A guard, arithmetic, and two clamps
This one mixes a guard, arithmetic, and clamps. The function leans on an early-return guard to drop junk inputs, runs a little arithmetic, then uses branchless-style clamps to keep the answer in bounds. No new instruction appears — the challenge is recognising machinery you've already built, plus one MWCC habit of welding a clamp's compare onto the arithmetic that just ran.
The trailing dot on add. or subf. records the result in cr0: LT, GT, and EQ indicate whether the signed result is less than, greater than, or equal to zero; SO copies the summary-overflow flag. A following branch can test those flags without a separate cmpwi against zero.
In step(pos, delta, limit), a non-positive limit gets ignored outright, pos moves up by delta, and the result ends up wedged inside [0, limit]:
cmpwi r5,0 # guard: limit <= 0 ?
blelr- # yes -> return pos untouched
add. r3,r3,r4 # pos += delta, and set cr0 from the result
bge- .check_hi # result >= 0 -> skip the floor
li r3,0 # floor: clamp up to 0
.check_hi:
cmpw r3,r5 # result vs limit
blelr- # <= limit -> done, return it
mr r3,r5 # otherwise saturate to limit
blr
The blelr- near the top is the guard, gone the moment it must be. Right after, add. does the arithmetic and, for free, arms the floor's bge-. The tail — cmpw into blelr- into mr — holds the value under the ceiling. You've written each move already; the real exercise is noticing them strung together.
Yours guards and clamps something different, but you read it the same way. The guard and its return value come first, the working arithmetic sits in the middle, and a clamp or two fences the result at the end. Spot a dotted op pressed against a branch with no compare in the gap, and the fusion is showing its face again.
Your task
Write func_8011fd9c to reproduce the target assembly.