[PATCH] D32665: [InstCombine] don't use DeMorgan's Law on integer constants

[Sanjay Patel via Phabricator via llvm-commits]

spatel created this revision.
Herald added subscribers: mcrosier, rengolin, aemerson.

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 https://reviews.llvm.org/D32255.

There are 2 or 3 problems with dyn_castNotVal, and I don't think we can reinstate https://reviews.llvm.org/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 get that too:
(~X | ~ Y) --> ~(X & Y)


https://reviews.llvm.org/D32665

Files:
  lib/Transforms/InstCombine/InstCombineAndOrXor.cpp
  test/Transforms/InstCombine/and-or.ll
  test/Transforms/InstCombine/assume2.ll

-------------- next part --------------
A non-text attachment was scrubbed...
Name: D32665.97172.patch
Type: text/x-patch
Size: 4433 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20170428/fb924834/attachment.bin>