The compilers are mostly doing the right thing (not always). The C standard specifies what is considered an effect on the outside world, i.e. file I/O and volatile accesses. For concurrent programming, there is also a memory model that specifies what other threads can see.

Here, the issue seems that compilers can reload variables. If this is a bug, then you already have a data race in your program which you can prevent with correct use of locks and/or atomics.