How should ICC2 inventory be reconciled with synthesis reports?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Short Answer
Don't compare one giant total from synthesis against one giant total from ICC2 and call it a day — that hides exactly where the discrepancy is coming from. Compare like-for-like scope and object class: registers against registers, combinational logic against combinational logic, memories against memories, macros against macros.
Technical Explanation
- Real handoff transformations legitimately change counts between the two tools — clock-gate merging, tie-cell insertion, buffer insertion for DRC fixing — so don't demand unexplained equality; demand explained differences.
- Separating the counts by object class turns "the numbers don't match" into "here's specifically what changed and why," which is a much more useful conversation with the synthesis team.
What To Check
- Warning sign: Sequential or macro counts differ substantially because the wrong revision or top was loaded.
- Inspect: Compare one named object across the related reports; the same object should tell a consistent story.
- Correct: Identify known transformations; otherwise stop and ask synthesis ownership to reconcile the handoff.
Command Checks & Actions
- sizeof_collection [get_cells -hierarchical *]: counts exactly the objects returned by a selector
- sizeof_collection [get_ports *]: counts exactly the objects returned by a selector
Run the commands in order. Each line answers a separate part of the check.
Healthy, Suspicious & Hard-stop Results
- Expected: Compare like-for-like scope and object classes, accounting for known handoff transformations rather than demanding unexplained equality.
- Stop before floorplanning when required logic or timing coverage is missing or unexplained.
Common Mistake
The Trap: Do not assume the check passed just because ICC2 continued. 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.
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. ”
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