diff --git a/c_gen/templates/loci_int.h b/c_gen/templates/loci_int.h
index b4df78c..241cf7b 100644
--- a/c_gen/templates/loci_int.h
+++ b/c_gen/templates/loci_int.h
@@ -46,6 +46,12 @@
 
 #include <loci/loci.h>
 
+#ifdef __GNUC__
+#define UNREACHABLE() __builtin_unreachable()
+#else
+#define UNREACHABLE()
+#endif
+
 /****************************************************************
  * Special case macros for calculating variable lengths and offsets
  ****************************************************************/