Remove opaque alpha channel special case

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

Index: third_party/WebKit/Source/platform/image-decoders/gif/GIFImageDecoder.cpp
diff --git a/third_party/WebKit/Source/platform/image-decoders/gif/GIFImageDecoder.cpp b/third_party/WebKit/Source/platform/image-decoders/gif/GIFImageDecoder.cpp
index 307e6a4f93d4c67f6793c152bb4172390813ec37..fb18e1e4f7758cb72c496630811252211e7d9a1d 100644
--- a/third_party/WebKit/Source/platform/image-decoders/gif/GIFImageDecoder.cpp
+++ b/third_party/WebKit/Source/platform/image-decoders/gif/GIFImageDecoder.cpp
@@ -157,10 +157,8 @@ bool GIFImageDecoder::haveDecodedRow(size_t frameIndex,
       if ((sourceValue != transparentPixel) &&
           (sourceValue < colorTable.size())) {
         *currentAddress = colorTableIter[sourceValue];
-      } else {
+      } else
         *currentAddress = 0;
-        m_currentBufferSawAlpha = true;
-      }
     }
   } else {
     for (; rowBegin != rowEnd; ++rowBegin, ++currentAddress) {
@@ -168,8 +166,6 @@ bool GIFImageDecoder::haveDecodedRow(size_t frameIndex,
       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.

     }
   }
 
@@ -191,10 +187,13 @@ bool GIFImageDecoder::frameComplete(size_t frameIndex) {
   if (!initFrameBuffer(frameIndex))
     return setFailed();
 
-  if (!m_currentBufferSawAlpha)
-    correctAlphaWhenFrameBufferSawNoAlpha(frameIndex);
+  ImageFrame& buffer = m_frameBufferCache[frameIndex];
+
+  const GIFFrameContext* frameContext = m_reader->frameContext(frameIndex);
+  const size_t transparentPixel = frameContext->transparentPixel();
+  buffer.setHasAlpha(transparentPixel != kNotFound);
 
-  m_frameBufferCache[frameIndex].setStatus(ImageFrame::FrameComplete);
+  buffer.setStatus(ImageFrame::FrameComplete);
 
   return true;
 }
@@ -269,10 +268,6 @@ void GIFImageDecoder::parse(GIFParseQuery 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() !=