Dont-Touch & Preserve Policy: Interview Drills

Interview drills

Interview Drills for Dont-Touch & Preserve Policy focuses on preserved object count, blocked optimization opportunities, ECO rework. The goal is to connect observed QoR movement to mechanism, ownership, and regression risk.

PROMPT
You observe preserved object count, blocked optimization opportunities, ECO rework in Dont-Touch & Preserve Policy. Walk through root cause and closure plan.

STRONG ANSWER
1. Names run context and affected QoR lane.
2. Explains dont_touch and preserve are surgical controls; overuse blocks optimization and underuse breaks intended structures such as CDC wrappers or DFT hooks.
3. Requests constraint audit, preserve list, optimization-block report.
4. Proposes one reversible fix and full regression scope.

WEAK ANSWER
Jumps to random tool switches without mechanism evidence.

Whiteboard diagram

Preserve policy decision tree

OBJECT MARKED preserve/dont_touch?
        |
  functional or signoff reason?
     /                \
   yes                 no
   |                   |
keep + audit       remove, re-compile

Root-cause narration

ROOT-CAUSE TREE — Dont-Touch & Preserve Policy

preserved object count, blocked optimization opportunities, ECO rework regressed
        |
   same RTL/constraints tag?
     /              \
   no                yes
   |                  |
input drift      transform side-effect
 /    \             /         \
SDC    libs      mapping      physical estimate
diff   diff      choice       mismatch

Synthesis deep dive

Constraint hygiene determines whether synthesis QoR is trustworthy.

Concept diagram

CONSTRAINT FLOW

RTL intent -> synthesis SDC -> compile checks -> QoR dashboard -> review gate

Metric graph

QOR HEALTH

constraint lint pass   ███████████████
unconstrained count    ███
policy violations      ██

Reports and artifacts

• compile QoR

• unconstrained endpoints

• constraint lint

• policy audit log

Mini case study

Nightly WNS improved but unconstrained paths jumped; strict SDC lint restored true QoR picture.

Debug branches

• Diff SDC first

• Audit dont_touch scope

• Track unconstrained trend weekly

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

dont_touch and preserve are surgical controls; overuse blocks optimization and underuse breaks intended structures such as CDC wrappers or DFT hooks.

Metric: preserved object count, blocked optimization opportunities, ECO rework