Scenario: QoR Storm: Expanded Case Study

Extended case study

Closure review: overnight WNS/TNS regression with no RTL intent change regressed after a change touching Scenario: QoR Storm.

Background

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

Symptoms observed

• overnight WNS/TNS regression with no RTL intent change 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 Scenario: QoR Storm: QoR storms test run-hygiene fundamentals: constraint diffs, library updates, and compile switches before speculative ECO.

Fix and validation

• Targeted synthesis tweak with audit trail

• Re-run scenario brief, run manifest diff, triage timeline

• Update dashboard and handoff notes

Lessons learned

• Compare like-for-like runs only

• Assign clear owner before ECO

• Keep rollback ready

CASE STUDY — Scenario: QoR Storm
baseline / regression / recovery QoR snapshot

Sequence under stress

SYNTHESIS FLOW — Scenario: QoR Storm

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

Primary metric: overnight WNS/TNS regression with no RTL intent change

Synthesis deep dive

Interview readiness comes from repeatable QoR reasoning under pressure.

Concept diagram

INTERVIEW ANSWER SHAPE

context -> metric -> mechanism -> fix -> regression -> ownership

Metric graph

PANEL SCORING

clarity      ███████
mechanism    ██████
regression   █████

Reports and artifacts

• mock scorecard

• scenario debrief

• Q&A coverage

• framework adherence

Mini case study

Candidate improved by replacing tool-command answers with evidence-driven closure narratives.

Debug branches

• State assumptions first

• Defend option tradeoffs

• Close with rollback/regression

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

QoR storms test run-hygiene fundamentals: constraint diffs, library updates, and compile switches before speculative ECO.

Metric: overnight WNS/TNS regression with no RTL intent change