Correct usage of volatile with std::atomic_ref<T>

Question

I'm having trouble wrapping my head around the correct usage of std::atomic_ref<int> with volatile.

Naively there are three possibilities:

std::atomic_ref<volatile int> ref1;
volatile std::atomic_ref<int> ref2;
volatile std::atomic_ref<volatile int> ref3;

When and do we want to use each one? The use-case I'm interested in is MMIO.

Answer 1

Unlike std::atomic<T>, std::atomic_ref<T> does not have volatile-qualified methods. So, you probably can't do much with a volatile std::atomic_ref<T> (whether T is itself volatile or not).

This makes sense given the quote

Like language references, constness is shallow for atomic_ref - it is possible to modify the referenced value through a const atomic_ref object.

Assuming cv-qualification is somewhat consistent, a shallowly-volatile atomic_ref is unlikely to be useful, and definitely isn't what you're asking for.

So, you want

std::atomic_ref<volatile int>

Note that it may be sufficient to just use std::atomic_ref<int>, but since the standard doesn't make any explicit guarantees about MMIO, you should probably consult your compiler documentation and/or check the code it generates.

Depending on std::atomic in this way is at least not portable. Specifically, this answer and its linked paper mention some ways in which std::atomic may be inadequate - you can check whether these are actual problems for you.

---

Follow-up to the main comment chain, which confuses MMIO and mmap'd IPC: these are completely different.

• Atomics control how loads & stores interact with the processor's native memory & coherence model.

  IPC happens entirely within the CPU & its memory controller, so atomics are sufficient.

  This is what OP actually wanted, and has no need for volatile.

• MMIO (as originally requested) is about addressing I/O devices outside the purview of the processor's coherence model.

  It isn't real memory, doesn't behave like real memory, and you may need volatile.

  This is what OP asked for, but not what they wanted.