[PATCH] D129293: [IndVars] Simplify instructions after replacing header phi with preheader value

[Nikita Popov via Phabricator via llvm-commits]

nikic added a comment.

@mstorsjo Thanks for the report! Here's a reduced test case:

  define i64 @test(i64 %arg) {
  entry:
    br label %loop1
  
  loop1:
    %iv1 = phi i64 [ 0, %entry ], [ 1, %loop1.latch ]
    br label %loop2
  
  loop2:
    %iv2 = phi i64 [ 0, %loop1 ], [ 1, %loop2 ]
    %res = add nuw nsw i64 %iv1, %iv2
    br i1 true, label %loop2, label %loop1.latch
  
  loop1.latch:
    %res.lcssa = phi i64 [ %res, %loop2 ]
    br i1 false, label %loop1, label %exit
  
  exit:
    %res.lcssa2 = phi i64 [ %res.lcssa, %loop1.latch ]
    ret i64 %res.lcssa2
  }

This gets reduced to:

  define i64 @t(i64 %arg) {
  entry:
    br label %loop1
  
  loop1:                                            ; preds = %loop1.latch, %entry
    br label %loop2
  
  loop2:                                            ; preds = %loop2, %loop1
    %iv2 = phi i64 [ 0, %loop1 ], [ 1, %loop2 ]
    br i1 true, label %loop2, label %loop1.latch
  
  loop1.latch:                                      ; preds = %loop2
    br i1 false, label %loop1, label %exit
  
  exit:                                             ; preds = %loop1.latch
    %res.lcssa2 = phi i64 [ %iv2, %loop1.latch ]
    ret i64 %res.lcssa2
  }

We need to make sure that we don't fold away any LCSSA phi nodes of inner loops.


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D129293/new/

https://reviews.llvm.org/D129293