add ABI target features *before* -Ctarget-features

Author: RalfJung

compiler/rustc_codegen_gcc/src/gcc_util.rs

@@ -154,12 +154,12 @@ pub(crate) fn global_gcc_features(sess: &Session, diagnostics: bool) -> Vec<Stri
     // we will silently correct them rather than silently producing wrong code.
     // (The target sanity check tries to catch this, but we can't know which features are
     // enabled in GCC by default so we can't be fully sure about that check.)
-    for feature in abi_enable {
-        all_rust_features.push((true, feature));
-    }
-    for feature in abi_disable {
-        all_rust_features.push((false, feature));
-    }
+    // We add these at the beginning of the list so that `-Ctarget-features` can
+    // still override it... that's unsound, but more compatible with past behavior.
+    all_rust_features.splice(
+        0..0,
+        abi_enable.iter().map(|&f| (true, f)).chain(abi_disable.iter().map(|&f| (false, f))),
+    );
 
     // Translate this into GCC features.
     let feats = all_rust_features

compiler/rustc_codegen_llvm/src/llvm_util.rs

@@ -779,12 +779,12 @@ pub(crate) fn global_llvm_features(
         // we will silently correct them rather than silently producing wrong code.
         // (The target sanity check tries to catch this, but we can't know which features are
         // enabled in LLVM by default so we can't be fully sure about that check.)
-        for feature in abi_enable {
-            all_rust_features.push((true, feature));
-        }
-        for feature in abi_disable {
-            all_rust_features.push((false, feature));
-        }
+        // We add these at the beginning of the list so that `-Ctarget-features` can
+        // still override it... that's unsound, but more compatible with past behavior.
+        all_rust_features.splice(
+            0..0,
+            abi_enable.iter().map(|&f| (true, f)).chain(abi_disable.iter().map(|&f| (false, f))),
+        );
 
         // Translate this into LLVM features.
         let feats = all_rust_features

compiler/rustc_target/src/target_features.rs

@@ -743,6 +743,9 @@ impl Target {
     /// the first list contains target features that must be enabled for ABI reasons,
     /// and the second list contains target feature that must be disabled for ABI reasons.
     ///
+    /// These features are automatically appended to whatever the target spec sats as default
+    /// features for the target.
+    ///
     /// All features enabled/disabled via `-Ctarget-features` and `#[target_features]` are checked
     /// against this. We also check any implied features, based on the information above. If LLVM
     /// implicitly enables more implied features than we do, that could bypass this check!

tests/codegen/target-feature-overrides.rs

@@ -39,8 +39,8 @@ pub unsafe fn banana() -> u32 {
 }
 
 // CHECK: attributes [[APPLEATTRS]]
-// COMPAT-SAME: "target-features"="+avx,+avx2,{{.*}}"
-// INCOMPAT-SAME: "target-features"="-avx2,-avx,+x87,+sse2,+avx,{{.*}}"
+// COMPAT-SAME: "target-features"="+x87,+sse2,+avx,+avx2,{{.*}}"
+// INCOMPAT-SAME: "target-features"="+x87,+sse2,-avx2,-avx,+avx,{{.*}}"
 // CHECK: attributes [[BANANAATTRS]]
-// COMPAT-SAME: "target-features"="+avx,+avx2,{{.*}}"
-// INCOMPAT-SAME: "target-features"="-avx2,-avx,+x87,+sse2"
+// COMPAT-SAME: "target-features"="+x87,+sse2,+avx,+avx2,{{.*}}"
+// INCOMPAT-SAME: "target-features"="+x87,+sse2,-avx2,-avx"