FFT is already integrated in the libraries, e.g.: "GNU multiple precision arithmetic library" https://gmplib.org/manual/Multiplication-Algorithms.html#Multipl

FFT is already integrated in the libraries, e.g.: "GNU multiple precision arithmetic library" https://gmplib.org/manual/Multiplication-Algorithms.html#Multiplication-Algorithms https://gmplib.org/manual/Multiplication-Algorithms.html#Mul... "NxN limb multiplications and squares are done using one of seven algorithms, as the size N increases. Algorithm Threshold ... FFT MUL_FFT_THRESHOLD"