On 31/05/2019 02:51, Jan Beulich wrote:
> Algorithmically this gets us in line with current Linux, where the same
> change did happen about 13 years ago. See in particular Linux commits
> f9b4192923 ("bitops: hweight() speedup") and 0136611c62 ("optimize
> hweight64 for x86_64").
>
> Kconfig changes for actually setting HAVE_FAST_MULTIPLY will follow.
>
> Take the opportunity and change generic_hweight64()'s return type to
> unsigned int.
>
> Suggested-by: Andrew Cooper <andrew.coop...@citrix.com>
> Signed-off-by: Jan Beulich <jbeul...@suse.com>
Reviewed-by: Andrew Cooper <andrew.coop...@citrix.com>
The code in Linux could do with a bit of cleanup. Do you have patches
prepared? I do have one further suggestion
> --- a/xen/include/xen/bitops.h
> +++ b/xen/include/xen/bitops.h
> @@ -153,41 +153,54 @@ static __inline__ int get_count_order(un
>
> static inline unsigned int generic_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);
> + w -= (w >> 1) & 0x55555555;
> + w = (w & 0x33333333) + ((w >> 2) & 0x33333333);
> + w = (w + (w >> 4)) & 0x0f0f0f0f;
> +
> +#ifdef CONFIG_HAS_FAST_MULTIPLY
> + return (w * 0x01010101) >> 24;
> +#else
> + w += w >> 8;
> +
> + return (w + (w >> 16)) & 0xff;
> +#endif
This would be slightly shorter, less liable to bitrot, and IMO cleaner,
to do
if ( IS_ENABLED(CONFIG_HAS_FAST_MULTIPLY) )
w = w * 0x01010101) >> 24;
else
w += w >> 8;
return w;
which removes the #ifdef-ary fully, and in particular, avoids having
different return logic.
~Andrew
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