Pass-by-Value or Pass-by-Reference for Trivialish Structs

struct A
{
    int x, y, z;
};

struct B
{
    A l, u;
};

struct C
{
    B t, f;
};

// sizeof(A) == 3*sizeof(int)
// sizeof(B) == 6*sizeof(int)
// sizeof(C) == 12*sizeof(int)

// foo() does not copy a. Should b be passed by value or by reference?
void foo(A a);
void foo(A const& a);

// bar() does not copy b. Should b be passed by value or by reference?
void bar(A a);
void bar(A const& a);

// buzz() does not copy c. Should c be passed by value or by reference?
void buzz(A c);
void buzz(A const& c);