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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
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
#include "dawn/common/RefCounted.h"
#include <cstddef>
#if defined(__has_feature)
#if __has_feature(thread_sanitizer)
#include <sanitizer/tsan_interface.h>
#endif
#endif
#include "dawn/common/Assert.h"
namespace dawn {
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);
RefCount::RefCount(uint64_t payload) : mRefCount(kRefCountIncrement + payload) {
ASSERT((payload & kPayloadMask) == payload);
}
uint64_t RefCount::GetValueForTesting() const {
return mRefCount >> kPayloadBits;
}
uint64_t RefCount::GetPayload() 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 RefCount::Increment() {
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:
// https://www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html
mRefCount.fetch_add(kRefCountIncrement, std::memory_order_relaxed);
}
bool RefCount::TryIncrement() {
uint64_t current = mRefCount.load(std::memory_order_relaxed);
bool success = false;
do {
if ((current & ~kPayloadMask) == 0u) {
return false;
}
// The relaxed ordering guarantees only the atomicity of the update. This is fine because:
// - If another thread's decrement happens before this increment, the increment should
// fail.
// - If another thread's decrement happens after this increment, the decrement shouldn't
// delete the object, because the ref count > 0.
// See Boost library for reference:
// https://github.com/boostorg/smart_ptr/blob/develop/include/boost/smart_ptr/detail/sp_counted_base_std_atomic.hpp#L62
success = mRefCount.compare_exchange_weak(current, current + kRefCountIncrement,
std::memory_order_relaxed);
} while (!success);
return true;
}
bool RefCount::Decrement() {
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:
// 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.
// https://github.com/google/sanitizers/issues/1415 There is false positive bug in TSAN
// when using standalone fence.
#if defined(__has_feature)
#if __has_feature(thread_sanitizer)
__tsan_acquire(&mRefCount);
#endif
#endif
std::atomic_thread_fence(std::memory_order_acquire);
return true;
}
return false;
}
RefCounted::RefCounted(uint64_t payload) : mRefCount(payload) {}
RefCounted::~RefCounted() = default;
uint64_t RefCounted::GetRefCountForTesting() const {
return mRefCount.GetValueForTesting();
}
uint64_t RefCounted::GetRefCountPayload() const {
return mRefCount.GetPayload();
}
void RefCounted::Reference() {
mRefCount.Increment();
}
void RefCounted::Release() {
if (mRefCount.Decrement()) {
DeleteThis();
}
}
void RefCounted::ReleaseAndLockBeforeDestroy() {
if (mRefCount.Decrement()) {
LockAndDeleteThis();
}
}
void RefCounted::DeleteThis() {
delete this;
}
void RefCounted::LockAndDeleteThis() {
DeleteThis();
}
} // namespace dawn