RFR(M): 8155949: Support relaxed semantics in cmpxchg

[Andrew Haley]

On 05/23/2016 10:29 AM, Doerr, Martin wrote:
> here's the new webrev:
> http://cr.openjdk.java.net/~mdoerr/8155949_relaxed_cas/webrev.04/
> 
> Btw.: The jbyte version of cmpxchg can be implemented on aarch like on ppc where we emulate the byte access by a 4 byte access (lwarx/stwcx). But that should better be done in a separate change.

The problem with Atomic::cmpxchg(jint) is still there.   This is
a significant performance regression with compressed OOPs, and must
be addressed.

Andrew.