Many constraints target objects optimised away by synthesis. Who owns the fix?
From PDVerse PnR Interview Handbook, part of the pdVerse Mentor Guide
Short Answer
When a pile of SDC constraints reference objects that synthesis already optimized away, the underlying problem is that the applied SDC and the delivered netlist have drifted out of sync — and that's a mapping/regeneration problem, not something PD should be quietly patching around. The instinct to write local wildcard fixes ("just widen the pattern match so the constraint applies to something") is exactly the wrong move — it papers over a real handoff defect instead of fixing it.
Technical Explanation
- Ownership belongs with whoever generates the SDC/netlist pairing — usually synthesis or the STA team regenerating constraints against the actual delivered netlist — because they're the ones who can correctly re-map or regenerate what changed.
- PD's job here is diagnosis, not repair: identify exactly which objects are stale, why (renamed, merged, removed during optimization), and hand that back with named objects so the fix happens at the source.
- This is the same discipline as "return to owner" readiness checks elsewhere in this set — recognizing when a problem is outside PD's authority to fix correctly is itself a core PD skill.
What To Check
- Warning sign: read_sdc reports empty collections for old hierarchy paths.
- Inspect: Keep at least two possible causes open, then use one named object and the active mode or scenario to separate them.
- Correct: Provide exact unresolved selectors and current object evidence to the constraint producer, then reload a corrected handoff.
Command Checks & Actions
- get_cells -hierarchical <pattern>: selects design cells
- get_pins -hierarchical <pattern>: selects instance pins
- read_sdc -syntax_only <file.sdc>: parses or applies the timing constraints
Run the commands in order. Each line answers a separate part of the check.
Healthy, Suspicious & Hard-stop Results
- Expected: The applied SDC must match the delivered netlist; mapping may need synthesis/STA regeneration, not local wildcard repair by PD.
- 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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