I believe volatile force the compiler to issue a real write to memory and can't be optimized away.

Except, of course, that the compiler can perfectly legally copy the variable behind the scenes and not overwrite the copy.

I mean it when I say you cannot.

I'm not sure what you mean here, if you have a volatile pointer that points to a memory buffer returned by malloc, how can the compiler prevents a write through the pointer from happening?

Edit: unless your point is a temporary copy can be spilled in memory and this copy will stay in memory and won't be overwritten?

Yes, your edit is correct.

Why not generate random data then read it? Or even a constant, then read it?

Because the compiler will optimize it out. Even if you return the random data / constant the compiler will optimize out the store to the variable and just pass it through directly.