2012/3/15 Richard Guenther <richard.guent...@gmail.com>:
> 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.
>
> 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)
...
The same for pattern X +- C1 CMP C2 to X CMP C2 +- C1.
The cases for '(X + Y) ==/!= (Z + X)' and co have the same issue or
are simply not present.
Sorry fold_const doesn't cover this at all.
Kai
