我们知道virtual来修饰子函数,可以让基类的指针指向扩展类,从而利用基类中的virtual函数可以复用扩展类中的同名函数。但是constrain呢?本小结就来详细说明一下constrain。
我们通过一个例子来简单声明一下:
情况一:
class bird;
rand bit [1:0] src;
rand bit [1:0] drc;
constraint src_stim{src ==3;}
constraint drc_stim{drc ==1;};
endclass
class parrot extends bird;
constraint drc_stim{drc ==2;};
endclass
program ex6_14_tb;
bird A;
parrot B;
initial begin
A = new();
B = new();
assert(A.randomize());
$display("A:src=%0d,drc=%0d",A.src,A.drc);
assert(B.randomize());
$display("A:src=%0d,drc=%0d",A.src,A.drc);
end
endprogram
打印结果是:
A:src=3,drc=1
B:src=3,drc=2
- 如果B extends A的话,那么约束其实是可以重写和继承的。
情况二:
class bird;
rand bit [1:0] src;
rand bit [1:0] drc;
constraint src_stim{src ==3;}
constraint drc_stim{drc ==1;};
endclass
class parrot extends bird;
constraint drc_stim{drc ==2;};
endclass
program ex6_14_tb;
bird A;
parrot B;
initial begin
A = new();
B = new();
A = B;
assert(A.randomize());
$display("A:src=%0d,drc=%0d",A.src,A.drc);
assert(B.randomize());
$display("A:src=%0d,drc=%0d",A.src,A.drc);
end
endprogram
打印结果:
A:src=3,drc=2
B:src=3,drc=2
- constrain,类似于virtual task,基类=扩展类;
- 如果改成B = A,即扩展类=基类,那么编译就会报错!!
情况三:
class bird;
rand bit [1:0] src;
rand bit [1:0] drc;
constraint src_stim{src ==3;}
constraint drc_stim{drc > 1;
drc < 6;};
endclass
class parrot extends bird;
constraint drc_stim{drc ==2;};
endclass
program ex6_14_tb;
bird A;
parrot B;
initial begin
A = new();
B = new();
assert(A.randomize() with {drc ==3;});
$display("A:src=%0d,drc=%0d",A.src,A.drc);
assert(B.randomize());
$display("A:src=%0d,drc=%0d",A.src,A.drc);
end
endprogram
打印结果:
A:src=3,drc=3
B:src=3,drc=2
-
randomize() with {} 是求解drc>1;drc<6;以及drc == 3的交集;
-
如果with里面的东西与原来的约束没有交集的话,那么会显示约束求解错误。
那么比如说我就有这样的需求呢?有两种解决方案:
方案一:class bird; rand bit [5:0] src; rand bit [5:0] drc; constraint src_stim{src ==3;} constraint drc_stim{ drc > 1; soft drc < 6;}; //****************** endclass class parrot extends bird; constraint drc_stim{drc ==2;}; endclass program ex6_14_tb; bird A; parrot B; initial begin A = new(); B = new(); assert(A.randomize() with {drc ==7;}); $display("A:src=%0d,drc=%0d",A.src,A.drc); assert(B.randomize()); $display("A:src=%0d,drc=%0d",A.src,A.drc); end endprogram
打印结果:
A:src=3,drc=7
B:src=3,drc=2
- 用soft来修饰约束,这样当with的约束和内部约束有冲突的时候,我们是以with里面的约束为准。
方案二:
class bird;
rand bit [5:0] src;
rand bit [5:0] drc;
constraint src_stim{src ==3;}
constraint drc_stim{
drc > 1;
drc < 6;};
endclass
class parrot extends bird;
constraint drc_stim{drc ==2;};
endclass
program ex6_14_tb;
bird A;
parrot B;
initial begin
A = new();
B = new();
A.drc_stim.constraint_mode(0); //*****************//
assert(A.randomize() with {drc ==7;});
$display("A:src=%0d,drc=%0d",A.src,A.drc);
assert(B.randomize());
$display("A:src=%0d,drc=%0d",A.src,A.drc);
end
endprogram
打印结果:
A:src=3,drc=7
B:src=3,drc=2
- constraint_mode来关闭对应的约束,然后with里面的约束是最终的约束。