Improving Circuit Mapping Performance Through MIG-based Synthesis for Carry Chains

Hard-wired carry chains in FPGAs are designed to improve efficiency of important arithmetic primitives. Although they are proven to be effective for arithmetic-rich functions, there are very few studies on the optimization opportunities of carry chains for general logic that is poor in arithmetic operations. Recently, Majority-Inverter Graphs (MIGs) were proposed for efficient Boolean logic optimization. MIGs open an opportunity for efficient mapping of critical paths onto hard carry chains, as the carry logic of a full adder is naturally a majority (MAJ) gate. In this paper, we propose an MIG-based synthesis method to exploit hard adders in FPGAs for the mapping of general logic. The proposed heuristic algorithm selects MAJ nodes to be mapped on the carry chains and the associated LUTs; then, the efficiency of carry chain mapping is examined theoretically for efficient LUT utilization. The experimental results show that, compared to traditional design flow Verilog-to-Routing (VTR 7.0), the proposed approach can improve delay by up to 25% with an average of 8%, while the channel width is reduced by up to 20% with an average of 6%.