[PATCH] D101727: Fix PR47960 - Incorrect transformation of fabs with nnan flag

[Krishna Kariya via Phabricator via llvm-commits]

Krishnakariya updated this revision to Diff 349553.
Krishnakariya marked 2 inline comments as done.
Krishnakariya added a comment.

I have made changes in FMF propagation logic. 
FMF like nnan and ninf should be propagated only if present in select instruction, while for other flags like arcp, reassoc, it doesn't really depend on select instruction.

Another thing that I want to point out is that after dropping nnan flag, the below test encounters an error.

Example link: https://alive2.llvm.org/ce/z/YEcHxG

  define double @select_fcmp_nnan_nsz_olt_zero(double %x) {
  %0:
    %ltzero = fcmp olt double %x, 0.000000
    %negx = fsub nnan nsz double -0.000000, %x
    %fabs = select i1 %ltzero, double %negx, double %x
    ret double %fabs
  }
  =>
  define double @select_fcmp_nnan_nsz_olt_zero(double %x) {
  %0:
    %1 = fabs nsz double %x
    ret double %1
  }
  Transformation doesn't verify!
  ERROR: Value mismatch
  
  Example:
  double %x = #x8000000000000000 (-0.0)
  
  Source:
  i1 %ltzero = #x0 (0)
  double %negx = #x0000000000000000 (+0.0)
  double %fabs = #x8000000000000000 (-0.0)
  
  Target:
  double %1 = #x0000000000000000 (+0.0)
  Source value: #x8000000000000000 (-0.0)
  Target value: #x0000000000000000 (+0.0)


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D101727/new/

https://reviews.llvm.org/D101727

Files:
  llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
  llvm/test/Transforms/InstCombine/fabs.ll

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