diff --git a/src/tools/pginclude/pgrminclude b/src/tools/pginclude/pgrminclude
index e9c3477b9da..5890c50a1a3 100755
--- a/src/tools/pginclude/pgrminclude
+++ b/src/tools/pginclude/pgrminclude
@@ -70,6 +70,8 @@ do
 		fi
 		echo "}" >>/tmp/$$.c
 
+		# Use -O1 to get warnings only generated by optimization,
+		# but -O2 is too slow.
 		cc -fsyntax-only -Werror -Wall -Wmissing-prototypes \
 			-Wmissing-declarations -I/pg/include -I/pg/backend \
 			-I/pg/interfaces/libpq -I`dirname $FILE` $CFLAGS -O1 -c /tmp/$$.c \