On the Relevance of Wire Load Models

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
• multiple iterations
• Statistical
• 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

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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
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7 Flow Variations

• 1. Area opt
• 2. Technology mapping
• 3. Timing opt
• 4. Placement
• 5. Post-pl timing opt
• 6. Global routing

• Structural WLM
• Library WLM
• Custom WLM
• No WLM
• No Pre Pl Opt
• No Post Pl Logic Opt
• No Post Pl Opt

12
7 Flow Variations

• 1. Area opt
• 2. Technology mapping
• 3. Timing opt
• Struct WLM
• 4. Placement
• 5. Post-pl timing opt
• 6. Global routing

• Structural WLM
• Library WLM
• Custom WLM
• No WLM
• No Pre Pl Opt
• No Post Pl Logic Opt
• No Post Pl Opt

13
7 Flow Variations

• 1. Area opt
• 2. Technology mapping
• 3. Timing opt
• Lib WLM
• 4. Placement
• 5. Post-pl timing opt
• 6. Global routing

• Structural WLM
• Library WLM
• Custom WLM
• No WLM
• No Pre Pl Opt
• No Post Pl Logic Opt
• No Post Pl Opt

14
7 Flow Variations

• 1. Area opt
• 2. Technology mapping
• 3. Timing opt
• Cust WLM
• 4. Placement
• 5. Post-pl timing opt
• 6. Global routing

• Structural WLM
• Library WLM
• Custom WLM
• No WLM
• No Pre Pl Opt
• No Post Pl Logic Opt
• No Post Pl Opt

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7 Flow Variations

• 1. Area opt
• 2. Technology mapping
• 3. Timing opt
• WLM 0
• 4. Placement
• 5. Post-pl timing opt
• 6. Global routing

• Structural WLM
• Library WLM
• Custom WLM
• No WLM
• No Pre Pl Opt
• No Post Pl Logic Opt
• No Post Pl Opt

16
7 Flow Variations

• 1. Area opt
• 2. Technology mapping
• 3. Timing opt
• 4. Placement
• 5. Post-pl timing opt
• 6. Global routing

• Structural WLM
• Library WLM
• Custom WLM
• No WLM
• No Pre Pl Opt
• No Post Pl Logic Opt
• No Post Pl Opt

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7 Flow Variations

• 1. Area opt
• 2. Technology mapping
• 3. Timing opt
• Lib WLM
• 4. Placement
• 5. Post-pl timing opt
• No logic opt
• 6. Global routing

• Structural WLM
• Library WLM
• Custom WLM
• No WLM
• No Pre Pl Opt
• No Post Pl Logic Opt
• No Post Pl Opt

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7 Flow Variations

• 1. Area opt
• 2. Technology mapping
• 3. Timing opt
• Lib WLM
• 4. Placement
• 5. Post-pl timing opt
• 6. Global routing

• Structural WLM
• Library WLM
• Custom WLM
• No WLM
• No Pre Pl Opt
• No Post Pl Logic Opt
• No Post Pl Opt

19
Flow Comparisons
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