Re: [patch tree-optimization]: Fix for PR 45397 part 2 of 2

[Kai Tietz <ktietz70 at googlemail dot com>]

2012/3/21 Richard Guenther <richard.guenther@gmail.com>:
> On Thu, Mar 15, 2012 at 3:45 PM, Kai Tietz <ktietz70@googlemail.com> wrote:
>> 2012/3/15 Richard Guenther <richard.guenther@gmail.com>:
>>> On Thu, Mar 15, 2012 at 2:46 PM, Kai Tietz <ktietz70@googlemail.com> wrote:
>>>> 2012/3/15 Richard Guenther <richard.guenther@gmail.com>:
>>>>> On Thu, Mar 15, 2012 at 2:09 PM, Kai Tietz <ktietz70@googlemail.com> wrote:
>>>>>> Hi,
>>>>>>
>>>>>> this is the second part of the patch for this problem. ?It adds some
>>>>>> basic simplifications for ==/!=
>>>>>> comparisons for eliminating redudant operands.
>>>>>>
>>>>>> It adds the following patterns:
>>>>>> ?-X ==/!= Z - X -> Z ==/!= 0.
>>>>>> ?~X ==/!= Z ^ X -> Z ==/!= ~0
>>>>>> ?X ==/!= X - Y -> Y == 0
>>>>>> ?X ==/!= X + Y -> Y == 0
>>>>>> ?X ==/!= X ^ Y -> Y == 0
>>>>>> ?(X - Y) ==/!= (Z - Y) -> X ==/!= Z
>>>>>> ?(Y - X) ==/!= (Y - Z) -> X ==/!= Z
>>>>>> ?(X + Y) ==/!= (X + Z) -> Y ==/!= Z
>>>>>> ?(X + Y) ==/!= (Z + X) -> Y ==/!= Z
>>>>>> ?(X ^ Y) ==/!= (Z ^ X) -> Y ==/!= Z
>>>>>
>>>>> Can you re-base this patch to work without the previous one? ?Also
>>>>> please coordinate with Andrew. ?Note that all of these(?) simplifications
>>>>> are already done by fold_comparison which we could share if you'd split
>>>>> out the EXPR_P op0/op1 cases with separated operands/code.
>>>>>
>>>>> Richard.
>>>>
>>>> Hmm, fold_comparison doesn't do the same thing as it checks for
>>>> possible overflow. ?This is true for comparisons not being ==/!= or
>>>> having operands of none-integral-type. ?But for ==/!= with integral
>>>> typed arguments ?the overflow doesn't matter at all. ?And exactly this
>>>> is what patch implements here.
>>>
>>> fold_comparison does not check for overflow for ==/!=.
>>>
>>>> This optimization of course is just desired in non-AST form, as we
>>>> otherwise loose information in FE. ?Therefore I didn't added it to
>>>> fold_const.
>>>
>>> Which pieces are not already in fold-const btw? ?forwprop already
>>> re-constructs trees for the defs of the lhs/rhs of a comparison.
>>>
>>> Richard.
>>
>> I have tried to use here instead a call to fold_build2 instead, and I
>> had to notice that it didn't optimized a single case (beside the - and
>> ~ case on both sides).
>>
>> I see in fold const for example in the pattern 'X +- C1 CMP Y +- C2'
>> to 'X CMP Y +- C2 +- C1' explicit the check for it.
>>
>> ...
>> /* Transform comparisons of the form X +- C1 CMP Y +- C2 to
>> ? X CMP Y +- C2 +- C1 for signed X, Y. ?This is valid if
>> ? the resulting offset is smaller in absolute value than the
>> ? original one. ?*/
>> if (TYPE_OVERFLOW_UNDEFINED (TREE_TYPE (arg0))
>> ? ?&& (TREE_CODE (arg0) == PLUS_EXPR || TREE_CODE (arg0) == MINUS_EXPR)
>> ...
>
> Because the transform is not valid if Y +- C2 +- C1 overflows. ?It is not valid
> because overflow is undefined, not because the comparison would do the
> wrong thing. ?You'd have to change the addition to unsigned.
>
>> The same for pattern X +- C1 CMP C2 to X CMP C2 +- C1.
>
> Well, this is obviously just a missed optimization in fold-const.c then. ?Mind
> conditionalizing the overflow check to codes not NE_EXPR or EQ_EXPR?
>
>> The cases for '(X + Y) ==/!= (Z + X)' and co have the same issue or
>> are simply not present.
>
> That's true. ?I suppose they were considered too special to worry about.
> Did you see these cases in real code?
>
>> Sorry fold_const doesn't cover this at all.
>
> It covers part of it.
>
>> Kai

Sure, the test code shown in this patch isn't that unusual.
Especially in gimple (by using different statements) such construct
are happening.

Eg.:

int f1 (int a, int b, int c)
{
  if ((a + b) == (c + a))
   return 1;
  return 0;
}

int f2 (int a, int b, int c)
{
  if ((a ^ b) == (a  ^ c))
   return 1;
  return 0;
}


int f2 (int a, int b)
{
  if (-a == (b - a))
   return 1;
  return 0;
}

In all those cases the use of variable should be optimized out.
Instead we are producing pretty weak code for those cases.

Kai