CODEWITHSUNDEEP
openclkernelargument-passing
Balanidhren
Balanidhren

Reputation: 31

Wrong value after setting (scalar) kernel argument OpenCL

I'm working on an OpenCL program, but the output is different each execution. I think it has to do with passing arguments to the kernel, because when I hardcode the values for a specific execution, the output is similar after each execution.

My kernel looks like this:

__kernel void sample_kernel(__global double *BufferA, int scalar1, int scalar2, int scalar3, ...) {

    for(int i = -1*scalar1; i < scalar1; i++) {
        for(int j = -1*scalar1; j < scalar1, j++) {
            if(scalar2 > 0 && scalar3 > 0) // do something.
        }
    }
}

And this is how I set the kernel arguments:

int scalar1 = 1;
int scalar2 = 2;
int scalar3 = 3;

Samplekernel.setArg(0, d_BufferA);
Samplekernel.setArg(1, sizeof(int), &scalar1);
Samplekernel.setArg(2, sizeof(int), &scalar2);
Samplekernel.setArg(3, sizeof(int), &scalar3);

The weird thing is that when I add...

if(scalar1 != 1) scalar1 = 1;
if(scalar2 != 2) scalar2 = 2;
if(scalar3 != 3) scalar3 = 3;

...in the kernel before the double for-loop, the output is correct.

I run my program on an Nvidia K20m GPU, OpenCL version 1.1. When I run my code on an Nvidia C2075 everything seems to work fine...

Does anybody have an idea what the problem might be? It looks like the value is not copied correctly or that is overwritten, but I don't access that value before the for-loops...

Thanks in advance!

Upvotes: 3

Views: 4507

Answers (1)

Matthias Holzapfel
Matthias Holzapfel

Reputation: 540

I looks like you are passing a pointer to an int to setArg

Samplekernel.setArg(1, sizeof(int), &scalar1);

and then, in you kernel paramater list, you have values of ints, not pointers:

__kernel void sample_kernel(__global double *BufferA, int scalar1, ...

You could either use pointers in the kernel parameter list, like so:

__kernel void sample_kernel(__global double *BufferA, global int *scalar1,

Or - and that is what I would suggest, since I could not find your version of kernel.setArg (...) in the C++ bindings specification but for some reason only in the implementation on khronos.org - directly copy the scalar like so:

Samplekernel.setArg(1, scalar1);

This also has the advantage that the variable is availabe in the private memory space of the kernel and not in the global space, as it would be when you specify a buffer as an argument.

The version of Kernel::setArg you are using might not copy the value but might only be used for host bound kernels, but I'm not shure of that.

Also, you may want to check the return value of setArg for errors.

Upvotes: 3

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