[Bug tree-optimization/69776] Wrong optimization with aliasing

[rguenth at gcc dot gnu.org]

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

--- Comment #5 from Richard Biener <rguenth at gcc dot gnu.org> ---
extern void *malloc (__SIZE_TYPE__);
extern void abort (void);

void __attribute__((noinline,noclone))
foo (int *pi)
{
  if (*pi != 1)
    abort ();
}

int
main()
{
  void *p = malloc(sizeof (double));
  int *pi = p;
  double *pd = p;

  *pi = 1;
  int a = *pi;
  *pd = 0;
  *pi = a;
  foo (pi);

  return 0;
}


FRE removes *pi = a because it stores the same value as *pi = 1 which is
found via

          val = vn_reference_lookup (gimple_assign_lhs (stmt),
                                     gimple_vuse (stmt), VN_WALK, NULL);

and thus treats the *pi = a store as a load, losing the fact that it
cannot TBAA disambiguate against other stores.