On second glance, surprisingly (or maybe not), this is not as easy to trigger as I thought. The somewhat messy interpreter loop I’m using still exhibits the problem on Clang 14 today (and I mean, I wrote a workaround for that a month ago, how much could it have changed in the meantime). Yet if I just write out the same union (every scalar type in C, basically) and make a function that assigns some things of that union type to some other things, I can’t get the bad codegen to manifest even on Clang 9.

So I’d need to do some work to make a small reproducer; then I’d have to figure out how to get access to their bug tracker, because apparently letting people register their own accounts is too easy, let alone just send mail to an address. I don’t know if I have that in me.

(On a more positive note, do you know that sometimes, if you add `default: __builtin_unreachable();` to your switch-threaded interpreter loop, Clang will look at it and replicate the dispatch at the end of each bytecode instruction, like in the standard technique for better branch prediction in handwritten assembly-language interpreters? When I noticed that I couldn’t stop imagining it going “It looks you’re writing an interpreter. Would you like some help?”.)