Calibrating your fear of big bad optimizing compilers

Right. The C++11/C11 memory model has just UB and doesn’t say which thread of execution experiences the UB. For use cases like kernel vs. userland, hypervisor vs. guest, Web browser UI vs. renderer, and Wasm host vs. Wasm program, you want to have things defined such that the more privileged side can’t experience UB if the less privileged side doesn’t follow the rules.