Goto in LuaJIT

吊炸天的 LuaJIT

Posted by qmsheng on September 20, 2016

Lua 在 5.2 之后的版本,加入了 goto 这个关键字,用来控制程序跳转到指定 label。我们可以利用这个特性,来模拟 continue 的实现。需要注意的是 goto 只能跳转到 label,而 ::name:: 的格式就可以设置一个 label

for i=1,5 do
    if i == 3 then
        goto continue
    end
    print(i)
    ::continue::
end

这样就简单实现了 continue。但是有些同学可能会有疑问,这是 Lua 5.2 的特性,我们都知道 OR 中使用的 Lua 5.1,那如何使用?

其实我们只需要使用 LuaJIT 就可以解决这个问题了。在 LuaJIT 的主页上有这个介绍:

LuaJIT supports some language and library extensions from Lua 5.2. Features that are unlikely to break existing code are unconditionally enabled:

  • goto and ::labels::.

也就是说 LuaJIT 把 5.2 的这个特性拿过来了。那我们来验证下:

[root@master:~]# cd /usr/local/openresty/luajit/bin/
[root@master:bin]# ./luajit
LuaJIT 2.1.0-beta1 -- Copyright (C) 2005-2015 Mike Pall. http://luajit.org/
JIT: ON SSE2 SSE3 SSE4.1 fold cse dce fwd dse narrow loop abc sink fuse
>
> for i=1,5 do
    if i == 3 then
        goto continue
    end
    print(i)
    ::continue::
end>> >> >> >> >> >>
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>

继续验证 OR :

[root@master:demo]# nl app/app.lua
     1	local lor = require("lor.index")
     2	local app = lor()

     3	app:get("/index", function(req,res,next)
     4	    res:send("hello")
     5	end)

     6	app:get("goto", function(req,res,next)
     7	    for i=1, 5 do
     8	        if i == 3 then
     9	            goto continue
    10	        end
    11	        ngx.say(i)
    12	        ::continue::
    13	    end
    14	end)

    15	return app

[root@master:demo]#
[root@master:demo]# http :8888/goto
HTTP/1.1 200 OK
Connection: keep-alive
Content-Type: text/plain
Date: Fri, 02 Dec 2016 02:55:43 GMT
Server: openresty/1.9.7.4
Transfer-Encoding: chunked
X-Powered-By: Lor Framework

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[root@master:demo]#

那么 goto 还有哪些玩法?尾递归优化使用 goto 来实现:

do
    local function fact(n)
        return fact_iter(n, 1)
    end

    function fact_iter(n, now)
        ::call::
        if n <= 1 then
            return now
        else
            -- return fact_iter(n-1, now*n)
            n, now = n-1, now*n
            goto call
        end
    end

    print(fact(5))
    print(fact(10000))
    print(fact(10000000))
end

然而感觉并没什么卵用。其他玩法需要各位同学自己去挖掘了。。不过需要注意的是 Lua 也有对 goto 的滥用限制:

  • A label is visible in the entire block where it is defined (including nested blocks, but not nested functions).
  • A goto may jump to any visible label as long as it does not enter into the scope of a local variable.