RFR: enqueue barrier + some other things

[Roman Kennke]

Am 20.06.2018 um 13:44 schrieb Roland Westrelin:
> 
>> In test_evacuation_in_progress() you are not reusing the gc_state load
>> from the test_heap_stable() test. I tried doing this, but got a crash.
>> Would C2 common up the 2 loads anyway? This seems useful.
> 
> The wb and enqueue barriers are 2 unrelated things by the time expansion
> happens. -XX:+ShenandoahCommonGCStateLoads would do that.
> 
>> What I *really* would like to have is to avoid the whole
>> evac-in-progress check at all, and only emit heap-stable-test for
>> traversal, for both WBs and EQBs. Would that be possible? As follow-up?
>> I tried to hack it by short-circuiting test_evacuation_in_progress after
>> the null-check, but it did not work (of course. it's C2 ;-) ).
> 
> Sure. What would be the flag that triggers that?
> ShenandoahStoreValEnqueueBarrier?

Yes, I think under this flag would be ok.

Roman