> > If it's okay (or desirable) to have the result in W as well as F, it can be
> > done safely in 4 cycles:
> >
> >         movlw   1
> >         btfss   scanbit,7
> >          rlf    scanbit,w
> >         movwf   scanbit
>
> Another variant.
>
>         movfw   scanbit
>         addwf   scanbit,f
>         skpnz
>         bsf     scanbit,0
>
> After this W contain previous scan and scanbit contain next scan.

Hmm... this has the same problem that the first four-cycler given before
had: if scanbit holds something other than a power of two, running the
routine repeatedly won't fix that.  On the other hand, looking at mine
above it's just plain broken.  Sigh.  Maybe too much egg nog.