Practical Reverse Engineering p. 79 #10

Question number 10 on page 79 of Practical Reverse Engineering is as follows:

Figure 2-16 is a function from Windows RT. Read MSDN if needed. Ignore the security PUSH/POP cookie routines.

Here is the disassembly of the function:

Figure 2-16. Practical Reverse Engineering. © 2014 by Bruce Dang

 The ARM processor is in Thumb state, but transfers out during some of the syscalls. The function queries different clock APIs depending on the size of the supplied struct.

size_t QueryChrono(size_t *bytes_copied, 
                   size_t max_size, 
                   struct *clock_info)
{
    /* MOVS R4, #0 */
    bytes_copied = 0;

    /* CMP R5, #0x10 */
    if (max_size >= 16)
    {
        SYSTEMTIME sysTime;           /* SUB SP, SP, #0x10 */
        GetSystemTime(&sysTime);      /* LDR R3, =__imp_GetSystemTime */

        /* LDR R3, [SP,#0x1C+var_1C] */
        /* LDR R3, [SP,#0x1C+var_18] */
        /* LDR R3, [SP,#0x1C+var_14] */
        /* LDR R3, [SP,#0x1C+var_10] */
        /* STR R3, [R6,#0xC] */
        memcpy(clock_info->sysTime0x0, &sysTime, sizeof(SYSTEMTIME));

        bytes_copied = 16;              /* MOVS R4, #0x10 */
    }

    /* SUBS R3, R5, R4 */
    /* CMP R3, #4 */
    if ((max_size - bytes_copied) >= 4)
    {
        /* LDR R3, =__imp_GetCurrentProcessId */
        /* STR R0, [R6,R4] */
        *(clock_info + bytes_copied) = GetCurrentProcessId();

        bytes_copied += 4;              /* ADDS R4, #4 */
    }


    /* SUBS R3, R5, R4 */
    /* CMP R3, #4 */
    if ((max_size - bytes_copied) >= 4)
    {
        /* LDR R3, =__imp_GetTickCount */
        /* STR R0, [R6,R4] */
        *(clock_info + bytes_copied) = GetTickCount();

        bytes_copied += 4;              /* ADDS R4, #4 */
    }


    /* SUBS R3, R5, R4 */
    /* CMP R3, #9 */
    if ((max_size - bytes_copied) >= 8)
    {
        /* MOV R0, SP */
        LARGE_INTEGER perfCount;

        /* LDR R3, =__imp_QueryPerformanceCounter */
        QueryPerformanceCounter(&perfCount);

        /* STR R3, [R6,R4] */
        /* STR R3, [R2,#4] */
        memcpy((clock_info + bytes_copied), &perfCount,
                sizeof(LARGE_INTEGER));

        bytes_copied += 8;              /* ADDS R4, #8 */
    }

    return bytes_copied;                /* MOV R0, R4 */
}