Workaround LLVM optimizer bug by not marking &mut pointers as noalias

LLVM's memory dependence analysis doesn't properly account for calls
that could unwind and thus effectively act as a branching point. This
can lead to stores that are only visible when the call unwinds being
removed, possibly leading to calls to drop() functions with b0rked
memory contents.

As there is no fix for this in LLVM yet and we want to keep
compatibility to current LLVM versions anyways, we have to workaround
this bug by omitting the noalias attribute on &mut function arguments.
Benchmarks suggest that the performance loss by this change is very
small.

Thanks to @RalfJung for pushing me towards not removing too many
noalias annotations and @alexcrichton for helping out with the test for
this bug.

Fixes #29485

1 files changed, 6 insertions, 3 deletions

diff --git a/src/test/codegen/function-arguments.rs b/src/test/codegen/function-arguments.rs
index 90ced88324e..c373cdb76c5 100644
--- a/src/test/codegen/function-arguments.rs
+++ b/src/test/codegen/function-arguments.rs
@@ -51,14 +51,16 @@ pub fn named_borrow<'r>(_: &'r i32) {
 pub fn unsafe_borrow(_: &UnsafeInner) {
 }
 
-// CHECK: @mutable_unsafe_borrow(%UnsafeInner* noalias dereferenceable(2))
+// CHECK: @mutable_unsafe_borrow(%UnsafeInner* dereferenceable(2))
 // ... unless this is a mutable borrow, those never alias
+// ... except that there's this LLVM bug that forces us to not use noalias, see #29485
 #[no_mangle]
 pub fn mutable_unsafe_borrow(_: &mut UnsafeInner) {
 }
 
-// CHECK: @mutable_borrow(i32* noalias dereferenceable(4))
+// CHECK: @mutable_borrow(i32* dereferenceable(4))
 // FIXME #25759 This should also have `nocapture`
+// ... there's this LLVM bug that forces us to not use noalias, see #29485
 #[no_mangle]
 pub fn mutable_borrow(_: &mut i32) {
 }
@@ -100,8 +102,9 @@ fn helper(_: usize) {
 fn slice(_: &[u8]) {
 }
 
-// CHECK: @mutable_slice(i8* noalias nonnull, [[USIZE]])
+// CHECK: @mutable_slice(i8* nonnull, [[USIZE]])
 // FIXME #25759 This should also have `nocapture`
+// ... there's this LLVM bug that forces us to not use noalias, see #29485
 #[no_mangle]
 fn mutable_slice(_: &mut [u8]) {
 }