Three values alive at once
When two computed values must both outlive calls, they take r31 and r30 (the first-declared survivor gets r31). Stack a third preserved value on top and a final call that consumes all of them, and you get the densest register juggling this chapter teaches: two saved-register slots, several survival moves, and a full argument marshalling pass right before the last bl.
Consider twoway(x, y), which converts each input with t() and combines the two results arithmetically:
stwu r1,-16(r1)
mflr r0
stw r0,20(r1)
stw r31,12(r1)
stw r30,8(r1)
mr r30,r4 # y kept for the second call
bl t # t(x)
mr r31,r3 # first result -> r31 (declared first)
mr r3,r30
bl t # t(y)
mullw r0,r31,r3 # first * second
subf r3,r31,r0 # ... - first
lwz r0,20(r1)
lwz r31,12(r1)
lwz r30,8(r1)
mtlr r0
addi r1,r1,16
blr
Here both t() results survive: the first lands in r31, the second is fresh in r3, and the tail is plain arithmetic. The r31/r30 pair and their paired stw/lwz at 12(r1) and 8(r1) are the signature of two live saved values.
The target assembly for fuse keeps the two-survivor structure but ends in a call instead of arithmetic — and it carries a third value through to that final call as well. After the two conversions, watch the registers get marshalled into r3, r4, r5 before the last bl. Recover which parameter feeds each conversion, which one is passed straight through, and the order they land in.
Your task
Write fuse, which converts two of its inputs and passes all the pieces to a final call, to reproduce the target assembly. conv and join are declared for you.