--- a/dom/media/webaudio/AudioBufferSourceNode.cpp
+++ b/dom/media/webaudio/AudioBufferSourceNode.cpp
@@ -233,16 +233,31 @@ public:
     for (uint32_t i = 0; i < aChannels; ++i) {
       float* baseChannelData = aOutput->ChannelFloatsForWrite(i);
       memcpy(baseChannelData + aOffsetWithinBlock,
              mBuffer->GetData(i) + mBufferPosition,
              aNumberOfFrames * sizeof(float));
     }
   }
 
+  // This has the same function as CopyFromInputBuffer
+  // but the input buffer is copied in reverse order
+  void ReverseCopyFromInputBuffer(AudioBlock* aOutput,
+                                  uint32_t aChannels,
+                                  uintptr_t aOffsetWithinBlock,
+                                  uint32_t aNumberOfFrames) {
+    for (uint32_t i = 0; i < aChannels; ++i) {
+      float* baseChannelData = aOutput->ChannelFloatsForWrite(i) + aOffsetWithinBlock;
+      const float* bufferData = mBuffer->GetData(i);
+      for (uint32_t j = 0; j < aNumberOfFrames; ++j) {
+        baseChannelData[j] = bufferData[mBufferPosition - j - 1];
+      }
+    }
+  }
+
   // Resamples input data to an output buffer, according to |mBufferSampleRate| and
   // the playbackRate/detune.
   // The number of frames consumed/produced depends on the amount of space
   // remaining in both the input and output buffer, and the playback rate (that
   // is, the ratio between the output samplerate and the input samplerate).
   void CopyFromInputBufferWithResampling(AudioBlock* aOutput,
                                          uint32_t aChannels,
                                          uint32_t* aOffsetWithinBlock,