[FFmpeg-devel] MPEG-2 Acceleration Refactor

Greg Hulands ghulands
Sun Jun 17 08:46:16 CEST 2007

Hi Michael,
On 16/06/2007, at 6:52 PM, Michael Niedermayer wrote:

> Hi
> On Sat, Jun 16, 2007 at 06:44:21PM -0700, Greg Hulands wrote:
>> Based on the START/STOP_TIMER stuff showing the patch is actually
>> faster I have tidied the patch a little more and removed the function
>> prototypes to the _fast functions at the top of the file.
>> If there are any other problems from stopping this getting committed,
>> please let me know.
> yes, why is it faster now and was slower before?

John Dalgliesh, the guy who originally wrote the patch on an older  
svn version contacted me today about the patch as he saw the thread  
on it and gave me some tips on the patch and what was wrong with it.  
It turned out that my editor was pre-indenting new lines with spaces  
so I just wasn't getting why you kept marking all the cosmetic problems.

As to the speed he said the following "But by far the most important  
is to explain why the patch changes the speed of the code at all!  
e.g. The inlining isn't working and the objects are the same but it  
fits into the cache better thus is faster. I can't think of another  
reason. You really do have to prove that the inlining is working...  
or at least that two separate versions of the function are indeed  
being generated. If inlining isn't working then macros might have to  
be resorted to."

I clean built ffmpeg with -s on gcc and then did a diff of the  
symbols dumped from the mpeg12.o files. I have attached these so you  
can look at them. I am not sure if that explains why it is faster or  
not. I know I have said this before, but I am out of my depth with  
things at the moment, so if you are able to hand hold me through this  
it would be greatly appreciated. If this isn't what is needed to  
determine why it is faster, could you outline what is required and  
how to do it please.

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