Everything at once
This target uses most of the chapter. A loop, a call inside it, values that have to survive both, a frame, and a pointer into that frame. Work through it in layers rather than top to bottom.
Start with the register layer. A call in a loop body means every loop-carried value — the cursor, the trip count, the accumulator — is live across a bl, so all of them move into r4-r7 before the loop starts. That is why a loop with a call in it has a push list and a loop without one usually does not:
0 push {r4, r5, r6, r7, lr}
2 mov r5, r0
4 mov r7, r1
6 mov r6, #0
8 mov r4, #0
10 cmp r6, r5
12 bge 30 ~>
14 ~>mov r0, r4
16 mov r1, r7
18 bl rateOf-4
22 add r6, r0
24 add r4, #1
26 cmp r4, r5
28 blt 14 ~>
30 ~>mov r0, r6
32 pop {r4, r5, r6, r7}
34 pop {r1}
36 bx r1
Four saved registers for four live values: the limit in r5, an argument that never changes in r7, the accumulator in r6, the counter in r4. The guard at 10-12 is the for condition tested once before entry, the back-edge at 26-28 is the same condition tested again, and r0 and r1 are rebuilt from the saved copies on every single trip because the call destroys them.
Now add the frame layer, and this is where the surprise is. You saw earlier in this chapter that a local whose address escapes stops living in a register — the memory slot becomes authoritative and the value has to be re-read after any call. Put such a local inside a loop and the cost is charged per trip: the accumulator can no longer be an add into a saved register, it becomes a load, an add, and a store, every iteration, for a pointer that is not used until after the loop is over. gcc does not sink the store or hoist the load out of the loop.
That is what the extra sub sp, #4 in your target is paying for, and it is worth seeing once, because it is the single clearest demonstration on this machine of why taking an address is never free.
The loop itself is the shape you already know from the loops chapter: a guard before the body, a ldmia rN!, {r0} walking the array one word at a time, and a counter rewritten to count down to zero because testing against zero is free. The example above counts up instead, and the reason is visible in its listing at address 14 — its counter is also one of the call's arguments, so gcc has to keep the real value of i around.
Your task
extern s32 weigh(s32 v); and extern void report(s32 *slot, s32 n); are declared for you. Write func_083177e0 to reproduce the target assembly.