Messages in this thread | | | Date | Thu, 26 Jan 2006 11:04:25 +0100 | From | Rutger Nijlunsing <> | Subject | Re: [PATCH 8/12] generic hweight{32,16,8}() |
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[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 ---------------------------------------------------------------------- - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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