Enable -Wimplicit-fallthrough.
Falling through switch cases on a clang build must now annotate the fallthrough
with the FALLTHROUGH_INTENDED macro.
Bug: 17731372
Change-Id: I836451cd5f96b01d1ababdbf9eef677fe8fa8324
diff --git a/runtime/base/macros.h b/runtime/base/macros.h
index b66d528..f5a38bb 100644
--- a/runtime/base/macros.h
+++ b/runtime/base/macros.h
@@ -181,6 +181,48 @@
template<typename T> void UNUSED(const T&) {}
#define UNREACHABLE __builtin_unreachable
+// The FALLTHROUGH_INTENDED macro can be used to annotate implicit fall-through
+// between switch labels:
+// switch (x) {
+// case 40:
+// case 41:
+// if (truth_is_out_there) {
+// ++x;
+// FALLTHROUGH_INTENDED; // Use instead of/along with annotations in
+// // comments.
+// } else {
+// return x;
+// }
+// case 42:
+// ...
+//
+// As shown in the example above, the FALLTHROUGH_INTENDED macro should be
+// followed by a semicolon. It is designed to mimic control-flow statements
+// like 'break;', so it can be placed in most places where 'break;' can, but
+// only if there are no statements on the execution path between it and the
+// next switch label.
+//
+// When compiled with clang in C++11 mode, the FALLTHROUGH_INTENDED macro is
+// expanded to [[clang::fallthrough]] attribute, which is analysed when
+// performing switch labels fall-through diagnostic ('-Wimplicit-fallthrough').
+// See clang documentation on language extensions for details:
+// http://clang.llvm.org/docs/LanguageExtensions.html#clang__fallthrough
+//
+// When used with unsupported compilers, the FALLTHROUGH_INTENDED macro has no
+// effect on diagnostics.
+//
+// In either case this macro has no effect on runtime behavior and performance
+// of code.
+#if defined(__clang__) && __cplusplus >= 201103L && defined(__has_warning)
+#if __has_feature(cxx_attributes) && __has_warning("-Wimplicit-fallthrough")
+#define FALLTHROUGH_INTENDED [[clang::fallthrough]] // NOLINT
+#endif
+#endif
+
+#ifndef FALLTHROUGH_INTENDED
+#define FALLTHROUGH_INTENDED do { } while (0)
+#endif
+
// Annotalysis thread-safety analysis support.
#if defined(__SUPPORT_TS_ANNOTATION__) || defined(__clang__)
#define THREAD_ANNOTATION_ATTRIBUTE__(x) __attribute__((x))