[FFmpeg-devel] [PATCH][RFC] Lagarith Decoder.

[Reimar Döffinger]

On Wed, Aug 12, 2009 at 02:12:55PM +0200, Michael Niedermayer wrote:
> On Mon, Aug 10, 2009 at 11:42:19PM -0600, Nathan Caldwell wrote:
> > On Sat, Aug 8, 2009 at 6:32 AM, Michael Niedermayer<michaelni at gmx.at> wrote:
> > >> +/* Fast round up to least power of 2 >= to x */
> > >> +static inline uint32_t clp2(uint32_t x)
> > >> +{
> > >> +    x--;
> > >> +    x |= (x >> 1);
> > >> +    x |= (x >> 2);
> > >> +    x |= (x >> 4);
> > >> +    x |= (x >> 8);
> > >> +    x |= (x >> 16);
> > >> +    return x+1;
> > >> +}
> > >
> > > is 1<<av_log2(x) faster?
> > 
> > Might be, but it gives different results, so it's a moot point.
> 
> 2<<av_log2(x-1)
> or whatever

Well, that all depends on what input range is needed.
E.g. for 0 the documentation does not match the behaviour
for the original function (returns 0 which is not even a
power of 2).
In the worst case, you'd have to do
return x > 1 ? 2 << av_log(x - 1) : x;
I think, which has a small but still existing chance of
being faster.