[FFmpeg-devel] manually hacked configure options increase h264 decoding speed significantly

[madshi]

Hey guys,

recently I've noticed that ffdshow decodes faster than my
own ffmpeg dlls. After some digging I found out that ffdshow
uses a couple of custom configure settings that noticably
improve h264 decoding performance. So now I'm wondering
why these options are not activated by default. Is there a
good reason for that?

I'm testing on win32, using MingW/GCC for compiling.

Here's the list of changes I'm currently doing manually to
config.h:

#define HAVE_CMOV 1
#define HAVE_EBP_AVAILABLE 1
#define HAVE_FAST_CLZ 0
#define HAVE_FAST_CMOV 1
#define HAVE_ISATTY 0
#define HAVE_MEMALIGN 1

These are all set differently. The biggest performance
gain comes from activating HAVE_EBP_AVAILABLE,
but the others seem to help a little bit, too.

Furthermore ffdshow also uses a modified mem.c with
"__mingw_aligned_malloc", "__mingw_aligned_realloc"
and "__mingw_aligned_free". Probably the modified mem.c
is necessary for being able to set HAVE_MEMALIGN? Not
sure.

I can't claim to understand what these options do exactly
and why they help h264 decoding performance, but I thought
I should post it here and ask for feedback. If there's no
disadvantage to these modified options, maybe ffmpeg can
use these options by default?

Best regards, Mathias.