[PATCH] D56875: [DAGCombiner] narrow vector binop with 2 insert subvector operands

[Eli Friedman via Phabricator via llvm-commits]

efriedma added a comment.

> 'xor undef, undef --> 0' (not undef).

Strictly speaking, `xor undef, undef ---> undef` is correct.  It's just that as a practical matter, we try to fold obvious cases to zero so we don't have to argue with people who write silly constructs like `__m128i a = _mm_xor_si128(a, a);`.  The fold here seems unlikely to cause problems in practice.


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  https://reviews.llvm.org/D56875/new/

https://reviews.llvm.org/D56875