Re: Static vs dynamic evaluation anomaly?

[Matt.Jaffe@gmail.com]

On Feb 7, 3:21 am, Georg Bauhaus <bauh...@futureapps.de> wrote:
> On Wed, 2007-02-07 at 11:00 +0100, Ludovic Brenta wrote:
> > Steve writes:
> > > Adam Beneschanwrote
> > >> On Feb 6, 9:29 am, MattJaffewrote:
>
> > >> The following program produces incorrect results on GNAT, or at least
> > >> on the version I'm using:
>
> > >> with Ada.Text_IO;
> > >...
> > >> end test271;
>
> > > One more data point... ObjectAda 7.2.2 gives the result:
>
> > > When J= 0, X*2**J = 31
> > > When J= 1, X*2**J = 62
> > > When J= 2, X*2**J = 60
> > > When J= 3, X*2**J = 56
> > > When J= 4, X*2**J = 48
> > > When J= 5, X*2**J = 32
>
> > > Which I believe is correct,
> > > Regards,
>
> > Actually I'll take back what I said earlier.  With Debian gnat
> > 4.1.1-22 I also get these seemingly correct results.
>
> It seems to depend on a compiler switch, could you try
>
> $ gnatmake -gnatp -s test271
> $ gnatmake -gnato -s test271
>
> -- Georg

You are correct; the anomaly does not appear when the -gnato option is
used for the compilation.  And since I have now looked up the
significance of the -gnato option, I now know why; sort of.  I am
disturbed, however, in that the default (no overflow checking) allows
the production and execution of code yielding what seem to me to be
semantically invalid results.  To allow the deliberately disabling,
for performance reasons, of a feature that insures semantic compliance
may be something that a compiler should allow to broaden its appeal;
but to have default behavior that is not compliant with the language's
semantics seems very odd to me.  Or am I mis-interpreting the
situation here?