* update post barrier cost estimate

src/hotspot/share/gc/g1/c2/g1BarrierSetC2.cpp

@@ -299,9 +299,13 @@ uint G1BarrierSetC2::estimated_barrier_size(const Node* node) const {
     nodes += 50;
   }
   if ((barrier_data & G1C2BarrierPost) != 0) {
-    // Approximate the number of nodes needed with the number of Assembly instructions
-    // since we do not have real nodes.
-    nodes += 12;
+    // Approximate the number of nodes needed; an if costs 4 nodes (Cmp, Bool,
+    // If, If projection), any other (Assembly) instruction is approximated with
+    // a cost of 1.
+    nodes +=   4  // base cost for the card write containing getting base offset, address calculation and the card write;
+             + 6  // same region check: Uncompress (new_val) oop, xor, shr, (cmp), jmp
+             + 4  // new_val is null check
+             + 4; // card not clean check.
   }
   return nodes;
 }