[FFmpeg-devel] ALT_BITSTREAM_READER vs. A32 on ARMv4

Måns Rullgård mans
Sat Dec 27 00:57:55 CET 2008

"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.

M?ns Rullg?rd
mans at mansr.com

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