Messages in this thread | | | Date | Wed, 30 Apr 2003 09:16:04 -0700 (PDT) | From | Linus Torvalds <> | Subject | Re: [RFC][PATCH] Faster generic_fls |
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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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