[PATCH] D63060: [InstCombine] usub.sat(a, b) + b => umax(a, b) (PR42178)

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri added a comment.

This seems correct:

  ----------------------------------------
    %sat = usub_sat i8 C1, C2
    %res = add i8 %sat, C2
  =>
    %sat = usub_sat i8 C1, C2
    %t0 = icmp ult i8 C1, C2
    %res = select i1 %t0, i8 C2, i8 C1
  
  Done: 1
  Optimization is correct!

But it seems to un-`undef` things:

  ----------------------------------------
    %sat = usub_sat i8 %a, %b
    %res = add i8 %sat, %b
  =>
    %sat = usub_sat i8 %a, %b
    %t0 = icmp ult i8 %a, %b
    %res = select i1 %t0, i8 %b, i8 %a
  
  ERROR: Value mismatch for i8 %res
  
  Example:
  i8 %a = undef
  i8 %b = #x10 (16)
  i8 %sat = undef
  i1 %t0 = undef
  Source value: undef
  Target value: #x02 (2)

?


Repository:
  rL LLVM

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