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28 #ifndef TALK_BASE_ATOMICOPS_H_
29 #define TALK_BASE_ATOMICOPS_H_
33 #include "talk/base/basictypes.h"
34 #include "talk/base/common.h"
35 #include "talk/base/logging.h"
36 #include "talk/base/scoped_ptr.h"
40 // A single-producer, single-consumer, fixed-size queue.
41 // All methods not ending in Unsafe can be safely called without locking,
42 // provided that calls to consumer methods (Peek/Pop) or producer methods (Push)
43 // only happen on a single thread per method type. If multiple threads need to
44 // read simultaneously or write simultaneously, other synchronization is
45 // necessary. Synchronization is also required if a call into any Unsafe method
46 // could happen at the same time as a call to any other method.
48 class FixedSizeLockFreeQueue {
50 // Atomic primitives and memory barrier
52 typedef uint32 Atomic32;
54 // Copied from google3/base/atomicops-internals-arm-v6plus.h
55 static inline void MemoryBarrier() {
56 asm volatile("dmb":::"memory");
59 // Adapted from google3/base/atomicops-internals-arm-v6plus.h
60 static inline void AtomicIncrement(volatile Atomic32* ptr) {
61 Atomic32 str_success, value;
66 "strex %0, %1, [%2]\n"
69 : "=&r"(str_success), "=&r"(value)
73 #elif !defined(SKIP_ATOMIC_CHECK)
74 #error "No atomic operations defined for the given architecture."
78 // Constructs an empty queue, with capacity 0.
79 FixedSizeLockFreeQueue() : pushed_count_(0),
83 // Constructs an empty queue with the given capacity.
84 FixedSizeLockFreeQueue(size_t capacity) : pushed_count_(0),
87 data_(new T[capacity]) {}
89 // Pushes a value onto the queue. Returns true if the value was successfully
90 // pushed (there was space in the queue). This method can be safely called at
91 // the same time as PeekFront/PopFront.
92 bool PushBack(T value) {
94 LOG(LS_WARNING) << "Queue capacity is 0.";
101 data_[pushed_count_ % capacity_] = value;
102 // Make sure the data is written before the count is incremented, so other
103 // threads can't see the value exists before being able to read it.
105 AtomicIncrement(&pushed_count_);
109 // Retrieves the oldest value pushed onto the queue. Returns true if there was
110 // an item to peek (the queue was non-empty). This method can be safely called
111 // at the same time as PushBack.
112 bool PeekFront(T* value_out) {
113 if (capacity_ == 0) {
114 LOG(LS_WARNING) << "Queue capacity is 0.";
121 *value_out = data_[popped_count_ % capacity_];
125 // Retrieves the oldest value pushed onto the queue and removes it from the
126 // queue. Returns true if there was an item to pop (the queue was non-empty).
127 // This method can be safely called at the same time as PushBack.
128 bool PopFront(T* value_out) {
129 if (PeekFront(value_out)) {
130 AtomicIncrement(&popped_count_);
136 // Clears the current items in the queue and sets the new (fixed) size. This
137 // method cannot be called at the same time as any other method.
138 void ClearAndResizeUnsafe(int new_capacity) {
139 capacity_ = new_capacity;
140 data_.reset(new T[new_capacity]);
145 // Returns true if there is no space left in the queue for new elements.
146 int IsFull() const { return pushed_count_ == popped_count_ + capacity_; }
147 // Returns true if there are no elements in the queue.
148 int IsEmpty() const { return pushed_count_ == popped_count_; }
149 // Returns the current number of elements in the queue. This is always in the
150 // range [0, capacity]
151 size_t Size() const { return pushed_count_ - popped_count_; }
153 // Returns the capacity of the queue (max size).
154 size_t capacity() const { return capacity_; }
157 volatile Atomic32 pushed_count_;
158 volatile Atomic32 popped_count_;
160 talk_base::scoped_ptr<T[]> data_;
161 DISALLOW_COPY_AND_ASSIGN(FixedSizeLockFreeQueue);
166 #endif // TALK_BASE_ATOMICOPS_H_