RFR: 8313574: GenShen: Completing a global mark should also complete an old mark

[William Kemper]

On Fri, 4 Aug 2023 18:17:29 GMT, William Kemper <wkemper at openjdk.org> wrote:

> The mark context is shared, but each generation maintains a separate flag (`ShenandoahGeneration::_is_marking_complete`) to record the status of the mark. The remembered set scan will use the mark bitmap only if the old generation mark is complete. Otherwise, it walks the old regions and requires them to be parseable.
> 
> Presently, we make the heap parseable after global marking finishes, but this work can be deferred to the start of an old mark if we had global mark record completion of old marking.

Converted to draft because of failures in Dacapo suite.

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PR Comment: https://git.openjdk.org/shenandoah/pull/304#issuecomment-1666136456