Make hint::cold_path #[cold] so that it works even if the MIR inliner can't inline it

Author: saethlin

library/core/src/hint.rs

@@ -778,6 +778,9 @@ pub const fn unlikely(b: bool) -> bool {
 #[stable(feature = "cold_path", since = "1.95.0")]
 #[rustc_const_stable(feature = "cold_path", since = "1.95.0")]
 #[inline(always)]
+// Even if for some reason the cold_path intrinsic is not visible to codegen, the coldness will
+// ensure that branches this is in are still known to be cold.
+#[cold]
 pub const fn cold_path() {
     crate::intrinsics::cold_path()
 }

tests/codegen-llvm/hint/cold_path.rs

@@ -50,4 +50,22 @@ pub fn test2(x: i32) {
     // CHECK: path_a
 }
 
+#[no_mangle]
+#[target_feature(enable = "sse2")]
+pub fn with_target_feature(x: bool) {
+    if x {
+        path_a();
+    } else {
+        cold_path();
+        path_b();
+    }
+
+    // CHECK-LABEL: @with_target_feature(
+    // CHECK: br i1 %x, label %bb1, label %bb2, !prof ![[NUM]]
+    // CHECK: bb2:
+    // CHECK: path_b
+    // CHECK: bb1:
+    // CHECK: path_a
+}
+
 // CHECK: ![[NUM]] = !{!"branch_weights", {{(!"expected", )?}}i32 2000, i32 1}