[PATCH] DAGCombiner: fold (xor (shl 1, x), -1) -> (rotl -2, x)

[Nadav Rotem]

David, the code and test case look great. The transformation is not trivial. Do you mind adding a line of comment that explains why -2 is used? Maybe an example input like 00000001 -> 11111101. Also a comment about how rotate can only pull in 1s and how large x values are undefined.


http://reviews.llvm.org/D8350

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