Definition sintv (v: aval) : Z * Z :=
  match v with
  | I n => (Int.signed n, Int.signed n)
  | Uns _ n =>
      if zlt n Int.zwordsize then (0, two_p n - 1) else (Int.min_signed, Int.max_signed)
  | Sgn _ n =>
      if zlt n Int.zwordsize
      then (let x := two_p (n-1) in (-x, x-1))
      else (Int.min_signed, Int.max_signed)
  | _ => (Int.min_signed, Int.max_signed)
  end.

Lemma sintv_sound: forall n v, vmatch (Vint n) v -> fst (sintv v) <= Int.signed n <= snd (sintv v).