Here's a version of the 15-bit to 30-bit patch
that adds in a souped-up version of Mario Pernici's
faster multiplication.
I did some testing of 100x100 digit and 1000x1000 digit
multiplies. On 32-bit x86:
100 x 100 digits : around 2.5 times faster
1000 x 1000 digits : around 3 times faster.
On x86_64, I'm getting more spectacular results:
100 x 100 digits : around 5 times faster
1000 x 1000 digits: around 7 times faster!
The idea of the faster multiplication is quite simple:
with 30-bit digits, one can fit a sum of 16 30-bit by
30-bit products in a uint64_t. This means that the
inner loop for the basecase grade-school multiplication
contains one fewer addition and no mask and shift.
[Victor, please don't delete the old longdigit4.patch!]