Code generation attributes

The following attributes are used for controlling code generation.

Optimization hints

The cold and inline attributes give suggestions to generate code in a way that may be faster than what it would do without the hint. The attributes are only hints, and may be ignored.

Both attributes can be used on functions. When applied to a function in a trait, they apply only to that function when used as a default function for a trait implementation and not to all trait implementations. The attributes have no effect on a trait function without a body.

The inline attribute

The inline attribute suggests that a copy of the attributed function should be placed in the caller, rather than generating code to call the function where it is defined.

Note: The rustc compiler automatically inlines functions based on internal heuristics. Incorrectly inlining functions can make the program slower, so this attribute should be used with care.

There are three ways to use the inline attribute:

  • #[inline] suggests performing an inline expansion.
  • #[inline(always)] suggests that an inline expansion should always be performed.
  • #[inline(never)] suggests that an inline expansion should never be performed.

The cold attribute

The cold attribute suggests that the attributed function is unlikely to be called.

The no_builtins attribute

The no_builtins attribute may be applied at the crate level to disable optimizing certain code patterns to invocations of library functions that are assumed to exist.

The target_feature attribute

The target_feature attribute may be applied to an unsafe function to enable code generation of that function for specific platform architecture features. It uses the MetaListNameValueStr syntax with a single key of enable whose value is a string of comma-separated feature names to enable.

#[target_feature(enable = "avx2")]
unsafe fn foo_avx2() {}

Each target architecture has a set of features that may be enabled. It is an error to specify a feature for a target architecture that the crate is not being compiled for.

It is undefined behavior to call a function that is compiled with a feature that is not supported on the current platform the code is running on.

Functions marked with target_feature are not inlined into a context that does not support the given features. The #[inline(always)] attribute may not be used with a target_feature attribute.

Available features

The following is a list of the available feature names.

x86 or x86_64

FeatureImplicitly EnablesDescription
aessse2AES — Advanced Encryption Standard
avxsse4.2AVX — Advanced Vector Extensions
avx2avxAVX2 — Advanced Vector Extensions 2
bmi1BMI1 — Bit Manipulation Instruction Sets
bmi2BMI2 — Bit Manipulation Instruction Sets 2
fmaavxFMA3 — Three-operand fused multiply-add
fxsrfxsave and fxrstor — Save and restore x87 FPU, MMX Technology, and SSE State
lzcntlzcnt — Leading zeros count
pclmulqdqsse2pclmulqdq — Packed carry-less multiplication quadword
popcntpopcnt — Count of bits set to 1
rdrandrdrand — Read random number
rdseedrdseed — Read random seed
shasse2SHA — Secure Hash Algorithm
sseSSE — Streaming SIMD Extensions
sse2sseSSE2 — Streaming SIMD Extensions 2
sse3sse2SSE3 — Streaming SIMD Extensions 3
sse4.1sse3SSE4.1 — Streaming SIMD Extensions 4.1
sse4.2sse4.1SSE4.2 — Streaming SIMD Extensions 4.2
ssse3sse3SSSE3 — Supplemental Streaming SIMD Extensions 3
xsavexsave — Save processor extended states
xsavecxsavec — Save processor extended states with compaction
xsaveoptxsaveopt — Save processor extended states optimized
xsavesxsaves — Save processor extended states supervisor

Additional information

See the target_feature conditional compilation option for selectively enabling or disabling compilation of code based on compile-time settings. Note that this option is not affected by the target_feature attribute, and is only driven by the features enabled for the entire crate.

See the is_x86_feature_detected macro in the standard library for runtime feature detection on the x86 platforms.

Note: rustc has a default set of features enabled for each target and CPU. The CPU may be chosen with the -C target-cpu flag. Individual features may be enabled or disabled for an entire crate with the -C target-feature flag.