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neohaskell-performance-design-review

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Design-time performance review of an approved contract-delta spec, before implementation. Runs when the spec's touches list intersects perf-sensitive capabilities. Produces the committed review record; measurement lives in the nightly bench harness, not here.

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Source SKILL.md: https://github.com/neohaskell/NeoHaskell/blob/HEAD/.claude/skills/neohaskell-performance-design-review/SKILL.md

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Performance design review (risk-tiered, design-time)

Rebuilt 2026-07-08 (Phase 5; predecessors archived in docs/archive/2026-07-ai-artifacts/claude-skills/ — note the archive's blanket-INLINE/UNPACK advice was CORRECTED against GHC reality during salvage; this file is the fixed version). Target: ~50k req/s event-sourcing services. Hot-path budgets: command intake <1ms, event apply <0.5ms, query <0.2ms, event persistence <1ms.

When this runs (mechanical, not judgment)

./dev spec-check --plan docs/changes/NNN-slug.md → design_reviews contains "perf" (spec touches: ∩ perf-sensitive capabilities). After spec approval, before implementation. Untagged specs skip this. This review is per-PR assurance by reasoning; per-night measurement is ./dev bench (nightly-bench.yml) — never conflate the two, and never demand benchmarks in a PR.

Compiler context (facts the review must not contradict)

  • Strict is ON globally: let-bindings, args, and constructor fields are already strict. Never advise "add strictness" / bangs / foldl'.
  • Strict does NOT cover: imported types (Prelude Maybe/lists/tuples stay lazy), nested patterns, or values inside containers (Map k v holds lazy vs). These are where leaks actually hide.
  • At -O GHC unboxes small strict fields itself (-funbox-small-strict-fields): UNPACK on a word-sized field of a single-constructor type is noise. UNPACK earns its place only on multi-word strict fields or where the unfolding shows re-boxing at lazy call sites — and that claim needs a profile.

Checklist (numbered — the record cites these IDs)

  • P1 Hot-path placement: which promised functions sit on command intake / event apply / query / persistence? Budget impact estimate for each. Not on a hot path → the rest of this list demotes to informational.
  • P2 Specialisation: exported polymorphic (typeclass-constrained) function called cross-module on a hot path → needs INLINABLE (GHC cannot specialise across modules without it). INLINE only for small functions that unlock fusion/RULES — never on >~10-line bodies.
  • P3 Laziness escapes: ~ opt-outs in hot-path types need a justifying comment; hot-path data relying on Prelude Maybe/list under the false assumption Strict covers it; recursive accumulators building non-flat structures (thunk-graph retention).
  • P4 Serialization: hand-written ToJSON on a hot codec (events, API responses, projections) must define toEncoding, not just toJSON (the default toEncoding via toJSON gives zero speedup; Generic/TH derivation emits a real one).
  • P5 Allocation: pack/unpack or encodeUtf8/decodeUtf8 round-trips more than once per request; [fmt|…|] inside tight loops; unfused Array.map f |> Array.map g chains; constants rebuilt inside loops.
  • P6 Contention: ConcurrentVar/TVar wrapping a whole Map with multiple writer threads serialises all writers — push toward ConcurrentMap (stm-containers) or per-key sharding. Single-writer or read-mostly state: the plain var is correct, the swap is overkill.
  • P7 Evidence discipline: any "this is faster" claim in the spec beyond removing the named anti-patterns requires measurement (a criterion/ tasty-bench number or a profile) — otherwise downgrade the claim and note it for the nightly harness. New perf-tagged surface should propose a bench-budgets entry (telemetry/bench-budgets.json).

Grounding filter (mandatory)

Same 4-question discipline as the security review: (1) measurable impact on the 50k req/s budget given the workload tier? (2) is the path actually exercised? (3) can the framework absorb the fix so users never see a pragma? (4) proportional — no pragma cascades on code no profile has shown hot? Findings failing any question are demoted to informational with the failed question named. Premature-optimization cascades (INLINE+UNPACK+SPECIALIZE everywhere) are a net negative: compile time, code size, lost fusion.

Output (the committed record)

Write docs/changes/NNN-slug.perf-review.md on the PR branch (same shape as the security record: checklist-ID table, grounding column, verdict, blocker count + resolution). Blockers amend the plan or park the run; zero-finding reviews still commit the record.