Stabilize #[optimize] attribute#2278
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This commit stabilizes the `#[optimize]` attribute, which allows for more fine-grained control of optimization levels. Tracking issue: rust-lang#54882 Reference PRs: rust-lang/reference#2278 cc @rust-lang/lang @rust-lang/lang-advisors @rust-lang/t-compiler > Describe each behavior being stabilized and give a short example of code that will now be accepted. ```rust // Instructs the optimization pipeline to prioritize smaller code size over speed. pub fn binary_size_sensitive() { // ... } // Instructs the optimization pipeline to prioritize execution speed over code size. pub fn performance_critical() { // ... } // Disables optimizations entirely for this item. pub fn debug_only() { // ... } ``` We might want to allow specifying a per-function optimization level (i.e. `#[optimize(level = 3)]`). To my understanding, backend support for this is not quite there yet (and this - or other - extensions have consequently not been implemented yet). Applying the attribute on a module level is also not implemented yet. - Description of the new attribute: rust-lang/reference#2278 - https://rust-lang.github.io/rfcs/2412-optimize-attr.html - Should we also implement optimize(always)? optimize(level=x)? Not yet. - Should there be any way to specify what global optimization for speed level is used in conjunction with the optimization for speed option (e.g. -Copt-level=s3 could be equivalent to -Copt-level=3 and #[optimize(size)] on the crate item). Not yet. The `optimize(none)` variant was added. The variants above were discussed here: rust-lang#54882 (comment) - `#[optimize(none)]` implies `#[inline(never)]` to be effective (rust-lang#136329) - As with all optimization-controlling flags, the effects on speed and binary size can be hard to predict: rust-lang#54882 (comment) - Applying the attribute to incompatible items emits an error: rust-lang#128458 The standard library itself uses this attribute in a few places - Wiring it up to the llvm codegen backend: https://github.com/rust-lang/rust/blob/c0bb140a37c81cf59a0b40c21c9413059644e294/compiler/rustc_codegen_llvm/src/attributes.rs#L416 - Catching usage on incompatible items: rust-lang#128458 - Basic tests: https://github.com/rust-lang/rust/blob/c0bb140a37c81cf59a0b40c21c9413059644e294/tests/ui/attributes/optimize.rs - Testing the effect on compilation: https://github.com/rust-lang/rust/blob/c0bb140a37c81cf59a0b40c21c9413059644e294/tests/codegen-llvm/optimize-attr-1.rs Trivial changes to rust-analyzer to support the new attribute as an inert attribute (part of this PR). As far as the AM is concerned, this attribute doesn't exist. Most of the work was not done by me, I'm just writing the stabilization report :-) @nagisa did the initial implementation, @clubby789 implemented optimize(none) and fixed the attribute being allowed in too many places.
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