diff --git a/auto.def b/auto.def
index ab0bb22..076d9e1 100644
--- a/auto.def
+++ b/auto.def
@@ -349,7 +349,7 @@ if {[find-an-executable cygpath] ne "" || $::tcl_platform(os)=="Windows NT"} {
 msg-checking "Debug build ??? "
 if {[opt-bool no-debug]} {
     msg-result "Non-debug build."
-    set cFlags "$cFlags -O2"
+    set cFlags "$cFlags -Os"
 } else {
     msg-result "Debug build enabled. Use --no-debug to build in non-debug mode."
     set cFlags "$cFlags -g -DDEBUG -O0"