RFR: Implement flag to generate write-barriers without membars

[Aleksey Shipilev]

On 01/19/2018 05:53 PM, Roman Kennke wrote:
> I extracted this from Traversal GC because it may be useful for other situations too. It introduces
> a flag ShenandoahWBWithMembar which enables to avoid generation of the load-load-membar in the
> write-barrier. This membar is not needed when evacuation is always turned off at safepoints (e.g.
> partial).
> 
> http://cr.openjdk.java.net/~rkennke/wbwithmembar/webrev.00/

Looks good to me, assuming nothing changed since Traversal GC code that Roland reviewed :)

Minor nit: can we make the option name consistent with other selective options. E.g. we have
ShenandoahWriteBarrierRB. So this one seems to be ShenandoahWriteBarrierMembar?

Thanks,
-Aleksey