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Xing Wei, Wai-Chung Tang, Yu-Liang Wu Department of Computer Science and Engineering The Chinese University of HongKong Cliff Sze, Charles Alpert IBM Austin Research Center Burnet Road, Austin, TX {csze, Mountain-Mover: An Intuitive Logic Shifting Heuristic for Improving Timing Slack Violating Paths ASPDAC 2013

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Outline Introduction The Mountain Mover Heuristic Preliminaries and problem definition Post-placement timing optimization using logic rewiring Experimental results Conclusion

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Introduction Traditionally, logic synthesis is done prior to physical synthesis which includes placement, routing and placement- based timing optimization [2]. However, under modern sub-micron VLSI technology, wire delay dominates the total path delay. It is observed that for industrial designs many critical paths fail to close on timing even after physical synthesis such as path straightening, buffering, Vt optimization and gate sizing.

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Introduction Increasing wire delay greatly increases the importance of the cell placement problem. As a result, placement engine has become the key player for timing optimization during physical synthesis of the design flow. In fact, multiple rounds of placement and timing optimization have to be iterated in order to identify true timing-critical paths which require an incremental placement engine to straighten them. Thus, post-placement logic synthesis is a must in order to further improve the path delay.

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The Mountain Mover Heuristic Design a heuristic being able to make decisions based on the whole timing picture during this post-placement logic synthesis, which avoids lots of futile logic restructures and makes the flow efficient. We build a slack distribution graph to help guide logic rewiring technique to shift logic resource efficiently. A slack distribution graph is a mapping from the locations of circuit cells to their slack values.

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The Mountain Mover Heuristic An example of such a graph of an industrial customer design created by an industrial EDA tool [13]

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The Mountain Mover Heuristic

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Preliminaries and problem definition A circuit delay is defined as the largest signal delay among all paths from any PI to any PO. The slack of a pin/gate is the difference (gap) between its arrival time and required arrival time. The slack of a path is the difference (gap) between the path delay and timing constraint. The slack of wire (i, j) is the worst slack between i and j. The worst negative slack (WNS) of the circuit is the largest violations among all slacks in the circuit.

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Logic rewiring Logic rewiring [14] is a circuit restructuring technique referring to the process of replacing a certain wire(TW) by another wire(AW) without changing the functionality of the circuit. The addition and removal of wires are sometimes accompanied by changes in logic gates corresponding to the AW and TW.

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An example of logic rewiring

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Problem definition Given a logic netlist, its physical placement, and the timing constraints, the restructuring-based post-placement timing optimization problem is to maximize the worst slack of the circuit by (i) restructuring local circuit while preserving the functionality (ii) keeping the placement free of overlaps.

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Post-placement timing optimization using logic rewiring The overall framework: First compute the slack of each gate/wire to construct the slack distribution graph and the slack mountain. Run the rewiring engine [11] to remove the target wire guided by the slack distribution graph. An overlapping-free incremental placement is performed after the logic restructuring to keep the placement legal. Run the STA to confirm the effectiveness of the selected AW in reducing WNS..

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Rule of Thumb : Boundary First The key idea of our scheme is to figure out a local slack mountain around the most critical path and first shift the logic from boundaries of the slack mountain to avoid getting stuck at local minimum.

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Shift Logic to Less Critical Area When we attempt to remove a target wire, the logic rewiring engine usually suggests more than one AWs to be chosen as the alternative logic. We use the slack distribution graph to guarantee the original logic is shifted from critical area to the less critical area.

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Shift Logic to Less Critical Area = (xg, yg):the gradient value of TW in the slack distribution graph. = (dx, dy) : the distance vector between TW and AW.

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Shift Logic to Less Critical Area The first inequality makes the alternative logic located in the less critical area. The second inequality ensures that the alternative logic is located along the gradient direction of TW.

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Incremental Placement The incremental placement will place the replaced gate at the same location. If there is any overlap after the gate change, a new gate like AND2 or OR2 is inserted instead of replacing the original gate and the new gate is added into the circuit while keeping the placement free of overlap.

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Slack Estimation We define the local worst negative slack (LWNS) and local total negative slack (LTNS) of a set of gates as below to help evaluate the AW candidates. pt(·) denotes a gate set in which each gate is a primary output and belongs to the fanout cone of a gate gi in G

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Slack Estimation

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We will adopt an AW if: 1. The LWNS of the rewired circuit is no worse than the original circuit. 2. The AW gains the most LTNS improvement among all AW candidates. The first condition guarantees that the delay of the rewired circuit is not degenerated. The second condition helps reducing the delay of all paths.

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Shifting logic for timing optimization

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Experimental result Initial placements were generated by Cadence SoC Encounter 10.1 Based on IWLS 2005 benchmark. The Cadence 180 nm generic library is used and the circuit timing is estimated by STA with the same library.

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Experimental result

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Conclusions We proposed a novel framework using logic rewiring to eliminate or improve slack violating paths. Extensive experiments upon IWLS2005 show that the scheme can improve 14.1% on circuit delays (averagely) and achieve 7X speedup compared to a recent work. We believe that the scheme can be integrated with other strategies including both logical and physical techniques to gain a further delay reduction.

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