Burst Accesses to Global Memory - 2021.2 English

Vitis Unified Software Platform Documentation: Application Acceleration Development (UG1393)

Document ID
UG1393
ft:locale
English (United States)
Release Date
2021-12-15
Version
2021.2 English

Accessing the global memory bank interface from the kernel has a large latency, so global memory transfer should be done in burst. For more information on burst transfers, refer to Optimizing Burst Transfers in the Vitis HLS User Guide (UG1399).

Tip: The Synthesis Summary report in Vitis HLS includes detailed information about burst transfers in the kernel. However, to review this report you will need to launch the tool.

To infer the burst, the following pipelined loop coding style is recommended.

hls::stream<datatype_t> str;

INPUT_READ: for(int i=0; i<INPUT_SIZE; i++) {
  #pragma HLS PIPELINE
  str.write(inp[i]); // Reading from Input interface
}

In the code example, a pipelined for loop is used to read data from the input memory interface, and writes to an internal hls::stream variable. The above coding style reads from the global memory bank in burst.

It is a recommended coding style to implement the for loop operation in the example above inside a separate function, and apply the dataflow optimization, as discussed in Dataflow Optimization. The code example below shows how this would look, letting the compiler establish dataflow between the read, execute, and write functions:

top_function(datatype_t * m_in, // Memory data Input
  datatype_t * m_out, // Memory data Output
  int inp1,     // Other Input
  int inp2) {   // Other Input
#pragma HLS DATAFLOW

hls::stream<datatype_t> in_var1;   // Internal stream to transfer
hls::stream<datatype_t> out_var1;  // data through the dataflow region

read_function(m_in, inp1, in_var1); // Read function contains pipelined for loop 
                           // to infer burst

execute_function(in_var1, out_var1, inp1, inp2); // Core compute function

write_function(out_var1, m_out); // Write function contains pipelined for loop 
                                 // to infer burst
}