Structuring & Buffering: Interview Drills

Interview drills

Interview Drills for Structuring & Buffering focuses on logic depth, fanout violations, buffer count, slew violations. The goal is to connect observed QoR movement to mechanism, ownership, and regression risk.

PROMPT
You observe logic depth, fanout violations, buffer count, slew violations in Structuring & Buffering. Walk through root cause and closure plan.

STRONG ANSWER
1. Names run context and affected QoR lane.
2. Explains Structuring rewrites logic cones and buffering controls transition/fanout; both directly affect timing, area, and power in different corners.
3. Requests fanout report, transition report, structure diff.
4. Proposes one reversible fix and full regression scope.

WEAK ANSWER
Jumps to random tool switches without mechanism evidence.

Whiteboard diagram

Structure vs buffer tradeoff

critical cone
  |-- restructure logic (depth)
  |-- add buffers (fanout/slew)

Choose by dominant delay mode.

Root-cause narration

ROOT-CAUSE TREE — Structuring & Buffering

logic depth, fanout violations, buffer count, slew violations regressed
        |
   same RTL/constraints tag?
     /              \
   no                yes
   |                  |
input drift      transform side-effect
 /    \             /         \
SDC    libs      mapping      physical estimate
diff   diff      choice       mismatch

Synthesis deep dive

Mapping converts logic intent into real PPA outcomes.

Concept diagram

MAPPING LOOP

boolean net -> library mapping -> optimization -> report and iterate

Metric graph

MAPPING IMPACT

depth reduction   ███████
fanout cleanup    █████
runtime overhead  ███

Reports and artifacts

• mapping summary

• critical path cell list

• fanout/slew report

• library coverage

Mini case study

Cell-heavy critical path improved after restructuring, while blanket buffering had worsened power.

Debug branches

• Depth issue -> structure

• Fanout issue -> buffers

• Library mismatch -> view audit

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

Structuring rewrites logic cones and buffering controls transition/fanout; both directly affect timing, area, and power in different corners.

Metric: logic depth, fanout violations, buffer count, slew violations