Messages in this thread | | | Subject | Re: [RFC][PATCH] Faster generic_fls | From | Falk Hueffner <> | Date | 30 Apr 2003 18:43:19 +0200 |
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Linus Torvalds <torvalds@transmeta.com> writes:
> 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_.
Right.
> Including those data-dependent branches.
Well, at least on Alpha, gcc can optimize them all away except one.
Note I'm not really arguing Linux should use a lookup table for fls; I'm just arguing putting it as default into libgcc for architectures that don't provide a special version (which aren't particularly many) isn't stupid. But probably I'm just biased, because I put it there :)
-- Falk - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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