SystemVerilog randomizer

SystemVerilog
1

Hello all,

I am having hard times with the SV randomizer, and I cannot find the problem with my code.

The code I am using is as follows:


class TestClass;
    rand bit [3:0]  opcode      ;
    rand bit [15:0] req_id      ;
    rand bit [7:0]  master_id   ;
    rand bit        solo        ;
    bit [2:0]       flags       ;
    bit [31:0]      rsv         ;

    constraint c_opcode {
        opcode >= 0 && opcode <= 2;
    }

    constraint c_master_id {
        master_id > 0;
    }

    constraint c_req_id {
        (opcode == 1) -> (req_id == 16'hfe0f);
        solve opcode before req_id;
    }

    function void display();
        $display("dbug: opcode=%0d,req_id=0x%h,master_id=%0d,solo=%0b,flags=%0b,rsv=0x%h", opcode, req_id, master_id, solo, flags, rsv);
    endfunction
endclass

module RandomTest;
    TestClass test;
    integer master_id_max;

    initial begin
        test = new();
        master_id_max = 1 + ($urandom % 16);
        $display("init: master_id_max=%0d", master_id_max);

        for(integer idx = 1; idx <= 1000; idx++) begin
            assert( test.randomize() with {
                master_id < master_id_max;
            } ) else $error("Something went wrong with the randomizer");
            test.display();
        end
    end
endmodule

I have tried to run the code using different seeds, and different seeds give very different results. For instance:

Case #1 xsim RandomTest -tclbatch xsim.in -sv_seed 87654
Results: no errors from the randomizer, but the distribution of the results is somehow strange. The value 0xfe0f appears many times (~25% of the iterations), and also when opcode is different than 1. The histogram of opcode field in this case is: opcode==0 50 times, opcode==1 433 times, opcode==2 65 times and opcode==3 72 times. Why opcode==1 is the most sampled? Do implication constraints bias the randomizer in some way?

Case #2: xsim RandomTest -tclbatch xsim.in -sv_seed 493046407
Results: Every call to randomize triggers the else branch of the assert, and part of the log (first 10 iterations) is given next:



****** xsim v2021.1 (64-bit)
  **** SW Build 3247384 on Thu Jun 10 19:36:07 MDT 2021
  **** IP Build 3246043 on Fri Jun 11 00:30:35 MDT 2021
    ** Copyright 1986-2021 Xilinx, Inc. All Rights Reserved.

source xsim.dir/work.RandomTest/xsim_script.tcl
# xsim {work.RandomTest} -autoloadwcfg -tclbatch {xsim.in} -sv_seed 493046407
Time resolution is 1 ps
source xsim.in
## run -all
init: master_id_max=1
WARNING: File: /home/user/docker-volumes/scorbetta-volume/sandbox/vivadio/sv_randomizer/test.sv Line: 37 : randomize failed to meet constraint.
Error: Something went wrong with the randomizer
Time: 0 ps  Iteration: 0  Process: /RandomTest/Initial31_0  Scope: RandomTest.Block36_1.Block36_2  File: /home/user/docker-volumes/scorbetta-volume/sandbox/vivadio/sv_randomizer/test.sv Line: 39
dbug: opcode=3,req_id=0x0000,master_id=176,solo=0,flags=0,rsv=0x00000000
WARNING: File: /home/user/docker-volumes/scorbetta-volume/sandbox/vivadio/sv_randomizer/test.sv Line: 37 : randomize failed to meet constraint.
Error: Something went wrong with the randomizer
Time: 0 ps  Iteration: 0  Process: /RandomTest/Initial31_0  Scope: RandomTest.Block36_1.Block36_2  File: /home/user/docker-volumes/scorbetta-volume/sandbox/vivadio/sv_randomizer/test.sv Line: 39
dbug: opcode=8,req_id=0x0000,master_id=48,solo=1,flags=0,rsv=0x00000000
WARNING: File: /home/user/docker-volumes/scorbetta-volume/sandbox/vivadio/sv_randomizer/test.sv Line: 37 : randomize failed to meet constraint.
Error: Something went wrong with the randomizer
Time: 0 ps  Iteration: 0  Process: /RandomTest/Initial31_0  Scope: RandomTest.Block36_1.Block36_2  File: /home/user/docker-volumes/scorbetta-volume/sandbox/vivadio/sv_randomizer/test.sv Line: 39
dbug: opcode=3,req_id=0x0000,master_id=63,solo=0,flags=0,rsv=0x00000000
WARNING: File: /home/user/docker-volumes/scorbetta-volume/sandbox/vivadio/sv_randomizer/test.sv Line: 37 : randomize failed to meet constraint.
Error: Something went wrong with the randomizer
Time: 0 ps  Iteration: 0  Process: /RandomTest/Initial31_0  Scope: RandomTest.Block36_1.Block36_2  File: /home/user/docker-volumes/scorbetta-volume/sandbox/vivadio/sv_randomizer/test.sv Line: 39
dbug: opcode=9,req_id=0x0000,master_id=186,solo=1,flags=0,rsv=0x00000000
WARNING: File: /home/user/docker-volumes/scorbetta-volume/sandbox/vivadio/sv_randomizer/test.sv Line: 37 : randomize failed to meet constraint.
Error: Something went wrong with the randomizer
Time: 0 ps  Iteration: 0  Process: /RandomTest/Initial31_0  Scope: RandomTest.Block36_1.Block36_2  File: /home/user/docker-volumes/scorbetta-volume/sandbox/vivadio/sv_randomizer/test.sv Line: 39
dbug: opcode=10,req_id=0x0000,master_id=133,solo=0,flags=0,rsv=0x00000000
WARNING: File: /home/user/docker-volumes/scorbetta-volume/sandbox/vivadio/sv_randomizer/test.sv Line: 37 : randomize failed to meet constraint.
Error: Something went wrong with the randomizer
Time: 0 ps  Iteration: 0  Process: /RandomTest/Initial31_0  Scope: RandomTest.Block36_1.Block36_2  File: /home/user/docker-volumes/scorbetta-volume/sandbox/vivadio/sv_randomizer/test.sv Line: 39
dbug: opcode=5,req_id=0x0000,master_id=8,solo=0,flags=0,rsv=0x00000000
WARNING: File: /home/user/docker-volumes/scorbetta-volume/sandbox/vivadio/sv_randomizer/test.sv Line: 37 : randomize failed to meet constraint.
Error: Something went wrong with the randomizer
Time: 0 ps  Iteration: 0  Process: /RandomTest/Initial31_0  Scope: RandomTest.Block36_1.Block36_2  File: /home/user/docker-volumes/scorbetta-volume/sandbox/vivadio/sv_randomizer/test.sv Line: 39
dbug: opcode=1,req_id=0x0000,master_id=197,solo=0,flags=0,rsv=0x00000000
WARNING: File: /home/user/docker-volumes/scorbetta-volume/sandbox/vivadio/sv_randomizer/test.sv Line: 37 : randomize failed to meet constraint.
Error: Something went wrong with the randomizer
Time: 0 ps  Iteration: 0  Process: /RandomTest/Initial31_0  Scope: RandomTest.Block36_1.Block36_2  File: /home/user/docker-volumes/scorbetta-volume/sandbox/vivadio/sv_randomizer/test.sv Line: 39
dbug: opcode=11,req_id=0x0000,master_id=0,solo=1,flags=0,rsv=0x00000000
WARNING: File: /home/user/docker-volumes/scorbetta-volume/sandbox/vivadio/sv_randomizer/test.sv Line: 37 : randomize failed to meet constraint.
Error: Something went wrong with the randomizer
Time: 0 ps  Iteration: 0  Process: /RandomTest/Initial31_0  Scope: RandomTest.Block36_1.Block36_2  File: /home/user/docker-volumes/scorbetta-volume/sandbox/vivadio/sv_randomizer/test.sv Line: 39
dbug: opcode=14,req_id=0x0000,master_id=222,solo=1,flags=0,rsv=0x00000000
WARNING: File: /home/user/docker-volumes/scorbetta-volume/sandbox/vivadio/sv_randomizer/test.sv Line: 37 : randomize failed to meet constraint.
Error: Something went wrong with the randomizer
Time: 0 ps  Iteration: 0  Process: /RandomTest/Initial31_0  Scope: RandomTest.Block36_1.Block36_2  File: /home/user/docker-volumes/scorbetta-volume/sandbox/vivadio/sv_randomizer/test.sv Line: 39
dbug: opcode=15,req_id=0x0000,master_id=250,solo=1,flags=0,rsv=0x00000000

What is wrong with the code?
Thank you for your help

Cheers
S

2

In reply to simon83:

The value generated for master_id_max is 1. While randomizing you are providing the inline constraint master_id < master_id_max , i.e., master_id should be less than 1, but it is conflicting with the constraint c_master_id {master_id > 0;}. Hence the randomize is failed.

3

In reply to shanthi:

Hello @shanthi, thanks for pointing that out :)

What about the distribution of the random values in Case#1? Any idea on how does the random engine work?

Thanks
S