Volatile wins.

GCC calls that out [0] - volatile means things in memory may not be what they appear to be, and that there are asynchronous things happening, so something that may not appear to be possible, may become so, because volatile is a side-effect.

So about the only optimisation allowed to happen, is combining multiple references.

Clang is similar:

> The compiler does not optimize out any accesses to variables declared volatile. The number of volatile reads and writes will be exactly as they appear in the C/C++ code, no more and no less and in the same order.

[0] https://www.gnu.org/software/c-intro-and-ref/manual/html_nod...

That's cool and all if you are writing GCC or Clang dialect C, but it doesn't change the fact that it is UB in the C standard.