3. Compile OmpSs-2@FPGA programs¶

To compile an OmpSs-2@FPGA program you should follow the general OmpSs-2 compilation procedure using the LLVM/Clang compiler. More information is provided in the OmpSs-2 User Guide (https://pm.bsc.es/ftp/ompss-2/doc/user-guide/llvm/index.html). The following sections detail the specific options of LLVM/Clang to generate the binaries, bitstream and boot files.

The entire list of LLVM/Clang options (for the FPGA phase) and AIT arguments are available here:

3.3. Binaries¶

To compile applications with LLVM/Clang you must add the flag -fompss-2 when using either:

clang++ for C++ applications.
clang for C applications.

3.4. Bitstream¶

Note

LLVM/Clang expects the Accelerator Integration Tool (AIT) to be available on the PATH, if not the linker will fail. Moreover, AIT expects Vitis HLS and Vivado to be available in the PATH.

Warning

Sourcing the Vivado settings.sh file may break the cross-compilation toolchain. Instead, just add the directory of vivado binaries in the PATH.

To generate the bitstream, you should enable the bitstream generation in the LLVM/Clang compiler (using the -fompss-fpga-wrapper-code flag) and provide the FPGA linker (aka AIT) flags with -fompss-fpga-ait-flags option. If the FPGA linker flags does not contain the -b (or --board) and -n (or --name) options, the linker phase will fail.

For example, to compile the dotproduct application, in debug mode, for the Alveo U200, with a target frequency of 300Mhz, you can use the following command:

clang -fompss-2 -fompss-fpga-wrapper-code \
  src/dotproduct.c -o dotproduct-d \
  -fompss-fpga-ait-flags "--board=alveo_u200 --clock=300 --name=dotproduct"

3.4.1. Shared memory port¶

By default, LLVM/Clang generates an independent port to access the main memory for each task argument. Moreover, the bit-width of those ports equals to the argument data type width. This can result in a huge interconnection network when there are several task accelerators or they have several non-scalar arguments.

This behavior can be modified to generate unique shared port to access the main memory between all task arguments. This is achieved with the -fompss-fpga-memory-port-width option of LLVM/Clang which defines the desired bit-width of the shared port. The value must be a common multiple of the bit-widths for all task arguments.

The usage of the LLVM/Clang variable to generate a 128 bit port in the previous dotproduct command will be like:

clang -fompss-2 -fompss-fpga-wrapper-code \
  src/dotproduct.c -o dotproduct-d \
  -fompss-fpga-memory-port-width 128 \
  -fompss-fpga-ait-flags "--board=alveo_u200 --clock=300 --name=dotproduct"

3.5. Boot Files¶

Some boards do not support loading the bitstream into the FPGA after the boot, therefore the boot files should be updated and the board rebooted. This step is not needed for the z7000 family of devices as the bitstream can be loaded after boot. AIT supports the generation of boot files for some boards but the step is disabled by default and should be enabled by hand.

First, you need to set the following environment variables:

PETALINUX_INSTALL. Petalinux installation directory.
PETALINUX_BUILD. Petalinux project directory. See Create boot files for ultrascale to have more information about how to setup a petalinux project build.

Then you can invoke AIT with the same options provided in -fompss-fpga-ait-flags and the following new options: --from_step=boot --to_step=boot. Also, you may directly add the --to_step=boot option in -fompss-fpga-ait-flags during the LLVM/Clang launch.