RFR (M): 8129558: Coalesce dead objects during removal of self-forwarded pointers

[Thomas Schatzl]

Hi all,

  can I have reviews for the first change in a set of changes that speed
up evacuation failure?

This changeset coalesces dead objects (either really dead or already
evacuated) in the regions where evacuation failure occurred, doing the
necessary cleanup (BOT update, mark bitmap update) on the coalesced
objects.

Since in a "normal" evacuation failure >99% of objects are dead in the
failing regions because G1 slightly overestimated allowable eden size,
so this change saves a significant amount of processing time in the
self-forwarding phase.

I do not have exact numbers just for this particular change, only that
at the end after applying all the changes I have we will be looking at
evacuation failure in that "normal" case taking almost no extra time.

Also cleaned up the code a little, e.g. removing unused parameters and
variables, etc.

CR:
https://bugs.openjdk.java.net/browse/JDK-8129558

Webrev:
http://cr.openjdk.java.net/~tschatzl/8129558/webrev

Testing:
jprt, gc nightly/vm.quick tests on Aurora

Thanks,
  Thomas