[PATCH] D69519: [InstCombine] make icmp vector canonicalization safe for constant with undef elements

[Sanjay Patel via Phabricator via llvm-commits]

spatel created this revision.
spatel added reviewers: lebedev.ri, nlopes, RKSimon.
Herald added subscribers: hiraditya, mcrosier.
Herald added a project: LLVM.

This is a fix for:
https://bugs.llvm.org/show_bug.cgi?id=43730
...and as shown there, we have an existing test case that shows a potential miscompile.

We could just bail out for vector constants that contain any undef elements, or we can do as shown here:
allow the transform, but replace the undefs with a safe value.

For the tests shown, this results in a full splat constant (no undefs) which is probably a win for further IR analysis because we conservatively don't match undefs in most cases. Codegen can probably recover these kinds of undef lanes via demanded elements analysis if that's profitable.


https://reviews.llvm.org/D69519

Files:
  llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
  llvm/test/Transforms/InstCombine/canonicalize-constant-low-bit-mask-and-icmp-eq-to-icmp-ule.ll
  llvm/test/Transforms/InstCombine/canonicalize-constant-low-bit-mask-and-icmp-sge-to-icmp-sle.ll
  llvm/test/Transforms/InstCombine/canonicalize-constant-low-bit-mask-and-icmp-sle-to-icmp-sle.ll
  llvm/test/Transforms/InstCombine/canonicalize-constant-low-bit-mask-and-icmp-uge-to-icmp-ule.ll
  llvm/test/Transforms/InstCombine/canonicalize-constant-low-bit-mask-and-icmp-ule-to-icmp-ule.ll
  llvm/test/Transforms/InstCombine/icmp-vec.ll
  llvm/test/Transforms/InstCombine/reuse-constant-from-select-in-icmp.ll

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