[Bug tree-optimization/58143] [4.8/4.9 regression] wrong code at -O3

["mikpelinux at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58143

--- Comment #26 from Mikael Pettersson <mikpelinux at gmail dot com> ---
(In reply to Bernd Edlinger from comment #24)
> Wall,
> 
> the logic behind the loop niter is really strange
> and the results are simply insane.
> 
> When you consider this example:
> 
> int a, b, c, d, e;
> 
> int
> main ()
> {
>   for (b = 4; b > -30; b--)
>     {
>     e = 2147483647 - b;
>     for (; c;)
>       for (;;)
>         {
>           if (d)
>             break;
>         }
>     }
>   return 0;
> }
> 
> then the reasoning is that an "undefined" value will
> be written to e in loop iteration #4.
> And therefore it does not matter what happens afterwards.
> But for the result of main() which will ultimately be 0
> the undefined value of e does not matter.
> 
> Why should a undefined value that is not used for anything
> create such a problem?

To re-emphasize what Richard wrote: it's undefined BEHAVIOUR not VALUE.  The
mere fact of causing undefined behaviour voids all guarantees about the future
behaviour of that process.  That the variable "e" above is unused is
immaterial.