Automatic Loop Pipelining - 2023.1 English

The config_compile -pipeline_loops command enables loops to be pipelined automatically based on the iteration count. All loops with a trip count greater than the specified limit are automatically pipelined. The default limit is 64.

Given the following example code:

loop1: for (i = 0; i < 5; i++) {
   // do something 5 times ... 
} 

If the -pipeline_loops option is set to 4, the for loop (loop1) will be pipelined. This is equivalent to adding the PIPELINE pragma as shown in the following code snippet:

loop1: for (i = 0; i < 5; i++) {
#pragma HLS PIPELINE II=1 
   // do something 5 times ... 
} 

However, for nested loops, the tool starts at the innermost loop, and determines if it can be pipelined, and then moves up the loop nest. In the example below, the -pipeline_loops threshold is still 4, and the inner most loop (loop1) is marked to be pipelined. Then the tool evaluates the parent loop of the loop pair (loop2) and finds that it can also be pipelined. In this case, the tool unrolls loop1 into loop2, and marks loop2 to be pipelined. Then it moves up to the parent loop (loop3) and repeats the analysis.

loop3: for (y = 0; y < 480; y++) {
   loop2: for (x = 0; x < 640; x++) {
      loop1: for (i = 0; i < 5; i++) {
         // 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.