Extension adds support for implementation-controlled subgroups.

#pragma OPENCL EXTENSION cl_khr_subgroups : enable


Subgroups behave similarly to workgroups with their own sets of builtins and synchronization primitives. Subgroups within a workgroup are independent, make forward progress with respect to each other and may map to optimized hardware structures where that makes sense.

If this extension is supported by an implementation, the string cl_khr_subgroups will be present in the CL_DEVICE_EXTENSIONS string described in table 4.3 (see clGetDeviceInfo).

Within a work-group work-items may be divided into sub-groups in an implementation-defined fashion. The mapping of work-items to sub-groups is implementation-defined and may be queried at runtime. While sub-groups may be used in multi-dimensional work-groups, each subgroup is 1-dimensional and any given work-item may query which sub-group it is a member of.

Work items are mapped into subgroups through a combination of compile-time decisions and the parameters of the dispatch. The mapping to subgroups is invariant for the duration of a kernel’s execution, across dispatches of a given kernel with the same launch parameters, and from one work-group to another within the dispatch (excluding the trailing edge work-groups in the presence of non-uniform work-group sizes). In addition, all sub-groups within a work-group will be the same size, apart from the sub-group with the maximum index which may be smaller if the size of the work-group is not evenly divisible by the size of the sub-groups

Sub-groups execute concurrently within a given work-group and make independent forward progress with respect to each other even in the absence of work-group barrier operations. Subgroups are able to internally synchronize using barrier operations without synchronizing with each other.

In the degenerate case, with the extension enabled, a single sub-group must be supported for each work-group. In this situation all sub-group scope functions alias their work-group level equivalents.

This extension enables the following functions:

clGetKernelSubGroupInfoKHR Returns information about the kernel object
get_sub_group_size Returns the number of work-items in the subgroup
get_max_sub_group_size Returns the maximum size of a subgroup within the dispatch
get_num_sub_groups Returns the number of subgroups in the current workgroup
get_enqueued_num_sub_groups Returns the number of enqueued subgroups
get_sub_group_id Returns the sub-group ID
get_sub_group_local_id Returns the unique work-item ID within the current subgroup
sub_group_barrier Sub-group barrier
sub_group_all Returns non-zero if a predicate evaluates true for all work-items
sub_group_any Returns non-zero if a predicate evaluates true for any work-items
sub_group_broadcast Broadcast a value to all work-items in a sub-group
sub_group_reduce_<op> Broadcast a value to all work-items in a sub-group
sub_group_scan_exclusive_<op> Do an exclusive scan operation
sub_group_scan_inclusive_<op> Do an inclusive scan operation
sub_group_reserve_read_pipe Reserve packet entries for reading from a pipe
sub_group_reserve_write_pipe Reserve packet entries for writing to a pipe
sub_group_commit_read_pipe Indicates that all reads associated with a reservation are completed
sub_group_commit_write_pipe Indicates that all writes associated with a reservation are completed
get_kernel_sub_group_count_for_ndrange Returns the number of subgroups in each workgroup of the dispatch
get_kernel_max_sub_group_size_for_ndrange Returns the maximum sub-group size for a block


OpenCL Specification

Also see

EXTENSION, clGetDeviceInfo, clGetKernelSubGroupInfoKHR, Work Item Functions, Sync Functions

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