ZERO_REGION_SIZE

[Warner Losh]

On Nov 15, 2012, at 3:11 PM, Patrick Kelsey wrote:

> On Thu, Nov 15, 2012 at 5:02 PM, Juli Mallett <jmallett at freebsd.org> wrote:
>> On Thu, Nov 15, 2012 at 1:07 PM, Warner Losh <imp at bsdimp.com> 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?
>>> 
>> 
>> Right.  And no widespread ctz/ffs MIPS instructions as far as I know.  We
>> could use pop/dpop on processors that support them to do non-iterative
>> ffs*, with a few additional instructions, though.
> 
> I was thinking something like this might work:
> 
> value to ffs is in r1, MSB is the index of the MSB (so, 31 or 63):
> 
> if (0 == r1) return(your_choice);
> r2 = r1 - 1;
> r2 = r1 ^ r2;
> return (MSB - clz(r2));


Turns out NetBSD has one that does exactly this...

Warner