Using HLS Streams for Streaming Data - 2021.2 English

Vitis High-Level Synthesis User Guide (UG1399)

Document ID
UG1399
ft:locale
English (United States)
Release Date
2021-12-15
Version
2021.2 English

One of the first enhancements which can be made to the earlier code is to use the HLS stream construct, typically referred to as an hls::stream. An hls::stream object can be used to store data samples in the same manner as an array. The data in an hls::stream can only be accessed sequentially. In the C/C++ code, the hls::stream behaves like a FIFO of infinite depth.

Code written using hls::stream will generally create designs in an FPGA which have high-performance and use few resources because an hls::stream enforces a coding style which is ideal for implementation in an FPGA.

Multiple reads of the same data from an hls::stream are impossible. Once the data has been read from an hls::stream it no longer exists in the stream. This helps remove this coding practice.

If the data from an hls::stream is required again, it must be cached. This is another good practice when writing code to be synthesized on an FPGA.

The hls::stream forces the C/C++ code to be developed in a manner which ideal for an FPGA implementation.

When an hls::stream is synthesized it is automatically implemented as a FIFO channel which is 1 element deep. This is the ideal hardware for connecting pipelined tasks.

There is no requirement to use hls::stream and the same implementation can be performed using arrays in the C/C++ code. The hls::stream construct does help enforce good coding practices.

With an hls::stream construct the outline of the new optimized code is as follows:

template<typename T, int K>
static void convolution_strm(
int width, 
int height,
hls::stream<T> &src, 
hls::stream<T> &dst,
const T *hcoeff, 
const T *vcoeff)
{

hls::stream<T> hconv("hconv");
hls::stream<T> vconv("vconv");
// These assertions let HLS know the upper bounds of loops
assert(height < MAX_IMG_ROWS);
assert(width < MAX_IMG_COLS);
assert(vconv_xlim < MAX_IMG_COLS - (K - 1));

// Horizontal convolution 
HConvH:for(int col = 0; col < height; col++) {
 HConvW:for(int row = 0; row < width; row++) {
   HConv:for(int i = 0; i < K; i++) {
 }
 }
}
// Vertical convolution 
VConvH:for(int col = 0; col < height; col++) {
 VConvW:for(int row = 0; row < vconv_xlim; row++) {
   VConv:for(int i = 0; i < K; i++) {
 }
}

Border:for (int i = 0; i < height; i++) {
 for (int j = 0; j < width; j++) {
 }
}

Some noticeable differences compared to the earlier code are:

  • The input and output data is now modeled as hls::stream.
  • Instead of a single local array of size HEIGHT*WDITH there are two internal hls::stream used to save the output of the horizontal and vertical convolutions.

In addition, some assert statements are used to specify the maximize of loop bounds. This is a good coding style which allows HLS to automatically report on the latencies of variable bounded loops and optimize the loop bounds.