Index: head/sys/sys/cdefs.h
===================================================================
--- head/sys/sys/cdefs.h
+++ head/sys/sys/cdefs.h
@@ -388,6 +388,12 @@
 #define	__alloc_size(x)
 #endif
 
+#if __has_builtin(__builtin_unreachable) || __GNUC_PREREQ__(4, 6)
+#define	__unreachable()	__builtin_unreachable()
+#else
+#define	__unreachable()	do {} while (/*CONSTCOND*/0)
+#endif
+
 #if __has_attribute(alloc_align) || __GNUC_PREREQ__(4, 9)
 #define	__alloc_align(x)	__attribute__((__alloc_align__(x)))
 #else