Where arguments come from
The rule for arguments is short: the first four arrive in r0, r1, r2, r3, left to right. Anything past the fourth goes on the stack, which is a later chapter's problem.
Notice that the first argument register and the return register are the same one. That overlap has a consequence worth seeing straight away. Here are two functions that each hand back one of their two arguments:
0 mov r0, r1
2 bx lr
That one returns its second argument: r1 holds it, r0 has to hold it, so one mov copies it across. The other function returns its first argument instead — and its entire body is this:
0 bx lr
Nothing at all. The value was already sitting in the register it needed to leave in, so the compiler emitted no instructions whatsoever. A bare bx lr is a complete, valid function, and when you see one you should immediately ask what the input and output would have to be for there to be no work to do.
Your task
Write identity, taking an s32 x, to reproduce the target assembly.