Dealing with SystemVerilog constraint solver failures &#

[cvc]

… Tuesday Technote on Solver Debug, Jijo PS, Srini TeamCVC www.cvcblr.com

Dealing with simple solver failure – looking for really “quick help”. It is a layered SystemVerilog code for a SAN Router. An inherited constraint in a testcase showed randomize() failure. Before you jump to conclusion on the simple nature of the problem – consider that this is the first time ever I look at this design/env as the original author moved out of the company (sign of good times ?) and am given to fix the code ASAP – in next 15 minutes that’s (sounds way too familiar, Huh?).

# Number of fware xactn 19
# ** Fatal: [Time 0 ns] Test cfg Solver failure
#    Time: 0 ns  Scope: san_rt_top.san_rt_test_pgm_0.b1.lp.a1 File: ../
rt_test_03.sv Line: 83
# ** Note: Data structure takes 9699728 bytes of memory
#          Process time 0.03 seconds
#          $finish    : ../test/san_rt_test_03.sv(83)
#    Time: 0 ns  Iteration: 2  Instance: /san_rt_top/san_rt_test_pgm_0

So what next? Consult our friendly Questa SolveDebug: add vsim –solvedebug and bang you go…

It does 2 things:

It prints the minimal set of conflicting constraints,
Creates a stand-alone test to reproduce the failure in a crisp testcase. See below:
Minimal set of constraints from user-code

# ../test/san_rt_test_03.sv(82): randomize() failed due to conflicts between the following constraints:
#     ../test/san_rt_test_03.sv(59): san_rt_test_cfg_0.cst_reasonable_fw_xactns_1 { (san_rt_test_cfg_0.no_of_fware_xactions > 32′h00001360); }
#     ../src/san_rt_fware_gen.sv(42): san_rt_test_cfg_0.cst_reasonable_fw_xactns { (san_rt_test_cfg_0.no_of_fware_xactions < 32′h00000032); }
# ** Fatal: [Time 0 ns] Test cfg Solver failure
#    Time: 0 ns  Scope: san_rt_top.san_rt_test_pgm_0.b1.lp.a1 File: ../test/san_rt_test_03.sv Line: 83
# ** Note: Data structure takes 9699728 bytes of memory
#          Process time 0.02 seconds
#          $finish    : ../test/san_rt_test_03.sv(83

Testcase being created by Questa (system verilog code, can be run standalone)

# ../test/san_rt_test_03.sv(82): randomize() failed; generating simplified testcase scenario…
# —– begin testcase —–
# module top;
#
# class TFoo;
#     rand bit [15:0] \san_rt_test_cfg_0.no_of_fware_xactions ;
#     constraint all_constraints {
#         // ../src/san_rt_fware_gen.sv(42): san_rt_test_cfg_0.cst_reasonable_fw_xactns { (san_rt_test_cfg_0.no_of_fware_xactions < 32′h00000032); }
#         (\san_rt_test_cfg_0.no_of_fware_xactions  < 32′h00000032);
#         // ../test/san_rt_test_03.sv(62): san_rt_test_cfg_0.small_tst_cst { (san_rt_test_cfg_0.no_of_fware_xactions < 32′h000013ec); }
#         (\san_rt_test_cfg_0.no_of_fware_xactions  < 32′h000013ec);
#         // ../test/san_rt_test_03.sv(59): san_rt_test_cfg_0.cst_reasonable_fw_xactns_1 { (san_rt_test_cfg_0.no_of_fware_xactions > 32′h00001360); }
#         (\san_rt_test_cfg_0.no_of_fware_xactions  > 32′h00001360);
#     }
# endclass
#
# TFoo f = new;
# int status;
#
# initial begin
#     status = f.randomize();
#     $display(status);
# end
#
# endmodule
# —– end testcase —–
#

Now that was easy to fix, simply override the test-specific constraint in the inherited test_cfg than “adding to it”. Glad I met my deadline for today!

Hats off Questa – wish it prints the vsim –solvefaildebug automatically on such failures to log file.

TeamCVC

www.cvcblr.com/blog