CODEWITHSUNDEEP
c++parallel-processinggpugpgpuc++-amp
Caesar Yu
Caesar Yu

Reputation: 43

C++ AMP nested loop

I'm working on a project that requires massive parallel computing. However, the tricky problem is that, the project contains a nested loop, like this:

for(int i=0; i<19; ++i){
    for(int j=0; j<57; ++j){
        //the computing section
    }
}

To achieve the highest gain, I need to parallelise those two levels of loops. Like this:

parallel_for_each{
    parallel_for_each{
        //computing section
    }
}

I tested and found that AMP doesn't support nested for loops. Anyone have any idea on this problem? Thanks

Upvotes: 4

Views: 1018

Answers (2)

Ade Miller
Ade Miller

Reputation: 13723

You could, as @High Performance Mark suggest collapse the two loops into one. However, you don't need to do this with C++ AMP because it supports 2 and 3 dimensional extents on arrays and array_views. You can the use an index as a multi-dimensional index.

array<float, 2> x(19,57);
parallel_for_each(x.extent, [=](index<2> idx) restrict(amp)
{
    x[idx] = func(x[idx]);
});

float func(const float v) restrict(amp) { return v * v; }

You can access the individual sub-indeces in idx using:

int row = idx[0]; 
int col = idx[1];

You should also consider the amount of work being done by computing section. If it is relatively small you may want to have each thread process more than one element of the array, x.

The following article is also worth reading as just like the CPU if your loops do not access memory efficiently it can have a big impact on performance. Arrays are Row Major in C++ AMP

Upvotes: 4

High Performance Mark
High Performance Mark

Reputation: 78324

So collapse the loops:

for(int ij=0; ij<19*57; ++ij){
        //if required extract i and j from ij
        //the computing section
    }
}

Upvotes: 0

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