Question

I have stored the text in char array a[textLength], and pattern in an array b[patternLength]

cl_char *a = (cl_char *) malloc(textLength*sizeof(cl_char));

for(int i =0; i<textLength;i++)
{
    a[i]=text[i];
    }

// A buffer object is a handle to a region of memory
cl_mem a_buffer = clCreateBuffer(context,
                                 CL_MEM_READ_ONLY | // buffer object read only for kernel
                                 CL_MEM_COPY_HOST_PTR, // copy data from memory referenced
                                 // by host pointer
                                 textLength*sizeof(cl_char), // size in bytes of buffer object
                                 a, // host pointer
                                 NULL); // no error code returned

// for text and pattern kernal arguments
cl_char *b = (cl_char *) malloc(patternLength*sizeof(cl_char));

for(int i =0; i<patternLength;i++)
{
   b[i]=pattern[i];
}

// A buffer object is a handle to a region of memory
/*cl_mem b_buffer = clCreateBuffer(context,
                                 CL_MEM_READ_ONLY | // buffer object read only for kernel
                                 CL_MEM_COPY_HOST_PTR, // copy data from memory referenced
                                 // by host pointer
                                 patternLength*sizeof(cl_char), // size in bytes of buffer object
                                 b, // host pointer
                                 NULL); // no error code returned */
cl_mem b_buffer = NULL;


    clSetKernelArg(kernel, 0, sizeof(a_buffer), (void*) &a_buffer);
clSetKernelArg(kernel, 1, sizeof(cl_mem), NULL);
clSetKernelArg(kernel, n, sizeof(cl_mem), &b_buffer);
    size_t global_work_size = numberofWorkItem;
    cl_int error= clEnqueueNDRangeKernel(queue, kernel,
                       1, NULL, // global work items dimensions and offset
                       &global_work_size, // number of global work items
                       &patternLength, // number of local work items
                       0, NULL, // don't wait on any events to complete
                       &timeEvent); // no event object returned

 I have read that in clSetKernelArg, for __local indentifiers, the arg_value should be NULL. I have done that by doing b_buffer=NULL;

But doing that would prevent the b_buffer from storing the value of b[] (pattern)
How do I do it??

Also,
If I am not wrong, the local_work_size can not be greater than the value given by CL_DEVICE_MAX_WORK_ITEM_SIZES. Because the local_work_size is constrained by the underlying device/hardware.
The global_work_size, on the other hand, can be as big as one wants.
Does it have to be a multiple of local_work_size ???
If yes, why??

Answer 1

Your error is in the clSetKernelArg line:

//incorrect
clSetKernelArg(kernel, n, sizeof(cl_mem), &b_buffer);

//correct
clSetKernelArg(kernel, n, sizeof(cl_char)*patternLength, NULL);

Local memory is cleared after kernel execution, so you won’t be able to get a copy of b_buffer using your method. Also, local memory is not assigned by the host. You need to copy from a global parameter to get it into the LDS.

To get the local data copied, you need to pass in a global cl_mem as well as the local parameter. The copy can be done at the end of the kernel and red back to the host with clEnqueueReadBuffer.

Update

Here’s a specific example of how to use a dynamic local buffer, and assign it the contents of a global buffer.

__kernel void copyBufferExample(__global int* srcBuff, __local int* localBuff, const int copyCount)
{
    int lid = get_local_id(0);
    int ls = get_local_size(0);
    int i;

    for(i=lid; i<copyCount; i+=ls){
        localBuff[i] = srcBuff[i];
    }

    //use localBuff here
    //copy result back to global memory if needed
}