Date: Mon, 09 Feb 2015 18:08:02 -0500 From: John Baldwin <jhb@freebsd.org> To: Jung-uk Kim <jkim@freebsd.org> Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org Subject: Re: svn commit: r278474 - head/sys/sys Message-ID: <5703806.fFzzT9N1y2@ralph.baldwin.cx> In-Reply-To: <54D93461.8010109@FreeBSD.org> References: <201502092103.t19L3OAn013792@svn.freebsd.org> <3501684.yzo0NMkRZd@ralph.baldwin.cx> <54D93461.8010109@FreeBSD.org>
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On Monday, February 09, 2015 05:27:45 PM Jung-uk Kim wrote: > On 02/09/2015 17:12, John Baldwin wrote: > > On Monday, February 09, 2015 04:55:52 PM Jung-uk Kim wrote: > >> On 02/09/2015 16:08, John Baldwin wrote: > >>> On Monday, February 09, 2015 09:03:24 PM John Baldwin wrote: > >>>> Author: jhb Date: Mon Feb 9 21:03:23 2015 New Revision: > >>>> 278474 URL: https://svnweb.freebsd.org/changeset/base/278474 > >>>> > >>>> Log: Use __builtin_popcnt() to implement a BIT_COUNT() > >>>> operation for bitsets and use this to implement CPU_COUNT() > >>>> to count the number of CPUs in a cpuset. > >>>> > >>>> MFC after: 2 weeks > >>> > >>> Yes, __builtin_popcnt() works with GCC 4.2. It should also > >>> allow the compiler to DTRT in userland uses of this if -msse4.2 > >>> is enabled. > >> > >> Back in 2012, when I submitted a similar patch, bde noted > >> __builtin_popcount*() cannot be used with GCC 4.2 for *kernel* > >> because it emits a library call. > >> > >> http://docs.freebsd.org/cgi/mid.cgi?20121116171923.L1135 > >> > >> FYI... > > > > Weird, I though I built a kernel with this in a tree that uses it > > in the igb(4) driver. We need a CPU_COUNT() no matter what, but if > > this emits a library call under GCC I will need to add the call. > > We could also adopt your bitcount header, though I think it is more > > consistent to keep the loop in BIT_COUNT() and use something that > > emulates popcountl() rather than directly using bitcount() in > > BIT_COUNT() (primarily because the rest of sys/bitset.h is > > structured that way: explicit loops in sys/bitset.h itself). > > I think you should back it out for now and move the discussion to arch > or hackers. I gave it up at the time but you may have better luck. :-) > > FYI, the following was the last version of my patch at the time. > > https://people.freebsd.org/~jkim/bitcount5.diff I could also just make it userland only for now? Rui wants to use it in userland. However, I can back it out if that is preferred. To be honest, I'm not sure how valuable it is at this point to expend a lot of effort to support GCC older than 3.4 (i.e. the non-builtin popcount approach). -- John Baldwin
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