OpenCL essential calls

Yesterday I distilled a “hello world” C program which uses the OpenCL framework. Let’s see the essential calls that that program makes.

Everything is in <OpenCL/opencl.h>. We make this available by compiling with -framework OpenCL.

First our program finds our GPU, using clGetDeviceIDs. (The Khronos site seems to be the canonical reference for OpenCL.) The clGetDeviceIDs call can find any and all available OpenCL devices, but we ask it for just one GPU device. We get back a cl_device_id, which is an opaque identifier type.

Next we create a context with clCreateContext. A context associates multiple devices, but we just give it one device: our GPU. We get back a cl_context. We pass this context in to many future calls.

Next we create a command queue with clCreateCommandQueue. We pass in our GPU’s device ID: all commands that we queue here will be processed by our GPU.

Next comes a set of three calls to compile our OpenCL kernel. First we do clCreateProgramWithSource, passing in the source code to our context. Our OpenCL source looks like:

__kernel void square(__global float* input, __global float* output, const unsigned int count) {
   int i = get_global_id(0);
   if(i < count) { output[i] = input[i] * input[i]; }
}

This gives us a cl_program. Our program is not yet built; to do that, we call clBuildProgram. Finally we extract a kernel from the program using clCreateKernel, passing the string "square". A kernel is a function in a program. We get the kernel for the function square, as type cl_kernel. We don’t refer to the cl_program again; we only use it to get the cl_kernel.

Next, we create two buffers - contiguous memory regions - with clCreateBuffer. One is an input buffer, the other an output buffer. We’ll use these to communicate with the kernel function: the buffers correspond to the input and output arguments to the kernel function. These buffers are managed by OpenCL. We specify the size of these buffers. Both are set to 1024 floats.

Next we write to the input buffer, in preparation for calling the kernel. We write to the input buffer with clEnqueueWriteBuffer. This command queues a command on the command queue we created earlier. The clEnqueueWriteBuffer command takes a buffer (our input buffer) and a pointer to some data which we want to copy to the buffer. We’ve pre-filled this buffer with the numbers 0..1023.

We specify that the clEnqueueWriteBuffer call should be blocking. This means the call will not return until the job has completed on the command queue.

Next we set the arguments to the kernel using clSetKernelArg. The kernel has three arguments, input, output and count. We refer to the arguments numerically: 0, 1, and 2. We set input to our input buffer (which we just filled), and we set output to the output buffer (which the kernel will fill when we call it).

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