How to porting a rather complicated c++ code (Geant4) into GPU ??

[ronaldo1018]

i, I'm pretty new to CUDA programming and I'm having a problem trying to port a part of Geant4 code into GPU.

Geant4 is a particle simulation tool based on c++ program.

The main reasons why we think it difficult is as following:

1. Geant4 simulation uses c++ instead of c programming.

2. Geant4's program structure is a multi-level class ( In other words, it uses class calls class method to
complete the work }

3. In order to let kernel function uses the class function we need to add Tag __device__ __host__ before each function. Could anyone gives some samrt ideas to add this Tag in front of every class's member function ?

4. Geant4's each class uses many class pointer variable. Hence, we need to use CUDA Unified Memory
Mechanism to handle it ( this is a rather difficult part)

And we have use a simple c++ program to test for it

   
    #include <iostream>
    #include <cuda_runtime.h>
    using namespace std;
    
     // Derived class
     class Rectangle
     {
        public:
           Rectangle()
          {
           
          }
          int getArea()
          {
             return (*width * *height);
          }
          int* width;
          int* height;
    };
   
    // Base class
    class Shape
    {
       public:
        Shape()
        {
        }
        Rectangle* rect;
    };
   
    __global__ void change_width(Shape* sha)
    {
        *(sha->rect->width) = 210;
        *(sha->rect->height) = 10;
        return;
    }
   
    int main(void)
    {
        Shape* sha;
        cudaMallocManaged(&sha,sizeof(Shape));
        cudaMallocManaged(&sha->rect, sizeof(Rectangle));
        cudaMallocManaged(&sha->rect->width,sizeof(int));
        cudaMallocManaged(&sha->rect->height,sizeof(int));
        *(sha->rect->width) = 20;
        *(sha->rect->height) = 10;
       change_width<<<1,1,0>>>(sha);
       cudaDeviceSynchronize();

       // Print the area of the object.
       cout << "Total area: " << sha->rect->getArea() << endl;
   
       return 0;
    }

In this code, we can port a 2-level class structure into GPU. And it works.
But you know , Geant4 is a rather big project. I am afraid that our simple idea could not fulfill the job.

Could anyone with good experience in porting c++ code to GPU give some good advice to me??
I really don't have idea how to handle for such a big program.
Thanks for all your help.

Sincerely,
KEVIN