Extend Sliding Window Table

[EvaOrLew Baxter]

I see that powlo.c and powm.c have tables used to choose the best k-ary sliding window for modular exponentiation. The table ends at 28161, so if the number of bits in the exponent is more than 28161 it always chooses k=10.

My application has exponents from about 74000 to 340000 bits and my (non-gmp) experiments show that k= 11 or k=12 is better than k=10. The improvement is minor (about 0.1% to 0.8%).

Is it safe for me to modify both powlo.c and powm.c to add entries into the table?

(I initially installed libgmp3-dev, but then built gmp and got a big improvement. One calculation of x^y mod z with 30000 digits takes 52 sec compared to 88 sec -- many thanks Torbjorn!)

If I modify the c files, do I now only need to 'make' and also 'make install'?

As for the best k, depending on bit size, is there some math used to optimize k, or did you do it experimentally?

Would you consider adding best k values in a future release of gmp?

Thanks
Lewis