8322097: Serial: Refactor CardTableRS::find_first_clean_card

Reviewed-by: tschatzl, iwalulya

src/hotspot/share/gc/serial/cardTableRS.cpp

@@ -382,20 +382,17 @@ CardTable::CardValue* CardTableRS::find_first_dirty_card(CardValue* const start_
   return end_card;
 }
 
-// Because non-objArray objs can be imprecisely-marked (only obj-start card is
-// dirty instead of the part containing old-to-young pointers), if the
-// obj-start of a non-objArray is dirty, all cards that obj completely resides
-// on are considered as dirty, since that obj will be iterated (scanned for
-// old-to-young pointers) as a whole.
+// Because non-objArray objs can be imprecisely marked (only the obj-start card
+// is dirty instead of the part containing old-to-young pointers), if the
+// obj-start of a non-objArray is dirty, all cards that the obj resides on,
+// except the final one, are unconditionally considered as dirty. This is
+// because that obj will be iterated (scanned for old-to-young pointers) as a
+// whole.
 template<typename Func>
 CardTable::CardValue* CardTableRS::find_first_clean_card(CardValue* const start_card,
                                                          CardValue* const end_card,
                                                          CardTableRS* ct,
                                                          Func& object_start) {
-
-  // end_card might be just beyond the heap, so need to use the _raw variant.
-  HeapWord* end_address = ct->addr_for_raw(end_card);
-
   for (CardValue* current_card = start_card; current_card < end_card; /* empty */) {
     if (is_dirty(current_card)) {
       current_card++;
@@ -418,21 +415,14 @@ CardTable::CardValue* CardTableRS::find_first_clean_card(CardValue* const start_
       return current_card;
     }
 
-    // This might be the last object in this area, avoid trying to access the
-    // card beyond the allowed area.
-    HeapWord* next_address = obj_start_addr + obj->size();
-    if (next_address >= end_address) {
-      break;
-    }
-
-    // Card occupied by next obj.
-    CardValue* next_obj_card = ct->byte_for(next_address);
-    if (is_clean(next_obj_card)) {
-      return next_obj_card;
+    // Final card occupied by obj.
+    CardValue* obj_final_card = ct->byte_for(obj_start_addr + obj->size() - 1);
+    if (is_clean(obj_final_card)) {
+      return obj_final_card;
     }
 
     // Continue the search after this known-dirty card...
-    current_card = next_obj_card + 1;
+    current_card = obj_final_card + 1;
   }
 
   return end_card;

src/hotspot/share/gc/shared/cardTable.hpp

@@ -83,14 +83,6 @@ protected:
     return cards_required(_whole_heap.word_size()) - 1;
   }
 
-  // Mapping from card marking array entry to address of first word without checks.
-  HeapWord* addr_for_raw(const CardValue* p) const {
-    // As _byte_map_base may be "negative" (the card table has been allocated before
-    // the heap in memory), do not use pointer_delta() to avoid the assertion failure.
-    size_t delta = p - _byte_map_base;
-    return (HeapWord*) (delta << _card_shift);
-  }
-
 private:
   void initialize_covered_region(void* region0_start, void* region1_start);
 
@@ -152,13 +144,16 @@ public:
     return byte_after(p);
   }
 
-  // Mapping from card marking array entry to address of first word.
+  // Mapping from card marking array entry to address of first word
   HeapWord* addr_for(const CardValue* p) const {
     assert(p >= _byte_map && p < _byte_map + _byte_map_size,
            "out of bounds access to card marking array. p: " PTR_FORMAT
            " _byte_map: " PTR_FORMAT " _byte_map + _byte_map_size: " PTR_FORMAT,
            p2i(p), p2i(_byte_map), p2i(_byte_map + _byte_map_size));
-    HeapWord* result = addr_for_raw(p);
+    // As _byte_map_base may be "negative" (the card table has been allocated before
+    // the heap in memory), do not use pointer_delta() to avoid the assertion failure.
+    size_t delta = p - _byte_map_base;
+    HeapWord* result = (HeapWord*) (delta << _card_shift);
     assert(_whole_heap.contains(result),
            "Returning result = " PTR_FORMAT " out of bounds of "
            " card marking array's _whole_heap = [" PTR_FORMAT "," PTR_FORMAT ")",