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Auto merge of #134794 - RalfJung:abi-required-target-features, r=workingjubilee

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Add a notion of "some ABIs require certain target features"

I think I finally found the right shape for the data and checks that I recently added in https://github.com/rust-lang/rust/pull/133099, https://github.com/rust-lang/rust/pull/133417, https://github.com/rust-lang/rust/pull/134337: we have a notion of "this ABI requires the following list of target features, and it is incompatible with the following list of target features". Both `-Ctarget-feature` and `#[target_feature]` are updated to ensure we follow the rules of the ABI.  This removes all the "toggleability" stuff introduced before, though we do keep the notion of a fully "forbidden" target feature -- this is needed to deal with target features that are actual ABI switches, and hence are needed to even compute the list of required target features.

We always explicitly (un)set all required and in-conflict features, just to avoid potential trouble caused by the default features of whatever the base CPU is. We do this *before* applying `-Ctarget-feature` to maintain backward compatibility; this poses a slight risk of missing some implicit feature dependencies in LLVM but has the advantage of not breaking users that deliberately toggle ABI-relevant target features. They get a warning but the feature does get toggled the way they requested.

For now, our logic supports x86, ARM, and RISC-V (just like the previous logic did). Unsurprisingly, RISC-V is the nicest. ;)

As a side-effect this also (unstably) allows *enabling* `x87` when that is harmless. I used the opportunity to mark SSE2 as required on x86-64, to better match the actual logic in LLVM and because all x86-64 chips do have SSE2. This infrastructure also prepares us for requiring SSE on x86-32 when we want to use that for our ABI (and for float semantics sanity), see https://github.com/rust-lang/rust/issues/133611, but no such change is happening in this PR.

r? `@workingjubilee`
This commit is contained in:
bors 2025-01-05 23:21:06 +00:00
commit feb32c6546
25 changed files with 837 additions and 793 deletions
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1
compiler/rustc_middle/src/middle/codegen_fn_attrs.rs
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@ -28,6 +28,7 @@ pub struct CodegenFnAttrs {
pub link_ordinal: Option<u16>,
/// The `#[target_feature(enable = "...")]` attribute and the enabled
/// features (only enabled features are supported right now).
/// Implied target features have already been applied.
pub target_features: Vec<TargetFeature>,
/// The `#[linkage = "..."]` attribute on Rust-defined items and the value we found.
pub linkage: Option<Linkage>,

2
compiler/rustc_middle/src/query/mod.rs
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@ -2357,7 +2357,7 @@ rustc_queries! {
}
/// Returns the Rust target features for the current target. These are not always the same as LLVM target features!
query rust_target_features(_: CrateNum) -> &'tcx UnordMap<String, rustc_target::target_features::StabilityComputed> {
query rust_target_features(_: CrateNum) -> &'tcx UnordMap<String, rustc_target::target_features::Stability> {
arena_cache
eval_always
desc { "looking up Rust target features" }