From owner-freebsd-mips@FreeBSD.ORG  Thu Nov 15 23:09:51 2012
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Date: Thu, 15 Nov 2012 17:10:01 -0600
From: Alan Cox <alc@rice.edu>
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To: Warner Losh <imp@bsdimp.com>
Subject: Re: ZERO_REGION_SIZE
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Cc: "Jayachandran C." <jchandra@freebsd.org>, mips@freebsd.org
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On 11/15/2012 3:07 PM, Warner Losh wrote:
> On Nov 15, 2012, at 1:13 PM, Alan Cox wrote:
>> P.S. I would encourage someone with hardware to look into implementing a
>> non-iterative ffs*() using (d)clz.  The MIPS pmap would benefit from
>> this.  Basically, most pmap_enter() calls are doing an ffs*().
> ffs finds the first bit set. clz counts the number of leading zeros and thus finds the last bit set.  Would a non-iterative fls* be helpful?

The code could probably use fls* in place of ffs*.  In any case, there's 
a standard trick for using (d)clz to implement a non-iterative ffs*.

Alan