Re: Is ther any sense in *= and matrices?

[Ole-Hjalmar Kristensen <oleh@vlinux.voxelvision.no>]

"John R. Strohm" <strohm@airmail.net> writes:

> "Russ" <18k11tm001@sneakemail.com> wrote in message
> news:bebbba07.0306082024.7cebb5df@posting.google.com...
> > tmoran@acm.org wrote in message news:<AmNEa.67177$d51.133670@sccrnsc01>...
> > > >Oh, really? I just did a test in C++ with 3x3 matrices. I added them
> > > >together 10,000,000 times using "+", then "+=". The "+=" version took
> > > >about 19 seconds, and the "+" version took about 55 seconds. That's
> > >   Would you be so kind as to post your code, and what C++ compiler
> > > and what hardware you used?  Your results seem quite different from
> > > other people's.  My old 900MHz Windows 2K machine using MSVC++ 5.0
> > > took 4.38 and 3.28 seconds, a factor of 1.33, and using Gnat 3.15p
> > > on the same machine took 1.38 and 0.85 seconds, a ratio of 1.6
> > > Clearly, there's something substantially different between our
> > > compilers/hardware/code.
> >
> > I'm using gcc 2.95.2 on a Sunblade 2000. I can't post the code, but it
> > is a pretty standard vector/matrix implementation in C++. Actually, it
> > is designed for very efficient indexing, perhaps at the expense of
> > slightly less efficient construction (it has a pointer for each row of
> > the matrix). That might explain part of the difference you are seeing,
> > but certainly not all. Perhaps your choice of C++ compiler is a factor
> > too.
> 
> I think the point that Tom is trying to make is that your results are so far
> out of line with expected reality that he suspects that your optimizations
> for indexing may in fact be pessimizations.
> 

Yes, on modern hardware, having a pointer for each row of the matrix
usually leads to slower code. The reason is that the indexing
operation by multilply/add is actually faster than fetching a pointer
from an array and then adding, thereby incurring an extra memory fetch
and cache pollution. The above holds for large matrices, I haven't
investigated the effects on small matrices.

> Without seeing your code, we aren't going to be able to tell very much.
> 
> There is also this.  You're on a Sunblade.  I'm guessing that is a PowerPC.

It's a SPARC.

 <snip>

-- 
Ole-Hj. Kristensen

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