Clean-up pass on C types:

- Ctypes: add useful functions on attributes; remove attrs in typeconv
    (because attributes are meaningless on r-values)
- C2C: fixed missing or redundant Evalof
- Cop: ignore attributes in ptr + int and ptr - int (meaningless on r-values);
    add sanity check between typeconv/classify_binarith and the C99 standard.
- cparser: fixed several cases where incorrect type annotations were put
    on expressions.


git-svn-id: https://yquem.inria.fr/compcert/svn/compcert/trunk@2457 fca1b0fc-160b-0410-b1d3-a4f43f01ea2e

4 files changed, 324 insertions, 91 deletions

diff --git a/cfrontend/C2C.ml b/cfrontend/C2C.ml
index f767372..8799bc0 100644
--- a/cfrontend/C2C.ml
+++ b/cfrontend/C2C.ml
@@ -200,9 +200,10 @@ a constant)"; Integers.Int.one in
 (** ** Translation of [va_arg] for variadic functions. *)
 
 let va_list_ptr e =
-  if CBuiltins.va_list_scalar
-  then Eaddrof(e, Tpointer(typeof e, noattr))
-  else e
+  if not CBuiltins.va_list_scalar then e else
+    match e with
+    | Evalof(e', _) -> Eaddrof(e', Tpointer(typeof e, noattr))
+    | _             -> error "bad use of a va_list object"; e
 
 let make_builtin_va_arg env ty e =
   let (helper, ty_ret) =
@@ -215,15 +216,14 @@ let make_builtin_va_arg env ty e =
         ("__compcert_va_float64", Tfloat(F64, noattr))
     | _ ->
         unsupported "va_arg at this type";
-        ("", Tvoid)
-  in
-    Ecast 
-      (Ecall(Evar (intern_string helper, 
-                   Tfunction(Tcons(Tpointer(Tvoid, noattr), Tnil), ty_ret, 
-                             cc_default)),
-             Econs(va_list_ptr e, Enil),
-             ty_ret),
-       ty)
+        ("", Tvoid) in
+  let ty_fun =
+    Tfunction(Tcons(Tpointer(Tvoid, noattr), Tnil), ty_ret, cc_default) in
+  Ecast 
+    (Ecall(Evalof(Evar(intern_string helper, ty_fun), ty_fun),
+           Econs(va_list_ptr e, Enil),
+           ty_ret),
+     ty)
 
 (** ** Translation functions *)
 
@@ -338,7 +338,7 @@ let convertTyp env t =
     | C.TStruct(id, a) ->
         let a' = convertAttr a in
         begin try
-          mergeTypAttr (Hashtbl.find compositeCache id) a'
+          merge_attributes (Hashtbl.find compositeCache id) a'
         with Not_found ->
           let flds =
             try
@@ -350,7 +350,7 @@ let convertTyp env t =
     | C.TUnion(id, a) ->
         let a' = convertAttr a in
         begin try
-          mergeTypAttr (Hashtbl.find compositeCache id) a'
+          merge_attributes (Hashtbl.find compositeCache id) a'
         with Not_found ->
           let flds =
             try
@@ -379,12 +379,6 @@ let convertTyp env t =
 
   in convertTyp [] t
 
-(*
-let rec convertTypList env = function
-  | [] -> Tnil
-  | t1 :: tl -> Tcons(convertTyp env t1, convertTypList env tl)
-*)
-
 let rec convertTypArgs env tl el =
   match tl, el with
   | _, [] -> Tnil
@@ -1062,10 +1056,11 @@ let convertProgram p =
   try
     let gl1 = convertGlobdecls (translEnv Env.empty p) [] (cleanupGlobals p) in
     let gl2 = globals_for_strings gl1 in
+    let p' = { AST.prog_defs = gl2;
+                AST.prog_main = intern_string "main" } in
     if !numErrors > 0
     then None
-    else Some { AST.prog_defs = gl2;
-                AST.prog_main = intern_string "main" }
+    else Some p'
   with Env.Error msg ->
     error (Env.error_message msg); None
 
diff --git a/cfrontend/Cop.v b/cfrontend/Cop.v
index f2550ef..83fe772 100644
--- a/cfrontend/Cop.v
+++ b/cfrontend/Cop.v
@@ -115,7 +115,7 @@ Definition classify_cast (tfrom tto: type) : classify_cast_cases :=
   end.
 
 (** Semantics of casts.  [sem_cast v1 t1 t2 = Some v2] if value [v1],
-  viewed with static type [t1], can be cast to type [t2],
+  viewed with static type [t1], can be converted  to type [t2],
   resulting in value [v2].  *)
 
 Definition cast_int_int (sz: intsize) (sg: signedness) (i: int) : int :=
@@ -266,7 +266,8 @@ Definition sem_cast (v: val) (t1 t2: type) : option val :=
 
 (** The following describes types that can be interpreted as a boolean:
   integers, floats, pointers.  It is used for the semantics of 
-  the [!] and [?] operators, as well as the [if], [while], [for] statements. *)
+  the [!] and [?] operators, as well as the [if], [while], 
+  and [for] statements. *)
 
 Inductive classify_bool_cases : Type :=
   | bool_case_i                           (**r integer *)
@@ -315,34 +316,6 @@ Definition bool_val (v: val) (t: type) : option bool :=
   | bool_default => None
   end.
 
-(** Common-sense relation between Boolean value and casting to [_Bool] type. *)
-
-Lemma cast_bool_bool_val:
-  forall v t,
-  sem_cast v t (Tint IBool Signed noattr) =
-  match bool_val v t with None => None | Some b => Some(Val.of_bool b) end.
-Proof.
-  intros.
-  assert (A: classify_bool t =
-    match t with
-    | Tint _ _ _ => bool_case_i
-    | Tpointer _ _ | Tcomp_ptr _ _ | Tarray _ _ _ | Tfunction _ _ _ => bool_case_p
-    | Tfloat _ _ => bool_case_f
-    | Tlong _ _ => bool_case_l
-    | _ => bool_default
-    end).
-  {
-    unfold classify_bool; destruct t; simpl; auto. destruct i; auto.
-  }
-  unfold bool_val. rewrite A. unfold sem_cast. destruct t; simpl; auto; destruct v; auto.
-  destruct (Int.eq i0 Int.zero); auto.
-  destruct (Int64.eq i Int64.zero); auto.
-  destruct (Float.cmp Ceq f0 Float.zero); auto.
-  destruct (Int.eq i Int.zero); auto. 
-  destruct (Int.eq i Int.zero); auto. 
-  destruct (Int.eq i Int.zero); auto. 
-  destruct (Int.eq i0 Int.zero); auto. 
-Qed.
 
 (** ** Unary operators *)
 
@@ -374,17 +347,6 @@ Definition sem_notbool (v: val) (ty: type) : option val :=
   | bool_default => None
   end.
 
-(** Common-sense relation between Boolean value and Boolean negation. *)
-
-Lemma notbool_bool_val:
-  forall v t,
-  sem_notbool v t =
-  match bool_val v t with None => None | Some b => Some(Val.of_bool (negb b)) end.
-Proof.
-  intros. unfold sem_notbool, bool_val. 
-  destruct (classify_bool t); auto; destruct v; auto; rewrite negb_involutive; auto.
-Qed.
-
 (** *** Opposite and absolute value *)
 
 Inductive classify_neg_cases : Type :=
@@ -503,6 +465,16 @@ Definition classify_binarith (ty1: type) (ty2: type) : binarith_cases :=
   | _, _ => bin_default
   end.
 
+(** The static type of the result. *)
+
+Definition binarith_result_type (c: binarith_cases) : option type :=
+  match c with
+  | bin_case_i sg => Some(Tint I32 sg noattr)
+  | bin_case_l sg => Some(Tlong sg noattr)
+  | bin_case_f sz => Some(Tfloat sz noattr)
+  | bin_default   => None
+  end.
+
 (** The type at which the computation is done.  Both arguments are
   converted to this type before the actual computation.  *)
 
@@ -549,43 +521,43 @@ Definition sem_binarith
 (** *** Addition *)
 
 Inductive classify_add_cases : Type :=
-  | add_case_pi(ty: type)(a: attr)     (**r pointer, int *)
-  | add_case_ip(ty: type)(a: attr)     (**r int, pointer *)
-  | add_case_pl(ty: type)(a: attr)     (**r pointer, long *)
-  | add_case_lp(ty: type)(a: attr)     (**r long, pointer *)
+  | add_case_pi(ty: type)     (**r pointer, int *)
+  | add_case_ip(ty: type)     (**r int, pointer *)
+  | add_case_pl(ty: type)     (**r pointer, long *)
+  | add_case_lp(ty: type)     (**r long, pointer *)
   | add_default.                       (**r numerical type, numerical type *)
 
 Definition classify_add (ty1: type) (ty2: type) :=
   match typeconv ty1, typeconv ty2 with
-  | Tpointer ty a, Tint _ _ _ => add_case_pi ty a
-  | Tint _ _ _, Tpointer ty a => add_case_ip ty a
-  | Tpointer ty a, Tlong _ _ => add_case_pl ty a
-  | Tlong _ _, Tpointer ty a => add_case_lp ty a
+  | Tpointer ty _, Tint _ _ _ => add_case_pi ty
+  | Tint _ _ _, Tpointer ty _ => add_case_ip ty
+  | Tpointer ty _, Tlong _ _ => add_case_pl ty
+  | Tlong _ _, Tpointer ty _ => add_case_lp ty
   | _, _ => add_default
   end.
 
 Definition sem_add (v1:val) (t1:type) (v2: val) (t2:type) : option val :=
   match classify_add t1 t2 with 
-  | add_case_pi ty _ =>                 (**r pointer plus integer *)
+  | add_case_pi ty =>                 (**r pointer plus integer *)
       match v1,v2 with
       | Vptr b1 ofs1, Vint n2 => 
         Some (Vptr b1 (Int.add ofs1 (Int.mul (Int.repr (sizeof ty)) n2)))
       | _,  _ => None
       end   
-  | add_case_ip ty _ =>                 (**r integer plus pointer *)
+  | add_case_ip ty =>                 (**r integer plus pointer *)
       match v1,v2 with
       | Vint n1, Vptr b2 ofs2 => 
         Some (Vptr b2 (Int.add ofs2 (Int.mul (Int.repr (sizeof ty)) n1)))
       | _,  _ => None
       end   
-  | add_case_pl ty _ =>                 (**r pointer plus long *)
+  | add_case_pl ty =>                 (**r pointer plus long *)
       match v1,v2 with
       | Vptr b1 ofs1, Vlong n2 => 
         let n2 := Int.repr (Int64.unsigned n2) in
         Some (Vptr b1 (Int.add ofs1 (Int.mul (Int.repr (sizeof ty)) n2)))
       | _,  _ => None
       end   
-  | add_case_lp ty _ =>                 (**r long plus pointer *)
+  | add_case_lp ty =>                 (**r long plus pointer *)
       match v1,v2 with
       | Vlong n1, Vptr b2 ofs2 => 
         let n1 := Int.repr (Int64.unsigned n1) in
@@ -603,28 +575,28 @@ Definition sem_add (v1:val) (t1:type) (v2: val) (t2:type) : option val :=
 (** *** Subtraction *)
 
 Inductive classify_sub_cases : Type :=
-  | sub_case_pi(ty: type)(a: attr)      (**r pointer, int *)
+  | sub_case_pi(ty: type)               (**r pointer, int *)
   | sub_case_pp(ty: type)               (**r pointer, pointer *)
-  | sub_case_pl(ty: type)(a: attr)      (**r pointer, long *)
+  | sub_case_pl(ty: type)               (**r pointer, long *)
   | sub_default.                        (**r numerical type, numerical type *)
 
 Definition classify_sub (ty1: type) (ty2: type) :=
   match typeconv ty1, typeconv ty2 with
-  | Tpointer ty a, Tint _ _ _ => sub_case_pi ty a
+  | Tpointer ty _, Tint _ _ _ => sub_case_pi ty
   | Tpointer ty _ , Tpointer _ _ => sub_case_pp ty
-  | Tpointer ty a, Tlong _ _ => sub_case_pl ty a
+  | Tpointer ty _, Tlong _ _ => sub_case_pl ty
   | _, _ => sub_default
   end.
 
 Definition sem_sub (v1:val) (t1:type) (v2: val) (t2:type) : option val :=
   match classify_sub t1 t2 with
-  | sub_case_pi ty attr =>            (**r pointer minus integer *)
+  | sub_case_pi ty =>            (**r pointer minus integer *)
       match v1,v2 with
       | Vptr b1 ofs1, Vint n2 => 
           Some (Vptr b1 (Int.sub ofs1 (Int.mul (Int.repr (sizeof ty)) n2)))
       | _,  _ => None
       end
-  | sub_case_pl ty attr =>            (**r pointer minus long *)
+  | sub_case_pl ty =>            (**r pointer minus long *)
       match v1,v2 with
       | Vptr b1 ofs1, Vlong n2 => 
           let n2 := Int.repr (Int64.unsigned n2) in
@@ -1177,5 +1149,234 @@ Proof.
   intros; eapply Mem.different_pointers_inject; eauto.
 Qed.
 
+(** * Some properties of operator semantics *)
+
+(** This section collects some common-sense properties about the type
+  classification and semantic functions above.  These properties are
+  not used in the CompCert semantics preservation proofs, but increase
+  confidence in the specification and its relation with the ISO C99 standard. *)
+
+(** Relation between Boolean value and casting to [_Bool] type. *)
+
+Lemma cast_bool_bool_val:
+  forall v t,
+  sem_cast v t (Tint IBool Signed noattr) =
+  match bool_val v t with None => None | Some b => Some(Val.of_bool b) end.
+Proof.
+  intros.
+  assert (A: classify_bool t =
+    match t with
+    | Tint _ _ _ => bool_case_i
+    | Tpointer _ _ | Tcomp_ptr _ _ | Tarray _ _ _ | Tfunction _ _ _ => bool_case_p
+    | Tfloat _ _ => bool_case_f
+    | Tlong _ _ => bool_case_l
+    | _ => bool_default
+    end).
+  {
+    unfold classify_bool; destruct t; simpl; auto. destruct i; auto.
+  }
+  unfold bool_val. rewrite A. unfold sem_cast. destruct t; simpl; auto; destruct v; auto.
+  destruct (Int.eq i0 Int.zero); auto.
+  destruct (Int64.eq i Int64.zero); auto.
+  destruct (Float.cmp Ceq f0 Float.zero); auto.
+  destruct (Int.eq i Int.zero); auto. 
+  destruct (Int.eq i Int.zero); auto. 
+  destruct (Int.eq i Int.zero); auto. 
+  destruct (Int.eq i0 Int.zero); auto. 
+Qed.
+
+(** Relation between Boolean value and Boolean negation. *)
 
+Lemma notbool_bool_val:
+  forall v t,
+  sem_notbool v t =
+  match bool_val v t with None => None | Some b => Some(Val.of_bool (negb b)) end.
+Proof.
+  intros. unfold sem_notbool, bool_val. 
+  destruct (classify_bool t); auto; destruct v; auto; rewrite negb_involutive; auto.
+Qed.
 
+(** Relation with the arithmetic conversions of ISO C99, section 6.3.1 *)
+
+Module ArithConv.
+
+(** This is the ISO C algebra of arithmetic types, without qualifiers.
+    [S] stands for "signed" and [U] for "unsigned".  *)
+
+Inductive int_type : Type :=
+  | _Bool
+  | Char | SChar | UChar
+  | Short | UShort
+  | Int | UInt
+  | Long | ULong
+  | Longlong | ULonglong.
+
+Inductive arith_type : Type :=
+  | I (it: int_type)
+  | Float
+  | Double
+  | Longdouble.
+
+Definition eq_int_type: forall (x y: int_type), {x=y} + {x<>y}.
+Proof. decide equality. Defined.
+
+Definition is_unsigned (t: int_type) : bool :=
+  match t with
+  | _Bool => true
+  | Char => false    (**r as in most of CompCert's target ABIs *)
+  | SChar => false
+  | UChar => true
+  | Short => false
+  | UShort => true
+  | Int => false
+  | UInt => true
+  | Long => false
+  | ULong => true
+  | Longlong => false
+  | ULonglong => true
+  end.
+
+Definition unsigned_type (t: int_type) : int_type :=
+  match t with
+  | Char => UChar
+  | SChar => UChar
+  | Short => UShort
+  | Int => UInt
+  | Long => ULong
+  | Longlong => ULonglong
+  | _ => t
+  end.
+
+Definition int_sizeof (t: int_type) : Z :=
+  match t with
+  | _Bool | Char | SChar | UChar => 1
+  | Short | UShort => 2
+  | Int | UInt | Long | ULong => 4
+  | Longlong | ULonglong => 8
+  end.
+
+(** 6.3.1.1 para 1: integer conversion rank *)
+
+Definition rank (t: int_type) : Z :=
+  match t with
+  | _Bool => 1
+  | Char | SChar | UChar => 2
+  | Short | UShort => 3
+  | Int | UInt => 4
+  | Long | ULong => 5
+  | Longlong | ULonglong => 6
+  end.
+
+(** 6.3.1.1 para 2: integer promotions, a.k.a. usual unary conversions *)
+
+Definition integer_promotion (t: int_type) : int_type :=
+  if zlt (rank t) (rank Int) then Int else t.
+
+(** 6.3.1.8: Usual arithmetic conversions, a.k.a. binary conversions.
+  This function returns the type to which the two operands must be
+  converted. *)
+
+Definition usual_arithmetic_conversion (t1 t2: arith_type) : arith_type :=
+  match t1, t2 with
+  (* First, if the corresponding real type of either operand is long
+     double, the other operand is converted, without change of type domain,
+     to a type whose corresponding real type is long double. *)
+  | Longdouble, _ | _, Longdouble => Longdouble
+  (* Otherwise, if the corresponding real type of either operand is
+     double, the other operand is converted, without change of type domain,
+     to a type whose corresponding real type is double. *)
+  | Double, _ | _, Double => Double
+  (* Otherwise, if the corresponding real type of either operand is
+     float, the other operand is converted, without change of type domain,
+     to a type whose corresponding real type is float. *)
+  | Float, _ | _, Float => Float
+  (* Otherwise, the integer promotions are performed on both operands. *)
+  | I i1, I i2 =>
+    let j1 := integer_promotion i1 in
+    let j2 := integer_promotion i2 in
+    (* Then the following rules are applied to the promoted operands:
+       If both operands have the same type, then no further conversion
+       is needed. *)
+    if eq_int_type j1 j2 then I j1 else
+    match is_unsigned j1, is_unsigned j2 with
+    (* Otherwise, if both operands have signed integer types or both
+       have unsigned integer types, the operand with the type of lesser
+       integer conversion rank is converted to the type of the operand with
+       greater rank. *)
+    | true, true | false, false =>
+        if zlt (rank j1) (rank j2) then I j2 else I j1
+    | true, false =>
+    (* Otherwise, if the operand that has unsigned integer type has
+       rank greater or equal to the rank of the type of the other operand,
+       then the operand with signed integer type is converted to the type of
+       the operand with unsigned integer type. *)
+        if zle (rank j2) (rank j1) then I j1 else
+    (* Otherwise, if the type of the operand with signed integer type
+       can represent all of the values of the type of the operand with
+       unsigned integer type, then the operand with unsigned integer type is
+       converted to the type of the operand with signed integer type. *)
+        if zlt (int_sizeof j1) (int_sizeof j2) then I j2 else
+    (* Otherwise, both operands are converted to the unsigned integer type
+       corresponding to the type of the operand with signed integer type. *)
+        I (unsigned_type j2)
+    | false, true =>
+    (* Same logic as above, swapping the roles of j1 and j2 *)
+        if zle (rank j1) (rank j2) then I j2 else
+        if zlt (int_sizeof j2) (int_sizeof j1) then I j1 else
+        I (unsigned_type j1)
+    end
+  end.
+
+(** Mapping ISO arithmetic types to CompCert types *)
+
+Definition proj_type (t: arith_type) : type :=
+  match t with
+  | I _Bool => Tint IBool Unsigned noattr
+  | I Char => Tint I8 Unsigned noattr
+  | I SChar => Tint I8 Signed noattr
+  | I UChar => Tint I8 Unsigned noattr
+  | I Short => Tint I16 Signed noattr
+  | I UShort => Tint I16 Unsigned noattr
+  | I Int => Tint I32 Signed noattr
+  | I UInt => Tint I32 Unsigned noattr
+  | I Long => Tint I32 Signed noattr
+  | I ULong => Tint I32 Unsigned noattr
+  | I Longlong => Tlong Signed noattr
+  | I ULonglong => Tlong Unsigned noattr
+  | Float => Tfloat F32 noattr
+  | Double => Tfloat F64 noattr
+  | Longdouble => Tfloat F64 noattr
+  end.
+
+(** Relation between [typeconv] and integer promotion. *)
+
+Lemma typeconv_integer_promotion:
+  forall i, typeconv (proj_type (I i)) = proj_type (I (integer_promotion i)).
+Proof.
+  destruct i; reflexivity.
+Qed.
+
+(** Relation between [classify_binarith] and arithmetic conversion. *)
+
+Lemma classify_binarith_arithmetic_conversion:
+  forall t1 t2,
+  binarith_result_type (classify_binarith (proj_type t1) (proj_type t2)) =
+  Some (proj_type (usual_arithmetic_conversion t1 t2)).
+Proof.
+  destruct t1; destruct t2; try reflexivity.
+- destruct it; destruct it0; reflexivity.
+- destruct it; reflexivity.
+- destruct it; reflexivity.
+- destruct it; reflexivity.
+- destruct it; reflexivity.
+- destruct it; reflexivity.
+- destruct it; reflexivity.
+Qed.
+
+End ArithConv.
+
+
+
+
+
+ 
diff --git a/cfrontend/Cshmgen.v b/cfrontend/Cshmgen.v
index c97e881..685fa71 100644
--- a/cfrontend/Cshmgen.v
+++ b/cfrontend/Cshmgen.v
@@ -203,16 +203,16 @@ Definition make_binarith (iop iopu fop lop lopu: binary_operation)
 
 Definition make_add (e1: expr) (ty1: type) (e2: expr) (ty2: type) :=
   match classify_add ty1 ty2 with
-  | add_case_pi ty _ =>
+  | add_case_pi ty =>
       let n := make_intconst (Int.repr (Ctypes.sizeof ty)) in
       OK (Ebinop Oadd e1 (Ebinop Omul n e2))
-  | add_case_ip ty _ =>
+  | add_case_ip ty =>
       let n := make_intconst (Int.repr (Ctypes.sizeof ty)) in
       OK (Ebinop Oadd e2 (Ebinop Omul n e1))
-  | add_case_pl ty _ =>
+  | add_case_pl ty =>
       let n := make_intconst (Int.repr (Ctypes.sizeof ty)) in
       OK (Ebinop Oadd e1 (Ebinop Omul n (Eunop Ointoflong e2)))
-  | add_case_lp ty _ =>
+  | add_case_lp ty =>
       let n := make_intconst (Int.repr (Ctypes.sizeof ty)) in
       OK (Ebinop Oadd e2 (Ebinop Omul n (Eunop Ointoflong e1)))
   | add_default =>
@@ -221,13 +221,13 @@ Definition make_add (e1: expr) (ty1: type) (e2: expr) (ty2: type) :=
 
 Definition make_sub (e1: expr) (ty1: type) (e2: expr) (ty2: type) :=
   match classify_sub ty1 ty2 with
-  | sub_case_pi ty _ =>
+  | sub_case_pi ty =>
       let n := make_intconst (Int.repr (Ctypes.sizeof ty)) in
       OK (Ebinop Osub e1 (Ebinop Omul n e2))
   | sub_case_pp ty =>
       let n := make_intconst (Int.repr (Ctypes.sizeof ty)) in
       OK (Ebinop Odivu (Ebinop Osub e1 e2) n)
-  | sub_case_pl ty _ =>
+  | sub_case_pl ty =>
       let n := make_intconst (Int.repr (Ctypes.sizeof ty)) in
       OK (Ebinop Osub e1 (Ebinop Omul n (Eunop Ointoflong e2)))
   | sub_default =>
diff --git a/cfrontend/Ctypes.v b/cfrontend/Ctypes.v
index 5cd032d..41d0dcb 100644
--- a/cfrontend/Ctypes.v
+++ b/cfrontend/Ctypes.v
@@ -145,18 +145,55 @@ Definition attr_of_type (ty: type) :=
   | Tcomp_ptr id a => a
   end.
 
+(** Change the top-level attributes of a type *)
+
+Definition change_attributes (f: attr -> attr) (ty: type) : type :=
+  match ty with
+  | Tvoid => ty
+  | Tint sz si a => Tint sz si (f a)
+  | Tlong si a => Tlong si (f a)
+  | Tfloat sz a => Tfloat sz (f a)
+  | Tpointer elt a => Tpointer elt (f a)
+  | Tarray elt sz a => Tarray elt sz (f a)
+  | Tfunction args res cc => ty
+  | Tstruct id fld a => Tstruct id fld (f a)
+  | Tunion id fld a => Tunion id fld (f a)
+  | Tcomp_ptr id a => Tcomp_ptr id (f a)
+  end.
+
+(** Erase the top-level attributes of a type *)
+
+Definition remove_attributes (ty: type) : type :=
+  change_attributes (fun _ => noattr) ty.
+
+(** Add extra attributes to the top-level attributes of a type *)
+
+Definition attr_union (a1 a2: attr) : attr :=
+  {| attr_volatile := a1.(attr_volatile) || a2.(attr_volatile);
+     attr_alignas :=
+       match a1.(attr_alignas), a2.(attr_alignas) with
+       | None, al => al
+       | al, None => al
+       | Some n1, Some n2 => Some (N.max n1 n2)
+       end 
+  |}.
+
+Definition merge_attributes (ty: type) (a: attr) : type :=
+  change_attributes (attr_union a) ty.
+
 Definition type_int32s := Tint I32 Signed noattr.
 Definition type_bool := Tint IBool Signed noattr.
 
 (** The usual unary conversion.  Promotes small integer types to [signed int32]
-  and degrades array types and function types to pointer types. *)
+  and degrades array types and function types to pointer types. 
+  Attributes are erased. *)
 
 Definition typeconv (ty: type) : type :=
   match ty with
-  | Tint (I8 | I16 | IBool) _ a => Tint I32 Signed a
-  | Tarray t sz a       => Tpointer t a
+  | Tint (I8 | I16 | IBool) _ _ => Tint I32 Signed noattr
+  | Tarray t sz a       => Tpointer t noattr
   | Tfunction _ _ _     => Tpointer ty noattr
-  | _                   => ty
+  | _                   => remove_attributes ty
   end.
 
 (** * Operations over types *)