Sensitivity guided net weighting for placement driven synthesis

Abstract

Net weighting is a key technique in large scale timing driven placement, which plays a crucial role for deep submicron physical synthesis and timing closure. A popular way to assign net weight is based on the slack of the nets, trying to minimize the worst negative slack (WNS) for the entire circuit. While WNS is an important optimization metric, another figure of merit (FOM), defined as the total slack difference compared to a certain slack threshold for all timing end points, is of equivalent importance to measure the overall timing closure result for highly complex modern ASIC and microprocessor designs. In this paper, we perform a comprehensive analysis of the slack and FOM sensitivities to the net weight, and propose a new net weighting scheme based on the slack and FOM sensitivities. Such sensitivity analysis implicitly takes potential physical synthesis effect into consideration. Experiment results on a set of industrial circuits are promising for both stand-alone timing driven placement and physical synthesis afterwards.