Case study 2 — cheap wins first: build flags and allocators

The playbook's ordering principle (SPEC §3.3, CLAUDE.md non-negotiable #3): build flags → LTO → PGO → allocator swap → then code changes. The cheapest classes are tried first because they carry the lowest cost-per-attempt and the lowest risk. Two of them produced Phase-0 results that frame the whole project's discipline.

Class 1 — link-time optimization (tokei) — ACCEPTED

phase0-tokei-003: set lto = true (fat LTO, up from thin) and codegen-units = 1 in tokei's release profile.

• Speedup: 1.104 median, 95% CI [1.085, 1.120] vs the pristine thin-LTO baseline.
• Byte-identical golden replay; no source change, so equivalence is by construction (same code, different codegen). Auto-accept tier.
• A pure build-config win worth +10% — the cheapest possible class, and on this target the single largest individual increment.

The lesson the ledger enforces: you do not go hand-optimize a hot loop before you have tried turning on the optimizer's own switches.

Class 2 — allocator swap (comrak) — NEEDS-HUMAN-REVIEW

phase0-comrak-002: swap the global allocator to mimalloc.

• Speedup: 1.046 median, 95% CI [1.033, 1.058] — a real, CI-clean +4.6% on comrak's Pro Git render.
• Byte-identical golden replay. And yet the verdict is not accepted — it is needs-human-review.

Why: an allocator swap changes the process's memory behavior globally. It is exactly the class of change SPEC §8 routes to a human — not because this instance is wrong, but because "the benchmark got faster" is not sufficient evidence that a global allocator change is safe under every workload, fragmentation pattern, and concurrency scenario the target will meet in production. The pipeline measured a real win and then refused to auto-ship it, tagging it for a human ruling instead.

Human ruling (results/rulings/phase0-comrak-002.md): ACCEPTED, scoped to the comrak CLI single-stream markdown-render workload — a real win for the measured use, deliberately not a blanket "always use mimalloc" mandate. The tier forced the judgment; the judgment is bounded and written down. The ledger row stays needs-human-review (append-only records what the pipeline decided); the ruling is the human layer on top.

What the pair demonstrates

The cheap classes are not lesser — class 1 produced this project's biggest single win. But "cheap to try" is not "safe to auto-accept." LTO is deterministic codegen: accept it. An allocator swap is a global behavioral change: measure it, prove the speedup, and still hand it to a human. The tier is decided by the nature of the change, never by the size of the number — the same rule that later caught a leaking teardown patch the bench loved (case study 3).