Warn on type punning that toggles scalar storage order

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

gcc/c/
	PR c/100653
	* c-decl.c (finish_struct): Warn for a union containing an aggregate
	field with a differing scalar storage order.
gcc/testsuite/
	* gcc.dg/sso-13.c: New test.

2 files changed, 31 insertions, 0 deletions

diff --git a/gcc/c/c-decl.c b/gcc/c/c-decl.c
index 53b2b5b..3c7b306 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.
diff --git a/gcc/testsuite/gcc.dg/sso-13.c b/gcc/testsuite/gcc.dg/sso-13.c
new file mode 100644
index 0000000..ddfde00
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/sso-13.c
@@ -0,0 +1,24 @@
+/* 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;