Date: Wed, 5 Nov 2003 22:47:00 -0800 (PST) From: Nate Lawson <nate@root.org> To: Peter Wemm <peter@FreeBSD.org> Cc: cvs-all@FreeBSD.org Subject: Re: cvs commit: src/sys/i386/i386 mp_machdep.c Message-ID: <20031105224619.Q17086@root.org> In-Reply-To: <20031106012459.0094816A542@hub.freebsd.org> References: <20031106012459.0094816A542@hub.freebsd.org>
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On Wed, 5 Nov 2003, Peter Wemm wrote: > Modified files: > sys/i386/i386 mp_machdep.c > Log: > OK, this might be a bit silly, but add another popcnt() candidate. > > Revision Changes Path > 1.221 +6 -0 src/sys/i386/i386/mp_machdep.c > > @@ -915,6 +915,12 @@ > * -mcpu=pentiumpro and -march=pentiumpro then gcc-3.1 will use > * an imull, and in that case it is faster. In most other cases > * it appears slightly slower. > + * > + * Another variant (also from fortune): > + * #define BITCOUNT(x) (((BX_(x)+(BX_(x)>>4)) & 0x0F0F0F0F) % 255) > + * #define BX_(x) ((x) - (((x)>>1)&0x77777777) \ > + * - (((x)>>2)&0x33333333) \ > + * - (((x)>>3)&0x11111111)) > */ > static __inline u_int32_t > popcnt(u_int32_t m) Ah, the O(1) bit count, a MSFT interview question. :) -Nate
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