Re: Move some bit and binary optimizations in simplify and match

[Bernd Schmidt <bschmidt@redhat.com>]

On 10/08/2015 08:03 PM, Joseph Myers wrote:
> On Thu, 8 Oct 2015, Bernd Schmidt wrote:
>
>> On 10/07/2015 11:54 AM, Hurugalawadi, Naveen wrote:
>>> 	Move Fold X & (X ^ Y) as X & ~Y to match.pd.
>>> 	Move Fold X & (Y ^ X) as ~Y & X to match.pd.
>>
>> I wonder if we shouldn't try to autogenerate patterns such as these. I did
>> something like that for a different project a long time ago. Generate
>> expressions up to a certain level of complexity, identify which ones are
>> equivalent, and pick the one with the lowest cost for simplifications...
>
> Any bitwise expression whose ultimate operands are X, Y, 0 and -1
> (possibly with conversions among types of the same width) could be
> canonicalized to one of: 0, -1, X, Y, ~X, ~Y, X^Y, X^~Y, A&B or A|B (where
> A is X or ~X and B is Y or ~Y).  I don't guarantee those are the best
> canonical forms, but if you're folding this sort of expression you ought
> to be able to make GCC fold all such expressions down to some such form
> (and fold away all equality comparisons among such expressions with
> constant value).

I was actually thinking of also doing this for more complex expressions 
with more than two different operands.


Bernd