svn commit: r278474 - head/sys/sys

Jung-uk Kim jkim at FreeBSD.org
Mon Feb 9 22:27:47 UTC 2015


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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

Jung-uk Kim
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