diff --git a/config/functions b/config/functions
index fa14e0e8e6..80b7ea4773 100644
--- a/config/functions
+++ b/config/functions
@@ -222,14 +222,17 @@ setup_toolchain() {
   fi
 
   # compiler optimization, descending priority: speed, size, default
-  if flag_enabled "speed" "no"; then
+  if [ "${BUILD_WITH_DEBUG}" = "yes" ]; then
+    TARGET_CFLAGS+=" $CFLAGS_OPTIM_SIZE"
+    TARGET_CXXFLAGS+=" $CXXFLAGS_OPTIM_SIZE"
+  elif flag_enabled "speed" "no"; then
     TARGET_CFLAGS+=" $CFLAGS_OPTIM_SPEED"
     TARGET_CXXFLAGS+=" $CXXFLAGS_OPTIM_SPEED"
   elif flag_enabled "size" "no"; then
     TARGET_CFLAGS+=" $CFLAGS_OPTIM_SIZE"
     TARGET_CXXFLAGS+=" $CXXFLAGS_OPTIM_SIZE"
   else
-    TARGET_CFLAGS+=" $CXXFLAGS_OPTIM_DEFAULT"
+    TARGET_CFLAGS+=" $CFLAGS_OPTIM_DEFAULT"
     TARGET_CXXFLAGS+=" $CXXFLAGS_OPTIM_DEFAULT"
   fi