[LLVMdev] Why can't comparisons with negative zero be simplified?

[Robison, Arch]

In InstCombineCompares.cpp, routine InstCombiner::FoldFCmp_IntToFP_Cst, there are these lines:

  // Comparisons with zero are a special case where we know we won't lose
  // information.
  bool IsCmpZero = RHS.isPosZero();

  // If the conversion would lose info, don't hack on this.
  if ((int)InputSize > MantissaWidth && !IsCmpZero)
    return nullptr;

Why check for positive zero instead of checking for any kind of zero?  My reading of IEEE 754-2008 is that floating-point comparison operations cannot distinguish a negative zero from a positive zero.  Further supporting this is that fact that http://llvm.org/docs/LangRef.html describes the difference between "ordered" and "unordered" as pertaining to QNAN operands,
with no mention of negative zero.

I tried fixing the issue, but then the following test in cast-int-fcmp-eq-0.ll fails:

; CHECK-LABEL: @i32_cast_cmp_oeq_int_n0_uitofp(
; CHECK: uitofp
; CHECK: fcmp oeq
define i1 @i32_cast_cmp_oeq_int_n0_uitofp(i32 %i) {
  %f = uitofp i32 %i to float
  %cmp = fcmp oeq float %f, -0.0
  ret i1 %cmp
}

Is this test really justified, or is it just reinforcing an oversight?

- Arch D. Robison
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