cntlzw

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

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

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 [off-list ref]
Genesi, Manager, Developer Relations

Scott Wood wrote:

Kevin Diggs wrote:

quoted

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
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