Optimizer: replace const reference with const object

Question

Chandler Carruth in his talk on compiler optimization said that compilers are terrible at optimizing functions with parameters passed by reference. I can understand that it is difficult when parameter is a non-const reference, since the compiler has to deal with memory, or when type of the parameter is complex (some weird structure or class). But if the parameter is const reference and a builtin type are there really any problems? Can optimizer replace const float& with const float? It can be even more helpful when use of SSE instructions is enabled since compiler will be able properly align data for them.

Answer 1

Can optimizer replace const float& with const float?

No, they cannot do that, because it may change the semantic of the program. A const reference is still a reference. It cannot be replaced by value. Consider this example:

void foo(const float& x, float a[]) {
    cout << x << endl;
    a[0] += 10.5;
    cout << x << endl;
}

int main() {
    float a[1] = { 3.25 };
    foo(a[0], a);
    return 0;
}

This prints

3.25
13.75

Demo 1

If you change const float& with const float, the result would be

3.25
3.25

Demo 2

The issue here is that a[0] is the same as x, but the connection is established by the caller, which is outside of optimizer's control.