8373203: Genshen: Non-strong reference leak in old gen

[earthling-amzn]

The generational mode for Shenandoah will collect referents for the generation being collected. For example, if we have a young reference pointing to an old referent, that young reference will be processed after we finish marking the old generation. This presents a problem for discovery.

When the young mark encounters a young reference with an old referent, it cannot discover it because old marking hasn't finished. However, if it does not discover it, the old referent will be strongly marked. This, in turn, will prevent the old generation from clearing the referent (if it even reaches it again during old marking).

To solve this, we let young reference processing discover the old reference by having it use the old generation reference processor to do so. This means the old reference processor can have a discovered list that contains young weak references. If any of these young references reside in a region that is collected, old reference processing will crash when it processes such a reference. Therefore, we add a method heal_discovered_lists to traverse the discovered lists after young evacuation is complete. The method will replace any forwarded entries in the discovered list with the forwardee.

This PR also extends whitebox testing support for Shenandoah, giving us the ability to trigger young/old collections and interrogate some properties of heaps and regions.

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Progress

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Issue

• JDK-8373203: Genshen: Non-strong reference leak in old gen (Bug - P4)

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[openjdk]

@earthling-amzn The following labels will be automatically applied to this pull request:

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[mlbridge]

Webrevs

• 03: Full - Incremental (74b7307c)
• 02: Full - Incremental (bc42a6ee)
• 01: Full - Incremental (1aaa4bfb)
• 00: Full (f8eb0cae)

[openjdk]

@earthling-amzn this pull request can not be integrated into master due to one or more merge conflicts. To resolve these merge conflicts and update this pull request you can run the following commands in the local repository for your personal fork:

git checkout fix-old-reference-processing
git fetch https://git.openjdk.org/jdk.git master
git merge FETCH_HEAD
# resolve conflicts and follow the instructions given by git merge
git commit -m "Merge master"
git push

[stefank]

I added a few style proposals to make the Shenandoah sections look more consistent with the surrounding style in the shared code.