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If we know the 2 halves of an oversized zext-in-reg are the same, don't create those halves independently. I tried several different approaches to fold this, but it's difficult to get right during legalization. In the default path, we are creating a generic shuffle that looks like an unpack high, but it can get transformed into a different mask (a blend), so it's not straightforward to match that. If we try to fold after it actually becomes an X86ISD::UNPCKH node, we can't be sure what the operand node is - it might be a generic shuffle, or it could be some x86-specific op. From the test output, we should be doing something like this for SSE4.1 as well, but I'd rather leave that as a follow-up since it involves changing lowering actions. Differential Revision: https://reviews.llvm.org/D59777 llvm-svn: 357129
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