gentype
work_group_reduce<op>
(
| gentype x) |
Description
Return result of reduction operation specified
by <op> for all values of x
specified by work-items in a work-group.
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.
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