CUDA parallelization - cuda

Question

I'm having trouble doing the parallelization on an array of numbers with CUDA.
So, for example if we have an array M containing numbers ( 1 , 2 , 3 , 4 , 5)
And If I were to remove the number 2 in the array and shift everything to the left,
the resulting array would be ( 1 , 3 , 4 , 5 , 5 )
where M[1] = M[2], M[2] = M[3] , M[3] = M[4]
And my question is how can we do this in parallel in cuda? Because when we parallel this
there might be a race condition where the number 2 (M[1]) might not be the first one to
act first, if M[2] were the first one to shift, the resulting array would become
( 1 , 4 , 4 , 5 , 5). Is there any method to handle this? I'm fairly new to cuda so I'm
not sure what to do...
My current code is as follows:
__global__ void gpu_shiftSeam(int *MCEnergyMat, int *seam, int width, int height, int currRow)
{
int i = blockIdx.x * blockDim.x + threadIdx.x;
int j = blockIdx.y * blockDim.y + threadIdx.y;
int index = i+width*j;
if(i < width && j <height)
{
//shift values of -1 to the side of the image
if(MCEnergyMat[i+width*j] == -1)
{
if(i+1 != width)
MCEnergyMat[index] = MCEnergyMat[index+1];
}
if(seam[j] < i)
{
if(i+1 != width)
MCEnergyMat[index] = MCEnergyMat[index+1];
}
}
}
Where seam[i] contains the index I would like to remove in the array. and MCEnergyMat is just a 1D array converted from a 2d array... However, my code does not work... and I believe race condition is the problem.
Thanks!

Answer 1

As talonmies notes in his comment, this sort of thing is called "stream compaction". Here's how you would do it with Thrust:
#include <thrust/device_vector.h>
#include <thrust/host_vector.h>
#include <thrust/remove.h>
#include <iostream>
int main()
{
int data[5] = {1,2,3,4,5};
thrust::device_vector<int> d_vec(data, data + 5);
// new_end points to the end of the sequence after 2 has been thrown out
thrust::device_vector<int>::iterator new_end =
thrust::remove(d_vec.begin(), d_vec.end(), 2);
// erase everything after the new end
d_vec.erase(new_end, d_vec.end());
// prove that it worked
thrust::host_vector<int> h_vec = d_vec;
std::cout << "result: ";
thrust::copy(h_vec.begin(), h_vec.end(), std::ostream_iterator<int>(std::cout, " "));
std::cout << std::endl;
return 0;
}
Here's the result:
$ nvcc test.cu -run
result: 1 3 4 5