BFM-decomp/cookbook/C0411.md

§370 ★★ — A HARD BOUND FROM sched.c, AND THE reorg SLOT-STEAL DIAGNOSTIC (P31 S68; main/func_8001BC6C, 69 ins, NOT closed — 33 → 28 over ~45 measured compiles)

This entry is a NEGATIVE result, and that is why it is worth its space: it tells you when to stop. A fable escalation drove this function to 28 and did not close it. What it PROVED is reusable.

THE HARD BOUND. sched.c schedule_select always fronts a ready load over an equal-priority ALU leaf (potential_hazard). Therefore no C spelling can emit an ALU chain before loads that are simultaneously-ready, same-priority leaves. If the target shows that order, the cause is NOT your source order — look for reorg slot-steals, hard-register dependency walls, or late in-block consumers before burning compiles on statement permutations.

THE SLOT-STEAL DIAGNOSTIC. A dying-operand ALU op sitting in a conditional branch's delay slot (or $a1,$a2,$a1) whose registers are untouched by the compare/address tail is reorg fill_simple_delay_slots stealing the FIRST ELIGIBLE insn scanning back from the branch (eligible = shares no register with anything between it and the branch).

THE PRECONDITION FOR REPRODUCING IT. The value must accumulate OUTSIDE the $v0-heavy tail, so spell the or-tree SPLIT, not fused:

hi  = c << 16;                 /* early */
hi |= (c << 8) | tag;          /* mid   */
color = hi | c;                /* late  */

The accumulator is then $a2 and is eligible. The fused one-statement form accumulates in $v0, is INELIGIBLE, and reorg steals a different insn instead.

Three supporting levers that each moved bytes: (1) pin the first-loaded field to $2 (register short f0 __asm__("$2")) — every consumer then reads $v0, creating post-reload anti-dependencies against all later $v0 writers; (2) two source reads of the same cell get two single-set temps, never one reused multi-set tmp (alloc then reuses $v0 for both loads); (3) fuse the colour expression into ONE late statement to fix a t5/t6 allocation swap.

REFUTED here, with measurements — do not re-derive:

• a dead-init f0 = 0; as a boost-kill: cse delete_dead_from_cse removes it before the final reg_scan (toplev.c:2925), so reg_n_sets stays 1 and it is a NO-OP;
• §350 re-ties on loads in a DENSE block: each re-tie is a real stream insn that re-anchors its load and delays the consumers — measured +4 to +9;
• the -fno-schedule-insns oracle: BOTH scheduling passes actively build the target here, so neither ablation approaches it. §353's pass-identification oracle does not discriminate when the residual is a multi-pass composition.

AND THE FRAMING LESSON (with §361). The residual is not a single sched1 tie, which is what the cheaper tier reported. Read off -dS/-dR dumps plus sched.c, it is a four-pass composition: sched1 birthing-boost layout → local-alloc birth order → sched2 hard-register anti/output webs (ADJUST_COST zeroes anti/output costs) → reorg slot-steal. A split+§350-re-tie variant (37) reproduces the target's slot, or-tree and sll 16-at-top EXACTLY, so the target is the union of one variant's load block and another's chain block — and no single spelling produced both. Per §361 the escalation first REMOVED the prior agent's pin and re-measured: the pin is exonerated for the head (identical head residual unpinned) and load-bearing only for the tail. That is the §361 procedure applied correctly, and it is why this diagnosis can be trusted where the previous one could not.