llvm dev - Jun 2016 - DAGCombiner folds (xor (and x, y), y) -> (and (not x), y). Can I do the reverse for a target?

Hi,

DAGCombiner currently folds  (xor (and x, y), y) -> (and (not x), y)

I was trying to do the reverse of this transformation, i.e., (and (xor x,
-1), y) -> (xor (and x, y), y), and recognized that this causes an infinite
loop. What's the advantage of doing the above folding? if I want to do the
reverse for a specific target where I can place that and make sure it
doesn't get reversed by the above folding?

Thanks,
Assem
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On Tue, Jun 21, 2016 at 12:49:14PM -0400, Assem Bsoul via llvm-dev
wrote:
> DAGCombiner currently folds  (xor (and x, y), y) -> (and (not x), y)
> 
> I was trying to do the reverse of this transformation, i.e., (and (xor x,
> -1), y) -> (xor (and x, y), y), and recognized that this causes an
infinite
> loop. What's the advantage of doing the above folding? if I want to do
the
> reverse for a specific target where I can place that and make sure it
> doesn't get reversed by the above folding?
The goal of the transform is to provide a simpler canonical form. E.g.
(not x) can be computed independently and it is also easier to reason
about. Why is it beneficial for your target to have the first form?

Joerg