Re: [RFC][PATCH] Faster generic_fls

[Thomas Schlichter <schlicht@uni-mannheim.de>]

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On May 2, Daniel Phillips wrote:
> On Friday 02 May 2003 02:10, Willy Tarreau wrote:
> > At first, I thought you had coded an FFS instead of an FLS. But I
> > realized it's valid, and is fast because one half of the numbers tested
> > will not even take one iteration.
>
> Aha, and duh.  At 1 million iterations, my binary search clobbers the shift
> version by a factor of four.  At 2**31 iterations, my version also wins, by
> 20%.
>
> I should note that the iterations parameter to my benchmark program is
> deeply flawed, since it changes the nature of the input set, not just the
> number of iterations.  Fortunately, all tests I've done have been with
> 2**32 iterations, giving a perfectly uniform distribution of input values.

That is what I posted in my first message in this thread... The shift 
algorithm only works fine for uniform distributed input values... But here is 
a version that behaves better for small values, too. I don't think it will 
reach the tree version but it should be much better that the old version!

int fls_shift(int x)
{
	int bit;

	if(x & 0xFFFF0000) {
		bit = 32;
 
		while(x > 0) {
			--bit;
			x <<= 1;
		}
	} else {
		bit = 0;
 
		while(x) {
			++bit;
			x >>= 1;
		}
	}

	return bit;
}

For me this version even speeds up the uniform distributed benchmark...

> For uniformly distributed inputs the simple shift loop does well, but for
> calculating floor(log2(x)) it's much worse than the fancier alternatives. 
> I suspect typical usage leans more to the latter than the former.

If this is the case the tree version will surely be the best!

But I think this topic is not worth any further work as this is not used very 
often... So this version will be my last one!

But this was a good example how suited algorithms can speed up benchmarks ;-)

> Regards,
>
> Daniel

Best regards
  Thomas

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