81 lines
3.5 KiB
C++
81 lines
3.5 KiB
C++
// Copyright 2017 The Dawn Authors
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// http://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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#include "dawn_native/RefCounted.h"
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#include "common/Assert.h"
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namespace dawn_native {
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static constexpr size_t kPayloadBits = 1;
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static constexpr uint64_t kPayloadMask = (uint64_t(1) << kPayloadBits) - 1;
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static constexpr uint64_t kRefCountIncrement = (uint64_t(1) << kPayloadBits);
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RefCounted::RefCounted(uint64_t payload) : mRefCount(kRefCountIncrement + payload) {
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ASSERT((payload & kPayloadMask) == payload);
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}
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RefCounted::~RefCounted() {
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}
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uint64_t RefCounted::GetRefCountForTesting() const {
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return mRefCount >> kPayloadBits;
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}
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uint64_t RefCounted::GetRefCountPayload() const {
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// We only care about the payload bits of the refcount. These never change after
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// initialization so we can use the relaxed memory order. The order doesn't guarantee
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// anything except the atomicity of the load, which is enough since any past values of the
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// atomic will have the correct payload bits.
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return kPayloadMask & mRefCount.load(std::memory_order_relaxed);
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}
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void RefCounted::Reference() {
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ASSERT((mRefCount & ~kPayloadMask) != 0);
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// The relaxed ordering guarantees only the atomicity of the update, which is enough here
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// because the reference we are copying from still exists and makes sure other threads
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// don't delete `this`.
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// See the explanation in the Boost documentation:
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// https://www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html
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mRefCount.fetch_add(kRefCountIncrement, std::memory_order_relaxed);
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}
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void RefCounted::Release() {
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ASSERT((mRefCount & ~kPayloadMask) != 0);
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// The release fence here is to make sure all accesses to the object on a thread A
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// happen-before the object is deleted on a thread B. The release memory order ensures that
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// all accesses on thread A happen-before the refcount is decreased and the atomic variable
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// makes sure the refcount decrease in A happens-before the refcount decrease in B. Finally
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// the acquire fence in the destruction case makes sure the refcount decrease in B
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// happens-before the `delete this`.
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//
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// See the explanation in the Boost documentation:
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// https://www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html
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uint64_t previousRefCount =
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mRefCount.fetch_sub(kRefCountIncrement, std::memory_order_release);
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// Check that the previous reference count was strictly less than 2, ignoring payload bits.
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if (previousRefCount < 2 * kRefCountIncrement) {
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// Note that on ARM64 this will generate a `dmb ish` instruction which is a global
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// memory barrier, when an acquire load on mRefCount (using the `ldar` instruction)
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// should be enough and could end up being faster.
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std::atomic_thread_fence(std::memory_order_acquire);
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delete this;
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}
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}
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} // namespace dawn_native
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