GCC Devirtualization and Inlining only first Interface

Question

i found a problem in the following code:

It looks like GCC can only devirtualize the first Interface I_Udc. The Interface GCC is not able to devirtualize the second interface I_Uac. If I write I_Uac first, the I_Uac call will be inlined.

Here is the code at compiler explorer: https://godbolt.org/z/z6WEoznfW

class I_Uac
{
    public:
    virtual float GetIUac() = 0;
};

class I_Udc
{
    public:
    virtual float GetIUdc() = 0;

};

class DataAcq final : public I_Udc, public I_Uac
{
    float GetIUac() 
    {
        return r;
    }

    float GetIUdc()
    {
        return m;
    }

    private:
        float r = 20;
        float m = 20;
};

DataAcq temp;

I_Uac& temp1 = temp;
I_Udc& temp2 = temp;


int main ()
{    
    volatile float r = temp1.GetIUac();
    volatile float m = temp2.GetIUdc();
}

So the question is how can i force the compiler to inline both function calls via the interface reference?

*"call non-virtual thunk to DataAcq::GetIUac()"*. seems devirtualized, but not inlined. — Jarod42, Apr 22 '21 at 12:25
If you use gcc master (called trunk on godbolt), it does inline. — Marc Glisse, Apr 23 '21 at 06:48

score 1 · Accepted Answer · answered Apr 22 '21 at 20:38

The best/only way I know to guarantee devirtualization is to not virtualize in the first place. A slight change to your classes allows for a CRTP approach, though it's a tad uglier, especially when referring to the base class types, it's entirely doable.

#include <cstdio>

template <typename DerivedT>
class I_Uac {
public:
    float GetIUac() { 
        return static_cast<DerivedT*>(this)->GetIUac();
    };
};

template <typename DerivedT>
class I_Udc {
    public:
    float GetIUdc() { 
        return static_cast<DerivedT*>(this)->GetIUdc();
    };
};

template <template <typename...> class... BaseTs>
class DataAcq final : public BaseTs<DataAcq<BaseTs...>>... {
public:
    float GetIUac() 
    {
        std::puts("GetIUac"); // much nicer in compiler explorer output
        return r;
    }

    float GetIUdc()
    {
        std::puts("GetIUdc");
        return m;
    }

    private:
        float r = 20;
        float m = 20;
};

DataAcq<I_Udc, I_Uac> temp;
I_Udc<decltype(temp)>& temp1 = temp;
I_Uac<decltype(temp)>& temp2 = temp;

int main ()
{    
    volatile float r = temp1.GetIUdc();
    volatile float m = temp2.GetIUac();
}

https://godbolt.org/z/EMExdoczs

GCC Devirtualization and Inlining only first Interface

1 Answers1