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

[Balbir Singh]

On 1/27/06, Akinobu Mita <mita@xxxxxxxxxxxxxxxx> 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.

> Anyway those hweight*() functions are copied from include/linux/bitops.h:
> generic_hweight*(). So you can optimize these functions.
>

You are right, even those functions can be optimized.

Balbir
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