Facilitating timing debug by logic path correspondence

Synthesis tools for high-performance VLSI designs employ aggressive logic optimization techniques in order to meet physical requirements such as area and cycle time. During these optimizations, the original structure of the design, which is usually written in a hardware description language (HDL), is lost. It is difficult, and often impossible, to relate signals after synthesis to the original signals in the HDL code. Some signals only lose their names while for others there are no equivalent counterparts in the design after synthesis. Debugging timing problems is based on timing reports which are usually represented in terms of the post-synthesis design. Hence, it is difficult to relate critical paths in the timing reports to the relevant paths in the HDL code when a logic fix is needed. In this paper, we propose a different approach for dealing with the correspondence problem: instead of trying to relate signals we relate paths. Given a critical path in a post-synthesis representation, our method is able to find all corresponding paths in the pre-synthesis (HDL) representation. As a result, locating the parts in the HDL which are relevant to the given timing problem becomes trivial. A novel Sat-based algorithm for dealing with the path-correspondence problem is described. Experimental results on various industrial high-end processor designs show the effectiveness of our algorithm in substantially reducing the amount of paths in the HDL which one will have to consider when debugging a given critical path.