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also do not add noalias on not-Unpin Box

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This commit is contained in:
Ralf Jung 2023-01-02 14:09:01 +01:00
parent ea541bc2ee
commit 1ef16874b5
6 changed files with 183 additions and 119 deletions
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9
compiler/rustc_ty_utils/src/abi.rs
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@ -261,7 +261,7 @@ fn adjust_for_rust_scalar<'tcx>(
// <https://github.com/rust-lang/unsafe-code-guidelines/issues/381>). If LLVM had a way
// to say "dereferenceable on entry" we could use it here.
attrs.pointee_size = match kind {
PointerKind::Box
PointerKind::Box { .. }
| PointerKind::SharedRef { frozen: false }
| PointerKind::MutableRef { unpin: false } => Size::ZERO,
PointerKind::SharedRef { frozen: true }
@ -278,17 +278,16 @@ fn adjust_for_rust_scalar<'tcx>(
// versions at all anymore. We still support turning it off using -Zmutable-noalias.
let noalias_mut_ref = cx.tcx.sess.opts.unstable_opts.mutable_noalias;
// `&mut` pointer parameters never alias other parameters,
// or mutable global data
//
// `&T` where `T` contains no `UnsafeCell<U>` is immutable, and can be marked as both
// `readonly` and `noalias`, as LLVM's definition of `noalias` is based solely on memory
// dependencies rather than pointer equality. However this only applies to arguments,
// not return values.
//
// `&mut T` and `Box<T>` where `T: Unpin` are unique and hence `noalias`.
let no_alias = match kind {
PointerKind::SharedRef { frozen } => frozen,
PointerKind::MutableRef { unpin } => unpin && noalias_mut_ref,
PointerKind::Box => noalias_for_box,
PointerKind::Box { unpin } => unpin && noalias_for_box,
};
// 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>).