Re: Filed PR jit/66812 for the code generation issue

[Dibyendu Majumdar <mobile@majumdar.org.uk>]

On 8 July 2015 at 21:32, Dibyendu Majumdar <mobile@majumdar.org.uk> wrote:
> On 8 July 2015 at 21:22, Dibyendu Majumdar <mobile@majumdar.org.uk> wrote:
>> Here is what I think the flow is.
>>
>> part 1
>> --------
>>
>> 1 [2] LOADNIL   0 0
>>
>> entry:
>>   cl = (struct ravi_LClosure *)L->ci->func->value_.gc;
>>   (void)raviV_op_loadnil (L->ci, (int)0, (int)0);
>>
>>
>> This sets register 0 (local variable IX) to NIL
>>
>> 2 [3] LOADK     1 -1 ; 10
>>
>>   (&L->ci->u.l.base[(int)1])->value_.i = (&cl->p->k[(int)0])->value_.i;
>>   (&L->ci->u.l.base[(int)1])->tt_ = (&cl->p->k[(int)0])->tt_;
>>
>> This sets the register at 1 (temporary) to 10.
>>
>> 3 [3] TEST     1 1
>> 4 [3] JMP       0 0 ; to 5
>>
>> The above two bytecodes go together. This says that if register 1 is
>> true then skip the JMP else JMP.
>> So we get:
>>
>>   comparison_0_5 = (&L->ci->u.l.base[(int)1])->tt_ == (int)0;
>>   comparison_0_6 = (&L->ci->u.l.base[(int)1])->tt_ == (int)1;
>>   comparison_0_7 = (&L->ci->u.l.base[(int)1])->value_.b == (int)0;
>>   isfalse_0_4 = comparison_0_5 || comparison_0_6 && comparison_0_7;
>>   if (!(isfalse_0_4)) goto OP_TEST_do_jmp_2_8; else goto OP_TEST_do_skip_2_9;
>>
>> The test is checking that register 1 is true (which it is as it has
>> the constant 10) - then goto skip_2_9.
>
> Since we skipped the jump the next instruction is
>
> 5 [3] LOADBOOL 1 0 0
>
> This is setting register 1 to false so we get:
>
> OP_TEST_do_skip_2_9:
>   goto jmp_5_1;
>
> jmp_5_1:
>   (&L->ci->u.l.base[(int)1])->value_.b = (int)0;
>   (&L->ci->u.l.base[(int)1])->tt_ = (int)1;
>
> Next instruction is:
>
> 6 [3] TEST     1 0
> 7 [3] JMP       0 1 ; to 9
>
> Now this is saying that if register is _false_ then skip JMP else JMP.
> Since we know that register 1 is false as above then the code should
> skip the jump.
>
>   comparison_0_11 = (&L->ci->u.l.base[(int)1])->tt_ == (int)0;
>   comparison_0_12 = (&L->ci->u.l.base[(int)1])->tt_ == (int)1;
>   comparison_0_13 = (&L->ci->u.l.base[(int)1])->value_.b == (int)0;
>   isfalse_0_10 = comparison_0_11 || comparison_0_12 && comparison_0_13;
>   if (!(!(isfalse_0_10))) goto OP_TEST_do_jmp_5_14; else goto
> OP_TEST_do_skip_5_15;
>
> So we go to skip_5_15:
>
> 8 [3] LOADBOOL 0 1 0
>
> OP_TEST_do_skip_5_15:
>   (&L->ci->u.l.base[(int)0])->value_.b = (int)1;
>   (&L->ci->u.l.base[(int)0])->tt_ = (int)1;
>   goto jmp_9_2;
>
> So you see that the local IX is set to false above which is what we
> want. Next we jump to 9_2 which is the start of the return statement.

part 3
-------

Here we essentially have a repeat of what was in entry block:

9 [3] LOADK     1 -1 ; 10
10 [3] TEST     1 1
11 [3] JMP       0 0 ; to 12

jmp_9_2:
  (&L->ci->u.l.base[(int)1])->value_.i = (&cl->p->k[(int)0])->value_.i;
  (&L->ci->u.l.base[(int)1])->tt_ = (&cl->p->k[(int)0])->tt_;
  comparison_0_17 = (&L->ci->u.l.base[(int)1])->tt_ == (int)0;
  comparison_0_18 = (&L->ci->u.l.base[(int)1])->tt_ == (int)1;
  comparison_0_19 = (&L->ci->u.l.base[(int)1])->value_.b == (int)0;
  isfalse_0_16 = comparison_0_17 || comparison_0_18 && comparison_0_19;
  if (!(isfalse_0_16)) goto OP_TEST_do_jmp_9_20; else goto OP_TEST_do_skip_9_21;

So the JMP is not taken as register 1 is _true_ and we go to 9_21:

12 [3] LOADBOOL 1 0 0

OP_TEST_do_skip_9_21:
  goto jmp_12_3;

jmp_12_3:
  (&L->ci->u.l.base[(int)1])->value_.b = (int)0;
  (&L->ci->u.l.base[(int)1])->tt_ = (int)1;

Above sets the register 1 to false and this is the return value.

Next the return instruction begins:

  L->top = &L->ci->u.l.base[(int)2];
  comparison_0_22 = cl->p->sizep > (int)0;
  if (comparison_0_22) goto OP_RETURN_if_sizep_gt_0_12_23; else goto
OP_RETURN_else_sizep_gt_0_12_24;


So as far as I can see the generated code is correct and matches the
Lua bytecode.

Regards
Dibyendu