Optimise propositional encoding of object_size

Avoid creating equalities over the postponed bitvector when the object
literals trivially aren't equal, and directly encode bitwise equality
when the object literals are trivially equal (and stop searching for a
matching object). In all other cases, avoid unnecessary Tseitin
variables to encode the postponed bitvector equality.

Author: tautschnig

src/solvers/flattening/bv_pointers.cpp

@@ -957,9 +957,26 @@ void bv_pointerst::do_postponed(
       PRECONDITION(size_bv.size() == postponed.bv.size());
 
       literalt l1=bv_utils.equal(bv, saved_bv);
+      if(l1.is_true())
+      {
+        for(std::size_t i = 0; i < postponed.bv.size(); ++i)
+          prop.set_equal(postponed.bv[i], size_bv[i]);
+        break;
+      }
+      else if(l1.is_false())
+        continue;
+#define COMPACT_OBJECT_SIZE_EQ
+#ifndef COMPACT_OBJECT_SIZE_EQ
       literalt l2=bv_utils.equal(postponed.bv, size_bv);
 
       prop.l_set_to_true(prop.limplies(l1, l2));
+#else
+      for(std::size_t i = 0; i < postponed.bv.size(); ++i)
+      {
+        prop.lcnf({!l1, !postponed.bv[i], size_bv[i]});
+        prop.lcnf({!l1, postponed.bv[i], !size_bv[i]});
+      }
+#endif
     }
   }
   else