Issue with packed structs and sizeof, alignof, offsetof in cparser/

CompCert has two implementations of sizeof, alignof and offsetof (byte offset of a struct field):
- the reference implementation, in Coq, from cfrontend/Ctypes.v
- the implementation used during elaboration, in OCaml, from cparser/Cutil.ml

The reference Coq implementation is used as much as possible, but sometimes during elaboration the size of a type must be computed (e.g. to compute array sizes), or the offset of a field (e.g. to evaluate __builtin_offsetof), in which case the OCaml implementation is used.

This causes issues with packed structs.  Currently, the cparser/Cutil.ml functions ignore the "packed" attribute on structs.  Their results disagree with the "true" sizes, alignments and offsets computed by the cfrontend/Ctypes.v functions after source-to-source transformation of packed structs as done in cparser/PackedStruct.ml.  For example:
```
struct __packed__(1) s { char c; short s; int i; };
assert (__builtin_offsetof(struct s, i) == 3);
assert (sizeof(struct s) = sizeof(char[sizeof(struct s)]));
```
The two assertions fail.  In the first assertion, __builtin_offsetof is elaborated to 4, because the packed attribute is ignored during elaboration.  In the second assertion, the type `char[sizeof(struct s)]` is elaborated to `char[8]`, again because the packed attribute is ignored during elaboration, while the other `sizeof(struct s)` is computed as 7 after the source-to-source transformation of packed structs.

This commit changes the cparser/Cutil.ml functions so that they take the packed attribute into account when computing sizeof, alignof, offsetof, and struct_layout.

Related changes:
* cparser/Cutil: add `packing_parameters` function to extract packing info from attributes
* cparser/Cutil: refactor and share more code between sizeof_struct, offsetof, and struct_layout
* cparser/Elab: check the alignment parameters given in packed attributes.  (The check was previously done in cparser/PackedStruct.ml but now it would come too late.)
* cparser/Elab: refactor the checking of alignment parameters between _Alignas, attribute((aligned)), __packed__, and attribute((packed)).
* cparser/PackedStructs: simplify the code, some functionality was moved to cparser/Cutil, other to cparser/Elab
* cfrontend/C2C: raise an "unsupported" error if a packed struct is defined and -fpacked-structs is not given.  Before, the packed attribute would be silently ignored, but now doing so would cause inconsistencies between cfrontend/ and cparser/.
* test/regression/packedstruct1.c: add tests to compare the sizes and the offsets produced by the elaborator with those obtained after elaboration.

1 files changed, 15 insertions, 29 deletions

diff --git a/cparser/PackedStructs.ml b/cparser/PackedStructs.ml
index e1287eb8..a2c91c0a 100644
--- a/cparser/PackedStructs.ml
+++ b/cparser/PackedStructs.ml
@@ -81,47 +81,33 @@ let transf_field_decl mfa swapped loc env struct_id f =
 (* Rewriting struct declarations *)
 
 let transf_struct_decl mfa msa swapped loc env struct_id attrs ml =
+  let attrs' =
+    remove_custom_attributes  ["packed";"__packed__"] attrs in
   let ml' =
     List.map (transf_field_decl mfa swapped loc env struct_id) ml in
-  if msa = 0 then (attrs, ml') else begin
-    let al' = (* natural alignment of the transformed struct *)
-      List.fold_left
-        (fun a f' -> max a (safe_alignof loc env f'.fld_typ))
-        1 ml' in
-      (set_alignas_attr (max msa al') attrs, ml')
+  if msa = 0 then (attrs', ml') else begin
+    (* [Cutil.composite_info_def] takes packing parameters into account.
+       Hence the alignment it returns is the correct alignment for
+       the transformed struct. *)
+    let ci = Cutil.composite_info_def env Struct attrs ml in
+    match ci.ci_alignof with
+    | None -> error loc "incomplete struct"; (attrs', ml')
+    | Some al -> (set_alignas_attr al attrs', ml')
   end
 
 (* Rewriting composite declarations *)
 
-let is_pow2 n = n > 0 && n land (n - 1) = 0
-
-let packed_param_value loc n =
-  let m = Int64.to_int n in
-  if n <> Int64.of_int m then
-    (error loc "__packed__ parameter `%Ld' is too large" n; 0)
-  else if m = 0 || is_pow2 m then
-    m
-  else
-    (error loc "__packed__ parameter `%Ld' must be a power of 2" n; 0)
-
 let transf_composite loc env su id attrs ml =
   match su with
   | Union -> (attrs, ml)
   | Struct ->
       let (mfa, msa, swapped) =
         match find_custom_attributes ["packed";"__packed__"] attrs with
-        | [] -> (0L, 0L, false)
-        | [[]] -> (1L, 0L, false)
-        | [[AInt n]] -> (n, 0L, false)
-        | [[AInt n; AInt p]] -> (n, p, false)
-        | [[AInt n; AInt p; AInt q]] -> (n, p, q <> 0L)
-        | _ ->
-          error loc "ill-formed or ambiguous __packed__ attribute";
-          (0L, 0L, false) in
-      let mfa = packed_param_value loc mfa in
-      let msa = packed_param_value loc msa in
-      let attrs' = remove_custom_attributes  ["packed";"__packed__"] attrs in
-      transf_struct_decl mfa msa swapped loc env id attrs' ml
+        | []  -> (0, 0, false)
+        | [_] -> Cutil.packing_parameters attrs
+        | _   -> error loc "multiple __packed__ attributes";
+                 (0, 0, false) in
+      transf_struct_decl mfa msa swapped loc env id attrs ml
 
 (* Accessor functions *)