RFR: 8333750: GenShen: Only instantiate young/old generations in generational mode [v3]

[Kelvin Nilsen]

On Fri, 7 Jun 2024 23:39:57 GMT, William Kemper <wkemper at openjdk.org> wrote:

>> Other modes should not instantiate or touch young/old generations.
>
> William Kemper has updated the pull request incrementally with three additional commits since the last revision:
> 
>  - Make age and youth const
>  - Do not age regions after completing old mark
>  - Detect cancellation of old GC after final mark.

Thanks for this refactoring.  All much deserved and important.  I wonder if the title of this PR should be a bit more general, since the refactoring appears to be more far reaching than just not instantiating young/old generations in non-generational mode.

src/hotspot/share/gc/shenandoah/shenandoahOldGC.cpp line 119:

> 117:     return false;
> 118:   }
> 119: 

This looks like it might be a bug fix, vs the rest of this PR, which is mostly isolation refactoring.  Should the bug fix be separated out into a different PR?

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Marked as reviewed by kdnilsen (Committer).

PR Review: https://git.openjdk.org/shenandoah/pull/444#pullrequestreview-2108335069
PR Review Comment: https://git.openjdk.org/shenandoah/pull/444#discussion_r1633573810