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

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`

2 files changed, 7 insertions, 4 deletions

diff --git a/compiler/rustc_codegen_ssa/src/target_features.rs b/compiler/rustc_codegen_ssa/src/target_features.rs
index 95a5e96fe46..8058cd1b178 100644
--- a/compiler/rustc_codegen_ssa/src/target_features.rs
+++ b/compiler/rustc_codegen_ssa/src/target_features.rs
@@ -190,7 +190,7 @@ pub(crate) fn provide(providers: &mut Providers) {
         },
         implied_target_features: |tcx, feature: Symbol| {
             let feature = feature.as_str();
-            UnordSet::from(tcx.sess.target.implied_target_features(std::iter::once(feature)))
+            UnordSet::from(tcx.sess.target.implied_target_features(feature))
                 .into_sorted_stable_ord()
                 .into_iter()
                 .map(|s| Symbol::intern(s))
diff --git a/compiler/rustc_codegen_ssa/src/traits/backend.rs b/compiler/rustc_codegen_ssa/src/traits/backend.rs
index ebcf118b903..65fd843e7a5 100644
--- a/compiler/rustc_codegen_ssa/src/traits/backend.rs
+++ b/compiler/rustc_codegen_ssa/src/traits/backend.rs
@@ -45,10 +45,13 @@ pub trait CodegenBackend {
 
     fn print(&self, _req: &PrintRequest, _out: &mut String, _sess: &Session) {}
 
-    /// Returns the features that should be set in `cfg(target_features)`.
+    /// Returns two feature sets:
+    /// - The first has the features that should be set in `cfg(target_features)`.
+    /// - The second is like the first, but also includes unstable features.
+    ///
     /// RUSTC_SPECIFIC_FEATURES should be skipped here, those are handled outside codegen.
-    fn target_features_cfg(&self, _sess: &Session, _allow_unstable: bool) -> Vec<Symbol> {
-        vec![]
+    fn target_features_cfg(&self, _sess: &Session) -> (Vec<Symbol>, Vec<Symbol>) {
+        (vec![], vec![])
     }
 
     fn print_passes(&self) {}