The thing about speculative execution and out of order execution to remember is that you always get the expected result from a single threaded perspective. If you throw in interrupts or threads, you know you must tread cautiously in C.

Few things have made me more nervous in my career than seeing a ISR making use of C++ standard library features. When I asked the engineer how they were sure this was safe and worked under the covers, I was immediately dismissed as "the compiler takes care of all that." The project only ever worked at -O0 and was still buggy. The engineer blamed the hardware.