Constraint Bug Patterns: Expanded Case Study

Extended case study

Signoff review: false violation count after SDC fix, constraint audit pass rate fails after an apparently minor change affecting Constraint Bug Patterns.

Background

Team had closure last week on full MMMC matrix. New run shows regression clustered in one path group.

Symptoms observed

• false violation count after SDC fix, constraint audit pass rate regression

• Disagreement between synth and STA WNS

• Hold violations after setup ECO

Investigation timeline

1. Hour 0: freeze netlist, SDC, SPEF, lib tags

2. Hour 1: diff constraints and run manifest

3. Hour 2: isolate corner/mode that fails

4. Hour 3: read worst path — cell vs net, clocks

5. Hour 4: classify root cause

6. Hour 5: minimal ECO or SDC fix

7. Hour 6: full regression + signoff memo

Root cause

Root cause tied to Constraint Bug Patterns: Most 'mystery' STA failures are wrong clocks, missing generated clocks, bad I/O delay, or over-broad exceptions.

Fix and validation

• Bounded ECO or audited SDC change

• Re-run SDC diff, constraint validator log, before/after path report

• Full MMMC + SI regression

Lessons learned

• Tag every run

• Diff constraints before blaming PD

• Setup and hold together

CASE STUDY — Constraint Bug Patterns
baseline WNS / regressed WNS / post-fix WNS

Sequence under stress

TIMING PATH — Constraint Bug Patterns

launch FF --logic stages--> capture FF
     |                           |
  launch clock                capture clock
     |                           |
  required: arrival < capture_edge - setup
  metric: false violation count after SDC fix, constraint audit pass rate

STA deep dive

Debug hygiene separates senior STA leads from report readers.

Concept diagram

DEBUG ORDER

1. same tags?  2. constraints?  3. view?  4. real path?  5. ECO

Metric graph

ROOT CAUSE MIX (typical)

constraint bugs  ████████  35%
view mismatch    ██████    25%
real path        ██████    25%
methodology      ████      15%

Reports and artifacts

• run manifest

• SDC diff

• SPEF tag

• scenario isolation

Mini case study

Mystery regression: SPEF corner tag typo in run script — not a design bug.

Debug branches

• Manifest every run

• War room by path group

Senior review question

Ask: what corner/mode/view proves this topic is closed or failing?

Key takeaways

• State corner, mode, view, and database tag with every slack claim.

• Run setup and hold plus MMMC regression after every ECO.

Common pitfalls

• Comparing STA runs with different SPEF or SDC tags.

• Broad false_path to green-wash violations.

• Setup-only ECO without hold check.

Principal STA review addendum

Most 'mystery' STA failures are wrong clocks, missing generated clocks, bad I/O delay, or over-broad exceptions.

Metric: false violation count after SDC fix, constraint audit pass rate