Wotan: Evaluating FPGA Architecture Routability without Benchmarks.

FPGA routing architectures consist of routing wires and programmable switches that together account for the majority of the fabric delay and area, making evaluation and optimization of an FPGA’s routing architecture very important. Routing architectures have traditionally been evaluated using a full synthesize, pack, place and route CAD flow over a suite of benchmark circuits. While the results are accurate, a full CAD flow has a long runtime and is often tuned to a specific FPGA architecture type, which limits exploration of different architecture options early in the design process. In this article, we present Wotan, a tool to quickly estimate routability for a wide range of architectures without the use of benchmark circuits. At its core, our routability predictor efficiently counts paths through the FPGA routing graph to (1) estimate the probability of node congestion and (2) estimate the probabilities to successfully route a randomized subset of (source, sink) pairs, which are then combined into an overall routability metric. We describe our predictor and present routability estimates for a range of 6-LUT and 4-LUT architectures using mixes of wire types connected in complex ways, showing a rank correlation of 0.91 with routability results from the full VPR CAD flow while requiring 18× less CPU effort.