Enable mutable noalias by default for LLVM 12

We don't have any known noalias bugs for LLVM 12 ... yet.

compiler/rustc_codegen_llvm/src/abi.rs

@@ -1,6 +1,7 @@
 use crate::builder::Builder;
 use crate::context::CodegenCx;
 use crate::llvm::{self, AttributePlace};
+use crate::llvm_util;
 use crate::type_::Type;
 use crate::type_of::LayoutLlvmExt;
 use crate::value::Value;
@@ -51,18 +52,15 @@ pub trait ArgAttributesExt {
 }
 
 fn should_use_mutable_noalias(cx: &CodegenCx<'_, '_>) -> bool {
-    // Previously we would only emit noalias annotations for LLVM >= 6 or in
-    // panic=abort mode. That was deemed right, as prior versions had many bugs
-    // in conjunction with unwinding, but later versions didn’t seem to have
-    // said issues. See issue #31681.
-    //
-    // Alas, later on we encountered a case where noalias would generate wrong
-    // code altogether even with recent versions of LLVM in *safe* code with no
-    // unwinding involved. See #54462.
-    //
-    // For now, do not enable mutable_noalias by default at all, while the
-    // issue is being figured out.
-    cx.tcx.sess.opts.debugging_opts.mutable_noalias.unwrap_or(false)
+    // LLVM prior to version 12 has known miscompiles in the presence of
+    // noalias attributes (see #54878). Only enable mutable noalias by
+    // default for versions we believe to be safe.
+    cx.tcx
+        .sess
+        .opts
+        .debugging_opts
+        .mutable_noalias
+        .unwrap_or_else(|| llvm_util::get_version() >= (12, 0, 0))
 }
 
 impl ArgAttributesExt for ArgAttributes {

compiler/rustc_session/src/options.rs

@@ -998,7 +998,7 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
     mir_opt_level: Option<usize> = (None, parse_opt_uint, [TRACKED],
         "MIR optimization level (0-4; default: 1 in non optimized builds and 2 in optimized builds)"),
     mutable_noalias: Option<bool> = (None, parse_opt_bool, [TRACKED],
-        "emit noalias metadata for mutable references (default: no)"),
+        "emit noalias metadata for mutable references (default: yes for LLVM >= 12, otherwise no)"),
     new_llvm_pass_manager: bool = (false, parse_bool, [TRACKED],
         "use new LLVM pass manager (default: no)"),
     nll_facts: bool = (false, parse_bool, [UNTRACKED],

src/test/codegen/function-arguments.rs

@@ -43,13 +43,13 @@ pub fn named_borrow<'r>(_: &'r i32) {
 pub fn unsafe_borrow(_: &UnsafeInner) {
 }
 
-// CHECK: @mutable_unsafe_borrow(i16* align 2 dereferenceable(2) %_1)
+// CHECK: @mutable_unsafe_borrow(i16* noalias align 2 dereferenceable(2) %_1)
 // ... unless this is a mutable borrow, those never alias
 #[no_mangle]
 pub fn mutable_unsafe_borrow(_: &mut UnsafeInner) {
 }
 
-// CHECK: @mutable_borrow(i32* align 4 dereferenceable(4) %_1)
+// CHECK: @mutable_borrow(i32* noalias align 4 dereferenceable(4) %_1)
 // FIXME #25759 This should also have `nocapture`
 #[no_mangle]
 pub fn mutable_borrow(_: &mut i32) {
@@ -94,7 +94,7 @@ pub fn helper(_: usize) {
 pub fn slice(_: &[u8]) {
 }
 
-// CHECK: @mutable_slice([0 x i8]* nonnull align 1 %_1.0, [[USIZE]] %_1.1)
+// CHECK: @mutable_slice([0 x i8]* noalias nonnull align 1 %_1.0, [[USIZE]] %_1.1)
 // FIXME #25759 This should also have `nocapture`
 #[no_mangle]
 pub fn mutable_slice(_: &mut [u8]) {