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, 3 insertions, 1 deletions

diff --git a/cfrontend/C2C.ml b/cfrontend/C2C.ml
index ce1f2c0e..ecaffbc6 100644
--- a/cfrontend/C2C.ml
+++ b/cfrontend/C2C.ml
@@ -610,9 +610,11 @@ let convertField env f =
   (intern_string f.fld_name, convertTyp env f.fld_typ)
 
 let convertCompositedef env su id attr members =
+  if Cutil.find_custom_attributes ["packed";"__packed__"] attr <> [] then
+    unsupported "packed struct (consider adding option -fpacked-structs)";
   let t = match su with
   | C.Struct ->
-      let layout = Cutil.struct_layout env members in
+      let layout = Cutil.struct_layout env attr members in
       List.iter (fun (a,b) -> Debug.set_member_offset id a b) layout;
       TStruct (id,attr)
   | C.Union -> TUnion (id,attr) in