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    SubjectRe: [PATCH 2/5] bitops: compile time optimization for hweight_long(CONSTANT)
    On Mon, Feb 08, 2010 at 01:35:41AM -0800, H. Peter Anvin wrote:
    > 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.

    The popcnt-replacement part of the alternative would be as fast as
    possible since we're adding the opcode there but the slow version would
    add the additional overhead of saving/restoring the registers before
    calling the software hweight implementation. I'll do some tracing to see
    what a change like that would cost on machines which don't have popcnt.

    Let me prep another version when I get back on Wed. (currently
    travelling) with all the stuff we discussed to see how it would turn.

    Thanks,
    Boris.


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