Re: [patch 14/44] generic hweight{64,32,16,8}()

Gabriel Paubert
2006-02-02T01:26:38Z

On Wed, Feb 01, 2006 at 06:02:38PM +0900, Akinobu Mita wrote:
>
> This patch introduces the C-language equivalents of the functions below:
>
> unsigned int hweight32(unsigned int w);
> unsigned int hweight16(unsigned int w);
> unsigned int hweight8(unsigned int w);
> unsigned long hweight64(__u64 w);
>
> In include/asm-generic/bitops/hweight.h
>
> This code largely copied from:
> include/linux/bitops.h
>
> Signed-off-by: Akinobu Mita <mita@miraclelinux.com>
> include/asm-generic/bitops/hweight.h | 54 +++++++++++++++++++++++++++++++++++
> 1 files changed, 54 insertions(+)
>
> Index: 2.6-git/include/asm-generic/bitops/hweight.h
> =================================> --- /dev/null
> +++ 2.6-git/include/asm-generic/bitops/hweight.h
> @@ -0,0 +1,54 @@
> +#ifndef _ASM_GENERIC_BITOPS_HWEIGHT_H_
> +#define _ASM_GENERIC_BITOPS_HWEIGHT_H_
> +
> +#include <asm/types.h>
> +
> +/**
> + * hweightN - returns the hamming weight of a N-bit word
> + * @x: the word to weigh
> + *
> + * The Hamming Weight of a number is the total number of bits set in it.
> + */
> +
> +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);
> +}

The first step can be implemented slightly better:

unsigned int res = w-((w>>1)&0x55555555);

as I found once on the web[1].

Several of the following steps can also be simplified
by omitting the masking when the result can't possibly
cause a carry to propagate too far.

This might also have a non negligible impact
on code size.

> +
> +static inline unsigned int hweight16(unsigned int w)
> +{
> + unsigned int res = (w & 0x5555) + ((w >> 1) & 0x5555);
> + res = (res & 0x3333) + ((res >> 2) & 0x3333);
> + res = (res & 0x0F0F) + ((res >> 4) & 0x0F0F);
> + return (res & 0x00FF) + ((res >> 8) & 0x00FF);
> +}
> +
> +static inline unsigned int hweight8(unsigned int w)
> +{
> + unsigned int res = (w & 0x55) + ((w >> 1) & 0x55);
> + res = (res & 0x33) + ((res >> 2) & 0x33);
> + return (res & 0x0F) + ((res >> 4) & 0x0F);
> +}
> +
> +static inline unsigned long hweight64(__u64 w)
> +{
> +#if BITS_PER_LONG = 32
> + return hweight32((unsigned int)(w >> 32)) + hweight32((unsigned int)w);
> +#elif BITS_PER_LONG = 64
> + u64 res;
> + res = (w & 0x5555555555555555ul) + ((w >> 1) & 0x5555555555555555ul);
> + res = (res & 0x3333333333333333ul) + ((res >> 2) & 0x3333333333333333ul);
> + res = (res & 0x0F0F0F0F0F0F0F0Ful) + ((res >> 4) & 0x0F0F0F0F0F0F0F0Ful);
> + res = (res & 0x00FF00FF00FF00FFul) + ((res >> 8) & 0x00FF00FF00FF00FFul);
> + res = (res & 0x0000FFFF0000FFFFul) + ((res >> 16) & 0x0000FFFF0000FFFFul);
> + return (res & 0x00000000FFFFFFFFul) + ((res >> 32) & 0x00000000FFFFFFFFul);
> +#else
> +#error BITS_PER_LONG not defined
> +#endif
> +}
> +
> +#endif /* _ASM_GENERIC_BITOPS_HWEIGHT_H_ */
>

Regards,
Gabriel

[1] It might be better to write the first line

unsigned res = w - ((w&0xaaaaaaaa)>>1);

but I can never remember what the C standard guarantess about
right shifts values (very little IIRC). I believe that it will
work properly on all architectures that GCC supports, however,
and that it will help on many.