chromium/v8/include/cppgc/internal/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 { … };

}  // namespace internal
}  // namespace cppgc

#endif  // INCLUDE_CPPGC_INTERNAL_ATOMIC_ENTRY_FLAG_H_