Always include global target features in function attributes

This ensures that information about target features configured with
`-C target-feature=...` or detected with `-C target-cpu=native` is
retained for subsequent consumers of LLVM bitcode.

This is crucial for linker plugin LTO, since this information is not
conveyed to the plugin otherwise.

compiler/rustc_codegen_llvm/src/attributes.rs

@@ -378,13 +378,12 @@ pub fn from_fn_attrs<'ll, 'tcx>(
         }
     }
 
-    if !function_features.is_empty() {
-        let global_features = cx.tcx.global_backend_features(()).iter().map(|s| &s[..]);
-        let val = global_features
-            .chain(function_features.iter().map(|s| &s[..]))
-            .intersperse(",")
-            .collect::<SmallStr<1024>>();
-        to_add.push(llvm::CreateAttrStringValue(cx.llcx, "target-features", &val));
+    let global_features = cx.tcx.global_backend_features(()).iter().map(|s| s.as_str());
+    let function_features = function_features.iter().map(|s| s.as_str());
+    let target_features =
+        global_features.chain(function_features).intersperse(",").collect::<SmallStr<1024>>();
+    if !target_features.is_empty() {
+        to_add.push(llvm::CreateAttrStringValue(cx.llcx, "target-features", &target_features));
     }
 
     attributes::apply_to_llfn(llfn, Function, &to_add);

src/test/codegen/target-feature-overrides.rs

@@ -29,7 +29,7 @@ pub unsafe fn apple() -> u32 {
     peach()
 }
 
-// target features same as global (not reflected or overriden in IR)
+// target features same as global
 #[no_mangle]
 pub unsafe fn banana() -> u32 {
 // CHECK-LABEL: @banana()
@@ -43,5 +43,5 @@ pub unsafe fn banana() -> u32 {
 // COMPAT-SAME: "target-features"="+avx2,+avx,+avx"
 // INCOMPAT-SAME: "target-features"="-avx2,-avx,+avx"
 // CHECK: attributes [[BANANAATTRS]]
-// CHECK-NOT: target-features
-// CHECK-SAME: }
+// COMPAT-SAME: "target-features"="+avx2,+avx"
+// INCOMPAT-SAME: "target-features"="-avx2,-avx"