diff --git a/hw3/bin/backend.ml b/hw3/bin/backend.ml
index 41e5b91..25bb6ad 100644
--- a/hw3/bin/backend.ml
+++ b/hw3/bin/backend.ml
@@ -173,7 +173,12 @@ let compile_operand (ctxt : ctxt) (dest : X86.operand) : Ll.operand -> ins = fun
      Your function should simply return 0 in those cases
 *)
 let rec size_ty (tdecls : (tid * ty) list) (t : Ll.ty) : int =
-  failwith "size_ty not implemented"
+  match t with
+  | Struct tl -> List.fold_left (fun acc st -> acc + size_ty tdecls st) 0 tl
+  | Array (sz, at) -> sz * size_ty tdecls at
+  | Namedt lb -> size_ty tdecls (lookup tdecls lb)
+  | I1 | I64 | Ptr _ -> 8
+  | Void | I8 | Fun _ -> 0
 ;;
 
 (* Generates code that computes a pointer value.