+/// Marks that the current location is not supposed to be reachable.
+/// In !NDEBUG builds, prints the message and location info to stderr.
+/// In NDEBUG builds, becomes an optimizer hint that the current location
+/// is not supposed to be reachable. On compilers that don't support
+/// such hints, prints a reduced message instead.
+///
+/// Use this instead of assert(0). It conveys intent more clearly and
+/// allows compilers to omit some unnecessary code.
+#ifndef NDEBUG
+#define llvm_unreachable(msg) \
+ ::llvm::llvm_unreachable_internal(msg, __FILE__, __LINE__)
+#elif defined(LLVM_BUILTIN_UNREACHABLE)
+#define llvm_unreachable(msg) LLVM_BUILTIN_UNREACHABLE
+#else
+#define llvm_unreachable(msg) ::llvm::llvm_unreachable_internal()