how to implement a "steps" of for loop in C++AMP

Question

Hi.

I've succeeded to implement for loop in C++ AMP.

But if the for loop has a steps more than 2.

e.g.  for(int i=5;i<100;i+=2) { (job)  }

in this case I implement like this for now,

parallel_for_each(extent,[=](index<1> idx) restrict(amp)

{

   int i=idx[0];

   if( i >=5 &&  i % 2==1)

   {

       (job)

   }

}

I am thinking this coding is a bit awkward.

Is there any more elegant coding? or this is only way to this?

thanx for reading .

Answer 1

This is a common indexing problem, where the parallel_for_each idx represents a single GPU thread but the computation may not use the same thread-like index.

I usually try to think of the extent as the number of worker threads I want to launch and then map the idx from the lambda to my problem's data indexes.

For the problem above you are 'wasting' the first 5 threads and every other one after that. That's not a very efficient use of GPU threads. What I would do is pre-compute the number of threads you will actually need.

Your above loop will execute: (100-5) / 2 times.

Consider the following for-loop as an equivalent:
int num = 100;
int skip = 5;
int num_iterations = (num - skip) / 2;

for(int idx = 0; idx < num_iterations; idx++) {
   int i = skip + (idx * 2);
   (job)
}

And therefore in C++AMP you can do:

parallel_for_each(extent<1>(num_iterations), [=](index<1> idx) restrict(amp) {
   int i = skip + (idx[0] * 2);
   (job)
}

This has the benefit of making sure you don't wasted GPU threads.

Let me know if this makes sense.

Answer 2

Without using "i" inside your job you can just use 95/2 consecusive iterations as-is:

extend<1> num((100 - 5) / 2);

If you use "i", do the [0, 95/2) and calculate "i":

int i = 5 + (idx[0] * 2)

Answer 3

Thank you! JoeM and Ethatron,

Wow ,Perfect Answer. Yes It does very make sense.

I have tried your code and it work as fine as my code.

Theres no performance differences between my code and your code.

I thought my code is slightly slow,but it was quite same.

anyway I use your code because it look more cool :)

Answer 4

Theres no performance differences between my code and your code.

Im so sorry,I was completely wrong..I saw wrong result...

Yes, my code is 2times slower.

Thanks for good response.

I feel I am so donkey...