Pipeline Constraint Violation - 2023.2 English

Description

Explanation

The II violation message is issued when the tool cannot satisfy the user-specified hard constraint. II violation conceptually occurs because of the hardware dependencies generated by the HLS from user code when the function/loop is pipelined with II=1. These dependencies are the same as the processor dependencies as explained in this wiki: https://en.wikipedia.org/wiki/Data_dependency

These dependencies can be categorized into three sections:

Memory dependencies

Memory dependencies arise when there is memory port contention. This contention only occurs if the loop is pipelined with II=1 and the number of access per loop iteration > 2. This limitation of memory ports is a hardware limit of a storage resource - BRAM/URAM.

So if the user tries to perform >2 access per loop iteration the tool won't be able to schedule the operations in one cycle. This can be visualized in Vitis High-Level Synthesis User Guide (UG1399)

Solution: Partition the array with regards to the user code or refactor the array to multiple dimensions.

Data dependencies

A data dependence is a property of the program, where the current instruction uses the value produced by an earlier instruction. There are 3 main scenarios where these dependencies can arise.

• The current loop iteration uses the value generated by an earlier loop iteration. and this value takes N cycles to be generated by the earlier iteration. This prevents the current iteration starting at the next cycle.
  • The following snippet displays the behavior.
  • In the below example because of the multiplication of a float variable the loop cannot schedule at II=1.

  float a;
  float b;
  for(...)
  {
  #pragma HLS pipeline II=1
  val *= a*b;
   
  }

• The current loop iteration uses the value generated by an earlier loop iteration but in infrequent terms. and this value takes N cycles to be generated by the earlier iteration. This prevents the current iteration starting at the next cycle.

• The current loop iteration exit condition uses the value generated by an earlier loop iteration. and this value takes N cycles to be generated by the earlier iteration. This prevents the current iteration starting at the next cycle.

  The following code snippet shows the behavior.

  float a;
  float b;
  for(...)
  {
  #pragma HLS pipeline II=1
  if(val==3.69)
      break;
  val *= a*b;
   
  }

Solution: These dependencies are visualized in the schedule viewer l to help the user understand. Break up these dependencies in the user code.

False dependencies

void histogram(int in[INPUT SIZE], int hist[VALUE SIZE]) {
int acc = 0;
int i, val;
int old = in[0];
#pragma HLS DEPENDENCE variable=hist intra RAW false
for(i = 0; i < INPUT SIZE; i++)
{
#pragma HLS PIPELINE II=1
val = in[i];
if(old == val)
{
acc = acc + 1;
}
else
{
hist[old] = acc;
acc = hist[val] + 1;
}
old = val;
}
hist[old] = acc;