[llvm-bugs] [Bug 25745] New: Problem in InstCombineSelect.cpp handling SPF_ABS/SPF_NABS

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=25745

            Bug ID: 25745
           Summary: Problem in InstCombineSelect.cpp handling
                    SPF_ABS/SPF_NABS
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: cvick at codeaurora.org
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

In InstCombineSelect.cpp, visitSelectInst calls matchSelectPattern.  If you
have a signed integer compare to a constant 0 or 1, matchSelectPattern returns
the flavor of SPF_ABS or SPF_NABS.  Then in the subsequent call to
getCmpPredicateForMinMax, those values are not handled, and you hit an
unreachable.

InstCombineSelect 1288,1297 (in visitSelectInst)


  Value *LHS, *RHS, *LHS2, *RHS2;
  Instruction::CastOps CastOp;
  SelectPatternResult SPR = matchSelectPattern(&SI, LHS, RHS, &CastOp);
  auto SPF = SPR.Flavor;

  if (SPF) {
    // Canonicalize so that type casts are outside select patterns.
    if (LHS->getType()->getPrimitiveSizeInBits() !=
        SI.getType()->getPrimitiveSizeInBits()) {
      CmpInst::Predicate Pred = getCmpPredicateForMinMax(SPF, SPR.Ordered);


InstCombineSelect 49,68

static CmpInst::Predicate getCmpPredicateForMinMax(SelectPatternFlavor SPF,
                                                 bool Ordered=false) {
switch (SPF) {
default:
  llvm_unreachable("unhandled!");

case SPF_SMIN:
  return ICmpInst::ICMP_SLT;
case SPF_UMIN:
  return ICmpInst::ICMP_ULT;
case SPF_SMAX:
  return ICmpInst::ICMP_SGT;
case SPF_UMAX:
  return ICmpInst::ICMP_UGT;
case SPF_FMINNUM:
  return Ordered ? FCmpInst::FCMP_OLT : FCmpInst::FCMP_ULT;
case SPF_FMAXNUM:
  return Ordered ? FCmpInst::FCMP_OGT : FCmpInst::FCMP_UGT;
}
}


ValueTracking.cpp 3960,3987 (in MatchSelectPattern)

if (ConstantInt *C1 = dyn_cast<ConstantInt>(CmpRHS)) {
  if ((CmpLHS == TrueVal && match(FalseVal, m_Neg(m_Specific(CmpLHS)))) ||
      (CmpLHS == FalseVal && match(TrueVal, m_Neg(m_Specific(CmpLHS))))) {

    // ABS(X) ==> (X >s 0) ? X : -X and (X >s -1) ? X : -X
    // NABS(X) ==> (X >s 0) ? -X : X and (X >s -1) ? -X : X
    if (Pred == ICmpInst::ICMP_SGT && (C1->isZero() || C1->isMinusOne())) {
      return {(CmpLHS == TrueVal) ? SPF_ABS : SPF_NABS, SPNB_NA, false};
    }

    // ABS(X) ==> (X <s 0) ? -X : X and (X <s 1) ? -X : X
    // NABS(X) ==> (X <s 0) ? X : -X and (X <s 1) ? X : -X
    if (Pred == ICmpInst::ICMP_SLT && (C1->isZero() || C1->isOne())) {
      return {(CmpLHS == FalseVal) ? SPF_ABS : SPF_NABS, SPNB_NA, false};
    }
  }

  // Y >s C ? ~Y : ~C == ~Y <s ~C ? ~Y : ~C = SMIN(~Y, ~C)
  if (const auto *C2 = dyn_cast<ConstantInt>(FalseVal)) {
    if (C1->getType() == C2->getType() && ~C1->getValue() == C2->getValue() &&
        (match(TrueVal, m_Not(m_Specific(CmpLHS))) ||
         match(CmpLHS, m_Not(m_Specific(TrueVal))))) {
      LHS = TrueVal;
      RHS = FalseVal;
      return {SPF_SMIN, SPNB_NA, false};
    }
  }
}

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