Emit noalias on &mut parameters by default

This used to be disabled due to LLVM bugs in the handling of
noalias information in conjunction with unwinding. However,
according to #31681 all known LLVM bugs have been fixed by
LLVM 6.0, so it's probably time to reenable this optimization.

Noalias annotations will not be emitted by default if either
-C panic=abort (as previously) or LLVM >= 6.0 (new).

-Z mutable-noalias=no is left as an escape-hatch to allow
debugging problems suspected to stem from this change.

1 files changed, 4 insertions, 6 deletions

diff --git a/src/test/codegen/function-arguments.rs b/src/test/codegen/function-arguments.rs
index 40a9ea5a181..e3fa7a7db39 100644
--- a/src/test/codegen/function-arguments.rs
+++ b/src/test/codegen/function-arguments.rs
@@ -10,6 +10,7 @@
 
 // compile-flags: -C no-prepopulate-passes
 // ignore-tidy-linelength
+// min-llvm-version 6.0
 
 #![crate_type = "lib"]
 #![feature(custom_attribute)]
@@ -52,16 +53,14 @@ pub fn named_borrow<'r>(_: &'r i32) {
 pub fn unsafe_borrow(_: &UnsafeInner) {
 }
 
-// CHECK: @mutable_unsafe_borrow(i16* dereferenceable(2) %arg0)
+// CHECK: @mutable_unsafe_borrow(i16* noalias dereferenceable(2) %arg0)
 // ... 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* dereferenceable(4) %arg0)
+// CHECK: @mutable_borrow(i32* noalias dereferenceable(4) %arg0)
 // 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) {
 }
@@ -103,9 +102,8 @@ pub fn helper(_: usize) {
 pub fn slice(_: &[u8]) {
 }
 
-// CHECK: @mutable_slice([0 x i8]* nonnull %arg0.0, [[USIZE]] %arg0.1)
+// CHECK: @mutable_slice([0 x i8]* noalias nonnull %arg0.0, [[USIZE]] %arg0.1)
 // 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_slice(_: &mut [u8]) {
 }