Hans Petter Selasky wrote this message on Wed, Dec 03, 2014 at 21:55 +0000:
> Author: hselasky
> Date: Wed Dec 3 21:55:44 2014
> New Revision: 275468
> URL: https://svnweb.freebsd.org/changeset/base/275468
>
> Log:
> Optimise the bit searching loops, by quickly skipping the 16 first set
> bits if all the 16 first bits are set. This way the worst case
> searching time is reduced from 32 to 16 cycles.
You could use ffs instead:
x = ffs(~map);
if (x) {
x--;
/* normal code */
}
This has the benefit of using a single instruction on platforms that
support it (bsfl on i386), though apparently, we haven't optimized this
for all platforms... arm has a version for int, but their ffsl does
the same linear search instead of just calling ffs, or at least
detecting if sizeof(long) == sizeof(int) and calling ffs...
--
John-Mark Gurney Voice: +1 415 225 5579
"All that I will do, has been done, All that I have, has not."
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