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The C and C++ Include Header Files
cat -n /usr/include/node/cppgc/internal/atomic-entry-flag.h
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 be 3 // 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 10 namespace cppgc { 11 namespace internal { 12 13 // A flag which provides a fast check whether a scope may be entered on the 14 // current thread, without needing to access thread-local storage or mutex. Can 15 // have false positives (i.e., spuriously report that it might be entered), so 16 // it is expected that this will be used in tandem with a precise check that the 17 // 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 atomic 25 // - each thread must observe its own operations in order 26 // - 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 equal 28 // number of exits as entries. 29 class 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 Enter 35 // and a call to Exit. Returns true if this thread or another thread may 36 // 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 internal 46 } // namespace cppgc 47 48 #endif // INCLUDE_CPPGC_INTERNAL_ATOMIC_ENTRY_FLAG_H_
