bjoernager/rust
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Move directly when Rc/Arc::make_mut splits from Weak

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When only other `Weak` references remain, we can directly move the data
into the new unique allocation as a plain memory copy.
This commit is contained in:
Josh Stone 2021-01-11 17:32:21 -08:00
parent d85df44e8d
commit f89f30fb2c
2 changed files with 13 additions and 14 deletions
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12
library/alloc/src/rc.rs
View file

@ -1048,14 +1048,16 @@ impl<T: Clone> Rc<T> {
} }
} else if Rc::weak_count(this) != 0 { } else if Rc::weak_count(this) != 0 {
// Can just steal the data, all that's left is Weaks // Can just steal the data, all that's left is Weaks
let mut rc = Self::new_uninit();
unsafe { unsafe {
let mut swap = Rc::new(ptr::read(&this.ptr.as_ref().value)); let data = Rc::get_mut_unchecked(&mut rc);
mem::swap(this, &mut swap); data.as_mut_ptr().copy_from_nonoverlapping(&**this, 1);
swap.inner().dec_strong();
this.inner().dec_strong();
// Remove implicit strong-weak ref (no need to craft a fake // Remove implicit strong-weak ref (no need to craft a fake
// Weak here -- we know other Weaks can clean up for us) // Weak here -- we know other Weaks can clean up for us)
swap.inner().dec_weak(); this.inner().dec_weak();
forget(swap); ptr::write(this, rc.assume_init());
} }
} }
// This unsafety is ok because we're guaranteed that the pointer // This unsafety is ok because we're guaranteed that the pointer

15
library/alloc/src/sync.rs
View file

@ -1392,17 +1392,14 @@ impl<T: Clone> Arc<T> {
// Materialize our own implicit weak pointer, so that it can clean // Materialize our own implicit weak pointer, so that it can clean
// up the ArcInner as needed. // up the ArcInner as needed.
let weak = Weak { ptr: this.ptr }; let _weak = Weak { ptr: this.ptr };
// mark the data itself as already deallocated // Can just steal the data, all that's left is Weaks
let mut arc = Self::new_uninit();
unsafe { unsafe {
// there is no data race in the implicit write caused by `read` let data = Arc::get_mut_unchecked(&mut arc);
// here (due to zeroing) because data is no longer accessed by data.as_mut_ptr().copy_from_nonoverlapping(&**this, 1);
// other threads (due to there being no more strong refs at this ptr::write(this, arc.assume_init());
// point).
let mut swap = Arc::new(ptr::read(&weak.ptr.as_ref().data));
mem::swap(this, &mut swap);
mem::forget(swap);
} }
} else { } else {
// We were the sole reference of either kind; bump back up the // We were the sole reference of either kind; bump back up the