Erlang的慣用法之一就是在訊息匹配的時候，如果需要唯一性，通常會通過make_ref搞個唯一的Ref來作為訊息的一部分來匹配。這個慣用法用在gen_server:call或者demonitor這樣的使用頻度很高的函式裡面。由於erlang的訊息匹配是再訊息佇列裡面挨個遍歷來匹配，特別是訊息佇列特別長的時候，會有很大的效能瓶頸。於是新的優化出現了，以下是編譯器beam_receive.erl裡面的解釋，寫的很清楚：

%%%                                                                                                                                                                    
%%% In code such as:                                                                                                                                                   
%%%                                                                                                                                                                    
%%%    Ref = make_ref(),        %Or erlang:monitor(process, Pid)                                                                                                       
%%%      .                                                                                                                                                             
%%%      .                                                                                                                                                             
%%%      .                                                                                                                                                             
%%%    receive                                                                                                                                                         
%%%       {Ref,Reply} -> Reply                                                                                                                                         
%%%    end.                                                                                                                                                            
%%%                                                                                                                                                                    
%%% we know that none of the messages that exist in the message queue                                                                                                  
%%% before the call to make_ref/0 can be matched out in the receive                                                                                                    
%%% statement. Therefore we can avoid going through the entire message                                                                                                 
%%% queue if we introduce two new instructions (here written as                                                                                                        
%%% BIFs in pseudo-Erlang):                                                                                                                                            
%%%                                                                                                                                                                    
%%%    recv_mark(SomeUniqInteger),                                                                                                                                     
%%%    Ref = make_ref(),                                                                                                                                               
%%%      .                                                                                                                                                             
%%%      .                                                                                                                                                             
%%%      .                                                                                                                                                             
%%%    recv_set(SomeUniqInteger),                                                                                                                                      
%%%    receive                                                                                                                                                         
%%%       {Ref,Reply} -> Reply                                                                                                                                         
%%%    end.                                                                                                                                                            
%%%                                                                                                                                                                    
%%% The recv_mark/1 instruction will save the current position and                                                                                                     
%%% SomeUniqInteger in the process context. The recv_set                                                                                                               
%%% instruction will verify that SomeUniqInteger is still stored                                                                                                       
%%% in the process context. If it is, it will set the current pointer                                                                                                  
%%% for the message queue (the next message to be read out) to the                                                                                                     
%%% position that was saved by recv_mark/1.                                                                                                                            
%%%    
%%% The remove_message instruction must be modified toinvalidate                                                                                                      
%%% the information stored by the previous recv_mark/1, in case there                                                                                                  
%%% is another receive executed between the calls to recv_mark/1 and                                                                                                   
%%% recv_set/1.                                                                                                                                                        
%%%                                                                                                                                                                    
%%% We use a reference to a label (i.e. a position in the loaded code)                                                                                                 
%%% as the SomeUniqInteger.                                                                                                                                            
%%%                                                   

結論： 點滴優化成就高效能。

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