[llvm] LoopVectorize/reduction: add negative test for vectorizable example (PR #68912)

[Mel Chen via llvm-commits]

https://github.com/Mel-Chen commented:

Nice case, thanks! This is popcount. In my opinion, it is possible to use loop idiom pass to convert to ctpop.

https://github.com/llvm/llvm-project/pull/68912