Difference in overflow while using byte and int

SystemVerilog
, ,
1

PROGRAM :–1

class array;
rand byte unsigned arr[];
constraint c_arr{
   arr.size() == 5;
   arr.sum() == 60; //inside {[1:1000]}; // if you want a positive result
   //foreach(arr[i]) arr[i] inside {[0:500]};
}
endclass

module tb;
array a=new();
int sum;
initial
repeat(10)
begin
a.randomize();
sum=0;
foreach(a.arr[i])
sum=sum+a.arr[i];
$display("sum =%0d elements of the array to sum up to <1000 ---->>  %0p",sum,a.arr);
end
endmodule

RESULT :–

sum =60 elements of the array to sum up to <1000 ---->> '{'h28, 'hb, 'h9, 'h0, 'h0}
sum =60 elements of the array to sum up to <1000 ---->> '{'h2, 'h0, 'h7, 'h0, 'h33}
sum =60 elements of the array to sum up to <1000 ---->> '{'h4, 'h0, 'h0, 'h35, 'h3}
sum =60 elements of the array to sum up to <1000 ---->> '{'hb, 'h10, 'h19, 'h8, 'h0}
sum =60 elements of the array to sum up to <1000 ---->> '{'h0, 'h37, 'h5, 'h0, 'h0}
sum =60 elements of the array to sum up to <1000 ---->> '{'h3, 'h0, 'h0, 'h38, 'h1}
sum =60 elements of the array to sum up to <1000 ---->> '{'h0, 'h5, 'h15, 'ha, 'h18}
sum =60 elements of the array to sum up to <1000 ---->> '{'ha, 'h0, 'h0, 'h0, 'h32}
sum =60 elements of the array to sum up to <1000 ---->> '{'h1a, 'h5, 'h2, 'h1b, 'h0}
sum =60 elements of the array to sum up to <1000 ---->> '{'h10, 'h1b, 'h11, 'h0, 'h0}

PROGRAM :–2

class array;
rand int unsigned arr[];
constraint c_arr{
   arr.size() == 5;
   arr.sum() == 60; //inside {[1:1000]}; // if you want a positive result
   //foreach(arr[i]) arr[i] inside {[0:500]};
}
endclass

module tb;
array a=new();
int sum;
initial
repeat(10)
begin
a.randomize();
sum=0;
foreach(a.arr[i])
sum=sum+a.arr[i];
$display("sum =%0d elements of the array to sum up to <1000 ---->>  %0p",sum,a.arr);
end
endmodule

RESULT:–

sum =60 elements of the array to sum up to <1000 ---->> '{'h4958832, 'h8ae8ecd3, 'hc4b9c064, 'h988848e0, 'h133f81f3}
sum =60 elements of the array to sum up to <1000 ---->> '{'h8e4ecd87, 'h4998543e, 'hb87f7c8b, 'h14cfff46, 'h5ac962a6}
sum =60 elements of the array to sum up to <1000 ---->> '{'he004869d, 'h390dda9c, 'h3670936e, 'hfbdf72e4, 'hb49d98b1}
sum =60 elements of the array to sum up to <1000 ---->> '{'h17d4fe53, 'h36b21455, 'hba7f13cd, 'h1d043a6a, 'hd9f59f5d}
sum =60 elements of the array to sum up to <1000 ---->> '{'h10d61bea, 'hd66d3faf, 'h23fe333c, 'h1f2447d9, 'hd59a298e}
sum =60 elements of the array to sum up to <1000 ---->> '{'h18028825, 'h8fb317cd, 'h7f98477f, 'hb7b3d42b, 'h20fe44a0}
sum =60 elements of the array to sum up to <1000 ---->> '{'hb6e94ce8, 'h3190c552, 'h750017d2, 'h59342392, 'h4951b29e}
sum =60 elements of the array to sum up to <1000 ---->> '{'hf0364918, 'hcd2504ea, 'hb7e8329a, 'h443f065b, 'h467d7945}
sum =60 elements of the array to sum up to <1000 ---->> '{'h8f09a442, 'h94eb9b2, 'h93c8b596, 'heff84f0e, 'he3e69da4}
sum =60 elements of the array to sum up to <1000 ---->> '{'h54d814c5, 'hb729f759, 'ha2c604ba, 'h52b1c34b, 'hfe862c19}

PROGRAM :–3

class array;
rand int unsigned arr[];
constraint c_arr{
   arr.size() == 5;
   arr.sum() < 60; //inside {[1:1000]}; // if you want a positive result
   //foreach(arr[i]) arr[i] inside {[0:500]};
}
endclass

module tb;
array a=new();
int sum;
initial
repeat(10)
begin
a.randomize();
sum=0;
foreach(a.arr[i])
sum=sum+a.arr[i];
$display("sum =%0d elements of the array to sum up to <1000 ---->>  %0p",sum,a.arr);
end
endmodule

RESULT:–
sum =54 elements of the array to sum up to <1000 ---->> '{'h0, 'h0, 'h0, 'h0, 'h36}
sum =59 elements of the array to sum up to <1000 ---->> '{'h0, 'h2b, 'h10, 'h0, 'h0}
sum =35 elements of the array to sum up to <1000 ---->> '{'h0, 'h0, 'h23, 'h0, 'h0}
sum =42 elements of the array to sum up to <1000 ---->> '{'h4, 'h26, 'h0, 'h0, 'h0}
sum =43 elements of the array to sum up to <1000 ---->> '{'h0, 'h2b, 'h0, 'h0, 'h0}
sum =38 elements of the array to sum up to <1000 ---->> '{'h0, 'h0, 'h0, 'h24, 'h2}
sum =38 elements of the array to sum up to <1000 ---->> '{'h19, 'h0, 'h7, 'h0, 'h6}
sum =18 elements of the array to sum up to <1000 ---->> '{'h8, 'h0, 'h0, 'h0, 'ha}
sum =34 elements of the array to sum up to <1000 ---->> '{'h13, 'h0, 'he, 'h1, 'h0}
sum =32 elements of the array to sum up to <1000 ---->> '{'h0, 'h0, 'h16, 'h0, 'ha}

Case 1:
For int : arr.sum() == 60; -->No overflow values are printed.
For int : arr.sum() < 60; -->Overflow values are printed.
Case 2:
Was expecting overflow for byte datatype also.

2

In reply to Chandra Shekar N:

Yes, Overflow is expected in byte case too. you may notice it if you run more simulation (run 100 instead of 10).

3

In reply to Rahulkumar:

Thank you for the reply, Rahul. I had tried it for even 100000, in int I was able to observe whereas for byte there is no overflow.

The attached file includes the byte file with 100000 simulation.

https://drive.google.com/open?id=1YpF0mlSqt4mCROPiIxxarDzocMZ6vSOp

4

In reply to Chandra Shekar N:




class array;
rand byte unsigned arr[];
constraint c_arr{
   arr.size() == 5;
   arr.sum() == 60; //inside {[1:1000]}; // if you want a positive result
   //foreach(arr[i]) arr[i] inside {[0:500]};
}
endclass
 
module tb;
array a=new();
int sum;
initial
repeat(10)
begin
a.randomize();
sum=0;
foreach(a.arr[i])
sum=sum+a.arr[i];
$display("sum =%0d elements of the array to sum up to <1000 ---->>  %0p",sum,a.arr);
end
endmodule


///OUTPUT
# KERNEL: sum =572 elements of the array to sum up to <1000 ---->>  '{237, 35, 97, 196, 7}
# KERNEL: sum =572 elements of the array to sum up to <1000 ---->>  '{115, 149, 160, 144, 4}
# KERNEL: sum =572 elements of the array to sum up to <1000 ---->>  '{141, 250, 16, 155, 10}
# KERNEL: sum =828 elements of the array to sum up to <1000 ---->>  '{129, 245, 119, 113, 222}
# KERNEL: sum =572 elements of the array to sum up to <1000 ---->>  '{182, 74, 28, 157, 131}
# KERNEL: sum =316 elements of the array to sum up to <1000 ---->>  '{93, 134, 30, 14, 45}
# KERNEL: sum =828 elements of the array to sum up to <1000 ---->>  '{213, 183, 237, 10, 185}
# KERNEL: sum =828 elements of the array to sum up to <1000 ---->>  '{239, 73, 114, 213, 189}
# KERNEL: sum =828 elements of the array to sum up to <1000 ---->>  '{18, 111, 224, 254, 221}
# KERNEL: sum =572 elements of the array to sum up to <1000 ---->>  '{142, 147, 67, 166, 50}
5

In reply to Chandra Shekar N:
I think you have a problem with your tool, it’s not giving all possible values that would make the constraint evaluate true.

6

Thanks Rahul and Dave for the replies, and for solving the issue. Yes it is problem with tool while evaluating with constraints.

Regards
Chandra Shekar N