RFR: Elide Evac-OOM protocol when enqueuing string deduplication candidates in traversal GC

[Zhengyu Gu]

Entering evac-oom protocol everytime to enqueuing a string deduplication 
candidate in traversal GC, has high cost. SpecJVM XmlTransform benchmark 
score was -10.5% worse than no deduplication, comparing to about -4.8% 
with adaptive GC.

We can avoid evac-oom protocol by ensuring that candidates are always 
to-space oops. With following patch, the scores are inline with adaptive GC.


Webrev: 
http://cr.openjdk.java.net/~zgu/shenandoah/traversal-dedup-elide-evac-oom/webrev.00/

Test:

   hotspot_gc_shenandoah (fastdebug and release)


Thanks,

-Zhengyu