[Ffmpeg-devel] [PATCH]: Too much alignment assumed by H264 decoder

Martin Boehme boehme
Mon Jan 2 10:04:45 CET 2006

Mike Melanson wrote:
> David S. Miller wrote:
>> I did the fix, some folks just don't like it.  To be honest, I don't
>> think it decreases performance all as much as is being implied.
>> The fact is that the object is 4-byte, not 8-byte aligned, on 32-bit
>> platforms.  So you can't legally do 8-byte memory accesses to them
>> in such cases.
>> With the way the code is now, it crashes on 32-bit platforms other
>> than x86 and x86_64 (which do not trap on unaligned memory accesses).
>> And because the x86 and x86_64 don't trap, but instead just eat a
>> couple extra cycles to perform the unaligned memory accesses, this
>> makes me believe that the "performance penalty" of my patch is
>> possibly a myth not fact.
>     Do you have any numbers to back this up? Per my understanding, 
> unaligned accessed for MMX instructions is possible but slightly slower. 
> IIRC, unaligned access on SSE2 instructions is not possible.

It is possible, but you have to use the MOVDQU (move double quad 
unaligned) instruction instead of MOVDQA (...aligned).


Martin B?hme
Inst. f. Neuro- and Bioinformatics
Ratzeburger Allee 160, D-23538 Luebeck
Phone: +49 451 500 5814
Fax:   +49 451 500 5502
boehme at inb.uni-luebeck.de

More information about the ffmpeg-devel mailing list