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. -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
