* cntlzw
@ 2008-06-03 20:06 Kevin Diggs
2008-06-03 20:11 ` cntlzw Scott Wood
0 siblings, 1 reply; 3+ messages in thread
From: Kevin Diggs @ 2008-06-03 20:06 UTC (permalink / raw)
To: linuxppc-dev
Hi,
x86 has bsf and bsr. PPC has cntlzw which I think is equivalent to bsr.
Anyone know of a sneaky algorithm to do bsf?
kevin
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: cntlzw
2008-06-03 20:06 cntlzw Kevin Diggs
@ 2008-06-03 20:11 ` Scott Wood
2008-06-06 9:53 ` cntlzw Matt Sealey
0 siblings, 1 reply; 3+ messages in thread
From: Scott Wood @ 2008-06-03 20:11 UTC (permalink / raw)
To: Kevin Diggs; +Cc: linuxppc-dev
Kevin Diggs wrote:
> Hi,
>
> x86 has bsf and bsr. PPC has cntlzw which I think is equivalent to
> bsr. Anyone know of a sneaky algorithm to do bsf?
wordsize - 1 - cntlzw(x & ~(x - 1));
-Scott
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: cntlzw
2008-06-03 20:11 ` cntlzw Scott Wood
@ 2008-06-06 9:53 ` Matt Sealey
0 siblings, 0 replies; 3+ messages in thread
From: Matt Sealey @ 2008-06-06 9:53 UTC (permalink / raw)
To: Scott Wood; +Cc: linuxppc-dev, Kevin Diggs
It's in the Compiler Writer's Guide with explanation; section 5-10.
http://www-01.ibm.com/chips/techlib/techlib.nsf/techdocs/852569B20050FF7785256996007558C6
There are lots of clever examples in there, it's definitely recommended
reading (I should make a cheat sheet of chapter 5 though so I can pin it
to my wall)
--
Matt Sealey <matt@genesi-usa.com>
Genesi, Manager, Developer Relations
Scott Wood wrote:
> Kevin Diggs wrote:
>> Hi,
>>
>> x86 has bsf and bsr. PPC has cntlzw which I think is equivalent to
>> bsr. Anyone know of a sneaky algorithm to do bsf?
>
> wordsize - 1 - cntlzw(x & ~(x - 1));
>
> -Scott
> _______________________________________________
> Linuxppc-dev mailing list
> Linuxppc-dev@ozlabs.org
> https://ozlabs.org/mailman/listinfo/linuxppc-dev
^ permalink raw reply [flat|nested] 3+ messages in thread
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