[FFmpeg-devel] ALT_BITSTREAM_READER vs. A32 on ARMv4

[Mike .]

> To: ffmpeg-devel at mplayerhq.hu
> From: mans at mansr.com
> Date: Sat, 27 Dec 2008 11:10:17 +0000
> Subject: Re: [FFmpeg-devel] ALT_BITSTREAM_READER vs. A32 on ARMv4
> 
> "Mike ." <giac2000 at hotmail.com> writes:
> 
> >> To: ffmpeg-devel at mplayerhq.hu
> >> From: mans at mansr.com
> >> Date: Fri, 26 Dec 2008 23:57:55 +0000
> >> Subject: Re: [FFmpeg-devel] ALT_BITSTREAM_READER vs. A32 on ARMv4
> >> 
> >> "Mike ." <giac2000 at hotmail.com> writes:
> >> 
> >> > Hi.
> >> >
> >> > I'm decoding wma on ARMv4 using the ALT_BITSTREAM_READER.  The list
> >> > indicates that the A32_BITSTREAM_READER should be faster, however in
> >> > my testing its actually a bit slower.  Is there anything I'm missing
> >> > (I simply forced the definition in bitstream.h to one or the other and
> >> > benchmarked)?  Perhaps this is normal for ARM7TDMI?
> >> >
> >> > Also, whats different about the two readers?  I've started digging
> >> > through them but I'm not really sure why they do things differently.
> >> 
> >> Using 32-bit aligned loads is often faster than several smaller loads
> >> on architectures that do not support unaligned accesses.  Sometimes
> >> extra processing overhead required to take advantage of this kills the
> >> improvement.  The bitstream readers are unfortunately rather sensitive
> >> to specifics of the CPU, so benchmarks are usually the only accurate
> >> way to tell which is faster.
> >> 
> >> Please try all three bitstream readers and report your results.
> >> 
> >
> > Thanks for the advice.
> >
> > My results (percent real time for a 192k WMAv2 track):
> >
> > LIBMPEG2_BITSTREAM_READER: 257.3%
> >
> > ALT_BITSTREAM_READER: 257.9% 
> >
> > A32_BITSTREAM_READER: 249.4%
> 
> This looks like A32_BITSTREAM_READER is the fastest on your CPU, or am
> I misinterpreting your numbers?

100% is real time, so having higher percentages means its decoding faster.

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