[FFmpeg-devel] [PATCH] Faster CABAC H.264 residual decoding

[Siarhei Siamashka]

On Sunday 27 April 2008, M?ns Rullg?rd wrote:
> matthieu castet <castet.matthieu at free.fr> writes:
> > Jason Garrett-Glaser wrote:
> >> On the advice of #ffmpeg-devel I have made a version with uint8_t
> >> arrays instead of int.
> >
> > Don't forget that some cpu (arm for example) don't have native 8 bits
> > operation. Everything is done in 32 bits, and 8 bits behavior is
> > emulated with extra operation.
>
> ARM has byte load and store instructions.  All ALU operations are
> 32-bit, except for certain multiplies.  I doubt this is a problem
> here.
>
> The only recent CPU I know of that lacks byte load/store is the first
> generation of the Alpha.

Probably he just wanted to say that reading bytes has higher latency 
(+1 cycle extra) than reading ints on at least some ARM cores (ARM9).
On the other hand, indexing bytes in array does not require shifted 
offset (which may also introduce some kind of penalty). Anyway, it 
is better to just run some benchmarks and get the numbers :)

-- 
Best regards,
Siarhei Siamashka