Comment by jstimpfle

I understand, that's why I said the code is obviously broken. The problem is not about order of evaluation. It's not about an UB arising from unsequenced volatile reads or whatever.

The problem is simply that the there are two volatile reads where only one was intended. It doesn't matter if there is UB or not. The code doesn't express the intention either way. All you need to know to understand that is that volatile might be modified concurrently (a little bit similar but not the same semantics as atomics).