"volatile" tells the compiler it is _not_ safe to optimise away any read or write, so it can't just optimise that section away at all.

> An object that has volatile-qualified type may be modified in ways unknown to the implementation or have other unknown side effects. Therefore any expression referring to such an object shall be evaluated strictly according to the rules of the abstract machine, as described in 5.1.2.3. Furthermore, at every sequence point the value last stored in the object shall agree with that prescribed by the abstract machine, except as modified by the unknown factors mentioned previously.

A compliant compiler is only free to optimise away, where it can determine there are no side-effects. But volatile in 5.1.2.3 has:

> Accessing a volatile object, modifying an object, modifying a file, or calling a function that does any of those operations are all side effects.