ClEnqueueCopyBuffer with offset 1

Question

To optimize a kernel i need to make a copy of a cl_mem object with an offset.

count_buffer3[n] = count_buffer[n+1] 

is the desired result

Looking at the specification of ClEnqueueCopyBuffer it seems to be possible with a simple argument.

cl_int clEnqueueCopyBuffer (    cl_command_queue command_queue,
    cl_mem src_buffer,
    cl_mem dst_buffer,
    size_t src_offset,
    size_t dst_offset,
    size_t cb,
    cl_uint num_events_in_wait_list,
    const cl_event *event_wait_list,
    cl_event *event)

My idea was to set dst_offset to 1. So copy_buffer[0] goes to copy_buffer[1] In my case the command looks like:

    clEnqueueCopyBuffer(command_queue, count_buffer, count_buffer3, 1, 0, (inCount1 + 1) * sizeof(int), NULL, NULL, NULL);

So i want to copy count_buffer to count_buffer3 with an offset of 1. The result should be like this:

count_buffer[1] = 2
count_buffer[2] = 12
count_buffer[3] = 26

count_buffer3[1] = 12
count_buffer3[2] = 26

Unfortunately, if my dst_offset is 1 like shown in the example my complete count_buffer3 object contains only "0" as int values.

If my offset is 0, the copy works fine and both count_buffers are identical.

Additional Information: Here are the init of the clmem objects:

cl_mem count_buffer3 = clCreateBuffer(context, CL_MEM_READ_WRITE, (inCount1 + 1) * sizeof(int), NULL, &err); errWrapper("create Buffer", err);

cl_mem count_buffer = clCreateBuffer(context, CL_MEM_READ_WRITE, (inCount1+1) * sizeof(int), NULL, &err); errWrapper("create Buffer", err);

I am using INtel INDE update 2 with visual Studio 2013

Am i doing sth wrong here, or should the copy with offset work like this?

Edit: i reduced the buffer size by one and the result changes. Instead of all "0" i get some very huge numbers.

example from debug:

count_buffer[0] = 0
count_buffer[1] = 31
count_buffer[2] = 31

count_buffer3[0] = 520093696
count_buffer3[1] = 520093696
count_buffer3[2] = 520093696

It is an improvement to "0" values, but still wrong. any ideas?

Thanks for the answer so far!

Answer 1

The offset is in bytes. You probably want an offset of sizeof count_buffer[0] and a size of (n - 1) * sizeof count_buffer[0]:

clEnqueueCopyBuffer(
    command_queue, count_buffer, count_buffer3, 
    sizeof(cl_int), 0, 
    inCount1 * sizeof(cl_int), 
    NULL, NULL, NULL);

Answer 2

It's very likely clEnqueueCopyBuffer returns an error which you don't check. According to the manual:

CL_INVALID_VALUE is returned if src_offset, dst_offset, cb, src_offset + cb, or dst_offset + cb require accessing elements outside the buffer memory objects.

which seems to be your case. You probably want to pass size to copy one less than the size of your buffer:

clEnqueueCopyBuffer(command_queue, count_buffer, count_buffer3, 1, 0, inCount1 * sizeof(int), NULL, NULL, NULL);
                                                                      ^^^^^^^^