Randomizing an array of transactions

UVM
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1

Hi,
I am a newbie with UVM/SV. I have a simple sequence_item with 2 random int variables. I am instantiating a dynamic array of the sequence_item in my sequence. Now I want to constrain the sequence_item in such a way that the random variables inside should not be the same. Below code gives run time Fatal error when it reaches the mentioned line. Solutions appreciated.

class my_transaction extends uvm_sequence_item;
rand int a;
rand int b;
– registered with factory and added field macros here –
endclass

class my_sequence extends uvm_sequence_item;
rand my_transaction items;
constraint c_txn {
items.size>0 && items.size<=10;
foreach(items[i]) {
items[i].ina != items[i].inb[1]; // FATAL ERROR on this line
}
}
endclass

=================================================

Transcript of log:

    items[i].ina != items[i].inb[1];
        |

ncsim: *E,RNDCNSTE (…/…my_seq_lib.sv,34|12): Randomization constraint has this error, which will cause the randomize function to return 0 and no new rand values will be set:
Null handle references in constraints are not supported.
if (!seq.do_not_randomize && !seq.randomize()) begin
|
ncsim: *W,SVRNDF (/tools/cadence/INCISIV14.10-s014/Linux/tools/methodology/UVM/CDNS-1.1d/sv/src/seq/uvm_sequencer_base.svh,1398|46): The randomize method call failed.
Observed simulation time : 0 FS + 99
UVM_FATAL /tools/cadence/INCISIV14.10-s014/Linux/tools/methodology/UVM/CDNS-1.1d/sv/src/seq/uvm_sequencer_base.svh(1400) @ 0: uvm_test_top.sa_env.sa_agent.sequencer [STRDEFSEQ] Randomization failed for default sequence ‘my_sequence’ for phase ‘main’

=======================================================

thanks
myashu

2

The randomize method will never construct class objects for you. You need to construct your array of items before calling randomize() or using the pre_randomize() method. The array needs to be as large as your constraints will allow, and the array size will get trimmed down to the size determined during randomization.

3

In reply to dave_59:

Thanks Dave,
Yes I need to add a pre_randomize() and create all the transactions in a foreach loop such that the number of created transactions are not less than the items.size() mentioned in the constraint block. I was able to contrain the variables of the transaction class with this.

thanks
Myashu

4

In reply to dave_59:

Can you help me with an example to implement an array of transactions?

5

In reply to Mohammed Essam Abd El Samee:

my_transaction tr[];
 
tr = new[5];
foreach(tr[ii]) begin 
    tr[ii] = my_transaction::type_id::create("tr");
    if (!tr[ii].randomize) `uvm_error()
   end
6

In reply to dave_59:

Thanks very much.

7

Hi Dave,

can you please let me know what will be the order of solving this array of objects. is it sequential? or parallel or random.

For example :-

typedef uvm_object temp1;
rand temp1 tr[10];

Will tr[0] solved first and then tr[1] solved later?

Thank you,
Sasi kiran

8

Without any constraints between the elements, the order does not matter. When there is a constraint connecting the elements, they are solved simultaniously. This is true for all random variables as well as elements of an unpacked array.