How to load coverage_db? - code-coverage

Question

I'm trying to write a functional coverage for my design. I wrote needed covergroups, but now I faced with difficulties to transport my coverage between test runs.
Here a few code examples:
`include "uvm_macros.svh"
package t;
import uvm_pkg::*;
class test extends uvm_test;
`uvm_component_utils(test)
byte unsigned data;
covergroup cg;
dat: coverpoint data;
endgroup
function new(string name, uvm_component parent);
super.new(name,parent);
cg = new();
endfunction: new
task run_phase(uvm_phase phase);
phase.raise_objection(this);
repeat(5) begin
#5data = $urandom();
#10 cg.sample();
end
phase.phase_done.set_drain_time(this, 50ns);
phase.drop_objection(this);
endtask
endclass: test
endpackage
module top();
import uvm_pkg::*;
import t::*;
initial begin
$load_coverage_db("coverage_report.db");
$set_coverage_db_name("coverage_report.db");
run_test();
end
endmodule
If I try to run test above, I get this error:
** Error: (vsim-6844) Covergroup '/t/test/cg' has no instance created in simulation, ignoring it
Clearly, the problem is that cg created after test start, while config_db loading when it's not created yet. So I placed $load_coverage_db in run_phase like this:
`include "uvm_macros.svh"
package t;
import uvm_pkg::*;
class test extends uvm_test;
`uvm_component_utils(test)
byte unsigned data;
covergroup cg;
dat: coverpoint data;
endgroup
function new(string name, uvm_component parent);
super.new(name,parent);
cg = new();
endfunction: new
task run_phase(uvm_phase phase);
$load_coverage_db("coverage_report.db");
phase.raise_objection(this);
repeat(5) begin
#5data = $urandom();
#10 cg.sample();
end
phase.phase_done.set_drain_time(this, 50ns);
phase.drop_objection(this);
endtask
endclass: test
endpackage
module top();
import uvm_pkg::*;
import t::*;
initial begin
$set_coverage_db_name("coverage_report.db");
run_test();
end
endmodule
Now, I'm getting this type of warning:
** Warning: (vsim-6841) Covergroup instance '/t::test::cg ' exists in simulation but not found in database
What I need to do, to get my old coverage in test?

Answer 1

After I already wrote a python script that creates and updates my own sqlite database after each run of tests from the coverage exported to a text file,I finally found out that there is a vcover merge command in questa sim that merges the coverage of all tests.
As far as I understand, this is the generally accepted practice: to keep the coverage of each of the tests and merge them together, rather than loading the total coverage into each of the new tests.
But there is still a problem that vcover has very poor help and there is practically no mention of it in the documentation.