RFR(M): 8154736: enhancement of cmpxchg and copy_to_survivor for ppc64

[Kim Barrett]

> On May 29, 2018, at 6:30 AM, Doerr, Martin <martin.doerr at sap.com> wrote:
> 
> Hi Kim,
> 
> I'm trying to understand how this is related to Michihiro's change. The else path of the initial test is not affected by it AFAICS.

The value being tested and the forwardee obtained in the else branch
are the result of some earlier CAS, e.g. some CAS (in this thread or
some other) has already won the race and the result is seen here.
Weakening the CAS fencing applies to that earlier CAS too, and so
affects this code path.