Three idioms, one function
Chapter capstone. Real functions don't use one idiom at a time — they stack a guard on an index on an update, and you read the stack from the top down. A worked example first, with the same spirit: tally reads two neighboring bytes, but only if the table exists:
s32 tally(u8 *v, s32 i) {
if (v == NULL) {
return 0;
}
return v[i] + v[i + 1];
}
0: bnez a0, 0x10 # table present? skip the bail-out
4: addu v1, a1, a0 # (slot) &v[i] — the byte-array duo, hoisted!
8: jr ra
c: or v0, zero, zero # (slot) the NULL result, 0
10: lbu t6, 1(v1) # v[i + 1] — neighbor via offset
14: lbu t7, 0(v1) # v[i]
18: addu v0, t6, t7
1c: jr ra
20: nop
The instructive line is the first delay slot: the address computation for v[i] runs before the guard has decided anything. If v is NULL the addu still executes — computing a garbage address is harmless as long as nothing loads from it, and IDO exploits that to keep the slot busy. Work in a slot may belong to the path that isn't taken; ask "who uses this register later?" rather than assuming slot work is universal.
From there it's pure vocabulary: duo with offsets 0 and 1 = byte neighbors, or v0, zero, zero in the early-out slot = return 0.
The target is the writing counterpart: guard a word table for NULL, address one element by variable index, and nudge it in place by the third argument. Guard, trio, sandwich — read each layer, then write them in order.
Your task
Write func_803f6350 to reproduce the target assembly.