[C] que fait cette fonction ?

/*  assume 32 bit int  */
 
 typedef int int32_t;
 typedef unsigned int u_int32_t;
 
 /*  assume little endian  */
 typedef union 
 {
   double value;
   struct 
   {
     u_int32_t lsw;
     u_int32_t msw;
   } parts;
 } ieee_double_shape_type;

 
 /* Get the more significant 32 bit int from a double.  */
 
 #define GET_HIGH_WORD(i,d)                                      \
 do {                                                            \
   ieee_double_shape_type gh_u;                                  \
   gh_u.value = (d);                                             \
   (i) = gh_u.parts.msw;                                         \
} while (0)

gh_u.value = (d);                                             \
   (i) = gh_u.parts.msw;