Unrolling Loops in C++ Classes - 2023.2 English

Vitis High-Level Synthesis User Guide (UG1399)

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2023.2 English
Important: When loops are used in C++ classes, care should be taken to ensure that the loop induction variable is not a data member of the class as this prevents the loop from being unrolled.

In this example, loop induction variable k is a member of class loop_class.

template <typename T0, typename T1, typename T2, typename T3, int N>
class loop_class {
 pe_mac<T0, T1, T2> mac;
 T0 areg;
 T0 breg;
 T2 mreg;
 T1 preg;
    T0 shift[N];
 int k;             // Class Member
   T0 shift_output;
 void exec(T1 *pcout, T0 *dataOut, T1 pcin, T3 coeff, T0 data, int col)
#pragma HLS inline off
 SRL:for (k = N-1; k >= 0; --k) {
#pragma HLS unroll // Loop will fail UNROLL
 if (k > 0) 
 shift[k] = shift[k-1];
 shift[k] = data;

    *dataOut = shift_output;
   shift_output = shift[N-1];

 *pcout = mac.exec1(shift[4*col], coeff, pcin);

For Vitis HLS to be able to unroll the loop as specified by the UNROLL pragma directive, the code should be rewritten to remove k as a class member and make it local to the exec function.