Exploiting non-critical Steiner tree branches for post-placement timing optimization

Abstract

The increasing impact of interconnections on the overall circuit performance renders physical design a crucial step to timing closure. Several techniques are used to optimize timing within the flow, such as gate sizing, buffer insertion, and timing-driven placement (TDP). Unfortunately, gate sizing and buffer insertion are not capable of modifying the length of interconnections. Although TDP is able to shorten critical interconnection by finding new legal locations for a subset of cells, it generally overlooks the impact of non-critical branches on the delay of critical cells. This work proposes a post-placement timing optimization technique to reduce the capacitive load of critical cells by shortening non-critical Steiner tree branches. To shorten such branches, our technique uses computational geometry for finding effective cell movements that consider maximum displacement constraints and macro blocks. Our experiments evaluate the capability of our technique to further reduce the timing violations from a TDP solution. We applied our technique on the solutions obtained by the top 3 teams in the ICCAD 2014 TDP Contest, where short and long displacement constraints are defined. For the short constraints, the average reductions assuming worst and total late negative slack metrics are 23% and 34%. Considering the long constraints, the average reductions are 62% and 67%. We also present extensions of our technique to tackle related physical design problems such as early violations reduction and electrical correction.