How do you compare timing between two floorplan alternatives?

Short Answer

A slack number is meaningless in isolation — before comparing anything, lock down everything except the floorplan itself: same netlist, same SDC, same scenarios/corners, same libraries, same pre-CTS clock latency/uncertainty assumptions, same coarse placement effort, and the same parasitic estimation method for both runs. Once the environment is controlled, compare the same named paths across both floorplans — pick a handful of representative register-to-register and I/O paths (especially known-critical ones) and track how their slack changes, rather than just comparing aggregate WNS/TNS, which can hide which specific paths got better or worse.

Technical Explanation

• For each path that changed meaningfully, dig into why: did the macro pin side change (forcing a longer detour), did the path now cross more hierarchy boundaries, does it pass through more feedthrough logic, did buffering opportunity improve or worsen because of new channel space?
• Physical distance is usually the root cause of a timing delta at this stage — floorplan A vs. B mostly differ in how far things are from each other, so a worse timing path in one usually traces back to a longer physical route or a worse-positioned macro pin.
• Congestion matters too, even for a "timing" comparison — a floorplan that looks timing-clean at the estimate stage but has hidden congestion can force much longer detours (and worse real timing) once actual routing happens, so don't treat this purely as a static-timing-in-isolation exercise.
• Document the comparison as a table: path name, floorplan A slack, floorplan B slack, delta, and the physical reason for the delta — this is what lets someone else trust the conclusion instead of just the final number.

Formula Or Decision Rule

Decision rule: attribute the delta to a physical cause; do not claim signoff closure.

What To Check

• Warning sign: One alternative reports better WNS but used another scenario or excluded a path.
• Inspect: choose one affected region, macro, row, pin, path, or net and trace the physical cause.
• Correct: Align contexts, rerun estimate_timing, and compare full path reports and congestion together.

Healthy, Suspicious & Hard-stop Results

• Healthy: Use the same netlist, SDC, scenarios, libraries, pre-CTS clock assumptions, coarse placement, and estimation method. Compare the same named paths and physical distances.
• Hard stop: required legality or physical feasibility is missing or unexplained.

What The Interviewer Is Testing

” to measurable physical evidence and an owned correction.

Follow-up Question & Model Response

"* Candidate Model Response: Model response: “I would change it if the same controlled rerun shows that one alternative reports better WNS but used another scenario or excluded a path. ”