Queue poping wrong value when otherwise unused

[nanduraj1]

Thanks for taking the time to report this.

Can you attach an example that shows the issue? (Must be openly licensed, ideally in test_regress format.)
queue_test.tar.gz

Test

module tb_top(
    input clk
);

    logic q [$];
    int cycle = 0;

    always@(posedge clk) begin
        q.push_front(1'b1);
        cycle <= cycle + 1'b1;
    end

    always@(posedge clk) begin
        if(cycle == 'd1) begin
            if (q.pop_back() == 1'b1) $display("Passed");
            else begin
                $display("Failed");
                $fatal;
            end
        end
    end

endmodule

#include "verilated.h"
#include "Vtb_top.h"
#include <ctime>

double sc_time_stamp() {return 2 * 1e9;}

int main(int argc, char **argv, char **env)
{
    Vtb_top* top = new Vtb_top;
    for (uint8_t d1=0, d2=0; true; d1++, d2++) {
        top->clk = !top->clk;
        top->eval();
        if (Verilated::gotFinish()) break;
    }
    top->final();
    exit(0);
}

Output

make
verilator -Mdir build_verilator tb_top.sv  -cc -no-decoration -O3 -CFLAGS -Wno-attributes -CFLAGS -O2  \
 +define+SIM_VERILATOR  -top-module tb_top -exe main.cpp
cd ./build_verilator && \
make -j -f Vtb_top.mk Vtb_top && \
cd -
make[1]: Entering directory '/home/ubuntu/Work/misc_verilog/verilator_exp/build_verilator'
ccache g++  -I.  -MMD -I/home/ubuntu/softwares/verilator/include -I/home/ubuntu/softwares/verilator/include/vltstd -DVM_COVERAGE=0 -DVM_SC=0 -DVM_TRACE=0 -Wno-sign-compare -Wno-uninitialized -Wno-unused-but-set-variable -Wno-unused-parameter -Wno-unused-variable -Wno-shadow     -Wno-attributes -O2  -Os -c -o main.o ../main.cpp
ccache g++  -I.  -MMD -I/home/ubuntu/softwares/verilator/include -I/home/ubuntu/softwares/verilator/include/vltstd -DVM_COVERAGE=0 -DVM_SC=0 -DVM_TRACE=0 -Wno-sign-compare -Wno-uninitialized -Wno-unused-but-set-variable -Wno-unused-parameter -Wno-unused-variable -Wno-shadow     -Wno-attributes -O2  -Os -c -o verilated.o /home/ubuntu/softwares/verilator/include/verilated.cpp
/usr/bin/perl /home/ubuntu/softwares/verilator/bin/verilator_includer -DVL_INCLUDE_OPT=include Vtb_top.cpp Vtb_top__Slow.cpp Vtb_top__Syms.cpp > Vtb_top__ALL.cpp
ccache g++  -I.  -MMD -I/home/ubuntu/softwares/verilator/include -I/home/ubuntu/softwares/verilator/include/vltstd -DVM_COVERAGE=0 -DVM_SC=0 -DVM_TRACE=0 -Wno-sign-compare -Wno-uninitialized -Wno-unused-but-set-variable -Wno-unused-parameter -Wno-unused-variable -Wno-shadow     -Wno-attributes -O2  -Os -c -o Vtb_top__ALL.o Vtb_top__ALL.cpp
ar -cr Vtb_top__ALL.a Vtb_top__ALL.o
ranlib Vtb_top__ALL.a
g++    main.o verilated.o Vtb_top__ALL.a      -o Vtb_top
make[1]: Leaving directory '/home/ubuntu/Work/misc_verilog/verilator_exp/build_verilator'
/home/ubuntu/Work/misc_verilog/verilator_exp
./build_verilator/Vtb_top
Failed
[2000000000] %Error: tb_top.sv:18: Assertion failed in TOP.tb_top
%Error: tb_top.sv:18: Verilog $stop
Aborting...
Makefile:53: recipe for target 'sim_verilator' failed
make: *** [sim_verilator] Aborted (core dumped)

What 'verilator --version' are you using? Did you try it with the git master version?

verilator version : Verilator 4.040 2020-08-15 rev v4.040-9-gc4f7bd6

Would you be willing to try to fix Verilator yourself with assistance?
yes

[wsnyder]

pop_back isn't correctly seen by verilator as using the queue, it only changes the queue, so Verilator removes q as it thinks it is unused. Will need some refactoring to fix this, so stay tuned.

Meanwhile, as a workaround for the moment, use

           $display("Failed %d", q.size);