Module Memdata


In-memory representation of values.

Require Import Coqlib.
Require Import AST.
Require Import Integers.
Require Import Floats.
Require Import Values.

Properties of memory chunks


Memory reads and writes are performed by quantities called memory chunks, encoding the type, size and signedness of the chunk being addressed. The following functions extract the size information from a chunk.

Definition size_chunk (chunk: memory_chunk) : Z :=
  match chunk with
  | Mint8signed => 1
  | Mint8unsigned => 1
  | Mint16signed => 2
  | Mint16unsigned => 2
  | Mint32 => 4
  | Mfloat32 => 4
  | Mfloat64 => 8
  end.

Lemma size_chunk_pos:
  forall chunk, size_chunk chunk > 0.
Proof.
  intros. destruct chunk; simpl; omega.
Qed.

Definition size_chunk_nat (chunk: memory_chunk) : nat :=
  nat_of_Z(size_chunk chunk).

Lemma size_chunk_conv:
  forall chunk, size_chunk chunk = Z_of_nat (size_chunk_nat chunk).
Proof.
  intros. destruct chunk; reflexivity.
Qed.

Lemma size_chunk_nat_pos:
  forall chunk, exists n, size_chunk_nat chunk = S n.
Proof.
  intros.
  generalize (size_chunk_pos chunk). rewrite size_chunk_conv.
  destruct (size_chunk_nat chunk).
  simpl; intros; omegaContradiction.
  intros; exists n; auto.
Qed.

Memory reads and writes must respect alignment constraints: the byte offset of the location being addressed should be an exact multiple of the natural alignment for the chunk being addressed. This natural alignment is defined by the following align_chunk function. Some target architectures (e.g. PowerPC and x86) have no alignment constraints, which we could reflect by taking align_chunk chunk = 1. However, other architectures have stronger alignment requirements. The following definition is appropriate for PowerPC, ARM and x86.

Definition align_chunk (chunk: memory_chunk) : Z :=
  match chunk with
  | Mint8signed => 1
  | Mint8unsigned => 1
  | Mint16signed => 2
  | Mint16unsigned => 2
  | _ => 4
  end.

Lemma align_chunk_pos:
  forall chunk, align_chunk chunk > 0.
Proof.
  intro. destruct chunk; simpl; omega.
Qed.

Lemma align_size_chunk_divides:
  forall chunk, (align_chunk chunk | size_chunk chunk).
Proof.
  intros. destruct chunk; simpl; try apply Zdivide_refl. exists 2; auto.
Qed.

Lemma align_chunk_compat:
  forall chunk1 chunk2,
  size_chunk chunk1 = size_chunk chunk2 -> align_chunk chunk1 = align_chunk chunk2.
Proof.
  intros chunk1 chunk2.
  destruct chunk1; destruct chunk2; simpl; congruence.
Qed.

Memory values


A ``memory value'' is a byte-sized quantity that describes the current content of a memory cell. It can be either:

Values stored in memory cells.

Inductive memval: Type :=
  | Undef: memval
  | Byte: byte -> memval
  | Pointer: block -> int -> nat -> memval.

Encoding and decoding integers


We define functions to convert between integers and lists of bytes of a given length

Fixpoint bytes_of_int (n: nat) (x: Z) {struct n}: list byte :=
  match n with
  | O => nil
  | S m => Byte.repr x :: bytes_of_int m (x / 256)
  end.

Fixpoint int_of_bytes (l: list byte): Z :=
  match l with
  | nil => 0
  | b :: l' => Byte.unsigned b + int_of_bytes l' * 256
  end.

Parameter big_endian: bool.

Definition rev_if_be (l: list byte) : list byte :=
  if big_endian then List.rev l else l.

Definition encode_int (sz: nat) (x: Z) : list byte :=
  rev_if_be (bytes_of_int sz x).

Definition decode_int (b: list byte) : Z :=
  int_of_bytes (rev_if_be b).

Length properties

Lemma length_bytes_of_int:
  forall n x, length (bytes_of_int n x) = n.
Proof.
  induction n; simpl; intros. auto. decEq. auto.
Qed.

Lemma rev_if_be_length:
  forall l, length (rev_if_be l) = length l.
Proof.
  intros; unfold rev_if_be; destruct big_endian.
  apply List.rev_length.
  auto.
Qed.

Lemma encode_int_length:
  forall sz x, length(encode_int sz x) = sz.
Proof.
  intros. unfold encode_int. rewrite rev_if_be_length. apply length_bytes_of_int.
Qed.

Decoding after encoding

Lemma int_of_bytes_of_int:
  forall n x,
  int_of_bytes (bytes_of_int n x) = x mod (two_p (Z_of_nat n * 8)).
Proof.
  induction n; intros.
  simpl. rewrite Zmod_1_r. auto.
Opaque Byte.wordsize.
  rewrite inj_S. simpl.
  replace (Zsucc (Z_of_nat n) * 8) with (Z_of_nat n * 8 + 8) by omega.
  rewrite two_p_is_exp; try omega.
  rewrite Zmod_recombine. rewrite IHn. rewrite Zplus_comm. reflexivity.
  apply two_p_gt_ZERO. omega. apply two_p_gt_ZERO. omega.
Qed.

Lemma rev_if_be_involutive:
  forall l, rev_if_be (rev_if_be l) = l.
Proof.
  intros; unfold rev_if_be; destruct big_endian.
  apply List.rev_involutive.
  auto.
Qed.

Lemma decode_encode_int:
  forall n x, decode_int (encode_int n x) = x mod (two_p (Z_of_nat n * 8)).
Proof.
  unfold decode_int, encode_int; intros. rewrite rev_if_be_involutive.
  apply int_of_bytes_of_int.
Qed.

Lemma decode_encode_int_1:
  forall x, Int.repr (decode_int (encode_int 1 (Int.unsigned x))) = Int.zero_ext 8 x.
Proof.
  intros. rewrite decode_encode_int.
  rewrite <- (Int.repr_unsigned (Int.zero_ext 8 x)).
  decEq. symmetry. apply Int.zero_ext_mod. compute; auto.
Qed.

Lemma decode_encode_int_2:
  forall x, Int.repr (decode_int (encode_int 2 (Int.unsigned x))) = Int.zero_ext 16 x.
Proof.
  intros. rewrite decode_encode_int.
  rewrite <- (Int.repr_unsigned (Int.zero_ext 16 x)).
  decEq. symmetry. apply Int.zero_ext_mod. compute; auto.
Qed.

Lemma decode_encode_int_4:
  forall x, Int.repr (decode_int (encode_int 4 (Int.unsigned x))) = x.
Proof.
  intros. rewrite decode_encode_int. transitivity (Int.repr (Int.unsigned x)).
  decEq. apply Zmod_small. apply Int.unsigned_range. apply Int.repr_unsigned.
Qed.

Lemma decode_encode_int_8:
  forall x, Int64.repr (decode_int (encode_int 8 (Int64.unsigned x))) = x.
Proof.
  intros. rewrite decode_encode_int. transitivity (Int64.repr (Int64.unsigned x)).
  decEq. apply Zmod_small. apply Int64.unsigned_range. apply Int64.repr_unsigned.
Qed.

A length-n encoding depends only on the low 8*n bits of the integer.

Lemma bytes_of_int_mod:
  forall n x y,
  Int.eqmod (two_p (Z_of_nat n * 8)) x y ->
  bytes_of_int n x = bytes_of_int n y.
Proof.
  induction n.
  intros; simpl; auto.
  intros until y.
  rewrite inj_S.
  replace (Zsucc (Z_of_nat n) * 8) with (Z_of_nat n * 8 + 8) by omega.
  rewrite two_p_is_exp; try omega.
  intro EQM.
  simpl; decEq.
  apply Byte.eqm_samerepr. red.
  eapply Int.eqmod_divides; eauto. apply Zdivide_factor_l.
  apply IHn.
  destruct EQM as [k EQ]. exists k. rewrite EQ.
  rewrite <- Z_div_plus_full_l. decEq. change (two_p 8) with 256. ring. omega.
Qed.

Lemma encode_int_8_mod:
  forall x y,
  Int.eqmod (two_p 8) x y ->
  encode_int 1%nat x = encode_int 1%nat y.
Proof.
  intros. unfold encode_int. decEq. apply bytes_of_int_mod. auto.
Qed.

Lemma encode_int_16_mod:
  forall x y,
  Int.eqmod (two_p 16) x y ->
  encode_int 2%nat x = encode_int 2%nat y.
Proof.
  intros. unfold encode_int. decEq. apply bytes_of_int_mod. auto.
Qed.

Encoding and decoding values


Definition inj_bytes (bl: list byte) : list memval :=
  List.map Byte bl.

Fixpoint proj_bytes (vl: list memval) : option (list byte) :=
  match vl with
  | nil => Some nil
  | Byte b :: vl' =>
      match proj_bytes vl' with None => None | Some bl => Some(b :: bl) end
  | _ => None
  end.

Remark length_inj_bytes:
  forall bl, length (inj_bytes bl) = length bl.
Proof.
  intros. apply List.map_length.
Qed.

Remark proj_inj_bytes:
  forall bl, proj_bytes (inj_bytes bl) = Some bl.
Proof.
  induction bl; simpl. auto. rewrite IHbl. auto.
Qed.

Lemma inj_proj_bytes:
  forall cl bl, proj_bytes cl = Some bl -> cl = inj_bytes bl.
Proof.
  induction cl; simpl; intros.
  inv H; auto.
  destruct a; try congruence. destruct (proj_bytes cl); inv H.
  simpl. decEq. auto.
Qed.

Fixpoint inj_pointer (n: nat) (b: block) (ofs: int) {struct n}: list memval :=
  match n with
  | O => nil
  | S m => Pointer b ofs m :: inj_pointer m b ofs
  end.

Fixpoint check_pointer (n: nat) (b: block) (ofs: int) (vl: list memval)
                       {struct n} : bool :=
  match n, vl with
  | O, nil => true
  | S m, Pointer b' ofs' m' :: vl' =>
      eq_block b b' && Int.eq_dec ofs ofs' && beq_nat m m' && check_pointer m b ofs vl'
  | _, _ => false
  end.

Definition proj_pointer (vl: list memval) : val :=
  match vl with
  | Pointer b ofs n :: vl' =>
      if check_pointer 4%nat b ofs vl then Vptr b ofs else Vundef
  | _ => Vundef
  end.

Definition encode_val (chunk: memory_chunk) (v: val) : list memval :=
  match v, chunk with
  | Vint n, (Mint8signed | Mint8unsigned) => inj_bytes (encode_int 1%nat (Int.unsigned n))
  | Vint n, (Mint16signed | Mint16unsigned) => inj_bytes (encode_int 2%nat (Int.unsigned n))
  | Vint n, Mint32 => inj_bytes (encode_int 4%nat (Int.unsigned n))
  | Vptr b ofs, Mint32 => inj_pointer 4%nat b ofs
  | Vfloat n, Mfloat32 => inj_bytes (encode_int 4%nat (Int.unsigned (Float.bits_of_single n)))
  | Vfloat n, Mfloat64 => inj_bytes (encode_int 8%nat (Int64.unsigned (Float.bits_of_double n)))
  | _, _ => list_repeat (size_chunk_nat chunk) Undef
  end.

Definition decode_val (chunk: memory_chunk) (vl: list memval) : val :=
  match proj_bytes vl with
  | Some bl =>
      match chunk with
      | Mint8signed => Vint(Int.sign_ext 8 (Int.repr (decode_int bl)))
      | Mint8unsigned => Vint(Int.zero_ext 8 (Int.repr (decode_int bl)))
      | Mint16signed => Vint(Int.sign_ext 16 (Int.repr (decode_int bl)))
      | Mint16unsigned => Vint(Int.zero_ext 16 (Int.repr (decode_int bl)))
      | Mint32 => Vint(Int.repr(decode_int bl))
      | Mfloat32 => Vfloat(Float.single_of_bits (Int.repr (decode_int bl)))
      | Mfloat64 => Vfloat(Float.double_of_bits (Int64.repr (decode_int bl)))
      end
  | None =>
      match chunk with
      | Mint32 => proj_pointer vl
      | _ => Vundef
      end
  end.

Lemma encode_val_length:
  forall chunk v, length(encode_val chunk v) = size_chunk_nat chunk.
Proof.
  intros. destruct v; simpl; destruct chunk;
  solve [ reflexivity
        | apply length_list_repeat
        | rewrite length_inj_bytes; apply encode_int_length ].
Qed.

Lemma check_inj_pointer:
  forall b ofs n, check_pointer n b ofs (inj_pointer n b ofs) = true.
Proof.
  induction n; simpl. auto.
  unfold proj_sumbool. rewrite dec_eq_true. rewrite dec_eq_true.
  rewrite <- beq_nat_refl. simpl; auto.
Qed.

Definition decode_encode_val (v1: val) (chunk1 chunk2: memory_chunk) (v2: val) : Prop :=
  match v1, chunk1, chunk2 with
  | Vundef, _, _ => v2 = Vundef
  | Vint n, Mint8signed, Mint8signed => v2 = Vint(Int.sign_ext 8 n)
  | Vint n, Mint8unsigned, Mint8signed => v2 = Vint(Int.sign_ext 8 n)
  | Vint n, Mint8signed, Mint8unsigned => v2 = Vint(Int.zero_ext 8 n)
  | Vint n, Mint8unsigned, Mint8unsigned => v2 = Vint(Int.zero_ext 8 n)
  | Vint n, Mint16signed, Mint16signed => v2 = Vint(Int.sign_ext 16 n)
  | Vint n, Mint16unsigned, Mint16signed => v2 = Vint(Int.sign_ext 16 n)
  | Vint n, Mint16signed, Mint16unsigned => v2 = Vint(Int.zero_ext 16 n)
  | Vint n, Mint16unsigned, Mint16unsigned => v2 = Vint(Int.zero_ext 16 n)
  | Vint n, Mint32, Mint32 => v2 = Vint n
  | Vint n, Mint32, Mfloat32 => v2 = Vfloat(Float.single_of_bits n)
  | Vint n, (Mfloat32 | Mfloat64), _ => v2 = Vundef
  | Vint n, _, _ => True (* nothing meaningful to say about v2 *)
  | Vptr b ofs, Mint32, Mint32 => v2 = Vptr b ofs
  | Vptr b ofs, _, _ => v2 = Vundef
  | Vfloat f, Mfloat32, Mfloat32 => v2 = Vfloat(Float.singleoffloat f)
  | Vfloat f, Mfloat32, Mint32 => v2 = Vint(Float.bits_of_single f)
  | Vfloat f, Mfloat64, Mfloat64 => v2 = Vfloat f
  | Vfloat f, (Mint8signed|Mint8unsigned|Mint16signed|Mint16unsigned|Mint32), _ => v2 = Vundef
  | Vfloat f, _, _ => True
  end.

Remark decode_val_undef:
  forall bl chunk, decode_val chunk (Undef :: bl) = Vundef.
Proof.
  intros. unfold decode_val. simpl. destruct chunk; auto.
Qed.

Lemma decode_encode_val_general:
  forall v chunk1 chunk2,
  decode_encode_val v chunk1 chunk2 (decode_val chunk2 (encode_val chunk1 v)).
Proof.
Opaque inj_pointer.
  intros.
  destruct v; destruct chunk1; simpl; try (apply decode_val_undef);
  destruct chunk2; unfold decode_val; auto; try (rewrite proj_inj_bytes).
  rewrite decode_encode_int_1. decEq. apply Int.sign_ext_zero_ext. compute; auto.
  rewrite decode_encode_int_1. decEq. apply Int.zero_ext_idem. compute; auto.
  rewrite decode_encode_int_1. decEq. apply Int.sign_ext_zero_ext. compute; auto.
  rewrite decode_encode_int_1. decEq. apply Int.zero_ext_idem. compute; auto.
  rewrite decode_encode_int_2. decEq. apply Int.sign_ext_zero_ext. compute; auto.
  rewrite decode_encode_int_2. decEq. apply Int.zero_ext_idem. compute; auto.
  rewrite decode_encode_int_2. decEq. apply Int.sign_ext_zero_ext. compute; auto.
  rewrite decode_encode_int_2. decEq. apply Int.zero_ext_idem. compute; auto.
  rewrite decode_encode_int_4. auto.
  rewrite decode_encode_int_4. auto.
  rewrite decode_encode_int_4. auto.
  rewrite decode_encode_int_4. decEq. apply Float.single_of_bits_of_single.
  rewrite decode_encode_int_8. decEq. apply Float.double_of_bits_of_double.
  change (proj_bytes (inj_pointer 4 b i)) with (@None (list byte)). simpl.
  unfold proj_pointer. generalize (check_inj_pointer b i 4%nat).
Transparent inj_pointer.
  simpl. intros EQ; rewrite EQ; auto.
Qed.

Lemma decode_encode_val_similar:
  forall v1 chunk1 chunk2 v2,
  type_of_chunk chunk1 = type_of_chunk chunk2 ->
  size_chunk chunk1 = size_chunk chunk2 ->
  decode_encode_val v1 chunk1 chunk2 v2 ->
  v2 = Val.load_result chunk2 v1.
Proof.
  intros until v2; intros TY SZ DE.
  destruct chunk1; destruct chunk2; simpl in TY; try discriminate; simpl in SZ; try omegaContradiction;
  destruct v1; auto.
Qed.

Lemma decode_val_type:
  forall chunk cl,
  Val.has_type (decode_val chunk cl) (type_of_chunk chunk).
Proof.
  intros. unfold decode_val.
  destruct (proj_bytes cl).
  destruct chunk; simpl; auto.
  destruct chunk; simpl; auto.
  unfold proj_pointer. destruct cl; try (exact I).
  destruct m; try (exact I).
  destruct (check_pointer 4%nat b i (Pointer b i n :: cl));
  exact I.
Qed.

Lemma encode_val_int8_signed_unsigned:
  forall v, encode_val Mint8signed v = encode_val Mint8unsigned v.
Proof.
  intros. destruct v; simpl; auto.
Qed.

Lemma encode_val_int16_signed_unsigned:
  forall v, encode_val Mint16signed v = encode_val Mint16unsigned v.
Proof.
  intros. destruct v; simpl; auto.
Qed.

Lemma encode_val_int8_zero_ext:
  forall n, encode_val Mint8unsigned (Vint (Int.zero_ext 8 n)) = encode_val Mint8unsigned (Vint n).
Proof.
  intros; unfold encode_val. decEq. apply encode_int_8_mod. apply Int.eqmod_zero_ext. compute; auto.
Qed.

Lemma encode_val_int8_sign_ext:
  forall n, encode_val Mint8signed (Vint (Int.sign_ext 8 n)) = encode_val Mint8signed (Vint n).
Proof.
  intros; unfold encode_val. decEq. apply encode_int_8_mod. apply Int.eqmod_sign_ext'. compute; auto.
Qed.

Lemma encode_val_int16_zero_ext:
  forall n, encode_val Mint16unsigned (Vint (Int.zero_ext 16 n)) = encode_val Mint16unsigned (Vint n).
Proof.
  intros; unfold encode_val. decEq. apply encode_int_16_mod. apply Int.eqmod_zero_ext. compute; auto.
Qed.

Lemma encode_val_int16_sign_ext:
  forall n, encode_val Mint16signed (Vint (Int.sign_ext 16 n)) = encode_val Mint16signed (Vint n).
Proof.
  intros; unfold encode_val. decEq. apply encode_int_16_mod. apply Int.eqmod_sign_ext'. compute; auto.
Qed.

Lemma decode_val_cast:
  forall chunk l,
  let v := decode_val chunk l in
  match chunk with
  | Mint8signed => v = Val.sign_ext 8 v
  | Mint8unsigned => v = Val.zero_ext 8 v
  | Mint16signed => v = Val.sign_ext 16 v
  | Mint16unsigned => v = Val.zero_ext 16 v
  | Mfloat32 => v = Val.singleoffloat v
  | _ => True
  end.
Proof.
  unfold decode_val; intros; destruct chunk; auto; destruct (proj_bytes l); auto.
  unfold Val.sign_ext. rewrite Int.sign_ext_idem; auto. compute; auto.
  unfold Val.zero_ext. rewrite Int.zero_ext_idem; auto. compute; auto.
  unfold Val.sign_ext. rewrite Int.sign_ext_idem; auto. compute; auto.
  unfold Val.zero_ext. rewrite Int.zero_ext_idem; auto. compute; auto.
  simpl. rewrite Float.singleoffloat_of_bits. auto.
Qed.

Pointers cannot be forged.

Definition memval_valid_first (mv: memval) : Prop :=
  match mv with
  | Pointer b ofs n => n = 3%nat
  | _ => True
  end.

Definition memval_valid_cont (mv: memval) : Prop :=
  match mv with
  | Pointer b ofs n => n <> 3%nat
  | _ => True
  end.

Inductive encoding_shape: list memval -> Prop :=
  | encoding_shape_intro: forall mv1 mvl,
      memval_valid_first mv1 ->
      (forall mv, In mv mvl -> memval_valid_cont mv) ->
      encoding_shape (mv1 :: mvl).

Lemma encode_val_shape:
  forall chunk v, encoding_shape (encode_val chunk v).
Proof.
  intros.
  destruct (size_chunk_nat_pos chunk) as [sz1 EQ].
  assert (A: encoding_shape (list_repeat (size_chunk_nat chunk) Undef)).
    rewrite EQ; simpl; constructor. exact I.
    intros. replace mv with Undef. exact I. symmetry; eapply in_list_repeat; eauto.
  assert (B: forall bl, length bl = size_chunk_nat chunk ->
          encoding_shape (inj_bytes bl)).
    intros. destruct bl; simpl in *. congruence.
    constructor. exact I. unfold inj_bytes. intros.
    exploit list_in_map_inv; eauto. intros [x [C D]]. subst mv. exact I.
  destruct v; auto; destruct chunk; simpl; auto; try (apply B; apply encode_int_length).
  constructor. red. auto.
  simpl; intros. intuition; subst mv; red; simpl; congruence.
Qed.

Lemma check_pointer_inv:
  forall b ofs n mv,
  check_pointer n b ofs mv = true -> mv = inj_pointer n b ofs.
Proof.
  induction n; destruct mv; simpl.
  auto.
  congruence.
  congruence.
  destruct m; try congruence. intro.
  destruct (andb_prop _ _ H). destruct (andb_prop _ _ H0).
  destruct (andb_prop _ _ H2).
  decEq. decEq. symmetry; eapply proj_sumbool_true; eauto.
  symmetry; eapply proj_sumbool_true; eauto.
  symmetry; apply beq_nat_true; auto.
  auto.
Qed.

Inductive decoding_shape: list memval -> Prop :=
  | decoding_shape_intro: forall mv1 mvl,
      memval_valid_first mv1 -> mv1 <> Undef ->
      (forall mv, In mv mvl -> memval_valid_cont mv /\ mv <> Undef) ->
      decoding_shape (mv1 :: mvl).

Lemma decode_val_shape:
  forall chunk mvl,
  List.length mvl = size_chunk_nat chunk ->
  decode_val chunk mvl = Vundef \/ decoding_shape mvl.
Proof.
  intros. destruct (size_chunk_nat_pos chunk) as [sz EQ].
  unfold decode_val.
  caseEq (proj_bytes mvl).
  intros bl PROJ. right. exploit inj_proj_bytes; eauto. intros. subst mvl.
  destruct bl; simpl in H. congruence. simpl. constructor.
  red; auto. congruence.
  unfold inj_bytes; intros. exploit list_in_map_inv; eauto. intros [b [A B]].
  subst mv. split. red; auto. congruence.
  intros. destruct chunk; auto. unfold proj_pointer.
  destruct mvl; auto. destruct m; auto.
  caseEq (check_pointer 4%nat b i (Pointer b i n :: mvl)); auto.
  intros. right. exploit check_pointer_inv; eauto. simpl; intros; inv H2.
  constructor. red. auto. congruence.
  simpl; intros. intuition; subst mv; simpl; congruence.
Qed.

Lemma encode_val_pointer_inv:
  forall chunk v b ofs n mvl,
  encode_val chunk v = Pointer b ofs n :: mvl ->
  chunk = Mint32 /\ v = Vptr b ofs /\ mvl = inj_pointer 3%nat b ofs.
Proof.
  intros until mvl.
  assert (A: list_repeat (size_chunk_nat chunk) Undef = Pointer b ofs n :: mvl ->
            chunk = Mint32 /\ v = Vptr b ofs /\ mvl = inj_pointer 3 b ofs).
    intros. destruct (size_chunk_nat_pos chunk) as [sz SZ]. rewrite SZ in H. simpl in H. discriminate.
  assert (B: forall bl, length bl <> 0%nat -> inj_bytes bl = Pointer b ofs n :: mvl ->
            chunk = Mint32 /\ v = Vptr b ofs /\ mvl = inj_pointer 3 b ofs).
    intros. destruct bl; simpl in *; congruence.
  unfold encode_val; destruct v; destruct chunk;
  (apply A; assumption) ||
  (apply B; rewrite encode_int_length; congruence) || idtac.
  simpl. intros EQ; inv EQ; auto.
Qed.

Lemma decode_val_pointer_inv:
  forall chunk mvl b ofs,
  decode_val chunk mvl = Vptr b ofs ->
  chunk = Mint32 /\ mvl = inj_pointer 4%nat b ofs.
Proof.
  intros until ofs; unfold decode_val.
  destruct (proj_bytes mvl).
  destruct chunk; congruence.
  destruct chunk; try congruence.
  unfold proj_pointer. destruct mvl. congruence. destruct m; try congruence.
  case_eq (check_pointer 4%nat b0 i (Pointer b0 i n :: mvl)); intros.
  inv H0. split; auto. apply check_pointer_inv; auto.
  congruence.
Qed.

Inductive pointer_encoding_shape: list memval -> Prop :=
  | pointer_encoding_shape_intro: forall mv1 mvl,
      ~memval_valid_cont mv1 ->
      (forall mv, In mv mvl -> ~memval_valid_first mv) ->
      pointer_encoding_shape (mv1 :: mvl).

Lemma encode_pointer_shape:
  forall b ofs, pointer_encoding_shape (encode_val Mint32 (Vptr b ofs)).
Proof.
  intros. simpl. constructor.
  unfold memval_valid_cont. red; intro. elim H. auto.
  unfold memval_valid_first. simpl; intros; intuition; subst mv; congruence.
Qed.

Lemma decode_pointer_shape:
  forall chunk mvl b ofs,
  decode_val chunk mvl = Vptr b ofs ->
  chunk = Mint32 /\ pointer_encoding_shape mvl.
Proof.
  intros. exploit decode_val_pointer_inv; eauto. intros [A B].
  split; auto. subst mvl. apply encode_pointer_shape.
Qed.

Compatibility with memory injections


Relating two memory values according to a memory injection.

Inductive memval_inject (f: meminj): memval -> memval -> Prop :=
  | memval_inject_byte:
      forall n, memval_inject f (Byte n) (Byte n)
  | memval_inject_ptr:
      forall b1 ofs1 b2 ofs2 delta n,
      f b1 = Some (b2, delta) ->
      ofs2 = Int.add ofs1 (Int.repr delta) ->
      memval_inject f (Pointer b1 ofs1 n) (Pointer b2 ofs2 n)
  | memval_inject_undef:
      forall mv, memval_inject f Undef mv.

Lemma memval_inject_incr:
  forall f f' v1 v2, memval_inject f v1 v2 -> inject_incr f f' -> memval_inject f' v1 v2.
Proof.
  intros. inv H; econstructor. rewrite (H0 _ _ _ H1). reflexivity. auto.
Qed.

decode_val, applied to lists of memory values that are pairwise related by memval_inject, returns values that are related by val_inject.

Lemma proj_bytes_inject:
  forall f vl vl',
  list_forall2 (memval_inject f) vl vl' ->
  forall bl,
  proj_bytes vl = Some bl ->
  proj_bytes vl' = Some bl.
Proof.
  induction 1; simpl. congruence.
  inv H; try congruence.
  destruct (proj_bytes al); intros.
  inv H. rewrite (IHlist_forall2 l); auto.
  congruence.
Qed.

Lemma check_pointer_inject:
  forall f vl vl',
  list_forall2 (memval_inject f) vl vl' ->
  forall n b ofs b' delta,
  check_pointer n b ofs vl = true ->
  f b = Some(b', delta) ->
  check_pointer n b' (Int.add ofs (Int.repr delta)) vl' = true.
Proof.
  induction 1; intros; destruct n; simpl in *; auto.
  inv H; auto.
  destruct (andb_prop _ _ H1). destruct (andb_prop _ _ H).
  destruct (andb_prop _ _ H5).
  assert (n = n0) by (apply beq_nat_true; auto).
  assert (b = b0) by (eapply proj_sumbool_true; eauto).
  assert (ofs = ofs1) by (eapply proj_sumbool_true; eauto).
  subst. rewrite H3 in H2; inv H2.
  unfold proj_sumbool. rewrite dec_eq_true. rewrite dec_eq_true.
  rewrite <- beq_nat_refl. simpl. eauto.
  congruence.
Qed.

Lemma proj_pointer_inject:
  forall f vl1 vl2,
  list_forall2 (memval_inject f) vl1 vl2 ->
  val_inject f (proj_pointer vl1) (proj_pointer vl2).
Proof.
  intros. unfold proj_pointer.
  inversion H; subst. auto. inversion H0; subst; auto.
  case_eq (check_pointer 4%nat b0 ofs1 (Pointer b0 ofs1 n :: al)); intros.
  exploit check_pointer_inject. eexact H. eauto. eauto.
  intro. rewrite H4. econstructor; eauto.
  constructor.
Qed.

Lemma proj_bytes_not_inject:
  forall f vl vl',
  list_forall2 (memval_inject f) vl vl' ->
  proj_bytes vl = None -> proj_bytes vl' <> None -> In Undef vl.
Proof.
  induction 1; simpl; intros.
  congruence.
  inv H; try congruence.
  right. apply IHlist_forall2.
  destruct (proj_bytes al); congruence.
  destruct (proj_bytes bl); congruence.
  auto.
Qed.

Lemma check_pointer_undef:
  forall n b ofs vl,
  In Undef vl -> check_pointer n b ofs vl = false.
Proof.
  induction n; intros; simpl.
  destruct vl. elim H. auto.
  destruct vl. auto.
  destruct m; auto. simpl in H; destruct H. congruence.
  rewrite IHn; auto. apply andb_false_r.
Qed.

Lemma proj_pointer_undef:
  forall vl, In Undef vl -> proj_pointer vl = Vundef.
Proof.
  intros; unfold proj_pointer.
  destruct vl; auto. destruct m; auto.
  rewrite check_pointer_undef. auto. auto.
Qed.

Theorem decode_val_inject:
  forall f vl1 vl2 chunk,
  list_forall2 (memval_inject f) vl1 vl2 ->
  val_inject f (decode_val chunk vl1) (decode_val chunk vl2).
Proof.
  intros. unfold decode_val.
  case_eq (proj_bytes vl1); intros.
  exploit proj_bytes_inject; eauto. intros. rewrite H1.
  destruct chunk; constructor.
  destruct chunk; auto.
  case_eq (proj_bytes vl2); intros.
  rewrite proj_pointer_undef. auto. eapply proj_bytes_not_inject; eauto. congruence.
  apply proj_pointer_inject; auto.
Qed.

Symmetrically, encode_val, applied to values related by val_inject, returns lists of memory values that are pairwise related by memval_inject.

Lemma inj_bytes_inject:
  forall f bl, list_forall2 (memval_inject f) (inj_bytes bl) (inj_bytes bl).
Proof.
  induction bl; constructor; auto. constructor.
Qed.

Lemma repeat_Undef_inject_any:
  forall f vl,
  list_forall2 (memval_inject f) (list_repeat (length vl) Undef) vl.
Proof.
  induction vl; simpl; constructor; auto. constructor.
Qed.

Lemma repeat_Undef_inject_self:
  forall f n,
  list_forall2 (memval_inject f) (list_repeat n Undef) (list_repeat n Undef).
Proof.
  induction n; simpl; constructor; auto. constructor.
Qed.

Theorem encode_val_inject:
  forall f v1 v2 chunk,
  val_inject f v1 v2 ->
  list_forall2 (memval_inject f) (encode_val chunk v1) (encode_val chunk v2).
Proof.
  intros. inv H; simpl.
  destruct chunk; apply inj_bytes_inject || apply repeat_Undef_inject_self.
  destruct chunk; apply inj_bytes_inject || apply repeat_Undef_inject_self.
  destruct chunk; try (apply repeat_Undef_inject_self).
  repeat econstructor; eauto.
  replace (size_chunk_nat chunk) with (length (encode_val chunk v2)).
  apply repeat_Undef_inject_any. apply encode_val_length.
Qed.

Definition memval_lessdef: memval -> memval -> Prop := memval_inject inject_id.

Lemma memval_lessdef_refl:
  forall mv, memval_lessdef mv mv.
Proof.
  red. destruct mv; econstructor.
  unfold inject_id; reflexivity. rewrite Int.add_zero; auto.
Qed.

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