Messages in this thread |  | | | Subject | Re: Alternative implementation of the generic __ffs | | From | Joe Perches <> | | Date | Fri, 18 Apr 2008 17:58:44 -0700 | |
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;
}
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