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OpenCL bug: 64-bit integer multiply produces incorrect result

I am running into an issue on my Intel GPU where the high 32 bits of a 64-bit product are all 0's. In particular it happens when:

 

1. One of the operands is an immediate value

and

2. The other operand is a 64-bit long less than 2^32-1, but large enough that the product should overflow into the higher 32 bits of the 64-bit result.

 

I have provided code to demonstrate the bug. It copies 0xfefefefe to the device (as a 64-bit long), and multiplies it by 0x1234 and copies back the result.

It produces this output on my machine:

Intel(R) Core(TM) i7-8850H CPU @ 2.60GHz
  Expected: 1221b9b9a798
  Actual:   1221b9b9a798
Intel(R) UHD Graphics 630
  Expected: 1221b9b9a798
  Actual:   b9b9a798
AMD Radeon Pro 560X Compute Engine
  Expected: 1221b9b9a798
  Actual:   1221b9b9a798

#include <iostream>
#include <vector>

#ifdef __APPLE__
#define CL_SILENCE_DEPRECATION
#include <OpenCL/opencl.h>
#else
#include <CL/cl.h>
#endif

const char *_kernelSource =
"__kernel void testKernel(__global long *ptr)\
{\
    if(get_global_id(0) == 0) {\
        ptr[0] *= 0x1234;\
    }\
}";


void run_test(cl_device_id device_id)
{
    cl_context ctx;
    cl_command_queue cmd;
    cl_program prog;
    cl_kernel kernel;
    cl_int err = 0;

    ctx = clCreateContext(0, 1, &device_id, NULL, NULL, &err);
    if (!ctx)
    {
        std::cout << "Error: Failed to create a compute context!" << std::endl;
        return;
    }

    cmd = clCreateCommandQueue(ctx, device_id, 0, &err);
    if (!cmd)
    {
        std::cout << "Error: Failed to create a command commands!" << std::endl;
        return;
    }

    size_t len = strlen(_kernelSource);
    prog = clCreateProgramWithSource(ctx, 1, &_kernelSource, &len, &err);
    if (!prog)
    {
        std::cout << "Error: Failed to create compute program!" << std::endl;
        return;
    }

    err = clBuildProgram(prog, 0, NULL, NULL, NULL, NULL);
    if (err != CL_SUCCESS)
    {
        size_t len;
        char buffer[2048] = {0};

        std::cout << "Error: Failed to build program executable! error code " << err << std::endl;
        clGetProgramBuildInfo(prog, device_id, CL_PROGRAM_BUILD_LOG, sizeof(buffer), buffer, &len);
        std::cout << "error: " << buffer << std::endl;
        return;
    }

    kernel = clCreateKernel(prog, "testKernel", &err);
    if (!kernel || err != CL_SUCCESS)
    {
        std::cout << "Error: Failed to create compute kernel!" << std::endl;
        return;
    }

    size_t global = 1;
    size_t local = 0;

    err = clGetKernelWorkGroupInfo(kernel, device_id, CL_KERNEL_WORK_GROUP_SIZE, sizeof(local), &local, NULL);
    if (err != CL_SUCCESS)
    {
        std::cout << "Error: Failed to retrieve kernel work group info!" << std::endl;
        return;
    }
    global = local;

    // Copy test value over to device
    uint64_t test_value = 0xfefefefe;

    cl_mem dev_ptr = clCreateBuffer(ctx, 0, sizeof(uint64_t), NULL, &err);
    clEnqueueWriteBuffer(cmd, dev_ptr, CL_TRUE, 0, sizeof(uint64_t), &test_value, 0, NULL, NULL);

    // Call the kernel
    clSetKernelArg(kernel, 0, sizeof(cl_mem), &dev_ptr);
    err = clEnqueueNDRangeKernel(cmd, kernel, 1, NULL, &global, &local, 0, NULL, NULL);
    if (err)
    {
        std::cout << "Error: Failed to execute kernel! Error code " << err << std::endl;
        return;
    }

    clFinish(cmd);

    // Copy result back from device
    std::cout << "  Expected: " << std::hex << (test_value * 0x1234)  << std::endl;
    uint64_t result;
    clEnqueueReadBuffer(cmd, dev_ptr, CL_TRUE, 0, sizeof(uint64_t), &result, 0, NULL, NULL);
    std::cout << "  Actual:   " << std::hex << result << std::endl;
    clFinish(cmd);


    // Cleanup
    clReleaseMemObject(dev_ptr);
    clReleaseProgram(prog);
    clReleaseKernel(kernel);
    clReleaseCommandQueue(cmd);
    clReleaseContext(ctx);

}

int main(int argc, char** argv)
{
    int err = 0;
      
    unsigned int num_platforms = 0;
   
    cl_platform_id platform_ids[10];
    std::vector<cl_device_id> device_ids;
    std::vector<std::string> device_names;

    err = clGetPlatformIDs(0, NULL, &num_platforms);

    if(num_platforms == 0) {
      std::cout << "No OpenCL platforms found" << std::endl;
      return 0; 
    }

    clGetPlatformIDs(num_platforms, platform_ids, NULL);

    // Get device ID's and names
    for(int i = 0; i < num_platforms; i++) {
      unsigned int num_devices = 0;
      cl_device_id ids[10];

      clGetDeviceIDs(platform_ids, CL_DEVICE_TYPE_ALL, 0, NULL, &num_devices);

      if(num_devices == 0) {
        continue;
      }

      clGetDeviceIDs(platform_ids, CL_DEVICE_TYPE_ALL, num_devices, ids, NULL);

      for(int j = 0; j < num_devices; j++) {
        char buf[256] = {0};
        size_t name_size;

        clGetDeviceInfo(ids, CL_DEVICE_NAME, sizeof(buf), buf, &name_size);
        device_ids.push_back(ids);
        device_names.push_back(std::string(buf));
      }
    }

    // Run the test on each device
    for(int i = 0; i < device_ids.size(); i++) {
      std::cout << device_names << std::endl;
      run_test(device_ids); 
    }

    return 0;
}

 

 

Thanks

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4 Replies
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Hi BenR,

Thanks for the clear example and the interest.

Can you confirm some platform details?

Are you on Linux* OS or Windows* OS?

For Intel Graphics Technology, which OpenCL runtime are you using? Which version?

If you're on a recent NEO Linux Debian/Ubuntu build, can you provide the versions of intel-gmmlib_X.X.X.X, intel-igc-core_X.X.X intel-igc-opencl_X.X.X, and intel-opencl_X.X.X packages? Or perhaps at least the Device Driver Version as reported from clGetDeviceInfo(...) or a platform interrogation tool like clinfo?

If it's Windows* OS can you provide the Device Driver Version as reported from clGetDeviceInfo(...) or a platform interrogation tool like clinfo? The driver revision for the package can be good as well. Reference for Windows* OS: https://downloadcenter.intel.com/product/80939/Graphics-Drivers.

Any deployment details that can be provided for enabling Intel Graphics Technology/Intel(R) UHD Graphics 630 are useful... thanks.

-MichaelC

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Hi Michael,

 

I originally discovered it on MacOS, but I have reproduced it on a Windows 10 laptop with an Intel HD Graphics 530.

This is the version information from clinfo.exe on the Windows machine:

  Device Name                                     Intel(R) HD Graphics 530
  Device Vendor                                   Intel(R) Corporation
  Device Vendor ID                                0x8086
  Device Version                                  OpenCL 2.0 
  Driver Version                                  21.20.16.4664
  Device OpenCL C Version                         OpenCL C 2.0 

 

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BenR,

Thanks again for the clear example and for following up on the version information. I've passed this sighting on to the dev team.  We'll post any issue updates to this thread.

 

-MichaelC

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For tracking... a source code change made it into March 2019 releases and newer: Reference.

 

-MichaelC

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