[cpp-threads] Failed compare-and-swap

[Herb Sutter]

I forgot to address one detail of what Hans wrote yesterday:

I wrote:
> > Summarizing:
> >
> > The point well illustrated by these examples is that if CAS
> > returns the old value (rather than a bool) it can be used to
> > perform an atomic load. If we allow that, then it should have
> > the same default acquire semantics as a load (unless
> > overridden in the weak/relaxed atomics of course).
> >
> > Lawrence and everyone, what do you think? Now that I see
> > this, it seems obvious in retrospect, and this question must
> > have come up before with CAS semantics. What do other systems do?

Hans wrote:
> I completely agree with this.  If a CAS can be used to read the old
> value (which it can, arguably if it only has a boolean result),

Why do you think a failed CAS can be used to read the old value if you have a boolean success/fail result?

> and it
> has at least acquire semantics, then it should still have acquire
> semantics even if it fails and the write doesn't take place.
> The read still succeeds.

It seems like there are two legitimate choices:

  - CAS returns the old value, in which case a failed CAS should at least by default have acquire semantics because it's a read.

  - CAS returns only a success/fail boolean, in which case a failed CAS need not have any ordering semantics.

Does that make sense? It seems we can happily choose either, but the choice affects the return type.

Herb