RFR(M): 8139867: Change how startsHumongous and continuesHumongous regions work in G1.

[David Lindholm]

Hi,

Please review the following patch that changes how startsHumongous and 
continuesHumongous regions work in G1. Before this patch the top and end 
of the startsHumongous regions were set so that they covered the entire 
object, pointing into the last continousHumongous region for this 
object. So used() and capacity() of the startsHumongous region could be 
substantially larger than a normal region size.

Now each region is "self-contained", so top and end will not point 
outside the region. This makes it possible to remove many special cases 
in the code, and the model is more clear. Also, one doesn't have to 
figure out if G1CollectedHeap::heap_region_containing_raw() or 
G1CollectedHeap::heap_region_containing() should be called (only the 
latter is left in the code).


Bug: https://bugs.openjdk.java.net/browse/JDK-8139867
Webrev: http://cr.openjdk.java.net/~david/JDK-8139867/webrev.00/

Testing: Passed JPRT.


Thanks,
David