Improve GIF decoding performance

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

Index: Source/core/platform/image-decoders/gif/GIFImageReader.h
diff --git a/Source/core/platform/image-decoders/gif/GIFImageReader.h b/Source/core/platform/image-decoders/gif/GIFImageReader.h
index 33a1bd7a4396c2da6b298d386fb71ac49d9a5dbd..4c322e3aeebfad9044da89c8a0275e32f7dee1ea 100644
--- a/Source/core/platform/image-decoders/gif/GIFImageReader.h
+++ b/Source/core/platform/image-decoders/gif/GIFImageReader.h
@@ -78,6 +78,7 @@ enum GIFState {
 };
 
 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 {
@@ -95,14 +96,13 @@ public:
         , 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; }
 
@@ -122,11 +122,9 @@ private:
 
     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;