls by first loading the address of the function into a register and then performing a subroutine call on this register. this switch is needed if the target function lies outside of the 64-megabyte addressing range of the offset-based version of the subroutine call instruction.
even if this switch is enabled, not all function calls are turned into long calls. the heuristic is that these are not turned into long calls:
- static functions,
- functions that have the
short_callattribute, - functions that are inside the scope of a
#pragma no_long_callsdirective, and - functions whose definitions have already been compiled within the current compilation unit.
the exceptions to this rule are that these are always turned into long calls:
- weak function definitions,
- functions with the
long_callattribute or thesectionattribute, and - functions that are within the scope of a
#pragma long_callsdirective.
no cap, that's the whole rulebook 🫶
Okay so this one's not enabled by default, bestie. If you wanna go back to the old default vibe, hit it with -mno-long-calls — or just drop your function calls inside a #pragma long_calls_off directive and it does the same thing. 🌸 And heads up: these switches do absolutely nothing to how the compiler generates code for function calls made through function pointers, so don't @ them for that. 🙈
-msingle-pic-base— treat the register used for PIC addressing as read-only instead of reloading it in the prologue of every single function. The runtime system is the one responsible for initializing this register with a good value before execution starts. It's giving ✨trust the process✨.-mpic-register=reg— you pick which register handles PIC addressing. For the standard PIC base case, the default is any suitable register determined by compiler (yes, really, it just vibes and picks one). For the single PIC base case, the default isR9if the target is EABI based or stack-checking is enabled — otherwise the default isR10. 👑-mpic-data-is-text-relative— assume the displacement between the text and data segments is fixed at static link time. This lets you use PC-relative addressing ops to reach data you know lives in the data segment. For non-VxWorks RTP targets this is enabled by default 💅 — and when you disable it on those targets, it'll enable-msingle-pic-baseby default. The way they're linked, iykyk.-mpoke-function-name— writes the name of every function into the text section, right before the function prologue. The generated code looks kinda like 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 you're doing a stack backtrace, the code can peek at 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 you know there's a function name embedded immediately before that location, and its length is "((pc[-3]) & 0xff000000)". Sleuthing era. 🔍
-mthumb-marm— pick your fighter: generate code that runs in ARM state, or in Thumb state. Default for most configs is ARM state, but that default can be switched by configuring GCC with the--with-mode=stateconfigure option. You can also override ARM/Thumb mode per-function using the"target("thumb")"and"target("arm")"function attributes or pragmas. 💖-mflip-thumb— flips ARM/Thumb modes on alternating functions. This one exists purely for regression testing of mixed Thumb/ARM code generation, and is not meant for ordinary use in compiling code. It's a lab experiment, not a lifestyle. 🧪-mtpcs-frame— generate a stack frame that's compliant with the Thumb Procedure Call Standard for all non-leaf functions. (A leaf function = one that doesn't call any other functions, periodt.) The default is-mno-tpcs-frame.-mtpcs-leaf-frame— generate a stack frame that's compliant with the Thumb Procedure Call Standard for all leaf functions. (Again: a leaf function is one that doesn't call any other functions.) The default is-mno-apcs-leaf-frame. 🍓-mcallee-super-interworking— gives every externally visible function in the file being compiled an ARM instruction set header that switches to Thumb mode before running the rest of the function. This lets those functions be called from non-interworking code. Heads up: this option is not valid in AAPCS configurations because interworking is already enabled by default there. 🚫-mcaller-super-interworking— lets calls via function pointers (including virtual functions) execute correctly no matter whether the target code was compiled for interworking or not. Small catch: there's a small overhead on the cost of executing a function pointer when this is enabled. Also not valid in AAPCS configurations, because interworking is enabled by default there. 💀-mtp=name— set the access model for the thread local storage pointer. The valid models are:soft— generates calls to"__aeabi_read_tp".cp15— fetches the thread pointer straight from"cp15"(supported in the arm6k architecture).auto— uses the best available method for the selected processor.
Default setting is auto. 🌈
-mtls-dialect=dialect— pick the dialect for accessing thread local storage. Two are supported: gnu and gnu2. Thegnudialect selects the original GNU scheme for supporting local and global dynamic TLS models. Thegnu2dialect selects the GNU descriptor scheme, which gives better performance for shared libraries. The GNU descriptor scheme is compatible with the original scheme, but it 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. Not everything gets the glow-up. ✨-mword-relocations— only generate absolute relocations on word-sized values (i.e.R_ARM_ABS32). This is enabled by default on targets (uClinux, SymbianOS) where the runtime loader imposes this restriction, and also when-fpicor-fPICis specified. Heads up bestie: this option conflicts with-mslow-flash-data. Pick a lane. 🙅-mfix-cortex-m3-ldrd— some Cortex-M3 cores can cause data corruption when"ldrd"instructions with overlapping destination and base registers are used. This option avoids generating those instructions entirely. It's enabled by default when-mcpu=cortex-m3is specified. Safety first, always. 🫶-munaligned-access-mno-unaligned-access— enables (or disables) reading and writing of 16- and 32-bit values from 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 all other architectures. If unaligned access isn't enabled, then words inside packed data structures get accessed a byte at a time (slow but honest 🐢). The ARM attribute"Tag_CPU_unaligned_access"is 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— enables using Neon to handle scalar 64-bit operations. Disabled by default because the cost of moving data from core registers to Neon is high — it's giving long-distance relationship energy. 😭-mslow-flash-data— assume loading data from flash is slower than fetching instructions, so literal loads get minimized for better performance. Only supported when compiling for ARMv7 M-profile, and off by default. It conflicts with-mword-relocations. 🚫-masm-syntax-unified— assume the inline assembler is using unified asm syntax. The default is currently off, which implies divided syntax. This option has no impact on Thumb2. However — plot twist — this may change in future releases of GCC. Divided syntax should be considered deprecated, so plan accordingly. ⚠️-mrestrict-it— restricts generation of IT blocks so they conform to the rules of ARMv8-A. IT blocks can only contain a single 16-bit instruction from a select set of instructions. This is on by default for ARMv8-A Thumb mode. Structured queen. 👑-mprint-tune-info— print CPU tuning information as a comment in the assembler file. This is an option used only for regression testing of the compiler, not intended for ordinary use in compiling code. Disabled by default. It's a dev tool, bestie, not a feature. 🔧-mverbose-cost-dump— enable verbose cost model dumping in the debug dump files. This one is provided for use in debugging the compiler itself. Compiler-dev shenanigans only. 🧚-mpure-code— don't allow constant data to be placed in code sections. Plus, when compiling for ELF object format, give all text sections the ELF processor-specific section attribute"SHF_ARM_PURECODE". This option is only available when generating non-pic code for M-profile targets. 🎀-mcmse— generate secure code as per the"ARMv8-M Security Extensions: Requirements on Development Tools Engineering Specification", which lives over at <https://developer.arm.com/documentation/ecm0359818/latest/>. Locked down and iconic. 🔒
🦋 AVR Options 🦋
These options are defined for AVR implementations, bestie. Tiny chips, big feelings. 💜
-mmcu=mcu— specify the Atmel AVR instruction set architecture (ISA) or MCU type. The default for this option isavr2. 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 the"MOVW"instruction.mcu="attiny13","attiny13a","attiny24","attiny24a","attiny25","attiny261","attiny261a","attiny2313","attiny2313a","attiny43u","attiny44","attiny44a","attiny45","attiny48","attiny441","attiny461","attiny461a","attiny4313","attiny84","attiny84a","attiny85","attiny87","attiny88","attiny828","attiny841","attiny861","attiny861a","ata5272","ata6616c","at86rf401"."avr3"— "Classic" devices with 16 KiB up to 64 KiB of program memory.mcu="at76c711","at43usb355"."avr31"— "Classic" devices with 128 KiB of program memory.mcu="atmega103","at43usb320"."avr35"— "Classic" devices with 16 KiB up to 64 KiB of program memory and with the"MOVW"instruction.mcu="attiny167","attiny1634","atmega8u2","atmega16u2","atmega32u2","ata5505","ata6617c","ata664251","at90usb82","at90usb162"."avr4"— "Enhanced" devices with up to 8 KiB of program memory.mcu="atmega48","atmega48a","atmega48p","atmega48pa","atmega48pb","atmega8","atmega8a","atmega8hva","atmega88","atmega88a","atmega88p","atmega88pa","atmega88pb","atmega8515","atmega8535","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","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". bestie that's a LOT of little chips, she ate 💅"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" devices with more than 64 KiB and up to 128 KiB of program memory.mcu="atxmega64a3","atxmega64a3u","atxmega64a4u","atxmega64b1","atxmega64b3","atxmega64c3","atxmega64d3","atxmega64d4"."avrxmega5"— "XMEGA" devices with more than 64 KiB and up to 128 KiB of program memory and more than 64 KiB of RAM.mcu="atxmega64a1","atxmega64a1u"."avrxmega6"— "XMEGA" devices with more than 128 KiB of program memory.mcu="atxmega128a3","atxmega128a3u","atxmega128b1","atxmega128b3","atxmega128c3","atxmega128d3","atxmega128d4","atxmega192a3","atxmega192a3u","atxmega192c3","atxmega192d3","atxmega256a3","atxmega256a3b","atxmega256a3bu","atxmega256a3u","atxmega256c3","atxmega256d3","atxmega384c3","atxmega384d3"."avrxmega7"— "XMEGA" devices with more than 128 KiB of program memory and more than 64 KiB of RAM.mcu="atxmega128a1","atxmega128a1u","atxmega128a4u"."avrtiny"— "TINY" Tiny core devices with 512 B up to 4 KiB of program memory.mcu="attiny4","attiny5","attiny9","attiny10","attiny20","attiny40". 🦋"avr1"— this ISA is implemented by the minimal AVR core and is supported for assembler only.mcu="attiny11","attiny12","attiny15","attiny28","at90s1200". assembler only bestie, do NOT try to compile C for these ones 😭
-mabsdata— assume that all data in static storage can be accessed byLDS/STSinstructions. This option only does anything on reduced Tiny devices like ATtiny40. See also theabsdataAVR Variable Attributes, variable attribute. 💅-maccumulate-args— accumulate outgoing function arguments and grab/release the stack space those outgoing args need once, up in the function prologue/epilogue. Without this option, outgoing arguments get pushed before calling a function and popped afterwards.
Popping the args after the function call can be expensive on AVR, so accumulating the stack space might give you smaller executables, because the arguments don't need to be removed from the stack after a call like that.
This option can lead to reduced code size for functions that make several calls to functions that take their arguments on the stack, like calls to printf-like functions. iconic honestly, smaller binaries are the vibe ✨
- *
-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 appropriate subroutines. Code size is smaller. 🎀-mgas-isr-prologues— interrupt service routines (ISRs) may use the__gcc_isrpseudo instruction supported by GNU Binutils. If this option is on, the feature can still be disabled for individual ISRs by means of theno_gccisrfunction attribute in AVR Function Attributes. This feature is activated per default if optimization is on (but not with-Og, see Optimize Options), and if GNU Binutils support PR21683 (https://sourceware.org/PR21683).-mint8— assumeintto be an 8-bit integer. This affects the sizes of all the types: acharis 1 byte, anintis 1 byte, alongis 2 bytes, and along longis 4 bytes. Please note that this option does not conform to the C standards, but it results in smaller code size. not standards-compliant bestie, she said what she said 💀-mmain-is-OS_task— do not save registers inmain. The effect is the same like attaching theOS_taskattribute (AVR Function Attributes) tomain. It is activated per default if optimization is on.- *
-mn-flash=num* — assume that the flash memory has a size of num times 64 KiB. -mno-interrupts— generated code is not compatible with hardware interrupts. Code size is smaller. no interrupt compatibility tho, keep that in mind 🙈-mrelax— try to replace theCALL(resp.JMP) instruction by the shorterRCALL(resp.RJMP) instruction if 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 performed by the linker because jump offsets aren't known before the code is located. Therefore, the assembler code generated by the compiler 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 that the device supports the Read-Modify-Write instructionsXCH,LAC,LASandLAT.-mshort-calls— assume thatRJMPandRCALLcan 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. hands off bestie, internal business 💅
-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 option by hand.
This option is used internally by the compiler to select and build multilibs for architectures avr2 and avr25. These 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 the compiler then knows if the device or architecture has an 8-bit stack pointer and thus no SPH register or not.
-mstrict-X— use address registerXin a way proposed by the hardware. This means thatXis only used in indirect, post-increment or pre-decrement addressing.
Without this option, the X register may be used in the same way as Y or Z, which then gets emulated by additional instructions. For example, loading a value with X+const addressing with a small non-negative const < 64 to a register Rn is performed as
adiw r26, const ; X += const
ld <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 to use 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-<mcu>to the compiler driver's command line. The user takes responsibility for supplying the sub-processes like compiler proper, assembler and linker with appropriate command line options. you're on your own with this one bestie, no cap 🙈-Waddr-space-convert— warn about conversions between address spaces in the case where the resulting address space is not contained in the incoming address space.-Wmisspelled-isr— warn if the ISR is misspelled, i.e. without the__vectorprefix. Enabled by default. she's always watching for your typos, queen behavior 👑
"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 that indirect jumps and calls can target any code address in the range of 64 Ki words. 16-bit pointers, 64 Ki words of reach, that's the whole tea ☕
So for indirect jumps on devices with more than 128 Ki bytes of program memory space, there’s this special function register called "EIND" that serves as the most significant part of the target address when "EICALL" or "EIJMP" instructions are used. 💫
Indirect jumps and calls on these devices are handled by the compiler like this, and bestie, there are some limitations:
- The compiler never sets
"EIND". 🙅♀️ - The compiler uses
"EIND"implicitly in"EICALL"/"EIJMP"instructions, or it might read"EIND"directly in order to emulate an indirect call/jump by means of a"RET"instruction. 👀 - The compiler assumes that
"EIND"never changes during the startup code or during the application. In particular,"EIND"is not saved/restored in function or interrupt service routine prologue/epilogue. 🚫 - For indirect calls to functions and computed goto, the linker generates stubs. Stubs are jump pads, sometimes also called trampolines. So the indirect call/jump jumps to such a stub. The stub contains a direct jump to the desired address. 🦋
- Linker relaxation must be turned on so that the linker generates the stubs correctly in all situations. See the compiler option
-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 with"EIND != 0", a custom linker script has to be used in order to place the sections whose name start with.trampolinesinto the segment where"EIND"points to. 🧚 - The startup code from libgcc never sets
"EIND". Notice that startup code is a blend of code from libgcc and AVR-LibC. For the impact of AVR-LibC on"EIND", see the AVR-LibC user manual (http://nongnu.org/avr-libc/user-manual/). 📖 - It is legitimate for user-specific startup code to set up
"EIND"early, for example by means of initialization code located in section.init3. Such code runs prior to general startup code that initializes RAM and calls constructors, but after the bit of startup code from AVR-LibC that sets"EIND"to the segment where the vector table is located. ✨
#include <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");
}The "__trampolines_start" symbol is defined in the linker script. 🎀
- Stubs are generated automatically by the linker if the following two conditions are met:
- The address of a label is taken by means of the
"gs"modifier (short for generate stubs) like so:
- 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.
- The compiler emits such
"gs"modifiers for code labels in the following 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 🦋
Some AVR devices support memories larger than the 64 KiB range that can be accessed with 16-bit pointers. To access memory locations outside this 64 KiB range, the content of a "RAMP" register is used as high part of the address: The "X", "Y", "Z" address register is concatenated with the "RAMPX", "RAMPY", "RAMPZ" special function register, respectively, to get a wide address. Similarly, "RAMPD" is used together with direct addressing. 🌈
- The startup code initializes the
"RAMP"special function registers with zero. 0️⃣ - If a AVR Named Address Spaces,named address space other than generic or
"__flash"is used, then"RAMPZ"is set as needed before the operation. ⚡ - If the device supports RAM larger than 64 KiB and the compiler needs to change
"RAMPZ"to accomplish an operation,"RAMPZ"is reset to zero after the operation. 🔄 - If the device comes with a specific
"RAMP"register, the ISR prologue/epilogue saves/restores that SFR and initializes it with zero in case the ISR code might (implicitly) use it. 🛟 - RAM larger than 64 KiB is not supported by GCC for AVR targets. If you use inline assembler to read from locations outside the 16-bit address range and change one of the
"RAMP"registers, you must reset it to zero after the access. ⚠️
AVR Built-in Macros 💖
GCC defines several built-in macros so that the user code can test for the presence or absence of features. Almost any of the following built-in macros are deduced from device capabilities and thus triggered by the -mmcu= command-line option. 👑
For even more AVR-specific built-in macros see AVR Named Address Spaces and AVR Built-in Functions. 🧚
__AVR_ARCH__— Build-in macro that resolves to a decimal number that identifies the architecture and depends on the-mmcu=mcuoption. Possible values are:
2, 25, 3, 31, 35, 4, 5, 51, 6
for mcu="avr2", "avr25", "avr3", "avr31", "avr35", "avr4", "avr5", "avr51", "avr6",
respectively and
100, 102, 103, 104, 105, 106, 107
for mcu="avrtiny", "avrxmega2", "avrxmega3", "avrxmega4", "avrxmega5", "avrxmega6", "avrxmega7", respectively. If mcu specifies a device, this built-in macro is set accordingly. For example, with -mmcu=atmega8 the macro is defined to 4.
__AVR_Device__— Setting-mmcu=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__,__AVR_3_BYTE_PC__— The device has the"EIJMP"and"EICALL"instructions. This is the case for devices with more than 128 KiB of program memory. This also means that the program counter (PC) is 3 bytes wide. 🧠__AVR_2_BYTE_PC__— The program counter (PC) is 2 bytes wide. This is the case for devices with up to 128 KiB of program memory. 🧮__AVR_HAVE_8BIT_SP__,__AVR_HAVE_16BIT_SP__— The stack pointer (SP) register is treated as 8-bit respectively 16-bit register by the compiler. The definition of these macros is affected by-mtiny-stack. 🥞__AVR_HAVE_SPH__,__AVR_SP8__— The device has the SPH (high part of stack pointer) special function register or has an 8-bit stack pointer, respectively. The definition of these macros is affected by-mmcu=and in the cases of-mmcu=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__,__AVR_ERRATA_SKIP_JMP_CALL__— Some AVR devices (AT90S8515, ATmega103) must not skip 32-bit instructions because of a hardware erratum. Skip instructions are"SBRS","SBRC","SBIS","SBIC"and"CPSE". The second macro is only defined if"__AVR_HAVE_JMP_CALL__"is also set. 💀__AVR_ISA_RMW__— The device has Read-Modify-Write instructions (XCH, LAC, LAS and LAT). 🔧__AVR_SFR_OFFSET__=offset— Instructions that can address I/O special function registers directly like"IN","OUT","SBI", etc. may use a different address as if addressed by an instruction to access RAM like"LD"or"STS". This offset depends on the device architecture and has to be subtracted from the RAM address in order to get the respective I/O address. 🧮__AVR_SHORT_CALLS__— The-mshort-callscommand line option is set. 📞"__AVR_PM_BASE_ADDRESS__=addr"— okay so some devices can read straight out of flash memory usingLD*instructions, and that flash shows up in the data address space sitting at an offset of"__AVR_PM_BASE_ADDRESS__". If that macro isn't defined, bestie, this feature just isn't available, periodt. If it is defined, the address space is linear and you don't need to shove.rodatainto RAM at all. The default linker description file handles all of this, and rn it's available for"avrtiny"and"avrxmega3". Even more convenient? You don't need to touch address spaces like"__flash"or features like the attribute"progmem"and"pgm_read_*". Iconic behavior honestly. ✨"__WITH_AVRLIBC__"— the compiler is configured to be used together with AVR-Libc. Peep the--with-avrlibcconfigure option. 🫶
🖤 Blackfin Options 🖤
-mcpu=cpu[-sirevision]— tells you the name of the target Blackfin processor. Right now, 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 optional sirevision is 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, no cap. If sirevision is any, literally all the 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, "__SILICON_REVISION__" is not defined. If sirevision is any, "__SILICON_REVISION__" is defined to be 0xffff. If you don't use this optional sirevision at all, GCC assumes the latest known silicon revision of the targeted Blackfin processor.
GCC defines a preprocessor macro for the specified cpu. For the bfin-elf toolchain, this option causes the hardware BSP provided by libgloss to be linked in if -msim is not given.
Without this option, bf532 is used as the processor by default.
Heads up: support forbf561is incomplete, fr fr. Forbf561, only the preprocessor macro is defined. 😭
-msim— says the program will be run on the simulator. This causes the simulator BSP provided by libgloss to be linked in. Only has an effect for thebfin-elftoolchain. Certain other options, like-mid-shared-libraryand-mfdpic, imply-msim.-momit-leaf-frame-pointer— don't keep the frame pointer in a register for leaf functions. This dodges the instructions needed to save, set up and restore frame pointers, and frees up an extra register in leaf functions. 💅-mspecld-anomaly— when enabled, the compiler makes sure the generated code doesn't contain speculative loads after jump instructions. If you use this option,"__WORKAROUND_SPECULATIVE_LOADS"is defined.-mno-specld-anomaly— don't generate extra code to prevent speculative loads from occurring.-mcsync-anomaly— when enabled, the compiler makes sure the generated code doesn't containCSYNCorSSYNCinstructions too soon after conditional branches. If you use this option,"__WORKAROUND_SPECULATIVE_SYNCS"is 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 knowing 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 this library or executable won't link against any other ID shared libraries. That lets the compiler use faster code for jumps and calls. 🏃♀️-mno-leaf-id-shared-library— do not assume that the code being compiled won't link against any ID shared libraries. Slower code 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 it's no more space- or time-efficient than just 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. You need this switch if the target function lies outside of the 24-bit addressing range of the offset-based version of the subroutine call instruction.
This feature is not enabled by default. Specifying -mno-long-calls restores the default behavior. Note that these switches have no effect on how the compiler generates code to handle function calls via function pointers.
-mfast-fp— link with the fast floating-point library. This library relaxes some of the IEEE floating-point standard's rules for checking inputs against Not-a-Number (NAN), all in the interest of performance.-minline-plt— enable inlining of PLT entries in function calls to functions that aren't 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 "coreb_main".
If this option isn't used, the single-core application programming model is used.
-mcorea— build a standalone application for Core A of BF561 when using the one-application-per-core programming model. Proper start files and link scripts are used to support Core A, and the macro"__BFIN_COREA"is defined. This option can only be used in conjunction with-mmulticore.-mcoreb— build a standalone application for Core B of BF561 when using the one-application-per-core programming model. Proper start files and link scripts are used to support Core B, and the macro"__BFIN_COREB"is defined. When this option is used,"coreb_main"should be used instead of"main". This option can only be used in conjunction with-mmulticore.-msdram— build a standalone application for SDRAM. Proper start files and link scripts are used to put the application into SDRAM, and the macro"__BFIN_SDRAM"is defined. The loader should initialize SDRAM before loading the application.-micplb— assume that ICPLBs are enabled at run time. This has an effect on certain anomaly workarounds. For Linux targets, the default is to assume ICPLBs are enabled; for standalone applications the default is off.
✨ C6X Options ✨
-march=name— this specifies the name of the target architecture. GCC uses this name to figure out 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 register"B14". 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 the"B14"register to access them.-msdata=none— make no use of the sections reserved for small data, and use absolute addresses to access all data. Put all initialized global and static data in the.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, iykyk.
-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— the options-metrax4and-metrax100are synonyms for-march=v3and-march=v8respectively. Slay.-mmul-bug-workaround,-mno-mul-bug-workaround— this one works around a bug in themulsandmuluinstructions, but only for the CPU models where that bug actually applies. It's active by default, so unless you're going out of your way to turn it off, you're already covered. 🛡️-mpdebug— turns on CRIS-specific verbose debug-related info in the assembly code. Bonus side quest: it also flips off the#NO_APPformatted-code indicator that gets handed to the assembler at the beginning of the assembly file. 🐛-mcc-init— no borrowing condition-code results from the previous instruction, bestie. Always emit compare and test instructions before condition codes get used. Clean slate energy every single time. ✨-mno-side-effects— don't emit instructions with side effects in addressing modes other than post-increment. That's the whole vibe. 🙅-mstack-align,-mno-stack-align,-mdata-align,-mno-data-align,-mconst-align,-mno-const-align— these options (and theirno-twins) arrange — or, for theno-ones, eliminate the arrangement — for the stack frame, individual data, and constants to be aligned for the maximum single data access size of whichever CPU model you picked. The default? 32-bit alignment. And ngl, ABI details like structure layout are not affected by any of these, so don't stress about that. 🎀-m32-bit,-m16-bit,-m8-bit— basically cousins of the stack-, data- and const-align options up there, except these arrange for the stack frame, writable data, and constants to all be 32-bit, 16-bit or 8-bit aligned respectively. Default is 32-bit alignment. Slay. 💅-mno-prologue-epilogue,-mprologue-epilogue— with-mno-prologue-epilogue, the normal function prologue and epilogue that set up the stack frame are just… omitted, and no return instructions or return sequences get generated in the code either. 🫣
⚠️ Only use this one if you're also visually inspecting the compiled code. No warnings or errors are generated when call-saved registers must be saved, or when storage for local variables needs to be allocated. It will simply not tell you. Fr fr.
-mno-gotplt,-mgotplt— with-fpicand-fPIC, don't generate (-mno-gotplt) or do generate (-mgotplt) instruction sequences that load addresses for functions from the PLT part of the GOT, rather than the traditional-on-other-architectures calls to the PLT. The default is-mgotplt. 🌸-melf— a legacy no-op option, only recognized with thecris-axis-elfandcris-axis-linux-gnutargets. It's giving "still here for the memories." 💀-mlinux— another legacy no-op, only recognized with thecris-axis-linux-gnutarget. 🙈-sim— recognized forcris-axis-elf, this arranges to link with input-output functions from a simulator library. Code, initialized data and zero-initialized data are all allocated consecutively. 🧚-sim2— like-sim, but it passes linker options to locate initialized data at0x40000000and zero-initialized data at0x80000000. Big addresses, big energy. 🔥
💖 CR16 Options 💖
These options are defined specifically for the CR16 ports. Very much their own thing.
-mmac— enables the use of multiply-accumulate instructions. Disabled by default, so you gotta opt in. 🍓-mcr16cplus,-mcr16c— generate code for the CR16C or CR16C+ architecture. CR16C+ is the default, iykyk. ⭐-msim— links the librarylibsim.a, which is compatible with simulator. Only applicable to the ELF compiler. 🔗-mint32— choose integer type as 32-bit wide. Periodt. 💅-mbit-ops— generatessbit/cbitinstructions for bit manipulations. Tiny ops, huge power. ✨-mdata-model=model— choose a data model. The choices for model arenear,farormedium.mediumis default. However —faris not valid with-mcr16c, because the CR16C architecture does not support the far data model. No cap. 🚫
🌈 C-SKY Options 🌈
GCC supports these options when compiling for C-SKY V2 processors. Let's get into it. 👑
-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. Yeah bestie, that's the whole list. 😭-mbig-endian,-EB,-mlittle-endian,-EL— select big- or little-endian code. The default is little-endian. 🦋-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, enable generation of double-precision float instructions. This is the default except when compiling for CK803. 🙃-mfdivdu,-mno-fdivdu— when-mhard-floatis in effect, enable generation offrecipd,fsqrtd, andfdivdinstructions. 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 extendedlrwinstruction. This option defaults to on for CK801 and off otherwise. ⚡-mistack,-mno-istack— enable interrupt stack instructions; the default is off.
The -mistack option is required to handle the interrupt and isr function attributes. 🚨
-mmp— enable multiprocessor instructions; the default is off. 🧩-mcp— enable coprocessor instructions; the default is off. 🤝-mcache— enable coprocessor instructions; the default is off. (Yes, it says coprocessor instructions again — we're reporting it exactly as it is, bestie. 💀)-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 numbered0-7to 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 numbered16-31. 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 usingpushandpopinstructions. This option defaults to on. 📚-mmultiple-stld,-mstm,-mno-multiple-stld,-mno-stm— generate code usingstmandldminstructions. 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 it's 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. Like, genuinely be careful with this one. 😭
🍎 Darwin Options 🍎
These options are defined for all architectures running the Darwin operating system. 🌴
FSF GCC on Darwin does not create "fat" object files; it creates an object file for the single architecture that GCC was built to target. Apple's GCC on Darwin does create "fat" files if multiple -arch options are used; it does so by running the compiler or linker multiple times and joining the results together with lipo. 🧬
The subtype of the file created (like ppc7400 or ppc970 or i686) is determined by the flags that specify the ISA that GCC is targeting, like -mcpu or -march. The -force_cpusubtype_ALL option can be used to override this. 🎚️
The Darwin tools vary in their behavior when presented with an ISA mismatch. The assembler, as, only permits instructions to be used that are valid for the subtype of the file it is generating, so you cannot put 64-bit instructions in a ppc750 object file. The linker for shared libraries, /usr/bin/libtool, fails and prints an error if asked to create a shared library with a less restrictive subtype than its input files (for instance, trying to put a ppc970 object file in a ppc7400 library). The linker for executables, ld, quietly gives the executable the most restrictive subtype of any of its input files. 🤐
-Fdir— add the framework directory dir to the head of the list of directories to be searched for header files. These directories are interleaved with those specified by-Ioptions and are scanned in a left-to-right order. 📂
A framework directory is literally just a directory that has frameworks in it 💖 and a framework is a directory with a Headers and/or PrivateHeaders directory sitting directly inside it, and the directory name ends in .framework. The name of the framework? It's that directory's name minus the .framework part, duh ✨. The headers that belong to the framework live in one of those two directories, and Headers gets searched first bestie, remember that. A subframework is a framework directory that's chillin' inside a framework's Frameworks directory. Includes of subframework headers can only show up in a header of a framework that contains the subframework, or in a sibling subframework header. Two subframeworks are siblings if they're in the same framework. A subframework should not have the same name as a framework — and if you violate that, you get a warning, so, like, don't 🙈. 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. iconic 🫶
-iframeworkdir— like-Fexcept the directory is treated as a system directory 👑. The main difference between this-iframeworkand-Fis that with-iframeworkthe compiler does not warn about constructs contained within header files found via dir. Heads up: this option is valid only for the C family of languages.-gused— emit debugging information for symbols that are used 💅. For stabs debugging format, this enables-feliminate-unused-debug-symbols. This is by default ON.-gfull— emit debugging information for all symbols and types. no cap, everything 🔥-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— enable kernel development mode 🧚. 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.-mone-byte-bool— override 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. fr fr, be careful with this one 😭
-mfix-and-continue-ffix-and-continue-findirect-data
Generate code suitable for fast turnaround development, such as to allow GDB to dynamically load .o files into already-running programs. -findirect-data and -ffix-and-continue are provided for backwards compatibility.
-all_load— loads all members of static archive libraries. See manld(1)for more information.-arch_errors_fatal— cause the errors having to do with files that have the wrong architecture to be fatal 💀.-bind_at_load— causes the output file to be marked such that the dynamic linker will bind all undefined references when the file is loaded or launched ✨.-bundle— produce a Mach-o bundle format file. See manld(1)for more information.-bundle_loader executable— this option specifies the executable that will load the build output file being linked. See manld(1)for more information.-dynamiclib— when passed this option, GCC produces a dynamic library instead of an executable when linking, using the Darwinlibtoolcommand 🎀.-force_cpusubtype_ALL— this causes GCC's output file to have theALLsubtype, instead of one controlled by the-mcpuor-marchoption.
And now bestie, here comes the big ol' list 🌈 — these next ones are all passed straight to the Darwin linker, and the Darwin linker man page describes them in detail, so I'm just gonna list them all out for you exactly as they are, no skips, no edits, periodt:
-allowable_client client_name-client_name-compatibility_version-current_version-dead_strip-dependency-file-dylib_file-dylinker_install_name-dynamic-exported_symbols_list-filelist-flat_namespace-force_flat_namespace-headerpad_max_install_names-image_base-init-install_name-keep_private_externs-multi_module-multiply_defined-multiply_defined_unused-noall_load-no_dead_strip_inits_and_terms-nofixprebinding-nomultidefs-noprebind-noseglinkedit-pagezero_size-prebind-prebind_all_twolevel_modules-private_bundle-read_only_relocs-sectalign-sectobjectsymbols-whyload-seg1addr-sectcreate-sectobjectsymbols-sectorder-segaddr-segs_read_only_addr-segs_read_write_addr-seg_addr_table-seg_addr_table_filename-seglinkedit-segprot-segs_read_only_addr-segs_read_write_addr-single_module-static-sub_library-sub_umbrella-twolevel_namespace-umbrella-undefined-unexported_symbols_list-weak_reference_mismatches-whatsloaded
yes bestie, some of them repeat in the original list —-sectobjectsymbols,-segs_read_only_addrand-segs_read_write_addreach show up twice. I'm keeping them exactly as they are, don't @ me 💅
These options are passed to the Darwin linker. The Darwin linker man page describes them in detail.
🌟 DEC Alpha Options 🌟
These -m options are defined for the DEC Alpha implementations:
-mno-soft-float-msoft-float
Use (do not use) the hardware floating-point instructions for floating-point operations 🧮. When -msoft-float is specified, functions in libgcc.a are used to perform floating-point operations. Unless they are replaced by routines that emulate the floating-point operations, or compiled in such a way as to call such emulations routines, these routines issue floating-point operations. If you are compiling for an Alpha without floating-point operations, you must ensure that the library is built so as not to call them.
Note that Alpha implementations without floating-point operations are required to have floating-point registers. wild, right? 😭
-mfp-reg-mno-fp-regs
Generate code that uses (does not use) the floating-point register set ✨. -mno-fp-regs implies -msoft-float. If the floating-point register set is not used, floating-point operands are passed in integer registers as if they were integers and floating-point results are passed in $0 instead of $f0. This is a non-standard calling sequence, so any function with a floating-point argument or return value called by code compiled with -mno-fp-regs must also be compiled with that option. iykyk 🔥
A typical use of this option is building a kernel that does not use, and hence need not save and restore, any floating-point registers.
-mieee— the Alpha architecture implements floating-point hardware optimized for maximum performance 🚀. It is mostly compliant with the IEEE floating-point standard. However, for full compliance, software assistance is required. This option generates code fully IEEE-compliant code except that the inexact-flag is not maintained (see below). If this option is turned on, the preprocessor macro"_IEEE_FP"is defined during compilation. The resulting code is less efficient but is able to correctly support denormalized numbers and exceptional IEEE values such as not-a-number and plus/minus infinity. Other Alpha compilers call this option-ieee_with_no_inexact.-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-fptm trap-mode. The trap mode can be set to one of four values:n— this is the default (normal) setting. The only traps that are enabled are the ones that cannot be disabled in software (e.g., division by zero trap).u— in addition to the traps enabled 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-fprm rounding-mode. The rounding-mode can be one of:n— normal IEEE rounding mode. Floating-point numbers are rounded towards the nearest machine number or towards the even machine number in case of a tie.m— round towards minus infinity.c— chopped rounding mode. Floating-point numbers are rounded towards zero.d— dynamic rounding mode. A field in the floating-point control register (fpcr, see Alpha architecture reference manual) controls the rounding mode in effect. The C library initializes this register for rounding towards plus infinity. Thus, unless your program modifies 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 precisions can be selected:p— program precision 💫 this is the default, and it means a trap handler can only clock which program caused a floating-point exception. that's it, that's the tea.f— function precision 🎯 now the trap handler can figure out which function caused the floating-point exception. upgrade!i— instruction precision 🔬 the trap handler can pin down the exact instruction that caused the floating-point exception. no cap, full detective mode.
Other Alpha compilers ship the equivalent options under the names -scope_safe and -resumption_safe. so if you're coming from one of those, hey bestie, same vibes 💅
-mieee-conformant— this one stamps the generated code as IEEE conformant ✨ but do NOT use it unless you also pass-mtrap-precision=iand either-mfp-trap-mode=suor-mfp-trap-mode=sui. its one and only effect? emitting the line.eflag 48in the function prologue of the generated assembly file. that's literally all it does, iykyk 🎀-mbuild-constants— normally GCC peeks at a 32- or 64-bit integer constant to see if it can build it out of smaller constants in two or three instructions 🧩 if it can't, it just drops the constant as a literal and generates code to load it from the data segment at run time.
use this option to make GCC construct all integer constants using code instead, even when that takes more instructions (the max is six, bestie, six 😭).
you'd typically reach for this when building a shared library dynamic loader. itself a shared library, it has to relocate itself in memory before it can find the variables and constants in its own data segment. self-discovery era 🦋
-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 the CPU type you named with-mcpu=supports — or, if you didn't name one, whatever the CPU GCC was built on supports. vibe check courtesy of the hardware ✨-mfloat-vax,-mfloat-ieee— generate code that uses (or does not use) VAX F and G floating-point arithmetic instead of IEEE single and double precision. pick your fighter 🥊-mexplicit-relocs,-mno-explicit-relocs— back in the day, older Alpha assemblers had no way to generate symbol relocations except through assembler macros 🙈 and using those macros means you can't get optimal instruction scheduling, tragic. GNU binutils as of version2.12supports a new syntax that lets the compiler explicitly mark which relocations apply to which instructions. this option is mostly useful for debugging, because GCC detects the assembler's capabilities when it's built and sets the default accordingly. it already knows, bestie 💅-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 get placed in a small data area (the".sdata"and".sbss"sections) and are accessed via 16-bit relocations off of the$gpregister. this caps the small data area at 64KB, but it lets those variables be accessed directly with a single instruction. slay 🎀
the default is -mlarge-data. with that, the data area is limited to just below 2GB. programs that need more than 2GB of data have to use "malloc" or "mmap" to allocate the data in the heap instead of in the program's data segment. (heap era, we love to see it 🫶)
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 the code of the entire program (or shared library) fits in 4MB, so it's reachable with a branch instruction ✨ and with-msmall-data, the compiler can assume all local symbols share the same$gpvalue, which cuts the number of instructions needed for a function call from 4 down to 1. glow-up fr fr 💖
the default is -mlarge-text.
-mcpu=cpu_type— set the instruction set and instruction scheduling parameters for machine type cpu_type 👑 you can give either the EV style name or the corresponding chip number. GCC supports scheduling parameters for the EV4, EV5 and EV6 family of processors and picks the default instruction-set values from the processor you specify. if you don't 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 🌈 note that -mcpu=native has no effect if GCC doesn't recognize the processor. it tried, okay 😭
-mtune=cpu_type— set only the instruction scheduling parameters for machine type cpu_type. the instruction set is not changed. scheduling-only era, very demure 💅
native toolchains also support the value native, which selects the best architecture option for the host processor. and same deal: -mtune=native has no effect if GCC doesn't 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 the application uses and on the size of the external cache on the machine. it's giving very "depends on your setup" energy.
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. heads up bestie:L3is only valid for EV5 📌
✨ 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. small but mighty, and also a lil risky 😌-mno-lsim— assume that runtime support has already been provided, so there's no need to put the simulator library (libsim.a) on the linker command line. you got this one handled, bestie 💪
🦋 FT32 Options 🦋
these options are defined specifically for the FT32 port 💜
-msim— specifies that the program will be run on the simulator. this makes an alternate runtime startup and library get linked in. you must not use this option when generating programs that will run on real hardware — for those you have to provide your own runtime library for whatever I/O functions are needed. simulator-only, no exceptions 🚫-mlra— enable Local Register Allocation. this is still experimental for FT32, so by default the compiler uses standard reload. it's in its experimental era 🙈-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. zip it, gorgeous 🤐✨-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. go big 👑-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. speed demon energy 🔥-msoft-float— use library routines for floating-point operations.-malloc-cc— dynamically allocate condition code registers.-mfixed-cc— don't try to dynamically allocate condition code registers; only use"icc0"and"fcc0". commitment issues? no. commitment only 💅-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. and with a bfin-elf target, this option implies-msim. multitasking fr fr ✨-minline-plt— enable inlining of PLT entries in function calls to functions that aren't 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. it just shows up when the vibe is right.-mTLS— assume a large TLS segment when generating thread-local code.-mtls— do not assume a large TLS segment when generating thread-local code.-mgprel-ro— enable the use of"GPREL"relocations in the FDPIC ABI for data that's known to be in read-only sections 🎀 it's enabled by default, except for-fpicor-fpie: even though it may help make the global offset table smaller, 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 avoids the need for a GOT entry for the referenced symbol, so it's more likely to be a win. if it isn't,-mno-gprel-rocan be used to disable it. situational, bestie, but we love a trade-off analysis 💖-multilib-library-pic— link with the (library, not FD) pic libraries. it's implied by-mlibrary-pic, as well as by-fPICand-fpicwithout-mfdpic. you should never have to use it explicitly. it's giving "auto, don't touch" 🚫💅-mlinked-fp— follow the EABI requirement of always creating a frame pointer whenever a stack frame is allocated. this option is enabled by default and can be disabled with-mno-linked-fp. frame pointer always invited to the party 🫶-mlong-calls— calls to functions outside the current compilation unit get made with indirect addressing. why do we care? because it means those functions can literally be placed anywhere in the 32-bit address space. iconic. 💅-malign-labels— tries to align labels to an 8-byte boundary by stuffing NOPs into the previous packet. heads up though: this only does anything when VLIW packing is enabled, and it does not create new packets — it just adds NOPs to ones that already exist. no new friends, just filler. 🙈-mlibrary-pic— generates position-independent EABI code. slay. ✨-macc-4— uses only the first four media accumulator registers. frugal queen. 🎀-macc-8— uses all eight media accumulator registers. go big or go home. 👑-mpack— packs VLIW instructions.-mno-pack— does not pack VLIW instructions.-mno-eflags— doesn't mark ABI switches ine_flags.-mcond-move— enables the use of conditional-move instructions (this is the default).
ngl bestie, this switch is mainly for debugging the compiler and will likely be removed in a future version. don't get attached. 😭
-mno-cond-move— disables the use of conditional-move instructions.
same warning fr fr: mainly for debugging the compiler, likely gone in a future version. 💀
-mscc— enables the use of conditional set instructions (default).
again — debugging-only energy, probably getting removed later. 🙈
-mno-scc— disables the use of conditional set instructions.
and again — mainly for debugging, likely removed in a future version. 💅
-mcond-exec— enables the use of conditional execution (default).
you already know: debugging the compiler, probably removed someday. 🦋
-mno-cond-exec— disables the use of conditional execution.
same disclaimer, same future-removal energy. ⭐
-mvliw-branch— runs a pass to pack branches into VLIW instructions (default).
debugging switch, likely removed in a future version. no cap. 💜
-mno-vliw-branch— doesn't run a pass to pack branches into VLIW instructions.
debugging switch, likely removed in a future version. periodt. 🌸
-mmulti-cond-exec— enables optimization of"&&"and"||"in conditional execution (default).
debugging switch, likely removed in a future version. 😭
-mno-multi-cond-exec— disables optimization of"&&"and"||"in conditional execution.
debugging switch, likely removed in a future version. 💀
-mnested-cond-exec— enables nested conditional execution optimizations (default).
debugging switch, likely removed in a future version. 🙈
-mno-nested-cond-exec— disables nested conditional execution optimizations.
debugging switch, likely removed in a future version. 💅
-moptimize-membar— this one removes redundant"membar"instructions from the compiler-generated code. it's enabled by default. lowkey a little housekeeper. 🧚-mno-optimize-membar— turns off that automatic removal of redundant"membar"instructions from the generated code.-mtomcat-stats— makesgasprint out tomcat statistics. stats girlies rise. 📊- *
-mcpu=cpu* — picks the processor type to generate code for. the possible values arefrv,fr550,tomcat,fr500,fr450,fr405,fr400,fr300andsimple. choose wisely bestie. ✨
💖 GNU/Linux Options 💖
these -m options are defined for GNU/Linux targets:
-mglibc— use the GNU C library. this is the default except on*-*-linux-*uclibc*,*-*-linux-*musl*and*-*-linux-*android*targets.-muclibc— use uClibc C library. this is the default on*-*-linux-*uclibc*targets.-mmusl— use the musl C library. this is the default on*-*-linux-*musl*targets.-mbionic— use Bionic C library. this is the default on*-*-linux-*android*targets.-mandroid— compile code compatible with 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. and finally, this option causes the preprocessor macro "__ANDROID__" to be defined. three jobs, one flag. 👑
-tno-android-cc— disables the compilation effects of-mandroid, i.e., it does not enable-mbionic,-fPIC,-fno-exceptionsand-fno-rttiby default.-tno-android-ld— disables the linking effects of-mandroid, i.e., it passes standard Linux linking options to the linker.
🌈 H8/300 Options 🌈
these -m options are defined for the H8/300 implementations:
-mrelax— shortens 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 a function with monitor attribute. the default option is-mexr. this option is valid only for H8S targets.-mno-exr— extended registers are not stored on stack before execution of a function with monitor attribute. the default option is-mno-exr. this option is valid only for H8S targets.-mint32— makes"int"data 32 bits by default.-malign-300— on the H8/300H and H8S, uses 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 instead. 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. it's giving one-way street. 🛑 -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. 🚨 use this one with care, bestie — 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. so like… tradeoffs. 🙈-mjump-in-delay— this option is ignored and provided for compatibility purposes only. it's just vibing. 💀-mdisable-fpregs— prevents 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— prevents the compiler from using indexing address modes. this avoids some rather obscure problems when compiling MIG generated code under MACH.-mno-space-regs— generates 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— generates 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— generates 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— enables the use of assembler directives only GAS understands.- *
-mschedule=cpu-type* — schedules code according to the constraints for the machine type cpu-type. the choices for cpu-type are7007100,7100LC,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— enables 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. it's giving chaotic. 😭-msoft-float— generates output containing library calls for floating point. ⚠️ Warning: the requisite libraries are not available for all HPPA targets. normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. you must make your own arrangements to provide suitable library functions for cross-compilation.
also: -msoft-float changes the calling convention in the output file, so it is only useful if you compile all of a program with this option. in particular, you need to compile libgcc.a, the library that comes with GCC, with -msoft-float in order for this to work. all or nothing bestie. 💅
-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— bestie, this one tells GCC to pass the GNU-flavored options told. Specifically it hands-sharedover toldwhen you're building a shared library. It's the default whenever GCC was configured, whether it said so out loud or just implied it, with the GNU linker. Important though: this option does NOT change whichldgets called — it only changes the parameters that get passed along to whateverldshows up. Theldthat actually gets called is decided by the--with-ldconfigure option, then GCC's program search path, and finally by your ownPATH. Wanna know which linker GCC is using? Print it withgcc -print-prog-name=ld. Heads up: this option is only a thing on the 64-bit HP-UX GCC, aka configured withhppa*64*-*-hpux*. 💅-mhp-ld— same energy, but for the HP linker. This passes-btoldwhen building a shared library, and passes+Accept TypeMismatchtoldon all links. It's the default when GCC was configured, explicitly or implicitly, with the HP linker. And again: it does not touch whichldis called, only what parameters thatldreceives. Theldin question is determined by the--with-ldconfigure option, GCC's program search path, and then yourPATH. Print it withgcc -print-prog-name=ld. Only available on 64-bit HP-UX GCC, i.e. configured withhppa*64*-*-hpux*. ✨-mlong-calls— okay so this generates code that uses long call sequences, which guarantees a call can always reach linker-generated stubs. 🦋
The default is to only generate long calls when the distance from the call site to the beginning of the function or translation unit (whichever applies) blows past a predefined limit set by the branch type being used. Those limits for normal calls are 7,600,000 and 240,000 bytes, respectively, for the PA 2.0 and PA 1.X architectures. Sibcalls are always capped at 240,000 bytes.
Distances get measured from the beginning of functions when you use the -ffunction-sections option, or when you use -mgas and -mno-portable-runtime together under HP-UX with the SOM linker.
Normally you do not want this option, because it degrades performance. That said, it can be useful in large applications, especially when partial linking is used to build the application.
Which type of long call gets used depends on what the assembler and linker can do, and on the type of code being generated. On systems that support long absolute calls, and long pic symbol-difference or pc-relative calls, the impact should be relatively small. But on 32-bit ELF systems in pic code, an indirect call is used and it's quite long. 😭
-munix=unix-std— generates compiler predefines and picks a startfile for the UNIX standard you name. Your choices forunix-stdare93,95and98.93is supported on all HP-UX versions.95is available on HP-UX 10.10 and later.98is available on HP-UX 11.11 and later. Defaults:93for HP-UX 10.00,95for HP-UX 10.10 through to 11.00, and98for HP-UX 11.11 and later.-munix=93gives you the same predefines as GCC 3.3 and 3.4.-munix=95gives you extra predefines forXOPEN_UNIXand_XOPEN_SOURCE_EXTENDED, plus the startfileunix95.o.-munix=98gives you extra predefines for_XOPEN_UNIX,_XOPEN_SOURCE_EXTENDED,_INCLUDE__STDC_A1_SOURCEand_INCLUDE_XOPEN_SOURCE_500, plus the startfileunix98.o.
⚠️ Fr fr, listen: this option changes the interfaces for various library routines. It also affects the operational behavior of the C library. So you need extreme care when using it.
Library code meant to work with more than one UNIX standard has to test, set and restore the variable __xpg4_extended_mask as appropriate. Most GNU software doesn't provide that capability. 🙈
-nolibdld— suppresses the generation of link options that searchlibdld.slwhen-staticis specified on HP-UX 10 and later.-static— the HP-UX implementation ofsetlocaleinlibchas a dependency onlibdld.sl. There is no archive version oflibdld.sl. So when-staticis specified, special link options are needed to resolve that dependency.
On HP-UX 10 and later, the GCC driver adds the necessary options to link with libdld.sl when -static is specified. This makes the resulting binary dynamic. On the 64-bit port, the linkers generate dynamic binaries by default anyway. The -nolibdld option can be used to stop the GCC driver from adding those link options.
-threads— adds support for multithreading with the dce thread library under HP-UX. This option sets flags for both the preprocessor and the linker. 🧵
IA-64 Options 👑
These are the -m options defined for the Intel IA-64 architecture. Periodt.
-mbig-endian— generates code for a big-endian target. This is the default for HP-UX.-mlittle-endian— generates 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— generates code that does not use a global pointer register. The result is not position independent code, and it 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— generates code that uses a single constant global pointer value. Useful when compiling kernel code.-mauto-pic— generates code that is self-relocatable. This implies-mconstant-gp. Useful when compiling firmware code.-minline-float-divide-min-latency— generates code for inline divides of floating-point values using the minimum latency algorithm.-minline-float-divide-max-throughput— generates 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— generates code for inline divides of integer values using the minimum latency algorithm.-minline-int-divide-max-throughput— generates 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— generates code for inline square roots using the minimum latency algorithm.-minline-sqrt-max-throughput— generates 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 you're 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 it doesn't always do so. 🤷-mfixed-range=register-range— generates code treating the given register range as fixed registers. A fixed register is one that the register allocator can't use. This is useful when compiling kernel code. A register range is specified as two registers separated by a dash. You can specify multiple register ranges separated by a comma.-mtls-size=tls-size— specify the 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 the 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 the 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 the 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)able speculative scheduling of the instructions that are dependent on the data speculative loads before reload. This is effective only with-msched-br-data-specenabled. The default setting is enabled.-msched-ar-in-data-spec,-mno-sched-ar-in-data-spec— (en/dis)able speculative scheduling of the instructions that are dependent on the data speculative loads after reload. This is effective only with-msched-ar-data-specenabled. The default setting is enabled.-msched-in-control-spec,-mno-sched-in-control-spec— (en/dis)able speculative scheduling of the instructions that are dependent on the control speculative loads. This is effective only with-msched-control-specenabled. The default setting is enabled.-mno-sched-prefer-non-data-spec-insns,-msched-prefer-non-data-spec-insns— turn this on and data-speculative instructions only get picked for the schedule when there's literally nothing else to choose from at that moment. So it makes data speculation way more conservative, bestie. Default? off. 💅-mno-sched-prefer-non-control-spec-insns,-msched-prefer-non-control-spec-insns— same vibe but for control-speculative instructions: they only get scheduled if there are no other choices available rn. Makes control speculation much more conservative. Default is disabled. 🎀-mno-sched-count-spec-in-critical-path,-msched-count-spec-in-critical-path— when it's on, speculative dependencies get counted while GCC is computing instruction priorities, which makes the speculation a little more conservative. Default setting: disabled. ✨-msched-spec-ldc— uses a simple data speculation check. On by default, no cap.-msched-control-spec-ldc— uses a simple check for control speculation. Also on by default. 💖-msched-stop-bits-after-every-cycle— drops a stop bit after every cycle while scheduling. Yep, on by default.-msched-fp-mem-deps-zero-cost— assumes floating-point stores and loads probably won't conflict when they're placed into the same instruction group. Disabled by default. 🙈-msel-sched-dont-check-control-spec— generates checks for control speculation in selective scheduling. This flag is disabled by default.- *
-msched-max-memory-insns=max-insns* — caps how many memory insns can live in one instruction group, and gives lower priority to the memory insns that come after and still try to schedule into that same group. Honestly super useful for dodging cache bank conflicts. Default value is1. 👑 -msched-max-memory-insns-hard-limit— makes the limit frommsched-max-memory-insnsa hard limit, so no more than that number in an instruction group, periodt. Without this, the limit is "soft" — meaning non-memory operations get preferred once you hit the limit, but memory operations can still sneak in and get scheduled. 🦋
🌸 LM32 Options 🌸
These -m options are defined for the LatticeMico32 architecture:
-mbarrel-shift-enabled— enables barrel-shift instructions.-mdivide-enabled— enables divide and modulus instructions.-mmultiply-enabled— enables multiply instructions.-msign-extend-enabled— enables sign extend instructions.-muser-enabled— enables user-defined instructions.
💜 M32C Options 💜
- *
-mcpu=name* — picks 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. 🫶 -msim— says the program is gonna run on the simulator, which links in an alternate runtime library that supports stuff like file I/O. > You must NOT use this option when you're generating programs that will run on real hardware — you have to bring 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 get used like real registers, so there's a whole tradeoff between GCC's ability to fit the code into the available registers and the performance penalty of using memory instead of registers. Note that **all modules in a program must be compiled with the same value for this option. And 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— assumes all objects live in the lower 16MB of memory (so their addresses can be loaded with theld24instruction), and assumes 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— assumes objects may be anywhere in the 32-bit address space (the compiler generatesseth/add3instructions to load their addresses), and assumes all subroutines are reachable with theblinstruction.-mmodel=large— assumes objects may be anywhere in the 32-bit address space (the compiler generatesseth/add3instructions to load their addresses), and assumes subroutines may not be reachable with theblinstruction (so the compiler generates the much slowerseth/add3/jlinstruction sequence). 😮💨-msdata=none— disables use of the small data area. Variables go 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— puts small global and static data in the small data area, but doesn't generate special code to reference them.-msdata=use— puts small global and static data in the small data area and generates special instructions to reference them.- *
-G*num — puts global and static objects that are 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. The-msdataoption must be set to one ofsdataorusefor this option to do anything at all.
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 — no incorrect code is generated. 💅
-mdebug— makes the M32R-specific code in the compiler display some statistics that might help when you're debugging programs.-malign-loops— aligns all loops to a 32-byte boundary.-mno-align-loops— doesn't enforce a 32-byte alignment for loops. This is the default.- *
-missue-rate=number* — issues number instructions per cycle. number can only be1or2. - *
-mbranch-cost=number* — number can only be1or2. If it's1, 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. The default is12. Valid numbers are between0and15inclusive. -mno-flush-trap— says the cache can't be flushed by using a trap.- *
-mflush-func=name* — specifies the name of the operating system function to call to flush the cache. The default is_flush_cache, but a function call is only used if a trap is not available. -mno-flush-func— indicates that there's 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:
Family : -mcpu arguments
51 : 51 51ac 51ag 51cn 51em 51je 51jf 51jg 51jm 51mm 51qe 51qm
5206 : 5202 5204 5206
5206e : 5206e
5208 : 5207 5208
5211a : 5210a 5211a
5213 : 5211 5212 5213
5216 : 5214 5216
52235 : 52230 52231 52232 52233 52234 52235
5225 : 5224 5225
52259 : 52252 52254 52255 52256 52258 52259
5235 : 5232 5233 5234 5235 523x
5249 : 5249
5250 : 5250
5271 : 5270 5271
5272 : 5272
5275 : 5274 5275
5282 : 5280 5281 5282 528x
53017 : 53011 53012 53013 53014 53015 53016 53017
5307 : 5307
5329 : 5327 5328 5329 532x
5373 : 5372 5373 537x
5407 : 5407
5475 : 5470 5471 5472 5473 5474 5475 547x 5480 5481 5482 5483 5484
5485-mcpu=cpu overrides -march=arch if arch is compatible with cpu. Other combinations of -mcpu and -march are rejected. 🚫
GCC defines the macro __mcf_cpu_cpu when ColdFire target cpu is selected. It also defines __mcf_family_family, where the value of family is given by the table above.
- *
-mtune=tune* — tune the code for a particular microarchitecture within the constraints set by-marchand-mcpu. The M680x0 microarchitectures are:68000,68010,68020,68030,68040,68060andcpu32. The ColdFire microarchitectures are:cfv1,cfv2,cfv3,cfv4andcfv4e.
You can also use -mtune=68020-40 for code that needs to run relatively well on 68020, 68030 and 68040 targets. -mtune=68020-60 is similar but includes 68060 targets as well. These two options select the same tuning decisions as -m68020-40 and -m68020-60 respectively.
GCC defines the macros __mcarch and __mcarch__ when tuning for 680x0 architecture arch. It also defines mcarch unless either -ansi or a non-GNU -std option is used. If GCC is tuning for a range of architectures, as selected by -mtune=68020-40 or -mtune=68020-60, it defines the macros for every architecture in the range. 🦋
GCC also defines the macro __muarch__ when tuning for ColdFire microarchitecture uarch, where uarch is one of the arguments given above. 💜
-m68000,-mc68000— bestie this makes output for a plain 68000. It's the default when the compiler was built for 68000-based systems, and it's literally the same thing as-march=68000.
Reach for this one on microcontrollers with a 68000 or EC000 core, ok? That includes the 68008, 68302, 68306, 68307, 68322, 68328 and 68356. ✨
-m68010— output for a 68010. Default when the compiler is configured for 68010-based systems. Equivalent to-march=68010. 💅-m68020,-mc68020— output for a 68020. Default when the compiler is configured for 68020-based systems. Equivalent to-march=68020. 🦋-m68030— output for a 68030. Default when the compiler is configured for 68030-based systems. Equivalent to-march=68030. 🌸-m68040— output for a 68040. Default when the compiler is configured for 68040-based systems. Equivalent to-march=68040.
Heads up: this option blocks the use of 68881/68882 instructions that would have to be emulated in software on the 68040. So if your 68040 doesn't have code to emulate those instructions, this is your girl. 👑
-m68060— output for a 68060. Default when the compiler is configured for 68060-based systems. Equivalent to-march=68060.
Same energy: this one blocks 68020 and 68881/68882 instructions that have to be emulated in software on the 68060. Use it if your 68060 has no emulation code for those. 💖
-mcpu32— output for a CPU32. Default when the compiler is configured for CPU32-based systems. Equivalent to-march=cpu32.
For microcontrollers with a CPU32 or CPU32+ core: the 68330, 68331, 68332, 68333, 68334, 68336, 68340, 68341, 68349 and 68360. periodt. ⭐
-m5200— output for a 520X ColdFire CPU. Default when the compiler is configured for 520X-based systems. Equivalent to-mcpu=5206, and ngl it's now deprecated in favor of that option.
For microcontrollers with a 5200 core: the MCF5202, MCF5203, MCF5204 and MCF5206.
-m5206e— output for a 5206e ColdFire CPU. This option is now deprecated in favor of the equivalent-mcpu=5206e. 💀-m528x— output for a member of the ColdFire 528X family. Now deprecated in favor of the equivalent-mcpu=528x.-m5307— output for a ColdFire 5307 CPU. Now deprecated in favor of the equivalent-mcpu=5307.-m5407— output for a ColdFire 5407 CPU. Now deprecated in favor of the equivalent-mcpu=5407.-mcfv4e— output for a ColdFire V4e family CPU (like 547x/548x). This includes hardware floating-point instructions, slay. Equivalent to-mcpu=547x, and yep, now deprecated in favor of that option. 🫶-m68020-40— output for a 68040, but without using any of the new instructions. The result is code that runs relatively efficiently on either a 68020/68881, a 68030, or a 68040. The generated code does use the 68881 instructions that get emulated on the 68040.
Equivalent to -march=68020 -mtune=68020-40. 🌈
-m68020-60— output for a 68060, without using any of the new instructions. Code runs relatively efficiently on either a 68020/68881, a 68030, or a 68040. The generated code does use the 68881 instructions emulated on the 68060.
Equivalent to -march=68020 -mtune=68020-60.
-mhard-float,-m68881— generate floating-point instructions. 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. Default for 68000, 68010, and 68832 targets. Also the default for ColdFire devices with no FPU.-mdiv,-mno-div— generate (or don't 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 comes from the target CPU (either the default CPU, or the one you named with-mcpu). For example, the default is "off" for-mcpu=5206and "on" for-mcpu=5206e.
When this option is enabled, GCC defines the macro __mcfhwdiv__.
-mshort— treat typeintas 16 bits wide, likeshort int. Also, parameters passed on the stack get aligned to a 16-bit boundary even on targets whose API mandates promotion to 32-bit.-mno-short— don't treat typeintas 16 bits wide. This is the default.-mnobitfield,-mno-bitfield— don't 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 where functions taking a fixed number of arguments return with thertdinstruction, which pops their arguments while returning. Saves one instruction in the caller since there's 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 alignsint,long,long long,float,double, andlong doublevariables on a 32-bit boundary (-malign-int) or a 16-bit boundary (-mno-align-int). Aligning on 32-bit boundaries gives code that runs somewhat faster on processors with 32-bit busses, at the cost 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, don't say nobody told you. 😭
-mpcrel— use the pc-relative addressing mode of the 68000 directly, instead of a global offset table. At present this option implies-fpic, allowing at most a 16-bit offset for pc-relative addressing.-fPICis 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 lets the data segment live 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 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* — specify the identification number of the ID-based shared library being compiled. A value of0generates more compact code; other values force the allocation of that number to the current library, but that's no more space- or time-efficient than just omitting this option. iykyk. 💜 -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 isn't needed;-fPICsuffices.
GCC normally uses a single instruction to load values from the GOT. While that's relatively efficient, it only works if the GOT is smaller than about 64k. Anything larger makes the linker report an error like:
relocation truncated to fit: R_68K_GOT16O foobarIf that 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 -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 inswitchtables. The default is to use 16-bit offsets.
🌈 MCore Options 🌈
These are the -m options defined for the Motorola M*Core processors. 🧚
-mhardlit,-mno-hardlit— inline constants into the code stream if it can be done in two instructions or less.-mdiv,-mno-div— use the divide instruction. (Enabled by default).-mrelax-immediate,-mno-relax-immediate— allow arbitrary-sized immediates in bit operations.-mwide-bitfields,-mno-wide-bitfields— always treat bit-fields asint-sized.-m4byte-functions,-mno-4byte-functions— force all functions to be aligned to a 4-byte boundary.-mcallgraph-data,-mno-callgraph-data— emit callgraph information.-mslow-bytes,-mno-slow-bytes— prefer word access when reading byte quantities. 🌸-mlittle-endian— generate code for a little-endian target.-mbig-endian— generate code for a little-endian target.-m210,-m340— generate code for the 210 processor. bestie that's the whole vibe, no notes 💅-mno-lsim— assume runtime support has already been provided, so omit the simulator library (libsim.a) from the linker command line. it's giving "i already brought snacks, don't add more" 🍓-mstack-increment=size— set the max amount for a single stack increment operation. big values can speed up programs whose functions need a <em>lot</em> of stack space, but heads up: they can also trigger a segmentation fault if the stack gets extended too much 😭 the default value is0x1000.
🧚 MeP Options 🧚
-mabsdiff— enables theabsinstruction, which is the absolute difference between two registers. slay ✨-mall-opts— enables all the optional instructions: average, multiply, divide, bit operations, leading zero, absolute difference, min/max, clip, and saturation. it's giving "yes to everything" energy 👑-maverage— enables theaveinstruction, which computes the average of two registers. mid-core of two numbers 🎀-mbased=n— variables of size n bytes or smaller get placed in the.basedsection by default. based variables use the$tpregister as a base register, and there's a 128-byte limit to the.basedsection. yes it's literally called.based, we love that for us 🫶-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 gets placed in. name may betiny,near, orfar.-mclip— enables theclipinstruction. note that-mclipis not useful unless you also pass-mminmax. they're a package deal, no cap 💀-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 plus a variety of coprocessors, optional instructions, and peripherals. theMeP-Integratortool (not part of GCC, bestie) 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. iconic 🌈-mdc— causes constant variables to be placed in the.nearsection.-mdiv— enables thedivanddivuinstructions.-meb— generate big-endian code.-mel— generate little-endian code.-mio-volatile— tells the compiler that any variable marked with theioattribute is to be considered volatile.-ml— causes variables to be assigned to the.farsection by default.-mleadz— enables theleadz(leading zero) instruction.-mm— causes variables to be assigned to the.nearsection by default.-mminmax— enables theminandmaxinstructions.-mmult— enables the multiplication and multiply-accumulate instructions.-mno-opts— disables all the optional instructions that-mall-optsturned on. the anti-slay button 🙈-mrepeat— enables therepeatanderepeatinstructions, used for low-overhead looping. loop de loop 💫-ms— causes all variables to default to the.tinysection. note that there's a 65536-byte limit to this section, and accesses to these variables use the%gpbase 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, likeas.-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, <em>excluding</em> 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 get allocated to the.tinysection. these variables use the$gpbase register. the default for this option is4, but note 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, usememcpy.-mno-clearbss— this option is deprecated. use-fno-zero-initialized-in-bssinstead.-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.-mxl-gp-opt— use GP-relative.sdata/.sbsssections.-mxl-multiply-high— use multiply high instructions for the high part of a 32x32 multiply.-mxl-float-convert— use hardware floating-point conversion instructions.-mxl-float-sqrt— use the 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.-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 the start of the text address instead of the GOT, since PC-relative addressing is not supported.xmdstub— for use with the Xilinx Microprocessor Debugger (XMD) based software intrusive debug agent calledxmdstub. 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.o, which does <em>not</em> contain a processor reset vector handler. this is suitable for transferring control on a processor reset to the bootloader rather than to 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. don't @ me, just use the long one bestie 💅
👑 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,mips64r5andmips64r6. 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,xlrandxlp. the special valuefrom-abiselects the most compatible architecture for the selected ABI (that is,mips1for 32-bit ABIs andmips3for 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 at 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 the target architecture as a string. the second has the form _MIPS_ARCH_foo, where foo is the capitalized value of _MIPS_ARCH. for example, -march=r2000 sets _MIPS_ARCH to r2000 and defines the macro _MIPS_ARCH_R2000.
note that the _MIPS_ARCH macro uses the processor names given above. in other words, it has the full prefix and does <em>not</em> abbreviate 000 as k. in the case of from-abi, the macro names the resolved architecture (either mips1 or mips3). it names the default architecture when no -march option is given. iykyk ✨
-mtune=arch— optimize for arch. among other things, this option controls the way instructions are scheduled, and the perceived cost of arithmetic operations. the list of arch values is the same as for-march.
when this option is not used, GCC optimizes for the processor specified by -march. by using -march and -mtune together, it's possible to generate code that runs on a family of processors, but optimize the code for one particular member of that family. main character energy for one specific chip in the fam 🫶
-mtune defines the macros _MIPS_TUNE and _MIPS_TUNE_foo, bestie — same energy as the -march ones we literally just talked about above. 💅
-mips1— Equivalent to-march=mips1.-mips2— Equivalent to-march=mips2.-mips3— Equivalent to-march=mips3.-mips4— Equivalent to-march=mips4.-mips32— Equivalent to-march=mips32.-mips32r3— Equivalent to-march=mips32r3.-mips32r5— Equivalent to-march=mips32r5.-mips32r6— Equivalent to-march=mips32r6.-mips64— Equivalent to-march=mips64.-mips64r2— Equivalent to-march=mips64r2.-mips64r3— Equivalent to-march=mips64r3.-mips64r5— Equivalent to-march=mips64r5.-mips64r6— Equivalent to-march=mips64r6.-mips16,-mno-mips16— Generate (or 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, using the "mips16" and "nomips16" attributes. per-function control?? that's the move 💖
-mflip-mips16— Generate MIPS16 code on alternating functions. This option is provided for regression testing of mixed MIPS16/non-MIPS16 code generation, and it is not intended for ordinary use in compiling user code. 🚧
bestie this one is a test-harness vibe only, do not ship it in your actual code, i'm begging 😭
-minterlink-compressed,-mno-interlink-compressed— Require (or 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. legacy behavior, still iconic ✨-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 <http://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. 🔥
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 (or do not generate) code that is suitable for SVR4-style dynamic objects.-mabicallsis the default for SVR4-based systems.-mshared,-mno-shared— Generate (or 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 efficiency queen 👑
-mshared is the default.
-mplt,-mno-plt— Assume (or 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 (or 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.
-mgp32— Assume that general-purpose registers are 32 bits wide.-mgp64— Assume that general-purpose registers are 64 bits wide.-mfp32— Assume that floating-point registers are 32 bits wide.-mfp64— Assume that floating-point registers are 64 bits wide.-mfpxx— Do not assume the width of floating-point registers.-mhard-float— Use floating-point coprocessor instructions.-msoft-float— Do not use floating-point coprocessor instructions. Implement floating-point calculations using library calls instead.-mno-float— Equivalent to-msoft-float, but additionally asserts that the program being compiled does not perform any floating-point operations. This option is presently supported only by some bare-metal MIPS configurations, where it may select a special set of libraries that lack all floating-point support (including, for example, the floating-point"printf"formats). If code compiled with-mno-floataccidentally contains floating-point operations, it is likely to suffer a link-time or run-time failure. 🚨
accidentally contains floating-point ops with -mno-float? yeah bestie that's a link-time or run-time L, be careful 💅
-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. correct in ALL cases?? that's the dream 🌸
-mnan=2008,-mnan=legacy— These options control the encoding of the special not-a-number (NaN) IEEE 754 floating-point data.
The -mnan=legacy option selects the legacy encoding. In this case quiet NaNs (qNaNs) are denoted by the first bit of their trailing significand field being 0, whereas signaling NaNs (sNaNs) are denoted by the first bit of their trailing significand field being 1.
The -mnan=2008 option selects the IEEE 754-2008 encoding. In this case qNaNs are denoted by the first bit of their trailing significand field being 1, whereas sNaNs are denoted by the first bit of their trailing significand field being 0.
The default is -mnan=legacy unless GCC has been configured with --with-nan=2008.
-mllsc,-mno-llsc— Use (or do not use)ll,sc, andsyncinstructions to implement atomic memory built-in functions. When neither option is specified, GCC uses the instructions if the target architecture supports them.
-mllsc is useful if the runtime environment can emulate the instructions and -mno-llsc can be useful when compiling for nonstandard ISAs. You can make either option the default by configuring GCC with --with-llsc and --without-llsc respectively. --with-llsc is the default for some configurations; see the installation documentation for details. 👑
-mdsp,-mno-dsp— Use (or do not use) revision 1 of the MIPS DSP ASE.
This option defines the preprocessor macro "__mips_dsp". It also defines "__mips_dsp_rev" to 1. ✨
-mdspr2,-mno-dspr2— Use (or do not use) revision 2 of the MIPS DSP ASE.
This option defines the preprocessor macros "__mips_dsp" and "__mips_dspr2". It also defines "__mips_dsp_rev" to 2. 🔥
-msmartmips,-mno-smartmips— turn the MIPS SmartMIPS ASE on (or off, bestie, your call).-mpaired-single,-mno-paired-single— use (or don't) paired-single floating-point instructions. Heads up tho: this one needs hardware floating-point support to be enabled first, no skipping that step. 🫶-mdmx,-mno-mdmx— use (or don't) MIPS Digital Media Extension instructions. And listen — this one is only usable when you're generating 64-bit code, and it also requires hardware floating-point support to be enabled. Both boxes checked or it's not happening. 💅-mips3d,-mno-mips3d— use (or don't) the MIPS-3D ASE. Cute lil fact:-mips3dimplies-mpaired-single, so it's bringing a friend along. 🎀-mmicromips,-mno-micromips— generate (or don't generate) microMIPS code.
Also! microMIPS code generation can be controlled per-function too, using the "micromips" and "nomicromips" attributes. So granular, so iconic. ✨
-mmt,-mno-mt— use (or don't) MT Multithreading instructions.-mmcu,-mno-mcu— use (or don't) the MIPS MCU ASE instructions.-meva,-mno-eva— use (or don't) the MIPS Enhanced Virtual Addressing instructions.-mvirt,-mno-virt— use (or don't) the MIPS Virtualization (VZ) instructions.-mxpa,-mno-xpa— use (or don't) the MIPS eXtended Physical Address (XPA) instructions.-mcrc,-mno-crc— use (or don't) the MIPS Cyclic Redundancy Check (CRC) instructions.-mginv,-mno-ginv— use (or don't) the MIPS Global INValidate (GINV) instructions.-mloongson-mmi,-mno-loongson-mmi— use (or don't) the MIPS Loongson MultiMedia extensions Instructions (MMI).-mloongson-ext,-mno-loongson-ext— use (or don't) the MIPS Loongson EXTensions (EXT) instructions.-mloongson-ext2,-mno-loongson-ext2— use (or don't) the MIPS Loongson EXTensions r2 (EXT2) instructions.-mlong64— force"long"types to be 64 bits wide. And if you're sitting there like "ok but what's the default tho" — go peek at-mlong32, it explains the default and how the pointer size gets determined. iykyk. 👑-mlong32— force"long","int", and pointer types to be 32 bits wide.
So here's the tea on defaults: the size of "int"s, "long"s and pointers depends on the ABI. All the supported ABIs use 32-bit "int"s. The n64 ABI uses 64-bit "long"s, and so does the 64-bit EABI; the others use 32-bit "long"s. Pointers are the same size as "long"s, or the same size as integer registers, whichever is smaller. Slay, but make it size-consistent. 💖
-msym32,-mno-sym32— assume (or don't assume) that all symbols have 32-bit values, regardless of the selected ABI. This one's a glow-up when you pair it with-mabi=64and-mno-abicalls, because it lets GCC generate shorter and faster references to symbolic addresses. Obsessed. ⭐-G num— put definitions of externally-visible data in a small data section if that data is no bigger thannumbytes. Then GCC can generate more efficient accesses to the data; the deets live in-mgpopt.
Oh and 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 static variables in C. And-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 wanna build large libraries with -mno-local-sdata, so that the libraries leave more room for the main program. Sharing is caring, bestie.
-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. And yup,-mextern-sdatais the default for all configurations.
Okay this next bit is important, don't scroll past it: if you compile a module Mod with -mextern-sdata -G num -mgpopt, and Mod references a variable Var that is no bigger than num bytes, you must make sure that Var is placed in a small data section. If Var is defined by another module, you must either compile that module with a high-enough -G setting or attach a "section" attribute to Var's definition. If Var is common, you must link the application with a high-enough -G setting.
The easiest way of satisfying these restrictions is to compile and link every module with the same -G option. But say you wanna build a library that supports several different small data limits — you can do that 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. Problem solved, main character energy. 🌟
-mgpopt,-mno-gpopt— use (or don't use) GP-relative accesses for symbols that are known to be in a small data section; peep-G,-mlocal-sdataand-mextern-sdata. And-mgpoptis the default for all configurations.
-mno-gpopt is the one for when the $gp register might not hold the value of "_gp". Like, 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 those situations, the boot monitor itself is usually compiled with -G0.) 😌
Also, -mno-gpopt implies -mno-local-sdata and -mno-extern-sdata. It's a package deal.
-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 it reduces the amount of RAM required when executing, and thus may be preferred for some embedded systems. Trade-offs, we love them. 🦋-muninit-const-in-rodata,-mno-uninit-const-in-rodata— put uninitialized"const"variables in the read-only data section. And heads up: this option is only meaningful in conjunction with-membedded-data. No solo acts here. 🎀-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's 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. Very specific, very chic.-mcode-readable=no— instructions must not access executable sections. This option can be useful on targets that are configured to have a dual instruction/data SRAM interface but that (unlike the M4K) do not automatically redirect PC-relative loads to the instruction RAM.
-msplit-addresses,-mno-split-addresses— enable (or disable) use of the"%hi()"and"%lo()"assembler relocation operators. Ngl tho: this option has been superseded by-mexplicit-relocsbut is retained for backwards compatibility. Still here for the nostalgia. 💜-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.
And -mexplicit-relocs is the default if GCC was configured to use an assembler that supports relocation operators. ✨
-mcheck-zero-division,-mno-check-zero-division— trap (or don't trap) on integer division by zero.
The default is -mcheck-zero-division. ⚠️
-mdivide-traps,-mdivide-breaks— MIPS systems check for division by zero by generating either a conditional trap or a break instruction. Using traps results in smaller code, but is only supported on MIPS II and later. Also, some versions of the Linux kernel have a bug that prevents trap from generating the proper signal ("SIGFPE"). Use-mdivide-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. And divide-by-zero checks can be completely disabled using -mno-check-zero-division. Choose your fighter. 😭
-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. Conditional slay. 🔥-mmemcpy,-mno-memcpy— force (or don't force) the use of"memcpy"for non-trivial block moves. The default is-mno-memcpy, which allows GCC to inline most constant-sized copies. 💅-mlong-calls,-mno-long-calls— disable (or don't disable) use of the"jal"instruction. Calling functions using"jal"is more efficient but requires the caller and callee to be in the same 256 megabyte segment.
This option has no effect on abicalls code. The default is -mno-long-calls.
-mmad,-mno-mad— enable (or disable) use of the"mad","madu"and"mul"instructions, as provided by the R4650 ISA. 🍓-mimadd,-mno-imadd— enable (or disable) use of the"madd"and"msub"integer instructions. The default is-mimaddon architectures that support"madd"and"msub"— except for the 74k architecture where it was found to generate slower code. Not the 74k catching strays. 💀-mfused-madd,-mno-fused-madd— enable (or disable) use of the floating-point multiply-accumulate instructions, when they are available. The default is-mfused-madd.
Okay so on the R8000 CPU, when multiply-accumulate instructions are used, the intermediate product is calculated to infinite precision and is not subject to the FCSR Flush to Zero bit. This may be undesirable in some circumstances. On other processors the result is numerically identical to the equivalent computation using separate multiply, add, subtract and negate instructions. Numbers stay numbers, periodt. ✨
-nocpp— tell the MIPS assembler to not run its preprocessor over user assembler files (with a.ssuffix) when assembling them.-mfix-24k,-mno-fix-24k— work around the 24K E48 (lost data on stores during refill) errata. And fun fact: the workarounds are implemented by the assembler rather than by GCC. Not GCC's problem, the assembler handles it. 🫶-mfix-r4000,-mno-fix-r4000— work around certain R4000 CPU errata:- A double-word or a variable shift may give an incorrect result if executed immediately after starting an integer division.
- A double-word or a variable shift may give an incorrect result if it's executed while an integer multiplication is still in progress. 😭
- An integer division may give an incorrect result if it's started in a delay slot of a taken branch or a jump. 💀
-mfix-r4400,-mno-fix-r4400— work around certain R4400 CPU errata, bestie, and here's the tea:- A double-word or a variable shift may give an incorrect result if it's executed immediately after starting an integer division.
-mfix-r10000,-mno-fix-r10000— work around certain R10000 errata, and babe, the list is giving chaos:ll/scsequences may not behave atomically on revisions prior to 3.0. They may deadlock on revisions 2.6 and earlier. 😳
Heads up: this option can only be used if the target architecture supports branch-likely instructions.-mfix-r10000is the default when-march=r10000is used;-mno-fix-r10000is the default otherwise.
-mfix-r5900,-mno-fix-r5900— do not try 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 — all because of a hardware bug in the R5900 chip. The workaround is implemented by the assembler rather than by GCC.-mfix-rm7000,-mno-fix-rm7000— work around the RM7000dmult/dmultuerrata. These workarounds are implemented by the assembler rather than by GCC.-mfix-vr4120,-mno-fix-vr4120— work 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— work 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. slay ✨-mfix-sb1,-mno-fix-sb1— work 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 so this one is a whole saga, let me explain 🫶
In common with many processors, the R10K tries to predict the outcome of a conditional branch and speculatively executes instructions from the "taken" branch. It later aborts these instructions if the predicted outcome is wrong. However, on the R10K, even aborted instructions can have side effects. 😱
This problem only affects kernel stores and, depending on the system, kernel loads. As an example, a speculatively-executed store may load the target memory into cache and mark the cache line as dirty, even if the store itself is later aborted. If a DMA operation writes to the same area of memory before the "dirty" line is flushed, the cached data overwrites the DMA-ed data. See the R10K processor manual for a full description, including other potential problems.
One workaround is to insert cache barrier instructions before every memory access that might be speculatively executed and that might have side effects even if aborted. -mr10k-cache-barrier=setting controls GCC's implementation of this workaround. It assumes that aborted accesses to any byte in the following regions does not have side effects:
- 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.
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 number3(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.-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 to 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.
-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're emitting 64-bit code, then we can use both FP pipes. Otherwise, we can only use one FP pipe. 💅
-mvr4130-align,-mno-vr4130-align— the VR4130 pipeline is two-way superscalar, but can only issue two instructions together if the first one is 8-byte aligned. When this option is enabled, GCC aligns pairs of instructions that it thinks should execute in parallel.
This option only has an effect when optimizing for the VR4130. It normally makes code faster, but at the expense of making it bigger. It is enabled by default at optimization level -O3.
-msynci,-mno-synci— enable (disable) generation 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's generally safe to use synci. However, on many multi-core (SMP) systems, it does not invalidate the instruction caches on all cores and may lead to undefined behavior. 😬
-mrelax-pic-calls,-mno-relax-pic-calls— try to turn PIC calls that are normally dispatched via register$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. With -mno-explicit-relocs, this optimization can be performed by the assembler and the linker alone without help from the compiler.
-mmcount-ra-address,-mno-mcount-ra-address— emit (do not emit) code that allows_mcountto modify the calling function's return address. When enabled, this option extends the usual_mcountinterface with a new ra-address parameter, which has typeintptr_t *and is passed in register$12._mcountcan then modify the return address by doing both of the following:- Returning the new address in register
$31. - Storing the new address in
*ra-address, if ra-address is nonnull.
- Returning the new address in register
The default is -mno-mcount-ra-address.
-mframe-header-opt,-mno-frame-header-opt— enable (disable) frame header optimization in the o32 ABI. When using the o32 ABI, calling functions will allocate 16 bytes on the stack for the called function to write out register arguments. When enabled, this optimization will suppress the allocation of the frame header if it can be determined that it is unused.
This optimization is off by default at all optimization levels. 🙈
-mlxc1-sxc1,-mno-lxc1-sxc1— when applicable, enable (disable) the generation oflwxc1,swxc1,ldxc1,sdxc1instructions. Enabled by default. ✨-mmadd4-mno-madd4When applicable, enable (disable) the generation of 4-operandmadd.s,madd.dand related instructions. Enabled by default, bestie — so it's already on, you don't gotta do anything. 💅
🌟 MMIX Options 🌟
These options are defined for the MMIX:
-mlibfuncs-mno-libfuncsSpecify that intrinsic library functions are being compiled, passing all values in registers, no matter the size. No matter the size, periodt.-mepsilon-mno-epsilonGenerate floating-point comparison instructions that compare with respect to therEepsilon register. ✨-mabi=mmixware-mabi=gnuGenerate code that passes function parameters and return values that (in the called function) are seen as registers$0and up, as opposed to the GNU ABI which uses global registers$231and up. Two whole different vibes for how your args travel, iykyk.-mzero-extend-mno-zero-extendWhen reading data from memory in sizes shorter than 64 bits, use (do not use) zero-extending load instructions by default, rather than sign-extending ones. 🦋-mknuthdiv-mno-knuthdivMake the result of a division yielding a remainder have the same sign as the divisor. With the default,-mno-knuthdiv, the sign of the remainder follows the sign of the dividend. Both methods are arithmetically valid, the latter being almost exclusively used. So like, both are fine, but everyone's doing the other one. 🙈-mtoplevel-symbols-mno-toplevel-symbolsPrepend (do not prepend) a:to all global symbols, so the assembly code can be used with thePREFIXassembly directive. 🎀-melfGenerate an executable in the ELF format, rather than the default mmo format used by the mmix simulator. ELF era unlocked.-mbranch-predict-mno-branch-predictUse (do not use) the probable-branch instructions, when static branch prediction indicates a probable branch. 💜-mbase-addresses-mno-base-addressesGenerate (do not generate) code that uses base addresses. Using a base address automatically generates a request (handled by the assembler and the linker) for a constant to be set up in a global register. The register is used for one or more base address requests within the range 0 to 255 from the value held in the register. The generally leads to short and fast code, but the number of different data items that can be addressed is limited. This means that a program that uses lots of static data may require-mno-base-addresses. 😭-msingle-exit-mno-single-exitForce (do not force) generated code to have a single exit point in each function. 🚪
🌈 MN10300 Options 🌈
These -m options are defined for Matsushita MN10300 architectures:
-mmult-bugGenerate code to avoid bugs in the multiply instructions for the MN10300 processors. This is the default.-mno-mult-bugDo not generate code to avoid bugs in the multiply instructions for the MN10300 processors. Living dangerously, ok.-mam33Generate code using features specific to the AM33 processor. ✨-mno-am33Do not generate code using features specific to the AM33 processor. This is the default.-mam33-2Generate code using features specific to the AM33/2.0 processor. 🦋-mam34Generate code using features specific to the AM34 processor. 👑-mtune=cpu-typeUse the timing characteristics of the indicated CPU type when scheduling instructions. This does not change the targeted processor type. The CPU type must be one ofmn10300,am33,am33-2oram34. Tuning only, bestie — it's giving "how you schedule", not "what you target".-mreturn-pointer-on-d0When 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-crt0Do not link in the C run-time initialization object file. 🙈-mrelaxIndicate to the linker that it should perform a relaxation optimization pass to shorten branches, calls and absolute memory addresses. This option only has an effect when used on the command line for the final link step.
⚠️ This option makes symbolic debugging impossible. Like, no cap. 💀
-mliwAllow the compiler to generate Long Instruction Word instructions if the target is the AM33 or later. This is the default. This option defines the preprocessor macro__LIW__. ✨-mno-liwDo not allow the compiler to generate Long Instruction Word instructions. This option defines the preprocessor macro__NO_LIW__. 🎀-msetlbAllow the compiler to generate the SETLB and Lcc instructions if the target is the AM33 or later. This is the default. This option defines the preprocessor macro__SETLB__. 💜-mno-setlbDo not allow the compiler to generate SETLB or Lcc instructions. This option defines the preprocessor macro__NO_SETLB__. 🦋
🧚 Moxie Options 🧚
-mebGenerate big-endian code. This is the default formoxie-*-*configurations.-melGenerate little-endian code. 🍓-mmul.xGeneratemul.xandumul.xinstructions. This is the default formoxiebox-*-*configurations.-mno-crt0Do not link in the C run-time initialization object file. 🙈
💫 MSP430 Options 💫
These options are defined for the MSP430:
-masm-hexForce assembly output to always use hex constants. Normally such constants are signed decimals, but this option is available for testsuite and/or aesthetic purposes. Purely for the aesthetic, honestly iconic. ✨-mmcu=Select the MCU to target. This is used to create a C preprocessor symbol based upon the MCU name, converted to upper case and pre- and post-fixed with__. This in turn is used by 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 is known to only support the 430 ISA then that is selected, otherwise the 430X ISA is selected. A generic MCU name of msp430 can also be used to select the 430 ISA. Similarly the generic msp430x MCU name selects the 430X ISA. 🦋
In addition an MCU-specific linker script is added to the linker command line. The script's name is the name of the MCU with .ld appended. Thus specifying -mmcu=xxx on the gcc command line defines the C preprocessor symbol __XXX__ and cause the linker to search for a script called xxx.ld. 💅
This option is also passed on to the assembler. 🎀
-mwarn-mcu-mno-warn-mcuThis 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. 💀-msimLink to the simulator runtime libraries and linker script. Overrides any scripts that would be selected by the-mmcu=option. 🌸-mlargeUse large-model addressing (20-bit pointers, 32-bitsize_t). 🔥-msmallUse small-model addressing (16-bit pointers, 16-bitsize_t). 🍓-mrelaxThis option is passed to the assembler and linker, and allows the linker to perform certain optimizations that cannot be done until the final link. ✨mhwmult=Describes the type of hardware multiply supported by the target. Accepted values arenonefor no hardware multiply,16bitfor the original 16-bit-only multiply supported by early MCUs.32bitfor the 16/32-bit multiply supported by later MCUs andf5seriesfor the 16/32-bit multiply supported by F5-series MCUs. A value ofautocan also be given. This tells GCC to deduce the hardware multiply support based upon the MCU name provided by the-mmcuoption. If no-mmcuoption is specified or if the MCU name is not recognized then no hardware multiply support is assumed.autois the default setting. 💜
Hardware multiplies are normally performed by calling a library routine. This saves space in the generated code. When compiling at -O3 or higher however the hardware multiplier is invoked inline. This makes for bigger, but faster code.
The hardware multiply routines disable interrupts whilst running and restore the previous interrupt state when they finish. This makes them safe to use inside interrupt handlers as well as in normal code. 🫶
-minrtEnable the use of a minimum runtime environment - no static initializers or constructors. This is intended for memory-constrained devices. The compiler includes special symbols in some objects that tell the linker and runtime which code fragments are required. 🧚-mcode-region=-mdata-region=These options tell the compiler where to place functions and data that do not have one of thelower,upper,eitherorsectionattributes. Possible values arelower,upper,eitherorany. The first three behave like the corresponding attribute. The fourth possible value -any- is the default. It leaves placement entirely up to the linker script and how it assigns the standard sections (.text,.data, etc) to the memory regions. 🌈-msilicon-errata=This option passes on a request to assembler to enable the fixes for the named silicon errata. Silicon said "oops" and we're fixing it. 😭-msilicon-errata-warn=This option passes on a request to the assembler to enable warning messages when a silicon errata might need to be applied. ⚠️
⭐ NDS32 Options ⭐
These options are defined for NDS32 implementations:
-mbig-endianGenerate code in big-endian mode. 🌸-mlittle-endianGenerate code in little-endian mode. 🦋-mreduced-regsUse reduced-set registers for register allocation. 🎀-mfull-regsUse full-set registers for register allocation. 👑-mcmovGenerate conditional move instructions. ✨-mno-cmovDo not generate conditional move instructions. 🙈-mext-perfGenerate performance extension instructions. 🔥-mno-ext-perfDo not generate performance extension instructions. 💀-mext-perf2Generate performance extension 2 instructions. 💜-mno-ext-perf2Do not generate performance extension 2 instructions. 😭-mext-stringGenerate string extension instructions. 🍓-mno-ext-stringDo not generate string extension instructions. 🌸-mv3pushGenerate v3 push25/pop25 instructions. 💅-mno-v3push— bestie, we are NOT generatingv3 push25/pop25instructions. nope. 🙅-m16-bit— yes generate 16-bit instructions ✨-mno-16-bit— and here's the anti, don't generate 16-bit instructions 💅-misr-vector-size=num— set the size of each interrupt vector. it literally has to be4or16, no in-between, pick one 🎀-mcache-block-size=num— set the size of each cache block. has to be a power of 2, between4and512. so like4,8,16... you get it 🌸-march=arch— say the name of the target architecture out loud, that's it 🦋-mcmodel=code-model— set the code model to one of these, bestie:small— all the data AND read-only data segments have to fit within 512KB addressing space. the text segment has to be within 16MB addressing space. cozy.medium— the data segment has to be within 512KB while the read-only data segment can stretch to within 4GB addressing space. the text segment should still be within 16MB addressing space.large— go big or go home: all the text and data segments can be within 4GB addressing space. 👑
-mctor-dtor— turn on the constructor/destructor feature. iconic. 💖-mrelax— this one just guides the linker to relax instructions. lowkey chill of it. 😌
🌈 Nios II Options 🌈
okay so these are the options defined for the Altera Nios II processor. let's get into it 💜
-G num— put global and static objects that are less than or equal tonumbytes into the small data or BSS sections instead of the normal data or BSS sections. the default value ofnumis8. periodt. 🎀-mgpopt=option-mgpopt-mno-gpopt— generate (or don't 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— same aslocal, but ALSO generate GP-relative accesses for small data objects that are external, weak, or common. > if you use this option, you must make sure that all parts of your program (including libraries) are compiled with the same-Gsetting. no cap. 🔥data— generate GP-relative accesses for ALL data objects in the program. > if you use this, 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, 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. 💀
so -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 one 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. theregexpis 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 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. theregexpis 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 (the default) or big-endian (experimental 👀) code, respectively.-march=arch— this specifies the name of the target Nios II architecture. GCC uses this name to figure out 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 bypasses 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 the"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— each-mcustom-insn=Noption enables use of a custom instruction with encodingNwhen generating code that usesinsn. for example,-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_lroundf"function 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 of X andsrc2into the most significant half of X.fwry— writesrc1into Y.frdxhi,frdxlo— read the most or least (respectively) significant half of X and store it indest.frdy— read the value of Y and 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 literally the same thing as typing-mcustom-fmuls=252 -mcustom-fadds=253 -mcustom-fsubs=254 -fsingle-precision-constant🫶-mcustom-fpu-cfg=60-2is giving-mcustom-fmuls=252 -mcustom-fadds=253 -mcustom-fsubs=254 -mcustom-fdivs=255 -fsingle-precision-constant✨-mcustom-fpu-cfg=72-3is giving-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💅
Custom instruction assignments you hand over with individual -mcustom-insn= options override the ones from -mcustom-fpu-cfg=, no matter what order you put them on the command line. Order does not matter bestie, the individual one wins, periodt. 👑
Also heads up: you can get more local control over picking a FPU configuration by using the target("custom-fpu-cfg=name") function attribute or pragma. 🎀
🌸 Altera Nios II ELF (bare-metal) extras 🌸
These extra -m options are here 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 it's typically used together with-msys-crt0=to point at where the alternate startup code from the HAL BSP lives. 🫶-msmallc— link with a limited version of the C library,-lsmallc, instead of 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, likereadandwrite. 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:
-m32,-m64— generate code for the 32-bit or 64-bit ABI. 👑-misa=ISA-string — generate code for the specified PTX ISA (e.g.sm_35). ISA strings **must be lower-case. Valid ISA strings includesm_30andsm_35. The default ISA issm_30. ✨-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 oralloca), and when global memory is used for underlying storage, it 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 a code generation variant that allows executing 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 arrayint __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 positiontid.ywith the current lane index to compute the master lane index. 🧚-mgomp— generate code for use in OpenMP offloading: enables-msoft-stackand-muniform-simtoptions, and selects the corresponding multilib variant. 💜
🎀 OpenRISC Options 🎀
These options are defined for OpenRISC:
- *
-mboard=name* — configure a board specific runtime. This gets passed to the linker for newlib board library linking. The default is"or1ksim". 🌸 -mnewlib— for compatibility, it's always newlib for elf now. 💀-mhard-div— generate code for hardware which supports divide instructions. This is the default. 👑-mhard-mul— generate code for hardware which supports multiply instructions. This is the default.-mcmov— generate code for hardware which supports the conditional move (l.cmov) instruction. ✨-mror— generate code for hardware which supports rotate right instructions.-msext— generate code for hardware which supports sign-extension instructions.-msfimm— generate code for hardware which supports set flag immediate (l.sf*i) instructions.-mshftimm— generate code for hardware which supports shift immediate related instructions (i.e.l.srai,l.srli,l.slli,1.rori). Note: to enable generation of thel.roriinstruction the-mrorflag must also be specified. 🫶-msoft-div— generate code for hardware which requires divide instruction emulation.-msoft-mul— generate code for hardware which requires multiply instruction emulation.
🌈 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 inac0(fr0in 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 old"reload"allocator 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's 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
⭐ 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— turn hardware integer division instructions on or off 💅. Lowkey needs the M extension, bestie. The default? Use them if the architecture you asked for actually has them. Simple as that ✨.-march=ISA-string — generate code for the given RISC-V ISA (likerv64im). The ISA string has to be lower-case, no cap 🙅♀️. Examples includerv64i,rv32g,rv32e, andrv32imaf.-mtune=processor-string — optimize the output for the given processor, specified by microarchitecture name 🎀. The permissible values for this option are:rocket,sifive-3-series,sifive-5-series,sifive-7-series, andsize.
When -mtune= isn't specified, the default is rocket 🚀.
The size choice is not intended for end-users, ok? It's what gets used when -Os is specified. It overrides the instruction cost info that -mtune= provides, but it does not override the pipeline info. That's the whole point: shrink the code size while still keeping good performance 💖.
-mpreferred-stack-boundary=num — attempt to keep the stack boundary aligned to a2raised to num byte boundary. If-mpreferred-stack-boundaryisn't specified, the default is4(that's 16 bytes, or 128-bits, iykyk) 🌸.
Warning bestie 🚨: 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. No mixing, fr fr.
-msmall-data-limit=n — put global and static data smaller than n bytes into a special section (on some targets) 📦.-msave-restore,-mno-save-restore— do or don't use smaller but slower prologue and epilogue code that uses library function calls 🦋. The default is to use fast inline prologues and epilogues — fast over smol, periodt.-mstrict-align,-mno-strict-align— do not or do generate unaligned memory accesses 😭 (yes the wording is like that, don't come for me). The default is set depending on whether the processor we're optimizing for supports fast unaligned access or not.-mcmodel=medlow— generate code for the medium-low code model ✨. The program and its statically defined symbols must lie within a single2 GiBaddress range and must lie between absolute addresses-2 GiBand+2 GiB. Programs can be statically or dynamically linked. This 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 single2 GiBaddress range. Programs can be statically or dynamically linked.-mexplicit-relocs,-mno-exlicit-relocs— use or do not use assembler relocation operators when dealing with symbolic addresses 🎯. The alternative is to use assembler macros instead, which may limit optimization. (And yes, that second flag is spelled-mno-exlicit-relocsup in here — copy it exactly, no autocorrecting 💀.)-mrelax,-mno-relax— take advantage of linker relaxations to reduce the number of instructions required to materialize symbol addresses 🌈. The default is to take advantage of linker relaxations — free wins, we love.-memit-attribute,-mno-emit-attribute— emit (or don't emit) RISC-V attribute to record extra information into ELF objects 📝. This feature requires at least binutils2.32.
🧚 RL78 Options 🧚
-msim— links in additional target libraries to support operation within a simulator 🖥️✨.-mmul=none,-mmul=g10,-mmul=g13,-mmul=g14,-mmul=rl78— specifies the type of hardware multiplication and division support to be used 💅. The simplest is"none", which uses software for both multiplication and division. This is the default. The"g13"value is for the hardware multiply/divide peripheral found on the RL78/G13 (S2 core) targets. The"g14"value selects the use of the multiplication and division instructions supported by the RL78/G14 (S3 core) parts. The value"rl78"is an alias for"g14"and the value"mg10"is an alias for"none".
In addition a C preprocessor macro is defined, based upon the setting of this option 🎀. Possible values are: "__RL78_MUL_NONE__", "__RL78_MUL_G13__" or "__RL78_MUL_G14__".
-mcpu=g10,-mcpu=g13,-mcpu=g14,-mcpu=rl78— specifies the RL78 core to target 🦋. The default is the G14 core, also known as an S3 core or just RL78. The G13 or S2 core does not have multiply or divide instructions — instead it uses a hardware peripheral for these operations. The G10 or S1 core does not have register banks, so it uses a different calling convention 👀.
If this option is set it also selects the type of hardware multiply support to use, unless this is overridden by an explicit -mmul=none option on the command line. So specifying -mcpu=g13 enables the use of the G13 hardware multiply peripheral, and specifying -mcpu=g10 disables the use of hardware multiplications altogether.
Note 📌, although the RL78/G14 core is the default target, specifying -mcpu=g14 or -mcpu=rl78 on the command line does change the behavior of the toolchain, since it also enables G14 hardware multiply support. If these options are not specified on the command line then software multiplication routines will be used even though the code targets the RL78 core. This is for backwards compatibility with older toolchains which did not have hardware multiply and divide support 🙈.
In addition a C preprocessor macro is defined, based upon the setting of this option 🌸. Possible values are: "__RL78_G10__", "__RL78_G13__" or "__RL78_G14__".
-mg10,-mg13,-mg14,-mrl78— these are aliases for the corresponding-mcpu=option 💖. They are provided for backwards compatibility.-mallregs— allow the compiler to use all of the available registers 🎉. By default registers"r24..r31"are reserved for use in interrupt handlers. With this option enabled these registers can be used in ordinary functions as well.-m64bit-doubles,-m32bit-doubles— make the"double"data type be 64 bits (-m64bit-doubles) or 32 bits (-m32bit-doubles) in size 💜. The default is-m32bit-doubles.-msave-mduc-in-interrupts,-mno-save-mduc-in-interrupts— specifies that interrupt handler functions should preserve the MDUC registers 🛡️. This is only necessary if normal code might use the MDUC registers, for example because it performs multiplication and division operations. The default is to ignore the MDUC registers as this makes the interrupt handlers faster ⚡. The target option-mg13needs to be passed for this to work, as this feature is only available on the G13 target (S2 core). The MDUC registers will only be saved if the interrupt handler performs a multiplication or division operation or it calls another function.
👑 IBM RS/6000 and PowerPC Options 👑
These -m options are defined for the IBM RS/6000 and PowerPC 💅:
-mpowerpc-gpopt,-mno-powerpc-gpopt-mpowerpc-gfxopt,-mno-powerpc-gfxopt-mpowerpc64,-mno-powerpc64-mmfcrf,-mno-mfcrf-mpopcntb,-mno-popcntb-mpopcntd,-mno-popcntd-mfprnd,-mno-fprnd-mcmpb,-mno-cmpb-mmfpgpr,-mno-mfpgpr-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 configuring GCC. Specifying the -mcpu=cpu_type overrides the specification of these options. We recommend you use the -mcpu=cpu_type option rather than the options listed above — trust, it's way less chaos 🫶.
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 -mmfpgpr option allows GCC to generate the FP move to/from general-purpose register instructions implemented on the POWER6X processor and other processors that support the extended PowerPC V2.05 architecture. The -mhard-dfp option allows GCC to generate the decimal floating-point instructions implemented on some POWER processors 🔥.
The -mpowerpc64 option allows GCC to generate the additional 64-bit instructions that are found in the full PowerPC64 architecture and to treat GPRs as 64-bit, doubleword quantities 💪. GCC defaults to -mno-powerpc64.
-mcpu=cpu_type — set architecture type, register usage, and instruction scheduling parameters for machine type cpu_type 🎀. Supported values for cpu_type are401,403,405,405fp,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,powerpc,powerpc64,powerpc64le,rs64, andnative🌈.
-mcpu=powerpc, -mcpu=powerpc64, and -mcpu=powerpc64le specify pure 32-bit PowerPC (either endian), 64-bit big endian PowerPC and 64-bit little endian PowerPC architecture machine types, with an appropriate, generic processor model assumed for scheduling purposes 👑
Specifying native as cpu type detects and selects the architecture option that corresponds to the host processor of the system performing the compilation 🔍. -mcpu=native has no effect if GCC does not recognize the processor 🙈.
The other options specify a specific processor. Code generated under those options runs best on that processor, and may not run at all on others 💀.
The -mcpu options automatically enable or disable the following options:
-maltivec -mfprnd -mhard-float -mmfcrf -mmultiple -mpopcntb
-mpopcntd -mpowerpc64 -mpowerpc-gpopt -mpowerpc-gfxopt -mmulhw
-mdlmzb -mmfpgpr -mvsx -mcrypto -mhtm -mpower8-fusion
-mpower8-vector -mquad-memory -mquad-memory-atomic -mfloat128
-mfloat128-hardwareokay so real talk: the exact set of options you get for any given CPU moves around between compiler versions — it literally depends on whatever setting the devs decided makes the nicest code for that chip. So don't read it as "this is what the hardware can do", because it is not that. 🙈 And if you want one specific option set to one specific value? Just drop it in after</> the -mcpu option, like this:
-mcpu=970 -mno-altivecso clean, so easy, slay. 💅
Oh and heads up: on AIX, -maltivec and -mpowerpc64 are not turned on or off by -mcpu rn, because AIX doesn't fully support them. But bestie, you can still flip them yourself one at a time if you're <em>sure</em> it'll work in your environment. 👑
-mtune=cpu_type— sets the instruction scheduling parameters for machine type cpu_type, but doesn't touch the architecture type or the register usage the way-mcpu=cpu_typedoes. Same cpu_type values work for-mtuneas for-mcpu. If you specify both, the code you get uses the architecture + registers from-mcpu, but the scheduling params from-mtune. teamwork fr fr 🫶-mcmodel=small— generates PowerPC64 code for the small model: the TOC is capped at64k.-mcmodel=medium— generates PowerPC64 code for the medium model: the TOC and other static data can total up to4Gin size. This is the default for 64-bit Linux.-mcmodel=large— generates PowerPC64 code for the large model: the TOC can be up to4G, and other data and code are only limited by the 64-bit address space. no cap, literally. ✨-maltivec,-mno-altivec— generate code that does (does not) use AltiVec instructions, and also lets you use built-in functions for more direct access to the AltiVec instruction set. You may also need to set-mabi=altivecto tweak the current ABI with AltiVec ABI enhancements.
When -maltivec is used, the element order for AltiVec intrinsics like vec_splat, vec_extract, and vec_insert matches array element order for the target's endianness. Translation: element zero is the leftmost element in a vector register on a big-endian platform, and the rightmost element in a vector register on a little-endian platform. 🌈
-mvrsave,-mno-vrsave— generates VRSAVE instructions when generating AltiVec code.-msecure-plt— generates code that letsldandld.sobuild executables and shared libraries with non-executable.pltand.gotsections. This is a PowerPC 32-bit SYSV ABI option. 🔒-mbss-plt— generates code that uses a BSS.pltsection thatld.sofills in, and needs.pltand.gotsections that are both writable and executable. This is a PowerPC 32-bit SYSV ABI option.-misel,-mno-isel— turns the generation of ISEL instructions on or off. switch flips, that's it. 🎀-mvsx,-mno-vsx— generate code that does (does not) use vector/scalar (VSX) instructions, and also enable built-in functions for more direct access to the VSX instruction set.-mcrypto,-mno-crypto— enable (disable) the built-in functions that give direct access to the cryptographic instructions added in version2.07of the PowerPC ISA. 🔐-mhtm,-mno-htm— enable (disable) the built-in functions for direct access to the Hardware Transactional Memory (HTM) instructions added in version2.07of the PowerPC ISA.-mpower8-fusion,-mno-power8-fusion— generate code that keeps (does not keep) some integer operations next to each other so the instructions can be fused together on power8 and later processors. bestie behaviour honestly 🫶-mpower8-vector,-mno-power8-vector— generate code that does (does not) use the vector and scalar instructions added in version2.07of the PowerPC ISA. Also enables built-in functions for more direct access to the vector instructions.-mquad-memory,-mno-quad-memory— generate code that does (does not) use the non-atomic quad word memory instructions. ⚠️-mquad-memoryrequires you to be in 64-bit mode.-mquad-memory-atomic,-mno-quad-memory-atomic— generate code that does (does not) use the atomic quad word memory instructions. ⚠️-mquad-memory-atomicalso requires 64-bit mode.-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.
The VSX instruction set (-mvsx, -mcpu=power7, -mcpu=power8), or -mcpu=power9, must be enabled to use IEEE 128-bit floating point support. And this support only works on PowerPC Linux systems.
The default for -mfloat128 is enabled on PowerPC Linux systems using the VSX instruction set, and disabled on other systems.
If you use the ISA 3.0 instruction set (-mpower9-vector or -mcpu=power9) on a 64-bit system, the IEEE 128-bit floating point support will also turn on generation of ISA 3.0 IEEE 128-bit floating point instructions. Otherwise — if you don't specify ISA 3.0 instruction generation, or you're targeting a 32-bit big endian system — IEEE 128-bit floating point gets done with software emulation. 💫
-mfloat128-hardware,-mno-float128-hardware— enable/disable using ISA 3.0 hardware instructions to support the__float128data type.
The default for -mfloat128-hardware is enabled on PowerPC Linux systems using the ISA 3.0 instruction set, and disabled on other systems.
-m32,-m64— generate code for 32-bit or 64-bit environments of Darwin and SVR4 targets (GNU/Linux included, bestie 🐧). 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, same as-mpowerpc64.-mfull-toc,-mno-fp-in-toc,-mno-sum-in-toc,-mminimal-toc— these mess with how the TOC (Table Of Contents) gets generated, and the TOC is created for every executable file.-mfull-tocis the default. In that case GCC allocates at least one TOC entry for every unique non-automatic variable reference in your program. GCC also drops floating-point constants in the TOC. But only16,384entries are available in the TOC. 💀
If the linker yells at you that you've overflowed the TOC space, you can shrink how much TOC space you're using with -mno-fp-in-toc and -mno-sum-in-toc. -mno-fp-in-toc stops GCC from putting floating-point constants in the TOC, and -mno-sum-in-toc forces GCC to generate code that calculates the sum of an address and a constant at run time instead of parking that sum in the TOC. You can specify one or both. Each one makes GCC produce very slightly slower and larger code, in exchange for conserving TOC space. trade-offs, bestie, always. 😭
If you still run out of TOC space even with both of those, use -mminimal-toc instead. This one makes GCC create only one TOC entry per file. And yeah, the code gets slower and larger, but it uses extremely little TOC space. Maybe only reach for this on files that contain less frequently-executed code. 🧚
-maix64,-maix32— enable 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bitlongtype, and all the infrastructure needed to support them.-maix64implies-mpowerpc64, while-maix32disables the 64-bit ABI and implies-mno-powerpc64. GCC defaults to-maix32.-mxl-compat,-mno-xl-compat— produce code that conforms more closely to IBM XL compiler semantics when using the AIX-compatible ABI. Passes floating-point arguments to prototyped functions beyond the register save area (RSA) on the stack in addition to argument FPRs. Doesn't assume the most significant double in a 128-bitlong doublevalue is properly rounded when comparing values and converting todouble. Uses XL symbol names forlong doublesupport routines.
Storytime 📖: the AIX calling convention was extended but not initially documented to handle an obscure K&R C case — calling a function that takes the address of its arguments with fewer arguments than declared. IBM XL compilers access floating-point arguments that don't fit in the RSA from the stack when a subroutine is compiled without optimization. And because always storing floating-point arguments on the stack is inefficient and rarely needed, this option is not enabled by default and is only necessary when you're calling subroutines compiled by IBM XL compilers without optimization. iykyk. 💅
-mpe— support IBM RS/6000 SP Parallel Environment (PE). Links an application written to use message passing with special startup code so the app can run. The system must have PE installed in the standard location/usr/lpp/ppe.poe/, or the specs file has to be overridden with the-specs=option to point at the right directory. 🚨 The Parallel Environment does not support threads, so-mpeand-pthreadare incompatible. pick one, bestie, you cannot have both. 😤-malign-natural,-malign-power— on AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux,-malign-naturaloverrides the ABI-defined alignment of larger types (floating-point doubles, for example) onto their natural size-based boundary.-malign-powertells GCC to follow the ABI-specified alignment rules instead. GCC defaults to the standard alignment defined in the ABI.
On 64-bit Darwin, natural alignment is the default, and -malign-power is not supported. 🙈
-msoft-float,-mhard-float— generate code that does not use (does use) the floating-point register set. Software floating-point emulation is provided if you use-msoft-float— and you gotta pass the option to GCC when you link, too. 🫶-mmultiple,-mno-multiple— tells the compiler to generate code that uses (or does not use, if you pick the-mno-one) the load multiple word instructions and the store multiple word instructions. Bestie, these are generated by default on POWER systems, and not generated on PowerPC systems. 🫶
Do not use -mmultiple on little-endian PowerPC systems, because those instructions literally do not work when the processor is in little-endian mode. The only exceptions are PPC740 and PPC750, which do permit these instructions in little-endian mode. ✨
-mupdate,-mno-update— generate code that uses (or doesn't 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.
Heads up though — if you use -mno-update, there's a small window between the time the stack pointer gets 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. 💀 Not the vibe you want.
-mavoid-indexed-addresses,-mno-avoid-indexed-addresses— generate code that tries to avoid (or, like, not avoid) using indexed load or store instructions. These instructions can incur a performance penalty on Power6 processors in certain situations, like when you're 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 (doesn't use) the floating-point multiply and accumulate instructions. These are generated by default if hardware floating point is used.
Real talk: the machine-dependent -mfused-madd option is now mapped to the machine-independent -ffp-contract=fast option, and -mno-fused-madd is mapped to -ffp-contract=off.
-mmulhw,-mno-mulhw— generate code that uses (doesn't 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 (doesn't 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 (or do, with-mbit-align) force structures and unions that contain bit-fields to be aligned to the base type of the bit-field.
For example, by default a structure containing nothing but 8 unsigned bit-fields of length 1 is aligned to a 4-byte boundary and has a size of 4 bytes. By using -mno-bit-align, the structure is aligned to a 1-byte boundary and is 1 byte in size. 🌸
-mno-strict-align,-mstrict-align— on System V.4 and embedded PowerPC systems, do not (or do) assume that unaligned memory references are handled by the system.-mrelocatable,-mno-relocatable— generate code that allows (doesn't 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 — that's 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. Also,-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 (or 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 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 arealtivec,no-altivec,ibmlongdouble,ieeelongdouble,elfv1,elfv2.-mabi=ibmlongdouble— change the current ABI to use IBM extended-precisionlong double.
This is not likely to work if your system defaults to using IEEE extended-precisionlong 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-precisionlong double.
This is not likely to work if your system defaults to using IBM extended-precisionlong 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. 💀
-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— okay so on embedded PowerPC systems this assumes the startup module is calledcrt0.oand the standard C libraries arelibads.aandlibc.a. slay, that's it 💅-myellowknife— same vibe, but on embedded PowerPC systems it assumes the startup module is calledcrt0.oand the standard C libraries arelibyk.aandlibc.a. not me remembering both 🫶-mvxworks— on System V.4 and embedded PowerPC systems, this says you're compiling for a VxWorks system. periodt.-memb— on embedded PowerPC systems, this sets 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 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 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. so if you're in your EABI era, you know ✨-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. 🚨 incompatible means pick one, bestie-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. default behavior said “choose your fighter” 😭-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. lowkey the no-r13 twist 👀-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. simple, clean, no small-data drama 💅-mreadonly-in-sdata— put read-only objects in the.sdatasection as well. This is the default. default queen 👑-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. ⚠️ same value everywhere or chaos-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. symbolic forms slay 💅-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 indlopen'dshared 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. 🚫 linking those is a no-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. 😭 precision said “not today”-mrecip=opt— this option controls which reciprocal estimate instructions may be used. opt is a comma-separated list of options, which may be preceded by a!to invert the option:all— enable all estimate instructions.default— enable the default instructions, equivalent to-mrecip.none— disable all estimate instructions, equivalent to-mno-recip.div— enable the reciprocal approximation instructions for both single and double precision.divf— enable the single-precision reciprocal approximation instructions.divd— enable the double-precision reciprocal approximation instructions.rsqrt— enable the reciprocal square root approximation instructions for both single and double precision.rsqrtf— enable the single-precision reciprocal square root approximation instructions.rsqrtd— enable the double-precision reciprocal square root approximation instructions.
So, for example, -mrecip=all,!rsqrtd enables all of the reciprocal estimate instructions, except for the FRSQRTE, XSRSQRTEDP, and XVRSQRTEDP instructions which handle the double-precision reciprocal square root calculations. iykyk 💅
-mrecip-precision,-mno-recip-precision— assume (do not assume) that the reciprocal estimate instructions provide higher-precision estimates than is mandated by the PowerPC ABI. Selecting-mcpu=power6,-mcpu=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— picks the ABI type for vectorizing intrinsics through an external library 🧚. The onlytypesupported rn ismass, which tells GCC to use IBM's Mathematical Acceleration Subsystem (MASS) libraries for vectorizing intrinsics with external libraries. When it's generating code forpower7, 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 tanhf4Both -ftree-vectorize and -funsafe-math-optimizations must also be enabled — like, no skipping steps bestie. And the MASS libraries must be specified at link time 💅.
-mfriz,-mno-friz— generate (or don't generate) thefrizinstruction when-funsafe-math-optimizationsis used to optimize rounding of floating-point values to 64-bit integer and back to floating point. Heads up though: thefrizinstruction 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 (or don't 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, the TOC value to be loaded in registerr2, and the static chain value to be loaded in registerr11. The-mpointers-to-nested-functionsis on by default 🫶.
Real talk: 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. That's not a vibe, that's a wall.
-msave-toc-indirect,-mno-save-toc-indirect— generate (or don't 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 (or don't 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 a canary atguard🐤. 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.
-mstack-protector-guard-symbol=symbol overrides the offset with a symbol reference to a canary in the TLS block.
✨ RX Options ✨
These command-line options are defined for RX targets 💖:
-m64bit-doubles,-m32bit-doubles— make thedoubledata 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— the hardware literally can't do the other one, bestie 🙈.-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 isRX600.
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 the 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 (usuallyr13) 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 an 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 valueNcan be between 0 and 4. A value of 1 means that registerr13is reserved for the exclusive use of fast interrupt handlers. A value of 2 reservesr13andr12. A value of 3 reservesr13,r12andr11, and a value of 4 reservesr13throughr10. A value of 0, the default, does not reserve any registers.-msave-acc-in-interrupts— specifies that interrupt handler functions should preserve the accumulator register. This is only necessary if normal code might use the accumulator register, for example because it performs 64-bit multiplications. The default is to ignore the accumulator as this makes the interrupt handlers faster 🏎️.-mpid,-mno-pid— enables the generation of position independent data. When enabled, any access to constant data is done via an offset from a base address held in a register. This allows the location of constant data to be determined at run time without requiring the executable to be relocated, which is a benefit to embedded applications with tight memory constraints. Data that can be modified is not affected by this option.
Note, using this feature reserves a register, usually r13, for the constant data base address. This can result in slower and/or larger code, especially in complicated functions 😮💨.
The actual register chosen to hold the constant data base address depends upon whether the -msmall-data-limit and/or the -mint-register command-line options are enabled. Starting with register r13 and proceeding downwards, registers are allocated first to satisfy the requirements of -mint-register, then -mpid and finally -msmall-data-limit. Thus it is possible for the small data area register to be r8 if both -mint-register=4 and -mpid are specified on the command line.
By default this feature is not enabled. The default can be restored via the -mno-pid command-line option.
-mno-warn-multiple-fast-interrupts,-mwarn-multiple-fast-interrupts— prevents GCC from issuing a warning message if it finds more than one fast interrupt handler when it is compiling a file. The default is to issue a warning for each extra fast interrupt handler found, as the RX only supports one such interrupt 🌈.-mallow-string-insns,-mno-allow-string-insns— toggles whether GCC gets to use the string manipulation instructionsSMOVF,SCMPU,SMOVB,SMOVU,SUNTILSWHILEand also theRMPAinstruction. Bestie, these instructions may prefetch data, and that is not safe if you're accessing an I/O register. (See section 12.2.7 of the RX62N Group User's Manual for more info, iykyk.)
The default is to allow these instructions, but here's the thing — it is not possible for GCC to reliably detect all circumstances where a string instruction might be used to access an I/O register, so their use cannot be disabled automatically. Instead it's all on the programmer to use the -mno-allow-string-insns option if their program accesses I/O space. No cap. 💅
When the instructions are enabled GCC defines the C preprocessor symbol __RX_ALLOW_STRING_INSNS__, otherwise it defines the symbol __RX_DISALLOW_STRING_INSNS__. Cute, right?
-mjsr,-mno-jsr— use only (or not only)JSRinstructions to access functions. This option can be used 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. The way that's a different thing entirely 😭
Note: the generic GCC command-line option-ffixed-reghas special significance to the RX port when used with the"interrupt"function attribute. This attribute indicates a function intended to process fast interrupts. GCC ensures that it only uses the registersr10,r11,r12and/orr13, and only provided that the normal use of the corresponding registers have been restricted via the-ffixed-regor-mint-registercommand-line options. Register discipline is the vibe. ✨
🌈 S/390 and zSeries Options 🌈
These are the -m options defined for the S/390 and zSeries architecture. Big mainframe energy, we love to see it. 👑
-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 do not 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 -mmo-backchain; however, use of the backchain for debugging purposes usually requires that the whole binary is built with -mbackchain. Note that the combination of -mbackchain, -mpacked-stack and -mhard-float is not supported. In order to build a linux kernel use -msoft-float.
The default is to not maintain the backchain.
-mpacked-stack,-mno-packed-stack— use (do not use) the packed stack layout. When-mno-packed-stackis specified, the compiler uses the all fields of the 96/160 byte register save area only for their default purpose; unused fields still take up stack space 😭. When-mpacked-stackis specified, register save slots are densely packed at the top of the register save area; unused space is reused for other purposes, allowing for more efficient use of the available stack space. However, when-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. Efficiency queen. ✨
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. Small but mighty... until it isn't. 💀-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. ABI drama, we're obsessed. 💅-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, 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* — tune to cpu-type everything applicable about the generated code, except for the ABI and the set of available instructions. The list of cpu-type values is the same as for-march. The default is the value used for-march. -mtpf-trace,-mno-tpf-trace— generate code that adds (does not add) in TPF OS specific branches to trace routines in the operating system. This option is off by default, even when compiling for the TPF OS.-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. Slay. 🔥- *
-mwarn-framesize=framesize* — emit a warning if the current function exceeds the given frame size. Because this is a compile-time check it doesn't need to be a real problem when the program runs. It is intended to identify functions that most probably cause a stack overflow. It is useful to be used in an environment with limited stack size e.g. the linux kernel. Heads up bestie, we're watching your frames. 👀 -mwarn-dynamicstack— emit a warning if the function callsallocaor uses dynamically-sized arrays. This is generally a bad idea with a limited stack size. 😭-mstack-guard=stack-guard,-mstack-size=stack-size — bestie, if you hand these two over, the S/390 back end drops extra instructions into the function prologue that fire a trap whenever the stack size climbs stack-guard bytes above stack-size (remember: the stack on S/390 grows downward, it's giving basement energy 🕳️). If you skip-mstack-guard, it just picks the smallest power of 2 that's bigger than the frame size of the compiled function. These are here to help you debug stack overflow problems — the extra code costs only a little overhead, so honestly you can run it on production-like systems without some huge performance degradation. But heads up: the values have to be exact powers of 2, and stack-size has to be greater than stack-guard without going over 64k. To stay efficient, that extra code assumes the stack starts at an address aligned to whatever value you gave stack-size. And-mstack-guardcan only be used alongside-mstack-size— they're a package deal 💖.-mhotpatch=pre-halfwords,post-halfwords — turn this on and a "hot-patching" function prologue gets generated for all functions in the compilation unit ✨. The function label gets prepended with the given number of two-byteNOPinstructions (pre-halfwords, max 1000000). After the label, 2 * post-halfwords bytes get appended, using the largest NOP-like instructions the architecture allows (post-halfwords, max 1000000).
If both arguments are zero, hotpatching is disabled. Periodt.
You can override this for individual functions with the "hotpatch" attribute 👑
🌸 Score Options 🌸
These ones 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 of"bcnz"instructions.-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, and there are... a lot of them. Buckle up bestie, we're going down the whole list 💅:
-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. Note that this changes the calling conventions, and thus some functions from the standard C library do not work unless you recompile it first with-mdalign⚠️.-mrelax— Shorten some address references at link time, when possible; uses the linker option-relax.-mbigtable— Use 32-bit offsets in"switch"tables. The default is to use 16-bit offsets.-mbitops— Enable the use of bit manipulation instructions on SH2A.-mfmovd— Enable the use of the instruction"fmovd". Check-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 the"MAC"register as call-clobbered, even if-mrenesasis given.-mieee,-mno-ieee— Control the IEEE compliance of floating-point comparisons, which affects the handling of cases where the result of a comparison is unordered. By default-mieeeis implicitly enabled. If-ffinite-math-onlyis enabled,-mno-ieeeis implicitly set, which results in 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 the"icbi"instruction. If the selected code generation option does not allow the use of the"icbi"instruction, and-musermodeis not in effect, the inlined code manipulates the instruction cache address array directly with an associative write. This not only requires privileged mode at run time, but it also fails if the cache line had been mapped via the TLB and has become unmapped 😭.-misize— Dump instruction size and location in the assembly code.-mpadstruct— This option is deprecated 💀. It pads structures to multiple of 4 bytes, which is incompatible with the SH ABI.-matomic-model=model — Sets the model of atomic operations and additional parameters as a comma separated list. For details on the atomic built-in functions see __atomic Builtins. The following models and parameters are supported:none— Disable compiler generated atomic sequences and emit library calls for atomic operations. This is the default if the target is not"sh*-*-linux*".soft-gusa— Generate GNU/Linux compatible gUSA software atomic sequences for the atomic built-in functions. The generated atomic sequences require additional support from the interrupt/exception handling code of the system and are only suitable for SH3* and SH4* single-core systems. This option is enabled by default when the target is"sh*-*-linux*"and SH3* or SH4*. When the target is SH4A, this option also partially utilizes the hardware atomic instructions"movli.l"and"movco.l"to create more efficient code, unless strict is specified.soft-tcb— Generate software atomic sequences that use a variable in the thread control block. This is a variation of the gUSA sequences which can also be used on SH1* and SH2* targets. The generated atomic sequences require additional support from the interrupt/exception handling code of the system and are only suitable for single-core systems. When using this model, the gbr-offset= parameter has to be specified as well.soft-imask— Generate software atomic sequences that temporarily disable interrupts by setting"SR.IMASK = 1111". This model works only when the program runs in privileged mode and is only suitable for single-core systems. Additional support from the interrupt/exception handling code of the system is not required. This model is enabled by default when the target is"sh*-*-linux*"and SH1* or SH2*.hard-llcs— Generate hardware atomic sequences using the"movli.l"and"movco.l"instructions only. This is only available on SH4A and is suitable for multi-core systems. Since the hardware instructions support only 32 bit atomic variables, access to 8 or 16 bit variables is emulated with 32 bit accesses. Code compiled with this option is also compatible with other software atomic model interrupt/exception handling systems if executed on an SH4A system. Additional support from the interrupt/exception handling code of the system is not required for this model ✨.
gbr-offset=— this one sets the offset, in bytes, of the variable inside the thread control block structure that the generated atomic sequences should use when you've gone with the soft-tcb model 🧵. Any other model? It just gets ignored, bestie. And the value has to be an integer multiple of four, chillin' in the range 0-1020.strict— no mixing allowed!! 🚫 This stops you from blending multiple atomic models even if they're compatible, and makes the compiler spit out atomic sequences for the model you specified and only that model. periodt.
-mtas— generates thetas.bopcode for__atomic_test_and_set. ⚠️ Heads up though: depending on your exact hardware and software configuration, this can degrade overall performance, because of the operand cache line flushes that thetas.binstruction implies. And on multi-core SH4A processors,tas.bmust be used with caution since it can result in data corruption for certain cache configurations. scary!! 💀-mprefergot— when you're generating position-independent code, emit function calls using the Global Offset Table instead of the Procedure Linkage Table. ✨-musermode,-mno-usermode— don't allow (allow) the compiler to generate privileged mode code. Specifying-musermodealso implies-mno-inline-ic_invalidateif the inlined code wouldn't work in user mode.-musermodeis the default when the target issh*-*-linux*. And if the target is SH1* or SH2*,-musermodehas no effect, since there's no user mode. 💅-multcost=number— set the cost to assume for a multiply insn.-mdiv=strategy— pick 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 instructiondiv1to 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 tocall-div1.call-table— calls a library function that uses a lookup table for small divisors and thediv1instruction 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 tocall-div1.
When a division strategy hasn't been specified, the default strategy gets selected based on the current target. For SH2A the default strategy is to use the divs and divu instructions instead of library function calls.
-maccumulate-outgoing-args— reserve space once for outgoing arguments in the function prologue rather than around each call. Generally beneficial for performance and size. Also needed for unwinding, so the stack frame doesn't change around conditional code. 🫶-mdivsi3_libfunc=name— set the name of the library function used for 32-bit signed division to name. This only affects the name used in the call division strategies, and the compiler still expects the same sets of input/output/clobbered registers as if this option were not present.-mfixed-range=register-range— generate code treating the given register range as fixed registers. A fixed register is one that the register allocator cannot use. Super handy when you're compiling kernel code. A register range is specified as two registers separated by a dash. And multiple register ranges can be specified, separated by a comma.-mbranch-cost=num— assume num to be the cost for a branch instruction. Higher numbers make the compiler try to generate more branch-free code if possible. If you don't specify it, the value gets selected depending on the processor type that's being compiled for.-mzdcbranch,-mno-zdcbranch— assume (do not assume) that zero displacement conditional branch instructionsbtandbfare 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 anopif a suitable instruction can't be found. By default this option is disabled. You can enable it to work around hardware bugs, like the ones found in the original SH7055. 🔧-mfused-madd,-mno-fused-madd— generate code that uses (does not use) the floating-point multiply and accumulate instructions. These instructions get 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.-mfsca,-mno-fsca— allow or disallow the compiler to emit thefscainstruction for sine and cosine approximations. The option-mfscamust be used in combination with-funsafe-math-optimizations. It's enabled by default when generating code for SH4A. Using-mno-fscadisables sine and cosine approximations even if-funsafe-math-optimizationsis in effect.-mfsrra,-mno-fsrra— allow or disallow the compiler to emit thefsrrainstruction for reciprocal square root approximations. The option-mfsrramust be used in combination with-funsafe-math-optimizationsand-ffinite-math-only. It's 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.-mpretend-cmove— prefer zero-displacement conditional branches for conditional move instruction patterns. This can result in faster code on the SH4 processor. 🏎️-mfdpic— generate code using the FDPIC ABI.
☀️ Solaris 2 Options ☀️
These -m options are supported on Solaris 2:
-mclear-hwcap— tells the compiler to remove the hardware capabilities generated by the Solaris assembler. This is only necessary when object files use ISA extensions that aren't supported by the current machine, but check at runtime whether or not to use them.-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. So instead of using -mimpure-text, you should compile all source code with -fpic or -fPIC. 💅
These switches are supported in addition to the above on Solaris 2:
-pthreads— this is a synonym for-pthread.
🌸 SPARC Options 🌸
These -m options are supported on the SPARC:
-mno-app-regs,-mapp-regs— specify-mapp-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's clobbered by function calls. This is the default.
To be fully SVR4 ABI-compliant at the cost of some performance loss, specify -mno-app-regs. You should compile libraries and system software with this option.
-mflat,-mno-flat— with-mflat, the compiler does not generate save/restore instructions and uses a "flat" or single register window model. This model is compatible with the regular register window model. The local registers and the input registers (0--5) are still treated as "call-saved" registers and get saved on the stack as needed.
With -mno-flat (the default), the compiler generates save/restore instructions (except for leaf functions). This is the normal operating mode. 🍃
-mfpu,-mhard-float— generate output containing floating-point instructions. This is the default. ✨-mno-fpu,-msoft-float— generate output containing library calls for floating point. ⚠️ Warning: the requisite libraries are not available for all SPARC targets. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation. The embedded targetssparc-*-aoutandsparclite-*-*do provide software floating-point support.
-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.
-mhard-quad-float— generate output containing quad-word (long double) floating-point instructions.-msoft-quad-float— generate output containing library calls for quad-word (long double) floating-point instructions. The functions called are those specified in the SPARC ABI. This is the default.
As of this writing, there are no SPARC implementations that have hardware support for the quad-word floating-point instructions. They all invoke a trap handler for one of these instructions, and then the trap handler emulates the effect of the instruction. Because of the trap handler overhead, this is much slower than calling the ABI library routines. Thus the -msoft-quad-float option is the default. 😭
-mno-unaligned-doubles,-munaligned-doubles— assume that doubles have 8-byte alignment. This is the default.
With -munaligned-doubles, GCC is like ok bestie, doubles get 8-byte alignment only if they're chillin' inside another type, or if they have an absolute address. Any other time? It just assumes 4-byte alignment. Turning this on dodges some rare compatibility drama with code that other compilers made. It's not the default tho, because it costs you performance — especially if you're doing floating-point stuff. 💅
-muser-mode,-mno-user-mode— don't generate code that can only run in supervisor mode. This only matters for thecasainstruction emitted for the LEON3 processor. This is the default. ✨-mfaster-structs,-mno-faster-structs— with-mfaster-structs, the compiler assumes structures should be 8-byte aligned. That unlocks pairs oflddandstdinstructions for copies in structure assignment, instead of twice as manyld/stpairs. BUT — and this is the tea — using this changed alignment directly violates the SPARC ABI. So it's only meant for targets where the dev is fully aware their resulting code is not in line with the rules of the ABI. 😭-mstd-struct-return,-mno-std-struct-return— with-mstd-struct-return, the compiler generates checking code in functions returning structures or unions, to detect size mismatches between the two sides of function calls, as per the 32-bit ABI.
The default is -mno-std-struct-return. This option has no effect in 64-bit mode. 🫶
-mlra,-mno-lra— enable Local Register Allocation. This has been the default for SPARC since GCC 7, so you gotta pass-mno-lraif you want the old Reload. 🎀-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,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 picks the best architecture option for the host processor. -mcpu=native has no effect if GCC doesn't recognize the processor.
Default instruction scheduling parameters are used for values that select an architecture and not an implementation. Those are v7, v8, sparclite, sparclet, v9.
Here's the list of each supported architecture and their supported implementations, bestie:
v7—cypress,leon3v7v8—supersparc,hypersparc,leon,leon3sparclitef930,f934,sparclite86x
sparclettsc701
v9—ultrasparc,ultrasparc3,niagara,niagara2,niagara3,niagara4,niagara7,m8
By default (unless configured otherwise), GCC generates code for the V7 variant of the SPARC architecture. With -mcpu=cypress, the compiler additionally optimizes it for the Cypress CY7C602 chip, as used in the SPARCStation/SPARCServer 3xx series. This is also appropriate for the older SPARCStation 1, 2, IPX etc. 🖥️
With -mcpu=v8, GCC generates code for the V8 variant of the SPARC architecture. The only difference from V7 code is that the compiler emits the integer multiply and integer divide instructions, which exist in SPARC-V8 but not in SPARC-V7. With -mcpu=supersparc, the compiler additionally optimizes it for the SuperSPARC chip, as used in the SPARCStation 10, 1000 and 2000 series.
With -mcpu=sparclite, GCC generates code for the SPARClite variant of the SPARC architecture. This adds the integer multiply, integer divide step and scan (ffs) instructions which exist in SPARClite but not in SPARC-V7. With -mcpu=f930, the compiler additionally optimizes it for the Fujitsu MB86930 chip, which is the original SPARClite, with no FPU. With -mcpu=f934, the compiler additionally optimizes it for the Fujitsu MB86934 chip, which is the more recent SPARClite with FPU. 🔥
With -mcpu=sparclet, GCC generates code for the SPARClet variant of the SPARC architecture. This adds the integer multiply, multiply/accumulate, integer divide step and scan (ffs) instructions which exist in SPARClet but not in SPARC-V7. With -mcpu=tsc701, the compiler additionally optimizes it for the TEMIC SPARClet chip.
With -mcpu=v9, GCC generates code for the V9 variant of the SPARC architecture. This adds 64-bit integer and floating-point move instructions, 3 additional floating-point condition code registers and conditional move instructions. With -mcpu=ultrasparc, the compiler additionally optimizes it for the Sun UltraSPARC I/II/IIi chips. With -mcpu=ultrasparc3, the compiler additionally optimizes it for the Sun UltraSPARC III/III+/IIIi/IIIi+/IV/IV+ chips. With -mcpu=niagara, the compiler additionally optimizes it for Sun UltraSPARC T1 chips. With -mcpu=niagara2, the compiler additionally optimizes it for Sun UltraSPARC T2 chips. With -mcpu=niagara3, the compiler additionally optimizes it for Sun UltraSPARC T3 chips. With -mcpu=niagara4, the compiler additionally optimizes it for Sun UltraSPARC T4 chips. With -mcpu=niagara7, the compiler additionally optimizes it for Oracle SPARC M7 chips. With -mcpu=m8, the compiler additionally optimizes it for Oracle M8 chips. 🌈
-mtune=cpu_type — set the instruction scheduling parameters for machine type cpu_type, but do not set the instruction set or register set that-mcpu=cpu_type does. 🦋
The same values for -mcpu=cpu_type can be used for -mtune=cpu_type, but the only useful values are those that select a particular CPU implementation. Those are cypress, supersparc, hypersparc, leon, leon3, leon3v7, 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. 💖-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.-mpopc,-mno-popc— with-mpopc, GCC generates code that takes advantage of the UltraSPARC Population Count instruction. The default is-mpopcwhen targeting a CPU that supports such an instruction, such as Niagara-2 and later.-msubxc,-mno-subxc— with-msubxc, GCC generates code that takes advantage of the UltraSPARC Subtract-Extended-with-Carry instruction. The default is-msubxcwhen targeting a CPU that supports such an instruction, such as Niagara-7 and later.-mfix-at697f— enable the documented workaround for the single erratum of the Atmel AT697F processor (which corresponds to erratum #13 of the AT697E processor).-mfix-ut699— enable the documented workarounds for the floating-point errata and the data cache nullify errata of the UT699 processor. 🛠️-mfix-ut700— enable the documented workaround for the back-to-back store errata of the UT699E/UT700 processor.-mfix-gr712rc— enable the documented workaround for the back-to-back store errata of the GR712RC processor.
These -m options are supported in addition to the above on SPARC-V9 processors in 64-bit environments, bestie: 💜
-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. ✨-mcmodel=which — set the code model to one of:medlow— the Medium/Low code model: 64-bit addresses, programs must be linked in the low 32 bits of memory. Programs can be statically or dynamically linked.medmid— the Medium/Middle code model: 64-bit addresses, programs must be linked in the low 44 bits of memory, the text and data segments must be less than 2GB in size and the data segment must be located within 2GB of the text segment.
medany— the Medium/Anywhere code model, bestie. 64-bit addresses, programs can be linked literally anywhere in memory, but the text and data segments have to be under 2GB in size, and the data segment must sit within 2GB of the text segment. Boundaries, we have them 💅embmedany— the Medium/Anywhere code model for embedded systems. 64-bit addresses, text and data segments must both be less than 2GB, and both can start anywhere in memory (decided at link time, so like… whenever the linker says so). The global register%g4points to the base of the data segment. Programs are statically linked and PIC is not supported. No cap. 🙈
-mmemory-model=mem-model
Set the memory model in force on the processor to one of:
default— the default memory model for the processor and operating system, iconic behaviour honestlyrmo— Relaxed Memory Orderpso— Partial Store Ordertso— Total Store Ordersc— Sequential Consistency
These memory models are formally defined in Appendix D of the SPARC-V9 architecture manual, and they're set in the processor's PSTATE.MM field. Like, that's the actual source, go read it, iykyk 📖
-mstack-bias
-mno-stack-bias
With -mstack-bias, GCC assumes the stack pointer — and the frame pointer, if one's even present — is offset by -2047, which you have to add back whenever you make stack frame references. This is the default in 64-bit mode, periodt. Otherwise? Assume there's no such offset at all. ✨
SPU Options 💖
These -m options are supported on the SPU:
-mwarn-reloc-merror-reloc— the loader for SPU does not handle dynamic relocations, and that's a whole thing. By default, GCC throws an error when it generates code that needs a dynamic relocation.-mno-error-relocdisables that error, and-mwarn-relocgenerates a warning instead. Pick your fighter. 😭-msafe-dma-munsafe-dma— instructions that initiate or test completion of DMA must not be reordered with respect to loads and stores of the memory being accessed. With-munsafe-dmayou have to use thevolatilekeyword to protect memory accesses, but that can lead to inefficient code in places where the memory is known to not change. Rather than mark the memory as volatile, you can use-msafe-dmato tell the compiler to treat the DMA instructions as potentially affecting all memory. Slay but careful 🫶-mbranch-hints— by default, GCC generates a branch hint instruction to avoid pipeline stalls for always-taken or probably-taken branches. A hint is not generated closer than 8 instructions away from its branch. There's little reason to disable them, except for debugging purposes, or to make an object a little bit smaller. Basically: don't touch unless you have a reason, bestie.-msmall-mem-mlarge-mem— by default, GCC generates code assuming addresses are never larger than 18 bits. With-mlarge-memcode is generated that assumes a full 32-bit address. Glow up for your addresses ✨-mstdmain— by default, GCC links against startup code that assumes the SPU-stylemainfunction interface (which has an unconventional parameter list, we don't talk about it). With-mstdmain, GCC links your program against startup code that assumes a C99-style interface tomain, including a local copy ofargvstrings. Very demure, very C99 💅-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. Kernel girlies, this one's for you 👑-mea32-mea64— compile code assuming that pointers to the PPU address space accessed via the__eanamed address space qualifier are either 32 or 64 bits wide. The default is 32 bits. As this is an ABI-changing option, all object code in an executable must be compiled with the same setting. No mixing, fr fr. 🙈-maddress-space-conversion-mno-address-space-conversion— allow/disallow treating the__eaaddress space as a superset of the generic address space. This enables explicit type casts between__eaand generic pointers as well as implicit conversions of generic pointers to__eapointers. The default is to allow address space pointer conversions. 🌈-mcache-size=cache-size— this option controls the version of libgcc that the compiler links to an executable and selects a software-managed cache for accessing variables in the__eaaddress space with a particular cache size. Possible options for cache-size are8,16,32,64and128. The default cache size is64KB. Big cache energy 🎀-matomic-updates-mno-atomic-updates— this option controls the version of libgcc that the compiler links to an executable and selects whether atomic updates to the software-managed cache of PPU-side variables are used. If you use atomic updates, changes to a PPU variable from SPU code using the__eanamed address space qualifier do not interfere with changes to other PPU variables residing in the same cache line from PPU code. If you do not use atomic updates, such interference may occur; however, writing back cache lines is more efficient. The default behavior is to use atomic updates. Tradeoffs, always tradeoffs 😭-mdual-nops-mdual-nops=n— by default, GCC inserts NOPs to increase dual issue when it expects it to increase performance. n can be a value from0to10. A smaller n inserts fewer NOPs.10is the default,0is the same as-mno-dual-nops. Disabled with-Os.-mhint-max-nops=n— maximum number of NOPs to insert for a branch hint. A branch hint must be at least 8 instructions away from the branch it is affecting. GCC inserts up to n NOPs to enforce this, otherwise it does not generate the branch hint. Rules are rules ✨-mhint-max-distance=n— the encoding of the branch hint instruction limits the hint to be within 256 instructions of the branch it is affecting. By default, GCC makes sure it is within 125. Underachieving on purpose, iconic 💅-msafe-hints— work around a hardware bug that causes the SPU to stall indefinitely. By default, GCC inserts thehbrpinstruction to make sure this stall won't happen. Hardware bugs are so not the vibe but okay 🙈
Options for System V 🌸
These additional options are available on System V Release 4 for compatibility with other compilers on those systems:
-G— create a shared object. It is recommended that-symbolicor-sharedbe used instead. (So like… old-school, bestie.)-Qy— identify the versions of each tool used by the compiler, in a.identassembler directive in the output.-Qn— refrain from adding.identdirectives to the output file (this is the default).-YP,dirs— search the directories dirs, and no others, for libraries specified with-l.-Ym,dir— look in the directory dir to find the M4 preprocessor. The assembler uses this option.
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 big, no cap 🔥-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. Pick a side bestie 🎀
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, honestly 💅)
V850 Options ⭐
These -m options are defined for V850 implementations:
-mlong-calls-mno-long-calls— treat all calls as being far away (near). If calls are assumed to be far away, the compiler always loads the function's address into a register, and calls indirect through the pointer.-mno-ep-mep— do not optimize (do optimize) basic blocks that use the same index pointer 4 or more times to copy pointer into theepregister, and use the shortersldandsstinstructions. The-mepoption is on by default if you optimize. Optimize girlies rise 🫶-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. Compact queen ✨-mtda=n— put static or global variables whose size is n bytes or less into the tiny data area that registereppoints to. The tiny data area can hold up to256bytes in total (128bytes for byte references).-msda=n— put static or global variables whose size is n bytes or less into the small data area that registergppoints to. The small data area can hold up to64kilobytes.-mzda=n— put static or global variables whose size is n bytes or less into the first32kilobytes 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— tells GCC the target processor is the V850E 👑 and if you use this flag, the preprocessor constant__v850e__gets defined, no cap.
okay bestie real talk: if none of-mv850or-mv850eor-mv850e1or-mv850e2or-mv850e2v3or-mv850e3v5are defined, then a default target processor is chosen and the relevant__v850*__preprocessor constant is defined. and the preprocessor constants__v850and__v851__are always defined, regardless of which processor variant is the target. always. 💖
-mdisable-callt,-mno-disable-callt— this option suppresses generation of theCALLTinstruction for the v850e, v850e1, v850e2, v850e2v3 and v850e3v5 flavors of the v850 architecture 🙈
it's enabled by default when the RH850 ABI is in use (see -mrh850-abi), and disabled by default when the GCC ABI is in use. and if CALLT instructions are being generated then the C preprocessor symbol __V850_CALLT__ is defined ✨
-mrelax,-mno-relax— passes on (or does not pass on) the-mrelaxcommand-line option to the assembler 🫶-mlong-jumps,-mno-long-jumps— disables (or re-enables) the generation of PC-relative jump instructions 🦋-msoft-float,-mhard-float— disables (or re-enables) the generation of hardware floating point instructions. this one only matters when the target architecture is V850E2V3 or higher 💅 and 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. so, yk, proceed accordingly.-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.
- functions are aligned to 32-bit boundaries, unless optimizing for size.
- the
-m8byte-aligncommand-line option is not supported. - the
-mdisable-calltcommand-line option is supported but not enabled by default.
- integer sized structures and unions are returned in register
when this version of the ABI is enabled the C preprocessor symbol __V850_GCC_ABI__ is defined ✨
-m8byte-align,-mno-8byte-align— enables support fordoubleandlong longtypes to be aligned on 8-byte boundaries 🎀 the default is to restrict the alignment of all objects to at most 4-bytes. when-m8byte-alignis in effect the C preprocessor symbol__V850_8BYTE_ALIGN__is defined.-mbig-switch— generates code suitable for big switch tables 🫶 use this option only if the assembler/linker complain about out of range branches within a switch table. only then. not vibes-based.-mapp-regs— this option causes r2 and r5 to be used in the code generated by the compiler 💅 this setting is the default.-mno-app-regs— this option causes r2 and r5 to be treated as fixed registers 🔒
VAX Options
these -m options are defined for the VAX:
-munix— do not output certain jump instructions (aobleqand so on) that the Unix assembler for the VAX cannot handle across long ranges 🙈-mgnu— do output those jump instructions, on the assumption that the GNU assembler is being used ✨-mg— output code for G-format floating-point numbers instead of D-format.
Visium Options
-mdebug— a program which performs file I/O and is destined to run on an MCM target should be linked with this option 💖 it causes the librarieslibc.aandlibdebug.ato be linked. the program should be run on the target under the control of the GDB remote debugging stub.-msim— a program which performs file I/O and is destined to run on the simulator should be linked with option 👑 this causes librarieslibc.aandlibsim.ato be linked.-mfpu,-mhard-float— generate code containing floating-point instructions. this is the default 🔥-mno-fpu,-msoft-float— generate code containing library calls for floating-point.
⚠️-msoft-floatchanges the calling convention in the output file; therefore, it is only useful if you compile all of a program with this option. in particular, you need to compilelibgcc.a, the library that comes with GCC, with-msoft-floatin order for this to work. all of it. no half measures bestie 😭
- *
-mcpu=cpu_type* — set 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 present for backward compatibility.
by default (unless configured otherwise), GCC generates code for the GR5 variant of the Visium architecture.
with -mcpu=gr6, GCC generates code for the GR6 variant of the Visium architecture 🌈 the only difference from GR5 code is that the compiler will generate block move instructions.
-mtune=cpu_type — set the instruction scheduling parameters for machine type cpu_type, but do *not set the instruction set or register set that the option-mcpu=cpu_type* would. so like, tuning only. scheduling vibes, not identity 🎀-msv-mode— generate code for the supervisor mode, where there are no restrictions on the access to general registers 👑 this is the default.-muser-mode— generate code for the user mode, where the access to some general registers is forbidden: on the GR5, registers r24 to r31 cannot be accessed in this mode; on the GR6, only registers r29 to r31 are affected 🔒
VMS Options
these -m options are defined for the VMS implementations:
-mvms-return-codes— return VMS condition codes 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 are 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.
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.i386— original Intel i386 CPU.i486— Intel i486 CPU. (no scheduling is implemented for this chip.) 🫠i586,pentium— Intel Pentium CPU with no MMX support.lakemont— Intel Lakemont MCU, based on Intel Pentium CPU.pentium-mmx— Intel Pentium MMX CPU, based on Pentium core with MMX instruction set support ✨pentiumpro— Intel Pentium Pro CPU.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 instruction set support.pentium3,pentium3m— Intel Pentium III CPU, based on Pentium Pro core with MMX and SSE instruction set support 💅pentium-m— Intel Pentium M; low-power version of Intel Pentium III CPU with MMX, SSE and SSE2 instruction set support 🔋 used by Centrino notebooks.pentium4,pentium4m— Intel Pentium 4 CPU with MMX, SSE and SSE2 instruction set support 🔥prescott— improved version of Intel Pentium 4 CPU with MMX, SSE, SSE2 and SSE3 instruction set support ✨nocona— improved version of Intel Pentium 4 CPU with 64-bit extensions, MMX, SSE, SSE2 and SSE3 instruction set support 🌈
core2— Intel Core 2 CPU, 64-bit extensions on deck, plusMMX,SSE,SSE2,SSE3andSSSE3instruction set support. 💖nehalem— Intel Nehalem CPU with 64-bit extensions, and it's servingMMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2andPOPCNTinstruction set support.westmere— Intel Westmere CPU, 64-bit extensions, and the full roster:MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,AESandPCLMULinstruction set support. ✨sandybridge— Intel Sandy Bridge CPU with 64-bit extensions,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,AVX,AESandPCLMULinstruction set support.ivybridge— Intel Ivy Bridge CPU with 64-bit extensions,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,AVX,AES,PCLMUL,FSGSBASE,RDRNDandF16Cinstruction set support. 🦋haswell— Intel Haswell CPU with 64-bit extensions,MOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,AVX,AVX2,AES,PCLMUL,FSGSBASE,RDRND,FMA,BMI,BMI2andF16Cinstruction set support.broadwell— Intel Broadwell CPU with 64-bit extensions,MOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,AVX,AVX2,AES,PCLMUL,FSGSBASE,RDRND,FMA,BMI,BMI2,F16C,RDSEEDADCXandPREFETCHWinstruction set support.skylake— Intel Skylake CPU with 64-bit extensions,MOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,AVX,AVX2,AES,PCLMUL,FSGSBASE,RDRND,FMA,BMI,BMI2,F16C,RDSEED,ADCX,PREFETCHW,CLFLUSHOPT,XSAVECandXSAVESinstruction set support. 👑bonnell— Intel Bonnell CPU with 64-bit extensions,MOVBE,MMX,SSE,SSE2,SSE3andSSSE3instruction set support.silvermont— Intel Silvermont CPU with 64-bit extensions,MOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,AES,PREFETCHW,PCLMULandRDRNDinstruction set support.goldmont— Intel Goldmont CPU with 64-bit extensions,MOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,AES,PREFETCHW,PCLMUL,RDRND,XSAVE,XSAVEC,XSAVES,XSAVEOPTandFSGSBASEinstruction set support.goldmont-plus— Intel Goldmont Plus 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,SGXandUMIPinstruction set support. ✨tremont— Intel Tremont 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,UMIP,GFNI-SSE,CLWB,MOVDIRI,MOVDIR64B,CLDEMOTEandWAITPKGinstruction set support. 🔥knl— Intel Knight's Landing CPU with 64-bit extensions,MOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,AVX,AVX2,AES,PCLMUL,FSGSBASE,RDRND,FMA,BMI,BMI2,F16C,RDSEED,ADCX,PREFETCHW,PREFETCHWT1,AVX512F,AVX512PF,AVX512ERandAVX512CDinstruction set support.knm— Intel Knights Mill CPU with 64-bit extensions,MOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,AVX,AVX2,AES,PCLMUL,FSGSBASE,RDRND,FMA,BMI,BMI2,F16C,RDSEED,ADCX,PREFETCHW,PREFETCHWT1,AVX512F,AVX512PF,AVX512ER,AVX512CD,AVX5124VNNIW,AVX5124FMAPSandAVX512VPOPCNTDQinstruction set support. 💅skylake-avx512— Intel Skylake Server CPU with 64-bit extensions,MOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,PKU,AVX,AVX2,AES,PCLMUL,FSGSBASE,RDRND,FMA,BMI,BMI2,F16C,RDSEED,ADCX,PREFETCHW,CLFLUSHOPT,XSAVEC,XSAVES,AVX512F,CLWB,AVX512VL,AVX512BW,AVX512DQandAVX512CDinstruction set support.cannonlake— Intel Cannonlake Server CPU with 64-bit extensions,MOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,PKU,AVX,AVX2,AES,PCLMUL,FSGSBASE,RDRND,FMA,BMI,BMI2,F16C,RDSEED,ADCX,PREFETCHW,CLFLUSHOPT,XSAVEC,XSAVES,AVX512F,AVX512VL,AVX512BW,AVX512DQ,AVX512CD,AVX512VBMI,AVX512IFMA,SHAandUMIPinstruction set support.icelake-client— Intel Icelake Client CPU with 64-bit extensions,MOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,PKU,AVX,AVX2,AES,PCLMUL,FSGSBASE,RDRND,FMA,BMI,BMI2,F16C,RDSEED,ADCX,PREFETCHW,CLFLUSHOPT,XSAVEC,XSAVES,AVX512F,AVX512VL,AVX512BW,AVX512DQ,AVX512CD,AVX512VBMI,AVX512IFMA,SHA,CLWB,UMIP,RDPID,GFNI,AVX512VBMI2,AVX512VPOPCNTDQ,AVX512BITALG,AVX512VNNI,VPCLMULQDQ,VAESinstruction set support. 🧚icelake-server— Intel Icelake Server CPU with 64-bit extensions,MOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,PKU,AVX,AVX2,AES,PCLMUL,FSGSBASE,RDRND,FMA,BMI,BMI2,F16C,RDSEED,ADCX,PREFETCHW,CLFLUSHOPT,XSAVEC,XSAVES,AVX512F,AVX512VL,AVX512BW,AVX512DQ,AVX512CD,AVX512VBMI,AVX512IFMA,SHA,CLWB,UMIP,RDPID,GFNI,AVX512VBMI2,AVX512VPOPCNTDQ,AVX512BITALG,AVX512VNNI,VPCLMULQDQ,VAES,PCONFIGandWBNOINVDinstruction set support.cascadelake— Intel Cascadelake CPU with 64-bit extensions,MOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,PKU,AVX,AVX2,AES,PCLMUL,FSGSBASE,RDRND,FMA,BMI,BMI2,F16C,RDSEED,ADCX,PREFETCHW,CLFLUSHOPT,XSAVEC,XSAVES,AVX512F,CLWB,AVX512VL,AVX512BW,AVX512DQ,AVX512CDandAVX512VNNIinstruction set support.tigerlake— Intel Tigerlake CPU with 64-bit extensions,MOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,PKU,AVX,AVX2,AES,PCLMUL,FSGSBASE,RDRND,FMA,BMI,BMI2,F16C,RDSEED,ADCX,PREFETCHW,CLFLUSHOPT,XSAVEC,XSAVES,AVX512F,AVX512VL,AVX512BW,AVX512DQ,AVX512CD,AVX512VBMI,AVX512IFMA,SHA,CLWB,UMIP,RDPID,GFNI,AVX512VBMI2,AVX512VPOPCNTDQ,AVX512BITALG,AVX512VNNI,VPCLMULQDQ,VAES,PCONFIG,WBNOINVD,MOVDIRI,MOVDIR64BandCLWBinstruction set support. 🎀k6— AMD K6 CPU withMMXinstruction set support.k6-2,k6-3— Improved versions of AMD K6 CPU withMMXand3DNow!instruction set support.athlon,athlon-tbird— AMD Athlon CPU withMMX,3dNOW!, enhanced3DNow!andSSEprefetch instructions support.athlon-4,athlon-xp,athlon-mp— Improved AMD Athlon CPU withMMX,3DNow!, enhanced3DNow!and fullSSEinstruction set support.k8,opteron,athlon64,athlon-fx— Processors based on the AMD K8 core with x86-64 instruction set support, including the AMD Opteron, Athlon 64, and Athlon 64 FX processors. (This supersetsMMX,SSE,SSE2,3DNow!, enhanced3DNow!and 64-bit instruction set extensions.)k8-sse3,opteron-sse3,athlon64-sse3— Improved versions of AMD K8 cores withSSE3instruction set support.amdfam10,barcelona— CPUs based on AMD Family 10h cores with x86-64 instruction set support. (This supersetsMMX,SSE,SSE2,SSE3,SSE4A,3DNow!, enhanced3DNow!,ABMand 64-bit instruction set extensions.) 🌸bdver1— CPUs based on AMD Family 15h cores with x86-64 instruction set support. (This supersetsFMA4,AVX,XOP,LWP,AES,PCL_MUL,CX16,MMX,SSE,SSE2,SSE3,SSE4A,SSSE3,SSE4.1,SSE4.2,ABMand 64-bit instruction set extensions.)bdver2— AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersetsBMI,TBM,F16C,FMA,FMA4,AVX,XOP,LWP,AES,PCL_MUL,CX16,MMX,SSE,SSE2,SSE3,SSE4A,SSSE3,SSE4.1,SSE4.2,ABMand 64-bit instruction set extensions.)bdver3— AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersetsBMI,TBM,F16C,FMA,FMA4,FSGSBASE,AVX,XOP,LWP,AES,PCL_MUL,CX16,MMX,SSE,SSE2,SSE3,SSE4A,SSSE3,SSE4.1,SSE4.2,ABMand 64-bit instruction set extensions.bdver4— AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersetsBMI,BMI2,TBM,F16C,FMA,FMA4,FSGSBASE,AVX,AVX2,XOP,LWP,AES,PCL_MUL,CX16,MOVBE,MMX,SSE,SSE2,SSE3,SSE4A,SSSE3,SSE4.1,SSE4.2,ABMand 64-bit instruction set extensions.znver1— AMD Family 17h core based CPUs with x86-64 instruction set support. (This supersetsBMI,BMI2,F16C,FMA,FSGSBASE,AVX,AVX2,ADCX,RDSEED,MWAITX,SHA,CLZERO,AES,PCL_MUL,CX16,MOVBE,MMX,SSE,SSE2,SSE3,SSE4A,SSSE3,SSE4.1,SSE4.2,ABM,XSAVEC,XSAVES,CLFLUSHOPT,POPCNT, and 64-bit instruction set extensions. 💜znver2— AMD Family 17h core based CPUs with x86-64 instruction set support. (This supersetsBMI,BMI2,,CLWB,F16C,FMA,FSGSBASE,AVX,AVX2,ADCX,RDSEED,MWAITX,SHA,CLZERO,AES,PCL_MUL,CX16,MOVBE,MMX,SSE,SSE2,SSE3,SSE4A,SSSE3,SSE4.1,SSE4.2,ABM,XSAVEC,XSAVES,CLFLUSHOPT,POPCNT, and 64-bit instruction set extensions.)btver1— CPUs based on AMD Family 14h cores with x86-64 instruction set support. (This supersetsMMX,SSE,SSE2,SSE3,SSSE3,SSE4A,CX16,ABMand 64-bit instruction set extensions.)btver2— CPUs based on AMD Family 16h cores with x86-64 instruction set support. This includesMOVBE,F16C,BMI,AVX,PCL_MUL,AES,SSE4.2,SSE4.1,CX16,ABM,SSE4A,SSSE3,SSE3,SSE2,SSE,MMXand 64-bit instruction set extensions. 🌈winchip-c6— IDT WinChip C6 CPU, dealt in same way as i486 with additionalMMXinstruction set support.winchip2— IDT WinChip 2 CPU, dealt in same way as i486 with additionalMMXand3DNow!instruction set support.c3— VIA C3 CPU withMMXand3DNow!instruction set support. (No scheduling is implemented for this chip.)c3-2— VIA C3-2 (Nehemiah/C5XL) CPU withMMXandSSEinstruction set support. (No scheduling is implemented for this chip.)c7— VIA C7 (Esther) CPU withMMX,SSE,SSE2andSSE3instruction set support. (No scheduling is implemented for this chip.) 🍓samuel-2— VIA Eden Samuel 2 CPU, and she comes with MMX + 3DNow! instruction set support. (No scheduling is implemented for this chip, bestie — so the compiler's just vibing, not choreographing.)nehemiah— VIA Eden Nehemiah CPU, holding MMX and SSE instruction set support. (No scheduling is implemented for this chip either.)esther— VIA Eden Esther CPU with MMX, SSE, SSE2 and SSE3 instruction set support. (Still no scheduling for this chip, ngl.)eden-x2— VIA Eden X2 CPU with x86-64, MMX, SSE, SSE2 and SSE3 instruction set support. (No scheduling 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 implemented for this chip.)nano— Generic VIA Nano CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. (No scheduling implemented for this chip.)nano-1000— VIA Nano 1xxx CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. (No scheduling implemented for this chip.)nano-2000— VIA Nano 2xxx CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. (No scheduling 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 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 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 implemented for this chip.)geode— AMD Geode embedded processor with MMX and 3DNow! instruction set support. ✨
-mtune=cpu-type— okay so this one tunes everything applicable about the generated code to cpu-type, except for the ABI and the set of available instructions. Like, while picking a specific cpu-type schedules things appropriately for that particular chip, the compiler does not generate any code that can't run on the default machine type unless you go and use a-march=cpu-typeoption. Example time: 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. And 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, fr fr.
As new processors are deployed in the marketplace, the behavior of this option will change. So 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 to perform 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. Deprecated!! She's giving legacy energy, use-mtuneinstead bestie. 💀-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 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 using387. 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— some 387 emulators do not support thesin,cosandsqrtinstructions for the 387. Specify this option to avoid generating those instructions. This option is overridden when-marchindicates that the target CPU always has an FPU and so the instruction does not need emulation. These instructions are not generated unless you also use the-funsafe-math-optimizationsswitch. 👑-malign-double-mno-align-double— control whether GCC alignsdouble,long double, andlong longvariables on a two-word boundary or a one-word boundary. Aligningdoublevariables on a two-word boundary produces code that runs somewhat faster on a Pentium at the expense of more memory.
On x86-64, -malign-double is enabled by default.
Warning: if you use the -malign-double switch, structures containing the above types are aligned differently than the published application binary interface specifications for the x86-32 and are not binary compatible with structures in code compiled without that switch. 💀
-m96bit-long-double-m128bit-long-double— these switches control the size oflong doubletype. The x86-32 application binary interface specifies the size to be 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, this is not possible. So specifying -m128bit-long-double aligns long double to a 16-byte boundary by padding the long double with an additional 32-bit zero. ✨
In the x86-64 compiler, -m128bit-long-double is the default choice as its ABI specifies that long double is aligned on 16-byte boundary.
Notice that neither of these options enable any extra precision over the x87 standard of 80 bits for a long double. 🦋
⚠️ Warning: if you override the default value for your target ABI, this changes the size of structures and arrays containinglong doublevariables, as well as modifying the function calling convention for functions takinglong double. Hence they are not binary-compatible with code compiled without that switch. like... bestie, ABI is a whole relationship, you can't just switch it up on one side 💔
-mlong-double-64-mlong-double-80-mlong-double-128— these three control how big thelong doubletype is. size queen energy but make it floating point ✨- A size of 64 bits makes
long doubleequivalent to thedoubletype. This is the default for the 32-bit Bionic C library. - A size of 128 bits makes
long doubleequivalent to the__float128type. This is the default for the 64-bit Bionic C library.
- A size of 64 bits makes
⚠️ Warning: if you override the default value for your target ABI, this changes the size of structures and arrays containinglong doublevariables, as well as modifying the function calling convention for functions takinglong double. Hence they are not binary-compatible with code compiled without that switch. (yes, same warning, it's that serious, it said it twice for a reason 🫶)
-malign-data=type— controls how GCC aligns variables. The supported values for type are:compat— uses increased alignment value, compatible with GCC 4.8 and earlier.abi— uses the alignment value as specified by the psABI.cacheline— uses increased alignment value to match the cache line size.
compat is the default. the safe lil default that just gets it 💅
-mlarge-data-threshold=threshold— when-mcmodel=mediumis specified, data objects larger than threshold get yeeted into 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. saves one whole instruction in the caller because there's no need to pop the arguments there. efficiency icon behavior 👑
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. no cap, it will Not Work 💀
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.) like bestie please just don't 🙈
-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. everyone on the same page or the whole thing falls apart fr fr
-msseregparm— uses SSE register passing conventions forfloatanddoublearguments 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. same rule, same drama 🎀
-mvect8-ret-in-mem— returns 8-byte vectors in memory instead of in MMX registers. This is the default on Solaris 8 and 9 and VxWorks, to match the ABI of the Sun Studio compilers until version 12. Later compiler versions (starting with Studio 12 Update 1) follow the ABI used by other x86 targets, which is the default on Solaris 10 and later. Only use this option if you need to remain compatible with existing code produced by those previous compiler versions or older versions of GCC. legacy support, we respect the elders 🫡-mpc32-mpc64-mpc80— set the 80387 floating-point precision to 32, 64 or 80 bits. precision? we love her ✨-mpc32→ the significands of results of floating-point operations are rounded to 24 bits (single precision).-mpc64→ rounds the significands of results of floating-point operations to 53 bits (double precision).-mpc80→ rounds the significands of results of floating-point operations to 64 bits (extended double precision), which is the default.
When this option is used, floating-point operations in higher precisions are not available to the programmer without setting the FPU control word explicitly. iykyk, go flip that control word yourself 💅
Setting the rounding of floating-point operations to less than the default 80 bits can speed some programs by 2% or more. slay, free speed! 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. catastrophic?? the name is NOT playing 💀
-mstackrealign— realign 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. legacy and modern, coexisting, we love a blended family 🌈-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 an 8 byte boundary. Since x86-64 ABI requires 16 byte stack alignment, this is ABI incompatible and intended to be used in a controlled environment where stack space is an 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. okay so this one is giving chaos, be careful bestie 😭
-mincoming-stack-boundary=num— assumes 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. the performance penalty jumpscare 🫣 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 these 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. just... use the default. please. for me 🫶
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. small stack? small boundary. balance ✨
Now bestie... buckle up, this next one is a LIST 💖
-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-mgfni-mvaes-mwaitpkg-mvpclmulqdq-mavx512bitalg-mmovdiri-mmovdir64b-mavx512vpopcntdq-mavx5124fmaps-mavx512vnni-mavx5124vnniw-mcldemote— these 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, AVX512VPOPCNTDQ, AVX5124FMAPS, AVX512VNNI, AVX5124VNNIW, or CLDEMOTE extended instruction sets. the way this list ate and left no crumbs 💅✨
Each has a corresponding -mno- option to disable use of these instructions. yes, the off switch exists, you're welcome 🎀
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. iykyk 🔥
To generate SSE/SSE2 instructions automatically from floating-point code (as opposed to 387 instructions), see -mfpmath=sse. automatic slay ✨
GCC depresses SSEx instructions when -mavx is used. Instead, it generates new AVX instructions or AVX equivalence for all SSEx instructions when needed. out with the old, in with the AVX, no cap 👑
These options let GCC flex those extended instructions in generated code, even without -mfpmath=sse. Big deal alert though: if your app does run-time CPU detection, you gotta compile separate files for each supported architecture with the right flags, okay? And specifically — the file with the CPU detection code should be compiled without these options. Like, no cap, don't do it.
-mdump-tune-features— tells GCC to dump out the names of the x86 performance tuning features plus their default settings. Those names? You can feed them straight into-mtune-ctrl=feature-list. ✨-mtune-ctrl=feature-list— this one does fine-grain control over x86 code generation features. feature-list is a comma-separated list of feature names. See also-mdump-tune-features. When you specify it, the feature gets turned on unless it's preceded with^— in which case it's turned off. 💅
Real talk warning: -mtune-ctrl=feature-list is meant for GCC developers. Using it may lead to code paths that testing never covers, and it can potentially cause compiler ICEs or runtime errors. Not a vibe for production, bestie.
-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 DF should be cleared on function entry, but some operating systems violate this by not clearing DF in their exception dispatchers. So the exception handler can get invoked with DF 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. In that case you can suppress generation of"cld"instructions with the-mno-cldcompiler option.-mvzeroupper— instructs GCC to emit a"vzeroupper"instruction before control flow transfers out of the function. Why? To minimize the AVX to SSE transition penalty and remove unnecessary"zeroupper"intrinsics. Two birds, one stone. 🦋-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.none— no extra limitations applied to GCC other than what the selected platform already defines.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— enables GCC to generate"CMPXCHG16B"instructions in 64-bit code, so it can implement compare-and-exchange operations on 16-byte aligned 128-bit objects. Super useful for atomic updates of data structures bigger than one machine word. The compiler uses this instruction to implement__syncBuiltins. But heads up — for__atomicBuiltins operating on 128-bit integers, a library call is always used.-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 are load and store instructions, respectively, for certain status flags. In 64-bit mode, the"SAHF"instruction is used to optimize thefmod,drem, andremainderbuilt-in functions — see Other Builtins for the tea. 🍓-mmovbe— enables use of the"movbe"instruction to implement__builtin_bswap32and__builtin_bswap64. Slay.-mshstk— the-mshstkoption 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.-mrecip— enables use of"RCPSS"and"RSQRTSS"instructions (plus their vectorized variants"RCPPS"and"RSQRTPS") with an extra Newton-Raphson step to boost precision, instead of"DIVSS"and"SQRTSS"(and their vectorized variants) for single-precision floating-point arguments. These instructions are generated only when-funsafe-math-optimizationsis enabled together with-ffinite-math-onlyand-fno-trapping-math. Note: 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. 😬
Also note that GCC implements 1.0f/sqrtf(x) in terms of "RSQRTSS" (or "RSQRTPS") already with -ffast-math (or the above option combo), so it doesn't need -mrecip.
And one more note: GCC emits the above sequence with the additional Newton-Raphson step for vectorized single-float division and vectorized sqrtf(x) already with -ffast-math (or the above option combo), so again, no -mrecip needed. 💅
-mrecip=opt— controls which reciprocal estimate instructions may be used. opt is a comma-separated list of options, and any of them may be preceded by a!to invert it: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. Iconic.
-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 the 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. That's a whole lotta names, bestie.
-mabi=name— generate code for the specified calling convention. Permissible values aresysvfor the ABI used on GNU/Linux and other systems, andmsfor the Microsoft ABI. The default is to use the Microsoft ABI when targeting Microsoft Windows and the SysV ABI on all other systems. You can control this behavior for specific functions by using the function attributes"ms_abi"and"sysv_abi".-mforce-indirect-call— forces all calls to functions to be indirect. Useful when you're using Intel Processor Trace, since it generates more precise timing information for function calls. ⏱️-mmanual-endbr— inserts ENDBR instruction at function entry only via the"cf_check"function attribute. Handy when used with the option-fcf-protection=branchto control ENDBR insertion at the function entry.-mcall-ms2sysv-xlogues— because of differences in 64-bit ABIs, 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 for saving and restoring these registers is emitted inline, which gives you fairly lengthy prologues and epilogues. Using-mcall-ms2sysv-xloguesemits prologues and epilogues that use stubs in the static portion of libgcc to do these saves and restores — reducing function size, at the cost of a few extra instructions. 💅-mtls-dialect=type— generate code to access thread-local storage using thegnuorgnu2conventions.gnuis the conservative default;gnu2is more efficient, but it may add compile- and run-time requirements that can't be satisfied on all systems. Tradeoff vibes.-mpush-args-mno-push-args— use PUSH operations to store outgoing parameters. This method is shorter and usually just as fast as the method using SUB/MOV operations, and it's enabled by default. In some cases disabling it may improve performance, thanks to improved scheduling and reduced dependencies.-maccumulate-outgoing-args— if enabled, the maximum amount of space required for outgoing arguments is computed in the function prologue. This is faster on most modern CPUs because of reduced dependencies, improved scheduling, and reduced stack usage when the preferred stack boundary isn't equal to 2. The drawback? A notable increase in code size. 📈 This switch implies-mno-push-args.-mthreads— support thread-safe exception handling on MinGW. Programs that rely on thread-safe exception handling must compile and link all code with the-mthreadsoption. When compiling,-mthreadsdefines-D_MT; when linking, it links in a special thread helper library-lmingwthrdwhich cleans up per-thread exception-handling data. 🧹-mms-bitfields-mno-ms-bitfields— enable/disable bit-field layout compatible with the native Microsoft Windows compiler.
okay so if you slap "packed" on a struct, or you're out here using bit-fields, then the Microsoft ABI might lay that structure out completely differently than the way GCC normally does it. 💅 And this matters big time when you're moving packed data between functions compiled with GCC and the native Microsoft compiler — whether that's through a function call or as data sitting in a file — because then you might actually need to access either format. 😭
This option is on by default for Microsoft Windows targets. You can also control this behavior locally with variable or type attributes — for more info, go peek at x86 Variable Attributes and x86 Type Attributes. 📚
The Microsoft structure layout algorithm is pretty simple actually, except for the bit-field packing part which is a whole vibe. The padding and alignment of members of structures — and whether a bit-field can straddle a storage-unit boundary — is 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
"aligned"attribute or the"pack"pragma), whichever is less. For structures, unions, and arrays, the alignment requirement is the largest alignment requirement of its members. Every object is allocated an offset so that:
offset % alignment_requirement == 0- 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","bar"is aligned as the type of the zero-length bit-field.
For example:
struct
{
char foo : 4;
short : 0;
char bar;
} t2;
struct
{
char foo : 4;
short : 0;
double bar;
} t3;For "t2", "bar" is placed at offset 2, rather than offset 1. Accordingly, the size of "t2" is 4. For "t3", the zero-length bit-field does not affect the alignment of "bar" or, as a result, the size of the structure.
Taking this into account, it is important to note the following:
- If a zero-length bit-field follows a normal bit-field, the type of the zero-length bit-field may affect the alignment of the structure as whole. For example,
"t2"has a size of 4 bytes, since the zero-length bit-field follows a normal bit-field, and is of type short. - 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— don't align the destination of inlined string operations. This switch reduces code size and improves performance if the destination is already aligned, but GCC just doesn't know about it. 🙈-minline-all-stringops— by default GCC only inlines string operations when the destination is known to be aligned to at least a 4-byte boundary. This enables more inlining and increases code size, but may improve performance of code that depends on fast"memcpy","strlen", and"memset"for short lengths. 🔥-minline-stringops-dynamically— for string operations of unknown size, use run-time checks with inline code for small blocks and a library call for large blocks. Bestie, it's giving adaptive energy. ✨-mstringop-strategy=alg— override the internal decision heuristic for the particular algorithm to use for inlining string operations. The allowed values for alg are:rep_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— override the internal decision heuristic to decide if"__builtin_memcpy"should be inlined and what inline algorithm to use when the expected size of the copy operation is known. strategy is a comma-separated list ofalg:max_size:dest_aligntriplets. alg is specified in-mstringop-strategy, max_size specifies the max byte size with which inline algorithm alg is allowed. For the last triplet, the max_size must be"-1". The max_size of the triplets in the list must be specified in increasing order. The minimal byte size for alg is 0 for the first triplet and"max_size + 1"of the preceding range.-mmemset-strategy=strategy— this option is similar to-mmemcpy-strategy=except that it controls"__builtin_memset"expansion instead. 💜-momit-leaf-frame-pointer— don't keep the frame pointer in a register for leaf functions. This avoids the instructions to save, set up, and restore frame pointers and makes an extra register available in leaf functions. The option-fomit-leaf-frame-pointerremoves 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 (%gsfor 32-bit,%fsfor 64-bit), or whether the thread base pointer must be added. Whether or not this is valid depends on the operating system, and whether it maps the segment to cover the entire TLS area.
For systems that use the GNU C Library, the default is on.
-msse2avx-mno-sse2avx— specify that the assembler should encode SSE instructions with VEX prefix. The option-mavxturns this on by default. ✨-mfentry-mno-fentry— if profiling is active (-pg), put the profiling counter call before the prologue. Note: on x86 architectures the attribute"ms_hook_prologue"isn't possible at the moment for-mfentryand-pg. ⚠️-mrecord-mcount-mno-record-mcount— if profiling is active (-pg), generate a__mcount_locsection that contains pointers to each profiling call. This is useful for automatically patching and out calls. 🎀-mnop-mcount-mno-nop-mcount— if profiling is active (-pg), generate the calls to the profiling functions as NOPs. This is useful when they should be patched in later dynamically. This is likely only useful together with-mrecord-mcount.-minstrument-return=type— instrument function exit in-pg-mfentryinstrumented functions with a call to the specified function. This only instruments true returns ending with ret, but not sibling calls ending with jump. Valid types arenoneto not instrument,callto generate a call to__return__, ornop5to generate a 5 byte nop. 💅-mrecord-return-mno-record-return— generate a__return_locsection pointing to all return instrumentation code.-mfentry-name=name— set the name of the__fentry__symbol called at function entry for-pg-mfentryfunctions.-mfentry-section=name— set the name of the section to record-mrecord-mcountcalls (default__mcount_loc).-mskip-rax-setup-mno-skip-rax-setup— when generating code for the x86-64 architecture with SSE extensions disabled,-mskip-rax-setupcan be used to skip setting up the RAX register when there are no variable arguments passed in vector registers.
Warning: 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. 💀
-m8bit-idiv-mno-8bit-idiv— on some processors, like Intel Atom, 8-bit unsigned integer divide is much faster than 32-bit/64-bit integer divide. This option generates a run-time check. If both dividend and divisor are within range of 0 to 255, 8-bit unsigned integer divide is used instead of 32-bit/64-bit integer divide. 🔥-mavx256-split-unaligned-load-mavx256-split-unaligned-store— split 32-byte AVX unaligned load and store. ✂️-mstack-protector-guard=guard-mstack-protector-guard-reg=reg-mstack-protector-guard-offset=offset— generate stack protection code using a canary at guard. Supported locations areglobalfor global canary ortlsfor per-thread canary in the TLS block (the default). 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— bestie, this one makes the compiler use only the general-purpose registers. No floating-point, no vector, no mask, no bound registers. Just the basics. Periodt.-mindirect-branch=choice — converts indirect call and jump according to choice. The default iskeep, which ✨keeps✨ indirect call and jump unmodified, unbothered, thriving. Then: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 attribute indirect_branch. iykyk.
💀 Heads up:-mcmodel=largeis incompatible with-mindirect-branch=thunkand-mindirect-branch=thunk-externsince the thunk function may not be reachable in the large code model. They are NOT friends. Do not invite them to the same party.
🌸 Also note that-mindirect-branch=thunk-externis compatible with-fcf-protection=branchsince the external thunk can be made to enable control-flow check. Compatible!! We love a healthy relationship.
-mfunction-return=choice — converts function return according to choice. Default iskeep, which keeps function return unmodified, living its truth. Then: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 attribute function_return.
✨ Note that-mindirect-return=thunk-externis compatible with-fcf-protection=branchsince the external thunk can be made to enable control-flow check.
💀 And note that-mcmodel=largeis incompatible with-mfunction-return=thunkand-mfunction-return=thunk-externsince the thunk function may not be reachable in the large code model.
-mindirect-branch-register— forces indirect call and jump via register. That's it. That's the whole vibe. 🎀
These -m switches are supported in addition to the above on x86-64 processors in 64-bit environments. Slay.
-m32-m64-mx32-m16-miamcu— generate code for a 16-bit, 32-bit or 64-bit environment. Okay so let's break it down, bestie:-m32setsint,long, and pointer types to 32 bits, and generates code that runs on any i386 system.-m64setsintto 32 bits andlongand pointer types to 64 bits, and generates code for the x86-64 architecture. For Darwin only,-m64also turns off the-fno-picand-mdynamic-no-picoptions. Special little guy.-mx32setsint,long, and pointer types to 32 bits, and generates code for the x86-64 architecture.-m16is the same as-m32, except for that it outputs the.code16gccassembly directive at the beginning of the assembly output so that the binary can run in 16-bit mode. Cute lil throwback.-miamcugenerates code which conforms to Intel MCU psABI. It requires the-m32option to be turned on. No-m32, no deal. 👑
-mno-red-zone— do not use a so-called "red zone" for x86-64 code. The red zone is mandated by the x86-64 ABI; it is a 128-byte area beyond the location of the stack pointer that is not modified by signal or interrupt handlers and therefore can be used for temporary data without adjusting the stack pointer. The flag-mno-red-zonedisables this red zone. So iconic of it to just say no. 🚫-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. The main character, honestly. 👑-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. Mandatory. No skips.-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. Free spirit energy. 🦋-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.
🪟 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. Default behavior, so convenient. ✨-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. Very demure, very DLL. 💅-mnop-fun-dllimport— this option is available for Cygwin and MinGW targets. It specifies that thedllimportattribute should be ignored. Just ignore it, bestie. 🙈-mthread— 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. Unicode girlies rise. 🌈-mwin32— this option is available for Cygwin and MinGW targets. It specifies that the typical Microsoft Windows predefined macros are to be set in the pre-processor, but does not influence the choice of runtime library/startup code.-mwindows— this option is available for Cygwin and MinGW targets. It specifies that a GUI application is to be generated by instructing the linker to set the PE header subsystem type appropriately. GUI mode activated. 🎀-fno-set-stack-executable— this option is available for MinGW targets. It specifies that the executable flag for the stack used by nested functions isn't set. This is necessary for binaries running in kernel mode of Microsoft Windows, as there the User32 API, which is used to set executable privileges, isn't available. Necessary, not optional. 🔥-fwritable-relocated-rdata— this option is available for MinGW and Cygwin targets. It specifies that relocated-data in read-only section is put into the.datasection. This is a necessary for older runtimes not supporting modification of.rdatasections for pseudo-relocation.-mpe-aligned-commons— this option is available for Cygwin and MinGW targets. It specifies that the GNU extension to the PE file format that permits the correct alignment of COMMON variables should be used when generating code. It is enabled by default if GCC detects that the target assembler found during configuration supports the feature.
See also under x86 Options for standard options.
⚡ Xstormy16 Options ⚡
These options are defined for Xstormy16:
-msim— choose startup files and linker script suitable for the simulator. Simulation slay. 🧚
🌸 Xtensa Options 🌸
These options are supported for Xtensa targets:
-mconst16-mno-const16— enable or disable use ofCONST16instructions for loading constant values. TheCONST16instruction is currently not a standard option from Tensilica. When enabled,CONST16instructions are always used in place of the standardL32Rinstructions. The use ofCONST16is enabled by default only if theL32Rinstruction is not available.-mfused-madd-mno-fused-madd— enable or disable use of fused multiply/add and multiply/subtract instructions in the floating-point option. This has no effect if the floating-point option is not also enabled. Disabling fused multiply/add and multiply/subtract instructions forces the compiler to use separate instructions for the multiply and add/subtract operations. This may be desirable in some cases where strict IEEE 754-compliant results are required: the fused multiply add/subtract instructions do not round the intermediate result, thereby producing results with more bits of precision than specified by the IEEE standard. Disabling fused multiply add/subtract instructions also ensures that the program output is not sensitive to the compiler's ability to combine multiply and add/subtract operations. Precision girlies, this one's for you. 💜-mserialize-volatile-mno-serialize-volatile— when this option is enabled, GCC insertsMEMWinstructions beforevolatilememory references to guarantee sequential consistency. The default is-mserialize-volatile. Use-mno-serialize-volatileto omit theMEMWinstructions. Order matters bestie, that's the whole point. ✨-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 code gets special treatment. 👑-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. Organized and cozy. 🎀-mauto-litpools,-mno-auto-litpools— these two control the whole literal-pool situation, and the default is-mno-auto-litpools, which puts literals in a separate section in the output file unless-mtext-section-literalsis used. Turn on-mauto-litpoolsand the literals get interspersed in the text section by the assembler instead. The compiler stops producing explicit".literal"directives and loads literals into registers withMOVIinstead ofL32R, so the assembler can do relaxation and place literals wherever it needs them. This option lets the assembler create several literal pools per function and assemble very big functions, which may not be possible with-mtext-section-literals. 🫶-mtarget-align,-mno-target-align— when this is on, GCC tells the assembler to automatically align instructions so branch penalties go down, at the cost of a little code density. The assembler tries to widen density instructions to align branch targets and the instructions that follow call instructions. If there aren't enough preceding safe density instructions to align a target, no widening happens. Default is-mtarget-align. These options do not touch the treatment of auto-aligned instructions likeLOOP, which the assembler always aligns, either by widening density instructions or by insertingNOPinstructions. Periodt. ✨-mlongcalls,-mno-longcalls— flip this on and GCC tells the assembler to translate direct calls into indirect calls, unless it can figure out that the target of a direct call is in the range the call instruction allows. This translation typically happens for calls to functions in other source files. Specifically, the assembler translates a directCALLinstruction into anL32Rfollowed by aCALLXinstruction. Default is-mno-longcalls. You want this in programs where the call target could potentially be out of range.
Heads up: this option is implemented in the assembler, not the compiler, so the assembly code GCC generates still shows direct call instructions — go look at the disassembled object code to see the actual instructions. And ngl, the assembler uses an indirect call for every cross-file call, not just the ones that really are out of range. 😭
zSeries Options
These are listed under
🌸 ENVIRONMENT 🌸
This bit describes several environment variables that affect how GCC operates. Some of them work by specifying directories or prefixes to use when searching for various kinds of files. Some are used to specify other aspects of the compilation environment. 💖
Note that you can also specify places to search using options such as -B, -I and -L. These take precedence over places specified using environment variables, which in turn take precedence over those specified by the configuration of GCC. Order of operations, bestie. 👑
LANG, LC_CTYPE, LC_MESSAGES, LC_ALL
These environment variables control the way that GCC uses localization information, which is how GCC works with different national conventions. GCC inspects the locale categories LC_CTYPE and LC_MESSAGES if it has been configured to do so. These locale categories can be set to any value supported by your installation. A typical value is en_GB.UTF-8 for English in the United Kingdom encoded in UTF-8. 🇬🇧
The LC_CTYPE environment variable specifies character classification. GCC uses it to determine the character boundaries in a string; this is needed for some multibyte encodings that contain quote and escape characters that are otherwise interpreted as a string end or escape. 🧐
The LC_MESSAGES environment variable specifies the language to use in diagnostic messages.
If the LC_ALL environment variable is set, it overrides the value of LC_CTYPE and LC_MESSAGES; otherwise, LC_CTYPE and LC_MESSAGES default to the value of the LANG environment variable. If none of these variables are set, GCC defaults to traditional C English behavior. Default behavior always coming in clutch. 💅
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. Pass the baton, bestie. 🏃♀️
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. Iconic little path-swap trick, honestly. ✨
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.
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. Sneaky but slay. 💜
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. Reproducible builds girlies, we love you. 🌈
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 @command{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-9/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.
okay so this one is lowkey important — "subtle defects" is dev-speak for "your program will act cursed and you'll spend three days debugging it bestie 😭". And yes, passing the flags when you didn't need to is totally harmless, so when in doubt, ship it 💅
👀 SEE ALSO 👀
Like, go read these too, they're all part of the family 🫶
…and also the Info entries for gcc, cpp, as, ld, binutils and gdb. the Info pages are where the real lore lives, fr fr ✨
💖 AUTHOR 💖
Wanna know who actually built this whole thing? See the Info entry for gcc, or hit up <http://gcc.gnu.org/onlinedocs/gcc/Contributors.html> for the full roster of contributors to GCC. it's a whole village and they ATE 👑
©️ COPYRIGHT ©️
Copyright (c) 1988-2019 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.tl;dr: you're literally allowed to copy, share and remix this doc, as long as you keep the license, the invariant sections and those cover texts intact. that's the deal, and it's a genuinely good deal 🌈
and that's a wrap on the whole gcc(1) page bestie!! 🎀 you made it through every single section, every flag, every footnote — main character energy, no cap 💅💖
this page is from gcc-9, dated 2022-05-27, and it's GCC(1) — the compiler's manual page, the one you just read the entire way through like a legend 🦋✨
go compile something iconic. mwah. 🫶
📦 where this page comes from
- ubuntu jammy: g++-9-sparc64-linux-gnu 9.5.0-1ubuntu1~22.04cross1
- 💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash on : a language model's rewrite of the original, under its terms