Re: [RFC][PATCH] Faster generic_fls

[Linus Torvalds]

On 30 Apr 2003, Falk Hueffner wrote:
> 
> gcc 3.4 will have a __builtin_ctz function which can be used for this.
> It will emit special instructions on CPUs that support it (i386, Alpha
> EV67), and use a lookup table on others, which is very boring, but
> also faster.

Classic mistake. Lookup tables are only faster in benchmarks, they are
almost always slower in real life. You only need to miss in the cache
_once_ on the lookup to lose all the time you won on the previous one
hundred calls.

"Small and simple" is almost always better than the alternatives. I 
suspect that's one reason why older versions of gcc often generate code 
that actually runs faster than newer versions: the newer versions _look_ 
like they do a better job, but..

			Linus