On 04 Dec 2014, at 01:54, John-Mark Gurney <j...@funkthat.com> wrote: > > 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...
Maybe it is easier to alias ffs() and friends to __builtin_ffs(), since the compilers we support have these builtins already. -Dimitry
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