[Bug c/69776] Wrong optimization with aliasing

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69776

Andrew Pinski <pinskia at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |RESOLVED
         Resolution|---                         |INVALID

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
>The last value is wrong, it should be 1.

Why do you think that? 

If we look at your code:


  void *p = malloc(10);
  int *pi = p;
  double *pd = p;


<< at this point p has no effective type.

  *pi = 1;
  printf("*pi = %d\n", *pi);

<< Now it has an effective type of int or a struct containing int

  int a = *pi;

<< a read 

  *pd = 0;

<< a write to a double, it does not alias int at all so it can be moved past
the next statement

  *pi = a;

<< store via an int

Since the order of pi and pd is not specified due to different aliasing of int
and double so either can be done first



  printf("p = %p\n", p);
  printf("*pi = %d\n", *pi);