lib/CodeGen/ItaniumCXXABI.cpp

//===------- ItaniumCXXABI.cpp - Emit LLVM Code from ASTs for a Module ----===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This provides C++ code generation targetting the Itanium C++ ABI.  The class
// in this file generates structures that follow the Itanium C++ ABI, which is
// documented at:
//  http://www.codesourcery.com/public/cxx-abi/abi.html
//  http://www.codesourcery.com/public/cxx-abi/abi-eh.html
//
// It also supports the closely-related ARM ABI, documented at:
// http://infocenter.arm.com/help/topic/com.arm.doc.ihi0041c/IHI0041C_cppabi.pdf
//
//===----------------------------------------------------------------------===//

#include "CGCXXABI.h"
#include "CGRecordLayout.h"
#include "CodeGenFunction.h"
#include "CodeGenModule.h"
#include "Mangle.h"
#include <clang/AST/Type.h>
#include <llvm/Target/TargetData.h>
#include <llvm/Value.h>

using namespace clang;
using namespace CodeGen;

namespace {
class ItaniumCXXABI : public CodeGen::CGCXXABI {
private:
  const llvm::IntegerType *PtrDiffTy;
protected:
  CodeGen::MangleContext MangleCtx;
  bool IsARM;

  // It's a little silly for us to cache this.
  const llvm::IntegerType *getPtrDiffTy() {
    if (!PtrDiffTy) {
      QualType T = CGM.getContext().getPointerDiffType();
      const llvm::Type *Ty = CGM.getTypes().ConvertTypeRecursive(T);
      PtrDiffTy = cast<llvm::IntegerType>(Ty);
    }
    return PtrDiffTy;
  }

public:
  ItaniumCXXABI(CodeGen::CodeGenModule &CGM, bool IsARM = false) :
    CGCXXABI(CGM), PtrDiffTy(0), MangleCtx(CGM.getContext(), CGM.getDiags()),
    IsARM(IsARM) { }

  CodeGen::MangleContext &getMangleContext() {
    return MangleCtx;
  }

  bool isZeroInitializable(const MemberPointerType *MPT);

  const llvm::Type *ConvertMemberPointerType(const MemberPointerType *MPT);

  llvm::Value *EmitLoadOfMemberFunctionPointer(CodeGenFunction &CGF,
                                               llvm::Value *&This,
                                               llvm::Value *MemFnPtr,
                                               const MemberPointerType *MPT);

  llvm::Value *EmitMemberPointerConversion(CodeGenFunction &CGF,
                                           const CastExpr *E,
                                           llvm::Value *Src);

  llvm::Constant *EmitMemberPointerConversion(llvm::Constant *C,
                                              const CastExpr *E);

  llvm::Constant *EmitNullMemberPointer(const MemberPointerType *MPT);

  llvm::Constant *EmitMemberPointer(const CXXMethodDecl *MD);
  llvm::Constant *EmitMemberPointer(const FieldDecl *FD);

  llvm::Value *EmitMemberPointerComparison(CodeGenFunction &CGF,
                                           llvm::Value *L,
                                           llvm::Value *R,
                                           const MemberPointerType *MPT,
                                           bool Inequality);

  llvm::Value *EmitMemberPointerIsNotNull(CodeGenFunction &CGF,
                                          llvm::Value *Addr,
                                          const MemberPointerType *MPT);
};

class ARMCXXABI : public ItaniumCXXABI {
public:
  ARMCXXABI(CodeGen::CodeGenModule &CGM) : ItaniumCXXABI(CGM, /*ARM*/ true) {}
};
}

CodeGen::CGCXXABI *CodeGen::CreateItaniumCXXABI(CodeGenModule &CGM) {
  return new ItaniumCXXABI(CGM);
}

CodeGen::CGCXXABI *CodeGen::CreateARMCXXABI(CodeGenModule &CGM) {
  return new ARMCXXABI(CGM);
}

const llvm::Type *
ItaniumCXXABI::ConvertMemberPointerType(const MemberPointerType *MPT) {
  if (MPT->isMemberDataPointer())
    return getPtrDiffTy();
  else
    return llvm::StructType::get(CGM.getLLVMContext(),
                                 getPtrDiffTy(), getPtrDiffTy(), NULL);
}

/// In the Itanium and ARM ABIs, method pointers have the form:
///   struct { ptrdiff_t ptr; ptrdiff_t adj; } memptr;
///
/// In the Itanium ABI:
///  - method pointers are virtual if (memptr.ptr & 1) is nonzero
///  - the this-adjustment is (memptr.adj)
///  - the virtual offset is (memptr.ptr - 1)
///
/// In the ARM ABI:
///  - method pointers are virtual if (memptr.adj & 1) is nonzero
///  - the this-adjustment is (memptr.adj >> 1)
///  - the virtual offset is (memptr.ptr)
/// ARM uses 'adj' for the virtual flag because Thumb functions
/// may be only single-byte aligned.
///
/// If the member is virtual, the adjusted 'this' pointer points
/// to a vtable pointer from which the virtual offset is applied.
///
/// If the member is non-virtual, memptr.ptr is the address of
/// the function to call.
llvm::Value *
ItaniumCXXABI::EmitLoadOfMemberFunctionPointer(CodeGenFunction &CGF,
                                               llvm::Value *&This,
                                               llvm::Value *MemFnPtr,
                                               const MemberPointerType *MPT) {
  CGBuilderTy &Builder = CGF.Builder;

  const FunctionProtoType *FPT =
    MPT->getPointeeType()->getAs<FunctionProtoType>();
  const CXXRecordDecl *RD =
    cast<CXXRecordDecl>(MPT->getClass()->getAs<RecordType>()->getDecl());

  const llvm::FunctionType *FTy =
    CGM.getTypes().GetFunctionType(CGM.getTypes().getFunctionInfo(RD, FPT),
                                   FPT->isVariadic());

  const llvm::IntegerType *ptrdiff = getPtrDiffTy();
  llvm::Constant *ptrdiff_1 = llvm::ConstantInt::get(ptrdiff, 1);

  llvm::BasicBlock *FnVirtual = CGF.createBasicBlock("memptr.virtual");
  llvm::BasicBlock *FnNonVirtual = CGF.createBasicBlock("memptr.nonvirtual");
  llvm::BasicBlock *FnEnd = CGF.createBasicBlock("memptr.end");

  // Extract memptr.adj, which is in the second field.
  llvm::Value *RawAdj = Builder.CreateExtractValue(MemFnPtr, 1, "memptr.adj");

  // Compute the true adjustment.
  llvm::Value *Adj = RawAdj;
  if (IsARM)
    Adj = Builder.CreateAShr(Adj, ptrdiff_1, "memptr.adj.shifted");

  // Apply the adjustment and cast back to the original struct type
  // for consistency.
  llvm::Value *Ptr = Builder.CreateBitCast(This, Builder.getInt8PtrTy());
  Ptr = Builder.CreateInBoundsGEP(Ptr, Adj);
  This = Builder.CreateBitCast(Ptr, This->getType(), "this.adjusted");

  // Load the function pointer.
  llvm::Value *FnAsInt = Builder.CreateExtractValue(MemFnPtr, 0, "memptr.ptr");

  // If the LSB in the function pointer is 1, the function pointer points to
  // a virtual function.
  llvm::Value *IsVirtual;
  if (IsARM)
    IsVirtual = Builder.CreateAnd(RawAdj, ptrdiff_1);
  else
    IsVirtual = Builder.CreateAnd(FnAsInt, ptrdiff_1);
  IsVirtual = Builder.CreateIsNotNull(IsVirtual, "memptr.isvirtual");
  Builder.CreateCondBr(IsVirtual, FnVirtual, FnNonVirtual);

  // In the virtual path, the adjustment left 'This' pointing to the
  // vtable of the correct base subobject.  The "function pointer" is an
  // offset within the vtable (+1 for the virtual flag on non-ARM).
  CGF.EmitBlock(FnVirtual);

  // Cast the adjusted this to a pointer to vtable pointer and load.
  const llvm::Type *VTableTy = Builder.getInt8PtrTy();
  llvm::Value *VTable = Builder.CreateBitCast(This, VTableTy->getPointerTo());
  VTable = Builder.CreateLoad(VTable, "memptr.vtable");

  // Apply the offset.
  llvm::Value *VTableOffset = FnAsInt;
  if (!IsARM) VTableOffset = Builder.CreateSub(VTableOffset, ptrdiff_1);
  VTable = Builder.CreateGEP(VTable, VTableOffset);

  // Load the virtual function to call.
  VTable = Builder.CreateBitCast(VTable, FTy->getPointerTo()->getPointerTo());
  llvm::Value *VirtualFn = Builder.CreateLoad(VTable, "memptr.virtualfn");
  CGF.EmitBranch(FnEnd);

  // In the non-virtual path, the function pointer is actually a
  // function pointer.
  CGF.EmitBlock(FnNonVirtual);
  llvm::Value *NonVirtualFn =
    Builder.CreateIntToPtr(FnAsInt, FTy->getPointerTo(), "memptr.nonvirtualfn");

  // We're done.
  CGF.EmitBlock(FnEnd);
  llvm::PHINode *Callee = Builder.CreatePHI(FTy->getPointerTo());
  Callee->reserveOperandSpace(2);
  Callee->addIncoming(VirtualFn, FnVirtual);
  Callee->addIncoming(NonVirtualFn, FnNonVirtual);
  return Callee;
}

/// Perform a derived-to-base or base-to-derived member pointer conversion.
///
/// Obligatory offset/adjustment diagram:
///         <-- offset -->          <-- adjustment -->
///   |--------------------------|----------------------|--------------------|
///   ^Derived address point     ^Base address point    ^Member address point
///
/// So when converting a base member pointer to a derived member pointer,
/// we add the offset to the adjustment because the address point has
/// decreased;  and conversely, when converting a derived MP to a base MP
/// we subtract the offset from the adjustment because the address point
/// has increased.
///
/// The standard forbids (at compile time) conversion to and from
/// virtual bases, which is why we don't have to consider them here.
///
/// The standard forbids (at run time) casting a derived MP to a base
/// MP when the derived MP does not point to a member of the base.
/// This is why -1 is a reasonable choice for null data member
/// pointers.
llvm::Value *
ItaniumCXXABI::EmitMemberPointerConversion(CodeGenFunction &CGF,
                                           const CastExpr *E,
                                           llvm::Value *Src) {
  assert(E->getCastKind() == CastExpr::CK_DerivedToBaseMemberPointer ||
         E->getCastKind() == CastExpr::CK_BaseToDerivedMemberPointer);

  if (isa<llvm::Constant>(Src))
    return EmitMemberPointerConversion(cast<llvm::Constant>(Src), E);

  CGBuilderTy &Builder = CGF.Builder;

  const MemberPointerType *SrcTy =
    E->getSubExpr()->getType()->getAs<MemberPointerType>();
  const MemberPointerType *DestTy = E->getType()->getAs<MemberPointerType>();

  const CXXRecordDecl *SrcDecl = SrcTy->getClass()->getAsCXXRecordDecl();
  const CXXRecordDecl *DestDecl = DestTy->getClass()->getAsCXXRecordDecl();

  bool DerivedToBase =
    E->getCastKind() == CastExpr::CK_DerivedToBaseMemberPointer;

  const CXXRecordDecl *BaseDecl, *DerivedDecl;
  if (DerivedToBase)
    DerivedDecl = SrcDecl, BaseDecl = DestDecl;
  else
    BaseDecl = SrcDecl, DerivedDecl = DestDecl;

  llvm::Constant *Adj =
    CGF.CGM.GetNonVirtualBaseClassOffset(DerivedDecl,
                                         E->path_begin(),
                                         E->path_end());
  if (!Adj) return Src;

  // For member data pointers, this is just a matter of adding the
  // offset if the source is non-null.
  if (SrcTy->isMemberDataPointer()) {
    llvm::Value *Dst;
    if (DerivedToBase)
      Dst = Builder.CreateNSWSub(Src, Adj, "adj");
    else
      Dst = Builder.CreateNSWAdd(Src, Adj, "adj");

    // Null check.
    llvm::Value *Null = llvm::Constant::getAllOnesValue(Src->getType());
    llvm::Value *IsNull = Builder.CreateICmpEQ(Src, Null, "memptr.isnull");
    return Builder.CreateSelect(IsNull, Src, Dst);
  }

  // The this-adjustment is left-shifted by 1 on ARM.
  if (IsARM) {
    uint64_t Offset = cast<llvm::ConstantInt>(Adj)->getZExtValue();
    Offset <<= 1;
    Adj = llvm::ConstantInt::get(Adj->getType(), Offset);
  }

  llvm::Value *SrcAdj = Builder.CreateExtractValue(Src, 1, "src.adj");
  llvm::Value *DstAdj;
  if (DerivedToBase)
    DstAdj = Builder.CreateNSWSub(SrcAdj, Adj, "adj");
  else
    DstAdj = Builder.CreateNSWAdd(SrcAdj, Adj, "adj");

  return Builder.CreateInsertValue(Src, DstAdj, 1);
}

llvm::Constant *
ItaniumCXXABI::EmitMemberPointerConversion(llvm::Constant *C,
                                           const CastExpr *E) {
  const MemberPointerType *SrcTy =
    E->getSubExpr()->getType()->getAs<MemberPointerType>();
  const MemberPointerType *DestTy =
    E->getType()->getAs<MemberPointerType>();

  bool DerivedToBase =
    E->getCastKind() == CastExpr::CK_DerivedToBaseMemberPointer;

  const CXXRecordDecl *DerivedDecl;
  if (DerivedToBase)
    DerivedDecl = SrcTy->getClass()->getAsCXXRecordDecl();
  else
    DerivedDecl = DestTy->getClass()->getAsCXXRecordDecl();

  // Calculate the offset to the base class.
  llvm::Constant *Offset =
    CGM.GetNonVirtualBaseClassOffset(DerivedDecl,
                                     E->path_begin(),
                                     E->path_end());
  // If there's no offset, we're done.
  if (!Offset) return C;

  // If the source is a member data pointer, we have to do a null
  // check and then add the offset.  In the common case, we can fold
  // away the offset.
  if (SrcTy->isMemberDataPointer()) {
    assert(C->getType() == getPtrDiffTy());

    // If it's a constant int, just create a new constant int.
    if (llvm::ConstantInt *CI = dyn_cast<llvm::ConstantInt>(C)) {
      int64_t Src = CI->getSExtValue();

      // Null converts to null.
      if (Src == -1) return CI;

      // Otherwise, just add the offset.
      int64_t OffsetV = cast<llvm::ConstantInt>(Offset)->getSExtValue();
      int64_t Dst = (DerivedToBase ? Src - OffsetV : Src + OffsetV);
      return llvm::ConstantInt::get(CI->getType(), Dst, /*signed*/ true);
    }

    // Otherwise, we have to form a constant select expression.
    llvm::Constant *Null = llvm::Constant::getAllOnesValue(C->getType());

    llvm::Constant *IsNull =
      llvm::ConstantExpr::getICmp(llvm::ICmpInst::ICMP_EQ, C, Null);

    llvm::Constant *Dst;
    if (DerivedToBase)
      Dst = llvm::ConstantExpr::getNSWSub(C, Offset);
    else
      Dst = llvm::ConstantExpr::getNSWAdd(C, Offset);

    return llvm::ConstantExpr::getSelect(IsNull, Null, Dst);
  }

  // The this-adjustment is left-shifted by 1 on ARM.
  if (IsARM) {
    int64_t OffsetV = cast<llvm::ConstantInt>(Offset)->getSExtValue();
    OffsetV <<= 1;
    Offset = llvm::ConstantInt::get(Offset->getType(), OffsetV);
  }

  llvm::ConstantStruct *CS = cast<llvm::ConstantStruct>(C);

  llvm::Constant *Values[2] = { CS->getOperand(0), 0 };
  if (DerivedToBase)
    Values[1] = llvm::ConstantExpr::getSub(CS->getOperand(1), Offset);
  else
    Values[1] = llvm::ConstantExpr::getAdd(CS->getOperand(1), Offset);

  return llvm::ConstantStruct::get(CGM.getLLVMContext(), Values, 2,
                                   /*Packed=*/false);
}


llvm::Constant *
ItaniumCXXABI::EmitNullMemberPointer(const MemberPointerType *MPT) {
  const llvm::Type *ptrdiff_t = getPtrDiffTy();

  // Itanium C++ ABI 2.3:
  //   A NULL pointer is represented as -1.
  if (MPT->isMemberDataPointer())
    return llvm::ConstantInt::get(ptrdiff_t, -1ULL, /*isSigned=*/true);

  llvm::Constant *Zero = llvm::ConstantInt::get(ptrdiff_t, 0);
  llvm::Constant *Values[2] = { Zero, Zero };
  return llvm::ConstantStruct::get(CGM.getLLVMContext(), Values, 2,
                                   /*Packed=*/false);
}

llvm::Constant *ItaniumCXXABI::EmitMemberPointer(const FieldDecl *FD) {
  // Itanium C++ ABI 2.3:
  //   A pointer to data member is an offset from the base address of
  //   the class object containing it, represented as a ptrdiff_t

  QualType ClassType = CGM.getContext().getTypeDeclType(FD->getParent());
  const llvm::StructType *ClassLTy =
    cast<llvm::StructType>(CGM.getTypes().ConvertType(ClassType));

  const CGRecordLayout &RL = CGM.getTypes().getCGRecordLayout(FD->getParent());
  unsigned FieldNo = RL.getLLVMFieldNo(FD);
  uint64_t Offset =
    CGM.getTargetData().getStructLayout(ClassLTy)->getElementOffset(FieldNo);

  return llvm::ConstantInt::get(getPtrDiffTy(), Offset);
}

llvm::Constant *ItaniumCXXABI::EmitMemberPointer(const CXXMethodDecl *MD) {
  assert(MD->isInstance() && "Member function must not be static!");
  MD = MD->getCanonicalDecl();

  CodeGenTypes &Types = CGM.getTypes();
  const llvm::Type *ptrdiff_t = getPtrDiffTy();

  // Get the function pointer (or index if this is a virtual function).
  llvm::Constant *MemPtr[2];
  if (MD->isVirtual()) {
    uint64_t Index = CGM.getVTables().getMethodVTableIndex(MD);

    // FIXME: We shouldn't use / 8 here.
    uint64_t PointerWidthInBytes =
      CGM.getContext().Target.getPointerWidth(0) / 8;
    uint64_t VTableOffset = (Index * PointerWidthInBytes);

    if (IsARM) {
      // ARM C++ ABI 3.2.1:
      //   This ABI specifies that adj contains twice the this
      //   adjustment, plus 1 if the member function is virtual. The
      //   least significant bit of adj then makes exactly the same
      //   discrimination as the least significant bit of ptr does for
      //   Itanium.
      MemPtr[0] = llvm::ConstantInt::get(ptrdiff_t, VTableOffset);
      MemPtr[1] = llvm::ConstantInt::get(ptrdiff_t, 1);
    } else {
      // Itanium C++ ABI 2.3:
      //   For a virtual function, [the pointer field] is 1 plus the
      //   virtual table offset (in bytes) of the function,
      //   represented as a ptrdiff_t.
      MemPtr[0] = llvm::ConstantInt::get(ptrdiff_t, VTableOffset + 1);
      MemPtr[1] = llvm::ConstantInt::get(ptrdiff_t, 0);
    }
  } else {
    const FunctionProtoType *FPT = MD->getType()->getAs<FunctionProtoType>();
    const llvm::Type *Ty;
    // Check whether the function has a computable LLVM signature.
    if (!CodeGenTypes::VerifyFuncTypeComplete(FPT)) {
      // The function has a computable LLVM signature; use the correct type.
      Ty = Types.GetFunctionType(Types.getFunctionInfo(MD), FPT->isVariadic());
    } else {
      // Use an arbitrary non-function type to tell GetAddrOfFunction that the
      // function type is incomplete.
      Ty = ptrdiff_t;
    }

    llvm::Constant *Addr = CGM.GetAddrOfFunction(MD, Ty);
    MemPtr[0] = llvm::ConstantExpr::getPtrToInt(Addr, ptrdiff_t);
    MemPtr[1] = llvm::ConstantInt::get(ptrdiff_t, 0);
  }

  return llvm::ConstantStruct::get(CGM.getLLVMContext(),
                                   MemPtr, 2, /*Packed=*/false);
}

/// The comparison algorithm is pretty easy: the member pointers are
/// the same if they're either bitwise identical *or* both null.
///
/// ARM is different here only because null-ness is more complicated.
llvm::Value *
ItaniumCXXABI::EmitMemberPointerComparison(CodeGenFunction &CGF,
                                           llvm::Value *L,
                                           llvm::Value *R,
                                           const MemberPointerType *MPT,
                                           bool Inequality) {
  CGBuilderTy &Builder = CGF.Builder;

  llvm::ICmpInst::Predicate Eq;
  llvm::Instruction::BinaryOps And, Or;
  if (Inequality) {
    Eq = llvm::ICmpInst::ICMP_NE;
    And = llvm::Instruction::Or;
    Or = llvm::Instruction::And;
  } else {
    Eq = llvm::ICmpInst::ICMP_EQ;
    And = llvm::Instruction::And;
    Or = llvm::Instruction::Or;
  }

  // Member data pointers are easy because there's a unique null
  // value, so it just comes down to bitwise equality.
  if (MPT->isMemberDataPointer())
    return Builder.CreateICmp(Eq, L, R);

  // For member function pointers, the tautologies are more complex.
  // The Itanium tautology is:
  //   (L == R) <==> (L.ptr == R.ptr /\ (L.ptr == 0 \/ L.adj == R.adj))
  // The ARM tautology is:
  //   (L == R) <==> (L.ptr == R.ptr /\
  //                  (L.adj == R.adj \/
  //                   (L.ptr == 0 /\ ((L.adj|R.adj) & 1) == 0)))
  // The inequality tautologies have exactly the same structure, except
  // applying De Morgan's laws.

  llvm::Value *LPtr = Builder.CreateExtractValue(L, 0, "lhs.memptr.ptr");
  llvm::Value *RPtr = Builder.CreateExtractValue(R, 0, "rhs.memptr.ptr");

  // This condition tests whether L.ptr == R.ptr.  This must always be
  // true for equality to hold.
  llvm::Value *PtrEq = Builder.CreateICmp(Eq, LPtr, RPtr, "cmp.ptr");

  // This condition, together with the assumption that L.ptr == R.ptr,
  // tests whether the pointers are both null.  ARM imposes an extra
  // condition.
  llvm::Value *Zero = llvm::Constant::getNullValue(LPtr->getType());
  llvm::Value *EqZero = Builder.CreateICmp(Eq, LPtr, Zero, "cmp.ptr.null");

  // This condition tests whether L.adj == R.adj.  If this isn't
  // true, the pointers are unequal unless they're both null.
  llvm::Value *LAdj = Builder.CreateExtractValue(L, 1, "lhs.memptr.adj");
  llvm::Value *RAdj = Builder.CreateExtractValue(R, 1, "rhs.memptr.adj");
  llvm::Value *AdjEq = Builder.CreateICmp(Eq, LAdj, RAdj, "cmp.adj");

  // Null member function pointers on ARM clear the low bit of Adj,
  // so the zero condition has to check that neither low bit is set.
  if (IsARM) {
    llvm::Value *One = llvm::ConstantInt::get(LPtr->getType(), 1);

    // Compute (l.adj | r.adj) & 1 and test it against zero.
    llvm::Value *OrAdj = Builder.CreateOr(LAdj, RAdj, "or.adj");
    llvm::Value *OrAdjAnd1 = Builder.CreateAnd(OrAdj, One);
    llvm::Value *OrAdjAnd1EqZero = Builder.CreateICmp(Eq, OrAdjAnd1, Zero,
                                                      "cmp.or.adj");
    EqZero = Builder.CreateBinOp(And, EqZero, OrAdjAnd1EqZero);
  }

  // Tie together all our conditions.
  llvm::Value *Result = Builder.CreateBinOp(Or, EqZero, AdjEq);
  Result = Builder.CreateBinOp(And, PtrEq, Result,
                               Inequality ? "memptr.ne" : "memptr.eq");
  return Result;
}

llvm::Value *
ItaniumCXXABI::EmitMemberPointerIsNotNull(CodeGenFunction &CGF,
                                          llvm::Value *MemPtr,
                                          const MemberPointerType *MPT) {
  CGBuilderTy &Builder = CGF.Builder;

  /// For member data pointers, this is just a check against -1.
  if (MPT->isMemberDataPointer()) {
    assert(MemPtr->getType() == getPtrDiffTy());
    llvm::Value *NegativeOne =
      llvm::Constant::getAllOnesValue(MemPtr->getType());
    return Builder.CreateICmpNE(MemPtr, NegativeOne, "memptr.tobool");
  }

  // In Itanium, a member function pointer is null if 'ptr' is null.
  llvm::Value *Ptr = Builder.CreateExtractValue(MemPtr, 0, "memptr.ptr");

  llvm::Constant *Zero = llvm::ConstantInt::get(Ptr->getType(), 0);
  llvm::Value *Result = Builder.CreateICmpNE(Ptr, Zero, "memptr.tobool");

  // In ARM, it's that, plus the low bit of 'adj' must be zero.
  if (IsARM) {
    llvm::Constant *One = llvm::ConstantInt::get(Ptr->getType(), 1);
    llvm::Value *Adj = Builder.CreateExtractValue(MemPtr, 1, "memptr.adj");
    llvm::Value *VirtualBit = Builder.CreateAnd(Adj, One, "memptr.virtualbit");
    llvm::Value *IsNotVirtual = Builder.CreateICmpEQ(VirtualBit, Zero,
                                                     "memptr.notvirtual");
    Result = Builder.CreateAnd(Result, IsNotVirtual);
  }

  return Result;
}

/// The Itanium ABI requires non-zero initialization only for data
/// member pointers, for which '0' is a valid offset.
bool ItaniumCXXABI::isZeroInitializable(const MemberPointerType *MPT) {
  return MPT->getPointeeType()->isFunctionType();
}