| /* |
| * Copyright 2017 Facebook, Inc. |
| * |
| * 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. |
| */ |
| |
| /* |
| * Drop-in replacement for std::call_once() with a fast path, which the GCC |
| * implementation lacks. The tradeoff is a slightly larger `once_flag' struct |
| * (8 bytes vs 4 bytes with GCC on Linux/x64). |
| * |
| * $ call_once_test --benchmark --bm_min_iters=100000000 --threads=16 |
| * ============================================================================ |
| * folly/test/CallOnceTest.cpp relative time/iter iters/s |
| * ============================================================================ |
| * StdCallOnceBench 3.54ns 282.82M |
| * FollyCallOnceBench 698.48ps 1.43G |
| * ============================================================================ |
| */ |
| |
| #pragma once |
| |
| #include <atomic> |
| #include <mutex> |
| #include <utility> |
| |
| #include <folly/Likely.h> |
| #include <folly/Portability.h> |
| #include <folly/SharedMutex.h> |
| |
| namespace folly { |
| |
| class once_flag { |
| public: |
| constexpr once_flag() noexcept = default; |
| once_flag(const once_flag&) = delete; |
| once_flag& operator=(const once_flag&) = delete; |
| |
| template <typename Callable, class... Args> |
| friend void call_once(once_flag& flag, Callable&& f, Args&&... args); |
| template <typename Callable, class... Args> |
| friend void call_once_impl_no_inline(once_flag& flag, |
| Callable&& f, |
| Args&&... args); |
| |
| private: |
| std::atomic<bool> called_{false}; |
| folly::SharedMutex mutex_; |
| }; |
| |
| template <class Callable, class... Args> |
| void FOLLY_ALWAYS_INLINE |
| call_once(once_flag& flag, Callable&& f, Args&&... args) { |
| if (LIKELY(flag.called_.load(std::memory_order_acquire))) { |
| return; |
| } |
| call_once_impl_no_inline( |
| flag, std::forward<Callable>(f), std::forward<Args>(args)...); |
| } |
| |
| // Implementation detail: out-of-line slow path |
| template <class Callable, class... Args> |
| void FOLLY_NOINLINE |
| call_once_impl_no_inline(once_flag& flag, Callable&& f, Args&&... args) { |
| std::lock_guard<folly::SharedMutex> lg(flag.mutex_); |
| if (flag.called_) { |
| return; |
| } |
| |
| std::forward<Callable>(f)(std::forward<Args>(args)...); |
| |
| flag.called_.store(true, std::memory_order_release); |
| } |
| } |