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.

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