(** ** Byte offset for a field of a structure *)

(** [field_offset env id fld] returns the byte offset for field [id]
  in a structure whose members are [fld].  Fields are laid out
  consecutively, and padding is inserted to align each field to the
  alignment for its type. *)

Fixpoint field_offset_rec (env: composite_env) (id: ident) (fld: members) (pos: Z)
                          {struct fld} : res Z :=
  match fld with
  | nil => Error (MSG "Unknown field " :: CTX id :: nil)
  | (id', t) :: fld' =>
      if ident_eq id id'
      then OK (align pos (alignof env t))
      else field_offset_rec env id fld' (align pos (alignof env t) + sizeof env t)
  end.Definition field_offset (env: composite_env) (id: ident) (fld: members) : res Z :=
  field_offset_rec env id fld 0.Fixpoint field_type (id: ident) (fld: members) {struct fld} : res type :=
  match fld with
  | nil => Error (MSG "Unknown field " :: CTX id :: nil)
  | (id', t) :: fld' => if ident_eq id id' then OK t else field_type id fld'
  end.(** Some sanity checks about field offsets.  First, field offsets are
  within the range of acceptable offsets. *)

Remark field_offset_rec_in_range:
  forall env id ofs ty fld pos,
  field_offset_rec env id fld pos = OK ofs -> field_type id fld = OK ty ->
  pos <= ofs /\ ofs + sizeof env ty <= sizeof_struct env pos fld.

Lemma field_offset_in_range: forall env fld id ofs ty, field_offset env id fld = OK ofs -> field_type id fld = OK ty -> 0 <= ofs /\ ofs + sizeof env ty <= sizeof_struct env 0 fld.