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

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

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.