Toward Effective Utilization of Timing Exceptions in Design Optimization

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Toward Effective Utilization of Timing Exceptions
in Design Optimization

• Kwangok Jeong, Andrew B. Kahng and Seokhyeong
  Kang
• VLSI CAD LABORATORY, UCSD

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Outline

• Timing exceptions vs. design outcomes
• Q1. Do timing exceptions help or hurt?
• Q2. Which exceptions give net benefit when
  inserted?
• Q3. When should exceptions be identified and
  applied?
• Guidelines to timing exceptions
• Conclusion and ongoing work

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Timing Exceptions vs. Design Outcomes

• Advantage
• Timing exceptions reduce pessimism in STA
• Help ease the task of timing closure step

Testcase AES cipher

• Adding exceptions doesnt always lead to better
  QoR
• Rutime increase with small improvement in TNS

runtime
area
TNS

• Our target
• Evaluate the impact of timing exceptions
• Give guidelines for designers / EDA vendors

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• Q1. Do timing exceptions help or hurt?
• Impact of timing exceptions in optimization

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Do Timing Exceptions Help or Hurt?

• Help
• Remove over-constraint
• Prevent excessive optimization
• Hurt
• Add complexity in optimization
• Prevent restructuring

• Restructuring of full-adder circuit

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Impact of Timing Exceptions in Optimization

• Quality of optimization in commercial tools after
  applying different types of exceptions
• Test case 4-bit ripple carry adder
• Timing is improved without through points
• FP and MCP have the same impacts on timing and
  area

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Experiment with Artificial Circuit

• Applying different number of FPs
• Optimization results in 2-stage and 8-stage
  circuit.

Restructured unit
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• Q2. Which exceptions give net benefit when
  inserted?
• Critical and effective timing exceptions

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Which exceptions give net benefit when inserted?

• Exceptions space according to format and
  criticality
• Critical / effective exceptions will give benefit
• Need to audit exceptions to obtain beneficial
  timing exceptions

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Critical Timing Exceptions

• WNS after applying top-k of exceptions (AES
  cipher)

• Timing slack is not improved by non-critical
  exceptions
• Critical timing exceptions without through
  (Critical MCP) effectively reduce the WNS

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Effective Timing Exceptions

• Ineffective exceptions
• Many false paths are not effective due to an
  interleaved true path
• Do not contribute on reducing constraints and
  increase the runtime

• Path A is ineffective false path because of a
  true path B

• Method for filtering out ineffective exceptions
• Use for metrics to quantify the effectiveness of
  timing exceptions

• c cell, p path
• sc (sc) slack of timing point c after (before)
  defining p as a false path
• nc number of timing point in p
• sc_end end point

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• Q3. When should exceptions be identified and
  applied?
• Design stages to extract and apply timing
  exceptions

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When should exceptions be identified and applied?

• Identify the most beneficial design stages for
  extraction / application
• Higher benefit can be obtained when exceptions
  are
• extracted as late as possible
• applied as early as possible

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Timing correlation between design stages

• Timing between synthesis and placement is not
  well correlated
• After placement, timing correlation is improved

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Design Stages to Extract and Apply TE

• QoR after applying false path exceptions in each
  stage

• Timing exceptions need to be extracted after
  placement or placement optimization stages.

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Outline

• Timing exceptions vs. design outcomes
• Q1. Do timing exceptions help or hurt?
• Q2. Which exceptions give net benefit when
  inserted?
• Q3. When should exceptions be identified and
  applied?
• Guidelines to timing exceptions
• Conclusion and ongoing work

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Guidelines to Timing Exceptions

• Recommended flow for timing exceptions

• Use only clearly effective timing exceptions.
• With through points, the optimization quality
  is not improved, and can even be degraded.
• The declaration form must be as compact as
  possible.
• Timing exceptions should be extracted after
  placement.
• Non-effective false paths should be filtered.

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Conclusion and Ongoing Work

• Impact of timing exceptions - inserting
  exceptions could be beneficial or harmful
• Critical and effective timing exceptions should
  be extracted and applied after the placement
  stage
• Ineffective false paths should be pruned for
  better QoR.
• Ongoing work
• Seek the ways of extracting and auditing
  consistently
• Seek quantified metrics of both user- and
  automatically-defined TE
• Pursue timing exception methodology for general
  SOC implementation

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• THANK YOU

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