[FFmpeg-devel] ALT_BITSTREAM_READER vs. A32 on ARMv4

[Måns Rullgård]

"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?

-- 
M?ns Rullg?rd
mans at mansr.com