Re: Bytecode hacks

[Mike Nelson <mikestar1313@...>]

On 5/21/2014 8:27 PM, Luiz Henrique de Figueiredo wrote:
> > I think there are patchs around for the t:[var]() syntax.
> Actually, this one seems simple. A quick hack is to just change
> suffixedexp in lparser.c (from Lua 5.2.3):
>
>         case ':': {  /* `:' NAME funcargs */
>           expdesc key;
>           luaX_next(ls);
> -        checkname(ls, &key);
> +        if (ls->t.token=='[') {
> +           luaK_exp2anyregup(fs, v);
> +           yindex(ls, &key);
> +        }
> +        else
> +           checkname(ls, &key);
>           luaK_self(fs, v, &key);
>           funcargs(ls, v, line);
>           break;
>
> I tested it only on this program. It works in some cases but not in others :-(
>
>        local t={}
>        t.f=print
>        t.ff=print
>        local v="f"
>        t:f(1)
>        t:[v](2)
>        t:[string.char(string.byte("f"))](3,32,33)
>        t:["f".."f"](4)
>        t:["f"..string.sub("afun",2,2)](5,52,53)
>        t:[(string.sub("afun",2,2))](6,62,63)
>
>        table: 0x107e04e40	1
>        table: 0x107e04e40	2
>        3	32	33
>        table: 0x107e04e40	4
>        table: 0x107e04e40	5	52	53
>        6	62	63
>
> It's been some time since I last hacked the parser and I must have
> overlooked something. Happy hacking.

The pathological cases in this patch can be corrected by concatenating 
the subscript with an empty string in the Lua script (NOT what a happy 
hacker wants).

This C code appears to detect these cases:
if (key.k==0x06) {/* I don't know what to do here */}
when inserted immediately after yindex(ls, &key);


The luac listing of these cases are all of this form:
    22    [10]    CALL         3 0 2
    23    [10]    SELF         4 0 3
    24    [10]    LOADK        5 -10    ; 3
    25    [10]    LOADK        6 -11    ; 32
    26    [10]    LOADK        7 -12    ; 33
    27    [10]    CALL         4 4 1

The C argument of SELF is always a register, the same register as the 
the A argument of the previous CALL.
Correct bytecode for the case above  should be like this:
    22    [10]    CALL         3 0 2
    23    [10]    SELF         4 0 3
    24    [10]    LOADK        6 -10    ; 3
    25    [10]    LOADK        7 -11    ; 32
    26    [10]    LOADK        8 -12    ; 33
    27    [10]    CALL         4 5 1

This should be useful for anybody attempting this parser hack, but I am 
at sea about how to fix it.

-- Mike