Does the C++ volatile keyword introduce a memory fence?

Question

I understand that volatile informs the compiler that the value may be changed, but in order to accomplish this functionality, does the compiler need to introduc

Answer 1

This is largely from memory, and based on pre-C++11, without threads. But having participated in discussions on threading in the committe, I can say that there was never an intent by the committee that volatile could be used for synchronization between threads. Microsoft proposed it, but the proposal didn't carry.

The key specification of volatile is that access to a volatile represents an "observable behavior", just like IO. In the same way the compiler cannot reorder or remove specific IO, it cannot reorder or remove accesses to a volatile object (or more correctly, accesses through an lvalue expression with volatile qualified type). The original intent of volatile was, in fact, to support memory mapped IO. The "problem" with this, however, is that it is implementation defined what constitutes a "volatile access". And many compilers implement it as if the definition was "an instruction which reads or writes to memory has been executed". Which is a legal, albeit useless definition, if the implementation specifies it. (I've yet to find the actual specification for any compiler.)

Arguably (and it's an argument I accept), this violates the intent of the standard, since unless the hardware recognizes the addresses as memory mapped IO, and inhibits any reordering, etc., you can't even use volatile for memory mapped IO, at least on Sparc or Intel architectures. Never the less, none of the comilers I've looked at (Sun CC, g++ and MSC) do output any fence or membar instructions. (About the time Microsoft proposed extending the rules for volatile, I think some of their compilers implemented their proposal, and did emit fence instructions for volatile accesses. I've not verified what recent compilers do, but it wouldn't surprise me if it depended on some compiler option. The version I checkd—I think it was VS6.0—didn't emit fences, however.)