[Bug tree-optimization/49367] missed optimization with __restrict field

[jason at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49367

--- Comment #2 from Jason Merrill <jason at gcc dot gnu.org> 2011-06-13 
18:11:04 UTC ---
(In reply to comment #1)
> As a1 and a2 are not restrict qualified they may point to the same object
> and thus the "two" restrict pointers are based on each other.

Marking them with restrict doesn't help:

typedef struct A
{
  int *__restrict p;
} A;

void g();

void f (A*__restrict a1, A*__restrict a2)
{
  *a1->p = 0;
  *a2->p = 1;
  if (*a1->p != 0)
    g();
}

int main()
{
  A a,b;
  f (&a,&b);
}

Saving the inside pointers into __restrict-qualified temporaries doesn't make
it work, either:

typedef struct A
{
  int *__restrict p;
} A;

void g();

void f (A* a1, A* a2)
{
  if (a1 == a2)
    return;

  int *__restrict a1p = a1->p;
  int *__restrict a2p = a2->p;

  *a1p = 0;
  *a2p = 1;
  if (*a1p != 0)
    g();
}

int main()
{
  A a,b;
  f (&a,&b);
}

But at this point, if I remove the __restrict from the declaration of p, it
works.

typedef struct A
{
  int * p;
} A;

void g();

void f (A* a1, A* a2)
{
  if (a1 == a2)
    return;

  int *__restrict a1p = a1->p;
  int *__restrict a2p = a2->p;

  *a1p = 0;
  *a2p = 1;
  if (*a1p != 0)
    g();
}

int main()
{
  A a,b;
  f (&a,&b);
}

It seems rather odd that the __restrict on p would prevent the optimization...