Re: Static vs dynamic evaluation anomaly?

[Stefan Lucks <lucks@th.informatik.uni-mannheim.de>]

On Thu, 8 Feb 2007, Jeffrey R. Carter wrote:

> Adam Beneschan wrote:
> > Hmmm.  This is certainly not the way I would expect a "perform this
> > check" or "don't perform this check" flag to work.  To me, if the
[...]
> I understand your logic; I'm not even saying I disagree. But perhaps the
> compiler writer's view is that it's OK to store the larger value as long
> as the extra bits are never used. Then the error is in the I/O, which is
> using the extra bits.

This is a valid point -- except that the extra bits are not just used by
the IO, but also, e.g., for comparisons:

with Ada.Text_IO;
use  Ada.Text_IO;
procedure test272 is
  type Six_Bits is mod 2**6;
  X: constant Six_Bits := 31;
  Z: constant Integer  := 31;
  T: integer;
begin
  for J in 0..9 loop
    T := (Integer(X) * 2**Integer(J)) mod Six_Bits'Modulus;
    Put (Integer 'Image         (J) &
         Six_Bits'Image  (X * 2**J) &
         Integer 'Image  (T)          );
    if (X * 2**J) = Six_Bits(T) then
      Put_Line(" equal");
    else
      Put_Line(" different!!!");
    end if;
  end loop;
end test272;

With "gnatmake" (no option) or "gnatmake -gnatp" I get wrong results:

./test272
 0 31 31 equal
 1 62 62 equal
 2 124 60 different!!!
 3 248 56 different!!!
 4 240 48 different!!!
 5 224 32 different!!!
 6 192 0 different!!!
 7 128 0 different!!!
 8 0 0 equal
 9 0 0 equal


-- 
Stefan Lucks      Th. Informatik, Univ. Mannheim, 68131 Mannheim, Germany
            e-mail: lucks@th.informatik.uni-mannheim.de
            home: http://th.informatik.uni-mannheim.de/people/lucks/
------  I  love  the  taste  of  Cryptanalysis  in  the  morning!  ------