Move decision aboute noalias into codegen_llvm

The frontend shouldn't be deciding whether or not to use mutable
noalias attributes, as this is a pure LLVM concern. Only provide
the necessary information and do the actual decision in
codegen_llvm.

2 files changed, 5 insertions, 2 deletions

diff --git a/compiler/rustc_target/src/abi/call/mod.rs b/compiler/rustc_target/src/abi/call/mod.rs
index 0deb1186b0f..2c3f7762759 100644
--- a/compiler/rustc_target/src/abi/call/mod.rs
+++ b/compiler/rustc_target/src/abi/call/mod.rs
@@ -65,7 +65,10 @@ mod attr_impl {
             const NoCapture = 1 << 2;
             const NonNull   = 1 << 3;
             const ReadOnly  = 1 << 4;
-            const InReg     = 1 << 8;
+            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.
+            const NoAliasMutRef = 1 << 6;
         }
     }
 }
diff --git a/compiler/rustc_target/src/abi/mod.rs b/compiler/rustc_target/src/abi/mod.rs
index b14b1ef00db..e2618da749f 100644
--- a/compiler/rustc_target/src/abi/mod.rs
+++ b/compiler/rustc_target/src/abi/mod.rs
@@ -1112,7 +1112,7 @@ pub enum PointerKind {
     /// `&T` where `T` contains no `UnsafeCell`, is `noalias` and `readonly`.
     Frozen,
 
-    /// `&mut T`, when we know `noalias` is safe for LLVM.
+    /// `&mut T` which is `noalias` but not `readonly`.
     UniqueBorrowed,
 
     /// `Box<T>`, unlike `UniqueBorrowed`, it also has `noalias` on returns.