问ARM GAS汇编程序中的反转字符串返回NULL

EN

        ldr r2, [r0], r3 /* start at end of string */
        add r3, #-1

reverse_string_func:
        PUSH {ip, lr}
        LDR r1, =input_string           @ pointer to first string.
        LDR r2, =reverse_string         @ pointer to second string
        mov r3, #0
mv2end:  /* try to find the end of the string */
        ldr r1, [r1], #1
        add r3, #1
        cmp r1, #0
        bne mv2end
        /* found end of string, move on */
string_copy:
        ldr r2, [r0], r3 /* start at end of string */
        add r3, #-1
        cmp r3, #0
        bne string_copy /* move back for next character */

        POP {ip, pc}

reverse_string_func:
            PUSH {ip, lr}
        LDR r1, =input_string           @ pointer to first string.
        LDR r4, =reverse_string         @ pointer to second string
        mov r3, #0
mv2end:  /* try to find the end of the string */
        ldrb r2, [r1, r3]
        add r3, #1
        cmp r2, #0
        bne mv2end
        /* found end of string, move on */
        add r3, #-2 /* on terminator, move back past terminator, and line feed */
string_copy:
        add r3, #-1
        ldrb r2, [r1, r3]
        strb r4, [r1, r3] @ <--- I believe I'm going wrong here, but how?
        cmp r3, #0
        bne string_copy /* move back for next character */
        POP {ip, pc}