gh-91851: Trivial optimizations in Fraction

[skirpichev]

NB: this is the first commit from #25518

• Issue: Optimize the Fraction class arithmetics for small components #91851

[hauntsaninja]

Thanks! Microbenchmarks confirm that this is about 2x as fast.

cc @JelleZijlstra

[mdickinson]

LGTM.

That's a pretty clunky implementation of round-ties-to-even in the ndigits is None case. Would something like the following be faster?

d = self._denominator
rounded, remainder = divmod(self._numerator + (d >> 1), d)
if not remainder and not (d & 1):  # tie case
    rounded &= -2
return rounded

EDIT: A bit more streamlined, and avoiding unnecessary not operations:

d = self._denominator
rounded, remainder = divmod(self._numerator + (d >> 1), d)
return rounded if remainder or d & 1 else rounded & -2

[mdickinson]

Would something like the following be faster?

Turns out not, at least on my machine.