c: Add support for byte arrays in C2Y

To get correct aliasing behavior requires that structures and unions
that contain a byte array, i.e. an array of non-atomic character
type (N3254), are marked with TYPE_TYPELESS_STORAGE.  This change
affects also earlier language modes.

gcc/c/
	* c-decl.cc (grokdeclarator, finish_struct): Set and
	propagate TYPE_TYPELESS_STORAGE.

gcc/testsuite/
	* gcc.dg/c2y-byte-alias-1.c: New test.
	* gcc.dg/c2y-byte-alias-2.c: New test.
	* gcc.dg/c2y-byte-alias-3.c: New test.

4 files changed, 149 insertions, 2 deletions

diff --git a/gcc/c/c-decl.cc b/gcc/c/c-decl.cc
index 97f1d346..8cef8f2 100644
--- a/gcc/c/c-decl.cc
+++ b/gcc/c/c-decl.cc
@@ -7502,12 +7502,18 @@ grokdeclarator (const struct c_declarator *declarator,
 	       modify the shared type, so we gcc_assert (itype)
 	       below.  */
 	      {
+		/* Identify typeless storage as introduced in C2Y
+		   and supported also in earlier language modes.  */
+		bool typeless = (char_type_p (type)
+				 && !(type_quals & TYPE_QUAL_ATOMIC))
+				|| (AGGREGATE_TYPE_P (type)
+				    && TYPE_TYPELESS_STORAGE (type));
+
 		addr_space_t as = DECODE_QUAL_ADDR_SPACE (type_quals);
 		if (!ADDR_SPACE_GENERIC_P (as) && as != TYPE_ADDR_SPACE (type))
 		  type = build_qualified_type (type,
 					       ENCODE_QUAL_ADDR_SPACE (as));
-
-		type = build_array_type (type, itype);
+		type = build_array_type (type, itype, typeless);
 	      }
 
 	    if (type != error_mark_node)
@@ -9659,6 +9665,10 @@ finish_struct (location_t loc, tree t, tree fieldlist, tree attributes,
       if (DECL_NAME (x)
 	  || RECORD_OR_UNION_TYPE_P (TREE_TYPE (x)))
 	saw_named_field = true;
+
+      if (AGGREGATE_TYPE_P (TREE_TYPE (x))
+	  && TYPE_TYPELESS_STORAGE (TREE_TYPE (x)))
+	TYPE_TYPELESS_STORAGE (t) = true;
     }
 
   detect_field_duplicates (fieldlist);
@@ -9859,6 +9869,7 @@ finish_struct (location_t loc, tree t, tree fieldlist, tree attributes,
       TYPE_FIELDS (x) = TYPE_FIELDS (t);
       TYPE_LANG_SPECIFIC (x) = TYPE_LANG_SPECIFIC (t);
       TYPE_TRANSPARENT_AGGR (x) = TYPE_TRANSPARENT_AGGR (t);
+      TYPE_TYPELESS_STORAGE (x) = TYPE_TYPELESS_STORAGE (t);
       C_TYPE_FIELDS_READONLY (x) = C_TYPE_FIELDS_READONLY (t);
       C_TYPE_FIELDS_VOLATILE (x) = C_TYPE_FIELDS_VOLATILE (t);
       C_TYPE_FIELDS_NON_CONSTEXPR (x) = C_TYPE_FIELDS_NON_CONSTEXPR (t);
diff --git a/gcc/testsuite/gcc.dg/c2y-byte-alias-1.c b/gcc/testsuite/gcc.dg/c2y-byte-alias-1.c
new file mode 100644
index 0000000..30bc2c0
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/c2y-byte-alias-1.c
@@ -0,0 +1,46 @@
+/* { dg-do run } */
+/* { dg-options "-std=c2y -O2" } */
+
+struct f { _Alignas(int) char buf[sizeof(int)]; };
+struct f2 { struct f x; };
+union g { _Alignas(int) char buf[sizeof(int)]; };
+
+[[gnu::noinline]]
+int foo(struct f *p, int *q)
+{
+	*q = 1;
+	*p = (struct f){ };
+	return *q;
+}
+
+[[gnu::noinline]]
+int foo2(struct f2 *p, int *q)
+{
+	*q = 1;
+	*p = (struct f2){ };
+	return *q;
+}
+
+[[gnu::noinline]]
+int bar(union g *p, int *q)
+{
+	*q = 1;
+	*p = (union g){ };
+	return *q;
+}
+
+
+int main()
+{
+	struct f p;
+	if (0 != foo(&p, (void*)&p.buf))
+		__builtin_abort();
+
+	struct f2 p2;
+	if (0 != foo2(&p2, (void*)&p2.x.buf))
+		__builtin_abort();
+
+	union g q;
+	if (0 != bar(&q, (void*)&q.buf))
+		__builtin_abort();
+}
diff --git a/gcc/testsuite/gcc.dg/c2y-byte-alias-2.c b/gcc/testsuite/gcc.dg/c2y-byte-alias-2.c
new file mode 100644
index 0000000..9bd2d18
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/c2y-byte-alias-2.c
@@ -0,0 +1,43 @@
+/* { dg-do run } */
+/* { dg-options "-std=c2y -O2" } */
+
+struct f2 {
+	struct f {
+		_Alignas(int) char buf[sizeof(int)];
+	} x[2];
+	int i;
+};
+
+[[gnu::noinline]]
+int foo2(struct f2 *p, int *q)
+{
+	*q = 1;
+	*p = (struct f2){ };
+	return *q;
+}
+
+struct g2 {
+	union g {
+		_Alignas(int) char buf[sizeof(int)];
+	} x[2];
+	int i;
+};
+
+[[gnu::noinline]]
+int bar2(struct g2 *p, int *q)
+{
+	*q = 1;
+	*p = (struct g2){ };
+	return *q;
+}
+
+int main()
+{
+	struct f2 p2;
+	if (0 != foo2(&p2, (void*)&p2.x[0].buf))
+		__builtin_abort();
+
+	struct g2 q2;
+	if (0 != bar2(&q2, (void*)&q2.x[0].buf))
+		__builtin_abort();
+}
diff --git a/gcc/testsuite/gcc.dg/c2y-byte-alias-3.c b/gcc/testsuite/gcc.dg/c2y-byte-alias-3.c
new file mode 100644
index 0000000..f88eab2
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/c2y-byte-alias-3.c
@@ -0,0 +1,47 @@
+/* { dg-do run } */
+/* { dg-options "-std=c2y -O2" } */
+
+struct f { _Alignas(int) typeof(volatile char) buf[sizeof(int)]; };
+struct f2 { struct f x; };
+union g { _Alignas(int) volatile char buf[sizeof(int)]; };
+
+
+[[gnu::noinline]]
+int foo(struct f *p, int *q)
+{
+	*q = 1;
+	*p = (struct f){ };
+	return *q;
+}
+
+[[gnu::noinline]]
+int foo2(struct f2 *p, int *q)
+{
+	*q = 1;
+	*p = (struct f2){ };
+	return *q;
+}
+
+[[gnu::noinline]]
+int bar(union g *p, int *q)
+{
+	*q = 1;
+	*p = (union g){ };
+	return *q;
+}
+
+
+int main()
+{
+	struct f p;
+	if (0 != foo(&p, (void*)&p.buf))
+		__builtin_abort();
+
+	struct f2 p2;
+	if (0 != foo2(&p2, (void*)&p2.x.buf))
+		__builtin_abort();
+
+	union g q;
+	if (0 != bar(&q, (void*)&q.buf))
+		__builtin_abort();
+}