At -O2 we should IMNSHO generate the same code for all 3 functions:
typedef union
{
  struct
  {
    int f1, f2, f3, f4, f5, f6, f7, f8;
    long int f9, f10;
    int f11;
  } f;
  char s[56];
  long int a;
} T;

void
foo (void)
{
  T t;
  t = (T) { { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 } };
  test (&t);
}

void
bar (void)
{
  T t = { { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 } };
  test (&t);
}

void
baz (void)
{
  T t;
  t = (const T) { { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 } };
  test (&t);
}

as nothing took the address of the compound literals, it was only used as initializer to some other variable.  But instead foo/baz clear a temporary (compound literal var) and then copy it over.  This is at least partly a regression from 3.2.x which cleared both the compound literal variable (wasn't
able to DSE it) and the t variable.

Without a union we generate better code:
typedef struct
{
  int f1, f2, f3, f4, f5, f6, f7, f8;
  long int f9, f10;
  int f11;
} T;

void
foo (void)
{
  T t;
  t = (T) { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 };
  test (&t);
}

void
bar (void)
{
  T t = { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 };
  test (&t);
}

void
baz (void)
{
  T t;
  t = (const T) { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 };
  test (&t);
}
but still bar uses on x86_64-linux rep stosq, while the other functions
store it by pieces.