Re: [Gnash-dev] Optimization to jpeg decoding

[Markus Gothe]

I'm not sure however counting down to zero instead of upwards would be  
feasible (saving, on x86, one instruction for every iteration).

//Markus

On 27 Nov 2008, at 02:32, dolphinling wrote:

> Hitachi's animation The Hard Drive is the New Bling  
> (Bling_Final.swf, available from e.g. http://dagobah.biz/flash/Bling_Final.swf) 
>  plays slowly in gnash. It contains a small number of very large  
> jpegs (up to 3422x2516 px), and a profile shows that there are over  
> 8 million calls to JpegImageInput::getWidth, nearly 2% of the total  
> cpu time (in a debug build with video and audio output disabled).
>
> All but 32 of those 8 million calls are from  
> ImageInput::readSWFJpeg3. This patch simply stores the result of  
> getWidth within readSWFJpeg3.
>
> This has not been run through make check--I don't have all the  
> packages for that. It has been tested on Bling_Final.swf, though,  
> and works fine.
>
> Jpeg decoding as a whole takes up just barely under 50% of the total  
> time (again, debug build run with -r0 so no output) so there may be  
> other improvements that could be made in the area. In fact,  
> boost::scoped_array<unsigned char>::operator[] is called nearly 25  
> million times from readSWFJpeg3, for 5.67% of the run time, but I  
> don't see a trivial way to eliminate those calls like with getWidth.
>
> Any comments or questions of course welcome.
>
> --
> dolphinling
> <http://dolphinling.net/>
> === modified file 'libbase/GnashImage.cpp'
> --- libbase/GnashImage.cpp      2008-10-27 16:05:13 +0000
> +++ libbase/GnashImage.cpp      2008-11-27 00:42:16 +0000
> @@ -20,7 +20,7 @@
> // Based on the public domain work of Thatcher Ulrich  
> <address@hidden> 2002
>
> #include <cstring>
> -#include <memory>                // for auto_ptr
> +#include <memory> // for auto_ptr
> #include <boost/scoped_array.hpp>
> #include <boost/shared_ptr.hpp>
>
> @@ -140,7 +140,7 @@
>     assert(x < _width);
>     assert(y < _height);
>
> -    boost::uint8_t*    data = scanline(y) + 4 * x;
> +    boost::uint8_t* data = scanline(y) + 4 * x;
>
>     data[0] = r;
>     data[1] = g;
> @@ -301,18 +301,23 @@
>     assert(j_in.get());
>
>     j_in->read();
> +
> +    // For large images, getWidth can be called millions of times,  
> so cache it
> +    // here. Cache getHeight for symmetry.
> +    const size_t j_in_width = j_in->getWidth();
> +    const size_t j_in_height = j_in->getHeight();
>
> -    im.reset(new ImageRGBA(j_in->getWidth(), j_in->getHeight()));
> +    im.reset(new ImageRGBA(j_in_width, j_in_height));
>
>     boost::scoped_array<boost::uint8_t> line (
> -            new boost::uint8_t[3 * j_in->getWidth()]);
> +            new boost::uint8_t[3 * j_in_width]);
>
> -    for (size_t y = 0; y < j_in->getHeight(); y++)
> +    for (size_t y = 0; y < j_in_height; y++)
>     {
>         j_in->readScanline(line.get());
>
> -        boost::uint8_t*        data = im->scanline(y);
> -        for (size_t x = 0; x < j_in->getWidth(); x++)
> +        boost::uint8_t* data = im->scanline(y);
> +        for (size_t x = 0; x < j_in_width; x++)
>         {
>             data[4*x+0] = line[3*x+0];
>             data[4*x+1] = line[3*x+1];
>
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