RFR: 8253081: G1 fails on stale objects in archived module graph in Open Archive regions

[Thomas Schatzl]

Hi all,

  can I have reviews for this change that changes the way how archive regions are managed in general and specifically by the G1 collector, fixing the crashes caused by adding the module graph into the archive in [JDK-8244778](https://bugs.openjdk.java.net/browse/JDK-8244778)?

Previously before the JDK-8244778 change, archived objects could always be assumed as live, and so the G1 collector did so, not caring about the archive region's contents at all. With JDK-8244778 however, archived objects could die, and keep stale references to objects outside of the archive regions, which obviously causes crashes when walking these objects.

With this change, open archive region contents are basically handled as any other objects; to support that, all open archive regions are now reachable via a single object array root. This hopefully also facilitates implementation in other collectors.

This allows us to remove quite a bit of special handling in G1 too; the only difference is that open archive regions will generally not be collected unless they are completely empty: we do want to profit from the sharing across VMs as much as possible.

Testing: tier1-5, one or two 6-8 runs

The appcds changes were done by @iklam. These changes are described in this document: https://wiki.openjdk.java.net/display/HotSpot/CDS+Archived+Heap+Improvements

Thanks,
  Thomas

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Commit messages:
 - Initial import

Changes: https://git.openjdk.java.net/jdk/pull/1163/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1163&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253081
  Stats: 657 lines in 32 files changed: 467 ins; 83 del; 107 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1163.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1163/head:pull/1163

PR: https://git.openjdk.java.net/jdk/pull/1163