Remove opaque alpha channel special case

third_party/WebKit/Source/platform/image-decoders/gif/GIFImageDecoder.cpp

 /*
  * Copyright (C) 2006 Apple Computer, Inc.  All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 139 matching lines @@
   // The loops below are almost identical. One writes a transparent pixel
   // and one doesn't based on the value of |writeTransparentPixels|.
   // The condition check is taken out of the loop to enhance performance.
   // This optimization reduces decoding time by about 15% for a 3MB image.
   if (writeTransparentPixels) {
     for (; rowBegin != rowEnd; ++rowBegin, ++currentAddress) {
       const size_t sourceValue = *rowBegin;
       if ((sourceValue != transparentPixel) &&
           (sourceValue < colorTable.size())) {
         *currentAddress = colorTableIter[sourceValue];
-      } else {
+      } else
         *currentAddress = 0;
-        m_currentBufferSawAlpha = true;
-      }
     }
   } else {
     for (; rowBegin != rowEnd; ++rowBegin, ++currentAddress) {
       const size_t sourceValue = *rowBegin;
       if ((sourceValue != transparentPixel) &&
           (sourceValue < colorTable.size()))
         *currentAddress = colorTableIter[sourceValue];
-      else
-        m_currentBufferSawAlpha = true;

[cblume, 2017/05/05 00:05:20]

I confirmed this line is making a difference and it is 100% because of it acting
like a prefetch for the cpu. I stepped through each piece and this had the
largest timing impact. I then replaced it with a __builtin_prefetch() and got
the same results.

This is because after this function returns, execution resumes in
GIFLZWContext::outputRow(), which starts to use memory around that location.

Since no where else in the code bases uses CPU prefetching (other than V8) and
we don't even do low-hanging cpu cache improvements like carefully laying out
structs, I'm going to go ahead and say now is a bad time to worry about the
effects of CPU caching. If we care, we should do that in a separate bug/CL. But
that means this is now within the noise floor.

     }
   }
 
   // Tell the frame to copy the row data if need be.
   if (repeatCount > 1)
     buffer.copyRowNTimes(xBegin, xEnd, yBegin, yEnd);
 
   buffer.setPixelsChanged(true);
   return true;
 }
 
 bool GIFImageDecoder::parseCompleted() const {
   return m_reader && m_reader->parseCompleted();
 }
 
 bool GIFImageDecoder::frameComplete(size_t frameIndex) {
   // Initialize the frame if necessary.  Some GIFs insert do-nothing frames,
   // in which case we never reach haveDecodedRow() before getting here.
   if (!initFrameBuffer(frameIndex))
     return setFailed();
 
-  if (!m_currentBufferSawAlpha)
-    correctAlphaWhenFrameBufferSawNoAlpha(frameIndex);
+  ImageFrame& buffer = m_frameBufferCache[frameIndex];
 
-  m_frameBufferCache[frameIndex].setStatus(ImageFrame::FrameComplete);
+  const GIFFrameContext* frameContext = m_reader->frameContext(frameIndex);
+  const size_t transparentPixel = frameContext->transparentPixel();
+  buffer.setHasAlpha(transparentPixel != kNotFound);
+
+  buffer.setStatus(ImageFrame::FrameComplete);
 
   return true;
 }
 
 void GIFImageDecoder::clearFrameBuffer(size_t frameIndex) {
   if (m_reader &&
       m_frameBufferCache[frameIndex].getStatus() == ImageFrame::FramePartial) {
     // Reset the state of the partial frame in the reader so that the frame
     // can be decoded again when requested.
     m_reader->clearDecodeState(frameIndex);
@@ skipping 54 matching lines @@
 
   if (!m_reader) {
     m_reader = WTF::makeUnique<GIFImageReader>(this);
     m_reader->setData(m_data);
   }
 
   if (!m_reader->parse(query))
     setFailed();
 }
 
-void GIFImageDecoder::onInitFrameBuffer(size_t frameIndex) {
-  m_currentBufferSawAlpha = false;
-}
-
 bool GIFImageDecoder::canReusePreviousFrameBuffer(size_t frameIndex) const {
   DCHECK(frameIndex < m_frameBufferCache.size());
   return m_frameBufferCache[frameIndex].getDisposalMethod() !=
          ImageFrame::DisposeOverwritePrevious;
 }
 
 }  // namespace blink