integrating logic retiming and register placement
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Integrating Logic Retiming and Register Placement
Tzu-Chieh Tien, Hsiao-Pin Su, Yu-Wen Tsay Yih-Chih Chou, and Youn-Long Lin
Department of Computer ScienceTsing Hua University
Hsin-Chu, Taiwan, R.O.C.
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Outline• Motivation• Key problems• Proposed System flow• Retiming algorithm• Interconnection delay estimation• Register placement• Experimental results• Conclusion and future work
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Motivation
• Wiring delay dominance in VDSM era.• Retiming cannot ignore wiring delay.• Wiring delay is layout dependent.
retiming
layout?
?
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Key Problems
• Post-Layout Retiming incorporating interconnection delay
• Interconnection delay estimation– delay change due to register insertion
or removal– tree topology dependence
• Post-retiming (register) placement
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Retiming Incorporating Interconnection Delay
• Given estimated interconnection delay, find a legal retiming such that the clock period of the circuit is optimal.
Delays a, b, and c must be estimated before retiming.
a
b c
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Previous Work• Soyata and Friedman (ICCAD’94):– branch-and-bound method– optimal solution in the first time– time consuming
• Lalgudi and Papaefthymiou (DAC’95): – integer linear programming method
• optimal solution– polynomial time algorithm if the circuit is one-
way extendible
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Interconnection Delay Estimation
• Given layout– Estimate wiring delay change due to
register insertion or removal– Tree-topology dependence of wiring
delay
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Effect of Register Insertion or Removal on Wiring Delay
(a) adding a register
(b) deleting a register
(c) an unchanged wire
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Tree-Topology-Dependence of Wiring Delay
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Post-Retiming (Register) Placement
• Post-retiming layout refinement to meet timing goal
• Need specific technique
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Proposed System Flow
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Retiming Incorporating Wiring Delay
• Extend a previous algorithm (proposed in pp.384-393, SASIMI’93) to incorporate the interconnection delay.
• Efficient, optimal for circuits satisfy the path monotonicity constraints.
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Interconnection Delay Estimation
(a) adding a register
(b) deleting a register
(c) an unchanged wire
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Predicting Location for the Inserted Register
• In the slot nearest the geometric center among its fan-in and fan-out cells.
G40392G42709
G42767
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Tree-Topology Selection
• Choosing the one with the most delay.
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Post-Retiming (Register) Placement
• Freeze all combinational cells• Relocate all registers in 3 steps– Slack assignment– Placement range calculation– Simultaneous register to slot assignment
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Slack Assignment
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Placement Range
G40391
G42612
G42552
0.1720.2150.110
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Simultaneous Register Place-ment by Bipartite MatchingG161G164G167G170G173G176G180G183G188G191G194
G170G173G176G180G183G188G191G218
register slot
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Experimental Results
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Conclusion• Retiming and layout are combined for
the first time.• Three sub-problems: retiming
incorporating interconnection delay, delay estimation, and post-retiming (register) placement.
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Future Work
• Tightly coupled simultaneous placement and retiming
• Retiming considering wire delay and number of registers
• Using more advanced delay model and considering more VDSM effects
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