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From: "John R. Strohm" <strohm@airmail.net>
Newsgroups: comp.lang.ada
Subject: Re: Is ther any sense in *= and matrices?
Date: Mon, 9 Jun 2003 00:17:24 -0500
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"Russ" <18k11tm001@sneakemail.com> wrote in message
news:bebbba07.0306082027.2c50eb6b@posting.google.com...
> "Bobby D. Bryant" <bdbryant@mail.utexas.edu> wrote in message
news:<pan.2003.06.08.22.56.37.297066@mail.utexas.edu>...
> > On Sun, 08 Jun 2003 11:51:31 -0700, Russ wrote:
> >
> > > "John R. Strohm" <strohm@airmail.net> wrote in message
> > > news:<bbumef$fnq@library2.airnews.net>...
> >
> > >> With all due respect, ladies and gentlemen, it has been known for a
> > >> very long time that the difference in "efficiency" between A := A + B
> > >> and A += B is lost in the noise floor compared to the improvements
that
> > >> can be gotten by improving the algorithms involved.
> > >
> > > 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
just
> > > shy of a factor of 3, folks. If that's your "noise floor," I can't
help
> > > wonder what kind of "algorithms" you are dealing with!
> >
> > I'm just curious why the compiler didn't generate the same code for both
> > versions.
>
> The compiler didn't generate the code. I did.

Why?

If you are hand-generating assembly code for a PowerPC processor, I would
bet a fair number of doughnuts that your cache utilization is shaky at best,
and THAT is what is generating your factor of 3 difference.