请大家帮我看一下这个宏定义
#define CALC_SUMD(s1, s2, sd) { \
(s1) = ((s1) & 0xffff) + ((s1) >> 16); \
(s2) = ((s2) & 0xffff) + ((s2) >> 16); \
/* Do it twice */ \
(s1) = ((s1) & 0xffff) + ((s1) >> 16); \
(s2) = ((s2) & 0xffff) + ((s2) >> 16); \
/* Because ~1 == -2, We really need ~1 == -1 */ \
if ((s1) > (s2)) (s2)--; \
(sd) = (s2) - (s1); \
(sd) = ((sd) & 0xffff) + ((sd) >> 16); }