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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 "base/atomicops.h"
     12 
     13 namespace base {
     14 
     15 typedef subtle::Atomic32 AtomicRefCount;
     16 
     17 // Increment a reference count by "increment", which must exceed 0.
     18 inline void AtomicRefCountIncN(volatile AtomicRefCount *ptr,
     19                                AtomicRefCount increment) {
     20   subtle::NoBarrier_AtomicIncrement(ptr, increment);
     21 }
     22 
     23 // Decrement a reference count by "decrement", which must exceed 0,
     24 // and return whether the result is non-zero.
     25 // Insert barriers to ensure that state written before the reference count
     26 // became zero will be visible to a thread that has just made the count zero.
     27 inline bool AtomicRefCountDecN(volatile AtomicRefCount *ptr,
     28                                AtomicRefCount decrement) {
     29   bool res = (subtle::Barrier_AtomicIncrement(ptr, -decrement) != 0);
     30   return res;
     31 }
     32 
     33 // Increment a reference count by 1.
     34 inline void AtomicRefCountInc(volatile AtomicRefCount *ptr) {
     35   base::AtomicRefCountIncN(ptr, 1);
     36 }
     37 
     38 // Decrement a reference count by 1 and return whether the result is non-zero.
     39 // Insert barriers to ensure that state written before the reference count
     40 // became zero will be visible to a thread that has just made the count zero.
     41 inline bool AtomicRefCountDec(volatile AtomicRefCount *ptr) {
     42   return base::AtomicRefCountDecN(ptr, 1);
     43 }
     44 
     45 // Return whether the reference count is one.  If the reference count is used
     46 // in the conventional way, a refrerence count of 1 implies that the current
     47 // thread owns the reference and no other thread shares it.  This call performs
     48 // the test for a reference count of one, and performs the memory barrier
     49 // needed for the owning thread to act on the object, knowing that it has
     50 // exclusive access to the object.
     51 inline bool AtomicRefCountIsOne(volatile AtomicRefCount *ptr) {
     52   bool res = (subtle::Acquire_Load(ptr) == 1);
     53   return res;
     54 }
     55 
     56 // Return whether the reference count is zero.  With conventional object
     57 // referencing counting, the object will be destroyed, so the reference count
     58 // should never be zero.  Hence this is generally used for a debug check.
     59 inline bool AtomicRefCountIsZero(volatile AtomicRefCount *ptr) {
     60   bool res = (subtle::Acquire_Load(ptr) == 0);
     61   return res;
     62 }
     63 
     64 }  // namespace base
     65 
     66 #endif  // BASE_ATOMIC_REF_COUNT_H_
     67