text: // Copyright (c) 2011 The Chromium Authors. All rights reserved.
  // Use of this source code is governed by a BSD-style license that can be
  // found in the LICENSE file.
  
  // This is a low level implementation of atomic semantics for reference
  // counting.  Please use butil/memory/ref_counted.h directly instead.
  //
  // The implementation includes annotations to avoid some false positives
  // when using data race detection tools.
  
  #ifndef BASE_ATOMIC_REF_COUNT_H_
  #define BASE_ATOMIC_REF_COUNT_H_
  
  #include "butil/atomicops.h"
  #include "butil/third_party/dynamic_annotations/dynamic_annotations.h"
  
  namespace butil {
  
  typedef subtle::Atomic32 AtomicRefCount;
  
  // Increment a reference count by "increment", which must exceed 0.
  inline void AtomicRefCountIncN(volatile AtomicRefCount *ptr,
                                 AtomicRefCount increment) {
    subtle::NoBarrier_AtomicIncrement(ptr, increment);
  }
  
  // Decrement a reference count by "decrement", which must exceed 0,
  // and return whether the result is non-zero.
  // Insert barriers to ensure that state written before the reference count
  // became zero will be visible to a thread that has just made the count zero.
  inline bool AtomicRefCountDecN(volatile AtomicRefCount *ptr,
                                 AtomicRefCount decrement) {
    ANNOTATE_HAPPENS_BEFORE(ptr);
    bool res = (subtle::Barrier_AtomicIncrement(ptr, -decrement) != 0);
    if (!res) {
      ANNOTATE_HAPPENS_AFTER(ptr);
    }
    return res;
  }
  
  // Increment a reference count by 1.
  inline void AtomicRefCountInc(volatile AtomicRefCount *ptr) {
    butil::AtomicRefCountIncN(ptr, 1);
  }
  
  // Decrement a reference count by 1 and return whether the result is non-zero.
  // Insert barriers to ensure that state written before the reference count
  // became zero will be visible to a thread that has just made the count zero.
  inline bool AtomicRefCountDec(volatile AtomicRefCount *ptr) {
    return butil::AtomicRefCountDecN(ptr, 1);
  }
  
  // Return whether the reference count is one.  If the reference count is used
  // in the conventional way, a refrerence count of 1 implies that the current
  // thread owns the reference and no other thread shares it.  This call performs
  // the test for a reference count of one, and performs the memory barrier
  // needed for the owning thread to act on the object, knowing that it has
  // exclusive access to the object.
  inline bool AtomicRefCountIsOne(volatile AtomicRefCount *ptr) {
    bool res = (subtle::Acquire_Load(ptr) == 1);
    if (res) {
      ANNOTATE_HAPPENS_AFTER(ptr);
    }
    return res;
  }
  
  // Return whether the reference count is zero.  With conventional object
  // referencing counting, the object will be destroyed, so the reference count
  // should never be zero.  Hence this is generally used for a debug check.
  inline bool AtomicRefCountIsZero(volatile AtomicRefCount *ptr) {
    bool res = (subtle::Acquire_Load(ptr) == 0);
    if (res) {
      ANNOTATE_HAPPENS_AFTER(ptr);
    }
    return res;
  }
  
  }  // namespace butil
  
  #endif  // BASE_ATOMIC_REF_COUNT_H_
  