RFR: Recompute generation accounts after fullgc [v2]

[Aleksey Shipilev]

On Thu, 30 Mar 2023 00:33:45 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

>> This fix addresses a problem found during verification following full gcs.  This recomputes the generation usage at the end of full GC as part of a 5th phase of full gc.
>> 
>> We may be able to remove this 5th phase at a later time after we find and fix whatever errors exist in generation accounting in the existing full gc implementation.
>
> Kelvin Nilsen has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Reviewer feedback: change function name to ...restore_generation_usage()

I think `phase4_compact_objects` does the "epilog" for the Full GC code, including setting the region and heap properties. Stands to reason the generation properties fixups should be there too? Also, `phase4_compact_objects` does "FullGC done: GLOBAL usage ..." logging, which in this version happens before we establish the usages for generations?

I'd propose to have the same `phase5_...` method you have here, but move everything starting from `// Reset complete bitmap...` from `phase4_...` into it.

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PR Review: https://git.openjdk.org/shenandoah/pull/235#pullrequestreview-1364563607