ConcurrentG1Refine uses ints for many of its members that should be unsigned types

The members:
  int _green_zone;
  int _yellow_zone;
  int _red_zone;

  int _thread_threshold_step;

of ConcurrentG1Refine should be some unsigned quantity, not signed. That will also allow removal of lots of safety "if (XY >= 0 && ...)" checks in the code.

There is a preliminary patch attached that fixes that.