uvm中start_item的源码

`start_item`函数是UVM中序列化对象的一种机制，用于将序列化对象插入到序列执行队列中。以下是UVM中`start_item`的源代码：

function bit uvm_sequence_base::start_item (uvm_sequence_item item,
                                            uvm_sequencer_base sequencer,
                                            bit set_priority = 0,
                                            string source_file = "",
                                            int source_line = 0);
   uvm_sequence_item item_copy;
   uvm_tlm_transaction_base t;
   uvm_component comp = get_parent();
   uvm_phase phase;

   if (!item)
      `uvm_fatal("NOITEM","Attempting to start a NULL sequence_item")
   if (!sequencer)
      `uvm_fatal("NOSEQ","Attempting to start a sequence_item with a NULL sequencer")

   if (comp is uvm_root) begin
      `uvm_warning("SEQROOT",{"Attempt to start sequence '",get_type_name(),
                       "' outside of a phase process. Ignored."})
      return 0;
   end

   // Create a copy of the item
   if(item.try_clone(item_copy) == 0) begin
      `uvm_error("CLONE", {"Unable to clone sequence item '",item.get_type_name(),"'"})
      return 0;
   end

   // Check for TLM analysis ports
   if(item_copy.get_type_name() == "uvm_tlm_analysis_port_base") begin
      t = item_copy;
      t.set_source_info(source_file, source_line);
      t.analysis_imp.connect(item_copy.analysis_export);
      t.write(t);
      return 1;
   end

   // Set the item's sequencer
   item_copy.set_sequencer(sequencer);

   // Set the item's source info
   item_copy.set_source_info(source_file, source_line);

   // Set the item's priority
   if (set_priority)
      item_copy.set_priority(m_priority);

   // If in a run phase, start the item
   phase = comp.get_phase();
   if (phase != null && phase.get_name() == "run") begin
      if (!sequencer.start_item(item_copy)) begin
         `uvm_error("NOSEQITEM", {"Failed to start sequence item ",
                         item_copy.get_type_name()," with sequencer ",
                         sequencer.get_type_name()})
         return 0;
      end
   end
   else
      m_req_q.push_back(item_copy);

   return 1;
endfunction

此函数接收四个参数：

- `item`：要插入序列执行队列中的序列化对象。
- `sequencer`：将序列化对象插入到其执行队列中的UVM序列器。
- `set_priority`：指示是否应将`item`的优先级设置为该序列的优先级。
- `source_file`和`source_line`：指示从哪个源代码文件和行开始创建序列化对象。

该函数首先检查传递的参数是否有效。然后，它会复制序列化对象并将其设置在传递的`sequencer`中。如果序列化对象是一个TLM分析端口，则该函数会将其连接到分析导出，并通过`write`函数写入分析对象。如果在运行阶段，则该函数将使用传递的`sequencer`启动序列化对象。否则，它将序列化对象插入到要求队列中。函数返回1表示成功，返回0表示失败。