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Rollup merge of #136147 - RalfJung:required-target-features-check-not-add, r=workingjubilee

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ABI-required target features: warn when they are missing in base CPU

Part of https://github.com/rust-lang/rust/pull/135408:
instead of adding ABI-required features to the target we build for LLVM, check that they are already there. Crucially we check this after applying `-Ctarget-cpu` and `-Ctarget-feature`, by reading `sess.unstable_target_features`. This means we can tweak the ABI target feature check without changing the behavior for any existing user; they will get warnings but the target features behave as before.

The test changes here show that we are un-doing the "add all required target features" part. Without the full #135408, there is no way to take a way an ABI-required target feature with `-Ctarget-cpu`, so we cannot yet test that part.

Cc ``@workingjubilee``
This commit is contained in:
León Orell Valerian Liehr 2025-01-29 03:12:21 +01:00 • committed by GitHub
commit 7e123e4940
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16 changed files with 91 additions and 140 deletions
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56
compiler/rustc_codegen_llvm/src/llvm_util.rs
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@ -319,7 +319,6 @@ pub fn target_features_cfg(sess: &Session, allow_unstable: bool) -> Vec<Symbol>
sess.target
.rust_target_features()
.iter()
.filter(|(_, gate, _)| gate.in_cfg())
.filter(|(feature, _, _)| {
// skip checking special features, as LLVM may not understand them
if RUSTC_SPECIAL_FEATURES.contains(feature) {
@ -388,9 +387,13 @@ pub fn target_features_cfg(sess: &Session, allow_unstable: bool) -> Vec<Symbol>
sess.target
.rust_target_features()
.iter()
.filter(|(_, gate, _)| gate.in_cfg())
.filter_map(|(feature, gate, _)| {
if sess.is_nightly_build() || allow_unstable || gate.requires_nightly().is_none() {
// The `allow_unstable` set is used by rustc internally to determined which target
// features are truly available, so we want to return even perma-unstable "forbidden"
// features.
if allow_unstable
|| (gate.in_cfg() && (sess.is_nightly_build() || gate.requires_nightly().is_none()))
{
Some(*feature)
} else {
None
@ -670,12 +673,6 @@ pub(crate) fn global_llvm_features(
// Will only be filled when `diagnostics` is set!
let mut featsmap = FxHashMap::default();
// Ensure that all ABI-required features are enabled, and the ABI-forbidden ones
// are disabled.
let abi_feature_constraints = sess.target.abi_required_features();
let abi_incompatible_set =
FxHashSet::from_iter(abi_feature_constraints.incompatible.iter().copied());
// Compute implied features
let mut all_rust_features = vec![];
for feature in sess.opts.cg.target_feature.split(',') {
@ -746,52 +743,11 @@ pub(crate) fn global_llvm_features(
}
}
// Ensure that the features we enable/disable are compatible with the ABI.
if enable {
if abi_incompatible_set.contains(feature) {
sess.dcx().emit_warn(ForbiddenCTargetFeature {
feature,
enabled: "enabled",
reason: "this feature is incompatible with the target ABI",
});
}
} else {
// FIXME: we have to request implied features here since
// negative features do not handle implied features above.
for &required in abi_feature_constraints.required.iter() {
let implied =
sess.target.implied_target_features(std::iter::once(required));
if implied.contains(feature) {
sess.dcx().emit_warn(ForbiddenCTargetFeature {
feature,
enabled: "disabled",
reason: "this feature is required by the target ABI",
});
}
}
}
// FIXME(nagisa): figure out how to not allocate a full hashset here.
featsmap.insert(feature, enable);
}
}
// To be sure the ABI-relevant features are all in the right state, we explicitly
// (un)set them here. This means if the target spec sets those features wrong,
// we will silently correct them rather than silently producing wrong code.
// (The target sanity check tries to catch this, but we can't know which features are
// enabled in LLVM by default so we can't be fully sure about that check.)
// We add these at the beginning of the list so that `-Ctarget-features` can
// still override it... that's unsound, but more compatible with past behavior.
all_rust_features.splice(
0..0,
abi_feature_constraints
.required
.iter()
.map(|&f| (true, f))
.chain(abi_feature_constraints.incompatible.iter().map(|&f| (false, f))),
);
// Translate this into LLVM features.
let feats = all_rust_features
.iter()