Thanks for the quick reply, however, I think you misunderstand my question and perhaps my knowledge of pointer arithmetic. As stated in the question, I am very aware of the literal meaning of the code, what I am looking for is a semantic understanding of what it does. The code from sun's fdlibm.h is as follows:
looking to understand what is found in hx and if I can reproduce the same effect in a different way with bit shifting. This is so I can port it to java.
#define __HI(x) *(1+(int*)&x)
double __ieee754_atan2(double y, double x)
hx = __HI(x);