On The Engineering of a Stable Force-Directed Placer
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Analytic and force-directed placement methods that simultaneously minimize wire length and spread cells are receiving renewed attention from both academia and industry. However, these methods are by no means trivial to implement---to date, published works have failed to provide sufficient engineering details to replicate results. This dissertation addresses the implementation of a generic force-directed placer entitled FDP. Specifically, this thesis provides (1) a description of efficient force computation for spreading cells, (2) an illustration of numerical instability in this method and a means to avoid the instability, (3) metrics for measuring cell distribution throughout the placement area, and (4) a complementary technique that aids in minimizing wire length. FDP is compared to Kraftwerk and other leading academic tools including Capo, Dragon, and mPG for both standard cell and mixed-size circuits. Wire lengths produced by FDP are found to be, on average, up to 9% and 3% better than Kraftwerk and Capo, respectively. All told, this thesis confirms the validity and applicability of the approach, and provides clarifying details of the intricacies surrounding the implementation of a force-directed global placer.