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

[Roman Kennke]

This seems slightly racy because it depends on GC not being cancelled,
which might well flip over once we reach enqueue_candidate().


> 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