Buffered Steiner Trees for Difficult Instances

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Buffered Steiner Trees for Difficult Instances

• Charles J. Alpert, Gopal Gandham, Milos Hrkic,
  John Lillis, Jiang Hu, Andrew B. Kahng, Bao Liu,
  Stephen T. Quay, Sachin S. Sapatnekar, Andrew J.
  Sullivan

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Min-Cost Steiner Tree
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Timing-driven Steiner Tree
Early 1990s
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Buffered Steiner Tree
Mid 1990s
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Blockage Aware Trees
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Ripping out Synthesis Trees

• Ugly pre-PD buffered trees
• Inverted sinks
• Trivial van Ginneken/Lillis change
• Normal sinks one positive candidate
• Inverted sinks one negative candidate
• No big deal

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Or Is It?
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But Look What Happens
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Buffer Aware Trees
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The Texas Two Step
Optimal
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C-Tree Algorithm

• A-, B-, H-, O-, P-, trees taken
• Cluster sinks by
• Polarity
• Manhattan distance
• Criticality
• Two-level tree
• Form tree for each cluster
• Form top-level tree

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C-Tree Example
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Net n8702 (44 sinks)
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Net n8702
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Flat C-Tree
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C-Tree 4 clusters
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C-Tree 2 clusters
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Difficult Steiner Instances

• Polarity/size makes it hard
• 3d trade-off
• Timing
• Buffers
• Wire length
• Work on it!