diff --git a/libcxx/include/__assert b/libcxx/include/__assert
--- a/libcxx/include/__assert
+++ b/libcxx/include/__assert
@@ -34,7 +34,10 @@
 #endif
 
 #if _LIBCPP_ENABLE_ASSERTIONS
-# define _LIBCPP_ASSERT(expression, message) ((expression) ? (void)0 : ::std::__libcpp_assertion_handler(__FILE__, __LINE__, #expression, message))
+# define _LIBCPP_ASSERT(expression, message)                                        \
+    (__builtin_expect(static_cast<bool>(expression), 1) ?                           \
+      (void)0 :                                                                     \
+      ::std::__libcpp_assertion_handler(__FILE__, __LINE__, #expression, message))
 #else
 # define _LIBCPP_ASSERT(x, m) ((void)0)
 #endif