From mboxrd@z Thu Jan 1 00:00:00 1970 From: Rutger Nijlunsing Date: Thu, 26 Jan 2006 10:04:25 +0000 Subject: Re: [PATCH 8/12] generic hweight{32,16,8}() Message-Id: <20060126100425.GA12345@nospam.com> List-Id: References: <20060125112625.GA18584@miraclelinux.com> <20060125113206.GD18584@miraclelinux.com> <20060125200250.GA26443@flint.arm.linux.org.uk> <20060125205907.GF9995@esmail.cup.hp.com> <20060126032713.GA9984@miraclelinux.com> <20060126033613.GG11138@miraclelinux.com> <661de9470601252312m1f9c9256peb79451e49fc8662@mail.gmail.com> In-Reply-To: <661de9470601252312m1f9c9256peb79451e49fc8662@mail.gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Balbir Singh Cc: Akinobu Mita , Grant Grundler , Linux Kernel Development , linux-ia64@vger.kernel.org [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 ----------------------------------------------------------------------