Intel® High Level Synthesis Compiler Standard Edition: Best Practices Guide

ID 683259
Date 12/18/2019
Document Table of Contents

4.7. Declare Variables in the Deepest Scope Possible

To reduce the FPGA hardware resources necessary for implementing a variable, declare the variable just before you use it in a loop. Declaring variables in the deepest scope possible minimizes data dependencies and FPGA hardware usage because the Intel® HLS Compiler Standard Edition does not need to preserve the variable data across loops that do not use the variables.

Consider the following example:

int a[N];
for (int i = 0; i < m; ++i)
    int b[N];
    for (int j = 0; j < n; ++j)
        // statements

The array a requires more resources to implement than the array b. To reduce hardware usage, declare array a outside the inner loop unless it is necessary to maintain the data through iterations of the outer loop.

Tip: Overwriting all values of a variable in the deepest scope possible also reduces the resources necessary to represent the variable.