5.2.1. Unrolling a Loop (unroll Pragma)

Intel® FPGA SDK for OpenCL™ Pro Edition: Programming Guide

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ID 683846

Date 12/13/2021

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5.2.1. Unrolling a Loop (unroll Pragma)

Loop unrolling involves replicating a loop body multiple times, and reducing the trip count of a loop. Unroll loops to reduce or eliminate loop control overhead on the FPGA. In cases where there are no loop-carried dependencies and the offline compiler can perform loop iterations in parallel, unrolling loops can also reduce latency and overhead on the FPGA.

To direct the offline compiler to unroll a loop, or explicitly not to unroll a loop, insert an unroll kernel pragma in the kernel code preceding a loop you want to unroll.

Note: Unrolling of nested loops with large bounds might generate huge number of instructions, which can lead to very long compile times.

Provide an unroll factor whenever possible. To specify an unroll factor N, insert the #pragma unroll <N> directive before a loop in your kernel code.
The offline compiler attempts to unroll the loop at most <N> times.
Consider the code fragment below. By assigning a value of 2 as the unroll factor, you direct the offline compiler to unroll the loop twice.
```
#pragma unroll 2
for(size_t k = 0; k < 4; k++)
{
   mac += data_in[(gid * 4) + k] * coeff[k];
}
```
To prevent a loop from unrolling, specify an unroll factor of 1 (that is, #pragma unroll 1).
To unroll a loop fully, you may omit the unroll factor by simply inserting the #pragma unroll directive before a loop in your kernel code.
The offline compiler attempts to unroll the loop fully if it understands the trip count. The offline compiler issues a warning if it cannot execute the unroll request.
Consider the following code fragment where the unroll factor is 2:
```
float data[N];
#pragma unroll 2
for (int i = 0; i < N; i++){
  data[i] = function(i, a);
}
```
The offline compiler partially unrolls the loop as shown in the following code fragment:
```
float data[N];
for (int i = 0; i < N; i += 2){
  data[i + 0] = function(i + 0, a);
  if (i + 1 < N){
    data[i + 1] = function(i + 1, a);
  }
}
```

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Intel® FPGA SDK for OpenCL™ Pro Edition: Programming Guide

5.2.1. Unrolling a Loop (unroll Pragma)