What does an empty collection during SDC loading indicate?
From PDVerse PnR Interview Handbook · pdVerse Mentor Guide
Short Answer
A selector found no current design objects, commonly because hierarchy, names, buses, or optimisation changed.
Technical Explanation
A collection is ICC2's set of selected objects. Empty means the pattern selected nothing, so a constraint may have quietly reached zero targets.
What To Check
• Warning sign: The command executes with a warning yet constrains zero ports or pins. • Inspect: Start with one named object in the report and trace it to the netlist, library, or SDC statement that created it. • Correct: Inspect the exact selector against the linked design and obtain a revision-correct constraint; never replace it immediately with *.
Command Checks & Actions
• get_ports <pattern>: selects boundary ports
• get_pins -hierarchical <pattern>: selects instance pins
Run the commands in order. Each line answers a separate part of the check.Healthy, Suspicious & Hard-stop Results
• Expected: A selector found no current design objects, commonly because hierarchy, names, buses, or optimisation changed. • Investigate: The command executes with a warning yet constrains zero ports or pins. • Stop before floorplanning when required logic or timing coverage is missing or unexplained. Inspect the exact selector against the linked design and obtain a revision-correct constraint; never replace it immediately with *.
Common Mistake
Do not assume the check passed just because ICC2 continued: The command executes with a warning yet constrains zero ports or pins. 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: The command executes with a warning yet constrains zero ports or pins.
Practical Example
Symptom: The command executes with a warning yet constrains zero ports or pins. Corrective path: Inspect the exact selector against the linked design and obtain a revision-correct constraint; never replace it immediately with *.
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