> #else
quoted text
> static inline int fls64(__u64 x)
> {
>         if (x == 0)
>                 return 0;
>         return __fls(x) + 1;

That would require a polymorphic macro __fls that adapts to 32bit and 64bit
arguments.  Not good C style.

quoted text
> This is the only reason that this define exists. With another
> name it would be fine. HWEIGHT_USE_MULTIPLIER?

AFAIK it only exists because some ancient sparc chips had incredibly
slow multipliers.

quoted text
> And my feeling is that this is exactly the reason why this is
> not a good version for a generic implementation in bitops.h. But
> I don't care much.

I bet most different approaches who might be slightly
faster for larger bit strings would make the one bit 
case slower.

-Andi

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