Re: [patch tree-ssa-reassoc.c]: Better reassoication for comparision and boolean-logic

[Kai Tietz <ktietz70@googlemail.com>]

2011/5/19 Richard Guenther <richard.guenther@gmail.com>:
> On Thu, May 19, 2011 at 2:59 PM, Kai Tietz <ktietz70@googlemail.com> wrote:
>> 2011/5/19 Richard Guenther <richard.guenther@gmail.com>:
>>> On Thu, May 19, 2011 at 2:48 PM, Kai Tietz <ktietz70@googlemail.com> wrote:
>>>> Hello,
>>>>
>>>> This patch improves reassociation folding for comparision. It expands
>>>> expressions within binary-AND/OR expression like (X | Y) == 0 to (X ==
>>>> 0 && Y == 0)
>>>> and (X | Y) != 0 to (X != 0 || Y != 0).  This is necessary to allow
>>>> better reassociation
>>>> on weak pre-folded logical expressions.  This unfolding gets undone
>>>> anyway later by pass,
>>>> so no disadvantage gets introduced.
>>>> Also while going through BB-list, it tries to do some little
>>>> type-sinking for SSA sequences
>>>> like "D1 = (type) bool1; D2 = (type) bool2; D3 = D1 & D2;' to 'D1 =
>>>> bool1 & bool2; D2 = (type) D1;'.
>>>> This folding has the advantage to see better through intermediate
>>>> results with none-boolean type.
>>>> The function eliminate_redundant_comparison () got reworded so, that
>>>> doesn't break in all cases.
>>>> It now continues to find duplicates and tries to find inverse variant
>>>> (folded to constant). By this
>>>> change we don't combine possible weak optimizations too fast, before
>>>> we can find and handle
>>>> inverse or duplicates.
>>>
>>> sinking casting belongs not here but instead to tree-ssa-forwprop.
>>> I'm not sure that a != 0 | b != 0 is the better canonical variant than
>>> a | b != 0 though.
>>>
>>> is_boolean_compatible_type_p looks like a strange remanent.
>>>
>>> Richard.
>>
>> Well, a | b != 0 is for sure more optimal, but for reassociation we
>> need to see the unfolded variant temporary. This is necessary as
>> fold-const can't see through SSA statements.  But this kind of
>> expansion should be reversed then by pass to the form (a | b) != 0
>> back.
>
> ?  fold-const shouldn't deal with this at all as we are in gimple and in
> SSA form.  Surely re-association comes to play only with chains of
> the above with more than two operands.
>
> Richard.
>
>>
>> Regards,
>> Kai
>>
>

The issue you can see by testcase binop_tor4.c, as here are the
intermediate variables d and e (with int type) are destroying the
reassociation pass. This testcase for example needs this sinking.

Kai