RFR: Add generations to freeset [v8]

[William Kemper]

On Thu, 20 Apr 2023 19:48:07 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

>> yes.  Putting the region back into the old_collector set at rebuild time just makes work for us, because then we'll have to rediscover that it needs to be retired, and will need to retire it, and will need to adjust the bounds again.
>
> I'm adding a comment to the code to clarify.

Should we have a more restrictive test when rebuilding the freeset? Rather than including regions with non-zero allocation capacity, include only regions with capacity greater than some minimum? (Minimum LAB size, e.g.)

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PR Review Comment: https://git.openjdk.org/shenandoah/pull/250#discussion_r1173082978