Eliminate copies of encoded image data

third_party/WebKit/Source/platform/image-decoders/ImageDecoder.cpp

Index: third_party/WebKit/Source/platform/image-decoders/ImageDecoder.cpp
diff --git a/third_party/WebKit/Source/platform/image-decoders/ImageDecoder.cpp b/third_party/WebKit/Source/platform/image-decoders/ImageDecoder.cpp
index b660c660eb881bd73c788b7b7b1f43c765f20169..9f196505bdbaf958d5c8687c270372be4b8b26a9 100644
--- a/third_party/WebKit/Source/platform/image-decoders/ImageDecoder.cpp
+++ b/third_party/WebKit/Source/platform/image-decoders/ImageDecoder.cpp
@@ -32,65 +32,51 @@
 
 namespace blink {
 
-static unsigned copyFromSharedBuffer(char* buffer, unsigned bufferLength, const SharedBuffer& sharedBuffer, unsigned offset)
-{
-    unsigned bytesExtracted = 0;
-    const char* moreData;
-    while (unsigned moreDataLength = sharedBuffer.getSomeData(moreData, offset)) {
-        unsigned bytesToCopy = std::min(bufferLength - bytesExtracted, moreDataLength);
-        memcpy(buffer + bytesExtracted, moreData, bytesToCopy);
-        bytesExtracted += bytesToCopy;
-        if (bytesExtracted == bufferLength)
-            break;
-        offset += bytesToCopy;
-    }
-    return bytesExtracted;
-}
-
-inline bool matchesJPEGSignature(char* contents)
+inline bool matchesJPEGSignature(const char* contents)
 {
     return !memcmp(contents, "\xFF\xD8\xFF", 3);
 }
 
-inline bool matchesPNGSignature(char* contents)
+inline bool matchesPNGSignature(const char* contents)
 {
     return !memcmp(contents, "\x89\x50\x4E\x47\x0D\x0A\x1A\x0A", 8);
 }
 
-inline bool matchesGIFSignature(char* contents)
+inline bool matchesGIFSignature(const char* contents)
 {
     return !memcmp(contents, "GIF87a", 6) || !memcmp(contents, "GIF89a", 6);
 }
 
-inline bool matchesWebPSignature(char* contents)
+inline bool matchesWebPSignature(const char* contents)
 {
     return !memcmp(contents, "RIFF", 4) && !memcmp(contents + 8, "WEBPVP", 6);
 }
 
-inline bool matchesICOSignature(char* contents)
+inline bool matchesICOSignature(const char* contents)
 {
     return !memcmp(contents, "\x00\x00\x01\x00", 4);
 }
 
-inline bool matchesCURSignature(char* contents)
+inline bool matchesCURSignature(const char* contents)
 {
     return !memcmp(contents, "\x00\x00\x02\x00", 4);
 }
 
-inline bool matchesBMPSignature(char* contents)
+inline bool matchesBMPSignature(const char* contents)
 {
     return !memcmp(contents, "BM", 2);
 }
 
-PassOwnPtr<ImageDecoder> ImageDecoder::create(const SharedBuffer& data, AlphaOption alphaOption, GammaAndColorProfileOption colorOptions)
+PassOwnPtr<ImageDecoder> ImageDecoder::create(const SegmentReader& data, AlphaOption alphaOption, GammaAndColorProfileOption colorOptions)
 {
     const unsigned longestSignatureLength = sizeof("RIFF????WEBPVP") - 1;
     ASSERT(longestSignatureLength == 14);
 
     size_t maxDecodedBytes = Platform::current() ? Platform::current()->maxDecodedImageBytes() : noDecodedImageByteLimit;
 
-    char contents[longestSignatureLength];
-    if (copyFromSharedBuffer(contents, longestSignatureLength, data, 0) < longestSignatureLength)
+    // No implementation of SegmentReader will return less than 14 bytes from a read from the beginning, unless there is no more data.

[Peter Kasting, 2016/03/23 02:42:58]

Does this comment matter?  It looks like we check the returned length below
anyway, so it doesn't seem like there's a problem even if this weren't true.

[scroggo_chromium, 2016/03/24 13:59:45]

Removed.

+    const char* contents;
+    if (data.getSomeData(contents, 0) < longestSignatureLength)
         return nullptr;
 
     if (matchesJPEGSignature(contents))