Implication（->） and If-Else constraint

可以使用蕴含运算符 (->) 和 if-else 构造进一步限制随机化。它们都表示两个变量之间的条件关系。隐含运算符用于声明隐含约束的表达式。例如：

constraint dataC { (data == 'hff) -> (addr == 0); }

相当于：

if (data == 'hff)  addr = 0;

看一个例子：

class dataP;
  rand bit [7:0] data;
  string    burst_mode ;
  constraint burst1 { (burst_mode == "short") -> (data < 16);}
                                             //implication
  constraint burstIf { if (burst_mode == "long") data > 16;
                  else if (burst_mode == "garbage") data == 0;}
endclass
module top;
  initial begin
    dataP d1;
    d1 = new();
    d1.burst_mode = "short";
    repeat(4) begin
      d1.randomize();
      $display("\t Short burst_mode = %s data = %0d",d1.burst_mode,d1.data);
    end
    $display("\n");
    d1.burst_mode = "long";
    repeat(4) begin
      d1.randomize();
      $display("\t Long burst_mode = %s data = %0d",d1.burst_mode,d1.data);
    end
    $display("\n");
    d1.burst_mode = "garbage";
    repeat(4) begin
      d1.randomize();
      $display("\t garbage burst_mode = %s data = %0d",d1.burst_mode,d1.data);
    end
  end
endmodule

仿真结果：

ompiler version S-2021.09; Runtime version S-2021.09; Aug 20 09:46 2022
Short burst_mode = short data = 13
Short burst_mode = short data = 6
Short burst_mode = short data = 1
Short burst_mode = short data = 2


Long burst_mode = long data = 36
Long burst_mode = long data = 109
Long burst_mode = long data = 28
Long burst_mode = long data = 47


garbage burst_mode = garbage data = 0
garbage burst_mode = garbage data = 0
garbage burst_mode = garbage data = 0
garbage burst_mode = garbage data = 0
V C S S i m u l a t i o n R e p o r t

当burst_mode == “short”时，data应该小于16。在模块“top”中，我们为字符串burst_mode分配值“short”，得到的data值均小于16。类似地，在 if-else 条件下，如果 burst_mode == long，我们看到data值大于 16，如果 burst_mode == “garbage”，data值被限制为0。