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From: Jean-Pierre Rosen <rosen@EMAIL.ENST.FR>
Subject: Re: Ada-95 for numerics?
Date: 1996/04/02
Message-ID: <199604021629.SAA00724@email.enst.fr>#1/1
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sender: Ada programming language <INFO-ADA@VM1.NoDak.EDU>
x-sender: rosen@email.enst.fr
comments: Gated by NETNEWS@AUVM.AMERICAN.EDU
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Date: 1996-04-02T00:00:00+00:00
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At 08:28 02/04/1996 GMT, you wrote:
>[...] So I started coding some of _my_ problems in Ada-95 and check
>for the speed. Yet, I am still a beginner in Ada-95, so maybe the coding style
>is somewhat unlucky...

Something you have to remember is that Ada has nice facilities for dealing
with arrays globally. For example, a typical benchmark trap is to translate:

DO 10 I=1,N
   DO 10 J=1,N
10 A(I,J) = B(I,J)

into:
for I in 1..N loop
   for J in 1..N loop
      A(I,J) := B(I,J)
   end loop;
end loop;

The correct translation is of course (assuming N is the size of the array):
A := B;

It is important, because since the compiler KNOWS that global assignment is
available, it will not spend a huge time trying to unwind loops, the way the
FORTRAN compiler does. And your FORTRAN might look faster...

In the same vein, you wrote :
>   function Unit (n : Integer) return Matrix is
>      C : Matrix (1..N, 1..N);
>   begin
>      for i in C'range(1) loop
>         for j in C'first(2)..i loop
>            C(i,j) := 0.0;
>            C(j,i) := 0.0;
>         end loop;
>         C(i,i) := 1.0;
>      end loop;
>      return (C);
>   end Unit;

This should be:
   function Unit (n : Integer) return Matrix is
      C : Matrix (1..N, 1..N) := (Others => (Others => 0.0));
   begin
      for i in C'range(1) loop
         C(i,i) := 1.0;
      end loop;
      return (C);
   end Unit;

If the compiler is not  very clever, generated code will be the same as with
what you wrote. But it is much easier for the compiler  to recognize that a
simple "move byte string" instruction is enough to do the trick.

Can't be worse, likely to be better....
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