What does report_clock_qor -type robustness actually measure, and why is -robustness_corner a required option, not optional?
From PDVerse PnR Interview Handbook · pdVerse Mentor Guide
Short Answer
The robustness metric is the ratio of a sink's latency at the reporting corner to its latency at a required -robustness_corner -- a direct cross-corner comparison, not an absolute number. -robustness_corner is required specifically because the metric is meaningless without a reference corner to compare against; a ratio needs two points, and the tool won't guess which corner should be the baseline.
Technical Explanation
- The robustness metric reported by -type robustness is a ratio: a sink's latency at the reporting corner divided by (or compared to) its latency at a required -robustness_corner.
- -robustness_corner is required, not optional -- the metric is fundamentally a cross-corner comparison, and without a specified reference corner there's nothing for the reporting corner's latency to be measured against.
- This directly operationalizes the Sum-vs-Pi tradeoff discussed elsewhere: a tree with poor robustness (a ratio far from 1) is more corner-dependent, exactly the risk profile a Sum tree's unbalanced structure carries more of than a Pi tree's symmetrical one.
Common Mistake
The Trap: Running report_clock_qor -type robustness without specifying -robustness_corner and expecting a meaningful default -- the option is required precisely because the metric has no meaning without a stated reference corner.
Follow-up Question & Model Response
"If a tree's robustness ratio is far from 1 at a given corner, what does that tell you about whether it was built as a Sum tree or a Pi tree?"
Candidate Model Response: A ratio far from 1 points toward a Sum-tree-style structure -- Sum trees minimize skew by per-path delay matching rather than structural symmetry, which is exactly the property that makes them more sensitive to which corner you're measuring at.
Practical Example
Debug Scenario: report_clock_qor -type robustness -robustness_corner WCS on a tree gives a ratio of 1.15 at the BCF corner -- meaning that sink's latency is 15% higher at BCF than at the WCS reference, a direct, quantified measure of that specific tree's cross-corner sensitivity.
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