Postlayout logic restructuring using alternative wires

In this paper, we propose a layout-driven synthesis approach for field programmable gate arrays (FPGA's). The approach attempts to identify alternative wires and alternative functions for wires that cannot be routed due to the limited routing resources in FPGA. The alternative wires (in the logic level) that can be routed through less congested areas substitute the unroutable wires without changing the circuit's functionality. Allowing the logic blocks to have alternative functions also increases the chance of successful routing. A redundancy addition and removal technique is used to identify such alternative wires. Experimental results are presented to demonstrate the usefulness of this approach. For a set of randomly selected benchmark circuits, on the average, 30-50% of wires have alternative wires. These results indicate that the routing flexibility can be substantially increased by considering these alternative wires. Our prototype system successfully completed routing for two AT&T designs that cannot be handled by an FPGA router alone. The proposed synthesis technique can also be applied to standard cell and gate array designs to reduce the routing area.