Synthesis SDC Contracts: Interview Drills

Interview drills

Interview Drills for Synthesis SDC Contracts focuses on compile WNS/TNS, unconstrained endpoint count, SDC lint error rate. The goal is to connect observed QoR movement to mechanism, ownership, and regression risk.

PROMPT
You observe compile WNS/TNS, unconstrained endpoint count, SDC lint error rate in Synthesis SDC Contracts. Walk through root cause and closure plan.

STRONG ANSWER
1. Names run context and affected QoR lane.
2. Explains Synthesis SDC defines clocks, IO assumptions, and optimization boundaries; weak constraints produce fake QoR wins and expensive downstream churn.
3. Requests compile report, SDC lint log, unconstrained path report.
4. Proposes one reversible fix and full regression scope.

WEAK ANSWER
Jumps to random tool switches without mechanism evidence.

Whiteboard diagram

Synthesis constraint contract

SDC CONTRACT

create_clock / generated_clock
set_input_delay / set_output_delay
exceptions (false/multicycle, audited)
compile QoR + unconstrained path report

Bad contract => misleading QoR.

Root-cause narration

ROOT-CAUSE TREE — Synthesis SDC Contracts

compile WNS/TNS, unconstrained endpoint count, SDC lint error rate 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

Synthesis SDC defines clocks, IO assumptions, and optimization boundaries; weak constraints produce fake QoR wins and expensive downstream churn.

Metric: compile WNS/TNS, unconstrained endpoint count, SDC lint error rate