enhancement of cmpxchg and copy_to_survivor for ppc64

[Andrew Haley]

On 04/18/2016 02:01 PM, Carsten Varming wrote:
> An important question is: Should the shared parts of hotspot move towards
> weaker memory models? If yes, then everybody should review code assuming
> the weaker semantics. If no, then there really isn't room for patches like
> this one :(.

This would surely be useful.  For example, the bitmap marking uses a
two-way acquire and release barrier at the moment, and I'm fairly sure
we don't need that.

I don't think this change should be #ifdef PPC64.  That disadvantages
other targets such as AArch64, to no advantage.  I understand that
moving this to shared code requires more work, but we should do at
least some of it in the JDK9 timeframe.

C++11 has a considerably greater variety of atomic memory operators
than the ones in HotSpot. Over time I believe we should migrate to
C++11-like operators in our code base. One way to do this would be to
create new operators which map in a simple way onto the standard ones.
The we can get rid of much of this inline assembly code.

Andrew.