Incremental Compile: Interview Drills

Interview drills

Interview Drills for Incremental Compile focuses on reuse ratio, localized QoR delta, compile turnaround time. The goal is to connect observed QoR movement to mechanism, ownership, and regression risk.

PROMPT
You observe reuse ratio, localized QoR delta, compile turnaround time in Incremental Compile. Walk through root cause and closure plan.

STRONG ANSWER
1. Names run context and affected QoR lane.
2. Explains Incremental compile reuses stable optimization regions while targeting changed cones, reducing churn and iteration cost.
3. Requests incremental reuse report, changed-cone map, QoR diff.
4. Proposes one reversible fix and full regression scope.

WEAK ANSWER
Jumps to random tool switches without mechanism evidence.

Whiteboard diagram

Incremental compile zones

stable cones: reuse
changed cones: re-optimize
global checks: timing/power consistency

Root-cause narration

ROOT-CAUSE TREE — Incremental Compile

reuse ratio, localized QoR delta, compile turnaround time 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

Incremental compile reuses stable optimization regions while targeting changed cones, reducing churn and iteration cost.

Metric: reuse ratio, localized QoR delta, compile turnaround time