c-lex.c (c_common_has_attribute): Handle attribute fallthrough.

	* c-lex.c (c_common_has_attribute): Handle attribute fallthrough.

	* system.h: Use __has_attribute to check whether the fallthrough
	attribute is supported.

	* g++.dg/cpp1z/feat-cxx1z.C: Test attribute fallthrough.

From-SVN: r240499

gcc/ChangeLog

 2016-09-26  Marek Polacek  <polacek@redhat.com>
 
+	* system.h: Use __has_attribute to check whether the fallthrough
+	attribute is supported.
+
+2016-09-26  Marek Polacek  <polacek@redhat.com>
+
 	* ipa-inline-analysis.c (find_foldable_builtin_expect): Use
 	gimple_call_internal_p.
 	* ipa-split.c (find_return_bb): Likewise.

gcc/c-family/ChangeLog

 2016-09-26  Marek Polacek  <polacek@redhat.com>
 
+	* c-lex.c (c_common_has_attribute): Handle attribute fallthrough.
+
+2016-09-26  Marek Polacek  <polacek@redhat.com>
+
 	PR c/7652
 	* c-common.c (c_common_attribute_table): Add fallthrough attribute.
 	(handle_fallthrough_attribute): New function.

gcc/c-family/c-lex.c

@@ -350,7 +350,8 @@ c_common_has_attribute (cpp_reader *pfile)
 	      else if (is_attribute_p ("deprecated", attr_name))
 		result = 201309;
 	      else if (is_attribute_p ("maybe_unused", attr_name)
-		       || is_attribute_p ("nodiscard", attr_name))
+		       || is_attribute_p ("nodiscard", attr_name)
+		       || is_attribute_p ("fallthrough", attr_name))
 		result = 201603;
 	      if (result)
 		attr_name = NULL_TREE;

gcc/system.h

@@ -746,8 +746,12 @@ extern void fancy_abort (const char *, int, const char *) ATTRIBUTE_NORETURN;
 #define gcc_unreachable() (fancy_abort (__FILE__, __LINE__, __FUNCTION__))
 #endif
 
-#if GCC_VERSION >= 7000
-# define gcc_fallthrough() __attribute__((fallthrough))
+#if GCC_VERSION >= 7000 && defined(__has_attribute)
+# if __has_attribute(fallthrough)
+#  define gcc_fallthrough() __attribute__((fallthrough))
+# else
+#  define gcc_fallthrough()
+# endif
 #else
 # define gcc_fallthrough()
 #endif

gcc/testsuite/ChangeLog

+2016-09-26  Marek Polacek  <polacek@redhat.com>
+
+	* g++.dg/cpp1z/feat-cxx1z.C: Test attribute fallthrough.
+
 2016-09-26  Martin Liska  <mliska@suse.cz>
 
 	* c-c++-common/ubsan/sanitize-recover-1.c: New test.

gcc/testsuite/g++.dg/cpp1z/feat-cxx1z.C

@@ -370,6 +370,12 @@
 #    error "__has_cpp_attribute(nodiscard) != 201603"
 #  endif
 
+#  if ! __has_cpp_attribute(fallthrough)
+#    error "__has_cpp_attribute(fallthrough)"
+#  elif __has_cpp_attribute(fallthrough) != 201603
+#    error "__has_cpp_attribute(fallthrough) != 201603"
+#  endif
+
 #else
 #  error "__has_cpp_attribute"
 #endif