How to run Apple's OpenCL “Hello World” example in XCode

问题

Apple provides an OpenCL "Hello World" example, which can be downloaded as a .zip file from the following page:

https://developer.apple.com/library/mac/samplecode/OpenCL_Hello_World_Example/Introduction/Intro.html

I downloaded it, opened the project in Xcode, and clicked Run. The build succeeded, but I got the following error message: Error: Failed to create a device group!

I would appreciate any advice on how to get a simple OpenCL example running on my Mac. In case it is diagnostically relevant: I'm running Mac OS 10.7.5 on an early 2011 MacBook Pro, and I have Xcode version 4.2 installed.

回答1:

Hooray, I worked it out myself. The hello.c file that Apple provides contains the following lines of code:

114    // Connect to a compute device
115    //
116    int gpu = 1;
117    err = clGetDeviceIDs(NULL, gpu ? CL_DEVICE_TYPE_GPU : CL_DEVICE_TYPE_CPU, 1, &device_id, NULL);
118    if (err != CL_SUCCESS)
119    {
120        printf("Error: Failed to create a device group!\n");
121        return EXIT_FAILURE;
122    }

The code is trying to get the id of a GPU device that supports OpenCL. The problem is that my machine (MacBook Pro, Early 2011) doesn't have a GPU that supports OpenCL. If the CL_DEVICE_TYPE_CPU flag is set instead, then the CPU is found, and this does support OpenCL. If the file is modified as follows:

116    int gpu = 0;

then I get the output: Computed '1024/1024' correct values!

回答2:

You can modify the code so that if there is no supported GPU then it fall back to CPU

// Connect to a compute device
//
int gpu = 1;
err = clGetDeviceIDs(NULL, gpu ? CL_DEVICE_TYPE_GPU : CL_DEVICE_TYPE_CPU, 1, &device_id, NULL);
if (err != CL_SUCCESS)
{
    printf("Failed to create a device group for gpu, so it will fall back to use cpu!\n");
    err = clGetDeviceIDs(NULL, CL_DEVICE_TYPE_CPU, 1, &device_id, NULL);
    if (err != CL_SUCCESS)
        return EXIT_FAILURE;
}


// Get some information about the returned device
cl_char vendor_name[1024] = {0};
cl_char device_name[1024] = {0};
size_t returned_size = 0;
err = clGetDeviceInfo(device_id, CL_DEVICE_VENDOR, sizeof(vendor_name),
                      vendor_name, &returned_size);
err |= clGetDeviceInfo(device_id, CL_DEVICE_NAME, sizeof(device_name),
                       device_name, &returned_size);
if (err != CL_SUCCESS)
    return EXIT_FAILURE;
printf("Connecting to %s %s...\n", vendor_name, device_name);

来源：https://stackoverflow.com/questions/18616599/how-to-run-apples-opencl-hello-world-example-in-xcode

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macos

opencl