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// Copyright 2017 The Dawn Authors
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.
#include "common/RefCounted.h"
#include "common/Assert.h"
#include <cstddef>
static constexpr size_t kPayloadBits = 1;
static constexpr uint64_t kPayloadMask = (uint64_t(1) << kPayloadBits) - 1;
static constexpr uint64_t kRefCountIncrement = (uint64_t(1) << kPayloadBits);
RefCounted::RefCounted(uint64_t payload) : mRefCount(kRefCountIncrement + payload) {
ASSERT((payload & kPayloadMask) == payload);
uint64_t RefCounted::GetRefCountForTesting() const {
return mRefCount >> kPayloadBits;
uint64_t RefCounted::GetRefCountPayload() const {
// We only care about the payload bits of the refcount. These never change after
// initialization so we can use the relaxed memory order. The order doesn't guarantee
// anything except the atomicity of the load, which is enough since any past values of the
// atomic will have the correct payload bits.
return kPayloadMask & mRefCount.load(std::memory_order_relaxed);
void RefCounted::Reference() {
ASSERT((mRefCount & ~kPayloadMask) != 0);
// 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:
mRefCount.fetch_add(kRefCountIncrement, std::memory_order_relaxed);
void RefCounted::Release() {
ASSERT((mRefCount & ~kPayloadMask) != 0);
// 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:
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.
void RefCounted::DeleteThis() {
delete this;