OWARU: Free space-aware timing-driven incremental placement

Abstract

This paper proposes a powerful new technique called "OWARU"1 that re-places and re-sizes multiple gates simultaneously to improve the most critical paths of a design. In essence, it is an incremental timing-driven placement technique integrated with gate sizing optimization that runs in conjunction with static timing analysis to guarantee a WYSIWYG 2 property. The OWARU technique offers several key advantages over previous techniques such as geometrical path straightening via the Bézier-curve algorithm, free space awareness to guarantee a legal placement solution, and an accurate true timing mode. The Bézier-curve geometric smoothing algorithm is extended with new anchor placement techniques to further improve the path placement. Free space aware placement algorithm is further enhanced with multiple gate optimization. The preliminary results are promising. We applied the OWARU technique at the end of industrial strength physical synthesis optimization on high performance microprocessor designs. The technique was extremely effective in improving the most critical path of the tested designs. On timing critical paths that were not fully closed from the previous physical synthesis optimization, the WS (worst slack) is improved by 5.3% of the total clock period and the TNS (total negative slack) improved by 91.3% on average.