Messages in this thread | | | Date | Fri, 18 Apr 2008 16:46:26 -0700 (PDT) | From | dean gaudet <> | Subject | Re: Alternative implementation of the generic __ffs |
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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@arctic.org> 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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