Improve GIF decoding performance

Source/core/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 116 matching lines @@
         m_reader->frameContext(index)->isHeaderDefined()) ?
         m_reader->frameContext(index)->delayTime() : 0;
 }
 
 bool GIFImageDecoder::setFailed()
 {
     m_reader.clear();
     return ImageDecoder::setFailed();
 }
 
-// FIXME: Can the intermediate |rowBuffer| be avoided?
-bool GIFImageDecoder::haveDecodedRow(size_t frameIndex, const Vector<unsigned char>& rowBuffer, size_t width, size_t rowNumber, unsigned repeatCount, bool writeTransparentPixels)
+bool GIFImageDecoder::haveDecodedRow(size_t frameIndex, Vector<unsigned char>::const_iterator rowBegin, size_t width, size_t rowNumber, unsigned repeatCount, bool writeTransparentPixels)
 {
     const GIFFrameContext* frameContext = m_reader->frameContext(frameIndex);
     // The pixel data and coordinates supplied to us are relative to the frame's
     // origin within the entire image size, i.e.
     // (frameContext->xOffset, frameContext->yOffset). There is no guarantee
     // that width == (size().width() - frameContext->xOffset), so
     // we must ensure we don't run off the end of either the source data or the
     // row's X-coordinates.
     const int xBegin = frameContext->xOffset();
     const int yBegin = frameContext->yOffset() + rowNumber;
     const int xEnd = std::min(static_cast<int>(frameContext->xOffset() + width), size().width());
     const int yEnd = std::min(static_cast<int>(frameContext->yOffset() + rowNumber + repeatCount), size().height());
-    if (rowBuffer.isEmpty() || (xBegin < 0) || (yBegin < 0) || (xEnd <= xBegin) || (yEnd <= yBegin))
+    if (!width || (xBegin < 0) || (yBegin < 0) || (xEnd <= xBegin) || (yEnd <= yBegin))
         return true;
 
     const GIFColorMap::Table& colorTable = frameContext->localColorMap().isDefined() ? frameContext->localColorMap().table() : m_reader->globalColorMap().table();
 
     if (colorTable.isEmpty())
         return true;
 
     GIFColorMap::Table::const_iterator colorTableIter = colorTable.begin();
 
     // Initialize the frame if necessary.
     ImageFrame& buffer = m_frameBufferCache[frameIndex];
     if ((buffer.status() == ImageFrame::FrameEmpty) && !initFrameBuffer(frameIndex))
         return false;
 
     const size_t transparentPixel = frameContext->transparentPixel();
-    Vector<unsigned char>::const_iterator rowBegin = rowBuffer.begin();
     Vector<unsigned char>::const_iterator rowEnd = rowBegin + (xEnd - xBegin);
     ImageFrame::PixelData* currentAddress = buffer.getAddr(xBegin, yBegin);
 
     // We may or may not need to write transparent pixels to the buffer.
     // If we're compositing against a previous image, it's wrong, and if
     // we're writing atop a cleared, fully transparent buffer, it's
     // unnecessary; but if we're decoding an interlaced gif and
     // displaying it "Haeberli"-style, we must write these for passes
     // beyond the first, or the initial passes will "show through" the
     // later ones.
@@ skipping 198 matching lines @@
 
     // Update our status to be partially complete.
     buffer->setStatus(ImageFrame::FramePartial);
 
     // Reset the alpha pixel tracker for this frame.
     m_currentBufferSawAlpha = false;
     return true;
 }
 
 } // namespace WebCore