[PATCH] D111744: [instcombine] PRE freeze to only potentially posion/undef operand of phi

[Philip Reames via Phabricator via llvm-commits]

reames created this revision.
reames added reviewers: nikic, aqjune, hyeongyukim, spatel.
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This extends the foldOpIntoPhi code used when visiting a freeze user of a phi to allow any non-undef/poison operand as opposed to only non-undef/poison constants.

While this works, I'm not sure this is the right way to solve this problem.  We're replacing a very cheap check with a bounded search of each phi operand, which has potential compile time implications.  (@nikic - Could you help quantify?)

This routine is fairly limited.  I went looking for a stronger routine (because really we're just folding the phi away when inst-simplify can prove there's an existing value for all but one operand), but was shocked not to find one (anywhere).  I've got a toy binop test case that shows we entirely miss the corresponding opportunity at O3 <https://reviews.llvm.org/owners/package/3/>.

I'm really curious on others take here.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D111744

Files:
  llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
  llvm/test/Transforms/InstCombine/freeze.ll

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