Synthesis & Logic Optimization · All levels

Constraints & QoR: Tricky Q&A

Senior interview and review questions for Constraints & QoR.

Section Q&A bank

Use these drills after completing all topics in Constraints & QoR. Answer with run context, metric, mechanism, owner, fix, and regression.

Why can unconstrained paths make QoR look better than reality?

diagram

[INT][SYNTH][CONSTRAINTS-QOR]

Q: Why can unconstrained paths make QoR look better than reality?

A:
Unconstrained endpoints are excluded from optimization objectives and reports, so apparent WNS can improve while true timing debt grows.

FOLLOW-UP TRAP: Treating unconstrained count as a secondary metric.

When should dont_touch be removed?

diagram

[INT][SYNTH][CONSTRAINTS-QOR]

Q: When should dont_touch be removed?

A:
When preservation reason is obsolete and optimization blockage outweighs structural protection; remove with audit and regression.

FOLLOW-UP TRAP: Leaving stale dont_touch through closure.

Q&A drill guide

diagram

CONTEXT -> METRIC -> MECHANISM -> FIX -> REGRESSION -> OWNERSHIP

Sketch while answering

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CONSTRAINT FLOW

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

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.

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