Definition sub (v w: aval) :=
  match v, w with
  | I i1, I i2 => I (Int.sub i1 i2)
  | Ptr p, I i => if Archi.ptr64 then Ifptr (poffset p) else Ptr (psub p (Ptrofs.of_int i))
  | Ptr p, _   => Ifptr (poffset p)
  | Ifptr p, I i => if Archi.ptr64 then Ifptr (plub (poffset p) (provenance w)) else Ifptr (psub p (Ptrofs.of_int i))
  | Ifptr p, _ => Ifptr (plub (poffset p) (provenance w))
  | _, _ => ntop2 v w
  end.

Lemma sub_sound: forall v w x y, vmatch v x -> vmatch w y -> vmatch (Val.sub v w) (sub x y).