The interfaces of primitives in this library are mostly HLS streams, with a single-bit stream along with the main data stream throughout the dataflow.
hls::stream<ap_uint<W> >& data_strm,
hls::stream<bool>& e_data_strm,
The benefits of this interface are
- Within an HLS dataflow region, all primitives connected via HLS streams can work in parallel, and this is the key to FPGA acceleration.
- Using the single-bit stream to mark end of operation can trigger stream consumer as soon as the first row data becomes available, without knowing how many rows will be generated later. Moreover, it can represent an empty table.
Some primitives work with separated columns, so their interface are naturally designed to receive multiple data streams, one for each column. As these columns are from the same table and always pass through same number of items, they would share one end signal stream in the interface.
Other primitives’ semantic may treat multiple-columns as one concept. For example, Hash-Join only cares about the key and the payload. For such primitives, combineCol and splitCol helpers can be used to combine or split columns. For more details on helper utilities, see 11. Glue Logic.