@@@ Iterative Fibonacci function in ARM assembly.
        .cpu cortex-m4
        .thumb
        .syntax unified
        .thumb_func
        .globl fib
fib:    mov r1, #1  @ XXX these two instructions are wide!  Why‽
        mov r2, #1
    1:  subs r0, #1
        itt lo
        movlo r0, r1
        bxlo lr
        mov r3, r2
        adds r2, r1
        mov r1, r3
        b 1b