Re: [PATCH 2/5] bitops: compile time optimization for hweight_long(CONSTANT)

[H. Peter Anvin]

On 02/08/2010 01:28 AM, Borislav Petkov wrote:
> Well, in the second version I did replace a 'call _hweightXX' with
> the actual popcnt opcode so the alternatives is only needed to do the
> replacement during boot. We might just as well do
>
> if (X86_FEATURE_POPCNT)
> 	__hw_popcnt()
> else
> 	__software_hweight()
>
> The only advantage of the alternatives is that it would save us the
> if-else test above each time we do cpumask_weight. However, the if-else
> approach is much more readable and obviates the need for all that macro
> magic and taking special care of calling c function from within asm. And
> since we do not call cpumask_weight all that often I'll honestly opt for
> alternative-less solution...

The highest performance will be gotten by alternatives, but it only make 
sense if they are inlined at the point of use... otherwise it's 
basically pointless.

	-hpa
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