WireMap: FPGA Technology Mapping for Improved Routability and Enhanced LUT Merging

This article presents a new technology mapper, WireMap. The mapper uses an edge flow heuristic to improve the routability of a mapped design. The heuristic is applied during the iterative mapping optimization to reduce the total number of pin-to-pin connections (or edges). On academic benchmark (ISCAS, MCNC, and ITC designs), the average edge reduction of 9.3% is achieved while maintaining depth and LUT count compared to state-of-the-art technology mapping. Placing and routing the resulting netlists leads to an 8.5% reduction in the total wirelength, a 6.0% reduction in minimum channel width, and a 2.3% reduction in critical path delay. This technique is applied in the Xilinx ISE Design tool to evaluate its effect on industrial Virtex5 circuits. In a set of 20 large designs, we find the edge reduction is 6.8% while total wirelength measured in the placer is reduced by 3.6%. Applying WireMap has an additional advantage of reducing an average number of inputs of LUTs without increasing the total LUT count and depth. The percentages of 5- and 6-LUTs in a typical design are reduced, while the percentages of 2-, 3-, and 4-LUTs are increased. These smaller LUTs can be merged into pairs and implemented using the dual-output LUT structure found in commercial FPGAs. For academic benchmarks, WireMap leads to 9.4% fewer dual-output LUTs after merging. For the industrial designs, WireMap leads to 6.3% fewer dual-output Virtex5 LUTs.