Change signature of target_features_cfg.

Currently it is called twice, once with `allow_unstable` set to true and
once with it set to false. This results in some duplicated work. Most
notably, for the LLVM backend, `LLVMRustHasFeature` is called twice for
every feature, and it's moderately slow. For very short running
compilations on platforms with many features (e.g. a `check` build of
hello-world on x86) this is a significant fraction of runtime.

This commit changes `target_features_cfg` so it is only called once, and
it now returns a pair of feature sets. This halves the number of
`LLVMRustHasFeature` calls.

compiler/rustc_codegen_cranelift/src/lib.rs

@@ -176,13 +176,9 @@ impl CodegenBackend for CraneliftCodegenBackend {
         }
     }
 
-    fn target_features_cfg(
-        &self,
-        sess: &Session,
-        _allow_unstable: bool,
-    ) -> Vec<rustc_span::Symbol> {
+    fn target_features_cfg(&self, sess: &Session) -> (Vec<Symbol>, Vec<Symbol>) {
         // FIXME return the actually used target features. this is necessary for #[cfg(target_feature)]
-        if sess.target.arch == "x86_64" && sess.target.os != "none" {
+        let target_features = if sess.target.arch == "x86_64" && sess.target.os != "none" {
             // x86_64 mandates SSE2 support and rustc requires the x87 feature to be enabled
             vec![sym::fsxr, sym::sse, sym::sse2, Symbol::intern("x87")]
         } else if sess.target.arch == "aarch64" {
@@ -196,7 +192,10 @@ impl CodegenBackend for CraneliftCodegenBackend {
             }
         } else {
             vec![]
-        }
+        };
+        // FIXME do `unstable_target_features` properly
+        let unstable_target_features = target_features.clone();
+        (target_features, unstable_target_features)
     }
 
     fn print_version(&self) {

compiler/rustc_codegen_gcc/src/lib.rs

@@ -259,8 +259,8 @@ impl CodegenBackend for GccCodegenBackend {
             .join(sess)
     }
 
-    fn target_features_cfg(&self, sess: &Session, allow_unstable: bool) -> Vec<Symbol> {
-        target_features_cfg(sess, allow_unstable, &self.target_info)
+    fn target_features_cfg(&self, sess: &Session) -> (Vec<Symbol>, Vec<Symbol>) {
+        target_features_cfg(sess, &self.target_info)
     }
 }
 
@@ -486,16 +486,17 @@ fn to_gcc_opt_level(optlevel: Option<OptLevel>) -> OptimizationLevel {
 /// Returns the features that should be set in `cfg(target_feature)`.
 fn target_features_cfg(
     sess: &Session,
-    allow_unstable: bool,
     target_info: &LockedTargetInfo,
-) -> Vec<Symbol> {
+) -> (Vec<Symbol>, Vec<Symbol>) {
     // TODO(antoyo): use global_gcc_features.
+    let f = |allow_unstable| {
         sess.target
             .rust_target_features()
             .iter()
             .filter_map(|&(feature, gate, _)| {
                 if allow_unstable
-                || (gate.in_cfg() && (sess.is_nightly_build() || gate.requires_nightly().is_none()))
+                    || (gate.in_cfg()
+                        && (sess.is_nightly_build() || gate.requires_nightly().is_none()))
                 {
                     Some(feature)
                 } else {
@@ -517,4 +518,9 @@ fn target_features_cfg(
             })
             .map(Symbol::intern)
             .collect()
+    };
+
+    let target_features = f(false);
+    let unstable_target_features = f(true);
+    (target_features, unstable_target_features)
 }

compiler/rustc_codegen_llvm/src/lib.rs

@@ -338,8 +338,8 @@ 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>(

compiler/rustc_codegen_llvm/src/llvm_util.rs

@@ -306,7 +306,7 @@ 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> {
+pub(crate) fn target_features_cfg(sess: &Session) -> (Vec<Symbol>, Vec<Symbol>) {
     let mut features: FxHashSet<Symbol> = Default::default();
 
     // Add base features for the target.
@@ -331,6 +331,9 @@ pub(crate) fn target_features_cfg(sess: &Session, allow_unstable: bool) -> Vec<S
                 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;
@@ -371,11 +374,7 @@ 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(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
@@ -387,16 +386,18 @@ pub(crate) fn target_features_cfg(sess: &Session, allow_unstable: bool) -> Vec<S
         }
     }
 
-    // Filter enabled features based on feature gates
+    // 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.
+                // 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()))
+                    || (gate.in_cfg()
+                        && (sess.is_nightly_build() || gate.requires_nightly().is_none()))
                 {
                     Some(Symbol::intern(feature))
                 } else {
@@ -405,6 +406,11 @@ pub(crate) fn target_features_cfg(sess: &Session, allow_unstable: bool) -> Vec<S
             })
             .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() {

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) {}

compiler/rustc_interface/src/util.rs

@@ -39,11 +39,11 @@ pub(crate) fn add_configuration(
 ) {
     let tf = sym::target_feature;
 
-    let unstable_target_features = codegen_backend.target_features_cfg(sess, true);
-    sess.unstable_target_features.extend(unstable_target_features.iter().cloned());
+    let (target_features, unstable_target_features) = codegen_backend.target_features_cfg(sess);
 
-    let target_features = codegen_backend.target_features_cfg(sess, false);
-    sess.target_features.extend(target_features.iter().cloned());
+    sess.unstable_target_features.extend(unstable_target_features.iter().copied());
+
+    sess.target_features.extend(target_features.iter().copied());
 
     cfg.extend(target_features.into_iter().map(|feat| (tf, Some(feat))));