VT Mix Strategy: Expanded Case Study

Extended case study

Closure review: HVT/SVT/LVT cell mix, setup/hold slack distribution, leakage budget regressed after a change touching VT Mix Strategy.

Background

Team previously had stable baseline across weekly compile runs and agreed guardbands.

Symptoms observed

• HVT/SVT/LVT cell mix, setup/hold slack distribution, leakage budget moved beyond threshold

• Mismatch between local and CI runs

• Owner disagreement on first fix

Investigation timeline

1. Hour 0: freeze manifests and compare run metadata

2. Hour 1: isolate section/topic-specific failure pattern

3. Hour 2: verify constraints and library view consistency

4. Hour 3: inspect transform and mapping deltas

5. Hour 4: choose one reversible fix

6. Hour 5: execute full regression matrix

7. Hour 6: publish closure memo and next actions

Root cause

Root cause centered on VT Mix Strategy: VT assignment trades delay against leakage; poor mix causes either timing misses or unnecessary static power burn.

Fix and validation

• Targeted synthesis tweak with audit trail

• Re-run VT distribution report, leakage breakdown, timing split report

• Update dashboard and handoff notes

Lessons learned

• Compare like-for-like runs only

• Assign clear owner before ECO

• Keep rollback ready

CASE STUDY — VT Mix Strategy
baseline / regression / recovery QoR snapshot

Sequence under stress

SYNTHESIS FLOW — VT Mix Strategy

RTL + constraints
      |
      v
elaboration + checks
      |
      v
mapping + optimization
      |
      v
QoR reports (timing/area/power)
      |
      v
incremental ECO + regression

Primary metric: HVT/SVT/LVT cell mix, setup/hold slack distribution, leakage budget

Synthesis deep dive

PPA closure is a constrained tradeoff problem, not a timing-only exercise.

Concept diagram

PPA TRIAD

timing target <-> power budget <-> area cap

Metric graph

PARETO PROGRESSION

early runs      o o
balanced point    o
over-tuned          o (risk)

Reports and artifacts

• VT mix

• leakage trend

• dynamic power trend

• PPA Pareto table

Mini case study

LVT-heavy fix recovered setup but violated leakage target; balanced VT strategy closed both.

Debug branches

• Timing-only claims need power check

• Validate hold on VT swaps

• Use Pareto memo

Senior review question

Ask: what evidence proves this QoR move is real and stable?

Key takeaways

• State exact run context (RTL, SDC, libs, switches) with every QoR claim.

• Re-run timing, area, and power regressions after each synthesis ECO.

Common pitfalls

• Comparing runs with mismatched constraints or library views.

• Timing-only fixes that violate power or area budgets.

• Skipping equivalence checks after structural changes.

Principal synthesis review addendum

VT assignment trades delay against leakage; poor mix causes either timing misses or unnecessary static power burn.

Metric: HVT/SVT/LVT cell mix, setup/hold slack distribution, leakage budget