RFR: Fix Region Promotion

[William Kemper]

On Wed, 5 May 2021 22:39:17 GMT, William Kemper <wkemper at openjdk.org> wrote:

>> src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp line 1293:
>> 
>>> 1291:       r->set_young_lab_flag();
>>> 1292:     }
>>> 1293:   }
>> 
>> Ah ok I see you handle TLABs here. Maybe move this into the ShenandoahTagGCLABClosure? What about PLABs?
>
> I think we aren't worried about PLABs here because they should only exist in old regions (i.e., ones that have already been promoted). I'll refactor this to not loop over the threads twice and add an assert that the thread's plab exists in an old region.

On second look, the `GCLABClosure` is only visiting the worker threads (they won't have TLABs).

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PR: https://git.openjdk.java.net/shenandoah/pull/36