Drew T
2.2 KiB
§428 ★★★ — A ZERO-BYTE CROSS-JUMP BARRIER: ADVANCE THE POINTER INSIDE EACH SWITCH ARM (P31 S72; main/func_80026D64, 218 ins, MATCH in 2 compiles)
Sharpens §5a/§336 and supplies the missing precondition to §162. The classic cross-jump defeat is
a volatile __asm__ barrier — visible, and something the original source cannot have contained. This
one costs zero bytes and is ordinary C.
The shape. A switch whose arms each advance a cursor. Written with one shared temp —
switch (kind) { case A: adv = *(u16 *)p * 2; break; case B: adv = *(u16 *)p * 4; break; ... }
p += adv; /* ONE converging `addu $s0,$s0,$v0` */
— gcc's cross_jump tail-merges the arms and you get LENGTH DRIFT (measured −23 instructions,
5 spurious merges). Write the advance inside each arm instead:
case A: p += *(u16 *)p * 2; break;
case B: p += *(u16 *)p * 4; break;
Why it works (jump.c:1988). The converging addu is now a label created by cross_jump
itself via get_label_before, so its INSN_UID is >= max_uid. jump.c's jump_chain search is
guarded by INSN_UID(JUMP_LABEL(insn)) < max_uid, so for a label born during this pass the
minimum=2 (jump-to-jump) search never runs at all. Only the minimum=1 path survives — which is
exactly the single merge the target has. The barrier is not a fence you add; it is a UID the
optimizer cannot look up.
Reach for it when: a switch/if-chain draft is byte-correct except that too many tails merged, and §5a's asm barrier would work but you want a spelling the original could plausibly have had.
The companion, and why the wave found it. Case SOURCE order still had to be fixed independently
(0x38 before 0x34, because the jtbl puts *0x18 at .L80026F3C and *0x1C at .L80026F58) —
§405-A: match_one compares .text only, so the agent verified the .rodata 29-word table
byte-identical against jtbl_80072BFC, plus 6 jal relocs at their exact offsets and 11
D_800A5E60 hi/lo pairs, before calling it done. That verification happened because the pack
carried the §426 carve note telling it to. Journal fuel earned its keep here.