Does the C++11 standard formally define acquire, r

问题:

In the C++11 standard, section 1.10/5 mentions, but does not formally define the terms acquire operation, release operation, and consume operation. It then goes on in Section 29 to use these terms to describe the actions of certain memory orderings, atomic operations, and memory fences. For instance, 29.3/1 on "Order and Consistency" states:

memory_order_release, memory_order_acq_rel, and memory_order_seq_cst: a store operation performs a release operation [emphasis added] on the affected memory location.

This type of language is repeated throughout section 29, but it bothers me a bit that all meanings for the memory_order enumerations are based on operation types that themselves do not seem to be formalized by the standard, yet must have some commonly agreed-to meaning for them to be effective as definitions.

Put a different way, if I said "A bar is a flipped foo", the concrete meaning of bar and foo are ambiguous since neither term is formally defined. Only their relative natures are defined.

Does the C++11 standard, or some other C++11 standards committee document formally define what exactly an acquire operation, release operation, etc. is, or are these simply commonly understood terms? If the latter, is there a good reference that is considered an industry standard for the meaning of these operations? I specifically ask because hardware memory consistency models are not created equal, and therefore I'm figuring there must be some commonly agreed-to reference that allows those implementing compilers, etc. to properly translate the semantics of these operations into native assembly commands.

回答1:

There's an informal summarized definition given in one of the notes:

performing a release operation on A forces prior side effects on other memory locations to become visible to other threads that later perform a consume or an acquire operation on A.

Besides that, the behavior of acquire and release operations is fully defined in 1.10, specifically in how they contribute to happens-before relationships. Any definition apart from behavior is useless.

回答2:

I don't see any formal definition of acquire/release semantics after a quick look through the standard, so my guess is that they are assumed to be commonly understood terms.

They can't define everything, after all.

It's not a definitive reference, but Raymond Chen has blogged about acquire/release semantics. His post includes a link to Microsoft's definition of acquire and release semantics, which you might also find useful.

回答3:

actually these operations are defined in section 1.10/5-12.

release/acquire pair corresponds to the happen before relationship; while release/consume pair to the dependency-ordered before relationship.

回答4:

I also take acquire/release semantics to be fairly definitive on it's own; although they're more hardware specific terms historically rather than programming terms.

But, I think section 1.10 para 5 and 6 seem to match all acquire/release semantics definitions I've read in other language standards as well as CPU definitions.

Regardless, one of the major points of C++11 was to define a modern memory model to support concurrent and multi-threaded code. I find it hard to believe they didn't get it right :)