Date: Wed, 3 Dec 2014 16:54:51 -0800 From: John-Mark Gurney <jmg@funkthat.com> To: Hans Petter Selasky <hselasky@freebsd.org> Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org Subject: Re: svn commit: r275468 - head/sys/dev/usb/controller Message-ID: <20141204005451.GM99957@funkthat.com> In-Reply-To: <201412032155.sB3LtjJN043364@svn.freebsd.org> References: <201412032155.sB3LtjJN043364@svn.freebsd.org>
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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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