Re: Why 'while' and 'repeat' loop need double block

[Dirk Laurie <dirk.laurie@...>]

2013/1/9 Roberto Ierusalimschy <roberto@inf.puc-rio.br>:
>> When parsing 'while' and 'repeat' statement, lua uses double block
>> to encapsulate the inner statement(one for 'loop').
>
> If I remember correctly, the inner block controls the scope of variables,
> while the outer block controls where to jump from a break. Consider
> this code:
>
>   while 1 do
>     local a
>     x = function () return a end
>   end
>
> and its opcodes:
>
>         1       [2]     LOADNIL         0 0
>         2       [3]     CLOSURE         1 0     ; 0x9f18fa0
>         3       [3]     MOVE            0 0
>         4       [3]     SETGLOBAL       1 -1    ; x
>         5       [3]     CLOSE           0
>         6       [3]     JMP             -6      ; to 1
>         7       [4]     RETURN          0 1
>

In Lua 5.2, the code is:

	1	[2]	LOADNIL  	0 0
	2	[3]	CLOSURE  	1 0	; 0x9198c18
	3	[3]	SETTABUP 	0 -1 1	; _ENV "x"
	4	[3]	JMP      	1 -4	; to 1
	5	[3]	JMP      	0 -5	; to 1
	6	[4]	RETURN   	0 1

The second JMP (never reached) does look mysterious…