Date: Tue, 10 May 2011 15:08:13 +0000 (UTC) From: Andriy Gapon <avg@FreeBSD.org> To: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: svn commit: r221741 - head/sys/sys Message-ID: <201105101508.p4AF8D4J049223@svn.freebsd.org>
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Author: avg Date: Tue May 10 15:08:13 2011 New Revision: 221741 URL: http://svn.freebsd.org/changeset/base/221741 Log: bitcount32: replace lengthy comment with a reference to SWAR MFC after: 5 days Modified: head/sys/sys/systm.h Modified: head/sys/sys/systm.h ============================================================================== --- head/sys/sys/systm.h Tue May 10 15:05:27 2011 (r221740) +++ head/sys/sys/systm.h Tue May 10 15:08:13 2011 (r221741) @@ -374,44 +374,8 @@ int alloc_unrl(struct unrhdr *uh); void free_unr(struct unrhdr *uh, u_int item); /* - * This is about as magic as it gets. fortune(1) has got similar code - * for reversing bits in a word. Who thinks up this stuff?? - * - * Yes, it does appear to be consistently faster than: - * while (i = ffs(m)) { - * m >>= i; - * bits++; - * } - * and - * while (lsb = (m & -m)) { // This is magic too - * m &= ~lsb; // or: m ^= lsb - * bits++; - * } - * Both of these latter forms do some very strange things on gcc-3.1 with - * -mcpu=pentiumpro and/or -march=pentiumpro and/or -O or -O2. - * There is probably an SSE or MMX popcnt instruction. - * - * I wonder if this should be in libkern? - * - * XXX Stop the presses! Another one: - * static __inline u_int32_t - * popcnt1(u_int32_t v) - * { - * v -= ((v >> 1) & 0x55555555); - * v = (v & 0x33333333) + ((v >> 2) & 0x33333333); - * v = (v + (v >> 4)) & 0x0F0F0F0F; - * return (v * 0x01010101) >> 24; - * } - * The downside is that it has a multiply. With a pentium3 with - * -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)) + * Population count algorithm using SWAR approach + * - "SIMD Within A Register". */ static __inline uint32_t bitcount32(uint32_t x)
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