Improve checks on C2x fallthrough attribute.

When adding C2x attribute support, some [[fallthrough]] support
appeared as a side-effect because of code for that attribute going
through separate paths from the normal attribute handling.

However, going through those paths without the normal attribute
handlers meant that certain checks, such as for the invalid usage
[[fallthrough()]], did not operate.  This patch improves checks by
adding this attribute to the standard attribute table, so that the
parser knows it expects no arguments, along with adding an explicit
check for "[[fallthrough]];" attribute-declarations at top level.  As
with other attributes, there are still cases where warnings should be
pedwarns because C2x constraints are violated, but this patch improves
the attribute handling.

Bootstrapped with no regressions on x86_64-pc-linux-gnu.

gcc/c:
	* c-decl.c (std_attribute_table): Add fallthrough.
	* c-parser.c (c_parser_declaration_or_fndef): Diagnose fallthrough
	attribute at top level.

gcc/c-family:
	* c-attribs.c (handle_fallthrough_attribute): Remove static.
	* c-common.h (handle_fallthrough_attribute): Declare.

gcc/testsuite:
	* gcc.dg/c2x-attr-fallthrough-2.c,
	gcc.dg/c2x-attr-fallthrough-3.c: New tests.

From-SVN: r278273

9 files changed, 82 insertions, 5 deletions

diff --git a/gcc/c-family/ChangeLog b/gcc/c-family/ChangeLog
index 571cf2b..c7420f6 100644
--- a/gcc/c-family/ChangeLog
+++ b/gcc/c-family/ChangeLog
@@ -1,5 +1,10 @@
 2019-11-15  Joseph Myers  <joseph@codesourcery.com>
 
+	* c-attribs.c (handle_fallthrough_attribute): Remove static.
+	* c-common.h (handle_fallthrough_attribute): Declare.
+
+2019-11-15  Joseph Myers  <joseph@codesourcery.com>
+
 	* c-attribs.c (handle_deprecated_attribute): Remove static.
 	* c-common.h (handle_deprecated_attribute): Declare.
 
diff --git a/gcc/c-family/c-attribs.c b/gcc/c-family/c-attribs.c
index 18b829f..4a59cdf 100644
--- a/gcc/c-family/c-attribs.c
+++ b/gcc/c-family/c-attribs.c
@@ -144,7 +144,6 @@ static tree handle_simd_attribute (tree *, tree, tree, int, bool *);
 static tree handle_omp_declare_target_attribute (tree *, tree, tree, int,
 						 bool *);
 static tree handle_designated_init_attribute (tree *, tree, tree, int, bool *);
-static tree handle_fallthrough_attribute (tree *, tree, tree, int, bool *);
 static tree handle_patchable_function_entry_attribute (tree *, tree, tree,
 						       int, bool *);
 static tree handle_copy_attribute (tree *, tree, tree, int, bool *);
@@ -4114,7 +4113,7 @@ handle_designated_init_attribute (tree *node, tree name, tree, int,
 /* Handle a "fallthrough" attribute; arguments as in struct
    attribute_spec.handler.  */
 
-static tree
+tree
 handle_fallthrough_attribute (tree *, tree name, tree, int,
 			      bool *no_add_attrs)
 {
diff --git a/gcc/c-family/c-common.h b/gcc/c-family/c-common.h
index ad40c15..f3478d3 100644
--- a/gcc/c-family/c-common.h
+++ b/gcc/c-family/c-common.h
@@ -1359,6 +1359,7 @@ extern void warn_for_multistatement_macros (location_t, location_t,
 extern bool attribute_takes_identifier_p (const_tree);
 extern tree handle_deprecated_attribute (tree *, tree, tree, int, bool *);
 extern tree handle_unused_attribute (tree *, tree, tree, int, bool *);
+extern tree handle_fallthrough_attribute (tree *, tree, tree, int, bool *);
 extern int parse_tm_stmt_attr (tree, int);
 extern int tm_attr_to_mask (tree);
 extern tree tm_mask_to_attr (int);
diff --git a/gcc/c/ChangeLog b/gcc/c/ChangeLog
index fdc9153..c2cab57 100644
--- a/gcc/c/ChangeLog
+++ b/gcc/c/ChangeLog
@@ -1,5 +1,11 @@
 2019-11-15  Joseph Myers  <joseph@codesourcery.com>
 
+	* c-decl.c (std_attribute_table): Add fallthrough.
+	* c-parser.c (c_parser_declaration_or_fndef): Diagnose fallthrough
+	attribute at top level.
+
+2019-11-15  Joseph Myers  <joseph@codesourcery.com>
+
 	* c-decl.c (std_attribute_table): New.
 	(c_init_decl_processing): Register attributes from
 	std_attribute_table.
diff --git a/gcc/c/c-decl.c b/gcc/c/c-decl.c
index a7f7c69..98b71ea 100644
--- a/gcc/c/c-decl.c
+++ b/gcc/c/c-decl.c
@@ -4343,6 +4343,8 @@ const struct attribute_spec std_attribute_table[] =
        affects_type_identity, handler, exclude } */
   { "deprecated", 0, 1, false, false, false, false,
     handle_deprecated_attribute, NULL },
+  { "fallthrough", 0, 0, false, false, false, false,
+    handle_fallthrough_attribute, NULL },
   { NULL, 0, 0, false, false, false, false, NULL, NULL }
 };
 
diff --git a/gcc/c/c-parser.c b/gcc/c/c-parser.c
index 721158a..5b290bf 100644
--- a/gcc/c/c-parser.c
+++ b/gcc/c/c-parser.c
@@ -1927,9 +1927,15 @@ c_parser_declaration_or_fndef (c_parser *parser, bool fndef_ok,
 	{
 	  if (fallthru_attr_p != NULL)
 	    *fallthru_attr_p = true;
-	  tree fn = build_call_expr_internal_loc (here, IFN_FALLTHROUGH,
-						  void_type_node, 0);
-	  add_stmt (fn);
+	  if (nested)
+	    {
+	      tree fn = build_call_expr_internal_loc (here, IFN_FALLTHROUGH,
+						      void_type_node, 0);
+	      add_stmt (fn);
+	    }
+	  else
+	    pedwarn (here, OPT_Wattributes,
+		     "%<fallthrough%> attribute at top level");
 	}
       else if (empty_ok && !(have_attrs
 			     && specs->non_std_attrs_seen_p))
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index 17494d0..8c6f891 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,5 +1,10 @@
 2019-11-15  Joseph Myers  <joseph@codesourcery.com>
 
+	* gcc.dg/c2x-attr-fallthrough-2.c,
+	gcc.dg/c2x-attr-fallthrough-3.c: New tests.
+
+2019-11-15  Joseph Myers  <joseph@codesourcery.com>
+
 	* gcc.dg/c2x-attr-deprecated-1.c, gcc.dg/c2x-attr-deprecated-2.c,
 	gcc.dg/c2x-attr-deprecated-3.c: New tests.
 
diff --git a/gcc/testsuite/gcc.dg/c2x-attr-fallthrough-2.c b/gcc/testsuite/gcc.dg/c2x-attr-fallthrough-2.c
new file mode 100644
index 0000000..e396a60
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/c2x-attr-fallthrough-2.c
@@ -0,0 +1,35 @@
+/* Test C2x attribute syntax.  Invalid use of fallthrough attribute.  */
+/* { dg-do compile } */
+/* { dg-options "-std=c2x -pedantic-errors -Wextra" } */
+
+[[fallthrough]]; /* { dg-error "'fallthrough' attribute at top level" } */
+
+int [[fallthrough]] x; /* { dg-warning "ignored" } */
+/* { dg-message "that appertains to a type-specifier" "appertains" { target *-*-* } .-1 } */
+
+int g () [[fallthrough]]; /* { dg-warning "ignored" } */
+/* { dg-message "that appertains to a type-specifier" "appertains" { target *-*-* } .-1 } */
+
+int
+f (int a)
+{
+  [[fallthrough]] int b = 2; /* { dg-warning "not followed by|ignored" } */
+  switch (a)
+    {
+    case 1:
+      b = 1; /* { dg-warning "may fall through" } */
+    case 2:
+      b = 2; /* { dg-warning "may fall through" } */
+      [[fallthrough()]]; /* { dg-error "does not take any arguments" } */
+    case 3:
+      b += 7;
+      break;
+    case 4:
+      b = 4; /* { dg-warning "may fall through" } */
+      [[fallthrough(1)]]; /* { dg-error "does not take any arguments|expected" } */
+    case 5:
+      b += 5;
+      break;
+    }
+  [[fallthrough]] return b; /* { dg-warning "ignored" } */
+}
diff --git a/gcc/testsuite/gcc.dg/c2x-attr-fallthrough-3.c b/gcc/testsuite/gcc.dg/c2x-attr-fallthrough-3.c
new file mode 100644
index 0000000..66fe820
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/c2x-attr-fallthrough-3.c
@@ -0,0 +1,18 @@
+/* Test C2x attribute syntax.  Invalid use of fallthrough attribute
+   outside switch or in bad context inside switch.  */
+/* { dg-do compile } */
+/* { dg-options "-std=c2x -pedantic-errors -Wextra" } */
+
+int
+f (int a)
+{
+  [[fallthrough]]; /* { dg-error "invalid use of attribute 'fallthrough'" } */
+  switch (a)
+    {
+    case 1:
+      a++;
+      [[fallthrough]]; /* { dg-warning "attribute 'fallthrough' not preceding a case label or default label" } */
+      a++;
+    }
+  return a;
+}