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

Rutger Nijlunsing
2006-01-26T10:04:25Z

[snip]
> > /*
> > * hweightN: returns the hamming weight (i.e. the number
> > * of bits set) of a N-bit word
> > */
> >
> > -#define hweight32(x) generic_hweight32(x)
> > -#define hweight16(x) generic_hweight16(x)
> > -#define hweight8(x) generic_hweight8(x)
> > +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;
>
> Similar optimizations can be applied to the routines below. Please see
> http://www-cs-faculty.stanford.edu/~knuth/mmixware.html errata and the code
> in mmix-arith.w for the complete set of optimizations and credits.
>
> http://www.jjj.de/fxt/fxtbook.pdf is another inspirational source for
> such algorithms.

Ah, the joys of bit twiddling!

http://graphics.stanford.edu/~seander/bithacks.html
...has some more.

--
Rutger Nijlunsing ---------------------------------- eludias ed dse.nl
never attribute to a conspiracy which can be explained by incompetence
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