BFM-decomp/cookbook/C0138.md

§127 — The -O0 regime: the CONSTANT-OFFSET FOLD, and why -O0 needs its own idiom set (P30 T3 wave, 15 targets)

The -O0 population is now large (the P30 T2 routing put 2,200+ open stubs into -O0 TUs), and a 15-target wave against it showed the cookbook is written almost entirely for -O2. The index fired on only 3 of 15 targets, and multiple agents INDEPENDENTLY re-derived the same idiom — the signature that a body of knowledge has a hole (§124's lesson applied to the index itself).

The idiom they kept re-deriving: the constant-offset fold

At -O0, gcc-2.7.2 folds a constant struct offset into the load's displacement but will NOT fold an indexed one:

p->f      /* -> lbu  $v0, 3($s0)          — offset folded into the load */
p[i]      /* -> addiu $v0,$s0,..; lw 0($v0) — address computed FIRST, then a 0-displacement load */

So when the target shows lbu 3(reg) you must write the member access; when it shows addiu + a 0-displacement load you must write the indexed form. At -O2 these converge and the distinction is invisible — which is why nothing in §1–§126 covers it.

The rest of the -O0 regime (write PLAIN C, and mean it)

• Frame-pointer prologue addu $fp,$sp,$zero (21F0A003) is the detector (§6/§116); every check needs --o0 or you chase a phantom SIZE-MISMATCH.
• Every local is spilled to the frame and reloaded at each use. A 0x18($fp) spill/reload pair is a real named local, not a compiler temp — declare it.
• Load-delay nops and redundant addu rd,rs,$zero copies are normal; do not "clean" them.
• Do not hand-optimize, do not add temporaries to help the compiler. At -O0 the C maps almost 1:1 to the asm; the usual -O2 steering levers (pins, live-length dials, statement reordering) are mostly inert and mostly a distraction.

§127a — §71 (sibling-first) is the strongest -O0 lever, and it beats the index

Several targets fell immediately to an already-banked sibling in the same TU. func_80184868 matched off the exact shape banked hours earlier in the same _o0d file:

s32 ret;  ret = func_8001ABBC(0, 0, &D_xxxxxxxx, 0, 0);  return ret;

Before drafting an -O0 function, read the banked functions in its own _o0* file. An -O0 TU is a near-uniform code regime, so a sibling's shape transfers far more reliably than at -O2.

§127b — the knowledge was in a SOURCE COMMENT, not the cookbook

Two agents reported their decisive levers came from the header comment at the top of src/ov_SC01_077/ov_SC01_077_o0.c, not from docs/. That comment is real, hard-won knowledge sitting where only someone already editing that file will find it. When a lever is discovered in a source comment, promote it to the cookbook and leave a pointer — otherwise every future agent pays to rediscover it, which is exactly what happened here across 12 of 15 targets.