diff --git a/parallel_hashmap/phmap.h b/parallel_hashmap/phmap.h
index 103f7ae..e59b02a 100644
--- a/parallel_hashmap/phmap.h
+++ b/parallel_hashmap/phmap.h
@@ -210,6 +210,7 @@ uint32_t TrailingZeros(T x) {
         return base_internal::CountTrailingZerosNonZero64(static_cast<uint64_t>(x));
     else
         return base_internal::CountTrailingZerosNonZero32(static_cast<uint32_t>(x));
+    PHMAP_BUILTIN_UNREACHABLE();
 }
 
 // --------------------------------------------------------------------------
diff --git a/parallel_hashmap/phmap_config.h b/parallel_hashmap/phmap_config.h
index b2e8654..a871ba0 100644
--- a/parallel_hashmap/phmap_config.h
+++ b/parallel_hashmap/phmap_config.h
@@ -650,6 +650,15 @@
     #define PHMAP_IF_CONSTEXPR(expr) if ((expr))
 #endif
 
+// ----------------------------------------------------------------------
+// builtin unreachable
+// ----------------------------------------------------------------------
+#if PHMAP_HAVE_BUILTIN(__builtin_unreachable)
+    #define PHMAP_BUILTIN_UNREACHABLE() __builtin_unreachable()
+#else
+    #define PHMAP_BUILTIN_UNREACHABLE() (void)0
+#endif
+
 // ----------------------------------------------------------------------
 // base/macros.h
 // ----------------------------------------------------------------------