[PATCH] D65718: [LangRef] Document forward-progress requirement

[Ralf via Phabricator via llvm-commits]

RalfJung added a comment.

> A fence is not an atomic operation on its own in C++. See http://eel.is/c++draft/atomics.fences where it says "is a synchronization operation *if* [...]".

Sure -- there is no operation that is guaranteed to synchronize in all executions, they all only "synchronize //if//".
But you are using "atomic operation" and "synchronization operation" as if they were the same term here. I do not think that is the case.
http://eel.is/c++draft/atomics.fences describes fences as part of the "atomic operations library", so I'd say we can safely assume that fences are an atomic operation.

> Relaxed atomics aren't synchronization operations in C++.

Indeed relaxed atomics don't sync on their own. Just like fences. Relaxed atomics can, however, be crucial part of the synchronization between two fences. And is there any doubt that relaxed atomics are *atomic*? Because that's all it takes to count as forward progress (http://eel.is/c++draft/intro.progress):

> perform a synchronization operation or an atomic operation.

So, as long as we agree that all fences and relaxed atomic accesses are all "atomic operations", whether they synchronize does not matter for the purposes of forward progress.


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