Work around pointer aliasing issue in Vec::extend_from_slice, extend_with_element #36355
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r? @aturon (rust_highfive has picked a reviewer for you, use r? to override) |
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…with_element Due to missing noalias annotations for &mut T in general (issue rust-lang#31681), in larger programs extend_from_slice and extend_with_element may both compile very poorly. What is observed is that the .set_len() calls are not lifted out of the loop, even for `Vec<u8>`. Use a local length variable for the Vec length instead, and use a scope guard to write this value back to self.len when the scope ends or on panic. Then the alias analysis is easy. This affects extend_from_slice, extend_with_element, the vec![x; n] macro, Write impls for Vec<u8>, BufWriter, etc (but may / may not have triggered since inlining can be enough for the compiler to get it right).
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Verified that it fixes the two issues. (More or less by finding programs that exhibit the regression behavior, and then that the new version fixes it). |
bluss
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Adding fixes #17844 since Vec::clone uses extend_from_slice |
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Thanks. I'm thrilled to see this fixed. |
bors
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…d, r=alexcrichton Work around pointer aliasing issue in Vec::extend_from_slice, extend_with_element Due to missing noalias annotations for &mut T in general (issue #31681), in larger programs extend_from_slice and extend_with_element may both compile very poorly. What is observed is that the .set_len() calls are not lifted out of the loop, even for `Vec<u8>`. Use a local length variable for the Vec length instead, and use a scope guard to write this value back to self.len when the scope ends or on panic. Then the alias analysis is easy. This affects extend_from_slice, extend_with_element, the vec![x; n] macro, Write impls for Vec<u8>, BufWriter, etc (but may / may not have triggered since inlining can be enough for the compiler to get it right). Fixes #32155 Fixes #33518 Closes #17844
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This ensures that the length of the SmallVec is updated even if the iterator panics in `next`. This uses `SetLenOnDrop` rather than setting the length inside the loop, because otherwise this suffers from the same optimization issue as rust-lang/rust#36355. Fixes servo#136.
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Don't leak on panic in extend This ensures that the length of the SmallVec is updated even if the iterator panics in `next`. This uses `SetLenOnDrop` rather than setting the length inside the loop, because otherwise this suffers from the same optimization issue as rust-lang/rust#36355. Fixes #136. <!-- Reviewable:start --> --- This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/rust-smallvec/137) <!-- Reviewable:end -->
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Due to missing noalias annotations for &mut T in general (issue #31681),
in larger programs extend_from_slice and extend_with_element may both
compile very poorly. What is observed is that the .set_len() calls are
not lifted out of the loop, even for
Vec<u8>.Use a local length variable for the Vec length instead, and use a scope
guard to write this value back to self.len when the scope ends or on
panic. Then the alias analysis is easy.
This affects extend_from_slice, extend_with_element, the vec![x; n]
macro, Write impls for Vec, BufWriter, etc (but may / may not
have triggered since inlining can be enough for the compiler to get it right).
Fixes #32155
Fixes #33518
Closes #17844