Question

I have noticed a number of kernel sources that look like this (found randomly by Googling):

__kernel void fill(__global float* array, unsigned int arrayLength, float val)
{
    if(get_global_id(0) < arrayLength)
    {
        array[get_global_id(0)] = val;
    }
}

My question is if that if-statement is actually necessary (assuming that "arrayLength" in this example is the same as the global work size).

In some of the more "professional" kernels I have seen, it is not present. It also seems to me that the hardware would do well to not assign kernels to nonsense coordinates.

However, I also know that processors work in groups. Hence, I can imagine that some processors of a group must do nothing (for example if you have 1 group of size 16, and a work size of 41, then the group would process the first 16 work items, then then next 16, then the next 9, with 7 processors not doing anything–do they get dummy kernels?).

I checked the spec., and the only relevant mention of "get_global_id" is the same as the online documentation, which reads:

    The global work-item ID specifies the work-item ID based on the number of global work-items specified to execute the kernel.

. . . based how?

So what is it? Is it safe to omit iff the array’s size is a multiple of the work group size? What?

Answer 1

You have the right answer already, I think. If the global size of your kernel execution is the same as the array length, then this if statement is useless.

In general, that type of check is only needed for cases where you’ve partitioned your data in such a way that you know you might execute extra work items relative to your array size. In my experience, you can almost always avoid such cases.