如何使用C ++ AMP并行化以下循环？

Question

I have the following loop in c++

dword result = 0;
for ( int i = 0; i < 16; i++ ) {
    result |= ( value[i] << (unsigned int)( i << 1 ) );
}

And I would like to parallelize it in amp. I know it might go slower then the actual non-parallelized version above, but I want to do it to learn something more about AMP.

My idea was to loop trough the value array in parallel:

And fill a new array with newarray[0] = value[0] << (unsigned int)(0 << 1 ) , newarray[1] = value[1] << (unsigned int)(1 << 1 ) , etc. Then I would OR the values in the array in parallel in a tree structure (see image).

I have tried to put this idea in some simple c++ amp code, but I don't succeed in it, so any help would be appreciated.

Thank you for your consideration of this matter, I look forward to a response.

Answer 1

The following code is part of what I think you need. This code will take a number of elements as input and preps the vector on the CPU, then it does the bit shift operations in parallel on the GPU. Then I set av[elements] back to 0 because I am using that element to store your final result. It's rough, but AMP is pretty restrictive about what data types can be processed on the GPU, so I just use an extra element of the existing array for it. After the bit shifting is done, I do another parallel for each for the bitwise OR function. This one also happens on the GPU, but it is less satisfactory because every operation is ORing any given element of the array with exactly the av[elements] element, so that will create a bottleneck. Your tree structure will make this part run much more quickly, but I was unable to figure out how to do that part easily. As it is, this program can process 100 million elements in a couple seconds on a fairly old computer. Apologies in advance for any best-practice violations in the code; I am a novice as well. The code follows:

#include <conio.h>
#include <amp.h>
#include <iostream>

using namespace concurrency;
using namespace std;

unsigned int doParallel(unsigned int);

unsigned int elements;

void main()
{
    int ch=NULL;
    cout<<"\nHow many elements to populate: ";
    cin>>elements;
    cout<<"The result is: "<<doParallel(elements);
    cout<<"\nPress 'X' to exit.";
    do
    {
        ch=_getch();
    } while (ch!='X' && ch!='x');
    exit(0);
}

unsigned int doParallel(unsigned int elements)
{
    vector<unsigned int> v(elements+1);

    for (unsigned int i = 0; i<elements+1;i++)
        {
            v[i]=i;
        }
    array_view<unsigned int,1> av(elements+1,v);

    parallel_for_each(av.extent,[=](index<1> idx)
        restrict(amp)
        {
            av[idx] = static_cast<unsigned int>(av[idx])<<1;
        });
    av[elements]=0;
    parallel_for_each(av.extent,[=](index<1> idx)
    restrict(amp)
    {
        av[elements] |= static_cast<unsigned int>(av[idx]);
    });

    return av[elements];
}