Date: Sun, 07 Jun 2015 17:02:32 +0200 From: Hans Petter Selasky <hps@selasky.org> To: Erich Dollansky <erichsfreebsdlist@alogt.com> Cc: "freebsd-hackers@freebsd.org" <freebsd-hackers@FreeBSD.org> Subject: Re: allow ffs & co. a binary search Message-ID: <55745D08.5000505@selasky.org> In-Reply-To: <20150607195245.62dc191f@B85M-HD3-0.alogt.com> References: <20150607081315.7c0f09fb@B85M-HD3-0.alogt.com> <5573EA5E.40806@selasky.org> <20150607195245.62dc191f@B85M-HD3-0.alogt.com>
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On 06/07/15 13:52, Erich Dollansky wrote: > Hi Hanns, > > On Sun, 07 Jun 2015 08:53:18 +0200 > Hans Petter Selasky <hps@selasky.org> wrote: > >> On 06/07/15 02:13, Erich Dollansky wrote: >>> >>> If you like the binary way I could give you the sources for the >>> complete family following your style to replace the older functions. >>> >> I think this is not the fastest way to do it. You first find the LSB, > > I never thought of this as I took the code a long, long time from > university and expanded it over time from 8 to 64 bits. > >> then you do a sumbits, which doesn't have any conditionals IF/ELSE >> and performs better with the CPU pipeline. I think the software ffs() >> is only used for platforms which doesn't have a hardware version btw: > > The code is not x86 specific. >> > This one is the same: > >> From my libmbin: >> >> int ffs(int value) >> { >> int retval = mbin_sumbits32(mbin_lsb32(value) - 1); >> if (retval == 32) >> retval = 0; >> else >> retval++; >> return (retval); >> } >> > Where is this? Should this also be in 10.1? Hi Erich, It is currently nowhere, just in my private SVN. Would you wrap up complete patch, and I can put it into Phabricator and let more people look at it. Does your patch cover both kernel and userspace? > > What I saw is that all CPUs except ARM uses the software version. Kernel too? --HPS
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