The earlier blog-post was about how to use Qt Creator with OpenCL. The examples are all around Images, but nowhere a simple Hello World. So here it is: AMD’s infamous OpenCL Hello World in Qt. Thank’s to linuxjunk for glueing the parts together.

int main(int argc, char *argv[]) {

    // Define the kernel. Take a good look what it does.
    QByteArray prog(
    "#pragma OPENCL EXTENSION cl_khr_byte_addressable_store : enablen" 
    "__constant char hw[] = "Hello World from Qt!"; n" 
    "__kernel void hello(__global char * out) {n" 
    "  size_t tid = get_global_id(0); n" 
    "  out[tid] = hw[tid]; n" 
    "}n"
    );

     // Get a context on the first available GPU.
     QCLContext context;
     if (!context.create(QCLDevice::GPU))
         qFatal("Could not create OpenCL context");

     // Allocate 100 bytes of memory on the Host.
     size_t mem_size = 100;
     char* outH = new char[mem_size];
     // Allocate buffer on the Device.
     QCLBuffer outCL = context.createBufferHost(outH, sizeof(char) * mem_size,
                                                QCLMemoryObject::WriteOnly);

     // Compile program against device
      QCLProgram program = context.buildProgramFromSourceCode(prog);

     // Create a kernel object, tell it we are using the kernel called "hello".
     QCLKernel kernel = program.createKernel("hello");

     // Set the necessary global memory. In this case it is the buffer-size.
     kernel.setGlobalWorkSize(outCL.size(), 1);

     // Turn on profiling.
     QCLCommandQueue queue = context.commandQueue();
     queue.setProfilingEnabled(true);

     // Queue the kernel up to run.
     // Give it an argument which is the memory we allocated above.
     QCLEvent event = kernel(outCL);

     // Use the event object above to block until processing has completed.
     event.waitForFinished();

     // Timing only works with profiling on. runTime is unsigned.
     printf(" time '%u'n", event.runTime());

     // Read the results out of the shared memory area.
     outCL.read(outH, mem_size);

     // Write to screen.
     printf(" result = '%s'", outH);
}

Have fun!