If variable width is more than 64 bit can we take coverage for that variable in SV?

Coverage
,
1

Recently I came across a scenario where the variable width is 256 bits so I have tried to take coverage of that variable by declaring bins as follows:
bins qrnb_mask = {{[ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_0000_0000_0000_0000:ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff]};}
But in coverage report, the coverage is not getting generated for the cover point.

2

In reply to Subhra Bera:

Realize that the auto_bin_max default is 64 bins, so this is the same as specifying

coverpoint my_variable[127:122] iff (my_variable[255:128] == '1);

Is that what you really want do?

3

In reply to dave_59:

Hi, Dave could you please elaborate? I know that if we do not define explicit bins the number of automatic bins gets created. In my case, I have coded in the following way:


covergroup QRNB;
option.per_instance=1;
mask_coverpoint:coverpoint qrnb_mask bins qrnb_mask [] = {{[ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_0000_0000_0000_0000 : ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff]};}
endgroup

In this above range no bins getting generated.

4

In reply to Subhra Bera:

How many bins are you expecting to see generated? What are the value ranges you expect for each generated bin?

5

In reply to dave_59:
In this case, I was expecting 2^64 bins which are too huge to calculate for the tool. This is what I understood. Correct me if I’m wrong. Values of bins I was expecting as follows:
qrnb_mask[0]=ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_0000_0000_0000_0000 ,
qrnb_mask[1]=ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_0000_0000_0000_0001 ,
qrnb_mask[2]=ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_0000_0000_0000_0002 ,
qrnb_mask[3]=ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_0000_0000_0000_0003 ,
.
.
.
.
qrnb_mask[F]=ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_0000_0000_0000_000F ,
.
.
.
qrnb_mask[FFFF_FFFF_FFFF_FFFF]=ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_ffff_FFFF_FFFF_FFFF_FFFF ,

6

In reply to Subhra Bera:

Hi, I wrote a simple test bench based on the suggestions by dave and the idea was to refute your observations that “coverage is not getting generated for the cover point”. I found the values are covered.

But I am not sure if coding 2^64 bins is a pragmatic approach. Rather divide values in pre-defined ranges based on constrained stimulus generated.

module test();
 class cov;
  bit [255:0] qrnb_mask ;
  event e;
  covergroup QRNB @(e.triggered);
    option.per_instance=1;
    CP_qrnb_mask : coverpoint qrnb_mask [63:0] iff (qrnb_mask [255:64] == '1) {
      bins qrnb_mask_b = {[64'h0 : 64'hffff_ffff_ffff_ffff]};
    }
  endgroup : QRNB
   
   function new();
     QRNB = new();
   endfunction : new
   
 endclass : cov
   cov c;
  initial begin
    c = new();
    std::randomize(c.qrnb_mask) with {c.qrnb_mask [255:64] == '1;};
    $display("qrnb_mask  = %0h",c.qrnb_mask);
    ->c.e;
  end
endmodule

Results:

CLASS - work.test/\test cov \


    SUMMARY
    =============================================
    |    Coverage Type    | Weight | Hits/Total |
    =============================================
    | Covergroup Coverage |      1 |   100.000% |
    |---------------------|--------|------------|
    | Types               |        |      1 / 1 |
    =============================================
    WEIGHTED AVERAGE: 100.000%


    COVERGROUP COVERAGE
    =======================================================================
    |             Covergroup              |   Hits   |  Goal /  | Status  |
    |                                     |          | At Least |         |
    =======================================================================
    | TYPE /test/cov/QRNB                 | 100.000% | 100.000% | Covered |
    =======================================================================
    | INSTANCE <UNNAMED1>                 | 100.000% | 100.000% | Covered |
    |-------------------------------------|----------|----------|---------|
    | COVERPOINT <UNNAMED1>::CP_qrnb_mask | 100.000% | 100.000% | Covered |
    |-------------------------------------|----------|----------|---------|
    | bin qrnb_mask_b                     |        1 |        1 | Covered |
    =======================================================================