Fappli_IEEE.v 51.8 KB
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(**
This file is part of the Flocq formalization of floating-point
arithmetic in Coq: http://flocq.gforge.inria.fr/

Copyright (C) 2010 Sylvie Boldo
#<br />#
Copyright (C) 2010 Guillaume Melquiond

This library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 3 of the License, or (at your option) any later version.

This library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
COPYING file for more details.
*)

(** * IEEE-754 arithmetic *)
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Require Import Fcore.
Require Import Fcalc_digits.
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Require Import Fcalc_round.
Require Import Fcalc_bracket.
Require Import Fcalc_ops.
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Require Import Fcalc_div.
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Require Import Fcalc_sqrt.
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Require Import Fprop_relative.
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Section AnyRadix.

Inductive full_float :=
  | F754_zero : bool -> full_float
  | F754_infinity : bool -> full_float
  | F754_nan : full_float
  | F754_finite : bool -> positive -> Z -> full_float.

Definition FF2R r x :=
  match x with
  | F754_finite s m e => F2R (Float r (cond_Zopp s (Zpos m)) e)
  | _ => R0
  end.

End AnyRadix.

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Section Binary.

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Variable prec emax : Z.
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Context (prec_gt_0_ : Prec_gt_0 prec).
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Hypothesis Hmax : (prec < emax)%Z.
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Let emin := (3 - emax - prec)%Z.
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Let fexp := FLT_exp emin prec.
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Instance fexp_correct : Valid_exp fexp := FLT_exp_valid emin prec.
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Instance fexp_monotone : Monotone_exp fexp := FLT_exp_monotone emin prec.
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Definition bounded_prec m e :=
  Zeq_bool (fexp (Z_of_nat (S (digits2_Pnat m)) + e)) e.

Definition bounded m e :=
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  andb (bounded_prec m e) (Zle_bool e (emax - prec)).
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Definition valid_binary x :=
  match x with
  | F754_finite _ m e => bounded m e
  | _ => true
  end.

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Inductive binary_float :=
  | B754_zero : bool -> binary_float
  | B754_infinity : bool -> binary_float
  | B754_nan : binary_float
  | B754_finite : bool ->
    forall (m : positive) (e : Z), bounded m e = true -> binary_float.

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Definition FF2B x :=
  match x as x return valid_binary x = true -> binary_float with
  | F754_finite s m e => B754_finite s m e
  | F754_infinity s => fun _ => B754_infinity s
  | F754_zero s => fun _ => B754_zero s
  | F754_nan => fun _ => B754_nan
  end.

Definition B2FF x :=
  match x with
  | B754_finite s m e _ => F754_finite s m e
  | B754_infinity s => F754_infinity s
  | B754_zero s => F754_zero s
  | B754_nan => F754_nan
  end.

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Definition radix2 := Build_radix 2 (refl_equal true).

Definition B2R f :=
  match f with
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  | B754_finite s m e _ => F2R (Float radix2 (cond_Zopp s (Zpos m)) e)
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  | _ => R0
  end.

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Theorem FF2R_B2FF :
  forall x,
  FF2R radix2 (B2FF x) = B2R x.
Proof.
now intros [sx|sx| |sx mx ex Hx].
Qed.

Theorem B2FF_FF2B :
  forall x Hx,
  B2FF (FF2B x Hx) = x.
Proof.
now intros [sx|sx| |sx mx ex] Hx.
Qed.

Theorem B2R_FF2B :
  forall x Hx,
  B2R (FF2B x Hx) = FF2R radix2 x.
Proof.
now intros [sx|sx| |sx mx ex] Hx.
Qed.

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Theorem match_FF2B :
  forall {T} fz fi fn ff x Hx,
  match FF2B x Hx return T with
  | B754_zero sx => fz sx
  | B754_infinity sx => fi sx
  | B754_nan => fn
  | B754_finite sx mx ex _ => ff sx mx ex
  end =
  match x with
  | F754_zero sx => fz sx
  | F754_infinity sx => fi sx
  | F754_nan => fn
  | F754_finite sx mx ex => ff sx mx ex
  end.
Proof.
now intros T fz fi fn ff [sx|sx| |sx mx ex] Hx.
Qed.

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Theorem canonic_bounded_prec :
  forall (sx : bool) mx ex,
  bounded_prec mx ex = true ->
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  canonic radix2 fexp (Float radix2 (cond_Zopp sx (Zpos mx)) ex).
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Proof.
intros sx mx ex H.
assert (Hx := Zeq_bool_eq _ _ H). clear H.
apply sym_eq.
simpl.
pattern ex at 2 ; rewrite <- Hx.
apply (f_equal fexp).
rewrite ln_beta_F2R_digits.
rewrite <- digits_abs.
rewrite Z_of_nat_S_digits2_Pnat.
now case sx.
now case sx.
Qed.

Theorem generic_format_B2R :
  forall x,
  generic_format radix2 fexp (B2R x).
Proof.
intros [sx|sx| |sx mx ex Hx] ; try apply generic_format_0.
simpl.
apply generic_format_canonic.
apply canonic_bounded_prec.
now destruct (andb_prop _ _ Hx) as (H, _).
Qed.

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Theorem binary_unicity :
  forall x y : binary_float,
  B2FF x = B2FF y ->
  x = y.
Proof.
intros [sx|sx| |sx mx ex Hx] [sy|sy| |sy my ey Hy] ; try easy.
(* *)
intros H.
now inversion H.
(* *)
intros H.
now inversion H.
(* *)
intros H.
inversion H.
clear H.
revert Hx.
rewrite H2, H3.
intros Hx.
apply f_equal.
apply eqbool_irrelevance.
Qed.

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Definition is_finite_strict f :=
  match f with
  | B754_finite _ _ _ _ => true
  | _ => false
  end.

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Theorem finite_binary_unicity :
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  forall x y : binary_float,
  is_finite_strict x = true ->
  is_finite_strict y = true ->
  B2R x = B2R y ->
  x = y.
Proof.
intros [sx|sx| |sx mx ex Hx] [sy|sy| |sy my ey Hy] ; try easy.
simpl.
intros _ _ Heq.
assert (Hs: sx = sy).
(* *)
revert Heq. clear.
case sx ; case sy ; try easy ;
  intros Heq ; apply False_ind ; revert Heq.
apply Rlt_not_eq.
apply Rlt_trans with R0.
now apply F2R_lt_0_compat.
now apply F2R_gt_0_compat.
apply Rgt_not_eq.
apply Rgt_trans with R0.
now apply F2R_gt_0_compat.
now apply F2R_lt_0_compat.
assert (mx = my /\ ex = ey).
(* *)
refine (_ (canonic_unicity _ fexp _ _ _ _ Heq)).
rewrite Hs.
now case sy ; intro H ; injection H ; split.
apply canonic_bounded_prec.
exact (proj1 (andb_prop _ _ Hx)).
apply canonic_bounded_prec.
exact (proj1 (andb_prop _ _ Hy)).
(* *)
revert Hx.
rewrite Hs, (proj1 H), (proj2 H).
intros Hx.
apply f_equal.
apply eqbool_irrelevance.
Qed.

Definition is_finite f :=
  match f with
  | B754_finite _ _ _ _ => true
  | B754_zero _ => true
  | _ => false
  end.

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Definition Bopp x :=
  match x with
  | B754_nan => x
  | B754_infinity sx => B754_infinity (negb sx)
  | B754_finite sx mx ex Hx => B754_finite (negb sx) mx ex Hx
  | B754_zero sx => B754_zero (negb sx)
  end.

Theorem Bopp_involutive :
  forall x, Bopp (Bopp x) = x.
Proof.
now intros [sx|sx| |sx mx ex Hx] ; simpl ; try rewrite Bool.negb_involutive.
Qed.

Theorem B2R_Bopp :
  forall x,
  B2R (Bopp x) = (- B2R x)%R.
Proof.
intros [sx|sx| |sx mx ex Hx] ; apply sym_eq ; try apply Ropp_0.
simpl.
rewrite opp_F2R.
now case sx.
Qed.

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Theorem bounded_lt_emax :
  forall mx ex,
  bounded mx ex = true ->
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  (F2R (Float radix2 (Zpos mx) ex) < bpow radix2 emax)%R.
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Proof.
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intros mx ex Hx.
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destruct (andb_prop _ _ Hx) as (H1,H2).
generalize (Zeq_bool_eq _ _ H1). clear H1. intro H1.
generalize (Zle_bool_imp_le _ _ H2). clear H2. intro H2.
generalize (ln_beta_F2R_digits radix2 (Zpos mx) ex).
destruct (ln_beta radix2 (F2R (Float radix2 (Zpos mx) ex))) as (e',Ex).
unfold ln_beta_val.
intros H.
apply Rlt_le_trans with (bpow radix2 e').
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change (Zpos mx) with (Zabs (Zpos mx)).
rewrite <- abs_F2R.
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apply Ex.
apply Rgt_not_eq.
now apply F2R_gt_0_compat.
apply bpow_le.
rewrite H. 2: discriminate.
revert H1. clear -H2.
rewrite Z_of_nat_S_digits2_Pnat.
change Fcalc_digits.radix2 with radix2.
unfold fexp, FLT_exp.
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generalize (digits radix2 (Zpos mx)).
intros ; zify ; subst.
clear -H H2. clearbody emin.
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omega.
Qed.

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Theorem B2R_lt_emax :
  forall x,
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  (Rabs (B2R x) < bpow radix2 emax)%R.
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Proof.
intros [sx|sx| |sx mx ex Hx] ; simpl ; try ( rewrite Rabs_R0 ; apply bpow_gt_0 ).
rewrite abs_F2R, abs_cond_Zopp.
now apply bounded_lt_emax.
Qed.

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Theorem bounded_canonic_lt_emax :
  forall mx ex,
  canonic radix2 fexp (Float radix2 (Zpos mx) ex) ->
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  (F2R (Float radix2 (Zpos mx) ex) < bpow radix2 emax)%R ->
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  bounded mx ex = true.
Proof.
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intros mx ex Cx Bx.
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apply andb_true_intro.
split.
unfold bounded_prec.
unfold canonic, Fexp in Cx.
rewrite Cx at 2.
rewrite Z_of_nat_S_digits2_Pnat.
change Fcalc_digits.radix2 with radix2.
unfold canonic_exponent.
rewrite ln_beta_F2R_digits. 2: discriminate.
now apply -> Zeq_is_eq_bool.
apply Zle_bool_true.
unfold canonic, Fexp in Cx.
rewrite Cx.
unfold canonic_exponent, fexp, FLT_exp.
destruct (ln_beta radix2 (F2R (Float radix2 (Zpos mx) ex))) as (e',Ex). simpl.
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apply Zmax_lub.
cut (e' - 1 < emax)%Z. clear ; omega.
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apply lt_bpow with radix2.
apply Rle_lt_trans with (2 := Bx).
change (Zpos mx) with (Zabs (Zpos mx)).
rewrite <- abs_F2R.
apply Ex.
apply Rgt_not_eq.
now apply F2R_gt_0_compat.
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unfold emin.
generalize (prec_gt_0 prec).
clear -Hmax ; omega.
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Qed.

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Record shr_record := { shr_m : Z ; shr_r : bool ; shr_s : bool }.

Definition shr_1 mrs :=
  let '(Build_shr_record m r s) := mrs in
  let s := orb r s in
  match m with
  | Z0 => Build_shr_record Z0 false s
  | Zpos xH => Build_shr_record Z0 true s
  | Zpos (xO p) => Build_shr_record (Zpos p) false s
  | Zpos (xI p) => Build_shr_record (Zpos p) true s
  | Zneg xH => Build_shr_record Z0 true s
  | Zneg (xO p) => Build_shr_record (Zneg p) false s
  | Zneg (xI p) => Build_shr_record (Zneg p) true s
  end.

Definition loc_of_shr_record mrs :=
  match mrs with
  | Build_shr_record _ false false => loc_Exact
  | Build_shr_record _ false true => loc_Inexact Lt
  | Build_shr_record _ true false => loc_Inexact Eq
  | Build_shr_record _ true true => loc_Inexact Gt
  end.

Definition shr_record_of_loc m l :=
  match l with
  | loc_Exact => Build_shr_record m false false
  | loc_Inexact Lt => Build_shr_record m false true
  | loc_Inexact Eq => Build_shr_record m true false
  | loc_Inexact Gt => Build_shr_record m true true
  end.

Theorem shr_m_shr_record_of_loc :
  forall m l,
  shr_m (shr_record_of_loc m l) = m.
Proof.
now intros m [|[| |]].
Qed.

Theorem loc_of_shr_record_of_loc :
  forall m l,
  loc_of_shr_record (shr_record_of_loc m l) = l.
Proof.
now intros m [|[| |]].
Qed.

Definition shr mrs e n :=
  match n with
  | Zpos p => (iter_pos p _ shr_1 mrs, (e + n)%Z)
  | _ => (mrs, e)
  end.

Theorem inbetween_shr_1 :
  forall x mrs e,
  (0 <= shr_m mrs)%Z ->
  inbetween_float radix2 (shr_m mrs) e x (loc_of_shr_record mrs) ->
  inbetween_float radix2 (shr_m (shr_1 mrs)) (e + 1) x (loc_of_shr_record (shr_1 mrs)).
Proof.
intros x mrs e Hm Hl.
refine (_ (new_location_even_correct (F2R (Float radix2 (shr_m (shr_1 mrs)) (e + 1))) (bpow radix2 e) 2 _ _ _ x (if shr_r (shr_1 mrs) then 1 else 0) (loc_of_shr_record mrs) _ _)) ; try easy.
2: apply bpow_gt_0.
2: now case (shr_r (shr_1 mrs)) ; split.
change (Z2R 2) with (bpow radix2 1).
rewrite <- bpow_plus.
rewrite (Zplus_comm 1), <- (F2R_bpow radix2 (e + 1)).
unfold inbetween_float, F2R. simpl.
rewrite Z2R_plus, Rmult_plus_distr_r.
replace (new_location_even 2 (if shr_r (shr_1 mrs) then 1%Z else 0%Z) (loc_of_shr_record mrs)) with (loc_of_shr_record (shr_1 mrs)).
easy.
clear -Hm.
destruct mrs as (m, r, s).
now destruct m as [|[m|m|]|m] ; try (now elim Hm) ; destruct r as [|] ; destruct s as [|].
rewrite (F2R_change_exp radix2 e).
2: apply Zle_succ.
unfold F2R. simpl.
rewrite <- 2!Rmult_plus_distr_r, <- 2!Z2R_plus.
rewrite Zplus_assoc.
replace (shr_m (shr_1 mrs) * 2 ^ (e + 1 - e) + (if shr_r (shr_1 mrs) then 1%Z else 0%Z))%Z with (shr_m mrs).
exact Hl.
ring_simplify (e + 1 - e)%Z.
change (2^1)%Z with 2%Z.
rewrite Zmult_comm.
clear -Hm.
destruct mrs as (m, r, s).
now destruct m as [|[m|m|]|m] ; try (now elim Hm) ; destruct r as [|] ; destruct s as [|].
Qed.

Theorem inbetween_shr :
  forall x m e l n,
  (0 <= m)%Z ->
  inbetween_float radix2 m e x l ->
  let '(mrs, e') := shr (shr_record_of_loc m l) e n in
  inbetween_float radix2 (shr_m mrs) e' x (loc_of_shr_record mrs).
Proof.
intros x m e l n Hm Hl.
destruct n as [|n|n].
now destruct l as [|[| |]].
2: now destruct l as [|[| |]].
unfold shr.
rewrite iter_nat_of_P.
rewrite Zpos_eq_Z_of_nat_o_nat_of_P.
induction (nat_of_P n).
simpl.
rewrite Zplus_0_r.
now destruct l as [|[| |]].
simpl iter_nat.
rewrite inj_S.
unfold Zsucc.
rewrite  Zplus_assoc.
revert IHn0.
apply inbetween_shr_1.
clear -Hm.
induction n0.
now destruct l as [|[| |]].
simpl.
revert IHn0.
generalize (iter_nat n0 shr_record shr_1 (shr_record_of_loc m l)).
clear.
intros (m, r, s) Hm.
now destruct m as [|[m|m|]|m] ; try (now elim Hm) ; destruct r as [|] ; destruct s as [|].
Qed.

Definition shr_fexp m e l :=
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  let d := match m with Z0 => m | Zpos p => Z_of_nat (S (digits2_Pnat p)) | Zneg p => Z_of_nat (S (digits2_Pnat p)) end in
  shr (shr_record_of_loc m l) e (fexp (d + e) - e).
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Theorem shr_truncate :
  forall m e l,
  (0 <= m)%Z ->
  shr_fexp m e l =
  let '(m', e', l') := truncate radix2 fexp (m, e, l) in (shr_record_of_loc m' l', e').
Proof.
intros m e l Hm.
case_eq (truncate radix2 fexp (m, e, l)).
intros (m', e') l'.
unfold shr_fexp.
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replace (match m with Z0 => m | Zpos p => Z_of_nat (S (digits2_Pnat p)) | Zneg p => Z_of_nat (S (digits2_Pnat p)) end)
  with (digits radix2 m).
2: now case m ; intros ; try rewrite Z_of_nat_S_digits2_Pnat.
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case_eq (fexp (digits radix2 m + e) - e)%Z.
(* *)
intros He.
unfold truncate.
rewrite He.
simpl.
intros H.
now inversion H.
(* *)
intros p Hp.
assert (He: (e <= fexp (digits radix2 m + e))%Z).
clear -Hp ; zify ; omega.
destruct (inbetween_float_ex radix2 m e l) as (x, Hx).
generalize (inbetween_shr x m e l (fexp (digits radix2 m + e) - e) Hm Hx).
assert (Hx0 : (0 <= x)%R).
apply Rle_trans with (F2R (Float radix2 m e)).
now apply F2R_ge_0_compat.
exact (proj1 (inbetween_float_bounds _ _ _ _ _ Hx)).
case_eq (shr (shr_record_of_loc m l) e (fexp (digits radix2 m + e) - e)).
intros mrs e'' H3 H4 H1.
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generalize (truncate_correct radix2 _ x m e l Hx0 Hx (or_introl _ He)).
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rewrite H1.
intros (H2,_).
rewrite <- Hp, H3.
assert (e'' = e').
change (snd (mrs, e'') = snd (fst (m',e',l'))).
rewrite <- H1, <- H3.
unfold truncate.
now rewrite Hp.
rewrite H in H4 |- *.
apply (f_equal (fun v => (v, _))).
destruct (inbetween_float_unique _ _ _ _ _ _ _ H2 H4) as (H5, H6).
rewrite H5, H6.
case mrs.
now intros m0 [|] [|].
(* *)
intros p Hp.
unfold truncate.
rewrite Hp.
simpl.
intros H.
now inversion H.
Qed.

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Inductive mode := mode_NE | mode_ZR | mode_DN | mode_UP | mode_NA.

Definition round_mode m :=
  match m with
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  | mode_NE => ZnearestE
  | mode_ZR => Ztrunc
  | mode_DN => Zfloor
  | mode_UP => Zceil
  | mode_NA => ZnearestA
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  end.
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Definition choice_mode m sx mx lx :=
  match m with
  | mode_NE => cond_incr (round_N (negb (Zeven mx)) lx) mx
  | mode_ZR => mx
  | mode_DN => cond_incr (round_sign_DN sx lx) mx
  | mode_UP => cond_incr (round_sign_UP sx lx) mx
  | mode_NA => cond_incr (round_N true lx) mx
  end.

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Global Instance valid_rnd_round_mode : forall m, Valid_rnd (round_mode m).
Proof.
destruct m ; unfold round_mode ; auto with typeclass_instances.
Qed.

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Definition overflow_to_inf m s :=
  match m with
  | mode_NE => true
  | mode_NA => true
  | mode_ZR => false
  | mode_UP => negb s
  | mode_DN => s
  end.

Definition binary_overflow m s :=
  if overflow_to_inf m s then F754_infinity s
  else F754_finite s (match (Zpower 2 prec - 1)%Z with Zpos p => p | _ => xH end) (emax - prec).

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Definition binary_round_sign mode sx mx ex lx :=
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  let '(mrs', e') := shr_fexp (Zpos mx) ex lx in
  let '(mrs'', e'') := shr_fexp (choice_mode mode sx (shr_m mrs') (loc_of_shr_record mrs')) e' loc_Exact in
  match shr_m mrs'' with
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  | Z0 => F754_zero sx
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  | Zpos m => if Zle_bool e'' (emax - prec) then F754_finite sx m e'' else binary_overflow mode sx
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  | _ => F754_nan (* dummy *)
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  end.

Theorem binary_round_sign_correct :
  forall mode x mx ex lx,
  inbetween_float radix2 (Zpos mx) ex (Rabs x) lx ->
  (ex <= fexp (digits radix2 (Zpos mx) + ex))%Z ->
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  valid_binary (binary_round_sign mode (Rlt_bool x 0) mx ex lx) = true /\
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  if Rlt_bool (Rabs (round radix2 fexp (round_mode mode) x)) (bpow radix2 emax) then
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    FF2R radix2 (binary_round_sign mode (Rlt_bool x 0) mx ex lx) = round radix2 fexp (round_mode mode) x
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  else
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    binary_round_sign mode (Rlt_bool x 0) mx ex lx = binary_overflow mode (Rlt_bool x 0).
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Proof with auto with typeclass_instances.
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intros m x mx ex lx Bx Ex.
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unfold binary_round_sign.
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rewrite shr_truncate. 2: easy.
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refine (_ (round_trunc_sign_any_correct _ _ (round_mode m) (choice_mode m) _ x (Zpos mx) ex lx Bx (or_introl _ Ex))).
refine (_ (truncate_correct_partial _ _ _ _ _ _ _ Bx Ex)).
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destruct (truncate radix2 fexp (Zpos mx, ex, lx)) as ((m1, e1), l1).
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rewrite loc_of_shr_record_of_loc, shr_m_shr_record_of_loc.
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set (m1' := choice_mode m (Rlt_bool x 0) m1 l1).
intros (H1a,H1b) H1c.
rewrite H1c.
assert (Hm: (m1 <= m1')%Z).
(* . *)
unfold m1', choice_mode, cond_incr.
case m ;
  try apply Zle_refl ;
  match goal with |- (m1 <= if ?b then _ else _)%Z =>
    case b ; [ apply Zle_succ | apply Zle_refl ] end.
assert (Hr: Rabs (round radix2 fexp (round_mode m) x) = F2R (Float radix2 m1' e1)).
(* . *)
rewrite <- (Zabs_eq m1').
replace (Zabs m1') with (Zabs (cond_Zopp (Rlt_bool x 0) m1')).
rewrite <- abs_F2R.
now apply f_equal.
apply abs_cond_Zopp.
apply Zle_trans with (2 := Hm).
apply Zlt_succ_le.
apply F2R_gt_0_reg with radix2 e1.
apply Rle_lt_trans with (1 := Rabs_pos x).
exact (proj2 (inbetween_float_bounds _ _ _ _ _ H1a)).
(* . *)
assert (Br: inbetween_float radix2 m1' e1 (Rabs (round radix2 fexp (round_mode m) x)) loc_Exact).
now apply inbetween_Exact.
destruct m1' as [|m1'|m1'].
(* . m1' = 0 *)
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rewrite shr_truncate. 2: apply Zle_refl.
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generalize (truncate_0 radix2 fexp e1 loc_Exact).
destruct (truncate radix2 fexp (Z0, e1, loc_Exact)) as ((m2, e2), l2).
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rewrite shr_m_shr_record_of_loc.
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intros Hm2.
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rewrite Hm2.
repeat split.
rewrite Rlt_bool_true.
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apply sym_eq.
case Rlt_bool ; apply F2R_0.
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rewrite abs_F2R, abs_cond_Zopp, F2R_0.
apply bpow_gt_0.
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(* . 0 < m1' *)
assert (He: (e1 <= fexp (digits radix2 (Zpos m1') + e1))%Z).
rewrite <- ln_beta_F2R_digits, <- Hr, ln_beta_abs.
2: discriminate.
rewrite H1b.
rewrite canonic_exponent_abs.
fold (canonic_exponent radix2 fexp (round radix2 fexp (round_mode m) x)).
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apply canonic_exponent_round...
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rewrite H1c.
case (Rlt_bool x 0).
apply Rlt_not_eq.
now apply F2R_lt_0_compat.
apply Rgt_not_eq.
now apply F2R_gt_0_compat.
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refine (_ (truncate_correct_partial _ _ _ _ _ _ _ Br He)).
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2: now rewrite Hr ; apply F2R_gt_0_compat.
refine (_ (truncate_correct_format radix2 fexp (Zpos m1') e1 _ _ He)).
2: discriminate.
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rewrite shr_truncate. 2: easy.
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destruct (truncate radix2 fexp (Zpos m1', e1, loc_Exact)) as ((m2, e2), l2).
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rewrite shr_m_shr_record_of_loc.
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intros (H3,H4) (H2,_).
destruct m2 as [|m2|m2].
elim Rgt_not_eq with (2 := H3).
rewrite F2R_0.
now apply F2R_gt_0_compat.
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rewrite F2R_cond_Zopp, H3, abs_cond_Ropp, abs_F2R.
simpl Zabs.
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case_eq (Zle_bool e2 (emax - prec)) ; intros He2.
assert (bounded m2 e2 = true).
apply andb_true_intro.
split.
unfold bounded_prec.
apply Zeq_bool_true.
rewrite Z_of_nat_S_digits2_Pnat.
rewrite <- ln_beta_F2R_digits.
apply sym_eq.
now rewrite H3 in H4.
discriminate.
exact He2.
apply (conj H).
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rewrite Rlt_bool_true.
apply F2R_cond_Zopp.
now apply bounded_lt_emax.
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rewrite (Rlt_bool_false _ (bpow radix2 emax)).
refine (conj _ (refl_equal _)).
unfold binary_overflow.
case overflow_to_inf.
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apply refl_equal.
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unfold valid_binary, bounded.
rewrite Zle_bool_refl.
rewrite Bool.andb_true_r.
apply Zeq_bool_true.
rewrite Z_of_nat_S_digits2_Pnat.
change Fcalc_digits.radix2 with radix2.
replace (digits radix2 (Zpos (match (Zpower 2 prec - 1)%Z with Zpos p => p | _ => xH end))) with prec.
unfold fexp, FLT_exp, emin.
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generalize (prec_gt_0 prec).
clear -Hmax ; zify ; omega.
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change 2%Z with (radix_val radix2).
case_eq (Zpower radix2 prec - 1)%Z.
simpl digits.
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generalize (Zpower_gt_1 radix2 prec (prec_gt_0 prec)).
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clear ; omega.
intros p Hp.
apply Zle_antisym.
cut (prec - 1 < digits radix2 (Zpos p))%Z. clear ; omega.
apply digits_gt_Zpower.
simpl Zabs. rewrite <- Hp.
cut (Zpower radix2 (prec - 1) < Zpower radix2 prec)%Z. clear ; omega.
apply lt_Z2R.
rewrite 2!Z2R_Zpower. 2: now apply Zlt_le_weak.
apply bpow_lt.
apply Zlt_pred.
now apply Zlt_0_le_0_pred.
apply digits_le_Zpower.
simpl Zabs. rewrite <- Hp.
apply Zlt_pred.
intros p Hp.
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generalize (Zpower_gt_1 radix2 _ (prec_gt_0 prec)).
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clear -Hp ; zify ; omega.
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apply Rnot_lt_le.
intros Hx.
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generalize (refl_equal (bounded m2 e2)).
unfold bounded at 2.
rewrite He2.
rewrite Bool.andb_false_r.
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rewrite bounded_canonic_lt_emax with (2 := Hx).
discriminate.
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unfold canonic.
now rewrite <- H3.
elim Rgt_not_eq with (2 := H3).
apply Rlt_trans with R0.
now apply F2R_lt_0_compat.
now apply F2R_gt_0_compat.
rewrite <- Hr.
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apply generic_format_abs...
apply generic_format_round...
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(* . not m1' < 0 *)
elim Rgt_not_eq with (2 := Hr).
apply Rlt_le_trans with R0.
now apply F2R_lt_0_compat.
apply Rabs_pos.
(* *)
apply Rlt_le_trans with (2 := proj1 (inbetween_float_bounds _ _ _ _ _ Bx)).
now apply F2R_gt_0_compat.
(* all the modes are valid *)
clear. case m.
exact inbetween_int_NE_sign.
exact inbetween_int_ZR_sign.
exact inbetween_int_DN_sign.
exact inbetween_int_UP_sign.
exact inbetween_int_NA_sign.
Qed.
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Definition Bsign x :=
  match x with
  | B754_nan => false
  | B754_zero s => s
  | B754_infinity s => s
  | B754_finite s _ _ _ => s
  end.

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Lemma Bmult_correct_aux :
  forall m sx mx ex (Hx : bounded mx ex = true) sy my ey (Hy : bounded my ey = true),
  let x := F2R (Float radix2 (cond_Zopp sx (Zpos mx)) ex) in
  let y := F2R (Float radix2 (cond_Zopp sy (Zpos my)) ey) in
  let z := binary_round_sign m (xorb sx sy) (mx * my) (ex + ey) loc_Exact in
  valid_binary z = true /\
  if Rlt_bool (Rabs (round radix2 fexp (round_mode m) (x * y))) (bpow radix2 emax) then
    FF2R radix2 z = round radix2 fexp (round_mode m) (x * y)
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  else
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    z = binary_overflow m (xorb sx sy).
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Proof.
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intros m sx mx ex Hx sy my ey Hy x y.
unfold x, y.
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rewrite <- mult_F2R.
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simpl.
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replace (xorb sx sy) with (Rlt_bool (F2R (Float radix2 (cond_Zopp sx (Zpos mx) * cond_Zopp sy (Zpos my)) (ex + ey))) 0).
apply binary_round_sign_correct.
constructor.
rewrite abs_F2R.
apply F2R_eq_compat.
rewrite Zabs_Zmult.
now rewrite 2!abs_cond_Zopp.
(* *)
change (Zpos (mx * my)) with (Zpos mx * Zpos my)%Z.
assert (forall m e, bounded m e = true -> fexp (digits radix2 (Zpos m) + e) = e)%Z.
clear. intros m e Hb.
destruct (andb_prop _ _ Hb) as (H,_).
apply Zeq_bool_eq.
now rewrite <- Z_of_nat_S_digits2_Pnat.
generalize (H _ _ Hx) (H _ _ Hy).
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clear x y sx sy Hx Hy H.
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unfold fexp, FLT_exp.
refine (_ (digits_mult_ge radix2 (Zpos mx) (Zpos my) _ _)) ; try discriminate.
refine (_ (digits_gt_0 radix2 (Zpos mx) _) (digits_gt_0 radix2 (Zpos my) _)) ; try discriminate.
generalize (digits radix2 (Zpos mx)) (digits radix2 (Zpos my)) (digits radix2 (Zpos mx * Zpos my)).
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clear -Hmax.
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unfold emin.
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intros dx dy dxy Hx Hy Hxy.
zify ; intros ; subst.
omega.
(* *)
case sx ; case sy.
apply Rlt_bool_false.
now apply F2R_ge_0_compat.
apply Rlt_bool_true.
now apply F2R_lt_0_compat.
apply Rlt_bool_true.
now apply F2R_lt_0_compat.
apply Rlt_bool_false.
now apply F2R_ge_0_compat.
Qed.
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Definition Bmult m x y :=
  match x, y with
  | B754_nan, _ => x
  | _, B754_nan => y
  | B754_infinity sx, B754_infinity sy => B754_infinity (xorb sx sy)
  | B754_infinity sx, B754_finite sy _ _ _ => B754_infinity (xorb sx sy)
  | B754_finite sx _ _ _, B754_infinity sy => B754_infinity (xorb sx sy)
  | B754_infinity _, B754_zero _ => B754_nan
  | B754_zero _, B754_infinity _ => B754_nan
  | B754_finite sx _ _ _, B754_zero sy => B754_zero (xorb sx sy)
  | B754_zero sx, B754_finite sy _ _ _ => B754_zero (xorb sx sy)
  | B754_zero sx, B754_zero sy => B754_zero (xorb sx sy)
  | B754_finite sx mx ex Hx, B754_finite sy my ey Hy =>
    FF2B _ (proj1 (Bmult_correct_aux m sx mx ex Hx sy my ey Hy))
  end.

Theorem Bmult_correct :
  forall m x y,
  if Rlt_bool (Rabs (round radix2 fexp (round_mode m) (B2R x * B2R y))) (bpow radix2 emax) then
    B2R (Bmult m x y) = round radix2 fexp (round_mode m) (B2R x * B2R y)
  else
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    B2FF (Bmult m x y) = binary_overflow m (xorb (Bsign x) (Bsign y)).
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Proof.
intros m [sx|sx| |sx mx ex Hx] [sy|sy| |sy my ey Hy] ;
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  try ( rewrite ?Rmult_0_r, ?Rmult_0_l, round_0, Rabs_R0, Rlt_bool_true ; [ apply refl_equal | apply bpow_gt_0 | auto with typeclass_instances ] ).
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simpl.
case Bmult_correct_aux.
intros H1 H2.
revert H2.
case Rlt_bool ; intros H2.
now rewrite B2R_FF2B.
now rewrite B2FF_FF2B.
Qed.

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Definition shl mx ex ex' :=
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  match (ex' - ex)%Z with
  | Zneg d => (shift_pos d mx, ex')
  | _ => (mx, ex)
  end.

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Theorem shl_correct :
  forall mx ex ex',
  let (mx', ex'') := shl mx ex ex' in
  F2R (Float radix2 (Zpos mx) ex) = F2R (Float radix2 (Zpos mx') ex'') /\
  (ex'' <= ex')%Z.
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Proof.
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intros mx ex ex'.
unfold shl.
case_eq (ex' - ex)%Z.
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(* d = 0 *)
intros H.
repeat split.
rewrite Zminus_eq with (1 := H).
apply Zle_refl.
(* d > 0 *)
intros d Hd.
repeat split.
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replace ex' with (ex' - ex + ex)%Z by ring.
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rewrite Hd.
pattern ex at 1 ; rewrite <- Zplus_0_l.
now apply Zplus_le_compat_r.
(* d < 0 *)
intros d Hd.
rewrite shift_pos_correct, Zmult_comm.
change (Zpower_pos 2 d) with (Zpower radix2 (Zpos d)).
change (Zpos d) with (Zopp (Zneg d)).
rewrite <- Hd.
split.
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replace (- (ex' - ex))%Z with (ex - ex')%Z by ring.
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apply F2R_change_exp.
apply Zle_0_minus_le.
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replace (ex - ex')%Z with (- (ex' - ex))%Z by ring.
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now rewrite Hd.
apply Zle_refl.
Qed.
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Theorem snd_shl :
  forall mx ex ex',
  (ex' <= ex)%Z ->
  snd (shl mx ex ex') = ex'.
Proof.
intros mx ex ex' He.
unfold shl.
case_eq (ex' - ex)%Z ; simpl.
intros H.
now rewrite Zminus_eq with (1 := H).
intros p.
clear -He ; zify ; omega.
intros.
apply refl_equal.
Qed.

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Definition shl_fexp mx ex :=
  shl mx ex (fexp (Z_of_nat (S (digits2_Pnat mx)) + ex)).

Theorem shl_fexp_correct :
  forall mx ex,
  let (mx', ex') := shl_fexp mx ex in
  F2R (Float radix2 (Zpos mx) ex) = F2R (Float radix2 (Zpos mx') ex') /\
  (ex' <= fexp (digits radix2 (Zpos mx') + ex'))%Z.
Proof.
intros mx ex.
unfold shl_fexp.
generalize (shl_correct mx ex (fexp (Z_of_nat (S (digits2_Pnat mx)) + ex))).
rewrite Z_of_nat_S_digits2_Pnat.
case shl.
intros mx' ex' (H1, H2).
split.
exact H1.
rewrite <- ln_beta_F2R_digits. 2: easy.
rewrite <- H1.
now rewrite ln_beta_F2R_digits.
Qed.

Definition binary_round_sign_shl m sx mx ex :=
  let '(mz, ez) := shl_fexp mx ex in binary_round_sign m sx mz ez loc_Exact.
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Theorem binary_round_sign_shl_correct :
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  forall m sx mx ex,
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  valid_binary (binary_round_sign_shl m sx mx ex) = true /\
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  let x := F2R (Float radix2 (cond_Zopp sx (Zpos mx)) ex) in
  if Rlt_bool (Rabs (round radix2 fexp (round_mode m) x)) (bpow radix2 emax) then
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    FF2R radix2 (binary_round_sign_shl m sx mx ex) = round radix2 fexp (round_mode m) x
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  else
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    binary_round_sign_shl m sx mx ex = binary_overflow m sx.
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Proof.
intros m sx mx ex.
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unfold binary_round_sign_shl.
generalize (shl_fexp_correct mx ex).
destruct (shl_fexp mx ex) as (mz, ez).
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intros (H1, H2).
simpl.
set (x := F2R (Float radix2 (cond_Zopp sx (Zpos mx)) ex)).
replace sx with (Rlt_bool x 0).
apply binary_round_sign_correct.
constructor.
unfold x.
now rewrite abs_F2R, abs_cond_Zopp.
exact H2.
unfold x.
case sx.
apply Rlt_bool_true.
now apply F2R_lt_0_compat.
apply Rlt_bool_false.
now apply F2R_ge_0_compat.
Qed.

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Definition Bplus m x y :=
  match x, y with
  | B754_nan, _ => x
  | _, B754_nan => y
  | B754_infinity sx, B754_infinity sy =>
    if Bool.eqb sx sy then x else B754_nan
  | B754_infinity _, _ => x
  | _, B754_infinity _ => y
  | B754_zero sx, B754_zero sy =>
    if Bool.eqb sx sy then x else
    match m with mode_DN => B754_zero true | _ => B754_zero false end
  | B754_zero _, _ => y
  | _, B754_zero _ => x
  | B754_finite sx mx ex Hx, B754_finite sy my ey Hy =>
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    let ez := Zmin ex ey in
    match Zplus (cond_Zopp sx (Zpos (fst (shl mx ex ez)))) (cond_Zopp sy (Zpos (fst (shl my ey ez)))) with
    | Z0 =>
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      match m with mode_DN => B754_zero true | _ => B754_zero false end
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    | Zpos mz =>
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      FF2B (binary_round_sign_shl m false mz ez) (proj1 (binary_round_sign_shl_correct _ _ _ _))
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    | Zneg mz =>
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      FF2B (binary_round_sign_shl m true mz ez) (proj1 (binary_round_sign_shl_correct _ _ _ _))
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    end
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  end.

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Theorem Bplus_correct :
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  forall m x y,
  is_finite x = true ->
  is_finite y = true ->
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  if Rlt_bool (Rabs (round radix2 fexp (round_mode m) (B2R x + B2R y))) (bpow radix2 emax) then
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    B2R (Bplus m x y) = round radix2 fexp (round_mode m) (B2R x + B2R y)
  else
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    (B2FF (Bplus m x y) = binary_overflow m (Bsign x) /\ Bsign x = Bsign y).
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Proof with auto with typeclass_instances.
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intros m [sx|sx| |sx mx ex Hx] [sy|sy| |sy my ey Hy] Fx Fy ; try easy.
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(* *)
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rewrite Rplus_0_r, round_0, Rabs_R0, Rlt_bool_true...
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simpl.
case (Bool.eqb sx sy) ; try easy.
now case m.
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apply bpow_gt_0.
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(* *)
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rewrite Rplus_0_l, round_generic, Rlt_bool_true...
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apply B2R_lt_emax.
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apply generic_format_B2R.
(* *)
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rewrite Rplus_0_r, round_generic, Rlt_bool_true...
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apply B2R_lt_emax.
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apply generic_format_B2R.
(* *)
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clear Fx Fy.
simpl.
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set (ez := Zmin ex ey).
set (mz := (cond_Zopp sx (Zpos (fst (shl mx ex ez))) + cond_Zopp sy (Zpos (fst (shl my ey ez))))%Z).
assert (Hp: (F2R (Float radix2 (cond_Zopp sx (Zpos mx)) ex) +
  F2R (Float radix2 (cond_Zopp sy (Zpos my)) ey))%R = F2R (Float radix2 mz ez)).
rewrite 2!F2R_cond_Zopp.
generalize (shl_correct mx ex ez).
generalize (shl_correct my ey ez).
generalize (snd_shl mx ex ez (Zle_min_l ex ey)).
generalize (snd_shl my ey ez (Zle_min_r ex ey)).
destruct (shl mx ex ez) as (mx', ex').
destruct (shl my ey ez) as (my', ey').
simpl.
intros H1 H2.
rewrite H1, H2.
clear H1 H2.
intros (H1, _) (H2, _).
rewrite H1, H2.
clear H1 H2.
rewrite <- 2!F2R_cond_Zopp.
unfold F2R. simpl.
now rewrite <- Rmult_plus_distr_r, <- Z2R_plus.
rewrite Hp.
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assert (Sz: (bpow radix2 emax <= Rabs (round radix2 fexp (round_mode m) (F2R (Float radix2 mz ez))))%R -> sx = Rlt_bool (F2R (Float radix2 mz ez)) 0 /\ sx = sy).
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(* . *)
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rewrite <- Hp.
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intros Bz.
destruct (Bool.bool_dec sx sy) as [Hs|Hs].
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(* .. *)
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refine (conj _ Hs).
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rewrite Hs.
apply sym_eq.
case sy.
apply Rlt_bool_true.
rewrite <- (Rplus_0_r 0).
apply Rplus_lt_compat.
now apply F2R_lt_0_compat.
now apply F2R_lt_0_compat.
apply Rlt_bool_false.
rewrite <- (Rplus_0_r 0).
apply Rplus_le_compat.
now apply F2R_ge_0_compat.
now apply F2R_ge_0_compat.
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(* .. *)
elim Rle_not_lt with (1 := Bz).
generalize (bounded_lt_emax _ _ Hx) (bounded_lt_emax _ _ Hy) (andb_prop _ _ Hx) (andb_prop _ _ Hy).
intros Bx By (Hx',_) (Hy',_).
generalize (canonic_bounded_prec sx _ _ Hx') (canonic_bounded_prec sy _ _ Hy').
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clear -Bx By Hs.
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intros Cx Cy.
destruct sx.
(* ... *)
destruct sy.
now elim Hs.
clear Hs.
apply Rabs_lt.
split.
apply Rlt_le_trans with (F2R (Float radix2 (cond_Zopp true (Zpos mx)) ex)).
rewrite <- opp_F2R.
now apply Ropp_lt_contravar.
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apply round_monotone_l...
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now apply generic_format_canonic.
pattern (F2R (Float radix2 (cond_Zopp true (Zpos mx)) ex)) at 1 ; rewrite <- Rplus_0_r.
apply Rplus_le_compat_l.
now apply F2R_ge_0_compat.
apply Rle_lt_trans with (2 := By).
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apply round_monotone_r...
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now apply generic_format_canonic.
rewrite <- (Rplus_0_l (F2R (Float radix2 (Zpos my) ey))).
apply Rplus_le_compat_r.
now apply F2R_le_0_compat.
(* ... *)
destruct sy.
2: now elim Hs.
clear Hs.
apply Rabs_lt.
split.
apply Rlt_le_trans with (F2R (Float radix2 (cond_Zopp true (Zpos my)) ey)).
rewrite <- opp_F2R.
now apply Ropp_lt_contravar.
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apply round_monotone_l...
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now apply generic_format_canonic.
pattern (F2R (Float radix2 (cond_Zopp true (Zpos my)) ey)) at 1 ; rewrite <- Rplus_0_l.
apply Rplus_le_compat_r.
now apply F2R_ge_0_compat.
apply Rle_lt_trans with (2 := Bx).
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apply round_monotone_r...
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now apply generic_format_canonic.
rewrite <- (Rplus_0_r (F2R (Float radix2 (Zpos mx) ex))).
apply Rplus_le_compat_l.
now apply F2R_le_0_compat.
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destruct mz as [|mz|mz].
(* . mz = 0 *)
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rewrite F2R_0, round_0, Rabs_R0, Rlt_bool_true...
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now case m.
apply bpow_gt_0.
(* . mz > 0 *)
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generalize (binary_round_sign_shl_correct m false mz ez).
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simpl.
case Rlt_bool_spec.
intros _ (Vz, Rz).
now rewrite B2R_FF2B.
intros Hz' (Vz, Rz).
specialize (Sz Hz').
refine (conj _ (proj2 Sz)).
rewrite B2FF_FF2B.
rewrite (proj1 Sz).
rewrite Rlt_bool_false.
exact Rz.
now apply F2R_ge_0_compat.
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(* . mz < 0 *)
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generalize (binary_round_sign_shl_correct m true mz ez).
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simpl.
case Rlt_bool_spec.
intros _ (Vz, Rz).
now rewrite B2R_FF2B.
intros Hz' (Vz, Rz).
specialize (Sz Hz').
refine (conj _ (proj2 Sz)).
rewrite B2FF_FF2B.
rewrite (proj1 Sz).
rewrite Rlt_bool_true.
exact Rz.
now apply F2R_lt_0_compat.
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Qed.
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Definition Bminus m x y := Bplus m x (Bopp y).

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Lemma Bdiv_correct_aux :
  forall m sx mx ex (Hx : bounded mx ex = true) sy my ey (Hy : bounded my ey = true),
  let x := F2R (Float radix2 (cond_Zopp sx (Zpos mx)) ex) in
  let y := F2R (Float radix2 (cond_Zopp sy (Zpos my)) ey) in
  let z :=
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    let '(mz, ez, lz) := Fdiv_core radix2 prec (Zpos mx) ex (Zpos my) ey in
    match mz with
    | Zpos mz => binary_round_sign m (xorb sx sy) mz ez lz
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    | _ => F754_nan (* dummy *)
    end in
  valid_binary z = true /\
  if Rlt_bool (Rabs (round radix2 fexp (round_mode m) (x / y))) (bpow radix2 emax) then
    FF2R radix2 z = round radix2 fexp (round_mode m) (x / y)
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  else
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    z = binary_overflow m (xorb sx sy).
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Proof.
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intros m sx mx ex Hx sy my ey Hy.
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refine (_ (Fdiv_core_correct radix2 prec (Zpos mx) ex (Zpos my) ey _ _ _)) ; try easy.
destruct (Fdiv_core radix2 prec (Zpos mx) ex (Zpos my) ey) as ((mz, ez), lz).
intros (Pz, Bz).
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simpl.
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replace (xorb sx sy) with (Rlt_bool (F2R (Float radix2 (cond_Zopp sx (Zpos mx)) ex) *
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  / F2R (Float radix2 (cond_Zopp sy (Zpos my)) ey)) 0).
unfold Rdiv.
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destruct mz as [|mz|mz].
(* . mz = 0 *)
elim (Zlt_irrefl prec).
now apply Zle_lt_trans with Z0.
(* . mz > 0 *)
apply binary_round_sign_correct.
rewrite Rabs_mult, Rabs_Rinv.
now rewrite 2!abs_F2R, 2!abs_cond_Zopp.
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case sy.
apply Rlt_not_eq.
now apply F2R_lt_0_compat.
apply Rgt_not_eq.
now apply F2R_gt_0_compat.
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revert Pz.
generalize (digits radix2 (Zpos mz)).
unfold fexp, FLT_exp.
clear.
intros ; zify ; subst.
omega.
(* . mz < 0 *)
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elim Rlt_not_le with (1 := proj2 (inbetween_float_bounds _ _ _ _ _ Bz)).
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apply Rle_trans with R0.
apply F2R_le_0_compat.
now case mz.
apply Rmult_le_pos.
now apply F2R_ge_0_compat.
apply Rlt_le.
apply Rinv_0_lt_compat.
now apply F2R_gt_0_compat.
(* *)
case sy ; simpl.
change (Zneg my) with (Zopp (Zpos my)).
rewrite <- opp_F2R.
rewrite <- Ropp_inv_permute.
rewrite Ropp_mult_distr_r_reverse.
case sx ; simpl.
apply Rlt_bool_false.
rewrite <- Ropp_mult_distr_l_reverse.
apply Rmult_le_pos.
rewrite opp_F2R.
now apply F2R_ge_0_compat.
apply Rlt_le.
apply Rinv_0_lt_compat.
now apply F2R_gt_0_compat.
apply Rlt_bool_true.
rewrite <- Ropp_0.
apply Ropp_lt_contravar.
apply Rmult_lt_0_compat.
now apply F2R_gt_0_compat.
apply Rinv_0_lt_compat.
now apply F2R_gt_0_compat.
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apply Rgt_not_eq.
now apply F2R_gt_0_compat.
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case sx.
apply Rlt_bool_true.
rewrite <- opp_F2R.
rewrite Ropp_mult_distr_l_reverse.
rewrite <- Ropp_0.
apply Ropp_lt_contravar.
apply Rmult_lt_0_compat.
now apply F2R_gt_0_compat.
apply Rinv_0_lt_compat.
now apply F2R_gt_0_compat.
apply Rlt_bool_false.
apply Rmult_le_pos.
now apply F2R_ge_0_compat.
apply Rlt_le.
apply Rinv_0_lt_compat.
now apply F2R_gt_0_compat.
Qed.

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Definition Bdiv m x y :=
  match x, y with
  | B754_nan, _ => x
  | _, B754_nan => y
  | B754_infinity sx, B754_infinity sy => B754_nan
  | B754_infinity sx, B754_finite sy _ _ _ => B754_infinity (xorb sx sy)
  | B754_finite sx _ _ _, B754_infinity sy => B754_infinity (xorb sx sy)
  | B754_infinity sx, B754_zero sy => B754_infinity (xorb sx sy)
  | B754_zero sx, B754_infinity sy => B754_zero (xorb sx sy)
  | B754_finite sx _ _ _, B754_zero sy => B754_infinity (xorb sx sy)
  | B754_zero sx, B754_finite sy _ _ _ => B754_zero (xorb sx sy)
  | B754_zero sx, B754_zero sy => B754_nan
  | B754_finite sx mx ex Hx, B754_finite sy my ey Hy =>
    FF2B _ (proj1 (Bdiv_correct_aux m sx mx ex Hx sy my ey Hy))
  end.

Theorem Bdiv_correct :
  forall m x y,
  B2R y <> R0 ->
  if Rlt_bool (Rabs (round radix2 fexp (round_mode m) (B2R x / B2R y))) (bpow radix2 emax) then
    B2R (Bdiv m x y) = round radix2 fexp (round_mode m) (B2R x / B2R y)
  else