Drew T
2.4 KiB
§395 ★★★ — FIVE NARROWING/PLACEMENT LEVERS FROM ONE 91-INSTRUCTION CRACK (P31 S69; byte-proven ov_SC06_018/func_80189E60, warm start 32 off → MATCH 91/91)
All five are TYPE or STATEMENT-POSITION changes. None is a register pin or a barrier, and four of them are invisible in the diff until you know to look for them.
(a) shorten_binary_op narrows a division to HImode, and a CAST DOES NOT STOP IT. Writing
short / 25 makes gcc-2.7.2 do the divide in HImode. An (s32) cast on the dividend does not
defeat it — get_narrower sees straight through the NOP_EXPR. The only lever that works is to
route the dividend through a real s32 TEMP, assigned in its own statement.
(b) A clamp local must be s16 to produce the target's plain MOVE. A HImode pseudo makes
b = <int expr> a plain move — which IS the target's addu $s3,$v0,$zero — and the subsequent
compare's sign-extend then reads the pre-copy $v0. All four s32 spellings coalesce that copy
away; a register pin restores the copy but cse then canonicalises the sll onto b's register and
the residual plateaus at 1. The type is the lever; the pin is a trap.
(c) Narrow the argument INSIDE the expression, not around it. (u16)(x | ((b << 8) | 0x3000))
makes convert_to_integer distribute the narrowing INTO the shift, emitting sll $a1,$s3,8. Casting
outside gives the sll 16/sra 8 pair instead. The trailing andi is then just the u16→int
promotion, not a mistake to chase.
(d) base = &D_XXXXXXXX must be the FIRST statement in the function. Anywhere else and
REG_ALLOC_ORDER hands it $a1 — costing two callee-saved registers and 50 mismatches. Note sched1
still sinks the lui/addiu into the multiply shadow afterwards, so the source position and the
emitted position differ: do not "fix" what looks like a misplaced address load.
(e) A parameter's declared width is a sched1 dial. u16 arg1 vs s32 arg1 + a mask moved
addu $s4,$a1,$zero by three slots. When a prologue copy sits a few instructions off and nothing
in the body explains it, try narrowing a parameter before touching the body.
Meta-lesson for the reach lanes: this function is a REACH EXEMPLAR (its cluster has six members). Five levers bought one crack — and the other five members inherit them for free via remap or a seeded card. Spend the deep effort on the exemplar, never on the sibling.