How do you find out that a false path or multicycle exception never matched any real path?

Short Answer

Writing set_false_path or set_multicycle_path (SDC) with a typo in a pin, instance, or clock name does not raise an error, PrimeTime simply applies the exception to zero paths and stays silent. report_exceptions -ignored (PT) lists every exception in the design that matched nothing, which is the only reliable way to catch this before it hides a real violation.

Technical Explanation

• SDC exception commands accept object lists built with get_pins, get_cells, or get_clocks (SDC), and an empty or wrong object list is valid Tcl, the command just runs with nothing to act on.
• report_exceptions (PT) on its own lists every active exception and, for most of them, the paths it actually covers.
• report_exceptions -ignored (PT) narrows the list to exceptions that PrimeTime accepted syntactically but that ended up covering zero real paths in the current design.
• An exception can go from matching real paths to matching nothing after a netlist change, a rename during synthesis, or an RTL restructure, even if the SDC file itself never changed.
• Because an ignored exception produces no warning by default, a path meant to be excluded stays fully checked, or a path meant to be relaxed to a multicycle still uses a single-cycle check, either way the report is silently different from what the designer intended.
• Running report_exceptions -ignored (PT) after any constraint or netlist change is the direct way to confirm every intended exception is still doing its job.

Follow-up Question & Model Response

Why would an exception that worked correctly for months suddenly show up in the ignored list after an unrelated RTL change?

Candidate Model Response: SDC exceptions written with get_pins or get_cells (SDC) usually reference object names from a specific netlist, and synthesis can rename or restructure logic when even a small piece of unrelated RTL changes upstream of it. If the renamed or removed instance was the one the exception's object list pointed to, the exception now matches nothing, even though the SDC text is untouched. This is why exceptions written against exact instance names are more fragile than ones written against a clock or a stable hierarchical pin. Re-running report_exceptions -ignored (PT) after any resynthesis catches this before it reaches signoff.