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| author | Guillaume Boisseau <Nadrieril@users.noreply.github.com> | 2024-03-02 20:13:22 +0100 |
|---|---|---|
| committer | GitHub <noreply@github.com> | 2024-03-02 20:13:22 +0100 |
| commit | b0ca9b5d445ea049f447c9fa03878922311f3c4a (patch) | |
| tree | 76014a6aca32960a73a6e0c50094036b2935879a /library/alloc/src | |
| parent | 8653c09087989489873a10054aeab43e5f9a5c87 (diff) | |
| parent | db535bad791fcab1e802728fe7e0ca95804192af (diff) | |
| download | rust-b0ca9b5d445ea049f447c9fa03878922311f3c4a.tar.gz rust-b0ca9b5d445ea049f447c9fa03878922311f3c4a.zip | |
Rollup merge of #121622 - dtolnay:wake, r=cuviper
Preserve same vtable pointer when cloning raw waker, to fix Waker::will_wake Fixes #121600. As `@jkarneges` identified in https://github.com/rust-lang/rust/issues/121600#issuecomment-1963041051, the issue is two different const promotions produce two statics at different addresses, which may or may not later be deduplicated by the linker (in this case not). Prior to #119863, the content of the statics was compared, and they were equal. After, the address of the statics are compared and they are not equal. It is documented that `will_wake` _"works on a best-effort basis, and may return false even when the Wakers would awaken the same task"_ so this PR fixes a quality-of-implementation issue, not a correctness issue.
Diffstat (limited to 'library/alloc/src')
| -rw-r--r-- | library/alloc/src/task.rs | 13 |
1 files changed, 13 insertions, 0 deletions
diff --git a/library/alloc/src/task.rs b/library/alloc/src/task.rs index b214f4946b1..b40768a52b6 100644 --- a/library/alloc/src/task.rs +++ b/library/alloc/src/task.rs @@ -136,6 +136,15 @@ impl<W: Wake + Send + Sync + 'static> From<Arc<W>> for RawWaker { #[inline(always)] fn raw_waker<W: Wake + Send + Sync + 'static>(waker: Arc<W>) -> RawWaker { // Increment the reference count of the arc to clone it. + // + // The #[inline(always)] is to ensure that raw_waker and clone_waker are + // always generated in the same code generation unit as one another, and + // therefore that the structurally identical const-promoted RawWakerVTable + // within both functions is deduplicated at LLVM IR code generation time. + // This allows optimizing Waker::will_wake to a single pointer comparison of + // the vtable pointers, rather than comparing all four function pointers + // within the vtables. + #[inline(always)] unsafe fn clone_waker<W: Wake + Send + Sync + 'static>(waker: *const ()) -> RawWaker { unsafe { Arc::increment_strong_count(waker as *const W) }; RawWaker::new( @@ -304,6 +313,10 @@ impl<W: LocalWake + 'static> From<Rc<W>> for RawWaker { #[inline(always)] fn local_raw_waker<W: LocalWake + 'static>(waker: Rc<W>) -> RawWaker { // Increment the reference count of the Rc to clone it. + // + // Refer to the comment on raw_waker's clone_waker regarding why this is + // always inline. + #[inline(always)] unsafe fn clone_waker<W: LocalWake + 'static>(waker: *const ()) -> RawWaker { unsafe { Rc::increment_strong_count(waker as *const W) }; RawWaker::new( |