How do renamed or optimised-away objects break constraints?
From PDVerse PnR Interview Handbook · pdVerse Mentor Guide
Short Answer
Post-synthesis hierarchy and object names may differ from the intent source, causing selectors to resolve incorrectly or not at all.
Technical Explanation
Treat this as a readiness check. It should produce named objects and a result that another engineer can repeat from the same handoff.
What To Check
• Warning sign: A path exception or generated clock targets an object removed by optimisation. • Inspect: Compare one named object across the related reports; the same object should tell a consistent story. • Correct: Use synthesis mapping/handoff evidence to update the constraint at its source and verify exact collection membership.
Command Checks & Actions
• get_cells -hierarchical <pattern>: selects design cells
• get_pins -hierarchical <pattern>: selects instance pins
Run the commands in order. Each line answers a separate part of the check.Healthy, Suspicious & Hard-stop Results
• Expected: Post-synthesis hierarchy and object names may differ from the intent source, causing selectors to resolve incorrectly or not at all. • Investigate: A path exception or generated clock targets an object removed by optimisation. • Stop before floorplanning when required logic or timing coverage is missing or unexplained. Use synthesis mapping/handoff evidence to update the constraint at its source and verify exact collection membership.
Common Mistake
Do not assume the check passed just because ICC2 continued: A path exception or generated clock targets an object removed by optimisation. Fix or narrowly justify the named objects, then rerun the same command.
What The Interviewer Is Testing
Be ready to explain why this matters before floorplanning: A path exception or generated clock targets an object removed by optimisation.
Follow-up Question & Model Response
Model response: “I would save the report, inspect one affected object in the correct block and scenario, and make or request this correction: Use synthesis mapping/handoff evidence to update the constraint at its source and verify exact collection membership. Then I would reopen the clean checkpoint and rerun the same evidence.”
Practical Example
Symptom: A path exception or generated clock targets an object removed by optimisation. Corrective path: Use synthesis mapping/handoff evidence to update the constraint at its source and verify exact collection membership.
Continue practising