RFR: Coalesce and fill old, pinned regions after old gen final mark

[Y. Srinivas Ramakrishna]

On Wed, 6 Jul 2022 00:19:25 GMT, William Kemper <wkemper at openjdk.org> wrote:

>> If it is selected as a candidate for an old evacuation, access to it will be protected by the mark bitmap (`heap->doing_mixed_evacuations()` will be true).
>> 
>> Hmm, we might not handle that case properly. Independent of the filling dead objects issue, I can see nothing to prevent old regions with a `pin_count() > 0` from being added to a mixed collection (which would be bad for other reasons). I shall convert this PR to draft until we have a solution that handles that scenario. Perhaps we can also address Ramki's concern that regions that become 'unpinned' before the mixed collection _should_ be included.
>
> The root of the issue is that mutator threads change `pin_count`, but only the gc threads transition (or synchronize) the pin count with `pinned` or `unpinned` states.

Modulo yr concerns re the transitions into and out of `_pinned` state, I'd have thought that `_cset_pinned` could be considered the moral equivalent of not being in `_cset`. I'd expect that if an attempt were made to pin an object in a region that is in state `_cset`, the caller (mutator) would be forced to evacuate the object and then pin the destination region before the pinning call returns?

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PR: https://git.openjdk.org/shenandoah/pull/149