name : atomic-entry-flag.h // Copyright 2020 the V8 project authors. All rights reserved. // Use of this source code is governed by a BSD-style license that can be // found in the LICENSE file. #ifndef INCLUDE_CPPGC_INTERNAL_ATOMIC_ENTRY_FLAG_H_ #define INCLUDE_CPPGC_INTERNAL_ATOMIC_ENTRY_FLAG_H_ #include <atomic> namespace cppgc { namespace internal { // A flag which provides a fast check whether a scope may be entered on the // current thread, without needing to access thread-local storage or mutex. Can // have false positives (i.e., spuriously report that it might be entered), so // it is expected that this will be used in tandem with a precise check that the // scope is in fact entered on that thread. // // Example: // g_frobnicating_flag.MightBeEntered() && // ThreadLocalFrobnicator().IsFrobnicating() // // Relaxed atomic operations are sufficient, since: // - all accesses remain atomic // - each thread must observe its own operations in order // - no thread ever exits the flag more times than it enters (if used correctly) // And so if a thread observes zero, it must be because it has observed an equal // number of exits as entries. class AtomicEntryFlag final { public: void Enter() { entries_.fetch_add(1, std::memory_order_relaxed); } void Exit() { entries_.fetch_sub(1, std::memory_order_relaxed); } // Returns false only if the current thread is not between a call to Enter // and a call to Exit. Returns true if this thread or another thread may // currently be in the scope guarded by this flag. bool MightBeEntered() const { return entries_.load(std::memory_order_relaxed) != 0; } private: std::atomic_int entries_{0}; }; } // namespace internal } // namespace cppgc #endif // INCLUDE_CPPGC_INTERNAL_ATOMIC_ENTRY_FLAG_H_