Practical Reverse Engineering p. 79 #7

Question number 7 on page 79 of Practical Reverse Engineering is as follows:

Figure 2-13 illustrates a common routine, but you may not have seen it implemented this way.

Here is the disassembly of the function:

Figure 2-13. Practical Reverse Engineering. © 2014 by Bruce Dang

The ARM processor is in Thumb state. We immediately recognize this is a strlen() routine. There is a bit field clear at the end, whose purpose is unclear. Here is how the function is implemented:

size_t strlen(const char *str)
{
    /* CBNZ R0, loc_100E1D8 */
    if (r0 == NULL)
        return 0;   /* MOVS R0, #0 */

    /* MOV R2, R0 */
    char *index = str;
    
    while (1)               /* loc_100E1E4 */
    {
        /* CMB R3, #0 */
        if (*index == '\0')
            break;

        ++index;            /* ADDS R2, #1 */
    }

    /* SUBS R0, R2, R0 */
    return (index - str);
}