[sh/8u] RFR 8267561: Shenandoah: Reference processing not properly setup for outside of cycle degenerated GC

[Zhengyu Gu]

Shenandoah 8u suffers the same problem as 11u, that could slowly leak 
heap memory due to not process weak references.

11u patch applies cleanly to Shenandoah 8u, however, it does not 
compile, as APIs are slightly different between 11u and 8u, fixed as follow:

diff -r 0a4911d2aac7 
src/share/vm/gc_implementation/shenandoah/shenandoahHeap.cpp
--- a/src/share/vm/gc_implementation/shenandoah/shenandoahHeap.cpp 
Tue May 25 13:04:23 2021 -0400
+++ b/src/share/vm/gc_implementation/shenandoah/shenandoahHeap.cpp 
Tue May 25 13:27:02 2021 -0400
@@ -1647,8 +1647,8 @@
          rp->set_active_mt_degree(_heap->workers()->active_workers());

          // enable ("weak") refs discovery
-        rp->enable_discovery(true /*verify_no_refs*/);
- 
rp->setup_policy(_heap->soft_ref_policy()->should_clear_all_soft_refs());
+        rp->enable_discovery(true /*verify_no_refs*/, true);
+ 
rp->setup_policy(_heap->collector_policy()->should_clear_all_soft_refs());
        }


Shenandoah 8u webrev: 
http://cr.openjdk.java.net/~zgu/JDK-8267561-8u/webrev.00/

Test:
   hotspot_gc_shenandoah

Thanks,

-Zhengyu