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Date:      Thu, 15 Nov 2012 17:10:01 -0600
From:      Alan Cox <alc@rice.edu>
To:        Warner Losh <imp@bsdimp.com>
Cc:        "Jayachandran C." <jchandra@freebsd.org>, mips@freebsd.org
Subject:   Re: ZERO_REGION_SIZE
Message-ID:  <50A57649.9020504@rice.edu>
In-Reply-To: <DF2CB7FC-C1ED-4F4C-A39A-60B5E7FC566D@bsdimp.com>
References:  <50A53391.4080909@rice.edu> <CAJ-Vmon-v3K7012ti_L6ao3wQHfQ99Jau1kJ%2BGhumbPjAuSDww@mail.gmail.com> <50A54CCD.8070409@rice.edu> <DF2CB7FC-C1ED-4F4C-A39A-60B5E7FC566D@bsdimp.com>

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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




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