[PATCH] D49229: [InstCombine] Fold redundant masking operations of shifted value

[Sanjay Patel via Phabricator via llvm-commits]

spatel added a comment.

> So all expensive operations have been eliminated, I do not see why it wouldn't fit in InstCombne. We detect a pattern and we reduce it.

Because the pattern that we are matching is larger than it needs to be (as the comment in the test file clearly shows - there is no 'or' in the minimal pattern). This problem of trying to make everything fit in instcombine has been discussed several times on llvm-dev in the last ~year. Eg:
http://lists.llvm.org/pipermail/llvm-dev/2017-September/117151.html

Did you look at (new)-GVN to see if it fits in there?

If this is in instcombine (in addition to missing the pattern when there is no 'or'), I think you have to limit the transform based on uses as Roman mentioned in an earlier comment.


https://reviews.llvm.org/D49229