[llvm] Zext sext undef in sdag (PR #115451)

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fengfeng09 wrote:

> > > This optimization is not correct for 'undef'. I think it would be correct for 'poison', but SelectionDAG doesn't have 'poison' today.
> > > See also https://blog.regehr.org/archives/1837
> > 
> > 
> > I see. Thanks for your explanation, so zext undef to a zero is by designed, is this right?
> 
> For (zext undef), the zero extended bits of the result must be 0. The bits that started out as undef can be any value. We choose 0 for simplicity.
> 
> For (sext undef), the sign extended bits must have the same value as the original sign bit. Because of undef, the original sign bit can be either 0 or 1. Folding to undef wouldn't guarantee the extended bits all match, so we fold to all 0s.
Got it, thanks.


https://github.com/llvm/llvm-project/pull/115451