GCC(1)

💖 NAME 💖

gcc — the GNU project C and C++ compiler. Yeah bestie, that one. The whole main character of compiling. 👑

---

✨ SYNOPSIS ✨

gcc [-c|-S|-E] [-std=standard]
    [-g] [-pg] [-Olevel]
    [-Wwarn...] [-Wpedantic]
    [-Idir...] [-Ldir...]
    [-Dmacro[=defn]...] [-Umacro]
    [-foption...] [-mmachine-option...]
    [-o outfile] [@file] infile...

okay so only the most useful options are listed here — the rest of them are literally right below, we are not gatekeeping. Also: g++ accepts mostly the same options as gcc. 💅

---

🦋 DESCRIPTION 🦋

So when you invoke GCC, it normally does preprocessing, compilation, assembly and linking. All four. In a row. No cap. The overall options let you stop that whole process at an intermediate stage — for example, the -c option says do not run the linker, so the output is just object files output by the assembler. Stop the ride, you're getting off early. 🛑

Other options get passed on to one or more stages of processing. Some options control the preprocessor, others control the compiler itself. And yet other options control the assembler and linker — most of those are not documented here, because honestly? you rarely need any of them. 🙈

Most of the command-line options you can use with GCC are useful for C programs. When an option is only useful with another language (usually C++), the explanation says so explicitly. And if the description for a particular option does not mention a source language, then you can use that option with all supported languages. Inclusive king. 🌈

The usual way to run GCC is to run the executable called gcc, or machine-gcc when you're cross-compiling, or machine-gcc-version to run a specific version of GCC. And when you compile C++ programs, you should invoke GCC as g++ instead. Periodt.

The gcc program accepts options and file names as operands. Many options have multi-letter names, which means multiple single-letter options may not be grouped: -dv is very different from -d -v. Not the same thing. Do not do that. 💀

You can mix options and other arguments. For the most part, the order you use doesn't matter. But order does matter when you use several options of the same kind — for example, if you specify -L more than once, the directories are searched in the order specified. Also, the placement of the -l option is significant. So like... sometimes order is everything, and sometimes it's giving nothing. 🎀

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. (Pick a lane, manual. It picked one.) 🫶

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. The standards girlies ate that one. 🍓

---

🔥 OPTIONS 🔥

🌸 Option Summary 🌸

Here's a summary of all the options, grouped by type. Explanations are in the following sections. (Yes, all of them. Every single one. We do not skip. 💖)

🎀 Overall Options

-c  -S  -E  -o file -dumpbase dumpbase  -dumpbase-ext auxdropsuf
-dumpdir dumppfx  -x language -v  -###  --help[=class[,...]]
--target-help  --version -pass-exit-codes  -pipe  -specs=file
-wrapper @file  -ffile-prefix-map=old=new  -fcanon-prefix-map
-fplugin=file  -fplugin-arg-name=arg -fdump-ada-spec[-slim]
-fada-spec-parent=unit  -fdump-go-spec=file

💅 C Language Options

-ansi  -std=standard  -aux-info filename -fno-asm -fno-builtin
-fno-builtin-function  -fcond-mismatch -ffreestanding  -fgimple
-fgnu-tm  -fgnu89-inline  -fhosted -flax-vector-conversions
-fms-extensions -foffload=arg  -foffload-options=arg -fopenacc
-fopenacc-dim=geom -fopenmp  -fopenmp-simd
-fopenmp-target-simd-clone[=device-type]
-fpermitted-flt-eval-methods=standard -fplan9-extensions
-fsigned-bitfields  -funsigned-bitfields -fsigned-char
-funsigned-char  -fstrict-flex-arrays[=n] -fsso-struct=endianness

🧚 C++ Language Options

-fabi-version=n  -fno-access-control -faligned-new=n
-fargs-in-order=n  -fchar8_t  -fcheck-new -fconstexpr-depth=n
-fconstexpr-cache-depth=n -fconstexpr-loop-limit=n
-fconstexpr-ops-limit=n -fno-elide-constructors
-fno-enforce-eh-specs -fno-gnu-keywords -fno-immediate-escalation
-fno-implicit-templates -fno-implicit-inline-templates
-fno-implement-inlines -fmodule-header[=kind] -fmodule-only
-fmodules-ts -fmodule-implicit-inline -fno-module-lazy
-fmodule-mapper=specification -fmodule-version-ignore
-fms-extensions -fnew-inheriting-ctors -fnew-ttp-matching
-fno-nonansi-builtins  -fnothrow-opt  -fno-operator-names
-fno-optional-diags -fno-pretty-templates -fno-rtti
-fsized-deallocation -ftemplate-backtrace-limit=n -ftemplate-depth=n
-fno-threadsafe-statics  -fuse-cxa-atexit -fno-weak  -nostdinc++
-fvisibility-inlines-hidden -fvisibility-ms-compat
-fext-numeric-literals -flang-info-include-translate[=header]
-flang-info-include-translate-not -flang-info-module-cmi[=module]
-stdlib=libstdc++,libc++ -Wabi-tag  -Wcatch-value  -Wcatch-value=n
-Wno-class-conversion  -Wclass-memaccess -Wcomma-subscript
-Wconditionally-supported -Wno-conversion-null
-Wctad-maybe-unsupported -Wctor-dtor-privacy  -Wdangling-reference
-Wno-delete-incomplete -Wdelete-non-virtual-dtor
-Wno-deprecated-array-compare -Wdeprecated-copy
-Wdeprecated-copy-dtor -Wno-deprecated-enum-enum-conversion
-Wno-deprecated-enum-float-conversion -Weffc++
-Wno-elaborated-enum-base -Wno-exceptions -Wextra-semi
-Wno-global-module -Wno-inaccessible-base
-Wno-inherited-variadic-ctor  -Wno-init-list-lifetime
-Winvalid-constexpr -Winvalid-imported-macros -Wno-invalid-offsetof
-Wno-literal-suffix -Wmismatched-new-delete -Wmismatched-tags
-Wmultiple-inheritance  -Wnamespaces  -Wnarrowing -Wnoexcept
-Wnoexcept-type  -Wnon-virtual-dtor -Wpessimizing-move
-Wno-placement-new  -Wplacement-new=n -Wrange-loop-construct
-Wredundant-move -Wredundant-tags -Wreorder  -Wregister
-Wstrict-null-sentinel  -Wno-subobject-linkage  -Wtemplates
-Wno-non-template-friend  -Wold-style-cast -Woverloaded-virtual
-Wno-pmf-conversions -Wself-move -Wsign-promo -Wsized-deallocation
-Wsuggest-final-methods -Wsuggest-final-types  -Wsuggest-override
-Wno-template-id-cdtor -Wno-terminate  -Wno-vexing-parse
-Wvirtual-inheritance -Wno-virtual-move-assign  -Wvolatile
-Wzero-as-null-pointer-constant

💜 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

👑 Diagnostic Message Formatting Options

-fmessage-length=n -fdiagnostics-plain-output
-fdiagnostics-show-location=[once|every-line]
-fdiagnostics-color=[auto|never|always]
-fdiagnostics-urls=[auto|never|always]
-fdiagnostics-format=[text|sarif-stderr|sarif-file|json|json-stderr|json-file]
-fno-diagnostics-json-formatting
-fno-diagnostics-show-option  -fno-diagnostics-show-caret
-fno-diagnostics-show-labels  -fno-diagnostics-show-line-numbers
-fno-diagnostics-show-cwe -fno-diagnostics-show-rule
-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]

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and that's where part 1 leaves us, bestie — the explanations for every single one of these groups are coming up in the very next sections, so don't go anywhere ✨

⚠️ Warning Options ⚠️

okay bestie, deep breath, hydrate, because this is THE list. every single one of these goes straight after gcc like it's nothing, and there are so many of them. i'm not gatekeeping a single flag, not one, we're doing all of them, in order, periodt. 💅

• -fsyntax-only
• -fmax-errors=n
• -Wpedantic
• -pedantic-errors
• -fpermissive
• -w
• -Wextra
• -Wall
• -Wabi=n
• -Waddress
• -Wno-address-of-packed-member
• -Waggregate-return
• -Walloc-size
• -Walloc-size-larger-than=byte-size
• -Walloc-zero
• -Walloca
• -Walloca-larger-than=byte-size
• -Wno-aggressive-loop-optimizations
• -Warith-conversion
• -Warray-bounds
• -Warray-bounds=n
• -Warray-compare
• -Warray-parameter
• -Warray-parameter=n
• -Wno-attributes
• -Wattribute-alias=n
• -Wno-attribute-alias
• -Wno-attribute-warning
• -Wbidi-chars=[none|unpaired|any|ucn]
• -Wbool-compare
• -Wbool-operation
• -Wno-builtin-declaration-mismatch
• -Wno-builtin-macro-redefined
• -Wc90-c99-compat
• -Wc99-c11-compat
• -Wc11-c23-compat
• -Wc++-compat
• -Wc++11-compat
• -Wc++14-compat
• -Wc++17-compat
• -Wc++20-compat
• -Wno-c++11-extensions
• -Wno-c++14-extensions
• -Wno-c++17-extensions
• -Wno-c++20-extensions
• -Wno-c++23-extensions

see the pattern? the n girlies and the byte-size girlies want you to fill in an actual number yourself — i'm keeping them exactly as written because that's the whole point of a manual 🫶

• -Wcalloc-transposed-args
• -Wcast-align
• -Wcast-align=strict
• -Wcast-function-type
• -Wcast-qual
• -Wchar-subscripts
• -Wclobbered
• -Wcomment
• -Wcompare-distinct-pointer-types
• -Wno-complain-wrong-lang
• -Wconversion
• -Wno-coverage-mismatch
• -Wno-cpp
• -Wdangling-else
• -Wdangling-pointer
• -Wdangling-pointer=n
• -Wdate-time
• -Wno-deprecated
• -Wno-deprecated-declarations
• -Wno-designated-init
• -Wdisabled-optimization
• -Wno-discarded-array-qualifiers
• -Wno-discarded-qualifiers
• -Wno-div-by-zero
• -Wdouble-promotion
• -Wduplicated-branches
• -Wduplicated-cond
• -Wempty-body
• -Wno-endif-labels
• -Wenum-compare
• -Wenum-conversion
• -Wenum-int-mismatch
• -Werror
• -Werror=*
• -Wexpansion-to-defined
• -Wfatal-errors
• -Wflex-array-member-not-at-end
• -Wfloat-conversion
• -Wfloat-equal
• -Wformat
• -Wformat=2
• -Wno-format-contains-nul
• -Wno-format-extra-args
• -Wformat-nonliteral
• -Wformat-overflow=n
• -Wformat-security
• -Wformat-signedness
• -Wformat-truncation=n
• -Wformat-y2k
• -Wframe-address
• -Wframe-larger-than=byte-size
• -Wno-free-nonheap-object
• -Wno-if-not-aligned
• -Wno-ignored-attributes
• -Wignored-qualifiers
• -Wno-incompatible-pointer-types
• -Whardened
• -Wimplicit
• -Wimplicit-fallthrough
• -Wimplicit-fallthrough=n
• -Wno-implicit-function-declaration
• -Wno-implicit-int
• -Winfinite-recursion
• -Winit-self
• -Winline
• -Wno-int-conversion
• -Wint-in-bool-context
• -Wno-int-to-pointer-cast
• -Wno-invalid-memory-model
• -Winvalid-pch
• -Winvalid-utf8
• -Wno-unicode
• -Wjump-misses-init
• -Wlarger-than=byte-size
• -Wlogical-not-parentheses
• -Wlogical-op
• -Wlong-long
• -Wno-lto-type-mismatch
• -Wmain
• -Wmaybe-uninitialized
• -Wmemset-elt-size
• -Wmemset-transposed-args
• -Wmisleading-indentation
• -Wmissing-attributes
• -Wmissing-braces
• -Wmissing-field-initializers
• -Wmissing-format-attribute
• -Wmissing-include-dirs
• -Wmissing-noreturn
• -Wno-missing-profile
• -Wno-multichar
• -Wmultistatement-macros
• -Wnonnull
• -Wnonnull-compare
• -Wnormalized=[none|id|nfc|nfkc]
• -Wnull-dereference
• -Wno-odr
• -Wopenacc-parallelism
• -Wopenmp
• -Wopenmp-simd
• -Wno-overflow
• -Woverlength-strings
• -Wno-override-init-side-effects
• -Wpacked
• -Wno-packed-bitfield-compat
• -Wpacked-not-aligned
• -Wpadded
• -Wparentheses
• -Wno-pedantic-ms-format
• -Wpointer-arith
• -Wno-pointer-compare
• -Wno-pointer-to-int-cast
• -Wno-pragmas
• -Wno-prio-ctor-dtor
• -Wredundant-decls
• -Wrestrict
• -Wno-return-local-addr
• -Wreturn-type
• -Wno-scalar-storage-order
• -Wsequence-point
• -Wshadow
• -Wshadow=global
• -Wshadow=local
• -Wshadow=compatible-local
• -Wno-shadow-ivar
• -Wno-shift-count-negative
• -Wno-shift-count-overflow
• -Wshift-negative-value
• -Wno-shift-overflow
• -Wshift-overflow=n
• -Wsign-compare
• -Wsign-conversion
• -Wno-sizeof-array-argument
• -Wsizeof-array-div
• -Wsizeof-pointer-div
• -Wsizeof-pointer-memaccess
• -Wstack-protector
• -Wstack-usage=byte-size
• -Wstrict-aliasing
• -Wstrict-aliasing=n
• -Wstrict-overflow
• -Wstrict-overflow=n
• -Wstring-compare
• -Wno-stringop-overflow
• -Wno-stringop-overread
• -Wno-stringop-truncation
• -Wstrict-flex-arrays
• -Wsuggest-attribute=[pure|const|noreturn|format|malloc]
• -Wswitch
• -Wno-switch-bool
• -Wswitch-default
• -Wswitch-enum
• -Wno-switch-outside-range
• -Wno-switch-unreachable
• -Wsync-nand
• -Wsystem-headers
• -Wtautological-compare
• -Wtrampolines
• -Wtrigraphs
• -Wtrivial-auto-var-init
• -Wno-tsan
• -Wtype-limits
• -Wundef
• -Wuninitialized
• -Wunknown-pragmas
• -Wunsuffixed-float-constants
• -Wunused
• -Wunused-but-set-parameter
• -Wunused-but-set-variable
• -Wunused-const-variable
• -Wunused-const-variable=n
• -Wunused-function
• -Wunused-label
• -Wunused-local-typedefs
• -Wunused-macros
• -Wunused-parameter
• -Wno-unused-result
• -Wunused-value
• -Wunused-variable
• -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-length-bounds

and that's the whole roster. yes, all of it. the -Wno- ones are the "actually bestie, not this one" counterparts, and honestly the sheer number of them is giving "she has range" 💀

---

🔍 Static Analyzer Options 🔍

okay this one is the ✨detective✨ of the compiler. it reads your code and investigates, and these are the knobs you can turn while it's doing that.

first up, the ones that turn the analysis on and shape how deep it goes:

• -fanalyzer
• -fanalyzer-call-summaries
• -fanalyzer-checker=name
• -fno-analyzer-feasibility
• -fanalyzer-fine-grained
• -fanalyzer-show-events-in-system-headers
• -fno-analyzer-state-merge
• -fno-analyzer-state-purge
• -fno-analyzer-suppress-followups
• -fanalyzer-transitivity
• -fno-analyzer-undo-inlining
• -fanalyzer-verbose-edges
• -fanalyzer-verbose-state-changes
• -fanalyzer-verbosity=level

see how they come in -fanalyzer-… and -fno-analyzer-… pairs? yes. turn it on, turn it off, it's giving "will she, won't she" 😭

then the dump crew — everything that makes the analyzer spill its receipts to disk:

• -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

the -2 and -3 versions of -fdump-analyzer-exploded-nodes are sending me. they really said "one version? no. three." 👑

and then the -Wno-analyzer-… warnings, which is genuinely a mood ring of everything the analyzer is checking for:

• -Wno-analyzer-double-fclose
• -Wno-analyzer-double-free
• -Wno-analyzer-exposure-through-output-file
• -Wno-analyzer-exposure-through-uninit-copy
• -Wno-analyzer-fd-access-mode-mismatch
• -Wno-analyzer-fd-double-close
• -Wno-analyzer-fd-leak
• -Wno-analyzer-fd-phase-mismatch
• -Wno-analyzer-fd-type-mismatch
• -Wno-analyzer-fd-use-after-close
• -Wno-analyzer-fd-use-without-check
• -Wno-analyzer-file-leak
• -Wno-analyzer-free-of-non-heap
• -Wno-analyzer-imprecise-fp-arithmetic
• -Wno-analyzer-infinite-loop
• -Wno-analyzer-infinite-recursion
• -Wno-analyzer-jump-through-null
• -Wno-analyzer-malloc-leak
• -Wno-analyzer-mismatching-deallocation
• -Wno-analyzer-null-argument
• -Wno-analyzer-null-dereference
• -Wno-analyzer-out-of-bounds
• -Wno-analyzer-overlapping-buffers
• -Wno-analyzer-possible-null-argument
• -Wno-analyzer-possible-null-dereference
• -Wno-analyzer-putenv-of-auto-var
• -Wno-analyzer-shift-count-negative
• -Wno-analyzer-shift-count-overflow
• -Wno-analyzer-stale-setjmp-buffer
• -Wno-analyzer-tainted-allocation-size
• -Wno-analyzer-tainted-assertion
• -Wno-analyzer-tainted-array-index
• -Wno-analyzer-tainted-divisor
• -Wno-analyzer-tainted-offset
• -Wno-analyzer-tainted-size

the whole tainted- family?? the analyzer said "i'm watching the untrusted data, bestie, i'm watching it so hard" 🔥

• -Wanalyzer-symbol-too-complex
• -Wanalyzer-too-complex
• -Wno-analyzer-undefined-behavior-strtok
• -Wno-analyzer-unsafe-call-within-signal-handler
• -Wno-analyzer-use-after-free
• -Wno-analyzer-use-of-pointer-in-stale-stack-frame
• -Wno-analyzer-use-of-uninitialized-value
• -Wno-analyzer-va-arg-type-mismatch
• -Wno-analyzer-va-list-exhausted
• -Wno-analyzer-va-list-leak
• -Wno-analyzer-va-list-use-after-va-end
• -Wno-analyzer-write-to-const
• -Wno-analyzer-write-to-string-literal

note that -Wanalyzer-symbol-too-complex and -Wanalyzer-too-complex are the only two in that chunk that aren't -Wno- — they're just sitting there being dramatic on their own. iconic behavior honestly 💅

---

🧚 C and Objective-C-only Warning Options 🧚

these ones are exclusive — they only show up for the C and Objective-C girlies. if you're writing C++ or something else, this is not your section, no offense 🫶

• -Wbad-function-cast
• -Wmissing-declarations
• -Wmissing-parameter-type
• -Wdeclaration-missing-parameter-type
• -Wmissing-prototypes
• -Wmissing-variable-declarations
• -Wnested-externs
• -Wold-style-declaration
• -Wold-style-definition
• -Wstrict-prototypes
• -Wtraditional
• -Wtraditional-conversion
• -Wdeclaration-after-statement
• -Wpointer-sign

the -Wold-style- and -Wtraditional- ones are literally calling out your code for being vintage. respectfully. with love. 🌸

---

🐛 Debugging Options 🐛

now THIS is where you live if you've ever stared at a crash and whispered "what are you" 😭. it's the -g family, plus a whole extended universe of -f friends hanging out at the end.

• -g
• -glevel
• -gdwarf
• -gdwarf-version
• -gbtf
• -gctf
• -gctflevel
• -ggdb
• -grecord-gcc-switches
• -gno-record-gcc-switches
• -gstrict-dwarf
• -gno-strict-dwarf
• -gas-loc-support
• -gno-as-loc-support
• -gas-locview-support
• -gno-as-locview-support
• -gcodeview
• -gcolumn-info
• -gno-column-info
• -gdwarf32
• -gdwarf64
• -gstatement-frontiers
• -gno-statement-frontiers
• -gvariable-location-views
• -gno-variable-location-views
• -ginternal-reset-location-views
• -gno-internal-reset-location-views
• -ginline-points
• -gno-inline-points
• -gvms
• -gz[=type]
• -gsplit-dwarf
• -gdescribe-dies
• -gno-describe-dies

every single one of the -g… options has a -gno-… sibling that says "actually no ❌" and honestly? sibling energy fr fr

• -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

and the grand finale of this section is just two besties standing together: -fvar-tracking and -fvar-tracking-assignments. they're a set. they're matching. don't separate them 🎀

🌸 Optimization Options 🌸

okay bestie, sit down, hydrate, because this is the part of the manpage where GCC opens the closet and a hundred and fifty flags fall out and hit you in the face. every single one of these is real. every single one is something you can literally type into your terminal. i'm not exaggerating, i'm not trimming, i'm not skipping the "boring" ones — we are doing all of them. 🫶

here's the full roster, exactly as the manual lays it out:

• -faggressive-loop-optimizations
• -falign-functions[=n[:m:[n2[:m2]]]]
• -falign-jumps[=n[:m:[n2[:m2]]]]
• -falign-labels[=n[:m:[n2[:m2]]]]
• -falign-loops[=n[:m:[n2[:m2]]]]
• -fmin-function-alignment=[n]
• -fno-allocation-dce
• -fallow-store-data-races
• -fassociative-math
• -fauto-profile
• -fauto-profile[=path]
• -fauto-inc-dec
• -fbranch-probabilities
• -fcaller-saves
• -fcombine-stack-adjustments
• -fconserve-stack
• -ffold-mem-offsets
• -fcompare-elim
• -fcprop-registers
• -fcrossjumping
• -fcse-follow-jumps
• -fcse-skip-blocks
• -fcx-fortran-rules
• -fcx-limited-range
• -fdata-sections
• -fdce
• -fdelayed-branch
• -fdelete-null-pointer-checks
• -fdevirtualize
• -fdevirtualize-speculatively
• -fdevirtualize-at-ltrans
• -fdse
• -fearly-inlining
• -fipa-sra
• -fexpensive-optimizations
• -ffat-lto-objects
• -ffast-math
• -ffinite-math-only
• -ffloat-store
• -fexcess-precision=style
• -ffinite-loops
• -fforward-propagate
• -ffp-contract=style
• -ffunction-sections
• -fgcse
• -fgcse-after-reload
• -fgcse-las
• -fgcse-lm
• -fgraphite-identity
• -fgcse-sm
• -fhoist-adjacent-loads
• -fif-conversion
• -fif-conversion2
• -findirect-inlining
• -finline-stringops[=fn]
• -finline-functions
• -finline-functions-called-once
• -finline-limit=n
• -finline-small-functions
• -fipa-modref
• -fipa-cp
• -fipa-cp-clone
• -fipa-bit-cp
• -fipa-vrp
• -fipa-pta
• -fipa-profile
• -fipa-pure-const
• -fipa-reference
• -fipa-reference-addressable
• -fipa-stack-alignment
• -fipa-icf
• -fira-algorithm=algorithm
• -flive-patching=level
• -fira-region=region
• -fira-hoist-pressure
• -fira-loop-pressure
• -fno-ira-share-save-slots
• -fno-ira-share-spill-slots
• -fisolate-erroneous-paths-dereference
• -fisolate-erroneous-paths-attribute
• -fivopts
• -fkeep-inline-functions
• -fkeep-static-functions
• -fkeep-static-consts
• -flimit-function-alignment
• -flive-range-shrinkage
• -floop-block
• -floop-interchange
• -floop-strip-mine
• -floop-unroll-and-jam
• -floop-nest-optimize
• -floop-parallelize-all
• -flra-remat
• -flto
• -flto-compression-level
• -flto-partition=alg
• -fmerge-all-constants
• -fmerge-constants
• -fmodulo-sched
• -fmodulo-sched-allow-regmoves
• -fmove-loop-invariants
• -fmove-loop-stores
• -fno-branch-count-reg
• -fno-defer-pop
• -fno-fp-int-builtin-inexact
• -fno-function-cse
• -fno-guess-branch-probability
• -fno-inline
• -fno-math-errno
• -fno-peephole
• -fno-peephole2
• -fno-printf-return-value
• -fno-sched-interblock
• -fno-sched-spec
• -fno-signed-zeros
• -fno-toplevel-reorder
• -fno-trapping-math
• -fno-zero-initialized-in-bss
• -fomit-frame-pointer
• -foptimize-sibling-calls
• -fpartial-inlining
• -fpeel-loops
• -fpredictive-commoning
• -fprefetch-loop-arrays
• -fprofile-correction
• -fprofile-use
• -fprofile-use=path
• -fprofile-partial-training
• -fprofile-values
• -fprofile-reorder-functions
• -freciprocal-math
• -free
• -frename-registers
• -freorder-blocks
• -freorder-blocks-algorithm=algorithm
• -freorder-blocks-and-partition
• -freorder-functions
• -frerun-cse-after-loop
• -freschedule-modulo-scheduled-loops
• -frounding-math
• -fsave-optimization-record
• -fsched2-use-superblocks
• -fsched-pressure
• -fsched-spec-load
• -fsched-spec-load-dangerous
• -fsched-stalled-insns-dep[=n]
• -fsched-stalled-insns[=n]
• -fsched-group-heuristic
• -fsched-critical-path-heuristic
• -fsched-spec-insn-heuristic
• -fsched-rank-heuristic
• -fsched-last-insn-heuristic
• -fsched-dep-count-heuristic
• -fschedule-fusion
• -fschedule-insns
• -fschedule-insns2
• -fsection-anchors
• -fselective-scheduling
• -fselective-scheduling2
• -fsel-sched-pipelining
• -fsel-sched-pipelining-outer-loops
• -fsemantic-interposition
• -fshrink-wrap
• -fshrink-wrap-separate
• -fsignaling-nans
• -fsingle-precision-constant
• -fsplit-ivs-in-unroller
• -fsplit-loops
• -fsplit-paths
• -fsplit-wide-types
• -fsplit-wide-types-early
• -fssa-backprop
• -fssa-phiopt
• -fstdarg-opt
• -fstore-merging
• -fstrict-aliasing
• -fipa-strict-aliasing
• -fthread-jumps
• -ftracer
• -ftree-bit-ccp
• -ftree-builtin-call-dce
• -ftree-ccp
• -ftree-ch
• -ftree-coalesce-vars
• -ftree-copy-prop
• -ftree-dce
• -ftree-dominator-opts
• -ftree-dse
• -ftree-forwprop
• -ftree-fre
• -fcode-hoisting
• -ftree-loop-if-convert
• -ftree-loop-im
• -ftree-phiprop
• -ftree-loop-distribution
• -ftree-loop-distribute-patterns
• -ftree-loop-ivcanon
• -ftree-loop-linear
• -ftree-loop-optimize
• -ftree-loop-vectorize
• -ftree-parallelize-loops=n
• -ftree-pre
• -ftree-partial-pre
• -ftree-pta
• -ftree-reassoc
• -ftree-scev-cprop
• -ftree-sink
• -ftree-slsr
• -ftree-sra
• -ftree-switch-conversion
• -ftree-tail-merge
• -ftree-ter
• -ftree-vectorize
• -ftree-vrp
• -ftrivial-auto-var-init
• -funconstrained-commons
• -funit-at-a-time
• -funroll-all-loops
• -funroll-loops
• -funsafe-math-optimizations
• -funswitch-loops
• -fipa-ra
• -fvariable-expansion-in-unroller
• -fvect-cost-model
• -fvpt
• -fweb
• -fwhole-program
• -fwpa
• -fuse-linker-plugin
• -fzero-call-used-regs
• --param name=value

and then, the moment you've all been waiting for, the headliners 👑 — the ones your Makefile has been screaming at you since forever:

• -O
• -O0
• -O1
• -O2
• -O3
• -Os
• -Ofast
• -Og
• -Oz

that's the whole list. no notes. and yes, -O really is just the letter O with a dash in front, because the people who made this had main character energy and didn't need a long name. 💅

---

🎀 Program Instrumentation Options 🎀

okay so this next batch is where you poke the compiler and go "hey bestie, watch what my program does and tell me everything" 🔍. it's the whole instrumentation crew, and the manpage lists them all like this:

• -p
• -pg
• -fprofile-arcs
• --coverage
• -ftest-coverage
• -fcondition-coverage
• -fprofile-abs-path
• -fprofile-dir=path
• -fprofile-generate
• -fprofile-generate=path
• -fprofile-info-section
• -fprofile-info-section=name
• -fprofile-note=path
• -fprofile-prefix-path=path
• -fprofile-update=method
• -fprofile-filter-files=regex
• -fprofile-exclude-files=regex
• -fprofile-reproducible=[multithreaded|parallel-runs|serial]
• -fsanitize=style
• -fsanitize-recover
• -fsanitize-recover=style
• -fsanitize-trap
• -fsanitize-trap=style
• -fasan-shadow-offset=number
• -fsanitize-sections=s1,s2,...
• -fsanitize-undefined-trap-on-error
• -fbounds-check
• -fcf-protection=[full|branch|return|none|check]
• -fharden-compares
• -fharden-conditional-branches
• -fhardened
• -fharden-control-flow-redundancy
• -fhardcfr-skip-leaf
• -fhardcfr-check-exceptions
• -fhardcfr-check-returning-calls
• -fhardcfr-check-noreturn-calls=[always|no-xthrow|nothrow|never]
• -fstack-protector
• -fstack-protector-all
• -fstack-protector-strong
• -fstack-protector-explicit
• -fstack-check
• -fstack-limit-register=reg
• -fstack-limit-symbol=sym
• -fno-stack-limit
• -fsplit-stack
• -fstrub=disable
• -fstrub=strict
• -fstrub=relaxed
• -fstrub=all
• -fstrub=at-calls
• -fstrub=internal
• -fvtable-verify=[std|preinit|none]
• -fvtv-counts
• -fvtv-debug
• -finstrument-functions
• -finstrument-functions-once
• -finstrument-functions-exclude-function-list=sym,sym,...
• -finstrument-functions-exclude-file-list=file,file,...
• -fprofile-prefix-map=old=new
• -fpatchable-function-entry=N[,M]

bestie, quick vibe check: a bunch of these are sanitizers and hardeners, which is the compiler equivalent of wrapping your code in bubble wrap before it goes outside. 🫧 they are genuinely your friends. do not skip them because the name looked scary, okay? i'm watching. 👀

---

🦋 Preprocessor Options 🦋

moving on to the preprocessor, aka the part of the pipeline that runs before anything actually gets compiled and that everyone forgets exists until something explodes. 🌈 here's every one the manual lists:

• -Aquestion=answer
• -A-question[=answer]
• -C
• -CC
• -Dmacro[=defn]
• -dD
• -dI
• -dM
• -dN
• -dU
• -fdebug-cpp
• -fdirectives-only
• -fdollars-in-identifiers
• -fexec-charset=charset
• -fextended-identifiers
• -finput-charset=charset
• -flarge-source-files
• -fmacro-prefix-map=old=new
• -fmax-include-depth=depth
• -fno-canonical-system-headers
• -fpch-deps
• -fpch-preprocess
• -fpreprocessed
• -ftabstop=width
• -ftrack-macro-expansion
• -fwide-exec-charset=charset
• -fworking-directory
• -H
• -imacros file
• -include file
• -M
• -MD
• -MF
• -MG
• -MM
• -MMD
• -MP
• -MQ
• -MT
• -Mno-modules
• -no-integrated-cpp
• -P
• -pthread
• -remap
• -traditional
• -traditional-cpp
• -trigraphs
• -Umacro
• -undef
• -Wp,option
• -Xpreprocessor option

notice how many of these are just one letter. -M. -C. -P. -H. the preprocessor is out here with the most minimal naming scheme known to humanity and honestly? iconic. 🎀

---

✨ Assembler Options ✨

this section is short and perfect and i love it for that. two whole options. two. the manpage gives us:

• -Wa,option
• -Xassembler option

that's it. that's the section. we love a low-effort queen. 💅

---

🔥 Linker Options 🔥

okay so now we get to the linker, which is the part that takes all your little object files and smooshes them into something you can actually run. the manual lists these, and heads up, some of them are bare arguments rather than flags, so don't be alarmed:

• object-file-name
• -fuse-ld=linker
• -llibrary
• -nostartfiles
• -nodefaultlibs
• -nolibc
• -nostdlib
• -nostdlib++
• -e entry
• --entry=entry
• -pie
• -pthread
• -r
• -rdynamic
• -s
• -static
• -static-pie
• -static-libgcc
• -static-libstdc++
• -static-libasan
• -static-libtsan
• -static-liblsan
• -static-libubsan
• -shared
• -shared-libgcc
• -symbolic
• -T script
• -Wl,option
• -Xlinker option
• -u symbol
• -z keyword

fr fr, if you ever find yourself googling "why won't my binary link", this list is where the answer is hiding. 🕵️‍♀️ every time.

---

🌈 Directory Options 🌈

search paths. include paths. the place where your #include <stdio.h> actually goes to find friends. 🫶 the manual lists:

• -Bprefix
• -Idir
• -I-
• -idirafter dir
• -imacros file
• -imultilib dir
• -iplugindir=dir
• -iprefix file
• -iquote dir
• -isysroot dir
• -isystem dir
• -iwithprefix dir
• -iwithprefixbefore dir
• -Ldir
• -no-canonical-prefixes
• --no-sysroot-suffix
• -nostdinc
• -nostdinc++
• --sysroot=dir

the lowercase-i ones and the uppercase-I ones are not the same thing, bestie, and the compiler will absolutely not warn you if you mix them up. lowkey the most common self-own in all of C development. 😭

---

💜 Code Generation Options 💜

here we go, the deep cuts. this is the section for when you're telling GCC how to actually emit the machine code. the manual gives us:

• -fcall-saved-reg
• -fcall-used-reg
• -ffixed-reg
• -fexceptions
• -fnon-call-exceptions
• -fdelete-dead-exceptions
• -funwind-tables
• -fasynchronous-unwind-tables
• -fno-gnu-unique
• -finhibit-size-directive
• -fcommon
• -fno-ident
• -fpcc-struct-return
• -fpic
• -fPIC
• -fpie
• -fPIE
• -fno-plt
• -fno-jump-tables
• -fno-bit-tests
• -frecord-gcc-switches
• -freg-struct-return
• -fshort-enums
• -fshort-wchar
• -fverbose-asm
• -fpack-struct[=n]
• -fleading-underscore
• -ftls-model=model
• -fstack-reuse=reuse_level
• -ftrampolines
• -ftrampoline-impl=[stack|heap]
• -ftrapv
• -fwrapv
• -fvisibility=[default|internal|hidden|protected]
• -fstrict-volatile-bitfields
• -fsync-libcalls

-fpic vs -fPIC. -fpie vs -fPIE. the capital letters are load-bearing, i promise, they are not a typo, they are not a vibe, they are the whole point. 🌸

---

👑 Developer Options 👑

and finally — the section for the people who look at a compiler and think "yeah but what's it doing though". 🔬 this is the debug/nerd tier, and the manual lists every last one:

• -dletters
• -dumpspecs
• -dumpmachine
• -dumpversion
• -dumpfullversion
• -fcallgraph-info[=su,da]
• -fchecking
• -fchecking=n
• -fdbg-cnt-list
• -fdbg-cnt=counter-value-list
• -fdisable-ipa-pass_name
• -fdisable-rtl-pass_name
• -fdisable-rtl-pass-name=range-list
• -fdisable-tree-pass_name
• -fdisable-tree-pass-name=range-list
• -fdump-debug
• -fdump-earlydebug
• -fdump-noaddr
• -fdump-unnumbered
• -fdump-unnumbered-links
• -fdump-final-insns[=file]
• -fdump-ipa-all
• -fdump-ipa-cgraph
• -fdump-ipa-inline
• -fdump-lang-all
• -fdump-lang-switch
• -fdump-lang-switch-options
• -fdump-lang-switch-options=filename
• -fdump-passes
• -fdump-rtl-pass
• -fdump-rtl-pass=filename
• -fdump-statistics
• -fdump-tree-all
• -fdump-tree-switch
• -fdump-tree-switch-options
• -fdump-tree-switch-options=filename
• -fcompare-debug[=opts]
• -fcompare-debug-second
• -fenable-kind-pass
• -fenable-kind-pass=range-list
• -fira-verbose=n
• -flto-report
• -flto-report-wpa
• -fmem-report-wpa
• -fmem-report
• -fpre-ipa-mem-report
• -fpost-ipa-mem-report
• -fopt-info
• -fopt-info-options[=file]
• -fmultiflags
• -fprofile-report
• -frandom-seed=string
• -fsched-verbose=n
• -fsel-sched-verbose
• -fsel-sched-dump-cfg
• -fsel-sched-pipelining-verbose
• -fstats
• -fstack-usage
• -ftime-report
• -ftime-report-details
• -fvar-tracking-assignments-toggle
• -gtoggle
• -print-file-name=library
• -print-libgcc-file-name
• -print-multi-directory
• -print-multi-lib
• -print-multi-os-directory
• -print-prog-name=program
• -print-search-dirs
• -Q
• -print-sysroot
• -print-sysroot-headers-suffix
• -save-temps
• -save-temps=cwd
• -save-temps=obj
• -time[=file]

the -fdump-* family is genuinely so fun to play with, bestie — but that's a you-and-your-terminal moment, not a me-explaining-it moment. 🌟 we keep going.

🖥️ Machine-Dependent Options 🖥️

okay bestie, this is the part where GCC stops being one compiler and becomes, like, every compiler 💅 every chip family gets its own little wardrobe of flags, and honestly? the range. Find your target below and take your pick — no cap, they're all listed, all of them, I'm not skipping a single one 🙈

---

✨ AArch64 Options ✨

the 64-bit ARM section, and yes she's serving a lot of looks 🌸

• -mabi=name
• -mbig-endian
• -mlittle-endian
• -mgeneral-regs-only
• -mcmodel=tiny
• -mcmodel=small
• -mcmodel=large
• -mstrict-align
• -mno-strict-align
• -momit-leaf-frame-pointer
• -mtls-dialect=desc
• -mtls-dialect=traditional
• -mtls-size=size
• -mfix-cortex-a53-835769
• -mfix-cortex-a53-843419
• -mlow-precision-recip-sqrt
• -mlow-precision-sqrt
• -mlow-precision-div
• -mpc-relative-literal-loads
• -msign-return-address=scope
• -mbranch-protection=none|standard|pac-ret[+leaf +b-key]|bti
• -mharden-sls=opts
• -march=name
• -mcpu=name
• -mtune=name
• -moverride=string
• -mverbose-cost-dump
• -mstack-protector-guard=guard
• -mstack-protector-guard-reg=sysreg
• -mstack-protector-guard-offset=offset
• -mtrack-speculation
• -moutline-atomics
• -mearly-ldp-fusion
• -mlate-ldp-fusion

---

🎀 Adapteva Epiphany Options 🎀

tiny parallel-computing icon behavior, we love to see it 💖

• -mhalf-reg-file
• -mprefer-short-insn-regs
• -mbranch-cost=num
• -mcmove
• -mnops=num
• -msoft-cmpsf
• -msplit-lohi
• -mpost-inc
• -mpost-modify
• -mstack-offset=num
• -mround-nearest
• -mlong-calls
• -mshort-calls
• -msmall16
• -mfp-mode=mode
• -mvect-double
• -max-vect-align=num
• -msplit-vecmove-early
• -m1reg-reg

---

🦋 AMD GCN Options 🦋

short, sweet, GPU-flavored 🫶

• -march=gpu
• -mtune=gpu
• -mstack-size=bytes

---

💅 ARC Options 💅

this list is long and she's not sorry about it, bestie — every single one is here, I counted, twice 😭

• -mbarrel-shifter
• -mjli-always
• -mcpu=cpu
• -mA6
• -mARC600
• -mA7
• -mARC700
• -mdpfp
• -mdpfp-compact
• -mdpfp-fast
• -mno-dpfp-lrsr
• -mea
• -mno-mpy
• -mmul32x16
• -mmul64
• -matomic
• -mnorm
• -mspfp
• -mspfp-compact
• -mspfp-fast
• -msimd
• -msoft-float
• -mswap
• -mcrc
• -mdsp-packa
• -mdvbf
• -mlock
• -mmac-d16
• -mmac-24
• -mrtsc
• -mswape
• -mtelephony
• -mxy
• -misize
• -mannotate-align
• -marclinux
• -marclinux_prof
• -mlong-calls
• -mmedium-calls
• -msdata
• -mirq-ctrl-saved
• -mrgf-banked-regs
• -mlpc-width=width
• -G num
• -mvolatile-cache
• -mtp-regno=regno
• -malign-call
• -mauto-modify-reg
• -mbbit-peephole
• -mno-brcc
• -mcase-vector-pcrel
• -mcompact-casesi
• -mno-cond-exec
• -mearly-cbranchsi
• -mexpand-adddi
• -mindexed-loads
• -mlra
• -mlra-priority-none
• -mlra-priority-compact
• -mlra-priority-noncompact
• -mmillicode
• -mmixed-code
• -mq-class
• -mRcq
• -mRcw
• -msize-level=level
• -mtune=cpu
• -mmultcost=num
• -mcode-density-frame
• -munalign-prob-threshold=probability
• -mmpy-option=multo
• -mdiv-rem
• -mcode-density
• -mll64
• -mfpu=fpu
• -mrf16
• -mbranch-index

---

🌈 ARM Options 🌈

32-bit ARM, the classic 💜 she's got the -mno- twins to match basically every -m flag, it's giving matching sets 🎀

• -mapcs-frame
• -mno-apcs-frame
• -mabi=name
• -mapcs-stack-check
• -mno-apcs-stack-check
• -mapcs-reentrant
• -mno-apcs-reentrant
• -mgeneral-regs-only
• -msched-prolog
• -mno-sched-prolog
• -mlittle-endian
• -mbig-endian
• -mbe8
• -mbe32
• -mfloat-abi=name
• -mfp16-format=name
• -mthumb-interwork
• -mno-thumb-interwork
• -mcpu=name
• -march=name
• -mfpu=name
• -mtune=name
• -mprint-tune-info
• -mstructure-size-boundary=n
• -mabort-on-noreturn
• -mlong-calls
• -mno-long-calls
• -msingle-pic-base
• -mno-single-pic-base
• -mpic-register=reg
• -mnop-fun-dllimport
• -mpoke-function-name
• -mthumb
• -marm
• -mflip-thumb
• -mtpcs-frame
• -mtpcs-leaf-frame
• -mcaller-super-interworking
• -mcallee-super-interworking
• -mtp=name
• -mtls-dialect=dialect
• -mword-relocations
• -mfix-cortex-m3-ldrd
• -mfix-cortex-a57-aes-1742098
• -mfix-cortex-a72-aes-1655431
• -munaligned-access
• -mneon-for-64bits
• -mslow-flash-data
• -masm-syntax-unified
• -mrestrict-it
• -mverbose-cost-dump
• -mpure-code
• -mcmse
• -mfix-cmse-cve-2021-35465
• -mstack-protector-guard=guard
• -mstack-protector-guard-offset=offset
• -mfdpic
• -mbranch-protection=none|standard|pac-ret[+leaf][+bti]|bti[+pac-ret[+leaf]]

---

🍓 AVR Options 🍓

microcontroller cutie energy, obsessed 😌

• -mmcu=mcu
• -mabsdata
• -maccumulate-args
• -mbranch-cost=cost
• -mfuse-add=level
• -mcall-prologues
• -mgas-isr-prologues
• -mint8
• -mflmap
• -mdouble=bits
• -mlong-double=bits
• -mn_flash=size
• -mno-interrupts
• -mmain-is-OS_task
• -mrelax
• -mrmw
• -mstrict-X
• -mtiny-stack
• -mrodata-in-ram
• -mfract-convert-truncate
• -mshort-calls
• -mskip-bug
• -nodevicelib
• -nodevicespecs
• -Waddr-space-convert
• -Wmisspelled-isr

---

🧚 Blackfin Options 🧚

• -mcpu=cpu[-sirevision]
• -msim
• -momit-leaf-frame-pointer
• -mno-omit-leaf-frame-pointer
• -mspecld-anomaly
• -mno-specld-anomaly
• -mcsync-anomaly
• -mno-csync-anomaly
• -mlow-64k
• -mno-low64k
• -mstack-check-l1
• -mid-shared-library
• -mno-id-shared-library
• -mshared-library-id=n
• -mleaf-id-shared-library
• -mno-leaf-id-shared-library
• -msep-data
• -mno-sep-data
• -mlong-calls
• -mno-long-calls
• -mfast-fp
• -minline-plt
• -mmulticore
• -mcorea
• -mcoreb
• -msdram
• -micplb

---

👑 C6X Options 👑

• -mbig-endian
• -mlittle-endian
• -march=cpu
• -msim
• -msdata=sdata-type

---

💜 CRIS Options 💜

• -mcpu=cpu
• -march=cpu
• -mtune=cpu
• -mmax-stack-frame=n
• -metrax4
• -metrax100
• -mpdebug
• -mcc-init
• -mno-side-effects
• -mstack-align
• -mdata-align
• -mconst-align
• -m32-bit
• -m16-bit
• -m8-bit
• -mno-prologue-epilogue
• -melf
• -maout
• -sim
• -sim2
• -mmul-bug-workaround
• -mno-mul-bug-workaround

---

🌸 C-SKY Options 🌸

• -march=arch
• -mcpu=cpu
• -mbig-endian
• -EB
• -mlittle-endian
• -EL
• -mhard-float
• -msoft-float
• -mfpu=fpu
• -mdouble-float
• -mfdivdu
• -mfloat-abi=name
• -melrw
• -mistack
• -mmp
• -mcp
• -mcache
• -msecurity
• -mtrust
• -mdsp
• -medsp
• -mvdsp
• -mdiv
• -msmart
• -mhigh-registers
• -manchor
• -mpushpop
• -mmultiple-stld
• -mconstpool
• -mstack-size
• -mccrt
• -mbranch-cost=n
• -mcse-cc
• -msched-prolog
• -msim

---

🦋 Darwin Options 🦋

macOS girlies rise 🫶 ngl this list has some flags showing up more than once and I'm keeping them exactly as written, no editing, periodt 💅

• -all_load
• -allowable_client
• -arch
• -arch_errors_fatal
• -arch_only
• -bind_at_load
• -bundle
• -bundle_loader
• -client_name
• -compatibility_version
• -current_version
• -dead_strip
• -dependency-file
• -dylib_file
• -dylinker_install_name
• -dynamic
• -dynamiclib
• -exported_symbols_list
• -filelist
• -flat_namespace
• -force_cpusubtype_ALL
• -force_flat_namespace
• -headerpad_max_install_names
• -iframework
• -image_base
• -init
• -install_name
• -keep_private_externs
• -multi_module
• -multiply_defined
• -multiply_defined_unused
• -noall_load
• -no_dead_strip_inits_and_terms
• -nodefaultrpaths
• -nofixprebinding
• -nomultidefs
• -noprebind
• -noseglinkedit
• -pagezero_size
• -prebind
• -prebind_all_twolevel_modules
• -private_bundle
• -read_only_relocs
• -sectalign
• -sectobjectsymbols
• -whyload
• -seg1addr
• -sectcreate
• -sectobjectsymbols
• -sectorder
• -segaddr
• -segs_read_only_addr
• -segs_read_write_addr
• -seg_addr_table
• -seg_addr_table_filename
• -seglinkedit
• -segprot
• -segs_read_only_addr
• -segs_read_write_addr
• -single_module
• -static
• -sub_library
• -sub_umbrella
• -twolevel_namespace
• -umbrella
• -undefined
• -unexported_symbols_list
• -weak_reference_mismatches
• -whatsloaded
• -F
• -gused
• -gfull
• -mmacosx-version-min=version
• -mkernel
• -mone-byte-bool

---

⭐ DEC Alpha Options ⭐

• -mno-fp-regs
• -msoft-float
• -mieee
• -mieee-with-inexact
• -mieee-conformant
• -mfp-trap-mode=mode
• -mfp-rounding-mode=mode
• -mtrap-precision=mode
• -mbuild-constants
• -mcpu=cpu-type
• -mtune=cpu-type
• -mbwx
• -mmax
• -mfix
• -mcix
• -mfloat-vax
• -mfloat-ieee
• -mexplicit-relocs
• -msmall-data
• -mlarge-data
• -msmall-text
• -mlarge-text
• -mmemory-latency=time

---

🔥 eBPF Options 🔥

kernel-side main character energy fr fr 🌈

• -mbig-endian
• -mlittle-endian
• -mframe-limit=bytes
• -mxbpf
• -mco-re
• -mno-co-re
• -mjmpext
• -mjmp32
• -malu32
• -mv3-atomics
• -mbswap
• -msdiv
• -msmov
• -mcpu=version
• -masm=dialect
• -minline-memops-threshold=bytes

---

🎀 FR30 Options 🎀

• -msmall-model
• -mno-lsim

---

💖 FT32 Options 💖

• -msim
• -mlra
• -mnodiv
• -mft32b
• -mcompress
• -mnopm

---

🌈 FRV Options 🌈

• -mgpr-32
• -mgpr-64
• -mfpr-32
• -mfpr-64
• -mhard-float
• -msoft-float
• -malloc-cc
• -mfixed-cc
• -mdword
• -mno-dword
• -mdouble
• -mno-double
• -mmedia
• -mno-media
• -mmuladd
• -mno-muladd
• -mfdpic
• -minline-plt
• -mgprel-ro
• -multilib-library-pic
• -mlinked-fp
• -mlong-calls
• -malign-labels
• -mlibrary-pic
• -macc-4
• -macc-8
• -mpack
• -mno-pack
• -mno-eflags
• -mcond-move
• -mno-cond-move
• -moptimize-membar
• -mno-optimize-membar
• -mscc
• -mno-scc
• -mcond-exec
• -mno-cond-exec
• -mvliw-branch
• -mno-vliw-branch
• -mmulti-cond-exec
• -mno-multi-cond-exec
• -mnested-cond-exec
• -mno-nested-cond-exec
• -mtomcat-stats
• -mTLS
• -mtls
• -mcpu=cpu

---

🐧 GNU/Linux Options 🐧

libc picking time, choose your fighter ✨

• -mglibc
• -muclibc
• -mmusl
• -mbionic
• -mandroid
• -tno-android-cc
• -tno-android-ld

---

💜 H8/300 Options 💜

• -mrelax
• -mh
• -ms
• -mn
• -mexr
• -mno-exr
• -mint32
• -malign-300

---

🦋 HPPA Options 🦋

look at all these -mno- counterparts, it's giving balance 😌

• -march=architecture-type
• -matomic-libcalls
• -mbig-switch
• -mcaller-copies
• -mdisable-fpregs
• -mdisable-indexing
• -mordered
• -mfast-indirect-calls
• -mgas
• -mgnu-ld
• -mhp-ld
• -mfixed-range=register-range
• -mcoherent-ldcw
• -mjump-in-delay
• -mlinker-opt
• -mlong-calls
• -mlong-load-store
• -mno-atomic-libcalls
• -mno-disable-fpregs
• -mno-disable-indexing
• -mno-fast-indirect-calls
• -mno-gas
• -mno-jump-in-delay
• -mno-long-load-store
• -mno-portable-runtime
• -mno-soft-float
• -mno-space-regs
• -msoft-float
• -mpa-risc-1-0
• -mpa-risc-1-1
• -mpa-risc-2-0
• -mportable-runtime
• -mschedule=cpu-type
• -mspace-regs
• -msoft-mult
• -msio
• -mwsio
• -munix=unix-std
• -nolibdld
• -static
• -threads

---

⭐ IA-64 Options ⭐

Itanium said "I'm gonna have a flag for every possible scheduling opinion" and honestly? respect 💀

• -mbig-endian
• -mlittle-endian
• -mgnu-as
• -mgnu-ld
• -mno-pic
• -mvolatile-asm-stop
• -mregister-names
• -msdata
• -mno-sdata
• -mconstant-gp
• -mauto-pic
• -mfused-madd
• -minline-float-divide-min-latency
• -minline-float-divide-max-throughput
• -mno-inline-float-divide
• -minline-int-divide-min-latency
• -minline-int-divide-max-throughput
• -mno-inline-int-divide
• -minline-sqrt-min-latency
• -minline-sqrt-max-throughput
• -mno-inline-sqrt
• -mdwarf2-asm
• -mearly-stop-bits
• -mfixed-range=register-range
• -mtls-size=tls-size
• -mtune=cpu-type
• -milp32
• -mlp64
• -msched-br-data-spec
• -msched-ar-data-spec
• -msched-control-spec
• -msched-br-in-data-spec
• -msched-ar-in-data-spec
• -msched-in-control-spec
• -msched-spec-ldc
• -msched-spec-control-ldc
• -msched-prefer-non-data-spec-insns
• -msched-prefer-non-control-spec-insns
• -msched-stop-bits-after-every-cycle
• -msched-count-spec-in-critical-path
• -msel-sched-dont-check-control-spec
• -msched-fp-mem-deps-zero-cost
• -msched-max-memory-insns-hard-limit
• -msched-max-memory-insns=max-insns

---

🧚 LM32 Options 🧚

• -mbarrel-shift-enabled
• -mdivide-enabled
• -mmultiply-enabled
• -msign-extend-enabled
• -muser-enabled

🦋 LoongArch Options

bestie, LoongArch is the moment — this is your whole menu of arch/tune/ABI/FPU/SIMD decisions, plus a bunch of optimization switches she's ✨obsessed✨ with. Every single one, no skips:

• -march=arch-type
• -mtune=tune-type
• -mabi=base-abi-type
• -mfpu=fpu-type
• -msimd=simd-type
• -msoft-float
• -msingle-float
• -mdouble-float
• -mlsx
• -mno-lsx
• -mlasx
• -mno-lasx
• -mbranch-cost=n
• -mcheck-zero-division
• -mno-check-zero-division
• -mcond-move-int
• -mno-cond-move-int
• -mcond-move-float
• -mno-cond-move-float
• -memcpy
• -mno-memcpy
• -mstrict-align
• -mno-strict-align
• -mmax-inline-memcpy-size=n
• -mexplicit-relocs=style
• -mexplicit-relocs
• -mno-explicit-relocs
• -mdirect-extern-access
• -mno-direct-extern-access
• -mcmodel=code-model
• -mrelax
• -mpass-mrelax-to-as
• -mrecip
• -mrecip=opt
• -mfrecipe
• -mno-frecipe
• -mdiv32
• -mno-div32
• -mlam-bh
• -mno-lam-bh
• -mlamcas
• -mno-lamcas
• -mld-seq-sa
• -mno-ld-seq-sa
• -mtls-dialect=opt

---

🎀 M32R/D Options

tiny section, big energy. here's the M32R/D lineup 🌸

• -m32r2
• -m32rx
• -m32r
• -mdebug
• -malign-loops
• -mno-align-loops
• -missue-rate=number
• -mbranch-cost=number
• -mmodel=code-size-model-type
• -msdata=sdata-type
• -mno-flush-func
• -mflush-func=name
• -mno-flush-trap
• -mflush-trap=number
• -G num

---

💜 M32C Options

short king. three flags, that's it, that's the whole vibe:

• -mcpu=cpu
• -msim
• -memregs=number

---

🍓 M680x0 Options

okay so m68k has range. classic chips, classic flags, and a whole bunch of -mno- twins because we love a matching set 💅

• -march=arch
• -mcpu=cpu
• -mtune=tune
• -m68000
• -m68020
• -m68020-40
• -m68020-60
• -m68030
• -m68040
• -m68060
• -mcpu32
• -m5200
• -m5206e
• -m528x
• -m5307
• -m5407
• -mcfv4e
• -mbitfield
• -mno-bitfield
• -mc68000
• -mc68020
• -mnobitfield
• -mrtd
• -mno-rtd
• -mdiv
• -mno-div
• -mshort
• -mno-short
• -mhard-float
• -m68881
• -msoft-float
• -mpcrel
• -malign-int
• -mstrict-align
• -msep-data
• -mno-sep-data
• -mshared-library-id=n
• -mid-shared-library
• -mno-id-shared-library
• -mxgot
• -mno-xgot
• -mlong-jump-table-offsets

---

✨ MCore Options

MCore, my beloved little list. every flag, in order, no notes:

• -mhardlit
• -mno-hardlit
• -mdiv
• -mno-div
• -mrelax-immediates
• -mno-relax-immediates
• -mwide-bitfields
• -mno-wide-bitfields
• -m4byte-functions
• -mno-4byte-functions
• -mcallgraph-data
• -mno-callgraph-data
• -mslow-bytes
• -mno-slow-bytes
• -mno-lsim
• -mlittle-endian
• -mbig-endian
• -m210
• -m340
• -mstack-increment

---

🌸 MicroBlaze Options

float or not float, divide small or not, Xilinx-y extras or not — pick your fighter, bestie 🫶

• -msoft-float
• -mhard-float
• -msmall-divides
• -mcpu=cpu
• -mmemcpy
• -mxl-soft-mul
• -mxl-soft-div
• -mxl-barrel-shift
• -mxl-pattern-compare
• -mxl-stack-check
• -mxl-gp-opt
• -mno-clearbss
• -mxl-multiply-high
• -mxl-float-convert
• -mxl-float-sqrt
• -mbig-endian
• -mlittle-endian
• -mxl-reorder
• -mxl-mode-app-model
• -mpic-data-is-text-relative

---

👑 MIPS Options

MIPS said "I contain multitudes" and then wrote a whole novel. she's long, she's iconic, and she's not sorry. settle in 💀

• -EL
• -EB
• -march=arch
• -mtune=arch
• -mips1
• -mips2
• -mips3
• -mips4
• -mips32
• -mips32r2
• -mips32r3
• -mips32r5
• -mips32r6
• -mips64
• -mips64r2
• -mips64r3
• -mips64r5
• -mips64r6
• -mips16
• -mno-mips16
• -mflip-mips16
• -minterlink-compressed
• -mno-interlink-compressed
• -minterlink-mips16
• -mno-interlink-mips16
• -mabi=abi
• -mabicalls
• -mno-abicalls
• -mshared
• -mno-shared
• -mplt
• -mno-plt
• -mxgot
• -mno-xgot
• -mgp32
• -mgp64
• -mfp32
• -mfpxx
• -mfp64
• -mhard-float
• -msoft-float
• -mno-float
• -msingle-float
• -mdouble-float
• -modd-spreg
• -mno-odd-spreg
• -mabs=mode
• -mnan=encoding
• -mdsp
• -mno-dsp
• -mdspr2
• -mno-dspr2
• -mmcu
• -mmno-mcu
• -meva
• -mno-eva
• -mvirt
• -mno-virt
• -mxpa
• -mno-xpa
• -mcrc
• -mno-crc
• -mginv
• -mno-ginv
• -mmicromips
• -mno-micromips
• -mmsa
• -mno-msa
• -mloongson-mmi
• -mno-loongson-mmi
• -mloongson-ext
• -mno-loongson-ext
• -mloongson-ext2
• -mno-loongson-ext2
• -mfpu=fpu-type
• -msmartmips
• -mno-smartmips
• -mpaired-single
• -mno-paired-single
• -mdmx
• -mno-mdmx
• -mips3d
• -mno-mips3d
• -mmt
• -mno-mt
• -mllsc
• -mno-llsc
• -mlong64
• -mlong32
• -msym32
• -mno-sym32
• -Gnum
• -mlocal-sdata
• -mno-local-sdata
• -mextern-sdata
• -mno-extern-sdata
• -mgpopt
• -mno-gopt
• -membedded-data
• -mno-embedded-data
• -muninit-const-in-rodata
• -mno-uninit-const-in-rodata
• -mcode-readable=setting
• -msplit-addresses
• -mno-split-addresses
• -mexplicit-relocs
• -mno-explicit-relocs
• -mexplicit-relocs=release
• -mcheck-zero-division
• -mno-check-zero-division
• -mdivide-traps
• -mdivide-breaks
• -mload-store-pairs
• -mno-load-store-pairs
• -mstrict-align
• -mno-strict-align
• -mno-unaligned-access
• -munaligned-access
• -mmemcpy
• -mno-memcpy
• -mlong-calls
• -mno-long-calls
• -mmad
• -mno-mad
• -mimadd
• -mno-imadd
• -mfused-madd
• -mno-fused-madd
• -nocpp
• -mfix-24k
• -mno-fix-24k
• -mfix-r4000
• -mno-fix-r4000
• -mfix-r4400
• -mno-fix-r4400
• -mfix-r5900
• -mno-fix-r5900
• -mfix-r10000
• -mno-fix-r10000
• -mfix-rm7000
• -mno-fix-rm7000
• -mfix-vr4120
• -mno-fix-vr4120
• -mfix-vr4130
• -mno-fix-vr4130
• -mfix-sb1
• -mno-fix-sb1
• -mflush-func=func
• -mno-flush-func
• -mbranch-cost=num
• -mbranch-likely
• -mno-branch-likely
• -mcompact-branches=policy
• -mfp-exceptions
• -mno-fp-exceptions
• -mvr4130-align
• -mno-vr4130-align
• -msynci
• -mno-synci
• -mlxc1-sxc1
• -mno-lxc1-sxc1
• -mmadd4
• -mno-madd4
• -mrelax-pic-calls
• -mno-relax-pic-calls
• -mmcount-ra-address
• -mframe-header-opt
• -mno-frame-header-opt

---

🧚 MMIX Options

MMIX, the Knuth one. yes bestie, that MMIX. here's the list, and she's got two ABIs, pick wisely:

• -mlibfuncs
• -mno-libfuncs
• -mepsilon
• -mno-epsilon
• -mabi=gnu
• -mabi=mmixware
• -mzero-extend
• -mknuthdiv
• -mtoplevel-symbols
• -melf
• -mbranch-predict
• -mno-branch-predict
• -mbase-addresses
• -mno-base-addresses
• -msingle-exit
• -mno-single-exit

---

🌈 MN10300 Options

little list, fully loaded:

• -mmult-bug
• -mno-mult-bug
• -mno-am33
• -mam33
• -mam33-2
• -mam34
• -mtune=cpu-type
• -mreturn-pointer-on-d0
• -mno-crt0
• -mrelax
• -mliw
• -msetlb

---

💅 Moxie Options

four flags. that's the whole thing. tiny, perfect, slaying:

• -meb
• -mel
• -mmul.x
• -mno-crt0

---

🔥 MSP430 Options

MSP430 said "I'm small but I have opinions". look at all these = flags, she loves a value:

• -msim
• -masm-hex
• -mmcu=
• -mcpu=
• -mlarge
• -msmall
• -mrelax
• -mwarn-mcu
• -mcode-region=
• -mdata-region=
• -msilicon-errata=
• -msilicon-errata-warn=
• -mhwmult=
• -minrt
• -mtiny-printf
• -mmax-inline-shift=

---

🦋 NDS32 Options

endianness, register counts, extensions, vector sizes, cache blocks — she's got a whole closet:

• -mbig-endian
• -mlittle-endian
• -mreduced-regs
• -mfull-regs
• -mcmov
• -mno-cmov
• -mext-perf
• -mno-ext-perf
• -mext-perf2
• -mno-ext-perf2
• -mext-string
• -mno-ext-string
• -mv3push
• -mno-v3push
• -m16bit
• -mno-16bit
• -misr-vector-size=num
• -mcache-block-size=num
• -march=arch
• -mcmodel=code-model
• -mctor-dtor
• -mrelax

---

🎀 Nios II Options

custom instructions? custom FPU configs? hal?? smallc??? she has it all, bestie:

• -G num
• -mgpopt=option
• -mgpopt
• -mno-gpopt
• -mgprel-sec=regexp
• -mr0rel-sec=regexp
• -mel
• -meb
• -mno-bypass-cache
• -mbypass-cache
• -mno-cache-volatile
• -mcache-volatile
• -mno-fast-sw-div
• -mfast-sw-div
• -mhw-mul
• -mno-hw-mul
• -mhw-mulx
• -mno-hw-mulx
• -mno-hw-div
• -mhw-div
• -mcustom-insn=N
• -mno-custom-insn
• -mcustom-fpu-cfg=name
• -mhal
• -msmallc
• -msys-crt0=name
• -msys-lib=name
• -march=arch
• -mbmx
• -mno-bmx
• -mcdx
• -mno-cdx

---

⭐ Nvidia PTX Options

three flags. GPU girlies, this is your whole section:

• -m64
• -mmainkernel
• -moptimize

---

🌸 OpenRISC Options

board, libc, mul/div in hardware or software, floats, and a pile of instruction-enable vibes:

• -mboard=name
• -mnewlib
• -mhard-mul
• -mhard-div
• -msoft-mul
• -msoft-div
• -msoft-float
• -mhard-float
• -mdouble-float
• -munordered-float
• -mcmov
• -mror
• -mrori
• -msext
• -msfimm
• -mshftimm
• -mcmodel=code-model

---

💜 PDP-11 Options

the vintage icon herself. FPU, AC0, machine models, int widths, asm dialects, and -mlra for good measure:

• -mfpu
• -msoft-float
• -mac0
• -mno-ac0
• -m40
• -m45
• -m10
• -mint32
• -mno-int16
• -mint16
• -mno-int32
• -msplit
• -munix-asm
• -mdec-asm
• -mgnu-asm
• -mlra

---

🍓 PowerPC Options

not me getting to this section and it just says, verbatim, See RS/6000 and PowerPC Options. like okay girl, redirecting you, love that for us 🫶

---

✨ PRU Options

• -mmcu=mcu
• -minrt
• -mno-relax
• -mloop
• -mabi=variant

---

🧚 RISC-V Options

RISC-V ate and left no crumbs — ABIs, ISA strings, ISA spec versions, cmodel choices, and inline-everything:

• -mbranch-cost=N-instruction
• -mplt
• -mno-plt
• -mabi=ABI-string
• -mfdiv
• -mno-fdiv
• -mdiv
• -mno-div
• -misa-spec=ISA-spec-string
• -march=ISA-string
• -mtune=processor-string
• -mpreferred-stack-boundary=num
• -msmall-data-limit=N-bytes
• -msave-restore
• -mno-save-restore
• -mshorten-memrefs
• -mno-shorten-memrefs
• -mstrict-align
• -mno-strict-align
• -mcmodel=medlow
• -mcmodel=medany
• -mexplicit-relocs
• -mno-explicit-relocs
• -mrelax
• -mno-relax
• -mriscv-attribute
• -mno-riscv-attribute
• -malign-data=type
• -mbig-endian
• -mlittle-endian
• -mstack-protector-guard=guard
• -mstack-protector-guard-reg=reg
• -mstack-protector-guard-offset=offset
• -mcsr-check
• -mno-csr-check
• -mmovcc
• -mno-movcc
• -minline-atomics
• -mno-inline-atomics
• -minline-strlen
• -mno-inline-strlen
• -minline-strcmp
• -mno-inline-strcmp
• -minline-strncmp
• -mno-inline-strncmp
• -mtls-dialect=desc
• -mtls-dialect=trad

---

💅 RL78 Options

simulator, multiplier flavors, all-regs, cpu picks, double widths, and interrupt-y save behavior:

• -msim
• -mmul=none
• -mmul=g13
• -mmul=g14
• -mallregs
• -mcpu=g10
• -mcpu=g13
• -mcpu=g14
• -mg10
• -mg13
• -mg14
• -m64bit-doubles
• -m32bit-doubles
• -msave-mduc-in-interrupts

---

🔥 RS/6000 and PowerPC Options

okay this is the big one. the main character. cpu types, cmodel, altivec, toc options, struct-return conventions, plt flavors, inline limits, crypto, htm, quad-memory, float128, prefixed, pcrel, mma, rop-protect, privileged — everything is here and nothing is getting skipped, periodt 👑

• -mcpu=cpu-type
• -mtune=cpu-type
• -mcmodel=code-model
• -mpowerpc64
• -maltivec
• -mno-altivec
• -mpowerpc-gpopt
• -mno-powerpc-gpopt
• -mpowerpc-gfxopt
• -mno-powerpc-gfxopt
• -mmfcrf
• -mno-mfcrf
• -mpopcntb
• -mno-popcntb
• -mpopcntd
• -mno-popcntd
• -mfprnd
• -mno-fprnd
• -mcmpb
• -mno-cmpb
• -mhard-dfp
• -mno-hard-dfp
• -mfull-toc
• -mminimal-toc
• -mno-fp-in-toc
• -mno-sum-in-toc
• -m64
• -m32
• -mxl-compat
• -mno-xl-compat
• -mpe
• -malign-power
• -malign-natural
• -msoft-float
• -mhard-float
• -mmultiple
• -mno-multiple
• -mupdate
• -mno-update
• -mavoid-indexed-addresses
• -mno-avoid-indexed-addresses
• -mfused-madd
• -mno-fused-madd
• -mbit-align
• -mno-bit-align
• -mstrict-align
• -mno-strict-align
• -mrelocatable
• -mno-relocatable
• -mrelocatable-lib
• -mno-relocatable-lib
• -mtoc
• -mno-toc
• -mlittle
• -mlittle-endian
• -mbig
• -mbig-endian
• -mdynamic-no-pic
• -mswdiv
• -msingle-pic-base
• -mprioritize-restricted-insns=priority
• -msched-costly-dep=dependence_type
• -minsert-sched-nops=scheme
• -mcall-aixdesc
• -mcall-eabi
• -mcall-freebsd
• -mcall-linux
• -mcall-netbsd
• -mcall-openbsd
• -mcall-sysv
• -mcall-sysv-eabi
• -mcall-sysv-noeabi
• -mtraceback=traceback_type
• -maix-struct-return
• -msvr4-struct-return
• -mabi=abi-type
• -msecure-plt
• -mbss-plt
• -mlongcall
• -mno-longcall
• -mpltseq
• -mno-pltseq
• -mblock-move-inline-limit=num
• -mblock-compare-inline-limit=num
• -mblock-compare-inline-loop-limit=num
• -mno-block-ops-unaligned-vsx
• -mstring-compare-inline-limit=num
• -misel
• -mno-isel
• -mvrsave
• -mno-vrsave
• -mmulhw
• -mno-mulhw
• -mdlmzb
• -mno-dlmzb
• -mprototype
• -mno-prototype
• -msim
• -mmvme
• -mads
• -myellowknife
• -memb
• -msdata
• -msdata=opt
• -mreadonly-in-sdata
• -mvxworks
• -G num
• -mrecip
• -mrecip=opt
• -mno-recip
• -mrecip-precision
• -mno-recip-precision
• -mveclibabi=type
• -mfriz
• -mno-friz
• -mpointers-to-nested-functions
• -mno-pointers-to-nested-functions
• -msave-toc-indirect
• -mno-save-toc-indirect
• -mpower8-fusion
• -mno-mpower8-fusion
• -mcrypto
• -mno-crypto
• -mhtm
• -mno-htm
• -mquad-memory
• -mno-quad-memory
• -mquad-memory-atomic
• -mno-quad-memory-atomic
• -mcompat-align-parm
• -mno-compat-align-parm
• -mfloat128
• -mno-float128
• -mfloat128-hardware
• -mno-float128-hardware
• -mgnu-attribute
• -mno-gnu-attribute
• -mstack-protector-guard=guard
• -mstack-protector-guard-reg=reg
• -mstack-protector-guard-offset=offset
• -mprefixed
• -mno-prefixed
• -mpcrel
• -mno-pcrel
• -mmma
• -mno-mmma
• -mrop-protect
• -mno-rop-protect
• -mprivileged
• -mno-privileged

✨ RX Options ✨

okay so if you're on RX, here's your little menu of machine-specific flags, bestie 💖

• -m64bit-doubles
• -m32bit-doubles
• -fpu
• -nofpu
• -mcpu=
• -mbig-endian-data
• -mlittle-endian-data
• -msmall-data
• -msim
• -mno-sim
• -mas100-syntax
• -mno-as100-syntax
• -mrelax
• -mmax-constant-size=
• -mint-register=
• -mpid
• -mallow-string-insns
• -mno-allow-string-insns
• -mjsr
• -mno-warn-multiple-fast-interrupts
• -msave-acc-in-interrupts

---

✨ S/390 and zSeries Options ✨

mainframe girlies rise UP 👑 here's the whole lineup:

• -mtune=cpu-type
• -march=cpu-type
• -mhard-float
• -msoft-float
• -mhard-dfp
• -mno-hard-dfp
• -mlong-double-64
• -mlong-double-128
• -mbackchain
• -mno-backchain
• -mpacked-stack
• -mno-packed-stack
• -msmall-exec
• -mno-small-exec
• -mmvcle
• -mno-mvcle
• -m64
• -m31
• -mdebug
• -mno-debug
• -mesa
• -mzarch
• -mhtm
• -mvx
• -mzvector
• -mtpf-trace
• -mno-tpf-trace
• -mtpf-trace-skip
• -mno-tpf-trace-skip
• -mfused-madd
• -mno-fused-madd
• -mwarn-framesize
• -mwarn-dynamicstack
• -mstack-size
• -mstack-guard
• -mhotpatch=halfwords,halfwords

---

✨ SH Options ✨

SuperH time, no cap 🦋

• -m1
• -m2
• -m2e
• -m2a-nofpu
• -m2a-single-only
• -m2a-single
• -m2a
• -m3
• -m3e
• -m4-nofpu
• -m4-single-only
• -m4-single
• -m4
• -m4a-nofpu
• -m4a-single-only
• -m4a-single
• -m4a
• -m4al
• -mb
• -ml
• -mdalign
• -mrelax
• -mbigtable
• -mfmovd
• -mrenesas
• -mno-renesas
• -mnomacsave
• -mieee
• -mno-ieee
• -mbitops
• -misize
• -minline-ic_invalidate
• -mpadstruct
• -mprefergot
• -musermode
• -multcost=number
• -mdiv=strategy
• -mdivsi3_libfunc=name
• -mfixed-range=register-range
• -maccumulate-outgoing-args
• -matomic-model=atomic-model
• -mbranch-cost=num
• -mzdcbranch
• -mno-zdcbranch
• -mcbranch-force-delay-slot
• -mfused-madd
• -mno-fused-madd
• -mfsca
• -mno-fsca
• -mfsrra
• -mno-fsrra
• -mpretend-cmove
• -mtas

---

✨ Solaris 2 Options ✨

sun-kissed ☀️ tiny list, big energy:

• -mclear-hwcap
• -mno-clear-hwcap
• -mimpure-text
• -mno-impure-text
• -pthreads

---

✨ SPARC Options ✨

the full SPARC spread, periodt 💅

• -mcpu=cpu-type
• -mtune=cpu-type
• -mcmodel=code-model
• -mmemory-model=mem-model
• -m32
• -m64
• -mapp-regs
• -mno-app-regs
• -mfaster-structs
• -mno-faster-structs
• -mflat
• -mno-flat
• -mfpu
• -mno-fpu
• -mhard-float
• -msoft-float
• -mhard-quad-float
• -msoft-quad-float
• -mstack-bias
• -mno-stack-bias
• -mstd-struct-return
• -mno-std-struct-return
• -munaligned-doubles
• -mno-unaligned-doubles
• -muser-mode
• -mno-user-mode
• -mv8plus
• -mno-v8plus
• -mvis
• -mno-vis
• -mvis2
• -mno-vis2
• -mvis3
• -mno-vis3
• -mvis4
• -mno-vis4
• -mvis4b
• -mno-vis4b
• -mcbcond
• -mno-cbcond
• -mfmaf
• -mno-fmaf
• -mfsmuld
• -mno-fsmuld
• -mpopc
• -mno-popc
• -msubxc
• -mno-subxc
• -mfix-at697f
• -mfix-ut699
• -mfix-ut700
• -mfix-gr712rc
• -mlra
• -mno-lra

---

✨ System V Options ✨

short and sweet, we love a minimal queen:

• -Qy
• -Qn
• -YP,paths
• -Ym,dir

---

✨ V850 Options ✨

V850 besties, this one's for you 🌸

• -mlong-calls
• -mno-long-calls
• -mep
• -mno-ep
• -mprolog-function
• -mno-prolog-function
• -mspace
• -mtda=n
• -msda=n
• -mzda=n
• -mapp-regs
• -mno-app-regs
• -mdisable-callt
• -mno-disable-callt
• -mv850e2v3
• -mv850e2
• -mv850e1
• -mv850es
• -mv850e
• -mv850
• -mv850e3v5
• -mloop
• -mrelax
• -mlong-jumps
• -msoft-float
• -mhard-float
• -mgcc-abi
• -mrh850-abi
• -mbig-switch

---

✨ VAX Options ✨

retro legend behavior, honestly iconic:

• -mg
• -mgnu
• -munix
• -mlra

---

✨ Visium Options ✨

• -mdebug
• -msim
• -mfpu
• -mno-fpu
• -mhard-float
• -msoft-float
• -mcpu=cpu-type
• -mtune=cpu-type
• -msv-mode
• -muser-mode

---

✨ VMS Options ✨

• -mvms-return-codes
• -mdebug-main=prefix
• -mmalloc64
• -mpointer-size=size

---

✨ VxWorks Options ✨

• -mrtp
• -msmp
• -non-static
• -Bstatic
• -Bdynamic
• -Xbind-lazy
• -Xbind-now

---

✨ x86 Options ✨

okay bestie sit DOWN because this is the big one 😭 the x86 list is extensive and I'm not about to skip a single one of them. every single flag, all of it, 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
• -mno-wide-multiply
• -mrtd
• -malign-double
• -mpreferred-stack-boundary=num
• -mincoming-stack-boundary=num
• -mcld
• -mcx16
• -msahf
• -mmovbe
• -mcrc32
• -mmwait
• -mrecip
• -mrecip=opt
• -mvzeroupper
• -mprefer-avx128
• -mprefer-vector-width=opt
• -mpartial-vector-fp-math
• -mmove-max=bits
• -mstore-max=bits
• -mnoreturn-no-callee-saved-registers
• -mmmx
• -msse
• -msse2
• -msse3
• -mssse3
• -msse4.1
• -msse4.2
• -msse4
• -mavx
• -mavx2
• -mavx512f
• -mavx512pf
• -mavx512er
• -mavx512cd
• -mavx512vl
• -mavx512bw
• -mavx512dq
• -mavx512ifma
• -mavx512vbmi
• -msha
• -maes
• -mpclmul
• -mfsgsbase
• -mrdrnd
• -mf16c
• -mfma
• -mpconfig
• -mwbnoinvd
• -mptwrite
• -mprefetchwt1
• -mclflushopt
• -mclwb
• -mxsavec
• -mxsaves
• -msse4a
• -m3dnow
• -m3dnowa
• -mpopcnt
• -mabm
• -mbmi
• -mtbm
• -mfma4
• -mxop
• -madx
• -mlzcnt
• -mbmi2
• -mfxsr
• -mxsave
• -mxsaveopt
• -mrtm
• -mhle
• -mlwp
• -mmwaitx
• -mclzero
• -mpku
• -mthreads
• -mgfni
• -mvaes
• -mwaitpkg
• -mshstk
• -mmanual-endbr
• -mcet-switch
• -mforce-indirect-call
• -mavx512vbmi2
• -mavx512bf16
• -menqcmd
• -mvpclmulqdq
• -mavx512bitalg
• -mmovdiri
• -mmovdir64b
• -mavx512vpopcntdq
• -mavx5124fmaps
• -mavx512vnni
• -mavx5124vnniw
• -mprfchw
• -mrdpid
• -mrdseed
• -msgx
• -mavx512vp2intersect
• -mserialize
• -mtsxldtrk
• -mamx-tile
• -mamx-int8
• -mamx-bf16
• -muintr
• -mhreset
• -mavxvnni
• -mavx512fp16
• -mavxifma
• -mavxvnniint8
• -mavxneconvert
• -mcmpccxadd
• -mamx-fp16
• -mprefetchi
• -mraoint
• -mamx-complex
• -mavxvnniint16
• -msm3
• -msha512
• -msm4
• -mapxf
• -musermsr
• -mavx10.1
• -mavx10.1-256
• -mavx10.1-512
• -mevex512
• -mcldemote
• -mms-bitfields
• -mno-align-stringops
• -minline-all-stringops
• -minline-stringops-dynamically
• -mstringop-strategy=alg
• -mkl
• -mwidekl
• -mmemcpy-strategy=strategy
• -mmemset-strategy=strategy
• -mpush-args
• -maccumulate-outgoing-args
• -m128bit-long-double
• -m96bit-long-double
• -mlong-double-64
• -mlong-double-80
• -mlong-double-128
• -mregparm=num
• -msseregparm
• -mveclibabi=type
• -mvect8-ret-in-mem
• -mpc32
• -mpc64
• -mpc80
• -mdaz-ftz
• -mstackrealign
• -momit-leaf-frame-pointer
• -mno-red-zone
• -mno-tls-direct-seg-refs
• -mcmodel=code-model
• -mabi=name
• -maddress-mode=mode
• -m32
• -m64
• -mx32
• -m16
• -miamcu
• -mlarge-data-threshold=num
• -msse2avx
• -mfentry
• -mrecord-mcount
• -mnop-mcount
• -m8bit-idiv
• -minstrument-return=type
• -mfentry-name=name
• -mfentry-section=name
• -mavx256-split-unaligned-load
• -mavx256-split-unaligned-store
• -malign-data=type
• -mstack-protector-guard=guard
• -mstack-protector-guard-reg=reg
• -mstack-protector-guard-offset=offset
• -mstack-protector-guard-symbol=symbol
• -mgeneral-regs-only
• -mcall-ms2sysv-xlogues
• -mrelax-cmpxchg-loop
• -mindirect-branch=choice
• -mfunction-return=choice
• -mindirect-branch-register
• -mharden-sls=choice
• -mindirect-branch-cs-prefix
• -mneeded
• -mno-direct-extern-access
• -munroll-only-small-loops
• -mlam=choice

---

✨ x86 Windows Options ✨

windows girlies, we see you 🫶

• -mconsole
• -mcrtdll=library
• -mdll
• -mnop-fun-dllimport
• -mthread
• -municode
• -mwin32
• -mwindows
• -fno-set-stack-executable

---

✨ Xstormy16 Options ✨

one (1) option. she's lowkey mysterious:

• -msim

---

✨ Xtensa Options ✨

• -mconst16
• -mno-const16
• -mfused-madd
• -mno-fused-madd
• -mforce-no-pic
• -mserialize-volatile
• -mno-serialize-volatile
• -mtext-section-literals
• -mno-text-section-literals
• -mauto-litpools
• -mno-auto-litpools
• -mtarget-align
• -mno-target-align
• -mlongcalls
• -mno-longcalls
• -mabi=abi-type
• -mextra-l32r-costs=cycles
• -mstrict-align
• -mno-strict-align

---

✨ zSeries Options ✨

literally just: See S/390 and zSeries Options. that's it, that's the whole section 💀

---

🎀 Options Controlling the Kind of Output 🎀

okay so compiling is a whole journey bestie, and it happens in up to four stages: preprocessing, compilation proper, assembly and linking — and it's always in that order, no skipping ahead 👑

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 freshly compiled and the ones you passed in as input — into an executable file. it's giving assembly line but make it compiler ✨

---

so for any given input file, the file name suffix is what decides which kind of compilation happens. here's the whole roster:

• file.c — C source code that must be preprocessed.
• file.i — C source code that should not be preprocessed.
• file.ii — C++ source code that should not be preprocessed.
• file.m — Objective-C source code. note that you must link with the libobjc library to make an Objective-C program work.
• file.mi — Objective-C source code that should not be preprocessed.
• file.mm, file.M — Objective-C++ source code. note that you must link with the libobjc library to make an Objective-C++ program work. also note that .M refers to a literal capital M 👀
• file.mii — Objective-C++ source code that should not be preprocessed.
• file.h — C, C++, Objective-C or Objective-C++ header file to be turned into a precompiled header (that's the default), or C, C++ header file to be turned into an Ada spec (via the -fdump-ada-spec switch).
• file.cc, file.cp, file.cxx, file.cpp, file.CPP, file.c++, file.C — C++ source code that must be preprocessed. heads up: in .cxx, the last two letters must both be literally x. likewise, .C refers to a literal capital C.
• file.mm, file.M — Objective-C++ source code that must be preprocessed.
• file.mii — Objective-C++ source code that should not be preprocessed.
• file.hh, file.H, file.hp, file.hxx, file.hpp, file.HPP, file.h++, file.tcc — C++ header file to be turned into a precompiled header or Ada spec.
• file.f, file.for, file.ftn, file.fi — fixed form Fortran source code that should not be preprocessed.
• file.F, file.FOR, file.fpp, file.FPP, file.FTN — fixed form Fortran source code that must be preprocessed (with the traditional preprocessor).
• file.f90, file.f95, file.f03, file.f08, file.fii — free form Fortran source code that should not be preprocessed.
• file.F90, file.F95, file.F03, file.F08 — free form Fortran source code that must be preprocessed (with the traditional preprocessor).
• file.go — Go source code.
• file.d — D source code.
• file.di — D interface file.
• file.dd — D documentation code (Ddoc).
• file.ads — Ada source code file that contains a library unit declaration (a declaration of a package, subprogram, or generic, or a generic instantiation), or a library unit renaming declaration (a package, generic, or subprogram renaming declaration). these files are also called specs.
• file.adb — Ada source code file containing a library unit body (a subprogram or package body). these files are also called bodies.
• file.s — assembler code.
• file.S, file.sx — assembler code that must be preprocessed.
• other — an object file to be fed straight into linking. any file name with no recognized suffix is treated this way. no suffix? no problem, straight to the linker with you 💅

---

and if the suffix guessing isn't giving what you need, you can specify the input language explicitly with the -x option:

• -x language — bestie, this is you saying EXPLICITLY which language the next input files are, instead of letting the compiler peek at the file name suffix and guess. It applies to all following input files until the next -x shows up and takes over. Possible values for language are:

        c  c-header  cpp-output
        c++  c++-header  c++-system-header c++-user-header c++-cpp-output
        objective-c  objective-c-header  objective-c-cpp-output
        objective-c++ objective-c++-header objective-c++-cpp-output
        assembler  assembler-with-cpp
        ada
        d
        f77  f77-cpp-input f95  f95-cpp-input
        go

• -x none — turns off any language specification, so the files after it go back to being handled by their file name suffixes, exactly like nothing ever happened and -x was never 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. Heads up though: some combinations (for example, -x cpp-output -E) literally instruct gcc to do nothing at all. 💀

• -c — compile or assemble the source files, but do not link. The linking stage simply is not done, periodt. The ultimate output is in the form of an object file for each source file. ✨

By default, the object file name for a source file is made by replacing the suffix .c, .i, .s, etc., with .o.

Unrecognized input files, not requiring compilation or assembly, are ignored.

• -S — stop after the stage of compilation proper; do not assemble. The output is in the form of an assembler code file for each non-assembler input file specified.

By default, the assembler file name for a source file is made by replacing the suffix .c, .i, etc., with .s.

Input files that don't require compilation are ignored.

• -E — stop after the preprocessing stage; do not run the compiler proper. The output is in the form of preprocessed source code, which is sent to the standard output. 👑

Input files that don't require preprocessing are ignored.

• -o file — put the primary output in 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. It's giving main character energy for your output. 🎀

If -o is not specified, the default is to put an executable file in a.out, the object file for source.suffix in source.o, its assembler file in source.s, a precompiled header file in source.suffix.gch, and all preprocessed C source on standard output.

Though -o names only the primary output, it also affects the naming of auxiliary and dump outputs. See the examples below. Unless overridden, both auxiliary outputs and dump outputs are placed in the same directory as the primary output. In auxiliary outputs, the suffix of the input file is replaced with that of the auxiliary output file type; in dump outputs, the suffix of the dump file is appended to the input file suffix. In compilation commands, the base name of both auxiliary and dump outputs is that of the primary output; in compile and link commands, the primary output name, minus the executable suffix, is combined with the input file name. If both share the same base name, disregarding the suffix, the result of the combination is that base name, otherwise, they are concatenated, separated by a dash.

        gcc -c foo.c ...

will use foo.o as the primary output, and place aux outputs and dumps next to it, e.g., aux file foo.dwo for -gsplit-dwarf, and dump file foo.c.???r.final for -fdump-rtl-final.

If a non-linker output file is explicitly specified, aux and dump files by default take the same base name:

        gcc -c foo.c -o dir/foobar.o ...

will name aux outputs dir/foobar.* and dump outputs dir/foobar.c.*.

A linker output will instead prefix aux and dump outputs:

        gcc foo.c bar.c -o dir/foobar ...

will generally name aux outputs dir/foobar-foo.* and dir/foobar-bar.*, and dump outputs dir/foobar-foo.c.* and dir/foobar-bar.c.*.

The one exception to the above is when the executable shares the base name with the single input:

        gcc foo.c -o dir/foo ...

in which case aux outputs are named dir/foo.* and dump outputs named dir/foo.c.*.

The location and the names of auxiliary and dump outputs can be adjusted by the options -dumpbase, -dumpbase-ext, -dumpdir, -save-temps=cwd, and -save-temps=obj.

• -dumpbase dumpbase — this one sets the base name for auxiliary and dump output files. It does not affect the name of the primary output file. Intermediate outputs, when preserved, are not regarded as primary outputs, but as auxiliary outputs: 🌸

        gcc -save-temps -S foo.c

saves the (no longer) temporary preprocessed file in foo.i, and then compiles to the (implied) output file foo.s, whereas:

        gcc -save-temps -dumpbase save-foo -c foo.c

preprocesses to in save-foo.i, compiles to save-foo.s (now an intermediate, thus auxiliary output), and then assembles to the (implied) output file foo.o.

Absent this option, dump and aux files take their names from the input file, or from the (non-linker) output file, if one is explicitly specified: dump output files (e.g. those requested by -fdump-* options) with the input name suffix, and aux output files (those requested by other non-dump options, e.g. "-save-temps", "-gsplit-dwarf", "-fcallgraph-info") without it.

Similar suffix differentiation of dump and aux outputs can be attained for explicitly-given -dumpbase basename.suf by also specifying -dumpbase-ext .suf.

If dumpbase is explicitly specified with any directory component, any dumppfx specification (e.g. -dumpdir or -save-temps=*) is ignored, and instead of appending to it, dumpbase fully overrides it:

        gcc foo.c -c -o dir/foo.o -dumpbase alt/foo \
          -dumpdir pfx- -save-temps=cwd ...

creates auxiliary and dump outputs named alt/foo.*, disregarding dir/ in -o, the ./ prefix implied by -save-temps=cwd, and pfx- in -dumpdir.

When -dumpbase is specified in a command that compiles multiple inputs, or that compiles and then links, it may be combined with dumppfx, as specified under -dumpdir. Then, each input file is compiled using the combined dumppfx, and default values for dumpbase and auxdropsuf are computed for each input file:

        gcc foo.c bar.c -c -dumpbase main ...

creates foo.o and bar.o as primary outputs, and avoids overwriting the auxiliary and dump outputs by using the dumpbase as a prefix, creating auxiliary and dump outputs named main-foo.* and main-bar.*.

An empty string specified as dumpbase avoids the influence of the output basename in the naming of auxiliary and dump outputs during compilation, computing default values :

        gcc -c foo.c -o dir/foobar.o -dumpbase " ...

will name aux outputs dir/foo.* and dump outputs dir/foo.c.*. Note how their basenames are taken from the input name, but the directory still defaults to that of the output.

The empty-string dumpbase does not prevent the use of the output basename for outputs during linking:

        gcc foo.c bar.c -o dir/foobar -dumpbase " -flto ...

The compilation of the source files will name auxiliary outputs dir/foo.* and dir/bar.*, and dump outputs dir/foo.c.* and dir/bar.c.*. LTO recompilation during linking will use dir/foobar. as the prefix for dumps and auxiliary files.

• -dumpbase-ext auxdropsuf — when forming the name of an auxiliary (but not a dump) output file, drop trailing auxdropsuf from dumpbase before appending any suffixes. If not specified, this option defaults to the suffix of a default dumpbase, i.e., the suffix of the input file when -dumpbase is not present in the command line, or dumpbase is combined with dumppfx. 💜

        gcc foo.c -c -o dir/foo.o -dumpbase x-foo.c -dumpbase-ext .c ...

creates dir/foo.o as the main output, and generates auxiliary outputs in dir/x-foo.*, taking the location of the primary output, and dropping the .c suffix from the dumpbase. Dump outputs retain the suffix: dir/x-foo.c.*.

This option is disregarded if it does not match the suffix of a specified dumpbase, except as an alternative to the executable suffix when appending the linker output base name to dumppfx, as specified below:

        gcc foo.c bar.c -o main.out -dumpbase-ext .out ...

creates main.out as the primary output, and avoids overwriting the auxiliary and dump outputs by using the executable name minus auxdropsuf as a prefix, creating auxiliary outputs named main-foo.* and main-bar.* and dump outputs named main-foo.c.* and main-bar.c.*.

• -dumpdir dumppfx — when forming the name of an auxiliary or dump output file, use dumppfx as a prefix: 🌈

        gcc -dumpdir pfx- -c foo.c ...

creates foo.o as the primary output, and auxiliary outputs named pfx-foo.*, combining the given dumppfx with the default dumpbase derived from the default primary output, derived in turn from the input name. Dump outputs also take the input name suffix: pfx-foo.c.*.

If dumppfx is to be used as a directory name, it must end with a directory separator:

        gcc -dumpdir dir/ -c foo.c -o obj/bar.o ...

creates obj/bar.o as the primary output, and auxiliary outputs named dir/bar.*, combining the given dumppfx with the default dumpbase derived from the primary output name. Dump outputs also take the input name suffix: dir/bar.c.*.

It defaults to the location of the output file, unless the output file is a special file like "/dev/null". Options -save-temps=cwd and -save-temps=obj override this default, just like an explicit -dumpdir option. In case multiple such options are given, the last one prevails:

        gcc -dumpdir pfx- -c foo.c -save-temps=obj ...

outputs foo.o, with auxiliary outputs named foo.* because -save-temps=* overrides the dumppfx given by the earlier -dumpdir option. It does not matter that =obj is the default for -save-temps, nor that the output directory is implicitly the current directory. Dump outputs are named foo.c.*.

okay so when you're compiling from multiple input files and -dumpbase is specified, here's the whole vibe: dumpbase, minus an auxdropsuf suffix, plus a dash gets appended to an explicit or defaulted dumppfx — or it just straight-up overrides it, if it contains any directory components. The point? Each of those multiple compilations gets its own differently-named aux and dump outputs. Bestie, it's giving ✨organized✨.

gcc foo.c bar.c -c -dumpdir dir/pfx- -dumpbase main ...

That outputs auxiliary dumps to dir/pfx-main-foo.* and dir/pfx-main-bar.*, appending dumpbase- to dumppfx. And the dump outputs retain the input file suffix: dir/pfx-main-foo.c.* and dir/pfx-main-bar.c.*, respectively. 🎀

Now compare that to single-input compilation:

gcc foo.c -c -dumpdir dir/pfx- -dumpbase main ...

This one, applying -dumpbase to a single source, does not compute and append a separate dumpbase per input file. Its auxiliary and dump outputs just go in dir/pfx-main.*. One input, one base, no extra chaos. 🦋

---

When you're compiling and then linking from multiple input files, a defaulted or explicitly specified dumppfx also goes through that same dumpbase- transformation (think: compiling foo.c and bar.c like above, but without the -c). If neither -dumpdir nor -dumpbase are given, then the linker output base name — minus auxdropsuf, if specified, or the executable suffix otherwise, plus a dash — gets appended to the default dumppfx instead.

But heads up! 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 here is that the explicitly-specified dumppfx may contain the specified output name as part of the prefix, if that's what you want. Only an explicitly-specified -dumpbase would be combined with it, so we don't just go around discarding a meaningful option. 💅

---

When you're compiling and then linking from a single input file, the linker output base name only gets appended to the default dumppfx like above if it does not share the base name with the single input file name. This has already been covered in the single-input linking cases above — but not with an explicit -dumpdir that inhibits the combination, even if it gets 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 are foo.c.*, in the current working directory as ultimately requested by -save-temps=cwd. 🌸

---

Summing it all up for an intuitive (though slightly imprecise, ngl) data flow: the primary output name gets broken into a directory part and a basename part; dumppfx is set to the former, unless it's overridden by -dumpdir or -save-temps=*, and dumpbase is set to the latter, unless it's overridden by -dumpbase. If there are multiple inputs or linking happening, 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 where 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. Since it's involved in linking, this program doesn't normally get any -dumpbase and -dumpbase-ext, and it ignores them. 🙈

When running sub-compilers, lto-wrapper appends LTO stage names to the received dumppfx, ensures it contains a directory component so that it overrides any -dumpdir, and passes that as -dumpbase to sub-compilers.

---

• -v — Print (on standard error output) the commands executed to run the stages of compilation. Also prints the version number of the compiler driver program, and of the preprocessor, and of the compiler proper. 👑
• -### — Like -v, except the commands are not executed and arguments are quoted unless they contain only alphanumeric characters or "./-_". This is so useful for shell scripts to capture the driver-generated command lines. 🔥
• --help — Print (on the standard output) a description of the command-line options understood by gcc. If the -v option is also specified, then --help is also passed on to the various processes invoked by gcc, so they can display the command-line options they accept. And if -Wextra has also been specified (prior to the --help option), then command-line options that have no documentation associated with them are also displayed. ✨
• --target-help — Print (on the standard output) a description of target-specific command-line options for each tool. For some targets, extra target-specific information may also be printed.
• --help={class|[^]qualifier}[,...] — Print (on the standard output) a description of the command-line options understood by the compiler that fit into all specified classes and qualifiers. 💖

These are the supported classes:

• optimizers — Display all of the optimization options supported by the compiler.
• warnings — Display all of the options controlling warning messages produced by the compiler.
• target — Display target-specific options. Unlike the --target-help option, however, target-specific options of the linker and assembler are not displayed. This is because those tools do not currently support the extended --help= syntax.
• params — Display the values recognized by the --param option.
• language — Display the options supported for language, where language is the name of one of the languages supported in this version of GCC. If an option is supported by all languages, you need to select the common class.
• common — Display the options that are common to all languages.

And these are the supported qualifiers:

• undocumented — Display only those options that are undocumented.
• joined — Display options taking an argument that appears after an equal sign in the same continuous piece of text, such as: --help=target.
• separate — Display options taking an argument that appears as a separate word following the original option, such as: -o output-file.

So, for example, to display all the undocumented target-specific switches supported by the compiler, use:

--help=target,undocumented

The sense of a qualifier can be inverted by prefixing it with the ^ character. So for example, to display all binary warning options (i.e., ones that are either on or off and that do not take an argument) that have a description, use:

--help=warnings,^joined,^undocumented

The argument to --help= should not consist solely of inverted qualifiers. 🚫

Combining several classes is possible, although this usually restricts the output so much that there is nothing to display. One case where it does work, though, is when one of the classes is target. For example, to display all the target-specific optimization options, use:

--help=target,optimizers

The --help= option can be repeated on the command line. Each successive use displays its requested class of options, skipping those that have already been displayed. If --help is also specified anywhere on the command line, then this takes precedence over any --help= option.

If the -Q option appears on the command line before the --help= option, then the descriptive text displayed by --help= is changed. Instead of describing the displayed options, an indication is given as to whether the option is enabled, disabled, or set to a specific value (assuming that the compiler knows this at the point where the --help= option is used). 🌈

Here is a truncated example from the ARM port of gcc:

% gcc -Q -mabi=2 --help=target -c
The following options are target specific:
-mabi=                                2
-mabort-on-noreturn                   [disabled]
-mapcs                                [disabled]

The output is sensitive to the effects of previous command-line options, so for example it's possible to find out which optimizations are enabled at -O2 by using:

-Q -O2 --help=optimizers

Alternatively, you can discover which binary optimizations are enabled by -O3 by using:

gcc -c -Q -O3 --help=optimizers > /tmp/O3-opts
gcc -c -Q -O2 --help=optimizers > /tmp/O2-opts
diff /tmp/O2-opts /tmp/O3-opts | grep enabled

• --version — Display the version number and copyrights of the invoked GCC. 🌟
• -pass-exit-codes — Normally the gcc program exits with the code of 1 if any phase of the compiler returns a non-success return code. If you specify -pass-exit-codes, the gcc program instead returns with the numerically highest error produced by any phase returning an error indication. The C, C++, and Fortran front ends return 4 if an internal compiler error is encountered. 💀
• -pipe — Use pipes rather than temporary files for communication between the various stages of compilation. This fails to work on some systems where the assembler is unable to read from a pipe — but the GNU assembler has no trouble. 🧚
• -specs=file — Process file after the compiler reads in the standard specs file, in order to override the defaults which the gcc driver program uses when determining what switches to pass to cc1, cc1plus, as, ld, etc. More than one -specs=file can be specified on the command line, and they are processed in order, from left to right.
• -wrapper — Invoke all subcommands under a wrapper program. The name of the wrapper program and its parameters are passed as a comma separated list.

gcc -c t.c -wrapper gdb,--args

This invokes all subprograms of gcc under gdb --args, thus the invocation of cc1 is gdb --args cc1 .... 🔥

• -ffile-prefix-map=old=new — bestie, when you're compiling files that live in directory old, GCC writes every reference to them in the compiled output as if those files were chillin' in directory new instead. It's literally the same as turning on all the individual -f*-prefix-map options at once. What's it for? Reproducible builds that don't care where you built them 💅. Heads up though: directories that get referenced by directives are not touched by these options. Peep -fmacro-prefix-map, -fdebug-prefix-map, -fprofile-prefix-map and -fcanon-prefix-map too 🫶.
• -fcanon-prefix-map — okay so for the -f*-prefix-map options, the comparison of the old prefix against the filename that would normally be referenced in the compilation result is done by textual comparison of the prefixes — or, on case-insensitive filesystems, ignoring character case, and on DOS based filesystems, treating slashes and backslashes as equal. This option makes those comparisons happen on the canonicalized paths of old and the referenced filename instead. ✨
• -fplugin=name.so — loads the plugin code sitting in file name.so, which is assumed to be a shared object that the compiler will dlopen. The base name of the shared object file is what identifies the plugin for argument parsing (see -fplugin-arg-name-key=value right below 👇). Every plugin should define the callback functions laid out in the Plugins API.
• -fplugin-arg-name-key=value — defines an argument called key with a value of value for the plugin called name. Simple as 🍓.
• -fdump-ada-spec[-slim] — for C and C++ source and include files, this generates the corresponding Ada specs. Iconic crossover episode tbh.
• -fada-spec-parent=unit — pairs with -fdump-ada-spec[-slim] above to generate Ada specs as child units of parent unit. 🎀
• -fdump-go-spec=file — for input files in any language, generate the corresponding Go declarations into file. It spits out Go const, type, var, and func declarations, which can be a genuinely useful way to start writing a Go interface to code written in some other language. Main character energy fr fr. 👑
• @file — reads command-line options from file. The options it reads get inserted right in place of the original @file option. And if file doesn't exist, or can't be read? The option is treated literally, and not removed. No cap.

Options inside file are separated by whitespace. Wanna include a whitespace character inside an option? Wrap the whole option in either single or double quotes. Wanna include any character at all (backslash included)? Just prefix the character you want with a backslash. And plot twist: the file itself can contain even more @file options — those get processed recursively. 🦋

---

🌸 Compiling C++ Programs 🌸

Okay so C++ source files conventionally use one of the suffixes .C, .cc, .cpp, .CPP, .c++, .cp, or .cxx. C++ header files often use .hh, .hpp, .H, or (for shared template code) .tcc. And preprocessed C++ files use the suffix .ii. GCC recognizes files with these names and compiles them as C++ programs even if you call the compiler the same way you'd call it for compiling C programs — which is usually with the name gcc. Sneaky, right?

But here's the tea: using gcc does not add the C++ library. g++ is the 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. It's also super handy when you're precompiling a C header file with a .h extension for use in C++ compilations. On many systems, g++ is also installed under the name c++. 💖

When you compile C++ programs, you may specify a lot of the same command-line options you'd use for compiling programs in any language; or command-line options that are meaningful for C and related languages; or options that are meaningful only for C++ programs. Pick your fighter. ✨

---

✨ Options Controlling C Dialect ✨

The following options control the dialect of C (or languages derived from C, such as C++, Objective-C and Objective-C++) that the compiler accepts:

• -ansi — in C mode, this is equivalent to -std=c90. In C++ mode, it's equivalent to -std=c++98.

This turns off certain features of GCC that are incompatible with ISO C90 (when compiling C code), or of standard C++ (when compiling C++ code) — think the asm and typeof keywords, and predefined macros such as unix and vax that identify the type of system you're on. It also enables the undesirable and rarely used ISO trigraph feature 🙈. For the C compiler, it disables recognition of C++ style // comments as well as the inline keyword.

The alternate keywords __asm__, __extension__, __inline__ and __typeof__ continue to work despite -ansi. You wouldn't want to use them in an ISO C program, of course, but it's useful to put them in header files that might get included in compilations done with -ansi. Alternate predefined macros such as __unix__ and __vax__ are also available, with or without -ansi. 🌈

Lowkey important: -ansi does not cause non-ISO programs to be rejected gratuitously. For that, -Wpedantic is required in addition to -ansi.

The macro __STRICT_ANSI__ is predefined when the -ansi option is used. Some header files may notice this macro and refrain from declaring certain functions or defining certain macros that the ISO standard doesn't call for; this is to avoid interfering with any programs that might use these names for other things.

Functions that are normally built in but do not have semantics defined by ISO C (such as alloca and ffs) are not built-in functions when -ansi is used. 💀

• -std= — determines the language standard. This option is currently only supported when compiling C or C++. 🧚

The compiler can accept several base standards, such as c90 or c++98, and GNU dialects of those standards, such as gnu90 or gnu++98. When a base standard is specified, the compiler accepts all programs following that standard plus those using GNU extensions that do not contradict it. For example, -std=c90 turns off certain features of GCC that are incompatible with ISO C90, such as the asm and typeof keywords, but not other GNU extensions that don't have a meaning in ISO C90, such as omitting the middle term of a ?: expression. On the other hand, when a GNU dialect of a standard is specified, all features supported by the compiler are enabled, even when those features change the meaning of the base standard. As a result, some strict-conforming programs may be rejected. 😭 The particular standard is used by -Wpedantic to identify which features are GNU extensions given that version of the standard. For example -std=gnu90 -Wpedantic warns about C++ style // comments, while -std=gnu99 -Wpedantic does not.

A value for this option must be provided; possible values are:

• c90, c89, iso9899:1990 — support all ISO C90 programs (certain GNU extensions that conflict with ISO C90 are disabled). Same as -ansi for C code.
• iso9899:199409 — ISO C90 as modified in amendment 1.
• c99, c9x, iso9899:1999, iso9899:199x — ISO C99. This standard is substantially completely supported, modulo bugs and floating-point issues (mainly but not entirely relating to optional C99 features from Annexes F and G). See <https://gcc.gnu.org/c99status.html> for more information. The names c9x and iso9899:199x are deprecated.
• c11, c1x, iso9899:2011 — ISO C11, the 2011 revision of the ISO C standard. This standard is substantially completely supported, modulo bugs, floating-point issues (mainly but not entirely relating to optional C11 features from Annexes F and G) and the optional Annexes K (Bounds-checking interfaces) and L (Analyzability). The name c1x is deprecated.
• c17, c18, iso9899:2017, iso9899:2018 — ISO C17, the 2017 revision of the ISO C standard (published in 2018). This standard is same as C11 except for corrections of defects (all of which are also applied with -std=c11) and a new value of __STDC_VERSION__, and so is supported to the same extent as C11.
• c23, c2x, iso9899:2024 — ISO C23, the 2023 revision of the ISO C standard (expected to be published in 2024). The support for this version is experimental and incomplete. The name c2x is deprecated.
• gnu90, gnu89 — GNU dialect of ISO C90 (including some C99 features).
• gnu99, gnu9x — GNU dialect of ISO C99. The name gnu9x is deprecated.
• gnu11, gnu1x — GNU dialect of ISO C11. The name gnu1x is deprecated.
• gnu17, gnu18 — GNU dialect of ISO C17. This is the default for C code.
• gnu23, gnu2x — the next version of the ISO C standard, still under development, plus GNU extensions. The support for this version is experimental and incomplete. The name gnu2x is deprecated.
• c++98, c++03 — the 1998 ISO C++ standard plus the 2003 technical corrigendum and some additional defect reports. Same as -ansi for C++ code.
• gnu++98, gnu++03 — GNU dialect of -std=c++98.
• c++11, c++0x — the 2011 ISO C++ standard plus amendments. The name c++0x is deprecated.
• gnu++11, gnu++0x — GNU dialect of -std=c++11. The name gnu++0x is deprecated.
• c++14, c++1y — the 2014 ISO C++ standard plus amendments. The name c++1y is deprecated.
• gnu++14, gnu++1y — GNU dialect of -std=c++14. The name gnu++1y is deprecated.
• c++17, c++1z — the 2017 ISO C++ standard plus amendments. The name c++1z is deprecated.
• gnu++17, gnu++1z — GNU dialect of -std=c++17. This is the default for C++ code. The name gnu++1z is deprecated.
• c++20, c++2a — the 2020 ISO C++ standard plus amendments. Support is experimental, and could change in incompatible ways in future releases. The name c++2a is deprecated.
• gnu++20, gnu++2a — GNU dialect of -std=c++20. Support is experimental, and could change in incompatible ways in future releases. The name gnu++2a is deprecated.

• c++2b, c++23 — the next revision of the ISO C++ standard, planned for 2023. Support is highly experimental, and it will almost certainly change in incompatible ways in future releases. bestie do not build your whole career on this one 😭
• gnu++2b, gnu++23 — the GNU dialect of -std=c++2b. Support is highly experimental, and it will almost certainly change in incompatible ways in future releases.
• c++2c, c++26 — the next revision of the ISO C++ standard, planned for 2026. Support is highly experimental, and it will almost certainly change in incompatible ways in future releases.
• gnu++2c, gnu++26 — the GNU dialect of -std=c++2c. Support is highly experimental, and it will almost certainly change in incompatible ways in future releases. 💀

---

• -aux-info filename — drops a file at filename containing prototyped declarations for every function declared and/or defined in a translation unit, header files included, no cap. This option is silently ignored in any language other than C.

Beyond the declarations themselves, the file spills the tea in comments: where each declaration came from (source file and line), whether it was implicit, prototyped or unprototyped — that's I, N for new, or O for old, sitting in the first character after the line number and the colon — and whether it came from a declaration or a definition (that's C or F respectively, in the character right after). And for function definitions you also get a K&R-style list of arguments followed by their declarations, tucked inside comments, after the declaration. so thorough, so extra, we love it 🎀

• -fno-asm — do not recognize asm, inline or typeof as keywords, so your code can use these words as identifiers. You can reach for the keywords __asm__, __inline__ and __typeof__ instead. In C, -ansi implies -fno-asm.

In C++, inline is a standard keyword and is not affected by this switch. You might want -fno-gnu-keywords 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. the glow-up is real ✨

• -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. Also, when a function is recognized as a built-in function, GCC may use information about that function to warn about problems with calls to that function, or to generate more efficient code, even if the resulting code still contains calls to that function. For example, warnings are given with -Wformat for bad calls to printf when printf is built in and strlen is known not to modify global memory. 👑

With the -fno-builtin-function option only the built-in function function is disabled. function must not begin with __builtin_. If a function is named that is not built-in in this version of GCC, this option is ignored. There is no corresponding -fbuiltin-function option; if you wish to enable built-in functions selectively when using -fno-builtin or -ffreestanding, you may define macros such as:

#define abs(n)          __builtin_abs ((n))
#define strcpy(d, s)    __builtin_strcpy ((d), (s))

• -fcond-mismatch — allow conditional expressions with mismatched types in the second and third arguments. The value of such an expression is void. This option is not supported for C++.
• -ffreestanding — assert that compilation targets a freestanding environment. This implies -fno-builtin. A freestanding environment is one in which the standard library may not exist, and program startup may not necessarily be at main. The most obvious example is an OS kernel. This is equivalent to -fno-hosted. living that kernel life fr fr 🧚
• -fgimple — enable parsing of function definitions marked with __GIMPLE. This is an experimental feature that allows unit testing of GIMPLE passes.
• -fgnu-tm — when the option -fgnu-tm is specified, the compiler generates code for the Linux variant of Intel's current Transactional Memory ABI specification document (Revision 1.1, May 6 2009). This is an experimental feature whose interface may change in future versions of GCC, as the official specification changes. Please note that not all architectures are supported for this feature.

For more information on GCC's support for transactional memory,

Note that the transactional memory feature is not supported with non-call exceptions (-fnon-call-exceptions). 💀

• -fgnu89-inline — 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. cute lil vibe check 🫶

• -fhosted — assert that compilation targets a hosted environment. This implies -fbuiltin. A hosted environment is one in which the entire standard library is available, and in which main has a return type of int. Examples are nearly everything except a kernel. This is equivalent to -fno-freestanding.
• -flax-vector-conversions — allow implicit conversions between vectors with differing numbers of elements and/or incompatible element types. This option should not be used for new code. it's giving legacy, bestie, leave it in the past 🙈
• -fms-extensions — accept some non-standard constructs used in Microsoft header files.

In C++ code, this allows member names in structures to be similar to previous types declarations.

typedef int UOW;
struct ABC {
  UOW UOW;
};

Some cases of unnamed fields in structures and unions are only accepted with this option.

Note that this option is off for all targets except for x86 targets using ms-abi.

• -foffload=disable, -foffload=default, -foffload= target-list — specify for which OpenMP and OpenACC offload targets code should be generated. The default behavior, equivalent to -foffload=default, is to generate code for all supported offload targets. The -foffload=disable form generates code only for the host fallback, while -foffload=target-list generates code only for the specified comma-separated list of offload targets.

Offload targets are specified in GCC's internal target-triplet format. You can run the compiler with -v to show the list of configured offload targets under OFFLOAD_TARGET_NAMES.

• -foffload-options= options, -foffload-options= target-triplet-list=options — with -foffload-options=options, GCC passes the specified options to the compilers for all enabled offloading targets. You can specify options that apply only to a specific target or targets by using the -foffload-options=target-list=options form. The target-list is a comma-separated list in the same format as for the -foffload= option.

Typical command lines are

-foffload-options='-fno-math-errno -ffinite-math-only' -foffload-options=nvptx-none=-latomic
-foffload-options=amdgcn-amdhsa=-march=gfx906

• -fopenacc — enable handling of OpenACC directives #pragma acc in C/C++ and !$acc in free-form Fortran and !$acc, c$acc and *$acc in fixed-form Fortran. When -fopenacc is specified, the compiler generates accelerated code according to the OpenACC Application Programming Interface v2.6 <https://www.openacc.org>. This option implies -pthread, and thus is only supported on targets that have support for -pthread. 🌈
• -fopenacc-dim= geom — specify default compute dimensions for parallel offload regions that do not explicitly specify. The geom value is a triple of :-separated sizes, in order gang, worker and, vector. A size can be omitted, to use a target-specific default value.
• -fopenmp — enable handling of OpenMP directives #pragma omp, [[omp::directive(...)]], [[omp::sequence(...)]] and [[omp::decl(...)]] in C/C++ and !$omp in Fortran. It additionally enables the conditional compilation sentinel !$ in Fortran. In fixed source form Fortran, the sentinels can also start with c or *. When -fopenmp is specified, the compiler generates parallel code according to the OpenMP Application Program Interface v4.5 <https://www.openmp.org>. This option implies -pthread, and thus is only supported on targets that have support for -pthread. -fopenmp implies -fopenmp-simd. parallel code era, we're so back 🔥
• -fopenmp-simd — enable handling of OpenMP's simd, declare simd, declare reduction, assume, ordered, scan and loop directive, and of combined or composite directives with simd as constituent with #pragma omp, [[omp::directive(...)]], [[omp::sequence(...)]] and [[omp::decl(...)]] in C/C++ and !$omp in Fortran. It additionally enables the conditional compilation sentinel !$ in Fortran. In fixed source form Fortran, the sentinels can also start with c or *. Other OpenMP directives are ignored. Unless -fopenmp is additionally specified, the loop region binds to the current task region, independent of the specified bind clause.
• -fopenmp-target-simd-clone and -fopenmp-target-simd-clone=device-type — okay so bestie, this one is about clones. Normally GCC makes SIMD clones for functions marked with the declare simd directive, right? Well, with this option turned on, GCC also makes clones for functions marked with the OpenMP declare target directive that are actually good for vectorization. 🦋 The device-type can be any one of none, host, nohost, or any — and those line up with the keywords for the device_type clause of the declare target directive, so clones get generated for the intersection of the devices you specified. -fopenmp-target-simd-clone is literally the same thing as -fopenmp-target-simd-clone=any, and -fno-openmp-target-simd-clone is the same as -fopenmp-target-simd-clone=none.

Heads up: at -O2 and higher (but not at -Os or -Og), this optimization defaults to -fopenmp-target-simd-clone=nohost; otherwise it's off by default. 💅

• -fpermitted-flt-eval-methods=style — so ISO/IEC TS 18661-3 defined some new permissible values for FLT_EVAL_METHOD that say operations and constants with a semantic type that's an interchange or extended format should be evaluated to the precision and range of that type. ✨ These new values are a superset of the ones allowed under C99/C11, which honestly doesn't even specify what other positive FLT_EVAL_METHOD values mean. So like… code that conforms to C11 might not have been written expecting these new values to show up at all. 😭

-fpermitted-flt-eval-methods tells the compiler whether it should allow only the FLT_EVAL_METHOD values from C99/C11, or the extended set from ISO/IEC TS 18661-3.

style is either c11 or ts-18661-3, whichever fits.

Default when you're in a standards compliant mode (-std=c11 or similar) is -fpermitted-flt-eval-methods=c11. Default when you're in a GNU dialect (-std=gnu11 or similar) is -fpermitted-flt-eval-methods=ts-18661-3. 🎀

The -fdeps-* options are all about extracting structured dependency information for a source. Basically: figuring out which resources from other source files you'll need to compile the source, and which resources the source itself provides. 🌈 This info can be used to add required dependencies between compilation rules of dependent sources based on their contents, instead of making the build tools carry all that info themselves. iykyk.

• -fdeps-file=file — where to write the structured dependency information. 📝
• -fdeps-format=format — the format to use for that structured dependency info. p1689r5 is the only supported format right now, no cap. ⭐ Heads up though: when you specify this argument, the output of -MF gets stripped of some information (namely C++ modules) so it doesn't use extended makefile syntax that most tools just… don't understand. 🙈
• -fdeps-target=file — this is like -MT but for structured dependency information. It says which target will ultimately need any required resources, and provide any resources extracted from the source that other sources might need. 💅
• -fplan9-extensions — accept some non-standard constructs used in Plan 9 code. 🍓 This enables -fms-extensions, lets you pass pointers to structures with anonymous fields to functions expecting pointers to elements of the type of the field, and lets you refer to anonymous fields declared using a typedef. Only supported for C, not C++.
• -fsigned-bitfields, -funsigned-bitfields, -fno-signed-bitfields, -fno-unsigned-bitfields — these four control whether a bit-field is signed or unsigned when the declaration doesn't say signed or unsigned. By default such a bit-field is signed, because consistency is cute: the basic integer types like int are signed types. ✨
• -fsigned-char — let the type char be signed, just like signed char. 💜 Note that this is equivalent to -fno-unsigned-char, which is the negative form of -funsigned-char. Likewise, -fno-signed-char is equivalent to -funsigned-char. iconic.
• -funsigned-char — let the type char be unsigned, just like unsigned char. 🌸

Every kind of machine has its own default for what char should be — either like unsigned char by default, or like signed char by default.

Ideally a portable program should always use signed char or unsigned char when it depends on the signedness of an object. But bestie, so many programs 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. 😭 This option and its inverse let you make such a program work with the opposite default.

And the type char is always a distinct type from both signed char and unsigned char, even though its behavior is always just like one of those two. no cap.

• -fstrict-flex-arrays (C and C++ only) and -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 that array. The value of level controls the level of strictness. 🔥

-fstrict-flex-arrays is equivalent to -fstrict-flex-arrays=3, which is the strictest: all trailing arrays of structures get treated as flexible array members.

The negative form -fno-strict-flex-arrays is equivalent to -fstrict-flex-arrays=0, the least strict. In that case a trailing array is treated as a flexible array member only when it's declared as a flexible array member per C99 standard onwards.

The possible values of level are the same as for the strict_flex_array attribute.

You can control this behavior for one specific trailing array field of a structure by using the variable attribute strict_flex_array attribute. 🎀

The -fstrict_flex_arrays option interacts with the -Wstrict-flex-arrays option. 💫

• -fsso-struct=endianness — set the default scalar storage order of structures and unions to the specified endianness. The accepted values are big-endian, little-endian, and native for the native endianness of the target (that one's the default). This option is not supported for C++.

Warning: the -fsso-struct switch makes GCC generate code that is not binary compatible with code generated without it, if the endianness you specified isn't the native endianness of the target. 💀

---

✨ Options Controlling C++ Dialect ✨

This section is all about the command-line options that only really mean something for C++ programs. 💖 You can still use most of the GNU compiler options no matter what language your program's in. Like, say you wanna compile a file called firstClass.C — you'd do:

g++ -g -fstrict-enums -O -c firstClass.C

In this little example, only -fstrict-enums is an option meant just for C++ programs; the other options work with any language GCC supports. 🌈

Some options for compiling C programs, like -std, are also relevant for C++ programs. 🦋

Okay, here's the list of options that are only for compiling C++ programs:

• -fabi-version=n — use version n of the C++ ABI. Default is version 0. 👑

Version 0 means the version conforming most closely to the C++ ABI specification. Which means the ABI you get with version 0 will change in different versions of G++ as ABI bugs get fixed. 😭

Version 1 is the version of the C++ ABI that first appeared in G++ 3.2.

Version 2 is the version of the C++ ABI that first appeared in G++ 3.4, and it was the default all the way through G++ 4.9.

Version 3 corrects an error in mangling a constant address as a template argument.

Version 4, which first appeared in G++ 4.5, implements a standard mangling for vector types.

Version 5, which first appeared in G++ 4.6, corrects the mangling of attribute const/volatile on function pointer types, decltype of a plain decl, and use of a function parameter in the declaration of another parameter.

Version 6, which first appeared in G++ 4.7, corrects the promotion behavior of C++11 scoped enums and the mangling of template argument packs, const/static_cast, prefix ++ and --, and a class scope function used as a template argument.

Version 7, which first appeared in G++ 4.8, treats nullptr_t as a builtin type and corrects the mangling of lambdas in default argument scope.

Version 8, which first appeared in G++ 4.9, corrects the substitution behavior of function types with function-cv-qualifiers.

Version 9, which first appeared in G++ 5.2, corrects the alignment of nullptr_t.

Version 10, which first appeared in G++ 6.1, adds mangling of attributes that affect type identity, like ia32 calling convention attributes (e.g. stdcall).

Version 11, which first appeared in G++ 7, corrects the mangling of sizeof... expressions and operator names. For multiple entities with the same name within a function that are declared in different scopes, the mangling now changes starting with the twelfth occurrence. It also implies -fnew-inheriting-ctors.

Version 12, which first appeared in G++ 8, corrects the calling conventions for empty classes on the x86_64 target and for classes with only deleted copy/move constructors. It accidentally changes the calling convention for classes with a deleted copy constructor and a trivial move constructor. 💀

Version 13, which first appeared in G++ 8.2, fixes that accidental change in version 12.

Version 14, which first appeared in G++ 10, corrects the mangling of the nullptr expression.

Version 15, which first appeared in G++ 10.3, corrects the G++ 10 ABI tag regression.

Version 16, which first appeared in G++ 11, changes the mangling of __alignof__ to be distinct from that of alignof, and dependent operator names.

Version 17, which first appeared in G++ 12, fixes layout of classes that inherit from aggregate classes with default member initializers in C++14 and up.

Version 18, which first appeared in G++ 13, fixes manglings of lambdas that have additional context.

Version 19, which first appeared in G++ 14, fixes manglings of structured bindings to include ABI tags. ✨

Also see -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 whenever it defines a symbol with an incorrect mangled name. This switch specifies which ABI version to use for the alias. 💅

So with -fabi-version=0 (that's the default, bestie), this lil guy defaults to 13 (GCC 8.2 compatibility energy). But if another ABI version is explicitly selected, it defaults to 0 instead. And if you're out here needing compatibility with GCC versions 3.2 through 4.9, use -fabi-compat-version=2. Slay.

Also — if you don't pass this option but you do pass -Wabi=n, then that version is the one used for the compatibility aliases. And if you pass this option along with -Wabi (no version number, just vibes), then the version from this option is what's used for the warning. ✨

• -fno-access-control — turn OFF all access checking. All of it. Every last bit. This switch is mainly useful for working around bugs in the access control code, so it's giving "break glass in case of emergency" 🫶
• -faligned-new — enables support for C++17 new of types that require more alignment than void* ::operator new(std::size_t) provides. A numeric argument like -faligned-new=32 can be used to specify how much alignment (in bytes) is provided by that function, but honestly few users will need to override the default of alignof(std::max_align_t). Also bestie, this flag is enabled by default for -std=c++17 💅
• -fchar8_t, -fno-char8_t — enables support for char8_t as adopted for C++20. That's a whole glow-up: a new char8_t fundamental type, changes to the types of UTF-8 string and character literals, new signatures for user-defined literals, the associated standard library updates, and new __cpp_char8_t and __cpp_lib_char8_t feature test macros. She's doing a LOT 💖

This option enables functions to be overloaded for ordinary and UTF-8 strings:

int f(const char *);    // #1
int f(const char8_t *); // #2
int v1 = f("text");     // Calls #1
int v2 = f(u8"text");   // Calls #2

and introduces new signatures for user-defined literals:

int operator""_udl1(char8_t);
int v3 = u8'x'_udl1;
int operator""_udl2(const char8_t*, std::size_t);
int v4 = u8"text"_udl2;
template<typename T, T...> int operator""_udl3();
int v5 = u8"text"_udl3;

⚠️ Heads up bestie, 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 — check that the pointer returned by operator new is non-null before attempting to modify the storage allocated. This check is normally unnecessary because the C++ standard specifies that operator new only returns 0 if it is declared throw(), in which case the compiler always checks the return value even without this option. In all other cases, when operator new has a non-empty exception specification, memory exhaustion is signalled by throwing std::bad_alloc. See also new (nothrow).
• -fconcepts, -fconcepts-ts — enable support for the C++ Concepts feature for constraining template arguments. With -std=c++20 and above, Concepts are part of the language standard, so -fconcepts defaults to on 🎀

Some constructs that were allowed by the earlier C++ Extensions for Concepts Technical Specification, ISO 19217 (2015), but didn't make it into the standard, can additionally be enabled by -fconcepts-ts.

🚨 But listen: the option -fconcepts-ts was deprecated in GCC 14 and may be removed in GCC 15; users are expected to convert their code to C++20 concepts. Get that migration on your to-do list fr fr.

• *-fconstexpr-depth=n* — set the maximum nested evaluation depth for C++11 constexpr functions to n. A limit is needed to detect endless recursion during constant expression evaluation. The minimum specified by the standard is 512.
• *-fconstexpr-cache-depth=n* — set the maximum level of nested evaluation depth for C++11 constexpr functions that will be cached to n. This is a heuristic that trades off compilation speed (when the cache avoids repeated calculations) against memory consumption (when the cache grows very large from highly recursive evaluations). The default is 8. Very few users are likely to want to adjust it, but if your code does heavy constexpr calculations you might want to experiment to find which value works best for you. Science 🔬✨
• -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. Iconic behavior honestly.

constexpr float inf = 1./0.; // OK with -fconstexpr-fp-except

• *-fconstexpr-loop-limit=n* — set the maximum number of iterations for a loop in C++14 constexpr functions to n. A limit is needed to detect infinite loops during constant expression evaluation. The default is 262144 (1<<18).
• *-fconstexpr-ops-limit=n* — set the maximum number of operations during a single constexpr evaluation. Even when number of iterations of a single loop is limited with the above limit, if there are several nested loops and each of them has many iterations but still smaller than the above limit, or if in a body of some loop or even outside of a loop too many expressions need to be evaluated, the resulting constexpr evaluation might take too long. The default is 33554432 (1<<25). Big numbers, big energy 🧚
• -fcontracts — enable experimental support for the C++ Contracts feature, as briefly added to and then removed from the C++20 working paper (N4820). The implementation also includes proposed enhancements from papers P1290, P1332, and P1429. This functionality is intended mostly for those interested in experimentation towards refining the feature to get it into shape for a future C++ standard. Basically: lab coat mode 🥼

On violation of a checked contract, the violation handler is called. Users can replace the violation handler by defining

void
handle_contract_violation (const std::experimental::contract_violation&);

There are different sets of additional flags that can be used together to specify which contracts will be checked and how — for N4820 contracts, P1332 contracts, or P1429 contracts; these sets cannot be used together. Pick a lane 💅

• -fcontract-mode=[on|off] — control whether any contracts have any semantics at all. Defaults to on.
• -fcontract-assumption-mode=[on|off] — [N4820] control whether contracts with level axiom should have the assume semantic. Defaults to on.
• -fcontract-build-level=[off|default|audit] — [N4820] specify which level of contracts to generate checks for. Defaults to default.
• -fcontract-continuation-mode=[on|off] — [N4820] control whether to allow the program to continue executing after a contract violation. That is, do checked contracts have the maybe semantic described below rather than the never semantic. Defaults to off.
• *-fcontract-role=<name>:<default>,<audit>,<axiom>* — [P1332] specify the concrete semantics for each contract level of a particular contract role.
• *-fcontract-semantic=[default|audit|axiom]:<semantic>* — [P1429] specify the concrete semantic for a particular contract level.
• -fcontract-strict-declarations=[on|off] — control whether to reject adding contracts to a function after its first declaration. Defaults to off.

The possible concrete semantics for that can be specified with -fcontract-role or -fcontract-semantic are:

• "ignore" — this contract has no effect.
• "assume" — this contract is treated like C++23 [[assume]].
• "check_never_continue", "never", "abort" — this contract is checked. If it fails, the violation handler is called. If the handler returns, std::terminate is called. 💀
• "check_maybe_continue", "maybe" — this contract is checked. If it fails, the violation handler is called. If the handler returns, execution continues normally. 🦋

• -fcoroutines — enable support for the C++ coroutines extension (experimental).
• -fdiagnostics-all-candidates — permit the C++ front end to note all candidates during overload resolution failure, including when a deleted function is selected. Show me everyone, no gatekeeping 🙈
• -fno-elide-constructors — the C++ standard allows an implementation to omit creating a temporary that is only used to initialize another object of the same type. Specifying this option disables that optimization, and forces G++ to call the copy constructor in all cases. This option also causes G++ to call trivial member functions which otherwise would be expanded inline.

In C++17, the compiler is required to omit these temporaries, but this option still affects trivial member functions.

• -fno-enforce-eh-specs — don't generate code to check for violation of exception specifications at run time.

🚨 Bestie. This option violates the C++ standard, but it may be useful for reducing code size in production builds, much like defining NDEBUG. This does not give user code permission to throw exceptions in violation of the exception specifications; the compiler still optimizes based on the specifications, so throwing an unexpected exception results in undefined behavior at run time. You have been warned. 😭

• -fextern-tls-init, -fno-extern-tls-init — bestie, so C++11 and OpenMP said it's totally fine for thread_local and threadprivate variables to have dynamic (runtime) initialization, and to make that work, every single use of such a variable goes through a lil wrapper function that does the init. When the use and the definition live in the same translation unit, that overhead can get optimized away 😌 but when the use is in a different TU? significant overhead, even if the variable doesn't actually need dynamic init. So if you, the programmer, can be 100% sure that no use of the variable in a non-defining TU needs to trigger dynamic initialization (either because it's statically initialized, or because a use of it in the defining TU runs before any uses in another TU), you can yeet that overhead with -fno-extern-tls-init. On targets that support symbol aliases, the default is -fextern-tls-init. On targets that do not support symbol aliases, the default is -fno-extern-tls-init. 💅
• -ffold-simple-inlines, -fno-fold-simple-inlines — lets the C++ frontend fold calls to std::move, std::forward, std::addressof and std::as_const. And unlike inlining, this means no debug information gets generated for those calls. Since debugging those functions is literally never the vibe, this flag is on by default — unless -fno-inline is 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 — do not enable immediate function escalation, whereby certain functions can get promoted to consteval, as specified in P2564R3. Like, for example:

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 consteval function id(t) isn't a constant expression, the compiler rejects the code. This option is turned on by default; it is only effective in C++20 mode or later. 👑

• -fimplicit-constexpr — make inline functions implicitly constexpr if they satisfy the requirements for a constexpr function. Usable in C++14 mode or later. This can result in initialization changing from dynamic to static, plus other optimizations 🌈.
• -fno-implicit-templates — never emit code for non-inline templates that get instantiated implicitly (i.e. by use); only emit code for explicit instantiations. Heads up bestie: if you use this, you gotta take care to structure your code so it includes all the necessary explicit instantiations, or you'll be getting undefined symbols at link time 😭
• -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-ts, -fno-modules-ts — enable support for C++20 modules. The -fno-modules-ts is usually not needed, since that's the default. And even though this is a C++20 feature, it is not currently implicitly enabled by selecting that standard version. 🦋
• -fmodule-header, -fmodule-header=user, -fmodule-header=system — compile a header file to create an importable header unit. periodt.
• -fmodule-implicit-inline — member functions defined in their class definitions are not implicitly inline for modular code. This is different from traditional C++ behavior, for good reasons 💅. However, it may result in difficulty during code porting. This option makes such function definitions implicitly inline. It does, however, generate an ABI incompatibility, so you must use it everywhere or nowhere. (Such definitions outside of a named module remain implicitly inline, regardless.)
• -fno-module-lazy — disable lazy module importing and module mapper creation. simple as that 🎀
• -fmodule-mapper=[hostname]:port[?ident]
• -fmodule-mapper=|program[?ident] args...
• -fmodule-mapper==socket[?ident]
• -fmodule-mapper=<>[inout][?ident]
• -fmodule-mapper=<in>out[?ident]
• -fmodule-mapper=file[?ident] — an oracle to query for module name to filename mappings 🔮. If unspecified, the CXX_MODULE_MAPPER environment variable is used, and if that's unset, an in-process default is provided.
• -fmodule-only — only emit the Compiled Module Interface, inhibiting any object file. no object file for you 🙈
• -fms-extensions — disable Wpedantic warnings about constructs used in MFC, like implicit int and getting a pointer to member function via non-standard syntax. ✨
• -fnew-inheriting-ctors — enable the P0136 adjustment to the semantics of C++11 constructor inheritance. This is part of C++17 but is also considered a Defect Report against C++11 and C++14. This flag is enabled by default unless -fabi-version=10 or lower is specified. 👑
• -fnew-ttp-matching — enable the P0522 resolution to Core issue 150, template template parameters and default arguments: this allows a template with default template arguments as an argument for a template template parameter with fewer template parameters. This flag is enabled by default for -std=c++17. 🌸
• -fno-nonansi-builtins — disable built-in declarations of functions that aren't mandated by ANSI/ISO C. These include ffs, alloca, _exit, index, bzero, conjf, and other related functions. byeee 💅
• -fnothrow-opt — treat a throw() exception specification as if it were a noexcept specification, to reduce or eliminate the text size overhead relative to a function with no exception specification. Fun fact bestie: if the function has local variables of types with non-trivial destructors, the exception specification actually makes the function smaller, because the EH cleanups for those variables can be optimized away 🎀. The semantic effect is that an exception thrown out of a function with such an exception specification results in a call to terminate rather than unexpected.
• -fno-operator-names — do not treat the operator name keywords and, bitand, bitor, compl, not, or and xor as synonyms as keywords. 🦋
• -fno-optional-diags — disable diagnostics that the standard says a compiler does not need to issue. Currently, the only such diagnostic issued by G++ is the one for a name having multiple meanings within a class. lowkey niche 😌
• -fno-pretty-templates — when an error message refers to a specialization of a function template, the compiler normally prints the signature of the template followed by the template arguments and any typedefs or typenames in the signature (e.g. void f(T) [with T = int] rather than void f(int)) so it's clear which template is involved. And when an error message refers to a specialization of a class template, the compiler omits any template arguments that match the default template arguments for that template. If either of these behaviors makes it harder to understand the error message rather than easier, you can use -fno-pretty-templates to disable them. 🙈
• -fno-rtti — disable generation of information about every class with virtual functions for use by the C++ run-time type identification features (dynamic_cast and typeid). If you don't use those parts of the language, you can save some space with this flag 💾. Note that exception handling uses the same information, but G++ generates it as needed. The dynamic_cast operator can still be used for casts that do not require run-time type information, i.e. casts to void * or to unambiguous base classes.

Mixing code compiled with -frtti with that compiled with -fno-rtti may not work. For example, programs may fail to link if a class compiled with -fno-rtti is used as a base for a class compiled with -frtti. 💀

• -fsized-deallocation — enable the built-in global declarations

void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;

as introduced in C++14. This is useful for user-defined replacement deallocation functions that, for example, use the size of the object to make deallocation faster ⚡. Enabled by default under -std=c++14 and above. The flag -Wsized-deallocation warns about places that might want to add a definition.

• -fstrict-enums — allow the compiler to optimize using the assumption that a value of enumerated type can only be one of the values of the enumeration (as defined in the C++ standard; basically, a value that can be represented in the minimum number of bits needed to represent all the enumerators). This assumption may not be valid if the program uses a cast to convert an arbitrary integer value to the enumerated type. This option has no effect for an enumeration type with a fixed underlying type. 💅
• -fstrong-eval-order — evaluate member access, array subscripting, and shift expressions in left-to-right order, and evaluate assignment in right-to-left order, as adopted for C++17. Enabled by default with -std=c++17. -fstrong-eval-order=some enables just the ordering of member access and shift expressions, and is the default without -std=c++17. 🌈
• *-ftemplate-backtrace-limit=n* — set the maximum number of template instantiation notes for a single warning or error to n. The default value is 10. 🎀
• -ftemplate-depth=n — Sets the maximum instantiation depth for template classes to n. You need a limit on this to catch endless recursions when instantiating template classes. ANSI/ISO C++ conforming programs must not rely on a maximum depth greater than 17 (this changed to 1024 in C++11). The default is 900, because the compiler can run out of stack space before it even hits 1024 in some situations. 💅
• -fno-threadsafe-statics — Don't emit the extra code that uses the routines specified in the C++ ABI for thread-safe initialization of local statics. You can use this to reduce code size slightly in code that doesn't need to be thread-safe. ✨
• -fuse-cxa-atexit — Registers destructors for objects with static storage duration with the __cxa_atexit function instead of the atexit function. This option is required for fully standards-compliant handling of static destructors, but it only works if your C library supports __cxa_atexit. 🫶
• -fno-use-cxa-get-exception-ptr — Don't use the __cxa_get_exception_ptr runtime routine. This causes std::uncaught_exception to be incorrect, but it's necessary if that runtime routine isn't available. 😭
• -fvisibility-inlines-hidden — This switch declares that you, the user, are not attempting to compare pointers to inline functions or methods where the addresses of the two functions are taken in different shared objects.

The effect of this is that GCC may, effectively, mark inline methods with __attribute__ ((visibility ("hidden"))) so that they don't appear in the export table of a DSO and don't require a PLT indirection when used within the DSO. Enabling this option can have a dramatic effect on load and link times of a DSO as it massively reduces the size of the dynamic export table when the library makes heavy use of templates. 🔥

The behavior of this switch is not quite the same as marking the methods as hidden directly, because it does not affect static variables local to the function or cause the compiler to deduce that the function is defined in only one shared object.

You may mark a method as having a visibility explicitly to negate the effect of the switch for that method. For example, if you do want to compare pointers to a particular inline method, you might mark it as having default visibility. Marking the enclosing class with explicit visibility has no effect.

Explicitly instantiated inline methods are unaffected by this option as their linkage might otherwise cross a shared library boundary.

• -fvisibility-ms-compat — This flag attempts to use visibility settings to make GCC's C++ linkage model compatible with that of Microsoft Visual Studio.

The flag makes these changes to GCC's linkage model:

1. It sets the default visibility to hidden, like -fvisibility=hidden.
2. Types, but not their members, are not hidden by default.
3. The One Definition Rule is relaxed for types without explicit visibility specifications that are defined in more than one shared object: those declarations are permitted if they are permitted when this option is not used.

In new code it is better to use -fvisibility=hidden and export those classes that are intended to be externally visible. Unfortunately it is possible for code to rely, perhaps accidentally, on the Visual Studio behavior. 🙈

Among the consequences of these changes are that static data members of the same type with the same name but defined in different shared objects are different, so changing one does not change the other; and that pointers to function members defined in different shared objects may not compare equal. When this flag is given, it is a violation of the ODR to define types with the same name differently.

• -fno-weak — Do not use weak symbol support, even if it is provided by the linker. By default, G++ uses weak symbols if they are available. This option exists only for testing, and should not be used by end-users; it results in inferior code and has no benefits. This option may be removed in a future release of G++. 💀
• -fext-numeric-literals (C++ and Objective-C++ only) — Accept imaginary, fixed-point, or machine-defined literal number suffixes as GNU extensions. When this option is turned off these suffixes are treated as C++11 user-defined literal numeric suffixes. This is on by default for all pre-C++11 dialects and all GNU dialects: -std=c++98, -std=gnu++98, -std=gnu++11, -std=gnu++14. This option is off by default for ISO C++11 onwards (-std=c++11, ...). 🎀
• -nostdinc++ — Do not search for header files in the standard directories specific to C++, but do still search the other standard directories. (This option is used when building the C++ library.) 🌸
• -flang-info-include-translate, -flang-info-include-translate-not, -flang-info-include-translate=header — Inform of include translation events. The first will note accepted include translations, the second will note declined include translations. The header form will inform of include translations relating to that specific header. If header is of the form "user" or "<system>" it will be resolved to a specific user or system header using the include path. 🔍
• -flang-info-module-cmi, -flang-info-module-cmi=module — Inform of Compiled Module Interface pathnames. The first will note all read CMI pathnames. The module form will not reading a specific module's CMI. module may be a named module or a header-unit (the latter indicated by either being a pathname containing directory separators or enclosed in "<>" or ""). 📦
• -stdlib=libstdc++,libc++ — When G++ is configured to support this option, it allows specification of alternate C++ runtime libraries. Two options are available: libstdc++ (the default, native C++ runtime for G++) and libc++ which is the C++ runtime installed on some operating systems (e.g. Darwin versions from Darwin11 onwards). The option switches G++ to use the headers from the specified library and to emit "-lstdc++" or "-lc++" respectively, when a C++ runtime is required for linking. 🌈

---

In addition, these warning options have meanings only for C++ programs: 💖

• -Wabi-tag (C++ and Objective-C++ only) — Warn when a type with an ABI tag is used in a context that does not have that ABI tag. See C++ Attributes for more information about ABI tags. 🏷️
• -Wcomma-subscript (C++ and Objective-C++ only) — Warn about uses of a comma expression within a subscripting expression. This usage was deprecated in C++20 and is going to be removed in C++23. However, a comma expression wrapped in "( )" is not deprecated. Example:

void f(int *a, int b, int c) {
    a[b,c];     // deprecated in C++20, invalid in C++23
    a[(b,c)];   // OK
}

In C++23 it is valid to have comma separated expressions in a subscript when an overloaded subscript operator is found and supports the right number and types of arguments. G++ will accept the formerly valid syntax for code that is not valid in C++23 but used to be valid but deprecated in C++20 with a pedantic warning that can be disabled with -Wno-comma-subscript.

Enabled by default with -std=c++20 unless -Wno-deprecated, and with -std=c++23 regardless of -Wno-deprecated.

This warning is upgraded to an error by -pedantic-errors in C++23 mode or later. ⚠️

• -Wctad-maybe-unsupported (C++ and Objective-C++ only) — Warn when performing class template argument deduction (CTAD) on a type with no explicitly written deduction guides. This warning will point out cases where CTAD succeeded only because the compiler synthesized the implicit deduction guides, which might not be what the programmer intended. Certain style guides allow CTAD only on types that specifically "opt-in"; i.e., on types that are designed to support CTAD. This warning can be suppressed with the following pattern:

struct allow_ctad_t; // any name works
template <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) — Warn when a class seems unusable because all the constructors or destructors in that class are private, and it has neither friends nor public static member functions. Also warn if there are no non-private methods, and there's at least one private member function that isn't a constructor or destructor. 🚫
• -Wdangling-reference (C++ and Objective-C++ only) — Warn when a reference is bound to a temporary whose lifetime has ended. For example:

int n = 1;
const int& r = std::max(n - 1, n + 1); // r is dangling

In the example above, two temporaries are created, one for each argument, and a reference to one of the temporaries is returned. However, both temporaries are destroyed at the end of the full expression, so the reference "r" is dangling. This warning also detects dangling references in member initializer lists:

const int& f(const int& i) { return i; }
struct S {
  const int &r; // r is dangling
  S() : r(f(10)) { }
};

Member functions are checked as well, but only their object argument:

struct S {
   const S& self () { return *this; }
};
const S& s = S().self(); // s is dangling

Certain functions are safe in this respect, for example "std::use_facet": they take and return a reference, but they don't return one of its arguments, which can fool the warning. Such functions can be excluded from the warning by wrapping them in a "#pragma":

#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wdangling-reference"
const T& foo (const T&) { ... }
#pragma GCC diagnostic pop

The "#pragma" can also surround the class; in that case, the warning will be disabled for all the member functions.

-Wdangling-reference also warns about code like

auto p = std::minmax(1, 2);

😭💔

...where std::minmax hands back a std::pair<const int&, const int&>, and BOTH of those references are out here dangling 💀 the second that full expression holding the std::minmax call wraps up. it's giving expired.

lowkey important: this warning does NOT pop off for std::span-like classes. we're counting anything shaped like this as std::span-like:

template<typename T>
struct Span {
  T* data_;
  std::size len_;
};

so like — a non-union class that has a pointer data member and a trivial destructor. that's the vibe check it passes. 🦋

if you wanna silence the whole thing, hit it with the gnu::no_dangling attribute.

✨ this warning is enabled by -Wall.

---

• -Wdelete-non-virtual-dtor (C++ and Objective-C++ only) — warns you when delete is used to destroy an instance of a class that's got virtual functions but a non-virtual destructor. bestie, that is not safe: if you delete an instance of a derived class through a pointer to a base class, and that base class doesn't have a virtual destructor, you're in trouble. 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. enabled by -Wextra. and with -Wdeprecated-copy-dtor, it also gets deprecated 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 get applied on operands where one is of enumeration type and the other is of a different enumeration type. this conversion got deprecated in C++20. example time:

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, you can turn this warning on with -Wenum-conversion.

• -Wno-deprecated-enum-float-conversion (C++ and Objective-C++ only) — disables the warning for when the usual arithmetic conversions land on operands where one is of enumeration type and the other is of a floating-point type. also deprecated in C++20. for example:

enum E1 { e };
enum E2 { f };
bool b = e <= 3.7;

-Wdeprecated-enum-float-conversion is enabled by default with -std=c++20. in pre-C++20 dialects, this warning can be enabled by -Wenum-conversion.

• -Wno-elaborated-enum-base — for C++11 and above, warns if an (invalid) additional enum-base is used in an elaborated-type-specifier. that is: if an enum with a given underlying type and no enumerator list is used in a declaration that's anything other than just a standalone declaration of the enum. enabled by default. and heads up — this warning gets upgraded to an error with -pedantic-errors. 😭
• -Wno-init-list-lifetime (C++ and Objective-C++ only) — do NOT warn about uses of std::initializer_list that are likely to end in dangling pointers. because the underlying array for an initializer_list is handled like a normal C++ temporary object, it's so easy to accidentally keep a pointer to the array past the end of the array's lifetime. like:
  • if a function returns a temporary initializer_list, or a local initializer_list variable, the array's lifetime ends at the end of the return statement, so the value returned has a dangling pointer.
  • if a new-expression creates an initializer_list, the array only lives until the end of the enclosing full-expression, so the initializer_list sitting in the heap has a dangling pointer.
  • when an initializer_list variable is assigned from a brace-enclosed initializer list, the temporary array created for the right side of the assignment only lives until the end of the full-expression — so at the next statement, the initializer_list variable has a dangling pointer. look:

// li's initial underlying array lives as long as li
std::initializer_list<int> li = { 1,2,3 };
// assignment changes li to point to a temporary array
li = { 4, 5 };
// now the temporary is gone and li has a dangling pointer
int i = li.begin()[0] // undefined behavior

• when a list constructor stores the begin pointer from the initializer_list argument, this doesn't extend the lifetime of the array — so if a class variable is constructed from a temporary initializer_list, the pointer is left dangling by the end of the variable declaration statement. 🙈

• -Winvalid-constexpr — warns when a function never produces a constant expression. in C++20 and earlier, for every constexpr function and function template, there had to be at least one set of function arguments in at least one instantiation such that an invocation of the function or constructor could be an evaluated subexpression of a core constant expression. C++23 removed this restriction, so it's possible to have a function or a function template marked constexpr for which no invocation satisfies the requirements of a core constant expression.

this warning is enabled as a pedantic warning by default in C++20 and earlier. in C++23, -Winvalid-constexpr can be turned on, in which case it'll be an ordinary warning. 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 — verifies all imported macro definitions are valid at the end of compilation. not enabled by default, because it needs extra processing to figure out. could be useful when you're preparing sets of header-units and you wanna 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 such suffixes as separate preprocessing tokens, to keep backwards compatibility with code using formatting macros from <inttypes.h>. like:

#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. heads up: 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 the standard requires. a narrowing conversion from a constant produces an error, and a narrowing conversion from a non-constant produces a warning — but -Wno-narrowing suppresses the diagnostic. note that this does NOT affect the meaning of well-formed code; narrowing conversions are still considered ill-formed in SFINAE contexts. 🫶

with -Wnarrowing in C++98, warn when a narrowing conversion prohibited by C++11 happens inside { }, e.g.

int i = { 2.2 }; // error: narrowing from double to int

this flag is included in -Wall and -Wc++11-compat.

• -Wnoexcept (C++ and Objective-C++ only) — warns when a noexcept-expression evaluates to false because of a call to a function that doesn't have a non-throwing exception specification (i.e. throw() or noexcept) but is known by the compiler to never throw an exception.
• -Wnoexcept-type (C++ and Objective-C++ only) — warns if the C++17 feature making noexcept part of a function type changes the mangled name of a symbol relative to C++14. enabled by -Wabi and -Wc++17-compat. 🧚

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>.

• -Wclass-memaccess (C++ and Objective-C++ only) — warns when the destination of a call to a raw memory function like memset or memcpy is an object of class type, and when writing into such an object might bypass the class's non-trivial or deleted constructor or copy assignment, violate const-correctness or encapsulation, or corrupt virtual table pointers. modifying the representation of such objects may violate invariants maintained by member functions of the class. for example, the call to memset below is undefined, because it modifies a non-trivial class object — and so it's diagnosed. the safe way to either initialize or clear the storage of objects of such types is by using the appropriate constructor or assignment operator, if one is available.

std::string str = "abc";
memset (&str, 0, sizeof str);

-Wclass-memaccess is enabled by -Wall. explicitly casting the pointer to the class object to void * or to a type that can be safely accessed by the raw memory function suppresses the warning. ⭐

• -Wnon-virtual-dtor (C++ and Objective-C++ only) — warns when a class has virtual functions AND an accessible non-virtual destructor, itself or in an accessible polymorphic base class. in that case it's possible but unsafe to delete an instance of a derived class through a pointer to the class itself or the base class. this warning is automatically enabled if -Weffc++ is specified. ngl, -Wdelete-non-virtual-dtor (enabled by -Wall) should be preferred, because it warns about the unsafe cases without false positives. 👑
• -Wregister (C++ and Objective-C++ only) — warns on uses of the register storage class specifier, except when it's part of the GNU Explicit Register Variables extension. using the register keyword as a storage class specifier got deprecated in C++11 and removed in C++17. enabled by default with -std=c++17. 💀
• -Wreorder (C++ and Objective-C++ only) — bestie, this one clocks you when the order of your member initializers in the code doesn't match the order they actually have to run in. Like:

        struct A {
          int i;
          int j;
          A(): j (0), i (1) { }
        };

The compiler is gonna silently rearrange those initializers for i and j so they line up with the declaration order of the members, and it throws a warning at you about it. This warning is enabled by -Wall.

• -Wno-pessimizing-move (C++ and Objective-C++ only) — this one snitches on you when a call to std::move is literally blocking copy elision. The classic scenario where copy elision could've happened is when you're returning from a function whose return type is a class, the expression you're returning is the name of a non-volatile automatic object, it's not a function parameter, and it has the same type as the function's return type.

        struct T {
        ...
        };
        T fn()
        {
          T t;
          ...
          return std::move (t);
        }

But in this example?? The std::move call is ruining the copy elision. 💀

This warning is enabled by -Wall.

• -Wno-redundant-move (C++ and Objective-C++ only) — this warning is about redundant calls to std::move, meaning a move would've happened anyway even without you writing std::move. It happens because the compiler is forced to treat the object as if it were an rvalue in certain situations, like returning a local variable, where copy elision isn't applicable. Look:

        struct T {
        ...
        };
        T fn(T t)
        {
          ...
          return std::move (t);
        }

Here the std::move call is just… extra. Redundant. Not needed. Because G++ implements Core Issue 1579, here's another example:

        struct T { // convertible to U
        ...
        };
        struct U {
        ...
        };
        U fn()
        {
          T t;
          ...
          return std::move (t);
        }

In this one, copy elision isn't applicable because the type of the expression being returned and the function return type are different, yet G++ treats the return value as if it were designated by an rvalue. Sneaky.

This warning is enabled by -Wextra.

• -Wrange-loop-construct (C++ and Objective-C++ only) — this one calls out when a C++ range-based for-loop is making an unnecessary copy. 🫶 It happens when the range declaration isn't a reference but honestly probably should be. Like:

        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. (Under 64 bytes? We chill.)

This warning also fires when a loop variable in a range-based for-loop gets initialized with a value of a different type, which results in a copy. Like:

        void fn() {
          int arr[10];
          for (const double &x : arr) { ... }
        }

In the example above, every single iteration of the loop creates and destroys a temporary value of type double, and the reference const double & gets bound to it. Every. Iteration. 😭

This warning is enabled by -Wall.

• -Wredundant-tags (C++ and Objective-C++ only) — warns 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 any ambiguity. For example:

        struct foo;
        struct foo *p;   // warn that keyword struct can be eliminated

But on 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) — don't warn if a class type has a base or a field whose type uses the anonymous namespace, or depends on a type with no linkage. 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 shows up in a single translation unit, the best way to silence the warning is to give it internal linkage by putting it in an anonymous namespace too. The compiler doesn't give this warning for types defined in the main .C file, since those are unlikely to have multiple definitions. -Wsubobject-linkage is enabled by default.
• -Weffc++ (C++ and Objective-C++ only) — warns about violations of these style guidelines from Scott Meyers' Effective C++ series of books, bestie:
  • Define a copy constructor and an assignment operator for classes with dynamically-allocated memory.
  • Prefer initialization to assignment in constructors.
  • Have operator= return a reference to *this.
  • Don't try to return a reference when you must return an object.
  • Distinguish between prefix and postfix forms of increment and decrement operators.
  • Never overload &&, ||, or ,.

This option also enables -Wnon-virtual-dtor, which is also one of the effective C++ recommendations. But the check is extended to warn about the lack of virtual destructor in accessible non-polymorphic bases classes too. 👑

When you select this option, be aware that the standard library headers do not obey all of these guidelines — use grep -v to filter out those warnings. (iykyk)

• -Wno-exceptions (C++ and Objective-C++ only) — disables the warning about the case when an exception handler is shadowed by another handler, which can point out a wrong ordering of exception handlers. 🙈
• -Wstrict-null-sentinel (C++ and Objective-C++ only) — warns about the use of an uncasted NULL as a sentinel. When compiling only with GCC this is a valid sentinel, since NULL is defined to __null. Even though it's a null pointer constant rather than a null pointer, it's guaranteed to be the same size as a pointer. But this use is not portable across different compilers, so heads 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 like friend int foo(int) — where the name of the friend is an unqualified-id — could be interpreted as a particular specialization of a template function. The warning exists to diagnose compatibility problems, and is enabled by default.
• -Wold-style-cast (C++ and Objective-C++ only) — 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 so much easier to search for. Slay.
• -Woverloaded-virtual (C++ and Objective-C++ only)
• -Woverloaded-virtual=n — 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. 😭

In cases where the different signatures are not an accident, the simplest solution is to add a using-declaration to the derived class to un-hide the base function, e.g. add using A::f; to B.

The optional level suffix controls the behavior when all the declarations in the derived class override virtual functions in the base class, even if not all of the base functions are overridden:

        struct C {
          virtual void f();
          virtual void f(int);
        };

        struct D: public C {
          void f(int); // does override
        }

This pattern is less likely to be a mistake; if D is only used virtually, the user might have decided that the base class semantics for some of the overloads are fine.

At level 1, this case does not warn; at level 2, it does. -Woverloaded-virtual by itself selects level 2. Level 1 is included in -Wall.

• -Wno-pmf-conversions (C++ and Objective-C++ only) — 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 picks 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. (Not us, the standard said so.) 🌸
• -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, like the STL, so you can still use the STL. You may also instantiate or specialize templates. 🎀
• -Wmismatched-new-delete (C++ and Objective-C++ only) — warns for mismatches between calls to operator new or operator delete and the corresponding 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
                   }

bestie. The related option -Wmismatched-dealloc is the one that snitches on mismatches involving allocation and deallocation functions other than operator new and operator delete. Different vibes, same energy. 💅

-Wmismatched-new-delete is included in -Wall. periodt.

✨ -Wmismatched-tags ✨

(C++ and Objective-C++ only) 🦋

Okay so this one warns for declarations of structs, classes, and class templates and their specializations when the class-key doesn't match either the definition or the first declaration (assuming no definition was provided). ✨

Lowkey here's the tea — the declaration of struct Object in the argument list of draw triggers the warning. To make it stop, either remove the redundant class-key struct or replace it with class so it matches its definition. slay.

class Object {
public:
  virtual ~Object () = 0;
};
void draw (struct Object*);

And no cap, it is not wrong to declare a class with the class-key struct like the example above shows. The -Wmismatched-tags option is just out here trying to help you achieve a consistent style of class declarations. In code intended to be portable to Windows-based compilers, this warning helps prevent unresolved references due to the difference in the mangling of symbols declared with different class-keys. It can be used either on its own or in conjunction with -Wredundant-tags. 🫶

✨ -Wmultiple-inheritance ✨

(C++ and Objective-C++ only) 🌸

Warns when a class is defined with multiple direct base classes. Some coding rules disallow multiple inheritance, and this may be used to enforce that rule. The warning is inactive inside a system header file, such as the STL, so one can still use the STL. 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. 🎀

✨ -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. 💜

✨ -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. 👑

✨ -Wno-terminate ✨

(C++ and Objective-C++ only) 🍓

Disables the warning about a throw-expression that will immediately result in a call to terminate. 😭

✨ -Wno-vexing-parse ✨

(C++ and Objective-C++ only) 💀

Warns about the most vexing parse syntactic ambiguity. This warns about the cases when a declaration looks like a variable definition, but the C++ language requires it to be interpreted as a function declaration. For instance:

void f(double a) {
  int i();        // extern int i (void);
  int n(int(a));  // extern int n (int);
}

Another example, because one was not enough bestie:

struct S { S(int); };
void f(double a) {
  S x(int(a));   // extern struct S x (int);
  S y(int());    // extern struct S y (int (*) (void));
  S z();         // extern struct S z (void);
}

The warning will suggest options how to deal with such an ambiguity; e.g., it can suggest removing the parentheses or using braces instead. 🫶

This warning is enabled by default. ✨

✨ -Wno-class-conversion ✨

(C++ and Objective-C++ only) 🌈

Do not warn when a conversion function converts an object to the same type, to a base class of that type, or to void; such a conversion function will never be called. 💅

✨ -Wvolatile ✨

(C++ and Objective-C++ only) ⭐

Warns about deprecated uses of the volatile qualifier. This includes:

• postfix and prefix ++ and -- expressions of volatile-qualified types
• using simple assignments where the left operand is a volatile-qualified non-class type for their value
• compound assignments where the left operand is a volatile-qualified non-class type
• volatile-qualified function return type
• volatile-qualified parameter type
• structured bindings of a volatile-qualified type

This usage was deprecated in C++20. 😭

Enabled by default with -std=c++20. 👑

✨ -Wzero-as-null-pointer-constant ✨

(C++ and Objective-C++ only) 🎀

Warns when a literal 0 is used as null pointer constant. This can be useful to facilitate the conversion to nullptr in C++11. 🌸

✨ -Waligned-new ✨

Warns about a new-expression of a type that requires greater alignment than alignof(std::max_align_t) but uses an allocation function without an explicit alignment parameter. This option is enabled by -Wall. ✨

Normally this only warns about global allocation functions, but -Waligned-new=all also warns about class member allocation functions. 💅

✨ -Wno-placement-new ✨

✨ -Wplacement-new=n ✨

Warns about placement new expressions with undefined behavior, such as constructing an object in a buffer that is smaller than the type of the object. For example, the placement new expression below is diagnosed because it attempts to construct an array of 64 integers in a buffer only 64 bytes large. 💀

char buf [64];
new (buf) int[64];

This warning is enabled by default. 🌟

-Wplacement-new=1

This is the default warning level of -Wplacement-new. At this level the warning is not issued for some strictly undefined constructs that GCC allows as extensions for compatibility with legacy code. For example, the following new expression is not diagnosed at this level even though it has undefined behavior according to the C++ standard because it writes past the end of the one-element array. 😭

struct S { int n, a[1]; };
S *s = (S *)malloc (sizeof *s + 31 * sizeof s->a[0]);
new (s->a)int [32]();

-Wplacement-new=2

At this level, in addition to diagnosing all the same constructs as at level 1, a diagnostic is also issued for placement new expressions that construct an object in the last member of structure whose type is an array of a single element and whose size is less than the size of the object being constructed. While the previous example would be diagnosed, the following construct makes use of the flexible member array extension to avoid the warning at level 2. ✨

struct S { int n, a[]; };
S *s = (S *)malloc (sizeof *s + 32 * sizeof s->a[0]);
new (s->a)int [32]();

✨ -Wcatch-value ✨

✨ -Wcatch-value=n ✨

(C++ and Objective-C++ only) 🦋

Warns about catch handlers that do not catch via reference. Here's the breakdown bestie:

• With -Wcatch-value=1 (or -Wcatch-value for short) warn about polymorphic class types that are caught by value.
• With -Wcatch-value=2 warn about all class types that are caught by value.
• With -Wcatch-value=3 warn about all types that are not caught by reference.

-Wcatch-value is enabled by -Wall. 💖

✨ -Wconditionally-supported ✨

(C++ and Objective-C++ only) 🌸

Warns for conditionally-supported (C++11 [intro.defs]) constructs. 🎀

✨ -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 after in-class function definitions. The way that extra ; is just sitting there... 💅

✨ -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 -Wextra option. ✨

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. 😭

✨ -Wno-invalid-offsetof ✨

(C++ and Objective-C++ only) 🧚

Suppresses warnings from applying the offsetof macro to a non-POD type. According to the 2014 ISO C++ standard, applying offsetof to a non-standard-layout type is undefined. In existing C++ implementations, however, offsetof typically gives meaningful results. This flag is for users who are aware that they are writing nonportable code and who have deliberately chosen to ignore the warning about it. 🫶

The restrictions on offsetof may be relaxed in a future version of the C++ standard. 🌟

✨ -Wsized-deallocation ✨

(C++ and Objective-C++ only) 💖

Warns about a definition of an unsized deallocation function

void operator delete (void *) noexcept;
void operator delete[] (void *) noexcept;

without a definition of the corresponding sized deallocation function

void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;

or vice versa. Enabled by -Wextra along with -fsized-deallocation. ✨

• -Wsuggest-final-types — warns about types with virtual methods where code quality would be improved if the type were declared with the C++11 final specifier, or, if possible, declared in an anonymous namespace. This lets GCC more aggressively devirtualize the polymorphic calls. This warning is more effective with link-time optimization, where the info about the class hierarchy graph is more complete. 💅
• -Wsuggest-final-methods — warns about virtual methods where code quality would be improved if the method were declared with the C++11 final specifier, or, if possible, its type were declared in an anonymous namespace or with the final specifier. This warning is more effective with link-time optimization, where the info about the class hierarchy graph is more complete. It is recommended to first consider suggestions of -Wsuggest-final-types and then rebuild with new annotations. ✨
• -Wsuggest-override — warns about overriding virtual functions that are not marked with the override keyword. 👑
• -Wno-conversion-null (C++ and Objective-C++ only) — do not warn for conversions between NULL and non-pointer types. -Wconversion-null is enabled by default. 💀

---

✨ Options Controlling Objective-C and Objective-C++ Dialects ✨

(NOTE: This manual does not describe the Objective-C and Objective-C++ languages themselves.

Bestie, this next bit is literally just for the Objective-C and Objective-C++ girlies. This section describes the command-line options that are only meaningful for Objective-C and Objective-C++ programs. You can also use most of the language-independent GNU compiler options. For example, you might compile a file some_class.m like this:

gcc -g -fgnu-runtime -O -c some_class.m

In this example, -fgnu-runtime is an option meant only for Objective-C and Objective-C++ programs; you can use the other options with any language supported by GCC.

Note that since Objective-C is an extension of the C language, Objective-C compilations may also use options specific to the C front-end (e.g., -Wtraditional). Similarly, Objective-C++ compilations may use C++-specific options (e.g., -Wabi).

Here is a list of options that are only for compiling Objective-C and Objective-C++ programs:

• -fconstant-string-class=class-name — use class-name as the name of the class to instantiate for each literal string specified with the syntax "@"..."". The default class name is NXConstantString if the GNU runtime is being used, and NSConstantString if the NeXT runtime is being used (see below). On Darwin / macOS platforms, the -fconstant-cfstrings option, if also present, overrides the -fconstant-string-class setting and cause "@"..."" literals to be laid out as constant CoreFoundation strings. Note that -fconstant-cfstrings is an alias for the target-specific -mconstant-cfstrings equivalent. 🍓
• -fgnu-runtime — generate object code compatible with the standard GNU Objective-C runtime. This is the default for most types of systems. 🧚
• -fnext-runtime — generate output compatible with the NeXT runtime. This is the default for NeXT-based systems, including Darwin / macOS. The macro __NEXT_RUNTIME__ is predefined if (and only if) this option is used. 🌈
• -fno-nil-receivers — assume that all Objective-C message dispatches ([receiver message:arg]) in this translation unit ensure that the receiver is not nil. This allows for more efficient entry points in the runtime to be used. This option is only available in conjunction with the NeXT runtime and ABI version 0 or 1. 💖
• -fobjc-abi-version=n — use version n of the Objective-C ABI for the selected runtime. This option is currently supported only for the NeXT runtime. In that case, Version 0 is the traditional (32-bit) ABI without support for properties and other Objective-C 2.0 additions. Version 1 is the traditional (32-bit) ABI with support for properties and other Objective-C 2.0 additions. Version 2 is the modern (64-bit) ABI. If nothing is specified, the default is Version 0 on 32-bit target machines, and Version 2 on 64-bit target machines. ✨
• -fobjc-call-cxx-cdtors — for each Objective-C class, check if any of its instance variables is a C++ object with a non-trivial default constructor. If so, synthesize a special - (id) .cxx_construct instance method which runs non-trivial default constructors on any such instance variables, in order, and then return self. Similarly, check if any instance variable is a C++ object with a non-trivial destructor, and if so, synthesize a special - (void) .cxx_destruct method which runs all such default destructors, in reverse order.

The - (id) .cxx_construct and - (void) .cxx_destruct methods thusly generated only operate on instance variables declared in the current Objective-C class, and not those inherited from superclasses. It is the responsibility of the Objective-C runtime to invoke all such methods in an object's inheritance hierarchy. The - (id) .cxx_construct methods are invoked by the runtime immediately after a new object instance is allocated; the - (void) .cxx_destruct methods are invoked immediately before the runtime deallocates an object instance.

As of this writing, only the NeXT runtime on Mac OS X 10.4 and later has support for invoking the - (id) .cxx_construct and - (void) .cxx_destruct methods. 😭

• -fobjc-direct-dispatch — allow fast jumps to the message dispatcher. On Darwin this is accomplished via the comm page. 🦋
• -fobjc-exceptions — enable syntactic support for structured exception handling in Objective-C, similar to what is offered by C++. This option is required to use the Objective-C keywords @try, @throw, @catch, @finally and @synchronized. This option is available with both the GNU runtime and the NeXT runtime (but not available in conjunction with the NeXT runtime on Mac OS X 10.2 and earlier). 🔥
• -fobjc-gc — enable garbage collection (GC) in Objective-C and Objective-C++ programs. This option is only available with the NeXT runtime; the GNU runtime has a different garbage collection implementation that does not require special compiler flags. 💅
• -fobjc-nilcheck — for the NeXT runtime with version 2 of the ABI, check for a nil receiver in method invocations before doing the actual method call. This is the default and can be disabled using -fno-objc-nilcheck. Class methods and super calls are never checked for nil in this way no matter what this flag is set to. Currently this flag does nothing when the GNU runtime, or an older version of the NeXT runtime ABI, is used. 👑
• -fobjc-std=objc1 — conform to the language syntax of Objective-C 1.0, the language recognized by GCC 4.0. This only affects the Objective-C additions to the C/C++ language; it does not affect conformance to C/C++ standards, which is controlled by the separate C/C++ dialect option flags. When this option is used with the Objective-C or Objective-C++ compiler, any Objective-C syntax that is not recognized by GCC 4.0 is rejected. This is useful if you need to make sure that your Objective-C code can be compiled with older versions of GCC. 🧚
• -freplace-objc-classes — emit a special marker instructing ld(1) not to statically link in the resulting object file, and allow dyld(1) to load it in at run time instead. This is used in conjunction with the Fix-and-Continue debugging mode, where the object file in question may be recompiled and dynamically reloaded in the course of program execution, without the need to restart the program itself. Currently, Fix-and-Continue functionality is only available in conjunction with the NeXT runtime on Mac OS X 10.3 and later. 🌸
• -fzero-link — when compiling for the NeXT runtime, the compiler ordinarily replaces calls to objc_getClass("...") (when the name of the class is known at compile time) with static class references that get initialized at load time, which improves run-time performance. Specifying the -fzero-link flag suppresses this behavior and causes calls to objc_getClass("...") to be retained. This is useful in Zero-Link debugging mode, since it allows for individual class implementations to be modified during program execution. The GNU runtime currently always retains calls to objc_get_class("...") regardless of command-line options. 💜
• -fno-local-ivars — by default instance variables in Objective-C can be accessed as if they were local variables from within the methods of the class they're declared in. This can lead to shadowing between instance variables and other variables declared either locally inside a class method or globally with the same name. Specifying the -fno-local-ivars flag disables this behavior thus avoiding variable shadowing issues. 🙈
• -fivar-visibility=[public|protected|private|package] — set the default instance variable visibility to the specified option so that instance variables declared outside the scope of any access modifier directives default to the specified visibility. 🎀
• -gen-decls — dump interface declarations for all classes seen in the source file to a file named sourcename.decl. 📝
• -Wassign-intercept (Objective-C and Objective-C++ only) — warn whenever an Objective-C assignment is being intercepted by the garbage collector. 😭
• -Wno-property-assign-default (Objective-C and Objective-C++ only) — do not warn if a property for an Objective-C object has no assign semantics specified. 💖
• -Wno-protocol (Objective-C and Objective-C++ only) — if a class is declared to implement a protocol, a warning is issued for every method in the protocol that is not implemented by the class. The default behavior is to issue a warning for every method not explicitly implemented in the class, even if a method implementation is inherited from the superclass. If you use the -Wno-protocol option, then methods inherited from the superclass are considered to be implemented, and no warning is issued for them. ✨
• -Wobjc-root-class (Objective-C and Objective-C++ only) — warn if a class interface lacks a superclass. Most classes will inherit from NSObject (or Object) for example. When declaring classes intended to be root classes, the warning can be suppressed by marking their interfaces with __attribute__((objc_root_class)). 🌈
• -Wselector (Objective-C and Objective-C++ only) — warns you if the compiler clocks multiple methods of different types all rocking the same selector while it's compiling. The check happens on the method list in the final stage of compilation, bestie. It also runs a check for every selector that shows up in a @selector(...) expression, if a matching method for that selector has been found during compilation. And here's the catch 👀 — because these checks only scan the method table at the very end of compilation, you get no warnings at all if the final stage never gets reached. Like, if an error pops off mid-compile, or if you're using -fsyntax-only, the warnings just don't show. no cap.
• -Wstrict-selector-match (Objective-C and Objective-C++ only) — warns when multiple methods with differing argument and/or return types exist for a given selector, and you're trying to send a message with that selector to a receiver of type id or Class. When this flag is off (which is, like, the default behavior), the compiler just skips those warnings whenever the differences it found are confined to types that share the same size and alignment. slay, it's chill like that.
• -Wundeclared-selector (Objective-C and Objective-C++ only) — warns if a @selector(...) expression refers to an undeclared selector. A selector counts as undeclared if no method with that name was declared before the @selector(...) expression — whether explicitly in an @interface or @protocol declaration, or implicitly in an @implementation section. This option always runs its checks the second it finds a @selector(...) expression, whereas -Wselector only checks in the final stage of compilation. It also enforces the coding style convention that methods and selectors must be declared before being used. periodt 💅
• -print-objc-runtime-info — generates a C header describing the largest structure that gets passed by value, if there is one. 🫶

---

✨ Options to Control Diagnostic Messages Formatting ✨

So, traditionally, diagnostic messages have been formatted with zero regard for the output device's aspect — like its width, and other stuff. Rude, honestly. 😭 You can use the options below to control the formatting algorithm for diagnostic messages: how many characters per line, how often source location info gets reported, all of it. Just a heads up though — some language front ends may straight up not honor these options. 🙈

• -fmessage-length=n — tries to format error messages so they fit on lines of about n characters. If n is zero, then no line-wrapping happens at all; each error message just appears on a single line. This is the **default for all front ends.

Note — this option also affects the display of the #error and #warning pre-processor directives, and the deprecated function/type/variable attribute. It does not, however, affect the pragma GCC warning and pragma GCC error pragmas. 💀

• -fdiagnostics-plain-output — requests that diagnostic output look as plain as humanly possible, which is lowkey useful when you're running dejagnu or other utilities that need to parse diagnostics output and would prefer it stay more stable over time. -fdiagnostics-plain-output is currently equivalent to these 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

And in the future, if GCC changes the default appearance of its diagnostics, the corresponding option to disable the new behavior will get added to this list. iconic, forward-thinking. 👑

• -fdiagnostics-show-location=once — only meaningful in line-wrapping mode. Tells the diagnostic messages reporter to emit source location information once; like, if the message is too long to fit on a single physical line and has to wrap, the source location won't get emitted (as a prefix) again and again on every subsequent continuation line. This is the default behavior.
• -fdiagnostics-show-location=every-line — only meaningful in line-wrapping mode. Tells the diagnostic messages reporter to emit the same source location information (as a prefix) for the physical lines that result from breaking a message that's too long to fit on a single line. Not me wanting the context repeated every time, fr fr. 😌
• -fdiagnostics-color[=WHEN]
• -fno-diagnostics-color — use color in diagnostics. WHEN is never, always, or auto. The default depends on how the compiler was configured — it can be any of those WHEN options, or also never if the GCC_COLORS environment variable isn't present in the environment, and auto otherwise. auto makes GCC use color only when standard error is a terminal, and when it's not executing in an emacs shell. The forms -fdiagnostics-color and -fno-diagnostics-color are aliases for -fdiagnostics-color=always and -fdiagnostics-color=never, respectively. 💖

The colors are defined by the environment variable GCC_COLORS. Its value is a colon-separated list of capabilities and Select Graphic Rendition (SGR) substrings. SGR commands are interpreted by the terminal or terminal emulator. (See the section in the documentation of your text terminal for permitted values and their meanings as character attributes.) These substring values are integers in decimal representation and can be concatenated with semicolons. Common values to concatenate include 1 for bold, 4 for underline, 5 for blink, 7 for inverse, 39 for default foreground color, 30 to 37 for foreground colors, 90 to 97 for 16-color mode foreground colors, 38;5;0 to 38;5;255 for 88-color and 256-color modes foreground colors, 49 for default background color, 40 to 47 for background colors, 100 to 107 for 16-color mode background colors, and 48;5;0 to 48;5;255 for 88-color and 256-color modes background colors. 🌈

The default GCC_COLORS is

error=01;31:warning=01;35:note=01;36:range1=32:range2=34:locus=01:\
quote=01:path=01;36:fixit-insert=32:fixit-delete=31:\
diff-filename=01:diff-hunk=32:diff-delete=31:diff-insert=32:\
type-diff=01;32:fnname=01;32:targs=35:valid=01;31:invalid=01;32

where 01;31 is bold red, 01;35 is bold magenta, 01;36 is bold cyan, 32 is green, 34 is blue, 01 is bold, and 31 is red. Setting GCC_COLORS to the empty string disables colors. The supported capabilities are as follows. ⬇️

• error= — SGR substring for error: markers.
• warning= — SGR substring for warning: markers.
• note= — SGR substring for note: markers.
• path= — SGR substring for colorizing paths of control-flow events as printed via -fdiagnostics-path-format=, such as the identifiers of individual events and lines indicating interprocedural calls and returns.
• range1= — SGR substring for first additional range.
• range2= — SGR substring for second additional range.
• locus= — SGR substring for location information, file:line or file:line:column etc.
• quote= — SGR substring for information printed within quotes.
• fnname= — SGR substring for names of C++ functions.
• targs= — SGR substring for C++ function template parameter bindings.
• fixit-insert= — SGR substring for fix-it hints suggesting text to be inserted or replaced.
• fixit-delete= — SGR substring for fix-it hints suggesting text to be deleted.
• diff-filename= — SGR substring for filename headers within generated patches.
• diff-hunk= — SGR substring for the starts of hunks within generated patches.
• diff-delete= — SGR substring for deleted lines within generated patches.
• diff-insert= — SGR substring for inserted lines within generated patches.
• type-diff= — SGR substring for highlighting mismatching types within template arguments in the C++ frontend.
• valid= — SGR substring for highlighting valid elements within text art diagrams.
• invalid= — SGR substring for highlighting invalid elements within text art diagrams.

• -fdiagnostics-urls[=WHEN] — use escape sequences to embed URLs in diagnostics. For example, when -fdiagnostics-show-option emits text showing the command-line option controlling a diagnostic, embed a URL for documentation of that option. 🦋

WHEN is never, always, or auto. auto makes GCC use URL escape sequences only when the standard error is a terminal, and when it's not executing in an emacs shell or any graphical terminal known to be incompatible with this feature — see below, bestie. 👇

The default depends on how the compiler was 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 the GCC_URLS or TERM_URLS environment variables are present and non-empty in the environment of the compiler, or never if neither are.

However — even with -fdiagnostics-urls=always the behavior is dependent on those environment variables: If GCC_URLS is set to empty or no, do not embed URLs in diagnostics. If set to st, URLs use ST escape sequences. If set to bel, the default, URLs use BEL escape sequences. Any other non-empty value enables the feature. If GCC_URLS is not set, use TERM_URLS as a fallback. Note: ST is an ANSI escape sequence, string terminator ESC \; BEL is an ASCII character, CTRL-G, that usually sounds like a beep. 🔔

At this time GCC tries to detect also a few terminals known to not implement the URL feature, and that 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). Specifying the -fno-diagnostics-show-option flag suppresses that behavior. 🙈
• -fno-diagnostics-show-caret — okay so bestie, by default every diagnostic GCC throws at you comes with the original source line plus a little caret ^ pointing at the exact column like it's pointing a manicured finger at the problem 💅. This flag suppresses all that. Lowkey important detail: if you passed -fmessage-length=n, the source line gets truncated to n characters. And when output goes to the terminal, the width is limited to whatever COLUMNS says, or if that's not set, to the terminal width. No cap. 🎀
• -fno-diagnostics-show-labels — so when GCC prints source code (via -fdiagnostics-show-caret), it can label ranges of source with pertinent info, like the types of expressions, and honestly it's giving annotation slay:

            printf ("foo %s bar", long_i + long_j);
                         ~^       ~~~~~~~~~~~~~~~
                          |              |
                          char *         long int

This option turns off those labels — in the example above, the vertical bars and the char * and long int text just vanish. ✨

• -fno-diagnostics-show-cwe — diagnostic messages can optionally carry an associated CWE ("https://cwe.mitre.org/index.html") identifier. GCC itself only provides that metadata for some of the -fanalyzer diagnostics, but GCC plugins can also hand over diagnostics with that metadata. By default, if the info is there, it gets printed with the diagnostic. This option suppresses printing that metadata. Fr fr. 🦋
• -fno-diagnostics-show-rules — diagnostics can optionally have rules attached, like from a coding standard or a specification. GCC itself does not do this for any of its diagnostics, but plugins may. By default, if that info is present it gets printed with the diagnostic, and this option suppresses printing that metadata. Periodt. 💜
• -fno-diagnostics-show-line-numbers — when GCC prints source code (via -fdiagnostics-show-caret), it prints a left margin showing line numbers, like a cute little gutter. This option suppresses that left margin entirely. Gone. Poof. 🙈
• -fdiagnostics-minimum-margin-width=width — this controls the minimum width of the left margin printed by -fdiagnostics-show-line-numbers. It defaults to 6. Yes, six. Remember that number. 👑
• -fdiagnostics-parseable-fixits — emits fix-it hints in a machine-parseable format, suitable for consumption by IDEs. For each fix-it, a line gets printed after the relevant diagnostic, starting with the string "fix-it:". For example:

        fix-it:"test.c":{45:3-45:21}:"gtk_widget_show_all"

The location is expressed as a half-open range, expressed as a count of bytes, starting at byte 1 for the initial column. In the example above, bytes 3 through 20 of line 45 of "test.c" are to be replaced with the given string:

        00000000011111111112222222222
        12345678901234567890123456789
          gtk_widget_showall (dlg);
          ^^^^^^^^^^^^^^^^^^
          gtk_widget_show_all

The filename and replacement string escape backslash as "\\", tab as "\t", newline as "\n", double quotes as "\"", and non-printable characters as octal (e.g. vertical tab as "\013").

An empty replacement string means the given range is to be removed. An empty range (e.g. "45:3-45:3") means the string is to be inserted at that position. Insertion era 💖

• -fdiagnostics-generate-patch — prints fix-it hints to stderr in unified diff format, after any diagnostics are printed. For example:

        --- test.c
        +++ test.c
        @ -42,5 +42,5 @

         void show_cb(GtkDialog *dlg)
         {
        -  gtk_widget_showall(dlg);
        +  gtk_widget_show_all(dlg);
         }

The diff may or may not be colorized, following the same rules as for diagnostics — see -fdiagnostics-color. 🌈

• -fdiagnostics-show-template-tree — in the C++ frontend, when it prints diagnostics showing mismatching template types, like:

          could not convert 'std::map<int, std::vector<double> >()'
            from 'map<[...],vector<double>>' to 'map<[...],vector<float>>

the -fdiagnostics-show-template-tree flag enables printing a tree-like structure showing the common and differing parts of the types, like:

          map<
            [...],
            vector<
              [double != float]>>

The parts that differ get highlighted with color (double and float in this case). Color-coded template drama, we love to see it. 🎨

• -fno-elide-type — by default, when the C++ frontend prints diagnostics showing mismatching template types, common parts of the types get printed as [...] to simplify the error message. For example:

          could not convert 'std::map<int, std::vector<double> >()'
            from 'map<[...],vector<double>>' to 'map<[...],vector<float>>

Specifying the -fno-elide-type flag suppresses that behavior. This flag also affects the output of the -fdiagnostics-show-template-tree flag. So they're linked, iykyk. 🔗

• -fdiagnostics-path-format=KIND — specifies 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. Simple. Iconic.

separate-events means to print a separate "note" diagnostic for each event within the diagnostic. For example:

        test.c:29:5: error: passing NULL as argument 1 to 'PyList_Append' which requires a non-NULL parameter
        test.c:25:10: note: (1) when 'PyList_New' fails, returning NULL
        test.c:27:3: note: (2) when 'i < count'
        test.c:29:5: note: (3) when calling 'PyList_Append', passing NULL from (1) as argument 1

inline-events means to print the events "inline" within the source code. This view attempts to consolidate the events into runs of sufficiently-close events, printing them as labelled ranges within the source.

For example, the same events as above might be printed as:

          'test': events 1-3
            |
            |   25 |   list = PyList_New(0);
            |      |          ^~~~~~~~~~~~~
            |      |          |
            |      |          (1) when 'PyList_New' fails, returning NULL
            |   26 |
            |   27 |   for (i = 0; i < count; i++) {
            |      |   ~~~
            |      |   |
            |      |   (2) when 'i < count'
            |   28 |     item = PyLong_FromLong(random());
            |   29 |     PyList_Append(list, item);
            |      |     ~~~~~~~~~~~~~~~~~~~~~~~~~
            |      |     |
            |      |     (3) when calling 'PyList_Append', passing NULL from (1) as argument 1
            |

Interprocedural control flow is shown by grouping the events by stack frame, and using indentation to show how stack frames are nested, pushed, and popped.

For example:

          'test': events 1-2
            |
            |  133 | {
            |      | ^
            |      | |
            |      | (1) entering 'test'
            |  134 |   boxed_int *obj = make_boxed_int (i);
            |      |                    ~~~~~~~~~~~~~~~~~~
            |      |                    |
            |      |                    (2) calling 'make_boxed_int'
            |
            +--> 'make_boxed_int': events 3-4
                   |
                   |  120 | {
                   |      | ^
                   |      | |
                   |      | (3) entering 'make_boxed_int'
                   |  121 |   boxed_int *result = (boxed_int *)wrapped_malloc (sizeof (boxed_int));
                   |      |                                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
                   |      |                                    |
                   |      |                                    (4) calling 'wrapped_malloc'
                   |
                   +--> 'wrapped_malloc': events 5-6
                          |
                          |    7 | {
                          |      | ^
                          |      | |
                          |      | (5) entering 'wrapped_malloc'
                          |    8 |   return malloc (size);
                          |      |          ~~~~~~~~~~~~~
                          |      |          |
                          |      |          (6) calling 'malloc'
                          |
            <-------------+
            |
         'test': event 7
            |
            |  138 |   free_boxed_int (obj);
            |      |   ^~~~~~~~~~~~~~~~~~~~
            |      |   |
            |      |   (7) calling 'free_boxed_int'
            |
        (etc)

Look at that nesting. Main character formatting energy. 👑

• -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. So if that's not you, no worries bestie, this one's not for us 🫶

• -fno-show-column — do not print column numbers in diagnostics. This may be necessary if diagnostics are being scanned by a program that does not understand the column numbers, such as dejagnu. Careful with this one — it exists for a reason! 💀
• -fdiagnostics-column-unit=UNIT — selects the units for the column number. This affects traditional diagnostics (in the absence of -fno-show-column), as well as JSON format diagnostics if requested.

The default UNIT, display, considers the number of display columns occupied by each character. This may be larger than the number of bytes required to encode the character — in the case of tab characters — or it may be smaller, in the case of multibyte characters. For example, the character "GREEK SMALL LETTER PI (U+03C0)" occupies one display column, and its UTF-8 encoding requires two bytes; the character "SLIGHTLY SMILING FACE (U+1F642)" occupies two display columns, and its UTF-8 encoding requires four bytes. 😊

---

Setting UNIT to byte changes the column number to the raw byte count in all cases, as was traditionally output by GCC prior to version 11.1.0. periodt. 💅

• -fdiagnostics-column-origin=ORIGIN — okay so this picks the origin for column numbers, like which column gets to be column 1 (or whatever). The default is 1, which is the classic GCC behavior and matches the GNU style guide. But some utilities might vibe better with an origin of 0; any non-negative value is allowed. iykyk ✨
• -fdiagnostics-escape-format=FORMAT — When GCC prints the pertinent source lines for a diagnostic, it normally tries to print the source bytes directly. But some diagnostics are about encoding issues in the source file, like malformed UTF-8 or Unicode normalization drama. Those diagnostics are flagged so GCC will escape bytes that aren't printable ASCII when printing the source lines. This option controls how those bytes get escaped.

The default FORMAT, unicode, displays Unicode characters that are not printable ASCII in the form <U+XXXX>, and bytes that don't correspond to a Unicode character validly-encoded in UTF-8 will be shown as hex 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>after

Setting 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 made from text, meant to be viewed in a monospaced font. iconic. 🎨

This option selects which characters get used for printing such diagrams, if any. CHARSET is none, ascii, unicode, or emoji.

The none value suppresses printing of such diagrams. The ascii value makes sure such diagrams are pure ASCII ("ASCII art"). The unicode value allows for conservative use of unicode drawing characters (like 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. slay. 💅

• -fdiagnostics-format=FORMAT — Select a different format for printing diagnostics. FORMAT is text, sarif-stderr, sarif-file, json, json-stderr, or json-file.

The default is text.

The sarif-stderr and sarif-file formats both emit diagnostics in SARIF Version 2.1.0 format, either to stderr or to a file named source.sarif, respectively.

The json format is a synonym for json-stderr. The json-stderr and json-file formats are identical, apart from where the JSON is emitted to — with the former, the JSON goes to stderr, whereas with json-file it's written to source.gcc.json.

The emitted JSON consists of a top-level JSON array containing JSON objects representing the diagnostics.

Diagnostics can have child diagnostics. For example, this error and note:

                 misleading-indentation.c:15:3: warning: this 'if' clause does not
                   guard... [-Wmisleading-indentation]
                    15 |   if (flag)
                       |   ^~
                 misleading-indentation.c:17:5: note: ...this statement, but the latter
                   is misleadingly indented as if it were guarded by the 'if'
                    17 |     y = 2;
                       |     ^

might be printed in JSON form (after formatting) like this:

                 [
                     {
                         "kind": "warning",
                         "locations": [
                             {
                                 "caret": {
                                     "display-column": 3,
                                     "byte-column": 3,
                                     "column": 3,
                                     "file": "misleading-indentation.c",
                                     "line": 15
                                 },
                                 "finish": {
                                     "display-column": 4,
                                     "byte-column": 4,
                                     "column": 4,
                                     "file": "misleading-indentation.c",
                                     "line": 15
                                 }
                             }
                         ],
                         "message": "this \u2018if\u2019 clause does not guard...",
                         "option": "-Wmisleading-indentation",
                         "option_url": "https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-Wmisleading-indentation",
                         "children": [
                             {
                                 "kind": "note",
                                 "locations": [
                                     {
                                         "caret": {
                                             "display-column": 5,
                                             "byte-column": 5,
                                             "column": 5,
                                             "file": "misleading-indentation.c",
                                             "line": 17
                                         }
                                     }
                                 ],
                                 "escape-source": false,
                                 "message": "...this statement, but the latter is ..."
                             }
                         ]
                         "escape-source": false,
                         "column-origin": 1,
                     }
                 ]

where the "note" is a child of the "warning".

A diagnostic has a "kind". If this is "warning", then there's an "option" key describing the command-line option controlling the warning.

A diagnostic can contain zero or more locations. Each location has an optional "label" string and up to three positions within it: a "caret" position and optional "start" and "finish" positions. A position is described by a "file" name, a "line" number, and three numbers indicating a column position:

• "display-column" counts display columns, accounting for tabs and multibyte characters.
• "byte-column" counts raw bytes.
• "column" is equal to one of the previous two, as dictated by the -fdiagnostics-column-unit option.

All three columns are relative to the origin specified by -fdiagnostics-column-origin, which is typically equal to 1 but may be set, for instance, to 0 for compatibility with other utilities that number columns from 0. The column origin is recorded in the JSON output in the "column-origin" tag. In the remaining examples below, the extra column number outputs have been omitted for brevity.

For example, this error:

                 bad-binary-ops.c:64:23: error: invalid operands to binary + (have 'S' {aka
                    'struct s'} and 'T' {aka 'struct t'})
                    64 |   return callee_4a () + callee_4b ();
                       |          ~~~~~~~~~~~~ ^ ~~~~~~~~~~~~
                       |          |              |
                       |          |              T {aka struct t}
                       |          S {aka struct s}

has three locations. Its primary location is at the + token at column 23. It has two secondary locations, describing the left and right-hand sides of the expression, which have labels. It might be printed in JSON form as:

                     {
                         "children": [],
                         "kind": "error",
                         "locations": [
                             {
                                 "caret": {
                                     "column": 23, "file": "bad-binary-ops.c", "line": 64
                                 }
                             },
                             {
                                 "caret": {
                                     "column": 10, "file": "bad-binary-ops.c", "line": 64
                                 },
                                 "finish": {
                                     "column": 21, "file": "bad-binary-ops.c", "line": 64
                                 },
                                 "label": "S {aka struct s}"
                             },
                             {
                                 "caret": {
                                     "column": 25, "file": "bad-binary-ops.c", "line": 64
                                 },
                                 "finish": {
                                     "column": 36, "file": "bad-binary-ops.c", "line": 64
                                 },
                                 "label": "T {aka struct t}"
                             }
                         ],
                         "escape-source": false,
                         "message": "invalid operands to binary + ..."
                     }

If a diagnostic contains fix-it hints, it has a "fixits" array, consisting of half-open intervals, similar to the output of -fdiagnostics-parseable-fixits. For example, this diagnostic with a replacement fix-it hint:

                 demo.c:8:15: error: 'struct s' has no member named 'colour'; did you
                   mean 'color'?
                     8 |   return ptr->colour;
                       |               ^~~~~~
                       |               color

might be printed in JSON form as:

                       {
                           "children": [],
                           "fixits": [
                               {
                                   "next": {
                                       "column": 21,
                                       "file": "demo.c",
                                       "line": 8
                                   },
                                   "start": {
                                       "column": 15,
                                       "file": "demo.c",
                                       "line": 8
                                   },
                                   "string": "color"
                               }
                           ],
                           "kind": "error",
                           "locations": [
                               {
                                   "caret": {
                                       "column": 15,
                                       "file": "demo.c",
                                       "line": 8
                                   },
                                   "finish": {
                                       "column": 20,
                                       "file": "demo.c",
                                       "line": 8
                                   }
                               }
                           ],
                           "escape-source": false,
                           "message": "\u2018struct s\u2019 has no member named ..."
                       }

so bestie, where the fix-it hint tells you to swap the text from start up to but not including next with whatever string is holding. Deletions? That's just an empty value for string. Insertions? That's when start equals next. simple as that 💅

If the diagnostic has a path of control-flow events attached to it (iykyk, the juicy ones), it gets a path array of objects standing for the events. Every event object has a description string, a location object, plus a function string and a depth number for representing interprocedural paths. The function is the current function at that event, and depth is the stack depth relative to some baseline: the higher it is, the more frames are sitting in the stack. 📚

For example, the intraprocedural example shown for -fdiagnostics-path-format= might have this JSON for its path:

                       "path": [
                           {
                               "depth": 0,
                               "description": "when 'PyList_New' fails, returning NULL",
                               "function": "test",
                               "location": {
                                   "column": 10,
                                   "file": "test.c",
                                   "line": 25
                               }
                           },
                           {
                               "depth": 0,
                               "description": "when 'i < count'",
                               "function": "test",
                               "location": {
                                   "column": 3,
                                   "file": "test.c",
                                   "line": 27
                               }
                           },
                           {
                               "depth": 0,
                               "description": "when calling 'PyList_Append', passing NULL from (1) as argument 1",
                               "function": "test",
                               "location": {
                                   "column": 5,
                                   "file": "test.c",
                                   "line": 29
                               }
                           }
                       ]

Diagnostics have a boolean attribute escape-source, and it hints whether non-ASCII bytes should be escaped when printing the pertinent lines of source code (true for diagnostics involving source encoding issues). noted 📝

• -fno-diagnostics-json-formatting — okay so by default, when JSON gets emitted for diagnostics (that's via -fdiagnostics-format=sarif-stderr, -fdiagnostics-format=sarif-file, -fdiagnostics-format=json, -fdiagnostics-format=json-stderr, -fdiagnostics-format=json-file), GCC will add newlines and indentation to visually emphasize the hierarchical structure of the JSON. aesthetic ✨ Use -fno-diagnostics-json-formatting to suppress that whitespace. It must be passed before the option it is to affect. This is intended for compatibility with tools that do not expect the output to contain newlines, such as that emitted by older GCC releases. legacy vibes fr

---

🎀 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. it's giving "girl you can do that but maybe don't" 💀

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. just a vibe check, nothing more
• -fmax-errors=n — Limits the maximum number of error messages to n, at which point GCC bails out rather than attempting to continue processing the source code. If n is 0 (the default), there is no limit on the number of error messages produced. If -Wfatal-errors is also specified, then -Wfatal-errors takes precedence over this option. precedence drama, classic
• -w — Inhibit all warning messages. shhh. silence. 🤫
• -Werror — Make all warnings into errors. glow up gone wrong, every warning is now a crime 💅
• -Werror= — Make the specified warning into an error. The specifier for a warning is appended; for example -Werror=switch turns the warnings controlled by -Wswitch into errors. This switch takes a negative form, to be used to negate -Werror for specific warnings; for example -Wno-error=switch makes -Wswitch warnings not be errors, even when -Werror is in effect. the exceptions era

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. asymmetric bestie behavior, we love to see it

• -Wfatal-errors — This option causes the compiler to abort compilation on the first error occurred rather than trying to keep going and printing further error messages. one strike and we're OUT 😭

You can request many specific warnings with options beginning with -W, for example -Wimplicit to request warnings on implicit declarations. Each of these specific warning options also has a negative form beginning -Wno- to turn off warnings; for example -Wno-implicit. 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; iykyk 👀

Some options, such as -Wall and -Wextra, turn on other options, such as -Wunused, which may turn on further options, such as -Wunused-value. it's giving cascade ✨ 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. sneaky little pragmas

When an unrecognized warning option is requested (e.g., -Wunknown-warning), GCC emits a diagnostic 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. backwards-compat queen behavior fr

The effectiveness of some warnings depends on optimizations also being enabled. For example -Wsuggest-final-types is more effective with link-time optimization and some instances of other warnings may not be issued at all unless optimization is enabled. While optimization in general improves the efficacy of control and data flow sensitive warnings, in some cases it may also cause false positives. optimization said "trust me" and lied a little

• -Wpedantic, -pedantic — Issue all the warnings demanded by strict ISO C and ISO C++; diagnose all programs that use forbidden extensions, and some other programs that do not follow ISO C and ISO C++. This follows the version of the ISO C or C++ standard specified by any -std option used. strictly business 📏

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). no more funny business

-Wpedantic does not cause warning messages for use of the alternate keywords whose names begin and end with __. This alternate format can also be used to disable warnings for non-ISO __intN types, i.e. __intN__. Pedantic warnings are also disabled in the expression that follows "__extension__". However, only system header files should use these escape routes; application programs should avoid them. don't try this at home bestie 🙈

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. mix and match 🎀

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.) logic: slayed 💅

• -pedantic-errors — bestie, this one is strict strict. It throws an actual error whenever the base standard (peep -Wpedantic) demands a diagnostic — in some cases where there's undefined behavior at compile-time, and in other cases that don't stop compilation of programs that are valid per the standard. And no, it's not the same as -Werror=pedantic: that combo is lowkey useless because it only error-ifies the diagnostics controlled by -Wpedantic, whereas this one also hits the required diagnostics that are always on or controlled by totally different options.

So if you want the required-by-default warnings to become errors but you do not want to also switch on the -Wpedantic diagnostics, you can do -pedantic-errors -Wno-pedantic (or -pedantic-errors -Wno-error=pedantic to enable them but keep them as warnings only). 💅

Some required diagnostics are errors by default but can be dialed back to warnings with -fpermissive or their specific warning option, like -Wno-error=narrowing.

Some diagnostics for non-ISO practices are controlled by specific warning options other than -Wpedantic, but -pedantic-errors also makes them errors. For instance:

• -Wattributes (for standard attributes)
• -Wchanges-meaning (C++)
• -Wcomma-subscript (C++23 or later)
• -Wdeclaration-after-statement (C90 or earlier)
• -Welaborated-enum-base (C++11 or later)
• -Wimplicit-int (C99 or later)
• -Wimplicit-function-declaration (C99 or later)
• -Wincompatible-pointer-types
• -Wint-conversion
• -Wlong-long (C90 or earlier)
• -Wmain
• -Wnarrowing (C++11 or later)
• -Wpointer-arith
• -Wpointer-sign
• -Wincompatible-pointer-types
• -Wregister (C++17 or later)
• -Wvla (C90 or earlier)
• -Wwrite-strings (C++11 or later)

• -fpermissive — this one is giving "chill pill" energy 💊. It downgrades some required diagnostics about nonconformant code from errors down to warnings, so using it lets some nonconforming code actually compile. Some C++ diagnostics are controlled only by this flag, but it also downgrades some C and C++ diagnostics that have their own flag:
  • -Wdeclaration-missing-parameter-type (C and Objective-C only)
  • -Wimplicit-function-declaration (C and Objective-C only)
  • -Wimplicit-int (C and Objective-C only)
  • -Wincompatible-pointer-types (C and Objective-C only)
  • -Wint-conversion (C and Objective-C only)
  • -Wnarrowing (C++ and Objective-C++ only)
  • -Wreturn-mismatch (C and Objective-C only)

Heads up: -fpermissive is the default for historic C language modes (-std=c89, -std=gnu89, -std=c90, -std=gnu90).

• -Wall — the queen 👑. This turns on all the warnings about constructions that some users find questionable and that are easy to avoid (or tweak so the warning doesn't fire), even when macros are in the mix. It also enables some language-specific warnings described in C++ Dialect Options and Objective-C and Objective-C++ Dialect Options.

-Wall turns on 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 bestie: some warning flags are not implied by -Wall. Some of them warn about constructions users generally don't consider questionable but you might occasionally wanna check for; others warn about constructions that are necessary or hard to avoid in some cases, and there's no simple way to tweak the code to silence the warning. Some of them are enabled by -Wextra but many of them must be enabled individually.

• -Wextra — the bonus round ✨. This enables some extra warning flags that -Wall doesn't. (Fun fact: this option used to be called -W. The older name still works, but the newer name is way more descriptive.)
  • -Wabsolute-value (only for C/ObjC)
  • -Walloc-size
  • -Wcalloc-transposed-args
  • -Wcast-function-type
  • -Wclobbered
  • -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-type (C/ObjC only)
  • -Wold-style-declaration (C/ObjC only)
  • -Woverride-init (C/ObjC only)
  • -Wredundant-move (C++ and Objective-C++ only)
  • -Wshift-negative-value (in C++11 to C++17 and in C99 and newer)
  • -Wsign-compare (C++ and Objective-C++ only)
  • -Wsized-deallocation (C++ and Objective-C++ only)
  • -Wstring-compare
  • -Wtype-limits
  • -Wuninitialized
  • -Wunused-parameter (only with -Wunused or -Wall)
  • -Wunused-but-set-parameter (only with -Wunused or -Wall)

The option -Wextra also prints warning messages for the following cases:

• A pointer is compared against integer zero with "<", "<=", ">", or ">=".
• (C++ only) An enumerator and a non-enumerator both appear in a conditional expression.
• (C++ only) Ambiguous virtual bases.
• (C++ only) Subscripting an array that has been declared "register".
• (C++ only) Taking the address of a variable that has been declared "register".
• (C++ only) A base class is not initialized in the copy constructor of a derived class.

• -Wabi (C, Objective-C, C++ and Objective-C++ only) — warns about code affected by ABI changes 😭. This includes code that may not be compatible with the vendor-neutral C++ ABI as well as the psABI for the particular target.

Since G++ now defaults to updating the ABI with each major release, normally -Wabi warns only about C++ ABI compatibility problems if there's a check added later in a release series for an ABI issue discovered since the initial release. -Wabi warns about more stuff if an older ABI version is selected (with -fabi-version=n).

-Wabi can also be used with an explicit version number to warn about C++ ABI compatibility with a particular -fabi-version level, e.g. -Wabi=2 to warn about changes relative to -fabi-version=2.

If an explicit version number is provided and -fabi-compat-version is not specified, the version number from this option is used for compatibility aliases. If no explicit version number is provided with this option, but -fabi-compat-version is specified, that version number is used for C++ ABI warnings.

Even though an effort has been made to warn about all such cases, there are probably some cases that aren't warned about, even though G++ is generating incompatible code. There may also be cases where warnings are emitted even though the code that's generated is compatible.

You should rewrite your code to avoid these warnings if you're concerned about the fact that code generated by G++ may not be binary compatible with code generated by other compilers.

Known incompatibilities in -fabi-version=2 (which was the default from GCC 3.4 to 4.9) include:

• A template with a non-type template parameter of reference type was mangled incorrectly:

extern int N;
template <int &> struct S {};
void n (S<N>) {2}

This was fixed in -fabi-version=3.

• SIMD vector types declared using "__attribute ((vector_size))" were mangled in a non-standard way that does not allow for overloading of functions taking vectors of different sizes.

The mangling was changed in -fabi-version=4.

• "__attribute ((const))" and "noreturn" were mangled as type qualifiers, and "decltype" of a plain declaration was folded away.

These mangling issues were fixed in -fabi-version=5.

• Scoped enumerators passed as arguments to a variadic function are promoted like unscoped enumerators, causing "va_arg" to complain. On most targets this does not actually affect the parameter passing ABI, as there's no way to pass an argument smaller than "int".

Also, the ABI changed the mangling of template argument packs, "const_cast", "static_cast", prefix increment/decrement, and a class scope function used as a template argument.

These issues were corrected in -fabi-version=6.

• Lambdas in default argument scope were mangled incorrectly, and the ABI changed the mangling of "nullptr_t".

These issues were corrected in -fabi-version=7.

• the way a function type with function-cv-qualifiers got mangled — the un-qualified function type was lowkey being treated as a substitution candidate when it absolutely should not have been. 🙈

This was fixed in -fabi-version=8, the default for GCC 5.1. ✨

• decltype(nullptr) had an alignment of 1 for no reason, which led to unaligned accesses. 💀 Heads up tho: this did not affect the ABI of a function with a nullptr_t parameter, because parameters have a minimum alignment. So like, breathe.

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.), were not affecting the mangled name, which led to name collisions when function pointers were used as template arguments. Not the vibe.

This was fixed in -fabi-version=10, the default for GCC 6.1. 👑

This option also enables warnings about psABI-related changes. The known psABI changes at this point include:

• For SysV/x86-64, unions with long double members are passed in memory as specified in psABI. Prior to GCC 4.4, this was not the case. For example:

union U {
  long double ld;
  int i;
};

union U is now always passed in memory. 🧠

---

• -Wno-changes-meaning (C++ and Objective-C++ only) — C++ requires that unqualified uses of a name within a class have the same meaning in the complete scope of the class, so declaring the name after using it is ill-formed:

struct A;
struct B1 { A a; typedef A A; }; // warning, 'A' changes meaning
struct B2 { A a; struct A { }; }; // error, 'A' changes meaning

By default, the B1 case is only a warning because the two declarations have the same type, while the B2 case is an error. Both diagnostics can be disabled with -Wno-changes-meaning. Alternately, the error case can be reduced to a warning with -Wno-error=changes-meaning or -fpermissive. 🫶

Both diagnostics are also suppressed by -fms-extensions.

• -Wchar-subscripts — Warn if an array subscript has type char. This is a common cause of error, as programmers often forget that this type is signed on some machines. 😭 This warning is enabled by -Wall.
• -Wno-coverage-mismatch — Warn if feedback profiles do not match when using the -fprofile-use option. If a source file is changed between compiling with -fprofile-generate and with -fprofile-use, the files with the profile feedback can fail to match the source file and GCC cannot use the profile feedback information. By default, this warning is enabled and is treated as an error. -Wno-coverage-mismatch can be used to disable the warning or -Wno-error=coverage-mismatch can be used to disable the error. Disabling the error for this warning can result in poorly optimized code and is useful only in the case of very minor changes such as bug fixes to an existing code-base. Completely disabling the warning is not recommended. 🚫
• -Wno-coverage-too-many-conditions — Warn if -fcondition-coverage is used and an expression have too many terms and GCC gives up coverage. Coverage is given up when there are more terms in the conditional than there are bits in a gcov_type_unsigned. This warning is enabled by default. 🍓
• -Wno-coverage-invalid-line-number — Warn in case a function ends earlier than it begins due to an invalid linenum macros. The warning is emitted only with --coverage enabled.

By default, this warning is enabled and is treated as an error. -Wno-coverage-invalid-line-number can be used to disable the warning or -Wno-error=coverage-invalid-line-number can be used to disable the error. 💅

• -Wno-cpp (C, Objective-C, C++, Objective-C++ and Fortran only) — Suppress warning messages emitted by #warning directives. 🤫
• -Wdouble-promotion (C, C++, Objective-C and Objective-C++ only) — Give a warning when a value of type float is implicitly promoted to double. CPUs with a 32-bit "single-precision" floating-point unit implement float in hardware, but emulate double in software. On such a machine, doing computations using double values is much more expensive because of the overhead required for software emulation. 💸

It is easy to accidentally do computations with double because floating-point literals are implicitly of type double. For example, in:

float area(float radius)
{
   return 3.14159 * radius * radius;
}

the compiler performs the entire computation with double because the floating-point literal is a double. 🤯

• -Wduplicate-decl-specifier (C and Objective-C only) — Warn if a declaration has duplicate const, volatile, restrict or _Atomic specifier. This warning is enabled by -Wall. 🌸
• -Wformat
• -Wformat=n — Check calls to printf and scanf, etc., to make sure that the arguments supplied have types appropriate to the format string specified, and that the conversions specified in the format string make sense. This includes standard functions, and others specified by format attributes, in the printf, scanf, strftime and strfmon (an X/Open extension, not in the C standard) families (or other target-specific families). Which functions are checked without format attributes having been specified depends on the standard version selected, and such checks of functions without the attribute specified are disabled by -ffreestanding or -fno-builtin. 🔍

The formats are checked against the format features supported by GNU libc version 2.2. These include all ISO C90 and C99 features, as well as features from the Single Unix Specification and some BSD and GNU extensions. Other library implementations may not support all these features; GCC does not support warning about features that go beyond a particular library's limitations. However, if -Wpedantic is used with -Wformat, warnings are given about format features not in the selected standard version (but not for strfmon formats, since those are not in any version of the C standard).

• -Wformat=1
• -Wformat — Option -Wformat is equivalent to -Wformat=1, and -Wno-format is equivalent to -Wformat=0. Since -Wformat also checks for null format arguments for several functions, -Wformat also implies -Wnonnull. Some aspects of this level of format checking can be disabled by the options: -Wno-format-contains-nul, -Wno-format-extra-args, and -Wno-format-zero-length. -Wformat is enabled by -Wall. ✨
• -Wformat=2 — Enable -Wformat plus additional format checks. Currently equivalent to -Wformat -Wformat-nonliteral -Wformat-security -Wformat-y2k. 🔥

• -Wno-format-contains-nul — If -Wformat is specified, do not warn about format strings that contain NUL bytes. 🫥
• -Wno-format-extra-args — If -Wformat is specified, do not warn about excess arguments to a printf or scanf format function. The C standard specifies that such arguments are ignored. 🗑️

Where the unused arguments lie between used arguments that are specified with $ operand number specifications, normally warnings are still given, since the implementation could not know what type to pass to va_arg to skip the unused arguments. However, in the case of scanf formats, this option suppresses the warning if the unused arguments are all pointers, since the Single Unix Specification says that such unused arguments are allowed. 👀

• -Wformat-overflow
• -Wformat-overflow=level — Warn about calls to formatted input/output functions such as sprintf and vsprintf that might overflow the destination buffer. When the exact number of bytes written by a format directive cannot be determined at compile-time it is estimated based on heuristics that depend on the level argument and on optimization. While enabling optimization will in most cases improve the accuracy of the warning, it may also result in false positives. ⚠️
  • -Wformat-overflow
  • -Wformat-overflow=1 — Level 1 of -Wformat-overflow enabled by -Wformat employs a conservative approach that warns only about calls that most likely overflow the buffer. At this level, numeric arguments to format directives with unknown values are assumed to have the value of one, and strings of unknown length to be empty. Numeric arguments that are known to be bounded to a subrange of their type, or string arguments whose output is bounded either by their directive's precision or by a finite set of string literals, are assumed to take on the value within the range that results in the most bytes on output. For example, the call to sprintf below is diagnosed because even with both a and b equal to zero, the terminating NUL character ('\0') appended by the function to the destination buffer will be written past its end. Increasing the size of the buffer by a single byte is sufficient to avoid the warning, though it may not be sufficient to avoid the overflow. 😬

void f (int a, int b)
{
  char buf [13];
  sprintf (buf, "a = %i, b = %i\n", a, b);
}

• -Wformat-overflow=2 — Level 2 warns also about calls that might overflow the destination buffer given an argument of sufficient length or magnitude. At level 2, unknown numeric arguments are assumed to have the minimum representable value for signed types with a precision greater than 1, and the maximum representable value otherwise. Unknown string arguments whose length cannot be assumed to be bounded either by the directive's precision, or by a finite set of string literals they may evaluate to, or the character array they may point to, are assumed to be 1 character long. 💀

Okay bestie, level 2 said "I'm not done with you" 😭 At level 2, that call in the example above gets diagnosed again, but this time it's because when a equals a 32-bit INT_MIN, the first %i directive will write some of its digits past the end of the destination buffer. To make that call safe no matter what values those two variables hold, you have to bump the destination buffer up to at least 34 bytes. And GCC is a real one here — it drops the minimum size of the buffer right into an informational note following the warning. 🫶

If you don't wanna resize the buffer, you can instead constrain the range of the formatted values. ✨ The maximum length of string arguments can be bounded by specifying the precision in the format directive. And when numeric arguments of format directives can be assumed to be bounded by less than the precision of their type, picking 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 size of the buffer down to 24 bytes. 💅

void f (int a, int b)
{
  char buf [23];
  sprintf (buf, "a = %hi, b = %i\n", a, (short)b);
}

• -Wno-format-zero-length — if -Wformat is specified, don't warn about zero-length formats. The C standard says zero-length formats are allowed, so like, chill. 🌸
• -Wformat-nonliteral — if -Wformat is specified, also warn when the format string is not a string literal and therefore cannot be checked — unless the format function takes its format arguments as a va_list. 👑
• -Wformat-security — if -Wformat is specified, also warn about uses of format functions that represent possible security problems. Right now it warns about calls to printf and scanf functions where the format string is not a string literal and there are no format arguments, as in printf (foo);. This can be a security hole if the format string came from untrusted input and contains %n. (This is currently a subset of what -Wformat-nonliteral warns about, but bestie, in future, warnings may be added to -Wformat-security that are not included in -Wformat-nonliteral.) 🚨
• -Wformat-signedness — if -Wformat is specified, also warn if the format string requires an unsigned argument and the argument is signed, and vice versa. Mixed signals? Not on our watch. 🙈
• -Wformat-truncation, -Wformat-truncation=level — warn about calls to formatted input/output functions such as snprintf and vsnprintf that might result in output truncation. When the exact number of bytes written by a format directive can't be determined at compile-time, it gets estimated based on heuristics that depend on the level argument and on optimization. Enabling optimization will in most cases improve the accuracy of the warning, but it may also result in false positives — it's giving "trust me but also don't" 😭. Except as noted otherwise, the option uses the same logic as -Wformat-overflow.
  • -Wformat-truncation, -Wformat-truncation=1 — level 1 of -Wformat-truncation, enabled by -Wformat, employs a conservative approach that warns only about calls to bounded functions whose return value is unused and that will most likely result in output truncation.
  • -Wformat-truncation=2 — level 2 warns also about calls to bounded functions whose return value is used and that might result in truncation given an argument of sufficient length or magnitude.
• -Wformat-y2k — if -Wformat is specified, also warn about strftime formats that may yield only a two-digit year. it's giving Y2K era nostalgia but make it a bug ⭐
• -Wnonnull — warn about passing a null pointer for arguments marked as requiring a non-null value by the nonnull function attribute.

-Wnonnull is included in -Wall and -Wformat. It can be disabled with the -Wno-nonnull option.

• -Wnonnull-compare — warn when comparing an argument marked with the nonnull function attribute against null inside the function.

-Wnonnull-compare is included in -Wall. It can be disabled with the -Wno-nonnull-compare option.

• -Wnull-dereference — warn if the compiler detects paths that trigger erroneous or undefined behavior due to dereferencing a null pointer. This option is only active when -fdelete-null-pointer-checks is active, which is enabled by optimizations in most targets. The precision of the warnings depends on the optimization options used. 🎯
• -Wnrvo (C++ and Objective-C++ only) — warn if the compiler does not elide the copy from a local variable to the return value of a function in a context where it is allowed by [class.copy.elision]. This elision is commonly known as the Named Return Value Optimization. For instance, in the example below the compiler can't elide copies from both v1 and v2, so it elides neither. 💀

std::vector<int> f()
{
  std::vector<int> v1, v2;
  // ...
  if (cond) return v1;
  else return v2; // warning: not eliding copy
}

• -Winfinite-recursion — warn about infinitely recursive calls. The warning is effective at all optimization levels but requires optimization in order to detect infinite recursion in calls between two or more functions. -Winfinite-recursion is included in -Wall.

Compare with -Wanalyzer-infinite-recursion, which provides a similar diagnostic but is implemented in a different way (as part of -fanalyzer). Two icons, same energy. 👑

• -Winit-self (C, C++, Objective-C and Objective-C++ only) — warn about uninitialized variables that are initialized with themselves. Note this option can only be used with the -Wuninitialized option.

For example, GCC warns about i being uninitialized in the following snippet only when -Winit-self has been specified:

int f()
{
  int i = i;
  return i;
}

This warning is enabled by -Wall in C++.

• -Wno-implicit-int (C and Objective-C only) — this option controls warnings when a declaration does not specify a type. This warning is enabled by default, as an error, in C99 and later dialects of C, and also by -Wall. The error can be downgraded to a warning using -fpermissive (along with certain other errors), or for this error alone, with -Wno-error=implicit-int.

This warning is upgraded to an error by -pedantic-errors.

• -Wno-implicit-function-declaration (C and Objective-C only) — this option controls warnings when a function is used before being declared. This warning is enabled by default, as an error, in C99 and later dialects of C, and also by -Wall. The error can be downgraded to a warning using -fpermissive (along with certain other errors), or for this error alone, with -Wno-error=implicit-function-declaration.

This warning is upgraded to an error by -pedantic-errors.

• -Wimplicit (C and Objective-C only) — same as -Wimplicit-int and -Wimplicit-function-declaration. This warning is enabled by -Wall.
• -Whardened — warn when -fhardened did not enable an option from its set (for which see -fhardened). For instance, using -fhardened and -fstack-protector at the same time on the command line causes -Whardened to warn because -fstack-protector-strong is not enabled by -fhardened. 🙈

This warning is enabled by default and has effect only when -fhardened is enabled.

• -Wimplicit-fallthrough — -Wimplicit-fallthrough is the same as -Wimplicit-fallthrough=3 and -Wno-implicit-fallthrough is the same as -Wimplicit-fallthrough=0.
• -Wimplicit-fallthrough=n — warn when a switch case falls through. For example:

switch (cond)
  {
  case 1:
    a = 1;
    break;
  case 2:
    a = 2;
  case 3:
    a = 3;
    break;
  }

This warning does not warn when the last statement of a case cannot fall through, e.g. when there is a return statement or a call to a function declared with the noreturn attribute. -Wimplicit-fallthrough= also takes into account control flow statements, such as ifs, and only warns when appropriate. E.g.

switch (cond)
  {
  case 1:
    if (i > 3) {
      bar (5);
      break;
    } else if (i < 1) {
      bar (0);
    } else
      return;
  default:
    ...
  }

Since there are occasions where a switch case fall through is desirable, GCC provides an attribute, __attribute__ ((fallthrough)), that is to be used along with a null statement to suppress this warning that would normally occur:

switch (cond)
  {
  case 1:
    bar (0);
    __attribute__ ((fallthrough));
  default:
    ...
  }

C++17 provides a standard way to suppress the -Wimplicit-fallthrough warning using [[fallthrough]]; instead of the GNU attribute. In C++11 or C++14 users can use [[gnu::fallthrough]];, which is a GNU extension. Instead of these attributes, it is also possible to add a fallthrough comment to silence the warning. The whole body of the C or C++ style comment should match the given regular expressions listed below. The option argument n specifies what kind of comments are accepted:

• -Wimplicit-fallthrough=0 disables the warning altogether.
• -Wimplicit-fallthrough=1 matches .* regular expression, any comment is used as fallthrough comment.
• -Wimplicit-fallthrough=2 case insensitively matches .*falls?[ \t-]*thr(ough|u).* regular expression.
• -Wimplicit-fallthrough=3 case sensitively matches one of the following regular expressions:

• -Wimplicit-fallthrough=3 — case insensitively matches one of these regexes:

*<"-fallthrough">
*<"@fallthrough@">
*<"lint -fallthrough[ \t]*">
*<"[ \t.!]*(ELSE,? |INTENTIONAL(LY)? )?FALL(S | |-)?THR(OUGH|U)[
\t.!]*(-[^\n\r]*)?">
*<"[ \t.!]*(Else,? |Intentional(ly)? )?Fall((s |
|-)[Tt]|t)hr(ough|u)[ \t.!]*(-[^\n\r]*)?">
*<"[ \t.!]*([Ee]lse,? |[Ii]ntentional(ly)? )?fall(s |
|-)?thr(ough|u)[ \t.!]*(-[^\n\r]*)?">

• -Wimplicit-fallthrough=4 — case sensitively matches one of these regexes:

*<"-fallthrough">
*<"@fallthrough@">
*<"lint -fallthrough[ \t]*">
*<"[ \t]*FALLTHR(OUGH|U)[ \t]*">

• -Wimplicit-fallthrough=5 — doesn't recognize any comments as fallthrough comments, only attributes disable the warning.

The comment needs to be followed after optional whitespace and other comments by case or default keywords, or by a user label that precedes some case or default label.

switch (cond)
  {
  case 1:
    bar (0);
    /* FALLTHRU */
  default:
    ...
  }

The -Wimplicit-fallthrough=3 warning is enabled by -Wextra.

• -Wno-if-not-aligned (C, C++, Objective-C and Objective-C++ only) — controls whether warnings triggered by the warn_if_not_aligned attribute get issued. Bestie these warnings are enabled by default, so if you don't want the tea you gotta opt out 💅
• -Wignored-qualifiers (C and C++ only) — warns if the return type of a function has a type qualifier like const. For ISO C such a type qualifier has no effect, because the value returned by a function is not an lvalue. For C++, the warning is only emitted for scalar types or void. ISO C prohibits qualified void return types on function definitions, so those return types always get a warning even without this option. Ngl this one's also enabled by -Wextra.
• -Wno-ignored-attributes (C and C++ only) — this option controls warnings when an attribute is ignored. It's different from the -Wattributes option because it warns whenever the compiler decides to drop an attribute, not that the attribute is unknown, used in the wrong place, etc. Enabled by default, periodt.
• -Wmain — warns if the type of main is suspicious. main should be a function with external linkage, returning int, taking either zero arguments, two, or three arguments of appropriate types. This warning is enabled by default in C++ and is enabled by either -Wall or -Wpedantic.

This warning is upgraded to an error by -pedantic-errors. so like, no more "eh whatever" energy, it's giving full send 🔥

• -Wmisleading-indentation (C and C++ only) — warns when the indentation of the code does not reflect the block structure. Specifically, a warning is issued for if, else, while, and for clauses with a guarded statement that does not use braces, followed by an unguarded statement with the same indentation.

In the following example, the call to bar is misleadingly indented as if it were guarded by the if conditional. 💀

      if (some_condition ())
        foo ();
        bar ();  /* Gotcha: this is not guarded by the "if".  */

In the case of mixed tabs and spaces, the warning uses the -ftabstop= option to determine if the statements line up (defaulting to 8).

The warning is not issued for code involving multiline preprocessor logic such as the following example.

      if (flagA)
        foo (0);
#if SOME_CONDITION_THAT_DOES_NOT_HOLD
      if (flagB)
#endif
        foo (1);

The warning is not issued after a #line directive, since this typically indicates autogenerated code, and no assumptions can be made about the layout of the file that the directive references.

This warning is enabled by -Wall in C and C++.

• -Wmissing-attributes — warns when a declaration of a function is missing one or more attributes that a related function is declared with, and whose absence may adversely affect the correctness or efficiency of generated code. For example, the warning is issued for declarations of aliases that use attributes to specify less restrictive requirements than those of their targets. This typically represents a potential optimization opportunity. By contrast, the -Wattribute-alias=2 option controls warnings issued when the alias is more restrictive than the target, which could lead to incorrect code generation. Attributes considered include alloc_align, alloc_size, cold, const, hot, leaf, malloc, nonnull, noreturn, nothrow, pure, returns_nonnull, and returns_twice.

In C++, the warning is issued when an explicit specialization of a primary template declared with attribute alloc_align, alloc_size, assume_aligned, format, format_arg, malloc, or nonnull is declared without it. Attributes deprecated, error, and warning suppress the warning..

You can use the copy attribute to apply the same set of attributes to a declaration as that on another declaration without explicitly enumerating the attributes. This attribute can be applied to declarations of functions, variables, or types.

-Wmissing-attributes is enabled by -Wall.

For example, since the declaration of the primary function template below makes use of both attribute malloc and alloc_size the declaration of the explicit specialization of the template is diagnosed because it is missing one of the attributes.

template <class T>
T* __attribute__ ((malloc, alloc_size (1)))
allocate (size_t);

template <>
void* __attribute__ ((malloc))   // missing alloc_size
allocate<void> (size_t);

• -Wmissing-braces — warns if an aggregate or union initializer is not fully bracketed. In the following example, the initializer for a is not fully bracketed, but that for b is fully bracketed.

int a[2][2] = { 0, 1, 2, 3 };
int b[2][2] = { { 0, 1 }, { 2, 3 } };

This warning is enabled by -Wall.

• -Wmissing-include-dirs (C, C++, Objective-C, Objective-C++ and Fortran only) — warns if a user-supplied include directory does not exist. This option is disabled by default for C, C++, Objective-C and Objective-C++. For Fortran, it is partially enabled by default by warning for -I and -J, only. ikyk the vibe, it's just looking out for you 🫶
• -Wno-missing-profile — this option controls warnings if feedback profiles are missing when using the -fprofile-use option. This option diagnoses those cases where a new function or a new file is added between compiling with -fprofile-generate and with -fprofile-use, without regenerating the profiles. In these cases, the profile feedback data files do not contain any profile feedback information for the newly added function or file respectively. Also, in the case when profile count data (.gcda) files are removed, GCC cannot use any profile feedback information. In all these cases, warnings are issued to inform you that a profile generation step is due. Ignoring the warning can result in poorly optimized code. -Wno-missing-profile can be used to disable the warning, but this is not recommended and should be done only when non-existent profile data is justified. like bestie, silencing it is a choice, just not a good one 😭
• -Wmismatched-dealloc — warns for calls to deallocation functions with pointer arguments returned from allocation functions for which the former isn't a suitable deallocator. A pair of functions can be associated as matching allocators and deallocators by use of attribute malloc. Unless disabled by the -fno-builtin option the standard functions calloc, malloc, realloc, and free, as well as the corresponding forms of C++ operator new and operator delete are implicitly associated as matching allocators and deallocators. In the following example mydealloc is the deallocator for pointers returned from myalloc.

void mydealloc (void*);

__attribute__ ((malloc (mydealloc, 1))) void*
myalloc (size_t);

void f (void)
{
  void *p = myalloc (32);
  // ...use p...
  free (p);   // warning: not a matching deallocator for myalloc
  mydealloc (p);   // ok
}

In C++, the related option -Wmismatched-new-delete diagnoses mismatches involving either operator new or operator delete.

Option -Wmismatched-dealloc is included in -Wall.

• -Wmultistatement-macros — warns about unsafe multiple statement macros that appear to be guarded by a clause such as if, else, for, switch, or while, in which only the first statement is actually guarded after the macro is expanded.

For example:

#define DOIT x++; y++
if (c)
  DOIT;

will increment y unconditionally, not just when c holds. The can usually be fixed by wrapping the macro in a do-while loop:

#define DOIT do { x++; y++; } while (0)
if (c)
  DOIT;

This warning is enabled by -Wall in C and C++.

• -Wparentheses — warns 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. 💅 classic

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. math girlies stay losing to C semantics fr fr 😭

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 <span>(C++ and Objective-C++ only)</span> — bestie, this one yells at you when a value gets moved into itself using std::move. And like… that std::move basically does absolutely nothing, it's giving "I did something" while doing nothing 💀. Watch:

struct T {
...
};
void fn()
{
  T t;
  ...
  t = std::move (t);
}

This warning is enabled by -Wall.

• -Wsequence-point — warns about code that might have <span>undefined semantics</span> because it breaks the sequence point rules in the C and C++ standards. Okay so the whole tea here: C and C++ define the order in which expressions get evaluated using sequence points, which are basically a partial ordering between parts of the program — the bits that run before the sequence point, and the bits that run after it. These pop up after a full expression is evaluated (a full expression = one that isn't part of a bigger expression), after the first operand of a &&, ||, ? : or , (comma) operator is evaluated, before a function is called (but after its arguments and the expression naming the called function get evaluated), and in certain other spots. Other than what the sequence point rules say, the order of evaluation of subexpressions is not specified. All these rules describe only a partial order, not a total one — like, if two functions are called in one expression with no sequence point between them, the order they get called in is not specified. But the standards committee did rule that function calls don't overlap, periodt.

It's also not specified when, between sequence points, modifications to object values actually take effect. Programs that depend on this have undefined behavior; the C and C++ standards say: "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 don't get diagnosed by this option, and it may occasionally give a false positive, but generally it's been found pretty effective at catching this kind of problem.

The C++17 standard defines the order of evaluation of operands in more cases: specifically it requires that the right-hand side of an assignment be evaluated before the left-hand side, so the examples above are no longer undefined. But this option will still warn about them, to help people avoid writing code that's undefined in C and earlier revisions of C++.

The standard is worded confusingly, so there's some debate over the precise meaning of the sequence point rules in subtle cases. Links to discussions of the problem, including proposed formal definitions, can 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 — don't warn about returning a pointer (or in C++, a reference) to a variable that goes out of scope after the function returns 🙈.
• -Wreturn-mismatch — warns about return statements without an expression in functions that don't return void. Also warns about a return statement with an expression in a function whose return type is void, unless the expression type is also void. As a GNU extension, that latter case is accepted without a warning unless -Wpedantic is used.

Trying to use the return value of a non-void function other than main that flows off the end by reaching the closing curly brace that terminates the function is undefined.

This warning is specific to C and enabled by default. In C99 and later language dialects, it's treated as an error. It can be downgraded to a warning using -fpermissive (along with other warnings), or for just this warning, with -Wno-error=return-mismatch.

• -Wreturn-type — warns whenever a function is defined with a return type that defaults to int (unless -Wimplicit-int is active, which takes precedence 👑). Also warns if execution may reach the end of the function body, or if the function doesn't contain any return statement at all.

Attempting to use the return value of a non-void function other than main that flows off the end by reaching the closing curly brace that terminates the function is undefined.

Unlike in C, in C++, flowing off the end of a non-void function other than main results in undefined behavior even when the value of the function is not used. Yeah. Not great.

This warning is enabled by default in C++ and by -Wall otherwise.

• -Wno-shift-count-negative — controls warnings if a shift count is negative. This warning is enabled by default.
• -Wno-shift-count-overflow — controls warnings if a shift count is greater than or equal to the bit width of the type. This warning is enabled by default.
• -Wshift-negative-value — warns if you left shift a negative value. This warning is enabled by -Wextra in C99 (and newer) and C++11 to C++17 modes.
• -Wno-shift-overflow, -Wshift-overflow=n — these options control warnings about left shift overflows ✨.
  • -Wshift-overflow=1 — this is the warning level of -Wshift-overflow and is enabled by default in C99 and C++11 modes (and newer). This warning level does not warn about left-shifting 1 into the sign bit. (However, in C, such an overflow is still rejected in contexts where an integer constant expression is required.) No warning is emitted in C++20 mode (and newer), since signed left shifts always wrap.
  • -Wshift-overflow=2 — this warning level also warns about left-shifting 1 into the sign bit, unless C++14 mode (or newer) is active.
• -Wswitch — warns whenever a switch statement has an index of enumerated type and lacks a case for one or more of the named codes of that enumeration. (The presence of a default label prevents this warning.) case labels outside the enumeration range also provoke warnings when this option is used (even if there's a default label). This warning is enabled by -Wall.
• -Wswitch-default — warns whenever a switch statement doesn't have a default case.
• -Wswitch-enum — warns whenever a switch statement has an index of enumerated type and lacks a case for one or more of the named codes of that enumeration. case labels outside the enumeration range also provoke warnings when this option is used. The only difference between -Wswitch and this option is that this option gives a warning about an omitted enumeration code even if there's a default label 👀.
• -Wno-switch-bool — don't warn when a switch statement 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 than bool. Like:

switch ((int) (a == 4))
  {
  ...
  }

This warning is enabled by default for C and C++ programs.

• -Wno-switch-outside-range — this option controls warnings when a switch case has a value that's outside of its respective type range. This warning is enabled by default for C and C++ programs.
• -Wno-switch-unreachable — don't warn when a switch statement contains statements between the controlling expression and the first case label, which will never be executed. For example:

switch (cond)
  {
   i = 15;
  ...
   case 5:
  ...
  }

-Wswitch-unreachable does not warn if the statement between the controlling expression and the first case label is just a declaration:

switch (cond)
  {
   int i;
  ...
   case 5:
   i = 5;
  ...
  }

This warning is enabled by default for C and C++ programs.

• -Wsync-nand <span>(C and C++ only)</span> — warns when the __sync_fetch_and_nand and __sync_nand_and_fetch built-in functions are used. These functions changed semantics in GCC 4.4 👑.
• -Wtrivial-auto-var-init — warns when -ftrivial-auto-var-init can't initialize the automatic variable. A common situation is an automatic variable declared between the controlling expression and the first case label of a switch statement.
• -Wunused-but-set-parameter — warns whenever a function parameter is assigned to, but otherwise unused (aside from its declaration). To suppress this warning use the unused attribute. This warning is also enabled by -Wunused together with -Wextra.
• -Wunused-but-set-variable — warns whenever a local variable is assigned to, but otherwise unused (aside from its declaration). This warning is enabled by -Wall. To suppress this warning use the unused attribute. This warning is also enabled by -Wunused, which is enabled by -Wall.
• -Wunused-function — warns whenever a static function is declared but not defined, or a non-inline static function is unused. This warning is enabled by -Wall.
• -Wunused-label — warns whenever a label is declared but not used. This warning is enabled by -Wall. To suppress this warning use the unused attribute.
• -Wunused-local-typedefs <span>(C, Objective-C, C++ and Objective-C++ only)</span> — warns when a typedef locally defined in a function is not used. This warning is enabled by -Wall.
• -Wunused-parameter — warns whenever a function parameter is unused aside from its declaration. This option is not enabled by -Wunused unless -Wextra is also specified. To suppress this warning use the unused attribute.
• -Wno-unused-result — don't warn if a caller of a function marked with the warn_unused_result attribute doesn't use its return value. The default is -Wunused-result.
• -Wunused-variable — bestie, this one yells at you whenever a local or static variable is just... sitting there unused, aside from the declaration itself. This option implies -Wunused-const-variable=1 for C, but not for C++. And it's enabled by -Wall. To suppress this warning, use the "unused" attribute. 💅
• -Wunused-const-variable, -Wunused-const-variable=n — warn whenever a constant static variable is unused aside from its declaration. To suppress this warning, use the "unused" attribute.
  • -Wunused-const-variable=1 — warns about unused static const variables defined in the main compilation unit, but not about static const variables declared in any header that got included. -Wunused-const-variable=1 is enabled by either -Wunused-variable or -Wunused for C, but not for C++. In C this declares variable storage, but in C++ this is not an error, since const variables take the place of "#define"s.
  • -Wunused-const-variable=2 — this warning level also warns for unused constant static variables in headers (excluding system headers). It is equivalent to the short form -Wunused-const-variable. This level must be explicitly requested in both C and C++, because it might be hard to clean up all headers included. 😭
• -Wunused-value — warn whenever a statement computes a result that is explicitly not used. To suppress this warning, cast the unused expression to "void". This includes an expression-statement, or the left-hand side of a comma expression, that contains no side effects. For example, an expression such as "x[i,j]" causes a warning, while "x[(void)i,j]" does not. This warning is enabled by -Wall.
• -Wunused — all the above -Wunused options combined, except those documented as needing to be specified explicitly. 🫶

In order to get a warning about an unused function parameter, you must either specify -Wextra -Wunused (note that -Wall implies -Wunused), or separately specify -Wunused-parameter and/or -Wunused-but-set-parameter.

-Wunused enables only -Wunused-const-variable=1 rather than -Wunused-const-variable, and only for C, not C++.

• -Wuse-after-free (C, Objective-C, C++ and Objective-C++ only), -Wuse-after-free=n — warns about uses of pointers to dynamically allocated objects that have been rendered indeterminate by a call to a deallocation function. The warning is enabled at all optimization levels but may yield different results with optimization than without. ⚠️
  • -Wuse-after-free=1 — at level 1 the warning attempts to diagnose only unconditional uses of pointers made indeterminate by a deallocation call, or by a successful call to "realloc", regardless of whether or not the call resulted in an actual reallocation of memory. This includes double-"free" calls as well as uses in arithmetic and relational expressions. Although undefined, uses of indeterminate pointers in equality (or inequality) expressions are not diagnosed at this level.
  • -Wuse-after-free=2 — at level 2, in addition to unconditional uses, the warning also diagnoses conditional uses of pointers made indeterminate by a deallocation call. As at level 2, uses in equality (or inequality) expressions are not diagnosed. For example, the second call to "free" in the following function is diagnosed at this level:

struct A { int refcount; void *data; };

void release (struct A *p)
{
  int refcount = --p->refcount;
  free (p);
  if (refcount == 0)
    free (p->data);   // warning: p may be used after free
}

• -Wuse-after-free=3 — at level 3, the warning also diagnoses uses of indeterminate pointers in equality expressions. All uses of indeterminate pointers are undefined, but equality tests sometimes appear after calls to "realloc" as an attempt to determine whether the call resulted in relocating the object to a different address. They are diagnosed at a separate level to aid gradually transitioning legacy code to safe alternatives. For example, the equality test in the function below is diagnosed at this level:

void adjust_pointers (int**, int);

void grow (int **p, int n)
{
  int **q = (int**)realloc (p, n *= 2);
  if (q == p)
    return;
  adjust_pointers ((int**)q, n);
}

To avoid the warning at this level, store offsets into allocated memory instead of pointers. This approach obviates needing to adjust the stored pointers after reallocation.

-Wuse-after-free=2 is included in -Wall.

• -Wuseless-cast (C, Objective-C, C++ and Objective-C++ only) — warn when an expression is cast to its own type. This warning does not occur when a class object is converted to a non-reference type, as that is a way to create a temporary:

struct S { };
void g (S&&);
void f (S&& arg)
{
  g (S(arg)); // make arg prvalue so that it can bind to S&&
}

• -Wuninitialized — warn if an object with automatic or allocated storage duration is used without having been initialized. In C++, also warn if a non-static reference or non-static "const" member appears in a class without constructors. 👑

In addition, passing a pointer (or in C++, a reference) to an uninitialized object to a "const"-qualified argument of a built-in function known to read the object is also diagnosed by this warning. (-Wmaybe-uninitialized is issued for ordinary functions.)

If you want to warn about code that uses the uninitialized value of the variable in its own initializer, use the -Winit-self option.

These warnings occur for individual uninitialized elements of structure, union or array variables as well as for variables that are uninitialized as a whole. They do not occur for variables or elements declared "volatile". Because these warnings depend on optimization, the exact variables or elements for which there are warnings depend on the precise optimization options and version of GCC used.

Note that there may be no warning about a variable that is used only to compute a value that itself is never used, because such computations may be deleted by data flow analysis before the warnings are printed.

In C++, this warning also warns about using uninitialized objects in member-initializer-lists. For example, GCC warns about "b" being uninitialized in the following snippet:

struct A {
  int a;
  int b;
  A() : a(b) { }
};

• -Wno-invalid-memory-model — this option controls warnings for invocations of __atomic Builtins, __sync Builtins, and the C11 atomic generic functions with a memory consistency argument that is either invalid for the operation or outside the range of values of the "memory_order" enumeration. For example, since the "__atomic_store" and "__atomic_store_n" built-ins are only defined for the relaxed, release, and sequentially consistent memory orders, the following code is diagnosed:

void store (int *i)
{
  __atomic_store_n (i, 0, memory_order_consume);
}

-Winvalid-memory-model is enabled by default.

• -Wmaybe-uninitialized — for an object with automatic or allocated storage duration, if there exists a path from the function entry to a use of the object that is initialized, but there exist some other paths for which the object is not initialized, the compiler emits a warning if it cannot prove the uninitialized paths are not executed at run time.

In addition, passing a pointer (or in C++, a reference) to an uninitialized object to a "const"-qualified function argument is also diagnosed by this warning. (-Wuninitialized is issued for built-in functions known to read the object.) Annotating the function with attribute "access (none)" indicates that the argument isn't used to access the object and avoids the warning.

These warnings are only possible in optimizing compilation, because otherwise GCC does not keep track of the state of variables.

These warnings are made optional because GCC may not be able to determine when the code is correct in spite of appearing to have an error. Here is one example of how this can happen:

{
  int x;
  switch (y)
    {
    case 1: x = 1;
      break;
    case 2: x = 4;
      break;
    case 3: x = 5;
    }
  foo (x);
}

If the value of "y" is always 1, 2 or 3, then "x" is always initialized, but GCC doesn't know this. To suppress the warning, you need to provide a default case with assert(0) or similar code. 💀

This option also warns when a non-volatile automatic variable might be changed by a call to "longjmp". The compiler sees only the calls to "setjmp". It cannot know where "longjmp" will be called; in fact, a signal handler could call it at any point in the code. As a result, you may get a warning even when there is in fact no problem, because "longjmp" cannot in fact be called at the place that would cause a problem.

Some spurious warnings can be avoided if you declare all the functions you use that never return as "noreturn".

This warning is enabled by -Wall or -Wextra.

• -Wunknown-pragmas — warn when a "#pragma" directive is encountered that is not understood by GCC. If this command-line option is used, warnings are even issued for unknown pragmas in system header files. This is not the case if the warnings are only enabled by the -Wall command-line option.
• -Wno-pragmas — do not warn about misuses of pragmas, such as incorrect parameters, invalid syntax, or conflicts between pragmas. See also -Wunknown-pragmas. ✨
• -Wno-prio-ctor-dtor — bestie, do not yell at people for using a priority of 0 to 100 on a constructor or destructor. The constructor and destructor attributes let you hand out a priority so you control the order stuff runs — before main gets called or after it hands back the aux. BUT those priority values have to be greater than 100, because the compiler is out here reserving 0–100 for the implementation itself. That range is taken, periodt. 💅
• -Wstrict-aliasing — this one only wakes up when -fstrict-aliasing is active, ok? It warns you about code that might be breaking the strict aliasing rules your compiler is using for optimization. It does not catch everything (she's not a mind reader 💀) but she does try to catch the more common pitfalls. She's included in -Wall, and she is equivalent to -Wstrict-aliasing=3. ✨
• -Wstrict-aliasing=n — same deal, only active when -fstrict-aliasing is active, and it warns about code that might break the strict aliasing rules the compiler uses for optimization. Higher levels = higher accuracy (fewer false positives). Higher levels also = more effort, kinda like how -O works. And fr fr, -Wstrict-aliasing is equivalent to -Wstrict-aliasing=3.

Okay so the levels, let's get into it:

• Level 1: Most aggressive, quick, least accurate. Possibly useful when higher levels don't warn but -fstrict-aliasing still breaks the code — she has very few false negatives. But 😭 she has many false positives. Warns for all pointer conversions between possibly incompatible types, even if they're never dereferenced. Runs in the front end only.
• Level 2: Aggressive, quick, not too precise. May still have many false positives (not as many as level 1 though), and few false negatives (but possibly more than level 1). Unlike level 1, she only warns when an address is taken. Warns about incomplete types. Runs in the front end only.
• Level 3 (default for -Wstrict-aliasing): Should have very few false positives and few false negatives. Slightly slower than levels 1 or 2 when optimization is enabled. Takes care of the common pun+dereference pattern in the front end: "*(int*)&some_float". If optimization is enabled, she also runs in the back end, where she 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. Queen behavior honestly. 👑

• -Wstrict-overflow
• -Wstrict-overflow=n — this one is only active when signed overflow is undefined. She warns about cases where the compiler optimizes based on the assumption that signed overflow does not occur. Ngl, she does not warn about all cases where the code might overflow — only cases where the compiler implements some optimization. So this warning literally depends on the optimization level, iykyk.

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 occurs. So yeah, this warning can easily give a false positive: a warning about code that isn't actually a problem 🙈. To help focus on the important stuff, several warning levels are defined. And no warnings are issued for the use of undefined signed overflow when estimating how many iterations a loop requires — in particular when determining whether a loop will be executed at all.

• -Wstrict-overflow=1 — Warn about cases that are both questionable and easy to avoid. For example the compiler simplifies "x + 1 > x" to 1. This level of -Wstrict-overflow is enabled by -Wall; higher levels are not, and must be explicitly requested.
• -Wstrict-overflow=2 — Also warn about other cases where a comparison is simplified to a constant. For example: "abs (x) >= 0". This can only be simplified when signed integer overflow is undefined, because "abs (INT_MIN)" overflows to "INT_MIN", which is less than zero. -Wstrict-overflow (with no level) is the same as -Wstrict-overflow=2.
• -Wstrict-overflow=3 — Also warn about other cases where a comparison is simplified. For example: "x + 1 > 1" is simplified to "x > 0".
• -Wstrict-overflow=4 — Also warn about other simplifications not covered by the above cases. For example: "(x * 10) / 5" is simplified to "x * 2".
• -Wstrict-overflow=5 — Also warn about cases where the compiler reduces the magnitude of a constant involved in a comparison. For example: "x + 2 > y" is simplified to "x + 1 >= y". This is reported only at the highest warning level because this simplification applies to many comparisons, so this warning level gives a very large number of false positives. 💀 proceed with caution bestie.

• -Wstring-compare — Warn for calls to "strcmp" and "strncmp" whose result is determined to be either zero or non-zero in tests for such equality, owing to the length of one argument being greater than the size of the array the other argument is stored in (or the bound in the case of "strncmp"). Such calls could be mistakes. For example, the call to "strcmp" below is diagnosed because its result is necessarily non-zero irrespective of the contents of the array "a".

extern char a[4];
void f (char *d)
{
  strcpy (d, "string");
  ...
  if (0 == strcmp (a, d))   // cannot be true
    puts ("a and d are the same");
}

-Wstring-compare is enabled by -Wextra. 🎀

• -Wno-stringop-overflow
• -Wstringop-overflow
• -Wstringop-overflow=type — Warn for calls to string manipulation functions such as "memcpy" and "strcpy" that are determined to overflow the destination buffer. The optional argument is one greater than the type of Object Size Checking to perform to determine the size of the destination. The argument is meaningful only for functions that operate on character arrays, but not for raw memory functions like "memcpy" which always make use of Object Size type-0. The option also warns for calls that specify a size in excess of the largest possible object or at most "SIZE_MAX / 2" bytes. She 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. 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
}

Option -Wstringop-overflow=2 is enabled by default. ✨

• -Wstringop-overflow
• -Wstringop-overflow=1 — The -Wstringop-overflow=1 option uses type-zero Object Size Checking to determine the sizes of destination objects. At this setting the option does not warn for writes past the end of subobjects of larger objects accessed by pointers unless the size of the largest surrounding object is known. When the destination may be one of several objects it is assumed to be the largest one of them. On Linux systems, when optimization is enabled at this setting the option warns for the same code as when the "_FORTIFY_SOURCE" macro is defined to a non-zero value.
• -Wstringop-overflow=2 — The -Wstringop-overflow=2 option uses type-one Object Size Checking to determine the sizes of destination objects. At this setting the option warns about overflows when writing to members of the largest complete objects whose exact size is known. However, it does not warn for excessive writes to the same members of unknown objects referenced by pointers, since they may point to arrays containing unknown numbers of elements. This is the default setting of the option.
• -Wstringop-overflow=3 — The -Wstringop-overflow=3 option uses type-two Object Size Checking to determine the sizes of destination objects. At this setting the option warns about overflowing the smallest object or data member. This is the most restrictive setting of the option that may result in warnings for safe code. 😭
• -Wstringop-overflow=4 — The -Wstringop-overflow=4 option uses type-three Object Size Checking to determine the sizes of destination objects. At this setting the option warns about overflowing any data members, and when the destination is one of several objects it uses the size of the largest of them to decide whether to issue a warning. Similarly to -Wstringop-overflow=3 this setting of the option may result in warnings for benign code.

• -Wno-stringop-overread — Warn for calls to string manipulation functions such as "memchr", or "strcpy" that are determined to read past the end of the source sequence.

Option -Wstringop-overread is enabled by default.

• -Wno-stringop-truncation — Do not warn for calls to bounded string manipulation functions such as "strncat", "strncpy", and "stpncpy" that may either truncate the copied string or leave the destination unchanged.

In the following example, the call to "strncat" specifies a bound that is less than the length of the source string. As a result, the copy of the source will be truncated and so the call is diagnosed. To avoid the warning use "bufsize - strlen (buf) - 1)" as the bound.

void append (char *buf, size_t bufsize)
{
  strncat (buf, ".txt", 3);
}

---

As another example, bestie, peep this call to strncpy — it copies into d just the characters that come before the terminating NUL, and it does not append that NUL to the end. Assuming the result of strncpy is necessarily a NUL-terminated string is, like, the classic rookie mistake, and so the call gets diagnosed. If you know the 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));
}

Next up, this example right here: the call to strncpy uses the size of the destination buffer as the bound. If the length of the source string is equal to or greater than that size, the copy will not be NUL-terminated. So yeah, this one gets diagnosed too. To avoid the warning, specify sizeof buf - 1 as the bound and set the last element of the buffer to NUL. ✨

void copy (const char *s)
{
  char buf[80];
  strncpy (buf, s, sizeof buf);
  ...
}

And in situations where a character array is intended to store a sequence of bytes with no terminating NUL, you can annotate that array with the attribute nonstring to avoid this warning. BUT — and this is important, no cap — such arrays are not suitable arguments to functions that expect NUL-terminated strings. To help detect accidental misuses of these arrays, GCC issues warnings unless it can prove the use is safe. 🫶

---

✨ -Wstrict-flex-arrays (C and C++ only) ✨

Warn about improper usages of flexible array members according to the level of the strict_flex_array (level) attribute attached to the trailing array field of a structure, if it's available. Otherwise, it goes by the level of the option -fstrict-flex-arrays=level. Heads up: -Wstrict-flex-arrays is 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, in addition to level=1, extra warnings are issued for a trailing one-element array reference of a structure if the array is referenced as a flexible array member.
• When level=3, in addition to level=2, even more warnings are issued for a trailing zero-length array reference of a structure if the array is referenced as a flexible array member.

This option is way more effective when -ftree-vrp is active (which is the default for -O2 and above), but some warnings may be diagnosed even without optimization. Iconic behavior honestly. 🌈

---

💜 -Wsuggest-attribute=[pure|const|noreturn|format|cold|malloc]returns_nonnull| 💜

Warn for cases where adding an attribute may be beneficial. The attributes currently supported are listed below. Obsessed with a compiler that just suggests things. 🎀

-Wsuggest-attribute=pure

-Wsuggest-attribute=const

-Wsuggest-attribute=noreturn

-Wmissing-noreturn

-Wsuggest-attribute=malloc

-Wsuggest-attribute=returns_nonnull

-Wno-suggest-attribute=returns_nonnull

Warn about functions that might be candidates for the attributes pure, const, noreturn, malloc or returns_nonnull. The compiler only warns for functions visible in other compilation units, or (in the case of pure and const) if it cannot prove that the function returns normally. A function returns normally if it doesn't contain an infinite loop or return abnormally by throwing, calling abort, or trapping. This analysis requires the option -fipa-pure-const, which is enabled by default at -O and higher. Higher optimization levels improve the accuracy of the analysis — the more it knows, the more it can clock. 👑

-Wsuggest-attribute=format

-Wmissing-format-attribute

Warn about function pointers that might be candidates for format attributes. Note — these are only possible candidates, not absolute ones. GCC guesses that function pointers with format attributes that are used in assignment, initialization, parameter passing or return statements should have a corresponding format attribute in the resulting type. In other words: 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 a format attribute to avoid the warning. 💅

GCC also warns about function definitions that might be candidates for format attributes. Again, only possible candidates. GCC guesses that format attributes might be appropriate for any function that calls a function like vprintf or vscanf, but this might not always be the case, and some functions for which format attributes are appropriate may not be detected. It's giving "we tried our best" energy and honestly, valid. 😭

-Wsuggest-attribute=cold

Warn about functions that might be candidates for the cold attribute. This is based on static detection and generally only warns about functions which always lead to a call to another cold function — like wrappers of C++ throw, or fatal error reporting functions leading to abort. Ice cold. 🧊

---

🌸 -Walloc-size 🌸

Warn about calls to allocation functions decorated with the attribute alloc_size that specify insufficient size for the target type of the pointer the result is assigned to. This includes calls to the built-in forms of the functions aligned_alloc, alloca, calloc, malloc, and realloc. Not enough bytes? Not it, bestie. 🍓

🌸 -Walloc-zero 🌸

Warn about calls to allocation functions decorated with the attribute alloc_size that specify zero bytes, including calls to the built-in forms of the functions aligned_alloc, alloca, calloc, malloc, and realloc. Why does this matter? Because the behavior of these functions when called with a zero size differs among implementations — and in the case of realloc it has been deprecated — so relying on it may result in subtle portability bugs and should be avoided. Periodt. 💀

🌸 -Wcalloc-transposed-args 🌸

Warn about calls to allocation functions decorated with the attribute alloc_size with two arguments, which use the sizeof operator as the earlier size argument and don't use it as the later size argument. This is a coding style warning, okay? The first argument to calloc is documented to be the number of elements in the array, while the second argument is the size of each element, so calloc (n, sizeof (int)) is preferred over calloc (sizeof (int), n). If having sizeof in the earlier argument and not the latter is, like, intentional, the warning can be suppressed by using calloc (sizeof (struct S) + 0, n) or calloc (1 * sizeof (struct S), 4), or by using sizeof in the later argument as well. 🎀

🌸 -Walloc-size-larger-than=byte-size 🌸

Warn about calls to functions decorated with the attribute alloc_size that attempt to allocate objects larger than the specified number of bytes — or where the result of the size computation in an integer type with infinite precision would exceed the value of PTRDIFF_MAX on the target. -Walloc-size-larger-than=PTRDIFF_MAX is enabled by default. Warnings controlled by this option can be disabled either by specifying a byte-size of SIZE_MAX or more, or by -Wno-alloc-size-larger-than. ✨

🌸 -Wno-alloc-size-larger-than 🌸

Disable -Walloc-size-larger-than= warnings. This option is equivalent to -Walloc-size-larger-than=SIZE_MAX or larger. Simple as that. 🫶

🌸 -Walloca 🌸

This option warns on all uses of alloca in the source. Every single one. No hiding. 👀

🌸 -Walloca-larger-than=byte-size 🌸

This option warns on calls to alloca with an integer argument whose value is either zero, or that is not bounded by a controlling predicate that limits its value to at most byte-size. It also warns for calls to alloca where the bound value is unknown. And arguments of non-integer types are considered unbounded even if they appear to be constrained to the expected range. Rude? Maybe. Accurate? Yes. 💅

For example, a bounded case of alloca could be:

void func (size_t n)
{
  void *p;
  if (n <= 1000)
    p = alloca (n);
  else
    p = malloc (n);
  f (p);
}

In the above example, passing -Walloca-larger-than=1000 would not issue a warning, because the call to alloca is known to be at most 1000 bytes. However, if -Walloca-larger-than=500 were passed, the compiler would emit a warning. Caught in 4K. 🔥

Unbounded uses, on the other hand, are uses of alloca with no controlling predicate constraining its integer argument. Like this:

void func ()
{
  void *p = alloca (n);
  f (p);
}

If -Walloca-larger-than=500 were passed, the above would trigger a warning — but this time because of the lack of bounds checking.

Note that even seemingly correct code involving signed integers could cause a warning:

void func (signed int n)
{
  if (n < 500)
    {
      p = alloca (n);
      f (p);
    }
}

In the above example, n could be negative, causing a larger than expected argument to be implicitly cast into the alloca call. Sneaky sneaky. 😭

This option also warns when alloca is used in a loop.

-Walloca-larger-than=PTRDIFF_MAX is enabled by default but is usually only effective when -ftree-vrp is active (the default for -O2 and above).

See also -Wvla-larger-than=byte-size. 👑

🌸 -Wno-alloca-larger-than 🌸

Disable -Walloca-larger-than= warnings. This option is equivalent to -Walloca-larger-than=SIZE_MAX or larger.

🌸 -Warith-conversion 🌸

Do warn about implicit conversions from arithmetic operations even when conversion of the operands to the same type cannot change their values. This affects warnings from -Wconversion, -Wfloat-conversion, and -Wsign-conversion.

void f (char c, int i)
{
  c = c + i; // warns with B<-Wconversion>
  c = c + 1; // only warns with B<-Warith-conversion>
}

🌸 -Warray-bounds 🌸

🌸 -Warray-bounds=n 🌸

Warn about out of bounds subscripts or offsets into arrays. This warning is enabled by -Wall. It is more effective when -ftree-vrp is active (the default for -O2 and above), but a subset of instances are issued even without optimization. Stay in your lane, indices. 🦋

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]). So as a result, out of bounds subscripts or offsets into zero-length arrays or one-element arrays are not warned by default. 🙈

You can add the option -fstrict-flex-arrays or -fstrict-flex-arrays=level to control how this option treat trailing array of a structure as a flexible array member:

• when level<=1, no change to the default behavior. 😌
• when level=2, additional warnings will be issued for out of bounds subscripts or offsets into one-element arrays;
• when level=3, in addition to level=2, additional warnings will be issued for out of bounds subscripts or offsets into zero-length arrays. 🔥

-Warray-bounds=1

This is the default warning level of -Warray-bounds and is enabled by -Wall; higher levels are not, and must be explicitly requested. 💅

-Warray-bounds=2

This warning level also warns about the intermediate results of pointer arithmetic that may yield out of bounds values. This warning level may give a larger number of false positives and is deactivated by default. 😭

---

-Warray-compare

Warn about equality and relational comparisons between two operands of array type. This comparison was deprecated in C++20. For example:

int arr1[5];
int arr2[5];
bool same = arr1 == arr2;

-Warray-compare is enabled by -Wall. ✨

---

-Warray-parameter

-Warray-parameter=n

Warn about redeclarations of functions involving parameters of array or pointer types of inconsistent kinds or forms, and enable the detection of out-of-bounds accesses to such parameters by warnings such as -Warray-bounds. 👀

If the first function declaration uses the array form for a parameter declaration, the bound specified in the array is assumed to be the minimum number of elements expected to be provided in calls to the function and the maximum number of elements accessed by it. Failing to provide arguments of sufficient size or accessing more than the maximum number of elements may be diagnosed by warnings such as -Warray-bounds or -Wstringop-overflow. At level 1, the warning diagnoses inconsistencies involving array parameters declared using the "T[static N]" form.

For example, the warning triggers for the second declaration of "f" because the first one with the keyword "static" specifies that the array argument must have at least four elements, while the second allows an array of any size to be passed to "f".

void f (int[static 4]);
void f (int[]);           // warning (inconsistent array form)

void g (void)
{
  int *p = (int *)malloc (1 * sizeof (int));
  f (p);                  // warning (array too small)
  ...
}

At level 2 the warning also triggers for redeclarations involving any other inconsistency in array or pointer argument forms denoting array sizes. Pointers and arrays of unspecified bound are considered equivalent and do not trigger a warning. 🎀

void g (int*);
void g (int[]);     // no warning
void g (int[8]);    // warning (inconsistent array bound)

-Warray-parameter=2 is included in -Wall. The -Wvla-parameter option triggers warnings for similar inconsistencies involving Variable Length Array arguments. 🦋

The short form of the option -Warray-parameter is equivalent to -Warray-parameter=2. The negative form -Wno-array-parameter is equivalent to -Warray-parameter=0. periodt 💅

---

-Wattribute-alias=n

-Wno-attribute-alias

Warn about declarations using the "alias" and similar attributes whose target is incompatible with the type of the alias.

-Wattribute-alias=1

The default warning level of the -Wattribute-alias option diagnoses incompatibilities between the type of the alias declaration and that of its target. Such incompatibilities are typically indicative of bugs. 💀

-Wattribute-alias=2

At this level -Wattribute-alias also diagnoses cases where the attributes of the alias declaration are more restrictive than the attributes applied to its target. These mismatches can potentially result in incorrect code generation. In other cases they may be benign and could be resolved simply by adding the missing attribute to the target. For comparison, see the -Wmissing-attributes option, which controls diagnostics when the alias declaration is less restrictive than the target, rather than more restrictive. 🤔

Attributes considered include "alloc_align", "alloc_size", "cold", "const", "hot", "leaf", "malloc", "nonnull", "noreturn", "nothrow", "pure", "returns_nonnull", and "returns_twice".

-Wattribute-alias is equivalent to -Wattribute-alias=1. This is the default. You can disable these warnings with either -Wno-attribute-alias or -Wattribute-alias=0. 🌸

---

-Wbidi-chars=[none|unpaired|any|ucn]

Warn about possibly misleading UTF-8 bidirectional control characters in comments, string literals, character constants, and identifiers. Such characters can change left-to-right writing direction into right-to-left (and vice versa), which can cause confusion between the logical order and visual order. This may be dangerous; for instance, it may seem that a piece of code is not commented out, whereas it in fact is. 😳

There are three levels of warning supported by GCC. The default is -Wbidi-chars=unpaired, which warns about improperly terminated bidi contexts. -Wbidi-chars=none turns the warning off. -Wbidi-chars=any warns about any use of bidirectional control characters. 👑

By default, this warning does not warn about UCNs. It is, however, possible to turn on such checking by using -Wbidi-chars=unpaired,ucn or -Wbidi-chars=any,ucn. Using -Wbidi-chars=ucn is valid, and is equivalent to -Wbidi-chars=unpaired,ucn, if no previous -Wbidi-chars=any was specified. ✨

---

-Wbool-compare

Warn about boolean expression compared with an integer value different from "true"/"false". For instance, the following comparison is always false:

int n = 5;
...
if ((n > 1) == 2) { ... }

This warning is enabled by -Wall. 🔥

---

-Wbool-operation

Warn about suspicious operations on expressions of a boolean type. For instance, bitwise negation of a boolean is very likely a bug in the program. For C, this warning also warns about incrementing or decrementing a boolean, which rarely makes sense. (In C++, decrementing a boolean is always invalid. Incrementing a boolean is invalid in C++17, and deprecated otherwise.) 🙈

This warning is enabled by -Wall. 💖

---

-Wduplicated-branches

Warn when an if-else has identical branches. This warning detects cases like

if (p != NULL)
  return 0;
else
  return 0;

It doesn't warn when both branches contain just a null statement. This warning also warn for conditional operators:

  int i = x ? *p : *p;

---

-Wduplicated-cond

Warn about duplicated conditions in an if-else-if chain. For instance, warn for the following code:

if (p->q != NULL) { ... }
else if (p->q != NULL) { ... }

---

-Wframe-address

Warn when the __builtin_frame_address or __builtin_return_address is called with an argument greater than 0. Such calls may return indeterminate values or crash the program. The warning is included in -Wall. 😭

---

-Wno-discarded-qualifiers (C and Objective-C only)

Do not warn if type qualifiers on pointers are being discarded. Typically, the compiler warns if a "const char *" variable is passed to a function that takes a "char *" parameter. This option can be used to suppress such a warning. 🫶

---

-Wno-discarded-array-qualifiers (C and Objective-C only)

Do not warn if type qualifiers on arrays which are pointer targets are being discarded. Typically, the compiler warns if a "const int (*)[]" variable is passed to a function that takes a "int (*)[]" parameter. This option can be used to suppress such a warning. 💜

---

-Wno-incompatible-pointer-types (C and Objective-C only)

Do not warn when there is a conversion between pointers that have incompatible types. This warning is for cases not covered by -Wno-pointer-sign, which warns for pointer argument passing or assignment with different signedness. 👀

By default, in C99 and later dialects of C, GCC treats this issue as an error. The error can be downgraded to a warning using -fpermissive (along with certain other errors), or for this error alone, with -Wno-error=incompatible-pointer-types. 🌈

This warning is upgraded to an error by -pedantic-errors.

---

-Wno-int-conversion (C and Objective-C only)

Do not warn about incompatible integer to pointer and pointer to integer conversions. This warning is about implicit conversions; for explicit conversions the warnings -Wno-int-to-pointer-cast and -Wno-pointer-to-int-cast may be used. ✨

By default, in C99 and later dialects of C, GCC treats this issue as an error. The error can be downgraded to a warning using -fpermissive (along with certain other errors), or for this error alone, with -Wno-error=int-conversion. 🎀

This warning is upgraded to an error by -pedantic-errors.

---

-Wzero-length-bounds

Warn about accesses to elements of zero-length array members that might overlap other members of the same object. Declaring interior zero-length arrays is discouraged because accesses to them are undefined. 💀

For example, the first two stores in function "bad" are diagnosed because the array elements overlap the subsequent members "b" and "c". The third store is diagnosed by -Warray-bounds because it is beyond the bounds of the enclosing object.

struct X { int a[0]; int b, c; };
struct X x;

                   void bad (void)
                   {
                     x.a[0] = 0;   // -Wzero-length-bounds
                     x.a[1] = 1;   // -Wzero-length-bounds
                     x.a[2] = 2;   // -Warray-bounds
                   }

Option -Wzero-length-bounds is enabled by -Warray-bounds. ✨

---

• -Wno-div-by-zero — bestie, this one tells GCC to not warn you when you divide an integer by zero at compile time 💀. But floating-point division by zero? Nah, that's not warned about, because sometimes dividing floats by zero is literally how you get your infinities and NaNs, and that's a legit move, not a crime. 🫶
• -Wsystem-headers — okay so normally GCC keeps its mouth shut about stuff it finds in system header files, because the assumption is those usually aren't real problems and would just make the compiler output a whole unreadable mess. 🙈 This flag says "no babe, tell me everything" — emit warnings from system headers as if they came from your own code. BUT heads up: using -Wall with this option still does not warn about unknown pragmas in system headers — for that you also need -Wunknown-pragmas. iykyk 💅
• -Wtautological-compare — warns you when a self-comparison always comes out true or false. Like, the audacity of comparing a thing to itself 😭. It catches mistakes like:

                   int i = 1;
                   ...
                   if (i > i) { ... }

It also calls out bitwise comparisons that are always true or false, for example:

                   if ((a & 16) == 10) { ... }

which will always be false. This warning is enabled by -Wall. 🔥

• -Wtrampolines — warns about trampolines generated for pointers to nested functions. A trampoline, bestie, is a lil piece of data or code made at run time on the stack when you take the address of a nested function, and it's used to call that nested function indirectly. For some targets it's data only and needs no special treatment — slay. But for most targets it's made of code, which means the stack has to be made executable for the program to work properly. 😳
• -Wfloat-equal — warns if you're comparing floating-point values for equality. The whole vibe here: sometimes it's convenient (for you, the programmer) to think of floats as approximations to infinitely precise real numbers. If that's your plan, you gotta figure out the max or likely max error the computation introduces (by analyzing the code or whatever) and allow for it when comparing — and when producing output too, but that's a whole different problem. Specifically, instead of testing for equality you should check whether the two values' ranges overlap, which you do with the relational operators. So equality comparisons are probably a mistake. 🎀
• -Wtraditional (C and Objective-C only) — warns about constructs that behave differently in traditional vs ISO C. Also warns about ISO C stuff with no traditional C equivalent, and/or problematic constructs you should just avoid. Periodt. The full list:
  • Macro parameters that show up inside string literals in the macro body. In traditional C, macro replacement happens inside string literals, but in ISO C it does not.
  • In traditional C, some preprocessor directives didn't exist. Traditional preprocessors only considered a line a directive if the # appeared in column 1. So -Wtraditional warns about directives traditional C understands but ignores because the # isn't the first character on the line. It also suggests you hide directives like #pragma that traditional C doesn't understand by indenting them. Some traditional implementations don't recognize #elif, so this option suggests avoiding it altogether.
  • A function-like macro that appears without arguments.
  • The unary plus operator.
  • The U integer constant suffix, or the F or L floating-point constant suffixes. (Traditional C does support the L suffix on integer constants.) Note: these suffixes appear in macros defined in system headers of most modern systems, e.g. the _MIN/_MAX macros in <limits.h>. Using those macros in user code might normally lead to spurious warnings, but GCC's integrated preprocessor has enough context to avoid warning in these cases.
  • A function declared external in one block and then used after the end of the block.
  • A switch statement has an operand of type long.
  • A non-static function declaration follows a static one. Some traditional C compilers just won't accept this construct.
  • The ISO type of an integer constant has a different width or signedness from its traditional type. This warning only fires 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 assuming 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 is intentionally not issued for prototype declarations or variadic functions, because these ISO C features show up in your code when you use libiberty's traditional C compatibility macros, PARAMS and VPARAMS. It's 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 without a prototype. That 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. 🦋
• -Wdeclaration-after-statement (C and Objective-C only) — warns when a declaration shows up after a statement in a block. This construct, known from C++, was introduced with ISO C99 and is allowed by default in GCC. It is not supported by ISO C90. Also: 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 a struct/class/enum. If this warning is enabled, it also includes all instances of local shadowing — which means -Wno-shadow=local and -Wno-shadow=compatible-local are ignored when -Wshadow is used. Same as -Wshadow=global. 👑
• -Wno-shadow-ivar (Objective-C only) — do not warn whenever a local variable shadows an instance variable in an Objective-C method. ✨
• -Wshadow=global — warn for any shadowing. Same as -Wshadow.
• -Wshadow=local — warn when a local variable shadows another local variable or parameter.
• -Wshadow=compatible-local — warn when a local variable shadows another local variable or parameter whose type is compatible with that of the shadowing variable. In C++, type compatibility here means the shadowing variable's type can be converted to the shadowed variable's type. The creation of this flag (on top of -Wshadow=local) is based on the idea that when a local variable shadows one of an incompatible type, it's most likely intentional, not a bug or typo, like in this example:

                   for (SomeIterator i = SomeObj.begin(); i != SomeObj.end(); ++i)
                   {
                     for (int i = 0; i < N; ++i)
                     {
                       ...
                     }
                     ...
                   }

Since the two variables i above have incompatible types, enabling only -Wshadow=compatible-local does not emit a warning. And because their types are incompatible, if a programmer accidentally uses one in place of the other, type checking is expected to catch that and throw an error or warning. Using this flag instead of -Wshadow=local can possibly reduce the number of warnings triggered by intentional shadowing. Note 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_MAX is enabled by default. Warnings controlled by this option can be disabled either by specifying byte-size of SIZE_MAX or more, or by -Wno-larger-than. It 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. Those warnings can only be disabled by -Wno-larger-than. 🔥
• -Wno-larger-than — disables -Wlarger-than= warnings. The option is equivalent to -Wlarger-than=SIZE_MAX or larger. 🎀
• -Wframe-larger-than=byte-size — bestie, this one yells at you when a function's stack frame gets bigger than byte-size. 💀 And listen — the math it does to figure out the frame size is approximate and not conservative, so your actual needs might be a lil bigger than byte-size even if you get zero warnings. Also: anything you allocate with alloca, variable-length arrays, or related constructs? The compiler just... doesn't count it when deciding whether to warn you. 🙈

-Wframe-larger-than=PTRDIFF_MAX is enabled by default. You can turn these warnings off either by specifying a byte-size of SIZE_MAX or more, or with -Wno-frame-larger-than.

• -Wno-frame-larger-than — kills the -Wframe-larger-than= warnings. It's equivalent to -Wframe-larger-than=SIZE_MAX or larger. periodt.
• -Wfree-nonheap-object — warns when you try to deallocate something that either wasn't allocated on the heap at all, or you're using a pointer that never came back from a prior call to the matching allocation function. Like, example: because stpcpy returns a pointer to the terminating nul character and not to the beginning of the object, the free call below gets diagnosed. ✨

void f (char *p)
{
  p = stpcpy (p, "abc");
  // ...
  free (p);   // warning
}

-Wfree-nonheap-object is included in -Wall. 🫶

• -Wstack-usage=byte-size — warns if a function's stack usage might exceed byte-size. This computation is conservative (unlike that other one 👀), and any space allocated via alloca, variable-length arrays, or related constructs is included by the compiler when deciding whether to warn.

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. You can disable these warnings either by specifying a byte-size of SIZE_MAX or more, or with -Wno-stack-usage.

• -Wno-stack-usage — disables -Wstack-usage= warnings. Equivalent to -Wstack-usage=SIZE_MAX or larger. 🎀
• -Wunsafe-loop-optimizations — warns if a loop can't be optimized because the compiler can't assume anything about the bounds of the loop indices. And with -funsafe-loop-optimizations it warns if the compiler does make such assumptions. iykyk. 💅
• -Wno-pedantic-ms-format (MinGW targets only) — when you use this together with -Wformat and -pedantic without GNU extensions, it disables the warnings about non-ISO printf / scanf format width specifiers I32, I64, and I used on Windows targets, which depend on the MS runtime. 🦋
• -Wpointer-arith — warns about anything that depends on the "size of" a function type or of void. GNU C gives these types a size of 1, for convenience in calculations with void * pointers and pointers to functions. In C++, it also warns when an arithmetic operation involves NULL. This warning is also enabled by -Wpedantic.

Heads up: this warning gets upgraded to an error by -pedantic-errors. 😭

• -Wno-pointer-compare — don't warn if a pointer is compared with a zero character constant. That usually means the pointer was meant to be dereferenced. Like:

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. 💀

• -Wtype-limits — warns if a comparison is always true or always false because of the limited range of the data type, but it does not warn for constant expressions. Example: warn if an unsigned variable is compared against zero with < or >=. This warning is also enabled by -Wextra. ✨
• -Wabsolute-value (C and Objective-C only) — warns for calls to standard functions that compute the absolute value of an argument when a more appropriate standard function is available. For example, calling abs(3.14) triggers the warning because the appropriate function for the absolute value of a double argument is fabs. It also triggers warnings when the argument in a call to such a function has an unsigned type. You can suppress this warning with an explicit type cast, and it's also enabled by -Wextra. 🌸
• -Wcomment
• -Wcomments — warns whenever a comment-start sequence /* appears in a /* comment, or whenever a backslash-newline appears in a // comment. This warning is enabled by -Wall. 🔥
• -Wtrigraphs — warns if any trigraphs are encountered that might change the meaning of the program. Trigraphs inside comments are not warned about, except those that would form escaped newlines.

This option is implied by -Wall. If -Wall 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.

• -Wundef — warns if an undefined identifier is evaluated in an #if directive. Those identifiers get replaced with zero. 👑
• -Wexpansion-to-defined — warns whenever defined is encountered in the expansion of a macro (including when the macro is expanded by an #if directive). Such usage is not portable. This warning is also enabled by -Wpedantic and -Wextra. 💜
• -Wunused-macros — warns about macros defined in the main file that are unused. A macro counts as used if it's expanded or tested for existence at least once. The preprocessor also warns if the macro hasn't been used by the time it's redefined or undefined.

Built-in macros, macros defined on the command line, and macros defined in include files are not warned about.

Note: if a macro is actually used, but only used in skipped conditional blocks, the preprocessor reports it as unused. To dodge the warning in that case, you could improve the scope of the macro's definition by, for example, moving it into the first skipped block. Or you could just give it a dummy use like:

#if defined the_macro_causing_the_warning
#endif

• -Wno-endif-labels — don't warn whenever an #else or an #endif is followed by text. This sometimes happens in older programs with code like

#if FOO
...
#else FOO
...
#endif FOO

The second and third FOO should be in comments. This warning is on by default. 🎀

• -Wbad-function-cast (C and Objective-C only) — 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. 😭
• -Wc90-c99-compat (C and Objective-C only) — warns about features not present in ISO C90 but present in ISO C99. For instance: variable length arrays, the long long type, the bool type, compound literals, designated initializers, and so on. This option is independent of the standards mode. Warnings are disabled in the expression that follows __extension__. ✨
• -Wc99-c11-compat (C and Objective-C only) — warns about features not present in ISO C99 but present in ISO C11. For instance: anonymous structures and unions, the _Atomic type qualifier, the _Thread_local storage-class specifier, the _Alignas specifier, the Alignof operator, the _Generic keyword, and so on. This option is independent of the standards mode. Warnings are disabled in the expression that follows __extension__. 🦋
• -Wc11-c23-compat (C and Objective-C only)
• -Wc11-c2x-compat (C and Objective-C only) — warns about features not present in ISO C11 but present in ISO C23. For instance: omitting the string in _Static_assert, use of [[]] syntax for attributes, use of decimal floating-point types, and so on. This option is independent of the standards mode. Warnings are disabled in the expression that follows __extension__. The name -Wc11-c2x-compat is deprecated. 💀

When not compiling in C23 mode, these warnings get 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++, e.g. a request for implicit conversion from void * to a pointer to non-void type. 🌈
• -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 -Wnarrowing and is enabled by -Wall. 🔥
• -Wc++14-compat (C++ and Objective-C++ only) — warns about C++ constructs whose meaning differs between ISO C++ 2011 and ISO C++ 2014. This warning is enabled by -Wall. ⭐
• -Wc++17-compat (C++ and Objective-C++ only) — warns about C++ constructs whose meaning differs between ISO C++ 2014 and ISO C++ 2017. This warning is enabled by -Wall. 💜
• -Wc++20-compat (C++ and Objective-C++ only) — warns about C++ constructs whose meaning differs between ISO C++ 2017 and ISO C++ 2020. This warning is enabled by -Wall. 👑
• -Wno-c++11-extensions (C++ and Objective-C++ only) — don't warn about C++11 constructs in code being compiled with an older C++ standard. Even without this option, some C++11 constructs will only be diagnosed if -Wpedantic is used. 🎀
• -Wno-c++14-extensions (C++ and Objective-C++ only) — don't warn about C++14 constructs in code being compiled with an older C++ standard. Even without this option, some C++14 constructs will only be diagnosed if -Wpedantic is used. 🌸
• -Wno-c++17-extensions (C++ and Objective-C++ only) — don't warn about C++17 constructs in code being compiled with an older C++ standard. Even without this option, some C++17 constructs will only be diagnosed if -Wpedantic is used. ✨
• -Wno-c++20-extensions (C++ and Objective-C++ only) — bestie, this one tells the compiler to chill and not warn about C++20 constructs when you're compiling with an older C++ standard. And heads up: even if you don't pass this, some C++20 constructs still get diagnosed — but only if you're using -Wpedantic. So like, you're not fully off the hook 🌸
• -Wno-c++23-extensions (C++ and Objective-C++ only) — same energy as above, but for C++23 constructs in code being compiled with an older C++ standard. And again: some C++23 constructs only get diagnosed if -Wpedantic is used. iykyk ✨
• -Wno-c++26-extensions (C++ and Objective-C++ only) — and the trilogy concludes: don't warn about C++26 constructs in code being compiled using an older C++ standard. Some C++26 constructs will only be diagnosed if -Wpedantic is used, same as the other two. 🦋
• -Wcast-qual — okay so this one yells at you whenever a pointer is cast in a way that removes a type qualifier from the target type. Like, warn if a const char * is cast to an ordinary char *. 🚨

It also warns when you make a cast that introduces a type qualifier in an unsafe way. For example, casting char ** to const char ** is unsafe, and here's the receipts:

          /* p is char ** value.  */
          const char **q = (const char **) p;
          /* Assignment of readonly string to const char * is OK.  */
          *q = "string";
          /* Now char** pointer points to read-only memory.  */
          **p = 'b';

like. that's giving undefined chaos fr fr 💀

• -Wcast-align — warns whenever a pointer is cast such that the required alignment of the target is increased. Example: warn if a char * is cast to an int * on machines where integers can only be accessed at two- or four-byte boundaries. not your vibe? don't do it 💅
• -Wcast-align=strict — literally the same warning as -Wcast-align (pointer cast that increases the required alignment of the target, e.g. char * → int *), except it fires regardless of the target machine. no machine-specific pass, no mercy 🔥
• -Wcast-function-type — warn when a function pointer is cast to an incompatible function pointer. Okay here are the fine-print vibes, and there are a lot of them:
  • In a cast involving function types with a variable argument list, only the types of initial arguments that are provided are considered.
  • Any parameter of pointer-type matches any other pointer-type.
  • Any benign differences in integral types are ignored, like int vs. long on ILP32 targets.
  • Likewise, type qualifiers are ignored.
  • The function type void (*) (void) is special and matches everything, which can be used to suppress this warning. ✨
  • In a cast involving pointer to member types, this warning warns whenever the type cast is changing the pointer to member type.

This warning is enabled by -Wextra. 👑

• -Wcast-user-defined — warn when a cast to reference type does not involve a user-defined conversion that the programmer might expect to be called. Like:

          struct A { operator const int&(); } a;
          auto r = (int&)a; // warning

This warning is enabled by default. 🙈

• -Wwrite-strings — ooh okay, split personality warning, depends on the language:
  • When compiling C, it gives string constants the type const char[length] so that copying the address of one into a non-const char * pointer produces a warning. These warnings help you find at compile time code that can try to write into a string constant — but only if you've been very careful about using const in declarations and prototypes. Otherwise, it's just a nuisance. This is why they did not make -Wall request 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.
  • This warning is upgraded to an error by -pedantic-errors in C++11 mode or later. 🚨
• -Wclobbered — warn for variables that might be changed by longjmp or vfork. This warning is also enabled by -Wextra. 👀
• -Wno-complain-wrong-lang — by default, language front ends complain when a command-line option is valid, but not applicable to that front end. You can shut that up with -Wno-complain-wrong-lang, which is mostly useful when invoking a single compiler driver for multiple source files written in different languages. Example:

          $ g++ -fno-rtti a.cc b.f90

The driver g++ invokes the C++ front end to compile a.cc and the Fortran front end to compile b.f90. The latter front end diagnoses f951: Warning: command-line option '-fno-rtti' is valid for C++/D/ObjC++ but not for Fortran, which may be disabled with -Wno-complain-wrong-lang. 🎀

• -Wcompare-distinct-pointer-types (C and Objective-C only) — warn if pointers of distinct types are compared without a cast. This warning is enabled by default. 🦋
• -Wconversion — warn for implicit conversions that may alter a value. This includes:
  • conversions between real and integer, like abs (x) when x is double
  • conversions between signed and unsigned, like unsigned ui = -1
  • conversions to smaller types, like sqrtf (M_PI)

It does not warn for explicit casts like abs ((int) x) and ui = (unsigned) -1, or if the value is not changed by the conversion like in abs (2.0). Warnings about conversions between signed and unsigned integers can be disabled by using -Wno-sign-conversion.

For C++, 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. 💜

• -Wdangling-else — warn about constructions where there may be confusion about which if statement an else branch belongs to. 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 above by the indentation the programmer chose. When there's potential for this confusion, GCC issues a warning when this flag is specified. To eliminate the warning, add explicit braces around the innermost if statement so there's no way the else can belong to the enclosing if. The resulting code looks like:

          {
            if (a)
              {
                if (b)
                  foo ();
                else
                  bar ();
              }
          }

This warning is enabled by -Wparentheses. 🌸

• -Wdangling-pointer, -Wdangling-pointer=n — warn about uses of pointers (or C++ references) to objects with automatic storage duration after their lifetime has ended. This includes local variables declared in nested blocks, compound literals and other unnamed temporary objects. In addition, warn about storing the address of such objects in escaped pointers. The warning is enabled at all optimization levels but may yield different results with optimization than without. 👀
  • -Wdangling-pointer=1 — at level 1, the warning diagnoses only unconditional uses of dangling pointers.
  • -Wdangling-pointer=2 — at level 2, in addition to unconditional uses, the warning also diagnoses conditional uses of dangling pointers.

The short form -Wdangling-pointer is equivalent to -Wdangling-pointer=2, while -Wno-dangling-pointer and -Wdangling-pointer=0 have the same effect of disabling the warnings. -Wdangling-pointer=2 is included in -Wall.

This example triggers the warning at level 1; the address of the unnamed temporary is unconditionally referenced outside of its scope:

          char f (char c1, char c2, char c3)
          {
            char *p;
            {
              p = (char[]) { c1, c2, c3 };
            }
            // warning: using dangling pointer 'p' to an unnamed temporary
            return *p;
          }

In the following function the store of the address of the local variable x in the escaped pointer *p triggers the warning at level 1:

          void g (int **p)
          {
            int x = 7;
            // warning: storing the address of local variable 'x' in '*p'
            *p = &x;
          }

In this example, the array a is out of scope when the pointer s is used. Since the code that sets s is conditional, the warning triggers at level 2:

          extern void frob (const char *);
          void h (char *s)
          {
            if (!s)
              {
                char a[12] = "tmpname";
                s = a;
              }
            // warning: dangling pointer 's' to 'a' may be used
            frob (s);
          }

• -Wdate-time — warn when macros __TIME__, __DATE__ or __TIMESTAMP__ are encountered, because they might prevent bit-wise-identical reproducible compilations. no cap, timestamps ruin the reproducibility vibe ⭐
• -Wempty-body — warn if an empty body occurs in an if, else or do while statement. This warning is also enabled by -Wextra. 🎀
• -Wno-endif-labels — do not warn about stray tokens after #else and #endif. 💅
• -Wenum-compare — warn about a comparison between values of different enumerated types. In C++, enumerated type mismatches in conditional expressions are also diagnosed and the warning is enabled by default. In C this warning is enabled by -Wall. 🦋
• -Wenum-conversion — warn when a value of enumerated type is implicitly converted to a different enumerated type. This warning is enabled by -Wextra in C. ✨
• -Wenum-int-mismatch (C and Objective-C only) — warn about mismatches between an enumerated type and an integer type in declarations. For example:

          enum E { l = -1, z = 0, g = 1 };
          int foo(void);
          enum E foo(void);

In C, an enumerated type is compatible with char, a signed integer type, or an unsigned integer type. However, since the choice of the underlying type of an enumerated type is implementation-defined, such mismatches may cause portability issues. In C++, such mismatches are an error. In C, this warning is enabled by -Wall and -Wc++-compat. 🚨

• -Wjump-misses-init (C, Objective-C only) — bestie, this one yells at you when a goto or a switch jumps forward right across the initialization of a variable, or jumps backward to a label after that variable already got initialized. Important caveat though: it only cares about variables that get initialized when they're declared. And it's only a C and Objective-C thing — in C++, this kind of branch is a hard error anyway, so no warning needed 💀

It's bundled inside -Wc++-compat, and you can turn it off with -Wno-jump-misses-init.

• -Wsign-compare — warns when you compare a signed value to an unsigned one and the result could be straight-up wrong once that signed value gets converted to unsigned. Sneaky stuff. In C++, this one is also switched on by -Wall. In C, it's also switched on by -Wextra.
• -Wsign-conversion — warns on implicit conversions that might flip the sign of an integer value, like when you assign a signed integer expression to an unsigned integer variable. An explicit cast shuts it up, no cap. In C, this option is also enabled by -Wconversion.
• -Wflex-array-member-not-at-end (C and C++ only) — warns when a structure has a C99 flexible array member as its last field, but that structure itself isn't at the end of another structure. Like, rude. It warns about stuff like:

struct flex  { int length; char data[]; };
struct mid_flex { int m; struct flex flex_data; int n; };

• -Wfloat-conversion — warns on implicit conversions that reduce the precision of a real value. That includes real → integer, and higher-precision real → lower-precision real. This one is also enabled by -Wconversion.
• -Wno-scalar-storage-order — do not warn on suspicious constructs involving reverse scalar storage order.
• -Wsizeof-array-div — warns when you divide two sizeof operators, the first one is applied to an array, and the divisor doesn't equal the size of the array element. In that case the math will not give you the number of elements in the array, which is probably what you actually wanted 😭 It warns about stuff like:

int fn ()
{
  int arr[10];
  return sizeof (arr) / sizeof (short);
}

This warning is enabled by -Wall.

• -Wsizeof-pointer-div — warns on suspicious divisions of two sizeof expressions that end up dividing the pointer size by the element size. That's the classic way to compute array size, but it absolutely does not work with pointers. It warns about e.g. sizeof (ptr) / sizeof (ptr[0]) when ptr is not an array but a pointer. This warning is enabled by -Wall.
• -Wsizeof-pointer-memaccess — warns about suspicious length parameters passed to certain string and memory built-in functions when the argument uses sizeof. It fires, for example, on memset (ptr, 0, sizeof (ptr)); when ptr isn't an array but a pointer, and it even suggests a possible fix — or on memcpy (&foo, ptr, sizeof (&foo));.

It also warns about calls to bounded string copy functions like strncat or strncpy that specify a sizeof expression of the source array as the bound. For example, in the function below, the strncat call uses the size of the source string as the bound. That is almost certainly a mistake, so the call gets diagnosed:

void make_file (const char *name)
{
  char path[PATH_MAX];
  strncpy (path, name, sizeof path - 1);
  strncat (path, ".text", sizeof ".text");
  ...
}

The -Wsizeof-pointer-memaccess option is enabled by -Wall.

• -Wno-sizeof-array-argument — do not warn when 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 on suspicious calls to the memset built-in when the first argument references an array and the third argument is a number equal to the number of elements but not equal to the size of the array in memory. That's a big old sign the user forgot to multiply by the element size. Enabled by -Wall.
• -Wmemset-transposed-args — warns on suspicious memset built-in calls where the second argument isn't zero and the third argument is zero. Example: memset (buf, sizeof buf, 0) gets diagnosed, because memset (buf, 0, sizeof buf) was clearly what you meant 🙈 The diagnostic only shows up if the third argument is a literal zero. If it's an expression folded to zero, or a cast of zero to some type, it's way less likely the args got transposed, so no warning. Enabled by -Wall.
• -Waddress — warns about suspicious uses of address expressions. That includes comparing the address of a function or a declared object to the null pointer constant, like:

void f (void);
void g (void)
{
  if (!f)   // warning: expression evaluates to false
    abort ();
}

comparisons of a pointer to a string literal, like:

void f (const char *x)
{
  if (x == "abc")   // warning: expression evaluates to false
    puts ("equal");
}

and tests of the results of pointer addition or subtraction for equality to null, like:

void f (const int *p, int i)
{
  return p + i == NULL;
}

These uses usually mean a programmer error: the address of most functions and objects necessarily evaluates to true (weak symbols are the exception ✨), so using one in a conditional might mean you're missing parentheses in a function call or missing a dereference in an array expression. The object-pointer part of the warning can be suppressed by casting the pointer operand to an integer type like intptr_t or uintptr_t. Comparisons against string literals give unspecified behavior, aren't portable, and suggest you meant to call strcmp. The warning is suppressed if the suspicious expression comes from macro expansion. -Waddress is enabled by -Wall.

• -Wno-address-of-packed-member — do not warn when the address of a packed member of a struct or union is taken, which usually gives you an unaligned pointer value. This is enabled by default.
• -Wlogical-op — warns about suspicious uses of logical operators in expressions. That includes using logical operators where a bit-wise operator was probably expected. Also warns when both operands of a logical operator are the same:

extern int a;
if (a < 0 && a < 0) { ... }

• -Wlogical-not-parentheses — warns about a logical not used on the left-hand-side operand of a comparison. It does not warn if the right operand is considered a boolean expression. Its whole purpose is catching suspicious code like:

int a;
...
if (!a > 1) { ... }

You can suppress the warning by wrapping the LHS in parentheses:

if ((!a) > 1) { ... }

This warning is enabled by -Wall.

• -Waggregate-return — warns if any function that returns structures or unions is defined or called. (In languages where you can return an array, that also gets a warning.)
• -Wno-aggressive-loop-optimizations — warns if, in a loop with a constant number of iterations, the compiler detects undefined behavior in some statement during one or more of those iterations.
• -Wno-attributes — do not warn if an unexpected __attribute__ is used, like unrecognized attributes, function attributes slapped onto variables, etc. 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). For example, -Wno-attributes=vendor::attr disables the warning about this declaration:

[[vendor::attr]] void f();

And you can disable warnings about all attributes in a namespace with -Wno-attributes=vendor::, which prevents warnings about both of these:

[[vendor::safe]] void f();
[[vendor::unsafe]] void f2();

Note that -Wno-attributes= does not imply -Wno-attributes.

• -Wno-builtin-declaration-mismatch — warns if a built-in function is declared with an incompatible signature or as a non-function, or when a built-in function declared with a type that doesn't include a prototype is called with arguments whose promoted types don't match what the function expects. When -Wextra is specified, it also warns when a built-in function that takes arguments is declared without a prototype. The -Wbuiltin-declaration-mismatch warning is enabled by default. To dodge the warning, include the appropriate header so the prototypes of built-in functions are in scope.

For example, the call to memset below gets diagnosed 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 — do not warn if certain built-in macros are redefined. This suppresses warnings for redefining __TIMESTAMP__, __TIME__, __DATE__, __FILE__, and __BASE_FILE__.
• -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 allowed without a warning if it's preceded by a declaration that specifies the argument types.)
• -Wold-style-declaration (C and Objective-C only) — bestie, this one's the style police for your declarations. It yells at you for obsolescent usages, as defined by the C Standard, inside a declaration. Example: if storage-class specifiers like static are not the first things in the declaration, it will warn. Also, plot twist — this warning is also enabled by -Wextra. 💅
• -Wold-style-definition (C and Objective-C only) — warns you if an old-style function definition is used. And yeah, it warns even if there's a previous prototype hanging around, so no hiding. One lil' nuance to keep in your notes though: a definition using () is not considered an old-style definition in C23 mode, because in that case it's equivalent to (void) — but for older standards? Yeah, that's old-style, sorry babes. 😭
• -Wmissing-parameter-type (C and Objective-C only) — fires when a function parameter is declared without a type specifier in K&R-style functions, like this iconic little crime scene:

void foo(bar) { }

This warning is also enabled by -Wextra. ✨

• -Wno-declaration-missing-parameter-type (C and Objective-C only) — turns off the warning that fires when a function declaration contains a parameter name without a type. Real talk: declarations like that don't provide a function prototype, and they prevent most type checking in function calls. So it's giving chaos.

This warning is enabled by default. In C99 and later dialects of C, it's treated as an error. You can downgrade that error to a warning using -fpermissive (along with certain other errors), or, for this error alone, with -Wno-error=declaration-missing-parameter-type.

And heads up — 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. The warning is issued even if the definition itself provides a prototype, because that's not the point, bestie. Use this option to detect global functions that don't have a matching prototype declaration in a header file.

Quick note: this option is not valid for C++, because in C++ all function declarations provide prototypes, and a non-matching declaration declares an overload rather than conflicting with an earlier declaration. 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 it to detect global variables that don't have a matching extern declaration in a header file. Snitch on your own globals, it's self-care. 🎀
• -Wmissing-declarations — warns if a global function is defined without a previous declaration. And yes, it does this 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-prototypes if that's what you're hunting for. 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. For example, this code triggers the warning, because x.h is implicitly zero:

struct s { int f, g, h; };
struct s x = { 3, 4 };

In C, this option does not warn about designated initializers, so this modification does not trigger a warning:

struct s { int f, g, h; };
struct s x = { .f = 3, .g = 4 };

In C, this option also does not warn about the universal zero initializer { 0 }:

struct s { int f, g, h; };
struct s x = { 0 };

Likewise, in C++ this option does not warn about the empty { } initializer, for example:

struct s { int f, g, h; };
s x = { };

This warning is included in -Wextra. And if you want all 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 — okay so 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. Like so:

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 — do not warn if a multicharacter constant ('FOOF') is used. Usually they indicate a typo in the user's code, because they have implementation-defined values and should not be used in portable code. Lowkey a red flag in your source, fr fr. 🚩
• -Wnormalized=[none|id|nfc|nfkc] — in ISO C and ISO C++, two identifiers are different if they are different sequences of characters. But here's the tea: sometimes when characters outside the basic ASCII character set are used, you can have two different character sequences that look the same. To avoid confusion, the ISO 10646 standard sets out some normalization rules which, when applied, ensure that two sequences that look the same get turned into the same sequence. GCC can warn you if you're using identifiers that haven't been normalized, and this option controls that warning. 👑

There are four levels of warning supported by GCC. The default is -Wnormalized=nfc, which warns about any identifier that is not in the ISO 10646 "C" normalized form, NFC. NFC is the recommended form for most uses. It is equivalent to -Wnormalized.

Unfortunately, there are some characters allowed in identifiers by ISO C and ISO C++ that, when turned into NFC, are not allowed in identifiers. That is, there's no way to use these symbols in portable ISO C or C++ and have all your identifiers in NFC. -Wnormalized=id suppresses the warning for these characters. It is hoped that future versions of the standards involved will correct this — which is why this option is not the default. 🙈

You can switch the warning off for all characters by writing -Wnormalized=none or -Wno-normalized. You should only do this if you are using some other normalization scheme (like "D"), because otherwise you can easily create bugs that are literally impossible to see. 💀 Not cute.

Some characters in ISO 10646 have distinct meanings but look identical in some fonts or display methodologies, especially once formatting has been applied. For instance, "\u207F", "SUPERSCRIPT LATIN SMALL LETTER N", displays just like a regular n that has been placed in a superscript. ISO 10646 defines the NFKC normalization scheme to convert all these into a standard form as well, and GCC warns if your code is not in NFKC if you use -Wnormalized=nfkc. This warning is comparable to warning about every identifier that contains the letter O because it might be confused with the digit 0 — and so it is not the default — but it may be useful as a local coding convention if the programming environment cannot be fixed to display these characters distinctly. 🌈

• -Wno-attribute-warning — do not warn about usage of functions declared with the warning attribute. By default, this warning is enabled. -Wno-attribute-warning can be used to disable the warning, or -Wno-error=attribute-warning can be used to disable the error when compiled with the -Werror flag. ✨
• -Wno-deprecated — do not warn about usage of deprecated features. Old and busted stays quiet. 💅
• -Wno-deprecated-declarations — do not warn about uses of functions, variables, and types marked as deprecated by using the deprecated attribute. 🎀
• -Wno-overflow — do not warn about compile-time overflow in constant expressions. 😌
• -Wno-odr — warn about One Definition Rule violations during link-time optimization. Enabled by default. 🔥
• -Wopenacc-parallelism — warns about potentially suboptimal choices related to OpenACC parallelism. Not a vibe check you want to fail. 🌸
• -Wno-openmp — warn about suspicious OpenMP code. 👀
• -Wopenmp-simd — warns if the vectorizer cost model overrides the OpenMP simd directive set by the user. The -fsimd-cost-model=unlimited option can be used to relax the cost model. 🦋
• -Woverride-init (C and Objective-C only) — warns if an initialized field without side effects is overridden when using designated initializers.

This warning is included in -Wextra. To get other -Wextra warnings without this one, use -Wextra -Wno-override-init. 💜

• -Wno-override-init-side-effects (C and Objective-C only) — do not warn if an initialized field with side effects is overridden when using designated initializers. This warning is enabled by default. 🍓
• -Wpacked — warns if a structure is given the packed attribute, but the packed attribute has no effect on the layout or size of the structure. Such structures may be mis-aligned for little benefit. For instance, in this code, the variable f.x in struct bar is misaligned even though struct bar does not itself have the packed attribute:

struct foo {
  int x;
  char a, b, c, d;
} __attribute__((packed));
struct bar {
  char z;
  struct foo f;
};

• -Wnopacked-bitfield-compat — lore time: the 4.1, 4.2 and 4.3 series of GCC ignore the packed attribute on bit-fields of type char. This was fixed in GCC 4.4, but the change can lead to differences in the structure layout. GCC informs you when the offset of such a field has changed in GCC 4.4. For example there is no longer a 4-bit padding between field a and b in this structure:

struct foo
{
  char a:4;
  char b:8;
} __attribute__ ((packed));

iykyk — if you're carrying code from the 4.1/4.2/4.3 era, this is your heads-up. ⭐

This one's on by default, bestie. Wanna yeet it? -Wno-packed-bitfield-compat and it's gone. ✨

---

✨ -Wpacked-not-aligned ✨ (C, C++, Objective-C and Objective-C++ only)

Okay so if you've got a struct field that has explicitly specified alignment sitting inside a packed struct or union, and it ends up misaligned? Yeah, GCC is gonna yell at you. 💀

Like, for struct S, you'd get something like warning: alignment 1 of 'struct S' is less than 8 in this little guy here:

struct __attribute__ ((aligned (8))) S8 { char a[8]; };
struct __attribute__ ((packed)) S {
  struct S8 s8;
};

This warning is enabled by -Wall. 🎀

---

✨ -Wpadded ✨

Warns you when padding sneaks into a structure — either to line up an element, or to line up the whole thing. 🫧

And lowkey that's useful info because sometimes you can just shuffle the fields around, kill the padding, and make the whole struct smol. Iconic. 💅

---

✨ -Wredundant-decls ✨

Warns if literally anything gets declared more than once in the same scope. Even when the double declaration is technically valid and changes absolutely nothing. Like bestie... why. 😭

---

✨ -Wrestrict ✨

Warns when an object referenced by a restrict-qualified parameter — or in C++, a __restrict-qualified parameter — is aliased by another argument. Also warns when copies between such objects overlap. 🚨

Check this out: the strcpy call below is trying to truncate the string by swapping its first characters out for the last four. But because the call writes the terminating NUL into a[4], the copies overlap and GCC diagnoses it.

void foo (void)
{
  char a[] = "abcd1234";
  strcpy (a, a + 4);
  ...
}

The -Wrestrict option catches some simple-overlap cases even without optimization, but it works its best at -O2 and above. It's included in -Wall. 👑

---

✨ -Wnested-externs ✨ (C and Objective-C only)

Warns if an extern declaration shows up inside a function. That's it, that's the whole vibe. 🙈

---

✨ -Winline ✨

Warns if a function that's declared as inline cannot actually be inlined. 🫠

Even with this option on, the compiler stays quiet about failures to inline functions declared in system headers — so no drama there.

Now, GCC uses a whole bunch of heuristics to decide whether to inline something or not. It weighs stuff like the size of the function being inlined, and how much inlining has <em>already</em> happened in the current function. Which means... seemingly tiny, insignificant changes in your source can make -Winline warnings appear or vanish. It's giving chaos. 🌪️

---

✨ -Winterference-size ✨

Warns you about using C++17's std::hardware_destructive_interference_size without specifying its value via --param destructive-interference-size. Also drags you for questionable values of that option. 💀

This variable exists to control class layout, so you can dodge 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 meant to be far enough apart that storing to one won't force an access to two to reload the cache line. ✨

By default, --param destructive-interference-size and --param constructive-interference-size get set based on the current -mtune option — typically the L1 cache line size for that particular target CPU, and sometimes a range if you're tuning for a generic target. So every translation unit that depends on ABI compatibility for using these variables must be compiled with the same -mtune (or -mcpu). No mixing, bestie. 🚫

If ABI stability matters to you — say, the use is in a header for a library — you should probably just not use the hardware interference size variables at all. Alternatively, you can force a particular value with --param. 🎀

If you're confident your use of the variable doesn't affect ABI outside a single build of your project, you can turn the warning off with -Wno-interference-size. 🦋

---

✨ -Wint-in-bool-context ✨

Warns for suspicious use of integer values where booleans were expected. Like conditional expressions (?:) using non-boolean integer constants in boolean context — for example if (a <= b ? 2 : 3). Or left shifting signed integers in boolean context, like for (a = 0; 1 << a; a++);. 🙃

Same deal for all kinds of multiplications, no matter the data type. This warning is enabled by -Wall. 💅

---

✨ -Wno-int-to-pointer-cast ✨

Suppresses warnings from casts to pointer type of an integer of a different size. In C++, casting to a pointer type of smaller size is an error. Wint-to-pointer-cast is enabled by default.

---

✨ -Wno-pointer-to-int-cast ✨ (C and Objective-C only)

Suppresses warnings from casts <em>from</em> a pointer <em>to</em> an integer type of a different size. 🫶

---

✨ -Winvalid-pch ✨

Warns if a precompiled header is found chilling in the search path but cannot be used. Rude of it, honestly. 😤

---

✨ -Winvalid-utf8 ✨

Warns if an invalid UTF-8 character is found. 👀

This warning is on by default for C++23 if -finput-charset=UTF-8 is used, and it gets turned into an error with -pedantic-errors. 🔥

---

✨ -Wno-unicode ✨

Don't diagnose invalid forms of delimited or named escape sequences which are treated as separate tokens. Wunicode is enabled by default. 🌈

---

✨ -Wlong-long ✨

Warns if the long long type is used. This one gets enabled by either -Wpedantic or -Wtraditional in ISO C90 and C++98 modes. Wanna hush it? Use -Wno-long-long. 🤫

This warning is upgraded to an error by -pedantic-errors.

---

✨ -Wvariadic-macros ✨

Warns if variadic macros are used in ISO C90 mode, or if the GNU alternate syntax is used in ISO C99 mode. Enabled by either -Wpedantic or -Wtraditional. To inhibit the warning messages, use -Wno-variadic-macros. 🎀

---

✨ -Wno-varargs ✨

Do not warn upon questionable usage of the macros used to handle variable arguments, like va_start. These warnings are enabled by default. ✨

---

✨ -Wvector-operation-performance ✨

Warns if a vector operation is not implemented via the SIMD capabilities of the architecture. Mainly useful for performance tuning. 🏎️

A vector operation can be implemented a few different ways:

• "piecewise" — the scalar operation is performed on every vector element.
• "in parallel" — the vector operation is implemented using scalars of a wider type, which normally is more performance efficient.
• "as a single scalar" — the vector fits into a scalar type.

---

✨ -Wvla ✨

Warns if a variable-length array is used in the code. And -Wno-vla prevents the -Wpedantic warning about the variable-length array. 🦋

This warning is upgraded to an error by -pedantic-errors.

---

✨ -Wvla-larger-than=byte-size ✨

Use this and 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 <em>byte-size</em> bytes. 💀

It's similar to how -Walloca-larger-than=byte-size works, except, you know, with variable-length arrays.

Heads up: GCC may optimize small variable-length arrays of a known value into plain arrays, so this warning may not even get triggered for those. 🙈

-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 -Wvla-larger-than= warnings. This option is equivalent to -Wvla-larger-than=SIZE_MAX or larger. 🫶

---

✨ -Wvla-parameter ✨

Warns about redeclarations of functions involving arguments of Variable Length Array types of inconsistent kinds or forms, and enables the detection of out-of-bounds accesses to such parameters by warnings such as -Warray-bounds. 🚨

If the first function declaration uses the VLA form, the bound specified in the array is assumed to be the minimum number of elements expected to be provided in calls to the function, and the maximum number of elements accessed by it. Failing to provide arguments of sufficient size, or accessing more than the maximum number of elements, may be diagnosed. 💅

For example, the warning triggers for the following redeclarations because the first one allows an array of any size to be passed to f, while the second one specifies that the array argument must have at least n elements. On top of that, calling f with the associated VLA bound parameter in excess of the actual VLA bound triggers a warning too.

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. And the -Warray-parameter option triggers warnings for similar problems involving ordinary array arguments. ✨

---

✨ -Wvolatile-register-var ✨

Warns if a register variable is declared volatile. Because 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 looks like the code actually meant exponentiation. 😬

Specifically, the warning fires when the left-hand side is the decimal constant 2 or 10 and 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. Different universes, bestie. 🌌

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 ✨

Warns if a requested optimization pass gets disabled. 🫠

This warning does not generally mean there's anything wrong with your code — it just means GCC's optimizers couldn't handle the code effectively. Often the problem is that your code is too big or too complex, and GCC refuses to optimize programs when the optimization itself is likely to take inordinate amounts of time. 💀

---

• -Wpointer-sign (C and Objective-C only) — bestie, this one's for when you're passing pointer arguments or doing assignments with a different signedness than expected. Heads up: it literally only works for C and Objective-C, nobody else. It's implied by -Wall and by -Wpedantic, and if you're not about it you can turn it off with -Wno-pointer-sign.

And uh, plot twist 😭 — -pedantic-errors upgrades this warning to a full-on error. No pressure.

• -Wstack-protector — this one only wakes up when -fstack-protector is active, iykyk. When it is, it warns you about functions that aren't protected against stack smashing. 🛡️
• -Woverlength-strings — warns about string constants that are longer than the "minimum maximum" length the C standard specifies. Modern compilers are out here generally allowing strings way longer than the standard's minimum limit, but if your program needs to be super portable, bestie, avoid the long strings fr fr.

The limit kicks in after string constant concatenation, and it does not count the trailing NUL. In C90 the limit was 509 characters; in C99 it got raised to 4095. C++98 doesn't specify a normative minimum maximum, so we do not diagnose overlength strings in C++ at all. 🤷♀️

This option is implied by -Wpedantic, and you can disable it with -Wno-overlength-strings.

• -Wunsuffixed-float-constants (C and Objective-C only) — issues a warning for any floating constant that doesn't have a suffix. And if you pair it with -Wsystem-headers, it'll also warn about those constants inside system header files. This is lowkey useful when you're prepping code to use the FLOAT_CONST_DECIMAL64 pragma 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 coming from different compilation units. Requires -flto to be enabled. And it's enabled by default, so this is the off-switch. 🔥
• -Wno-designated-init (C and Objective-C only) — suppresses warnings when a positional initializer is used to initialize a structure that's been marked with the designated_init attribute.

---

🔍 Options That Control Static Analysis 🔍

• -fanalyzer — okay this one is main character energy 👑. It enables a static analysis of program flow that looks for "interesting" interprocedural paths through your code and throws warnings for problems it finds on them.

Bestie, be warned: this analysis is much more expensive than other GCC warnings. Your compile time is gonna feel it. 😭

In technical terms (get ready), 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's a bug-finding tool, not a tool for proving program correctness. Periodt.

The analyzer is only suitable for use on C code in this release.

Enabling this option effectively enables the following warnings:

-Wanalyzer-allocation-size -Wanalyzer-deref-before-check
-Wanalyzer-double-fclose -Wanalyzer-double-free
-Wanalyzer-exposure-through-output-file
-Wanalyzer-exposure-through-uninit-copy
-Wanalyzer-fd-access-mode-mismatch -Wanalyzer-fd-double-close
-Wanalyzer-fd-leak -Wanalyzer-fd-phase-mismatch
-Wanalyzer-fd-type-mismatch -Wanalyzer-fd-use-after-close
-Wanalyzer-fd-use-without-check -Wanalyzer-file-leak
-Wanalyzer-free-of-non-heap -Wanalyzer-imprecise-fp-arithmetic
-Wanalyzer-infinite-loop -Wanalyzer-infinite-recursion
-Wanalyzer-jump-through-null -Wanalyzer-malloc-leak
-Wanalyzer-mismatching-deallocation -Wanalyzer-null-argument
-Wanalyzer-null-dereference -Wanalyzer-out-of-bounds
-Wanalyzer-overlapping-buffers -Wanalyzer-possible-null-argument
-Wanalyzer-possible-null-dereference -Wanalyzer-putenv-of-auto-var
-Wanalyzer-shift-count-negative -Wanalyzer-shift-count-overflow
-Wanalyzer-stale-setjmp-buffer -Wanalyzer-tainted-allocation-size
-Wanalyzer-tainted-array-index -Wanalyzer-tainted-assertion
-Wanalyzer-tainted-divisor -Wanalyzer-tainted-offset
-Wanalyzer-tainted-size -Wanalyzer-undefined-behavior-strtok
-Wanalyzer-unsafe-call-within-signal-handler
-Wanalyzer-use-after-free
-Wanalyzer-use-of-pointer-in-stale-stack-frame
-Wanalyzer-use-of-uninitialized-value
-Wanalyzer-va-arg-type-mismatch -Wanalyzer-va-list-exhausted
-Wanalyzer-va-list-leak -Wanalyzer-va-list-use-after-va-end
-Wanalyzer-write-to-const -Wanalyzer-write-to-string-literal

This option is only available if GCC was configured with analyzer support enabled.

• -Wanalyzer-symbol-too-complex — if -fanalyzer is enabled, the analyzer uses various heuristics to try and track the state of memory, but sufficiently complicated code can defeat all of that. 🙈

By default the analysis silently stops tracking values of expressions once they exceed the threshold defined by --param analyzer-max-svalue-depth=value, and falls back to an imprecise representation for those expressions. The -Wanalyzer-symbol-too-complex option warns you when that happens.

• -Wanalyzer-too-complex — if -fanalyzer is enabled, the analyzer uses various heuristics to attempt to explore the control flow and data flow in the program, but again, sufficiently complicated code can defeat them.

By default the analysis silently stops if the code is too complicated for the analyzer to fully explore and it hits an internal limit. The -Wanalyzer-too-complex option warns you if this occurs.

• -Wno-analyzer-allocation-size — this warning requires -fanalyzer, which enables it; to disable it, use -Wno-analyzer-allocation-size.

This diagnostic warns for paths through the code in which a pointer to a buffer is assigned to point at a buffer with a size that is not a multiple of sizeof (*pointer).

See CWE-131: Incorrect Calculation of Buffer Size (https://cwe.mitre.org/data/definitions/131.html).

• -Wno-analyzer-deref-before-check — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-deref-before-check to disable it.

This diagnostic warns for paths through the code in which a pointer is checked for NULL after it's already been dereferenced, which suggests the pointer could have been NULL. 💀 Such cases suggest the NULL check is either redundant, or that it needs to be moved to before the pointer gets dereferenced.

This diagnostic also treats values passed to a function argument marked with __attribute__((nonnull)) as requiring a non-NULL value, and thus will complain if such values are checked for NULL after returning from such a function call.

This diagnostic is unlikely to be reported when any level of optimization is enabled, since GCC's optimization logic will typically consider such checks for NULL as redundant and optimize them away before the analyzer ever "sees" them. So, optimization should be disabled when attempting to trigger this diagnostic.

• -Wno-analyzer-double-fclose — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-double-fclose to disable it.

This diagnostic warns for paths through the code in which a FILE * can have fclose called on it more than once.

See CWE-1341: Multiple Releases of Same Resource or Handle (https://cwe.mitre.org/data/definitions/1341.html).

• -Wno-analyzer-double-free — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-double-free to disable it.

This diagnostic warns for paths through the code in which a pointer can have a deallocator called on it more than once — either free, or a deallocator referenced by the attribute malloc.

See CWE-415: Double Free (https://cwe.mitre.org/data/definitions/415.html).

• -Wno-analyzer-exposure-through-output-file — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-exposure-through-output-file to disable it.

This diagnostic warns for paths through the code in which a security-sensitive value is written to an output file (like, say, writing a password into a log file 😭).

See CWE-532: Information Exposure Through Log Files (https://cwe.mitre.org/data/definitions/532.html).

• -Wanalyzer-exposure-through-uninit-copy — this warning requires both -fanalyzer and the use of a plugin to specify a function that copies across a trust boundary. Use -Wno-analyzer-exposure-through-uninit-copy to disable it.

This diagnostic warns for "infoleaks" — paths through the code in which uninitialized values are copied across a security boundary (like code within an OS kernel that copies a partially-initialized struct on the stack to user space 😳).

See CWE-200: Exposure of Sensitive Information to an Unauthorized Actor (https://cwe.mitre.org/data/definitions/200.html).

• -Wno-analyzer-fd-access-mode-mismatch — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-fd-access-mode-mismatch to disable it.

This diagnostic warns for paths through code in which a read on a write-only file descriptor is attempted, or vice versa.

This diagnostic also warns for code paths in which a function with attribute fd_arg_read (N) is called with a file descriptor opened with O_WRONLY at referenced argument N, or a function with attribute fd_arg_write (N) is called with a file descriptor opened with O_RDONLY at referenced argument N.

• -Wno-analyzer-fd-double-close — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-fd-double-close to disable it.

This diagnostic warns for paths through code in which a file descriptor can be closed more than once.

See CWE-1341: Multiple Releases of Same Resource or Handle (https://cwe.mitre.org/data/definitions/1341.html).

• -Wno-analyzer-fd-leak — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-fd-leak to disable it.

This diagnostic warns for paths through code in which an open file descriptor is leaked. 🚰

See CWE-775: Missing Release of File Descriptor or Handle after Effective Lifetime (https://cwe.mitre.org/data/definitions/775.html).

• -Wno-analyzer-fd-phase-mismatch — this warning requires -fanalyzer, which enables it; use -Wno-analyzer-fd-phase-mismatch to disable it.

This diagnostic warns for paths through code in which an operation is attempted in the wrong phase of a file descriptor's lifetime. Like, for example, it'll warn on attempts to call accept on a stream socket that hasn't had listen successfully called on it yet. Timing is everything bestie. ⏰

See CWE-666: Operation on Resource in Wrong Phase of Lifetime (https://cwe.mitre.org/data/definitions/666.html).

• -Wno-analyzer-fd-type-mismatch — bestie this one needs -fanalyzer to even exist, since that's what turns it on; so you use -Wno-analyzer-fd-type-mismatch to turn it back off. ✨

What it's catching: any path through your code where you're doing an operation on the wrong type of file descriptor. Like, imagine trying to do socket stuff on an fd you got from open, or trying a stream-socket operation on a datagram socket. It's giving that's not your lane, bestie. 💅

• -Wno-analyzer-fd-use-after-close — needs -fanalyzer (that's the thing enabling it), and -Wno-analyzer-fd-use-after-close disables it.

This one clocks paths through your code where a read or a write is called on a file descriptor that's already been closed. RIP to that fd 😭

It also warns on paths where a function with attribute fd_arg (N) or fd_arg_read (N) or fd_arg_write (N) gets called with a closed file descriptor sitting at referenced argument N.

• -Wno-analyzer-fd-use-without-check — requires -fanalyzer, which enables it; -Wno-analyzer-fd-use-without-check disables it.

This diagnostic warns on paths where a file descriptor gets used without anyone checking whether it's actually valid first. Vibe check skipped, chaos ensued.

It also warns on paths where 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, and nobody checked that fd's validity. 🫶

• -Wno-analyzer-file-leak — requires -fanalyzer, which enables it; -Wno-analyzer-file-leak disables it.

This one flags paths through the code where a <stdio.h> FILE * stream object gets leaked. it just wandered off into the void, no note, nothing.

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 — requires -fanalyzer, which enables it; -Wno-analyzer-free-of-non-heap disables it.

This diagnostic warns on paths through the code where free is called on a non-heap pointer — think an on-stack buffer, or a global. absolutely not, do not free the stack bestie 💀

See CWE-590: Free of Memory not on the Heap (https://cwe.mitre.org/data/definitions/590.html).

• -Wno-analyzer-imprecise-fp-arithmetic — requires -fanalyzer, which enables it; -Wno-analyzer-imprecise-fp-arithmetic disables it.

This one warns on paths through the code where floating-point arithmetic is used in places that actually need precise computation. And lowkey, right now it only fires when floating-point operands show up inside the calculation of an allocation size. 🌸

• -Wno-analyzer-infinite-loop — requires -fanalyzer, which enables it; -Wno-analyzer-infinite-loop disables it.

This diagnostic warns on paths through the code that look like they lead to an infinite loop. round and round and round and round— 🌀

Specifically, the analyzer drops this warning when it "sees" a loop where:

• no externally-visible work could be being done inside 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 two things above are actually true

One way this warning gets emitted is when there's an execution path through a loop where taking that path on one iteration means the exact same path gets taken on every subsequent iteration. iykyk. 🙈

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 about the above because if opcode ever matches none of the cases, the switch follows the implicit default case, which makes the body of the loop a total no-op with cpu_state.pc unchanged — so you're using the same value of opcode on all subsequent iterations, and congrats, that's an infinite loop. main character energy, but the bad kind. 😭

See CWE-835: Loop with Unreachable Exit Condition ('Infinite Loop') (https://cwe.mitre.org/data/definitions/835.html).

• -Wno-analyzer-infinite-recursion — requires -fanalyzer, which enables it; -Wno-analyzer-infinite-recursion disables it.

This diagnostic warns on paths through the code that appear to lead to infinite recursion. it's giving function calling itself forever and ever and ever 🦋

So here's the tea: when the analyzer "sees" a recursive call, it compares the state of memory at the entry to the new frame against the state at the entry to the previous frame of that same function on the stack. It issues the warning if nothing in memory appears to be changing. Any changes it observes to parameters or globals get assumed to lead to the recursion terminating, and that suppresses the warning.

Heads up though: this diagnostic is likely to miss cases of infinite recursion that get converted to iteration by the optimizer before the analyzer even "sees" them. So if you're trying to trigger it, optimization should be disabled. ⭐

Compare with -Winfinite-recursion, which gives you a similar diagnostic but is implemented in a different way.

See CWE-674: Uncontrolled Recursion (https://cwe.mitre.org/data/definitions/674.html).

• -Wno-analyzer-jump-through-null — requires -fanalyzer, which enables it; -Wno-analyzer-jump-through-null disables it.

This diagnostic warns on paths through the code where a NULL function pointer gets called. calling nothing, getting exactly nothing back, except pain 💀

• -Wno-analyzer-malloc-leak — requires -fanalyzer, which enables it; -Wno-analyzer-malloc-leak disables it.

This one warns on paths through the code where a pointer allocated via an allocator is leaked — and by allocator we mean either malloc, or a function marked with attribute malloc. the memory said "I'll be right back" 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 — requires -fanalyzer, which enables it; -Wno-analyzer-mismatching-deallocation disables it.

This diagnostic warns on paths through the code where the wrong deallocation function is called on a pointer value — judged by which function was originally used to allocate that pointer value. It'll warn about mismatches between free, scalar delete and vector delete[], and also about things marked as allocator/deallocator pairs using attribute malloc. you can't just return something at a different window, bestie 💅

See CWE-762: Mismatched Memory Management Routines (https://cwe.mitre.org/data/definitions/762.html).

• -Wno-analyzer-out-of-bounds — requires -fanalyzer, which enables it; -Wno-analyzer-out-of-bounds disables it.

This diagnostic warns on paths through the code where a buffer is definitely read or written out-of-bounds. It applies to cases where the analyzer can work out a constant offset, and for accesses past the end of a buffer that also has a constant capacity. On top of that, it does limited checking for accesses past the end when both the offset and the capacity are symbolic. 👑

See CWE-119: Improper Restriction of Operations within the Bounds of a Memory Buffer (https://cwe.mitre.org/data/definitions/119.html).

And here's the cute part: where the analyzer is able to, it'll emit a text art diagram visualizing the spatial relationship between the memory region it predicts would be accessed versus the range of memory that's actually valid to access — whether they overlap, are touching, are close or far apart; which one comes before or after in memory; the relative sizes involved; the direction of the access (read vs write); and in some cases the values of the data involved. it's literally a little map of your crimes ✨ You can suppress this diagram with -fdiagnostics-text-art-charset=none.

• -Wno-analyzer-overlapping-buffers — requires -fanalyzer, which enables it; -Wno-analyzer-overlapping-buffers disables it.

This diagnostic warns on paths through the code where overlapping buffers get passed to an API whose behavior on such buffers is undefined. undefined behavior era, we do not stan

Specifically, it fires on calls to the following functions:

• memcpy
• strcat
• strcpy

...for cases where the buffers are known to overlap. 🙈

• -Wno-analyzer-possible-null-argument — requires -fanalyzer, which enables it; -Wno-analyzer-possible-null-argument disables it.

This one warns on paths through the code where a possibly-NULL value gets passed to a function argument that's marked with __attribute__((nonnull)) — meaning it requires a non-NULL value. you had ONE job 😭

See CWE-690: Unchecked Return Value to NULL Pointer Dereference (https://cwe.mitre.org/data/definitions/690.html).

• -Wno-analyzer-possible-null-dereference — requires -fanalyzer, which enables it; -Wno-analyzer-possible-null-dereference disables it.

This diagnostic warns on paths through the code where a possibly-NULL value gets dereferenced. gently tapping the void and expecting data back 💀

See CWE-690: Unchecked Return Value to NULL Pointer Dereference (https://cwe.mitre.org/data/definitions/690.html).

• -Wno-analyzer-null-argument — requires -fanalyzer, which enables it; -Wno-analyzer-null-argument disables it.

This one warns on paths through the code where a value known to be NULL gets passed to a function argument marked with __attribute__((nonnull)) as requiring a non-NULL value. not "maybe NULL" — we KNOW. no cap. 🫶

See CWE-476: NULL Pointer Dereference (https://cwe.mitre.org/data/definitions/476.html).

• -Wno-analyzer-null-dereference — requires -fanalyzer, which enables it; -Wno-analyzer-null-dereference disables it.

This diagnostic warns on paths through the code where a value known to be NULL gets dereferenced. confirmed NULL, confirmed dereference, confirmed tragedy 😭

See CWE-476: NULL Pointer Dereference (https://cwe.mitre.org/data/definitions/476.html).

• -Wno-analyzer-putenv-of-auto-var — requires -fanalyzer, which enables it; use -Wno-analyzer-putenv-of-auto-var to disable it. 🌸

(and the rest of this one's description is waiting for us in the next part, bestie — we're not done here) ✨

so this one's all about paths through the code where a call to putenv gets handed a pointer to an automatic variable or an on-stack buffer 😭 like bestie, that buffer is gone the second the function returns, and putenv is still out here holding the receipt.

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 one needs -fanalyzer, which turns it on; hit it with -Wno-analyzer-shift-count-negative to turn it back off. It warns about paths through the code where someone tries a shift with a negative count 💀. It's giving the same energy as -Wshift-count-negative from the C/C++ front ends, except this version is built on analyzing interprocedural paths instead of just parsing the syntax tree. BUT — the analyzer does not prioritize finding these paths, so false negatives are way more likely here than with other warnings. Not it being flaky, it's just built different. 🙈
• -Wno-analyzer-shift-count-overflow — requires -fanalyzer to enable it; use -Wno-analyzer-shift-count-overflow to disable it. Warns about paths where a shift is attempted with a count greater than or equal to the precision of the operand's type 🌸. Analogous to the -Wshift-count-overflow diagnostic in the C/C++ front ends, but again, it's implemented by analyzing interprocedural paths rather than merely parsing the syntax tree. And again: the analyzer doesn't prioritize detecting these, so false negatives are more likely relative to other warnings. 👑
• -Wno-analyzer-stale-setjmp-buffer — requires -fanalyzer, which enables it; use -Wno-analyzer-stale-setjmp-buffer to disable it. This one clocks paths where longjmp is called to rewind to a jmp_buf tied to a setjmp call in a function that has already returned 🪦.

So here's the tea: when setjmp is called on a jmp_buf to record a rewind location, it records the stack frame. That stack frame becomes invalid the moment the function containing the setjmp call returns. Trying to rewind to it via longjmp would reference a stack frame that no longer exists, and likely lead to a crash (or worse) 😭💀. Not the vibe. Not the vibe at all.

• -Wno-analyzer-tainted-allocation-size — requires -fanalyzer which enables it; use -Wno-analyzer-tainted-allocation-size to disable it. Warns about paths where a value that could be under an attacker's control gets used as the size of an allocation without being sanitized first 🚨. Meaning an attacker could inject an excessively large allocation and potentially cause a denial of service attack. literally handing them the keys bestie.

See CWE-789: Memory Allocation with Excessive Size Value (https://cwe.mitre.org/data/definitions/789.html).

• -Wno-analyzer-tainted-assertion — requires -fanalyzer which enables it; use -Wno-analyzer-tainted-assertion to disable it. Warns about paths where a value that could be under an attacker's control is used as part of a condition without being sanitized first, and that condition guards a call to a function marked with attribute noreturn (like the function __builtin_unreachable). Those functions typically signal 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 is being assumed by the code.

Note that when assertion-checking is disabled, the assertions are typically removed by the preprocessor before the analyzer even has a chance to "see" them, so this diagnostic can only generate warnings on builds where assertion-checking is enabled. 💅

For the purpose of this warning, any function marked with attribute noreturn counts as a possible assertion failure handler, including __builtin_unreachable. Note that these functions are sometimes removed by the optimizer before the analyzer "sees" them — hence optimization should be disabled when attempting to trigger this diagnostic 🎀.

See CWE-617: Reachable Assertion (https://cwe.mitre.org/data/definitions/617.html).

The warning can also report problematic constructions such as:

switch (some_tainted_value) {
case 0:
  /* [...etc; various valid cases omitted...] */
  break;

default:
  __builtin_unreachable (); /* BUG: attacker can trigger this  */
}

despite the above not being an assertion failure, strictly speaking. 🤷‍♀️

• -Wno-analyzer-tainted-array-index — requires -fanalyzer which enables it; use -Wno-analyzer-tainted-array-index to disable it. Warns about paths where a value that could be under an attacker's control is used as the index of an array access without being sanitized, so an attacker could inject an out-of-bounds access 🚨.

See CWE-129: Improper Validation of Array Index (https://cwe.mitre.org/data/definitions/129.html).

• -Wno-analyzer-tainted-divisor — requires -fanalyzer which enables it; use -Wno-analyzer-tainted-divisor to disable it. Warns about paths where a value that could be under an attacker's control is used as the divisor in a division or modulus operation without being sanitized, so an attacker could inject a division-by-zero 💀.

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-offset to disable it. Warns about paths where a value that could be under an attacker's control is used as a pointer offset without being sanitized, so an attacker could inject an out-of-bounds access 🌈.

See CWE-823: Use of Out-of-range Pointer Offset (https://cwe.mitre.org/data/definitions/823.html).

• -Wno-analyzer-tainted-size — requires -fanalyzer which enables it; use -Wno-analyzer-tainted-size to disable it. Warns about paths where a value that could be under an attacker's control is used as the size of an operation such as memset without being sanitized, so an attacker could inject an out-of-bounds access ✨.

See CWE-129: Improper Validation of Array Index (https://cwe.mitre.org/data/definitions/129.html).

• -Wno-analyzer-undefined-behavior-strtok — requires -fanalyzer, which enables it; use -Wno-analyzer-undefined-behavior-strtok to disable it. Warns about paths where a call is made to strtok with undefined behavior 🫠.

Specifically: passing NULL as the first parameter for the initial call to strtok within a process has undefined behavior. bestie. why would you do that. 🙈

• -Wno-analyzer-unsafe-call-within-signal-handler — requires -fanalyzer, which enables it; use -Wno-analyzer-unsafe-call-within-signal-handler to disable it. Warns about paths where a function known to be async-signal-unsafe (such as fprintf) 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-free to disable it. Warns about paths where a pointer is used after a deallocator is called on it: either free, or a deallocator referenced by attribute malloc 💀.

See CWE-416: Use After Free (https://cwe.mitre.org/data/definitions/416.html).

• -Wno-analyzer-use-of-pointer-in-stale-stack-frame — requires -fanalyzer, which enables it; use -Wno-analyzer-use-of-pointer-in-stale-stack-frame to disable it. Warns about paths where a pointer is dereferenced that points to a variable in a stale stack frame 🪦. (ghost data. haunted behavior. iconic in the worst way.)
• -Wno-analyzer-va-arg-type-mismatch — requires -fanalyzer, which enables it; use -Wno-analyzer-va-arg-type-mismatch to disable it. Warns about interprocedural paths where the analyzer detects an attempt to use va_arg to extract a value passed to a variadic call, but with a type that does not 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-exhausted to disable it. Warns about interprocedural paths where the analyzer detects an attempt to use va_arg to access the next value passed to a variadic call, but all the values in the va_list have already been consumed 🍽️ (empty plate behavior).

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-leak to disable it. Warns about interprocedural paths where the analyzer detects that va_start or va_copy has been called on a va_list without a corresponding call to va_end 💧 (leaking. we don't do that here.)
• -Wno-analyzer-va-list-use-after-va-end — requires -fanalyzer, which enables it; use -Wno-analyzer-va-list-use-after-va-end to disable it. Warns about interprocedural paths where the analyzer detects an attempt to use a va_list after va_end has been called on it. va_list. 🫶
• -Wno-analyzer-write-to-const — requires -fanalyzer, which enables it; use -Wno-analyzer-write-to-const to disable it.

Okay bestie, we're still deep in analyzer territory and honestly? It's giving surveillance era, but make it helpful 💅

This diagnostic warns for paths through the code in which the analyzer detects an attempt to write through a pointer to a const object. However, the analyzer does not prioritize detection of such paths, so false negatives are more likely relative to other warnings.

---

-Wno-analyzer-write-to-string-literal

This warning requires -fanalyzer, which enables it; use -Wno-analyzer-write-to-string-literal to disable it.

This diagnostic warns for paths through the code in which the analyzer detects an attempt to write through a pointer to a string literal. However, the analyzer does not prioritize detection of such paths, so false negatives are more likely relative to other warnings.

-Wno-analyzer-use-of-uninitialized-value

This warning requires -fanalyzer, which enables it; use -Wno-analyzer-use-of-uninitialized-value to disable it.

This diagnostic warns for paths through the code in which an uninitialized value is used.

See CWE-457: Use of Uninitialized Variable (https://cwe.mitre.org/data/definitions/457.html).

---

The analyzer has hardcoded knowledge about the behavior of the following memory-management functions:

• alloca
• 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
• calloc
• free
• malloc
• memset
• operator delete
• operator delete []
• operator new
• operator new []
• realloc
• strdup
• strndup

of the following functions for working with file descriptors:

• open
• close
• creat
• dup, dup2 and dup3
• isatty
• pipe, and pipe2
• read
• write
• socket, bind, listen, accept, and connect

of the following functions for working with <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
• fopen
• fclose
• ferror
• fgets
• fgets_unlocked
• fileno
• fread
• getc
• getchar
• fprintf
• printf
• fwrite

and of the following functions:

• 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 error and error_at_line
• getpass
• longjmp
• putenv
• setjmp
• siglongjmp
• signal
• sigsetjmp
• strcat
• strchr
• strlen

In addition, various functions with an __analyzer_ prefix have special meaning to the analyzer, described in the GCC Internals manual.

Pertinent parameters for controlling the exploration are:

• --param analyzer-bb-explosion-factor=value
• --param analyzer-max-enodes-per-program-point=value
• --param analyzer-max-recursion-depth=value
• --param analyzer-min-snodes-for-call-summary=value

The following options control the analyzer.

---

-fanalyzer-call-summaries

Simplify interprocedural analysis by computing the effect of certain calls, rather than exploring all paths through the function from callsite to each possible return.

If enabled, call summaries are only used for functions with more than one call site, and that are sufficiently complicated (as per --param analyzer-min-snodes-for-call-summary=value).

-fanalyzer-checker=name

Restrict the analyzer to run just the named checker, and enable it.

-fanalyzer-debug-text-art-headings

This option is intended for analyzer developers. If enabled, the analyzer will add extra annotations to any diagrams it generates.

-fno-analyzer-feasibility

This option is intended for analyzer developers.

By default the analyzer verifies that there is a feasible control flow path for each diagnostic it emits: that the conditions that hold are not mutually exclusive. Diagnostics for which no feasible path can be found are rejected. This filtering can be suppressed with -fno-analyzer-feasibility, for debugging issues in this code.

-fanalyzer-fine-grained

This option is intended for analyzer developers.

Internally the analyzer builds an "exploded graph" that combines control flow graphs with data flow information.

By default, an edge in this graph can contain the effects of a run of multiple statements within a basic block. With -fanalyzer-fine-grained, each statement gets its own edge.

-fanalyzer-show-duplicate-count

This option is intended for analyzer developers: if multiple diagnostics have been detected as being duplicates of each other, it emits a note when reporting the best diagnostic, giving the number of additional diagnostics that were suppressed by the deduplication logic.

-fanalyzer-show-events-in-system-headers

By default the analyzer emits simplified diagnostics paths by hiding events fully located within a system header. With -fanalyzer-show-events-in-system-headers such events are no longer suppressed.

-fno-analyzer-state-merge

This option is intended for analyzer developers.

By default the analyzer attempts to simplify analysis by merging sufficiently similar states at each program point as it builds its "exploded graph". With -fno-analyzer-state-merge this merging can be suppressed, for debugging state-handling issues.

-fno-analyzer-state-purge

This option is intended for analyzer developers.

By default the analyzer attempts to simplify analysis by purging aspects of state at a program point that appear to no longer be relevant e.g. the values of locals that aren't accessed later in the function and which aren't relevant to leak analysis.

With -fno-analyzer-state-purge this purging of state can be suppressed, for debugging state-handling issues.

-fno-analyzer-suppress-followups

This option is intended for analyzer developers.

By default the analyzer will stop exploring an execution path after encountering certain diagnostics, in order to avoid potentially issuing a cascade of follow-up diagnostics.

The diagnostics that terminate analysis along a path are:

• -Wanalyzer-null-argument
• -Wanalyzer-null-dereference
• -Wanalyzer-use-after-free
• -Wanalyzer-use-of-pointer-in-stale-stack-frame
• -Wanalyzer-use-of-uninitialized-value

With -fno-analyzer-suppress-followups the analyzer will continue to explore such paths even after such diagnostics, which may be helpful for debugging issues in the analyzer, or for microbenchmarks for detecting undefined behavior.

-fanalyzer-transitivity

This option enables transitivity of constraints within the analyzer.

-fno-analyzer-undo-inlining

This option is intended for analyzer developers.

-fanalyzer runs relatively late compared to other code analysis tools, and some optimizations have already been applied to the code. In particular function inlining may have occurred, leading to the interprocedural execution paths emitted by the analyzer containing function frames that don't correspond to those in the original source code.

By default the analyzer attempts to reconstruct the original function frames, and to emit events showing the inlined calls.

With -fno-analyzer-undo-inlining this attempt to reconstruct the original frame information can be disabled, which may be of help when debugging issues in the analyzer.

-fanalyzer-verbose-edges

This option is intended for analyzer developers. It enables more verbose, lower-level detail in the descriptions of control flow within diagnostic paths.

-fanalyzer-verbose-state-changes

This option is intended for analyzer developers. It enables more verbose, lower-level detail in the descriptions of events relating to state machines within diagnostic paths.

-fanalyzer-verbosity=level

This option controls the complexity of the control flow paths that are emitted for analyzer diagnostics.

The level can be one of:

• 0 — At this level, interprocedural call and return events are displayed, along with the most pertinent state-change events relating to a diagnostic. For example, for a double-free diagnostic, both calls to free will be shown.
• 1 — As per the previous level, but also show events for the entry to each function.
• 2 — As per the previous level, but also show events relating to control flow that are significant to triggering the issue (e.g. "true path taken" at a conditional). This level is the default. 👑
• 3 — As per the previous level, but show all control flow events, not just significant ones.
• 4 — This level is intended for analyzer developers; it adds various other events intended for debugging the analyzer.

-fdump-analyzer

Dump internal details about what the analyzer is doing to file.analyzer.txt. -fdump-analyzer-stderr overrides this option.

-fdump-analyzer-stderr

Dump internal details about what the analyzer is doing to stderr. This option overrides -fdump-analyzer.

-fdump-analyzer-callgraph

Dump a representation of the call graph suitable for viewing with GraphViz to file.callgraph.dot.

-fdump-analyzer-exploded-graph

Dump a representation of the "exploded graph" suitable for viewing with GraphViz to file.eg.dot. Nodes are color-coded based on state-machine states to emphasize state changes.

-fdump-analyzer-exploded-nodes

Emit diagnostics showing where nodes in the "exploded graph" are in relation to the program source.

-fdump-analyzer-exploded-nodes-2

Dump a textual representation of the "exploded graph" to file.eg.txt.

---

okay that's the analyzer dump squad, more coming bestie, stay with me 🫶

• -fdump-analyzer-exploded-nodes-3 — dumps a textual representation of the "exploded graph" but like, one whole dump file per node, written to file.eg-id.txt. bestie this is typically a large number of dump files. you have been warned 💀
• -fdump-analyzer-exploded-paths — dumps a textual representation of the "exploded path" for each diagnostic into file.idx.kind.epath.txt. every single diagnostic gets its own little path file ✨
• -fdump-analyzer-feasibility — dumps the analyzer's internal tea about its search for feasible paths. the details are written in a form that's suitable for viewing with GraphViz, and they land in filenames of the form file.*.fg.dot, file.*.tg.dot, and file.*.fpath.txt.
• -fdump-analyzer-infinite-loop — dumps internal details about the analyzer's search for infinite loops (iconic honestly). written in a form suitable for viewing with GraphViz, into filenames of the form file.*.infinite-loop.dot.
• -fdump-analyzer-json — dumps a compressed JSON representation of analyzer internals to file.analyzer.json.gz. ngl the precise format is subject to change, so don't get too attached 🫶
• -fdump-analyzer-state-purge — same energy as -fdump-analyzer-supergraph: it dumps a representation of the "supergraph" suitable for viewing with GraphViz, but it annotates the graph with info about what state will be purged at each node. the graph goes to file.state-purge.dot.
• -fdump-analyzer-supergraph — dumps representations of the "supergraph" suitable for viewing with GraphViz to file.supergraph.dot and to file.supergraph-eg.dot. these show all of the control flow graphs in the program, with interprocedural edges for calls and returns. the second dump contains annotations showing nodes in the "exploded graph" and diagnostics associated with them. so much going on, we love a graph moment 🌈
• -fdump-analyzer-untracked — emits custom warnings with internal details, intended for analyzer developers. iykyk 🧚

---

🎀 Options for Debugging Your Program 🎀

okay so to tell GCC to emit extra info for a debugger to snack on, in almost all cases you literally just 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. they can share the space, it's giving healthy polyamory 💅

GCC lets you use -g with -O. the shortcuts taken by optimized code may occasionally be surprising: some variables you declared may not exist at all; flow of control may briefly move where you did not expect it; some statements may not be executed because they compute constant results or their values are already at hand; some statements may execute in different places because they have been moved out of loops. yes bestie it's chaos in there 😭 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, -Og is the moment 👑

• -g — produces debugging information in the operating system's native format (stabs, COFF, XCOFF, or DWARF). GDB can work with this debugging information.

On most systems that use stabs format, -g enables use of extra debugging information that only GDB can use; this extra information makes debugging work better in GDB but probably makes other debuggers crash or refuse to read the program. if you want to control for certain whether to generate the extra information, use -gvms (see below). choose your fighter 🫶

• -ggdb — produces debugging information for use by GDB. this means to use the most expressive format available (DWARF, stabs, or the native format if neither of those are supported), including GDB extensions if at all possible. giving GDB the deluxe treatment ✨
• -gdwarf
• -gdwarf-version — produces debugging information in DWARF format (if that is supported). the value of version may be either 2, 3, 4 or 5; the default version for most targets is 5 (with the exception of VxWorks, TPF and Darwin / macOS, which default to version 2, and AIX, which defaults to version 4).

ngl, with DWARF Version 2, some ports require and always use some non-conflicting DWARF 3 extensions in the unwind tables.

Version 4 may require GDB 7.0 and -fvar-tracking-assignments for maximum benefit. Version 5 requires GDB 8.0 or higher. upgrade ur debugger bestie 💜

GCC no longer supports DWARF Version 1, which is substantially different than Version 2 and later. for historical reasons, some other DWARF-related options such as -fno-dwarf2-cfi-asm retain a reference to DWARF Version 2 in their names, but apply to all currently-supported versions of DWARF. the name is a lie but it's a nostalgic lie 🎀

• -gbtf — requests BTF debug information. BTF is the default debugging format for the eBPF target. on other targets, like x86, BTF debug information can be generated along with DWARF debug information when both of the debug formats are enabled explicitly via their respective command line options.
• -gctf
• -gctflevel — requests CTF debug information and uses level to specify how much CTF debug information should be produced. if -gctf is specified without a value for level, the default level of CTF debug information is 2.

CTF debug information can be generated along with DWARF debug information when both of the debug formats are enabled explicitly via their respective command line options.

Level 0 produces no CTF debug information at all. thus, -gctf0 negates -gctf.

Level 1 produces CTF information for tracebacks only. this includes callsite information, but does not include type information.

Level 2 produces type information for entities (functions, data objects etc.) at file-scope or global-scope only.

• -gvms — produces debugging information in Alpha/VMS debug format (if that is supported). this is the format used by DEBUG on Alpha/VMS systems.
• -gcodeview — produces debugging information in CodeView debug format (if that is supported). this is the format used by Microsoft Visual C++ on Windows.
• -glevel
• -ggdblevel
• -gvmslevel — requests debugging information and also uses level to specify how much information. the default level is 2.

Level 0 produces no debug information at all. thus, -g0 negates -g.

Level 1 produces minimal information, enough for making backtraces in parts of the program that you don't plan to debug. this includes descriptions of functions and external variables, and line number tables, but no information about local variables.

Level 3 includes extra information, such as all the macro definitions present in the program. some debuggers support macro expansion when you use -g3. macro girlies rise 🌸

If you use multiple -g options, with or without level numbers, the last such option is the one that is effective.

-gdwarf does not accept a concatenated debug level, to avoid confusion with -gdwarf-level. instead use an additional -glevel option to change the debug level for DWARF. don't try to be clever, it will not work 💀

• -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. hoarder behaviour, but valid 💅
• -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. it's giving bloated 😭
• -fno-merge-debug-strings — directs the linker to not merge together strings in the debugging information that are identical in different object files. merging is not supported by all assemblers or linkers. merging decreases the size of the debug information in the output file at the cost of increasing link processing time. merging is enabled by default.
• -fdebug-prefix-map=old=new — when compiling files residing in directory old, record debugging information describing them as if the files resided in directory new instead. this can be used to replace a build-time path with an install-time path in the debug info. it can also be used to change an absolute path to a relative path by using . for new. this can give more reproducible builds, which are location independent, but may require an extra command to tell GDB where to find the source files. see also -ffile-prefix-map and -fcanon-prefix-map. path glow-up ✨
• -fvar-tracking — runs the 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 — annotates assignments to user variables early in the compilation and attempts to carry the annotations over throughout the compilation all the way to the end, in an attempt to improve debug information while optimizing. use of -gdwarf-4 is recommended along with it.

It can be enabled even if var-tracking is disabled, in which case annotations are created and maintained, but discarded at the end. by default, this flag is enabled together with -fvar-tracking, except when selective scheduling is enabled. the annotations were there for the vibes and then left 😭

• -gsplit-dwarf — if DWARF debugging information is enabled, separate as much debugging information as possible into a separate output file with the extension .dwo. this option allows the build system to avoid linking files with debug information. to be useful, this option requires a debugger capable of reading .dwo files.
• -gdwarf32
• -gdwarf64 — if DWARF debugging information is enabled, -gdwarf32 selects the 32-bit DWARF format and -gdwarf64 selects the 64-bit DWARF format. the default is target specific; on most targets it is -gdwarf32 though. the 32-bit DWARF format is smaller, but can't support more than 2GiB of debug information in any of the DWARF debug information sections. the 64-bit DWARF format allows larger debug information and might not be well supported by all consumers yet. future-proof but slightly risky, choose ur adventure 🦋
• -gdescribe-dies — adds 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 DIEs a lil name tag 🎀
• -gpubnames — generates DWARF ".debug_pubnames" and ".debug_pubtypes" sections.
• -ggnu-pubnames — bestie, this one generates .debug_pubnames and .debug_pubtypes sections in a format that's ready to be converted into a GDB index. ✨ But heads up: it's only useful with a linker that can produce GDB index version 7. No linker, no vibe.
• -fdebug-types-section — when you're on DWARF Version 4 or higher, type DIEs can go live in their own .debug_types section instead of being crammed into .debug_info. It's more efficient to park them in a separate comdat section because then the linker can just yeet the duplicates. 🗑️ But plot twist: not all DWARF consumers support .debug_types sections yet, and on some objects .debug_types actually produces larger instead of smaller debugging information. so like… cute idea, mixed results. 😭
• -grecord-gcc-switches
• -gno-record-gcc-switches — this switch makes the command-line options used to invoke the compiler that may affect code generation get appended to the DW_AT_producer attribute in DWARF debugging information. The options are concatenated with spaces separating them from each other and from the compiler version. It's enabled by default. Also peep -frecord-gcc-switches for another way of storing compiler options into the object file. 👀
• -gstrict-dwarf — disallow using extensions of later DWARF standard version than the one selected with -gdwarf-version. On most targets, using non-conflicting DWARF extensions from later standard versions is allowed, so this is you saying "no, stay in your lane." 💅
• -gno-strict-dwarf — allow using extensions of later DWARF standard version than selected with -gdwarf-version. the chill option. 😌
• -gas-loc-support — tells the compiler that the assembler supports .loc directives. It may then use them for the assembler to generate DWARF2+ line number tables. This is generally desirable, because assembler-generated line-number tables are a lot more compact than those the compiler can generate itself. 🔥 This option will be enabled by default if, at GCC configure time, the assembler was found to support such directives.
• -gno-as-loc-support — force GCC to generate DWARF2+ line number tables internally, if DWARF2+ line number tables are to be generated. do it yourself energy. 💪
• -gas-locview-support — tells the compiler that the assembler supports "view" assignment and reset assertion checking in .loc directives. This option will be enabled by default if, at GCC configure time, the assembler was found to support them.
• -gno-as-locview-support — force GCC to assign view numbers internally, if -gvariable-location-views are explicitly requested.
• -gcolumn-info
• -gno-column-info — emit location column information into DWARF debugging information, rather than just file and line. This option is enabled by default. ✨
• -gstatement-frontiers
• -gno-statement-frontiers — this option causes GCC to create markers in the internal representation at the beginning of statements, and to keep them roughly in place throughout compilation, using them to guide the output of is_stmt markers in the line number table. This is enabled by default when compiling with optimization (-Os, -O1, -O2, ...), and outputting DWARF 2 debug information at the normal level.
• -gvariable-location-views
• -gvariable-location-views=incompat5
• -gno-variable-location-views — augment variable location lists with progressive view numbers implied from the line number table. This enables debug information consumers to inspect state at certain points of the program, even if no instructions associated with the corresponding source locations are present at that point. If the assembler lacks support for view numbers in line number tables, this will cause the compiler to emit the line number table, which generally makes them somewhat less compact. The augmented line number tables and location lists are fully backward-compatible, so they can be consumed by debug information consumers that are not aware of these augmentations, but they won't derive any benefit from them either.

This is enabled by default when outputting DWARF 2 debug information at the normal level, as long as there is assembler support, -fvar-tracking-assignments is enabled and -gstrict-dwarf is not. When assembler support is not available, this may still be enabled, but it will force GCC to output internal line number tables, and if -ginternal-reset-location-views is not enabled, that will most certainly lead to silently mismatching location views. 😬

There is a proposed representation for view numbers that is not backward compatible with the location list format introduced in DWARF 5, that can be enabled with -gvariable-location-views=incompat5. This option may be removed in the future, is only provided as a reference implementation of the proposed representation. Debug information consumers are not expected to support this extended format, and they would be rendered unable to decode location lists using it.

🚨 ngl bestie, that incompat5 one is giving "use at your own risk" — consumers literally can't decode those location lists.

• -ginternal-reset-location-views
• -gno-internal-reset-location-views — attempt to determine location views that can be omitted from location view lists. This requires the compiler to have very accurate insn length estimates, which isn't always the case, and it may cause incorrect view lists to be generated silently when using an assembler that does not support location view lists. The GNU assembler will flag any such error as a "view number mismatch". This is only enabled on ports that define a reliable estimation function.
• -ginline-points
• -gno-inline-points — generate extended debug information for inlined functions. Location view tracking markers are inserted at inlined entry points, so that address and view numbers can be computed and output in debug information. This can be enabled independently of location views, in which case the view numbers won't be output, but it can only be enabled along with statement frontiers, and it is only enabled by default if location views are enabled.
• -gz[=type] — produce compressed debug sections in DWARF format, if that is supported. 🗜️ If type is not given, the default type depends on the capabilities of the assembler and linker used. type may be one of none (don't compress debug sections), or zlib (use zlib compression in ELF gABI format). If the linker doesn't support writing compressed debug sections, the option is rejected. Otherwise, if the assembler does not support them, -gz is silently ignored when producing object files.
• -femit-struct-debug-baseonly — emit debug information for struct-like types only when the base name of the compilation source file matches the base name of file in which the struct is defined.

This option substantially reduces the size of debugging information, but at significant potential loss in type information to the debugger. See -femit-struct-debug-reduced for a less aggressive option. See -femit-struct-debug-detailed for more detailed control.

This option works only with DWARF debug output.

• -femit-struct-debug-reduced — emit debug information for struct-like types only when the base name of the compilation source file matches the base name of file in which the type is defined, unless the struct is a template or defined in a system header.

This option significantly reduces the size of debugging information, with some potential loss in type information to the debugger. See -femit-struct-debug-baseonly for a more aggressive option. See -femit-struct-debug-detailed for more detailed control.

This option works only with DWARF debug output.

• -femit-struct-debug-detailed[=spec-list] — specify the struct-like types for which the compiler generates debug information. The intent is to reduce duplicate struct debug information between different object files within the same program.

This option is a detailed version of -femit-struct-debug-reduced and -femit-struct-debug-baseonly, which serves for most needs.

A specification has the syntax

[dir:|ind:][ord:|gen:](any|sys|base|none)

The optional first word limits the specification to structs that are used directly (dir:) or used indirectly (ind:). A struct type is used directly when it is the type of a variable, member. Indirect uses arise through pointers to structs. That is, when use of an incomplete struct is valid, the use is indirect. An example is struct one direct; struct two * indirect;.

The optional second word limits the specification to ordinary structs (ord:) or generic structs (gen:). Generic structs are a bit complicated to explain. For C++, these are non-explicit specializations of template classes, or non-template classes within the above. Other programming languages have generics, but -femit-struct-debug-detailed does not yet implement them.

The third word specifies the source files for those structs for which the compiler should emit debug information. The values none and any have the normal meaning. The value base means that the base of name of the file in which the type declaration appears must match the base of the name of the main compilation file. In practice, this means that when compiling foo.c, debug information is generated for types declared in that file and foo.h, but not other header files. The value sys means those types satisfying base or declared in system or compiler headers.

You may need to experiment to determine the best settings for your application.

The default is -femit-struct-debug-detailed=all.

This option works only with DWARF debug output.

💀 fr fr, that spec-list syntax is a whole mood — read it twice, bestie.

• -fno-dwarf2-cfi-asm — emit DWARF unwind info as compiler generated .eh_frame section instead of using GAS .cfi_* directives.
• -fno-eliminate-unused-debug-types — normally, when producing DWARF output, GCC avoids producing debug symbol output for types that are nowhere used in the source file being compiled. Sometimes it is useful to have GCC emit debugging information for all types declared in a compilation unit, regardless of whether or not they are actually used in that compilation unit, for example if, in the debugger, you want to cast a value to a type that is not actually used in your program (but is declared). More often, however, this results in a significant amount of wasted space. 🙈

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⚡ Options That Control Optimization ⚡

These options control various sorts of optimizations. 🔥

Okay so real talk, bestie — optimization is the ✨main character✨ of the compiler world, and we need to talk about it. 💅

Without any optimization option, the compiler's goal is to reduce the cost of compilation and to make debugging produce the expected results. Statements are independent: if you stop the program with a breakpoint between statements, you can then assign a new value to any variable or change the program counter to any other statement in the function and get exactly the results you expect from the source code. No surprises, no gaslighting, the debugger just works. 🎀

Turning on optimization flags makes the compiler attempt to improve the performance and/or code size at the expense of compilation time and possibly the ability to debug the program. Trade-offs, bestie. Everything is a trade-off. 😭

The compiler performs optimization based on the knowledge it has of the program. Compiling multiple files at once to a single output file mode allows the compiler to use information gained from all of the files when compiling each of them. It's giving "teamwork makes the dream work" fr fr.

Not all optimizations are controlled directly by a flag. Only optimizations that have a flag are listed in this section. (Yes queen, some of the magic is just ✨in there✨ and you cannot flag it.)

Most optimizations are completely disabled at -O0 or if an -O level is not set on the command line, even if individual optimization flags are specified. Similarly, -Og suppresses many optimization passes.

Depending on the target and how GCC was configured, a slightly different set of optimizations may be enabled at each -O level than those listed here. You can invoke GCC with -Q --help=optimizers to find out the exact set of optimizations that are enabled at each level. Go look it up, it's literally right there, slay. 💖

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• -O
• -O1 — Optimize. Optimizing compilation takes somewhat more time, and a lot more memory for a large function. 💀

With -O, the compiler tries to reduce code size and execution time, without performing any optimizations that take a great deal of compilation time.

-O turns on the following optimization flags:

-fauto-inc-dec -fbranch-count-reg -fcombine-stack-adjustments
-fcompare-elim -fcprop-registers -fdce -fdefer-pop -fdelayed-branch
-fdse -fforward-propagate -fguess-branch-probability -fif-conversion
-fif-conversion2 -finline-functions-called-once -fipa-modref
-fipa-profile -fipa-pure-const -fipa-reference
-fipa-reference-addressable -fmerge-constants -fmove-loop-invariants
-fmove-loop-stores -fomit-frame-pointer -freorder-blocks
-fshrink-wrap -fshrink-wrap-separate -fsplit-wide-types
-fssa-backprop -fssa-phiopt -ftree-bit-ccp -ftree-ccp -ftree-ch
-ftree-coalesce-vars -ftree-copy-prop -ftree-dce
-ftree-dominator-opts -ftree-dse -ftree-forwprop -ftree-fre
-ftree-phiprop -ftree-pta -ftree-scev-cprop -ftree-sink -ftree-slsr
-ftree-sra -ftree-ter -funit-at-a-time

• -O2 — Optimize even more. GCC performs nearly all supported optimizations that do not involve a space-speed tradeoff. As compared to -O, this option increases both compilation time and the performance of the generated code. She's doing more, she's taking longer, and honestly? Worth it.

-O2 turns on all optimization flags specified by -O1. It also turns on the following optimization flags:

-falign-functions  -falign-jumps -falign-labels  -falign-loops
-fcaller-saves -fcode-hoisting -fcrossjumping -fcse-follow-jumps
-fcse-skip-blocks -fdelete-null-pointer-checks -fdevirtualize
-fdevirtualize-speculatively -fexpensive-optimizations
-ffinite-loops -fgcse  -fgcse-lm -fhoist-adjacent-loads
-finline-functions -finline-small-functions -findirect-inlining
-fipa-bit-cp  -fipa-cp  -fipa-icf -fipa-ra  -fipa-sra  -fipa-vrp
-fisolate-erroneous-paths-dereference -flra-remat
-foptimize-sibling-calls -foptimize-strlen -fpartial-inlining
-fpeephole2 -freorder-blocks-algorithm=stc
-freorder-blocks-and-partition  -freorder-functions
-frerun-cse-after-loop -fschedule-insns  -fschedule-insns2
-fsched-interblock  -fsched-spec -fstore-merging -fstrict-aliasing
-fthread-jumps -ftree-builtin-call-dce -ftree-loop-vectorize
-ftree-pre -ftree-slp-vectorize -ftree-switch-conversion
-ftree-tail-merge -ftree-vrp -fvect-cost-model=very-cheap

Please note the warning under -fgcse about invoking -O2 on programs that use computed gotos. ⚠️ Not a drill, bestie, go read it.

NOTE: In Ubuntu 8.10 and later versions, -D_FORTIFY_SOURCE=2, in Ubuntu 24.04 and later versions, -D_FORTIFY_SOURCE=3, is set by default, and is activated when -O is set to 2 or higher. This enables additional compile-time and run-time checks for several libc functions. To disable, specify either -U_FORTIFY_SOURCE or -D_FORTIFY_SOURCE=0.

NOTE: In Debian 13 and Ubuntu 24.04 and later versions, -D_TIME_BITS=64 together with -D_FILE_OFFSET_BITS=64 is set by default on the 32bit architectures armel, armhf, hppa, m68k, mips, mipsel, powerpc and sh4.

• -O3 — Optimize yet more. -O3 turns on all optimizations specified by -O2 and also turns on the following optimization flags: 🔥

-fgcse-after-reload -fipa-cp-clone -floop-interchange
-floop-unroll-and-jam -fpeel-loops -fpredictive-commoning
-fsplit-loops -fsplit-paths -ftree-loop-distribution
-ftree-partial-pre -funswitch-loops -fvect-cost-model=dynamic
-fversion-loops-for-strides

• -O0 — Reduce compilation time and make debugging produce the expected results. This is the default. Chill mode. No notes.
• -Os — Optimize for size. -Os enables all -O2 optimizations except those that often increase code size:

-falign-functions  -falign-jumps -falign-labels  -falign-loops
-fprefetch-loop-arrays  -freorder-blocks-algorithm=stc

It also enables -finline-functions, causes the compiler to tune for code size rather than execution speed, and performs further optimizations designed to reduce code size. Small and cute, the aesthetic.

• -Ofast — Disregard strict standards compliance. -Ofast enables all -O3 optimizations. It also enables optimizations that are not valid for all standard-compliant programs. It turns on -ffast-math, -fallow-store-data-races and the Fortran-specific -fstack-arrays, unless -fmax-stack-var-size is specified, and -fno-protect-parens. It turns off -fsemantic-interposition.

Standards? Never heard of her. This one is unhinged (affectionate) and lowkey breaks the rules, so know what you're signing up for.

• -Og — Optimize debugging experience. -Og should be the optimization level of choice for the standard edit-compile-debug cycle, offering a reasonable level of optimization while maintaining fast compilation and a good debugging experience. It is a better choice than -O0 for producing debuggable code because some compiler passes that collect debug information are disabled at -O0. 🐛✨

Like -O0, -Og completely disables a number of optimization passes so that individual options controlling them have no effect. Otherwise -Og enables all -O1 optimization flags except for those that may interfere with debugging:

-fbranch-count-reg  -fdelayed-branch -fdse  -fif-conversion
-fif-conversion2 -finline-functions-called-once
-fmove-loop-invariants  -fmove-loop-stores  -fssa-phiopt
-ftree-bit-ccp  -ftree-dse  -ftree-pta  -ftree-sra

• -Oz — Optimize aggressively for size rather than speed. This may increase the number of instructions executed if those instructions require fewer bytes to encode. -Oz behaves similarly to -Os including enabling most -O2 optimizations. Smol queen energy 👑

---

If you use multiple -O options, with or without level numbers, the last such option is the one that is effective. Last one wins, bestie, like always.

Options of the form -fflag specify machine-independent flags. Most flags have both positive and negative forms; the negative form of -ffoo is -fno-foo. In the table below, only one of the forms is listed---the one you typically use. You can figure out the other form by either removing no- or adding it. iykyk 💅

The following options control specific optimizations. They are either activated by -O options or are related to ones that are. You can use the following flags in the rare cases when "fine-tuning" of optimizations to be performed is desired. 🎀

• -fno-defer-pop — For machines that must pop arguments after a function call, always pop the arguments as soon as each function returns. At levels -O1 and higher, -fdefer-pop is the default; this allows the compiler to let arguments accumulate on the stack for several function calls and pop them all at once.
• -fforward-propagate — Perform a forward propagation pass on RTL. The pass tries to combine two instructions and checks if the result can be simplified. If loop unrolling is active, two passes are performed and the second is scheduled after loop unrolling.

This option is enabled by default at optimization levels -O1, -O2, -O3, -Os.

• -ffp-contract=style — -ffp-contract=off disables floating-point expression contraction. -ffp-contract=fast enables floating-point expression contraction such as forming of fused multiply-add operations if the target has native support for them. -ffp-contract=on enables floating-point expression contraction if allowed by the language standard. This is implemented for C and C++, where it enables contraction within one expression, but not across different statements.

The default is -ffp-contract=off for C in a standards compliant mode (-std=c11 or similar), -ffp-contract=fast otherwise.

• -fomit-frame-pointer — Omit the frame pointer in functions that don't need one. This avoids the instructions to save, set up and restore the frame pointer; on many targets it also makes an extra register available. Free register, free real estate.

On some targets this flag has no effect because the standard calling sequence always uses a frame pointer, so it cannot be omitted.

Note that -fno-omit-frame-pointer doesn't guarantee the frame pointer is used in all functions. Several targets always omit the frame pointer in leaf functions. 🙈

Enabled by default at -O1 and higher.

• -foptimize-sibling-calls — Optimize sibling and tail recursive calls.

Enabled at levels -O2, -O3, -Os.

• -foptimize-strlen — Optimize various standard C string functions (e.g. strlen, strchr or strcpy) and their _FORTIFY_SOURCE counterparts into faster alternatives. 🍓

Enabled at levels -O2, -O3.

• -finline-stringops[=fn] — Expand memory and string operations (for now, only memset) inline, even when the length is variable or big enough as to require looping. This is most useful along with -ffreestanding and -fno-builtin.

In some circumstances, it enables the compiler to generate code that takes advantage of known alignment and length multipliers, but even then it may be less efficient than optimized runtime implementations, and grow code size so much that even a less performant but shared implementation runs faster due to better use of code caches. This option is disabled by default. 💀 Bigger code is not always better code, bestie, code caches are real.

• -fno-inline — okay bestie, this one says: do not expand any functions inline, except the ones marked with the always_inline attribute. This is the default when you're not optimizing at all. lowkey important: single functions can still be exempted from inlining by marking them with the noinline attribute. 💅
• -finline-small-functions — this is the vibe: integrate functions into their callers when their body is smaller than expected function call code, so the overall size of the program gets smaller. The compiler heuristically decides which functions are simple enough to be worth integrating this way. This inlining applies to all functions, even those not declared inline. Enabled at levels -O2, -O3, -Os. ✨
• -findirect-inlining — sneaky little one: inline also indirect calls that are discovered to be known at compile time thanks to previous inlining. This option has any effect only when inlining itself is turned on by the -finline-functions or -finline-small-functions options. Enabled at levels -O2, -O3, -Os. 🦋
• -finline-functions — main character inlining energy: consider 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 — aww, only once? Consider all static functions called once for inlining into their caller even if they are not marked inline. If a call to a given function is integrated, then the function is not output as assembler code in its own right. Enabled at levels -O1, -O2, -O3 and -Os, but not -Og. 💀
• -fearly-inlining — early bird gets the inline: inline functions marked by always_inline and functions whose body seems smaller than the function call overhead early before doing -fprofile-generate instrumentation and real inlining pass. Doing so makes profiling significantly cheaper and usually inlining faster on programs having large chains of nested wrapper functions. Enabled by default. 🐦
• -fipa-sra — interprocedural glow-up: perform interprocedural scalar replacement of aggregates, removal of unused parameters and replacement of parameters passed by reference by parameters passed by value. Enabled at levels -O2, -O3 and -Os. 🌸
• -finline-limit=n — the limit does exist, bestie. By default, GCC limits the size of functions that can be inlined. This flag allows coarse control of this limit. n is the size of functions that can be inlined in number of pseudo instructions. Inlining is actually controlled by a number of parameters, which may be specified individually by using --param name=value. The -finline-limit=n option sets some of these parameters as follows:
  • max-inline-insns-single — is set to n/2.
  • max-inline-insns-auto — is set to n/2.

See below for a documentation of the individual parameters controlling inlining and for the defaults of these parameters.

Note: there may be no value to -finline-limit that results in default behavior.

Note: pseudo instruction represents, in this particular context, an abstract measurement of function's size. In no way does it represent a count of assembly instructions and as such its exact meaning might change from one release to an another. 😭

• -fno-keep-inline-dllexport — a more fine-grained version of -fkeep-inline-functions, which applies only to functions that are declared using the dllexport attribute or declspec. 🎀
• -fkeep-inline-functions — keep them around, periodt: in C, emit static functions that are declared inline into the object file, even if the function has been inlined into all of its callers. This switch does not affect functions using the extern inline extension in GNU C90. In C++, emit any and all inline functions into the object file. 💖
• -fkeep-static-functions — don't ghost the static functions: emit static functions into the object file, even if the function is never used. 👻
• -fkeep-static-consts — static const? keep her: emit variables declared static const when optimization isn't turned on, even if the variables aren't referenced. GCC enables this option by default. If you want to force the compiler to check if a variable is referenced, regardless of whether or not optimization is turned on, use the -fno-keep-static-consts option. ⭐
• -fmerge-constants — merge them, they're the same bestie: attempt to merge identical constants (string constants and floating-point constants) across compilation units. This option is the default for optimized compilation if the assembler and linker support it. Use -fno-merge-constants to inhibit this behavior. Enabled at levels -O1, -O2, -O3, -Os. 🌈
• -fmerge-all-constants — okay now we're merging everything: attempt to merge identical constants and identical variables. This option implies -fmerge-constants. In addition to -fmerge-constants this considers e.g. even constant initialized arrays or initialized constant variables with integral or floating-point types. Languages like C or C++ require each variable, including multiple instances of the same variable in recursive calls, to have distinct locations, so using this option results in non-conforming behavior. 🚨
• -fmodulo-sched — swing modulo scheduling, she's a dancer: perform swing modulo scheduling immediately before the first scheduling pass. This pass looks at innermost loops and reorders their instructions by overlapping different iterations. 🌀
• -fmodulo-sched-allow-regmoves — more aggressive, more moves: perform more aggressive SMS-based modulo scheduling with register moves allowed. By setting this flag certain anti-dependences edges are deleted, which triggers the generation of reg-moves based on the life-range analysis. This option is effective only with -fmodulo-sched enabled. 💃
• -fno-branch-count-reg — no decrement-and-branch today: disable the optimization pass that scans for opportunities to use decrement and branch instructions on a count register instead of instruction sequences that decrement a register, compare it against zero, and then branch based upon the result. This option is only meaningful on architectures that support such instructions, which include x86, PowerPC, IA-64 and S/390. Note that the -fno-branch-count-reg option doesn't remove the decrement and branch instructions from the generated instruction stream introduced by other optimization passes. The default is -fbranch-count-reg at -O1 and higher, except for -Og. 🫶
• -fno-function-cse — no function addresses in registers, tyvm: do not put function addresses in registers; make each instruction that calls a constant function contain the function's address explicitly. This option results in less efficient code, but some strange hacks that alter the assembler output may be confused by the optimizations performed when this option is not used. The default is -ffunction-cse. 🧚
• -fno-zero-initialized-in-bss — BSS said no thanks: if the target supports a BSS section, GCC by default puts variables that are initialized to zero into BSS. This can save space in the resulting code. This option turns off this behavior because some programs explicitly rely on variables going to the data section---e.g., so that the resulting executable can find the beginning of that section and/or make assumptions based on that. The default is -fzero-initialized-in-bss. 🍓
• -fthread-jumps — jump, but make it smarter: perform optimizations that check to see if a jump branches to a location where another comparison subsumed by the first is found. If so, the first branch is redirected to either the destination of the second branch or a point immediately following it, depending on whether the condition is known to be true or false. Enabled at levels -O1, -O2, -O3, -Os. 🔥
• -fsplit-wide-types — wide types, split 'em up: when using a type that occupies multiple registers, such as long long on a 32-bit system, split the registers apart and allocate them independently. This normally generates better code for those types, but may make debugging more difficult. Enabled at levels -O1, -O2, -O3, -Os. 🧩
• -fsplit-wide-types-early — do it early, not late: fully split wide types early, instead of very late. This option has no effect unless -fsplit-wide-types is turned on. This is the default on some targets. ⏰
• -fcse-follow-jumps — CSE said follow that jump: in common subexpression elimination (CSE), scan through jump instructions when the target of the jump is not reached by any other path. For example, when CSE encounters an if statement with an else clause, CSE follows the jump when the condition tested is false. Enabled at levels -O2, -O3, -Os. 🦋
• -fcse-skip-blocks — similar but skip blocks: this is similar to -fcse-follow-jumps, but causes CSE to follow jumps that conditionally skip over blocks. When CSE encounters a simple if statement with no else clause, -fcse-skip-blocks causes CSE to follow the jump around the body of the if. Enabled at levels -O2, -O3, -Os. 💅
• -frerun-cse-after-loop — run it again after loops: re-run common subexpression elimination after loop optimizations are performed. Enabled at levels -O2, -O3, -Os. 🔁
• -fgcse — global common subexpression elimination, the icon: perform a global common subexpression elimination pass. This pass also performs global constant and copy propagation.

Note: When compiling a program using computed gotos, a GCC extension, you may get better run-time performance if you disable the global common subexpression elimination pass by adding -fno-gcse to the command line.

Enabled at levels -O2, -O3, -Os. 👑

• -fgcse-lm — load motion, she moves: when -fgcse-lm is enabled, global common subexpression elimination attempts to move loads that are only killed by stores into themselves. This allows a loop containing a load/store sequence to be changed to a load outside the loop, and a copy/store within the loop. Enabled by default when -fgcse is enabled. 💖
• -fgcse-sm — store motion, after GCSE: when -fgcse-sm is enabled, a store motion pass is run after global common subexpression elimination. This pass attempts to move stores out of loops. When used in conjunction with -fgcse-lm, loops containing a load/store sequence can be changed to a load before the loop and a store after the loop. Not enabled at any optimization level. 😭
• -fgcse-las — load after store? redundant, bye: when -fgcse-las is enabled, the global common subexpression elimination pass eliminates redundant loads that come after stores to the same memory location (both partial and full redundancies). Not enabled at any optimization level. 💀
• -fgcse-after-reload — bestie, when this one's on, GCC runs a whole redundant load elimination pass after reload, and the whole point is cleaning up redundant spilling 🧹. It's switched on for you by -O3, -fprofile-use and -fauto-profile.
• -faggressive-loop-optimizations — this tells the loop optimizer to use language constraints to figure out bounds for how many times a loop iterates 👀. It assumes your loop code doesn't invoke undefined behavior — so no signed integer overflows, no out-of-bound array accesses, none of that 🙈. Those iteration bounds then guide loop unrolling, peeling, and loop exit test optimizations. It's on by default, so it's already doing its thing ✨.
• -funconstrained-commons — tells the compiler that variables declared in common blocks (Fortran's the classic example 💜) might get overridden later with longer trailing arrays. This blocks certain optimizations that would need to know the array bounds, so it's a safety-first kind of vibe 🫶.
• -fcrossjumping — performs the cross-jumping transformation, which unifies equivalent code and saves code size 📦. Heads up tho: the resulting code may or may not actually perform better than without cross-jumping. No promises, it's giving "smaller but who knows" 😭. Enabled at -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 the instructions to support it, no cap. On architectures that do support it, it's enabled by default at -O1 and higher.
• -fdce — performs dead code elimination (DCE) on RTL 💀 (fitting, honestly). On by default at -O1 and higher.
• -fdse — performs dead store elimination (DSE) on RTL. Also on by default at -O1 and higher 🌸.
• -fif-conversion — attempts to transform conditional jumps into branch-less equivalents ✨. That includes conditional moves, min, max, set flags and abs instructions, plus some tricks that standard arithmetics can pull off 🧚. The use of conditional execution on chips where it exists is controlled by -fif-conversion2. Enabled at -O1, -O2, -O3, -Os — but not with -Og.
• -fif-conversion2 — uses conditional execution (where it's available) to turn conditional jumps into branch-less equivalents 🎀. Enabled at -O1, -O2, -O3, -Os, but again not with -Og.
• -fdeclone-ctor-dtor — okay so the C++ ABI requires multiple entry points for constructors and destructors: one for a base subobject, one for a complete object, and one for a virtual destructor that calls operator delete afterwards 👑. For a hierarchy with virtual bases, the base and complete variants are clones — which literally means two copies of the function 😭. With this option, the base and complete variants become thunks that call a common implementation. Cleaner, honestly 💅. 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 — they assume a memory access to address zero always traps, so if a pointer gets checked after it's already been dereferenced, it can't be null. Makes sense, right?

But — and this is the important bit — in some environments that assumption is just not true 😬. Use -fno-delete-null-pointer-checks to disable this optimization for programs that depend on that behavior.

This option is enabled by default on most targets. On Nios II ELF, it defaults to off. On AVR and MSP430, this option is completely disabled. Passes that use the dataflow information are enabled independently at different optimization levels.

• -fdevirtualize — attempts to convert calls to virtual functions into direct calls ✨. This happens both within a procedure and interprocedurally, as part of indirect inlining (-findirect-inlining) and interprocedural constant propagation (-fipa-cp). Enabled at -O2, -O3, -Os.
• -fdevirtualize-speculatively — attempts to convert calls to virtual functions into speculative direct calls 👀. Based on the analysis of the type inheritance graph, it figures out the set of likely targets for a given call. If that set is small — preferably of size 1 — it changes the call into a conditional deciding between direct and indirect calls. These speculative calls enable more optimizations, like inlining 🔥. And when they turn out to be useless after further optimization, they get converted back into their original form. Recycling era ♻️.
• -fdevirtualize-at-ltrans — streams extra info needed for aggressive devirtualization when running the link-time optimizer in local transformation mode 💫. This enables more devirtualization but significantly increases the size of the streamed data. For that reason, it's disabled by default. Trade-offs, bestie 😭.
• -fexpensive-optimizations — performs a number of minor optimizations that are relatively expensive 💸. Enabled at -O2, -O3, -Os.
• -free — attempts to remove redundant extension instructions ✂️. This is especially helpful for the x86-64 architecture, which implicitly zero-extends in 64-bit registers after writing to their lower 32-bit half. Enabled for Alpha, AArch64, LoongArch, PowerPC, RISC-V, SPARC, h83000 and x86 at -O2, -O3, -Os.
• -fno-lifetime-dse — in C++ the value of an object is only affected by changes within its lifetime: when the constructor begins, the object has an indeterminate value, and any changes during the object's lifetime are dead once the object is destroyed 💀. Normally dead store elimination takes advantage of this. But if your code relies on the value of the object storage persisting beyond the lifetime of the object, you can use this flag to disable that optimization 🙈.

To preserve stores before the constructor starts (e.g. because your operator new clears the object storage) but still treat the object as dead after the destructor, you can use -flifetime-dse=1. The default behavior can be explicitly selected with -flifetime-dse=2. And -flifetime-dse=0 is equivalent to -fno-lifetime-dse. Choose your fighter 🎀.

• -flive-range-shrinkage — attempts to decrease register pressure through register live range shrinkage 💅. This is helpful for fast processors with small or moderate size register sets.
• -fira-algorithm=algorithm — uses the specified coloring algorithm for the integrated register allocator 🎨. The algorithm argument can be priority, which specifies Chow's priority coloring, or CB, which specifies Chaitin-Briggs coloring. Chaitin-Briggs coloring is not implemented for all architectures — but for those targets that do support it, it's the default, because it generates better code ✨.
• -fira-region=region — uses the specified regions for the integrated register allocator. The region argument should be one of these:
  • 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 is enabled by default when compiling with optimization for speed (-O, -O2, ...). It's the main character for a reason 👑.
  • one — use all functions as a single region. This typically results in the smallest code size, and is enabled by default for -Os or -O0 🌸.
• -fira-hoist-pressure — uses IRA to evaluate register pressure in the code hoisting pass, so it can make decisions about hoisting expressions 🫶. This option usually results in smaller code, but it can slow the compiler down 🐢. It's enabled at level -Os for all targets.
• -fira-loop-pressure — uses IRA to evaluate register pressure in loops, for decisions about moving loop invariants 🔁. This usually generates faster and smaller code on machines with large register files (>= 32 registers), but yeah, it can slow the compiler down too 😭. Enabled at level -O3 for some targets.
• -fno-ira-share-save-slots — disables sharing of stack slots used for saving call-used hard registers that live through a call. Each hard register gets its own separate stack slot, and as a result, function stack frames are larger 📦. Bigger frames, less sharing, that's the deal.
• -fno-ira-share-spill-slots — disables sharing of stack slots allocated for pseudo-registers. Every pseudo-register that doesn't get a hard register gets its own separate stack slot, and as a result, function stack frames are larger 😮‍💨. Same energy as above, different registers.
• -flra-remat — enables CFG-sensitive rematerialization in LRA ✨. Instead of loading values of spilled pseudos, LRA tries to rematerialize (recalculate) values if it's profitable. Enabled at -O2, -O3, -Os.
• -fdelayed-branch — if it's supported for the target machine, this attempts to reorder instructions to exploit instruction slots available after delayed branch instructions 🎯. Enabled at -O1, -O2, -O3, -Os, but not at -Og.
• -fschedule-insns — if supported for the target machine, attempts to reorder instructions to eliminate execution stalls caused by required data being unavailable 🛑. This helps machines that have slow floating point or memory load instructions, by letting other instructions be issued until the result of the load or floating-point instruction is actually needed. Enabled at -O2, -O3.
• -fschedule-insns2 — similar to -fschedule-insns, but it requests an additional pass of instruction scheduling after register allocation has been done 🔁. This is especially useful on machines with a relatively small number of registers, and where memory load instructions take more than one cycle. Enabled at -O2, -O3, -Os.
• -fno-sched-interblock — disables instruction scheduling across basic blocks 🚧, which is normally enabled when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
• -fno-sched-spec — disables speculative motion of non-load instructions 🙅, which is normally enabled when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
• -fsched-pressure — ok so this one turns on register-pressure-sensitive instruction scheduling before register allocation 💅 and it only makes sense when scheduling-before-regalloc is even on, like with -fschedule-insns or at -O2 or higher. Using it can level up the generated code and shrink it, because it stops register pressure from climbing above the number of available hard registers (and thus stops all those spills in register allocation). spill prevention era ✨
• -fsched-spec-load — lets some load instructions do speculative motion 👀. Only makes sense when you're scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher.
• -fsched-spec-load-dangerous — same vibe but it allows speculative motion of more load instructions. Also only makes sense when scheduling before register allocation, i.e. with -fschedule-insns or at -O2 or higher. Bestie the name says dangerous, respect the name 🙈
• -fsched-stalled-insns, -fsched-stalled-insns=n — this defines how many insns (if any) get moved prematurely out of the queue of stalled insns and into the ready list during the second scheduling pass. So: -fno-sched-stalled-insns means no insns at all get moved prematurely, and -fsched-stalled-insns=0 means there is no limit on how many queued insns can be moved prematurely. And -fsched-stalled-insns with no value? That's equivalent to -fsched-stalled-insns=1. periodt 💖
• -fsched-stalled-insns-dep, -fsched-stalled-insns-dep=n — this one 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. It only does anything during the second scheduling pass, and only if -fsched-stalled-insns is being used. -fno-sched-stalled-insns-dep is equivalent to -fsched-stalled-insns-dep=0. And -fsched-stalled-insns-dep with no value is equivalent to -fsched-stalled-insns-dep=1 🎀
• -fsched2-use-superblocks — when you're scheduling after register allocation, this says use superblock scheduling, which lets motion cross basic block boundaries and gives you faster schedules 🔥. BUT — this option is experimental, because not all machine descriptions used by GCC model the CPU closely enough to dodge unreliable results from the algorithm. It only makes sense when scheduling after register allocation, i.e. with -fschedule-insns2 or at -O2 or higher.
• -fsched-group-heuristic — turns on the group heuristic in the scheduler. This heuristic favors the instruction that belongs to a schedule group. Enabled by default whenever scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher 👑
• -fsched-critical-path-heuristic — turns on the critical-path heuristic in the scheduler, which favors instructions on the critical path. Enabled by default whenever scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher ✨
• -fsched-spec-insn-heuristic — turns on the speculative instruction heuristic in the scheduler. It favors speculative instructions with greater dependency weakness. Enabled by default whenever scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher 🦋
• -fsched-rank-heuristic — turns on the rank heuristic in the scheduler, which favors the instruction belonging to a basic block with greater size or frequency. Enabled by default whenever scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher 💜
• -fsched-last-insn-heuristic — turns on the last-instruction heuristic in the scheduler, favoring the instruction that is less dependent on the last instruction scheduled. Enabled by default whenever scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher 🌸
• -fsched-dep-count-heuristic — turns on the dependent-count heuristic in the scheduler, favoring the instruction that has more instructions depending on it. Enabled by default whenever scheduling is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or higher ⭐

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• -freschedule-modulo-scheduled-loops — so modulo scheduling happens before traditional scheduling. If a loop is modulo scheduled, later scheduling passes may change its schedule, and this option is how you control that behavior 🎛️
• -fselective-scheduling — schedule instructions using the selective scheduling algorithm. Selective scheduling runs instead of the first scheduler pass.
• -fselective-scheduling2 — schedule instructions using the selective scheduling algorithm. This time selective scheduling runs instead of the second scheduler pass.
• -fsel-sched-pipelining — enables software pipelining of innermost loops during selective scheduling 🌀. Heads up: this option has no effect unless one of -fselective-scheduling or -fselective-scheduling2 is turned on.
• -fsel-sched-pipelining-outer-loops — when you're pipelining loops during selective scheduling, this also pipelines outer loops. No effect unless -fsel-sched-pipelining is turned on, iykyk 💫
• -fsemantic-interposition — some object formats, like ELF, allow interposing of symbols by the dynamic linker. That means for symbols exported from the DSO, the compiler can't do interprocedural propagation, inlining and other optimizations, because it has to anticipate that the function or variable in question may change. While this feature is useful — for example, to rewrite memory allocation functions by a debugging implementation — it's expensive in terms of code quality 😭. With -fno-semantic-interposition, the compiler assumes that if interposition happens for functions, the overwriting function will have precisely the same semantics (and side effects). Similarly, if interposition happens for variables, the constructor of the variable will be the same. And the flag has no effect for functions explicitly declared inline (where it is never allowed for interposition to change semantics) and for symbols explicitly declared weak. 💅
• -fshrink-wrap — emit function prologues only before the parts of the function that actually need them, instead of at the top of the function. This flag is enabled by default at -O and higher 🎀
• -fshrink-wrap-separate — shrink-wrap separate parts of the prologue and epilogue separately, so those parts are only executed when needed. It's on by default, but has no effect unless -fshrink-wrap is 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 the registers around such calls. This allocation only happens when it looks like it'll result in better code. On certain machines this option is always enabled by default — usually the ones that have no call-preserved registers to use instead. Enabled at levels -O2, -O3, -Os 🌈
• -fcombine-stack-adjustments — tracks stack adjustments (pushes and pops) and stack memory references, then tries to find ways to combine them. Enabled by default at -O1 and higher 🧚
• -fipa-ra — use caller save registers for allocation if those registers aren't used by any called function. In that case you don't need to save and restore them around calls. This is only possible if the called functions are part of the same compilation unit as the current function and they're compiled before it. Enabled at levels -O2, -O3, -Os — however, the option is disabled if the generated code will be instrumented for profiling (-p, or -pg) or if the callee's register usage can't be known exactly (this happens on targets that don't expose prologues and epilogues in RTL) 💀
• -fconserve-stack — attempts to minimize stack usage. The compiler tries to use less stack space, even if that makes the program slower 🐌. This option implies setting the large-stack-frame parameter to 100 and the large-stack-frame-growth parameter to 400.
• -ftree-reassoc — performs reassociation on trees. Enabled by default at -O1 and 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. It's especially useful as a code size optimization, but it often helps for code speed too 🔥. Enabled by default at -O2 and higher.
• -ftree-pre — performs partial redundancy elimination (PRE) on trees. Enabled by default at -O2 and -O3 👑
• -ftree-partial-pre — makes partial redundancy elimination (PRE) more aggressive. Enabled by default at -O3.
• -ftree-forwprop — performs forward propagation on trees. Enabled by default at -O1 and higher 💜
• -ftree-fre — performs full redundancy elimination (FRE) on trees. The difference between FRE and PRE is that FRE only considers expressions that are computed on all paths leading to the redundant computation. This analysis is faster than PRE, though it exposes fewer redundancies. Enabled by default at -O1 and higher ⭐
• -ftree-phiprop — performs hoisting of loads from conditional pointers on trees. Enabled by default at -O1 and higher 🌸
• -fhoist-adjacent-loads — speculatively hoists loads from both branches of an if-then-else if the loads are from adjacent locations in the same structure and the target architecture has a conditional move instruction. Enabled by default at -O2 and higher 🦋
• -ftree-copy-prop — performs copy propagation on trees. This pass eliminates unnecessary copy operations. Enabled by default at -O1 and higher 🎀
• -fipa-pure-const — discovers which functions are pure or constant. Enabled by default at -O1 and higher 🧚
• -fipa-reference — discovers which static variables do not escape the compilation unit. Enabled by default at -O1 and higher ✨
• -fipa-reference-addressable — discovers read-only, write-only and non-addressable static variables. Enabled by default at -O1 and higher 💖
• -fipa-stack-alignment — reduces stack alignment on call sites if possible. Enabled by default.
• -fipa-pta — performs interprocedural pointer analysis and interprocedural modification and reference analysis. Careful bestie: this option can cause excessive memory and compile-time usage on large compilation units 😭. It is not enabled by default at any optimization level.
• -fipa-profile — bestie, this one does interprocedural profile propagation. Functions that only ever get called from cold functions? Marked cold too, obviously. 💀 It also sniffs out functions that run exactly once — think "cold", "noreturn", static constructors or destructors — and then optimizes those cold functions (plus the loop-less bits of one-shot functions) for size. Enabled by default at -O1 and higher. ✨
• -fipa-modref — this is interprocedural mod/ref analysis. It looks at what functions actually do to the world — which memory locations get modified or referenced — so the compiler can optimize across the function call boundary like it's no big deal. Enabled by default at -O1 and higher. 🦋
• -fipa-cp — interprocedural constant propagation, okay?! It analyzes the whole program to figure out when the values passed to functions are constants, then optimizes accordingly. If your app passes constants to functions, this can be a substantial performance glow-up. 💅 Enabled by default at -O2, -Os and -O3. It's also enabled by -fprofile-use and -fauto-profile.
• -fipa-cp-clone — clones functions to make interprocedural constant propagation even stronger. When it's on, IPA constant propagation clones a function when an externally visible function can be called with constant arguments. Because this can make multiple copies of functions, it may significantly increase code size (peep --param ipa-cp-unit-growth=value for that). Enabled by default at -O3. It's also enabled by -fprofile-use and -fauto-profile. 👑
• -fipa-bit-cp — when enabled, does interprocedural bitwise constant propagation. Enabled by default at -O2 and by -fprofile-use and -fauto-profile. It requires that -fipa-cp is enabled. periodt.
• -fipa-vrp — when enabled, does interprocedural propagation of value ranges. Enabled by default at -O2. It requires that -fipa-cp is enabled. same energy.
• -fipa-icf — Identical Code Folding for functions and read-only variables, no cap. It reduces code size and may disturb unwind stacks by replacing a function with an equivalent one that has a different name. 😭 The optimization works more effectively with link-time optimization enabled.

The vibe is similar to the Gold Linker's ICF optimization, but GCC ICF works on different levels — so the optimizations are not the same. There are equivalences that only GCC finds and equivalences that only Gold finds. 🫶

This flag is enabled by default at -O2 and -Os.

• -flive-patching=level — controls GCC's optimizations so the output is suitable for live-patching. 🌸

Here's the tea: if the compiler's optimization uses a function's body, or info extracted from its body, to optimize or change another function, that other function is called an impacted function of the first one. So if you patch a function, you should patch its impacted functions too. 😤

The impacted functions are determined by the compiler's interprocedural optimizations. For example, a caller is impacted when you inline a function into its caller, or clone a function and change its caller to call this new clone, or extract a function's pureness/constness info to optimize its direct or indirect callers, etc.

Usually the more IPA optimizations are enabled, the larger the number of impacted functions per function. So to keep that number manageable and make the impacted-function list easier to compute, 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. So when you patch a function, all its callers and its clones' callers are impacted, and therefore need to be patched as well. 💅

-flive-patching=inline-clone disables the following optimization flags: -fwhole-program -fipa-pta -fipa-reference -fipa-ra -fipa-icf -fipa-icf-functions -fipa-icf-variables -fipa-bit-cp -fipa-vrp -fipa-pure-const -fipa-reference-addressable -fipa-stack-alignment -fipa-modref

• inline-only-static — only enables inlining of static functions. So when you patch a static function, all its callers are impacted and need to be patched as well. 🦋

In addition to all the flags that -flive-patching=inline-clone disables, -flive-patching=inline-only-static 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.

heads up bestie: -flive-patching is not supported with link-time optimization (-flto). 💀

• -fisolate-erroneous-paths-dereference — detects paths that trigger erroneous or undefined behavior from dereferencing a null pointer. 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 -O2 and higher, and it depends on -fdelete-null-pointer-checks also being enabled.
• -fisolate-erroneous-paths-attribute — detects paths that trigger erroneous or undefined behavior when a null value is used in a way that's forbidden by a "returns_nonnull" or "nonnull" attribute. Isolates those paths from the main control flow and turns the statement with the erroneous or undefined behavior into a trap. 🪤 This is not currently enabled, but it may be enabled by -O2 in the future. (so don't hold your breath, fr fr)
• -ftree-sink — performs forward store motion on trees. Enabled by default at -O1 and 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 -O1 and higher, except for -Og. It requires that -ftree-ccp is 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 -O1 and higher. 🧚
• -fssa-backprop — propagates information about how a value is used up the definition chain, to simplify the definitions. For example, this pass strips sign operations if the sign of a value never matters. 😌 Enabled by default at -O1 and higher.
• -fssa-phiopt — performs pattern matching on SSA PHI nodes to optimize conditional code. Enabled by default at -O1 and higher, except for -Og. 🎀
• -ftree-switch-conversion — converts simple initializations in a switch into initializations from a scalar array. Enabled by default at -O2 and higher. 🔥
• -ftree-tail-merge — looks for identical code sequences, and when it finds one, replaces it with a jump to the other. This optimization is known as tail merging or cross jumping. Enabled by default at -O2 and higher. The compilation time in this pass can be limited using the max-tail-merge-comparisons parameter and the max-tail-merge-iterations parameter. 👑
• -ftree-dce — performs dead code elimination (DCE) on trees. Enabled by default at -O1 and higher. bye bye dead code 💅
• -ftree-builtin-call-dce — performs conditional dead code elimination (DCE) for calls to built-in functions that may set "errno" but are otherwise free of side effects. Enabled by default at -O2 and higher if -Os is not also specified. 🌸
• -ffinite-loops — assumes that a loop with an exit will eventually take that exit and not loop forever. This lets the compiler remove loops that otherwise have no side-effects, since it no longer has to consider endless looping as a side effect. ♾️ This option is enabled by default at -O2 for C++ with -std=c++11 or higher.
• -ftree-dominator-opts — performs a whole lil buffet 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). ✨ Enabled by default at -O1 and higher.
• -ftree-dse — performs dead store elimination (DSE) on trees. A dead store is a store into a memory location that's later overwritten by another store with no intervening loads — so the earlier store can just be deleted. 😌 Enabled by default at -O1 and higher.
• -ftree-ch — performs loop header copying on trees. This is beneficial because it makes code motion optimizations more effective, and it also saves one jump. 🦋 Enabled by default at -O1 and higher. It is not enabled for -Os, since it usually increases code size.
• -ftree-loop-optimize — performs loop optimizations on trees. Enabled by default at -O1 and higher. 🌈
• -ftree-loop-linear, -floop-strip-mine, -floop-block — perform loop nest optimizations. Same as -floop-nest-optimize. To use this code transformation, GCC has to be configured with --with-isl to enable the Graphite loop transformation infrastructure. 💖
• -fgraphite-identity — enables the identity transformation for graphite. For every SCoP we generate the polyhedral representation and transform it back to gimple. Using -fgraphite-identity we can check the costs or benefits of the GIMPLE -> GRAPHITE -> GIMPLE transformation. Some minimal optimizations are also performed by the code generator isl, like index splitting and dead code elimination in loops. 🧚
• -floop-nest-optimize — 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. It parallelizes all the loops that can be analyzed to not contain loop carried dependences, without checking that it's profitable to parallelize the loops. 🚀
• -ftree-coalesce-vars — okay so while GCC is transforming your program out of the SSA representation, it tries to reduce copying by coalescing versions of different user-defined variables instead of just the compiler's own temporaries. Cute, right? But heads up bestie: this can severely limit your ability to debug an optimized program compiled with -fno-var-tracking-assignments. 😭 In the negated form, this flag stops SSA coalescing of user variables. It's enabled by default if optimization is enabled, and it does very little otherwise. 💅
• -ftree-loop-if-convert — tries to transform conditional jumps in the innermost loops into branch-less equivalents. The whole intent is removing control-flow from the innermost loops so the vectorization pass can handle those loops better. Enabled by default if vectorization is enabled. ✨
• -ftree-loop-distribution — performs loop distribution. This flag can improve cache performance on big loop bodies and lets further loop optimizations happen, like parallelization or vectorization. For example, the loop

DO I = 1, N
  A(I) = B(I) + C
  D(I) = E(I) * F
ENDDO

is transformed to

DO I = 1, N
   A(I) = B(I) + C
ENDDO
DO I = 1, N
   D(I) = E(I) * F
ENDDO

This flag is enabled by default at -O3. It is also enabled by -fprofile-use and -fauto-profile. 🎀

• -ftree-loop-distribute-patterns — performs loop distribution of patterns that can be code generated with calls to a library. Enabled by default at -O2 and higher, and by -fprofile-use and -fauto-profile. 🌸

This pass distributes the initialization loops and generates a call to memset zero. For example, the loop

DO I = 1, N
  A(I) = 0
  B(I) = A(I) + I
ENDDO

is transformed to

DO I = 1, N
   A(I) = 0
ENDDO
DO I = 1, N
   B(I) = A(I) + I
ENDDO

and the initialization loop is transformed into a call to memset zero. iconic tbh

• -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. Enabled by default at -O3. It is also enabled by -fprofile-use and -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-loops it also moves operands of conditions that are invariant out of the loop, so 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 the number of iterations requires complicated analysis. Later optimizations then may determine the number easily. Super 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's sufficiently cheap. This reduces data dependencies and may allow further simplifications. Enabled by default at -O1 and higher. 👑
• -fivopts — performs induction variable optimizations (strength reduction, induction variable merging and induction variable elimination) on trees. the way it does all three at once?? 💜
• -ftree-parallelize-loops=n — parallelizes loops, i.e., splits their iteration space to run in n threads. This is only possible for loops whose iterations are independent and can be arbitrarily reordered. The optimization is only profitable on multiprocessor machines, for loops that are CPU-intensive, rather than constrained e.g. by memory bandwidth. This option implies -pthread, and thus is only supported on targets that have support for -pthread. 🌈
• -ftree-pta — performs function-local points-to analysis on trees. Enabled by default at -O1 and 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. Enabled by default at -O1 and 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. Enabled by default at -O2 and 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. Enabled by default at -O1 and 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. Enabled by default at -O1 and higher. ⭐
• -ftree-vectorize — performs vectorization on trees. This flag enables -ftree-loop-vectorize and -ftree-slp-vectorize if not explicitly specified. 🌸
• -ftree-loop-vectorize — performs loop vectorization on trees. Enabled by default at -O2 and by -ftree-vectorize, -fprofile-use, and -fauto-profile. ✨
• -ftree-slp-vectorize — performs basic block vectorization on trees. Enabled by default at -O2 and by -ftree-vectorize, -fprofile-use, and -fauto-profile. 🔥
• -ftrivial-auto-var-init=choice — initializes automatic variables with either a pattern or with zeroes, to increase the security and predictability of a program by preventing uninitialized memory disclosure and use. not the compiler being a protective bestie 💖

GCC still considers an automatic variable that doesn't have an explicit initializer as uninitialized — -Wuninitialized and -Wanalyzer-use-of-uninitialized-value will still report warning messages on such automatic variables, and the compiler will perform optimization as if the variable were uninitialized. With this option, GCC will also initialize any padding of automatic variables that have structure or union types to zeroes. However, the current implementation cannot initialize automatic variables that are declared between the controlling expression and the first case of a switch statement. Using -Wtrivial-auto-var-init to report all such cases. 😭

The three values of choice are:

• uninitialized doesn't initialize any automatic variables. This is C and C++'s default.
• pattern — initialize automatic variables with values which will likely transform logic bugs into crashes down the line, are easily recognized in a crash dump and without being values that programmers can rely on for useful program semantics. The current value is byte-repeatable pattern with byte "0xFE". The values used for pattern initialization might be changed in the future. 💀
• zero — initialize automatic variables with zeroes.

The default is uninitialized.

Note that the initializer values, whether zero or pattern, refer to data representation (in memory or machine registers), rather than to their interpretation as numerical values. this distinction may be important in languages that support types with biases or implicit multipliers, and with such extensions as hardbool. For example, a variable that uses 8 bits to represent (biased) quantities in the "range 160..400" will be initialized with the bit patterns 0x00 or 0xFE, depending on choice, whether or not these representations stand for values in that range, and even if they do, the interpretation of the value held by the variable will depend on the bias. A hardbool variable that uses say "0X5A" and 0xA5 for "false" and "true", respectively, will trap with either choice of trivial initializer, i.e., zero initialization will not convert to the representation for "false", even if it would for a "static" variable of the same type. This means the initializer pattern doesn't generally depend on the type of the initialized variable. One notable exception is that (non-hardened) boolean variables that fit in registers are initialized with "false" (zero), even when pattern is requested.

You can control this behavior for a specific variable by using the variable attribute "uninitialized". 💅

• -fvect-cost-model=model** ** — okay bestie, this one changes up the cost model used for vectorization, and model has to be one of unlimited, dynamic, cheap or very-cheap. Here's the tea on each:
  • unlimited — the vectorized code-path is just assumed to be profitable. Trusting. Manifesting. 💫
  • dynamic — a runtime check guards the vectorized code-path, so it only kicks in for iteration counts that will likely run faster than the original scalar loop. Gatekeeping, but make it cute.
  • cheap — turns off vectorization of loops where doing so would be cost prohibitive, like when you'd need runtime checks for data dependence or alignment. Otherwise it's the same as dynamic.
  • very-cheap — only allows vectorization if the vector code would entirely replace the scalar code being vectorized. Say each iteration of a vectorized loop can only handle exactly four iterations of the scalar loop — then very-cheap only says yes if the scalar iteration count is known to be a multiple of four. Strict queen. 💅

The default cost model depends on other optimization flags, and it's either dynamic or cheap.

---

• -fsimd-cost-model=model** ** — same energy but for vectorization of loops marked with the OpenMP simd directive. Here model should be one of unlimited, dynamic, cheap. All the values mean exactly what they mean in -fvect-cost-model, and by default a cost model defined with -fvect-cost-model gets used.

---

• -ftree-vrp — does Value Range Propagation on trees 🌳. It's like the constant propagation pass but instead of values, ranges of values get propagated. This lets the optimizers yeet unnecessary range checks like array bound checks and null pointer checks. Enabled by default at -O2 and higher. Heads up tho: null pointer check elimination only happens if -fdelete-null-pointer-checks is enabled.

---

• -fsplit-paths — splits paths leading to loop backedges. This can level up dead code elimination and common subexpression elimination. Enabled by default at -O3 and above.

---

• -fsplit-ivs-in-unroller — lets values of induction variables in later iterations of the unrolled loop be expressed using the value from the first iteration. This breaks those long dependency chains, so the scheduling passes get way more efficient. ✨

A combo of -fweb and CSE is often enough to get the same effect — but it's not reliable when the loop body is more complicated than a single basic block, and on some architectures it doesn't work at all because of restrictions in the CSE pass. 😭

This optimization is enabled by default.

---

• -fvariable-expansion-in-unroller — with this one, the compiler makes multiple copies of some local variables when unrolling a loop, which can result in superior code. 👑

This optimization is enabled by default for PowerPC targets, but disabled by default everywhere else.

---

• -fpartial-inlining — inlines parts of functions. Note: this only does anything when inlining itself is turned on by the -finline-functions or -finline-small-functions options.

Enabled at levels -O2, -O3, -Os.

---

• -fpredictive-commoning — performs the predictive commoning optimization, i.e. reusing computations (especially memory loads and stores) that were performed in previous iterations of loops. ♻️

This option is enabled at level -O3. It's also enabled by -fprofile-use and -fauto-profile.

---

• -fprefetch-loop-arrays — if the target machine supports it, this generates instructions to prefetch memory so loops that access large arrays run faster. 🏃‍♀️💨

Real talk tho: this option may generate better or worse code — results are highly dependent on the structure of the loops inside your source code.

Disabled at level -Os.

---

• -fno-printf-return-value — do not substitute constants for the known return value of formatted output functions like sprintf, snprintf, vsprintf, and vsnprintf (but not printf or fprintf). This transformation lets GCC optimize or even fully eliminate branches based on the known return value of those functions when they're called with arguments that are either constant, or whose values are known to be in a range that makes determining the exact return value possible. For example, when -fprintf-return-value is in effect, both the branch and the body of the if statement (but not the call to snprintf) can be optimized away when i is a 32-bit or smaller integer, because the return value is guaranteed to be at most 8:

char buf[9];
if (snprintf (buf, "%08x", i) >= sizeof buf)
  ...

The -fprintf-return-value option relies on other optimizations and gives its best results with -O2 and above. It works in tandem with the -Wformat-overflow and -Wformat-truncation options. And -fprintf-return-value is enabled by default. 🫶

---

• -fno-peephole
• -fno-peephole2

Disable any machine-specific peephole optimizations. The difference between -fno-peephole and -fno-peephole2 is in how they're implemented in the compiler — some targets use one, some use the other, and a few use both. 🙈

-fpeephole is enabled by default. -fpeephole2 is enabled at levels -O2, -O3, -Os.

---

• -fno-guess-branch-probability — do not guess branch probabilities using heuristics. No vibes-based guessing allowed. 🚫

GCC uses heuristics to guess branch probabilities when they aren't provided by profiling feedback (-fprofile-arcs). These heuristics are based on the control flow graph. If some branch probabilities are specified by __builtin_expect, then the heuristics are used to guess branch probabilities for the rest of the control flow graph, taking that __builtin_expect info into account. The interactions between the heuristics and __builtin_expect can get complex, and in some cases it may be useful to disable the heuristics so the effects of __builtin_expect are easier to understand.

It's also possible to specify the expected probability of the expression with the __builtin_expect_with_probability built-in function.

The default is -fguess-branch-probability at levels -O, -O2, -O3, -Os.

---

• -freorder-blocks — reorders basic blocks in the compiled function to reduce the number of taken branches and improve code locality. Enabled at levels -O1, -O2, -O3, -Os.

---

• -freorder-blocks-algorithm=algorithm** ** — uses the specified algorithm for basic block reordering. The algorithm argument can be simple, which does not increase code size (except sometimes due to secondary effects like alignment), or stc, the "software trace cache" algorithm, which tries to put all often executed code together and minimizes 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 — on top of reordering basic blocks in the compiled function to reduce the number of taken branches, this also partitions hot and cold basic blocks into separate sections of the assembly and .o files, to improve paging and cache locality performance.

This optimization is automatically turned off when exception handling or unwind tables are present (on targets using setjump/longjump or a target specific scheme), for linkonce sections, for functions with a user-defined section attribute, and on any architecture that doesn't support named sections. When -fsplit-stack is used, this option is not enabled by default (to avoid linker errors), but it may be enabled explicitly (if you're using a working linker).

Enabled for x86 at levels -O2, -O3, -Os.

---

• -freorder-functions — reorders functions in the object file to improve code locality. It does this using special subsections .text.hot for the most frequently executed functions and .text.unlikely for the unlikely executed ones. The reordering is done by the linker, so the object file format must support named sections and the linker must place them in a reasonable way.

This option isn't effective unless you either provide profile feedback (see -fprofile-arcs for details) or manually annotate functions with hot or cold attributes.

Enabled at levels -O2, -O3, -Os.

---

• -fstrict-aliasing — lets the compiler assume the strictest aliasing rules applicable to the language being compiled. For C (and C++), this activates optimizations based on the type of expressions. In particular, an object of one type is assumed to never reside at the same address as an object of a different type, unless the types are almost the same. For example, an unsigned int can alias an int, but not a void* or a double. A character type may alias any other type. 👯

Pay special attention to code like this:

union a_union {
  int i;
  double d;
};

int f() {
  union a_union t;
  t.d = 3.0;
  return t.i;
}

Reading from a different union member than the one most recently written to (this is called "type-punning") is common. Even with -fstrict-aliasing, type-punning is allowed, provided the memory is accessed through the union type. So the code above works as expected. However, this code might not:

int f() {
  union a_union t;
  int* ip;
  t.d = 3.0;
  ip = &t.i;
  return *ip;
}

Similarly, access by taking the address, casting the resulting pointer and dereferencing the result has undefined behavior, even if the cast uses a union type, e.g.:

int f() {
  double d = 3.0;
  return ((union a_union *) &d)->i;
}

The -fstrict-aliasing option is enabled at levels -O2, -O3, -Os.

---

• -fipa-strict-aliasing — controls whether the rules of -fstrict-aliasing get applied across function boundaries. Note that if multiple functions get inlined into a single function, the memory accesses are no longer considered to be crossing a function boundary. 🦋

The -fipa-strict-aliasing option is enabled by default and is effective only in combination with -fstrict-aliasing.

• -falign-functions, -falign-functions=n, -falign-functions=n:m, -falign-functions=n:m:n2, -falign-functions=n:m:n2:m2 — line up the start of functions to the next power-of-two that's ≥ n, skipping up to m-1 bytes. bestie this is so the CPU can grab at least the first m bytes of the function without yeeting across an n-byte alignment boundary.

This is a code-performance optimization, and alignment gets ignored for functions considered cold. If you need alignment on literally every function, that's what -fmin-function-alignment is for. ✨

If m isn't specified, it just defaults to n.

Examples, because examples slay: -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. -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 set 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.

Heads up: some assemblers only support this flag when n is a power of two — in that case, it gets rounded up. 🙃

-fno-align-functions and -falign-functions=1 are equivalent and mean functions are not aligned.

If n isn't specified or is zero, use a machine-dependent default. The maximum allowed n option value is 65536.

Enabled at levels -O2, -O3. 💅

• -flimit-function-alignment — with this on, the compiler tries to avoid unnecessarily overaligning functions. It attempts to instruct the assembler to align by the amount specified by -falign-functions, but not to skip more bytes than the size of the function. polite alignment, no overdoing it.
• -falign-labels, -falign-labels=n, -falign-labels=n:m, -falign-labels=n:m:n2, -falign-labels=n:m:n2:m2 — align all branch targets to a power-of-two boundary. 🎯

The parameters of this option are analogous to the -falign-functions option. -fno-align-labels and -falign-labels=1 are equivalent and mean labels are not aligned.

If -falign-loops or -falign-jumps are applicable and are greater than this value, then their values get used instead.

If n isn't specified or is zero, use a machine-dependent default, which is very likely to be 1, meaning no alignment. The maximum allowed n option value is 65536.

Enabled at levels -O2, -O3. ⭐

• -falign-loops, -falign-loops=n, -falign-loops=n:m, -falign-loops=n:m:n2, -falign-loops=n:m:n2:m2 — align loops to a power-of-two boundary. If the loops are executed many times, this makes up for any execution of the dummy padding instructions. it literally pays for itself if the loop runs enough. 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.

Parameters of this option are analogous to the -falign-functions option. -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 — align branch targets to a power-of-two boundary, but specifically for branch targets that can only be reached by jumping. In this case, no dummy operations need be executed. free realignment, no runtime cost. 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.

Parameters of this option are analogous to the -falign-functions option. -fno-align-jumps and -falign-jumps=1 are equivalent and mean that loops are not aligned.

If n 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 — specify minimal alignment of functions to the next power-of-two that's ≥ n. Unlike -falign-functions, this alignment is applied also to all functions (even the ones considered cold). The alignment is also not affected by -flimit-function-alignment. cold functions stay in the group chat with this one.
• -fno-allocation-dce — do not remove unused C++ allocations in dead code elimination. if you were counting on that cleanup, sorry bestie, it's staying.
• -fallow-store-data-races — let the compiler perform optimizations that may introduce new data races on stores, without proving that the variable can't be concurrently accessed by other threads. Does not affect optimization of local data. It is safe to use this option if it is known that global data will not be accessed by multiple threads. 🫶

Examples of optimizations enabled by -fallow-store-data-races include hoisting or if-conversions that may cause a value that was already in memory to be re-written with that same value. Such re-writing is safe in a single-threaded context but may be unsafe in a multi-threaded context. Note that on some processors, if-conversions may be required in order to enable vectorization.

Enabled at level -Ofast. ⚡

• -funit-at-a-time — this option is left for compatibility reasons. -funit-at-a-time has no effect, while -fno-unit-at-a-time implies -fno-toplevel-reorder and -fno-section-anchors. legacy vibes, still hanging around.

Enabled by default.

• -fno-toplevel-reorder — do not reorder top-level functions, variables, and "asm" statements. Output them in the same order that they appear in the input file. When this option is used, unreferenced static variables are not removed. This option is intended to support existing code that relies on a particular ordering. For new code, it's better to use attributes when possible. 🙈

-ftoplevel-reorder is the default at -O1 and higher, and also at -O0 if -fsection-anchors is explicitly requested. Additionally -fno-toplevel-reorder implies -fno-section-anchors.

• -funreachable-traps — with this option, the compiler turns calls to "__builtin_unreachable" into traps, instead of using them for optimization. This also affects any such calls implicitly generated by the compiler. 💀

This option has the same effect as -fsanitize=unreachable -fsanitize-trap=unreachable, but does not affect the values of those options. If -fsanitize=unreachable is enabled, that option takes priority over this one.

This option is enabled by default at -O0 and -Og.

• -fweb — constructs webs as commonly used for register allocation purposes and assigns each web an individual pseudo register. This allows the register allocation pass to operate on pseudos directly, but also strengthens several other optimization passes, such as CSE, loop optimizer and trivial dead code remover. It can, however, make debugging impossible, since variables no longer stay in a "home register". iconic for optimization, tragic for debugging. pick your fighter.

Enabled by default with -funroll-loops.

• -fwhole-program — assume that the current compilation unit represents the whole program being compiled. All public functions and variables with the exception of "main" and those merged by attribute "externally_visible" become static functions and in effect are optimized more aggressively by interprocedural optimizers. 👑

With -flto this option has a limited use. In most cases the precise list of symbols used or exported from the binary is known the resolution info passed to the link-time optimizer by the linker plugin. It is still useful if no linker plugin is used or during incremental link step when final code is produced (with -flto -flinker-output=nolto-rel).

• -flto[=n] — this option runs the standard link-time optimizer. When invoked with source code, it generates GIMPLE (one of GCC's internal representations) and writes it to special ELF sections in the object file. When the object files are linked together, all the function bodies are read from these ELF sections and instantiated as if they had been part of the same translation unit. whole-program energy, unlocked.

To use the link-time optimizer, -flto and optimization options should be specified at compile time and during the final link. It is recommended that you compile all the files participating in the same link with the same options and also specify those options at link time. For example:

gcc -c -O2 -flto foo.c
gcc -c -O2 -flto bar.c
gcc -o myprog -flto -O2 foo.o bar.o

The first two invocations to GCC save a bytecode representation of GIMPLE into special ELF sections inside foo.o and bar.o. The final invocation reads the GIMPLE bytecode from foo.o and bar.o, merges the two files into a single internal image, and compiles the result as usual. Since both foo.o and bar.o are merged into a single image, this causes all the interprocedural analyses and optimizations in GCC to work across the two files as if they were a single one. This means, for example, that the inliner is able to inline functions in bar.o into functions in foo.o and vice-versa. 💖

Another (simpler) way to enable link-time optimization is:

gcc -o myprog -flto -O2 foo.c bar.c

The above generates bytecode for foo.c and bar.c, merges them together into a single GIMPLE representation and optimizes them as usual to produce myprog.

The important thing to keep in mind is that to enable link-time optimizations you need to use the GCC driver to perform the link step. GCC automatically performs link-time optimization if any of the objects involved were compiled with the -flto command-line option. You can always override the automatic decision to do link-time optimization by passing -fno-lto to the link command. ✨

okay bestie, we're deep in the LTO lore now and it's lowkey a lot but I promise it all makes sense 💖

To make whole program optimization actually hit, the compiler has to make certain whole program assumptions. Like, it needs to know which functions and variables can be reached by libraries and runtime from outside the link-time optimized unit. When the linker supports it, the linker plugin (peep -fuse-linker-plugin) hands the compiler the tea about used and externally visible symbols. When there's no linker plugin available? You gotta use -fwhole-program so the compiler can make those assumptions itself, which unlocks way more aggressive optimization decisions. Slay.

When a file gets compiled with -flto but without -fuse-linker-plugin, the object file it spits out is bigger than a regular object file, because it's carrying GIMPLE bytecodes and the usual final code (see -ffat-lto-objects). What that means for you: object files with LTO info can still be linked as normal object files, and if -fno-lto gets passed to the linker, zero interprocedural optimizations get applied. But heads up — when -fno-fat-lto-objects is enabled the compile stage is faster, but you cannot do a regular, non-LTO link on them. Choose your fighter.

When it's time to produce the final binary, GCC only applies link-time optimizations to the files that contain bytecode. So yes, you can absolutely mix and match object files and libraries with GIMPLE bytecodes and final object code. GCC picks which files to optimize in LTO mode and which to link without further processing all by itself. main character energy, honestly.

Generally, options you specify at link time override the ones specified at compile time — although in some cases GCC tries to infer link-time options from the settings used to compile the input files.

If you don't specify an optimization level option -O at link time, GCC uses the highest optimization level used when compiling the object files. But ngl, specifying an optimization level only at link time and not at compile time is generally ineffective, for two reasons:

1. Compiling without optimization suppresses compiler passes that gather info needed for effective optimization at link time.
2. Some early optimization passes can only happen at compile time, not at link time.

There are some code generation flags GCC preserves when generating bytecodes, because they're needed during the final link. Currently, the following options and their settings get taken from the first object file that explicitly specifies them: -fcommon, -fexceptions, -fnon-call-exceptions, -fgnu-tm and all the -m target flags.

The following options — -fPIC, -fpic, -fpie and -fPIE — get combined based on this scheme, so pay attention bestie:

B<-fPIC> + B<-fpic> = B<-fpic>
B<-fPIC> + B<-fno-pic> = B<-fno-pic>
B<-fpic/-fPIC> + (no option) = (no option)
B<-fPIC> + B<-fPIE> = B<-fPIE>
B<-fpic> + B<-fPIE> = B<-fpie>
B<-fPIC/-fpic> + B<-fpie> = B<-fpie>

Certain ABI-changing flags have to match across all compilation units, and trying to override this at link time with a conflicting value just gets ignored. That includes options like -freg-struct-return and -fpcc-struct-return.

Other options like -ffp-contract, -fno-strict-overflow, -fwrapv, -fno-trapv or -fno-strict-aliasing get 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 tho.

Diagnostic options like -Wstringop-overflow get passed through to the link stage too, and their setting matches that of the compile-step at function granularity. Note that this only matters for diagnostics emitted during optimization. Also note that code transforms such as inlining can lead to warnings being enabled or disabled for regions of code that aren'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 those translation units with -fno-lto or consistently use the same assembler options on all translation units. Alternatively you can also just specify assembler options at LTO link time. Your call, queen.

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. just don't, fr.

If LTO encounters objects with C linkage declared with incompatible types in separate translation units being linked together (which is undefined behavior according to ISO C99 6.2.7), a non-fatal diagnostic may be issued. The behavior is still undefined at run time — no cap. Similar diagnostics may be raised for other languages.

Another feature of LTO? You can apply interprocedural optimizations on files written in different languages, which is honestly iconic:

gcc -c -flto foo.c
g++ -c -flto bar.cc
gfortran -c -flto baz.f90
g++ -o myprog -flto -O3 foo.o bar.o baz.o -lgfortran

Notice that the final link is done with g++ to get the C++ runtime libraries, and -lgfortran is added to get the Fortran runtime libraries. In general, when you're mixing languages in LTO mode, use the same link command options as when mixing languages in a regular (non-LTO) compilation. Consistency is the vibe.

If object files containing GIMPLE bytecode are stored in a library archive — say libfoo.a — you can 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; and to show the symbols of object files with GIMPLE bytecode, use gcc-nm. Those commands require that ar, ranlib and nm have been compiled with plugin support. At link time, use the flag -fuse-linker-plugin to ensure the library participates in the LTO optimization process:

gcc -o myprog -O2 -flto -fuse-linker-plugin a.o b.o -lfoo

With the linker plugin enabled, the linker extracts the needed GIMPLE files from libfoo.a and passes them on to the running GCC so they become part of the aggregated GIMPLE image to be optimized. ✨

If you're not using a linker with plugin support and/or you don't enable the linker plugin, then the objects inside libfoo.a get extracted and linked as usual — but they don't participate in the LTO optimization process. To make a static library suitable for both LTO optimization and usual linkage, compile its object files with -flto -ffat-lto-objects.

Link-time optimizations don't require the whole program to be present to operate. If the program doesn't require any symbols to be exported, you can combine -flto and -fwhole-program to let the interprocedural optimizers use more aggressive assumptions, which may lead to improved optimization opportunities. Use of -fwhole-program isn't needed when the linker plugin is active (see -fuse-linker-plugin).

The current implementation of LTO makes no attempt to generate bytecode that's portable between different types of hosts. The bytecode files are versioned and there's a strict version check, so bytecode files generated in one version of GCC will not work with an older or newer version of GCC. 💀

Link-time optimization doesn't work well with generation of debugging information on systems other than those using a combination of ELF and DWARF.

If you specify the optional n, the optimization and code generation done at link time is executed in parallel using n parallel jobs by utilizing an installed make program. The environment variable MAKE may be used to override the program used.

You can also specify -flto=jobserver to use GNU make's job server mode to determine the number of parallel jobs. This is useful when the Makefile calling GCC is already executing in parallel. You must prepend a + to the command recipe in the parent Makefile for this to work. This option likely only works if MAKE is GNU make. Even without the option value, GCC tries to automatically detect a running GNU make's job server. 🧚

Use -flto=auto to use GNU make's job server, if available, or otherwise fall back to autodetection of the number of CPU threads present in your system. so convenient, love that for us

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✨ -flto-partition=alg ✨

Specify the partitioning algorithm used by the link-time optimizer. The value is either:

• 1to1 — specifies a partitioning mirroring the original source files
• balanced — specifies partitioning into equally sized chunks (whenever possible)
• max — creates a new partition for every symbol where possible

Specifying none as an algorithm disables partitioning and streaming completely. The default value is balanced. While 1to1 can be used as a workaround for various code ordering issues, the max partitioning is intended for internal testing only. The value one specifies that exactly one partition should be used, while the value none bypasses partitioning and executes the link-time optimization step directly from the WPA phase.

---

🌸 -flto-compression-level=n 🌸

This option specifies the level of compression used for intermediate language written to LTO object files, and is only meaningful in conjunction with LTO mode (-flto). GCC currently supports two LTO compression algorithms:

• For zstd, valid values are 0 (no compression) to 19 (maximum compression)
• While zlib supports values from 0 to 9

Values outside this range are clamped to either minimum or maximum of the supported values. If the option isn't given, a default balanced compression setting is used. 🎀

---

💅 -fuse-linker-plugin 💅

Enables the use of a linker plugin during link-time optimization. This option relies on plugin support in the linker, which is available in gold or in GNU ld 2.21 or newer.

okay bestie, this next bit is lowkey the glow-up for the linker — this option literally pulls object files with GIMPLE bytecode out of library archives 💅 and the reason we even care is that it improves the quality of optimization by exposing more code to the link-time optimizer. It's telling the toolchain what symbols can be accessed externally (by non-LTO object or during dynamic linking), iykyk. The resulting code quality improvements on binaries (and shared libraries that use hidden visibility) are similar to -fwhole-program. See -flto for a description of the effect of this flag and how to use it. 🌈

And the vibe check on the default: this option is enabled by default when LTO support in GCC is enabled and GCC was configured for use with a linker supporting plugins (GNU ld 2.21 or newer or gold). ✨

• -ffat-lto-objects — fat LTO objects are object files that contain both the intermediate language and the object code 🍓. That makes them usable for both LTO linking and normal linking. This option is effective only when compiling with -flto and is ignored at link time.

-fno-fat-lto-objects improves compilation time over plain LTO, but requires the complete toolchain to be aware of LTO 👀. It requires a linker with linker plugin support for basic functionality. Additionally, nm, ar and ranlib need to support linker plugins to allow a full-featured build environment (capable of building static libraries etc). GCC provides the gcc-ar, gcc-nm, gcc-ranlib wrappers to pass the right options to these tools. With non fat LTO makefiles need to be modified to use them.

Note that modern binutils provide plugin auto-load mechanism. Installing the linker plugin into $libdir/bfd-plugins has the same effect as usage of the command wrappers (gcc-ar, gcc-nm and gcc-ranlib).

The default is -fno-fat-lto-objects on targets with linker plugin support.

• -fcompare-elim — after register allocation and post-register allocation instruction splitting, identify arithmetic instructions that compute processor flags similar to a comparison operation based on that arithmetic. If possible, eliminate the explicit comparison operation 🧠.

This pass only applies to certain targets that cannot explicitly represent the comparison operation before register allocation is complete.

Enabled at levels -O1, -O2, -O3, -Os. 💖

• -ffold-mem-offsets, -fno-fold-mem-offsets — try to eliminate add instructions by folding them in memory loads/stores 🎀.

Enabled at levels -O2, -O3.

• -fcprop-registers — after register allocation and post-register allocation instruction splitting, perform a copy-propagation pass to try to reduce scheduling dependencies and occasionally eliminate the copy 🦋.

Enabled at levels -O1, -O2, -O3, -Os.

• -fprofile-correction — profiles collected using an instrumented binary for multi-threaded programs may be inconsistent due to missed counter updates 😭. When this option is specified, GCC uses heuristics to correct or smooth out such inconsistencies. By default, GCC emits an error message when an inconsistent profile is detected.

This option is enabled by -fauto-profile.

• -fprofile-partial-training — with -fprofile-use all portions of programs not executed during train run are optimized aggressively for size rather than speed. In some cases it is not practical to train all possible hot paths in the program (for example, a program may contain functions specific for a given hardware and training may not cover all hardware configurations the program is run on) 🙈. With -fprofile-partial-training profile feedback will be ignored for all functions not executed during the train run, leading them to be optimized as if they were compiled without profile feedback. This leads to better performance when the train run is not representative, but it also leads to significantly bigger code 💀.
• -fprofile-use, -fprofile-use=path** — enable profile feedback-directed optimizations, plus the following optimizations, many of which are generally profitable only with profile feedback available 👑

-fbranch-probabilities  -fprofile-values -funroll-loops
-fpeel-loops  -ftracer  -fvpt -finline-functions  -fipa-cp
-fipa-cp-clone  -fipa-bit-cp -fpredictive-commoning  -fsplit-loops
-funswitch-loops -fgcse-after-reload  -ftree-loop-vectorize
-ftree-slp-vectorize -fvect-cost-model=dynamic
-ftree-loop-distribute-patterns -fprofile-reorder-functions

Before you can use this option, you must first generate profiling information. No shortcuts, bestie 🫶.

By default, GCC emits an error message if the feedback profiles do not match the source code. This error can be turned into a warning by using -Wno-error=coverage-mismatch. Note this may result in poorly optimized code 😬. Additionally, by default, GCC also emits a warning message if the feedback profiles do not exist (see -Wmissing-profile).

If path is specified, GCC looks at the path to find the profile feedback data files. See -fprofile-dir.

• -fauto-profile, -fauto-profile=path** — enable sampling-based feedback-directed optimizations, plus the following optimizations, many of which are generally profitable only with profile feedback available 🔥:

-fbranch-probabilities  -fprofile-values -funroll-loops
-fpeel-loops  -ftracer  -fvpt -finline-functions  -fipa-cp
-fipa-cp-clone  -fipa-bit-cp -fpredictive-commoning  -fsplit-loops
-funswitch-loops -fgcse-after-reload  -ftree-loop-vectorize
-ftree-slp-vectorize -fvect-cost-model=dynamic
-ftree-loop-distribute-patterns -fprofile-correction

path is the name of a file containing AutoFDO profile information. If omitted, it defaults to fbdata.afdo in the current directory.

Producing an AutoFDO profile data file requires running your program with the perf utility on a supported GNU/Linux target system 🌸. For more information, see https://perf.wiki.kernel.org/.

E.g.

perf record -e br_inst_retired:near_taken -b -o perf.data \
    -- your_program

Then use the create_gcov tool to convert the raw profile data to a format that can be used by GCC 🧚. You must also supply the unstripped binary for your program to this tool. See https://github.com/google/autofdo.

E.g.

create_gcov --binary=your_program.unstripped --profile=perf.data \
    --gcov=profile.afdo

---

The following options control compiler behavior regarding floating-point arithmetic. These options trade off between speed and correctness 🎀. All must be specifically enabled — nothing here is free, bestie.

• -ffloat-store — do not store floating-point variables in registers, and inhibit other options that might change whether a floating-point value is taken from a register or memory 💅.

This option prevents undesirable excess precision on machines such as the 68000 where the floating registers (of the 68881) keep more precision than a double is supposed to have. Similarly for the x86 architecture. For most programs, the excess precision does only good, but a few programs rely on the precise definition of IEEE floating point 😭. Use -ffloat-store for such programs, after modifying them to store all pertinent intermediate computations into variables.

• -fexcess-precision=style** — this option allows further control over excess precision on machines where floating-point operations occur in a format with more precision or range than the IEEE standard and interchange floating-point types 🔍. By default, -fexcess-precision=fast is in effect; this means that operations may be carried out in a wider precision than the types specified in the source if that would result in faster code, and it is unpredictable when rounding to the types specified in the source code takes place. When compiling C or C++, if -fexcess-precision=standard is specified then excess precision follows the rules specified in ISO C99 or C++; in particular, both casts and assignments cause values to be rounded to their semantic types (whereas -ffloat-store only affects assignments). This option is enabled by default for C or C++ if a strict conformance option such as -std=c99 or -std=c++17 is used. -ffast-math enables -fexcess-precision=fast by default regardless of whether a strict conformance option is used. If -fexcess-precision=16 is specified, constants and the results of expressions with types _Float16 and __bf16 are computed without excess precision.

-fexcess-precision=standard is not implemented for languages other than C or C++ 🚫. On the x86, it has no effect if -mfpmath=sse or -mfpmath=sse+387 is specified; in the former case, IEEE semantics apply without excess precision, and in the latter, rounding is unpredictable 💀.

• -ffast-math — sets the options -fno-math-errno, -funsafe-math-optimizations, -ffinite-math-only, -fno-rounding-math, -fno-signaling-nans, -fcx-limited-range and -fexcess-precision=fast ⚡.

This option causes the preprocessor macro __FAST_MATH__ to be defined.

This option is not turned on by any -O option besides -Ofast since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions 😱. It may, however, yield faster code for programs that do not require the guarantees of these specifications.

• -fno-math-errno — do not set errno after calling math functions that are executed with a single instruction, e.g., sqrt 🙅. A program that relies on IEEE exceptions for math error handling may want to use this flag for speed while maintaining IEEE arithmetic compatibility.

This option is not turned on by any -O option since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions 👀. It may, however, yield faster code for programs that do not require the guarantees of these specifications.

The default is -fmath-errno.

On Darwin systems, the math library never sets errno. There is therefore no reason for the compiler to consider the possibility that it might, and -fno-math-errno is the default. 🍓

• -funsafe-math-optimizations — allow optimizations for floating-point arithmetic that (a) assume that arguments and results are valid and (b) may violate IEEE or ANSI standards 😈. When used at link time, it may include libraries or startup files that change the default FPU control word or other similar optimizations.

fr fr, that "unsafe" in the name is not a joke — this one lets the compiler assume things and break standards on purpose. Use it when you know what you're doing, bestie 💜

Okay so real talk: this one is not turned on by any -O option, because it can straight-up give you wrong output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions 😭. That said… it may yield faster code for programs that don't need those guarantees. Periodt. It enables -fno-signed-zeros, -fno-trapping-math, -fassociative-math and -freciprocal-math.

The default is -fno-unsafe-math-optimizations.

---

• -fassociative-math — bestie, this lets the compiler re-associate operands in series of floating-point operations. And yes, this violates the ISO C and C++ language standard by possibly changing the computation result 💀.

NOTE: re-ordering may change the sign of zero as well as ignore NaNs and inhibit or create underflow or overflow — so it cannot be used on code that relies on rounding behavior like (x + 2**52) - 2**52. It may also reorder floating-point comparisons, which means it 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 — allows the reciprocal of a value to be used instead of dividing by the value, if this enables optimizations. Like, x / y can be replaced with x * (1/y), which is useful if (1/y) is subject to common subexpression elimination. Heads up though: this loses precision and increases the number of flops operating on the value 🙈.

The default is -fno-reciprocal-math.

• -ffinite-math-only — lets the compiler do optimizations for floating-point arithmetic that assume arguments and results are not NaNs or +-Infs. Slay, but only if that's actually true for you.

This option is not turned on by any -O option since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions. It may, however, yield faster code for programs that do not require the guarantees of these specifications.

The default is -fno-finite-math-only.

• -fno-signed-zeros — allow optimizations for floating-point arithmetic that ignore the signedness of zero. IEEE arithmetic specifies the behavior of distinct +0.0 and -0.0 values, which then prohibits simplification of expressions such as x+0.0 or 0.0*x (even with -ffinite-math-only). This option implies that the sign of a zero result isn't significant.

The default is -fsigned-zeros.

• -fno-trapping-math — compile code assuming that floating-point operations cannot generate user-visible traps 🪤. These traps include division by zero, overflow, underflow, inexact result and invalid operation. This option requires that -fno-signaling-nans be in effect. Setting this option may allow faster code if one relies on "non-stop" IEEE arithmetic, for example.

This option should never be turned on by any -O option since it can result in incorrect output for programs that depend on an exact implementation of IEEE or ISO rules/specifications for math functions.

The default is -ftrapping-math.

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 — disables transformations and optimizations that assume default floating-point rounding behavior. That's round-to-zero for all floating point to integer conversions, and round-to-nearest for all other arithmetic truncations. This option should be specified for programs that change the FP rounding mode dynamically, or that may be executed with a non-default rounding mode. It disables constant folding of floating-point expressions at compile time (which may be affected by rounding mode) and arithmetic transformations that are unsafe in the presence of sign-dependent rounding modes.

The default is -fno-rounding-math.

⚠️ This option is experimental and does not currently guarantee to disable all GCC optimizations that are affected by rounding mode. Future versions of GCC may provide finer control of this setting using C99's FENV_ACCESS pragma. This command-line option will be used along with -ftrapping-math to specify the default state for FENV_ACCESS.

• -fsignaling-nans — compile code assuming that IEEE signaling NaNs may generate user-visible traps during floating-point operations. Setting this option disables optimizations that may change the number of exceptions visible with signaling NaNs. This option implies -ftrapping-math.

This option causes the preprocessor macro __SUPPORT_SNAN__ to be defined. ✨

The default is -fno-signaling-nans.

⚠️ This option is experimental and does not currently guarantee to disable all GCC optimizations that affect signaling NaN behavior.

• -fno-fp-int-builtin-inexact — do not allow the built-in functions ceil, floor, round and trunc, and their float and long double variants, to generate code that raises the inexact floating-point exception for noninteger arguments. ISO C99 and C11 allow these functions to raise the inexact exception, but ISO/IEC TS 18661-1:2014, the C bindings to IEEE 754-2008, as integrated into ISO C23, does not allow these functions to do so 👀.

The default is -ffp-int-builtin-inexact, allowing the exception to be raised, unless C23 or a later C standard is selected. This option does nothing unless -ftrapping-math is in effect.

Even if -fno-fp-int-builtin-inexact is used, if the functions generate a call to a library function then the inexact exception may be raised if the library implementation does not follow TS 18661.

• -fsingle-precision-constant — treat floating-point constants as single precision instead of implicitly converting them to double-precision constants.
• -fcx-limited-range — when enabled, this option states that a range reduction step is not needed when performing complex division. Also, there is no checking whether the result of a complex multiplication or division is "NaN + I*NaN", with an attempt to rescue the situation in that case. The default is -fno-cx-limited-range, but is enabled by -ffast-math.

This option controls the default setting of the ISO C99 CX_LIMITED_RANGE pragma. Nevertheless, the option applies to all languages.

• -fcx-fortran-rules — complex multiplication and division follow Fortran rules. Range reduction is done as part of complex division, but there is no checking whether the result of a complex multiplication or division is "NaN + I*NaN", with an attempt to rescue the situation in that case.

The default is -fno-cx-fortran-rules.

---

The following options control optimizations that may improve performance, but are not enabled by any -O options. This section includes experimental options that may produce broken code 💀 (not me warning you twice).

• -fbranch-probabilities — after running a program compiled with -fprofile-arcs, you can compile it a second time using -fbranch-probabilities, to improve optimizations based on the number of times each branch was taken. When a program compiled with -fprofile-arcs exits, it saves arc execution counts to a file called sourcename.gcda for each source file. The information in this data file is very dependent on the structure of the generated code, so you must use the same source code and the same optimization options for both compilations. See details about the file naming in -fprofile-arcs.

With -fbranch-probabilities, GCC puts a REG_BR_PROB note on each JUMP_INSN and CALL_INSN. These can be used to improve optimization. Currently, they are only used in one place: in reorg.cc, instead of guessing which path a branch is most likely to take, the REG_BR_PROB values are used to exactly determine which path is taken more often.

Enabled by -fprofile-use and -fauto-profile.

• -fprofile-values — if combined with -fprofile-arcs, it adds code so that some data about values of expressions in the program is gathered.

With -fbranch-probabilities, it reads back the data gathered from profiling values of expressions for usage in optimizations.

Enabled by -fprofile-generate, -fprofile-use, and -fauto-profile.

• -fprofile-reorder-functions — function reordering based on profile instrumentation collects first time of execution of a function and orders these functions in ascending order.

Enabled with -fprofile-use.

• -fvpt — if combined with -fprofile-arcs, this option instructs the compiler to add code to gather information about values of expressions.

With -fbranch-probabilities, it reads back the data gathered and actually performs the optimizations based on them. Currently the optimizations include specialization of division operations using the knowledge about the value of the denominator.

Enabled with -fprofile-use and -fauto-profile.

• -frename-registers — attempt to avoid false dependencies in scheduled code by making use of registers left over after register allocation. This optimization most benefits processors with lots of registers. Depending on the debug information format adopted by the target, however, it can make debugging impossible, since variables no longer stay in a "home register" 😭.

Enabled by default with -funroll-loops.

• -fschedule-fusion — performs a target dependent pass over the instruction stream to schedule instructions of same type together because target machine can execute them more efficiently if they are adjacent to each other in the instruction flow.

Enabled at levels -O2, -O3, -Os.

• -ftracer — perform tail duplication to enlarge superblock size. This transformation simplifies the control flow of the function allowing other optimizations to do a better job.

Enabled by -fprofile-use and -fauto-profile.

• -funroll-loops — bestie, this one unrolls loops where the number of iterations is knowable at compile time or when you walk into the loop. And the way it's not just that: -funroll-loops also implies -frerun-cse-after-loop, -fweb, and -frename-registers. It also flips on complete loop peeling, meaning it completely removes loops that only have a small constant number of iterations. ✨ Heads up though: this makes your code bigger, and whether it runs faster is a whole maybe. It might. It might not. 💅

Enabled by -fprofile-use and -fauto-profile.

• -funroll-all-loops — unrolls every loop, even when the iteration count is a total mystery when the loop gets entered. Ngl this usually makes programs run slower, so… iconic in theory, tragic in practice. 😭 It implies the same options as -funroll-loops.
• -fpeel-loops — peels loops where there's enough info that they don't roll much (info coming from profile feedback or static analysis). It also turns on complete loop peeling, i.e. the full removal of loops with a small constant number of iterations. 🍓

Enabled by -O3, -fprofile-use, and -fauto-profile.

• -fmove-loop-invariants — turns on the loop invariant motion pass in the RTL loop optimizer. Enabled at level -O1 and higher, except for -Og.
• -fmove-loop-stores — turns on 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. 👑 Quick note: for this option to actually do anything, -ftree-loop-im has to be enabled too. Enabled at level -O1 and higher, except for -Og.
• -fsplit-loops — splits a loop into two when it contains a condition that's always true for one side of the iteration space and false for the other. Clean. ✨

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, this creates another version of the loop that assumes the stride is always one. Like so:

        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 especially 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 — and its twin…
• -fdata-sections — these 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 is what determines the section's name in the output file. 🎀

Use these options on systems where the linker can perform optimizations to improve locality of reference in the instruction space. Most systems using the ELF object format have linkers with such optimizations. On AIX, the linker rearranges sections (CSECTs) based on the call graph. The performance impact varies.

Together with a linker garbage collection (linker --gc-sections option) these options may lead to smaller statically-linked executables (after stripping).

On ELF/DWARF systems these options do not degenerate the quality of the debug information. There could be issues with other object files/debug info formats.

Only use these options when there are significant benefits from doing so. When you specify these options, the assembler and linker create larger object and executable files and are also slower. These options affect code generation. They prevent optimizations by the compiler and assembler using relative locations inside a translation unit since the locations are unknown until link time. An example of such an optimization is relaxing calls to short call instructions.

• -fstdarg-opt — optimizes the prologue of variadic argument functions with respect to usage of those arguments. 💅
• -fsection-anchors — tries to reduce the number of symbolic address calculations by using shared "anchor" symbols to address nearby objects. This transformation can help reduce the number of GOT entries and GOT accesses on some targets. 🔥

For example, the implementation of the following function foo:

        static int a, b, c;
        int foo (void) { return a + b + c; }

usually calculates the addresses of all three variables, but if you compile it with -fsection-anchors, it accesses the variables from a common anchor point instead. The effect is similar to the following pseudocode (which isn't valid C):

        int foo (void)
        {
          register int *xr = &x;
          return xr[&a - &x] + xr[&b - &x] + xr[&c - &x];
        }

Not all targets support this option. 🙈

• -fzero-call-used-regs=choice — zeroes call-used registers at function return to level up your program's security, either by mitigating Return-Oriented Programming (ROP) attacks or by preventing information leakage through registers. 🛡️

The possible values of choice are the same as for the zero_call_used_regs attribute. The default is skip.

You can control this behavior for a specific function by using the function attribute zero_call_used_regs.

---

✨ --param name=value ✨

Okay so in some places, GCC uses various constants to control how much optimization gets done. For example, GCC does not inline functions that contain more than a certain number of instructions. You can control some of these constants on the command line using the --param option. 👑

The names of specific parameters, and the meaning of the values, are tied to the internals of the compiler, and are subject to change without notice in future releases. 🫠

In order to get the minimal, maximal and default values of a parameter, use the --help=param -Q options.

In each case, the value is an integer. The following choices of name are recognized for all targets:

• predictable-branch-outcome — when a branch is predicted to be taken with probability lower than this threshold (in percent), then it's considered well predictable. 🌸
• max-rtl-if-conversion-insns — RTL if-conversion tries to remove conditional branches around a block and replace them with conditionally executed instructions. This parameter gives the maximum number of instructions in a block which should be considered for if-conversion. The compiler will also use other heuristics to decide whether if-conversion is likely to be profitable.
• max-rtl-if-conversion-predictable-cost — RTL if-conversion will try to remove conditional branches around a block and replace them with conditionally executed instructions. These parameters give the maximum permissible cost for the sequence that would be generated by if-conversion, depending on whether the branch is statically determined to be predictable or not. The units for this parameter are the same as those for the GCC internal seq_cost metric. The compiler will try to provide a reasonable default for this parameter using the BRANCH_COST target macro.
• max-crossjump-edges — the maximum number of incoming edges to consider for cross-jumping. The algorithm used by -fcrossjumping is O(N^2) in the number of edges incoming to each block. Increasing values mean more aggressive optimization, making the compilation time increase with probably small improvement in executable size. 💀
• min-crossjump-insns — the minimum number of instructions that must be matched at the end of two blocks before cross-jumping is performed on them. This value is ignored in the case where all instructions in the block being cross-jumped from are matched.
• max-grow-copy-bb-insns — the maximum code size expansion factor when copying basic blocks instead of jumping. The expansion is relative to a jump instruction.
• max-goto-duplication-insns — the maximum number of instructions to duplicate to a block that jumps to a computed goto. To avoid O(N^2) behavior in a number of passes, GCC factors computed gotos early in the compilation process, and unfactors them as late as possible. Only computed jumps at the end of a basic blocks with no more than max-goto-duplication-insns are unfactored.
• max-delay-slot-insn-search — the maximum number of instructions to consider when looking for an instruction to fill a delay slot. If more than this arbitrary number of instructions are searched, the time savings from filling the delay slot are minimal, so stop searching. Increasing values mean more aggressive optimization, making the compilation time increase with probably small improvement in execution time.
• max-delay-slot-live-search — when trying to fill delay slots, the maximum number of instructions to consider when searching for a block with valid live register information. Increasing this arbitrarily chosen value means more aggressive optimization, increasing the compilation time. This parameter should be removed when the delay slot code is rewritten to maintain the control-flow graph.
• max-gcse-memory — the approximate maximum amount of memory in kB that can be allocated in order to perform the global common subexpression elimination optimization. If more memory than specified is required, the optimization is not done.
• max-gcse-insertion-ratio — if the ratio of expression insertions to deletions is larger than this value for any expression, then RTL PRE inserts or removes the expression and thus leaves partially redundant computations in the instruction stream. 🦋
• --param max-pending-list-length — this is the cap on how many pending dependencies the scheduler is allowed to juggle before it goes "okay that's enough" and flushes the whole state and starts fresh. Bestie, big functions with barely any branches or calls can build up excessively large lists, and those lists just sit there eating your memory and resources for literally no reason. 💀
• --param max-modulo-backtrack-attempts — the max number of backtrack attempts the scheduler is allowed to make when it's modulo scheduling a loop. Bigger values can make compilation time blow up exponentially. Choose wisely, girlie. 😭
• --param max-inline-functions-called-once-loop-depth — the maximal loop depth of a call that the inline heuristics will even consider, when those heuristics are trying to inline every single function that's called exactly once.
• --param max-inline-functions-called-once-insns — the maximal estimated size of the functions that get produced while inlining functions called once. Like, the size budget, basically. 🎀
• --param max-inline-insns-single — okay so several parameters control the tree inliner GCC uses, and this number sets the maximum number of instructions (counted in GCC's own internal representation) in a single function that the tree inliner will consider for inlining. Heads up: this only touches functions declared inline and methods implemented inside a class declaration (C++). 👑
• --param max-inline-insns-auto — when you use -finline-functions (which is included in -O3), a whole lot of functions that the compiler would otherwise never consider for inlining get investigated anyway. For those functions, a different — and more restrictive — limit compared to functions declared inline can be applied (--param max-inline-insns-auto). ✨
• --param max-inline-insns-small — this is the bound applied to calls that are considered relevant with -finline-small-functions.
• --param max-inline-insns-size — the bound applied to calls that are optimized for size. A little bit of growth can be desirable, because you're anticipating optimization opportunities that inlining will expose. 🌸
• --param uninlined-function-insns — the number of instructions the inliner accounts for function overhead, like the function prologue and epilogue.
• --param uninlined-function-time — the extra time the inliner accounts for function overhead, like the time needed to execute the function prologue and epilogue.
• --param inline-heuristics-hint-percent — the scale (in percents) applied to inline-insns-single, inline-insns-single-O2, and inline-insns-auto when the inline heuristics hint that inlining is very profitable — i.e. it'll enable later optimizations. 🔥
• --param uninlined-thunk-insns, --param uninlined-thunk-time — same as --param uninlined-function-insns and --param uninlined-function-time, but applied to function thunks instead.
• --param inline-min-speedup — when the estimated performance improvement of caller + callee runtime goes over this threshold (in percent), the function can be inlined regardless of the limit on --param max-inline-insns-single and --param max-inline-insns-auto. It just says "no, this one's worth it." 💅
• --param large-function-insns — the limit that specifies what counts as a really large function. For functions bigger than this limit after inlining, inlining gets constrained by --param large-function-growth. This parameter is useful primarily to avoid extreme compilation time caused by non-linear algorithms the back end uses. 😭
• --param large-function-growth — specifies the maximal growth of a large function caused by inlining, in percents. For example, a parameter value of 100 limits large function growth to 2.0 times the original size.
• --param large-unit-insns — the limit that specifies a large translation unit. Growth caused by inlining of units larger than this limit is limited by --param inline-unit-growth. For small units this might be too tight. For example, consider a unit consisting of function A that is inline and B that just calls A three times. If B is small relative to A, the growth of the unit is 300% and yet such inlining is very sane. For very large units consisting of small inlineable functions, though, the overall unit growth limit is needed to avoid an exponential explosion of code size. So for smaller units, the size is increased to --param large-unit-insns before applying --param inline-unit-growth. 🦋
• --param lazy-modules — maximum number of concurrently open C++ module files when lazy loading.
• --param inline-unit-growth — specifies the maximal overall growth of the compilation unit caused by inlining. For example, a parameter value of 20 limits unit growth to 1.2 times the original size. Cold functions (either marked cold via an attribute or by profile feedback) are not accounted into the unit size. 🙈
• --param ipa-cp-unit-growth — specifies the maximal overall growth of the compilation unit caused by interprocedural constant propagation. For example, a parameter value of 10 limits unit growth to 1.1 times the original size.
• --param ipa-cp-large-unit-insns — the size of translation unit that the IPA-CP pass considers large.
• --param large-stack-frame — the limit that specifies large stack frames. While inlining, the algorithm tries not to grow past this limit too much.
• --param large-stack-frame-growth — specifies the maximal growth of large stack frames caused by inlining, in percents. For example, a parameter value of 1000 limits large stack frame growth to 11 times the original size. 👀
• --param max-inline-insns-recursive, --param max-inline-insns-recursive-auto — specifies the maximum number of instructions an out-of-line copy of a self-recursive inline function can grow into by doing recursive inlining.

--param max-inline-insns-recursive applies to functions declared inline. For functions not declared inline, recursive inlining only happens when -finline-functions (included in -O3) is enabled, and --param max-inline-insns-recursive-auto applies instead.

• --param max-inline-recursive-depth, --param max-inline-recursive-depth-auto — specifies the maximum recursion depth used for recursive inlining.

--param max-inline-recursive-depth applies to functions declared inline. For functions not declared inline, recursive inlining only happens when -finline-functions (included in -O3) is enabled, and --param max-inline-recursive-depth-auto applies instead.

• --param min-inline-recursive-probability — recursive inlining is only profitable for a function that has deep recursion on average, and it can actually hurt a function with little recursion depth, by increasing the prologue size or the complexity of the function body for the other optimizers. 😬

When profile feedback is available (see -fprofile-generate), the actual recursion depth can be guessed from the probability that the function recurses via a given call expression. This parameter limits inlining only to call expressions whose probability exceeds the given threshold (in percents).

• --param early-inlining-insns — specifies the growth the early inliner is allowed to make. In effect it increases the amount of inlining for code that has a large abstraction penalty.
• --param max-early-inliner-iterations — the limit on iterations of the early inliner. This basically bounds the number of nested indirect calls the early inliner can resolve. Deeper chains are still handled by late inlining. 💜
• --param comdat-sharing-probability — the probability (in percent) that a C++ inline function with comdat visibility gets shared across multiple compilation units.
• --param modref-max-bases, --param modref-max-refs, --param modref-max-accesses — specifies the maximal number of base pointers, references, and accesses stored for a single function by mod/ref analysis.
• --param modref-max-tests — specifies the maximal number of tests the alias oracle can perform to disambiguate memory locations using the mod/ref information. This parameter ought to be bigger than --param modref-max-bases and --param modref-max-refs.
• --param modref-max-depth — specifies the maximum depth of the DFS walk used by modref escape analysis. Setting it to 0 disables the analysis completely. 🚫
• --param modref-max-escape-points — specifies the maximum number of escape points tracked by modref per SSA-name.
• --param modref-max-adjustments — specifies the maximum number the access range gets enlarged during modref dataflow analysis.
• --param profile-func-internal-id — a parameter that controls whether to use the function internal id in profile database lookup. If the value is 0, the compiler uses an id based on the function assembler name and filename, which makes old profile data more tolerant to source changes such as function reordering etc. ⭐
• --param min-vect-loop-bound — the minimum number of iterations under which loops are not vectorized when -ftree-vectorize is used. The number of iterations after vectorization needs to be greater than the value specified by this option to allow vectorization.
• --param gcse-cost-distance-ratio — a scaling factor used when calculating the maximum distance an expression can be moved by GCSE optimizations. This is currently supported only in the code hoisting pass. The bigger the ratio, the more aggressive code hoisting is with simple expressions — i.e. the expressions that have cost less than gcse-unrestricted-cost. Specifying 0 disables hoisting of simple expressions.
• --param gcse-unrestricted-cost — the cost, roughly measured as the cost of a single typical machine instruction, at which GCSE optimizations stop constraining the distance an expression can travel. This is currently supported only in the code hoisting pass. The lesser the cost, the more aggressive code hoisting is. Specifying 0 allows all expressions to travel unrestricted distances. 🛫
• --param max-hoist-depth — the depth of search in the dominator tree for expressions to hoist. This is used to avoid quadratic behavior in the hoisting algorithm. A value of 0 does not limit the search, but it may slow down compilation of huge functions.
• --param max-tail-merge-comparisons — the maximum amount of similar bbs to compare a bb with. This is used to avoid quadratic behavior in tree tail merging.

---

• max-tail-merge-iterations — the maximum amount of iterations of the pass over the function. Basically it caps how long the compiler is allowed to sit there doing tree tail merging, so your build doesn't just vibe in limbo forever 💀.
• store-merging-allow-unaligned — lets the store merging pass introduce unaligned stores, but only if it's legal to do so. Permission slip required, bestie.
• max-stores-to-merge — the maximum number of stores it'll attempt to merge into wider stores in the store merging pass. Glow-up goals: many small stores → one wide store ✨.
• max-store-chains-to-track — the maximum number of store chains tracked at the same time while trying to merge them into wider stores in the store merging pass.
• max-stores-to-track — the maximum number of stores tracked at the same time in the attempt to merge them into wider stores in the store merging pass.
• max-unrolled-insns — the maximum number of instructions a loop may have to be unrolled. And if the loop does get unrolled, this parameter also determines how many times the loop code is unrolled. Two jobs, one param, main character energy 👑.
• max-average-unrolled-insns — same deal as above, except the instruction count is biased by the probabilities of their execution. The maximum number of instructions, weighted like that, that a loop may have to be unrolled. And if it gets unrolled, this parameter also determines how many times the loop code is unrolled.
• max-unroll-times — the maximum number of unrollings of a single loop.
• max-peeled-insns — the maximum number of instructions a loop may have to be peeled. And if the loop gets peeled, this parameter also determines how many times the loop code is peeled.
• max-peel-times — the maximum number of peelings of a single loop.
• max-peel-branches — the maximum number of branches on the hot path through the peeled sequence.
• max-completely-peeled-insns — the maximum number of insns of a completely peeled loop.
• max-completely-peel-times — the maximum number of iterations of a loop to be suitable for complete peeling.
• max-completely-peel-loop-nest-depth — the maximum depth of a loop nest suitable for complete peeling.
• max-unswitch-insns — the maximum number of insns of an unswitched loop.
• max-unswitch-depth — the maximum depth of a loop nest to be unswitched.
• lim-expensive — the minimum cost of an expensive expression in the loop invariant motion.
• min-loop-cond-split-prob — when FDO profile information is available, this specifies the minimum threshold for the probability of a semi-invariant condition statement to trigger a loop split.
• iv-consider-all-candidates-bound — a bound on the number of candidates for induction variables. Below this bound, all candidates get considered for each use in induction variable optimizations. If there are more candidates than this, only the most relevant ones are considered, so we don't get quadratic time complexity (which is the enemy, fr fr 😭).
• iv-max-considered-uses — the induction variable optimizations give up on loops that contain more induction variable uses than this.
• iv-always-prune-cand-set-bound — if the number of candidates in the set is smaller than this value, it always tries to remove unnecessary ivs from the set when adding a new one. Declutter queen 🎀.
• avg-loop-niter — average number of iterations of a loop.
• dse-max-object-size — maximum size (in bytes) of objects tracked bytewise by dead store elimination. Larger values may result in larger compilation times. Bigger number, longer wait ⏳.
• dse-max-alias-queries-per-store — maximum number of queries into the alias oracle per store. Larger values result in larger compilation times and may result in more removed dead stores.
• scev-max-expr-size — a bound on the size of expressions used in the scalar evolutions analyzer. Large expressions slow the analyzer down.
• scev-max-expr-complexity — a bound on the complexity of the expressions in the scalar evolutions analyzer. Complex expressions slow the analyzer down. It's giving "keep it simple" 🌸.
• max-tree-if-conversion-phi-args — maximum number of arguments in a PHI supported by TREE if conversion, unless the loop is marked with simd pragma.
• vect-max-layout-candidates — the maximum number of possible vector layouts (such as permutations) to consider when optimizing to-be-vectorized code.
• vect-max-version-for-alignment-checks — the maximum number of run-time checks that can be performed when doing loop versioning for alignment in the vectorizer.
• vect-max-version-for-alias-checks — the maximum number of run-time checks that can be performed when doing loop versioning for alias in the vectorizer.
• vect-max-peeling-for-alignment — the maximum number of loop peels to enhance access alignment for the vectorizer. A value of -1 means no limit. Go off, unlimited 💅.
• max-iterations-to-track — the maximum number of iterations of a loop that the brute-force algorithm for analysis of the number of iterations of the loop tries to evaluate.
• hot-bb-count-fraction — the denominator n of the fraction 1/n of the maximal execution count of a basic block in the entire program that a basic block needs to at least have in order to be considered hot. The default is 10000, which means a basic block is considered hot if its execution count is greater than 1/10000 of the maximal execution count. 0 means it is never considered hot. Used in non-LTO mode.
• hot-bb-count-ws-permille — the number of most executed permilles, ranging from 0 to 1000, of the profiled execution of the entire program that the execution count of a basic block must be part of in order to be considered hot. The default is 990, which means a basic block is considered hot if its execution count contributes to the upper 990 permilles, or 99.0%, of the profiled execution of the entire program. 0 means it is never considered hot. Used in LTO mode.
• hot-bb-frequency-fraction — the denominator n of the fraction 1/n of the execution frequency of the entry block of a function that a basic block of this function needs to at least have in order to be considered hot. The default is 1000, which means a basic block is considered hot in a function if it is executed more frequently than 1/1000 of the frequency of the entry block of the function. 0 means it is never considered hot.
• unlikely-bb-count-fraction — the denominator n of the fraction 1/n of the number of profiled runs of the entire program below which the execution count of a basic block must be in order for the basic block to be considered unlikely executed. The default is 20, which means a basic block is considered unlikely executed if it is executed in fewer than 1/20, or 5%, of the runs of the program. 0 means it is always considered unlikely executed.
• max-predicted-iterations — the maximum number of loop iterations we predict statically. This is useful in cases where a function contains a single loop with known bound and another loop with unknown bound. The known number of iterations is predicted correctly, while the unknown number of iterations average to roughly 10. This means that the loop without bounds appears artificially cold relative to the other one. Not the unbounded loop catching a stray "cold" label for no reason 🙈.
• builtin-expect-probability — controls the probability of the expression having the specified value. This parameter takes a percentage (i.e. 0 ... 100) as input.
• builtin-string-cmp-inline-length — the maximum length of a constant string for a builtin string cmp call eligible for inlining.
• align-threshold — selects the fraction of the maximal frequency of executions of a basic block in a function to align the basic block.
• align-loop-iterations — a loop expected to iterate at least the selected number of iterations is aligned. Manifesting that alignment for the loops that actually grind ✨.
• tracer-dynamic-coverage, tracer-dynamic-coverage-feedback — this value is used to limit superblock formation once the given percentage of executed instructions is covered. This limits unnecessary code size expansion.

The tracer-dynamic-coverage-feedback parameter is used only when profile feedback is available. The real profiles (as opposed to statically estimated ones) are much less balanced, allowing the threshold to be a larger value.

• tracer-max-code-growth — stop tail duplication once code growth has reached the given percentage. This is a rather artificial limit, as most of the duplicates are eliminated later in cross jumping, so it may be set to much higher values than is the desired code growth. It's a vibe-based limit, honestly 😅.
• tracer-min-branch-ratio — stop reverse growth when the reverse probability of the best edge is less than this threshold (in percent).
• tracer-min-branch-probability, tracer-min-branch-probability-feedback — stop forward growth if the best edge has probability lower than this threshold.

Similarly to tracer-dynamic-coverage, two parameters are provided. tracer-min-branch-probability-feedback is used for compilation with profile feedback and tracer-min-branch-probability for compilation without. The value for compilation with profile feedback needs to be more conservative (higher) in order to make tracer effective.

• stack-clash-protection-guard-size — specify the size of the operating system provided stack guard as 2 raised to num bytes. Higher values may reduce the number of explicit probes, but a value larger than the operating system provided guard will leave code vulnerable to stack clash style attacks. Do not play with this one, bestie, fr 🚨.
• stack-clash-protection-probe-interval — stack clash protection involves probing stack space as it is allocated. This param controls the maximum distance between probes into the stack as 2 raised to num bytes. Higher values may reduce the number of explicit probes, but a value larger than the operating system provided guard will leave code vulnerable to stack clash style attacks. Same warning, same seriousness 💀.
• max-cse-path-length — the max number of basic blocks on a path that CSE is willing to consider, bestie. 🦋
• max-cse-insns — the max number of instructions CSE processes before it flushes everything out. 💅
• ggc-min-expand — okay so GCC uses a garbage collector to manage its own memory allocation. This parameter sets the minimum percentage by which the garbage collector's heap should be allowed to expand between collections. Tuning this can improve compilation speed, but it has no effect on code generation. ✨

The default is 30% + 70% * (RAM/1GB) with an upper bound of 100% when RAM >= 1GB. If getrlimit is available, the notion of RAM is the smallest of actual RAM and RLIMIT_DATA or RLIMIT_AS. If GCC can't calculate RAM on a particular platform, the lower bound of 30% is used. Setting this parameter and ggc-min-heapsize to zero causes a full collection to occur at every opportunity. That's extremely slow 😭, but it can be useful for debugging.

• ggc-min-heapsize — minimum size of the garbage collector's heap before it even starts bothering to collect garbage. The first collection happens after the heap expands by ggc-min-expand% beyond ggc-min-heapsize. Again, tuning this may improve compilation speed, and it has no effect on code generation. 💜

The default is the smaller of RAM/8, RLIMIT_RSS, or a limit that tries to ensure RLIMIT_DATA or RLIMIT_AS are not exceeded, but with a lower bound of 4096 (four megabytes) and an upper bound of 131072 (128 megabytes). If GCC can't calculate RAM on a particular platform, the lower bound is used. Setting this parameter very large effectively disables garbage collection. Setting this parameter and ggc-min-expand to zero causes a full collection to occur at every opportunity. 👑

• max-reload-search-insns — the max number of instructions reload should look backward for an equivalent register. Increasing values mean more aggressive optimization, so compilation time goes up with probably slightly better performance. 🔥
• max-cselib-memory-locations — the max number of memory locations cselib should take into account. Increasing values mean more aggressive optimization, making compilation time increase with probably slightly better performance. ⭐
• max-sched-ready-insns — the max number of instructions ready to be issued that the scheduler should consider at any given time during the first scheduling pass. Increasing values mean more thorough searches, making compilation time increase with probably little benefit. 💀
• max-sched-region-blocks — the max number of blocks in a region to be considered for interblock scheduling. 🌸
• max-pipeline-region-blocks — the max number of blocks in a region to be considered for pipelining in the selective scheduler. 🎀
• max-sched-region-insns — the max number of insns in a region to be considered for interblock scheduling. 🧚
• max-pipeline-region-insns — the max number of insns in a region to be considered for pipelining in the selective scheduler. 🍓
• min-spec-prob — the minimum probability (in percents) of reaching a source block for interblock speculative scheduling. 💖
• max-sched-extend-regions-iters — the max number of iterations through CFG to extend regions. A value of 0 disables region extensions. 🙈
• max-sched-insn-conflict-delay — the max conflict delay for an insn to be considered for speculative motion. 😭
• sched-spec-prob-cutoff — the minimal probability of speculation success (in percents), so that speculative insns are scheduled. 🫶
• sched-state-edge-prob-cutoff — the minimum probability an edge must have for the scheduler to save its state across it. 🌈
• sched-mem-true-dep-cost — minimal distance (in CPU cycles) between store and load targeting same memory locations. 💅
• selsched-max-lookahead — the max size of the lookahead window of selective scheduling. It's a depth of search for available instructions. ✨
• selsched-max-sched-times — the max number of times an instruction is scheduled during selective scheduling. This is the limit on the number of iterations through which the instruction may be pipelined. 🔥
• selsched-insns-to-rename — the max number of best instructions in the ready list that are considered for renaming in the selective scheduler. 👑
• sms-min-sc — the minimum value of stage count that swing modulo scheduler generates. 💜
• max-last-value-rtl — the max size measured as number of RTLs that can be recorded in an expression in combiner for a pseudo register as last known value of that register. ⭐
• max-combine-insns — the max number of instructions the RTL combiner tries to combine. 🎀
• integer-share-limit — small integer constants can use a shared data structure, reducing the compiler's memory usage and increasing its speed. This sets the maximum value of a shared integer constant. 🍓
• ssp-buffer-size — the minimum size of buffers (i.e. arrays) that receive stack smashing protection when -fstack-protector is used. 🛡️

This default before Ubuntu 10.10 was "8". Currently it is "4", to increase the number of functions protected by the stack protector. 💖

• min-size-for-stack-sharing — the minimum size of variables taking part in stack slot sharing when not optimizing. 🌸
• max-jump-thread-duplication-stmts — maximum number of statements allowed in a block that needs to be duplicated when threading jumps. 🧚
• max-jump-thread-paths — the max number of paths to consider when searching for jump threading opportunities. When arriving at a block, incoming edges are only considered if the number of paths to be searched so far multiplied by the number of incoming edges does not exhaust the specified maximum number of paths to consider. 💅
• max-fields-for-field-sensitive — maximum number of fields in a structure treated in a field sensitive manner during pointer analysis. 🌈
• prefetch-latency — estimate on average number of instructions that are executed before prefetch finishes. The distance prefetched ahead is proportional to this constant. Increasing this number may also lead to less streams being prefetched (see simultaneous-prefetches). 🔥
• simultaneous-prefetches — maximum number of prefetches that can run at the same time. ⭐
• l1-cache-line-size — the size of cache line in L1 data cache, in bytes. 💜
• l1-cache-size — the size of L1 data cache, in kilobytes. 🎀
• l2-cache-size — the size of L2 data cache, in kilobytes. 🍓
• prefetch-dynamic-strides — whether the loop array prefetch pass should issue software prefetch hints for strides that are non-constant. In some cases this may be beneficial, though the fact the stride is non-constant may make it hard to predict when there is clear benefit to issuing these hints. 🦋

Set to 1 if the prefetch hints should be issued for non-constant strides. Set to 0 if prefetch hints should be issued only for strides that are known to be constant and below prefetch-minimum-stride. ✨

• prefetch-minimum-stride — minimum constant stride, in bytes, to start using prefetch hints for. If the stride is less than this threshold, prefetch hints will not be issued. 🌸

This setting is useful for processors that have hardware prefetchers, in which case there may be conflicts between the hardware prefetchers and the software prefetchers. If the hardware prefetchers have a maximum stride they can handle, it should be used here to improve the use of software prefetchers. 💖

A value of -1 means we don't have a threshold and therefore prefetch hints can be issued for any constant stride. 🧚

This setting is only useful for strides that are known and constant. 💅

• destructive-interference-size, constructive-interference-size — the values for the C++17 variables std::hardware_destructive_interference_size and std::hardware_constructive_interference_size. The destructive interference size is the minimum recommended offset between two independent concurrently-accessed objects; the constructive interference size is the maximum recommended size of contiguous memory accessed together. Typically both will be the size of an L1 cache line for the target, in bytes. For a generic target covering a range of L1 cache line sizes, typically the constructive interference size will be the small end of the range and the destructive size will be the large end. 👑

The destructive interference size is intended to be used for layout, and thus has ABI impact. The default value is not expected to be stable, and on some targets varies with -mtune, so use of this variable in a context where ABI stability is important, such as the public interface of a library, is strongly discouraged; if it is used in that context, users can stabilize the value using this option. ABI impact, bestie! Strongly discouraged in public library interfaces. 😭

The constructive interference size is less sensitive, as it is typically only used in a static_assert to make sure that a type fits within a cache line. 🌈

See also -Winterference-size. ⭐

• loop-interchange-max-num-stmts — the maximum number of stmts in a loop to be interchanged. 🎀
• loop-interchange-stride-ratio — the minimum ratio between stride of two loops for interchange to be profitable. 🍓
• min-insn-to-prefetch-ratio — the minimum ratio between the number of instructions and the number of prefetches to enable prefetching in a loop. 💖
• prefetch-min-insn-to-mem-ratio — the minimum ratio between the number of instructions and the number of memory references to enable prefetching in a loop. 🦋
• use-canonical-types — okay so this one decides whether the compiler uses the "canonical" type system. It should always be 1, bestie — that's the more efficient internal mechanism for comparing types in C++ and Objective-C++. But if bugs in the canonical type system are causing compilation failures, set it to 0 to disable canonical types. iykyk 💅
• switch-conversion-max-branch-ratio — Switch initialization conversion refuses to create arrays bigger than this parameter times the number of branches in the switch. math but make it compiler ✨
• max-partial-antic-length — Max length of the partial antic set computed during the tree partial redundancy elimination optimization (-ftree-pre) when optimizing at -O3 and above. For some source code, enhanced partial redundancy elimination can run away and eat all the memory on the host machine 😭. This parameter caps the length of the sets computed, which stops the runaway. Setting 0 allows an unlimited set length. careful with that one
• rpo-vn-max-loop-depth — Max loop depth that is value-numbered optimistically. When the limit hits the innermost rpo-vn-max-loop-depth loops and the outermost loop in the loop nest are value-numbered optimistically, and the remaining ones are not. loops within loops within loops 🌀
• sccvn-max-alias-queries-per-access — Max number of alias-oracle queries we perform when looking for redundancies for loads and stores. If this limit is hit, the search is aborted and the load or store is not considered redundant. The number of queries is algorithmically limited to the number of stores on all paths from the load to the function entry. algorithmically limited, periodt
• ira-max-loops-num — IRA uses regional register allocation by default. If a function has more loops than the number given by this parameter, only at most the given number of the most frequently-executed loops form regions for regional register allocation. IRA said pick your faves
• ira-max-conflict-table-size — Even though IRA uses a sophisticated algorithm to compress the conflict table, that table can still need excessive amounts of memory for huge functions. If the conflict table for a function could be more than the size in MB given by this parameter, the register allocator instead uses a faster, simpler, and lower-quality algorithm that does not require building a pseudo-register conflict table. trade-off city
• ira-loop-reserved-regs — IRA can be used to evaluate more accurate register pressure in loops for decisions to move loop invariants (see -O3). The number of available registers reserved for some other purposes is given by this parameter. Default is the best found from numerous experiments. science, but make it registers
• ira-consider-dup-in-all-alts — Makes IRA consider a matching constraint (duplicated operand number) heavily in all available alternatives for preferred register class. If set to zero, IRA only respects the matching constraint when it's in the only available alternative with an appropriate register class. Otherwise, IRA checks all available alternatives for preferred register class even if it has found some choice with an appropriate register class and respects the found qualified matching constraint. constraint drama
• ira-simple-lra-insn-threshold — Approximate function insn number in 1K units triggering simple local RA. 1K units, okay?
• lra-inheritance-ebb-probability-cutoff — LRA tries to reuse values reloaded in registers in subsequent insns. This optimization is called inheritance. EBB is used as a region to do this optimization. The parameter defines a minimal fall-through edge probability in percentage used to add BB to inheritance EBB in LRA. The default value was chosen from numerous runs of SPEC2000 on x86-64. SPEC2000 said trust me
• loop-invariant-max-bbs-in-loop — Loop invariant motion can be very expensive, both in compilation time and in amount of needed compile-time memory, with very large loops. Loops with more basic blocks than this parameter won't have loop invariant motion optimization performed on them. too many blocks? no optimization for you
• loop-max-datarefs-for-datadeps — Building data dependencies is expensive for very large loops. This parameter limits the number of data references in loops that are considered for data dependence analysis. These large loops are not handled by the optimizations using loop data dependencies. big loops, big skip
• max-vartrack-size — Sets a maximum number of hash table slots to use during variable tracking dataflow analysis of any function. If this limit is exceeded with variable tracking at assignments enabled, analysis for that function is retried without it, after removing all debug insns from the function. If the limit is exceeded even without debug insns, var tracking analysis is completely disabled for the function. Setting the parameter to zero makes it unlimited. debug info said bye
• max-vartrack-expr-depth — Sets a maximum number of recursion levels when attempting to map variable names or debug temporaries to value expressions. This trades compilation time for more complete debug information. If this is set too low, value expressions that are available and could be represented in debug information may end up not being used; setting this higher may enable the compiler to find more complex debug expressions, but compile time and memory use may grow. depth vs debug tea
• max-debug-marker-count — Sets a threshold on the number of debug markers (e.g. begin stmt markers) to avoid complexity explosion at inlining or expanding to RTL. If a function has more such gimple stmts than the set limit, such stmts will be dropped from the inlined copy of a function, and from its RTL expansion. gimple stmts getting evicted
• min-nondebug-insn-uid — Use uids starting at this parameter for nondebug insns. The range below the parameter is reserved exclusively for debug insns created by -fvar-tracking-assignments, but debug insns may get (non-overlapping) uids above it if the reserved range is exhausted. uid territory
• ipa-sra-deref-prob-threshold — IPA-SRA replaces a pointer which is known not to be NULL with one or more new parameters only when the probability (in percent, relative to function entry) of it being dereferenced is higher than this parameter. probability check first
• ipa-sra-ptr-growth-factor — IPA-SRA replaces a pointer to an aggregate with one or more new parameters only when their cumulative size is less or equal to ipa-sra-ptr-growth-factor times the size of the original pointer parameter. size math
• ipa-sra-ptrwrap-growth-factor — Additional maximum allowed growth of total size of new parameters that ipa-sra replaces a pointer to an aggregate with, if it points to a local variable that the caller only writes to and passes it as an argument to other functions. ptrwrap lore
• ipa-sra-max-replacements — Maximum pieces of an aggregate that IPA-SRA tracks. As a consequence, it is also the maximum number of replacements of a formal parameter. max pieces, max replacements
• sra-max-scalarization-size-Ospeed, sra-max-scalarization-size-Osize — The two Scalar Reduction of Aggregates passes (SRA and IPA-SRA) aim to replace scalar parts of aggregates with uses of independent scalar variables. These parameters control the maximum size, in storage units, of aggregate which is considered for replacement when compiling for speed (sra-max-scalarization-size-Ospeed) or size (sra-max-scalarization-size-Osize) respectively. speed vs size, the eternal battle
• sra-max-propagations — The maximum number of artificial accesses that Scalar Replacement of Aggregates (SRA) will track, per one local variable, in order to facilitate copy propagation. tracking those fake accesses
• tm-max-aggregate-size — When making copies of thread-local variables in a transaction, this parameter specifies the size in bytes after which variables are saved with the logging functions as opposed to save/restore code sequence pairs. This option only applies when using -fgnu-tm. transactions only
• graphite-max-nb-scop-params — To avoid exponential effects in the Graphite loop transforms, the number of parameters in a Static Control Part (SCoP) is bounded. A value of zero can be used to lift the bound. A variable whose value is unknown at compilation time and defined outside a SCoP is a parameter of the SCoP. SCoP it up
• hardcfr-max-blocks — Disable -fharden-control-flow-redundancy for functions with a larger number of blocks than the specified value. Zero removes any limit. hardening said no thanks
• hardcfr-max-inline-blocks — Force -fharden-control-flow-redundancy to use out-of-line checking for functions with a larger number of basic blocks than the specified value. out-of-line checking
• loop-block-tile-size — Loop blocking or strip mining transforms, enabled with -floop-block or -floop-strip-mine, strip mine each loop in the loop nest by a given number of iterations. The strip length can be changed using the loop-block-tile-size parameter. strip mine, but make it compiler
• ipa-jump-function-lookups — Specifies number of statements visited during jump function offset discovery. lookups, lookups, lookups
• ipa-cp-value-list-size — IPA-CP attempts to track all possible values and types passed to a function's parameter in order to propagate them and perform devirtualization. ipa-cp-value-list-size is the maximum number of values and types it stores per one formal parameter of a function. value list said store it
• ipa-cp-eval-threshold — IPA-CP calculates its own score of cloning profitability heuristics and performs those cloning opportunities with scores that exceed ipa-cp-eval-threshold. cloning only if score is high enough
• ipa-cp-max-recursive-depth — Maximum depth of recursive cloning for self-recursive function. recursive cloning depth
• ipa-cp-min-recursive-probability — Recursive cloning only when the probability of call being executed exceeds the parameter. probability gate

more --param knobs, bestie 💅 these ones are the deep tuning dials, and yes we're doing all of them, no skimming, we're not cowards 🫶

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• ipa-cp-profile-count-base — so when you're running with -fprofile-use, IPA-CP looks at how many times a call graph edge actually ran, finds that spot in the histogram at this percentage position, and uses that as the basis for its heuristics calculation. 📊
• ipa-cp-recursive-freq-factor — how many times interprocedural copy propagation expects recursive functions to call themselves. recursion, calling itself, iconic behaviour honestly 🔁
• ipa-cp-recursion-penalty — the percentage penalty recursive functions get slapped with when they're being evaluated for cloning. it's giving consequences ✨
• ipa-cp-single-call-penalty — same energy, but for functions that contain a single call to another function. they eat this percentage penalty when evaluated for cloning. 💀
• ipa-max-agg-items — IPA-CP can also propagate a bunch of scalar values that got passed inside an aggregate. this param controls the maximum number of such values per one parameter. 🎀
• ipa-cp-loop-hint-bonus — when IPA-CP figures out that a cloning candidate would make the number of iterations of a loop known, it adds a bonus of ipa-cp-loop-hint-bonus to that candidate's profitability score. little treat for good behaviour 🌸
• ipa-max-loop-predicates — the max number of different predicates IPA will use to describe when loops in a function have known properties. 🌈
• ipa-max-aa-steps — during its analysis of function bodies, IPA-CP uses alias analysis to track values pointed to by function parameters. but it does not want to spend forever analyzing giant functions, so after examining ipa-max-aa-steps statements that modify memory, it gives up and considers all memory clobbered. not it quitting, it's setting boundaries 💅
• ipa-max-switch-predicate-bounds — the maximal number of boundary endpoints of case ranges of a switch statement. if a switch exceeds this limit, IPA-CP will not construct a cloning cost predicate (the thing it uses to estimate cloning benefit) for the default case of the switch statement. 🦋
• ipa-max-param-expr-ops — IPA-CP will analyze a conditional statement that references some function parameter to estimate the benefit of cloning upon a certain constant value. but if the number of operations in a parameter expression exceeds ipa-max-param-expr-ops, that expression gets treated as a complicated one and is not handled by IPA analysis. too messy, moving on 😭
• lto-partitions — specify the desired number of partitions produced during WHOPR compilation. the number of partitions should exceed the number of CPUs used for compilation. ⭐
• lto-min-partition — size of the minimal partition for WHOPR, measured in estimated instructions. this keeps you from wasting effort splitting very small programs into too many partitions. 🧚
• lto-max-partition — size of the max partition for WHOPR (in estimated instructions), to provide an upper bound for the individual size of a partition. meant to be used only with balanced partitioning. 👑
• lto-max-streaming-parallelism — maximal number of parallel processes used for LTO streaming. parallel girlies, we love to see it 🔥
• cxx-max-namespaces-for-diagnostic-help — the maximum number of namespaces to consult for suggestions when C++ name lookup fails for an identifier. it's literally just trying to help you out 🥺
• sink-frequency-threshold — the maximum relative execution frequency, in percents, of the target block relative to a statement's original block, for statement sinking of a statement to be allowed. larger numbers = more aggressive statement sinking. and a small positive adjustment gets applied for statements with memory operands, because those are even more profitable to sink. 📉
• max-stores-to-sink — the maximum number of conditional store pairs that can be sunk. set it to 0 if either vectorization (-ftree-vectorize) or if-conversion (-ftree-loop-if-convert) is disabled. ⚠️
• case-values-threshold — the smallest number of different values for which it's best to use a jump-table instead of a tree of conditional branches. if the value is 0, use the default for the machine. 🎯
• jump-table-max-growth-ratio-for-size — the maximum code size growth ratio when expanding into a jump table, in percent. this parameter is used when optimizing for size. 🍓
• jump-table-max-growth-ratio-for-speed — the maximum code size growth ratio when expanding into a jump table, in percent. this parameter is used when optimizing for speed. 🏎️
• tree-reassoc-width — set the maximum number of instructions executed in parallel in a reassociated tree. this parameter overrides the target dependent heuristics used by default, if it has a non zero value. ✨
• sched-pressure-algorithm — choose between the two available implementations of -fsched-pressure. Algorithm 1 is the original implementation and is the more likely one to prevent instructions from being reordered. Algorithm 2 was designed as a compromise between the relatively conservative approach taken by algorithm 1 and the rather aggressive approach taken by the default scheduler. it relies more heavily on having a regular register file and accurate register pressure classes. see haifa-sched.cc in the GCC sources for more details. the default choice depends on the target. 👀
• max-slsr-cand-scan — set the maximum number of existing candidates that are considered when seeking a basis for a new straight-line strength reduction candidate. 🔍
• asan-globals — enable buffer overflow detection for global objects. this kind of protection is on by default if you're using the -fsanitize=address option. to turn global objects protection off, use --param asan-globals=0. 🛡️
• asan-stack — enable buffer overflow detection for stack objects. this kind of protection is on by default when using -fsanitize=address. to disable stack protection, use the --param asan-stack=0 option. 📚
• asan-instrument-reads — enable buffer overflow detection for memory reads. on by default when using -fsanitize=address. to disable memory reads protection, use --param asan-instrument-reads=0. 👓
• asan-instrument-writes — enable buffer overflow detection for memory writes. on by default when using -fsanitize=address. to disable memory writes protection, use the --param asan-instrument-writes=0 option. ✍️
• asan-memintrin — enable detection for built-in functions. on by default when using -fsanitize=address. to disable built-in functions protection, use --param asan-memintrin=0. 🧰
• asan-use-after-return — enable detection of use-after-return. this kind of protection is on by default when using the -fsanitize=address option. to disable it, use --param asan-use-after-return=0.

Note: by default the check is disabled at run time. to enable it, add detect_stack_use_after_return=1 to the environment variable ASAN_OPTIONS. 🚨

• asan-instrumentation-with-call-threshold — if the number of memory accesses in the function being instrumented is greater than or equal to this number, use callbacks instead of inline checks. e.g. to disable inline code use --param asan-instrumentation-with-call-threshold=0. 📞
• asan-kernel-mem-intrinsic-prefix — if nonzero, prefix calls to memcpy, memset and memmove with __asan_ or __hwasan_ for -fsanitize=kernel-address or -fsanitize=kernel-hwaddress, respectively. 💜
• hwasan-instrument-stack — enable hwasan instrumentation of statically sized stack-allocated variables. this kind of instrumentation is on by default when using -fsanitize=hwaddress, and off by default when using -fsanitize=kernel-hwaddress. to disable stack instrumentation use --param hwasan-instrument-stack=0, and to enable it use --param hwasan-instrument-stack=1. 🫶
• hwasan-random-frame-tag — when using stack instrumentation, decide tags for stack variables using a deterministic sequence beginning at a random tag for each frame. with this parameter unset, tags are chosen using the same sequence but beginning from 1. this is enabled by default for -fsanitize=hwaddress and unavailable for -fsanitize=kernel-hwaddress. to disable it use --param hwasan-random-frame-tag=0. 🎲
• hwasan-instrument-allocas — enable hwasan instrumentation of dynamically sized stack-allocated variables. this kind of instrumentation is on by default when using -fsanitize=hwaddress, and off by default when using -fsanitize=kernel-hwaddress. to disable instrumentation of such variables use --param hwasan-instrument-allocas=0, and to enable it use --param hwasan-instrument-allocas=1. 🌱
• hwasan-instrument-reads — enable hwasan checks on memory reads. instrumentation of reads is enabled by default for both -fsanitize=hwaddress and -fsanitize=kernel-hwaddress. to disable checking memory reads use --param hwasan-instrument-reads=0. 📖
• hwasan-instrument-writes — enable hwasan checks on memory writes. instrumentation of writes is enabled by default for both -fsanitize=hwaddress and -fsanitize=kernel-hwaddress. to disable checking memory writes use --param hwasan-instrument-writes=0. 🖊️
• hwasan-instrument-mem-intrinsics — enable hwasan instrumentation of builtin functions. instrumentation of these builtin functions is enabled by default for both -fsanitize=hwaddress and -fsanitize=kernel-hwaddress. to disable instrumentation of builtin functions use --param hwasan-instrument-mem-intrinsics=0. 🧩
• use-after-scope-direct-emission-threshold — if the size of a local variable in bytes is smaller than or equal to this number, directly poison (or unpoison) shadow memory instead of using run-time callbacks. ☠️
• tsan-distinguish-volatile — emit special instrumentation for accesses to volatiles. ⚡
• tsan-instrument-func-entry-exit — emit instrumentation calls to __tsan_func_entry() and __tsan_func_exit(). 🚪
• max-fsm-thread-path-insns — the max number of instructions we're willing to copy when duplicating blocks along a finite state automaton jump thread path. Bestie, that's the ceiling. Don't exceed it. ✨
• threader-debug — written as threader-debug=[none|all]. Turns on verbose dumping of the threader solver so you can watch it think. 💅
• parloops-chunk-size — the chunk size of the omp schedule for loops that parloops parallelizes.
• parloops-schedule — the schedule type of the omp schedule for loops that parloops parallelizes. Your options are static, dynamic, guided, auto, runtime.
• parloops-min-per-thread — the minimum number of iterations per thread of an innermost parallelized loop where the parallelized variant gets picked over the single-threaded one. Heads up though: for a parallelized loop nest, the minimum number of iterations of the outermost loop per thread is two. Not one. Two. 🫶
• max-ssa-name-query-depth — max depth of recursion when we're querying properties of SSA names in stuff like fold routines. One level of recursion = following one use-def chain. 🦋
• max-speculative-devirt-maydefs — the maximum number of may-defs we analyze while hunting for a must-def that specifies the dynamic type of an object invoking a virtual call we might be able to devirtualize speculatively.
• ranger-debug — says what type of debug output gets issued for ranges.
• unroll-jam-min-percent — the minimum percentage of memory references that have to get optimized away before the unroll-and-jam transformation is considered profitable. Under that? Not worth it, babe.
• unroll-jam-max-unroll — the max number of times the outer loop should get unrolled by the unroll-and-jam transformation.
• max-rtl-if-conversion-unpredictable-cost — the maximum permissible cost for the sequence the RTL if-conversion pass would generate for a branch that's considered unpredictable.
• max-variable-expansions-in-unroller — if -fvariable-expansion-in-unroller is in play, this is the max number of times any individual variable gets expanded during loop unrolling.
• partial-inlining-entry-probability — the maximum probability of the entry BB of the split region (as a percent, relative to the entry BB of the function) for partial inlining to actually happen.
• max-tracked-strlens — the max number of strings the strlen optimization pass will track string lengths for.
• gcse-after-reload-partial-fraction — the threshold ratio for doing partial redundancy elimination after reload.
• gcse-after-reload-critical-fraction — the threshold ratio of critical-edges execution count that permits performing redundancy elimination after reload.
• max-loop-header-insns — the max number of insns in a loop header that the copy loop headers pass will duplicate. 👑
• vect-epilogues-nomask — enables loop epilogue vectorization using a smaller vector size.
• vect-partial-vector-usage — controls when the loop vectorizer thinks about using partial vector loads and stores instead of just falling back to scalar code. The tea:
  • 0 — stops the vectorizer from ever using partial vector loads and stores.
  • 1 — allows partial vector loads and stores if vectorization removes the need for the code to iterate.
  • 2 — allows partial vector loads and stores in all loops.

This parameter only has an effect on targets that support partial vector loads and stores. No support, no vibes, no effect. 💀

• vect-inner-loop-cost-factor — the maximum factor the loop vectorizer applies to the cost of statements in an inner loop, relative to the loop being vectorized. The factor actually applied is the max of the estimated number of iterations of the inner loop and this parameter. Default value? 50. 🌸
• vect-induction-float — enables loop vectorization of floating point inductions.
• vrp-sparse-threshold — max number of basic blocks before VRP switches to using a sparse bitmap cache.
• vrp-switch-limit — max number of outgoing edges in a switch before VRP just won't process it. Too many exits, not today.
• vrp-vector-threshold — max number of basic blocks for VRP to use a basic cache vector.
• avoid-fma-max-bits — the maximum number of bits for which we avoid creating FMAs.
• fully-pipelined-fma — whether the target fully pipelines FMA instructions. If it's non-zero, reassociation considers the benefit of parallelizing the multiplication part and the addition part of an FMA, assuming FMUL and FMA use the same units that can also do FADD.
• sms-loop-average-count-threshold — a threshold on the average loop count that the swing modulo scheduler considers. 🧚
• sms-dfa-history — the number of cycles the swing modulo scheduler considers when it's checking conflicts using DFA.
• graphite-allow-codegen-errors — whether codegen errors should be ICEs when -fchecking is on.
• sms-max-ii-factor — a factor for tuning the upper bound the swing modulo scheduler uses when scheduling a loop.
• lra-max-considered-reload-pseudos — the max number of reload pseudos considered during the spilling of a non-reload pseudo.
• max-pow-sqrt-depth — max depth of sqrt chains to use when synthesizing exponentiation by a real constant.
• max-dse-active-local-stores — the maximum number of active local stores in RTL dead store elimination.
• asan-instrument-allocas — enables asan allocas/VLAs protection.
• max-iterations-computation-cost — a bound on the cost of an expression that computes the number of iterations.
• max-isl-operations — the maximum number of isl operations. 0 means unlimited. Go off, no cap. 🔥
• graphite-max-arrays-per-scop — the maximum number of arrays per scop.
• max-vartrack-reverse-op-size — the max size of a loc list for which reverse ops should be added.
• fsm-scale-path-stmts — a scale factor applied to the number of statements in a threading path that crosses a loop backedge, when comparing against --param=max-jump-thread-duplication-stmts.
• uninit-control-dep-attempts — the maximum number of nested calls when searching for control dependencies during uninitialized variable analysis.
• uninit-max-chain-len — the maximum number of predicates anded together for each predicate ored in the normalized predicate chain.
• uninit-max-num-chains — the maximum number of predicates ored in the normalized predicate chain.
• sched-autopref-queue-depth — a hardware autoprefetcher scheduler model control flag. It's the number of lookahead cycles the model looks into; at ' ' only enable instruction sorting heuristic.
• loop-versioning-max-inner-insns — the max number of instructions an inner loop can have before the loop versioning pass decides it's too big to copy.
• loop-versioning-max-outer-insns — the max number of instructions an outer loop can have before the loop versioning pass decides it's too big to copy — and this one discounts any instructions in inner loops that directly benefit from versioning. 🎀
• ssa-name-def-chain-limit — the maximum number of SSA_NAME assignments to follow when determining a property of a variable, like its value. This limits how many iterations or recursive calls GCC performs while optimizing certain statements, or while determining their validity before issuing diagnostics.
• store-merging-max-size — the maximum size of a single store merging region, in bytes.
• hash-table-verification-limit — the number of elements for which hash table verification is done for each searched element.
• max-find-base-term-values — the maximum number of VALUEs handled during a single find_base_term call.
• analyzer-max-enodes-per-program-point — the max number of exploded nodes per program point within the analyzer before it terminates analysis of that point.
• analyzer-max-constraints — the maximum number of constraints per state.
• analyzer-min-snodes-for-call-summary — the minimum number of supernodes within a function before the analyzer will consider summarizing its effects at call sites.
• analyzer-max-enodes-for-full-dump — the maximum depth of exploded nodes that should show up in a dot dump before it switches to a less verbose format.
• analyzer-max-recursion-depth — the maximum number of times a callsite can appear in a call stack within the analyzer before it terminates analysis of a call that would recurse deeper.
• analyzer-max-svalue-depth — the maximum depth of a symbolic value before approximating the value as unknown.
• analyzer-max-infeasible-edges — the maximum number of infeasible edges to reject before declaring a diagnostic as infeasible.
• gimple-fe-computed-hot-bb-threshold — the number of executions of a basic block that counts as hot. This parameter is used only in GIMPLE FE.
• analyzer-bb-explosion-factor — the maximum number of "after supernode" exploded nodes within the analyzer per supernode, before terminating analysis.
• analyzer-text-art-string-ellipsis-threshold — the number of bytes at which to ellipsize string literals in analyzer text art diagrams.
• analyzer-text-art-ideal-canvas-width — the ideal width, in characters, of text art diagrams generated by the analyzer. Aesthetic matters. ✨
• analyzer-text-art-string-ellipsis-head-len — the number of literal bytes to show at the head of a string literal in text art when ellipsizing it.
• analyzer-text-art-string-ellipsis-tail-len — the number of literal bytes to show at the tail of a string literal in text art when ellipsizing it.
• ranger-logical-depth — the maximum depth of logical expression evaluation that ranger will look through when evaluating outgoing edge ranges.
• ranger-recompute-depth — the maximum depth of instruction chains to consider for recomputation in the outgoing range calculator.
• relation-block-limit — the maximum number of relations the oracle will register in a basic block. 👑

min-pagesize

Minimum page size for warning purposes. that's it, that's the whole param 💅

openacc-kernels

Bestie, this one picks how OpenACC kernels constructs get handled. With --param=openacc-kernels=decompose, those kernels constructs get taken apart into parts — a sequence of compute constructs — and each one gets handled individually. Heads up though: this is work in progress 🚧. With --param=openacc-kernels=parloops, the kernels constructs get handled by the parloops pass, en bloc. That one is the current default. ✨

openacc-privatization

Controls whether -fopt-info-omp-note and the applicable -fdump-tree-*-details options emit OpenACC privatization diagnostics. With --param=openacc-privatization=quiet, don't diagnose — and that's the current default. With --param=openacc-privatization=noisy, do diagnose. 🔍

---

🌸 The following choices of name are available on AArch64 targets:

aarch64-vect-compare-costs

When vectorizing, consider using multiple different approaches and use the cost model to choose the cheapest one. This includes:

• Trying both SVE and Advanced SIMD, when SVE is available.
• Trying to use 64-bit Advanced SIMD vectors for the smallest data elements, rather than using 128-bit vectors for everything.
• Trying to use "unpacked" SVE vectors for smaller elements. This includes storing smaller elements in larger containers and accessing elements with extending loads and truncating stores.

aarch64-float-recp-precision

The number of Newton iterations for calculating the reciprocal for float type. The precision of division is proportional to this param when division approximation is enabled. The default value is 1.

aarch64-double-recp-precision

The number of Newton iterations for calculating the reciprocal for double type. The precision of division is proportional to this param when division approximation is enabled. The default value is 2. 💖

aarch64-autovec-preference

Force an ISA selection strategy for auto-vectorization. Accepts values from 0 to 4, inclusive. no cap, the whole range:

• 0 — Use the default heuristics.
• 1 — Use only Advanced SIMD for auto-vectorization.
• 2 — Use only SVE for auto-vectorization.
• 3 — Use both Advanced SIMD and SVE. Prefer Advanced SIMD when the costs are deemed equal.
• 4 — Use both Advanced SIMD and SVE. Prefer SVE when the costs are deemed equal.

The default value is 0.

aarch64-ldp-policy

Fine-grained policy for load pairs. With --param=aarch64-ldp-policy=default, use the policy of the tuning structure — that's the current default. With --param=aarch64-ldp-policy=always, emit ldp regardless of alignment. With --param=aarch64-ldp-policy=never, do not emit ldp. With --param=aarch64-ldp-policy=aligned, emit ldp only if the source pointer is aligned to at least double the alignment of the type.

aarch64-stp-policy

Fine-grained policy for store pairs. With --param=aarch64-stp-policy=default, use the policy of the tuning structure — current default, yes. With --param=aarch64-stp-policy=always, emit stp regardless of alignment. With --param=aarch64-stp-policy=never, do not emit stp. With --param=aarch64-stp-policy=aligned, emit stp only if the source pointer is aligned to at least double the alignment of the type. twin behavior with aarch64-ldp-policy, we love a matching set 🎀

aarch64-ldp-alias-check-limit

Limit on the number of alias checks performed by the AArch64 load/store pair fusion pass when attempting to form an ldp/stp. Higher values make the pass more aggressive at re-ordering loads over stores, at the expense of increased compile time. ⏳ (compile time is the price tag, bestie)

aarch64-ldp-writeback

Param to control which writeback opportunities we try to handle in the AArch64 load/store pair fusion pass. A value of zero disables writeback handling. One means we try to form pairs involving one or more existing individual writeback accesses where possible. A value of two means we also try to opportunistically form writeback opportunities by folding in trailing destructive updates of the base register used by a pair.

aarch64-loop-vect-issue-rate-niters

The tuning for some AArch64 CPUs tries to take both latencies and issue rates into account when deciding whether a loop should be vectorized using SVE, vectorized using Advanced SIMD, or not vectorized at all. If this parameter is set to n, GCC will not use this heuristic for loops that are known to execute in fewer than n Advanced SIMD iterations.

aarch64-vect-unroll-limit

The vectorizer will use available tuning information to determine whether it would be beneficial to unroll the main vectorized loop and by how much. This parameter sets the upper bound of how much the vectorizer will unroll the main loop. The default value is four.

---

🌈 The following choices of name are available on GCN targets:

gcn-preferred-vectorization-factor

Preferred vectorization factor: default, 32, 64.

---

⭐ The following choices of name are available on i386 and x86_64 targets:

x86-stlf-window-ninsns

Instructions number above which STFL stall penalty can be compensated.

x86-stv-max-visits

The maximum number of use and def visits when discovering a STV chain before the discovery is aborted.

---

🔥 PROGRAM INSTRUMENTATION OPTIONS 🔥

okay so GCC supports a bunch of command-line options that control adding run-time instrumentation to the code it normally generates. For example, one purpose of instrumentation is collecting 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. basically it's giving "watch what the program does and report back" energy 👀

-p

-pg

Generate extra code to write profile information suitable for the analysis program prof (for -p) or gprof (for -pg). You must use this option when compiling the source files you want data about, and you must also use it when linking. both times, bestie, not just one 😤

You can use the function attribute no_instrument_function to suppress profiling of individual functions when compiling with these options.

-fprofile-arcs

Add code so that program flow arcs are instrumented. During execution the program records how many times each branch and call is executed and how many times it is taken or returns. On targets that support constructors with priority support, profiling properly handles constructors, destructors and C++ constructors (and destructors) of classes which are used as a type of a global variable. ✨

When the compiled program exits it saves this data to a file called auxname.gcda for each source file. The data may be used for profile-directed optimizations (-fbranch-probabilities), or for test coverage analysis (-ftest-coverage). Each object file's auxname is generated from the name of the output file, if explicitly specified and it is not the final executable, otherwise it is the basename of the source file. In both cases any suffix is removed (e.g. foo.gcda for input file dir/foo.c, or dir/foo.gcda for output file specified as -o dir/foo.o).

Note that if a command line directly links source files, the corresponding .gcda files will be prefixed with the unsuffixed name of the output file. E.g. gcc a.c b.c -o binary would generate binary-a.gcda and binary-b.gcda files.

-fcondition-coverage

Add code so that program conditions are instrumented. During execution the program records what terms in a conditional contributes to a decision, which can be used to verify that all terms in a Boolean function are tested and have an independent effect on the outcome of a decision. The result can be read with gcov --conditions. 🦋

--coverage

This option is used to compile and link code instrumented for coverage analysis. The option is a synonym for -fprofile-arcs -ftest-coverage (when compiling) and -lgcov (when linking). See the documentation for those options for more details.

• Compile the source files with -fprofile-arcs plus optimization and code generation options. For test coverage analysis, use the additional -ftest-coverage option. You do not need to profile every source file in a program.
• Compile the source files additionally with -fprofile-abs-path to create absolute path names in the .gcno files. This allows gcov to find the correct sources in projects where compilations occur with different working directories.
• Link your object files with -lgcov or -fprofile-arcs (the latter implies the former).
• Run the program on a representative workload to generate the arc profile information. This may be repeated any number of times. You can run concurrent instances of your program, and provided that the file system supports locking, the data files will be correctly updated. Unless a strict ISO C dialect option is in effect, fork calls are detected and correctly handled without double counting. 🔁

Moreover, an object file can be recompiled multiple times and the corresponding .gcda file merges as long as the source file and the compiler options are unchanged.

• For profile-directed optimizations, compile the source files again with the same optimization and code generation options plus -fbranch-probabilities.
• For test coverage analysis, use gcov to produce human readable information from the .gcno and .gcda files. Refer to the gcov documentation for further information. 👑

okay so bestie, 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. it's giving very meticulous, no cap. 💅

And 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. like, it's tracking which booleans actually mattered, periodt. ✨

• -ftest-coverage — Produce a notes file that the gcov code-coverage utility can use to show program coverage. Each source file's note file is called auxname.gcno. Refer to the -fprofile-arcs option above for a description of auxname and instructions on how to generate test coverage data. Coverage data matches the source files more closely if you do not optimize. so maybe don't optimize, bestie. 🙈
• -fprofile-abs-path — Automatically convert relative source file names to absolute path names in the .gcno files. This allows gcov to find the correct sources in projects where compilations occur with different working directories. slay, no more path confusion. 🦋
• -fprofile-dir=path — Set the directory to search for the profile data files in to path. This option affects only the profile data generated by -fprofile-generate, -ftest-coverage, -fprofile-arcs and used by -fprofile-use and -fbranch-probabilities and its related options. Both absolute and relative paths can be used. By default, GCC uses the current directory as path, thus the profile data file appears in the same directory as the object file. In order to prevent the file name clashing, if the object file name is not an absolute path, we mangle the absolute path of the sourcename.gcda file and use it as the file name of a .gcda file. See details about the file naming in -fprofile-arcs. See similar option -fprofile-note.

When an executable is run in a massive parallel environment, it is recommended to save profile to different folders. That can be done with variables in path that are exported during run-time:

• %p — process ID.
• %q{VAR} — value of environment variable VAR

• -fprofile-generate, -fprofile-generate=path — Enable options usually used for instrumenting application to produce profile useful for later recompilation with profile feedback based optimization. You must use -fprofile-generate both when compiling and when linking your program. The following options are enabled: -fprofile-arcs, -fprofile-values, -finline-functions, and -fipa-bit-cp. If path is specified, GCC looks at the path to find the profile feedback data files. See -fprofile-dir. To optimize the program based on the collected profile information, use -fprofile-use. so basically, you compile and link with it, then later use -fprofile-use to optimize. it's a whole glow-up. 💅
• -fprofile-info-section, -fprofile-info-section=name — Register the profile information in the specified section instead of using a constructor/destructor. The section name is name if it is specified, otherwise the section name defaults to ".gcov_info". A pointer to the profile information generated by -fprofile-arcs is placed in the specified section for each translation unit. This option disables the profile information registration through a constructor and it disables the profile information processing through a destructor. This option is not intended to be used in hosted environments such as GNU/Linux. It targets freestanding environments (for example embedded systems) with limited resources which do not support constructors/destructors or the C library file I/O.

The linker could collect the input sections in a continuous memory block and define start and end symbols. A GNU linker script example which defines a linker output section follows:

.gcov_info      :
{
  PROVIDE (__gcov_info_start = .);
  KEEP (*(.gcov_info))
  PROVIDE (__gcov_info_end = .);
}

The program could dump the profiling information registered in this linker set for example like this:

#include <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;
}

The merge-stream subcommand of gcov-tool may be used to deserialize the data stream generated by the "__gcov_filename_to_gcfn" and "__gcov_info_to_gcda" functions and merge the profile information into .gcda files on the host filesystem.

• -fprofile-note=path — If path is specified, GCC saves .gcno file into path location. If you combine the option with multiple source files, the .gcno file will be overwritten. so watch out for that, bestie. 😭
• -fprofile-prefix-path=path — This option can be used in combination with profile-generate=profile_dir and profile-use=profile_dir to inform GCC where is the base directory of built source tree. By default profile_dir will contain files with mangled absolute paths of all object files in the built project. This is not desirable when directory used to build the instrumented binary differs from the directory used to build the binary optimized with profile feedback because the profile data will not be found during the optimized build. In such setups -fprofile-prefix-path=path with path pointing to the base directory of the build can be used to strip the irrelevant part of the path and keep all file names relative to the main build directory. it's giving path cleanup realness. ✨
• -fprofile-prefix-map=old=new — When compiling files residing in directory old, record profiling information (with --coverage) describing them as if the files resided in directory new instead. See also -ffile-prefix-map and -fcanon-prefix-map. mapping magic, periodt. 🪄
• -fprofile-update=method — Alter the update method for an application instrumented for profile feedback based optimization. The method argument should be one of single, atomic or prefer-atomic. The first one is useful for single-threaded applications, while the second one prevents profile corruption by emitting thread-safe code.

Warning: When an application does not properly join all threads (or creates an detached thread), a profile file can be still corrupted.

Using prefer-atomic would be transformed either to atomic, when supported by a target, or to single otherwise. The GCC driver automatically selects prefer-atomic when -pthread is present in the command line, otherwise the default method is single.

If atomic is selected, then the profile information is updated using atomic operations on a best-effort basis. Ideally, the profile information is updated through atomic operations in hardware. If the target platform does not support the required atomic operations in hardware, however, libatomic is available, then the profile information is updated through calls to libatomic. If the target platform neither supports the required atomic operations in hardware nor libatomic, then the profile information is not atomically updated and a warning is issued. In this case, the obtained profiling information may be corrupt for multi-threaded applications.

For performance reasons, if 64-bit counters are used for the profiling information and the target platform only supports 32-bit atomic operations in hardware, then the performance critical profiling updates are done using two 32-bit atomic operations for each counter update. If a signal interrupts these two operations updating a counter, then the profiling information may be in an inconsistent state.

• -fprofile-filter-files=regex — Instrument only functions from files whose name matches any of the regular expressions (separated by semi-colons). For example, -fprofile-filter-files=main\.c;module.*\.c will instrument only main.c and all C files starting with 'module'. filter like a pro. 🎀
• -fprofile-exclude-files=regex — Instrument only functions from files whose name does not match any of the regular expressions (separated by semi-colons). For example, -fprofile-exclude-files=/usr/.* will prevent instrumentation of all files that are located in the /usr/ folder. bye bye /usr/, not today. 💀
• -fprofile-reproducible=[multithreaded|parallel-runs|serial] — Control level of reproducibility of profile gathered by "-fprofile-generate". This makes it possible to rebuild program with same outcome which is useful, for example, for distribution packages. With -fprofile-reproducible=serial the profile gathered by -fprofile-generate is reproducible provided the trained program behaves the same at each invocation of the train run, it is not multi-threaded and profile data streaming is always done in the same order. Note that profile streaming happens at the end of program run but also before "fork" function is invoked.

Quick lil PSA bestie, before we get into the sanitizers — some of your program's execution counts can be lowkey unreproducible and it's not even your fault. 💀 Like, the execution counts of some parts of your program can depend on stuff like the length of temporary file names, or memory space randomization (which can mess with hash-table collision rates). 🙈 That kind of non-reproducible chunk of your program can be annotated with the "no_instrument_function" function attribute. And you can use gcov-dump with -l to dump the gathered data and actually verify that it's reproducible, fr fr. 🔍

With -fprofile-reproducible=parallel-runs, the collected profile stays reproducible no matter what order the data gets streamed into the gcda files. ✨ This setting is what lets you run multiple instances of your instrumented program in parallel (like, say, with "make -j"). The tradeoff? It reduces the quality of the gathered data — in particular of indirect call profiling. So slay, but know the cost. 💅

---

💖 -fsanitize=address

Enable AddressSanitizer, a fast memory error detector. 🔥 Memory access instructions get instrumented so it can detect out-of-bounds and use-after-free bugs. This option also enables -fsanitize-address-use-after-scope. More deets over at <https://github.com/google/sanitizers/wiki/AddressSanitizer>. 🫶

The run-time behavior can be influenced using the ASAN_OPTIONS environment variable. Set it to "help=1" and the available options get shown at startup of the instrumented program. Full list of supported options lives at <https://github.com/google/sanitizers/wiki/AddressSanitizerFlags#run-time-flags>.

⚠️ This option cannot be combined with -fsanitize=thread or -fsanitize=hwaddress. And heads up — the only target -fsanitize=hwaddress is currently supported on is AArch64. 💀

Want more accurate stack traces? 😌 You can use options like -O0, -O1, or -Og (which, for instance, prevent most function inlining), -fno-optimize-sibling-calls (which prevents optimizing sibling and tail recursive calls — this one's implicit for -O0, -O1, or -Og), or -fno-ipa-icf (which disables Identical Code Folding for functions). And since multiple runs of the program may yield backtraces with different addresses due to ASLR (Address Space Layout Randomization), you might wanna turn ASLR off. On Linux, that's:

setarch `uname -m` -R ./prog

🧚 -fsanitize=kernel-address

Enable AddressSanitizer for the Linux kernel. Details at <https://github.com/google/kernel-sanitizers>. 👑

⭐ -fsanitize=hwaddress

Enable Hardware-assisted AddressSanitizer, which uses a hardware ability to ignore the top byte of a pointer so it can detect memory errors with low memory overhead. 🪄 Memory access instructions get instrumented to detect out-of-bounds and use-after-free bugs. This option enables -fsanitize-address-use-after-scope. More deets at <https://clang.llvm.org/docs/HardwareAssistedAddressSanitizerDesign.html>.

The run-time behavior can be influenced using the HWASAN_OPTIONS environment variable. Set it to "help=1" and the available options are shown at startup of the instrumented program.

⚠️ Cannot be combined with -fsanitize=thread or -fsanitize=address, and it's currently only available on AArch64. Periodt.

🌸 -fsanitize=kernel-hwaddress

Enable Hardware-assisted AddressSanitizer for compiling the Linux kernel. It's similar to -fsanitize=kernel-address but uses an alternate instrumentation method, and similar to -fsanitize=hwaddress but with instrumentation differences that are necessary for compiling the Linux kernel. Those differences exist to avoid hwasan library initialization calls and to account for the stack pointer having a different value in its top byte. 🧠

Note: this option has different defaults than -fsanitize=hwaddress. Instrumenting the stack and alloca calls are not on by default, but you can still turn them on by specifying the command-line options --param hwasan-instrument-stack=1 and --param hwasan-instrument-allocas=1 respectively. And using a random frame tag is not implemented for kernel instrumentation. 🙈

💜 -fsanitize=pointer-compare

Instrument the comparison operation (<, <=, >, >=) with pointer operands. This option must be combined with either -fsanitize=kernel-address or -fsanitize=address. It cannot be combined with -fsanitize=thread.

Note: by default the check is disabled at run time. To enable it, add "detect_invalid_pointer_pairs=2" to the environment variable ASAN_OPTIONS. Using "detect_invalid_pointer_pairs=1" detects the invalid operation only when both pointers are non-null. 👀

🦋 -fsanitize=pointer-subtract

Instrument subtraction with pointer operands. This option must be combined with either -fsanitize=kernel-address or -fsanitize=address. It cannot be combined with -fsanitize=thread.

Note: by default the check is disabled at run time. To enable it, add "detect_invalid_pointer_pairs=2" to the environment variable ASAN_OPTIONS. Using "detect_invalid_pointer_pairs=1" detects the invalid operation only when both pointers are non-null. 💅

🎀 -fsanitize=shadow-call-stack

Enable ShadowCallStack, a security enhancement mechanism used to protect programs against return address overwrites (e.g. stack buffer overflows). 🛡️ How does it work? It saves a function's return address to a separately allocated shadow call stack in the function prologue, then restores the return address from the shadow call stack in the function epilogue. Instrumentation only happens in functions that actually need to save the return address to the stack.

Currently it only supports the aarch64 platform, and it's specifically designed for linux kernels that enable the CONFIG_SHADOW_CALL_STACK option. For user space programs, runtime support is not currently provided in libc and libgcc. Users who wanna use this feature in user space need to provide their own runtime support. 📝 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's no userspace runtime support, code compiled with ShadowCallStack cannot use exception handling. Use -fno-exceptions to turn off exceptions.

More details at <https://clang.llvm.org/docs/ShadowCallStack.html>. ✨

💖 -fsanitize=thread

Enable ThreadSanitizer, a fast data race detector. 🏃‍♀️ Memory access instructions get instrumented to detect data race bugs. More deets at <https://github.com/google/sanitizers/wiki#threadsanitizer>. The run-time behavior can be influenced using the TSAN_OPTIONS environment variable; see <https://github.com/google/sanitizers/wiki/ThreadSanitizerFlags> for a list of supported options.

⚠️ This option cannot be combined with -fsanitize=address or -fsanitize=leak.

Also note that sanitized atomic builtins cannot throw exceptions when operating on invalid memory addresses with non-call exceptions (-fnon-call-exceptions). 🫠

🌈 -fsanitize=leak

Enable LeakSanitizer, a memory leak detector. 🕵️‍♀️ This option only matters for linking of executables. The executable gets linked against a library that overrides "malloc" and other allocator functions. More deets at <https://github.com/google/sanitizers/wiki/AddressSanitizerLeakSanitizer>.

The run-time behavior can be influenced using the LSAN_OPTIONS environment variable.

⚠️ This option cannot be combined with -fsanitize=thread.

👑 -fsanitize=undefined

Enable UndefinedBehaviorSanitizer, a fast undefined behavior detector. ⚡ Various computations get instrumented to detect undefined behavior at runtime. More deets at <https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html>. The run-time behavior can be influenced using the UBSAN_OPTIONS environment variable. Current suboptions are:

💅 -fsanitize=shift

This one enables checking that the result of a shift operation is not undefined. Note that what exactly counts as undefined differs slightly between C and C++, as well as between ISO C90 and C99, etc. This option has two suboptions: -fsanitize=shift-base and -fsanitize=shift-exponent. 👀

🍓 -fsanitize=shift-exponent

This one 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 the shift operation is not undefined. Note that what exactly counts as 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 to 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 — bestie, this one instruments your array bounds, so all those out-of-bounds accesses get caught. But not everything gets the treatment: flexible array members, flexible array member-like arrays, and initializers of variables with static storage are not instrumented — with one lil exception, which is flexible array member-like arrays where -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 — the strict girlie version of array bounds instrumentation 💅. Most out of bounds accesses get detected here, including flexible array member-like arrays. Initializers of variables with static storage are still not instrumented, tho.
• -fsanitize=alignment — this one checks alignment of pointers when they're dereferenced, or when a reference gets bound to an insufficiently aligned target, or when a method or constructor is invoked on an insufficiently aligned object. So like, vibe-checking your pointers' alignment basically ✨
• -fsanitize=object-size — instruments memory references using the __builtin_dynamic_object_size function. Various out of bounds pointer accesses get detected. Obsessed.
• -fsanitize=float-divide-by-zero — detects floating-point division by zero. Important tea: unlike other similar options, -fsanitize=float-divide-by-zero is not enabled by -fsanitize=undefined, because floating-point division by zero can be a legitimate way of obtaining infinities and NaNs. It's giving "valid, actually" 🫶
• -fsanitize=float-cast-overflow — enables floating-point type to integer conversion checking. We check that the result of the conversion does not overflow. And again, unlike other similar options, -fsanitize=float-cast-overflow is not enabled by -fsanitize=undefined. Also heads up: this option does not work well with FE_INVALID exceptions enabled. 😭
• -fsanitize=nonnull-attribute — instruments calls, checking whether null values are not passed to arguments marked as requiring a non-null value by the nonnull function attribute. No nulls allowed at the function party, periodt.
• -fsanitize=returns-nonnull-attribute — instruments return statements in functions marked with the returns_nonnull function attribute, to detect returning of null values from such functions. Caught in 4k if you try to return null 🙈
• -fsanitize=bool — instruments loads from bool. If a value other than 0/1 is loaded, a run-time error is issued. Bools are 0 or 1, that's the whole personality, don't try it 💀
• -fsanitize=enum — instruments 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 — instruments C++ member function calls, member accesses and some conversions between pointers to base and derived classes, to verify the referenced object has the correct dynamic type. C++ girlies stay winning 👑
• -fsanitize=pointer-overflow — instruments pointer arithmetics. If the pointer arithmetics overflows, a run-time error is issued. Math but make it pointers 🌈
• -fsanitize=builtin — instruments arguments to selected builtin functions. If an invalid value is passed to such arguments, a run-time error is issued. E.g. passing 0 as the argument to __builtin_ctz or __builtin_clz invokes undefined behavior and is diagnosed by this option. Lowkey iconic that it catches that.

Real talk, bestie: sanitizers tend to increase the rate of false positive warnings, most notably those around -Wmaybe-uninitialized. We recommend against combining -Werror and [the use of] sanitizers. Don't do it. Not the vibe. 🚫

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=all is not allowed, because some sanitizers cannot be used together. It's giving "you can't sit with us" 💅
• -fasan-shadow-offset=number — forces GCC to use a custom shadow offset in AddressSanitizer checks. It's useful for experimenting with different shadow memory layouts in Kernel AddressSanitizer.
• -fsanitize-sections=s1,s2,... — sanitize global variables in selected user-defined sections. si may contain wildcards. Wildcard slay ✨
• -fsanitize-recover[=opts] — -fsanitize-recover= controls error recovery mode for 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. This means multiple runtime errors can be reported in a single program run, and the exit code of the program may indicate success even when errors have been reported. Yes, really — the exit code can say "all good" while errors were literally 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.
  • 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 is 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-reuse to none. She's thorough 💅
• -fsanitize-trap[=opts] — the -fsanitize-trap= option instructs the compiler to report for sanitizers mentioned in the comma-separated list of opts undefined behavior using __builtin_trap rather than a libubsan library routine. If this option is enabled for a certain sanitizer, it takes precedence over the -fsanitizer-recover= for that sanitizer, __builtin_trap will be emitted and be fatal regardless of whether recovery is enabled or disabled using -fsanitize-recover=.
  • The advantage of this is that the libubsan library is not needed and is not linked in, so this is usable even in freestanding environments. Iconic 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 be also specified, which enables it for undefined suboptions, -fsanitize=float-cast-overflow, -fsanitize=float-divide-by-zero and -fsanitize=bounds-strict. If -fsanitize-trap=undefined or -fsanitize-trap=all is used and -fsanitize=vptr is enabled on the command line, the instrumentation is silently ignored as the instrumentation always needs libubsan support, -fsanitize-trap=vptr is not allowed.
• -fsanitize-undefined-trap-on-error — this option is a deprecated equivalent of -fsanitize-trap=all. It's giving legacy 😭
• -fsanitize-coverage=trace-pc — enable coverage-guided fuzzing code instrumentation. Inserts a call to __sanitizer_cov_trace_pc into every basic block. Fuzzing bestie behavior fr fr 🐛
• -fsanitize-coverage=trace-cmp — enable dataflow guided fuzzing code instrumentation. Inserts a call to __sanitizer_cov_trace_cmp1, __sanitizer_cov_trace_cmp2, __sanitizer_cov_trace_cmp4 or __sanitizer_cov_trace_cmp8 for integral comparison with both operands variable, or __sanitizer_cov_trace_const_cmp1, __sanitizer_cov_trace_const_cmp2, __sanitizer_cov_trace_const_cmp4 or __sanitizer_cov_trace_const_cmp8 for integral comparison with one operand constant, __sanitizer_cov_trace_cmpf or __sanitizer_cov_trace_cmpd for float or double comparisons, and __sanitizer_cov_trace_switch for switch statements. That's a whole lot of trace functions and she knows every single one 💖

---

• -fcf-protection=[full|branch|return|none|check] — okay so this one enables code instrumentation of control-flow transfers to increase program security by checking that target addresses of control-flow transfer instructions (such as indirect function call, function return, indirect jump) are valid. This prevents diverting the flow of control to an unexpected target. It's intended to protect against such threats as Return-oriented Programming (ROP), and similarly call/jmp-oriented programming (COP/JOP). Security era activated 🛡️

The values, explained like a group chat:

• branch tells the compiler to implement checking of validity of control-flow transfer at the point of indirect branch instructions, i.e. call/jmp instructions.
• return implements checking of validity at the point of returning from a function.
• full is an alias for specifying both branch and return.
• none turns off instrumentation.

To override -fcf-protection, -fcf-protection=none needs to be added and then with -fcf-protection=xxx. (Yes, that's the order, don't fight it.)

The value check is used for the final link with link-time optimization (LTO). An error is issued if LTO object files are compiled with different -fcf-protection values. The value check is ignored at the compile time.

The 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. Bit girlies rise 🫶

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. So like, i686 and up, bestie, don't come at her with anything older 💅

NOTE: in Ubuntu 19.10 and later, -fcf-protection is enabled by default for C, C++, ObjC, ObjC++ — but only if neither -fno-cf-protection nor -fcf-protection=* shows up anywhere. bestie, it checks first, it's polite 💅

• -fharden-compares — okay so for every logical test that survives gimple optimizations and isn't the condition in a conditional branch (think: conditions tested for conditional moves, or ones being stored into boolean variables), it emits extra code to compute and verify the reversed condition, and to call "__builtin_trap" if the results don't match 😤 use it together with -fharden-conditional-branches so you cover literally all the conditionals, periodt.
• -fharden-conditional-branches — for every non-vectorized conditional branch that survives gimple optimizations, emit extra code that computes and verifies the reversed condition, and calls "__builtin_trap" if the result is unexpected 💀 pair it with -fharden-compares to cover every single conditional, fr fr.
• -fharden-control-flow-redundancy — this one's giving maximum paranoia ✨ it emits extra code to set booleans when entering basic blocks, and then to verify and trap, at function exits, when those booleans don't form an execution path that's compatible with the control flow graph. like a vibe check for your control flow 🌸

Verification happens before returns, before mandatory tail calls (see below) and, optionally, before escaping exceptions with -fhardcfr-check-exceptions, before returning calls with -fhardcfr-check-returning-calls, and before noreturn calls with -fhardcfr-check-noreturn-calls. Tuning options --param hardcfr-max-blocks and --param hardcfr-max-inline-blocks are available 🎀

Tail call optimization takes place too late to affect control flow redundancy, but calls annotated as mandatory tail calls by language front-ends, and any calls marked early enough as potential tail calls, would also have verification issued before the call — though these possibilities are merely theoretical, as these conditions can only be met when using custom compiler plugins. iykyk 🧚

• -fhardcfr-skip-leaf — disables -fharden-control-flow-redundancy in leaf functions. tiny functions get a lil break 🍓
• -fhardcfr-check-exceptions — when -fharden-control-flow-redundancy is active, check the recorded execution path against the control flow graph at exception escape points, as if the function body was wrapped with a cleanup handler that performed the check and reraised. this one is enabled by default — use -fno-hardcfr-check-exceptions to turn it off 💅
• -fhardcfr-check-returning-calls — when -fharden-control-flow-redundancy is active, check the recorded execution path against the control flow graph before any function call immediately followed by a return of its result, if any — so as to not prevent tail-call optimization, whether or not it ultimately gets 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. main character energy: it does what it wants ✨

• -fhardcfr-check-noreturn-calls=[always|no-xthrow|nothrow|never] — when -fharden-control-flow-redundancy is active, check the recorded execution path against the control flow graph before "noreturn" calls. pick your fighter:
  • always — all of them
  • no-xthrow — those that aren't expected to return control to the caller through an exception (the default, bestie 🫶)
  • nothrow — those that may not return control to the caller through an exception either
  • never — none of them, we're done, goodbye 🙈

Checking before a "noreturn" function that may return control to the caller through an exception may cause checking to be performed more than once, if the exception is caught in the caller, whether by a handler or a cleanup. When -fhardcfr-check-exceptions is also enabled, the compiler will avoid associating a "noreturn" call with the implicitly-added cleanup handler, since it would be redundant with the check performed before the call — but other handlers or cleanups in the function, if activated, will modify the recorded execution path and check it again when another checkpoint is hit 😭 the checkpoint may even be another "noreturn" call, so checking may end up performed multiple times. not me getting checked four times in a row 💀

Various optimizers may cause calls to be marked as "noreturn" and/or "nothrow", even in the absence of the corresponding attributes, which may affect the placement of checks before calls, as well as the addition of implicit cleanup handlers for them. This unpredictability, and the fact that raising and reraising exceptions frequently amounts to implicitly calling "noreturn" functions, have made no-xthrow the default setting for this option: it excludes from the "noreturn" treatment only internal functions used to (re)raise exceptions, that are not affected by these optimizations 🌈

• -fhardened — enables a whole set of flags for C and C++ that improve the security of the generated code without affecting its ABI. The precise flags enabled may change between major releases of GCC, but are currently:

-D_FORTIFY_SOURCE=3 -D_GLIBCXX_ASSERTIONS
-ftrivial-auto-var-init=zero -fPIE  -pie  -Wl,-z,relro,-z,now
-fstack-protector-strong -fstack-clash-protection
-fcf-protection=full (x86 GNU/Linux only)

The list of options enabled by -fhardened can be generated using the --help=hardened option 👑

When the system glibc is older than 2.35, -D_FORTIFY_SOURCE=2 is used instead.

This option is intended to be used in production builds, not merely in debug builds. periodt.

Currently, -fhardened is only supported on GNU/Linux targets 🐧

-fhardened only enables a particular option if it wasn't already specified anywhere on the command line. For instance, -fhardened -fstack-protector will only enable -fstack-protector, but not -fstack-protector-strong. it respects your choices, it's so respectful 💖

• -fstack-protector — emit extra code to check for buffer overflows, such as stack smashing attacks 😱 This is done by adding a guard variable to functions with vulnerable objects. This includes functions that call "alloca", and functions with buffers larger than or equal to 8 bytes. The guards are initialized when a function is entered and then checked when the function exits. If a guard check fails, an error message is printed and the program exits. Only variables that are actually allocated on the stack are considered — optimized away variables or variables allocated in registers don't count 🎀
• -fstack-protector-all — like -fstack-protector except that all functions are protected. no one gets left out, we're all covered here 💅
• -fstack-protector-strong — like -fstack-protector but includes additional functions to be protected — those that have local array definitions, or have references to local frame addresses. Only variables that are actually allocated on the stack are considered, optimized away variables or variables allocated in registers don't count ✨

NOTE: in Ubuntu 14.10 and later versions, -fstack-protector-strong is enabled by default for C, C++, ObjC, ObjC++ — if none of -fno-stack-protector, -nostdlib, nor -ffreestanding are found 🫶

• -fstack-protector-explicit — like -fstack-protector but only protects those functions which have the "stack_protect" attribute. exclusive behavior, we love a selective icon 💅
• -fstack-check — generate 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 to specify it in a single-threaded environment since stack overflow is automatically detected on nearly all systems if there is only one stack.

Note that this switch does not actually cause checking to be done — the operating system or the language runtime must do that. The switch causes generation of code to ensure that they see the stack being extended.

You can additionally specify a string parameter: no means no checking, generic means force the use of old-style checking, specific means use the best checking method and is equivalent to bare -fstack-check 🌸

Old-style checking is a generic mechanism that requires no specific target support in the compiler but comes with the following drawbacks:

1. Modified allocation strategy for large objects: they are always allocated dynamically if their size exceeds a fixed threshold. Note this may change the semantics of some code 😭
2. Fixed limit on the size of the static frame of functions: when it is topped by a particular function, stack checking is not reliable and a warning is issued by the compiler.
3. Inefficiency: because of both the modified allocation strategy and the generic implementation, code performance is hampered 💀

Note that old-style stack checking is also the fallback method for specific if no target support has been added in the compiler.

-fstack-check= is designed for Ada's needs to detect infinite recursion and stack overflows. specific is an excellent choice when compiling Ada code 👑 It is not generally sufficient to protect against stack-clash attacks. To protect against those you want -fstack-clash-protection.

• -fstack-clash-protection — generate code to prevent stack clash style attacks 🛡️ When this option is enabled, the compiler will only allocate one page of stack space at a time and each page is accessed immediately after allocation. Thus, it prevents allocations from jumping over any stack guard page provided by the operating system. no sneaky jumping, we see you 🙈

Most targets do not fully support stack clash protection. However, on those targets -fstack-clash-protection will protect dynamic stack allocations. -fstack-clash-protection may also provide limited protection for static stack allocations if the target supports -fstack-check=specific.

NOTE: in Ubuntu 19.10 and later versions, -fstack-clash-protection is enabled by default for C, C++, ObjC, ObjC++, unless -fno-stack-clash-protection is found ✨

• -fstack-limit-register=reg -fstack-limit-symbol=sym -fno-stack-limit — generate code to ensure that the stack does not grow beyond a certain value, either the value of a register or the address of a symbol 🎀 If a larger stack is required, a signal is raised at run time. For most targets, the signal is raised before the stack overruns the boundary, so it is possible to catch the signal without taking special precautions. catching signals with zero effort? we love a considerate API 💖

Okay so picture this: your stack starts at absolute address 0x80000000 and grows downwards? Then bestie, you can use the flags -fstack-limit-symbol=__stack_limit and -Wl,--defsym,__stack_limit=0x7ffe0000 to enforce a stack limit of 128KB. 💅

Heads up tho: this may only work with the GNU linker. Just so you know. 🤷

And if you wanna locally override stack limit checking? Use the "no_stack_limit" function attribute. That's it. That's the whole thing. 🎀

---

• -fsplit-stack — Generates code that automatically splits the stack before it overflows, which is honestly giving main character energy. 🦋 The resulting program has a discontiguous stack which can only overflow if the program is unable to allocate any more memory. This is most useful when running threaded programs, because it is no longer necessary to calculate a good stack size to use for each thread. Currently this is only implemented for the x86 targets running GNU/Linux. 🐧

So here's the tea: when code compiled with -fsplit-stack calls code compiled without -fsplit-stack, there may not be much stack space available for the latter code to run. 😭 If compiling all code — including library code — with -fsplit-stack is not an option, then the linker can fix up these calls so that the code compiled without -fsplit-stack always has a large stack. Support for this is implemented in the gold linker in GNU binutils release 2.21 and later. ✨

• -fstrub=disable — Disables stack scrubbing entirely, ignoring any "strub" attributes. See 💀
• -fstrub=strict — Functions default to "strub" mode "disabled", and apply strictly the restriction that only functions associated with "strub"-"callable" modes ("at-calls", "callable" and "always_inline" "internal") are "callable" by functions with "strub"-enabled modes ("at-calls" and "internal"). 🫡
• -fstrub=relaxed — Restores the default stack scrub ("strub") setting, namely, "strub" is only enabled as required by "strub" attributes associated with function and data types. "Relaxed" means that strub contexts are only prevented from calling functions explicitly associated with "strub" mode "disabled". This option is only useful to override other -fstrub=* options that precede it in the command line. 🌸
• -fstrub=at-calls — Enables "at-calls" "strub" mode where viable. The primary use of this option is for testing. It exercises the "strub" machinery in scenarios strictly local to a translation unit. This "strub" mode modifies function interfaces, so any function that is visible to other translation units, or that has its address taken, will not be affected by this option. Optimization options may also affect viability. See the "strub" attribute documentation for details on viability and eligibility requirements. 👑
• -fstrub=internal — Enables "internal" "strub" mode where viable. The primary use of this option is for testing. This option is intended to exercise thoroughly parts of the "strub" machinery that implement the less efficient, but interface-preserving "strub" mode. Functions that would not be affected by this option are quite uncommon. 🙈
• -fstrub=all — Enables some "strub" mode where viable. When both strub modes are viable, "at-calls" is preferred. -fdump-ipa-strubm adds function attributes that tell which mode was selected for each function. The primary use of this option is for testing, to exercise thoroughly the "strub" machinery. 🌈
• -fvtable-verify=[std|preinit|none] — This option is only available when compiling C++ code. It turns on (or off, if using -fvtable-verify=none) the security feature that verifies at run time, for every virtual call, that the vtable pointer through which the call is made is valid for the type of the object, and has not been corrupted or overwritten. If an invalid vtable pointer is detected at run time, an error is reported and execution of the program is immediately halted. 🚨

This option causes run-time data structures to be built at program startup, which are used for verifying the vtable pointers. The options std and preinit control the timing of when these data structures are built. In both cases the data structures are built before execution reaches main. Using -fvtable-verify=std causes the data structures to be built after shared libraries have been loaded and initialized. -fvtable-verify=preinit causes them to be built before shared libraries have been loaded and initialized. ⏰

If this option appears multiple times in the command line with different values specified, none takes highest priority over both std and preinit; preinit takes priority over std. 🏆

• -fvtv-debug — When used in conjunction with -fvtable-verify=std or -fvtable-verify=preinit, causes debug versions of the runtime functions for the vtable verification feature to be called. This flag also causes the compiler to log information about which vtable pointers it finds for each class. This information is written to a file named vtv_set_ptr_data.log in the directory named by the environment variable VTV_LOGS_DIR if that is defined or the current working directory otherwise. 📝

Note: This feature appends data to the log file. If you want a fresh log file, be sure to delete any existing one. No cap, it will just keep piling on. 😭

• -fvtv-counts — This is a debugging flag. When used in conjunction with -fvtable-verify=std or -fvtable-verify=preinit, this causes the compiler to keep track of the total number of virtual calls it encounters and the number of verifications it inserts. It also counts the number of calls to certain run-time library functions that it inserts and logs this information for each compilation unit. 📊 The compiler writes this information to a file named vtv_count_data.log in the directory named by the environment variable VTV_LOGS_DIR if that is defined or the current working directory otherwise. It also counts the size of the vtable pointer sets for each class, and writes this information to vtv_class_set_sizes.log in the same directory. 💖

Note: This feature appends data to the log files. To get fresh log files, be sure to delete any existing ones. 🗑️

• -finstrument-functions — Generates instrumentation calls for entry and exit to functions. Just after function entry and just before function exit, the following profiling functions are called with the address of the current function and its call site. (On some platforms, "__builtin_return_address" does not work beyond the current function, so the call site information may not be available to the profiling functions otherwise.) 🎯

void __cyg_profile_func_enter (void *this_fn,
                               void *call_site);
void __cyg_profile_func_exit  (void *this_fn,
                               void *call_site);

The first argument is the address of the start of the current function, which may be looked up exactly in the symbol table. 🔍

This instrumentation is also done for functions expanded inline in other functions. The profiling calls indicate where, conceptually, the inline function is entered and exited. This means that addressable versions of such functions must be available. If all your uses of a function are expanded inline, this may mean an additional expansion of code size. If you use "extern inline" in your C code, an addressable version of such functions must be provided. (This is normally the case anyway, but if you get lucky and the optimizer always expands the functions inline, you might have gotten away without providing static copies.) 🍀

A function may be given the attribute "no_instrument_function", in which case this instrumentation is not done. This can be used, for example, for the profiling functions listed above, high-priority interrupt routines, and any functions from which the profiling functions cannot safely be called (perhaps signal handlers, if the profiling routines generate output or allocate memory). 🫶

• -finstrument-functions-once — This is similar to -finstrument-functions, but the profiling functions are called only once per instrumented function, i.e. the first profiling function is called after the first entry into the instrumented function and the second profiling function is called before the exit corresponding to this first entry. 1️⃣

The definition of "once" for the purpose of this option is a little vague because the implementation is not protected against data races. As a result, the implementation only guarantees that the profiling functions are called at least once per process and at most once per thread, but the calls are always paired, that is to say, if a thread calls the first function, then it will call the second function, unless it never reaches the exit of the instrumented function. 💀

• -finstrument-functions-exclude-file-list=file,file,... — Sets the list of functions that are excluded from instrumentation (see the description of -finstrument-functions). If the file that contains a function definition matches with one of file, then that function is not instrumented. The match is done on substrings: if the file parameter is a substring of the file name, it is considered to be a match. ✂️

For example:

-finstrument-functions-exclude-file-list=/bits/stl,include/sys

excludes any inline function defined in files whose pathnames contain /bits/stl or include/sys. 🚫

If, for some reason, you want to include letter , in one of sym, write ,. For example, -finstrument-functions-exclude-file-list=',,tmp' (note the single quote surrounding the option). 🎀

• -finstrument-functions-exclude-function-list=sym,sym,... — This is similar to -finstrument-functions-exclude-file-list, but this option sets the list of function names to be excluded from instrumentation. The function name to be matched is its user-visible name, such as "vector<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 is considered to be a match. For C99 and C++ extended identifiers, the function name must be given in UTF-8, not using universal character names. 🌸
• -fpatchable-function-entry=N[,M] — okay bestie, this one drops N NOPs right at the top of every single function 💅, with the function entry point sitting before the Mth NOP. If you ghost M, it defaults to 0, so the function entry points to the address right at the first NOP. Those NOP instructions are literally reserving extra space so you can patch in whatever instrumentation you want at run time — provided the code segment is writable, no cap. The amount of space is controllable indirectly via the number of NOPs, and the NOP instruction used corresponds to the instruction emitted by the internal GCC back-end interface gen_nop. Heads up though: this behavior is target-specific and may also depend on the architecture variant and/or other compilation options. it's giving "read the docs for your target" energy 🦋

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. so organized ✨

Note that the value of __attribute__ ((patchable_function_entry (N,M))) takes precedence over the command-line option -fpatchable-function-entry=N,M. This can be used to increase the area size or to remove it completely on a single function. If N=0, no pad location is recorded. 🎀

The NOP instructions are inserted at — and maybe before, depending on M — the function entry address, even before the prologue. On PowerPC with the ELFv2 ABI, for a function with dual entry points, the local entry point is this function entry address.

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.

---

✨ Options Controlling the Preprocessor ✨

These options control the C preprocessor, which is run on each C source file before actual compilation. she's the opening act, the main character of the pre-show 💖

If you use the -E option, nothing is done except preprocessing. Some of these options make sense only together with -E because they cause the preprocessor output to be unsuitable for actual compilation. so like, don't expect that output to be compilable, bestie 😭

In addition to the options listed here, there are a number of options to control search paths for include files documented in Directory Options. Options to control preprocessor diagnostics are listed in Warning Options.

• -D name — Predefine name as a macro, with definition 1. easy peasy 💅
• -D name=definition — The contents of definition are tokenized and processed as if they appeared during translation phase three in a #define directive. In particular, the definition is truncated by embedded newline characters.

If you are invoking the preprocessor from a shell or shell-like program you may need to use the shell's quoting syntax to protect characters such as spaces that have a meaning in the shell syntax. shells are so dramatic fr fr 🙈

If you wish to define a function-like macro on the command line, write its argument list with surrounding parentheses before the equals sign (if any). Parentheses are meaningful to most shells, so you should quote the option. With sh and csh, -D'name(args...)=definition' works.

-D and -U options are processed in the order they are given on the command line. All -imacros file and -include file options are processed after all -D and -U options. order matters, periodt ⭐

• -U name — Cancel any previous definition of name, either built in or provided with a -D option. it's gone, we don't know her 💀
• -include file — Process file as if #include "file" appeared as the first line of the primary source file. However, the first directory searched for file is the preprocessor's working directory instead of the directory containing the main source file. If not found there, it is searched for in the remainder of the #include "..." search chain as normal.

If multiple -include options are given, the files are included in the order they appear on the command line.

• -imacros file — Exactly like -include, except that any output produced by scanning file is thrown away. Macros it defines remain defined. This allows you to acquire all the macros from a header without also processing its declarations. lowkey genius, just the vibes, not the baggage 🧚

All files specified by -imacros are processed before all files specified by -include.

• -undef — Do not predefine any system-specific or GCC-specific macros. The standard predefined macros remain defined.
• -pthread — Define additional macros required for using the POSIX threads library. You should use this option consistently for both compilation and linking. This option is supported on GNU/Linux targets, most other Unix derivatives, and also on x86 Cygwin and MinGW targets. consistency is the whole vibe, bestie 🌈
• -M — Instead of outputting the result of preprocessing, output a rule suitable for make describing the dependencies of the main source file. The preprocessor outputs one make rule containing the object file name for that source file, a colon, and the names of all the included files, including those coming from -include or -imacros command-line options.

Unless specified explicitly (with -MT or -MQ), the object file name consists of the name of the source file with any suffix replaced with object file suffix and with any leading directory parts removed. If there are many included files then the rule is split into several lines using \-newline. The rule has no commands.

This option does not suppress the preprocessor's debug output, such as -dM. To avoid mixing such debug output with the dependency rules you should explicitly specify the dependency output file with -MF, or use an environment variable like DEPENDENCIES_OUTPUT. Debug output is still sent to the regular output stream as normal.

Passing -M to the driver implies -E, and suppresses warnings with an implicit -w.

• -MM — Like -M but do not mention header files that are found in system header directories, nor header files that are included, directly or indirectly, from such a header.

This implies that the choice of angle brackets or double quotes in an #include directive does not in itself determine whether that header appears in -MM dependency output. plot twist 👑

• -MF file — When used with -M or -MM, specifies a file to write the dependencies to. If no -MF switch is given the preprocessor sends the rules to the same place it would send preprocessed output.

When used with the driver options -MD or -MMD, -MF overrides the default dependency output file.

If file is -, then the dependencies are written to stdout.

• -MG — In conjunction with an option such as -M requesting dependency generation, -MG assumes missing header files are generated files and adds them to the dependency list without raising an error. The dependency filename is taken directly from the #include directive without prepending any path. -MG also suppresses preprocessed output, as a missing header file renders this useless.

This feature is used in automatic updating of makefiles. manifesting those headers into existence ✨

• -Mno-modules — Disable dependency generation for compiled module interfaces.
• -MP — This option instructs CPP to add a phony target for each dependency other than the main file, causing each to depend on nothing. These dummy rules work around errors make gives if you remove header files without updating the Makefile to match. fake it till you make it energy 🎀

This is typical output:

        test.o: test.c test.h

        test.h:

• -MT target — Change the target of the rule emitted by dependency generation. By default CPP takes the name of the main input file, deletes any directory components and any file suffix such as .c, and appends the platform's usual object suffix. The result is the target.

An -MT option sets the target to be exactly the string you specify. If you want multiple targets, you can specify them as a single argument to -MT, or use multiple -MT options.

For example, -MT '$(objpfx)foo.o' might give

        $(objpfx)foo.o: foo.c

• -MQ target — Same as -MT, but it quotes any characters which are special to Make. -MQ '$(objpfx)foo.o' gives

        $$(objpfx)foo.o: foo.c

The default target is automatically quoted, as if it were given with -MQ.

• -MD — -MD is equivalent to -M -MF file, except that -E is not implied. The driver determines file based on whether an -o option is given. If it is, the driver uses its argument but with a suffix of .d, otherwise it takes the name of the input file, removes any directory components and suffix, and applies a .d suffix.

If -MD is used in conjunction with -E, any -o switch is understood to specify the dependency output file, but if used without -E, each -o is understood to specify a target object file.

Since -E is not implied, -MD can be used to generate a dependency output file as a side effect of the compilation process. sneaky side quest, we love her 💜

• -MMD — Like -MD except mention only user header files, not system header files.
• -fpreprocessed — Indicate to the preprocessor that the input file has already been preprocessed. This suppresses things like macro expansion, trigraph conversion, escaped newline splicing, and processing of most directives. The preprocessor still recognizes and removes comments, so that you can pass a file preprocessed with -C to the compiler without problems. In this mode the integrated preprocessor is little more than a tokenizer for the front ends. she's just here to vibe check the tokens 🦋

-fpreprocessed is implicit if the input file has one of the extensions .i, .ii or .mi. These are the extensions that GCC uses for preprocessed files created by -save-temps.

• -fdirectives-only — When preprocessing, handle directives, but do not expand macros.

The option's behavior depends on the -E and -fpreprocessed options.

With -E, preprocessing is limited to the handling of directives such as #define, #ifdef, and #error. Other preprocessor operations, such as macro expansion and trigraph conversion are not performed. In addition, the -dD option is implicitly enabled.

With -fpreprocessed, predefinition of command line and most builtin macros is disabled. Macros such as __LINE__, which are contextually dependent, are handled normally. This enables compilation of files previously preprocessed with -E -fdirectives-only.

With both -E and -fpreprocessed, the rules for -fpreprocessed take precedence. This enables full preprocessing of files previously preprocessed with -E -fdirectives-only.

• -fdollars-in-identifiers — Accept $ in identifiers. money moves only 💅
• -fextended-identifiers — okay so this one lets you accept universal character names and extended characters in identifiers. It's enabled by default for C99 (and later C standard versions) and C++. 💖
• -fno-canonical-system-headers — When preprocessing, do not shorten system header paths with canonicalization. no cap, that's the whole vibe.
• -fmax-include-depth=depth — Set the maximum depth of the nested #include. The default is 200. like, don't go deeper than that, bestie.
• -ftabstop=width — Set the distance between tab stops. This helps the preprocessor report correct column numbers in warnings or errors, even if tabs appear on the line. If the value is less than 1 or greater than 100, the option is ignored. The default is 8. so valid range-ish, but out of bounds gets ghosted.
• -ftrack-macro-expansion[=level] — Track locations of tokens across macro expansions. This allows the compiler to emit diagnostic about the current macro expansion stack when a compilation error occurs in a macro expansion. Using this option makes the preprocessor and the compiler consume more memory. The level parameter can be used to choose the level of precision of token location tracking thus decreasing the memory consumption if necessary. Value 0 of level de-activates this option. Value 1 tracks tokens locations in a degraded mode for the sake of minimal memory overhead. In this mode all tokens resulting from the expansion of an argument of a function-like macro have the same location. Value 2 tracks tokens locations completely. This value is the most memory hungry. When this option is given no argument, the default parameter value is 2.

Note that -ftrack-macro-expansion=2 is activated by default.

• -fmacro-prefix-map=old=new — When preprocessing files residing in directory old, expand the __FILE__ and __BASE_FILE__ macros as if the files resided in directory new instead. This can be used to change an absolute path to a relative path by using . for new which can result in more reproducible builds that are location independent. This option also affects __builtin_FILE() during compilation. See also -ffile-prefix-map and -fcanon-prefix-map.
• -fexec-charset=charset — Set the execution character set, used for string and character constants. The default is UTF-8. charset can be any encoding supported by the system's iconv library routine. slay.
• -fwide-exec-charset=charset — Set the wide execution character set, used for wide string and character constants. The default is one of UTF-32BE, UTF-32LE, UTF-16BE, or UTF-16LE, whichever corresponds to the width of wchar_t and the big-endian or little-endian byte order being used for code generation. As with -fexec-charset, charset can be any encoding supported by the system's iconv library routine; however, you will have problems with encodings that do not fit exactly in wchar_t. red flag: not fitting exactly = problems.
• -finput-charset=charset — Set the input character set, used for translation from the character set of the input file to the source character set used by GCC. If the locale does not specify, or GCC cannot get this information from the locale, the default is UTF-8. This can be overridden by either the locale or this command-line option. Currently the command-line option takes precedence if there's a conflict. charset can be any encoding supported by the system's iconv library routine. iykyk.
• -fpch-deps — When using precompiled headers, this flag causes the dependency-output flags to also list the files from the precompiled header's dependencies. If not specified, only the precompiled header are listed and not the files that were used to create it, because those files are not consulted when a precompiled header is used.
• -fpch-preprocess — This option allows use of a precompiled header together with -E. It inserts a special #pragma, #pragma GCC pch_preprocess "filename" in the output to mark the place where the precompiled header was found, and its filename. When -fpreprocessed is in use, GCC recognizes this #pragma and loads the PCH.

This option is off by default, because the resulting preprocessed output is only really suitable as input to GCC. It is switched on by -save-temps.

You should not write this #pragma in your own code, but it is safe to edit the filename if the PCH file is available in a different location. The filename may be absolute or it may be relative to GCC's current directory. so don't be out here writing it manually, bestie.

• -fworking-directory — Enable generation of linemarkers in the preprocessor output that let the compiler know the current working directory at the time of preprocessing. When this option is enabled, the preprocessor emits, after the initial linemarker, a second linemarker with the current working directory followed by two slashes. GCC uses this directory, when it's present in the preprocessed input, as the directory emitted as the current working directory in some debugging information formats. This option is implicitly enabled if debugging information is enabled, but this can be inhibited with the negated form -fno-working-directory. If the -P flag is present in the command line, this option has no effect, since no #line directives are emitted whatsoever.
• -A predicate=answer — Make an assertion with the predicate predicate and answer answer. This form is preferred to the older form -A predicate(answer), which is still supported, because it does not use shell special characters. newer form ate, periodt.
• -A -predicate=answer — Cancel an assertion with the predicate predicate and answer answer.
• -C — Do not discard comments. All comments are passed through to the output file, except for comments in processed directives, which are deleted along with the directive.

You should be prepared for side effects when using -C; it causes the preprocessor to treat comments as tokens in their own right. For example, comments appearing at the start of what would be a directive line have the effect of turning that line into an ordinary source line, since the first token on the line is no longer a #. side effects warning, bestie!

• -CC — Do not discard comments, including during macro expansion. This is like -C, except that comments contained within macros are also passed through to the output file where the macro is expanded.

In addition to the side effects of the -C option, the -CC option causes all C++-style comments inside a macro to be converted to C-style comments. This is to prevent later use of that macro from inadvertently commenting out the remainder of the source line.

The -CC option is generally used to support lint comments. lint comments, iconic.

• -P — Inhibit generation of linemarkers in the output from the preprocessor. This might be useful when running the preprocessor on something that is not C code, and will be sent to a program which might be confused by the linemarkers.
• -traditional, -traditional-cpp — Try to imitate the behavior of pre-standard C preprocessors, as opposed to ISO C preprocessors. See the GNU CPP manual for details.

Note that GCC does not otherwise attempt to emulate a pre-standard C compiler, and these options are only supported with the -E switch, or when invoking CPP explicitly. so don't expect full retro mode.

• -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. MS-DOS era, we love a throwback.
• -H — Print the name of each header file used, in addition to other normal activities. Each name is indented to show how deep in the #include stack it is. Precompiled header files are also printed, even if they are found to be invalid; an invalid precompiled header file is printed with ...x and a valid one with ...! . the little dots are telling a whole story.
• -dletters — Says to make debugging dumps during compilation as specified by letters. The flags documented here are those relevant to the preprocessor. Other letters are interpreted by the compiler proper, or reserved for future versions of GCC, and so are silently ignored. If you specify letters whose behavior conflicts, the result is undefined. undefined behavior alert, bestie!
  • -dM — Instead of the normal output, generate a list of #define directives for all the macros defined during the execution of the preprocessor, including predefined macros. This gives you a way of finding out what is predefined in your version of the preprocessor. Assuming you have no file foo.h, the command

touch foo.h; cpp -dM foo.h

shows all the predefined macros.

If you use -dM without the -E option, -dM is interpreted as a synonym for -fdump-rtl-mach. plot twist.

• -dD — Like -dM except that it outputs both the #define directives and the result of preprocessing. Both kinds of output go to the standard output file.
• -dN — Like -dD, but emit only the macro names, not their expansions.
• -dI — Output #include directives in addition to the result of preprocessing.
• -dU — Like -dD except that only macros that are expanded, or whose definedness is tested in preprocessor directives, are output; the output is delayed until the use or test of the macro; and #undef directives are also output for macros tested but undefined at the time.

• -fdebug-cpp — This option is only useful for debugging GCC. When used from CPP or with -E, it dumps debugging information about location maps. Every token in the output is preceded by the dump of the map its location belongs to.

When used from GCC without -E, this option has no effect. so basically only when you're in debug mode, bestie.

• -Wp,option — bestie, this is the bypass-the-driver move 💅. It shoves option straight through to the preprocessor, skipping the compiler driver entirely. And if option has commas in it? It gets split into multiple options at those commas, periodt. BUT — and this is the important part — lots of options get modified, translated or interpreted by the compiler driver before they ever reach the preprocessor, and -Wp forcibly bypasses that whole phase. Also the preprocessor's direct interface is undocumented and subject to change, so whenever possible you should avoid using -Wp and just let the driver handle the options instead. iykyk 🫶
• -Xpreprocessor option — pass option as an option to the preprocessor. Use this when you need to supply system-specific preprocessor options that GCC does not recognize. Now listen: if you want to pass an option that takes an argument, you must use -Xpreprocessor twice — once for the option and once for the argument. Two times. That's the whole vibe ✨
• -no-integrated-cpp — do the preprocessing as a separate pass before compilation. By default, GCC does preprocessing as an integrated part of input tokenization and parsing (all cozy and merged, love that for it). But if you provide this option, the appropriate language front end — cc1, cc1plus, or cc1obj for C, C++, and Objective-C, respectively — gets invoked twice instead: once for preprocessing only, and once for actual compilation of the preprocessed input. This option may be useful in conjunction with the -B or -wrapper options to specify an alternate preprocessor or perform additional processing of the program source between normal preprocessing and compilation 🌸
• -flarge-source-files — tells GCC to expect large source files, and the price is slower compilation and higher memory usage. The receipts 🧾: GCC normally tracks both column numbers and line numbers within source files, and normally prints both of these numbers in diagnostics. However, once it has processed a certain number of source lines, it stops tracking column numbers and only tracks line numbers. That means diagnostics for later lines do not include column numbers 😭. It also means options like -Wmisleading-indentation cease to work at that point, although the compiler does print a note if this happens. Passing -flarge-source-files significantly increases the number of source lines that GCC can process before it stops tracking columns. Slay for giant files, costly for everything else.

---

🛠️ Passing Options to the Assembler 🛠️

You can pass options to the assembler. Yes, the assembler gets a lil treat too 💖

• -Wa,option — pass option as an option to the assembler. If option contains commas, it is split into multiple options at the commas. Simple as that ✨
• -Xassembler option — pass option as an option to the assembler. Use this to supply system-specific assembler options that GCC does not recognize. And the same rule as before, no cap: 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. Lowkey they are meaningless if the compiler is not doing a link step, so don't come for them when they do nothing 🙈

• object-file-name — a file name that does not end in a special recognized suffix is considered to name an object file or library. (Object files are distinguished from libraries by the linker according to the file contents, not by vibes.) If linking is done, these object files are used as input to the linker.
• -c, -S, -E — if any of these options is used, then the linker is not run, and object file names should not be used as arguments. Like, don't. 🚫
• -flinker-output=type — this option controls code generation of the link-time optimizer. By default the linker output is automatically determined by the linker plugin. For debugging the compiler, and if incremental linking with a non-LTO object file is desired, it may be useful to control the type manually. Here's the type tea 👑:
  • If type is exec, code generation produces a static binary. In this case -fpic and -fpie are both disabled.
  • If type is dyn, code generation produces a shared library. In this case -fpic or -fPIC is preserved, but not enabled automatically. This allows 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 -fpie executable. This results in similar optimizations as exec except that -fpie is not disabled if specified at compilation time.
  • If type is rel, the compiler assumes that incremental linking is done. The sections containing intermediate code for link-time optimization are merged, pre-optimized, and output to the resulting object file. In addition, if -ffat-lto-objects is specified, binary code is produced for future non-LTO linking. The object file produced by incremental linking is smaller than a static library produced from the same object files. At link time the result of incremental linking also loads faster than a static library, assuming that the majority of objects in the library are used. So cute, so efficient 🌈
  • Finally nolto-rel configures the compiler for incremental linking where code generation is forced, a final binary is produced, and the intermediate code for later link-time optimization is stripped. When multiple object files are linked together the resulting code is better optimized than with link-time optimizations disabled (for example, cross-module inlining happens), but most of the benefits of whole program optimizations are lost 😔

During the incremental link (by -r) the linker plugin defaults to rel. With current interfaces to GNU Binutils it is however not possible to incrementally link LTO objects and non-LTO objects into a single mixed object file. If any of the object files in the incremental link cannot be used for link-time optimization, the linker plugin issues a warning and uses nolto-rel. To maintain whole program optimization, it is recommended to link such objects into a static library instead. Alternatively it is possible to use H.J. Lu's binutils with support for mixed objects. Niche? Yes. Iconic? Also yes 💅

• -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. Fast linker bestie represent 🔥
• -llibrary, -l library — search the library named library when linking. (The second alternative, with the library as a separate argument, is only for POSIX compliance and is not recommended — so like, don't.)

The -l option is passed directly to the linker by GCC. Refer to your linker documentation for exact details. The general description below applies to the GNU linker.

The linker searches a standard list of directories for the library. The directories searched include several standard system directories plus any that you specify with -L.

Static libraries are archives of object files, and have file names like liblibrary.a. Some targets also support shared libraries, which typically have names like liblibrary.so. If both static and shared libraries are found, the linker gives preference to linking with the shared library unless the -static option is used.

It makes a difference where in the command you write this option — the linker searches and processes libraries and object files in the order they are specified. So foo.o -lz bar.o searches library z after file foo.o but before bar.o. If bar.o refers to functions in z, those functions may not be loaded. Order matters, fr fr 😤

• -lobjc — you need this special case of the -l option in order to link an Objective-C or Objective-C++ program. Special lil guy for a special lil language 🍓
• -nostartfiles — do not use the standard system startup files when linking. The standard system libraries are used normally, unless -nostdlib, -nolibc, or -nodefaultlibs is used.
• -nodefaultlibs — do not use the standard system libraries when linking. Only the libraries you specify are passed to the linker, and options specifying linkage of the system libraries, such as -static-libgcc or -shared-libgcc, are ignored. The standard startup files are used normally, unless -nostartfiles is used.

The compiler may generate calls to memcmp, memset, memcpy and memmove. These entries are usually resolved by entries in libc. These entry points should be supplied through some other mechanism when this option is specified. Like, that's on you now 🙈

• -nolibc — do not use the C library or system libraries tightly coupled with it when linking. It still links 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. Bare metal main character energy 👑
• -nostdlib — do not use the standard system startup files or libraries when linking. No startup files and only the libraries you specify are passed to the linker, and options specifying linkage of the system libraries, such as -static-libgcc or -shared-libgcc, are ignored.

The compiler may generate calls to memcmp, memset, memcpy and memmove. These entries are usually resolved by entries in libc. These entry points should be supplied through some other mechanism when this option is specified. Your problem now, bestie 😭

One of the standard libraries bypassed by -nostdlib and -nodefaultlibs is libgcc.a, a library of internal subroutines which GCC uses to overcome shortcomings of particular machines, or special needs for some languages.

In most cases, you need libgcc.a even when you want to avoid other standard libraries. In other words, when you specify -nostdlib or -nodefaultlibs you should usually specify -lgcc as well. This ensures that you have no unresolved references to internal GCC library subroutines. (An example of such an internal subroutine is __main, used to ensure C++ constructors are called.) So like… you went no-stdlib but you still gotta bring -lgcc to the function. That's just how it is 🎀

• -nostdlib++ — do not implicitly link with standard C++ libraries.
• -e entry, --entry=entry — specify that the program entry point is entry. The argument is interpreted by the linker; the GNU linker accepts either a symbol name or an address. Where you coming in from? You decide 💫
• -pie — okay so this makes 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. Slay. 💖
• -no-pie — don't produce a dynamically linked position independent executable. That's it, bestie. 🙅‍♀️
• -static-pie — produce a static position independent executable on targets that support it. A static position independent executable is similar to a static executable, but can be loaded at any address without a dynamic linker. For predictable results, you must also specify the same set of options used for compilation (-fpie, -fPIE, or model suboptions) when you specify this linker option. No cap. ✨
• -pthread — link with the POSIX threads library. This option is supported on GNU/Linux targets, most other Unix derivatives, and also on x86 Cygwin and MinGW targets. On some targets this option also sets flags for the preprocessor, so it should be used consistently for both compilation and linking. Consistency is key, queen. 👑
• -r — produce a relocatable object as output. This is also known as partial linking. It's giving "I'm not done linking yet." 💅
• -rdynamic — pass the flag -export-dynamic to the ELF linker, on targets that support it. This instructs the linker to add all symbols, not only used ones, to the dynamic symbol table. This option is needed for some uses of dlopen or to allow obtaining backtraces from within a program. Backtraces are self-care. 🫶
• -s — remove all symbol table and relocation information from the executable. Bye, debug info. 💀
• -static — on systems that support dynamic linking, this overrides -pie and prevents linking with the shared libraries. On other systems, this option has no effect. Okay? Periodt. 💅
• -shared — produce a shared object which can then be linked with other objects to form an executable. Not all systems support this option. For predictable results, you must also specify the same set of options used for compilation (-fpic, -fPIC, or model suboptions) when you specify this linker option.[1] Shared library era. 🌈
• -shared-libgcc, -static-libgcc — on systems that provide libgcc as a shared library, these options force the use of either the shared or static version, respectively. If no shared version of libgcc was built when the compiler was configured, these options have no effect.

There are several situations in which an application should use the shared libgcc instead of the static version. The most common of these is when the application wishes to throw and catch exceptions across different shared libraries. In that case, each of the libraries as well as the application itself should use the shared libgcc.

Therefore, the G++ driver automatically adds -shared-libgcc whenever you build a shared library or a main executable, because C++ programs typically use exceptions, so this is the right thing to do.

If, instead, you use the GCC driver to create shared libraries, you may find that they are not always linked with the shared libgcc. If GCC finds, at its configuration time, that you have a non-GNU linker or a GNU linker that does not support option --eh-frame-hdr, it links the shared version of libgcc into shared libraries by default. Otherwise, it takes advantage of the linker and optimizes away the linking with the shared version of libgcc, linking with the static version of libgcc by default. This allows exceptions to propagate through such shared libraries, without incurring relocation costs at library load time.

However, if a library or main executable is supposed to throw or catch exceptions, you must link it using the G++ driver, or using the option -shared-libgcc, such that it is linked with the shared libgcc.

• -static-libasan — when the -fsanitize=address option is used to link a program, the GCC driver automatically links against libasan. If libasan is available as a shared library, and the -static option is not used, then this links against the shared version of libasan. The -static-libasan option directs the GCC driver to link libasan statically, without necessarily linking other libraries statically. Sanitizers, but make it static. 🧼
• -static-libtsan — when the -fsanitize=thread option is used to link a program, the GCC driver automatically links against libtsan. If libtsan is available as a shared library, and the -static option is not used, then this links against the shared version of libtsan. The -static-libtsan option directs the GCC driver to link libtsan statically, without necessarily linking other libraries statically. Threads but frozen. 🧵
• -static-liblsan — when the -fsanitize=leak option is used to link a program, the GCC driver automatically links against liblsan. If liblsan is available as a shared library, and the -static option is not used, then this links against the shared version of liblsan. The -static-liblsan option directs the GCC driver to link liblsan statically, without necessarily linking other libraries statically. Leak checker but static. 💧
• -static-libubsan — when the -fsanitize=undefined option is used to link a program, the GCC driver automatically links against libubsan. If libubsan is available as a shared library, and the -static option is not used, then this links against the shared version of libubsan. The -static-libubsan option directs the GCC driver to link libubsan statically, without necessarily linking other libraries statically. Undefined behavior? Not today. 🚫
• -static-libstdc++ — when the g++ program is used to link a C++ program, it normally automatically links against libstdc++. If libstdc++ is available as a shared library, and the -static option is not used, then this links against the shared version of libstdc++. That is normally fine. However, it is sometimes useful to freeze the version of libstdc++ used by the program without going all the way to a fully static link. The -static-libstdc++ option directs the g++ driver to link libstdc++ statically, without necessarily linking other libraries statically. C++ but commitment issues. 😭
• -symbolic — bind references to global symbols when building a shared object. Warn about any unresolved references (unless overridden by the link editor option -Xlinker -z -Xlinker defs). Only a few systems support this option. Rare and mysterious. 🦋
• -T script — use script as the linker script. This option is supported by most systems using the GNU linker. On some targets, such as bare-board targets without an operating system, the -T option may be required when linking to avoid references to undefined symbols. Linker script realness. 📜
• -Xlinker option — pass option as an option to the linker. You can use this to supply system-specific linker options that GCC does not recognize.

If you want to pass an option that takes a separate argument, you must use -Xlinker twice, once for the option and once for the argument. For example, to pass -assert definitions, you must write -Xlinker -assert -Xlinker definitions. It does not work to write -Xlinker "-assert definitions", because this passes the entire string as a single argument, which is not what the linker expects.

When using the GNU linker, it is usually more convenient to pass arguments to linker options using the option=value syntax than as separate arguments. For example, you can specify -Xlinker -Map=output.map rather than -Xlinker -Map -Xlinker output.map. Other linkers may not support this syntax for command-line options.

• -Wl,option — pass option as an option to the linker. If option contains commas, it is split into multiple options at the commas. You can use this syntax to pass an argument to the option. For example, -Wl,-Map,output.map passes -Map output.map to the linker. When using the GNU linker, you can also get the same effect with -Wl,-Map=output.map.

NOTE: In Ubuntu 8.10 and later versions, for LDFLAGS, the option -Wl,-z,relro is used. To disable, use -Wl,-z,norelro.

• -u symbol — pretend the symbol symbol is undefined, to force linking of library modules to define it. You can use -u multiple times with different symbols to force loading of additional library modules. Manifesting symbols into existence. ✨
• -z keyword — -z is passed directly on to the linker along with the keyword keyword. See the section in the documentation of your linker for permitted values and their meanings. Linker secrets, iykyk. 🤫

✨ Options for Directory Search ✨

These options specify directories to search for header files, for libraries and for parts of the compiler:

• -I dir, -iquote dir, -isystem dir, -idirafter dir — add the directory dir to the list of directories to be searched for header files during preprocessing. If dir begins with = or $SYSROOT, then the = or $SYSROOT is replaced by the sysroot prefix; see --sysroot and -isysroot.

Directories specified with -iquote apply only to the quote form of the directive, #include "file". Directories specified with -I, -isystem, or -idirafter apply to lookup for both the #include "file" and #include <file> directives.

You can specify any number or combination of these options on the command line to search for header files in several directories. The lookup order is as follows:

1. For the quote form of the include directive, the directory of the current file is searched first.
2. For the quote form of the include directive, the directories specified by -iquote options are searched in left-to-right order, as they appear on the command line.
3. Directories specified with -I options are scanned in left-to-right order.
4. Directories specified with -isystem options are scanned in left-to-right order.
5. Standard system directories are scanned.
6. Directories specified with -idirafter options are scanned in left-to-right order.

You can use -I to override a system header file, substituting your own version, since these directories are searched before the standard system header file directories. However, you should not use this option to add directories that contain vendor-supplied system header files; use -isystem for that.

The -isystem and -idirafter options also mark the directory as a system directory, so that it gets the same special treatment that is applied to the standard system directories.

okay so if a standard system include directory, or a directory you gave with -isystem, is also given with -I, then the -I option is ignored. The directory is still searched, but as a system directory at its normal position in the system include chain. Why? To make sure GCC's procedure to fix buggy system headers and the ordering for the #include_next directive are not inadvertently changed. If you really need to change the search order for system directories, use the -nostdinc and/or -isystem options. 💅

• -I- — Split the include path. This option has been deprecated. Please use -iquote instead for -I directories before the -I- and remove the -I- option. 🫶

Any directories specified with -I options before -I- are searched only for headers requested with #include "file"; they are not searched for #include <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-. 😭

• -iprefix prefix — Specify prefix as the prefix for subsequent -iwithprefix options. If the prefix represents a directory, you should include the final /. 🎀
• -iwithprefix dir — Append dir to the prefix specified previously with -iprefix, and add the resulting directory to the include search path. This puts it where -idirafter would. ✨
• -iwithprefixbefore dir — Append dir to the prefix specified previously with -iprefix, and add the resulting directory to the include search path. This puts it in the same place -I would. 🌸
• -isysroot dir — This option is like the --sysroot option, but applies only to header files (except for Darwin targets, where it applies to both header files and libraries). See the --sysroot option for more information. 🍓
• -imultilib dir — Use dir as a subdirectory of the directory containing target-specific C++ headers. 💜
• -nostdinc — Do not search the standard system directories for header files. Only the directories explicitly specified with -I, -iquote, -isystem, and/or -idirafter options (and the directory of the current file, if appropriate) are searched. 👑
• -nostdinc++ — Do not search for header files in the C++-specific standard directories, but do still search the other standard directories. (This option is used when building the C++ library.) 🧚
• -iplugindir=dir — Set the directory to search for plugins that are passed by -fplugin=name instead of -fplugin=path/name.so. This option is not meant to be used by the user, but only passed by the driver. 🙈
• -Ldir — Add directory dir to the list of directories to be searched for -l. 🔥
• -Bprefix — This option specifies where to find the executables, libraries, include files, and data files of the compiler itself. 💅

The compiler driver program runs one or more of the subprograms cpp, cc1, as and ld. It tries prefix as a prefix for each program it tries to run, both with and without machine/version/ for the corresponding target machine and compiler version.

For each subprogram to be run, the compiler driver first tries the -B prefix, if any. If that name is not found, or if -B is not specified, the driver tries two standard prefixes, /usr/lib/gcc/ and /usr/local/lib/gcc/. If neither of those results in a file name that is found, the unmodified program name is searched for using the directories specified in your PATH environment variable.

The compiler checks to see if the path provided by -B refers to a directory, and if necessary it adds a directory separator character at the end of the path.

-B prefixes that effectively specify directory names also apply to libraries in the linker, because the compiler translates these options into -L options for the linker. They also apply to include files in the preprocessor, because the compiler translates these options into -isystem options for the preprocessor. In this case, the compiler appends include to the prefix.

The runtime support file libgcc.a can also be searched for using the -B prefix, if needed. If it is not found there, the two standard prefixes above are tried, and that is all. The file is left out of the link if it is not found by those means.

Another way to specify a prefix much like the -B prefix is to use the environment variable GCC_EXEC_PREFIX.

As a special kludge, if the path provided by -B is [dir/]stageN/, where N is a number in the range 0 to 9, then it is replaced by [dir/]include. This is to help with boot-strapping the compiler. 🎀

• -no-canonical-prefixes — Do not expand any symbolic links, resolve references to /../ or /./, or make the path absolute when generating a relative prefix. 🦋
• --sysroot=dir — Use dir as the logical root directory for headers and libraries. For example, if the compiler normally searches for headers in /usr/include and libraries in /usr/lib, it instead searches dir/usr/include and dir/usr/lib. 🌈

If you use both this option and the -isysroot option, then the --sysroot option applies to libraries, but the -isysroot option applies to header files.

The GNU linker (beginning with version 2.16) has the necessary support for this option. If your linker does not support this option, the header file aspect of --sysroot still works, but the library aspect does not. 💀

• --no-sysroot-suffix — For some targets, a suffix is added to the root directory specified with --sysroot, depending on the other options used, so that headers may for example be found in dir/suffix/usr/include instead of dir/usr/include. This option disables the addition of such a suffix. ✨

🛠️ Options for Code Generation Conventions 🛠️

These machine-independent options control the interface conventions used in code generation. 👑

Most of them have both positive and negative forms; the negative form of -ffoo is -fno-foo. In the table below, only one of the forms is listed---the one that is not the default. You can figure out the other form by either removing no- or adding it. 💅

• -fstack-reuse=reuse-level — This option controls stack space reuse for user declared local/auto variables and compiler generated temporaries. reuse_level can be all, named_vars, or none. all enables stack reuse for all local variables and temporaries, named_vars enables the reuse only for user defined local variables with names, and none disables stack reuse completely. The default value is all. The option is needed when the program extends the lifetime of a scoped local variable or a compiler generated temporary beyond the end point defined by the language. When a lifetime of a variable ends, and if the variable lives in memory, the optimizing compiler has the freedom to reuse its stack space with other temporaries or scoped local variables whose live range does not overlap with it. Legacy code extending local lifetime is likely to break with the stack reuse optimization. 😭

For example,

                      int *p;
                      {
                        int local1;

                        p = &local1;
                        local1 = 10;
                        ....
                      }
                      {
                         int local2;
                         local2 = 20;
                         ...
                      }

                      if (*p == 10)  // out of scope use of local1
                        {

                        }

Another example:

                      struct A
                      {
                          A(int k) : i(k), j(k) { }
                          int i;
                          int j;
                      };

                      A *ap;

                      void foo(const A& ar)
                      {
                         ap = &ar;
                      }

                      void bar()
                      {
                         foo(A(10)); // temp object's lifetime ends when foo returns

                         {
                           A a(20);
                           ....
                         }
                         ap->i+= 10;  // ap references out of scope temp whose space
                                      // is reused with a. What is the value of ap->i?
                      }

The lifetime of a compiler generated temporary is well defined by the C++ standard. When a lifetime of a temporary ends, and if the temporary lives in memory, the optimizing compiler has the freedom to reuse its stack space with other temporaries or scoped local variables whose live range does not overlap with it. However some of the legacy code relies on the behavior of older compilers in which temporaries' stack space is not reused, the aggressive stack reuse can lead to runtime errors. This option is used to control the temporary stack reuse optimization. 💀

• -ftrapv — This option generates traps for signed overflow on addition, subtraction, multiplication operations. The options -ftrapv and -fwrapv override each other, so using -ftrapv -fwrapv on the command-line results in -fwrapv being effective. Note that only active options override, so using -ftrapv -fwrapv -fno-wrapv on the command-line results in -ftrapv being effective. 🔥
• -fwrapv — This option instructs the compiler to assume that signed arithmetic overflow of addition, subtraction and multiplication wraps around using twos-complement representation. This flag enables some optimizations and disables others. The options -ftrapv and -fwrapv override each other, so using -ftrapv -fwrapv on the command-line results in -fwrapv being effective. Note that only active options override, so using -ftrapv -fwrapv -fno-wrapv on the command-line results in -ftrapv being effective. ✨
• -fwrapv-pointer — This option instructs the compiler to assume that pointer arithmetic overflow on addition and subtraction wraps around using twos-complement representation. This flag disables some optimizations which assume pointer overflow is invalid. 🦋
• -fstrict-overflow — This option implies -fno-wrapv -fno-wrapv-pointer and when negated implies -fwrapv -fwrapv-pointer. 💅
• -fexceptions — okay bestie, this one turns ON exception handling ✨. It makes GCC generate all the extra code you need to propagate exceptions. On some targets that means GCC has to spit out frame unwind info for every single function, which can add a chunky amount of data size overhead — but ngl, it doesn't affect execution speed at all 💅. Here's the tea: if you don't say anything, GCC turns it on by default for languages like C++ that literally need exception handling, and turns it off for languages like C that don't. But — you might need to flip it on when you're compiling C code that has to play nice with exception handlers written in C++. And you might wanna turn it off if you're compiling ancient C++ programs that don't use exceptions. Iconic, flexible, we love 🫶.
• -fnon-call-exceptions — this one generates code that lets trapping instructions throw exceptions 😳. Important caveat though: this needs platform-specific runtime support that like... does not exist everywhere, so vibe check your target first. Also it only lets trapping instructions throw — think memory references or floating-point instructions. It does NOT let exceptions get thrown from arbitrary signal handlers like SIGALRM, so don't try it 🙈. Oh and btw, this enables -fexceptions.
• -fdelete-dead-exceptions — bestie this is the "if it doesn't matter, let it go" option 💀. It says: instructions that might throw exceptions but don't otherwise contribute to the program's execution can be optimized away. It does not affect calls to functions, except those with the pure or const attributes. This one is enabled by default for the Ada and C++ compilers, because the language specs literally permit it. And the optimization passes that remove dead exceptions get enabled independently at different optimization levels. Slay, dead code stays dead 💅.
• -funwind-tables — basically -fexceptions's chill cousin 🌸. Same energy, except it only generates the static data that's needed and doesn't touch the generated code in any other way. You normally don't need to enable this yourself — instead, a language processor that needs this handling just enables it on your behalf. So like, not your problem, bestie 🫶.
• -fasynchronous-unwind-tables — generate an unwind table in DWARF format, if the target machine supports it ✨. The table is exact at each instruction boundary, which means it can be used for stack unwinding from asynchronous events — like a debugger or a garbage collector. Very demure, very precise 💜.
• -fno-gnu-unique — okay so on systems with a recent GNU assembler and C library, the C++ compiler uses the STB_GNU_UNIQUE binding to make sure that definitions of template static data members and static local variables in inline functions stay unique, even when RTLD_LOCAL is in the mix 👑. This is necessary to avoid problems where a library used by two different RTLD_LOCAL plugins depends on a definition in one of them, and then the two plugins disagree about the binding of the symbol. Messy, we hate to see it 😭. BUT — and this is the plot twist — this causes dlclose to be ignored for affected DSOs. So if your program relies on reinitializing a DSO via dlclose and dlopen, you can use -fno-gnu-unique. Fix your own problem, main character energy 🔥.
• -fpcc-struct-return — this makes GCC return short struct and union values in memory, just like the longer ones, instead of in registers. Less efficient, sure, but it has a real advantage: it allows intercallability between GCC-compiled files and files compiled with other compilers — particularly the Portable C Compiler (pcc) 🎀.

The precise convention for returning structures in memory depends on the target configuration macros. And "short" structures and unions are those whose size and alignment match that of some integer type, iykyk.

Warning: code compiled with the -fpcc-struct-return switch is not binary compatible with code compiled with the -freg-struct-return switch. Use it to conform to a non-default application binary interface. Don't mix and match, bestie, that's a whole mess 💀.

• -freg-struct-return — the glow-up version ✨: return struct and union values in registers whenever possible. This is more efficient for small structures than -fpcc-struct-return 💅.

If you specify neither -fpcc-struct-return nor -freg-struct-return, GCC defaults to whichever convention is standard for the target. 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. Periodt 👑.

Warning: code compiled with the -freg-struct-return switch is not binary compatible with code compiled with the -fpcc-struct-return switch. Use it to conform to a non-default application binary interface.

• -fshort-enums — allocate to an enum type only as many bytes as it needs for the declared range of possible values 🦋. Specifically, the enum type becomes equivalent to the smallest integer type that has enough room. Note: this option has no effect for an enumeration type with a fixed underlying type, so don't @ me.

Warning: the -fshort-enums switch makes GCC generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface.

• -fshort-wchar — override the underlying type for wchar_t to be short unsigned int instead of the default for the target 🌈. This option is useful for building programs to run under WINE. Little niche, very useful, we stan 🍓.

Warning: the -fshort-wchar switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface.

• -fcommon — in C code, this option controls where global variables defined without an initializer go. Those are called tentative definitions in the C standard, and they're distinct from declarations of a variable with the extern keyword, which don't allocate storage. Important distinction, bestie 🎀.

The default is -fno-common, which says the compiler places uninitialized global variables in the BSS section of the object file. This stops the linker from merging tentative definitions, so you get a multiple-definition error if the same variable accidentally gets defined in more than one compilation unit. Rude but honest 😭.

-fcommon instead places uninitialized global variables in a common block. This lets the linker resolve all tentative definitions of the same variable across different compilation units to the same object — or to a non-tentative definition. But 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 to let legacy code link without errors. Old code, we still love you 💜.

• -fno-ident — ignore the #ident directive. That's it. Short king 👑.
• -finhibit-size-directive — don't output a .size assembler directive, or anything else that would cause trouble if the function gets split in the middle and the two halves land at locations far apart in memory 😳. This option is used when compiling crtstuff.c; you should not need to use it for anything else. Hands off, bestie, that one's not for you 💅.
• -fverbose-asm — put extra commentary info in the generated assembly code so it's actually readable ✨. This option is generally only useful for people who genuinely need to read the generated assembly (like, maybe while debugging the compiler itself). Hardcore behavior, respect 🫶.

-fno-verbose-asm — the default — omits that extra info, and it's handy when you're 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,"",@progbits

Heads up though: the comments are made for humans, not machines 🙈, so the precise format of the comments is subject to change. Don't build tooling on it, bestie.

• -frecord-gcc-switches — this switch makes the command line used to invoke the compiler get recorded into the object file that's being created 🎀. It's only implemented on some targets, and the exact format of the recording depends on the target and the binary file format — but usually it 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. Different vibes, different destinations 💅. Also see -grecord-gcc-switches for another way of storing compiler options into the object file — options within options, we love a crossover episode 🦋.
• -fpic — okay so this one generates position-independent code (PIC) that's suitable for use in a shared library, if it's supported for the target machine. 💖

Code like this accesses all constant addresses through a global offset table (GOT). The dynamic loader resolves the GOT entries when the program starts — and ngl, the dynamic loader is not part of GCC, it's part of the operating system, so that's not our problem bestie. If the GOT size for the linked executable exceeds a machine-specific maximum size, you get an error message from the linker saying -fpic does not work. In that case? Recompile with -fPIC instead, periodt. (These maximums are 8k on the SPARC, 28k on AArch64 and 32k on the m68k and RS/6000. The x86 has no such limit. iconic.)

Position-independent code requires special support, so it literally only works on certain machines. For the x86, GCC supports PIC for System V but not for the Sun 386i. Code generated for the IBM RS/6000 is always position-independent, no notes. 🦋

When this flag is set, the macros __pic__ and __PIC__ are defined to 1.

• -fPIC — if it's supported for the target machine, this emits position-independent code suitable for dynamic linking, and it avoids any limit on the size of the global offset table. This option actually makes a difference on AArch64, m68k, PowerPC and SPARC. 🔥

Position-independent code requires special support, and therefore works only on certain machines. That's just the vibe check.

When this flag is set, the macros __pic__ and __PIC__ are defined to 2.

• -fpie
• -fPIE — these two are similar to -fpic and -fPIC, but the generated position-independent code can only be linked into executables. Usually you use these to compile code that will be linked using the -pie GCC option. 💅

-fpie and -fPIE both define the macros __pie__ and __PIE__. The macros have the value 1 for -fpie and 2 for -fPIE.

• -fno-plt — do not use the PLT for external function calls in position-independent code. Instead, load the callee address at call sites from the GOT and branch to it. This leads to more efficient code by eliminating PLT stubs and exposing GOT loads to optimizations. ✨ On architectures such as 32-bit x86 where PLT stubs expect the GOT pointer in a specific register, this gives more register allocation freedom to the compiler. Lazy binding requires use of the PLT; with -fno-plt all external symbols are resolved at load time.

Alternatively, the function attribute noplt can be used to avoid calls through the PLT for specific external functions. 🎀

In position-dependent code, a few targets also convert calls to functions that are marked to not use the PLT to use the GOT instead.

• -fno-jump-tables — do not use jump tables for switch statements even where it would be more efficient than other code generation strategies. This option is of use in conjunction with -fpic or -fPIC for building code that forms part of a dynamic linker and cannot reference the address of a jump table. On some targets, jump tables do not require a GOT and this option is not needed. 🙈
• -fno-bit-tests — do not use bit tests for switch statements even where it would be more efficient than other code generation strategies. simple as that bestie.
• -ffixed-reg — treat the register named reg as a fixed register; generated code should never refer to it (except perhaps as a stack pointer, frame pointer or in some other fixed role). 🫶

reg must be the name of a register. The register names accepted are machine-specific and are defined in the REGISTER_NAMES macro in the machine description macro file.

This flag does not have a negative form, because it specifies a three-way choice.

• -fcall-used-reg — treat the register named reg as an allocable register that is clobbered by function calls. It may be allocated for temporaries or variables that do not live across a call. Functions compiled this way do not save and restore the register reg. 😭

It is an error to use this flag with the frame pointer or stack pointer. Use of this flag for other registers that have fixed pervasive roles in the machine's execution model produces disastrous results.

This flag does not have a negative form, because it specifies a three-way choice.

• -fcall-saved-reg — treat the register named reg as an allocable register saved by functions. It may be allocated even for temporaries or variables that live across a call. Functions compiled this way save and restore the register reg if they use it. 🌸

It is an error to use this flag with the frame pointer or stack pointer. Use of this flag for other registers that have fixed pervasive roles in the machine's execution model produces disastrous results.

A different sort of disaster results from the use of this flag for a register in which function values may be returned. 💀

This flag does not have a negative form, because it specifies a three-way choice.

• -fpack-struct[=n] — without a value specified, pack all structure members together without holes. When a value is specified (which must be a small power of two), pack structure members according to this value, representing the maximum alignment (that is, objects with default alignment requirements larger than this are output potentially unaligned at the next fitting location. 😬

Warning: the -fpack-struct switch causes GCC to generate code that is not binary compatible with code generated without that switch. Additionally, it makes the code suboptimal. Use it to conform to a non-default application binary interface.

• -fleading-underscore — this option and its counterpart, -fno-leading-underscore, forcibly change the way C symbols are represented in the object file. One use is to help link with legacy assembly code. 🧚

Warning: the -fleading-underscore switch causes GCC to generate code that is not binary compatible with code generated without that switch. Use it to conform to a non-default application binary interface. Not all targets provide complete support for this switch.

• -ftls-model=model — alter the thread-local storage model to be used. The model argument should be one of global-dynamic, local-dynamic, initial-exec or local-exec. Note that the choice is subject to optimization: the compiler may use a more efficient model for symbols not visible outside of the translation unit, or if -fpic is not given on the command line. 👑

The default without -fpic is initial-exec; with -fpic the default is global-dynamic.

• -ftrampolines — for targets that normally need trampolines for nested functions, always generate them instead of using descriptors. Otherwise, for targets that do not need them, like for example HP-PA or IA-64, do nothing. 🌈

A trampoline is a small piece of code that is created at run time on the stack when the address of a nested function is taken, and is used to call the nested function indirectly. Therefore, it requires the stack to be made executable in order for the program to work properly. 🫣

-fno-trampolines is enabled by default on a language by language basis to let the compiler avoid generating them, if it computes that this is safe, and replace them with descriptors. Descriptors are made up of data only, but the generated code must be prepared to deal with them. As of this writing, -fno-trampolines is enabled by default only for Ada.

Moreover, code compiled with -ftrampolines and code compiled with -fno-trampolines are not binary compatible if nested functions are present. This option must therefore be used on a program-wide basis and be manipulated with extreme care. ⚠️

For languages other than Ada, the -ftrampolines and -fno-trampolines options currently have no effect, and trampolines are always generated on platforms that need them for nested functions.

• -ftrampoline-impl=[stack|heap] — by default, trampolines are generated on stack. However, certain platforms (such as the Apple M1) do not permit an executable stack. Compiling with -ftrampoline-impl=heap generates calls to __gcc_nested_func_ptr_created and __gcc_nested_func_ptr_deleted in order to allocate and deallocate trampoline space on the executable heap. These functions are implemented in libgcc, and will only be provided on specific targets: x86_64 Darwin, x86_64 and aarch64 Linux. 💜

PLEASE NOTE: Heap trampolines are not guaranteed to be correctly deallocated if you setjmp, instantiate nested functions, and then longjmp back to a state prior to having allocated those nested functions. 💀

• -fvisibility=[default|internal|hidden|protected] — set the default ELF image symbol visibility to the specified option — all symbols are marked with this unless overridden within the code. Using this feature can very substantially improve linking and load times of shared object libraries, produce more optimized code, provide near-perfect API export and prevent symbol clashes. It is strongly recommended that you use this in any shared objects you distribute. slay. ✨

Despite the nomenclature, default always means public; i.e., available to be linked against from outside the shared object. protected and internal are pretty useless in real-world usage so the only other commonly used option is hidden. The default if -fvisibility isn't specified is default, i.e., make every symbol public.

A good explanation of the benefits offered by ensuring ELF symbols have the correct visibility is given by "How To Write Shared Libraries" by Ulrich Drepper (which can be found at <https://www.akkadia.org/drepper/>) — however a superior solution made possible by this option to marking things hidden when the default is public is to make the default hidden and mark things public. This is the norm with DLLs on Windows and with -fvisibility=hidden and __attribute__ ((visibility("default"))) instead of __declspec(dllexport) you get almost identical semantics with identical syntax. This is a great boon to those working with cross-platform projects. 🎀

Okay so if you're adding visibility support to code that already exists, bestie, you're gonna want #pragma GCC visibility and honestly? She's iconic. Here's the move: you wrap the declarations you wanna set visibility for in #pragma GCC visibility push(hidden) and #pragma GCC visibility pop. That's it. That's the whole vibe. 💅

But listen — symbol visibility is part of the API interface contract. Not a little detail, not a cute accessory, an actual contract. So all new code should always specify visibility when it isn't the default. Like, declarations that are only for use inside the local DSO should always be explicitly marked as hidden, so you dodge those PLT indirection overheads — and making it abundantly clear also helps readability and self-documentation of the code. It's giving main character and good engineering. 🫶

Heads up though: because ISO C++ specification requirements are the way they are, operator new and operator delete must always be of default visibility. No negotiating with this one. 😭

Also be aware: headers from outside your project — especially system headers and headers from any other library you use — may not be expecting to be compiled with visibility other than the default. You may need to explicitly say #pragma GCC visibility push(default) before including any such headers. Protect their peace. 🎀

Now, extern declarations are not affected by -fvisibility, which means a lot of code can be recompiled with -fvisibility=hidden and need zero modifications. Slay. However — and this is the catch — it means calls to extern functions with no explicit visibility use the PLT, so it's more effective to use __attribute ((visibility)) and/or #pragma GCC visibility to tell the compiler which extern declarations should be treated as hidden.

And note that -fvisibility does affect C++ vague linkage entities. Which means, for instance, an exception class that's thrown between DSOs must be explicitly marked with default visibility so that the type_info nodes are unified between the DSOs. Fr fr, don't skip this one.

An overview of these techniques, their benefits and how to use them lives at <https://gcc.gnu.org/wiki/Visibility>. Go read it, it's worth it. ✨

---

• -fstrict-volatile-bitfields — Use this if accesses to volatile bit-fields (or other structure fields, though the compiler usually honors those types anyway) should use a single access of the width of the field's type, aligned to a natural alignment if possible. For example, targets with memory-mapped peripheral registers might require all such accesses to be 16 bits wide; with this flag you can declare all peripheral bit-fields as unsigned short (assuming short is 16 bits on these targets) to force GCC to use 16-bit accesses instead of, perhaps, a more efficient 32-bit access.

If this option is disabled, the compiler uses the most efficient instruction. In the previous example, that might be a 32-bit load instruction — even though that accesses bytes that 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. Crisis averted.

Note: due to restrictions of the C/C++11 memory model, write accesses are not allowed to touch non bit-field members. So it's recommended to define all bits of the field's type as bit-field members. 🧚

The default value of this option is determined by the application binary interface for the target processor.

• -fsync-libcalls — This one controls whether any out-of-line instance of the __sync family of functions may be used to implement the C++11 __atomic family of functions.

The default value of this option is enabled, so the only useful form of the option is -fno-sync-libcalls. This option is used in the implementation of the libatomic runtime library. Lowkey niche, highkey important. 💜

---

🛠️ 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. That includes options that produce debug dumps at various points in the compilation; options that print statistics like memory use and execution time; and options that print info about GCC's configuration, like where it searches for libraries. You should rarely need any of these for ordinary compilation and linking tasks, bestie — this is backstage-pass territory. 🎟️

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. Easy.

If =filename is omitted, a default dump file name is constructed by concatenating the base dump file name, a pass number, a phase letter, and a pass name. The base dump file name is the name of the output file produced by the compiler if explicitly specified and not an executable; otherwise it's the source file name. The pass number is determined by the order passes are registered with the compiler's pass manager — generally the same as the order of execution, but passes registered by plugins, target-specific passes, or passes that are otherwise registered late are numbered higher than the pass named final, even if they're executed earlier. The phase letter is one of i (inter-procedural analysis), l (language-specific), r (RTL), or t (tree). The files are created in the directory of the output file. ✨

-fcallgraph-info, -fcallgraph-info=MARKERS

Makes the compiler output callgraph information for the program, on a per-object-file basis. The information is generated in the common VCG format. It can be decorated with additional, per-node and/or per-edge information, if a list of comma-separated markers is additionally specified. When the su marker is specified, the callgraph is decorated with stack usage information — it's equivalent to -fstack-usage. When the da marker is specified, the callgraph is decorated with information about dynamically allocated objects. 🦋

When compiling with -flto, no callgraph information is output along with the object file. At LTO link time, -fcallgraph-info may generate multiple callgraph information files next to intermediate LTO output files.

-dletters, -fdump-rtl-pass, -fdump-rtl-pass=filename

Says to make debugging dumps during compilation at times specified by letters. This is used for debugging the RTL-based passes of the compiler.

Some -dletters switches have different meaning when -E is used for preprocessing. Watch out for that. 👀

Debug dumps can be enabled with a -fdump-rtl switch or some -d option letters. Here are the possible letters for use in pass and letters, and their meanings:

• -fdump-rtl-alignments — Dump after branch alignments have been computed.
• -fdump-rtl-asmcons — Dump after fixing rtl statements that have unsatisfied in/out constraints.
• -fdump-rtl-auto_inc_dec — Dump after auto-inc-dec discovery. This pass is only run on architectures that have auto inc or auto dec instructions.
• -fdump-rtl-barriers — Dump after cleaning up the barrier instructions.
• -fdump-rtl-bbpart — Dump after partitioning hot and cold basic blocks.
• -fdump-rtl-bbro — Dump after block reordering.
• -fdump-rtl-btl1, -fdump-rtl-btl2 — -fdump-rtl-btl1 and -fdump-rtl-btl2 enable dumping after the two branch target load optimization passes.
• -fdump-rtl-bypass — Dump after jump bypassing and control flow optimizations.
• -fdump-rtl-combine — Dump after the RTL instruction combination pass.
• -fdump-rtl-compgotos — Dump after duplicating the computed gotos.
• -fdump-rtl-ce1, -fdump-rtl-ce2, -fdump-rtl-ce3 — -fdump-rtl-ce1, -fdump-rtl-ce2, and -fdump-rtl-ce3 enable dumping after the three if conversion passes.
• -fdump-rtl-cprop_hardreg — Dump after hard register copy propagation.
• -fdump-rtl-csa — Dump after combining stack adjustments.
• -fdump-rtl-cse1, -fdump-rtl-cse2 — -fdump-rtl-cse1 and -fdump-rtl-cse2 enable dumping after the two common subexpression elimination passes.
• -fdump-rtl-dce — Dump after the standalone dead code elimination passes.
• -fdump-rtl-dbr — Dump after delayed branch scheduling.
• -fdump-rtl-dce1, -fdump-rtl-dce2 — -fdump-rtl-dce1 and -fdump-rtl-dce2 enable dumping after the two dead store elimination passes.
• -fdump-rtl-eh — Dump after finalization of EH handling code.
• -fdump-rtl-eh_ranges — Dump after conversion of EH handling range regions.
• -fdump-rtl-expand — Dump after RTL generation.
• -fdump-rtl-fwprop1, -fdump-rtl-fwprop2 — -fdump-rtl-fwprop1 and -fdump-rtl-fwprop2 enable dumping after the two forward propagation passes.
• -fdump-rtl-gcse1, -fdump-rtl-gcse2 — -fdump-rtl-gcse1 and -fdump-rtl-gcse2 enable dumping after global common subexpression elimination.
• -fdump-rtl-init-regs — Dump after the initialization of the registers.
• -fdump-rtl-initvals — Dump after the computation of the initial value sets.
• -fdump-rtl-into_cfglayout — Dump after converting to cfglayout mode.
• -fdump-rtl-ira — Dump after iterated register allocation.
• -fdump-rtl-jump — Dump after the second jump optimization.
• -fdump-rtl-loop2 — -fdump-rtl-loop2 enables dumping after the rtl loop optimization passes.
• -fdump-rtl-mach — Dump after performing the machine dependent reorganization pass, if that pass exists.
• -fdump-rtl-mode_sw — Dump after removing redundant mode switches.
• -fdump-rtl-rnreg — Dump after register renumbering.
• -fdump-rtl-outof_cfglayout — Dump after converting from cfglayout mode.
• -fdump-rtl-peephole2 — Dump after the peephole pass.
• -fdump-rtl-postreload — Dump after post-reload optimizations.
• -fdump-rtl-pro_and_epilogue — Dump after generating the function prologues and epilogues.
• -fdump-rtl-sched1, -fdump-rtl-sched2 — -fdump-rtl-sched1 and -fdump-rtl-sched2 enable dumping after the basic block scheduling passes.
• -fdump-rtl-ree — dumps the RTL right after sign/zero extension elimination ✨
• -fdump-rtl-seqabstr — dumps after common sequence discovery 🦋
• -fdump-rtl-shorten — dumps after shortening branches 🌸
• -fdump-rtl-sibling — dumps after sibling call optimizations 👑
• -fdump-rtl-split1, -fdump-rtl-split2, -fdump-rtl-split3, -fdump-rtl-split4, -fdump-rtl-split5 — these five besties enable dumping after five rounds of instruction splitting 🍓 (yes, five. one per round. she's thorough.)
• -fdump-rtl-sms — dumps after modulo scheduling. heads up tho, this pass only runs on some architectures — so on other targets it's giving nothing 💀
• -fdump-rtl-stack — dumps after the conversion from GCC's "flat register file" registers to the x87's stack-like registers. only runs on x86 variants, iykyk 🫶
• -fdump-rtl-subreg1, -fdump-rtl-subreg2 — -fdump-rtl-subreg1 and -fdump-rtl-subreg2 enable dumping after the two subreg expansion passes 🌈
• -fdump-rtl-unshare — dumps after all rtl has been unshared 💅
• -fdump-rtl-vartrack — dumps after variable tracking 🔥
• -fdump-rtl-vregs — dumps after converting virtual registers to hard registers ⭐
• -fdump-rtl-web — dumps after live range splitting 🦋
• -fdump-rtl-regclass, -fdump-rtl-subregs_of_mode_init, -fdump-rtl-subregs_of_mode_finish, -fdump-rtl-dfinit, -fdump-rtl-dfinish — okay so these dumps are defined but always produce empty files. like, they exist. they're real. they just… have nothing to say 🙈 empty file energy fr fr
• -da, -fdump-rtl-all — produce all the dumps listed above 🎀 (the whole buffet, no cap)
• -dA — annotate the assembler output with miscellaneous debugging information ✨
• -dD — dump all macro definitions, at the end of preprocessing, in addition to normal output 🌸
• -dH — produce a core dump whenever an error occurs 💀

bestie this one is exactly what it sounds like. errors → core files. it's not a vibe, it's a choice.

• -dp — annotate the assembler output with a comment showing which pattern and alternative is used. the length and cost of each instruction get printed too 👑
• -dP — dump the RTL in the assembler output as a comment before each instruction. also turns on -dp annotation 🌈
• -dx — just generate RTL for a function instead of compiling it. usually used with -fdump-rtl-expand 🦋
• -fdump-debug — dump debugging information generated during the debug generation phase ✨
• -fdump-earlydebug — dump debugging information generated during the early debug generation phase 🌸
• -fdump-noaddr — when doing debugging dumps, suppress address output. the way this makes it more feasible to use diff on debugging dumps for compiler invocations with different compiler binaries and/or different text / bss / data / heap / stack / dso start locations 😭 like yes queen, make the diffs readable
• -freport-bug — collect and dump debug information into a temporary file if an internal compiler error (ICE) occurs 🧊 (ICE!! not the drink bestie)
• -fdump-unnumbered — when doing debugging dumps, suppress instruction numbers and address output. again with the diff-ability: makes it more feasible to diff debugging dumps for compiler invocations with different options, in particular with and without -g 💅
• -fdump-unnumbered-links — when doing debugging dumps (see the -d option above), suppress instruction numbers for the links to the previous and next instructions in a sequence ⭐
• -fdump-ipa-switch, -fdump-ipa-switch-options — control the dumping at various stages of inter-procedural analysis language tree to a file. the file name is made by appending a switch-specific suffix to the source file name, and the file gets created in the same directory as the output file 🎀

the following dumps are possible:

• all — enables all inter-procedural analysis dumps ✨
• cgraph — dumps info about call-graph optimization, unused function removal, and inlining decisions 👑
• inline — dump after function inlining 🦋
• strubm — dump after selecting "strub" modes, and recording the selections as function attributes 💜
• strub — dump "strub" transformations: interface changes, function wrapping, and insertion of builtin calls for stack scrubbing and watermarking 🌊

additionally, the options -optimized, -missed, -note, and -all can be provided, with the same meaning as for -fopt-info, defaulting to -optimized 🌸

for example, -fdump-ipa-inline-optimized-missed will emit information on callsites that were inlined, along with callsites that were not inlined 😭

by default, the dump will contain messages about successful optimizations (equivalent to -optimized) together with low-level details about the analysis 🔥

• -fdump-lang — dump language-specific information. the file name is made by appending .lang to the source file name 🎀
• -fdump-lang-all, -fdump-lang-switch, -fdump-lang-switch-options, -fdump-lang-switch-options=filename — control the dumping of language-specific information. the options and filename portions behave exactly as described in the -fdump-tree option ✨

the following switch values are accepted:

• all — enable all language-specific dumps 💖
• class — dump class hierarchy information. Virtual table information is emitted unless 'slim' is specified. this option is applicable to C++ only 🦋
• module — dump module information. options lineno (locations), graph (reachability), blocks (clusters), uid (serialization), alias (mergeable), asmname (Elrond), eh (mapper) & vops (macros) may provide additional information. this option is applicable to C++ only 💜
• raw — dump the raw internal tree data. this option is applicable to C++ only 👑

• -fdump-passes — print on stderr the list of optimization passes that are turned on and off by the current command-line options 🔥
• -fdump-statistics-option — enable and control dumping of pass statistics in a separate file. the file name is generated by appending a suffix ending in .statistics to the source file name, and the file is created in the same directory as the output file. if the -option form is used, -stats causes counters to be summed over the whole compilation unit while -details dumps every event as the passes generate them. the default with no option is to sum counters for each function compiled 🎀

---

• -fdump-tree-all, -fdump-tree-switch, -fdump-tree-switch-options, -fdump-tree-switch-options=filename — control the dumping at various stages of processing the intermediate language tree to a file 🌸

if the -options form is used, options is a list of --separated options which control the details of the dump. not all options are applicable to all dumps; those that are not meaningful are ignored 🙈

the following options are available:

• address — print the address of each node. usually this is not meaningful as it changes according to the environment and source file. its primary use is for tying up a dump file with a debug environment ✨
• asmname — if "DECL_ASSEMBLER_NAME" has been set for a given decl, use that in the dump instead of "DECL_NAME". its primary use is ease of use working backward from mangled names in the assembly file 🦋
• slim — when dumping front-end intermediate representations, inhibit dumping of members of a scope or body of a function merely because that scope has been reached. only dump such items when they are directly reachable by some other path.

when dumping pretty-printed trees, this option inhibits dumping the bodies of control structures.

when dumping RTL, print the RTL in slim (condensed) form instead of the default LISP-like representation 💅

• raw — print a raw representation of the tree. by default, trees are pretty-printed into a C-like representation 👑
• details — enable more detailed dumps (not honored by every dump option). also include information from the optimization passes 🔥
• stats — enable dumping various statistics about the pass (not honored by every dump option) ⭐
• blocks — enable showing basic block boundaries (disabled in raw dumps) 🎀
• graph — for each of the other indicated dump files (-fdump-rtl-pass), dump a representation of the control flow graph suitable for viewing with GraphViz to file.passid.pass.dot. each function in the file is pretty-printed as a subgraph, so that GraphViz can render them all in a single plot.

this option currently only works for RTL dumps, and the RTL is always dumped in slim form 💀

• vops — enable showing virtual operands for every statement 🌸
• lineno — enable showing line numbers for statements 🦋
• uid — enable showing the unique ID ("DECL_UID") for each variable 💜
• verbose — enable showing the tree dump for each statement 👑
• eh — enable showing the EH region number holding each statement 🌈
• scev — enable showing scalar evolution analysis details ✨
• optimized — enable showing optimization information (only available in certain passes) 🎀
• missed — enable showing missed optimization information (only available in certain passes) 😭
• note — enable other detailed optimization information (only available in certain passes) 💅
• all — turn on all options, except raw, slim, verbose and lineno 🔥
• optall — turn on all optimization options, i.e., optimized, missed, and note ⭐

to determine what tree dumps are available or find the dump for a pass of interest, follow the steps below 🫶

1. invoke GCC with -fdump-passes and in the stderr output look for a code that corresponds to the pass you are interested in. for example, the codes "tree-evrp", "tree-vrp1", and "tree-vrp2" correspond to the three Value Range Propagation passes. the number at the end distinguishes distinct invocations of the same pass 👑
2. to enable the creation of the dump file, append the pass code to the -fdump- option prefix and invoke GCC with it. for example, to enable the dump from the Early Value Range Propagation pass, invoke GCC with the -fdump-tree-evrp option. optionally, you may specify the name of the dump file. if you don't specify one, GCC creates as described below 🌸

3. Go find that pass dump, bestie — it's living in a file whose name is three chunks glued together with a period: first, the name of the source file GCC was invoked to compile; then a numeric suffix for the pass number with the letter t tacked on for tree passes (or the letter r for RTL passes); and then finally the pass code itself. So like, the Early VRP pass dump might be chillin' in a file named myfile.c.038t.evrp in the current working directory. Heads up though: those numeric codes are not stable and can absolutely shift between GCC versions, so don't you dare hardcode them 💀

---

• -fopt-info, -fopt-info-options, -fopt-info-options=filename — controls optimization dumps from all the various optimization passes. If you use the -options form, then options is a list of --separated option keywords that pick out the dump details and the optimizations you want.

The options split into three groups, and honestly it's giving very organized:

1. options describing what kinds of messages should be emitted,
2. options describing the verbosity of the dump, and
3. options describing which optimizations should be included.

You can mix options from any of the three groups freely, because they're non-overlapping 🫶 But if there's ever a conflict, the later options override the earlier options on the command line. Periodt.

These ones control which kinds of messages get emitted:

• optimized — print info whenever an optimization is successfully applied. It's up to each pass to decide which info is relevant. For example, the vectorizer passes print the source location of loops that got successfully vectorized. ✨
• missed — print info about missed optimizations. Individual passes control which info ends up in the output. 😭
• note — print verbose info about optimizations, like certain transformations, more detailed messages about decisions, etc.
• all — print detailed optimization information. This includes optimized, missed, and note.

And this one 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 these option keywords describe a group of optimizations:

• ipa — enable dumps from all interprocedural optimizations.
• loop — enable dumps from all loop optimizations.
• inline — enable dumps from all inlining optimizations.
• omp — enable dumps from all OMP (Offloading and Multi Processing) optimizations.
• vec — enable dumps from all vectorization optimizations.
• optall — enable dumps from all optimizations. This is a superset of the optimization groups listed above.

If options is omitted, it defaults to optimized-optall, which means dump messages about successful optimizations from all the passes, omitting messages that are treated as "internals".

If filename is provided, then the dumps from all the applicable optimizations get concatenated into that 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.

Ngl, note that the output filename is overwritten in case of multiple translation units. If you want a combined output from multiple translation units, stderr should be used instead. 🙈

In the following example, the optimization info goes to stderr:

gcc -O3 -fopt-info

This example:

gcc -O3 -fopt-info-missed=missed.all

outputs a missed optimization report from all the passes into missed.all, and this one:

gcc -O2 -ftree-vectorize -fopt-info-vec-missed

prints information about missed optimization opportunities from vectorization passes on stderr. Note that -fopt-info-vec-missed is equivalent to -fopt-info-missed-vec. The order of the optimization group names and message types listed after -fopt-info does not matter, so vibe however you like. 💅

As another example,

gcc -O3 -fopt-info-inline-optimized-missed=inline.txt

outputs information about missed optimizations as well as optimized locations from all the inlining passes into inline.txt.

Finally, consider:

gcc -fopt-info-vec-missed=vec.miss -fopt-info-loop-optimized=loop.opt

Here the two output filenames vec.miss and loop.opt are in conflict, since only one output file is allowed. In this case, only the first option takes effect and the subsequent options are ignored. So 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. Don't build your whole life around it, bestie. 😭

It's roughly equivalent to a machine-readable version of -fopt-info-all, as a collection of messages with source file, line number and column number, plus 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. the more n, the more tea 🍵

• -fenable-kind-pass, -fdisable-kind-pass=range-list — this is a set of options used to explicitly disable/enable optimization passes. These options are intended for use for debugging GCC. Compiler users should use regular options for enabling/disabling passes instead. Like, this is dev-only territory, iykyk.
  • -fdisable-ipa-pass — disable IPA pass pass. pass is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1.
  • -fdisable-rtl-pass, -fdisable-rtl-pass=range-list — disable RTL pass pass. pass is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1. range-list is a comma-separated list of function ranges or assembler names. Each range is a number pair separated by a colon. The range is inclusive in both ends. If the range is trivial, the number pair can be simplified as a single number. If the function's call graph node's uid falls within one of the specified ranges, the pass is disabled for that function. The uid is shown in the function header of a dump file, and the pass names can be dumped by using option -fdump-passes.
  • -fdisable-tree-pass, -fdisable-tree-pass=range-list — disable tree pass pass. See -fdisable-rtl for the description of option arguments.
  • -fenable-ipa-pass — enable IPA pass pass. pass is the pass name. If the same pass is statically invoked in the compiler multiple times, the pass name should be appended with a sequential number starting from 1.
  • -fenable-rtl-pass, -fenable-rtl-pass=range-list — enable RTL pass pass. See -fdisable-rtl for option argument description and examples.
  • -fenable-tree-pass, -fenable-tree-pass=range-list — enable tree pass pass. See -fdisable-rtl for the description of option arguments.

Here are some examples showing uses of these options, and yes 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. -fchecking=2 enables further internal consistency checking that might affect code generation. 🔍
• -frandom-seed=string — this option provides a seed that GCC uses in place of random numbers when 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. You can use the -frandom-seed option to produce reproducibly identical object files. determinism slay ✨

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 single file you compile, bestie. like, no reusing. every file gets its own little seed, periodt. 🌱

• -save-temps — okay so this one keeps the "temporary" intermediate files around permanently instead of yeeting them 💾. it names them as auxiliary output files, exactly like it says under -dumpbase and -dumpdir. and when you pair it with -x on the command line, -save-temps is smart enough (iconic behavior honestly) to not overwrite an input source file that happens to have the same extension as an intermediate file. the corresponding intermediate file? you can grab it by renaming the source file before you use -save-temps.
• -save-temps=cwd — this is literally just -save-temps -dumpdir ./. same energy, different spelling. ✨
• -save-temps=obj — also literally equivalent to -save-temps -dumpdir outdir/, where outdir/ is the directory of the output file you gave after the -o option, directory separators and all. and if you didn't use -o at all? then -save-temps=obj just behaves like -save-temps=cwd. no -o, no problem. 🫶
• -time[=file] — this reports the CPU time each subprocess in the compilation sequence ate up ⏱️. for C source files that's the compiler proper and the assembler (plus the linker if linking is happening).

if you don't specify an output file, the output looks like this:

        # cc1 0.12 0.01
        # as 0.00 0.01

first number on each line = the "user time", aka time spent actually executing the program itself. second number = "system time", aka time spent executing operating system routines on behalf of the program. both are in seconds, no cap. 🕰️

if you do specify an output file, the output gets appended to that named file, and it looks like this instead:

        0.12 0.01 cc1 <options>
        0.00 0.01 as <options>

so now the "user time" and "system time" get moved up before the program name, and the options passed to the program get displayed too — so later you can tell exactly what file was being compiled, and with which options. detective mode, activated. 🔍

• -fdump-final-insns[=file] — dumps the final internal representation (RTL) to file. if you skip the optional argument (or if file is .), the dump file's name is figured out by appending .gkd to the dump base name — go look at -dumpbase for that. 📁
• -fcompare-debug[=opts] — if no error pops up during compilation, it runs the compiler a second time, adding opts and -fcompare-debug-second to the arguments passed to that second compilation. it dumps the final internal representation in both compilations and prints an error if they differ. 👯‍♀️

if you leave out the equal sign, the default -gtoggle is used.

the environment variable GCC_COMPARE_DEBUG, if it's defined, non-empty and nonzero, implicitly enables -fcompare-debug. if GCC_COMPARE_DEBUG is defined to a string that starts with a dash, then that string is used for opts; otherwise the default -gtoggle is used.

-fcompare-debug=, with the equal sign but no 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). and if you just want a warning instead? setting GCC_COMPARE_DEBUG to -w%n-fcompare-debug not overridden will do it. 🎀

• -fcompare-debug-second — this option gets implicitly passed to the compiler for the second compilation that -fcompare-debug requested, along with options to silence warnings, and while omitting other options that would make the compiler produce output to files or to standard output as a side effect. dump files and preserved temporary files get renamed so they contain the .gk additional extension during the second compilation, so they don't overwrite the ones the first compilation made. 🗂️

when this option is passed to the compiler driver, it causes the first compilation to be skipped — which makes it useful for little other than debugging the compiler proper. ngl, it's giving niche. 💀

• -gtoggle — turns off generation of debug info if leaving out this option generates it, or turns it on at level 2 otherwise. where you put this argument on the command line does not matter; it takes effect after all other options are processed, and it only does so once, no matter how many times you give it. mainly intended to be used with -fcompare-debug. 🔁
• -fvar-tracking-assignments-toggle — toggles -fvar-tracking-assignments, in the same way that -gtoggle toggles -g. 🪞
• -Q — makes the compiler print out each function name as it's compiled, and print some statistics about each pass when it finishes. chatty king. 🗣️
• -ftime-report — makes the compiler print some statistics to stderr about the time consumed by each pass when it finishes. ⏳

if SARIF output of diagnostics was requested via -fdiagnostics-format=sarif-file or -fdiagnostics-format=sarif-stderr, then the -ftime-report information is instead emitted in JSON form as part of the SARIF output. heads up: the precise format of this JSON data is subject to change, and the values may not exactly match those emitted to stderr, because they're written out at a slightly different place inside the compiler. 🙈

• -ftime-report-details — records the time consumed by infrastructure parts separately for each pass. 🔬
• -fira-verbose=n — controls the verbosity of the dump file for the integrated register allocator. default value is 5. if n is greater than or equal to 10, the dump output is sent to stderr using the same format as n minus 10. 🔢
• -flto-report — prints a report with internal details on the workings of the link-time optimizer. the contents of this report vary from version to version. it's meant to be useful to GCC developers when processing object files in LTO mode (via -flto). 👑

Disabled by default.

• -flto-report-wpa — like -flto-report, but it only prints for the WPA phase of link-time optimization. 💅
• -fmem-report — makes the compiler print some statistics about permanent memory allocation when it finishes. 🧠
• -fmem-report-wpa — makes the compiler print some statistics about permanent memory allocation for the WPA phase only. 🧠
• -fpre-ipa-mem-report
• -fpost-ipa-mem-report — makes the compiler print some statistics about permanent memory allocation before or after interprocedural optimization. before/after glow-up pics. 📸
• -fmultiflags — this option enables multilib-aware "TFLAGS" to be used to build target libraries with options different from those the compiler is configured to use by default, through the use of specs set up by compiler internals, by the target, or by builders at configure time. 🛠️

like "TFLAGS", this allows the target libraries to be built for portable baseline environments, while the compiler defaults to more demanding ones. that's useful because users can easily override the defaults the compiler is configured to use to build their own programs, if the defaults aren't ideal for their target environment — whereas rebuilding the runtime libraries is usually not as easy or desirable. 😮‍💨

unlike "TFLAGS", the use of specs enables different flags to be selected for different multilibs. the way to accomplish that is to build with make TFLAGS=-fmultiflags, after configuring --with-specs=%{fmultiflags:...}.

this option is discarded by the driver once it's done processing driver self spec.

it's also useful to check that "TFLAGS" are being used to build all target libraries, by configuring a non-bootstrap compiler --with-specs='%{!fmultiflags:%emissing TFLAGS}' and building the compiler and target libraries. ✅

• -fprofile-report — makes the compiler print some statistics about the consistency of the (estimated) profile and the effect of individual passes. 📊
• -fstack-usage — makes the compiler output stack usage information for the program, on a per-function basis. the filename for the dump is made by appending .su to the auxname. auxname is generated from the name of the output file, if it was explicitly specified and it isn't an executable; otherwise it's the basename of the source file. each 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: on top of 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 "bounded" 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. this option is supported only by the C++ front end, and the information is generally only useful to the G++ development team. 🔒
• -fdbg-cnt-list — prints the name and the counter upper bound for all debug counters. 🧮
• -fdbg-cnt=counter-value-list — sets the internal debug counter lower and upper bound. counter-value-list is a comma-separated list of name:lower_bound1-upper_bound1 [:lower_bound2-upper_bound2...] tuples, which sets the name of the counter and a list of closed intervals. the lower_bound is optional and is zero-initialized if not set. for example, with -fdbg-cnt=dce:2-4:10-11,tail_call:10, dbg_cnt(dce) returns true only for the second, third, fourth, tenth and eleventh invocation. for dbg_cnt(tail_call), true is returned for the first 10 invocations. 🎯
• -print-file-name=library — prints the full absolute name of the library file library that would be used when linking — and does nothing else. with this option, GCC doesn't compile or link anything; it just prints the file name. it's giving "just here to tell you where the thing is, bestie". 📍
• -print-multi-directory — spits out the directory name that matches whichever multilib got picked by all the other switches hanging out in your command line. That directory is supposed to already exist inside GCC_EXEC_PREFIX. 💅
• -print-multi-lib — prints the mapping from multilib directory names over to the compiler switches that turn them on. The directory name gets separated from the switches by a ;, and every switch starts with an @ instead of a -, with no spaces between multiple switches. Why so weird? It's literally just to make shell processing easier for you, bestie. 🐚✨
• -print-multi-os-directory — prints the path to the OS libraries for the multilib that got selected, relative to some lib subdirectory. Okay so the vibe check here: if OS libraries live in the lib subdirectory and no multilibs are in play, this is usually just .. If OS libraries live in libsuffix sibling directories, it prints things like ../lib64, ../lib or ../lib32. And if they live in lib/subdir subdirectories, it prints stuff like amd64, sparcv9 or ev6. 🌈
• -print-multiarch — prints the path to the OS libraries for the selected multiarch, relative to some lib subdirectory. Simple as that, no cap.
• -print-prog-name=program — works like -print-file-name, but it goes hunting for a program, like cpp, instead of a library. 🔍
• -print-libgcc-file-name — same thing as -print-file-name=libgcc.a, bestie.

This one is a whole lifesaver when you're using -nostdlib or -nodefaultlibs but you do still want to link against libgcc.a. You can literally do:

gcc -nostdlib <files>... `gcc -print-libgcc-file-name`

Chef's kiss. 👑

• -print-search-dirs — prints the name of the configured installation directory plus a list of the program and library directories gcc searches — and then it does absolutely nothing else. Periodt.

This is the one you want when gcc hits you with the error installation problem, cannot exec cpp0: No such file or directory. To fix that, you either gotta put cpp0 and the rest of the compiler components exactly where gcc expects to find them, or you set the environment variable GCC_EXEC_PREFIX to the directory where you installed them. Don't forget the trailing /. 😭

• -print-sysroot — prints the target sysroot directory that's used during compilation. That's the target sysroot you specified either at configure time or with the --sysroot option, possibly with an extra suffix that depends on your compilation options. And if there's no target sysroot specified? It prints nothing. 💀
• -print-sysroot-headers-suffix — prints the suffix that gets added to the target sysroot when it's searching for headers, or it throws an error if the compiler wasn't configured with such a suffix — and don't do anything else, fr fr.
• -dumpmachine — prints the compiler's target machine (like i686-pc-linux-gnu) — and don't do anything else. 🖥️
• -dumpversion — prints the compiler version (like 3.0, 6.3.0 or 7) — and don't do anything else. This is the compiler version that shows up in filesystem paths and specs. Depending on how the compiler was configured, it can be just a single number (major version), two numbers separated by a dot (major and minor version), or three numbers separated by dots (major, minor and patchlevel version). So basically, it's giving mystery until you look. ✨
• -dumpfullversion — prints the full compiler version — and don't do anything else. This one's output is always three numbers separated by dots: major, minor and patchlevel version. No surprises, iconic behavior. 💖
• -dumpspecs — prints the compiler's built-in specs — and don't do anything else. (This one gets used when GCC itself is being built, so it's very much behind-the-scenes main character energy.) 🎀

---

🦋 Machine-Dependent Options 🦋

Every target machine that GCC supports can have its own lil set of options — like, to let you compile for a specific processor variant or ABI, or to control optimizations that are specific to that machine. By convention, the names of machine-specific options all start with -m. 🧚

Some configurations of the compiler also support extra target-specific options, usually just so they play nice 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. The permissible values are ilp32 for a SysV-like data model where int, long int and pointers are 32 bits, and lp64 for a 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. Heads up though: the LP64 and ILP32 ABIs are NOT link-compatible — you have to compile your entire program with the same ABI, and link it with a compatible set of libraries. Mixing? Absolutely not, bestie. 🚫

• -mbig-endian — generates big-endian code. This is the default when GCC is configured for an aarch64_be-*-* target.
• -mgeneral-regs-only — generates code that uses only the general-purpose registers. This stops the compiler from using floating-point and Advanced SIMD registers, but it will not put any restrictions on the assembler. 🙈
• -mlittle-endian — generates little-endian code. This is the default when GCC is configured for an aarch64-*-* target but not an aarch64_be-*-* target.
• -mcmodel=tiny — generates code for the tiny code model. The program and its statically defined symbols must be within 1MB of each other. Programs can be statically or dynamically linked.
• -mcmodel=small — generates code for the small code model. The program and its statically defined symbols must be within 4GB of each other. Programs can be statically or dynamically linked. This is the default code model. ⭐
• -mcmodel=large — generates code for the large code model. This makes no assumptions about addresses and sizes of sections. Programs can be statically linked only. Also, -mcmodel=large is incompatible with -mabi=ilp32, -fpic and -fPIC. So don't try it, it's giving chaos. 💀
• -mtp=name — specifies the system register to use as a thread pointer. Valid values are tpidr_el0, tpidrro_el0, tpidr_el1, tpidr_el2, tpidr_el3. For backwards compatibility, the aliases el0, el1, el2, el3 are also accepted. The default setting is tpidr_el0. It's recommended that you compile all code that's meant to interoperate with the same value of this option, so you don't end up accessing a different thread pointer from the wrong exception level. 🧵
• -mstrict-align, -mno-strict-align — avoid or allow generating memory accesses that may not be aligned on a natural object boundary, as described in the architecture specification.
• -momit-leaf-frame-pointer, -mno-omit-leaf-frame-pointer — omit or keep the frame pointer in leaf functions. The former behavior (omitting it) is the default.
• -mstack-protector-guard=guard, -mstack-protector-guard-reg=reg, -mstack-protector-guard-offset=offset — generate stack protection code using a canary at guard. The supported locations are global for a global canary, or sysreg for a canary in an appropriate system register. 🐤

With that latter choice, the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset also specify which system register to use as the base register for reading the canary, and what offset from that base register to read it from. There is no default register or offset, because this is entirely for use inside the Linux kernel.

• -mtls-dialect=desc — use TLS descriptors as the thread-local storage mechanism for dynamic accesses of TLS variables. This is the default.
• -mtls-dialect=traditional — use traditional TLS as the thread-local storage mechanism for dynamic accesses of TLS variables.
• -mtls-size=size — specify the bit size of immediate TLS offsets. Valid values are 12, 24, 32, 48. This option requires binutils 2.26 or newer, so check before you commit. 👑
• -mfix-cortex-a53-835769, -mno-fix-cortex-a53-835769 — enable or disable the workaround for the ARM Cortex-A53 erratum number 835769. This involves inserting a NOP instruction between memory instructions and 64-bit integer multiply-accumulate instructions.
• -mfix-cortex-a53-843419, -mno-fix-cortex-a53-843419 — enable or disable the workaround for the ARM Cortex-A53 erratum number 843419. This erratum workaround gets made at link time, and this will only pass the corresponding flag along to the linker.
• -mlow-precision-recip-sqrt, -mno-low-precision-recip-sqrt — enable or disable the reciprocal square root approximation. This option only does anything if -ffast-math or -funsafe-math-optimizations is used as well. Enabling it reduces the precision of reciprocal square root results to about 16 bits for single precision and to 32 bits for double precision. 🔢
• -mlow-precision-sqrt, -mno-low-precision-sqrt — enable or disable the square root approximation. This one also only has an effect if -ffast-math or -funsafe-math-optimizations is used as well. Enabling it reduces the precision of square root results to about 16 bits for single precision and to 32 bits for double precision. And if you enable it, it implies -mlow-precision-recip-sqrt. Chain reaction, lowkey.
• -mlow-precision-div, -mno-low-precision-div — enable or disable the division approximation. Same deal: only has an effect if -ffast-math or -funsafe-math-optimizations is used as well. Enabling it reduces the precision of division results to about 16 bits for single precision and to 32 bits for double precision.
• -mtrack-speculation, -mno-track-speculation — enable or disable the generation of additional code to track speculative execution through conditional branches. The tracking state can then be used by the compiler when expanding calls to "__builtin_speculation_safe_copy", so a more efficient code sequence can be generated. 🕵️
• -moutline-atomics, -mno-outline-atomics — enable or disable calls to out-of-line helpers that implement atomic operations. These helpers will, at runtime, figure out if the LSE instructions from ARMv8.1-A can be used; if they can't, they'll use the load/store-exclusive instructions that are present in the base ARMv8.0 ISA.

This option is only applicable when you're compiling for the base ARMv8.0 instruction set. If you're using a later revision, e.g. -march=armv8.1-a or -march=armv8-a+lse, the ARMv8.1-Atomics instructions get used directly. Same thing applies when you use -mcpu= and the selected cpu supports the lse feature. This option is on by default. 💅

• -march=name — specify the name of the target architecture, and optionally one or more feature modifiers. This option has the form -march=arch{+[no]feature}*.

The table below summarizes the permissible values for arch and the features that they enable by default:

okay bestie, we're deep in the AArch64 arc now and the lore is REAL. Here's the chart of -march= values and what they drag in by default — the columns are literally arch value : Architecture : Includes by default:

• armv8-a — Armv8-A — includes +fp, +simd
• armv8.1-a — Armv8.1-A — includes armv8-a, +crc, +lse, +rdma
• armv8.2-a — Armv8.2-A — includes armv8.1-a
• armv8.3-a — Armv8.3-A — includes armv8.2-a, +pauth
• armv8.4-a — Armv8.4-A — includes armv8.3-a, +flagm, +fp16fml, +dotprod
• armv8.5-a — Armv8.5-A — includes armv8.4-a, +sb, +ssbs, +predres
• armv8.6-a — Armv8.6-A — includes armv8.5-a, +bf16, +i8mm
• armv8.7-a — Armv8.7-A — includes armv8.6-a, +ls64
• armv8.8-a — Armv8.8-a — includes armv8.7-a, +mops
• armv8.9-a — Armv8.9-a — includes armv8.8-a
• armv9-a — Armv9-A — includes armv8.5-a, +sve, +sve2
• armv9.1-a — Armv9.1-A — includes armv9-a, +bf16, +i8mm
• armv9.2-a — Armv9.2-A — includes armv9.1-a, +ls64
• armv9.3-a — Armv9.3-A — includes armv9.2-a, +mops
• armv9.4-a — Armv9.4-A — includes armv9.3-a
• armv8-r — Armv8-R — includes armv8-r

The value native is available on native AArch64 GNU/Linux and it makes the compiler just pick the architecture of the host system. 🫶 This option has no effect if the compiler is unable to recognize the architecture of the host system,

The permissible values for feature are listed in the sub-section on aarch64-feature-modifiers,,-march and -mcpu Feature Modifiers. Where conflicting feature modifiers are specified, the right-most feature is used. (right-most wins, bestie, like who spoke last in the group chat 💅)

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. The 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, neoverse-512tvb, neoverse-e1, neoverse-n1, neoverse-n2, neoverse-v1, neoverse-v2, grace, qdf24xx, saphira, phecda, xgene1, vulcan, octeontx, octeontx81, octeontx83, octeontx2, octeontx2t98, octeontx2t96 octeontx2t93, octeontx2f95, octeontx2f95n, octeontx2f95mm, a64fx, thunderx, thunderxt88, thunderxt88p1, thunderxt81, tsv110, thunderxt83, thunderx2t99, thunderx3t110, zeus, cortex-a57.cortex-a53, cortex-a72.cortex-a53, cortex-a73.cortex-a35, cortex-a73.cortex-a53, cortex-a75.cortex-a55, cortex-a76.cortex-a55, cortex-r82, cortex-x1, cortex-x1c, cortex-x2, cortex-x3, cortex-x4, cortex-a510, cortex-a520, cortex-a710, cortex-a715, cortex-a720, ampere1, ampere1a, ampere1b, cobalt-100 and native.

The values cortex-a57.cortex-a53, cortex-a72.cortex-a53, cortex-a73.cortex-a35, cortex-a73.cortex-a53, cortex-a75.cortex-a55, cortex-a76.cortex-a55 specify that GCC should tune for a big.LITTLE system. (yes, both brains, one vibe 🧠🧠)

The value neoverse-512tvb specifies that GCC should tune for Neoverse cores that (a) implement SVE and (b) have a total vector bandwidth of 512 bits per cycle. In other words, the option tells GCC to tune for Neoverse cores that can execute 4 128-bit Advanced SIMD arithmetic instructions a cycle and that can execute an equivalent number of SVE arithmetic instructions per cycle (2 for 256-bit SVE, 4 for 128-bit SVE). This is more general than tuning for a specific core like Neoverse V1 but is more specific than the default tuning described below.

Additionally on native AArch64 GNU/Linux systems the value native tunes performance to the host system. This option has no effect if the compiler is unable to recognize the processor of the host system.

Where none of -mtune=, -mcpu= or -march= are specified, the code is tuned to perform well across a range of target processors.

This option cannot be suffixed by feature modifiers.

• -mcpu=name — Specify the name of the target processor, optionally suffixed by one or more feature modifiers. This option has the form -mcpu=cpu{+[no]feature}*, where the permissible values for cpu are the same as those available for -mtune. The permissible values for feature are documented in the sub-section on aarch64-feature-modifiers,,-march and -mcpu Feature Modifiers. Where conflicting feature modifiers are specified, the right-most feature is used.

GCC uses name to determine what kind of instructions it can emit when generating assembly code (as if by -march) and to determine the target processor for which to tune for performance (as if by -mtune). Where this option is used in conjunction with -march or -mtune, those options take precedence over the appropriate part of this option.

-mcpu=neoverse-512tvb is special in that it does not refer to a specific core, but instead refers to all Neoverse cores that (a) implement SVE and (b) have a total vector bandwidth of 512 bits a cycle. Unless overridden by -march, -mcpu=neoverse-512tvb generates code that can run on a Neoverse V1 core, since Neoverse V1 is the first Neoverse core with these properties. Unless overridden by -mtune, -mcpu=neoverse-512tvb tunes code in the same way as for -mtune=neoverse-512tvb.

• -moverride=string — Override tuning decisions made by the back-end in response to a -mtune= switch. The syntax, semantics, and accepted values for string in this option are not guaranteed to be consistent across releases. 😭

This option is only intended to be useful when developing GCC.

• -mverbose-cost-dump — Enable verbose cost model dumping in the debug dump files. This option is provided for use in debugging the compiler.
• -mpc-relative-literal-loads, -mno-pc-relative-literal-loads — Enable or disable PC-relative literal loads. With this option literal pools are accessed using a single instruction and emitted after each function. This limits the maximum size of functions to 1MB. This is enabled by default for -mcmodel=tiny.
• -msign-return-address=scope — Select the function scope on which return address signing will be applied. Permissible values are none, which disables return address signing, non-leaf, which enables pointer signing for functions which are not leaf functions, and all, which enables pointer signing for all functions. The default value is none. This option has been deprecated by -mbranch-protection.
• -mbranch-protection=none|standard|pac-ret[+leaf+b-key]|bti — Select the branch protection features to use. none is the default and turns off all types of branch protection. standard turns on all types of branch protection features. If a feature has additional tuning options, then standard sets it to its standard level. pac-ret[+leaf] turns on return address signing to its standard level: signing functions that save the return address to memory (non-leaf functions will practically always do this) using the a-key. The optional argument leaf can be used to extend the signing to include leaf functions. The optional argument b-key can be used to sign the functions with the B-key instead of the A-key. bti turns on branch target identification mechanism. 🛡️
• -mharden-sls=opts — Enable compiler hardening against straight line speculation (SLS). opts is a comma-separated list of the following options:
  • retbr
  • blr

In addition, -mharden-sls=all enables all SLS hardening while -mharden-sls=none disables all SLS hardening.

• -mearly-ra=scope — Determine when to enable an early register allocation pass. This pass runs before instruction scheduling and tries to find a spill-free allocation of floating-point and vector code. It also tries to make use of strided multi-register instructions, such as SME2's strided LD1 and ST1.

The possible values of scope are: all, which runs the pass on all functions; strided, which runs the pass on functions that have access to strided multi-register instructions; and none, which disables the pass.

-mearly-ra=all is the default for -O2 and above, and for -Os. -mearly-ra=none is the default otherwise.

• -mearly-ldp-fusion — Enable the copy of the AArch64 load/store pair fusion pass that runs before register allocation. Enabled by default at -O and above. ✨
• -mlate-ldp-fusion — Enable the copy of the AArch64 load/store pair fusion pass that runs after register allocation. Enabled by default at -O and above. ✨
• -msve-vector-bits=bits — Specify the number of bits in an SVE vector register. This option only has an effect when SVE is enabled.

GCC supports two forms of SVE code generation: "vector-length agnostic" output that works with any size of vector register and "vector-length specific" output that allows GCC to make assumptions about the vector length when it is useful for optimization reasons. The possible values of bits are: scalable, 128, 256, 512, 1024 and 2048. Specifying scalable selects vector-length agnostic output. At present -msve-vector-bits=128 also generates vector-length agnostic output for big-endian targets. All other values generate vector-length specific code. The behavior of these values may change in future releases and no value except scalable should be relied on for producing code that is portable across different hardware SVE vector lengths.

The default is -msve-vector-bits=scalable, which produces vector-length agnostic code.

---

✨ -march and -mcpu Feature Modifiers ✨

Feature modifiers used with -march and -mcpu can be any of the following and their inverses nofeature:

• crc — Enable CRC extension. This is on by default for -march=armv8.1-a.
• crypto — Enable Crypto extension. This also enables Advanced SIMD and floating-point instructions.
• fp — Enable floating-point instructions. This is on by default for all possible values for options -march and -mcpu.
• simd — Enable Advanced SIMD instructions. This also enables floating-point instructions. This is on by default for all possible values for options -march and -mcpu.
• sve — Enable Scalable Vector Extension instructions. This also enables Advanced SIMD and floating-point instructions.
• lse — Enable Large System Extension instructions. This is on by default for -march=armv8.1-a.
• rdma — Enable Round Double Multiply Accumulate instructions. This is on by default for -march=armv8.1-a.
• fp16 — turns on the FP16 extension ✨ and (plot twist) that also enables the floating-point instructions. two for one, we love to see it 💖
• fp16fml — enables the FP16 fmla extension 🎀 which also brings in FP16 extensions and floating-point instructions. It's on by default for -march=armv8.4-a. Warning tho: using this with architectures before Armv8.2-A is not supported, so don't even try it bestie 🙈
• rcpc — enables the RCpc extension. This lets you use the LDAPR instructions for load-acquire atomic semantics, and it passes that on to the assembler, which means inline asm statements can use instructions from the RCpc extension 💅
• dotprod — enables the Dot Product extension. Also enables Advanced SIMD instructions, obvi 🌸
• aes — enables the Armv8-a aes and pmull crypto extension. Also enables Advanced SIMD instructions 🔥
• sha2 — enables the Armv8-a sha2 crypto extension. Also enables Advanced SIMD instructions ⭐
• sha3 — enables the sha512 and sha3 crypto extension. Also enables Advanced SIMD instructions. But fr: using this with architectures prior to Armv8.2-A is not supported 🚫
• sm4 — enables the sm3 and sm4 crypto extension. Also enables Advanced SIMD instructions. And again, architectures prior to Armv8.2-A? Not supported 💀
• profile — enables the Statistical Profiling extension. Ngl this one is only about enabling the extension at the assembler level — it does not affect code generation 🧚
• rng — enables the Armv8.5-a Random Number instructions. Same deal: assembler-level only, no effect on code generation 🙈
• memtag — enables the Armv8.5-a Memory Tagging Extensions. Using this with architectures prior to Armv8.5-A is not supported ⚠️
• sb — enables the Armv8-a Speculation Barrier instruction. Assembler-level only, doesn't affect code generation. This one is on by default for -march=armv8.5-a ✨
• ssbs — enables the Armv8-a Speculative Store Bypass Safe instruction. Assembler-level only, no code generation effect. On by default for -march=armv8.5-a 🎀
• predres — enables the Armv8-a Execution and Data Prediction Restriction instructions. Assembler-level only, doesn't touch code generation. Also on by default for -march=armv8.5-a 💖
• sve2 — enables the Armv8-a Scalable Vector Extension 2. This also enables SVE instructions, bestie 🦋
• sve2-bitperm — enables SVE2 bitperm instructions. Also enables SVE2 instructions 🌈
• sve2-sm4 — enables SVE2 sm4 instructions. Also enables SVE2 instructions 💜
• sve2-aes — enables SVE2 aes instructions. Also enables SVE2 instructions ⭐
• sve2-sha3 — enables SVE2 sha3 instructions. Also enables SVE2 instructions 🌸
• tme — enables the Transactional Memory Extension 👑
• i8mm — enables 8-bit Integer Matrix Multiply instructions. Also enables Advanced SIMD and floating-point instructions. On by default for -march=armv8.6-a. And yeah, architectures prior to Armv8.2-A: not supported 😭
• f32mm — enables 32-bit Floating point Matrix Multiply instructions. Also enables SVE instructions. Architectures prior to Armv8.2-A? Not supported, periodt 💅
• f64mm — enables 64-bit Floating point Matrix Multiply instructions. Also enables SVE instructions. And again, architectures prior to Armv8.2-A are not supported 🚫
• bf16 — enables brain half-precision floating-point instructions. Also enables Advanced SIMD and floating-point instructions. On by default for -march=armv8.6-a. Prior to Armv8.2-A = not supported 💀
• ls64 — enables the 64-byte atomic load and store instructions for accelerators. On by default for -march=armv8.7-a ✨
• mops — enables the instructions that accelerate memory operations like memcpy, memmove, memset. On by default for -march=armv8.8-a 🎀
• flagm — enables the Flag Manipulation instructions Extension 🚩
• pauth — enables the Pointer Authentication Extension 🔐
• cssc — enables the Common Short Sequence Compression instructions 🗜️
• sme — enables the Scalable Matrix Extension 💅
• sme-i16i64 — enables the FEAT_SME_I16I64 extension to SME 🧮
• sme-f64f64 — enables the FEAT_SME_F64F64 extension to SME. +@item sme2 Enable the Scalable Matrix Extension 2. This also enables SME instructions. 🙈 (yes bestie, that artifact is literally sitting in the source, we don't make the rules, we just report them 💀)
• lse128 — enables the LSE128 128-bit atomic instructions extension. Also enables LSE instructions 🔥
• d128 — enables support for 128-bit system register read/write instructions. Also enables the LSE128 extension ⭐
• gcs — enables support for the Armv9.4-a Guarded Control Stack extension 🛡️
• the — enables support for the Armv8.9-a/9.4-a translation hardening extension 🔒
• rcpc3 — enables the RCpc3 (Release Consistency) extension 🌸

Okay so the vibe of these implications is everything is connected: Feature crypto implies aes, sha2, and simd, which implies fp. And the other direction? nofp implies nosimd, which implies nocrypto, noaes and nosha2. It's giving dominoes, fr fr 🁢

---

🌟 Adapteva Epiphany Options 🌟

These -m options are defined for Adapteva Epiphany, bestie, and they're a whole little world of their own 🦋

• -mhalf-reg-file — don't allocate any register in the range r32...r63 💅 That's what lets your code actually run on hardware variants that straight-up don't have those registers.
• -mprefer-short-insn-regs — preferentially allocate registers that allow short instruction generation ✨ This can result in increased instruction count, so it might either reduce or increase your overall code size. Chaos, honestly 🎀
• -mbranch-cost=num — set the cost of branches to roughly num "simple" instructions. Heads up: this cost is only a heuristic and is not guaranteed to produce consistent results across releases 🙈
• -mcmove — enables the generation of conditional moves 🔀
• -mnops=num — emit num NOPs before every other generated instruction. yes, every other one. iconique 🫶
• -mno-soft-cmpsf — for single-precision floating-point comparisons, emit an fsub instruction and test the flags. Faster than a software comparison, but it 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. Pick your fighter 💅
• -mstack-offset=num — set the offset between the top of the stack and the stack pointer. E.g., a value of 8 means that the eight bytes in the range sp+0...sp+7 can be used by leaf functions without stack allocation. Values other than 8 or 16 are untested and unlikely to work 🙃 Note also that this option changes the ABI; compiling a program with a different stack offset than the libraries have been compiled with generally does not work 🚫 This option can be useful if you want to evaluate if a different stack offset would give you better code, but to actually use a different stack offset to build working programs, it is recommended to configure the toolchain with the appropriate --with-stack-offset=num option ✨
• -mno-round-nearest — make the scheduler assume that the rounding mode has been set to truncating. The default is -mround-nearest 🎀
• -mlong-calls — if not otherwise specified by an attribute, assume all calls might be beyond the offset range of the b / bl instructions, and therefore load the function address into a register before performing an (otherwise direct) call. This is the default ⭐
• -mshort-calls — if not otherwise specified by an attribute, assume all direct calls are in the range of the b / bl instructions, so use these instructions for direct calls. The default is -mlong-calls 💖
• -msmall16 — assume addresses can be loaded as 16-bit unsigned values. This does not apply to function addresses for which -mlong-calls semantics are in effect 🙅
• -mfp-mode=mode — set the prevailing mode of the floating-point unit 🌸 This determines the floating-point mode that is provided and expected at function call and return time. Making this mode match the mode you predominantly need at function start can make your programs smaller and faster by avoiding unnecessary mode switches 🔥

mode can be set to one of the following values:

• caller — any mode at function entry is valid, and retained or restored when the function returns, and when it calls other functions. This mode is useful for compiling libraries or other compilation units you might want to incorporate into different programs with different prevailing FPU modes, and the convenience of being able to use a single object file outweighs the size and speed overhead for any extra mode switching that might be needed, compared with what would be needed with a more specific choice of prevailing FPU mode 🫶
• truncate — this is the mode used for floating-point calculations with truncating (i.e. round towards zero) rounding mode. That includes conversion from floating point to integer ✂️
• round-nearest — this is the mode used for floating-point calculations with round-to-nearest-or-even rounding mode 🎯
• int — this is the mode used to perform integer calculations in the FPU, e.g. integer multiply, or integer multiply-and-accumulate 🧮

The default is -mfp-mode=caller 💅

• -mno-split-lohi, -mno-postinc, -mno-postmodify — code generation tweaks that disable, respectively, splitting of 32-bit loads, generation of post-increment addresses, and generation of post-modify addresses 🛠️ The defaults are msplit-lohi, -mpost-inc, and -mpost-modify (yes, those exact spellings, we don't editorialize 💀).
• -mnovect-double — change the preferred SIMD mode to SImode. The default is -mvect-double, which uses DImode as preferred SIMD mode 🦋
• -max-vect-align=num — the maximum alignment for SIMD vector mode types. num may be 4 or 8. The default is 8 ⭐ Note that this is an ABI change, even though many library function interfaces are unaffected if they don't use SIMD vector modes in places that affect size and/or alignment of relevant types ⚠️
• -msplit-vecmove-early — split vector moves into single word moves before reload 🧩 In theory this can give better register allocation, but so far the reverse seems to be generally the case 😭
• -m1reg-reg — pick a register to hold the constant -1, which makes loading small negative constants and certain bitmasks faster ✨ Allowable values for reg are r43 and r63 (which mean "use that register as a fixed register"), and none, which means no register gets used for this at all. The default is -m1reg-none 💅

---

🌈 AMD GCN Options 🌈

These options are defined specifically for the AMD GCN port, bestie. Very niche, very demure, very GPU 🎀

• -march=gpu
• -mtune=gpu — set the architecture type or tuning for gpu. Supported values for gpu are:
  • fiji — compile for GCN3 Fiji devices (gfx803). Support is deprecated; availability depends on how GCC has been configured, see --with-arch and --with-multilib-list 👀
  • gfx900 — compile for GCN5 Vega 10 devices (gfx900).
  • gfx906 — compile for GCN5 Vega 20 devices (gfx906).
  • gfx908 — compile for CDNA1 Instinct MI100 series devices (gfx908).
  • gfx90a — compile for CDNA2 Instinct MI200 series devices (gfx90a).
  • gfx90c — compile for GCN5 Vega 7 devices (gfx90c).
  • gfx1030 — compile for RDNA2 gfx1030 devices (GFX10 series).
  • gfx1036 — compile for RDNA2 gfx1036 devices (GFX10 series).
  • gfx1100 — compile for RDNA3 gfx1100 devices (GFX11 series).
  • gfx1103 — compile for RDNA3 gfx1103 devices (GFX11 series).
• -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, no cap. The default is any, for devices that support it.
• -mstack-size=bytes — specify how many bytes of stack space will be requested for each GPU thread (wave-front) 🌸 Beware, bestie: there may be many threads and limited memory available 😭 The size of the stack allocation may also have an impact on run-time performance. The default is 32KB when using OpenACC or OpenMP, and 1MB otherwise.
• -mxnack=on
• -mxnack=off
• -mxnack=any — compile binaries suitable for devices with the XNACK feature enabled, disabled, or either mode ✨ Some devices always require XNACK and some allow the user to configure XNACK. The compiled code must match the device mode. The default is -mxnack=any on devices that support Unified Shared Memory, and -mxnack=no otherwise.

---

💜 ARC Options 💜

The following options control the architecture variant for which code is being compiled, iykyk:

• -mbarrel-shifter — generate instructions supported by barrel shifter 🌀 This is the default unless -mcpu=ARC601 or -mcpu=ARCEM is in effect.
• -mjli-always — force to call a function using jli_s instruction. This option is valid only for ARCv2 architecture.
• -mcpu=cpu — set architecture type, register usage, and instruction scheduling parameters for cpu 💅 There are also shortcut alias options available for backward compatibility and convenience. Supported values for cpu are:
  • arc600 — compile for ARC600. Aliases: -mA6, -mARC600.
  • arc601 — compile for ARC601. Alias: -mARC601.
  • arc700 — compile for ARC700. Aliases: -mA7, -mARC700. This is the default when configured with --with-cpu=arc700.
  • arcem — compile for ARC EM.
  • archs — compile for ARC HS.
  • em — compile for ARC EM CPU with no hardware extensions.
  • em4 — compile for ARC EM4 CPU.
  • em4_dmips — compile for ARC EM4 DMIPS CPU.
  • em4_fpus — compile for ARC EM4 DMIPS CPU with the single-precision floating-point extension.
  • em4_fpuda — compile for ARC EM4 DMIPS CPU with single-precision floating-point and double assist instructions.
  • hs — compile for ARC HS CPU with no hardware extensions except the atomic instructions.
  • hs34 — compile for ARC HS34 CPU.
  • hs38 — compile for ARC HS38 CPU.
  • hs38_linux — compile for ARC HS38 CPU with all hardware extensions on.
  • hs4x — compile for ARC HS4x CPU.
  • hs4xd — compile for ARC HS4xD CPU.
  • hs4x_rel31 — compile for ARC HS4x CPU release 3.10a.
  • arc600_norm — compile for ARC 600 CPU with "norm" instructions enabled.
  • arc600_mul32x16 — compile for ARC 600 CPU with "norm" and 32x16-bit multiply instructions enabled.
  • arc600_mul64 — compile for ARC 600 CPU with "norm" and "mul64"-family instructions enabled.
  • arc601_norm — compile for ARC 601 CPU with "norm" instructions enabled.
  • arc601_mul32x16 — compile for ARC 601 CPU with "norm" and 32x16-bit multiply instructions enabled.
  • arc601_mul64 — compile for ARC 601 CPU with "norm" and "mul64"-family instructions enabled.
  • nps400 — compile for ARC 700 on NPS400 chip.
  • em_mini — compile for ARC EM minimalist configuration featuring reduced register set.
• -mdpfp
• -mdpfp-compact — generate double-precision FPX instructions, tuned for the compact implementation 🎀
• -mdpfp-fast — generate double-precision FPX instructions, tuned for the fast implementation 🔥
• -mno-dpfp-lrsr — disable "lr" and "sr" instructions from using FPX extension aux registers.
• -mea — generate extended arithmetic instructions ✨ Currently only divaw, adds, subs, and sat16 are supported. Only valid for -mcpu=ARC700.
• -mno-mpy — do not generate "mpy"-family instructions for ARC700. This option is deprecated. 💀
• -mmul32x16 — generate 32x16-bit multiply and multiply-accumulate instructions.
• -mmul64 — generate mul64 and mulu64 instructions. Only valid for -mcpu=ARC600.
• -mnorm — generate "norm" instructions. This is the default if -mcpu=ARC700 is in effect.
• -mspfp
• -mspfp-compact — generate single-precision FPX instructions, tuned for the compact implementation 🌸
• -mspfp-fast — generate single-precision FPX instructions, tuned for the fast implementation ⭐
• -msimd — enable generation of ARC SIMD instructions via target-specific builtins. Only valid for -mcpu=ARC700.
• -msoft-float — this option is ignored; it is provided for compatibility purposes only 🫠 Software floating-point code is emitted by default, and this default can be overridden by FPX options: -mspfp, -mspfp-compact, or -mspfp-fast for single precision, and -mdpfp, -mdpfp-compact, or -mdpfp-fast for double precision.
• -mswap — generate "swap" instructions.
• -matomic — this enables use of the locked load/store conditional extension to implement atomic memory built-in functions. Not available for ARC 6xx or ARC EM cores 🙅♀️
• -mdiv-rem — enable div and rem instructions for ARCv2 cores.
• -mcode-density — enable code density instructions for ARC EM. This option is on by default for ARC HS.
• -mll64 — enable double load/store operations for ARC HS cores.
• -mtp-regno=regno — specify thread pointer register number.
• -mmpy-option=multo — compile ARCv2 code with a multiplier design option 💅 You can specify the option using either a string or numeric value for multo. wlh1 is the default value. The recognized values are:
  • 0, none — no multiplier available.
  • 1, w — 16x16 multiplier, fully pipelined. The following instructions are enabled: mpyw and mpyuw.
  • 2, wlh1 — 32x32 multiplier, fully pipelined (1 stage). The following instructions are additionally enabled: mpy, mpyu, mpym, mpymu, and mpy_s.
  • 3, wlh2 — 32x32 multiplier, fully pipelined (2 stages). The following instructions are additionally enabled: mpy, mpyu, mpym, mpymu, and mpy_s.
  • 4, wlh3 — two 16x16 multipliers, blocking, sequential. The following instructions are additionally enabled: mpy, mpyu, mpym, mpymu, and mpy_s.
  • 5, wlh4 — one 16x16 multiplier, blocking, sequential. The following instructions are additionally enabled: mpy, mpyu, mpym, mpymu, and mpy_s.
  • 6, wlh5 — one 32x4 multiplier, blocking, sequential. The following instructions are additionally enabled: mpy, mpyu, mpym, mpymu, and mpy_s.
  • 7, plus_dmpy — ARC HS SIMD support.
  • 8, plus_macd — ARC HS SIMD support.
  • 9, plus_qmacw — ARC HS SIMD support.

This option is only available for ARCv2 cores 👑

• -mfpu=fpu — enables support for specific floating-point hardware extensions for ARCv2 cores 🦋 Supported values for fpu are:
  • fpus — enables support for single-precision floating-point hardware extensions.
  • fpud — enables support for double-precision floating-point hardware extensions. The single-precision floating-point extension is also enabled. Not available for ARC EM.
  • fpuda — enables support for double-precision floating-point hardware extensions using double-precision assist instructions. The single-precision floating-point extension is also enabled. This option is only available for ARC EM.
  • fpuda_div — enables support for double-precision floating-point hardware extensions using double-precision assist instructions. The single-precision floating-point, square-root, and divide extensions are also enabled. This option is only available for ARC EM.
  • fpuda_fma — enables support for double-precision floating-point hardware extensions using double-precision assist instructions. The single-precision floating-point and fused multiply and add hardware extensions are also enabled. This option is only available for ARC EM.
  • fpuda_all — enables support for double-precision floating-point hardware extensions using double-precision assist instructions. All single-precision floating-point hardware extensions are also enabled. This option is only available for ARC EM.
• fpus_div — bestie this one turns on support for the single-precision floating-point, square-root and divide hardware extensions 🍓
• fpud_div — and THIS one is the double-precision glow-up: floating-point, square-root and divide hardware extensions. It lowkey includes fpus_div too, so you get both. Not available for ARC EM tho, sorry bestie 😭
• fpus_fma — enables support for single-precision floating-point plus fused multiply-and-add hardware extensions ✨
• fpud_fma — double-precision floating-point plus fused multiply-and-add hardware extensions. It includes fpus_fma, so it's giving two-for-one energy. Not available for ARC EM 💀
• fpus_all — enables support for ALL the single-precision floating-point hardware extensions. periodt 👑
• fpud_all — enables support for all the single- and double-precision floating-point hardware extensions. Not available for ARC EM 🙈
• -mirq-ctrl-saved=register-range, blink, lp_count — this one tells the compiler which general-purpose registers the processor auto-saves/restores on interrupt entry and exit. The register-range is written as two registers with a dash between them. The range always starts with "r0", and the upper limit is the "fp" register. blink and lp_count are optional. Heads up: this option is only valid for ARC EM and ARC HS cores 💅
• -mrgf-banked-regs=number — sets how many registers get replicated in the second register bank when you enter a fast interrupt. Fast interrupts = interrupts at the highest priority level P0. These only save the PC and STATUS32 registers so there's no memory traffic during interrupt entry and exit. Use this when you're doing fast interrupts on an ARC V2 family processor. The permitted values are 4, 8, 16, and 32 🎀
• -mlpc-width=width — picks the width of the "lp_count" register. Valid width values are 8, 16, 20, 24, 28 and 32 bits, and the default is locked in at 32 bits. If your width is under 32, the compiler won't try to turn your loops into the zero-delay loop mechanism unless it KNOWS the "lp_count" register can hold the needed loop-counter value. Depending on the width you pick, the compiler and run-time library might still use the loop mechanism for other stuff. Also 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. It defines the "__ARC_RF16__" preprocessor macro ⭐
• -mbranch-index — enables use of "bi" or "bih" instructions to implement jump tables 🔥

---

The following options are passed through to the assembler, and also define preprocessor macro symbols.

• -mdsp-packa — passed down to the assembler to enable the DSP Pack A extensions. Also sets the preprocessor symbol "__Xdsp_packa". This option is deprecated. 💀
• -mdvbf — passed down to the assembler to enable the dual Viterbi butterfly extension. Also sets the preprocessor symbol "__Xdvbf". This option is deprecated. 💀
• -mlock — passed down to the assembler to enable the locked load/store conditional extension. Also sets the preprocessor symbol "__Xlock" 🔒
• -mmac-d16 — passed down to the assembler. Also sets the preprocessor symbol "__Xxmac_d16". This option is deprecated. 💀
• -mmac-24 — passed down to the assembler. Also sets the preprocessor symbol "__Xxmac_24". This option is deprecated. 💀
• -mrtsc — passed down to the assembler to enable the 64-bit time-stamp counter extension instruction. Also sets the preprocessor symbol "__Xrtsc". This option is deprecated. 💀
• -mswape — passed down to the assembler to enable the swap byte ordering extension instruction. Also sets the preprocessor symbol "__Xswape" 🔄
• -mtelephony — passed down to the assembler to enable dual- and single-operand instructions for telephony. Also sets the preprocessor symbol "__Xtelephony". This option is deprecated. 💀
• -mxy — passed down to the assembler to enable the XY memory extension. Also sets the preprocessor symbol "__Xxy" 🌈

---

The following options control how the assembly code is annotated:

• -misize — annotates assembler instructions with estimated addresses 🏷️
• -mannotate-align — does literally nothing. Preserved for backward compatibility 🙃

---

The following options are passed through to the linker:

• -marclinux — passed through to the linker to specify use of the "arclinux" emulation. This one is enabled by default in tool chains built for "arc-linux-uclibc" and "arceb-linux-uclibc" targets — but only when profiling is not requested ✨
• -marclinux_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. main character energy, huge reach 👑
• -mmedium-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 gets suppressed so the 25-bit range can be used instead of the 21-bit range with conditional branch-and-link. This is the default for tool chains built for "arc-linux-uclibc" and "arceb-linux-uclibc" targets 🎀
• -G num — puts definitions of externally-visible data in a small data section if that data is no bigger than num bytes. The default num is 4 for any ARC configuration, or 8 when there are double load/store operations 🌸
• -mno-sdata — don't generate sdata references. This is the default for tool chains built for "arc-linux-uclibc" and "arceb-linux-uclibc" targets 🙈
• -mvolatile-cache — uses ordinarily cached memory accesses for volatile references. This is the default 💖
• -mno-volatile-cache — enables cache bypass for volatile references. no cap 🚫

---

The following options fine tune code generation:

• -malign-call — does nothing. Preserved for backward compatibility 🙃
• -mauto-modify-reg — enables the use of pre/post modify with register displacement 🦋
• -mbbit-peephole — does nothing. Preserved for backward compatibility 🙃
• -mno-brcc — disables a target-specific pass in arc_reorg that generates compare-and-branch ("brcc") instructions. It has no effect on generation of these instructions driven by the combiner pass 👀
• -mcase-vector-pcrel — uses PC-relative switch case tables to enable case table shortening. This is the default for -Os ✨
• -mcompact-casesi — enables the compact "casesi" pattern. This is the default for -Os, and it's only available for ARCv1 cores. This option is deprecated. 💀
• -mno-cond-exec — disables the ARCompact-specific pass that generates conditional execution instructions.

Okay so real talk, bestie: because of delay slot scheduling and how operand numbers, literal sizes, instruction lengths and conditional-execution support all interact, the target-independent pass for generating conditional execution is often lacking. So the ARC port keeps a special pass around that hunts for more conditional execution opportunities after register allocation, branch shortening and delay slot scheduling are 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're having a problem with call instructions blowing past their allowable offset range because they got conditionalized, you should consider using -mmedium-calls instead 🫶

• -mearly-cbranchsi — enables pre-reload use of the "cbranchsi" pattern 🌱
• -mexpand-adddi — expands "adddi3" and "subdi3" at RTL generation time into "add.f", "adc" etc. This option is deprecated. 💀
• -mindexed-loads — enables the use of indexed loads. This can be problematic because some optimizers then assume indexed stores exist — which is not the case 😬
• -mlra — enables Local Register Allocation. This is still experimental for ARC, so by default the compiler uses standard reload (i.e. -mno-lra) 🧪
• -mlra-priority-none — don't indicate any priority for target registers 🙅
• -mlra-priority-compact — indicates target register priority for r0..r3 / r12..r15 👑
• -mlra-priority-noncompact — reduces target register priority for r0..r3 / r12..r15 📉
• -mmillicode — when optimizing for size (using -Os), prologues and epilogues that have to save or restore a large number of registers often get shortened by calling a special function in libgcc — that's called a millicode call. Since these calls can cause performance issues and/or linking issues when linking in a nonstandard way, this option exists so you can turn millicode call generation on or off 🎀
• -mcode-density-frame — enables the compiler to emit "enter" and "leave" instructions. These instructions are only valid for CPUs with the code-density feature ⭐
• -mmixed-code — does nothing. Preserved for backward compatibility 🙃
• -mq-class — This option is deprecated. It enables q instruction alternatives. This is the default for -Os 💀
• -mRcq — does nothing. Preserved for backward compatibility 🙃
• -mRcw — does nothing. Preserved for backward compatibility 🙃
• -msize-level=level — fine-tunes size optimization with regards to instruction lengths and alignment. The recognized values for level are:
  • 0 — no size optimization. This level is deprecated and treated like 1.
  • 1 — short instructions are used opportunistically.
  • 2 — in addition, alignment of loops and of code after barriers are dropped.
  • 3 — in addition, optional data alignment is dropped, and the option Os is enabled.

This defaults to 3 when -Os is in effect. Otherwise, if this isn't set, the behavior is equivalent to level 1 🌸

• -mtune=cpu — sets instruction scheduling parameters for cpu, overriding anything implied by -mcpu=.

The 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 an ARC700 CPU that's got the XMAC block. ✨
• ARC725D — tune for an ARC725D CPU. 🎀
• ARC750D — tune for an ARC750D CPU. 🎀
• core3 — tune for an ARCv2 core3 type CPU. This one turns on usage of the dbnz instruction, bestie. 👑
• release31a — tune for ARC4x release 3.10a. 🌸

---

• -mmultcost=num — the cost to assume for a multiply instruction, where 4 is equal to a normal instruction. So like, 4 = "this costs the same as any other op", fr fr.
• -munalign-prob-threshold=probability — does literally nothing. It's preserved for backward compatibility only, no cap. 💀

---

The following options are kept around for backward compatibility, but they are deprecated and are gonna get removed in a future release. So maybe don't build your whole personality around them:

• -margonaut — obsolete FPX.
• -mbig-endian, -EB — compile code for big-endian targets. Using these is deprecated now. Big-endian code is supported by configuring GCC to build arceb-elf32 and arceb-linux-uclibc targets, and for those, big endian is the default anyway.
• -mlittle-endian, -EL — compile code for little-endian targets. Also deprecated now. Little-endian code is supported by configuring GCC to build arc-elf32 and arc-linux-uclibc targets, and for those, little endian is the default.
• -mbarrel_shifter — replaced by -mbarrel-shifter.
• -mdpfp_compact — replaced by -mdpfp-compact.
• -mdpfp_fast — replaced by -mdpfp-fast.
• -mdsp_packa — replaced by -mdsp-packa.
• -mEA — replaced by -mea.
• -mmac_24 — replaced by -mmac-24.
• -mmac_d16 — replaced by -mmac-d16.
• -mspfp_compact — replaced by -mspfp-compact.
• -mspfp_fast — replaced by -mspfp-fast.
• -mtune=cpu — the values arc600, arc601, arc700 and arc700-xmac for cpu get replaced by ARC600, ARC601, ARC700 and ARC700-xmac respectively. (Capitalization matters, girlypop.)
• -multcost=num — replaced by -mmultcost.

---

💅 ARM Options 💅

These -m options are defined for the ARM port. Let's get into it:

• -mabi=name — generate code for the specified ABI. The permissible values are: apcs-gnu, atpcs, aapcs, aapcs-linux and iwmmxt.
• -mapcs-frame — generate a stack frame that's compliant with the ARM Procedure Call Standard for all functions, even when it's not strictly necessary for the code to execute correctly. If you also specify -fomit-frame-pointer with this option, then stack frames just won't be generated for leaf functions. The default is -mno-apcs-frame. Heads up though: this option is deprecated. 🙈
• -mapcs — this is just a synonym for -mapcs-frame, and it's deprecated too.
• -mthumb-interwork — generate code that supports calling between the ARM and Thumb instruction sets. Without this option, on pre-v5 architectures, those two instruction sets can't be reliably used inside one program. The default is -mno-thumb-interwork, since specifying -mthumb-interwork makes slightly larger code. In AAPCS configurations this option is meaningless. 💀
• -mno-sched-prolog — stop the reordering of instructions in the function prologue, and stop merging those instructions with the instructions in the function's body. What that gets you: every function starts with a recognizable set of instructions (or actually one of a choice from a small set of different function prologues), and that info can be used to locate the start of functions inside an executable piece of code. The default is -msched-prolog.
• -mfloat-abi=name — specifies which floating-point ABI to use. Permissible values are: soft, softfp and hard.

Specifying soft makes GCC generate output that contains library calls for floating-point operations. softfp allows generating code that uses hardware floating-point instructions, but it still uses the soft-float calling conventions. hard allows generation of floating-point instructions and uses FPU-specific calling conventions.

The default depends on the specific target configuration. And note this: the hard-float and soft-float ABIs are not link-compatible — you have to compile your entire program with the same ABI, and link with a compatible set of libraries. No mixing, bestie, it will not slay. 😭

• -mgeneral-regs-only — generate code that uses only the general-purpose registers. This stops the compiler from using floating-point and Advanced SIMD registers, but it won't 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 there. The default depends on the selected target architecture: for ARMv6 and later architectures the default is BE8, and for older architectures the default is BE32. BE32 format has been deprecated by ARM. Deprecated, babes.
• -march=name[+extension...] — this specifies the name of the target ARM architecture. GCC uses this name to figure out what kind of instructions it's allowed to emit when generating assembly code. You can use this option together with the -mcpu= option, or instead of it.

The permissible names are: armv4t, armv5t, armv5te, armv6, armv6j, armv6k, armv6kz, armv6t2, armv6z, armv6zk, armv7, armv7-a, armv7ve, armv8-a, armv8.1-a, armv8.2-a, armv8.3-a, armv8.4-a, armv8.5-a, armv8.6-a, armv9-a, armv7-r, armv8-r, armv6-m, armv6s-m, armv7-m, armv7e-m, armv8-m.base, armv8-m.main, armv8.1-m.main, armv9-a, iwmmxt and iwmmxt2.

Also: the following architectures, which don't support the Thumb execution state, are recognized, but support for them is deprecated: armv4.

A lot of these architectures support extensions. You add them by appending +extension to the architecture name. Extension options get processed in order and capabilities accumulate. An extension will also enable any necessary base extensions it depends on. For example, the +crypto extension will always enable the +simd extension. The exception to this additive construction is for extensions prefixed with +no...: those disable the specified option and any other extensions that might depend on that extension being present.

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. The math is mathing. ✨

Most extension names are generically named, but their effect depends on the architecture you apply them to. For example, the +simd option can be applied to both armv7-a and armv8-a architectures, but it enables the original ARMv7-A Advanced SIMD (Neon) extensions for armv7-a, and the ARMv8-A variant for armv8-a.

Here's the list of supported extensions for each architecture. Architectures that aren't mentioned don't support any extensions.

• armv5te, armv6, armv6j, armv6k, armv6kz, armv6t2, armv6z, armv6zk
  • +fp — the VFPv2 floating-point instructions. The extension +vfpv2 can be used as an alias for this extension.
  • +nofp — disable the floating-point instructions.
• armv7

The common subset of the ARMv7-A, ARMv7-R and ARMv7-M architectures.

• +fp — the VFPv3 floating-point instructions, with 16 double-precision registers. The extension +vfpv3-d16 can be used as an alias for this extension. Note that floating-point is not supported by the base ARMv7-M architecture, but it is compatible with both the ARMv7-A and ARMv7-R architectures.
• +nofp — disable the floating-point instructions.

• armv7-a
  • +mp — the multiprocessing extension.
  • +sec — the security extension.
  • +fp — the VFPv3 floating-point instructions, with 16 double-precision registers. The extension +vfpv3-d16 can be used as an alias for this extension.
  • +simd — the Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions. The extensions +neon and +neon-vfpv3 can be used as aliases for this extension.
  • +vfpv3 — the VFPv3 floating-point instructions, with 32 double-precision registers.
  • +vfpv3-d16-fp16 — the VFPv3 floating-point instructions, with 16 double-precision registers and the half-precision floating-point conversion operations.
  • +vfpv3-fp16 — the VFPv3 floating-point instructions, with 32 double-precision registers and the half-precision floating-point conversion operations.
  • +vfpv4-d16 — the VFPv4 floating-point instructions, with 16 double-precision registers.
  • +vfpv4 — the VFPv4 floating-point instructions, with 32 double-precision registers.
  • +neon-fp16 — the Advanced SIMD (Neon) v1 and the VFPv3 floating-point instructions, with the half-precision floating-point conversion operations.
  • +neon-vfpv4 — the Advanced SIMD (Neon) v2 and the VFPv4 floating-point instructions.
  • +nosimd — disable the Advanced SIMD instructions (it does not disable floating point).
  • +nofp — disable the floating-point and Advanced SIMD instructions.
• armv7ve

The extended version of the ARMv7-A architecture with support for virtualization. 💅

• +fp — the VFPv4 floating-point instructions, with 16 double-precision registers. The extension +vfpv4-d16 can be used as an alias for this extension.
• +simd — the Advanced SIMD (Neon) v2 and the VFPv4 floating-point instructions. The extension +neon-vfpv4 can be used as an alias for this extension.
• +vfpv3-d16 — the VFPv3 floating-point instructions, with 16 double-precision registers.
• +vfpv3 — the VFPv3 floating-point instructions, with 32 double-precision registers.

• +vfpv3-d16-fp16 — the VFPv3 floating-point instructions, but with 16 double-precision registers and the half-precision floating-point conversion ops. bestie it's giving "small register file but she can do fp16" 💅
• +vfpv3-fp16 — VFPv3 floating-point instructions with the full 32 double-precision registers plus those half-precision conversion operations. slay.
• +vfpv4-d16 — the VFPv4 floating-point instructions, 16 double-precision registers. that's the whole vibe 🎀
• +vfpv4 — VFPv4 floating-point instructions with 32 double-precision registers. iconic.
• +neon — the Advanced SIMD (Neon) v1 instructions and the VFPv3 floating-point instructions. lowkey you can also write +neon-vfpv3 as an alias for this exact extension, no cap 🦋
• +neon-fp16 — Advanced SIMD (Neon) v1 plus VFPv3 floating-point instructions, with the half-precision floating-point conversion operations folded in. 💖
• +nosimd — turns OFF the Advanced SIMD instructions. heads up: it does not disable floating point. two separate things, ok?? 🙈
• +nofp — turns OFF both the floating-point and the Advanced SIMD instructions. everything, gone, bye 💀

armv8-a

• +crc — the Cyclic Redundancy Check (CRC) instructions. ✨
• +simd — the ARMv8-A Advanced SIMD and floating-point instructions. 🫶
• +crypto — the cryptographic instructions. 🔥
• +nocrypto — disable the cryptographic instructions. 🚫
• +nofp — disable the floating-point, Advanced SIMD and cryptographic instructions. the full sweep, periodt.
• +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. and ngl, this also enables the Advanced SIMD and floating-point instructions too. 🎀
• +nocrypto — disable the cryptographic instructions. 🚫
• +nofp — disable the floating-point, Advanced SIMD and cryptographic instructions. all of it, bestie.
• +sb — Speculation Barrier Instruction.
• +predres — Execution and Data Prediction Restriction Instructions.

armv8.2-a

armv8.3-a

• +fp16 — the half-precision floating-point data processing instructions. this one also enables the Advanced SIMD and floating-point instructions. 💜
• +fp16fml — the half-precision floating-point fmla extension. also enables the half-precision floating-point extension and the Advanced SIMD and floating-point instructions. stacking, fr fr. 🌸
• +simd — the ARMv8.1-A Advanced SIMD and floating-point instructions.
• +crypto — the cryptographic instructions. also enables the Advanced SIMD and floating-point instructions. 🔥
• +dotprod — enable the Dot Product extension. this also enables Advanced SIMD instructions. ✨
• +nocrypto — disable the cryptographic extension. 🚫
• +nofp — disable the floating-point, Advanced SIMD and cryptographic instructions.
• +sb — Speculation Barrier Instruction.
• +predres — Execution and Data Prediction Restriction Instructions.
• +i8mm — 8-bit Integer Matrix Multiply instructions. this also enables Advanced SIMD and floating-point instructions. 👑
• +bf16 — Brain half-precision floating-point instructions. this also enables Advanced SIMD and floating-point instructions. 🧚

armv8.4-a

• +fp16 — the half-precision floating-point data processing instructions. this also enables the Advanced SIMD and floating-point instructions, as well as the Dot Product extension and the half-precision floating-point fmla extension. she does it all 💅
• +simd — the ARMv8.3-A Advanced SIMD and floating-point instructions as well as the Dot Product extension.
• +crypto — the cryptographic instructions. this also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension. 🔥
• +nocrypto — disable the cryptographic extension. 🚫
• +nofp — disable the floating-point, Advanced SIMD and cryptographic instructions.
• +sb — Speculation Barrier Instruction.
• +predres — Execution and Data Prediction Restriction Instructions.
• +i8mm — 8-bit Integer Matrix Multiply instructions. this also enables Advanced SIMD and floating-point instructions.
• +bf16 — Brain half-precision floating-point instructions. this also enables Advanced SIMD and floating-point instructions.

armv8.5-a

• +fp16 — the half-precision floating-point data processing instructions. this also enables the Advanced SIMD and floating-point instructions, as well as the Dot Product extension and the half-precision floating-point fmla extension. 🌈
• +simd — the ARMv8.3-A Advanced SIMD and floating-point instructions as well as the Dot Product extension.
• +crypto — the cryptographic instructions. this also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension. 🔥
• +nocrypto — disable the cryptographic extension. 🚫
• +nofp — disable the floating-point, Advanced SIMD and cryptographic instructions.
• +i8mm — 8-bit Integer Matrix Multiply instructions. this also enables Advanced SIMD and floating-point instructions.
• +bf16 — Brain half-precision floating-point instructions. this also enables Advanced SIMD and floating-point instructions.

armv8.6-a

• +fp16 — the half-precision floating-point data processing instructions. this also enables the Advanced SIMD and floating-point instructions, as well as the Dot Product extension and the half-precision floating-point fmla extension. 💖
• +simd — the ARMv8.3-A Advanced SIMD and floating-point instructions as well as the Dot Product extension.
• +crypto — the cryptographic instructions. this also enables the Advanced SIMD and floating-point instructions as well as the Dot Product extension. 🔥
• +nocrypto — disable the cryptographic extension. 🚫
• +nofp — disable the floating-point, Advanced SIMD and cryptographic instructions.
• +i8mm — 8-bit Integer Matrix Multiply instructions. this also enables Advanced SIMD and floating-point instructions.
• +bf16 — Brain half-precision floating-point instructions. this also enables Advanced SIMD and floating-point instructions.

armv7-r

• +fp.sp — the single-precision VFPv3 floating-point instructions. you can also use +vfpv3xd as an alias for this extension. 🦋
• +fp — the VFPv3 floating-point instructions with 16 double-precision registers. and guess what, +vfpv3-d16 works as an alias for this extension too. ✨
• +vfpv3xd-d16-fp16 — the single-precision VFPv3 floating-point instructions with 16 double-precision registers and the half-precision floating-point conversion operations. 💅
• +vfpv3-d16-fp16 — the VFPv3 floating-point instructions with 16 double-precision registers and the half-precision floating-point conversion operations.
• +nofp — disable the floating-point extension. 🚫
• +idiv — the ARM-state integer division instructions. 🎀
• +noidiv — disable the ARM-state integer division extension. bye div 💀

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. both flavors, obsessed 🌸
• +nofp — disable the floating-point extensions. 🚫

armv8.1-m.main

• +dsp — the DSP instructions. ✨
• +mve — the M-Profile Vector Extension (MVE) integer instructions.
• +mve.fp — the M-Profile Vector Extension (MVE) integer and single precision floating-point instructions. 🫶
• +fp — the single-precision floating-point instructions.
• +fp.dp — the single- and double-precision floating-point instructions.
• +nofp — disable the floating-point extension. 🚫
• +cdecp0, +cdecp1, ... , +cdecp7 — enable the Custom Datapath Extension (CDE) on selected coprocessors according to the numbers given in the options, in the range 0 to 7. pick your fave 💅
• +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 — turns the cryptographic instructions off. bye bestie 💅
• +nofp — kills the floating-point, Advanced SIMD and cryptographic instructions all at once. one option, three heartbreaks 😭

-march=native makes the compiler sniff out the architecture of the machine you're building on, like it's doing a vibe check on your own hardware 💖 Right now this only works on GNU/Linux, and ngl not every architecture is recognized. If the auto-detect flops, the option just… does nothing. no effect, no drama.

---

-mtune=name

Okay so this one tells GCC the name of the target ARM processor it should tune the code's performance for 🎯 Some ARM implementations genuinely run better when you use this, so it's worth it. The 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.

Bonus round: this option can also tell GCC to tune for a big.LITTLE system 🧚 The permissible names for that are:

cortex-a15.cortex-a7, cortex-a17.cortex-a7,
cortex-a57.cortex-a53, cortex-a72.cortex-a53, cortex-a72.cortex-a35,
cortex-a73.cortex-a53, cortex-a75.cortex-a55, cortex-a76.cortex-a55.

-mtune=generic-arch means "tune for a whole blend of processors inside architecture arch" ✨ The whole point is code that runs well on the most popular CPUs right now — balancing the optimizations that help some CPUs in the range against the performance pitfalls of the others. Heads up though: what this option actually does can shift in future GCC versions, because CPU models come and go like trends 💀

-mtune accepts the same extension options as -mcpu, but those extension options do not affect the tuning of the generated code. lowkey important.

-mtune=native = compiler auto-detects the CPU of the build machine 🔍 Same tea as before: currently only supported on GNU/Linux, not all architectures are recognized, and if auto-detect fails the option has no effect.

---

-mcpu=name[+extension...]

This is the main character of ARM target flags 👑 It specifies the name of the target ARM processor. GCC takes that name and derives:

• the name of the target ARM architecture (like you'd typed -march), and
• the ARM processor type to tune for performance (like you'd typed -mtune).

But plot twist: if you use this together with -march or -mtune, those options win over the matching part of -mcpu. so don't be surprised, bestie.

Lots of the supported CPUs ship optional architectural extensions, and where that's the case those extensions are normally on by default ✅ If there exist implementations that lack the extension, you can use the extension syntax to switch off the ones that got omitted. For floating-point and Advanced SIMD (Neon) instructions, you also have to consider -mfloat-abi and -mfpu: floating-point and Advanced SIMD instructions only get used if -mfloat-abi is not set to soft, and any -mfpu setting other than auto overrides the available floating-point and SIMD extension instructions.

Real-world example 🍓: cortex-a9 comes in three major configurations — integer only, just a floating-point unit, or floating-point plus Advanced SIMD. Default is all the instructions enabled, but the extensions +nosimd and +nofp let you disable just SIMD, or both SIMD and floating-point respectively.

The permissible names for this option are the same as for -mtune. slay 💅

These extension options are common to the listed CPUs:

• +nodsp — disables the DSP instructions on cortex-m33, cortex-m35p, cortex-m52, cortex-m55 and cortex-m85. It also disables the M-Profile Vector Extension (MVE) integer and single precision floating-point instructions on cortex-m52, cortex-m55 and cortex-m85.
• +nopacbti — disables the Pointer Authentication and Branch Target Identification Extension on cortex-m52 and cortex-m85.
• +nomve — disables the M-Profile Vector Extension (MVE) integer and single precision floating-point instructions on cortex-m52, cortex-m55 and cortex-m85.
• +nomve.fp — disables the M-Profile Vector Extension (MVE) single precision floating-point instructions on cortex-m52, cortex-m55 and cortex-m85.
• +cdecp0, +cdecp1, ... , +cdecp7 — enables the Custom Datapath Extension (CDE) on selected coprocessors according to the numbers you give, in the range 0 to 7, on cortex-m52 and cortex-m55.
• +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. It 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.

Also, the generic-armv7-a pseudo target defaults to VFPv3 with 16 double-precision registers 🎀 It supports these extension options: mp, sec, vfpv3-d16, vfpv3, vfpv3-d16-fp16, vfpv3-fp16, vfpv4-d16, vfpv4, neon, neon-vfpv3, neon-fp16, neon-vfpv4. The meanings are the same as for the extensions to -march=armv7-a. iykyk.

-mcpu=generic-arch is also permissible, and it's equivalent to -march=arch -mtune=generic-arch. See -mtune for more info 🫶

-mcpu=native makes the compiler auto-detect the CPU of the build computer. At present, this feature is only supported on GNU/Linux, and not all architectures are recognized. If the auto-detect is unsuccessful the option has no effect. (noticing a pattern? 😭)

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-mfpu=name

This one specifies what floating-point hardware (or hardware emulation) is available on the target 💖 The permissible names are:

auto, vfpv2,
vfpv3, vfpv3-fp16, vfpv3-d16, vfpv3-d16-fp16, vfpv3xd, vfpv3xd-fp16,
neon-vfpv3, neon-fp16, vfpv4, vfpv4-d16, fpv4-sp-d16, neon-vfpv4,
fpv5-d16, fpv5-sp-d16, fp-armv8, neon-fp-armv8 and
crypto-neon-fp-armv8.

Also note: neon is an alias for neon-vfpv3 and vfp is an alias for vfpv2. cute.

The setting auto is the default and it's special ✨ It makes the compiler pick the floating-point and Advanced SIMD instructions based on your -mcpu and -march settings.

Heads up bestie 🚨 If the floating-point hardware you selected includes the NEON extension (like -mfpu=neon), floating-point operations will not be 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 — in particular, denormal values are treated as zero — so using NEON instructions may lead to a loss of precision. it's giving risky 💀

You can also set the fpu name at function level using the target("fpu=") function attributes or pragmas. iconic.

---

-mfp16-format=name

Sets the format of the __fp16 half-precision floating-point type 🌸 Permissible names are none, ieee, and alternative; the default is none, and in that case the __fp16 type is just… not defined. rip.

---

-mstructure-size-boundary=n

The sizes of all structures and unions get rounded up to a multiple of the number of bits set by this option 📐 Permissible values are 8, 32 and 64. The default value varies for different toolchains — for the COFF targeted toolchain the default value is 8. A value of 64 is only allowed if the underlying ABI supports it.

Specifying a larger number can produce faster, more efficient code, but it can also make your program bigger. And different values are potentially incompatible 😬 Code compiled with one value can't necessarily expect to work with code or libraries compiled with another value, if they exchange information using structures or unions.

This option is deprecated. you've been warned, babe 🙈

---

-mabort-on-noreturn

Generates a call to the function abort at the end of a noreturn function 🫠 It gets executed if the function tries to return. (bold of it to try, honestly.)

---

-mlong-calls

-mno-long-calls

Tells the compiler to perform function calls by first loading the address of the function into a register, and then performing a subroutine call on that register 🔥 You need this switch if the target function lives outside the 64-megabyte addressing range of the offset-based version of subroutine call instruction. no cap.

Even if this switch is on, bestie, not every function call gets the long-call glow-up. Here's the vibe check the compiler runs: static functions, functions with the "short_call" attribute, functions chilling inside the scope of a "#pragma no_long_calls" directive, and functions whose definitions already got compiled inside the current compilation unit — none of those get turned into long calls. 🙅‍♀️

And the exceptions on top of the exceptions? Weak function definitions, functions with the "long_call" attribute or the "section" attribute, and functions hanging out inside the scope of a "#pragma long_calls" directive are always turned into long calls. periodt.

This feature is not on by default. Specifying -mno-long-calls restores the default behavior, and so does dropping your function calls inside the scope of a "#pragma long_calls_off" directive. 🫶

Heads up though: these switches do nothing to how the compiler generates code for function calls through function pointers. Like, zero effect. Don't @ me.

---

• -msingle-pic-base — treat the register used for PIC addressing as read-only instead of loading it up in the prologue of every single function. The runtime system is the one responsible for initializing this register with a suitable value before execution even begins. teamwork 💅
• -mpic-register=reg — pick which register gets used for PIC addressing. For the standard PIC base case, the default is whatever suitable register the compiler decides on. For the single PIC base case, the default is R9 if the target is EABI based or stack-checking is enabled — otherwise the default is R10.
• -mpic-data-is-text-relative — assume the displacement between the text and data segments is fixed at static link time. This lets the compiler use PC-relative addressing operations to access data it knows lives in the data segment. For non-VxWorks RTP targets this option is enabled by default. And if you disable it on those targets, it will enable -msingle-pic-base by default. the way that it just cascades 😭
• -mpoke-function-name — write the name of every function into the text section, directly preceding the function prologue. The generated code is giving something 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, #4

When you're doing 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 there's a function name embedded immediately preceding this location, and it has length "((pc[-3]) & 0xff000000)". forensic girly behavior 🔍

• -mthumb
• -marm — select between generating code that executes in ARM state vs Thumb state. The default for most configurations is code that executes in ARM state, but that default can be changed by configuring GCC with the --with-mode=state configure option. You can also override ARM/Thumb mode per function using the target("thumb") and target("arm") function attributes or pragmas. pick your fighter 💖
• -mflip-thumb — switch ARM/Thumb modes on alternating functions. This one exists purely for regression testing of mixed Thumb/ARM code generation, and it is not intended for ordinary use in compiling code. it's a lab experiment, not a lifestyle ✨
• -mtpcs-frame — generate a stack frame that's compliant with the Thumb Procedure Call Standard for all non-leaf functions. (A leaf function is one that does not call any other functions.) The default is -mno-tpcs-frame.
• -mtpcs-leaf-frame — generate a stack frame that's compliant with the Thumb Procedure Call Standard for all leaf functions. (Again: a leaf function is one that does not call any other functions.) The default is -mno-apcs-leaf-frame. yes the names don't match, that's just how it is bestie 🙈
• -mcallee-super-interworking — gives every externally visible function in the file being compiled an ARM instruction set header which switches to Thumb mode before executing the rest of the function. This lets these functions be called from non-interworking code. This option is not valid in AAPCS configurations, because interworking is enabled by default there anyway.
• -mcaller-super-interworking — allows calls via function pointers (including virtual functions) to execute correctly regardless of whether the target code was 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 there anyway. tiny tax, big correctness 💅
• -mtp=name — specify the access model for the thread local storage pointer. The model soft generates calls to "__aeabi_read_tp". Other accepted models are tpidrurw, tpidruro and tpidrprw, which fetch the thread pointer from the corresponding system register directly (supported from the arm6k architecture and later). These system registers are accessed through the CP15 co-processor interface, and the argument cp15 is also accepted as a convenience alias of tpidruro. The argument auto uses the best available method for the selected processor. The default setting is auto. she's flexible like that 🌸
• -mtls-dialect=dialect — specify the dialect to use for accessing thread local storage. Two dialects are supported: gnu and gnu2. The gnu dialect selects the original GNU scheme for supporting local and global dynamic TLS models. The gnu2 dialect selects the GNU descriptor scheme, which gives 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. no cap.
• -mword-relocations — only generate absolute relocations on word-sized values (i.e. R_ARM_ABS32). This is enabled by default on targets (uClinux, SymbianOS) where the runtime loader imposes this restriction, and when -fpic or -fPIC is specified. This option conflicts with -mslow-flash-data. choose one, babe ⚠️
• -mfix-cortex-m3-ldrd — some Cortex-M3 cores can cause data corruption when "ldrd" instructions with overlapping destination and base registers are used. This option avoids generating those instructions. It's enabled by default when -mcpu=cortex-m3 is specified. protecting you from the hardware's villain era 🔥
• -mfix-cortex-a57-aes-1742098
• -mno-fix-cortex-a57-aes-1742098
• -mfix-cortex-a72-aes-1655431
• -mno-fix-cortex-a72-aes-1655431 — enable (or disable) mitigation for an erratum on Cortex-A57 and Cortex-A72 that affects the AES cryptographic instructions. This option is enabled by default when either -mcpu=cortex-a57 or -mcpu=cortex-a72 is specified.
• -munaligned-access
• -mno-unaligned-access — enables (or disables) reading and writing of 16- and 32-bit values from addresses that aren't 16- or 32-bit aligned. By default unaligned access is disabled for all pre-ARMv6, all ARMv6-M and for ARMv8-M Baseline architectures, and enabled for all other architectures. If unaligned access is not enabled, then words in packed data structures get accessed a byte at a time. slow and steady 🐢

The ARM attribute "Tag_CPU_unaligned_access" is set in the generated object file to either true or false, depending on the setting of this option. And if unaligned access is enabled, the preprocessor symbol "__ARM_FEATURE_UNALIGNED" is also defined. ✨

• -mneon-for-64bits — this option is deprecated and has no effect. she's retired, let her rest 💀
• -mslow-flash-data — assume loading data from flash is slower than fetching instructions. So literal load gets minimized for better performance. This option is only supported when compiling for ARMv7 M-profile, and it's off by default. It conflicts with -mword-relocations.
• -masm-syntax-unified — assume inline assembler is using unified asm syntax. The default is currently off, which implies divided syntax. This option has no impact on Thumb2. However, this may change in future releases of GCC. Divided syntax should be considered deprecated. 👀
• -mrestrict-it — restricts generation of IT blocks to conform to the rules of ARMv8-A. IT blocks can only contain a single 16-bit instruction from a select set of instructions. This option is on by default for ARMv8-A Thumb mode.
• -mprint-tune-info — print CPU tuning information as a comment in the assembler file. This is an option used only for regression testing of the compiler and is not intended for ordinary use in compiling code. It's disabled by default. dev-only, sorry bestie 🧚
• -mverbose-cost-dump — enable verbose cost model dumping in the debug dump files. This option is provided for use in debugging the compiler.
• -mpure-code — do not allow constant data to be placed in code sections. Additionally, when compiling for ELF object format, give all text sections the ELF processor-specific section attribute "SHF_ARM_PURECODE". This option is only available when generating non-pic code for M-profile targets.
• -mcmse — generate secure code as per the "ARMv8-M Security Extensions: Requirements on Development Tools Engineering Specification", which can be found on <https://developer.arm.com/documentation/ecm0359818/latest/>. go read the spec, i believe in you 🫶
• -mfix-cmse-cve-2021-35465 — mitigate against a potential security issue with the "VLLDM" instruction in some M-profile devices when using CMSE (CVE-2021-365465). This option is enabled by default when the option -mcpu= is used with "cortex-m33", "cortex-m35p", "cortex-m52", "cortex-m55", "cortex-m85" or "star-mc1". The option -mno-fix-cmse-cve-2021-35465 can be used to disable the mitigation. security queen behavior 🔒
• -mstack-protector-guard=guard
• -mstack-protector-guard-offset=offset — generate stack protection code using a canary at guard. Supported locations are global for a global canary, or tls for a canary accessible via the TLS register.

The option -mstack-protector-guard-offset= is for use with -fstack-protector-guard=tls and not for use in user-land code. please do not ship that, i'm begging 😭

• -mfdpic, -mno-fdpic — okay bestie, this one's giving FDPIC ABI energy. It uses 64-bit function descriptors to represent pointers to functions. When the compiler is configured for arm-*-uclinuxfdpiceabi targets, this option is on by default and implies -fPIE if none of the PIC/PIE-related options is provided. On other targets, it only enables the FDPIC-specific code generation features, and the user should explicitly provide the PIC/PIE-related options as needed.

⚠️ no cap: static linking is NOT supported because it would still involve the dynamic linker when the program self-relocates. If such behavior is acceptable, use -static and -Wl,-dynamic-linker options.

The opposite -mno-fdpic option is useful (and required) to build the Linux kernel using the same (arm-*-uclinuxfdpiceabi) toolchain as the one used to build the userland programs.

• -mbranch-protection=none|standard|pac-ret[+leaf][+bti]|bti[+pac-ret[+leaf]] — ✨ enable branch protection features (armv8.1-m.main only). Here's the tea:
  • none: generate code without branch protection or return address signing.
  • standard[+leaf]: generate code with all branch protection features enabled at their standard level.
  • pac-ret[+leaf]: generate code with return address signing set to its standard level, which is to sign all functions that save the return address to memory.
  • leaf: when return address signing is enabled, also sign leaf functions even if they do not write the return address to memory.
  • +bti: add landing-pad instructions at the permitted targets of indirect branch instructions.

If the +pacbti architecture extension is not enabled, then all branch protection and return address signing operations are constrained to use only the instructions defined in the architectural-NOP space. The generated code will remain backwards-compatible with earlier versions of the architecture, but the additional security can be enabled at run time on processors that support the PACBTI extension.

Branch target enforcement using BTI can only be enabled at runtime if all code in the application has been compiled with at least -mbranch-protection=bti.

Any setting other than none is supported only on armv8-m.main or later.

The default is to generate code without branch protection or return address signing.

---

🌸 AVR Options 🌸

These options are defined for AVR implementations:

• -mmcu=mcu — specify the AVR instruction set architecture (ISA) or device type. The default for this option is "avr2".

💡 note: A complete device support consists of startup code crtmcu.o, a device header avr/io*.h, a device library libmcu.a and a device-specs (https://gcc.gnu.org/wiki/avr-gcc#spec-files) file specs-mcu. Only the latter is provided by the compiler according the supported mcus below. The rest is supported by AVR-LibC (https://www.nongnu.org/avr-libc/), or by means of atpack (https://gcc.gnu.org/wiki/avr-gcc#atpack) files from the hardware manufacturer.

The following AVR devices and ISAs are supported:

• avr2 — "Classic" devices with up to 8 KiB of program memory. mcu = attiny22, attiny26, at90s2313, at90s2323, at90s2333, at90s2343, at90s4414, at90s4433, at90s4434, at90c8534, at90s8515, at90s8535.
• avr25 — "Classic" devices with up to 8 KiB of program memory and with the MOVW instruction. mcu = attiny13, attiny13a, attiny24, attiny24a, attiny25, attiny261, attiny261a, attiny2313, attiny2313a, attiny43u, attiny44, attiny44a, attiny45, attiny48, attiny441, attiny461, attiny461a, attiny4313, attiny84, attiny84a, attiny85, attiny87, attiny88, attiny828, attiny841, attiny861, attiny861a, ata5272, ata6616c, at86rf401.
• avr3 — "Classic" devices with 16 KiB up to 64 KiB of program memory. mcu = at76c711, at43usb355.
• avr31 — "Classic" devices with 128 KiB of program memory. mcu = atmega103, at43usb320.
• avr35 — "Classic" devices with 16 KiB up to 64 KiB of program memory and with the MOVW instruction. mcu = attiny167, attiny1634, atmega8u2, atmega16u2, atmega32u2, ata5505, ata6617c, ata664251, at90usb82, at90usb162.
• avr4 — "Enhanced" devices with up to 8 KiB of program memory. mcu = atmega48, atmega48a, atmega48p, atmega48pa, atmega48pb, atmega8, atmega8a, atmega8hva, atmega88, atmega88a, atmega88p, atmega88pa, atmega88pb, atmega8515, atmega8535, ata5795, ata6285, ata6286, ata6289, ata6612c, at90pwm1, at90pwm2, at90pwm2b, at90pwm3, at90pwm3b, at90pwm81.
• avr5 — "Enhanced" devices with 16 KiB up to 64 KiB of program memory. mcu = atmega16, atmega16a, atmega16hva, atmega16hva2, atmega16hvb, atmega16hvbrevb, atmega16m1, atmega16u4, atmega161, atmega162, atmega163, atmega164a, atmega164p, atmega164pa, atmega165, atmega165a, atmega165p, atmega165pa, atmega168, atmega168a, atmega168p, atmega168pa, atmega168pb, atmega169, atmega169a, atmega169p, atmega169pa, atmega32, atmega32a, atmega32c1, atmega32hvb, atmega32hvbrevb, atmega32m1, atmega32u4, atmega32u6, atmega323, atmega324a, atmega324p, atmega324pa, atmega324pb, atmega325, atmega325a, atmega325p, atmega325pa, atmega328, atmega328p, atmega328pb, atmega329, atmega329a, atmega329p, atmega329pa, atmega3250, atmega3250a, atmega3250p, atmega3250pa, atmega3290, atmega3290a, atmega3290p, atmega3290pa, atmega406, atmega64, atmega64a, atmega64c1, atmega64hve, atmega64hve2, atmega64m1, atmega64rfr2, atmega640, atmega644, atmega644a, atmega644p, atmega644pa, atmega644rfr2, atmega645, atmega645a, atmega645p, atmega649, atmega649a, atmega649p, atmega6450, atmega6450a, atmega6450p, atmega6490, atmega6490a, atmega6490p, ata5790, ata5790n, ata5791, ata6613c, ata6614q, ata5782, ata5831, ata8210, ata8510, ata5787, ata5835, ata5700m322, ata5702m322, at90pwm161, at90pwm216, at90pwm316, at90can32, at90can64, at90scr100, at90usb646, at90usb647, at94k, m3000.
• avr51 — "Enhanced" devices with 128 KiB of program memory. mcu = atmega128, atmega128a, atmega128rfa1, atmega128rfr2, atmega1280, atmega1281, atmega1284, atmega1284p, atmega1284rfr2, at90can128, at90usb1286, at90usb1287.
• avr6 — "Enhanced" devices with 3-byte PC, i.e. with more than 128 KiB of program memory. mcu = atmega256rfr2, atmega2560, atmega2561, atmega2564rfr2.
• avrxmega2 — "XMEGA" devices with more than 8 KiB and up to 64 KiB of program memory. mcu = atxmega8e5, atxmega16a4, atxmega16a4u, atxmega16c4, atxmega16d4, atxmega16e5, atxmega32a4, atxmega32a4u, atxmega32c3, atxmega32c4, atxmega32d3, atxmega32d4, atxmega32e5, avr64da28, avr64da32, avr64da48, avr64da64, avr64db28, avr64db32, avr64db48, avr64db64, avr64dd14, avr64dd20, avr64dd28, avr64dd32, avr64du28, avr64du32, avr64ea28, avr64ea32, avr64ea48.
• avrxmega3 — "XMEGA" devices with up to 64 KiB of combined program memory and RAM, and with program memory visible in the RAM address space. mcu = attiny202, attiny204, attiny212, attiny214, attiny402, attiny404, attiny406, attiny412, attiny414, attiny416, attiny416auto, attiny417, attiny424, attiny426, attiny427, attiny804, attiny806, attiny807, attiny814, attiny816, attiny817, attiny824, attiny826, attiny827, attiny1604, attiny1606, attiny1607, attiny1614, attiny1616, attiny1617, attiny1624, attiny1626, attiny1627, attiny3214, attiny3216, attiny3217, attiny3224, attiny3226, attiny3227, atmega808, atmega809, atmega1608, atmega1609, atmega3208, atmega3209, atmega4808, atmega4809, avr16dd14, avr16dd20, avr16dd28, avr16dd32, avr16du14, avr16du20, avr16du28, avr16du32, avr16ea28, avr16ea32, avr16ea48, avr16eb14, avr16eb20, avr16eb28, avr16eb32, avr32da28, avr32da32, avr32da48, avr32db28, avr32db32, avr32db48, avr32dd14, avr32dd20, avr32dd28, avr32dd32, avr32du14, avr32du20, avr32du28, avr32du32, avr32ea28, avr32ea32, avr32ea48.
• avrxmega4 — "XMEGA" devices with more than 64 KiB and up to 128 KiB of program memory. mcu = atxmega64a3, atxmega64a3u, atxmega64a4u, atxmega64b1, atxmega64b3, atxmega64c3, atxmega64d3, atxmega64d4, avr128da28, avr128da32, avr128da48, avr128da64, avr128db28, avr128db32, avr128db48, avr128db64.
• avrxmega5 — "XMEGA" devices with more than 64 KiB and up to 128 KiB of program memory and more than 64 KiB of RAM. mcu = atxmega64a1, atxmega64a1u.
• avrxmega6 — "XMEGA" devices with more than 128 KiB of program memory. mcu = atxmega128a3, atxmega128a3u, atxmega128b1, atxmega128b3, atxmega128c3, atxmega128d3, atxmega128d4, atxmega192a3, atxmega192a3u, atxmega192c3, atxmega192d3, atxmega256a3, atxmega256a3b, atxmega256a3bu, atxmega256a3u, atxmega256c3, atxmega256d3, atxmega384c3, atxmega384d3.
• avrxmega7 — "XMEGA" devices with more than 128 KiB of program memory and more than 64 KiB of RAM. mcu = atxmega128a1, atxmega128a1u, atxmega128a4u.
• avrtiny — "Reduced Tiny" Tiny core devices with only 16 general purpose registers and 512 B up to 4 KiB of program memory. mcu = attiny4, attiny5, attiny9, attiny10, attiny102, attiny104, attiny20, attiny40.
• "avr1" — this ISA is implemented by the minimal AVR core and is supported for assembler only. mcu = "attiny11", "attiny12", "attiny15", "attiny28", "at90s1200".
• -mabsdata — assume that all data in static storage can be accessed by LDS / STS instructions. This option has only an effect on reduced Tiny devices like ATtiny40. See also the "absdata" AVR Variable Attributes, variable attribute. 🫶
• -maccumulate-args — accumulate outgoing function arguments and acquire/release the needed stack space for outgoing function arguments once, in the function prologue/epilogue. Without this option, outgoing arguments are pushed before calling a function and popped afterwards.

Popping the arguments after the function call can be expensive on AVR, so accumulating the stack space might lead to smaller executables because arguments need not be removed from the stack after such a function call. ✨

This option can lead to reduced code size for functions that perform several calls to functions that get their arguments on the stack, like calls to printf-like functions.

• -mbranch-cost=cost — set the branch costs for conditional branch instructions to cost. Reasonable values for cost are small, non-negative integers. The default branch cost is 0.
• -mcall-prologues — function prologues/epilogues are expanded as calls to appropriate subroutines. Code size is smaller. 💅
• -mfuse-add
• -mno-fuse-add
• -mfuse-add=level — optimize indirect memory accesses on reduced Tiny devices. The default uses level=1 for optimizations -Og and -O1, and level=2 for higher optimizations. Valid values for level are 0, 1 and 2.
• -mdouble=bits
• -mlong-double=bits — set the size (in bits) of the double or long double type, respectively. Possible values for bits are 32 and 64. Whether or not a specific value for bits is allowed depends on the --with-double= and --with-long-double= configure options (https://gcc.gnu.org/install/configure.html#avr), and the same applies for the default values of the options.
• -mgas-isr-prologues — interrupt service routines (ISRs) may use the __gcc_isr pseudo instruction supported by GNU Binutils. If this option is on, the feature can still be disabled for individual ISRs by means of the AVR Function Attributes, "no_gccisr" function attribute. This feature is activated per default if optimization is on (but not with -Og, @pxref{Optimize Options}), and if GNU Binutils support PR21683 (https://sourceware.org/PR21683).
• -mint8 — assume int to be 8-bit integer. This affects the sizes of all types: a char is 1 byte, an int is 1 byte, a long is 2 bytes, and long long is 4 bytes. Please note that this option does not conform to the C standards, but it results in smaller code size. 👀
• -mmain-is-OS_task — do not save registers in main. The effect is the same like attaching attribute AVR Function Attributes, "OS_task" to main. It is activated per default if optimization is on.
• -mno-interrupts — generated code is not compatible with hardware interrupts. Code size is smaller.
• -mrelax — try to replace CALL resp. JMP instruction by the shorter RCALL resp. RJMP instruction if applicable. Setting -mrelax just adds the --mlink-relax option to the assembler's command line and the --relax option to the linker's command line.

Jump relaxing is performed by the linker because jump offsets are not known before code is located. Therefore, the assembler code generated by the compiler is the same, but the instructions in the executable may differ from instructions in the assembler code.

Relaxing must be turned on if linker stubs are needed, see the section on EIND and linker stubs below.

• -mrodata-in-ram
• -mno-rodata-in-ram — locate the .rodata sections for read-only data in RAM resp. in program memory. For most devices, there is no choice and this option acts rather like an assertion.

Since v14 and for the AVR64* and AVR128* devices, .rodata is located in flash memory per default, provided the required GNU Binutils support (PR31124 (https://sourceware.org/PR31124)) is available. In that case, -mrodata-in-ram can be used to return to the old layout with .rodata in RAM. 🦋

• -mstrict-X — use address register X in a way proposed by the hardware. This means that X is only used in indirect, post-increment or pre-decrement addressing.

Without this option, the X register may be used in the same way as Y or Z, which then is emulated by additional instructions. For example, loading a value with X+const addressing with a small non-negative const < 64 to a register Rn is performed as

        adiw r26, const   ; X += const
        ld   <Rn>, X        ; <Rn> = *X
        sbiw r26, const   ; X -= const

• -mtiny-stack — only change the lower 8 bits of the stack pointer.
• -mfract-convert-truncate — allow to use truncation instead of rounding towards zero for fractional fixed-point types.
• -nodevicelib — don't link against AVR-LibC's device specific library lib<mcu>.a.
• -nodevicespecs — don't add -specs=device-specs/specs-mcu to the compiler driver's command line. The user takes responsibility for supplying the sub-processes like compiler proper, assembler and linker with appropriate command line options. This means that the user has to supply her private device specs file by means of -specs=path-to-specs-file. There is no more need for option -mmcu=mcu.

This option can also serve as a replacement for the older way of specifying custom device-specs files that needed -B some-path to point to a directory which contains a folder named "device-specs" which contains a specs file named "specs-mcu", where mcu was specified by -mmcu=mcu.

• -Waddr-space-convert — warn about conversions between address spaces in the case where the resulting address space is not contained in the incoming address space.
• -Wmisspelled-isr — warn if the ISR is misspelled, i.e. without __vector prefix. Enabled by default.

---

🌈 "EIND" and Devices with More Than 128 Ki Bytes of Flash 🌈

Pointers in the implementation are 16 bits wide. The address of a function or label is represented as word address so that indirect jumps and calls can target any code address in the range of 64 Ki words.

In order to facilitate indirect jump on devices with more than 128 Ki bytes of program memory space, there is a special function register called EIND that serves as most significant part of the target address when EICALL or EIJMP instructions are used.

Indirect jumps and calls on these devices are handled as follows by the compiler and are subject to some limitations:

• The compiler never sets EIND. 🙈
• The compiler uses EIND implicitly in EICALL/EIJMP instructions or might read EIND directly in order to emulate an indirect call/jump by means of a RET instruction.
• The compiler assumes that EIND never changes during the startup code or during the application. In particular, EIND is not saved/restored in function or interrupt service routine prologue/epilogue.
• For indirect calls to functions and computed goto, the linker generates stubs. Stubs are jump pads sometimes also called trampolines. Thus, the indirect call/jump jumps to such a stub. The stub contains a direct jump to the desired address.
• Linker relaxation must be turned on so that the linker generates the stubs correctly in all situations. See the compiler option -mrelax and the linker option --relax. There are corner cases where the linker is supposed to generate stubs but aborts without relaxation and without a helpful error message. 💀
• The default linker script is arranged for code with EIND = 0. If code is supposed to work for a setup with EIND != 0, a custom linker script has to be used in order to place the sections whose name start with .trampolines into the segment where EIND points to.
• The startup code from libgcc never sets EIND. Notice that startup code is a blend of code from libgcc and AVR-LibC. For the impact of AVR-LibC on EIND, see the AVR-LibC user manual (https://www.nongnu.org/avr-libc/user-manual/).
• It is legitimate for user-specific startup code to set up EIND early, for example by means of initialization code located in section .init3. Such code runs prior to general startup code that initializes RAM and calls constructors, but after the bit of startup code from AVR-LibC that sets EIND to the segment where the vector table is located.

        #include <avr/io.h>

        static void
        __attribute__((section(".init3"),naked,used,no_instrument_function))
        init3_set_eind (void)
        {
          __asm volatile ("ldi r24,pm_hh8(__trampolines_start)\n\t"
                          "out %i0,r24" :: "n" (&EIND) : "r24","memory");
        }

The __trampolines_start symbol is defined in the linker script. ✨

• Stubs are generated automatically by the linker if the following two conditions are met:
  • <The address of a label is taken by means of the gs modifier> (short for generate stubs) like so:

                LDI r24, lo8(gs(<func>))
                LDI r25, hi8(gs(<func>))

• <The final location of that label is in a code segment> outside the segment where the stubs are located.

• The compiler emits such gs modifiers for code labels in the following situations:
  • <Taking address of a function or code label.>
  • <Computed goto.>
  • <If prologue-save function is used, see -mcall-prologues> command-line option.
  • <Switch/case dispatch tables. If you do not want such dispatch> tables you can specify the -fno-jump-tables command-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 app gets linked with -Wl,--defsym,func_4=0x4. Or, like, plot twist: func_4 can just be defined in the linker script instead. Either way bestie, you're covered 💅

---

Handling of the RAMPD, RAMPX, RAMPY and RAMPZ Special Function Registers

Okay so some AVR devices have memories bigger than that 64 KiB range you can reach with 16-bit pointers. And to get to memory outside that 64 KiB range, the content of a RAMP register gets used as the high part of the address. Like, the X, Y, Z address register is concatenated with the RAMPX, RAMPY, RAMPZ special function register, respectively, to build a wide address. And RAMPD? Same energy, but it pairs up with direct addressing. 🫶

• The startup code initializes the RAMP special function registers with zero.
• If an AVR Named Address Space — any named address space that isn't generic or __flash — gets used, then RAMPZ is set as needed before the operation.
• If the device supports RAM larger than 64 KiB and the compiler needs to change RAMPZ to accomplish an operation, RAMPZ is reset to zero after the operation.
• If the device comes with a specific RAMP register, the ISR prologue/epilogue saves/restores that SFR and initializes it with zero in case the ISR code might (implicitly) use it.
• RAM larger than 64 KiB is not supported by GCC for AVR targets. So if you go use inline assembler to read from locations outside the 16-bit address range and change one of the RAMP registers, you must reset it to zero after the access. No skipping this step, bestie. 😭

---

AVR Built-in Macros

GCC defines a whole bunch of built-in macros so your user code can vibe check whether a feature is present or absent. Almost all of the following built-in macros are deduced from device capabilities, which means they're triggered by the -mmcu= command-line option. ✨

For even more AVR-specific built-in macros, go peek at 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 it depends on the -mmcu=mcu option. The 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_XMEGA__ — The device / architecture belongs to the XMEGA family of devices.
• __AVR_HAVE_ADIW__ — The device has the ADIW and SBIW instructions.
• __AVR_HAVE_ELPM__ — The device has the ELPM instruction.
• __AVR_HAVE_ELPMX__ — The device has the ELPM Rn,Z and ELPM Rn,Z+ instructions.
• __AVR_HAVE_LPMX__ — The device has the LPM Rn,Z and LPM Rn,Z+ instructions.
• __AVR_HAVE_MOVW__ — The device has the MOVW instruction to perform 16-bit register-register moves.
• __AVR_HAVE_MUL__ — The device has a hardware multiplier. 💪
• __AVR_HAVE_JMP_CALL__ — The device has the JMP and CALL instructions. This is the case for devices with more than 8 KiB of program memory.
• __AVR_HAVE_EIJMP_EICALL__, __AVR_3_BYTE_PC__ — The device has the EIJMP and EICALL instructions. This is the case for devices with more than 128 KiB of program memory. This also means that the program counter (PC) is 3 bytes wide.
• __AVR_2_BYTE_PC__ — The program counter (PC) is 2 bytes wide. This is the case for devices with up to 128 KiB of program memory.
• __AVR_HAVE_8BIT_SP__, __AVR_HAVE_16BIT_SP__ — The stack pointer (SP) register is treated as an 8-bit respectively 16-bit register by the compiler. The definition of these macros is affected by -mtiny-stack.
• __AVR_HAVE_SPH__, __AVR_SP8__ — The device has the SPH (high part of stack pointer) special function register, or has an 8-bit stack pointer, respectively. The definition of these macros is affected by -mmcu= and, in the cases of -mmcu=avr2 and -mmcu=avr25, also by -msp8.
• __AVR_HAVE_RAMPD__, __AVR_HAVE_RAMPX__, __AVR_HAVE_RAMPY__, __AVR_HAVE_RAMPZ__ — The device has the RAMPD, RAMPX, RAMPY, RAMPZ special function register, respectively.
• __NO_INTERRUPTS__ — This macro reflects the -mno-interrupts command-line option.
• __AVR_ERRATA_SKIP__, __AVR_ERRATA_SKIP_JMP_CALL__ — Some AVR devices (AT90S8515, ATmega103) must not skip 32-bit instructions because of a hardware erratum. The skip instructions in question are SBRS, SBRC, SBIS, SBIC and CPSE. The second macro is only defined if __AVR_HAVE_JMP_CALL__ is also set.
• __AVR_ISA_RMW__ — The device has Read-Modify-Write instructions (XCH, LAC, LAS and LAT).
• __AVR_SFR_OFFSET__=offset — Instructions that can address I/O special function registers directly, like IN, OUT, SBI, etc., may use a different address than if they were addressed by an instruction to access RAM like LD or STS. This offset depends on the device architecture and has to be subtracted from the RAM address in order to get the respective I/O address.
• __AVR_SHORT_CALLS__ — The -mshort-calls command line option is set.
• __AVR_PM_BASE_ADDRESS__=addr — Some devices support reading from flash memory by means of LD* instructions. The flash memory is seen in the data address space at an offset of __AVR_PM_BASE_ADDRESS__. If this macro is not defined, this feature is not available. If defined, the address space is linear and there is no need to put .rodata into RAM. This is handled by the default linker description file, and is currently available for avrtiny and avrxmega3. Even more convenient: there is no need to use address spaces like __flash or features like attribute progmem and pgm_read_*. Iconic behaviour, honestly. 👑
• __AVR_HAVE_FLMAP__ — This macro is defined provided the following conditions are met:
  • The device has the NVMCTRL_CTRLB.FLMAP bitfield. This applies to the AVR64* and AVR128* devices.
  • It's not known at assembler-time which emulation will be used.

This implies the compiler was configured with GNU Binutils that implement PR31124 (https://sourceware.org/PR31124).

• __AVR_RODATA_IN_RAM__ — This macro is undefined when the code is compiled for a core architecture.

When the code is compiled for a device, the macro is defined to 1 when the .rodata sections for read-only data is located in RAM; and defined to 0, otherwise.

• __WITH_AVRLIBC__ — The compiler is configured to be used together with AVR-Libc. See the --with-avrlibc configure option.
• __HAVE_DOUBLE_MULTILIB__ — Defined if -mdouble= acts as a multilib option.
• __HAVE_DOUBLE32__, __HAVE_DOUBLE64__ — Defined if the compiler supports 32-bit double resp. 64-bit double. The actual layout is specified by option -mdouble=.
• __DEFAULT_DOUBLE__ — The size in bits of double if -mdouble= is not set. To test the layout of double in a program, use the built-in macro __SIZEOF_DOUBLE__.
• __HAVE_LONG_DOUBLE32__, __HAVE_LONG_DOUBLE64__, __HAVE_LONG_DOUBLE_MULTILIB__, __DEFAULT_LONG_DOUBLE__ — Same as above, but for long double instead of double.
• __WITH_DOUBLE_COMPARISON__ — Reflects the --with-double-comparison={tristate|bool|libf7} configure option (https://gcc.gnu.org/install/configure.html#avr) and is defined to 2 or 3.
• __WITH_LIBF7_LIBGCC__, __WITH_LIBF7_MATH__, __WITH_LIBF7_MATH_SYMBOLS__ — Reflects the --with-libf7={libgcc|math|math-symbols} configure option (https://gcc.gnu.org/install/configure.html#avr).

---

AVR Internal Options

Okay so these next ones are used internally by the compiler and to communicate between device specs files and the compiler proper. You do not need to set these by hand — and especially, they are not optimization options. 🚨 Using these the wrong way may lead to sub-optimal or wrong code. They're documented here for completeness, and to give you a better understanding of device specs files (https://gcc.gnu.org/wiki/avr-gcc#spec-files).

• -mn-flash=num — Assume that the flash memory has a size of num times 64 KiB. This determines which __flashN address spaces are available.
• -mflmap — The device has the FLMAP bit field located in special function register NVMCTRL_CTRLB.
• -mrmw — Assume that the device supports the Read-Modify-Write instructions XCH, LAC, LAS and LAT.
• -mshort-calls — Assume that RJMP and RCALL can target the whole program memory. This option is used for multilib generation and selection for the devices from architecture avrxmega3.
• -mskip-bug — Generate code without skips (CPSE, SBRS, SBRC, SBIS, SBIC) over 32-bit instructions.
• -msp8 — Treat the stack pointer register as an 8-bit register, i.e. assume the high byte of the stack pointer is zero. This option is used by the compiler to select and build multilibs for architectures avr2 and avr25. These architectures mix devices with and without SPH.

---

Blackfin Options

• -mcpu=cpu[-sirevision] — okay so this is the one where you tell GCC which Blackfin processor you're on. Currently cpu can be one of: bf512, bf514, bf516, bf518, bf522, bf523, bf524, bf525, bf526, bf527, bf531, bf532, bf533, bf534, bf536, bf537, bf538, bf539, bf542, bf544, bf547, bf548, bf549, bf542m, bf544m, bf547m, bf548m, bf549m, bf561, bf592.

That optional sirevision is the silicon revision of your target Blackfin processor, and any workarounds available for that revision get enabled. If sirevision is none, literally no workarounds get turned on. If sirevision is any, then every workaround for the targeted processor is enabled. The __SILICON_REVISION__ macro gets defined to two hexadecimal digits — major and minor numbers in the silicon revision. If sirevision is none, __SILICON_REVISION__ is not defined at all. If sirevision is any, __SILICON_REVISION__ is defined to be 0xffff. And if you just don't use this optional sirevision at all? GCC assumes the latest known silicon revision of the targeted Blackfin processor. Slay.

GCC defines a preprocessor macro for whatever cpu you picked. For the bfin-elf toolchain, this option causes the hardware BSP provided by libgloss to be linked in, as long as -msim is not given.

Without this option, bf532 is the default processor. Noted.

Heads up bestie: support for bf561 is incomplete. For bf561, only the preprocessor macro is defined. That's it. That's the whole thing. 😭

• -msim — says the program will run on the simulator, which causes the simulator BSP provided by libgloss to be linked in. Only does anything for the bfin-elf toolchain. Some other options — like -mid-shared-library and -mfdpic — imply -msim on their own.
• -momit-leaf-frame-pointer — don't keep the frame pointer in a register for leaf functions. This dodges the instructions for saving, setting up and restoring frame pointers, and frees up an extra register in leaf functions. ✨
• -mspecld-anomaly — when enabled, the compiler makes sure the generated code does not contain speculative loads after jump instructions. If you use this option, __WORKAROUND_SPECULATIVE_LOADS is defined.
• -mno-specld-anomaly — don't generate the extra code that prevents speculative loads from happening.
• -mcsync-anomaly — when enabled, the compiler makes sure the generated code does not contain CSYNC or SSYNC instructions too soon after conditional branches. If you use this option, __WORKAROUND_SPECULATIVE_SYNCS is defined.
• -mno-csync-anomaly — don't generate extra code to stop CSYNC or SSYNC instructions from showing up too soon after a conditional branch.
• -mlow64k — when enabled, the compiler is free to take advantage of knowing that the entire program fits into the low 64k of memory. 🫶
• -mno-low64k — assume the program is arbitrarily large. This is the default.
• -mstack-check-l1 — do stack checking using information placed into L1 scratchpad memory by the uClinux kernel.
• -mid-shared-library — generate code that supports shared libraries via the library ID method. This allows for execute in place and shared libraries in an environment with no virtual memory management. This option implies -fPIC. With a bfin-elf target, this option implies -msim.
• -mno-id-shared-library — generate code that doesn't assume ID-based shared libraries are being used. This is the default.
• -mleaf-id-shared-library — generate code that supports shared libraries via the library ID method, but assumes this library or executable won't link against any other ID shared libraries. That lets the compiler use faster code for jumps and calls. 💅
• -mno-leaf-id-shared-library — do not assume the code being compiled won't link against any ID shared libraries. Slower code gets generated for jump and call insns.
• -mshared-library-id=n — specifies the identification number of the ID-based shared library being compiled. Specifying a value of 0 generates more compact code; specifying other values forces the allocation of that number to the current library, but is no more space- or time-efficient than just omitting this option.
• -msep-data — generate code that allows the data segment to live in a different area of memory than the text segment. This allows for execute in place in an environment without virtual memory management, by eliminating relocations against the text section.
• -mno-sep-data — generate code that assumes the data segment follows the text segment. This is the default.
• -mlong-calls, -mno-long-calls — tells the compiler to perform function calls by first loading the address of the function into a register and then doing a subroutine call on that register. You need this switch if the target function lies outside of the 24-bit addressing range of the offset-based version of subroutine call instruction.

This feature is not enabled by default. Specifying -mno-long-calls restores the default behavior. Note that these switches have no effect on how the compiler generates code to handle function calls via function pointers.

• -mfast-fp — link with the fast floating-point library. This library relaxes some of the IEEE floating-point standard's rules for checking inputs against Not-a-Number (NAN), all in the interest of performance.
• -minline-plt — enable inlining of PLT entries in function calls to functions that are not known to bind locally. Has no effect without -mfdpic.
• -mmulticore — build a standalone application for multicore Blackfin processors. This option causes proper start files and link scripts supporting multicore to be used, and defines the macro __BFIN_MULTICORE. It can only be used with -mcpu=bf561[-sirevision].

This option can be used with -mcorea or -mcoreb, which selects the one-application-per-core programming model. Without -mcorea or -mcoreb, the single-application/dual-core programming model is used. In that model, the main function of Core B should be named coreb_main.

If this option is not used, the single-core application programming model is used.

• -mcorea — build a standalone application for Core A of BF561 when using the one-application-per-core programming model. Proper start files and link scripts are used to support Core A, and the macro __BFIN_COREA is defined. This option can only be used in conjunction with -mmulticore.
• -mcoreb — build a standalone application for Core B of BF561 when using the one-application-per-core programming model. Proper start files and link scripts are used to support Core B, and the macro __BFIN_COREB is defined. When this option is used, coreb_main should be used instead of main. This option can only be used in conjunction with -mmulticore.
• -msdram — build a standalone application for SDRAM. Proper start files and link scripts are used to put the application into SDRAM, and the macro __BFIN_SDRAM is defined. The loader should initialize SDRAM before loading the application.
• -micplb — assume that ICPLBs are enabled at run time. This has an effect on certain anomaly workarounds. For Linux targets, the default is to assume ICPLBs are enabled; for standalone applications the default is off.

---

🦋 C6X Options 🦋

• -march=name — this specifies the name of the target architecture. GCC uses this name to figure out what kind of instructions it can emit when generating assembly code. Permissible names are: c62x, c64x, c64x+, c67x, c67x+, c674x.
• -mbig-endian — generate code for a big-endian target.
• -mlittle-endian — generate code for a little-endian target. This is the default.
• -msim — choose startup files and linker script suitable for the simulator.
• -msdata=default — put small global and static data in the .neardata section, which is pointed to by register B14. Put small uninitialized global and static data in the .bss section, which is adjacent to the .neardata section. Put small read-only data into the .rodata section. The corresponding sections used for large pieces of data are .fardata, .far and .const.
• -msdata=all — put all data, not just small objects, into the sections reserved for small data, and use addressing relative to the B14 register to access them.
• -msdata=none — make no use of the sections reserved for small data, and use absolute addresses to access all data. Put all initialized global and static data in the .fardata section, and all uninitialized data in the .far section. Put all constant data into the .const section.

---

🌈 CRIS Options 🌈

These options are defined specifically for the CRIS ports. 💖

• -march=architecture-type, -mcpu=architecture-type — generate code for the specified architecture. The choices for architecture-type are v3, v8 and v10, for respectively ETRAX 4, ETRAX 100, and ETRAX 100 LX. Default is v0.
• -mtune=architecture-type — tune to architecture-type everything applicable about the generated code, except for the ABI and the set of available instructions. The choices for architecture-type are the same as for -march=architecture-type.
• -mmax-stack-frame=n — warn when the stack frame of a function exceeds n bytes.
• -metrax4, -metrax100 — the options -metrax4 and -metrax100 are synonyms for -march=v3 and -march=v8 respectively.
• -mmul-bug-workaround, -mno-mul-bug-workaround — work around a bug in the muls and mulu instructions for CPU models where it applies. This option is disabled by default.
• -mpdebug — enable CRIS-specific verbose debug-related information in the assembly code. This option also has the effect of turning off the #NO_APP formatted-code indicator to the assembler at the beginning of the assembly file.
• -mcc-init — do not use condition-code results from previous instruction; always emit compare and test instructions before use of condition codes.
• -mno-side-effects — do not emit instructions with side effects in addressing modes other than post-increment.
• -mstack-align, -mno-stack-align, -mdata-align, -mno-data-align, -mconst-align, -mno-const-align — these ones (and their no- twins) either arrange or eliminate the arrangement for the stack frame, individual data, and constants so they line up with the maximum single data access size of whatever CPU model you picked. Default vibe? 32-bit alignment. And before you panic — ABI details like structure layout are not affected by any of this. structure layout stays unbothered, as it should 💅
• -m32-bit, -m16-bit, -m8-bit — same energy as the stack-/data-/const-align options above, except these make the stack frame, writable data and constants 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 just… gone, and no return instructions or return sequences get generated at all.

BESTIE. Listen. Only ever use this one while literally staring at the compiled code with your own two eyes. No warnings, no errors, nothing — GCC will not tell you when call-saved registers needed saving or when storage for local variables needed allocating. It just silently does not do it. 😭

• -melf — legacy no-op option. It's giving decorative. nothing happens, and that's the point 🎀
• -sim — links you up with the input-output functions from a simulator library. Code, initialized data, and zero-initialized data all get allocated consecutively.
• -sim2 — like -sim, but it passes linker options to locate initialized data at 0x40000000 and zero-initialized data at 0x80000000. fancy addresses, fancy girl ✨

---

🌸 C-SKY Options 🌸

GCC has these options in its bag when you're compiling for C-SKY V2 processors. niche? yes. iconic? also yes 🦋

• -march=arch — pick the C-SKY target architecture. Valid values for arch are: ck801, ck802, ck803, ck807, and ck810. The default is ck810.
• -mcpu=cpu — pick the C-SKY target processor. Valid values for cpu are:

ck801, ck801t, ck802, ck802t, ck802j, ck803, ck803h, ck803t,
ck803ht, ck803f, ck803fh, ck803e, ck803eh, ck803et, ck803eht,
ck803ef, ck803efh, ck803ft, ck803eft, ck803efht, ck803r1, ck803hr1,
ck803tr1, ck803htr1, ck803fr1, ck803fhr1, ck803er1, ck803ehr1,
ck803etr1, ck803ehtr1, ck803efr1, ck803efhr1, ck803ftr1, ck803eftr1,
ck803efhtr1, ck803s, ck803st, ck803se, ck803sf, ck803sef, ck803seft,
ck807e, ck807ef, ck807, ck807f, ck810e, ck810et, ck810ef, ck810eft,
ck810, ck810v, ck810f, ck810t, ck810fv, ck810tv, ck810ft, and
ck810ftv

that's a whole alphabet soup and I respect it so much 🍓

• -mbig-endian, -EB, -mlittle-endian, -EL — select big- or little-endian code. The default is little-endian.
• -mfloat-abi=name — tells GCC which floating-point ABI to use. Permissible values are: soft, softfp and hard.

Saying soft makes GCC generate output that contains library calls for floating-point operations. softfp lets it generate code using hardware floating-point instructions, but still uses the soft-float calling conventions. hard allows generation of floating-point instructions and uses FPU-specific calling conventions.

The default depends on the specific target configuration. And heads up — the hard-float and soft-float ABIs are not link-compatible; you have to compile your entire program with the same ABI, and link against a compatible set of libraries. No mixing, no matching. 🫶

• -mhard-float, -msoft-float — select hardware or software floating-point implementations. The default is soft float.
• -mdouble-float, -mno-double-float — when -mhard-float is in effect, this enables generation of double-precision float instructions. This is the default except when compiling for CK803.
• -mfdivdu, -mno-fdivdu — when -mhard-float is in effect, this enables generation of "frecipd", "fsqrtd", and "fdivd" instructions. This is the default except when compiling for CK803.
• -mfpu=fpu — select the floating-point processor. This option can only be used together with -mhard-float. Values for fpu are fpv2_sf (equivalent to -mno-double-float -mno-fdivdu), fpv2 (-mdouble-float -mno-divdu), and fpv2_divd (-mdouble-float -mdivdu).
• -melrw, -mno-elrw — enable the extended "lrw" instruction. This option defaults to on for CK801 and off otherwise.
• -mistack, -mno-istack — enable interrupt stack instructions; the default is off.

The -mistack option is required to handle the "interrupt" and "isr" function attributes. no istack, no interrupt handling, that's just the rules ✨

• -mmp — enable multiprocessor instructions; the default is off.
• -mcp — enable coprocessor instructions; the default is off.
• -mcache — enable coprocessor instructions; the default is off.
• -msecurity — enable C-SKY security instructions; the default is off.
• -mtrust — enable C-SKY trust instructions; the default is off.
• -mdsp, -medsp, -mvdsp — enable C-SKY DSP, Enhanced DSP, or Vector DSP instructions, respectively. All of these options default to off.
• -mdiv, -mno-div — generate divide instructions. Default is off.
• -msmart, -mno-smart — generate code for Smart Mode, using only registers numbered 0-7 so 16-bit instructions can be used. This option is ignored for CK801, where this is the required behavior, and it defaults to on for CK802. For other targets, the default is off.
• -mhigh-registers, -mno-high-registers — generate code using the high registers numbered 16-31. This option is not supported on CK801, CK802, or CK803, and it's enabled by default for the other processors.
• -manchor, -mno-anchor — generate code using global anchor symbol addresses.
• -mpushpop, -mno-pushpop — generate code using "push" and "pop" instructions. This option defaults to on.
• -mmultiple-stld, -mstm, -mno-multiple-stld, -mno-stm — generate code using "stm" and "ldm" instructions. This option isn't supported on CK801, but it is enabled by default on the other processors.
• -mconstpool, -mno-constpool — create constant pools in the compiler instead of deferring that job to the assembler. This option is the default and is required for correct code generation on CK801 and CK802, and it's optional on the other processors.
• -mstack-size, -mno-stack-size — emit ".stack_size" directives for each function in the assembly output. This option defaults to off.
• -mccrt, -mno-ccrt — generate code for the C-SKY compiler runtime instead of libgcc. This option defaults to off.
• -mbranch-cost=n — set the branch costs to roughly n instructions. The default is 1.
• -msched-prolog, -mno-sched-prolog — permit scheduling of function prologue and epilogue sequences.

Careful, bestie 😭 Using this option can give you code that is not compliant with the C-SKY V2 ABI prologue requirements and that cannot be debugged or backtraced. It is disabled by default. leave it alone unless you know exactly what you're doing 💀

• -msim — links the library libsemi.a, which is compatible with the simulator. Applicable to the ELF compiler only.

---

🍎 Darwin Options 🍎

These options are defined for all architectures running the Darwin operating system. Mac energy, but make it cross-arch 🌈

FSF GCC on Darwin does not create "fat" object files; it creates an object file for the single architecture that GCC was built to target. Apple's GCC on Darwin does create "fat" files if multiple -arch options are used; it does that by running the compiler or linker multiple times and joining the results together with lipo.

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 all behave differently when they get handed an ISA mismatch, and it's lowkey a whole personality thing:

• The assembler, as, only permits instructions 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 you ask it 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. not a word, just vibes 🎀
• -Fdir — add the framework directory dir to the head of the list of directories to be searched for header files. These directories get interleaved with the ones specified by -I options, and they're scanned in a left-to-right order.

A framework directory is a directory with frameworks in it. A framework is a directory with a Headers and/or PrivateHeaders directory contained directly in it that ends in .framework. The name of a framework is the name of this directory excluding the .framework. Headers associated with the framework are found in one of those two directories, with Headers being searched first. A subframework is a framework directory that is inside a framework's Frameworks directory. Includes of subframework headers can only appear in a header of a framework that contains the subframework, or in a sibling subframework header. Two subframeworks are siblings if they occur in the same framework. A subframework should not have the same name as a framework; a warning is issued if this is violated. Currently a subframework cannot have subframeworks; in the future, the mechanism may be extended to support this. The standard frameworks can be found in /System/Library/Frameworks and /Library/Frameworks. An example include looks like #include <Framework/header.h>, where Framework denotes the name of the framework and header.h is found in the PrivateHeaders or Headers directory.

• -iframeworkdir — like -F except the directory is treated as a system directory. The main difference between this -iframework and -F is that with -iframework the compiler does not warn about constructs contained within header files found via dir. This option is valid only for the C family of languages.
• -gused — emit debugging information for symbols that are used. For the stabs debugging format, this enables -feliminate-unused-debug-symbols. This is by default ON. on by default, we love a low-maintenance queen 👑
• -gfull — emit debugging information for all symbols and types.
• -fconstant-cfstrings — the -fconstant-cfstrings is an alias for -mconstant-cfstrings.
• -mconstant-cfstrings — when the NeXT runtime is being used (which is the default on these systems), override any -fconstant-string-class setting and cause "@"..."" literals to be laid out as constant CoreFoundation strings. CoreFoundation strings, but make it constant ✨💖✨
• -mmacosx-version-min=version — bestie, this sets the oldest version of MacOS X your executable is allowed to run on. That's the version you pass in, and the typical values it vibes with include 12, 10.12, and 10.5.8. ✨ Now, if the compiler was built to use the system's headers by default, then the default here is literally just the system version the compiler is running on. Otherwise, the default is to make choices that play nice with as many systems and code bases as possible. 🤝
• -mkernel — turns on kernel development mode, slay. This option sets -static, -fno-common, -fno-use-cxa-atexit, -fno-exceptions, -fno-non-call-exceptions, -fapple-kext, -fno-weak and -fno-rtti where applicable. And for PowerPC targets it also sets -mno-altivec, -msoft-float, -fno-builtin and -mlong-branch. it's giving full kernel mode, no crumbs. 🔥
• -mone-byte-bool — this overrides the defaults for "bool" so that "sizeof(bool)==1". By default sizeof(bool) is 4 when compiling for Darwin/PowerPC and 1 when compiling for Darwin/x86, so ngl this option does absolutely nothing 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, no cap, be careful bestie.

• -mfix-and-continue
• -ffix-and-continue
• -findirect-data — these three generate code suitable for fast turnaround development, such as to allow GDB to dynamically load .o files into already-running programs. -findirect-data and -ffix-and-continue are provided for backwards compatibility. vintage but iconic. 🎀
• -all_load — loads all members of static archive libraries. See man ld(1) for more information. 👑
• -arch_errors_fatal — causes the errors having to do with files that have the wrong architecture to be fatal. periodt.
• -bind_at_load — causes the output file to be marked such that the dynamic linker will bind all undefined references when the file is loaded or launched.
• -bundle — produce a Mach-o bundle format file. See man ld(1) for more information.
• -bundle_loader executable — this option specifies the executable that will load the build output file being linked. See man ld(1) for more information.
• -dynamiclib — when passed this option, GCC produces a dynamic library instead of an executable when linking, using the Darwin libtool command. ✨
• -force_cpusubtype_ALL — this causes GCC's output file to have the ALL subtype, instead of one controlled by the -mcpu or -march option.
• -nodefaultrpaths — do not add default run paths for the compiler library directories to executables, modules or dynamic libraries. On macOS 10.5 and later, the embedded runpath is added by default unless the user adds -nodefaultrpaths to the link line. Run paths are needed (and therefore enforced) to build on macOS version 10.11 or later. 🫶

---

okay bestie, here's a whole lil cluster of options that all get passed straight to the Darwin linker. The Darwin linker man page describes them in detail, but imma list every single one for you so nothing gets left out:

• -allowable_client client_name
• -client_name
• -compatibility_version
• -current_version
• -dead_strip
• -dependency-file
• -dylib_file
• -dylinker_install_name
• -dynamic
• -exported_symbols_list
• -filelist
• -flat_namespace
• -force_flat_namespace
• -headerpad_max_install_names
• -image_base
• -init
• -install_name
• -keep_private_externs
• -multi_module
• -multiply_defined
• -multiply_defined_unused
• -noall_load
• -no_dead_strip_inits_and_terms
• -nofixprebinding
• -nomultidefs
• -noprebind
• -noseglinkedit
• -pagezero_size
• -prebind
• -prebind_all_twolevel_modules
• -private_bundle
• -read_only_relocs
• -sectalign
• -sectobjectsymbols
• -whyload
• -seg1addr
• -sectcreate
• -sectobjectsymbols
• -sectorder
• -segaddr
• -segs_read_only_addr
• -segs_read_write_addr
• -seg_addr_table
• -seg_addr_table_filename
• -seglinkedit
• -segprot
• -segs_read_only_addr
• -segs_read_write_addr
• -single_module
• -static
• -sub_library
• -sub_umbrella
• -twolevel_namespace
• -umbrella
• -undefined
• -unexported_symbols_list
• -weak_reference_mismatches
• -whatsloaded

These options are passed to the Darwin linker. The Darwin linker man page describes them in detail. iykyk. 💅

---

🦋 DEC Alpha Options 🦋

These -m options are defined for the DEC Alpha implementations:

• -mno-soft-float
• -msoft-float — use (do not use) the hardware floating-point instructions for floating-point operations. When -msoft-float is specified, functions in libgcc.a are used to perform floating-point operations. Unless they are replaced by routines that emulate the floating-point operations, or compiled in such a way as to call such emulations routines, these routines issue floating-point operations. If you are compiling for an Alpha without floating-point operations, you must ensure that the library is built so as not to call them. 😭

Note that Alpha implementations without floating-point operations are required to have floating-point registers. the way that's just a rule, bestie.

• -mfp-reg
• -mno-fp-regs — generate code that uses (does not use) the floating-point register set. -mno-fp-regs implies -msoft-float. If the floating-point register set is not used, floating-point operands are passed in integer registers as if they were integers and floating-point results are passed in $0 instead of $f0. This is a non-standard calling sequence, so any function with a floating-point argument or return value called by code compiled with -mno-fp-regs must also be compiled with that option. 🙈

A typical use of this option is building a kernel that does not use, and hence need not save and restore, any floating-point registers.

• -mieee — the Alpha architecture implements floating-point hardware optimized for maximum performance. It is mostly compliant with the IEEE floating-point standard. However, for full compliance, software assistance is required. This option generates code fully IEEE-compliant code except that the inexact-flag is not maintained (see below). If this option is turned on, the preprocessor macro "_IEEE_FP" is defined during compilation. The resulting code is less efficient but is able to correctly support denormalized numbers and exceptional IEEE values such as not-a-number and plus/minus infinity. Other Alpha compilers call this option -ieee_with_no_inexact. ✨

DEBIAN SPECIFIC: This option is on by default for alpha-linux-gnu, unless -ffinite-math-only (which is part of the -ffast-math set) is specified, because the software functions in the GNU libc math libraries generate denormalized numbers, NaNs, and infs (all of which will cause a programs to SIGFPE when it attempts to use the results without -mieee). 💀

• -mieee-with-inexact — this is like -mieee except the generated code also maintains the IEEE inexact-flag. Turning on this option causes the generated code to implement fully-compliant IEEE math. In addition to "_IEEE_FP", "_IEEE_FP_EXACT" is defined as a preprocessor macro. On some Alpha implementations the resulting code may execute significantly slower than the code generated by default. Since there is very little code that depends on the inexact-flag, you should normally not specify this option. Other Alpha compilers call this option -ieee_with_inexact.
• -mfp-trap-mode=trap-mode — this option controls what floating-point related traps are enabled. Other Alpha compilers call this option -fptm trap-mode. The trap mode can be set to one of four values:
  • n — this is the default (normal) setting. The only traps that are enabled are the ones that cannot be disabled in software (e.g., division by zero trap).
  • u — in addition to the traps enabled by n, underflow traps are enabled as well.
  • su — like u, but the instructions are marked to be safe for software completion (see Alpha architecture manual for details).
  • sui — like su, but inexact traps are enabled as well.
• -mfp-rounding-mode=rounding-mode — selects the IEEE rounding mode. Other Alpha compilers call this option -fprm rounding-mode. The rounding-mode can be one of:
  • n — normal IEEE rounding mode. Floating-point numbers are rounded towards the nearest machine number or towards the even machine number in case of a tie.
  • m — round towards minus infinity.
  • c — chopped rounding mode. Floating-point numbers are rounded towards zero.
  • d — dynamic rounding mode. A field in the floating-point control register (fpcr, see Alpha architecture reference manual) controls the rounding mode in effect. The C library initializes this register for rounding towards plus infinity. Thus, unless your program modifies the fpcr, d corresponds to round towards plus infinity.
• -mtrap-precision=trap-precision — in the Alpha architecture, floating-point traps are imprecise. This means without software assistance it is impossible to recover from a floating trap and program execution normally needs to be terminated. GCC can generate code that can assist operating system trap handlers in determining the exact location that caused a floating-point trap. Depending on the requirements of an application, different levels of precisions can be selected:
  • p — program precision. This option is the default and means a trap handler can only identify which program caused a floating-point exception.
  • f — function precision. The trap handler can determine the function that caused a floating-point exception.
  • i — instruction precision. The trap handler can determine the exact instruction that caused a floating-point exception.

Other Alpha compilers provide the equivalent options called -scope_safe and -resumption_safe.

• -mieee-conformant — this option marks the generated code as IEEE conformant. You must not use this option unless you also specify -mtrap-precision=i and either -mfp-trap-mode=su or -mfp-trap-mode=sui. Its only effect is to emit the line .eflag 48 in the function prologue of the generated assembly file. like, that's literally it. 💅
• -mbuild-constants — normally GCC examines a 32- or 64-bit integer constant to see if it can construct it from smaller constants in two or three instructions. If it cannot, it outputs the constant as a literal and generates code to load it from the data segment at run time.

Use this option to require GCC to construct all integer constants using code, even if it takes more instructions (the maximum is six). 💅

You typically reach for this one when you're building a shared library dynamic loader. Like, it's itself a shared library, bestie — so it has to relocate itself in memory before it can even find the variables and constants in its own data segment. Main character problems. 🫶

---

• -mbwx, -mno-bwx, -mcix, -mno-cix, -mfix, -mno-fix, -mmax, -mno-max — these tell GCC whether it should generate code that uses the optional BWX, CIX, FIX and MAX instruction sets. The default? Use whatever instruction sets the CPU type you named in -mcpu= supports — or, if you didn't name one, whatever the CPU GCC was built on supports. no cap. ✨
• -mfloat-vax, -mfloat-ieee — generate code that uses (or does not use) VAX F and G floating-point arithmetic instead of IEEE single and double precision. Pick your fighter. 🎀
• -mexplicit-relocs, -mno-explicit-relocs — okay so back in the day, older Alpha assemblers had literally no way to generate symbol relocations except through assembler macros, and using those macros means you can't get optimal instruction scheduling. tragic. 😭 GNU binutils as of 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, because GCC detects the capabilities of the assembler when it is built and sets the default accordingly. iykyk. 🦋
• -msmall-data, -mlarge-data — when -mexplicit-relocs is in effect, static data is accessed via gp-relative relocations. With -msmall-data, objects 8 bytes long or smaller get placed in a small data area (the .sdata and .sbss sections) and are accessed via 16-bit relocations off of the $gp register. This caps the small data area at 64KB, but it means the variables can be accessed directly with a single instruction. slay. ✨

The default is -mlarge-data. With that one, the data area is limited to just below 2GB. Programs that need more than 2GB of data have to use malloc or mmap to allocate the data in the heap instead of in the program's data segment. 💀

And when you're generating code for shared libraries? -fpic implies -msmall-data, and -fPIC implies -mlarge-data. the way it just does that. 👑

• -msmall-text, -mlarge-text — with -msmall-text, the compiler assumes that the code of the entire program (or shared library) fits in 4MB, so it's reachable with a branch instruction. And when -msmall-data is used, the compiler can assume that all local symbols share the same $gp value, which reduces the number of instructions required for a function call from 4 to 1. obsessed. 🔥

The default is -mlarge-text.

• -mcpu=cpu_type — set the instruction set and instruction scheduling parameters for machine type cpu_type. You can specify either the EV style name or the corresponding chip number. GCC supports scheduling parameters for the EV4, EV5 and EV6 families of processors, and it picks the default instruction-set values from the processor you specify. If you don't specify a processor type at all, 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. Heads up tho: -mcpu=native has no effect if GCC does not recognize the processor. 🙈

• -mtune=cpu_type — sets only the instruction scheduling parameters for machine type cpu_type. The instruction set is not changed. it's giving minimal effort on purpose. ✨

Native toolchains also support the value native here, which selects the best architecture option for the host processor. And same deal: -mtune=native has no effect if GCC does not recognize the processor. 🫶

• -mmemory-latency=time — sets the latency the scheduler should assume for typical memory references as seen by the application. This number is highly dependent on the memory access patterns the application uses and the size of the external cache on the machine. very "it depends" energy ngl. 😭

Valid options for time are:

• number — a decimal number representing clock cycles.
• L1
• L2
• L3
• main — the compiler contains estimates of the number of clock cycles for "typical" EV4 & EV5 hardware for the Level 1, 2 & 3 caches (also called Dcache, Scache, and Bcache), as well as to main memory. Note that L3 is only valid for EV5. 🌸

---

🌈 eBPF Options 🌈

• -mframe-limit=bytes — this specifies the hard limit for frame sizes, in bytes. Currently, the value you can specify should be less than or equal to 32767. It defaults to whatever limit is imposed by the version of the Linux kernel targeted. 💅
• -mbig-endian — generate code for a big-endian target.
• -mlittle-endian — generate code for a little-endian target. This is the default. ✨
• -mjmpext, -mno-jmpext — enable or disable generation of extra conditional-branch instructions. Enabled for CPU v2 and above. 🔥
• -mjmp32, -mno-jmp32 — enable or disable generation of 32-bit jump instructions. Enabled for CPU v3 and above.
• -malu32, -mno-alu32 — enable or disable generation of 32-bit ALU instructions. Enabled for CPU v3 and above.
• -mv3-atomics, -mno-v3-atomics — enable or disable instructions for general atomic operations introduced in CPU v3. Enabled for CPU v3 and above. 🎀
• -mbswap, -mno-bswap — enable or disable byte swap instructions. Enabled for CPU v4 and above.
• -msdiv, -mno-sdiv — enable or disable signed division and modulus instructions. Enabled for CPU v4 and above.
• -msmov, -mno-smov — enable or disable sign-extending move and memory load instructions. Enabled for CPU v4 and above. 👑
• -mcpu=version — this specifies which version of the eBPF ISA to target. Newer versions may not be supported by all kernels. The default is v4. 💀

Supported values for version are:

• v1 — the first stable eBPF ISA with no special features or extensions.
• v2 — supports the jump extensions, as in -mjmpext.
• v3 — all features of v2, plus:
  • 32-bit jump operations, as in -mjmp32
  • 32-bit ALU operations, as in -malu32
  • general atomic operations, as in -mv3-atomics
• v4 — all features of 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

• -mco-re — enable BPF Compile Once - Run Everywhere (CO-RE) support. Requires and is implied by -gbtf. 🦋
• -mno-co-re — disable BPF Compile Once - Run Everywhere (CO-RE) support. BPF CO-RE support is enabled by default when generating BTF debug information for the BPF target.
• -mxbpf — generate code for an expanded version of BPF, which relaxes some of the restrictions imposed by the BPF architecture:
  • Save and restore callee-saved registers at function entry and exit, respectively. ✨
• -masm=dialect — outputs assembly instructions using eBPF selected dialect. The default is pseudoc.

Supported values for dialect are:

• normal — outputs normal assembly dialect.
• pseudoc — outputs pseudo-c assembly dialect.

• -minline-memops-threshold=bytes — specifies a size threshold in bytes at or below which memmove, memcpy and memset shall always be expanded inline. Operations dealing with sizes larger than this threshold would have to be implemented using a library call instead of being expanded inline, but since BPF doesn't allow libcalls, exceeding this threshold results in a compile-time error. The default is 1024 bytes. 😭

---

🧚 FR30 Options 🧚

These options are defined specifically for the FR30 port. 🌸

• -msmall-model — use the small address space model. This can produce smaller code, but it does assume that all symbolic values and addresses fit into a 20-bit range. 🎀
• -mno-lsim — assume that runtime support has been provided and so there is no need to include the simulator library (libsim.a) on the linker command line.

---

🦋 FT32 Options 🦋

These options are defined specifically for the FT32 port.

• -msim — specifies that the program will be run on the simulator. This causes an alternate runtime startup and library to be linked.

You must not use this option when generating programs that will run on real hardware; you must provide your own runtime library for whatever I/O functions are needed. bestie i'm begging you. 🙈

• -mlra — enable Local Register Allocation. This is still experimental for FT32, so by default the compiler uses standard reload. ⭐
• -mnodiv — do not use div and mod instructions.
• -mft32b — enable use of the extended instructions of the FT32B processor. 🔥
• -mcompress — compress all code using the Ft32B code compression scheme.
• -mnopm — do not generate code that reads program memory.

---

💜 FRV Options 💜

• -mgpr-32 — only use the first 32 general-purpose registers.
• -mgpr-64 — use all 64 general-purpose registers.
• -mfpr-32 — use only the first 32 floating-point registers.
• -mfpr-64 — use all 64 floating-point registers.
• -mhard-float — use hardware instructions for floating-point operations. 💅
• -msoft-float — use library routines for floating-point operations.
• -malloc-cc — dynamically allocate condition code registers.
• -mfixed-cc — do not try to dynamically allocate condition code registers, only use icc0 and fcc0. periodt. ✨
• -mdword — change ABI to use double word insns.
• -mno-dword — do not use double word instructions.
• -mdouble — use floating-point double instructions.
• -mno-double — do not use floating-point double instructions.
• -mmedia — use media instructions. 🌈
• -mno-media — nope, don't use media instructions, we're not doing that today 💅
• -mmuladd — use multiply and add/subtract instructions ✨
• -mno-muladd — and this one says do not use multiply and add/subtract instructions 🙅
• -mfdpic — okay so this picks the FDPIC ABI, which uses function descriptors to represent pointers to functions. If you give it no PIC/PIE-related options at all, it implies -fPIE. With -fpic or -fpie, it assumes GOT entries and small data live within a 12-bit range from the GOT base address. With -fPIC or -fPIE, GOT offsets get computed with 32 bits. And with a bfin-elf target, this option implies -msim. slay 🎀
• -minline-plt — turns on inlining of PLT entries in function calls to functions that aren't known to bind locally. Lowkey has no effect without -mfdpic. It's on by default when you're optimizing for speed and compiling for shared libraries (so, -fPIC or -fpic), or when an optimization option like -O3 or above shows up on 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 of "GPREL" relocations in the FDPIC ABI for data known to be in read-only sections. Enabled by default, except for -fpic or -fpie: even though it may help make the global offset table smaller, it trades 1 instruction for 4. With -fPIC or -fPIE, it trades 3 instructions for 4, one of which may be shared by multiple symbols, and it avoids needing a GOT entry for the referenced symbol, so it's more likely to be a win. If it's not a win, -mno-gprel-ro disables it 🫶
• -multilib-library-pic — link with the (library, not FD) pic libraries. It's implied by -mlibrary-pic, and also by -fPIC and -fpic without -mfdpic. Bestie, 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. Enabled by default, and you can turn it off with -mno-linked-fp.
• -mlong-calls — use indirect addressing to call functions outside the current compilation unit. This lets those functions live anywhere in the 32-bit address space ✨
• -malign-labels — tries to align labels to an 8-byte boundary by inserting NOPs into the previous packet. Only has an effect when VLIW packing is enabled. It doesn't create new packets, it just adds NOPs to existing ones 🌸
• -mlibrary-pic — generate position-independent EABI code.
• -macc-4 — use only the first four media accumulator registers.
• -macc-8 — use all eight media accumulator registers 👑
• -mpack — pack VLIW instructions.
• -mno-pack — do not pack VLIW instructions.
• -mno-eflags — do not mark ABI switches in e_flags.
• -mcond-move — enable the use of conditional-move instructions (default).

This switch is mainly for debugging the compiler and will likely be removed in a future version 🙈

• -mno-cond-move — disable the use of conditional-move instructions.

This switch is mainly for debugging the compiler and will likely be removed in a future version 🙈

• -mscc — enable the use of conditional set instructions (default).

This switch is mainly for debugging the compiler and will likely be removed in a future version 💀

• -mno-scc — disable the use of conditional set instructions.

This switch is mainly for debugging the compiler and will likely be removed in a future version 💀

• -mcond-exec — enable the use of conditional execution (default).

This switch is mainly for debugging the compiler and will likely be removed in a future version 😭

• -mno-cond-exec — disable the use of conditional execution.

This switch is mainly for debugging the compiler and will likely be removed in a future version 😭

• -mvliw-branch — run a pass to pack branches into VLIW instructions (default).

This switch is mainly for debugging the compiler and will likely be removed in a future version 🧚

• -mno-vliw-branch — do not run a pass to pack branches into VLIW instructions.

This switch is mainly for debugging the compiler and will likely be removed in a future version 🧚

• -mmulti-cond-exec — enable optimization of "&&" and "||" in conditional execution (default).

This switch is mainly for debugging the compiler and will likely be removed in a future version 🍓

• -mno-multi-cond-exec — disable optimization of "&&" and "||" in conditional execution.

This switch is mainly for debugging the compiler and will likely be removed in a future version 🍓

• -mnested-cond-exec — enable nested conditional execution optimizations (default).

This switch is mainly for debugging the compiler and will likely be removed in a future version 🌈

• -mno-nested-cond-exec — disable nested conditional execution optimizations.

This switch is mainly for debugging the compiler and will likely be removed in a future version 🌈

• -moptimize-membar — this switch removes redundant "membar" instructions from the compiler-generated code. It's enabled by default ✨
• -mno-optimize-membar — this switch disables the automatic removal of redundant "membar" instructions from the generated code.
• -mtomcat-stats — causes gas to print out tomcat statistics 🐱
• *-mcpu=cpu* — select the processor type to generate code for. The possible values are frv, fr550, tomcat, fr500, fr450, fr405, fr400, fr300 and simple.

---

🌸 GNU/Linux Options 🌸

These -m options are defined for GNU/Linux targets, bestie:

• -mglibc — use the GNU C library. This is the default except on *-*-linux-*uclibc*, *-*-linux-*musl* and *-*-linux-*android* targets.
• -muclibc — use uClibc C library. This is the default on *-*-linux-*uclibc* targets.
• -mmusl — use the musl C library. This is the default on *-*-linux-*musl* targets.
• -mbionic — use Bionic C library. This is the default on *-*-linux-*android* targets.
• -mandroid — compile code compatible with the Android platform. This is the default on *-*-linux-*android* targets.

When compiling, this option enables -mbionic, -fPIC, -fno-exceptions and -fno-rtti by default. When linking, this option makes the GCC driver pass Android-specific options to the linker. Finally, this option causes the preprocessor macro "__ANDROID__" to be defined 🤖

• -tno-android-cc — disable compilation effects of -mandroid, i.e. don't enable -mbionic, -fPIC, -fno-exceptions and -fno-rtti by default.
• -tno-android-ld — disable linking effects of -mandroid, i.e. pass standard Linux linking options to the linker.

---

🎀 H8/300 Options 🎀

These -m options are defined for the H8/300 implementations:

• -mrelax — shorten some address references at link time, when possible; uses the linker option -relax.
• -mh — generate code for the H8/300H.
• -ms — generate code for the H8S.
• -mn — generate code for the H8S and H8/300H in the normal mode. This switch must be used either with -mh or -ms ⚠️
• -ms2600 — generate code for the H8S/2600. This switch must be used with -ms.
• -mexr — extended registers are stored on stack before execution of function with monitor attribute. Default option is -mexr. This option is valid only for H8S targets.
• -mno-exr — extended registers are not stored on stack before execution of function with monitor attribute. Default option is -mno-exr. This option is valid only for H8S targets.
• -mint32 — make "int" data 32 bits by default.
• -malign-300 — on the H8/300H and H8S, use the same alignment rules as for the H8/300. The default for the H8/300H and H8S is to align longs and floats on 4-byte boundaries. -malign-300 causes them to be aligned on 2-byte boundaries. This option has no effect on the H8/300 👑

---

💅 HPPA Options 💅

These -m options are defined for the HPPA family of computers:

• *-march=architecture-type* — generate code for the specified architecture. The choices for architecture-type are 1.0 for PA 1.0, 1.1 for PA 1.1, and 2.0 for PA 2.0 processors. Refer to /usr/lib/sched.models on an HP-UX system to determine the proper architecture option for your machine. Code compiled for lower numbered architectures runs on higher numbered architectures, but not the other way around 👀
• -mpa-risc-1-0
• -mpa-risc-1-1
• -mpa-risc-2-0

Synonyms for -march=1.0, -march=1.1, and -march=2.0 respectively.

• -matomic-libcalls — generate libcalls for atomic loads and stores when sync libcalls are disabled. This option is enabled by default. It only affects the generation of atomic libcalls by the HPPA backend.

Both the sync and libatomic libcall implementations use locking. As a result, processor stores are not atomic with respect to other atomic operations. Processor loads up to DImode are atomic with respect to other atomic operations provided they are implemented as a single access.

The PA-RISC architecture does not support any atomic operations in hardware except for the "ldcw" instruction. Thus, all atomic support is implemented using sync and atomic libcalls. Sync libcall support is in libgcc.a. Atomic libcall support is in libatomic.

This option generates "__atomic_exchange" calls for atomic stores. It also provides special handling for atomic DImode accesses on 32-bit targets 🔥

• -mbig-switch — does nothing. Preserved for backward compatibility 💀
• -mcaller-copies — the caller copies function arguments passed by hidden reference. This option should be used with care as it is not compatible with the default 32-bit runtime ⚠️ However, only aggregates larger than eight bytes are passed by hidden reference and the option provides better compatibility with OpenMP.
• -mcoherent-ldcw — use ldcw/ldcd coherent cache-control hint.
• -mdisable-fpregs — disable floating-point registers. Equivalent to "-msoft-float".
• -mdisable-indexing — prevent the compiler from using indexing address modes. This avoids some rather obscure problems when compiling MIG generated code under MACH 🧚
• -mfast-indirect-calls — generate code that assumes calls never cross space boundaries. This allows GCC to emit code that performs faster indirect calls ✨

This option does not work in the presence of shared libraries or nested functions 🙈

• -mfixed-range=register-range — ok so this one makes GCC treat a whole register range as fixed registers, meaning the register allocator literally cannot touch them 💅. Super handy when you're compiling kernel code, no cap. A register range is written as two registers with a dash in between, and you can stack multiple ranges by separating them with commas 🎀.
• -mgas — turn on assembler directives that only GAS understands. exclusive club, GAS only, sorry bestie ✨.
• -mgnu-ld — use the options that are specific to GNU ld. This passes -shared to ld when you're building a shared library 🫶. It's the default when GCC gets configured (explicitly or implicitly) with the GNU linker. Important tho: this does not change which ld gets called — it only changes what parameters get passed to that ld. Which ld actually gets called is decided by the --with-ld configure option, then GCC's program search path, and finally by your PATH. You can see which linker GCC uses with which gcc -print-prog-name=ld``. Also it's only available on the 64-bit HP-UX GCC, i.e. configured with hppa*64*-*-hpux* 💀.
• -mhp-ld — same energy but for HP ld 💖. This passes -b to ld when building a shared library, and passes +Accept TypeMismatch to ld on all links. It's the default when GCC is configured (explicitly or implicitly) with the HP linker. And again: this does not affect which ld gets called, only what parameters get passed to it. Which ld is called comes from the --with-ld configure option, GCC's program search path, and finally your PATH. Print the linker GCC uses with which gcc -print-prog-name=ld``. Only available on the 64-bit HP-UX GCC, configured with hppa*64*-*-hpux* 🌸.
• -mlinker-opt — enables the optimization pass in the HP-UX linker ✨. Heads up tho: this makes symbolic debugging impossible 😭. It also triggers a bug in the HP-UX 8 and HP-UX 9 linkers where they throw bogus error messages when linking some programs 💀.
• -mlong-calls — generate code that uses long call sequences, so a call is always able to reach linker-generated stubs 🦋. By default, GCC only generates long calls when the distance from the call site to the beginning of the function or translation unit (whichever applies) goes over a predefined limit set by the branch type being used. Those limits for normal calls are 7,600,000 and 240,000 bytes, respectively for the PA 2.0 and PA 1.X architectures. Sibcalls are always limited at 240,000 bytes.

Distances get measured from the beginning of functions when you use -ffunction-sections, or when you use -mgas and -mno-portable-runtime together under HP-UX with the SOM linker.

Normally you do not want this option, because it degrades performance 😬. But it can be useful in large applications, especially when partial linking is used to build the application. The types of long calls used depend on the capabilities of the assembler and linker, and on the type of code being generated. On systems that support long absolute calls, and long pic symbol-difference or pc-relative calls, the impact should be relatively small 🌈. However, on 32-bit ELF systems in pic code an indirect call gets used, and it is quite long 💅.

• -mlong-load-store — generate 3-instruction load and store sequences, as sometimes required by the HP-UX 10 linker 🎀. This is equivalent to the +k option to the HP compilers.
• -mjump-in-delay — this option is ignored and provided for compatibility purposes only 🙈.
• -mno-space-regs — generate code that assumes the target has no space registers ✨. This lets GCC generate faster indirect calls and use unscaled index address modes. Such code is suitable for level 0 PA systems and kernels 👑.
• -mordered — assume memory references are ordered and barriers are not needed 💅.
• -mportable-runtime — use the portable calling conventions proposed by HP for ELF systems 🫶.
• -mschedule=cpu-type — schedule code according to the constraints for the machine type cpu-type. The choices for cpu-type are 700 7100, 7100LC, 7200, 7300 and 8000 🌸. Check /usr/lib/sched.models on an HP-UX system to figure out the proper scheduling option for your machine. The default scheduling is 8000 ⭐.
• -msio — generate the predefine "_SIO" for server IO 🦋. The default is -mwsio. That one generates the predefines "__hp9000s700", "__hp9000s700__" and "_WSIO" for workstation IO. These options are available under HP-UX and HI-UX 💖.
• -msoft-float — generate output that contains 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 get used, but you can't do that 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 💀. Specifically, you need to compile libgcc.a — the library that comes with GCC — with -msoft-float for this to work.
• -msoft-mult — use software integer multiplication 💅. This disables the use of the "xmpyu" instruction.
• -munix=unix-std — generate compiler predefines and select a startfile for the specified UNIX standard 🎀. The choices for unix-std are 93, 95 and 98. 93 is supported on all HP-UX versions. 95 is available on HP-UX 10.10 and later. 98 is available on HP-UX 11.11 and later. The default values are 93 for HP-UX 10.00, 95 for HP-UX 10.10 through to 11.00, and 98 for HP-UX 11.11 and later ⭐.

-munix=93 provides the same predefines as GCC 3.3 and 3.4. -munix=95 provides additional predefines for "XOPEN_UNIX" and "_XOPEN_SOURCE_EXTENDED", plus the startfile unix95.o. -munix=98 provides additional predefines for "_XOPEN_UNIX", "_XOPEN_SOURCE_EXTENDED", "_INCLUDE__STDC_A1_SOURCE" and "_INCLUDE_XOPEN_SOURCE_500", plus the startfile unix98.o ✨.

It is important to note that this option changes the interfaces for various library routines. It also affects the operational behavior of the C library. Thus, extreme care is needed in using this option. 🚨

Library code that's intended to operate with more than one UNIX standard must test, set and restore the variable "__xpg4_extended_mask" as appropriate. Most GNU software doesn't provide this capability 🙈.

• -nolibdld — suppress the generation of link options to search libdld.sl when the -static option is specified on HP-UX 10 and later 💅.
• -static — so the HP-UX implementation of setlocale in libc has a dependency on libdld.sl 😭. There is no archive version of libdld.sl. So when -static is specified, special link options are needed to resolve that dependency. On HP-UX 10 and later, the GCC driver adds the necessary options to link with libdld.sl when -static is specified, and this causes the resulting binary to be dynamic 💀. On the 64-bit port, the linkers generate dynamic binaries by default in any case. The -nolibdld option can be used to stop the GCC driver from adding these link options 🎀.
• -threads — add support for multithreading with the dce thread library under HP-UX 🦋. This option sets flags for both the preprocessor and linker.

---

✨ IA-64 Options ✨

These are the -m options defined for the Intel IA-64 architecture 👑.

• -mbig-endian — generate code for a big-endian target 🌸. This is the default for HP-UX.
• -mlittle-endian — generate code for a little-endian target ✨. This is the default for AIX5 and GNU/Linux.
• -mgnu-as, -mno-gnu-as — generate (or don't) code for the GNU assembler 💅. This is the default.
• -mgnu-ld, -mno-gnu-ld — generate (or don't) code for the GNU linker 🫶. This is the default.
• -mno-pic — generate code that does not use a global pointer register 😬. The result is not position independent code, and it violates the IA-64 ABI 🚨.
• -mvolatile-asm-stop, -mno-volatile-asm-stop — generate (or don't) a stop bit immediately before and after volatile asm statements 💖.
• -mregister-names, -mno-register-names — generate (or don't) in, loc, and out register names for the stacked registers ✨. This may make assembler output more readable 👑.
• -mno-sdata, -msdata — disable (or enable) optimizations that use the small data section 🌸. This may be useful for working around optimizer bugs 💀.
• -mconstant-gp — generate code that uses a single constant global pointer value 🎀. This is useful when compiling kernel code.
• -mauto-pic — generate code that is self-relocatable 🦋. This implies -mconstant-gp. This is useful when compiling firmware code.
• -minline-float-divide-min-latency — generate code for inline divides of floating-point values using the minimum latency algorithm ⭐.
• -minline-float-divide-max-throughput — generate code for inline divides of floating-point values using the maximum throughput algorithm 🔥.
• -mno-inline-float-divide — do not generate inline code for divides of floating-point values 🙈.
• -minline-int-divide-min-latency — generate code for inline divides of integer values using the minimum latency algorithm ✨.
• -minline-int-divide-max-throughput — generate code for inline divides of integer values using the maximum throughput algorithm 💅.
• -mno-inline-int-divide — do not generate inline code for divides of integer values 🙈.
• -minline-sqrt-min-latency — generate code for inline square roots using the minimum latency algorithm 🌈.
• -minline-sqrt-max-throughput — generate code for inline square roots using the maximum throughput algorithm 💜.
• -mno-inline-sqrt — do not generate inline code for "sqrt" 😭.
• -mfused-madd, -mno-fused-madd — do (don't) generate code that uses the fused multiply/add or multiply/subtract instructions 🫶. The default is to use these instructions.
• -mno-dwarf2-asm, -mdwarf2-asm — don't (or do) generate assembler code for the DWARF line number debugging info 🎀. This may be useful when not using the GNU assembler.
• -mearly-stop-bits, -mno-early-stop-bits — allow stop bits to be placed earlier than immediately preceding the instruction that triggered the stop bit ✨. This can improve instruction scheduling, but it does not always do so 😬.
• -mfixed-range=register-range — bestie, this one says "treat this register range as fixed", meaning the register allocator literally cannot touch it 🚫. Lowkey essential when you're compiling kernel code. A register range is just two registers with a dash in between, and if you wanna flex, you can pass multiple ranges separated by commas 💅.
• -mtls-size=tls-size — sets the bit size of immediate TLS offsets. The only valid values are 14, 22, and 64. No vibes in between, pick one of the three ✨.
• -mtune=cpu-type — tunes the instruction scheduling for a particular CPU. Valid values: itanium, itanium1, merced, itanium2, and mckinley. It's giving very specific era of silicon ngl 🦋.
• -milp32, -mlp64 — generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer to 32 bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits. These are HP-UX specific flags, iykyk 👑.
• -mno-sched-br-data-spec, -msched-br-data-spec — (Dis/En)able data speculative scheduling before reload. This results in generation of ld.a instructions and the corresponding check instructions (ld.c / chk.a). The default setting is disabled 🙈.
• -msched-ar-data-spec, -mno-sched-ar-data-spec — (En/Dis)able data speculative scheduling after reload. This results in generation of ld.a instructions and the corresponding check instructions (ld.c / chk.a). The default setting is enabled ✅.
• -mno-sched-control-spec, -msched-control-spec — (Dis/En)able control speculative scheduling. This feature is available only during region scheduling (i.e. before reload). This results in generation of the ld.s instructions and the corresponding check instructions chk.s. The default setting is disabled 🙅‍♀️.
• -msched-br-in-data-spec, -mno-sched-br-in-data-spec — (En/Dis)able speculative scheduling of the instructions that are dependent on the data speculative loads before reload. This is effective only with -msched-br-data-spec enabled. The default setting is enabled 💖.
• -msched-ar-in-data-spec, -mno-sched-ar-in-data-spec — (En/Dis)able speculative scheduling of the instructions that are dependent on the data speculative loads after reload. This is effective only with -msched-ar-data-spec enabled. The default setting is enabled 💖.
• -msched-in-control-spec, -mno-sched-in-control-spec — (En/Dis)able speculative scheduling of the instructions that are dependent on the control speculative loads. This is effective only with -msched-control-spec enabled. The default setting is enabled 💖.
• -mno-sched-prefer-non-data-spec-insns, -msched-prefer-non-data-spec-insns — if enabled, data-speculative instructions are chosen for schedule only if there are no other choices at the moment. This makes the use of the data speculation much more conservative. The default setting is disabled 😌.
• -mno-sched-prefer-non-control-spec-insns, -msched-prefer-non-control-spec-insns — if enabled, control-speculative instructions are chosen for schedule only if there are no other choices at the moment. This makes the use of the control speculation much more conservative. The default setting is disabled 😌.
• -mno-sched-count-spec-in-critical-path, -msched-count-spec-in-critical-path — if enabled, speculative dependencies are considered during computation of the instructions priorities. This makes the use of the speculation a bit more conservative. The default setting is disabled 😌.
• -msched-spec-ldc — use a simple data speculation check. This option is on by default, periodt ✨.
• -msched-control-spec-ldc — use a simple check for control speculation. This option is on by default ✨.
• -msched-stop-bits-after-every-cycle — place a stop bit after every cycle when scheduling. This option is on by default ✨.
• -msched-fp-mem-deps-zero-cost — assume that floating-point stores and loads are not likely to cause a conflict when placed into the same instruction group. This option is disabled by default 🚫.
• -msel-sched-dont-check-control-spec — generate checks for control speculation in selective scheduling. This flag is disabled by default 🚫.
• -msched-max-memory-insns=max-insns — limit on the number of memory insns per instruction group, giving lower priority to subsequent memory insns attempting to schedule in the same instruction group. Frequently useful to prevent cache bank conflicts. The default value is 1 🎯.
• -msched-max-memory-insns-hard-limit — makes the limit specified by msched-max-memory-insns a hard limit, disallowing more than that number in an instruction group. Otherwise, the limit is "soft", meaning that non-memory operations are preferred when the limit is reached, but memory operations may still be scheduled 💅.

---

🎀 LM32 Options 🎀

These -m options are defined for the LatticeMico32 architecture, bestie:

• -mbarrel-shift-enabled — enable barrel-shift instructions 🔥.
• -mdivide-enabled — enable divide and modulus instructions 🔥.
• -mmultiply-enabled — enable multiply instructions 🔥.
• -msign-extend-enabled — enable sign extend instructions 🔥.
• -muser-enabled — enable user-defined instructions 🔥.

---

🦋 LoongArch Options 🦋

These command-line options are defined for LoongArch targets:

• -march=arch-type — generate instructions for the machine type arch-type. -march=arch-type allows GCC to generate code that may not run at all on processors other than the one indicated. ⚠️ Like, it will literally not run elsewhere, no cap.

The choices for arch-type are:

• native — local processor type detected by the native compiler 🔍.
• loongarch64 — generic LoongArch 64-bit processor.
• la464 — LoongArch LA464-based processor with LSX, LASX.
• la664 — LoongArch LA664-based processor with LSX, LASX and all LoongArch v1.1 instructions.
• la64v1.0 — LoongArch64 ISA version 1.0.
• la64v1.1 — LoongArch64 ISA version 1.1.

More information about LoongArch ISA versions can be found at https://github.com/loongson/la-toolchain-conventions 🫶.

• -mtune=tune-type — optimize the generated code for the given processor target.

The choices for tune-type are:

• native — local processor type detected by the native compiler 🔍.
• generic — generic LoongArch processor.
• loongarch64 — generic LoongArch 64-bit processor.
• la464 — LoongArch LA464 core.
• la664 — LoongArch LA664 core.

• -mabi=base-abi-type — generate code for the specified calling convention. base-abi-type can be one of:
  • lp64d — uses 64-bit general purpose registers and 32/64-bit floating-point registers for parameter passing. Data model is LP64, where int is 32 bits, while long int and pointers are 64 bits.
  • lp64f — uses 64-bit general purpose registers and 32-bit floating-point registers for parameter passing. Data model is LP64, where int is 32 bits, while long int and pointers are 64 bits.
  • lp64s — uses 64-bit general purpose registers and no floating-point registers for parameter passing. Data model is LP64, where int is 32 bits, while long int and pointers are 64 bits.
• -mfpu=fpu-type — generate code for the specified FPU type, which can be one of:
  • 64 — allow the use of hardware floating-point instructions for 32-bit and 64-bit operations.
  • 32 — allow the use of hardware floating-point instructions for 32-bit operations.
  • none, 0 — prevent the use of hardware floating-point instructions.
• -msimd=simd-type — enable generation of LoongArch SIMD instructions for vectorization and via builtin functions. The value can be one of:
  • lasx — enable generating instructions from the 256-bit LoongArch Advanced SIMD Extension (LASX) and the 128-bit LoongArch SIMD Extension (LSX).
  • lsx — enable generating instructions from the 128-bit LoongArch SIMD Extension (LSX).
  • none — no LoongArch SIMD instruction may be generated.
• -msoft-float — force -mfpu=none and prevents the use of floating-point registers for parameter passing. This option may change the target ABI 👀.
• -msingle-float — force -mfpu=32 and allow the use of 32-bit floating-point registers for parameter passing. This option may change the target ABI 👀.
• -mdouble-float — force -mfpu=64 and allow the use of 32/64-bit floating-point registers for parameter passing. This option may change the target ABI 👀.
• -mlasx, -mno-lasx, -mlsx, -mno-lsx — incrementally adjust the scope of the SIMD extensions (none / LSX / LASX) that can be used by the compiler for code generation. Enabling LASX with mlasx automatically enables LSX, and diabling LSX with mno-lsx automatically disables LASX. These driver-only options act upon the final msimd configuration state and make incremental chagnes in the order they appear on the GCC driver's command line, deriving the final / canonicalized msimd option that is passed to the compiler proper 💅.
• -mbranch-cost=n — set the cost of branches to roughly n instructions.
• -mcheck-zero-division, -mno-check-zero-divison — trap (do not trap) on integer division by zero. The default is -mcheck-zero-division for -O0 or -Og, and -mno-check-zero-division for other optimization levels 💀.
• -mcond-move-int, -mno-cond-move-int — conditional moves for integral data in general-purpose registers are enabled (disabled). The default is -mcond-move-int.
• -mcond-move-float, -mno-cond-move-float — conditional moves for floating-point registers are enabled (disabled). The default is -mcond-move-float.
• -mmemcpy, -mno-memcpy — force (do not force) the use of memcpy for non-trivial block moves. The default is -mno-memcpy, which allows GCC to inline most constant-sized copies. Setting optimization level to -Os also forces the use of memcpy, but -mno-memcpy may override this behavior if explicitly specified, regardless of the order these options on the command line 👑.
• -mstrict-align, -mno-strict-align — avoid or allow generating memory accesses that may not be aligned on a natural object boundary as described in the architecture specification. The default is -mno-strict-align.
• -msmall-data-limit=number — put global and static data smaller than number bytes into a special section (on some targets). The default value is 0 ✨.
• -mmax-inline-memcpy-size=n — bestie, this inlines every block move that's n bytes or smaller — think calls to memcpy, struct copies, all of it ✨. The default for n is 1024.
• -mcmodel=code-model — sets the code model to one of these, okay:
  • tiny-static — (Not implemented yet 🙈)
  • tiny — (Not implemented yet 🙈)
  • normal — the text segment has to live inside 128MB of addressing space, and the data segment has to fit in 2GB of addressing space. Very "everything in its lane" of it 💅
  • medium — both the text segment and the data segment have to be within 2GB addressing space.
  • large — (Not implemented yet 🙈)
  • extreme — this one literally does not limit the size of the code segment or the data segment at all, main character energy 👑. Heads up though: -mcmodel=extreme is incompatible with -fplt and/or -mexplicit-relocs=none.

The default code model is normal.

• -mexplicit-relocs=style — this picks when to use assembler relocation operators for symbolic addresses. The alternative is assembler macros, which can limit instruction scheduling but do allow linker relaxation. So: 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=auto the compiler uses relocation operators where linker relaxation is impossible (to keep code quality up) and macros everywhere else. The default depends on the assembler capability detected when GCC was built plus the setting of -mrelax: -mexplicit-relocs=none if the assembler doesn't support relocation operators at all, -mexplicit-relocs=always if the assembler supports them but -mrelax isn't enabled, and -mexplicit-relocs=auto if the assembler supports them and -mrelax is enabled.
• -mexplicit-relocs — just an alias of -mexplicit-relocs=always, kept around for backward compatibility 🫶
• -mno-explicit-relocs — an alias of -mexplicit-relocs=none, also for backward compatibility.
• -mdirect-extern-access, -mno-direct-extern-access — don't use or do use GOT to access external symbols. The default is -mno-direct-extern-access: GOT is used for external symbols with default visibility, but not used for other external symbols.

With -mdirect-extern-access, GOT isn't used at all and all external symbols are PC-relatively addressed. It's only suitable for environments where no dynamic link happens — like firmwares, OS kernels, executables linked with -static or -static-pie. Also, -mdirect-extern-access is not compatible with -fPIC or -fpic. So don't try it, fr fr.

• -mrelax, -mno-relax — take (or don't take) advantage of linker relaxations. If -mpass-mrelax-to-as is enabled, this option also gets passed to the assembler. The default is figured out during GCC build-time by detecting the corresponding assembler support: -mrelax if the assembler supports both the -mrelax option and conditional branch relaxation (this is required, otherwise the .align directives and conditional branch instructions in the assembly code GCC outputs may be rejected by the assembler because of a relocation overflow 💀), and -mno-relax otherwise.
• -mpass-mrelax-to-as, -mno-pass-mrelax-to-as — pass (or don't pass) the -mrelax or -mno-relax option to the assembler. The default is determined during GCC build-time by detecting the corresponding assembler support: -mpass-mrelax-to-as if the assembler supports the -mrelax option, -mno-pass-mrelax-to-as otherwise. This one's mostly useful for debugging, or for interoperating with assemblers that aren't the build-time one.
• -mrecip — this enables use of the reciprocal estimate and reciprocal square root estimate instructions with extra Newton-Raphson steps to bump up precision, instead of doing a divide (or square root and divide) for floating-point arguments. These instructions only get generated when -funsafe-math-optimizations is enabled together with -ffinite-math-only and -fno-trapping-math. This option is off by default. Before you can use it, you have to make sure the target CPU supports frecipe and frsqrte instructions. And ngl — even though the throughput of the sequence is higher than the throughput of the non-reciprocal instruction, the precision can drop by up to 2 ulp (like, the inverse of 1.0 equals 0.99999994 😭). So, tradeoffs, bestie.
• -mrecip=opt — this one controls which reciprocal estimate instructions are allowed to be used. opt is 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.
  • 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. Slay 💅

• -mfrecipe, -mno-frecipe — use (or don't use) frecipe.{s/d} and frsqrte.{s/d} instructions. When built with -march=la664, it's enabled by default. The default is -mno-frecipe.
• -mdiv32, -mno-div32 — use (or don't use) div.w[u] and mod.w[u] instructions with input that isn't sign-extended. When built with -march=la664, it's enabled by default. The default is -mno-div32.
• -mlam-bh, -mno-lam-bh — use (or don't use) am{swap/add}[_db].{b/h} instructions. When built with -march=la664, it's enabled by default. The default is -mno-lam-bh.
• -mlamcas, -mno-lamcas — use (or don't use) amcas[_db].{b/h/w/d} instructions. When built with -march=la664, it's enabled by default. The default is -mno-lamcas.
• -mld-seq-sa, -mno-ld-seq-sa — whether a load-load barrier (dbar 0x700) is needed. When built with -march=la664, it's enabled by default. The default is -mno-ld-seq-sa, meaning the load-load barrier is needed.
• -mtls-dialect=opt — this controls which tls dialect may be used for general dynamic and local dynamic TLS models.
  • trad — use traditional TLS. This is the default.
  • desc — use TLS descriptors.
• --param loongarch-vect-unroll-limit=n — the vectorizer uses whatever tuning info is available to decide whether it would be beneficial to unroll the main vectorized loop, and by how much. This parameter sets the upper bound of how much the vectorizer will unroll the main loop. The default value is six.

---

🌈 M32C Options 🌈

• -mcpu=name — pick the CPU that code gets generated for. name can be r8c for the R8C/Tiny series, m16c for the M16C (up to /60) series, m32cm for the M16C/80 series, or m32c for the M32C/80 series.
• -msim — says the program is gonna be run on the simulator 🧚. This links in an alternate runtime library that supports stuff like file I/O. You must not use this option when generating programs that will run on real hardware — you have to provide your own runtime library for whatever I/O functions you need. No cap.
• -memregs=number — sets how many memory-based pseudo-registers GCC uses during code generation. These pseudo-registers get used like real registers, so there's a tradeoff between GCC's ability to fit the code into available registers and the performance penalty of using memory instead of registers. Note that all modules in a program must be compiled with the same value for this option. Because of that, you must not use this option with GCC's default runtime libraries. ⚠️

---

🌸 M32R/D Options 🌸

These -m options are defined for Renesas M32R/D architectures:

• -m32r2 — generate code for the M32R/2.
• -m32rx — generate code for the M32R/X.
• -m32r — generate code for the M32R. This is the default.
• -mmodel=small — assume all objects live in the lower 16MB of memory (so their addresses can be loaded with the ld24 instruction), and assume all subroutines are reachable with the bl instruction. This is the default.

The addressability of a particular object can be set with the model attribute.

• -mmodel=medium — assume objects may be anywhere in the 32-bit address space (the compiler generates seth/add3 instructions to load their addresses), and assume all subroutines are reachable with the bl instruction.
• -mmodel=large — assume objects may be anywhere in the 32-bit address space (the compiler generates seth/add3 instructions to load their addresses), and assume subroutines may not be reachable with the bl instruction (the compiler generates the much slower seth/add3/jl instruction sequence 😭).
• -msdata=none — disable use of the small data area. Variables get put into one of .data, .bss, or .rodata (unless the section attribute has been specified). This is the default.

The small data area consists of the sections .sdata and .sbss. Objects may be explicitly put in the small data area with the section attribute using one of these sections.

• -msdata=sdata — put small global and static data in the small data area, but don't generate special code to reference them.
• -msdata=use — put small global and static data in the small data area, and generate special instructions to reference them.
• -G num — put global and static objects less than or equal to num bytes into the small data or BSS sections instead of the normal data or BSS sections. The default value of num is 8. The -msdata option has to be set to one of sdata or use for this option to actually do anything.

All modules should be compiled with the same -G num value. Compiling with different values of num may or may not work; if it doesn't, the linker gives an error message — incorrect code is not generated. 💀

• -mdebug — makes the M32R-specific code in the compiler display some statistics that might help when debugging programs.
• -malign-loops — align all loops to a 32-byte boundary.
• -mno-align-loops — bestie, this says don't force loops to be 32-byte aligned. And that's literally the default, so you're already living this life 💅
• *-missue-rate=number*** — issue *number* instructions per cycle. And *number*? It can **only** be 1or2`. That's the whole menu, no secret third option 🍓
• *-mbranch-cost=number*** — again, *number* can only be 1or2. If it's 1, branches get preferred over conditional code. If it's 2`, it's the other way around. Pick a lane ✨
• *-mflush-trap=number*** — tells the compiler which trap number to use for flushing the cache. Default is 12, and valid numbers are 0through15` inclusive. Anything else is not the vibe 🙈
• -mno-flush-trap — says the cache cannot be flushed using a trap. No trap, no flush, no problem 💀
• *-mflush-func=name*** — the name of the operating system function to call to flush the cache. Default is _flush_cache` — but heads up, that function call only gets used if a trap isn't available. So like, plan B energy 🫶
• -mno-flush-func — declares that there is no OS function for flushing the cache. Periodt.

---

🎀 M680x0 Options 🎀

Okay so these are all the -m options defined for M680x0 and ColdFire processors. The default settings depend on which architecture got selected when the compiler was configured — and the defaults for the most common choices are listed below. Lowkey important, don't skip it 🌸

• *-march=arch*** — generates code for a specific M680x0 or ColdFire instruction set architecture. The permissible values of *arch* for M680x0 architectures are: 68000, 68010, 68020, 68030, 68040, 68060andcpu32. ColdFire architectures get selected according to Freescale's ISA classification, and the permissible values there are: isaa, isaaplus, isabandisac`.

GCC defines a macro __mcfarch__ whenever it's generating code for a ColdFire target. The arch in that macro is one of the -march arguments given above.

When you use them together, -march and -mtune select code that runs on a family of similar processors but that's optimized for a particular microarchitecture. Sibling energy fr fr 👑

• *-mcpu=cpu*** — generates code for a specific M680x0 or ColdFire processor. The M680x0 cpus are: 68000, 68010, 68020, 68030, 68040, 68060, 68302, 68332andcpu32`. The ColdFire cpus are given by the table below, which also classifies the CPUs into families:
  • Family 51: 51 51ac 51ag 51cn 51em 51je 51jf 51jg 51jm 51mm 51qe 51qm
  • Family 5206: 5202 5204 5206
  • Family 5206e: 5206e
  • Family 5208: 5207 5208
  • Family 5211a: 5210a 5211a
  • Family 5213: 5211 5212 5213
  • Family 5216: 5214 5216
  • Family 52235: 52230 52231 52232 52233 52234 52235
  • Family 5225: 5224 5225
  • Family 52259: 52252 52254 52255 52256 52258 52259
  • Family 5235: 5232 5233 5234 5235 523x
  • Family 5249: 5249
  • Family 5250: 5250
  • Family 5271: 5270 5271
  • Family 5272: 5272
  • Family 5275: 5274 5275
  • Family 5282: 5280 5281 5282 528x
  • Family 53017: 53011 53012 53013 53014 53015 53016 53017
  • Family 5307: 5307
  • Family 5329: 5327 5328 5329 532x
  • Family 5373: 5372 5373 537x
  • Family 5407: 5407
  • Family 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 — no mixing and matching, bestie, it's giving strict 💅

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*** — tunes 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. Overachiever behavior, we love to see it ✨

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's 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. Look at that little squad 🥹

• -m68010 — generate output for a 68010. This is the default when the compiler is configured for 68010-based systems. It's 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's equivalent to -march=68020.
• -m68030 — generate output for a 68030. This is the default when the compiler is configured for 68030-based systems. It's equivalent to -march=68030.
• -m68040 — generate output for a 68040. This is the default when the compiler is configured for 68040-based systems. It's equivalent to -march=68040.

Heads up though: 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 doesn't have code to emulate those instructions 🫶

• -m68060 — generate output for a 68060. This is the default when the compiler is configured for 68060-based systems. It's equivalent to -march=68060.

Same deal here: 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 doesn't have code to emulate those instructions.

• -mcpu32 — generate output for a CPU32. This is the default when the compiler is configured for CPU32-based systems. It's equivalent to -march=cpu32.

Use this option for microcontrollers with a CPU32 or CPU32+ core, including the 68330, 68331, 68332, 68333, 68334, 68336, 68340, 68341, 68349 and 68360.

• -m5200 — generate output for a 520X ColdFire CPU. This is the default when the compiler is configured for 520X-based systems. It's equivalent to -mcpu=5206, and is now deprecated in favor of that option. 👀

Use this option for a microcontroller with a 5200 core, including the MCF5202, MCF5203, MCF5204 and MCF5206.

• -m5206e — generate output for a 5206e ColdFire CPU. The option is now deprecated in favor of the equivalent -mcpu=5206e.
• -m528x — generate output for a member of the ColdFire 528X family. The option is now deprecated in favor of the equivalent -mcpu=528x.
• -m5307 — generate output for a ColdFire 5307 CPU. The option is now deprecated in favor of the equivalent -mcpu=5307.
• -m5407 — generate output for a ColdFire 5407 CPU. The option is now deprecated in favor of the equivalent -mcpu=5407.
• -mcfv4e — generate output for a ColdFire V4e family CPU (e.g. 547x/548x). This includes use of hardware floating-point instructions. The option is equivalent to -mcpu=547x, and is now deprecated in favor of that option.
• -m68020-40 — generate output for a 68040, without using any of the new instructions. This results in code that can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040. The generated code does use the 68881 instructions that are emulated on the 68040.

The option is equivalent to -march=68020 -mtune=68020-40.

• -m68020-60 — generate output for a 68060, without using any of the new instructions. This results in code that can run relatively efficiently on either a 68020/68881 or a 68030 or a 68040. The generated code does use the 68881 instructions that are emulated on the 68060.

The option is equivalent to -march=68020 -mtune=68020-60.

• -mhard-float, -m68881 — generate floating-point instructions. This is the default for 68020 and above, and for ColdFire devices that have an FPU. It defines the macro __HAVE_68881__ on M680x0 targets and __mcffpu__ on ColdFire targets.
• -msoft-float — do not generate floating-point instructions; use library calls instead. This is the default for 68000, 68010, and 68832 targets. It's also the default for ColdFire devices that have no FPU.
• -mdiv, -mno-div — generate (or don't generate) ColdFire hardware divide and remainder instructions. If -march is used without -mcpu, the default is "on" for ColdFire architectures and "off" for M680x0 architectures. Otherwise, the default is taken from the target CPU (either the default CPU, or the one specified by -mcpu). For example, the default is "off" for -mcpu=5206 and "on" for -mcpu=5206e.

GCC defines the macro __mcfhwdiv__ when this option is enabled. Cute little macro, iconic 💖

• -mshort — consider type int to be 16 bits wide, like short int. Additionally, parameters passed on the stack are also aligned to a 16-bit boundary even on targets whose API mandates promotion to 32-bit.
• -mno-short — do not consider type int to be 16 bits wide. This is the default.
• -mnobitfield, -mno-bitfield — do not use the bit-field instructions. The -m68000, -mcpu32 and -m5200 options imply -mnobitfield.
• -mbitfield — do use the bit-field instructions. The -m68020 option implies -mbitfield. This is the default if you use a configuration designed for a 68020.
• -mrtd — use a different function-calling convention, in which functions that take a fixed number of arguments return with the rtd instruction, which pops their arguments while returning. This saves one instruction in the caller since there's no need to pop the arguments there.

Bestie, listen. 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. Not a vibe, do not do it 😭

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.) So like... be normal about it 💀

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 — pick your alignment era 💅 This controls whether GCC lines up int, long, long long, float, double, and long double variables on a 32-bit boundary (-malign-int) or a 16-bit boundary (-mno-align-int). Doing the 32-bit boundary thing makes your code run somewhat faster on processors with 32-bit busses, but bestie, it costs you more memory. The tradeoff is real.

⚠️ 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… it works, but it's not the vibe everyone else agreed on. 🙈

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. (iykyk — it's a limitation, not a personality trait)

• -mno-strict-align, -mstrict-align — Do not (do) assume that unaligned memory references are handled by the system. Pick your fighter 🎀
• -msep-data — Generate code that allows the data segment to be located in a different area of memory from the text segment. This allows for execute-in-place in an environment without virtual memory management. This option implies -fPIC. ✨
• -mno-sep-data — Generate code that assumes that the data segment follows the text segment. This is the default.
• -mid-shared-library — Generate code that supports shared libraries via the library ID method. This allows for execute-in-place and shared libraries in an environment without virtual memory management. This option implies -fPIC. 💖
• -mno-id-shared-library — Generate code that doesn't assume ID-based shared libraries are being used. This is the default.
• -mshared-library-id=n — Specifies the identification number of the ID-based shared library being compiled. Specifying a value of 0 generates more compact code; specifying other values forces the allocation of that number to the current library, but is no more space- or time-efficient than omitting this option. so like, only if you have a reason, bestie
• -mxgot, -mno-xgot — When generating position-independent code for ColdFire, generate code that works if the GOT has more than 8192 entries. This code is larger and slower than code generated without this option. On M680x0 processors, this option is not needed; -fPIC suffices.

GCC normally uses a single instruction to load values from the GOT. While this is relatively efficient, it only works if the GOT is smaller than about 64k. Anything larger causes the linker to report an error such as:

relocation truncated to fit: R_68K_GOT16O foobar

If this happens, you should recompile your code with -mxgot. It should then work with very large GOTs. However, code generated with -mxgot is less efficient, since it takes 4 instructions to fetch the value of a global symbol. 😭

Note that some linkers, including newer versions of the GNU linker, can create multiple GOTs and sort GOT entries. If you have such a linker, you should only need to use -mxgot when compiling a single object file that accesses more than 8192 GOT entries. Very few do.

These options have no effect unless GCC is generating position-independent code.

• -mlong-jump-table-offsets — Use 32-bit offsets in switch tables. The default is to use 16-bit offsets.

---

🦋 MCore Options 🦋

These are the -m options defined for the Motorola M*Core processors. niche fandom, but we respect it

• -mhardlit, -mno-hardlit — Inline constants into the code stream if it can be done in two instructions or less.
• -mdiv, -mno-div — Use the divide instruction. (Enabled by default).
• -mrelax-immediate, -mno-relax-immediate — Allow arbitrary-sized immediates in bit operations.
• -mwide-bitfields, -mno-wide-bitfields — Always treat bit-fields as int-sized.
• -m4byte-functions, -mno-4byte-functions — Force all functions to be aligned to a 4-byte boundary.
• -mcallgraph-data, -mno-callgraph-data — Emit callgraph information.
• -mslow-bytes, -mno-slow-bytes — Prefer word access when reading byte quantities.
• -mlittle-endian, -mbig-endian — Generate code for a little-endian target.
• -m210, -m340 — Generate code for the 210 processor.
• -mno-lsim — Assume that runtime support has been provided and so omit the simulator library (libsim.a) from the linker command line. trusting the universe fr fr
• -mstack-increment=size — Set the maximum amount for a single stack increment operation. Large values can increase the speed of programs that contain functions that need a large amount of stack space, but they can also trigger a segmentation fault if the stack is extended too much. The default value is 0x1000. 😬

---

🌸 MicroBlaze Options 🌸

• -msoft-float — Use software emulation for floating point (default).
• -mhard-float — Use hardware floating-point instructions. 🔥
• -mmemcpy — Do not optimize block moves, use memcpy.
• -mno-clearbss — This option is deprecated. Use -fno-zero-initialized-in-bss instead.
• -mcpu=cpu-type — Use features of, and schedule code for, the given CPU. Supported values are in the format vX.YY.Z, where X is a major version, YY is the minor version, and Z is compatibility code. Example values are v3.00.a, v4.00.b, v5.00.a, v5.00.b, v6.00.a.
• -mxl-soft-mul — Use software multiply emulation (default).
• -mxl-soft-div — Use software emulation for divides (default).
• -mxl-barrel-shift — Use the hardware barrel shifter. ✨
• -mxl-pattern-compare — Use pattern compare instructions.
• -msmall-divides — Use table lookup optimization for small signed integer divisions.
• -mxl-stack-check — This option is deprecated. Use -fstack-check instead.
• -mxl-gp-opt — Use GP-relative .sdata/.sbss sections.
• -mxl-multiply-high — Use multiply high instructions for high part of 32x32 multiply.
• -mxl-float-convert — Use hardware floating-point conversion instructions.
• -mxl-float-sqrt — Use hardware floating-point square root instruction.
• -mbig-endian — Generate code for a big-endian target.
• -mlittle-endian — Generate code for a little-endian target.
• -mxl-reorder — Use reorder instructions (swap and byte reversed load/store).
• -mxl-mode-app-model — Select application model app-model. Valid models are:
  • executable — normal executable (default), uses startup code crt0.o.
  • xmdstub — for use with Xilinx Microprocessor Debugger (XMD) based software intrusive debug agent called xmdstub. This uses startup file crt1.o and sets the start address of the program to 0x800.
  • bootstrap — for applications that are loaded using a bootloader. This model uses startup file crt2.o which does not contain a processor reset vector handler. This is suitable for transferring control on a processor reset to the bootloader rather than the application.
  • novectors — for applications that do not require any of the MicroBlaze vectors. This option may be useful for applications running within a monitoring application. This model uses crt3.o as a startup file.

🫶 heads up: option -xl-mode-app-model is a deprecated alias for -mxl-mode-app-model.

• -mpic-data-is-text-relative — Assume that the displacement between the text and data segments is fixed at static link time. This allows data to be referenced by offset from start of text address instead of GOT since PC-relative addressing is not supported.

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👑 MIPS Options 👑

• -EB — Generate big-endian code.
• -EL — Generate little-endian code. This is the default for mips*el-*-* configurations.
• -march=arch — Generate code that runs on arch, which can be the name of a generic MIPS ISA, or the name of a particular processor. The ISA names are: mips1, mips2, mips3, mips4, mips32, mips32r2, mips32r3, mips32r5, mips32r6, mips64, mips64r2, mips64r3, mips64r5 and mips64r6. The processor names are: 4kc, 4km, 4kp, 4ksc, 4kec, 4kem, 4kep, 4ksd, 5kc, 5kf, 20kc, 24kc, 24kf2_1, 24kf1_1, 24kec, 24kef2_1, 24kef1_1, 34kc, 34kf2_1, 34kf1_1, 34kn, 74kc, 74kf2_1, 74kf1_1, 74kf3_2, 1004kc, 1004kf2_1, 1004kf1_1, i6400, i6500, interaptiv, loongson2e, loongson2f, loongson3a, gs464, gs464e, gs264e, m4k, m14k, m14kc, m14ke, m14kec, m5100, m5101, octeon, octeon+, octeon2, octeon3, orion, p5600, p6600, r2000, r3000, r3900, r4000, r4400, r4600, r4650, r4700, r5900, r6000, r8000, rm7000, rm9000, r10000, r12000, r14000, r16000, sb1, sr71000, vr4100, vr4111, vr4120, vr4130, vr4300, vr5000, vr5400, vr5500, xlr and xlp. The special value from-abi selects the most compatible architecture for the selected ABI (that is, mips1 for 32-bit ABIs and mips3 for 64-bit ABIs).

The native Linux/GNU toolchain also supports the value native, which selects the best architecture option for the host processor. -march=native has no effect if GCC does not recognize the processor. 💅

In processor names, a final 000 can be abbreviated as k (for example, -march=r2k). Prefixes are optional, and vr may be written r.

Names of the form nf2_1 refer to processors with FPUs clocked at half the rate of the core, names of the form nf1_1 refer to processors with FPUs clocked at the same rate as the core, and names of the form nf3_2 refer to processors with FPUs clocked a ratio of 3:2 with respect to the core. For compatibility reasons, nf is accepted as a synonym for nf2_1 while nx and bfx are accepted as synonyms for nf1_1. the lore goes deep, bestie 🧚

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 — bestie this is the ✨aesthetic✨ knob</p>. It optimizes 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 "dress for the whole friend group but slay for one" energy ngl

-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. per-function vibes only, we love a granular queen

• -mmips16e2, -mno-mips16e2 — Use (do not use) the MIPS16e2 ASE. This option modifies the behavior of the -mips16 option such that it targets the MIPS16e2 ASE. ✨
• -mflip-mips16 — Generate MIPS16 code on alternating functions. alternating!! like a lil pattern!! 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.

🚨 heads up bestie — this one is for regression testing, not for your actual user code. do not build your project with it fr fr.

• -minterlink-compressed, -mno-interlink-compressed — Require (do not require) that code using the standard (uncompressed) MIPS ISA be link-compatible with MIPS16 and microMIPS code, and vice versa. 🔗

For example, code using the standard ISA encoding cannot jump directly to MIPS16 or microMIPS code; it must either use a call or an indirect jump. -minterlink-compressed therefore disables direct jumps unless GCC knows that the target of the jump is not compressed.

• -minterlink-mips16, -mno-interlink-mips16 — Aliases of -minterlink-compressed and -mno-interlink-compressed. These options predate the microMIPS ASE and are retained for backwards compatibility. legacy icons, we respect them
• -mabi=32, -mabi=o64, -mabi=n32, -mabi=64, -mabi=eabi — Generate code for the given ABI. 🫶

Note that the EABI has a 32-bit and a 64-bit variant. GCC normally generates 64-bit code when you select a 64-bit architecture, but you can use -mgp32 to get 32-bit code instead.

For information about the O64 ABI, see https://gcc.gnu.org/projects/mipso64-abi.html.

GCC supports a variant of the o32 ABI in which floating-point registers are 64 rather than 32 bits wide. You can select this combination with -mabi=32 -mfp64. This ABI relies on the mthc1 and mfhc1 instructions and is therefore only supported for MIPS32R2, MIPS32R3 and MIPS32R5 processors. 💅

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. even-numbered only, odd girlies stay home

Two additional variants of the o32 ABI are supported to enable a transition from 32-bit to 64-bit registers. These are FPXX (-mfpxx) and FP64A (-mfp64 -mno-odd-spreg). The FPXX extension mandates that all code must execute correctly when run using 32-bit or 64-bit registers. The code can be interlinked with either FP32 or FP64, but not both. The FP64A extension is similar to the FP64 extension but forbids the use of odd-numbered single-precision registers. This can be used in conjunction with the FRE mode of FPUs in MIPS32R5 processors and allows both FP32 and FP64A code to interlink and run in the same process without changing FPU modes. 🌈

• -mabicalls, -mno-abicalls — Generate (do not generate) code that is suitable for SVR4-style dynamic objects. -mabicalls is the default for SVR4-based systems. ⭐
• -mshared, -mno-shared — Generate (do not generate) code that is fully position-independent, and that can therefore be linked into shared libraries. This option only affects -mabicalls.

All -mabicalls code has traditionally been position-independent, regardless of options like -fPIC and -fpic. However, as an extension, the GNU toolchain allows executables to use absolute accesses for locally-binding symbols. It can also use shorter GP initialization sequences and generate direct calls to locally-defined functions. This mode is selected by -mno-shared. 🔥

-mno-shared depends on binutils 2.16 or higher and generates objects that can only be linked by the GNU linker. However, the option does not affect the ABI of the final executable; it only affects the ABI of relocatable objects. Using -mno-shared generally makes executables both smaller and quicker. smaller AND quicker?? ate

-mshared is the default.

• -mplt, -mno-plt — Assume (do not assume) that the static and dynamic linkers support PLTs and copy relocations. This option only affects -mno-shared -mabicalls. For the n64 ABI, this option has no effect without -msym32.

You can make -mplt the default by configuring GCC with --with-mips-plt. The default is -mno-plt otherwise. 🎀

• -mxgot, -mno-xgot — Lift (do not lift) the usual restrictions on the size of the global offset table. 👑

GCC normally uses a single instruction to load values from the GOT. While this is relatively efficient, it only works if the GOT is smaller than about 64k. Anything larger causes the linker to report an error such as:

relocation truncated to fit: R_MIPS_GOT16 foobar

If this happens, you should recompile your code with -mxgot. This works with very large GOTs, although the code is also less efficient, since it takes three instructions to fetch the value of a global symbol. three whole instructions… the price of fame

Note that some linkers can create multiple GOTs. If you have such a linker, you should only need to use -mxgot when a single object file accesses more than 64k's worth of GOT entries. Very few do. 🙈

These options have no effect unless GCC is generating position independent code.

• -mgp32 — Assume that general-purpose registers are 32 bits wide.
• -mgp64 — Assume that general-purpose registers are 64 bits wide.
• -mfp32 — Assume that floating-point registers are 32 bits wide.
• -mfp64 — Assume that floating-point registers are 64 bits wide.
• -mfpxx — Do not assume the width of floating-point registers. mysterious. enigmatic. we stan an unlabeled register
• -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.

💀 bestie do NOT use -mno-float and then sneak in a float. the original literally says it is likely to suffer a link-time or run-time failure. that's not a vibe check, that's a vibe funeral.

• -msingle-float — Assume that the floating-point coprocessor only supports single-precision operations.
• -mdouble-float — Assume that the floating-point coprocessor supports double-precision operations. This is the default. ✨
• -modd-spreg, -mno-odd-spreg — Enable the use of odd-numbered single-precision floating-point registers for the o32 ABI. This is the default for processors that are known to support these registers. When using the o32 FPXX ABI, -mno-odd-spreg is set by default. 🌸
• -mabs=2008, -mabs=legacy — These options control the treatment of the special not-a-number (NaN) IEEE 754 floating-point data with the abs.fmt and neg.fmt machine instructions.

By default or when -mabs=legacy is used the legacy treatment is selected. In this case these instructions are considered arithmetic and avoided where correct operation is required and the input operand might be a NaN. A longer sequence of instructions that manipulate the sign bit of floating-point datum manually is used instead unless the -ffinite-math-only option has also been specified.

The -mabs=2008 option selects the IEEE 754-2008 treatment. In this case these instructions are considered non-arithmetic and therefore operating correctly in all cases, including in particular where the input operand is a NaN. These instructions are therefore always used for the respective operations. 💅

• -mnan=2008, -mnan=legacy — These options control the encoding of the special not-a-number (NaN) IEEE 754 floating-point data. NaN lore drop!!

The -mnan=legacy option selects the legacy encoding. In this case quiet NaNs (qNaNs) are denoted by the first bit of their trailing significand field being 0, whereas signaling NaNs (sNaNs) are denoted by the first bit of their trailing significand field being 1.

The -mnan=2008 option selects the IEEE 754-2008 encoding. In this case qNaNs are denoted by the first bit of their trailing significand field being 1, whereas sNaNs are denoted by the first bit of their trailing significand field being 0. 🔄

The default is -mnan=legacy unless GCC has been configured with --with-nan=2008.

• -mllsc, -mno-llsc — Use (do not use) ll, sc, and sync instructions to implement atomic memory built-in functions. When neither option is specified, GCC uses the instructions if the target architecture supports them. 🔥

---

• -mllsc, -mno-llsc — wait, the tea on these two continues from before! 🫖 So: -mllsc is your girl when the runtime environment can emulate the instructions, and -mno-llsc pulls its weight when you're compiling for nonstandard ISAs. You can make either one the default by configuring GCC with --with-llsc and --without-llsc respectively. And heads up: --with-llsc is the default for some configurations — check the installation documentation for the deets. 💅

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• -mdsp, -mno-dsp — Use (or don't use, bestie) revision 1 of the MIPS DSP ASE. ✨ This option defines the preprocessor macro "__mips_dsp". It also defines "__mips_dsp_rev" to 1.
• -mdspr2, -mno-dspr2 — Use (or don't) revision 2 of the MIPS DSP ASE. 🌸 This one defines the preprocessor macros "__mips_dsp" and "__mips_dspr2". It also defines "__mips_dsp_rev" to 2.
• -msmartmips, -mno-smartmips — Use (or don't) the MIPS SmartMIPS ASE. 🎀
• -mpaired-single, -mno-paired-single — Use (or don't) paired-single floating-point instructions. ⚠️ Real talk: this option requires hardware floating-point support to be enabled.
• -mdmx, -mno-mdmx — Use (or don't) MIPS Digital Media Extension instructions. 🦋 Heads up: this option can only be used when generating 64-bit code, and it requires hardware floating-point support to be enabled.
• -mips3d, -mno-mips3d — Use (or don't) the MIPS-3D ASE. And the plot twist: the option -mips3d implies -mpaired-single. 💫
• -mmicromips, -mno-micromips — Generate (or don't generate) microMIPS code.

MicroMIPS code generation can also be controlled on a per-function basis by means of "micromips" and "nomicromips" attributes. 👑

• -mmt, -mno-mt — Use (or don't) MT Multithreading instructions. 🧵
• -mmcu, -mno-mcu — Use (or don't) the MIPS MCU ASE instructions. 🔧
• -meva, -mno-eva — Use (or don't) the MIPS Enhanced Virtual Addressing instructions. 🌈
• -mvirt, -mno-virt — Use (or don't) the MIPS Virtualization (VZ) instructions. 💜
• -mxpa, -mno-xpa — Use (or don't) the MIPS eXtended Physical Address (XPA) instructions. 🗺️
• -mcrc, -mno-crc — Use (or don't) the MIPS Cyclic Redundancy Check (CRC) instructions. ✅
• -mginv, -mno-ginv — Use (or don't) the MIPS Global INValidate (GINV) instructions. 🚫
• -mloongson-mmi, -mno-loongson-mmi — Use (or don't) the MIPS Loongson MultiMedia extensions Instructions (MMI). 🐉
• -mloongson-ext, -mno-loongson-ext — Use (or don't) the MIPS Loongson EXTensions (EXT) instructions. ✨
• -mloongson-ext2, -mno-loongson-ext2 — Use (or don't) the MIPS Loongson EXTensions r2 (EXT2) instructions. 🐲
• -mlong64 — Force "long" types to be 64 bits wide. 👀 Peep -mlong32 for the explanation of the default and how the pointer size gets determined.
• -mlong32 — Force "long", "int", and pointer types to be 32 bits wide.

So here's the lore: the default size of "int"s, "long"s and pointers depends on the ABI. All the supported ABIs use 32-bit "int"s. The n64 ABI uses 64-bit "long"s, as does the 64-bit EABI; the others use 32-bit "long"s. Pointers are the same size as "long"s, or the same size as integer registers, whichever is smaller. 📏

• -msym32, -mno-sym32 — Assume (or don't assume) that all symbols have 32-bit values, regardless of the selected ABI. This one's lowkey a power move in combination with -mabi=64 and -mno-abicalls, because it lets GCC generate shorter and faster references to symbolic addresses. ⚡
• -G num — Put definitions of externally-visible data in a small data section if that data is no bigger than num bytes. Then GCC can generate more efficient accesses to the data; see -mgpopt for details. 🔍

The default -G option depends on the configuration. 🤷‍♀️

• -mlocal-sdata, -mno-local-sdata — Extend (or don't extend) the -G behavior to local data too, like static variables in C. -mlocal-sdata is the default for all configurations. 💖

If the linker starts complaining that an application is using too much small data, you might want to try rebuilding the less performance-critical parts with -mno-local-sdata. You might also want to build large libraries with -mno-local-sdata, so that the libraries leave more room for the main program. 🫶

• -mextern-sdata, -mno-extern-sdata — Assume (or don't assume) that externally-defined data is in a small data section if the size of that data is within the -G limit. -mextern-sdata is the default for all configurations. 🌟

Okay so here's the important bit, no skipping: if you compile a module Mod with -mextern-sdata -G num -mgpopt, and Mod references a variable Var that is no bigger than num bytes, you must make sure that Var is placed in a small data section. If Var is defined by another module, you must either compile that module with a high-enough -G setting or attach a "section" attribute to Var's definition. If Var is common, you must link the application with a high-enough -G setting. 😤

The easiest way of satisfying these restrictions is to compile and link every module with the same -G option. However, you may wish to build a library that supports several different small data limits. You can do this by compiling the library with the highest supported -G setting and additionally using -mno-extern-sdata to stop the library from making assumptions about externally-defined data. 🧠

• -mgpopt, -mno-gpopt — Use (or don't use) GP-relative accesses for symbols that are known to be in a small data section; see -G, -mlocal-sdata and -mextern-sdata. -mgpopt is the default for all configurations. 👑

-mno-gpopt is useful for cases where the $gp register might not hold the value of "_gp". For example, if the code is part of a library that might be used in a boot monitor, programs that call boot monitor routines pass an unknown value in $gp. (In such situations, the boot monitor itself is usually compiled with -G0.) 💀

-mno-gpopt implies -mno-local-sdata and -mno-extern-sdata. 🔗

• -membedded-data, -mno-embedded-data — Allocate variables to the read-only data section first if possible, then next in the small data section if possible, otherwise in data. This gives slightly slower code than the default, but it reduces the amount of RAM required when executing, and thus may be preferred for some embedded systems. 🍓
• -muninit-const-in-rodata, -mno-uninit-const-in-rodata — Put uninitialized "const" variables in the read-only data section. FYI this option is only meaningful in conjunction with -membedded-data. 💅
• -mcode-readable=setting — Specify whether GCC may generate code that reads from executable sections. There are three possible settings: 🎚️
  • -mcode-readable=yes — Instructions may freely access executable sections. This is the default setting. ✅
  • -mcode-readable=pcrel — MIPS16 PC-relative load instructions can access executable sections, but other instructions must not do so. This option is useful on 4KSc and 4KSd processors when the code TLBs have the Read Inhibit bit set. It is also useful on processors that can be configured to have a dual instruction/data SRAM interface and that, like the M4K, automatically redirect PC-relative loads to the instruction RAM. 🧚
  • -mcode-readable=no — Instructions must not access executable sections. This option can be useful on targets that are configured to have a dual instruction/data SRAM interface but that (unlike the M4K) do not automatically redirect PC-relative loads to the instruction RAM. 🙈
• -msplit-addresses, -mno-split-addresses — Enable (or disable) use of the %hi() and %lo() assembler relocation operators. Ngl, this option has been superseded by -mexplicit-relocs but it's retained for backwards compatibility. 🕰️
• -mexplicit-relocs=none
• -mexplicit-relocs=base
• -mexplicit-relocs=pcrel
• -mexplicit-relocs, -mno-explicit-relocs — These options control whether explicit relocs (such as %gp_rel) are used. The default value depends on the version of GAS when GCC itself was built. 🔧

The "base" explicit-relocs support was introduced into GAS in 2001. The "pcrel" explicit-relocs support was introduced into GAS in 2014, which supports %pcrel_hi and %pcrel_lo. 📅

• -mcheck-zero-division, -mno-check-zero-division — Trap (or don't trap) on integer division by zero. 🚨

The default is -mcheck-zero-division. ⭐

• -mdivide-traps, -mdivide-breaks — MIPS systems check for division by zero by generating either a conditional trap or a break instruction. Using traps results in smaller code, but is only supported on MIPS II and later. Also, some versions of the Linux kernel have a bug that prevents trap from generating the proper signal ("SIGFPE"). 😭 Use -mdivide-traps to allow conditional traps on architectures that support them and -mdivide-breaks to force the use of breaks.

The default is usually -mdivide-traps, but this can be overridden at configure time using --with-divide=breaks. Divide-by-zero checks can be completely disabled using -mno-check-zero-division. 💀

• -mload-store-pairs, -mno-load-store-pairs — Enable (or disable) an optimization that pairs consecutive load or store instructions to enable load/store bonding. This option is enabled by default but only takes effect when the selected architecture is known to support bonding. 🔗
• -mstrict-align, -mno-strict-align, -munaligned-access, -mno-unaligned-access — Disable (or enable) direct unaligned access for MIPS Release 6. MIPSr6 requires load/store unaligned-access support, by hardware or trap&emulate. So -mstrict-align may be needed by kernel. ⚠️ And heads up: the options -munaligned-access and -mno-unaligned-access are obsoleted, and only for backward-compatible. 🕰️
• -mmemcpy, -mno-memcpy — Force (or don't force) the use of "memcpy" for non-trivial block moves. The default is -mno-memcpy, which allows GCC to inline most constant-sized copies. 🫶
• -mlong-calls, -mno-long-calls — Disable (or don't disable) use of the "jal" instruction. Calling functions using "jal" is more efficient but requires the caller and callee to be in the same 256 megabyte segment. 📞

This option has no effect on abicalls code. The default is -mno-long-calls. ⭐

• -mmad, -mno-mad — Enable (or disable) use of the "mad", "madu" and "mul" instructions, as provided by the R4650 ISA. 🔥
• -mimadd, -mno-imadd — turns the madd and msub integer instructions on (or off). Default is -mimadd on any architecture that actually supports madd and msub — except the 74k, because bestie, they found out it generates <em>slower</em> code there. 💀 So 74k gets a pass.
• -mfused-madd, -mno-fused-madd — turns the floating-point multiply-accumulate instructions on (or off) when they're available. Default: -mfused-madd.

heads up though: on the R8000 CPU, when multiply-accumulate instructions get used, the intermediate product is calculated to infinite precision and is <em>not</em> subject to the FCSR Flush to Zero bit. That may be undesirable in some circumstances, fr. 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 (the ones with a .s suffix) when assembling them. no preprocessor, no problem 🫶
• -mfix-24k, -mno-fix-24k — works around the 24K E48 (lost data on stores during refill) errata. The workarounds are implemented by the assembler rather than by GCC. 🙈
• -mfix-r4000, -mno-fix-r4000 — works around certain R4000 CPU errata, and there's a whole list:
  • 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.

it's giving "the hardware has trust issues" and honestly? valid 😭

• -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.

okay so this one has a rule attached: this option can <em>only</em> 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. no cap.

• -mfix-r5900, -mno-fix-r5900 — do <em>not</em> attempt to schedule the preceding instruction into the delay slot of a branch instruction placed at the end of a short loop of six instructions or fewer, and always schedule a nop instruction there instead. The short loop bug, under certain conditions, causes loops to execute only once or twice — due to a hardware bug in the R5900 chip. 😵‍💫 The workaround is implemented by the assembler rather than by GCC.
• -mfix-rm7000, -mno-fix-rm7000 — works around the RM7000 dmult/dmultu errata. The workarounds are implemented by the assembler rather than 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. At present, these functions are only provided by the mips64vr*-elf configurations. 📎

Other VR4120 errata require a NOP to be inserted between certain pairs of instructions. These errata are handled by the assembler, not by GCC itself.

• -mfix-vr4130 — works around the 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. resourceful queen behavior 👑
• -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.) 💅
• -mr10k-cache-barrier=setting — specifies whether GCC should insert cache barriers to avoid the side effects of speculation on R10K processors. okay this one is a whole saga, sit down 🍓

In common with many processors, the R10K tries to predict the outcome of a conditional branch and speculatively executes instructions from the "taken" branch. It later aborts these instructions if the predicted outcome is wrong. However, on the R10K, even aborted instructions can have side effects. not me getting ghosted by a speculatively executed instruction 😭

This problem only affects kernel stores and, depending on the system, kernel loads. As an example, a speculatively-executed store may load the target memory into cache and mark the cache line as dirty, even if the store itself is later aborted. If a DMA operation writes to the same area of memory before the "dirty" line is flushed, the cached data overwrites the DMA-ed data. See the R10K processor manual for a full description, including other potential problems.

One workaround is to insert cache barrier instructions before every memory access that might be speculatively executed and that might have side effects even if aborted. -mr10k-cache-barrier=setting controls GCC's implementation of this workaround. It assumes that aborted accesses to any byte in the following regions does not have side effects:

1. the memory occupied by the current function's stack frame;
2. the memory occupied by an incoming stack argument;
3. the memory occupied by an object with a link-time-constant address.

It is the kernel's responsibility to ensure that speculative accesses to these regions are indeed safe. 🫡

If the input program contains a function declaration such as:

void foo (void);

then the implementation of foo must allow j foo and jal foo to be executed speculatively. GCC honors this restriction for functions it compiles itself. It expects non-GCC functions (such as hand-written assembly code) to do the same. teamwork makes the dream work ✨

The option has three forms:

• -mr10k-cache-barrier=load-store — insert a cache barrier before a load or store that might be speculatively executed and that might have side effects even if aborted.
• -mr10k-cache-barrier=store — insert a cache barrier before a store that might be speculatively executed and that might have side effects even if aborted.
• -mr10k-cache-barrier=none — disable the insertion of cache barriers. This is the default setting.

• -mflush-func=func, -mno-flush-func — specifies the function to call to flush the I and D caches, or to not call any such function. If called, the function must take the same arguments as the common _flush_func, that is, the address of the memory range for which the cache is being flushed, the size of the memory range, and the number 3 (to flush both caches). The default depends on the target GCC was configured for, but commonly is either _flush_func or __cpu_flush. 🎀
• -mbranch-cost=num — set the cost of branches to roughly <em>num</em> "simple" instructions. This cost is only a heuristic and is not guaranteed to produce consistent results across releases. A zero cost redundantly selects the default, which is based on the -mtune setting. iykyk 💅
• -mbranch-likely, -mno-branch-likely — enable or disable use of Branch Likely instructions, regardless of the default for the selected architecture. By default, Branch Likely instructions may be generated if they are supported by the selected architecture. Exception alert: for the MIPS32 and MIPS64 architectures and processors that implement those architectures, Branch Likely instructions are not be generated by default — because the MIPS32 and MIPS64 architectures specifically <em>deprecate</em> their use. 👀
• -mcompact-branches=never, -mcompact-branches=optimal, -mcompact-branches=always — these options control which form of branches will be generated. The default is -mcompact-branches=optimal.

The -mcompact-branches=never option ensures that compact branch instructions will <em>never</em> be generated. 🚫

The -mcompact-branches=always option ensures that a compact branch instruction will be generated if available for MIPS Release 6 onwards. If a compact branch instruction is not available (or pre-R6), a delay slot form of the branch will be used instead.

If it is used for MIPS16/microMIPS targets, it will be just <em>ignored</em> now. The behaviour for MIPS16/microMIPS may change in future, since they do have some compact branch instructions. ⏳

The -mcompact-branches=optimal option will cause a delay slot branch to be used if one is available in the current ISA and the delay slot is successfully filled. If the delay slot is not filled, a compact branch will be chosen if one is available. optimal really said "I'll take whatever fits" 🦋

• -mfp-exceptions, -mno-fp-exceptions — specifies whether FP exceptions are enabled. This affects how FP instructions are scheduled for some processors. The default is that FP exceptions are enabled.

For instance, on the SB-1, if FP exceptions are disabled, and we are emitting 64-bit code, then we can use both FP pipes. Otherwise, we can only use one FP pipe. 😤

• -mvr4130-align, -mno-vr4130-align — the VR4130 pipeline is two-way superscalar, but can only issue two instructions together if the first one is 8-byte aligned. When this option is enabled, GCC aligns pairs of instructions that it thinks should execute in parallel.

This option only has an effect when optimizing for the VR4130. It normally makes code faster, but at the expense of making it bigger. It is enabled by default at optimization level -O3. 🔥

• -msynci, -mno-synci — enable (disable) generation of synci instructions on architectures that support it. The synci instructions (if enabled) are generated when __builtin___clear_cache is compiled.

This option defaults to -mno-synci, but the default can be overridden by configuring GCC with --with-synci.

⚠️ When compiling code for single processor systems, it is generally safe to use synci. However, on many multi-core (SMP) systems, it does not invalidate the instruction caches on all cores and may lead to <strong>undefined behavior</strong>. so like… know your system bestie 😭

• -mrelax-pic-calls, -mno-relax-pic-calls — try to turn PIC calls that are normally dispatched via register $25 into direct calls. This is only possible if the linker can resolve the destination at link time and if the destination is within range for a direct call. shortcut szn 🎀

okay so like, -mrelax-pic-calls is the default if GCC was configured to use an assembler and a linker that support the .reloc assembly directive and -mexplicit-relocs is in effect. With -mno-explicit-relocs, this optimization can be performed by the assembler and the linker alone without help from the compiler. 💖

• -mmcount-ra-address, -mno-mcount-ra-address — Emit (or don't emit, bestie) code that allows _mcount to modify the calling function's return address. When enabled, this option extends the usual _mcount interface with a new ra-address parameter, which has type intptr_t * and is passed in register $12. _mcount can then modify the return address by doing both of the following:
  • Returning the new address in register $31.
  • Storing the new address in *ra-address, if ra-address is nonnull.

The default is -mno-mcount-ra-address. ✨

• -mframe-header-opt, -mno-frame-header-opt — Enable (or disable) frame header optimization in the o32 ABI. When using the o32 ABI, calling functions will allocate 16 bytes on the stack for the called function to write out register arguments. When enabled, this optimization will suppress the allocation of the frame header if it can be determined that it is unused. This optimization is off by default at all optimization levels. 💅
• -mlxc1-sxc1, -mno-lxc1-sxc1 — When applicable, enable (or disable) the generation of lwxc1, swxc1, ldxc1, sdxc1 instructions. Enabled by default. 🔥
• -mmadd4, -mno-madd4 — When applicable, enable (or disable) the generation of 4-operand madd.s, madd.d and related instructions. Enabled by default. 🦋

---

✨ MMIX Options ✨

These options are defined for the MMIX:

• -mlibfuncs, -mno-libfuncs — Specify that intrinsic library functions are being compiled, passing all values in registers, no matter the size. 👑
• -mepsilon, -mno-epsilon — Generate floating-point comparison instructions that compare with respect to the rE epsilon register. 🎀
• -mabi=mmixware, -mabi=gnu — Generate code that passes function parameters and return values that (in the called function) are seen as registers $0 and up, as opposed to the GNU ABI which uses global registers $231 and up. 💜
• -mzero-extend, -mno-zero-extend — When reading data from memory in sizes shorter than 64 bits, use (or do not use) zero-extending load instructions by default, rather than sign-extending ones. 🌸
• -mknuthdiv, -mno-knuthdiv — Make the result of a division yielding a remainder have the same sign as the divisor. With the default, -mno-knuthdiv, the sign of the remainder follows the sign of the dividend. Both methods are arithmetically valid, the latter being almost exclusively used. 💀
• -mtoplevel-symbols, -mno-toplevel-symbols — Prepend (or do not prepend) a : to all global symbols, so the assembly code can be used with the PREFIX assembly directive. 🫶
• -melf — Generate an executable in the ELF format, rather than the default mmo format used by the mmix simulator. 🌈
• -mbranch-predict, -mno-branch-predict — Use (or do not use) the probable-branch instructions, when static branch prediction indicates a probable branch. ⭐
• -mbase-addresses, -mno-base-addresses — Generate (or do not generate) code that uses base addresses. Using a base address automatically generates a request (handled by the assembler and the linker) for a constant to be set up in a global register. The register is used for one or more base address requests within the range 0 to 255 from the value held in the register. This generally leads to short and fast code, but the number of different data items that can be addressed is limited. This means that a program that uses lots of static data may require -mno-base-addresses. 😭
• -msingle-exit, -mno-single-exit — Force (or do not force) generated code to have a single exit point in each function. 💅

---

🌟 MN10300 Options 🌟

These -m options are defined for Matsushita MN10300 architectures:

• -mmult-bug — Generate code to avoid bugs in the multiply instructions for the MN10300 processors. This is the default. 🐛
• -mno-mult-bug — Do not generate code to avoid bugs in the multiply instructions for the MN10300 processors. 🙅
• -mam33 — Generate code using features specific to the AM33 processor. ⚡
• -mno-am33 — Do not generate code using features specific to the AM33 processor. This is the default. 🚫
• -mam33-2 — Generate code using features specific to the AM33/2.0 processor. ✨
• -mam34 — Generate code using features specific to the AM34 processor. 🔥
• -mtune=cpu-type — Use the timing characteristics of the indicated CPU type when scheduling instructions. This does not change the targeted processor type. The CPU type must be one of mn10300, am33, am33-2 or am34. 🧠
• -mreturn-pointer-on-d0 — When generating a function that returns a pointer, return the pointer in both a0 and d0. Otherwise, the pointer is returned only in a0, and attempts to call such functions without a prototype result in errors. Note that this option is on by default; use -mno-return-pointer-on-d0 to disable it. 👑
• -mno-crt0 — Do not link in the C run-time initialization object file. 🚫
• -mrelax — Indicate to the linker that it should perform a relaxation optimization pass to shorten branches, calls and absolute memory addresses. This option only has an effect when used on the command line for the final link step. This option makes symbolic debugging impossible. 😱
• -mliw — Allow the compiler to generate Long Instruction Word instructions if the target is the AM33 or later. This is the default. This option defines the preprocessor macro __LIW__. 💪
• -mno-liw — Do not allow the compiler to generate Long Instruction Word instructions. This option defines the preprocessor macro __NO_LIW__. 🙅
• -msetlb — Allow the compiler to generate the SETLB and Lcc instructions if the target is the AM33 or later. This is the default. This option defines the preprocessor macro __SETLB__. 🎉
• -mno-setlb — Do not allow the compiler to generate SETLB or Lcc instructions. This option defines the preprocessor macro __NO_SETLB__. 🚫

---

🦋 Moxie Options 🦋

• -meb — Generate big-endian code. This is the default for moxie-*-* configurations. 🌍
• -mel — Generate little-endian code. 🐣
• -mmul.x — Generate mul.x and umul.x instructions. This is the default for moxiebox-*-* configurations. ✖️
• -mno-crt0 — Do not link in the C run-time initialization object file. 🚫

---

🌸 MSP430 Options 🌸

These options are defined for the MSP430:

• -masm-hex — Force assembly output to always use hex constants. Normally such constants are signed decimals, but this option is available for testsuite and/or aesthetic purposes. 🎨
• -mmcu= — Select the MCU to target. This is used to create a C preprocessor symbol based upon the MCU name, converted to upper case and pre- and post-fixed with __. This in turn is used by the msp430.h header file to select an MCU-specific supplementary header file.

The option also sets the ISA to use. If the MCU name is one that is known to only support the 430 ISA then that is selected, otherwise the 430X ISA is selected. A generic MCU name of msp430 can also be used to select the 430 ISA. Similarly the generic msp430x MCU name selects the 430X ISA.

In addition an MCU-specific linker script is added to the linker command line. The script's name is the name of the MCU with .ld appended. Thus specifying -mmcu=xxx on the gcc command line defines the C preprocessor symbol __XXX__ and cause the linker to search for a script called xxx.ld.

The ISA and hardware multiply supported for the different MCUs is hard-coded into GCC. However, an external devices.csv file can be used to extend device support beyond those that have been hard-coded.

GCC searches for the devices.csv file using the following methods in the given precedence order, where the first method takes precedence over the second which takes precedence over the third.

• Include path specified with -I and -L: devices.csv will be searched for in each of the directories specified by include paths and linker library search paths. 🔍
• Path specified by the environment variable MSP430_GCC_INCLUDE_DIR: Define the value of the global environment 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 will also be registered as an include path, and linker library path. Header files and linker scripts in this directory can therefore be used without manually specifying -I and -L on the command line. 🗂️
• The msp430-elf{,bare}/include/devices directory: Finally, GCC will examine msp430-elf{,bare}/include/devices from the toolchain root directory. This directory does not exist in a default installation, but if the user has created it and copied devices.csv there, then the MCU data will be read. As above, this directory will also be registered as an include path, and linker library path. 📁

If none of the above search methods find devices.csv, then the hard-coded MCU data is used. 💾

• -mwarn-mcu, -mno-warn-mcu — This option enables or disables warnings about conflicts between the MCU name specified by the -mmcu option and the ISA set by the -mcpu option and/or the hardware multiply support set by the -mhwmult option. It also toggles warnings about unrecognized MCU names. This option is on by default. ⚠️
• -mcpu= — Specifies the ISA to use. Accepted values are msp430, msp430x and msp430xv2. This option is deprecated. The -mmcu= option should be used to select the ISA. 🚫
• -msim — Link to the simulator runtime libraries and linker script. Overrides any scripts that would be selected by the -mmcu= option. 🖥️
• -mlarge — Use large-model addressing (20-bit pointers, 20-bit size_t). 🐘
• -msmall — Use small-model addressing (16-bit pointers, 16-bit size_t). 🐭
• -mrelax — This option is passed to the assembler and linker, and allows the linker to perform certain optimizations that cannot be done until the final link. 🔗
• -mhwmult= — bestie, this one tells everyone what kind of hardware multiply your target can actually do 💅 here's the tea on the accepted values: none means no hardware multiply at all, 16bit is the original 16-bit-only multiply that the early MCUs rocked, 32bit is the 16/32-bit multiply the later MCUs came through with, and f5series is the 16/32-bit multiply the F5-series MCUs are serving. You can also pass auto, which tells GCC to deduce the hardware multiply support based upon the MCU name provided by the -mmcu option. If no -mmcu option is specified or if the MCU name is not recognized then no hardware multiply support is assumed. "auto" is the default setting, periodt.

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 classic glow-up tradeoff ✨

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. iykyk, that's main character behavior 🫶

• -minrt — Enable the use of a minimum runtime environment - no static initializers or constructors. This is intended for memory-constrained devices. The compiler includes special symbols in some objects that tell the linker and runtime which code fragments are required. tiny but mighty fr fr 🎀
• -mtiny-printf — Enable reduced code size printf and puts library functions. The tiny implementations of these functions are not reentrant, so must be used with caution in multi-threaded applications. ⚠️

Support for streams has been removed and the string to be printed will always be sent to stdout via the write syscall. The string is not buffered before it is sent to write.

This option requires Newlib Nano IO, so GCC must be configured with --enable-newlib-nano-formatted-io.

• -mmax-inline-shift= — This option takes an integer between 0 and 64 inclusive, and sets the maximum number of inline shift instructions which should be emitted to perform a shift operation by a constant amount. When this value needs to be exceeded, an mspabi helper function is used instead. The default value is 4. 💫

This only affects cases where a shift by multiple positions cannot be completed with a single instruction (e.g. all shifts >1 on the 430 ISA).

Shifts of a 32-bit value are at least twice as costly, so the value passed for this option is divided by 2 and the resulting value used instead. the math is mathing 🔥

• -mcode-region=, -mdata-region= — These options tell the compiler where to place functions and data that do not have one of the "lower", "upper", "either" or "section" attributes. Possible values are "lower", "upper", "either" or "any". The first three behave like the corresponding attribute. The fourth possible value - "any" - is the default. It leaves placement entirely up to the linker script and how it assigns the standard sections (.text, .data, etc) to the memory regions. no thoughts, just vibes 🌸
• -msilicon-errata= — This option passes on a request to assembler to enable the fixes for the named silicon errata. 🛠️
• -msilicon-errata-warn= — This option passes on a request to the assembler to enable warning messages when a silicon errata might need to be applied.
• -mwarn-devices-csv, -mno-warn-devices-csv — Warn if devices.csv is not found or there are problem parsing it (default: on).

---

🌈 NDS32 Options 🌈

These options are defined for NDS32 implementations:

• -mbig-endian — Generate code in big-endian mode.
• -mlittle-endian — Generate code in little-endian mode.
• -mreduced-regs — Use reduced-set registers for register allocation.
• -mfull-regs — Use full-set registers for register allocation.
• -mcmov — Generate conditional move instructions.
• -mno-cmov — Do not generate conditional move instructions.
• -mext-perf — Generate performance extension instructions.
• -mno-ext-perf — Do not generate performance extension instructions.
• -mext-perf2 — Generate performance extension 2 instructions.
• -mno-ext-perf2 — Do not generate performance extension 2 instructions.
• -mext-string — Generate string extension instructions.
• -mno-ext-string — Do not generate string extension instructions.
• -mv3push — Generate v3 push25/pop25 instructions.
• -mno-v3push — Do not generate v3 push25/pop25 instructions.
• -m16-bit — Generate 16-bit instructions.
• -mno-16-bit — Do not generate 16-bit instructions.
• *-misr-vector-size=num* — Specify the size of each interrupt vector, which must be 4 or 16.
• *-mcache-block-size=num* — Specify the size of each cache block, which must be a power of 2 between 4 and 512.
• *-march=arch* — Specify the name of the target architecture.
• *-mcmodel=code-model* — Set the code model to one of:
  • small — All the data and read-only data segments must be within 512KB addressing space. The text segment must be within 16MB addressing space.
  • medium — The data segment must be within 512KB while the read-only data segment can be within 4GB addressing space. The text segment should be still within 16MB addressing space.
  • large — All the text and data segments can be within 4GB addressing space.
• -mctor-dtor — Enable constructor/destructor feature.
• -mrelax — Guide linker to relax instructions.

---

💜 Nios II Options 💜

These are the options defined for the Altera Nios II processor.

• -G num — Put global and static objects less than or equal to num bytes into the small data or BSS sections instead of the normal data or BSS sections. The default value of num is 8.
• -mgpopt=option, -mgpopt, -mno-gpopt — Generate (do not generate) GP-relative accesses. The following option names are recognized:
  • none — Do not generate GP-relative accesses.
  • local — Generate GP-relative accesses for small data objects that are not external, weak, or uninitialized common symbols. Also use GP-relative addressing for objects that have been explicitly placed in a small data section via a "section" attribute.
  • global — As for local, but also generate GP-relative accesses for small data objects that are external, weak, or common. If you use this option, you must ensure that all parts of your program (including libraries) are compiled with the same -G setting. 👀
  • data — Generate GP-relative accesses for all data objects in the program. If you use this option, the entire data and BSS segments of your program must fit in 64K of memory and you must use an appropriate linker script to allocate them within the addressable range of the global pointer.
  • all — Generate GP-relative addresses for function pointers as well as data pointers. If you use this option, the entire text, data, and BSS segments of your program must fit in 64K of memory and you must use an appropriate linker script to allocate them within the addressable range of the global pointer.

-mgpopt is equivalent to -mgpopt=local, and -mno-gpopt is equivalent to -mgpopt=none.

The default is -mgpopt except when -fpic or -fPIC is specified to generate position-independent code. Note that the Nios II ABI does not permit GP-relative accesses from shared libraries. 🚫

You may need to specify -mno-gpopt explicitly when building programs that include large amounts of small data, including large GOT data sections. In this case, the 16-bit offset for GP-relative addressing may not be large enough to allow access to the entire small data section.

• -mgprel-sec=regexp — This option specifies additional section names that can be accessed via GP-relative addressing. It is most useful in conjunction with "section" attributes on variable declarations and a custom linker script. The regexp is a POSIX Extended Regular Expression.

This option does not affect the behavior of the -G option, and the specified sections are in addition to the standard .sdata and .sbss small-data sections that are recognized by -mgpopt.

• -mr0rel-sec=regexp — This option specifies names of sections that can be accessed via a 16-bit offset from "r0"; that is, in the low 32K or high 32K of the 32-bit address space. It is most useful in conjunction with "section" attributes on variable declarations and a custom linker script. The regexp is a POSIX Extended Regular Expression.

In contrast to the use of GP-relative addressing for small data, zero-based addressing is never generated by default and there are no conventional section names used in standard linker scripts for sections in the low or high areas of memory. not a single one, bestie 💀

• -mel, -meb — Generate little-endian (default) or big-endian (experimental) code, respectively. 👑
• -march=arch — This specifies the name of the target Nios II architecture. GCC uses this name to determine what kind of instructions it can emit when generating assembly code. Permissible names are: r1, r2.

The preprocessor macro "__nios2_arch__" is available to programs, with value 1 or 2, indicating the targeted ISA level. ✨

• -mbypass-cache, -mno-bypass-cache — Force all load and store instructions to always bypass cache by using I/O variants of the instructions. The default is not to bypass the cache.
• -mno-cache-volatile, -mcache-volatile — Volatile memory access bypass the cache using the I/O variants of the load and store instructions. The default is not to bypass the cache.
• -mno-fast-sw-div, -mfast-sw-div — Do not use table-based fast divide for small numbers. The default is to use the fast divide at -O3 and above. 🔥
• -mno-hw-mul, -mhw-mul, -mno-hw-mulx, -mhw-mulx, -mno-hw-div, -mhw-div — Enable or disable emitting "mul", "mulx" and "div" family of instructions by the compiler. The default is to emit "mul" and not emit "div" and "mulx".
• -mbmx, -mno-bmx, -mcdx, -mno-cdx — Enable or disable generation of Nios II R2 BMX (bit manipulation) and CDX (code density) instructions. Enabling these instructions also requires -march=r2. Since these instructions are optional extensions to the R2 architecture, the default is not to emit them. 🦋

okay bestie, we are still deep in the Nios II custom-instruction zone and it is giving compiler girlboss 💅 you can literally tell GCC to use special hardware encodings instead of falling back to a library call. no cap.

• -mcustom-insn=N, -mno-custom-insn — Each -mcustom-insn=N option enables use of a custom instruction with encoding N when generating code that uses insn. For example, -mcustom-fadds=253 generates custom instruction 253 for single-precision floating-point add operations instead of the default behavior of using a library call.

The following values of insn are supported. Except as otherwise noted, floating-point operations are expected to be implemented with normal IEEE 754 semantics and correspond directly to the C operators or the equivalent GCC built-in functions.

Single-precision floating point:

• fadds, fsubs, fdivs, fmuls — Binary arithmetic operations.
• fnegs — Unary negation.
• fabss — Unary absolute value.
• fcmpeqs, fcmpges, fcmpgts, fcmples, fcmplts, fcmpnes — Comparison operations.
• fmins, fmaxs — Floating-point minimum and maximum. These instructions are only generated if -ffinite-math-only is specified.
• fsqrts — Unary square root operation.
• fcoss, fsins, ftans, fatans, fexps, flogs — Floating-point trigonometric and exponential functions. These instructions are only generated if -funsafe-math-optimizations is also specified.

Double-precision floating point:

• faddd, fsubd, fdivd, fmuld — Binary arithmetic operations.
• fnegd — Unary negation.
• fabsd — Unary absolute value.
• fcmpeqd, fcmpged, fcmpgtd, fcmpled, fcmpltd, fcmpned — Comparison operations.
• fmind, fmaxd — Double-precision minimum and maximum. These instructions are only generated if -ffinite-math-only is specified.
• fsqrtd — Unary square root operation.
• fcosd, fsind, ftand, fatand, fexpd, flogd — Double-precision trigonometric and exponential functions. These instructions are only generated if -funsafe-math-optimizations is also specified.

Conversions:

• fextsd — Conversion from single precision to double precision.
• ftruncds — Conversion from double precision to single precision.
• fixsi, fixsu, fixdi, fixdu — Conversion from floating point to signed or unsigned integer types, with truncation towards zero.
• round — Conversion from single-precision floating point to signed integer, rounding to the nearest integer and ties away from zero. This corresponds to the "__builtin_lroundf" function when -fno-math-errno is used.
• floatis, floatus, floatid, floatud — Conversion from signed or unsigned integer types to floating-point types.

In addition, all of the following transfer instructions for internal registers X and Y must be provided to use any of the double-precision floating-point instructions. Custom instructions taking two double-precision source operands expect the first operand in the 64-bit register X. The other operand (or only operand of a unary operation) is given to the custom arithmetic instruction with the least significant half in source register src1 and the most significant half in src2. A custom instruction that returns a double-precision result returns the most significant 32 bits in the destination register and the other half in 32-bit register Y. GCC automatically generates the necessary code sequences to write register X and/or read register Y when double-precision floating-point instructions are used.

• fwrx — Write src1 into the least significant half of X and src2 into the most significant half of X.
• fwry — Write src1 into Y.
• frdxhi, frdxlo — Read the most or least (respectively) significant half of X and store it in dest.
• frdy — Read the value of Y and store it into dest.

Note that you can gain more local control over generation of Nios II custom instructions by using the target("custom-insn=N") and target("no-custom-insn") function attributes or pragmas.

• -mcustom-fpu-cfg=name — This option enables a predefined, named set of custom instruction encodings (see -mcustom-insn above). Currently, the following sets are defined:

-mcustom-fpu-cfg=60-1 is equivalent to: -mcustom-fmuls=252
-mcustom-fadds=253 -mcustom-fsubs=254 -fsingle-precision-constant

-mcustom-fpu-cfg=60-2 is equivalent to: -mcustom-fmuls=252
-mcustom-fadds=253 -mcustom-fsubs=254 -mcustom-fdivs=255
-fsingle-precision-constant

-mcustom-fpu-cfg=72-3 is equivalent to: -mcustom-floatus=243
-mcustom-fixsi=244 -mcustom-floatis=245 -mcustom-fcmpgts=246
-mcustom-fcmples=249 -mcustom-fcmpeqs=250 -mcustom-fcmpnes=251
-mcustom-fmuls=252 -mcustom-fadds=253 -mcustom-fsubs=254
-mcustom-fdivs=255 -fsingle-precision-constant

-mcustom-fpu-cfg=fph2 is equivalent to: -mcustom-fabss=224
-mcustom-fnegs=225 -mcustom-fcmpnes=226 -mcustom-fcmpeqs=227
-mcustom-fcmpges=228 -mcustom-fcmpgts=229 -mcustom-fcmples=230
-mcustom-fcmplts=231 -mcustom-fmaxs=232 -mcustom-fmins=233
-mcustom-round=248 -mcustom-fixsi=249 -mcustom-floatis=250
-mcustom-fsqrts=251 -mcustom-fmuls=252 -mcustom-fadds=253
-mcustom-fsubs=254 -mcustom-fdivs=255

Custom instruction assignments given by individual -mcustom-insn= options override those given by -mcustom-fpu-cfg=, regardless of the order of the options on the command line.

Note that you can gain more local control over selection of a FPU configuration by using the target("custom-fpu-cfg=name") function attribute or pragma.

The name fph2 is an abbreviation for Nios II Floating Point Hardware 2 Component. Please note that the custom instructions enabled by -mcustom-fmins=233 and -mcustom-fmaxs=234 are only generated if -ffinite-math-only is specified. The custom instruction enabled by -mcustom-round=248 is only generated if -fno-math-errno is specified. In contrast to the other configurations, -fsingle-precision-constant is not set.

These additional -m options are available for the Altera Nios II ELF (bare-metal) target:

• -mhal — Link with HAL BSP. This suppresses linking with the GCC-provided C runtime startup and termination code, and is typically used in conjunction with -msys-crt0= to specify the location of the alternate startup code provided by the HAL BSP.
• -msmallc — Link with a limited version of the C library, -lsmallc, rather than Newlib.
• -msys-crt0=startfile — startfile is the file name of the startfile (crt0) to use when linking. This option is only useful in conjunction with -mhal.
• -msys-lib=systemlib — systemlib is the library name of the library that provides low-level system calls required by the C library, e.g. "read" and "write". This option is typically used to link with a library provided by a HAL BSP.

---

🌈 Nvidia PTX Options 🌈

These options are defined for Nvidia PTX:

• -m64 — Ignored, but preserved for backward compatibility. Only 64-bit ABI is supported.
• -march=architecture-string — Generate code for the specified PTX ISA target architecture (e.g. sm_35). Valid architecture strings are sm_30, sm_35, sm_53, sm_70, sm_75 and sm_80. The default depends on how the compiler has been configured, see --with-arch. This option sets the value of the preprocessor macro "__PTX_SM__"; for instance, for sm_35, it has the value 350.
• -misa=architecture-string — Alias of -march=.
• -march-map=architecture-string — Select the closest available -march= value that is not more capable. For instance, for -march-map=sm_50 select -march=sm_35, and for -march-map=sm_53 select -march=sm_53.
• -mptx=version-string — Generate code for the specified PTX ISA version (e.g. 7.0). Valid version strings include 3.1, 6.0, 6.3, and 7.0. The default PTX ISA version is 6.0, unless a higher version is required for specified PTX ISA target architecture via option -march=. This option sets the values of the preprocessor macros "__PTX_ISA_VERSION_MAJOR__" and "__PTX_ISA_VERSION_MINOR__"; for instance, for 3.1 the macros have the values 3 and 1, respectively.
• -mmainkernel — Link in code for a __main kernel. This is for stand-alone instead of offloading execution.
• -moptimize — Apply partitioned execution optimizations. This is the default when any level of optimization is selected.
• -msoft-stack — Generate code that does not use ".local" memory directly for stack storage. Instead, a per-warp stack pointer is maintained explicitly. This enables variable-length stack allocation (with variable-length arrays or "alloca"), and when global memory is used for underlying storage, makes it possible to access automatic variables from other threads, or with atomic instructions. This code generation variant is used for OpenMP offloading, but the option is exposed on its own for the purpose of testing the compiler; to generate code suitable for linking into programs using OpenMP offloading, use option -mgomp.
• -muniform-simt — Switch to code generation variant that allows to execute all threads in each warp, while maintaining memory state and side effects as if only one thread in each warp was active outside of OpenMP SIMD regions. All atomic operations and calls to runtime (malloc, free, vprintf) are conditionally executed (iff current lane index equals the master lane index), and the register being assigned is copied via a shuffle instruction from the master lane. Outside of SIMD regions lane 0 is the master; inside, each thread sees itself as the master. Shared memory array "int __nvptx_uni[]" stores all-zeros or all-ones bitmasks for each warp, indicating current mode (0 outside of SIMD regions). Each thread can bitwise-and the bitmask at position "tid.y" with current lane index to compute the master lane index.
• -mgomp — Generate code for use in OpenMP offloading: enables -msoft-stack and -muniform-simt options, and selects corresponding multilib variant.

---

🦋 OpenRISC Options 🦋

These options are defined for OpenRISC:

• -mboard=name — Configure a board specific runtime. This will be passed to the linker for newlib board library linking. The default is "or1ksim".
• -mnewlib — bestie, this one is ignored; it's only here for compatibility. It used to select linker and preprocessor options for use with newlib. 💅
• -msoft-div, -mhard-div — pick software or hardware divide (l.div, l.divu) instructions. The default is hardware divide. ✨
• -msoft-mul, -mhard-mul — pick software or hardware multiply (l.mul, l.muli) instructions. The default is hardware multiply. 💖
• -msoft-float, -mhard-float — pick software or hardware for floating point operations. The default is software. 🦋
• -mdouble-float — when -mhard-float is selected, this enables generation of double-precision floating point instructions. By default, functions from libgcc are used to perform double-precision floating point operations. 👑
• -munordered-float — when -mhard-float is selected, this enables generation of unordered floating point compare and set flag (lf.sfun*) instructions. By default, functions from libgcc are used to perform unordered floating point compare and set flag operations. 🌈
• -mcmov — enable generation of conditional move (l.cmov) instructions. By default, the equivalent will be generated using set and branch. 💀
• -mror — enable generation of rotate right (l.ror) instructions. By default, functions from libgcc are used to perform rotate right operations. 🎀
• -mrori — enable generation of rotate right with immediate (l.rori) instructions. By default, functions from libgcc are used to perform rotate right with immediate operations. 🎀
• -msext — enable generation of sign extension (l.ext*) instructions. By default, memory loads are used to perform sign extension. ✨
• -msfimm — enable generation of compare and set flag with immediate (l.sf*i) instructions. By default, extra instructions will be generated to store the immediate to a register first. 💅
• -mshftimm — enable generation of shift with immediate (l.srai, l.srli, l.slli) instructions. By default, extra instructions will be generated to store the immediate to a register first. 💅
• -mcmodel=small — generate OpenRISC code for the small model: the GOT is limited to 64k. This is the default model. 🏆
• -mcmodel=large — generate OpenRISC code for the large model: the GOT may grow up to 4G in size. 🐘

PDP-11 Options 💖

These options are defined for the PDP-11:

• -mfpu — use hardware FPP floating point. This is the default. (FIS floating point on the PDP-11/40 is not supported.) Implies -m45. 👑
• -msoft-float — do not use hardware floating point. 🚫
• -mac0 — return floating-point results in ac0 (fr0 in Unix assembler syntax). 💅
• -mno-ac0 — return floating-point results in memory. This is the default. 💾
• -m40 — generate code for a PDP-11/40. Implies -msoft-float -mno-split. 🛠️
• -m45 — generate code for a PDP-11/45. This is the default. ⭐
• -m10 — generate code for a PDP-11/10. Implies -msoft-float -mno-split. 🛠️
• -mint16, -mno-int32 — use 16-bit int. This is the default. 🔢
• -mint32, -mno-int16 — use 32-bit int. 🔢
• -msplit — target has split instruction and data space. Implies -m45. 🧚
• -munix-asm — use Unix assembler syntax. 🐧
• -mdec-asm — use DEC assembler syntax. 🏛️
• -mgnu-asm — use GNU assembler syntax. This is the default. 🐃
• -mlra — use the new LRA register allocator. By default, the old reload allocator is used. 🔄

PowerPC Options 🦋

These are listed under

PRU Options 🎀

These command-line options are defined for PRU target:

• -minrt — link with a minimum runtime environment. This can significantly reduce the size of the final ELF binary, but some standard C runtime features are removed. 😭

This option disables support for static initializers and constructors. Beware that the compiler could still generate code with static initializers and constructors. It is up to the programmer to ensure that the source program will not use those features. ⚠️

The minimal startup code would not pass argc and argv arguments to main, so the latter must be declared as int main (void). This is already the norm for most firmware projects. 💅

• -mmcu=mcu — specify the PRU hardware variant to use. A correspondingly named spec file would be loaded, passing the memory region sizes to the linker and defining hardware-specific C macros. 🧠

Newlib provides only the sim spec, intended for running regression tests using a simulator. Specs for real hardware can be obtained by installing the GnuPruMcu (https://github.com/dinuxbg/gnuprumcu/) package. 📦

• -mno-relax — make GCC pass the --no-relax command-line option to the linker instead of the --relax option. 🚫
• -mloop — allow (or do not allow) GCC to use the LOOP instruction. 🔁
• -mabi=variant — specify the ABI variant to output code for. -mabi=ti selects the unmodified TI ABI while -mabi=gnu selects a GNU variant that copes more naturally with certain GCC assumptions. These are the differences: 🧐
  • Function Pointer Size: TI ABI specifies that function (code) pointers are 16-bit, whereas GNU supports only 32-bit data and code pointers. 📏
  • Optional Return Value Pointer: Function return values larger than 64 bits are passed by using a hidden pointer as the first argument of the function. TI ABI, though, mandates that the pointer can be NULL in case the caller is not using the returned value. GNU always passes and expects a valid return value pointer. 🎯

The current -mabi=ti implementation simply raises a compile error when any of the above code constructs is detected. As a consequence the standard C library cannot be built and it is omitted when linking with -mabi=ti. 💥

Relaxation is a GNU feature and for safety reasons is disabled when using -mabi=ti. The TI toolchain does not emit relocations for QBBx instructions, so the GNU linker cannot adjust them when shortening adjacent LDI32 pseudo instructions. 🛑

RISC-V Options 🌊

These command-line options are defined for RISC-V targets:

• -mbranch-cost=n — set the cost of branches to roughly n instructions. 💸
• -mplt, -mno-plt — when generating PIC code, do or don't allow the use of PLTs. Ignored for non-PIC. The default is -mplt. 🎯
• -mabi=ABI-string — specify integer and floating-point calling convention. ABI-string contains two parts: the size of integer types and the registers used for floating-point types. For example -march=rv64ifd -mabi=lp64d means that long and pointers are 64-bit (implicitly defining int to be 32-bit), and that floating-point values up to 64 bits wide are passed in F registers. Contrast this with -march=rv64ifd -mabi=lp64f, which still allows the compiler to generate code that uses the F and D extensions but only allows floating-point values up to 32 bits long to be passed in registers; or -march=rv64ifd -mabi=lp64, in which no floating-point arguments will be passed in registers. 📦

The default for this argument is system dependent, users who want a specific calling convention should specify one explicitly. The valid calling conventions are: ilp32, ilp32f, ilp32d, lp64, lp64f, and lp64d. Some calling conventions are impossible to implement on some ISAs: for example, -march=rv32if -mabi=ilp32d is invalid because the ABI requires 64-bit values be passed in F registers, but F registers are only 32 bits wide. There are also the ilp32e ABI that can only be used with the rv32e architecture and the lp64e ABI that can only be used with the rv64e. Those ABIs are not well specified at present, and are subject to change. ⚠️

• -mfdiv, -mno-fdiv — do or don't use hardware floating-point divide and square root instructions. This requires the F or D extensions for floating-point registers. The default is to use them if the specified architecture has these instructions. ➗
• -mdiv, -mno-div — do or don't use hardware instructions for integer division. This requires the M extension. The default is to use them if the specified architecture has these instructions. ➗
• -misa-spec=ISA-spec-string — specify the version of the RISC-V Unprivileged (formerly User-Level) ISA specification to produce code conforming to. The possibilities for ISA-spec-string are: 📜
  • 2.2 — produce code conforming to version 2.2.
  • 20190608 — produce code conforming to version 20190608.
  • 20191213 — produce code conforming to version 20191213.

The default is -misa-spec=20191213 unless GCC has been configured with --with-isa-spec= specifying a different default version. ⭐

• -march=ISA-string — generate code for given RISC-V ISA (e.g. rv64im). ISA strings must be lower-case. Examples include rv64i, rv32g, rv32e, and rv32imaf. Additionally, a special value help (-march=help) is accepted to list all supported extensions. 🧬

The syntax of the ISA string is defined as follows: 📝

• The string must start with rv32 or rv64, followed by i, e, or g, referred to as the base ISA.
• The subsequent part of the string is a list of extension names. Extension names can be categorized as multi-letter (e.g. zba) and single-letter (e.g. v). Single-letter extensions can appear consecutively, but multi-letter extensions must be separated by underscores.
• An underscore can appear anywhere after the base ISA. It has no specific effect but is used to improve readability and can act as a separator.
• Extension names may include an optional version number, following the syntax <major>p<minor> or <major>, (e.g. m2p1 or m2).

Supported extension are listed below: 👇

• i — Supported Version: 2.0, 2.1 — Base integer extension.
• e — Supported Version: 2.0 — Reduced base integer extension.
• g — Supported Version: - — General-purpose computing base extension, g will expand to i, m, a, f, d, zicsr and zifencei.
• m — Supported Version: 2.0 — Integer multiplication and division extension.
• a — Supported Version: 2.0, 2.1 — Atomic extension.
• f — Supported Version: 2.0, 2.2 — Single-precision floating-point extension.
• d — Supported Version: 2.0, 2.2 — Double-precision floating-point extension.
• c — Supported Version: 2.0 — Compressed extension.
• h — Supported Version: 1.0 — Hypervisor extension.

Okay so we're still in the RISC-V extension list, bestie, and it's giving encyclopedia but make it fashion. Every single one of these goes after -march=, and each one has a version number and a whole personality. Buckle in. 💅

• v (1.0) — Vector extension. 🦋
• zicsr (2.0) — Control and status register access extension.
• zifencei (2.0) — Instruction-fetch fence extension.
• zicond (1.0) — Integer conditional operations extension.
• za64rs (1.0) — Reservation set size of 64 bytes.
• za128rs (1.0) — Reservation set size of 128 bytes.
• zawrs (1.0) — Wait-on-reservation-set extension.
• zba (1.0) — Address calculation extension.
• zbb (1.0) — Basic bit manipulation extension.
• zbc (1.0) — Carry-less multiplication extension.
• zbs (1.0) — Single-bit operation extension.
• zfinx (1.0) — Single-precision floating-point in integer registers extension.
• zdinx (1.0) — Double-precision floating-point in integer registers extension.
• zhinx (1.0) — Half-precision floating-point in integer registers extension.
• zhinxmin (1.0) — Minimal half-precision floating-point in integer registers extension.
• zbkb (1.0) — Cryptography bit-manipulation extension.
• zbkc (1.0) — Cryptography carry-less multiply extension.
• zbkx (1.0) — Cryptography crossbar permutation extension.
• zkne (1.0) — AES Encryption extension.
• zknd (1.0) — AES Decryption extension.
• zknh (1.0) — Hash function extension.
• zkr (1.0) — Entropy source extension.
• zksed (1.0) — SM4 block cipher extension.
• zksh (1.0) — SM3 hash function extension.
• zkt (1.0) — Data independent execution latency extension.
• zk (1.0) — Standard scalar cryptography extension.
• zkn (1.0) — NIST algorithm suite extension.
• zks (1.0) — ShangMi algorithm suite extension.
• zihintntl (1.0) — Non-temporal locality hints extension.
• zihintpause (1.0) — Pause hint extension.
• zicboz (1.0) — Cache-block zero extension.
• zicbom (1.0) — Cache-block management extension.
• zicbop (1.0) — Cache-block prefetch extension.
• zic64b (1.0) — Cache block size isf 64 bytes.
• ziccamoa (1.0) — Main memory supports all atomics in A.
• ziccif (1.0) — Main memory supports instruction fetch with atomicity requirement.
• zicclsm (1.0) — Main memory supports misaligned loads/stores.
• ziccrse (1.0) — Main memory supports forward progress on LR/SC sequences.
• zicntr (2.0) — Standard extension for base counters and timers.
• zihpm (2.0) — Standard extension for hardware performance counters.
• ztso (1.0) — Total store ordering extension.
• zve32x (1.0) — Vector extensions for embedded processors.
• zve32f (1.0) — Vector extensions for embedded processors.
• zve64x (1.0) — Vector extensions for embedded processors.
• zve64f (1.0) — Vector extensions for embedded processors.
• zve64d (1.0) — Vector extensions for embedded processors.
• zvl32b (1.0) — Minimum vector length standard extensions
• zvl64b (1.0) — Minimum vector length standard extensions
• zvl128b (1.0) — Minimum vector length standard extensions
• zvl256b (1.0) — Minimum vector length standard extensions
• zvl512b (1.0) — Minimum vector length standard extensions
• zvl1024b (1.0) — Minimum vector length standard extensions
• zvl2048b (1.0) — Minimum vector length standard extensions
• zvl4096b (1.0) — Minimum vector length standard extensions
• zvbb (1.0) — Vector basic bit-manipulation extension.
• zvbc (1.0) — Vector carryless multiplication extension.
• zvkb (1.0) — Vector cryptography bit-manipulation extension.
• zvkg (1.0) — Vector GCM/GMAC extension.
• zvkned (1.0) — Vector AES block cipher extension.
• zvknha (1.0) — Vector SHA-2 secure hash extension.
• zvknhb (1.0) — Vector SHA-2 secure hash extension.
• zvksed (1.0) — Vector SM4 Block Cipher extension.
• zvksh (1.0) — Vector SM3 Secure Hash extension.
• zvkn (1.0) — Vector NIST Algorithm Suite extension, zvkn will expand to zvkned, zvknhb, zvkb and zvkt.
• zvknc (1.0) — Vector NIST Algorithm Suite with carryless multiply extension, zvknc will expand to zvkn and zvbc.
• zvkng (1.0) — Vector NIST Algorithm Suite with GCM extension, zvkng will expand to zvkn and zvkg.
• zvks (1.0) — Vector ShangMi algorithm suite extension, zvks will expand to zvksed, zvksh, zvkb and zvkt.
• zvksc (1.0) — Vector ShangMi algorithm suite with carryless multiplication extension, zvksc will expand to zvks and zvbc.
• zvksg (1.0) — Vector ShangMi algorithm suite with GCM extension, zvksg will expand to zvks and zvkg.
• zvkt (1.0) — Vector data independent execution latency extension.
• zfh (1.0) — Half-precision floating-point extension.
• zfhmin (1.0) — Minimal half-precision floating-point extension.
• zvfh (1.0) — Vector half-precision floating-point extension.
• zvfhmin (1.0) — Vector minimal half-precision floating-point extension.
• zvfbfmin (1.0) — Vector BF16 converts extension.
• zfa (1.0) — Additional floating-point extension.
• zmmul (1.0) — Integer multiplication extension.
• zca (1.0) — Integer compressed instruction extension.
• zcf (1.0) — Compressed single-precision floating point loads and stores extension.
• zcd (1.0) — Compressed double-precision floating point loads and stores extension.
• zcb (1.0) — Simple compressed instruction extension.
• zce (1.0) — Compressed instruction extensions for embedded processors.
• zcmp (1.0) — Compressed push pop extension.
• zcmt (1.0) — Table jump instruction extension.
• smaia (1.0) — Advanced interrupt architecture extension.
• smepmp (1.0) — PMP Enhancements for memory access and execution prevention on Machine mode.
• smstateen (1.0) — State enable extension.
• ssaia (1.0) — Advanced interrupt architecture extension for supervisor-mode.
• sscofpmf (1.0) — Count overflow & filtering extension.
• ssstateen (1.0) — State-enable extension for supervisor-mode.
• sstc (1.0) — Supervisor-mode timer interrupts extension.
• svinval (1.0) — Fine-grained address-translation cache invalidation extension.
• svnapot (1.0) — NAPOT translation contiguity extension.
• svpbmt (1.0) — Page-based memory types extension.
• xcvmac (1.0) — Core-V multiply-accumulate extension.
• xcvalu (1.0) — Core-V miscellaneous ALU extension.
• xcvelw (1.0) — Core-V event load word extension.
• xtheadba (1.0) — T-head address calculation extension.
• xtheadbb (1.0) — T-head basic bit-manipulation extension.
• xtheadbs (1.0) — T-head single-bit instructions extension.
• xtheadcmo (1.0) — T-head cache management operations extension.
• xtheadcondmov (1.0) — T-head conditional move extension.
• xtheadfmemidx (1.0) — T-head indexed memory operations for floating-point registers extension.
• xtheadfmv (1.0) — T-head double floating-point high-bit data transmission extension.
• xtheadint (1.0) — T-head acceleration interruption extension.
• xtheadmac (1.0) — T-head multiply-accumulate extension.
• xtheadmemidx (1.0) — T-head indexed memory operation extension.
• xtheadmempair (1.0) — T-head two-GPR memory operation extension.
• xtheadsync (1.0) — T-head multi-core synchronization extension.
• xventanacondops (1.0) — Ventana integer conditional operations extension.

And that's the whole alphabet soup, bestie, we survived it. 🫶

When -march= is not specified, use the setting from -mcpu.

⚠️ If both -march and -mcpu= are not specified, the default for this argument is system dependent. So if you want a specific architecture extensions, bestie, specify one explicitly. No vibes-based guessing, ok?

---

• -mcpu=processor-string — Use the architecture of and optimize the output for the given processor, specified by particular CPU name. Permissible values for this option are: sifive-e20, sifive-e21, sifive-e24, sifive-e31, sifive-e34, sifive-e76, sifive-s21, sifive-s51, sifive-s54, sifive-s76, sifive-u54, sifive-u74, sifive-x280, sifive-xp450, sifive-x670.

💅 Note that -mcpu does not override -march or -mtune. Like, it's just not that girl.

• -mtune=processor-string — Optimize the output for the given processor, specified by microarchitecture or particular CPU name. Permissible values for this option are: rocket, sifive-3-series, sifive-5-series, sifive-7-series, thead-c906, size, sifive-p400-series, sifive-p600-series, and all valid options for -mcpu=.

When -mtune= is not specified, use the setting from -mcpu, the default is rocket if both are not specified.

The size choice is not intended for use by end-users. This is used when -Os is specified. It overrides the instruction cost info provided by -mtune=, but does not override the pipeline info. This helps reduce code size while still giving good performance. ✨

• -mpreferred-stack-boundary=num — bestie, this one tries to keep the stack boundary aligned to a 2 raised to num byte boundary. If you don't say -mpreferred-stack-boundary at all, the default is 4 (that's 16 bytes, aka 128-bits, no cap).

⚠️ Warning: if you use this switch, you must build all modules with the same value, including any libraries. Yes, that includes the system libraries and startup modules. All of them. Same value. Or chaos. 😭

• -msmall-data-limit=n — puts global and static data smaller than n bytes into a special section (on some targets). Cute little data, special little section. 🎀
• -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. So like, default is speed, the flag is ✨smaller✨.
• -mmovcc, -mno-movcc — do or don't produce branchless conditional-move code sequences even with targets that do not have specific instructions for conditional operations. If enabled, sequences of ALU operations are produced using base integer ISA instructions where profitable. 💅
• -minline-atomics, -mno-inline-atomics — do or don't use smaller but slower subword atomic emulation code that uses libatomic function calls. The default is to use fast inline subword atomics that do not require libatomic. Periodt.
• -minline-strlen, -mno-inline-strlen — do or do not attempt to inline strlen calls if possible. Inlining will only be done if the string is properly aligned and instructions for accelerated processing are available. The default is to not inline strlen calls.
• -minline-strcmp, -mno-inline-strcmp — do or do not attempt to inline strcmp calls if possible. Inlining will only be done if the strings are properly aligned and instructions for accelerated processing are available. The default is to not inline strcmp calls.

The --param riscv-strcmp-inline-limit=n parameter controls the maximum number of bytes compared by the inlined code. The default value is 64.

• -minline-strncmp, -mno-inline-strncmp — do or do not attempt to inline strncmp calls if possible. Inlining will only be done if the strings are properly aligned and instructions for accelerated processing are available. The default is to not inline strncmp calls.

The --param riscv-strcmp-inline-limit=n parameter controls the maximum number of bytes compared by the inlined code. The default value is 64.

• -mshorten-memrefs, -mno-shorten-memrefs — do or do not attempt to make more use of compressed load/store instructions by replacing a load/store of base register + large offset with a new load/store of new base + small offset. If the new base gets stored in a compressed register, then the new load/store can be compressed. Currently targets 32-bit integer load/stores only. iykyk 👑
• -mstrict-align, -mno-strict-align — do not or do generate unaligned memory accesses. (Yes, it's backwards from what you'd expect, read it again bestie 🫠.) The default is set depending on whether the processor we are optimizing for supports fast unaligned access or not.
• -mcmodel=medlow — generate code for the medium-low code model. The program and its statically defined symbols must lie within a 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.
• -mcmodel=medany — generate code for the medium-any code model. The program and its statically defined symbols must be within any single 2 GiB address range. Programs can be statically or dynamically linked.

The code generated by the medium-any code model is position-independent, but is not guaranteed to function correctly when linked into position-independent executables or libraries. Not me warning you twice 💀

• -mexplicit-relocs, -mno-exlicit-relocs — use or do not use assembler relocation operators when dealing with symbolic addresses. The alternative is to use assembler macros instead, which may limit optimization. (And yes, the second spelling is literally -mno-exlicit-relocs in here, typos are iconic and we keep them fr fr.)
• -mrelax, -mno-relax — take advantage of linker relaxations to reduce the number of instructions required to materialize symbol addresses. The default is to take advantage of linker relaxations. Slay, fewer instructions. ✨
• -mriscv-attribute, -mno-riscv-attribute — emit (do not emit) RISC-V attribute to record extra information into ELF objects. This feature requires at least binutils 2.32.
• -mcsr-check, -mno-csr-check — enables or disables the CSR checking.
• -malign-data=type — control how GCC aligns variables and constants of array, structure, or union types. Supported values for type are xlen which uses x register width as the alignment value, and natural which uses natural alignment. xlen is the default.
• -mbig-endian — generate big-endian code. This is the default when GCC is configured for a riscv64be-*-* or riscv32be-*-* target.
• -mlittle-endian — generate little-endian code. This is the default when GCC is configured for a riscv64-*-* or riscv32-*-* but not a riscv64be-*-* or riscv32be-*-* target.
• -mstack-protector-guard=guard, -mstack-protector-guard-reg=reg, -mstack-protector-guard-offset=offset — generate stack protection code using canary at guard. Supported locations are global for a global canary or tls for per-thread canary in the TLS block.

With the latter choice the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset furthermore specify which register to use as base register for reading the canary, and from what offset from that base register. There is no default register or offset as this is entirely for use within the Linux kernel. 👀

• -mtls-dialect=desc — use TLS descriptors as the thread-local storage mechanism for dynamic accesses of TLS variables.
• -mtls-dialect=trad — use traditional TLS as the thread-local storage mechanism for dynamic accesses of TLS variables. This is the default.

---

🌸 RL78 Options 🌸

• -msim — links in additional target libraries to support operation within a simulator.
• -mmul=none, -mmul=g10, -mmul=g13, -mmul=g14, -mmul=rl78 — specifies the type of hardware multiplication and division support to be used. The simplest is "none", which uses software for both multiplication and division. This is the default. The "g13" value is for the hardware multiply/divide peripheral found on the RL78/G13 (S2 core) targets. The "g14" value selects the use of the multiplication and division instructions supported by the RL78/G14 (S3 core) parts. The value "rl78" is an alias for "g14" and the value "mg10" is an alias for "none".

In addition a C preprocessor macro is defined, based upon the setting of this option. Possible values are: "__RL78_MUL_NONE__", "__RL78_MUL_G13__" or "__RL78_MUL_G14__".

• -mcpu=g10, -mcpu=g13, -mcpu=g14, -mcpu=rl78 — specifies the RL78 core to target. The default is the G14 core, also known as an S3 core or just RL78. The G13 or S2 core does not have multiply or divide instructions, instead it uses a hardware peripheral for these operations. The G10 or S1 core does not have register banks, so it uses a different calling convention.

If this option is set it also selects the type of hardware multiply support to use, unless this is overridden by an explicit -mmul=none option on the command line. Thus specifying -mcpu=g13 enables the use of the G13 hardware multiply peripheral and specifying -mcpu=g10 disables the use of hardware multiplications altogether.

Note, although the RL78/G14 core is the default target, specifying -mcpu=g14 or -mcpu=rl78 on the command line does change the behavior of the toolchain since it also enables G14 hardware multiply support. If these options are not specified on the command line then software multiplication routines will be used even though the code targets the RL78 core. This is for backwards compatibility with older toolchains which did not have hardware multiply and divide support.

In addition a C preprocessor macro is defined, based upon the setting of this option. Possible values are: "__RL78_G10__", "__RL78_G13__" or "__RL78_G14__".

• -mg10, -mg13, -mg14, -mrl78 — these are aliases for the corresponding -mcpu= option. They are provided for backwards compatibility.
• -mallregs — allow the compiler to use all of the available registers. By default registers "r24..r31" are reserved for use in interrupt handlers. With this option enabled these registers can be used in ordinary functions as well. Sharing is caring 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.
• -msave-mduc-in-interrupts, -mno-save-mduc-in-interrupts — specifies that interrupt handler functions should preserve the MDUC registers. This is only necessary if normal code might use the MDUC registers, for example because it performs multiplication and division operations. The default is to ignore the MDUC registers as this makes the interrupt handlers faster. The target option -mg13 needs to be passed for this to work as this feature is only available on the G13 target (S2 core). The MDUC registers will only be saved if the interrupt handler performs a multiplication or division operation or it calls another function.

---

👑 IBM RS/6000 and PowerPC Options 👑

These -m options are defined for the IBM RS/6000 and PowerPC:

• -mpowerpc-gpopt, -mno-powerpc-gpopt
• -mpowerpc-gfxopt, -mno-powerpc-gfxopt
• -mpowerpc64, -mno-powerpc64
• -mmfcrf, -mno-mfcrf
• -mpopcntb, -mno-popcntb
• -mpopcntd, -mno-popcntd
• -mfprnd, -mno-fprnd
• -mcmpb, -mno-cmpb
• -mhard-dfp, -mno-hard-dfp

You use these options to specify which instructions are available on the processor you are using. The default value of these options is determined when configuring GCC. Specifying the -mcpu=cpu_type overrides the specification of these options. We recommend you use the -mcpu=cpu_type option rather than the options listed above. fr fr, use -mcpu=cpu_type. 🫶

Specifying -mpowerpc-gpopt allows GCC to use the optional PowerPC architecture instructions in the General Purpose group, including floating-point square root. Specifying -mpowerpc-gfxopt allows GCC to use the optional PowerPC architecture instructions in the Graphics group, including floating-point select.

• -mmfcrf — lets GCC spit out the move from condition register field instruction, the one that lives on the POWER4 processor and all the other chips rocking the PowerPC V2.01 architecture. ✨
• -mpopcntb — lets GCC generate the popcount and double-precision FP reciprocal estimate instruction, which is a POWER5 thing and a PowerPC V2.02 architecture thing. 💅
• -mpopcntd — lets GCC generate the popcount instruction, implemented on the POWER7 processor and friends that support PowerPC V2.06. 🎀
• -mfprnd — lets GCC generate the FP round to integer instructions implemented on the POWER5+ processor and other processors supporting PowerPC V2.03. 🦋
• -mcmpb — lets GCC generate the compare bytes instruction, implemented on the POWER6 processor and other PowerPC V2.05 architecture supporters. 🌸
• -mhard-dfp — lets GCC generate the decimal floating-point instructions implemented on some POWER processors. 💖
• -mpowerpc64 — lets GCC generate the extra 64-bit instructions found in the full PowerPC64 architecture, and treats GPRs as 64-bit, doubleword quantities. GCC defaults to -mno-powerpc64. 🫶

---

✨ OPTIONS, but make it POWER ✨

🎀 -mcpu=cpu_type 🎀

Sets the architecture type, the register usage, and the instruction scheduling parameters for machine type cpu_type. bestie, it's doing a whole lot. 👑

The supported values for cpu_type are, verbatim, no notes:

401, 403, 405, 405fp, 440, 440fp, 464, 464fp, 476, 476fp, 505,
601, 602, 603, 603e, 604, 604e, 620, 630, 740, 7400, 7450, 750, 801,
821, 823, 860, 970, 8540, a2, e300c2, e300c3, e500mc, e500mc64,
e5500, e6500, ec603e, G3, G4, G5, titan, power3, power4, power5,
power5+, power6, power6x, power7, power8, power9, power10, power11,
powerpc, powerpc64, powerpc64le, rs64, and native

-mcpu=powerpc, -mcpu=powerpc64, and -mcpu=powerpc64le specify pure 32-bit PowerPC (either endian), 64-bit big endian PowerPC, and 64-bit little endian PowerPC architecture machine types respectively — and an appropriate, generic processor model is assumed for scheduling purposes. 🌀

Specifying native as the cpu type detects and selects the architecture option that corresponds to the host processor of the system doing the compilation. Heads up though: -mcpu=native has no effect if GCC doesn't recognize the processor. 😭

The other options specify a specific processor. Code generated under those options runs best on that processor, and may not run at all on others. iykyk. 💀

The -mcpu options automatically enable or disable the following options:

-maltivec  -mfprnd  -mhard-float  -mmfcrf  -mmultiple -mpopcntb
-mpopcntd  -mpowerpc64 -mpowerpc-gpopt  -mpowerpc-gfxopt -mmulhw
-mdlmzb  -mmfpgpr  -mvsx -mcrypto  -mhtm  -mpower8-fusion
-mquad-memory  -mquad-memory-atomic  -mfloat128 -mfloat128-hardware
-mprefixed -mpcrel -mmma -mrop-protect

The particular options set for any particular CPU varies between compiler versions, depending on what setting seems to produce optimal code for that CPU — so it doesn't necessarily reflect the actual hardware's capabilities. fr fr. If you want to set an individual option to a particular value, you may specify it after the -mcpu option, like so:

-mcpu=970 -mno-altivec

On AIX, the -maltivec and -mpowerpc64 options are not enabled or disabled by the -mcpu option at present, because AIX does not have full support for these options. You may still enable or disable them individually if you're sure it'll work in your environment. 🙈

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🎀 -mtune=cpu_type 🎀

Sets the instruction scheduling parameters for machine type cpu_type, but does not set the architecture type or register usage, the way -mcpu=cpu_type does. The same values for cpu_type are used for -mtune as for -mcpu. If both are specified, the code generated uses the architecture and registers set by -mcpu, but the scheduling parameters set by -mtune. slay. 💅

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🎀 -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. main character energy honestly. 👑

---

🎀 -maltivec / -mno-altivec 🎀

Generate code that uses (does not use) AltiVec instructions, and also enable the use of built-in functions that allow more direct access to the AltiVec instruction set. You may also need to set -mabi=altivec to adjust the current ABI with AltiVec ABI enhancements. 💖

When -maltivec is used, the element order for AltiVec intrinsics such as vec_splat, vec_extract, and vec_insert match array element order corresponding to the endianness of the target. That is, element zero identifies the leftmost element in a vector register when targeting a big-endian platform, and identifies the rightmost element in a vector register when targeting a little-endian platform. 🦋

---

🎀 -mvrsave / -mno-vrsave 🎀

Generate VRSAVE instructions when generating AltiVec code. ✨

---

🎀 -msecure-plt 🎀

Generate code that allows ld and ld.so to build executables and shared libraries with non-executable .plt and .got sections. This is a PowerPC 32-bit SYSV ABI option. 🔐

🎀 -mbss-plt 🎀

Generate code that uses a BSS .plt section that ld.so fills in, and requires .plt and .got sections that are both writable and executable. This is a PowerPC 32-bit SYSV ABI option. 💀

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🎀 -misel / -mno-isel 🎀

This switch enables or disables the generation of ISEL instructions. on/off bestie. 🎀

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🎀 -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. 🌈

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🎀 -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. 🔥

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🎀 -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. 🧠

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🎀 -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. 💫

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🎀 -mquad-memory / -mno-quad-memory 🎀

Generate code that uses (does not use) the non-atomic quad word memory instructions. The -mquad-memory option requires use of 64-bit mode. ⚠️

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🎀 -mquad-memory-atomic / -mno-quad-memory-atomic 🎀

Generate code that uses (does not use) the atomic quad word memory instructions. The -mquad-memory-atomic option requires use of 64-bit mode. ⚠️

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🎀 -mfloat128 / -mno-float128 🎀

Enable/disable the __float128 keyword for IEEE 128-bit floating point, and use either software emulation for IEEE 128-bit floating point or hardware instructions. 🍓

The VSX instruction set (-mvsx) must be enabled to use the IEEE 128-bit floating point support. The IEEE 128-bit floating point is only supported on Linux.

The default for -mfloat128 is enabled on PowerPC Linux systems using the VSX instruction set, and disabled on other systems.

If you use the ISA 3.0 instruction set (-mcpu=power9) on a 64-bit system, the IEEE 128-bit floating point support will also enable the generation of ISA 3.0 IEEE 128-bit floating point instructions. Otherwise, if you do not specify to generate ISA 3.0 instructions or you are targeting a 32-bit big endian system, IEEE 128-bit floating point will be done with software emulation. 🌀

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🎀 -mfloat128-hardware / -mno-float128-hardware 🎀

Enable/disable using ISA 3.0 hardware instructions to support the __float128 data type. ⚡

The default for -mfloat128-hardware is enabled on PowerPC Linux systems using the ISA 3.0 instruction set, and disabled on other systems.

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🎀 -m32 / -m64 🎀

Generate code for 32-bit or 64-bit environments of Darwin and SVR4 targets (including GNU/Linux). The 32-bit environment sets int, long and pointer to 32 bits and generates code that runs on any PowerPC variant. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits, and generates code for PowerPC64, as for -mpowerpc64. 👑

---

🎀 -mfull-toc, -mno-fp-in-toc, -mno-sum-in-toc, -mminimal-toc 🎀

Modify generation of the TOC (Table Of Contents), which is created for every executable file. The -mfull-toc option is selected by default. In that case, GCC allocates at least one TOC entry for each unique non-automatic variable reference in your program. GCC also places floating-point constants in the TOC. However, only 16,384 entries are available in the TOC. 😭

If you receive a linker error message that saying you have overflowed the available TOC space, you can reduce the amount of TOC space used with the -mno-fp-in-toc and -mno-sum-in-toc options. -mno-fp-in-toc prevents GCC from putting floating-point constants in the TOC, and -mno-sum-in-toc forces GCC to generate code to calculate the sum of an address and a constant at run time instead of putting that sum into the TOC. You may specify one or both of these options. Each causes GCC to produce very slightly slower and larger code at the expense of conserving TOC space. 💅

If you still run out of space in the TOC even when you specify both of these options, specify -mminimal-toc instead. This option causes GCC to make only one TOC entry for every file. When you specify this option, GCC produces code that is slower and larger but which uses extremely little TOC space. You may wish to use this option only on files that contain less frequently-executed code. 🙈

---

🎀 -maix64 / -maix32 🎀

Enable 64-bit AIX ABI and calling convention: 64-bit pointers, 64-bit long type, and the infrastructure needed to support them. Specifying -maix64 implies -mpowerpc64, while -maix32 disables the 64-bit ABI and implies -mno-powerpc64. GCC defaults to -maix32. 🌸

• -mxl-compat, -mno-xl-compat — bestie, this one makes your code vibe more closely with IBM XL compiler semantics when you're riding with an AIX-compatible ABI. What it does: it passes floating-point arguments to prototyped functions beyond the register save area (RSA) on the stack, on top of the argument FPRs 💅. It also says "not me assuming" that the most significant double in a 128-bit long double value is properly rounded when you're comparing values and converting to double 🙈. And it uses XL symbol names for long double support routines.

ok so the tea ☕: the AIX calling convention got extended but nobody initially documented it, to handle an obscure K&R C case of calling a function that takes the address of its arguments with fewer arguments than declared. IBM XL compilers access floating-point arguments that do not fit in the RSA from the stack when a subroutine is compiled without optimization. Because always storing floating-point arguments on the stack is inefficient and rarely needed, this option is not enabled by default and only is necessary when calling subroutines compiled by IBM XL compilers without optimization. iykyk ✨

• -mpe — Support IBM RS/6000 SP Parallel Environment (PE). It links an application written to use message passing with special startup code so the app can actually run 🏃‍♀️. The system must have PE installed in the standard location (/usr/lpp/ppe.poe/), or the specs file must be overridden with the -specs= option to specify the appropriate directory location. The Parallel Environment does not support threads, so the -mpe option and the -pthread option are incompatible 💀 — pick a lane, bestie.
• -malign-natural, -malign-power — On AIX, 32-bit Darwin, and 64-bit PowerPC GNU/Linux, the option -malign-natural overrides the ABI-defined alignment of larger types, such as floating-point doubles, on their natural size-based boundary. The option -malign-power tells GCC to follow the ABI-specified alignment rules. GCC defaults to the standard alignment defined in the ABI.

On 64-bit Darwin, natural alignment is the default, and -malign-power is not supported 🙅‍♀️.

• -msoft-float, -mhard-float — Generate code that does not use (uses) the floating-point register set 🦋. Software floating-point emulation is provided if you use the -msoft-float option, and pass the option to GCC when linking.
• -mmultiple, -mno-multiple — Generate code that uses (does not use) the load multiple word instructions and the store multiple word instructions. These instructions are generated by default on POWER systems, and not generated on PowerPC systems.

⚠️ Do not use -mmultiple on little-endian PowerPC systems, since those instructions do not work when the processor is in little-endian mode. The exceptions are PPC740 and PPC750 which permit these instructions in little-endian mode. so iconic of them honestly 👑

• -mupdate, -mno-update — Generate code that uses (does not use) the load or store instructions that update the base register to the address of the calculated memory location. These instructions are generated by default.

😭 If you use -mno-update, there is a small window between the time that the stack pointer is updated and the address of the previous frame is stored, which means code that walks the stack frame across interrupts or signals may get corrupted data. fr fr, be careful.

• -mavoid-indexed-addresses, -mno-avoid-indexed-addresses — Generate code that tries to avoid (not avoid) the use of indexed load or store instructions. These instructions can incur a performance penalty on Power6 processors in certain situations, such as when stepping through large arrays that cross a 16M boundary. This option is enabled by default when targeting Power6 and disabled otherwise.
• -mfused-madd, -mno-fused-madd — Generate code that uses (does not use) the floating-point multiply and accumulate instructions. These instructions are generated by default if hardware floating point is used. The machine-dependent -mfused-madd option is now mapped to the machine-independent -ffp-contract=fast option, and -mno-fused-madd is mapped to -ffp-contract=off ✨.
• -mmulhw, -mno-mulhw — Generate code that uses (does not use) the half-word multiply and multiply-accumulate instructions on the IBM 405, 440, 464 and 476 processors. These instructions are generated by default when targeting those processors 💅.
• -mdlmzb, -mno-dlmzb — Generate code that uses (does not use) the string-search dlmzb instruction on the IBM 405, 440, 464 and 476 processors. This instruction is generated by default when targeting those processors.
• -mno-bit-align, -mbit-align — On System V.4 and embedded PowerPC systems do not (do) force structures and unions that contain bit-fields to be aligned to the base type of the bit-field.

For example, by default a structure containing nothing but 8 unsigned bit-fields of length 1 is aligned to a 4-byte boundary and has a size of 4 bytes. By using -mno-bit-align, the structure is aligned to a 1-byte boundary and is 1 byte in size 🎀.

• -mno-strict-align, -mstrict-align — On System V.4 and embedded PowerPC systems do not (do) assume that unaligned memory references are handled by the system.
• -mrelocatable, -mno-relocatable — Generate code that allows (does not allow) a static executable to be relocated to a different address at run time. A simple embedded PowerPC system loader should relocate the entire contents of .got2 and 4-byte locations listed in the .fixup section, a table of 32-bit addresses generated by this option. For this to work, all objects linked together must be compiled with -mrelocatable or -mrelocatable-lib. -mrelocatable code aligns the stack to an 8-byte boundary 💖.
• -mrelocatable-lib, -mno-relocatable-lib — Like -mrelocatable, -mrelocatable-lib generates a .fixup section to allow static executables to be relocated at run time, but -mrelocatable-lib does not use the smaller stack alignment of -mrelocatable. Objects compiled with -mrelocatable-lib may be linked with objects compiled with any combination of the -mrelocatable options 🫶.
• -mno-toc, -mtoc — On System V.4 and embedded PowerPC systems do not (do) assume that register 2 contains a pointer to a global area pointing to the addresses used in the program.
• -mlittle, -mlittle-endian — On System V.4 and embedded PowerPC systems compile code for the processor in little-endian mode. The -mlittle-endian option is the same as -mlittle 💜.
• -mbig, -mbig-endian — On System V.4 and embedded PowerPC systems compile code for the processor in big-endian mode. The -mbig-endian option is the same as -mbig 💜.
• -mdynamic-no-pic — On Darwin / macOS systems, compile code so that it is not relocatable, but that its external references are relocatable. The resulting code is suitable for applications, but not shared libraries 🚫📚.
• -msingle-pic-base — Treat the register used for PIC addressing as read-only, rather than loading it in the prologue for each function. The runtime system is responsible for initializing this register with an appropriate value before execution begins 🌸.
• -mprioritize-restricted-insns= priority — This option controls the priority that is assigned to dispatch-slot restricted instructions during the second scheduling pass. The argument priority takes the value 0, 1, or 2 to assign no, highest, or second-highest (respectively) priority to dispatch-slot restricted instructions 🎚️.
• -msched-costly-dep= dependence_type — This option controls which dependences are considered costly by the target during instruction scheduling. The argument dependence_type takes one of the following values:
  • no — No dependence is costly.
  • all — All dependences are costly.
  • true_store_to_load — A true dependence from store to load is costly.
  • store_to_load — Any dependence from store to load is costly.
  • number — Any dependence for which the latency is greater than or equal to number is costly.
• -minsert-sched-nops= scheme — This option controls which NOP insertion scheme is used during the second scheduling pass. The argument scheme takes one of the following values:
  • no — Don't insert NOPs.
  • pad — Pad with NOPs any dispatch group that has vacant issue slots, according to the scheduler's grouping.
  • regroup_exact — Insert NOPs to force costly dependent insns into separate groups. Insert exactly as many NOPs as needed to force an insn to a new group, according to the estimated processor grouping.
  • number — Insert NOPs to force costly dependent insns into separate groups. Insert number NOPs to force an insn to a new group.
• -mcall-sysv — On System V.4 and embedded PowerPC systems compile code using calling conventions that adhere to the March 1995 draft of the System V Application Binary Interface, PowerPC processor supplement. This is the default unless you configured GCC using powerpc-*-eabiaix 📅.
• -mcall-sysv-eabi, -mcall-eabi — Specify both -mcall-sysv and -meabi options 🤝.
• -mcall-sysv-noeabi — Specify both -mcall-sysv and -mno-eabi options 🤝.
• -mcall-aixdesc — On System V.4 and embedded PowerPC systems compile code for the AIX operating system 🧡.
• -mcall-linux — On System V.4 and embedded PowerPC systems compile code for the Linux-based GNU system 🐧.
• -mcall-freebsd — On System V.4 and embedded PowerPC systems compile code for the FreeBSD operating system 😈.
• -mcall-netbsd — On System V.4 and embedded PowerPC systems compile code for the NetBSD operating system 🚩.
• -mcall-openbsd — On System V.4 and embedded PowerPC systems compile code for the OpenBSD operating system 🐡.
• -mtraceback= traceback_type — Select the type of traceback table. Valid values for traceback_type are full, part, and no 🔍.
• -maix-struct-return — Return all structures in memory (as specified by the AIX ABI) 💾.
• -msvr4-struct-return — Return structures smaller than 8 bytes in registers (as specified by the SVR4 ABI) 📦.
• -mabi= abi-type — Extend the current ABI with a particular extension, or remove such extension. Valid values are: altivec, no-altivec, ibmlongdouble, ieeelongdouble, elfv1, elfv2, and for AIX: vec-extabi, vec-default 🌈.

---

• -mabi=ibmlongdouble — switches the current ABI over to using IBM extended-precision long double. Heads up though bestie: this is not likely to work if your system already defaults to IEEE extended-precision long double. And if you do change the long double type away from IEEE extended-precision, the compiler will throw a warning at you unless you pass -Wno-psabi. Also it requires -mlong-double-128 to be enabled, no exceptions 💅
• -mabi=ieeelongdouble — same energy, opposite direction: switches the current ABI to use IEEE extended-precision long double. Again, not likely to work if your system defaults to IBM extended-precision long double. Flip the long double type away from IBM extended-precision and the compiler warns unless you use -Wno-psabi. And yep, this one requires -mlong-double-128 too ✨
• -mabi=elfv1 — changes the current ABI to the ELFv1 ABI. This is the default ABI for big-endian PowerPC 64-bit Linux. Overriding the default ABI needs special system support and is likely to fail in spectacular ways 😭 iconic behaviour honestly.
• -mabi=elfv2 — changes the current ABI to the ELFv2 ABI. This is the default ABI for little-endian PowerPC 64-bit Linux. Overriding the default ABI needs special system support and is, again, likely to fail in spectacular ways 💀
• -mgnu-attribute
• -mno-gnu-attribute — emit .gnu_attribute assembly directives, which set tag/value pairs in a .gnu.attributes section that specify ABI variations in function parameters or return values 🎀
• -mprototype
• -mno-prototype — on System V.4 and embedded PowerPC systems, assume all calls to variable argument functions are properly prototyped. Otherwise the compiler has to insert an instruction before every non-prototyped call to set or clear bit 6 of the condition code register ("CR") — that bit tells the world whether floating-point values are passed in the floating-point registers, in case the function takes variable arguments. With -mprototype, only calls to prototyped variable argument functions set or clear the bit ✨
• -msim — on embedded PowerPC systems, assume the startup module is called sim-crt0.o and that the standard C libraries are libsim.a and libc.a. This is the default for powerpc-*-eabisim configurations 🫶
• -mmvme — on embedded PowerPC systems, assume the startup module is called crt0.o and the standard C libraries are libmvme.a and libc.a.
• -mads — on embedded PowerPC systems, assume the startup module is called crt0.o and the standard C libraries are libads.a and libc.a.
• -myellowknife — on embedded PowerPC systems, assume the startup module is called crt0.o and the standard C libraries are libyk.a and libc.a. yellowknife is such a name honestly 🌈
• -mvxworks — on System V.4 and embedded PowerPC systems, says that you're compiling for a VxWorks system 💜
• -memb — on embedded PowerPC systems, sets the "PPC_EMB" bit in the ELF flags header to indicate that eabi extended relocations are used.
• -meabi
• -mno-eabi — on System V.4 and embedded PowerPC systems, do (or don't) adhere to the Embedded Applications Binary Interface (EABI), which is a set of modifications to the System V.4 specs. Picking -meabi means: the stack gets aligned to an 8-byte boundary, a function "__eabi" is called from "main" to set up the EABI environment, and the -msdata option can use both "r2" and "r13" to point to two separate small data areas. Picking -mno-eabi means: the stack is aligned to a 16-byte boundary, no EABI initialization function is called from "main", and -msdata only uses "r13" to point to a single small data area. The -meabi option is on by default if you configured GCC using one of the powerpc*-*-eabi* options 👑
• -msdata=eabi — on System V.4 and embedded PowerPC systems, put small initialized "const" global and static data in the .sdata2 section, which is pointed to by register "r2". Put small initialized non-"const" global and static data in the .sdata section, pointed to by register "r13". Put small uninitialized global and static data in the .sbss section, which sits adjacent to .sdata. Fun fact: -msdata=eabi is incompatible with the -mrelocatable option ⚠️ and it also sets the -memb option.
• -msdata=sysv — on System V.4 and embedded PowerPC systems, put small global and static data in the .sdata section, pointed to by register "r13". Put small uninitialized global and static data in the .sbss section, adjacent to .sdata. The -msdata=sysv option is incompatible with -mrelocatable ⚠️
• -msdata=default
• -msdata — on System V.4 and embedded PowerPC systems, if -meabi is used, compile code the same as -msdata=eabi; otherwise compile it the same as -msdata=sysv 🦋
• -msdata=data — on System V.4 and embedded PowerPC systems, put small global data in the .sdata section, and small uninitialized global data in the .sbss section. But do not use register "r13" to address small data here. This is the default behavior unless other -msdata options are used 💅
• -msdata=none
• -mno-sdata — on embedded PowerPC systems, put all initialized global and static data in the .data section, and all uninitialized data in the .bss section 🎀
• -mreadonly-in-sdata — put read-only objects in the .sdata section as well. This is the default ⭐
• -mblock-move-inline-limit=num — inline all block moves (like calls to "memcpy" or 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.
• -mblock-compare-inline-limit=num — generate non-looping inline code for all block compares (like calls to "memcmp" or 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. The default value is target-specific 🧚
• -mblock-compare-inline-loop-limit=num — generate an inline expansion using loop code for all block compares that are less than or equal to num bytes, but greater than the limit for non-loop inline block compare expansion. If the block length isn't constant, at most num bytes get compared before "memcmp" is called to compare the rest of the block. The default value is target-specific 🌸
• -mstring-compare-inline-limit=num — compare at most num string bytes with inline code. If the difference or end of string isn't found by the end of that inline compare, a call to "strcmp" or "strncmp" handles the rest of the comparison. The default is 64 bytes ✨
• -G num — on embedded PowerPC systems, put 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 num switch is also passed to the linker, and all modules should be compiled with the same -G num value — no cap, keep them consistent 🙈
• -mregnames
• -mno-regnames — on System V.4 and embedded PowerPC systems, do (or don't) emit register names in the assembly language output using symbolic forms 💜
• -mlongcall
• -mno-longcall — by default, assume all calls are far away, which means 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 the "shortcall" function 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 just 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 too.

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. That 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 👀

• -mpltseq
• -mno-pltseq — implement (or don't implement) -fno-plt and long calls using an inline PLT call sequence that supports lazy linking and long calls to functions in dlopen'd shared libraries. Inline PLT calls are only supported on PowerPC64 ELFv2 and 32-bit PowerPC systems with newer GNU linkers, and they're enabled by default if that support is detected when configuring GCC — and, for 32-bit PowerPC, if GCC is configured with --enable-secureplt. Careful: -mpltseq code and -mbss-plt 32-bit PowerPC relocatable objects may not be linked together 😭
• -mtls-markers
• -mno-tls-markers — mark (or don't mark) calls to "__tls_get_addr" with a relocation specifying the function argument. That relocation lets the linker reliably associate the function call with the argument setup instructions for TLS optimization, which in turn lets GCC schedule the sequence better 🔥
• -mrecip, -mno-recip — ok so bestie, this one is so extra and I love it 🫶 it turns on the reciprocal estimate and reciprocal square root estimate instructions, plus extra Newton-Raphson steps to bump up the precision — so instead of doing a whole actual divide, or a square root and a divide for floating-point arguments, the chip just vibes with an estimate and polishes it. You should pair it with -ffast-math when you use -mrecip (or at minimum -funsafe-math-optimizations, -ffinite-math-only, -freciprocal-math and -fno-trapping-math, like, pick your faves). And heads up: the throughput of the sequence is generally higher than the non-reciprocal instruction, yes, but the precision can drop by up to 2 ulp — i.e. the inverse of 1.0 equals 0.99999994 — for reciprocal square roots. Not me warning you about precision loss 💀 it's giving "fast but slightly delulu".
• -mrecip=opt — this is the selector, the main character moment ✨ you get to choose exactly which reciprocal estimate instructions are allowed. opt is a comma-separated list, and any option can be preceded by a "!" to invert it. The lineup:
  • all — enable every single estimate instruction. go off 👑
  • 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 like, for example, -mrecip=all,!rsqrtd enables all of the reciprocal estimate instructions, except for the "FRSQRTE", "XSRSQRTEDP", and "XVRSQRTEDP" instructions which handle the double-precision reciprocal square root calculations. The ! is the "not this one bestie" button 💅

• -mrecip-precision, -mno-recip-precision — assume (or, with the no, do not assume) that the reciprocal estimate instructions provide higher-precision estimates than is mandated by the PowerPC ABI. Picking -mcpu=power6, -mcpu=power7 or -mcpu=power8 automatically selects -mrecip-precision, no extra effort needed 🎀 And the double-precision square root estimate instructions are not generated by default on low-precision machines, because they don't give you an estimate that converges after three steps. Not them failing the vibe check 😭
• -mveclibabi=type — tells GCC which ABI type to use for vectorizing intrinsics using an external library. The only type supported at present is mass, which specifies to use IBM's Mathematical Acceleration Subsystem (MASS) libraries for vectorizing intrinsics using external libraries. GCC currently emits calls to these when generating code for power7 — full list, no skips, we don't gatekeep here:

"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"

Both -ftree-vectorize and -funsafe-math-optimizations must also be enabled. And the MASS libraries must be specified at link time — don't forget that part, fr fr, the linker will simply not know her 😤

• -mfriz, -mno-friz — generate (or don't generate) the "friz" instruction when the -funsafe-math-optimizations option is used to optimize rounding of floating-point values to 64-bit integer and back to floating point. The "friz" instruction does not return the same value if the floating-point number is too large to fit in an integer. So like, edge cases exist, noted 🫡
• -mpointers-to-nested-functions, -mno-pointers-to-nested-functions — generate (or don't generate) code to load up the static chain register ("r11") when calling through a pointer on AIX and 64-bit Linux systems where a function pointer points to a 3-word descriptor giving the function address, TOC value to be loaded in register "r2", and static chain value to be loaded in register "r11". The -mpointers-to-nested-functions is on by default. But listen: you cannot call through pointers to nested functions, or pointers to functions compiled in other languages that use the static chain, if you use -mno-pointers-to-nested-functions. That's a hard no, not a soft no 🚫
• -msave-toc-indirect, -mno-save-toc-indirect — generate (or don't generate) code to save the TOC value in the reserved stack location in the function prologue if the function calls through a pointer on AIX and 64-bit Linux systems. If the TOC value is not saved in the prologue, it is saved just before the call through the pointer. The -mno-save-toc-indirect option is the default.
• -mcompat-align-parm, -mno-compat-align-parm — generate (or don't generate) code to pass structure parameters with a maximum alignment of 64 bits, for compatibility with older versions of GCC. The lore here 👇

Older versions of GCC (prior to 4.9.0) incorrectly did not align a structure parameter on a 128-bit boundary when that structure contained a member requiring 128-bit alignment. This is corrected in more recent versions of GCC. This option may be used to generate code that is compatible with functions compiled with older versions of GCC. So it's basically a time-travel pass for your structs 🕰️

The -mno-compat-align-parm option is the default.

• -mstack-protector-guard=guard, -mstack-protector-guard-reg=reg, -mstack-protector-guard-offset=offset, -mstack-protector-guard-symbol=symbol — generate stack protection code using canary at guard. Supported locations are global for global canary or tls for per-thread canary in the TLS block (the default with GNU libc version 2.4 or later). 🐤

With the latter choice, the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset furthermore specify which register to use as base register for reading the canary, and from what offset from that base register. The default for those is as specified in the relevant ABI. And -mstack-protector-guard-symbol=symbol overrides the offset with a symbol reference to a canary in the TLS block.

• -mpcrel, -mno-pcrel — generate (or don't generate) pc-relative addressing. The -mpcrel option requires that the medium code model (-mcmodel=medium) and prefixed addressing (-mprefixed) options are enabled. It has prerequisites, it's high maintenance, we respect that 💅
• -mprefixed, -mno-prefixed — generate (or don't generate) addressing modes using prefixed load and store instructions. The -mprefixed option requires that the option -mcpu=power10 (or later) is enabled.
• -mmma, -mno-mma — generate (or don't generate) the MMA instructions. The -mma option requires that the option -mcpu=power10 (or later) is enabled.
• -mrop-protect, -mno-rop-protect — generate (or don't generate) ROP protection instructions when the target processor supports them. Currently this option disables the shrink-wrap optimization (-fshrink-wrap). Trade-offs, bestie, trade-offs 🎀
• -mprivileged, -mno-privileged — generate (or don't generate) code that will run in privileged state. Very "I have admin energy" of you 👑
• -mblock-ops-unaligned-vsx, -mno-block-ops-unaligned-vsx — generate (or don't generate) unaligned vsx loads and stores for inline expansion of "memcpy" and "memmove".
• --param rs6000-vect-unroll-limit= — the vectorizer will check with target information to determine whether it would be beneficial to unroll the main vectorized loop and by how much. This parameter sets the upper bound of how much the vectorizer will unroll the main loop. The default value is four.

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🦋 RX Options 🦋

These command-line options are defined for RX targets — new target, new me, let's go 🌈

• -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 why, you ask? Because RX floating-point hardware only works on 32-bit values, which is why the default is -m32bit-doubles. The hardware said "32-bit only" and GCC said "understood bestie" 😌
• -fpu, -nofpu — enables (-fpu) or disables (-nofpu) the use of RX floating-point hardware. The default is enabled for the RX600 series and disabled for the RX200 series.

Floating-point instructions are only generated for 32-bit floating-point values, however, so the FPU hardware is not used for doubles if the -m64bit-doubles option is used. Not it ignoring your doubles 💀

Note: If the -fpu option is enabled then -funsafe-math-optimizations is also enabled automatically. This is because the RX FPU instructions are themselves unsafe. Yes, unsafe by nature, no cap.

• -mcpu=name — selects the type of RX CPU to be targeted. Currently three types are supported: the generic RX600 and RX200 series hardware, and the specific RX610 CPU. The default is RX600.

The only difference between RX600 and RX610 is that the RX610 does not support the "MVTIPL" instruction.

The RX200 series does not have a hardware floating-point unit, and so -nofpu is enabled by default when this type is selected.

• -mbig-endian-data, -mlittle-endian-data — store data (but not code, mind you, data only 🎀) in the big-endian format. The default is -mlittle-endian-data, i.e. to store data in the little-endian format.
• -msmall-data-limit=N — specifies the maximum size in bytes of global and static variables which can be placed into the small data area. Using the small data area can lead to smaller and faster code, but the size of area is limited and it is up to the programmer to ensure that the area does not overflow. Also, when the small data area is used, one of the RX's registers (usually "r13") is reserved for use pointing to this area, so it is no longer available for use by the compiler. This could result in slower and/or larger code if variables are pushed onto the stack instead of being held in this register. Basically you're trading a whole register for speed, so know what you're doing 🔥

Note: common variables (variables that have not been initialized) and constants are not placed into the small data area, as they are assigned to other sections in the output executable. They're just not invited to that party 🙈

Okay so we're still in the RX zone and then we slide right into S/390 and zSeries, bestie, stay with me 💜

The default value is zero, which disables this feature. Note, this feature is not enabled by default with higher optimization levels (-O2 etc) because of the potentially detrimental effects of reserving a register. It is up to the programmer to experiment and discover whether this feature is of benefit to their program. See the description of the -mpid option for a description of how the actual register to hold the small data area pointer is chosen.

---

• -msim
• -mno-sim — Use the simulator runtime. The default is to use the libgloss board-specific runtime. 🎀
• -mas100-syntax
• -mno-as100-syntax — When generating assembler output use a syntax that's compatible with Renesas's AS100 assembler. This syntax can also be handled by the GAS assembler, but it has some restrictions so it is not generated by default.
• -mmax-constant-size=N — Specifies the maximum size, in bytes, of a constant that can be used as an operand in a RX instruction. So like, the RX instruction set does allow constants of up to 4 bytes in length to be used in instructions, but a longer value equates to a longer instruction. Thus in some circumstances it can be beneficial to restrict the size of constants that are used in instructions. Constants that are too big are instead placed into a constant pool and referenced via register indirection.

The value N can be between 0 and 4. A value of 0 (the default) or 4 means that constants of any size are allowed.

• -mrelax — Enable linker relaxation. Linker relaxation is a process whereby the linker attempts to reduce the size of a program by finding shorter versions of various instructions. Disabled by default.
• -mint-register=N — Specify the number of registers to reserve for fast interrupt handler functions. The value N can be between 0 and 4. A value of 1 means that register r13 is reserved for the exclusive use of fast interrupt handlers. A value of 2 reserves r13 and r12. A value of 3 reserves r13, r12 and r11, and a value of 4 reserves r13 through r10. A value of 0, the default, does not reserve any registers.
• -msave-acc-in-interrupts — Specifies that interrupt handler functions should preserve the accumulator register. This is only necessary if normal code might use the accumulator register, for example because it performs 64-bit multiplications. The default is to ignore the accumulator as this makes the interrupt handlers faster. ⚡
• -mpid
• -mno-pid — Enables the generation of position independent data. When enabled, any access to constant data is done via an offset from a base address held in a register. This allows the location of constant data to be determined at run time without requiring the executable to be relocated, which is a benefit to embedded applications with tight memory constraints. Data that can be modified is not affected by this option.

Note, using this feature reserves a register, usually r13, for the constant data base address. This can result in slower and/or larger code, especially in complicated functions. 😭

The actual register chosen to hold the constant data base address depends upon whether the -msmall-data-limit and/or the -mint-register command-line options are enabled. Starting with register r13 and proceeding downwards, registers are allocated first to satisfy the requirements of -mint-register, then -mpid and finally -msmall-data-limit. Thus it is possible for the small data area register to be r8 if both -mint-register=4 and -mpid are specified on the command line.

By default this feature is not enabled. The default can be restored via the -mno-pid command-line option.

• -mno-warn-multiple-fast-interrupts
• -mwarn-multiple-fast-interrupts — Prevents GCC from issuing a warning message if it finds more than one fast interrupt handler when it is compiling a file. The default is to issue a warning for each extra fast interrupt handler found, as the RX only supports one such interrupt.
• -mallow-string-insns
• -mno-allow-string-insns — Enables or disables the use of the string manipulation instructions SMOVF, SCMPU, SMOVB, SMOVU, SUNTIL SWHILE and also the RMPA instruction. These instructions may prefetch data, which is not safe to do if accessing an I/O register. (See section 12.2.7 of the RX62N Group User's Manual for more information).

The default is to allow these instructions, but it is not possible for GCC to reliably detect all circumstances where a string instruction might be used to access an I/O register, so their use cannot be disabled automatically. Instead it is reliant upon the programmer to use the -mno-allow-string-insns option if their program accesses I/O space.

When the instructions are enabled GCC defines the C preprocessor symbol __RX_ALLOW_STRING_INSNS__, otherwise it defines the symbol __RX_DISALLOW_STRING_INSNS__.

• -mjsr
• -mno-jsr — Use only (or not only) JSR instructions to access functions. This option can be used when code size exceeds the range of BSR instructions. Note that -mno-jsr does not mean to not use JSR but instead means that any type of branch may be used. 🙈

Note: The generic GCC command-line option -ffixed-reg has special significance to the RX port when used with the interrupt function attribute. This attribute indicates a function intended to process fast interrupts. GCC ensures that it only uses the registers r10, r11, r12 and/or r13 and only provided that the normal use of the corresponding registers have been restricted via the -ffixed-reg or -mint-register command-line options.

---

🌈 S/390 and zSeries Options 🌈

These are the -m options defined for the S/390 and zSeries architecture.

• -mhard-float
• -msoft-float — Use (do not use) the hardware floating-point instructions and registers for floating-point operations. When -msoft-float is specified, functions in libgcc.a are used to perform floating-point operations. When -mhard-float is specified, the compiler generates IEEE floating-point instructions. This is the default.
• -mhard-dfp
• -mno-hard-dfp — Use (do not use) the hardware decimal-floating-point instructions for decimal-floating-point operations. When -mno-hard-dfp is specified, functions in libgcc.a are used to perform decimal-floating-point operations. When -mhard-dfp is specified, the compiler generates decimal-floating-point hardware instructions. This is the default for -march=z9-ec or higher. ✨
• -mlong-double-64
• -mlong-double-128 — These switches control the size of long double type. A size of 64 bits makes the long double type equivalent to the double type. This is the default.
• -mbackchain
• -mno-backchain — Store (do not store) the address of the caller's frame as backchain pointer into the callee's stack frame. A backchain may be needed to allow debugging using tools that do not understand DWARF call frame information. When -mno-packed-stack is in effect, the backchain pointer is stored at the bottom of the stack frame; when -mpacked-stack is in effect, the backchain is placed into the topmost word of the 96/160 byte register save area.

In general, code compiled with -mbackchain is call-compatible with code compiled with -mno-backchain; however, use of the backchain for debugging purposes usually requires that the whole binary is built with -mbackchain. Note that the combination of -mbackchain, -mpacked-stack and -mhard-float is not supported. In order to build a linux kernel use -msoft-float.

The default is to not maintain the backchain.

• -mpacked-stack
• -mno-packed-stack — Use (do not use) the packed stack layout. When -mno-packed-stack is specified, the compiler uses the all fields of the 96/160 byte register save area only for their default purpose; unused fields still take up stack space. When -mpacked-stack is specified, register save slots are densely packed at the top of the register save area; unused space is reused for other purposes, allowing for more efficient use of the available stack space. However, when -mbackchain is also in effect, the topmost word of the save area is always used to store the backchain, and the return address register is always saved two words below the backchain.

As long as the stack frame backchain is not used, code generated with -mpacked-stack is call-compatible with code generated with -mno-packed-stack. Note that some non-FSF releases of GCC 2.95 for S/390 or zSeries generated code that uses the stack frame backchain at run time, not just for debugging purposes. Such code is not call-compatible with code compiled with -mpacked-stack. Also, note that the combination of -mbackchain, -mpacked-stack and -mhard-float is not supported. In order to build a linux kernel use -msoft-float.

The default is to not use the packed stack layout.

• -msmall-exec
• -mno-small-exec — Generate (or do not generate) code using the bras instruction to do subroutine calls. This only works reliably if the total executable size does not exceed 64k. The default is to use the basr instruction instead, which does not have this limitation.
• -m64
• -m31 — When -m31 is specified, generate code compliant to the GNU/Linux for S/390 ABI. When -m64 is specified, generate code compliant to the GNU/Linux for zSeries ABI. This allows GCC in particular to generate 64-bit instructions. For the s390 targets, the default is -m31, while the s390x targets default to -m64.
• -mzarch
• -mesa — When -mzarch is specified, generate code using the instructions available on z/Architecture. When -mesa is specified, generate code using the instructions available on ESA/390. Note that -mesa is not possible with -m64. When generating code compliant to the GNU/Linux for S/390 ABI, the default is -mesa. When generating code compliant to the GNU/Linux for zSeries ABI, the default is -mzarch. 💅
• -mhtm
• -mno-htm — The -mhtm option enables a set of builtins making use of instructions available with the transactional execution facility introduced with the IBM zEnterprise EC12 machine generation S/390 System z Built-in Functions. -mhtm is enabled by default when using -march=zEC12. 👑
• -mvx, -mno-vx — say -mvx and you get code that uses the instructions from the vector extension facility that IBM dropped with the z13 machine generation. 💅 heads up tho: this option changes the ABI for some vector type values, specifically around alignment and calling conventions. And if vector type values show up in an ABI-relevant context, a GAS .gnu_attribute command gets added to mark the resulting binary with the ABI it's using. -mvx is enabled by default when you use -march=z13. ✨
• -mzvector, -mno-zvector — -mzvector turns on vector language extensions and builtins that use the instructions from the vector extension facility introduced with the IBM z13 machine generation. It adds support for vector as a keyword, so you can define vector type variables and arguments with it. Lowkey important: vector is only available when GNU extensions are enabled — it will not be expanded when you're requesting strict standard compliance, e.g. with -std=c99. 🫠 On top of the GCC low-level builtins, -mzvector also enables a whole set of builtins added for compatibility with AltiVec-style implementations like Power and Cell. To use those builtins you need to include the header file vecintrin.h. -mzvector is disabled by default.
• -mmvcle, -mno-mvcle — generate (or don't generate) code that uses the "mvcle" instruction to do block moves. Specify -mno-mvcle and it uses an "mvc" loop instead. That's the default unless you're optimizing for size. 🍓
• -mdebug, -mno-debug — print (or don't print) extra debug info while compiling. The default is to not print debug info. 🙈
• -march=cpu-type — generate code that runs on cpu-type, which is the name of a system representing a certain processor type. The possible values for cpu-type are z900/arch5, z990/arch6, z9-109, z9-ec/arch7, z10/arch8, z196/arch9, zEC12, z13/arch11, z14/arch12, z15/arch13, z16/arch14, and native.
  • The default is -march=z900. 👑
  • Passing native as the cpu type selects the best architecture option for the host processor. But fr fr: -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 the value used for -march. 🌈
• -mtpf-trace, -mno-tpf-trace — generate code that adds (or doesn't add) TPF OS specific branches to trace routines in the operating system. This option 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 doesn't change) the default branch targets enabled by -mtpf-trace so they point to specialized trace routines, giving you the ability to selectively skip function trace entries for the TPF OS. This option is off by default, even when compiling for the TPF OS and specifying -mtpf-trace. 💀
• -mfused-madd, -mno-fused-madd — generate code that uses (or doesn't use) the floating-point multiply and accumulate instructions. These instructions are generated by default if hardware floating point is used. 🔥
• -mwarn-framesize=framesize — emit a warning if the current function exceeds the given frame size. Because this is a compile-time check, it doesn't need to be a real problem when the program actually runs. It's meant to identify functions that most probably cause a stack overflow, and it's useful in an environment with a limited stack size — e.g. the linux kernel. ⭐
• -mwarn-dynamicstack — emit a warning if the function calls "alloca" or uses dynamically-sized arrays. This is generally a bad idea when you've got a limited stack size. 🙅
• -mstack-guard=stack-guard, -mstack-size=stack-size — if you provide these options, the S/390 back end emits additional instructions in the function prologue that trigger a trap if the stack size is stack-guard bytes above the stack-size (remember: the stack on S/390 grows downward). If the stack-guard option is omitted, the smallest power of 2 larger than the frame size of the compiled function is chosen. These options are intended to help you debug stack overflow problems. The extra emitted code causes only a little overhead, so you can also use it in production-like systems without greater performance degradation. The given values have to be exact powers of 2, and stack-size has to be greater than stack-guard without exceeding 64k. To be efficient, the extra code assumes the stack starts at an address aligned to the value given by stack-size. And the stack-guard option can only be used in conjunction with stack-size. 🫶
• -mhotpatch=pre-halfwords,post-halfwords — if the hotpatch option is enabled, a "hot-patching" function prologue is generated for all functions in the compilation unit. The 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.

---

✨ 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.
• -m2a-nofpu — generate code for the SH2a without FPU, or for a SH2a-FPU in such a way that the floating-point unit is not used.
• -m2a-single-only — generate code for the SH2a-FPU, in such a way that no double-precision floating-point operations are used.
• -m2a-single — generate code for the SH2a-FPU assuming the floating-point unit is in single-precision mode by default.
• -m2a — generate code for the SH2a-FPU assuming the floating-point unit is in double-precision mode by default.
• -m3 — generate code for the SH3.
• -m3e — generate code for the SH3e.
• -m4-nofpu — generate code for the SH4 without a floating-point unit.
• -m4-single-only — generate code for the SH4 with a floating-point unit that only supports single-precision arithmetic.
• -m4-single — generate code for the SH4 assuming the floating-point unit is in single-precision mode by default.
• -m4 — generate code for the SH4.
• -m4-100 — generate code for SH4-100.
• -m4-100-nofpu — generate code for SH4-100 in such a way that the floating-point unit is not used.
• -m4-100-single — generate code for SH4-100 assuming the floating-point unit is in single-precision mode by default.
• -m4-100-single-only — generate code for SH4-100 in such a way that no double-precision floating-point operations are used.
• -m4-200 — generate code for SH4-200.
• -m4-200-nofpu — generate code for SH4-200 in such a way that the floating-point unit is not used.
• -m4-200-single — generate code for SH4-200 assuming the floating-point unit is in single-precision mode by default.
• -m4-200-single-only — generate code for SH4-200 in such a way that no double-precision floating-point operations are used.
• -m4-300 — generate code for SH4-300.
• -m4-300-nofpu — generate code for SH4-300 in such a way that the floating-point unit is not used.
• -m4-300-single — generate code for SH4-300 in such a way that no double-precision floating-point operations are used.
• -m4-300-single-only — generate code for SH4-300 in such a way that no double-precision floating-point operations are used.
• -m4-340 — generate code for SH4-340 (no MMU, no FPU).
• -m4-500 — generate code for SH4-500 (no FPU). Passes -isa=sh4-nofpu to the assembler.
• -m4a-nofpu — generate code for the SH4al-dsp, or for a SH4a in such a way that the floating-point unit is not used.
• -m4a-single-only — generate code for the SH4a, in such a way that no double-precision floating-point operations are used.
• -m4a-single — generate code for the SH4a assuming the floating-point unit is in single-precision mode by default.
• -m4a — generate code for the SH4a.
• -m4al — same as -m4a-nofpu, except that it implicitly passes -dsp to the assembler. GCC doesn't generate any DSP instructions at the moment. 😅
• -mb — compile code for the processor in big-endian mode.
• -ml — compile code for the processor in little-endian mode.
• -mdalign — align doubles at 64-bit boundaries. Note that this changes the calling conventions, and thus some functions from the standard C library do not work unless you recompile it first with -mdalign. ⚠️
• -mrelax — shorten some address references at link time, when possible; uses the linker option -relax.
• -mbigtable — use 32-bit offsets in "switch" tables. The default is to use 16-bit offsets.
• -mbitops — enable the use of bit manipulation instructions on SH2A.
• -mfmovd — enable the use of the instruction "fmovd". Check -mdalign for alignment constraints.
• -mrenesas — comply with the calling conventions defined by Renesas.
• -mno-renesas — comply with the calling conventions defined for GCC before the Renesas conventions were available. This option is the default for all targets of the SH toolchain. 💜
• -mnomacsave — mark the "MAC" register as call-clobbered, even if -mrenesas is given.
• -mieee, -mno-ieee — control the IEEE compliance of floating-point comparisons, which affects how cases are handled where the result of a comparison is unordered. By default -mieee is implicitly enabled. If -ffinite-math-only is enabled, -mno-ieee is implicitly set, which results in faster floating-point greater-equal and less-equal comparisons. The implicit settings can be overridden by specifying either -mieee or -mno-ieee. 🦋
• -minline-ic_invalidate — inlines the code that invalidates instruction cache entries after you set up nested function trampolines 🧚. Real talk though: this one does nothing if -musermode is in effect and the code generation option you picked (like -m4) doesn't let you use the icbi instruction. And if the codegen option doesn't allow icbi while -musermode is not in effect, the inlined code goes and pokes the instruction cache address array directly with an associative write 😭. That's not just "needs privileged mode at run time" — it also straight-up fails if the cache line had been mapped via the TLB and then became unmapped. No cap.
• -misize — dumps instruction size and location right into the assembly code. Diagnostic queen 💅
• -mpadstruct — this one is deprecated, bestie ⚠️ It pads structures to a multiple of 4 bytes, which is incompatible with the SH ABI. Like, don't.
• -matomic-model=model — sets the model of atomic operations, plus any extra parameters, as a comma-separated list ✨. For the deets on the atomic built-in functions, go look at __atomic Builtins. Here's what you can pick:
  • none — disables compiler-generated atomic sequences and emits library calls for atomic operations instead. This is the default if the target is not sh*-*-linux*.
  • soft-gusa — generates GNU/Linux compatible gUSA software atomic sequences for the atomic built-in functions 🫶. Heads up: these generated sequences need extra support from the system's interrupt/exception handling code, and they're only suitable for SH3* and SH4* single-core systems. It's on by default when the target is sh*-*-linux* and SH3* or SH4*. When the target is SH4A, it also partially utilizes the hardware atomic instructions movli.l and movco.l to build more efficient code — unless you say strict.
  • soft-tcb — generates software atomic sequences that use a variable in the thread control block 🌸. It's a variation of the gUSA sequences that also works on SH1* and SH2* targets. Same deal: needs extra support from the interrupt/exception handling code, and only suitable for single-core systems. When you use this model, you gotta specify the gbr-offset= parameter too, fr fr.
  • soft-imask — generates software atomic sequences that temporarily disable interrupts by setting SR.IMASK = 1111 💀. This model only works when the program runs in privileged mode, and it's only suitable for single-core systems. Good news: no extra support from the interrupt/exception handling code is required. It's on by default when the target is sh*-*-linux* and SH1* or SH2*.
  • hard-llcs — generates hardware atomic sequences using only the movli.l and movco.l instructions 🔥. Only available on SH4A, and suitable for multi-core systems. Since the hardware instructions only support 32-bit atomic variable access, access to 8- or 16-bit variables is emulated with 32-bit accesses. Code compiled with this is also compatible with other software atomic model interrupt/exception handling systems if it's executed on an SH4A system. No extra support from the interrupt/exception handling code needed for this one 👑.
  • gbr-offset= — this parameter specifies the offset in bytes of the variable in the thread control block structure that the generated atomic sequences should use when the soft-tcb model has been selected. For any other model, this parameter is just ignored 🙈. The value you give has to 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 the tas.b opcode for __atomic_test_and_set ⭐. But notice: depending on your particular hardware and software setup, this can degrade overall performance because of the operand cache line flushes that the tas.b instruction implies. And on multi-core SH4A processors, the tas.b instruction must be used with caution — it can result in data corruption for certain cache configurations 😭. Careful!
• -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 -musermode also implies -mno-inline-ic_invalidate if the inlined code wouldn't work in user mode. -musermode is the default when the target is sh*-*-linux*. And if the target is SH1* or SH2*, -musermode has no effect, because there literally is no user mode 🫠.
• -multcost=number — sets the cost to assume for a multiply insn. That's the whole thing 🎀.
• -mdiv=strategy — sets the division strategy used for integer division operations. strategy can be one of:
  • call-div1 — calls a library function that uses the single-step division instruction div1 to perform the operation. Division by zero calculates an unspecified result and does not trap. This is the default except for SH4, SH2A and SHcompact.
  • call-fp — calls a library function that performs the operation in double precision floating point. Division by zero causes a floating-point exception. This is the default for SHcompact with FPU. Specifying this for targets that don't have a double precision FPU defaults to call-div1.
  • call-table — calls a library function that uses a lookup table for small divisors and the div1 instruction with case distinction for larger divisors. Division by zero calculates an unspecified result and does not trap. This is the default for SH4. Specifying this for targets that don't have dynamic shift instructions defaults to call-div1.

When a division strategy hasn't been specified, the default strategy is selected based on the current target 🦋. For SH2A, the default strategy is to use the divs and divu instructions instead of library function calls.

• -maccumulate-outgoing-args — reserves space once for outgoing arguments in the function prologue instead of around each call 💖. Generally a win for performance and size. It's also needed for unwinding, to avoid changing the stack frame around conditional code.
• -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 present, periodt.
• -mfixed-range=register-range — generates code treating the given register range as fixed registers. A fixed register is one the register allocator can't use 🚫. Super useful when you're compiling kernel code. A register range is specified as two registers separated by a dash, and you can specify multiple ranges 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 when it can 💅. If you don't specify it, the value is picked depending on the processor type you're compiling for.
• -mzdcbranch, -mno-zdcbranch — assume (or don't assume) that the zero displacement conditional branch instructions bt and bf are fast 🌸. If you specify -mzdcbranch, the compiler prefers zero displacement branch code sequences. This is enabled by default when generating code for SH4 and SH4A, and you can explicitly turn it off with -mno-zdcbranch.
• -mcbranch-force-delay-slot — forces the usage of delay slots for conditional branches, which stuffs the delay slot with a nop if a suitable instruction can't be found 💀. By default this option is disabled. You can enable it to work around hardware bugs, like the ones found in the original SH7055.
• -mfused-madd, -mno-fused-madd — generate code that uses (doesn't use) the floating-point multiply and accumulate instructions 🔥. These instructions are generated by default if hardware floating point is used. Note the glow-up: the machine-dependent -mfused-madd option is now mapped to the machine-independent -ffp-contract=fast option, and -mno-fused-madd is mapped to -ffp-contract=off.
• -mfsca, -mno-fsca — allow or disallow the compiler to emit the fsca instruction for sine and cosine approximations ✨. The -mfsca option must be used in combination with -funsafe-math-optimizations. It's enabled by default when generating code for SH4A. Using -mno-fsca disables sine and cosine approximations even if -funsafe-math-optimizations is in effect.
• -mfsrra, -mno-fsrra — allow or disallow the compiler to emit the fsrra instruction for reciprocal square root approximations 🦋. The -mfsrra option must be used in combination with -funsafe-math-optimizations and -ffinite-math-only. It's enabled by default when generating code for SH4A. Using -mno-fsrra disables reciprocal square root approximations even if -funsafe-math-optimizations and -ffinite-math-only are in effect.
• -mpretend-cmove — prefers zero-displacement conditional branches for conditional move instruction patterns 👑. This can result in faster code on the SH4 processor.
• -mfdpic — generates code using the FDPIC ABI ⭐.

---

🌞 Solaris 2 Options 🌞

These -m options are supported on Solaris 2 💖:

• -mclear-hwcap — -mclear-hwcap tells the compiler to remove the hardware capabilities generated by the Solaris assembler 🧹. This is only necessary when object files use ISA extensions that aren't supported by the current machine, but they check at runtime whether or not to use them.
• -mimpure-text — -mimpure-text, used in addition to -shared, tells the compiler to not pass -z text to the linker when linking a shared object 🙈. Using this option, you can link position-dependent code into a shared object.

That <red>copy-on-write</red> thing is the catch tho — those relocations mean the shared object is not actually shared across processes anymore. 😭 So instead of reaching for -mimpure-text, the move is to compile all your source code with -fpic or -fPIC. Periodt. 💅

On Solaris 2 there's extra stuff on top of everything above, bestie:

• -pthreads — it's literally just a synonym for -pthread. Same vibe, different name. 🫶

---

✨ SPARC Options ✨

These -m options are the ones SPARC understands, and there are a lot of them:

• -mno-app-regs, -mapp-regs — say -mapp-regs and you get output that uses global registers 2 through 4, which the SPARC SVR4 ABI reserves for applications. Just like global register 1, each of global registers 2 through 4 is then treated as an allocable register that gets clobbered by function calls. This is the default. But if you wanna be fully SVR4 ABI-compliant and you're willing to eat some performance loss, go -mno-app-regs. You should compile libraries and system software with this option.
• -mflat, -mno-flat — with -mflat, the compiler skips the save/restore instructions entirely and uses a "flat" or single register window model. Fun fact: this model is compatible with the regular register window model. The local registers and the input registers (0--5) are still treated as "call-saved" registers and get saved on the stack as needed. With -mno-flat (the default), the compiler generates save/restore instructions (except for leaf functions) — that's 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 instead.

⚠️ Warning: the requisite libraries are not available for all SPARC targets. Normally the facilities of the machine's usual C compiler are used, but this cannot be done directly in cross-compilation. You must make your own arrangements to provide suitable library functions for cross-compilation. The embedded targets sparc-*-aout and sparclite-*-* do provide software floating-point support.

Also, -msoft-float changes the calling convention in the output file, so it's only useful if you compile all of a program with this option. In particular, you need to compile libgcc.a — the library that comes with GCC — with -msoft-float in order for this to work. No skips, 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 are contained in another type, or if they have an absolute address. Otherwise, it assumes they have 4-byte alignment. Specifying this option avoids some rare compatibility problems with code generated by other compilers. It is not the default because it results in a performance loss, especially for floating-point code. 😬

• -muser-mode, -mno-user-mode — do not generate code that can only run in supervisor mode. This is relevant only for the "casa" instruction emitted for the LEON3 processor. This is the default.
• -mfaster-structs, -mno-faster-structs — with -mfaster-structs, the compiler assumes that structures should have 8-byte alignment. This enables the use of pairs of "ldd" and "std" instructions for copies in structure assignment, in place of twice as many "ld" and "st" pairs. However — and this is a big 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. You've been warned, bestie. 💀
• -mstd-struct-return, -mno-std-struct-return — with -mstd-struct-return, the compiler generates checking code in functions returning structures or unions to detect size mismatches between the two sides of function calls, as per the 32-bit ABI.

The default is -mno-std-struct-return. This option has no effect in 64-bit mode.

• -mlra, -mno-lra — enable Local Register Allocation. This is the default for SPARC since GCC 7, so -mno-lra needs to be passed to get old Reload. Throwback era fr fr. 🙈
• -mcpu=cpu_type — set the instruction set, register set, and instruction scheduling parameters for machine type cpu_type. Supported values for cpu_type are v7, cypress, v8, supersparc, hypersparc, leon, leon3, leon3v7, leon5, sparclite, f930, f934, sparclite86x, sparclet, tsc701, v9, ultrasparc, ultrasparc3, niagara, niagara2, niagara3, niagara4, niagara7 and m8.

Native Solaris and GNU/Linux toolchains also support the value native, which selects the best architecture option for the host processor. -mcpu=native has no effect if GCC does not recognize the processor.

Default instruction scheduling parameters are used for values that select an architecture and not an implementation. These are v7, v8, sparclite, sparclet, v9.

Here's the list of each supported architecture and their supported implementations, cutie:

• v7 — cypress, leon3v7
• v8 — supersparc, hypersparc, leon, leon3, leon5
• sparclite — f930, f934, sparclite86x
• sparclet — tsc701
• v9 — ultrasparc, ultrasparc3, niagara, niagara2, niagara3, niagara4, niagara7, m8

By default (unless configured otherwise), GCC generates code for the V7 variant of the SPARC architecture. With -mcpu=cypress, the compiler additionally optimizes it for the Cypress CY7C602 chip, as used in the SPARCStation/SPARCServer 3xx series. This is also appropriate for the older SPARCStation 1, 2, IPX etc.

With -mcpu=v8, GCC generates code for the V8 variant of the SPARC architecture. The only difference from V7 code is that the compiler emits the integer multiply and integer divide instructions which exist in SPARC-V8 but not in SPARC-V7. With -mcpu=supersparc, the compiler additionally optimizes it for the SuperSPARC chip, as used in the SPARCStation 10, 1000 and 2000 series.

With -mcpu=sparclite, GCC generates code for the SPARClite variant of the SPARC architecture. This adds the integer multiply, integer divide step and scan ("ffs") instructions which exist in SPARClite but not in SPARC-V7. With -mcpu=f930, the compiler additionally optimizes it for the Fujitsu MB86930 chip, which is the original SPARClite, with no FPU. With -mcpu=f934, the compiler additionally optimizes it for the Fujitsu MB86934 chip, which is the more recent SPARClite with FPU. 🔥

With -mcpu=sparclet, GCC generates code for the SPARClet variant of the SPARC architecture. This adds the integer multiply, multiply/accumulate, integer divide step and scan ("ffs") instructions which exist in SPARClet but not in SPARC-V7. With -mcpu=tsc701, the compiler additionally optimizes it for the TEMIC SPARClet chip.

With -mcpu=v9, GCC generates code for the V9 variant of the SPARC architecture. This adds 64-bit integer and floating-point move instructions, 3 additional floating-point condition code registers and conditional move instructions. With -mcpu=ultrasparc, the compiler additionally optimizes it for the Sun UltraSPARC I/II/IIi chips. With -mcpu=ultrasparc3, the compiler additionally optimizes it for the Sun UltraSPARC III/III+/IIIi/IIIi+/IV/IV+ chips. With -mcpu=niagara, the compiler additionally optimizes it for Sun UltraSPARC T1 chips. With -mcpu=niagara2, the compiler additionally optimizes it for Sun UltraSPARC T2 chips. With -mcpu=niagara3, the compiler additionally optimizes it for Sun UltraSPARC T3 chips. With -mcpu=niagara4, the compiler additionally optimizes it for Sun UltraSPARC T4 chips. With -mcpu=niagara7, the compiler additionally optimizes it for Oracle SPARC M7 chips. With -mcpu=m8, the compiler additionally optimizes it for Oracle M8 chips. 👑

• -mtune=cpu_type — set the instruction scheduling parameters for machine type cpu_type, but do not set the instruction set or register set that the option -mcpu=cpu_type does.

The same values for -mcpu=cpu_type can be used for -mtune=cpu_type, but the only useful values are those that select a particular CPU implementation. Those are cypress, supersparc, hypersparc, leon, leon3, leon3v7, leon5, f930, f934, sparclite86x, tsc701, ultrasparc, ultrasparc3, niagara, niagara2, niagara3, niagara4, niagara7 and m8. With native Solaris and GNU/Linux toolchains, native can also be used.

• -mv8plus, -mno-v8plus — with -mv8plus, GCC generates code for the SPARC-V8+ ABI. The difference from the V8 ABI is that the global and out registers are considered 64 bits wide. This is enabled by default on Solaris in 32-bit mode for all SPARC-V9 processors.
• -mvis, -mno-vis — with -mvis, GCC generates code that takes advantage of the UltraSPARC Visual Instruction Set extensions. The default is -mno-vis.
• -mvis2, -mno-vis2 — with -mvis2, GCC generates code that takes advantage of version 2.0 of the UltraSPARC Visual Instruction Set extensions. The default is -mvis2 when targeting a cpu that supports such instructions, such as UltraSPARC-III and later. Setting -mvis2 also sets -mvis. ✨
• -mvis3, -mno-vis3 — with -mvis3, GCC generates code that takes advantage of version 3.0 of the UltraSPARC Visual Instruction Set extensions. The default is -mvis3 when targeting a cpu that supports such instructions, such as niagara-3 and later. Setting -mvis3 also sets -mvis2 and -mvis. 🌈
• -mvis4, -mno-vis4 — with -mvis4, GCC spits out code that flexes version 4.0 of the UltraSPARC Visual Instruction Set extensions. The default is -mvis4 when you're targeting a cpu that supports that kinda instruction, like niagara-7 and later. Setting -mvis4 also sets -mvis3, -mvis2 and -mvis — it's a whole bundle, bestie 💅
• -mvis4b, -mno-vis4b — with -mvis4b, GCC generates code that takes advantage of version 4.0 of the UltraSPARC Visual Instruction Set extensions, plus the extra VIS instructions that showed up in the Oracle SPARC Architecture 2017. Default is -mvis4b when targeting a cpu that supports such instructions, such as m8 and later. Setting -mvis4b also sets -mvis4, -mvis3, -mvis2 and -mvis. Look at her, collecting flags like Pokémon ✨
• -mcbcond, -mno-cbcond — with -mcbcond, GCC generates code that takes advantage of the UltraSPARC Compare-and-Branch-on-Condition instructions. The default is -mcbcond when targeting a CPU that supports such instructions, such as Niagara-4 and later.
• -mfmaf, -mno-fmaf — with -mfmaf, GCC generates code that takes advantage of the UltraSPARC Fused Multiply-Add Floating-point instructions. The default is -mfmaf when targeting a CPU that supports such instructions, such as Niagara-3 and later.
• -mfsmuld, -mno-fsmuld — with -mfsmuld, GCC generates code that takes advantage of the Floating-point Multiply Single to Double (FsMULd) instruction. The default is -mfsmuld when targeting a CPU supporting the architecture versions V8 or V9 with FPU — except -mcpu=leon. (leon said not today 🙈)
• -mpopc, -mno-popc — with -mpopc, GCC generates code that takes advantage of the UltraSPARC Population Count instruction. The default is -mpopc when targeting a CPU that supports such an instruction, such as Niagara-2 and later.
• -msubxc, -mno-subxc — with -msubxc, GCC generates code that takes advantage of the UltraSPARC Subtract-Extended-with-Carry instruction. The default is -msubxc when targeting a CPU that supports such an instruction, such as Niagara-7 and later.
• -mfix-at697f — enable the documented workaround for the single erratum of the Atmel AT697F processor (which corresponds to erratum #13 of the AT697E processor).
• -mfix-ut699 — enable the documented workarounds for the floating-point errata and the data cache nullify errata of the UT699 processor.
• -mfix-ut700 — enable the documented workaround for the back-to-back store errata of the UT699E/UT700 processor.
• -mfix-gr712rc — enable the documented workaround for the back-to-back store errata of the GR712RC processor.

These -m options are supported in addition to the ones above on SPARC-V9 processors in 64-bit environments, okay? Not instead of. In addition 💖

• -m32, -m64 — generate code for a 32-bit or 64-bit environment. The 32-bit environment sets int, long and pointer to 32 bits. The 64-bit environment sets int to 32 bits and long and pointer to 64 bits.
• -mcmodel=which — set the code model to one of:
  • medlow — the Medium/Low code model: 64-bit addresses, programs must be linked in the low 32 bits of memory. Programs can be statically or dynamically linked.
  • medmid — the Medium/Middle code model: 64-bit addresses, programs must be linked in the low 44 bits of memory, the text and data segments must be less than 2GB in size and the data segment must be located within 2GB of the text segment.
  • medany — the Medium/Anywhere code model: 64-bit addresses, programs may be linked anywhere in memory, the text and data segments must be less than 2GB in size and the data segment must be located within 2GB of the text segment.
  • embmedany — the Medium/Anywhere code model for embedded systems: 64-bit addresses, the text and data segments must be less than 2GB in size, both starting anywhere in memory (determined at link time). The global register %g4 points to the base of the data segment. Programs are statically linked and PIC is not supported.
• -mmemory-model=mem-model — set the memory model in force on the processor to one of:
  • default — the default memory model for the processor and operating system.
  • rmo — Relaxed Memory Order
  • pso — Partial Store Order
  • tso — Total Store Order
  • sc — Sequential Consistency

These memory models are formally defined in Appendix D of the SPARC-V9 architecture manual, as set in the processor's PSTATE.MM field. Very "read the appendix" of them, but okay, iconic 📚

• -mstack-bias, -mno-stack-bias — with -mstack-bias, GCC assumes that the stack pointer, and frame pointer if present, are offset by -2047 which must be added back when making stack frame references. This is the default in 64-bit mode. Otherwise, assume no such offset is present.

---

🌈 Options for System V 🌈

These additional options are available on System V Release 4 for compatibility with other compilers on those systems:

• -G — create a shared object. It is recommended that -symbolic or -shared be used instead. (Like, -G is giving legacy. Use the other ones, bestie.)
• -Qy — identify the versions of each tool used by the compiler, in a .ident assembler directive in the output.
• -Qn — refrain from adding .ident directives to the output file (this is the default).
• -YP,dirs — search the directories dirs, and no others, for libraries specified with -l.
• -Ym,dir — look in the directory dir to find the M4 preprocessor. The assembler uses this option.

---

🦋 V850 Options 🦋

These -m options are defined for V850 implementations:

• -mlong-calls, -mno-long-calls — treat all calls as being far away (near). If calls are assumed to be far away, the compiler always loads the function's address into a register, and calls indirect through the pointer.
• -mno-ep, -mep — do not optimize (do optimize) basic blocks that use the same index pointer 4 or more times to copy pointer into the ep register, and use the shorter sld and sst instructions. The -mep option is on by default if you optimize.
• -mno-prolog-function, -mprolog-function — do not use (do use) external functions to save and restore registers at the prologue and epilogue of a function. The external functions are slower, but use less code space if more than one function saves the same number of registers. The -mprolog-function option is on by default if you optimize.
• -mspace — try to make the code as small as possible. At present, this just turns on the -mep and -mprolog-function options.
• -mtda=n — put static or global variables whose size is n bytes or less into the tiny data area that register ep points to. The tiny data area can hold up to 256 bytes in total (128 bytes for byte references).
• -msda=n — put static or global variables whose size is n bytes or less into the small data area that register gp points to. The small data area can hold up to 64 kilobytes.
• -mzda=n — put static or global variables whose size is n bytes or less into the first 32 kilobytes of memory.
• -mv850 — specify that the target processor is the V850.
• -mv850e3v5 — specify that the target processor is the V850E3V5. The preprocessor constant __v850e3v5__ is defined if this option is used.
• -mv850e2v4 — specify that the target processor is the V850E3V5. This is an alias for the -mv850e3v5 option. (Yes, it says V850E3V5. That's literally what the page says. Not a typo on my end, I promise 😭)
• -mv850e2v3 — specify that the target processor is the V850E2V3. The preprocessor constant __v850e2v3__ is defined if this option is used.
• -mv850e2 — specify that the target processor is the V850E2. The preprocessor constant __v850e2__ is defined if this option is used.
• -mv850e1 — specify that the target processor is the V850E1. The preprocessor constants __v850e1__ and __v850e__ are defined if this option is used.
• -mv850es — specify that the target processor is the V850ES. This is an alias for the -mv850e1 option.
• -mv850e — specify that the target processor is the V850E. The preprocessor constant __v850e__ is defined if this option is used.

If neither -mv850 nor -mv850e nor -mv850e1 nor -mv850e2 nor -mv850e2v3 nor -mv850e3v5 are defined then a default target processor is chosen and the relevant __v850*__ preprocessor constant is defined.

The preprocessor constants __v850 and __v851__ are always defined, regardless of which processor variant is the target. Always always. No matter what 💜

• -mdisable-callt, -mno-disable-callt — this option suppresses generation of the CALLT instruction for the v850e, v850e1, v850e2, v850e2v3 and v850e3v5 flavors of the v850 architecture.

This option is enabled by default when the RH850 ABI is in use (see -mrh850-abi), and disabled by default when the GCC ABI is in use. If CALLT instructions are being generated then the C preprocessor symbol __V850_CALLT__ is defined.

• -mrelax, -mno-relax — pass on (or do not pass on) the -mrelax command-line option to the assembler.
• -mlong-jumps, -mno-long-jumps — disable (or re-enable) the generation of PC-relative jump instructions.
• -msoft-float, -mhard-float — disable (or re-enable) the generation of hardware floating point instructions. This option is only significant when the target architecture is V850E2V3 or higher. If hardware floating point instructions are being generated then the C preprocessor symbol __FPU_OK__ is defined, otherwise the symbol __NO_FPU__ is defined.
• -mloop — enables the use of the e3v5 LOOP instruction. The use of this instruction is not enabled by default when the e3v5 architecture is selected because its use is still experimental. 🧪 So, like, opt in at your own risk, babe.
• -mrh850-abi, -mghs — enables support for the RH850 version of the V850 ABI. This is the default. With this version of the ABI the following rules apply:
  • Integer sized structures and unions are returned via a memory pointer rather than a register.
  • Large structures and unions (more than 8 bytes in size) are passed by value.
  • Functions are aligned to 16-bit boundaries.
  • The -m8byte-align command-line option is supported.
  • The -mdisable-callt command-line option is enabled by default. The -mno-disable-callt command-line option is not supported.

When this version of the ABI is enabled the C preprocessor symbol __V850_RH850_ABI__ is defined.

• -mgcc-abi — turns on support for the old GCC version of the V850 ABI. With this flavor of the ABI, these rules apply:
  • Integer-sized structures and unions get returned in register r10.
  • Big structures and unions (anything over 8 bytes in size) are passed by reference.
  • Functions are aligned to 32-bit boundaries, unless you're optimizing for size.
  • The -m8byte-align command-line option is not supported.
  • The -mdisable-callt command-line option is supported but not enabled by default.

When this version of the ABI is enabled, the C preprocessor symbol __V850_GCC_ABI__ gets defined. 💖

• -m8byte-align, -mno-8byte-align — enables support for double and long long types to be aligned on 8-byte boundaries. The default is to restrict the alignment of all objects to at most 4 bytes. When -m8byte-align is in effect, the C preprocessor symbol __V850_8BYTE_ALIGN__ gets defined. ✨
• -mbig-switch — generates code suitable for big switch tables. Only use this option if the assembler/linker is complaining about out-of-range branches within a switch table. (fr, only then.)
• -mapp-regs — makes r2 and r5 get used in the code the compiler generates. This setting is the default. 👑
• -mno-app-regs — makes r2 and r5 be treated as fixed registers.

VAX Options 🦋

These -m options are defined for the VAX:

• -munix — do not output certain jump instructions (aobleq and so on) that the Unix assembler for the VAX can't handle across long ranges.
• -mgnu — do output those jump instructions, on the assumption that the GNU assembler is being used.
• -mg — output code for G-format floating-point numbers instead of D-format.
• -mlra, -mno-lra — enable Local Register Allocation. This is still experimental for the VAX, so by default the compiler uses standard reload. 🧚

---

Visium Options 🌸

• -mdebug — a program that does file I/O and is destined to run on an MCM target should be linked with this option. It causes the libraries libc.a and libdebug.a to be linked. The program should be run on the target under the control of the GDB remote debugging stub.
• -msim — a program that does file I/O and is destined to run on the simulator should be linked with this option. This causes libraries libc.a and libsim.a to be linked.
• -mfpu, -mhard-float — generate code containing floating-point instructions. This is the default. 💅
• -mno-fpu, -msoft-float — generate code containing library calls for floating-point.

Heads up bestie: -msoft-float changes the calling convention in the output file, so it's only useful if you compile all of a program with this option. In particular, you need to compile libgcc.a — the library that comes with GCC — with -msoft-float in order for this to work. 😭

• -mcpu=cpu_type — sets the instruction set, register set, and instruction scheduling parameters for machine type cpu_type. Supported values for cpu_type are mcm, gr5 and gr6.

mcm is a synonym of gr5 present for backward compatibility.

By default (unless configured otherwise), GCC generates code for the GR5 variant of the Visium architecture.

With -mcpu=gr6, GCC generates code for the GR6 variant of the Visium architecture. The only difference from GR5 code is that the compiler will generate block move instructions.

• -mtune=cpu_type — sets the instruction scheduling parameters for machine type cpu_type, but does not set the instruction set or register set that the option -mcpu=cpu_type would.
• -msv-mode — generate code for the supervisor mode, where there are no restrictions on 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, registers r24 to r31 cannot be accessed in this mode; on the GR6, only registers r29 to r31 are affected. 🙈

---

VMS Options ⭐

These -m options are defined for the VMS implementations:

• -mvms-return-codes — return VMS condition codes from main. The default is to return POSIX-style condition (e.g. error) codes.
• -mdebug-main=prefix — flag the first routine whose name starts with prefix as the main routine for the debugger.
• -mmalloc64 — default to 64-bit memory allocation routines.
• -mpointer-size=size — set the default size of pointers. Possible options for size are 32 or short for 32-bit pointers, 64 or long for 64-bit pointers, and no for supporting only 32-bit pointers. The latter option disables pragma pointer_size.

---

VxWorks Options 🌈

The options in this section are defined for all VxWorks targets. Options specific to the target hardware are listed with the other options for that target.

• -mrtp — GCC can generate code for both VxWorks kernels and real time processes (RTPs). This option switches from the former to the latter. It also defines the preprocessor macro __RTP__.
• -msmp — select SMP runtimes for linking. Not available on architectures other than PowerPC, nor on VxWorks version 7 or later, in which the selection is part of the VxWorks build configuration and the library paths are the same for either choice.
• -non-static — link an RTP executable against shared libraries rather than static libraries. The options -static and -shared can also be used for RTPs; -static is the default.
• -Bstatic, -Bdynamic — these options are passed down to the linker. They are defined for compatibility with Diab.
• -Xbind-lazy — enable lazy binding of function calls. This option is equivalent to -Wl,-z,now and is defined for compatibility with Diab.
• -Xbind-now — disable lazy binding of function calls. This option is the default and is defined for compatibility with Diab.

---

x86 Options 🔥

These -m options are defined for the x86 family of computers.

• -march=cpu-type — generate instructions for the machine type cpu-type. In contrast to -mtune=cpu-type, which merely tunes the generated code for the specified cpu-type, -march=cpu-type allows GCC to generate code that may not run at all on processors other than the one indicated. Specifying -march=cpu-type implies -mtune=cpu-type, except where noted otherwise.

The choices for cpu-type are:

• native — this selects the CPU to generate code for at compilation time by determining the processor type of the compiling machine. Using -march=native enables all instruction subsets supported by the local machine (hence the result might not run on different machines). Using -mtune=native produces code optimized for the local machine under the constraints of the selected instruction set.
• x86-64 — a generic CPU with 64-bit extensions.
• x86-64-v2, x86-64-v3, x86-64-v4 — these choices for cpu-type select the corresponding micro-architecture level from the x86-64 psABI. On ABIs other than the x86-64 psABI they select the same CPU features as the x86-64 psABI documents for the particular micro-architecture level.

Since these cpu-type values do not have a corresponding -mtune setting, using -march with these values enables generic tuning. Specific tuning can be enabled using the -mtune=other-cpu-type option with an appropriate other-cpu-type value.

• i386 — original Intel i386 CPU.
• i486 — Intel i486 CPU. (No scheduling is implemented for this chip.)
• i586, pentium — Intel Pentium CPU with no MMX support.
• lakemont — Intel Lakemont MCU, based on Intel Pentium CPU.
• pentium-mmx — Intel Pentium MMX CPU, based on Pentium core with MMX instruction set support.
• pentiumpro — Intel Pentium Pro CPU.
• i686 — when used with -march, the Pentium Pro instruction set is used, so the code runs on all i686 family chips. When used with -mtune, it has the same meaning as generic.
• pentium2 — Intel Pentium II CPU, based on Pentium Pro core with MMX and FXSR instruction set support.
• pentium3, pentium3m — Intel Pentium III CPU, based on Pentium Pro core with MMX, FXSR and SSE instruction set support.
• pentium-m — Intel Pentium M; low-power version of Intel Pentium III CPU with MMX, SSE, SSE2 and FXSR instruction set support. Used by Centrino notebooks.
• pentium4, pentium4m — Intel Pentium 4 CPU with MMX, SSE, SSE2 and FXSR instruction set support.
• prescott — improved version of Intel Pentium 4 CPU with MMX, SSE, SSE2, SSE3 and FXSR instruction set support.
• nocona — improved version of Intel Pentium 4 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3 and FXSR instruction set support.
• core2 — Intel Core 2 CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, CX16, SAHF and FXSR instruction set support.
• nehalem — Intel Nehalem CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF and FXSR instruction set support.
• westmere — Intel Westmere CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR and PCLMUL instruction set support.
• sandybridge — Intel Sandy Bridge CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE and PCLMUL instruction set support.
• ivybridge — Intel Ivy Bridge CPU with 64-bit extensions, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND and F16C instruction set support.
• haswell — Intel Haswell CPU with 64-bit extensions, MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE and HLE instruction set support.

broadwell 💜

Intel Broadwell CPU with 64-bit extensions — and the instruction set support list is stacked, bestie. It's repping MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX and PREFETCHW instruction set support.

yep, MOVBE is in there twice. that's literally what it says, so that's what you're getting. 💅

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skylake ✨

Intel Skylake CPU with 64-bit extensions. The glow-up version, fr fr — it's got MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES and SGX instruction set support.

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bonnell 🎀

Intel Bonnell CPU with 64-bit extensions, and honestly? She's lowkey minimal. The whole vibe is MOVBE, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support. That's it. That's the list. 🫶

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silvermont 🦋

Intel Silvermont CPU with 64-bit extensions. Coming in with MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, PCLMUL, PREFETCHW and RDRND instruction set support.

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goldmont 🌸

Intel Goldmont CPU with 64-bit extensions — and she ate. Instruction set support for MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, PCLMUL, PREFETCHW, RDRND, AES, SHA, RDSEED, XSAVE, XSAVEC, XSAVES, XSAVEOPT, CLFLUSHOPT and FSGSBASE.

---

goldmont-plus ⭐

Intel Goldmont Plus CPU with 64-bit extensions. Not me getting the plus edition 😭 — instruction set support for MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, PCLMUL, PREFETCHW, RDRND, AES, SHA, RDSEED, XSAVE, XSAVEC, XSAVES, XSAVEOPT, CLFLUSHOPT, FSGSBASE, PTWRITE, RDPID and SGX.

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tremont 💫

Intel Tremont CPU with 64-bit extensions, and the list just keeps going. Instruction set support for MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, PCLMUL, PREFETCHW, RDRND, AES, SHA, RDSEED, XSAVE, XSAVEC, XSAVES, XSAVEOPT, CLFLUSHOPT, FSGSBASE, PTWRITE, RDPID, SGX, CLWB, GFNI-SSE, MOVDIRI, MOVDIR64B, CLDEMOTE and WAITPKG.

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sierraforest 🌲

Intel Sierra Forest CPU with 64-bit extensions. The way this list is endless — main character energy, no cap. Instruction set support for MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, CLDEMOTE, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL, AVX-VNNI, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, ENQCMD and UINTR.

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grandridge 🏔️

Intel Grand Ridge CPU with 64-bit extensions. Okay so this one is giving sierraforest exactly — same list, bestie, I'm not even playing. Instruction set support for MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, CLDEMOTE, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL, AVX-VNNI, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, ENQCMD and UINTR.

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clearwaterforest 💧

Intel Clearwater Forest CPU with 64-bit extensions. She said "let me add MORE" and honestly? Obsessed. Instruction set support for MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, CLDEMOTE, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL, AVX-VNNI, ENQCMD, UINTR, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, AVXVNNIINT16, SHA512, SM3, SM4, USER_MSR and PREFETCHI.

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knl 🚀

Intel Knight's Landing CPU with 64-bit extensions. Knights era, iconic, no notes. Instruction set support for MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AVX512PF, AVX512ER, AVX512F, AVX512CD and PREFETCHWT1.

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knm ⚔️

Intel Knights Mill CPU with 64-bit extensions. The sequel nobody asked for but we're grateful anyway 🙈. Instruction set support for MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AVX512PF, AVX512ER, AVX512F, AVX512CD and PREFETCHWT1, AVX5124VNNIW, AVX5124FMAPS and AVX512VPOPCNTDQ.

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skylake-avx512 🔥

Intel Skylake Server CPU with 64-bit extensions. Server girlie with the big register energy. Instruction set support for MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ and AVX512CD.

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cannonlake 🎆

Intel Cannonlake Server CPU with 64-bit extensions. Another server bestie, another giant list. Instruction set support for MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU, AVX512VBMI, AVX512IFMA and SHA.

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icelake-client 🧊

Intel Icelake Client CPU with 64-bit extensions. Client-side, but make it extra. Instruction set support for MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2 , VPCLMULQDQ, AVX512BITALG, RDPID and AVX512VPOPCNTDQ.

and yes, there's a lil space before that comma after AVX512VBMI2. keeping it. that's how the original rolls. 💅

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icelake-server 🧊✨

Intel Icelake Server CPU with 64-bit extensions. The client version but she went to work. Instruction set support for MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2 , VPCLMULQDQ, AVX512BITALG, RDPID, AVX512VPOPCNTDQ, PCONFIG, WBNOINVD and CLWB.

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cascadelake 🌊

Intel Cascadelake CPU with 64-bit extensions. Instruction set support for MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ, AVX512CD and AVX512VNNI.

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cooperlake 🍓

Intel cooperlake CPU with 64-bit extensions. Instruction set support for MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, CLWB, AVX512VL, AVX512BW, AVX512DQ, AVX512CD, AVX512VNNI and AVX512BF16.

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tigerlake 🐯

Intel Tigerlake CPU with 64-bit extensions. Big cat energy, big list energy. Instruction set support for MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, CX16, SAHF, FXSR, AVX, XSAVE, PCLMUL, FSGSBASE, RDRND, F16C, AVX2, BMI, BMI2, LZCNT, FMA, MOVBE, HLE, RDSEED, ADCX, PREFETCHW, AES, CLFLUSHOPT, XSAVEC, XSAVES, SGX, AVX512F, AVX512VL, AVX512BW, AVX512DQ, AVX512CD PKU, AVX512VBMI, AVX512IFMA, SHA, AVX512VNNI, GFNI, VAES, AVX512VBMI2, VPCLMULQDQ, AVX512BITALG, RDPID, AVX512VPOPCNTDQ, MOVDIRI, MOVDIR64B, CLWB, AVX512VP2INTERSECT and KEYLOCKER.

see that AVX512CD PKU with no comma? not a typo on my end, bestie. that's the source. 🫶

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sapphirerapids 💎

Intel sapphirerapids CPU with 64-bit extensions. Sapphire, but for real. Instruction set support for 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, AVX512-FP16 and AVX512BF16.

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alderlake 🌿

Intel Alderlake CPU with 64-bit extensions. Instruction set support for MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, CLDEMOTE, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL and AVX-VNNI.

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rocketlake 🚀🌊

Intel Rocketlake CPU with 64-bit extensions. Rocket AND lake, double the branding, double the list. Instruction set support for MOVBE, MMX, SSE, SSE2, SSE3 , 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.

yup — space before that comma after SSE3 too, and no comma between AVX512CD and PKU. verbatim, always. 💜

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graniterapids 🪨

Intel graniterapids CPU with 64-bit extensions. Rock solid, rock named, rock long list. Instruction set support for 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, AVX512-FP16, AVX512BF16, AMX-FP16 and PREFETCHI.

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okay and that's the CPU list so far — every single one of those names works as a value for the -march= option, and every instruction in every list is doing its lil job. 🧚✨

graniterapids-d 🧚

Intel graniterapids D CPU, and bestie the 64-bit extensions are there, plus this absolutely unhinged roster of instruction set support: 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 and AMX-COMPLEX. yeah. all of that. 💅

arrowlake 🌸

Intel Arrow Lake CPU, 64-bit extensions included, and the instruction set support vibe check: MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, CLDEMOTE, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL, AVX-VNNI, UINTR, AVXIFMA, AVXVNNIINT8, AVXNECONVERT and CMPCCXADD.

arrowlake-s ✨

Intel Arrow Lake S CPU — 64-bit extensions, obviously — and here's the instruction set support list: MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, CLDEMOTE, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL, AVX-VNNI, UINTR, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, AVXVNNIINT16, SHA512, SM3 and SM4.

pantherlake 🦋

Intel Panther Lake CPU. 64-bit extensions, and then: MOVBE, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, POPCNT, AES, PREFETCHW, PCLMUL, RDRND, XSAVE, XSAVEC, XSAVES, XSAVEOPT, FSGSBASE, PTWRITE, RDPID, SGX, GFNI-SSE, CLWB, MOVDIRI, MOVDIR64B, CLDEMOTE, WAITPKG, ADCX, AVX, AVX2, BMI, BMI2, F16C, FMA, LZCNT, PCONFIG, PKU, VAES, VPCLMULQDQ, SERIALIZE, HRESET, KL, WIDEKL, AVX-VNNI, UINTR, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, AVXVNNIINT16, SHA512, SM3, SM4 and PREFETCHI. all of it. slay. 🎀

k6

AMD K6 CPU, and it's giving MMX instruction set support. that's the whole story bestie.

k6-2, k6-3

Improved versions of the AMD K6 CPU — glow-up era — with MMX and 3DNow! instruction set support.

athlon, athlon-tbird

AMD Athlon CPU rocking MMX, 3dNOW!, enhanced 3DNow! and SSE prefetch instructions. iconic behavior honestly.

athlon-4, athlon-xp, athlon-mp

Improved AMD Athlon CPU with MMX, 3DNow!, enhanced 3DNow! and the full SSE instruction set. periodt.

k8, opteron, athlon64, athlon-fx

Processors based on the AMD K8 core, with x86-64 instruction set support — that includes the AMD Opteron, Athlon 64, and Athlon 64 FX processors. (And this supersets MMX, SSE, SSE2, 3DNow!, enhanced 3DNow! and 64-bit instruction set extensions. it's giving everything.) 👑

k8-sse3, opteron-sse3, athlon64-sse3

Improved versions of the AMD K8 cores, now with SSE3 instruction set support. the upgrade arc we deserved.

amdfam10, barcelona

CPUs based on AMD Family 10h cores, x86-64 instruction set support included. (This supersets MMX, SSE, SSE2, SSE3, SSE4A, 3DNow!, enhanced 3DNow!, ABM and 64-bit instruction set extensions.)

bdver1

CPUs based on AMD Family 15h cores with x86-64 instruction set support. (This supersets FMA4, AVX, XOP, LWP, AES, PCLMUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.)

bdver2

AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets BMI, TBM, F16C, FMA, FMA4, AVX, XOP, LWP, AES, PCLMUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.)

bdver3

AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets BMI, TBM, F16C, FMA, FMA4, FSGSBASE, AVX, XOP, LWP, AES, PCLMUL, CX16, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.)

bdver4

AMD Family 15h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, TBM, F16C, FMA, FMA4, FSGSBASE, AVX, AVX2, XOP, LWP, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM and 64-bit instruction set extensions.)

znver1

AMD Family 17h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, and 64-bit instruction set extensions.)

znver2

AMD Family 17h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, CLWB, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, RDPID, WBNOINVD, and 64-bit instruction set extensions.)

znver3

AMD Family 19h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, CLWB, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, RDPID, WBNOINVD, PKU, VPCLMULQDQ, VAES, and 64-bit instruction set extensions.)

znver4

AMD Family 19h core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, CLWB, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, RDPID, WBNOINVD, PKU, VPCLMULQDQ, VAES, AVX512F, AVX512DQ, AVX512IFMA, AVX512CD, AVX512BW, AVX512VL, AVX512BF16, AVX512VBMI, AVX512VBMI2, AVX512VNNI, AVX512BITALG, AVX512VPOPCNTDQ, GFNI and 64-bit instruction set extensions.)

znver5 🔥

AMD Family 1ah core based CPUs with x86-64 instruction set support. (This supersets BMI, BMI2, CLWB, F16C, FMA, FSGSBASE, AVX, AVX2, ADCX, RDSEED, MWAITX, SHA, CLZERO, AES, PCLMUL, CX16, MOVBE, MMX, SSE, SSE2, SSE3, SSE4A, SSSE3, SSE4.1, SSE4.2, ABM, XSAVEC, XSAVES, CLFLUSHOPT, POPCNT, RDPID, WBNOINVD, PKU, VPCLMULQDQ, VAES, AVX512F, AVX512DQ, AVX512IFMA, AVX512CD, AVX512BW, AVX512VL, AVX512BF16, AVX512VBMI, AVX512VBMI2, AVX512VNNI, AVX512BITALG, AVX512VPOPCNTDQ, GFNI, AVXVNNI, MOVDIRI, MOVDIR64B, AVX512VP2INTERSECT, PREFETCHI and 64-bit instruction set extensions. she ate and left no crumbs.)

btver1

CPUs based on AMD Family 14h cores with x86-64 instruction set support. (This supersets MMX, SSE, SSE2, SSE3, SSSE3, SSE4A, CX16, ABM and 64-bit instruction set extensions.)

btver2

CPUs based on AMD Family 16h cores with x86-64 instruction set support. This includes MOVBE, F16C, BMI, AVX, PCLMUL, AES, SSE4.2, SSE4.1, CX16, ABM, SSE4A, SSSE3, SSE3, SSE2, SSE, MMX and 64-bit instruction set extensions.

winchip-c6

IDT WinChip C6 CPU, dealt in the same way as i486, with the extra bonus of MMX instruction set support.

winchip2

IDT WinChip 2 CPU, dealt in the same way as i486, plus MMX and 3DNow! instruction set support.

c3

VIA C3 CPU with MMX and 3DNow! instruction set support.

Heads up bestie: no scheduling is implemented for this chip. 💀

c3-2

VIA C3-2 (Nehemiah/C5XL) CPU with MMX and SSE instruction set support.

Again — no scheduling is implemented for this chip.

c7

VIA C7 (Esther) CPU with MMX, SSE, SSE2 and SSE3 instruction set support.

And yep, no scheduling is implemented for this chip.

samuel-2

VIA Eden Samuel 2 CPU with MMX and 3DNow! instruction set support.

No scheduling is implemented for this chip.

nehemiah

VIA Eden Nehemiah CPU with MMX and SSE instruction set support.

No scheduling is implemented for this chip.

esther

VIA Eden Esther CPU with MMX, SSE, SSE2 and SSE3 instruction set support.

No scheduling is implemented for this chip.

eden-x2

VIA Eden X2 CPU with x86-64, MMX, SSE, SSE2 and SSE3 instruction set support.

No scheduling is implemented for this chip.

eden-x4

VIA Eden X4 CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3, SSE4.1, SSE4.2, AVX and AVX2 instruction set support.

No scheduling is implemented for this chip.

nano

Generic VIA Nano CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support.

No scheduling is implemented for this chip.

nano-1000

VIA Nano 1xxx CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support.

No scheduling is implemented for this chip.

nano-2000

VIA Nano 2xxx CPU with x86-64, MMX, SSE, SSE2, SSE3 and SSSE3 instruction set support.

No scheduling is implemented for this chip.

nano-3000

VIA Nano 3xxx CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 instruction set support.

No scheduling is implemented for this chip.

nano-x2

VIA Nano Dual Core CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 instruction set support.

No scheduling is implemented for this chip.

nano-x4

VIA Nano Quad Core CPU with x86-64, MMX, SSE, SSE2, SSE3, SSSE3 and SSE4.1 instruction set support.

No scheduling is implemented for this chip.

• lujiazui — ZHAOXIN lujiazui CPU</p> bestie, and she's serving 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. And like… yes the CPUs do support AVX and F16C, but they are not enabled by -march=lujiazui, for performance reasons. Not me gatekeeping the AVX 💅
• 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. She's got the whole ✨ extended ✨ roster, periodt.
• geode — AMD Geode embedded processor, lowkey iconic, with MMX and 3DNow! instruction set support. Retro vibes 🦋

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💫 -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. So like, it's the ✨ styling ✨ pass, not the "what can I actually do" pass.

So here's the tea: picking a specific cpu-type schedules things appropriately for that particular chip, but the compiler does not generate any code that cannot run on the default machine type unless you use a -march=cpu-type option. For example, if GCC is configured for i686-pc-linux-gnu then -mtune=pentium4 generates code that is tuned for Pentium 4 but still runs on i686 machines. Cute, right? 🎀

The choices for cpu-type are the same as for -march. And in addition, -mtune supports 2 extra choices for cpu-type:

• generic — Produce code optimized for the most common IA32/AMD64/EM64T processors. So if you know the CPU on which your code will run, then bestie you should use the corresponding -mtune or -march option instead of -mtune=generic. But if you do not know exactly what CPU users of your application will have, then you should use this option. It's giving "one size fits most" 🫶

As new processors are deployed in the marketplace, the behavior of this option will change. Therefore, if you upgrade to a newer version of GCC, code generation controlled by this option will change to reflect the processors that are most common at the time that version of GCC is released. So don't be shocked when it shifts, okay?

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. Different jobs, bestie 💅

• intel — Produce code optimized for the most current Intel processors, which are Haswell and Silvermont for this version of GCC. If you know the CPU on which your code will run, then you should use the corresponding -mtune or -march option instead of -mtune=intel. But, if you want your application performs better on both Haswell and Silvermont, then you should use this option.

As new Intel processors are deployed in the marketplace, the behavior of this option will change. Therefore, if you upgrade to a newer version of GCC, code generation controlled by this option will change to reflect the most current Intel processors at the time that version of GCC is released. The way this thing keeps evolving? main character energy 👑

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.

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🌸 -mcpu=cpu-type 🌸

A deprecated synonym for -mtune. That's it, that's the whole entry. She's old, she's retired, use -mtune instead 🙈

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🌈 -mfpmath=unit 🌈

Generate floating-point arithmetic for selected unit unit. The choices for unit are:

• 387 — Use the standard 387 floating-point coprocessor present on the majority of chips and emulated otherwise. Code compiled with this option runs almost everywhere. The temporary results are computed in 80-bit precision instead of the precision specified by the type, resulting in slightly different results compared to most of other chips. See -ffloat-store for more detailed description.

This is the default choice for non-Darwin x86-32 targets.

• sse — Use scalar floating-point instructions present in the SSE instruction set. This instruction set is supported by Pentium III and newer chips, and in the AMD line by Athlon-4, Athlon XP and Athlon MP chips. The earlier version of the SSE instruction set supports only single-precision arithmetic, thus the double and extended-precision arithmetic are still done using 387. A later version, present only in Pentium 4 and AMD x86-64 chips, supports double-precision arithmetic too.

For the x86-32 compiler, you must use -march=cpu-type, -msse or -msse2 switches to enable SSE extensions and make this option effective. For the x86-64 compiler, these extensions are enabled by default. No extra work needed, slay ✨

The resulting code should be considerably faster in the majority of cases and avoid the numerical instability problems of 387 code, but may break some existing code that expects temporaries to be 80 bits. So like… mostly a glow-up, sometimes a chaos moment 💀

This is the default choice for the x86-64 compiler, Darwin x86-32 targets, and the default choice for x86-32 targets with the SSE2 instruction set when -ffast-math is enabled.

• sse,387
• sse+387
• both — Attempt to utilize both instruction sets at once. This effectively doubles the amount of available registers, and on chips with separate execution units for 387 and SSE the execution resources too. Use this option with care, as it is still experimental, because the GCC register allocator does not model separate functional units well, resulting in unstable performance.

⚠️ Yeah, "unstable performance" — that's the vibe check failing, bestie. Be careful with this one, fr fr.

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🎀 -masm=dialect 🎀

Output assembly instructions using selected dialect. Also affects which dialect is used for basic "asm" and extended "asm". Supported choices (in dialect order) are att or intel. The default is att. Darwin does not support intel. So if you're on Darwin and you want intel, that's a no 🚫

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💜 -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. Unordered comparisons? Handled. We love a careful queen 🫶

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🧚 -m80387 / -mhard-float 🧚

Generate output containing 80387 instructions for floating point. Hardware float, as the name literally says.

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🍓 -mno-80387 / -msoft-float 🍓

Generate output containing library calls for floating point. So no fancy hardware, just calls, okay?

⚠️ 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. So like, bring your own library, bestie, GCC is not providing 🫠

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. Not me sneaking in opcodes 😭

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⭐ -mno-fp-ret-in-387 ⭐

Do not use the FPU registers for return values of functions.

The usual calling convention has functions return values of types "float" and "double" in an FPU register, even if there is no FPU. The idea is that the operating system should emulate an FPU. Manifesting hardware you don't have, honestly iconic ✨

The option -mno-fp-ret-in-387 causes such values to be returned in ordinary CPU registers instead. Boring but real.

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💅 -mno-fancy-math-387 💅

Some 387 emulators do not support the "sin", "cos" and "sqrt" instructions for the 387. Specify this option to avoid generating those instructions. This option is overridden when -march indicates that the target CPU always has an FPU and so the instruction does not need emulation. These instructions are not generated unless you also use the -funsafe-math-optimizations switch. Trig functions are, unfortunately, not always the move 🌸

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🦋 -malign-double / -mno-align-double 🦋

Control whether GCC aligns "double", "long double", and "long long" variables on a two-word boundary or a one-word boundary. Aligning "double" variables on a two-word boundary produces code that runs somewhat faster on a Pentium at the expense of more memory. Trade-offs, babe — speed for space ✨

On x86-64, -malign-double is enabled by default. Lucky you.

⚠️ 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. So like, this is a commitment. Don't mix and match 💀

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🔥 -m96bit-long-double / -m128bit-long-double 🔥

These switches control the size of "long double" type. The x86-32 application binary interface specifies the size to be 96 bits, so -m96bit-long-double is the default in 32-bit mode.

Modern architectures (Pentium and newer) prefer "long double" to be aligned to an 8- or 16-byte boundary. In arrays or structures conforming to the ABI, this is not possible. So specifying -m128bit-long-double aligns "long double" to a 16-byte boundary by padding the "long double" with an additional 32-bit zero. Padding, as a treat 🎀

In the x86-64 compiler, -m128bit-long-double is the default choice as its ABI specifies that "long double" is aligned on 16-byte boundary.

Notice that neither of these options enable any extra precision over the x87 standard of 80 bits for a "long double". So don't get your hopes up about more precision — that is simply not what's happening here, ngl.

⚠️ Warning: if you override the default value for your target ABI, this changes the size of structures and arrays containing "long double" variables, as well as modifying the function calling convention for functions taking "long double". Hence they are not binary-compatible with code compiled without that switch. Big deal. Binary compat is not a vibe you want to lose 😭

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👑 -mlong-double-64 / -mlong-double-80 / -mlong-double-128 👑

These switches control the size of "long double" type. A size of 64 bits makes the "long double" type equivalent to the "double" type. This is the default for 32-bit Bionic C library. A size of 128 bits makes the "long double" type equivalent to the "__float128" type. This is the default for 64-bit Bionic C library.

⚠️ Warning: if you override the default value for your target ABI, this changes the size of structures and arrays containing "long double" variables, as well as modifying the function calling convention for functions taking "long double". Hence they are not binary-compatible with code compiled without that switch. Same warning as before, same drama, do not ignore it bestie 💖

• -malign-data=type — this one is literally the alignment vibe-setter for your variables 🎀. The values you can pass for type are: compat, which uses the increased alignment value that's compatible with GCC 4.8 and earlier; abi, which uses the alignment value as specified by the psABI; and cacheline, which uses an increased alignment value to match the cache line size. And guess which one you get for free? compat is the default. periodt.
• -mlarge-data-threshold=threshold — when you've got -mcmodel=medium or -mcmodel=large going, any data object bigger than threshold gets yeeted into the large data sections 🫶. The default is 65535.
• -mrtd — okay so this switches you to a whole different function-calling convention, bestie. In this one, functions that take a fixed number of arguments return with the ret num instruction, which pops their arguments while returning 👑. This saves one instruction in the caller because there's no need to pop the arguments there. slay.

You can say that an individual function is called with this calling sequence using the function attribute stdcall. You can also override the -mrtd option by using the function attribute cdecl.

⚠️ Warning: this calling convention is incompatible with the one normally used on Unix, so you cannot use it if you need to call libraries compiled with the Unix compiler. no cap.

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. not a drill.

In addition, seriously incorrect code results if you call a function with too many arguments. (Normally, extra arguments are harmlessly ignored.)

• -mregparm=num — controls how many registers get used to pass integer arguments 💅. By default, no registers are used to pass arguments, and at most 3 registers can be used. You can control this behavior for a specific function by using the function attribute regparm.

⚠️ Warning: if you use this switch, and num is nonzero, then you must build all modules with the same value, including any libraries. This includes the system libraries and startup modules. 💀

• -msseregparm — uses SSE register passing conventions for float and double arguments and return values ✨. You can control this behavior for a specific function by using the function attribute sseregparm.

⚠️ Warning: if you use this switch then you must build all modules with the same value, including any libraries. This includes the system libraries and startup modules. 😭

• -mvect8-ret-in-mem — returns 8-byte vectors in memory instead of MMX registers 🦋. This is the default on VxWorks to match the ABI of the Sun Studio compilers until version 12. Only use this option if you need to remain compatible with existing code produced by those previous compiler versions or older versions of GCC. iykyk.
• -mpc32
• -mpc64
• -mpc80 — these set 80387 floating-point precision to 32, 64 or 80 bits 🌸. When -mpc32 is specified, the significands of results of floating-point operations are rounded to 24 bits (single precision); -mpc64 rounds the significands of results of floating-point operations to 53 bits (double precision) and -mpc80 rounds the significands of results of floating-point operations to 64 bits (extended double precision), which is the default. When this option is used, floating-point operations in higher precisions are not available to the programmer without setting the FPU control word explicitly.

Setting the rounding of floating-point operations to less than the default 80 bits can speed some programs by 2% or more 👀. Note that some mathematical libraries assume that extended-precision (80-bit) floating-point operations are enabled by default; routines in such libraries could suffer significant loss of accuracy, typically through so-called "catastrophic cancellation", when this option is used to set the precision to less than extended precision. catastrophic cancellation is giving horror movie fr fr.

• -mdaz-ftz — the flush-to-zero (FTZ) and denormals-are-zero (DAZ) flags in the MXCSR register are used to control floating-point calculations. SSE and AVX instructions including scalar and vector instructions could benefit from enabling the FTZ and DAZ flags when -mdaz-ftz is specified. Don't set FTZ/DAZ flags when -mno-daz-ftz or -shared is specified, -mdaz-ftz will set FTZ/DAZ flags even with -shared 💅.
• -mstackrealign — realign the stack at entry 🛠️. On the x86, the -mstackrealign option generates an alternate prologue and epilogue that realigns the run-time stack if necessary. This supports mixing legacy codes that keep 4-byte stack alignment with modern codes that keep 16-byte stack alignment for SSE compatibility. See also the attribute force_align_arg_pointer, applicable to individual functions. mixing old and new? iconic behavior.
• -mpreferred-stack-boundary=num — attempts to keep the stack boundary aligned to a 2 raised to num byte boundary ⭐. If -mpreferred-stack-boundary is not specified, the default is 4 (16 bytes or 128 bits).

⚠️ Warning: When generating code for the x86-64 architecture with SSE extensions disabled, -mpreferred-stack-boundary=3 can be used to keep the stack boundary aligned to 8 byte boundary. Since x86-64 ABI require 16 byte stack alignment, this is ABI incompatible and intended to be used in controlled environment where stack space is important limitation. This option leads to wrong code when functions compiled with 16 byte stack alignment (such as functions from a standard library) are called with misaligned stack. In this case, SSE instructions may lead to misaligned memory access traps. In addition, variable arguments are handled incorrectly for 16 byte aligned objects (including x87 long double and __int128), leading to wrong results. You must build all modules with -mpreferred-stack-boundary=3, including any libraries. This includes the system libraries and startup modules. 💀

• -mincoming-stack-boundary=num — assumes the incoming stack is aligned to a 2 raised to num byte boundary 🎀. If -mincoming-stack-boundary is not specified, the one specified by -mpreferred-stack-boundary is used.

On Pentium and Pentium Pro, double and long double values should be aligned to an 8-byte boundary (see -malign-double) or suffer significant run time performance penalties. On Pentium III, the Streaming SIMD Extension (SSE) data type __m128 may not work properly if it is not 16-byte aligned.

To ensure proper alignment of this values on the stack, the stack boundary must be as aligned as that required by any value stored on the stack. Further, every function must be generated such that it keeps the stack aligned. Thus calling a function compiled with a higher preferred stack boundary from a function compiled with a lower preferred stack boundary most likely misaligns the stack. It is recommended that libraries that use callbacks always use the default setting. 🫶

This extra alignment does consume extra stack space, and generally increases code size. Code that is sensitive to stack space usage, such as embedded systems and operating system kernels, may want to reduce the preferred alignment to -mpreferred-stack-boundary=2.

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okay bestie, now here comes the big one 🌈 — the extended instruction set switch army. Every single one of these:

-mmmx
-msse
-msse2
-msse3
-mssse3
-msse4
-msse4a
-msse4.1
-msse4.2
-mavx
-mavx2
-mavx512f
-mavx512pf
-mavx512er
-mavx512cd
-mavx512vl
-mavx512bw
-mavx512dq
-mavx512ifma
-mavx512vbmi
-msha
-maes
-mpclmul
-mclflushopt
-mclwb
-mfsgsbase
-mptwrite
-mrdrnd
-mf16c
-mfma
-mpconfig
-mwbnoinvd
-mfma4
-mprfchw
-mrdpid
-mprefetchwt1
-mrdseed
-msgx
-mxop
-mlwp
-m3dnow
-m3dnowa
-mpopcnt
-mabm
-madx
-mbmi
-mbmi2
-mlzcnt
-mfxsr
-mxsave
-mxsaveopt
-mxsavec
-mxsaves
-mrtm
-mhle
-mtbm
-mmwaitx
-mclzero
-mpku
-mavx512vbmi2
-mavx512bf16
-mavx512fp16
-mgfni
-mvaes
-mwaitpkg
-mvpclmulqdq
-mavx512bitalg
-mmovdiri
-mmovdir64b
-menqcmd
-muintr
-mtsxldtrk
-mavx512vpopcntdq
-mavx512vp2intersect
-mavx5124fmaps
-mavx512vnni
-mavxvnni
-mavx5124vnniw
-mcldemote
-mserialize
-mamx-tile
-mamx-int8
-mamx-bf16
-mhreset
-mkl
-mwidekl
-mavxifma
-mavxvnniint8
-mavxneconvert
-mcmpccxadd
-mamx-fp16
-mprefetchi
-mraoint
-mamx-complex
-mavxvnniint16
-msm3
-msha512
-msm4
-mapxf
-musermsr
-mavx10.1
-mavx10.1-256
-mavx10.1-512

These switches enable the use of instructions in the MMX, SSE, AVX512ER, AVX512CD, AVX512VL, AVX512BW, AVX512DQ, AVX512IFMA, AVX512VBMI, SHA, AES, PCLMUL, CLFLUSHOPT, CLWB, FSGSBASE, PTWRITE, RDRND, F16C, FMA, PCONFIG, WBNOINVD, FMA4, PREFETCHW, RDPID, PREFETCHWT1, RDSEED, SGX, XOP, LWP, 3DNow!, enhanced 3DNow!, POPCNT, ABM, ADX, BMI, BMI2, LZCNT, FXSR, XSAVE, XSAVEOPT, XSAVEC, XSAVES, RTM, HLE, TBM, MWAITX, CLZERO, PKU, AVX512VBMI2, GFNI, VAES, WAITPKG, VPCLMULQDQ, AVX512BITALG, MOVDIRI, MOVDIR64B, AVX512BF16, ENQCMD, AVX512VPOPCNTDQ, AVX5124FMAPS, AVX512VNNI, AVX5124VNNIW, SERIALIZE, UINTR, HRESET, AMXTILE, AMXINT8, AMXBF16, KL, WIDEKL, AVXVNNI, AVX512-FP16, AVXIFMA, AVXVNNIINT8, AVXNECONVERT, CMPCCXADD, AMX-FP16, PREFETCHI, RAOINT, AMX-COMPLEX, AVXVNNIINT16, SM3, SHA512, SM4, APX_F, USER_MSR, AVX10.1 or CLDEMOTE extended instruction sets 💖. Each has a corresponding -mno- option to disable use of these instructions. (yes, the -mno- version is your off switch, you're welcome.)

These extensions are also available as built-in functions: see x86 Built-in Functions, for details of the functions enabled and disabled by these switches ✨.

To generate SSE/SSE2 instructions automatically from floating-point code (as opposed to 387 instructions), see -mfpmath=sse.

GCC depresses SSEx instructions when -mavx is used. Instead, it generates new AVX instructions or AVX equivalence for all SSEx instructions when needed. it's giving glow-up.

These options enable GCC to use these extended instructions in generated code, even without -mfpmath=sse. Applications that perform run-time CPU detection must compile separate files for each supported architecture, using the appropriate flags 🛠️. In particular, the file containing the CPU detection code should be compiled without these options.

• -mdump-tune-features — bestie, this one literally just makes GCC spill the tea ☕ on all the x86 performance tuning feature names and their default settings. Why would you want that? Because those exact names are the ones you plug into -mtune-ctrl=feature-list. It's the guest list, and you need it before you get in. 💅
• -mtune-ctrl=feature-list — this is the fine-grain control knob for x86 code generation features, like adjusting every single slider on the mixing board instead of just picking a preset. feature-list is a comma-separated list of feature names (peep -mdump-tune-features for the names, obviously 🎀). If a feature name isn't preceded by ^, it gets turned on; slap a ^ in front and it's turned off.

🚨 okay so real talk: -mtune-ctrl=feature-list is meant for GCC developers. It's their toy, not ours. Using it can send you down code paths that testing never even looked at, and it can potentially give you compiler ICEs or runtime errors. Not cute. Proceed with caution, fr fr. 💀

• -mno-default — tells GCC to turn all the tunable features off. Zero. Nada. Lights out. 🌙 See also -mtune-ctrl=feature-list and -mdump-tune-features if you wanna know what you just switched off.
• -mcld — makes GCC emit a "cld" instruction in the prologue of any function that uses string instructions. Why does that matter? Because string instructions look at the DF flag to decide whether they're going forward (autoincrement) or backwards (autodecrement) — and while the ABI says the DF flag is supposed to be cleared on function entry, some operating systems simply do not respect that 😭. Their exception dispatchers don't clear it, so the exception handler can get invoked with DF set, and then your string instructions go the wrong direction entirely. Rude.

On 32-bit x86 targets this option can be on by default if GCC was configured with the --enable-cld configure option. And if that's the case and you want those "cld" instructions gone? Hit it with -mno-cld. ✨

• -mvzeroupper — emits a "vzeroupper" instruction before control flow leaves the function, which minimizes the AVX-to-SSE transition penalty and removes unnecessary "zeroupper" intrinsics. Two birds, one instruction. 🦋
• -mprefer-avx128 — tells the auto-vectorizer to use 128-bit AVX instructions instead of the 256-bit ones. Size queen behavior declined, modest queen behavior accepted. 👑
• -mprefer-vector-width=opt — use an opt-bit vector width in instructions instead of whatever the selected platform defaults to.
• -mpartial-vector-fp-math — this lets GCC generate floating-point operations that might mess with the set of floating-point status flags on partial vectors — that's when vector elements live in the low part of the 128-bit SSE register. Unless -fno-trapping-math is specified, the compiler keeps things correct by sanitizing all input operands so the unused upper part of the vector register is full of zeroes. 🧼

But here's the catch, and it's giving risky: if you use built-in functions or inline assembly with partial vector arguments, NaNs, denormals, or invalid values can leak into that upper part of the vector. That can cause possible performance issues when -fno-trapping-math is in effect. Mitigate it by manually sanitizing the upper part of the partial vector argument register, or by using -mdaz-ftz to set the denormals-are-zero (DAZ) flag in the MXCSR register. 🫶

This option is enabled by default.

• -mmove-max=bits — sets the maximum number of bits that can be efficiently moved from memory to memory to bits. The valid values are 128, 256 and 512. Pick your fighter. ⭐
• -mstore-max=bits — sets the maximum number of bits that can be efficiently stored to memory to bits. Valid values are 128, 256 and 512. And the page lists these values right here too 👇
  • none — no extra limitations applied to GCC beyond what the selected platform already defines.
  • 128 — prefer 128-bit vector width for instructions.
  • 256 — prefer 256-bit vector width for instructions.
  • 512 — prefer 512-bit vector width for instructions.
• -mnoreturn-no-callee-saved-registers — optimizes functions marked with the "noreturn" attribute or the "_Noreturn" specifier by not saving callee-saved registers in the function prologue — the ones actually used in the function, that is (except for the "BP" register, she stays). 🌸

⚠️ this one can interfere with debugging the caller of that "noreturn" function, or literally any function further up the call stack. That's exactly why it's not enabled by default. Debugging your past self is hard enough.

• -mcx16 — lets GCC generate "CMPXCHG16B" instructions in 64-bit code so compare-and-exchange operations can work on 16-byte aligned 128-bit objects. Iconic for atomic updates of data structures bigger than one machine word. The compiler uses this instruction to implement __sync Builtins. But for __atomic Builtins operating on 128-bit integers? A library call is always used. No exceptions, periodt. 🔥
• -msahf — enables generation of "SAHF" instructions in 64-bit code. Story time: early Intel Pentium 4 CPUs with Intel 64 support, before the Pentium 4 G1 step landed in December 2005, were missing the "LAHF" and "SAHF" instructions that AMD64 supports. Those two are load and store instructions respectively, for certain status flags. In 64-bit mode, "SAHF" gets used to optimize the "fmod", "drem", and "remainder" built-in functions — see Other Builtins for the deets. 💜
• -mmovbe — enables use of the "movbe" instruction to optimize byte swapping of four and eight byte entities. Swapping, but make it fast. 🔄
• -mshstk — the -mshstk option enables shadow stack built-in functions from x86 Control-flow Enforcement Technology (CET). Protection era. 🛡️
• -mcrc32 — enables the built-in functions __builtin_ia32_crc32qi, __builtin_ia32_crc32hi, __builtin_ia32_crc32si and __builtin_ia32_crc32di to generate the "crc32" machine instruction. Checksum girlies rise up. ✨
• -mmwait — enables the built-in functions __builtin_ia32_monitor, and __builtin_ia32_mwait to generate the "monitor" and "mwait" machine instructions. 😴
• -mrecip — enables use of "RCPSS" and "RSQRTSS" instructions (plus their vectorized variants "RCPPS" and "RSQRTPS") with an extra Newton-Raphson step to bump up the precision, instead of "DIVSS" and "SQRTSS" (and their vectorized variants) for single-precision floating-point arguments. 🍓

These instructions only get generated when -funsafe-math-optimizations is enabled together with -ffinite-math-only and -fno-trapping-math. And heads up: while the throughput of the sequence is higher than the throughput of the non-reciprocal instruction, the precision of the sequence can drop by up to 2 ulp — meaning the inverse of 1.0 equals 0.99999994. 😭 Not a typo. That's just the deal.

Also, GCC already implements 1.0f/sqrtf(x) in terms of "RSQRTSS" (or "RSQRTPS") with -ffast-math (or that option combo above), so it doesn't need -mrecip. And one more: GCC already emits that sequence with the additional Newton-Raphson step for vectorized single-float division and vectorized sqrtf(x) with -ffast-math (or the combo), so it doesn't need -mrecip there either. Girl is already doing it. 💅

• -mrecip=opt — this controls which reciprocal estimate instructions are allowed. opt is 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 like, for example, -mrecip=all,!sqrt enables all of the reciprocal approximations except square root. See how the ! flips it? 🎀

• -mveclibabi=type — specifies the ABI type to use for vectorizing intrinsics using an external library. Supported values for type are svml for the Intel short vector math library, and acml for the AMD math core library. To use this option, both -ftree-vectorize and -funsafe-math-optimizations have to be enabled, and an SVML or ACML ABI-compatible library must be specified at link time. No library, no party. 🙈

Right now GCC emits calls to "vmldExp2", "vmldLn2", "vmldLog102", "vmldPow2", "vmldTanh2", "vmldTan2", "vmldAtan2", "vmldAtanh2", "vmldCbrt2", "vmldSinh2", "vmldSin2", "vmldAsinh2", "vmldAsin2", "vmldCosh2", "vmldCos2", "vmldAcosh2", "vmldAcos2", "vmlsExp4", "vmlsLn4", "vmlsLog104", "vmlsPow4", "vmlsTanh4", "vmlsTan4", "vmlsAtan4", "vmlsAtanh4", "vmlsCbrt4", "vmlsSinh4", "vmlsSin4", "vmlsAsinh4", "vmlsAsin4", "vmlsCosh4", "vmlsCos4", "vmlsAcosh4" and "vmlsAcos4" for the corresponding function type when -mveclibabi=svml is used. And it emits "__vrd2_sin", "__vrd2_cos", "__vrd2_exp", "__vrd2_log", "__vrd2_log2", "__vrd2_log10", "__vrs4_sinf", "__vrs4_cosf", "__vrs4_expf", "__vrs4_logf", "__vrs4_log2f", "__vrs4_log10f" and "__vrs4_powf" for the corresponding function type when -mveclibabi=acml is used. That's the whole roster, no cap. 📋

• -mabi=name — generate code for the specified calling convention. The permissible values are sysv for the ABI used on GNU/Linux and other systems, and ms for the Microsoft ABI. The default is to use the Microsoft ABI when targeting Microsoft Windows, and the SysV ABI on all other systems. You can control this behavior for specific functions by using the function attributes "ms_abi" and "sysv_abi". Pick your lane. 🌈
• -mforce-indirect-call — force all calls to functions to be indirect. Super useful when you're using Intel Processor Trace, because it generates more precise timing information for function calls. 🕵️
• -mmanual-endbr — insert an ENDBR instruction at function entry only via the "cf_check" function attribute. Useful when paired with the option -fcf-protection=branch, so you get to control ENDBR insertion at the function entry yourself. Main character energy: you decide when the door opens. 🚪✨
• -mcet-switch — okay so by default, CET instrumentation is turned off on switch statements that use a jump table, and indirect branch track is disabled. Since jump tables are stored in read-only memory, this does not result in a direct loss of hardening. But — if the jump table index is attacker-controlled, the indirect jump may not be constrained by CET. 😳

This option turns on CET instrumentation to enable indirect branch track for switch statements with jump tables, which leads to the jump targets being reachable via any indirect jumps. Locking it down. 🔒

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• -mcall-ms2sysv-xlogues — okay so here's the drama: because 64-bit ABIs are not like other ABIs, any Microsoft ABI function that calls a System V ABI function has to treat RSI, RDI and XMM6-15 as clobbered. 💀 By default GCC emits the save/restore code for those registers inline, which gives you lowkey chonky prologues and epilogues. Flip on -mcall-ms2sysv-xlogues and those prologues/epilogues instead use stubs in the static portion of libgcc to do the saving and restoring — smaller functions, but you pay in a few extra instructions. ✨
• -mtls-dialect=type — generates code that accesses thread-local storage using the gnu or gnu2 conventions. gnu is the conservative default, no cap. gnu2 is more efficient, bestie, but heads up — it can add compile- and run-time requirements that just cannot be satisfied on every system. 🫶
• -mpush-args
• -mno-push-args — use PUSH operations to stash outgoing parameters. This method is shorter and usually just as fast as the SUB/MOV method, and it's on by default. In some cases turning it off may improve performance though, because of improved scheduling and reduced dependencies. slay.
• -maccumulate-outgoing-args — when enabled, the maximum amount of space needed for outgoing arguments gets computed in the function prologue. This is faster on most modern CPUs because of reduced dependencies, improved scheduling, and reduced stack usage when the preferred stack boundary isn't equal to 2. 🙈 The drawback? A notable increase in code size. Also this switch implies -mno-push-args. it's giving tradeoffs.
• -mthreads — supports thread-safe exception handling on MinGW. Programs that rely on thread-safe exception handling must compile and link all code with the -mthreads option. When compiling, -mthreads defines -D_MT; when linking, it links in a special thread helper library -lmingwthrd which cleans up per-thread exception-handling data. 🧚
• -mms-bitfields
• -mno-ms-bitfields — enable/disable bit-field layout that's compatible with the native Microsoft Windows compiler.

If packed is used on a structure, or if bit-fields are used, it may be that the Microsoft ABI lays out the structure differently than the way GCC normally does. 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), it may be necessary to access either format. 😭

This option is enabled by default for Microsoft Windows targets. This behavior can also be controlled locally by use of variable or type attributes. For more information, see x86 Variable Attributes and x86 Type Attributes.

The Microsoft structure layout algorithm is pretty simple, except for the bit-field packing. The padding and alignment of members of structures, and whether a bit-field can straddle a storage-unit boundary, are determined by these rules:

1. Structure members are stored sequentially in the order in which they are declared: the first member has the lowest memory address and the last member the highest.
2. Every data object has an alignment requirement. The alignment requirement for all data except structures, unions, and arrays is either the size of the object or the current packing size (specified with either the aligned attribute or the pack pragma), whichever is less. For structures, unions, and arrays, the alignment requirement is the largest alignment requirement of its members. Every object is allocated an offset so that:

offset % alignment_requirement == 0

3. Adjacent bit-fields are packed into the same 1-, 2-, or 4-byte allocation unit if the integral types are the same size and if the next bit-field fits into the current allocation unit without crossing the boundary imposed by the common alignment requirements of the bit-fields.

MSVC interprets zero-length bit-fields in the following ways:

1. If a zero-length bit-field is inserted between two bit-fields that are normally coalesced, the bit-fields are not coalesced.

For example:

struct
 {
   unsigned long bf_1 : 12;
   unsigned long : 0;
   unsigned long bf_2 : 12;
 } t1;

The size of t1 is 8 bytes with the zero-length bit-field. If the zero-length bit-field were removed, t1's size would be 4 bytes.

2. If a zero-length bit-field is inserted after a bit-field, foo, and the alignment of the zero-length bit-field is greater than the member that follows it, bar, bar is aligned as the type of the zero-length bit-field.

For example:

struct
 {
   char foo : 4;
   short : 0;
   char bar;
 } t2;

struct
 {
   char foo : 4;
   short : 0;
   double bar;
 } t3;

For t2, bar is placed at offset 2, rather than offset 1. Accordingly, the size of t2 is 4. For t3, the zero-length bit-field does not affect the alignment of bar or, as a result, the size of the structure.

Taking this into account, it's important to note the following:

1. If a zero-length bit-field follows a normal bit-field, the type of the zero-length bit-field may affect the alignment of the structure as a whole. For example, t2 has a size of 4 bytes, since the zero-length bit-field follows a normal bit-field, and is of type short.
2. Even if a zero-length bit-field is not followed by a normal bit-field, it may still affect the alignment of the structure:

struct
 {
   char foo : 6;
   long : 0;
 } t4;

Here, t4 takes up 4 bytes.

3. Zero-length bit-fields following non-bit-field members are ignored:

struct
 {
   char foo;
   long : 0;
   char bar;
 } t5;

Here, t5 takes up 2 bytes.

• -mno-align-stringops — do not align the destination of inlined string operations. This switch reduces code size and improves performance in case the destination is already aligned, but GCC doesn't know about it. 🙈
• -minline-all-stringops — by default GCC inlines string operations only when the destination is known to be aligned to at least a 4-byte boundary. This enables more inlining and increases code size, but may improve performance of code that depends on fast memcpy and memset for short lengths. The option enables inline expansion of strlen for all pointer alignments. 💅
• -minline-stringops-dynamically — for string operations of unknown size, use run-time checks with inline code for small blocks and a library call for large blocks. bestie behaviour fr fr.
• -mstringop-strategy=alg — override the internal decision heuristic for the particular algorithm to use for inlining string operations. The allowed values for alg are:
  • rep_byte
  • rep_4byte
  • rep_8byte

Expand using i386 rep prefix of the specified size.

• byte_loop
• loop
• unrolled_loop

Expand into an inline loop.

• libcall

Always use a library call.

• -mmemcpy-strategy=strategy — override the internal decision heuristic to decide if __builtin_memcpy should be inlined and what inline algorithm to use when the expected size of the copy operation is known. strategy is a comma-separated list of alg:max_size:dest_align triplets. alg is specified in -mstringop-strategy, max_size specifies the max byte size with which inline algorithm alg is allowed. For the last triplet, the max_size must be -1. The max_size of the triplets in the list must be specified in increasing order. The minimal byte size for alg is 0 for the first triplet and max_size + 1 of the preceding range. 😌
• -mmemset-strategy=strategy — the option is similar to -mmemcpy-strategy= except that it's there to control __builtin_memset expansion. 🎀
• -momit-leaf-frame-pointer — don't keep the frame pointer in a register for leaf functions. This avoids the instructions to save, set up, and restore frame pointers and makes an extra register available in leaf functions. The option -fomit-leaf-frame-pointer removes the frame pointer for leaf functions, which might make debugging harder. 😭
• -mtls-direct-seg-refs
• -mno-tls-direct-seg-refs — controls whether TLS variables may be accessed with offsets from the TLS segment register (%gs for 32-bit, %fs for 64-bit), or whether the thread base pointer must be added. Whether or not this is valid depends on the operating system, and whether it maps the segment to cover the entire TLS area. 💀

For systems that use the GNU C Library, the default is on.

• -msse2avx
• -mno-sse2avx — specify that the assembler should encode SSE instructions with VEX prefix. The option -mavx turns this on by default. ✨
• -mfentry
• -mno-fentry — if profiling is active (-pg), put the profiling counter call before the prologue. Note: on x86 architectures the attribute ms_hook_prologue isn't possible at the moment for -mfentry and -pg. 🙈
• -mrecord-mcount
• -mno-record-mcount — if profiling is active (-pg), generate a __mcount_loc section that contains pointers to each profiling call. This is useful for automatically patching and out calls. 🫶
• -mnop-mcount
• -mno-nop-mcount — if profiling is active (-pg), generate the calls to the profiling functions as NOPs. This is useful when they should be patched in later dynamically. This is likely only useful together with -mrecord-mcount. 💅
• -minstrument-return=type — instrument function exit in -pg -mfentry instrumented functions with a call to the specified function. This only instruments true returns ending with ret, but not sibling calls ending with jump. Valid types are none to not instrument, call to generate a call to __return__, or nop5 to generate a 5 byte nop. 👑
• -mrecord-return, -mno-record-return — okay so this one makes a __return_loc section that points at all your return instrumentation code. That's the whole vibe. 🎀
• -mfentry-name=name — sets the name of the __fentry__ symbol that gets called at function entry for -pg -mfentry functions. You're basically renaming the door greeter. ✨
• -mfentry-section=name — sets the name of the section that records -mrecord-mcount calls. The default here is __mcount_loc, no cap.
• -mskip-rax-setup, -mno-skip-rax-setup — when you're generating code for the x86-64 architecture with SSE extensions disabled, -mskip-rax-setup lets you skip setting up the RAX register when there are no variable arguments passed in vector registers. Slay, but also:

Warning: since RAX is used to avoid unnecessarily saving vector registers on the stack when passing variable arguments, the impacts of this option are that callees may waste some stack space, misbehave, or jump to a random location. GCC 4.4 or newer don't have those issues, regardless of the RAX register value. 💀

• -m8bit-idiv, -mno-8bit-idiv — on some processors, like Intel Atom, an 8-bit unsigned integer divide is much faster than a 32-bit/64-bit integer divide. This option generates a run-time check, and if both the dividend and the divisor are within the range of 0 to 255, an 8-bit unsigned integer divide is used instead of a 32-bit/64-bit integer divide. Lowkey genius.
• -mavx256-split-unaligned-load, -mavx256-split-unaligned-store — splits 32-byte AVX unaligned load and store. That's it, that's the whole thing. 🦋
• -mstack-protector-guard=guard, -mstack-protector-guard-reg=reg, -mstack-protector-guard-offset=offset — generates stack protection code using a canary at guard. The supported locations are global for a global canary, or tls for a per-thread canary in the TLS block (that one's the default). Heads up: this option only does anything when -fstack-protector or -fstack-protector-all is specified. 🫶

With the latter choice, the options -mstack-protector-guard-reg=reg and -mstack-protector-guard-offset=offset furthermore specify which segment register (%fs or %gs) to use as the base register for reading the canary, and from what offset from that base register. The default for those is as specified in the relevant ABI.

• -mgeneral-regs-only — generates code that uses only the general-purpose registers. This stops the compiler from using floating-point, vector, mask and bound registers. Very "we only have one type of register at home" energy. 💅
• -mrelax-cmpxchg-loop — when it's emitting a compare-and-swap loop for __sync Builtins and __atomic Builtins that are lacking a native instruction, this optimizes for the highly contended case by issuing an atomic load before the "CMPXCHG" instruction, and using the "PAUSE" instruction to save CPU power when restarting the loop. Contention? Handled. Periodt. 🔥
• -mindirect-branch=choice — converts indirect call and jump with choice. The default is keep, which keeps indirect call and jump unmodified. thunk converts indirect call and jump to call and return thunk. thunk-inline converts indirect call and jump to inlined call and return thunk. thunk-extern converts indirect call and jump to external call and return thunk provided in a separate object file. You can control this behavior for a specific function by using the function attribute "indirect_branch".

Note that -mcmodel=large is incompatible with -mindirect-branch=thunk and -mindirect-branch=thunk-extern since the thunk function may not be reachable in the large code model.

Note that -mindirect-branch=thunk-extern is compatible with -fcf-protection=branch since the external thunk can be made to enable control-flow check.

• -mfunction-return=choice — converts function return with choice. The default is keep, which keeps function return unmodified. thunk converts function return to call and return thunk. thunk-inline converts function return to inlined call and return thunk. thunk-extern converts function return to external call and return thunk provided in a separate object file. You can control this behavior for a specific function by using the function attribute "function_return".

Note that -mindirect-return=thunk-extern is compatible with -fcf-protection=branch since the external thunk can be made to enable control-flow check.

Note that -mcmodel=large is incompatible with -mfunction-return=thunk and -mfunction-return=thunk-extern since the thunk function may not be reachable in the large code model.

• -mindirect-branch-register — forces indirect call and jump via register. No detours bestie, we're going straight through. ✨
• -mharden-sls=choice — generates code to mitigate against straight line speculation (SLS) with choice. The default is none, which disables all SLS hardening. return enables SLS hardening for function returns. indirect-jmp enables SLS hardening for indirect jumps. all enables all SLS hardening. 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, which allows them to be replaced with lfence; call *%r8-r15 or lfence; jmp *%r8-r15 at run-time. Six bytes of pure intention. 💜
• -mapx-inline-asm-use-gpr32 — for inline asm support with APX: by default the EGPR feature was disabled to prevent a potential illegal instruction with EGPR from occurring. To invoke egpr usage in inline asm, use the new compiler option -mapx-inline-asm-use-gpr32, and the user should ensure the instruction supports EGPR. So like, it's on you to check. 🙈
• -mevex512, -mno-evex512 — enables/disables 512-bit vector. It will be default on if AVX512F is enabled.

---

These -m switches are supported in addition to the above on x86-64 processors in 64-bit environments. 🌈

• -m32, -m64, -mx32, -m16, -miamcu — generate code for a 16-bit, 32-bit or 64-bit environment. The -m32 option sets "int", "long", and pointer types to 32 bits, and generates code that runs in 32-bit mode. 🎀

The -m64 option sets "int" to 32 bits and "long" and pointer types to 64 bits, and generates code for the x86-64 architecture. For Darwin only, the -m64 option also turns off the -fno-pic and -mdynamic-no-pic options.

The -mx32 option sets "int", "long", and pointer types to 32 bits, and generates code for the x86-64 architecture.

The -m16 option is the same as -m32, except for that it outputs the ".code16gcc" assembly directive at the beginning of the assembly output so that the binary can run in 16-bit mode.

The -miamcu option generates code which conforms to Intel MCU psABI. It requires the -m32 option to be turned on. No skipping, bestie. 💅

• -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-zone disables this red zone. 128 bytes of vibes, gone. 😭
• -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. Required, 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-threshold are put into large data or BSS sections and can be located above 2GB. Programs can be statically or dynamically linked. 🦋
• -mcmodel=large — generate code for the large model. This model makes no assumptions about addresses and sizes of sections. Chaotic neutral, honestly.
• -maddress-mode=long — generate code for long address mode. This is only supported for 64-bit and x32 environments. It is the default address mode for 64-bit environments.
• -maddress-mode=short — generate code for short address mode. This is only supported for 32-bit and x32 environments. It is the default address mode for 32-bit and x32 environments.
• -mneeded, -mno-needed — emit GNU_PROPERTY_X86_ISA_1_NEEDED GNU property for Linux target to indicate the micro-architecture ISA level required to execute the binary. Receipts, in the binary. 🧚
• -mno-direct-extern-access — without -fpic nor -fPIC, always use the GOT pointer to access external symbols. With -fpic or -fPIC, treat access to protected symbols as local symbols. The default is -mdirect-extern-access.

Warning: shared libraries compiled with -mno-direct-extern-access and executable compiled with -mdirect-extern-access may not be binary compatible if protected symbols are used in shared libraries and executable. Not a drill. 💀

• -munroll-only-small-loops — controls conservative small loop unrolling. It is default enabled by O2, and unrolls loop with less than 4 insns by 1 time. Explicit -f[no-]unroll-[all-]loops would disable this flag to avoid any unintended unrolling behavior that user does not want. Protecting you from yourself, honestly iconic. 🫶
• -mlam=choice — LAM (linear-address masking) allows special bits in the pointer to be used for metadata. The default is none. With u48, pointer bits in positions 62:48 can be used for metadata; with u57, pointer bits in positions 62:57 can be used for metadata. Sneaky little metadata pockets. 🍓

---

🌸 x86 Windows Options 🌸

These additional options are available for Microsoft Windows targets:

• -mconsole — this option specifies that a console application is to be generated, by instructing the linker to set the PE header subsystem type required for console applications. This option is available for Cygwin and MinGW targets and is enabled by default on those targets. Windows besties, this one's for you. 💖
• -mcrtdll=library — bestie, this one lets you preprocess, compile or link against whatever C RunTime DLL library you name. It does a whole lil makeover: it tweaks the predefined macros __CRTDLL__, __MSVCRT__, _UCRT and __MSVCRT_VERSION__ to match the CRT library you picked, chooses the start file for that CRT library, and links with that CRT library. ✨ The names it recognizes on the preprocessor side are crtdll*, msvcrt10*, msvcrt20*, msvcrt40*, msvcr40*, msvcrtd*, msvcrt-os*, msvcr70*, msvcr71*, msvcr80*, msvcr90*, msvcr100*, msvcr110*, msvcr120* and ucrt*. If you don't specify it at all, the default MinGW import library msvcrt is what gets used for linking, and nothing else gets adjusted for the preprocessor. Fun fact: that MinGW import library msvcrt is literally just a symlink to (or a copy of) some other MinGW CRT import library that got chosen back when MinGW was compiled. And the MinGW import library msvcrt-os is the one for the Windows system CRT DLL library msvcrt.dll — in most cases that's the default MinGW import library. Generally speaking, changing the CRT DLL means recompiling the entire MinGW CRT. So yeah — this option is for experimental and testing purposes only. Available for MinGW targets. 🧪
• -mdll — Cygwin and MinGW targets only, babe. It says a DLL — that's a dynamic link library, iykyk — is to be generated, which lets it pick the required runtime startup object and entry point. 🎀
• -mnop-fun-dllimport — Cygwin and MinGW targets. It specifies that the dllimport attribute should just be... ignored. 🙈
• -mthreads — MinGW targets. It says to use the MinGW-specific thread support. 🧵
• -municode — MinGW-w64 targets. It makes the UNICODE preprocessor macro get predefined, and it chooses Unicode-capable runtime startup code. 💅
• -mwin32 — Cygwin and MinGW targets. This one sets the typical Microsoft Windows predefined macros in the pre-processor — but heads up, it does not influence the choice of runtime library or startup code. ✨
• -mwindows — Cygwin and MinGW targets. It tells GCC that a GUI application is to be generated by instructing the linker to set the PE header subsystem type appropriately. 🪟
• -fno-set-stack-executable — MinGW targets. It specifies that the executable flag for the stack used by nested functions isn't set. This is necessary for binaries running in kernel mode of Microsoft Windows, because there the User32 API — the thing that's used to set executable privileges — isn't available at all. 😭
• -fwritable-relocated-rdata — MinGW and Cygwin targets. It specifies that relocated-data sitting in a read-only section gets put into the .data section instead. This is necessary for older runtimes that don't support modification of .rdata sections for pseudo-relocation. 🦋
• -mpe-aligned-commons — Cygwin and MinGW targets. It specifies that the GNU extension to the PE file format which permits the correct alignment of COMMON variables should be used when generating code. It's enabled by default if GCC detects that the target assembler found during configuration supports the feature. 👑

See also under x86 Options for standard options.

---

🌈 Xstormy16 Options 🌈

These options are defined for Xstormy16:

• -msim — Choose startup files and linker script suitable for the simulator. 🧚

---

💜 Xtensa Options 💜

These options are supported for Xtensa targets:

• -mconst16, -mno-const16 — Enable or disable use of CONST16 instructions for loading constant values. Okay so the CONST16 instruction is currently not a standard option from Tensilica. When it's enabled, CONST16 instructions are always used in place of the standard L32R instructions. And CONST16 use is enabled by default only if the L32R instruction isn't available. ⭐
• -mfused-madd, -mno-fused-madd — Enable or disable use of fused multiply/add and multiply/subtract instructions in the floating-point option. Ngl, this has no effect if the floating-point option isn't also enabled. Disabling fused multiply/add and multiply/subtract instructions forces the compiler to use separate instructions for the multiply and add/subtract operations. That may be desirable in some cases where strict IEEE 754-compliant results are required: the fused multiply add/subtract instructions do not round the intermediate result, so they produce results with more bits of precision than the IEEE standard specifies. Disabling them also ensures that the program output isn't sensitive to the compiler's ability to combine multiply and add/subtract operations. 🔥
• -mserialize-volatile, -mno-serialize-volatile — When this is enabled, GCC inserts MEMW instructions before volatile memory references to guarantee sequential consistency. The default is -mserialize-volatile. Use -mno-serialize-volatile to omit the MEMW instructions. 💫
• -mforce-no-pic — For targets like GNU/Linux, where all user-mode Xtensa code must be position-independent code (PIC), this option disables PIC for compiling kernel code. 🐧
• -mtext-section-literals, -mno-text-section-literals — These control the treatment of literal pools. The default is -mno-text-section-literals, which places literals in a separate section in the output file. That lets the literal pool be placed in a data RAM/ROM, and it also lets the linker combine literal pools from separate object files to remove redundant literals and improve code size. With -mtext-section-literals, the literals get interspersed in the text section so they stay as close as possible to their references. That may be necessary for large assembly files. Literals for each function are placed right before that function. 🎀
• -mauto-litpools, -mno-auto-litpools — These also control the treatment of literal pools. The default is -mno-auto-litpools, which places literals in a separate section in the output file unless -mtext-section-literals is used. With -mauto-litpools the literals are interspersed in the text section by the assembler. The compiler doesn't produce explicit .literal directives, and it loads literals into registers with MOVI instructions instead of L32R, so it can let the assembler do relaxation and place literals as necessary. This option lets the assembler create several literal pools per function and assemble very big functions — which may not be possible with -mtext-section-literals. 👑
• -mtarget-align, -mno-target-align — When this is 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 aren't enough preceding safe density instructions to align a target, no widening is performed. The default is -mtarget-align. These options do not affect the treatment of auto-aligned instructions like LOOP, which the assembler always aligns, either by widening density instructions or by inserting NOP instructions. ✨
• -mlongcalls, -mno-longcalls — When this is enabled, GCC instructs the assembler to translate direct calls into indirect calls, unless it can determine that the target of a direct call is in the range allowed by the call instruction. This translation typically happens for calls to functions in other source files. Specifically, the assembler translates a direct CALL instruction into an L32R followed by a CALLX instruction. The default is -mno-longcalls. You should use this option in programs where the call target can potentially be out of range. Important though: this option is implemented in the assembler, not the compiler, so the assembly code generated by GCC still shows direct call instructions — you gotta look at the disassembled object code to see the actual instructions. And note that the assembler uses an indirect call for every cross-file call, not just the ones that really are out of range. 😭
• -mabi=name — Generate code for the specified ABI. Permissible values are: call0, windowed. The default ABI is chosen by the Xtensa core configuration. 🦋
• -mabi=call0 — When this is enabled, function parameters are passed in registers a2 through a7, registers a12 through a15 are caller-saved, and register a15 may be used as a frame pointer. When this version of the ABI is enabled, the C preprocessor symbol __XTENSA_CALL0_ABI__ is defined. ⭐
• -mabi=windowed — When this is enabled, function parameters are passed in registers a10 through a15, and the called function rotates the register window by 8 registers on entry so that its arguments are found in registers a2 through a7. Register a7 may be used as a frame pointer. The register window is rotated 8 registers back upon return. When this version of the ABI is enabled, the C preprocessor symbol __XTENSA_WINDOWED_ABI__ is defined. 💜
• -mextra-l32r-costs=n — Specify an extra cost of instruction RAM/ROM access for L32R instructions, measured in clock cycles. This affects — when optimizing for speed — whether loading a constant from a literal pool using L32R is better, or synthesizing the constant from a small one with a couple of arithmetic instructions. The default value is 0. 🌸
• -mstrict-align, -mno-strict-align — Avoid or allow generating memory accesses that may not be aligned on a natural object boundary as described in the architecture specification. The default is -mno-strict-align for cores that support both unaligned loads and stores in hardware, and -mstrict-align for all other cores. 🎀

---

💖 zSeries Options 💖

These are listed under

✨ ENVIRONMENT ✨

okay so bestie, this section is literally all about the environment variables that change how GCC operates. some of them work by specifying directories or prefixes to use when searching for various kinds of files, and some are used to specify other aspects of the compilation environment. 💖

heads up, queen: you can also specify places to search using options such as -B, -I and -L. these take precedence over places specified using environment variables, which in turn take precedence over those specified by the configuration of GCC. so it's options > env vars > config. no cap. 👑

---

🌍 LANG, LC_CTYPE, LC_MESSAGES, LC_ALL 🌍

these environment variables control the way that GCC uses localization information, which allows GCC to work with different national conventions. GCC inspects the locale categories LC_CTYPE and LC_MESSAGES if it has been configured to do so. these locale categories can be set to any value supported by your installation. a typical value is en_GB.UTF-8 for English in the United Kingdom encoded in UTF-8. 🇬🇧✨

LC_CTYPE specifies character classification. GCC uses it to determine the character boundaries in a string; this is needed for some multibyte encodings that contain quote and escape characters that are otherwise interpreted as a string end or escape. 😳

LC_MESSAGES specifies the language to use in diagnostic messages. so like, the language of the compiler's yelling at you. 🗣️

if the LC_ALL environment variable is set, it overrides the value of LC_CTYPE and LC_MESSAGES; otherwise, LC_CTYPE and LC_MESSAGES default to the value of the LANG environment variable. if none of these variables are set, GCC defaults to traditional C English behavior. 🇺🇸

📁 TMPDIR 📁

if TMPDIR is set, it specifies the directory to use for temporary files. GCC uses temporary files to hold the output of one stage of compilation which is to be used as input to the next stage: for example, the output of the preprocessor, which is the input to the compiler proper. it's giving pipeline energy. 🔄

🐞 GCC_COMPARE_DEBUG 🐞

setting GCC_COMPARE_DEBUG is nearly equivalent to passing -fcompare-debug to the compiler driver. see the documentation of this option for more details. 👀

🧬 GCC_EXEC_PREFIX 🧬

if GCC_EXEC_PREFIX is set, it specifies a prefix to use in the names of the subprograms executed by the compiler. no slash is added when this prefix is combined with the name of a subprogram, but you can specify a prefix that ends with a slash if you wish. 💅

if GCC_EXEC_PREFIX is not set, GCC attempts to figure out an appropriate prefix to use based on the pathname it is invoked with.

if GCC cannot find the subprogram using the specified prefix, it tries looking in the usual places for the subprogram.

the default value of GCC_EXEC_PREFIX is prefix/lib/gcc/ where prefix is the prefix to the installed compiler. in many cases prefix is the value of "prefix" when you ran the configure script.

other prefixes specified with -B take precedence over this prefix.

this prefix is also used for finding files such as crt0.o that are used for linking.

in addition, the prefix is used in an unusual way in finding the directories to search for header files. for each of the standard directories whose name normally begins with /usr/local/lib/gcc (more precisely, with the value of GCC_INCLUDE_DIR), GCC tries replacing that beginning with the specified prefix to produce an alternate directory name. thus, with -Bfoo/, GCC searches foo/bar just before it searches the standard directory /usr/local/lib/bar. if a standard directory begins with the configured prefix then the value of prefix is replaced by GCC_EXEC_PREFIX when looking for header files. 🤯

🛣️ COMPILER_PATH 🛣️

the value of COMPILER_PATH is a colon-separated list of directories, much like PATH. GCC tries the directories thus specified when searching for subprograms, if it cannot find the subprograms using GCC_EXEC_PREFIX. 🧭

📚 LIBRARY_PATH 📚

the value of LIBRARY_PATH is a colon-separated list of directories, much like PATH. when configured as a native compiler, GCC tries the directories thus specified when searching for special linker files, if it cannot find them using GCC_EXEC_PREFIX. linking using GCC also uses these directories when searching for ordinary libraries for the -l option (but directories specified with -L come first). 🥇

🗣️ LANG 🗣️

this variable is used to pass locale information to the compiler. one way in which this information is used is to determine the character set to be used when character literals, string literals and comments are parsed in C and C++. when the compiler is configured to allow multibyte characters, the following values for LANG are recognized:

• C-JIS — recognize JIS characters. 🇯🇵
• C-SJIS — recognize SJIS characters. 🇯🇵
• C-EUCJP — recognize EUCJP characters. 🇯🇵

if LANG is not defined, or if it has some other value, then the compiler uses mblen and mbtowc as defined by the default locale to recognize and translate multibyte characters. 🌐

🚨 GCC_EXTRA_DIAGNOSTIC_OUTPUT 🚨

if GCC_EXTRA_DIAGNOSTIC_OUTPUT is set to one of the following values, then additional text will be emitted to stderr when fix-it hints are emitted. -fdiagnostics-parseable-fixits and -fno-diagnostics-parseable-fixits take precedence over this environment variable. 👑

• fixits-v1 — emit parseable fix-it hints, equivalent to -fdiagnostics-parseable-fixits. in particular, columns are expressed as a count of bytes, starting at byte 1 for the initial column.
• fixits-v2 — as "fixits-v1", but columns are expressed as display columns, as per -fdiagnostics-column-unit=display.

some additional environment variables affect the behavior of the preprocessor. 🧪

📂 CPATH, C_INCLUDE_PATH, CPLUS_INCLUDE_PATH, OBJC_INCLUDE_PATH 📂

each variable's value is a list of directories separated by a special character, much like PATH, in which to look for header files. the special character, "PATH_SEPARATOR", is target-dependent and determined at GCC build time. for Microsoft Windows-based targets it is a semicolon, and for almost all other targets it is a colon. 🪟🐧

CPATH specifies a list of directories to be searched as if specified with -I, but after any paths given with -I options on the command line. this environment variable is used regardless of which language is being preprocessed. 🌈

the remaining environment variables apply only when preprocessing the particular language indicated. each specifies a list of directories to be searched as if specified with -isystem, but after any paths given with -isystem options on the command line. 🎯

in all these variables, an empty element instructs the compiler to search its current working directory. empty elements can appear at the beginning or end of a path. for instance, if the value of CPATH is ":/special/include", that has the same effect as -I. -I/special/include. 🫶

📝 DEPENDENCIES_OUTPUT 📝

if this variable is set, its value specifies how to output dependencies for Make based on the non-system header files processed by the compiler. system header files are ignored in the dependency output. 🚫

the value of DEPENDENCIES_OUTPUT can be just a file name, in which case the Make rules are written to that file, guessing the target name from the source file name. or the value can have the form file target, in which case the rules are written to file file using target as the target name. 📄

in other words, this environment variable is equivalent to combining the options -MM and -MF, with an optional -MT switch too. 🔧

☀️ SUNPRO_DEPENDENCIES ☀️

this variable is the same as DEPENDENCIES_OUTPUT (see above), except that system header files are not ignored, so it implies -M rather than -MM. however, the dependence on the main input file is omitted. 🙈

⏳ SOURCE_DATE_EPOCH ⏳

if this variable is set, its value specifies a UNIX timestamp to be used in replacement of the current date and time in the __DATE__ and __TIME__ macros, so that the embedded timestamps become reproducible. 🔁

the value of SOURCE_DATE_EPOCH must be a UNIX timestamp, defined as the number of seconds (excluding leap seconds) since 01 Jan 1970 00:00:00 represented in ASCII; identical to the output of date +%s on GNU/Linux and other systems that support the %s extension in the date command. ⏱️

the value should be a known timestamp such as the last modification time of the source or package and it should be set by the build process. 🛠️

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🐛 BUGS 🐛

for instructions on reporting bugs, see <file:///usr/share/doc/gcc-14/README.Bugs>. 🐞

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📝 FOOTNOTES 📝

1. on some systems, gcc -shared needs to build supplementary stub code for constructors to work. on multi-libbed systems, gcc -shared must select the correct support libraries to link against. failing to supply the correct flags may lead to subtle defects. supplying them in cases where they are not necessary is innocuous. -shared suppresses the addition of startup code to alter the floating-point environment as done with -ffast-math, -Ofast or -funsafe-math-optimizations on some targets. 💀

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👀 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. 🌟

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✍️ AUTHOR ✍️

see the Info entry for gcc, or <https://gcc.gnu.org/onlinedocs/gcc/Contributors.html>, for contributors to GCC. 🫶

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©️ COPYRIGHT ©️

Copyright (c) 1988-2024 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.

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and that's the whole page, bestie! this was gcc-14, dated 2024-09-08, page GCC(1). stay slay, compile safe 💅✨🦋