Re: Alternative implementation of the generic __ffs

[dean gaudet]

On Fri, 18 Apr 2008, Alexander van Heukelum wrote:

> On Mon, Apr 07, 2008 at 12:25:50PM +0200, Alexander van Heukelum wrote:
> > On Sun, 6 Apr 2008 13:22:58 -0700 (PDT), "dean gaudet" <dean@xxxxxxxxxx> said:
> > > On Sun, 6 Apr 2008, Alexander van Heukelum wrote:
> > > > The current generic implementation of ffz is O(lg(n)) already
> > > 
> > > it's O(lg(n)) time... the operations all depend on each other.
> > > 
> > > the implementation i pointed to is O(lg(n)) code space... and the time 
> > > depends on how parallel the machine is, they're not dependent on each 
> > > other.
> > 
> > Indeed. The worst dependencies are in the sum of all the partial
> > results in this implementation. And addition is associative, so
> > partial results can be written as ((a+b)+(c+d))+(e+f). Assuming
> > perfect parallel execution this would lead to O(ln(ln(n))). Good.
> 
> Hello all,
> 
> I've implemented ffs (find first set bit) like it is shown
> in http://www.hackersdelight.org/ (see revisions, page 21).

sweet!  thanks for doing this.


> static ATTR int __ffs32_new(unsigned int value)
> {
> 	int x0, x1, x2, x3, x4;
> 
> 	value &= -value;
> 	x0 = (value & 0x55555555) ? 0 : 1;
> 	x1 = (value & 0x33333333) ? 0 : 2;
> 	x2 = (value & 0x0f0f0f0f) ? 0 : 4;
> 	x3 = (value & 0x00ff00ff) ? 0 : 8;
> 	x4 = (value & 0x0000ffff) ? 0 : 16;

technically you can compute x4 with the original value prior to isolating 
the least-significant one-bit -- the compiler probably can't figure this 
out on its own though, so it's probably worth hoisting it manually.


> 	return x0 | x1 | x2 | x3 | x4;

i'm never sure if it's better to use | or + here... i bet it depends on 
what native operations the processor has... and depends on how ?: are 
implemented.

-dean
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