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
rustccompiler 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.
Note:
#[inline]in every form is a hint, with no requirements on the language to place a copy of the attributed function in the caller.
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 a 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.
#![allow(unused)] fn main() { #[cfg(target_feature = "avx2")] #[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
This platform requires that #[target_feature] is only applied to unsafe
functions.
| Feature | Implicitly Enables | Description |
|---|---|---|
aes | sse2 | AES — Advanced Encryption Standard |
avx | sse4.2 | AVX — Advanced Vector Extensions |
avx2 | avx | AVX2 — Advanced Vector Extensions 2 |
bmi1 | BMI1 — Bit Manipulation Instruction Sets | |
bmi2 | BMI2 — Bit Manipulation Instruction Sets 2 | |
fma | avx | FMA3 — Three-operand fused multiply-add |
fxsr | fxsave and fxrstor — Save and restore x87 FPU, MMX Technology, and SSE State | |
lzcnt | lzcnt — Leading zeros count | |
pclmulqdq | sse2 | pclmulqdq — Packed carry-less multiplication quadword |
popcnt | popcnt — Count of bits set to 1 | |
rdrand | rdrand — Read random number | |
rdseed | rdseed — Read random seed | |
sha | sse2 | SHA — Secure Hash Algorithm |
sse | SSE — Streaming SIMD Extensions | |
sse2 | sse | SSE2 — Streaming SIMD Extensions 2 |
sse3 | sse2 | SSE3 — Streaming SIMD Extensions 3 |
sse4.1 | ssse3 | SSE4.1 — Streaming SIMD Extensions 4.1 |
sse4.2 | sse4.1 | SSE4.2 — Streaming SIMD Extensions 4.2 |
ssse3 | sse3 | SSSE3 — Supplemental Streaming SIMD Extensions 3 |
xsave | xsave — Save processor extended states | |
xsavec | xsavec — Save processor extended states with compaction | |
xsaveopt | xsaveopt — Save processor extended states optimized | |
xsaves | xsaves — Save processor extended states supervisor |
wasm32 or wasm64
This platform allows #[target_feature] to be applied to both safe and
unsafe functions.
| Feature | Description |
|---|---|
simd128 | WebAssembly simd proposal |
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:
rustchas a default set of features enabled for each target and CPU. The CPU may be chosen with the-C target-cpuflag. Individual features may be enabled or disabled for an entire crate with the-C target-featureflag.
The track_caller attribute
The track_caller attribute may be applied to any function with "Rust" ABI
with the exception of the entry point fn main. When applied to functions and methods in
trait declarations, the attribute applies to all implementations. If the trait provides a
default implementation with the attribute, then the attribute also applies to override implementations.
When applied to a function in an extern block the attribute must also be applied to any linked
implementations, otherwise undefined behavior results. When applied to a function which is made
available to an extern block, the declaration in the extern block must also have the attribute,
otherwise undefined behavior results.
Behavior
Applying the attribute to a function f allows code within f to get a hint of the Location of
the "topmost" tracked call that led to f's invocation. At the point of observation, an
implementation behaves as if it walks up the stack from f's frame to find the nearest frame of an
unattributed function outer, and it returns the Location of the tracked call in outer.
#![allow(unused)] fn main() { #[track_caller] fn f() { println!("{}", std::panic::Location::caller()); } }
Note:
coreprovidescore::panic::Location::callerfor observing caller locations. It wraps thecore::intrinsics::caller_locationintrinsic implemented byrustc.
Note: because the resulting
Locationis a hint, an implementation may halt its walk up the stack early. See Limitations for important caveats.
Examples
When f is called directly by calls_f, code in f observes its callsite within calls_f:
#![allow(unused)] fn main() { #[track_caller] fn f() { println!("{}", std::panic::Location::caller()); } fn calls_f() { f(); // <-- f() prints this location } }
When f is called by another attributed function g which is in turn called by calls_g, code in
both f and g observes g's callsite within calls_g:
#![allow(unused)] fn main() { #[track_caller] fn f() { println!("{}", std::panic::Location::caller()); } #[track_caller] fn g() { println!("{}", std::panic::Location::caller()); f(); } fn calls_g() { g(); // <-- g() prints this location twice, once itself and once from f() } }
When g is called by another attributed function h which is in turn called by calls_h, all code
in f, g, and h observes h's callsite within calls_h:
#![allow(unused)] fn main() { #[track_caller] fn f() { println!("{}", std::panic::Location::caller()); } #[track_caller] fn g() { println!("{}", std::panic::Location::caller()); f(); } #[track_caller] fn h() { println!("{}", std::panic::Location::caller()); g(); } fn calls_h() { h(); // <-- prints this location three times, once itself, once from g(), once from f() } }
And so on.
Limitations
This information is a hint and implementations are not required to preserve it.
In particular, coercing a function with #[track_caller] to a function pointer creates a shim which
appears to observers to have been called at the attributed function's definition site, losing actual
caller information across virtual calls. A common example of this coercion is the creation of a
trait object whose methods are attributed.
Note: The aforementioned shim for function pointers is necessary because
rustcimplementstrack_callerin a codegen context by appending an implicit parameter to the function ABI, but this would be unsound for an indirect call because the parameter is not a part of the function's type and a given function pointer type may or may not refer to a function with the attribute. The creation of a shim hides the implicit parameter from callers of the function pointer, preserving soundness.