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From: John Baldwin <jhb@freebsd.org>
To: Jung-uk Kim <jkim@freebsd.org>
Subject: Re: svn commit: r278474 - head/sys/sys
Date: Mon, 09 Feb 2015 17:12:02 -0500
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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).

-- 
John Baldwin