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On the Relevance of Wire Load Models

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Presentation on theme: "On the Relevance of Wire Load Models"— Presentation transcript:

1 On the Relevance of Wire Load Models
Kenneth D. Boese, Cadence Design Systems, San Jose Andrew B. Kahng, UCSD CSE and ECE Depts., La Jolla Stefanus Mantik, UCLA CS Dept., Los Angeles

2 Why WLMs? Synthesis needs placement parasitics
Placement needs synthesized netlist Chicken vs. egg WLMs estimate parastics pre-placement Wire cap = f(net fanout) But WLMs no longer accurate! (?)

3 Motivating Questions How inaccurate are WLMs?
Are WLMs good for optimization? Are WLMs good for estimation? How use WLMs in optimization flow?

4 Previous Works “Bad” WLMs motivate post-placement synthesis
e.g., Lu et al. [7] Need for constructive estimators Scheffer and Nequist [8] “law of small numbers” Accuracy vs. fidelity Boese et al. [1,2]

5 Kinds of WLMs Custom Statistical Structural multiple iterations
from library Structural look at adjacent nets

6 Historical Design Flows
No WLM interconnect insignificant Custom WLMs requires iterations Statistical WLMs fewer iterations Add post-placement optimization IPO, PB-Opt Post-placement only (integrated tool) PKS, Physical Compiler, Blast Fusion

7 How Accurate Are WLMs? High Deviations  WLMs Inaccurate
Normalized to Ave. Net Cap

8 Accuracy: Skew Measures
What if long nets not critical? timing-driven placement buffer insertion So ... Must Test WLMs in Actual Flows

9 Are WLMs Good for Optimization?
Sufficient? Necessary? Compare different flows real tool (PKS) real designs

10 Post-placement timing opt
Default PKS Flow RTL Generic Netlist Area opt Technology mapping Mapped Netlist Timing opt Placement Placed Netlist Post-placement timing opt Global routing Routed Netlist

11 7 Flow Variations Structural WLM Library WLM Custom WLM No WLM
No Pre Pl Opt No Post Pl Logic Opt No Post Pl Opt 1. Area opt 2. Technology mapping 3. Timing opt 4. Placement 5. Post-pl timing opt 6. Global routing

12 7 Flow Variations Structural WLM Library WLM Custom WLM No WLM
No Pre Pl Opt No Post Pl Logic Opt No Post Pl Opt 1. Area opt 2. Technology mapping 3. Timing opt Struct WLM 4. Placement 5. Post-pl timing opt 6. Global routing

13 7 Flow Variations Structural WLM Library WLM Custom WLM No WLM
No Pre Pl Opt No Post Pl Logic Opt No Post Pl Opt 1. Area opt 2. Technology mapping 3. Timing opt Lib WLM 4. Placement 5. Post-pl timing opt 6. Global routing

14 7 Flow Variations Structural WLM Library WLM Custom WLM No WLM
No Pre Pl Opt No Post Pl Logic Opt No Post Pl Opt 1. Area opt 2. Technology mapping 3. Timing opt Cust WLM 4. Placement 5. Post-pl timing opt 6. Global routing

15 7 Flow Variations Structural WLM Library WLM Custom WLM No WLM
No Pre Pl Opt No Post Pl Logic Opt No Post Pl Opt 1. Area opt 2. Technology mapping 3. Timing opt WLM = 0 4. Placement 5. Post-pl timing opt 6. Global routing

16 7 Flow Variations Structural WLM Library WLM Custom WLM No WLM
No Pre Pl Opt No Post Pl Logic Opt No Post Pl Opt 1. Area opt 2. Technology mapping 3. Timing opt 4. Placement 5. Post-pl timing opt 6. Global routing

17 7 Flow Variations Structural WLM Library WLM Custom WLM No WLM
No Pre Pl Opt No Post Pl Logic Opt No Post Pl Opt 1. Area opt 2. Technology mapping 3. Timing opt Lib WLM 4. Placement 5. Post-pl timing opt No logic opt 6. Global routing

18 7 Flow Variations Structural WLM Library WLM Custom WLM No WLM
No Pre Pl Opt No Post Pl Logic Opt No Post Pl Opt 1. Area opt 2. Technology mapping 3. Timing opt Lib WLM 4. Placement 5. Post-pl timing opt 6. Global routing

19 Flow Comparisons Struct WLM Library WLM Custom WLM No preOpt No WLM
No postLogicOpt No postOpt Struct WLM Library WLM Custom WLM No preOpt No WLM No postLogicOpt No postOpt

20 Implications WLMs not sufficient
post-place opt needed WLMs not necessary for good slacks (?) WLMs reduce running time Some improvement from Post Place Logic Opts

21 Do WLMs Give Good Estimates?
WLM slack vs. groute slack Custom WLM

22 How Should We Use WLMs? How much optimism? Which WLM best?
Structural (?) no extra input Multi-start? different WLMs / optimism levels is it randomness?

23 Caveats This tool, this version, these designs
Global not detailed or extracted routing No clock tree Routability ignored detours, strict Die size fixed Some designs pre-optimized

24 Conclusions / Future Work
Strict accuracy not right question WLMs improve run time NOT design quality (?) WLMs may be ballpark estimators Post-place opt indispensible small gains from post-place logic opt Sensitivity analysis of placement and post-place opt

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