RFR: Improve humongous remset scan

[Kelvin Nilsen]

On Tue, 14 Jun 2022 20:09:14 GMT, Kelvin Nilsen <kdnilsen at openjdk.org> wrote:

> These changes allow humongous objects residing within the remembered set to be scanned by muitiple concurrent GC worker threads, allowing more efficient parallelization of the remembered set scanning effort.

src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp line 2471:

> 2469:                 HeapWord* start_of_scan = r->bottom();
> 2470:                 size_t card_size_in_words = CardTable::card_size_in_words();
> 2471:                 size_t words_to_scan = end_of_object - start_of_scan;

With some "last minute" editing, I introduced an error here.  This code should scan to end of region or end of object, whichever comes first.  William is correct to point out that we are rescanning portions of humongous objects multiple times.

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PR: https://git.openjdk.org/shenandoah/pull/144