static void set_last(objArrayOop pic, oop first, oop second) {
   int size = pic->length();
   pic->obj_at_put(size--, second);
   pic->obj_at_put(size,   first);
 }
 static void set_first(objArrayOop pic, oop first, oop second) {
   pic->obj_at_put(1, first);
   pic->obj_at_put(2, second);
 }
 static void set_second(objArrayOop pic, oop first, oop second) {
   pic->obj_at_put(3, first);
   pic->obj_at_put(4, second);
 }
 static void deallocate(objArrayOop pic) {
   int entry = (pic->length()/2) - 1;
   oop first = free_list()->obj_at(entry);
   pic->obj_at_put(1, first);
   free_list()->obj_at_put(entry, pic);
 }