RFR: 8337511: Implement JEP 404: Generational Shenandoah (Experimental) [v7]

[William Kemper]

On Thu, 21 Nov 2024 18:30:31 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> Yes, in single gen mode we set `MARKING` and `YOUNG_MARKING` together. See https://github.com/openjdk/shenandoah/blob/master/src/hotspot/share/gc/shenandoah/shenandoahGlobalGeneration.cpp#L81
>
> We should really either predicate it on generational mode (we already poll it a few lines below) and check for `MARKING` specifically, or assert it.

I'll make some adjustments here. Note that `OLD_MARKING` also implies (and sets) `MARKING`, and - unlike `YOUNG_MARKING` - is not exclusive with `EVACUTION` and `UPDATEREFS`. Perhaps an explanatory comment is the best option here?

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PR Review Comment: https://git.openjdk.org/jdk/pull/21273#discussion_r1857098111