GMP 4.3 multiplication performance

[Niels Möller]

Torbjorn Granlund <tg at gmplib.org> writes:

> That seems somewhat unorthodox.
>
> Why override HAVE_NATIVE_mpn_lshift with a new meaning (and what should
> defining it to empty mean, that it exists or that it does not exist?)?
>
> To what should mpn_sub_lshift default?

I don't know. How would you suggest that one writes code that wants to
use mpn_sub_lshift, but fall back to either submul_1 or lshift + sub
depending on the target machine? That's going to be the case for
practically every use of sub_lshift, and a single #ifdef per call site
is ugly enough.

My thinking was that if mpn_submul_1 is the best way to compute
sub_lshift on a particular machine, then mpn_submul_1 on that machine
can be considered as a decent native implementation of *both* submul_1
and sub_lshift.

Regards,
/Niels