Drew T
1.1 KiB
§422 ★★ — QImode ARITHMETIC VIA (u8)(x - K), AND flag ^ 1 NEEDS ITS OWN TEMP (P31 S71; byte-proven resident/func_800D06E8, 344 ins)
1. A raw-$a0 addiu on a byte value comes from doing the arithmetic in QImode.
mips.h defines no PROMOTE_MODE, so a u8 local really is QImode. Write the range test in QImode —
(u8)(c - 3) < 2 — and widen_operand hands the subtraction a paradoxical subreg (the raw
$a0), after which combine.c:9246's A - C1 vs C2 rule drops the truncation before the sltiu.
Every earlier probe on this function used (u32)(c - 3), which re-widens and loses it. The cast
is not cosmetic: it selects the mode the arithmetic happens in.
2. s1 = (cmp) ^ 1 puts the comparison in the DESTINATION pseudo. That is expand-time subtarget
reuse, and it produces a $s1/$v0 pair the target does not have. Give the flag its own temp:
t = (u32)(r - 0x64) < 0x1E;
s1 = t ^ 1;
Both are one-line respellings that no permuter reaches, because both change which RTL the expander builds rather than the order of anything.