If a signed or unsigned long long type is used in the OpenCL™ host code function clSetKernelArg() as shown below
unsigned long long data = 1;
clSetKernelArg(kernel, 0, sizeof(unsigned long long), (void*)&data);
then an error like the following may occur when the host code is compiled using the Intel® SDK for OpenCL™.
Context callback: Argument size is the wrong size
Failed to set kernel arg 0
This error did not appear for this case in versions earlier than 18.1 of the Intel® SDK for OpenCL™. The error now appears because the size of an unsigned long long type was changed from 8 to 16 in the underlying kernel compiler, but the host call sizeof(unsigned long long) still returns 8.
Types signed / unsigned long long do not have a defined size in C99 or OpenCL™ version 1.X so it's allowed for the host and device to use different sizes for the type. As such, one should never use it as the type of an argument to the kernel. It's not guaranteed to be portable between compilers, devices, or even compiler versions. In the OpenCL™ 2.0 specification, a long long type is defined as 128 bits, but the C99 ambiguity remains. The OpenCL™ specification does not add a cl_* compatibility type, so it is not possible to use a long long type as a scalar argument safely.