Re: Lua 5.2 OP_LOADNIL instruction

[liam mail <liam.list@...>]

On 28 October 2011 12:58, Roberto Ierusalimschy <roberto@inf.puc-rio.br> wrote:
>> I was wondering why Lua 5.2 does not preform the same optimisation for
>> bytecode which 5.1 did when a there is not an operation before an
>> OP_LOADNIL, for 5.1 the generator would remove the load nil
>> instructions. For example a simple file which just contains a local
>> which will be set to nil:
>> $ cat ./op_loadnil.lua
>> local a
>>
>> For 5.1 produces the following:
>> $ luac -l ./op_loadnil.lua
>> main <./op_loadnil.lua:0,0> (1 instruction, 4 bytes at 0x100101050)
>> 0+ params, 2 slots, 0 upvalues, 1 local, 0 constants, 0 functions
>>       1       [1]     RETURN          0 1
>>
>> Yet for 5.2 it leaves in the instruction:
>> $ ./luac -l ./op_loadnil.lua
>> main <./op_loadnil.lua:0,0> (2 instructions at 0x100100ee0)
>> 0+ params, 2 slots, 1 upvalue, 1 local, 0 constants, 0 functions
>>       1       [1]     LOADNIL         0 0
>>       2       [1]     RETURN          0 1
>>
>> Would someone please shine some light on this and any implications
>> that may follow it.
>
> Lua 5.1 fills with nils all slots of a function before calling it:
>
> ldo.c:296:
>    for (st = L->top; st < ci->top; st++)
>      setnilvalue(st);
>
> So, it does not need any initial LOADNIL instruction, because all slots
> are already nil. On the ohter hand, it always pays the price, even if
> these nils are nor necessary (e.g., when we declare our locals with
> proper initialization).
>
> Lua 5.2 fills with nils only the missing arguments:
>
> ldo.c:329:
>      for (; n < p->numparams; n++)
>        setnilvalue(L->top++);  /* complete missing arguments */
>
> So, slots are not nil when the function start and LOADNIL instructions
> cannot be removed.
>
> (This change also has some small implications for the garbage collector.)
>
> -- Roberto
>
>

Thanks Roberto.

Liam