From 62c92241a69cd4597650d8408744ff922ca34245 Mon Sep 17 00:00:00 2001 From: Xavier Leroy Date: Wed, 8 May 2019 16:05:56 +0200 Subject: Define integer sign extension for zero bits Just ensure sign_ext 0 x = zero. This simplifies some statements and proofs. --- lib/Zbits.v | 58 ++++++++++++++++++++++++++++++---------------------------- 1 file changed, 30 insertions(+), 28 deletions(-) (limited to 'lib/Zbits.v') diff --git a/lib/Zbits.v b/lib/Zbits.v index 459e891b..fb40ccb5 100644 --- a/lib/Zbits.v +++ b/lib/Zbits.v @@ -557,7 +557,7 @@ Definition Zzero_ext (n: Z) (x: Z) : Z := Definition Zsign_ext (n: Z) (x: Z) : Z := Z.iter (Z.pred n) (fun rec x => Zshiftin (Z.odd x) (rec (Z.div2 x))) - (fun x => if Z.odd x then -1 else 0) + (fun x => if Z.odd x && zlt 0 n then -1 else 0) x. Lemma Ziter_base: @@ -606,32 +606,34 @@ Proof. Qed. Lemma Zsign_ext_spec: - forall n x i, 0 <= i -> 0 < n -> + forall n x i, 0 <= i -> Z.testbit (Zsign_ext n x) i = Z.testbit x (if zlt i n then i else n - 1). Proof. - intros n0 x i I0 N0. - revert x i I0. pattern n0. apply Zlt_lower_bound_ind with (z := 1). - - unfold Zsign_ext. intros. - destruct (zeq x 1). - + subst x; simpl. - replace (if zlt i 1 then i else 0) with 0. - rewrite Ztestbit_base. - destruct (Z.odd x0). - apply Ztestbit_m1; auto. - apply Ztestbit_0. - destruct (zlt i 1); omega. - + set (x1 := Z.pred x). replace x1 with (Z.succ (Z.pred x1)). - rewrite Ziter_succ. rewrite Ztestbit_shiftin. - destruct (zeq i 0). - * subst i. rewrite zlt_true. rewrite Ztestbit_base; auto. omega. - * rewrite H. unfold x1. destruct (zlt (Z.pred i) (Z.pred x)). - rewrite zlt_true. rewrite (Ztestbit_eq i x0); auto. rewrite zeq_false; auto. omega. - rewrite zlt_false. rewrite (Ztestbit_eq (x - 1) x0). rewrite zeq_false; auto. - omega. omega. omega. unfold x1; omega. omega. - * omega. - * unfold x1; omega. - * omega. - - omega. + intros n0 x i I0. unfold Zsign_ext. + unfold proj_sumbool; destruct (zlt 0 n0) as [N0|N0]; simpl. +- revert x i I0. pattern n0. apply Zlt_lower_bound_ind with (z := 1); [ | omega ]. + unfold Zsign_ext. intros. + destruct (zeq x 1). + + subst x; simpl. + replace (if zlt i 1 then i else 0) with 0. + rewrite Ztestbit_base. + destruct (Z.odd x0); [ apply Ztestbit_m1; auto | apply Ztestbit_0 ]. + destruct (zlt i 1); omega. + + set (x1 := Z.pred x). replace x1 with (Z.succ (Z.pred x1)) by omega. + rewrite Ziter_succ by (unfold x1; omega). rewrite Ztestbit_shiftin by auto. + destruct (zeq i 0). + * subst i. rewrite zlt_true. rewrite Ztestbit_base; auto. omega. + * rewrite H by (unfold x1; omega). + unfold x1; destruct (zlt (Z.pred i) (Z.pred x)). + ** rewrite zlt_true by omega. + rewrite (Ztestbit_eq i x0) by omega. + rewrite zeq_false by omega. auto. + ** rewrite zlt_false by omega. + rewrite (Ztestbit_eq (x - 1) x0) by omega. + rewrite zeq_false by omega. auto. +- rewrite Ziter_base by omega. rewrite andb_false_r. + rewrite Z.testbit_0_l, Z.testbit_neg_r. auto. + destruct (zlt i n0); omega. Qed. (** [Zzero_ext n x] is [x modulo 2^n] *) @@ -661,7 +663,7 @@ Qed. (** Relation between [Zsign_ext n x] and (Zzero_ext n x] *) Lemma Zsign_ext_zero_ext: - forall n, 0 < n -> forall x, + forall n, 0 <= n -> forall x, Zsign_ext n x = Zzero_ext n x - (if Z.testbit x (n - 1) then two_p n else 0). Proof. intros. apply equal_same_bits; intros. @@ -698,12 +700,12 @@ Proof. assert (C: two_p n = 2 * two_p (n - 1)). { rewrite <- two_p_S by omega. f_equal; omega. } rewrite Zzero_ext_spec, zlt_true in B by omega. - rewrite Zsign_ext_zero_ext by auto. rewrite B. + rewrite Zsign_ext_zero_ext by omega. rewrite B. destruct (zlt (Zzero_ext n x) (two_p (n - 1))); omega. Qed. Lemma eqmod_Zsign_ext: - forall n x, 0 < n -> + forall n x, 0 <= n -> eqmod (two_p n) (Zsign_ext n x) x. Proof. intros. rewrite Zsign_ext_zero_ext by auto. -- cgit