BFM-decomp/cookbook/C0413.md

§372 ★★★ — THE COPY-CAPTURE PAIR, AND THE ONE ZERO-BYTE EDIT THAT DEFEATS BOTH (P31 S68; byte-proven main/func_8003491C, 78 ins, fable escalation closed 5 → 0)

THE TELL. A REGALLOC-PERM residual whose wrong-register rows read the destination of a nearby MATCHING copy insn (map $dst > $src) — e.g. your narrow/compare reads $a2 where the target reads $v0, while the addu $a2,$v0,$zero copy itself matches. This is NOT an allocation tie, and pins make it worse.

TWO SEPARATE PASSES re-base uses of a value onto a copy's DESTINATION:

1. cse.c make_regs_eqv — after (set P X), P becomes the quantity's CANONICAL register whenever X is a non-fixed hard reg ("prefer pseudo to hard") or P outlives the basic block with a later last-use. canon_reg then rewrites every LATER same-EBB use of the value to P.
2. local-alloc.c optimize_reg_copy_1 (live at -O2 via flag_expensive_optimizations) — a single_set REG-REG copy whose source does NOT die in the copy (e.g. after sched1 hoists the copy above the source's last use) forward-substitutes dest for src up to src's death.

BOTH DIE TO THE SAME EDIT. Spell the copy as a PLUS:

register s32 zr __asm__("$0");
P = X + zr;

SET_SRC is no longer a reg-reg copy (kills #2) and cse records no register equivalence (kills #1) — and it emits the byte-identical canonical move encoding addu $rd,$rs,$zero, so it is zero-cost and safe to leave in the banked source.

ORDERING COROLLARY. Uses that must read the SOURCE register (a narrow, a compare) must precede the copy in SOURCE order — canon_reg walks source order — and sched1 will still hoist the dependence-free copy above them to match a copy-first target schedule.

AND WHERE THE TARGET KEEPS A LIVE COPY OF AN INCOMING ARGUMENT (addu $aN,$a0,$zero at entry), the same + zr on the ENTRY def is what keeps that copy insn alive at all.

Relationship to §368 — read both, they are different animals. §368's tell is a wrong-register row whose COMMUTATIVE OPERANDS are also swapped, and its lever is an uncolorable single-set local routed through reload. Here the rows are pure shift/slti with no commutative operands, the tell did not hold, and the RTL dumps named copy-capture instead. The escalation was told to CHECK whether §368's tell applied rather than assume it, reported that it did not, and found the real cause — which is the procedure §361 asks for, working as intended.