请教一个函数的功能

Question

本人水平比较菜，下面这个函数怎么看都看不出功能，主要是X7在这里的功能是什么。请教热心人士帮忙解答一下。

__inline int32 sad_4pixel(int32 src1, int32 src2, int32 mask)
    {
        int32 x7;

        x7 = src2 ^ src1;       /* check odd/even combination */
        if ((uint32)src2 >= (uint32)src1)
        {
            src1 = src2 - src1;     /* subs */
        }
        else
        {
            src1 = src1 - src2;
        }
        x7 = x7 ^ src1;     /* only odd bytes need to add carry */
        x7 = mask & ((uint32)x7 >> 1);
        x7 = (x7 << 8) - x7;
        src1 = src1 + (x7 >> 7); /* add 0xFF to the negative byte, add back carry */
        src1 = src1 ^(x7 >> 7);   /* take absolute value of negative byte */

        return src1;
    }