Here's GMP's current tuning (for Skylake CPUs, from https://gmplib.org/devel/thres/ https://gmplib.org/devel/thres/) Basecase: up to 520 digits (27 limbs x 19.

Here's GMP's current tuning (for Skylake CPUs, from https://gmplib.org/devel/thres/ https://gmplib.org/devel/thres/) Basecase: up to 520 digits (27 limbs x 19.26 digits per 64-bit limb = 520) Karatsuba: 520 - 1425 digits Toom3: 1425 - 4045 digits Toom4: 4045 - 6068 digits Toom6: 6068 - 8053 digits Toom8: 8053 - 120835 digits FFT: 120835+ digits