Semantics of VarHandle CAS methods

[Martin Buchholz]

On Thu, Jun 30, 2016 at 10:54 AM, Hans Boehm <hboehm at google.com> wrote:

>
> On Thu, Jun 30, 2016 at 7:59 AM, Martin Buchholz <martinrb at google.com>
> wrote:
> > One concrete proposal that removes a method:
> >
> > Replace
> > weakCompareAndSetAcquire and weakCompareAndSetRelease with
> > weakCompareAndSetAcquireRelease
>
> Using standard implementation recipes, the AcquireRelease version is
> significantly
> more expensive on Power and ARM, and there are many cases (e.g.
> implementing
> lock acquisition or lock release) where you only need one.  I'd be
> hesitant to do that.
>

You've mostly convinced me.

The C++ standard is not entirely clear on this, but a cas with m_o_release
does a relaxed read, and similarly cas with m_o_acquire does a relaxed
write, corresponding to our weakCompareAndSetRelease
and weakCompareAndSetAcquire.

Let's see if I can argue against my own
suggestion weakCompareAndSetAcquireRelease:
you usually read the old value before the cas, and if you want the acquire,
you generally want it then!
So the natural sequence would be:

r1 = x.getAcquire();
r2 = f(r1); // reads through r1
x.weakCompareAndSetRelease(r1, r2);