Do an exclusive prefix-sum operation of all values in work-items in the work-group

gentype work_group_scan_exclusive_<op>(gentype x)

Description

Do an exclusive scan operation specified by <op> of all values specified by work-items in the work-group. The scan results are returned for each work-item.

The scan order is defined by increasing 1D linear global ID within the work-group.

General information about work-group functions

This built-in function must be encountered by all work-items in a work-group executing the kernel. We use the generic type name gentype to indicate the built-in data types half (if the cl_khr_fp16 extension is supported), int, uint, long, ulong, float or double (if double precision is supported) as the type for the arguments.

The <op> in work_group_reduce_<op>, work_group_scan_exclusive_<op> and work_group_scan_inclusive_<op> defines the operator and can be add, min or max.

The inclusive scan operation takes a binary operator op with an identity I and n (where n is the size of the work-group) elements [a0, a1, …​ an-1] and returns [a0, (a0 op a1), …​ (a0 op a1 op …​ op an-1)]. If <`op`> = add, the identity I is 0. If <`op`> = min, the identity I is INT_MAX, UINT_MAX, LONG_MAX, ULONG_MAX, for int, uint, long, ulong types and is +INF for floating-point types. Similarly if <`op`> = max, the identity I is INT_MIN, 0, LONG_MIN, 0 and -INF.

Consider the following example:

void foo(int *p)
{
    ...
    int prefix_sum_val = work_group_scan_inclusive_add(
              p[get_local_id(0)]);
}

For the example above, let’s assume that the work-group size is 8 and p points to the following elements [3 1 7 0 4 1 6 3]. Work-item 0 calls work_group_scan_inclusive_add with 3 and returns 3. Work-item 1 calls work_group_scan_inclusive_add with 1 and returns 4. The full set of values returned by work_group_scan_inclusive_add for work-items 0 …​ 7 are [3 4 11 11 14 16 22 25].

The exclusive scan operation takes a binary associative operator op with an identity I and n (where n is the size of the work-group) elements [a0, a1, …​ an-1] and returns [I, a0, (a0 op a1), …​ (a0 op a1 op …​ op an-2)]. For the example above, the exclusive scan add operation on the ordered set [3 1 7 0 4 1 6 3] would return [0 3 4 11 11 14 16 22].

Note
The order of floating-point operations is not guaranteed for the work_group_reduce_<op>, work_group_scan_inclusive_<op> and work_group_scan_exclusive_<op> built-in functions that operate on half, float and double data types. The order of these floating-point operations is also non-deterministic for a given workgroup.

Also see

Specification