Description
The functionality described in this section requires
support for OpenCL C 2.0, or OpenCL C 3.0 or newer and the
__opencl_c_ feature.
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This section describes built-in functions that perform collective options
across a work-group.
These built-in functions 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 [1], int, uint, long
[2], ulong, float or double
[3] as the type for the arguments.
| Function | Description |
|---|---|
int work_group_all(int predicate) |
Evaluates predicate for all work-items in the work-group and returns a non-zero value if predicate evaluates to non-zero for all work-items in the work-group. |
int work_group_any(int predicate) |
Evaluates predicate for all work-items in the work-group and returns a non-zero value if predicate evaluates to non-zero for any work-items in the work-group. |
gentype work_group_broadcast(gentype a, size_t local_id) |
Broadcast the value of a for work-item identified by local_id to all work-items in the work-group. Behavior is undefined when the value of local_id is not equivalent for all work-items in the work-group. Behavior is undefined when local_id is greater or equal to the work-group size in the corresponding dimension. |
gentype work_group_reduce_<op>(gentype x) |
Return result of reduction operation specified by <op> for all values of x specified by work-items in a work-group. |
gentype work_group_scan_exclusive_<op>(gentype x) |
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. |
gentype work_group_scan_inclusive_<op>(gentype x) |
Do an inclusive 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. |
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 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)].
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 15 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)].
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.
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 15 16 22].
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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 |
Document Notes
For more information, see the OpenCL C Specification
This page is extracted from the OpenCL C Specification. Fixes and changes should be made to the Specification, not directly.