Improved gcd_1 code

[Niels Möller]

Torbjorn Granlund <tg at gmplib.org> writes:

> I think a 16 byte zerotab is too small, and should be expanded.  (But if
> a tiny tab is to be used, I suppose extraction from a magic limb
> constant is better.)  Well, zerotab is unconditionally disabled now and
> a loop is used.

I don't quite remember, but I think I only tried that small zerotab,
found it wasn't an improvement.

But I'm not sure which loop you're referring to, when zerotab is
disabled, it's using plain count_trailing_zeros macro.

Another alternative for a branch-free count_trailing_zeros would be to
base it on an unrolled popcount (of (x-1) & ~x). 

> a count_trailing_zeros_gcd should be added which is to be optimised
> for small counts.

Makes sense. I wonder if that should loop, or return MIN(SOME_LIMIT,
correct ctz) leaving to the caller to loop.

Regards,
/Niels

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