Re: efficient vector/matrix operations in Ada

[Jacob Sparre Andersen <sparre@nbi.dk>]

Russ:

I haven't looked at the code generated by "my" compiler, so
I will just come with some general remarks...

> Larry Hazel <lhazel@mindspring.com> wrote in message news:<3B783732.A3A1EB82@mindspring.com>...
[...]
> > Seems to me if a compiler can't generate the most efficient code for
> >
> > A = B + C + D
> >
> >   or
> >
> > A := B + C + D
> >
> > in any language, you need a different compiler.

It doesn't always make a large difference, but I would
definitely prefer a compiler which could do the
optimisation.

> I'm no compiler expert, but I seriously doubt that you are correct
> here. Come to think of it, I'll just say that you are flat out
> wrong--and I'll eat crow if you are right.

I think it is time to go hunting now...

> The difference (in C++)
> between
> 
>     A = B + C + D;
> and
>     A = B; A += C; A += D;
> 
> is more fundamental than a compiler can account for. Don't forget that
> "+" and "+=" are user-defined operators to start with. The former says
> to add B, C, and D, place the result in a temporary matrix, and copy
> that matrix by value back into A. The latter says to copy B into A,
> Add C directly to the result, then add D directly to that result. The
> latter needs no temporary matrices, whereas the former requires a
> temporary for each "+" operator. A special preprocessor could possibly
> convert the former to the latter, but I don't see how a compiler could
> possibly do it.

It shouldn't be too hard for a compiler to figure out if a
user-defined operator, "+", can do the calculation "A := A +
B" without having to allocate an intermediate buffer (at
least all the cases, where the operation is simply a loop
over all elements in the composite type).

If you know that, then it is pretty simple to generate the
two versions of the assignment and compare their execution
times.

> This is all old hat in the C++ community. Ada is a great language, but
> it missed the boat here.

Sorry?

What's the difference between:

   A += B;

and

   Add (Target => A,
        Addend => B);

(except for a few extra characters)?

> By the way, if you think the arithmetic operators should be called
> "+:=", "-:=", etc., I give up.

When you come to the point, where this kind of optimisations
are necessary, it really doesn't matter if the operators are
called "+:=", ":+", "+=" or are written as a procedure call.

Jacob
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