[PATCH] InstCombine ((x | ~y) ^ (~x ^ y)) to (x & ~y)

[Richard Smith]

On Fri, Aug 22, 2014 at 11:14 AM, David Majnemer <david.majnemer at gmail.com>
wrote:

> This can be generalized:
>
> `(X | (Y ^ Z)) ^ ((X ^ Z) ^ Y)` -> `X & (Y ^ Z)`


Is matching that really the right approach? I would (perhaps naively) think
we should instead CSE that to

  (X | A) ^ X ^ A   (with A = Y ^ Z)

... and we should be able to perform these more primitive transformations:

 (X | A) ^ X -> ~X & A
 (~X & A) ^ A -> X & A

... which together reduce your expression to X & (Y ^ Z). Perhaps one of
those takes us further from canonical form, though. What are the rules for
canonicity here?
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