Summary of C++ for Efficient Hardware - 2022.1 English

Vitis High-Level Synthesis User Guide (UG1399)

Document ID
Release Date
2022.1 English

Minimize data input reads. Once data has been read into the block it can easily feed many parallel paths but the input ports can be bottlenecks to performance. Read data once and use a local cache if the data must be reused.

Minimize accesses to arrays, especially large arrays. Arrays are implemented in block RAM which like I/O ports only have a limited number of ports and can be bottlenecks to performance. Arrays can be partitioned into smaller arrays and even individual registers but partitioning large arrays will result in many registers being used. Use small localized caches to hold results such as accumulations and then write the final result to the array.

Seek to perform conditional branching inside pipelined tasks rather than conditionally execute tasks, even pipelined tasks. Conditionals will be implemented as separate paths in the pipeline. Allowing the data from one task to flow into with the conditional performed inside the next task will result in a higher performing system.

Minimize output writes for the same reason as input reads: ports are bottlenecks. Replicating addition ports simply pushes the issue further out into the system.

For C++ code which processes data in a streaming manner consider using hls::streams or hls::stream_of_blocks, as these will enforce good coding practices. It is much more productive to design an algorithm in C which will result in a high-performance FPGA implementation than debug why the FPGA is not operating at the performance required.