diff --git a/src/stdafx.h b/src/stdafx.h
index cf9fe92f99..631f64cca7 100644
--- a/src/stdafx.h
+++ b/src/stdafx.h
@@ -431,6 +431,14 @@ assert_compile(SIZE_MAX >= UINT32_MAX);
 	#define CloseConnection OTTD_CloseConnection
 #endif /* __APPLE__ */
 
+#ifdef __GNUC__
+#define likely(x)       __builtin_expect(!!(x), 1)
+#define unlikely(x)     __builtin_expect(!!(x), 0)
+#else
+#define likely(x)       (x)
+#define unlikely(x)     (x)
+#endif
+
 void NORETURN CDECL usererror(const char *str, ...) WARN_FORMAT(1, 2);
 void NORETURN CDECL error(const char *str, ...) WARN_FORMAT(1, 2);
 void NORETURN CDECL assert_msg_error(int line, const char *file, const char *expr, const char *str, ...) WARN_FORMAT(4, 5);
@@ -442,13 +450,13 @@ void NORETURN CDECL assert_msg_error(int line, const char *file, const char *exp
  */
 #if (defined(_MSC_VER) && defined(NDEBUG) && defined(WITH_ASSERT)) || (!defined(_MSC_VER) && !defined(NDEBUG) && !defined(_DEBUG))
 	#undef assert
-	#define assert(expression) if (!(expression)) error("Assertion failed at line %i of %s: %s", __LINE__, __FILE__, #expression);
+	#define assert(expression) if (unlikely(!(expression))) error("Assertion failed at line %i of %s: %s", __LINE__, __FILE__, #expression);
 #endif
 
 /* Asserts are enabled if NDEBUG isn't defined, or if we are using MSVC and WITH_ASSERT is defined. */
 #if !defined(NDEBUG) || (defined(_MSC_VER) && defined(WITH_ASSERT))
 	#define OTTD_ASSERT
-	#define assert_msg(expression, ...) if (!(expression)) assert_msg_error(__LINE__, __FILE__, #expression, __VA_ARGS__);
+	#define assert_msg(expression, ...) if (unlikely(!(expression))) assert_msg_error(__LINE__, __FILE__, #expression, __VA_ARGS__);
 #else
 	#define assert_msg(expression, ...)
 #endif