pragma HLS unroll

Description

Unroll loops to create multiple independent operations rather than a single collection of operations. The UNROLL pragma transforms loops by creating multiples copies of the loop body in the RTL design, which allows some or all loop iterations to occur in parallel.

Loops in the C/C++ functions are kept rolled by default. When loops are rolled, synthesis creates the logic for one iteration of the loop, and the RTL design executes this logic for each iteration of the loop in sequence. A loop is executed for the number of iterations specified by the loop induction variable. The number of iterations might also be impacted by logic inside the loop body (for example, break conditions or modifications to a loop exit variable). Using the UNROLL pragma you can unroll loops to increase data access and throughput.

The UNROLL pragma allows the loop to be fully or partially unrolled. Fully unrolling the loop creates a copy of the loop body in the RTL for each loop iteration, so the entire loop can be run concurrently. Partially unrolling a loop lets you specify a factor N, to create N copies of the loop body and reduce the loop iterations accordingly. To unroll a loop completely, the loop bounds must be known at compile time. This is not required for partial unrolling.

Partial loop unrolling does not require N to be an integer factor of the maximum loop iteration count. Vivado HLS adds an exit check to ensure that partially unrolled loops are functionally identical to the original loop. For example, given the following code:
for(int i = 0; i < X; i++) {
  pragma HLS unroll factor=2
  a[i] = b[i] + c[i];
}
Loop unrolling by a factor of 2 effectively transforms the code to look like the following code where the break construct is used to ensure the functionality remains the same, and the loop exits at the appropriate point:
for(int i = 0; i < X; i += 2) {
  a[i] = b[i] + c[i];
  if (i+1 >= X) break;
  a[i+1] = b[i+1] + c[i+1];
}

Because the maximum iteration count X is a variable, Vivado HLS may not be able to determine its value and so adds an exit check and control logic to partially unrolled loops. However, if you know that the specified unrolling factor, 2 in this example, is an integer factor of the maximum iteration count X, the skip_exit_check option lets you remove the exit check and associated logic. This helps minimize the area and simplify the control logic.

Tip: When the use of pragmas like DATA_PACK, ARRAY_PARTITION, or ARRAY_RESHAPE, let more data be accessed in a single clock cycle, Vivado HLS automatically unrolls any loops consuming this data, if doing so improves the throughput. The loop can be fully or partially unrolled to create enough hardware to consume the additional data in a single clock cycle. This feature is controlled using the config_unroll command. See config_unroll in the Vivado Design Suite User Guide: High-Level Synthesis (UG902) for more information.

Syntax

Place the pragma in the C/C++ source within the body of the loop to unroll.

#pragma HLS unroll factor=<N> region skip_exit_check

Where:

  • factor=<N>: Specifies a non-zero integer indicating that partial unrolling is requested. The loop body is repeated the specified number of times, and the iteration information is adjusted accordingly. If factor= is not specified, the loop is fully unrolled.
  • region: An optional keyword that unrolls all loops within the body (region) of the specified loop, without unrolling the enclosing loop itself.
  • skip_exit_check: An optional keyword that applies only if partial unrolling is specified with factor=. The elimination of the exit check is dependent on whether the loop iteration count is known or unknown:
    • Fixed (known) bounds: No exit condition check is performed if the iteration count is a multiple of the factor. If the iteration count is not an integer multiple of the factor, the tool:
      1. Prevents unrolling.
      2. Issues a warning that the exit check must be performed to proceed.
    • Variable (unknown) bounds: The exit condition check is removed as requested. You must ensure that:
      1. The variable bounds is an integer multiple of the specified unroll factor.
      2. No exit check is in fact required.

Example 1

The following example fully unrolls loop_1 in function foo. Place the pragma in the body of loop_1 as shown:
loop_1: for(int i = 0; i < N; i++) {
  #pragma HLS unroll
  a[i] = b[i] + c[i];
}

Example 2

This example specifies an unroll factor of 4 to partially unroll loop_2 of function foo, and removes the exit check:
void foo (...) {
  int8 array1[M];
  int12 array2[N];
  ...
  loop_2: for(i=0;i<M;i++) {
    #pragma HLS unroll skip_exit_check factor=4
    array1[i] = ...;  
    array2[i] = ...;
    ...
  }
  ...
}

Example 3

The following example fully unrolls all loops inside loop_1 in function foo, but not loop_1 itself due to the presence of the region keyword:
void foo(int data_in[N], int scale, int data_out1[N], int data_out2[N]) {
  int temp1[N];
  loop_1: for(int i = 0; i < N; i++) {  
    #pragma HLS unroll region
    temp1[i] = data_in[i] * scale;
      loop_2: for(int j = 0; j < N; j++) {
        data_out1[j] = temp1[j] * 123;
      }
      loop_3: for(int k = 0; k < N; k++) {
        data_out2[k] = temp1[k] * 456;
      }
  }
}

See Also