If the operand in ST is outside the acceptable range, the result of FYL2XP1 is undefined.
The FYL2XP1 instruction provides improved accuracy over FYL2X when computing the logarithms of numbers very close to 1. When ε is small, more significant digits can be retained by providing ε as an argument to FYL2XP1 than by providing 1+ε as an argument to FYL2X.