Messages in this thread | | | Date | Fri, 27 Jan 2006 11:10:29 +0530 | From | Balbir Singh <> | Subject | Re: [PATCH 8/12] generic hweight{32,16,8}() |
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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.
> 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.
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