Side-channels are optional signals, which
are part of the AXI4-Stream standard. The
side-channel signals can be directly referenced and controlled in the C/C++ code using a
struct, provided that the member elements of the struct match the names of the AXI4-Stream side-channel signals. The AXI4-Stream side-channel signals are considered data
signals and are registered whenever
TDATA is
registered. There are significant differences in how this is handled in Vivado HLS vs
Vitis HLS. The AXI4-Stream Interfaces with Side-Channels section in the user guide (UG1399) details the
limitations and restrictions in Vitis HLS.