6728478: Assertion at parallel promotion from young to old generation

Summary: The fix avoids a call to address_for_index() in this particular situation where it is not known if the passed index is in bounds.
Reviewed-by: tonyp

src/share/vm/gc_implementation/parNew/parGCAllocBuffer.cpp

@@ -229,7 +229,7 @@ void ParGCAllocBufferWithBOT::retire(bool end_of_gc, bool retain) {
     HeapWord* first_card_start = _bsa->address_for_index(first_card_index);
     if (first_card_start < pre_top) {
       HeapWord* second_card_start =
-        _bsa->address_for_index(first_card_index + 1);
+        _bsa->inc_by_region_size(first_card_start);
 
       // Ensure enough room to fill with the smallest block
       second_card_start = MAX2(second_card_start, pre_top + AlignmentReserve);

src/share/vm/memory/blockOffsetTable.hpp

@@ -199,6 +199,12 @@ public:
   // "index" in "_offset_array".
   HeapWord* address_for_index(size_t index) const;
 
+  // Return the address "p" incremented by the size of
+  // a region.  This method does not align the address
+  // returned to the start of a region.  It is a simple
+  // primitive.
+  HeapWord* inc_by_region_size(HeapWord* p) const { return p + N_words; }
+
   // Shared space support
   void serialize(SerializeOopClosure* soc, HeapWord* start, HeapWord* end);
 };