[cpp-threads] SC on PPC

[Nelson, Clark]

> So your proposal is to have causality
> be triggered by release but not acquire? Or do you need both 
> acquire and releases
> to trigger causality? Do you foresee this changing on the 
> next version of the model?
> 
> I realize that this is not so interesting if all you have is
> acquire/release
> operations, but once you include relaxed operations (and fences) it is
> important
> to clearly define how they interact with each other.

My apologies if I'm misunderstanding your point, but you seem to be
saying something with which I would strongly disagree. So just in case
you are....

It is *not* important for the standard to clearly define how every
possible operation interacts with every other possible operation. What
*is* important is to specify rules for how portable multi-threading code
can be written. If, for example, the rules were to say it is necessary
to use both acquire and release to properly communicate between threads,
it could just say that failing to use both results in undefined
behavior. The exact behavior of improperly written code doesn't need to
be specified.

Clark