8360288: Shenandoah crash at size_given_klass in op_degenerated

Reviewed-by: phh
Backport-of: 3b44d7bfa4d78e3ec715fce1863e052852f33180

Author: William Kemper

src/hotspot/share/gc/shenandoah/shenandoahHeap.cpp

@@ -1039,26 +1039,23 @@ void ShenandoahHeap::print_heap_regions_on(outputStream* st) const {
   }
 }
 
-void ShenandoahHeap::trash_humongous_region_at(ShenandoahHeapRegion* start) {
+size_t ShenandoahHeap::trash_humongous_region_at(ShenandoahHeapRegion* start) const {
   assert(start->is_humongous_start(), "reclaim regions starting with the first one");
-
-  oop humongous_obj = cast_to_oop(start->bottom());
-  size_t size = humongous_obj->size();
-  size_t required_regions = ShenandoahHeapRegion::required_regions(size * HeapWordSize);
-  size_t index = start->index() + required_regions - 1;
-
   assert(!start->has_live(), "liveness must be zero");
 
-  for(size_t i = 0; i < required_regions; i++) {
-    // Reclaim from tail. Otherwise, assertion fails when printing region to trace log,
-    // as it expects that every region belongs to a humongous region starting with a humongous start region.
-    ShenandoahHeapRegion* region = get_region(index --);
-
-    assert(region->is_humongous(), "expect correct humongous start or continuation");
+  // Do not try to get the size of this humongous object. STW collections will
+  // have already unloaded classes, so an unmarked object may have a bad klass pointer.
+  ShenandoahHeapRegion* region = start;
+  size_t index = region->index();
+  do {
+    assert(region->is_humongous(), "Expect correct humongous start or continuation");
     assert(!region->is_cset(), "Humongous region should not be in collection set");
-
     region->make_trash_immediate();
-  }
+    region = get_region(++index);
+  } while (region != nullptr && region->is_humongous_continuation());
+
+  // Return number of regions trashed
+  return index - start->index();
 }
 
 class ShenandoahCheckCleanGCLABClosure : public ThreadClosure {

src/hotspot/share/gc/shenandoah/shenandoahHeap.hpp

@@ -651,7 +651,7 @@ class ShenandoahHeap : public CollectedHeap {
   static inline void atomic_clear_oop(narrowOop* addr,       oop compare);
   static inline void atomic_clear_oop(narrowOop* addr, narrowOop compare);
 
-  void trash_humongous_region_at(ShenandoahHeapRegion *r);
+  size_t trash_humongous_region_at(ShenandoahHeapRegion *r) const;
 
 private:
   void trash_cset_regions();