Add call_once, wrapper around std::call_once with a fast path

[folly.git] / folly / CallOnce.h

diff --git a/folly/CallOnce.h b/folly/CallOnce.h
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+++ b/folly/CallOnce.h
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+/*
+ * Copyright 2016 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>
+
+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};
+  std::once_flag std_once_flag_;
+};
+
+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::call_once(flag.std_once_flag_,
+                 std::forward<Callable>(f),
+                 std::forward<Args>(args)...);
+  flag.called_.store(true, std::memory_order_release);
+}
+}