Change OpenCL function to C++

Question

I am trying to write a code in C++, but after some search on the internet, I found one OpenCL based code is doing exactly the same thing as I want to do in C++. But since this is the first time I see a OpenCL code, I don't know how to change the following functions into c++:

const __global float4 *in_buf;

int x = get_global_id(0);
int y = get_global_id(1);

float result = y * get_global_size(0);

Is 'const __global float4 *in_buf' equivalent to 'const float *in_buf' in c++? And how to change the above other functions? Could anyone help? Thanks.

Oak · Accepted Answer · 2014-02-19 08:29:31Z

3

In general, you should take a look at the OpenCL specification (I'm assuming it's written in OpenCL 1.x) to better understand functions, types and how a kernel works.

Specifically for your question:

get_global_id returns the id of the current work item, and get_global_size returns the total number of work items. Since an OpenCL work-item is roughly equivalent to a single iteration in a sequential language, the equivalent of OpenCL's:

int x = get_global_id(0);
int y = get_global_id(1);
// do something with x and y
float result = y * get_global_size(0);

Will be C's:

for (int x = 0; x < dim0; x++) {
    for (int y = 0; y < dim1; y++) {
        // do something with x and y
        float result = y * dim0;
    }
}

As for float4 it's a vector type of 4 floats, roughly equivalent to C's float[4] (except that it supports many additional operators, such as vector arithmetic). Of course in this case it's a buffer, so an appropriate type would be float** or float[4]* - or better yet, just pack them together into a float* buffer and then load 4 at a time.

Feel free to ignore the __global modifier.

answered Feb 19, 2014 at 8:29

Oak

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2 Comments

Angew is no longer proud of SO Over a year ago

+1; but I'd say a float4 is more closely equivalent to std::array<float, 4>- it can be assigned and copied.

huseyin tugrul buyukisik Over a year ago

Maybe it is equivalent to __m128 intrinsic variable?

ssavec · Accepted Answer · 2014-02-19 08:33:11Z

2

const __global float4 *in_buf is not equivalent to const float *in_buf. The OpenCL uses vector variables, e.g. floatN, where N is e.g. 2,4,8. So float4 is in fact struct { float w, float x, float y, float z} with lot of tricks available to express vector operations.

get_global_id(0) gives you the iterator variable, so essentially replace every get_global_id(dim) with for(int x = 0; x< max[dim]; x++)

answered Feb 19, 2014 at 8:33

ssavec

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