RFR: 8373720: GenShen: Count live-at-old mark using Snapshot at Beginning [v5]

[Y. Srinivas Ramakrishna]

On Wed, 17 Dec 2025 16:04:07 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

>> Live memory in old is measured as of the start of old-generation concurrent marking.
>> 
>> Memory promoted during concurrent old marking (memory above TAMS for the old-generation heap regions) and memory regions promoted in place during concurrent old marking are excluded from the total live data at start of old marking.
>
> Kelvin Nilsen has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Add some comments

LGTM 🚢

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Marked as reviewed by ysr (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/28837#pullrequestreview-3589916438