The following
OpenCL™
kernel discussion is based on the
information provided in the C/C++ Kernels topic. The same
programming techniques for accelerating the performance of a kernel apply to both C/C++ and
OpenCL kernels. However, the OpenCL kernel uses the __attribute syntax in
place of pragmas. For details of the available attributes, refer to OpenCL Attributes.
The following code examples show some of the elements of an OpenCL kernel for the Vitis™ application acceleration development flow. This is not intended to be a primer on OpenCL or kernel development, but to merely highlight some of the key difference between OpenCL and C/C++ kernels.
Kernel Signature
In C/C++ kernels, the kernel is identified on the Vitis compiler command line using the v++ --kernel option. However, in OpenCL
code, the __kernel keyword identifies a kernel in the
code. You can have multiple kernels defined in a single .cl file, and the Vitis compiler will
compile all of the kernels, unless you specify the --kernel option to identify which kernel to compile.
__kernel __attribute__ ((reqd_work_group_size(1, 1, 1)))
void apply_watermark(__global const TYPE * __restrict input,
__global TYPE * __restrict output, int width, int height) {
{
...
}apply_watermark, can be found
in the Global Memory Two Banks (CL) example in theVitis Accel Examples GitHub repository.In the example above, you can see the watermark kernel has two pointer
type arguments: input and output, and has two scalar type int arguments: width and height.
In C/C++ kernels, these arguments would need to be identified with the
HLS INTERFACE pragmas. However, in the OpenCL kernel, the Vitis compiler, and
Vitis HLS recognize the kernel arguments, and compile
them as needed: pointer arguments into m_axi interfaces,
and scalar arguments into s_axilite interfaces.