ExpressionSemantics.v 11.8 KB
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From Coq
     Require Import Reals.Reals.

From Flover.Infra
     Require Import RealRationalProps RationalSimps Ltacs.

From Flover.Infra
     Require Export ExpressionAbbrevs.
(**
  Finally, define an error function that computes an errorneous value for a
  given type. For a fixed-point datatype, truncation is used and any
  floating-point type is perturbed. As we need not compute on this function we
  define it in Prop.
**)
Definition perturb (rVal:R) (m:mType) (delta:R) :R :=
  match m with
  (* The Real-type has no error *)
  |REAL => rVal
  (* Fixed-point numbers have an absolute error *)
  |F w f => rVal + delta
  (* Floating-point numbers have a relative error *)
  | _ => rVal * (1 + delta)
  end.

Hint Unfold perturb.

(**
Define expression evaluation relation parametric by an "error" epsilon.
The result value exprresses float computations according to the IEEE standard,
using a perturbation of the real valued computation by (1 + delta), where
|delta| <= machine epsilon.
**)
Open Scope R_scope.

Inductive eval_expr (E:env)
          (Gamma: expr R -> option mType)
  :(expr R) -> R -> mType -> Prop :=
| Var_load m x v:
    Gamma (Var R x) = Some m ->
    E x = Some v ->
    eval_expr E Gamma (Var R x) v m
| Const_dist m n delta:
    Gamma (Const m n) = Some m ->
    Rabs delta <= mTypeToR m ->
    eval_expr E Gamma (Const m n) (perturb n m delta) m
| Unop_neg m mN f1 v1:
    Gamma (Unop Neg f1) = Some mN ->
    isCompat m mN = true ->
    eval_expr E Gamma f1 v1 m ->
    eval_expr E Gamma (Unop Neg f1) (evalUnop Neg v1) mN
| Unop_inv m mN f1 v1 delta:
    Gamma (Unop Inv f1) = Some mN ->
    isCompat m mN = true ->
    Rabs delta <= mTypeToR mN ->
    eval_expr E Gamma f1 v1 m ->
    (~ v1 = 0)%R  ->
    eval_expr E Gamma (Unop Inv f1) (perturb (evalUnop Inv v1) mN delta) mN
| Downcast_dist m m1 f1 v1 delta:
    Gamma (Downcast m f1) = Some m ->
    isMorePrecise m1 m = true ->
    Rabs delta <= mTypeToR m ->
    eval_expr E Gamma f1 v1 m1 ->
    eval_expr E Gamma (Downcast m f1) (perturb v1 m delta) m
| Binop_dist m1 m2 op f1 f2 v1 v2 delta m:
    Gamma (Binop op f1 f2) = Some m ->
    isJoin m1 m2 m = true ->
    Rabs delta <= mTypeToR m ->
    eval_expr E Gamma f1 v1 m1 ->
    eval_expr E Gamma f2 v2 m2 ->
    ((op = Div) -> (~ v2 = 0)%R) ->
    eval_expr E Gamma (Binop op f1 f2) (perturb (evalBinop op v1 v2) m delta) m
| Fma_dist m1 m2 m3 m f1 f2 f3 v1 v2 v3 delta:
    Gamma (Fma f1 f2 f3) = Some m ->
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    isJoin3 m1 m2 m3 m = true ->
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    Rabs delta <= mTypeToR m ->
    eval_expr E Gamma f1 v1 m1 ->
    eval_expr E Gamma f2 v2 m2 ->
    eval_expr E Gamma f3 v3 m3 ->
    eval_expr E Gamma (Fma f1 f2 f3) (perturb (evalFma v1 v2 v3) m delta) m.

Close Scope R_scope.

Hint Constructors eval_expr.

(** *)
(*   Show some simpler (more general) rule lemmata *)
(* **)
Lemma Const_dist' m n delta v m' E Gamma:
  Rle (Rabs delta) (mTypeToR m') ->
  v = perturb n m delta ->
  Gamma (Const m n) = Some m ->
  m' = m ->
  eval_expr E Gamma (Const m n) v m'.
Proof.
  intros; subst; auto.
Qed.

Hint Resolve Const_dist'.

Lemma Unop_neg' m mN f1 v1 v m' E Gamma:
  eval_expr E Gamma f1 v1 m ->
  v = evalUnop Neg v1 ->
  Gamma (Unop Neg f1) = Some mN ->
  isCompat m mN = true ->
  m' = mN ->
  eval_expr E Gamma (Unop Neg f1) v m'.
Proof.
  intros; subst; eauto.
Qed.

Hint Resolve Unop_neg'.

Lemma Unop_inv' m mN f1 v1 delta v m' E Gamma:
  Rle (Rabs delta) (mTypeToR m') ->
  eval_expr E Gamma f1 v1 m ->
  (~ v1 = 0)%R  ->
  v = perturb (evalUnop Inv v1) mN delta ->
  Gamma (Unop Inv f1) = Some mN ->
  isCompat m mN = true ->
  m' = mN ->
  eval_expr E Gamma (Unop Inv f1) v m'.
Proof.
  intros; subst; eauto.
Qed.

Hint Resolve Unop_inv'.

Lemma Downcast_dist' m m1 f1 v1 delta v m' E Gamma:
  isMorePrecise m1 m = true ->
  Rle (Rabs delta) (mTypeToR m') ->
  eval_expr E Gamma f1 v1 m1 ->
  v = (perturb v1 m delta) ->
  Gamma (Downcast m f1) = Some m ->
  m' = m ->
  eval_expr E Gamma (Downcast m f1) v m'.
Proof.
  intros; subst; eauto.
Qed.

Hint Resolve Downcast_dist'.

Lemma Binop_dist' m1 m2 op f1 f2 v1 v2 delta v m m' E Gamma:
  Rle (Rabs delta) (mTypeToR m') ->
  eval_expr E Gamma f1 v1 m1 ->
  eval_expr E Gamma f2 v2 m2 ->
  ((op = Div) -> (~ v2 = 0)%R) ->
  v = perturb (evalBinop op v1 v2) m' delta ->
  Gamma (Binop op f1 f2) = Some m ->
  isJoin m1 m2 m = true ->
  m = m' ->
  eval_expr E Gamma (Binop op f1 f2) v m'.
Proof.
  intros; subst; eauto.
Qed.

Hint Resolve Binop_dist'.

Lemma Fma_dist' m1 m2 m3 f1 f2 f3 v1 v2 v3 delta v m' E Gamma m:
  Rle (Rabs delta) (mTypeToR m') ->
  eval_expr E Gamma f1 v1 m1 ->
  eval_expr E Gamma f2 v2 m2 ->
  eval_expr E Gamma f3 v3 m3 ->
  v = perturb (evalFma v1 v2 v3) m' delta ->
  Gamma (Fma f1 f2 f3) = Some m ->
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  isJoin3 m1 m2 m3 m = true ->
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  m = m' ->
  eval_expr E Gamma (Fma f1 f2 f3) v m'.
Proof.
  intros; subst; eauto.
Qed.

Hint Resolve Fma_dist'.

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Lemma Gamma_def e E Gamma v m:
  eval_expr E Gamma e v m ->
  Gamma e = Some m.
Proof.
  destruct e; intros * eval_e; inversion eval_e; subst; auto.
Qed.

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Lemma toRMap_eval_REAL f:
  forall v E Gamma m, eval_expr E (toRMap Gamma) (toREval f) v m -> m = REAL.
Proof.
  induction f; intros * eval_f; inversion eval_f; subst;
  repeat
    match goal with
    | H: context[toRMap _ _] |- _ => unfold toRMap in H
    | H: context[match ?Gamma ?v with | _ => _ end ] |- _ => destruct (Gamma v) eqn:?
    | H: Some ?m1 = Some ?m2 |- _ => inversion H; try auto
    | H: None = Some ?m |- _ => inversion H
    end; try auto.
Qed.

(**
  If |delta| <= 0 then perturb v delta is exactly v.
**)
Lemma delta_0_deterministic (v:R) m (delta:R):
  (Rabs delta <= 0)%R ->
  perturb v m delta = v.
Proof.
  intros abs_0; apply Rabs_0_impl_eq in abs_0; subst.
  unfold perturb. destruct m; lra.
Qed.

(**
Evaluation with 0 as machine epsilon is deterministic
**)
Lemma meps_0_deterministic (f:expr R) (E:env) Gamma:
  forall v1 v2,
  eval_expr E (toRMap Gamma) (toREval f) v1 REAL ->
  eval_expr E (toRMap Gamma) (toREval f) v2 REAL ->
  v1 = v2.
Proof.
  induction f;
    intros v1 v2 ev1 ev2.
  - inversion ev1; inversion ev2; subst.
    rewrite H1 in H6.
    inversion H6; auto.
  - inversion ev1; inversion ev2; subst.
    simpl in *; subst; auto.
  - inversion ev1; inversion ev2; subst; try congruence.
    + rewrite (IHf v0 v3); [ auto | |];
        destruct m, m0; cbn in *; congruence.
    + cbn in *. Flover_compute; rewrite (IHf v0 v3); [auto | | ];
                  destruct m, m0; cbn in *; congruence.
  - inversion ev1; inversion ev2; subst.
    assert (m0 = REAL) by (eapply toRMap_eval_REAL; eauto).
    assert (m3 = REAL) by (eapply toRMap_eval_REAL; eauto).
    assert (m1 = REAL) by (eapply toRMap_eval_REAL; eauto).
    assert (m2 = REAL) by (eapply toRMap_eval_REAL; eauto).
    subst.
    rewrite (IHf1 v0 v4); try auto.
    rewrite (IHf2 v3 v5); try auto.
  - inversion ev1; inversion ev2; subst.
    assert (m0 = REAL) by (eapply toRMap_eval_REAL; eauto).
    assert (m1 = REAL) by (eapply toRMap_eval_REAL; eauto).
    assert (m2 = REAL) by (eapply toRMap_eval_REAL; eauto).
    assert (m3 = REAL) by (eapply toRMap_eval_REAL; eauto).
    assert (m4 = REAL) by (eapply toRMap_eval_REAL; eauto).
    assert (m5 = REAL) by (eapply toRMap_eval_REAL; eauto).
    subst.
    rewrite (IHf1 v0 v5); try auto.
    rewrite (IHf2 v3 v6); try auto.
    rewrite (IHf3 v4 v7); try auto.
  - inversion ev1; inversion ev2; subst.
    apply REAL_least_precision in H2;
      apply REAL_least_precision in H9; subst.
    rewrite (IHf v0 v3); try auto.
Qed.

(**
Helping lemmas. Needed in soundness proof.
For each evaluation of using an arbitrary epsilon, we can replace it by
evaluating the subexprressions and then binding the result values to different
variables in the Environment.
 **)
Lemma binary_unfolding b f1 f2 E v1 v2 m1 m2 m Gamma delta:
  (b = Div -> ~(v2 = 0 )%R) ->
  (Rabs delta <= mTypeToR m)%R ->
  eval_expr E Gamma f1 v1 m1 ->
  eval_expr E Gamma f2 v2 m2 ->
  eval_expr E Gamma (Binop b f1 f2) (perturb (evalBinop b v1 v2) m delta) m ->
  eval_expr (updEnv 2 v2 (updEnv 1 v1 emptyEnv))
            (updDefVars (Binop b (Var R 1) (Var R 2)) m
           (updDefVars (Var R 2) m2 (updDefVars (Var R 1) m1 Gamma)))
             (Binop b (Var R 1) (Var R 2)) (perturb (evalBinop b v1 v2) m delta) m.
Proof.
  intros no_div_zero err_v eval_f1 eval_f2 eval_float.
  inversion eval_float; subst.
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  rewrite H2 in *.
  eapply Gamma_def in eval_f1; eapply Gamma_def in H6.
  rewrite H6 in eval_f1; inversion eval_f1; subst.
  eapply Gamma_def in eval_f2; eapply Gamma_def in H8;
    rewrite H8 in eval_f2; inversion eval_f2; subst.
  eapply Binop_dist' with (v1:=v1) (v2:=v2) (delta:=delta); try eauto.
  unfold updDefVars.
  unfold R_orderedExps.compare; rewrite R_orderedExps.exprCompare_refl; auto.
Qed.
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Lemma fma_unfolding f1 f2 f3 E v1 v2 v3 m1 m2 m3 m Gamma delta:
  (Rabs delta <= mTypeToR m)%R ->
  eval_expr E Gamma f1 v1 m1 ->
  eval_expr E Gamma f2 v2 m2 ->
  eval_expr E Gamma f3 v3 m3 ->
  eval_expr E Gamma (Fma f1 f2 f3) (perturb (evalFma v1 v2 v3) m delta) m ->
  eval_expr (updEnv 3 v3 (updEnv 2 v2 (updEnv 1 v1 emptyEnv)))
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            (updDefVars (Fma (Var R 1) (Var R 2) (Var R 3) ) m
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            (updDefVars (Var R 3) m3 (updDefVars (Var R 2) m2
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                                                 (updDefVars (Var R 1) m1 Gamma))))
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             (Fma (Var R 1) (Var R 2) (Var R 3)) (perturb (evalFma v1 v2 v3) m delta) m.
Proof.
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  intros err_v eval_f1 eval_f2 eval_f3 eval_float.
  inversion eval_float; subst.
  repeat
    (match goal with
     | [H1: eval_expr ?E ?Gamma ?f ?v1 ?m1, H2: eval_expr ?E ?Gamma ?f ?v2 ?m2 |- _] =>
       assert (m1 = m2)
         by (eapply Gamma_def in H1; eapply Gamma_def in H2;
             rewrite H2 in H1; inversion H1; subst; auto);
       revert H1 H2
     end).
  intros; subst.
  rewrite H2.
  eapply Fma_dist' with (v1:=v1) (v2:=v2) (v3:=v3) (delta:=delta); try eauto.
  - eapply Var_load; eauto.
  - eapply Var_load; eauto.
  - eapply Var_load; eauto.
  - cbn; auto.
Qed.
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Lemma eval_eq_env e:
  forall E1 E2 Gamma v m,
    (forall x, E1 x = E2 x) ->
    eval_expr E1 Gamma e v m ->
    eval_expr E2 Gamma e v m.
Proof.
  induction e; intros;
    (match_pat (eval_expr _ _ _ _ _) (fun H => inversion H; subst; simpl in H));
    try eauto.
  eapply Var_load; auto.
  rewrite <- (H n); auto.
Qed.

Lemma eval_expr_ignore_bind e:
  forall x v m Gamma E,
    eval_expr E Gamma e v m ->
    ~ NatSet.In x (usedVars e) ->
    forall m_new v_new,
    eval_expr (updEnv x v_new E) (updDefVars (Var R x) m_new Gamma) e v m.
Proof.
  induction e; intros * eval_e no_usedVar *; cbn in *;
    inversion eval_e; subst; try eauto.
  - assert (n <> x).
    { hnf. intros. subst. apply no_usedVar; set_tac. }
    rewrite <- Nat.eqb_neq in H.
    eapply Var_load.
    + unfold updDefVars.
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      cbn.
      apply beq_nat_false in H.
      destruct (n ?= x)%nat eqn:?; try auto.
      apply Nat.compare_eq in Heqc; subst; congruence.
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    + unfold updEnv.
      rewrite H; auto.
  - eapply Binop_dist'; eauto;
      [ eapply IHe1 | eapply IHe2];
      eauto;
      hnf; intros; eapply no_usedVar;
      set_tac.
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  - eapply Fma_dist'; eauto;
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      [eapply IHe1 | eapply IHe2 | eapply IHe3];
      eauto;
      hnf; intros; eapply no_usedVar;
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        set_tac.
Qed.
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Lemma swap_Gamma_eval_expr e E vR m Gamma1 Gamma2:
  (forall n, Gamma1 n = Gamma2 n) ->
  eval_expr E Gamma1 e vR m ->
  eval_expr E Gamma2 e vR m.
Proof.
  revert E vR Gamma1 Gamma2 m;
    induction e; intros * Gamma_eq eval_e;
      inversion eval_e; subst; simpl in *;
        [ eapply Var_load
        | eapply Const_dist'
        | eapply Unop_neg'
        | eapply Unop_inv'
        | eapply Binop_dist'
        | eapply Fma_dist'
        | eapply Downcast_dist' ]; try eauto;
          rewrite <- Gamma_eq; auto.
Qed.

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Lemma Rmap_updVars_comm Gamma n m:
  forall x,
    updDefVars n REAL (toRMap Gamma) x = toRMap (updDefVars n m Gamma) x.
Proof.
  unfold updDefVars, toRMap; simpl.
  intros x; destruct (R_orderedExps.compare x n); auto.
Qed.