http://benji3up2kxewkqfcq7buxk2xd6zwy3zggnurkrm3l4cvwy2iipvyyad.onion/mirrors/gmpdoc/Subquadratic-GCD.html
GCD is then implemented as a loop around HGCD, similarly to Lehmer’s
algorithm. Where Lehmer repeatedly chops off the top two limbs, calls mpn_hgcd2 , and applies the resulting matrix to the full numbers, the
sub-quadratic GCD chops off the most significant third of the limbs (the
proportion is a tuning parameter, and 1/3 seems to be more efficient
than, e.g., 1/2 ), calls mpn_hgcd , and applies the resulting
matrix.