要随机产生一个4bit的 -1 ~ -7的数：
val = $urandom_range(-1,-7);

要随机产生一个4bit的 0 ~ -7的数：
val = $urandom_range(-0,-7); // 这样写错误


需要写成
std::randomize(val) with { val inside {0,[9:15]};};

参考：

https://www.cnblogs.com/-9-8/p/4414449.html

http://www.itkeyword.com/doc/1250864225704845x672/difference-between-stdrandomize-and-class-based-randomize

https://blog.51cto.com/13824643/2145849  randsequence / randcase

Example:

constraint cons_value{
    num1 < 1024;
    num2 > 0;
    (rd_flag == 1'b1) -> data_queue.size == num1 - num1;
    data_queue.sum == 1024;
    if(mode == 1'b0) { num dist {[1:10]:/20,[11:12]:/60};}
    if(gen_mode == 1){ 
        sim inside {0,1,2};
        (sim == 2) -> a == 4;   
    } else if (gen_mode == 4) {
        b ==5;
    }
    foreach(data_queue[i]){
        data_queue[i] inside {[0:data]};
        if(wr_flag == 1） {
            data_queue[i] % 9 == 0;
        }
    }
}

#val变量中4个bit为1，其余为0.
void'(std::randomize(val) with { $countones(val) inside {tr.num} ; }); 

assert(std::randomize(val_a) with {val_a inside {[0,10],[100,1000]}；}；

sucess = std::randomize(val_1,val_2,val_3) with { 
   val_1 inside { A ,B ,C};
   val_2 dist { A := 2 ,B := 5 ,C := 4 };
   val_3 inside {[0:{32{1'b}}]}; 
};

if(  sucess == 0 ) begin
   `uvm_fatal("TEST", " randomization failed")
end

class rand_value extends uvm_sequence_item;
    randc int value;
    constraint c_value {value inside {[1:16]};}
endclass

rand_value r_value;
r_value = new();
repeat(16) begin
    //r_value = new(); #放在内部错误，无法遍历到
    r_value.randomize();
    $display("value:%0d",r_value.vaule);
end

post_randomize()

post_randomize 用于有先后关系的随机化

class Package_Gen;
    rand bit [7:0] value[];
    rand bit [7:0] index[];
    
    package_type cmd_s;

    constraint cons_package {
                             value.size inside {[0:8]};
                             index.size inside {[0:1024]};
                             index.size % 16 == 0;
                            }

    function void post_randomize();
        cmd_s.block_value = value.size;
        cmd_s.index_value = index.size;
    endfunciton
endclass

class c1;
rand int randnum;
int hist[$];
constraint cstr1 {randnum inside {[0:10]};};
constraint cstr2 {!(randnum inside {hist});};
function void post_randomize();
  hist.push_back(randnum);
endfunction
endclass

module m1;

initial begin
c1 i1;
i1 = new();
  repeat(10) begin
    assert(i1.randomize());
    $display("m1::proc2.i1 randnum %0d", i1.randnum);
  end
end
endmodule