IMPORTANT!: This is a compiler hint which the compiler may ignore.

The work-group size in the OpenCL standard defines the size of the ND range space that can be handled by a single invocation of a kernel compute unit. When OpenCL kernels are submitted for execution on an OpenCL device, they execute within an index space, called an ND range, which can have 1, 2, or 3 dimensions. See "OpenCL Execution Model" in SDAccel Environment Optimization Guide (UG1207) for more information.

OpenCL kernel functions are executed exactly one time for each point in the ND range index space. This unit of work for each point in the ND range is called a work-item. Unlike for loops in C, where loop iterations are executed sequentially and in-order, an OpenCL runtime and device is free to execute work-items in parallel and in any order.

Work-items are organized into work-groups, which are the unit of work scheduled onto compute units. The optional work_group_size_hint attribute is part of the OpenCL Language Specification, and is a hint to the compiler that indicates the work-group size value most likely to be specified by the local_work_size argument to clEnqueueNDRangeKernel. This allows the compiler to optimize the generated code according to the expected value.

TIP: In the case of an FPGA implementation, the specification of the reqd_work_group_size attribute instead of the work_group_size_hint is highly recommended as it can be used for performance optimization during the generation of the custom logic for a kernel.


Place this attribute before the kernel definition, or before the primary function specified for the kernel:
__attribute__((work_group_size_hint(X, Y, Z)))
  • X, Y, Z: Specifies the ND range of the kernel. This represents each dimension of a three dimensional matrix specifying the size of the work-group for the kernel.


The following example is a hint to the compiler that the kernel will most likely be executed with a work-group size of 1:

__attribute__((work_group_size_hint(1, 1, 1)))
__kernel void

See Also