Unconditionally preserve all marks during GC

[rkennke]

When storing the Klass* in the object header, we need to preserve all marks during GC, because otherwise we'd be loosing the Klass*.

This means that the PreservedMarks structure is repurposed as a full (reverse) forwarding table. Reverse because instead of mapping object->forwardee, and leaving the mark alone, it maps object->mark and stores the forwardee in the header, which might be a better idea because it means we can have a most compact table.

Shenandoah doesn't have this problem, except during full-GC, and ZGC has its own forwarding table.

This change will likely affect performance, but I haven't checked by how much, yet. It's probably more useful to see this in the context of actually reduced header.
Testing:

• tier1
• tier2

---

Progress

• Change must not contain extraneous whitespace
• Change must be properly reviewed

Reviewers

• Aleksey Shipilev (@shipilev - Committer)

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[mlbridge]

Webrevs

• 01: Full - Incremental (a63acf4d)
• 00: Full (0675ff94)

[shipilev]

I have questions about the asserts.

[shipilev]

Looks fine to me as experimental code.

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Unconditionally preserve all marks during GC

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[rkennke]

Looks fine to me as experimental code.

Thanks.
However, I noticed that we (will) have another problem during GC: For sliding compaction, while objects are forwarded, we will also require the Klass* at the same time e.g. for heap traversal. Which means, we cannot simply override the Klass* in the header, not even temporarily. Solutions to this could be avoiding klass-based traversal (e.g. using bitmaps, but serial/parallel GC don't use a bitmap), or an external forwarding table. If we go for the forwarding table, which is what I'm currently leaning to, we don't need the preserved-marks anymore. I need to think this through a little bit.

[rkennke]

I am closing this because it is no longer needed.