opencl Kernel Basics Kernel Skelleton
Example
Lets walk through the most simple kernel there is and some variations of it
__kernel void myKernel() {
}
A kernel which can be started from your main code is identified by the __kernel keyword. A Kernel function can only have return type void.
__kernel void myKernel(float a, uint b, byte c) {
}
Of course you can create more functions which are not exposed as kernels. In this case you can just omit the __kernel modifier.
A function may expose variables like any other C/C++ function would. The only difference is when you want to reference memory. This applies to all pointers whether they are arguments or used in code.
float* ptr;
is a pointer to a memory region only the executing thread has access to. In fact it is the same as
__private float* ptr;
There are four different memory region modifiers available. Inside the kernel you usually don't have to worry about it, but for arguments this is essential.
- __global: This modifier refers to a pointer which is placed in the global memory
- __constant: refers to a constant memory pointer
- __local: refers to a shared memory pointer
- __private: refers to a local memory pointer
In addition we can define how we want to access the memory
- no modifier: read and write
- __read_only
- __write_only
Those flags have to match the way we allocated the memory buffer back on our host.
