RefCounted: use more precise barriers
This improves the DrawCallPerfRun/Vulkan_NoReuseBindGroups benchmark by
2% on an Intel processor but should be a bigger improvement on ARM.
The change was inspired by the Boost documentation at
https://www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html
Chromium's base::AtomicRefCount implementation and Rust's core::Arc
implementation.
BUG=dawn:304
Change-Id: I7ca71f34af20fd267cf2efc63871ff330b1dcc7c
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/14482
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Reviewed-by: David Turner <digit@google.com>
diff --git a/src/dawn_native/RefCounted.cpp b/src/dawn_native/RefCounted.cpp
index 6782c14..9b4c1c2 100644
--- a/src/dawn_native/RefCounted.cpp
+++ b/src/dawn_native/RefCounted.cpp
@@ -43,14 +43,36 @@
void RefCounted::Reference() {
ASSERT((mRefCount & ~kPayloadMask) != 0);
- mRefCount += kRefCountIncrement;
+
+ // The relaxed ordering guarantees only the atomicity of the update, which is enough here
+ // because the reference we are copying from still exists and makes sure other threads
+ // don't delete `this`.
+ // See the explanation in the Boost documentation:
+ // https://www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html
+ mRefCount.fetch_add(kRefCountIncrement, std::memory_order_relaxed);
}
void RefCounted::Release() {
ASSERT((mRefCount & ~kPayloadMask) != 0);
- mRefCount -= kRefCountIncrement;
- if (mRefCount < kRefCountIncrement) {
+ // The release fence here is to make sure all accesses to the object on a thread A
+ // happen-before the object is deleted on a thread B. The release memory order ensures that
+ // all accesses on thread A happen-before the refcount is decreased and the atomic variable
+ // makes sure the refcount decrease in A happens-before the refcount decrease in B. Finally
+ // the acquire fence in the destruction case makes sure the refcount decrease in B
+ // happens-before the `delete this`.
+ //
+ // See the explanation in the Boost documentation:
+ // https://www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html
+ uint64_t previousRefCount =
+ mRefCount.fetch_sub(kRefCountIncrement, std::memory_order_release);
+
+ // Check that the previous reference count was strictly less than 2, ignoring payload bits.
+ if (previousRefCount < 2 * kRefCountIncrement) {
+ // Note that on ARM64 this will generate a `dmb ish` instruction which is a global
+ // memory barrier, when an acquire load on mRefCount (using the `ldar` instruction)
+ // should be enough and could end up being faster.
+ std::atomic_thread_fence(std::memory_order_acquire);
delete this;
}
}
