Re: [PATCH] speed up on find_first_bit for i386 (let compiler dothe work)

From: Steven Rostedt
Date: Thu Jul 28 2005 - 12:02:22 EST

On Thu, 2005-07-28 at 17:34 +0100, Maciej W. Rozycki wrote:

> Since you're considering GCC-generated code for ffs(), ffz() and friends,
> how about trying __builtin_ffs(), __builtin_clz() and __builtin_ctz() as
> apropriate? Reasonably recent GCC may actually be good enough to use the
> fastest code depending on the processor submodel selected.

I can change the find_first_bit to use __builtin_ffs, but how would you
implement the ffz? The clz and ctz only count the number of leading or
trailing zeros respectively, it doesn't find the first zero. Of course a
__builtin_ctz(~x) would but this might take longer than what we already
have. I'll go ahead and try it and see. But I still don't have a
decent benchmark on this. I'll start looking into the kernel and see how
it's used, and see if I can find a proper benchmark.

-- Steve

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