Correct way to sum 128bit numbers represented by 4 32bit unsigned integers?

Question

I'm creating a kernel in C++ / CUDA in which I need to sum two 128bit numbers represented as follows:

struct Int128 {
    unsigned int bit0_31;
    unsigned int bit32_63;
    unsigned int bit64_95;
    unsigned int bit96_127;
};

Int128 sum(Int128 a, Int128 b) {
    ...?
}

What is the most efficient C++ way to achieve this? Thanks.

P.S. Overflow to 129bit can be ignored.