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Clarify comment about Arc::as_ptr

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Andy Leiserson 2025-10-22 16:37:55 -07:00 committed by Teodor Tanasoaia
parent 50a26133ec
commit a60e71c765
1 changed files with 7 additions and 6 deletions
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13
wgpu-core/src/id.rs
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@ -161,12 +161,13 @@ impl<T: Marker> Hash for PointerId<T> {
impl<T: StorageItem> From<&Arc<T>> for PointerId<T::Marker> {
fn from(arc: &Arc<T>) -> Self {
// Since the memory representation of `Arc<T>` is just a pointer, it
// would be nice to use that pointer as the trace ID, since many
// `into_trace` implementations would then be no-ops at runtime.
// Unfortunately, because `Arc::as_ptr` returns a pointer to the
// contained data, and `Arc` stores reference counts before the data,
// we are adding an offset to the pointer here.
// Since the memory representation of `Arc<T>` is just a pointer to
// `ArcInner<T>`, it would be nice to use that pointer as the trace ID,
// since many `into_trace` implementations would then be no-ops at
// runtime. However, `Arc::as_ptr` returns a pointer to the contained
// data, not to the `ArcInner`. The `ArcInner` stores the reference
// counts before the data, so the machine code for this conversion has
// to add an offset to the pointer.
PointerId::PointerId(Arc::as_ptr(arc) as usize, PhantomData)
}
}