[PATCH] D124552: CodeGenPrepare: Replace constant PHI arguments with switch condition value

[Matthias Braun via Phabricator via llvm-commits]

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We often see code like the following after running SCCP:

  switch (x) { case 42: phi(42, ...); }

This tends to produce bad code as we currently materialize the constant
phi-argument in the switch-block. This increases register pressure and if the
pattern repeats for `n` case statements, we end up generating `n` constant
values.

This changes CodeGenPrepare to catch this pattern and revert it back to:

  switch (x) { case 42: phi(x, ...); }

This fixes a part of https://github.com/llvm/llvm-project/issues/55153


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D124552

Files:
  llvm/lib/CodeGen/CodeGenPrepare.cpp
  llvm/test/CodeGen/X86/ragreedy-hoist-spill.ll
  llvm/test/CodeGen/X86/speculative-load-hardening-indirect.ll
  llvm/test/Transforms/CodeGenPrepare/X86/switch-phi-const.ll

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