Re: Behavior of logical operators

[GrayFace <sergroj@...>]

On 13.03.2011 19:06, Gavin Wraith wrote:
> I use lua5.2-alpha with the following patch (marked by
> #if 0 ... #endif) in lcode.c which seems to cure the problem:

It's the one from http://www.lua.org/bugs.html#5.1.4-3
I have it in my 5.1.4 and it doesn't help.

I tried to dig into the problem a little, it's my first experience with 
Lua VM instructions. Here's the code generated for this function:

return (1 or 2) and true or false

    1    [3]    LOADK        0 -1    ; 1  -- R(0) := 1
    2    [3]    TEST         0 0 1        -- do nothing, go to 3
    3    [3]    JMP          5    ; to 9   -- go to 9
    4    [3]    JMP          3    ; to 8
    5    [3]    LOADBOOL     0 0 0
    6    [3]    JMP          2    ; to 9
    7    [3]    LOADBOOL     0 0 1
    8    [3]    LOADBOOL     0 1 0
    9    [3]    RETURN       0 2          -- return R(0)
    10    [4]    RETURN       0 1
constants (1) for 0162CC10:
    1    1

To me it looks like the instruction #3 is a leftover from (1 or 2), but 
that's just a blind guess.

From my further tests it looks like the patch does more harm than good. 
It doesn't fix the problem of and/or optimization, but just disables it 
under certain conditions. As a result, this common construction has a 
useless comparison in its generated code in case 'aa' is true:

return aa and true or false

    1    [2]    GETUPVAL     0 0    ; aa
    2    [2]    TEST         0 0 0
    3    [2]    JMP          3    ; to 7
    4    [2]    LOADBOOL     0 1 0
    5    [2]    TEST         0 0 1     -- <= useless
    6    [2]    JMP          1    ; to 8
    7    [2]    LOADBOOL     0 0 0
    8    [2]    RETURN       0 2
    9    [3]    RETURN       0 1
upvalues (1) for 0162CC10:
    0    aa

Compare it to unpatched 5.1.4:

    1    [3]    GETUPVAL     0 0    ; aa
    2    [3]    TEST         0 0 0
    3    [3]    JMP          1    ; to 5
    4    [3]    JMP          3    ; to 8
    5    [3]    LOADBOOL     0 0 0
    6    [3]    JMP          2    ; to 9
    7    [3]    LOADBOOL     0 0 1
    8    [3]    LOADBOOL     0 1 0
    9    [3]    RETURN       0 2
    10    [4]    RETURN       0 1
upvalues (1) for 0162CC10:
    0    aa

Some useless jumping around here, but no useless TEST instructions.

I also found that "A no-frills introduction to Lua 5 VM instructions" 
has wrong description for TEST instruction.
It says:
"if (R(B) <=> C) then R(A) := R(B) else pc++"
and treats "<=>" sign as "~=". In fact, here's how it works in 5.1.4:
"if (R(B) == C) then R(A) := R(B) else pc++".

--
Best regards,
Sergey Rozhenko                 mailto:sergroj@mail.ru