diff --git a/src/hb.hh b/src/hb.hh
index 30b3de499..c6b3cc021 100644
--- a/src/hb.hh
+++ b/src/hb.hh
@@ -255,8 +255,8 @@ extern "C" void  hb_free_impl(void *ptr);
 #endif
 
 #if defined(__OPTIMIZE__) && hb_has_builtin(__builtin_expect)
-#define likely(expr) (__builtin_expect (!!(expr), 1))
-#define unlikely(expr) (__builtin_expect (!!(expr), 0))
+#define likely(expr) (__builtin_expect (bool(expr), 1))
+#define unlikely(expr) (__builtin_expect (bool(expr), 0))
 #else
 #define likely(expr) (expr)
 #define unlikely(expr) (expr)