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From: Andriy Gapon <avg@FreeBSD.org>
Date: Tue, 10 May 2011 15:08:13 +0000 (UTC)
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Subject: svn commit: r221741 - head/sys/sys
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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)