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80 lines
2.7 KiB
C++
80 lines
2.7 KiB
C++
/*
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* Copyright 2016 Facebook, Inc.
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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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*/
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/*
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* Drop-in replacement for std::call_once() with a fast path, which the GCC
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* implementation lacks. The tradeoff is a slightly larger `once_flag' struct
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* (8 bytes vs 4 bytes with GCC on Linux/x64).
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*
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* $ call_once_test --benchmark --bm_min_iters=100000000 --threads=16
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* ============================================================================
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* folly/test/CallOnceTest.cpp relative time/iter iters/s
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* ============================================================================
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* StdCallOnceBench 3.54ns 282.82M
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* FollyCallOnceBench 698.48ps 1.43G
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* ============================================================================
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*/
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#pragma once
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#include <atomic>
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#include <mutex>
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#include <utility>
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#include <folly/Likely.h>
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#include <folly/Portability.h>
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namespace folly {
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class once_flag {
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public:
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constexpr once_flag() noexcept = default;
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once_flag(const once_flag&) = delete;
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once_flag& operator=(const once_flag&) = delete;
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template <typename Callable, class... Args>
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friend void call_once(once_flag& flag, Callable&& f, Args&&... args);
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template <typename Callable, class... Args>
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friend void call_once_impl_no_inline(once_flag& flag,
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Callable&& f,
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Args&&... args);
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private:
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std::atomic<bool> called_{false};
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std::once_flag std_once_flag_;
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};
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template <class Callable, class... Args>
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void FOLLY_ALWAYS_INLINE
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call_once(once_flag& flag, Callable&& f, Args&&... args) {
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if (LIKELY(flag.called_.load(std::memory_order_acquire))) {
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return;
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}
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call_once_impl_no_inline(
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flag, std::forward<Callable>(f), std::forward<Args>(args)...);
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}
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// Implementation detail: out-of-line slow path
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template <class Callable, class... Args>
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void FOLLY_NOINLINE
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call_once_impl_no_inline(once_flag& flag, Callable&& f, Args&&... args) {
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std::call_once(flag.std_once_flag_,
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std::forward<Callable>(f),
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std::forward<Args>(args)...);
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flag.called_.store(true, std::memory_order_release);
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}
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}
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