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/*
* Copyright (c) 2015 The WebRTC project authors. All Rights Reserved.
*
* Use of this source code is governed by a BSD-style license
* that can be found in the LICENSE file in the root of the source
* tree. An additional intellectual property rights grant can be found
* in the file PATENTS. All contributing project authors may
* be found in the AUTHORS file in the root of the source tree.
*/
#ifndef RTC_BASE_SWAP_QUEUE_H_
#define RTC_BASE_SWAP_QUEUE_H_
#include <stddef.h>
#include <atomic>
#include <utility>
#include <vector>
#include "absl/base/attributes.h"
#include "rtc_base/checks.h"
namespace webrtc {
namespace internal {
// (Internal; please don't use outside this file.)
template <typename T>
bool NoopSwapQueueItemVerifierFunction(const T&) {
return true;
}
} // namespace internal
// Functor to use when supplying a verifier function for the queue.
template <typename T,
bool (*QueueItemVerifierFunction)(const T&) =
internal::NoopSwapQueueItemVerifierFunction>
class SwapQueueItemVerifier {
public:
bool operator()(const T& t) const { return QueueItemVerifierFunction(t); }
};
// This class is a fixed-size queue. A single producer calls Insert() to insert
// an element of type T at the back of the queue, and a single consumer calls
// Remove() to remove an element from the front of the queue. It's safe for the
// producer and the consumer to access the queue concurrently, from different
// threads.
//
// To avoid the construction, copying, and destruction of Ts that a naive
// queue implementation would require, for each "full" T passed from
// producer to consumer, SwapQueue<T> passes an "empty" T in the other
// direction (an "empty" T is one that contains nothing of value for the
// consumer). This bidirectional movement is implemented with swap().
//
// // Create queue:
// Bottle proto(568); // Prepare an empty Bottle. Heap allocates space for
// // 568 ml.
// SwapQueue<Bottle> q(N, proto); // Init queue with N copies of proto.
// // Each copy allocates on the heap.
// // Producer pseudo-code:
// Bottle b(568); // Prepare an empty Bottle. Heap allocates space for 568 ml.
// loop {
// b.Fill(amount); // Where amount <= 568 ml.
// q.Insert(&b); // Swap our full Bottle for an empty one from q.
// }
//
// // Consumer pseudo-code:
// Bottle b(568); // Prepare an empty Bottle. Heap allocates space for 568 ml.
// loop {
// q.Remove(&b); // Swap our empty Bottle for the next-in-line full Bottle.
// Drink(&b);
// }
//
// For a well-behaved Bottle class, there are no allocations in the
// producer, since it just fills an empty Bottle that's already large
// enough; no deallocations in the consumer, since it returns each empty
// Bottle to the queue after having drunk it; and no copies along the
// way, since the queue uses swap() everywhere to move full Bottles in
// one direction and empty ones in the other.
template <typename T, typename QueueItemVerifier = SwapQueueItemVerifier<T>>
class SwapQueue {
public:
// Creates a queue of size size and fills it with default constructed Ts.
explicit SwapQueue(size_t size) : queue_(size) {
RTC_DCHECK(VerifyQueueSlots());
}
// Same as above and accepts an item verification functor.
SwapQueue(size_t size, const QueueItemVerifier& queue_item_verifier)
: queue_item_verifier_(queue_item_verifier), queue_(size) {
RTC_DCHECK(VerifyQueueSlots());
}
// Creates a queue of size size and fills it with copies of prototype.
SwapQueue(size_t size, const T& prototype) : queue_(size, prototype) {
RTC_DCHECK(VerifyQueueSlots());
}
// Same as above and accepts an item verification functor.
SwapQueue(size_t size,
const T& prototype,
const QueueItemVerifier& queue_item_verifier)
: queue_item_verifier_(queue_item_verifier), queue_(size, prototype) {
RTC_DCHECK(VerifyQueueSlots());
}
// Resets the queue to have zero content while maintaining the queue size.
// Just like Remove(), this can only be called (safely) from the
// consumer.
void Clear() {
// Drop all non-empty elements by resetting num_elements_ and incrementing
// next_read_index_ by the previous value of num_elements_. Relaxed memory
// ordering is sufficient since the dropped elements are not accessed.
next_read_index_ += std::atomic_exchange_explicit(
&num_elements_, size_t{0}, std::memory_order_relaxed);
if (next_read_index_ >= queue_.size()) {
next_read_index_ -= queue_.size();
}
RTC_DCHECK_LT(next_read_index_, queue_.size());
}
// Inserts a "full" T at the back of the queue by swapping *input with an
// "empty" T from the queue.
// Returns true if the item was inserted or false if not (the queue was full).
// When specified, the T given in *input must pass the ItemVerifier() test.
// The contents of *input after the call are then also guaranteed to pass the
// ItemVerifier() test.
ABSL_MUST_USE_RESULT bool Insert(T* input) {
RTC_DCHECK(input);
RTC_DCHECK(queue_item_verifier_(*input));
// Load the value of num_elements_. Acquire memory ordering prevents reads
// and writes to queue_[next_write_index_] to be reordered to before the
// load. (That element might be accessed by a concurrent call to Remove()
// until the load finishes.)
if (std::atomic_load_explicit(&num_elements_, std::memory_order_acquire) ==
queue_.size()) {
return false;
}
using std::swap;
swap(*input, queue_[next_write_index_]);
// Increment the value of num_elements_ to account for the inserted element.
// Release memory ordering prevents the reads and writes to
// queue_[next_write_index_] to be reordered to after the increment. (Once
// the increment has finished, Remove() might start accessing that element.)
const size_t old_num_elements = std::atomic_fetch_add_explicit(
&num_elements_, size_t{1}, std::memory_order_release);
++next_write_index_;
if (next_write_index_ == queue_.size()) {
next_write_index_ = 0;
}
RTC_DCHECK_LT(next_write_index_, queue_.size());
RTC_DCHECK_LT(old_num_elements, queue_.size());
return true;
}
// Removes the frontmost "full" T from the queue by swapping it with
// the "empty" T in *output.
// Returns true if an item could be removed or false if not (the queue was
// empty). When specified, The T given in *output must pass the ItemVerifier()
// test and the contents of *output after the call are then also guaranteed to
// pass the ItemVerifier() test.
ABSL_MUST_USE_RESULT bool Remove(T* output) {
RTC_DCHECK(output);
RTC_DCHECK(queue_item_verifier_(*output));
// Load the value of num_elements_. Acquire memory ordering prevents reads
// and writes to queue_[next_read_index_] to be reordered to before the
// load. (That element might be accessed by a concurrent call to Insert()
// until the load finishes.)
if (std::atomic_load_explicit(&num_elements_, std::memory_order_acquire) ==
0) {
return false;
}
using std::swap;
swap(*output, queue_[next_read_index_]);
// Decrement the value of num_elements_ to account for the removed element.
// Release memory ordering prevents the reads and writes to
// queue_[next_write_index_] to be reordered to after the decrement. (Once
// the decrement has finished, Insert() might start accessing that element.)
std::atomic_fetch_sub_explicit(&num_elements_, size_t{1},
std::memory_order_release);
++next_read_index_;
if (next_read_index_ == queue_.size()) {
next_read_index_ = 0;
}
RTC_DCHECK_LT(next_read_index_, queue_.size());
return true;
}
// Returns the current number of elements in the queue. Since elements may be
// concurrently added to the queue, the caller must treat this as a lower
// bound, not an exact count.
// May only be called by the consumer.
size_t SizeAtLeast() const {
// Acquire memory ordering ensures that we wait for the producer to finish
// inserting any element in progress.
return std::atomic_load_explicit(&num_elements_, std::memory_order_acquire);
}
private:
// Verify that the queue slots complies with the ItemVerifier test. This
// function is not thread-safe and can only be used in the constructors.
bool VerifyQueueSlots() {
for (const auto& v : queue_) {
RTC_DCHECK(queue_item_verifier_(v));
}
return true;
}
// TODO(peah): Change this to use std::function() once we can use C++11 std
// lib.
QueueItemVerifier queue_item_verifier_;
// Only accessed by the single producer.
size_t next_write_index_ = 0;
// Only accessed by the single consumer.
size_t next_read_index_ = 0;
// Accessed by both the producer and the consumer and used for synchronization
// between them.
std::atomic<size_t> num_elements_{0};
// The elements of the queue are acced by both the producer and the consumer,
// mediated by num_elements_. queue_.size() is constant.
std::vector<T> queue_;
SwapQueue(const SwapQueue&) = delete;
SwapQueue& operator=(const SwapQueue&) = delete;
};
} // namespace webrtc
#endif // RTC_BASE_SWAP_QUEUE_H_