[PATCH] D90382: [InstCombine] foldSelectRotate - generalize to foldSelectFunnelShift

[Nuno Lopes via Phabricator via llvm-commits]

nlopes added a comment.

Alive2 says this test is incorrect (because `select` blocks poison and funnel shift doesn't):

  define i8 @fshr_select(i8 %x, i8 %y, i8 %shamt) {
  %0:
    %cmp = icmp eq i8 %shamt, 0
    %sub = sub i8 8, %shamt
    %shr = lshr i8 %y, %shamt
    %shl = shl i8 %x, %sub
    %or = or i8 %shl, %shr
    %r = select i1 %cmp, i8 %y, i8 %or
    ret i8 %r
  }
  =>
  define i8 @fshr_select(i8 %x, i8 %y, i8 %shamt) {
  %0:
    %r = fshr i8 %x, i8 %y, i8 %shamt
    ret i8 %r
  }
  Transformation doesn't verify!
  ERROR: Target is more poisonous than source
  
  Example:
  i8 %x = poison
  i8 %y = any
  i8 %shamt = #x00 (0)
  
  Source:
  i1 %cmp = #x1 (1)
  i8 %sub = #x08 (8)
  i8 %shr = any
  i8 %shl = poison
  i8 %or = poison
  i8 %r = any
  
  Target:
  i8 %r = poison
  Source value: any
  Target value: poison

https://web.ist.utl.pt/nuno.lopes/alive2/index.php?hash=bdd9860ccefdbd55&test=Transforms%2FInstCombine%2Ffunnel.ll


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D90382/new/

https://reviews.llvm.org/D90382