Definition subl (v w: aval) :=
  match v, w with
  | L i1, L i2 => L (Int64.sub i1 i2)
  | Ptr p, L i => if Archi.ptr64 then Ptr (psub p (Ptrofs.of_int64 i)) else Ifptr (poffset p)
  | Ptr p, _   => Ifptr (poffset p)
  | Ifptr p, L i => if Archi.ptr64 then Ifptr (psub p (Ptrofs.of_int64 i)) else Ifptr (plub (poffset p) (provenance w))
  | Ifptr p, _ => Ifptr (plub (poffset p) (provenance w))
  | _, _ => ntop2 v w
  end.

Lemma subl_sound: forall v w x y, vmatch v x -> vmatch w y -> vmatch (Val.subl v w) (subl x y).