[PATCH] D43367: [X86] Turn selects with constant condition into vector shuffles during DAG combine

[Sanjay Patel via Phabricator via llvm-commits]

spatel added reviewers: efriedma, zvi, andreadb.
spatel added a comment.

If we can get rid of SHRUNKBLEND, that would be great.

But we need to be careful about undef here. That caused trouble when we added the equivalent canonicalization in InstCombine. See:
https://bugs.llvm.org/show_bug.cgi?id=32486 / https://reviews.llvm.org/D31980

We ended up with this logic in IR, so I think we want the same in the DAG:

  // Undef in a select condition (choose one of the operands) does not mean
  // the same thing as undef in a shuffle mask (any value is acceptable), so
  // give up.


https://reviews.llvm.org/D43367