On 30 Apr 2003, Falk Hueffner wrote:
> Linus Torvalds <torvalds@transmeta.com> 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
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This archive was generated by hypermail 2b29 : Wed Apr 30 2003 - 22:00:35 EST