Re: [patch tree-optimization]: [3 of 3]: Boolify compares & more

[Richard Guenther <richard.guenther@gmail.com>]

On Fri, Jul 8, 2011 at 4:35 PM, Kai Tietz <ktietz70@googlemail.com> wrote:
> 2011/7/8 Richard Guenther <richard.guenther@gmail.com>:
>> On Thu, Jul 7, 2011 at 6:28 PM, Kai Tietz <ktietz70@googlemail.com> wrote:
>>> 2011/7/7 Paolo Bonzini <bonzini@gnu.org>:
>>>> On 07/07/2011 06:07 PM, Kai Tietz wrote:
>>>>>
>>>>> +  /* We redo folding here one time for allowing to inspect more
>>>>> +     complex reductions.  */
>>>>> +  substitute_and_fold (op_with_constant_singleton_value_range,
>>>>> +                      vrp_fold_stmt, false);
>>>>> +  /* We need to mark this second pass to avoid re-entering of same
>>>>> +     edges for switch statments.  */
>>>>> +  in_second_pass = true;
>>>>>    substitute_and_fold (op_with_constant_singleton_value_range,
>>>>>                       vrp_fold_stmt, false);
>>>>> +  in_second_pass = false;
>>>>
>>>> This needs a much better explanation.
>>>>
>>>> Paolo
>>>
>>> Well, I can work on a better comment.  The complex reduction I mean
>>> here are cases like
>>>
>>> int x;
>>> int y;
>>> _Bool D1;
>>> _Bool D2;
>>> _Bool D3;
>>> int R;
>>>
>>> D1 = x[0..1] != 0;
>>> D2 = y[0..1] != 0;
>>> D3 = D1 & D2
>>> R = (int) D3
>>>
>>> (testcase is already present. See tree-ssa/vrp47.c).
>>>
>>> As VRP in first pass produces (and replaces) to:
>>>
>>> D1 = (_Bool) x[0..1];
>>> D2 = (_Bool) y[0..1];
>>> D3 = D1 & D2
>>> R = (int) D3
>>>
>>> Just in the second pass the reduction
>>>
>>> R = x[0..1] & y[0..1]
>>
>> So why wouldn't that happen during the first pass?  The first
>> pass could change the IL to
>>
>>  D1 = x[0..1] != 0;
>>  D2 = y[0..1] != 0;
>>  D3 = D1 & D2;
>>  R = x & y;
>>
>> if D3 only has a single use.
> No, as D3 would need a type change, and this isn't possible.  If it
> wasn't absolutely clear, this patch to VRP is necessary after patch 2,
> as here D1, D2, and D3 have bool-type, and just R is of type int.

In your example x,y and R are int, so it works with re-using R.

>>> can happen.  In general it is sad that VRP can't insert during pass
>>> new statements right now.  This would cause issues in range-tables,
>>> which aren't designed for insertations.  As otherwise, we could do
>>> also simplify things like
>>>
>>> D1 = x[0..1] != 0;
>>> D2 = y[0..1] == 0;
>>> D3 = D1 & D2
>>> R = (int) D3
>>>
>>> to
>>> R = x[0..1] & (y[0..1] ^ 1)
>>
>> Why that ^ 1?  And why does that confuse the range tables
>> if you re-use R?
> Because we would need to insert a new statement and this isn't allowed
> in VRP. See the comments in VRP and substitute_and_fold.  VRP
> disallows to remove statements or to insert new ones.

That's not a hard limitation.

>>> Regards,
>>> Kai
>