c-family, tree: Allow nonstring attribute on multidimensional arrays [PR117178]

As requested in the PR117178 thread, the following patch allows nonstring
attribute also on multi-dimensional arrays (with cv char/signed char/unsigned
char as innermost element type) and pointers to such multi-dimensional arrays
or pointers to single-dimensional cv char/signed char/unsigned char arrays.
Given that (unfortunately) nonstring is a decl attribute rather than type
attribute, I think restricting it to single-dimensional arrays makes no
sense, even multi-dimensional ones can be used for storage of non-nul
terminated strings.

I really don't know what the kernel plans are, whether
they'll go with -Wno-unterminated-string-initialization added in Makefiles,
or whether the plan is to use nonstring attributes to quiet the warning.
In the latter case, some of the nonstring attributes will need to be
conditional on gcc version, because gcc before this patch will reject it
on multidimensional arrays.

2025-03-08  Jakub Jelinek  <[email protected]>

	PR c/117178
gcc/
	* tree.cc (get_attr_nonstring_decl): Look through all ARRAY_REFs, not
	just one and handle COMPONENT_REF and MEM_REF after skipping those
	rather than only when there wasn't ARRAY_REF.  Formatting fix.
gcc/c-family/
	* c-attribs.cc (handle_nonstring_attribute): Allow the attribute also
	on multi-dimensional arrays with char/signed char/unsigned char
	element type or pointers to such single and multi-dimensional arrays.
gcc/testsuite/
	* c-c++-common/attr-nonstring-7.c: Remove one xfail.
	* c-c++-common/attr-nonstring-9.c: New test.
	* c-c++-common/attr-nonstring-10.c: New test.
	* c-c++-common/attr-nonstring-11.c: New test.
	* c-c++-common/attr-nonstring-12.c: New test.
	* c-c++-common/attr-nonstring-13.c: New test.
	* c-c++-common/attr-nonstring-14.c: New test.
	* c-c++-common/attr-nonstring-15.c: New test.
	* c-c++-common/attr-nonstring-16.c: New test.

Author: jakubjelinek

gcc/c-family/c-attribs.cc

@@ -5117,8 +5117,9 @@ handle_nonstring_attribute (tree *node, tree name, tree ARG_UNUSED (args),
       if (POINTER_TYPE_P (type) || TREE_CODE (type) == ARRAY_TYPE)
 	{
 	  /* Accept the attribute on arrays and pointers to all three
-	     narrow character types.  */
-	  tree eltype = TREE_TYPE (type);
+	     narrow character types, including multi-dimensional arrays
+	     or pointers to them.  */
+	  tree eltype = strip_array_types (TREE_TYPE (type));
 	  eltype = TYPE_MAIN_VARIANT (eltype);
 	  if (eltype == char_type_node
 	      || eltype == signed_char_type_node

gcc/testsuite/c-c++-common/attr-nonstring-10.c

@@ -0,0 +1,75 @@
+/* Test to exercise attribute "nonstring".
+   { dg-do compile }
+   { dg-options "-O2 -Wattributes -Wstringop-truncation -ftrack-macro-expansion=0" }  */
+
+#define ATTR(list) __attribute__ (list)
+#define NONSTR     ATTR ((nonstring))
+#define strncpy(d, s, n) (__builtin_strncpy ((d), (s), (n)), sink (d))
+
+void sink (void*);
+
+/* Global string with an known bound.  */
+extern char gns3[][3] NONSTR;
+
+/* Global non-string pointers.  */
+extern NONSTR char (*pns_1)[1];
+extern char (* NONSTR pns_2)[2];
+extern char (*pns_3)[3] NONSTR;
+
+struct MemArrays
+{
+  NONSTR char ma3[2][3];
+  char NONSTR ma4[3][4];
+  char ma5[4][5] NONSTR;
+};
+
+
+void test_array (const char *s, unsigned n)
+{
+  const char s7[] = "1234567";
+
+  strncpy (gns3[2], "", 0);
+  strncpy (gns3[2], "a", 1);
+  strncpy (gns3[2], "a", 2);
+  strncpy (gns3[2], "a", 3);
+  strncpy (gns3[2], "ab", 3);
+  strncpy (gns3[2], "abc", 3);
+}
+
+
+void test_pointer (const char *s, unsigned n)
+{
+  const char s7[] = "1234567";
+
+  strncpy (*pns_1, "a", 1);
+  strncpy (*pns_2, "ab", 2);
+  strncpy (*pns_3, "abc", 3);
+  strncpy (*pns_3, s7, 3);
+
+  strncpy (*pns_1, s, 1);
+  strncpy (*pns_2, s, 1);
+  strncpy (*pns_3, s, 1);
+
+  strncpy (*pns_1, s, n);
+  strncpy (*pns_2, s, n);
+  strncpy (*pns_3, s, n);
+}
+
+
+void test_member_array (struct MemArrays *p, const char *s, unsigned n)
+{
+  const char s7[] = "1234567";
+
+  strncpy (p->ma3[1], "", 0);
+  strncpy (p->ma3[1], "a", 1);
+  strncpy (p->ma4[2], "ab", 2);
+  strncpy (p->ma5[3], "abc", 3);
+
+  strncpy (p->ma3[1], s, 1);
+  strncpy (p->ma4[2], s, 1);
+  strncpy (p->ma5[3], s, 1);
+
+  strncpy (p->ma3[1], s7, n);
+  strncpy (p->ma4[2], s7, n);
+  strncpy (p->ma5[3], s7, n);
+}