volatile is not a hint, it's a requirement. You couldn't control hardware without it (compiler: "you're only writing this variable, never reading it. I'll skip those writes to speed things up". Programmer: "why isn't my program writing anything to this I/O port?")
I suggest reading the C standard, where you'll find that, despite the many words used about volatile, it doesn't really guarantee you anything at all. I especially love this sentence:
"What constitutes an access to an object that has volatile-qualified type is implementation-defined."
The behavior of I/O ports and the meaning of any attempt to access them is beyond the scope of the standards. The "requirements" that exist for volatile are simply an absurdity outside the (new in C11/C++11) threading context, since the implementation is not actually required to do anything useful.
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volatile is not a hint, it's a requirement. You couldn't control hardware without it (compiler: "you're only writing this variable, never reading it. I'll skip those writes to speed things up". Programmer: "why isn't my program writing anything to this I/O port?")
I suggest reading the C standard, where you'll find that, despite the many words used about volatile, it doesn't really guarantee you anything at all. I especially love this sentence:
The behavior of I/O ports and the meaning of any attempt to access them is beyond the scope of the standards. The "requirements" that exist for volatile are simply an absurdity outside the (new in C11/C++11) threading context, since the implementation is not actually required to do anything useful.