Unroll Loop

Syntax

#pragma HLS loop unroll factor(<int>)

Description

Specifies a loop to be unrolled.

Parameters

The factor indicates how many times to unroll the loop. If it is not specified, or specified as N (the total number of loop iterations), the loop will be fully unrolled. If it is specified as 2, the loop will be unrolled 2 times, where the number of loop iterations will be halved and the loop body will be replicated twice. If it is specified as 1, the loop will NOT be unrolled.

Parameter	Value	Optional	Default	Description
factor	Integer	Yes	N (fully unroll)	Unroll count

Position

Before the beginning of the loop.

Note

If there is a loop label, the pragma should be placed after the label.

Examples

Fully unroll a loop.

#pragma HLS loop unroll
for (int i = 0; i < 10; i++) {
  ...
}

Unroll the loop by 2 times only.

LOOP_LABEL:
#pragma HLS loop unroll factor(2)
while (i < 10) {
  ...
}

Small loops may be unrolled even without the unroll pragma. Make sure the loop is not unrolled.

#pragma HLS loop unroll factor(1)
for (int i = 0; i < 10; i++) {
  ...
}

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