[PATCH] D109440: [InstCombine] remove a buggy set of zext-icmp transforms

[Sanjay Patel via Phabricator via llvm-commits]

spatel created this revision.
spatel added reviewers: lebedev.ri, nikic, efriedma.
Herald added subscribers: hiraditya, mcrosier.
spatel requested review of this revision.
Herald added a project: LLVM.

The motivating case is an infinite loop shown with a reduced test from:
https://llvm.org/PR51762

To solve this, I'm proposing we delete the most obviously broken part of this code.

The bug example shows a fundamental problem: we ask computeKnownBits if a transform will be profitable, alter the code by creating new instructions, then rely on computeKnownBits to return the same answer to actually eliminate instructions.

But there's no guarantee that the results will be the same between the 1st and 2nd calls. In the infinite loop example, we get different answers, so we add instructions that conflict with some other transform, and we're stuck.

There's at least one other problem visible in the test diff for `@zext_or_masked_bit_test_uses`: the code doesn't check uses properly, so we can end up with extra instructions created.

Last, it's not clear if this set of transforms actually improves analysis or codegen. I spot-checked a few targets and don't see a clear win:
https://godbolt.org/z/x87EWovso

If we do see a regression from this change, codegen seems like the right place to add a cmp -> bit-hack fold.

If this is too big of a step, we could limit the computeKnownBits calls by not passing a context instruction and/or limiting the recursion. I checked that those would stop the infinite loop for PR51762, but that won't guarantee that some other example does not fall into the same loop.


https://reviews.llvm.org/D109440

Files:
  llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
  llvm/lib/Transforms/InstCombine/InstCombineInternal.h
  llvm/test/Transforms/InstCombine/zext-or-icmp.ll
  llvm/test/Transforms/InstCombine/zext.ll

-------------- next part --------------
A non-text attachment was scrubbed...
Name: D109440.371336.patch
Type: text/x-patch
Size: 11857 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20210908/f4e5c38d/attachment.bin>