BFM-decomp/docs/accelerators.md

Accelerators — what we learned late that a future decomp should know on day one

Purpose (Drew, 2026-08-07): we are building a reusable Claude Code decomp workflow. The PhaseEnds record what happened; docs/decision-log.md (R31) records why we pivoted. Neither answers the question this file exists for: "what should phase 1 of the NEXT game do differently?"

How to add an entry (same session as the discovery — R30 timing): what it is · when we found it · when it could have been found · what it would have saved · and the honest prerequisite, because some levers genuinely cannot exist before something else is in place. Mark those PREREQ rather than pretending they were available on day one — the prerequisite is then the real advice.

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A1 — Instrument-integrity BEFORE measurement (R35), as a standing habit

Found: phase 26 (the tooling-integrity audit), then re-confirmed in 27, 28, and again at S46. Could have been found: phase 1. What it cost us: four separate strategic reversals. A "families bank ≈0%" verdict that steered two phases was a missing build step. A "~3% h_seq ceiling" was an -O0 compile-flag artifact. Four hidden overlays and ~39 code modules were invisible because a glob only looked at one payload layer. A whole class of "the compiler beat us" walls were our own scanners.

The rule for a new project: any number that is about to scope real work — a coverage %, a "wall", a "cheap win", a 0-hit scan — gets its instrument verified before the number is believed. Cheapest version: every scanner asserts its own coverage against an over-approximating candidate set (R32), and every oracle that is structurally blind to a class of error gets a second, disagreeing oracle (R34).

A2 — The whole-binary byte-gate + parallel drafting harness

Found: phase 12 (of ~30). Could have been found: immediately after the first byte-identical build (phase 5–6). What it would have saved: phases 6–11 matched functions essentially by hand. The pattern — many cheap agents draft C, and an incorruptible whole-binary byte-gate accepts only byte-identical output — took the resident engine 1.4% → 85.6% in a single session once it existed. It also makes model quality a throughput question instead of a correctness risk, which is what makes cheap models usable at all.

The rule: build the gate before the drafting. The gate is what makes everything after it safe.

A3 — Cross-binary dedup propagation (match once → stamp ×N)

Found: phases 11–15. Could have been found: as soon as a second binary existed (phase 10). PREREQ: ≥2 binaries onboarded and signed. What it would have saved: this is the project's economic engine — one match banks up to 138 instances. Every match made before it existed was worth ×1.

The rule: the moment you have two binaries, ask "how much identical code do they share?" and build the propagation path before harvesting.

A4 — Read the compiler's SOURCE once, into a codegen map

Found: phase 23. Could have been found: phase 6, right after the compiler triple was pinned. PREREQ: the exact compiler identified. What it would have saved: phases 17–22 re-derived the same allocator/scheduler behaviour per function, and repeatedly concluded "unsteerable" for classes that a map later dissolved. Reading the passes once and writing a residual → C-lever | intrinsic→brute-force catalog converted per-function reverse-engineering into cheap lookup, usable by cheap models.

The rule: pin the compiler, then spend one deliberate session reading its allocator, scheduler and CSE passes into a lookup table. Do it before the first "this one is impossible" verdict, not after the twentieth.

A5 — A static scanner must model the compiler's ADDRESSING forms (S46, cookbook §155c)

Found: S46 (phase 30). Could have been found: the first time we scanned for cross-references (phase 3). What it cost us: a register-tracked scan for "who references address X" was blind to gcc's indexed global-array read (lui … addu index … lh lo(base)), where the address lives in the lui and the load with the index add between. It returned zero — and zero was read as a fact about the game. It produced an evening lost to a phantom hunt (S45 p4), four failed static scans (S45 p5/p6), and a written, wrong structural conclusion in memory-map.md ("the index never appears in CODE at all — invisible to any fleet-wide code scan"). It appears in code; we couldn't see it.

The rule: before trusting a scanner's negative, hand-disassemble one known-good case and confirm the scanner sees it. An exactly-zero result is more often a decoder gap than a discovery (§155b). Keep a control the scanner must reproduce, and fail the tool if it can't (tools/idxtab_map.py does).

A6 — Regroup the residue by STRUCTURE before calling it unique

Found: phases 24–25. Could have been found: as soon as a fleet of similar binaries existed. What it would have saved: a "36,000 unique hand-decompiles" frontier was a grouping artifact; regrouped by instruction skeleton, 90% collapsed into ~986 families. The endgame arithmetic changed completely. (Caveat, byte-proven later: structural families are TEMPLATES, not free dedup — cracking one makes the rest fast, not free.)

The rule: when the remaining work looks like a wall of unique functions, try a looser fingerprint before accepting the estimate.

A7 — The load map (tools/idxtab_map.py, docs/idxtab-map.md)

Found: S46 (phase 30). Could it have helped earlier? Partly — and the honest answer matters. PREREQ: the per-binary loader wrapper identified + the fleet onboarded + ≥2 byte-proved control rows. Those did not exist before ~phase 27, so the map itself could not have.

But the thing that blocked it (A5) existed from day one, and the map's shape is worth copying early: once you can name the loader wrapper, build payload → owning binary → load address as a controlled, regenerable table. Onboarding a new binary needs exactly one fact — its load address — and this project spent multiple sessions per binary deriving that by hand (a TLO-law oracle, a runtime tracer, four failed value scans, "the parked five"). A map that answers it as a lookup, gated by controls, is worth building the day the wrapper is understood.

And its limit, recorded so it isn't over-trusted: absence from the map is NOT evidence a payload is dead — byte-proved loaders are absent from that route too, because several load routes exist.

A8 — Parallelism defaults for fleet-wide tools (processes vs threads; batch what you already know)

Found: S46 (phase 30). Could have been found: the first time a tool iterated the fleet — phase 13, when check-all first existed. Measured: dedup_propagate 24 min → 11.4 min, same 29 functions, +62 more member instances, R22 213/213 both ways.

Four rules, each learned from a specific failure:

1. Return every verdict a sweep already computed. The byte-gate built all 141 overlays and returned only the first failure; the recovery loop then paid a full sweep to rediscover each of the next 137. Convergence went ~138 rounds → 1–3 rounds with no extra builds. A previous session's run died 92 minutes in looking like it was nearly done — it had barely started.
2. PROCESSES for CPU-bound work, threads only for subprocess waits. 138 "independent parallel" searches on a ThreadPoolExecutor kept 0–4 builds alive at load 3 on a 32-core box: the work was regex over 15k-line files and every thread queued on the GIL. The identical logic in a ProcessPoolExecutor: 14–29 builds, load 34.75. Threads remain correct for the byte-gate itself (each is a subprocess.run that releases the GIL).
3. Longest-first scheduling. ex.map starts in list order, so the giant items landing last left 31 cores watching one build for ~25 s of every 56 s sweep. Sort by cost descending; re-sort the results into the caller's order so the verdict is bit-identical to the serial one.
4. Per-item search beats lock-step sweeps when items are independent — and the independence must be argued, not assumed. Here the shared header carries every macro regardless of which sites are instantiated, so writing it once leaves each overlay owning only its own files and build dir.

Two traps it exposed: a fixed temp path (.run/dpcc/t.c) is a correctness bug the day anything runs concurrently (the same fake-isolation class as a shared --work dir); and a pool submitted all at once shares no learning between workers — seed with one item in-process, then fan out with the result, or every worker repeats the same expensive search.

The acceptance test is the transferable part: not a stopwatch — a regression. Revert to the pre-run state, re-run the identical command, require the same output plus a byte-verified fleet. That is what surfaced the over-exclusion in the old path: the faster run was also strictly more correct, and a timing comparison would never have shown it.

A9 — Validate the target list, and let an empty tier TERMINATE the pipeline

Found: S46 (phase 30), the hard way. Could have been found: the first multi-tier wave. Measured cost: a 47-target wave burned 9.7M tokens for 20 matches; ~29 targets were not real, and 87 of 119 agents were spent proving phantoms absent.

Two independent defects that multiplied:

1. No validity gate on the target list. It was derived by joining sig data against corpus.stubs with no check that each address was a real function boundary with an actual .s on disk. Four defect classes, every one diagnosed by the agents after the money was spent: NO-ASM, MID-BODY (an address ~545 instructions inside another function's carve, on a load-delay nop), OUT-OF-RANGE (before the binary's vram start), and ALREADY-DONE (banked in an earlier phase). → tools/validate_targets.py now names each class and exits non-zero.
2. The pipeline short-circuited on MATCH but not on SKIPPED. done = r.status === 'MATCH' meant a non-existent target was not "done": it fell through tier 1 → tier 2 → tier 3, and three agents each re-investigated the same phantom — the third at the most expensive model in the stack. One invalid target cost 3× instead of 1×. → done = (status === 'MATCH' || status === 'SKIPPED'). A tier with nothing to work on must terminate the pipeline, not pass its emptiness downstream.

And the part that stings: wave_snapshot's R32 coverage assertion refused this exact list, reporting 24 of 57 files found. That was the instrument saying the list was wrong. It was read as a path-convention problem and routed around via a different resolver. Same session, fifth instrument warning — and the only one that was correct and overridden.

The rule: before a wave, every target passes a validity gate; and in any cascade, a tier that cannot act ends the chain. Cheap targets must fail cheaply, or the cheapest thing in the pool becomes the most expensive.