1// Copyright (c) 2011 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/memory/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
14#include "base/atomicops.h"
15
16namespace base {
17
18typedef subtle::Atomic32 AtomicRefCount;
19
20// Increment a reference count by "increment", which must exceed 0.
21inline void AtomicRefCountIncN(volatile AtomicRefCount *ptr,
22                               AtomicRefCount increment) {
23  subtle::NoBarrier_AtomicIncrement(ptr, increment);
24}
25
26// Decrement a reference count by "decrement", which must exceed 0,
27// and return whether the result is non-zero.
28// Insert barriers to ensure that state written before the reference count
29// became zero will be visible to a thread that has just made the count zero.
30inline bool AtomicRefCountDecN(volatile AtomicRefCount *ptr,
31                               AtomicRefCount decrement) {
32  bool res = (subtle::Barrier_AtomicIncrement(ptr, -decrement) != 0);
33  return res;
34}
35
36// Increment a reference count by 1.
37inline void AtomicRefCountInc(volatile AtomicRefCount *ptr) {
38  base::AtomicRefCountIncN(ptr, 1);
39}
40
41// Decrement a reference count by 1 and return whether the result is non-zero.
42// Insert barriers to ensure that state written before the reference count
43// became zero will be visible to a thread that has just made the count zero.
44inline bool AtomicRefCountDec(volatile AtomicRefCount *ptr) {
45  return base::AtomicRefCountDecN(ptr, 1);
46}
47
48// Return whether the reference count is one.  If the reference count is used
49// in the conventional way, a refrerence count of 1 implies that the current
50// thread owns the reference and no other thread shares it.  This call performs
51// the test for a reference count of one, and performs the memory barrier
52// needed for the owning thread to act on the object, knowing that it has
53// exclusive access to the object.
54inline bool AtomicRefCountIsOne(volatile AtomicRefCount *ptr) {
55  bool res = (subtle::Acquire_Load(ptr) == 1);
56  return res;
57}
58
59// Return whether the reference count is zero.  With conventional object
60// referencing counting, the object will be destroyed, so the reference count
61// should never be zero.  Hence this is generally used for a debug check.
62inline bool AtomicRefCountIsZero(volatile AtomicRefCount *ptr) {
63  bool res = (subtle::Acquire_Load(ptr) == 0);
64  return res;
65}
66
67}  // namespace base
68
69#endif  // BASE_ATOMIC_REF_COUNT_H_
70