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1// Copyright 2020 the V8 project authors. All rights reserved.2// Use of this source code is governed by a BSD-style license that can be3// found in the LICENSE file.4 5#ifndef INCLUDE_CPPGC_INTERNAL_ATOMIC_ENTRY_FLAG_H_6#define INCLUDE_CPPGC_INTERNAL_ATOMIC_ENTRY_FLAG_H_7 8#include <atomic>9 10namespace cppgc {11namespace internal {12 13// A flag which provides a fast check whether a scope may be entered on the14// current thread, without needing to access thread-local storage or mutex.  Can15// have false positives (i.e., spuriously report that it might be entered), so16// it is expected that this will be used in tandem with a precise check that the17// scope is in fact entered on that thread.18//19// Example:20//   g_frobnicating_flag.MightBeEntered() &&21//   ThreadLocalFrobnicator().IsFrobnicating()22//23// Relaxed atomic operations are sufficient, since:24// - all accesses remain atomic25// - each thread must observe its own operations in order26// - no thread ever exits the flag more times than it enters (if used correctly)27// And so if a thread observes zero, it must be because it has observed an equal28// number of exits as entries.29class AtomicEntryFlag final {30 public:31  void Enter() { entries_.fetch_add(1, std::memory_order_relaxed); }32  void Exit() { entries_.fetch_sub(1, std::memory_order_relaxed); }33 34  // Returns false only if the current thread is not between a call to Enter35  // and a call to Exit. Returns true if this thread or another thread may36  // currently be in the scope guarded by this flag.37  bool MightBeEntered() const {38    return entries_.load(std::memory_order_relaxed) != 0;39  }40 41 private:42  std::atomic_int entries_{0};43};44 45}  // namespace internal46}  // namespace cppgc47 48#endif  // INCLUDE_CPPGC_INTERNAL_ATOMIC_ENTRY_FLAG_H_49