Re: GNAT does not consistently raise an exception to log(0.0)

[Jerry <lanceboyle@qwest.net>]

On Saturday, July 13, 2013 2:37:20 AM UTC-7, AdaMagica wrote:
> On Saturday, July 13, 2013 2:52:08 AM UTC+2, Jerry wrote:
> > with Ada.Numerics.Long_Elementary_Functions; 
> > use  Ada.Numerics.Long_Elementary_Functions;
> > procedure Bomb_Log is
> >     x : Long_Float;
> > begin
> >     x := log(0.0);
> > end Bomb_Log;
> >
> > However, under the same conditions as above, the following program raises the exception always:
> >
> > with Ada.Numerics.Long_Elementary_Functions; 
> > use  Ada.Numerics.Long_Elementary_Functions;
> > with Ada.Text_IO;
> > use  Ada.Text_IO;
> > procedure Bomb_Log is
> >     x : Long_Float;
> > begin
> >     x := log(0.0);
> >     Put_Line(Long_Float'Image(x));
> > end Bomb_Log;
> 
> That's easy. In the first version, the compiler optimizes X away, since it's not used for any external effects.

Yes, as Simon has also shown with the generated code differences.
> 
> In the second version, you output an invalid value.
No--I only attempt to output an invalid value. The Put_Line is never executed because the exception is raised before that, at the log(0.0) level.

> See RM on allowed optimizations.

I suppose that would be 11.6(5):
http://www.adaic.org/resources/add_content/standards/12rm/html/RM-11-6.html

Jerry