[Bug target/79671] [7 Regression] mapnik miscompilation on armv7hl since r235622

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79671

--- Comment #26 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
What about PODs or say int?  Let's say I have a class with union in it and in
that union say float and char array, then use placement new an int with some
value into the char array in the union.  What will then happen if I default
copy the class (default assignment operator or copy constructor)?  The
following doesn't fail, but I think true_dependence still returns 0 for
(mem/c:SI (plus:DI (reg/f:DI 7 sp)
        (const_int 4 [0x4])) [4 MEM[(int *)&b + 4B]+0 S4 A32])
and
(mem:DI (reg/v/f:DI 0 ax [orig:87 p ] [87]) [1 MEM[(const struct B &)p_4]+0 S8
A32])
where the former is from the placement new store of int and the load is from
the structure assignment.

inline void* operator new(__SIZE_TYPE__, void *p) { return p; }
struct B
{
  float x;
  union U
  {
    float a;
    char b[sizeof (int)];
  } u;
  float y;
};

__attribute__((noinline, noclone)) void
bar (B &x, B &y)
{
  if (x.x != 0 || x.y != 3 || y.x != 0 || y.y != 3)
    __builtin_abort ();
  int f;
  __builtin_memcpy (&f, x.u.b, sizeof (int));
  if (f != 81)
    __builtin_abort ();
  __builtin_memcpy (&f, y.u.b, sizeof (int));
  if (f != 81)
    __builtin_abort ();
}

__attribute__((noinline, noclone)) 
B *
baz (B &x)
{
  return &x;
}

__attribute__((noinline, noclone)) void
foo (int x)
{
  B b { 0.0f, {}, 3.0f }, c;
  B *p = baz (b);
  new (b.u.b) int (x);
  c = *p;
  bar (*p, c);
}

int
main ()
{
  foo (81);
}