diff --git a/m4/curl-compilers.m4 b/m4/curl-compilers.m4
index ddd44acb53..dba2c755f9 100644
--- a/m4/curl-compilers.m4
+++ b/m4/curl-compilers.m4
@@ -107,7 +107,7 @@ AC_DEFUN([CURL_CHECK_COMPILER_CLANG], [
     compiler_num=`(expr $clangvhi "*" 100 + $clangvlo) 2>/dev/null`
     flags_dbg_yes="-g"
     flags_opt_all="-O -O0 -O1 -O2 -Os -O3 -O4"
-    flags_opt_yes="-Os"
+    flags_opt_yes="-O2"
     flags_opt_off="-O0"
   else
     AC_MSG_RESULT([no])