code_opt.md

Vitis™ Hardware Acceleration Tutorials See Vitis™ Development Environment on xilinx.com

NOTE: In this section, the TSP design is implemented with four parallel memory lookups to fetch distances.

Load the Project into Vitis HLS

Open a terminal, and navigate to the build directory. Launch the following command which will open the graphical interface of Vitis HLS and configure the project based on the settings included in the hls_opt.tcl Tcl file:

user@server:~$ cd ./build
user@server:~$ vitis_hls -p hls_opt.tcl &

When the tool comes up, on the left-hand side, locate the Explorer pane, expand proj->Source and double-click tsp_opt.cpp to inspect the source code. The testbench is found below in proj->TestBench in the file is tsp_TB.cpp.

Review the Code Changes

There are now four arrays for the distances:

uint16_t distances_0[N][N];
uint16_t distances_1[N][N];
uint16_t distances_2[N][N];
uint16_t distances_3[N][N];

The incoming distance data points are still read one at a time, but they are copied into all four memories:

loop_distances: for (int i = 0; i < N*N; ++i)
  {
    uint16_t val;
    streamDistances >> val;
    distances_0[i/N][i%N] = val;
    distances_1[i/N][i%N] = val;
    distances_2[i/N][i%N] = val;
    distances_3[i/N][i%N] = val;
  }

The loop_compute main loop continuously increments by four and distributes the four values to copies of the compute function. Each evaluates a route:

loop_compute: for( unsigned long int i_ = 0; i_ < factorialN; i_ += 4 )
  {
    #pragma HLS pipeline II=1
    candidate0 = std::min(candidate0, compute(i_+0, distances_0));
    candidate1 = std::min(candidate1, compute(i_+1, distances_1));
    candidate2 = std::min(candidate2, compute(i_+2, distances_2));
    candidate3 = std::min(candidate3, compute(i_+3, distances_3));
  }

Final determination of the shortest distance:

    // Determine shortest between the four candidates
    shortestDistance = std::min({ candidate0, candidate1,
                                  candidate2, candidate3 });

Running C Simulation and C Synthesis

First, you will run C simulation to confirm the optimized design works as expected. Edit the tsp.h file to make sure the number of cities is small (N=5) for faster simulation run times and Run C Simulation.

After reviewing the results of simulation, increase the number of cities again (N=13) in the tsp.h file for C synthesis so that you can compare results with the original design. Run C Synthesis.

The following figure shows the C synthesis report in the Vitis HLS GUI (the Performance and Resource Estimates section):

You will notice that:

• The latency for the tsp function is now less than a second.
• The loop distance trip count is unchanged as expected since the input data is same.
• The tripcount for the main loop (loop_compute) is now a fourth of factorial 12 (12!/4) thanks to the parallel execution of the compute function.
• The new Loop 3 originates from the final std::min call that returns the smallest of the four results obtained.

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