Add Mac OS X synchronized audio I/O back-end

media/base/audio_fifo.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "media/base/audio_fifo.h"
 
 #include "base/logging.h"
 
+using base::subtle::Atomic32;
+using base::subtle::NoBarrier_Store;
+using base::subtle::MemoryBarrier;
+
 namespace media {
 
 // Given current position in the FIFO, the maximum number of elements in the
 // FIFO and the size of the input; this method provides two output results:
 // |size| and |wrap_size|. These two results can then be utilized for memcopy
 // operations to and from the FIFO.
 // Under "normal" circumstances, |size| will be equal to |in_size| and
 // |wrap_size| will be zero. This case corresponding to the non-wrapping case
 // where we have not yet reached the "edge" of the FIFO. If |pos| + |in_size|
 // exceeds the total size of the FIFO, we must wrap around and start reusing
@@ skipping 14 matching lines @@
 // Updates the read/write position with |step| modulo the maximum number of
 // elements in the FIFO to ensure that the position counters wraps around at
 // the endpoint.
 static int UpdatePos(int pos, int step, int max_size) {
   return ((pos + step) % max_size);
 }
 
 AudioFifo::AudioFifo(int channels, int frames)
     : audio_bus_(AudioBus::Create(channels, frames)),
       max_frames_(frames),
-      frames_(0),
+      frames_pushed_(0),
+      frames_consumed_(0),
       read_pos_(0),
       write_pos_(0) {}
 
 AudioFifo::~AudioFifo() {}
 
+int AudioFifo::frames() const {
+  MemoryBarrier();
+  return frames_pushed_ - frames_consumed_;

[DaleCurtis, 2012/09/17 21:17:36]

Seems like this could overflow on 32-bit systems pretty easily?

[Chris Rogers, 2012/09/17 22:00:42]

It can, but the math still works out properly if either one or both overflows in
either direction.

+}
+
 void AudioFifo::Push(const AudioBus* source) {
   DCHECK(source);
   DCHECK_EQ(source->channels(), audio_bus_->channels());
 
   // Ensure that there is space for the new data in the FIFO.
   const int source_size = source->frames();
-  CHECK_LE(source_size + frames_, max_frames_);
+  CHECK_LE(source_size + frames(), max_frames_);
 
   // Figure out if wrapping is needed and if so what segment sizes we need
   // when adding the new audio bus content to the FIFO.
   int append_size = 0;
   int wrap_size = 0;
   GetSizes(write_pos_, max_frames(), source_size, &append_size, &wrap_size);
 
   // Copy all channels from the source to the FIFO. Wrap around if needed.
   for (int ch = 0; ch < source->channels(); ++ch) {
     float* dest = audio_bus_->channel(ch);
     const float* src = source->channel(ch);
 
     // Append part of (or the complete) source to the FIFO.
     memcpy(&dest[write_pos_], &src[0], append_size * sizeof(src[0]));
     if (wrap_size > 0) {
       // Wrapping is needed: copy remaining part from the source to the FIFO.
       memcpy(&dest[0], &src[append_size], wrap_size * sizeof(src[0]));
     }
   }
 
-  frames_ += source_size;
-  DCHECK_LE(frames_, max_frames());
+  // Ensure the data is *really* written before updating |frames_pushed_|.
+  MemoryBarrier();
+
+  Atomic32 new_frames_pushed = frames_pushed_ + source_size;
+  NoBarrier_Store(&frames_pushed_, new_frames_pushed);
+
+  DCHECK_LE(frames(), max_frames());
   write_pos_ = UpdatePos(write_pos_, source_size, max_frames());
 }
 
 void AudioFifo::Consume(AudioBus* destination,
                         int start_frame,
                         int frames_to_consume) {
   DCHECK(destination);
   DCHECK_EQ(destination->channels(), audio_bus_->channels());
 
   // It is not possible to ask for more data than what is available in the FIFO.
-  CHECK_LE(frames_to_consume, frames_);
+  CHECK_LE(frames_to_consume, frames());
 
   // A copy from the FIFO to |destination| will only be performed if the
   // allocated memory in |destination| is sufficient.
   CHECK_LE(frames_to_consume + start_frame, destination->frames());
 
   // Figure out if wrapping is needed and if so what segment sizes we need
   // when removing audio bus content from the FIFO.
   int consume_size = 0;
   int wrap_size = 0;
   GetSizes(read_pos_, max_frames(), frames_to_consume,
            &consume_size, &wrap_size);
 
   // For all channels, remove the requested amount of data from the FIFO
   // and copy the content to the destination. Wrap around if needed.
   for (int ch = 0; ch < destination->channels(); ++ch) {
     float* dest = destination->channel(ch);
     const float* src = audio_bus_->channel(ch);
 
     // Copy a selected part of the FIFO to the destination.
     memcpy(&dest[start_frame], &src[read_pos_], consume_size * sizeof(src[0]));
     if (wrap_size > 0) {
       // Wrapping is needed: copy remaining part to the destination.
       memcpy(&dest[consume_size + start_frame], &src[0],
              wrap_size * sizeof(src[0]));
     }
   }
 
-  frames_ -= frames_to_consume;
+  Atomic32 new_frames_consumed = frames_consumed_ + frames_to_consume;
+  NoBarrier_Store(&frames_consumed_, new_frames_consumed);
+
   read_pos_ = UpdatePos(read_pos_, frames_to_consume, max_frames());
 }
 
 void AudioFifo::Clear() {
-  frames_ = 0;
+  frames_pushed_ = 0;
+  frames_consumed_ = 0;
   read_pos_ = 0;
   write_pos_ = 0;
 }
 
 }  // namespace media