Why inspect hierarchy before applying SDC?

Short Answer

SDC commands like create_clock, set_input_delay, or set_false_path all target objects by path — and those paths only exist if they match what ICC2 actually loaded, not what the RTL author originally wrote. Synthesis routinely renames, flattens, or removes hierarchy: a module boundary that existed in RTL might be gone after optimization, or an instance name might have picked up a synthesis-tool suffix.

Technical Explanation

• If your SDC selector (get_cells, get_pins, get_ports) references an old path that no longer exists, the command silently matches nothing — no error, just an empty collection — and that constraint never gets applied.
• The design ends up underconstrained without any obvious warning: paths that should have been false-pathed or given multicycle treatment get analyzed as ordinary single-cycle paths, and you find out only when timing numbers look wrong or too good.
• The fix is simple discipline: run report_hierarchy / inspect report_design right after import, before sourcing SDC, so you know the real object namespace you're targeting.
• This is exactly the same failure mode described elsewhere for library and unit mismatches — the tool interpreted something differently than you assumed, and the only defense is checking what it actually loaded before trusting downstream numbers.

What To Check

• Warning sign: Constraints target pre-synthesis paths or a renamed block and resolve to nothing.
• Inspect: Start with one named object in the report and trace it to the netlist, library, or SDC statement that created it.
• Correct: Update the constraint producer's mapping or handoff for the real netlist; avoid broad wildcards as a shortcut.

Healthy, Suspicious & Hard-stop Results

• Expected: SDC object paths depend on the post-synthesis hierarchy and naming that ICC2 actually loaded.
• Stop before floorplanning when required logic or timing coverage is missing or unexplained.

What The Interviewer Is Testing

Be ready to explain why this matters before floorplanning.