[fer]

Literally the volatile keyword in Java is to make the Java compiler aware in order to insert memory barriers. That only guarantees consistent reads or writes, but it doesn't make it thread safe (i.e. write after read), that's what atomics are for.

Also not only compilers reorder things, most processors nowadays do OoOE; even if the order from the compiler is perfect in theory, different latencies for different instruction operands may lead to execute later things earlier not to stall the CPU.

[zozbot234]

Note that this is only true of the Java/C# volatile keyword. The volatile keyword in C/C++ is solely about direct access in hardware to memory-mapped locations, for such purposes as controlling external devices; it is entirely unrelated to the C11 memory model for concurrency, does not provide the same guarantees, and should never be used for that purpose.