[C] Warn on type punning that toggles scalar storage order

[Eric Botcazou]

Hi,

As documented in the manual, we do not support type punning that toggles the
scalar storage order, so this adds a warning for the simple case of unions.

Tested on x86-64/Linux, OK for the mainline?


2021-05-26  Eric Botcazou  <ebotca...@adacore.com>

        PR c/100653
        * c-decl.c (finish_struct): Warn for a union containing an aggregate
        field with a differing scalar storage order.


2021-05-26  Eric Botcazou  <ebotca...@adacore.com>

        * gcc.dg/sso-13.c: New test.

-- 
Eric Botcazou
diff --git a/gcc/c/c-decl.c b/gcc/c/c-decl.c
index 53b2b5b637d..3c7b306809d 100644
--- a/gcc/c/c-decl.c
+++ b/gcc/c/c-decl.c
@@ -8853,6 +8853,13 @@ finish_struct (location_t loc, tree t, tree fieldlist, tree attributes,
 		= c_build_qualified_type (fmain_type, TYPE_QUALS (ftype));
 	    }
 	}
+
+      if (TREE_CODE (t) == UNION_TYPE
+	  && AGGREGATE_TYPE_P (TREE_TYPE (field))
+	  && TYPE_REVERSE_STORAGE_ORDER (t)
+	     != TYPE_REVERSE_STORAGE_ORDER (TREE_TYPE (field)))
+	warning_at (DECL_SOURCE_LOCATION (field), OPT_Wscalar_storage_order,
+		    "type punning toggles scalar storage order");
     }
 
   /* Now we have the truly final field list.

/* Test support of scalar_storage_order attribute */

/* { dg-do compile } */

#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
#define REV_ENDIANNESS __attribute__((scalar_storage_order("big-endian")))
#else
#define REV_ENDIANNESS __attribute__((scalar_storage_order("little-endian")))
#endif

typedef struct tIp6Addr
{
    unsigned int s6_addr32[4];
} tIp6Addr;

struct _tBeTimNetAddr
{
    unsigned char isIPv4;
    union
    {
        unsigned int addr;
        tIp6Addr addr6;   /* { dg-warning "type punning toggles" } */
    } REV_ENDIANNESS u;
} REV_ENDIANNESS;