RFR/C: 8218922: SA: Enable best-effort implementation of live regions iteration for ZGC

[Stefan Karlsson]

Hi all,

Please review / comment on this patch to enable a best-effort live heap 
region iteration implementation in ZGC.

http://cr.openjdk.java.net/~stefank/8218922/webrev.01/
https://bugs.openjdk.java.net/browse/JDK-8218922

The SA has functionally that relies on live heap region information from 
the GCs. This is problematic when dead objects are left in the heap, and 
their classes have been unloaded.

Because of this ZGC has so far not implemented this feature. However, we 
could provide a best-effort implementation that most likely will work if 
classes are not unloaded (or class unloading is turned off), and 
otherwise might fail to fully parse and report all live heap regions.

For active, non-relocating pages the patch simply returns [start, top) 
and for pages being actively relocated, it reports regions containing 
the non-forwarded objects, the other objects are either dead or could be 
found in one of the to-regions.

With this patch I'm able to get heap histograms with ZGC.

Maybe this is enough to enable a bit more SA debugging capabilities when 
running with ZGC? What do you think, should we bring in this change?

To be able to implement this more cleanly I've restructured the live 
region collection, and pushed GC specific code into the specific GCs. 
There are some extra usage of generics to make the code a bit easier to 
read and develop.

Thanks,
StefanK