[PATCH] D122913: [InstCombine] Simplify PHI node whose type and type of its cond differ

[Hirochika Matsumoto via Phabricator via llvm-commits]

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Currently LLVM optimizes the code below:

  llvm
  define i8 @foo(i8 %cond) {
  entry:
    switch i8 %cond, label %default [
    i8 -1, label %sw.-1
    i8 0, label %sw.0
    i8 1, label %sw.1
    ]
  
  default:
    unreachable
  
  sw.-1:
    br label %merge
  
  sw.0:
    br label %merge
  
  sw.1:
    br label %merge
  
  merge:
    %ret = phi i8 [ 1, %sw.1 ], [ 0, %sw.0 ], [ -1, %sw.-1 ]
    ret i8 %ret
  }

to:

  llvm
  define i8 @foo(i8 %cond) {
    ret i8 %cond
  }

But once we make @foo return larger types, say i32 it no longer
optimizes. This patch covers such cases.

Fixes https://github.com/llvm/llvm-project/issues/54561


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D122913

Files:
  llvm/lib/Transforms/InstCombine/InstCombinePHI.cpp
  llvm/test/Transforms/InstCombine/pr54561.ll

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