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

[AdaMagica <christ-usch.grein@t-online.de>]

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.

In the second version, you output an invalid value.

See RM on allowed optimizations.