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| author | bors <bors@rust-lang.org> | 2023-01-03 04:54:03 +0000 |
|---|---|---|
| committer | bors <bors@rust-lang.org> | 2023-01-03 04:54:03 +0000 |
| commit | 442f997f98ac9f16f60ba3a7109f884dbf8d370c (patch) | |
| tree | 644e0d2b3c45e2e04c5071831e540038bc11e443 | |
| parent | 481c9bad80754670197cf28d8c6d43825d4404f0 (diff) | |
| parent | e7cad62257e5b0d59ca18515a522e90924177330 (diff) | |
| download | rust-442f997f98ac9f16f60ba3a7109f884dbf8d370c.tar.gz rust-442f997f98ac9f16f60ba3a7109f884dbf8d370c.zip | |
Auto merge of #106371 - RalfJung:no-ret-position-noalias, r=nikic
do not add noalias in return position `noalias` as a return attribute in LLVM indicates that the returned pointer does not alias anything else that is reachable from the caller, *including things reachable before this function call*. This is clearly not the case with a function like `fn id(Box<T>) -> Box<T>`, so we cannot use this attribute. Fixes https://github.com/rust-lang/unsafe-code-guidelines/issues/385 (including an actual miscompilation that `@comex` managed to produce).
| -rw-r--r-- | compiler/rustc_ty_utils/src/abi.rs | 6 | ||||
| -rw-r--r-- | src/test/codegen/function-arguments.rs | 2 |
2 files changed, 5 insertions, 3 deletions
diff --git a/compiler/rustc_ty_utils/src/abi.rs b/compiler/rustc_ty_utils/src/abi.rs index d644cbccea1..73d2d278f93 100644 --- a/compiler/rustc_ty_utils/src/abi.rs +++ b/compiler/rustc_ty_utils/src/abi.rs @@ -273,9 +273,11 @@ fn adjust_for_rust_scalar<'tcx>( | PointerKind::UniqueBorrowed | PointerKind::UniqueBorrowedPinned => false, PointerKind::UniqueOwned => noalias_for_box, - PointerKind::Frozen => !is_return, + PointerKind::Frozen => true, }; - if no_alias { + // We can never add `noalias` in return position; that LLVM attribute has some very surprising semantics + // (see <https://github.com/rust-lang/unsafe-code-guidelines/issues/385#issuecomment-1368055745>). + if no_alias && !is_return { attrs.set(ArgAttribute::NoAlias); } diff --git a/src/test/codegen/function-arguments.rs b/src/test/codegen/function-arguments.rs index 44fee952307..0f9e90f6ba7 100644 --- a/src/test/codegen/function-arguments.rs +++ b/src/test/codegen/function-arguments.rs @@ -145,7 +145,7 @@ pub fn raw_struct(_: *const S) { // `Box` can get deallocated during execution of the function, so it should // not get `dereferenceable`. -// CHECK: noalias noundef nonnull align 4 {{i32\*|ptr}} @_box({{i32\*|ptr}} noalias noundef nonnull align 4 %x) +// CHECK: noundef nonnull align 4 {{i32\*|ptr}} @_box({{i32\*|ptr}} noalias noundef nonnull align 4 %x) #[no_mangle] pub fn _box(x: Box<i32>) -> Box<i32> { x |