Array Reshaping - 2023.2 English

Vitis High-Level Synthesis User Guide (UG1399)

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2023.2 English

The ARRAY_RESHAPE directive reforms the array with a vertical mode of remapping, and is used to reduce the number of block RAM consumed while providing parallel access to the data.

Given the following example code:

void foo (...) {
int  array1[N];
int  array2[N];
int  array3[N];
#pragma HLS ARRAY_RESHAPE variable=array1 type=block factor=2 dim=1
#pragma HLS ARRAY_RESHAPE variable=array2 type=cyclic factor=2 dim=1
#pragma HLS ARRAY_RESHAPE variable=array3 type=complete dim=1

The ARRAY_RESHAPE directive transforms the arrays into the form shown in the following figure.

Figure 1. Array Reshaping

The ARRAY_RESHAPE directive allows more data to be accessed in a single clock cycle. In cases where more data can be accessed in a single clock cycle, Vitis HLS might automatically unroll any loops consuming this data, if doing so will improve 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 syn.unroll.tripcount_threshold configuration command as described Unroll Setting.

In the following example, any loops with a tripcount of less than 16 will be automatically unrolled if doing so improves the throughput.