(** * Relating the memory states *)

(** The [magree] predicate is a variant of [Mem.extends] where we
  allow the contents of the two memory states to differ arbitrarily
  on some locations.  The predicate [P] is true on the locations whose
  contents must be in the [lessdef] relation. *)

Definition locset := block -> Z -> Prop.Record magree (m1 m2: mem) (P: locset) : Prop := mk_magree {
  ma_perm:
    forall b ofs k p,
    Mem.perm m1 b ofs k p -> Mem.perm m2 b ofs k p;
  ma_perm_inv:
    forall b ofs k p,
    Mem.perm m2 b ofs k p -> Mem.perm m1 b ofs k p \/ ~Mem.perm m1 b ofs Max Nonempty;
  ma_memval:
    forall b ofs,
    Mem.perm m1 b ofs Cur Readable ->
    P b ofs ->
    memval_lessdef (ZMap.get ofs (PMap.get b (Mem.mem_contents m1)))
                   (ZMap.get ofs (PMap.get b (Mem.mem_contents m2)));
  ma_nextblock:
    Mem.nextblock m2 = Mem.nextblock m1
}.Record magree (m1 m2: mem) (P: locset) : Prop := mk_magree {
  ma_perm:
    forall b ofs k p,
    Mem.perm m1 b ofs k p -> Mem.perm m2 b ofs k p;
  ma_perm_inv:
    forall b ofs k p,
    Mem.perm m2 b ofs k p -> Mem.perm m1 b ofs k p \/ ~Mem.perm m1 b ofs Max Nonempty;
  ma_memval:
    forall b ofs,
    Mem.perm m1 b ofs Cur Readable ->
    P b ofs ->
    memval_lessdef (ZMap.get ofs (PMap.get b (Mem.mem_contents m1)))
                   (ZMap.get ofs (PMap.get b (Mem.mem_contents m2)));
  ma_nextblock:
    Mem.nextblock m2 = Mem.nextblock m1
}.Record magree (m1 m2: mem) (P: locset) : Prop := mk_magree {
  ma_perm:
    forall b ofs k p,
    Mem.perm m1 b ofs k p -> Mem.perm m2 b ofs k p;
  ma_perm_inv:
    forall b ofs k p,
    Mem.perm m2 b ofs k p -> Mem.perm m1 b ofs k p \/ ~Mem.perm m1 b ofs Max Nonempty;
  ma_memval:
    forall b ofs,
    Mem.perm m1 b ofs Cur Readable ->
    P b ofs ->
    memval_lessdef (ZMap.get ofs (PMap.get b (Mem.mem_contents m1)))
                   (ZMap.get ofs (PMap.get b (Mem.mem_contents m2)));
  ma_nextblock:
    Mem.nextblock m2 = Mem.nextblock m1
}.Record magree (m1 m2: mem) (P: locset) : Prop := mk_magree {
  ma_perm:
    forall b ofs k p,
    Mem.perm m1 b ofs k p -> Mem.perm m2 b ofs k p;
  ma_perm_inv:
    forall b ofs k p,
    Mem.perm m2 b ofs k p -> Mem.perm m1 b ofs k p \/ ~Mem.perm m1 b ofs Max Nonempty;
  ma_memval:
    forall b ofs,
    Mem.perm m1 b ofs Cur Readable ->
    P b ofs ->
    memval_lessdef (ZMap.get ofs (PMap.get b (Mem.mem_contents m1)))
                   (ZMap.get ofs (PMap.get b (Mem.mem_contents m2)));
  ma_nextblock:
    Mem.nextblock m2 = Mem.nextblock m1
}.Record magree (m1 m2: mem) (P: locset) : Prop := mk_magree {
  ma_perm:
    forall b ofs k p,
    Mem.perm m1 b ofs k p -> Mem.perm m2 b ofs k p;
  ma_perm_inv:
    forall b ofs k p,
    Mem.perm m2 b ofs k p -> Mem.perm m1 b ofs k p \/ ~Mem.perm m1 b ofs Max Nonempty;
  ma_memval:
    forall b ofs,
    Mem.perm m1 b ofs Cur Readable ->
    P b ofs ->
    memval_lessdef (ZMap.get ofs (PMap.get b (Mem.mem_contents m1)))
                   (ZMap.get ofs (PMap.get b (Mem.mem_contents m2)));
  ma_nextblock:
    Mem.nextblock m2 = Mem.nextblock m1
}.Record magree (m1 m2: mem) (P: locset) : Prop := mk_magree {
  ma_perm:
    forall b ofs k p,
    Mem.perm m1 b ofs k p -> Mem.perm m2 b ofs k p;
  ma_perm_inv:
    forall b ofs k p,
    Mem.perm m2 b ofs k p -> Mem.perm m1 b ofs k p \/ ~Mem.perm m1 b ofs Max Nonempty;
  ma_memval:
    forall b ofs,
    Mem.perm m1 b ofs Cur Readable ->
    P b ofs ->
    memval_lessdef (ZMap.get ofs (PMap.get b (Mem.mem_contents m1)))
                   (ZMap.get ofs (PMap.get b (Mem.mem_contents m2)));
  ma_nextblock:
    Mem.nextblock m2 = Mem.nextblock m1
}.

Lemma magree_valid_access: forall m1 m2 (P: locset) chunk b ofs p, magree m1 m2 P -> Mem.valid_access m1 chunk b ofs p -> Mem.valid_access m2 chunk b ofs p.