struct MatMul {
    const float* A;
    const float* B;
    float* C;
    int    N;

    void operator()(int n) const {
        int     i = n / N;
        int     j = n % N;
        float acc = 0.0f;
        for (int k = 0; k < N; ++k)
            acc += A[i * N + k] * B[k * N + j];
        C[n] = acc;
    }
};