cg_llvm: Use fewer FFI calls to check the target CPU's features

compiler/rustc_codegen_llvm/src/back/owned_mc_subtarget_info.rs

@@ -4,7 +4,7 @@ use std::ptr::NonNull;
 use rustc_data_structures::small_c_str::SmallCStr;
 
 use crate::diagnostics::LlvmError;
-use crate::llvm;
+use crate::{llvm, llvm_util};
 
 /// Responsible for safely creating and disposing llvm::MCSubtargetInfo via ffi functions.
 /// Not cloneable as there is no clone function for llvm::MCSubtargetInfo.
@@ -28,11 +28,20 @@ impl OwnedMCSubtargetInfo {
             .ok_or_else(|| LlvmError::CreateMCSubtargetInfo { triple: SmallCStr::from(triple) })
     }
 
-    pub(crate) fn has_feature(&self, feature: &CStr) -> bool {
-        // SAFETY: `new` ensures we have a valid pointer created by
+    pub(crate) fn has_features(&self, features: llvm_util::LLVMFeature<'_>) -> bool {
+        // Convert to a feature string expected by LLVM's `SubtargetFeatures`.
+        // All of the required LLVM features must be enabled.
+        // (The trailing comma is OK, as `SubtargetFeatures::Split` discards empty substrings.)
+        let features = features.into_iter().flat_map(|feat| ["+", feat, ","]).collect::<String>();
+
+        // SAFETY: `info_unique` is a valid pointer created by
         // `llvm::LLVMRustCreateMCSubtargetInfo`.
         unsafe {
-            llvm::LLVMRustMCSubtargetInfoHasFeature(self.info_unique.as_ref(), feature.as_ptr())
+            llvm::LLVMRustMCSubtargetInfoCheckFeatures(
+                self.info_unique.as_ref(),
+                features.as_ptr(),
+                features.len(),
+            )
         }
     }
 }

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -2422,9 +2422,10 @@ unsafe extern "C" {
         Features: *const c_char,
     ) -> *mut MCSubtargetInfo;
 
-    pub(crate) fn LLVMRustMCSubtargetInfoHasFeature(
+    pub(crate) fn LLVMRustMCSubtargetInfoCheckFeatures(
         MCInfo: &MCSubtargetInfo,
-        Feature: *const c_char,
+        Features: *const c_uchar, // See "PTR_LEN_STR".
+        FeaturesLen: usize,
     ) -> bool;
 
     pub(crate) fn LLVMRustDisposeMCSubtargetInfo(MCInfo: ptr::NonNull<MCSubtargetInfo>);

compiler/rustc_codegen_llvm/src/llvm_util.rs

@@ -361,17 +361,9 @@ pub(crate) fn target_config(sess: &EarlySession) -> TargetConfig {
             // do *not* consider the `-Ctarget-feature`s here (that's why we passed `for_cfg: true`
             // to `global_llvm_features` above) because that will be handled later in
             // `internal_target_features`.
-            if let Some(feat) = to_llvm_features(&sess.target, feature) {
-                // All the LLVM features this expands to must be enabled.
-                for llvm_feature in feat {
-                    let cstr = SmallCStr::new(llvm_feature);
-                    // `has_feature` 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 !mc_subtarget_info.has_feature(&cstr) {
-                        return false;
-                    }
-                }
+            if let Some(feat) = to_llvm_features(&sess.target, feature)
+                && mc_subtarget_info.has_features(feat)
+            {
                 true
             } else {
                 false

compiler/rustc_llvm/llvm-wrapper/PassWrapper.cpp

@@ -107,9 +107,10 @@ LLVMRustCreateMCSubtargetInfo(const char *TripleStr, const char *CPU,
 #endif
 }
 
-extern "C" bool LLVMRustMCSubtargetInfoHasFeature(MCSubtargetInfo *MCInfo,
-                                                  const char *Feature) {
-  return MCInfo->checkFeatures(std::string("+") + Feature);
+extern "C" bool LLVMRustMCSubtargetInfoCheckFeatures(MCSubtargetInfo *MCInfo,
+                                                     const char *Features,
+                                                     size_t FeaturesLen) {
+  return MCInfo->checkFeatures(StringRef{Features, FeaturesLen});
 }
 
 extern "C" void LLVMRustDisposeMCSubtargetInfo(MCSubtargetInfo *MCInfo) {