RFR: 8141637: Parallelize single threaded heap region iteration during Pre Evacuate Collection Set

[Stefan Johansson]

Hi,

Please review this fix to parallelize parts of the G1 young gc preparation.

Issue: https://bugs.openjdk.java.net/browse/JDK-8141637
Webrev: http://cr.openjdk.java.net/~sjohanss/8141637/00/

Summary
To sub-phases of the "Pre Evacuate Collection Set" phase, "Region 
Register" and "Prepare Heap Roots" previously did single threaded 
iteration of all regions in the heap. For heaps with a lot of regions 
this become a problem so this change groups these iterations into one 
task that is executed in parallel.

Testing
Manual performance testing show good results on large heaps and no 
problem on small heaps. Functional tests using mach5 tier 1-4.

Thanks,
Stefan