Absolutely, isalnum et al are probably never going to be a bottleneck. I was curious whether it even made a difference at all, after all, the naive implementati

Absolutely, isalnum et al are probably never going to be a bottleneck. I was curious whether it even made a difference at all, after all, the naive implementation doesn't really do a lot. And I fully agree with you that this glibc code is awful. As evidenced by the Plan 9 implementation, table lookups can be done simple as well, and probably perform just the same.