Re: [RFC][PATCH] Faster generic_fls

[Linus Torvalds]

On 30 Apr 2003, Falk Hueffner wrote:
> Linus Torvalds <[e-mail]> writes:
> 
> > There is _never_ any excuse to use a lookup table for something that
> > can be calculated with a few simple instructions. That's just
> > stupid.
> 
> Well, the "few simple instructions" are 28 instructions on Alpha for
> example, including 6 data-dependent branches. So I don't think it's
> *that* stupid.

You're comparing apples to oranges.

Clearly you're not going to make _one_ load to get fls, since having a 
4GB lookup array for a 32-bit fls would be "somewhat" wasteful.

So the lookup table would probably look up just the last 8 bits.

So the lookup table version is several instructions in itself, doing about
half of what the calculating version needs to do _anyway_. Including those
data-dependent branches.

		Linus