Re: bitcount32: replace lengthy comment with SWAR reference

On Sat, May 07, 2011 at 12:58:11PM +0300, Andriy Gapon wrote:
> 
> I think that the SWAR reference should be more concise and should lead an
> interested reader to more information on the topic (and more up-to-date as well).
> 
SWAR acronim seems to be a wikipedia invention. Without the abbreviation,
the trimmed down comment looks better, IMHO.

>     bitcount32: replace lengthy comment with SWAR reference
> 
> diff --git a/sys/sys/systm.h b/sys/sys/systm.h
> index 48bb33c..5218988 100644
> --- a/sys/sys/systm.h
> +++ b/sys/sys/systm.h
> _at__at_ -383,44 +383,8 _at__at_ 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)
> 
> -- 
> Andriy Gapon
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