Unrelated to your actual point, with which I agree with, the problem here is not just UB. It's that timing behaviour is not part of the language specification,

Unrelated to your actual point, with which I agree with, the problem here is not just UB. It's that timing behaviour is not part of the language specification, so compilers don't necessarily uphold it, same for other machine specific semantics, like reachability of symbols or access patterns to random addresses.