On Thu, Mar 15, 2012 at 2:46 PM, Kai Tietz <ktiet...@googlemail.com> wrote:
> 2012/3/15 Richard Guenther <richard.guent...@gmail.com>:
>> On Thu, Mar 15, 2012 at 2:09 PM, Kai Tietz <ktiet...@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.

## Advertising

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 can rework the patch so that it works without the other one.
>
> Regards,
> Kai