Re: In place matrix multiplication?

[Bill Findlay <yaldnifw@blueyonder.co.uk>]

On 1/6/03 13:54, in article
BxnCa.16707$Mu3.366629@newsfep4-glfd.server.ntli.net, "chris.danx"
<spamoff.danx@ntlworld.com> wrote:

> I've simplified it by just using a floating point type, rather than an
> abstraction (the abstraction provided only additional complexity and
> little benefit otherwise).

Matrix multiplication is useful with matrices having integer elements and
boolean elements (e.g. representing relations, and graph-theory adjacency
and path length).

It would be interesting to know how well versions instantiated with float,
integer and boolean types compare in terms of performance with non-generic
versions, especially using GNAT.

> Now I'm wondering if the cost of the more
> complex code is more expensive than doing an out of place multiplication
> followed by a copy back to the original array.  I guess that'd depend on
> the code and the compiler.  How do I find out which is cheaper in time
> for a given compiler (GNAT)?

For large (square) matrices, in-place lets you run a 50% bigger problem in
the same amount of cache. It also much improves locality of reference.
I would guess that these effects would more than outweigh the extra copying
done by the in-place version. But I could be wrong. You need to run some
tests and see ...

-- 
Bill-Findlay chez blue-yonder.co.uk ("-" => "")

P.S. I hope the exams went well!