Passing Host Function as a function pointer in __g

Question

I am currently developing a GPU version of a CPU function (e.g. function Calc(int a, int b, double* c, souble* d, CalcInvFunction GetInv )), in which a host function is passes as a function pointer(e.g. in above example GetInv is the host function of CalcInvFunction type). My question is, if i have to put Calc() function entirely in GPU, i have to pass the GetInv function as a function pointer argument in device function/kernel function, and is that possible?

Answer 1

Yes, for a GPU implementation of Calc, you should pass the GetInv as a __device__ function pointer.

It is possible, here are some worked examples:

Ex. 1

Ex. 2

Ex. 3

Most of the above examples demonstrate bringing the device function pointer all the way back to the host code. This may not be necessary for your particular case. But it should be fairly obvious from above how to grab a __device__ function pointer (in device code) and use it in a kernel.

Answer 2

Finally, i have been able to pass a host function as a function pointer in cuda kernel function (__global__ function). Thanks to Robert Crovella and njuffa for the answer. I have been able to pass a class member function(cpu function) as a function pointer to a cuda kernel. But, the main problem is, i can only pass the static class member function. I am not being able to pass the function not declared as static. For Example:

/**/ __host__ __device__ static int CellfunPtr( void*ptr, int a ); /**/

The above function work because this member function is declared as static member function. If i do not declare this member function as a static member as , /**/ __host__ __device__ int CellfunPtr( void*ptr, int a ); /**/

then it doesnt work.

The complete code has four files.

---

1. First file

/*start of fundef.h file*/

typedef int (*pFunc_t)(void* ptr, int N);

/*end of fundef.h file*/

---

2. Second file

/*start of solver.h file*/

    class CalcVars {

       int eqnCount;
       int numCell;                      
       int numTri;
       int numTet;

    public:
       double* cellVel; 
       double* cellPre;

    /** Constructor */

    CalcVars(
        const int eqnCount_,             
        const int numCell_,          
        const int numTri_,             
        const int numTet_                
    );

    /** Destructor */

    ~CalcVars(void);

    public:

      void 
          CalcAdv();


      __host__ __device__ 
      static int 
          CellfunPtr(
          void*ptr, int a
    );

    };

/*end of solver.h file*/

---

3. Third file

/*start of solver.cu file*/

     #include "solver.h"
     __device__ pFunc_t pF1_d = CalcVars::CellfunPtr;

    pFunc_t pF1_h ;


    __global__ void kernel(int*a, pFunc_t func, void* thisPtr_){
        int tid = threadIdx.x;
        a[tid] = (*func)(thisPtr_, a[tid]); 
    };

    /* Constructor */

    CalcVars::CalcVars(
        const int eqnCount_,             
        const int numCell_,          
        const int numTri_,             
        const int numTet_   

    )
    {
        this->eqnCount = eqnCount_;
        this->numCell = numCell_;
        this->numTri = numTri_;

        this->cellVel = (double*) calloc((size_t) eqnCount, sizeof(double)); 
        this->cellPre = (double*) calloc((size_t) eqnCount, sizeof(double)); 

    }

    /* Destructor */

    CalcVars::~CalcVars(void)
    {
       free(this->cellVel);
       free(this->cellPre);

    }


    void 
    CalcVars::CalcAdv(
    ){

        /*int b1 = 0;

        b1 = CellfunPtr(this, 1);*/

       int Num = 50;
       int *a1, *a1_dev;

        a1 = (int *)malloc(Num*sizeof(int));

        cudaMalloc((void**)&a1_dev, Num*sizeof(int));

        for(int i = 0; i <Num; i++){
            a1[i] = i;
        }

        cudaMemcpy(a1_dev, a1, Num*sizeof(int), cudaMemcpyHostToDevice);

        //copy addresses of device functions to host 
        cudaMemcpyFromSymbol(&pF1_h, pF1_d, sizeof(pFunc_t));


        kernel<<<1,42>>>(a1_dev, pF1_h, this);

        cudaDeviceSynchronize();

        cudaMemcpy(a1, a1_dev, Num*sizeof(int), cudaMemcpyDeviceToHost);


    };


    int 
    CalcVars::CellfunPtr(
        void* ptr, int a
    ){
        //CalcVars* ClsPtr = (CalcVars*)ptr;
        printf("Printing from CPU function\n");
        //int eqn_size = ClsPtr->eqnCount;
        //printf("The number is %d",eqn_size);
        return a-1;

    };

/*end of solver.cu file*/

---

4. Fourth file

/*start of main.cpp file*/

    #include "solver.h"


    int main(){

        int n_Eqn, n_cell, n_tri, n_tetra;
        n_Eqn = 100;
        n_cell = 200;
        n_tri = 300;
        n_tetra = 400;

       CalcVars* calcvars;

       calcvars = new CalcVars(n_Eqn, n_cell, n_tri, n_tetra );

       calcvars->CalcAdv();

       system("pause");

    }

/*end of main.cpp file*/