| // 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> |
| |
| #include "dawn/common/Assert.h" |
| |
| 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::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. |
| 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::DeleteThis() { |
| delete this; |
| } |