Automatic Loop Pipelining - 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

The config_compile configuration enables loops to be pipelined automatically based on the iteration count. This configuration is accessed through the menu Solution > Solution Setting > General > Add > config_compile.

The pipeline_loops option sets the iteration limit. All loops with an iteration count below this limit are automatically pipelined. The default is 64.

Given the following example code:

for (y = 0; y < 480; y++) {
  for (x = 0; x < 640; x++) {
    for (i = 0; i < 5; i++) {
      // do something 5 times
 …     ...
    }
  }
}

If the pipeline_loops option is set to 6, the innermost for loop in the above code snippet will be automatically pipelined. This is equivalent to the following code snippet:

for (y = 0; y < 480; y++) {
  for (x = 0; x < 640; x++) {
    for (i = 0; i < 5; i++) {
#pragma HLS PIPELINE II=1
      // do something 5 times
 …    ...
    }
  }
}

If there are loops in the design for which you do not want to use automatic pipelining, apply the PIPELINE directive with the off option to that loop. The off option prevents automatic loop pipelining.

Important: Vitis HLS applies the config_compile pipeline_loops option after performing all user-specified directives. For example, if Vitis HLS applies a user-specified UNROLL directive to a loop, the loop is first unrolled, and automatic loop pipelining cannot be applied.