RFR: Add generations to freeset [v8]

[Kelvin Nilsen]

On Fri, 21 Apr 2023 18:34:07 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

>> What you suggest might be slightly more efficient, but the code is probably cleaner if we just remove "recompute_bounds()" and compute the bounds incrementally as part of the rebuild() and reserve() process.  This happens only twice per GC cycle (at start of evac, and after update-refs).  In the next revision of this PR, I will offer the "simplified" API.
>
> Actually, I did not remove recompute_bounds() because we currently depend on a side effect of this function, which is to decide whether to allocate old_colllector regions left-to-right or right-to-left.  I've placed a comment in the code to clarify a future TODO action.  Is it ok to leave as is for now?  (Or should I try to sort this out right now?)

In the very latest review, I've removed recompute bounds entirely.  The only thing we do at the end of rebuild is decide whether to use right-to-left or left-to-right bias for allocation from OldCollector set.

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PR Review Comment: https://git.openjdk.org/shenandoah/pull/250#discussion_r1180443071