Dataflow Canonical Rules – 1 - 2022.2 English

Vitis HLS Messaging (UG1448)

Document ID
UG1448
Release Date
2022-12-16
Version
2022.2 English

Description

This message reports that the dataflow region is not following canonical rules.

Solution

Vitis HLS transforms the region to apply DATAFLOW optimization. It is recommended to write the code inside this region (referred to as the canonical region) using canonical forms.

The canonical form for a function where functions are not inlined:

void dataflow(Input0, Input1, Output0, Output1)
{
#pragma HLS dataflow
UserDataType C0, C1, C2;
func1(read Input0, read Input1, write C0, write C1);
func2(read C0, read C1, write C2);
func3(read C2, write Output0, write Output1);
}

For the for loop (where no function inside is inlined), the integral loop variable requires:

  1. Initial value declared in the loop header and set to 0.
  2. The loop condition is a positive numerical constant or constant function argument.
  3. Increment by 1.
  4. Dataflow pragma needs to be inside the loop.
  5. Should have a single loop counter.
void dataflow(Input0, Input1, Output0, Output1)
{
for (int i = 0; i < N; i++)
{
#pragma HLS dataflow
UserDataType C0, C1, C2;
func1(read Input0, read Input1, write C0, write C1);
func2(read C0, read C0, read C1, write C2);
func3(read C2, write Output0, write Output1);
}
}