.global Sum
    .text
    .align  2

//  x0 is the pointer to data
//  x1 is the length and is reused as `end`
//  x2 is the sum
//  x3 is the current dereferenced value

Sum:
    mov     x2, xzr
    add     x1, x0, x1, lsl 3
    b       2f

1:  ldr     x3, [x0], 8
    add     x2, x2, x3
2:  cmp     x0, x1
    blt     1b

    mov     x0, x2
    ret

    .end