Re: [PATCH 8/12] generic hweight{32,16,8}()

Akinobu Mita
2006-01-27T06:40:34Z

On Fri, Jan 27, 2006 at 11:10:29AM +0530, Balbir Singh wrote:
> On 1/27/06, Akinobu Mita <mita@miraclelinux.com> wrote:
> > On Thu, Jan 26, 2006 at 12:42:09PM +0530, Balbir Singh wrote:
> >
> > > > +static inline unsigned int hweight32(unsigned int w)
> > > > +{
> > > > + unsigned int res = (w & 0x55555555) + ((w >> 1) & 0x55555555);
> > > > + res = (res & 0x33333333) + ((res >> 2) & 0x33333333);
> > > > + res = (res & 0x0F0F0F0F) + ((res >> 4) & 0x0F0F0F0F);
> > > > + res = (res & 0x00FF00FF) + ((res >> 8) & 0x00FF00FF);
> > > > + return (res & 0x0000FFFF) + ((res >> 16) & 0x0000FFFF);
> > > > +}
> > > > +
> > >
> > > This can be replaced with
> > >
> > > register int res=w;
> > > res=res-((res>>1)&0x55555555);
> > > res=(res&0x33333333)+((res>>2)&0x33333333);
> > > res=(res+(res>>4))&0x0f0f0f0f;
> > > res=res+(res>>8);
> > > return (res+(res>>16)) & 0xff;
> >
> > Probably you are right.
> > Unfortunately, it is difficult for me to prove that sane equivalence.
> >
>
> Well, a proof is not difficult. This is a well tested proven piece of
> code published by Don Knuth. If you need a proof, I can provide one.

Thanks, I want.