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This is the fold that causes the infinite loop in BoringSSL (https://github.com/google/boringssl/blob/master/crypto/cipher/e_rc2.c) when we fix instcombine demanded bits to prefer 'not' ops as in D32255. There are 2 or 3 problems with dyn_castNotVal, and I don't think we can reinstate D32255 until dyn_castNotVal is completely eliminated. 1. As shown here, it transforms 'not' into random xor. This transform is harmful to SCEV and codegen because 'not' can often be folded while random xor cannot. 2. It does not transform vector constants. This is actually a good thing, but if you don't believe the above argument, then we shouldn't have excluded vectors. 3. It tries to avoid transforming not(not(X)). That's nice, but it doesn't match the greedy nature of instcombine. If we DeMorganize a pattern that has an extra 'not' in it: ~(~(~X) & Y) --> (~X | ~Y) That's just another case of DeMorgan, so we should trust that we'll fold that pattern too: (~X | ~ Y) --> ~(X & Y) Differential Revision: https://reviews.llvm.org/D32665 git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@301923 91177308-0d34-0410-b5e6-96231b3b80d8
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