Ignored exceptions appear in the report. What does that mean?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Short Answer
Just because an exception (a false path, multicycle path, or case-analysis setting) is sitting in your SDC doesn't mean it's doing anything — the tool can load it, parse it fine, and still never apply it to a single path. One common reason: it's overridden. SDC exceptions have precedence rules, and a more specific or more recently applied exception on the same path segment can silently take priority over an earlier, broader one.
Technical Explanation
- One common reason: it's overridden.
- Another reason: it's redundant. If a
set_false_path -through Xalready covers everything a second, narrower false path was trying to declare, the second one has nothing left to do. - A third reason: it's malformed — the object selector (
get_pins,get_cells) matches zero objects because of a naming/hierarchy mismatch (the same class of bug as applying SDC before checking hierarchy), so the exception exists syntactically but targets nothing real. - g. a case-analysis setting that already removes a path from active analysis makes a false path on that same path a no-op.
- Treat every "ignored exception" report as a readiness check, not noise: trace each one back to a named object and a reproducible reason, so another engineer looking at the same handoff gets the same answer — don't just suppress the message.
What To Check
- Warning sign: A carefully written multicycle or false path has no effect.
- Inspect: Keep at least two possible causes open, then use one named object and the active mode or scenario to separate them.
- Correct: Inspect ignored and dominant relationships, object resolution, and precedence; correct or remove stale intent with the STA owner.
Command Checks & Actions
- report_exceptions -ignored: shows timing exceptions and their scope
- report_exceptions -dominant: shows timing exceptions and their scope
Run the commands in order. Each line answers a separate part of the check.
Healthy, Suspicious & Hard-stop Results
- Expected: An exception may be overridden, redundant, malformed, or dominated by another constraint; its presence in SDC does not mean it affects analysis.
- 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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