RFR (S): 7080389: G1: refactor marking code in evacuation pause copy closures

[John Cuthbertson]

Hi Everyone,

Can I have a couple of volunteers review these refactoring changes to 
the marking code used during evacuation pauses (both initial mark pauses 
and regular evacuation pauses when marking is active) - the change can 
be found at http://cr.openjdk.java.net/~johnc/7080389/webrev.0/.

The refactoring changes fix an issue that was seen with the code changes 
for 6486945.

During an initial mark pause, during root scanning, one thread had 
successfully forwarded an object and had started to copy it. While the 
object was being copied to its new location, another thread saw that the 
object had been forwarded and, after checking that the new location was 
unmarked, successfully marked the new location. The first thread would 
finish the copying, see that the new location was marked and skip the 
mark. The situation I ran into was that I was attempting to obtain the 
size of the new object just after it was marked (by the thread doing the 
marking) and the old object had not yet been fully copied to its new 
location.

With these refactoring changes, the thread that successfully forwards an 
object in the collection set will mark the forwardee after copying - 
allowing me to safely obtain it's size.

Testing: several runs of the GC test suite with a marking threshold of 
10 and 20%, Kitchensink, and jprt.

Thanks,

JohnC