RFR: 8310680: GenShen: In-place region promotions may fail [v3]

[William Kemper]

On Fri, 23 Jun 2023 21:08:54 GMT, William Kemper <wkemper at openjdk.org> wrote:

>> There are a few small changes here:
>> * In `op-update-refs-final`, only active regions are aged.
>> * In `op-final-roots`, a region's age may be reset outside of an aging cycle.
>> * When the field used to save `top` before promotion is set, it indicates the region has been chosen for in-place-promotion.
>
> William Kemper has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Assert that promotion conditions hold, rather than rechecking

Right - the sentinel value is meant to remove duplication of the logic. I added a comment explaining this in the code.

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PR Comment: https://git.openjdk.org/shenandoah/pull/290#issuecomment-1605035792