Proof with auto with typeclass_instances.
intros x y Fx Fy Zx.
case (Zle_or_lt (ln_beta beta (x/Z2R beta)) (ln_beta beta y)); intros H1.
pose (e:=cexp (x / Z2R beta)).
destruct (ex_shift x e) as (nx, Hnx); try exact Fx.
apply monotone_exp.
rewrite <- (ln_beta_minus1 x Zx); omega.
destruct (ex_shift y e) as (ny, Hny); try assumption.
apply monotone_exp...
destruct (round_repr_same_exp beta fexp rnd (nx+ny) e) as (n,Hn).
exists n.
apply trans_eq with (F2R (Float beta n e)).
rewrite <- Hn; f_equal.
rewrite Hnx, Hny; unfold F2R; simpl; rewrite Z2R_plus; ring.
unfold F2R; simpl.
rewrite ulp_neq_0; try easy.
apply Rmult_integral_contrapositive_currified; try assumption.
apply Rinv_neq_0_compat.
apply Rgt_not_eq.
apply radix_pos.
(* *)
destruct (ex_shift (round beta fexp rnd (x + y)) (cexp (x/Z2R beta))) as (n,Hn).
apply generic_format_round...
apply Zle_trans with (cexp (x+y)).
apply monotone_exp.
rewrite <- ln_beta_minus1 by easy.
rewrite <- (ln_beta_abs beta (x+y)).
(* . *)
assert (U: (Rabs (x+y) = Rabs x + Rabs y)%R \/ (y <> 0 /\ Rabs (x+y) = Rabs x - Rabs y)%R).
assert (V: forall x y, (Rabs y <= Rabs x)%R ->
(Rabs (x+y) = Rabs x + Rabs y)%R \/ (y <> 0 /\ Rabs (x+y) = Rabs x - Rabs y)%R).
clear; intros x y.
case (Rle_or_lt 0 y); intros Hy.
case Hy; intros Hy'.
case (Rle_or_lt 0 x); intros Hx.
intros _; rewrite (Rabs_pos_eq y) by easy.
rewrite (Rabs_pos_eq x) by easy.
left; apply Rabs_pos_eq.
now apply Rplus_le_le_0_compat.
rewrite (Rabs_pos_eq y) by easy.
rewrite (Rabs_left x) by easy.
intros H; right; split.
now apply Rgt_not_eq.
rewrite Rabs_left1.
ring.
apply Rplus_le_reg_l with (-x)%R; ring_simplify; assumption.
intros _; left.
now rewrite <- Hy', Rabs_R0, 2!Rplus_0_r.
case (Rle_or_lt 0 x); intros Hx.
rewrite (Rabs_left y) by easy.
rewrite (Rabs_pos_eq x) by easy.
intros H; right; split.
now apply Rlt_not_eq.
rewrite Rabs_pos_eq.
ring.
apply Rplus_le_reg_l with (-y)%R; ring_simplify; assumption.
intros _; left.
rewrite (Rabs_left y) by easy.
rewrite (Rabs_left x) by easy.
rewrite Rabs_left1.
ring.
lra.
apply V; left.
apply ln_beta_lt_pos with beta.
now apply Rabs_pos_lt.
rewrite <- ln_beta_minus1 in H1; try assumption.
rewrite 2!ln_beta_abs; omega.
(* . *)
destruct U as [U|U].
rewrite U; apply Zle_trans with (ln_beta beta x).
omega.
rewrite <- ln_beta_abs.
apply ln_beta_le.
now apply Rabs_pos_lt.
apply Rplus_le_reg_l with (-Rabs x)%R; ring_simplify.
apply Rabs_pos.
destruct U as (U',U); rewrite U.
rewrite <- ln_beta_abs.
apply ln_beta_minus_lb.
now apply Rabs_pos_lt.
now apply Rabs_pos_lt.
rewrite 2!ln_beta_abs.
assert (ln_beta beta y < ln_beta beta x - 1)%Z.
now rewrite (ln_beta_minus1 x Zx).
omega.
apply canonic_exp_round_ge...
intros K.
apply round_plus_eq_zero in K...
contradict H1; apply Zle_not_lt.
rewrite <- (ln_beta_minus1 x Zx).
replace y with (-x)%R.
rewrite ln_beta_opp; omega.
lra.
exists n.
rewrite ulp_neq_0.
assumption.
apply Rmult_integral_contrapositive_currified; try assumption.
apply Rinv_neq_0_compat.
apply Rgt_not_eq.
apply radix_pos.
Qed.
Context {exp_not_FTZ : Exp_not_FTZ fexp}.

Theorem round_plus_ge_ulp :
forall x y, format x -> format y ->
round beta fexp rnd (x+y) = 0%R \/
(ulp beta fexp (x/Z2R beta) <= Rabs (round beta fexp rnd (x+y)))%R.