Hello World (OpenCL Kernel)

This example is a simple OpenCL application. It will highlight the basic flow of an OpenCL application.

KEY CONCEPTS: OpenCL Host API

This example introduces the basic structure of host and kernel code. Kernel vadd optimizes a simple vector addition.

Kernel arguments with global keyword specify the inputs read from the global memory and outputs written back to it.

If the number of loop iterations are unknown, tool cannot report the loop latency. In these cases, xcl_loop_tripcount can be used to specify minimum and maximum number of iterations to help the tool to analyze how the loop’s latency contributes to the total latency of the application.

__attribute__((xcl_loop_tripcount(c_len, c_len)))

Loop iterations are done sequentially on a CPU, i.e. it will take multiple clock cycles for a new iteration to begin execution. FPGA can pipeline these iterations to increase the throughput of the application by beginning a new iteration every cycle. xcl_pipeline_loop attribute is used to implement pipeline and to mention the Initiation interval (No. of clock cycles before a new loop iteration can be started).

__attribute__((xcl_pipeline_loop(1)))

Rather than reading individual items for addition, buffers are created in kernel’s local memory and multiple items are read in a single burst. This is done to achieve low memory access latency and also for efficient use of bandwidth provided by the AXI interface. Similarly, results are stored in a buffer and are written to global memory in a burst. The for loops used have the following requirements to implement burst read/write: - Pipeline the loop : Loop pipeline must have II (Initiation interval) = 1 - Aligned memory : Memory addresses for read/write should be contiguous.

__attribute__((xcl_pipeline_loop(1)))
     readB: for (int j = 0 ; j < size ; j++) {
             arrayB[j] = b[i+j]; }

EXCLUDED PLATFORMS

Platforms containing following strings in their names are not supported for this example :

nodma

DESIGN FILES

Application code is located in the src directory. Accelerator binary files will be compiled to the xclbin directory. The xclbin directory is required by the Makefile and its contents will be filled during compilation. A listing of all the files in this example is shown below

src/host.cpp
src/vector_addition.cl

Access these files in the github repo by clicking here.

COMMAND LINE ARGUMENTS

Once the environment has been configured, the application can be executed by

./cl_helloworld <vector_addition XCLBIN>