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, 6 insertions, 1 deletions

diff --git a/cparser/Cutil.mli b/cparser/Cutil.mli
index fb875b0d..5ae5dcd7 100644
--- a/cparser/Cutil.mli
+++ b/cparser/Cutil.mli
@@ -36,6 +36,11 @@ val incl_attributes : attributes -> attributes -> bool
 val alignas_attribute : attributes -> int
   (* Extract the value of the [_Alignas] attributes, if any.
      Return 0 if none, a (positive) power of two alignment if some. *)
+val packing_parameters : attributes -> int * int * bool
+  (* Extract the value of the [__packed__] attributes, if any.
+     Return a triple
+     (maximum field alignment, minimum struct alignment, byte swapping).
+     Alignments of 0 mean default alignment. *)
 val find_custom_attributes : string list -> attributes -> attr_arg list list
   (* Extract arguments of custom [Attr] attributes whose names appear
      in the given list of names. *)
@@ -127,7 +132,7 @@ val composite_info_decl:
 val composite_info_def:
   Env.t -> struct_or_union -> attributes -> field list -> Env.composite_info
 val struct_layout:
-  Env.t -> field list -> (string * int) list
+  Env.t -> attributes -> field list -> (string * int) list
 val offsetof:
   Env.t -> typ -> field -> int
 (* Compute the offset of a struct member *)