Update LLVM comments around NoAliasMutRef

Author: cuviper

compiler/rustc_middle/src/ty/layout.rs

@@ -3060,9 +3060,10 @@ impl<'tcx> LayoutCx<'tcx, TyCtxt<'tcx>> {
                     // LLVM's definition of `noalias` is based solely on memory
                     // dependencies rather than pointer equality
                     //
-                    // Due to miscompiles in LLVM < 12, we apply a separate NoAliasMutRef attribute
-                    // for UniqueBorrowed arguments, so that the codegen backend can decide
-                    // whether or not to actually emit the attribute.
+                    // Due to past miscompiles in LLVM, we apply a separate NoAliasMutRef attribute
+                    // for UniqueBorrowed arguments, so that the codegen backend can decide whether
+                    // or not to actually emit the attribute. It can also be controlled with the
+                    // `-Zmutable-noalias` debugging option.
                     let no_alias = match kind {
                         PointerKind::Shared | PointerKind::UniqueBorrowed => false,
                         PointerKind::UniqueOwned => true,

compiler/rustc_target/src/abi/call/mod.rs

@@ -68,8 +68,10 @@ mod attr_impl {
             const NonNull   = 1 << 3;
             const ReadOnly  = 1 << 4;
             const InReg     = 1 << 5;
-            // NoAlias on &mut arguments can only be used with LLVM >= 12 due to miscompiles
-            // in earlier versions. FIXME: Remove this distinction once possible.
+            // Due to past miscompiles in LLVM, we use a separate attribute for
+            // &mut arguments, so that the codegen backend can decide whether
+            // or not to actually emit the attribute. It can also be controlled
+            // with the `-Zmutable-noalias` debugging option.
             const NoAliasMutRef = 1 << 6;
         }
     }