全局变量和类 - 2021.2 Chinese

赛灵思不建议在类中使用全局变量。此类变量可能阻止执行某些最优化。在以下代码示例中，类用于创建筛选工具组件（polyd_cell 类用作为执行移位、乘法和累加运算的组件）。

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++;
}

在 polyd_cell 类中包含用于数据移位的 SHIFT 循环。如果 SHIFT 循环中使用的循环索引 k 已被移除并替换为对应 k 的全局索引（如上示例所示，但带有 static int k 注释），则 Vitis HLS 无法对使用 polyd_cell 类的任何循环或函数进行流水打拍。Vitis HLS 将发出如下消息：

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

使用局部非全局变量来建立循环索引即可确保 Vitis HLS 能执行所有最优化操作。