Global Variables and Classes - 2021.2 English

Vitis High-Level Synthesis User Guide (UG1399)

Document ID
UG1399
ft:locale
English (United States)
Release Date
2021-12-15
Version
2021.2 English

Xilinx does not recommend using global variables in classes. They can prevent some optimizations from occurring. In the following code example, a class is used to create the component for a filter (class polyd_cell is used as a component that performs shift, multiply and accumulate operations).

typedef long long acc_t;
typedef int mult_t;
typedef char data_t;
typedef char coef_t;

#define TAPS 3
#define PHASES 4
#define DATA_SAMPLES 256
#define CELL_SAMPLES 12

// Use k on line 73 static int k;

template <typename T0, typename T1, typename T2, typename T3, int N>
class polyd_cell {
private:
public:
     T0 areg;
     T0 breg;
     T2 mreg;
     T1 preg;
 T0 shift[N];
     int k;   //line 73
 T0 shift_output;
     void exec(T1 *pcout, T0 *dataOut, T1 pcin, T3 coeff, T0 data, int col)
     {
     Function_label0:;

     if (col==0) {
   SHIFT:for (k = N-1; k >= 0; --k) {
      if (k > 0) 
        shift[k] = shift[k-1];
      else 
        shift[k] = data;
     }
     *dataOut = shift_output;
     shift_output = shift[N-1];
     }
     *pcout = (shift[4*col]* coeff) + pcin;

     }
};

// Top-level function with class instantiated
void cpp_class_data (
 acc_t *dataOut,
 coef_t coeff1[PHASES][TAPS],
 coef_t  coeff2[PHASES][TAPS],
 data_t  dataIn[DATA_SAMPLES],
 int  row
) {

 acc_t pcin0 = 0;
 acc_t pcout0, pcout1;
 data_t dout0, dout1;
 int col;
 static acc_t accum=0;
 static int sample_count = 0;
 static polyd_cell<data_t, acc_t, mult_t, coef_t, CELL_SAMPLES> 
polyd_cell0;
 static polyd_cell<data_t, acc_t, mult_t, coef_t, CELL_SAMPLES> 
polyd_cell1;

 COL:for (col = 0; col <= TAPS-1; ++col) {

 polyd_cell0.exec(&pcout0,&dout0,pcin0,coeff1[row][col],dataIn[sample_count],

col);

 polyd_cell1.exec(&pcout1,&dout1,pcout0,coeff2[row][col],dout0,col);

 
    if ((row==0) && (col==2)) {
        *dataOut = accum;
        accum = pcout1;
    } else {
        accum = pcout1 + accum;
 }

 }
 sample_count++;
}

Within class polyd_cell there is a loop SHIFT used to shift data. If the loop index k used in loop SHIFT was removed and replaced with the global index for k (shown earlier in the example, but commented static int k), Vitis HLS is unable to pipeline any loop or function in which class polyd_cell was used. Vitis HLS would issue the following message:

@W [XFORM-503] Cannot unroll loop 'SHIFT' in function 'polyd_cell<char, long long, 
int, char, 12>::exec' completely: variable loop bound.

Using local non-global variables for loop indexing ensures that Vitis HLS can perform all optimizations.