[PATCH] D121243: [InstCombine] Preserve FMF in foldLogicOfFCmps.

[Nuno Lopes via Phabricator via llvm-commits]

nlopes added a comment.

In D121243#3368365 <https://reviews.llvm.org/D121243#3368365>, @nlopes wrote:

>> For this transform only -- because we are guaranteed to repeat the values in each fcmp -- I was expecting that we could 'or' the relevant flags. But there's a surprising corner case with `true` and `false` fcmp predicates according to Alive2:
>> https://alive2.llvm.org/ce/z/Nffn3L
>>
>> The behavior -- blocking poison via predicate? -- does not seem to be documented in the LangRef.
>
> oops, that seems to be a bug in Alive2.
> We convert `fcmp true` to `true`, but that is wrong as we lose the fast-math flags. Let me fix that.

Fixed (not poison anymore):

  define i1 @src(double %a, double %b) {
  %0:
    %cmp = fcmp ninf true double %a, %b
    %cmp1 = fcmp nnan oge double %a, %b
    %retval = and i1 %cmp, %cmp1
    ret i1 %retval
  }
  =>
  define i1 @tgt(double %a, double %b) {
  %0:
    %r = fcmp nnan ninf oeq double %a, %b
    ret i1 %r
  }
  Transformation doesn't verify!
  
  ERROR: Value mismatch
  
  Example:
  double %a = #x0015250a74408022 (0.000000000000?)
  double %b = #x88018b0635082009 (-0.000000000000?)
  
  Source:
  i1 %cmp = #x1 (1)
  i1 %cmp1 = #x1 (1)
  i1 %retval = #x1 (1)
  
  Target:
  i1 %r = #x0 (0)
  Source value: #x1 (1)
  Target value: #x0 (0)


Repository:
  rG LLVM Github Monorepo

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https://reviews.llvm.org/D121243