Still, implementation-defined behavior is a progress over undefined
behaviour.
Even if it's not set in stone, the good thing about
implementation-defined behaviour is that it's known to be non-random.
So it can be checked, either with autoconf, or with a macro.
Would it be acceptable to have both variants of the code and decide
among them with such a check?
It feels a bit overkill for such a little function, but it is the only
way that I see to achieve both need for speed and for well-defined-ness.
I guess it won't kill the bootstrap-ubsan errors though.