Here's the thing: isalnum has never been a bottleneck in my code. The sum amount of runtime that glibc's optimizations have saved my programs is almost certainly significantly less than the amount of time I had to spend untangling glibc's bullshit when it broke. Isn't "programmer time is more expensive than computer time" the lie that we've built this whole mess on anyway?

And even if you want to use a LUT, other implementations have incorporated a LUT without this segfault issue, for instance plan 9:

https://code.9front.org/hg/plan9front/file/00397858f3d8/sys/...

https://code.9front.org/hg/plan9front/file/00397858f3d8/sys/...