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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).
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commit
442f997f98
@ -273,9 +273,11 @@ fn adjust_for_rust_scalar<'tcx>(
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| PointerKind::UniqueBorrowed
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| PointerKind::UniqueBorrowedPinned => false,
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PointerKind::UniqueOwned => noalias_for_box,
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PointerKind::Frozen => !is_return,
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PointerKind::Frozen => true,
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};
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if no_alias {
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// We can never add `noalias` in return position; that LLVM attribute has some very surprising semantics
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// (see <https://github.com/rust-lang/unsafe-code-guidelines/issues/385#issuecomment-1368055745>).
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if no_alias && !is_return {
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attrs.set(ArgAttribute::NoAlias);
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}
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@ -145,7 +145,7 @@ pub fn raw_struct(_: *const S) {
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// `Box` can get deallocated during execution of the function, so it should
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// not get `dereferenceable`.
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// CHECK: noalias noundef nonnull align 4 {{i32\*|ptr}} @_box({{i32\*|ptr}} noalias noundef nonnull align 4 %x)
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// CHECK: noundef nonnull align 4 {{i32\*|ptr}} @_box({{i32\*|ptr}} noalias noundef nonnull align 4 %x)
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#[no_mangle]
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pub fn _box(x: Box<i32>) -> Box<i32> {
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x
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