Drew T
2.3 KiB
§66d-5 — residual_class's "structural ⇒ permuter CPU is waste" is WRONG for schedule permutations (measured, Phase 29 SESSION-18)
tools/residual_class.py documents its buckets (lines 55–58) as:
structural — local mutation CANNOT introduce it (a different access width, a flipped branch, a multi-instruction shape change). Spending permuter CPU here is waste; it wants an idiom.
Byte-measured counterexample. func_8014D820 was classed OPCODE-MIXED [structural], and later
WIDTH [structural] sig=WIDTH/lhu!=sh, at every waypoint — and the permuter moved it 16 → 14 → 12
→ 10 across three weight profiles. Six points of gain inside a bucket whose documented guidance is
"don't run the permuter." Meanwhile the guidance the bucket does give ("read it") was exhausted:
eleven source-shape attempts on the same window, all inert or worse.
Why the classifier is fooled. It reasons position-by-position. A pure schedule permutation
changes what lands at every index in the affected window — so a block whose instructions are all
present but reordered presents as "different operations, no single family" (OPCODE-MIXED) or as a
width flip (WIDTH), because index i now holds a sh where the target holds an lhu. The signature
of a reorder is indistinguishable, per-position, from a genuine shape change. The classifier's
SHIFT-DRIFT rule catches this only when ONE shift point re-aligns the tail; a permutation within a
window with matching endpoints re-aligns nowhere.
The corrected routing rule:
[permuter]⇒ search, as before.[structural]⇒ read first — the idiom is the cheap win when there is one.- But
structuralis NOT a permuter veto. If reading fails 2–3 times AND the instruction COUNTS match AND the same multiset of operations appears in the window in a different order, treat it as a schedule permutation and run the three profiles (§66d-4). Instruction-count equality is the tell: a genuine shape change usually changes the count; a permutation never does.
Cost of the old reading: this is a project-wide mis-route, not a one-off — every schedule-permuted residual that ever landed in OPCODE-MIXED/WIDTH was steered away from the one tool that moves it. Worth a targeted re-check of backlog entries carrying those two class tags with equal instruction counts.