[flush function of uvm_tlm_fifo class does not work as expected]

Question: Why do you say "before UVM 2017 v1.1 Library Code for IEEE 1800.2 flush function is correct?

Answer:

the below code is flush function of uvm_tlm_fifo.svh in 1800.2-2020-2.0 Library Code

care for the Line.14, it used mailbox's in-build function try_get();

1  virtual function void flush();
2    T t;
3    bit r;
4
5    m_clear_zombie_gets();
6    
7    if( m.num() > 0 && m_pending_blocked_gets.size() != 0 ) begin
8      uvm_report_error("flush failed" ,
9		       "there are blocked gets preventing the flush", UVM_NONE);
10      return;
11    end
12
13    r = 1; 
14    while( r ) r = m.try_get( t ) ;
15    
16  
17   endfunction

the below code is flush function of uvm_tlm_fifo.svh in UVM 2017 v1.1 Library Code for IEEE 1800.2

the critical code is Line.6, it used the function try_get() in class uvm_tlm_fifo, but not the mailbox in-build function!

1  virtual function void flush();
2    T t;
3    bit r;
4
5    r = 1; 
6    while( r ) r = try_get( t ) ;
7    
8    if( m.num() > 0 && m_pending_blocked_gets != 0 ) begin
9      uvm_report_error("flush failed" ,
10		       "there are blocked gets preventing the flush", UVM_NONE);
11    end
12  
13  endfunction

And in all UVM Version Library Code, the try_get() function in class uvm_tlm_fifo which is as below are all the same.

The one of differences between function try_get() in class uvm_tlm_fifo and the mailbox is the return value type!

the function try_get() in class uvm_tlm_fifo return type is bit, and the mailbox try_get() function return type is int.

  virtual function bit try_get( output T t );
    if( !m.try_get( t ) ) begin
      return 0;
    end

    get_ap.write( t );
    return 1;
  endfunction 

So in UVM-1.1d, UVM-1.2, and  UVM 2017 v1.1 Library Code for IEEE 1800.2, the flush function of uvm_tlm_fifo is correct. But after UVM 2017 v1.1 Library Code for IEEE 1800.2, the flush function of uvm_tlm_fifo is wrong. 

I have a question: Why change the line code from

while( r ) r = try_get( t ) ;

to 

while( r ) r = m.try_get( t ) ;

after UVM 2017 v1.1 Library Code for IEEE 1800.2 ?

[flush function of uvm_tlm_fifo class does not work as expected]

Hi,

      I am trying to use flush function in uvm_tlm_fifo class to clear all transactions in uvm_tlm_fifo, but it can not work correctly. 

      the code of flush function in uvm_tlm_fifo class is as below.

  // Function -- NODOCS -- flush
  //
  // Removes all entries from the FIFO, after which <used> returns 0
  // and <is_empty> returns 1.

  virtual function void flush();
    T t;
    bit r;

    m_clear_zombie_gets();
    
    if( m.num() > 0 && m_pending_blocked_gets.size() != 0 ) begin
      uvm_report_error("flush failed" ,
		       "there are blocked gets preventing the flush", UVM_NONE);
      return;
    end

    r = 1; 
    while( r ) r = m.try_get( t ) ;
    
  
  endfunction

       For " while( r ) r = m.try_get( t ) ; ",  in the while loop it used mailbox m in-build function try_get().

       Reference to Systemverilog IEEE 1800-2017 LRM 15.4.6 try_get():

       If a message is available and the message type is equivalent to the type of the message variable, the message is retrieved, and the method returns a positive integer.

       m.try_get(t) returns a positive integer number, and it will be assigned to var r, but var r is bit type, so int number bit0 will be assigned to var r actually. In that case, r may be assigned to zero, and while loop can go out even when mailbox m still has transaction in it. That is the case I meet, the flush function does not work as expected.

      I also download previous release of UVM code. I found  before UVM 2017 v1.1 Library Code for IEEE 1800.2 flush function is correct. the code is as below:

  // Function -- NODOCS -- flush
  //
  // Removes all entries from the FIFO, after which <used> returns 0
  // and <is_empty> returns 1.

  virtual function void flush();
    T t;
    bit r;

    r = 1; 
    while( r ) r = try_get( t ) ;
    
    if( m.num() > 0 && m_pending_blocked_gets != 0 ) begin
      uvm_report_error("flush failed" ,
		       "there are blocked gets preventing the flush", UVM_NONE);
    end
  
  endfunction