The exponent computation in the function prec in Preloaded relies on a non-standard compiler extension:

"It's undefined behavior to read from the member of the union that wasn't most recently written. Many compilers implement, as a non-standard language extension, the ability to read inactive members of a union." (https://en.cppreference.com/w/cpp/language/union).

I recommend to compute the exponent with the frexp function.