Scheduling Separator - 2024.1 English

AI Engine-ML Kernel and Graph Programming Guide (UG1603)

Document ID
UG1603
Release Date
2024-06-06
Version
2024.1 English

The AI Engine-ML compiler can reorder data movements and expressions in a basic block. When data dependence is not seen by the compiler, or when you want to intentionally insert a sequential point in the code, the pragma chess_separator_scheduler() can be used to separate parts of the code, as follows:

func(...){
  //code block 1
  chess_separator_scheduler();
  //code block 2
}

The chess_separator_scheduler(N) pragma is another form of the chess_separator_scheduler() pragma, where N indicates additional N cycles between two blocks. N can be positive or negative. With a negative offset, you can allow a partial overlap (up to absolute N cycles) between two blocks.

For example, the compiler does not have knowledge about dependence between the different streams of the kernel. It can schedule different stream reads or writes in the same cycle. If any stream read or write stalls the kernel, it relies on an external source to supply or consume the data to or from the kernel. In the following example code, the stream write (to out) and read (from receive_back) can be scheduled in the same cycle.

void producer(output_stream<int32> *out, input_stream<int32> *receive_back){
  int32 data;
  ...
  writeincr(out,data); //schedule in the same cycle
  readincr(receive_back); //schedule in the same cycle
  ...
}

The above kernel will stall if there is no data read from stream receive_back. Thus, there will be no data sent to stream out. If an external source must receive data from producer before sending data to receive_back, the kernel stalls and cannot be recovered. To schedule the stream operations in different cycles, the chess_separator_scheduler(N) can be added, as follows:

void producer(output_stream<int32> *out, input_stream<int32> *receive_back){
  int32 data;
  ...
  writeincr(out,data);
  //Make sure read occurs after write and data is sent out before a possible stall
  chess_separator_scheduler(1);
  readincr(receive_back); 
  ...
}