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Move decision aboute noalias into codegen_llvm

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The frontend shouldn't be deciding whether or not to use mutable
noalias attributes, as this is a pure LLVM concern. Only provide
the necessary information and do the actual decision in
codegen_llvm.
This commit is contained in:
Nikita Popov 2021-03-18 21:50:28 +01:00
parent f82664191d
commit dfc4cafe8e
4 changed files with 65 additions and 40 deletions
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31
compiler/rustc_middle/src/ty/layout.rs
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@ -2327,24 +2327,7 @@ where
PointerKind::Shared
}
}
hir::Mutability::Mut => {
// Previously we would only emit noalias annotations for LLVM >= 6 or in
// panic=abort mode. That was deemed right, as prior versions had many bugs
// in conjunction with unwinding, but later versions didn’t seem to have
// said issues. See issue #31681.
//
// Alas, later on we encountered a case where noalias would generate wrong
// code altogether even with recent versions of LLVM in *safe* code with no
// unwinding involved. See #54462.
//
// For now, do not enable mutable_noalias by default at all, while the
// issue is being figured out.
if tcx.sess.opts.debugging_opts.mutable_noalias {
PointerKind::UniqueBorrowed
} else {
PointerKind::Shared
}
}
hir::Mutability::Mut => PointerKind::UniqueBorrowed,
};
cx.layout_of(ty).to_result().ok().map(|layout| PointeeInfo {
@ -2775,10 +2758,14 @@ where
// 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
//
// Due to miscompiles in LLVM < 12, we apply a separate NoAliasMutRef attribute
// for UniqueBorrowed arguments, so that the codegen backend can decide
// whether or not to actually emit the attribute.
let no_alias = match kind {
PointerKind::Shared => false,
PointerKind::Shared | PointerKind::UniqueBorrowed => false,
PointerKind::UniqueOwned => true,
PointerKind::Frozen | PointerKind::UniqueBorrowed => !is_return,
PointerKind::Frozen => !is_return,
};
if no_alias {
attrs.set(ArgAttribute::NoAlias);
@ -2787,6 +2774,10 @@ where
if kind == PointerKind::Frozen && !is_return {
attrs.set(ArgAttribute::ReadOnly);
}
if kind == PointerKind::UniqueBorrowed && !is_return {
attrs.set(ArgAttribute::NoAliasMutRef);
}
}
}
};