Drew T
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§106 — Persist the MEASUREMENT, derive the POLICY: a stored route let a stale file out-vote the live table (Phase 29 T54, residual_class._ROUTE)
residual_class answers two different questions in one pass. klass is a measurement — it comes
from comparing two instruction streams and is expensive. (profile, bucket) is a policy — a table
lookup over klass that says which tool should work on it. autopsy persisted both to
.run/autopsy/residuals.jsonl, and verdicts() read both back.
That made the corpus authoritative for a decision the table owns, with two consequences:
- a route correction was inert. Editing
_ROUTEchanged nothing until someone re-ran the whole collect — so the fix and its effect were separated by an expensive step that is easy to skip. - a weeks-old row could silently contradict the live table, and nothing would ever report the disagreement (there is no oracle comparing a stored policy against the current one — R34's blind spot).
The fix: re-derive the route at read time from the stored klass + detail. The measurement stays
persisted; the policy is looked up fresh on every read.
The subtlety that makes this a technique rather than a one-liner: one route is magnitude-dependent.
LENGTH-DRIFT is permuter-shaped only when |delta| <= 2 AND explains == "tail" (§60b). A naive
re-derivation from klass alone would silently demote those rows. Both inputs are already in detail,
so route_for(klass, detail) reproduces the override exactly — verified at 1610/1610 against the
stored corpus with the table UNCHANGED, before the table was edited. Prove the derivation is faithful
first, then change the policy; otherwise a bug in the derivation is indistinguishable from the intended
change.
The route change itself (the reason this came up): ADDRESSING was routed to the permuter, which
contradicted residual_class' own bucket definition — "structural — local mutation CANNOT introduce
it … it wants a C-level idiom." The §10/§20 hoist-vs-remat shape is a multi-instruction change with a
documented deterministic recipe (gcc-2.7.2-map/cse_expr.md §2). Measured: both admitted ADDRESSING
targets plateaued under a §31-directed permuter, and the class was 32% of the entire admission pool
(18 of 56). After the fix: 56 → 38, exactly 18 rows changed, all ADDRESSING, nothing else moved.
And the bound, kept in the comment where the next reader will hit it: T31's finding 4 byte-tested
the §2 recipe on func_80132F40 across six variants and it never closed. structural here does not
promise a free fix — it means "a search over local mutations is the wrong tool; try the documented
idiom", exactly what WIDTH / BRANCH-POLARITY / IMM-OFFSET already mean.
The law: persist what was measured; derive what was decided. If a stored field can be recomputed from other stored fields plus a table, storing it converts a future correction into a silent no-op — and the staler the file, the more confidently it lies.