The index that becomes a cursor
Written out literally, a[i] is an address calculation: take the base, scale the index by the element size, add. Done that way it costs a shift and an add every trip, and it keeps the base and the index both live for the whole loop. gcc usually avoids it. It notices the address moves by a fixed amount each trip and keeps the address in a register instead of the index, advancing it by the element size at the end of the body. The scaling is gone and so, usually, is the index.
Here is a loop that fills a halfword array with 32:
0 cmp r1, #0
2 ble 16 ~>
4 mov r2, #32
6 ~>strh r2, [r0, #0]
8 add r0, #2
10 sub r1, #1
12 cmp r1, #0
14 bne 6 ~>
16 ~>bx lr
The body is a store through r0 at offset zero, then add r0, #2. Nothing multiplies anything. r0 arrived as the array pointer and is being walked forward across the loop, and the count in r1 is doing the countdown from the previous lesson, so the two of them together carry all the state the loop needs.
The stride is the element size, and that makes it the most useful number in the listing. add rP, #2 means halfwords; #1 means bytes; #4 means words. Read the stride and you know the pointer type before you have read anything else.
mov r2, #32 sits above the loop rather than inside it. Thumb's strh has no immediate source operand, so the value needs a register, and since it never changes gcc materializes it once in the preheader — the space between the guard and the loop top. Constants that appear there are loop-invariant values hoisted out of the body.
One variation to expect: sometimes the cursor is the argument register itself, as here, and sometimes gcc copies the pointer into a different register first with a mov after the guard. It makes the copy when the body needs the argument register for something else, such as holding a loaded value.
Your target's loop is built the same way, with a load where this one has a store. Read its stride first, then work out what the body does with each value it brings in.
Your task
Write func_081811a4 to reproduce the target assembly.