GCC(1)
🎀 GCC(1) — GNU project C and C++ compiler 🎀
🌸 NAME
gcc — GNU project C and C++ compiler
✨ SYNOPSIS
gcc [-c|-S|-E] [-std=standard]
[-g] [-pg] [-Olevel]
[-Wwarn...] [-Wpedantic]
[-Idir...] [-Ldir...]
[-Dmacro[=defn]...] [-Umacro]
[-foption...] [-mmachine-option...]
[-o outfile] [@file] infile...Only the most useful options are listed here; see below for the remainder. g++ accepts mostly the same options as gcc.
💖 DESCRIPTION
Okay so when you invoke gcc, it normally does preprocessing, compilation, assembly and linking — the whole pipeline, bestie. The overall options allow you to stop this process at an intermediate stage. For example, the -c option says not to run the linker. Then the output consists of object files output by the assembler. Slay.
Other options are passed on to one or more stages of processing. Some options control the preprocessor and others the compiler itself. Yet other options control the assembler and linker; most of these are not documented here, since you rarely need to use any of them. No cap.
Most of the command-line options that you can use with GCC are useful for C programs; when an option is only useful with another language (usually C++), the explanation says so explicitly. If the description for a particular option does not mention a source language, you can use that option with all supported languages. Periodt.
The usual way to run GCC is to run the executable called gcc, or machine-gcc when cross-compiling, or machine-gcc-version to run a specific version of GCC. When you compile C++ programs, you should invoke GCC as g++ instead. Like, don't forget the plus-plus, bestie.
The gcc program accepts options and file names as operands. Many options have multi-letter names; therefore multiple single-letter options may not be grouped: -dv is very different from -d -v. Completely different energy, fr fr.
You can mix options and other arguments. For the most part, the order you use doesn't matter. Order does matter when you use several options of the same kind; for example, if you specify -L more than once, the directories are searched in the order specified. Also, the placement of the -l option is significant. Lowkey important.
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. That's the vibe.
Some options take one or more arguments typically separated either by a space or by the equals sign (=) from the option name. Unless documented otherwise, an argument can be either numeric or a string. Numeric arguments must typically be small unsigned decimal or hexadecimal integers. Hexadecimal arguments must begin with the 0x prefix. Arguments to options that specify a size threshold of some sort may be arbitrarily large decimal or hexadecimal integers followed by a byte size suffix designating a multiple of bytes such as kB and KiB for kilobyte and kibibyte, respectively, MB and MiB for megabyte and mebibyte, GB and GiB for gigabyte and gigibyte, and so on. Such arguments are designated by byte-size in the following text. Refer to the NIST, IEC, and other relevant national and international standards for the full listing and explanation of the binary and decimal byte size prefixes.
🎀 OPTIONS
🌈 Option Summary
Here is a summary of all the options, grouped by type. Explanations are in the following sections. Obsessed with this organization, honestly.
🛠️ Overall Options
-c-S-E-o file-dumpbase dumpbase-dumpbase-ext auxdropsuf-dumpdir dumppfx-x language-v-###--help[=class[,...]]--target-help--version-pass-exit-codes-pipe-wrapper @file-ffile-prefix-map=old=new-fcanon-prefix-map-fplugin=file-fplugin-arg-name=arg-fdump-ada-spec[-slim]-fada-spec-parent=unit-fdump-go-spec=file--assemble--compile--dumpbase dumpbase--dumpbase-ext auxdropsuf--dumpdir dumppfx--language=language--output=file--pass-exit-codes--pipe--preprocess--verbose
🧚 C Language Options
-ansi-std=standard-aux-info filename-fno-asm-fno-builtin-fno-builtin-function-fcond-mismatch-ffreestanding-fgimple-fgnu-tm-fgnu89-inline-fhosted-flax-vector-conversions-fms-extensions-fpermitted-flt-eval-methods=standard-fplan9-extensions-fsigned-bitfields-funsigned-bitfields-fsigned-char-funsigned-char-fstrict-flex-arrays[=n]-fsso-struct=endianness--ansi
💅 C++ Language Options
--compile-std-module-fabi-compat-version=n-fabi-version=n-fno-access-control-faligned-new=[n]-fno-assume-sane-operators-new-delete-fchar8_t-fcheck-new-fconcepts-fconcepts-diagnostics-depth=n-fconstexpr-depth=n-fconstexpr-cache-depth=n-fconstexpr-loop-limit=n-fconstexpr-ops-limit=n-fcontracts-fcontract-evaluation-semantic=[ignore|observe|enforce|quick_enforce]-fcontracts-conservative-ipa-fcontract-checks-outlined-fcontract-disable-optimized-checks-fcontracts-client-check=[none|pre|all]-fcontracts-definition-check=[on|off]-fcoroutines-fdiagnostics-all-candidates-fno-elide-constructors-fno-enforce-eh-specs-fext-numeric-literals-fno-gnu-keywords-fno-immediate-escalation-fno-implement-inlines-fimplicit-constexpr-fno-implicit-inline-templates-fno-implicit-templates-fmodule-header[=kind]-fmodule-implicit-inline-fno-module-lazy-fmodule-mapper=specification-fmodule-only-fmodules-fms-extensions-fnew-inheriting-ctors-fnew-ttp-matching-fno-nonansi-builtins-fnothrow-opt-fno-operator-names-fno-optional-diags-fno-pretty-templates-frange-for-ext-temps-freflection-fno-rtti-fsized-deallocation-fstrict-enums-fstrong-eval-order[=kind]-ftemplate-backtrace-limit=n-ftemplate-depth=n-fno-threadsafe-statics-fuse-cxa-atexit-fno-use-cxa-get-exception-ptr-fno-weak-nostdinc++-fvisibility-inlines-hidden-fvisibility-ms-compat-flang-info-include-translate[=header]-flang-info-include-translate-not-flang-info-module-cmi[=module]-stdlib=libstdc++,libc++-Wabbreviated-auto-in-template-arg-Wabi-tag-Waligned-new[=kind]-Wcatch-value-Wcatch-value=n-Wno-class-conversion-Wclass-memaccess-Wcomma-subscript-Wconditionally-supported-Wno-conversion-null-Wctad-maybe-unsupported-Wctor-dtor-privacy-Wdangling-reference-Wno-defaulted-function-deleted-Wno-delete-incomplete-Wdelete-non-virtual-dtor-Wno-deprecated-array-compare-Wdeprecated-copy-Wdeprecated-copy-dtor-Wno-deprecated-enum-enum-conversion-Wno-deprecated-enum-float-conversion-Wno-deprecated-literal-operator-Wdeprecated-variadic-comma-omission-Weffc++-Wno-elaborated-enum-base-Wno-exceptions-Wno-expose-global-module-tu-local-Wno-external-tu-local-Wextra-semi-Wno-global-module-Wno-inaccessible-base-Wno-inherited-variadic-ctor-Wno-init-list-lifetime-Winvalid-constexpr-Winvalid-imported-macros-Wno-invalid-offsetof-Wno-literal-suffix-Wmismatched-new-delete-Wmismatched-tags-Wmultiple-inheritance-Wnamespaces-Wnarrowing-Wnoexcept-Wnoexcept-type-Wnon-virtual-dtor-Wpessimizing-move-Wno-placement-new-Wplacement-new=n-Wrange-loop-construct-Wredundant-move-Wredundant-tags-Wreorder-Wregister-Wno-sfinae-incomplete-Wstrict-null-sentinel-Wno-subobject-linkage-Wtemplates-Wno-non-c-typedef-for-linkage-Wno-non-template-friend-Wold-style-cast-Woverloaded-virtual-Wno-pmf-conversions-Wself-move-Wsign-promo-Wsized-deallocation-Wsuggest-final-methods-Wsuggest-final-types-Wsuggest-override-Wno-template-body-Wno-template-id-cdtor-Wtemplate-names-tu-local-Wno-terminate-Wno-vexing-parse-Wvirtual-inheritance-Wno-virtual-move-assign-Wvolatile
🦋 Objective-C and Objective-C++ Language Options
-fconstant-string-class=class-name-fgnu-runtime-fnext-runtime-fno-nil-receivers-fobjc-abi-version=n-fobjc-call-cxx-cdtors-fobjc-direct-dispatch-fobjc-exceptions-fobjc-gc-fobjc-nilcheck-fobjc-std=objc1-fno-local-ivars-fivar-visibility=[public|protected|private|package]-freplace-objc-classes-fzero-link-gen-decls-Wassign-intercept-Wno-property-assign-default-Wno-protocol-Wobjc-root-class-Wselector-Wstrict-selector-match-Wundeclared-selector
🔥 OpenMP and OpenACC Options
-foffload=arg-foffload-options=arg-fopenacc-fopenacc-dim=geom-fopenmp-fopenmp-simd-fopenmp-target-simd-clone[=device-type]
🍓 Diagnostic Message Formatting Options
-fmessage-length=n-fdiagnostics-plain-output-fdiagnostics-show-location=[once|every-line]-fdiagnostics-color=[auto|never|always]-fdiagnostics-urls=[auto|never|always]-fdiagnostics-format=[text|sarif-stderr|sarif-file]-fdiagnostics-add-output=DIAGNOSTICS-OUTPUT-SPEC-fdiagnostics-set-output=DIAGNOSTICS-OUTPUT-SPEC-fno-diagnostics-json-formatting-fno-diagnostics-show-option-fno-diagnostics-show-caret-fno-diagnostics-show-event-links-fno-diagnostics-show-labels-fno-diagnostics-show-line-numbers-fno-diagnostics-show-cwe-fno-diagnostics-show-rules-fno-diagnostics-show-highlight-colors-fno-diagnostics-show-nesting-fno-diagnostics-show-nesting-locations-fdiagnostics-show-nesting-levels-fdiagnostics-minimum-margin-width=width-fdiagnostics-parseable-fixits-fdiagnostics-generate-patch-fdiagnostics-show-template-tree-fno-elide-type-fdiagnostics-path-format=[none|separate-events|inline-events]-fdiagnostics-show-path-depths-fno-show-column-fdiagnostics-column-unit=[display|byte]-fdiagnostics-column-origin=origin-fdiagnostics-escape-format=[unicode|bytes]-fdiagnostics-text-art-charset=[none|ascii|unicode|emoji]-fdiagnostics-show-context[=depth]
⚠️ Warning Options ⚠️
okay bestie, this is the big one. this is the entire roll call of warning flags, and yes, I mean every single one, so buckle in 🫶 these are the flags that make your compiler go "girl…" before it even thinks about producing a binary. no skipping, no summarising, we're doing the whole roster like a group chat attendance list.
- the starter pack, the ones everyone name-drops:
-fsyntax-only,-fmax-errors=n,-Wpedantic,-pedantic-errors,-fpermissive,-w,-Wextra,-Wall - the allocation + address gang:
-Wabi=n,-Waddress,-Wno-address-of-packed-member,-Waggregate-return,-Walloc-size,-Walloc-size-larger-than=byte-size,-Walloc-zero,-Walloca,-Walloca-larger-than=byte-size,-Wauto-profile,-Wno-aggressive-loop-optimizations,-Warith-conversion - array behaviour, all up in your business:
-Warray-bounds,-Warray-bounds=n,-Warray-compare,-Warray-parameter,-Warray-parameter=n,-Wno-attributes,-Wattribute-alias=n,-Wno-attribute-alias,-Wno-attribute-warning,-Wbidi-chars=[none|unpaired|any|ucn] - bools and the whole compatibility timeline, literally from C90 to C2y:
-Wbool-compare,-Wbool-operation,-Wno-builtin-declaration-mismatch,-Wno-builtin-macro-redefined,-Wc90-c99-compat,-Wc99-c11-compat,-Wc11-c23-compat,-Wc23-c2y-compat,-Wc++-compat,-Wc++11-compat,-Wc++14-compat,-Wc++17-compat,-Wc++20-compat,-Wc++26-compat,-Wno-c++11-extensions,-Wno-c++14-extensions,-Wno-c++17-extensions,-Wno-c++20-extensions,-Wno-c++23-extensions - casts, chars, comments, conversions:
-Wcalloc-transposed-args,-Wcannot-profile,-Wcast-align,-Wcast-align=strict,-Wcast-function-type,-Wcast-qual,-Wchar-subscripts,-Wclobbered,-Wcomment,-Wcompare-distinct-pointer-types,-Wno-complain-wrong-lang,-Wconversion,-Wno-coverage-mismatch,-Wno-cpp - dangling stuff and everything deprecated (we still love her):
-Wdangling-else,-Wdangling-pointer,-Wdangling-pointer=n,-Wdate-time,-Wno-deprecated,-Wno-deprecated-declarations,-Wno-designated-init,-Wno-deprecated-openmp,-Wdisabled-optimization,-Wno-discarded-array-qualifiers,-Wno-discarded-qualifiers,-Wno-div-by-zero,-Wdouble-promotion,-Wduplicated-branches,-Wduplicated-cond - empties, enums, and the
-Werrorroyalty 👑:-Wempty-body,-Wno-endif-labels,-Wenum-compare,-Wenum-conversion,-Wenum-int-mismatch,-Werror,-Werror=*,-Wexpansion-to-defined,-Wfatal-errors,-Wflex-array-member-not-at-end,-Wfloat-conversion,-Wfloat-equal - the format family, an entire dynasty:
-Wformat,-Wformat=2,-Wno-format-contains-nul,-Wno-format-diag,-Wno-format-extra-args,-Wformat-nonliteral,-Wformat-overflow=n,-Wformat-security,-Wformat-signedness,-Wformat-truncation=n,-Wformat-y2k - frames, headers, ignored things, implicit things:
-Wframe-address,-Wframe-larger-than=byte-size,-Wno-free-nonheap-object,-Wheader-guard,-Wno-if-not-aligned,-Wno-ignored-attributes,-Wignored-qualifiers,-Wno-incompatible-pointer-types,-Whardened,-Wimplicit,-Wimplicit-fallthrough,-Wimplicit-fallthrough=n,-Wno-implicit-function-declaration,-Wno-implicit-int,-Winfinite-recursion,-Winit-self,-Winline,-Wno-int-conversion - int-in-bool, invalid pch, invalid utf8, and friends:
-Wint-in-bool-context,-Wno-int-to-pointer-cast,-Wno-invalid-memory-model,-Winvalid-pch,-Winvalid-utf8,-Wno-unicode,-Wjump-misses-init,-Wkeyword-macro,-Wlarger-than=byte-size,-Wleading-whitespace=kind,-Wlogical-not-parentheses,-Wlogical-op,-Wlong-long,-Wno-lto-type-mismatch - main, memset, misleading indentation, and the missing-* crew:
-Wmain,-Wmaybe-uninitialized,-Wmemset-elt-size,-Wmemset-transposed-args,-Wmisleading-indentation,-Wmissing-attributes,-Wmissing-braces,-Wmissing-field-initializers,-Wmissing-format-attribute,-Wmissing-include-dirs,-Wmissing-noreturn,-Wmusttail-local-addr,-Wmaybe-musttail-local-addr,-Wno-missing-profile,-Wno-multichar,-Wmultistatement-macros - nonnull, normalized, null-deref, and the open* squad:
-Wnonnull,-Wnonnull-compare,-Wnormalized=[none|id|nfc|nfkc],-Wnull-dereference,-Wno-odr,-Wopenacc-parallelism,-Wopenmp,-Wopenmp-simd,-Wno-overflow,-Woverlength-strings,-Wno-override-init-side-effects - packed, padded, parentheses, pointers, pragmas:
-Wpacked,-Wno-packed-bitfield-compat,-Wpacked-not-aligned,-Wpadded,-Wparentheses,-Wno-pedantic-ms-format,-Wpointer-arith,-Wno-pointer-compare,-Wno-pointer-to-int-cast,-Wno-pragmas,-Wno-pragma-once-outside-header,-Wno-prio-ctor-dtor,-Wno-psabi - redundant, restrict, return-type, and the shadow variants (pick your flavour 🍓):
-Wredundant-decls,-Wrestrict,-Wno-return-local-addr,-Wreturn-type,-Wno-scalar-storage-order,-Wsequence-point,-Wshadow,-Wshadow=global,-Wshadow=local,-Wshadow=compatible-local,-Wno-shadow-ivar - shifts, signs, and sizeof:
-Wno-shift-count-negative,-Wno-shift-count-overflow,-Wshift-negative-value,-Wno-shift-overflow,-Wshift-overflow=n,-Wsign-compare,-Wsign-conversion,-Wno-sizeof-array-argument,-Wsizeof-array-div,-Wsizeof-pointer-div,-Wsizeof-pointer-memaccess - stack stuff, strict-everything, stringop, suggest-attribute:
-Wstack-protector,-Wstack-usage=byte-size,-Wstrict-aliasing,-Wstrict-aliasing=n,-Wstrict-overflow,-Wstrict-overflow=n,-Wstring-compare,-Wno-stringop-overflow,-Wno-stringop-overread,-Wno-stringop-truncation,-Wstrict-flex-arrays,-Wsuggest-attribute=attribute-name - the switch kingdom 👑:
-Wswitch,-Wno-switch-bool,-Wswitch-default,-Wswitch-enum,-Wno-switch-outside-range,-Wno-switch-unreachable,-Wsync-nand,-Wsystem-headers - tautological takes, trailing whitespace, trampolines, trigraphs:
-Wtautological-compare,-Wtrailing-whitespace,-Wtrailing-whitespace=kind,-Wtrampolines,-Wtrigraphs,-Wtrivial-auto-var-init,-Wno-tsan,-Wtype-limits,-Wundef,-Wuninitialized,-Wunknown-pragmas,-Wunsuffixed-float-constants,-Wunterminated-string-initialization - the entire unused cinematic universe (she contains multitudes):
-Wunused,-Wunused-but-set-parameter,-Wunused-but-set-parameter=n,-Wunused-but-set-variable,-Wunused-but-set-variable=n,-Wunused-const-variable,-Wunused-const-variable=n,-Wunused-function,-Wunused-label,-Wunused-local-typedefs,-Wunused-macros,-Wunused-parameter,-Wno-unused-result,-Wunused-value,-Wunused-variable - use-after-free, useless-cast, varargs, vla, volatile, write-strings, and the tail end:
-Wuse-after-free,-Wuse-after-free=n,-Wuseless-cast,-Wno-varargs,-Wvariadic-macros,-Wvector-operation-performance,-Wvla,-Wvla-larger-than=byte-size,-Wno-vla-larger-than,-Wvolatile-register-var,-Wwrite-strings,-Wno-xor-used-as-pow,-Wzero-as-null-pointer-constant,-Wzero-length-bounds - the double-dash girlies, aka the long-form aliases ✨:
--all-warnings,--extra-warnings,--no-warnings,--pedantic,--pedantic-errors
like. that's the whole list. it's giving "I have a warning flag for every emotion" and honestly? iconic 💅
🔍 Static Analyzer Options 🔍
so the static analyzer is basically the friend who reads your code and goes "bestie… this pointer? it's not okay." here's her whole toolkit — every flag, no cap 🦋
- the core switches:
-fanalyzer,-fanalyzer-assume-nothrow,-fanalyzer-call-summaries,-fanalyzer-checker=name,-fno-analyzer-feasibility,-fanalyzer-show-events-in-system-headers,-fno-analyzer-state-merge,-fno-analyzer-state-purge,-fno-analyzer-suppress-followups,-fanalyzer-transitivity,-fno-analyzer-undo-inlining,-fanalyzer-verbose-edges,-fanalyzer-verbose-state-changes,-fanalyzer-verbosity=level - the dump squad (she keeps receipts):
-fdump-analyzer,-fdump-analyzer-callgraph,-fdump-analyzer-exploded-graph,-fdump-analyzer-exploded-nodes,-fdump-analyzer-exploded-nodes-2,-fdump-analyzer-exploded-nodes-3,-fdump-analyzer-exploded-paths,-fdump-analyzer-feasibility,-fdump-analyzer-infinite-loop,-fdump-analyzer-json,-fdump-analyzer-state-purge,-fdump-analyzer-stderr,-fdump-analyzer-supergraph,-fdump-analyzer-untracked - file-descriptor drama, double-frees, leaks, and the exposure of things:
-Wno-analyzer-double-fclose,-Wno-analyzer-double-free,-Wno-analyzer-exposure-through-output-file,-Wno-analyzer-exposure-through-uninit-copy,-Wno-analyzer-fd-access-mode-mismatch,-Wno-analyzer-fd-double-close,-Wno-analyzer-fd-leak,-Wno-analyzer-fd-phase-mismatch,-Wno-analyzer-fd-type-mismatch,-Wno-analyzer-fd-use-after-close,-Wno-analyzer-fd-use-without-check,-Wno-analyzer-file-leak,-Wno-analyzer-free-of-non-heap,-Wno-analyzer-imprecise-fp-arithmetic,-Wno-analyzer-infinite-loop,-Wno-analyzer-infinite-recursion,-Wno-analyzer-jump-through-null,-Wno-analyzer-malloc-leak,-Wno-analyzer-mismatching-deallocation - null stuff, out-of-bounds, overlapping buffers, putenv of auto-var (scream):
-Wno-analyzer-null-argument,-Wno-analyzer-null-dereference,-Wno-analyzer-out-of-bounds,-Wno-analyzer-overlapping-buffers,-Wno-analyzer-possible-null-argument,-Wno-analyzer-possible-null-dereference,-Wno-analyzer-putenv-of-auto-var,-Wno-analyzer-shift-count-negative,-Wno-analyzer-shift-count-overflow,-Wno-analyzer-stale-setjmp-buffer - the tainted-* family and other trust issues:
-Wno-analyzer-tainted-allocation-size,-Wno-analyzer-tainted-assertion,-Wno-analyzer-tainted-array-index,-Wno-analyzer-tainted-divisor,-Wno-analyzer-tainted-offset,-Wno-analyzer-tainted-size,-Wno-analyzer-throw-of-unexpected-type,-Wanalyzer-symbol-too-complex,-Wanalyzer-too-complex - undefined behaviour, unsafe signal handlers, use-after-free, stale stack frames, uninitialized values:
-Wno-analyzer-undefined-behavior-ptrdiff,-Wno-analyzer-undefined-behavior-strtok,-Wno-analyzer-unsafe-call-within-signal-handler,-Wno-analyzer-use-after-free,-Wno-analyzer-use-of-pointer-in-stale-stack-frame,-Wno-analyzer-use-of-uninitialized-value - varargs chaos + writing to things you absolutely should not write to 💀:
-Wno-analyzer-va-arg-type-mismatch,-Wno-analyzer-va-list-exhausted,-Wno-analyzer-va-list-leak,-Wno-analyzer-va-list-use-after-va-end,-Wno-analyzer-write-to-const,-Wno-analyzer-write-to-string-literal
🌸 C and Objective-C-only Warning Options 🌸
these ones are ✨exclusive✨ — C and Objective-C only, so if you're a C++ main character these aren't for you, and that's okay 🎀
-Wbad-function-cast,-Wdeprecated-non-prototype,-Wfree-labels,-Wmissing-declarations,-Wmissing-parameter-name,-Wmissing-parameter-type,-Wdeclaration-missing-parameter-type,-Wmissing-prototypes,-Wmissing-variable-declarations,-Wmultiple-parameter-fwd-decl-lists,-Wnested-externs,-Wold-style-declaration,-Wold-style-definition,-Wstrict-prototypes,-Wtraditional,-Wtraditional-conversion,-Wdeclaration-after-statement,-Wpointer-sign
like the -Wold-style-* ones are literally the "your ancestors called and they want their declarations back" flags 😭
🐛 Debugging Options 🐛
debugging era!! this is the section where you learn what your binary is actually doing instead of just vibing and hoping 🦋 every flag below, exactly as it comes:
- the
-gcinematic universe:-g,-glevel,-gdwarf,-gdwarf-version,-gbtf,-gctf,-gctflevel,-gno-prune-btf,-ggdb,-gno-record-gcc-switches,-gstrict-dwarf - location tracking, codeview, column info, dwarf32/dwarf64:
-gas-loc-support,-gas-locview-support,-gcodeview,-gcolumn-info,-gdwarf32,-gdwarf64,-gno-statement-frontiers,-gno-variable-location-views,-gvariable-location-views=incompat5,-ginternal-reset-location-views,-ginline-points,-gvms,-gz[=type],-gsplit-dwarf,-gdescribe-dies - the
-fdebug family, aka "please keep / please don't keep this info":-fdebug-prefix-map=old=new,-fdebug-types-section,-fno-eliminate-unused-debug-types,-femit-struct-debug-baseonly,-femit-struct-debug-reduced,-femit-struct-debug-detailed[=spec-list],-fno-eliminate-unused-debug-symbols,-femit-class-debug-always,-fno-merge-debug-strings,-fno-dwarf2-cfi-asm,-fvar-tracking,-fvar-tracking-assignments,-fvar-tracking-uninit - and the long-form shortcut at the end:
--debug
lowkey the way this section alone has more flags than most people will use in a lifetime 💀 but that's the vibe, she's thorough, she contains multitudes 💜
⚡ Optimization Options ⚡
okay so bestie. this is the section. the one you scroll past with your thumb going nyoom because there are like four hundred flags in here and every single one starts with -f 💀. it's giving compiler-girl autocomplete-core. do NOT feel bad if your eyes glaze over, that's normal, that's expected, that is literally the correct response.
here's the thing though — i'm gonna give you every single one of them, in order, exactly as they're written, because a manpage is not a vibe, a manpage is a contract 🫶. so buckle up, sip your lil drink, and let's go ✨
-faggressive-loop-optimizations -falign-functions[=n[:m:[n2[:m2]]]]
-falign-jumps[=n[:m:[n2[:m2]]]] -falign-labels[=n[:m:[n2[:m2]]]]
-falign-loops[=n[:m:[n2[:m2]]]] -fmin-function-alignment=[n]
-fno-allocation-dce -fallow-store-data-races -fassociative-math
-fauto-profile -fauto-profile[=path] -fauto-profile-inlining
-fauto-inc-dec -fbranch-probabilities -fcaller-saves
-fcombine-stack-adjustments -fconserve-stack -ffold-mem-offsets
-fcompare-elim -fcprop-registers -fcrossjumping -fcse-follow-jumps
-fcse-skip-blocks -fcx-fortran-rules -fcx-limited-range -fcx-method
-fdata-sections -fdce -fdelayed-branch
-fdelete-null-pointer-checks -fdep-fusion -fdevirtualize
-fdevirtualize-speculatively -fdevirtualize-at-ltrans -fdse
-fearly-inlining -fexcess-precision=style -fexpensive-optimizations
-fext-dce -ffast-math -ffat-lto-objects -ffinite-loops
-ffinite-math-only -ffloat-store -fforward-propagate
-ffp-contract=style -ffp-int-builtin-inexact -ffunction-sections
-ffuse-ops-with-volatile-access -fgcse -fgcse-after-reload
-fgcse-las -fgcse-lm -fgraphite-identity -fgcse-smsee those [=n[:m:[n2[:m2]]]] brackets up top? that's not a typo, that's not me mashing the keyboard, that's genuinely how the alignment flags are written 😭 four nested optional things. she's got layers. she contains multitudes. the -falign-jumps, -falign-labels, -falign-loops girlies all share the same bracket situation, and -fmin-function-alignment=[n] is up there too with its own single bracket. copy those characters exactly or the compiler will simply not hear you 💅
now the middle chunk, where we start getting into the inlining and IPA flags — -fipa- everything, all the little friends, they're all here:
-fhoist-adjacent-loads -fif-conversion -fif-conversion2
-findirect-inlining -finline-atomics -finline-functions
-finline-functions-called-once -finline-limit=n
-finline-small-functions -finline-stringops[=fn] -fipa-modref
-fipa-cp -fipa-cp-clone -fipa-bit-cp -fipa-vrp -fipa-pta
-fipa-profile -fipa-pure-const -fipa-reference
-fipa-reference-addressable -fipa-reorder-for-locality -fipa-sra
-fipa-stack-alignment -fipa-icf -fipa-icf-functions
-fipa-icf-variables -fira-algorithm=algorithm
-flate-combine-instructions -flifetime-dse -flive-patching=level
-fira-region=region -fira-hoist-pressure -fira-loop-pressure
-fno-ira-share-save-slots -fno-ira-share-spill-slots
-fisolate-erroneous-paths-dereference
-fisolate-erroneous-paths-attribute -fivopts
-fkeep-inline-functions -fkeep-static-functions
-fkeep-static-consts -flimit-function-alignment
-flive-range-shrinkage -floop-block -floop-interchange
-floop-strip-mine -floop-unroll-and-jam -floop-nest-optimize
-floop-parallelize-all -flra-remat -fltonotice how -fisolate-erroneous-paths-dereference and -fisolate-erroneous-paths-attribute are on their own lil lines because they're just that long? iconic behavior honestly. and -fira-algorithm=algorithm is not a stutter, bestie, that's literally the flag — the word algorithm is the placeholder for the actual algorithm name. do not "fix" it. do not get cute with it. 🙈
also hi -flto, you tiny three-letter legend, you're about to spawn a whole family of friends in the next block 👑
-flto-compression-level=n -flto-toplevel-asm-heuristics
-flto-partition=alg -flto-incremental=path
-flto-incremental-cache-size=n -fmalloc-dce -fmerge-all-constants
-fmerge-constants -fmodulo-sched -fmodulo-sched-allow-regmoves
-fmove-loop-invariants -fmove-loop-stores -fno-branch-count-reg
-fno-defer-pop -fno-function-cse -fno-guess-branch-probability
-fno-inline -fno-math-errno -fno-peephole -fno-peephole2
-fno-printf-return-value -fno-sched-interblock -fno-sched-spec
-fno-signed-zeros -fno-toplevel-reorder -fno-trapping-math
-fno-zero-initialized-in-bss -fomit-frame-pointer -foptimize-crc
-foptimize-sibling-calls -fpartial-inlining -fpeel-loops
-fpredictive-commoning -fprefetch-loop-arrays -fprofile-correction
-fprofile-use -fprofile-use=path -fprofile-partial-training
-fprofile-values -fprofile-reorder-functions -freciprocal-math
-free -frename-registers -freorder-blocks
-freorder-blocks-algorithm=algorithm -freorder-blocks-and-partitionokay -flto-partition=alg — yes, the placeholder there is really alg, not algorithm, while -fira-algorithm and -freorder-blocks-algorithm both use algorithm. inconsistent? yes ma'am. are we changing it? absolutely not, that's the spec, that's the text, we transcribe and we move on 💀
and a whole run of -fno- girlies in the middle — -fno-defer-pop, -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 — they're all here, they're all accounted for, nobody got left behind 🫶
-freorder-functions -frerun-cse-after-loop
-freschedule-modulo-scheduled-loops -frounding-math
-fsave-optimization-record -fsched2-use-superblocks
-fsched-pressure -fsched-spec-load -fsched-spec-load-dangerous
-fsched-stalled-insns-dep[=n] -fsched-stalled-insns[=n]
-fsched-group-heuristic -fsched-critical-path-heuristic
-fsched-spec-insn-heuristic -fsched-rank-heuristic
-fsched-last-insn-heuristic -fsched-dep-count-heuristic
-fschedule-fusion -fschedule-insns -fschedule-insns2
-fsection-anchors -fselective-scheduling -fselective-scheduling2
-fsel-sched-pipelining -fsel-sched-pipelining-outer-loops
-fsemantic-interposition -fshrink-wrap -fshrink-wrap-separate
-fsignaling-nans -fsingle-precision-constant
-fsplit-ivs-in-unroller -fsplit-loops
-fspeculatively-call-stored-functions -fsplit-paths
-fsplit-wide-types -fsplit-wide-types-early -fssa-backprop-fsched-spec-load-dangerous — she has dangerous right there in the name, no cap, and yes that is the actual full flag name, i'm not being dramatic (i'm always being a little dramatic but not about this) 🔥
also pls appreciate the six -fsched-*-heuristic friends in a row: -fsched-group-heuristic, -fsched-critical-path-heuristic, -fsched-spec-insn-heuristic, -fsched-rank-heuristic, -fsched-last-insn-heuristic, -fsched-dep-count-heuristic. that's a whole friend group standing in a line for a photo 📸
-fssa-phiopt -fstdarg-opt -fstore-merging -fstrict-aliasing
-fipa-strict-aliasing -fthread-jumps -ftracer -ftree-bit-ccp
-ftree-builtin-call-dce -ftree-ccp -ftree-ch -ftree-coalesce-vars
-ftree-copy-prop -ftree-cselim -ftree-dce -ftree-dominator-opts
-ftree-dse -ftree-forwprop -ftree-fre -fcode-hoisting
-ftree-loop-if-convert -ftree-loop-im -ftree-phiprop
-ftree-loop-distribution -ftree-loop-distribute-patterns
-ftree-loop-ivcanon -ftree-loop-linear -ftree-loop-optimize
-ftree-loop-vectorize -ftree-parallelize-loops[=n] -ftree-pre
-ftree-partial-pre -ftree-pta -ftree-reassoc -ftree-scev-cprop
-ftree-sink -ftree-slsr -ftree-sra -ftree-switch-conversion
-ftree-tail-merge -ftree-ter -ftree-vectorize -ftree-vrp
-ftrivial-auto-var-init -funconstrained-commons -funit-at-a-time
-funroll-all-loops -funroll-loops -funsafe-math-optimizations
-funswitch-loops -fipa-ra -fvariable-expansion-in-unroller
-fvect-cost-model -fvpt -fweb -fwhole-program -fwpa
-fuse-linker-plugin -fzero-call-used-regs -O -O0 -O1 -O2 -O3
-Os -Ofast -Og -Oz --optimizeand there they are at the bottom, the main characters 🌈 — -O, -O0, -O1, -O2, -O3, -Os, -Ofast, -Og, -Oz, and long-form --optimize. the whole -O family, all nine of them, chillin at the end of the flag parade like the popular kids arriving fashionably late to the party 👑
that's the optimization list. all of it. you survived. i'm proud of you fr fr 🎀
🧪 Program Instrumentation Options 🧪
okay so this section is the measure-y one — profiling, sanitizers, stack hardening, all the "i would like to know what my program is actually doing" energy 🔍. it is also long. like, longer than you'd think. the -fsanitize= stuff lives here, the -fprofile- stuff lives here, the -fstack-protector family lives here, the strub girlies live here. everybody's here. it's a party.
-p -pg -fprofile-arcs -coverage -ftest-coverage
-fcondition-coverage -fpath-coverage -fprofile -fprofile-abs-path
-fprofile-dir=path -fprofile-generate -fprofile-generate=path
-fprofile-info-section -fprofile-info-section=name
-fprofile-note=path -fprofile-prefix-path=path
-fprofile-update=method -fprofile-filter-files=regex
-fprofile-exclude-files=regex
-fprofile-reproducible=[multithreaded|parallel-runs|serial]
-fsanitize=style -fsanitize-recover -fsanitize-recover=style
-fsanitize-trap -fsanitize-trap=style -fasan-shadow-offset=number
-fsanitize-sections=s1,s2,... -fsanitize-undefined-trap-on-error
-fcf-protection -fcf-protection=[full|branch|return|none|check]quick heads up bestie: -fsanitize-trap -fsanitize-trap=style has a triple space situation in there between the two, that's just how it's typeset, not a secret third thing 💀 Also note -fcf-protection appears twice — once bare, once with the full =[full|branch|return|none|check] value list. both real. both yours. don't collapse them 🫶
-fharden-compares -fharden-conditional-branches -fhardened
-fharden-control-flow-redundancy -fhardcfr-skip-leaf
-fhardcfr-check-exceptions -fhardcfr-check-returning-calls
-fhardcfr-check-noreturn-calls=[always|no-xthrow|nothrow|never]
-fstack-protector -fstack-protector-all -fstack-protector-strong
-fstack-protector-explicit -fstack-check -fstack-limit-register=reg
-fstack-limit-symbol=sym -fno-stack-limit -fsplit-stack
-fstrub=disable -fstrub=strict -fstrub=relaxed -fstrub=all
-fstrub=at-calls -fstrub=internal
-fvtable-verify=[std|preinit|none] -fvtv-counts -fvtv-debug
-finstrument-functions -finstrument-functions-once
-finstrument-functions-exclude-function-list=sym,sym,...
-finstrument-functions-exclude-file-list=file,file,...
-fprofile-prefix-map=old=new -fpatchable-function-entry=N[,M]
--coverage --profilethe -fhardcfr-check-noreturn-calls=[always|no-xthrow|nothrow|never] value list is a whole vibe — four options and one of them is no-xthrow with the hyphen, keep that hyphen, it matters 😤
and -fstrub= has six forms: disable, strict, relaxed, all, at-calls, internal. six. count 'em. i did 💅
-fprofile-prefix-map=old=new — yes, exactly like that, old and new as placeholders. i'm not gonna sit here and editorialize about it, i'm just telling you it's in the list and it's staying in the list.
and the two long-form stragglers at the bottom: --coverage and --profile. cute. love them. ✨
🔤 Preprocessor Options 🔤
this is the preprocessor corner, where -D and -U and the entire -M alphabet mafia live 🅰️. huge -M-family presence here — -M, -MD, -MF, -MG, -MM, -MMD, -MP, -MQ, -MT — plus a whole set of long-form -- options. and yes -Mno-modules is in there, sitting right between -MT and -no-integrated-cpp, looking a little out of place, doing its best 🥺
-C -CC -Dmacro[=defn] -dD -dI -dM -dN -dU -fdebug-cpp
-fdirectives-only -fdollars-in-identifiers -fexec-charset=charset
-fextended-identifiers -finput-charset=charset
-fmacro-prefix-map=old=new -fmax-include-depth=depth
-fno-canonical-system-headers -fpch-deps -fpch-preprocess
-fpreprocessed -ftabstop=width -ftrack-macro-expansion
-fwide-exec-charset=charset -fworking-directory -H -imacros file
-include file -M -MD -MF -MG -MM -MMD -MP -MQ -MT
-Mno-modules -no-integrated-cpp -P -pthread -remap -traditional
-traditional-cpp -trigraphs -Umacro -undef -Wp,option
-Xpreprocessor option --comments --comments-in-macros
--define-macro=macro[=defn] --dependencies --dump=letters
--imacros=file --include=file --no-integrated-cpp
--no-line-commands --print-missing-file-dependencies --traditional
--traditional-cpp --trigraphs --trace-includes
--undefine-macro=macro --user-dependencies --write-dependencies
--write-user-dependenciesokay -C and -CC are two different flags and you must not merge them in your brain OR your terminal, they are sisters not twins 🫶
also -Dmacro[=defn] and -Umacro — the placeholders macro and defn are literally part of how it's written, so when you see them in the docs, sub in your own.
and look at the tail end: --comments, --comments-in-macros, --define-macro=macro[=defn], --dependencies, --dump=letters, --imacros=file, --include=file, --no-integrated-cpp, --no-line-commands, --print-missing-file-dependencies, --traditional, --traditional-cpp, --trigraphs, --trace-includes, --undefine-macro=macro, --user-dependencies, --write-dependencies, --write-user-dependencies. that's a lot of long forms. gcc really said "what if every short option had a dramatic longer sibling" and honestly? slay 💅
note -pthread shows up in here too — it's in this section's list, that's just where it's listed, we don't argue with the manpage, we transcribe the manpage 🎀
🔧 Assembler Options 🔧
three. literally three. after the absolute wall of text that was the last two sections, the assembler section is a lil vacation 🌸
-Wa,option -Xassembler option --for-assembler=option-Wa,option (comma, no space), -Xassembler option (space, no comma), --for-assembler=option (equals sign, long form). three flags, three different punctuation situations, all correct, all exact 🦋
🔗 Linker Options 🔗
okay this one's fun because it starts with object-file-name — which is not a flag at all, it's a placeholder for, you know, an object file name. but it's listed first anyway because that's the order, and we respect the order 👑
the static-lib family is all here: -static-libgcc, -static-libstdc++, -static-libasan, -static-libhwasan, -static-liblsan, -static-libtsan, -static-libubsan. seven of them. sanitiser girlies get their own static versions and honestly they deserve it 🫶
object-file-name -flink-libatomic -fuse-ld=linker -llibrary
-nostartfiles -nodefaultlibs -nolibc -nostdlib -nostdlib++ -e
entry -pie -pthread -r -rdynamic -s -static -static-pie
-static-libgcc -static-libstdc++ -static-libasan -static-libhwasan
-static-liblsan -static-libtsan -static-libubsan -shared
-shared-libgcc -symbolic -T script -Wl,option -Xlinker option -u
symbol -Tbss=addr -Tdata=addr -Ttext=addr -N -n -t -Z -z
keyword --entry=entry --for-linker=option --force-link=symbol
--no-standard-library --pie --static --static-pie --symbolicsee how -e is on one line and its argument entry is on the next? same with -u / symbol and -T / script and -z / keyword. that's just the line wrapping, the flag and its argument are together, don't let the newline gaslight you 😭
and look at the single-letter girlies down there at the end: -N, -n, -t, -Z, -z. case matters! -N is not -n, -Z is not -z, these are four different flags and one pair is capitalized. ngl this is the most stressful part of any manpage 💀
the long forms at the bottom get their own lil row: --entry=entry, --for-linker=option, --force-link=symbol, --no-standard-library, --pie, --static, --static-pie, --symbolic ✨
📁 Directory Options 📁
the -I and -L section!! search paths, sysroots, prefix stuff, and a whole bunch of --include-* long forms that all sound the same and are all different, i promise 🌈
-Bprefix -Idir -I- -idirafter dir -imacros file -imultilib dir
-imultiarch dir -iplugindir=dir -iprefix file -iquote dir
-isysroot dir -isystem dir -iwithprefix dir -iwithprefixbefore dir
-Ldir -no-canonical-prefixes --no-sysroot-suffix -nostdinc
-nostdinc++ --embed-dir=dir --embed-directory=dir --include-barrier
--include-directory=dir --include-directory-after=dir
--include-prefix=prefix --include-with-prefix=prefix
--include-with-prefix-after=prefix
--include-with-prefix-before=prefix --no-canonical-prefixes
--no-standard-includes --prefix=prefix --sysroot=dirokay important detail: -I- is a real one, that's capital-I followed by a dash, and it's different from -Idir where dir is a placeholder. one is punctuation, one is a stand-in. do not mix them up 🙈
also -iprefix file takes a file placeholder, but -iplugindir=dir uses an = with a dir placeholder, and -Bprefix has no separator at all — the prefix is glued right onto the -B. three different argument styles in one section. that's the gcc experience, bestie 💅
and notice --no-canonical-prefixes appears twice in this list — once with the short-ish ordering and once again near the bottom. yes really. it's in the original twice, it's in mine twice. i don't make the rules, i just enforce them 😤
-nostdinc, -nostdinc++, --no-standard-includes, --no-sysroot-suffix, --sysroot=dir, --prefix=prefix — all present and correct 🎀
🧬 Code Generation Options 🧬
last section of this part!! this is the "how should the actual assembly come out" corner — calling conventions, PIC/PIE, visibility, TLS models, and the stack-reuse situation 🏗️
-fcall-saved-reg -fcall-used-reg -ffixed-reg -fexceptions
-fnon-call-exceptions -fdelete-dead-exceptions -funwind-tables
-fasynchronous-unwind-tables -fno-gnu-unique
-finhibit-size-directive -fcommon -fno-ident -fpcc-struct-return
-fpic -fPIC -fpie -fPIE -fno-plt -fno-jump-tables
-fno-bit-tests -frecord-gcc-switches -freg-struct-return
-fshort-enums -fshort-wchar -fverbose-asm -fpack-struct[=n]
-fleading-underscore -ftls-model=model -fstack-reuse=reuse_level
-ftrampolines -ftrampoline-impl=[stack|heap] -ftrapv -fwrapv
-fwrapv-pointer -fvisibility=[default|internal|hidden|protected]
-fstrict-volatile-bitfields -fsync-libcalls
-fzero-init-padding-bits=value -Qy -Qnokay the PIC/PIE quartet: -fpic, -fPIC, -fpie, -fPIE. case is the whole entire point 🔥 lowercase and uppercase are separate flags, they mean different things, and if you type the wrong one you will simply have a different experience. this is the most case-sensitive cluster on the whole page and yes i'm a lil stressed about it for you 😭
-ftrampoline-impl=[stack|heap] with the two values, -fvisibility=[default|internal|hidden|protected] with four — those bracket lists are part of the option, keep the pipes, keep the brackets, keep the spelling 🫶
and at the very very end: -Qy and -Qn. capital Q. lowercase y or n. two flags. tiny legends. that's how this part of the page ends 🌸
okay that's where this chunk runs out of room bestie — there is so much more of this page coming, like genuinely so much, so don't you dare close the terminal yet 💖 we've done optimization, instrumentation, preprocessor, assembler, linker, directory, and code generation as lists, and the rest is still ahead. stretch your fingers, hydrate, and scroll on 🦋
Developer Options 💻✨
Okay bestie, this is the dev-options block and it is long. I’m keeping the whole roster exactly as written — every dash, bracket, equals sign, and weird spelling. No summaries, no “helpful” renaming. 🫶
-dletters-dumpspecs-dumpmachine-dumpversion-dumpfullversion-fcallgraph-info[=su,da]-fchecking-fchecking=n-fdbg-cnt-list-fdbg-cnt=counter-value-list-fdisable-ipa-pass_name-fdisable-rtl-pass_name-fdisable-rtl-pass-name=range-list-fdisable-tree-pass_name-fdisable-tree-pass-name=range-list-fdump-debug-fdump-earlydebug-fdump-noaddr-fdump-unnumbered-fdump-unnumbered-links-fdump-final-insns[=file]-fdump-internal-locations-fdump-ipa-all-fdump-ipa-cgraph-fdump-ipa-inline-fdump-lang-all-fdump-lang-switch-fdump-lang-switch-options-fdump-lang-switch-options=filename-fdump-passes-fdump-rtl-pass-fdump-rtl-pass=filename-fdump-statistics-fdump-tree-all-fdump-tree-switch-fdump-tree-switch-options-fdump-tree-switch-options=filename-fcompare-debug[=opts]-fcompare-debug-second-fenable-kind-pass-fenable-kind-pass=range-list-fira-verbose=n-flto-report-flto-report-wpa-fmem-report-wpa-fmem-report-fpre-ipa-mem-report-fpost-ipa-mem-report-fopt-info-fopt-info-options[=file]-fmultiflags-fprofile-report-frandom-seed=string-fsched-verbose=n-fsel-sched-verbose-fsel-sched-dump-cfg-fsel-sched-pipelining-verbose-fstats-fstack-usage-ftime-report-ftime-report-details-fvar-tracking-assignments-toggle-gtoggle-print-autofdo-gcov-version-print-file-name=library-print-libgcc-file-name-print-multi-directory-print-multi-lib-print-multi-os-directory-print-multiarch-print-prog-name=program-print-search-dirs-Q-print-sysroot-print-sysroot-headers-suffix-save-temps-save-temps=cwd-save-temps=obj-specs=file-time[=file]--dump=letters--print-autofdo-gcov-version--print-file-name=library--print-libgcc-file-file-name--print-multi-directory--print-multi-lib--print-multi-os-directory--print-multiarch--print-prog-name=program--print-search-dirs--print-sysroot--print-sysroot-headers-suffix--save-temps--specs=file--param name=value
Target-Specific Options 🌍🎯
Now the architecture-specific corner. Same rule: every flag stays exactly as printed, even when it looks like a keyboard smash. 💅
AArch64 Options (AArch64 Options) 🦋
-mabi=name-mbig-endian-mlittle-endian-menable-sysreg-checking-mgeneral-regs-only-mcmodel=tiny-mcmodel=small-mcmodel=large-mstrict-align-momit-leaf-frame-pointer-mtls-dialect=desc-mtls-dialect=traditional-mtls-size=size-mtp=name-mfix-cortex-a53-835769-mfix-cortex-a53-843419-mlow-precision-recip-sqrt-mlow-precision-sqrt-mlow-precision-div-mmax-vectorization-mautovec-preference=name-mpc-relative-literal-loads-msign-return-address=scope-mbranch-protection=features-mharden-sls=opts-march=name-mcpu=name-mtune=name-moverride=string-mstack-protector-guard=guard-mstack-protector-guard-reg=sysreg-mstack-protector-guard-offset=offset-mtrack-speculation-moutline-atomics-mearly-ra-mearly-ldp-fusion-mlate-ldp-fusion-mnarrow-gp-writes-msve-vector-bits=bits
Adapteva Epiphany Options (Adapteva Epiphany Options) 🧚
-mhalf-reg-file-mprefer-short-insn-regs-mbranch-cost=num-mcmove-mnops=num-msoft-cmpsf-msplit-lohi-mpost-inc-mpost-modify-mstack-offset=num-mround-nearest-mlong-calls-mshort-calls-msmall16-mfp-mode=mode-mmay-round-for-trunc-mfp-iarith-mvect-double-max-vect-align=num-msplit-vecmove-early-m1reg-reg
AMD GCN Options (AMD GCN Options) 🔥
-march=gpu-mtune=gpu-mgang-private-size=bytes-msram-ecc=[on|off|any]-mxnack=[on|off|any]-Wopenacc-dims
ARC Options (ARC Options) 🌈
-mbarrel-shifter-mjli-always-mcpu=cpu-mA6-mARC600-mA7-mARC700-mdpfp-mdpfp-compact-mdpfp-fast-mno-dpfp-lrsr-mea-mmul32x16-mmul64-matomic-mnorm-mspfp-mspfp-compact-mspfp-fast-msimd-msoft-float-mswap-mlock-mswape-mxy-misize-marclinux-marclinux_prof-mlong-calls-mmedium-calls-msdata-mirq-ctrl-saved-mrgf-banked-regs-mlpc-width=width-G num-mvolatile-cache-mtp-regno=regno-mbitops-mcmem-munaligned-access-mauto-modify-reg-mno-brcc-mcase-vector-pcrel-mno-cond-exec-mearly-cbranchsi-mindexed-loads-mlra-priority-none-mlra-priority-compact-mlra-priority-noncompact-mmillicode-msize-level=level-mtune=cpu-mmultcost=num-mcode-density-frame-mmpy-option=multo-mdiv-rem-mcode-density-mll64-mfpu=fpu-mrf16-mbranch-index
ARM Options (ARM Options) 💜
-mapcs-frame-mapcs-mabi=name-mgeneral-regs-only-mno-sched-prolog-mlittle-endian-mbig-endian-mbe8-mbe32-mfloat-abi=name-mfp16-format=name-mthumb-interwork-mcpu=name-march=name-mfpu=name-mtune=name-mstructure-size-boundary=n-mabort-on-noreturn-mlong-calls-msingle-pic-base-mpic-register=reg-mpic-data-is-text-relative-mnop-fun-dllimport-mpoke-function-name-mthumb-marm-mtpcs-frame-mtpcs-leaf-frame-mcaller-super-interworking-mcallee-super-interworking-mtp=name-mtls-dialect=dialect-mword-relocations-mfix-cortex-m3-ldrd-mfix-cortex-a57-aes-1742098-mfix-cortex-a72-aes-1655431-munaligned-access-mslow-flash-data-masm-syntax-unified-mrestrict-it-mpure-code-mcmse-mfix-cmse-cve-2021-35465-mstack-protector-guard=guard-mstack-protector-guard-offset=offset-mfdpic-mbranch-protection=features-mvectorize-with-neon-quad-mvectorize-with-neon-double
AVR Options (AVR Options) 🍓
-mmcu=mcu-mabsdata-maccumulate-args-mcvt-mbranch-cost=cost-mfuse-add=level-mfuse-move=level-mfuse-move2-mcall-prologues-mgas-isr-prologues-mint8-mflmap-mdouble=bits-mlong-double=bits-mno-call-main-mn_flash=size-mfract-convert-truncate-mno-interrupts-mmain-is-OS_task-mrelax-mpmem-wrap-around-mrmw-mstrict-X-mtiny-stack-mrodata-in-ram-msplit-bit-shift-msplit-ldst-mshort-calls-mskip-bug-muse-nonzero-bits-nodevicelib-nodevicespecs-Waddr-space-convert-Wmisspelled-isr
Blackfin Options (Blackfin Options) 🖤
-mcpu=cpu[-sirevision]-msim-momit-leaf-frame-pointer-mspecld-anomaly-mcsync-anomaly-mlow-64k-mstack-check-l1-mid-shared-library-mleaf-id-shared-library-mshared-library-id=n-msep-data-mlong-calls-mfast-fp-minline-plt-mmulticore-mcorea-mcoreb-msdram-micplb
C6X Options (C6X Options) ⭐
-mbig-endian-mlittle-endian-march=cpu-msim-msdata=sdata-type-mdsbt-mlong-calls
CRIS Options (CRIS Options) 🎀
-mcpu=cpu-march=cpu-mtune=cpu-mmax-stackframe=n-metrax4-metrax100-mpdebug-mcc-init-mno-side-effects-mstack-align-mdata-align-mconst-align-m32-bit-m16-bit-m8-bit-mno-prologue-epilogue-mbest-lib-options-moverride-best-lib-options-mtrap-using-break8-mtrap-unaligned-atomic-munaligned-atomic-may-use-library-sim-sim2-mmul-bug-workaround
C-SKY Options (C-SKY Options) 🦋
-march=arch-mcpu=cpu-mbig-endian-mlittle-endian-mfpu=fpu-mdouble-float-mfdivdu-mfloat-abi=name-melrw-mistack-mmp-mcp-mcache-msecurity-mtrust-mdsp-medsp-mvdsp-mdiv-msmart-mhigh-registers-manchor-mpushpop-mmultiple-stld-mconstpool-mstack-size-mccrt-mbranch-cost=n-msched-prolog-msim
Cygwin and MinGW Options (Cygwin and MinGW Options) 🌸
-mconsole-mcrtdll=library-mdll-mnop-fun-dllimport-mthreads-municode-mwin32-mwindows-fno-set-stack-executable-fwritable-relocated-rdata-mpe-aligned-commons-muse-libstdc-wrappers
Darwin Options (Darwin Options) 🍎
-all_load-allowable_client-arch name-arch_errors_fatal-asm_macosx_version_min=version-bind_at_load-bundle-bundle_loader-client_name-compatibility_version-current_version-dead_strip-dependency-file-dylib_file-dylinker-dylinker_install_name-dynamic-dynamiclib-exported_symbols_list-fapple-kext-fconstant-cfstrings-ffix-and-continue-filelist-findirect-data-flat_namespace-force_cpusubtype_ALL-force_flat_namespace-framework name-gfull-gused-headerpad_max_install_names-iframework-image_base-init symbol-name-install_name-keep_private_externs-matt-stubs-mconstant-cfstrings-mdynamic-no-pic-mfix-and-continue-mkernel-mmacosx-version-min=version-mone-byte-bool-msymbol-stubs-mtarget-linker[=]version-nodefaultexport-nodefaultrpaths-pagezero_size-preload-read_only_relocs-sectalign-sectcreate-seg_addr_table-seg1addr-segaddr-segprot-segs_read_only_addr-segs_read_write_addr-sub_library-sub_umbrella-twolevel_namespace-twolevel_namespace_hints-umbrella-undefined-unexported_symbols_list-weak_framework name-weak_reference_mismatches-whatsloaded-whyload-F-ObjC-ObjC++-Wnonportable-cfstrings
DEC Alpha Options (DEC Alpha Options) 🌟
-mno-fp-regs-msoft-float-mieee-mieee-with-inexact-mieee-conformant-mfp-trap-mode=mode-mfp-rounding-mode=mode-mtrap-precision=mode-mbuild-constants-mcpu=cpu-type-mtune=cpu-type-mbwx-mmax-mfix-mcix-msafe-bwa-msafe-partial-mfloat-vax-mfloat-ieee-mexplicit-relocs-msmall-data-mlarge-data-msmall-text-mlarge-text-mmemory-latency=time-mtls-kernel-mtls-size=bitsize-mlong-double-128-mlong-double-64
eBPF Options (eBPF Options) 🧚
-mbig-endian-mlittle-endian-mframe-limit=bytes-mxbpf-mco-re-mjmpext-mjmp32-malu32-mv3-atomics-mbswap-msdiv-msmov-mcpu=version-masm=dialect-minline-memops-threshold=bytes-Wco-re
FR30 Options (FR30 Options) 🎀
-msmall-model-mno-lsim
FRV Options (FRV Options) 💀
-mgpr-32-mgpr-64-mfpr-32-mfpr-64-mhard-float-msoft-float-malloc-cc-mfixed-cc-mdword-mdouble-mmedia-mmuladd-mfdpic-minline-plt-mgprel-ro-multilib-library-pic-mlinked-fp-mlong-calls-malign-labels-mlibrary-pic-macc-4-macc-8-mpack-mno-eflags-mno-cond-move-mno-optimize-membar-mno-scc-mno-cond-exec-mno-vliw-branch-mno-multi-cond-exec-mno-nested-cond-exec-mtomcat-stats-mTLS-mtls-mcpu=cpu
FT32 Options (FT32 Options) 🦋
-msim-mnodiv-mft32b-mcompress-mnopm
GNU/Linux Options (GNU/Linux Options) 🐧
-mglibc-muclibc-mmusl-mbionic-mandroid-tno-android-cc-tno-android-ld
H8/300 Options (H8/300 Options) 🌸
-mrelax-mh-ms-mn-msx-ms2600-mquickcall-mslowbyte-mexr-mint32-malign-300
🧡 HPPA Options (HPPA Options) 🧡
Hewlett-Packard PA-RISC girlies, this one's for you — the whole flag family, no gatekeeping: -march=architecture-type, -mno-atomic-libcalls, -mcaller-copies, -mdisable-fpregs, -mdisable-indexing, -mordered, -mfast-indirect-calls, -mgas, -mgnu-ld, -mhp-ld, -mfixed-range=register-range, -mcoherent-ldcw, -mlinker-opt, -mlong-calls, -mlong-load-store, -mno-space-regs, -msoft-float, -mpa-risc-1-0, -mpa-risc-1-1, -mpa-risc-2-0, -mportable-runtime, -mschedule=cpu-type, -msoft-mult, -msio, -mwsio, -munix=unix-std, -nolibdld, -static, -threads.
💜 IA-64 Options (IA-64 Options) 💜
Itanium besties, your list is long and honestly kind of iconic for it: -mbig-endian, -mlittle-endian, -mgnu-as, -mgnu-ld, -mno-pic, -mvolatile-asm-stop, -mregister-names, -msdata, -mconstant-gp, -mauto-pic, -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-stop-bits-after-every-cycle, -msched-count-spec-in-critical-path, -msel-sched-dont-check-control-spec, -msched-fp-mem-deps-zero-cost, -msched-max-memory-insns-hard-limit, -msched-max-memory-insns=max-insns.
🦋 LM32 Options (LM32 Options) 🦋
Small but mighty, lowkey underrated: -mbarrel-shift-enabled, -mdivide-enabled, -mmultiply-enabled, -msign-extend-enabled, -muser-enabled.
🌸 LoongArch Options (LoongArch Options) 🌸
The LoongArch glow-up is real, and here's the full roster: -march=arch-type, -mtune=tune-type, -mabi=base-abi-type, -mfpu=fpu-type, -msimd=simd-type, -msoft-float, -msingle-float, -mdouble-float, -mlsx, -mlasx, -mbranch-cost=n, -maddr-reg-reg-cost=n, -mcheck-zero-division, -mbreak-code=code, -mcond-move-int, -mcond-move-float, -memcpy, -mstrict-align, -G num, -mmax-inline-memcpy-size=n, -mexplicit-relocs=style, -mexplicit-relocs, -mno-explicit-relocs, -mdirect-extern-access, -mcmodel=code-model, -mrelax, -mpass-mrelax-to-as, -mrecip, -mrecip=opt, -mfrecipe, -mdiv32, -mlam-bh, -mlamcas, -mld-seq-sa, -mscq, -mtls-dialect=opt, -mannotate-tablejump.
🎀 LynxOS Options (LynxOS Options) 🎀
Short and sweet, we love a minimal queen: -mshared, -mthreads, -mlegacy-threads.
🍓 M32C Options (M32C Options) 🍓
Tiny chip energy: -mcpu=cpu, -msim, -memregs=number.
💅 M32R/D Options (M32R/D Options) 💅
Okay so this family has range: -m32r2, -m32rx, -m32r, -mdebug, -malign-loops, -missue-rate=number, -mbranch-cost=number, -mmodel=code-size-model-type, -msdata=sdata-type, -mno-flush-func, -mflush-func=name, -mno-flush-trap, -mflush-trap=number, -G num.
🔥 M680x0 Options (M680x0 Options) 🔥
Motorola 68000 stans rise up, this list is giving retro main character energy: -march=arch, -mcpu=cpu, -mtune=tune, -m68000, -m68020, -m68020-40, -m68020-60, -m68030, -m68040, -m68060, -m68302, -m68332, -m68851, -mcpu32, -mfidoa, -m5200, -m5206e, -m528x, -m5307, -m5407, -mcfv4e, -mbitfield, -mc68000, -mc68020, -mrtd, -mdiv, -mshort, -mhard-float, -m68881, -msoft-float, -mpcrel, -malign-int, -mstrict-align, -msep-data, -mshared-library-id=n, -mid-shared-library, -mxgot, -mxtls, -mlong-jump-table-offsets.
✨ MCore Options (MCore Options) ✨
A whole little buffet of choices, no cap: -mhardlit, -mdiv, -mrelax-immediates, -mwide-bitfields, -m4byte-functions, -mcallgraph-data, -mslow-bytes, -mno-lsim, -mlittle-endian, -mbig-endian, -m210, -m340, -mstack-increment.
🌈 MicroBlaze Options (MicroBlaze Options) 🌈
Soft core, hard flex. The full set: -msoft-float, -mhard-float, -msmall-divides, -mcpu=cpu, -mmemcpy, -mxl-soft-mul, -mxl-soft-div, -mxl-barrel-shift, -mxl-pattern-compare, -mxl-gp-opt, -mxl-multiply-high, -mxl-float-convert, -mxl-float-sqrt, -mbig-endian, -mlittle-endian, -mxl-reorder, -mxl-mode-app-model, -mxl-prefetch, -mpic-data-is-text-relative.
👑 MIPS Options (MIPS Options) 👑
Bestie. Sit down. This is the iconic long list and I'm not skipping a single one, because MIPS ate and left no crumbs:
-EL, -EB, -mel, -meb, -march=arch, -mtune=arch, -mips1, -mips2, -mips3, -mips4, -mips32, -mips32r2, -mips32r3, -mips32r5, -mips32r6, -mips64, -mips64r2, -mips64r3, -mips64r5, -mips64r6, -mips16, -mmips16e2, -mflip-mips16, -minterlink-compressed, -minterlink-mips16, -mabi=abi, -mabicalls, -mshared, -mplt, -mxgot, -mgp32, -mgp64, -mfp32, -mfpxx, -mfp64, -mhard-float, -msoft-float, -mno-float, -msingle-float, -mdouble-float, -modd-spreg, -mabs=mode, -mnan=encoding, -mdsp, -mdspr2, -mmcu, -meva, -mvirt, -mxpa, -mcrc, -mginv, -mmicromips, -mmsa, -mloongson-mmi, -mloongson-ext, -mloongson-ext2, -mfpu=fpu-type, -msmartmips, -mpaired-single, -mdmx, -mips3d, -mmt, -mllsc, -mlong64, -mlong32, -msym32, -Gnum, -mno-local-sdata, -mno-extern-sdata, -mno-gopt, -membedded-data, -muninit-const-in-rodata, -mcode-readable=setting, -mno-data-in-code, -mcode-xonly, -msplit-addresses, -mexplicit-relocs, -mexplicit-relocs=release, -mno-check-zero-division, -mdivide-traps, -mdivide-breaks, -mno-load-store-pairs, -mstrict-align, -mno-unaligned-access, -mmemcpy, -mlong-calls, -mmad, -mimadd, -mno-fused-madd, -nocpp, -mfix-24k, -mfix-r4000, -mfix-r4400, -mfix-r5900, -mfix-r10000, -mfix-rm7000, -mfix-vr4120, -mfix-vr4130, -mfix-sb1, -mfix4300, -mr10k-cache-barrier=setting, -mflush-func=func, -mno-flush-func, -mbranch-cost=num, -mbranch-likely, -mcompact-branches=policy, -mno-fp-exceptions, -mvr4130-align, -msynci, -mno-lxc1-sxc1, -mno-madd4, -mno-relax-pic-calls, -mmcount-ra-address, -mframe-header-opt.
Whew. Slay. 💅
🧚 MMIX Options (MMIX Options) 🧚
Knuth girlies, this one's your moment: -mlibfuncs, -mepsilon, -mabi=gnu, -mabi=mmixware, -mzero-extend, -mknuthdiv, -mtoplevel-symbols, -melf, -mbranch-predict, -mbase-addresses, -msingle-exit, -mset-data-start=address, -mset-program-start=address, -mno-set-program-start.
🫶 MN10300 Options (MN10300 Options) 🫶
Panasonic core rep, and yes there's a flag literally called -mmult-bug, love that for us: -mmult-bug, -mno-mult-bug, -mam33, -mam33-2, -mam34, -mtune=cpu-type, -mno-return-pointer-on-d0, -mno-crt0, -mrelax, -mno-liw, -mno-setlb.
🦄 Moxie Options (Moxie Options) 🦄
Four icons, that's it, that's the list: -meb, -mel, -mmul.x, -mno-crt0.
💖 MSP430 Options (MSP430 Options) 💖
Embedded cuties, this is a whole vibe: -msim, -masm-hex, -mmcu=name, -mlarge, -msmall, -mrelax, -mwarn-mcu, -mwarn-devices-csv, -mcode-region=where, -mdata-region=where, -muse-lower-region-prefix, -msilicon-errata=name[,name...], -msilicon-errata-warn=name[,name...], -mhwmult=type, -minrt, -mtiny-printf, -mmax-inline-shift=n.
🌟 NDS32 Options (NDS32 Options) 🌟
Andes core besties, get comfy, it's a long one: -mbig-endian, -mlittle-endian, -EB, -EL, -mabi=name, -mfloat-abi=name, -mreduced-regs, -mfull-regs, -malways-align, -malign-functions, -mfp-as-gp, -mcmov, -mhw-abs, -mext-perf, -mext-perf2, -mext-string, -mext-dsp, -mext-fpu-fma, -mext-fpu-sp, -mext-fpu-dp, -mv3push, -m16-bit, -mvh, -misr-vector-size=num, -misr-secure=num, -mcache-block-size=num, -march=arch, -mcpu=cpu, -mconfig-fpu=num, -mconfig-mul=type, -mconfig-register-ports=kind, -mcmodel=code-model, -mctor-dtor, -mrelax, -mrelax-hint, -msched-prolog-epilog, -mno-ret-in-naked-func, -malways-save-lp, -munaligned-access, -minline-asm-r15.
⭐ Nvidia PTX Options (Nvidia PTX Options) ⭐
GPU girlies, your flags are here: -m64, -march=arch, -misa=arch, -march-map=arch, -mptx=version, -mmainkernel, -moptimize, -msoft-stack, -msoft-stack-reserve-local=size, -muniform-simt, -mgomp.
🦋 OpenRISC Options (OpenRISC Options) 🦋
Open source royalty fr: -mboard=name, -mhard-mul, -mhard-div, -msoft-mul, -msoft-div, -msoft-float, -mhard-float, -mdouble-float, -munordered-float, -mcmov, -mror, -mrori, -msext, -msfimm, -mshftimm, -mcmodel=code-model.
🎀 PDP-11 Options (PDP-11 Options) 🎀
Vintage queen behavior, we respect it: -mfpu, -msoft-float, -mac0, -m40, -m45, -m10, -mint32, -mint16, -msplit, -munix-asm, -mdec-asm, -mgnu-asm, -mlra.
🍓 Picolibc Options (Picolibc Options) 🍓
Ooh these are the fancy long-flag ones, so cute: --oslib=library, --crt0=[none|minimal|hosted|semihost], --printf=[d|f|l|i|m], --scanf=[d|f|l|i|m].
🌸 PowerPC Options 🌸
Not me gatekeeping — PowerPC keeps it simple and just points you elsewhere: See RS/6000 and PowerPC Options. 🫶
💜 PRU Options (PRU Options) 💜
Programmable real-time unit besties: -mmcu=mcu, -minrt, -mno-relax, -mloop, -mmul, -mfillzero, -mabi=variant.
🔥 RISC-V Options (RISC-V Options) 🔥
Okay the RISC-V girlies are thriving and the list is giving extensive: -mbranch-cost=N-instruction, -mabi=ABI-string, -mfdiv, -mdiv, -mno-fence-tso, -misa-spec=ISA-spec-string, -march=[ISA|Profile|Profile_ISA|processor-string], -mcpu=processor-string, -mtune=processor-string, -mpreferred-stack-boundary=num, -msmall-data-limit=N-bytes, -msave-restore, -mno-shorten-memrefs, -mstrict-align, -mscalar-strict-align, -mno-vector-strict-align, -mcmodel=medlow, -mcmodel=medany, -mcmodel=large, -mexplicit-relocs, -mrelax, -mriscv-attribute, -malign-data=type, -mbig-endian, -mlittle-endian, -mstack-protector-guard=guard, -mstack-protector-guard-reg=reg, -mstack-protector-guard-offset=offset, -mcsr-check, -momit-leaf-frame-pointer, -mmovcc, -mno-inline-atomics, -mno-inline-strlen, -mno-inline-strcmp, -mno-inline-strncmp, -mstringop-strategy=strategy, -mtls-dialect=desc, -mtls-dialect=trad, -mrvv-vector-bits=value, -mrvv-max-lmul=value, -madjust-lmul-cost, -mmax-vectorization, -mno-autovec-segment.
🌈 RL78 Options (RL78 Options) 🌈
Last but never least, the RL78 cuties: -msim, -mallregs, -mrelax, -mes0, -mmul=none, -mmul=g13, -mmul=g14, -mmul=rl78, -mcpu=g10, -mcpu=g13, -mcpu=g14, -mcpu=rl78, -mg10, -mg13, -mg14, -mrl78, -msave-mduc-in-interrupts.
🍓 RS/6000 and PowerPC Options
okay bestie, this is the full RS/6000 and PowerPC flag lineup — every single one, no skipping, no cap:
-mcpu=cpu-type -mtune=cpu-type -mcmodel=code-model -mprofile-kernel
-mpowerpc64 -maltivec -mpowerpc-gpopt -mpowerpc-gfxopt -mmfcrf
-mpopcntb -mpopcntd -mfprnd -mcmpb -mhard-dfp -mfull-toc
-mminimal-toc -mno-fp-in-toc -mno-sum-in-toc -maix64 -maix32
-m64 -m32 -mxl-compat -mpe -malign-power -malign-natural
-msoft-float -mhard-float -mmultiple -mupdate
-mavoid-indexed-addresses -mfused-madd -mbit-align -mbit-word
-mstrict-align -mrelocatable -mrelocatable-lib -mlittle
-mlittle-endian -mbig -mbig-endian -mdynamic-no-pic
-msingle-pic-base -mprioritize-restricted-insns=priority
-msched-costly-dep=dependence_type -minsert-sched-nops=scheme
-mcall-aixdesc -mcall-eabi -mcall-freebsd -mcall-linux
-mcall-netbsd -mcall-openbsd -mcall-sysv -mcall-sysv-eabi
-mcall-sysv-noeabi -mtraceback=traceback_type -maix-struct-return
-msvr4-struct-return -mabi=abi-type -msecure-plt -mbss-plt
-msplit-patch-nops -mregnames -mlongcall -mpltseq -mtls-markers
-mblock-move-inline-limit=num -mblock-compare-inline-limit=num
-mblock-compare-inline-loop-limit=num -mblock-ops-unaligned-vsx
-mstring-compare-inline-limit=num -misel -mvsx -mvrsave -mmulhw
-mdlmzb -mprototype -msim -mmvme -mads -myellowknife -memb
-meabi -msdata -msdata=opt -mreadonly-in-sdata -mvxworks -G num
-mrecip -mrecip=opt -mno-recip -mrecip-precision -mveclibabi=type
-mfriz -mpointers-to-nested-functions -msave-toc-indirect
-mpower8-fusion -mcrypto -mhtm -mquad-memory -mquad-memory-atomic
-mcompat-align-parm -mfloat128 -mfloat128-hardware -mgnu-attribute
-mstack-protector-guard=guard -mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset -mprefixed -mpcrel -mmma
-mdense-math -mrop-protect -mprivileged -mno-splat-word-constant
-mno-splat-float-constant -mno-ieee128-constant
-mno-warn-altivec-longlike, that's the whole vibe check 💅 scroll it, don't skim it.
🧚 RX Options
RX time!! here's the list, go off:
-m64bit-doubles -m32bit-doubles -fpu
-nofpu -mcpu=name -mbig-endian-data -mlittle-endian-data
-msmall-data-limit=N -msim -mas100-syntax -mrelax
-mmax-constant-size=N -mint-register=N -mpid
-mno-allow-string-insns -mjsr -mno-warn-multiple-fast-interrupts
-msave-acc-in-interrupts -mlra🎀 S/390 and zSeries Options
big mainframe energy only 👑 here's every flag:
-mtune=cpu-type -march=cpu-type -mhard-float -msoft-float
-mhard-dfp -mlong-double-64 -mlong-double-128 -mbackchain
-mpacked-stack -msmall-exec -mmvcle -m64 -m31 -mdebug -mesa
-mzarch -mhtm -mvx -mzvector -mtpf-trace -mtpf-trace-skip -mmain
-mfused-madd -mno-fused-madd -mwarn-framesize=framesize
-mwarn-dynamicstack -mstack-size=stack-size -mno-stack-size
-mstack-guard=stack-guard -mno-stack-guard
-mstack-protector-guard=guard -mstack-protector-guard-record
-mhotpatch=pre-halfwords,post-halfwords
-mno-pic-data-is-text-relative -mindirect-branch=choice
-mindirect-branch-jump=choice -mindirect-branch-call=choice
-mfunction-return=choice -mfunction-return-mem=choice
-mfunction-return-reg=choice -mindirect-branch-table -mfentry
-mrecord-mcount -mnop-mcount -mpreserve-args -munaliged-symbols(yes, -munaliged-symbols is spelled exactly like that in here — I'm not the one fixing it, I'm just the messenger 💀)
🦋 SH Options
SH bestie!! the list is long long, so hydrate first:
-m1 -m2 -m2e -m2a -m2a-nofpu -m2a-single
-m2a-single-only -m3 -m3e -m4 -m4-nofpu -m4-single
-m4-single-only -m4-100 -m4-100-nofpu -m4-100-single
-m4-100-single-only -m4-200 -m4-200-nofpu -m4-200-single
-m4-200-single-only -m4-300 -m4-300-nofpu -m4-300-single
-m4-300-single-only -m4-340 -m4-400 -m4-500 -m4a -m4al
-m4a-nofpu -m4a-single -m4a-single-only -mb -ml -mdalign
-mrelax -mbigtable -mbitops -mfmovd -mrenesas -mnomacsave -mieee
-misize -minline-ic_invalidate -mprefergot -musermode
-multcost=number -mdiv=strategy -mdivsi3_libfunc=name
-mfixed-range=register-range -maccumulate-outgoing-args
-matomic-model=atomic-model -mbranch-cost=num -mzdcbranch
-mcbranch-force-delay-slot -mfsca -mfsrra -mpretend-cmove -mfdpic
-mtas -mlraate, honestly 💅
🌸 Solaris 2 Options
short and sweet, we love a concise queen:
-mclear-hwcap
-mno-clear-hwcap -mimpure-text -mno-impure-text -gsctf⭐ SPARC Options
SPARC lineup, all of it, no edits:
-mcpu=cpu-type -mtune=cpu-type
-mcmodel=code-model -mmemory-model=mem-model -m32 -m64 -mptr32
-mptr64 -mapp-regs -mfaster-structs -mflat -mfpu -mhard-float
-msoft-float -mhard-quad-float -msoft-quad-float -mstack-bias
-mstd-struct-return -munaligned-doubles -muser-mode -mv8plus -mvis
-mvis2 -mvis3 -mvis3b -mvis4 -mvis4b -mcbcond -mfmaf -mfsmuld
-mpopc -msubxc -mfix-at697f -mfix-ut699 -mfix-ut700
-mfix-gr712rc💜 System V Options
only two, tiny icon behaviour:
-YP,paths-Ym,dir
🌈 V850 Options
okay the V850 list is giving "I brought my whole friend group":
-mlong-calls -mep -mprolog-function
-mspace -mtda=n -msda=n -mzda=n -mv850 -mv850e3v5 -m850e2v4
-mv850e2v3 -mv850e2 -mv850e1 -mv850es -mv850e -mdisable-callt
-mrelax -mlong-jumps -msoft-float -mhard-float -mloop -mrh850-abi
-mghs -mgcc-abi -m8byte-align -mbig-switch -mapp-regs
-msmall-sld -mno-strict-align -mjump-tables-in-data-section🫶 VAX Options
retro legend behaviour, here you go:
-munix -mgnu -md -md-float -mg
-mg-float -mlra -mvaxc-alignment -mqmath🙈 Visium Options
-mdebug -msim -mfpu -mhard-float
-msoft-float -mcpu=cpu-type -mtune=cpu-type -msv-mode -muser-mode🍓 VMS Options
four flags, zero notes:
-mvms-return-codes -mdebug-main=prefix
-mmalloc64 -mpointer-size=size🧚 VxWorks Options
-mrtp -msmp -mvthreads
-non-static -Bstatic -Bdynamic -Xbind-lazy -Xbind-now👑 x86 Options
ALRIGHT bestie, deep breath — this is the x86 list, and it is the main character of option lists. Not one flag skipped, fr fr:
-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 -mieee-fp -mrtd -malign-double
-mpreferred-stack-boundary=num -mincoming-stack-boundary=num -mcld
-mcx16 -msahf -mmovbe -mcrc32 -mmwait -mrecip -mrecip=opt
-mvzeroupper -mstv -mprefer-avx128 -mprefer-vector-width=opt
-mpartial-vector-fp-math -mmove-max=bits -mstore-max=bits
-mnoreturn-no-callee-saved-registers -mmmx -msse -msse2 -msse3
-mssse3 -msse4.1 -msse4.2 -msse4 -mavx -mavx2 -mavx512f
-mavx512cd -mavx512vl -mavx512bw -mavx512dq -mavx512ifma
-mavx512vbmi -msha -maes -mpclmul -mfsgsbase -mrdrnd -mf16c
-mfma -mpconfig -mwbnoinvd -mptwrite -mclflushopt -mclwb
-mxsavec -mxsaves -msse4a -m3dnow -m3dnowa -mpopcnt -mabm
-mbmi -mtbm -mfma4 -mxop -madx -mlzcnt -mbmi2 -mfxsr -mxsave
-mxsaveopt -mrtm -mhle -mlwp -mmwaitx -mclzero -mpku -mgfni
-mvaes -mwaitpkg -mshstk -mmanual-endbr -mcet-switch
-mforce-indirect-call -mavx512vbmi2 -mavx512bf16 -menqcmd
-mvpclmulqdq -mavx512bitalg -mmovdiri -mmovdir64b
-mavx512vpopcntdq -mavx512vnni -mprfchw -mrdpid -mrdseed -msgx
-mavx512vp2intersect -mserialize -mtsxldtrk -mamx-tile -mamx-int8
-mamx-bf16 -muintr -mhreset -mavxvnni -mamx-fp8 -mavx512fp16
-mavxifma -mavxvnniint8 -mavxneconvert -mcmpccxadd -mamx-fp16
-mprefetchi -mraoint -mamx-complex -mavxvnniint16 -msm3 -msha512
-msm4 -mapxf -musermsr -mavx10.1 -mavx10.2 -mamx-avx512
-mamx-tf32 -mmovrs -mamx-movrs -mavx512bmm -mcldemote
-mms-bitfields -mno-align-stringops -minline-all-stringops
-minline-stringops-dynamically -mstringop-strategy=alg -mkl
-mwidekl -mmemcpy-strategy=strategy -mmemset-strategy=strategy
-mpush-args -maccumulate-outgoing-args -m128bit-long-double
-m96bit-long-double -mlong-double-64 -mlong-double-80
-mlong-double-128 -mregparm=num -msseregparm -mveclibabi=type
-mvect8-ret-in-mem -mpc32 -mpc64 -mpc80 -mdaz-ftz -mstackrealign
-mstack-arg-probe -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 -mrecord-return
-mfentry-name=name -mfentry-section=name -mskip-rax-setup
-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 -mtls-dialect=type -mrelax-cmpxchg-loop
-mindirect-branch=choice -mfunction-return=choice
-mindirect-branch-register -mharden-sls=choice
-mindirect-branch-cs-prefix -mapx-inline-asm-use-gpr32 -mgather
-mscatter -mneeded -mno-direct-extern-access
-munroll-only-small-loops -mdispatch-scheduler -mlam=choiceliterally iconic 🔥
🎀 x86 Windows Options
okay this one's not a list, it's a handoff — it just says: See Cygwin and MinGW Options.
🌸 Xstormy16 Options
one (1) flag. humble. understated. here it is:
-msim🌈 Xtensa Options
last of the arch dumps, and she's cute:
-mconst16 -mforce-no-pic
-mno-serialize-volatile -mtext-section-literals -mauto-litpools
-mno-target-align -mlongcalls -mabi=abi-type
-mextra-l32r-costs=cycles -mstrict-align💜 zSeries Options
and this one's also a redirect: See S/390 and zSeries Options. (we already scrolled that one together, bestie, you're fine 🫶)
✨ Options Controlling the Kind of Output ✨
okay so here's the whole pipeline, and it's honestly not that deep: compilation can involve up to four stages — preprocessing, compilation proper, assembly and linking — and they always happen in that order. No vibes-based reordering, periodt.
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 — the ones that just got newly compiled and the ones you passed in as input — into an executable file. that's the whole glow-up arc from "text file" to "runs on your machine". 💅
For any given input file, the file name suffix is what decides which kind of compilation happens. it's giving "your 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.file.mm,file.M— Objective-C++ source code. note that you must link with thelibobjclibrary to make an Objective-C++ program work. and yes,.Mrefers to a literal capital M — uppercase matters, bestie, don't let autocorrect gaslight you.file.mii— Objective-C++ source code that should not be preprocessed.file.h— C, C++, Objective-C or Objective-C++ header file to be turned into a precompiled header (that's the default), or C, C++ header file to be turned into an Ada spec (via the-fdump-ada-specswitch).
little heads up 🫶 — those "must be preprocessed" vs. "should not be preprocessed" splits are doing real work. pick the wrong suffix and you're not getting the build you thought you asked for.
okay so the file-suffix lore keeps going, and honestly it’s giving “the compiler knows what you meant from the vibes of your extension” 💅
file.cc,file.cp,file.cxx,file.cpp,file.CPP,file.c++,file.C— C++ source code that must be preprocessed. Note that in.cxx, the last two letters must both be literallyx. Likewise,.Cmeans a literal capitalC.file.mm,file.M— Objective-C++ source code that must be preprocessed.file.mii— Objective-C++ source code that should not be preprocessed.file.hh,file.H,file.hp,file.hxx,file.hpp,file.HPP,file.h++,file.tcc— C++ header file to be turned into a precompiled header or Ada spec.file.f,file.for,file.ftn,file.fi— Fixed form Fortran source code that should not be preprocessed.file.F,file.FOR,file.fpp,file.FPP,file.FTN— Fixed form Fortran source code that must be preprocessed (with the traditional preprocessor).file.f90,file.f95,file.f03,file.f08,file.fii— Free form Fortran source code that should not be preprocessed.file.F90,file.F95,file.F03,file.F08— Free form Fortran source code that must be preprocessed (with the traditional preprocessor).file.cob,file.COB,file.cbl,file.CBL— COBOL source code.file.go— Go source code.file.d— D source code.file.di— D interface file.file.dd— D documentation code (Ddoc).file.ads— Ada source code file that contains a library unit declaration (a declaration of a package, subprogram, or generic, or a generic instantiation), or a library unit renaming declaration (a package, generic, or subprogram renaming declaration). Such files are also called specs.file.adb— Ada source code file containing a library unit body (a subprogram or package body). Such files are also called bodies.file.s— Assembler code.file.S,file.sx— Assembler code that must be preprocessed.other— An object file to be fed straight into linking. Any file name with no recognized suffix is treated this way.
You can specify the input language explicitly with the -x option:
-x language,--language=language,--language language— Specify explicitly the language for the following input files (rather than letting the compiler choose a default based on the file name suffix). This option applies to all following input files until the next-xoption. Possible values forlanguageare:
c c-header cpp-output
c++ c++-header c++-system-header c++-user-header c++-cpp-output
c++-system-module
objective-c objective-c-header objective-c-cpp-output objc-cpp-output
objective-c++ objective-c++-header objective-c++-cpp-output
objc++-cpp-output
assembler assembler-with-cpp
ada adascil adawhy
cobol
d
f77 f77-cpp-input f95 f95-cpp-input
go
modula-2 modula-2-cpp-output
rust
algol68
ltoNote that -x does not imply a particular language standard. For example -x f77 may also require -std=legacy for some older source codes.
-x none— Turn off any specification of a language, so that subsequent files are handled according to their file name suffixes (as if-xhas not been used at all).
If you only want some of the stages of compilation, you can use -x (or filename suffixes) to tell gcc where to start, and one of the options -c, -S, or -E to say where gcc is to stop. Note that some combinations (for example, -x cpp-output -E) instruct gcc to do nothing at all. 💀
-c,--compile— Compile or assemble the source files, but do not link. The linking stage simply is not done. The ultimate output is in the form of an object file for each source file.
By default, the object file name for a source file is made by replacing the suffix .c, .i, .s, etc., with .o.
Unrecognized input files, not requiring compilation or assembly, are ignored.
-S,--assemble— Stop after the stage of compilation proper; do not assemble. The output is in the form of an assembler code file for each non-assembler input file specified.
By default, the assembler file name for a source file is made by replacing the suffix .c, .i, etc., with .s.
Input files that don’t require compilation are ignored.
-E,--preprocess— Stop after the preprocessing stage; do not run the compiler proper. The output is in the form of preprocessed source code, which is sent to the standard output.
Input files that don’t require preprocessing are ignored.
-o file,--output=file,--output file— Place the primary output in file file. This applies to whatever sort of output is being produced, whether it be an executable file, an object file, an assembler file or preprocessed C code.
If -o is not specified, the default is to put an executable file in a.out, the object file for source.suffix in source.o, its assembler file in source.s, a precompiled header file in source.suffix.gch, and all preprocessed C source on standard output.
Though -o names only the primary output, it also affects the naming of auxiliary and dump outputs. See the examples below. Unless overridden, both auxiliary outputs and dump outputs are placed in the same directory as the primary output. In auxiliary outputs, the suffix of the input file is replaced with that of the auxiliary output file type; in dump outputs, the suffix of the dump file is appended to the input file suffix. In compilation commands, the base name of both auxiliary and dump outputs is that of the primary output; in compile and link commands, the primary output name, minus the executable suffix, is combined with the input file name. If both share the same base name, disregarding the suffix, the result of the combination is that base name, otherwise, they are concatenated, separated by a dash.
gcc -c foo.c ...will use foo.o as the primary output, and place aux outputs and dumps next to it, e.g., aux file foo.dwo for -gsplit-dwarf, and dump file foo.c.???r.final for -fdump-rtl-final.
If a non-linker output file is explicitly specified, aux and dump files by default take the same base name:
gcc -c foo.c -o dir/foobar.o ...will name aux outputs dir/foobar.* and dump outputs dir/foobar.c.*.
A linker output will instead prefix aux and dump outputs:
gcc foo.c bar.c -o dir/foobar ...will generally name aux outputs dir/foobar-foo.* and dir/foobar-bar.*, and dump outputs dir/foobar-foo.c.* and dir/foobar-bar.c.*.
The one exception to the above is when the executable shares the base name with the single input:
gcc foo.c -o dir/foo ...in which case aux outputs are named dir/foo.* and dump outputs named dir/foo.c.*.
The location and the names of auxiliary and dump outputs can be adjusted by the options -dumpbase, -dumpbase-ext, -dumpdir, -save-temps=cwd, and -save-temps=obj.
-dumpbase dumpbase,--dumpbase dumpbase— This option sets the base name for auxiliary and dump output files. It does not affect the name of the primary output file. Intermediate outputs, when preserved, are not regarded as primary outputs, but as auxiliary outputs:
gcc -save-temps -S foo.csaves the (no longer) temporary preprocessed file in foo.i, and then compiles to the (implied) output file foo.s, whereas:
gcc -save-temps -dumpbase save-foo -c foo.cpreprocesses into save-foo.i, compiles to save-foo.s (now an intermediate, thus auxiliary output), and then assembles to the (implied) output file foo.o.
Absent this option, dump and aux files take their names from the input file, or from the (non-linker) output file, if one is explicitly specified: dump output files (e.g. those requested by -fdump-* options) with the input name suffix, and aux output files (those requested by other non-dump options, e.g. -save-temps, -gsplit-dwarf, -fcallgraph-info) without it.
Similar suffix differentiation of dump and aux outputs can be attained for explicitly-given -dumpbase basename.suf by also specifying -dumpbase-ext .suf.
If dumpbase is explicitly specified with any directory component, any dumppfx specification (e.g. -dumpdir or -save-temps=*) is ignored, and instead of appending to it, dumpbase fully overrides it:
gcc foo.c -c -o dir/foo.o -dumpbase alt/foo \
-dumpdir pfx- -save-temps=cwd ...creates auxiliary and dump outputs named alt/foo.*, disregarding dir/ in -o, the ./ prefix implied by -save-temps=cwd, and pfx- in -dumpdir.
When -dumpbase is specified in a command that compiles multiple inputs, or that compiles and then links, it may be combined with dumppfx, as specified under -dumpdir. Then, each input file is compiled using the combined dumppfx, and default values for dumpbase and auxdropsuf are computed for each input file:
gcc foo.c bar.c -c -dumpbase main ...creates foo.o and bar.o as primary outputs, and avoids overwriting the auxiliary and dump outputs by using the dumpbase as a prefix, creating auxiliary and dump outputs named main-foo.* and main-bar.*.
An empty string specified as dumpbase avoids the influence of the output basename in the naming of auxiliary and dump outputs during compilation, computing default values:
gcc -c foo.c -o dir/foobar.o -dumpbase " ...will name aux outputs dir/foo.* and dump outputs dir/foo.c.*. Note how their basenames are taken from the input name, but the directory still defaults to that of the output.
The empty-string dumpbase does not prevent the use of the output basename for outputs during linking:
gcc foo.c bar.c -o dir/foobar -dumpbase " -flto ...The compilation of the source files will name auxiliary outputs dir/foo.* and dir/bar.*, and dump outputs dir/foo.c.* and dir/bar.c.*. LTO recompilation during linking will use dir/foobar as the prefix for dumps and auxiliary files. 🌈
-dumpbase-ext auxdropsuf,--dumpbase-ext auxdropsuf— okay so bestie, when you're forming the name of an auxiliary output file (but not a dump file, important), drop the trailing auxdropsuf from dumpbase before you append any suffixes. If you don't specify this option, it defaults to the suffix of a default dumpbase, i.e., the suffix of the input file when-dumpbaseisn't present on the command line, or dumpbase is combined with dumppfx.
gcc foo.c -c -o dir/foo.o -dumpbase x-foo.c -dumpbase-ext .c ...This creates dir/foo.o as the main output, and generates auxiliary outputs in dir/x-foo.*, taking the location of the primary output, and dropping the .c suffix from the dumpbase. Dump outputs retain the suffix: dir/x-foo.c.*.
This option is disregarded if it does not match the suffix of a specified dumpbase, except as an alternative to the executable suffix when appending the linker output base name to dumppfx, as specified below:
gcc foo.c bar.c -o main.out -dumpbase-ext .out ...This creates main.out as the primary output, and avoids overwriting the auxiliary and dump outputs by using the executable name minus auxdropsuf as a prefix, creating auxiliary outputs named main-foo.* and main-bar.* and dump outputs named main-foo.c.* and main-bar.c.*.
-dumpdir dumppfx,--dumpdir dumppfx— when forming the name of an auxiliary or dump output file, use dumppfx as a prefix:
gcc -dumpdir pfx- -c foo.c ...This creates foo.o as the primary output, and auxiliary outputs named pfx-foo.*, combining the given dumppfx with the default dumpbase derived from the default primary output, derived in turn from the input name. Dump outputs also take the input name suffix: pfx-foo.c.*.
If dumppfx is to be used as a directory name, it must end with a directory separator:
gcc -dumpdir dir/ -c foo.c -o obj/bar.o ...This creates obj/bar.o as the primary output, and auxiliary outputs named dir/bar.*, combining the given dumppfx with the default dumpbase derived from the primary output name. Dump outputs also take the input name suffix: dir/bar.c.*.
It defaults to the location of the output file, unless the output file is a special file like "/dev/null". Options -save-temps=cwd and -save-temps=obj override this default, just like an explicit -dumpdir option. In case multiple such options are given, the last one prevails:
gcc -dumpdir pfx- -c foo.c -save-temps=obj ...This outputs foo.o, with auxiliary outputs named foo.* because -save-temps=* overrides the dumppfx given by the earlier -dumpdir option. It does not matter that =obj is the default for -save-temps, nor that the output directory is implicitly the current directory. Dump outputs are named foo.c.*.
When compiling from multiple input files, if -dumpbase is specified, dumpbase, minus a auxdropsuf suffix, and a dash are appended to (or override, if containing any directory components) an explicit or defaulted dumppfx, so that each of the multiple compilations gets differently-named aux and dump outputs.
gcc foo.c bar.c -c -dumpdir dir/pfx- -dumpbase main ...This outputs auxiliary dumps to dir/pfx-main-foo.* and dir/pfx-main-bar.*, appending dumpbase- to dumppfx. Dump outputs retain the input file suffix: dir/pfx-main-foo.c.* and dir/pfx-main-bar.c.*, respectively. Contrast with the single-input compilation:
gcc foo.c -c -dumpdir dir/pfx- -dumpbase main ...That, applying -dumpbase to a single source, does not compute and append a separate dumpbase per input file. Its auxiliary and dump outputs go in dir/pfx-main.*.
When compiling and then linking from multiple input files, a defaulted or explicitly specified dumppfx also undergoes the dumpbase- transformation above (e.g. the compilation of foo.c and bar.c above, but without -c). If neither -dumpdir nor -dumpbase are given, the linker output base name, minus auxdropsuf, if specified, or the executable suffix otherwise, plus a dash is appended to the default dumppfx instead. Note, however, that unlike earlier cases of linking:
gcc foo.c bar.c -dumpdir dir/pfx- -o main ...This does not append the output name main to dumppfx, because -dumpdir is explicitly specified. The goal is that the explicitly-specified dumppfx may contain the specified output name as part of the prefix, if desired; only an explicitly-specified -dumpbase would be combined with it, in order to avoid simply discarding a meaningful option.
When compiling and then linking from a single input file, the linker output base name will only be appended to the default dumppfx as above if it does not share the base name with the single input file name. This has been covered in single-input linking cases above, but not with an explicit -dumpdir that inhibits the combination, even if overridden by -save-temps=*:
gcc foo.c -dumpdir alt/pfx- -o dir/main.exe -save-temps=cwd ...Auxiliary outputs are named foo.*, and dump outputs foo.c.*, in the current working directory as ultimately requested by -save-temps=cwd.
Summing it all up for an intuitive though slightly imprecise data flow: the primary output name is broken into a directory part and a basename part; dumppfx is set to the former, unless overridden by -dumpdir or -save-temps=*, and dumpbase is set to the latter, unless overridden by -dumpbase. If there are multiple inputs or linking, this dumpbase may be combined with dumppfx and taken from each input file. Auxiliary output names for each input are formed by combining dumppfx, dumpbase minus suffix, and the auxiliary output suffix; dump output names are only different in that the suffix from dumpbase is retained.
When it comes to auxiliary and dump outputs created during LTO recompilation, a combination of dumppfx and dumpbase, as given or as derived from the linker output name but not from inputs, even in cases in which this combination would not otherwise be used as such, is passed down with a trailing period replacing the compiler-added dash, if any, as a -dumpdir option to lto-wrapper; being involved in linking, this program does not normally get any -dumpbase and -dumpbase-ext, and it ignores them.
When running sub-compilers, lto-wrapper appends LTO stage names to the received dumppfx, ensures it contains a directory component so that it overrides any -dumpdir, and passes that as -dumpbase to sub-compilers.
-v,--verbose— print (on standard error output) the commands executed to run the stages of compilation. Also print the version number of the compiler driver program and of the preprocessor and the compiler proper. 💅-###— like-vexcept the commands are not executed and arguments are quoted unless they contain only alphanumeric characters or./-_. This is useful for shell scripts to capture the driver-generated command lines. 🫶--help— print (on the standard output) a description of the command-line options understood bygcc. If the-voption is also specified then--helpis also passed on to the various processes invoked bygcc, so that they can display the command-line options they accept. If the-Wextraoption has also been specified (prior to the--helpoption), then command-line options that have no documentation associated with them are also displayed. ✨--target-help— print (on the standard output) a description of target-specific command-line options for each tool. For some targets extra target-specific information may also be printed. 🎯--help={class|[^]qualifier}[,...]— print (on the standard output) a description of the command-line options understood by the compiler that fit into all 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 however, target-specific options of the linker and assembler are not displayed. This is because those tools do not currently support the extended--help=syntax. 🎯params— display the values recognized by the--paramoption. 🎛️language— display the options supported for language, where language is the name of one of the languages supported in this version of GCC. If an option is supported by all languages, one needs to selectcommonclass. 🌐common— display the options that are common to all languages. 🌍
These are the supported qualifiers:
undocumented— display only those options that are undocumented. 🙈joined— display options taking an argument that appears after an equal sign in the same continuous piece of text, such as:--help=target. 🔗separate— display options taking an argument that appears as a separate word following the original option, such as:-o output-file. ✂️
Thus for example to display all the undocumented target-specific switches supported by the compiler, use:
--help=target,undocumentedThe sense of a qualifier can be inverted by prefixing it with the ^ character, so for example to display all binary warning options (i.e., ones that are either on or off and that do not take an argument) that have a description, use:
--help=warnings,^joined,^undocumentedThe argument to --help= should not consist solely of inverted qualifiers. 🚫
Combining several classes is possible, although this usually restricts the output so much that there is nothing to display. One case where it does work, however, is when one of the classes is target. For example, to display all the target-specific optimization options, use:
--help=target,optimizersThe --help= option can be repeated on the command line. Each successive use displays its requested class of options, skipping those that have already been displayed. If --help is also specified anywhere on the command line then this takes precedence over any --help= option. 👑
Oh, and real quick before we move on: if the -Q option shows up on the command line before the --help= option, then the descriptive text that --help= displays gets changed. Instead of describing the displayed options, it gives you an indication of whether the option is enabled, disabled, or set to a specific value (assuming the compiler actually knows this at the point where the --help= option is used). slay, very diagnostic of it 💅
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 is 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. iconic little identity check 🪪✨-pass-exit-codes,--pass-exit-codes— normally the gcc program exits with the code of1if any phase of the compiler returns a non-success return code. If you specify-pass-exit-codes, the gcc program instead returns with the numerically highest error produced by any phase returning an error indication. The C, C++, and Fortran front ends return4if an internal compiler error is encountered. numbers, bestie, they matter 🔢-pipe,--pipe— use pipes rather than temporary files for communication between the various stages of compilation. This fails to work on some systems where the assembler is unable to read from a pipe; but the GNU assembler has no trouble. pipes >> temp files, lowkey a vibe 🚰-wrapper— invoke all subcommands under a wrapper program. The name of the wrapper program and its parameters are passed as a comma separated list.
gcc -c t.c -wrapper gdb,--argsThis invokes all subprograms of gcc under gdb --args, thus the invocation of cc1 is gdb --args cc1 .... wrapped like a lil compiler gift 🎁
-ffile-prefix-map=old=new— when compiling files residing in directoryold, record any references to them in the result of the compilation as if the files resided in directorynewinstead. 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. Directories referenced by directives are not affected by these options. See also-fmacro-prefix-map,-fdebug-prefix-map,-fprofile-prefix-mapand-fcanon-prefix-map. reproducible builds are THAT girl 🧚-fcanon-prefix-map— for the-f*-prefix-mapoptions, normally comparison of theoldprefix against the filename that would normally be referenced in the result of the compilation is done using textual comparison of the prefixes, or ignoring character case for case insensitive filesystems and considering slashes and backslashes as equal on DOS based filesystems. The-fcanon-prefix-mapcauses such comparisons to be done on canonicalized paths ofoldand the referenced filename. canonicalized, not just textually compared — important distinction fr 🎀-fplugin=name.so— load the plugin code in filename.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. plug me in, bestie 🔌-fplugin-arg-name-key=value— define an argument calledkeywith a value ofvaluefor the plugin calledname. names and values, very demure 🫶-fdump-ada-spec[-slim]— for C and C++ source and include files, generate corresponding Ada specs. Ada specs, we love an unexpected crossover 💫-fada-spec-parent=unit— in conjunction with-fdump-ada-spec[-slim]above, generate Ada specs as child units of parentunit. family tree but make it Ada 🌳-fdump-go-spec=file— for input files in any language, generate corresponding Go declarations infile. 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. Go gals stay winning 🦋@file— read command-line options fromfile. The options read are inserted in place of the original@fileoption. Iffiledoes 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. recursive little snack, obsessed 🍓
💻 Compiling C++ Programs 💻
C++ source files conventionally use one of the suffixes .C, .cc, .cpp, .CPP, .c++, .cp, or .cxx; C++ header files often use .hh, .hpp, .H, or (for shared template code) .tcc; preprocessed C++ files use the suffix .ii; and C++20 module interface units sometimes use .ixx, .cppm, .cxxm, .c++m, or .ccm. suffix lore, honestly crucial 📚
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++. g++ girlies rise 💖
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,--ansi— in C mode, this is equivalent to-std=c90. In C++ mode, it is equivalent to-std=c++98. vintage, but make it official 🕰️-std=— determine the language standard. This option is currently only supported when compiling C or C++. the standard-setter, literally 👑
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). Seehttps://gcc.gnu.org/projects/c-status.htmlfor more information. The namesc9xandiso9899:199xare deprecated. deprecated names, we still respect you 🙈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.c23,c2x,iso9899:2024— ISO C23, the 2023 revision of the ISO C standard (published in 2024). The namec2xis deprecated. new standard smell ✨c2y— the next version of the ISO C standard, still under development. The support for this version is experimental and incomplete. experimental, bestie — do not expect full slay 💀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.gnu23,gnu2x— GNU dialect of ISO C23. This is the default for C code. The namegnu2xis deprecated. default C code behavior, main character energy 👑gnu2y— the next version of the ISO C standard, still under development, plus GNU extensions. The support for this version is experimental and incomplete. The namegnu2xis deprecated. yes, it saysgnu2xthere — we don’t make the rules, we just relay them 🫶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. deprecated but not forgotten, periodt 💅
c++14,c++1y— the 2014 ISO C++ standard plus amendments 💅. The namec++1yis deprecated, so maybe don't use it unless you're feeling vintage.gnu++14,gnu++1y— GNU dialect of-std=c++14. The namegnu++1yis also deprecated 🫶.c++17,c++1z— the 2017 ISO C++ standard plus amendments. Yep,c++1zis deprecated too.gnu++17,gnu++1z— GNU dialect of-std=c++17. Andgnu++1zis deprecated, no cap.c++20,c++2a— the 2020 ISO C++ standard plus amendments. Heads up bestie: C++20 modules support is still experimental and needs to be enabled with the-fmodulesoption. The namec++2ais deprecated 🙈.gnu++20,gnu++2a— GNU dialect of-std=c++20. This is the default for C++ code, main character energy honestly. C++20 modules support is still experimental and needs-fmodules. The namegnu++2ais deprecated.c++23,c++2b— the 2023 ISO C++ standard plus amendments, published in 2024 📅. Support is experimental, and could change in incompatible ways in future releases. The namec++2bis deprecated.gnu++23,gnu++2b— GNU dialect of-std=c++23. Support is experimental, and could change in incompatible ways in future releases. The namegnu++2bis deprecated.c++2c,c++26— the next revision of the ISO C++ standard, planned for 2026 🔮. Support is highly experimental, and will almost certainly change in incompatible ways in future releases. Like, do not build your whole life on this one.gnu++2c,gnu++26— GNU dialect of-std=c++2c. Support is highly experimental, and will almost certainly change in incompatible ways in future releases 💀.
-aux-infofilename — writes prototyped declarations to filename for all functions declared and/or defined in a translation unit, including the ones hiding in header files 👀. This option is silently ignored in any language other than C.
Besides the declarations, the file also tells you, in comments, the origin of each declaration — source file and line — whether the declaration was implicit, prototyped or unprototyped (I, N for new or O for old, respectively, in the first character after the line number and the colon), and whether it came from a declaration or a definition (C or F, respectively, in the following character). In the case of function definitions, a K&R-style list of arguments followed by their declarations is also provided, inside comments, after the declaration 🎀.
-fno-asm— does not recognize"asm","inline"or"typeof"as a keyword, so your code can use these words as identifiers 🦋. You can use the keywords"__asm__","__inline__"and"__typeof__"instead. In C,-ansiimplies-fno-asm.
In C++, "inline" is a standard keyword and is not affected by this switch. You may want to use the -fno-gnu-keywords flag instead, which disables "typeof" but not "asm" and "inline". In C99 mode (-std=c99 or -std=gnu99), this switch only affects the "asm" and "typeof" keywords, since "inline" is a standard keyword in ISO C99. In C23 mode (-std=c23 or -std=gnu23), this switch only affects the "asm" keyword, since "typeof" is a standard keyword in ISO C23 ✨.
-fno-builtin-fno-builtin-function— don't recognize built-in functions that do not begin with__builtin_as prefix 🙅.
GCC normally generates special code to handle certain built-in functions more efficiently — for instance, calls to "alloca" may become single instructions which adjust the stack directly, and calls to "memcpy" may become inline copy loops 🔥. The resulting code is often both smaller and faster, but since the function calls no longer appear as such, you cannot set a breakpoint on those calls, nor can you change the behavior of the functions by linking with a different library 😭. In addition, when a function is recognized as a built-in function, GCC may use information about that function to warn about problems with calls to that function, or to generate more efficient code, even if the resulting code still contains calls to that function. For example, warnings are given with -Wformat for bad calls to "printf" when "printf" is built in and "strlen" is known not to modify global memory 🧚.
With the -fno-builtin-function option only the built-in function function is disabled. function must not begin with __builtin_. If a function is named that is not built-in in this version of GCC, this option is ignored. There is no corresponding -fbuiltin-function option; if you wish to enable built-in functions selectively when using -fno-builtin or -ffreestanding, you may define macros such as:
#define abs(n) __builtin_abs ((n))
#define strcpy(d, s) __builtin_strcpy ((d), (s))-fcond-mismatch— allows 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++.-ffreestanding— asserts that compilation targets a freestanding environment 🌱. This implies-fno-builtin. A freestanding environment is one in which the standard library may not exist, and program startup may not necessarily be at"main"— the most obvious example being an OS kernel 👑. This is equivalent to-fno-hosted.-fgimple— enables parsing of function definitions marked with"__GIMPLE"✨. This is an experimental feature that allows unit testing of GIMPLE passes.-fgnu-tm— when you slap-fgnu-tmon, the compiler generates code for the Linux variant of Intel's current Transactional Memory ABI specification document (Revision 1.1, May 6 2009) 🕰️. This is an experimental feature whose interface may change in future versions of GCC, as the official specification changes. Please note that not all architectures are supported for this feature 💀.
For more information on GCC's support for transactional memory,
Note that the transactional memory feature is not supported with non-call exceptions (-fnon-call-exceptions).
-fgnu89-inline— tells GCC to use the traditional GNU semantics for"inline"functions when in C99 mode 🎀.
Using this option is roughly equivalent to adding the "gnu_inline" function attribute to all inline functions.
The option -fno-gnu89-inline explicitly tells GCC to use the C99 semantics for "inline" when in C99 or gnu99 mode — i.e., it specifies the default behavior. This option is not supported in -std=c90 or -std=gnu90 mode.
The preprocessor macros "__GNUC_GNU_INLINE__" and "__GNUC_STDC_INLINE__" may be used to check which semantics are in effect for "inline" functions 🌸.
-fhosted— asserts that compilation targets a hosted environment 🏠. This implies-fbuiltin. A hosted environment is one in which the entire standard library is available, and in which"main"has a return type of"int". Examples are nearly everything except a kernel. This is equivalent to-fno-freestanding.-flax-vector-conversions— allows implicit conversions between vectors with differing numbers of elements and/or incompatible element types 😬. This option should not be used for new code, periodt.-fms-extensions— accepts some non-standard constructs used in Microsoft header files 🪟.
In C++ code, this allows member names in structures to be similar to previous types declarations:
typedef int UOW;
struct ABC {
UOW UOW;
};Some cases of unnamed fields in structures and unions are only accepted with this option 👀.
Note that this option is off for all targets except for x86 targets using ms-abi.
-fpermitted-flt-eval-methods=style — ISO/IEC TS 18661-3 defines new permissible values for"FLT_EVAL_METHOD"that indicate operations and constants with a semantic type that is an interchange or extended format should be evaluated to the precision and range of that type 🧮. These new values are a superset of those permitted under C99/C11, which does not specify the meaning of other positive values of"FLT_EVAL_METHOD". As such, code conforming to C11 may not have been written expecting the possibility of the new values 😭.
-fpermitted-flt-eval-methods specifies whether the compiler should allow only the values of "FLT_EVAL_METHOD" specified in C99/C11, or the extended set of values specified in ISO/IEC TS 18661-3.
style is either "c11" or "ts-18661-3" as appropriate 💅.
The default when in a standards compliant mode (-std=c11 or similar) is -fpermitted-flt-eval-methods=c11. The default when in a GNU dialect (-std=gnu11 or similar) is -fpermitted-flt-eval-methods=ts-18661-3 ✨.
The -fdeps-* options are used to extract structured dependency information for a source 🔍. This involves determining what resources provided by other source files will be required to compile the source as well as what resources are provided by the source. This information can be used to add required dependencies between compilation rules of dependent sources based on their contents rather than requiring such information be reflected within the build tools as well 🫶.
-fdeps-file=file — where to write structured dependency information 📝.-fdeps-format=format — the format to use for structured dependency information.p1689r5is the only supported format right now. Note that when this argument is specified, the output of-MFis stripped of some information (namely C++ modules) so that it does not use extended makefile syntax not understood by most tools 👀.-fdeps-target=file — analogous to-MTbut for structured dependency information. This indicates the target which will ultimately need any required resources and provide any resources extracted from the source that may be required by other sources 🎯.-fplan9-extensions— accepts some non-standard constructs used in Plan 9 code 🐇.
This enables -fms-extensions, permits passing pointers to structures with anonymous fields to functions that expect pointers to elements of the type of the field, and permits referring to anonymous fields declared using a typedef ✨. This is only supported for C, not C++.
-fsigned-bitfields,-funsigned-bitfields,-fno-signed-bitfields,-fno-unsigned-bitfields— these four decide whether a bit-field is signed or unsigned when the declaration just… doesn't saysignedorunsigned🙈. The default is signed, and that's literally for consistency reasons: the basic integer types likeintare signed types, so it just tracks. No cap.-fsigned-char,-funsigned-char— okay so-fsigned-charlets the typecharbe signed, likesigned char, while-funsigned-charlets the typecharbe unsigned, likeunsigned char✨.
Bestie, note this: -fsigned-char 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. Same coin, different side 💅.
Every target GCC supports has a default for what char should be, and it's typically spelled out in the processor-specific ABI document for that target. So it's either like unsigned char by default or like signed char by default — depends on the machine.
Ideally, a portable program should always use signed char or unsigned char whenever it depends on the signedness of an object. Like, ideally. But sooo many programs out there were written using plain char and expecting it to be signed, or expecting it to be unsigned, depending on the machines they were written for 😭. These options are your little rescue squad for making such a program work with the opposite default.
And 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. Periodt.
-fstrict-flex-arrays(C and C++ only),-fstrict-flex-arrays=level(C and C++ only) — these control when to treat the trailing array of a structure as a flexible array member, for the purpose of accessing the elements of such an array. The value of level controls the level of strictness 👑
-fstrict-flex-arrays is equivalent to -fstrict-flex-arrays=3, which is the strictest — a trailing array is treated as a flexible array member only when it is declared as a flexible array member per the C99 standard onwards.
The negative form -fno-strict-flex-arrays is equivalent to -fstrict-flex-arrays=0, which is the least strict. In this case all trailing arrays of structures are treated as flexible array members 🦋.
There are two more levels in between 0 and 3, and they exist to support older code that uses the GCC zero-length array extension ([0]) or one-element array as flexible array members ([1]). When level is 1, the trailing array is treated as a flexible array member when it is declared as either [], [0], or [1]. When level is 2, the trailing array is treated as a flexible array member when it is declared as either [], or [0] — yeah, [1] is out at that point, iykyk.
You can control this behavior for a specific trailing array field of a structure by using the variable attribute strict_flex_array attribute 🔥.
Also, the -fstrict_flex_arrays option interacts with the -Wstrict-flex-arrays option. They're in each other's orbit.
-fsso-struct=endianness— sets the default scalar storage order of structures and unions to the specified endianness. The accepted values arebig-endian,little-endianandnativefor the native endianness of the target (which is the default). This option is not supported for C++ 🫶.
⚠️ Warning: the -fsso-struct switch causes GCC to generate code that is not binary compatible with code generated without it if the specified endianness is not the native endianness of the target. Like, actually important, don't skip this one 💀.
🦋 Options Controlling C++ Dialect 🦋
This section describes the command-line options that are only meaningful for C++ programs. You can also use most of the GNU compiler options regardless of what language your program is in — GCC is flexible like that 💖. For example, you might compile a file firstClass.C like this:
g++ -g -fstrict-enums -O -c firstClass.CIn this example, only -fstrict-enums is an option meant only for C++ programs; you can use the other options with any language supported by GCC.
Some options for compiling C programs, such as -std, are also relevant for C++ programs 👀.
Here is a list of options that are only for compiling C++ programs:
--compile-std-module— builds the compiled module interfaces for the<bits/stdc++.h>header unit and thestdandstd.compatmodules before compiling any source files explicitly specified on the command line. This is intended to be useful for building simple programs that useimport std;with a single command — one and done, iconic ✨.-fabi-version=n— use version n of the C++ ABI. The default is version0.
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
1is the version of the C++ ABI that first appeared in G++ 3.2. - Version
2is the version of the C++ ABI that first appeared in G++ 3.4, and was the default through G++ 4.9. - Version
3corrects 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 treatsnullptr_tas a builtin type and corrects the mangling of lambdas in default argument scope. - Version
8, which first appeared in G++ 4.9, corrects the substitution behavior of function types with function-cv-qualifiers. - Version
9, which first appeared in G++ 5.2, corrects the alignment ofnullptr_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 ofsizeof...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. Oops 😭. - Version
13, which first appeared in G++ 8.2, fixes the accidental change in version12. - Version
14, which first appeared in G++ 10, corrects the mangling of thenullptrexpression. - Version
15, which first appeared in G++ 10.3, corrects G++ 10 ABI tag regression. - Version
16, which first appeared in G++ 11, changes the mangling of__alignof__to be distinct from that ofalignof, and dependent operator names. - Version
17, which first appeared in G++ 12, fixes layout of classes that inherit from aggregate classes with default member initializers in C++14 and up. - Version
18, which first appeared in G++ 13, fixes manglings of lambdas that have additional context. - Version
19, which first appeared in G++ 14, fixes manglings of structured bindings to include ABI tags, handling of cv-qualified[[no_unique_address]]members, and adds mangling of C++20 constraints on function templates. - Version
20, which first appeared in G++ 15, fixes manglings of lambdas in static data member initializers. - Version
21, which first appeared in G++ 16, fixes unnecessary captures in noexcept lambdas (c++/119764), layout of a base class with all explicitly defaulted constructors (c++/120012), and mangling of class and array objects with implicitly zero-initialized non-trailing subobjects (c++/121231).
See also -Wabi 💜.
-fabi-compat-version=n— on targets that support strong aliases, G++ works around mangling changes by creating an alias with the correct mangled name when defining a symbol with an incorrect mangled name. This switch specifies which ABI version to use for the alias 🎀.
With -fabi-version=0 (the default), this defaults to 13 (GCC 8.2 compatibility). If another ABI version is explicitly selected, this defaults to 0. For compatibility with GCC versions 3.2 through 4.9, use -fabi-compat-version=2.
If this option is not provided but -Wabi=n is, that version is used for compatibility aliases. If this option is provided along with -Wabi (without the version), the version from this option is used for the warning 👑.
-fno-access-control— turns off all access checking. This switch is mainly useful for working around bugs in the access control code 🙈. Very "let me in" energy.-faligned-new,-faligned-new=alignment— 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, standards.
-fno-assume-sane-operators-new— okay so the C++ standard lets you replace the globalnew,new[],deleteanddelete[]operators, but honestly a ton of C++ programs just… don't, and use the implementation-provided version like good little defaults. And on top of that, the standard lets the compiler straight-up omit those calls if they're made from new or delete expressions (and by extension, the same is assumed if__builtin_operator_newor__builtin_operator_deletefunctions are used). This option gives you the aux cord for some optimizations around calls to those operators.
With -fassume-sane-operators-new-delete GCC may assume that calls to the replaceable global operators from new or delete expressions, or from __builtin_operator_new or __builtin_operator_delete calls, don't read or modify any global variables, or variables whose address could escape to the operators (that's global state; except for errno for the new and new[] operators 💅). This lets most optimizations happen across those calls, and it's something the implementation-provided operators satisfy — unless malloc implementation details are observable in the code, or unless malloc hooks are used. But it might not be satisfied if a program replaces those operators, bestie. This behavior is enabled by default.
With -fno-assume-sane-operators-new-delete GCC must assume all these calls (whether from new or delete expressions or called directly) may read and write global state unless proven otherwise (e.g. when GCC compiles their implementation). Use this option if those operators are or may be replaced and code needs to expect such behavior.
-fchar8_t,-fno-char8_t— turns on support forchar8_tas adopted for C++20 ✨. 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.
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— makes the compiler check that the pointer returned byoperator newis non-null before it tries to modify the storage allocated. Ngl this check is normally unnecessary, because the C++ standard specifies thatoperator newonly returns 0 if it is declaredthrow(), and in that 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 alsonew (nothrow).
-fconcepts— enables support for the C++ Concepts feature for constraining template arguments 🎀. With-std=c++20and above, Concepts are part of the language standard, so-fconceptsdefaults to on.
Some constructs that were allowed by the earlier C++ Extensions for Concepts Technical Specification, ISO 19217 (2015), but didn't make it into the standard, could additionally be enabled by -fconcepts-ts.
💀 Heads up: the option -fconcepts-ts was deprecated in GCC 14 and removed in GCC 15. Users are expected to convert their code to C++20 concepts. Periodt.
- *
-fconcepts-diagnostics-depth=n* — sets the maximum error replay depth during recursive diagnosis of a constraint satisfaction failure. The default is1.
- *
-fconstexpr-depth=n* — sets the maximum nested evaluation depth for C++11 constexpr functions to n. A limit is needed so the compiler can detect endless recursion during constant expression evaluation 🌀. The minimum specified by the standard is512.
- *
-fconstexpr-cache-depth=n* — sets the maximum level of nested evaluation depth for C++11 constexpr functions that will be cached to n. It's a heuristic that trades off compilation speed (when the cache avoids repeated calculations) against memory consumption (when the cache grows very large from highly recursive evaluations). The default is8. Very few users are likely to want to adjust it, but if your code does heavy constexpr calculations you might want to experiment to find which value works best for you 🔥.
-fconstexpr-fp-except— Annex F of the C standard specifies that IEC559 floating point exceptions encountered at compile time should not stop compilation. C++ compilers have historically not followed this guidance, instead treating floating point division by zero as non-constant even though it has a well defined value 🙈. This flag tells the compiler to give Annex F priority over other rules saying that a particular operation is undefined.
constexpr float inf = 1./0.; // OK with -fconstexpr-fp-except- *
-fconstexpr-loop-limit=n* — sets the maximum number of iterations for a loop in C++14 constexpr functions to n. A limit is needed to detect infinite loops during constant expression evaluation 💫. The default is262144(1<<18).
- *
-fconstexpr-ops-limit=n* — sets the maximum number of operations during a single constexpr evaluation. Even when the number of iterations of a single loop is limited with the above limit, if there are several nested loops and each of them has many iterations but still smaller than the above limit, or if in a body of some loop — or even outside of a loop — too many expressions need to be evaluated, the resulting constexpr evaluation might take too long 😴. The default is33554432(1<<25).
-fcontracts— enables support for the C++ Contracts feature, as specified in the C++26 working draft (N5003) 👑
On violation of an enforced or observed contract (see -fcontract-evaluation-semantic below), a violation handler is called; the standard library provides a default handler that emits information about the contract that failed.
Users can replace the default violation handler by defining:
void
handle_contract_violation (const std::contracts::contract_violation&);- *
-fcontract-evaluation-semantic=semantic* — sets the semantic with which contracts will be evaluated to semantic.
Available values for the evaluation mode are:
"ignored"— the contract checks will be elided, with no checking added at either compile or runtime. Nothing to see here 💅"observed"— the contract checks are performed (at runtime) and, if one fails, a handler is called that allows actions such as logging or emitting run-time warnings. When the handler returns, the execution of the code will continue. At compile-time the checks are performed forconstexprevaluations, and in this case a diagnostic is emitted if the check fails."enforce"— the contract checks are performed and the handler is called if one fails. When the handler returns, the execution of the code is terminated, preventing it from continuing past the failed case 💀. At compile-time the checks are applied toconstexprand, if one fails, the program is ill-formed; an error will be emitted."quick_enforce"— the contract checks are performed (at runtime) and, if one fails, the execution is terminated immediately (without calling any handler). At compile-time there is no difference in behaviour between this option and the"enforce"case, since no handler is invoked at compile time.
-fcontracts-conservative-ipa— this prevents inter-procedural analysis from taking action when the body of an inline function is visible in a given TU. This allows for the case that contract evaluation conditions are permitted to differ between TUs, which means that such actions would be potentially incorrect 🚨.
-fcontract-checks-outlined— by default, contract checks forpreandpostcondition checks are expanded in-line into function bodies. This option causes a small function to be created instead (containing the checks) that is called in the relevant position 🦋. This permits different criteria to be applied to the compilation of the checking and the remainder of the function body.
-fcontract-disable-optimized-checks— whenpreandpostcondition checks are outlined (using-fcontract-checks-outlined), this option disables the optimisation steps for those functions, which can avoid unwanted elision of checking steps 🙌.
- *
-fcontracts-client-check=mode* — this option enables insertion of checks at call sites (caller-side checking) ✨
The value of mode determines which contract checks are made:
"none"— no caller/client side checking (default)"pre"— preconditions are checked at the caller/client side."all"— both pre and post-conditions are checked at the caller/client side.
- *
-fcontracts-definition-check=mode* — this option introduces the ability to disable callee/definition-side checks, which are otherwise enabled by default when the contracts feature is active 🎯
The variable mode has the values "on" and "off".
-fcoroutines— turns on support for the C++ coroutines extension ✨ and if you're on-std=c++20or above, coroutines are literally part of the language standard, so-fcoroutinesis on by default. no extra work needed, bestie 💅-fdiagnostics-all-candidates— lets the C++ front end spill all the candidates during an overload resolution failure, including when a deleted function is the one that got picked. more info = less crying at your terminal 😭-fdump-lang-,-fdump-lang-switch,-fdump-lang-switch-options,-fdump-lang-switch-options=filename— these control dumping of C++-specific info 🎀 theoptionsandfilenameparts behave exactly like they do in-fdump-tree. accepted switch values, bestie:all— enable literally all of the ones below 👑class— dump class hierarchy info. virtual table info gets emitted too unless you say'slim'🦋module— dump module info. the optionslineno(locations),graph(reachability),blocks(clusters),uid(serialization),alias(mergeable),asmname(Elrond),eh(mapper) &vops(macros) may give you extra deets 💖raw— dumps the raw internal tree data, no cap 🌈tinst— dumps the sequence of template instantiations, indented to show how deep the recursion goes 🌀 thelinenooption adds the source location where the instantiation was triggered, and thedetailsoption also dumps pre-instantiation substitutions like the ones performed during template argument deduction. lines in the.tinstdump start withIfor an instantiation,Sfor another substitution, andR[IS]for the reopened context of a deferred instantiation ⭐
-fno-elide-constructors— okay so the C++ standard lets an implementation skip creating a temporary that's only used to initialize another object of the same type. this option says nope, no skipping 😤 it disables that optimization and forces G++ to call the copy constructor in ALL cases. it also makes G++ call trivial member functions that would otherwise be expanded inline. heads up: in C++17 the compiler is required to omit these temporaries, but this option still affects trivial member functions 💀-fno-enforce-eh-specs— don't generate code that checks for exception-specification violations at run time 🙈 this one straight up violates the C++ standard, but it can be useful for shrinking code size in production builds, kind of like defining"NDEBUG". it does not give your user code permission to throw exceptions that violate the exception specifications — the compiler still optimizes based on those specs, so throwing an unexpected exception = undefined behavior at run time 😱 be careful fr fr-fextern-tls-init,-fno-extern-tls-init— the C++11 and OpenMP standards allow"thread_local"and"threadprivate"variables to have dynamic (runtime) initialization ✨ to support that, any use of such a variable goes through a wrapper function that does whatever initialization is needed. when the use and the definition are in the same translation unit, that overhead can be optimized away, but when the use is in a different translation unit 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 a use in the defining TU will run before any uses in another TU), you can dodge 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 🎀
-ffold-simple-inlines,-fno-fold-simple-inlines— lets the C++ frontend fold calls to"std::move","std::forward","std::addressof","std::to_underlying"and"std::as_const"🫶 unlike inlining, this means no debug info gets generated for those calls. and since these functions are rarely interesting to debug, the flag is on by default unless-fno-inlineis active 💖-fno-gnu-keywords— don't recognize"typeof"as 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-immediate-escalation— don't enable immediate function escalation, the thing where certain functions can get promoted toconsteval, as specified in P2564R3 🤯 example time:
consteval int id(int i) { return i; }
constexpr int f(auto t)
{
return t + id(t); // id causes f<int> to be promoted to consteval
}
void g(int i)
{
f (3);
}this compiles in C++20: "f" is an immediate-escalating function (because of the "auto" it's a function template, and it's declared "constexpr") and id(t) is an immediate-escalating expression, so "f" gets promoted to "consteval" 🔥 consequently, the call to id(t) is in an immediate context, so it doesn't have to produce a constant (that's the mechanism that allows consteval function composition ✨). but with -fno-immediate-escalation, "f" is not promoted to "consteval", and since the call to the consteval function id(t) isn't a constant expression, the compiler rejects the code 💀
this option is turned on by default; it's only effective in C++20 mode or later 💜
-fimplicit-constexpr— makes inline functions implicitlyconstexprif they satisfy the requirements for a constexpr function ✨ you can use this in C++14 mode or later. it can result in initialization changing from dynamic to static, plus other optimizations 👑-fno-implicit-templates— never emit code for non-inline templates that are instantiated implicitly (i.e. by use); only emit code for explicit instantiations 😤 if you use this, you must structure your code to include all the necessary explicit instantiations, otherwise you'll get undefined symbols at link time 😭 pls don't skip this step-fno-implicit-inline-templates— don't emit code for implicit instantiations of inline templates either 🫶 the default handles inlines differently so that compiles with and without optimization need the same set of explicit instantiations 💅-fno-implement-inlines— to save space, don't 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 💀-fmodules,-fno-modules— enable support for C++20 modules ✨-fno-modulesis usually not needed since that's the default. and even though this is a C++20 feature, it's not currently implicitly enabled just by selecting that standard version — you gotta ask for it 🙈-fmodule-header,-fmodule-header=user,-fmodule-header=system— compile a header file to create an importable header unit 💖-fmodule-implicit-inline— member functions defined in their class definitions are not implicitly inline for modular code 🦋 this differs from traditional C++ behavior, and for good reasons. however, it may make code porting a struggle 😭 this option makes such function definitions implicitly inline. but heads up: it generates an ABI incompatibility, so you must use it everywhere or nowhere 😱 (such definitions outside of a named module remain implicitly inline regardless 💅)-fno-module-lazy— disables lazy module importing and module mapper creation 🙈-fmodule-mapper=[hostname]:port[?ident],-fmodule-mapper=|program[?ident] args...,-fmodule-mapper==socket[?ident],-fmodule-mapper=<>[inout][?ident],-fmodule-mapper=<in>out[?ident],-fmodule-mapper=file[?ident]— an oracle you can query for module-name-to-filename mappings 👑 if unspecified, theCXX_MODULE_MAPPERenvironment variable is used, and if that's unset, an in-process default is provided ✨-fmodule-only— only emit the Compiled Module Interface, blocking any object file 💅-fms-extensions— disableWpedanticwarnings about constructs used in MFC, like implicitintand getting a pointer to member function via non-standard syntax 🎀-fnew-inheriting-ctors— enables the P0136 adjustment to the semantics of C++11 constructor inheritance ✨ this is part of C++17 but also considered a Defect Report against C++11 and C++14. this flag is enabled by default unless-fabi-version=10or lower is specified 💖-fnew-ttp-matching— enables the P0522 resolution to Core issue 150, template template parameters and default arguments: it lets a template with default template arguments be used as an argument for a template template parameter with fewer template parameters 🦋 this flag is enabled by default for-std=c++17⭐-fno-nonansi-builtins— disables built-in declarations of functions that aren't mandated by ANSI/ISO C 🙈 these include"ffs","alloca","_exit","index","bzero","conjf", and other related functions 💀-fnothrow-opt— treats athrow()exception specification as if it were a"noexcept"specification, to reduce or eliminate the text size overhead compared to a function with no exception specification 🎀 if the function has local variables of types with non-trivial destructors, the exception specification actually makes the function smaller, because the EH cleanups for those variables can be optimized away ✨ the semantic effect: an exception thrown out of a function with such an exception specification results in a call to"terminate"rather than"unexpected"💅-fno-operator-names— don't treat the operator name keywords"and","bitand","bitor","compl","not","or"and"xor"as synonyms for keywords 🫶-fno-optional-diags— disables diagnostics that the standard says a compiler doesn't need to issue 🙈 currently, the only such diagnostic G++ issues is the one for a name having multiple meanings within a class 💜-fno-pretty-templates— okay so error messages are about to get real specific. When an error message refers to a specialization of a function template, the compiler normally prints the signature of the template followed by the template arguments and any typedefs or typenames in the signature (e.g.void f(T) [with T = int]rather thanvoid f(int)) so that it’s clear which template is involved. When an error message refers to a specialization of a class template, the compiler omits any template arguments that match the default template arguments for that template. If either of these behaviors make it harder to understand the error message rather than easier, you can use-fno-pretty-templatesto disable them. fr fr, sometimes less pretty is more useful.-frange-for-ext-temps— Enable lifetime extension of C++ range based for temporaries. With-std=c++23and above this is part of the language standard, so lifetime of the temporaries is extended until the end of the loop by default. This option allows enabling that behavior also in earlier versions of the standard. lowkey a time-travel pass for older standards ✨-freflection— Enable experimental C++26 Reflection. experimental means bestie, don’t ship your whole personality on it yet 💅-fno-rtti— Disable generation of information about every class with virtual functions for use by the C++ run-time type identification features (dynamic_castandtypeid). If you don’t use those parts of the language, you can save some space by using this flag. Note that exception handling uses the same information, but G++ generates it as needed. Thedynamic_castoperator can still be used for casts that do not require run-time type information, i.e. casts tovoid *or to unambiguous base classes.
Mixing code compiled with -frtti with that compiled with -fno-rtti may not work. For example, programs may fail to link if a class compiled with -fno-rtti is used as a base for a class compiled with -frtti. do not mix these casually, that link error will humble you 😭
-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. size-aware deallocation is giving main character energy, ngl.
-fstrict-enums— Allow the compiler to optimize using the assumption that a value of enumerated type can only be one of the values of the enumeration (as defined in the C++ standard; basically, a value that can be represented in the minimum number of bits needed to represent all the enumerators). This assumption may not be valid if the program uses a cast to convert an arbitrary integer value to the enumerated type. This option has no effect for an enumeration type with a fixed underlying type. it’s an optimization vibe check, so only use it if your enums are actually behaving.-fstrong-eval-order,-fstrong-eval-order=kind— Evaluate member access, array subscripting, and shift expressions in left-to-right order, and evaluate assignment in right-to-left order, as adopted for C++17.-fstrong-eval-orderis equivalent to-fstrong-eval-order=all, and is enabled by default with-std=c++17or later.-fstrong-eval-order=some— enables just the ordering of member access and shift expressions, and is the default for C++ dialects prior to C++17.-fstrong-eval-order=none— is equivalent to-fno-strong-eval-order.
order matters, bestie. this is literally the “who goes first” of expression evaluation.
-ftemplate-backtrace-limit=n— Set the maximum number of template instantiation notes for a single warning or error to n. The default value is 10. because nobody needs a 400-note template essay in their terminal 😭-ftemplate-depth=n— Set the maximum instantiation depth for template classes to n. A limit on the template instantiation depth is needed to detect endless recursions during template class instantiation. ANSI/ISO C++ conforming programs must not rely on a maximum depth greater than 17 (changed to 1024 in C++11). The default value is 900, as the compiler can run out of stack space before hitting 1024 in some situations. recursion without a limit is not slay, it’s a stack crash 💀-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. only if you genuinely don’t need thread safety, okay? this is not a “just for fun” flag.-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. standards-compliant static destructor cleanup is lowkey iconic.-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 is not available. like okay if your runtime is missing it, you do what you gotta do, but bestie, thatstd::uncaught_exceptionresult is not to be trusted.-fvisibility-inlines-hidden— This switch declares that the user does not attempt to compare pointers to inline functions or methods where the addresses of the two functions are taken in different shared objects.
The effect of this is that GCC may, effectively, mark inline methods with __attribute__ ((visibility ("hidden"))) so that they do not appear in the export table of a DSO and do not require a PLT indirection when used within the DSO. Enabling this option can have a dramatic effect on load and link times of a DSO as it massively reduces the size of the dynamic export table when the library makes heavy use of templates.
The behavior of this switch is not quite the same as marking the methods as hidden directly, because it does not affect static variables local to the function or cause the compiler to deduce that the function is defined in only one shared object.
You may mark a method as having a visibility explicitly to negate the effect of the switch for that method. For example, if you do want to compare pointers to a particular inline method, you might mark it as having default visibility. Marking the enclosing class with explicit visibility has no effect.
Explicitly instantiated inline methods are unaffected by this option as their linkage might otherwise cross a shared library boundary. this is the “stop exporting every tiny inline thing” glow-up for template-heavy DSOs.
-fvisibility-ms-compat— This flag attempts to use visibility settings to make GCC’s C++ linkage model compatible with that of Microsoft Visual Studio.
The flag makes these changes to GCC’s linkage model:
- 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 is better to use -fvisibility=hidden and export those classes that are intended to be externally visible. Unfortunately it is possible for code to rely, perhaps accidentally, on the Visual Studio behavior.
Among the consequences of these changes are that static data members of the same type with the same name but defined in different shared objects are different, so changing one does not change the other; and that pointers to function members defined in different shared objects may not compare equal. When this flag is given, it is a violation of the ODR to define types with the same name differently. ODR violation is not a cute look, bestie. be careful.
-fno-weak— Do not use weak symbol support, even if it is provided by the linker. By default, G++ uses weak symbols if they are available. This option exists only for testing, and should not be used by end-users; it results in inferior code and has no benefits. This option may be removed in a future release of G++. literally says “only for testing” and “inferior code” — please do not make this your everyday flag 😭-fext-numeric-literals(C++ and Objective-C++ only) — Accept imaginary, fixed-point, or machine-defined literal number suffixes as GNU extensions. When this option is turned off these suffixes are treated as C++11 user-defined literal numeric suffixes. This is on by default for all pre-C++11 dialects and all GNU dialects:-std=c++98,-std=gnu++98,-std=gnu++11,-std=gnu++14. This option is off by default for ISO C++11 onwards (-std=c++11, ...). GNU extension mode says “sure, cute suffix, come on in.” ISO C++11 mode says “that’s a user-defined literal now.”-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.) C++-specific include paths take a little break, but the other standard dirs stay.-flang-info-include-translate,-flang-info-include-translate-not,-flang-info-include-translate=header— Inform of include translation events. The first will note accepted include translations, the second will note declined include translations. The header form will inform of include translations relating to that specific header. If header is of the form"user"or"<system>"it will be resolved to a specific user or system header using the include path. basically receipts for include translations, iykyk ✨-flang-info-module-cmi,-flang-info-module-cmi=module— Inform of Compiled Module Interface pathnames. The first will note all read CMI pathnames. The module form will note reading a specific module’s CMI. module may be a named module or a header-unit (the latter indicated by either being a pathname containing directory separators or enclosed in<>or""). module CMI path receipts, because sometimes you need to know exactly which compiled module interface got read.-stdlib=libstdc++,libc++— When G++ is configured to support this option, it allows specification of alternate C++ runtime libraries. Two options are available:libstdc++(the default, native C++ runtime for G++) andlibc++which is the C++ runtime installed on some operating systems (e.g. Darwin versions from Darwin11 onwards). The option switches G++ to use the headers from the specified library and to emit-lstdc++or-lc++respectively, when a C++ runtime is required for linking. choose your runtime era, bestie. default is libstdc++, but libc++ is out here on some systems too.
In addition, these warning options have meanings only for C++ programs:
-Wabi-tag(C++ and Objective-C++ only) — Warn when a type with an ABI tag is used in a context that does not have that ABI tag. See C++ Attributes for more information about ABI tags. ABI tag mismatch warning is the compiler saying “bestie, this context does not match the tag.”-Wno-abbreviated-auto-in-template-arg— Disable the error for anautoplaceholder type used within a template argument list to declare a C++20 abbreviated function template, e.g.
void f(S<auto>);this suppresses the error for that specific C++20 abbreviated function template situation. no cap.
This feature was proposed in the Concepts TS but never got adopted into C++20; in the standard, a placeholder in a parameter declaration has to appear as a decl-specifier. Bestie, you can also downgrade that error to a warning with -fpermissive or -Wno-error=abbreviated-auto-in-template-arg. 💅
-Wcomma-subscript(C++ and Objective-C++ only) — okay so this one warns about using a comma expression inside a subscripting expression. That usage was deprecated in C++20 and is going to be removed in C++23. But a comma expression wrapped in( )is not deprecated, so don’t panic. Example:
void f(int *a, int b, int c) {
a[b,c]; // deprecated in C++20, invalid in C++23
a[(b,c)]; // OK
}In C++23 it is valid to have comma separated expressions in a subscript when an overloaded subscript operator is found and supports the right number and types of arguments. G++ will accept the formerly valid syntax for code that is not valid in C++23 but used to be valid but deprecated in C++20 with a pedantic warning that can be disabled with -Wno-comma-subscript.
Enabled by default with -std=c++20 unless -Wno-deprecated, and after -std=c++23 regardless of -Wno-deprecated. Before -std=c++20, enabled with explicit -Wdeprecated.
This warning is upgraded to an error by -pedantic-errors in C++23 mode or later. 😭
-Wctad-maybe-unsupported(C++ and Objective-C++ only) — this one warns when you’re performing class template argument deduction (CTAD) on a type with no explicitly written deduction guides. It points out cases where CTAD succeeded only because the compiler synthesized the implicit deduction guides, which might not be what the programmer intended. Certain style guides allow CTAD only on types that specifically "opt-in"; i.e., on types that are designed to support CTAD. This warning can be suppressed with the following pattern:
struct allow_ctad_t; // any name works
template <typename T> struct S {
S(T) { }
};
// Guide with incomplete parameter type will never be considered.
S(allow_ctad_t) -> S<void>;-Wctor-dtor-privacy(C++ and Objective-C++ only) — warns when a class seems unusable because all the constructors or destructors in that class are private, and it has neither friends nor public static member functions. Also warn if there are no non-private methods, and there’s at least one private member function that isn’t a constructor or destructor. 💀-Wdangling-reference(C++ and Objective-C++ only) — warns when a reference is bound to a temporary whose lifetime has ended. For example:
int n = 1;
const int& r = std::max(n - 1, n + 1); // r is danglingIn the example above, two temporaries are created, one for each argument, and a reference to one of the temporaries is returned. However, both temporaries are destroyed at the end of the full expression, so the reference r is dangling. This warning also detects dangling references in member initializer lists:
const int& f(const int& i) { return i; }
struct S {
const int &r; // r is dangling
S() : r(f(10)) { }
};Member functions are checked as well, but only their object argument:
struct S {
const S& self () { return *this; }
};
const S& s = S().self(); // s is danglingCertain functions are safe in this respect, for example std::use_facet: they take and return a reference, but they don’t return one of its arguments, which can fool the warning. Such functions can be excluded from the warning by wrapping them in a #pragma:
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wdangling-reference"
const T& foo (const T&) { ... }
#pragma GCC diagnostic popThe #pragma can also surround the class; in that case, the warning will be disabled for all the member functions.
-Wdangling-reference also warns about code like
auto p = std::minmax(1, 2);where std::minmax returns std::pair<const int&, const int&>, and both references dangle after the end of the full expression that contains the call to std::minmax.
The warning does not warn for std::span-like classes. We consider classes of the form:
template<typename T>
struct Span {
T* data_;
std::size len_;
};as std::span-like; that is, the class is a non-union class that has a pointer data member and a trivial destructor.
The warning can be disabled by using the gnu::no_dangling attribute.
This warning is enabled by -Wextra. ✨
-Wdelete-non-virtual-dtor(C++ and Objective-C++ only) — warns whendeleteis 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) — warns 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.-Wdeprecated-copy-dtor(C++ and Objective-C++ only) — similar to-Wdeprecated-copy, but also deprecate if the class has a user-provided destructor.-Wno-deprecated-enum-enum-conversion(C++ and Objective-C++ only) — disables the warning about the case when the usual arithmetic conversions are applied on operands where one is of enumeration type and the other is of a different enumeration type. This conversion was deprecated in C++20. For example:
enum E1 { e };
enum E2 { f };
int k = f - e;-Wdeprecated-enum-enum-conversion is enabled by default with -std=c++20. In pre-C++20 dialects, this warning can be enabled by -Wenum-conversion or -Wdeprecated.
-Wno-deprecated-enum-float-conversion(C++ and Objective-C++ only) — disables the warning about the case when the usual arithmetic conversions are applied on operands where one is of enumeration type and the other is of a floating-point type. This conversion was deprecated in C++20. For example:
enum E1 { e };
enum E2 { f };
bool b = e <= 3.7;-Wdeprecated-enum-float-conversion is enabled by default with -std=c++20. In pre-C++20 dialects, this warning can be enabled by -Wenum-conversion or -Wdeprecated.
-Wdeprecated-literal-operator(C++ and Objective-C++ only) — warns that the declaration of a user-defined literal operator with a space before the suffix is deprecated. This warning is enabled by default in C++23, or with explicit-Wdeprecated.
string operator "" _i18n(const char*, std::size_t); // deprecated
string operator ""_i18n(const char*, std::size_t); // preferred-Wdeprecated-variadic-comma-omission(C++ and Objective-C++ only) — warns that omitting a comma before the varargs...at the end of a function parameter list is deprecated. This warning is enabled by default in C++26, or with explicit-Wdeprecated.
void f1(int...); // deprecated
void f1(int, ...); // preferred
template <typename ...T>
void f2(T...); // ok
template <typename ...T>
void f3(T......); // deprecated-Wno-elaborated-enum-base— for C++11 and above, warn if an (invalid) additional enum-base is used in an elaborated-type-specifier. That is, if an enum with given underlying type and no enumerator list is used in a declaration other than just a standalone declaration of the enum. Enabled by default. This warning is upgraded to an error with-pedantic-errors.-Wno-init-list-lifetime(C++ and Objective-C++ only) — do not warn about uses ofstd::initializer_listthat are likely to result in dangling pointers. Since the underlying array for aninitializer_listis handled like a normal C++ temporary object, it 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 localinitializer_listvariable, the array’s lifetime ends at the end of the return statement, so the value returned has a dangling pointer. - If a new-expression creates an
initializer_list, the array only lives until the end of the enclosing full-expression, so theinitializer_listin the heap has a dangling pointer. - When an
initializer_listvariable 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 theinitializer_listvariable has a dangling pointer.
- If a function returns a temporary
// 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
beginpointer from theinitializer_listargument, this doesn’t extend the lifetime of the array, so if a class variable is constructed from a temporaryinitializer_list, the pointer is left dangling by the end of the variable declaration statement. 😭
-Winvalid-constexpr— warns when a function never produces a constant expression. In C++20 and earlier, for everyconstexprfunction and function template, there must be at least one set of function arguments in at least one instantiation such that an invocation of the function or constructor could be an evaluated subexpression of a core constant expression. C++23 removed this restriction, so it’s possible to have a function or a function template markedconstexprfor which no invocation satisfies the requirements of a core constant expression. 💀
Okay so back to -Winvalid-constexpr for a sec — this one is enabled as a pedantic warning by default in C++20 and earlier. In C++23, -Winvalid-constexpr can be turned on, in which case it'll be an ordinary warning. Like, for example:
void f (int& i);
constexpr void
g (int& i)
{
// Warns by default in C++20, in C++23 only with -Winvalid-constexpr.
f(i);
}-Winvalid-imported-macros— bestie, this one verifies that all imported macro definitions are valid at the end of compilation. It is not enabled by default, because it requires additional processing to figure out. It may be useful when you're preparing sets of header-units and want to make sure the macros stay consistent. 🎀-Wno-literal-suffix(C++ and Objective-C++ only) — do not warn when a string or character literal is followed by a ud-suffix that doesn't begin with an underscore. As a conforming extension, GCC treats those suffixes as separate preprocessing tokens, so that backwards compatibility with code using formatting macros from<inttypes.h>stays alive. 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.
This option also controls warnings when a user-defined literal operator is declared with a literal suffix identifier that doesn't begin with an underscore. Literal suffix identifiers that don't begin with an underscore are reserved for future standardization — so, like, not yours to claim, no cap.
These warnings are enabled by default. 💅
-Wno-narrowing(C++ and Objective-C++ only) — for C++11 and later standards, narrowing conversions are diagnosed by default, as required by the standard. A narrowing conversion from a constant produces an error, and a narrowing conversion from a non-constant produces a warning, but-Wno-narrowingsuppresses the diagnostic. Important vibe check: this does not affect the meaning of well-formed code — narrowing conversions are still considered ill-formed in SFINAE contexts.
With -Wnarrowing in C++98, warn when a narrowing conversion prohibited by C++11 occurs within { }, e.g.
int i = { 2.2 }; // error: narrowing from double to intThis flag is included in -Wall and -Wc++11-compat.
-Wnoexcept(C++ and Objective-C++ only) — warn when a noexcept-expression evaluates tofalsebecause of a call to a function that doesn't have a non-throwing exception specification (i.e.throw()ornoexcept) but is known by the compiler to never throw an exception. Slightly unhinged of it, honestly, but slay. ✨-Wnoexcept-type(C++ and Objective-C++ only) — warn if the C++17 feature that makesnoexceptpart of a function type changes the mangled name of a symbol relative to C++14. Enabled by-Wabiand-Wc++17-compat.
As an example:
template <class T> void f(T t) { t(); };
void g() noexcept;
void h() { f(g); }In C++14, f calls f<void(*)()>, but in C++17 it calls f<void(*)()noexcept>. The way the name literally changed 😭
-Wclass-memaccess(C++ and Objective-C++ only) — warn when the destination of a call to a raw memory function likememsetormemcpyis an object of class type, and when writing into such an object might bypass the class's non-trivial or deleted constructor or copy assignment, violate const-correctness or encapsulation, or corrupt virtual table pointers. Big nope energy. Modifying the representation of such objects may violate invariants maintained by member functions of the class. For example, the call tomemsetbelow 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 these types is by using the appropriate constructor or assignment operator, if one is available.
std::string str = "abc";
memset (&str, 0, sizeof str);The -Wclass-memaccess option is enabled by -Wall. Explicitly casting the pointer to the class object to void * or to a type that can be safely accessed by the raw memory function suppresses the warning.
-Wnon-virtual-dtor(C++ and Objective-C++ only) — warn when a class has virtual functions and an accessible non-virtual destructor itself or in an accessible polymorphic base class, in which case it'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. The-Wdelete-non-virtual-dtoroption (enabled by-Wall) should be preferred, because it warns about the unsafe cases without false positives. 👑-Wregister(C++ and Objective-C++ only) — warn on uses of theregisterstorage class specifier, except when it's part of the GNU Explicit Register Variables extension. The use of theregisterkeyword as a storage class specifier has been deprecated in C++11 and removed in C++17. Enabled by default with-std=c++17. ripregister, you had a good run 🫶-Wreorder(C++ and Objective-C++ only) — warn when the order of member initializers given in the code doesn't match the order in which they must be executed. For instance:
struct A {
int i;
int j;
A(): j (0), i (1) { }
};The compiler rearranges the member initializers for i and j to match the declaration order of the members, emitting a warning to that effect. This warning is enabled by -Wall.
-Wno-pessimizing-move(C++ and Objective-C++ only) — this warning warns when a call tostd::moveprevents copy elision. That's right, the thing you added to be fast is making it slow. 💀 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 tostd::move; that is, when a move operation would have been performed even without thestd::movecall. 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. literally extra 🤭
-Wrange-loop-construct(C++ and Objective-C++ only) — this warning warns when a C++ range-based for-loop is creating an unnecessary copy. That can happen when the range declaration isn't a reference, but probably should be. For example:
struct S { char arr[128]; };
void fn () {
S arr[5];
for (const auto x : arr) { ... }
}It does not warn when the type being copied is a trivially-copyable type whose size is less than 64 bytes. Small copy, no drama.
This warning also warns when a loop variable in a range-based for-loop is initialized with a value of a different type, resulting in a copy. For example:
void fn() {
int arr[10];
for (const double &x : arr) { ... }
}In the example above, in every iteration of the loop a temporary value of type double is created and destroyed, to which the reference const double & is bound. Every. Single. Iteration. 😭
This warning is enabled by -Wall.
-Wredundant-tags(C++ and Objective-C++ only) — warn about redundant class-key and enum-key in references to class types and enumerated types in contexts where the key can be eliminated without causing an ambiguity. For example:
struct foo;
struct foo *p; // warn that keyword struct can be eliminatedOn the other hand, in this example there's no warning:
struct foo;
void foo (); // "hides" struct foo
void bar (struct foo&); // no warning, keyword struct is necessary-Wno-subobject-linkage(C++ and Objective-C++ only) — do not warn if a class type has a base or a field whose type uses the anonymous namespace or depends on a type with no linkage. Here's the tea: 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.Cfile, as those are unlikely to have multiple definitions.-Wsubobject-linkageis enabled by default.-Weffc++(C++ and Objective-C++ only) — warn about violations of the following style guidelines from Scott Meyers' Effective C++ series of books. Iconic reading list, ngl 📚- 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,.
That paragraph up top is still riding with whichever warning bundle it belongs to, so consider it the encore: this option also turns on -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 you pick this option, be aware that the standard library headers do not obey all of these guidelines; use grep -v to filter out those warnings. 💅
-Wno-exceptions(C++ and Objective-C++ only) — turns off the warning for when an exception handler is shadowed by another handler, which can point out a wrong ordering of exception handlers. Bestie, that ordering matters 😭-Wsfinae-incomplete(C++ and Objective-C++ only) — warns about a class that is found to be incomplete, or a function withautoreturn type that has not yet been deduced, in a context where that causes substitution failure rather than an error, and then the class or function is defined later in the translation unit. This is problematic because template instantiations or concept checks could have different results if they first occur either before or after the definition.
This warning is enabled by default. -Wsfinae-incomplete=2 adds a warning at the point of substitution failure, to make it easier to track down problems flagged by the default mode. Level 2 is literally the detective mode 🔍
-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. so like, it works at home but don't take it traveling ✈️-Wno-non-c-typedef-for-linkage(C++ and Objective-C++ only) — disables the pedwarn for unnamed classes with a typedef name for linkage purposes containing C++ specific members, base classes, default member initializers or lambda expressions, including those on nested member classes.
typedef struct {
int a; // non-static data members are ok
struct T { int b; }; // member classes too
enum E { E1, E2, E3 }; // member enumerations as well
int c = 42; // default member initializers are not ok
struct U : A { int c; }; // classes with base classes are not ok
typedef int V; // typedef is not ok
using W = int; // using declaration is not ok
decltype([](){}) x; // lambda expressions not ok
} S;In all these cases, the tag name S should be added after the struct keyword. tiny fix, huge glow-up ✨
-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. ancient lore fr fr 🏺-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. the way that new-style casts are just safer and grep-able… we stan 💅-Woverloaded-virtual, *-Woverloaded-virtual=n* ** (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); // does not override
};the "A" class version of "f" is hidden in "B", and code like:
B* b;
b->f();fails to compile. and you're just standing there like 😭
In cases where the different signatures are not an accident, the simplest solution is to add a using-declaration to the derived class to un-hide the base function, e.g. add "using A::f;" to "B".
The optional level suffix controls the behavior when all the declarations in the derived class override virtual functions in the base class, even if not all of the base functions are overridden:
struct C {
virtual void f();
virtual void f(int);
};
struct D: public C {
void f(int); // does override
}This pattern is less likely to be a mistake; if D is only used virtually, the user might have decided that the base class semantics for some of the overloads are fine.
At level 1, this case does not warn; at level 2, it does. -Woverloaded-virtual by itself selects level 2. Level 1 is included in -Wall. pick your level, pick your vibe 🎚️
-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. the standard said "no" and we simply do not argue 👑-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. loophole: intact 🦋-Wmismatched-new-delete(C++ and Objective-C++ only) — warns for mismatches between calls to"operator new"or"operator delete"and the corresponding call to the allocation or deallocation function. This includes invocations of C++"operator delete"with pointers returned from either mismatched forms of"operator new", or from other functions that allocate objects for which the"operator delete"isn't a suitable deallocator, as well as calls to other deallocation functions with pointers returned from"operator new"for which the deallocation function isn't suitable.
For example, the "delete" expression in the function below is diagnosed because it doesn't match the array form of the "new" expression the pointer argument was returned from. Similarly, the call to "free" is also diagnosed.
void f ()
{
int *a = new int[n];
delete a; // warning: mismatch in array forms of expressions
char *p = new char[n];
free (p); // warning: mismatch between new and free
}The related option -Wmismatched-dealloc diagnoses mismatches involving allocation and deallocation functions other than "operator new" and "operator delete".
-Wmismatched-new-delete is included in -Wall. new/delete matching is a love story, don't break them up 💔
-Wmismatched-tags(C++ and Objective-C++ only) — warns for declarations of structs, classes, and class templates and their specializations with a class-key that does not match either the definition or the first declaration if no definition is provided.
For example, the declaration of "struct Object" in the argument list of "draw" triggers the warning. To avoid it, either remove the redundant class-key "struct" or replace it with "class" to match its definition.
class Object {
public:
virtual ~Object () = 0;
};
void draw (struct Object*);It is not wrong to declare a class with the class-key "struct" as the example above shows. The -Wmismatched-tags option is intended to help achieve a consistent style of class declarations. In code that is intended to be portable to Windows-based compilers the warning helps prevent unresolved references due to the difference in the mangling of symbols declared with different class-keys. The option can be used either on its own or in conjunction with -Wredundant-tags. consistency is a personality trait 🎀
-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.-Wno-virtual-move-assign— suppresses warnings about inheriting from a virtual base with a non-trivial C++11 move assignment operator. This is dangerous because if the virtual base is reachable along more than one path, it 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. > bestie this one is genuinely dangerous — double-move means both objects end up moved-from, and that's giving undefined-vibes horror movie 💀-Wnamespaces— warns when a namespace definition is opened. Some coding rules disallow namespaces, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL. One may also use using directives and qualified names. so you can stillusing namespace std;in your heart 🫶-Wno-template-body(C++ and Objective-C++ only) — disables diagnosing errors when parsing a template, and instead issues an error only upon instantiation of the template. This flag can also be used to downgrade such errors into warnings withWno-error=or-fpermissive. procrastination, but for diagnostics 😌-Wno-template-id-cdtor(C++ and Objective-C++ only) — disables the warning about the use of simple-template-id as the declarator-id of a constructor or destructor, which became invalid in C++20 via DR 2237. For example:
template<typename T> struct S {
S<T>(); // should be S();
~S<T>(); // should be ~S();
};-Wtemplate-id-cdtor is enabled by default with -std=c++20; it is also enabled by -Wc++20-compat. C++20 said "that syntax is cancelled" and honestly? fair 🌸
-Wtemplate-names-tu-local— ok so bestie, picture this: a template body hides an exposure of a translation-unit-local entity, and GCC is like "girl, I'm warning you about this." 💅 Like, in most cases, referring to a translation-unit-local entity (think: an internal linkage declaration) inside an entity that gets emitted into a module's CMI is literally an error. BUT — plot twist — inside the initializer of a variable, or in the body of a non-inline function, that's not an exposure, so no error gets emitted. 🫶
Why do we even care? Because variable or function templates can accidentally become unusable if they reference such an entity — since other translation units that import the template will never be able to instantiate it. 😭 This warning attempts to detect the cases where that might occur. And the presence of an explicit instantiation silences the warning. ✨
This flag is enabled by -Wextra.
-Wno-expose-global-module-tu-local— so an exposure of a translation-unit-local entity from a module interface is invalid, periodt, because it may cause ODR violations and manifest in link errors or other unexpected behaviour. 💀 However! Many existing libraries declare TU-local entities in their interface, and avoiding exposures of these entities may be difficult in some cases.
As an extension, GCC allows exposures of internal variables and functions that were declared in the global module fragment. This warning indicates when such an invalid exposure has occurred, and it can be silenced using diagnostic pragmas either at the site of the exposure, or at the point of declaration of the internal declaration. This also applies to export using declarations naming such entities. 👑
When combined with -Wtemplate-names-tu-local, GCC will also warn about non-exposure references to TU-local entities in template bodies. Such templates can still be instantiated in other TUs, but the above risks regarding exposures of translation-unit-local entities apply. 🙈
This warning is enabled by default, and is upgraded to an error by -pedantic-errors.
-Wno-external-tu-local— warns when you're naming a TU-local entity outside of the translation unit it was declared in. Such declarations will be ignored during name lookup, bestie. This can occur when performing ADL from a template declared in the same TU as the internal function:
export module M;
template <typename T> void foo(T t) {
bar(t);
}
struct S {} s;
static void bar(S) {} // internal linkage
// instantiating foo(s) from outside this TU can see ::bar,
// but naming it there is ill-formed.You can work around this by making bar attached to the global module, using extern "C++". 🎀
This warning is enabled by default, and is upgraded to an error by -pedantic-errors.
-Wno-terminate(C++ and Objective-C++ only) — turns off the warning about a throw-expression that will immediately result in a call toterminate. Like, immediately?? rude. 😭-Wno-vexing-parse(C++ and Objective-C++ only) — warns about the most vexing parse syntactic ambiguity. Yes, that's its actual name, iconic behaviour honestly. 🌈 This warns about the cases when a declaration looks like a variable definition, but the C++ language requires it to be interpreted as a function declaration. For instance:
void f(double a) {
int i(); // extern int i (void);
int n(int(a)); // extern int n (int);
}Another example:
struct S { S(int); };
void f(double a) {
S x(int(a)); // extern struct S x (int);
S y(int()); // extern struct S y (int (*) (void));
S z(); // extern struct S z (void);
}The warning will suggest options how to deal with such an ambiguity; e.g., it can suggest removing the parentheses or using braces instead. helpful king 💖
This warning is enabled by default.
-Wno-class-conversion(C++ and Objective-C++ only) — do not warn when a conversion function converts an object to the same type, to a base class of that type, or tovoid; such a conversion function will never be called. ghosted. 👻-Wvolatile(C++ and Objective-C++ only) — warns about deprecated uses of thevolatilequalifier. This includes postfix and prefix++and--expressions ofvolatile-qualified types, using simple assignments where the left operand is avolatile-qualified non-class type for their value, compound assignments where the left operand is avolatile-qualified non-class type,volatile-qualified function return type,volatile-qualified parameter type, and structured bindings of avolatile-qualified type. This usage was deprecated in C++20. ⚠️
Enabled by default with -std=c++20. Before -std=c++20, enabled with explicit -Wdeprecated.
-Waligned-new-Waligned-new=[none|global|all]— 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. highkey thorough 💅
-Wno-placement-new-Wplacement-new=n— warns about placement new expressions with undefined behavior, such as constructing an object in a buffer that is smaller than the type of the object. For example, the placement new expression below is diagnosed because it attempts to construct an array of 64 integers in a buffer only 64 bytes large. 😭
char buf [64];
new (buf) int[64];This warning is enabled by default.
-Wplacement-new=1— this is the default warning level of-Wplacement-new. At this level the warning is not issued for some strictly undefined constructs that GCC allows as extensions for compatibility with legacy code. For example, the 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, in addition to diagnosing all the same constructs as at level 1, a diagnostic is also issued for placement new expressions that construct an object in the last member of structure whose type is an array of a single element and whose size is less than the size of the object being constructed. While the previous example would be diagnosed, the following construct makes use of the flexible member array extension to avoid the warning at level 2.
struct S { int n, a[]; };
S *s = (S *)malloc (sizeof *s + 32 * sizeof s->a[0]);
new (s->a)int [32]();-Wcatch-value-Wcatch-value=n(C++ and Objective-C++ only) — 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. 🦋-Wconditionally-supported(C++ and Objective-C++ only) — warns for conditionally-supported (C++11 [intro.defs]) constructs. iykyk ✨-Wno-defaulted-function-deleted(C++ and Objective-C++ only) — warns when an explicitly defaulted function is deleted by the compiler. That can occur when the function's declared type does not match the type of the function that would have been implicitly declared. This warning is enabled by default.-Wno-delete-incomplete(C++ and Objective-C++ only) — do not warn when deleting a pointer to incomplete type, which may cause undefined behavior at runtime. 💀 This warning is enabled by default.-Wextra-semi(C++, Objective-C++ only) — warns about redundant semicolons. There are various contexts in which an extra semicolon can occur. One is a semicolon after in-class function definitions, which is valid in all C++ dialects (and is never a pedwarn):
struct S {
void foo () {};
};Another is an extra semicolon at namespace scope, which has been allowed since C++11 (therefore is a pedwarn in C++98):
struct S {
};
;And yet another is an extra semicolon in class definitions, which has been allowed since C++11 (therefore is a pedwarn in C++98):
struct S {
int a;
;
};-Wno-global-module(C++ and Objective-C++ only) — disables the diagnostic for when the global module fragment of a module unit does not consist only of preprocessor directives. 🎀-Wno-inaccessible-base(C++, Objective-C++ only) — this option controls warnings when a base class is inaccessible in a class derived from it due to ambiguity. The warning is enabled by default. Note that the warning for ambiguous virtual bases is enabled by the-Wextraoption. 👀
struct A { int a; };
struct B : A { };
struct C : B, A { };-Wno-inherited-variadic-ctor— suppresses warnings about use of C++11 inheriting constructors when the base class inherited from has a C variadic constructor; the warning is on by default because the ellipsis is not inherited. the way that ellipsis just does NOT get passed down 😤-Wno-invalid-offsetof(C++ and Objective-C++ only) — suppresses warnings from applying theoffsetofmacro to a non-POD type. According to the 2014 ISO C++ standard, applyingoffsetofto a non-standard-layout type is undefined. In existing C++ implementations, however,offsetoftypically gives meaningful results. This flag is for users who are aware that they are writing nonportable code and who have deliberately chosen to ignore the warning about it. 👑
The restrictions on offsetof may be relaxed in a future version of the C++ standard.
-Wsized-deallocation(C++ and Objective-C++ only) — 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 the other way around, whichever mismatch you've got going on. This one's enabled by -Wextra along with -fsized-deallocation ✨
-Wsuggest-final-types— this one snitches on types with virtual methods where your code would be so much better if the type were declared with the C++11finalspecifier, or (if you can swing it) tucked into an anonymous namespace. Why do we care? Because then GCC can go full send on devirtualizing those polymorphic calls 🔥 Heads up: this warning hits different with link-time optimization, where the class hierarchy graph info is way more complete. iykyk.-Wsuggest-final-methods— same energy but for virtual methods: it warns when a method would be better off declared with the C++11finalspecifier, or if possible its type were declared in an anonymous namespace or withfinal. Also more effective with link-time optimization (complete class hierarchy graph, etc.). Pro tip from the page itself: consider the-Wsuggest-final-typessuggestions first, then rebuild with your new annotations 🫶-Wsuggest-override— warns about overriding virtual functions that forgot to be marked with theoverridekeyword. Bestie. Just add the keyword 💅-Wno-conversion-null(C++ and Objective-C++ only) — do not warn for conversions betweenNULLand non-pointer types. And just so you know:-Wconversion-nullis on by default, so this is you actively telling it to chill.
Options Controlling Objective-C and Objective-C++ Dialects 🍎
(NOTE: This manual does not describe the Objective-C and Objective-C++ languages themselves.
Real talk — this section is only about the command-line options that actually mean something for Objective-C and Objective-C++ programs. The good news: you can still use most of the language-independent GNU compiler options too. Like, you could compile a file some_class.m like this:
gcc -g -fgnu-runtime -O -c some_class.mSee what's happening there? -fgnu-runtime is an option that's purely for Objective-C and Objective-C++ programs; the other options are fair game for any language GCC supports 🌈
Also worth clocking: since Objective-C is an extension of C, Objective-C compilations may also use options specific to the C front-end (e.g. -Wtraditional). Same deal — Objective-C++ compilations may use C++-specific options (e.g. -Wabi) 💖
Here's the list of options that are only for compiling Objective-C and Objective-C++ programs:
-fconstant-string-class=class-name— use class-name as the name of the class to instantiate for each literal string you write with the@"..."syntax. Default class name isNXConstantStringif the GNU runtime is being used, andNSConstantStringif the NeXT runtime is being used (see below 👇). On Darwin / macOS platforms, the-fconstant-cfstringsoption, if it's also present, overrides the-fconstant-string-classsetting and causes@"..."literals to be laid out as constant CoreFoundation strings. FYI-fconstant-cfstringsis an alias for the target-specific-mconstant-cfstringsequivalent ✨-fgnu-runtime— generate object code compatible with the standard GNU Objective-C runtime. This is the default for most types of systems, so it's giving "default main character energy" 💅-fnext-runtime— generate output compatible with the NeXT runtime. This is the default for NeXT-based systems, including Darwin / macOS. The macro__NEXT_RUNTIME__is predefined if (and only if!) this option is used 👑-fno-nil-receivers— assume that all Objective-C message dispatches ([receiver message:arg]) in this translation unit ensure that the receiver is notnil. That lets the runtime use more efficient entry points. Only available in conjunction with the NeXT runtime and ABI version 0 or 1 🦋-fobjc-abi-version=n— use version n of the Objective-C ABI for the selected runtime. Currently only supported for the NeXT runtime (sorry bestie). 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 you specify nothing, 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, it synthesizes a special- (id) .cxx_constructinstance method which runs non-trivial default constructors on any such instance variables, in order, and then returnsself. Same vibe on the way out: it checks if any instance variable is a C++ object with a non-trivial destructor, and if so synthesizes a special- (void) .cxx_destructmethod which runs all such default destructors, in reverse order 😭
The - (id) .cxx_construct and - (void) .cxx_destruct methods thusly generated only operate on instance variables declared in the current Objective-C class, and not those inherited from superclasses. It's the Objective-C runtime's job 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. No cap 🧚
-fobjc-direct-dispatch— allow fast jumps to the message dispatcher. On Darwin this is accomplished via the comm page 🚀-fobjc-exceptions— enable syntactic support for structured exception handling in Objective-C, similar to what C++ offers. You need this option to use the Objective-C keywords@try,@throw,@catch,@finallyand@synchronized. 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 — ancient, bestie) 💀-fobjc-gc— enable garbage collection (GC) in Objective-C and Objective-C++ programs. Only available with the NeXT runtime; the GNU runtime has a different garbage collection implementation that does not require special compiler flags 🍓-fobjc-nilcheck— for the NeXT runtime with version 2 of the ABI, check for a nil receiver in method invocations before doing the actual method call. This is the default, and you can turn it off with-fno-objc-nilcheck. Class methods and super calls are never checked for nil this way, no matter what this flag is set to. Also, currently this flag does literally nothing when the GNU runtime (or an older version of the NeXT runtime ABI) is used 🙈-fobjc-std=objc1— conform to the language syntax of Objective-C 1.0, the language recognized by GCC 4.0. This only affects the Objective-C additions to the C/C++ language; it does not affect conformance to C/C++ standards, which is controlled by the separate C/C++ dialect option flags. When you use this with the Objective-C or Objective-C++ compiler, any Objective-C syntax that GCC 4.0 doesn't recognize gets rejected. Super useful if you need to make sure your Objective-C code can be compiled with older versions of GCC 🌸-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 with the Fix-and-Continue debugging mode, where the object file in question may be recompiled and dynamically reloaded in the course of program execution, without needing to restart the program itself. Currently, Fix-and-Continue functionality is only available in conjunction with the NeXT runtime on Mac OS X 10.3 and later 💜-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. Useful in Zero-Link debugging mode, since it allows 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. That 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 🎀-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 that visibility ✨-gen-decls— dump interface declarations for all classes seen in the source file to a file namedsourcename.decl📝-Wassign-intercept(Objective-C and Objective-C++ only) — warn whenever an Objective-C assignment is being intercepted by the garbage collector 👀-Wno-property-assign-default(Objective-C and Objective-C++ only) — do not warn if a property for an Objective-C object has no assign semantics specified. Silence is golden 💅-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 🦋-Wobjc-root-class(Objective-C and Objective-C++ only) — warns you if a class interface is out here with no superclass 😭 like bestie most classes will inherit fromNSObject(orObject) for example. But when you're intentionally declaring a root class, you can hush the warning by marking the interface with__attribute__((objc_root_class))✨-Wselector(Objective-C and Objective-C++ only) — warns if multiple methods of different types for the same selector show up during compilation 👀 the check runs on the list of methods in the final stage of compilation. On top of that, a check happens for each selector appearing in a@selector(...)expression, and a corresponding method for that selector has been found during compilation. And because these checks scan the method table only at the end of compilation, no warnings get produced if the final stage isn't reached — like if an error is found during compilation, or because-fsyntax-onlyis being used 💅-Wstrict-selector-match(Objective-C and Objective-C++ only) — warns if multiple methods with differing argument and/or return types are found for a given selector when you attempt to send a message using this selector to a receiver of typeidorClass. When this flag is off (which is the default behavior, no cap), the compiler omits such warnings if any differences it finds are confined to types that share the same size and alignment 🫶-Wundeclared-selector(Objective-C and Objective-C++ only) — warns if a@selector(...)expression referring to an undeclared selector is found. A selector counts as undeclared if no method with that name has been declared before the@selector(...)expression — either explicitly in an@interfaceor@protocoldeclaration, or implicitly in an@implementationsection. This option always performs its checks the moment a@selector(...)expression is found, while-Wselectoronly performs its checks in the final stage of compilation. It also enforces that coding style convention that methods and selectors must be declared before being used 👑-print-objc-runtime-info— generates a C header describing the largest structure that is passed by value, if any 📦
✨ Options Controlling OpenMP and OpenACC ✨
GCC supports OpenMP extensions to the C, C++, and Fortran languages with the -fopenmp option 🌈 similarly, OpenACC extensions are supported in all three languages with -fopenacc.
-foffload=disable-foffload=default-foffload=target-list— specify for which OpenMP and OpenACC offload targets code should be generated 🎯 the default behavior, equivalent to-foffload=default, is to generate code for all supported offload targets. The-foffload=disableform generates code only for the host fallback, while-foffload=target-listgenerates code only for the specified comma-separated list of offload targets.
Offload targets are specified in GCC's internal target-triplet format, iykyk 💅 you can run the compiler with -v to show the list of configured offload targets under OFFLOAD_TARGET_NAMES 👀
-foffload-options=options-foffload-options=target-triplet-list=options— with-foffload-options=options, GCC passes the specified options to the compilers for all enabled offloading targets 🫶 you can specify options that apply only to a specific target or targets by using the-foffload-options=target-list=optionsform. The target-list is a comma-separated list in the same format as for the-foffload=option.
Typical command lines are
-foffload-options='-fno-math-errno -ffinite-math-only' \
-foffload-options=nvptx-none=-latomic
-foffload-options=amdgcn-amdhsa=-march=gfx906-fopenacc— enables handling of OpenACC directives#pragma accin C/C++ and!$accin free-form Fortran and!$acc,c$accand*$accin fixed-form Fortran 🌸 this option implies-pthread, and thus is only supported on targets that have support for-pthread.-fopenacc-dim=geom— specify default compute dimensions for parallel offload regions that don't explicitly specify them 📐 the geom value is a triple of:-separated sizes, in order gang, worker, and vector. A size can be omitted, to use a target-specific default value ✨-fopenmp— enables handling of OpenMP directives#pragma omp,[[omp::decl(...)]],[[omp::directive(...)]], and[[omp::sequence(...)]]in C/C++ 🔥 in Fortran, it enables!$ompand the conditional compilation sentinel!$. In fixed source form Fortran, the sentinels can also start withcor*.
This option implies -pthread, and thus is only supported on targets that have support for -pthread. -fopenmp implies -fopenmp-simd 💫
-fopenmp-simd— enables handling of OpenMP'ssimd,declare simd,declare reduction,assume,ordered,scanandloopdirective, and of combined or composite directives withsimdas constituent with#pragma omp,[[omp::directive(...)]],[[omp::sequence(...)]]and[[omp::decl(...)]]in C/C++ and!$ompin Fortran 🌟 it additionally enables the conditional compilation sentinel!$in Fortran. In fixed source form Fortran, the sentinels can also start withcor*. Other OpenMP directives are ignored. Unless-fopenmpis additionally specified, theloopregion binds to the current task region, independent of the specifiedbindclause 👑-fopenmp-target-simd-clone-fopenmp-target-simd-clone=device-type— in addition to generating SIMD clones for functions marked with thedeclare simddirective, GCC also generates clones for functions marked with the OpenMPdeclare targetdirective that are suitable for vectorization when this option is in effect 🧚 the device-type may be one ofnone,host,nohost, andany, which correspond to keywords for thedevice_typeclause of thedeclare targetdirective; clones are generated for the intersection of devices specified.-fopenmp-target-simd-cloneis equivalent to-fopenmp-target-simd-clone=anyand-fno-openmp-target-simd-cloneis equivalent to-fopenmp-target-simd-clone=none.
At -O2 and higher (but not -Os or -Og) this optimization defaults to -fopenmp-target-simd-clone=nohost; otherwise it is disabled by default 💅
✨ Options to Control Diagnostic Messages Formatting ✨
Traditionally, diagnostic messages have been formatted irrespective of the output device's aspect (e.g. its width, ...) 😩 you can use the options described below to control the formatting algorithm for diagnostic messages — e.g. how many characters per line, how often source location information should be reported. Note that some language front ends may not honor these options 🫶
-fmessage-length=n— try to format error messages so that they fit on lines of about n characters 📏 if n is zero, then no line-wrapping is done; each error message appears on a single line. This is the default for all front ends.
Note — this option also affects the display of the#errorand#warningpre-processor directives, and the deprecated function/type/variable attribute. It does not however affect thepragma GCC warningandpragma GCC errorpragmas ⚠️
-fdiagnostics-plain-output— this option requests that diagnostic output look as plain as possible, which may be useful when running dejagnu or other utilities that need to parse diagnostics output and prefer that it remain more stable over time 🧐-fdiagnostics-plain-outputis currently equivalent to the following options:-fno-diagnostics-show-caret-fno-diagnostics-show-line-numbers-fdiagnostics-color=never-fdiagnostics-urls=never-fdiagnostics-path-format=separate-events-fdiagnostics-text-art-charset=none-fno-diagnostics-show-event-links-fno-diagnostics-show-nesting. In the future, if GCC changes the default appearance of its diagnostics, the corresponding option to disable the new behavior will be added to this list 👀-fdiagnostics-show-location=once— only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit source location information once; that is, in case the message is too long to fit on a single physical line and has to be wrapped, the source location won't be emitted (as prefix) again, over and over, in subsequent continuation lines 🎀 this is the default behavior.-fdiagnostics-show-location=every-line— only meaningful in line-wrapping mode. Instructs the diagnostic messages reporter to emit the same source location information (as prefix) for physical lines that result from the process of breaking a message which is too long to fit on a single line 🔁-fdiagnostics-color[=WHEN]-fno-diagnostics-color— use color in diagnostics 🎨 WHEN isnever,always, orauto. The default depends on how the compiler has been configured, it can be any of the above WHEN options or alsoneverifGCC_COLORSenvironment variable isn't present in the environment, andautootherwise.automakes GCC use color only when the standard error is a terminal, and when not executing in an emacs shell 💅 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:path=01;36:fixit-insert=32:fixit-delete=31:\
diff-filename=01:diff-hunk=32:diff-delete=31:diff-insert=32:\
type-diff=01;32:fnname=01;32:targs=35:valid=01;31:invalid=01;32\
highlight-a=01;32:highlight-b=01;34So real quick, bestie, in case you're squinting at that: 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. And if you set GCC_COLORS to the empty string? Colors are gone, off, cancelled. ✨
Here's every capability you can set, and what each one actually paints 💅
error=— the SGR substring for error: markers.warning=— the SGR substring for warning: markers.note=— the SGR substring for note: markers.path=— the SGR substring for colorizing paths of control-flow events as printed via-fdiagnostics-path-format=, like the identifiers of individual events and the lines showing interprocedural calls and returns.range1=— the SGR substring for the first additional range.range2=— the SGR substring for the second additional range.locus=— the SGR substring for location info, sofile:lineorfile:line:columnetc.quote=— the SGR substring for information printed within quotes.fnname=— the SGR substring for names of C++ functions.targs=— the SGR substring for C++ function template parameter bindings.fixit-insert=— the SGR substring for fix-it hints suggesting text to be inserted or replaced.fixit-delete=— the SGR substring for fix-it hints suggesting text to be deleted.diff-filename=— the SGR substring for filename headers within generated patches.diff-hunk=— the SGR substring for the starts of hunks within generated patches.diff-delete=— the SGR substring for deleted lines within generated patches.diff-insert=— the SGR substring for inserted lines within generated patches.type-diff=— the SGR substring for highlighting mismatching types within template arguments in the C++ frontend.valid=— the SGR substring for highlighting valid elements within text art diagrams.invalid=— the SGR substring for highlighting invalid elements within text art diagrams.highlight-a=,highlight-b=— SGR substrings for contrasting two different things within diagnostics, like a pair of mismatching types. Peep-fdiagnostics-show-highlight-colorsfor the tea 🫖
🦋 -fdiagnostics-urls[=WHEN]
Use escape sequences to embed URLs right into diagnostics. Like, when -fdiagnostics-show-option emits text showing the command-line option that controls a diagnostic, this embeds a URL for the documentation of that option. Iconic, honestly.
WHEN is never, always, or auto. auto makes GCC use URL escape sequences only when standard error is a terminal, and when it's not executing in an emacs shell or any graphical terminal that's known to be incompatible with this feature — see below, bestie 👇
The default depends on how the compiler has been configured. It can be any of the above WHEN options.
GCC can also be configured (via the --with-diagnostics-urls=auto-if-env configure-time option) so that the default is affected by environment variables. Under such a configuration, GCC defaults to using auto if either GCC_URLS or TERM_URLS environment variables are present and non-empty in the environment of the compiler, or never if neither are.
BUT — and this is the plot twist 😭 — even with -fdiagnostics-urls=always the behavior is dependent on those environment variables:
- If
GCC_URLSis set to empty orno, do not embed URLs in diagnostics. - If set to
st, URLs use ST escape sequences. - If set to
bel, the default, URLs use BEL escape sequences. - Any other non-empty value enables the feature.
- If
GCC_URLSis not set, useTERM_URLSas a fallback.
Note: ST is an ANSI escape sequence, string terminatorESC \, BEL is an ASCII character,CTRL-Gthat usually sounds like a beep. 💀
At this time GCC tries to detect also a few terminals that are known to not implement the URL feature, and have bugs or at least had bugs in some versions that are still in use, where the URL escapes are likely to misbehave, i.e. print garbage on the screen. That list is currently xfce4-terminal, certain known to be buggy gnome-terminal versions, the linux console, and mingw. This check can be skipped with -fdiagnostics-urls=always.
🙈 -fno-diagnostics-show-option
By default, each diagnostic emitted includes text indicating the command-line option that directly controls the diagnostic (if such an option is known to the diagnostic machinery, that is). Specifying the -fno-diagnostics-show-option flag suppresses that behavior. Bye bye option names 👋
🎀 -fno-diagnostics-show-caret
By default, each diagnostic emitted includes the original source line and a caret ^ indicating the column. This option suppresses this information. The source line is truncated to n characters, if the -fmessage-length=n option is given. When the output is done to the terminal, the width is limited to the width given by the COLUMNS environment variable or, if not set, to the terminal width.
🌸 -fno-diagnostics-show-labels
By default, when printing source code (via -fdiagnostics-show-caret), diagnostics can label ranges of source code with pertinent information, such as the types of expressions:
printf ("foo %s bar", long_i + long_j);
~^ ~~~~~~~~~~~~~~~
| |
char * long intThis option suppresses the printing of these labels (in the example above, the vertical bars and the char * and long int text). Lowkey tragic, those labels were slaying.
⭐ -fno-diagnostics-show-event-links
By default, when printing execution paths (via -fdiagnostics-path-format=inline-events), GCC will print lines connecting related events, such as the line connecting events 1 and 2 in:
3 | if (p)
| ^
| |
| (1) following `false' branch (when `p' is NULL)... ->-+
| |
| |
|+------------------------------------------------------------+
4 || return 0;
5 || return *p;
|| ~
|| |
|+-------->(2) ...to here
| (3) dereference of NULL `p'This option suppresses the printing of such connector lines.
💜 -fno-diagnostics-show-cwe
Diagnostic messages can optionally have an associated CWE (https://cwe.mitre.org/index.html) identifier. GCC itself only provides such metadata for some of the -fanalyzer diagnostics. GCC plugins may also provide diagnostics with such metadata. By default, if this information is present, it will be printed with the diagnostic. This option suppresses the printing of this metadata.
🧚 -fno-diagnostics-show-rules
Diagnostic messages can optionally have rules associated with them, such as from a coding standard, or a specification. GCC itself does not do this for any of its diagnostics, but plugins may do so. By default, if this information is present, it will be printed with the diagnostic. This option suppresses the printing of this metadata.
🌈 -fno-diagnostics-show-highlight-colors
GCC can use color for emphasis and contrast when printing diagnostic messages and quoting the user's source.
For example, in
demo.c: In function `test_bad_format_string_args':
../../src/demo.c:25:18: warning: format `%i' expects argument of type `int', but argument 2 has type `const char *' [-Wformat=]
25 | printf("hello %i", msg);
| ~^ ~~~
| | |
| int const char *
| %s- the
%iand"int"in the message and the"int"in the quoted source are colored usinghighlight-a(bold green by default), and - the
"const char *"in the message and in the quoted source are both colored usinghighlight-b(bold blue by default).
The intent is to draw the reader's eyes to the relationships between the various aspects of the diagnostic message and the source, using color to group related elements and distinguish between mismatching ones. Honestly? Serving visual hierarchy 👑
This additional colorization is enabled by default if color printing is enabled (as per -fdiagnostics-color=), but it can be separately disabled via -fno-diagnostics-show-highlight-colors.
🍓 -fno-diagnostics-show-line-numbers
By default, when printing source code (via -fdiagnostics-show-caret), a left margin is printed, showing line numbers. This option suppresses this left margin. No numbers, no margin, just vibes.
🦋 -fdiagnostics-minimum-margin-width=width
This option controls the minimum width of the left margin printed by -fdiagnostics-show-line-numbers. It defaults to 6.
💅 -fdiagnostics-show-context[=depth], -fno-diagnostics-show-context
With this option, the compiler might print the interesting control flow chain that guards the basic block of the statement which has the warning. depth is the maximum depth of the control flow chain.
Currently, the list of the impacted warning options includes: -Warray-bounds, -Wstringop-overflow, -Wstringop-overread, -Wstringop-truncation. and -Wrestrict. More warning options might be added to this list in future releases. (Yes, that period before the "and" is in the original — we keep it, no cap 😌)
The forms -fdiagnostics-show-context and -fno-diagnostics-show-context are aliases for -fdiagnostics-show-context=1 and -fdiagnostics-show-context=0, respectively.
🎀 -fdiagnostics-parseable-fixits
Emit fix-it hints in a machine-parseable format, suitable for consumption by IDEs. For each fix-it, a line will be printed after the relevant diagnostic, starting with the string fix-it:. For example:
fix-it:"test.c":{45:3-45:21}:"gtk_widget_show_all"Bestie, your IDE is gonna eat this up 🫶
Okay so the location is a half-open range 💅 — it’s counted in bytes, and byte 1 is the initial column. In the example above, bytes 3 through 20 of line 45 of test.c are getting 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 \", and non-printable characters as octal (e.g. vertical tab as \013). 🫶
An empty replacement string means “bestie, remove that range.” An empty range (e.g. 45:3-45:3) means “insert the string right at this position.” ✨
-fdiagnostics-generate-patch— prints fix-it hints tostderrin unified diff format, after any diagnostics are printed. For example:
--- test.c
+++ test.c
@ -42,5 +42,5 @
void show_cb(GtkDialog *dlg)
{
- gtk_widget_showall(dlg);
+ gtk_widget_show_all(dlg);
}The diff may or may not be colorized, following the same rules as for diagnostics (see -fdiagnostics-color). 🎀
-fdiagnostics-show-template-tree— in the C++ frontend, when printing diagnostics showing mismatching template types, such as:
could not convert 'std::map<int, std::vector<double> >()'
from 'map<[...],vector<double>>' to 'map<[...],vector<float>>the -fdiagnostics-show-template-tree flag enables printing a tree-like structure showing the common and differing parts of the types, such as:
map<
[...],
vector<
[double != float]>>The parts that differ are highlighted with color ("double" and "float" in this case). 🌈
-fno-elide-type— by default when the C++ frontend prints diagnostics showing mismatching template types, common parts of the types are printed as[...]to simplify the error message. For example:
could not convert 'std::map<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. 🔥
-fdiagnostics-path-format=KIND— specify how to print paths of control-flow events for diagnostics that have such a path associated with them.
KIND is none, separate-events, or inline-events, the default.
none means to not print diagnostic paths.
separate-events means to print a separate "note" diagnostic for each event within the diagnostic. For example:
test.c:29:5: error: passing NULL as argument 1 to 'PyList_Append' which requires a non-NULL parameter
test.c:25:10: note: (1) when 'PyList_New' fails, returning NULL
test.c:27:3: note: (2) when 'i < count'
test.c:29:5: note: (3) when calling 'PyList_Append', passing NULL from (1) as argument 1inline-events means to print the events "inline" within the source code. This view attempts to consolidate the events into runs of sufficiently-close events, printing them as labelled ranges within the source.
For example, the same events as above might be printed as:
'test': events 1-3
25 | list = PyList_New(0);
| ^~~~~~~~~~~~~
| |
| (1) when 'PyList_New' fails, returning NULL
26 |
27 | for (i = 0; i < count; i++) {
| ~~~
| |
| (2) when 'i < count'
28 | item = PyLong_FromLong(random());
29 | PyList_Append(list, item);
| ~~~~~~~~~~~~~~~~~~~~~~~~~
| |
| (3) when calling 'PyList_Append', passing NULL from (1) as argument 1Interprocedural control flow is shown by grouping the events by stack frame, and using indentation to show how stack frames are nested, pushed, and popped.
For example:
'test': events 1-2
|
| 133 | {
| | ^
| | |
| | (1) entering 'test'
| 134 | boxed_int *obj = make_boxed_int (i);
| | ~~~~~~~~~~~~~~~~~~
| | |
| | (2) calling 'make_boxed_int'
|
+--> 'make_boxed_int': events 3-4
|
| 120 | {
| | ^
| | |
| | (3) entering 'make_boxed_int'
| 121 | boxed_int *result = (boxed_int *)wrapped_malloc (sizeof (boxed_int));
| | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| | |
| | (4) calling 'wrapped_malloc'
|
+--> 'wrapped_malloc': events 5-6
|
| 7 | {
| | ^
| | |
| | (5) entering 'wrapped_malloc'
| 8 | return malloc (size);
| | ~~~~~~~~~~~~~
| | |
| | (6) calling 'malloc'
|
<-------------+
|
'test': event 7
|
| 138 | free_boxed_int (obj);
| | ^~~~~~~~~~~~~~~~~~~~
| | |
| | (7) calling 'free_boxed_int'
|
(etc)-fdiagnostics-show-path-depths— this option provides additional information when printing control-flow paths associated with a diagnostic.
If this option is provided, then the stack depth will be printed for each run of events within -fdiagnostics-path-format=inline-events. If provided with -fdiagnostics-path-format=separate-events, then the stack depth and function declaration will be appended when printing each event.
This is intended for use by GCC developers and plugin developers when debugging diagnostics that report interprocedural control flow. 🧚
-fno-show-column— do not print column numbers in diagnostics. This may be necessary if diagnostics are being scanned by a program that does not understand the column numbers, such asdejagnu. 🙈-fdiagnostics-column-unit=UNIT— select the units for the column number. This affects traditional diagnostics (in the absence of-fno-show-column).
The default UNIT, display, considers the number of display columns occupied by each character. This may be larger than the number of bytes required to encode the character, in the case of tab characters, or it may be smaller, in the case of multibyte characters. For example, the character "GREEK SMALL LETTER PI (U+03C0)" occupies one display column, and its UTF-8 encoding requires two bytes; the character "SLIGHTLY SMILING FACE (U+1F642)" occupies two display columns, and its UTF-8 encoding requires four bytes. 🍓
Setting UNIT to byte changes the column number to the raw byte count in all cases, as was traditionally output by GCC prior to version 11.1.0.
-fdiagnostics-column-origin=ORIGIN— select the origin for column numbers, i.e. the column number assigned to the first column. The default value of1corresponds to traditional GCC behavior and to the GNU style guide. Some utilities may perform better with an origin of0; any non-negative value may be specified. 💖-fdiagnostics-escape-format=FORMAT— when GCC prints pertinent source lines for a diagnostic it normally attempts to print the source bytes directly. However, some diagnostics relate to encoding issues in the source file, such as malformed UTF-8, or issues with Unicode normalization. These diagnostics are flagged so that GCC will escape bytes that are not printable ASCII when printing their pertinent source lines.
This option controls how such bytes should be escaped.
The default FORMAT, unicode, displays Unicode characters that are not printable ASCII in the form <U+XXXX>, and bytes that do not correspond to a Unicode character validly encoded in UTF-8 will be displayed as hexadecimal in the form <XX>.
For example, a source line containing the string before followed by the Unicode character U+03C0 ("GREEK SMALL LETTER PI", with UTF-8 encoding 0xCF 0x80) followed by the byte 0xBF (a stray UTF-8 trailing byte), followed by the string after will be printed for such a diagnostic as:
before<U+03C0><BF>afterSetting FORMAT to bytes will display all non-printable-ASCII bytes in the form <XX>, thus showing the underlying encoding of non-ASCII Unicode characters. For the example above, the following will be printed:
before<CF><80><BF>after-fdiagnostics-text-art-charset=CHARSET— some diagnostics can contain"text art"diagrams: visualizations created from text, intended to be viewed in a monospaced font.
This option selects which characters should be used for printing such diagrams, if any. CHARSET is none, ascii, unicode, or emoji.
The none value suppresses the printing of such diagrams. The ascii value will ensure that such diagrams are pure ASCII ("ASCII art"). The unicode value will allow for conservative use of unicode drawing characters (such as box-drawing characters). The emoji value further adds the possibility of emoji in the output (such as emitting U+26A0 WARNING SIGN followed by U+FE0F VARIATION SELECTOR-16 to select the emoji variant of the character). 👑
The default is emoji, except when the environment variable LANG is set to C, in which case the default is ascii. 🌸
-fno-diagnostics-show-nesting— okay so some GCC diagnostics are literally tree-shaped: they have an internal structure of nested sub-diagnostics, like when C++ templates get instantiated and everything goes sideways 😭. By default GCC shows that nesting with indentation and bullet points, moves location info onto separate lines so the structure is actually legible, and drops the redundant repeated bits. This flag suppresses all of it — the indentation, the reformatting, the elision — restoring an older "look" for the diagnostics. ✨-fno-diagnostics-show-nesting-locations— whenfdiagnostics-show-nestingis enabled, file names and line and column numbers get printed on separate lines from the messages. This flag disables that location info altogether. Heads up bestie: this option exists for use by GCC developers, for writing DejaGnu test cases. 🫶-fdiagnostics-show-nesting-levels— whenfdiagnostics-show-nestingis enabled, this one also prints numbers showing how deep the nesting goes. Another GCC-developer option, for debugging nested diagnostics — but plugin authors might lowkey find it useful too. 🔍-fdiagnostics-format=FORMAT— picks a different format for printing diagnostics.FORMATistext,sarif-stderrorsarif-file.
Using this option replaces any extra "output sinks" added by -fdiagnostics-add-output=, or the one set by -fdiagnostics-set-output=.
The default is text. 💅
The sarif-stderr and sarif-file formats both emit diagnostics in SARIF Version 2.1.0 format — either to stderr, or to a file named source.sarif, respectively.
-fdiagnostics-add-output=DIAGNOSTICS-OUTPUT-SPEC— adds an additional "output sink" for emitting diagnostics. More sinks, more slay. 💖
DIAGNOSTICS-OUTPUT-SPEC should specify a scheme, optionally followed by : and one or more KEY=VALUE pairs, in this form:
<SCHEME>
<SCHEME>:<KEY>=<VALUE>
<SCHEME>:<KEY>=<VALUE>,<KEY2>=<VALUE2>etc.
Schemes, keys, or values with a name prefixed experimental may change or be removed without notice. Keys can be per-scheme, or related to GCC as a whole. ⚠️
SCHEME can be:
text— emit diagnostics tostderrusing GCC's classic text output format.- Supported keys for the
textscheme are:color=[yes|no]— override colorization settings from-fdiagnostics-colorfor this text output.show-nesting=[yes|no]— enables a mode that emphasizes hierarchical relationships within diagnostics messages, as per-fdiagnostics-show-nesting. Defaults toyes.show-nesting-locations=[yes|no]— ifshow-nesting=yes, then by default locations are shown; set this key tonoto disable printing such locations. This exists for use by GCC developers, for writing DejaGnu test cases.show-nesting-levels=[yes|no]— this is a debugging option for use withshow-nesting=yes. Set this key toyesto print explicit nesting levels in the output. This exists for use by GCC developers.
- Supported keys for the
sarif— emit diagnostics to a file in SARIF format.- Supported keys for the
sarifscheme are:file=FILENAME— specify the filename to write the SARIF output to, potentially with a leading absolute or relative path. If not specified, it defaults tosource.sarif.serialization=[json]— specify the serialization format to use when writing out the SARIF. Currently this can only bejson, but it's present as an extension point for experimenting with other serializations.version=[2.1|2.2-prerelease]— specify the version of SARIF to use for the output. If not specified, defaults to2.1.2.2-prereleaseuses an unofficial draft of the future SARIF 2.2 specification and should only be used for experimentation in this release.
- There's also this key intended for use by GCC developers, rather than end-users, and subject to change or removal without notice:
state-graphs=[yes|no]— this is a debugging feature and defaults tono. Ifstate-graphs=yes, then attempt to capture detailed state information from-fanalyzerin the generated SARIF.
- Supported keys for the
experimental-html— emit diagnostics to a file in HTML format. This scheme is experimental, and may go away in future GCC releases 🫠. The keys and details of the output are also subject to change.- Supported keys for the
experimental-htmlscheme are:css=[yes|no]— add an embedded<style>to the generated HTML. Defaults to yes.file=FILENAME— specify the filename to write the HTML output to, potentially with a leading absolute or relative path. If not specified, it defaults tosource.html.javascript=[yes|no]— add an embedded<script>to the generated HTML providing a barebones UI for viewing results. Defaults to yes.
- There are also these keys intended for use by GCC developers, rather than end-users, and subject to change or removal without notice:
show-state-diagrams=[yes|no]— this is a debugging feature and defaults tono. Ifshow-state-diagrams=yes, then attempt to usedotto generate SVG diagrams in the generated HTML, visualizing the state at each event in a diagnostic path. These are visible by pressing j and k to single-step forward and backward through events. Enabling this option will slow down HTML generation.show-graph-dot-src=[yes|no]— this is a debugging feature and defaults tono. Ifshow-graph-dot-src=yesthen ifshow-state-diagrams=yes, the generated state diagrams will also show the.dotsource input to GraphViz used for the diagram.show-graph-sarif=[yes|no]— this is a debugging feature and defaults tono. Ifshow-graph-sarif=yesthen ifshow-state-diagrams=yes, the generated state diagrams will also show a SARIF representation of the state.
- Supported keys for the
As well as scheme-specific keys, the following GCC-related key is usable on sinks of any scheme:
cfgs=[yes|no]— ifcfgs=yesfor a sink, then GCC will attempt to send information to that sink about the control flow graphs for the functions it is compiling. Text sinks ignore the information. SARIF sinks will add the graphs withintheRun.graphs. HTML sinks will generate SVG displaying the graphs. The precise form of the information is subject to change without notice. 🌀
For example,
-fdiagnostics-add-output=sarif:version=2.1,file=foo.2.1.sarif
-fdiagnostics-add-output=sarif:version=2.2-prerelease,file=foo.2.2.sarifwould add a pair of outputs, each writing to a different file, using versions 2.1 and 2.2 of the SARIF standard respectively. Two files, two vibes. 🎀
In EBNF:
<diagnostics-output-specifier> = <diagnostics-output-name>
| <diagnostics-output-name>, ":", <key-value-pairs>;
<diagnostics-output-name> = "text" | "sarif" | "experimental-html";
<key-value-pairs> = <key-value-pair>
| <key-value-pair> "," <key-value-pairs>;
<key-value-pair> = <key> "=" <value>;
<key> = ? string without a '=' ? ;
<value> = ? string without a ',' ? ;-fdiagnostics-set-output=DIAGNOSTICS-OUTPUT-SPEC— works similarly to-fdiagnostics-add-output=but instead of adding an extra "output sink" for diagnostics, it replaces all existing output sinks, such as ones from-fdiagnostics-format=,-fdiagnostics-add-output=, or a prior call to-fdiagnostics-set-output=. Out with the old, in with the new 💅-fno-diagnostics-json-formatting— by default, when JSON is emitted for diagnostics (via-fdiagnostics-format=sarif-stderror-fdiagnostics-format=sarif-file), GCC adds newlines and indentation to visually emphasize the hierarchical structure of the JSON. This flag suppresses that whitespace.
It must be passed before the option it is to affect. ⚠️
This is intended for compatibility with tools that don't expect the output to contain newlines, such as that emitted by older GCC releases.
🌸 Options to Request or Suppress Warnings 🌸
Warnings are diagnostic messages that report constructions that are not inherently erroneous but that are risky or suggest there may have been an error. Basically: not a hard no, but a "bestie… are you sure?" 🥺
The following language-independent options do not enable specific warnings but control the kinds of diagnostics produced by GCC.
-fsyntax-only— check the code for syntax errors, but don't do anything beyond that. Pure vibe check, no building. ✋-fmax-errors=n— limits the maximum number of error messages to n, at which point GCC bails out rather than attempting to continue processing the source code. If n is0(the default), there is no limit on the number of error messages produced. If-Wfatal-errorsis also specified, then-Wfatal-errorstakes precedence over this option. 👑-w,--no-warnings— inhibit all warning messages. Not me silencing the haters 😌-Werror— turn all warnings into errors. Every little thing is a crisis now. 💀-Werror=— turn the specified warning into an error. The specifier for a warning is appended; for example-Werror=switchturns the warnings controlled by-Wswitchinto errors. This switch takes a negative form, to be used to negate-Werrorfor specific warnings; for example-Wno-error=switchmakes-Wswitchwarnings not be errors, even when-Werroris in effect.
The warning message for each controllable warning includes the option that controls the warning. That option can then be used with -Werror= and -Wno-error= as described above. (Printing of the option in the warning message can be disabled using the -fno-diagnostics-show-option flag.)
Note that specifying -Werror=foo automatically implies -Wfoo. However, -Wno-error=foo does not imply anything. One-sided like that 🙈
-Wfatal-errors— okay so bestie, this is the “abort on the first error” button ⭐. It causes the compiler to abort compilation on the first error that occurred rather than trying to keep going and printing further error messages. It’s giving “we’re done here” 💅.
Now, the warning-option lore: you can request many specific warnings with options beginning with -W, for example -Wunused-variable to request warnings on declarations of variables that are never used. Each of these specific warning options also has a negative form beginning with -Wno- to turn off warnings; for example, -Wno-unused-variable. This manual lists only one of the two forms, whichever is not the default. For further language-specific options also refer to C++ Dialect Options and Objective-C and Objective-C++ Dialect Options. Additional warnings can be produced by enabling the static analyzer.
Some options, such as -Wall and -Wextra, turn on other options, such as -Wunused, which may turn on further options, such as -Wunused-variable. The combined effect of positive and negative forms is that more specific options have priority over less specific ones, independently of their position in the command line. For options of the same specificity, the last one takes effect. Options enabled or disabled via pragmas take effect as if they appeared at the end of the command line.
When an unrecognized warning option is requested (e.g., -Wunknown-warning), GCC gives an error stating that the option is not recognized. However, if the -Wno- form is used, the behavior is slightly different: no diagnostic is produced for -Wno-unknown-warning unless other diagnostics are being produced. This allows the use of new -Wno- options with old compilers, but if something goes wrong, the compiler warns that an unrecognized option is present. Vibe check: recognized warnings only, bestie ✨.
The effectiveness of some warnings depends on optimizations also being enabled. For example, -Wsuggest-final-types is more effective with link-time optimization. Some other warnings may not be issued at all unless optimization is enabled. While optimization in general improves the efficacy of warnings about control and data-flow problems, in some cases it may also cause false positives 💀.
-Wpedantic,-pedantic,--pedantic— obsessed with this one 👑. It issues all the warnings demanded by strict ISO C and ISO C++; diagnoses all programs that use forbidden extensions, and some other programs that do not follow ISO C and ISO C++. This follows the version of the ISO C or C++ standard specified by any-stdoption used.
Valid ISO C and ISO C++ programs should compile properly with or without this option (though a rare few require -ansi or a -std option specifying the version of the standard) 🫶. However, without this option, certain GNU extensions and traditional C and C++ features are supported as well. With this option, they are diagnosed (or rejected with -pedantic-errors).
-Wpedantic does not cause warning messages for use of the alternate keywords whose names begin and end with __. This alternate format can also be used to disable warnings for non-ISO __intN types, i.e. __intN__. Pedantic warnings are also disabled in the expression that follows __extension__.
okay but only system header files should use these escape routes; application programs should avoid them. iykyk 🙈.
Some warnings about non-conforming programs are controlled by options other than -Wpedantic; in many cases they are implied by -Wpedantic but can be disabled separately by their specific option, e.g. -Wpedantic -Wno-pointer-sign.
Where the standard specified with -std represents a GNU extended dialect of C, such as gnu90 or gnu99, there is a corresponding base standard, the version of ISO C on which the GNU extended dialect is based. Warnings from -Wpedantic are given where they are required by the base standard. (It does not make sense for such warnings to be given only for features not in the specified GNU C dialect, since by definition the GNU dialects of C include all features the compiler supports with the given option, and there would be nothing to warn about.) the way that whole paragraph just explained itself—iconic 💜.
-pedantic-errors,--pedantic-errors— gives an error whenever the base standard (see-Wpedantic) requires a diagnostic, in some cases where there is undefined behavior at compile-time 😭 and in some other cases that do not prevent compilation of programs that are valid according to the standard.
This is not equivalent to -Werror=pedantic: the latter option is unlikely to be useful, as it only makes errors of the diagnostics that are controlled by -Wpedantic, whereas this option also affects required diagnostics that are always enabled or controlled by options other than -Wpedantic. periodt 💅.
If you want the required diagnostics that are warnings by default to be errors instead, but don’t also want to enable the -Wpedantic diagnostics, you can specify -pedantic-errors -Wno-pedantic (or -pedantic-errors -Wno-error=pedantic to enable them but only as warnings).
Some required diagnostics are errors by default, but can be reduced to warnings using -fpermissive or their specific warning option, e.g. -Wno-error=narrowing.
Some diagnostics for non-ISO practices are controlled by specific warning options other than -Wpedantic, but are also made errors by -pedantic-errors. For instance:
-Wattributes— for standard attributes-Wchanges-meaning— C++-Wcomma-subscript— C++23 or later-Wdeclaration-after-statement— C90 or earlier-Welaborated-enum-base— C++11 or later-Wimplicit-int— C99 or later-Wimplicit-function-declaration— C99 or later-Wincompatible-pointer-types-Wint-conversion-Wlong-long— C90 or earlier-Wmain-Wnarrowing— C++11 or later-Wpointer-arith-Wpointer-sign-Wincompatible-pointer-types— yep, it’s listed twice in the original list, no cap 🫶-Wregister— C++17 or later-Wvla— C90 or earlier-Wwrite-strings— C++11 or later
-fpermissive— lowkey the forgiveness flag 🍓. It downgrades some required diagnostics about nonconformant code from errors to warnings. Thus, using-fpermissiveallows some nonconforming code to compile. Some C++ diagnostics are controlled only by this flag, but it also downgrades some C and C++ diagnostics that have their own flag:-Wabbreviated-auto-in-template-arg— C++ and Objective-C++ only-Wdeclaration-missing-parameter-type— C and Objective-C only-Wimplicit-function-declaration— C and Objective-C only-Wimplicit-int— C and Objective-C only-Wincompatible-pointer-types— C and Objective-C only-Wint-conversion— C and Objective-C only-Wnarrowing— C++ and Objective-C++ only-Wreturn-mismatch— C and Objective-C only-Wtemplate-body— C++ and Objective-C++ only
The -fpermissive option is the default for historic C language modes (-std=c89, -std=gnu89, -std=c90, -std=gnu90). historic C modes stay permissive, fr fr 🧚.
-Wall,--all-warnings— main character energy warning bundle 🌸. 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 inC++ Dialect OptionsandObjective-C and Objective-C++ Dialect Options.
-Wall turns on the following warning flags:
-Waddress-Waligned-new— C++ and Objective-C++ only-Warray-bounds=1— only with-O2-Warray-compare-Warray-parameter=2-Wbool-compare-Wbool-operation-Wc++11-compat-Wc++14-compat-Wc++17compat-Wc++20compat-Wcatch-value— C++ and Objective-C++ only-Wchar-subscripts-Wclass-memaccess— C++ and Objective-C++ only-Wcomment-Wdangling-else-Wdangling-pointer=2-Wdelete-non-virtual-dtor— C++ and Objective-C++ only-Wduplicate-decl-specifier— C and Objective-C only-Wenum-compare— in C/ObjC; this is on by default in C++-Wenum-int-mismatch— C and Objective-C only-Wformat=1-Wformat-contains-nul-Wformat-diag-Wformat-extra-args-Wformat-overflow=1-Wformat-truncation=1-Wformat-zero-length-Wframe-address-Wimplicit— C and Objective-C only-Wimplicit-function-declaration— C and Objective-C only-Wimplicit-int— C and Objective-C only-Winfinite-recursion-Winit-self— C++ and Objective-C++ only-Wint-in-bool-context-Wlogical-not-parentheses-Wmain— only for C/ObjC and unless-ffreestanding-Wmaybe-uninitialized-Wmemset-elt-size-Wmemset-transposed-args-Wmisleading-indentation— only for C/C++-Wmismatched-dealloc-Wmismatched-new-delete— C++ and Objective-C++ only-Wmissing-attributes-Wmissing-braces— only for C/ObjC-Wmultistatement-macros-Wnarrowing— C++ and Objective-C++ only-Wnonnull-Wnonnull-compare-Wopenmp-simd— C and C++ only-Woverloaded-virtual=1— C++ and Objective-C++ only-Wpacked-not-aligned-Wparentheses-Wpessimizing-move— C++ and Objective-C++ only-Wpointer-sign— only for C/ObjC-Wrange-loop-construct— C++ and Objective-C++ only-Wreorder— C++ and Objective-C++ only-Wrestrict-Wreturn-type-Wself-move— C++ and Objective-C++ only-Wsequence-point-Wsign-compare— C++ and Objective-C++ only-Wsizeof-array-div-Wsizeof-pointer-div-Wsizeof-pointer-memaccess-Wstrict-aliasing-Wstrict-overflow=1-Wswitch-Wtautological-compare-Wtrigraphs-Wuninitialized-Wunknown-pragmas-Wunused-Wunused-but-set-variable-Wunused-const-variable=1— only for C/ObjC-Wunused-function-Wunused-label-Wunused-local-typedefs-Wunused-value-Wunused-variable-Wuse-after-free=2-Wvla-parameter-Wvolatile-register-var-Wzero-length-bounds
Note that some warning flags are not implied by -Wall. Some of them warn about constructions that users generally do not consider questionable, but which occasionally you might wish to check for; others warn about constructions that are necessary or hard to avoid in some cases, and there is no simple way to modify the code to suppress the warning. Some of them are enabled by -Wextra but many of them must be enabled individually. no cap, this list is long but every flag ate 🔥.
-Wextra,--extra-warnings— the extra glow-up 🦋. This enables some extra warning flags that are not enabled by-Wall. (This option used to be called-W. The older name is still supported, but the newer name is more descriptive.)-Wabsolute-value(only for C/ObjC) ·-Walloc-size·-Wcalloc-transposed-args·-Wcast-function-type·-Wclobbered·-Wdangling-reference(C++ only) ·-Wdeprecated-copy(C++ and Objective-C++ only) ·-Wempty-body·-Wenum-conversion(only for C/ObjC) ·-Wexpansion-to-defined·-Wignored-qualifiers(only for C/C++) ·-Wimplicit-fallthrough=3·-Wmaybe-uninitialized·-Wmissing-field-initializers·-Wmissing-parameter-name(C/ObjC only) ·-Wmissing-parameter-type(C/ObjC only) ·-Wold-style-declaration(C/ObjC only) ·-Wmultiple-parameter-fwd-decl-lists(C/ObjC only) ·-Woverride-init(C/ObjC only) ·-Wredundant-move(C++ and Objective-C++ only) ·-Wshift-negative-value(in C++11 to C++17 and in C99 and newer) ·-Wsign-compare(C++ and Objective-C++ only) ·-Wsized-deallocation(C++ and Objective-C++ only) ·-Wstring-compare·-Wtype-limits·-Wuninitialized·-Wunterminated-string-initialization(C/ObjC only) ·-Wunused-parameter(only with-Wunusedor-Wall) ·-Wunused-but-set-parameter(only with-Wunusedor-Wall) — okay that's the whole squad, all of them turned on by-Wextra💅
And bestie, -Wextra is not done serving looks. It also hits you with warnings for these situations:
- A pointer gets compared against integer zero with
<,<=,>, or>=🫠 - (C++ only) An enumerator and a non-enumerator both show up in a conditional expression
- (C++ only) Ambiguous virtual bases
- (C++ only) Subscripting an array that's been declared
register - (C++ only) Taking the address of a variable declared
register - (C++ only) A base class isn't initialized in the copy constructor of a derived class
-Wabi (C, Objective-C, C++ and Objective-C++ only)
This one warns you about code that's affected by ABI changes ✨ — meaning code that might not be compatible with the vendor-neutral C++ ABI, or with the psABI for your particular target. Those psABI-flavoured warnings are also their own separate thing under -Wpsabi, which -Wabi implies, so yeah, package deal 🎀
So here's the tea: since G++ now defaults to updating the ABI with each major release, -Wabi normally only warns about C++ ABI compatibility problems if there's a check added later in a release series for some ABI issue discovered since the initial release. But you get more warnings if you pick an older ABI version with -fabi-version=n.
You can also hand -Wabi an explicit version number to warn about C++ ABI compatibility with a particular -fabi-version level — like -Wabi=2 to warn about changes relative to -fabi-version=2 💫
If you do give an explicit version number and -fabi-compat-version isn't specified, the version number from this option gets used for compatibility aliases. Conversely, if you don't give an explicit version number here but do specify -fabi-compat-version, then that version number is what's used for C++ ABI warnings. Teamwork, honestly 🫶
Real talk though: they've made an effort to warn about all these cases, but there are probably some that slip through unwarned even while G++ is out there generating incompatible code 😭 And sometimes warnings fire even when the generated code is compatible. It's giving imperfect science.
If you're stressed about G++-generated code maybe not being binary compatible with code from other compilers, you should rewrite your code to dodge these warnings. Periodt.
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))andnoreturnwere mangled as type qualifiers, anddecltypeof a plain declaration was folded away.
These mangling issues were fixed in -fabi-version=5.
- Scoped enumerators passed as arguments to a variadic function are promoted like unscoped enumerators, causing
va_argto complain 🔥 On most targets this doesn't actually affect the parameter passing ABI, since there's no way to pass an argument smaller thanint.
Also, the ABI changed the mangling of template argument packs, const_cast, static_cast, prefix increment/decrement, and a class scope function used as a template argument.
These issues were corrected in -fabi-version=6.
- Lambdas in default argument scope were mangled incorrectly, and the ABI changed the mangling of
nullptr_t.
These issues were corrected in -fabi-version=7.
- When mangling a function type with function-cv-qualifiers, the un-qualified function type was incorrectly treated as a substitution candidate.
This was fixed in -fabi-version=8, the default for GCC 5.1.
decltype(nullptr)incorrectly had an alignment of 1, leading to unaligned accesses 😬 Note that this did not affect the ABI of a function with anullptr_tparameter, 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 — like ia32 calling conventions on a function type (
stdcall,regparm, etc.) — did not affect the mangled name, leading to name collisions when function pointers were used as template arguments.
This was fixed in -fabi-version=10, the default for GCC 6.1.
-Wpsabi (C, Objective-C, C++ and Objective-C++ only)
-Wpsabi turns on warnings about processor-specific ABI changes 💅 — stuff like changes in alignment requirements, or how function arguments get passed. On several targets, including AArch64, ARM, x86, MIPS, RS6000/PowerPC, and S/390, these details have changed between different versions of GCC and/or different versions of the C or C++ language standards in ways that affect the binary compatibility of compiled code. With -Wpsabi, GCC can spot potentially incompatible usages and warn you about them 👑
-Wpsabi is enabled by default, and it's also implied by -Wabi. Double coverage, we love to see it 💖
-Wno-changes-meaning (C++ and Objective-C++ only)
C++ requires that unqualified uses of a name within a class have the same meaning in the complete scope of the class — so declaring the name after using it is ill-formed 🙈:
struct A;
struct B1 { A a; typedef A A; }; // warning, 'A' changes meaning
struct B2 { A a; struct A { }; }; // error, 'A' changes meaningBy default, the B1 case is only a warning because the two declarations have the same type, while the B2 case is an error. Both diagnostics can be disabled with -Wno-changes-meaning. Alternately, the error case can be reduced to a warning with -Wno-error=changes-meaning or -fpermissive ✨
Both diagnostics are also suppressed by -fms-extensions.
-Wchar-subscripts
Warns if an array subscript has type char 🎀 This is such a common cause of error, because programmers keep forgetting that this type is signed on some machines. @xref{Characters implementation}, and char type signedness.
This warning is enabled by -Wall. And when it's enabled, the warning fires regardless of whether char is unsigned by default on the target — and it's also not affected by the -fsigned-char or -funsigned-char options. No cap, it does what it wants 💀
-Wno-coverage-mismatch
Warns if feedback profiles don't match when you're using the -fprofile-use option 😭 If a source file changes between compiling with -fprofile-generate and compiling with -fprofile-use, the files with the profile feedback can fail to match the source file and GCC just can't use the profile feedback information.
By default, this warning is enabled and treated as an error. So you can use -Wno-coverage-mismatch to disable the warning, or -Wno-error=coverage-mismatch to disable the error. But listen — disabling the error for this warning can result in poorly optimized code, and it's only useful in the case of very minor changes like bug fixes to an existing code-base. Completely disabling the warning is not recommended, bestie 🫠
-Wno-coverage-too-many-conditions
Warns if -fcondition-coverage is used and an expression has too many terms, so GCC gives up on coverage 😮💨 Coverage gets abandoned when there are more terms in the conditional than there are bits in a gcov_type_unsigned. This warning is enabled by default.
-Wno-coverage-too-many-paths
Warns if -fpath-coverage is used and a function has too many paths, so GCC gives up on coverage ✋ Giving up is controlled by -fpath-coverage-limit. Enabled by default.
-Wno-coverage-invalid-line-number
Warns in case a function ends earlier than it begins, thanks to invalid linenum macros 🌀 The warning is emitted only with --coverage enabled.
By default this warning is enabled and treated as an error. -Wno-coverage-invalid-line-number disables the warning, or -Wno-error=coverage-invalid-line-number disables the error. Pick your fighter 💅
-Wno-cpp (C, Objective-C, C++, Objective-C++ and Fortran only)
Suppresses warning messages emitted by #warning directives 🤫 That's it. That's the whole vibe.
-Wdouble-promotion (C, C++, Objective-C and Objective-C++ only)
Gives you a warning when a value of type float gets implicitly promoted to double ✨ Here's why that matters: CPUs with a 32-bit "single-precision" floating-point unit implement float in hardware, but emulate double in software. On a machine like that, doing computations with double values is so much more expensive because of all the overhead that software emulation needs 🔥
And it's genuinely 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. Sneaky little thing, honestly 😤
-Wduplicate-decl-specifier (C and Objective-C only)
Warns if a declaration has a duplicate const, volatile, restrict or _Atomic specifier 😌 This warning is enabled by -Wall.
-Wformat,-Wformat=n — bestie, this one is a vibe check for yourprintfandscanfcalls 💅 It checks that the arguments you're feeding in have types that actually match the format string you gave, and that the conversions in that format string make sense at all. This covers the standard functions plus anything that's been given format attributes, across theprintf,scanf,strftimeandstrfmonfamilies (heads up:strfmonis an X/Open extension, not in the C standard), or other target-specific families. Which functions get checked even without explicit format attributes depends on the standard version you selected — and those no-attribute checks get switched off entirely by-ffreestandingor-fno-builtin. Ngl that part trips people up.
The formats get checked against what GNU libc version 2.2 supports. That includes every ISO C90 and C99 feature, plus stuff from the Single Unix Specification and some BSD and GNU extensions. Other library implementations might not support all of that — and GCC does not warn you about features that go beyond a particular library's limits. However (and this is the key bit ✨), if you use -Wpedantic together with -Wformat, you do get warnings about format features that aren't in the standard version you selected — but not for strfmon formats, since those aren't in any version of the C standard at all. 🫶
-Wformat=1,-Wformat— Option-Wformatis exactly equivalent to-Wformat=1, and-Wno-formatis exactly equivalent to-Wformat=0. Since-Wformatalso checks for null format arguments for several functions, it also implies-Wnonnull. You can turn off some aspects of this level of checking with-Wno-format-contains-nul,-Wno-format-diag,-Wno-format-extra-args, and-Wno-format-zero-length. Oh and-Wformatis enabled by-Wallalready, so it's probably lurking in your build rn. 🙈-Wformat=2— Turns on-Wformatplus extra format checks. Currently it's equivalent to-Wformat -Wformat-nonliteral -Wformat-security -Wformat-y2k. The deluxe bundle, literally. 👑
-Wno-format-contains-nul— If-Wformatis on, this tells it not to warn about format strings that contain NUL bytes. Silence is golden 🎀-Wno-format-diag— If-Wformatis on, skip the warnings about format strings that are unsuitable for GCC diagnostics.-Wno-format-extra-args— If-Wformatis on, don't warn about excess arguments passed to aprintforscanfformat function. The C standard says such arguments are just ignored, so like... whatever 💀
Where the unused arguments sit between used arguments that are specified with $ operand number specifications, warnings are normally still given, because the implementation couldn't know what type to pass to va_arg to skip past the unused ones. 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 such unused arguments are allowed. Cute loophole 🌸
-Wformat-overflow,-Wformat-overflow=level — Warns about 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, it gets estimated using heuristics that depend on the level argument and on optimization. Turning on optimization usually makes the warning more accurate, but it can also produce false positives. So like, it's giving helpful but occasionally dramatic 😭-Wformat-overflow,-Wformat-overflow=1— Level 1 of-Wformat-overflow(the one enabled by-Wformat) plays it safe and only warns about calls that will most likely overflow the buffer. At this level, numeric arguments to format directives with unknown values are assumed to be one, and strings of unknown length are assumed to be empty. Numeric arguments 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 in that range that produces the most output bytes. For example, thesprintfcall below 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 it might not be enough to dodge the actual overflow. 💀
void f (int a, int b)
{
char buf [13];
sprintf (buf, "a = %i, b = %i\n", a, b);
}-Wformat-overflow=2— Level 2 also warns about calls that might overflow the destination buffer when given an argument of sufficient length or magnitude. At level 2, unknown numeric arguments are assumed to be 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 bounded either by the directive's precision, or by a finite set of string literals they might evaluate to, or the character array they might point at, are assumed to be 1 character long.
At level 2, the call in the example above gets diagnosed again — but this time because with a equal to a 32-bit INT_MIN, the first %i directive writes some of its digits past the end of the destination buffer. To make the call safe no matter what the two variables hold, the destination buffer needs to be at least 34 bytes. GCC includes the minimum buffer size in an informational note right after the warning, which is genuinely so helpful of it 🫶
An alternative to growing the destination buffer is constraining the range of the 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 bounded by less than the precision of their type, choosing an appropriate length modifier for the format specifier reduces 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 drops the maximum required buffer size to 24 bytes. Slay, precision. ✨
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 on, don't warn about zero-length formats. The C standard says zero-length formats are allowed, so like... let the girl live 🎀-Wformat-nonliteral— If-Wformatis on, also warn when the format string isn't a string literal (so it can't be checked) — unless the format function takes its format arguments as ava_list.-Wformat-security— If-Wformatis on, also warn about uses of format functions that could represent security problems. Right now this warns about calls toprintfandscanffunctions where the format string isn't a string literal and there are no format arguments, likeprintf (foo);. That can 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 aren't included in-Wformat-nonliteral. So keep an eye out, bestie.)-Wformat-signedness— If-Wformatis on, also warn when the format string requires an unsigned argument but the argument is signed, and vice versa. Matching matters 💅-Wformat-truncation,-Wformat-truncation=level — Warns about calls to formatted input/output functions likesnprintfandvsnprintfthat might truncate the output. When the exact number of bytes written by a format directive can't be determined at compile-time, it's estimated with heuristics that depend on the level argument and on optimization. Enabling optimization usually makes the warning more accurate, but it can also cause false positives. Except where noted otherwise, this option uses the same logic as-Wformat-overflow.-Wformat-truncation,-Wformat-truncation=1— Level 1 of-Wformat-truncation(enabled by-Wformat) plays it safe and only warns 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 truncate given an argument of sufficient length or magnitude.
-Wformat-y2k— If-Wformatis on, also warn aboutstrftimeformats that might yield only a two-digit year. It's giving Y2K anxiety all over again 😭-Wnonnull— Warns about passing a null pointer for arguments that are marked as requiring a non-null value by thenonnullfunction attribute.
-Wnonnull is included in -Wall and -Wformat. You can disable it with -Wno-nonnull.
-Wnonnull-compare— Warns when you compare an argument marked with thenonnullfunction attribute against null inside the function. Like girl, you literally said it's never null 😭
-Wnonnull-compare is included in -Wall. You can disable it with -Wno-nonnull-compare.
-Wnull-dereference— Warns if the compiler spots paths that trigger erroneous or undefined behavior by dereferencing a null pointer. This option is only active when-fdelete-null-pointer-checksis active, which optimizations enable on most targets. How precise the warnings are depends on the optimization options you used. 🦋-Wno-musttail-local-addr— Don't warn about passing a pointer (or in C++, a reference) to a local variable or label as an argument to amusttailcall. Those variables go out of scope before the tail call instruction, so it's fine, ok? ✨-Wmaybe-musttail-local-addr— Warns when the address of a local variable can escape to amusttailcall — unless it already goes out of scope before themusttailcall.
int foo (int *); int
bar (int *x)
{
if (x[0] == 1)
{
int a = 42;
foo (&a);
/* Without the C<musttail> attribute this call would not
be tail called, because address of the C<a> variable escapes
and the second foo call could dereference it. With the attribute
the local variables are assumed to go out of scope immediately
before the tail call instruction and the compiler warns about
this. */
[[gnu::musttail]] return foo (nullptr);
}
else
{
{
int a = 42;
foo (&a);
}
/* The C<a> variable isn't already in scope, so even when it
escaped, even without C<musttail> attribute it would be
undefined behavior to dereference it and the compiler could
turn this into a tail call. No warning is diagnosed here. */
[[gnu::musttail]] return foo (nullptr);
}
}OK so real talk: that warning? It's switched on by -Wextra. 💖
-Wnrvo(C++ and Objective-C++ only) — bestie, this one warns you when the compiler does not elide the copy from a local variable into a function's return value, in a spot where[class.copy.elision]says it's allowed to. We call that elision the Named Return Value Optimization around here (iykyk 💅). Look at the example: the compiler can't elide copies from bothv1andv2, so it elides neither, and you get the warning. Rude but fair.
std::vector<int> f()
{
std::vector<int> v1, v2;
// ...
if (cond) return v1;
else return v2; // warning: not eliding copy
}-Winfinite-recursion— warns you about infinitely recursive calls 🔁 (yes, your function calling itself forever, we've all been there). Heads up though: the warning works at all optimization levels, but it needs optimization to actually catch infinite recursion in calls between two or more functions. It's included in-Wall. And ngl, don't confuse it with-Wanalyzer-infinite-recursion— that one gives a similar diagnostic but is implemented a whole different way (as part of-fanalyzer).-Winit-self(C, C++, Objective-C and Objective-C++ only) — warns when an uninitialized variable is initialized with itself 😭 the audacity. Quick note: this option only does anything when you also use-Wuninitialized. In the snippet below, GCC only calls out"i"being uninitialized when-Winit-selfis specified:
int f()
{
int i = i;
return i;
}This warning is enabled by -Wall in C++. ✨
-Wno-implicit-int(C and Objective-C only) — controls warnings for when a declaration just... doesn't specify a type 💀. Enabled by default, as an error, in C99 and later dialects of C, and also by-Wall. You can downgrade the error to a warning with-fpermissive(which does that for certain other errors too), or for this error alone with-Wno-error=implicit-int. And-pedantic-errorsupgrades this warning to an error. 🎀-Wno-implicit-function-declaration(C and Objective-C only) — same energy, but for when a function is used before it's declared 🙈. Also on by default, as an error, in C99 and later C dialects, and by-Wall. Downgrade it with-fpermissive(along with certain other errors) or just this one with-Wno-error=implicit-function-declaration.-pedantic-errorsupgrades it to an error.-Wimplicit(C and Objective-C only) — literally the same as-Wimplicit-intand-Wimplicit-function-declarationcombined 🫶. Enabled by-Wall.-Whardened— warns when-fhardeneddid not enable an option from its set (go peek at-fhardenedfor that list). Example: if you throw-fhardenedand-fstack-protectoron the command line at the same time,-Whardenedwill warn, because-fstack-protector-strongwon't get enabled by-fhardened👀. This warning is enabled by default, but it only does anything when-fhardenedis enabled.-Wimplicit-fallthrough— shorthand city:-Wimplicit-fallthroughis exactly the same as-Wimplicit-fallthrough=3, and-Wno-implicit-fallthroughis exactly the same as-Wimplicit-fallthrough=0. 💅- *
-Wimplicit-fallthrough=n* — warns when aswitchcase falls through 🍂. Like this:
switch (cond)
{
case 1:
a = 1;
break;
case 2:
a = 2;
case 3:
a = 3;
break;
}It does not warn when the last statement of a case can't fall through — say there's a return, or a call to a function declared with the noreturn attribute. -Wimplicit-fallthrough= also takes control flow statements like ifs into account, and only warns when it's actually appropriate. Example:
switch (cond)
{
case 1:
if (i > 3) {
bar (5);
break;
} else if (i < 1) {
bar (0);
} else
return;
default:
...
}Since sometimes you want a case to fall through, GCC gives you an attribute, "__attribute__ ((fallthrough))", which you use alongside a null statement to suppress the warning that would normally fire:
switch (cond)
{
case 1:
bar (0);
__attribute__ ((fallthrough));
default:
...
}C++17 and C23 have a standard way to shush the -Wimplicit-fallthrough warning: use "[[fallthrough]];" instead of the GNU attribute 🌈. On C++11 or C++14 you can use "[[gnu::fallthrough]];", which is a GNU extension. And instead of any of these attributes, you can drop a fallthrough comment to silence the warning — the whole body of the C or C++ style comment has to match the given regular expressions listed below. The option argument n says what kind of comments are accepted:
-Wimplicit-fallthrough=0disables the warning altogether.-Wimplicit-fallthrough=1matches the".*"regular expression, so literally any comment counts as a fallthrough comment.-Wimplicit-fallthrough=2case insensitively matches the".*falls?[ \t-]*thr(ough|u).*"regular expression.-Wimplicit-fallthrough=3case sensitively matches one of these regular expressions:"-fallthrough""@fallthrough@""lint -fallthrough[ \t]*""[ \t.!]*(ELSE,? |INTENTIONAL(LY)? )?FALL(S | |-)?THR(OUGH|U)[ \t.!]*(-[^\n\r]*)?""[ \t.!]*(Else,? |Intentional(ly)? )?Fall((s | |-)[Tt]|t)hr(ough|u)[ \t.!]*(-[^\n\r]*)?""[ \t.!]*([Ee]lse,? |[Ii]ntentional(ly)? )?fall(s | |-)?thr(ough|u)[ \t.!]*(-[^\n\r]*)?"
-Wimplicit-fallthrough=4case sensitively matches one of these regular expressions:"-fallthrough""@fallthrough@""lint -fallthrough[ \t]*""[ \t]*FALLTHR(OUGH|U)[ \t]*"
-Wimplicit-fallthrough=5doesn't recognize any comments as fallthrough comments — only attributes disable the warning.
Heads up on placement: the comment needs to be followed (after optional whitespace and other comments) by the "case" or "default" keywords, or by a user label that comes before some "case" or "default" label 🎀
switch (cond)
{
case 1:
bar (0);
/* FALLTHRU */
default:
...
}Oh and btw, the -Wimplicit-fallthrough=3 warning is enabled by -Wextra. ✨
-Wno-if-not-aligned(C, C++, Objective-C and Objective-C++ only) — controls whether warnings triggered by the"warn_if_not_aligned"attribute get issued or not. These warnings are enabled by default 👑-Wignored-qualifiers(C and C++ only) — warns when a function's return type has a type qualifier like"const". For ISO C that qualifier does literally nothing, because the value returned by a function isn't an lvalue 🫠. For C++, the warning is only emitted for scalar types or"void". And ISO C straight-up prohibits qualified"void"return types on function definitions, so those return types always get a warning even without this option. Also,-Wextraenables this warning too. 💜-Wno-ignored-attributes(C and C++ only) — controls warnings when an attribute is ignored. Don't mix it up with-Wattributes: this one warns whenever the compiler decides to drop an attribute, not that the attribute is unknown or used in the wrong place or whatever. Enabled by default. 🦋-Wmain— warns if the type of"main"looks suspicious 👀."main"should be a function with external linkage, returningint, taking either zero arguments, two, or three arguments of appropriate types. Enabled by default in C++, and enabled by either-Wallor-Wpedantic.-pedantic-errorsupgrades this warning to an error. 🎀-Wmisleading-indentation(C and C++ only) — warns when your indentation doesn't match your block structure 💀 the classic. Specifically, it fires for"if","else","while", and"for"clauses that have a guarded statement without braces, followed by an unguarded statement with the same indentation. In this example the call to"bar"is misleadingly indented like the"if"is guarding it:
if (some_condition ())
foo ();
bar (); /* Gotcha: this is not guarded by the "if". */When tabs and spaces get mixed up, the warning uses the -ftabstop= option to figure out whether the statements line up (defaulting to 8). And it does not fire for code involving multiline preprocessor logic, like the example that follows. 🙈
Okay bestie, picture this little gremlin of a code chunk:
if (flagA)
foo (0);
#if SOME_CONDITION_THAT_DOES_NOT_HOLD
if (flagB)
#endif
foo (1);The warning is not issued after a #line directive, since this typically indicates autogenerated code, and no assumptions can be made about the layout of the file that the directive references.
This warning is enabled by -Wall in C and C++. ✨
-Wmissing-attributes— Warn when a declaration of a function is missing one or more attributes that a related function is declared with and whose absence may adversely affect the correctness or efficiency of generated code. For example, the warning is issued for declarations of aliases that use attributes to specify less restrictive requirements than those of their targets. This typically represents a potential optimization opportunity. By contrast, the-Wattribute-alias=2option controls warnings issued when the alias is more restrictive than the target, which could lead to incorrect code generation. Attributes considered includealloc_align,alloc_size,cold,const,hot,leaf,malloc,nonnull,noreturn,nothrow,pure,returns_nonnull, andreturns_twice.
In C++, the warning is issued when an explicit specialization of a primary template declared with attribute alloc_align, alloc_size, assume_aligned, format, format_arg, malloc, or nonnull is declared without it. Attributes deprecated, error, and warning suppress the warning..
You can use the copy attribute to apply the same set of attributes to a declaration as that on another declaration without explicitly enumerating the attributes. This attribute can be applied to declarations of functions, variables, or types. Common Attributes.
-Wmissing-attributes is enabled by -Wall.
For example, since the declaration of the primary function template below makes use of both attribute malloc and alloc_size the declaration of the explicit specialization of the template is diagnosed because it is missing one of the attributes.
template <class T>
T* __attribute__ ((malloc, alloc_size (1)))
allocate (size_t);
template <>
void* __attribute__ ((malloc)) // missing alloc_size
allocate<void> (size_t);-Wmissing-braces— Warn if an aggregate or union initializer is not fully bracketed. In the following example, the initializer forais not fully bracketed, but that forbis fully bracketed.
int a[2][2] = { 0, 1, 2, 3 };
int b[2][2] = { { 0, 1 }, { 2, 3 } };This warning is enabled by -Wall.
-Wmissing-include-dirs(C, C++, Objective-C, Objective-C++ and Fortran only) — Warn if a user-supplied include directory does not exist. This option is disabled by default for C, C++, Objective-C and Objective-C++. For Fortran, it is partially enabled by default by warning for-Iand-J, only.-Wno-missing-profile— This option controls warnings if feedback profiles are missing when using the-fprofile-useoption. This option diagnoses those cases where a new function or a new file is added between compiling with-fprofile-generateand with-fprofile-use, without regenerating the profiles. In these cases, the profile feedback data files do not contain any profile feedback information for the newly added function or file respectively. Also, in the case when profile count data (.gcda) files are removed, GCC cannot use any profile feedback information. In all these cases, warnings are issued to inform you that a profile generation step is due. Ignoring the warning can result in poorly optimized code.-Wno-missing-profilecan be used to disable the warning, but this is not recommended and should be done only when non-existent profile data is justified.-Wmismatched-dealloc— Warn for calls to deallocation functions with pointer arguments returned from allocation functions for which the former isn't a suitable deallocator. A pair of functions can be associated as matching allocators and deallocators by use of attributemalloc. Unless disabled by the-fno-builtinoption the standard functionscalloc,malloc,realloc, andfree, as well as the corresponding forms of C++operator newandoperator deleteare implicitly associated as matching allocators and deallocators. In the following examplemydeallocis the deallocator for pointers returned frommyalloc.
void mydealloc (void*);
__attribute__ ((malloc (mydealloc, 1))) void*
myalloc (size_t);
void f (void)
{
void *p = myalloc (32);
// ...use p...
free (p); // warning: not a matching deallocator for myalloc
mydealloc (p); // ok
}In C++, the related option -Wmismatched-new-delete diagnoses mismatches involving either operator new or operator delete.
Option -Wmismatched-dealloc is included in -Wall.
-Wmultistatement-macros— Warn about unsafe multiple statement macros that appear to be guarded by a clause such asif,else,for,switch, orwhile, in which only the first statement is actually guarded after the macro is expanded.
For example:
#define DOIT x++; y++
if (c)
DOIT;will increment y unconditionally, not just when c holds. You can usually fix this by wrapping the macro in a do-while loop:
#define DOIT do { x++; y++; } while (0)
if (c)
DOIT;This warning is enabled by -Wall in C and C++.
-Wparentheses— Warn if parentheses are omitted in certain contexts, such as when there is an assignment in a context where a truth value is expected, or when operators are nested whose precedence people often get confused about.
Also warn if a comparison like x<=y<=z appears; this is equivalent to (x<=y ? 1 : 0) <= z, which is a different interpretation from that of ordinary mathematical notation.
Also warn for dangerous uses of the GNU extension to ?: with omitted middle operand. When the condition in the ?: operator is a boolean expression, the omitted value is always 1. Often programmers expect it to be a value computed inside the conditional expression instead.
For C++ this also warns for some cases of unnecessary parentheses in declarations, which can indicate an attempt at a function call instead of a declaration:
{
// Declares a local variable called mymutex.
std::unique_lock<std::mutex> (mymutex);
// User meant std::unique_lock<std::mutex> lock (mymutex);
}This warning is enabled by -Wall.
-Wno-self-move(C++ and Objective-C++ only) — This warning warns when a value is moved to itself withstd::move. Such astd::movetypically has no effect.
struct T {
...
};
void fn()
{
T t;
...
t = std::move (t);
}This warning is enabled by -Wall.
-Wsequence-point— Warn about code that may have undefined semantics because of violations of sequence point rules in the C and C++ standards. This is the “bestie, pls don’t write that” warning.
The C and C++ standards define the order in which expressions in a C/C++ program are evaluated in terms of sequence points, which represent a partial ordering between the execution of parts of the program: those executed before the sequence point, and those executed after it. These occur after the evaluation of a full expression (one which is not part of a larger expression), after the evaluation of the first operand of a &&, ||, ? : or , (comma) operator, before a function is called (but after the evaluation of its arguments and the expression denoting the called function), and in certain other places. Other than as expressed by the sequence point rules, the order of evaluation of subexpressions of an expression is not specified. All these rules describe only a partial order rather than a total order, since, for example, if two functions are called within one expression with no sequence point between them, the order in which the functions are called is not specified. However, the standards committee have ruled that function calls do not overlap.
It is not specified when between sequence points modifications to the values of objects take effect. Programs whose behavior depends on this have undefined behavior; the C and C++ standards specify that “Between the previous and next sequence point an object shall have its stored value modified at most once by the evaluation of an expression. Furthermore, the prior value shall be read only to determine the value to be stored.”. If a program breaks these rules, the results on any particular implementation are entirely unpredictable.
Examples of code with undefined behavior are a = a++;, a[n] = b[n++] and a[i++] = i;. Some more complicated cases are not diagnosed by this option, and it may give an occasional false positive result, but in general it has been found fairly effective at detecting this sort of problem in programs.
The C++17 standard will define the order of evaluation of operands in more cases: in particular it requires that the right-hand side of an assignment be evaluated before the left-hand side, so the above examples are no longer undefined. But this option will still warn about them, to help people avoid writing code that is undefined in C and earlier revisions of C++.
The standard is worded confusingly, therefore there is some debate over the precise meaning of the sequence point rules in subtle cases. Links to discussions of the problem, including proposed formal definitions, may be found on the GCC readings page, at https://gcc.gnu.org/readings.html.
This warning is enabled by -Wall for C and C++.
-Wno-return-local-addr— Do not warn about returning a pointer (or in C++, a reference) to a variable that goes out of scope after the function returns.-Wreturn-mismatch— ok so this one's the vibe check on yourreturnstatements 💅 It yells at you when a function that does not returnvoidhas areturnstatement with no expression in it. And also — plot twist — when a function whose return type isvoidhas areturnstatement with an expression, unless that expression's type is alsovoid. BUT as a GNU extension, that second case gets accepted with no warning at all unless-Wpedanticis on. Sneaky.
Using the return value of a non-void function (other than main) that just... flows off the end by hitting the closing curly brace that terminates the function? That's undefined, bestie. Not a vibe.
This warning is C-only and it's on by default. In C99 and later dialects it's straight-up treated as an error. You can downgrade it to a warning with -fpermissive (which downgrades a bunch of other stuff too), or if you only want to soften this one, use -Wno-error=return-mismatch.
-Wreturn-type— warns whenever a function is defined with a return type that defaults toint(unless-Wimplicit-intis active, 'cause that one takes precedence). Also warns if execution might reach the end of the function body, or if the function has noreturnstatement at all 😭
Attempting to use the return value of a non-void function other than main that flows off the end by reaching the closing curly brace that terminates the function is undefined.
And unlike C, in C++ flowing off the end of a non-void function other than main is undefined behavior even when you never use the value 💀
Enabled by default in C++, and by -Wall otherwise.
-Wno-shift-count-negative— controls warnings when a shift count is negative. Enabled by default.-Wno-shift-count-overflow— controls warnings when a shift count is greater than or equal to the bit width of the type. Enabled by default.-Wshift-negative-value— warns if you're left shifting a negative value 🙈 Enabled by-Wextrain C99 (and newer) and C++11 through C++17 modes.-Wno-shift-overflow,-Wshift-overflow=n— these bad boys control the warnings about left shift overflows ✨-Wshift-overflow=1— this is the warning level of-Wshift-overflow, enabled by default in C99 and C++11 modes (and newer). At this level it does not warn about left-shifting 1 into the sign bit. (Tho in C, that overflow is still rejected in contexts where an integer constant expression is required — just so you know 🫶) No warning is emitted in C++20 mode (and newer), 'cause signed left shifts always wrap there.-Wshift-overflow=2— this level also warns about left-shifting 1 into the sign bit, unless C++14 mode (or newer) is active.
-Wswitch— warns whenever aswitchstatement has an index of enumerated type and is missing acasefor one or more of the named codes of that enumeration. (Adefaultlabel prevents this warning, iykyk.)caselabels that don't correspond to enumerators also provoke warnings with this option, unless the enumeration is marked with theflag_enumattribute. Enabled by-Wall.-Wswitch-default— warns whenever aswitchstatement doesn't have adefaultcase at all. Like, hello?? give it a default 🌸-Wswitch-enum— warns whenever aswitchstatement has an index of enumerated type and lacks acasefor one or more of the named codes of that enumeration.caselabels that don't correspond to enumerators also provoke warnings here too, unless the enumeration is marked with theflag_enumattribute. The only difference between-Wswitchand this option is that this one gives you a warning about an omitted enumeration code even if there's adefaultlabel 👑-Wno-switch-bool— do not warn when aswitchstatement has an index of boolean type and the case values are outside the range of a boolean type. You can also suppress this warning by casting the controlling expression to a type other thanbool. For example:
switch ((int) (a == 4))
{
...
}This warning is enabled by default for C and C++ programs 💜
-Wno-switch-outside-range— controls warnings when aswitchcase has a value outside of its respective type range. Enabled by default for C and C++ programs.-Wno-switch-unreachable— do not warn when aswitchstatement contains statements between the controlling expression and the first case label, which will never ever be executed 😭 For example:
switch (cond)
{
i = 15;
...
case 5:
...
}-Wswitch-unreachable does not warn if the statement between the controlling expression and the first case label is just a declaration:
switch (cond)
{
int i;
...
case 5:
i = 5;
...
}This warning is enabled by default for C and C++ programs.
-Wsync-nand(C and C++ only) — warns when the__sync_fetch_and_nandand__sync_nand_and_fetchbuilt-in functions are used. Those functions changed semantics in GCC 4.4, so like... watch out ⭐-Wtrivial-auto-var-init— warns when-ftrivial-auto-var-initcannot initialize the automatic variable. A common situation is an automatic variable declared between the controlling expression and the first case label of aswitchstatement 🦋-Wunused-but-set-parameter— this is literally the same as-Wunused-but-set-parameter=3, and-Wno-unused-but-set-parameteris the same as-Wunused-but-set-parameter=0.-Wunused-but-set-parameter=n— warns whenever a function parameter is assigned to but otherwise unused (aside from its declaration).
To suppress this warning, use the unused attribute.
-Wunused-but-set-parameter=0 disables the warning. With -Wunused-but-set-parameter=1 all uses except initialization and left hand side of assignment which is not further used disable the warning. With -Wunused-but-set-parameter=2 additionally uses of parameter in ++ and -- operators don't count as uses. And finally with -Wunused-but-set-parameter=3 additionally uses in parm @= rhs outside of rhs don't count as uses. See the -Wunused-but-set-variable=n option for examples 👀
This -Wunused-but-set-parameter=3 warning is also enabled by -Wunused together with -Wextra.
-Wunused-but-set-variable— same deal: this is the same as-Wunused-but-set-variable=3, and-Wno-unused-but-set-variableis the same as-Wunused-but-set-variable=0.-Wunused-but-set-variable=n— warns whenever a local variable is assigned to but otherwise unused (aside from its declaration). This-Wunused-but-set-variable=3warning is enabled by-Wall.
To suppress this warning use the unused attribute.
-Wunused-but-set-variable=0 disables the warning. With -Wunused-but-set-variable=1 all uses except initialization and left hand side of assignment which is not further used disable the warning. With -Wunused-but-set-variable=2 additionally uses of variable in ++ and -- operators don't count as uses. And finally with -Wunused-but-set-variable=3 additionally uses in parm @= rhs outside of rhs don't count as uses.
This -Wunused-but-set-variable=3 warning is also enabled by -Wunused, which is enabled by -Wall.
void foo (void)
{
int a = 1; // B<-Wunused-variable> warning
int b = 0; // Warning for <n> >= 1
b = 1; b = 2;
int c = 0; // Warning for <n> >= 2
++c; c--; --c; c++;
int d = 0; // Warning for <n> >= 3
d += 4;
int e = 0; // No warning, cast to void
(void) e;
int f = 0; // No warning, f used
int g = f = 5;
(void) g;
int h = 0; // No warning, preincrement result used
int i = ++h;
(void) i;
int j = 0; // No warning, postdecrement result used
int k = j--;
(void) k;
int l = 0; // No warning, l used
int m = l |= 2;
(void) m;
}-Wunused-function— warns whenever a static function is declared but not defined, or a non-inline static function is unused. Enabled by-Wall.-Wunused-label— warns whenever a label is declared but not used. Enabled by-Wall.
To suppress this warning use the unused attribute 🎀
-Wunused-local-typedefs(C, Objective-C, C++ and Objective-C++ only) — warns when a typedef locally defined in a function isn't used. Enabled by-Wall.-Wunused-parameter— warns whenever a function parameter is unused aside from its declaration. This option is not enabled by-Wunusedunless-Wextrais also specified, fyi.
To suppress this warning use the unused attribute.
-Wno-unused-result— do not warn if a caller of a function marked with attributewarn_unused_resultdoesn't use its return value. The default is-Wunused-result.-Wunused-variable— warns whenever a local or static variable is unused aside from its declaration. This option implies-Wunused-const-variable=1for C, but not for C++. Enabled by-Wall.
To suppress this warning use the unused attribute.
-Wunused-const-variable,-Wunused-const-variable=n— warns whenever a constant static variable is unused aside from its declaration.
To suppress this warning use the unused attribute.
-Wunused-const-variable=1— warns about unused static const variables defined in the main compilation unit, but not about static const variables declared in any header included.
-Wunused-const-variable=1 is enabled by either -Wunused-variable or -Wunused for C, but not for C++. In C this declares variable storage, but in C++ it's not an error since const variables take the place of #defines 🔥
-Wunused-const-variable=2— okay so this warning level also catches unused constantstaticvariables chilling in headers (system headers are excluded, bestie). It’s equivalent to the short form-Wunused-const-variable. You have to explicitly request this level in both C and C++ because it might be hard to clean up all the headers included, no cap. 💅-Wunused-value— warns whenever a statement computes a result that is explicitly not used. To suppress this warning, cast the unused expression tovoid. This includes an expression-statement or the left-hand side of a comma expression that contains no side effects. For example, an expression likex[i,j]causes a warning, whilex[(void)i,j]does not. This warning is enabled by-Wall.-Wunused— all the above-Wunusedoptions combined, except those documented as needing to be specified explicitly. To get a warning about an unused function parameter, you must either specify-Wextra -Wunused(note that-Wallimplies-Wunused), or separately specify-Wunused-parameterand/or-Wunused-but-set-parameter. Also:-Wunusedenables only-Wunused-const-variable=1rather than-Wunused-const-variable, and only for C, not C++. 🎀-Wuse-after-free,-Wuse-after-free=n— (C, Objective-C, C++ and Objective-C++ only) warn about uses of pointers to dynamically allocated objects that have been rendered indeterminate by a call to a deallocation function. The warning is enabled at all optimization levels but may yield different results with optimization than without.-Wuse-after-freeis equivalent to-Wuse-after-free=1.-Wuse-after-free=1— at level 1 the warning attempts to diagnose only unconditional uses of pointers made indeterminate by a deallocation call or a successful call torealloc, regardless of whether or not the call resulted in an actual reallocation of memory. This includes double-freecalls as well as uses in arithmetic and relational expressions. Although undefined, uses of indeterminate pointers in equality (or inequality) expressions are not diagnosed at this level.-Wuse-after-free=2— at level 2, in addition to unconditional uses, the warning also diagnoses conditional uses of pointers made indeterminate by a deallocation call. As at level 2, uses in equality (or inequality) expressions are not diagnosed. For example, the second call tofreein the following function is diagnosed at this level:
struct A { int refcount; void *data; };
void release (struct A *p)
{
int refcount = --p->refcount;
free (p);
if (refcount == 0)
free (p->data); // warning: p may be used after free
}-Wuse-after-free=3— at level 3, the warning also diagnoses uses of indeterminate pointers in equality expressions. All uses of indeterminate pointers are undefined, but equality tests sometimes appear after calls toreallocas an attempt to determine whether the call resulted in relocating the object to a different address. They are diagnosed at a separate level to aid gradually transitioning legacy code to safe alternatives. For example, the equality test in the function below is diagnosed at this level:
void adjust_pointers (int**, int);
void grow (int **p, int n)
{
int **q = (int**)realloc (p, n *= 2);
if (q == p)
return;
adjust_pointers ((int**)q, n);
}To avoid the warning at this level, store offsets into allocated memory instead of pointers. This approach obviates needing to adjust the stored pointers after reallocation.
-Wuse-after-free=2 is included in -Wall.
-Wuseless-cast— (C, Objective-C, C++ and Objective-C++ only) warn when an expression is cast to its own type. This warning does not occur when a class object is converted to a non-reference type, as that is a way to create a temporary:
struct S { };
void g (S&&);
void f (S&& arg)
{
g (S(arg)); // make arg prvalue so that it can bind to S&&
}-Wuninitialized— warn if an object with automatic or allocated storage duration is used without having been initialized. In C++, also warn if a non-static reference or non-staticconstmember appears in a class without constructors. In addition, passing a pointer (or in C++, a reference) to an uninitialized object to aconst-qualified argument of a built-in function known to read the object is also diagnosed by this warning. (-Wmaybe-uninitializedis issued for ordinary functions.) If you want to warn about code that uses the uninitialized value of the variable in its own initializer, use the-Winit-selfoption. These warnings occur for individual uninitialized elements of structure, union or array variables as well as for variables that are uninitialized as a whole. They do not occur for variables or elements declaredvolatile. Because these warnings depend on optimization, the exact variables or elements for which there are warnings depend on the precise optimization options and version of GCC used. Note that there may be no warning about a variable that is used only to compute a value that itself is never used, because such computations may be deleted by data flow analysis before the warnings are printed. In C++, this warning also warns about using uninitialized objects in member-initializer-lists. For example, GCC warns aboutbbeing uninitialized in the following snippet:
struct A {
int a;
int b;
A() : a(b) { }
};-Wno-invalid-memory-model— this option controls warnings for invocations of__atomicBuiltins,__syncBuiltins, and the C11 atomic generic functions with a memory consistency argument that is either invalid for the operation or outside the range of values of thememory_orderenumeration. For example, since the__atomic_storeand__atomic_store_nbuilt-ins are only defined for the relaxed, release, and sequentially consistent memory orders, the following code is diagnosed:
void store (int *i)
{
__atomic_store_n (i, 0, memory_order_consume);
}-Winvalid-memory-model is enabled by default.
-Wmaybe-uninitialized— for an object with automatic or allocated storage duration, if there exists a path from the function entry to a use of the object that is initialized, but there exist some other paths for which the object is not initialized, the compiler emits a warning if it cannot prove the uninitialized paths are not executed at run time. In addition, passing a pointer (or in C++, a reference) to an uninitialized object to aconst-qualified function argument is also diagnosed by this warning. (-Wuninitializedis issued for built-in functions known to read the object.) Annotating the function with attributeaccess (none)indicates that the argument isn’t used to access the object and avoids the warning. These warnings are only possible in optimizing compilation, because otherwise GCC does not keep track of the state of variables. On the other hand,-Wmaybe-uninitializedis known not to warn in many situations (false negatives) due to optimizations taking advantage of undefinedness of uninitialized uses like constant propagation. These warnings are made optional because GCC may not be able to determine when the code is correct in spite of appearing to have an error. Here is one example of how this can happen:
{
int x;
switch (y)
{
case 1: x = 1;
break;
case 2: x = 4;
break;
case 3: x = 5;
}
foo (x);
}If the value of y is always 1, 2 or 3, then x is always initialized, but GCC doesn’t know this. To suppress the warning, you need to provide a default case with assert(0) or similar code. This option also warns when a non-volatile automatic variable might be changed by a call to longjmp. The compiler sees only the calls to setjmp. It cannot know where longjmp will be called; in fact, a signal handler could call it at any point in the code. As a result, you may get a warning even when there is in fact no problem because longjmp cannot in fact be called at the place that would cause a problem. Some spurious warnings can be avoided if you declare all the functions you use that never return as noreturn. This warning is enabled by -Wall or -Wextra.
-Wunknown-pragmas— warn when a#pragmadirective is encountered that is not understood by GCC. If this command-line option is used, warnings are even issued for unknown pragmas in system header files. This is not the case if the warnings are only enabled by the-Wallcommand-line option.-Wno-pragmas— do not warn about misuses of pragmas, such as incorrect parameters, invalid syntax, or conflicts between pragmas. See also-Wunknown-pragmas.-Wno-pragma-once-outside-header— do not warn when#pragma onceis used in a file that is not a header file, such as a main file.-Wno-prio-ctor-dtor— do not warn if a priority from 0 to 100 is used for constructor or destructor. The use of constructor and destructor attributes allow you to assign a priority to the constructor/destructor to control its order of execution beforemainis called or after it returns. The priority values must be greater than 100 as the compiler reserves priority values between0--100for the implementation. 🫶-Wstrict-aliasing— okay so this one only wakes up when-fstrict-aliasingis on, iykyk? It yells at code that might break the strict aliasing rules the compiler is using for optimization. Heads up tho: it does not catch every case, it just tries to catch the messy common pitfalls. It's included in-Wall, and it's literally the same thing as-Wstrict-aliasing=3. 💅-Wstrict-aliasing=n— same deal, only active when-fstrict-aliasingis active. Warns about code that might break the strict aliasing rules the compiler uses for optimization. Higher levels = higher accuracy (fewer false positives), but also more effort, kinda like how-Oworks. And yes,-Wstrict-aliasingis equivalent to-Wstrict-aliasing=3.- Level 1 — most aggressive, quick, least accurate 😭. Possibly useful when higher levels don't warn but
-fstrict-aliasingstill breaks the code, because it has very few false negatives. Unfortunately 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 tho), 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 (the default for
-Wstrict-aliasing) — should have very few false positives and few false negatives ✨. Slightly slower than levels 1 or 2 when optimization is enabled. Takes care of the common pun+dereference pattern in the front end:"*(int*)&some_float". If optimization is enabled it also runs in the back end, where it deals with multiple statement cases using flow-sensitive points-to information. Only warns when the converted pointer is dereferenced. Does not warn about incomplete types.
- Level 1 — most aggressive, quick, least accurate 😭. Possibly useful when higher levels don't warn but
-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 bestie: 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 actually happens. So this warning can easily give a false positive: a warning about code that isn't actually a problem. To help focus on important issues, several warning levels are defined. No warnings are issued for the use of undefined signed overflow when estimating how many iterations a loop requires, in particular when determining whether a loop will be executed at all. 💀
-Wstrict-overflow=1— warn about cases that are both questionable and easy to avoid. For example the compiler simplifies"x + 1 > x"to 1. This level of-Wstrict-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, because"abs (INT_MIN)"overflows to"INT_MIN", which is less than zero.-Wstrict-overflow(with no level) is the same as-Wstrict-overflow=2.-Wstrict-overflow=3— also warn about other cases where a comparison is simplified. For example:"x + 1 > 1"is simplified to"x > 0".-Wstrict-overflow=4— also warn about other simplifications not covered by the above cases. For example:"(x * 10) / 5"is simplified to"x * 2".-Wstrict-overflow=5— also warn about cases where the compiler reduces the magnitude of a constant involved in a comparison. For example:"x + 2 > y"is simplified to"x + 1 >= y". This is reported only at the highest warning level because this simplification applies to many comparisons, so this warning level gives a very large number of false positives. 😭
-Wstring-compare— warns for calls tostrcmpandstrncmpwhose result is determined to be either zero or non-zero in tests for such equality, owing to the length of one argument being greater than the size of the array the other argument is stored in (or the bound in the case ofstrncmp). Such calls could be mistakes. For example, the call tostrcmpbelow is diagnosed because its result is necessarily non-zero irrespective of the contents of the arraya. 🙈
extern char a[4];
void f (char *d)
{
strcpy (d, "string");
...
if (0 == strcmp (a, d)) // cannot be true
puts ("a and d are the same");
}-Wstring-compare is enabled by -Wextra.
-Wno-stringop-overflow,-Wstringop-overflow,-Wstringop-overflow=type— warns for code that can be statically determined to cause buffer overflows or memory overruns, like calls tomemcpyandstrcpythat 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. That argument is only meaningful for string functions that operate on character arrays; raw memory functions likememcpyalways use type-zero Object Size Checking.
The option also warns for calls that specify a size in excess of the largest possible object or at most SIZE_MAX / 2 bytes. 🔥
It produces the best results with optimization enabled but can detect a small subset of simple buffer overflows even without optimization in calls to the GCC built-in functions like __builtin_memcpy that correspond to the standard functions. In any case, the option warns about just a subset of buffer overflows detected by the corresponding overflow checking built-ins, such as __builtin___memcpy_chk, which can perform run-time checking if the access cannot be identified as safe at compile time.
For example, the option issues a warning for the strcpy call below because it copies at least 5 characters (the string "blue" including the terminating NUL) into the buffer of size 4:
enum Color { blue, purple, yellow };
const char* f (enum Color clr)
{
static char buf [4];
const char *str;
switch (clr)
{
case blue: str = "blue"; break;
case purple: str = "purple"; break;
case yellow: str = "yellow"; break;
}
return strcpy (buf, str); // warning here
}The effect of this option is not limited to string or memory manipulation functions. In this example, a warning is diagnosed because a 1-element array is passed to a function requiring at least a 4-element array argument:
void f (int[static 4]);
void g (void)
{
int *p = (int *) malloc (1 * sizeof(int));
f (p); // warning here
}Option -Wstringop-overflow=2 is enabled by default. 👑
-Wstringop-overflow,-Wstringop-overflow=1— uses type-zero Object Size Checking to determine the sizes of destination objects. At this setting the option does not warn for writes past the end of subobjects of larger objects accessed by pointers unless the size of the largest surrounding object is known. When the destination may be one of several objects it is assumed to be the largest one of them. On Linux systems, when optimization is enabled at this setting the option warns for the same code as when the_FORTIFY_SOURCEmacro is defined to a non-zero value.-Wstringop-overflow=2— uses type-one Object Size Checking to determine the sizes of destination objects. At this setting the option warns about overflows when writing to members of the largest complete objects whose exact size is known. However, it does not warn for excessive writes to the same members of unknown objects referenced by pointers since they may point to arrays containing unknown numbers of elements. This is the default setting of the option. ✨-Wstringop-overflow=3— uses type-two Object Size Checking to determine the sizes of destination objects. At this setting the option warns about overflowing the smallest object or data member. This is the most restrictive setting of the option that may result in warnings for safe code. 😬-Wstringop-overflow=4— uses type-three Object Size Checking to determine the sizes of destination objects. At this setting the option warns about overflowing any data members, and when the destination is one of several objects it uses the size of the largest of them to decide whether to issue a warning. Similarly to-Wstringop-overflow=3this setting of the option may result in warnings for benign code.
-Wno-stringop-overread— warns for calls to string manipulation functions such asmemchr, orstrcpythat are determined to read past the end of the source sequence. 🫶
Option -Wstringop-overread is enabled by default.
-Wno-stringop-truncation— do not warn for calls to bounded string manipulation functions such asstrncat,strncpy, andstpncpythat may either truncate the copied string or leave the destination unchanged. 💅
In the following example, the call to strncat specifies a bound that is less than the length of the source string. As a result, the copy of the source will be truncated and so the call is diagnosed. To avoid the warning use "bufsize - strlen (buf) - 1)" as the bound.
void append (char *buf, size_t bufsize)
{
strncat (buf, ".txt", 3);
}So anyway, back to strncpy shenanigans 💅 — another iconically cursed example: the following call to strncpy copies to d just the characters before the terminating NUL, and doesn't append the NUL to the end. Assuming the result of strncpy is necessarily a NUL-terminated string is a super common mistake, which is exactly why this call gets diagnosed. If your result isn't supposed to be NUL-terminated, call memcpy instead and dodge the warning entirely ✨
void copy (char *d, const char *s)
{
strncpy (d, s, strlen (s));
}And in this next one, the call to strncpy uses the size of the destination buffer as the bound. If the source string's length is equal to or greater than that size, the copy will not be NUL-terminated 😭 so yeah, diagnosed again. To escape the warning, pass 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);
...
}Okay but if a character array is intentionally storing a sequence of bytes with no terminating NUL, you can annotate that array with attribute nonstring to avoid this warning 🫶. That said — such arrays are not suitable arguments to functions that expect NUL-terminated strings, so GCC will still issue warnings unless it can prove the use is safe, all to help catch accidental misuse 👑
-Wstrict-flex-arrays(C and C++ only) — warns about improper usages of flexible array members, based on the level of thestrict_flex_array (level)attribute attached to the trailing array field of a structure if it's available, otherwise based on the level of the option-fstrict-flex-arrays=level. Heads up:-Wstrict-flex-arraysis only effective when level is greater than 0 ⭐- When
level=1, warnings are issued for a trailing array reference of a structure that has 2 or more elements, if the trailing array is referenced as a flexible array member. - When
level=2, on top oflevel=1, extra warnings are issued for a trailing one-element array reference of a structure if that array is referenced as a flexible array member. - When
level=3, on top oflevel=2, even more warnings fire for a trailing zero-length array reference of a structure if the array is referenced as a flexible array member. - This option is way more effective when
-ftree-vrpis active (that's the default for-O2and above), but some warnings may still get diagnosed even with no optimization at all 💫
- When
-Wsuggest-attribute=attribute-name— warns in cases where adding an attribute might be a glow-up. The *attribute-name*s currently supported are listed right below 👇-Wsuggest-attribute=pure-Wsuggest-attribute=const-Wsuggest-attribute=noreturn-Wmissing-noreturn-Wsuggest-attribute=malloc-Wsuggest-attribute=returns_nonnull- Warns about functions that might be candidates for the
pure,const,noreturn,mallocorreturns_nonnullattributes. 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 and doesn't return abnormally by throwing, callingabort, or trapping 👀. This analysis requires option-fipa-pure-const, which is enabled by default at-Oand higher. Higher optimization levels make the analysis more accurate, slay.
- Warns about functions that might be candidates for the
-Wsuggest-attribute=format-Wmissing-format-attribute- Warns about function pointers that might be candidates for
formatattributes. Ngl, these are only possible candidates, not absolute ones. 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 should also have aformatattribute to avoid the warning 💅 - GCC also warns about function definitions that might be candidates for
formatattributes. Again — only possible candidates. GCC guessesformatattributes might be appropriate for any function that calls something likevprintforvscanf, but that's not always true, and some functions whereformatattributes would be appropriate may not get detected 🙈
- Warns about function pointers that might be candidates for
-Wsuggest-attribute=cold- Warns about functions that might be candidates for the
coldattribute. This one's based on static detection and generally only flags functions that always lead to a call to anothercoldfunction — like wrappers around C++throw, or fatal error reporting functions leading toabort💀
- Warns about functions that might be candidates for the
-Walloc-size— warns about calls to allocation functions decorated with attributealloc_sizethat specify insufficient size for the target type of the pointer the result is assigned to — including calls to the built-in forms ofaligned_alloc,alloca,calloc,malloc, andrealloc🎀-Walloc-zero— warns about calls to allocation functions decorated with attributealloc_sizethat specify zero bytes, including the built-in forms ofaligned_alloc,alloca,calloc,malloc, andrealloc. Why? Because the behavior of these functions when called with a zero size differs between implementations (and forreallocit's literally been deprecated 😭), so relying on it may cause subtle portability bugs and should be avoided, periodt.-Wcalloc-transposed-args— warns about calls to allocation functions decorated with attributealloc_sizethat take two arguments, where thesizeofoperator is used as the earlier size argument and not as the later one. This is a coding style warning 💅. The first argument tocallocis documented as the number of elements in the array, while the second is the size of each element — socalloc (n, sizeof (int))is preferred overcalloc (sizeof (int), n). If havingsizeofin the earlier argument and not the latter is intentional, you can suppress the warning withcalloc (sizeof (struct S) + 0, n)orcalloc (1 * sizeof (struct S), 4), or just usesizeofin the later argument too ✨-Walloc-size-larger-than=byte-size— warns about calls to functions decorated with attributealloc_sizethat try to allocate objects larger than the specified number of bytes, or where the result of the size computation (in an integer type with infinite precision) would exceed the value ofPTRDIFF_MAXon the target.-Walloc-size-larger-than=PTRDIFF_MAXis enabled by default. You can disable these warnings either by specifying a byte-size ofSIZE_MAXor more, or with-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 cap 🔥-Wauto-profile— outputs warnings about auto-profile inconsistencies 👀-Wcannot-profile— warns when profiling instrumentation was requested but couldn't be applied to a certain function 😭-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. Arguments of non-integer types count as unbounded even if they look like they're constrained to the expected range 🙈- For example, a bounded case of
allocacould look like this:
- For example, a bounded case of
void func (size_t n)
{
void *p;
if (n <= 1000)
p = alloca (n);
else
p = malloc (n);
f (p);
}- In the example above, passing
-Walloca-larger-than=1000would not issue a warning, because the call toallocais known to be at most 1000 bytes. But if-Walloca-larger-than=500were passed, the compiler would emit a warning 💅 - Unbounded uses, on the other hand, are uses of
allocawith no controlling predicate constraining its integer argument. Like this:
void func ()
{
void *p = alloca (n);
f (p);
}- If
-Walloca-larger-than=500were passed, the above would trigger a warning — this time because of the lack of bounds checking 🎀 - Note that even seemingly correct code involving signed integers could cause a warning:
void func (signed int n)
{
if (n < 500)
{
p = alloca (n);
f (p);
}
}- In the above example,
ncould be negative, which causes a larger-than-expected argument to be implicitly cast into theallocacall 😭 - This option also warns when
allocais used in a loop. -Walloca-larger-than=PTRDIFF_MAXis enabled by default, but it's usually only effective when-ftree-vrpis active (the default for-O2and above).- See also
-Wvla-larger-than=byte-size👀
-Wno-alloca-larger-than— disables-Walloca-larger-than=warnings. Equivalent to-Walloca-larger-than=SIZE_MAXor larger 🫶-Warith-conversion— do warn about implicit conversions from arithmetic operations even when converting the operands to the same type can't change their values. This affects warnings from-Wconversion,-Wfloat-conversion, and-Wsign-conversion✨
void f (char c, int i)
{
c = c + i; // warns with -Wconversion
c = c + 1; // only warns with -Warith-conversion
}-Warray-bounds,-Warray-bounds=n— warns about out of bounds subscripts or offsets into arrays. Enabled by-Wall. It is more effective when-ftree-vrpis active (the default for-O2and above) but a subset of instances are issued even without optimization.
By default, the trailing array of a structure will be treated as a flexible array member by -Warray-bounds or -Warray-bounds=n if it is declared as either a flexible array member per C99 standard onwards ([]), a GCC zero-length array extension ([0]), or an one-element array ([1]). As a result, out of bounds subscripts or offsets into zero-length arrays or one-element arrays are not warned by default.
You can add the option -fstrict-flex-arrays or -fstrict-flex-arrays=level to control how this option treat trailing array of a structure as a flexible array member:
- when
level<=1, no change to the default behavior. - when
level=2, additional warnings will be issued for out of bounds subscripts or offsets into one-element arrays; - when
level=3, in addition tolevel=2, additional warnings will be issued for out of bounds subscripts or offsets into zero-length arrays. -Warray-bounds=1— this is the default warning level of-Warray-boundsand is enabled by-Wall; higher levels are not, and must be explicitly requested. ✨-Warray-bounds=2— this warning level also warns about the intermediate results of pointer arithmetic that may yield out of bounds values. This warning level may give a larger number of false positives and is deactivated by default.
-Wunterminated-string-initialization(C and Objective-C only) — warns about character arrays initialized as unterminated character sequences with a string literal, unless the declaration being initialized has the"nonstring"attribute. For example:
char arr[3] = "foo"; /* Warning. */
char arr2[3] __attribute__((nonstring)) = "bar"; /* No warning. */This warning is enabled by -Wextra. If -Wc++-compat is enabled, the warning has slightly different wording and warns even if the declaration being initialized has the "nonstring" warning, as in C++ such initializations are an error.
-Warray-compare— warns about equality and relational comparisons between two operands of array type. This comparison was deprecated in C++20. For example:
int arr1[5];
int arr2[5];
bool same = arr1 == arr2;-Warray-compare is enabled by -Wall.
-Warray-parameter,-Warray-parameter=n— warns about redeclarations of functions involving parameters of array or pointer types of inconsistent kinds or forms, and enable the detection of out-of-bounds accesses to such parameters by warnings such as-Warray-bounds.
If the first function declaration uses the array form for a parameter declaration, the bound specified in the array is assumed to be the minimum number of elements expected to be provided in calls to the function and the maximum number of elements accessed by it. Failing to provide arguments of sufficient size or accessing more than the maximum number of elements may be diagnosed by warnings such as -Warray-bounds or -Wstringop-overflow. At level 1, the warning diagnoses inconsistencies involving array parameters declared using the "T[static N]" form.
For example, the warning triggers for the second declaration of "f" because the first one with the keyword "static" specifies that the array argument must have at least four elements, while the second allows an array of any size to be passed to "f".
void f (int[static 4]);
void f (int[]); // warning (inconsistent array form)
void g (void)
{
int *p = (int *)malloc (1 * sizeof (int));
f (p); // warning (array too small)
...
}At level 2 the warning also triggers for redeclarations involving any other inconsistency in array or pointer argument forms denoting array sizes. Pointers and arrays of unspecified bound are considered equivalent and do not trigger a warning.
void g (int*);
void g (int[]); // no warning
void g (int[8]); // warning (inconsistent array bound)-Warray-parameter=2 is included in -Wall. The -Wvla-parameter option triggers warnings for similar inconsistencies involving Variable Length Array arguments.
The short form of the option -Warray-parameter is equivalent to -Warray-parameter=2. The negative form -Wno-array-parameter is equivalent to -Warray-parameter=0.
-Wattribute-alias=n,-Wno-attribute-alias— warns about declarations using the"alias"and similar attributes whose target is incompatible with the type of the alias.-Wattribute-alias=1— the default warning level of the-Wattribute-aliasoption diagnoses incompatibilities between the type of the alias declaration and that of its target. Such incompatibilities are typically indicative of bugs.-Wattribute-alias=2— at this level-Wattribute-aliasalso diagnoses cases where the attributes of the alias declaration are more restrictive than the attributes applied to its target. These mismatches can potentially result in incorrect code generation. In other cases they may be benign and could be resolved simply by adding the missing attribute to the target. For comparison, see the-Wmissing-attributesoption, which controls diagnostics when the alias declaration is less restrictive than the target, rather than more restrictive.
Attributes considered include "alloc_align", "alloc_size", "cold", "const", "hot", "leaf", "malloc", "nonnull", "noreturn", "nothrow", "pure", "returns_nonnull", and "returns_twice". 👑
-Wattribute-alias is equivalent to -Wattribute-alias=1. This is the default. You can disable these warnings with either -Wno-attribute-alias or -Wattribute-alias=0.
-Wbidi-chars=[none|unpaired|any|ucn]— warns about possibly misleading UTF-8 bidirectional control characters in comments, string literals, character constants, and identifiers. Such characters can change left-to-right writing direction into right-to-left (and vice versa), which can cause confusion between the logical order and visual order. This may be dangerous; for instance, it may seem that a piece of code is not commented out, whereas it in fact is. 😭
There are three levels of warning supported by GCC. The default is -Wbidi-chars=unpaired, which warns about improperly terminated bidi contexts. -Wbidi-chars=none turns the warning off. -Wbidi-chars=any warns about any use of bidirectional control characters.
By default, this warning does not warn about UCNs. It is, however, possible to turn on such checking by using -Wbidi-chars=unpaired,ucn or -Wbidi-chars=any,ucn. Using -Wbidi-chars=ucn is valid, and is equivalent to -Wbidi-chars=unpaired,ucn, if no previous -Wbidi-chars=any was specified.
-Wbool-compare— warns about boolean expression compared with an integer value different from"true"/"false". For instance, the following comparison is always false:
int n = 5;
...
if ((n > 1) == 2) { ... }This warning is enabled by -Wall.
-Wbool-operation— warns about suspicious operations on expressions of a boolean type. For instance, bitwise negation of a boolean is very likely a bug in the program. For C, this warning also warns about incrementing or decrementing a boolean, which rarely makes sense. (In C++, decrementing a boolean is always invalid. Incrementing a boolean is invalid in C++17, and deprecated otherwise.)
This warning is enabled by -Wall.
-Wduplicated-branches— warns when an if-else has identical branches. This warning detects cases like:
if (p != NULL)
return 0;
else
return 0;It doesn't warn when both branches contain just a null statement. This warning also warn for conditional operators:
int i = x ? *p : *p;-Wduplicated-cond— warns about duplicated conditions in an if-else-if chain. For instance, warn for the following code:
if (p->q != NULL) { ... }
else if (p->q != NULL) { ... }-Wframe-address— warns when the__builtin_frame_addressor__builtin_return_addressis called with an argument greater than0. Such calls may return indeterminate values or crash the program. The warning is included in-Wall.-Wno-discarded-qualifiers(C and Objective-C only) — do not warn if type qualifiers on pointers are being discarded. Typically, the compiler warns if aconst char *variable is passed to a function that takes achar *parameter. This option can be used to suppress such a warning.-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.-Wno-incompatible-pointer-types(C and Objective-C only) — do not warn when there is a conversion between pointers that have incompatible types. This warning is for cases not covered by-Wno-pointer-sign, which warns for pointer argument passing or assignment with different signedness.
By default, in C99 and later dialects of C, GCC treats this issue as an error. The error can be downgraded to a warning using -fpermissive (along with certain other errors), or for this error alone, with -Wno-error=incompatible-pointer-types.
This warning is upgraded to an error by -pedantic-errors.
-Wno-int-conversion(C and Objective-C only) — do not warn about incompatible integer to pointer and pointer to integer conversions. This warning is about implicit conversions; for explicit conversions the warnings-Wno-int-to-pointer-castand-Wno-pointer-to-int-castmay be used. 🫶
So like, by default in C99 and every later dialect of C, GCC treats this whole situation as an error, bestie. The error can be downgraded to a warning with -fpermissive (along with certain other errors), or — if you just wanna forgive this one specific thing — with -Wno-error=int-conversion. Also heads up: this warning gets upgraded to a full error by -pedantic-errors. 💅
-Wzero-as-null-pointer-constant— Warns when a literal0is used as a null pointer constant. Just0. NotNULL, notnullptr, the number. Iconic mistake energy, honestly.-Wzero-length-bounds— Warns about accesses to elements of zero-length array members that might overlap other members of the same object. Declaring interior zero-length arrays is discouraged because accessing them is undefined — like, genuinely undefined behavior, not cute undefined. 😭
For example, the first two stores in function bad get diagnosed because the array elements overlap the subsequent members b and c. The third store gets diagnosed by -Warray-bounds instead, because it's beyond the bounds of the enclosing object.
struct X { int a[0]; int b, c; };
struct X x;
void bad (void)
{
x.a[0] = 0; // -Wzero-length-bounds
x.a[1] = 1; // -Wzero-length-bounds
x.a[2] = 2; // -Warray-bounds
}This option is enabled by -Warray-bounds, so if you've got that one on, you get this one for free. 🎁
-Wno-div-by-zero— Do not warn about compile-time integer division by zero. And floating-point division by zero? Not warned about at all, because that can legitimately be how you obtain infinities and NaNs. slay, IEEE 754. 🌈-Wsystem-headers— Prints warning messages for constructs found in system header files. Normally warnings from system headers are suppressed, on the assumption that they usually don't indicate real problems and would only make the compiler output harder to read — which, honestly, fair. Using this command-line option tells GCC to emit warnings from system headers as if they occurred in user code. ⚠️ But watch out: using-Walltogether with this option does not warn about unknown pragmas in system headers — for that,-Wunknown-pragmasmust also be used. iykyk.-Wtautological-compare— Warns if a self-comparison always evaluates to true or false. This one catches all sorts of silly mistakes like:
int i = 1;
...
if (i > i) { ... }It also warns about bitwise comparisons that always evaluate to true or false. For instance:
if ((a & 16) == 10) { ... }will always be false. 💀 This warning is enabled by -Wall.
-Wtrailing-whitespace-Wtrailing-whitespace=kind— Warns about trailing whitespace at the end of lines, including inside comments, but *excluding trailing whitespace in raw string literals (they get a pass, they're special like that ✨). Plain-Wtrailing-whitespaceis equivalent to-Wtrailing-whitespace=blanksand warns just about trailing space and horizontal tab characters.-Wtrailing-whitespace=anywarns about those *or trailing form feed or vertical tab characters.-Wno-trailing-whitespaceor-Wtrailing-whitespace=nonedisables the warning, which is the default. This is a coding style warning, so it's purely aesthetic judgment — but we all know trailing whitespace is a crime. 🎀-Wleading-whitespace=kind— Warns about style issues in leading whitespace, but *not about the amount of indentation. Some projects use coding styles where only spaces are used for indentation, others use only tabs, others use zero or more tabs (for multiples of-ftabstop=n) followed by zero or fewer than *nspaces. No warning is emitted on lines which contain solely whitespace (although a-Wtrailing-whitespace=warning might be emitted), and no warnings are emitted inside raw string literals. Warnings are also emitted for leading whitespace inside multi-line comments.-Wleading-whitespace=spaces— warns about leading whitespace other than spaces, for projects that want to indent just by spaces.-Wleading-whitespace=tabs— warns about leading whitespace other than horizontal tabs, for projects that want to indent just by horizontal tabs.-Wleading-whitespace=blanks— warns about leading whitespace other than spaces and horizontal tabs, or about horizontal tab after a space in the leading whitespace, or aboutnor more consecutive spaces in leading whitespace (wherenis the argument of-ftabstop=n, 8 by default).-Wleading-whitespace=none— disables the warning, which is the default.
Also a coding style warning. Pick your fighter. 🥊
-Wtrampolines— Warns about trampolines generated for pointers to nested functions. A trampoline is a small piece of data or code that's created at run time on the stack when the address of a nested function is taken, and it's used to call the nested function indirectly. For some targets it's made up of data only, so no special treatment needed. But for most targets it's made up of code, which means the stack has to be made executable for the program to work properly. 😬 Yeah. Executable stack. That's the whole vibe.-Wfloat-equal— Warns if floating-point values are used in equality comparisons. The idea behind this: sometimes it's convenient (for the programmer) to consider floating-point values as approximations to infinitely precise real numbers. If you're doing that, 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 whether the two values have ranges that overlap — and that's done with the relational operators, so equality comparisons are probably mistaken. Bestie,==on floats is giving "I trust the FPU" and the FPU has betrayed us all. 🙈-Wtraditional(C and Objective-C only) — Warns about certain constructs that behave differently in traditional C and ISO C. Also warns about ISO C constructs that have no traditional C equivalent, and/or problematic constructs that should be avoided. The whole list is a mood:- Macro parameters that appear within string literals in the macro body. In traditional C, macro replacement takes place within string literals, but in ISO C it does not.
- In traditional C, some preprocessor directives did not exist. Traditional preprocessors only considered a line to be a directive if the
#appeared in column 1 on the line. Therefore-Wtraditionalwarns about directives that traditional C understands but ignores because the#does not appear as the first character on the line. It also suggests you hide directives like#pragmanot understood by traditional C by indenting them. Some traditional implementations do not recognize#elif, so this option suggests avoiding it altogether. - A function-like macro that appears without arguments.
- The unary plus operator.
- The
Uinteger constant suffix, or theForLfloating-point constant suffixes. (Traditional C does support theLsuffix on integer constants.) Note: these suffixes appear in macros defined in the system headers of most modern systems, e.g. the_MIN/_MAXmacros in<limits.h>. Use of these macros in user code might normally lead to spurious warnings, however GCC's integrated preprocessor has enough context to avoid warning in these cases. 🧠 - A function declared external in one block and then used after the end of the block.
- A
switchstatement has an operand of typelong. - A non-
staticfunction declaration follows astaticone. This construct is not accepted by some traditional C compilers. - The ISO type of an integer constant has a different width or signedness from its traditional type. This warning is only issued if the base of the constant is ten. I.e. hexadecimal or octal values, which typically represent bit patterns, are not warned about.
- Usage of ISO string concatenation is detected.
- Initialization of automatic aggregates.
- Identifier conflicts with labels. Traditional C lacks a separate namespace for labels.
- Initialization of unions. If the initializer is zero, the warning is omitted. This is done under the assumption that the zero initializer in user code appears conditioned on e.g.
__STDC__to avoid missing initializer warnings and relies on default initialization to zero in the traditional C case. - Conversions by prototypes between fixed/floating-point values and vice versa. The absence of these prototypes when compiling with traditional C causes serious problems. This is a subset of the possible conversion warnings; for the full set use
-Wtraditional-conversion. - Use of ISO C style function definitions. This warning intentionally is not issued for prototype declarations or variadic functions because these ISO C features appear in your code when using libiberty's traditional C compatibility macros,
PARAMSandVPARAMS. This warning is also bypassed for nested functions because that feature is already a GCC extension and thus not relevant to traditional C compatibility.
-Wtraditional-conversion(C and Objective-C only) — Warns if a prototype causes a type conversion that's different from what would happen to the same argument in the absence of a prototype. This includes conversions of fixed point to floating and vice versa, and conversions changing the width or signedness of a fixed-point argument — except when it's the same as the default promotion. Subtle. Petty. Traditional C behavior in a nutshell. 💅-Wdeclaration-after-statement(C and Objective-C only) — Warns when a declaration is found after a statement in a block. This construct, known from C++, was introduced with ISO C99 and is by default allowed in GCC. It is not supported by ISO C90. So if you're mixing your declarations and statements like it's no big deal, this one will side-eye you. 👀 This warning is upgraded to an error by-pedantic-errors.-Wshadow— Warns whenever a local variable or type declaration shadows another variable, parameter, type, class member (in C++), or instance variable (in Objective-C), or whenever a built-in function is shadowed. Note that in C++, the compiler warns if a local variable shadows an explicit typedef, but not if it shadows astruct/class/enum. If this warning is enabled, it includes all instances of local shadowing too — which means-Wno-shadow=localand-Wno-shadow=compatible-localare ignored when-Wshadowis used. Same as-Wshadow=global. No cap, shadowing abwith anotherbis highkey chaotic. 🌸-Wno-shadow-ivar(Objective-C only) — bestie, this one tells the compiler to not warn when a local variable goes and shadows an instance variable inside an Objective-C method. No drama, no warning, we move on. 🎀-Wshadow=global— warns for any shadowing at all. Literally the same thing as plain-Wshadow. Same vibe, different name. ✨-Wshadow=local— warns when a local variable shadows another local variable or a parameter. So like, when one local steps on another local's turf. 👀-Wshadow=compatible-local— warns when a local variable shadows another local variable or a parameter whose type is compatible with the type of the shadowing variable. In C++, "type compatible" here means the type of the shadowing variable can be converted to the type of the shadowed variable. The whole reason this flag exists (on top of-Wshadow=local) is the idea that when a local shadows another of incompatible type, it's almost definitely intentional — not a bug, not a typo. Exhibit A, bestie:
for (SomeIterator i = SomeObj.begin(); i != SomeObj.end(); ++i)
{
for (int i = 0; i < N; ++i)
{
...
}
...
}Since those two i variables up there have incompatible types, enabling only -Wshadow=compatible-local emits no warning. And because their types are incompatible, if a programmer accidentally uses one in place of the other, type checking is expected to catch it and emit an error or warning. So using this flag instead of -Wshadow=local can possibly reduce the number of warnings triggered by intentional shadowing. Heads up though — this also means shadowing const char *i by char *i does not emit a warning. 🙈
This warning is also enabled by -Wshadow=local.
-Wlarger-than=byte-size — warns whenever an object is defined whose size 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.
Also warns for calls to bounded functions like memchr or strnlen that specify a bound greater than the largest possible object, which is PTRDIFF_MAX bytes by default. These warnings can only be disabled by -Wno-larger-than. 💅
-Wno-larger-than— disables-Wlarger-than=warnings. This option is equivalent to-Wlarger-than=SIZE_MAXor larger. Poof, gone. 🦋-Wframe-larger-than=byte-size — warns if the size of a function frame exceeds byte-size. And okay, important caveat: the computation done to determine the stack frame size is approximate and not conservative. The actual requirements may be somewhat greater than byte-size even if you do not get a warning. Plus, any space allocated viaalloca, variable-length arrays, or related constructs is not included by the compiler when determining whether or not to issue a warning. So like, don't @ me if it's bigger than it said. 😭
-Wframe-larger-than=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-frame-larger-than.
-Wno-frame-larger-than— disables-Wframe-larger-than=warnings. Equivalent to-Wframe-larger-than=SIZE_MAXor larger. ✨-Wfree-nonheap-object— warns when you're attempting to deallocate an object that either wasn't allocated on the heap, or was allocated using a pointer that was not returned from a prior call to the corresponding allocation function. For example: because the call tostpcpyreturns a pointer to the terminating nul character and not to the beginning of the object, thatfreecall down there gets diagnosed. Caught in 4K 💀:
void f (char *p)
{
p = stpcpy (p, "abc");
// ...
free (p); // warning
}-Wfree-nonheap-object is included in -Wall.
-Wstack-usage=byte-size — warns if the stack usage of a function might exceed byte-size. Unlike the frame one, the computation done here to determine stack usage is conservative. And any space allocated viaalloca, variable-length arrays, or related constructs is included by the compiler when deciding whether or not to warn. Iconic of it honestly. 👑
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. Equivalent to-Wstack-usage=SIZE_MAXor larger. 🌸-Wunsafe-loop-optimizations— warns if the loop cannot be optimized because the compiler cannot assume anything about the bounds of the loop indices. And with-funsafe-loop-optimizations, warns if the compiler does make such assumptions. Either way the compiler is being messy about it. 🔥-Wno-pedantic-ms-format(MinGW targets only) — when used together with-Wformatand-pedanticwithout GNU extensions, this option disables the warnings about the non-ISOprintf/scanfformat width specifiersI32,I64, andIused on Windows targets, which depend on the MS runtime. Niche but we respect it. 🫶-Wpointer-arith— warns about anything that depends on the "size of" a function type or ofvoid. GNU C assigns these types a size of1, for convenience in calculations withvoid *pointers and pointers to functions. In C++, it also warns when an arithmetic operation involvesNULL. This warning is also enabled by-Wpedantic.
This warning is upgraded to an error by -pedantic-errors. So like, it's serious-serious now. ⚠️
-Wno-pointer-compare— do not warn if a pointer is compared with a zero character constant. That usually means the pointer was meant to be dereferenced. Example:
const char *p = foo ();
if (p == '\0')
return 42;Note that the code above is invalid in C++11. 🙈
This warning is enabled by default.
-Wno-tsan— disables warnings about unsupported features in ThreadSanitizer.
ThreadSanitizer does not support std::atomic_thread_fence and can report false positives. So yeah, heads up. 💀
-Wtype-limits— warns if a comparison is always true or always false due to the limited range of the data type, but does not warn for constant expressions. For example, it warns if an unsigned variable is compared against zero with<or>=. This warning is also enabled by-Wextra. 💅-Wabsolute-value(C and Objective-C only) — warns for calls to standard functions that compute the absolute value of an argument when a more appropriate standard function is available. For example, callingabs(3.14)triggers the warning because the appropriate function to call to compute the absolute value of adoubleargument isfabs. This option also triggers warnings when the argument in a call to such a function has an unsigned type. The warning can be suppressed with an explicit type cast, and it's also enabled by-Wextra. Math but make it correct 😌✨-Wcomment-Wcomments— warns whenever a comment-start sequence/*appears in a/*comment, or whenever a backslash-newline appears in a//comment. This warning is enabled by-Wall. Nested-comment jump scare fr fr. 😭-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 still get the other -Wall warnings, use -trigraphs -Wall -Wno-trigraphs. Big brain combo. 🧠🎀
-Wundef— warns if an undefined identifier is evaluated in an#ifdirective. Such identifiers are replaced with zero. Lowkey the sneakiest bug source, so slay that this exists. 👑-Wexpansion-to-defined— warns wheneverdefinedis 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 counts as 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. We don't snitch on those. 🙅♀️
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
#endifSneaky but valid 💅
-Wno-endif-labels— do not warn whenever an#elseor an#endifis followed by text. This sometimes happens in older programs with code of the form:
#if FOO
...
#else FOO
...
#endif FOOThe second and third FOO should be in comments. This warning is on by default. Vintage code energy 🕰️
-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. Like bestie… what are you doing. 😭-Wc90-c99-compat(C and Objective-C only) — warns about features not present in ISO C90, but present in ISO C99. For instance, it warns about use of variable length arrays,long longtype,booltype, compound literals, designated initializers, and so on. This option is independent of the standards mode. Warnings are disabled in the expression that follows__extension__. Time-travel compatibility check ✨-Wc99-c11-compat(C and Objective-C only) — warns about features not present in ISO C99, but present in ISO C11. For instance, it warns about use of anonymous structures and unions,_Atomictype qualifier,_Thread_localstorage-class specifier,_Alignasspecifier,Alignofoperator,_Generickeyword, and so on. This option is independent of the standards mode. Warnings are disabled in the expression that follows__extension__. Same energy, newer era 🦋-Wc11-c23-compat,-Wc11-c2x-compat(C and Objective-C only) — bestie, this one's your "you're writing C11 but dipping into C23" detector 💅. It yells about features that aren't in ISO C11 but are in ISO C23 — think omitting the string in_Static_assert, using[[]]syntax for attributes, decimal floating-point types, and so on. It's independent of the standards mode, and warnings are disabled in the expression that follows__extension__. Also heads up: the name-Wc11-c2x-compatis deprecated. When you're not compiling in C23 mode, these warnings get upgraded to errors by-pedantic-errors. 😭-Wc23-c2y-compat(C and Objective-C only) — (yes, the original lists this one twice, and honestly? iconic of it). It warns about features not present in ISO C23 but present in ISO C2Y — for example, warn about_Genericselecting with a type name instead of an expression. This option is independent of the standards mode, and warnings are disabled in the expression that follows__extension__. When you're not compiling in C2Y mode, these warnings are upgraded to errors by-pedantic-errors.-Wc++-compat(C and Objective-C only) — warns about ISO C constructs that fall outside the common subset of ISO C and ISO C++, like when you request an implicit conversion fromvoid *to a pointer to a non-voidtype. C++ girlies know the struggle 🫶.-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. 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. Enabled by-Wall.-Wc++20-compat(C++ and Objective-C++ only) — warns about C++ constructs whose meaning differs between ISO C++ 2017 and ISO C++ 2020. Enabled by-Wall.-Wc++26-compat(C++ and Objective-C++ only) — warns about C++ constructs whose meaning differs between ISO C++ 2023 and the upcoming ISO C++ 2026. Enabled by-Wall. Future vibes only 🔮.-Wno-c++11-extensions(C++ and Objective-C++ only) — do not warn about C++11 constructs in code being compiled using an older C++ standard. Even without this option, some C++11 constructs will only be diagnosed if-Wpedanticis used.-Wno-c++14-extensions(C++ and Objective-C++ only) — do not warn about C++14 constructs in code being compiled using an older C++ standard. Even without this option, some C++14 constructs will only be diagnosed if-Wpedanticis used.-Wno-c++17-extensions(C++ and Objective-C++ only) — do not warn about C++17 constructs in code being compiled using an older C++ standard. Even without this option, some C++17 constructs will only be diagnosed if-Wpedanticis used.-Wno-c++20-extensions(C++ and Objective-C++ only) — do not warn about C++20 constructs in code being compiled using an older C++ standard. Even without this option, some C++20 constructs will only be diagnosed if-Wpedanticis used.-Wno-c++23-extensions(C++ and Objective-C++ only) — do not warn about C++23 constructs in code being compiled using an older C++ standard. Even without this option, some C++23 constructs will only be diagnosed if-Wpedanticis used.-Wno-c++26-extensions(C++ and Objective-C++ only) — do not warn about C++26 constructs in code being compiled using an older C++ standard. Even without this option, some C++26 constructs will only be diagnosed if-Wpedanticis used.-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 *. It also warns when you make a cast that introduces a type qualifier in an unsafe way — for example, castingchar **toconst char **is unsafe, like in this example:
/* p is char ** value. */
const char **q = (const char **) p;
/* Assignment of readonly string to const char * is OK. */
*q = "string";
/* Now char** pointer points to read-only memory. */
**p = 'b';-Wcast-align— 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.
Warn whenever a pointer is cast such that the required alignment of the target is increased. For example, warn if a char * is cast to an int * regardless of the target machine. (Yep, the manpage says it twice — we're keeping both, no cap 💀.)
-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. 🔥-Wcast-user-defined— warns when a cast to reference type does not involve a user-defined conversion that the programmer might expect to be called.
struct A { operator const int&(); } a;
auto r = (int&)a; // warningThis warning is enabled by default.
-Wwrite-strings— when compiling C, gives 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's just a nuisance. This is why we did not make-Wallrequest these warnings. 😌
When compiling C++, it warns about the deprecated conversion from string literals to char *. This warning is enabled by default for C++ programs. It's upgraded to an error by -pedantic-errors in C++11 mode or later.
-Wclobbered— warns for variables that might be changed bylongjmporvfork. This warning is also enabled by-Wextra. 😳-Wno-complain-wrong-lang— by default, language front ends complain when a command-line option is valid, but not applicable to that front end. This may be disabled with-Wno-complain-wrong-lang, which is mostly useful when invoking a single compiler driver for multiple source files written in different languages, for example:
$ g++ -fno-rtti a.cc b.f90The driver g++ invokes the C++ front end to compile a.cc and the Fortran front end to compile b.f90. The latter front end diagnoses f951: Warning: command-line option '-fno-rtti' is valid for C++/D/ObjC++ but not for Fortran, which may be disabled with -Wno-complain-wrong-lang.
This option can also be used to disable warnings like cc1plus: note: CTF debug info requested, but not supported for 'GNU C++20' frontend produced by -gctf or -gsctf for unsupported languages. 🫶
-Wcompare-distinct-pointer-types(C and Objective-C only) — warns if pointers of distinct types are compared without a cast. This warning is enabled by default.-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++, it also warns for confusing overload resolution for user-defined conversions; and conversions that never use a type conversion operator: conversions to void, the same type, a base class or a reference to them. Warnings about conversions between signed and unsigned integers are disabled by default in C++ unless -Wsign-conversion is explicitly enabled.
Warnings about conversion from arithmetic on a small type back to that type are only given with -Warith-conversion.
-Wconversion— (yes, listed again in the original, and this one's a whole different warning 👀) warns for conversions to or fromtime_tthat are not able to carry the whole 64bit value range.-Wdangling-else— warns about constructions where there may be confusion as to 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). This is often not what the programmer expected, as illustrated in the above example by the indentation the programmer chose. When there's the potential for this confusion, GCC issues a warning when this flag is specified. To eliminate the warning, add explicit braces around the innermost if statement so there is no way the else can belong to the enclosing if. The resulting code looks like this:
{
if (a)
{
if (b)
foo ();
else
bar ();
}
}This warning is enabled by -Wparentheses. 🎀
-Wdangling-pointer,-Wdangling-pointer=n— warns about uses of pointers (or C++ references) to objects with automatic storage duration after their lifetime has ended 💀. This includes local variables declared in nested blocks, compound literals and other unnamed temporary objects. In addition, it warns about storing the address of such objects in escaped pointers. The warning is enabled at all optimization levels but may yield different results with optimization than without.-Wdangling-pointer=1— level 1 is the shy version: it only calls out unconditional uses of dangling pointers. like, you literally couldn't avoid it, bestie.-Wdangling-pointer=2— level 2 steps it up, no cap: on top of the unconditional uses, it also snitches on conditional uses of dangling pointers. ✨
so the short form -Wdangling-pointer is exactly equivalent to -Wdangling-pointer=2, while -Wno-dangling-pointer and -Wdangling-pointer=0 do the same glow-down and just disable the warnings entirely. also -Wdangling-pointer=2 is included in -Wall. 💅
this next example triggers the warning at level 1 — the address of the unnamed temporary is unconditionally referenced outside of its scope. very much giving "I left my charger at home":
char f (char c1, char c2, char c3)
{
char *p;
{
p = (char[]) { c1, c2, c3 };
}
// warning: using dangling pointer 'p' to an unnamed temporary
return *p;
}and in the following function, the store of the address of the local variable "x" in the escaped pointer *p triggers the warning at level 1:
void g (int **p)
{
int x = 7;
// warning: storing the address of local variable 'x' in '*p'
*p = &x;
}last one 🫶 — here the array a is out of scope when the pointer s is used. since the code that sets "s" is conditional, the warning triggers at level 2:
extern void frob (const char *);
void h (char *s)
{
if (!s)
{
char a[12] = "tmpname";
s = a;
}
// warning: dangling pointer 's' to 'a' may be used
frob (s);
}-Wdate-time— warns you when the macros"__TIME__","__DATE__"or"__TIMESTAMP__"show up, because they might prevent bitwise-identical reproducible compilations. literally the enemy of reproducible builds 😭-Wempty-body— warns if there's an empty body in an"if","else"or"do while"statement. this warning is also enabled by-Wextra. like bestie, you pressed send on a message with nothing in it 📭-Wno-endif-labels— do not warn about stray tokens after"#else"and"#endif". mind your business mode: on.-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. apples to oranges, fr fr 🍓-Wenum-conversion— warns when a value of enumerated type gets implicitly converted to a different enumerated type. this warning is enabled by-Wextrain C. identity theft! 🙈-Wenum-int-mismatch(C and Objective-C only) — warns about mismatches between an enumerated type and an integer type in declarations. for example:
enum E { l = -1, z = 0, g = 1 };
int foo(void);
enum E foo(void);in C, an enumerated type is compatible with "char", a signed integer type, or an unsigned integer type. however, since the choice of the underlying type of an enumerated type is implementation-defined, such mismatches may cause portability issues. in C++, such mismatches are an error. in C, this warning is enabled by -Wall and -Wc++-compat.
-Wjump-misses-init(C, Objective-C only) — warns if a"goto"statement or a"switch"statement jumps forward across the initialization of a variable, or jumps backward to a label after the variable has been initialized. this only warns about variables that are initialized when they are declared. this warning is only supported for C and Objective-C; in C++ this sort of branch is an error in any case. icon behavior tbh 💀
-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. in C++, this warning is also enabled by-Wall. in C, it is also enabled by-Wextra. the classic "wait, which one is negative" moment 🌀-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.-Wflex-array-member-not-at-end(C and C++ only) — warns when a structure containing a C99 flexible array member as the last field is not at the end of another structure. this warning warns e.g. about:
struct flex { int length; char data[]; };
struct mid_flex { int m; struct flex flex_data; int n; };the flexible array said "I'm the last one, periodt" and then this struct was like "no 💅".
-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. your 3.14159265 is now 3.14, hope you're happy 😭-Wno-scalar-storage-order— do not warn on suspicious constructs involving reverse scalar storage order.-Wsizeof-array-div— warns about divisions of two sizeof operators when the first one is applied to an array and the divisor does not equal the size of the array element. in such a case, the computation will not yield the number of elements in the array, which is likely what the user intended. this warning warns e.g. about:
int fn ()
{
int arr[10];
return sizeof (arr) / sizeof (short);
}this warning is enabled by -Wall. ma'am that does not divide out, and that's on math 🧮✨
-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. about"sizeof (ptr) / sizeof (ptr[0])"if"ptr"is not an array, but a pointer. this warning is enabled by-Wall. plot twist: it was never an array 🎀-Wsizeof-pointer-memaccess— warns for suspicious length parameters to certain string and memory built-in functions if the argument uses"sizeof". this warning triggers for example for"memset (ptr, 0, sizeof (ptr));"if"ptr"is not an array, but a pointer, and suggests a possible fix, or about"memcpy (&foo, ptr, sizeof (&foo));".
-Wsizeof-pointer-memaccess also warns about calls to bounded string copy functions like "strncat" or "strncpy" that specify as the bound a "sizeof" expression of the source array. for example, in the following function the call to "strncat" specifies the size of the source string as the bound. that is almost certainly a mistake and so the call is diagnosed.
void make_file (const char *name)
{
char path[PATH_MAX];
strncpy (path, name, sizeof path - 1);
strncat (path, ".text", sizeof ".text");
...
}the -Wsizeof-pointer-memaccess option is enabled by -Wall. caught in 4k 📸
-Wno-sizeof-array-argument— do not warn when the"sizeof"operator is applied to a parameter that is declared as an array in a function definition. this warning is enabled by default for C and C++ programs.-Wmemset-elt-size— warns for suspicious calls to the"memset"built-in function, if the first argument references an array, and the third argument is a number equal to the number of elements, but not equal to the size of the array in memory. this indicates that the user has omitted a multiplication by the element size. this warning is enabled by-Wall. you forgot to multiply, bestie, and the compiler noticed 🔥-Wmemset-transposed-args— warns for suspicious calls to the"memset"built-in function where the second argument is not zero and the third argument is zero. for example, the call"memset (buf, sizeof buf, 0)"is diagnosed because"memset (buf, 0, sizeof buf)"was meant instead. the diagnostic is only emitted if the third argument is a literal zero. otherwise, if it is an expression that is folded to zero, or a cast of zero to some type, it is far less likely that the arguments have been mistakenly transposed and no warning is emitted. this warning is enabled by-Wall. you flipped them like a pancake 🥞-Waddress— warns about suspicious uses of address expressions. these include comparing the address of a function or a declared object to the null pointer constant, such as in:
void f (void);
void g (void)
{
if (!f) // warning: expression evaluates to false
abort ();
}comparisons of a pointer to a string literal, such as in:
void f (const char *x)
{
if (x == "abc") // warning: expression evaluates to false
puts ("equal");
}and tests of the results of pointer addition or subtraction for equality to null, such as in:
void f (const int *p, int i)
{
return p + i == NULL;
}such uses typically indicate a programmer error: the address of most functions and objects necessarily evaluates to true (the exception are weak symbols), so their use in a conditional might indicate missing parentheses in a function call or a missing dereference in an array expression. the subset of the warning for object pointers can be suppressed by casting the pointer operand to an integer type such as "intptr_t" or "uintptr_t". comparisons against string literals result in unspecified behavior and are not portable, and suggest the intent was to call "strcmp". the warning is suppressed if the suspicious expression is the result of macro expansion. the -Waddress warning is enabled by -Wall. bestie the address of a function is never null, I promise you 🙏
-Wno-address-of-packed-member— do not warn when the address of packed member of struct or union is taken, which usually results in an unaligned pointer value. this is enabled by default. alignment found crying in the corner 😭-Wlogical-op— warns about suspicious uses of logical operators in expressions. this includes using logical operators in contexts where a bitwise 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) { ... }like girl, 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) { ... }and yeah, that's the vibe check failing 😭 you can shush the warning by wrapping the LHS (left-hand side) in parens, like:
if ((!a) > 1) { ... }This warning is switched on by -Wall.
-Waggregate-return— warns you if any function that returns a structure or union gets defined or called. (And in languages where you're allowed to return an array? yeah, that gets a warning too. no escape 🫠)-Wno-aggressive-loop-optimizations— says "please don't warn me" when the compiler spots undefined behavior in a loop that has a constant number of iterations. Just so you know:-Waggressive-loop-optimizationsis on by default. 👑-Wno-attributes— don't warn when an unexpected"__attribute__"shows up — think unrecognized attributes, function attributes slapped onto variables, that whole situation. Important tho: this does not stop errors for incorrect use of supported attributes.
Warnings about ill-formed uses of standard attributes get upgraded to errors by -pedantic-errors. ⚠️
Also, with -Wno-attributes=, you can suppress warnings about unknown scoped attributes (in C++11 and C23). Example: -Wno-attributes=vendor::attr disables the warning for this declaration:
[[vendor::attr]] void f();And you can kill warnings for every attribute in a namespace by using -Wno-attributes=vendor:: — which stops the warning for both of these declarations:
[[vendor::safe]] void f();
[[vendor::unsafe]] void f2();Heads up bestie:-Wno-attributes=does not imply-Wno-attributes. They're different things, fr fr.
-Wno-builtin-declaration-mismatch— warns when a built-in function is declared with an incompatible signature, or declared as a non-function, or when a built-in function declared with a type that doesn't include a prototype gets called with arguments whose promoted types don't match what the function expects. When-Wextrais specified it also warns when a built-in function that takes arguments is declared without a prototype. The-Wbuiltin-declaration-mismatchwarning is enabled by default. Wanna dodge it? include the appropriate header so the prototypes of built-in functions are in scope. ✨
Like, the call to "memset" below gets diagnosed by this 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 gets diagnosed too:
extern void* memset ();
void f (void *d)
{
memset (d, '\0', 32);
}-Wno-builtin-macro-redefined— no warnings when certain built-in macros get redefined. This suppresses warnings for redefinition of"__TIMESTAMP__","__TIME__","__DATE__","__FILE__", and"__BASE_FILE__". 🎀-Wkeyword-macro— warns if a keyword is defined as a macro or undefined. For C++, identifiers with special meaning or standard attribute identifiers are diagnosed as well. This warning is enabled by default for C++26 if"-Wpedantic"is given, and emits a pedwarn in that case. 💅-Wfree-labels(C and Objective-C only) — warns if a label is applied to a non-statement, or appears at the end of a compound statement. Such labels are allowed by C23 and later dialects of C, and are available as a GCC extension in all other dialects.
This warning is also enabled by -Wc11-c23-compat. And it gets turned into an error if you're building for a C version before C23, courtesy of -pedantic-errors. 🔥
-Wheader-guard— ok this one's a whole saga, stay with me 🫶. It warns if a valid preprocessor header multiple inclusion guard has a"#define"directive right after the"#ifndef"or"#if !defined"directive for the multiple inclusion guard, which defines a different macro from the guard macro with a similar name, the actual multiple inclusion guard macro isn't defined at the corresponding"#ifndef"directive at the end of the header, and the"#define"directive defines an object-like macro with empty definition. In that case it's often just a misspelled guard name — either in the"#ifndef"or"#if !defined"directive, or in the subsequent"#define"directive. This warning is enabled by-Wall. 🕵️-Wstrict-prototypes(C and Objective-C only) — warns if a function is declared or defined without specifying the argument types. (An old-style function definition is permitted without a warning if it's preceded by a declaration that specifies the argument types — so like, you can still get away with it if you do it in the right order 👀)-Wold-style-declaration(C and Objective-C only) — warns for obsolescent usages, according to the C Standard, inside a declaration. For example, it warns if storage-class specifiers like"static"aren't the first things in a declaration. This warning is also enabled by-Wextra.-Wold-style-definition(C and Objective-C only) — warns if an old-style function definition is used. You get a warning even if there's a previous prototype. A definition using()is not considered an old-style definition in C23 mode, because it's equivalent to(void)in that case — but it is considered an old-style definition for older standards. 🌈-Wmultiple-parameter-fwd-decl-lists(C and Objective-C only) — warns if more than one list of forward declarations of parameters appears in a function prototype. This warning is also enabled by-Wextra.-Wdeprecated-non-prototype(C and Objective-C only) — warns if a function declared with an empty parameter list()is called with one or more arguments, or if a function definition with one or more parameters shows up after such a declaration. Both cases are errors in C23 and later dialects of C. 😱
This warning is also enabled by -Wc11-c23-compat.
-Wmissing-parameter-name(C and Objective-C only) — warns if a function definition omits a parameter name, specifying only its type. Fun fact: you can use this to document that a parameter is unused in the definition. It's part of C23 and later dialects of C, and available as a GCC extension in all other dialects. ✨
This warning is also enabled by -Wc11-c23-compat. It's turned into an error if you're building for a C version before C23, by -pedantic-errors.
-Wmissing-parameter-type(C and Objective-C only) — catches a function parameter declared without a type specifier in K&R-style functions:
void foo(bar) { }This warning is also enabled by -Wextra. 💀
-Wno-declaration-missing-parameter-type(C and Objective-C only) — don't warn if a function declaration contains a parameter name without a type. Because like, such function declarations don't provide a function prototype and they prevent most type checking in function calls. 🙈
This warning is enabled by default. In C99 and later dialects of C, it's treated as an error. The error can be downgraded to a warning using -fpermissive (along with certain other errors), or for this error alone, with -Wno-error=declaration-missing-parameter-type.
This warning is upgraded to an error by -pedantic-errors.
-Wmissing-prototypes(C and Objective-C only) — warns if a global function is defined without a previous prototype declaration. Yeah, this warning fires even if the definition itself provides a prototype. Use this option to detect global functions that don't have a matching prototype declaration in a header file. 🎯
This option is not valid for C++, because all function declarations provide prototypes and a non-matching declaration declares an overload rather than conflicting with an earlier declaration. Use -Wmissing-declarations to detect missing declarations in C++.
-Wmissing-variable-declarations(C and Objective-C only) — warns if a global variable is defined without a previous declaration. Use this option to detect global variables that don't have a matching extern declaration in a header file. 🦋-Wmissing-declarations— warns if a global function is defined without a previous declaration. Does it even if the definition itself provides a prototype. Use this option to detect global functions that aren't declared in header files. In C, no warnings are issued for functions with previous non-prototype declarations — use-Wmissing-prototypesto detect missing prototypes. In C++, no warnings are issued for function templates, or for inline functions, or for functions in anonymous namespaces. 💅-Wmissing-field-initializers— warns if a structure's initializer has some fields missing. Like, this code triggers it, because"x.h"is implicitly zero:
struct s { int f, g, h; };
struct s x = { 3, 4 };In C this option does not warn about designated initializers, so this modification doesn't trigger a warning:
struct s { int f, g, h; };
struct s x = { .f = 3, .g = 4 };In C this option also doesn't warn about the universal zero initializer { 0 }:
struct s { int f, g, h; };
struct s x = { 0 };Likewise, in C++ this option doesn't warn about the empty { } initializer, for example:
struct s { int f, g, h; };
s x = { };This warning is included in -Wextra. Wanna get the other -Wextra warnings without this one? use -Wextra -Wno-missing-field-initializers. 🎀
-Wno-missing-requires— by default, the compiler warns about a concept-id appearing as a C++20 simple-requirement:
bool satisfied = requires { C<T> };Here satisfied will be true if C<T> is a valid expression, which it is for all T. Presumably the user meant to write:
bool satisfied = requires { requires C<T> };so that satisfied is only true if concept C is satisfied for type T. 😭
This warning can be disabled with -Wno-missing-requires.
-Wno-missing-template-keyword— the member access tokens.,->and::must be followed by the"template"keyword if the parent object is dependent and the member being named is a template.
template <class X>
void DoStuff (X x)
{
x.template DoSomeOtherStuff<X>(); // Good.
x.DoMoreStuff<X>(); // Warning, x is dependent.
}In rare cases it's possible to get false positives. To silence this, wrap the expression in parentheses. For example, the following is treated as a template, even where m and N are integers:
void NotATemplate (my_class t)
{
int N = 5; bool test = t.m < N > (0); // Treated as a template.
test = (t.m < N) > (0); // Same meaning, but not treated as a template.
}This warning can be disabled with -Wno-missing-template-keyword.
-Wno-multichar— ok so this one tells GCC to not complain if you've got a multicharacter constant like'FOOF'in your code. Usually those are just a typo, bestie — they have implementation-defined values, so like... don't put them in portable code, periodt. 🙈-Wnormalized=[none|id|nfc|nfkc]— this is the unicode-vibes checker ✨ In ISO C and ISO C++, two identifiers are different if they're different sequences of characters. But sometimes when you use characters outside basic ASCII, you can end up with two different character sequences that look exactly the same. 😭 To avoid the confusion, the ISO 10646 standard lays out some normalization rules — apply them and two sequences that look the same get turned into the same sequence. GCC can warn you if your identifiers aren't normalized, and this option controls that warning.
There are four levels of warning that GCC supports:
- The default is
-Wnormalized=nfc, which warns about any identifier that's not in the ISO 10646 "C" normalized form, NFC. NFC is the recommended form for most uses. It's equivalent to just-Wnormalized. - Unfortunately there are some characters allowed in identifiers by ISO C and ISO C++ that, once turned into NFC, are not allowed in identifiers. Like, there's literally no way to use these symbols in portable ISO C or C++ and have all your identifiers in NFC.
-Wnormalized=idsuppresses the warning for these characters. Fingers crossed that future versions of the standards involved fix this — which is why this option is not the default. 🫶 - You can switch the warning off for all characters by writing
-Wnormalized=noneor-Wno-normalized. Only do this if you're using some other normalization scheme (like "D"), because otherwise you can easily create bugs that are literally impossible to see. 💀 - Some characters in ISO 10646 have distinct meanings but look identical in some fonts or display methodologies — especially once formatting has been applied. For instance,
"\u207F", "SUPERSCRIPT LATIN SMALL LETTER N", displays just like a regular "n" that's been placed in a superscript. ISO 10646 defines the NFKC normalization scheme to convert all these into a standard form too, and GCC warns if your code isn't in NFKC when you use-Wnormalized=nfkc. Ngl, this warning is comparable to warning about every identifier containing the letter O because it might get confused with the digit 0 — so it's not the default — but it may 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 thewarningattribute. By default this warning is enabled.-Wno-attribute-warningdisables the warning, or-Wno-error=attribute-warningdisables the error when you compile with the-Werrorflag. 🎀-Wno-deprecated— don't warn about usage of deprecated features. In C++, explicitly specifying-Wdeprecatedalso enables warnings about some features that are deprecated in later language standards — specifically-Wcomma-subscript,-Wvolatile,-Wdeprecated-enum-float-conversion,-Wdeprecated-enum-enum-conversion,-Wdeprecated-literal-operator, and-Wdeprecated-variadic-comma-omission. 👑-Wno-deprecated-declarations— don't warn about uses of functions, variables, or types marked as deprecated by using thedeprecatedattribute.-Wno-deprecated-openmp— don't warn about deprecated OpenMP code. 🧚-Wno-overflow— don't warn about compile-time overflow in constant expressions.-Wno-odr— warn about One Definition Rule violations during link-time optimization. Enabled by default. (Yes bestie, the name is a whole jump-scare, I know. 😭)-Wopenacc-parallelism— warn about potentially suboptimal choices related to OpenACC parallelism.-Wno-openmp— warn about suspicious OpenMP code. 🔍-Wopenmp-simd— warn if the vectorizer cost model overrides the OpenMP simd directive set by the user. The-fsimd-cost-model=unlimitedoption can be used to relax the cost model. 🌈-Woverride-init(C and Objective-C only) — warn if an initialized field without side effects is overridden when using designated initializers. This warning is included in-Wextra. To get the other-Wextrawarnings without this one, use-Wextra -Wno-override-init. 💅-Wno-override-init-side-effects(C and Objective-C only) — do not warn if an initialized field with side effects is overridden when using designated initializers. This warning is enabled by default.-Wpacked— warn if a structure is given thepackedattribute, but the attribute has no effect on the layout or size of the structure. Those structures may be mis-aligned for basically no benefit, which is giving pointless chaos. For instance, in this code, the variablef.xinstruct baris misaligned even thoughstruct bardoesn't itself have thepackedattribute:
struct foo {
int x;
char a, b, c, d;
} __attribute__((packed));
struct bar {
char z;
struct foo f;
};-Wnopacked-bitfield-compat— ok storytime 📖 the 4.1, 4.2 and 4.3 series of GCC ignore thepackedattribute on bit-fields of typechar. This was fixed in GCC 4.4, but the change can lead to differences in the structure layout. GCC lets you know when the offset of such a field has 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) — warn if a structure field with explicitly specified alignment in a packed struct or union is misaligned. For example, a warning will be issued onstruct S, like,warning: alignment 1 of 'struct S' is less than 8, in this code:
struct __attribute__ ((aligned (8))) S8 { char a[8]; };
struct __attribute__ ((packed)) S {
struct S8 s8;
};This warning is enabled by -Wall. ⭐
-Wpadded— warn if padding is included in a structure, either to align an element of the structure or to align the whole structure. Sometimes when this happens you can literally just rearrange the fields of the structure to reduce the padding and make the whole thing smaller. slay 🎀-Wredundant-decls— warn if anything is declared more than once in the same scope, even in cases where the multiple declaration is valid and changes nothing. (Redundant? in my codebase? it's more likely than you think 💀)-Wrestrict— warn when an object referenced by arestrict-qualified parameter (or, in C++, a__restrict-qualified parameter) is aliased by another argument, or when copies between such objects overlap. For example, the call to thestrcpyfunction below attempts to truncate the string by replacing its initial characters with the last four. But because the call writes the terminating NUL intoa[4], the copies overlap and the call is diagnosed. 🚨
void foo (void)
{
char a[] = "abcd1234";
strcpy (a, a + 4);
...
}The -Wrestrict option detects some instances of simple overlap even without optimization, but it works best at -O2 and above. It's included in -Wall.
-Wnested-externs(C and Objective-C only) — warn if anexterndeclaration is encountered within a function. (bestie... why is it in there...) 🙈-Winline— warn if a function that's declared as inline can't be inlined. Even with this option, tho, the compiler doesn't warn about failures to inline functions declared in system headers.
The compiler uses a whole variety of heuristics to decide whether or not to inline a function. For example, it takes into account how big the function being inlined is and how much inlining has already been done in the current function. So like, seemingly insignificant changes in the source program can make -Winline warnings appear or disappear. it's giving fragile 💅
-Winterference-size-Wno-interference-size— warn about questionable uses of the C++17constexprvariablesstd::hardware_destructive_interference_sizeandstd::hardware_constructive_interference_size.
These variables are meant to be used for controlling class layout, to avoid false sharing in concurrent code:
struct independent_fields {
alignas(std::hardware_destructive_interference_size)
std::atomic<int> one;
alignas(std::hardware_destructive_interference_size)
std::atomic<int> two;
};Here one and two are intended to be far enough apart that stores to one won't require accesses to the other to reload the cache line. iconic behavior tbh ✨
By default, these variables get values based on the current -mtune option — typically the L1 cache line size for the particular target CPU, sometimes a range if you're tuning for a generic target. So all translation units that depend on ABI compatibility for the use of these variables must be compiled with the same -mtune (or -mcpu). 🫶
If ABI stability is important — like if the use is in a header for a library — you should probably not use the hardware interference size variables at all. ngl, just don't.
These warnings are enabled by default. If you're confident that your use of these variables doesn't affect ABI outside a single build of your project, you can turn the warning off with -Wno-interference-size.
-Wint-in-bool-context— warn for suspicious use of integer values where boolean values are expected. Like conditional expressions (?:) using non-boolean integer constants in boolean context, likeif (a <= b ? 2 : 3). Or left shifting of signed integers in boolean context, likefor (a = 0; 1 << a; a++);. Likewise for all kinds of multiplications regardless of the data type. This warning is enabled by-Wall. 🔥-Wno-int-to-pointer-cast— shuts up the warning you get when you cast an integer to a pointer type of a different size. ok but in C++ specifically, casting to a pointer type of smaller size is straight-up an error, not a warning 💀.Wint-to-pointer-castis enabled by default, bestie.-Wno-pointer-to-int-cast(C and Objective-C only) — kills the warning for casts going the other way: pointer → integer of a different size. only C and Objective-C get this one, iykyk.-Winvalid-pch— warn if a precompiled header turns up in the search path but literally cannot be used. it's giving "found it but it's not serving" 😭.-Winvalid-utf8— warn when an invalid UTF-8 character shows up. it's on by default for C++23 if you're using-finput-charset=UTF-8, and it gets upgraded to an error with-pedantic-errors.-Wno-unicode— don't diagnose invalid forms of delimited or named escape sequences that get treated as separate tokens.Wunicodeis enabled by default, so this is the off switch ✨.-Wlong-long— warn if thelong longtype is used at all. this one gets enabled by either-Wpedanticor-Wtraditionalin ISO C90 and C++98 modes. to inhibit the warning messages, use-Wno-long-long.
this warning is upgraded to an error by -pedantic-errors. so like, behave 🫶.
-Wvariadic-macros— warn if variadic macros are used in ISO C90 mode, or if the GNU alternate syntax is used in ISO C99 mode. enabled by either-Wpedanticor-Wtraditional. to inhibit the warning messages, use-Wno-variadic-macros.-Wno-varargs— don't warn on questionable usage of the macros that handle variable arguments, likeva_start. these warnings are enabled by default, so this is you going "no <em>thought</em> actually" 🙈.-Wvector-operation-performance— warn if a vector operation is not implemented via the SIMD capabilities of the architecture. mainly useful for performance tuning, bestie. a vector operation can be implemented:- "piecewise" — the scalar operation is performed on every vector element
- "in parallel" — the vector operation is implemented using scalars of wider type, which normally is more performance efficient
- "as a single scalar" — the vector fits into a scalar type
-Wvla— warn if a variable-length array is used in the code.-Wno-vlaprevents the-Wpedanticwarning of the variable-length array.
this warning is upgraded to an error by -pedantic-errors 🔥.
-Wvla-larger-than=_byte-size_ — with this option, the compiler warns for declarations of variable-length arrays whose size is either unbounded, or bounded by an argument that lets the array size exceed byte-size bytes. it's similar to how-Walloca-larger-than=byte-size works, but with variable-length arrays.
ngl though: GCC may optimize small variable-length arrays of a known value into plain arrays, so this warning may not get triggered for such arrays. so don't be shocked if it's quiet 👑
-Wvla-larger-than=PTRDIFF_MAX is enabled by default but is typically only effective when -ftree-vrp is active (default for -O2 and above).
see also -Walloca-larger-than=byte-size.
-Wno-vla-larger-than— disables the-Wvla-larger-than=warnings. the option is equivalent to-Wvla-larger-than=SIZE_MAXor larger. so basically "i don't wanna hear it" 💅.-Wvla-parameter— warns about redeclarations of functions involving arguments of Variable Length Array types of inconsistent kinds or forms, and enables the detection of out-of-bounds accesses to such parameters by warnings such as-Warray-bounds. two-for-one slay.
here's the tea: if the first function declaration uses the VLA form, the bound specified in the array is assumed to be the minimum number of elements expected to be provided in calls to the function, and the maximum number of elements accessed by it. failing to provide arguments of sufficient size, or accessing more than the maximum number of elements, may be diagnosed.
for example, the warning triggers for the following redeclarations because the first one allows an array of any size to be passed to f while the second one specifies that the array argument must have at least n elements. additionally, calling f with the associated VLA bound parameter in excess of the actual VLA bound triggers a warning as well.
void f (int n, int[n]);
// warning: argument 2 previously declared as a VLA
void f (int, int[]);
void g (int n)
{
if (n > 4)
return;
int a[n];
// warning: access to a by f may be out of bounds
f (sizeof a, a);
...
}-Wvla-parameter is included in -Wall. the -Warray-parameter option triggers warnings for similar problems involving ordinary array arguments.
-Wvolatile-register-var— warn if a register variable is declared volatile. the volatile modifier does not inhibit all optimizations that may eliminate reads and/or writes to register variables. this warning is enabled by-Wall.-Wno-xor-used-as-pow(C, C++, Objective-C and Objective-C++ only) — disables warnings about uses of^, the exclusive or operator, where it appears the code meant exponentiation. specifically, the warning occurs when the left-hand side is the decimal constant2or10and the right-hand side is also a decimal constant.
because in C and C++, ^ means exclusive or — whereas in some other languages (e.g. TeX and some versions of BASIC) it means exponentiation. classic mixup 😭
you can silence this warning by converting one of the operands to hexadecimal, as well as by compiling with -Wno-xor-used-as-pow.
-Wdisabled-optimization— warn if a requested optimization pass is disabled. this does not generally indicate that there's 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 giving "i could but at what cost" 💀.-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's implied by-Walland by-Wpedantic, which can be disabled with-Wno-pointer-sign.
this warning is upgraded to an error by -pedantic-errors.
-Wstack-protector— this option is only active when-fstack-protectoris active. it warns about functions that are not protected against stack smashing. so it's the snitch telling on the unprotected ones 🫣.-Woverlength-strings— warn about string constants longer than the "minimum maximum" length specified in the C standard. modern compilers generally allow string constants that are much longer than the standard's minimum limit, but very portable programs should avoid using longer strings.
the limit applies after string constant concatenation, and does not count the trailing NUL. in C90 the limit was 509 characters; in C99 it was raised to 4095. C++98 does not specify a normative minimum maximum, so we do not diagnose overlength strings in C++.
this option is implied by -Wpedantic, and can be disabled with -Wno-overlength-strings.
-Wunsuffixed-float-constants(C and Objective-C only) — issue a warning for any floating constant that does not have a suffix. when used together with-Wsystem-headersit warns about such constants in system header files too. this can be useful when preparing code to use with theFLOAT_CONST_DECIMAL64pragma from the decimal floating-point extension to C99.-Wno-lto-type-mismatch— during link-time optimization, don't warn about type mismatches in global declarations from different compilation units. requires-fltoto be enabled. enabled by default ✨.-Wno-designated-init(C and Objective-C only) — suppress warnings when a positional initializer is used to initialize a structure that has been marked with thedesignated_initattribute.
✨ Options That Control Static Analysis ✨
-fanalyzer— this option enables a static analysis of program flow which looks for "interesting" interprocedural paths through the code, and issues warnings for problems found on them. 👑
this analysis is much more expensive than other GCC warnings. like, expensive expensive.
in technical terms, it performs coverage-guided symbolic execution of the code being compiled. it is neither sound nor complete: it can have false positives and false negatives. it is a bug-finding tool, rather than a tool for proving program correctness. important distinction, bestie 🫶
the analyzer is only suitable for use on C code in this release.
enabling this option effectively enables the following warnings:
-Wanalyzer-allocation-size -Wanalyzer-deref-before-check
-Wanalyzer-double-fclose -Wanalyzer-double-free
-Wanalyzer-exposure-through-output-file
-Wanalyzer-exposure-through-uninit-copy
-Wanalyzer-fd-access-mode-mismatch -Wanalyzer-fd-double-close
-Wanalyzer-fd-leak -Wanalyzer-fd-phase-mismatch
-Wanalyzer-fd-type-mismatch -Wanalyzer-fd-use-after-close
-Wanalyzer-fd-use-without-check -Wanalyzer-file-leak
-Wanalyzer-free-of-non-heap -Wanalyzer-imprecise-fp-arithmetic
-Wanalyzer-infinite-loop -Wanalyzer-infinite-recursion
-Wanalyzer-jump-through-null -Wanalyzer-malloc-leak
-Wanalyzer-mismatching-deallocation -Wanalyzer-null-argument
-Wanalyzer-null-dereference -Wanalyzer-out-of-bounds
-Wanalyzer-overlapping-buffers -Wanalyzer-possible-null-argument
-Wanalyzer-possible-null-dereference -Wanalyzer-putenv-of-auto-var
-Wanalyzer-shift-count-negative -Wanalyzer-shift-count-overflow
-Wanalyzer-stale-setjmp-buffer -Wanalyzer-tainted-allocation-size
-Wanalyzer-tainted-array-index -Wanalyzer-tainted-assertion
-Wanalyzer-tainted-divisor -Wanalyzer-tainted-offset
-Wanalyzer-tainted-size -Wanalyzer-throw-of-unexpected-type
-Wanalyzer-undefined-behavior-ptrdiff
-Wanalyzer-undefined-behavior-strtok
-Wanalyzer-unsafe-call-within-signal-handler
-Wanalyzer-use-after-free
-Wanalyzer-use-of-pointer-in-stale-stack-frame
-Wanalyzer-use-of-uninitialized-value
-Wanalyzer-va-arg-type-mismatch -Wanalyzer-va-list-exhausted
-Wanalyzer-va-list-leak -Wanalyzer-va-list-use-after-va-end
-Wanalyzer-write-to-const -Wanalyzer-write-to-string-literalreal talk: this option is only available if GCC was configured with analyzer support enabled. so if it's not there, it's not you, it's the build 💅
-Wanalyzer-symbol-too-complex— if-fanalyzeris enabled, the analyzer uses various heuristics to attempt to track the state of memory, but these can be defeated by sufficiently complicated code.
Okay so bestie, real talk: by default the analyzer just... silently gives up tracking the values of your expressions if they get too big for its little internal limit, and falls back to an imprecise representation for those expressions. Like it's not gonna throw hands, it's just gonna squint and go "eh, close enough". The -Wanalyzer-symbol-too-complex option warns you when this happens. 💅
-Wanalyzer-too-complex— if-fanalyzeris enabled, the analyzer busts out various heuristics to try to explore the control flow and data flow in your program, but ngl, sufficiently complicated code can absolutely defeat them.
By default, the analysis silently stops if the code is too complicated for the analyzer to fully explore and it reaches an internal limit. The -Wanalyzer-too-complex option warns if this occurs. So if your analyzer is being suspiciously quiet, this is your vibe check. 👀
-Wno-analyzer-allocation-size— this warning requires-fanalyzer, which enables it; to disable it, use-Wno-analyzer-allocation-size.
This diagnostic warns for paths through the code in which a pointer to a buffer is assigned to point at a buffer with a size that is not a multiple of sizeof (*pointer). Math crimes!! 🚨
See CWE-131: Incorrect Calculation of Buffer Size (https://cwe.mitre.org/data/definitions/131.html).
-Wno-analyzer-deref-before-check— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-deref-before-checkto disable it.
This diagnostic warns for paths through the code in which a pointer is checked for NULL after it has already been dereferenced, suggesting that the pointer could have been NULL. It's giving "locking the door after the burglar already left". Such cases suggest that the check for NULL is either redundant, or that it needs to be moved to before the pointer is dereferenced.
This diagnostic also considers values passed to a function argument marked with __attribute__((nonnull)) as requiring a non-NULL value, and thus will complain if such values are checked for NULL after returning from such a function call.
Heads up bestie: this diagnostic is unlikely to be reported when any level of optimization is enabled, as GCC's optimization logic will typically consider such checks for NULL as being redundant, and optimize them away before the analyzer "sees" them. Hence optimization should be disabled when attempting to trigger this diagnostic. iykyk ✨
-Wno-analyzer-double-fclose— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-double-fcloseto disable it.
This diagnostic warns for paths through the code in which a FILE * can have fclose called on it more than once. once is iconic, twice is a crime 🎀
See CWE-1341: Multiple Releases of Same Resource or Handle (https://cwe.mitre.org/data/definitions/1341.html).
-Wno-analyzer-double-free— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-double-freeto disable it.
This diagnostic warns for paths through the code in which a pointer can have a deallocator called on it more than once, either free, or a deallocator referenced by attribute malloc.
See CWE-415: Double Free (https://cwe.mitre.org/data/definitions/415.html). 💀 free() said "no more, we're done" and you called again anyway.
-Wno-analyzer-exposure-through-output-file— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-exposure-through-output-fileto disable it.
This diagnostic warns for paths through the code in which a security-sensitive value is written to an output file (such as writing a password to a log file). NOT the password in the log file, bestie 😭 that's giving main character energy in the worst way.
See CWE-532: Information Exposure Through Log Files (https://cwe.mitre.org/data/definitions/532.html).
-Wanalyzer-exposure-through-uninit-copy— this warning requires both-fanalyzerand the use of a plugin to specify a function that copies across a "trust boundary". Use-Wno-analyzer-exposure-through-uninit-copyto disable it.
This diagnostic warns for "infoleaks" — paths through the code in which uninitialized values are copied across a security boundary (such as code within an OS kernel that copies a partially-initialized struct on the stack to user space). We are literally leaking the vibes. 🙈
See CWE-200: Exposure of Sensitive Information to an Unauthorized Actor (https://cwe.mitre.org/data/definitions/200.html).
-Wno-analyzer-fd-access-mode-mismatch— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-fd-access-mode-mismatchto disable it.
This diagnostic warns for paths through code in which a read on a write-only file descriptor is attempted, or vice versa. girl. girl. it's write-only. what are you doing.
This diagnostic also warns for code paths in a which a function with attribute fd_arg_read (N) is called with a file descriptor opened with O_WRONLY at referenced argument N or a function with attribute fd_arg_write (N) is called with a file descriptor opened with O_RDONLY at referenced argument N.
-Wno-analyzer-fd-double-close— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-fd-double-closeto disable it.
This diagnostic warns for paths through code in which a file descriptor can be closed more than once. close it once and let it rest, queen 💅
See CWE-1341: Multiple Releases of Same Resource or Handle (https://cwe.mitre.org/data/definitions/1341.html).
-Wno-analyzer-fd-leak— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-fd-leakto disable it.
This diagnostic warns for paths through code in which an open file descriptor is leaked. it's giving "left the tap running and walked out of the house". 🚰
See CWE-775: Missing Release of File Descriptor or Handle after Effective Lifetime (https://cwe.mitre.org/data/definitions/775.html).
-Wno-analyzer-fd-phase-mismatch— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-fd-phase-mismatchto disable it.
This diagnostic warns for paths through code in which an operation is attempted in the wrong phase of a file descriptor's lifetime. For example, it will warn on attempts to call accept on a stream socket that has not yet had listen successfully called on it. you cannot skip steps, babe, this is not a montage ⭐
See CWE-666: Operation on Resource in Wrong Phase of Lifetime (https://cwe.mitre.org/data/definitions/666.html).
-Wno-analyzer-fd-type-mismatch— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-fd-type-mismatchto disable it.
This diagnostic warns for paths through code in which an operation is attempted on the wrong type of file descriptor. For example, it will warn on attempts to use socket operations on a file descriptor obtained via open, or when attempting to use a stream socket operation on a datagram socket. wrong genre of fd entirely, no cap 🦋
-Wno-analyzer-fd-use-after-close— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-fd-use-after-closeto disable it.
This diagnostic warns for paths through code in which a read or write is called on a closed file descriptor. it's CLOSED. the relationship is OVER. 💀
This diagnostic also warns for paths through code in which a function with attribute fd_arg (N) or fd_arg_read (N) or fd_arg_write (N) is called with a closed file descriptor at referenced argument N.
-Wno-analyzer-fd-use-without-check— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-fd-use-without-checkto disable it.
This diagnostic warns for paths through code in which a file descriptor is used without being checked for validity. trust issues, but make it C 🙈
This diagnostic also warns for paths through code in which a function with attribute fd_arg (N) or fd_arg_read (N) or fd_arg_write (N) is called with a file descriptor, at referenced argument N, without being checked for validity.
-Wno-analyzer-file-leak— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-file-leakto disable it.
This diagnostic warns for paths through the code in which a <stdio.h> FILE * stream object is leaked.
See CWE-775: Missing Release of File Descriptor or Handle after Effective Lifetime (https://cwe.mitre.org/data/definitions/775.html).
-Wno-analyzer-free-of-non-heap— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-free-of-non-heapto disable it.
This diagnostic warns for paths through the code in which free is called on a non-heap pointer (e.g. an on-stack buffer, or a global). free() does NOT do returns, bestie. no receipts, no refunds.
See CWE-590: Free of Memory not on the Heap (https://cwe.mitre.org/data/definitions/590.html).
-Wno-analyzer-imprecise-fp-arithmetic— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-imprecise-fp-arithmeticto disable it.
This diagnostic warns for paths through the code in which floating-point arithmetic is used in locations where precise computation is needed. This diagnostic only warns on use of floating-point operands inside the calculation of an allocation size at the moment. (yes, at the moment — stay tuned ✨)
-Wno-analyzer-infinite-loop— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-infinite-loopto disable it.
This diagnostics warns for paths through the code which appear to lead to an infinite loop. we love a loop, we do NOT love this loop 🌀
Specifically, the analyzer will issue this warning when it "sees" a loop in which:
- no externally-visible work could be being done within the loop
- there is no way to escape from the loop
- the analyzer is sufficiently confident about the program state throughout the loop to know that the above are true
One way for this warning to be emitted is when there is an execution path through a loop for which taking the path on one iteration implies that the same path will be taken on all subsequent iterations.
For example, consider:
while (1)
{
char opcode = *cpu_state.pc;
switch (opcode)
{
case OPCODE_FOO:
handle_opcode_foo (&cpu_state);
break;
case OPCODE_BAR:
handle_opcode_bar (&cpu_state);
break;
}
}The analyzer will complain for the above case because if opcode ever matches none of the cases, the switch will follow the implicit default case, making the body of the loop be a "no-op" with cpu_state.pc unchanged, and thus using the same value of opcode on all subseqent iterations, leading to an infinite loop. iconic fail, honestly 😭
See CWE-835: Loop with Unreachable Exit Condition ('Infinite Loop') (https://cwe.mitre.org/data/definitions/835.html).
-Wno-analyzer-infinite-recursion— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-infinite-recursionto disable it.
This diagnostic is here to warn you about paths through the code that look like they lead to infinite recursion 💀
So here's how it works, bestie: when the analyzer literally sees a recursive call, it compares the state of memory at the entry to the new frame with the state at the entry to the previous frame of that same function on the stack. If nothing in memory appears to be changing, it issues the warning. But if it spots any changes to parameters or globals, it assumes the recursion will terminate and it suppresses the warning. That's the whole vibe check 🫶
Heads up: this diagnostic is likely to miss cases of infinite recursion that get converted to iteration by the optimizer before the analyzer "sees" them. So optimization should be disabled when you're trying to trigger this diagnostic. No cap.
Compare with -Winfinite-recursion, which gives you a similar diagnostic but is implemented in a totally different way. Siblings, not twins 💅
See CWE-674: Uncontrolled Recursion ("https://cwe.mitre.org/data/definitions/674.html").
-Wno-analyzer-jump-through-null— this warning requires-fanalyzer, which is what enables it; use-Wno-analyzer-jump-through-nullto disable it.
This diagnostic warns you about paths through the code where a NULL function pointer gets called. Yeah. That one. 😭
-Wno-analyzer-malloc-leak— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-malloc-leakto disable it.
This diagnostic warns about paths through the code where a pointer allocated via an allocator is leaked: either malloc, or a function marked with attribute malloc. Your memory said bye and never came back 🦋
See CWE-401: Missing Release of Memory after Effective Lifetime ("https://cwe.mitre.org/data/definitions/401.html").
-Wno-analyzer-mismatching-deallocation— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-mismatching-deallocationto disable it.
This diagnostic warns about paths through the code where the wrong deallocation function is called on a pointer value, based on which function was used to allocate that pointer value. The diagnostic will warn about mismatches between free, scalar delete and vector delete[], and those marked as allocator/deallocator pairs using attribute malloc. It's giving "you used the wrong key on the wrong lock" energy 🔥
See CWE-762: Mismatched Memory Management Routines ("https://cwe.mitre.org/data/definitions/762.html").
-Wno-analyzer-out-of-bounds— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-out-of-boundsto disable it.
This diagnostic warns about paths through the code where a buffer is definitely read or written out-of-bounds. It applies for cases where the analyzer is able to determine a constant offset, and for accesses past the end of a buffer, also with a constant capacity. Further, the diagnostic does limited checking for accesses past the end when the offset as well as the capacity is symbolic. Bestie went out of bounds, and it's not cute 🙈
See CWE-119: Improper Restriction of Operations within the Bounds of a Memory Buffer ("https://cwe.mitre.org/data/definitions/119.html").
For cases where the analyzer is able, it will emit a text art diagram visualizing the spatial relationship between the memory region that the analyzer predicts would be accessed, versus the range of memory that's valid to access: whether they overlap, are touching, are close or far apart; which one is before or after in memory, the relative sizes involved, the direction of the access (read vs write), and, in some cases, the values of data involved. Honestly kind of iconic of it. You can suppress this diagram using -fdiagnostics-text-art-charset=none.
-Wno-analyzer-overlapping-buffers— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-overlapping-buffersto disable it.
This diagnostic warns about paths through the code where overlapping buffers are passed to an API for which the behavior on such buffers is undefined.
Specifically, the diagnostic fires on calls to the following functions:
memcpystrcatstrcpy
for cases where the buffers are known to overlap. Personal space, please ✨
-Wno-analyzer-possible-null-argument— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-possible-null-argumentto disable it.
This diagnostic warns about paths through the code where a possibly-NULL value is passed to a function argument marked with __attribute__((nonnull)) as requiring a non-NULL value.
See CWE-690: Unchecked Return Value to NULL Pointer Dereference ("https://cwe.mitre.org/data/definitions/690.html").
-Wno-analyzer-possible-null-dereference— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-possible-null-dereferenceto disable it.
This diagnostic warns about paths through the code where a possibly-NULL value is dereferenced. Possibly. It's the suspense for me 😭
See CWE-690: Unchecked Return Value to NULL Pointer Dereference ("https://cwe.mitre.org/data/definitions/690.html").
-Wno-analyzer-null-argument— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-null-argumentto disable it.
This diagnostic warns about paths through the code where a value known to be NULL is passed to a function argument marked with __attribute__((nonnull)) as requiring a non-NULL value. No "possibly" about it, this one's confirmed 💀
See CWE-476: NULL Pointer Dereference ("https://cwe.mitre.org/data/definitions/476.html").
-Wno-analyzer-null-dereference— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-null-dereferenceto disable it.
This diagnostic warns about paths through the code where a value known to be NULL is dereferenced. Periodt.
See CWE-476: NULL Pointer Dereference ("https://cwe.mitre.org/data/definitions/476.html").
-Wno-analyzer-putenv-of-auto-var— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-putenv-of-auto-varto disable it.
This diagnostic warns about paths through the code where a call to putenv is passed a pointer to an automatic variable or an on-stack buffer. That stack frame is leaving, and it's taking your pointer with it 🧚
See POS34-C. Do not call putenv() with a pointer to an automatic variable as the argument ("https://wiki.sei.cmu.edu/confluence/x/6NYxBQ").
-Wno-analyzer-shift-count-negative— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-shift-count-negativeto disable it.
This diagnostic warns about paths through the code where a shift is attempted with a negative count. It's analogous to the -Wshift-count-negative diagnostic implemented in the C/C++ front ends, but it's implemented based on analyzing interprocedural paths, rather than merely parsing the syntax tree. However — and this is the part to remember — the analyzer does not prioritize detection of such paths, so false negatives are more likely relative to other warnings. Lowkey shy about catching them 🙈
-Wno-analyzer-shift-count-overflow— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-shift-count-overflowto disable it.
This diagnostic warns about paths through the code where a shift is attempted with a count greater than or equal to the precision of the operand's type. It's analogous to the -Wshift-count-overflow diagnostic implemented in the C/C++ front ends, but it's implemented based on analyzing interprocedural paths, rather than merely parsing the syntax tree. However, the analyzer does not prioritize detection of such paths, so false negatives are more likely relative to other warnings. Same disclaimer, same energy 💅
-Wno-analyzer-stale-setjmp-buffer— this warning requires-fanalyzer, which enables it; use-Wno-analyzer-stale-setjmp-bufferto disable it.
This diagnostic warns about paths through the code where longjmp is called to rewind to a jmp_buf relating to a setjmp call in a function that has already returned.
So when setjmp is called on a jmp_buf to record a rewind location, it records the stack frame. That stack frame becomes invalid when the function containing the setjmp call returns. Attempting to rewind to it via longjmp would reference a stack frame that no longer exists, and likely lead to a crash (or worse). And bestie, "or worse" is doing a LOT of heavy lifting there 😭
-Wno-analyzer-tainted-allocation-size— this warning requires-fanalyzerwhich enables it; use-Wno-analyzer-tainted-allocation-sizeto disable it.
This diagnostic warns about paths through the code where a value that could be under an attacker's control is used as the size of an allocation without being sanitized — so an attacker could inject an excessively large allocation and potentially cause a denial of service attack. Ma'am, that number is not yours 🔥
See CWE-789: Memory Allocation with Excessive Size Value ("https://cwe.mitre.org/data/definitions/789.html").
-Wno-analyzer-tainted-assertion— this warning requires-fanalyzerwhich enables it; use-Wno-analyzer-tainted-assertionto disable it.
This diagnostic warns about paths through the code where a value that could be under an attacker's control is used as part of a condition without being first sanitized, and that condition guards a call to a function marked with attribute noreturn (such as the function __builtin_unreachable). Such functions typically indicate abnormal termination of the program, like assertion failure handlers. For example:
assert (some_tainted_value < SOME_LIMIT);In such cases:
- when assertion-checking is enabled: an attacker could trigger a denial of service by injecting an assertion failure
- when assertion-checking is disabled, such as by defining
NDEBUG, an attacker could inject data that subverts the process, since it presumably violates a precondition that's being assumed by the code.
Note that when assertion-checking is disabled, the assertions are typically removed by the preprocessor before the analyzer has a chance to "see" them, so this diagnostic can only generate warnings on builds in which assertion-checking is enabled. Gone before the analyzer even got a look 💔
For the purpose of this warning, any function marked with attribute noreturn is considered as a possible assertion failure handler, including __builtin_unreachable. Note that these functions are sometimes removed by the optimizer before the analyzer "sees" them. Hence optimization should be disabled when attempting to trigger this diagnostic. Same rule as before: turn the optimizer off if you want the receipts.
See CWE-617: Reachable Assertion ("https://cwe.mitre.org/data/definitions/617.html").
The warning can also report problematic constructions such as
switch (some_tainted_value) {
case 0:
/* [...etc; various valid cases omitted...] */
break;
default:
__builtin_unreachable (); /* BUG: attacker can trigger this */
}...even though, strictly speaking, that's not an assertion failure. 💀 Yeah bestie, the analyzer is still gonna clock it. Feral behavior honestly.
-Wno-analyzer-tainted-array-index— this warning needs-fanalyzerturned on (that's what enables it, babes); use-Wno-analyzer-tainted-array-indexto switch it off. This diagnostic clocks paths through the code where a value that could be under an attacker's control gets used as the index of an array access without being sanitized — which means an attacker could inject an out-of-bounds access. 😭 See CWE-129: Improper Validation of Array Index (https://cwe.mitre.org/data/definitions/129.html).-Wno-analyzer-tainted-divisor— needs-fanalyzer(it's the enabler, periodt); use-Wno-analyzer-tainted-divisorto disable. This diagnostic warns for paths where a value potentially under an attacker's control is used as the divisor in a division or modulus operation without being sanitized — so an attacker could inject a division-by-zero. See CWE-369: Divide By Zero (https://cwe.mitre.org/data/definitions/369.html).-Wno-analyzer-tainted-offset— requires-fanalyzer, which enables it; use-Wno-analyzer-tainted-offsetto turn it off. This one warns for paths where an attacker-controllable value is used as a pointer offset without sanitization — attacker could inject an out-of-bounds access. 😬 See CWE-823: Use of Out-of-range Pointer Offset (https://cwe.mitre.org/data/definitions/823.html).-Wno-analyzer-tainted-size— requires-fanalyzerwhich enables it; use-Wno-analyzer-tainted-sizeto disable. This diagnostic warns for paths where an attacker-controllable value is used as the size of an operation such asmemsetwithout being sanitized — attacker could inject an out-of-bounds access. See CWE-129: Improper Validation of Array Index (https://cwe.mitre.org/data/definitions/129.html).-Wno-analyzer-throw-of-unexpected-type— requires-fanalyzerwhich enables it; use-Wno-analyzer-throw-of-unexpected-typeto disable. Heads up: dynamic exception specifications are only available in C++14 and earlier, so iykyk. 🫶 This diagnostic warns for paths where an exception is thrown from a function with a dynamic exception specification and the exception doesn't comply with that specification.-Wno-analyzer-undefined-behavior-ptrdiff— requires-fanalyzer, which enables it; use-Wno-analyzer-undefined-behavior-ptrdiffto disable. This diagnostic warns for paths where a pointer subtraction happens but the pointers refer to different chunks of memory. That code is relying on undefined behavior, bestie — pointer subtraction is only defined when both pointers point within (or just after) the same array. 🚨 See CWE-469: Use of Pointer Subtraction to Determine Size (https://cwe.mitre.org/data/definitions/469.html).-Wno-analyzer-undefined-behavior-strtok— requires-fanalyzer, which enables it; use-Wno-analyzer-undefined-behavior-strtokto disable. This diagnostic warns for paths where a call tostrtokis made with undefined behavior. Specifically: passingNULLas the first parameter for the initial call tostrtokwithin a process has undefined behavior. Not it. 🙈-Wno-analyzer-unsafe-call-within-signal-handler— requires-fanalyzer, which enables it; use-Wno-analyzer-unsafe-call-within-signal-handlerto disable. This diagnostic warns for paths where a function known to be async-signal-unsafe (likefprintf) gets called from a signal handler. See CWE-479: Signal Handler Use of a Non-reentrant Function (https://cwe.mitre.org/data/definitions/479.html).-Wno-analyzer-use-after-free— requires-fanalyzer, which enables it; use-Wno-analyzer-use-after-freeto disable. This diagnostic warns for paths where a pointer is used after a deallocator was called on it — eitherfree, or a deallocator referenced by the attributemalloc. 💀 See CWE-416: Use After Free (https://cwe.mitre.org/data/definitions/416.html).-Wno-analyzer-use-of-pointer-in-stale-stack-frame— requires-fanalyzer, which enables it; use-Wno-analyzer-use-of-pointer-in-stale-stack-frameto disable. This diagnostic warns for paths where a pointer gets dereferenced that points to a variable in a stale stack frame. Ma'am, that memory has moved on. 😭-Wno-analyzer-va-arg-type-mismatch— requires-fanalyzer, which enables it; use-Wno-analyzer-va-arg-type-mismatchto disable. This diagnostic warns for interprocedural paths where the analyzer catches an attempt to useva_argto extract a value passed to a variadic call, but the type used doesn't match the type of the expression passed to the call. See CWE-686: Function Call With Incorrect Argument Type (https://cwe.mitre.org/data/definitions/686.html).-Wno-analyzer-va-list-exhausted— requires-fanalyzer, which enables it; use-Wno-analyzer-va-list-exhaustedto disable. This diagnostic warns for interprocedural paths where the analyzer catches an attempt to useva_argto access the next value passed to a variadic call, but all the values in theva_listhave already been consumed. Nothing left in the bag, babe. 🎀 See CWE-685: Function Call With Incorrect Number of Arguments (https://cwe.mitre.org/data/definitions/685.html).-Wno-analyzer-va-list-leak— requires-fanalyzer, which enables it; use-Wno-analyzer-va-list-leakto disable. This diagnostic warns for interprocedural paths where the analyzer catches thatva_startorva_copywas called on ava_listwithout a corresponding call tova_end. Close your loops, bestie. ✨-Wno-analyzer-va-list-use-after-va-end— requires-fanalyzer, which enables it; use-Wno-analyzer-va-list-use-after-va-endto disable. This diagnostic warns for interprocedural paths where the analyzer catches an attempt to use ava_listafterva_endhas been called on it.va_list. 🫠-Wno-analyzer-write-to-const— requires-fanalyzer, which enables it; use-Wno-analyzer-write-to-constto disable. This diagnostic warns for paths where the analyzer catches an attempt to write through a pointer to aconstobject. Small caveat though: the analyzer does not prioritize detecting these paths, so false negatives are more likely compared to other warnings. 👀-Wno-analyzer-write-to-string-literal— requires-fanalyzer, which enables it; use-Wno-analyzer-write-to-string-literalto disable. This diagnostic warns for paths where the analyzer catches an attempt to write through a pointer to a string literal. Same caveat: the analyzer doesn't prioritize detection of these paths, so false negatives are more likely relative to other warnings. 🙃-Wno-analyzer-use-of-uninitialized-value— requires-fanalyzer, which enables it; use-Wno-analyzer-use-of-uninitialized-valueto disable. This diagnostic warns for paths where an uninitialized value is used. See CWE-457: Use of Uninitialized Variable (https://cwe.mitre.org/data/definitions/457.html).
💖 what the analyzer just knows, no notes needed
The analyzer has hardcoded knowledge about the behavior of these memory-management functions:
alloca- The built-in functions
__builtin_alloc,__builtin_alloc_with_align,__builtin_calloc,__builtin_free,__builtin_malloc,__builtin_memcpy,__builtin_memcpy_chk,__builtin_memset,__builtin_memset_chk,__builtin_realloc,__builtin_stack_restore, and__builtin_stack_save callocfreemallocmemsetoperator deleteoperator delete []operator newoperator new []reallocstrdupstrndup
...and of these functions for working with file descriptors:
openclosecreatdup,dup2anddup3isattypipe, andpipe2readwritesocket,bind,listen,accept, andconnect
...and of these functions for working with <stdio.h> streams:
- The built-in functions
__builtin_fprintf,__builtin_fprintf_unlocked,__builtin_fputc,__builtin_fputc_unlocked,__builtin_fputs,__builtin_fputs_unlocked,__builtin_fwrite,__builtin_fwrite_unlocked,__builtin_printf,__builtin_printf_unlocked,__builtin_putc,__builtin_putchar,__builtin_putchar_unlocked,__builtin_putc_unlocked,__builtin_puts,__builtin_puts_unlocked,__builtin_vfprintf, and__builtin_vprintf fopenfcloseferrorfgetsfgets_unlockedfilenofreadgetcgetcharfprintfprintffwrite
...and of these functions, which is giving main character energy honestly:
- The built-in functions
__builtin_expect,__builtin_expect_with_probability,__builtin_strchr,__builtin_strcpy,__builtin_strcpy_chk,__builtin_strlen,__builtin_va_copy, and__builtin_va_start - The GNU extensions
erroranderror_at_line getpasslongjmpputenvsetjmpsiglongjmpsignalsigsetjmpstrcatstrchrstrlen
Also, various functions with an__analyzer_prefix have special meaning to the analyzer — they're described in the GCC Internals manual. And various internal parameters, settable via--param, are used to control the exploration; those are also documented in the GCC Internals manual. 📚
The following options control the analyzer.
-fanalyzer-assume-nothrow— okay so by default, if-fexceptionsis on, the analyzer is a suspicious queen and assumes that calling any function without thenothrowattribute could throw an exception 😭. That's actually helpful — it can catch execution paths that leak because an exception yeeted past the clean-up code — but it can also give you false positives when you're using headers where the author never bothered adding thenothrowattribute.
But if -fanalyzer-assume-nothrow is enabled? The analyzer just trusts that external functions do not throw exceptions 💅. Super useful for cutting down the "noise" from the analyzer when you turn on -fexceptions on C code — but lowkey it could hide true issues if something the C code calls actually could raise an exception. So like, be aware of the trade-off, bestie 🫶.
-fanalyzer-call-summaries— simplifies interprocedural analysis by computing the effect of certain calls instead of exploring all the paths through the function from callsite to every possible return ✨. If enabled, call summaries are only used for functions with more than one call site, and that are sufficiently complicated. So like, only the messy ones get the summary treatment.-fanalyzer-checker=name— restricts the analyzer to run just the named checker, and enables it 👑. One checker, one job, no distractions.-fanalyzer-debug-text-art-headings— this one's for the analyzer developers 🧚. If enabled, the analyzer adds extra annotations to any diagrams it generates. Fancy little labels on the art, basically.-fno-analyzer-feasibility— for analyzer developers again 🔧. By default the analyzer double-checks that there's a feasible control flow path for every diagnostic it emits — like, the conditions that hold are not mutually exclusive. Diagnostics where no feasible path can be found get rejected. This filtering can be suppressed with-fno-analyzer-feasibility, for debugging issues in this code.-fanalyzer-fine-grained— does literally nothing 😭. Preserved for backward compatibility. It's giving ghost of options past, iconic in its own useless way.-fanalyzer-show-duplicate-count— analyzer developer option: if multiple diagnostics have been detected as being duplicates of each other, it emits a note when reporting the best diagnostic 🎀, giving the number of additional diagnostics that were suppressed by the deduplication logic. So you know how many lil' duplicates got filtered out.-fanalyzer-show-events-in-system-headers— by default the analyzer emits simplified diagnostic paths by hiding events fully located within a system header 🙈. With-fanalyzer-show-events-in-system-headersthose events are no longer suppressed. Everything's out in the open now.-fno-analyzer-simplify-supergraph— analyzer devs only 💅. By default the analyzer performs various simplifications to the program supergraph before analyzing it. With-fno-analyzer-simplify-supergraphthat simplification can be suppressed, for debugging issues with it.-fno-analyzer-state-merge— dev option again ✨. By default the analyzer simplifies analysis by merging sufficiently similar states at each program point as it builds its "exploded graph". With-fno-analyzer-state-mergethis merging can be suppressed, for debugging state-handling issues.-fno-analyzer-state-purge— same energy, dev-only 🔍. By default the analyzer simplifies analysis by purging aspects of state at a program point that appear to no longer be relevant — e.g. the values of locals that aren't accessed later in the function and which aren't relevant to leak analysis. With-fno-analyzer-state-purgethis purging of state can be suppressed, for debugging state-handling issues.-fno-analyzer-suppress-followups— analyzer devs, this one's yours 🧚. By default the analyzer stops exploring an execution path after encountering certain diagnostics, so it doesn't potentially issue a whole cascade of follow-up diagnostics. The diagnostics that terminate analysis along a path are:-Wanalyzer-null-argument-Wanalyzer-null-dereference-Wanalyzer-use-after-free-Wanalyzer-use-of-pointer-in-stale-stack-frame-Wanalyzer-use-of-uninitialized-value
With -fno-analyzer-suppress-followups the analyzer keeps exploring such paths even after those diagnostics — which may be helpful for debugging issues in the analyzer, or for microbenchmarks for detecting undefined behavior. Chaos mode, activated 💀.
-fanalyzer-transitivity— enables transitivity of constraints within the analyzer 🦋. Constraints connecting to constraints, the transitive property stays slaying.-fno-analyzer-undo-inlining— analyzer developer option 🌸. So-fanalyzerruns relatively late compared to other code analysis tools, and some optimizations have already been applied to the code. In particular function inlining may have occurred, leading to the interprocedural execution paths emitted by the analyzer containing function frames that don't correspond to those in the original source code 😭.
By default the analyzer tries to reconstruct the original function frames, and to emit events showing the inlined calls ✨. With -fno-analyzer-undo-inlining that attempt to reconstruct the original frame information can be disabled, which may help when debugging issues in the analyzer.
-fanalyzer-verbose-edges— for analyzer developers 💅. It enables more verbose, lower-level detail in the descriptions of control flow within diagnostic paths. More words, more detail, more edge lore.-fanalyzer-verbose-state-changes— also for analyzer devs 🎀. It enables more verbose, lower-level detail in the descriptions of events relating to state machines within diagnostic paths.-fanalyzer-verbosity=level— controls the complexity of the control flow paths that are emitted for analyzer diagnostics 👑. The level can be one of:0— at this level, interprocedural call and return events are displayed, along with the most pertinent state-change events relating to a diagnostic. For example, for a double-freediagnostic, both calls tofreewill be shown.1— as per the previous level, but also shows events for the entry to each function.2— as per the previous level, but also shows events relating to control flow that are significant to triggering the issue (e.g. "true path taken" at a conditional). This level is the default.3— as per the previous level, but shows all control flow events, not just significant ones.4— this level is intended for analyzer developers; it adds various other events intended for debugging the analyzer 🔥.
-fdump-analyzer— dumps internal details about what the analyzer is doing tofile.analyzer.txt📝.-fdump-analyzer-stderroverrides this option.-fdump-analyzer-stderr— dumps internal details about what the analyzer is doing to stderr. This option overrides-fdump-analyzer💅.-fdump-analyzer-callgraph— dumps a representation of the call graph suitable for viewing with GraphViz tofile.callgraph.dot🌈.-fdump-analyzer-exploded-graph— dumps a representation of the "exploded graph" suitable for viewing with GraphViz tofile.eg.dot✨. Nodes are color-coded based on state-machine states to emphasize state changes. Aesthetic and informative.-fdump-analyzer-exploded-nodes— emits diagnostics showing where nodes in the "exploded graph" are in relation to the program source 🔍.-fdump-analyzer-exploded-nodes-2— dumps a textual representation of the "exploded graph" tofile.eg.txt📄.-fdump-analyzer-exploded-nodes-3— dumps a textual representation of the "exploded graph" to one dump file per node, tofile.eg-id.txt😭. This is typically a large number of dump files. Like, a lot. Prepare your directory bestie.-fdump-analyzer-exploded-paths— dumps a textual representation of the "exploded path" for each diagnostic tofile.idx.kind.epath.txt🦋.-fdump-analyzer-feasibility— dumps internal details about the analyzer's search for feasible paths. The details are written in a form suitable for viewing with GraphViz to filenames of the formfile.*.fg.dot,file.*.tg.dot, andfile.*.fpath.txt✨.-fdump-analyzer-infinite-loop— dumps internal details about the analyzer's search for infinite loops. The details are written in a form suitable for viewing with GraphViz to filenames of the formfile.*.infinite-loop.dot🌀.-fdump-analyzer-json— dumps a compressed JSON representation of analyzer internals tofile.analyzer.json.gz📦. The precise format is subject to change — so like, don't get too attached, iykyk.-fdump-analyzer-state-purge— as per-fdump-analyzer-supergraph, dumps a representation of the "supergraph" suitable for viewing with GraphViz, but annotates the graph with information on what state will be purged at each node 🎀. The graph is written tofile.state-purge.dot.-fdump-analyzer-supergraph— dumps representations of the "supergraph" suitable for viewing with GraphViz tofile.supergraph.index.kind.dot🌸. These show all of the control flow graphs in the program, at various stages of the analysis. The precise set of dumps and what they show is subject to change.-fdump-analyzer-untracked— emits custom warnings with internal details intended for analyzer developers 💅.
🐛 Options for Debugging Your Program 🐛
To tell GCC to emit extra information for use by a debugger, in almost all cases you need only to add -g to your other options 🫶. Some debug formats can co-exist (like DWARF with CTF) when each of them is enabled explicitly by adding the respective command line option to your other options. Cute lil' debug format hangout.
GCC allows you to use -g with -O ✨. But bestie, the shortcuts taken by optimized code may occasionally be surprising:
- some variables you declared may not exist at all 😭
- flow of control may briefly move where you did not expect it
- some statements may not be executed because they compute constant results or their values are already at hand
- some statements may execute in different places because they have been moved out of loops
Nevertheless it is possible to debug optimized output 🔥. This makes it reasonable to use the optimizer for programs that might have bugs.
If you are not using some other optimization option, consider using -Og with -g 👑. With no -O option at all, some compiler passes that collect information useful for debugging do not run at all, so -Og may result in a better debugging experience. Fr fr, it's the glow-up your debug session deserves.
-g,--debug— produces debugging information in the operating system's native format (stabs,COFF,XCOFF, orDWARF) 💖. GDB can work with this debugging information. Slay.-ggdb— this one's for the GDB girlies 💅 it asks for the most expressive debug format available — so DWARF, or stabs, or the native format if neither of those is supported — and it includes GDB extensions whenever that's even possible.-gdwarf,-gdwarf-version— ok so this produces debugging info in DWARF format, if that's supported.versioncan be2,3,4or5, and the default for most targets is5— with the exceptions of VxWorks, TPF and Darwin / macOS, which all default to version2, and AIX, which defaults to version4.
Heads up: with DWARF Version 2, some ports require and always use some non-conflicting DWARF 3 extensions in the unwind tables. Not a vibe, just a fact 🫶
Version 4 may require GDB 7.0 and -fvar-tracking-assignments for maximum benefit. Version 5 requires GDB 8.0 or higher.
GCC no longer supports DWARF Version 1, which is substantially different from 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. legacy naming, we've all been there 😭
-gbtf— request BTF debug information. BTF is the default debugging format for the eBPF target. On other targets, like x86, BTF debug information can be generated along with DWARF debug information when both of the debug formats are enabled explicitly via their respective command line options. team players fr fr 🤝-gprune-btf,-gno-prune-btf— prune BTF information before emission. When pruning, only type information for types used by global variables and file-scope functions will be emitted. If compiling for the BPF target with BPF CO-RE enabled, type information will also be emitted for types used in BPF CO-RE relocations. In addition, struct and union types which are only referred to via pointers from members of other struct or union types shall be pruned and replaced withBTF_KIND_FWD, as though those types were only present in the input as forward declarations.
This option substantially reduces the size of produced BTF information, but at significant loss in the amount of detailed type information. It is primarily useful when compiling for the BPF target, to minimize the size of the resulting object, and to eliminate BTF information which is not immediately relevant to the BPF program loading process.
This option is enabled by default for the BPF target when generating BTF information. she's the default, periodt ✨
-gctf,-gctflevel— request CTF debug information and uselevelto specify how much CTF debug information should be produced. If-gctfis specified without a value forlevel, the default level of CTF debug information is2.
CTF debug information can be generated along with DWARF debug information when both of the debug formats are enabled explicitly via their respective command line options.
Level 0 produces no CTF debug information at all — thus, -gctf0 negates -gctf. 💀
Level 1 produces CTF information for tracebacks only. This includes callsite information, but does not include type information.
Level 2 produces type information for entities (functions, data objects etc.) at file-scope or global-scope only.
-gvms— produce debugging information in Alpha/VMS debug format (if that is supported). This is the format used byDEBUGon Alpha/VMS systems. retro icon behavior 🎀-gcodeview— produce debugging information in CodeView debug format (if that is supported). This is the format used by Microsoft Visual C++ on Windows. Windows besties, this one's yours 🪟-glevel,-ggdblevel,-gvmslevel— request debugging information and also uselevelto specify how much information. The default level is2.
Level 0 produces no debug information at all. Thus, -g0 negates -g. literally nothing, ghosted 👻
Level 1 produces minimal information, enough for making backtraces in parts of the program that you don't plan to debug. This includes descriptions of functions and external variables, and line number tables, but no information about local variables.
Level 2 is the normal setting to produce debuggable code. The debug output includes all the information from level 1, plus information about local variables and typedefs to allow stepping through code and examining its state. For C++ code, additional information is also emitted to assist in debugging namespace, class, and template functions. this is the main character setting 💅
Level 3 includes extra information, such as all the macro definitions present in the program. Some debuggers support macro expansion when you use -g3.
If you use multiple -g options, with or without level numbers, the last such option is the one that is effective. last one wins, no cap 🧚
-gdwarf does not accept a concatenated debug level, to avoid confusion with -gdwarf-level. Instead use an additional -glevel option to change the debug level for DWARF.
-fno-eliminate-unused-debug-symbols— by default, no debug information is produced for symbols that are not actually used. Use this option if you want debug information for all symbols. collector energy, no symbol left behind 🍓-femit-class-debug-always— instead of emitting debugging information for a C++ class in only one object file, emit it in all object files using the class. This option should be used only with debuggers that are unable to handle the way GCC normally emits debugging information for classes, because using this option increases the size of debugging information by as much as a factor of two. she's expensive, use her wisely 💸-fno-merge-debug-strings— direct the linker to not merge together strings in the debugging information that are identical in different object files. Merging is not supported by all assemblers or linkers. Merging decreases the size of the debug information in the output file at the cost of increasing link processing time. Merging is enabled by default.-fdebug-prefix-map=old=new— when compiling files residing in directoryold, record debugging information describing them as if the files resided in directorynewinstead. This can be used to replace a build-time path with an install-time path in the debug info. It can also be used to change an absolute path to a relative path by using.fornew. This can give more reproducible builds, which are location independent, but may require an extra command to tell GDB where to find the source files. See also-ffile-prefix-mapand-fcanon-prefix-map. path glow-up ✨-fvar-tracking— run variable tracking pass. It computes where variables are stored at each position in code. Better debugging information is then generated (if the debugging information format supports this information).
It is enabled by default when compiling with optimization (-Os, -O, -O2, ...), debugging information (-g) and the debug info format supports it.
-fvar-tracking-assignments— annotate assignments to user variables early in the compilation and attempt to carry the annotations over throughout the compilation all the way to the end, in an attempt to improve debug information while optimizing. Use of-gdwarf-4is recommended along with it.
It can be enabled even if var-tracking is disabled, in which case annotations are created and maintained, but discarded at the end. all that effort just to get tossed 😭
By default, this flag is enabled together with -fvar-tracking, except when selective scheduling is enabled.
-fvar-tracking-uninit,-fno-var-tracking-uninit— perform variable tracking and also mark uninitialized variables in the debug information. This flag is enabled by default by-fvar-tracking; it also implies-fvar-tracking. To do variable tracking without marking uninitialized variables, use-fvar-tracking -fno-var-tracking-uninit.-gsplit-dwarf— if DWARF debugging information is enabled, separate as much debugging information as possible into a separate output file with the extension.dwo. This option allows the build system to avoid linking files with debug information. To be useful, this option requires a debugger capable of reading.dwofiles. she lives in her own file now, independent queen 🦋-gdwarf32,-gdwarf64— if DWARF debugging information is enabled, the-gdwarf32selects the 32-bit DWARF format and the-gdwarf64selects the 64-bit DWARF format. The default is target specific, on most targets it is-gdwarf32though. The 32-bit DWARF format is smaller, but can't support more than 2GiB of debug information in any of the DWARF debug information sections. The 64-bit DWARF format allows larger debug information and might not be well supported by all consumers yet. big DWARF energy but read the fine print bestie 🙈-gdescribe-dies— add description attributes to some DWARF DIEs that have no name attribute, such as artificial variables, external references and call site parameter DIEs. giving the unnamed a lil personality 🎀-gpubnames— generate DWARF.debug_pubnamesand.debug_pubtypessections.-ggnu-pubnames— generate.debug_pubnamesand.debug_pubtypessections in a format suitable for conversion into a GDB index. This option is only useful with a linker that can produce GDB index version 7.-gno-pubnames— don't generate DWARF.debug_pubnamesand.debug_pubtypessections. the anti--gpubnames, we respect her boundaries 💀-fdebug-types-section— when using DWARF Version 4 or higher, type DIEs can be put into their own.debug_typessection instead of making them part of the.debug_infosection. It is more efficient to put them in a separate comdat section since the linker can then remove duplicates. But not all DWARF consumers support.debug_typessections yet and on some objects.debug_typesproduces larger instead of smaller debugging information. so it's giving efficiency sometimes, check your consumer first 🔥-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 information. The options are concatenated with spaces separating them from each other and from the compiler version. It is enabled by default. See also-frecord-gcc-switchesfor another way of storing compiler options into the object file. receipts, but in the binary 🧾✨-gstrict-dwarf— bestie, this one says nope to using extensions from any later DWARF standard version than whatever you picked with-gdwarf-version. (On most targets, non-conflicting DWARF extensions from later standards are allowed — this shuts that down.)-gno-strict-dwarf— the chill version: allow extensions of later DWARF standard versions than the one selected with-gdwarf-version. 💅-gas-loc-support— tells the compiler "hey, the assembler eats.locdirectives" so it can use them and let the assembler generate the DWARF2+ line number tables. This is generally desirable because assembler-generated line-number tables are a lot more compact than the ones the compiler cooks up itself. Enabled by default if, at GCC configure time, the assembler is found to support such directives. ✨-gno-as-loc-support— forces GCC to generate those DWARF2+ line number tables internally, if DWARF2+ line number tables are to be generated at all.-gas-locview-support— tells the compiler the assembler supports "view" assignment and reset assertion checking in.locdirectives. Enabled by default if, at GCC configure time, the assembler is found to support them. 🎀-gno-as-locview-support— forces GCC to assign view numbers internally, if-gvariable-location-viewsare explicitly requested.-gcolumn-info,-gno-column-info— emit location column info into DWARF debugging information, not just file and line. This one is enabled by default. 🦋-gstatement-frontiers,-gno-statement-frontiers— makes GCC drop markers into the internal representation at the beginning of statements and keep them roughly in place through the whole compilation, using them to guide the output of"is_stmt"markers in the line number table. Enabled by default when compiling with optimization (-Os,-O1,-O2, ...) and outputting DWARF 2 debug information at the normal level.-gvariable-location-views,-gvariable-location-views=incompat5,-gno-variable-location-views— augments variable location lists with progressive view numbers implied from the line number table. This lets debug-info consumers peek at state at certain points in the program even when no instructions tied to the corresponding source locations are present at that point. If the assembler doesn't support view numbers in line number tables, this makes the compiler 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 consumers that don't know about the augmentations can still eat them up — they just won't get any benefit from them either. 🙈
This is enabled by default when outputting DWARF 2 debug information at the normal level, as long as there's assembler support, -fvar-tracking-assignments is enabled and -gstrict-dwarf is not. When assembler support isn't available, it may still be enabled, but it forces GCC to output internal line number tables, and if -ginternal-reset-location-views is not enabled, that most certainly leads to silently mismatching location views. 😭
There's a proposed representation for view numbers that is not backward compatible with the location list format introduced in DWARF 5, which you can turn on with -gvariable-location-views=incompat5. This option may be removed in the future — it's only provided as a reference implementation of the proposed representation. Debug info consumers are not expected to support this extended format, and they'd be rendered unable to decode location lists that use it. 💀
-ginternal-reset-location-views,-gno-internal-reset-location-views— tries to figure out which location views can be left out of location view lists. This needs the compiler to have very accurate instruction length estimates, which isn't always the case, and it may silently generate incorrect view lists when you're using an assembler that doesn't support location view lists. The GNU assembler flags any such error as a"view number mismatch". Only enabled on ports that define a reliable estimation function.-ginline-points,-gno-inline-points— generates extended debug information for inlined functions. Location view tracking markers get inserted at inlined entry points so address and view numbers can be computed and output in the debug info. You can enable this independently of location views — in which case the view numbers just 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. 🧚-gz[=type]— produce compressed debug sections in DWARF format, if that's supported. If type isn't given, the default depends on what the assembler and linker you're using can do. type may be one ofnone(don't compress debug sections),zlib(use zlib compression in ELF gABI format), orzstd(use zstd compression in ELF gABI format). If the linker doesn't support writing compressed debug sections, the option gets rejected. Otherwise, if the assembler doesn't support them,-gzis silently ignored when producing object files.-femit-struct-debug-baseonly— emit debug information for struct-like types only when the base name of the compilation source file matches the base name of the file where the struct is defined.
This substantially shrinks the debugging information, but at a significant potential loss of type info for the debugger. Check out -femit-struct-debug-reduced for a less aggressive option, and -femit-struct-debug-detailed for more fine-grained control.
Works only with DWARF debug output.
-femit-struct-debug-reduced— emit debug information for struct-like types only when the base name of the compilation source file matches the base name of the file where the type is defined, unless the struct is a template or defined in a system header.
This significantly shrinks the debugging information, with some potential loss of type info for the debugger. See -femit-struct-debug-baseonly for a more aggressive option, and -femit-struct-debug-detailed for more detailed control.
Works only with DWARF debug output.
-femit-struct-debug-detailed[=spec-list]— pick exactly which struct-like types the compiler generates debug information for. The whole point is reducing duplicate struct debug info across different object files in the same program.
This is the detailed version of -femit-struct-debug-reduced and -femit-struct-debug-baseonly, which cover 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 use of an incomplete struct is valid, the use is indirect. Example: struct one direct; struct two * indirect;.
The optional second word limits the spec to ordinary structs (ord:) or generic structs (gen:). Generic structs are a bit complicated to explain. For C++, these are non-explicit specializations of template classes, or non-template classes within the above. Other programming languages have generics, but -femit-struct-debug-detailed doesn't implement them yet.
The third word specifies the source files for those structs the compiler should emit debug information for. none and any mean what you'd expect. base means the base name of the file where the type declaration appears must match the base name of the main compilation file — in practice, when compiling foo.c, debug info is generated for types declared in that file and foo.h, but not other headers. 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. 🔬
The default is -femit-struct-debug-detailed=all.
Works only with DWARF debug output.
-fno-dwarf2-cfi-asm— emit DWARF unwind info as a compiler-generated.eh_framesection instead of using GAS.cfi_*directives.-fno-eliminate-unused-debug-types— normally, when producing DWARF output, GCC avoids emitting debug symbol output for types that are nowhere used in the source file being compiled. But sometimes it's genuinely useful to have GCC emit debug info for all types declared in a compilation unit, whether or not they're actually used there — like when, in the debugger, you want to cast a value to a type that isn't actually used in your program (but is declared). More often, though, this just wastes a significant amount of space. 🗑️
Options That Control Optimization 🔥
These options control various sorts of optimizations. Slay.
Without any optimization option, the compiler's whole goal is to reduce the cost of compilation and 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. Iconic, honestly.
Turning on optimization flags makes the compiler attempt to improve performance and/or code size at the expense of compilation time and possibly the ability to debug the program. Trade-offs, bestie.
The compiler optimizes based on the knowledge it has of the program. Compiling multiple files at once to a single output file mode lets the compiler use info gained from all of the files when compiling each of them. Main character energy.
Not all optimizations are controlled directly by a flag. Only optimizations that have a flag are listed in this section.
Heads up: most optimizations are completely disabled at-O0or if an-Olevel isn't set on the command line — even if you specify individual optimization flags. Similarly,-Ogsuppresses many optimization passes. 💀
So like, depending on the target and how GCC was configured, the exact set of optimizations you get at each -O level might be slightly different than what's listed here. Want to know the exact tea? Run GCC with -Q --help=optimizers and it'll spill which optimizations are on at each level. 💅
-O,-O1,--optimize— Optimize, bestie. Optimizing compilation takes a little more time and a lot more memory for a big function.
With -O, the compiler tries to reduce code size and execution time without doing any of the optimizations that eat a ton of compile time.
-O is the recommended level for big machine-generated code — it's the sensible balance between compile time and memory use. Higher levels do optimizations with way more algorithmic complexity than -O.
-O turns on these flags:
-fauto-inc-dec -fbranch-count-reg -fcombine-stack-adjustments
-fcompare-elim -fcprop-registers -fdce -fdefer-pop -fdelayed-branch
-fdse -fforward-propagate -fguess-branch-probability -fif-conversion
-fif-conversion2 -finline-functions-called-once -fipa-modref
-fipa-profile -fipa-pure-const -fipa-reference
-fipa-reference-addressable -fivopts -fmerge-constants
-fmove-loop-invariants -fmove-loop-stores -fomit-frame-pointer
-freorder-blocks -fshrink-wrap -fshrink-wrap-separate
-fsplit-wide-types -fssa-backprop -fssa-phiopt -ftree-bit-ccp
-ftree-ccp -ftree-ch -ftree-coalesce-vars -ftree-copy-prop
-ftree-dce -ftree-dominator-opts -ftree-dse -ftree-forwprop
-ftree-fre -ftree-phiprop -ftree-pta -ftree-scev-cprop -ftree-sink
-ftree-slsr -ftree-sra -ftree-ter -funit-at-a-time-O2— Optimize even more. GCC does nearly all the supported optimizations that don't involve a space-speed tradeoff. Compared to-O, this bumps up both compile time and the performance of the generated code.
-O2 turns on everything from -O1, plus these:
-falign-functions -falign-jumps -falign-labels -falign-loops
-fcaller-saves -fcode-hoisting -fcrossjumping -fcse-follow-jumps
-fcse-skip-blocks -fdelete-null-pointer-checks -fdep-fusion
-fdevirtualize -fdevirtualize-speculatively
-fexpensive-optimizations -ffinite-loops -fgcse -fgcse-lm
-fhoist-adjacent-loads -finline-functions -finline-small-functions
-findirect-inlining -fipa-bit-cp -fipa-cp -fipa-icf -fipa-ra
-fipa-sra -fipa-vrp -fisolate-erroneous-paths-dereference
-flra-remat -foptimize-crc -foptimize-sibling-calls
-foptimize-strlen -fpartial-inlining -fpeephole2
-freorder-blocks-algorithm=stc -freorder-blocks-and-partition
-freorder-functions -frerun-cse-after-loop -fschedule-insns
-fschedule-insns2 -fsched-interblock -fsched-spec
-fspeculatively-call-stored-functions -fstore-merging
-fstrict-aliasing -fthread-jumps -ftree-builtin-call-dce
-ftree-loop-vectorize -ftree-pre -ftree-slp-vectorize
-ftree-switch-conversion -ftree-tail-merge -ftree-vrp
-fvect-cost-model=very-cheap⚠️ Heads up: read the warning under-fgcsebefore you go running-O2on programs that use computed gotos. Not kidding, bestie.
-O3— Optimize yet more.-O3turns on all the-O2optimizations and also these:
-fgcse-after-reload -fipa-cp-clone -floop-interchange
-floop-unroll-and-jam -fpeel-loops -fpredictive-commoning
-fsplit-loops -fsplit-paths -ftree-loop-distribution
-ftree-partial-pre -funswitch-loops -fvect-cost-model=dynamic
-fversion-loops-for-strides-O0— Reduce compile time and make debugging actually produce the results you expect. This is the default, btw. 🙈
At -O0, GCC completely disables most optimization passes — they don't run even if you explicitly enable them on the command line, or if -Q --help=optimizers lists them as enabled by default. So many optimizations GCC does depend on code analysis/canonicalization passes that -O enables, and running individual optimization passes in isolation just wouldn't be useful. It's giving "don't bother, the vibe isn't there."
-Os— Optimize for size.-Osenables all-O2optimizations except the ones that often make code bigger:
-falign-functions -falign-jumps -falign-labels -falign-loops
-fprefetch-loop-arrays -freorder-blocks-algorithm=stcIt also enables -finline-functions, makes the compiler tune for code size instead of execution speed, and does even more optimizations designed to shrink your code. Small and cute 🎀
-Ofast— Disregard strict standards compliance, we're going feral.-Ofastenables all-O3optimizations, plus optimizations that are not valid for all standard-compliant programs. It turns on-ffast-math,-fallow-store-data-races, and the Fortran-specific-fstack-arrays(unless-fmax-stack-var-sizeis specified), and-fno-protect-parens. It turns off-fsemantic-interposition.-Og— Optimize while keeping the debugging experience in mind.-Ogshould be your go-to level for the standard edit-compile-debug cycle — a reasonable blend of optimization, fast compilation, and debugging experience, especially for code with a high abstraction penalty. Unlike-O0, it enables-fvar-tracking-assignmentsand-fvar-tracking, which handle debug info in function prologues and epilogues better than-O0.
Like -O0, -Og completely skips a bunch of optimization passes, so individual options controlling them have no effect. Otherwise -Og enables all -O1 flags except the ones known to greatly interfere with debugging:
-fbranch-count-reg -fdelayed-branch -fdse -fif-conversion
-fif-conversion2 -finline-functions-called-once
-fmove-loop-invariants -fmove-loop-stores -fssa-phiopt
-ftree-bit-ccp -ftree-dse -ftree-pta -ftree-sra-Oz— Optimize aggressively for size rather than speed. Lowkey this may increase the number of instructions executed if those instructions need fewer bytes to encode.-Ozbehaves similarly to-Os, including enabling most-O2optimizations.
If you use multiple -O options, with or without level numbers, the last one wins. Periodt. 👑
Options of the form -fflag specify machine-independent flags. Most flags have both a positive and a negative form; the negative form of -ffoo is -fno-foo. In the table below, only one of the forms is listed — the one you'd typically use. You can figure out the other form by either removing no- or adding it. iykyk 💅
The following options control specific optimizations. They're either activated by -O options or related to ones that are. Use these flags in the rare cases when you want to "fine-tune" which optimizations get performed. Very main character of you. ✨
-fno-defer-pop— For machines that must pop arguments after a function call, always pop the args as soon as each function returns. At-O1and higher,-fdefer-popis the default; that lets the compiler let arguments pile up on the stack across several function calls and pop them all at once. Efficient 💅-fforward-propagate— Do 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 run and the second is scheduled after loop unrolling.
This one's enabled by default at -O1, -O2, -O3, and -Os.
-favoid-store-forwarding,-fno-avoid-store-forwarding— So many CPUs stall for many cycles when a load partially depends on previous smaller stores 😭 This pass tries to detect those cases and dodge the penalty by reordering the load and store and then fixing up the loaded value.
Disabled by default.
-ffp-contract=style —-ffp-contract=offdisables floating-point expression contraction.-ffp-contract=fastenables floating-point expression contraction, like forming fused multiply-add operations if the target natively supports them.-ffp-contract=onenables floating-point expression contraction if the language standard allows it — implemented for C and C++, where it enables contraction within one expression but not across different statements.
The default is -ffp-contract=off for C in a standards-compliant mode (-std=c11 or similar), and -ffp-contract=fast otherwise.
-ffp-int-builtin-inexact— Allow the built-in functionsceil,floor,roundandtrunc, plus theirfloatandlong doublevariants, to generate code that raises theinexactfloating-point exception for noninteger arguments. ISO C99 and C11 allow these functions to raise theinexactexception, but ISO/IEC TS 18661-1:2014 — the C bindings to IEEE 754-2008, as integrated into ISO C23 — does not allow these functions to do that.
The default is -fno-fp-int-builtin-inexact, which disallows raising the exception, unless C17 or an earlier C standard is selected. This option does nothing unless -ftrapping-math is in effect.
🫶 Even if-fno-fp-int-builtin-inexactis used, if the functions generate a call to a library function, then theinexactexception may still be raised if the library implementation doesn't follow TS 18661. Not our fault, bestie.
-fomit-frame-pointer— Omit the frame pointer in functions that don't need one. This skips the instructions that save, set up, and restore the frame pointer; on many targets it also frees up an extra register. Free real estate 🌈
On some targets this flag has no effect at all, because the standard calling sequence always uses a frame pointer, so it can't be omitted.
Note that -fno-omit-frame-pointer doesn't guarantee the frame pointer is used in all functions — several targets always omit the frame pointer in leaf functions.
Enabled by default at -O1 and higher.
-foptimize-crc— Detect loops that calculate CRC (doing polynomial long division) and replace them with a faster implementation. It detects 8, 16, 32, and 64 bit CRC, with a constant polynomial without the leading 1 bit, for both bit-forward and bit-reversed cases. If the target supports a CRC instruction and the polynomial in your source matches the polynomial used in the CRC instruction, it generates that CRC instruction. Otherwise, if the target supports a carry-less-multiplication instruction, it generates CRC using that; otherwise it generates table-based CRC. So many options, so iconic 🔥
Enabled by default at -O2 and higher.
-foptimize-sibling-calls— Optimize sibling and tail recursive calls.-foptimize-strlen— bestie, this one takes your standard C string girlies (likestrlen,strchr, orstrcpy) and their_FORTIFY_SOURCEcounterparts and turns them into faster alternatives. glow-up fr ✨
Enabled at levels -O2, -O3.
-finline-atomics,-fno-inline-atomics— inlines__atomicoperations when a lock-free instruction sequence is available. This optimization is enabled by default — she's already on 💅-finline-stringops[=fn]— expands memory and string operations (for now, onlymemset) inline, even when the length is variable or big enough that it'd need looping. Most useful along with-ffreestandingand-fno-builtin.
Okay real talk though: in some circumstances it lets the compiler generate code that takes advantage of known alignment and length multipliers, but even then it may be less efficient than optimized runtime implementations, and it can grow code size so much that even a less performant but shared implementation runs faster due to better use of code caches 🫠 This option is disabled by default.
-fno-inline— do not expand any functions inline apart from those marked with thealways_inlineattribute. This is the default when not optimizing.
Single functions can be exempted from inlining by marking them with the noinline attribute. 🚫
-finline-small-functions— integrates functions into their callers when their body is smaller than expected function call code (so overall size of program gets smaller). The compiler heuristically decides which functions are simple enough to be worth integrating this way. This inlining applies to all functions, even those not declared inline. iconic tbh 👑
Enabled at levels -O2, -O3, -Os.
-findirect-inlining— inlines also indirect calls that are discovered to be known at compile time thanks to previous inlining. Heads up: 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 are 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 declared static, then the function is normally not output as assembler code in its own right.
Enabled at levels -O2, -O3, -Os. Also enabled by -fprofile-use and -fauto-profile. ✨
-finline-functions-called-once— considers allstaticfunctions called once for inlining into their caller even if they are 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, -O3 and -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 inlining faster on programs having large chains of nested wrapper functions. she's efficient 💅
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. spring cleaning but make it compilers 🧹
Enabled at levels -O2, -O3 and -Os.
-finline-limit=n— by default, GCC limits the size of functions that can be inlined. This flag allows coarse control of this limit. n is the size of functions that can be inlined in number of pseudo instructions.
Inlining is actually controlled by a number of internal parameters, which are documented in the GCC Internals manual and should not normally be set directly. ngl, don't poke the internals 🙈
Note: there may be no value to -finline-limit that results in default behavior.
Note: pseudo instruction represents, in this particular context, an abstract measurement of function's size. In no way does it represent a count of assembly instructions and as such its exact meaning might change from one release to an another. so don't come for me if it's different next version, k? 😭
-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. fancy and specific 🎀-fkeep-inline-functions— in C, emitstaticfunctions that are declaredinlineinto the object file, even if the function has been inlined into all of its callers. This switch does not affect functions using theextern inlineextension in GNU C90. In C++, emit any and all inline functions into the object file. keep her, she's staying 💖-fkeep-static-functions— emitstaticfunctions into the object file, even if the function is never used. unemployed but still invited to the party 🫶-fkeep-static-consts— emit variables declaredstatic constwhen optimization isn't turned on, even if the variables aren't referenced.
GCC enables this option by default. If you want to force the compiler to check if a variable is referenced, regardless of whether or not optimization is turned on, use the -fno-keep-static-consts option.
-fmerge-constants— attempts to merge identical constants (string constants and floating-point constants) across compilation units. twinning fr 👯♀️
This option is the default for optimized compilation if the assembler and linker support it. Use -fno-merge-constants to inhibit this behavior.
Enabled at levels -O1, -O2, -O3, -Os.
-fmerge-all-constants— attempts to merge identical constants and identical variables.
This option implies -fmerge-constants. In addition to -fmerge-constants this considers e.g. even constant initialized arrays or initialized constant variables with integral or floating-point types. Languages like C or C++ require each variable, including multiple instances of the same variable in recursive calls, to have distinct locations, so using this option results in non-conforming behavior. ⚠️ like, it's giving rulebreaker and not in the cute way
-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. choreography but for instructions 💃-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. iykyk 🌀-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. so they're still lurking 💀
The default is -fbranch-count-reg at -O1 and higher, except for -Og.
-fno-function-cse— do not put function addresses in registers; make each instruction that calls a constant function contain the function's address explicitly. more typing, less efficiency 🫠
This option results in less efficient code, but some strange hacks that alter the assembler output may be confused by the optimizations performed when this option is not used.
The default is -ffunction-cse
-ffuse-ops-with-volatile-access— allow limited optimization of operations with volatile memory access when doing so does not change the semantics outlined in (the reference here got cut off in the source, bestie — not me, the page did that 💀)
The default is -ffuse-ops-with-volatile-access
-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. slay, free real estate ✨
This option turns off this behavior because some programs explicitly rely on variables going to the data section---e.g., so that the resulting executable can find the beginning of that section and/or make assumptions based on that. some programs are like that, we love them anyway 🫶
The default is -fzero-initialized-in-bss except in Ada.
-fthread-jumps— performs optimizations that check to see if a jump branches to a location where another comparison subsumed by the first is found. If so, the first branch is redirected to either the destination of the second branch or a point immediately following it, depending on whether the condition is known to be true or false. shortcut queen 👑
Enabled at levels -O1, -O2, -O3, -Os.
-fsplit-wide-types— when using a type that occupies multiple registers, such aslong longon a 32-bit system, split the registers apart and allocate them independently. This normally generates better code for those types, but may make debugging more difficult. trade-offs, always the trade-offs 😭
Enabled at levels -O1, -O2, -O3, -Os.
-fsplit-wide-types-early— fully split wide types early, instead of very late. This option has no effect unless-fsplit-wide-typesis turned on. no solo career for this one 💅
This is the default on some targets.
-fcse-follow-jumps— in common subexpression elimination (CSE), scan through jump instructions when the target of the jump is not reached by any other path. For example, when CSE encounters anifstatement with anelseclause, CSE follows the jump when the condition tested is false. she follows the plot 🌸
Enabled at levels -O2, -O3, -Os.
-fcse-skip-blocks— this is similar to-fcse-follow-jumps, but causes CSE to follow jumps that conditionally skip over blocks. When CSE encounters a simpleifstatement with no else clause,-fcse-skip-blockscauses CSE to follow the jump around the body of theif. skipping the filler episodes fr ⏭️
Enabled at levels -O2, -O3, -Os.
-frerun-cse-after-loop— re-runs common subexpression elimination after loop optimizations are performed. round two, bestie 🔁
Enabled at levels -O2, -O3, -Os.
-fgcse— performs a global common subexpression elimination pass. This pass also performs global constant and copy propagation. main character energy for real 👑
Note: When compiling a program using computed gotos, a GCC extension, you may get better run-time performance if you disable the global common subexpression elimination pass by adding -fno-gcse to the command line.
Enabled at levels -O2, -O3, -Os.
-fgcse-lm— okay so when this one's on, global common subexpression elimination goes looking for loads that are only killed by stores into themselves 🎯. Which means a loop holding a load/store sequence can get rewritten into a load outside the loop plus a copy/store inside it. Slay.
It's on by default whenever -fgcse is on 💖.
-fgcse-sm— turning this on runs a store motion pass after global common subexpression elimination, and this pass is literally just trying to move stores out of loops 🦋. Team it up with-fgcse-lmand loops with a load/store sequence can become a load before the loop and a store after the loop. Iconic behavior.
Not enabled at any optimization level, bestie. You have to ask for it.
-fgcse-las— when enabled, the global common subexpression elimination pass yeets redundant loads that come after stores to the same memory location — and yes, that covers both partial and full redundancies 💅.
Not enabled at any optimization level either. Fend for yourself.
-fgcse-after-reload— this one does a redundant load elimination pass after reload, and its whole purpose in life is cleaning up redundant spilling 🧹✨.
Enabled by -O3, -fprofile-use and -fauto-profile.
-faggressive-loop-optimizations— this tells the loop optimizer to use language constraints to derive bounds for how many iterations a loop has 🔥. Which means it's assuming your loop code doesn't invoke undefined behavior — like, say, causing signed integer overflows or out-of-bound array accesses 😭. Those derived iteration bounds get used to guide loop unrolling and peeling and loop exit test optimizations. On by default, no cap.-funconstrained-commons— tells the compiler that variables declared in common blocks (think Fortran) might later get overridden with longer trailing arrays 🫶. Which unfortunately blocks certain optimizations that depend on knowing the array bounds. Tradeoffs, bestie.-fcrossjumping— performs the cross-jumping transformation, which unifies equivalent code and saves code size ✨. Warning though: the resulting code may or may not perform better than without cross-jumping. It's giving "we'll see" 💀.
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-O1and higher on architectures that do support it.-fdce— performs dead code elimination (DCE) on RTL. Enabled by default at-O1and higher 💜.-fdse— performs dead store elimination (DSE) on RTL. Also enabled by default at-O1and higher 🌸.-fif-conversion— attempts to transform conditional jumps into branch-less equivalents, which includes using conditional moves, min, max, set flags and abs instructions, plus some tricks doable by standard arithmetics 🧚. Whether conditional execution actually gets used on chips that have it is controlled by-fif-conversion2.
Enabled at levels -O1, -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 -O1, -O2, -O3, -Os, but not with -Og.
-fdeclone-ctor-dtor— okay so the C++ ABI requires multiple entry points for constructors and destructors: one for a base subobject, one for a complete object, and one for a virtual destructor that callsoperator deleteafterwards 😮💨. For a hierarchy with virtual bases, the base and complete variants are clones — meaning two whole copies of the function. With this option, the base and complete variants get changed into thunks that call a common implementation. Chef's kiss.
Enabled by -Os.
-fdelete-null-pointer-checks— assumes programs can't safely dereference null pointers, and that no code or data element lives at address zero 🚫. This 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 null pointer checks — and those assume a memory access to address zero always traps, so if a pointer is checked after it's already been dereferenced, it can't be null.
Lowkey important caveat: in some environments this assumption just isn't true 🙈. Use -fno-delete-null-pointer-checks to disable this optimization for programs that depend on that behavior.
Enabled by default on most targets. On AVR and MSP430 it's completely disabled. And the passes using that dataflow info get enabled independently at different optimization levels, iykyk.
-fdevirtualize— attempts to convert calls to virtual functions into direct calls 👑. It does this both within a procedure and interprocedurally, as part of indirect inlining (-findirect-inlining) and interprocedural constant propagation (-fipa-cp). Enabled at levels-O2,-O3,-Os.-fdevirtualize-speculatively— attempts to convert calls to virtual functions into speculative direct calls ⭐. Based on analysis of the type inheritance graph, it figures out for a given call the set of likely targets; if that set is small — preferably size 1 — it changes the call into a conditional deciding between direct and indirect calls. These speculative calls enable more optimizations, like inlining. And when they look useless after further optimization, they get converted back into their original form. Recycling, we love to see it 💖.-fdevirtualize-at-ltrans— streams extra information needed for aggressive devirtualization when running the link-time optimizer in local transformation mode 🦋. This enables more devirtualization but significantly increases the size of streamed data, which is exactly why it's disabled by default. Bigger data, bigger problems.-fexpensive-optimizations— performs a number of minor optimizations that are relatively expensive 💸.
Enabled at levels -O2, -O3, -Os.
-fext-dce,-fno-ext-dce— performs dead code elimination on zero and sign extensions, with special dataflow analysis 🔍. Cute and thorough.-free— attempts to remove redundant extension instructions, and this is especially helpful for x86-64, which implicitly zero-extends in 64-bit registers after writing to their lower 32-bit half 🎀.
Enabled for Alpha, AArch64, LoongArch, PowerPC, RISC-V, SPARC, h83000 and x86 at levels -O2, -O3, -Os. (Yes, h83000 exactly like that, don't come for me.)
-fno-lifetime-dse— get comfy, this one's a whole story 🫖. 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 object's lifetime are dead once it's destroyed. Normally dead store elimination takes full advantage of this. But if your code relies on the value of the object storage persisting beyond the object's lifetime, you can use this flag to disable the optimization.
Want to preserve stores before the constructor starts (say, because your operator new clears the object storage) but still treat the object as dead after the destructor? 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, which is helpful for fast processors with small or moderate size register sets 💅.-fira-algorithm=algorithm — use the specified coloring algorithm for the integrated register allocator 🎨. The algorithm argument can bepriority, which specifies Chow's priority coloring, orCB, which specifies Chaitin-Briggs coloring. Chaitin-Briggs coloring isn't implemented for all architectures, but for the targets that do support it, it's the default because it generates better code. Periodt.-fira-region=region — use the specified regions for the integrated register allocator. The region argument should be one of these besties:all— use all loops as register allocation regions. This can give the best results for machines with a small and/or irregular register set 🔥.mixed— use all loops except loops with small register pressure as the regions. This value usually gives the best results in most cases and for most architectures, and it's 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 it's enabled by default for-Osor-O0🎀.
-fira-hoist-pressure— uses IRA to evaluate register pressure in the code hoisting pass when deciding whether to hoist expressions 🌸. Usually gives smaller code, but it can slow the compiler down. Enabled at level-Osfor all targets.-fira-loop-pressure— uses IRA to evaluate register pressure in loops when deciding to move loop invariants 🌀. Usually generates faster and smaller code on machines with large register files (>= 32 registers), but yeah, it can slow the compiler down. Enabled at level-O3for some targets.-fno-ira-share-save-slots— disables sharing of stack slots used for saving call-used hard registers living through a call 😤. Each hard register gets its own separate stack slot, and as a result function stack frames get bigger.-fno-ira-share-spill-slots— disables sharing of stack slots allocated for pseudo-registers. Each pseudo-register that doesn't get a hard register gets its own separate stack slot, and as a result function stack frames get bigger 📦.-flra-remat— enables CFG-sensitive rematerialization in LRA 🧠. Instead of loading values of spilled pseudos, LRA tries to rematerialize (recalculate) values when it's profitable. Enabled at levels-O2,-O3,-Os.-fdelayed-branch— if the target machine supports it, attempts to reorder instructions to exploit instruction slots available after delayed branch instructions 🎯. Enabled at levels-O1,-O2,-O3,-Os, but not at-Og.-fschedule-insns— okay so if the target machine supports it, this one tries to reorder your instructions around so you don't sit there stalled waiting for data that isn't ready yet 💅 it's literally a lifesaver for machines with slow floating point or memory load instructions, because it lets other instructions sneak in and get issued while the load or FP result is still cooking. Conventionally it's turned on at-O2and-O3, but bestie, many targets override that. Like on x86 it's disabled at all levels, and on AArch64 it's only enabled at-O3. iykyk.-fschedule-insns2— same vibe as-fschedule-insns, but it asks for a whole extra pass of instruction scheduling after register allocation has happened. This one eats 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. That scheduling is normally enabled when it's scheduling before register allocation, i.e. with-fschedule-insnsor at-O2or higher.-fno-sched-spec— turns off speculative motion of non-load instructions. Again, normally enabled when scheduling before register allocation, i.e. with-fschedule-insnsor at-O2or higher.-fsched-pressure— enables register pressure sensitive instruction scheduling before register allocation. Only makes sense when scheduling before register allocation is on, 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 in register allocation that follow). slay 🎀-fsched-spec-load— allows speculative motion of some load instructions. Only makes sense when scheduling before register allocation, i.e. with-fschedule-insnsor at-O2or higher.-fsched-spec-load-dangerous— allows speculative motion of more load instructions 👀 (yes, "dangerous" is right there in the name, not me being dramatic). Only makes sense when scheduling before register allocation, i.e. with-fschedule-insnsor at-O2or higher.-fsched-stalled-insns, *-fsched-stalled-insns=n*** — defines how many insns (if any) can be moved prematurely out of the queue of stalled insns and into the ready list during the second scheduling pass.-fno-sched-stalled-insnsmeans none 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. 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, and-fsched-stalled-insns-depwith no value is equivalent to-fsched-stalled-insns-dep=1`.-fsched2-use-superblocks— when scheduling after register allocation, use superblock scheduling. This lets motion happen across basic block boundaries, which gives you faster schedules ✨ It's experimental though — 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, which favors the instruction belonging to a schedule group. On by default whenever scheduling is enabled, i.e. with-fschedule-insnsor-fschedule-insns2or at-O2or higher.-fsched-critical-path-heuristic— enables the critical-path heuristic, which favors instructions on the critical path. On 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, which favors speculative instructions with greater dependency weakness. On by default when scheduling is enabled, i.e. with-fschedule-insnsor-fschedule-insns2or at-O2or higher.-fsched-rank-heuristic— enables the rank heuristic, which favors the instruction belonging to a basic block with greater size or frequency. On 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, which favors the instruction that's less dependent on the last instruction scheduled. On 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, which favors the instruction that has more instructions depending on it. On by default when scheduling is enabled, i.e. with-fschedule-insnsor-fschedule-insns2or at-O2or higher.-fspeculatively-call-stored-functions— tries to convert indirect calls of function pointers into pointers loaded from a structure field, but only if all visible stores to that field store just a single candidate. When it does that, it turns the call into a conditional deciding between the direct call and the original indirect one. These speculative calls often unlock more optimizations, like inlining 🦋 If they look useless after further optimization, they get converted back into their original form. no cap.-freschedule-modulo-scheduled-loops— modulo scheduling happens before traditional scheduling. So if a loop is modulo scheduled, later scheduling passes might go and change its schedule 😭 use this option to control that behavior.-fselective-scheduling— schedules instructions using the selective scheduling algorithm. Selective scheduling runs instead of the first scheduler pass.-fselective-scheduling2— schedules instructions using the selective scheduling algorithm, but this time it runs instead of the second scheduler pass.-fsel-sched-pipelining— enables software pipelining of innermost loops during selective scheduling. 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. Has no effect unless-fsel-sched-pipeliningis turned on.-fsemantic-interposition— some object formats, like ELF, allow interposing of symbols by the dynamic linker. Which means for symbols exported from the DSO, the compiler can't do interprocedural propagation, inlining and other optimizations, because the function or variable in question might change. While that feature is useful (say, to rewrite memory allocation functions with a debugging implementation), it's expensive in terms of code quality 💀 With-fno-semantic-interpositionthe compiler assumes that if interposition happens for functions, the overwriting function will have precisely the same semantics (and side effects). Similarly, if interposition happens for variables, the constructor of the variable will be the same. The flag has no effect for functions explicitly declared inline (where interposition is never allowed to change semantics) and for symbols explicitly declared weak.-fshrink-wrap— emits function prologues only before the parts of the function that actually need them, instead of at the top of the function. Enabled by default at-Oand higher.-fshrink-wrap-separate— shrink-wraps separate parts of the prologue and epilogue separately, so those parts are only executed when needed. On by default, but has no effect unless-fshrink-wrapis also turned on and the target supports this.-fcaller-saves— enables allocation of 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 to result 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— uses 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. 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 (which happens on targets that don't expose prologues and epilogues in RTL).-fconserve-stack— attempts to minimize stack usage. The compiler tries to use less stack space, even if that makes the program slower 🐢 This option implies setting thelarge-stack-frameparameter to100and thelarge-stack-frame-growthparameter to400.-ftree-reassoc— performs reassociation on trees. Enabled by default at-O1and higher.-fcode-hoisting— performs code hoisting. Code hoisting tries to move the evaluation of expressions executed on all paths to the function exit as early as possible. Especially useful as a code size optimization, but 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— does forward propagation on trees 💅 this one's on by default at-O1and up, no notes.-ftree-fre— full redundancy elimination (FRE) on trees, bestie. The lil' difference from PRE? FRE only looks at expressions that get computed on all the paths leading to the redundant computation. So it's faster than PRE, yeah, but it lowkey exposes fewer redundancies. Trade-off city. On by default at-O1and higher ✨-ftree-phiprop— hoists loads from conditional pointers, on trees. Enabled by default at-O1and higher 🎀-fhoist-adjacent-loads— speculatively hoists loads from both branches of an if-then-else, but only if those loads come from adjacent locations in the same structure and the target architecture has a conditional move instruction. Very picky, very demure. On by default at-O2and higher 🦋-ftree-copy-prop— copy propagation on trees. This pass yeets the unnecessary copy operations. Enabled by default at-O1and higher, periodt.-fipa-pure-const— figures out which functions are pure or constant. Enabled by default at-O1and higher 🌸-fipa-reference— figures out which static variables don't escape the compilation unit. Enabled by default at-O1and higher.-fipa-reference-addressable— figures out which static variables are read-only, write-only and non-addressable. Enabled by default at-O1and higher 💫-fipa-reorder-for-locality— groups call chains close together in the binary layout so code locality improves and jump distances between frequently called functions get minimized. Okay so how is this different from-freorder-functions? That one groups all hot/normal/cold/never-executed functions into separate sections; this pass looks at the call chains between functions and groups them together instead. And how is it different from-fprofile-reorder-functions? That one focuses on program startup, while this one aims to improve code locality throughout the entire runtime of the program 👑
Heads up though: this option is incompatible with an explicit -flto-partition= option, because it enforces a custom partitioning scheme. If you're using this, it's recommended you also use profile feedback — but otherwise this option is not enabled by default. Just so we're clear 🙈
-fipa-stack-alignment— reduces stack alignment on call sites if possible. Enabled by default. Slay.-fipa-pta— does interprocedural pointer analysis plus interprocedural modification and reference analysis. Warning though bestie: this can cause excessive memory and compile-time usage on large compilation units 😭 It is not enabled by default at any optimization level. Not even one. None.-fipa-profile— interprocedural profile propagation. Functions that are only called from cold functions get marked cold. Also, functions that execute once (things like"cold","noreturn", static constructors or destructors) get identified. Then cold functions — and the loop-less parts of functions that execute once — get optimized for size. Enabled by default at-O1and higher ✨-fipa-modref— interprocedural mod/ref analysis. It analyzes the side effects of functions (like which memory locations get modified or referenced) and that enables better optimization across the function call boundary. Enabled by default at-O1and higher 🫶-fipa-cp— interprocedural constant propagation. It analyzes the program to figure out when the values passed to functions are constants, then optimizes accordingly. If your app passes constants to functions, this can substantially increase performance 🔥 Enabled by default at-O2,-Osand-O3. It's also enabled by-fprofile-useand-fauto-profile.-fipa-cp-clone— function cloning, to make interprocedural constant propagation stronger. When it's enabled, IPA constant propagation does function cloning whenever an externally visible function can be called with constant arguments. And because this optimization can create multiple copies of functions, it may significantly increase code size 💀 Enabled by default at-O3. It's also enabled by-fprofile-useand-fauto-profile.-fipa-bit-cp— when enabled, does interprocedural bitwise constant propagation. Enabled by default at-O2, and by-fprofile-useand-fauto-profile. It requires that-fipa-cpis enabled. Teamwork, iykyk 🎀-fipa-vrp— when enabled, does interprocedural propagation of value ranges. Enabled by default at-O2. It requires that-fipa-cpis enabled. Same deal 💅-fipa-icf-functions,-fipa-icf-variables,-fipa-icf— Identical Code Folding for functions (-fipa-icf-functions), for read-only variables (-fipa-icf-variables), or for both (-fipa-icf) 🌈 This optimization reduces code size, and it may disturb unwind stacks by replacing a function with an equivalent one that has a different name. Noted, not ignored. The optimization works more effectively with link-time optimization enabled.
So it's similar to the Gold Linker's ICF optimization, but GCC ICF works on different levels, which means the optimizations are not the same — there are equivalences that only GCC finds, and equivalences that only Gold finds. Different flavors, both iconic.
-fipa-icf is enabled by default at -O2 and -Os.
-flate-combine-instructions— enables two instruction combination passes that run relatively late in the compilation process. One of the passes runs before register allocation, and the other runs after register allocation. The main aim of these passes is to substitute definitions into all uses. Most targets enable this flag by default at-O2and-Os⭐- *
-flive-patching=level* — controls GCC's optimizations so the output is suitable for live-patching 🩹
Okay so real talk, here's the concept. If the compiler's optimization uses a function's body — or information extracted from its body — to optimize/change another function, that other function is called an impacted function of the former. So if a function gets patched, its impacted functions should be patched too. Fr fr.
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. Etcetera, etcetera.
Usually, the more IPA optimizations are enabled, the larger the number of impacted functions for each function. And to control the number of impacted functions — and to 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 these:
inline-clone— only enables inlining and cloning optimizations, which includes inlining, cloning, interprocedural scalar replacement of aggregates and partial inlining. As a result, when you patch a function, all its callers and its clones' callers are impacted, so they need to be patched as well.
-flive-patching=inline-clone disables these optimization flags: -fwhole-program -fipa-pta -fipa-reference -fipa-ra -fipa-icf -fipa-icf-functions -fipa-icf-variables -fipa-bit-cp -fipa-vrp -fipa-pure-const -fipa-reference-addressable -fipa-stack-alignment -fipa-modref
inline-only-static— only enables inlining of static functions. So when you patch a static function, all its callers are impacted and need to be patched as well.
On top of every flag that -flive-patching=inline-clone disables, -flive-patching=inline-only-static also disables these 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:-flive-patchingis not supported with link-time optimization (-flto). Do not try it, bestie 😭
-fisolate-erroneous-paths-dereference— detects paths that trigger erroneous or undefined behavior by dereferencing a null pointer (with-fdelete-null-pointer-checksenabled) or by dividing by zero. It isolates those paths from the main control flow and turns the statement with the erroneous or undefined behavior into a trap 🪤 Enabled by default at-O2and higher.-fisolate-erroneous-paths-attribute— detects paths that trigger erroneous or undefined behavior because a null value is used in a way that's forbidden by a"returns_nonnull"or"nonnull"attribute. Same treatment: isolate those paths from the main control flow and turn the statement with the erroneous or undefined behavior into a trap. This is not currently enabled, but it may be enabled by-O2in the future 👀-ftree-sink— performs forward store motion on trees. Enabled by default at-O1and 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, which is the one exception, don't come for me. 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-O1and higher ✨-fssa-backprop— propagates information about a value's uses up the definition chain, so the definitions get simplified. For example, this pass strips sign operations if the sign of a value never matters. Enabled by default at-O1and higher 🦋-fssa-phiopt— does pattern matching on SSA PHI nodes to optimize conditional code. Enabled by default at-O1and higher — except for-Og, again the exception 💅-ftree-switch-conversion— converts simple initializations in aswitchinto initializations from a scalar array. Enabled by default at-O2and higher 🌟-ftree-tail-merge— hunts for identical code sequences, and when it finds them, it replaces one with a jump to the other. This optimization is known as tail merging, or cross jumping, depending on who you ask. Enabled by default at-O2and higher. The compilation time in this pass can be limited using themax-tail-merge-comparisonsparameter and themax-tail-merge-iterationsparameter 🎀-ftree-cselim— does conditional store elimination on trees, bestie ✨. It’s enabled by default at-O1and higher on targets that have conditional move instructions.-ftree-dce— performs dead code elimination (DCE) on trees. This flag is enabled by default at-O1and higher. lowkey essential 💅.-ftree-builtin-call-dce— performs conditional dead code elimination (DCE) for calls to built-in functions that may seterrnobut are otherwise free of side effects. It’s enabled by default at-O2and higher if-Osis not also specified 🫶.-ffinite-loops— assumes that a loop with an exit will eventually take the exit and not loop indefinitely. This lets the compiler remove loops that otherwise have no side-effects, not considering eventual endless looping as such 👑. This option is enabled by default at-O2for C++ with-std=c++11or higher.-ftree-dominator-opts— does a variety of simple scalar cleanups (constant/copy propagation, redundancy elimination, range propagation and expression simplification) based on a dominator tree traversal. It also performs jump threading (to reduce jumps to jumps) 😭. This flag is enabled by default at-O1and higher.-ftree-dse— performs dead store elimination (DSE) on trees. A dead store is a store into a memory location that is later overwritten by another store without any intervening loads. In this case the earlier store can be deleted 🎀. This flag is enabled by default at-O1and higher.-ftree-ch— performs loop header copying on trees. This is beneficial since it increases effectiveness of code motion optimizations. It also saves one jump 🔥. This flag is enabled by default at-O1and higher. It is not enabled for-Os, since it usually increases code size.-ftree-loop-optimize— performs loop optimizations on trees. This flag is enabled by default at-O1and higher ✨.-ftree-loop-linear,-floop-strip-mine,-floop-block— perform loop nest optimizations. Same as-floop-nest-optimize. To use this code transformation, GCC has to be configured with--with-islto enable the Graphite loop transformation infrastructure 💅.-fgraphite-identity— enables the identity transformation for graphite. For every SCoP we generate the polyhedral representation and transform it back to gimple. Using-fgraphite-identitywe can check the costs or benefits of the GIMPLE -> GRAPHITE -> GIMPLE transformation. Some minimal optimizations are also performed by the code generator isl, like index splitting and dead code elimination in loops 🌸.-floop-nest-optimize— enables the isl based loop nest optimizer. This is a generic loop nest optimizer based on the Pluto optimization algorithms. It calculates a loop structure optimized for data-locality and parallelism. This option is experimental 👀.-floop-parallelize-all— uses the Graphite data dependence analysis to identify loops that can be parallelized. Parallelize all the loops that can be analyzed to not contain loop carried dependences without checking that it is profitable to parallelize the loops 🫶.-ftree-coalesce-vars— while transforming the program out of the SSA representation, attempts to reduce copying by coalescing versions of different user-defined variables, instead of just compiler temporaries. This may severely limit the ability to debug an optimized program compiled with-fno-var-tracking-assignments😭. In the negated form, this flag prevents SSA coalescing of user variables. This option is enabled by default if optimization is enabled, and it does very little otherwise.-ftree-loop-if-convert— attempts to transform conditional jumps in the innermost loops to branch-less equivalents. The intent is to remove control-flow from the innermost loops in order to improve the ability of the vectorization pass to handle these loops 💖. This is enabled by default if vectorization is enabled.-ftree-loop-distribution— performs loop distribution. This flag can improve cache performance on big loop bodies and allow further loop optimizations, like parallelization or vectorization, to take place. For example, the loop
DO I = 1, N
A(I) = B(I) + C
D(I) = E(I) * F
ENDDOis transformed to
DO I = 1, N
A(I) = B(I) + C
ENDDO
DO I = 1, N
D(I) = E(I) * F
ENDDOThis flag is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile 👑.
-ftree-loop-distribute-patterns— performs loop distribution of patterns that can be code generated with calls to a library. This flag is enabled by default at-O2and higher, and by-fprofile-useand-fauto-profile✨.
This pass distributes the initialization loops and generates a call to memset zero. For example, the loop
DO I = 1, N
A(I) = 0
B(I) = A(I) + I
ENDDOis transformed to
DO I = 1, N
A(I) = 0
ENDDO
DO I = 1, N
B(I) = A(I) + I
ENDDOand the initialization loop is transformed into a call to memset zero 🎀.
-floop-interchange— performs loop interchange outside of graphite. This flag can improve cache performance on loop nest and allow further loop optimizations, like vectorization, to take place. For example, the loop
for (int i = 0; i < N; i++)
for (int j = 0; j < N; j++)
for (int k = 0; k < N; k++)
c[i][j] = c[i][j] + a[i][k]*b[k][j];is transformed to
for (int i = 0; i < N; i++)
for (int k = 0; k < N; k++)
for (int j = 0; j < N; j++)
c[i][j] = c[i][j] + a[i][k]*b[k][j];This flag is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile 🔥.
-floop-unroll-and-jam— 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 💅. This flag is enabled by default at-O3. It is 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 number of iterations in loops for which determining number of iterations requires complicated analysis. Later optimizations then may determine the number easily. Useful especially in connection with unrolling 🫶.-ftree-scev-cprop— performs final value replacement. If a variable is modified in a loop in such a way that its value when exiting the loop can be determined using only its initial value and the number of loop iterations, replace uses of the final value by such a computation, provided it is sufficiently cheap. This reduces data dependencies and may allow further simplifications. Enabled by default at-O1and higher 😭.-fivopts— performs induction variable optimizations (strength reduction, induction variable merging and induction variable elimination) on trees. Enabled by default at-O1and higher ⭐.-ftree-parallelize-loops,-ftree-parallelize-loops=n— parallelize loops, i.e., split their iteration space to run in multiple threads. This is only possible for loops whose iterations are independent and can be arbitrarily reordered. The optimization is only profitable on multiprocessor machines, for loops that are CPU-intensive, rather than constrained e.g. by memory bandwidth 🔥. This option implies-pthread, and thus is only supported on targets that have support for-pthread.
When a positive value n is specified, the number of threads is fixed at compile time and cannot be changed after compilation. The compiler generates "#pragma omp parallel num_threads(n)" 👑
When used without =n (i.e., -ftree-parallelize-loops), the number of threads is determined at program execution time via the OMP_NUM_THREADS environment variable. If OMP_NUM_THREADS is not set, the OpenMP runtime automatically detects the number of available processors and uses that value. This enables creating binaries that adapt to different hardware configurations without recompilation ✨.
-ftree-pta— performs function-local points-to analysis on trees. This flag is enabled by default at-O1and higher, except for-Og🎀.-ftree-sra— performs scalar replacement of aggregates. This pass replaces structure references with scalars to prevent committing structures to memory too early. This flag is enabled by default at-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 to reduce the number of instructions. This is 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 are replaced at their use location with their defining expression. This results in non-GIMPLE code, but gives the expanders much more complex trees to work on resulting in better RTL generation 💅. This is enabled by default at-O1and higher.-ftree-slsr— performs straight-line strength reduction on trees. This recognizes related expressions involving multiplications and replaces them by less expensive calculations when possible. This is enabled by default at-O1and higher 🫶.-ftree-vectorize— performs vectorization on trees. This flag enables-ftree-loop-vectorizeand-ftree-slp-vectorizeif not explicitly specified ✨.-ftree-loop-vectorize— performs loop vectorization on trees. This flag is enabled by default at-O2and by-ftree-vectorize,-fprofile-use, and-fauto-profile👑.-ftree-slp-vectorize— okay so this one does basic block vectorization on trees, bestie ✨ It’s enabled by default at-O2and by-ftree-vectorize,-fprofile-use, and-fauto-profile. slay.-ftrivial-auto-var-init=choice— initialize automatic variables or temporary objects with either a pattern or with zeroes, which increases the security and predictability of a program by preventing uninitialized memory disclosure and use 🛡️ GCC still considers an automatic variable that doesn’t have an explicit initializer as uninitialized, and-Wuninitializedand-Wanalyzer-use-of-uninitialized-valuewill still report warning messages on such automatic variables or temporary objects, and the compiler will perform optimization as if the variable were uninitialized. With this option, GCC will also initialize any padding of automatic variables or temporary objects that have structure or union types to zeroes. However, the current implementation cannot initialize automatic variables whose initialization is bypassed throughswitchorgotostatement. Using-Wtrivial-auto-var-initto report all such cases 👀
The three values of choice are:
uninitialized— doesn’t initialize any automatic variables.pattern— initialize automatic variables with values which will likely transform logic bugs into crashes down the line, are easily recognized in a crash dump, and without being values that programmers can rely on for useful program semantics. The current value is byte-repeatable pattern with byte0xFE. The values used forpatterninitialization might be changed in the future.zero— initialize automatic variables with zeroes.
The default is uninitialized except for C++26, in which case if -ftrivial-auto-var-init= is not specified at all automatic variables or temporary objects are zero initialized, but zero initialization of padding bits does not happen 🌸
Note that the initializer values, whether zero or pattern, refer to data representation (in memory or machine registers), rather than to their interpretation as numerical values. This distinction may be important in languages that support types with biases or implicit multipliers, and with such extensions as hardbool. For example, a variable that uses 8 bits to represent (biased) quantities in the range 160..400 will be initialized with the bit patterns 0x00 or 0xFE, depending on choice, whether or not these representations stand for values in that range, and even if they do, the interpretation of the value held by the variable will depend on the bias. A hardbool variable that uses say 0x5A and 0xA5 for false and true, respectively, will trap with either choice of trivial initializer, i.e., zero initialization will not convert to the representation for false, even if it would for a static variable of the same type. This means the initializer pattern doesn’t generally depend on the type of the initialized variable. One notable exception is that (non-hardened) boolean variables that fit in registers are initialized with false (zero), even when pattern is requested 👑
You can control this behavior for a specific variable by using the variable attribute uninitialized standard attribute or the C++26 [[indeterminate]]. periodt.
-fvect-cost-model=model— alter the cost model used for vectorization, bestie 💅 Themodelargument should be one ofunlimited,dynamic,cheaporvery-cheap. With theunlimitedmodel the vectorized code-path is assumed to be profitable, while with thedynamicmodel a runtime check guards the vectorized code-path to enable it only for iteration counts that will likely execute faster than when executing the original scalar loop. 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 is equal to thedynamicmodel. Thevery-cheapmodel disables vectorization of loops when any runtime check for data dependence or alignment is required; it also disables vectorization of epilogue loops but otherwise is equal to thecheapmodel 🦋
The default cost model depends on other optimization flags and is either dynamic or cheap.
-fsimd-cost-model=model— alter the cost model used for vectorization of loops marked with the OpenMP simd directive ✨ Themodelargument should be one ofunlimited,dynamic,cheap. All values ofmodelhave the same meaning as described in-fvect-cost-modeland by default a cost model defined with-fvect-cost-modelis used.-ftree-vrp— perform Value Range Propagation on trees, lowkey iconic 🌈 This is similar to the constant propagation pass, but instead of values, ranges of values are propagated. This allows the optimizers to remove unnecessary range checks like array bound checks and null pointer checks. This is enabled by default at-O2and higher. Null pointer check elimination is only done if-fdelete-null-pointer-checksis enabled.-fsplit-paths— split paths leading to loop backedges 🎀 This can improve dead code elimination and common subexpression elimination. This is enabled by default at-O3and above.-fsplit-ivs-in-unroller— enables expression of values of induction variables in later iterations of the unrolled loop using the value in the first iteration. This breaks long dependency chains, thus improving efficiency of the scheduling passes 💖
A combination of -fweb and CSE is often sufficient to obtain the same effect. However, that is not reliable in cases where the loop body is more complicated than a single basic block. It also does not work at all on some architectures due to restrictions in the CSE pass 😭
This optimization is enabled by default.
-fvariable-expansion-in-unroller— with this option, the compiler creates multiple copies of some local variables when unrolling a loop, which can result in superior code ✨
This optimization is enabled by default for PowerPC targets, but disabled by default otherwise.
-fpartial-inlining— inline parts of functions 🫶 This option has any effect only when inlining itself is turned on by the-finline-functionsor-finline-small-functionsoptions.
Enabled at levels -O2, -O3, -Os.
-fpredictive-commoning— perform predictive commoning optimization, i.e., reusing computations (especially memory loads and stores) performed in previous iterations of loops 🔥
This option is enabled at level -O3. It is also enabled by -fprofile-use and -fauto-profile.
-fprefetch-loop-arrays— if supported by the target machine, generate instructions to prefetch memory to improve the performance of loops that access large arrays 🌸
This option may generate better or worse code; results are highly dependent on the structure of loops within the source code 👀
Disabled at level -Os.
-fno-printf-return-value— do not substitute constants for known return value of formatted output functions such assprintf,snprintf,vsprintf, andvsnprintf(but notprintfoffprintf) 🙈 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-valueis in effect, both the branch and the body of theifstatement (but not the call tosnprint) can be optimized away wheniis a 32-bit or smaller integer because the return value is guaranteed to be at most 8.
char buf[9];
if (snprintf (buf, "%08x", i) >= sizeof buf)
...The -fprintf-return-value option relies on other optimizations and yields best results with -O2 and above. It works in tandem with the -Wformat-overflow and -Wformat-truncation options. The -fprintf-return-value option is enabled by default 💖
-fno-peephole,-fno-peephole2— disable any machine-specific peephole optimizations 🎀 The difference between-fno-peepholeand-fno-peephole2is in how they are implemented in the compiler; some targets use one, some use the other, a few use both.
-fpeephole is enabled by default. -fpeephole2 enabled at levels -O2, -O3, -Os.
-fno-guess-branch-probability— do not guess branch probabilities using heuristics, ngl 🫶
GCC uses heuristics to guess branch probabilities if they are not provided by profiling feedback (-fprofile-arcs). These heuristics are based on the control flow graph. If some branch probabilities are specified by __builtin_expect, then the heuristics are used to guess branch probabilities for the rest of the control flow graph, taking the __builtin_expect info into account. The interactions between the heuristics and __builtin_expect can be complex, and in some cases, it may be useful to disable the heuristics so that the effects of __builtin_expect are easier to understand.
It is also possible to specify expected probability of the expression with __builtin_expect_with_probability built-in function.
The default is -fguess-branch-probability at levels -O, -O2, -O3, -Os.
-freorder-blocks— reorder basic blocks in the compiled function in order to reduce number of taken branches and improve code locality ✨
Enabled at levels -O1, -O2, -O3, -Os.
-freorder-blocks-algorithm=algorithm— use the specified algorithm for basic block reordering 💅 Thealgorithmargument can besimple, which does not increase code size (except sometimes due to secondary effects like alignment), orstc, the “software trace cache” algorithm, which tries to put all often executed code together, minimizing the number of branches executed by making extra copies of code.
The default is simple at levels -O1, -Os, and stc at levels -O2, -O3.
-freorder-blocks-and-partition— in addition to reordering basic blocks in the compiled function, in order to reduce number of taken branches, partitions hot and cold basic blocks into separate sections of the assembly and.ofiles, to improve paging and cache locality performance 🔥
...and this optimization lowkey ghosts you (turns itself off) when exception handling or unwind tables are in the picture (on targets that use setjump/longjump or some target-specific scheme), for linkonce sections, for functions with a user-defined section attribute, and on any architecture that doesn't do named sections. Oh and when -fsplit-stack is on, it's not enabled by default (so you don't get linker errors 😭), but you can turn it on explicitly if your linker is actually functional.
Enabled for x86 at levels -O2, -O3, -Os. 💅
-freorder-functions— shuffles functions around in the object file so your code locality is cute and cozy. Unlike-fipa-reorder-for-locality, this one's priority is grouping all functions in a category (hot/normal/cold/never-executed) together. It pulls this off using special subsections".text.hot"for the most-executed functions and".text.unlikely"for the ones that basically never run. The linker does the actual reordering, so your object file format has to support named sections and the linker has to place them somewhere sensible.
This one is giving nothing unless you either hand over profile feedback (peep -fprofile-arcs for the deets) or manually annotate functions with "hot" or "cold" attributes. No vibes = no effect, fr.
Enabled at levels -O2, -O3, -Os.
-fstrict-aliasing— lets the compiler assume the strictest aliasing rules that apply to whatever language you're compiling. For C (and C++), this activates optimizations based on the type of expressions. Specifically: you're not allowed to access an object of one type through an expression of a different type, unless the types are compatible, differ only in signedness or qualifiers, or the expression has a character type. Accessing scalar objects via a corresponding vector type is also allowed. 👑
Example time: an "unsigned int" can alias an "int", but it can not alias a "void*" or a "double". A character type may alias any other type. Slay.
Pay extra attention to code like this:
union a_union {
int i;
double d;
};
int f() {
union a_union t;
t.d = 3.0;
return t.i;
}Reading from a different union member than the one you most recently wrote to (this is called "type-punning", iykyk) is super common. Even with -fstrict-aliasing, type-punning is allowed in C, as long as the memory is accessed through the union type. In ISO C++, type-punning through a union type is undefined behavior, but GCC supports it as an extension. 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;
}Same energy: access by taking the address, casting the resulting pointer, and dereferencing the result has undefined behavior, even if the cast uses a union type, like:
int f() {
double d = 3.0;
return ((union a_union *) &d)->i;
}The -fstrict-aliasing option is enabled at levels -O2, -O3, -Os.
-fipa-strict-aliasing— controls whether the rules of-fstrict-aliasingget applied across function boundaries. Heads up: if multiple functions get inlined into a single function, those memory accesses are no longer considered to be crossing a function boundary. Boundaries? Gone. Poof. ✨
The -fipa-strict-aliasing option is enabled by default and is effective only in combination with -fstrict-aliasing.
-falign-functions,-falign-functions=n,-falign-functions=n:m,-falign-functions=n:m:n2,-falign-functions=n:m:n2:m2— lines up the start of functions to the next power-of-two boundary that's greater than or equal to n, skipping up to m-1 bytes. This makes sure at least the first m bytes of the function can be fetched by the CPU without crossing an n-byte alignment boundary. It's a code-performance optimization, and alignment is ignored for functions considered cold 🥶. If you need alignment on all the functions, use-fmin-function-alignment.
If m isn't specified, it defaults to n.
Examples, bestie, memorize these: -falign-functions=32 aligns functions to the next 32-byte boundary, -falign-functions=24 aligns to the next 32-byte boundary only if that can be done by skipping 23 bytes or less, and -falign-functions=32:7 aligns to the next 32-byte boundary only if that can be done by skipping 6 bytes or less. 🎯
The second pair of n2:m2 values lets you specify a secondary alignment: -falign-functions=64:7:32:3 aligns to the next 64-byte boundary if that can be done by skipping 6 bytes or less, otherwise it aligns to the next 32-byte boundary if that can be done by skipping 2 bytes or less. If m2 isn't specified, it defaults to n2.
Some assemblers only support this flag when n is a power of two — in that case it gets rounded up.
-fno-align-functions and -falign-functions=1 are equivalent and mean functions are not aligned at all. Chaotic. Love that for you.
If n isn't 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— with this one on, the compiler tries to avoid unnecessarily overaligning functions. It attempts to tell the assembler to align by the amount specified by-falign-functions, but not to skip more bytes than the size of the function. Polite. Considerate. Iconic. 🎀-falign-labels,-falign-labels=n,-falign-labels=n:m,-falign-labels=n:m:n2,-falign-labels=n:m:n2:m2— aligns all branch targets to a power-of-two boundary.
The parameters of this option are analogous to -falign-functions. -fno-align-labels and -falign-labels=1 are equivalent and mean labels are not aligned.
If -falign-loops or -falign-jumps are applicable and are greater than this value, then their values get used instead. Hierarchy, honestly.
If n isn't specified or is zero, use a machine-dependent default which is very likely to be 1, meaning no alignment. The maximum allowed n option value is 65536.
Enabled at levels -O2, -O3.
-falign-loops,-falign-loops=n,-falign-loops=n:m,-falign-loops=n:m:n2,-falign-loops=n:m:n2:m2— aligns loops to a power-of-two boundary. If the loops get executed a bunch of times, that makes up for any execution of the dummy padding instructions. This is a code-performance optimization, and alignment is ignored for loops considered cold 🧊.
If -falign-labels is greater than this value, then its value is used instead.
The parameters of this option are analogous to -falign-functions. -fno-align-loops and -falign-loops=1 are equivalent and mean loops are not aligned. The maximum allowed n option value is 65536.
If n isn't specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3.
-falign-jumps,-falign-jumps=n,-falign-jumps=n:m,-falign-jumps=n:m:n2,-falign-jumps=n:m:n2:m2— aligns branch targets to a power-of-two boundary, but only for branch targets you can only reach by jumping. In that case no dummy operations need to be executed, so it's free real estate. This is a code-performance optimization, and alignment is ignored for jumps considered cold ❄️.
If -falign-labels is greater than this value, then its value is used instead.
The parameters of this option are analogous to -falign-functions. -fno-align-jumps and -falign-jumps=1 are equivalent and mean that loops are not aligned. (Yes, it says loops. Copying the lore exactly, don't come for me 💀.)
If n isn't specified or is zero, use a machine-dependent default. The maximum allowed n option value is 65536.
Enabled at levels -O2, -O3.
-fmin-function-alignment— specifies the minimal alignment of functions to the next power-of-two greater than or equal to n. Unlike-falign-functions, this alignment gets applied to all functions (even the ones considered cold). The alignment is also not affected by-flimit-function-alignment.-fno-allocation-dce— do not remove unused C++ allocations (using operator"new"and operator"delete") during dead code elimination. Leave my allocations alone 😤.
See also -fmalloc-dce.
-fallow-store-data-races— allows the compiler to perform optimizations that may introduce new data races on stores, without proving the variable can't be concurrently accessed by other threads. It does not affect optimization of local data. It's safe to use this option if you know global data won't be accessed by multiple threads. 👀
Examples of optimizations enabled by -fallow-store-data-races include hoisting or if-conversions that may cause a value already in memory to be re-written with that same value. That re-writing is safe in a single-threaded context but may be unsafe in a multi-threaded context. Note that on some processors, if-conversions may be required in order to enable vectorization.
Enabled at level -Ofast.
Bestie, real talk: that one is literally inviting data races to the function. Only ship it if you're certain about your threads, okay? 🫶
-funit-at-a-time— this option is left for compatibility reasons.-funit-at-a-timehas no effect, while-fno-unit-at-a-timeimplies-fno-toplevel-reorderand-fno-section-anchors. It's giving legacy. It's giving "we keep it around for your ex's codebase".
Enabled by default.
-fno-toplevel-reorder— do not reorder top-level functions, variables, and"asm"statements. Output them in the same order they appear in the input file. When this option is used, unreferenced static variables are not removed. This option is intended to support existing code that relies on a particular ordering. For new code, it's better to use attributes when possible. Glow up, use attributes. ✨
-ftoplevel-reorder is the default at -O1 and higher, and also at -O0 if -fsection-anchors is explicitly requested. Additionally -fno-toplevel-reorder implies -fno-section-anchors.
-funreachable-traps— with this option, the compiler turns calls to"__builtin_unreachable"into traps, instead of using them for optimization. This also affects any such calls implicitly generated by the compiler. Unreachable? Not in this house. 🪤
This option has the same effect as -fsanitize=unreachable -fsanitize-trap=unreachable, but does not affect the values of those options. If -fsanitize=unreachable is enabled, that option takes priority over this one.
This option is enabled by default at -O0 and -Og.
-fweb— okay so this one builds webs the way register allocation usually wants them, then gives every single web its own personal pseudo register. That means the register allocation pass can work straight on pseudos, and it also gives a glow-up to a bunch of other optimization passes like CSE, the loop optimizer, and the trivial dead code remover. Slay for optimization, but bestie, it can absolutely make debugging impossible, because variables stop staying in a "home register". So like… iconic for release builds, tragic for debug vibes.
It’s enabled by default with -funroll-loops.
-fwhole-program— this one tells GCC to assume the compilation unit you’re compiling is literally the whole entire program. All public functions and variables become static functions — exceptmainand the ones merged by the attributeexternally_visible— and then interprocedural optimizers can go feral and optimize them more aggressively. Main character assumption energy fr fr.
With -flto, this option has limited use. In most cases the precise list of symbols used or exported from the binary is known from the resolution info that the linker plugin passes to the link-time optimizer. It’s still useful if no linker plugin is used, or during an incremental link step when final code is produced (with -flto -flinker-output=nolto-rel).
-flto[=n]— this is the standard link-time optimizer, and she is kind of a big deal 💅. When you invoke it with source code, it generates GIMPLE (one of GCC’s internal representations) and writes it into special ELF sections in the object file. When those object files get linked together, all the function bodies are read from those ELF sections and instantiated as if they had been part of the same translation unit. It’s giving “separate files? never heard of her.”
To use the link-time optimizer, -flto and optimization options should be specified at compile time and during the final link. It’s 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, all the interprocedural analyses and optimizations in GCC work across the two files as if they were a single one. That means, for example, the inliner can inline functions in bar.o into functions in foo.o and vice versa. Obsessed.
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.
To make whole-program optimization effective, it is necessary to make certain 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 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. So like… pick your fighter.
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. Flexible queen behavior.
Generally, options specified at link time override those specified at compile time, although in some cases GCC attempts to infer link-time options from the settings used to compile the input files.
If you do not specify an optimization level option -O at link time, then GCC uses the highest optimization level used when compiling the object files. Note that it is generally ineffective to specify an optimization level option only at link time and not at compile time, for two reasons. First, compiling without optimization suppresses compiler passes that gather information needed for effective optimization at link time. Second, some early optimization passes can be performed only at compile time and not at link time.
There are some code generation flags preserved by GCC when generating bytecodes, because they need to be used during the final link. Currently, the following options and their settings are taken from the first object file that explicitly specifies them: -fcommon, -fexceptions, -fnon-call-exceptions, -fgnu-tm, and all the -m target flags.
The following options -fPIC, -fpic, -fpie and -fPIE are combined based on this scheme:
-fPIC + -fpic = -fpic
-fPIC + -fno-pic = -fno-pic
-fpic/-fPIC + (no option) = (no option)
-fPIC + -fPIE = -fPIE
-fpic + -fPIE = -fpie
-fPIC/-fpic + -fpie = -fpieCertain ABI-changing flags are required to match in all compilation units, and trying to override this at link time with a conflicting value is ignored. This includes options such as -freg-struct-return and -fpcc-struct-return.
Other options such as -ffp-contract, -fno-strict-overflow, -fwrapv, -fno-trapv or -fno-strict-aliasing are passed through to the link stage and merged conservatively for conflicting translation units. Specifically -fno-strict-overflow, -fwrapv and -fno-trapv take precedence; and for example -ffp-contract=off takes precedence over -ffp-contract=fast. You can override them at link time.
Diagnostic options such as -Wstringop-overflow are passed through to the link stage and their setting matches that of the compile-step at function granularity. Note that this matters only for diagnostics emitted during optimization. Note that code transforms such as inlining can lead to warnings being enabled or disabled for regions if the code isn’t consistent with the setting at compile time.
When you need to pass options to the assembler via -Wa or -Xassembler, make sure to either compile such translation units with -fno-lto or consistently use the same assembler options on all translation units. You can alternatively also specify assembler options at LTO link time.
To enable debug info generation, you need to supply -g at compile time. If any of the input files at link time were built with debug info generation enabled, the link will enable debug info generation as well. Any elaborate debug info settings like the dwarf level -gdwarf-5 need to be explicitly repeated at the linker command line, and mixing different settings in different translation units is discouraged. No cap.
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’s possible to apply interprocedural optimizations on files written in different languages:
gcc -c -flto foo.c
g++ -c -flto bar.cc
gfortran -c -flto baz.f90
g++ -o myprog -flto -O3 foo.o bar.o baz.o -lgfortranNotice that the final link is done with g++ to get the C++ runtime libraries, and -lgfortran is added to get the Fortran runtime libraries. In general, when mixing languages in LTO mode, you should use the same link command options as when mixing languages in a regular, non-LTO compilation.
If object files containing GIMPLE bytecode are stored in a library archive — say libfoo.a — it’s possible to extract and use them in an LTO link if you’re using a linker with plugin support. To create static libraries suitable for LTO, use gcc-ar and gcc-ranlib instead of ar and ranlib; to show the symbols of object files with GIMPLE bytecode, use gcc-nm. Those commands require that ar, ranlib and nm have been compiled with plugin support. At link time, use the flag -fuse-linker-plugin to ensure that the library participates in the LTO optimization process:
gcc -o myprog -O2 -flto -fuse-linker-plugin a.o b.o -lfooWith the linker plugin enabled, the linker extracts the needed GIMPLE files from libfoo.a and passes them on to the running GCC to make them part of the aggregated GIMPLE image to be optimized.
If you are not using a linker with plugin support and/or do not enable the linker plugin, then the objects inside libfoo.a are extracted and linked as usual, but they do not participate in the LTO optimization process. In order to make a static library suitable for both LTO optimization and usual linkage, compile its object files with -flto -ffat-lto-objects.
Link-time optimizations do not require the presence of the whole program to operate. Periodt. If the program does not require any symbols to be exported, it is possible to combine -flto and -fwhole-program to allow the interprocedural optimizers to use more aggressive assumptions, which may lead to improved optimization opportunities. Slay. Use of -fwhole-program is not needed when linker plugin is active (see -fuse-linker-plugin).
The current implementation of LTO makes no attempt to generate bytecode that is portable between different types of hosts. The bytecode files are versioned and there is a strict version check, so bytecode files generated in one version of GCC do not work with an older or newer version of GCC. No cap, that version check is strict.
Link-time optimization does not work well with generation of debugging information on systems other than those using a combination of ELF and DWARF.
If you specify the optional n, the optimization and code generation done at link time is executed in parallel using n parallel jobs by utilizing an installed make program. The environment variable MAKE may be used to override the program used.
You can also specify -flto=jobserver to use GNU make's job server mode to determine the number of parallel jobs. This is useful when the Makefile calling GCC is already executing in parallel. You must prepend a + to the command recipe in the parent Makefile for this to work. Heads up: this option likely only works if MAKE is GNU make. Even without the option value, GCC tries to automatically detect a running GNU make's job server.
Use -flto=auto to use GNU make's job server, if available, or otherwise fall back to autodetection of the number of CPU threads present in your system. Main character energy: auto-detect.
-flto-partition=alg— Specify the partitioning algorithm used by the link-time optimizer. The value is either1to1to specify a partitioning mirroring the original source files, orbalancedto specify partitioning into equally sized chunks (whenever possible), ormaxto create a new partition for every symbol where possible, orcacheto balance chunk sizes while keeping related symbols together for better caching in incremental LTO. Specifyingnoneas an algorithm disables partitioning and streaming completely. The default value isbalanced. While1to1can be used as an 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-incremental=path— Enable incremental LTO, with its cache in the given existing directory. Can significantly shorten edit-compile cycles with LTO. When used with LTO (-flto), the output of translation units inside LTO is cached. Cached translation units are likely to be encountered again when recompiling with small code changes, leading to recompile time reduction. Multiple GCC instances can use the same cache in parallel.-flto-incremental-cache-size=n— Specifies number of cache entries in incremental LTO after which to prune old entries. This is a soft limit, temporarily there may be more entries.-flto-compression-level=n— This option specifies the level of compression used for intermediate language written to LTO object files, and is only meaningful in conjunction with LTO mode (-flto). GCC currently supports two LTO compression algorithms. Forzstd, valid values are0(no compression) to19(maximum compression), whilezlibsupports values from0to9. Values outside this range are clamped to either minimum or maximum of the supported values. If the option is not given, a default balanced compression setting is used.-flto-toplevel-asm-heuristics— Enables heuristics to find symbols used in top-level basic"asm". This will restrict link-time optimizations that could cause renaming or deletion of such symbols which would result in missing symbol errors by linker. This flag is intended for projects that have not converted to using top-level extended"asm", which specify the usage directly without any false positives. The heuristics are simple and do not parse the assembly. The heuristics scan through top-level assembly for all possible identifiers; if an identifier is found among declared symbols, the symbol will be marked to restrict link-time optimizations. Static symbols disable more optimizations. Identifiers followed by':'disable more optimizations as well, because they might be a locally defined symbol in assembly, even when the declaration is marked'extern'.-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 ingoldor 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-fltofor 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 orgold).-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-objectsimproves 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,arandranlibneed to support linker plugins to allow a full-featured build environment (capable of building static libraries etc). GCC provides thegcc-ar,gcc-nm,gcc-ranlibwrappers 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-pluginshas the same effect as usage of the command wrappers (gcc-ar,gcc-nmandgcc-ranlib). The default is-fno-fat-lto-objectson targets with linker plugin support.-fcompare-elim— After register allocation and post-register allocation instruction splitting, identify arithmetic instructions that compute processor flags similar to a comparison operation based on that arithmetic. If possible, eliminate the explicit comparison operation. This pass only applies to certain targets that cannot explicitly represent the comparison operation before register allocation is complete. Enabled at levels-O1,-O2,-O3,-Os.-ffold-mem-offsets,-fno-fold-mem-offsets— Try to eliminate add instructions by folding them in memory loads/stores. Enabled at levels-O2,-O3.-fcprop-registers— After register allocation and post-register allocation instruction splitting, perform a copy-propagation pass to try to reduce scheduling dependencies and occasionally eliminate the copy. Enabled at levels-O1,-O2,-O3,-Os.-fprofile-correction— Profiles collected using an instrumented binary for multi-threaded programs may be inconsistent due to missed counter updates. When this option is specified, GCC uses heuristics to correct or smooth out such inconsistencies. By default, GCC emits an error message when an inconsistent profile is detected. This option is enabled by-fauto-profile.-fprofile-partial-training— With-fprofile-useall portions of programs not executed during training runs are optimized aggressively for size rather than speed. In some cases it is not practical to train all possible hot paths in the program. (For example, it may contain functions specific to a given hardware and training may not cover all hardware configurations the program later runs on.) With-fprofile-partial-trainingprofile feedback is ignored for all functions not executed during the training runs, causing them to be optimized as if they were compiled without profile feedback. This leads to better performance when the training is not representative at the cost of significantly bigger code.-fprofile-use,-fprofile-use=path— Enable profile feedback-directed optimizations, and the following optimizations, many of which are generally profitable only with profile feedback available:
-fbranch-probabilities -fprofile-values -funroll-loops
-fpeel-loops -ftracer -fvpt -finline-functions -fipa-cp
-fipa-cp-clone -fipa-bit-cp -fpredictive-commoning -fsplit-loops
-funswitch-loops -fgcse-after-reload -ftree-loop-vectorize
-ftree-slp-vectorize -fvect-cost-model=dynamic
-ftree-loop-distribute-patterns -fprofile-reorder-functionsBefore you can use this option, you must first generate profiling information.
By default, GCC emits an error message if the feedback profiles do not match the source code. This error can be turned into a warning by using -Wno-error=coverage-mismatch. Note this may result in poorly optimized code. Additionally, by default, GCC also emits a warning message if the feedback profiles do not exist (see -Wmissing-profile).
If path is specified, GCC looks at the path to find the profile feedback data files. See -fprofile-dir.
-fauto-profile,-fauto-profile=path— Enable sampling-based feedback-directed optimizations, and the following optimizations, many of which are generally profitable only with profile feedback available:
-fbranch-probabilities -fprofile-values -funroll-loops
-fpeel-loops -ftracer -fvpt -finline-functions -fipa-cp
-fipa-cp-clone -fipa-bit-cp -fpredictive-commoning -fsplit-loops
-funswitch-loops -fgcse-after-reload -ftree-loop-vectorize
-ftree-slp-vectorize -fvect-cost-model=dynamic
-ftree-loop-distribute-patterns -fprofile-correction- path is the name of a file holding AutoFDO profile info. Leave it out and it defaults to
fbdata.afdoin whatever your current directory happens to be. 🎀
To actually make an AutoFDO profile data file you gotta run your program with the perf utility on a GNU/Linux target system that supports it. More deets over at https://perfwiki.github.io/main/.
Like, for example:
perf record -e br_inst_retired:near_taken -b -o perf.data \
-- your_programThen you use the create_gcov tool to convert that raw profile data into a format GCC can actually eat. You also have to hand this tool the unstripped binary for your program — no stripped binaries, bestie. See https://github.com/google/autofdo.
E.g.:
create_gcov --binary=your_program.unstripped --profile=perf.data \
--gcov=profile.afdo-fauto-profile-inlining— when auto-profile is available, this inlines all the relevant functions that were inlined in the tran run before it reads the profile feedback. It literally improves the context sensitivity of the profile. Enabled by default. ✨
🌸 Floating-point arithmetic options, but make it a vibe check 🌸
Okay so these next ones control how the compiler behaves around floating-point arithmetic. They're all trading speed against correctness, and here's the thing: all of them must be specifically enabled. None of that automatic stuff. 💅
-fexcess-precision=style
This one lets you control excess precision on machines where floating-point operations happen in a format with more precision or range than the IEEE standard and interchange floating-point types. Iconic example of such a target? x87 floating point on x86 processors, which uses an 80-bit representation internally instead of the 64-bit IEEE format. For most programs the excess precision is harmless, but some programs genuinely rely on what the C or C++ language standards require for handling IEEE values.
By default -fexcess-precision=fast is in effect — meaning operations may be carried out in a wider precision than the types specified in the source if that gets you faster code, and honestly it is unpredictable when the rounding to the source-specified types actually takes place. When compiling C or C++, if -fexcess-precision=standard is specified, then excess precision follows the rules in ISO C99 or C++; in particular both casts and assignments cause values to be rounded to their semantic types (whereas -ffloat-store only affects assignments). This option is enabled by default for C or C++ if a strict conformance option like -std=c99 or -std=c++17 is used. -ffast-math enables -fexcess-precision=fast by default regardless of whether a strict conformance option is used. And if -fexcess-precision=16 is specified, constants and the results of expressions with types _Float16 and __bf16 are computed without excess precision. 🧚
Heads up:-fexcess-precision=standardis not implemented for languages other than C or C++. On the x86, it has no effect if-mfpmath=sseor-mfpmath=sse+387is specified; in the former case IEEE semantics apply without excess precision, and in the latter, rounding is unpredictable. 💀
-ffloat-store
Do not store floating-point variables in registers, and inhibit other options that might change whether a floating-point value gets taken from a register or memory. Ngl this one has generally been subsumed by -fexcess-precision=standard, which is more general. And if you do use -ffloat-store, you may need to modify your program to explicitly store intermediate computations in temporary variables, since -ffloat-store handles rounding to IEEE format only on assignments and not casts the way -fexcess-precision=standard does.
-ffast-math
Sets all of these bad boys: -fno-math-errno, -funsafe-math-optimizations, -ffinite-math-only, -fno-rounding-math, -fno-signaling-nans, -fcx-limited-range and -fexcess-precision=fast.
This option causes the preprocessor macro __FAST_MATH__ to be defined.
Lowkey important: this option is not turned on by any-Ooption besides-Ofast, because 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. 🙈
-fno-math-errno
Do not set errno after calling math functions that are executed with a single instruction, e.g. sqrt. A program that relies on IEEE exceptions for math error handling may want to use this flag for speed while maintaining IEEE arithmetic compatibility.
This option is not turned on by any-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 require the guarantees of these specifications.
The default is -fmath-errno.
On Darwin systems, the math library never sets errno. So there's literally no reason for the compiler to consider the possibility that it might, and -fno-math-errno is the default there. 🍓
-funsafe-math-optimizations
Allow optimizations for floating-point arithmetic that (a) assume arguments and results are valid and (b) may violate IEEE or ANSI standards. When used at link time, it may 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 besides-Ofast, since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that don't require the guarantees of these specifications.
It enables -fno-signed-zeros, -fno-trapping-math, -fassociative-math and -freciprocal-math.
The default is -fno-unsafe-math-optimizations.
-fassociative-math
Allow re-association of operands in series of floating-point operations. This violates the ISO C and C++ language standard by possibly changing computation result. NOTE: re-ordering may change the sign of zero as well as ignore NaNs and inhibit or create underflow or overflow — and thus cannot be used on code that relies on rounding behavior like (x + 2**52) - 2**52. 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-zeros and -fno-trapping-math be in effect. Moreover, it doesn't make much sense with -frounding-math. For Fortran the option is automatically enabled when both -fno-signed-zeros and -fno-trapping-math are in effect. 🦋
The default is -fno-associative-math.
-freciprocal-math
Allow the reciprocal of a value to be used instead of dividing by the value, if that enables optimizations. For example x / y can be replaced with x * (1/y), which is useful if (1/y) is subject to common subexpression elimination. Note that this loses precision and increases the number of flops operating on the value. 😭
The default is -fno-reciprocal-math.
-ffinite-math-only
Allow optimizations for floating-point arithmetic that assume arguments and results are not NaNs or +-Infs.
This option 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 require the guarantees of these specifications.
The default is -fno-finite-math-only.
-fno-signed-zeros
Allow optimizations for floating-point arithmetic that ignore the signedness of zero. IEEE arithmetic specifies the behavior of distinct +0.0 and -0.0 values, which then prohibits simplification of expressions such as x+0.0 or 0.0*x (even with -ffinite-math-only). This option implies that the sign of a zero result isn't significant. 💅
The default is -fsigned-zeros.
-fno-trapping-math
Compile code assuming that floating-point operations cannot generate user-visible traps. These traps include division by zero, overflow, underflow, inexact result and invalid operation. This option requires that -fno-signaling-nans be in effect. Setting this option may allow faster code if you rely on "non-stop" IEEE arithmetic, for example.
This option 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.
The default is -ftrapping-math.
Future versions of GCC may provide finer control of this setting using C99's FENV_ACCESS pragma. This command-line option will be used along with -frounding-math to specify the default state for FENV_ACCESS. ⭐
-frounding-math
Disable transformations and optimizations that assume default floating-point rounding behavior (round-to-nearest). 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.
Fair warning babe: 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 along with-ftrapping-mathto 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.
Again: this option is experimental and does not currently guarantee to disable all GCC optimizations that affect signaling NaN behavior. 💀
-fsingle-precision-constant— bestie, this one treats floating-point constants as single precision instead of quietly promoting them todouble-precision constants. No sneaky upgrades 💅-fcx-limited-range— when this is on, it basically says “range reduction? don’t need it” for complex division 😌✨. It also skips checking whether a complex multiply or divide produced"NaN + I*NaN"and trying to rescue that mess. This option is turned on by-ffast-math.
It controls the default setting of the ISO C99 "CX_LIMITED_RANGE" pragma. But like, it applies to all languages anyway. Periodt.
-fcx-fortran-rules— complex multiplication and division follow Fortran rules here 🧚. Range reduction is done as part of complex division, but there’s no checking whether a complex multiply or divide gave you"NaN + I*NaN"and no attempt to rescue it. That’s the vibe.-fcx-method=method — complex multiply/divide follows whatever method you name. The method argument should be one oflimited-range,fortranorstdc👑
The default is to honor language-specific constraints, which means fortran for Fortran and stdc otherwise. iykyk.
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. 💀 So like… be careful, bestie.
-fbranch-probabilities— after running a program compiled with-fprofile-arcs, you can compile it a second time using-fbranch-probabilitiesto improve optimizations based on how many times each branch was taken 🎀. When a program compiled with-fprofile-arcsexits, it saves arc execution counts to a file calledsourcename.gcdafor each source file. The info in that data file is very dependent on the structure of the generated code, so you must use the same source code and the same optimization options for both compilations. See details about the file naming in-fprofile-arcs.
With -fbranch-probabilities, GCC puts a REG_BR_PROB note on each JUMP_INSN and CALL_INSN. These can be used to improve optimization. Currently they’re only used in one place: in reorg.cc, instead of guessing which path a branch is most likely to take, the REG_BR_PROB values are used to exactly determine which path is taken more often. Slay, honestly.
Enabled by -fprofile-use and -fauto-profile.
-fprofile-values— if combined with-fprofile-arcs, it adds code so some data about values of expressions in the program gets gathered 📊✨.
With -fbranch-probabilities, it reads back the data gathered from profiling values of expressions for usage in optimizations.
Enabled by -fprofile-generate, -fprofile-use, and -fauto-profile.
-fprofile-reorder-functions— function reordering based on profile instrumentation collects the first time of execution of a function and orders these functions in ascending order, aiming to optimize program startup through more efficient loading of text segments 🚀. Enabled with-fprofile-use.-fvpt— if combined with-fprofile-arcs, this option tells 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. Math girlie behavior fr fr.
Enabled with -fprofile-use and -fauto-profile.
-frename-registers— attempt to avoid false dependencies in scheduled code by making use of registers left over after register allocation 🎀. This optimization most benefits processors with lots of registers. Depending on the debug information format adopted by the target, however, it can make debugging impossible, since variables no longer stay in a"home register". Not cute for debugging.
Enabled by default with -funroll-loops.
-fschedule-fusion— performs a target-dependent pass over the instruction stream to schedule instructions of the same type together, because the target machine can execute them more efficiently if they’re adjacent to each other in the instruction flow 🧚♀️✨.
Enabled at levels -O2, -O3, -Os.
-fdep-fusion— detects macro-op fusible pairs consisting of single-use instructions and their uses, and places such pairs together in the instruction stream to increase fusion opportunities in hardware 🔥. This pass is executed once before register allocation, and another time before register renaming.
Enabled at levels -O2, -O3, -Os.
-ftracer— performs tail duplication to enlarge superblock size 🦋. This transformation simplifies the control flow of the function, allowing other optimizations to do a better job.
Enabled by -fprofile-use and -fauto-profile.
-funroll-loops— unrolls 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. No promises, bestie 😭.
Enabled by -fprofile-use and -fauto-profile.
-funroll-all-loops— unrolls all loops, even if their number of iterations is uncertain when the loop is entered 💀. This usually makes programs run more slowly.-funroll-all-loopsimplies the same options as-funroll-loops.-fpeel-loops— peels loops for which there’s enough information that they don’t roll much (from profile feedback or static analysis) 🍓. It also turns on complete loop peeling (i.e. complete removal of loops with a small constant number of iterations).
Enabled by -O3, -fprofile-use, and -fauto-profile.
-fmalloc-dce— controls whether"malloc"(and its variants such as"calloc"or"strdup") can be optimized away, provided its return value is only used as a parameter of a"free"call or compared with"NULL"🧠✨. If-fmalloc-dce=1is used, only calls to"free"are allowed; with-fmalloc-dce=2, comparisons with a"NULL"pointer are also considered safe to remove.
The default is -fmalloc-dce=2. See also -fallocation-dce.
-fmove-loop-invariants— enables the loop invariant motion pass in the RTL loop optimizer 🌈. Enabled at level-O1and higher, except for-Og.-fmove-loop-stores— enables the loop store motion pass in the GIMPLE loop optimizer 💜. This moves invariant stores to after the end of the loop in exchange for carrying the stored value in a register across the iteration. Note: for this option to have an effect,-ftree-loop-imhas to be enabled as well. Enabled at level-O1and higher, except for-Og.-fsplit-loops— splits a loop into two if it contains a condition that’s always true for one side of the iteration space and false for the other ✂️✨.
Enabled by -fprofile-use and -fauto-profile.
-funswitch-loops— moves branches with loop-invariant conditions out of the loop, with duplicates of the loop on both branches (modified according to the 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, it creates another version of the loop that assumes the stride is always one 👑. For example:
for (int i = 0; i < n; ++i)
x[i * stride] = ...;becomes:
if (stride == 1)
for (int i = 0; i < n; ++i)
x[i] = ...;
else
for (int i = 0; i < n; ++i)
x[i * stride] = ...;This is particularly useful for assumed-shape arrays in Fortran, where (for example) it allows better vectorization assuming contiguous accesses 🦋. This flag is enabled by default at -O3. It’s also enabled by -fprofile-use and -fauto-profile.
-ffunction-sections,-fdata-sections— place each function or data item into its own section in the output file if the target supports arbitrary sections 🌸. The name of the function or the name of the data item determines the section’s name in the output file.
Use these options on systems where the linker can perform optimizations to improve locality of reference in the instruction space. Most systems using the ELF object format have linkers with such optimizations. On AIX, the linker rearranges sections (CSECTs) based on the call graph. The performance impact varies.
Together with a linker garbage collection (linker --gc-sections option), these options may lead to smaller statically-linked executables (after stripping) 🎀.
On ELF/DWARF systems these options do not degenerate the quality of the debug information. There could be issues with other object files/debug info formats.
Only use these options when there are significant benefits from doing so. When you specify these options, the assembler and linker create larger object and executable files and are also slower. These options affect code generation. They prevent optimizations by the compiler and assembler using relative locations inside a translation unit, since the locations are unknown until link time. An example of such an optimization is relaxing calls to short call instructions. So like… trade-offs, bestie 💅
-fstdarg-opt— optimizes the prologue of variadic argument functions with respect to usage of those arguments ✨.-fsection-anchors— tries to reduce the number of symbolic address calculations by using shared"anchor"symbols to address nearby objects 🎯. This transformation can help reduce the number of GOT entries and GOT accesses on some targets.
For example, the implementation of the following function "foo":
static int a, b, c;
int foo (void) { return a + b + c; }usually calculates the addresses of all three variables, but if you compile it with -fsection-anchors, it accesses the variables from a common anchor point instead. The effect is similar to the following pseudocode (which isn’t valid C):
int foo (void)
{
register int *xr = &x;
return xr[&a - &x] + xr[&b - &x] + xr[&c - &x];
}Not all targets support this option. 🙈
-fzero-call-used-regs=choice— zeroes out the call-used registers when a function returns, which bumps up your program's security by either mitigating Return-Oriented Programming (ROP) attacks or stopping info from leaking through registers 🛡️✨
The possible values of choice are the same as for the "zero_call_used_regs" attribute. The default is skip.
You can control this behavior for a specific function by using the function attribute "zero_call_used_regs". iykyk 💅
🎀 Program Instrumentation Options 🎀
GCC supports a number of command-line options that control adding run-time instrumentation to the code it normally generates. Like, one purpose of instrumentation is collect profiling statistics for use in finding program hot spots, code coverage analysis, or profile-guided optimizations. Another class of program instrumentation is adding run-time checking to detect programming errors like invalid pointer dereferences or out-of-bounds array accesses, as well as deliberately hostile attacks such as stack smashing or C++ vtable hijacking. There is also a general hook which can be used to implement other forms of tracing or function-level instrumentation for debug or program analysis purposes. It's giving "we see everything" energy 👀🔥
-p,--profile,-fprofile,-pg— generate extra code to write profile information suitable for the analysis programprof(for-p,--profile, and-fprofile) orgprof(for-pg). You must use this option when compiling the source files you want data about, and you must also use it when linking. both times, bestie, no skipping 🫶
You can use the function attribute "no_instrument_function" to suppress profiling of individual functions when compiling with these options.
-fprofile-arcs— add code so that program flow arcs are instrumented. During execution the program records how many times each branch and call is executed and how many times it is taken or returns. On targets that support constructors with priority support, profiling properly handles constructors, destructors and C++ constructors (and destructors) of classes which are used as a type of a global variable. slay 🦋
When the compiled program exits it saves this data to a file called auxname.gcda for each source file. The data may be used for profile-directed optimizations (-fbranch-probabilities), or for test coverage analysis (-ftest-coverage). Each object file's auxname is generated from the name of the output file, if explicitly specified and it is not the final executable, otherwise it is the basename of the source file. In both cases any suffix is removed (e.g. foo.gcda for input file dir/foo.c, or dir/foo.gcda for output file specified as -o dir/foo.o).
Note that if a command line directly links source files, the corresponding .gcda files will be prefixed with the unsuffixed name of the output file. E.g. gcc a.c b.c -o binary would generate binary-a.gcda and binary-b.gcda files. sneaky but iconic 💀
-fcondition-coverage— add code so that program conditions are instrumented. During execution the program records what terms in a conditional contributes to a decision, which can be used to verify that all terms in a Boolean function are tested and have an independent effect on the outcome of a decision. The result can be read withgcov --conditions. periodt 💜-fpath-coverage— add code so that the paths taken are tracked. During execution the program records the prime paths taken. The number of paths grows very fast with complexity, and to avoid exploding compile times GCC will give up instrumentation if the approximate number of paths exceeds the limit controlled by-fpath-coverage-limit. The result can be read withgcov --prime-paths --prime-paths-lines --prime-paths-source, no cap 🌈-fpath-coverage-limit=limit — the threshold at which point-fpath-coveragegives up on instrumenting a function. This limit is approximate and conservative, as GCC uses a pessimistic heuristic which slightly overcounts the running number of paths, and gives up if the threshold is reached before finding all the paths. This option is not for fine grained control over which functions to instrument — rather it is intended to limit the effect of path explosion and keep compile times reasonable. The default is250000. bestie the paths will absolutely multiply, it's giving exponential 😭--coverage,-coverage— this option is used to compile and link code instrumented for coverage analysis. The options-coverageand--coverageare equivalent; both are a synonym for-fprofile-arcs -ftest-coverage(when compiling) and-lgcov(when linking). See the documentation for those options for more details. the way these two are literally the same thing 💅- 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 allows gcov to 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,
"fork"calls are detected and correctly handled without double counting. fr fr, no double counting 🍓
- Compile the source files with
Moreover, an object file can be recompiled multiple times and the corresponding .gcda file merges as long as the source file and the compiler options are unchanged.
- For profile-directed optimizations, compile the source files again with the same optimization and code generation options plus
-fbranch-probabilities. - For test coverage analysis, use gcov to produce human readable information from the
.gcnoand.gcdafiles. Refer to the gcov documentation for further information.
With -fprofile-arcs, for each function of your program GCC creates a program flow graph, then finds a spanning tree for the graph. Only arcs that are not on the spanning tree have to be instrumented: the compiler adds code to count the number of times that these arcs are executed. When an arc is the only exit or only entrance to a block, the instrumentation code can be added to the block; otherwise, a new basic block must be created to hold the instrumentation code. efficient queen ✨
With -fcondition-coverage, for each conditional in your program GCC creates a bitset and records the exercised boolean values that have an independent effect on the outcome of that expression.
With -fpath-coverage, GCC finds and enumerates and records the taken prime paths of each function, unless the number of paths would exceed the limit controlled by -fpath-coverage-limit. If the limit is exceeded the function is not instrumented as if -fpath-coverage was not used. A prime path is the longest sequence of unique blocks, except possibly the first and last, which is not a subpath of any other path. okay that's actually kind of a beautiful definition ngl 🧚
-ftest-coverage— produce a notes file that the gcov code-coverage utility can use to show program coverage. Each source file's note file is called auxname.gcno. Refer to the-fprofile-arcsoption above for a description of auxname and instructions on how to generate test coverage data. Coverage data matches the source files more closely if you do not optimize. yes bestie, resist the-O2, I know it's hard 💀-fprofile-abs-path— automatically convert relative source file names to absolute path names in the.gcnofiles. This allows gcov to find the correct sources in projects where compilations occur with different working directories. absolute paths, absolute queen 👑-fprofile-dir=path — set the directory to search for the profile data files in to path. This option affects only the profile data generated by-fprofile-generate,-ftest-coverage,-fprofile-arcsand used by-fprofile-useand-fbranch-probabilitiesand its related options. Both absolute and relative paths can be used. By default, GCC uses the current directory as path, thus the profile data file appears in the same directory as the object file. In order to prevent the file name clashing, if the object file name is not an absolute path, we mangle the absolute path of the sourcename.gcdafile and use it as the file name of a.gcdafile. See details about the file naming in-fprofile-arcs. See similar option-fprofile-note. 🎀
When an executable is run in a massive parallel environment, it is recommended to save profile to different folders. That can be done with variables in path that are exported during run-time:
%p— process ID.%q{VAR}— value of environment variable VAR
-fprofile-generate,-fprofile-generate=path — enable options usually used for instrumenting application to produce profile useful for later recompilation with profile feedback based optimization. You must use-fprofile-generateboth when compiling and when linking your program. both. times. 😤
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. go get that feedback loop bestie 🔥
-fprofile-info-section,-fprofile-info-section=name — register the profile information in the specified section instead of using a constructor/destructor. The section name is name if it is specified, otherwise the section name defaults to".gcov_info". A pointer to the profile information generated by-fprofile-arcsis placed in the specified section for each translation unit. This option disables the profile information registration through a constructor and it disables the profile information processing through a destructor. This option is not intended to be used in hosted environments such as GNU/Linux. It targets freestanding environments (for example embedded systems) with limited resources which do not support constructors/destructors or the C library file I/O. tiny embedded systems deserve love too 🫶🌸
The linker is lowkey able to gather all those input sections into one continuous memory block and then define start and end symbols for you — here's a GNU linker script example that defines a linker output section, bestie:
.gcov_info :
{
PROVIDE (__gcov_info_start = .);
KEEP (*(.gcov_info))
PROVIDE (__gcov_info_end = .);
}And then your program can dump all the profiling info registered in that linker set, like, literally like this:
#include <gcov.h>
#include <stdio.h>
#include <stdlib.h>
extern const struct gcov_info *const __gcov_info_start[];
extern const struct gcov_info *const __gcov_info_end[];
static void
dump (const void *d, unsigned n, void *arg)
{
const unsigned char *c = d;
for (unsigned i = 0; i < n; ++i)
printf ("%02x", c[i]);
}
static void
filename (const char *f, void *arg)
{
__gcov_filename_to_gcfn (f, dump, arg );
}
static void *
allocate (unsigned length, void *arg)
{
return malloc (length);
}
static void
dump_gcov_info (void)
{
const struct gcov_info *const *info = __gcov_info_start;
const struct gcov_info *const *end = __gcov_info_end;
/* Obfuscate variable to prevent compiler optimizations. */
__asm__ ("" : "+r" (info));
while (info != end)
{
void *arg = NULL;
__gcov_info_to_gcda (*info, filename, dump, allocate, arg);
putchar ('\n');
++info;
}
}
int
main (void)
{
dump_gcov_info ();
return 0;
}Okay and then the merge-stream subcommand of gcov-tool is the girlypop that deserializes the data stream those __gcov_filename_to_gcfn and __gcov_info_to_gcda functions made, and merges the profile info into .gcda files over on your host filesystem. Slay. 💅
-fprofile-note=path — if you actually give it a path, GCC saves the.gcnofile right there in that location. ⚠️ But heads up: if you combine this option with multiple source files, the.gcnofile gets overwritten. Like, one file, bestie, or it's giving chaos.-fprofile-prefix-path=path — this one's a team player: use it together withprofile-generate=profile_dir andprofile-use=profile_dir to tell GCC where the base directory of your built source tree lives. By default profile_dir ends up holding files with mangled absolute paths of every object file in the built project, and that's not the vibe when the directory you built the instrumented binary in is different from the one where you build the binary optimized with profile feedback — because then the profile data will not be found during the optimized build 😭. In those setups,-fprofile-prefix-path=path (with path pointing at the base directory of the build) strips off the irrelevant part of the path and keeps all the file names relative to the main build directory. Iconic.-fprofile-prefix-map=old=new — when you're compiling files that live in directory old, record profiling information (with--coverage) describing them as if they lived in directory new instead. Cute little identity crisis. See also-ffile-prefix-mapand-fcanon-prefix-map.-fprofile-update=method — changes the update method for an app that's instrumented for profile-feedback-based optimization. method should be one ofsingle,atomicorprefer-atomic. The first one is the move for single-threaded apps, while the second one stops profile corruption by emitting thread-safe code. 🔥
Warning: when an app doesn't properly join all its threads (or spawns a detached thread), the profile file can still get corrupted. No cap.
Using prefer-atomic gets transformed into either atomic (when the target supports it) or single (otherwise). The GCC driver automatically picks prefer-atomic whenever -pthread is on the command line — otherwise the default method is single.
If atomic is selected, the profile info gets updated using atomic operations on a best-effort basis. Ideally that means atomic operations in hardware, but if the target platform doesn't support the required atomic ops in hardware and libatomic is available, then it's updated through calls to libatomic. And if the target platform supports neither the required hardware atomics nor libatomic, then the profile info is not atomically updated and a warning gets issued — in which case the profiling information you get may be corrupt for multi-threaded apps. 💀
For performance reasons, if 64-bit counters are used for the profiling info and the target only supports 32-bit atomic operations in hardware, then the performance-critical profiling updates are done using two 32-bit atomic operations per counter update. And if a signal interrupts those two operations while a counter is being updated, the profiling information may end up in an inconsistent state. 🙈
-fprofile-filter-files=regex — only instrument functions from files whose name matches any of the regular expressions (separated by semi-colons).
For example, -fprofile-filter-files=main\.c;module.*\.c will instrument only main.c and all C files starting with 'module'.
-fprofile-exclude-files=regex — only instrument functions from files whose name does not match any of the regular expressions (separated by semi-colons).
For example, -fprofile-exclude-files=/usr/.* will prevent instrumentation of all files located in the /usr/ folder. Bye bestie ✨
-fprofile-reproducible=[multithreaded|parallel-runs|serial]— controls the level of reproducibility of the profile gathered by-fprofile-generate. This means you can rebuild the program with the same outcome, which is useful for, say, distribution packages.
With -fprofile-reproducible=serial the profile gathered by -fprofile-generate is reproducible provided the trained program behaves the same at each invocation of the train run, it's not multi-threaded, and profile data streaming is always done in the same order. Note that profile streaming happens at the end of the program run but also before the fork function is invoked.
Note that it's quite common for execution counts of some part of a program to depend on, like, the length of temporary file names or memory space randomization (which may affect hash-table collision rate). Such non-reproducible parts of programs may be annotated by the no_instrument_function function attribute. gcov-dump with -l can dump the gathered data and verify it's actually reproducible.
With -fprofile-reproducible=parallel-runs the collected profile stays reproducible regardless of the order data gets streamed into the gcda files. This setting makes it possible to run multiple instances of the instrumented program in parallel (like with make -j). Ngl though, it reduces the quality of the gathered data — in particular of indirect call profiling.
-fsanitize=address— enables AddressSanitizer, a fast memory error detector 🕵️♀️. Memory access instructions get instrumented to detect out-of-bounds and use-after-free bugs. The option enables-fsanitize-address-use-after-scope. Seehttps://github.com/google/sanitizers/wiki/AddressSanitizerfor more details. The run-time behavior can be influenced with theASAN_OPTIONSenvironment variable — when it's set tohelp=1, the available options are shown at startup of the instrumented program. Seehttps://github.com/google/sanitizers/wiki/AddressSanitizerFlags#run-time-flagsfor a list of supported options. This option cannot be combined with-fsanitize=threador-fsanitize=hwaddress.
Heads up bestie: the only targets-fsanitize=hwaddressis currently supported on are x86-64 (only with-mlam=u48or-mlam=u57options) and AArch64 — in both cases only in ABIs with 64-bit pointers. Similarly,-fsanitize=memtag-stackis currently only supported on AArch64 ABIs with 64-bit pointers.
When you compile with -fsanitize=address, you should also use -g to produce more meaningful output. For more accurate stack traces you can use options such as -O0, -O1, or -Og (which, for instance, prevent most function inlining), -fno-optimize-sibling-calls (which prevents optimizing sibling and tail recursive calls; this option is implicit for -O0, -O1, or -Og), or -fno-ipa-icf (which disables Identical Code Folding for functions). Using -fno-omit-frame-pointer also improves stack traces. And since multiple runs of the program may yield backtraces with different addresses due to ASLR (Address Space Layout Randomization), you may wanna turn ASLR off — on Linux you can do that with setarch uname -m -R ./prog. 🐧
-fsanitize=kernel-address— enables AddressSanitizer for the Linux kernel. Seehttps://github.com/google/kernel-sanitizersfor more details.-fsanitize=hwaddress— enables Hardware-assisted AddressSanitizer, which uses a hardware ability to ignore the top byte of a pointer so it can detect memory errors with low memory overhead. Memory access instructions get instrumented to detect out-of-bounds and use-after-free bugs. This option enables-fsanitize-address-use-after-scope. Seehttps://clang.llvm.org/docs/HardwareAssistedAddressSanitizerDesign.htmlfor more details. The run-time behavior can be influenced with theHWASAN_OPTIONSenvironment variable — when set tohelp=1, the available options are shown at startup of the instrumented program. This option cannot be combined with-fsanitize=threador-fsanitize=address, and it's currently only available on AArch64. 🌸
-fsanitize=kernel-hwaddress— okay bestie, this enables Hardware-assisted AddressSanitizer for compiling the Linux kernel 💅 It’s similar to-fsanitize=kernel-addressbut uses an alternate instrumentation method, and similar to-fsanitize=hwaddressbut with instrumentation differences that are necessary for compiling the Linux kernel. Those differences are there to avoidhwasanlibrary initialization calls and to account for the stack pointer having a different value in its top byte.
⚠️ Note: this option has different defaults than-fsanitize=hwaddress. Instrumenting the stack andallocacalls are not on by default. Using a random frame tag is not implemented for kernel instrumentation. No cap.
-fsanitize=memtag-stack— this uses Memory Tagging Extension instructions instead of instrumentation to allow the detection of memory errors 🎀 Similar toHWASAN, it is also a probabilistic method. This option is available only on those AArch64 architectures that support Memory Tagging Extensions.-fsanitize=pointer-compare— instruments comparison operations (<,<=,>,>=) 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, add"detect_invalid_pointer_pairs=2"to the environment variableASAN_OPTIONS. Using"detect_invalid_pointer_pairs=1"detects invalid operation only when both pointers are non-null.
-fsanitize=pointer-subtract— instruments 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, add"detect_invalid_pointer_pairs=2"to the environment variableASAN_OPTIONS. Using"detect_invalid_pointer_pairs=1"detects invalid operation only when both pointers are non-null.
-fsanitize=shadow-call-stack— enables ShadowCallStack, a security enhancement mechanism used to protect programs against return address overwrites (e.g. stack buffer overflows) 🔥 It works by saving a function’s return address to a separately allocated shadow call stack in the function prologue and restoring the return address from the shadow call stack in the function epilogue. Instrumentation only occurs in functions that need to save the return address to the stack.
Currently it only supports the aarch64 platform. It is specifically designed for linux kernels that enable the CONFIG_SHADOW_CALL_STACK option. For user space programs, runtime support is not currently provided in libc and libgcc. Users who want to use this feature in user space need to provide their own support for the runtime. It should be noted that this may cause the ABI rules to be broken.
On aarch64, the instrumentation makes use of the platform register "x18". This generally means that any code that may run on the same thread as code compiled with ShadowCallStack must be compiled with the flag -ffixed-x18, otherwise functions compiled without -ffixed-x18 might clobber "x18" and so corrupt the shadow stack pointer.
Also, because there is no userspace runtime support, code compiled with ShadowCallStack cannot use exception handling. Use -fno-exceptions to turn off exceptions.
See https://clang.llvm.org/docs/ShadowCallStack.html for more details.
-fsanitize=thread— enables ThreadSanitizer, a fast data race detector 🌈 Memory access instructions are instrumented to detect data race bugs. Seehttps://github.com/google/sanitizers/wiki#threadsanitizerfor more details. The run-time behavior can be influenced using theTSAN_OPTIONSenvironment variable; seehttps://github.com/google/sanitizers/wiki/ThreadSanitizerFlagsfor a list of supported options. The option cannot be combined with-fsanitize=address,-fsanitize=leak.
When compiling with -fsanitize=thread, you should also use -g to produce more meaningful output.
💀 Note that sanitized atomic builtins cannot throw exceptions when operating on invalid memory addresses with non-call exceptions (-fnon-call-exceptions).
-fsanitize=leak— enables LeakSanitizer, a memory leak detector 🧚 This option only matters for linking of executables. The executable is linked against a library that overrides"malloc"and other allocator functions. Seehttps://github.com/google/sanitizers/wiki/AddressSanitizerLeakSanitizerfor more details. The run-time behavior can be influenced using theLSAN_OPTIONSenvironment variable. The option cannot be combined with-fsanitize=thread.-fsanitize=undefined— enables UndefinedBehaviorSanitizer, a fast undefined behavior detector ⭐ Various computations are instrumented to detect undefined behavior at runtime. Seehttps://clang.llvm.org/docs/UndefinedBehaviorSanitizer.htmlfor more details. The run-time behavior can be influenced using theUBSAN_OPTIONSenvironment variable. Current suboptions are:-fsanitize=shift— this option enables checking that the result of a shift operation is not undefined. Note that what exactly is considered undefined differs slightly between C and C++, as well as between ISO C90 and C99, etc. This option has two suboptions,-fsanitize=shift-baseand-fsanitize=shift-exponent.-fsanitize=shift-exponent— this option enables checking that the second argument of a shift operation is not negative and is smaller than the precision of the promoted first argument.-fsanitize=shift-base— if the second argument of a shift operation is within range, check that the result of a shift operation is not undefined. Note that what exactly is considered undefined differs slightly between C and C++, as well as between ISO C90 and C99, etc.-fsanitize=integer-divide-by-zero— detect integer division by zero.-fsanitize=unreachable— with this option, the compiler turns the"__builtin_unreachable"call into a diagnostics message call instead. When reaching the"__builtin_unreachable"call, the behavior is undefined.-fsanitize=vla-bound— this option instructs the compiler to check that the size of a variable length array is positive.-fsanitize=null— this option enables pointer checking. Particularly, the application built with this option turned on will issue an error message when it tries to dereference a NULL pointer, or if a reference (possibly an rvalue reference) is bound to a NULL pointer, or if a method is invoked on an object pointed by a NULL pointer.-fsanitize=return— this option enables return statement checking. Programs built with this option turned on will issue an error message when the end of a non-void function is reached without actually returning a value. This option works in C++ only.-fsanitize=signed-integer-overflow— this option enables signed integer overflow checking. We check that the result of"+","*", and both unary and binary"-"does not overflow in the signed arithmetics. This also detects"INT_MIN / -1"signed division. Note, integer promotion rules must be taken into account. That is, the following is not an overflow:
signed char a = SCHAR_MAX;
a++;-fsanitize=bounds— this option enables instrumentation of array bounds. Various out of bounds accesses are detected. Flexible array members, flexible array member-like arrays, and initializers of variables with static storage are not instrumented, with the exception of flexible array member-like arrays for which"-fstrict-flex-arrays"or"-fstrict-flex-arrays="options or"strict_flex_array"attributes say they shouldn’t be treated like flexible array member-like arrays.-fsanitize=bounds-strict— this option enables strict instrumentation of array bounds. Most out of bounds accesses are detected, including flexible array member-like arrays. Initializers of variables with static storage are not instrumented.-fsanitize=alignment— this option enables checking of alignment of pointers when they are dereferenced, or when a reference is bound to insufficiently aligned target, or when a method or constructor is invoked on insufficiently aligned object.-fsanitize=object-size— this option enables instrumentation of memory references using the"__builtin_dynamic_object_size"function. Various out of bounds pointer accesses are detected.-fsanitize=float-divide-by-zero— detect floating-point division by zero. Unlike other similar options,-fsanitize=float-divide-by-zerois not enabled by-fsanitize=undefined, since floating-point division by zero can be a legitimate way of obtaining infinities and NaNs.-fsanitize=float-cast-overflow— this option enables floating-point type to integer conversion checking. We check that the result of the conversion does not overflow. Unlike other similar options,-fsanitize=float-cast-overflowis not enabled by-fsanitize=undefined. This option does not work well with"FE_INVALID"exceptions enabled.-fsanitize=nonnull-attribute— this option enables instrumentation of calls, checking whether null values are not passed to arguments marked as requiring a non-null value by the"nonnull"function attribute.-fsanitize=returns-nonnull-attribute— this option enables instrumentation of return statements in functions marked with"returns_nonnull"function attribute, to detect returning of null values from such functions.-fsanitize=bool— this option enables instrumentation of loads from bool. If a value other than 0/1 is loaded, a run-time error is issued.-fsanitize=enum— this option enables instrumentation of loads from an enum type. If a value outside the range of values for the enum type is loaded, a run-time error is issued.
-fsanitize=vptr— okay so this one instruments C++ member function calls, member accesses, and some conversions between pointers to base and derived classes, all so it can verify the referenced object actually has the correct dynamic type. iykyk 💅-fsanitize=pointer-overflow— instruments pointer arithmetic. if the pointer arithmetic overflows, a run-time error gets issued. that's it, that's the vibe 🫶-fsanitize=builtin— instruments arguments to selected builtin functions. pass an invalid value to one of those arguments and boom, run-time error. e.g. passing0as the argument to__builtin_ctzor__builtin_clzinvokes undefined behavior and this option diagnoses it. 🔥
⚠️ heads up bestie: sanitizers tend to increase the rate of false positive warnings, most notably the ones around-Wmaybe-uninitialized. we recommend against combining-Werrorand [the use of] sanitizers. not the combo you want fr fr
also, while -ftrapv causes traps for signed overflows to be emitted, -fsanitize=undefined gives a diagnostic message instead. this currently works only for the C family of languages. 💀
-fno-sanitize=all— disables all previously enabled sanitizers. and no,-fsanitize=allis not allowed, because some sanitizers simply cannot be used together. the audacity of them 😭-fasan-shadow-offset=number— forces GCC to use a custom shadow offset in AddressSanitizer checks. useful for experimenting with different shadow memory layouts in Kernel AddressSanitizer. ✨-fsanitize-sections=s1,s2,...— sanitize global variables in selected user-defined sections.simay contain wildcards. 👑-fsanitize-recover[=opts]—<pink>okay this one is a whole thing, stay with me</pink>.-fsanitize-recover=controls error recovery mode for the sanitizers mentioned in the 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. which means multiple runtime errors can be reported in a single program run, and the exit code of the program may indicate success even when errors have been reported. 😳
the -fno-sanitize-recover= option can be used to alter this behavior: only the first detected error is reported, and the program then exits with a non-zero exit code. much more dramatic, honestly.
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 are 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 the 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's equivalent to specifying an opts list of:
undefined,float-cast-overflow,float-divide-by-zero,bounds-strict-fsanitize-address-use-after-scope— enables sanitization of local variables to detect use-after-scope bugs. the option sets-fstack-reusetonone. 💜-fsanitize-trap[=opts]— instructs the compiler to report for the sanitizers mentioned in the comma-separated list of opts undefined behavior using__builtin_traprather than alibubsanlibrary routine. if this option is enabled for a certain sanitizer, it takes precedence over-fsanitizer-recover=for that sanitizer —__builtin_trapwill be emitted and be fatal regardless of whether recovery is enabled or disabled using-fsanitize-recover=. no cap. 🔥
the advantage of this is that the libubsan library is not needed and is not linked in, so this is usable even in freestanding environments. slay for the freestanding girlies ✨
currently this feature works with -fsanitize=undefined (and its suboptions except for -fsanitize=vptr), -fsanitize=float-cast-overflow, -fsanitize=float-divide-by-zero and -fsanitize=bounds-strict. -fsanitize-trap=all can also be specified, which enables it for undefined suboptions, -fsanitize=float-cast-overflow, -fsanitize=float-divide-by-zero and -fsanitize=bounds-strict. if -fsanitize-trap=undefined or -fsanitize-trap=all is used and -fsanitize=vptr is enabled on the command line, the instrumentation is silently ignored, as the instrumentation always needs libubsan support — and -fsanitize-trap=vptr is not allowed. 🙈
-fsanitize-undefined-trap-on-error— this is the deprecated equivalent of-fsanitize-trap=all. giving "old news, still works" energy fr 💀-fsanitize-coverage=trace-pc— enable coverage-guided fuzzing code instrumentation. inserts a call to__sanitizer_cov_trace_pcinto every basic block. 🦋-fsanitize-coverage=trace-cmp— enable dataflow guided fuzzing code instrumentation. inserts a call to__sanitizer_cov_trace_cmp1,__sanitizer_cov_trace_cmp2,__sanitizer_cov_trace_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. the way that is a lot of function names 😭
-fcf-protection=[full|branch|return|none|check],-fcf-protection— enable code instrumentation to increase program security by checking that target addresses of control-flow transfer instructions (such as indirect function call, function return, indirect jump) are valid. this prevents diverting the flow of control to an unexpected target. it's intended to protect against threats like Return-oriented Programming (ROP), and similarly call/jmp-oriented programming (COP/JOP). main character security energy 👑
the -fcf-protection= keywords are interpreted as follows:
- the value
branchtells the compiler to implement checking of validity of control-flow transfer at the point of indirect branch instructions, i.e. call/jmp instructions. - the value
returnimplements checking of validity at the point of returning from a function. - the value
fullis an alias for specifying bothbranchandreturn. - the value
checkis used for the final link with link-time optimization (LTO). an error is issued if LTO object files are compiled with different-fcf-protectionvalues. the valuecheckis ignored at the compile time. - the value
noneturns off instrumentation.
-fcf-protection is an alias for -fcf-protection=full. to override a previous -fcf-protection option on the command line, add -fcf-protection=none and then -fcf-protection=kind. 🎀
the macro __CET__ is defined when -fcf-protection is used. the first bit of __CET__ is set to 1 for the value branch, and the second bit of __CET__ is set to 1 for the return. ✨
you can also use the nocf_check attribute to identify which functions and calls should be skipped from instrumentation. 🫶
currently the x86 GNU/Linux target provides an implementation based on Intel Control-flow Enforcement Technology (CET) which works for i686 processor or newer. 💅
-fharden-compares— for every logical test that survives gimple optimizations and is not the condition in a conditional branch (for example, conditions tested for conditional moves, or to store in boolean variables), emit extra code to compute and verify the reversed condition, and to call__builtin_trapif the results do not match. use with-fharden-conditional-branchesto cover all conditionals. 💖-fharden-conditional-branches— for every non-vectorized conditional branch that survives gimple optimizations, emit extra code to compute and verify the reversed condition, and to call__builtin_trapif the result is unexpected. use with-fharden-comparesto cover all conditionals. 🌸-fharden-control-flow-redundancy— emit extra code to set booleans when entering basic blocks, and to verify and trap, at function exits, when the booleans do not form an execution path that is compatible with the control flow graph. 🌈
verification takes place before returns, before mandatory tail calls (see below) and, optionally, before escaping exceptions with -fhardcfr-check-exceptions, before returning calls with -fhardcfr-check-returning-calls, and before noreturn calls with -fhardcfr-check-noreturn-calls. 🔍
tail call optimization takes place too late to affect control flow redundancy, but calls annotated as mandatory tail calls by language front-ends, and any calls marked early enough as potential tail calls, would also have verification issued before the call — but these possibilities are merely theoretical, as these conditions can only be met when using custom compiler plugins. very "trust me bro, it's theoretical" 😭
-fhardcfr-skip-leaf— disable-fharden-control-flow-redundancyin leaf functions. short and sweet 🍓-fhardcfr-check-exceptions— when-fharden-control-flow-redundancyis active, check the recorded execution path against the control flow graph at exception escape points, as if the function body was wrapped with a cleanup handler that performed the check and reraised. this option is enabled by default; use-fno-hardcfr-check-exceptionsto disable it. 💜-fhardcfr-check-returning-calls— when-fharden-control-flow-redundancyis active, check the recorded execution path against the control flow graph before any function call immediately followed by a return of its result, if any, so as to not prevent tail-call optimization, whether or not it is ultimately optimized to a tail call. 🦋
this option is enabled by default whenever sibling call optimizations are enabled (see -foptimize-sibling-calls), but it can be enabled (or disabled, using its negated form) explicitly, regardless of the optimizations. 🔥
-fhardcfr-check-noreturn-calls=[always|no-xthrow|nothrow|never]— okay so when-fharden-control-flow-redundancyis doing its thing, this one decides whether we check the recorded execution path against the control flow graph beforenoreturncalls. Your options, bestie:always— check before, like, all of them 💅no-xthrow— check before the ones that aren't expected to return control to the caller through an exception (this is the default, iykyk)nothrow— check before the ones that may not return control to the caller through an exception eithernever— check before none of them, we simply don't ✨
So here's the tea: if you check before a noreturn function that may return control to the caller through an exception, the checking might happen more than once if that exception gets caught in the caller — whether by a handler or a cleanup. 😭 And when -fhardcfr-check-exceptions is also enabled, the compiler avoids associating a noreturn call with the implicitly-added cleanup handler, since that would be redundant with the check performed before the call — but other handlers or cleanups in the function, if they get activated, will modify the recorded execution path and check it again when another checkpoint is hit. And the checkpoint may even be another noreturn call, so the checking may end up performed multiple times. RIP.
Also: various optimizers may cause calls to be marked as noreturn and/or nothrow even when the corresponding attributes aren't there, which can affect where checks go before calls, plus the addition of implicit cleanup handlers for them. This unpredictability, combined with the fact that raising and reraising exceptions frequently amounts to implicitly calling noreturn functions, is exactly why no-xthrow is the default setting for this option — it excludes from noreturn treatment only the internal functions used to (re)raise exceptions, which aren't affected by those optimizations. Slay. 🫶
-fhardened— enables a set of flags for C and C++ that make the generated code more secure without affecting its ABI. The precise flags may change between major releases of GCC, but currently they are:
-D_FORTIFY_SOURCE=3 -D_GLIBCXX_ASSERTIONS
-ftrivial-auto-var-init=zero -fPIE -pie -Wl,-z,relro,-z,now
-fstack-protector-strong -fstack-clash-protection
-fcf-protection=full (x86 GNU/Linux only)You can generate the list of options enabled by -fhardened using the --help=hardened option. ✨
When the system glibc is older than 2.35, -D_FORTIFY_SOURCE=2 gets used instead. Fun.
This option is intended for production builds, not merely debug builds. 💅
Currently, -fhardened is only supported on GNU/Linux targets.
And here's the important bit: -fhardened only enables a particular option if it wasn't already specified anywhere on the command line. So like, -fhardened -fstack-protector will only enable -fstack-protector, but not -fstack-protector-strong. No cap.
-fstack-protector— emits extra code to check for buffer overflows, like stack smashing attacks. 🛡️ It does this by adding a guard variable to functions with vulnerable objects — that includes functions that callalloca, and functions with buffers larger than or equal to 8 bytes. The guards get initialized when a function is entered and then checked when the function exits. If a guard check fails, an error message is printed and the program exits. Only variables that are actually allocated on the stack count, so optimized-away variables or variables allocated in registers don't. Periodt.-fstack-protector-all— like-fstack-protectorexcept that all functions are protected. All of them. Every single one. 👑-fstack-protector-strong— like-fstack-protectorbut it includes additional functions to be protected: those that have local array definitions, or have references to local frame addresses. Same deal as before — only variables actually allocated on the stack count, optimized-away variables or register-allocated variables don't. 💖-fstack-protector-explicit— like-fstack-protectorbut it only protects those functions which have thestack_protectattribute. Very exclusive, very VIP. 🎀-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 since stack overflow is automatically detected on nearly all systems if there's only one stack.
Note that this switch doesn't actually cause checking to be done — the operating system or the language runtime has to do that. The switch just generates code to ensure that they see the stack being extended. Smart, right?
You can additionally specify a string parameter: no means no checking, generic means force the use of old-style checking, and 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 it comes with some drawbacks, and I need you to hear me out:
- Modified allocation strategy for large objects: they're 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's 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's not generally sufficient to protect against stack-clash attacks though — for those you want -fstack-clash-protection. 💅
-fstack-clash-protection— generates code to prevent stack clash style attacks. When this option is enabled, the compiler only allocates 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. 🛡️✨
Most targets don't fully support stack clash protection. However, on those targets -fstack-clash-protection will protect dynamic stack allocations. It may also provide limited protection for static stack allocations if the target supports -fstack-check=specific.
-fstack-limit-register=reg,-fstack-limit-symbol=sym,-fno-stack-limit— generate code to ensure the stack doesn't 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 you can catch the signal without taking special precautions. 🫶
For instance, if the stack starts at absolute address 0x80000000 and grows downwards, you can use the flags -fstack-limit-symbol=__stack_limit and -Wl,--defsym,__stack_limit=0x7ffe0000 to enforce a stack limit of 128KB. Note that this may only work with the GNU linker.
You can locally override stack limit checking by using the no_stack_limit function attribute.
-fsplit-stack— generates code to automatically split the stack before it overflows. The resulting program has a discontiguous stack which can only overflow if the program is unable to allocate any more memory. This is most useful when running threaded programs, since it's no longer necessary to calculate a good stack size to use for each thread. Currently this is 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 isn't an option, then the linker can fix up these calls so that the code compiled without -fsplit-stack always has a large stack. Support for this is implemented in the gold linker in GNU Binutils release 2.21 and later. ✨
-fstrub=disable— disables stack scrubbing entirely, ignoring anystrubattributes. Just, like, nope. 🙈-fstrub=strict— functions default tostrubmodedisabled, and it applies strictly the restriction that only functions associated withstrub-callablemodes (at-calls,callableandalways_inlineinternal) arecallableby functions withstrub-enabled modes (at-callsandinternal). Very strict. Very serious. 💅-fstrub=relaxed— restores the default stack scrub (strub) setting, namely,strubis only enabled as required bystrubattributes associated with function and data types. "Relaxed" means strub contexts are only prevented from calling functions explicitly associated withstrubmodedisabled. This option is only useful to override other-fstrub=*options that precede it in the command line. 🦋-fstrub=at-calls— enablesat-callsstrubmode where viable. The primary use of this option is for testing — it exercises thestrubmachinery in scenarios strictly local to a translation unit. Thisstrubmode modifies function interfaces, so any function that's visible to other translation units, or that has its address taken, will not be affected by this option. Optimization options may also affect viability. See thestrubattribute documentation for details on viability and eligibility requirements. 👑-fstrub=internal— enablesinternalstrubmode where viable. The primary use of this option is for testing. It's intended to exercise thoroughly parts of thestrubmachinery that implement the less efficient, but interface-preservingstrubmode. Functions that wouldn't be affected by this option are quite uncommon. 😭-fstrub=all— enables somestrubmode where viable. When both strub modes are viable,at-callsis preferred.-fdump-ipa-strubmadds function attributes that tell which mode was selected for each function. The primary use of this option is for testing, to exercise thoroughly thestrubmachinery. ✨🔥-fvtable-verify=[std|preinit|none]— bestie, this one is C++ only, no cap. It flips on (or off, with-fvtable-verify=none) the security feature that checks at run time, for every virtual call, that the vtable pointer used for the call is valid for the object’s type and hasn’t been corrupted or overwritten. If it spots a bad vtable pointer at run time, it reports an error and immediately halts the program. 💀
This option makes run-time data structures get built at program startup, and those are what verify the vtable pointers. The std and preinit values control when those structures get built. In both cases they’re built before execution reaches "main". With -fvtable-verify=std, the structures get built after shared libraries have been loaded and initialized. With -fvtable-verify=preinit, they get built before shared libraries have been loaded and initialized.
If this option shows up multiple times on the command line with different values, none has highest priority over both std and preinit; preinit takes priority over std.
-fvtv-debug— when you use this with-fvtable-verify=stdor-fvtable-verify=preinit, it calls debug versions of the runtime functions for the vtable verification feature. It also makes the compiler log info about which vtable pointers it finds for each class. That info goes into a file namedvtv_set_ptr_data.log, in the directory named by the environment variableVTV_LOGS_DIRif that’s defined, or in the current working directory otherwise. 🎀
Note: this feature appends data to the log file. If you want a fresh log file, delete any existing one first, bestie.
-fvtv-counts— this is a debugging flag, periodt. Used with-fvtable-verify=stdor-fvtable-verify=preinit, it makes the compiler 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 info for each compilation unit. The compiler writes it to a file namedvtv_count_data.log, in the directory named byVTV_LOGS_DIRif that’s defined, or the current working directory otherwise. It also counts the size of the vtable pointer sets for each class and writes that intovtv_class_set_sizes.login the same directory. ✨
Note: this feature appends data to the log files. To get fresh log files, delete any existing ones first.
-finstrument-functions— generates instrumentation calls for entry to and exit from functions. Right after function entry and right before function exit, these profiling functions get called with the address of the current function and its call site. (On some platforms,"__builtin_return_address"doesn’t work beyond the current function, so the call-site info 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, and you can look that up exactly in the symbol table.
This instrumentation also happens for functions expanded inline in other functions. The profiling calls show where, conceptually, the inline function is entered and exited. That means addressable versions of those functions have to be available. If all your uses of a function get expanded inline, this may mean extra code size. If you use "extern inline" in your C code, an addressable version of those functions has to be provided. (That’s normally the case anyway, but if you get lucky and the optimizer always expands the functions inline, you might have gotten away without providing static copies.)
A function can be given the attribute "no_instrument_function", and then this instrumentation isn’t done. You can use that for, like, the profiling functions listed above, high-priority interrupt routines, and any functions from which the profiling functions can’t safely be called (maybe signal handlers, if the profiling routines generate output or allocate memory).
-finstrument-functions-once— similar to-finstrument-functions, but the profiling functions get called only once per instrumented function. That means the first profiling function is called after the first entry into the instrumented function, and the second profiling function is called before the exit corresponding to that first entry. 🌸
The definition of "once" for this option is a little vague because the implementation isn’t protected against data races. So the implementation only guarantees that the profiling functions get called at least once per process and at most once per thread — but the calls are always paired. That is: if a thread calls the first function, then it will call the second function, unless it never reaches the exit of the instrumented function.
-finstrument-functions-exclude-file-list=file,file,...— sets the list of functions excluded from instrumentation (see-finstrument-functions). If the file that contains a function definition matches one of file, that function isn’t instrumented. The match is done on substrings: if the file parameter is a substring of the file name, it counts as a match. 💅
For example:
-finstrument-functions-exclude-file-list=/bits/stl,include/sysexcludes any inline function defined in files whose pathnames contain /bits/stl or include/sys.
If, for some reason, you want to include letter , in one of sym, write ,. For example, -finstrument-functions-exclude-file-list=',,tmp' (note the single quote surrounding the option).
-finstrument-functions-exclude-function-list=sym,sym,...— similar to-finstrument-functions-exclude-file-list, but this one sets the list of function names excluded from instrumentation. The function name matched is its user-visible name, likevector<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 counts as a match. For C99 and C++ extended identifiers, the function name has to be given in UTF-8, not using universal character names. 🦋-fpatchable-function-entry=N[,M]— generatesNNOPs right at the beginning of each function, with the function entry point before theMth NOP. IfMis omitted, it defaults to0, so the function entry points to the address just at the first NOP. Those NOP instructions reserve extra space that you can use to patch in whatever instrumentation you want at run time — as long as the code segment is writable. The amount of space is controllable indirectly via the number of NOPs; the NOP instruction used corresponds to the instruction emitted by the internal GCC back-end interface"gen_nop". This behavior is target-specific and may also depend on the architecture variant and/or other compilation options. 👑
For run-time identification, the starting addresses of these areas — which correspond to their respective function entries minus M — are additionally collected in the "__patchable_function_entries" section of the resulting binary.
Note that the value of "__attribute__ ((patchable_function_entry (N,M)))" takes precedence over the command-line option -fpatchable-function-entry=N,M. You can use that to increase the area size or remove it completely on a single function. If "N=0", no pad location is recorded.
The NOP instructions are inserted at — and maybe before, depending on M — the function entry address, even before the prologue. On PowerPC with the ELFv2 ABI, for a function with dual entry points, the local entry point is this function entry address by default. See the -msplit-patch-nops option to change this.
The maximum value of N and M is 65535. On PowerPC with the ELFv2 ABI, for a function with dual entry points, the supported values for M are 0, 2, 6 and 14 when not using -msplit-patch-nops.
✨ Options Controlling the Preprocessor ✨
These options control the C preprocessor, which runs on each C source file before actual compilation. 🧚
If you use the -E option, nothing is done except preprocessing. Some of these options only make sense together with -E, because they make the preprocessor output unsuitable for actual compilation.
In addition to the options listed here, there are a bunch of options controlling search paths for include files documented in Directory Options. Options controlling preprocessor diagnostics are listed in Warning Options.
-D name,--define-macro=name,--define-macro name— predefines name as a macro, with definition1. 🍓-D name=definition,--define-macro=name=definition,--define-macro 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 the shell’s quoting syntax to protect characters such as spaces that mean something in the shell syntax.
If you want 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’re given on the command line. All -imacros file and -include file options are processed after all -D and -U options. 💖
-U name,--undefine-macro=name,--undefine-macro name— cancels any previous definition of name, whether it was built in or provided with a-Doption. Bye-bye macro! 👋-includefile,--include=file,--includefile — bestie this is literally "process file as if#include "file"was the very first line of the primary source file" ✨ but with a plot twist: the first directory searched for file is the preprocessor's working directory, NOT the directory holding the main source file. If it's not there, then it gets searched in the rest of the#include "..."chain like normal. And if you give multiple-includeoptions, the files are included in the order they show up on the command line. order matters, iykyk 🫶-imacrosfile,--imacros=file,--imacrosfile — exactly like-include, except any output produced by scanning file gets yeeted into the void 🗑️. The macros it defines stay defined tho. So this is the slay way to grab all the macros from a header without also processing its declarations. And important: all files specified by-imacrosare processed before all files specified by-include. no cap 💅-undef— do not predefine any system-specific or GCC-specific macros. The standard predefined macros stay defined tho (those are iconic and not going anywhere 👑).-pthread— defines the extra macros you need for using the POSIX threads library. Use this option consistently for both compilation AND linking, fr fr — don't be out here doing one and not the other 😭. This option is supported on GNU/Linux targets, most other Unix derivatives, and also on x86 Cygwin and MinGW targets.-M,--dependencies— 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 the 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. So to avoid mixing that 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.
Also: passing-Mto the driver implies-E, and suppresses warnings with an implicit-w. sneaky but true 🙈
-MM,--user-dependencies— like-Mbut do NOT mention header files 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. (yes, that's the whole point, don't @ me 🦋)
-MFfile — 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.
And if file is -, then the dependencies are written to stdout. cute little dash, huge power 💅
-MG,--print-missing-file-dependencies— in conjunction with an option such as-Mthat requests 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, since a missing header file renders that useless (no thoughts, head empty 💀).
This feature is used in automatic updating of makefiles.
-Mno-modules— disable dependency generation for compiled module interfaces. simple as that, queen 🌸-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. lowkey a lifesaver 🫶
This is typical output:
test.o: test.c test.h
test.h:-MTtarget — 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. your choice bestie 💜
For example, -MT '$(objpfx)foo.o' might give
$(objpfx)foo.o: foo.c-MQtarget — same as-MT, but it quotes any characters which are special to Make. So-MQ '$(objpfx)foo.o'gives
$$(objpfx)foo.o: foo.cThe default target is automatically quoted, as if it were given with -MQ. icon behavior honestly 👑
-MD,--write-dependencies—-MDis equivalent to-M -MF file, except that-Eis not implied. The driver determines file based on whether an-ooption is given. If it is, the driver uses its argument but with a suffix of.d, otherwise it takes the name of the input file, removes any directory components and suffix, and applies a.dsuffix ✨
If -MD is used in conjunction with -E, any -o switch is understood to specify the dependency output file, but if used without -E, each -o is understood to specify a target object file. context is everything, iykyk 🦋
Since -E is not implied, -MD can be used to generate a dependency output file as a side effect of the compilation process. multitasking queen 💅
-MMD,--write-user-dependencies— like-MDexcept it mentions only user header files, not system header files. keep it user-only, we don't need the system drama 🙈-fpreprocessed— tells the preprocessor that the input file has already been preprocessed. This suppresses things like macro expansion, trigraph conversion, escaped newline splicing, and processing of most directives. The preprocessor still recognizes and removes comments, so you can pass a file preprocessed with-Cto the compiler without problems 🎀. In this mode the integrated preprocessor is little more than a tokenizer for the front ends.
-fpreprocessed is implicit if the input file has one of the extensions .i, .ii or .mi. These are the extensions that GCC uses for preprocessed files created by -save-temps.
-fdirectives-only— when preprocessing, handle directives, but do NOT expand macros. the way this option is so specific 😭
The option's behavior depends on the -E and -fpreprocessed options:
- With
-E, preprocessing is limited to the handling of directives such as#define,#ifdef, and#error. Other preprocessor operations, such as macro expansion and trigraph conversion, are not performed. In addition, the-dDoption 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
-Eand-fpreprocessed, the rules for-fpreprocessedtake precedence. This enables full preprocessing of files previously preprocessed with-E -fdirectives-only.
-fdollars-in-identifiers— accept$in identifiers. yes literally just that. dollars in your identifiers, go off 💸-fextended-identifiers— accept universal character names and extended characters in identifiers. This option is enabled by default for C99 (and later C standard versions) and C++. it's giving inclusivity 🌈-fno-canonical-system-headers— when preprocessing, do not shorten system header paths with canonicalization. leave those paths long and dramatic, we respect them 💜-fmax-include-depth=depth — set the maximum depth of the nested#include. The default is200. that's your limit bestie, no infinite include inception 🌀-fsearch-include-path[=kind]— look for input files on the#includepath, not just the current directory. This is particularly useful with C++20 modules, for which both header units and module interface units need to be compiled directly:
g++ -c -std=c++20 -fmodules -fsearch-include-path bits/stdc++.h bits/std.cckind defaults to user, which looks on the #include "..." search path; you can also explicitly specify system for the #include <...> search path. pick your lane ✨
-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 than1or greater than100, the option is ignored (rude but ok). The default is8.-ftrack-macro-expansion[=level]— track locations of tokens across macro expansions. This allows the compiler to emit diagnostics about the current macro expansion stack when a compilation error occurs in a macro expansion. Using this option makes the preprocessor and the compiler consume more memory 😭. The level parameter can be used to choose the level of precision of token location tracking, thus decreasing memory consumption if necessary:- Value
0of level de-activates this option. - Value
1tracks token locations in a degraded mode for the sake of minimal memory overhead. In this mode all tokens resulting from the expansion of an argument of a function-like macro have the same location. - Value
2tracks token locations completely. This value is the most memory hungry.
- Value
When this option is given no argument, the default parameter value is 2.
Note that -ftrack-macro-expansion=2 is activated by default. so you're already living that memory-hungry life, bestie 💀
-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-mapand-fcanon-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. so basically any vibe youriconvcan handle 💅-fwide-exec-charset=charset— Sets the wide execution character set, bestie: the one used for wide string and character constants. The default is one ofUTF-32BE,UTF-32LE,UTF-16BE, orUTF-16LE, whichever corresponds to the width ofwchar_tand the big-endian or little-endian byte order being used for code generation. As with-fexec-charset,charsetcan be any encoding supported by the system'siconvlibrary routine; however, you will have problems with encodings that do not fit exactly inwchar_t. 💅-finput-charset=charset— Sets the input character set, used for translation from the character set of the input file to the source character set used by GCC. The default isUTF-8.charsetcan be any encoding supported by the system'siconvlibrary routine. ✨
If the input character set is UTF-8, warnings about ill-formed code unit sequences are issued if -Winvalid-utf8 is enabled. Otherwise no diagnostics are issued when the input character set matches the execution character set. If they are different, ill-formed code unit sequences result in an error during transcoding to the execution character set.
-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 gets listed, and not the files that were used to create it, because those files are not consulted when a precompiled header is used. 🫶-fpch-preprocess— This option allows use of a precompiled header together with-E. It inserts a special#pragma,#pragma GCC pch_preprocess "filename"in the output to mark the place where the precompiled header was found, and its filename. When-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.
Do not, bestie: 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. 🎀-C,--comments— 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,--comments-in-macros— 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,--no-line-commands— 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,-traditional-cpp,--traditional-cpp— Try to imitate the behavior of pre-standard C preprocessors, as opposed to ISO C preprocessors. See the GNU CPP manual for details. 👑
Note that GCC does not otherwise attempt to emulate a pre-standard C compiler, and these options are only supported with the -E switch, or when invoking CPP explicitly.
-trigraphs,--trigraphs— Support ISO C trigraphs. These are three-character sequences, all starting with??, that are defined by ISO C to stand for single characters. For example,??/stands for\, so'??/n'is a character constant for a newline. 🌈
The nine trigraphs and their replacements are
Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??-
Replacement: [ ] { } # \ ^ | ~By default, GCC ignores trigraphs, but in standard-conforming modes it converts them. See the -std and -ansi options.
-remap— Enable special code to work around file systems which only permit very short file names, such as MS-DOS. 💀-H,--trace-includes— 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,--dump=letters,--dump letters— Says to make debugging dumps during compilation as specified byletters. The flags documented here are those relevant to the preprocessor. Other letters are interpreted by the compiler proper, or reserved for future versions of GCC, and so are silently ignored. If you specify letters whose behavior conflicts, the result is undefined. 😳-dM,--dump=M— 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 gives you a 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.
-dD,--dump=D— Like-dMexcept that it outputs both the#definedirectives and the result of preprocessing. Both kinds of output go to the standard output file.-dN,--dump=N— Like-dD, but emit only the macro names, not their expansions.-dI,--dump=I— Output#includedirectives in addition to the result of preprocessing.-dU,--dump=U— 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.
-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 passoptiondirectly through to the preprocessor. Ifoptioncontains 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. 🍓-Xpreprocessor option— Passoptionas an option to the preprocessor. You can use this to supply system-specific preprocessor options that GCC does not recognize. 💜
If you want to pass an option that takes an argument, you must use -Xpreprocessor twice, once for the option and once for the argument.
-no-integrated-cpp,--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. ✨
✨ Passing Options to the Assembler ✨
You can pass options to the assembler, bestie. 🎀
-Wa,option— Passoptionas an option to the assembler. Ifoptioncontains commas, it is split into multiple options at the commas.-Xassembler option,--for-assembler=option,--for-assembler option— Passoptionas an option to the assembler. You can use this to supply system-specific assembler options that GCC does not recognize. 💅
If you want to pass an option that takes an argument, you must use -Xassembler twice, once for the option and once for the argument.
✨ Options for Linking ✨
These options come into play when the compiler links object files into an executable output file. They are meaningless if the compiler is not doing a link step. 🌟
🧸 object-file-name — the files that aren't special-suffixed
Okay so here's the vibe check on random file names: if a file name doesn't end in a special recognized suffix, GCC just assumes it's naming an object file or a library. That's it, that's the rule ✨
(Object files vs. libraries get told apart by the linker, based on what's actually inside the file contents — not by the name, bestie.)
And if linking actually happens? Those object files get used as input to the linker. Simple as that 🫶
🎀 -c, -S, -E — the "no linker today, thanks" squad
If any of these options is used, then the linker is not run, and object file names should not be used as arguments. Periodt 💅
-c
-S
-E🦋 -flink-libatomic — libatomic? linked, if we can
Enables linking of libatomic if it's supported by the target — and it's enabled by default, no cap 😌
The negative form -fno-link-libatomic can be used to explicitly disable linking of libatomic. Iconic symmetry.
💀 -flinker-output=type — manually steering the link-time optimizer
This option controls code generation of the link-time optimizer. By default the linker output is automatically determined by the linker plugin — it just knows, main character energy. But for debugging the compiler, and if incremental linking with a non-LTO object file is desired, it may be useful to control the type manually. So here are your flavors 🍓
- If type is
exec, code generation produces a static binary. In this case-fpicand-fpieare both disabled. - If type is
dyn, code generation produces a shared library. In this case-fpicor-fPICis preserved, but not enabled automatically. This allows you to build shared libraries without position-independent code on architectures where this is possible, i.e. on x86. - If type is
pie, code generation produces an-fpieexecutable. This results in similar optimizations asexecexcept that-fpieis not disabled if specified at compilation time. - If type is
rel, the compiler assumes that incremental linking is done. The sections containing intermediate code for link-time optimization are merged, pre-optimized, and output to the resulting object file. In addition, if-ffat-lto-objectsis specified, binary code is produced for future non-LTO linking. The object file produced by incremental linking is smaller than a static library produced from the same object files. At link time the result of incremental linking also loads faster than a static library — assuming that the majority of objects in the library are used. Slay, lowkey efficient 💖 - Finally
nolto-relconfigures the compiler for incremental linking where code generation is forced, a final binary is produced, and the intermediate code for later link-time optimization is stripped. When multiple object files are linked together the resulting code is better optimized than with link-time optimizations disabled (for example, cross-module inlining happens), but most of the benefits of whole-program optimizations are lost. Trade-offs, bestie 😭
During the incremental link (by-r) the linker plugin defaults torel. GNU Binutils 2.44 or later is needed to incrementally link LTO objects and non-LTO objects into a single mixed object file. If any of the object files in an incremental link cannot be used for link-time optimization, the linker plugin issues a warning and usesnolto-rel. To maintain whole-program optimization, link such objects into a static library instead.
🔥 -fuse-ld= — pick your linker like you pick your iced coffee
-fuse-ld=bfd— use the bfd linker instead of the default linker.-fuse-ld=gold— use the gold linker instead of the default linker.-fuse-ld=lld— use the LLVM lld linker instead of the default linker.-fuse-ld=mold— use the Modern Linker (mold) instead of the default linker.-fuse-ld=wild— use the Wild linker (wild) instead of the default linker.
Five linkers, one flag, we love options ✨
🌸 -llibrary / -l library — the library-searching icon
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 — it's giving legacy, ngl.)
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 👑
⚠️ Order matters, bestie!! 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. Sofoo.o -lz bar.osearches libraryzafter filefoo.obut beforebar.o. Ifbar.orefers to functions inz, those functions may not be loaded. Cue the sad trombone 💀
🧚 -lobjc — the special-case bestie
You need this special case of the -l option in order to link an Objective-C or Objective-C++ program. That's the whole note. Short and iconic 🎀
🌈 -nostartfiles — no startup files, thank you
Do not use the standard system startup files when linking. The standard system libraries are used normally — unless -nostdlib, -nolibc, or -nodefaultlibs is used. Stack those flags at your own risk, iykyk.
🫶 -nodefaultlibs — no default libs
Do not use the standard system libraries when linking. Only the libraries you specify are passed to the linker, and options specifying linkage of the system libraries, such as -static-libgcc or -shared-libgcc, are ignored. The standard startup files are used normally, unless -nostartfiles is used.
The compiler may generate calls to memcmp, memset, memcpy and memmove. These entries are usually resolved by entries in libc. These entry points should be supplied through some other mechanism when this option is specified 👀
💜 -nolibc — no C library at all
Do not use the C library or system libraries tightly coupled with it when linking. Still link with the startup files, libgcc or toolchain provided language support libraries such as libgnat, libgfortran or libstdc++ — unless options preventing their inclusion are used as well.
This typically removes -lc from the link command line, as well as system libraries that normally go with it and become meaningless when absence of a C library is assumed — for example -lpthread or -lm in some configurations. This is intended for bare-board targets when there is indeed no C library available. Respect the bare metal 🦋
⭐ -nostdlib / --no-standard-libraries — the full "no thanks"
Do not use the standard system startup files or libraries when linking. No startup files and only the libraries you specify are passed to the linker, and options specifying linkage of the system libraries, such as -static-libgcc or -shared-libgcc, are ignored.
The compiler may generate calls to memcmp, memset, memcpy and memmove. These entries are usually resolved by entries in libc. These entry points should be supplied through some other mechanism when this option is specified.
One of the standard libraries bypassed by -nostdlib and -nodefaultlibs is libgcc.a, a library of internal subroutines which GCC uses to overcome shortcomings of particular machines, or special needs for some languages.
🚨 In most cases, you needlibgcc.aeven when you want to avoid other standard libraries. In other words, when you specify-nostdlibor-nodefaultlibsyou should usually specify-lgccas well. This ensures that you have no unresolved references to internal GCC library subroutines. (An example of such an internal subroutine is__main, used to ensure C++ constructors are called.)
🌟 -nostdlib++ — no implicit C++ libs
Do not implicitly link with standard C++ libraries. That's it, that's the whole thing 😌
👑 -e entry / --entry=entry / --entry entry — where do we start?
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. Either way, you're the one calling the shots 💅
-e entry
--entry=entry
--entry entry🎀 -pie / --pie — position independent, dynamically linked
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. Don't skip this step, bestie, fr fr.
💀 -no-pie — absolutely not
Don't produce a dynamically linked position independent executable. The vibe is the opposite of -pie.
🦋 -static-pie / --static-pie — the static PIE combo
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. Self-sufficient AND flexible, we love that for them ✨
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. Same rule as before — consistent or bust.
🌸 -pthread — threads, but make it a linker flag
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. Half-applying it is giving chaos 😭
🧚 -r — relocatable, also known as partial linking
Produce a relocatable object as output. Also known as partial linking. Cute nickname, real functionality 🫶
🔥 -rdynamic — export all the symbols
Pass the flag -export-dynamic to the ELF linker, on targets that support it. This instructs the linker to add all symbols, not only used ones, to the dynamic symbol table.
This option is needed for some uses of dlopen or to allow obtaining backtraces from within a program. Backtraces are self-care, honestly 💖
🎀 -s — strip it all away
Remove all symbol table and relocation information from the executable. Lean. Minimal. Ate. 💅
⭐ -static / --static — no shared libs for you
On systems that support dynamic linking, this overrides -pie and prevents linking with the shared libraries. On other systems, this option has no effect. Nothing. Nada. Zero notes.
🌈 -shared / --shared — make a shared object
Produce a shared object which can then be linked with other objects to form an executable. Not all systems support this option, so, y'know, know your target 🦋
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]
(Yes, the footnote marker is real, it's [1], we don't make the rules.)
👑 -shared-libgcc / -static-libgcc — pick your libgcc flavor
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 — sorry bestie, nothing to pick from 😔
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. Teamwork, but make it linkage 💜
So yeah — the G++ driver just automatically adds -shared-libgcc whenever you build a shared library or a main executable, because C++ programs typically use exceptions, and that's the correct move. It's giving "we got you" energy fr fr.
If you use the GCC driver to create shared libraries instead, you might find they're not always linked with the shared libgcc 😬. Here's the tea: if GCC finds, at its configuration time, that you have a non-GNU linker or a GNU linker that doesn't support option --eh-frame-hdr, it links the shared version of libgcc into shared libraries by default. Otherwise, it takes advantage of the linker and optimizes away the linking with the shared version of libgcc, linking with the static version of libgcc by default. This lets exceptions propagate through such shared libraries, without incurring relocation costs at library load time. Iconic behaviour honestly.
But — and this is the important part bestie — 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. No exceptions (pun intended 💀).
-static-libasan— when the-fsanitize=addressoption is used to link a program, the GCC driver automatically links againstlibasan. Iflibasanis available as a shared library, and the-staticoption is not used, then this links against the shared version oflibasan. The-static-libasanoption directs the GCC driver to linklibasanstatically, without necessarily linking other libraries statically. Just asan, static, only asan 🎀-static-libhwasan— when the-fsanitize=hwaddressoption is used to link a program, the GCC driver automatically links againstlibhwasan. Iflibhwasanis available as a shared library, and the-staticoption is not used, then this links against the shared version oflibhwasan. The-static-libhwasanoption directs the GCC driver to linklibhwasanstatically, without necessarily linking other libraries statically.-static-liblsan— when the-fsanitize=leakoption is used to link a program, the GCC driver automatically links againstliblsan. Ifliblsanis available as a shared library, and the-staticoption is not used, then this links against the shared version ofliblsan. The-static-liblsanoption directs the GCC driver to linkliblsanstatically, without necessarily linking other libraries statically.-static-libtsan— when the-fsanitize=threadoption is used to link a program, the GCC driver automatically links againstlibtsan. Iflibtsanis available as a shared library, and the-staticoption is not used, then this links against the shared version oflibtsan. The-static-libtsanoption directs the GCC driver to linklibtsanstatically, without necessarily linking other libraries statically.-static-libubsan— when the-fsanitize=undefinedoption is used to link a program, the GCC driver automatically links againstlibubsan. Iflibubsanis available as a shared library, and the-staticoption is not used, then this links against the shared version oflibubsan. The-static-libubsanoption directs the GCC driver to linklibubsanstatically, without necessarily linking other libraries statically.-static-libstdc++— when theg++program is used to link a C++ program, it normally automatically links againstlibstdc++. Iflibstdc++is available as a shared library, and the-staticoption is not used, then this links against the shared version oflibstdc++. That's normally fine, no notes 💅. However, it is sometimes useful to freeze the version oflibstdc++used by the program without going all the way to a fully static link. The-static-libstdc++option directs theg++driver to linklibstdc++statically, without necessarily linking other libraries statically. Snapshot the vibes, not the whole universe ✨
-symbolic,--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 — so like, vibe check your platform first 🙈-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. Classic quoting trap, we've all been there 😭
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, so like, respect their boundaries 🫶
-Wl,option,--for-linker=*option*,--for-linkeroption — 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. Commas are basically the group chat separator here 💬- *
-u*symbol,--force-link=symbol,--force-link*symbol*** — pretend the symbol *symbol* is undefined, to force linking of library modules to define it. You can use-u` multiple times with different symbols to force loading of additional library modules. Summon the symbols, make them appear 🔥 -Tbss=addr,-Tdata=addr,-Ttext=addr,-N,-n,-t,-Z,-zkeyword — these options are passed through to the linker without interpretation by GCC. Refer to your linker documentation for the meanings of these options. Not GCC's business, GCC is just the messenger 🦋
✨ Options for Directory Search ✨
These options specify directories to search for header files, for libraries and for parts of the compiler. Basically: where do we go looking for stuff 👀
-Idir,-iquotedir,-isystemdir,-idirafterdir,--include-directory-after=dir,--include-directory-afterdir,--include-directory=dir,--include-directorydir — add the directory dir to the list of directories to be searched for header files during preprocessing.--include-directoryis an alias for-I, while--include-directory-afteris an alias for-idirafter. 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. Quote gang vs angle bracket gang 🎀
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 are searched in left-to-right order, as they appear on the command line. - Directories specified with
-Ioptions are scanned in left-to-right order. - Directories specified with
-isystemoptions are scanned in left-to-right order. - Standard system directories are scanned.
- Directories specified with
-idirafteroptions are scanned in left-to-right order.
You can use -I to override a system header file, substituting your own version, since these directories are searched before the standard system header file directories. However, you should not use this option to add directories that contain vendor-supplied system header files; use -isystem for that. Respect the vendor, bestie 🙏
The -isystem and -idirafter options also mark the directory as a system directory, so that it gets the same special treatment that is applied to the standard system directories.
If a standard system include directory, or a directory specified with -isystem, is also specified with -I, the -I option is ignored. The directory is still searched but as a system directory at its normal position in the system include chain. This is to ensure that GCC's procedure to fix buggy system headers and the ordering for the #include_next directive are not inadvertently changed. If you really need to change the search order for system directories, use the -nostdinc and/or -isystem options. Don't fight the system include chain unless you truly have to 😤
-I-,--include-barrier— split the include path. This option has been deprecated. Please use-iquoteinstead for-Idirectories before the-I-and remove the-I-option. It's giving legacy behaviour, we're moving on 💀
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.
In addition, -I- inhibits the use of the directory of the current file directory as the first search directory for #include "file". There is no way to override this effect of -I-. Like, absolutely no override, periodt.
-iprefixprefix,--include-prefix=prefix,--include-prefixprefix — specify prefix as the prefix for subsequent-iwithprefixoptions. If the prefix represents a directory, you should include the final/. Don't forget the slash, it's the whole plot twist 🔪-iwithprefixdir,-iwithprefixbeforedir,--include-with-prefix=prefix,--include-with-prefixprefix,--include-with-prefix-after=prefix,--include-with-prefix-afterprefix,--include-with-prefix-before=prefix,--include-with-prefix-beforeprefix — 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.
--include-with-prefix and --include-with-prefix-after are both aliases for -iwithprefix, while --include-with-prefix-before is an alias for -iwithprefixbefore. So many aliases, so little time 🌸
-isysroot dir— okay so this is basically--sysroot's little sibling 🫶 it only applies to header files... except on Darwin targets, where it applies to both header files and libraries. See the--sysrootoption for more info 💅-imultilib dir— usediras a subdirectory of the directory that holds the target-specific C++ headers 🎀-imultiarch dir— same vibe but for architecture-specific C++ headers: usediras a subdirectory of the directory containing them ✨-nostdinc,--no-standard-includes— do not search the standard system directories for header files 🚫 only the directories you explicitly named with-I,-iquote,-isystem, and/or-idirafterget searched (plus the directory of the current file, if appropriate). That's it, that's the list 👑-nostdinc++— don't search for header files in the C++-specific standard directories, but still search the other standard directories 💜 (this one's used when building the C++ library, iykyk)--embed-dir=dir,--embed-directory=dir,--embed-directory dir— append thedirdirectory to the list of searched directories for the"#embed"preprocessing directive or the"__has_embed"macro 🌸 and ngl there are no default directories for"#embed"— you gotta supply them yourself, bestie.
If dir begins with = or $SYSROOT, then that = or $SYSROOT gets replaced by the sysroot prefix 🦋 see --sysroot and -isysroot.
-iplugindir=dir— set the directory to search for plugins that get passed by-fplugin=nameinstead of-fplugin=path/name.so💅 real talk: this option is not meant to be used by you, the user — it's only passed by the driver. Hands off 🙈-Ldir,--library-directory=dir,--library-directory dir— add directorydirto the list of directories searched for-l🎀-Bprefix,--prefix=prefix,--prefix prefix— this one tells the compiler where to find its own executables, libraries, include files, and data files 👑
The compiler driver program runs one or more of the subprograms cpp, cc1, as and ld. It tries prefix as a prefix for each program it tries to run — both with and without machine/version/ for the corresponding target machine and compiler version 🌀
For each subprogram to be run, the compiler driver first tries the -B prefix, if any. If that name isn't found, or if -B wasn't specified, 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 gets searched for using the directories in your PATH environment variable.
The compiler checks whether the path from -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's all — the file is left out of the link if it's not found by those means 😭
Another way to specify a prefix much like the -B prefix is to use the environment variable GCC_EXEC_PREFIX 💜
As a special kludge, if the path provided by -B is [dir/]stageN/, where N is a number in the range 0 to 9, then it's replaced by [dir/]include 🦋 this is to help with boot-strapping the compiler. Weird little hack, iconic behavior honestly.
-no-canonical-prefixes,--no-canonical-prefixes— do not expand any symbolic links, do not resolve references to/../or/./, and do not make the path absolute when generating a relative prefix 🙅♀️--sysroot=dir,--sysroot dir— usediras 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 --sysroot applies to libraries, but -isysroot applies to header files 💅
The GNU linker (beginning with version 2.16) has the necessary support for this option. If your linker doesn't support it, the header file aspect of --sysroot still works — but the library aspect does not 💀
--no-sysroot-suffix— for some targets, a suffix gets added to the root directory specified with--sysroot, depending on the other options used, so headers may for example be found indir/suffix/usr/includeinstead ofdir/usr/include🌀 this option disables adding that 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's not the default 🫶 you can figure out the other form by either removing no- or adding it. Easy math.
-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 temporariesnamed_varsenables reuse only for user-defined local variables with namesnonedisables stack reuse completely
The default value is all ✨ The option is needed when the program extends the lifetime of a scoped local variable or a compiler-generated temporary beyond the end point defined by the language 😬 When the 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 doesn't overlap with it. Legacy code that extends 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 the 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 doesn't overlap with it. But some legacy code relies on the behavior of older compilers where temporaries' stack space is not reused — and aggressive stack reuse can lead to runtime errors 😭 this option is used to control the temporary stack reuse optimization.
-ftrapv— generates traps for signed overflow on addition, subtraction, and 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— tells the compiler to assume that signed arithmetic overflow of addition, subtraction, and multiplication wraps around using twos-complement representation 🌀 this flag enables some optimizations and disables others. Same override rules as above:-ftrapvand-fwrapvoverride each other, so-ftrapv -fwrapvon the command-line results in-fwrapvbeing effective. And again — only active options override, so-ftrapv -fwrapv -fno-wrapvon the command-line results in-ftrapvbeing effective 💅-fwrapv-pointer— tells 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🌀 little implication chain, love that for us.-fexceptions— enable exception handling 💖 generates the 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 doesn't affect execution 🫶 if you don't 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 don't normally require it. But you may need to enable it 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're 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 doesn't exist everywhere 😬 moreover, it only allows trapping instructions to throw exceptions — i.e. memory references or floating-point instructions. It does not allow exceptions to be thrown from arbitrary signal handlers such as"SIGALRM"💀 this enables-fexceptions.-fdelete-dead-exceptions— okay so this one tells GCC it's totally fine to yeet instructions that might throw exceptions but don't actually do anything else for the program's execution. Like, if a throw is giving nothing to the plot, it's getting cut. 💀 Important caveat though: this doesn't affect calls to functions, except the ones with thepureorconstattributes. This is on by default for the Ada and C++ compilers, since the language specs say that's allowed. And the optimization passes that actually remove dead exceptions? They get enabled independently at different optimization levels. So like, this flag just gives permission — the vibe check happens elsewhere.-funwind-tables— similar to-fexceptions, except it only generates the static data that's needed and doesn't touch the generated code in literally any other way. You normally don't need to turn this on yourself; instead, a language processor that needs this handling turns it on for you. Let the language processor be the main character here. 🫶-fasynchronous-unwind-tables— generates an unwind table in DWARF format, if the target machine supports it. The table is exact at every single instruction boundary, so it can be used for stack unwinding from asynchronous events — like a debugger or a garbage collector just rolling up uninvited. ✨-fno-gnu-unique— okay so on systems with a 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 stay unique even whenRTLD_LOCALis in the picture. Why does that matter? Because otherwise you get problems when a library is used by two differentRTLD_LOCALplugins, each depending on a definition in one of them, and then they're disagreeing with each other about the binding of the symbol. Messy. 😭
But — and this is the plot twist — it causes dlclose to be ignored for the affected DSOs. So if your program relies on reinitializing a DSO via dlclose and dlopen, you can use -fno-gnu-unique. That's your escape hatch. 🚪
-fpcc-struct-return— returnsshortstructandunionvalues in memory like the longer ones, instead of in registers. Yeah it's less efficient, but the flex is that it allows intercallability between GCC-compiled files and files compiled with other compilers — especially the Portable C Compiler (pcc). 🤝
The precise convention for returning structures in memory depends on the target configuration macros.
And "short structures and unions" means ones whose size and alignment match that of some integer type. No cap.
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. Don't mix these, bestie. 🙈
-freg-struct-return— returnsstructandunionvalues in registers when possible. For small structures this is more efficient than-fpcc-struct-return. Efficient king. 👑
If you specify neither -fpcc-struct-return nor -freg-struct-return, GCC defaults to whichever convention is standard for the target. And if there's no standard convention? GCC defaults to -fpcc-struct-return, except on targets where GCC is the principal compiler. In those cases we get to choose the standard, and we chose the more efficient register return alternative. Slay. 💅
Warning: code compiled with the-freg-struct-returnswitch is not binary compatible with code compiled with the-fpcc-struct-returnswitch. Use it to conform to a non-default application binary interface. Same warning, other direction, still serious. 💀
-fshort-enums— allocates to anenumtype only as many bytes as it needs for the declared range of possible values. Specifically, theenumtype becomes equivalent to the smallest integer type that has enough room. Tiny enum, tiny footprint, iconic. 🎀 One thing though: this option has no effect for an enumeration type with a fixed underlying type.
Warning: the -fshort-enums switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface.
-fshort-wchar— overrides the underlying type forwchar_tto beshort unsigned intinstead of whatever the target's default is. This one's useful for building programs to run under WINE. 🍷
Warning: the -fshort-wchar switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface.
-fcommon— okay so in C code, this option controls where global variables that are defined without an initializer get placed. Those are called tentative definitions in the C standard. And just to be clear: tentative definitions are distinct from declarations of a variable with theexternkeyword — those don't allocate storage at all. 📝
The default is -fno-common, which means the compiler places uninitialized global variables in the BSS section of the object file. And that inhibits the merging of tentative definitions by the linker, so if you accidentally define the same variable in more than one compilation unit, you get a multiple-definition error. Rude but honest.
-fcommon places uninitialized global variables in a common block instead. That lets the linker resolve all tentative definitions of the same variable in different compilation units to the same object — or to a non-tentative definition. Cute, right? Except this behavior is inconsistent with C++, and on many targets it implies a speed and code size penalty on global variable references. 💀 It's mainly useful for enabling legacy code to link without errors. So like, a rescue mission for old code, not a lifestyle choice.
-fno-ident/-Qy/-Qn—-fno-identsuppresses emission of.identassembler directives and causes the#identpreprocessor directive to be ignored. And for the lore:-Qyand-Qnare obsolete synonyms for-fidentand-fno-ident, respectively. Old names, same energy. 🕰️-finhibit-size-directive— don't output a.sizeassembler directive, or anything else that would cause trouble if the function gets split in the middle and the two halves end up placed at locations far apart in memory. This option is used when compilingcrtstuff.c— and you should not need to use it for anything else. Literally not for you, bestie. It's got one job. 🚫-fverbose-asm— puts extra commentary info in the generated assembly code to make it more readable. Honestly this is only of use to people who actually need to read the generated assembly (maybe while debugging the compiler itself). Niche but valid. 🔍
-fno-verbose-asm — the default — omits the extra info 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,"",@progbitsAnd here's the fine print: the comments are intended for humans rather than machines, and hence the precise format of the comments is subject to change. So don't build your whole parser on top of them, k? 🙏
-frecord-gcc-switches— this switch causes the command line used to invoke the compiler to be recorded into the object file being created. It's only implemented on some targets, and the exact format of the recording is target and binary file format dependent, but it usually takes the form of a section containing ASCII text. This switch is related to-fverbose-asm, but that one only records info in the assembler output file as comments, so it never reaches the object file. See also-grecord-gcc-switchesfor another way of storing compiler options into the object file. Two routes, same destination vibe. 🗂️-fpic— generates position-independent code (PIC) suitable for use in a shared library, if it's 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 just so we're clear, the dynamic loader is not part of GCC, it's part of the operating system. 🖥️
If the GOT size for the linked executable exceeds a machine-specific maximum size, you get an error message from the linker saying -fpic doesn't work. In that case, recompile with -fPIC instead. The maximums, for the record: 8k on the SPARC, 28k on AArch64, and 32k on the m68k and RS/6000. The x86 has no such limit. Lucky them. ✨
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. And code generated for the IBM RS/6000 is always position-independent — it doesn't even have to try. 💅
When this flag is set, the macros __pic__ and __PIC__ are defined to 1.
-fPIC— bestie, if the target machine supports it, this emits position-independent code, which is what you want for dynamic linking and it dodges any limit on the size of the global offset table. It actually makes a difference on AArch64, m68k, PowerPC and SPARC.
Position-independent code needs special support, so it only works on certain machines. Periodt.
When this flag is set, the macros __pic__ and __PIC__ get defined to 2. 🫶
-fpie,-fPIE— these are like the lowkey cousins of-fpicand-fPIC, except the position-independent code they generate can only be linked into executables. Usually you'd use these to compile code that's gonna get linked with the-pieGCC option.
Both -fpie and -fPIE define the macros __pie__ and __PIE__. The value is 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. It's giving efficient — you eliminate PLT stubs and expose GOT loads to optimizations. On architectures like 32-bit x86, where PLT stubs expect the GOT pointer in a specific register, this hands the compiler way more register allocation freedom. ✨
Lazy binding requires the PLT, so heads up: with -fno-plt all external symbols are resolved at load time.
Alternatively you can use the function attribute noplt to skip calls through the PLT for specific external functions only.
In position-dependent code, a few targets also convert calls to functions that are marked to not use the PLT into GOT calls instead.
-fno-jump-tables— do not use jump tables forswitchstatements, even where that would be more efficient than other code generation strategies. This one's for when you're building code that's part of a dynamic linker and cannot reference the address of a jump table — pair it with-fpicor-fPIC. Ngl, on some targets jump tables don't need a GOT at all, and then this option isn't needed. 🙈-fno-bit-tests— do not use bit tests forswitchstatements, even where it would be more efficient than other code generation strategies. That's it, that's the vibe.-ffixed-reg— treat the register named reg as a fixed register; generated code should never refer to it (except maybe as a stack pointer, frame pointer, or in some other fixed role).
reg must be the name of a register. The accepted register names are machine-specific and are defined in the REGISTER_NAMES macro in the machine description macro file.
This flag does not have a negative form, because it specifies a three-way choice. 💀
-fcall-used-reg— treat the register named reg as an allocable register that gets clobbered by function calls. It may be allocated for temporaries or variables that don't live across a call. Functions compiled this way do not save and restore the register reg.
It is an error to use this flag with the frame pointer or stack pointer. And using it for other registers that have fixed pervasive roles in the machine's execution model produces disastrous results. 😭
This flag does not have a negative form, because it specifies a three-way choice.
-fcall-saved-reg— treat the register named reg as an allocable register saved by functions. It may be allocated even for temporaries or variables that live across a call. Functions compiled this way save and restore the register reg if they use it.
It is an error to use this flag with the frame pointer or stack pointer. Use of this flag for other registers that have fixed pervasive roles in the machine's execution model produces disastrous results.
A different sort of disaster results from using this flag for a register in which function values may be returned. Do not do that. 🙏
This flag does not have a negative form, because it specifies a three-way choice.
-fpack-struct[=n]— without a value specified, pack all structure members together without holes. When a value n is given (it must be a small power of two), pack structure members according to that value, representing the maximum alignment — that is, objects with default alignment requirements larger than this are output potentially unaligned at the next fitting location.
Warning: the -fpack-struct switch 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 helping you 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. And not all targets provide complete support for this switch. 🫠
-ftls-model=model— alters 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 that aren't visible outside of the translation unit, or if-fpicisn't given on the command line.
The default without -fpic is initial-exec; with -fpic the default is global-dynamic. ✨
-ftrampolines— for targets that normally need trampolines for nested functions, always generate them instead of using descriptors. Otherwise, for targets that don't need them — like for example HP-PA or IA-64 — do nothing.
A trampoline is a small piece of code created at run time on the stack when the address of a nested function is taken, and it's used to call the nested function indirectly. So yeah, it requires the stack to be made executable for the program to work properly. 😳
-fno-trampolines is enabled by default on a language-by-language basis, so the compiler can 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, bestie. Extremely.
For languages other than Ada, the -ftrampolines and -fno-trampolines options currently have no effect, and trampolines are always generated on platforms that need them for nested functions.
-ftrampoline-impl=[stack|heap]— by default, trampolines are generated on the stack. But some platforms (such as the Apple M1) do not permit an executable stack. Compiling with-ftrampoline-impl=heapgenerates calls to__gcc_nested_func_ptr_createdand__gcc_nested_func_ptr_deletedin order to allocate and deallocate trampoline space on the executable heap. Those functions are implemented in libgcc and will only be provided on specific targets: x86_64 Darwin, x86_64 and aarch64 Linux.
PLEASE NOTE: heap trampolines are not guaranteed to be correctly deallocated if yousetjmp, instantiate nested functions, and thenlongjmpback to a state prior to having allocated those nested functions. 💀
-fvisibility=[default|internal|hidden|protected]— set the default ELF image symbol visibility to the specified option — all symbols are marked with this unless overridden within the code. Using this feature can very substantially improve linking and load times of shared object libraries, produce more optimized code, provide near-perfect API export, and prevent symbol clashes. It is strongly recommended that you use this in any shared objects you distribute. 👑
Despite the nomenclature, default always means public — i.e. available to be linked against from outside the shared object. protected and internal are pretty useless in real-world usage, so the only other commonly used option is hidden. If -fvisibility isn't specified, the default is default, i.e. make every symbol public.
A good explanation of the benefits of ensuring ELF symbols have the correct visibility is given by How To Write Shared Libraries by Ulrich Drepper (which can be found at https://www.akkadia.org/drepper/) — however, a superior solution made possible by this option is, instead of marking things hidden when the default is public, to make the default hidden and mark things public. This is the norm with DLLs on Windows, and with -fvisibility=hidden plus __attribute__ ((visibility("default"))) instead of __declspec(dllexport) you get almost identical semantics with identical syntax. Absolute win for everyone working on cross-platform projects. 🫶
For those adding visibility support to existing code, you may find #pragma GCC visibility of use. It works by you enclosing the declarations you wish to set visibility for with (for example) #pragma GCC visibility push(hidden) and #pragma GCC visibility pop. Bear in mind that symbol visibility should be viewed as part of the API interface contract, and thus all new code should always specify visibility when it isn't the default; i.e. declarations only for use within the local DSO should always be marked explicitly as hidden so as to avoid PLT indirection overheads — making this abundantly clear also aids readability and self-documentation of the code. Note that due to ISO C++ specification requirements, operator new and operator delete must always be of default visibility. 💅
Be aware that headers from outside your project — in particular system headers and headers from any other library you use — may not be expecting to be compiled with visibility other than the default. You may need to explicitly say #pragma GCC visibility push(default) before including any such headers. 🙈
Okay so extern declarations are literally not affected by -fvisibility, which means a whole lot of code can get recompiled with -fvisibility=hidden and need zero modifications. 💅 But plot twist: this means calls to extern functions with no explicit visibility still use the PLT, so it’s way more effective to use __attribute ((visibility)) and/or #pragma GCC visibility to tell the compiler which extern declarations should be treated as hidden.
Also note that -fvisibility does affect C++ vague linkage entities. That means, for example, an exception class thrown between DSOs must be explicitly marked with default visibility so the type_info nodes get unified between the DSOs. ✨
For an overview of these techniques, their benefits, and how to use them, peep https://gcc.gnu.org/wiki/Visibility.
-fstrict-volatile-bitfields— bestie, you want this option if accesses to volatile bit-fields (or other structure fields, though the compiler usually honors those types anyway) should use a single access of the width of the field’s type, aligned to a natural alignment if possible. For example, targets with memory-mapped peripheral registers might require all such accesses to be 16 bits wide; with this flag you can declare all peripheral bit-fields asunsigned short(assumingshortis 16 bits on these targets) to force GCC to use 16-bit accesses instead of, perhaps, a more efficient 32-bit access.
If this option is disabled, the compiler uses the most efficient instruction. In the previous example, that might be a 32-bit load instruction, even though that accesses bytes that do not contain any portion of the bit-field, or memory-mapped registers unrelated to the one being updated. 😭
In some cases, such as when the packed attribute is applied to a structure field, it may not be possible to access the field with a single read or write that is correctly aligned for the target machine. In this case GCC falls back to generating multiple accesses rather than code that will fault or truncate the result at run time.
Note: due to restrictions of the C/C++11 memory model, write accesses are not allowed to touch non bit-field members. It is therefore recommended to define all bits of the field’s type as bit-field members. 🫶
The default value of this option is determined by the application binary interface for the target processor.
-fsync-libcalls— this option controls whether any out-of-line instance of the__syncfamily of functions may be used to implement the C++11__atomicfamily of functions. 👑
The default value of this option is enabled, so the only useful form is -fno-sync-libcalls. This option is used in the implementation of the libatomic runtime library.
-fzero-init-padding-bits=value— guarantees zero initialization of padding bits in automatic variable initializers. Certain languages guarantee zero initialization of padding bits in certain cases, e.g. C23 when using empty initializers ({}), or C++ when using zero-initialization, or C guarantees that fields not specified in an initializer have their padding bits zero initialized. This option lets you change when padding bits in initializers are guaranteed to be zero initialized. The default is-fzero-init-padding-bits=standard, which makes no further guarantees than the corresponding standard. 🌸 E.g.
struct A { char a; unsigned long long b; char c; };
union B { char a; unsigned long long b; };
struct A a = {}; // C23 guarantees padding bits are zero.
struct A b = { 1, 2, 3 }; // No guarantees.
union B c = {}; // C23 guarantees padding bits are zero.
union B d = { 1 }; // No guarantees.-fzero-init-padding-bits=unions guarantees zero initialization of padding bits in unions on top of what the standards guarantee, if the initializer of a union is empty (then all bits of the union are zero initialized) or if the initialized member of the union is smaller than the size of the union (in that case it guarantees padding bits outside of the initialized member are zero initialized). This was the GCC behavior before GCC 15 and in the above example guarantees zero initialization of the last sizeof (unsigned long long) - 1 bytes in the union. 💖
-fzero-init-padding-bits=all guarantees additionally zero initialization of padding bits of other aggregates, so the padding in between b.a and b.b (if any) and tail padding in the structure (if any).
💻 GCC Developer Options 💻
This section describes command-line options that are primarily of interest to GCC developers, including options to support compiler testing and investigation of compiler bugs and compile-time performance problems. This includes options that produce debug dumps at various points in the compilation; options that print statistics such as memory use and execution time; and options that print information about GCC’s configuration, such as where it searches for libraries. You should rarely need any of these options for ordinary compilation and linking tasks. 😌
Many developer options that cause GCC to dump output to a file take an optional =filename suffix. You can specify stdout or - to dump to standard output, and stderr for standard error. ✨
If =filename is omitted, a default dump file name is constructed by concatenating the base dump file name, a pass number, phase letter, and pass name. The base dump file name is the name of the output file produced by the compiler if explicitly specified and not an executable; otherwise it is the source file name. The pass number is determined by the order passes are registered with the compiler’s pass manager. This is generally the same as the order of execution, but passes registered by plugins, target-specific passes, or passes that are otherwise registered late are numbered higher than the pass named final, even if they are executed earlier. The phase letter is one of i (inter-procedural analysis), l (language-specific), r (RTL), or t (tree). The files are created in the directory of the output file. 🎀
-fcallgraph-info,-fcallgraph-info=MARKERS— makes the compiler output callgraph information for the program, on a per-object-file basis. The information is generated in the common VCG format. It can be decorated with additional, per-node and/or per-edge information, if a list of comma-separated markers is additionally specified. When thesumarker is specified, the callgraph is decorated with stack usage information; it is equivalent to-fstack-usage. When thedamarker is specified, the callgraph is decorated with information about dynamically allocated objects. 🦋
When compiling with -flto, no callgraph information is output along with the object file. At LTO link time, -fcallgraph-info may generate multiple callgraph information files next to intermediate LTO output files.
-dletters,--dump=letters,--dump letters,-fdump-rtl-pass,-fdump-rtl-pass-options,-fdump-rtl-pass-options=filename— says to make debugging dumps during compilation at times specified byletterswhen using-dor bypasswhen using-fdump-rtl. This is used for debugging the RTL-based passes of the compiler. 👑
Some -dletters switches have different meaning when -E is used for preprocessing.
The -options form allows greater control over the details of the dump. See -fdump-tree.
Here are actual instances of command-line options following these patterns 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— these two enable dumping after the two branch target load optimization passes.-fdump-rtl-bypass— dump after jump bypassing and control flow optimizations.-fdump-rtl-combine— dump after the RTL instruction combination pass.-fdump-rtl-compgotos— dump after duplicating the computed gotos.-fdump-rtl-ce1,-fdump-rtl-ce2,-fdump-rtl-ce3— these enable dumping after the three if conversion passes.-fdump-rtl-cprop_hardreg— dump after hard register copy propagation.-fdump-rtl-csa— dump after combining stack adjustments.-fdump-rtl-cse1,-fdump-rtl-cse2— these enable dumping after the two common subexpression elimination passes.-fdump-rtl-dce— dump after the standalone dead code elimination passes.-fdump-rtl-dbr— dump after delayed branch scheduling.-fdump-rtl-dce1,-fdump-rtl-dce2— these enable dumping after the two dead store elimination passes.-fdump-rtl-eh— dump after finalization of EH handling code.-fdump-rtl-eh_ranges— dump after conversion of EH handling range regions.-fdump-rtl-expand— dump after RTL generation.-fdump-rtl-fwprop1,-fdump-rtl-fwprop2— these enable dumping after the two forward propagation passes.-fdump-rtl-gcse1,-fdump-rtl-gcse2— these enable dumping after global common subexpression elimination.-fdump-rtl-init-regs— dump after the initialization of the registers.-fdump-rtl-initvals— dump after the computation of the initial value sets.-fdump-rtl-into_cfglayout— dump after converting to cfglayout mode.-fdump-rtl-ira— dump after iterated register allocation.-fdump-rtl-jump— dump after the second jump optimization.-fdump-rtl-loop2— enables dumping after the rtl loop optimization passes. 🌈-fdump-rtl-mach— dumps the tea after the machine dependent reorganization pass runs (if that pass even exists on your target, bestie 💅).-fdump-rtl-mode_sw— dump after it removes those redundant mode switches. bye redundant switches 👋-fdump-rtl-rnreg— dump after register renumbering. numbers get a lil makeover ✨-fdump-rtl-outof_cfglayout— dump after converting out of cfglayout mode.-fdump-rtl-peephole2— dump after the peephole pass. yes it's called that, i don't make the rules 🫶-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. two of them, one for each 💫-fdump-rtl-ree— dump after sign/zero extension elimination.-fdump-rtl-seqabstr— dump after common sequence discovery.-fdump-rtl-shorten— dump after shortening branches.-fdump-rtl-split1,-fdump-rtl-split2,-fdump-rtl-split3,-fdump-rtl-split4,-fdump-rtl-split5— these options enable dumping after five rounds of instruction splitting. five. whole. rounds. 🔥-fdump-rtl-sms— dump after modulo scheduling. lowkey niche though: this pass is only run on some architectures 😭-fdump-rtl-stack— dump after conversion from GCC's "flat register file" registers to the x87's stack-like registers. also architecture-locked, sorry: 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-vartrack— dump after variable tracking. it's literally tracking the variables 📍-fdump-rtl-vregs— dump after converting virtual registers to hard registers.-fdump-rtl-web— dump after live range splitting.-fdump-rtl-regclass,-fdump-rtl-subregs_of_mode_init,-fdump-rtl-subregs_of_mode_finish,-fdump-rtl-dfinit,-fdump-rtl-dfinish— okay so these dumps are defined but always produce empty files. yes queen, an empty file, that's the whole bit 💀 don't ask, just accept.-da,--dump=a,-fdump-rtl-all— produce all the dumps listed above. all of them. every single one. main character energy 👑-dA,--dump=A— annotate the assembler output with miscellaneous debugging information.-dD,--dump=D— dump all macro definitions, at the end of preprocessing, in addition to normal output.-dH,--dump=H— produce a core dump whenever an error occurs. no cap, it really does that 🙈-dp,--dump=p— annotate the assembler output with a comment indicating which pattern and alternative is used. and the length and cost of each instruction are also printed. so much lore in the comments 🎀-dP,--dump=P— dump the RTL in the assembler output as a comment before each instruction. also turns on-dpannotation, package deal ✨-dx,--dump=x— just generate RTL for a function instead of compiling it. usually used with-fdump-rtl-expand.
-fdump-debug— dump debugging information generated during the debug generation phase.-fdump-earlydebug— dump debugging information generated during the early debug generation phase. early bird gets the debug 🦋-fdump-noaddr— when doing debugging dumps, suppress address output. why do we care? because this makes it more feasible to usediffon debugging dumps for compiler invocations with different compiler binaries and/or different text / bss / data / heap / stack / dso start locations. diff-ability is the vibe check ✅-freport-bug— collect and dump debug information into a temporary file if an internal compiler error (ICE) occurs. it's giving "save the receipts for the group chat" 📸-fdump-unnumbered— when doing debugging dumps, suppress instruction numbers and address output. this makes it more feasible to usediffon debugging dumps for compiler invocations with different options, in particular with and without-g.-fdump-unnumbered-links— when doing debugging dumps (see-doption above), suppress instruction numbers for the links to the previous and next instructions in a sequence.-fdump-internal-locations— dump detailed information about GCC's internal representation of source code locations. it's literally the receipts of where everything came from 🧚
🌈 the ipa dumps 🌈
-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. everything. all at once. iconic 💅cgraph— dumps information about call-graph optimization, unused function removal, and inlining decisions.inline— dump after function inlining.strubm— dump after selecting "strub" modes, and recording the selections as function attributes.strub— dump "strub" transformations: interface changes, function wrapping, and insertion of builtin calls for stack scrubbing and watermarking.
additionally, the options -optimized, -missed, -note, and -all can be provided, with the same meaning as for -fopt-info, defaulting to -optimized.
for example, -fdump-ipa-inline-optimized-missed will emit information on callsites that were inlined, along with callsites that were not inlined. both. the slay and the flop. 😭
by default, the dump will contain messages about successful optimizations (equivalent to -optimized) together with low-level details about the analysis.
-fdump-ipa-clones— create a dump file containing information about creation of call graph node clones and removals of call graph nodes during inter-procedural optimizations and transformations. its main intended use is that tools that create live-patches can determine the set of functions that need to be live-patched to completely replace a particular function (see-flive-patching). the file name is generated by appending suffix"ipa-clones"to the source file name, and the file is created in the same directory as the output file. each entry in the file is on a separate line containing semicolon separated fields. ok so be ready for the field breakdown, it's long but it's important 🫶
in the case of call graph clone creation, the individual fields are:
- String
"Callgraph clone". - Name of the function being cloned as it is presented to the assembler.
- A number that uniquely represents the function being cloned in the call graph. note that the number is unique only within a compilation unit or within whole-program analysis but is likely to be different in the two phases.
- The file name of the source file where the function is defined.
- The line on which the function definition is located.
- The column where the function definition is located.
- Name of the new function clone as it is presented to the assembler.
- A number that uniquely represents the new function clone in the call graph. note that the number is unique only within a compilation unit or within whole-program analysis but is likely to be different in the two phases.
- The file name of the source file where the source code location of the new clone points to.
- The line to which the source code location of the new clone points to.
- The column to which the source code location of the new clone points to.
- A string that determines the reason for cloning.
in the case of call graph clone removal, the individual fields are:
- String
"Callgraph removal". - Name of the function being removed as it would be presented to the assembler.
- A number that uniquely represents the function being cloned in the call graph. note that the number is unique only within a compilation unit or within whole-program analysis but is likely to be different in the two phases.
- The file name of the source file where the function is defined.
- The line on which the function definition is located.
- The column where the function definition is located.
yes field 3 in the removal list says "the function being cloned" even though it's a removal. that's what the page says, we don't rewrite the lore 💀
-fdump-lang— dump language-specific information. the file name is made by appending.langto the source file name.-fdump-lang-all,-fdump-lang-switch,-fdump-lang-switch-options,-fdump-lang-switch-options=filename— control the dumping of language-specific information. the options and filename portions behave as described in the-fdump-treeoption.-fdump-tree-allenables all language-specific dumps; other options vary with the language. for instance, see-fdump-passes— print on stderr the list of optimization passes that are turned on and off by the current command-line options. a whole guest list of passes, on stderr 🎀-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. working backwards is such a mood 🔍
okay so slim is giving “don’t dump every little thing just because you stepped into a scope” 💅 — when you’re dumping front-end intermediate representations, it inhibits dumping members of a scope or body of a function merely because that scope has been reached. Only dump those items when they’re directly reachable by some other path. ✨
When dumping pretty-printed trees, this option also inhibits dumping the bodies of control structures.
When dumping RTL, it prints 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. 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).folding— enable dumping information about match-and-simplify (match.pd) patterns, when they are applied.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 thestderroutput 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.
next up: -fopt-info and its little family of forms, aka “tell me what the optimizer is doing” mode 👑
-fopt-info-fopt-info-options-fopt-info-options=filename
Controls optimization dumps from various optimization passes. If the -options form is used, options is a list of - separated option keywords to select the dump details and optimizations.
The options can be divided into three groups:
- 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. ⚠️
The following options control which kinds of messages should be emitted:
optimized— print information when an optimization is successfully applied. It is up to a pass to decide which information is relevant. For example, the vectorizer passes print the source location of loops which are successfully vectorized.missed— print information about missed optimizations. Individual passes control which information to include in the output.note— print verbose information about optimizations, such as certain transformations, more detailed messages about decisions etc.all— print detailed optimization information. This includesoptimized,missed, andnote.
The following option controls the dump verbosity:
internals— by default, only “high-level” messages are emitted. This option enables additional, more detailed, messages, which are likely to only be of interest to GCC developers.
One or more of the following option keywords can be used to describe a group of optimizations:
ipa— enable dumps from all interprocedural optimizations.loop— enable dumps from all loop optimizations.inline— enable dumps from all inlining optimizations.omp— enable dumps from all OMP (Offloading and Multi Processing) optimizations.vec— enable dumps from all vectorization optimizations.optall— enable dumps from all optimizations. This is a superset of the optimization groups listed above.
If options is omitted, it defaults to optimized-optall, which means to dump messages about successful optimizations from all the passes, omitting messages that are treated as “internals”.
If the filename is provided, then the dumps from all the applicable optimizations are concatenated into the filename. Otherwise the dump is output onto stderr. Though multiple -fopt-info options are accepted, only one of them can include a filename. If other filenames are provided then all but the first such option are ignored.
Note that the output filename is overwritten in case of multiple translation units. If a combined output from multiple translation units is desired, stderr should be used instead. 😭
In the following example, the optimization info is output to stderr:
gcc -O3 -fopt-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. ✨
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. 💀
-fsave-optimization-record — write a SRCFILE.opt-record.json.gz file detailing what optimizations were performed, for those optimizations that support -fopt-info.
This option is experimental and the format of the data within the compressed JSON file is subject to change. 🫶
It is roughly equivalent to a machine-readable version of -fopt-info-all, as a collection of messages with source file, line number and column number, with the following additional data for each message:
- the execution count of the code being optimized, along with metadata about whether this was from actual profile data, or just an estimate, allowing consumers to prioritize messages by code hotness,
- the function name of the code being optimized, where applicable,
- the “inlining chain” for the code being optimized, so that when a function is inlined into several different places (which might themselves be inlined), the reader can distinguish between the copies,
- objects identifying those parts of the message that refer to expressions, statements or symbol-table nodes, which of these categories they are, and, when available, their source code location,
- the GCC pass that emitted the message, and
- the location in GCC’s own code from which the message was emitted.
Additionally, some messages are logically nested within other messages, reflecting implementation details of the optimization passes.
-fsched-verbose=n — on targets that use instruction scheduling, this option controls the amount of debugging output the scheduler prints to the dump files. 🌈
For n greater than zero, -fsched-verbose outputs the same information as -fdump-rtl-sched1 and -fdump-rtl-sched2. For n greater than one, it also 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.
-fenable-kind-pass and -fdisable-kind-pass=range-list — this is a set of options that are used to explicitly disable/enable optimization passes. These options are intended for use for debugging GCC. Compiler users should use regular options for enabling/disabling passes instead. 💜
-fdisable-ipa-pass— turn off the IPA pass called pass, bestie. pass is literally the pass name, okay? And if that same pass gets statically invoked in the compiler like, multiple times, you gotta tack on a sequential number starting from 1. ✨-fdisable-rtl-pass-fdisable-rtl-pass=range-list— turn off the RTL pass pass. Same deal: pass is the pass name, and if it's statically invoked multiple times, append a sequential number starting from 1. Nowrange-listis a comma-separated list of function ranges or assembler names, and each range is a number pair separated by a colon — inclusive on both ends, no cap. If the range is trivial you can just simplify the number pair into a single number. So if a function's call graph node's uid falls inside one of the ranges you specified, the pass is disabled for that function. The uid shows up in the function header of a dump file, and the pass names can be dumped with-fdump-passes, iykyk. 💅-fdisable-tree-pass-fdisable-tree-pass=range-list— turn off the tree pass pass. See-fdisable-rtlfor the description of the option arguments, because it's the same vibe. 🦋-fenable-ipa-pass— turn on the IPA pass pass. pass is the pass name; if the same pass is statically invoked in the compiler multiple times, append a sequential number starting from 1. Slay. ✨-fenable-rtl-pass-fenable-rtl-pass=range-list— turn on the RTL pass pass. See-fdisable-rtlfor the option argument description and examples, since it's literally the same shape. 💖-fenable-tree-pass-fenable-tree-pass=range-list— turn on the tree pass pass. See-fdisable-rtlfor the description of option arguments. Periodt. 👑
Here are some examples showing uses of these options, and honestly they ate:
# 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, so no promises there.-fchecking=2enables further internal consistency checking — and heads up, that one might affect code generation. 🙈-frandom-seed=string— this option gives GCC a seed to use in place of random numbers when it's generating certain symbol names that have to be different in every compiled file. It's also used to place unique stamps in coverage data files and the object files that produce them. So yeah — you can use-frandom-seedto produce reproducibly identical object files. 🎀
The string can either be a number (decimal, octal or hex) or an arbitrary string (in which case it's converted to a number by computing CRC32).
The string should be different for every file you compile. Not optional, bestie.
-save-temps--save-temps— store the usual "temporary" intermediate files permanently, naming them as auxiliary output files, as specified described under-dumpbaseand-dumpdir. 🫶
When it's used in combination with the -x command-line option, -save-temps is sensible enough to avoid overwriting an input source file with the same extension as an intermediate file. The corresponding intermediate file may be obtained by renaming the source file before using -save-temps. Iconic behaviour honestly.
-save-temps=cwd— equivalent to-save-temps -dumpdir ./.🌸-save-temps=obj— equivalent to-save-temps -dumpdir outdir/, whereoutdir/is the directory of the output file specified after the-ooption, including any directory separators. If the-ooption isn't used, the-save-temps=objswitch behaves like-save-temps=cwd. 🦋-specs=file--specs=file--specs file— process file after the compiler reads in the standard specs file, so you can override the defaults that thegccdriver program uses when determining what switches to pass tocc1,cc1plus,as,ld, etc. More than one-specs=filecan be specified on the command line, and they're processed in order, from left to right. Main character energy for real. 👑-time[=file]— report the CPU time taken by each subprocess in the compilation sequence. For C source files, this is the compiler proper and assembler (plus the linker if linking is done). ⏱️
Without the specification of an output file, the output looks like this:
# cc1 0.12 0.01
# as 0.00 0.01The 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. Lowkey so useful for debugging. 💜
-fdump-final-insns[=file]— dump the final internal representation (RTL) to file. If the optional argument is omitted (or if file is"."), the name of the dump file is determined by appending".gkd"to the dump base name, see-dumpbase. ✨-fcompare-debug[=opts]— if no error occurs during compilation, run the compiler a second time, adding opts and-fcompare-debug-secondto the arguments passed to the second compilation. Dump the final internal representation in both compilations, and print an error if they differ. Double-checking the fit, basically. 🔥
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's used for opts, otherwise the default -gtoggle is used.
-fcompare-debug=, with the equal sign but without opts, is equivalent to -fno-compare-debug, which disables the dumping of the final representation and the second compilation, preventing even GCC_COMPARE_DEBUG from taking effect.
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. Honest.
-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's given. This is mainly intended to be used with-fcompare-debug. 💅-fvar-tracking-assignments-toggle— toggle-fvar-tracking-assignments, in the same way that-gtoggletoggles-g. Symmetry, we love to see it. 🎀-Q— when used on the command line prior to--help=,-Qacts as a modifier to the help output. 🌈
Otherwise, this option makes the compiler print out each function name as it is compiled, and print some statistics about each pass when it finishes. Chatty queen.
-ftime-report— makes the compiler print some statistics tostderrabout the time consumed by each pass when it finishes. 💖
If SARIF output of diagnostics was requested via -fdiagnostics-format=sarif-file or -fdiagnostics-format=sarif-stderr then the -ftime-report information is instead emitted in JSON form as part of SARIF output. The precise format of this JSON data is subject to change, and the values may not exactly match those emitted to stderr due to being written out at a slightly different place within the compiler. 🙈
-ftime-report-details— record the time consumed by infrastructure parts separately for each pass. Detail-oriented icon. ⭐-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 tostderrusing the same format as n minus 10. Math! 🧚-flto-report— prints a report with internal details on the workings of the link-time optimizer. The contents of this report vary from version to version. It's meant to be useful to GCC developers when processing object files in LTO mode (via-flto). 🌸
Disabled by default. Sad but true.
-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. Memory check, love that for us. 💜-fmem-report-wpa— makes the compiler print some statistics about permanent memory allocation for the WPA phase only. ✨-fpre-ipa-mem-report-fpost-ipa-mem-report— makes the compiler print some statistics about permanent memory allocation before or after interprocedural optimization. Before or after, your call. 🔥-fmultiflags— this option enables multilib-aware "TFLAGS" to be used to build target libraries with options different from those the compiler is configured to use by default, through the use of specs set up by compiler internals, by the target, or by builders at configure time. Big config energy. 👑
Like TFLAGS, this is the whole “build target libraries for portable baseline environments while the compiler defaults to more demanding ones” situation. And honestly? It's useful because users can easily override the defaults the compiler is configured to use when building their own programs — if those defaults aren't ideal for their target environment — whereas rebuilding the runtime libraries is usually not as easy or desirable. 💅
Unlike TFLAGS, using specs lets different flags get selected for different multilibs. The way to do that: build with make TFLAGS=-fmultiflags, after configuring --with-specs=%{fmultiflags:...}. ✨
Heads-up: this option is discarded by the driver once it's done processing driver self spec.
It's also useful for checking that TFLAGS are being used to build all target libraries — configure a non-bootstrap compiler with --with-specs='%{!fmultiflags:%emissing TFLAGS}' and build the compiler and target libraries. 🛠️
-fprofile-report— makes the compiler print some statistics about how consistent the (estimated) profile is and what effect individual passes have. 📊-fstack-usage— makes the compiler output stack usage info for the program, on a per-function basis. The dump filename is made by appending.suto the auxname. The auxname is generated from the name of the output file, if it was explicitly specified and it is not an executable; otherwise it's the basename of the source file. An entry is made up of three fields:- The name of the function.
- A number of bytes.
- One or more qualifiers:
static,dynamic,bounded.
The qualifier static means the function manipulates the stack statically: a fixed number of bytes are allocated for the frame on function entry and released on function exit; no stack adjustments are otherwise made in the function. The second field is this fixed number of bytes.
The qualifier dynamic means the function manipulates the stack dynamically: in addition to the static allocation described above, stack adjustments are made in the body of the function, for example to push/pop arguments around function calls. If the qualifier bounded is also present, the amount of these adjustments is bounded at compile time and the second field is an upper bound of the total amount of stack used by the function. If it is not present, the amount of these adjustments is not bounded at compile time and the second field only represents the bounded part. 📚
-fstats— emits statistics about front-end processing at the end of the compilation. Only the C++ front end supports this option, and the information is generally only useful to the G++ development team. 👩💻-fdbg-cnt-list— prints the name and the counter upper bound for all debug counters. 🔢-fdbg-cnt=counter-value-list— sets the internal debug counter lower and upper bound. counter-value-list is a comma-separated list ofname:lower_bound1-upper_bound1 [:lower_bound2-upper_bound2...]tuples which sets the name of the counter and list of closed intervals. Thelower_boundis optional and is zero initialized if not set. For example, with-fdbg-cnt=dce:2-4:10-11,tail_call:10,dbg_cnt(dce)returns true only for second, third, fourth, tenth and eleventh invocation. Fordbg_cnt(tail_call)true is returned for first 10 invocations. 🎛️-print-autofdo-gcov-version,--print-autofdo-gcov-version— prints the current version of GCOV being used by the AutoFDO infrastructure. 🧬-print-file-name=library,--print-file-name=library,--print-file-name library— prints 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-multi-directory— prints the directory name corresponding to the multilib selected by any other switches present in the command line. This directory is supposed to exist inGCC_EXEC_PREFIX. 🗂️-print-multi-lib,--print-multi-lib— prints the mapping from multilib directory names to compiler switches that enable them. The directory name is separated from the switches by;, and each switch starts with an@instead of the-, without spaces between multiple switches. This is supposed to ease shell processing. 🐚-print-multi-os-directory,--print-multi-os-directory— prints the path to OS libraries for the selected multilib, relative to somelibsubdirectory. If OS libraries are present in thelibsubdirectory and no multilibs are used, this is usually just., if OS libraries are present inlibsuffixsibling directories this prints e.g.../lib64,../libor../lib32, or if OS libraries are present inlib/subdirsubdirectories it prints e.g.amd64,sparcv9orev6. 🧭-print-multiarch,--print-multiarch— prints the path to OS libraries for the selected multiarch, relative to somelibsubdirectory. 🌍-print-prog-name=program,--print-prog-name=program,--print-prog-name program— like-print-file-name, but searches for a program such ascpp. 🔎-print-libgcc-file-name,--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`-print-search-dirs,--print-search-dirs— prints the name of the configured installation directory and a list of program and library directories gcc searches — and don't do anything else. 🗺️
This is useful when gcc prints the error message installation problem, cannot exec cpp0: No such file or directory. To resolve this you either need to put cpp0 and the other compiler components where gcc expects to find them, or you can set the environment variable GCC_EXEC_PREFIX to the directory where you installed them. Don't forget the trailing /. 🚨
-print-sysroot,--print-sysroot— prints the target sysroot directory that is used during compilation. This is the target sysroot specified either at configure time or using the--sysrootoption, possibly with an extra suffix that depends on compilation options. If no target sysroot is specified, the option prints nothing. 📦-print-sysroot-headers-suffix,--print-sysroot-headers-suffix— prints the suffix added to the target sysroot when searching for headers, or gives an error if the compiler is not configured with such a suffix — and don't do anything else. 🏷️-dumpmachine— prints the compiler's target machine (for example,i686-pc-linux-gnu) — and don't do anything else. 💻-dumpversion— prints the compiler version (for example,3.0,6.3.0or7) — and don't do anything else. This is the compiler version used in filesystem paths and specs. Depending on how the compiler has been configured it can be just a single number (major version), two numbers separated by a dot (major and minor version) or three numbers separated by dots (major, minor and patchlevel version). 🔢-dumpfullversion— prints the full compiler version — and don't do anything else. The output is always three numbers separated by dots, major, minor and patchlevel version. 🎯-dumpspecs— prints the compiler's built-in specs — and don't do anything else. (This is used when GCC itself is being built.) 📜--param name=value,--param=name=value— this is how you poke at GCC's internal parameters. 👑
GCC by convention uses parameters that can be specified on the command line instead of hard-wired constants to represent arbitrary compiler limits or heuristics. Many parameters are related to optimization; for example, GCC does not inline functions that contain more than a certain number of instructions. The static analyzer similarly uses parameters to limit complexity, link-time optimization uses parameters to control partitioning, and so on. Other parameters control aspects of GCC that are completely internal, such as its memory allocation and garbage collection strategy. Still others control target-specific behavior.
The --param option provides a uniform interface for specifying values for these compiler parameters. However, 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. You should not depend on parameter settings for correct compilation of your program. They are exposed via the command line for the convenience of developers in debugging compilation problems or, in some cases, to provide workarounds for compiler bugs.
Warning bestie: parameter names and value meanings are tied to compiler internals and can change without notice. Do not depend on parameter settings for correct compilation of your program. 🙈
🎯 Target-Specific Options 🎯
Each target machine supported by GCC can have its own options — for example, to allow you to compile for a particular processor variant or ABI, or to control optimizations specific to that machine. Similarly, GCC also has options that are specific to particular operating systems or runtime environments on the target. 🌈
By convention, the names of machine-specific options start with -m. Some configurations of the compiler also support additional target-specific options, usually for compatibility with other compilers on the same platform. 🤝
🌸 AArch64 Options 🌸
These options are defined for AArch64 implementations:
-mabi=name— generates code for the specified data model. Permissible values areilp32for SysV-like data model where int, long int and pointers are 32 bits, andlp64for SysV-like data model where int is 32 bits, but long int and pointers are 64 bits. 🧮
The default depends on the specific target configuration. Note that the LP64 and ILP32 ABIs are not link-compatible; you must compile your entire program with the same ABI, and link with a compatible set of libraries. 🔗
Deprecated alert: the ilp32 model is deprecated. 💀
-mbig-endian— generates big-endian code. This is the default when GCC is configured for anaarch64_be-*-*target. 🐘-mlittle-endian— generates little-endian code. This is the default when GCC is configured for anaarch64-*-*but not anaarch64_be-*-*target. 🐭-menable-sysreg-checking— generates an error message if an attempt is made to access a system register which is not available on the target architecture. 🚫-mgeneral-regs-only— bestie, this one tells GCC to keep it humble: generate code that uses only the general-purpose registers. It stops the compiler from touching floating-point and Advanced SIMD registers, but heads up — it puts zero restrictions on the assembler. The assembler does whatever it wants, periodt.-mcmodel=tiny— generate code for the tiny code model. The program and its statically defined symbols must be within1MBof each other. Programs can be statically or dynamically linked. small but mighty 💅-mcmodel=small— generate code for the small code model: the program and its statically defined symbols must be within4GBof each other. Statically or dynamically linked, dealer's choice. This is the default code model ✨-mcmodel=large— generate code for the large code model. It makes no assumptions about addresses and sizes of sections. Programs can be statically linked only. And ngl,-mcmodel=largeis incompatible with-mabi=ilp32,-fpicand-fPIC— they do not get along, do not invite them to the same party 🙈-mtp=name— pick which system register gets to be the thread pointer. The valid values aretpidr_el0,tpidrro_el0,tpidr_el1,tpidr_el2,tpidr_el3. For backwards compatibility the aliasesel0,el1,el2,el3are also accepted. Default setting istpidr_el0. And the recommendation fr: compile all code that's meant to interoperate with the same value of this option, otherwise you'll be accessing a different thread pointer from the wrong exception level and that's giving chaos 😭-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. That's the whole vibe check 🦋-momit-leaf-frame-pointer,-mno-omit-leaf-frame-pointer— omit or keep the frame pointer in leaf functions. The former behavior (omitting it) is the default. slay 🎀-mstack-protector-guard=guard,-mstack-protector-guard-reg=reg,-mstack-protector-guard-offset=offset— generate stack protection code using a canary at guard. Supported locations areglobalfor a global canary, orsysregfor a canary in an appropriate system register.
With that latter choice, -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 — this is entirely for use within the Linux kernel, iykyk 👑
-mtls-dialect=desc— use TLS descriptors as the thread-local storage mechanism for dynamic accesses of TLS variables. This is the default, so it's kind of the main character here 💖-mtls-dialect=traditional— use traditional TLS as the thread-local storage mechanism for dynamic accesses of TLS variables. retro era 🌸-mtls-size=size— specify the bit size of immediate TLS offsets. Valid values are12,24,32,48. Heads up: this option requires binutils 2.26 or newer ⭐-mfix-cortex-a53-835769,-mno-fix-cortex-a53-835769— enable or disable the workaround for the ARM Cortex-A53 erratum number835769. This involves inserting aNOPinstruction between memory instructions and 64-bit integer multiply-accumulate instructions. The flag will be ignored if an architecture or cpu is specified on the command line which does not need the workaround — it's not being dramatic, it just doesn't apply 💀-mfix-cortex-a53-843419,-mno-fix-cortex-a53-843419— enable or disable the workaround for the ARM Cortex-A53 erratum number843419. Lowkey important detail: this erratum workaround is made at link time, and this will only pass the corresponding flag to the linker. Also ignored if an architecture or cpu is specified on the command line which does not need the workaround 🫶-mlow-precision-recip-sqrt,-mno-low-precision-recip-sqrt— enable or disable the reciprocal square root approximation. It only has an effect if-ffast-mathor-funsafe-math-optimizationsis used as well. Enabling this reduces precision of reciprocal square root results to about16bits for single precision and to32bits for double precision. precision? never met her 💅-mlow-precision-sqrt,-mno-low-precision-sqrt— enable or disable the square root approximation. Again, only has an effect if-ffast-mathor-funsafe-math-optimizationsis used as well. Enabling reduces precision of square root results to about16bits for single precision and32bits for double precision. And if enabled, it implies-mlow-precision-recip-sqrt— it drags its friend along 🌈-mlow-precision-div,-mno-low-precision-div— enable or disable the division approximation. Same deal: only has an effect if-ffast-mathor-funsafe-math-optimizationsis used as well. Enabling reduces precision of division results to about16bits for single precision and32bits for double precision ✨-mtrack-speculation,-mno-track-speculation— enable or disable generation of additional code to track speculative execution through conditional branches. That tracking state can then be used by the compiler when expanding calls to__builtin_speculation_safe_valueto permit a more efficient code sequence to be generated. very demure, very mindful 🧚-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.
This option is only applicable when compiling for the base ARMv8.0 instruction set. If using a later revision, e.g. -march=armv8.1-a or -march=armv8-a+lse, the ARMv8.1-Atomics instructions will be used directly. Same applies when using -mcpu= when the selected cpu supports the lse feature. This option is on by default 👑
-mmax-vectorization,-mno-max-vectorization— enable or disable an override to the vectorizer cost model making vectorization always appear profitable. This option can be combined with-mautovec-preference, allowing precise control over which ISA will be used for auto-vectorization. And unlike-fno-vect-cost-modelor-fvect-cost-model=unlimited, this option does not turn off cost comparison between different vector modes. nuance, bestie 🔥-mautovec-preference=name— force an ISA selection strategy for auto-vectorization. The possible values of name are:default— use the default heuristics.asimd-only— use only Advanced SIMD for auto-vectorization.sve-only— use only SVE for auto-vectorization.prefer-asimd— use both Advanced SIMD and SVE. Prefer Advanced SIMD when the costs are deemed equal.prefer-sve— use both Advanced SIMD and SVE. Prefer SVE when the costs are deemed equal.
For best performance it is highly recommended to use-mcpuor-mtuneinstead. This parameter should only be used for code exploration. like, it's a field trip, not a lifestyle 🙈
-march=name— specify the name of the target architecture and, optionally, one or more feature modifiers. The option has the form-march=arch{+[no]feature}*.
The table below summarizes the permissible values for arch and the features that they enable by default:
arch value : Architecture : Includes by default
armv8-a : Armv8-A : +fp, +simd
armv8.1-a : Armv8.1-A : armv8-a, +crc, +lse, +rdma
armv8.2-a : Armv8.2-A : armv8.1-a
armv8.3-a : Armv8.3-A : armv8.2-a, +pauth, +fcma, +jscvt
armv8.4-a : Armv8.4-A : armv8.3-a, +flagm, +fp16fml, +dotprod,
+rcpc2
armv8.5-a : Armv8.5-A : armv8.4-a, +sb, +ssbs, +predres, +frintts,
+flagm2
armv8.6-a : Armv8.6-A : armv8.5-a, +bf16, +i8mm
armv8.7-a : Armv8.7-A : armv8.6-a, +wfxt, +xs
armv8.8-a : Armv8.8-a : armv8.7-a, +mops
armv8.9-a : Armv8.9-a : armv8.8-a
armv9-a : Armv9-A : armv8.5-a, +sve, +sve2
armv9.1-a : Armv9.1-A : armv9-a, +bf16, +i8mm
armv9.2-a : Armv9.2-A : armv9.1-a, +wfxt, +xs
armv9.3-a : Armv9.3-A : armv9.2-a, +mops
armv9.4-a : Armv9.4-A : armv9.3-a, +sve2p1
armv9.5-a : Armv9.5-A : armv9.4-a, cpa, +faminmax, +lut
armv8-r : Armv8-R : armv8-rThe value native is available on native AArch64 GNU/Linux and causes the compiler to pick the architecture of the host system. This option has no effect if the compiler is unable to recognize the architecture of the host system. When -march=native is given and no other -mcpu or -mtune is given, then GCC will pick the host CPU as the CPU to tune for as well as select the architecture features from. That is, -march=native is treated as -mcpu=native. main character energy, honestly 👑
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 in wins, no debates 💅
GCC uses name to determine what kind of instructions it can emit when generating assembly code. If -march is specified without either of -mtune or -mcpu also being specified, the code is tuned to perform well across a range of target processors implementing the target architecture 🌸
-mtune=name— specify the name of the target processor for which GCC should tune the performance of the code. Permissible values for this option are:
generic, cortex-a35, cortex-a53, cortex-a55, cortex-a57,
cortex-a72, cortex-a73, cortex-a75, cortex-a76, cortex-a76ae,
cortex-a77, cortex-a65, cortex-a65ae, cortex-a34, cortex-a78,
cortex-a78ae, cortex-a78c, ares, exynos-m1, emag, falkor, oryon-1,
neoverse-512tvb, neoverse-e1, neoverse-n1, neoverse-n2, neoverse-v1,
neoverse-v2, grace, neoverse-v3, neoverse-v3ae, armagicpu,
neoverse-n3, olympus, rigel, cortex-a725, cortex-x925, qdf24xx,
saphira, phecda, xgene1, vulcan, octeontx, octeontx81, octeontx83,
octeontx2, octeontx2t98, octeontx2t96 octeontx2t93, octeontx2f95,
octeontx2f95n, octeontx2f95mm, a64fx, fujitsu-monaka, thunderx,
thunderxt88, thunderxt88p1, thunderxt81, tsv110, hip12, thunderxt83,
thunderx2t99, thunderx3t110, zeus, cortex-a57.cortex-a53,
cortex-a72.cortex-a53, cortex-a73.cortex-a35, cortex-a73.cortex-a53,
cortex-a75.cortex-a55, cortex-a76.cortex-a55, cortex-r82,
cortex-r82ae, cortex-x1, cortex-x1c, cortex-x2, cortex-x3,
cortex-x4, cortex-a510, cortex-a520, cortex-a520ae, cortex-a710,
cortex-a715, cortex-a720, cortex-a720ae, ampere1, ampere1a,
ampere1b, ampere1c, cobalt-100, apple-m1, apple-m2, apple-m3,
apple-m4, apple-m5, c1-nano, c1-pro, c1-premium c1-ultra and native.yes that's the whole list, every single one, we don't skip entries in this house 🫶
cortex-a57.cortex-a53,cortex-a72.cortex-a53,cortex-a73.cortex-a35,cortex-a73.cortex-a53,cortex-a75.cortex-a55,cortex-a76.cortex-a55,apple-m1,apple-m2,apple-m3,gb10— okay so all of these values are basically saying "bestie, tune for a big.LITTLE system" 💅 like that's the whole vibe check.neoverse-512tvb— this one's for the Neoverse girlies that (a) implement SVE and (b) have a total vector bandwidth of 512 bits per cycle 🌀 In other words, it 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 (so 2 for 256-bit SVE, 4 for 128-bit SVE). Lowkey it's more general than tuning for a specific core like Neoverse V1, but more specific than the default tuning described below ✨
Also, on native AArch64 GNU/Linux systems, the value native tunes performance to the host system 🫶 This option literally does nothing if the compiler can't recognize the processor of the host system (rip).
Where none of -mtune=, -mcpu= or -march= are specified, the code is tuned to perform well across a range of target processors 🌈
Heads up bestie: this option cannot be suffixed by feature modifiers. Periodt. 💀
💜 -mcpu=name
Specify the name of the target processor, optionally suffixed by one or more feature modifiers ✨ This option has the form -mcpu=cpu{+[no]feature}*, where the permissible values for cpu are the same as those available for -mtune. The permissible values for feature are documented in the sub-section on __aarch64-feature-modifiers,,-march and -mcpu Feature Modifiers__. Where conflicting feature modifiers are specified, the right-most feature is used — so like, left-to-right, the last one wins, iykyk 👑
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 -mcpu=neoverse-512tvb is special — it does not refer to a specific core, it refers to all Neoverse cores that (a) implement SVE and (b) have a total vector bandwidth of 512 bits a cycle 🌀 Unless it's overridden by -march, -mcpu=neoverse-512tvb generates code that can run on a Neoverse V1 core, since Neoverse V1 is the first Neoverse core with these properties. And unless overridden by -mtune, -mcpu=neoverse-512tvb tunes code in the same way as for -mtune=neoverse-512tvb ✨
🌸 -moverride=string
Override tuning decisions made by the back-end in response to a -mtune= switch 🛠️ The syntax, semantics, and accepted values for string in this option are not guaranteed to be consistent across releases — so like, don't get attached, bestie 😭
This option is only intended to be useful when developing GCC. That's it. That's the whole audience. 🙈
🦋 -mpc-relative-literal-loads
🦋 -mno-pc-relative-literal-loads
Enable or disable PC-relative literal loads ✨ With this option, literal pools are accessed using a single instruction and emitted after each function. This limits the maximum size of functions to 1MB 🫶 This is enabled by default for -mcmodel=tiny.
🎀 -msign-return-address=scope
Select the function scope on which return address signing will be applied 💫 The permissible values are:
none— disables return address signingnon-leaf— enables pointer signing for functions which are not leaf functionsall— enables pointer signing for all functions
The default value is none.
Ngl this option has been deprecated by -mbranch-protection 💀 so maybe don't build your whole personality on it.
👑 -mbranch-protection=features
Select the branch protection features to use 🛡️ features can have one of the following forms:
none— the default, and turns off all types of branch protectionstandard— turns on all types of branch protection features. If a feature has additional tuning options, thenstandardsets it to its standard level ✨pac-ret— turns on return address signing to its standard level: signing functions that save the return address to memory (non-leaf functions practically always do this) using the A-key 🔑pac-ret+leaf— extends thepac-retsigning to include leaf functions 🍃pac-ret+b-keyorpac-ret+leaf+b-key— can be used to sign the functions with the B-key instead of the A-key 🗝️bti— turns on branch target identification mechanism 🎯gcs— turns on guarded control stack compatible code generation 🧚
💜 -mharden-sls=opts
Enable compiler hardening against straight line speculation (SLS) 🛡️ opts is a comma-separated list of the following options:
retbrblr
In addition, -mharden-sls=all enables all SLS hardening while -mharden-sls=none disables all SLS hardening ✨
🌈 -mearly-ra=scope
Determine when to enable an early register allocation pass 👀 This pass runs before instruction scheduling and tries to find a spill-free allocation of floating-point and vector code. It also tries to make use of strided multi-register instructions, such as SME2's strided LD1 and ST1 🌀
The possible values of scope are:
all— runs the pass on all functionsstrided— runs the pass on functions that have access to strided multi-register instructionsnone— disables the pass
-mearly-ra=all is the default for -O2 and above, and for -Os 🌸 -mearly-ra=none is the default otherwise.
🦋 -mearly-ldp-fusion
🦋 -mno-early-ldp-fusion
Enable the copy of the AArch64 load/store pair fusion pass that runs before register allocation ✨ Enabled by default at -O and above 💅
🎀 -mlate-ldp-fusion
🎀 -mno-late-ldp-fusion
Enable the copy of the AArch64 load/store pair fusion pass that runs after register allocation ✨ Enabled by default at -O and above 💅
⭐ -mnarrow-gp-writes
Enable conversion of 64-bit general purpose register writes to equivalent 32-bit operations when the upper 32 bits are known to be zero 🧠 This pass can be controlled with -mnarrow-gp-writes and is active at -O2 and above, but not enabled by default — except for -mcpu=olympus 👑
🌀 -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:
scalable12825651210242048
Specifying scalable selects vector-length agnostic output ⭐ At present -msve-vector-bits=128 also generates vector-length agnostic output for big-endian targets. All other values generate vector-length specific code.
The behavior of these values may change in future releases, and no value except scalable should be relied on for producing code that is portable across different hardware SVE vector lengths 😭
The default is -msve-vector-bits=scalable, which produces vector-length agnostic code 🫶
🧚 -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 FP16fmlaextension. This also enables FP16 extensions and floating-point instructions. This option is enabled by default for-march=armv8.4-a. Use of this option with architectures prior to Armv8.2-A is not supported 💀rcpc— Enable the RCpc extension. This enables the use of theLDAPRinstructions for load-acquire atomic semantics, and passes it on to the assembler, enabling inline asm statements to use instructions from the RCpc extension 🛠️dotprod— Enable the Dot Product extension. This also enables Advanced SIMD instructions 🎯aes— Enable the Armv8-aaesandpmullcrypto extension. This also enables Advanced SIMD instructions 🔐sha2— Enable the Armv8-asha2crypto extension. This also enables Advanced SIMD instructions 🔒sha3— Enable thesha512andsha3crypto extension. This also enables Advanced SIMD instructions. Use of this option with architectures prior to Armv8.2-A is not supported 💀sm4— Enable thesm3andsm4crypto extension. This also enables Advanced SIMD instructions. Use of this option with architectures prior to Armv8.2-A is not supported 💀profile— Enable the Statistical Profiling extension. This option is only to enable the extension at the assembler level and does not affect code generation 📊rng— Enable the Armv8.5-a Random Number instructions. This option is only to enable the extension at the assembler level and does not affect code generation 🎲memtag— Enable the Armv8.5-a Memory Tagging Extensions. Use of this option with architectures prior to Armv8.5-A is not supported 🏷️sb— Enable the Armv8-a Speculation Barrier instruction. This option is only to enable the extension at the assembler level and does not affect code generation. This option is enabled by default for-march=armv8.5-a🚧ssbs— Enable the Armv8-a Speculative Store Bypass Safe instruction. This option is only to enable the extension at the assembler level and does not affect code generation. This option is enabled by default for-march=armv8.5-a🛡️predres— turns on the Armv8-a Execution and Data Prediction Restriction instructions. Heads up bestie: this only enables the extension at the assembler level and does not affect code generation. It's on by default for-march=armv8.5-a. ✨sve2— enables the Armv8-a Scalable Vector Extension 2. This also enables SVE instructions.sve2-bitperm— enables SVE2 bitperm instructions. This also enables SVE2 instructions.sve2-sm4— enables SVE2 sm4 instructions. This also enables SVE2 instructions.sve2-aes— enables SVE2 aes instructions. This also enables SVE2 instructions.sve2-sha3— enables SVE2 sha3 instructions. This also enables SVE2 instructions.sve2p1— enables SVE2.1 instructions. This also enables SVE2 instructions.tme— enable the Transactional Memory Extension. 🎀i8mm— enables 8-bit Integer Matrix Multiply instructions. This also enables Advanced SIMD and floating-point instructions. On by default for-march=armv8.6-a. Using it with architectures prior to Armv8.2-A is not supported — don't do it, it's giving chaos. 💀f32mm— enables 32-bit Floating point Matrix Multiply instructions. This also enables SVE instructions. Use with architectures prior to Armv8.2-A is not supported.f64mm— enables 64-bit Floating point Matrix Multiply instructions. This also enables SVE instructions. Use with architectures prior to Armv8.2-A is not supported.bf16— enables brain half-precision floating-point instructions. This also enables Advanced SIMD and floating-point instructions. On by default for-march=armv8.6-a. Use with architectures prior to Armv8.2-A is not supported.ls64— enables the 64-byte atomic load and store instructions for accelerators.mops— enables the instructions to accelerate memory operations likememcpy,memmove,memset. On by default for-march=armv8.8-a.flagm— enables the Flag Manipulation instructions Extension.flagm2— enables the FlagM2 flag conversion instructions.pauth— enables the Pointer Authentication Extension. 🔥cssc— enables the Common Short Sequence Compression instructions.cmpbr— enables the shorter compare and branch instructions,cbb,cbhandcb.sme— enable the Scalable Matrix Extension.sme-i16i64— enables the FEAT_SME_I16I64 extension to SME. This also enables SME instructions.sme-f64f64— enables the FEAT_SME_F64F64 extension to SME. This also enables SME instructions.sme2— enables the Scalable Matrix Extension 2. This also enables SME instructions.sme-f8f16— enables the FEAT_SME_F8F16 extension to SME. This also enables SME2 and FP8 instructions.sme-f8f32— enables the FEAT_SME_F8F32 extension to SME. This also enables SME2 and FP8 instructions.sme-b16b16— enables the FEAT_SME_B16B16 extension to SME. This also enables SME2 and SVE_B16B16 instructions.sme-f16f16— enables the FEAT_SME_F16F16 extension to SME. This also enables SME2 instructions.sme2p1— enables the Scalable Matrix Extension version 2.1. This also enables SME2 instructions.fcma— enables the complex number SIMD extensions.jscvt— enables thefjcvtzsJavaScript conversion instruction. slay 💅frintts— enables floating-point round to integral value instructions.wfxt— enableswfetandwfitinstructions.xs— enable the XS memory attribute extension.lse128— enables the LSE128 128-bit atomic instructions extension. This also enables LSE instructions.d128— enables support for 128-bit system register read/write instructions. This also enables the LSE128 extension.gcs— enable support for Armv9.4-a Guarded Control Stack extension.the— enable support for Armv8.9-a/9.4-a translation hardening extension.rcpc2— enables the RCpc2 extension.rcpc3— enables the RCpc3 (Release Consistency) extension.fp8— enable the fp8 (8-bit floating point) extension.fp8fma— enable the fp8 (8-bit floating point) multiply accumulate extension.ssve-fp8fma— enable the fp8 (8-bit floating point) multiply accumulate extension in streaming mode.fp8dot4— enable the fp8 (8-bit floating point) to single-precision 4-way dot product extension.ssve-fp8dot4— enable the fp8 (8-bit floating point) to single-precision 4-way dot product extension in streaming mode.fp8dot2— enable the fp8 (8-bit floating point) to half-precision 2-way dot product extension.ssve-fp8dot2— enable the fp8 (8-bit floating point) to half-precision 2-way dot product extension in streaming mode.faminmax— enable the Floating Point Absolute Maximum/Minimum extension.lut— enable the Lookup Table extension.sme-lutv2— enable the SME Lookup Table v2 (LUTv2) extension.cpa— enable the Checked Pointer Arithmetic instructions.sve-b16b16— enable the SVE non-widening brain floating-point (bf16) extension. Quick note: this only has an effect whensve2orsme2are also enabled. 🦋sve-bfscale— enable the SVE_BFSCALE extension.poe2— enable the Permission Overlays Extension 2.tev— enable the TIndex Exception-like Vector Extension.tlbid— enable the TLBI Domains Extension.gcie— enable the GICv5 (Generic Interrupt Controller) CPU Interface Extension.lscp— enable the load acquire and store release pair extension.mops-go— enable tag only variants of MOPS instructions. This also enables the instructions to accelerate memory operations and Armv8.5-a Memory Tagging Extensions.sve2p3— enable SVE2.3. This also enables SVE2.2 instructions.sme2p3— enable SME2.3. This also enables SME2.2 instructions.f16f32dot— enables Armv9.7-a f16f32dot instructions. This also enables Advanced SIMD, floating-point instructions and Armv8.2-a FP16 instructions.sve-b16mm— enable the SVE B16MM Extension. This also enables SVE instructions, Advanced SIMD and floating-point instructions.mtetc— enable Data cache tag block operations. This also enables Armv8.5-a Memory Tagging Extensions.f16f32mm— enables Armv9.7-a f16f32mm instructions. This also enables Advanced SIMD, floating-point instructions and Armv8.2-a FP16 instructions.f16mm— enables f16mm instructions. This also enables Advanced SIMD, floating-point instructions and Armv8.2-a FP16 instructions.
Feature crypto implies aes, sha2, and simd, which implies fp. Conversely, nofp implies nosimd, which implies nocrypto, noaes and nosha2. 🌈 iykyk, the implication chain is lowkey iconic.
🫶 Adapteva Epiphany Options 🫶
These -m options are defined for Adapteva Epiphany:
-mhalf-reg-file,-mno-half-reg-file— don't allocate any register in the ranger32...r63. That's what lets the code run on hardware variants that lack these registers. 💜-mprefer-short-insn-regs,-mno-prefer-short-insn-regs— preferentially allocate registers that allow short instruction generation. Fair warning bestie: this can result in an increased instruction count, so it may either reduce or increase overall code size. It's giving "it depends".-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. No cap.-mcmove,-mno-cmove— enable the generation of conditional moves.- *
-mnops=num* — emit num NOPs before every other generated instruction. ✨ -mno-soft-cmpsf,-msoft-cmpsf— for single-precision floating-point comparisons, emit anfsubinstruction and test the flags. This is faster than a software comparison, but can get incorrect results in the presence of NaNs, or when two different small numbers are compared such that their difference is calculated as zero. 😭 The default is-msoft-cmpsf, which uses slower, but IEEE-compliant, software comparisons.-mstack-offset=num — set the offset between the top of the stack and the stack pointer. E.g., a value of 8 means that the eight bytes in the rangesp+0...sp+7can be used by leaf functions without stack allocation. Values other than 8 or 16 are untested and unlikely to work. Also note: this option *changes the ABI — compiling a program with a different stack offset than the libraries have been compiled with generally does not work. 🫠 This option can be useful if you want to evaluate if a different stack offset would give you better code, but to actually use a different stack offset to build working programs, it is recommended to configure the toolchain with the appropriate--with-stack-offset=num* option.-mno-round-nearest,-mround-nearest—-mno-round-nearestmakes the scheduler assume that the rounding mode has been set to truncating. The default is-mround-nearest.-mlong-calls— if not otherwise specified by an attribute, assume all calls might be beyond the offset range of theb/blinstructions, and therefore load the function address into a register before performing a (otherwise direct) call. This is the default.-mshort-calls— if not otherwise specified by an attribute, assume all direct calls are in the range of theb/blinstructions, so use these instructions for direct calls.
The default is -mlong-calls. Note that -mlong-calls is equivalent to -mno-short-calls, and similarly -mno-long-calls is equivalent to -mshort-calls. 🎀
-msmall16,-mno-small16— assume addresses can be loaded as 16-bit unsigned values. This does not apply to function addresses for which-mlong-callssemantics are in effect.- *
-mfp-mode=mode* — set the prevailing mode of the floating-point unit. This determines the floating-point mode that is provided and expected at function call and return time. Making this mode match the mode you predominantly need at function start can make your programs smaller and faster by avoiding unnecessary mode switches. 🔥
mode can be set to one the following values:
caller— honestly the ✨chill✨ mode: any mode is valid when you enter a function, and it gets kept or restored when the function returns, and again when it calls other functions. Perfect for compiling libraries or other compilation units you might drop into different programs that run with different prevailing FPU modes — the convenience of one single object file outweighs the size and speed overhead of any extra mode switching you might need, compared to what a more specific prevailing FPU mode would cost you. 🫶truncate— the mode for floating-point calculations with truncating rounding (that's round-towards-zero, bestie, iykyk). Also covers conversion from floating point to integer. 💅round-nearest— the mode for floating-point calculations with round-to-nearest-or-even rounding. 🎀int— the mode for doing integer calculations in the FPU, like integer multiply, or integer multiply-and-accumulate. 🔥
The default is -mfp-mode=caller.
-mmay-round-for-trunc,-mno-may-round-for-trunc— lets floating point to integer truncation be replaced with rounding to save mode switching. Disabled by default. 🙈-mfp-iarith,-mno-fp-iarith— enables use of the floating-point unit for integer add and subtract. Disabled by default.-msplit-lohi,-mno-split-lohi,-mpost-inc,-mno-post-inc,-mpost-modify,-mno-post-modify— code generation tweaks that control, 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. ⭐-mno-vect-double— change the preferred SIMD mode to SImode. The default is-mvect-double, which uses DImode as preferred SIMD mode.-max-vect-align=num— the maximum alignment for SIMD vector mode types. num may be4or8. The default is8.
Heads up, gorgeous: 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,-mno-split-vecmove-early— split vector moves into single word moves before reload. In theory this can give better register allocation, but so far the reverse seems to be generally the case. 💀-m1reg-reg— specify a register to hold the constant-1, which makes loading small negative constants and certain bitmasks faster. Allowable values for reg arer43andr63, which specify use of that register as a fixed register, andnone, which means that no register is used for this purpose. The default is-m1reg-none. ✨
💜 AMD GCN Options 💜
These options are defined specifically for the AMD GCN port. Obsessed, honestly.
-march=gpu,-mtune=gpu— set architecture type or tuning for gpu. Supported values for gpu are:gfx900— compile for GCN5 Vega 10 devices (gfx900).gfx902— compile for GCN5 Vegagfx902devices. (Experimental)gfx904— compile for GCN5 Vegagfx904devices. (Experimental)gfx906— compile for GCN5 Vega 20 devices (gfx906).gfx908— compile for CDNA1 Instinct MI100 series devices (gfx908).gfx909— compile for GCN5 Vegagfx909devices. (Experimental)gfx90a— compile for CDNA2 Instinct MI200 series devices (gfx90a).gfx90c— compile for GCN5 Vega 7 devices (gfx90c).gfx942— compile for CDNA3 Instinct MI300 series devices (gfx942). (Experimental)gfx950— compile for the CDNA3gfx950devices. (Experimental)gfx9-generic— compile generic code for Vega devices, executable on the following subset of GFX9 devices:gfx900,gfx902,gfx904,gfx906,gfx909andgfx90c.gfx9-4-generic— compile generic code for CDNA3 devices, executable on the following subset of GFX9 devices:gfx942andgfx950. (Experimental)gfx1030— compile for RDNA2gfx1030devices (GFX10 series).gfx1031— compile for RDNA2gfx1031devices (GFX10 series). (Experimental)gfx1032— compile for RDNA2gfx1032devices (GFX10 series). (Experimental)gfx1033— compile for RDNA2gfx1033devices (GFX10 series). (Experimental)gfx1034— compile for RDNA2gfx1034devices (GFX10 series). (Experimental)gfx1035— compile for RDNA2gfx1035devices (GFX10 series). (Experimental)gfx1036— compile for RDNA2gfx1036devices (GFX10 series).gfx10-3-generic— compile generic code for GFX10-3 devices, executable ongfx1030,gfx1031,gfx1032,gfx1033,gfx1034,gfx1035, andgfx1036.gfx1100— compile for RDNA3gfx1100devices (GFX11 series).gfx1101— compile for RDNA3gfx1101devices (GFX11 series). (Experimental)gfx1102— compile for RDNA3gfx1102devices (GFX11 series). (Experimental)gfx1103— compile for RDNA3gfx1103devices (GFX11 series).gfx1150— compile for RDNA3gfx1150devices (GFX11 series). (Experimental)gfx1151— compile for RDNA3gfx1151devices (GFX11 series). (Experimental)gfx1152— compile for RDNA3gfx1152devices (GFX11 series). (Experimental)gfx1153— compile for RDNA3gfx1153devices (GFX11 series). (Experimental)gfx11-generic— compile generic code for GFX11 devices, executable ongfx1100,gfx1101,gfx1102,gfx1103,gfx1150,gfx1151,gfx1152, andgfx1153.
-mgang-private-size=bytes— set the amount of local data-share (LDS) memory to reserve for gang-private variables. The default is512. 🍓-msram-ecc=on,-msram-ecc=off,-msram-ecc=any— compile binaries suitable for devices with the SRAM-ECC feature enabled, disabled, or either mode. This feature can be enabled per-process on some devices. The compiled code must match the device mode. The default isany, for devices that support it. 🌈-mxnack=on,-mxnack=off,-mxnack=any— compile binaries suitable for devices with the XNACK feature enabled, disabled, or either mode. Some devices always require XNACK and some allow the user to configure XNACK. The compiled code must match the device mode. The default is-mxnack=anyon devices that support Unified Shared Memory, and-mxnack=nootherwise. ⭐-Wopenacc-dims,-Wno-openacc-dims— control warnings about invalid OpenACC dimensions.
🦋 ARC Options 🦋
The following options control the architecture variant for which code is being compiled:
-mbarrel-shifter— generate instructions supported by barrel shifter. This is the default unless-mcpu=ARC601or-mcpu=ARCEMis in effect. 💅-mjli-always— force to call a function usingjli_sinstruction. This option is valid only for ARCv2 architecture.-mcpu=cpu— set architecture type, register usage, and instruction scheduling parameters for cpu. There are also shortcut alias options available for backward compatibility and convenience. Supported values for cpu are:arc600— compile for ARC600. Aliases:-mA6,-mARC600.arc601— compile for ARC601. Alias:-mARC601.arc700— compile for ARC700. Aliases:-mA7,-mARC700. This is the default when configured with--with-cpu=arc700.arcem— compile for ARC EM.archs— compile for ARC HS.em— compile for ARC EM CPU with no hardware extensions.em4— compile for ARC EM4 CPU.em4_dmips— compile for ARC EM4 DMIPS CPU.em4_fpus— compile for ARC EM4 DMIPS CPU with the single-precision floating-point extension.em4_fpuda— compile for ARC EM4 DMIPS CPU with single-precision floating-point and double assist instructions.hs— compile for ARC HS CPU with no hardware extensions except the atomic instructions.hs34— compile for ARC HS34 CPU.hs38— compile for ARC HS38 CPU.hs38_linux— compile for ARC HS38 CPU with all hardware extensions on.hs4x— compile for ARC HS4x CPU.hs4xd— compile for ARC HS4xD CPU.hs4x_rel31— compile for ARC HS4x CPU release 3.10a.arc600_norm— compile for ARC 600 CPU with "norm" instructions enabled.arc600_mul32x16— compile for ARC 600 CPU with "norm" and 32x16-bit multiply instructions enabled.arc600_mul64— compile for ARC 600 CPU with "norm" and "mul64"-family instructions enabled.arc601_norm— compile for ARC 601 CPU with "norm" instructions enabled.arc601_mul32x16— compile for ARC 601 CPU with "norm" and 32x16-bit multiply instructions enabled.arc601_mul64— compile for ARC 601 CPU with "norm" and "mul64"-family instructions enabled.nps400— compile for ARC 700 on NPS400 chip.em_mini— compile for ARC EM minimalist configuration featuring reduced register set.
-mdpfp,-mdpfp-compact— generate double-precision FPX instructions, tuned for the compact implementation. 💖-mdpfp-fast— generate double-precision FPX instructions, tuned for the fast implementation.-mno-dpfp-lrsr,-mdpfp-lrsr— control whether "lr" and "sr" instructions use FPX extension aux registers. This is enabled by default. ✨-mea— generate extended arithmetic instructions. Currently only "divaw", "adds", "subs", and "sat16" are supported. Only valid for-mcpu=ARC700. 🔥-mmul32x16— generate 32x16-bit multiply and multiply-accumulate instructions.-mmul64— generate "mul64" and "mulu64" instructions. Only valid for-mcpu=ARC600.-mnorm— generate "norm" instructions. This is the default if-mcpu=ARC700is in effect.-mspfp,-mspfp-compact— generate single-precision FPX instructions, tuned for the compact implementation.-mspfp-fast— generate single-precision FPX instructions, tuned for the fast implementation. 🧚-msimd— unlocks ARC SIMD instruction generation through target-specific builtins ✨ Only valid with-mcpu=ARC700— anywhere else it's just vibes, no cap.-msoft-float— okay so this one is literally ignored 😭 it exists purely for compatibility. Software floating-point code gets emitted by default, and that default can be overridden by the FPX options:-mspfp,-mspfp-compact, or-mspfp-fastfor single precision, and-mdpfp,-mdpfp-compact, or-mdpfp-fastfor double precision.-mswap— generates"swap"instructions 💅-matomic— turns on the locked load/store conditional extension so your atomic memory built-in functions can actually do their thing. Not available for ARC 6xx or ARC EM cores, bestie 🙈-mdiv-rem— enables the"div"and"rem"instructions for ARCv2 cores 🔥-mcode-density,-mno-code-density— enables code density instructions for ARC EM. On by default for ARC HS, periodt.-mll64— enables double load/store operations for ARC HS cores 🎀- *
-mtp-regno=regno* — specifies the thread pointer register number. -mbitops— enables use of NPS400 bit operations ⭐-mcmem— enables use of the NPS400xld/xstextension.- *
-mmpy-option=multo* — compiles ARCv2 code with a multiplier design option. You can pass multo as either a string or a numeric value.wlh1is the default value 💖 The recognized values are:0/none— no multiplier available.1/w— 16x16 multiplier, fully pipelined. Enables these instructions:"mpyw"and"mpyuw".2/wlh1— 32x32 multiplier, fully pipelined (1 stage). Additionally enables:"mpy","mpyu","mpym","mpymu", and"mpy_s".3/wlh2— 32x32 multiplier, fully pipelined (2 stages). Additionally enables:"mpy","mpyu","mpym","mpymu", and"mpy_s".4/wlh3— two 16x16 multipliers, blocking, sequential. Additionally enables:"mpy","mpyu","mpym","mpymu", and"mpy_s".5/wlh4— one 16x16 multiplier, blocking, sequential. Additionally enables:"mpy","mpyu","mpym","mpymu", and"mpy_s".6/wlh5— one 32x4 multiplier, blocking, sequential. Additionally enables:"mpy","mpyu","mpym","mpymu", and"mpy_s".7/plus_dmpy— ARC HS SIMD support.8/plus_macd— ARC HS SIMD support.9/plus_qmacw— ARC HS SIMD support.
This option is only available for ARCv2 cores, iykyk 🦋
- *
-mfpu=fpu* — enables support for specific floating-point hardware extensions for ARCv2 cores. Supported values for fpu are:fpus— enables support for single-precision floating-point hardware extensions.fpud— enables support for double-precision floating-point hardware extensions. The single-precision floating-point extension is also enabled. Not available for ARC EM.fpuda— enables support for double-precision floating-point hardware extensions using double-precision assist instructions. The single-precision floating-point extension is also enabled. 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. 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. 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. Only available for ARC EM.fpus_div— enables support for single-precision floating-point, square-root and divide hardware extensions.fpud_div— enables support for double-precision floating-point, square-root and divide hardware extensions. This option includes optionfpus_div. Not available for ARC EM.fpus_fma— enables support for single-precision floating-point and fused multiply and add hardware extensions.fpud_fma— enables support for double-precision floating-point and fused multiply and add hardware extensions. This option includes optionfpus_fma. Not available for ARC EM.fpus_all— enables support for all single-precision floating-point hardware extensions.fpud_all— enables support for all single- and double-precision floating-point hardware extensions. Not available for ARC EM.
- *
-mirq-ctrl-saved=register-range*, blink, lp_count — specifies general-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. blink and lp_count are optional. Only valid for ARC EM and ARC HS cores. - *
-mrgf-banked-regs=number* — specifies how many registers get replicated in the second register bank on entry to a fast interrupt. Fast interrupts are the interrupts with the highest priority level P0 — they only save the PC and STATUS32 registers to avoid memory transactions during interrupt entry and exit sequences. Use this when you're running fast interrupts on an ARC V2 family processor. Permitted values are4,8,16, and32. - *
-mlpc-width=width* — specifies the width of the"lp_count"register. Valid values for width are8,16,20,24,28and32bits. The default width is fixed to 32 bits. If the width is less than 32, the compiler won't try to transform loops in your program to use the zero-delay loop mechanism unless it knows the"lp_count"register can hold the required loop-counter value. Depending on the width you specify, the compiler and run-time library might keep using the loop mechanism for various needs. This option defines the macro"__ARC_LPC_WIDTH__"with the value of width. -mrf16— tells the compiler to generate code for a 16-entry register file. This option defines the"__ARC_RF16__"preprocessor macro 💜-mbranch-index— enables use of"bi"or"bih"instructions to implement jump tables.
The following options get passed straight through to the assembler, and they also define preprocessor macro symbols 🌈
-mlock— passed down to the assembler to enable the locked load/store conditional extension. Also sets the preprocessor symbol"__Xlock".-mswape— passed down to the assembler to enable the swap byte ordering extension instruction. Also sets the preprocessor symbol"__Xswape".-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— annotates assembler instructions with estimated addresses.
The following options are passed through to the linker ✨
-marclinux,-mno-arclinux— passed through to the linker to specify use of the"arclinux"emulation. Enabled by default in tool chains built for"arc-linux-uclibc"and"arceb-linux-uclibc"targets when profiling is not requested.-marclinux_prof,-mno-arclinux_prof— passed through to the linker to specify use of the"arclinux_prof"emulation. 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— generates calls as register indirect calls, which gives you access to the full 32-bit address range 🔥-mmedium-calls,-mno-medium-calls— don't use less than a 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 so the 25-bit range can be used instead of the 21-bit range you'd get with conditional branch-and-link. This is the default for tool chains built for"arc-linux-uclibc"and"arceb-linux-uclibc"targets.-Gnum — puts 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,-mno-volatile-cache— control how volatile references are accessed. The default is-mvolatile-cache, which uses ordinary cached memory accesses for volatile references. Use-mno-volatile-cacheto enable cache bypass for volatile references 💅
The following options fine tune code generation 🌸
-mauto-modify-reg— enables the use of pre/post modify with register displacement.-mno-brcc,-mbrcc— controls a target-specific pass inarc_reorgthat generates compare-and-branch ("brcc") instructions, which is enabled by default. It has no effect on generation of these instructions driven by the combiner pass, ngl.-mcase-vector-pcrel— uses PC-relative switch case tables so case tables can be shortened. This is the default for-Os.-mno-cond-exec— disables the ARCompact-specific pass that generates conditional execution instructions.
Due to delay slot scheduling and interactions between operand numbers, literal sizes, instruction lengths, and the support for conditional execution, the target-independent pass that generates conditional execution is often lowkey lacking 😭 so the ARC port has kept a special pass around that tries to find more conditional execution generation opportunities after register allocation, branch shortening, and delay slot scheduling have all been done. This pass generally (but not always!) improves performance and code size, at the cost of extra compilation time — which is exactly 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— enables pre-reload use of thecbranchsipattern. ✨-mindexed-loads— enables the use of indexed loads. This can be problematic because some optimizers then assume that indexed stores exist, which is not the case 🙈 so like, be aware.-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 a call to a special function in libgcc; this is referred to as a millicode call. Since these calls can pose performance issues, and/or cause linking issues when linking in a nonstandard way, this option is provided to turn millicode call generation on or off. 💅-mcode-density-frame— this option enables the compiler to emitenterandleaveinstructions. These instructions are only valid for CPUs with the code-density feature 🔥-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.core3— tune for ARCv2 core3 type CPU. This option enables usage of thedbnzinstruction.release31a— tune for ARC4x release 3.10a.
-mmultcost=num — cost to assume for a multiply instruction, with4being equal to a normal instruction.
ARM Options
These -m options are defined for the ARM port, bestie 💖 get comfy, there's a lot:
-mabi=name — generate code for the specified ABI. Permissible values are:apcs-gnu,atpcs,aapcsandaapcs-linux.-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. (yes, also deprecated, we don't make the rules 💀)-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. 🦋-msched-prolog,-mno-sched-prolog— allow or prevent the reordering of instructions in the function prologue, or the merging of those instructions with the instructions in the function's body. With-mno-sched-prolog, 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
softcauses GCC to generate output containing library calls for floating-point operations. softfpallows the generation of code using hardware floating-point instructions, but still uses the soft-float calling conventions.hardallows generation of floating-point instructions and uses FPU-specific calling conventions.
- Specifying
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. No mixing and matching, periodt.
-mgeneral-regs-only— generate code which uses only the general-purpose registers. This will prevent the compiler from using floating-point and Advanced SIMD registers, but will not impose any restrictions on the assembler. ✨-mlittle-endian— generate code for a processor running in little-endian mode. This is the default for all standard configurations.-mbig-endian— generate code for a processor running in big-endian mode; the default is to compile code for a little-endian processor.-mbe8,-mbe32— when linking a big-endian image, select between BE8 and BE32 formats. The option has no effect for little-endian images and is ignored. The default is dependent on the selected target architecture. For ARMv6 and later architectures the default is BE8; for older architectures the default is BE32. BE32 format has been deprecated by ARM. 💀-march=name[+extension...]— this specifies the name of the target ARM architecture. GCC uses this name to determine what kind of instructions it can emit when generating assembly code. This option can be used in conjunction with or instead of the-mcpu=option.
Permissible names are: armv4t, armv5t, armv5te, armv6, armv6j, armv6k, armv6kz, armv6t2, armv6z, armv6zk, armv7, armv7-a, armv7ve, armv8-a, armv8.1-a, armv8.2-a, armv8.3-a, armv8.4-a, armv8.5-a, armv8.6-a, armv9-a, armv7-r, armv8-r, armv6-m, armv6s-m, armv7-m, armv7e-m, armv8-m.base, armv8-m.main, armv8.1-m.main, iwmmxt and iwmmxt2.
Additionally, the following architectures, which lack support for the Thumb execution state, are recognized but support is deprecated: armv4.
Many of the architectures support extensions. These can be added by appending +extension to the architecture name. Extension options are processed in order and capabilities accumulate. An extension will also enable any necessary base extensions upon which it depends. For example, the +crypto extension will always enable the +simd extension. The exception to the additive construction is for extensions that are prefixed with +no...: these extensions disable the specified option and any other extensions that may depend on the presence of that extension. 🧚
For example, -march=armv7-a+simd+nofp+vfpv4 is equivalent to writing -march=armv7-a+vfpv4 since the +simd option is entirely disabled by the +nofp option that follows it.
Most extension names are generically named, but have an effect that is dependent upon the architecture to which it is applied. For example, the +simd option can be applied to both armv7-a and armv8-a architectures, but will enable the original ARMv7-A Advanced SIMD (Neon) extensions for armv7-a and the ARMv8-A variant for armv8-a. 🌈
The list below has the supported extensions for each architecture. Architectures not mentioned do not support any extensions.
armv5te, armv6, armv6j, armv6k, armv6kz, armv6t2, armv6z, armv6zk
+fp— the VFPv2 floating-point instructions. The extension+vfpv2can be used as an alias for this extension.+nofp— disable the floating-point instructions.
armv7
- The common subset of the ARMv7-A, ARMv7-R and ARMv7-M architectures.
+fp— the VFPv3 floating-point instructions, with 16 double-precision registers. The extension+vfpv3-d16can be used as an alias for this extension. Note that floating-point is not supported by the base ARMv7-M architecture, but is compatible with both the ARMv7-A and ARMv7-R architectures.+nofp— disable the floating-point instructions.
armv7-a
+mp— the multiprocessing extension.+sec— the security extension.+fp— the VFPv3 floating-point instructions, with 16 double-precision registers. The extension+vfpv3-d16can be used as an alias for this extension.+simd— the Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions. The extensions+neonand+neon-vfpv3can be used as aliases for this extension.+vfpv3— the VFPv3 floating-point instructions, with 32 double-precision registers.+vfpv3-d16-fp16— the VFPv3 floating-point instructions, with 16 double-precision registers and the half-precision floating-point conversion operations.+vfpv3-fp16— the VFPv3 floating-point instructions, with 32 double-precision registers and the half-precision floating-point conversion operations.+vfpv4-d16— the VFPv4 floating-point instructions, with 16 double-precision registers.
+vfpv4— the VFPv4 floating-point instructions, with 32 double-precision registers. yes bestie, thirty-two 🫶+neon-fp16— the Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions, plus the half-precision floating-point conversion operations.+neon-vfpv4— the Advanced SIMD (Neon) v2 and the VFPv4 floating-point instructions. iconic combo ✨+nosimd— turns the Advanced SIMD instructions off. heads up tho: it does not disable floating point, they're separate vibes 💅+nofp— disables the floating-point and Advanced SIMD instructions. both. gone. 💀
armv7ve
the extended version of the ARMv7-A architecture, with support for virtualization. main character energy fr 👑
+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— disables the Advanced SIMD instructions (does not disable floating point).+nofp— disables the floating-point and Advanced SIMD instructions.
armv8-a
+crc— the Cyclic Redundancy Check (CRC) instructions. checksum girlie 🎀+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.+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— disables the cryptographic instructions.+nofp— disables 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
okay so these two share the exact same list, keeping them together so you don't have to read it twice 💖
+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— enables the Dot Product extension. this also enables Advanced SIMD instructions.+nocrypto— disables the cryptographic extension.+nofp— disables the floating-point, Advanced SIMD and cryptographic instructions.+sb— Speculation Barrier Instruction.+predres— Execution and Data Prediction Restriction Instructions.+i8mm— 8-bit Integer Matrix Multiply instructions. this also enables Advanced SIMD and floating-point instructions.+bf16— Brain half-precision floating-point instructions. this also enables Advanced SIMD and floating-point instructions.
armv8.4-a
+fp16— the half-precision floating-point data processing instructions. this also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension and the half-precision floating-point fmla extension.+simd— the ARMv8.3-A Advanced SIMD and floating-point instructions as well as the Dot Product extension.+crypto— the cryptographic instructions. this also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension.+nocrypto— disables the cryptographic extension.+nofp— disables the floating-point, Advanced SIMD and cryptographic instructions.+sb— Speculation Barrier Instruction.+predres— Execution and Data Prediction Restriction Instructions.+i8mm— 8-bit Integer Matrix Multiply instructions. this also enables Advanced SIMD and floating-point instructions.+bf16— Brain half-precision floating-point instructions. this also enables Advanced SIMD and floating-point instructions.
armv8.5-a
+fp16— the half-precision floating-point data processing instructions. this also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension and the half-precision floating-point fmla extension.+simd— the ARMv8.3-A Advanced SIMD and floating-point instructions as well as the Dot Product extension.+crypto— the cryptographic instructions. this also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension.+nocrypto— disables the cryptographic extension.+nofp— disables the floating-point, Advanced SIMD and cryptographic instructions.+i8mm— 8-bit Integer Matrix Multiply instructions. this also enables Advanced SIMD and floating-point instructions.+bf16— Brain half-precision floating-point instructions. this also enables Advanced SIMD and floating-point instructions.
armv8.6-a
+fp16— the half-precision floating-point data processing instructions. this also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension and the half-precision floating-point fmla extension.+simd— the ARMv8.3-A Advanced SIMD and floating-point instructions as well as the Dot Product extension.+crypto— the cryptographic instructions. this also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension.+nocrypto— disables the cryptographic extension.+nofp— disables the floating-point, Advanced SIMD and cryptographic instructions.+i8mm— 8-bit Integer Matrix Multiply instructions. this also enables Advanced SIMD and floating-point instructions.+bf16— Brain half-precision floating-point instructions. this also enables Advanced SIMD and floating-point instructions.
nope, you're not hallucinating — every single one of these lists is genuinely repeated in the docs like that. i don't make the rules, i just make them cute 🌸
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— disables the floating-point extension.+idiv— the ARM-state integer division instructions.+noidiv— disables 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— disables the floating-point extensions.
armv8.1-m.main
+dsp— the DSP instructions. 🎛️+mve— the M-Profile Vector Extension (MVE) integer instructions.+mve.fp— the M-Profile Vector Extension (MVE) integer and single precision floating-point instructions.+fp— the single-precision floating-point instructions.
+fp.dp The single- and double-precision floating-point instructions.
+nofp Disable the floating-point extension.
+cdecp0, +cdecp1, ... , +cdecp7 Enable the Custom Datapath Extension (CDE) on selected coprocessors according to the numbers given in the options in the range 0 to 7.
+pacbti Enable the Pointer Authentication and Branch Target Identification Extension.
armv8-m.main +dsp The DSP instructions.
+nodsp Disable the DSP extension.
+fp The single-precision floating-point instructions.
+fp.dp The single- and double-precision floating-point instructions.
+nofp Disable the floating-point extension.
+cdecp0, +cdecp1, ... , +cdecp7 Enable the Custom Datapath Extension (CDE) on selected coprocessors according to the numbers given in the options in the range 0 to 7.
armv8-r +crc The Cyclic Redundancy Check (CRC) instructions.
+fp.sp The single-precision FPv5 floating-point instructions.
+simd The ARMv8-A Advanced SIMD and floating-point instructions.
+crypto The cryptographic instructions.
+nocrypto Disable the cryptographic instructions.
+nofp Disable the floating-point, Advanced SIMD and cryptographic instructions.
-march=native causes the compiler to auto-detect the architecture of the build computer. At present, this feature is only supported on GNU/Linux, and not all architectures are recognized. If the auto-detect is unsuccessful the option has no effect.
-march=unset causes the compiler to ignore any -march=... options that appear earlier on the command line and behave as if the option was never passed. This is useful to avoid warnings about conflicting CPU and architecture options when the two produce different architecture specifications.
-mtune=name This option specifies the name of the target ARM processor for which GCC should tune the performance of the code. For some ARM implementations better performance can be obtained by using this option. Permissible names are: arm7tdmi, arm7tdmi-s, arm710t, arm720t, arm740t, strongarm, strongarm110, strongarm1100, strongarm1110, arm8, arm810, arm9, arm9e, arm920, arm920t, arm922t, arm946e-s, arm966e-s, arm968e-s, arm926ej-s, arm940t, arm9tdmi, arm10tdmi, arm1020t, arm1026ej-s, arm10e, arm1020e, arm1022e, arm1136j-s, arm1136jf-s, mpcore, mpcorenovfp, arm1156t2-s, arm1156t2f-s, arm1176jz-s, arm1176jzf-s, generic-armv7-a, cortex-a5, cortex-a7, cortex-a8, cortex-a9, cortex-a12, cortex-a15, cortex-a17, cortex-a32, cortex-a35, cortex-a53, cortex-a55, cortex-a57, cortex-a72, cortex-a73, cortex-a75, cortex-a76, cortex-a76ae, cortex-a77, cortex-a78, cortex-a78ae, cortex-a78c, cortex-a710, ares, cortex-r4, cortex-r4f, cortex-r5, cortex-r7, cortex-r8, cortex-r52, cortex-r52plus, cortex-m0, cortex-m0plus, cortex-m1, cortex-m3, cortex-m4, cortex-m7, cortex-m23, cortex-m33, cortex-m35p, cortex-m52, cortex-m55, cortex-m85, cortex-x1, cortex-x1c, cortex-m1.small-multiply, cortex-m0.small-multiply, cortex-m0plus.small-multiply, exynos-m1, marvell-pj4, neoverse-n1, neoverse-n2, neoverse-v1, xscale, iwmmxt, iwmmxt2, ep9312, fa526, fa626, fa606te, fa626te, fmp626, fa726te, star-mc1, xgene1.
Additionally, this option can specify that GCC should tune the performance of the code for a big.LITTLE system. Permissible names are: cortex-a15.cortex-a7, cortex-a17.cortex-a7, cortex-a57.cortex-a53, cortex-a72.cortex-a53, cortex-a72.cortex-a35, cortex-a73.cortex-a53, cortex-a75.cortex-a55, cortex-a76.cortex-a55.
-mtune=generic-arch specifies that GCC should tune the performance for a blend of processors within architecture arch. The aim is to generate code that run well on the current most popular processors, balancing between optimizations that benefit some CPUs in the range, and avoiding performance pitfalls of other CPUs. The effects of this option may change in future GCC versions as CPU models come and go.
-mtune permits the same extension options as -mcpu, but the extension options do not affect the tuning of the generated code.
-mtune=native causes the compiler to auto-detect the CPU of the build computer. At present, this feature is only supported on GNU/Linux, and not all architectures are recognized. If the auto-detect is unsuccessful the option has no effect.
-mcpu=name[+extension...] This specifies the name of the target ARM processor. GCC uses this name to derive the name of the target ARM architecture (as if specified by -march) and the ARM processor type for which to tune for performance (as if specified by -mtune). Where this option is used in conjunction with -march or -mtune, those options take precedence over the appropriate part of this option.
Many of the supported CPUs implement optional architectural extensions. Where this is so the architectural extensions are normally enabled by default. If implementations that lack the extension exist, then the extension syntax can be used to disable those extensions that have been omitted. For floating-point and Advanced SIMD (Neon) instructions, the settings of the options -mfloat-abi and -mfpu must also be considered: floating-point and Advanced SIMD instructions will only be used if -mfloat-abi is not set to soft; and any setting of -mfpu other than auto will override the available floating-point and SIMD extension instructions.
For example, cortex-a9 can be found in three major configurations: integer only, with just a floating-point unit or with floating-point and Advanced SIMD. The default is to enable all the instructions, but the extensions +nosimd and +nofp can be used to disable just the SIMD or both the SIMD and floating-point instructions respectively.
Permissible names for this option are the same as those for -mtune.
The following extension options are common to the listed CPUs:
+nodsp Disable the DSP instructions on cortex-m33, cortex-m35p, cortex-m52, cortex-m55 and cortex-m85. Also disable the M-Profile Vector Extension (MVE) integer and single precision floating-point instructions on cortex-m52, cortex-m55 and cortex-m85.
+nopacbti Disable the Pointer Authentication and Branch Target Identification Extension on cortex-m52 and cortex-m85.
+nomve Disable the M-Profile Vector Extension (MVE) integer and single precision floating-point instructions on cortex-m52, cortex-m55 and cortex-m85.
+nomve.fp Disable the M-Profile Vector Extension (MVE) single precision floating-point instructions on cortex-m52, cortex-m55 and cortex-m85.
+cdecp0, +cdecp1, ... , +cdecp7 Enable the Custom Datapath Extension (CDE) on selected coprocessors according to the numbers given in the options in the range 0 to 7 on cortex-m52, cortex-m55, cortex-m85 and star-mc1.
+nofp Disables the floating-point instructions on arm9e, arm946e-s, arm966e-s, arm968e-s, arm10e, arm1020e, arm1022e, arm926ej-s, arm1026ej-s, cortex-r5, cortex-r7, cortex-r8, cortex-m4, cortex-m7, cortex-m33, cortex-m35p, cortex-m52, cortex-m55 and cortex-m85. Disables the floating-point and SIMD instructions on generic-armv7-a, cortex-a5, cortex-a7, cortex-a8, cortex-a9, cortex-a12, cortex-a15, cortex-a17, cortex-a15.cortex-a7, cortex-a17.cortex-a7, cortex-a32, cortex-a35, cortex-a53 and cortex-a55.
+nofp.dp Disables the double-precision component of the floating-point instructions on cortex-r5, cortex-r7, cortex-r8, cortex-r52, cortex-r52plus and cortex-m7.
+nosimd Disables the SIMD (but not floating-point) instructions on generic-armv7-a, cortex-a5, cortex-a7 and cortex-a9.
+crypto Enables the cryptographic instructions on cortex-a32, cortex-a35, cortex-a53, cortex-a55, cortex-a57, cortex-a72, cortex-a73, cortex-a75, exynos-m1, xgene1, cortex-a57.cortex-a53, cortex-a72.cortex-a53, cortex-a73.cortex-a35, cortex-a73.cortex-a53 and cortex-a75.cortex-a55.
Additionally the generic-armv7-a pseudo target defaults to VFPv3 with 16 double-precision registers. It supports the following extension options: mp, sec, vfpv3-d16, vfpv3, vfpv3-d16-fp16, vfpv3-fp16, vfpv4-d16, vfpv4, neon, neon-vfpv3, neon-fp16, neon-vfpv4. The meanings are the same as for the extensions to -march=armv7-a.
-mcpu=generic-arch is also permissible, and is equivalent to -march=arch -mtune=generic-arch. See -mtune for more information.
-mcpu=native causes the compiler to auto-detect the CPU of the build computer. At present, this feature is only supported on GNU/Linux, and not all architectures are recognized. If the auto-detect is unsuccessful the option has no effect.
-mcpu=unset causes the compiler to ignore any -mcpu=... options that appear earlier on the command line and behave as if the option was never passed. This is useful to avoid warnings about conflicting CPU and architecture options when the two produce different architecture specifications.
-mfpu=name This specifies what floating-point hardware (or hardware emulation) is available on the target. Permissible names are: auto, vfpv2, vfpv3, vfpv3-fp16, vfpv3-d16, vfpv3-d16-fp16, vfpv3xd, vfpv3xd-fp16, neon-vfpv3, neon-fp16, vfpv4, vfpv4-d16, fpv4-sp-d16, neon-vfpv4, fpv5-d16, fpv5-sp-d16, fp-armv8, neon-fp-armv8 and crypto-neon-fp-armv8. Note that neon is an alias for neon-vfpv3 and vfp is an alias for vfpv2.
The setting auto is the default and is special. It causes the compiler to select the floating-point and Advanced SIMD instructions based on the settings of -mcpu and -march.
Like, the NEON thing — bestie, listen up — if the floating-point hardware you picked includes the NEON extension (e.g. -mfpu=neon), just know that floating-point operations are not generated by GCC's auto-vectorization pass unless -funsafe-math-optimizations is also specified. Why? Because NEON hardware doesn't fully implement the IEEE 754 standard for floating-point arithmetic (denormal values get treated as zero, rude), so using NEON instructions may lead to a loss of precision. 😭
You can also set the fpu name at function level by using the target("fpu=") function attributes or pragmas. cute, right? 💅
-mfp16-format=name— specifies the format of the__fp16half-precision floating-point type. Permissible names arenone,ieee, andalternative; the default isnone, and in that case the__fp16type is just… not defined. gone. 💀-mstructure-size-boundary=n— rounds the sizes of all structures and unions up to a multiple of the number of bits you set here. Permissible values are8,32and64. The default value varies for different toolchains — for the COFF targeted toolchain the default value is8. A value of64is only allowed if the underlying ABI supports it. Specifying a larger number can produce faster, more efficient code, but can also increase the size of the program (trade-offs, always trade-offs 🫶). Different values are potentially incompatible: code compiled with one value cannot necessarily expect to work with code or libraries compiled with another value, if they exchange information using structures or unions. This option is deprecated.-mabort-on-noreturn— generates a call to the functionabortat the end of anoreturnfunction. It gets executed if the function tries to return. (like, sir, you were told you don't return 💀)-mlong-calls,-mno-long-calls— tells the compiler to perform function calls by first loading the address of the function into a register and then performing a subroutine call on this register. This switch is needed if the target function lies outside of the 64-megabyte addressing range of the offset-based version of subroutine call instruction.
Even if this switch is enabled, not all function calls are turned into long calls. The heuristic is that static functions, functions that have the short_call attribute, functions that are inside the scope of a #pragma no_long_calls directive, and functions whose definitions have already been compiled within the current compilation unit are not turned into long calls. The exceptions to this rule are that weak function definitions, functions with the long_call attribute or the section attribute, and functions that are within the scope of a #pragma long_calls directive are always turned into long calls. (exceptions to the exceptions, iconic ✨)
This feature is not enabled by default. Specifying -mno-long-calls restores the default behavior, as does placing the function calls within the scope of a #pragma long_calls_off directive. Note these switches have no effect on how the compiler generates code to handle function calls via function pointers. fr fr.
-msingle-pic-base— treats the register used for PIC addressing as read-only, rather than loading it in the prologue for each function. The runtime system is responsible for initializing this register with an appropriate value before execution begins. (so, like, that's on the runtime, not on GCC 🎀)-mpic-register=reg— specifies the register to be used for PIC addressing. For the standard PIC base case, the default is any suitable register determined by the compiler. 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— assumes the displacement between the text and data segments is fixed at static link time. This permits using PC-relative addressing operations to access data known to be in the data segment. For non-VxWorks RTP targets, this option is enabled by default. When disabled on such targets, it will enable-msingle-pic-baseby default. (sneaky little chain reaction, love that for us 💜)-mpoke-function-name— writes the name of each function into the text section, directly preceding 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 performing a stack backtrace, code can inspect the value of pc stored at fp + 0. If the trace function then looks at location pc - 12 and the top 8 bits are set, then we know that there is a function name embedded immediately preceding this location and has length ((pc[-3]) & 0xff000000). iykyk 🧚
-mthumb,-marm— selects between generating code that executes in ARM and Thumb states. The default for most configurations is to generate code that executes in ARM state, but the default can be changed by configuring GCC with the--with-mode=stateconfigure option.
You can also override the ARM and Thumb mode for each function by using the target("thumb") and target("arm") function attributes or pragmas. per-function vibes, we stan 💅
-mtpcs-frame— generates a stack frame that is compliant with the Thumb Procedure Call Standard for all non-leaf functions. (A leaf function is one that does not call any other functions.) The default is-mno-tpcs-frame.-mtpcs-leaf-frame— generates a stack frame that is compliant with the Thumb Procedure Call Standard for all leaf functions. (Again: a leaf function is one that does not call any other functions.) The default is-mno-apcs-leaf-frame. (yes, the default name is spelled differently from the option, no I will not be explaining that, it's just how it is 🙈)-mcallee-super-interworking— gives all externally visible functions in the file being compiled an ARM instruction set header which switches to Thumb mode before executing the rest of the function. This allows these functions to be called from non-interworking code. This option is not valid in AAPCS configurations because interworking is enabled by default.-mcaller-super-interworking— allows calls via function pointers (including virtual functions) to execute correctly regardless of whether the target code has been compiled for interworking or not. There's a small overhead in the cost of executing a function pointer if this option is enabled. This option is not valid in AAPCS configurations because interworking is enabled by default.-mtp=name— specifies the access model for the thread local storage pointer. The modelsoftgenerates calls to__aeabi_read_tp. Other accepted models aretpidrurw,tpidruroandtpidrprw, which fetch the thread pointer from the corresponding system register directly (supported from thearm6karchitecture and later). These system registers are accessed through theCP15co-processor interface and the argumentcp15is also accepted as a convenience alias oftpidruro. The argumentautouses the best available method for the selected processor. The default setting isauto. 🦋-mtls-dialect=dialect— specifies the dialect to use for accessing thread local storage. Two dialects are supported---gnuandgnu2. Thegnudialect selects the original GNU scheme for supporting local and global dynamic TLS models. Thegnu2dialect selects the GNU descriptor scheme, which provides better performance for shared libraries. The GNU descriptor scheme is compatible with the original scheme, but does require new assembler, linker and library support. Initial and local exec TLS models are unaffected by this option and always use the original scheme. (so it's giving "new but also old", love a compatible era 🌸)-mword-relocations— only generate absolute relocations on word-sized values (i.e.R_ARM_ABS32). This is enabled by default on targets (uClinux,SymbianOS) where the runtime loader imposes this restriction, and when-fpicor-fPICis specified. This option conflicts with-mslow-flash-data.-mfix-cortex-m3-ldrd— some Cortex-M3 cores can cause data corruption whenldrdinstructions with overlapping destination and base registers are used. This option avoids generating these instructions. This option is enabled by default when-mcpu=cortex-m3is specified. (silently protecting you, main character energy 👑)-mfix-cortex-a57-aes-1742098,-mno-fix-cortex-a57-aes-1742098,-mfix-cortex-a72-aes-1655431,-mno-fix-cortex-a72-aes-1655431— enable (disable) mitigation for an erratum on Cortex-A57 and Cortex-A72 that affects the AES cryptographic instructions. This option is enabled by default when either-mcpu=cortex-a57or-mcpu=cortex-a72is specified. (erratum is just a fancy word for "oops, hardware bug" 🔥)-munaligned-access,-mno-unaligned-access— enables (or disables) reading and writing of 16- and 32-bit values from addresses that are not 16- or 32-bit aligned. By default unaligned access is disabled for all pre-ARMv6, all ARMv6-M and for ARMv8-M Baseline architectures, and enabled for all other architectures. If unaligned access is not enabled then words in packed data structures are accessed a byte at a time. (slow and steady, but mostly slow 😭)
The ARM attribute Tag_CPU_unaligned_access is set in the generated object file to either true or false, depending upon the setting of this option. If unaligned access is enabled then the preprocessor symbol __ARM_FEATURE_UNALIGNED is also defined. ⭐
-mslow-flash-data— assumes loading data from flash is slower than fetching instructions. Therefore literal load is minimized for better performance. This option is only supported when compiling for ARMv7 M-profile and is off by default. It conflicts with-mword-relocations.-masm-syntax-unified— assumes inline assembler is using unified asm syntax. The default is currently off, which implies divided syntax. This option has no impact on Thumb2. However, this may change in future releases of GCC. Divided syntax should be considered deprecated. (so, like, move on bestie 💀)-mrestrict-it— restricts generation of IT blocks to conform to the rules of ARMv8-A. IT blocks can only contain a single 16-bit instruction from a select set of instructions. This option is on by default for ARMv8-A Thumb mode. 🎀-mpure-code— do not allow constant data to be placed in code sections. Additionally, when compiling for ELF object format, give all text sections the ELF processor-specific section attributeSHF_ARM_PURECODE. This option is only available when generating non-pic code for M-profile targets. ✨-mcmse— generates secure code exactly as laid out in the "ARMv8-M Security Extensions: Requirements on Development Tools Engineering Specification", which lives over at <https://developer.arm.com/documentation/ecm0359818/latest/> 💅-mfix-cmse-cve-2021-35465— patches over a potential security oopsie with theVLLDMinstruction in some M-profile devices when you're using CMSE (CVE-2021-365465) 😭 this one's automatically on when you pass-mcpu=withcortex-m33,cortex-m35p,cortex-m52,cortex-m55,cortex-m85orstar-mc1. And if you ever need to turn the mitigation off, that's what-mno-fix-cmse-cve-2021-35465is for 🫶-mstack-protector-guard=guard</>-mstack-protector-guard-offset=offset</> — makes stack protection code with the canary sitting at guard 🐤 the supported locations areglobal(a global canary) ortls(a canary you reach through the TLS register). heads up: the-mstack-protector-guard-offset=option is meant for use with-fstack-protector-guard=tlsand not for user-land code, ok?? 🙈-mfdpic,-mno-fdpic— picks the FDPIC ABI, which represents pointers to functions with 64-bit function descriptors ✨ when the compiler is configured forarm-*-uclinuxfdpiceabitargets this option is on by default and implies-fPIEif none of the PIC/PIE-related options is given. on other targets it only enables the FDPIC-specific code generation features, and you gotta explicitly hand over the PIC/PIE-related options yourself as needed 🫡
ngl static linking is not supported here, because it would still involve the dynamic linker when the program self-relocates 😬 if that behavior is acceptable to you, use the-staticand-Wl,-dynamic-linkeroptions.
The opposite -mno-fdpic option is useful (and actually required) for building the Linux kernel with the same (arm-*-uclinuxfdpiceabi) toolchain that you used to build the userland programs 💀
-mbranch-protection=features</> — turns on branch protection features (armv8.1-m.main only) 👑
features can take one of these forms:
none— generates code with no branch protection or return address signing.standard— generates code with all branch protection features enabled at their standard level.pac-ret— generates code with return address signing at its standard level, which means signing all functions that save the return address to memory.pac-ret+leaf— extends the pac-ret signing to also cover leaf functions, even if they don't write the return address to memory.bti— adds landing-pad instructions at the permitted targets of indirect branch instructions.
If support for the +pacbti architecture extension isn't enabled (like, say, via -march=), then all branch protection and return address signing operations are constrained to use only the instructions defined in the architectural-NOP space 🧚 the generated code stays backwards-compatible with earlier versions of the architecture, but that extra security can be enabled at run time on processors that support the PACBTI extension.
Branch target enforcement using BTI can only be enabled at runtime if all the code in the application has been compiled with at least -mbranch-protection=bti — no sneaky exceptions bestie.
Any setting other than none is supported only on armv8-m.main or later ⭐
The default? generate code with no branch protection or return address signing, periodt.
-mvectorize-with-neon-quad,-mvectorize-with-neon-double— controls whether vectorization uses NEON quad-word or double-word registers 🦋 the default is-mvectorize-with-neon-quad.
🌈 AVR Options 🌈
These options are defined for AVR implementations:
-mmcu=mcu</> — specifies the AVR instruction set architecture (ISA) or device type 🎀 the default for this option isavr2.
Here's every AVR device and ISA that's supported 👇
Note: full device support is made up of startup codecrtmcu.o, a device headeravr/io*.h, a device librarylibmcu.a, and a device-specs filespecs-mcu. Only that last one is provided by the compiler, according to the supported *mcu*s listed below 😭 the rest comes from AVR-LibC (<https://github.com/avrdudes/avr-libc/>), or viaatpackfiles from the hardware manufacturer (<https://gcc.gnu.org/wiki/avr-gcc#atpack>).
avr2— "Classic" devices with up to 8 KiB of program memory. mcu =attiny22,attiny26,at90s2313,at90s2323,at90s2333,at90s2343,at90s4414,at90s4433,at90s4434,at90c8534,at90s8515,at90s8535.avr25— "Classic" devices with up to 8 KiB of program memory and with theMOVWinstruction. mcu =attiny13,attiny13a,attiny24,attiny24a,attiny25,attiny261,attiny261a,attiny2313,attiny2313a,attiny43u,attiny44,attiny44a,attiny45,attiny48,attiny441,attiny461,attiny461a,attiny4313,attiny84,attiny84a,attiny85,attiny87,attiny88,attiny828,attiny841,attiny861,attiny861a,ata5272,ata6616c,at86rf401.avr3— "Classic" devices with 16 KiB up to 64 KiB of program memory. mcu =at76c711,at43usb355.avr31— "Classic" devices with 128 KiB of program memory. mcu =atmega103,at43usb320.avr35— "Classic" devices with 16 KiB up to 64 KiB of program memory and with theMOVWinstruction. mcu =attiny167,attiny1634,atmega8u2,atmega16u2,atmega32u2,ata5505,ata6617c,ata664251,at90usb82,at90usb162.avr4— "Enhanced" devices with up to 8 KiB of program memory. mcu =atmega48,atmega48a,atmega48p,atmega48pa,atmega48pb,atmega8,atmega8a,atmega8hva,atmega88,atmega88a,atmega88p,atmega88pa,atmega88pb,atmega8515,atmega8535,ata5795,ata6285,ata6286,ata6289,ata6612c,at90pwm1,at90pwm2,at90pwm2b,at90pwm3,at90pwm3b,at90pwm81.avr5— "Enhanced" devices with 16 KiB up to 64 KiB of program memory. mcu =atmega16,atmega16a,atmega16hva,atmega16hva2,atmega16hvb,atmega16hvbrevb,atmega16m1,atmega16u4,atmega161,atmega162,atmega163,atmega164a,atmega164p,atmega164pa,atmega165,atmega165a,atmega165p,atmega165pa,atmega168,atmega168a,atmega168p,atmega168pa,atmega168pb,atmega169,atmega169a,atmega169p,atmega169pa,atmega32,atmega32a,atmega32c1,atmega32hvb,atmega32hvbrevb,atmega32m1,atmega32u4,atmega32u6,atmega323,atmega324a,atmega324p,atmega324pa,atmega324pb,atmega325,atmega325a,atmega325p,atmega325pa,atmega328,atmega328p,atmega328pb,atmega329,atmega329a,atmega329p,atmega329pa,atmega3250,atmega3250a,atmega3250p,atmega3250pa,atmega3290,atmega3290a,atmega3290p,atmega3290pa,atmega406,atmega64,atmega64a,atmega64c1,atmega64hve,atmega64hve2,atmega64m1,atmega64rfr2,atmega640,atmega644,atmega644a,atmega644p,atmega644pa,atmega644rfr2,atmega645,atmega645a,atmega645p,atmega649,atmega649a,atmega649p,atmega6450,atmega6450a,atmega6450p,atmega6490,atmega6490a,atmega6490p,ata5790,ata5790n,ata5791,ata6613c,ata6614q,ata5782,ata5831,ata8210,ata8510,ata5787,ata5835,ata5700m322,ata5702m322,at90pwm161,at90pwm216,at90pwm316,at90can32,at90can64,at90scr100,at90usb646,at90usb647,at94k,m3000.avr51— "Enhanced" devices with 128 KiB of program memory. mcu =atmega128,atmega128a,atmega128rfa1,atmega128rfr2,atmega1280,atmega1281,atmega1284,atmega1284p,atmega1284rfr2,at90can128,at90usb1286,at90usb1287.avr6— "Enhanced" devices with a 3-byte PC, i.e. more than 128 KiB of program memory. mcu =atmega256rfr2,atmega2560,atmega2561,atmega2564rfr2.avrxmega2— "XMEGA" devices with more than 8 KiB and up to 64 KiB of program memory. mcu =atxmega8e5,atxmega16a4,atxmega16a4u,atxmega16c4,atxmega16d4,atxmega16e5,atxmega32a4,atxmega32a4u,atxmega32c3,atxmega32c4,atxmega32d3,atxmega32d4,atxmega32e5,avr64da28,avr64da28s,avr64da32,avr64da32s,avr64da48,avr64da48s,avr64da64,avr64da64s,avr64db28,avr64db32,avr64db48,avr64db64,avr64dd14,avr64dd20,avr64dd28,avr64dd32,avr64du28,avr64du32,avr64ea28,avr64ea32,avr64ea48,avr64sd28,avr64sd32,avr64sd48.
avrxmega3— "XMEGA" girlies with up to 64 KiB of combined program memory and RAM, where the program memory is literally visible in the RAM address space 👀
mcu = "attiny202", "attiny204", "attiny212", "attiny214",
"attiny402", "attiny404", "attiny406", "attiny412", "attiny414",
"attiny416", "attiny416auto", "attiny417", "attiny424",
"attiny426", "attiny427", "attiny804", "attiny806", "attiny807",
"attiny814", "attiny816", "attiny817", "attiny824", "attiny826",
"attiny827", "attiny1604", "attiny1606", "attiny1607",
"attiny1614", "attiny1616", "attiny1617", "attiny1624",
"attiny1626", "attiny1627", "attiny3214", "attiny3216",
"attiny3217", "attiny3224", "attiny3226", "attiny3227",
"atmega808", "atmega809", "atmega1608", "atmega1609",
"atmega3208", "atmega3209", "atmega4808", "atmega4809",
"avr16dd14", "avr16dd20", "avr16dd28", "avr16dd32", "avr16du14",
"avr16du20", "avr16du28", "avr16du32", "avr16ea28", "avr16ea32",
"avr16ea48", "avr16eb14", "avr16eb20", "avr16eb28", "avr16eb32",
"avr16la14", "avr16la20", "avr16la28", "avr16la32", "avr32da28",
"avr32da28s", "avr32da32", "avr32da32s", "avr32da48",
"avr32da48s", "avr32db28", "avr32db32", "avr32db48",
"avr32dd14", "avr32dd20", "avr32dd28", "avr32dd32", "avr32du14",
"avr32du20", "avr32du28", "avr32du32", "avr32ea28", "avr32ea32",
"avr32ea48", "avr32eb14", "avr32eb20", "avr32eb28", "avr32eb32",
"avr32la14", "avr32la20", "avr32la28", "avr32la32", "avr32sd20",
"avr32sd28", "avr32sd32".avrxmega4— "XMEGA" besties holding more than 64 KiB and up to 128 KiB of program memory 💾
mcu = "atxmega64a3", "atxmega64a3u", "atxmega64a4u", "atxmega64b1",
"atxmega64b3", "atxmega64c3", "atxmega64d3", "atxmega64d4",
"avr128da28", "avr128da28s", "avr128da32", "avr128da32s",
"avr128da48", "avr128da48s", "avr128da64", "avr128da64s",
"avr128db28", "avr128db32", "avr128db48", "avr128db64".avrxmega5— "XMEGA" devices with more than 64 KiB and up to 128 KiB of program memory and more than 64 KiB of RAM — she's got range ✨
mcu = "atxmega64a1", "atxmega64a1u".avrxmega6— "XMEGA" girlies with more than 128 KiB of program memory, absolute units fr 💅
mcu = "atxmega128a3", "atxmega128a3u", "atxmega128b1", "atxmega128b3",
"atxmega128c3", "atxmega128d3", "atxmega128d4", "atxmega192a3",
"atxmega192a3u", "atxmega192c3", "atxmega192d3", "atxmega256a3",
"atxmega256a3b", "atxmega256a3bu", "atxmega256a3u",
"atxmega256c3", "atxmega256d3", "atxmega384c3", "atxmega384d3".avrxmega7— "XMEGA" with more than 128 KiB of program memory and more than 64 KiB of RAM — the main character of the XMEGA lineup 👑
mcu = "atxmega128a1", "atxmega128a1u", "atxmega128a4u".avrtiny— "Reduced Tiny" Tiny core devices, and listen, they're tiny tiny: only 16 general purpose registers and 512 B up to 4 KiB of program memory 🐜
mcu = "attiny4", "attiny5", "attiny9", "attiny10", "attiny102",
"attiny104", "attiny20", "attiny40".avr1— this ISA is implemented by the minimal AVR core and it's supported for assembler only, no cap, that's just the vibe 🫶
mcu = "attiny11", "attiny12", "attiny15", "attiny28", "at90s1200".-mabsdata— okay so this one assumes that all data in static storage can be accessed byLDS/STSinstructions. It has an effect only on reduced Tiny devices like ATtiny40. Also go peek at the"absdata"AVR Attributes, variable attribute 💖-mcvt— use a compact vector table. Some devices support a CVT with only four entries:0=Reset,1=NMI,2=Prio1 IRQ,3=Prio0 IRQs. This option links startup code fromcrtmcu-cvt.oinstead of the usualcrtmcu.o. And bestie, it does double duty — apart from providing a compact vector table, the startup code will set bitCPUINT_CTRLA.CPUINT_CVT, which enables the CVT on the device 🔥
When you do not want the startup code to set CPUINT_CTRLA.CPUINT_CVT, then you can satisfy symbol __init_cvt so that the respective code is no more pulled in from libmcu.a. For example, you can link with:
-Wl,--defsym,__init_cvt=0The CVT startup code is available since AVR-LibC v2.3 (https://github.com/avrdudes/avr-libc/issues/1010).
-mdouble=bits,-mlong-double=bits— set the size (in bits) of thedoubleorlong doubletype, respectively 🌸 Possible values for bits are32and64. Whether or not a specific value for bits is allowed depends on the--with-double=and--with-long-double=configure options (https://gcc.gnu.org/install/configure.html#avr), and the same applies for the default values of the options. iykyk 💫-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 the AVR Attributes,"no_gccisr"function attribute. This feature is activated per default if optimization is on (but not with-Og, @pxref{Optimize Options}), and if GNU Binutils support PR21683 (https://sourceware.org/PR21683) 🎀-mint8— assumeintto be an 8-bit integer. This affects the sizes of all types: acharis 1 byte, anintis 1 byte, alongis 2 bytes, andlong longis 4 bytes 😭 Please note that this option does not conform to the C standards, but it results in smaller code size. So like... tiny code, tiny chaos, your call bestie 💀-mmain-is-OS_task— do not save registers inmain. The effect is the same like attaching attribute AVR Attributes,"OS_task"tomain. It is activated per default if optimization is on ✨-mno-call-main— don't runmainby means of
XCALL main
XJMP exitInstead, put main in section .init9 (https://avrdudes.github.io/avr-libc/avr-libc-user-manual/mem_sections.html#sec_dot_init) so that no call is required. By setting this option the user asserts that main will not return 👀
This option can be used for devices with very limited resources in order to save a few bytes of code and stack space. It will work as expected since AVR-LibC v2.3 (https://github.com/avrdudes/avr-libc/issues/1012). With older versions, there will be no performance gain.
-mno-interrupts— generated code is not compatible with hardware interrupts. Code size is smaller. That's the whole trade, no extra lore 🫠-mrelax— try to replaceCALLresp.JMPinstruction by the shorterRCALLresp.RJMPinstruction 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 are not known before code is located. Therefore, the assembler code generated by the compiler is the same, but the instructions in the executable may differ from instructions in the assembler code. It's giving glow-up, not rewrite ✨
Relaxing must be turned on if linker stubs are needed, see the section on EIND and linker stubs below. Don't skip that part, bestie 🚨
-mpmem-wrap-around— enable program counter wrap-around in linker relaxation 🌀-mrodata-in-ram,-mno-rodata-in-ram— locate the.rodatasections for read-only data in RAM resp. in program memory. For most devices, there is no choice and this option acts rather like an assertion 🙈
Since v14 and for the AVR64* and AVR128* devices, .rodata is located in flash memory per default, provided the required GNU Binutils support (PR31124 (https://sourceware.org/PR31124)) is available. In that case, -mrodata-in-ram can be used to return to the old layout with .rodata in RAM 🎀
-mtiny-stack— only change the lower 8 bits of the stack pointer. That's it. That's the whole option 🐜-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 librarylibmcu.a🚫
Notice that since AVR-LibC v2.3, that library contains code that is essential for the correct functioning of a program. In particular, it contains parts of the startup code like:
__init_sp(https://github.com/avrdudes/avr-libc/issues/1011) to initialize the stack pointer with symbol__stack__init_cvt(https://github.com/avrdudes/avr-libc/issues/1010) to set up the hardware to use a compact vector table with-mcvt__call_main(https://github.com/avrdudes/avr-libc/issues/1012) to callmainandexit__do_flmap_init(https://github.com/avrdudes/avr-libc/issues/931) to set up FLMAP according to symbol__flmap
Like, this is load-bearing bestie, be careful 💀
-nodevicespecs— don't add-specs=device-specs/specs-mcuto the compiler driver's command line. The user takes responsibility for supplying the sub-processes like compiler proper, assembler and linker with appropriate command line options 😤 This means that the user has to supply her private device specs file by means of-specs=path-to-specs-file. There is no more need for option-mmcu=mcu✨
This option can also serve as a replacement for the older way of specifying custom device-specs files that needed -B some-path to point to a directory which contains a folder named device-specs which contains a specs file named specs-mcu, where mcu was specified by -mmcu=mcu. Ngl that's a whole scavenger hunt, so congrats on the upgrade 🎉
-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__vectorprefix. Enabled by default. She's always watching your spelling, periodt 📝
🚀 AVR Optimization Options 🚀
The following options are pure optimization options 🧚 Options -mgas-isr-prologues, -mmain-is-OS_task, -mno-call-main and -mrelax from above are only almost optimization options, since there are rare occasions where their different code generation matters 👀
-maccumulate-args— accumulate outgoing function arguments and acquire/release the needed stack space for outgoing function arguments once in function prologue/epilogue 💖 Without this option, outgoing arguments are pushed before calling a function and popped afterwards. See also the-fdefer-popOptimize Options, optimization option.
Popping the arguments after the function call can be expensive on AVR so that accumulating the stack space might lead to smaller executables because arguments need not be removed from the stack after such a function call. Basically: less popping, more slaying 🔥
That's the whole point of this one, bestie — it can lead to reduced code size for functions that perform several calls to functions that get their arguments on the stack, like calls to printf-like functions. 💖
-mbranch-cost=cost— sets the branch costs for conditional branch instructions to cost. Reasonable values for cost are small, non-negative integers. The default branch cost is0.-mcall-prologues— function prologues/epilogues get expanded as calls to the appropriate subroutines. Code size is smaller. 🎀-mfuse-add,-mno-fuse-add,-mfuse-add=level— optimizes indirect memory accesses on reduced Tiny devices. The default useslevel=1for optimizations-Ogand-O1, andlevel=2for higher optimizations. Valid values for level are0,1and2.-mfuse-move,-mno-fuse-move,-mfuse-move=level— runs a post reload optimization pass that tries to fuse move instructions and to split multi-byte instructions into 8-bit operations. The default useslevel=3for optimization-O1, andlevel=23for higher optimizations. Valid values for level are in the range0 ... 23, which is a 3:2:2:2 mixed radix value. Each digit controls some aspect of the optimization. 🌸-mfuse-move2— runs a post combine optimization pass that tries to fuse move instructions.-mstrict-X— uses 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-msplit-bit-shift— splits multi-byte shifts with a constant offset into a shift with a byte offset and a residual shift with a non-byte offset. This optimization is turned on per default for-O2and higher, including-Osbut excluding-Oz. Splitting of shifts with a constant offset that is a multiple of8is controlled by-mfuse-move. ✨-msplit-ldst— splits multi-byte loads and stores into several byte loads and stores. This optimization is turned on per default for-O2and higher.-muse-nonzero-bits— enables optimizations that are only possible when some bits in a register are always zero. This optimization is turned on per default for-O2and higher. 🔥
🌈 EIND and Devices with More Than 128 Ki Bytes of Flash
Okay so pointers in the implementation are 16 bits wide, right? 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.
In order to facilitate indirect jump on devices with more than 128 Ki bytes of program memory space, there's a special function register called EIND that serves as the most significant part of the target address when EICALL or EIJMP instructions are used. 👑
Indirect jumps and calls on these devices are handled as follows by the compiler, and are subject to some limitations:
- The compiler never sets
EIND. 💀 - The compiler uses
EINDimplicitly inEICALL/EIJMPinstructions, or might readEINDdirectly in order to emulate an indirect call/jump by means of aRETinstruction. - The compiler assumes that
EINDnever changes during the startup code or during the application. In particular,EINDis not saved/restored in function or interrupt service routine prologue/epilogue. - For indirect calls to functions and computed goto, the linker generates stubs. Stubs are jump pads sometimes also called trampolines. So the indirect call/jump jumps to 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 withEIND != 0, a custom linker script has to be used in order to place the sections whose name start with.trampolinesinto the segment whereEINDpoints to. - The startup code from libgcc never sets
EIND. Notice that startup code is a blend of code from libgcc and AVR-LibC. For the impact of AVR-LibC onEIND, see the AVR-LibC user manual (https://avrdudes.github.io/avr-libc/avr-libc-user-manual/). - It is legitimate for user-specific startup code to set up
EINDearly — for example by means of initialization code located in section.init3. Such code runs prior to general startup code that initializes RAM and calls constructors, but after the bit of startup code from AVR-LibC that setsEINDto the segment where the vector table is located.
#include <avr/io.h>
static void
__attribute__((section(".init3"),naked,used,no_instrument_function))
init3_set_eind (void)
{
__asm volatile ("ldi r24,pm_hh8(__trampolines_start)\n\t"
"out %i0,r24" :: "n" (&EIND) : "r24","memory");
}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
gsmodifier (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
gsmodifiers 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 the high part of the address: the X, Y, Z address register is concatenated with the RAMPX, RAMPY, RAMPZ special function register, respectively, to get a wide address. Similarly, RAMPD is used together with direct addressing. 🌟
- The startup code initializes the
RAMPspecial function registers with zero. - If a AVR Named Address Spaces, named address space other than generic or
__flashis used, thenRAMPZis set as needed before the operation. - If the device supports RAM larger than 64 KiB and the compiler needs to change
RAMPZto accomplish an operation,RAMPZis reset to zero after the operation. - If the device comes with a specific
RAMPregister, 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
RAMPregisters, you must reset it to zero after the access. 🙈
⭐ AVR Built-in Macros
GCC defines several built-in macros so that your code can test for the presence or absence of features. Almost any of the following built-in macros are deduced from device capabilities and thus triggered by the -mmcu= command-line option. 🎀
For even more AVR-specific built-in macros see AVR Named Address Spaces and AVR Built-in Functions.
__AVR_ARCH__
Build-in macro that resolves to a decimal number that identifies the architecture and depends on the -mmcu=mcu option. Possible values are:
2, 25, 3, 31, 35, 4, 5, 51, 6
for mcu=avr2, avr25, avr3, avr31, avr35, avr4, avr5, avr51, avr6, respectively, and
100, 102, 103, 104, 105, 106, 107
for mcu=avrtiny, avrxmega2, avrxmega3, avrxmega4, avrxmega5, avrxmega6, avrxmega7, respectively. If mcu specifies a device, this built-in macro is set accordingly. For example, with -mmcu=atmega8 the macro is defined to 4. 🔥
__AVR_Device__
Setting -mmcu=device defines this built-in macro, which reflects the device's name. For example, -mmcu=atmega8 defines the built-in macro __AVR_ATmega8__, -mmcu=attiny261a defines __AVR_ATtiny261A__, etc.
The built-in macros' names follow the scheme __AVR_Device__ where Device is the device name as from the AVR user manual. The difference between Device in the built-in macro and device in -mmcu=device is that the latter is always lowercase. 💅
If device is not a device but only a core architecture like avr51, this macro is not defined.
__AVR_DEVICE_NAME__
Setting -mmcu=device defines this built-in macro to the device's name. For example, with -mmcu=atmega8 the macro is defined to atmega8. 🦋
If device is not a device but only a core architecture like avr51, this macro is not defined.
__AVR_CVT__
The code is being compiled with option -mcvt to use a compact vector table. ✨
__AVR_XMEGA__— the device / architecture is part of the XMEGA family of devices. slay 🦋__AVR_HAVE_ADIW__— the device has theADIWandSBIWinstructions. 💅__AVR_HAVE_ELPM__— the device has theELPMinstruction. ✨__AVR_HAVE_ELPMX__— the device has theELPM Rn,ZandELPM Rn,Z+instructions. 🎀__AVR_HAVE_LPMX__— the device has theLPM Rn,ZandLPM Rn,Z+instructions. 🦋__AVR_HAVE_MOVW__— the device has theMOVWinstruction to perform 16-bit register-register moves. 💫__AVR_HAVE_MUL__— the device has a hardware multiplier. 🔥__AVR_HAVE_JMP_CALL__— the device has theJMPandCALLinstructions. This is the case for devices with more than8 KiBof program memory. 👑__AVR_HAVE_EIJMP_EICALL__,__AVR_3_BYTE_PC__— the device has theEIJMPandEICALLinstructions. This is the case for devices with more than128 KiBof program memory. This also means the program counter (PC) is3bytes wide. 🌈__AVR_2_BYTE_PC__— the program counter (PC) is2bytes wide. This is the case for devices with up to128 KiBof program memory. 🍓__AVR_HAVE_8BIT_SP__,__AVR_HAVE_16BIT_SP__— the stack pointer (SP) register is treated as an 8-bit or 16-bit register, respectively, by the compiler. The definition of these macros is affected by-mtiny-stack. 💜__AVR_HAVE_SPH__,__AVR_SP8__— the device has theSPH(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=avr25, also by-msp8. 🧚__AVR_HAVE_RAMPD__,__AVR_HAVE_RAMPX__,__AVR_HAVE_RAMPY__,__AVR_HAVE_RAMPZ__— the device has theRAMPD,RAMPX,RAMPY, andRAMPZspecial 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 areSBRS,SBRC,SBIS,SBIC, andCPSE. 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, andLAT). 🔧__AVR_SFR_OFFSET__=offset— instructions that can address I/O special function registers directly, likeIN,OUT,SBI, etc., may use a different address than if addressed by an instruction to access RAM likeLDorSTS. This offset depends on the device architecture and has to be subtracted from the RAM address in order to get the respective I/O address. 🎯__AVR_SHORT_CALLS__— the-mshort-callscommand-line option is set. ⭐__AVR_PM_BASE_ADDRESS__=addr— some devices support reading from flash memory by means ofLD*instructions. The flash memory is seen in the data address space at an offset of__AVR_PM_BASE_ADDRESS__. If this macro is not defined, this feature is not available. If defined, the address space is linear and there is no need to put.rodatainto RAM. This is handled by the default linker description file, and is currently available foravrtinyandavrxmega3. Even more convenient: there is no need to use address spaces like__flashor features like attributeprogmemandpgm_read_*. 🫶__AVR_HAVE_FLMAP__— this macro is defined provided the following conditions are met:- the device has the
NVMCTRL_CTRLB.FLMAPbitfield. This applies to theAVR64*andAVR128*devices. 💅 - it’s not known at assembler-time which emulation will be used.
- the device has the
This implies the compiler was configured with GNU Binutils that implement PR31124 (https://sourceware.org/PR31124). 🧚
__AVR_RODATA_IN_RAM__— this macro is undefined when the code is compiled for a core architecture. When the code is compiled for a device, the macro is defined to1when the.rodatasections for read-only data is located in RAM; and defined to0, otherwise. 🌿__WITH_AVRLIBC__— the compiler is configured to be used together with AVR-LibC. See the--with-avrlibcconfigure option. 💖__HAVE_SIGNAL_N__— the compiler supports thesignal(num)andinterrupt(num)AVRAttributes,,functionattributes with an argumentnumthat specifies the number of the interrupt service routine. 📣__HAVE_DOUBLE_MULTILIB__— defined if-mdouble=acts as a multilib option. 🎀__HAVE_DOUBLE32__,__HAVE_DOUBLE64__— defined if the compiler supports 32-bitdoubleresp. 64-bitdouble. The actual layout is specified by option-mdouble=. ✨__DEFAULT_DOUBLE__— the size in bits ofdoubleif-mdouble=is not set. To test the layout ofdoublein a program, use the built-in macro__SIZEOF_DOUBLE__. 🦋__HAVE_LONG_DOUBLE32__,__HAVE_LONG_DOUBLE64__,__HAVE_LONG_DOUBLE_MULTILIB__,__DEFAULT_LONG_DOUBLE__— same as above, but forlong doubleinstead ofdouble. 💜__WITH_DOUBLE_COMPARISON__— reflects the--with-double-comparison={tristate|bool|libf7}configure option (https://gcc.gnu.org/install/configure.html#avr) and is defined to2or3. 🌈__WITH_LIBF7_LIBGCC__,__WITH_LIBF7_MATH__,__WITH_LIBF7_MATH_SYMBOLS__— reflects the--with-libf7={libgcc|math|math-symbols}configure option (https://gcc.gnu.org/install/configure.html#avr). 🍓
AVR Internal Options 🔧
Okay bestie, these next ones are used internally by the compiler and to communicate between device specs files and the compiler proper. You don’t need to set these options by hand — in particular, they are not optimization options. 👀
Using these options in the wrong way may lead to sub-optimal or wrong code. fr fr, be careful. They are documented for completeness, and in order to get a better understanding of device specs (https://gcc.gnu.org/wiki/avr-gcc#spec-files) files. 💅
-mn-flash=num— assume that the flash memory has a size ofnumtimes64 KiB. This determines which__flashNaddress spaces are available. 🍓-mflmap— the device has theFLMAPbit field located in special function registerNVMCTRL_CTRLB. 🌸-mrmw— assume that the device supports the Read-Modify-Write instructionsXCH,LAC,LAS, andLAT. 🔥-mshort-calls— assume thatRJMPandRCALLcan target the whole program memory. This option is used for multilib generation and selection for the devices from architectureavrxmega3. 💫-mskip-bug— generate code without skips (CPSE,SBRS,SBRC,SBIS,SBIC) over 32-bit instructions. 🙈-msp8— treat the stack pointer register as an 8-bit register, i.e. assume the high byte of the stack pointer is zero. This option is used by the compiler to select and build multilibs for architecturesavr2andavr25. These architectures mix devices with and withoutSPH. 🧚
Blackfin Options 🖤
-mcpu=cpu[-sirevision]— specifies the name of the target Blackfin processor. Currently,cpucan 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. 👑
The optional sirevision specifies the silicon revision of the target Blackfin processor. Any workarounds available for the targeted silicon revision are enabled. If sirevision is none, no workarounds are enabled. If sirevision is any, all workarounds for the targeted processor are enabled. The __SILICON_REVISION__ macro is defined to two hexadecimal digits representing the major and minor numbers in the silicon revision. If sirevision is none, the __SILICON_REVISION__ is not defined. If sirevision is any, the __SILICON_REVISION__ is defined to be 0xffff. If this optional sirevision is not used, GCC assumes the latest known silicon revision of the targeted Blackfin processor. 🦋
GCC defines a preprocessor macro for the specified cpu. For the bfin-elf toolchain, this option causes the hardware BSP provided by libgloss to be linked in if -msim is not given. ✨
Without this option, bf532 is used as the processor by default. 🌸
Note that support for bf561 is incomplete. For bf561, only the preprocessor macro is defined. 💀
-msim— specifies that the program will be run on the simulator. This causes the simulator BSP provided by libgloss to be linked in. This option has effect only forbfin-elftoolchain. Certain other options, such as-mid-shared-libraryand-mfdpic, imply-msim. 🎮-momit-leaf-frame-pointer— don’t keep the frame pointer in a register for leaf functions. This avoids the instructions to save, set up, and restore frame pointers and makes an extra register available in leaf functions. 💅-mspecld-anomaly,-mno-specld-anomaly— when enabled, the compiler ensures that the generated code does not contain speculative loads after jump instructions. If this option is used,__WORKAROUND_SPECULATIVE_LOADSis defined. ⚡-mcsync-anomaly,-mno-csync-anomaly— when enabled, the compiler ensures that the generated code does not containCSYNCorSSYNCinstructions too soon after conditional branches. If this option is used,__WORKAROUND_SPECULATIVE_SYNCSis defined. 🔄-mlow64k,-mno-low64k— when enabled, the compiler is free to take advantage of the knowledge that the entire program fits into the low64kof memory. The default behavior is to assume that the program is arbitrarily large (-mno-low64k). 🧠-mstack-check-l1— do stack checking using information placed into L1 scratchpad memory by the uClinux kernel. 🛡️-mid-shared-library,-mno-id-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. The default is-mno-id-shared-library, to generate code that doesn’t assume ID-based shared libraries are being used. 🫶-mleaf-id-shared-library,-mno-leaf-id-shared-library— generate code that supports shared libraries via the library ID method, but assumes that this library or executable won’t link against any other ID shared libraries. That allows the compiler to use faster code for jumps and calls. 🚀-mno-leaf-id-shared-library— this is the default, and it means GCC assumes the code you're compiling won't link against any ID shared libraries 💅. Downside? Jump and callinsnsget slower code generated for them. Trade-off city.-mshared-library-id=n— tells the compiler the identification number of the ID-based shared library being built. Settingnto0gives you more compact code 🎀. Setting it to any other value forces that number onto the current library, but honestly? It's no more space- or time-efficient than just not using this option at all. So like… do with that what you will.-msep-data,-mno-sep-data— this one's fun: it generates code that lets the data segment live in a whole different area of memory from the text segment ✨. Big deal because it enables execute in place in environments with no virtual memory management, by eliminating relocations against the text section. The default is-mno-sep-data, which tells GCC to assume the data segment follows the text segment. Classic default behavior, no cap.-mlong-calls,-mno-long-calls— makes the compiler do function calls the long way: first load the function's address into a register, then do a subroutine call on that register 🫶. You need this switch when the target function lives outside the 24-bit addressing range of the offset-based version of the subroutine call instruction. It's not enabled by default, and-mno-long-callsputs you back on default behavior. Heads up tho: these switches do nothing to how the compiler generates code for function calls through function pointers. Periodt.-mfast-fp— links against 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. Speed over strictness, basically.-minline-plt— enables inlining of PLT entries in function calls to functions that aren't known to bind locally. Real talk: it has no effect without-mfdpic. 💀-mmulticore— builds a standalone application for multicore Blackfin processors 🌈. This pulls in the proper start files and link scripts that support multicore, and it defines the macro"__BFIN_MULTICORE". It can only be used with-mcpu=bf561[-sirevision].
You can pair it with -mcorea or -mcoreb, which selects the one-application-per-core programming model. Without either of those, you get the single-application/dual-core programming model, and in that model the main function of Core B should be named "coreb_main" 👑
If you don't use this option at all, the single-core application programming model is what you get.
-mcorea— builds a standalone application for Core A of BF561 when you're in the one-application-per-core programming model 🎀. Proper start files and link scripts are used to support Core A, and the macro"__BFIN_COREA"gets defined. Can only be used together with-mmulticore.-mcoreb— same vibe, but for Core B of BF561 in the one-application-per-core programming model 🦋. Proper start files and link scripts support Core B, and"__BFIN_COREB"is defined. When you use this one,"coreb_main"should be used instead of"main". Also only valid with-mmulticore.-msdram— builds a standalone application for SDRAM ✨. Proper start files and link scripts put the application into SDRAM, and the macro"__BFIN_SDRAM"is defined. Note: the loader should initialize SDRAM before loading the application. That's on the loader, not on GCC, iykyk.-micplb— assumes ICPLBs are enabled at run time 💜. This affects 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— specifies the name of the target architecture, and GCC uses it to figure out what kind of instructions it can emit when generating assembly code. Permissible names are:c62x,c64x,c64x+,c67x,c67x+,c674x. Pick your fighter 🫶-mbig-endian— generates code for a big-endian target.-mlittle-endian— generates code for a little-endian target. This is the default 🌸-msim— chooses startup files and linker script suitable for the simulator.-msdata=default— puts small global and static data in the.neardatasection, which is pointed to by register"B14"👑. Small uninitialized global and static data goes in the.bsssection, which sits adjacent to.neardata. Small read-only data goes into.rodata. The sections used for large pieces of data are.fardata,.farand.const.-msdata=all— puts all data, not just the small objects, into the sections reserved for small data, and uses addressing relative to the"B14"register to access them. All in, no crumbs 💅-msdata=none— makes no use of the small-data sections at all and uses absolute addresses to access all data. All initialized global and static data goes in.fardata, all uninitialized data goes in.far, and all constant data goes into.const.-mdsbt— compiles for the DSBT shared library ABI 🦋. This option is required to compile with-fpicor-fPIC, and it implies-fpic. So like, non-negotiable.-mlong-calls— avoids generating PC-relative calls; uses indirection instead.
🎀 CRIS Options 🎀
These options are defined specifically for the CRIS ports, bestie 💖
-march=architecture-type,-mcpu=architecture-type— generates code for the specified architecture. The choices for architecture-type arev3,v8andv10, for respectively ETRAX 4, ETRAX 100, and ETRAX 100 LX. Default isv0.-mtune=architecture-type— tunes everything applicable about the generated code to architecture-type, except for the ABI and the set of available instructions ✨. The choices for architecture-type are the same as for-march=architecture-type.-mmax-stackframe=n— warns you when the stack frame of a function exceedsnbytes.-metrax4,-metrax100— these two are synonyms for-march=v3and-march=v8respectively. Shortcuts, slay.-mmul-bug-workaround,-mno-mul-bug-workaround— works around a bug in the"muls"and"mulu"instructions for CPU models where it applies 💀. This option is disabled by default.-mpdebug— enables CRIS-specific verbose debug-related information in the assembly code. It also has the effect of turning off the#NO_APPformatted-code indicator to the assembler at the beginning of the assembly file. Two-for-one.-mcc-init— don't use condition-code results from a previous instruction; always emit compare and test instructions before using condition codes. Safety first, no cap.-mno-side-effects— don't emit instructions with side effects in addressing modes other than post-increment.-mstack-align,-mno-stack-align,-mdata-align,-mno-data-align,-mconst-align,-mno-const-align— these options (and their no- forms) arrange (or eliminate the arrangement, respectively) for the stack frame, individual data and constants to be aligned for the maximum single data access size for the chosen CPU model 🌸. The default is to arrange for 32-bit alignment. ABI details such as structure layout are not affected by these options, so don't come for me.-m32-bit,-m16-bit,-m8-bit— similar to the stack-, data- and const-align options above, these arrange for stack frame, writable data and constants to all be 32-bit, 16-bit or 8-bit aligned 🎀. The default is 32-bit alignment.-mno-prologue-epilogue,-mprologue-epilogue— with-mno-prologue-epilogue, the normal function prologue and epilogue that set up the stack frame are omitted, and no return instructions or return sequences are generated in the code 😭.
Use this option only together with visual inspection of the compiled code: no warnings or errors are generated when call-saved registers must be saved, or storage for local variables needs to be allocated. Like, that's the whole warning. Don't skip it.
-mbest-lib-options,-moverride-best-lib-options—-mbest-lib-optionsselects the most feature-enabling options allowed by other options ✨. This option has no no- form, but-moverride-best-lib-optionsdisables it regardless of the relative order of the two options on the command line.-mtrap-using-break8— emits traps asbreak 8🔥. This is the default for CRIS v3 and up. If disabled, calls to"abort"are used instead.-mtrap-unaligned-atomic— emits checks that causebreak 8instructions to execute when applying atomic builtins on misaligned memory.-munaligned-atomic-may-use-library— handles atomic builtins that may be applied to unaligned data by calling library functions. This option overrides-mtrap-unaligned-atomic.-sim— arranges to link with input-output functions from a simulator library 🌈. Code, initialized data and zero-initialized data are allocated consecutively.-sim2— like-sim, but passes linker options to locate initialized data at0x40000000and zero-initialized data at0x80000000. Big addresses, big energy.
🦋 C-SKY Options 🦋
GCC supports these options when compiling for C-SKY V2 processors 💜
-march=arch— specifies the C-SKY target architecture. Valid values for arch are:ck801,ck802,ck803,ck807, andck810. The default isck810✨-mcpu=cpu— specifies 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. Yes, that's the whole list, and yes, I typed every single one 💅-mbig-endian,-mlittle-endian— selects big- or little-endian code. The default is little-endian 🎀-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 🔥 Choose wisely.
okay so the default here lowkey depends on the exact target configuration, bestie. and note this: the hard-float and soft-float ABIs are not link-compatible — you literally have to compile your entire program with the same ABI and link it with a compatible set of libraries. no mixing, fr fr.
-mdouble-float,-mno-double-float— when-mhard-floatis in effect, this enables generation of double-precision float instructions. it's the default except when you're compiling for CK803.-mfdivdu,-mno-fdivdu— when-mhard-floatis in effect, this enables generation of"frecipd","fsqrtd", and"fdivd"instructions. it's the default except when compiling for CK803.-mfpu=fpu— select the floating-point processor. this option can only be used with-mhard-float. values for fpu arefpv2_sf(equivalent to-mno-double-float -mno-fdivdu),fpv2(-mdouble-float -mno-divdu), andfpv2_divd(-mdouble-float -mdivdu).-melrw,-mno-elrw— enable the extended"lrw"instruction. defaults to on for CK801 and off otherwise.-mistack,-mno-istack— enable interrupt stack instructions; default is off. the-mistackoption is required to handle the"interrupt"and"isr"function attributes.-mmp,-mno-mp— enable multiprocessor instructions; default is off.-mcp,-mno-cp— enable coprocessor instructions; default is off.-mcache,-mno-cache— enable coprocessor instructions; default is off. (yes, it says coprocessor instructions again — that's the actual text, bestie 💅)-msecurity,-mno-security— enable C-SKY security instructions; default is off.-mtrust,-mno-trust— enable C-SKY trust instructions; default is off.-mdsp,-mno-dsp— enable C-SKY DSP instructions; default is off.-medsp,-mno-edsp— enable C-SKY Enhanced DSP instructions; default is off.-mvdsp,-mno-vdsp— enable C-SKY Vector DSP instructions; default is off.
all of these DSP-ish options default to off, periodt.
-mdiv,-mno-div— generate divide instructions. default is off.-msmart,-mno-smart— generate code for Smart Mode, using only registers numbered 0-7 to allow use of 16-bit instructions. this option is ignored for CK801 where this is the required behavior, and it defaults to on for CK802. for other targets, default is off.-mhigh-registers,-mno-high-registers— generate code using the high registers numbered 16-31. not supported on CK801, CK802, or CK803, and enabled by default for other processors.-manchor,-mno-anchor— generate code using global anchor symbol addresses.-mpushpop,-mno-pushpop— generate code using"push"and"pop"instructions. defaults to on.-mmultiple-stld,-mno-multiple-stld— generate code using"stm"and"ldm"instructions. 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 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_size"directives for each function in the assembly output. defaults to off.-mccrt,-mno-ccrt— generate code for the C-SKY compiler runtime instead of libgcc. defaults to off.-mbranch-cost=n— set the branch costs to roughly n instructions. default is 1.-msched-prolog,-mno-sched-prolog— permit scheduling of function prologue and epilogue sequences. using this option can result in code that is not compliant with the C-SKY V2 ABI prologue requirements and that cannot be debugged or backtraced. it's disabled by default.-msim,-mno-sim— links the librarylibsemi.awhich is in compatible with simulator. applicable to ELF compiler only.
🪟 Cygwin and MinGW Options 🪟
these additional options are available for Microsoft Windows targets:
-mconsole— specifies that a console application is to be generated, by instructing the linker to set the PE header subsystem type required for console applications. available for Cygwin and MinGW targets and enabled by default on those targets.-mcrtdll=library— preprocess, compile or link with the specified C RunTime DLL library. this option adjusts predefined macros"__CRTDLL__","__MSVCRT__","_UCRT"and"__MSVCRT_VERSION__"for the specified CRT library, chooses the start file for the CRT library and links with the CRT library. recognized CRT library names for the preprocessor are:"crtdll*","msvcrt10*","msvcrt20*","msvcrt40*","msvcr40*","msvcrtd*","msvcrt-os*","msvcr70*","msvcr71*","msvcr80*","msvcr90*","msvcr100*","msvcr110*","msvcr120*"and"ucrt*". if this option isn't specified then the default MinGW import library"msvcrt"is used for linking and no other adjustment for the preprocessor is done. MinGW import library"msvcrt"is just a symlink to (or a copy of) another MinGW CRT import library chosen during MinGW compilation. MinGW import library"msvcrt-os"is for Windows system CRT DLL library"msvcrt.dll"and in most cases is the default MinGW import library. generally speaking, changing the CRT DLL requires recompiling the entire MinGW CRT. this option is for experimental and testing purposes only. available for MinGW targets.-mdll— available for Cygwin and MinGW targets. specifies that a DLL — a dynamic link library — is to be generated, enabling the selection of the required runtime startup object and entry point.-mnop-fun-dllimport— available for Cygwin and MinGW targets. specifies that the"dllimport"attribute should be ignored.-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.-municode— available for MinGW-w64 targets. causes the"UNICODE"preprocessor macro to be predefined, and chooses Unicode-capable runtime startup code.-mwin32— available for Cygwin and MinGW targets. 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— available for Cygwin and MinGW targets. specifies that a GUI application is to be generated by instructing the linker to set the PE header subsystem type appropriately.-fno-set-stack-executable— available for MinGW targets. specifies that the executable flag for the stack used by nested functions isn't set. necessary for binaries running in kernel mode of Microsoft Windows, as there the User32 API, which is used to set executable privileges, isn't available.-fwritable-relocated-rdata— available for MinGW and Cygwin targets. specifies that relocated-data in read-only section is put into the".data"section. necessary for older runtimes not supporting modification of".rdata"sections for pseudo-relocation.-mpe-aligned-commons— available for Cygwin and MinGW targets. specifies that the GNU extension to the PE file format that permits the correct alignment of COMMON variables should be used when generating code. enabled by default if GCC detects that the target assembler found during configuration supports the feature.-muse-libstdc-wrappers— use Cygwin DLL wrappers to support C++ operators"new"and"delete".
see also under x86 Options for standard options. 🫶
🍎 Darwin Options 🍎
these options are defined for all architectures running the Darwin operating system.
FSF GCC on Darwin does not create "fat" object files; it creates an object file for the single architecture that GCC was built to target. Apple's GCC on Darwin does create "fat" files if multiple -arch options are used; it does so by running the compiler or linker multiple times and joining the results together with lipo.
the subtype of the file created (like ppc7400 or ppc970 or i686) is determined by the flags that specify the ISA that GCC is targeting, like -mcpu or -march. the -force_cpusubtype_ALL option can be used to override this.
the Darwin tools vary in their behavior when presented with an ISA mismatch. the assembler, as, only permits instructions to be used that are valid for the subtype of the file it is generating, so you cannot put 64-bit instructions in a ppc750 object file. the linker for shared libraries, /usr/bin/libtool, fails and prints an error if asked to create a shared library with a less restrictive subtype than its input files (for instance, trying to put a ppc970 object file in a ppc7400 library). the linker for executables, ld, quietly gives the executable the most restrictive subtype of any of its input files.
-Fdir— add the framework directory dir to the head of the list of directories to be searched for header files. these directories are interleaved with those specified by-Ioptions and are scanned in a left-to-right order. 💖
A framework directory is literally just a directory that has frameworks in it — that's it, that's the vibe. A framework is a directory with a Headers and/or PrivateHeaders directory sitting directly inside it, and the name has to end in .framework. The framework's name? It's the directory name minus the .framework, periodt. Headers belonging to the framework live in one of those two directories, and Headers gets searched first (first come, first served, bestie 🎀). A subframework is a framework directory chillin' inside a framework's Frameworks directory. Includes of subframework headers can only show up in a header of the framework that contains the subframework, or in a sibling subframework header. Two subframeworks are siblings if they occur in the same framework 💫. A subframework should not have the same name as a framework — you'll get a warning if you violate this, so don't 🙈. Currently a subframework cannot have subframeworks; in the future, the mechanism may be extended to support this (so like, watch this space ✨). The standard frameworks live 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. Chef's kiss 💅
-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. This option is valid only for the C family of languages.-arch name— Generate output for architecture name. As described above, GCC generates output for the architecture it was configured for, using its usual options to select subarchitecture variants. The-archoption is accepted for compatibility, but an error is diagnosed if name is inconsistent with GCC's own idea of the target architecture. (So it's like, accepted but also it's gonna call you out if you're wrong 😭)-dependency-file filename— Alias for the preprocessor option-MF filename.-fapple-kext— Generate code for Darwin loadable kernel extensions.-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 skips, no crumbs.-matt-stubs,-mno-att-stubs— Enable AT&T-style PIC stubs. This is the default when supported by the target architecture (currently x86 only).-mconstant-cfstrings,-fconstant-cfstrings— When the NeXT runtime is being used (the default on these systems), override any-fconstant-string-classsetting and cause@"..."literals to be laid out as constant CoreFoundation strings. Note:-fconstant-cfstringsis an alias for-mconstant-cfstrings.-mdynamic-no-pic— Generate code suitable for executables (not shared libraries). This option is incompatible with-fpic,-fPIC,-fpie, or-fPIE. Pick a lane bestie 🫶-mfix-and-continue,-ffix-and-continue,-findirect-data— Generate code suitable for fast turnaround development, such as to allow GDB to dynamically load.ofiles into already-running programs.-findirect-dataand-ffix-and-continueare provided for backwards compatibility.-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.-mmacosx-version-min=version,-asm_macosx_version_min=version— The-mmacosx-version-minoption specifies that the earliest version of MacOS X that this executable will run on is version. Typical values supported for version include12,10.12, and10.5.8. If the compiler was built to use the system's headers by default, then the default for this option is the system version on which the compiler is running, otherwise the default is to make choices that are compatible with as many systems and code bases as possible.-asm_macosx_version_min=versionis similar, but the GCC driver passes its version information only to the assembler.-mone-byte-bool— Override the defaults forboolso thatsizeof(bool)==1. By defaultsizeof(bool)is4when compiling for Darwin/PowerPC and1when compiling for Darwin/x86, so this option has no effect on x86.
⚠️ Warning: The -mone-byte-bool switch causes GCC to generate code that is not binary compatible with code generated without that switch. Using this switch may require recompiling all other modules in a program, including system libraries. Use this switch to conform to a non-default data model. Like, proceed with caution, bestie 💀
-msymbol-stubs,-mno-symbol-stubs— Force generation of external symbol indirection stubs for PIC references. By default, this option is enabled automatically if the target linker version (-mtarget-linker) is old enough to require them.-mtarget-linker=version,-mtarget-linker version— Specify the target ld64 version, overriding any version specified in the GCC configuration. Newer linker versions support improved code generation in some cases, for example for PIC code. Upgrade your linker, upgrade your life ✨-ObjC— Equivalent to-x objective-c; specifies that the input is Objective-C source code.-ObjC++— Equivalent to-x objective-c++; specifies that the input is Objective-C++ source code.-Wnonportable-cfstrings,-Wno-nonportable-cfstrings— Warn if constant CoreFoundation string objects contain non-portable characters. This warning is enabled by default.-all_load— Loads all members of static archive libraries. See man ld(1) for more information.-arch_errors_fatal— Cause the errors having to do with files that have the wrong architecture to be fatal. No mercy 🔥-bind_at_load— Causes the output file to be marked such that the dynamic linker will bind all undefined references when the file is loaded or launched.-bundle— Produce a Mach-o bundle format file. See man ld(1) for more information.-bundle_loader executable— This option specifies the executable that will load the build output file being linked. See man ld(1) for more information.-dynamiclib— When passed this option, GCC produces a dynamic library instead of an executable when linking, using the Darwinlibtoolcommand.-force_cpusubtype_ALL— This causes GCC's output file to have theALLsubtype, instead of one controlled by the-mcpuor-marchoption.-nodefaultrpaths— Do not add default run paths for the compiler library directories to executables, modules or dynamic libraries. On macOS 10.5 and later, the embedded runpath is added by default unless the user adds-nodefaultrpathsto the link line. Run paths are needed (and therefore enforced) to build on macOS version 10.11 or later.-nodefaultexport— Do not add default symbol exports to modules or dynamic libraries.
And then there's this whole squad of Darwin linker flags — GCC literally just hands these off to the Darwin linker, and the Darwin linker man page describes them in detail. Group chat forwarding, basically 💌:
-allowable_client client_name-client_name-compatibility_version-current_version-dead_strip-dylib_file-dylinker-dylinker_install_name-dynamic-exported_symbols_list-filelist-flat_namespace-force_flat_namespace-framework-headerpad_max_install_names-image_base-init symbol-name-install_name-keep_private_externs-pagezero_size-preload-read_only_relocs-sectalign-sectcreate-seg_addr_table-seg1addr-segaddr-segprot-segs_read_only_addr-segs_read_write_addr-sub_library-sub_umbrella-twolevel_namespace-twolevel_namespace_hints-umbrella-undefined-unexported_symbols_list-weak_framework-weak_reference_mismatches-whatsloaded-whyload
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 — yes, the DEC Alpha, she's vintage, she's iconic, she ate and left no crumbs 👑
-mno-soft-float,-msoft-float— Use (do not use) the hardware floating-point instructions for floating-point operations. When-msoft-floatis specified, functions inlibgcc.aare used to perform floating-point operations. Unless they are replaced by routines that emulate the floating-point operations, or compiled in such a way as to call such 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. (Yes, required to have them even if they don't use them. The audacity 💀)-mfp-regs,-mno-fp-regs— Generate code that uses (does not use) the floating-point register set.-mno-fp-regsimplies-msoft-float. If the floating-point register set is not used, floating-point operands are passed in integer registers as if they were integers and floating-point results are passed in$0instead of$f0. This is a non-standard calling sequence, so any function with a floating-point argument or return value called by code compiled with-mno-fp-regsmust also be compiled with that option. Fr fr, no cap — mismatch this and everything breaks 😭 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_FPis 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. Slower but correct? That's the tradeoff, babe 💅-mieee-with-inexact— okay so this one is like-mieeebut extra: the generated code also maintains the IEEE inexact-flag. Turning it on makes the code implement fully-compliant IEEE math 💅. On top of_IEEE_FP, it also defines_IEEE_FP_EXACTas a preprocessor macro. Heads up though: on some Alpha implementations the resulting code may run significantly slower than the default code. And since there's very little code out there that actually depends on the inexact-flag, you should normally not specify this option. Other Alpha compilers call this one-ieee_with_inexact.- *
-mfp-trap-mode=trap-mode* — this controls which floating-point related traps are enabled. Other Alpha compilers call it-fptmtrap-mode. The trap mode can be set to one of four values:n— the default (normal) setting. Only the traps that cannot be disabled in software are enabled (e.g., division by zero trap).u— on top of the traps enabled byn, underflow traps are enabled as well.su— likeu, but the instructions are marked to be safe for software completion (see the Alpha architecture manual for the deets).sui— likesu, but inexact traps are enabled as well.
- *
-mfp-rounding-mode=rounding-mode* — selects the IEEE rounding mode. Other Alpha compilers call this-fprmrounding-mode. The rounding-mode can be one of:n— normal IEEE rounding mode. Floating-point numbers are rounded towards the nearest machine number, or towards the even machine number in case of a tie.m— round towards minus infinity.c— chopped rounding mode. Floating-point numbers are rounded towards zero.d— dynamic rounding mode. A field in the floating-point control register (fpcr, see the Alpha architecture reference manual) controls the rounding mode in effect. The C library initializes this register for rounding towards plus infinity. So unless your program modifies thefpcr,dcorresponds to round towards plus infinity ✨.
-mtrap-precision=trap-precision — in the Alpha architecture, floating-point traps are **imprecise 😭. Which means without software assistance it's impossible to recover from a floating trap, and program execution normally needs to be terminated. GCC can generate code that assists operating system trap handlers in determining the exact location that caused a floating-point trap. Depending on what your app needs, you can pick different levels of precision:p— program precision. This option is the default and means a trap handler can only identify which program caused a floating-point exception.f— function precision. The trap handler can determine the function that caused a floating-point exception.i— instruction precision. The trap handler can determine the exact instruction that caused a floating-point exception.
Other Alpha compilers provide the equivalent options called -scope_safe and -resumption_safe.
-mieee-conformant— marks the generated code as IEEE conformant 👑. > But bestie, careful: you must not use this option unless you also specify-mtrap-precision=iand either-mfp-trap-mode=suor-mfp-trap-mode=sui. Its only effect is to emit the line.eflag 48in the function prologue of the generated assembly file.-mbuild-constants— normally GCC examines a 32- or 64-bit integer constant to see if it can construct it from smaller constants in two or three instructions. If it can't, it outputs the constant as a literal and generates code to load it from the data segment at run time. Use this option to require GCC to construct all integer constants using code, even if it takes more instructions (the maximum is six). You typically use this option to build a shared library dynamic loader — itself a shared library, it has to relocate itself in memory before it can find the variables and constants in its own data segment 🌀.-mbwx,-mno-bwx,-mcix,-mno-cix,-mfix,-mno-fix,-mmax,-mno-max— these indicate whether GCC should generate code to use the optional BWX, CIX, FIX and MAX instruction sets. The default is to use the instruction sets supported by the CPU type specified via the-mcpu=option, or that of the CPU on which GCC was built if none is specified.-msafe-bwa,-mno-safe-bwa— indicate whether, in the absence of the optional BWX instruction set, GCC should generate multi-thread and async-signal safe code for byte and aligned word memory accesses.-msafe-partial,-mno-safe-partial— indicate whether GCC should generate multi-thread and async-signal safe code for partial memory accesses. That includes piecemeal accesses to unaligned data, plus block accesses to leading and trailing parts of aggregate types or other objects in memory that don't respectively start and end on an aligned 64-bit data boundary.-mfloat-vax,-mfloat-ieee— generate code that uses (does not use) VAX F and G floating-point arithmetic instead of IEEE single and double precision.-mexplicit-relocs,-mno-explicit-relocs— older Alpha assemblers gave you no way to generate symbol relocations except via assembler macros, and using those macros doesn't allow optimal instruction scheduling 😤. GNU Binutils as of version 2.12 supports a new syntax that lets the compiler explicitly mark which relocations should apply to which instructions. This option is mostly useful for debugging, since GCC detects the capabilities of the assembler when it is built and sets the default accordingly.-msmall-data,-mlarge-data— when-mexplicit-relocsis in effect, static data is accessed via gp-relative relocations. When-msmall-datais used, objects 8 bytes long or smaller are placed in a small data area (the.sdataand.sbsssections) and are accessed via 16-bit relocations off of the$gpregister. This limits the size of the small data area to 64KB, but lets the variables be directly accessed via a single instruction 💫.
The default is -mlarge-data. With this option the data area is limited to just below 2GB. Programs that require more than 2GB of data must use malloc or mmap to allocate the data in the heap instead of in the program's data segment.
When generating code for shared libraries, -fpic implies -msmall-data and -fPIC implies -mlarge-data.
-msmall-text,-mlarge-text— when-msmall-textis used, the compiler assumes that the code of the entire program (or shared library) fits in 4MB, and is thus reachable with a branch instruction. When-msmall-datais used, the compiler can assume that all local symbols share the same$gpvalue, and thus reduce the number of instructions required for a function call from 4 to 1 🔥. The default is-mlarge-text.- *
-mcpu=cpu_type* — sets the instruction set and instruction scheduling parameters for machine type cpu_type. You can specify either the EV style name or the corresponding chip number. GCC supports scheduling parameters for the EV4, EV5 and EV6 family of processors and chooses the default values for the instruction set from the processor you specify. If you 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. -mcpu=native has no effect if GCC doesn't recognize the processor.
- *
-mtune=cpu_type* — sets only the instruction scheduling parameters for machine type cpu_type. The instruction set is not changed, periodt. Native toolchains also support the valuenative, which selects the best architecture option for the host processor.-mtune=nativehas 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 used by the application and the size of the external cache on the machine. Valid options for time are:- number — a decimal number representing clock cycles.
L1,L2,L3,main— the compiler contains estimates of the number of clock cycles for "typical" EV4 & EV5 hardware for the Level 1, 2 & 3 caches (also called Dcache, Scache, and Bcache), as well as to main memory. Note thatL3is only valid for EV5 👀.
-mtls-kernel— emitrdvalinstead ofrduniqfor thread pointer.- *
-mtls-size=bitsize* — specify bit size of immediate TLS offsets. Valid values for bitsize are16,32, and64; it defaults to32. -mlong-double-128,-mlong-double-64— specify the size of thelong doubletype. Note that-mlong-double-128is incompatible with VAX floating point 🫶.
🦋 eBPF Options 🦋
- *
-mframe-limit=bytes* — this specifies the hard limit for frame sizes, in bytes. Currently, the value you can specify should be less than or equal to32767. Defaults to whatever limit is imposed by the version of the Linux kernel targeted. -mbig-endian— generate code for a big-endian target.-mlittle-endian— generate code for a little-endian target. This is the default ✨.-mjmpext,-mno-jmpext— enable or disable generation of extra conditional-branch instructions. Enabled for CPU v2 and above.-mjmp32,-mno-jmp32— enable or disable generation of 32-bit jump instructions. Enabled for CPU v3 and above.-malu32,-mno-alu32— turns generation of 32-bit ALU instructions on or off. It's on already for CPU v3 and above ✨-mv3-atomics,-mno-v3-atomics— enables/disables the instructions for general atomic operations that showed up in CPU v3. On by default for CPU v3 and above 💅-mbswap,-mno-bswap— byte swap instructions, yes or no. Enabled for CPU v4 and above 🦋-msdiv,-mno-sdiv— signed division and modulus instructions. Enabled for CPU v4 and above.-msmov,-mno-smov— sign-extending move and sign-extending memory load instructions. Enabled for CPU v4 and above.-mcpu=version— this is the "which version of the eBPF ISA are we even targeting" knob. Lowkey important: newer versions might not be supported by every kernel out there. The default isv4.
The supported values for version are:
v1— the very first stable eBPF ISA, no special features, no extensions. The OG 🎀v2— supports the jump extensions, same as-mjmpext.v3— everything in v2, plus:- 32-bit jump operations, as in
-mjmp32 - 32-bit ALU operations, as in
-malu32 - general atomic operations, as in
-mv3-atomics
- 32-bit jump operations, as in
v4— everything in v3, plus:- Byte swap instructions, as in
-mbswap - Signed division and modulus instructions, as in
-msdiv - Sign-extending move and memory load instructions, as in
-msmov
- Byte swap instructions, as in
-mco-re,-mno-co-re— Enable or disable BPF Compile Once - Run Everywhere (CO-RE) support. BPF CO-RE support is enabled by default when generating BTF debug information for the BPF target (-gbtf). 🧚-mxbpf— generates code for an expanded version of BPF that relaxes some of the restrictions the BPF architecture imposes:- Save and restore callee-saved registers at function entry and exit, respectively.
-masm=dialect— outputs assembly instructions using the eBPF-selected dialect. Default ispseudoc.
Supported values for dialect are:
normal— outputs normal assembly dialect.pseudoc— outputs pseudo-c assembly dialect.
-minline-memops-threshold=bytes— sets a size threshold in bytes, at or below whichmemmove,memcpyandmemsetshall always be expanded inline. Operations dealing with sizes larger than this threshold would have to be implemented using a library call instead of being expanded inline, but since BPF doesn't allow libcalls, going over this threshold results in a compile-time error 😭. The default is1024bytes.-Wco-re— turns on warnings for scenarios where-mco-reis enabled but it may not be possible to generate CO-RE compatible code.
🌈 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 all symbolic values and addresses fit into a 20-bit range 🫶-mno-lsim— assume that runtime support has been provided, so there's no need to include the simulator library (libsim.a) on the linker command line.
🦋 FRV Options
-mgpr-32— only use the first 32 general-purpose registers.-mgpr-64— use all 64 general-purpose registers.-mfpr-32— use only the first 32 floating-point registers.-mfpr-64— use all 64 floating-point registers.-mhard-float— use hardware instructions for floating-point operations.-msoft-float— use library routines for floating-point operations.-malloc-cc— dynamically allocate condition code registers.-mfixed-cc— do not try to dynamically allocate condition code registers, only useicc0andfcc0.-mdword,-mno-dword— control whether the ABI uses double-word instructions.-mdouble,-mno-double— enable or disable use of floating-point double instructions.-mmedia,-mno-media— enable or disable use of media instructions.-mmuladd,-mno-muladd— enable or disable use of multiply and add/subtract instructions.-mfdpic— selects the FDPIC ABI, which uses function descriptors to represent pointers to functions. Without any PIC/PIE-related options, it implies-fPIE. With-fpicor-fpie, it assumes GOT entries and small data are within a 12-bit range from the GOT base address; with-fPICor-fPIE, GOT offsets are computed with 32 bits. With abfin-elftarget, this option implies-msim. ✨-minline-plt— enables 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.-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— enables the use ofGPRELrelocations 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's not,-mno-gprel-rocan be used to disable it 💅-multilib-library-pic— link with the (library, not FD) pic libraries. It's implied by-mlibrary-pic, as well as by-fPICand-fpicwithout-mfdpic. You should never have to use it explicitly.-mlinked-fp— follows the EABI requirement of always creating a frame pointer whenever a stack frame is allocated. This option is enabled by default and can be disabled with-mno-linked-fp.-mlong-calls— use indirect addressing to call functions outside the current compilation unit. This lets the functions be placed anywhere within the 32-bit address space 👑-malign-labels— try to align labels to an 8-byte boundary by inserting NOPs into the previous packet. This option only has an effect when VLIW packing is enabled. It doesn't create new packets; it merely adds NOPs to existing ones.-mlibrary-pic— generate position-independent EABI code.-macc-4— use only the first four media accumulator registers.-macc-8— use all eight media accumulator registers.-mpack,-mno-pack— enable or disable packing VLIW instructions.-mno-eflags— do not mark ABI switches ine_flags.-mcond-move,-mno-cond-move— enable or disable the use of conditional-move instructions; it's enabled by default.
This switch is mainly for debugging the compiler and will likely be removed in a future version. 💀
-mscc,-mno-scc— enable or disable the use of conditional set instructions; it's enabled by default.
This switch is mainly for debugging the compiler and will likely be removed in a future version. 💀
-mcond-exec,-mno-cond-exec— enable or disable the use of conditional execution; it's enabled by default.
This switch is mainly for debugging the compiler and will likely be removed in a future version. 💀
-mvliw-branch,-mno-vliw-branch— enable or disable an optimization pass to pack branches into VLIW instructions; it's enabled by default.
This switch is mainly for debugging the compiler and will likely be removed in a future version. 💀
-mmulti-cond-exec,-mno-multi-cond-exec— enable or disable optimization of&&and||in conditional execution; it's enabled by default.
This switch is mainly for debugging the compiler and will likely be removed in a future version. 💀
-mnested-cond-exec,-mno-nested-cond-exec— enable or disable nested conditional execution optimizations; it's enabled by default.
This switch is mainly for debugging the compiler and will likely be removed in a future version. 💀
-moptimize-membar,-mno-optimize-membar— this switch removes redundantmembarinstructions from the compiler-generated code. It's enabled by default. 🔥-mtomcat-stats— causegasto print out tomcat statistics.-mcpu=cpu— select the processor type to generate code for. Possible values arefrv,fr550,tomcat,fr500,fr450,fr405,fr400,fr300andsimple.
🎀 FT32 Options
These options are defined specifically for the FT32 port.
-msim— specifies that the program will be run on the simulator. This causes an alternate runtime startup and library to be linked in. You must not use this option when generating programs that will run on real hardware; you have to provide your own runtime library for whatever I/O functions are needed. No cap 🫶-mnodiv— do not usedivandmodinstructions.-mft32b— enable use of the extended instructions of the FT32B processor.-mcompress— compress all code using the Ft32B code compression scheme.-mnopm— do not generate code that reads program memory.
🌸 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,-mno-android— 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. 🤖
This option can be disabled completely with -mno-android.
-tno-android-cc— disable the compilation effects of-mandroid, i.e., do not enable-mbionic,-fPIC,-fno-exceptionsand-fno-rttiby default.-tno-android-ld— disable the linking effects of-mandroid, i.e., pass standard Linux linking options to the linker. ✨
🌸 H8/300 Options 🌸
These -m options are defined for the H8/300 implementations, bestie:
-mrelax— shortens some address references at link time when it can; it does this by using the linker option-relax.-mh— generates code for the H8/300H.-ms— generates code for the H8S.-mn— generates code for the H8S and H8/300H in normal mode. Heads up: this switch must be used together with either-mhor-ms.-msx— generates H8SX code.-ms2600— generates code for the H8S/2600. And this switch must be used with-ms.-mquickcall— uses registers for passing arguments.-mslowbyte— treats access to byte-sized memory as slow.-mexr,-mno-exr— stores extended registers on the stack before functions with the"monitor"attribute execute. Default is-mexr. 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 the H8/300. The default for the H8/300H and H8S is to align longs and floats on 4-byte boundaries, and-malign-300makes them align 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— generates code for the specified architecture. Your choices for architecture-type are1.0for PA 1.0,1.1for PA 1.1, and2.0for PA 2.0 processors. Go peek at/usr/lib/sched.modelson an HP-UX system to figure out the proper architecture option for your machine. Also important: code compiled for lower numbered architectures runs on higher numbered architectures, but not the other way around. 💅-mpa-risc-1-0,-mpa-risc-1-1,-mpa-risc-2-0— these are synonyms for-march=1.0,-march=1.1, and-march=2.0, respectively.-matomic-libcalls,-mno-atomic-libcalls— generate libcalls for atomic loads and stores when sync libcalls are disabled. This option is enabled by default, and it only affects the generation of atomic libcalls by the HPPA backend.
Both the sync and libatomic libcall implementations use locking, so as a result, processor stores are not atomic with respect to other atomic operations. Processor loads up to DImode are atomic with respect to other atomic operations, provided they're implemented as a single access.
The PA-RISC architecture doesn't support any atomic operations in hardware except for the "ldcw" instruction. So like, all atomic support is implemented using sync and atomic libcalls. Sync libcall support lives in libgcc.a, and atomic libcall support lives in libatomic.
This option generates "__atomic_exchange" calls for atomic stores, and it also provides special handling for atomic DImode accesses on 32-bit targets.
-mcaller-copies— the caller copies function arguments passed by hidden reference. Use this one with care, because it is not compatible with the default 32-bit runtime. That said, only aggregates larger than eight bytes get passed by hidden reference, and the option gives better compatibility with OpenMP.-mcoherent-ldcw— uses theldcw/ldcdcoherent cache-control hint.-mdisable-fpregs— disables floating-point registers. Equivalent to-msoft-float.-mdisable-indexing— stops the compiler from using indexing address modes. This avoids some rather obscure problems when compiling MIG generated code under MACH.-mfast-indirect-calls— generates code that assumes calls never cross space boundaries, which lets GCC emit code that does faster indirect calls.
Ngl though, this option does not work in the presence of shared libraries or nested functions.
-mfixed-range=register-range— generates code treating the given register range as fixed registers. A fixed register is one the register allocator cannot use — super useful when compiling kernel code. A register range is specified as two registers separated by a dash, and you can specify multiple register ranges separated by a comma.-mgas— enables the use of assembler directives that only GAS understands.-mgnu-ld— uses options specific to GNU ld. It passes-sharedto ld when building a shared library, and it's the default when GCC is configured, explicitly or implicitly, with the GNU linker. Lowkey key detail: this option does not affect which ld gets called; it only changes what parameters are passed to that ld. The ld that's called is determined by the--with-ldconfigure option, GCC's program search path, and finally by the user'sPATH. You can print the linker GCC uses withwhich \gcc -print-prog-name=ld`. This option is only available on the 64-bit HP-UX GCC, i.e. configured withhppa*64*--hpux`.-mhp-ld— uses options specific to HP ld. It passes-bto ld when building a shared library, and passes+Accept TypeMismatchto ld on all links. It's the default when GCC is configured, explicitly or implicitly, with the HP linker. Same deal as before: this option does not affect which ld is called; it only changes what parameters are passed to that ld. The ld that's called is determined by the--with-ldconfigure option, GCC's program search path, and finally by the user'sPATH. You can print the linker GCC uses withwhich \gcc -print-prog-name=ld`. This option is only available on the 64-bit HP-UX GCC, i.e. configured withhppa*64*--hpux`.-mlinker-opt— enables the optimization pass in the HP-UX linker. Just know this makes symbolic debugging impossible. 😭-mlong-calls— generates code that uses long call sequences. This makes sure a call is always able to reach linker generated stubs. The default is to generate long calls only when the distance from the call site to the beginning of the function or translation unit (as the case may be) exceeds a predefined limit set by the branch type being used. The limits for normal calls are 7,600,000 and 240,000 bytes, respectively for the PA 2.0 and PA 1.X architectures. Sibling calls are always limited at 240,000 bytes.
Distances are measured from the beginning of functions when using the -ffunction-sections option, or when using the -mgas and -mno-portable-runtime options together under HP-UX with the SOM linker.
It's normally not desirable to use this option since it degrades performance — however, it may be useful in large applications, particularly when partial linking is used to build the application.
The types of long calls used depends on the capabilities of the assembler and linker, and the type of code being generated. The impact on systems that support long absolute calls, and long PIC symbol-difference or PC-relative calls, should be relatively small. But an indirect call is used on 32-bit ELF systems in PIC code, and it is quite long. 💀
-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.-mno-space-regs— generates code that assumes the target has no space registers. This lets GCC generate faster indirect calls and use unscaled index address modes.
Such code is suitable for level 0 PA systems and kernels. ✨
-mordered— assumes memory references are ordered and barriers aren't needed.-mportable-runtime— uses the portable calling conventions proposed by HP for ELF systems.-mschedule=cpu-type— schedules code according to the constraints for the machine type cpu-type. The choices for cpu-type are7007100,7100LC,7200,7300and8000. Go check/usr/lib/sched.modelson an HP-UX system to determine the proper scheduling option for your machine. The default scheduling is8000.-msio,-mwsio—-msiogenerates the predefine"_SIO"for server IO. The default is-mwsio, which generates the predefines"__hp9000s700","__hp9000s700__"and"_WSIO"for workstation IO. These options are available under HP-UX and HI-UX.-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 can't be done directly in cross-compilation — you have to make your own arrangements to provide suitable library functions for cross-compilation.
Also -msoft-float changes the calling convention in the output file, so it's only useful if you compile all of a program with this option. In particular, you need to compile libgcc.a (the library that comes with GCC) with -msoft-float for this to work.
-msoft-mult— uses software integer multiplication.
This disables the use of the "xmpyu" instruction.
-munix=unix-std— generates compiler predefines and selects a startfile for the specified UNIX standard. The choices for unix-std are93,95and98.93is supported on all HP-UX versions,95is available on HP-UX 10.10 and later, and98is available on HP-UX 11.11 and later. The default values are93for HP-UX 10.00,95for HP-UX 10.10 through to 11.00, and98for HP-UX 11.11 and later.
-munix=93 provides the same predefines as GCC 3.3 and 3.4. -munix=95 provides additional predefines for "XOPEN_UNIX" and "_XOPEN_SOURCE_EXTENDED", and the startfile unix95.o. -munix=98 provides additional predefines for "_XOPEN_UNIX", "_XOPEN_SOURCE_EXTENDED", "_INCLUDE__STDC_A1_SOURCE" and "_INCLUDE_XOPEN_SOURCE_500", and the startfile unix98.o.
It's important to note that this option changes the interfaces for various library routines, and it also affects the operational behavior of the C library. So like, extreme care is needed when using this option, fr fr. 🫶
Library code intended to operate with more than one UNIX standard must test, set and restore the variable "__xpg4_extended_mask" as appropriate. Most GNU software doesn't provide this capability.
-nolibdld— suppresses the generation of link options to searchlibdld.slwhen the-staticoption is specified on HP-UX 10 and later.-static— the HP-UX C library implementation of"setlocale"has a dependency onlibdld.sl. There isn't an archive version oflibdld.sl, so when the-staticoption is specified, special link options are needed to resolve this dependency.
On HP-UX 10 and later, the GCC driver adds the necessary options to link with libdld.sl when the -static option is specified. Yeah, plot twist — that makes the resulting binary dynamic 🫠. And on the 64-bit port, the linkers generate dynamic binaries by default in any case. The -nolibdld option can be used to stop the GCC driver from adding these link options. Cool cool cool.
-threads— adds support for multithreading with thedcethread library under HP-UX. This one sets flags for both the preprocessor and the linker. Teamwork, bestie 💅.
IA-64 Options
These are the -m options defined for the Intel IA-64 architecture. Niche? Yes. Iconic? Also yes ✨.
-mbig-endian— generate code for a big-endian target. This is the default for HP-UX.-mlittle-endian— generate code for a little-endian target. This is the default for AIX5 and GNU/Linux.-mgnu-as,-mno-gnu-as— generate (or don't) code for the GNU assembler. It's the default.-mgnu-ld,-mno-gnu-ld— generate (or don't) code for the GNU linker. Also the default.-mno-pic— generate code that doesn't use a global pointer register. Heads up: the result is not position independent code, and it violates the IA-64 ABI 😭. Like, technically naughty. You've been warned.-mvolatile-asm-stop,-mno-volatile-asm-stop— generate (or don't) a stop bit immediately before and after volatileasmstatements.-mregister-names,-mno-register-names— generate (or don't)in,loc, andoutregister names for the stacked registers. Lowkey makes assembler output more readable 🎀.-mno-sdata,-msdata— disable (or enable) optimizations that use the small data section. Can be useful for working around optimizer bugs.-mconstant-gp— generate code that uses a single constant global pointer value. Usefully iconic when compiling kernel code.-mauto-pic— generate code that is self-relocatable. This implies-mconstant-gp. Useful when compiling firmware code.-minline-float-divide-min-latency— generate code for inline divides of floating-point values using the minimum latency algorithm.-minline-float-divide-max-throughput— generate code for inline divides of floating-point values using the maximum throughput algorithm.-mno-inline-float-divide— do not generate inline code for divides of floating-point values.-minline-int-divide-min-latency— generate code for inline divides of integer values using the minimum latency algorithm.-minline-int-divide-max-throughput— generate code for inline divides of integer values using the maximum throughput algorithm.-mno-inline-int-divide— do not generate inline code for divides of integer values.-minline-sqrt-min-latency— generate code for inline square roots using the minimum latency algorithm.-minline-sqrt-max-throughput— generate code for inline square roots using the maximum throughput algorithm.-mno-inline-sqrt— do not generate inline code forsqrt.-mno-dwarf2-asm,-mdwarf2-asm— don't (or do) generate assembler code for the DWARF line number debugging info. Might come in handy 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 ngl it doesn't always do so 🙈.- *
-mfixed-range=register-range* — generate code treating the given register range as fixed registers. A fixed register is one the register allocator cannot use, which is useful when compiling kernel code. A register range is specified as two registers separated by a dash. Multiple register ranges can be specified separated by a comma. - *
-mtls-size=tls-size* — specify bit size of immediate TLS offsets. Valid values are14,22, and64. - *
-mtune=cpu-type* — tune the instruction scheduling for a particular CPU. Valid values areitanium,itanium1,merced,itanium2, andmckinley. -milp32,-mlp64— generate code for a 32-bit or 64-bit environment. The 32-bit environment setsint,longand pointer to 32 bits. The 64-bit environment setsintto 32 bits andlongand pointer to 64 bits. These are HP-UX specific flags.-mno-sched-br-data-spec,-msched-br-data-spec— (dis/en)able data speculative scheduling before reload. This results in generation ofld.ainstructions and the corresponding check instructions (ld.c/chk.a). The default setting is disabled.-msched-ar-data-spec,-mno-sched-ar-data-spec— (en/dis)able data speculative scheduling after reload. Also generatesld.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). It results in generation of theld.sinstructions and the corresponding check instructionschk.s. The default setting is disabled.-msched-br-in-data-spec,-mno-sched-br-in-data-spec— (en/dis)able speculative scheduling of the instructions that are dependent on the data speculative loads before reload. Only effective 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. Only effective 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. Only effective with-msched-control-specenabled. The default setting is enabled.-mno-sched-count-spec-in-critical-path,-msched-count-spec-in-critical-path— if enabled, speculative dependencies are considered during computation of the instructions priorities. Makes the use of speculation a bit more conservative. The default setting is disabled.-msched-spec-ldc,-mno-sched-spec-ldc— use a simple data speculation check. This option is on by default.-msched-control-spec-ldc,-mno-sched-control-spec-ldc— use a simple check for control speculation. This option is on by default.-msched-stop-bits-after-every-cycle,-mno-sched-stop-bits-after-every-cycle— place a stop bit after every cycle when scheduling. This option is on by default.-msched-fp-mem-deps-zero-cost,-mno-sched-fp-mem-deps-zero-cost— assume that floating-point stores and loads are not likely to cause a conflict when placed into the same instruction group. This option is disabled by default.-msel-sched-dont-check-control-spec,-mno-sel-sched-dont-check-control-spec— generate checks for control speculation in selective scheduling. This flag is disabled by default.- *
-msched-max-memory-insns=max-insns* — limit on the number of memory insns per instruction group, giving lower priority to subsequent memory insns attempting to schedule in the same instruction group. Frequently useful to prevent cache bank conflicts. The default value is1. -msched-max-memory-insns-hard-limit,-mno-sched-max-memory-insns-hard-limit— makes the limit specified bymsched-max-memory-insnsa hard limit, disallowing more than that number in an instruction group. Otherwise the limit is "soft", meaning non-memory operations are preferred when the limit is reached, but memory operations may still be scheduled. ✨
LM32 Options
These -m options are defined for the LatticeMico32 architecture 💜:
-mbarrel-shift-enabled— enable barrel-shift instructions.-mdivide-enabled— enable divide and modulus instructions.-mmultiply-enabled— enable multiply instructions.-msign-extend-enabled— enable sign extend instructions.-muser-enabled— enable user-defined instructions.
LoongArch Options
These command-line options are defined for LoongArch targets 🐉:
-march=arch-type— generate instructions for the machine typearch-type.-march=arch-typeallows GCC to generate code that **may not run at all on processors other than the one indicated 😳. Fr fr, read that twice.
The choices for arch-type are:
native— local processor type detected by the native compiler.loongarch64— generic LoongArch 64-bit processor.la464— LoongArch LA464-based processor with LSX, LASX.la664— LoongArch LA664-based processor with LSX, LASX and all LoongArch v1.1 instructions.la64v1.0— LoongArch64 ISA version 1.0.la64v1.1— LoongArch64 ISA version 1.1.la32v1.0— LoongArch32 ISA version 1.0.la32rv1.0— LoongArch32 Reduced ISA version 1.0.
More information about LoongArch ISA versions can be found at <https://github.com/loongson/la-toolchain-conventions>. Go read, bestie 📚.
- *
-mtune=tune-type* — optimize the generated code for the given processor target.
The choices for tune-type are:
native— local processor type detected by the native compiler.generic— generic LoongArch processor.loongarch64— generic LoongArch 64-bit processor.la464— LoongArch LA464 core.la664— LoongArch LA664 core.loongarch32— generic LoongArch 32-bit processor.
- *
-mabi=base-abi-type* — generate code for the specified calling convention.base-abi-typecan be one of:lp64d— uses 64-bit general purpose registers and 32/64-bit floating-point registers for parameter passing. Data model is LP64, whereintis 32 bits, whilelong intand pointers are 64 bits.lp64f— uses 64-bit general purpose registers and 32-bit floating-point registers for parameter passing. Data model is LP64, whereintis 32 bits, whilelong intand pointers are 64 bits.lp64s— uses 64-bit general purpose registers and no floating-point registers for parameter passing. Data model is LP64, whereintis 32 bits, whilelong intand pointers are 64 bits.
- *
-mfpu=fpu-type* — generate code for the specified FPU type, which can be one of:
More -mfpu values — the list is literally still going:
64— allow the use of hardware floating-point instructions for 32-bit and 64-bit operations. 💅32— allow the use of hardware floating-point instructions for 32-bit operations.none,0— prevent the use of hardware floating-point instructions. 🚫-msimd=simd-type— enable generation of LoongArch SIMD instructions for vectorization and via builtin functions.simd-typecan be one of:lasx— enable generating instructions from the 256-bit LoongArch Advanced SIMD Extension (LASX) and the 128-bit LoongArch SIMD Extension (LSX).lsx— enable generating instructions from the 128-bit LoongArch SIMD Extension (LSX).none— no LoongArch SIMD instruction may be generated.
-msoft-float— force-mfpu=noneand prevent the use of floating-point registers for parameter passing. This option may change the target ABI. 😭-msingle-float— force-mfpu=32and allow the use of 32-bit floating-point registers for parameter passing. This option may change the target ABI.-mdouble-float— force-mfpu=64and allow the use of 32/64-bit floating-point registers for parameter passing. This option may change the target ABI.-mlasx,-mno-lasx,-mlsx,-mno-lsx— incrementally adjust the scope of the SIMD extensions (none / LSX / LASX) that can be used by the compiler for code generation. Enabling LASX with-mlasxautomatically enables LSX, and disabling LSX with-mno-lsxautomatically disables LASX. These driver-only options act upon the final-msimdconfiguration state and make incremental changes in the order they appear on the GCC driver’s command line, deriving the final / canonicalized-msimdoption that is passed to the compiler proper. ✨-mbranch-cost=n— set the cost of branches to roughlyninstructions.-maddr-reg-reg-cost=n— set the cost ofADDRESS_REG_REGto the value calculated byn.-mcheck-zero-division,-mno-check-zero-divison— trap (do not trap) on integer division by zero. The default is-mcheck-zero-divisionfor-O0or-Og, and-mno-check-zero-divisionfor other optimization levels. (Yes, the negated option is spelled-mno-check-zero-divisonin the manual — keep it exactly like that, bestie. 🙈)-mbreak-code=code— emit abreakcode instruction for irrecoverable traps from__builtin_trapor inserted by the compiler (for example an erroneous path isolated with-fisolate-erroneous-paths-dereference), or anamswap.w $r0, $r1, $r0instruction which will cause the hardware to trigger an Instruction Not-defined Exception ifcodeis negative or greater than 32767. The default is-1, meaning to use theamswap.winstruction. 💀-mcond-move-int,-mno-cond-move-int— conditional moves for integral data in general-purpose registers are enabled (disabled). The default is-mcond-move-int.-mcond-move-float,-mno-cond-move-float— conditional moves for floating-point registers are enabled (disabled). The default is-mcond-move-float.-mmemcpy,-mno-memcpy— force (do not force) the use ofmemcpyfor non-trivial block moves. The default is-mno-memcpy, which allows GCC to inline most constant-sized copies. Setting optimization level to-Osalso forces the use ofmemcpy, but-mno-memcpymay override this behavior if explicitly specified, regardless of the order these options on the command line.-mstrict-align,-mno-strict-align— avoid or allow generating memory accesses that may not be aligned on a natural object boundary as described in the architecture specification. The default is-mno-strict-align.-G num— put global and static data smaller thannumbytes into a small data section. The default value is0.-mmax-inline-memcpy-size=n— inline all block moves (such as calls tomemcpyor structure copies) less than or equal tonbytes. The default value ofnis1024.-mcmodel=code-model— set the code model to one of:tiny-static— (Not implemented yet)tiny— (Not implemented yet)normal— the text segment must be within 128MB addressing space. The data segment must be within 2GB addressing space.medium— the text segment and data segment must be within 2GB addressing space. This is the default code model unless GCC has been configured with--with-cmodel=specifying a different default code model.large— (Not implemented yet)extreme— this mode does not limit the size of the code segment and data segment. The-mcmodel=extremeoption is incompatible with-fpltand/or-mexplicit-relocs=none.
-mexplicit-relocs=style,-mexplicit-relocs,-mno-explicit-relocs— set when to use assembler relocation operators when dealing with symbolic addresses. The alternative is to use assembler macros instead, which may limit instruction scheduling but allow linker relaxation. With-mexplicit-relocs=none, the assembler macros are always used; with-mexplicit-relocs=always, the assembler relocation operators are always used; and with-mexplicit-relocs=autothe compiler uses the relocation operators where linker relaxation is impossible to improve the code quality, and macros elsewhere. The default value is determined with the assembler capability detected during GCC build-time and the setting of-mrelax:-mexplicit-relocs=noneif the assembler does not support relocation operators at all.-mexplicit-relocs=alwaysif the assembler supports relocation operators but-mrelaxis not enabled.-mexplicit-relocs=autoif the assembler supports relocation operators and-mrelaxis enabled.
For backward compatibility, -mexplicit-relocs is equivalent to -mexplicit-relocs=always, while -mno-explicit-relocs is equivalent to -mexplicit-relocs=none. iconic backwards-compat behavior, honestly 🌈
-mdirect-extern-access,-mno-direct-extern-access— control use of the GOT to access external symbols. The default is-mno-direct-extern-access: the GOT is used for external symbols with default visibility, but not used for other external symbols. With-mdirect-extern-access, the GOT is not used and all external symbols are PC-relatively addressed. It is only suitable for environments where no dynamic link is performed, like firmwares, OS kernels, executables linked with-staticor-static-pie.-mdirect-extern-accessis not compatible with-fPICor-fpic. ⚠️-mrelax,-mno-relax— take (do not take) advantage of linker relaxations. If-mpass-mrelax-to-asis enabled, this option is also passed to the assembler. The default is determined during GCC build-time by detecting corresponding assembler support:-mrelaxif the assembler supports both the-mrelaxoption and the conditional branch relaxation (it’s required or the.aligndirectives and conditional branch instructions in the assembly code outputted by GCC may be rejected by the assembler because of a relocation overflow).-mno-relaxotherwise.
-mpass-mrelax-to-as,-mno-pass-mrelax-to-as— pass (do not pass) the-mrelaxor-mno-relaxoption to the assembler. The default is determined during GCC build-time by detecting corresponding assembler support:-mpass-mrelax-to-asif the assembler supports the-mrelaxoption,-mno-pass-mrelax-to-asotherwise. This option is mostly useful for debugging, or interoperation with assemblers different from the build-time one. 🛠️-mrecip— this option enables use of the reciprocal estimate and reciprocal square root estimate instructions with additional Newton-Raphson steps to increase precision instead of doing a divide or square root and divide for floating-point arguments. These instructions are generated only when-funsafe-math-optimizationsis enabled together with-ffinite-math-onlyand-fno-trapping-math. This option is off by default. Before you can use this option, you gotta make sure the target CPU supports thefrecipeandfrsqrteinstructions. Note that while the throughput of the sequence is higher than the throughput of the non-reciprocal instruction, the precision of the sequence can be decreased by up to 2 ulp (i.e. the inverse of 1.0 equals 0.99999994). 😭 math girlies stay strong-mrecip=opt— this option controls which reciprocal estimate instructions may be used.optis a comma-separated list of options, which may be preceded by a!to invert the option:all— enable all estimate instructions.default— enable the default instructions, equivalent to-mrecip.none— disable all estimate instructions, equivalent to-mno-recip.div— enable the approximation for scalar division.vec-div— enable the approximation for vectorized division.sqrt— enable the approximation for scalar square root.vec-sqrt— enable the approximation for vectorized square root.rsqrt— enable the approximation for scalar reciprocal square root.vec-rsqrt— enable the approximation for vectorized reciprocal square root.
So, for example, -mrecip=all,!sqrt enables all of the reciprocal approximations, except for scalar square root. 💅
-mfrecipe,-mno-frecipe— use (do not use)frecipe.{s/d}andfrsqrte.{s/d}instructions. When compiling with-march=la664, it is enabled by default. Otherwise the default is-mno-frecipe.-mdiv32,-mno-div32— use (do not use)div.w[u]andmod.w[u]instructions with input not sign-extended. When compiling with-march=la664, it is enabled by default. Otherwise the default is-mno-div32.-mlam-bh,-mno-lam-bh— use (do not use)am{swap/add}[_db].{b/h}instructions. When compiling with-march=la664, it is enabled by default. Otherwise the default is-mno-lam-bh.-mlamcas,-mno-lamcas— use (do not use)amcas[_db].{b/h/w/d}instructions. When compiling with-march=la664, it is enabled by default. Otherwise the default is-mno-lamcas.-mld-seq-sa,-mno-ld-seq-sa— whether a same-address load-load barrier (dbar 0x700) is needed. When compiling with-march=la664, it is enabled by default. Otherwise the default is-mno-ld-seq-sa, the load-load barrier is needed. 🦋-mscq,-mno-scq— use (do not use) the 16-byte conditional store instructionsc.q. The default is-mscqif the machine type specified with-march=supports this instruction,-mno-scqotherwise.-mtls-dialect=opt— bestie, this one picks which TLS dialect gets used for the general dynamic and local dynamic TLS models. Theoptargument is one of these two:trad— use traditional TLS. This is the default, the OG, the one you probably want.desc— use TLS descriptors.
-mannotate-tablejump,-mno-annotate-tablejump— okay so this creates an annotation section called.discard.tablejump_annotateto correlate thejirlinstruction and the jump table when a jump table is used to optimize theswitchstatement. Some external tools — like, for example,objtoolfrom the Linux kernel building system — literally need that annotation to analyze the control flow. The default is-mno-annotate-tablejump. 🫶
🌸 LynxOS Options 🌸
These options are available for LynxOS targets. niche but iconic.
-mshared— use shared libraries. The default is to link with static libraries, fyi.-mthreads— support multi-threading. slay.-mlegacy-threads— support legacy multi-threading. ngl, this option is incompatible with-mthreads, so pick a lane bestie.
🎀 M32C Options 🎀
-mcpu=name— selects the CPU for which code is generated.namemay 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— specifies that the program will be run on the simulator. This causes an alternate runtime library to be linked in which supports, like, for example, file I/O.
⚠️ You must not use this option when generating programs that will run on real hardware — you must provide your own runtime library for whatever I/O functions are needed. no cap, this one matters.
-memregs=number— specifies the number of memory-based pseudo-registers GCC uses during code generation. These pseudo-registers are used like real registers, so there's a whole tradeoff between GCC's ability to fit the code into available registers and the performance penalty of using memory instead of registers. Note that all modules in a program must be compiled with the same value for this option. Because of that, you must not use this option with GCC's default runtime libraries. 💀
✨ M32R/D Options ✨
These -m options are defined for Renesas M32R/D architectures:
-m32r2— generate code for the M32R/2.-m32rx— generate code for the M32R/X.-m32r— generate code for the M32R. This is the default, bestie.-mmodel=small— assume all objects live in the lower 16MB of memory (so that their addresses can be loaded with theld24instruction), and assume all subroutines are reachable with theblinstruction. This is the default. The addressability of a particular object can be set with themodelattribute.-mmodel=medium— assume objects may be anywhere in the 32-bit address space (the compiler generatesseth/add3instructions to load their addresses), and assume all subroutines are reachable with theblinstruction.-mmodel=large— assume objects may be anywhere in the 32-bit address space (the compiler generatesseth/add3instructions to load their addresses), and assume subroutines may not be reachable with theblinstruction (the compiler generates the much slowerseth/add3/jlinstruction sequence). 😭-msdata=none— disable use of the small data area. Variables are put into one of.data,.bss, or.rodata(unless thesectionattribute has been specified). This is the default. The small data area consists of sections.sdataand.sbss. Objects may be explicitly put in the small data area with thesectionattribute using one of these sections.-msdata=sdata— put small global and static data in the small data area, but don't generate special code to reference them.-msdata=use— put small global and static data in the small data area, and generate special instructions to reference them. fancy.-G num— put global and static objects less than or equal tonumbytes into the small data or BSS sections instead of the normal data or BSS sections. The default value ofnumis8. The-msdataoption must be set to one ofsdataorusefor this option to have any effect.
All modules should be compiled with the same -G num value. Compiling with different values of num may or may not work; if it doesn't, the linker gives an error message — incorrect code is not generated. good to know fr.
-mdebug— makes the M32R-specific code in the compiler display some statistics that might help in debugging programs.-malign-loops,-mno-align-loops— align all loops to a 32-byte boundary. This option is disabled by default.-missue-rate=number— issuenumberinstructions per cycle.numbercan only be1or2.-mbranch-cost=number—numbercan only be1or2. If it's1then branches are preferred over conditional code; if it's2, then the opposite applies.-mflush-trap=number— specifies the trap number to use to flush the cache. The default is12. Valid numbers are between0and15inclusive.-mno-flush-trap— specifies that the cache cannot be flushed by using a trap.-mflush-func=name— specifies the name of the operating system function to call to flush the cache. The default is_flush_cache, but a function call is only used if a trap is not available.-mno-flush-func— indicates that there is no OS function for flushing the cache.
💜 M680x0 Options 💜
These are the -m options defined for M680x0 and ColdFire processors. The default settings depend on which architecture was selected when the compiler was configured; the defaults for the most common choices are given below.
-march=arch— generate code for a specific M680x0 or ColdFire instruction set architecture. Permissible values ofarchfor 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 M680x0cpus are:68000,68010,68020,68030,68040,68060,68302,68332andcpu32. The ColdFirecpus 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— generate output for a 68000. This is the default when the compiler is configured for 68000-based systems. It is equivalent to-march=68000.
Use this option for microcontrollers with a 68000 or EC000 core, including the 68008, 68302, 68306, 68307, 68322, 68328 and 68356. 🦋
-m68010— generate output for a 68010. This is the default when the compiler is configured for 68010-based systems. It is equivalent to-march=68010.-m68020,-mc68020— generate output for a 68020. This is the default when the compiler is configured for 68020-based systems. It is equivalent to-march=68020.-m68030— generate output for a 68030. This is the default when the compiler is configured for 68030-based systems. It is equivalent to-march=68030.-m68040— generate output for a 68040. This is the default when the compiler is configured for 68040-based systems. It is equivalent to-march=68040.
This option inhibits the use of 68881/68882 instructions that have to be emulated by software on the 68040. Use this option if your 68040 does not have code to emulate those instructions. iykyk.
-m68060— generate output for a 68060. This is the default when the compiler is configured for 68060-based systems. It is equivalent to-march=68060.
This option inhibits the use of 68020 and 68881/68882 instructions that have to be emulated by software on the 68060. Use this option if your 68060 does not have code to emulate those instructions.
-m68302— generate code for a 68302.-m68332— generate code for a 68332.-m68851— so the 68851 was this external MMU for the 68020 processor, right? GCC will accept this option... and then literally not use it. It's giving "I hear you but I'm not doing anything about it" 💀-mcpu32— generates output for a CPU32. This is the default when the compiler is configured for CPU32-based systems, and it's equivalent to-march=cpu32.
Use this one for microcontrollers with a CPU32 or CPU32+ core — including the 68330, 68331, 68332, 68333, 68334, 68336, 68340, 68341, 68349 and 68360. That's a whole squad bestie 🫶
-mfidoa— generates code for a Fido A. That's it. That's the whole thing. 🐕-m5200— generates output for a 520X ColdFire CPU. Default when the compiler is configured for 520X-based systems. It's equivalent to-mcpu=5206, and ngl it's now deprecated in favor of that option (so like, use-mcpu=5206instead, bestie).
Use this for microcontrollers with a 5200 core, including the MCF5202, MCF5203, MCF5204 and MCF5206 ✨
-m5206e— generates output for a 5206e ColdFire CPU. This option is now deprecated in favor of the equivalent-mcpu=5206e.-m528x— generates output for a member of the ColdFire 528X family. This option is now deprecated in favor of the equivalent-mcpu=528x.-m5307— generates output for a ColdFire 5307 CPU. This option is now deprecated in favor of the equivalent-mcpu=5307.-m5407— generates output for a ColdFire 5407 CPU. This option is now deprecated in favor of the equivalent-mcpu=5407.-mcfv4e— generates output for a ColdFire V4e family CPU (e.g. 547x/548x). This includes use of hardware floating-point instructions. The option is equivalent to-mcpu=547x, and is now deprecated in favor of that option.-m68020-40— generates output for a 68040, without using any of the new instructions. The result? Code that can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040. The generated code does use the 68881 instructions that are emulated on the 68040. Sneaky but iconic 💅
The option is equivalent to -march=68020 -mtune=68020-40.
-m68020-60— generates output for a 68060, without using any of the new instructions. This results in code that can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040. The generated code does use the 68881 instructions that are emulated on the 68060.
The option is equivalent to -march=68020 -mtune=68020-60.
-mhard-float,-m68881— generate floating-point instructions. This is the default for 68020 and above, and for ColdFire devices that have an FPU. It defines the macro__HAVE_68881__on M680x0 targets and__mcffpu__on ColdFire targets. Macro-defining queen 👑-msoft-float— do not generate floating-point instructions; use library calls instead. This is the default for 68000, 68010, and 68832 targets. It's also the default for ColdFire devices that have no FPU.-mdiv,-mno-div— generate (do not generate) ColdFire hardware divide and remainder instructions. If-marchis used without-mcpu, the default is "on" for ColdFire architectures and "off" for M680x0 architectures. Otherwise, the default is taken from the target CPU (either the default CPU, or the one specified by-mcpu). For example, the default is "off" for-mcpu=5206and "on" for-mcpu=5206e.
GCC defines the macro __mcfhwdiv__ when this option is enabled.
-mshort,-mno-short,-mnoshort— consider typeintto be 16 bits wide, likeshort int. Additionally, parameters passed on the stack are also aligned to a 16-bit boundary even on targets whose API mandates promotion to 32-bit. This option is disabled by default.-mbitfield,-mno-bitfield,-mnobitfield— control use of the bit-field instructions. The-m68000,-mcpu32and-m5200options imply-mnobitfield; the-m68020option implies-mbitfield.-mrtd,-mno-rtd,-mnortd— control use of a different function-calling convention, in which functions that take a fixed number of arguments return with thertdinstruction, which pops their arguments while returning. This saves one instruction in the caller since there's no need to pop the arguments there. Efficiency slay ✨
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 variables on 32-bit boundaries produces code that runs somewhat faster on processors with 32-bit busses — at the expense of more memory. Tradeoffs, bestie, tradeoffs 🎀
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. So like... be aware 👀
Use the pc-relative addressing mode of the 68000 directly, instead of using a global offset table. At present, this option implies -fpic, allowing at most a 16-bit offset for pc-relative addressing. -fPIC is not presently supported with -mpcrel, though this could be supported for 68020 and higher processors.
-mno-strict-align,-mstrict-align— do not (do) assume that unaligned memory references are handled by the system.-msep-data,-mno-sep-data— with-msep-data, generate code that allows the data segment to be located in a different area of memory from the text segment. This allows for execute-in-place in an environment without virtual memory management. This option implies-fPIC.
This option is disabled by default; GCC generates code that assumes that the data segment follows the text segment.
-mid-shared-library,-mno-id-shared-library— if enabled, 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.
This option is disabled by default.
-mshared-library-id=n— specifies the identification number of the ID-based shared library being compiled. Specifying a value of0generates more compact code; specifying other values forces the allocation of that number to the current library, but is no more space- or time-efficient than omitting this option.-mxgot,-mno-xgot— when generating position-independent code for ColdFire, generate code that works if the GOT has more than 8192 entries. This code is larger and slower than code generated without this option. On M680x0 processors, this option is not needed;-fPICsuffices.
GCC normally uses a single instruction to load values from the GOT. While this is relatively efficient, it only works if the GOT is smaller than about 64k. Anything larger causes the linker to report an error such as:
relocation truncated to fit: R_68K_GOT16O foobarIf this happens, you should recompile your code with -mxgot. It should then work with very large GOTs. However, code generated with -mxgot is less efficient, since it takes 4 instructions to fetch the value of a global symbol. Not the vibe but sometimes you gotta 💀
Note that some linkers, including newer versions of the GNU linker, can create multiple GOTs and sort GOT entries. If you have such a linker, you should only need to use -mxgot when compiling a single object file that accesses more than 8192 GOT entries. Very few do.
These options have no effect unless GCC is generating position-independent code.
-mxtls— support TLS segment larger than 64K. The considerations for using this option are similar to those for-mxgotabove; the default 16-bit offset addressing for TLS is more efficient, but this may be inadequate for some programs.-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. Let's gooo 🫶
-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-immediates,-mno-relax-immediates— 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,-mbig-endian— generate code for a little- or big-endian target, respectively. The default is big-endian. Little-endian code is supported only with-m340.-m210— generate code for the 210 processor.-m340— generate code for the 340 processor.-mno-lsim— assume that runtime support has been provided and so omit the simulator library (libsim.a) from the linker command line.-mstack-increment=size— set the maximum amount for a single stack increment operation. Large values can increase the speed of programs that contain functions that need a large amount of stack space, but they can also trigger a segmentation fault if the stack is extended too much. The default value is0x1000.
🌸 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.-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. Version numbers but make it fashion 💜
okay bestie we're still in MicroBlaze hours and honestly? the vibe is immaculate. grab a snack 🍓, we're finishing this architecture and then it's MIPS o'clock 💅
-mxl-soft-mul,-mno-xl-soft-mul— turns on software multiply emulation. it's on by default, so no cap, that's just how it ships ✨-mxl-soft-div,-mno-xl-soft-div— software emulation for divides. also enabled by default 🫶-mxl-barrel-shift— use the hardware barrel shifter, periodt 🔥-mxl-pattern-compare— use pattern compare instructions 🎀-msmall-divides— table lookup optimization for small signed integer divisions. it's giving "work smarter not harder" 💅-mxl-gp-opt— use GP-relative.sdata/.sbsssections ⭐-mxl-multiply-high— multiply high instructions for the high part of a 32x32 multiply. math queen behavior 😭-mxl-float-convert— hardware floating-point conversion instructions 🦋-mxl-float-sqrt— 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— picks the application model app-model. the valid models are, and yes we're listing every single one because i love you:executable— normal executable (the default), uses startup codecrt0.o✨xmdstub— for use with the Xilinx Microprocessor Debugger (XMD) based software intrusive debug agent called xmdstub. this one 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 not contain a processor reset vector handler. suitable for transferring control on a processor reset to the bootloader rather than the application 🥾novectors— for applications that do not require any of the MicroBlaze vectors. this option may be useful for applications running within a monitoring application. it usescrt3.oas a startup file 👀
-mxl-prefetch— enable insertion of prefetch ("wic") instructions at call sites 🔮-mpic-data-is-text-relative— assume that the displacement between the text and data segments is fixed at static link time. this lets data be referenced by offset from start of text address instead of the GOT, since PC-relative addressing is not supported. it's giving "we make it work" 💀
🦋 MIPS Options 🦋
bestie. breathe. it's a lot of options. but they're all so good, i promise.
-EB,-meb— generate big-endian code 🌈-EL,-mel— generate little-endian code. this is the default formips*el-*-*configurations 🧚-march=arch — generate code that runs on arch, which can be the name of a generic MIPS ISA, or the name of a particular processor.
the ISA names are: mips1, mips2, mips3, mips4, mips32, mips32r2, mips32r3, mips32r5, mips32r6, mips64, mips64r2, mips64r3, mips64r5 and mips64r6.
the processor names are: 4kc, 4km, 4kp, 4ksc, 4kec, 4kem, 4kep, 4ksd, 5kc, 5kf, 20kc, 24kc, 24kf2_1, 24kf1_1, 24kec, 24kef2_1, 24kef1_1, 34kc, 34kf2_1, 34kf1_1, 34kn, 74kc, 74kf2_1, 74kf1_1, 74kf3_2, 1004kc, 1004kf2_1, 1004kf1_1, i6400, i6500, interaptiv, loongson2e, loongson2f, loongson3a, gs464, gs464e, gs264e, m4k, m14k, m14kc, m14ke, m14kec, m5100, m5101, octeon, octeon+, octeon2, octeon3, orion, p5600, p6600, r2000, r3000, r3900, r4000, r4400, r4600, r4650, r4700, r5900, r6000, r8000, rm7000, rm9000, r10000, r12000, r14000, r16000, sb1, sr71000, vr4100, vr4111, vr4120, vr4130, vr4300, vr5000, vr5400, vr5500, xlr and xlp. allegrex, The special value from-abi selects the most compatible architecture for the selected ABI (that is, mips1 for 32-bit ABIs and mips3 for 64-bit ABIs) 💖
the native Linux/GNU toolchain also supports the value native, which selects the best architecture option for the host processor. -march=native has no effect if GCC does not recognize the processor ⚠️
in processor names, a final 000 can be abbreviated as k (for example, -march=r2k). prefixes are optional, and vr may be written r 🎀
names of the form nf2_1 refer to processors with FPUs clocked at half the rate of the core, names of the form nf1_1 refer to processors with FPUs clocked at the same rate as the core, and names of the form nf3_2 refer to processors with FPUs clocked a ratio of 3:2 with respect to the core. for compatibility reasons, nf is accepted as a synonym for nf2_1 while nx and bfx are accepted as synonyms for nf1_1 ✨
GCC defines two macros based on the value of this option. the first is "_MIPS_ARCH", which gives the name of target architecture, as a string. the second has the form "_MIPS_ARCH_foo", where foo is the capitalized value of "_MIPS_ARCH". for example, -march=r2000 sets "_MIPS_ARCH" to "r2000" and defines the macro "_MIPS_ARCH_R2000" 🫶
note that the "_MIPS_ARCH" macro uses the processor names given above. in other words, it has the full prefix and does not abbreviate 000 as k. in the case of from-abi, the macro names the resolved architecture (either "mips1" or "mips3"). it names the default architecture when no -march option is given 👑
-mtune=arch — optimize for arch. among other things, this option controls the way instructions are scheduled, and the perceived cost of arithmetic operations. the list of arch values is the same as for-march💅
when this option is not used, GCC optimizes for the processor specified by -march. by using -march and -mtune together, it is possible to generate code that runs on a family of processors, but optimize the code for one particular member of that family. it's giving "one outfit, whole friend group" 🌸
-mtune defines the macros "_MIPS_TUNE" and "_MIPS_TUNE_foo", which work in the same way as the -march ones described above ⭐
-mips1— equivalent to-march=mips1✨-mips2— equivalent to-march=mips2✨-mips3— equivalent to-march=mips3✨-mips4— equivalent to-march=mips4✨-mips32— equivalent to-march=mips32✨-mips32r3— equivalent to-march=mips32r3✨-mips32r5— equivalent to-march=mips32r5✨-mips32r6— equivalent to-march=mips32r6✨-mips64— equivalent to-march=mips64✨-mips64r2— equivalent to-march=mips64r2✨-mips64r3— equivalent to-march=mips64r3✨-mips64r5— equivalent to-march=mips64r5✨-mips64r6— equivalent to-march=mips64r6✨-mips16,-mno-mips16— generate (do not generate) MIPS16 code. if GCC is targeting a MIPS32 or MIPS64 architecture, it makes use of the MIPS16e ASE 🧚
MIPS16 code generation can also be controlled on a per-function basis by means of "mips16" and "nomips16" attributes 🎀
-mmips16e2,-mno-mips16e2— use (do not use) the MIPS16e2 ASE. this option modifies the behavior of the-mips16option such that it targets the MIPS16e2 ASE 🔥-mflip-mips16— generate MIPS16 code on alternating functions. this option is provided for regression testing of mixed MIPS16/non-MIPS16 code generation, and is not intended for ordinary use in compiling user code ⚠️ (like, genuinely, this is a lab-coat option, not a fit-check option 💀)-minterlink-compressed,-mno-interlink-compressed— require (do not require) that code using the standard (uncompressed) MIPS ISA be link-compatible with MIPS16 and microMIPS code, and vice versa 🦋
for example, code using the standard ISA encoding cannot jump directly to MIPS16 or microMIPS code; it must either use a call or an indirect jump. -minterlink-compressed therefore disables direct jumps unless GCC knows that the target of the jump is not compressed 🎯
-minterlink-mips16,-mno-interlink-mips16— aliases of-minterlink-compressedand-mno-interlink-compressed. these options predate the microMIPS ASE and are retained for backwards compatibility 👑-mabi=32,-mabi=o64,-mabi=n32,-mabi=64,-mabi=eabi— generate code for the given ABI 💅
note that the EABI has a 32-bit and a 64-bit variant. GCC normally generates 64-bit code when you select a 64-bit architecture, but you can use -mgp32 to get 32-bit code instead 🫶
for information about the O64 ABI, see https://gcc.gnu.org/projects/mipso64-abi.html 🌈
GCC supports a variant of the o32 ABI in which floating-point registers are 64 rather than 32 bits wide. you can select this combination with -mabi=32 -mfp64. this ABI relies on the "mthc1" and "mfhc1" instructions and is therefore only supported for MIPS32R2, MIPS32R3 and MIPS32R5 processors ⭐
the register assignments for arguments and return values remain the same, but each scalar value is passed in a single 64-bit register rather than a pair of 32-bit registers. for example, scalar floating-point values are returned in $f0 only, not a $f0/$f1 pair. the set of call-saved registers also remains the same in that the even-numbered double-precision registers are saved ✨
two additional variants of the o32 ABI are supported to enable a transition from 32-bit to 64-bit registers. these are FPXX (-mfpxx) and FP64A (-mfp64 -mno-odd-spreg). the FPXX extension mandates that all code must execute correctly when run using 32-bit or 64-bit registers. the code can be interlinked with either FP32 or FP64, but not both. the FP64A extension is similar to the FP64 extension but forbids the use of odd-numbered single-precision registers. this can be used in conjunction with the "FRE" mode of FPUs in MIPS32R5 processors and allows both FP32 and FP64A code to interlink and run in the same process without changing FPU modes 🧚
-mabicalls,-mno-abicalls— generate (do not generate) code that is suitable for SVR4-style dynamic objects.-mabicallsis the default for SVR4-based systems 💖-mshared,-mno-shared— generate (do not generate) code that is fully position-independent, and that can therefore be linked into shared libraries. this option only affects-mabicalls🎀
all -mabicalls code has traditionally been position-independent, regardless of options like -fPIC and -fpic. however, as an extension, the GNU toolchain allows executables to use absolute accesses for locally-binding symbols. it can also use shorter GP initialization sequences and generate direct calls to locally-defined functions. this mode is selected by -mno-shared 🔥
-mno-shared depends on binutils 2.16 or higher and generates objects that can only be linked by the GNU linker. however, the option does not affect the ABI of the final executable; it only affects the ABI of relocatable objects. using -mno-shared generally makes executables both smaller and quicker 🌟
-msharedis the default. 💅-mplt,-mno-plt— assume (or don't assume, your call bestie) that the static and dynamic linkers support PLTs and copy relocations. Heads up tho: this option only does anything with-mno-shared-mabicalls. And for the n64 ABI? It's literally a no-op unless-msym32is also in the mix 🙈
Wanna make -mplt the default? Configure GCC with --with-mips-plt. Otherwise the default is -mno-plt. that's the whole vibe ✨
-mxgot,-mno-xgot— lift (or don't lift) the usual restrictions on the size of the global offset table. 🫶
So normally GCC uses a single instruction to load values from the GOT — cute and efficient, right? But it only works if the GOT is smaller than about 64k. Anything bigger and the linker just gives up and yells at you with something like:
relocation truncated to fit: R_MIPS_GOT16 foobarIf that happens, recompile your code with -mxgot — it works with very large GOTs, but ngl the code is also less efficient, since it takes three instructions to fetch the value of a global symbol 😭 tradeoffs!!
Note that some linkers can create multiple GOTs. If you've got one of those, you should only need -mxgot when a single object file accesses more than 64k's worth of GOT entries. Very few do. (like... barely anyone. chill.)
Also these options do absolutely nothing 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. mysterious. iconique. 💅-mhard-float— use floating-point coprocessor instructions. 🔥-msoft-float— do not use floating-point coprocessor instructions. Implement floating-point calculations using library calls instead.-mno-float— equivalent to-msoft-float, but additionally asserts that the program being compiled does not perform any floating-point operations. This option is presently supported only by some bare-metal MIPS configurations, where it may select a special set of libraries that lack all floating-point support (including, for example, the floating-point"printf"formats).
⚠️ If code compiled with -mno-float accidentally contains floating-point operations, it is likely to suffer a link-time or run-time failure. like it WILL break, bestie, no cap 💀
-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. So these instructions are therefore always used for the respective operations. slay ✨
-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. (yes they're literally swapped, that's the whole point 💀)
The default is -mnan=legacy unless GCC has been configured with --with-nan=2008.
-mllsc,-mno-llsc— use (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. iykyk 📚
-mdsp,-mno-dsp— use (do not use) revision 1 of the MIPS DSP ASE. This option defines the preprocessor macro"__mips_dsp". It also defines"__mips_dsp_rev"to1. ⭐-mdspr2,-mno-dspr2— use (do not use) revision 2 of the MIPS DSP ASE. This option defines the preprocessor macros"__mips_dsp"and"__mips_dspr2". It also defines"__mips_dsp_rev"to2. ⭐-msmartmips,-mno-smartmips— use (do not use) the MIPS SmartMIPS ASE.-mpaired-single,-mno-paired-single— use (do not use) paired-single floating-point instructions. This option requires hardware floating-point support to be enabled. 🔥-mdmx,-mno-mdmx— use (do not use) MIPS Digital Media Extension instructions. This option can only be used when generating 64-bit code and requires hardware floating-point support to be enabled.-mips3d,-mno-mips3d— use (do not use) the MIPS-3D ASE. The option-mips3dimplies-mpaired-single. 🎀-mmicromips,-mno-micromips— generate (do not generate) microMIPS code.
MicroMIPS code generation can also be controlled on a per-function basis by means of "micromips" and "nomicromips" attributes. fancy 💅
-mmt,-mno-mt— use (do not use) MT Multithreading instructions.-mmcu,-mno-mcu— use (do not use) the MIPS MCU ASE instructions.-meva,-mno-eva— use (do not use) the MIPS Enhanced Virtual Addressing instructions.-mvirt,-mno-virt— use (do not use) the MIPS Virtualization (VZ) instructions.-mxpa,-mno-xpa— use (do not use) the MIPS eXtended Physical Address (XPA) instructions.-mcrc,-mno-crc— use (do not use) the MIPS Cyclic Redundancy Check (CRC) instructions.-mginv,-mno-ginv— use (do not use) the MIPS Global INValidate (GINV) instructions.-mmsa,-mno-msa— use (do not use) the MIPS MSA extension instructions.-mloongson-mmi,-mno-loongson-mmi— use (do not use) the MIPS Loongson MultiMedia extensions Instructions (MMI).-mloongson-ext,-mno-loongson-ext— use (do not use) the MIPS Loongson EXTensions (EXT) instructions.-mloongson-ext2,-mno-loongson-ext2— use (do not use) the MIPS Loongson EXTensions r2 (EXT2) instructions. 🌈
-mlong64— force"long"types to be 64 bits wide. See-mlong32for an explanation of the default and the way that the pointer size is determined. 🦋-mlong32— force"long","int", and pointer types to be 32 bits wide.
The default size of "int"s, "long"s and pointers depends on the ABI. All the supported ABIs use 32-bit "int"s. The n64 ABI uses 64-bit "long"s, as does the 64-bit EABI; the others use 32-bit "long"s. Pointers are the same size as "long"s, or the same size as integer registers, whichever is smaller. math is mathing ✨
-msym32,-mno-sym32— assume (do not assume) that all symbols have 32-bit values, regardless of the selected ABI. This option is useful in combination with-mabi=64and-mno-abicallsbecause it allows GCC to generate shorter and faster references to symbolic addresses. obsessed with the efficiency honestly 🔥-G num— put definitions of externally-visible data in a small data section if that data is no bigger than num bytes. GCC can then generate more efficient accesses to the data; see-mgpoptfor details. 👑
The default -G option depends on the configuration.
-mlocal-sdata,-mno-local-sdata— extend (do not extend) the-Gbehavior to local data too, such as to static variables in C.-mlocal-sdatais the default for all configurations. 💖
If the linker complains that an application is using too much small data, you might want to try rebuilding the less performance-critical parts with -mno-local-sdata. You might also want to build large libraries with -mno-local-sdata, so that the libraries leave more room for the main program. sharing is caring 🫶
-mextern-sdata,-mno-extern-sdata— assume (do not assume) that externally-defined data is in a small data section if the size of that data is within the-Glimit.-mextern-sdatais the default for all configurations. 🌸
If you compile a module Mod with -mextern-sdata -G num -mgpopt, and Mod references a variable Var that is no bigger than num bytes, you must make sure that Var is placed in a small data section. If Var is defined by another module, you must either compile that module with a high-enough -G setting or attach a "section" attribute to Var's definition. If Var is common, you must link the application with a high-enough -G setting. 😭 rules are rules
The easiest way of satisfying these restrictions is to compile and link every module with the same -G option. However, you may wish to build a library that supports several different small data limits. You can do this by compiling the library with the highest supported -G setting and additionally using -mno-extern-sdata to stop the library from making assumptions about externally-defined data. big brain energy fr fr 🧠✨
-mgpopt,-mno-gpopt— use (do not use) GP-relative accesses for symbols that are known to be in a small data section; see-G,-mlocal-sdataand-mextern-sdata.-mgpoptis the default for all configurations. 💜-mno-gpopt— lowkey the fix for when$gpmight not actually hold the value of"_gp". Like, picture this: the code is part of a library that might get used in a boot monitor, and programs calling boot monitor routines pass along an unknown value in$gp. (In those situations, the boot monitor itself is usually compiled with-G0.) Ngl,-mno-gpoptimplies-mno-local-sdataand-mno-extern-sdata. ✨-membedded-data,-mno-embedded-data— this one says put your variables in the read-only data section first if possible, then in the small data section if possible, otherwise in data. It's giving slightly slower code than the default, but it reduces the amount of RAM you need while executing, so some embedded systems are literally obsessed with it. 🎀-muninit-const-in-rodata,-mno-uninit-const-in-rodata— drops uninitialized"const"variables into the read-only data section. Heads up bestie: this option is only meaningful when you pair it with-membedded-data. 💅-mcode-readable=setting— tells GCC whether it may generate code that reads from executable sections. Three possible settings, no more no less:-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 all the other instructions must not do so. Useful on 4KSc and 4KSd processors when the code TLBs have the Read Inhibit bit set. Also useful on processors that can be configured to have a dual instruction/data SRAM interface and that, like the M4K, automatically redirect PC-relative loads to the instruction RAM. ⭐-mcode-readable=no— instructions must not access executable sections. This one's useful on targets configured with a dual instruction/data SRAM interface but that (unlike the M4K) do not automatically redirect PC-relative loads to the instruction RAM.
On SDE targets, -mcode-data-in-code is available as a traditional alias for -mcode-readable=no, and -mcode-xonly is the alias for -mcode-readable=pcrel. iykyk 🫶
-msplit-addresses,-mno-split-addresses— enable (disable) use of the%hi()and%lo()assembler relocation operators. Fair warning: this option has been superseded by-mexplicit-relocs, it's just still hanging around for backwards compatibility. 😭-mexplicit-relocs=none,-mexplicit-relocs=base,-mexplicit-relocs=pcrel,-mexplicit-relocs,-mno-explicit-relocs— these control whether explicit relocs (such as%gp_rel) are used. The default value depends on the version of GAS that was around when GCC itself was built. 🙈
The "base" explicit-relocs support was introduced into GAS in 2001. The "pcrel" explicit-relocs support was introduced into GAS in 2014, and it supports %pcrel_hi and %pcrel_lo. ✨
-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. Traps give you smaller code, but they're 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 that can be overridden at configure time with --with-divide=breaks. And you can completely disable divide-by-zero checks with -mno-check-zero-division. 💀
-mload-store-pairs,-mno-load-store-pairs— enable (disable) an optimization that pairs consecutive load or store instructions so load/store bonding can happen. It's enabled by default, but it only actually does anything when the selected architecture is known to support bonding. ⭐-mstrict-align,-mno-strict-align,-munaligned-access,-mno-unaligned-access— disable (enable) direct unaligned access for MIPS Release 6. MIPSr6 requires load/store unaligned-access support, either by hardware or by trapping and emulation. In that emulation case,-mstrict-alignmay be needed by the operating system kernel. And just so you know:-munaligned-accessand-mno-unaligned-accessare obsolete, only provided for backward compatibility. 🦋-mmemcpy,-mno-memcpy— force (don't force) the use of"memcpy"for non-trivial block moves. The default is-mno-memcpy, which lets GCC inline most constant-sized copies. 💅-mlong-calls,-mno-long-calls— disable (don't disable) use of the"jal"instruction. Calling functions with"jal"is more efficient, but it requires the caller and callee to be in the same 256 megabyte segment. 🫶
This option has no effect on abicalls code. The default is -mno-long-calls. ✨
-mmad,-mno-mad— enable (disable) use of the"mad","madu"and"mul"instructions, as provided by the R4650 ISA. 🔥-mimadd,-mno-imadd— enable (disable) use of the"madd"and"msub"integer instructions. The default is-mimaddon architectures that support"madd"and"msub"— except for the 74k architecture, where it was found to generate slower code. 💀-mfused-madd,-mno-fused-madd— enable (disable) use of the floating-point multiply-accumulate instructions, when they're available. The default is-mfused-madd. 🎀
On the R8000 CPU, when multiply-accumulate instructions are used, the intermediate product is calculated to infinite precision and is not subject to the FCSR Flush to Zero bit. That may be undesirable in some circumstances. On other processors, the result is numerically identical to the equivalent computation using separate multiply, add, subtract and negate instructions. 🌸
-nocpp— tells the MIPS assembler to not run its preprocessor over user assembler files (with a.ssuffix) when assembling them. 🙈-mfix-24k,-mno-fix-24k— works around the 24K E48 errata (lost data on stores during refill). These workarounds are implemented by the assembler, not by GCC. 💜-mfix-r4000,-mno-fix-r4000— works around certain R4000 CPU errata:- A double-word or a variable shift may give an incorrect result if executed immediately after starting an integer division.
- A double-word or a variable shift may give an incorrect result if executed while an integer multiplication is in progress.
- An integer division may give an incorrect result if started in a delay slot of a taken branch or a jump.
-mfix-r4400,-mno-fix-r4400— works around certain R4400 CPU errata:- A double-word or a variable shift may give an incorrect result if executed immediately after starting an integer division.
-mfix-r10000,-mno-fix-r10000— works around certain R10000 errata:"ll"/"sc"sequences may not behave atomically on revisions prior to 3.0. They may deadlock on revisions 2.6 and earlier. 💀
This option can only be used if the target architecture supports branch-likely instructions. -mfix-r10000 is the default when -march=r10000 is used; -mno-fix-r10000 is the default otherwise. ⭐
-mfix-r5900,-mno-fix-r5900— do not attempt to schedule the preceding instruction into the delay slot of a branch instruction placed at the end of a short loop of six instructions or fewer, and always schedule a"nop"instruction there instead. Under certain conditions the short loop bug causes loops to execute only once or twice, thanks to a hardware bug in the R5900 chip. This workaround is implemented by the assembler, not by GCC. 🫶-mfix-rm7000,-mno-fix-rm7000— works around the RM7000"dmult"/"dmultu"errata. These workarounds are implemented by the assembler, not by GCC. 🌸-mfix-vr4120,-mno-fix-vr4120— works around certain VR4120 errata:"dmultu"does not always produce the correct result."div"and"ddiv"do not always produce the correct result if one of the operands is negative.
The workarounds for the division errata rely on special functions in libgcc.a. Right now, those functions are only provided by the "mips64vr*-elf" configurations.
Other VR4120 errata require a NOP to be inserted between certain pairs of instructions. Those errata are handled by the assembler, not by GCC itself. ✨
-mfix-vr4130,-mno-fix-vr4130— works around the VR4130"mflo"/"mfhi"errata. The workarounds are implemented by the assembler rather than by GCC — although GCC avoids using"mflo"and"mfhi"if the VR4130"macc","macchi","dmacc"and"dmacchi"instructions are available instead. 💅-mfix-sb1,-mno-fix-sb1— works around certain SB-1 CPU core errata. (This flag currently works around the SB-1 revision 2"F1"and"F2"floating-point errata.) 🎀-mfix4300,-mno-fix4300— works around a bug in early VR4300 silicon that causes multiplies with certain operands to corrupt immediately following multiplies. 😭-mr10k-cache-barrier=setting— tells GCC whether it should insert cache barriers to avoid the side effects of speculation on R10K processors.
Like a lot of processors, the R10K tries to predict the outcome of a conditional branch and speculatively executes instructions from the "taken" branch. It later aborts those instructions if the predicted outcome was wrong. But on the R10K, even aborted instructions can have side effects. 💀
This problem only affects kernel stores and, depending on the system, kernel loads. For 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.
So like, it's the kernel's job (not ours bestie, we're just the compiler 💅) to make sure speculative accesses to these regions are actually safe. No cap.
If the input program has a function declaration like this:
void foo (void);then the implementation of foo must allow j foo and jal foo to be executed speculatively. GCC is a good citizen and honors this restriction for functions it compiles itself, and it just expects non-GCC functions (hand-written assembly, we're looking at you 👀) to do the same. Play nice or the vibe check fails.
The option comes in three forms, bestie:
-mr10k-cache-barrier=load-store— inserts a cache barrier before a load or store that might be speculatively executed and that might have side effects even if aborted. Girl, the safety net is real. 🛟-mr10k-cache-barrier=store— inserts a cache barrier before a store that might be speculatively executed and that might have side effects even if aborted. Just stores, we're keeping it cute.-mr10k-cache-barrier=none— disables inserting cache barriers entirely. And this is the default setting, so out of the box you get zero barriers. 🙈
-mflush-func=func,-mno-flush-func— tells GCC which function to call to flush the I and D caches, or says "nope, don't call anything." If it does get called, the function has to take the same arguments as the common_flush_func, and those are: the address of the memory range for which the cache is being flushed, the size of that memory range, and the number3(which means flush both caches, cute). The default depends on what target GCC was configured for, but it's commonly either_flush_funcor__cpu_flush. 🧹✨-mbranch-cost=num— sets the cost of branches to roughly num "simple" instructions. Heads up though: this cost is only a heuristic and is not guaranteed to produce consistent results across releases (so don't come for us when it shifts, k?). A zero cost redundantly selects the default, which is based on the-mtunesetting. 💀-mbranch-likely,-mno-branch-likely— enables or disables use of Branch Likely instructions, regardless of whatever the default is for the architecture you picked. By default, Branch Likely instructions may be generated if the selected architecture supports them. There's one exception though: for the MIPS32 and MIPS64 architectures and the processors that implement those architectures, Branch Likely instructions are not generated by default, because MIPS32 and MIPS64 specifically deprecate their use. It's giving "we retired that trend." 🫠-mcompact-branches=never,-mcompact-branches=optimal,-mcompact-branches=always— these control which form of branches get generated. The default is-mcompact-branches=optimal.- The
-mcompact-branches=neveroption ensures that compact branch instructions are never generated. Periodt. - The
-mcompact-branches=alwaysoption ensures a compact branch instruction is generated if it's available for MIPS Release 6 onwards. If a compact branch instruction is not available (or it's pre-R6), a delay slot form of the branch is used instead. - If it's used for MIPS16/microMIPS targets, it's just ignored right now 🙃. The behavior for MIPS16/microMIPS may change in the future, since they do have some compact branch instructions.
- The
-mcompact-branches=optimaloption causes 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 is chosen if one is available. Optimizing icon behavior. 👑
- The
-mfp-exceptions,-mno-fp-exceptions— specifies whether FP exceptions are enabled. This affects how FP instructions get 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. One pipe is giving sad single-lane energy ngl. 😭-mvr4130-align,-mno-vr4130-align— okay so the VR4130 pipeline is two-way superscalar, but it 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. Glow-up for your instruction pairs. ✨ 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's enabled by default at optimization level-O3.-msynci,-mno-synci— enable (or 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. So like, be careful bestie. 💀
-mrelax-pic-calls,-mno-relax-pic-calls— tries 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-callsis the default if GCC was configured to use an assembler and a linker that support the.relocassembly directive and-mexplicit-relocsis in effect. With-mno-explicit-relocs, this optimization can be performed by the assembler and the linker alone — no compiler help needed. Teamwork makes the dream work. 🫶-mmcount-ra-address,-mno-mcount-ra-address— emit (or don't 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. Then_mcountcan modify the return address by doing both of these:- Returning the new address in register
$31. - Storing the new address in
*ra-address, if ra-address is nonnull. The default is-mno-mcount-ra-address. 🙅♀️
- Returning the new address in register
-mframe-header-opt,-mno-frame-header-opt— enable (or disable) frame header optimization in the o32 ABI. When using the o32 ABI, calling functions allocate16bytes on the stack for the called function to write out register arguments. When enabled, this optimization suppresses the allocation of the frame header if it can be determined that it's unused. Cleanup era. 🧼 This optimization is off by default at all optimization levels. Even-O3. Yeah.-mlxc1-sxc1,-mno-lxc1-sxc1— when applicable, enable (or disable) the generation oflwxc1,swxc1,ldxc1,sdxc1instructions. Enabled by default. Slay. ⭐-mmadd4,-mno-madd4— when applicable, enable (or disable) the generation of 4-operandmadd.s,madd.dand related instructions. Enabled by default. Four operands? Iconic. 👑
🌈 MMIX Options 🌈
These options are defined for the MMIX, bestie:
-mlibfuncs,-mno-libfuncs— specify that intrinsic library functions are being compiled, passing all values in registers, no matter the size. Every value gets the VIP register treatment. 💅-mepsilon,-mno-epsilon— generate floating-point comparison instructions that compare with respect to therEepsilon register.-mabi=mmixware,-mabi=gnu— generate code that passes function parameters and return values that (in the called function) are seen as registers$0and up, as opposed to the GNU ABI which uses global registers$231and up. Two different friend groups, same function. 🤝-mzero-extend,-mno-zero-extend— when reading data from memory in sizes shorter than 64 bits, use (or don't use) zero-extending load instructions by default, rather than sign-extending ones.-mknuthdiv,-mno-knuthdiv— makes 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. Don't ask which one's the "right" one, they both ate. 😌-mtoplevel-symbols,-mno-toplevel-symbols— prepend (or don't prepend) a:to all global symbols, so the assembly code can be used with thePREFIXassembly directive.-melf— generate an executable in the ELF format, rather than the default mmo format used by the mmix simulator. ✨-mbranch-predict,-mno-branch-predict— use (or don't use) the probable-branch instructions, when static branch prediction indicates a probable branch.-mbase-addresses,-mno-base-addresses— generate (or don't 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 range0to255from the value held in the register. This generally leads to short and fast code, but the number of different data items that can be addressed is limited. Which means a program that uses lots of static data may require-mno-base-addresses. Trade-offs, always trade-offs. 😮💨-msingle-exit,-mno-single-exit— force (or don't force) generated code to have a single exit point in each function. One door out, main character energy. 🚪-mset-data-start=address— when linking, set the start of the data section to address.-mset-program-start=address,-mno-set-program-start— when linking, set the program start address to address. It defaults to0x100unless-mno-set-program-startis used to suppress this option entirely. Note it's-mno-set-program-startwith no=address, okay? Don't add one. 🫶
🍓 MN10300 Options 🍓
These -m options are defined for Matsushita MN10300 architectures:
-mmult-bug,-mno-mult-bug— when enabled, generate code to avoid bugs in the multiply instructions for the MN10300 processors. This is the default. Avoiding bugs by default? We love a proactive queen. 👑-mam33,-mno-am33— generate code using features specific to the AM33 processor. The default is-mno-am33.-mam33-2,-mno-am33-2— generate code using features specific to the AM33/2.0 processor. The default is-mno-am33-2.-mam34,-mno-am34— generate code using features specific to the AM34 processor. The default is-mno-am34.-mtune=cpu-type— use the timing characteristics of the indicated CPU type when scheduling instructions. This does not change the targeted processor type (important distinction, bestie 🫵). The CPU type must be one ofmn10300,am33,am33-2oram34.-mreturn-pointer-on-d0,-mno-return-pointer-on-d0— okay so when the compiler makes a function that returns a pointer, this returns that pointer in botha0andd0. Otherwise the pointer comes back only ina0, and attempts to call such functions without a prototype result in errors. 💀 Heads up: this option is on by default; use-mno-return-pointer-on-d0to disable it.-mno-crt0— do not link in the C run-time initialization object file. That's the whole vibe.-mrelax— tells the linker to perform a relaxation optimization pass to shorten branches, calls and absolute memory addresses. It only has an effect when used on the command line for the final link step. Warning: this option makes symbolic debugging impossible 😭, so maybe don't do it while you're actively debugging, bestie.-mliw— allows the compiler to generate Long Instruction Word instructions if the target is the AM33 or later. Enabled by default. It defines the preprocessor macro__LIW__.-mno-liw— turns that off and defines the preprocessor macro__NO_LIW__.-msetlb— allows the compiler to generateSETLBandLccinstructions if the target is the AM33 or later. Enabled by default. It defines the preprocessor macro__SETLB__.-mno-setlb— turns that off and defines the preprocessor macro__NO_SETLB__.
💖 Moxie Options 💖
Moxie girlies, this section is short and slay:
-meb— generate big-endian code. This is the default formoxie-*-*configurations.-mel— generate little-endian code.-mmul.x— generatemul.xandumul.xinstructions. This option is enabled by default formoxiebox-*-*configurations.-mno-mul.x— disables those instructions.-mno-crt0— do not link in the C run-time initialization object file.
🌸 MSP430 Options 🌸
These options are defined for the MSP430:
-masm-hex— force assembly output to always use hex constants. Normally such constants are signed decimals, but this option is available for testsuite and/or aesthetic purposes. 💅-mmcu=name— 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__. That symbol is then 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 causes the linker to search for a script called xxx.ld.
The ISA and hardware multiply supported for the different MCUs is hard-coded into GCC. However, an external devices.csv file can be used to extend device support beyond those that have been hard-coded.
GCC searches for the devices.csv file using the following methods in the given precedence order, where the first method takes precedence over the second which takes precedence over the third.
🔎 Include path specified with -I and -L
devices.csv is searched for in each of the directories specified by include paths and linker library search paths.
🔎 Path specified by the environment variable MSP430_GCC_INCLUDE_DIR
Define the value of the global environment variable MSP430_GCC_INCLUDE_DIR to the full path to the directory containing devices.csv, and GCC will search this directory for devices.csv. If devices.csv is found, this directory is also registered as an include path and linker library path. Header files and linker scripts in this directory can therefore be used without manually specifying -I and -L on the command line.
🔎 The msp430-elf{,bare}/include/devices directory
Finally, GCC examines msp430-elf{,bare}/include/devices from the toolchain root directory. This directory does not exist in a default installation, but if you have created it and copied devices.csv there, then the MCU data is read. As above, this directory is also registered as an include path and linker library path.
If none of the above search methods find devices.csv, then the hard-coded MCU data is used.
-mwarn-mcu,-mno-warn-mcu— this 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.-msim— link to the simulator runtime libraries and linker script. Overrides any scripts that would be selected by the-mmcu=option.-mlarge— use large-model addressing (20-bit pointers, 20-bitsize_t).-msmall— use small-model addressing (16-bit pointers, 16-bitsize_t).-mrelax— this option is passed to the assembler and linker, and allows the linker to perform certain optimizations that cannot be done until the final link.mhwmult=type— describes the type of hardware multiply supported by the target. Accepted values fortypearenonefor 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."auto"is the default setting.
Hardware multiplies are normally performed by calling a library routine. This saves space in the generated code. When compiling at -O3 or higher however the hardware multiplier is invoked inline. This makes for bigger, but faster code.
The hardware multiply routines disable interrupts whilst running and restore the previous interrupt state when they finish. This makes them safe to use inside interrupt handlers as well as in normal code.
-minrt— enable the use of a minimum runtime environment without support for static initializers or constructors. This is intended for memory-constrained devices. The compiler includes special symbols in some objects that tell the linker and runtime which code fragments are required.-mtiny-printf— enable reduced code sizeprintfandputslibrary functions. The tiny implementations of these functions are not reentrant, so must be used with caution in multi-threaded applications.
Support for streams has been removed and the string to be printed are always sent to stdout via the write syscall. The string is not buffered before it is sent to write.
This option requires Newlib Nano IO, so GCC must be configured with --enable-newlib-nano-formatted-io.
-mmax-inline-shift=n— this option takes an integernbetween 0 and 64 inclusive, and sets the maximum number of inline shift instructions which should be emitted to perform a shift operation by a constant amount. When this value needs to be exceeded, anmspabihelper function is used instead. The default value is 4.
This only affects cases where a shift by multiple positions cannot be completed with a single instruction (e.g. all shifts >1 on the 430 ISA).
Shifts of a 32-bit value are at least twice as costly, so the value passed for this option is divided by 2 and the resulting value used instead.
-mcode-region=where,-mdata-region=where— these options tell the compiler where to place functions and data that do not have one of thelower,upper,eitherorsectionattributes. Possible values forwherearelower,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.-muse-lower-region-prefix— add the.lowerprefix to section names when compiling with-mcode-region=loweror-mdata-region=lower. Disabled by default.-msilicon-errata=name[,name...]— this option passes on a request to assembler to enable the fixes for the named silicon errata. Refer to the assembler documentation for details.-msilicon-errata-warn=name[,name...]— this option passes on a request to the assembler to enable warning messages when a named silicon errata might need to be applied. Refer to the assembler documentation for details.-mwarn-devices-csv,-mno-warn-devices-csv— warn ifdevices.csvis not found or there are problems parsing it (default: on).
🦋 NDS32 Options 🦋
These options are defined for NDS32 implementations:
-mbig-endian,-EB— generate code in big-endian mode.-mlittle-endian,-EL— generate code in little-endian mode.-mabi=2,-mfloat-abi=soft— use the soft floating-point ABI.-mabi=2fp+,-mfloat-abi=hard— use the hard floating-point ABI.-mreduced-regs— use reduced-set registers for register allocation.-mfull-regs— use full-set registers for register allocation.-malways-align— always align function entry, jump targets, and return address.-malign-functions— align function entry to 4 bytes.-mfp-as-gp,-mno-fp-as-gp— enable/disable fp-as-gp optimization.-mcmov,-mno-cmov— enable/disable generation of conditional move instructions.-mhw-abs,-mno-hw-abs— enable/disable generation of hardware abs instructions.-mext-perf,-mno-ext-perf— enable/disable generation of performance extension instructions.-mext-perf2,-mno-ext-perf2— enable/disable generation of performance extension 2 instructions.-mext-string,-mno-ext-string— enable/disable generation of string extension instructions.-mext-dsp,-mno-ext-dsp— enable/disable generation of DSP extension instructions.-mext-fpu-fma,-mno-ext-fpu-fma— enable/disable generation of floating-point multiply-accumulation instructions.-mext-fpu-sp,-mno-ext-fpu-sp— turns generation of single-precision floating-point instructions on/off ✨-mext-fpu-dp,-mno-ext-fpu-dp— same vibe but for double-precision floating-point instructions, on/off 🎀-mv3push,-mno-v3push— enable/disable generation ofv3 push25/pop25instructions, cute and efficient 💅-m16-bit,-mno-16-bit— enable/disable generation of 16-bit instructions 🐣-mvh,-mno-vh— enable/disable Virtual Hosting support, lowkey fancy 🦋-misr-vector-size=num— sets the size of each interrupt vector. bestie, it must be4or16, no in-between 👑-misr-secure=num— sets the security level ofc-isrfor the whole file (like, the entire file, no cap) 🔥-mcache-block-size=num— sets the size of each cache block. has to be a power of 2, between4and512⭐-march=arch— pick the target architecture. arch is one ofv2,v3,v3j,v3m,v3f, orv3s🌸-mcpu=cpu— pick the CPU to compile for. cpu is one ofn6,n650,n7,n705,n8,n801,sn8,sn801,s8,s801,e8,e801,n820,s830,e830,n9,n903,n903a,n968,n968a,n10,n1033,n1033a,n1033-fpu,n1033-spu,n1068,n1068a,n1068-fpu,n1068a-fpu,d10,d1088,d1088-fpu,d1088-spu,n15,d15,n15s,d15s,n15f,d15f,n12,n1213,n1233,n1233-fpu,n1233-spu,n13,n1337,n1337-fpu,n1337-spu, orsimple— yes that's a lot, go off 💀-mconfig-fpu=num— sets an FPU configuration value from0to7.0--3is as the FPU spec says, and4--7correspond to0--3🧚-mconfig-mul=type— configures the multiply instruction. type is one offast,fast1(equivalent tofast),fast2, orslow. the default isfast1💜-mconfig-register-ports=kind— sets the numbers of read/write ports forn9/n10cores. value should be3r2wor2r1w🎀-mcmodel=code-model— sets the code model, pick one:small— all the data and read-only data segments must be within512KBaddressing space. the text segment must be within16MBaddressing space 🐥medium— the data segment must be within512KBwhile the read-only data segment can be within4GBaddressing space. the text segment should still be within16MBaddressing space 🌼large— all the text and data segments can be within4GBaddressing space, sky's the limit fr fr 🌈
the default is large on GNU/Linux targets and medium on other ELF targets, iykyk 👑
-mctor-dtor,-mno-ctor-dtor— enable/disable the constructor/destructor feature 🎀-mrelax,-mno-relax— enable/disable the linker option to relax instructions, chill vibes only 🧘-mrelax-hint,-mno-relax-hint— enable/disable insertion of hints for the linker to do relaxation 👀-msched-prolog-epilog,-mno-sched-prolog-epilog— enable/disable scheduling of a function's prologue and epilogue sequence 📋-mret-in-naked-func,-mno-ret-in-naked-func— enable/disable generation of return instructions in naked functions. this one is enabled by default, just so u know 💫-malways-save-lp,-mno-always-save-lp— enable/disable always saving$lpon the stack 🫶-munaligned-access,-mno-unaligned-access— enable/disable unaligned word and halfword accesses to packed data 🙈-minline-asm-r15,-mno-inline-asm-r15— allow/disallow use ofr15for inline asm 👑
🌈 Nvidia PTX Options 🌈
these ones are defined for Nvidia PTX, bestie:
-m64— ignored, but preserved for backward compatibility. only the 64-bit ABI is supported, periodt 💅-march=architecture-string— generates code for the specified PTX ISA target architecture. valid architecture strings aresm_30,sm_35,sm_37,sm_50,sm_52,sm_53,sm_61,sm_70,sm_75,sm_80, andsm_89. the default depends on how the compiler has been configured, see--with-arch✨
this option sets the value of the preprocessor macro __PTX_SM__; for instance, for sm_35, it has the value 350 🔢
-misa=architecture-string— alias of-march=, same energy 🪞-march-map=architecture-string— selects the closest available-march=value that is not more capable. for instance, for-march-map=sm_50select-march=sm_37, and for-march-map=sm_53select-march=sm_53🎯-mptx=version-string— generates code for the specified PTX ISA version. valid version strings are3.1,4.0,4.1,4.2,5.0,6.0,6.3,7.0,7.3, and7.8. the default PTX ISA version is the one that added support for the selected PTX ISA target architecture, see-march=, but at least6.3, or7.3for-march=sm_52and higher 🌟
this option sets the values of the preprocessor macros __PTX_ISA_VERSION_MAJOR__ and __PTX_ISA_VERSION_MINOR__; for instance, for 3.1 the macros have the values 3 and 1, respectively 🧮
-mmainkernel— links in code for a__mainkernel. this is for stand-alone instead of offloading execution 🚀-moptimize,-mno-optimize— enable/disable partitioned execution optimizations. this option is enabled by default when any level of optimization is selected 🔥-msoft-stack,-mno-soft-stack— okay this one's a whole paragraph so stay with me:
for -mno-soft-stack (the default, unless -mgomp has been specified), use PTX "native" stacks, that is, generate code that uses .local memory or PTX alloca directly for stack storage. unless -mptx=7.3 or higher and -march=sm_52 or higher are active, variable-length arrays and dynamically allocating memory on the stack with alloca are not supported 😭
for -msoft-stack (implied by -mgomp), generate code that does not use .local memory or PTX alloca directly for stack storage. instead, a per-warp stack pointer is maintained explicitly. this enables variable-length stack allocation (with variable-length arrays or alloca), and when global memory is used for underlying storage, makes it possible to access automatic variables from other threads, or with atomic instructions. this code generation variant is used for OpenMP offloading, but the option is exposed on its own for the purpose of testing the compiler; to generate code suitable for linking into programs using OpenMP offloading, use option -mgomp 💖
-msoft-stack-reserve-local=size— sets the size of.localmemory used for the stack when the exact amount isn't known. defaults to128🍓-muniform-simt,-mno-uniform-simt— enable/disable a code generation variant that allows execution of 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 (0outside of SIMD regions). each thread can bitwise-and the bitmask at positiontid.ywith current lane index to compute the master lane index 🧠-mgomp,-mno-gomp— enable/disable generation of code for use in OpenMP offloading.-mgompenables-msoft-stackand-muniform-simtoptions, and selects a corresponding multilib variant 🫶
🦋 OpenRISC Options 🦋
these are defined for OpenRISC, slay:
-mboard=name— configures a board specific runtime. this gets passed to the linker for newlib board library linking. the default isor1ksim🎀-msoft-div,-mhard-div— select software or hardware divide (l.div,l.divu) instructions. the default is hardware divide ⚡-msoft-mul,-mhard-mul— select software or hardware multiply (l.mul,l.muli) instructions. the default is hardware multiply ✖️-msoft-float,-mhard-float— select software or hardware for floating point operations. the default is software 🌊-mdouble-float— when-mhard-floatis selected, enables generation of double-precision floating point instructions. by default functions fromlibgccare used to perform double-precision floating point operations 🎯-munordered-float— when-mhard-floatis selected, enables generation of unordered floating point compare and set flag (lf.sfun*) instructions. by default functions fromlibgccare used to perform unordered floating point compare and set flag operations 🌸-mcmov— enables generation of conditional move (l.cmov) instructions. by default the equivalents are generated using set and branch 🍀-mror— enables generation of rotate right (l.ror) instructions. by default functions fromlibgccare used to perform rotate right operations 🔄-mrori— enables generation of rotate right with immediate (l.rori) instructions. by default functions fromlibgccare used to perform rotate right with immediate operations 🔁-msext— enables generation of sign extension (l.ext*) instructions. by default memory loads are used to perform sign extension 💫-msfimm— enables generation of compare and set flag with immediate (l.sf*i) instructions. by default extra instructions are generated to store the immediate to a register first 🧩-mshftimm— enables generation of shift with immediate (l.srai,l.srli,l.slli) instructions. by default extra instructions are generated to store the immediate to a register first 🧚-mcmodel=small— generate OpenRISC code for the small model: the GOT is limited to64kand function call jumps are limited to64Moffsets. this is the default model 👑-mcmodel=large— generate OpenRISC code for the large model: the GOT may grow up to4Gin size and function call jumps can target the full4Gaddress space 🌈
💜 PDP-11 Options 💜
defined for the PDP-11, vintage icon behavior:
-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,-mno-ac0— with-mac0, return floating-point results inac0(fr0in Unix assembler syntax). the default,-mno-ac0, is to return floating-point results in memory 🧠-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— makeint16 bits, bestie. This is the default, so like, it's already the vibe. ✨-mint32,-mno-int16— makeinta chunky 32 bits instead. 💅-msplit— the target's got split instruction and data space (so code and data live in different neighborhoods, iykyk). Also lowkey implies-m45.-munix-asm— assembler syntax? Unix flavored, please. 🍓-mdec-asm— assembler syntax? DEC flavored. Vintage queen.-mgnu-asm— assembler syntax? GNU flavored. This is the default, no cap. 👑-mlra— use the new LRA register allocator. By default you get the oldreloadallocator instead, which is giving "fine but make it legacy."
🫶 Picolibc Options 🫶
Okay so these ones control how stuff gets compiled and linked when you're using Picolibc. Pay attention bestie:
--oslib=library— after the C library gets searched, search the library called library too. This lets symbols that the C library left undefined get defined by this one. The C library,libgcc, and this library all get sandwiched between--start-groupand--end-groupflags so each one can refer to symbols in the others (mutual support, we love to see it 🤝). For lots of targets, Picolibc ships a semihost variant (you spell it--oslib=semihost) that gives you enough basic OS functionality to support console and file I/O — both when running in an emulator and when you're using an in-circuit debugger.--crt0=[none|minimal|hosted|semihost]— swaps the defaultcrt0.oname out forcrt0-variant.o. Here's the tea on each variant:nonegives you literally zero startup code, which frees you up to supply your own (main character energy).minimaldoes basic memory setup but does not invoke any constructors.- When you don't pass any
-crt0option at all, the default initialization code adds calls to all constructors. hostedadds a call toexitoncemainreturns.semihostreaches into a command-line parameter supplied via the semihosting interface and splits it at whitespace boundaries, then passes the resulting array of strings tomainasargcandargv. On some targets — including AArch64, ARC, ARM, LoongArch, M680x0, RISC-V, SH, and x86 —semihostalso traps hardware exceptions and prints info to the console. ⚠️ Heads up:--crt0=semihostdepends on APIs provided by--oslib=semihost.
--printf=[d|f|l|i|m]— picks yourprintfvariant. Picolibc gives you five wholeprintfvariants, each with decreasing functionality and decreasing code size (the tradeoff is real 😭).d— the default level, offering full C17 and POSIX.1-2024 conformance. Slay.f— same feature set, but it supportsfloatvalues instead ofdouble, and those get passed using theprintf_floatmacro.l— elides all floating-point and POSIX positional parameter support.i— limits integers to ones no larger thanlong.m— removes support for most formatting options, including width and precision. The formats and arguments are still parsed correctly, but the output just... doesn't respect those parameters. 🙈
--scanf=[d|f|l|i|m]— same deal but forscanf. Five variants, decreasing functionality, decreasing code size:d— the default, full C17 and POSIX.1-2024 conformance.f— removes support fordoublevalues.l— elides all floating point support.i— limits integers to ones no larger thanlong.m— removes support for%[conversion specifiers.
🌈 PowerPC Options 🌈
These are listed under
(that's genuinely all it says, bestie — the cross-reference is doing its own thing)
💀 PRU Options 💀
These command-line options are defined for the PRU target:
-minrt— link with a minimum runtime environment. This can significantly shrink your final ELF binary, but heads up: some standard C runtime features get removed in the process.- This option disables support for static initializers and constructors. ⚠️ And beware — the compiler could still generate code with static initializers and constructors. It's on you, the programmer, to make sure your source program doesn't use those features. No cap, that's your job now.
- The minimal startup code doesn't pass
argcandargvarguments tomain, somainhas to be declared asint main (void). Which honestly is already the norm for most firmware projects, so like, you're probably fine. 💅
-mmcu=mcu— names the PRU hardware variant you're targeting. A spec file with a corresponding name gets loaded, which passes the memory region sizes through to the linker and defines hardware-specific C macros.- Newlib only provides the
simspec, intended for running regression tests using a simulator. Want specs for real hardware? Get them by installing the GnuPruMcu (https://github.com/dinuxbg/gnuprumcu/) package. 🫶
- Newlib only provides the
-mno-relax— makes GCC pass the--no-relaxcommand-line option to the linker instead of the--relaxoption.-mloop,-mno-loop— allow (or don't allow) GCC to use theLOOPinstruction. 🌀-mmul,-mno-mul— allow (or don't allow) GCC to use the PRU multiplier unit. ✖️-mfillzero,-mno-fillzero— allow (or don't allow) GCC to use theFILLandZEROinstructions.-mabi=variant— specify which ABI variant to output code for.-mabi=tiselects the unmodified TI ABI, while-mabi=gnuselects a GNU variant that copes more naturally with certain GCC assumptions. Here's exactly how they differ:
Function Pointer Size — TI ABI says function (code) pointers are 16-bit, whereas GNU supports only 32-bit data and code pointers.
Optional Return Value Pointer — return values larger than 64 bits get passed using a hidden pointer as the function's first argument. But TI ABI mandates that this pointer can be NULL in the case where the caller isn't using the returned value. GNU always passes and expects a valid return value pointer.
Size Of Struct Containing Bit-fields — TI ABI mandates that struct size is determined by the bit-field type if it contains any. GNU, on the other hand, allocates the smallest amount of bytes that would fit the bit-field.
For example, TI ABI reserves 4 bytes for this struct, whereas GNU reserves a single byte:
struct S { int i:1; };Access Size For Volatile Bit-fields — TI ABI mandates that volatile bit-fields are accessed using their type. In contrast, the GNU ABI uses the smallest integer type fitting the bit-field.
For example, TI ABI requires a single load of 4 bytes for the following bit-field. GNU generates a load of 1 byte:
struct S { volatile int i:1; };The current -mabi=ti implementation simply raises a compile error whenever any of the above code constructs is detected. As a consequence, the standard C library cannot be built, and it's omitted when you link with -mabi=ti. 😭
Also — relaxation is a GNU feature, and for safety reasons it's disabled when you use -mabi=ti. The TI toolchain doesn't emit relocations for QBBx instructions, so the GNU linker can't adjust them when shortening adjacent LDI32 pseudo instructions.
🦋 RISC-V Options 🦋
These command-line options are defined for RISC-V targets:
-mbranch-cost=n— set the cost of branches to roughly n instructions.-mabi=ABI-string— specify the integer and floating-point calling convention. The ABI-string has two parts: the size of integer types, and the registers used for floating-point types. For example,-march=rv64ifd -mabi=lp64dmeanslongand pointers are 64-bit (implicitly definingintto be 32-bit), and that floating-point values up to 64 bits wide get passed in F registers. Contrast that with-march=rv64ifd -mabi=lp64f, which still lets the compiler 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, where no floating-point arguments get passed in registers at all. 🤯
The default for this argument is system dependent — so if you want a specific calling convention, specify one explicitly, bestie. The valid calling conventions are: ilp32, ilp32f, ilp32d, lp64, lp64f, and lp64d. Some calling conventions are genuinely 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's also the ilp32e ABI, which can only be used with the rv32e architecture, and the lp64e ABI, which can only be used with rv64e. Those ABIs aren't well-specified at present and are subject to change — you've been warned.
-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 architecture you specified has these instructions. ✨-mfence-tso,-mno-fence-tso— do or don't use thefence.tsoinstruction, which is unimplemented on some processors (including those from T-Head 🙈). Iffence.tsoisn't available, then a stronger fence gets used instead.-mdiv,-mno-div— do or don't use hardware instructions for integer division. This requires the M extension. The default is to use them if the architecture you specified has these instructions.-misa-spec=ISA-spec-string— specify which version of the RISC-V Unprivileged (formerly User-Level) ISA specification your generated code should conform to. The possibilities for ISA-spec-string are:
2.2 — produce code conforming to version 2.2.
20190608 — produce code conforming to version 20190608.
20191213 — produce code conforming to version 20191213.
The default is -misa-spec=20191213 unless GCC has been configured with --with-isa-spec= specifying a different default version.
-march=[ISA|Profile|Profile_ISA|processor-string]— generate code for the given RISC-V ISA, profile, or a combination of them (e.g.rv64im rvi20u64 rvi20u64_zbb). Names of ISAs and profiles must be lower case, periodt. Examples includerv64i,rv32g,rv32e,rv32imaf,rva22u64andrva23u64. To combine a named profile with optional RISC-V ISA extensions, give the profile first and then append the extension name(s) using an underscore as a delimiter (e.g.rvi20u64_zca_zcb rva23u64_zacas). Plus, a special valuehelp(-march=help) is accepted to list all supported extensions. 🎀
-march=unset causes the compiler to ignore any -march=... options that appear earlier on the command line, behaving as if the option was never passed. This is useful for making sure the architecture comes from the -mcpu option — and an error results if there's no -mcpu option given when -march=unset is used.
The syntax of the ISA string is defined as follows:
rv32/rv64— the string must start with one of these, theni,e, org— that bit is called the base ISA.- After that comes the extension list, bestie. Extension names come in two flavors: multi-letter (like
zba) and single-letter (likev). Single-letter ones can be stacked right next to each other with no separator, but multi-letter ones have to be separated by underscores. - An underscore can go anywhere after the base ISA. It doesn't actually do anything — it's purely for readability, acting as a separator. Aesthetic underscore 💅
- Extension names can carry an optional version number, written as
<major>p<minor>or just<major>— so likem2p1orm2.
Here's the full roster of supported extensions, no cap, all of them:
g— version-— the general-purpose computing base extension;gexpands toi,m,a,f,d,zicsrandzifencei.e— version2.0— reduced base integer extension.i— versions2.0,2.1— base integer extension.m— version2.0— integer multiplication and division extension.a— versions2.0,2.1— atomic extension.f— versions2.0,2.2— single-precision floating-point extension.d— versions2.0,2.2— double-precision floating-point extension.c— version2.0— compressed extension.b— version1.0— standard extension for bit manipulation functions.v— version1.0— vector extension.h— version1.0— hypervisor extension.zic64b— version1.0— cache block size is 64 bytes.zicbom— version1.0— cache-block management extension.zicbop— version1.0— cache-block prefetch extension.zicboz— version1.0— cache-block zero extension.ziccamoa— version1.0— main memory supports all atomics inA.ziccif— version1.0— main memory supports instruction fetch with atomicity requirement.zicclsm— version1.0— main memory supports misaligned loads/stores.ziccrse— version1.0— main memory supports forward progress on LR/SC sequences.zicfilp— version1.0— control-flow integrity landing pad extension.zicfiss— version1.0— control-flow integrity shadow stack extension.zicntr— version2.0— standard extension for base counters and timers.zicond— version1.0— integer conditional operations extension.zicsr— version2.0— control and status register access extension.zifencei— version2.0— instruction-fetch fence extension.zihintntl— version1.0— non-temporal locality hints extension.zihintpause— version2.0— pause hint extension.zihpm— version2.0— standard extension for hardware performance counters.zimop— version1.0— may-be-operations extension.zilsd— version1.0— load/store pair instructions extension.zmmul— version1.0— integer multiplication extension.za128rs— version1.0— reservation set size of 128 bytes.za64rs— version1.0— reservation set size of 64 bytes.zaamo— version1.0— atomic memory operations extension.zabha— version1.0— byte and halfword atomic memory operations extension.zacas— version1.0— atomic compare-and-swap instructions extension.zalrsc— version1.0— load-reserved/store-conditional subset of theAextension.zawrs— version1.0— wait-on-reservation-set extension.zama16b— version1.0— misaligned loads, stores, and AMOs that are fully contained within a naturally-aligned 16-byte boundary are atomic.zfa— version1.0— additional floating-point extension.zfbfmin— version1.0— minimal BF16 support extension.zfh— version1.0— half-precision floating-point extension.zfhmin— version1.0— minimal half-precision floating-point extension.zfinx— version1.0— single-precision floating-point in integer registers extension.zdinx— version1.0— double-precision floating-point in integer registers extension.zca— version1.0— integer compressed instruction extension.zcb— version1.0— simple compressed instruction extension.zcd— version1.0— compressed double-precision floating point loads and stores extension.zce— version1.0— compressed instruction extensions for embedded processors.zcf— version1.0— compressed single-precision floating point loads and stores extension.zcmop— version1.0— compressed may-be-operations extension.zcmp— version1.0— compressed push pop extension.zcmt— version1.0— table jump instruction extension.zclsd— version1.0— compressed load/store pair instructions extension.zba— version1.0— address calculation extension.zbb— version1.0— basic bit manipulation extension.zbc— version1.0— carry-less multiplication extension.zbkb— version1.0— cryptography bit-manipulation extension.zbkc— version1.0— cryptography carry-less multiply extension.zbkx— version1.0— cryptography crossbar permutation extension.zbs— version1.0— single-bit operation extension.zk— version1.0— standard scalar cryptography extension.zkn— version1.0— NIST algorithm suite extension.zknd— version1.0— AES Decryption extension.zkne— version1.0— AES Encryption extension.zknh— version1.0— hash function extension.zkr— version1.0— entropy source extension.zks— version1.0— ShangMi algorithm suite extension.zksed— version1.0— SM4 block cipher extension.zksh— version1.0— SM3 hash function extension.zkt— version1.0— data independent execution latency extension.ztso— version1.0— total store ordering extension.zvbb— version1.0— vector basic bit-manipulation extension.zvbc— version1.0— vector carry-less multiplication extension.zve32f— version1.0— vector extensions for embedded processors.zve32x— version1.0— vector extensions for embedded processors.zve64d— version1.0— vector extensions for embedded processors.zve64f— version1.0— vector extensions for embedded processors.zve64x— version1.0— vector extensions for embedded processors.zvfbfmin— version1.0— vector BF16 converts extension.zvfbfwma— version1.0— vector BF16 widening multiply/add extension.zvfh— version1.0— vector half-precision floating-point extension.zvfhmin— version1.0— vector minimal half-precision floating-point extension.zvkb— version1.0— vector cryptography bit-manipulation extension.zvkg— version1.0— vector GCM/GMAC extension.zvkn— version1.0— vector NIST Algorithm Suite extension,zvknwill expand to.zvknc— version1.0— vector NIST Algorithm Suite with carry-less multiply extension,zvknc.zvkned— version1.0— vector AES block cipher extension.zvkng— version1.0— vector NIST Algorithm Suite with GCM extension,zvkngwill expand.zvknha— version1.0— vector SHA-2 secure hash extension.zvknhb— version1.0— vector SHA-2 secure hash extension.zvks— version1.0— vector ShangMi algorithm suite extension,zvkswill expand.zvksc— version1.0— vector ShangMi algorithm suite with carry-less multiplication extension.zvksed— version1.0— vector SM4 block cipher extension.zvksg— version1.0— vector ShangMi algorithm suite with GCM extension.zvksh— version1.0— vector SM3 secure hash extension.zvkt— version1.0— vector data independent execution latency extension.zvl1024b— version1.0— minimum vector length standard extensions.zvl128b— version1.0— minimum vector length standard extensions.zvl16384b— version1.0— minimum vector length standard extension.zvl2048b— version1.0— minimum vector length standard extensions.zvl256b— version1.0— minimum vector length standard extensions.zvl32768b— version1.0— minimum vector length standard extension.zvl32b— version1.0— minimum vector length standard extensions.zvl4096b— version1.0— minimum vector length standard extensions.zvl512b— version1.0— minimum vector length standard extensions.zvl64b— version1.0— minimum vector length standard extensions.zvl65536b— version1.0— minimum vector length standard extension.zvl8192b— version1.0— minimum vector length standard extension.zhinx— version1.0— half-precision floating-point in integer registers extension.zhinxmin— version1.0— minimal half-precision floating-point in integer registers extension.sdtrig— version1.0— debug triggers extension.sha— version1.0— the augmented hypervisor extension.shcounterenw— version1.0— support writeable enables for any supported counter.shgatpa— version1.0—SvNNx4mode supported for all modes supported bysatp.shlcofideleg— version1.0— delegating LCOFI interrupts to VS-mode.shtvala— version1.0— thehtvalregister provides all needed values.shvstvala— version1.0— thevstvalregister provides all needed values.shvstvecd— version1.0— thevstvecregister supports direct mode.shvsatpa— version1.0— thevsatpregister supports all modes supported bysatp.smaia— version1.0— advanced interrupt architecture extension.smcntrpmf— version1.0— cycle and instret privilege mode filtering.smcsrind— version1.0— machine-level indirect CSR access.smepmp— version 1.0, and it's giving PMP enhancements for memory access and execution prevention on machine mode ✨smmpm— version 1.0, Supervisor-mode pointer masking extensionsmnpm— version 1.0, Supervisor-mode pointer masking extension (yep, same description as the one above, not a typo, they're siblings 💅)smrnmi— version 1.0, Resumable non-maskable interruptssmstateen— version 1.0, State enable extensionsmdbltrp— version 1.0, Double trap extensionsssaia— version 1.0, Advanced interrupt architecture extension for supervisor modessccptr— version 1.0, Main memory supports page table readssscofpmf— version 1.0, Count overflow and filtering extensionsscounterenw— version 1.0, Support writeable enables for any supported countersscsrind— version 1.0, Supervisor-mode indirect CSR accessssnpm— version 1.0, Supervisor-mode pointer masking extensionsspm— version 1.0, Supervisor-mode pointer masking extensionssstateen— version 1.0, Supervisor-mode state-enable extensionsstc— version 1.0, Supervisor-mode timer interrupts extensionsstvala— version 1.0, Stval provides all needed valuessstvecd— version 1.0, Stvec supports direct modessstrict— version 1.0, Unimplemented reserved encodings raise illegal instruction exceptions and no non-conforming extensions are present — strict queen behavior fr fr 👑ssdbltrp— version 1.0, Double trap extensionsssu64xl— version 1.0, UXLEN=64 must be supportedsupm— version 1.0, User-mode pointer masking extensionsvinval— version 1.0, Fine-grained address-translation cache invalidation extensionsvnapot— version 1.0, NAPOT translation contiguity extensionsvpbmt— version 1.0, Page-based memory types extensionsvvptc— version 1.0, Extension for obviating memory-management instructions after marking PTEs validsvadu— version 1.0, Hardware updating of A/D bits extensionsvade— version 1.0, Cause exception when hardware updating of A/D bits is disabledsvbare— version 1.0, Satp mode bare is supportedxcvalu— version 1.0, Core-V miscellaneous ALU extensionxcvbi— version 1.0, Core-V immediate branch extensionxcvelw— version 1.0, Core-V event load word extensionxcvmac— version 1.0, Core-V multiply-accumulate extensionxcvsimd— version 1.0, Core-V SIMD extensionxsfcease— version 1.0, SiFive CEASE instruction extensionxsfvcp— version 1.0, SiFive VCIX vector coprocessor extensionxsfvfnrclipxfqf— version 1.0, SiFive FP32-to-int8 ranged clip instructions (say that name three times fast, i dare you 😭)xsfvqmaccdod— version 1.0, SiFive int8 matrix multiplication extensionxsfvqmaccqoq— version 1.0, SiFive int8 matrix multiplication extensionxtheadba— version 1.0, T-head address calculation extensionxtheadbb— version 1.0, T-head basic bit-manipulation extensionxtheadbs— version 1.0, T-head single-bit instructions extensionxtheadcmo— version 1.0, T-head cache management operations extensionxtheadcondmov— version 1.0, T-head conditional move extensionxtheadfmemidx— version 1.0, T-head indexed memory operations for floating-point registers extensionxtheadfmv— version 1.0, T-head double floating-point high-bit data transmission extensionxtheadint— version 1.0, T-head acceleration interruption extensionxtheadmac— version 1.0, T-head multiply-accumulate extensionxtheadmemidx— version 1.0, T-head indexed memory operation extensionxtheadmempair— version 1.0, T-head two-GPR memory operation extensionxtheadsync— version 1.0, T-head multi-core synchronization extensionxtheadvector— version 1.0, T-head vector extensionxventanacondops— version 1.0, Ventana integer conditional operations extensionxmipscmov— version 1.0, Mips conditional move extensionxmipscbop— version 1.0, Mips prefetch extensionxandesperf— version 5.0, Andes performance extensionxandesbfhcvt— version 5.0, Andes bfloat16 conversion extensionxandesvbfhcvt— version 5.0, Andes vector bfloat16 conversion extensionxandesvsintload— version 5.0, Andes vector INT4 load extensionxandesvpackfph— version 5.0, Andes vector packed FP16 extensionxandesvdot— version 5.0, Andes vector dot product extensionxsmtvdot— version 1.0, SpacemiT vector dot product extension
Okay so, when -march= is not specified, GCC uses the setting from -mcpu. Bestie behavior 🫶
And if both -march and -mcpu= are not specified, the default for this argument is system dependent — so if you want a specific architecture extension, you should specify one explicitly. No guessing games, just say what you want 💅
Also! When the RISC-V specifications define an extension as depending on other extensions, GCC implicitly adds the dependent extensions to the enabled extension set if they weren't added explicitly. So it's got your back, lowkey ✨
👑 Core Name
-mcpu=processor-string— Use architecture of and optimize the output for the given processor, specified by particular CPU name. Permissible values for this option are:
sifive-e20,
sifive-e21,
sifive-e24,
sifive-e31,
sifive-e34,
sifive-e76,
sifive-s21,
sifive-s51,
sifive-s54,
sifive-s76,
sifive-u54,
sifive-u74,
sifive-x280,
sifive-p450,
sifive-p670,
thead-c906,
xt-c908,
xt-c908v,
xt-c910,
xt-c910v2,
xt-c920,
xt-c920v2,
tt-ascalon-x,
tt-ascalon-d8,
xiangshan-nanhu,
xiangshan-kunminghu,
mips-p8700,
andes-n22,
andes-n25,
andes-a25,
andes-nx25,
andes-ax25,
andes-a27,
andes-ax27,
andes-n225,
andes-d23,
andes-n45,
andes-nx45,
andes-a45,
andes-ax45,
spacemit-x60.Heads up bestie: note that-mcpudoes not override-marchor-mtune. It's not that kind of option 🙈
🎀 Tune Name
-mtune=processor-string— Optimize the output for the given processor, specified by microarchitecture or particular CPU name. Permissible values for this option are:
generic,
rocket,
sifive-3-series,
sifive-5-series,
sifive-7-series,
sifive-p400-series,
sifive-p600-series,
tt-ascalon-x,
tt-ascalon-d8,
thead-c906,
xt-c908,
xt-c908v,
xt-c910,
xt-c910v2,
xt-c920,
xt-c920v2,
xiangshan-nanhu,
xiangshan-kunminghu,
spacemit-x60,
generic-ooo,
size,
mips-p8700,
andes-25-series,
andes-23-series,
andes-45-series,
and all valid options for -mcpu=.When -mtune= is not specified, GCC uses the setting from -mcpu. The default is generic if neither is specified. Simple as that 💖
The size choice is not intended for use by end-users. This is used when -Os is specified. It overrides the instruction cost info provided by -mtune=, but does not override the pipeline info. This helps reduce code size while still giving good performance. Iconic tradeoff behavior, honestly ⭐
-mpreferred-stack-boundary=num— Attempt to keep the stack boundary aligned to a 2 raised to num byte boundary. If-mpreferred-stack-boundaryis not specified, the default is4(16 bytes or 128-bits).
⚠️ Warning, and I mean it: 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, no vibes-based builds, ok? 😭
-msmall-data-limit=n— Put global and static data smaller than n bytes into a special section (on some targets). Tiny data gets its own little section, cute 🎀-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. Slow and small vs. fast and inline — pick your fighter 💅-mmovcc,-mno-movcc— Do or don't produce branchless conditional-move code sequences even with targets that do not have specific instructions for conditional operations. If enabled, sequences of ALU operations are produced using base integer ISA instructions where profitable. No branches, no drama 🌸-minline-atomics,-mno-inline-atomics— Do or don't use smaller but slower subword atomic emulation code that useslibatomicfunction calls. The default is to use fast inline subword atomics that do not requirelibatomic. Fewer library calls = main character energy ✨-minline-strlen,-mno-inline-strlen— Do or do not attempt to inlinestrlencalls if possible. Inlining can only be done if the string is properly aligned and instructions for accelerated processing are available. The default is to inlinestrlencalls. Aligned strings only, bestie 🦋-minline-strcmp,-mno-inline-strcmp— Do or do not attempt to inlinestrcmpcalls if possible. Inlining can only be done if the strings are properly aligned and instructions for accelerated processing are available. The default is to inlinestrcmpcalls.-minline-strncmp,-mno-inline-strncmp— Do or do not attempt to inlinestrncmpcalls if possible. Inlining can only be done if the strings are properly aligned and instructions for accelerated processing are available. The default is to inlinestrncmpcalls. The whole string-inlining trio, slaying in order 💜-mstringop-strategy=strategy— Specify a particular strategy for inlining string and memory operations. strategy may be one ofauto,libcall,scalar, orvector. Four choices, zero confusion 🎀-mshorten-memrefs,-mno-shorten-memrefs— Do or do not attempt to make more use of compressed load/store instructions by replacing a load/store ofbase register + large offsetwith a new load/store ofnew base + small offset. If the new base gets stored in a compressed register, then the new load/store can be compressed. Currently targets 32-bit integer load/stores only. Compression hack, but make it legal 🔥-mstrict-align,-mno-strict-align— do not / do generate unaligned memory accesses, bestie. The default just depends on whether the processor you're optimizing for has fast unaligned access or not. That's it, that's the vibe check 🌸-mscalar-strict-align,-mno-scalar-strict-align— same deal: do not / do generate unaligned memory accesses, default set by whether the target processor supports fast unaligned access. Lowkey it's just an alias for-mstrict-align, so don't overthink it 💅-mvector-strict-align,-mno-vector-strict-align— do not / do generate unaligned vector memory accesses. Default is off unless the processor you're optimizing for explicitly supports element-misaligned vector memory access ✨-mmax-vectorization,-mno-max-vectorization— turn on/off an override to the vectorizer cost model that makes vectorization always appear profitable. Important: unlike-fno-vect-cost-modelor-fvect-cost-model=unlimited, this does not turn off cost comparison between different vector modes. She's different, okay 😌-mcmodel=medlow— generate code for the medium-low code model. The program and its statically defined symbols must lie within a single 2 GiB address range, and must lie between absolute addresses -2 GiB and +2 GiB. Programs can be statically or dynamically linked. This is the default code model unless GCC was configured with--with-cmodel=specifying a different default. Periodt 🎀-mcmodel=medany— generate code for the medium-any code model. The program and its statically defined symbols must be within any single 2 GiB address range. Programs can be statically or dynamically linked.
Heads up bestie: the code generated by the medium-any code model is position-independent, but it's not guaranteed to function correctly when linked into position-independent executables or libraries. Read that twice 😭
-mcmodel=large— generate code for a large code model, which has no restrictions on size or placement of symbols. Main character energy, honestly 👑-mexplicit-relocs,-mno-explicit-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. Choose your fighter 🦋-mrelax,-mno-relax— take advantage of linker relaxations to reduce the number of instructions required to materialize symbol addresses, or don't. Default is to take advantage of linker relaxations, we love an efficient queen ✨-mriscv-attribute,-mno-riscv-attribute— emit (or don't emit) the RISC-V attribute to record extra information into ELF objects. FYI this feature requires at least binutils 2.32, so check your toolchain first 🧚-mcsr-check,-mno-csr-check— enables or disables CSR checking. That's the whole thing 💅-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. Free real estate 🔥-malign-data=type— control how GCC aligns variables and constants of array, structure, or union types. Supported values for type:xlen— uses x register width as the alignment valuenatural— uses natural alignment
xlen is the default, no cap 🫶
-mbig-endian— generate big-endian code. This is the default when GCC is configured for ariscv64be-*-*orriscv32be-*-*target.
Okay so this one's a little experimental 🚨: support for RISC-V big-endian is experimental. The ABI is not yet stable and could change in incompatible ways in future releases. You've been warned, bestie 💀
-mlittle-endian— generate little-endian code. This is the default when GCC is configured for ariscv64-*-*orriscv32-*-*but not ariscv64be-*-*orriscv32be-*-*target 🌈-mstack-protector-guard=guard,-mstack-protector-guard-reg=reg,-mstack-protector-guard-offset=offset— generate stack protection code using a canary at guard. Supported locations areglobalfor a global canary, ortlsfor a per-thread canary in the TLS block 🎀
With the latter choice (tls), 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.
Ngl there is no default register or offset, because this is entirely for use within the Linux kernel. Kernel business only 🐧
-mtls-dialect=desc— use TLS descriptors as the thread-local storage mechanism for dynamic accesses of TLS variables.-mtls-dialect=trad— use traditional TLS as the thread-local storage mechanism for dynamic accesses of TLS variables. This is the default, the classic, the icon 💅-mrvv-vector-bits=value— specify how the number of bits for an RVV vector register, as taken from the-march=option, is interpreted. The value parameter is specified as a string keyword, and may be one ofscalableorzvl. The default isscalable, which tells GCC to interpret the number as a minimum, whilezvltells GCC to use exactly the number of bits specified. Pick your interpretation bestie ✨-mrvv-max-lmul=value— this option allows explicit control over the maximum length multiplier (LMUL) used when generating code for the RISC-V Vector Extensions (RVV). The value parameter is specified as a string keyword and may be one ofm1,m2,m4,m8, ordynamic. The default ism1for compatibility with existing hardware that does not support the other options. Respect the elders 🫶-madjust-lmul-cost,-mno-adjust-lmul-cost— this option adjusts the cost model used to schedule vector instructions to multiply the latency of instructions by the RVV length multiplier, LMUL. It is disabled by default. Off by default, so don't come for me 😌-mautovec-segment,-mno-autovec-segment— enable or disable generation of vector segment load/store instructions. This option is enabled by default 🦋
🎀 RL78 Options 🎀
-msim— links in additional target libraries to support operation within a simulator. Simulator vibes only 🧚-mmul=none,-mmul=g10,-mmul=g13,-mmul=g14,-mmul=rl78— specifies the type of hardware multiplication and division support to be used.noneis the simplest — it uses software for both multiplication and division. This is the default.g13is for the hardware multiply/divide peripheral found on the RL78/G13 (S2 core) targets.g14selects the use of the multiplication and division instructions supported by the RL78/G14 (S3 core) parts.rl78is an alias forg14, andmg10is an alias fornone.
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 👑
Okay this next bit is important so pay attention bestie 😭: although the RL78/G14 core is the default target, specifying-mcpu=g14or-mcpu=rl78on 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 are used even though the code targets the RL78 core. This is for backward 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're provided for backwards compatibility, bless 💅-mallregs— allow the compiler to use all of the available registers. By default, registersr24..r31are reserved for use in interrupt handlers. With this option enabled, these registers can be used in ordinary functions as well. Sharing is caring 🫶-mrelax,-mno-relax— enable/disable assembler and linker relaxation. This is enabled by default at-Os✨-mes0— assumeESis zero throughout program execution, and use it to address read-only data. Manifesting zero energy 🕯️-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, since this feature is only available on the G13 target (S2 core). The MDUC registers are only saved if the interrupt handler performs a multiplication or division operation or it calls another function. So many conditions bestie 😭
👑 IBM RS/6000 and PowerPC Options 👑
These -m options are defined for the IBM RS/6000 and PowerPC, bestie. Buckle up 🎢
-mcpu=cpu_type— set architecture type, register usage, and instruction scheduling parameters for machine type cpu_type. Supported values for cpu_type are:
401, 403, 405, 405fp, 440, 440fp, 464, 464fp, 476, 476fp, 505, 601, 602, 603, 603e, 604, 604e, 620, 630, 740, 7400, 7450, 750, 801, 821, 823, 860, 970, 8540, a2, e300c2, e300c3, e500mc, e500mc64, e5500, e6500, ec603e, G3, G4, G5, titan, power3, power4, power5, power5+, power6, power6x, power7, power8, power9, power10, power11, future, powerpc, powerpc64, powerpc64le, rs64, and native.
-mcpu=powerpc, -mcpu=powerpc64, and -mcpu=powerpc64le specify pure 32-bit PowerPC (either endian), 64-bit big endian PowerPC and 64-bit little endian PowerPC architecture machine types, with an appropriate, generic processor model assumed for scheduling purposes.
Specifying native as cpu type detects and selects the architecture option that corresponds to the host processor of the system performing the compilation. FYI: -mcpu=native has no effect if GCC does not recognize the processor 🙈
-mcpu=cpu_type(the other options, the ones that name a specific processor) — code generated with those runs best on that processor and might not run at all on others 😭
The -mcpu options auto-enable or auto-disable these ones for you:
-maltivec -mfprnd -mhard-float -mmfcrf -mmultiple -mpopcntb
-mpopcntd -mpowerpc64 -mpowerpc-gpopt -mpowerpc-gfxopt -mmulhw
-mdlmzb -mmfpgpr -mvsx -mcrypto -mhtm -mpower8-fusion
-mquad-memory -mquad-memory-atomic -mfloat128 -mfloat128-hardware
-mprefixed -mpcrel -mmma -mdense-math -mrop-protectOkay so the exact set of options any given CPU flips on varies between compiler versions — it's whatever setting seems to produce optimal code for that CPU, and it does not necessarily reflect what the actual hardware can do. iykyk.
Want to hand-set one of them? Just slap it after the -mcpu option, like -mcpu=970 -mno-altivec. Slay.
On AIX, -maltivec and -mpowerpc64 are not enabled or disabled by -mcpu at present, because AIX doesn't have full support for these options. You can still flip them individually if you're sure it'll work in your environment 🫶.
In other cases, the manpage's official recommendation: use the -mcpu=cpu_type option rather than the options that control generation of specific instructions. Periodt.
-mtune=cpu_type— sets the instruction scheduling parameters for machine type cpu_type, but does not set the architecture type or register usage like-mcpu=cpu_typedoes. Same values for cpu_type as-mcpu. If you specify both, the code generated uses the architecture and registers set by-mcpu, but the scheduling parameters set by-mtune💅-mpowerpc64,-mno-powerpc64— this one lets GCC generate the additional 64-bit instructions found in the full PowerPC64 architecture, and treat GPRs as 64-bit, doubleword quantities. GCC defaults to-mno-powerpc64. Ngl, default is the no-fun one.-mcmodel=small— generate PowerPC64 code for the small model: the TOC is limited to 64k 🍓-mcmodel=medium— generate PowerPC64 code for the medium model: the TOC and other static data may be up to a total of 4G in size. This is the default for 64-bit Linux ✨-mcmodel=large— generate PowerPC64 code for the large model: the TOC may be up to 4G in size. Other data and code is only limited by the 64-bit address space 🔥-mprofile-kernel— available on PowerPC64 GNU/Linux targets. When used with-pg, it causes calls tomcountto be inserted before the function prologue instead of after it. Little order-of-operations moment 💅-maltivec,-mno-altivec— generate code that uses (does not use) AltiVec instructions, and also enable the use of built-in functions that allow more direct access to the AltiVec instruction set. You may also need to set-mabi=altivecto adjust the current ABI with AltiVec ABI enhancements 👑
When -maltivec is used, the element order for AltiVec intrinsics such as vec_splat, vec_extract, and vec_insert matches array element order corresponding to the endianness of the target. That is: element zero identifies the leftmost element in a vector register when targeting a big-endian platform, and identifies the rightmost element in a vector register when targeting a little-endian platform. The way endianness just flips your whole mental model 😭
-mpowerpc-gpopt,-mno-powerpc-gpopt— specifying-mpowerpc-gpoptallows GCC to use the optional PowerPC architecture instructions in the General Purpose group, including floating-point square root 🧚-mpowerpc-gfxopt,-mno-powerpc-gfxopt—-mpowerpc-gfxoptallows GCC to use the optional PowerPC architecture instructions in the Graphics group, including floating-point select 🌈-mmfcrf,-mno-mfcrf— 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 ⭐-mpopcntb,-mno-popcntb— 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 💜-mpopcntd,-mno-popcntd— allows GCC to generate the popcount instruction implemented on the POWER7 processor and other processors that support the PowerPC V2.06 architecture 🦋-mfprnd,-mno-fprnd— 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 🎀-mcmpb,-mno-cmpb— allows GCC to generate the compare bytes instruction implemented on the POWER6 processor and other processors that support the PowerPC V2.05 architecture 🌸-mhard-dfp,-mno-hard-dfp— allows GCC to generate the decimal floating-point instructions implemented on some POWER processors 🔥-mvrsave,-mno-vrsave— generate VRSAVE instructions when generating AltiVec code 💅-msecure-plt— generate code that allowsldandld.soto build executables and shared libraries with non-executable.pltand.gotsections. This is a PowerPC 32-bit SYSV ABI option 👑-mbss-plt— generate code that uses a BSS.pltsection thatld.sofills in, and requires.pltand.gotsections that are both writable and executable. This is a PowerPC 32-bit SYSV ABI option. (Writable and executable, bestie — that's a whole vibe check 🫶)-msplit-patch-nops— when adding NOPs for a patchable area via the-fpatchable-function-entryoption, emit the "before" NOPs in front of the global entry point and the "after" NOPs after the local entry point. This makes the sequence of NOPs not consecutive when a global entry point is generated. Without this option the NOPs are emitted directly before and after the local entry point, making them consecutive but moving global and local entry point further apart. If only a single entry point is generated this option has no effect 🌈-misel,-mno-isel— this switch enables or disables the generation of ISEL instructions 💜-mvsx,-mno-vsx— generate code that uses (does not use) vector/scalar (VSX) instructions, and also enable the use of built-in functions that allow more direct access to the VSX instruction set ✨-mcrypto,-mno-crypto— enable the use (disable) of the built-in functions that allow direct access to the cryptographic instructions that were added in version 2.07 of the PowerPC ISA 🔐 (well, 🔐 in spirit — emoji not in the original but the vibe is real)-mhtm,-mno-htm— enable (disable) the use of the built-in functions that allow direct access to the Hardware Transactional Memory (HTM) instructions that were added in version 2.07 of the PowerPC ISA 🧚-mpower8-fusion,-mno-power8-fusion— generate code that keeps (does not keeps) some integer operations adjacent so that the instructions can be fused together on power8 and later processors 😭 (yes, "does not keeps" is verbatim, I don't make the rules)-mquad-memory,-mno-quad-memory— generate code that uses (does not use) the non-atomic quad word memory instructions. The-mquad-memoryoption requires use of 64-bit mode 🫶-mquad-memory-atomic,-mno-quad-memory-atomic— generate code that uses (does not use) the atomic quad word memory instructions. The-mquad-memory-atomicoption requires use of 64-bit mode ⭐-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) must be enabled to use the IEEE 128-bit floating point support. The IEEE 128-bit floating point is only supported on Linux.
The default for -mfloat128 is enabled on PowerPC Linux systems using the VSX instruction set, and disabled on other systems.
If you use the ISA 3.0 instruction set (-mcpu=power9) on a 64-bit system, the IEEE 128-bit floating point support also enables the generation of ISA 3.0 IEEE 128-bit floating point instructions 🔥 Otherwise, if you don't specify generation of ISA 3.0 instructions or you're targeting a 32-bit big-endian system, IEEE 128-bit floating point is handled 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 (including GNU/Linux). The 32-bit environment setsint,longand pointer to 32 bits and generates code that runs on any PowerPC variant. The 64-bit environment setsintto 32 bits andlongand pointer to 64 bits, and generates code for PowerPC64, as for-mpowerpc64👑-mfull-toc,-mno-fp-in-toc,-mno-sum-in-toc,-mminimal-toc— modify generation of the TOC (Table Of Contents), which is created for every executable file 🦋
-mfull-toc is selected by default. In that case, GCC allocates at least one TOC entry for each unique non-automatic variable reference in your program. GCC also places floating-point constants in the TOC. However, only 16,384 entries are available in the TOC. 💀
If you get a linker error message saying you've overflowed the available TOC space, you can reduce the amount of TOC space used with the -mno-fp-in-toc and -mno-sum-in-toc options. -mno-fp-in-toc prevents GCC from putting floating-point constants in the TOC, and -mno-sum-in-toc forces GCC to generate code to calculate the sum of an address and a constant at run time instead of putting that sum into the TOC. You may specify one or both of these options. Each causes GCC to produce very slightly slower and larger code at the expense of conserving TOC space 😭
If you still run out of space in the TOC even when you specify both of these options, specify -mminimal-toc instead. This option causes GCC to make only one TOC entry for every file. When you specify this option, GCC produces code that is slower and larger but which uses extremely little TOC space. You may wish to use this option only on files that contain less frequently-executed code 🫶
-maix64,-maix32— turns on the 64-bit AIX ABI + calling convention: 64-bit pointers, 64-bitlong, and all the scaffolding that has to exist to make those work. Saying-maix64implies-mpowerpc64, and-maix32kills the 64-bit ABI and implies-mno-powerpc64. GCC's default?-maix32. slay.-mxl-compat,-mno-xl-compat— makes your code act more like IBM XL compiler semantics when you're using the AIX-compatible ABI. Specifically it passes floating-point arguments to prototyped functions beyond the register save area (RSA) on the stack in addition to the argument FPRs. It also refuses to assume that the most significant double in a 128-bitlong doublevalue is properly rounded when comparing values and converting todouble. Oh, and it uses XL symbol names forlong doublesupport routines.
okay so the lore: the AIX calling convention got extended at some point but nobody documented it initially, and the reason was this super 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. But constantly shoving floating-point args onto the stack is inefficient and like, barely ever needed, so this option is not enabled by default, and it's only necessary when you're calling subroutines compiled by IBM XL compilers without optimization. iykyk 🫶
-mpe— supports the IBM RS/6000 SP Parallel Environment (PE). Links an application written for message passing with special startup code so the app can actually run. The system has to have PE installed in the standard location (/usr/lpp/ppe.poe/), or else you override the specs file with the-specs=option to point at wherever it actually lives. And bestie, the Parallel Environment does not support threads, so-mpeand-pthreadare incompatible. pick a lane 💅-malign-natural,-malign-power— on AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux,-malign-naturaloverrides the ABI-defined alignment of bigger types (floating-point doubles, we're looking at you) onto their natural size-based boundary.-malign-powertells GCC to follow the ABI-specified alignment rules like a good little compiler. 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. soz ✨
-msoft-float,-mhard-float— generates code that does not use (or does use) the floating-point register set. Software floating-point emulation is provided if you use-msoft-float, and you have to pass the option to GCC when linking too — don't forget the link step 🙈-mmultiple,-mno-multiple— generates code that uses (or doesn't use) the load multiple word instructions and the store multiple word instructions. These are generated by default on POWER systems, and not generated on PowerPC systems. Do not use-mmultipleon little-endian PowerPC systems, because those instructions don't work when the processor is in little-endian mode. The exceptions are PPC740 and PPC750, which permit these instructions in little-endian mode. special little guys 💖-mupdate,-mno-update— generates 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 are generated by default. Fair warning though: if you use-mno-update, there's a small window between the moment the stack pointer is updated and the moment the address of the previous frame is stored — which means code that walks the stack frame across interrupts or signals may get corrupted data. 😭 that's not a vibe.-mavoid-indexed-addresses,-mno-avoid-indexed-addresses— generates code that tries to avoid (or not avoid) the use of 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. Enabled by default when targeting Power6, disabled otherwise. know your target 👑-mfused-madd,-mno-fused-madd— generates code that uses (or doesn't use) the floating-point multiply and accumulate instructions. These are generated by default if hardware floating point is used. The machine-dependent-mfused-maddoption is now mapped to the machine-independent-ffp-contract=fastoption, and-mno-fused-maddis mapped to-ffp-contract=off. glow-up ✨-mmulhw,-mno-mulhw— generates code that uses (or doesn't use) the half-word multiply and multiply-accumulate instructions on the IBM 405, 440, 464 and 476 processors. Generated by default when targeting those processors.-mdlmzb,-mno-dlmzb— generates code that uses (or 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, does not (or does) force structures and unions that contain bit-fields to be aligned to the base type of the bit-field.
Example time 🎀 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, that structure is aligned to a 1-byte boundary and is 1 byte in size. it's giving shrinkage, but make it intentional.
-mbit-word,-mno-bit-word— allows bit-fields to cross word boundaries.-mno-strict-align,-mstrict-align— on System V.4 and embedded PowerPC systems, does not (or does) assume that unaligned memory references are handled by the system.-mrelocatable,-mno-relocatable— generates code that allows (or 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.got2plus the 4-byte locations listed in the.fixupsection — that's a table of 32-bit addresses generated by this option. For any of 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 so static executables can be relocated at run time, but-mrelocatable-libdoesn't use the smaller stack alignment of-mrelocatable. The nice part? Objects compiled with-mrelocatable-libmay be linked with objects compiled with any combination of the-mrelocatableoptions. mix and match bestie 🧚-mlittle,-mlittle-endian— on System V.4 and embedded PowerPC systems, compiles 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, compiles code for the processor in big-endian mode. The-mbig-endianoption is the same as-mbig.-mdynamic-no-pic— on Darwin / macOS systems, compiles code so it's not relocatable, but its external references are relocatable. The resulting code is suitable for applications, but not shared libraries. read the room 😌-msingle-pic-base— treats the register used for PIC addressing as read-only, instead of loading it in the prologue for each function. The runtime system is responsible for initializing this register with an appropriate value before execution begins. that's on the runtime, not on you 🫡-mprioritize-restricted-insns=priority — controls the priority assigned to dispatch-slot restricted instructions during the second scheduling pass. The argument priority takes the value0,1, or2, to assign no, highest, or second-highest (respectively) priority to dispatch-slot restricted instructions.-msched-costly-dep=dependence_type — controls which dependences are considered costly by the target during instruction scheduling. The argument dependence_type takes one of these 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 — controls which NOP insertion scheme is used during the second scheduling pass. The argument scheme takes one of these 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. Inserts 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. Inserts number NOPs to force an insn to a new group.
-mcall-sysv— on System V.4 and embedded PowerPC systems, compiles 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— specifies both-mcall-sysvand-meabi.-mcall-sysv-noeabi— specifies both-mcall-sysvand-mno-eabi.-mcall-aixdesc— on System V.4 and embedded PowerPC systems, compiles code for the AIX operating system.-mcall-linux— on System V.4 and embedded PowerPC systems, compiles code for the Linux-based GNU system.-mcall-freebsd— on System V.4 and embedded PowerPC systems, compiles code for the FreeBSD operating system.-mcall-netbsd— on System V.4 and embedded PowerPC systems, compiles code for the NetBSD operating system.-mcall-openbsd— on System V.4 and embedded PowerPC systems, compiles code for the OpenBSD operating system. collecting OSes like pokémon at this point 🌈-mtraceback=traceback_type — selects the type of traceback table. Valid values for traceback_type arefull,part, andno.-maix-struct-return— returns all structures in memory (as specified by the AIX ABI).-msvr4-struct-return— returns structures smaller than 8 bytes in registers (as specified by the SVR4 ABI). tiny structs go fast 💨-mabi=abi-type— bestie this one extends the current ABI with a specific extension, or yeets one out. The valid values are:altivec,no-altivec,ibmlongdouble,ieeelongdouble,elfv1,elfv2, and for AIX specifically:vec-extabi,vec-default. That's the whole menu, no substitutions 💅-mabi=ibmlongdouble— switches the current ABI to use IBM extended-precisionlong double. Heads up though: this is not likely to work if your system defaults to IEEE extended-precisionlong double😭. And if you change thelong doubletype away from IEEE extended-precision, the compiler will throw a warning at you unless you pass-Wno-psabi. Also it requires-mlong-double-128to be enabled. So many conditions, but okay.-mabi=ieeelongdouble— the mirror image, bestie: changes the current ABI to use IEEE extended-precisionlong double. Same vibe check as above — not likely to work if your system defaults to IBM extended-precisionlong double. If you switch thelong doubletype away from IBM extended-precision, the compiler issues a warning unless you use-Wno-psabi. Requires-mlong-double-128to be enabled. ✨-mabi=elfv1— changes the current ABI to the ELFv1 ABI. This is the default ABI for big-endian PowerPC 64-bit Linux. Fair warning though: overriding the default ABI requires special system support and is likely to fail in spectacular ways 💀 like, spectacular spectacular. Don't say I didn't warn you.-mabi=elfv2— changes the current ABI to the ELFv2 ABI. This is the default ABI for little-endian PowerPC 64-bit Linux. Same energy as its sibling: overriding the default ABI requires special system support and is likely to fail in spectacular ways 😭. The drama is built in.-mgnu-attribute,-mno-gnu-attribute— emits.gnu_attributeassembly directives that set up tag/value pairs in a.gnu.attributessection, which specify ABI variations in function parameters or return values. Little metadata moment, we love to see it 🎀-mprototype,-mno-prototype— on System V.4 and embedded PowerPC systems, this assumes all calls to variable argument functions are properly prototyped. If they're not, the compiler has to insert an instruction before every non-prototyped call to set or clear bit 6 of the condition code register (CR), so it knows 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 that bit. Efficiency era 🔥-msim— on embedded PowerPC systems, assumes the startup module is calledsim-crt0.oand the standard C libraries arelibsim.aandlibc.a. This is the default forpowerpc-*-eabisimconfigurations. Cute defaults, no notes 💖-mmvme— on embedded PowerPC systems, assumes the startup module is calledcrt0.oand the standard C libraries arelibmvme.aandlibc.a.-mads— on embedded PowerPC systems, assumes the startup module is calledcrt0.oand the standard C libraries arelibads.aandlibc.a.-myellowknife— on embedded PowerPC systems, assumes the startup module is calledcrt0.oand the standard C libraries arelibyk.aandlibc.a. Not the name I expected but honestly iconic 🌸-mvxworks— on System V.4 and embedded PowerPC systems, specifies that you're compiling for a VxWorks system. Slay, embedded real-time vibes ✨-memb— on embedded PowerPC systems, sets thePPC_EMBbit in the ELF flags header to say that eabi extended relocations are used. Bit set, era defined 💅-meabi,-mno-eabi— on System V.4 and embedded PowerPC systems, these do (or don't) adhere to the Embedded Applications Binary Interface (EABI), which is a set of modifications to the System V.4 specifications. Picking-meabimeans the stack is aligned to an 8-byte boundary, a function__eabiis called frommainto set up the EABI environment, and-msdatacan use bothr2andr13to point to two separate small data areas. Picking-mno-eabimeans the stack is aligned to a 16-byte boundary, no EABI initialization function is called frommain, and-msdataonly usesr13to point at a single small data area. And-meabiis on by default if you configured GCC using one of thepowerpc*-*-eabi*options. Two data areas or one, choose your fighter 🫶-msdata=eabi— on System V.4 and embedded PowerPC systems, puts small initializedconstglobal and static data in the.sdata2section, pointed to by registerr2. Small initialized non-constglobal and static data goes in.sdata, pointed to by registerr13. Small uninitialized global and static data lands in.sbss, which is adjacent to.sdata. Important:-msdata=eabiis incompatible with-mrelocatable, and it also sets the-memboption. Chain reaction behavior, we respect it 🦋-msdata=sysv— on System V.4 and embedded PowerPC systems, puts small global and static data in.sdata, pointed to by registerr13. Small uninitialized global and static data goes in.sbss, right next to.sdata. Also incompatible with-mrelocatable, no cap 🧚-msdata=default,-msdata— on System V.4 and embedded PowerPC systems: if-meabiis used, it compiles the same as-msdata=eabi; otherwise it compiles the same as-msdata=sysv. The "it depends" queen 👑-msdata=data— on System V.4 and embedded PowerPC systems, puts small global data in.sdataand small uninitialized global data in.sbss. But like — do not use registerr13to address small data here. This is the default behavior unless other-msdataoptions are used. Default era, but read the fine print 😌-msdata=none,-mno-sdata— on embedded PowerPC systems, puts all initialized global and static data in the.datasection, and all uninitialized data in the.bsssection. Everything in its normal little home 🏠-mreadonly-in-sdata— puts read-only objects in the.sdatasection too. This is the default. Simple, clean, default 💜-mblock-move-inline-limit=num— inlines all block moves (think calls tomemcpyor structure copies) that are 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. Math but make it compiler ✨-mblock-compare-inline-limit=num— generates non-looping inline code for all block compares (like calls tomemcmpor structure compares) that are less than or equal to num bytes. If num is 0, all inline expansion (non-loop and loop) of block compare is disabled. Default value is target-specific. Zero means off, bestie, that's the whole tea ☕-mblock-compare-inline-loop-limit=num— generates 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 isn't constant, at most num bytes get compared beforememcmpis called to compare the rest of the block. Default is target-specific. The in-between queen 🎀-mstring-compare-inline-limit=num— compares at most num string bytes with inline code. If the difference or the end of the string isn't found by the end of the inline compare, a call tostrcmporstrncmphandles the rest of the comparison. The default is 64 bytes. Teamwork makes the dream work fr fr 💅-G num— on embedded PowerPC systems, puts global and static items that are 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 also gets passed to the linker, and all modules should be compiled with the same-G numvalue — consistency is key, bestie, don't mix and match 🙈-mregnames,-mno-regnames— on System V.4 and embedded PowerPC systems, these do (or don't) emit register names in the assembly language output using symbolic forms. Symbolic era, we love a readable output ⭐-mlongcall,-mno-longcall— by default, assumes all calls are far away, so a longer and more expensive calling sequence is required. This is needed for calls farther than 32 megabytes (33,554,432 bytes) from the current location. A short call gets generated if the compiler knows the call can't be that far away. This setting can be overridden by theshortcallfunction attribute, or by#pragma longcall(0).
Some linkers can detect out-of-range calls and generate glue code on the fly. On those systems, long calls are unnecessary and generate slower code. As of this writing, the AIX linker can do this, and so can the GNU linker for PowerPC/64. It's 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 gets 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 to emit 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. Future tea, noted 🔮
-mpltseq,-mno-pltseq— implements (or doesn't implement)-fno-pltand long calls using an inline PLT call sequence that supports lazy linking and long calls to functions indlopen'd shared libraries. Inline PLT calls are only supported on PowerPC64 ELFv2 and 32-bit PowerPC systems with newer GNU linkers, and they're enabled by default if the support is detected when configuring GCC, and, for 32-bit PowerPC, if GCC is configured with--enable-secureplt. Careful:-mpltseqcode and-mbss-plt32-bit PowerPC relocatable objects may not be linked together. Don't mix them, bestie, it's giving chaos 💀-mtls-markers,-mno-tls-markers— marks (or doesn't mark) calls to__tls_get_addrwith a relocation specifying the function argument. That relocation lets the linker reliably associate a function call with the argument setup instructions for TLS optimization, which in turn lets GCC schedule the sequence better. Linker and compiler besties fr fr 🫶-mrecip,-mno-recip— bestie, this lets GCC go full main-character mode and use the reciprocal estimate and reciprocal square root estimate instructions, with extra Newton-Raphson steps on top to beef up precision, instead of straight-up doing a divide (or a square root plus divide) for floating-point args. 💅 When you turn on-mrecipyou should also be running-ffast-math(or at the very least-funsafe-math-optimizations,-ffinite-math-only,-freciprocal-mathand-fno-trapping-math). Heads up though, no cap: even though the throughput of this sequence is generally higher than the non-reciprocal version, the precision can drop by up to 2 ulp — like, the inverse of1.0can literally come out as0.99999994for reciprocal square roots. 😭 So it's giving "fast but slightly delulu precision."-mrecip=opt— this one's the vibe controller for which reciprocal estimate instructions are allowed.optis a comma-separated list, and any option in it can be prefixed with"!"to invert it. 🌈 Here's the whole menu:all— enable every estimate instruction, periodt.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 check it, like: -mrecip=all,!rsqrtd enables all of the reciprocal estimate instructions except the "FRSQRTE", "XSRSQRTEDP", and "XVRSQRTEDP" instructions — those are the ones handling the double-precision reciprocal square root math. 🦋
-mrecip-precision,-mno-recip-precision— assume (or don't assume, bestie) that the reciprocal estimate instructions hand you higher-precision estimates than the PowerPC ABI says they have to. Picking-mcpu=power6,-mcpu=power7or-mcpu=power8automatically selects-mrecip-precision. 🎀 And the double-precision square root estimate instructions are not generated by default on low-precision machines — because they don't serve up an estimate that actually converges after three steps. 💀-mveclibabi=type— tells GCC which ABI type to use when vectorizing intrinsics through an external library. Currently the only supported type ismass, which means "use IBM's Mathematical Acceleration Subsystem (MASS) libraries to vectorize intrinsics via external libraries." ✨ Right now GCC emits calls to"acosd2","acosf4","acoshd2","acoshf4","asind2","asinf4","asinhd2","asinhf4","atan2d2","atan2f4","atand2","atanf4","atanhd2","atanhf4","cbrtd2","cbrtf4","cosd2","cosf4","coshd2","coshf4","erfcd2","erfcf4","erfd2","erff4","exp2d2","exp2f4","expd2","expf4","expm1d2","expm1f4","hypotd2","hypotf4","lgammad2","lgammaf4","log10d2","log10f4","log1pd2","log1pf4","log2d2","log2f4","logd2","logf4","powd2","powf4","sind2","sinf4","sinhd2","sinhf4","sqrtd2","sqrtf4","tand2","tanf4","tanhd2", and"tanhf4"when generating code forpower7. Both-ftree-vectorizeand-funsafe-math-optimizationsalso have to be enabled, and the MASS libraries must be specified at link time. 🔥-mfriz,-mno-friz— generate (or don't generate) the"friz"instruction when-funsafe-math-optimizationsis used to optimize rounding floating-point values to 64-bit integer and back to floating point. ⚠️ Ngl, the"friz"instruction does not return the same value if the floating-point number is too large to fit in an integer. So, like, careful.-mpointers-to-nested-functions,-mno-pointers-to-nested-functions— generate (or don't) 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 register"r2", and the static chain value to be loaded in register"r11". 🫶-mpointers-to-nested-functionsis on by default. Big warning though: you cannot call through pointers to nested functions — or pointers to functions compiled in other languages that use the static chain — if you use-mno-pointers-to-nested-functions. 😭-msave-toc-indirect,-mno-save-toc-indirect— generate (or don't) 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 isn't saved in the prologue, it gets saved right 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) code that passes structure parameters with a maximum alignment of 64 bits, for compatibility with older versions of GCC. 🎀 So here's the lore: older GCC versions (before4.9.0) incorrectly did not align a structure parameter on a 128-bit boundary when that structure contained a member requiring 128-bit alignment. That's been fixed in more recent GCC versions. This option lets you generate code compatible with functions compiled by those older GCC versions. And-mno-compat-align-parmis the default, bestie.-mstack-protector-guard=guard,-mstack-protector-guard-reg=reg,-mstack-protector-guard-offset=offset— generate stack protection code using the canary at guard. The supported locations areglobalfor a global canary, ortlsfor a per-thread canary in the TLS block (which is the default with GNU libc version 2.4 or later). 🦋
And with that latter choice, -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset also specify which register to use as the base register for reading the canary, and what offset from that base register to read it at. The defaults for those are as specified in the relevant ABI. ✨
-mpcrel,-mno-pcrel— generate (or don't) pc-relative addressing. The-mpcreloption requires that the medium code model (-mcmodel=medium) and prefixed addressing (-mprefixed) options are enabled. 💅-mprefixed,-mno-prefixed— generate (or don't) addressing modes that use prefixed load and store instructions. The-mprefixedoption requires that-mcpu=power10(or later) is enabled. 🌸-mmma,-mno-mma— generate (or don't) the MMA instructions. The-mmaoption requires that-mcpu=power10(or later) is enabled. (Yeah, the manpage literally says-mmathere — we're just passing that along, iykyk. 🙈)-mdense-math,-mno-dense-math— generate (or don't) Dense Math Facility (DMF) instructions. The-mdense-mathoption requires that-mcpu=future(or later) is enabled. 👑-mrop-protect,-mno-rop-protect— generate (or don't) ROP protection instructions when the target processor supports them. Currently this option disables the shrink-wrap optimization (-fshrink-wrap). 😭-mprivileged,-mno-privileged— generate (or don't) code that runs in privileged state. 🎀-mblock-ops-unaligned-vsx,-mno-block-ops-unaligned-vsx— generate (or don't) unaligned vsx loads and stores for inline expansion of"memcpy"and"memmove". ⭐-msplat-word-constant,-mno-splat-word-constant— generate (or don't) code that uses the"XXSPLTIW"instruction. This option is enabled by default. 💖-msplat-float-constant,-mno-splat-float-constant— generate (or don't) code that uses the"XXSPLTIDP"instruction. This option is enabled by default. 💖-mieee128-constant,-mno-ieee128-constant— generate (or don't) code that uses the"LXVKQ"instruction. This option is enabled by default. 💖-mwarn-altivec-long,-mno-warn-altivec-long— enable or disable warnings about deprecatedvector long ...Altivec type usage. This option is enabled by default. 🔔
🌸 RX Options 🌸
These command-line options are the ones defined for RX targets, bestie:
-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. 👀 And like, note that RX floating-point hardware only works on 32-bit values — which is literally why the default is-m32bit-doubles.-fpu,-nofpu— enables (-fpu) or disables (-nofpu) 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 though — 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. Why? Because the RX FPU instructions are themselves unsafe. 😭 No cap.
-mcpu=name— selects the type of RX CPU to target. Currently three types are supported: the generic RX600 and RX200 series hardware, plus 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 has no hardware floating-point unit, so -nofpu is enabled by default when this type is selected.
-mbig-endian-data,-mlittle-endian-data— store data (but not code!) in big-endian format. The default is-mlittle-endian-data, i.e. storing data in little-endian format. 🌈-msmall-data-limit=N— specifies the maximum size in bytes of global and static variables that 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's up to the programmer to make sure the area doesn't overflow. 🙈 Also, when the small data area is used, one of the RX's registers (usually"r13") is reserved for pointing at this area, so it's no longer available for the compiler to use. That could make your code slower and/or larger if variables get pushed onto the stack instead of living in this register. 💀
Note: common variables (variables that haven't been initialized) and constants are not placed into the small data area, because they get assigned to other sections in the output executable. 🫶
The default value is zero, and zero means this whole thing is off. 🫶 Also note: this feature does not get switched on automatically at higher optimization levels (-O2 etc) — because reserving a register can genuinely hurt you, and that's a whole vibe nobody signed up for. So it's on you, the programmer, to experiment and figure out whether your program actually benefits. ✨ And for how the actual register holding the small data area pointer gets picked? Go read the -mpid option description, it's all spelled out there. 👑
-msim,-mno-sim— use the simulator runtime. Default is the libgloss board-specific runtime, no cap.-mas100-syntax,-mno-as100-syntax— when you're generating assembler output, use a syntax that's compatible with Renesas's AS100 assembler. The GAS assembler can handle this syntax too, but it comes with some restrictions, which is exactly why it's not generated by default. 🙈-mmax-constant-size=N— sets the max size, in bytes, of a constant that can be used as an operand in an RX instruction. Okay so here's the tea: the RX instruction set does allow constants up to 4 bytes long inside instructions, but a longer value = a longer instruction. So sometimes it's genuinely better to restrict how big those in-instruction constants get. Constants that are too chonky get yeeted into a constant pool and referenced via register indirection instead. 🎀Ncan be between 0 and 4. A value of 0 (that's the default) or 4 means constants of any size are allowed.-mrelax— enable linker relaxation, bestie. Linker relaxation is when the linker tries to shrink your program by finding shorter versions of various instructions. Disabled by default. 💅-mint-register=N— specify how many registers to reserve for fast interrupt handler functions.Ncan be between 0 and 4. Value1means register"r13"is reserved exclusively for fast interrupt handlers. Value2reserves"r13"and"r12". Value3reserves"r13","r12"and"r11", and value4reserves"r13"all the way through"r10". Value0— the default — reserves nothing at all. 🌸-msave-acc-in-interrupts— says interrupt handler functions should preserve the accumulator register. You only need this if normal code might use the accumulator register, like, say, because it's doing 64-bit multiplications. Default is to ignore the accumulator, because that makes interrupt handlers faster. 🔥-mpid,-mno-pid— enables generation of position-independent data. When it's on, any access to constant data happens via an offset from a base address held in a register. That means the location of constant data can be figured out at run time without needing the executable to be relocated — which is a huge win for embedded apps with tight memory constraints. Data that can be modified is not affected by this option. ⭐
Okay but heads up: using this feature reserves a register, usually "r13", for the constant data base address. That can mean slower and/or bigger code, especially in complicated functions. 😭
Which register actually ends up holding the constant data base address depends on whether -msmall-data-limit and/or the -mint-register command-line options are enabled. Starting at register "r13" and going downwards, registers get allocated first to satisfy -mint-register, then -mpid, and finally -msmall-data-limit. So yes — it is genuinely possible for the small data area register to be "r8" if you pass both -mint-register=4 and -mpid on the command line. 🫶
By default this feature is off. You can restore that default via the -mno-pid command-line option.
-mno-warn-multiple-fast-interrupts,-mwarn-multiple-fast-interrupts— stops GCC from throwing a warning message if it finds more than one fast interrupt handler while compiling a file. The default is to issue a warning for each extra fast interrupt handler found, because the RX only supports one such interrupt. 💀-mallow-string-insns,-mno-allow-string-insns— enables or disables use of the string manipulation instructions"SMOVF","SCMPU","SMOVB","SMOVU","SUNTIL""SWHILE"and also the"RMPA"instruction. These instructions may prefetch data, which is not safe if you're accessing an I/O register. (See section 12.2.7 of the RX62N Group User's Manual for more info.)
The default is to allow these instructions — but GCC can't reliably detect every circumstance where a string instruction might be used to access an I/O register, so their use can't be disabled automatically. It's on the programmer (that's you, bestie 💅) to use -mno-allow-string-insns if their program accesses I/O space.
When the instructions are enabled, GCC defines the C preprocessor symbol "__RX_ALLOW_STRING_INSNS__"; otherwise it defines "__RX_DISALLOW_STRING_INSNS__". ✨
-mjsr,-mno-jsr— use only (or not only)"JSR"instructions to access functions. This one's for when code size exceeds the range of"BSR"instructions. And ngl,-mno-jsrdoes not mean "don't use"JSR"" — it means any type of branch may be used. 🙈-mlra— use the new LRA register allocator. By default the old"reload"allocator is used.
Note: the generic GCC command-line option-ffixed-reghas special significance to the RX port when used with the"interrupt"function attribute. That attribute marks a function intended to process fast interrupts. GCC makes sure it only uses the registers"r10","r11","r12"and/or"r13"— and only provided that the normal use of the corresponding registers has been restricted via the-ffixed-regor-mint-registercommand-line options. 👑
🦋 S/390 and zSeries Options 🦋
These are the -m options defined for the S/390 and zSeries architecture. Let's get into it 💖
-mhard-float,-msoft-float— use (or don't use) the hardware floating-point instructions and registers for floating-point operations. When-msoft-floatis specified, functions inlibgcc.aare used to do the floating-point operations. When-mhard-floatis specified, the compiler generates IEEE floating-point instructions. That one's the default. ✨-mhard-dfp,-mno-hard-dfp— use (or don't use) the hardware decimal-floating-point instructions for decimal-floating-point operations. When-mno-hard-dfpis specified, functions inlibgcc.ado the 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 the"long double"type. A size of 64 bits makes the"long double"type equivalent to the"double"type. That's the default.-mbackchain,-mno-backchain— store (or don't store) the address of the caller's frame as a backchain pointer into the callee's stack frame. A backchain may be needed so you can debug with tools that don't understand DWARF call frame info. When-mno-packed-stackis in effect, the backchain pointer goes at the bottom of the stack frame; when-mpacked-stackis in effect, the backchain is placed into the topmost word of the 96/160 byte register save area. 🫶
In general, code compiled with -mbackchain is call-compatible with code compiled with -mno-backchain; however, using the backchain for debugging usually requires that the whole binary is built with -mbackchain. And note 📝 — the combination of -mbackchain, -mpacked-stack and -mhard-float is not supported. If you're building a linux kernel, use -msoft-float.
Default is to not maintain the backchain.
-mpacked-stack,-mno-packed-stack— use (or don't use) the packed stack layout. When-mno-packed-stackis specified, the compiler uses all fields of the 96/160 byte register save area only for their default purpose; unused fields still take up stack space. When-mpacked-stackis specified, register save slots are densely packed at the top of the register save area, and unused space gets reused for other purposes, allowing more efficient use of the available stack space. But when-mbackchainis also in effect, the topmost word of the save area is always used to store the backchain, and the return address register is always saved two words below the backchain. 🌸
As long as the stack frame backchain isn't used, code generated with -mpacked-stack is call-compatible with code generated with -mno-packed-stack. Heads up though: 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. That code is not call-compatible with code compiled with -mpacked-stack. Also — say it with me — the combination of -mbackchain, -mpacked-stack and -mhard-float is not supported. In order to build a linux kernel use -msoft-float. 💅
Default is to not use the packed stack layout.
-msmall-exec,-mno-small-exec— generate (or don't generate) code using the"bras"instruction to do subroutine calls. This only works reliably if the total executable size doesn't exceed 64k. The default is to use the"basr"instruction instead, which doesn't have that limitation. 😌-m64,-m31— when-m31is specified, generate code compliant with the GNU/Linux for S/390 ABI. When-m64is specified, generate code compliant with the GNU/Linux for zSeries ABI. This lets GCC in particular generate 64-bit instructions. For the s390 targets, the default is-m31, while s390x targets default to-m64. ⚠️ Note:-m31is deprecated and support will be removed. Like, fr fr, don't build your whole thing on it.-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 with the GNU/Linux for S/390 ABI, the default is-mesa. When generating code compliant with the GNU/Linux for zSeries ABI, the default is-mzarch. 🦋-mhtm,-mno-htm— turning on-mhtmgives you a whole set of builtins that use the instructions from the transactional execution facility that showed up with the IBM zEnterprise EC12 machine generation S/390 System z Built-in Functions ✨ it's on by default whenever you're using-march=zEC12.-mvx,-mno-vx— with-mvxyou get code built from the instructions in the vector extension facility, which arrived with the IBM z13 machine generation 🦋 heads up tho: this option changes the ABI for some vector type values when it comes to alignment and calling conventions. If vector type values are used in an ABI-relevant context, a GAS.gnu_attributecommand gets added to mark the resulting binary with the ABI in use.-mvxis on by default with-march=z13.-mzvector,-mno-zvector—-mzvectorenables vector language extensions and builtins that use instructions from the vector extension facility from the IBM z13 machine generation 💅 it adds support forvectorbeing usable as a keyword to define vector type variables and arguments.vectoris only available when GNU extensions are enabled — it does not expand when you're requesting strict standard compliance, e.g. with-std=c99. On top of the GCC low-level builtins,-mzvectorenables a set of builtins added for compatibility with AltiVec-style implementations like Power and Cell. To use those builtins you must include the header filevecintrin.h.-mzvectoris disabled by default.-mmvcle,-mno-mvcle— generate (or don't generate) code that uses themvcleinstruction to do block moves. With-mno-mvcleyou get amvcloop instead. That's the default unless you're optimizing for size.-mdebug,-mno-debug— print (or don't print) extra debug information while compiling. Default is to not print debug information, bestie.- *
-march=cpu-type* — generate code that runs on cpu-type, which is the name of a system representing a certain processor type 👑 possible values for cpu-type arez900/arch5,z990/arch6,z9-109,z9-ec/arch7,z10/arch8,z196/arch9,zEC12,z13/arch11,z14/arch12,z15/arch13,z16/arch14,z17/arch15, andnative.
The default is -march=z900.
Specifying native as the cpu type lets you pick the best architecture option for the host processor. -march=native has no effect if GCC doesn't recognize the processor 🫶
- *
-mtune=cpu-type* — tune everything applicable about the generated code to cpu-type, 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 whatever value you used for-march. -mtpf-trace,-mno-tpf-trace— generate code that adds (or does not add) TPF OS specific branches to trace routines in the operating system. This one is off by default, even when you're compiling for the TPF OS 🙈-mtpf-trace-skip,-mno-tpf-trace-skip— generate code that changes (or does not change) the default branch targets enabled by-mtpf-traceso they point at specialized trace routines that can selectively skip function trace entries for the TPF OS. Off by default, even when compiling for the TPF OS and specifying-mtpf-trace.-mmain— for the TPF OS target, use this when linking to add the startup code and other linker options so you produce a main object.-mfused-madd,-mno-fused-madd— generate code that uses (or doesn't use) the floating-point multiply and accumulate instructions. These get generated by default if hardware floating point is used.- *
-mwarn-framesize=framesize* — emit a warning if the current function goes over the given frame size. Since this is a compile-time check it doesn't have to be a real problem when the program actually runs — it's meant to flag functions that most probably cause a stack overflow. Super useful for environments with a limited stack size, e.g. the Linux kernel 🍓
A value of zero disables this warning, and that's the default.
-mwarn-dynamicstack,-mno-warn-dynamicstack— emit a warning if the function callsallocaor uses dynamically-sized arrays. Generally a bad idea when your stack size is limited, ngl.-mstack-guard=stack-guard,-mstack-size=stack-size,-mno-stack-guard,-mno-stack-size— when these are enabled, the S/390 back end emits extra instructions in the function prologue that trigger a trap if the stack size is stack-guard bytes above stack-size (remember the stack on S/390 grows downward 💀). If you leave out the stack-guard option, the smallest power of 2 larger than the frame size of the compiled function gets chosen.
These options are meant to help debug stack overflow problems. The extra emitted code adds only a little overhead, so it can also be used in production-like systems without a big performance hit.
The given values have to be exact powers of 2, and stack-size has to be greater than stack-guard without exceeding 64k. To stay efficient, the extra code assumes the stack starts at an address aligned to the value given by stack-size.
The -mstack-guard= option can only be used together with -mstack-size=. You can override these on the command line with -mno-stack-guard and -mno-stack-size, respectively.
-mhotpatch=pre-halfwords,post-halfwords — if the hotpatch option is enabled, a "hot-patching" function prologue is generated for *all functions in the compilation unit 🔥 the function label gets prepended with the given number of two-byte NOP instructions (pre-halfwords*, maximum 1000000). After the label, 2 * post-halfwords bytes are appended, using the largest NOP-like instructions the architecture allows (maximum 1000000).
If both arguments are zero, hotpatching is disabled.
This option can be overridden for individual functions with the hotpatch attribute.
-mpic-data-is-text-relative,-mno-pic-data-is-text-relative— when compiling for S/390 with-fpicor-fPIC, GCC normally emits code that uses relative addressing between code and data. But for hotpatching you might need to introduce new.textparts while still using the existing.dataand.bsssections ✨ in that case, use-mno-pic-data-is-text-relativeto force addressing through the GOT instead.-mstack-protector-guard=guard,-mstack-protector-guard-record— generate stack protection code using a canary at guard. Supported locations areglobalfor a global canary, ortlsfor a per-thread canary in the TLS block (that's the default).
The option -mstack-protector-guard-record results in the generation of section __stack_protector_loc, which contains pointers to all the instructions that load the address of the global guard. So this option only has an effect in conjunction with -mstack-protector-guard=global 💅 the intended use is the Linux kernel.
- *
-mindirect-branch=choice*,-mindirect-branch-jump=choice,-mindirect-branch-call=choice,-mfunction-return=choice,-mfunction-return-mem=choice,-mfunction-return-reg=choice — this whole group of options tackles security vulnerabilities related to speculative execution and indirect branch prediction by replacing indirect branches and function returns with direct branches to a thunk 🛡️
Specifying a choice of keep causes generation of the default code. thunk triggers generation of out-of-line thunks and replaces the formerly indirect branch with a direct branch to the thunk. thunk-extern does the branch replacement just like thunk, but doesn't emit the thunks. thunk-inline is only available with -mindirect-branch-jump — you should prefer it over thunk in user-space applications so stack unwinding and exception handling stay correct 😭
-mindirect-branch sets the value of -mindirect-branch-jump and -mindirect-branch-call. -mfunction-return sets the value of -mfunction-return-reg and -mfunction-return-mem.
All of these options can also be set per-function using function attributes.
-mindirect-branch-table,-mno-indirect-branch-table— this one's useful alongside-mindirect-branchand-mfunction-return. When enabled, it generates tables pointing to the branch locations that were patched by those options. The tables get emitted in sections named.s390_indirect_jump,.s390_indirect_call,.s390_return_reg, and.s390_return_mem⭐ each section is an array of 32-bit elements, and each entry holds the offset from the entry to the patched location.-mfentry,-mno-fentry— when used together with the-pgoption, emit profiling code using the__fentry__hook that GLIBC 2.29 or later provides. This is only available for 64-bit code. If this option isn't enabled, profiling uses the less efficient_mcounthook instead.-mrecord-mcount,-mno-record-mcount— when enabled, generate a__mcount_locsection containing the locations of all generated profiling calls to_mcountor__fentry__.mnop-mcount,mno-nop-mcount— instead of generating calls to_mcountor__fentry__with-pg, insert a sequence of no-op instructions of the same length right where the calls would've been inserted 🎀 pair it with-mrecord-mcountto patch the call sequences into the object file without recompiling it.-mpreserve-args,-mno-preserve-args— when enabled, save all argument registers to the stack and generate the corresponding CFI information. Handy for applications that want to implement their own stack unwinding and need access to function arguments.-munaligned-symbols,-mno-unaligned-symbols— assume that external symbols with a natural alignment of 1 are potentially unaligned. By default, all symbols without explicit alignment are assumed to be aligned on a 2-byte boundary, as the IBM Z ABI mandates 🌸
🦋 SH Options 🦋
These -m options are defined for the SH implementations, bestie:
-m1— generate code for the SH1 ✨-m2— generate code for the SH2 ✨-m2e— generate code for the SH2e ✨
okay so we're still deep in SH options and honestly bestie there are SO many of them 😭 but every single one matters so let's go, no skipping 💅
-m2a-nofpu— generates code for the SH2a with no FPU at all, or for an SH2a-FPU in a way where the floating-point unit just... doesn't get used 🚫-m2a-single-only— code for the SH2a-FPU, but in such a way that no double-precision floating-point operations are used-m2a-single— code for the SH2a-FPU, assuming the floating-point unit is in single-precision mode by default-m2a— code for the SH2a-FPU, assuming the floating-point unit is in double-precision mode by default-m3— generates code for the SH3-m3e— generates code for the SH3e-m4-nofpu— generates code for the SH4 with no floating-point unit-m4-single-only— generates code for the SH4 with an FPU that only supports single-precision arithmetic, periodt-m4-single— generates code for the SH4 assuming the floating-point unit is in single-precision mode by default-m4— generates code for the SH4-m4-100— generates code for SH4-100-m4-100-nofpu— generates code for SH4-100 in such a way that the floating-point unit is not used-m4-100-single— generates code for SH4-100 assuming the floating-point unit is in single-precision mode by default-m4-100-single-only— generates code for SH4-100 in such a way that no double-precision floating-point operations are used-m4-200— generates code for SH4-200-m4-200-nofpu— generates code for SH4-200 in such a way that the floating-point unit is not used-m4-200-single— generates code for SH4-200 assuming the floating-point unit is in single-precision mode by default-m4-200-single-only— generates code for SH4-200 in such a way that no double-precision floating-point operations are used-m4-300— generates code for SH4-300-m4-300-nofpu— generates code for SH4-300 in such a way that the floating-point unit is not used-m4-300-single— generates code for SH4-300 in such a way that no double-precision floating-point operations are used-m4-300-single-only— generates code for SH4-300 in such a way that no double-precision floating-point operations are used-m4-340— generates code for SH4-340 (no MMU, no FPU)-m4-400— generates code for SH4-400 (no MMU, no FPU)-m4-500— generates code for SH4-500 (no FPU). Also passes-isa=sh4-nofputo the assembler ✨-m4a-nofpu— generates code for the SH4al-dsp, or for an SH4a in such a way that the floating-point unit is not used-m4a-single-only— generates code for the SH4a, in such a way that no double-precision floating-point operations are used-m4a-single— generates code for the SH4a assuming the floating-point unit is in single-precision mode by default-m4a— generates code for the SH4a-m4al— same as-m4a-nofpu, except it implicitly passes-dspto the assembler. Heads up: GCC doesn't generate any DSP instructions at the moment 🙈-mb— compiles code for the processor in big-endian mode-ml— compiles code for the processor in little-endian mode-mdalign— aligns doubles at 64-bit boundaries. Real talk though, this changes the calling conventions, and thus some functions from the standard C library do not work unless you recompile it first with-mdalign⚠️-mrelax— shortens some address references at link time when possible; uses the linker option-relax-mbigtable— uses 32-bit offsets inswitchtables. The default is to use 16-bit offsets.-mbitops— enables the use of bit manipulation instructions on SH2A-mfmovd— enables the use of the instructionfmovd. Check-mdalignfor alignment constraints 👀-mrenesas,-mno-renesas— comply with the calling conventions defined by Renesas. The default for all targets of the SH toolchain is-mno-renesas, which uses the calling conventions defined for GCC before the Renesas conventions were available.-mnomacsave— marks theMACregister 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,-mno-inline-ic_invalidate— inline code to invalidate instruction cache entries after setting up nested function trampolines. This option has no effect if-musermodeis in effect and the selected code generation option (e.g.-m4) does not allow the use of theicbiinstruction. If the selected code generation option does not allow the use of theicbiinstruction, and-musermodeis not in effect, the inlined code manipulates the instruction cache address array directly with an associative write. 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— dumps instruction size and location in the assembly code-matomic-model=model— sets the model of atomic operations plus additional parameters as a comma-separated list. For the deets on the atomic built-in functions, see __atomic Builtins. These are the models and parameters you can pick from:none— disables compiler generated atomic sequences and emits library calls for atomic operations. This is the default if the target is notsh*-*-linux*.soft-gusa— generates GNU/Linux-compatible gUSA software atomic sequences for the atomic built-in functions. These 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 issh*-*-linux*and SH3* or SH4*. When the target is SH4A, this option also partially utilizes the hardware atomic instructionsmovli.landmovco.lto create more efficient code, unlessstrictis specified.soft-tcb— generates software atomic sequences that use a variable in the thread control block. It's a variation of the gUSA sequences, and it can also be used on SH1* and SH2* targets. The generated atomic sequences require additional support from the interrupt/exception handling code of the system and are only suitable for single-core systems. When using this model, thegbr-offset=parameter has to be specified as well.soft-imask— generates software atomic sequences that temporarily disable interrupts by settingSR.IMASK = 1111. This model works only when the program runs in privileged mode and is only suitable for single-core systems. No additional support from the interrupt/exception handling code of the system is required. This model is enabled by default when the target issh*-*-linux*and SH1* or SH2*.hard-llcs— generates hardware atomic sequences using themovli.landmovco.linstructions 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. No additional support from the interrupt/exception handling code of the system is required for this model.gbr-offset=— this parameter specifies the offset in bytes of the variable in the thread control block structure that should be used by the generated atomic sequences when thesoft-tcbmodel has been selected. For other models this parameter is ignored. The specified value must be an integer multiple of four and in the range 0-1020.strict— this parameter prevents mixed usage of multiple atomic models, even if they're compatible, and makes the compiler generate atomic sequences of the specified model only.
-mtas— generates thetas.bopcode for__atomic_test_and_set. Notice that depending on the particular hardware and software configuration this can degrade overall performance due to the operand cache line flushes that are implied by thetas.binstruction. On multi-core SH4A processors, thetas.binstruction must be used with caution since it can result in data corruption for certain cache configurations 😬-mprefergot— when generating position-independent code, emits 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 would not work in user mode.-musermodeis the default when the target issh*-*-linux*. If the target is SH1* or SH2*,-musermodehas no effect, since there is no user mode.-multcost=number— sets the cost to assume for a multiply instruction-mdiv=strategy— sets 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 the small divisors and thediv1instruction with case distinction for the bigger ones 🌸. Dividing by zero calculates an unspecified result and does not trap, iykyk. This is the default for SH4. Specifying this for targets that don't have dynamic shift instructions defaults tocall-div1.
When you haven't specified a division strategy, the default gets picked based on the current target, periodt. For SH2A the default strategy is to use the divs and divu instructions instead of library function calls ✨.
🎀 the rest of the SH goodies 🎀
-maccumulate-outgoing-args,-mno-accumulate-outgoing-args— reserves space once for outgoing arguments in the function prologue instead of around every single call. This is generally a glow-up for performance and size, and it's also needed for unwinding so you don't change the stack frame around conditional code.-maccumulate-outgoing-argsis enabled the default 💅.-mdivsi3_libfunc=name — sets 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 — the compiler still expects the same sets of input/output/clobbered registers as if this option weren't there at all. No cap 🧚.- *
-mfixed-range=register-range* — generates code treating the given register range as fixed registers. A fixed register is one the register allocator cannot use — lowkey useful when compiling kernel code 👑. A register range is specified as two registers separated by a dash. Multiple register ranges can be specified separated by a comma. - *
-mbranch-cost=num* — assumes num to be the cost for a branch instruction. Higher numbers make the compiler try to generate more branch-free code if it can. If you don't specify it, the value is selected depending on the processor type you're compiling for. -mzdcbranch,-mno-zdcbranch— assume (or do not assume) that the zero displacement conditional branch instructionsbtandbfare fast. If-mzdcbranchis specified, the compiler prefers zero displacement branch code sequences. Enabled by default when generating code for SH4 and SH4A. It can be explicitly disabled by specifying-mno-zdcbranch⭐.-mcbranch-force-delay-slot— forces 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, as found in the original SH7055 💀.-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. 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. 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,-mno-pretend-cmove— prefer, or don't prefer, zero-displacement conditional branches for conditional move instruction patterns. This can result in faster code on the SH4 processor 🔥.-mfdpic,-mno-fdpic— generate code using the FDPIC ABI.-mlra,-mno-lra— use the new LRA register allocator. By default, the oldreloadallocator is used 🫶.
☀️ Solaris 2 Options ☀️
These options are supported on Solaris 2, bestie:
-mclear-hwcap,-mno-clear-hwcap—-mclear-hwcaptells the compiler to remove the hardware capabilities generated by the Solaris assembler. This is only necessary when object files use ISA extensions not supported by the current machine, but check at runtime whether or not to use them 💜.-mimpure-text,-mno-impure-text—-mimpure-text, used in addition to-shared, tells the compiler to not pass-z textto the linker when linking a shared object. Using this option, you can link position-dependent code into a shared object.
-mimpure-text suppresses the relocations remain against allocatable but non-writable sections linker error message. However 😭 — the necessary relocations trigger copy-on-write, and the shared object is not actually shared across processes. Instead of using -mimpure-text, you should compile all source code with -fpic or -fPIC. Do not come for me, that's literally what it says.
-gsctf— generate Solaris CTF. Needs to be used both for compilation and linking. Seectf(7)for more info. Only supported since Solaris 11.4 SRU 84, where the necessary toolchain support was added 🦋.
🌞 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 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 doesn't generate save/restore instructions and uses a "flat" or single register window model. This model is compatible with the regular register window model. The local registers and the input registers (0--5) are still treated as call-saved registers and are saved on the stack as needed ✨.
With -mno-flat (the default), the compiler generates save/restore instructions (except for leaf functions). This is the normal operating mode.
-mfpu,-mhard-float— generate output containing floating-point instructions. This is the default 💖.-mno-fpu,-msoft-float— generate output containing library calls for floating point. Warning 🚨: the requisite libraries are not available for all SPARC targets. Normally the facilities of the machine's usual C compiler are used, but this can't 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 for this to work. No shortcuts, bestie 😤.
-mhard-quad-float— generate output containing quad-word (long double) floating-point instructions.-msoft-quad-float— generate output containing library calls for quad-word (long double) floating-point instructions. The functions called are those specified in the SPARC ABI. This is the default.
As of this writing, there are no SPARC implementations that have hardware support for the quad-word floating-point instructions 😭. They all invoke a trap handler for one of these instructions, and then the trap handler emulates the effect of the instruction. Because of the trap handler overhead, this is much slower than calling the ABI library routines. Thus the -msoft-quad-float option is the default.
-mno-unaligned-doubles,-munaligned-doubles— assume that doubles have 8-byte alignment. This is the default.
With -munaligned-doubles, GCC assumes that doubles have 8-byte alignment only if they're contained in another type, or if they have an absolute address. Otherwise it assumes they have 4-byte alignment. Specifying this option avoids some rare compatibility problems with code generated by other compilers 🧚. It's not the default because it results in a performance loss, especially for floating-point code 💀.
-muser-mode,-mno-user-mode— do not generate code that can only run in supervisor mode. This is relevant only for thecasainstruction emitted for the LEON3 processor. This is the default 🌸.-mfaster-structs,-mno-faster-structs— with-mfaster-structs, the compiler assumes structures should have 8-byte alignment. This enables the use of pairs oflddandstdinstructions for copies in structure assignment, in place of twice as manyldandstpairs 🔥. However, the use of this changed alignment directly violates the SPARC ABI. So it's intended only for use on targets where the developer acknowledges that their resulting code is not directly in line with the rules of the ABI 🙈.-mstd-struct-return,-mno-std-struct-return— with-mstd-struct-return, the compiler generates checking code in functions returning structures or unions to detect size mismatches between the two sides of function calls, as per the 32-bit ABI.
The default is -mno-std-struct-return. This option has no effect in 64-bit mode.
- *
-mcpu=cpu_type* — set the instruction set, register set, and instruction scheduling parameters for machine type cpu_type. Supported values for cpu_type arev7,cypress,v8,supersparc,hypersparc,leon,leon3,leon3v7,leon5,sparclite,f930,f934,sparclite86x,sparclet,tsc701,v9,ultrasparc,ultrasparc3,niagara,niagara2,niagara3,niagara4,niagara7andm8👑.
Native Solaris and GNU/Linux toolchains also support the value native, which selects the best architecture option for the host processor. -mcpu=native has no effect if GCC doesn't recognize the processor.
Default instruction scheduling parameters are used for values that select an architecture and not an implementation. These are v7, v8, sparclite, sparclet, v9.
Here's a list of each supported architecture and their supported implementations 🦋:
v7—cypress,leon3v7v8—supersparc,hypersparc,leon,leon3,leon5sparclite—f930,f934,sparclite86xsparclet—tsc701v9—ultrasparc,ultrasparc3,niagara,niagara2,niagara3,niagara4,niagara7,m8
okay bestie, SPARC time! 💅 By default (unless you configured it otherwise), GCC is gonna generate code for the V7 variant of the SPARC architecture. No cap.
-mcpu=cypress— the compiler also optimizes 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. ✨-mcpu=v8— 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. 🌀-mcpu=supersparc— the compiler additionally optimizes for the SuperSPARC chip, as used in the SPARCStation 10, 1000 and 2000 series. 🔥-mcpu=sparclite— generates code for the SPARClite variant. This adds the integer multiply, integer divide step and scan ("ffs") instructions which exist in SPARClite but not in SPARC-V7. 🌸-mcpu=f930— the compiler additionally optimizes for the Fujitsu MB86930 chip, which is the original SPARClite, with no FPU. 😭-mcpu=f934— the compiler additionally optimizes for the Fujitsu MB86934 chip, which is the more recent SPARClite with FPU. 💖-mcpu=sparclet— generates code for the SPARClet variant. This adds the integer multiply, multiply/accumulate, integer divide step and scan ("ffs") instructions which exist in SPARClet but not in SPARC-V7. 🦋-mcpu=tsc701— the compiler additionally optimizes for the TEMIC SPARClet chip. 🎀-mcpu=v9— generates code for the V9 variant. This adds 64-bit integer and floating-point move instructions, 3 additional floating-point condition code registers and conditional move instructions. 👑-mcpu=ultrasparc— the compiler additionally optimizes for the Sun UltraSPARC I/II/IIi chips. ⭐-mcpu=ultrasparc3— the compiler additionally optimizes for the Sun UltraSPARC III/III+/IIIi/IIIi+/IV/IV+ chips. 🌈-mcpu=niagara— the compiler additionally optimizes for Sun UltraSPARC T1 chips. 💜-mcpu=niagara2— the compiler additionally optimizes for Sun UltraSPARC T2 chips. 💀-mcpu=niagara3— the compiler additionally optimizes for Sun UltraSPARC T3 chips. 🙈-mcpu=niagara4— the compiler additionally optimizes for Sun UltraSPARC T4 chips. 🍓-mcpu=niagara7— the compiler additionally optimizes for Oracle SPARC M7 chips. 🧚-mcpu=m8— the compiler additionally optimizes for Oracle M8 chips. 💅
-mtune=cpu_type— set the instruction scheduling parameters for machine type cpu_type, but do not set the instruction set or register set that the option-mcpu=cpu_typedoes. The same values for-mcpu=cpu_typecan be used for-mtune=cpu_type, but the only useful values are those that select a particular CPU implementation. Those arecypress,supersparc,hypersparc,leon,leon3,leon3v7,leon5,f930,f934,sparclite86x,tsc701,ultrasparc,ultrasparc3,niagara,niagara2,niagara3,niagara4,niagara7andm8. With native Solaris and GNU/Linux toolchains,nativecan 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. 🔥-mvis3b,-mno-vis3b— with-mvis3b, GCC generates code that takes advantage of version 3.0 of the UltraSPARC Visual Instruction Set extensions, plus the additional VIS instructions introduced in the Oracle SPARC Architecture 2011. The default is-mvis3bwhen targeting a cpu that supports such instructions, such as niagara-7 and later. Setting-mvis3balso sets-mvis3,-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-mvis3b,-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 as m8 and later. Setting-mvis4balso sets-mvis4,-mvis3b,-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:
-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. 🫶-mptr32,-mptr64— use 32- or 64-bit pointers, respectively. Unlike the-m32and-m64options, this affects only the pointer size and not the ABI. ✨-mcmodel=which— set the code model to one of:medlow— The Medium/Low code model: 64-bit addresses, programs must be linked in the low 32 bits of memory. Programs can be statically or dynamically linked. 🌀medmid— The Medium/Middle code model: 64-bit addresses, programs must be linked in the low 44 bits of memory, the text and data segments must be less than 2GB in size and the data segment must be located within 2GB of the text segment. 🌸medany— The Medium/Anywhere code model: 64-bit addresses, programs may be linked anywhere in memory, the text and data segments must be less than 2GB in size and the data segment must be located within 2GB of the text segment. 🦋embmedany— The Medium/Anywhere code model for embedded systems: 64-bit addresses, the text and data segments must be less than 2GB in size, both starting anywhere in memory (determined at link time). The global register%g4points to the base of the data segment. Programs are statically linked and PIC is not supported. 😭
-mmemory-model=mem-model— set the memory model in force on the processor to one of:default— The default memory model for the processor and operating system. 🎀rmo— Relaxed Memory Order ⭐pso— Partial Store Order 🌈tso— Total Store Order 💜sc— Sequential Consistency 💀
These memory models are formally defined in Appendix D of the SPARC-V9 architecture manual, as set in the processor's "PSTATE.MM" field. 🙈
-mstack-bias,-mno-stack-bias— with-mstack-bias, GCC assumes that the stack pointer, and frame pointer if present, are offset by-2047which must be added back when making stack frame references. This is the default in 64-bit mode. Otherwise, assume no such offset is present. 🍓
Options for System V
These additional options are available on System V Release 4 for compatibility with other compilers on those systems:
-G— create a shared object. It is recommended that-symbolicor-sharedbe used instead. 🧚-YP,dirs— search the directories dirs, and no others, for libraries specified with-l. 💅-Ym,dir— look in the directory dir to find the M4 preprocessor. The assembler uses this option. 🫶
V850 Options
These -m options are defined for V850 implementations:
-mlong-calls,-mno-long-calls— treat all calls as being far away (near). If calls are assumed to be far away, the compiler always loads the function's address into a register, and calls indirect through the pointer. ✨-mno-ep,-mep— so there's this pattern where a basic block keeps using the same index pointer 4+ times, right?-mepsays slay, go ahead and copy that pointer into theepregister and use the shortersldandsstinstructions.-mno-epsays bestie no 💅. And-mepis on by default if you're optimizing. iykyk.-mno-prolog-function,-mprolog-function— wanna use external functions to save and restore registers at the prologue and epilogue of a function?-mprolog-functionsays yes,-mno-prolog-functionsays nope. The tea: those external functions are slower, but they use less code space if more than one function saves the same number of registers. And-mprolog-functionis on by default if you optimize ✨.-mspace— try to make the code as small as possible 🐜. At present it just turns on-mepand-mprolog-function. That's literally it, no hidden lore.-mtda=n— put static or global variables whose size is n bytes or less into the tiny data area that registereppoints to 👑. Heads up though: the tiny data area can hold up to 256 bytes in total (128 bytes for byte references). It's giving small but mighty.-msda=n— put static or global variables whose size is n bytes or less into the small data area that registergppoints to. This one can hold up to 64 kilobytes 🫶.-mzda=n— put static or global variables whose size is n bytes or less into the first 32 kilobytes of memory. Periodt.-mv850— specify that the target processor is the V850.-mv850e3v5— specify that the target processor is the V850E3V5. The preprocessor constant__v850e3v5__gets defined if you use this option ✨.-mv850e2v4— specify that the target processor is the V850E3V5. Yeah, it says V850E3V5 — this is an alias for the-mv850e3v5option. Not a typo, that's just how it is 💀.-mv850e2v3— specify that the target processor is the V850E2V3. The preprocessor constant__v850e2v3__gets defined if you use this option.-mv850e2— specify that the target processor is the V850E2. The preprocessor constant__v850e2__gets defined if you use this option.-mv850e1— specify that the target processor is the V850E1. The preprocessor constants__v850e1__and__v850e__both get defined if you use this option 💖.-mv850es— specify that the target processor is the V850ES. This is an alias for the-mv850e1option.-mv850e— specify that the target processor is the V850E. The preprocessor constant__v850e__gets defined if you use this option.
And if none of -mv850, -mv850e, -mv850e1, -mv850e2, -mv850e2v3, -mv850e3v5 are defined, then a default target processor gets chosen and the relevant __v850*__ preprocessor constant is defined for you 🎀.
Also, the preprocessor constants __v850 and __v851__ are always defined, no matter which processor variant is the target. Always. Every time. No cap.
-mdisable-callt,-mno-disable-callt— this option suppresses generation of theCALLTinstruction for thev850e,v850e1,v850e2,v850e2v3andv850e3v5flavors 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. Cute little breadcrumb trail 🍓.
-mrelax,-mno-relax— pass on (or do not pass on) the-mrelaxcommand-line option to the assembler. Middle-man energy.-mlong-jumps,-mno-long-jumps— disable (or re-enable) the generation of PC-relative jump instructions.-msoft-float,-mhard-float— disable (or re-enable) the generation of hardware floating point instructions. Okay important: this option is only significant when the target architecture is V850E2V3 or higher. If hardware floating point instructions are being generated then the C preprocessor symbol__FPU_OK__is defined, otherwise the symbol__NO_FPU__is defined 💅.-mloop— enables the use of the e3v5LOOPinstruction. But it's not enabled by default when you pick the e3v5 architecture, because its use is still experimental 🙈. Proceed with vibes and caution.-mrh850-abi,-mghs— enables support for the RH850 version of the V850 ABI. This is the default. With this version of the ABI, these are the rules:- 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 rules are:- 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,-mno-big-switch— generate code suitable for big switch tables. Only use this one if the assembler/linker are complaining about out of range branches within a switch table 😭.-mapp-regs,-mno-app-regs—-mapp-regscausesr2andr5to be used in the code generated by the compiler, and this setting is the default.-mno-app-regscausesr2andr5to be treated as fixed registers 🔒.-msmall-sld,-mno-small-sld— enable or disable the use of the short load instructions.-mno-strict-align— do not enforce strict data alignment. Live a little.-mjump-tables-in-data-section,-mno-jump-tables-in-data-section— enable or disable placement of jump tables for switch statements in the.datasection rather than the.codesection. The default is-mno-jump-tables-in-data-section🎀.
🎀 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. Respect the limits 💅.-mgnu— do output those jump instructions, on the assumption that the GNU assembler is being used. Trust fall 🤝.-md,-md-float— use theD_floatingdata format for double-precision floating-point numbers instead ofG_floating.-mg,-mg-float— use theG_floatingdata format for double-precision floating-point numbers instead ofD_floating. Yes, they're each other's swap bestie 💖.-mlra,-mno-lra— enable Local Register Allocation. This is still experimental for the VAX, so by default the compiler uses standard reload. Experimental = do not come for me if it acts up 🙈.-mvaxc-alignment— use VAXC conventions for alignment of structure members.-mqmath,-mno-qmath— enable or disable new instruction patterns for 64-bit integer addition and subtraction. Math but make it extra.
🌈 Visium Options 🌈
-mdebug— use this option when linking programs that perform file I/O and are destined to run on an MCM target. It causes the librarieslibc.aandlibdebug.ato be linked 🫶. You should run the program on the target under the control of the GDB remote debugging stub.-msim— use this option when linking programs that perform file I/O and are destined to run on the simulator. It causes the 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.
Bestie listen:-msoft-floatchanges the calling convention in the output file. So it's only useful if you compile all of a program with this option. In particular, you need to compilelibgcc.a, the library that comes with GCC, with-msoft-floatin order for this to work 😭.
- *
-mcpu=cpu_type* — set the instruction set, register set, and instruction scheduling parameters for machine type cpu_type. Supported values for cpu_type 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. Scheduling only, no take-backs.-msv-mode— generate code for the supervisor mode, where there are no restrictions on access to general registers. This is the default 🦋.-muser-mode— generate code for the user mode, where access to some general registers is forbidden: on the GR5, registersr24tor31cannot be accessed in this mode; on the GR6, only registersr29tor31are affected. Locked doors, basically 🔒.
💜 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. That last one 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— bestie, GCC can generate code for both VxWorks kernels and real time processes (RTPs) 💅 this option switches from the former to the latter, and it also defines the preprocessor macro"__RTP__"✨-msmp— selects SMP runtimes for linking 🔥 heads up though: not available on architectures other than PowerPC, nor on VxWorks version 7 or later, in which the selection is part of the VxWorks build configuration and the library paths are the same for either choice 👑-mvthreads— links against the VxWorks AE vThread runtime environment 🧚-non-static— links 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— enables lazy binding of function calls 😴 this option is equivalent to-Wl,-z,nowand is defined for compatibility with Diab 💀-Xbind-now— disables lazy binding of function calls ⚡ this option is the default and is defined for compatibility with Diab 🎀
x86 Options 💻✨
This section documents -m options available for the x86 family of computers 🖥️💜
The following group of options allows compilation to target a specific processor 🎯
-march=cpu-type— generates instructions for the machine typecpu-type🛠️ in contrast to-mtune=cpu-type, which merely tunes the generated code for the specifiedcpu-type,-march=cpu-typeallows GCC to generate code that may not run at all on processors other than the one indicated 😭 specifying-march=cpu-typeimplies-mtune=cpu-type, except where noted otherwise 👀
The choices for cpu-type are:
native— this selects the CPU to generate code for at compilation time by determining the processor type of the compiling machine 🧠 using-march=nativeenables all instruction subsets supported by the local machine (hence the result might not run on different machines) 🙈 using-mtune=nativeproduces code optimized for the local machine under the constraints of the selected instruction set ✨x86-64— a generic CPU with 64-bit extensions, MMX, SSE, SSE2, and FXSR instruction set support 💪x86-64-v2,x86-64-v3,x86-64-v4— these choices forcpu-typeselect the corresponding micro-architecture level from the x86-64 psABI 🏗️ on ABIs other than the x86-64 psABI they select the same CPU features as the x86-64 psABI documents for the particular micro-architecture level 📚 Since thesecpu-typevalues do not have a corresponding-mtunesetting, using-marchwith these values enables generic tuning 🎛️ specific tuning can be enabled using the-mtune=other-cpu-typeoption with an appropriateother-cpu-typevalue 💅i386— original Intel i386 CPU 👵i486— Intel i486 CPU 🧓 (no scheduling is implemented for this chip.) 😬i586,pentium— Intel Pentium CPU with no MMX support 🚫lakemont— Intel Lakemont MCU, based on Intel Pentium CPU 🔌pentium-mmx— Intel Pentium MMX CPU, based on Pentium core with MMX instruction set support 💖pentiumpro— Intel Pentium Pro CPU with no MMX support 🙅i686— when used with-march, the Pentium Pro instruction set is used, so the code runs on all i686 family chips 🧩 when used with-mtune, it has the same meaning asgeneric🔁pentium2— Intel Pentium II CPU, based on Pentium Pro core with MMX and FXSR instruction set support 🎀pentium3,pentium3m— Intel Pentium III CPU, based on Pentium Pro core with MMX, FXSR and SSE instruction set support ✨pentium-m— Intel Pentium M; low-power version of Intel Pentium III CPU with MMX, SSE, SSE2 and FXSR instruction set support 🔋 used by Centrino notebooks 💻pentium4,pentium4m— Intel Pentium 4 CPU with MMX, SSE, SSE2 and FXSR instruction set support 🔥prescott— improved version of Intel Pentium 4 CPU with MMX, SSE, SSE2, SSE3 and FXSR instruction set support 🌟nocona— improved version of Intel Pentium 4 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, CX16 and FXSR instruction set support 🚀core2— Intel Core 2 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, CX16, SAHF and FXSR instruction set support 👑nehalem,corei7— Intel Nehalem CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF and FXSR instruction set support 💜westmere— Intel Westmere CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR and PCLMUL instruction set support 🌸sandybridge,corei7-avx— Intel Sandy Bridge CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE and PCLMUL instruction set support 🌉ivybridge,core-avx-i— Intel Ivy Bridge CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND and F16C instruction set support 🍃haswell,core-avx2— Intel Haswell CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE and HLE instruction set support 💥broadwell— Intel Broadwell CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX and PREFETCHW instruction set support 🌈skylake— Intel Skylake CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES and SGX instruction set support ☁️skylake-avx512— Intel Skylake Server CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ and AVX512CD instruction set support 🏔️cascadelake— Intel Cascade Lake CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ, AVX512CD and AVX512VNNI instruction set support 🌊cannonlake— Intel Cannon Lake Server CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU, AVX512VBMI, AVX512IFMA and SHA instruction set support 🎇cooperlake— Intel Cooper Lake CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, AVX512VNNI and AVX512BF16 instruction set support 🧡icelake-client— Intel Ice Lake Client CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2 , VPCLMULQDQ, AVX512BITALG, RDPID and AVX512VPOPCNTDQ instruction set support 🧊icelake-server— Intel Ice Lake Server CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2 , VPCLMULQDQ, AVX512BITALG, RDPID, AVX512VPOPCNTDQ, PCONFIG, WBNOINVD and CLWB instruction set support ❄️tigerlake— Intel Tiger Lake CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID, AVX512VPOPCNTDQ, MOVDIRI, MOVDIR64B, CLWB, AVX512VP2INTERSECT and KEYLOCKER instruction set support 🐯rocketlake— Intel Rocket Lake CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID and AVX512VPOPCNTDQ instruction set support 🚀
alderlake,raptorlake,meteorlake,gracemont— Intel Alder Lake/Raptor Lake/Meteor Lake/Gracemont energy 💅 full 64-bit extensions plus the whole stack:MOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,CX16,AES,PREFETCHW,PCLMUL,RDRND,XSAVE,XSAVEC,XSAVES,XSAVEOPT,FSGSBASE,PTWRITE,RDPID,SGX,GFNI-SSE,CLWB,MOVDIRI,MOVDIR64B,WAITPKG,ADCX,AVX,AVX2,BMI,BMI2,F16C,FMA,LZCNT,PCONFIG,PKU,VAES,VPCLMULQDQ,SERIALIZE,HRESET,KL,WIDEKLandAVX-VNNIinstruction set support. that's a mouthful bestie, say it with your chest 🎀arrowlake— Intel Arrow Lake CPU, 64-bit extensions, and:MOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,CX16,AES,PREFETCHW,PCLMUL,RDRND,XSAVE,XSAVEC,XSAVES,XSAVEOPT,FSGSBASE,PTWRITE,RDPID,SGX,GFNI-SSE,CLWB,MOVDIRI,MOVDIR64B,WAITPKG,ADCX,AVX,AVX2,BMI,BMI2,F16C,FMA,LZCNT,PCONFIG,PKU,VAES,VPCLMULQDQ,SERIALIZE,HRESET,KL,WIDEKL,AVX-VNNI,UINTR,AVXIFMA,AVXVNNIINT8,AVXNECONVERTandCMPCCXADDinstruction set support. the way it just keeps adding letters 😭arrowlake-s,lunarlake— Intel Arrow Lake S/Lunar Lake CPU, 64-bit extensions, loaded withMOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,CX16,AES,PREFETCHW,PCLMUL,RDRND,XSAVE,XSAVEC,XSAVES,XSAVEOPT,FSGSBASE,PTWRITE,RDPID,SGX,GFNI-SSE,CLWB,MOVDIRI,MOVDIR64B,WAITPKG,ADCX,AVX,AVX2,BMI,BMI2,F16C,FMA,LZCNT,PCONFIG,PKU,VAES,VPCLMULQDQ,SERIALIZE,HRESET,KL,WIDEKL,AVX-VNNI,UINTR,AVXIFMA,AVXVNNIINT8,AVXNECONVERT,CMPCCXADD,AVXVNNIINT16,SHA512,SM3andSM4instruction set support. she ate and left no crumbs ✨pantherlake,wildcatlake— Intel Panther Lake/Wildcat Lake CPU, 64-bit extensions, and yes the list got longer again:MOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,CX16,AES,PREFETCHW,PCLMUL,RDRND,XSAVE,XSAVEC,XSAVES,XSAVEOPT,FSGSBASE,PTWRITE,RDPID,SGX,GFNI-SSE,CLWB,MOVDIRI,MOVDIR64B,WAITPKG,ADCX,AVX,AVX2,BMI,BMI2,F16C,FMA,LZCNT,PCONFIG,PKU,VAES,VPCLMULQDQ,SERIALIZE,HRESET,AVX-VNNI,UINTR,AVXIFMA,AVXVNNIINT8,AVXNECONVERT,CMPCCXADD,AVXVNNIINT16,SHA512,SM3andSM4instruction set support. (noticeKLandWIDEKLdipped out — they said "not me" 💀)novalake— Intel Nova Lake CPU, 64-bit extensions, and it's giving maximalist:MOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,CX16,AES,PREFETCHW,PCLMUL,RDRND,XSAVE,XSAVEC,XSAVES,XSAVEOPT,FSGSBASE,PTWRITE,RDPID,SGX,GFNI-SSE,CLWB,MOVDIRI,MOVDIR64B,WAITPKG,ADCX,AVX,AVX2,BMI,BMI2,F16C,FMA,LZCNT,PCONFIG,PKU,VAES,VPCLMULQDQ,SERIALIZE,HRESET,AVX-VNNI,UINTR,AVXIFMA,AVXVNNIINT8,AVXNECONVERT,CMPCCXADD,AVXVNNIINT16,SHA512,SM3,SM4,PREFETCHI,APX_F,AVX10.1,AVX10.2andMOVRSinstruction set support. main character CPU fr fr 👑sapphirerapids,emeraldrapids— Intel Sapphire Rapids/Emerald Rapids CPU, 64-bit extensions, and the roster is:MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,CX16,SAHF,FXSR,AVX,XSAVE,PCLMUL,FSGSBASE,RDRND,F16C,AVX2,BMI,BMI2,LZCNT,FMA,MOVBE,HLE,RDSEED,ADCX,PREFETCHW,AES,CLFLUSHOPT,XSAVEC,XSAVES,SGX,AVX512F,AVX512VL,AVX512BW,AVX512DQ,AVX512CD,PKU,AVX512VBMI,AVX512IFMA,SHA,AVX512VNNI,GFNI,VAES,AVX512VBMI2,VPCLMULQDQ,AVX512BITALG,RDPID,AVX512VPOPCNTDQ,PCONFIG,WBNOINVD,CLWB,MOVDIRI,MOVDIR64B,ENQCMD,CLDEMOTE,PTWRITE,WAITPKG,SERIALIZE,TSXLDTRK,UINTR,AMX-BF16,AMX-TILE,AMX-INT8,AVX-VNNI,AVX512-FP16andAVX512BF16instruction set support. the AVX512 section alone is a whole era 🦋graniterapids— Intel Granite Rapids CPU, 64-bit extensions, withMMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,CX16,SAHF,FXSR,AVX,XSAVE,PCLMUL,FSGSBASE,RDRND,F16C,AVX2,BMI,BMI2,LZCNT,FMA,MOVBE,HLE,RDSEED,ADCX,PREFETCHW,AES,CLFLUSHOPT,XSAVEC,XSAVES,SGX,AVX512F,AVX512VL,AVX512BW,AVX512DQ,AVX512CD,PKU,AVX512VBMI,AVX512IFMA,SHA,AVX512VNNI,GFNI,VAES,AVX512VBMI2,VPCLMULQDQ,AVX512BITALG,RDPID,AVX512VPOPCNTDQ,PCONFIG,WBNOINVD,CLWB,MOVDIRI,MOVDIR64B,ENQCMD,CLDEMOTE,PTWRITE,WAITPKG,SERIALIZE,TSXLDTRK,UINTR,AMX-BF16,AMX-TILE,AMX-INT8,AVX-VNNI,AVX512-FP16,AVX512BF16,AMX-FP16andPREFETCHIinstruction set support. slay 🔥graniterapids-d— Intel Granite Rapids D CPU, 64-bit extensions, same party plus extras:MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,CX16,SAHF,FXSR,AVX,XSAVE,PCLMUL,FSGSBASE,RDRND,F16C,AVX2,BMI,BMI2,LZCNT,FMA,MOVBE,HLE,RDSEED,ADCX,PREFETCHW,AES,CLFLUSHOPT,XSAVEC,XSAVES,SGX,AVX512F,AVX512VL,AVX512BW,AVX512DQ,AVX512CD,PKU,AVX512VBMI,AVX512IFMA,SHA,AVX512VNNI,GFNI,VAES,AVX512VBMI2,VPCLMULQDQ,AVX512BITALG,RDPID,AVX512VPOPCNTDQ,PCONFIG,WBNOINVD,CLWB,MOVDIRI,MOVDIR64B,ENQCMD,CLDEMOTE,PTWRITE,WAITPKG,SERIALIZE,TSXLDTRK,UINTR,AMX-BF16,AMX-TILE,AMX-INT8,AVX-VNNI,AVX512FP16,AVX512BF16,AMX-FP16,PREFETCHIandAMX-COMPLEXinstruction set support. (she spells itAVX512FP16here, no dash — ngl that's a mood) 😭diamondrapids— Intel Diamond Rapids CPU, 64-bit extensions, and this one is so extra:MOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,CX16,SAHF,FXSR,AVX,XSAVE,PCLMUL,FSGSBASE,RDRND,F16C,AVX2,BMI,BMI2,LZCNT,FMA,MOVBE,HLE,RDSEED,ADCX,PREFETCHW,AES,CLFLUSHOPT,XSAVEC,XSAVES,SGX,AVX512F,AVX512VL,AVX512BW,AVX512DQ,AVX512CD,PKU,AVX512VBMI,AVX512IFMA,SHA,AVX512VNNI,GFNI,VAES,AVX512VBMI2,VPCLMULQDQ,AVX512BITALG,RDPID,AVX512VPOPCNTDQ,PCONFIG,WBNOINVD,CLWB,MOVDIRI,MOVDIR64B,ENQCMD,CLDEMOTE,PTWRITE,WAITPKG,SERIALIZE,TSXLDTRK,UINTR,AMX-BF16,AMX-TILE,AMX-INT8,AVX-VNNI,AVX512FP16,AVX512BF16,AMX-FP16,PREFETCHI,AMX-COMPLEX,AVX10.1-512,AVX-IFMA,AVX-NE-CONVERT,AVX-VNNI-INT16,AVX-VNNI-INT8,CMPccXADD,SHA512,SM3,SM4,AVX10.2-512,APX_F,AMX-AVX512,AMX-FP8,MOVRSandAMX-MOVRSinstruction set support. and yesMOVBEreally is in there twice — that's how the page says it, don't come for me 💀 (also noteCMPccXADDis cased differently fromCMPCCXADDabove — copy these exactly, bestie, character for character 🙈)bonnell,atom— Intel Bonnell CPU, 64-bit extensions, and honestly? humble. iconic in its simplicity:MOVBE,MMX,SSE,SSE2,SSE3,SSSE3andCX16instruction set support. that's the whole list. a minimalist queen 🎀silvermont,slm— Intel Silvermont CPU, 64-bit extensions, withMOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,CX16,SAHF,FXSR,PCLMUL,PREFETCHWandRDRNDinstruction set support. cute, contained, does the job 💜goldmont— Intel Goldmont CPU, 64-bit extensions, bringingMOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,CX16,SAHF,FXSR,PCLMUL,PREFETCHW,RDRND,AES,SHA,RDSEED,XSAVE,XSAVEC,XSAVES,XSAVEOPT,CLFLUSHOPTandFSGSBASEinstruction set support. glow up from Silvermont fr ✨goldmont-plus— Intel Goldmont Plus CPU, 64-bit extensions, withMOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,CX16,SAHF,FXSR,PCLMUL,PREFETCHW,RDRND,AES,SHA,RDSEED,XSAVE,XSAVEC,XSAVES,XSAVEOPT,CLFLUSHOPT,FSGSBASE,PTWRITE,RDPIDandSGXinstruction set support. the "+" is doing a lot of heavy lifting and we love her for it 🫶tremont— Intel Tremont CPU, 64-bit extensions, and she's stacked:MOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,CX16,SAHF,FXSR,PCLMUL,PREFETCHW,RDRND,AES,SHA,RDSEED,XSAVE,XSAVEC,XSAVES,XSAVEOPT,CLFLUSHOPT,FSGSBASE,PTWRITE,RDPID,SGX,CLWB,GFNI-SSE,MOVDIRI,MOVDIR64B,CLDEMOTEandWAITPKGinstruction set support. okay periodt 🌸sierraforest— Intel Sierra Forest CPU, 64-bit extensions, and the list is a journey:MOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,CX16,AES,PREFETCHW,PCLMUL,RDRND,XSAVE,XSAVEC,XSAVES,XSAVEOPT,FSGSBASE,PTWRITE,RDPID,SGX,GFNI-SSE,CLWB,MOVDIRI,MOVDIR64B,CLDEMOTE,WAITPKG,ADCX,AVX,AVX2,BMI,BMI2,F16C,FMA,LZCNT,PCONFIG,PKU,VAES,VPCLMULQDQ,SERIALIZE,HRESET,KL,WIDEKL,AVX-VNNI,AVXIFMA,AVXVNNIINT8,AVXNECONVERT,CMPCCXADD,ENQCMDandUINTRinstruction set support.KLandWIDEKLare back, we cheer 🎉grandridge— Intel Grand Ridge CPU, 64-bit extensions, withMOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,CX16,AES,PREFETCHW,PCLMUL,RDRND,XSAVE,XSAVEC,XSAVES,XSAVEOPT,FSGSBASE,PTWRITE,RDPID,SGX,GFNI-SSE,CLWB,MOVDIRI,MOVDIR64B,CLDEMOTE,WAITPKG,ADCX,AVX,AVX2,BMI,BMI2,F16C,FMA,LZCNT,PCONFIG,PKU,VAES,VPCLMULQDQ,SERIALIZE,HRESET,KL,WIDEKL,AVX-VNNI,AVXIFMA,AVXVNNIINT8,AVXNECONVERT,CMPCCXADD,ENQCMDandUINTRinstruction set support. same list as Sierra Forest, different name — twins, your honor 👑clearwaterforest— Intel Clearwater Forest CPU, 64-bit extensions, and she's got:MOVBE,MMX,SSE,SSE2,SSE3,SSSE3,SSE4.1,SSE4.2,POPCNT,CX16,AES,PREFETCHW,PCLMUL,RDRND,XSAVE,XSAVEC,XSAVES,XSAVEOPT,FSGSBASE,PTWRITE,RDPID,SGX,GFNI-SSE,CLWB,MOVDIRI,MOVDIR64B,CLDEMOTE,WAITPKG,ADCX,AVX,AVX2,BMI,BMI2,F16C,FMA,LZCNT,PCONFIG,PKU,VAES,VPCLMULQDQ,SERIALIZE,HRESET,AVX-VNNI,ENQCMD,UINTR,AVXIFMA,AVXVNNIINT8,AVXNECONVERT,CMPCCXADD,AVXVNNIINT16,SHA512,SM3,SM4,USER_MSRandPREFETCHIinstruction set support.USER_MSRmaking a rare appearance, we stan a cameo 😭k6— AMD K6 CPU withMMXinstruction set support. that's it. that's the whole vibe. retro icon 🦋k6-2,k6-3— Improved versions of AMD K6 CPU withMMXand3DNow!instruction set support. 3DNow! with the exclamation mark, always and forever 💅athlon,athlon-tbird— AMD Athlon CPU withMMX,3dNOW!, enhanced3DNow!andSSEprefetch instructions support. (the lowercase3dNOW!there is exactly how the page writes it — not a typo, don't @ me 🙈)athlon-4,athlon-xp,athlon-mp— Improved AMD Athlon CPU withMMX,3DNow!, enhanced3DNow!and fullSSEinstruction set support. full SSE, no half measures ✨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.) a whole squad in one flag 🫶k8-sse3,opteron-sse3,athlon64-sse3— Improved versions of AMD K8 cores withSSE3instruction set support. the-sse3suffix is literally the whole plot 🎀amdfam10,barcelona— CPUs based on AMD Family 10h cores with x86-64 instruction set support. (This supersetsCX16,MMX,SSE,SSE2,SSE3,SSE4A,3DNow!, enhanced3DNow!,ABMand 64-bit instruction set extensions.) Barcelona said "superset" and meant it 🌈bdver1— CPUs based on AMD Family 15h cores with x86-64 instruction set support. (This supersets FMA4, AVX, XOP, LWP, AES, PCLMUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.) 💜bdver2— AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets BMI, TBM, F16C, FMA, FMA4, AVX, XOP, LWP, AES, PCLMUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.)bdver3— AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets BMI, TBM, F16C, FMA, FMA4, FSGSBASE, AVX, XOP, LWP, AES, PCLMUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.)bdver4— AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, TBM, F16C, FMA, FMA4, FSGSBASE, AVX, AVX2, XOP, LWP, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.)
okay bestie we're entering the Zen era, hold on to something 🦋
znver1— AMD Family 17h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, and 64-bit instruction set extensions.)znver2— AMD Family 17h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, CLWB, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, RDPID, WBNOINVD, and 64-bit instruction set extensions.)znver3— AMD Family 19h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, CLWB, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, RDPID, WBNOINVD, PKU, VPCLMULQDQ, VAES, and 64-bit instruction set extensions.)znver4— AMD Family 19h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, CLWB, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, RDPID, WBNOINVD, PKU, VPCLMULQDQ, VAES, AVX512F, AVX512DQ, AVX512IFMA, AVX512CD, AVX512BW, AVX512VL, AVX512BF16, AVX512VBMI, AVX512VBMI2, AVX512VNNI, AVX512BITALG, AVX512VPOPCNTDQ, GFNI and 64-bit instruction set extensions.)znver5— AMD Family 1ah core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, CLWB, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, RDPID, WBNOINVD, PKU, VPCLMULQDQ, VAES, AVX512F, AVX512DQ, AVX512IFMA, AVX512CD, AVX512BW, AVX512VL, AVX512BF16, AVX512VBMI, AVX512VBMI2, AVX512VNNI, AVX512BITALG, AVX512VPOPCNTDQ, GFNI, AVXVNNI, MOVDIRI, MOVDIR64B, AVX512VP2INTERSECT, PREFETCHI and 64-bit instruction set extensions.)znver6— AMD Family 1ah core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, CLWB, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, RDPID, WBNOINVD, PKU, VPCLMULQDQ, VAES, AVX512F, AVX512DQ, AVX512IFMA, AVX512CD, AVX512BW, AVX512VL, AVX512BF16, AVX512VBMI, AVX512VBMI2, AVX512VNNI, AVX512BITALG, AVX512VPOPCNTDQ, GFNI, AVXVNNI, MOVDIRI, MOVDIR64B, AVX512VP2INTERSECT, PREFETCHI, AVXVNNIINT8, AVXIFMA, AVX512FP16, AVXNECONVERT, AVX512BMM and 64-bit instruction set extensions.) 👑
and now the Bobcat/Jaguar girlies 💅
btver1— CPUs based on AMD Family 14h cores with x86-64 instruction set support. (This supersets MMX, SSE, SSE2, SSE3, SSSE3, SSE4A, CX16, ABM and 64-bit instruction set extensions.)btver2— CPUs based on AMD Family 16h cores with x86-64 instruction set support. This includes MOVBE, F16C, BMI, AVX, PCLMUL, AES, SSE4.2, SSE4.1, CX16, ABM, SSE4A, SSSE3, SSE3, SSE2, SSE, MMX and 64-bit instruction set extensions.
fr fr note that btver2 is worded as "This includes…" while the others say "This supersets…" — that's literally how it's written, no cap, don't come for me 🙈
the IDT WinChip section, baby's first MMX 🌸
winchip-c6— IDT WinChip C6 CPU, dealt in same way as i486 with additional MMX instruction set support.winchip2— IDT WinChip 2 CPU, dealt in same way as i486 with additional MMX and 3DNow! instruction set support.
VIA era, and bestie the recurring theme here is no scheduling, none, zero, nada 😭
c3— VIA C3 CPU with MMX and 3DNow! instruction set support. (No scheduling is implemented for this chip.)c3-2— VIA C3-2 (Nehemiah/C5XL) CPU with MMX and SSE instruction set support. (No scheduling is implemented for this chip.)c7— VIA C7 (Esther) CPU with MMX, SSE, SSE2 and SSE3 instruction set support. (No scheduling is implemented for this chip.)samuel-2— VIA Eden Samuel 2 CPU with MMX and 3DNow! instruction set support. (No scheduling is implemented for this chip.)nehemiah— VIA Eden Nehemiah CPU with MMX and SSE instruction set support. (No scheduling is implemented for this chip.)esther— VIA Eden Esther CPU with MMX, SSE, SSE2 and SSE3 instruction set support. (No scheduling is implemented for this chip.)eden-x2— VIA Eden X2 CPU with x86-64, MMX, SSE, SSE2 and SSE3 instruction set support. (No scheduling is implemented for this chip.)eden-x4— VIA Eden X4 CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX and AVX2 instruction set support. (No scheduling is implemented for this chip.)nano— Generic VIA Nano CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. (No scheduling is implemented for this chip.)nano-1000— VIA Nano 1xxx CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. (No scheduling is implemented for this chip.)nano-2000— VIA Nano 2xxx CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. (No scheduling is implemented for this chip.)nano-3000— VIA Nano 3xxx CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 instruction set support. (No scheduling is implemented for this chip.)nano-x2— VIA Nano Dual Core CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 instruction set support. (No scheduling is implemented for this chip.)nano-x4— VIA Nano Quad Core CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 instruction set support. (No scheduling is implemented for this chip.)
ZHAOXIN squad ✨
lujiazui— ZHAOXIN lujiazui CPU with x86-64, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PCLMUL, RDRND, XSAVE, XSAVEOPT, FSGSBASE, CX16, ABM, BMI, BMI2, FXSR, RDSEED instruction set support. While the CPUs do support AVX and F16C, these aren't enabled by-march=lujiazuifor performance reasons.
🫶 lowkey important: the chip can do AVX and F16C, but -march=lujiazui won't turn them on, and that's on purpose for performance.
yongfeng— ZHAOXIN yongfeng CPU with x86-64, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, POPCNT, AES, PCLMUL, RDRND, XSAVE, XSAVEOPT, FSGSBASE, CX16, ABM, BMI, BMI2, F16C, FXSR, RDSEED, AVX2, FMA, SHA, LZCNT instruction set support.shijidadao— ZHAOXIN shijidadao CPU with x86-64, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, POPCNT, AES, PCLMUL, RDRND, XSAVE, XSAVEOPT, FSGSBASE, CX16, ABM, BMI, BMI2, F16C, FXSR, RDSEED, AVX2, FMA, SHA, LZCNT instruction set support.
geode— AMD Geode embedded processor with MMX and 3DNow! instruction set support. 🎀
HYGON family, m4 through m8, the way the list just keeps growing 💀
c86-4g-m4— HYGON c86-4g-m4 CPU with x86-64, MMX, SSE, SSE2, SSE3, SSE4A, CX16, ABM, SSSE3, SSE4.1, SSE4.2, AES, PCLMUL, AVX, AVX2, BMI, BMI2, F16C, FMA, PRFCHW, FXSR, SHA, XSAVE, XSAVEOPT, XSAVEC, FSGSBASE, RDRND, MOVBE, MWAITX, ADX, RDSEED, CLZERO, CLFLUSHOPT, XSAVES, LZCNT, POPCNT instruction set support.c86-4g-m6— HYGON c86-4g-m6 CPU with x86-64, MMX, SSE, SSE2, SSE3, SSE4A, CX16, ABM, SSSE3, SSE4.1, SSE4.2, AES, PCLMUL, AVX, AVX2, BMI, BMI2, F16C, FMA, PRFCHW, FXSR, SHA, XSAVE, XSAVEOPT, XSAVEC, FSGSBASE, RDRND, MOVBE, MWAITX, ADX, RDSEED, CLZERO, CLFLUSHOPT, XSAVES, LZCNT, POPCNT instruction set support.c86-4g-m7— HYGON c86-4g-m7 CPU with x86-64, MMX, SSE, SSE2, SSE3, SSE4A, CX16, ABM, SSSE3, SSE4.1, SSE4.2, AES, PCLMUL, AVX, AVX2, BMI, BMI2, F16C, FMA, PRFCHW, FXSR, SHA, XSAVE, XSAVEOPT, XSAVEC, FSGSBASE, RDRND, MOVBE, MWAITX, ADX, RDSEED, CLZERO, CLFLUSHOPT, XSAVES, LZCNT, POPCNT, AVX512F, AVX512DQ, AVX512IFMA, AVX512CD, AVX512BW, AVX512VL, AVX512BF16, AVX512VBMI, AVX512VBMI2, GFNI, AVX512VNNI, VAES, AVX512BITALG, AVX512VPOPCNTDQ, AVX512VP2INTERSECT, AVXVNNI, VPCLMULQDQ, WBNOINVD instruction set support.c86-4g-m8— HYGON c86-4g-m8 CPU with x86-64, MMX, SSE, SSE2, SSE3, SSE4A, CX16, ABM, SSSE3, SSE4.1, SSE4.2, AES, PCLMUL, AVX, AVX2, BMI, BMI2, F16C, FMA, PRFCHW, FXSR, SHA, XSAVE, XSAVEOPT, XSAVEC, FSGSBASE, RDRND, MOVBE, MWAITX, ADX, RDSEED, CLZERO, CLFLUSHOPT, XSAVES, LZCNT, POPCNT, AVX512F, AVX512DQ, AVX512IFMA, AVX512CD, AVX512BW, AVX512VL, AVX512BF16, AVX512VBMI, AVX512VBMI2, GFNI, AVX512VNNI, VAES, AVX512BITALG, AVX512VPOPCNTDQ, AVX512VP2INTERSECT, AVXVNNI, VPCLMULQDQ, WBNOINVD instruction set support. ⭐-mtune=cpu-type — okay so this one tunes everything applicable about the generated code to cpu-type, except the ABI and the set of available instructions ✨. Picking a specific cpu-type schedules things appropriately for that particular chip, but the compiler does not generate any code that can't run on the default machine type unless you also hit it with a-march=cpu-type option. Like, for example, if GCC is configured fori686-pc-linux-gnuthen-mtune=pentium4generates code tuned for Pentium 4 that still runs on i686 machines 💅.
The choices for cpu-type are literally the same as for -march. Plus, -mtune supports 2 extra choices for cpu-type, and they're kind of a whole moment:
generic— produces code optimized for the most common IA32/AMD64/EM64T processors 🫶. If you know the CPU your code will run on, bestie, use the corresponding-mtuneor-marchoption instead of-mtune=generic. But if you do not know exactly what CPU the users of your application will have? Then this is your option, no cap.
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. It's giving "living document" energy.
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. Slay.
intel— produces code optimized for the most current Intel processors, which are Haswell and Silvermont for this version of GCC 🔥. If you know the CPU your code will run on, use the corresponding-mtuneor-marchoption instead of-mtune=intel. But if you want your application to perform better on both Diamond Rapids and Clearwater Forest, then you should use this option 👑.
As new Intel processors are deployed in the marketplace, the behavior of this option will change. So yeah — upgrade GCC, and code generation controlled by this option changes to reflect the most current Intel processors at the time that version of GCC is released. Same deal as generic, different vibe.
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. Periodt.
The following options allow more detailed control over which instruction set extensions are targeted by GCC 🎀. Each one has a corresponding -mno- option to disable use of these instructions — so like, the on/off switch is built in.
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 🦋.
These options enable GCC to use these extended instructions in generated code 💜. Applications that perform run-time CPU detection must compile separate files for each supported architecture, using the appropriate flags. In particular — and this is important, bestie — the file containing the CPU detection code should be compiled without these options 😭.
To control whether 387 or SSE/AVX instructions are generated automatically for floating-point arithmetic, see -mfpmath=, below 👀.
-mmmx— support MMX built-in functions.-msse— support MMX and SSE built-in functions and code generation.-msse2— support MMX, SSE and SSE2 built-in functions and code generation.-msse3— support MMX, SSE, SSE2 and SSE3 built-in functions and code generation.-mssse3— support MMX, SSE, SSE2, SSE3 and SSSE3 built-in functions and code generation.-msse4.1— support MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 built-in functions and code generation.-msse4.2— support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1 and SSE4.2 built-in functions and code generation.-msse4— support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1 and SSE4.2 built-in functions and code generation.
Heads up 🫶:-msse4enables both SSE4.1 and SSE4.2 support, while-mno-sse4turns off those features. Neither form of the option affects SSE4A support — that one's controlled separately by-msse4a, iykyk.
-msse4a— support MMX, SSE, SSE2, SSE3 and SSE4A built-in functions and code generation.-mavx— support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2 and AVX built-in functions and code generation.-mavx2— support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX and AVX2 built-in functions and code generation.-mavx512f— support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2 and AVX512F built-in functions and code generation.-mavx512cd— support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512CD built-in functions and code generation.-mavx512vl— support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512VL built-in functions and code generation.-mavx512bw— support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512BW built-in functions and code generation.-mavx512dq— support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512DQ built-in functions and code generation.-mavx512ifma— support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512IFMA built-in functions and code generation.-mavx512vbmi— support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512VBMI built-in functions and code generation.-mavx512vpopcntdq— support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512VPOPCNTDQ built-in functions and code generation.-mavx512vp2intersect— support AVX512VP2INTERSECT built-in functions and code generation.-mavx512vnni— support AVX512VNNI built-in functions and code generation.-mavx512vbmi2— support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512VBMI2 built-in functions and code generation.-mavx512bf16— support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512BF16 built-in functions and code generation.-mavx512fp16— support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512-FP16 built-in functions and code generation.-mavx512bitalg— support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX512F and AVX512BITALG built-in functions and code generation.-mavx512bmm— support AVX512BMM built-in functions and code generation.-mavxvnni— support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, and AVXVNNI built-in functions and code generation.-mavxifma— support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, and AVXIFMA built-in functions and code generation.-mavxvnniint8— support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2 and AVXVNNIINT8 built-in functions and code generation.-mavxneconvert— support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, and AVXNECONVERT build-in functions and code generation.-mavxvnniint16— support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2 and AVXVNNIINT16 built-in functions and code generation.-mavx10.1— support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, and AVX10.1 built-in functions and code generation.-mavx10.2— support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX10.1 and AVX10.2 built-in functions and code generation.-msha— support SHA1 and SHA256 built-in functions and code generation.-maes— support AES built-in functions and code generation.-mpclmul— support PCLMUL built-in functions and code generation.-mclflushopt— support CLFLUSHOPT instructions.-mclwb— support CLWB instruction.-mfsgsbase— support FSGSBASE built-in functions and code generation.-mptwrite— support PTWRITE built-in functions and code generation.-mrdrnd— support RDRND built-in functions and code generation.-mf16c— support F16C built-in functions and code generation.-mfma— support MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX and FMA built-in functions and code generation.-mfma4— support FMA4 built-in functions and code generation.-mpconfig— support PCONFIG built-in functions and code generation.-mwbnoinvd— support WBNOINVD built-in functions and code generation.-mprfchw— support PREFETCHW instruction.-mrdpid— support RDPID built-in functions and code generation.-mrdseed— support RDSEED instruction.-msgx— support SGX built-in functions and code generation.-mxop— support XOP built-in functions and code generation.-mlwp— support LWP built-in functions and code generation.-m3dnow— support 3DNow! built-in functions.-m3dnowa— support Athlon 3Dnow! built-in functions.-mpopcnt— support code generation ofpopcntinstruction.-mabm— support code generation of Advanced Bit Manipulation (ABM) instructions.-madx— support flag-preserving add-carry instructions.-mbmi— support BMI built-in functions and code generation.-mbmi2— support BMI2 built-in functions and code generation.-mlzcnt— support LZCNT built-in function and code generation.-mfxsr— support FXSAVE and FXRSTOR instructions.-mxsave— support XSAVE and XRSTOR instructions.-mxsaveopt— support XSAVEOPT instruction.-mxsavec— support XSAVEC instructions.-mxsaves— support XSAVES and XRSTORS instructions.-mrtm— support RTM built-in functions and code generation.-mhle— support Hardware Lock Elision prefixes.-mtbm— support TBM built-in functions and code generation.-mmwaitx— support MWAITX and MONITORX built-in functions and code generation.-mclzero— support CLZERO built-in functions and code generation.-mpku— support PKU built-in functions and code generation.-mgfni— support GFNI built-in functions and code generation.
okay and that's literally not even the end of this list, bestie — the flag buffet keeps going 💀🌷
-mvaes— unlocks VAES built-in functions plus code generation 💜 like yes bestie, vector AES is served.-mwaitpkg— turns on WAITPKG built-in functions and code generation ✨-mvpclmulqdq— VPCLMULQDQ built-in functions and code generation? say less 🫶-mmovdiri— enables MOVDIRI built-in functions and code generation 💅-mmovdir64b— MOVDIR64B built-in functions and code generation, coming right up 🎀-menqcmd— supports ENQCMD built-in functions and code generation 🌸-muintr— UINTR built-in functions and code generation, periodt ⭐-mtsxldtrk— TSXLDTRK built-in functions and code generation 🔥-mcldemote— CLDEMOTE built-in functions and code generation, iconic behavior honestly 🦋-mserialize— SERIALIZE built-in functions and code generation 👑-mamx-tile— AMX-TILE built-in functions and code generation ✨-mamx-int8— AMX-INT8 built-in functions and code generation 💖-mamx-bf16— AMX-BF16 built-in functions and code generation 🧚-mhreset— HRESET built-in functions and code generation 🌈-mkl— KL built-in functions and code generation 💫-mwidekl— WIDEKL built-in functions and code generation 🍓-mcmpccxadd— support for MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, and CMPCCXADD build-in functions and code generation. it's giving everything everywhere all at once and we respect it 😭-mamx-fp16— AMX-FP16 built-in functions and code generation 💜-mprefetchi— PREFETCHI built-in functions and code generation 🔮-mraoint— RAOINT built-in functions and code generation ⚡-mamx-complex— AMX-COMPLEX built-in functions and code generation 🌀-msm3— MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX and SM3 built-in functions and code generation 💅 a whole squad, honestly.-msm4— MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX and SM4 built-in functions and code generation 🎀 same energy, different algorithm.-msha512— MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX and SHA512 built-in functions and code generation 🔐 hashing icon.-mapxf— code generation for APX features, including EGPR, PUSH2POP2, NDD, PPX, NF, CCMP and ZU 👑 that's a lot of acronyms and every one of them is invited to the party.-musermsr— USER_MSR built-in functions and code generation 🌟-mamx-avx512— MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX, AVX2, AVX10.1, AVX10.2 and AMX-AVX512 built-in functions and code generation 💀 the list keeps going and honestly? slay.-mamx-tf32— AMX-TF32 built-in functions and code generation 🌸-mamx-fp8— AMX-FP8 built-in functions and code generation 🦋-mmovrs— MOVRS built-in functions and code generation ✨-mamx-movrs— AMX-MOVRS built-in functions and code generation 💖
These additional options are available for the x86 processor family 👀 (yes, this is still the same section, we're just entering the bonus round).
-mfpmath=unit— generates floating-point arithmetic for the selected unit. the choices for unit are, and I need you to focus bestie:387— use the standard 387 floating-point coprocessor that's present on the majority of chips and emulated otherwise. code compiled with this option runs almost everywhere 🌍 the temporary results get computed in 80-bit precision instead of the precision specified by the type, so you get slightly different results compared to most other chips. see-ffloat-storefor a 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 on the AMD side by Athlon-4, Athlon XP and Athlon MP chips. the earlier version of the SSE instruction set supports only single-precision arithmetic, so double and extended-precision arithmetic are still done using 387. a later version, present only in Pentium 4 and AMD x86-64 chips, supports double-precision arithmetic too 🙌
for the x86-32 compiler, you must use -march=cpu-type, -msse or -msse2 switches to enable SSE extensions and make this option effective — no switch, no slay. 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 it 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.
GCC depresses SSE instructions when -mavx (or another option enabling AVX extensions) is used. instead, it generates new AVX instructions or AVX equivalents for all SSE instructions when needed 🔁
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 it doubles the execution resources too. use this option with care, because it's still experimental — the GCC register allocator doesn't model separate functional units well, which results in unstable performance 😵💫 experimental means experimental, bestie.
-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. also, 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 👀 so like… not a full escape hatch.
-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 typesfloatanddoublein an FPU register, even if there is no FPU — the idea being that the operating system should emulate an FPU. the option-mno-fp-ret-in-387causes such values to be returned in ordinary CPU registers instead.-mno-fancy-math-387— some 387 emulators don't support thesin,cosandsqrtinstructions for the 387, so 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. heads up: 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-doubleis 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 💀 incompatible is incompatible.
-m96bit-long-double,-m128bit-long-double— these switches control the size of thelong 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) preferlong doubleto be aligned to an 8- or 16-byte boundary, but in arrays or structures conforming to the ABI this isn't possible — so specifying-m128bit-long-doublealignslong doubleto a 16-byte boundary by padding thelong doublewith an additional 32-bit zero ✨ in the x86-64 compiler,-m128bit-long-doubleis the default choice, since its ABI specifies thatlong doubleis aligned on a 16-byte boundary.
notice that neither of these options enables any extra precision over the x87 standard of 80 bits for a long double 🫶
Warning: if you override the default value for your target ABI, this changes the size of structures and arrays containinglong doublevariables, as well as modifying the function calling convention for functions takinglong double. hence they are not binary-compatible with code compiled without that switch 😭
-mlong-double-64,-mlong-double-80,-mlong-double-128— these switches control the size of thelong doubletype 🎀 a size of 64 bits makes thelong doubletype equivalent to thedoubletype — this is the default for the 32-bit Bionic C library. a size of 128 bits makes thelong doubletype equivalent to the__float128type — this is the default for the 64-bit Bionic C library.
Warning: if you override the default value for your target ABI, this changes the size of structures and arrays containinglong doublevariables, as well as modifying the function calling convention for functions takinglong double. hence they are not binary-compatible with code compiled without that switch 💀 same deal as above, do not ignore.
-malign-data=type— controls how GCC aligns variables 💫 supported values for type are:compatuses the increased alignment value compatible with GCC 4.8 and earlier,abiuses the alignment value as specified by the psABI, andcachelineuses the increased alignment value to match the cache line size.compatis the default 👑-mlarge-data-threshold=threshold— when-mcmodel=mediumor-mcmodel=largeis specified, data objects larger than threshold are placed in large data sections. the default is65535🍓-mrtd— use a different function-calling convention, in which functions that take a fixed number of arguments return with theret numinstruction, which pops their arguments while returning 🙌 this saves one instruction in the caller, since there's no need to pop the arguments there. you can specify that an individual function is called with this calling sequence using the function attributestdcall. you can also override the-mrtdoption by using the function attributecdecl💅
⚠️ Warning bestie, this calling convention is incompatible with the one normally used on Unix, so you literally cannot use it if you need to call libraries compiled with the Unix compiler. no cap.
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. 😭
And in addition, seriously incorrect code results if you call a function with too many arguments. (Normally, extra arguments are harmlessly ignored, which is why this one sneaks up on you. 🙈)
-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. like, all of them. no exceptions. 💀
-msseregparm— use 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 deal as above, bestie, it's giving "commit to the bit or don't start". 💅
-mvect8-ret-in-mem— return 8-byte vectors in memory instead of MMX registers. This is the default on VxWorks to match the ABI of the Sun Studio compilers until version 12. Only use this option if you need to remain compatible with existing code produced by those previous compiler versions or older versions of GCC. 🫶-mpc32,-mpc64,-mpc80— set 80387 floating-point precision to 32, 64 or 80 bits. When-mpc32is specified, the significands of results of floating-point operations are rounded to 24 bits (single precision);-mpc64rounds the significands of results of floating-point operations to 53 bits (double precision) and-mpc80rounds the significands of results of floating-point operations to 64 bits (extended double precision), which is the default. When this option is used, floating-point operations in higher precisions are not available to the programmer without setting the FPU control word explicitly. 😤
Setting the rounding of floating-point operations to less than the default 80 bits can speed some programs by 2% or more. Note that some mathematical libraries assume that extended-precision (80-bit) floating-point operations are enabled by default; routines in such libraries could suffer significant loss of accuracy, typically through so-called "catastrophic cancellation", when this option is used to set the precision to less than extended precision. so like… free 2%, but the math girlies will come for you. ⭐
-mdaz-ftz,-mno-daz-ftz— the flush-to-zero (FTZ) and denormals-are-zero (DAZ) flags in the MXCSR register are used to control floating-point calculations. The-Ofast,-ffast-math, or-funsafe-math-optimizationsoptions normally link in startup code that sets these flags except when building a shared library (-shared). You can use the-mdaz-ftzand-mno-daz-ftzoptions to explicitly enable or disable setting these flags, regardless of other options passed to GCC. 🔥-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. 🧚-mstack-arg-probe— emit stack probing code in the function prologue. short king of an option, honestly. 🎀-mpreferred-stack-boundary=num— attempt to keep the stack boundary aligned to a 2 raised to num byte boundary. If-mpreferred-stack-boundaryis not specified, the default is 4 (16 bytes or 128 bits). ✨
⚠️ Warning: when generating code for the x86-64 architecture with SSE extensions disabled,-mpreferred-stack-boundary=3can be used to keep the stack boundary aligned to 8 byte boundary. Since x86-64 ABI require 16 byte stack alignment, this is ABI incompatible and intended to be used in controlled environment where stack space is important limitation. This option leads to wrong code when functions compiled with 16 byte stack alignment (such as functions from a standard library) are called with misaligned stack. In this case, SSE instructions may lead to misaligned memory access traps. In addition, variable arguments are handled incorrectly for 16 byte aligned objects (including x87long doubleand__int128), leading to wrong results. You must build all modules with-mpreferred-stack-boundary=3, including any libraries. This includes the system libraries and startup modules. 😭 this one is a whole situation, do not do it casually.
-mincoming-stack-boundary=num— assume the incoming stack is aligned to a 2 raised to num byte boundary. If-mincoming-stack-boundaryis not specified, the one specified by-mpreferred-stack-boundaryis used. 🌈
On Pentium and Pentium Pro, double and long double values should be aligned to an 8-byte boundary (see -malign-double) or suffer significant run time performance penalties. On Pentium III, the Streaming SIMD Extension (SSE) data type __m128 may not work properly if it is not 16-byte aligned. 💀
To ensure proper alignment of this values on the stack, the stack boundary must be as aligned as that required by any value stored on the stack. Further, every function must be generated such that it keeps the stack aligned. Thus calling a function compiled with a higher preferred stack boundary from a function compiled with a lower preferred stack boundary most likely misaligns the stack. It is recommended that libraries that use callbacks always use the default setting. iykyk. 🫶
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. 🍓
-mdump-tune-features— this option instructs GCC to dump the names of the x86 performance tuning features and default settings. The names can be used in-mtune-ctrl=feature-list. 👑- *
-mtune-ctrl=feature-list* — this option is used to do fine-grain control of x86 code generation features. feature-list is a comma-separated list of feature names. See also-mdump-tune-features. When specified, the feature is turned on if it is not preceded with^; otherwise, it is turned off.-mtune-ctrl=feature-list is intended to be used by GCC developers. Using it may lead to code paths not covered by testing and can potentially result in compiler ICEs or runtime errors. 😳 absolute main character option but also absolutely not for casual use. -mno-default— this option instructs GCC to turn off all tunable features. See also-mtune-ctrl=feature-list and-mdump-tune-features. the "everything off, we ball" option. 💅-mcld— this option instructs GCC to emit acldinstruction in the prologue of functions that use string instructions. String instructions depend on the DF flag to select between autoincrement or autodecrement mode. While the ABI specifies the DF flag to be cleared on function entry, some operating systems violate this specification by not clearing the DF flag in their exception dispatchers. The exception handler can be invoked with the DF flag set, which leads to wrong direction mode when string instructions are used. This option can be enabled by default on 32-bit x86 targets by configuring GCC with the--enable-cldconfigure option. Generation ofcldinstructions can be suppressed with the-mno-cldcompiler option in this case. 😤 rude OS behaviour, honestly.-mvzeroupper— this option instructs GCC to emit avzeroupperinstruction before a transfer of control flow out of the function to minimize the AVX to SSE transition penalty as well as remove unnecessaryzeroupperintrinsics. 🔥-mstv,-mno-stv— enable/disable Scalar to Vectorization pass transforming 64-bit integer computation into vector ones. This optimization is restricted to-O2and higher. ⭐-mprefer-avx128— this option instructs GCC to use 128-bit AVX instructions instead of 256-bit AVX instructions in the auto-vectorizer. 🌸- *
-mprefer-vector-width=opt* — this option instructs GCC to use opt-bit vector width in instructions instead of default on the selected platform. 🦋 -mpartial-vector-fp-math,-mno-partial-vector-fp-math— this option enables GCC to generate floating-point operations that might affect the set of floating-point status flags on partial vectors, where vector elements reside in the low part of the 128-bit SSE register. Unless-fno-trapping-mathis specified, the compiler guarantees correct behavior by sanitizing all input operands to have zeroes in the unused upper part of the vector register. Note that by using built-in functions or inline assembly with partial vector arguments, NaNs, denormal or invalid values can leak into the upper part of the vector, causing possible performance issues when-fno-trapping-mathis in effect. These issues can be mitigated by manually sanitizing the upper part of the partial vector argument register or by using-mdaz-ftzto set denormals-are-zero (DAZ) flag in the MXCSR register. 😭
This option is enabled by default. ✨
- *
-mmove-max=bits* — this option instructs GCC to set the maximum number of bits can be moved from memory to memory efficiently to bits. The valid bits are128,256and512. 💜 - *
-mstore-max=bits* — this option instructs GCC to set the maximum number of bits can be stored to memory efficiently to bits. The valid bits are128,256and512. 💜
and the values in play 👇
none— no extra limitations applied to GCC other than defined by the selected platform. easy breezy. 🎀128— prefer 128-bit vector width for instructions.256— prefer 256-bit vector width for instructions.512— prefer 512-bit vector width for instructions. 👑-mnoreturn-no-callee-saved-registers— bestie, this one optimizes functions with thenoreturnattribute or the_Noreturnspecifier by not saving the callee-saved registers that get used in the function (except for theBPregister, she's not going anywhere). Lowkey though, it can interfere with debugging the caller of thenoreturnfunction, or any function further up the call stack, so it's not enabled by default 🙈-mcx16— lets GCC generateCMPXCHG16Binstructions in 64-bit code so it can do compare-and-exchange on 16-byte aligned 128-bit objects. Perfect for atomic updates of data structures that are bigger than one machine word ✨ The compiler uses this instruction to implement__syncBuiltins. BUT — and this is the plot twist — for__atomicBuiltins operating on 128-bit integers, a library call is always used. No exceptions 💅-msahf— enables generation ofSAHFinstructions in 64-bit code. History lesson: early Intel Pentium 4 CPUs with Intel 64 support, before the Pentium 4 G1 step in December 2005, were missing theLAHFandSAHFinstructions that AMD64 supports. Those two are load and store instructions (respectively) for certain status flags. In 64-bit mode,SAHFis used to optimize thefmod,drem, andremainderbuilt-in functions — see Other Builtins for the details 🧚-mmovbe— enables use of themovbeinstruction to optimize byte swapping of four and eight byte entities. Little-endian-to-big-endian, slay 🦋-mshstk— the-mshstkoption enables shadow stack built-in functions from x86 Control-flow Enforcement Technology (CET).-mcrc32— enables the built-in functions__builtin_ia32_crc32qi,__builtin_ia32_crc32hi,__builtin_ia32_crc32si, and__builtin_ia32_crc32diso they generate thecrc32machine instruction.-mmwait— enables the built-in functions__builtin_ia32_monitorand__builtin_ia32_mwaitto generate themonitorandmwaitmachine instructions.-mrecip— enables use ofRCPSSandRSQRTSSinstructions (and their vectorized glow-upsRCPPSandRSQRTPS) with an extra Newton-Raphson step to bump up precision, instead ofDIVSSandSQRTSS(and their vectorized variants) for single-precision floating-point arguments. These instructions only get generated when-funsafe-math-optimizationsis enabled together with-ffinite-math-onlyand-fno-trapping-math. Heads up: while the throughput of the sequence is higher than the throughput of the non-reciprocal instruction, the precision can drop by up to 2 ulp — i.e. the inverse of 1.0 equals0.99999994😭
Also note that GCC implements 1.0f/sqrtf(x) in terms of RSQRTSS (or RSQRTPS) already with -ffast-math (or the option combination above), and doesn't need -mrecip.
Also also note that GCC emits the above sequence with an additional Newton-Raphson step for vectorized single-float division and vectorized sqrtf(x) already with -ffast-math (or the above option combination), and doesn't need -mrecip.
-mrecip=opt— this one controls which reciprocal estimate instructions may be used.optis a comma-separated list of options, and any of them can 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. iykyk 💅
-mveclibabi=type— specifies the ABI type to use for vectorizing intrinsics using an external library. Supported values fortypearesvmlfor the Intel short vector math library,aoclfor the math library (LibM) from AMD Optimizing CPU Libraries (AOCL), andacmlfor the end-of-life AMD core math library (to which AOCL-LibM is the successor — rip acml, you had a good run 🫡). 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.
It emits calls to amd_vrs4_acosf, amd_vrs16_acosf, amd_vrd8_asin, amd_vrs4_asinf, amd_vrs8_asinf, amd_vrs16_asinf, amd_vrd2_atan, amd_vrd8_atan, amd_vrs4_atanf, amd_vrs8_atanf, amd_vrs16_atanf, amd_vrd2_cos, amd_vrd4_cos, amd_vrd8_cos, amd_vrs4_cosf, amd_vrs8_cosf, amd_vrs16_cosf, amd_vrs4_coshf, amd_vrs8_coshf, amd_vrd2_erf, amd_vrd4_erf, amd_vrd8_erf, amd_vrs4_erff, amd_vrs8_erff, amd_vrs16_erff, amd_vrd2_exp, amd_vrd4_exp, amd_vrd8_exp, amd_vrs4_expf, amd_vrs8_expf, amd_vrs16_expf, amd_vrd2_exp10, amd_vrs4_exp10f, amd_vrd2_exp2, amd_vrd4_exp2, amd_vrd8_exp2, amd_vrs4_exp2f, amd_vrs8_exp2f, amd_vrs16_exp2f, amd_vrs4_expm1f, amd_vrd2_log, amd_vrd4_log, amd_vrd8_log, amd_vrs4_logf, amd_vrs8_logf, amd_vrs16_logf, amd_vrd2_log10, amd_vrs4_log10f, amd_vrs8_log10f, amd_vrs16_log10f, amd_vrd2_log1p, amd_vrs4_log1pf, amd_vrd2_log2, amd_vrd4_log2, amd_vrd8_log2, amd_vrs4_log2f, amd_vrs8_log2f, amd_vrs16_log2f, amd_vrd2_pow, amd_vrd4_pow, amd_vrd8_pow, amd_vrs4_powf, amd_vrs8_powf, amd_vrs16_powf, amd_vrd2_sin, amd_vrd4_sin, amd_vrd8_sin, amd_vrs4_sinf, amd_vrs8_sinf, amd_vrs16_sinf, amd_vrd2_tan, amd_vrd4_tan, amd_vrd8_tan, amd_vrs16_tanf, amd_vrs4_tanhf, amd_vrs8_tanhf, and amd_vrs16_tanhf for the corresponding function type when -mveclibabi=aocl is used.
And it emits calls to __vrd2_sin, __vrd2_cos, __vrd2_exp, __vrd2_log, __vrd2_log2, __vrd2_log10, __vrs4_sinf, __vrs4_cosf, __vrs4_expf, __vrs4_logf, __vrs4_log2f, __vrs4_log10f, and __vrs4_powf for the corresponding function type when -mveclibabi=acml is used.
-mabi=name— generate code for the specified calling convention. The permissible values aresysvfor the ABI used on GNU/Linux and other systems, andmsfor the Microsoft ABI. Default behavior: Microsoft ABI when targeting Microsoft Windows, and SysV ABI on literally every other system. You can control this per-function using the function attributesms_abiandsysv_abi💖-masm=dialect— output assembly instructions using the selected dialect. It also affects which dialect is used for basicasmand extendedasm. Supported choices (in dialect order) areattorintel. The default isatt. And Darwin does not supportintel💀-mforce-indirect-call— forces all calls to functions to be indirect. Useful when you're using Intel Processor Trace, because it generates more precise timing information for function calls.-mmanual-endbr— inserts anENDBRinstruction at function entry only via thecf_checkfunction attribute. Handy when used with-fcf-protection=branchto control ENDBR insertion at the function entry 👑-mcet-switch— by default, CET instrumentation is turned off on switch statements that use a jump table, and indirect branch track is disabled. Since jump tables are stored in read-only memory, that doesn't cause a direct loss of hardening. But if the jump table index is attacker-controlled, the indirect jump may not be constrained by CET 👀 This option turns on CET instrumentation to enable indirect branch track for switch statements with jump tables, which leads to the jump targets reachable via any indirect jumps.-mcall-ms2sysv-xlogues— because 64-bit ABIs differ, any Microsoft ABI function that calls a System V ABI function must considerRSI,RDI, andXMM6-15as clobbered. By default, the code for saving and restoring these registers is emitted inline, which gives you pretty lengthy prologues and epilogues 😮💨 Using-mcall-ms2sysv-xloguesemits prologues and epilogues that use stubs in the static portion oflibgccto perform those saves and restores — so function size goes down, 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 cannot be satisfied on all systems. Don't say nobody warned you 🫶-mpush-args,-mno-push-args— use PUSH operations to store outgoing parameters. This method is shorter and usually equally fast as the method using SUB/MOV operations, and it's enabled by default. In some cases, disabling it may improve performance because of improved scheduling and reduced dependencies.-maccumulate-outgoing-args— if enabled, the maximum amount of space required for outgoing arguments is computed in the function prologue. This is faster on most modern CPUs because of reduced dependencies, improved scheduling, and reduced stack usage when the preferred stack boundary is not equal to 2. The drawback is a notable increase in code size. Heads up: this switch implies-mno-push-args.-mms-bitfields,-mno-ms-bitfields— enable/disable bit-field layout that's compatible with the native Microsoft Windows compiler.
If packed is used on a structure, or if bit-fields are used, the Microsoft ABI may lay out the structure differently than the way GCC normally does. Especially when you're moving packed data between functions compiled with GCC and the native Microsoft compiler (either via function call or as data in a file), you may need to access either format 🌸
Okay so this part is all about how structs get laid out in memory, and then a whole pile of -m switches, and honestly? It's giving "compiler internals but make it fashion." Let's go 💅
This behavior is on by default for Microsoft Windows targets. You can also control it locally with variable attributes or type attributes — for the full tea, see x86 Attributes.
The Microsoft structure layout algorithm is pretty simple, except for the bit-field packing part (of course it's the bit-field part, it's always the bit-field part 😭). The padding and alignment of struct members, and whether a bit-field can straddle a storage-unit boundary, get decided 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. No reordering, no vibes-based shuffling, just vibes in declared order 🫶
- 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. Cozy little bit-field roommates ✨
Now, MSVC has opinions about zero-length bit-fields, and here's how it interprets them:
- If a zero-length bit-field is inserted between two bit-fields that are normally coalesced, the bit-fields are not coalesced. Rude, but okay.
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. That one little : 0 is doing a lot of heavy lifting 💀
- 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, thenbaris 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. So t3 is just living its truth 🙈
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 a whole. For example,
t2has a size of 4 bytes, since the zero-length bit-field follows a normal bit-field, and is of typeshort. - 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. Ignored. Not invited to the function. 🧚
-mno-align-stringops— Don't align the destination of inlined string operations. This switch reduces code size and improves performance in case the destination is already aligned, but GCC doesn't know about it. (It's not psychic, bestie 😭)-minline-all-stringops— By default GCC inlines string operations only when the destination is known to be aligned to at least a 4-byte boundary. This enables more inlining and increases code size, but may improve performance of code that depends on fastmemcpyandmemsetfor short lengths. The option enables inline expansion ofstrlenfor all pointer alignments. More inlining, more code, more speed — the classic trade 🔥-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. Best of both worlds, honestly iconic.- *
-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 i386repprefix 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_memcpyshould be inlined and what inline algorithm to use when the expected size of the copy operation is known. strategy is a comma-separated list of alg:max_size:dest_align triplets. alg is specified in-mstringop-strategy, max_size specifies the max byte size with which inline algorithm alg is allowed. For the last triplet, the max_size must be-1. The max_size of the triplets in the list must be specified in increasing order. The minimal byte size for alg is0for the first triplet andmax_size + 1of the preceding range. Triplets! In ascending order! Respect the order, bestie 🎀 - *
-mmemset-strategy=strategy* — This option is similar to-mmemcpy-strategy=except that it controls the__builtin_memsetexpansion. -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. Note that the option-momit-leaf-frame-pointerremoves the frame pointer for leaf functions, which might make debugging harder. Faster, yes — but debugging gets lowkey painful 😬-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. Not possible. At the moment. We don't make the rules 💀-mrecord-mcount-mno-record-mcount— If profiling is active (-pg), generate a section that contains pointers to each profiling call; this is useful for automatically patching the calls. You can use themfentry-section=option to set the name of the section; it defaults to__mcount_loc.-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. NOP now, patch later — main character energy 🌟-minstrument-return=type — With-pg -mfentry, instrument function exits according to type, which may be one ofnoneto not instrument,callto generate a call to__return__, ornop5to generate a 5-byte nop sequence. This option only instruments **true returns ending with aret, not sibling calls via a jump. Only real returns get the treatment, periodt.-mrecord-return-mno-record-return— Generate a__return_locsection pointing to all return instrumentation code.- *
-mfentry-name=name* — Set name of__fentry__symbol called at function entry for-pg -mfentryfunctions. - *
-mfentry-section=name* — Set name of section to record-mrecord-mcountcalls. The default is__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 RAX register when there are no variable arguments passed in vector registers.-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 of0to255, 8-bit unsigned integer divide is used instead of 32-bit/64-bit integer divide. Small numbers, big speedup 💜-mavx256-split-unaligned-load-mavx256-split-unaligned-store— Split 32-byte AVX unaligned load and store.- *
-mstack-protector-guard=guard* - *
-mstack-protector-guard-reg=reg* - *
-mstack-protector-guard-offset=offset* - *
-mstack-protector-guard-symbol=symbol* — Generate stack protection code using canary at guard. Supported locations 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.
-mstack-protector-guard-symbol=symbol overrides the offset with a symbol reference to a canary in the TLS block. 🐤 (canary pun, I had to)
-mgeneral-regs-only— Generate code that uses only the general-purpose registers. This prevents the compiler from using floating-point, vector, mask and bound registers. Basic registers only, no fancy stuff, we're keeping it humble 🎀-mrelax-cmpxchg-loop— okay so when GCC has to emit a compare-and-swap loop for__syncBuiltins and__atomicBuiltins and there's no native instruction to do it, this makes it optimize for the highly contended case 💅 how? by issuing an atomic load before theCMPXCHGinstruction, and using thePAUSEinstruction to save CPU power when it restarts the loop. cute and efficient, we love her.-mindirect-branch=choice** — converts indirect call and jump according to choice. the default iskeep, which literally keeps indirect call and jump unmodified (no thoughts, head empty).thunkconverts indirect call and jump to call and return thunk.thunk-inlineconverts indirect call and jump to inlined call and return thunk.thunk-externconverts indirect call and jump to external call and return thunk provided in a separate object file. you can control this behavior for a specific function by using the function attributeindirect_branch✨
🚨 heads up bestie:-mcmodel=largeis incompatible with-mindirect-branch=thunkand-mindirect-branch=thunk-externsince the thunk function may not be reachable in the large code model. no cap.
💖 also note that-mindirect-branch=thunk-externis compatible with-fcf-protection=branchsince the external thunk can be made to enable control-flow check.
-mfunction-return=choice** — converts function return according to choice. default iskeep, which keeps function return unmodified.thunkconverts function return to call and return thunk.thunk-inlineconverts function return to inlined call and return thunk.thunk-externconverts function return to external call and return thunk provided in a separate object file. you can control this behavior for a specific function by using the function attributefunction_return🎀
✨ note that-mindirect-return=thunk-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— force indirect call and jump via register. that's it, that's the whole vibe 🦋-mharden-sls=choice** — generates code to mitigate against straight line speculation (SLS) with choice. the default isnone, which disables all SLS hardening.returnenables SLS hardening for function returns.indirect-jmpenables SLS hardening for indirect jumps.allenables all SLS hardening. pick your fighter 💀-mindirect-branch-cs-prefix— adds a CS prefix to call and jmp to indirect thunk with branch target in r8-r15 registers, so that the call and jmp instruction length is 6 bytes. why do we care? because this allows them to potentially be replaced withlfence; call *%r8-r15orlfence; jmp *%r8-r15at run time 🔥 iconic runtime glow-up.-mapx-inline-asm-use-gpr32— by default, EGPR usage is disabled in inline asm constraints as GCC cannot be aware of whether the asm instructions support GP32 or not. so if your inline asm can handle EGPR, use-mapx-inline-asm-use-gpr32👑-mgather,-mscatter— enable vectorization for gather and scatter instructions, respectively. respectively is doing so much work in that sentence lmao 🌸
so these -m switches are supported in addition to the above on x86-64 processors in 64-bit environments 💜
-m32,-m64,-mx32,-m16— generate code for a 16-bit, 32-bit or 64-bit environment.- the
-m32option setsint,long, and pointer types to 32 bits, and generates code that runs in 32-bit mode. - the
-m64option setsintto 32 bits andlongand pointer types to 64 bits, and generates code for the x86-64 architecture. for Darwin only, the-m64option also turns off the-fno-picand-mdynamic-no-picoptions 🍓 - the
-mx32option setsint,long, and pointer types to 32 bits, and generates code for the x86-64 architecture. - the
-m16option is 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.
- the
-miamcu— generates code that conforms to Intel MCU psABI. it requires the-m32option to be turned on. it's giving "you can't sit with us unless you're 32-bit" 🙈-mno-red-zone— do not use a so-called "red zone" for x86-64 code. the red zone is mandated by the x86-64 ABI; it's a 128-byte area beyond the location of the stack pointer that is not modified by signal or interrupt handlers and therefore can be used for temporary data without adjusting the stack pointer. the flag-mno-red-zonedisables this red zone. bye red zone 👋-mcmodel=small— generate code for the small code model: the program and its symbols must be linked in the lower 2 GB of the address space. pointers are 64 bits. programs can be statically or dynamically linked. this is the default code model 💖-mcmodel=kernel— generate code for the kernel code model. the kernel runs in the negative 2 GB of the address space. this model has to be used for Linux kernel code. non-negotiable fr fr-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's 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's the default address mode for 32-bit and x32 environments. (yes the symmetry ate 💅)-mneeded,-mno-needed— emitGNU_PROPERTY_X86_ISA_1_NEEDEDGNU property for Linux target to indicate the micro-architecture ISA level required to execute the binary. very "please read the requirements before you @ me" of it 🎀-mno-direct-extern-access— without-fpicor-fPIC, always use the GOT pointer to access external symbols. with-fpicor-fPIC, treat access to protected symbols as local symbols. the default is-mdirect-extern-access.
⚠️ Warning: shared libraries compiled with-mno-direct-extern-accessand executable compiled with-mdirect-extern-accessmay not be binary compatible if protected symbols are used in shared libraries and executable. like, they will simply not vibe together 😭
-munroll-only-small-loops,-mno-unroll-only-small-loops— controls conservative small loop unrolling. it's enabled by default with-O2, and unrolls loops with less than 4 instructions by 1 time. this gives better utilization of the instruction decoding pipeline on modern processors 🔥 you can disable this with-mno-unroll-only-small-loops, and it's also disabled if the more general options-funroll-loopsor-funroll-all-loopsare either enabled or explicitly disabled. (yes, explicitly disabled counts too. the audacity.)-mdispatch-scheduler— enable instruction scheduling. this is only supported onbdver1,bdver2,bdver3,bdver4, andznver1processors, and additionally requires-fschedule-insns -fsched-pressure. exclusive guest list only 🫶-mlam=choice** — LAM (linear-address masking) allows special bits in the pointer to be used for metadata. the default isnone. withu48, pointer bits in positions 62:48 can be used for metadata; withu57, pointer bits in positions 62:57 can be used for metadata 👑
💻 x86 Windows Options
(no options listed under this one in this stretch of the page — she's just standing there, main character energy, guarding the next section 😌)
🦋 Xstormy16 Options
these options are defined for Xstormy16:
-msim— choose startup files and linker script suitable for the simulator. sim mode activated ✨
🎀 Xtensa Options
these options are supported for Xtensa targets:
-mconst16,-mno-const16— enable or disable use ofCONST16instructions for loading constant values. when enabled,CONST16instructions are always used in place of the standardL32Rinstructions. the use ofCONST16is enabled by default only if theL32Rinstruction is not available 💜-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.-mforce-no-pic,-mno-force-no-pic— for targets like GNU/Linux, where all user-mode Xtensa code must be position-independent code (PIC), this option disables PIC for compiling kernel code 🧚-mtext-section-literals,-mno-text-section-literals— these options control the treatment of literal pools. the default is-mno-text-section-literals, which places literals in a separate section in the output file. this allows the literal pool to be placed in a data RAM/ROM, and it also allows the linker to combine literal pools from separate object files to remove redundant literals and improve code size 🌸 with-mtext-section-literals, the literals are interspersed in the text section in order to keep them as close as possible to their references. this may be necessary for large assembly files. literals for each function are placed right before that function. besties staying close, iykyk 💖-mauto-litpools, **-mno-auto-litpools** — these options control the treatment of literal pools. the default is-mno-auto-litpools, which places literals in a separate section in the output file unless-mtext-section-literalsis used. with-mauto-litpoolsthe literals are interspersed in the text section **by the assembler**. compiler does not produce explicit.literaldirectives and loads literals into registers withMOVIinstructions instead ofL32Rto let the assembler do relaxation and place literals as necessary ✨ this option allows assembler to create several literal pools per function and assemble very big functions, which may not be possible with-mtext-section-literals`. big function rights 🫶-mtarget-align,-mno-target-align— when this option is enabled, GCC instructs the assembler to automatically align instructions to reduce branch penalties at the expense of some code density. the assembler attempts to widen density instructions to align branch targets and the instructions following call instructions 🎯 if there are not enough preceding safe density instructions to align a target, no widening is performed. the default is-mtarget-align. these options do not affect the treatment of auto-aligned instructions likeLOOP, which the assembler always aligns, either by widening density instructions or by inserting NOP instructions.LOOPsaid "i align myself, thanks" 💅-mlongcalls/-mno-longcalls— okay so this one is a whole vibe 💅: when it's enabled, GCC tells the assembler to flip direct calls into indirect calls unless it can prove the target of the direct call is inside the range the call instruction can actually reach. That swap usually happens for calls to functions sitting in other source files. Concretely? The assembler turns a directCALLinstruction into anL32Rfollowed by aCALLX. Default is-mno-longcalls. Reach for this option in programs where the call target could legitimately be out of range. 🫶
Heads up bestie, this is implemented in the assembler, not the compiler — so the assembly code GCC hands you still shows direct call instructions. Go look at the disassembled object code if you wanna see the actual instructions. And ngl, the assembler uses an indirect call for every cross-file call, not just the ones that are genuinely out of range. 😭
-mabi=name — generates code for the specified ABI. The permissible values are:call0,windowed. The default ABI is chosen by the Xtensa core configuration. ✨-mabi=call0— with this enabled, function parameters get passed in registersa2througha7, registersa12througha15are caller-saved, and registera15may be used as a frame pointer. When this version of the ABI is enabled, the C preprocessor symbol__XTENSA_CALL0_ABI__is defined. 🎀-mabi=windowed— with this enabled, function parameters get passed in registersa10througha15, and the called function rotates its register window by 8 registers on entry so its arguments land in registersa2througha7. Registera7may be used as a frame pointer. The register window is rotated 8 registers back on return. When this version of the ABI is enabled, the C preprocessor symbol__XTENSA_WINDOWED_ABI__is defined. 🌸-mextra-l32r-costs=n — specifies an extra cost of instruction RAM/ROM access forL32Rinstructions, measured in clock cycles. When optimizing for speed, this affects whether a constant gets loaded from the literal pool usingL32Ror synthesized from a small one with a couple of arithmetic instructions. The default value is0. ⭐-mstrict-align/-mno-strict-align— avoid (or allow) generating memory accesses that may not be aligned on a natural object boundary, as described in the architecture specification. The default is-mno-strict-alignfor cores that support both unaligned loads and stores in hardware, and-mstrict-alignfor all other cores. 🦋
🌈 zSeries Options 🌈
These are listed under
🌈 ENVIRONMENT 🌈
okay bestie, this whole section is about the little env vars that quietly boss GCC around behind the scenes 💅 Some of them are literally just "here are some directories/prefixes, go look in them," and some of them change how the compilation environment itself behaves. Both flavors, all here.
also real quick: you can always just say where to look with options like-B,-Iand-L, and those take precedence over anything you set with environment variables. And env vars in turn take precedence over whatever GCC's own configuration says. Priorities, she has them ✨
🎀 LANG, LC_CTYPE, LC_MESSAGES, LC_ALL
These four control how GCC uses localization info, aka how it vibes with different national conventions for diagnostic and informational output. GCC peeks at the locale categories LC_CTYPE and LC_MESSAGES if it's been configured to do so. You can set these categories to any value your installation supports — a classic example being en_GB.UTF-8 for English in the United Kingdom, encoded in UTF-8. 🇬🇧
So the hierarchy is: if LC_ALL is set, it overrides LC_CTYPE and LC_MESSAGES. Otherwise, LC_CTYPE and LC_MESSAGES default to the value of the LANG environment variable. And if none of these are set at all? GCC just defaults to traditional C English behavior. simple as that 🫶
⚠️ important, don't skip this: these environment variables do not affect the encodings of input files and strings in output files produced by GCC. Those are controlled by-finput-charsetand-fexec-charset, respectively. Different jobs, different vars, don't mix them up 😭
🧚 TMPDIR
If TMPDIR is set, it tells GCC which directory to use for temporary files. And why does GCC need temp files? Because it holds the output of one stage of compilation that's gonna be the input to the next stage — like, for example, the output of the preprocessor, which is the input to the compiler proper. pipeline girlie behavior ✨
🔍 GCC_DIAGNOSTICS_LOG
If GCC_DIAGNOSTICS_LOG is set, extra info about the diagnostics subsystem gets emitted. Set it to an empty value and the info goes to stderr; set it to anything else and GCC will try to open that file and write the info there.
🚨 the exact content and format of this info is subject to change — it's meant for GCC developers, not for end-users. iykyk, this one's not for us 💀
🦋 GCC_COMPARE_DEBUG
Setting GCC_COMPARE_DEBUG is nearly equivalent to passing -fcompare-debug to the compiler driver. Peep the documentation for that option if you want the full lore 📖
👑 GCC_EXEC_PREFIX
If it's set, GCC_EXEC_PREFIX gives a prefix to slap onto the names of the subprograms the compiler executes. Heads up: no slash is added when this prefix gets combined with the subprogram name — but you're free to specify a prefix that ends with a slash yourself if that's your thing 💅
If GCC_EXEC_PREFIX is not set, GCC tries to figure out an appropriate prefix based on the pathname it was invoked with. And if GCC can't find the subprogram using the prefix you gave it, it goes looking in the usual places instead. She's resourceful.
The default value of GCC_EXEC_PREFIX is prefix/lib/gcc/ where prefix is the prefix of 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 one.
This prefix is also used for finding files like crt0.o that are used for linking.
And in a genuinely unusual twist: the prefix is used in a weird way when finding 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 make an alternate directory name. So with -Bfoo/, GCC searches foo/bar just before it searches the standard directory /usr/local/lib/bar. And if a standard directory begins with the configured prefix, then the value of prefix gets replaced by GCC_EXEC_PREFIX when looking for header files. wild, i know 😭
⭐ COMPILER_PATH
The value of COMPILER_PATH is a colon-separated list of directories, much like PATH. GCC tries the directories you list when searching for subprograms, but only if it can't find the subprograms using GCC_EXEC_PREFIX. backup plan energy 🫶
💖 LIBRARY_PATH
LIBRARY_PATH is also a colon-separated list of directories, again much like PATH. When GCC is configured as a native compiler, it tries these directories when searching for special linker files — but only if it can't find them using GCC_EXEC_PREFIX. And linking with GCC also uses these directories when hunting for ordinary libraries for the -l option (though directories specified with -L come first, obvs 🎀).
🌸 GCC_EXTRA_DIAGNOSTIC_OUTPUT
Set GCC_EXTRA_DIAGNOSTIC_OUTPUT to one of the values below and extra text gets emitted to stderr whenever fix-it hints are emitted. Also, -fdiagnostics-parseable-fixits and -fno-diagnostics-parseable-fixits take precedence over this environment variable — so the command line wins, always.
fixits-v1— emit parseable fix-it hints, equivalent to-fdiagnostics-parseable-fixits. In particular, columns are expressed as a count of bytes, starting at byte 1 for the initial column.fixits-v2— same asfixits-v1, but the columns are expressed as display columns, as per-fdiagnostics-column-unit=display.
🔥 EXPERIMENTAL_SARIF_SOCKET
If EXPERIMENTAL_SARIF_SOCKET is set in the environment, the compiler will attempt to connect to a UNIX domain stream socket with that name, and send an "OnSarifResult" JSON-RPC 2.0 notification to it for every diagnostic that occurs — where the value of the notification is a SARIF "result" object. 🤯
😭 the compiler will fail immediately if EXPERIMENTAL_SARIF_SOCKET is set and it can't connect to it. no soft landing, no plan B.
💀 This feature is experimental and subject to change or removal without notice. literally read the name bestie, it's right there.
Alright, and on top of all that, some additional environment variables affect how the preprocessor behaves 🧚
🌈 CPATH, C_INCLUDE_PATH, CPLUS_INCLUDE_PATH, OBJC_INCLUDE_PATH
Each of these variables holds a list of directories separated by a special character, much like PATH, in which to look for header files. That special character, "PATH_SEPARATOR", is target-dependent and determined at GCC build time: for Microsoft Windows-based targets it's a semicolon, and for almost all other targets it's a colon.
CPATHspecifies a list of directories to be searched as if they were specified with-I, but after any paths given with-Ioptions on the command line. This one gets used regardless of which language is being preprocessed 💅- The remaining variables only apply when preprocessing the particular language indicated. Each one specifies a list of directories to be searched as if specified with
-isystem, but after any paths given with-isystemoptions on the command line.
In all of these variables, an empty element tells the compiler to search its current working directory. Empty elements can sit at the beginning or the end of a path. For instance, if CPATH is :/special/include, that has the same effect as -I. -I/special/include. sneaky but iconic ✨
🍓 DEPENDENCIES_OUTPUT
Set this and 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 can be:
- just a file name — in which case the Make rules get written to that file, and the target name is guessed from the source file name 🎀
- the form
file target— in which case the rules get 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 thrown in too.
🐚 SUNPRO_DEPENDENCIES
Same deal 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. subtle difference, big consequences 🫶
⏰ SOURCE_DATE_EPOCH
If this is set, its value specifies a UNIX timestamp to be used in place of the current date and time in the "__DATE__" and "__TIME__" macros — so the embedded timestamps become reproducible. reproducible builds girlies, this one's for 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. That's identical to the output of date +%s on GNU/Linux and other systems that support the %s extension in the date command.
It should be a known timestamp — like the last modification time of the source or package — and it should be set by the build process 🛠️
💀 BUGS
For instructions on how to report bugs, see <https://bugs.opensuse.org/>. go snitch, bestie, it's how things get fixed 🫶
📎 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 aren't necessary is innocuous, so like, better safe than sorry. Also:-sharedsuppresses the addition of startup code that alters the floating-point environment, as is done with-ffast-math,-Ofastor-funsafe-math-optimizationson some targets.
🫶 SEE ALSO
gpl(7), gfdl(7), fsf-funding(7), cpp(1), gcov(1), as(1), ld(1), gdb(1) and the Info entries for gcc, cpp, as, ld, binutils and gdb.
👑 AUTHOR
See the Info entry for gcc, or <https://gcc.gnu.org/onlinedocs/gcc/Contributors.html>, for contributors to GCC. the whole squad, honestly 💖
📜 COPYRIGHT
Copyright (c) 1988-2026 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.aaaand that's the whole page, bestie 🎉 you made it to the very end — iconic behavior fr fr. this bad girl is gcc-16, dated 2026-08-07, filed under GCC(1). slay, go compile something pretty 💅✨💖
📦 where this page comes from
- opensuse-leap 16.0: gcc16 16.2.0+git9497-160000.2.1 · GPL-3.0-or-later
- opensuse-leap 16.0: gcc16 16.2.0+git9497-160000.2.1 · GPL-3.0-or-later
- opensuse-leap 16.0: gcc16 16.2.0+git9497-160000.2.1 · GPL-3.0-or-later
- opensuse-leap 16.0: gcc16 16.2.0+git9497-160000.2.1 · GPL-3.0-or-later
- 💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash-1017 on : a language model's rewrite of the original, under its terms