原创文章，转载请注明： 转载自Erlang非业余研究

本文链接地址: R14A添加新指令优化Ref消息的接收

Erlang的惯用法之一就是在消息匹配的时候，如果需要唯一性，通常会通过make_ref搞个唯一的Ref来作为消息的一部分来匹配。这个惯用法用在gen_server:call或者demonitor这样的使用频度很高的函数里面。由于erlang的消息匹配是再消息队列里面挨个遍历来匹配，特别是消息队列特别长的时候，会有很大的性能瓶颈。于是新的优化出现了，以下是编译器beam_receive.erl里面的解释，写的很清楚：

%%%
%%% 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.
%%%

结论： 点滴优化成就高性能。