Speeding up technology-independent timing optimization by network partitioning

Technology-independent timing optimization is an important problem in logic synthesis. Although many promising techniques have been proposed in the past, unfortunately they are quite slow and thus impractical for large networks. In this paper, we propose DEPART, a delay-based partitioner-cum-optimizer, which purports to solve this problem. Given a combinational logic network that is to be optimized for timing, DEPART divides it into sub-networks using timing information and a constraint on the maximum number of gates allowed in a single sub-network. These sub-networks are then dispatched, one by one, to a standard timing optimizer. The optimized sub-networks are re-glued, generating an optimized network. The challenge is how to partition the original network into sub-networks so that the final solution quality after partitioning and optimization is comparable to that from the timing optimizer. We propose a partitioning technique that is timing-driven and is simple yet effective. We compare DEPART with speed_up, a state-of-the-art timing optimization tool, and with various partitioning techniques such as min-cut based and region growing, on a suite of large industrial and ISCAS circuits. On more than half of the benchmarks, DEPART yields run-time improvements of 20 to 450 times over a normal invocation of speed_up (the overall average improvement being 8 times), without compromising the solution quality much. Min-cut and region growing partitioning schemes, not being timing-driven, perform poorly in terms of the final circuit delay.