[PATCH] D107180: [InstCombine] Prevent unnecessary sinks caused by the Freeze

[Hyeongyu Kim via Phabricator via llvm-commits]

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The Freeze is added to Cond in fixing the miscomputation of loop unswitch(D106041 <https://reviews.llvm.org/D106041>).
However, more sinks are generated in Instcombine by Freeze that added in D106041 <https://reviews.llvm.org/D106041>.

Below is a situation where D106041 <https://reviews.llvm.org/D106041> is not applied(bug exists in LU).
If D106041 <https://reviews.llvm.org/D106041> is not applied, `load(ptr)` will not sink after InstCombine is performed.

    v = load(ptr)
    cond = icmp eq(v, 0)
    br body
  
  body:
    br(cond, ..., ...)
  =>
    v = load(ptr)
    br body
  
  body:
    cond = icmp eq(v, 0)
    br(cond, ..., ...)

But when we add the Freeze to `Cond`(to fix LU), `load(ptr)` is sink to body block.
And this is not the intended situation.

    v = load(ptr)
    cond = icmp eq(v, 0)
    cond.fr = freeze(cond)
    br body
  
  body:
    br(cond.fr, ..., ...)
  =>
    br body
  
  body:
    v = load(ptr)
    v.fr = freeze(v)
    cond = icmp eq(v, 0)
    br(cond, ..., ...)

Once this patch is applied, the load will not sink, but only Freeze and icmp will sink as below.

    v = load(ptr)
    cond = icmp eq(v, 0)
    cond.fr = freeze(cond)
    br body
  
  body:
    br(cond.fr, ..., ...)
  =>
    v = load(ptr)
    br body
  
  body:
    v.fr = freeze(v)
    cond = icmp eq(v, 0)
    br(cond, ..., ...)


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D107180

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

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