BFM-decomp/cookbook/C0087.md

§76 — The allocno CLASS (local vs global) is the dominant regalloc lever, and C reaches it ONLY through declaration scope and variable reuse (Phase 29 SESSION-19, behemoth #3 func_8017F510 1,511 ins, 97 → MATCH, pin-free)

Behemoth #3 sat at 97/1511 mismatched after a full pass at a lower reasoning tier that had already localized every mismatch, exhausted ~10 C-shape levers, and run 3,663 permuter candidates at base 97 with zero improvement. The residual was not in the C-randomisation search space — and the reason is the reusable lesson: four of the five decisions were local-vs-global allocno CLASS choices, which C expresses only through declaration scope and variable reuse. Statement order, expression shape, register pins and random search cannot reach them.

The mechanism, with citations

• local-alloc.c:472 admits a pseudo to LOCAL allocation only when REG_BASIC_BLOCK >= 0 && REG_N_DEATHS == 1. A function-scope variable used in four emit arms has 4 deaths ⇒ it becomes a global allocno ⇒ combine_regs (local-alloc.c:1825) can no longer tie its producer chain into it (global-alloc has no coalescing), so an in-place sra/sll/addu chain on that register is unreachable. Declaring the same variable inside each arm makes it four 1-death local pseudos and the chain ties.
• global.c:668-671 re-marks every pseudo that local-alloc already placed (reg_renumber[i] >= 0) as a hard register for global-alloc's conflict scan. So a local pseudo's placement removes a register from the global pool, and that shows up as a hard-reg number in the ;; N conflicts: tail of the .greg dump. In this function a global otp in $a2 pushed a per-tail constant into $a0, which made the target's otp = $a0 structurally impossible — not merely lower-priority. Read the ;; N conflicts: hard-reg tail before reasoning about priorities.
• global.c:594 allocno_compare ranks by floor_log2(n_refs)*n_refs/live_length, with refs counted × loop depth. Reusing an existing variable as a second temp (§45-A / RC-14 MERGE) raises its ref count — here cb 27 → 39 refs — and flips the grant ORDER of a 3-way colouring (cb→$a1, c3→$a2, tp→$a3 instead of tp→$a2, cb→$a1, c3→$a3). Recomputing that formula off the .lreg dump reproduces the ;; N regs to allocate: order exactly, so you can PREDICT a flip instead of searching for one.

The attribution primitive (use this before calling anything a scheduling residual)

Compile the draft with -fno-schedule-insns and again with -fno-schedule-insns2. If a transposed instruction pair keeps source order under both, the ordering was fixed at RTL expansion, not by either scheduler — so the lever is source statement order, and no amount of sched.c reasoning applies. That test converted this function's residual A from "needs a third form of a scheduling tie-break" into a one-line source swap.

Two diagnosis traps this function proved

1. A "scheduling" diff can be a register grant in disguise. Residual B (an OT tag materialised before a two-insn constant) was never a sched.c choice: once the unlit colour temp was granted $a1, the 0xFFFFFF constant — also wanting $a1 — could not be materialised until that temp died, so sched2 slid it below the tag by itself. Fixing the allocation fixed the "schedule".
2. A "single seed cascade" model can be wrong even when the cascade is real. The prior dossier attributed ~93 of the 97 to one seed (c3 must win $a2). c3 turned out to have no lever of its own — it moves only as a side-effect of cb out-ranking tp. The cascade was real; the seed was not the thing you can steer. Steer the ranked variable, not the symptom variable.

Practice

1. When a residual survives a full permuter sweep at its measured base, stop searching and ask which allocno CLASS decisions the C is failing to express. Search cannot reach a class choice.
2. Sweep declaration scope at three granularities — function / innermost-if / per-ARM — for every temp in the divergent block. Per-arm is a distinct granularity from per-case and is frequently the one that matters (here: per-case = neutral, per-arm = the crack).
3. Then sweep variable REUSE (merge two temps into one) to move allocno_compare priority.
4. Keep the measured result of every lever; behemoth #3's session produced a ~50-row do-not-re-buy table (.run/giants/s19_f510_report.md) that is worth more than the match itself.