atomic_ref_count.h revision 3345a6884c488ff3a535c2c9acdd33d74b37e311 
1// Copyright (c) 2006-2008 The Chromium 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// This is a low level implementation of atomic semantics for reference
6// counting.  Please use base/ref_counted.h directly instead.
7//
8// The implementation includes annotations to avoid some false positives
9// when using data race detection tools.
10
11#ifndef BASE_ATOMIC_REF_COUNT_H_
12#define BASE_ATOMIC_REF_COUNT_H_
13#pragma once
14
15#include "base/atomicops.h"
16#include "base/third_party/dynamic_annotations/dynamic_annotations.h"
17
18namespace base {
19
20typedef subtle::Atomic32 AtomicRefCount;
21
22// Increment a reference count by "increment", which must exceed 0.
23inline void AtomicRefCountIncN(volatile AtomicRefCount *ptr,
24                               AtomicRefCount increment) {
25  subtle::NoBarrier_AtomicIncrement(ptr, increment);
26}
27
28// Decrement a reference count by "decrement", which must exceed 0,
29// and return whether the result is non-zero.
30// Insert barriers to ensure that state written before the reference count
31// became zero will be visible to a thread that has just made the count zero.
32inline bool AtomicRefCountDecN(volatile AtomicRefCount *ptr,
33                               AtomicRefCount decrement) {
34  ANNOTATE_HAPPENS_BEFORE(ptr);
35  bool res = (subtle::Barrier_AtomicIncrement(ptr, -decrement) != 0);
36  if (!res) {
37    ANNOTATE_HAPPENS_AFTER(ptr);
38  }
39  return res;
40}
41
42// Increment a reference count by 1.
43inline void AtomicRefCountInc(volatile AtomicRefCount *ptr) {
44  base::AtomicRefCountIncN(ptr, 1);
45}
46
47// Decrement a reference count by 1 and return whether the result is non-zero.
48// Insert barriers to ensure that state written before the reference count
49// became zero will be visible to a thread that has just made the count zero.
50inline bool AtomicRefCountDec(volatile AtomicRefCount *ptr) {
51  return base::AtomicRefCountDecN(ptr, 1);
52}
53
54// Return whether the reference count is one.  If the reference count is used
55// in the conventional way, a refrerence count of 1 implies that the current
56// thread owns the reference and no other thread shares it.  This call performs
57// the test for a reference count of one, and performs the memory barrier
58// needed for the owning thread to act on the object, knowing that it has
59// exclusive access to the object.
60inline bool AtomicRefCountIsOne(volatile AtomicRefCount *ptr) {
61  bool res = (subtle::Acquire_Load(ptr) == 1);
62  if (res) {
63    ANNOTATE_HAPPENS_AFTER(ptr);
64  }
65  return res;
66}
67
68// Return whether the reference count is zero.  With conventional object
69// referencing counting, the object will be destroyed, so the reference count
70// should never be zero.  Hence this is generally used for a debug check.
71inline bool AtomicRefCountIsZero(volatile AtomicRefCount *ptr) {
72  bool res = (subtle::Acquire_Load(ptr) == 0);
73  if (res) {
74    ANNOTATE_HAPPENS_AFTER(ptr);
75  }
76  return res;
77}
78
79}  // namespace base
80
81#endif  // BASE_ATOMIC_REF_COUNT_H_
82