From: ni...@lysator.liu.se (Niels Möller) Date: Fri, 04 Jan 2013 22:29:58 +0100
> David Miller <da...@davemloft.net> writes: > >> If it's needed for sub_n, then yes that's a bit difficult. I was >> trying to figure out ways to fabricate the needed calculations >> using just subcc and addxc/addxcc but haven't come up with anything >> just yet. > > You could always do the two's complement of one of the operands on the > fly, and then use the same add with carry instructions as in add_n. > > I'm thinking aloud, so I'm sorry if I get this wrong, but I think it's > best to handle the unlikely case of low zero limbs up front. Then it's a > plain negate of the first non-zero limb, and a plain complement for the > remaining limbs; the important thing here is that the negation generates > no additional carries to propagate. > > So compared to add_n, you just get an additional xor with -1 in the loop > (and not on the loop's critical path). I can't guess whether or not that > will be visible in the execution time. Thanks I'll give this a try! _______________________________________________ gmp-devel mailing list gmp-devel@gmplib.org http://gmplib.org/mailman/listinfo/gmp-devel
