a question

Timothee Ewart Timothee.Ewart at unige.ch
Wed Feb 20 23:11:40 CET 2013

thank you very much both of you.

Le 20 févr. 2013 à 18:42, Torbjorn Granlund <tg at gmplib.org> a écrit :

> Your message is a bit vague, but I'll try to rely as well as I can.
> So, you're using twos complement bignums.  Is the n you mention fixed?

Yes I am using only twos complement method. And  I can give you more infos of what I did.

As C++ dev, I am working for physicists who have with fixed size (128 -512 bits) pb.
I designed integer class which is instantiated by the "user" (multiple of 64 bit word), with meta programming  I let the compiler
generate specific ASM solver to every operations. (one more time, my pbs  are fixed size, I start to work in 256 bit I finish at 256 bit).

For the multiplication, I integrated 3 different one,  integer<n bits> *= long, integer <n bits> *= integer <n bits>, and a free function multiply which return integer <2 * nbits> 
The sign was really a pb for the last one ... but presently the performance are very good. 

> If not, how come you're interested in just n x n -> 2n multiply, it
> seems you would need n x m -> mn multiply.

> If n is just a word, there are some tricks of converting between signed
> n x n -> 2n and unsigned dito.  See GMP's longlong.h, somewhere not to
> far from the end.

Great  I will have a look

> If n is arbitrary, I think it might be best to multiply the absolute
> values.

well what I do more and less presently

> -- 
> Torbjörn



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