Net weighting for timing-driven placement has been very popular in industry and academia. It has various advantages such as low complexity, high flexibility and ease of implementation. Existing net weighting algorithms, however, are often ad-hoc. There is generally no known good net weighting algorithms. In this paper, we present a novel net weighting algorithm based on the concept of path-counting, and apply it in timing-driven FPGA placement application. Theoretically this is the first ever known accurate, all-path counting algorithm. Experimental data shows that compared with the weighting algorithm used in state-of-the-art FPGA placement package VPR[1], this new algorithm can achieve the longest path delay reduction of up to 38.8%, 15.6% on average with no runtime overhead and only a 4.1% increase in total wirelength.
