clang attribute wrappers

Summary: adding GCC compatible wrappers for clang attributes to folly.

Reviewed By: yfeldblum

Differential Revision: D2706913

fb-gh-sync-id: 17db34089d48a6a01c20904321a47ff98141c2fe

folly/CppAttributes.h

+/*
+ * Copyright 2015 Facebook, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/**
+ * GCC compatible wrappers around clang attributes.
+ *
+ * @author Dominik Gabi
+ */
+
+#ifndef FOLLY_BASE_ATTRIBUTES_H_
+#define FOLLY_BASE_ATTRIBUTES_H_
+
+#ifndef __has_cpp_attribute
+#define FOLLY_HAS_CPP_ATTRIBUTE(x) 0
+#else
+#define FOLLY_HAS_CPP_ATTRIBUTE(x) __has_cpp_attribute(x)
+#endif
+
+/**
+ * Fallthrough to indicate that `break` was left out on purpose in a switch
+ * statement, e.g.
+ *
+ * switch (n) {
+ *   case 22:
+ *   case 33:  // no warning: no statements between case labels
+ *     f();
+ *   case 44:  // warning: unannotated fall-through
+ *     g();
+ *     FOLLY_FALLTHROUGH; // no warning: annotated fall-through
+ * }
+ */
+#if FOLLY_HAS_CPP_ATTRIBUTE(clang::fallthrough)
+#define FOLLY_FALLTHROUGH [[clang::fallthrough]]
+#else
+#define FOLLY_FALLTHROUGH
+#endif
+
+#endif /* FOLLY_BASE_ATTRIBUTES_H_ */

folly/Makefile.am

@@ -30,6 +30,7 @@ nobase_follyinclude_HEADERS = \
 	AtomicLinkedList.h \
 	AtomicStruct.h \
 	AtomicUnorderedMap.h \
+	CppAttributes.h \
 	Baton.h \
 	Benchmark.h \
 	Bits.h \