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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 #ifndef BASE_ATOMIC_REF_COUNT_H_
9 #define BASE_ATOMIC_REF_COUNT_H_
10 
11 #include <atomic>
12 
13 namespace base {
14 
15 class AtomicRefCount {
16  public:
AtomicRefCount()17   constexpr AtomicRefCount() : ref_count_(0) {}
AtomicRefCount(int initial_value)18   explicit constexpr AtomicRefCount(int initial_value)
19       : ref_count_(initial_value) {}
20 
21   // Increment a reference count.
Increment()22   void Increment() { Increment(1); }
23 
24   // Increment a reference count by "increment", which must exceed 0.
Increment(int increment)25   void Increment(int increment) {
26     ref_count_.fetch_add(increment, std::memory_order_relaxed);
27   }
28 
29   // Decrement a reference count, 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.
Decrement()32   bool Decrement() {
33     // TODO(jbroman): Technically this doesn't need to be an acquire operation
34     // unless the result is 1 (i.e., the ref count did indeed reach zero).
35     // However, there are toolchain issues that make that not work as well at
36     // present (notably TSAN doesn't like it).
37     return ref_count_.fetch_sub(1, std::memory_order_acq_rel) != 1;
38   }
39 
40   // Return whether the reference count is one.  If the reference count is used
41   // in the conventional way, a refrerence count of 1 implies that the current
42   // thread owns the reference and no other thread shares it.  This call
43   // performs the test for a reference count of one, and performs the memory
44   // barrier needed for the owning thread to act on the object, knowing that it
45   // has exclusive access to the object.
IsOne()46   bool IsOne() const { return ref_count_.load(std::memory_order_acquire) == 1; }
47 
48   // Return whether the reference count is zero.  With conventional object
49   // referencing counting, the object will be destroyed, so the reference count
50   // should never be zero.  Hence this is generally used for a debug check.
IsZero()51   bool IsZero() const {
52     return ref_count_.load(std::memory_order_acquire) == 0;
53   }
54 
55   // Returns the current reference count (with no barriers). This is subtle, and
56   // should be used only for debugging.
SubtleRefCountForDebug()57   int SubtleRefCountForDebug() const {
58     return ref_count_.load(std::memory_order_relaxed);
59   }
60 
61  private:
62   std::atomic_int ref_count_;
63 };
64 
65 }  // namespace base
66 
67 #endif  // BASE_ATOMIC_REF_COUNT_H_
68