diff --git a/passes/opt/opt_clean.cc b/passes/opt/opt_clean.cc
index 466808216..6600ffa25 100644
--- a/passes/opt/opt_clean.cc
+++ b/passes/opt/opt_clean.cc
@@ -156,6 +156,9 @@ bool compare_signals(RTLIL::SigBit &s1, RTLIL::SigBit &s2, SigPool &regs, SigPoo
 	if (w1->port_input != w2->port_input)
 		return w2->port_input;
 
+	if ((w1->port_input && w1->port_output) != (w2->port_input && w2->port_output))
+		return !(w2->port_input && w2->port_output);
+
 	if (w1->name[0] == '\\' && w2->name[0] == '\\') {
 		if (regs.check_any(s1) != regs.check_any(s2))
 			return regs.check_any(s2);