[FFmpeg-devel] ALT_BITSTREAM_READER vs. A32 on ARMv4
Sat Dec 27 12:10:17 CET 2008
"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?
mans at mansr.com
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