ECO Regression Discipline: Interview Drills

Interview drills

Interview Drills for ECO Regression Discipline focuses on post-ECO regressions caught, corner coverage, signoff approval latency. The goal is to connect observed QoR movement to mechanism, ownership, and regression risk.

PROMPT
You observe post-ECO regressions caught, corner coverage, signoff approval latency in ECO Regression Discipline. Walk through root cause and closure plan.

STRONG ANSWER
1. Names run context and affected QoR lane.
2. Explains Regression discipline prevents late-stage surprises by enforcing setup/hold, power, and equivalence checks after every ECO wave.
3. Requests regression matrix, signoff checklist, release memo.
4. Proposes one reversible fix and full regression scope.

WEAK ANSWER
Jumps to random tool switches without mechanism evidence.

Whiteboard diagram

Regression matrix

[ ] setup corners
[ ] hold corners
[ ] power checks
[ ] equivalence
[ ] handoff report update

Root-cause narration

ROOT-CAUSE TREE — ECO Regression Discipline

post-ECO regressions caught, corner coverage, signoff approval latency regressed
        |
   same RTL/constraints tag?
     /              \
   no                yes
   |                  |
input drift      transform side-effect
 /    \             /         \
SDC    libs      mapping      physical estimate
diff   diff      choice       mismatch

Synthesis deep dive

Incremental and ECO synthesis protect schedule only with strict regression discipline.

Concept diagram

INCREMENTAL ECO LOOP

identify delta -> localized compile -> patch -> regression -> release

Metric graph

ECO CYCLE TIME

full compile      ███████████
incremental       █████

Reports and artifacts

• incremental reuse

• ECO patch list

• LEC status

• regression matrix

Mini case study

Localized compile cut turnaround 40%, but skipped LEC caused rollback.

Debug branches

• Reuse ratio is not enough

• Patch locality check

• Always include equivalence hooks

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

Regression discipline prevents late-stage surprises by enforcing setup/hold, power, and equivalence checks after every ECO wave.

Metric: post-ECO regressions caught, corner coverage, signoff approval latency