hls_max_template

An efficient C++14 description of a max process which finds the maximum of a real array.

Used in High Level Synthesis, produce the most efficient maximum possible by doing reduction at compile time.

Currently, it produces log2(N) (+1 if N is odd) stages of integer_part(N/2) comparisons, can be easily unrolled and make use of pipelines.

Content

This repository provides a header which defines two recursive template classes with a process public method that find the maximum of an array of any size and any type. max_template can be used for all possible sizes while max_pow2 works only with sizes that are power of 2. max_struct is used to implement max_template and may not be used directly.

Usage

After including the header (with #include "max.hpp"), just call the process method like that:

const T max_array = max_template<SIZE>::process(array);

Where T is a comparable class — where operators < and > are implemented — e.g. int, float or ap_uint, SIZE is an unsigned integer known at compile time (e.g. a literal, a constexpr or a preprocessor constant) and array is a C-style array containing SIZE elements T. max_pow2 is used the same. However, note that it cannot work (even if it compiles) if SIZE is not a power of 2.

It must be compiled with --std=c++14 (or gnu++14 or another equivalent).

Including

Including the max.hpp header is all that is needed to use it in another HLS project.

Testbench

The prototype of a testbench is provided, and the files can be tested with make testbench.

Xilinx® Vitis HLS

A TCL script is provided in hls_files and can be used with Xilinx® Vitis HLS. Just do in a shell:

cd hls_files
vitis_hls -f script.tcl

By default, no IP are created. You may need to adjust the part and the clock targeted.

It has been successfully tested on Vitis HLS version 2020.2 and may or may not work with other versions.

Perspectives

A template argument to produce either the current description or a more resources friendly (Higher latency and/or lower throughput) may be added in the future. Another template argument specifying the most comparisons per stages is also considered.