bjoernager/rust
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Auto merge of #137586 - nnethercote:SetImpliedBits, r=bjorn3

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Speed up target feature computation

The LLVM backend calls `LLVMRustHasFeature` twice for every feature. In short-running rustc invocations, this accounts for a surprising amount of work.

r? `@bjorn3`
This commit is contained in:
bors 2025-03-11 12:05:16 +00:00
commit ebf0cf75d3
9 changed files with 120 additions and 113 deletions
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4
compiler/rustc_codegen_llvm/src/lib.rs
View file

@ -341,8 +341,8 @@ impl CodegenBackend for LlvmCodegenBackend {
llvm_util::print_version();
}
fn target_features_cfg(&self, sess: &Session, allow_unstable: bool) -> Vec<Symbol> {
target_features_cfg(sess, allow_unstable)
fn target_features_cfg(&self, sess: &Session) -> (Vec<Symbol>, Vec<Symbol>) {
target_features_cfg(sess)
}
fn codegen_crate<'tcx>(

115
compiler/rustc_codegen_llvm/src/llvm_util.rs
View file

@ -306,45 +306,44 @@ pub(crate) fn to_llvm_features<'a>(sess: &Session, s: &'a str) -> Option<LLVMFea
/// Must express features in the way Rust understands them.
///
/// We do not have to worry about RUSTC_SPECIFIC_FEATURES here, those are handled outside codegen.
pub(crate) fn target_features_cfg(sess: &Session, allow_unstable: bool) -> Vec<Symbol> {
let mut features: FxHashSet<Symbol> = Default::default();
pub(crate) fn target_features_cfg(sess: &Session) -> (Vec<Symbol>, Vec<Symbol>) {
// Add base features for the target.
// We do *not* add the -Ctarget-features there, and instead duplicate the logic for that below.
// The reason is that if LLVM considers a feature implied but we do not, we don't want that to
// show up in `cfg`. That way, `cfg` is entirely under our control -- except for the handling of
// the target CPU, that is still expanded to target features (with all their implied features) by
// LLVM.
// the target CPU, that is still expanded to target features (with all their implied features)
// by LLVM.
let target_machine = create_informational_target_machine(sess, true);
// Compute which of the known target features are enabled in the 'base' target machine.
// We only consider "supported" features; "forbidden" features are not reflected in `cfg` as of now.
features.extend(
sess.target
.rust_target_features()
.iter()
.filter(|(feature, _, _)| {
// skip checking special features, as LLVM may not understand them
if RUSTC_SPECIAL_FEATURES.contains(feature) {
return true;
}
// check that all features in a given smallvec are enabled
if let Some(feat) = to_llvm_features(sess, feature) {
for llvm_feature in feat {
let cstr = SmallCStr::new(llvm_feature);
if !unsafe { llvm::LLVMRustHasFeature(target_machine.raw(), cstr.as_ptr()) }
{
return false;
}
// Compute which of the known target features are enabled in the 'base' target machine. We only
// consider "supported" features; "forbidden" features are not reflected in `cfg` as of now.
let mut features: FxHashSet<Symbol> = sess
.target
.rust_target_features()
.iter()
.filter(|(feature, _, _)| {
// skip checking special features, as LLVM may not understand them
if RUSTC_SPECIAL_FEATURES.contains(feature) {
return true;
}
if let Some(feat) = to_llvm_features(sess, feature) {
for llvm_feature in feat {
let cstr = SmallCStr::new(llvm_feature);
// `LLVMRustHasFeature` is moderately expensive. On targets with many
// features (e.g. x86) these calls take a non-trivial fraction of runtime
// when compiling very small programs.
if !unsafe { llvm::LLVMRustHasFeature(target_machine.raw(), cstr.as_ptr()) } {
return false;
}
true
} else {
false
}
})
.map(|(feature, _, _)| Symbol::intern(feature)),
);
true
} else {
false
}
})
.map(|(feature, _, _)| Symbol::intern(feature))
.collect();
// Add enabled features
// Add enabled and remove disabled features.
for (enabled, feature) in
sess.opts.cg.target_feature.split(',').filter_map(|s| match s.chars().next() {
Some('+') => Some((true, Symbol::intern(&s[1..]))),
@ -360,7 +359,7 @@ pub(crate) fn target_features_cfg(sess: &Session, allow_unstable: bool) -> Vec<S
#[allow(rustc::potential_query_instability)]
features.extend(
sess.target
.implied_target_features(std::iter::once(feature.as_str()))
.implied_target_features(feature.as_str())
.iter()
.map(|s| Symbol::intern(s)),
);
@ -371,11 +370,7 @@ pub(crate) fn target_features_cfg(sess: &Session, allow_unstable: bool) -> Vec<S
// `features.contains` below.
#[allow(rustc::potential_query_instability)]
features.retain(|f| {
if sess
.target
.implied_target_features(std::iter::once(f.as_str()))
.contains(&feature.as_str())
{
if sess.target.implied_target_features(f.as_str()).contains(&feature.as_str()) {
// If `f` if implies `feature`, then `!feature` implies `!f`, so we have to
// remove `f`. (This is the standard logical contraposition principle.)
false
@ -387,25 +382,31 @@ pub(crate) fn target_features_cfg(sess: &Session, allow_unstable: bool) -> Vec<S
}
}
// Filter enabled features based on feature gates
sess.target
.rust_target_features()
.iter()
.filter_map(|(feature, gate, _)| {
// 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
}
})
.filter(|feature| features.contains(&Symbol::intern(feature)))
.map(|feature| Symbol::intern(feature))
.collect()
// Filter enabled features based on feature gates.
let f = |allow_unstable| {
sess.target
.rust_target_features()
.iter()
.filter_map(|(feature, gate, _)| {
// 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(Symbol::intern(feature))
} else {
None
}
})
.filter(|feature| features.contains(&feature))
.collect()
};
let target_features = f(false);
let unstable_target_features = f(true);
(target_features, unstable_target_features)
}
pub(crate) fn print_version() {
@ -682,7 +683,7 @@ pub(crate) fn global_llvm_features(
for feature in sess.opts.cg.target_feature.split(',') {
if let Some(feature) = feature.strip_prefix('+') {
all_rust_features.extend(
UnordSet::from(sess.target.implied_target_features(std::iter::once(feature)))
UnordSet::from(sess.target.implied_target_features(feature))
.to_sorted_stable_ord()
.iter()
.map(|&&s| (true, s)),