[PATCH] D124988: [DAGCombiner] Fold (sext/zext undef) -> 0 and aext(undef) -> undef.

[Craig Topper via Phabricator via llvm-commits]

craig.topper added a comment.

In D124988#3494483 <https://reviews.llvm.org/D124988#3494483>, @jrtc27 wrote:

> What's the justification for 0 rather than undef? That the high bits need to be guaranteed equal even for undef input?

For zext the high bits need to be 0 and for sext the high bits need to match. The result of the fold needs to produce a value that is possible given the undefined input we received. If we folded to undef that would be a larger set of values than were possible. We're allowed to assume a specific value for the input. So here we've assume the input is 0.


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