| // 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. | |
| 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 | |