8386287: GenShen: assert(starts_object(card_index)) failed: Can't get last start because no object starts here

Reviewed-by: kdnilsen
Backport-of: 1251526588314059c6c0bc132c1a052ef04c83a6

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

@@ -144,7 +144,7 @@ void ShenandoahGenerationalFullGC::account_for_region(ShenandoahHeapRegion* r, s
 void ShenandoahGenerationalFullGC::maybe_coalesce_and_fill_region(ShenandoahHeapRegion* r) {
   if (r->is_pinned() && r->is_old() && r->is_active() && !r->is_humongous()) {
     r->begin_preemptible_coalesce_and_fill();
-    r->oop_coalesce_and_fill(false);
+    r->oop_coalesce_and_fill(/* cancellable = */ false, /* do_card_table_updates = */ false);
   }
 }
 

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

@@ -452,7 +452,7 @@ void ShenandoahHeapRegion::print_on(outputStream* st) const {
 }
 
 // oop_iterate without closure, return true if completed without cancellation
-bool ShenandoahHeapRegion::oop_coalesce_and_fill(bool cancellable) {
+bool ShenandoahHeapRegion::oop_coalesce_and_fill(bool cancellable, bool do_card_table_updates) {
 
   assert(!is_humongous(), "No need to fill or coalesce humongous regions");
   if (!is_active()) {
@@ -489,7 +489,16 @@ bool ShenandoahHeapRegion::oop_coalesce_and_fill(bool cancellable) {
       size_t fill_size = next_marked_obj - obj_addr;
       assert(fill_size >= ShenandoahHeap::min_fill_size(), "previously allocated object known to be larger than min_size");
       ShenandoahHeap::fill_with_object(obj_addr, fill_size);
-      heap->old_generation()->card_scan()->coalesce_objects(obj_addr, fill_size);
+      if (do_card_table_updates) {
+        heap->old_generation()->card_scan()->coalesce_objects(obj_addr, fill_size);
+      } else {
+        // A humongous object allocation failure during evacuation will skip the degenerated cycle and
+        // jump straight to a full GC. If this region is pinned when the full GC cycle starts, it will
+        // not be compacted. Therefore, if the region is old, we must fill in any unmarked objects. However,
+        // promoted objects will not have been registered yet, so we cannot use the card table here.
+        assert(heap->is_full_gc_in_progress(), "Can only skip card table updates during a full GC");
+      }
+
       obj_addr = next_marked_obj;
     }
     if (cancellable && heap->cancelled_gc()) {

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

@@ -447,7 +447,7 @@ public:
   // This is used by old-gen GC following concurrent marking to make old-gen HeapRegions parsable. Old regions must be
   // parsable because the mark bitmap is not reliable during the concurrent old mark.
   // Return true iff region is completely coalesced and filled.  Returns false if cancelled before task is complete.
-  bool oop_coalesce_and_fill(bool cancellable);
+  bool oop_coalesce_and_fill(bool cancellable, bool do_card_table_updates = true);
 
   // Invoke closure on every reference contained within the humongous object that spans this humongous
   // region if the reference is contained within a DIRTY card and the reference is no more than words following

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

@@ -28,8 +28,6 @@
 #include "gc/shenandoah/shenandoahOldGeneration.hpp"
 #include "gc/shenandoah/shenandoahPLAB.hpp"
 #include "logging/log.hpp"
-#include "runtime/globals.hpp"
-#include "runtime/javaThread.hpp"
 #include "utilities/copy.hpp"
 
 ShenandoahPLAB::ShenandoahPLAB() :
@@ -46,9 +44,7 @@ ShenandoahPLAB::ShenandoahPLAB() :
 }
 
 ShenandoahPLAB::~ShenandoahPLAB() {
-  if (_plab != nullptr) {
-    delete _plab;
-  }
+  delete _plab;
 }
 
 void ShenandoahPLAB::subtract_from_promoted(size_t increment) {
@@ -119,7 +115,7 @@ HeapWord* ShenandoahPLAB::allocate_slow(size_t size, bool is_promotion) {
   }
 
   if (_plab->words_remaining() < plab_min_size) {
-    // Retire current PLAB. This takes care of any PLAB book-keeping.
+    // Retire current PLAB. This takes care of any PLAB bookkeeping.
     retire();
 
     size_t actual_size = 0;
@@ -191,10 +187,12 @@ void ShenandoahPLAB::retire() {
     log_debug(gc, plab)("Retire PLAB, unexpend unpromoted: %zu", not_promoted * HeapWordSize);
     _heap->old_generation()->unexpend_promoted(not_promoted);
   }
-  const size_t original_waste = _plab->waste();
-  HeapWord* const top = _plab->top();
 
   // plab->retire() overwrites unused memory between plab->top() and plab->hard_end() with a dummy object to make memory parsable.
-  // It adds the size of this unused memory, in words, to plab->waste().
+  // We do _not_ need to register this remnant object with the card table because all paths where a PLAB object would
+  // be created are covered by a subsequent phase in the cycle. For the concurrent and degenerated cycles, all PLABs
+  // are retired in preparation for update-references. All objects in these PLABs will be registered by update-card-tables.
+  // For a full GC, the entire remembered set will be rebuilt in the final phase. Note also that an empty TLAB will _not_
+  // create a filler object when it is retired.
   _plab->retire();
 }

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

@@ -388,7 +388,7 @@ public:
      _used(0), _committed(0), _garbage(0), _regions(0), _humongous_waste(0), _trashed_regions(0), _trashed_used(0)
   {
     _region_size_bytes = ShenandoahHeapRegion::region_size_bytes();
-    // Retired regions are not necessarily filled, thouugh their remnant memory is considered used.
+    // Retired regions are not necessarily filled, though their remnant memory is considered used.
     _min_free_size = PLAB::min_size() * HeapWordSize;
   };