6.8. 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 Pro Edition does not need to preserve the variable data across loops that do not use the variables. - 2025-12-16

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 Pro 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.