RFR (M): 8025813: G1: Code root marking unbalanced

[Thomas Schatzl]

Hi all,

  could I have reviews for the following change that parallelizes code
root scan on a per region's code root chunk basis instead of on the code
root set of an entire region basis.

This fixes some imbalances when scanning the code roots with some
regions containing 90% of code roots.

The idea is to keep around a current code root chunk pointer for every
region, that is claimed by the threads using a CAS.

Webrev:
http://cr.openjdk.java.net/~tschatzl/8025813/webrev.1/

Bugs:
https://bugs.openjdk.java.net/browse/JDK-8025813

Testing:
jprt

Thanks,
Thomas