Fix issue #639

passes/equiv/equiv_make.cc

@@ -40,6 +40,8 @@ struct EquivMakeWorker
 	pool<SigBit> undriven_bits;
 	SigMap assign_map;
 
+	dict<SigBit, pool<Cell*>> bit2driven; // map: bit <--> and its driven cells
+
 	void read_blacklists()
 	{
 		for (auto fn : blacklists)
@@ -278,12 +280,20 @@ struct EquivMakeWorker
 			}
 		}
 
+		init_bit2driven();
+
+	 	pool<Cell*> visited_cells;
 		for (auto c : cells_list)
 		for (auto &conn : c->connections())
 			if (!ct.cell_output(c->type, conn.first)) {
 				SigSpec old_sig = assign_map(conn.second);
 				SigSpec new_sig = rd_signal_map(old_sig);
+				
+				visited_cells.clear();
 				if (old_sig != new_sig) {
+					if (check_signal_in_fanout(visited_cells, old_sig, new_sig)) 
+						continue;
+
 					log("Changing input %s of cell %s (%s): %s -> %s\n",
 							log_id(conn.first), log_id(c), log_id(c->type),
 							log_signal(old_sig), log_signal(new_sig));
@@ -378,6 +388,54 @@ struct EquivMakeWorker
 		}
 	}
 
+	void init_bit2driven()
+	{
+		for (auto cell : equiv_mod->cells()) {
+			if (!ct.cell_known(cell->type) && !cell->type.in("$dff", "$_DFF_P_", "$_DFF_N_", "$ff", "$_FF_"))
+				continue;
+			for (auto &conn : cell->connections())
+			{
+				if (yosys_celltypes.cell_input(cell->type, conn.first))
+					for (auto bit : assign_map(conn.second))
+					{
+						bit2driven[bit].insert(cell);
+					}
+			}
+		}
+	}
+
+	bool check_signal_in_fanout(pool<Cell*> visited_cells, SigBit source_bit, SigBit target_bit)
+	{
+		if (source_bit == target_bit)
+			return true;
+
+		if (bit2driven.count(source_bit) == 0)
+			return false;
+
+		auto driven_cells = bit2driven.at(source_bit);
+		for (auto driven_cell: driven_cells)
+		{
+			if (visited_cells.count(driven_cell) > 0)
+				continue;
+
+			visited_cells.insert(driven_cell);
+
+			for (auto &conn: driven_cell->connections())
+			{
+				if (yosys_celltypes.cell_input(driven_cell->type, conn.first))
+					continue;
+
+				for (auto bit: conn.second) {
+					bool is_in_fanout = check_signal_in_fanout(visited_cells, bit, target_bit);
+					if (is_in_fanout == true)
+						return true;
+				}
+			}
+		}
+
+		return false;
+	}
+
 	void run()
 	{
 		copy_to_equiv();