Messages in this thread | | | Date | Fri, 18 Apr 2008 18:11:22 -0700 (PDT) | From | dean gaudet <> | Subject | Re: Alternative implementation of the generic __ffs |
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On Fri, 18 Apr 2008, Harvey Harrison wrote:
> On Fri, 2008-04-18 at 17:58 -0700, Joe Perches wrote: > > On Fri, 2008-04-18 at 17:20 -0700, dean gaudet wrote: > > > any reasonable compiler should figure out the two are the same... but i > > > really prefer spelling out the lack of dependencies of the computations by > > > breaking it out per-bit. > > > > It seems gcc 4.3 (-Os or -O2) isn't a reasonable compiler. > > > > I think this might be best: > > > > int ffs32(unsigned int value) > > { > > int x; > > > > value &= -value; > > if (!(value & 0x55555555)) > > x = 1; > > else > > x = 0; > > if (!(value & 0x33333333)) > > x |= 2; > > if (!(value & 0x0f0f0f0f)) > > x |= 4; > > if (!(value & 0x00ff00ff)) > > x |= 8; > > if (!(value & 0x0000ffff)) > > x |= 16; > > > > return x; > > } > > > > That produces the shortest assembly for me, also uses the fewest > registers.
unfortunately it kind of defeats the purpose of the original code... which is high parallelism / no-dependencies.
have you benchmarked it?
-dean
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