RFR: Generational support for weak roots and references

[Zhengyu Gu]

On Thu, 22 Jul 2021 22:53:56 GMT, William Kemper <wkemper at openjdk.org> wrote:

> ### Summary
> The LRB for non-strong references is modified to permit resurrection of objects outside the generation being collected. In other words, resurrection is only blocked for unmarked objects in the generation being collected.
> 
> Each `ShenandoahGeneration` has its own reference processor instance. In some cases, a reference from the old generation may end up on the young generation discovered list if the reference points to a young referent (this would happen if the old reference is in the remembered set). However, young references that point to referents in the old generation are _not_ discovered. This has the effect of strongly marking the old generation referent. This also avoids the case of having young references on the old generation discovered list being evacuated/relocated while they wait for old generation reference processing (although we believe this case would be handled correctly by the existing update references code).

Would it be simpler to encapsulate generational info in MarkingContext for is_marked_xxx tests? e.g. for young marking context, everything in old gen returns is_marked_strong = true?

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PR: https://git.openjdk.java.net/shenandoah/pull/53