Improve GIF decoding performance

Source/core/platform/image-decoders/gif/GIFImageReader.h

 /* -*- Mode: C; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
 /* ***** BEGIN LICENSE BLOCK *****
  * Version: MPL 1.1/GPL 2.0/LGPL 2.1
  *
  * The contents of this file are subject to the Mozilla Public License Version
  * 1.1 (the "License"); you may not use this file except in compliance with
  * the License. You may obtain a copy of the License at
  * http://www.mozilla.org/MPL/
  *
  * Software distributed under the License is distributed on an "AS IS" basis,
@@ skipping 29 matching lines @@
 
 // Define ourselves as the clientPtr.  Mozilla just hacked their C++ callback class into this old C decoder,
 // so we will too.
 #include "core/platform/SharedBuffer.h"
 #include "core/platform/image-decoders/gif/GIFImageDecoder.h"
 #include "wtf/OwnPtr.h"
 #include "wtf/PassOwnPtr.h"
 #include "wtf/Vector.h"
 
 #define MAX_LZW_BITS          12
 #define MAX_BYTES           4097 /* 2^MAX_LZW_BITS+1 */

[Peter Kasting, 2013/08/29 23:46:25]

Of the uses of this value in this header, all but |stack| -- which you're
removing -- really should have been using 2^MAX_LZW_BITS anyway, since they want
to have an entry for each possible dictionary code, and don't care about the
length of the sequence encodable by the dictionary.  This is also true of all
but one spot in the .cpp file.

Therefore, I suggest replacing this with:

#define MAX_DICTIONARY_ENTRY_BITS 12
#define MAX_DICTIONARY_ENTRIES 4096 // 2^MAX_DICTIONARY_ENTRY_BITS

...and then using that everywhere in this header, and almost everywhere in the
.cpp file.  The exception is in prepareToDecode(), which I comment separately
about there.

[Alpha Left Google, 2013/08/30 23:25:36]

Done.

 #define MAX_COLORS           256
 #define GIF_COLORS             3

[Peter Kasting, 2013/08/29 23:46:25]

Nit: While here, can you name this BYTES_PER_COLORMAP_ENTRY?

[Alpha Left Google, 2013/08/30 23:25:36]

Done.

 
 const int cLoopCountNotSeen = -2;
 
 // List of possible parsing states.
 enum GIFState {
     GIFType,
     GIFGlobalHeader,
     GIFGlobalColormap,
     GIFImageStart,
     GIFImageHeader,
     GIFImageColormap,
     GIFImageBody,
     GIFLZWStart,
     GIFLZW,
     GIFSubBlock,
     GIFExtension,
     GIFControlExtension,
     GIFConsumeBlock,
     GIFSkipBlock,
     GIFDone,
     GIFCommentExtension,
     GIFApplicationExtension,
     GIFNetscapeExtensionBlock,
     GIFConsumeNetscapeExtension,
     GIFConsumeComment
 };
 
 struct GIFFrameContext;
+typedef Vector<unsigned char> GIFRow;

[Peter Kasting, 2013/08/29 23:46:25]

Nit: Maybe instead of declaring this here we can declare it in
GIFImageDecoder.h, so we can use it to replace the explicit Vector<unsigned
char>s that appear in that file (and the .cpp file).

[Alpha Left Google, 2013/08/30 23:25:36]

Done.

 
 // LZW decoder state machine.
 class GIFLZWContext {
     WTF_MAKE_FAST_ALLOCATED;
 public:
     GIFLZWContext(WebCore::GIFImageDecoder* client, const GIFFrameContext* frameContext)
         : codesize(0)
         , codemask(0)
         , clearCode(0)
         , avail(0)
         , oldcode(0)
         , firstchar(0)
         , bits(0)
         , datum(0)
         , ipass(0)
         , irow(0)
         , rowsRemaining(0)
-        , stackp(0)
         , rowIter(0)
         , m_client(client)
         , m_frameContext(frameContext)
     { }
 
     bool prepareToDecode();
-    bool outputRow();
+    bool outputRow(GIFRow::const_iterator rowBegin);
     bool doLZW(const unsigned char* block, size_t bytesInBlock);
     bool hasRemainingRows() { return rowsRemaining; }
 
 private:
     // LZW decoding states and output states.
     int codesize;
     int codemask;
     int clearCode; // Codeword used to trigger dictionary reset.
     int avail; // Index of next available slot in dictionary.
     int oldcode;
     unsigned char firstchar;
     int bits; // Number of unread bits in "datum".
     int datum; // 32-bit input buffer.
     int ipass; // Interlace pass; Ranges 1-4 if interlaced.
     size_t irow; // Current output row, starting at zero.
     size_t rowsRemaining; // Rows remaining to be output.
 
     unsigned short prefix[MAX_BYTES];

[Peter Kasting, 2013/08/29 23:46:25]

These three arrays don't actually need to be MAX_BYTES long; they can simply be
(2 ^ MAX_LZW_BITS) long.

[Alpha Left Google, 2013/08/30 23:25:36]

Done.

     unsigned char suffix[MAX_BYTES];
-    unsigned char stack[MAX_BYTES];
-    Vector<unsigned char> rowBuffer; // Single scanline temporary buffer.
-
-    size_t stackp; // Current stack pointer.
-    Vector<unsigned char>::iterator rowIter;
+    unsigned short suffixLength[MAX_BYTES];
+    GIFRow rowBuffer; // Single scanline temporary buffer.
+    GIFRow::iterator rowIter;
 
     // Initialized during construction and read-only.
     WebCore::GIFImageDecoder* m_client;
     const GIFFrameContext* m_frameContext;
 };
 
 // Data structure for one LZW block.
 struct GIFLZWBlock {
     WTF_MAKE_FAST_ALLOCATED;
 public:
@@ skipping 218 matching lines @@
     GIFColorMap m_globalColorMap;
     int m_loopCount; // Netscape specific extension block to control the number of animation loops a GIF renders.
 
     Vector<OwnPtr<GIFFrameContext> > m_frames;
 
     RefPtr<WebCore::SharedBuffer> m_data;
     bool m_parseCompleted;
 };
 
 #endif