Code generation - The Rust Reference (original) (raw)

The Rust Reference

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 atrait, 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.

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 naked attribute

The naked attribute prevents the compiler from emitting a function prologue and epilogue for the attributed function.

[attributes.codegen.naked.body]

The function body must consist of exactly one naked_asm! macro invocation.

No function prologue or epilogue is generated for the attributed function. The assembly code in the naked_asm! block constitutes the full body of a naked function.

The naked attribute is an unsafe attribute. Annotating a function with #[unsafe(naked)] comes with the safety obligation that the body must respect the function’s calling convention, uphold its signature, and either return or diverge (i.e., not fall through past the end of the assembly code).

The assembly code may assume that the call stack and register state are valid on entry as per the signature and calling convention of the function.

The assembly code may not be duplicated by the compiler except when monomorphizing polymorphic functions.

Note

Guaranteeing when the assembly code may or may not be duplicated is important for naked functions that define symbols.

The unused_variables lint is suppressed within naked functions.

The inline attribute cannot by applied to a naked function.

The track_caller attribute cannot be applied to a naked function.

The testing attributes cannot be applied to a naked function.

The no_builtins attribute

The nobuiltins 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 targetfeature 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 ofenable whose value is a string of comma-separated feature names to enable.

#![allow(unused)]
fn main() {
#[cfg(target_feature = "avx2")]
#[target_feature(enable = "avx2")]
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.

Closures defined within a target_feature-annotated function inherit the attribute from the enclosing function.

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, _except_if the platform explicitly documents this to be safe.

The following restrictions apply unless otherwise specified by the platform rules below:

• Safe #[target_feature] functions (and closures that inherit the attribute) can only be safely called within a caller that enables all the target_features that the callee enables. This restriction does not apply in an unsafe context.
• Safe #[target_feature] functions (and closures that inherit the attribute) can only be coerced to safe function pointers in contexts that enable all the target_features that the coercee enables. This restriction does not apply to unsafe function pointers.

Implicitly enabled features are included in this rule. For example an sse2 function can call ones marked with sse.

#![allow(unused)]
fn main() {
#[cfg(target_feature = "sse2")] {
#[target_feature(enable = "sse")]
fn foo_sse() {}

fn bar() {
    // Calling `foo_sse` here is unsafe, as we must ensure that SSE is
    // available first, even if `sse` is enabled by default on the target
    // platform or manually enabled as compiler flags.
    unsafe {
        foo_sse();
    }
}

#[target_feature(enable = "sse")]
fn bar_sse() {
    // Calling `foo_sse` here is safe.
    foo_sse();
    || foo_sse();
}

#[target_feature(enable = "sse2")]
fn bar_sse2() {
    // Calling `foo_sse` here is safe because `sse2` implies `sse`.
    foo_sse();
}
}
}

A function with a #[target_feature] attribute never implements the Fn family of traits, although closures inheriting features from the enclosing function do.

The #[target_feature] attribute is not allowed on the following places:

• the main function
• a panic_handler function
• safe trait methods
• safe default functions in traits

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

Executing code with unsupported features is undefined behavior on this platform. Hence on this platform usage of #[target_feature] functions follows theabove restrictions.

aarch64

On this platform the usage of #[target_feature] functions follows theabove restrictions.

Further documentation on these features can be found in the ARM Architecture Reference Manual, or elsewhere on developer.arm.com.

Note

The following pairs of features should both be marked as enabled or disabled together if used:

• paca and pacg, which LLVM currently implements as one feature.

riscv32 or riscv64

On this platform the usage of #[target_feature] functions follows theabove restrictions.

Further documentation on these features can be found in their respective specification. Many specifications are described in the RISC-V ISA Manual or in another manual hosted on the RISC-V GitHub Account.

wasm32 or wasm64

Safe #[target_feature] functions may always be used in safe contexts on Wasm platforms. It is impossible to cause undefined behavior via the#[target_feature] attribute because attempting to use instructions unsupported by the Wasm engine will fail at load time without the risk of being interpreted in a way different from what the compiler expected.

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 or is_aarch64_feature_detected macros in the standard library for runtime feature detection on these 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.

The track_caller attribute

The track_caller attribute may be applied to any function with "Rust" ABIwith 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

Because the resulting Location is 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 rustc implements track_caller in 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.

The instruction_set attribute

The instructionset attribute may be applied to a function to control which instruction set the function will be generated for.

This allows mixing more than one instruction set in a single program on CPU architectures that support it.

It uses the MetaListPath syntax, and a path comprised of the architecture family name and instruction set name.

It is a compilation error to use the instruction_set attribute on a target that does not support it.

On ARM

For the ARMv4T and ARMv5te architectures, the following are supported:

• arm::a32 — Generate the function as A32 “ARM” code.
• arm::t32 — Generate the function as T32 “Thumb” code.

#[instruction_set(arm::a32)]
fn foo_arm_code() {}

#[instruction_set(arm::t32)]
fn bar_thumb_code() {}

Using the instruction_set attribute has the following effects:

• If the address of the function is taken as a function pointer, the low bit of the address will be set to 0 (arm) or 1 (thumb) depending on the instruction set.
• Any inline assembly in the function must use the specified instruction set instead of the target default.