cppgc/internal/atomic-entry-flag.h

0001 // Copyright 2020 the V8 project authors. All rights reserved.
0002 // Use of this source code is governed by a BSD-style license that can be
0003 // found in the LICENSE file.
0004
0005 #ifndef INCLUDE_CPPGC_INTERNAL_ATOMIC_ENTRY_FLAG_H_
0006 #define INCLUDE_CPPGC_INTERNAL_ATOMIC_ENTRY_FLAG_H_
0007
0008 #include <atomic>
0009
0010 namespace cppgc {
0011 namespace internal {
0012
0013 // A flag which provides a fast check whether a scope may be entered on the
0014 // current thread, without needing to access thread-local storage or mutex.  Can
0015 // have false positives (i.e., spuriously report that it might be entered), so
0016 // it is expected that this will be used in tandem with a precise check that the
0017 // scope is in fact entered on that thread.
0018 //
0019 // Example:
0020 //   g_frobnicating_flag.MightBeEntered() &&
0021 //   ThreadLocalFrobnicator().IsFrobnicating()
0022 //
0023 // Relaxed atomic operations are sufficient, since:
0024 // - all accesses remain atomic
0025 // - each thread must observe its own operations in order
0026 // - no thread ever exits the flag more times than it enters (if used correctly)
0027 // And so if a thread observes zero, it must be because it has observed an equal
0028 // number of exits as entries.
0029 class AtomicEntryFlag final {
0030  public:
0031   void Enter() { entries_.fetch_add(1, std::memory_order_relaxed); }
0032   void Exit() { entries_.fetch_sub(1, std::memory_order_relaxed); }
0033
0034   // Returns false only if the current thread is not between a call to Enter
0035   // and a call to Exit. Returns true if this thread or another thread may
0036   // currently be in the scope guarded by this flag.
0037   bool MightBeEntered() const {
0038     return entries_.load(std::memory_order_relaxed) != 0;
0039   }
0040
0041  private:
0042   std::atomic_int entries_{0};
0043 };
0044
0045 }  // namespace internal
0046 }  // namespace cppgc
0047
0048 #endif  // INCLUDE_CPPGC_INTERNAL_ATOMIC_ENTRY_FLAG_H_