RFR: 8280958: G1/Parallel: Unify marking code structure

[Thomas Schatzl]

Hi all,

  can I have reviews for this change that makes G1 and Parallel Full GC marking code structure a bit more similar both terms of structure as well as naming?

This allows easier comparison and unless we are going to really merge them easier to do changes. They are still a bit different for several reasons:
  * G1 Full GC can disable class unloading while Parallel Full GC always unloads classes
  * different code structure in calls: Parallel GC drains the task queues after every kind of root, G1 only does at the end of all roots
  * root processing between the two collectors is very different

All these are potential follow-ups.

Thanks,
  Thomas

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Commit messages:
 - Initial change

Changes: https://git.openjdk.java.net/jdk/pull/7561/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7561&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8280958
  Stats: 43 lines in 6 files changed: 18 ins; 8 del; 17 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7561.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7561/head:pull/7561

PR: https://git.openjdk.java.net/jdk/pull/7561