Formal Coverage Metrics: Beyond Green Proof Counts

What this topic teaches

Formal Coverage Metrics: Beyond Green Proof Counts converts formal concepts into release-ready verification decisions. Formal closure quality depends on metric meaning, not property pass volume.

Senior-engineer framing question

When non-vacuous closure rate, counterexample turnaround, and residual-risk trend by requirement class regresses, can you isolate the first failing assumption/property boundary, prove causality, assign owner, and close with auditable risk?

FORMAL EXECUTION FLOW - Formal Coverage Metrics: Beyond Green Proof Counts

requirement intent and risk class
      |
      v
property and assumption modeling
      |
      v
proof engine exploration and trace extraction
      |
      v
counterexample classification and fix hypothesis
      |
      v
re-proof, coverage audit, and signoff decision

Evidence to collect

• Primary metric: non-vacuous closure rate, counterexample turnaround, and residual-risk trend by requirement class.

• Primary artifact: closure packet for Formal Coverage Metrics: Beyond Green Proof Counts: assumptions audit, proof status matrix, and replay-ready divergence trace.

• Owners to include: formal verification owner, rtl owner, Formal Coverage & Signoff owner.

• One reproducible failing trace and one stable comparator run.

• One fixed metadata run with assumptions and tool settings locked.

Ownership layers

OWNERSHIP LAYERS - Formal Coverage Metrics: Beyond Green Proof Counts

+----------------------+--------------------------------+--------------------------------+
| Team                 | Primary responsibility         | Closure artifact               |
+----------------------+--------------------------------+--------------------------------+
| formal verification owner | property and model integrity      | assumptions and proof packet   |
| rtl owner | implementation root-cause closure | RTL fix and replay evidence    |
| Formal Coverage & Signoff owner | signoff governance and rollout    | risk memo + acceptance gates   |
+----------------------+--------------------------------+--------------------------------+

Decision matrix

EVIDENCE MATRIX - Formal Coverage Metrics: Beyond Green Proof Counts

+-----------------------------+--------------------------------+--------------------------------+---------------------------+
| Evidence                    | Tells you                      | Does not prove                 | Next action               |
+-----------------------------+--------------------------------+--------------------------------+---------------------------+
| property status by class    | closure shape by requirement   | model realism                  | pair with cover reachability |
| vacuity and trigger checks  | assertion meaningfulness       | full legal-path exploration    | inspect assumptions       |
| counterexample traces       | concrete divergence path       | complete bug-space closure     | classify and replay       |
| assumption audit trail      | model boundary confidence      | implementation correctness     | review spec traceability  |
| before/after trend packet   | mitigation movement quality    | long-window stability          | run broader matrix        |
+-----------------------------+--------------------------------+--------------------------------+---------------------------+

Key takeaways

• Start with first-divergence classification before broad model edits.

• Tie each claim to one proving artifact and one owner action.

• Close with residual-risk statement and rollback-safe criteria.

Common pitfalls

• Treating green status as correctness without vacuity and reachability audits.

• Changing assumptions and RTL together, destroying causality.

• Declaring closure without replaying representative legal scenarios.

Formal deep dive

Signoff quality is requirement-centric and must integrate proof status, reachability, bounded limits, and waiver governance.

Concept diagram

FORMAL SIGNOFF PYRAMID

requirements -> properties and covers -> quality metrics -> waiver governance -> release decision

Metric graph

SIGNOFF CONFIDENCE TREND

fully proven critical    ███████
bounded-only critical    ████
unexplained cover gaps   ███

Metrics and artifacts to collect

• requirement-to-proof closure map

• critical cover reachability and gap aging

• bounded-only risk register

• waiver debt with owner and expiry

Mini case study

A release review blocked signoff until bounded-only properties were paired with explicit residual-risk and replay plans.

Debug branches

• Separate status color from proof quality dimensions.

• Treat unreachable critical covers as signoff blockers.

• Document bounded-horizon rationale with architecture limits.

Senior review question

Ask: which requirement intent is proven, under which assumptions, and what residual risk remains?

Key takeaways

• Tie each proof claim to assumption boundaries and reachability evidence.

• Prefer minimal reversible fixes and preserve legal behavior visibility.

Common pitfalls

• Treating runtime reduction as proof-quality improvement without audits.

• Declaring closure while critical covers remain unreachable.

• Using broad waivers instead of first-divergence root-cause ownership.