Converting thrust::iterators to and from raw pointers

Question

I want to use Thrust library to calculate prefix sum of device array in CUDA. My array is allocated with cudaMalloc(). My requirement is as follows:

main()  
{  
     Launch kernel 1 on data allocated through cudaMalloc()  
     // This kernel will poplulate some data d.  
     Use thrust to calculate prefix sum of d.  
     Launch kernel 2 on prefix sum.  
}

I want to use Thrust somewhere between my kernels so I need method to convert pointers to device iterators and back.What is wrong in following code?

int main()                                                        
{                                                                 
    int *a;                                                   
    cudaMalloc((void**)&a,N*sizeof(int));   
    thrust::device_ptr<int> d=thrust::device_pointer_cast(a);  
    thrust::device_vector<int> v(N);                    
    thrust::exclusive_scan(a,a+N,v);                          
    return 0;                                                  
}                     

Answer 1

A complete working example from your latest edit would look like this:

#include <thrust/device_ptr.h>
#include <thrust/device_vector.h>
#include <thrust/scan.h>
#include <thrust/fill.h>
#include <thrust/copy.h>
#include <cstdio>

int main()                                                        
{                                                                 
    const int N = 16;
    int * a;
    cudaMalloc((void**)&a, N*sizeof(int));   
    thrust::device_ptr<int> d = thrust::device_pointer_cast(a);  
    thrust::fill(d, d+N, 2);
    thrust::device_vector<int> v(N);                    
    thrust::exclusive_scan(d, d+N, v.begin());

    int v_[N];
    thrust::copy(v.begin(), v.end(), v_);
    for(int i=0; i<N; i++)
        printf("%d %d\n", i, v_[i]);     

    return 0;                                                  
}

The things you got wrong:

1. N not defined anywhere
2. passing the raw device pointer a rather than the device_ptr d as the input iterator to exclusive_scan
3. passing the device_vector v to exclusive_scan rather than the appropriate iterator v.begin()

Attention to detail was all that is lacking to make this work. And work it does:

$ nvcc -arch=sm_12 -o thrust_kivekset thrust_kivekset.cu 
$ ./thrust_kivekset

0 0
1 2
2 4
3 6
4 8
5 10
6 12
7 14
8 16
9 18
10 20
11 22
12 24
13 26
14 28
15 30

---

Edit:

thrust::device_vector.data() will return a thrust::device_ptr which points to the first element of the vector. thrust::device_ptr.get() will return a raw device pointer. Therefore

cudaMemcpy(v_, v.data().get(), N*sizeof(int), cudaMemcpyDeviceToHost);

and

thrust::copy(v, v+N, v_);

are functionally equivalent in this example.

Answer 2

Convert your raw pointer obtained from cudaMalloc() to a thrust::device_ptr using thrust::device_pointer_cast. Here's an example from the Thrust docs:

#include <thrust/device_ptr.h>
#include <thrust/fill.h>
#include <cuda.h>

int main(void)
{
    size_t N = 10;

    // obtain raw pointer to device memory
    int * raw_ptr;
    cudaMalloc((void **) &raw_ptr, N * sizeof(int));

    // wrap raw pointer with a device_ptr 
    thrust::device_ptr<int> dev_ptr = thrust::device_pointer_cast(raw_ptr);

    // use device_ptr in Thrust algorithms
    thrust::fill(dev_ptr, dev_ptr + N, (int) 0);    

    // access device memory transparently through device_ptr
    dev_ptr[0] = 1;

    // free memory
    cudaFree(raw_ptr);

    return 0;
}

Use thrust::inclusive_scan or thrust::exclusive_scan to compute the prefix sum.

http://code.google.com/p/thrust/wiki/QuickStartGuide#Prefix-Sums