Sequential Loops - 2021.2 English

void adder(unsigned int *in, unsigned int *out, int inc, int size) {

  unsigned int in_internal[MAX_SIZE];
  unsigned int out_internal[MAX_SIZE];
  mem_rd: for (int i = 0 ; i < size ; i++){
    #pragma HLS PIPELINE
    // Reading from the input vector "in" and saving to internal variable
    in_internal[i] = in[i];
  }
  compute: for (int i=0; i<size; i++) {
  #pragma HLS PIPELINE
    out_internal[i] = in_internal[i] + inc;
  } 

  mem_wr: for(int i=0; i<size; i++) {
  #pragma HLS PIPELINE
    out[i] = out_internal[i];
  }
}

In the previous example, three sequential loops are shown: mem_rd, compute, and mem_wr.

• The mem_rd loop reads input vector data from the memory interface and stores it in internal storage.
• The main compute loop reads from the internal storage and performs an increment operation and saves the result to another internal storage.
• The mem_wr loop writes the data back to memory from the internal storage.

This code example is using two separate loops for reading and writing from/to the memory input/output interfaces to infer burst read/write.

By default, these loops are executed sequentially without any overlap. First, the mem_rd loop finishes reading all the input data before the compute loop starts its operation. Similarly, the compute loop finishes processing the data before the mem_wr loop starts to write the data. However, the execution of these loops can be overlapped, allowing the compute (or mem_wr) loop to start as soon as there is enough data available to feed its operation, before the mem_rd (or compute) loop has finished processing its data.

The loop execution can be overlapped using dataflow optimization as described in Dataflow Optimization.