(* Calls forward_if with the current precondition to which the provided conditions are added *)
(* QQQ TODO does this already exist? Add to library? *)
Ltac forward_if_diff add := match add with
| (PROPx ?P2 (LOCALx ?Q2 (SEPx ?R2))) => match goal with
  | |- semax ?Delta (PROPx ?P1 (LOCALx ?Q1 (SEPx ?R1))) _ _ =>
    let P3 := fresh "P3" in let Q3 := fresh "Q3" in let R3 := fresh "R3" in
    pose (P3 := P1 ++ P2); pose (Q3 := Q1 ++ Q2); pose (R3 := R1 ++ R2);
    simpl in P3, Q3, R3;
    forward_if (PROPx P3 (LOCALx Q3 (SEPx R3)));
    subst P3 Q3 R3
  end
end.

Lemma add_no_overflow: forall n i log (Hn: 1 <= n < 256) (Hlog : forall j : Z, 1 <= j < 256 -> Znth j log = log3 (Int.repr j)) (H1: forall i : Z, Int.unsigned (Znth i RSb) <= Byte.max_unsigned) (H3 : (if Int.eq (Znth i RSb) Int.zero then Int.zero else Int.one) <> Int.zero), Int.min_signed <= Int.signed (Int.repr (Znth n log)) + Int.signed (Int.repr (Znth (Int.unsigned (Znth i RSb)) log)) <= Int.max_signed.