gcd_11 asm

[Niels Möller]

Hi,

I've had a look at the latest gcd_11 asm, and it's really neat,
including naturally getting %rdx zero on return.

One question: the bd2 and bd4 versions use

L(top): rep;bsf %rdx, %rcx  C tzcnt!

I've not seen this before, but a quick web search indicates that tzcnt
is the same as bsf, except that it has a well defined result also when
the input is zero. But in these loops, we should get to this instruction
only for non-zero %rdx. So are there any other subtleties?

Regards,
/Niels

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