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.