mantbits
mantbits(U *d)
i = fivesbits[e] + mantbits(&rv) <= P;
ldbl_mant_type mantbits, sq_mantbits;
mantbits = ((ldbl_mant_type)1 << (mantdig / 2)) - 1;
mantbits > ((ldbl_mant_type)1 << (mantdig / 2 - 1));
i++, mantbits--) {
sq_mantbits = mantbits * mantbits;
x = ldexpl(mantbits, (exp - 2 - mantdig) / 2);