http://benji3up2kxewkqfcq7buxk2xd6zwy3zggnurkrm3l4cvwy2iipvyyad.onion/mirrors/gmpdoc/Assembly-Floating-Point.html
With
some care though six 21x 32->53 bit products can be
used, if one of the lower two 21-bit pieces also uses the sign bit. For the mpn_mul_1 family of functions on a 64-bit machine, the
invariant single limb is split at the start, into 3 or 4 pieces. Inside the
loop, the bignum operand is split into 32-bit pieces.