C++ TargetLowering类代码示例

/// ComputeValueVTs - Given an LLVM IR type, compute a sequence of
/// EVTs that represent all the individual underlying
/// non-aggregate types that comprise it.
///
/// If Offsets is non-null, it points to a vector to be filled in
/// with the in-memory offsets of each of the individual values.
///
void llvm::ComputeValueVTs(const TargetLowering &TLI, Type *Ty,
                           SmallVectorImpl<EVT> &ValueVTs,
                           SmallVectorImpl<uint64_t> *Offsets,
                           uint64_t StartingOffset) {
  // Given a struct type, recursively traverse the elements.
  if (StructType *STy = dyn_cast<StructType>(Ty)) {
    const StructLayout *SL = TLI.getDataLayout()->getStructLayout(STy);
    for (StructType::element_iterator EB = STy->element_begin(),
                                      EI = EB,
                                      EE = STy->element_end();
         EI != EE; ++EI)
      ComputeValueVTs(TLI, *EI, ValueVTs, Offsets,
                      StartingOffset + SL->getElementOffset(EI - EB));
    return;
  }
  // Given an array type, recursively traverse the elements.
  if (ArrayType *ATy = dyn_cast<ArrayType>(Ty)) {
    Type *EltTy = ATy->getElementType();
    uint64_t EltSize = TLI.getDataLayout()->getTypeAllocSize(EltTy);
    for (unsigned i = 0, e = ATy->getNumElements(); i != e; ++i)
      ComputeValueVTs(TLI, EltTy, ValueVTs, Offsets,
                      StartingOffset + i * EltSize);
    return;
  }
  // Interpret void as zero return values.
  if (Ty->isVoidTy())
    return;
  // Base case: we can get an EVT for this LLVM IR type.
  ValueVTs.push_back(TLI.getValueType(Ty));
  if (Offsets)
    Offsets->push_back(StartingOffset);
}

/// Get the EVTs and ArgFlags collections that represent the legalized return
/// type of the given function.  This does not require a DAG or a return value,
/// and is suitable for use before any DAGs for the function are constructed.
/// TODO: Move this out of TargetLowering.cpp.
void llvm::GetReturnInfo(Type* ReturnType, AttributeSet attr,
                         SmallVectorImpl<ISD::OutputArg> &Outs,
                         const TargetLowering &TLI) {
  SmallVector<EVT, 4> ValueVTs;
  ComputeValueVTs(TLI, ReturnType, ValueVTs);
  unsigned NumValues = ValueVTs.size();
  if (NumValues == 0) return;

  for (unsigned j = 0, f = NumValues; j != f; ++j) {
    EVT VT = ValueVTs[j];
    ISD::NodeType ExtendKind = ISD::ANY_EXTEND;

    if (attr.hasAttribute(AttributeSet::ReturnIndex, Attribute::SExt))
      ExtendKind = ISD::SIGN_EXTEND;
    else if (attr.hasAttribute(AttributeSet::ReturnIndex, Attribute::ZExt))
      ExtendKind = ISD::ZERO_EXTEND;

    // FIXME: C calling convention requires the return type to be promoted to
    // at least 32-bit. But this is not necessary for non-C calling
    // conventions. The frontend should mark functions whose return values
    // require promoting with signext or zeroext attributes.
    if (ExtendKind != ISD::ANY_EXTEND && VT.isInteger()) {
      MVT MinVT = TLI.getRegisterType(ReturnType->getContext(), MVT::i32);
      if (VT.bitsLT(MinVT))
        VT = MinVT;
    }

    unsigned NumParts = TLI.getNumRegisters(ReturnType->getContext(), VT);
    MVT PartVT = TLI.getRegisterType(ReturnType->getContext(), VT);

    // 'inreg' on function refers to return value
    ISD::ArgFlagsTy Flags = ISD::ArgFlagsTy();
    if (attr.hasAttribute(AttributeSet::ReturnIndex, Attribute::InReg))
      Flags.setInReg();

    // Propagate extension type if any
    if (attr.hasAttribute(AttributeSet::ReturnIndex, Attribute::SExt))
      Flags.setSExt();
    else if (attr.hasAttribute(AttributeSet::ReturnIndex, Attribute::ZExt))
      Flags.setZExt();

    for (unsigned i = 0; i < NumParts; ++i)
      Outs.push_back(ISD::OutputArg(Flags, PartVT, /*isFixed=*/true, 0, 0));
  }
}

/// IsOperandAMemoryOperand - Check to see if all uses of OpVal by the specified
/// inline asm call are due to memory operands.  If so, return true, otherwise
/// return false.
static bool IsOperandAMemoryOperand(CallInst *CI, InlineAsm *IA, Value *OpVal,
                                    const TargetLowering &TLI) {
  std::vector<TargetLowering::AsmOperandInfo> TargetConstraints = TLI.ParseConstraints(ImmutableCallSite(CI));
  for (unsigned i = 0, e = TargetConstraints.size(); i != e; ++i) {
    TargetLowering::AsmOperandInfo &OpInfo = TargetConstraints[i];
    
    // Compute the constraint code and ConstraintType to use.
    TLI.ComputeConstraintToUse(OpInfo, SDValue());

    // If this asm operand is our Value*, and if it isn't an indirect memory
    // operand, we can't fold it!
    if (OpInfo.CallOperandVal == OpVal &&
        (OpInfo.ConstraintType != TargetLowering::C_Memory ||
         !OpInfo.isIndirect))
      return false;
  }

  return true;
}

bool llvm::isInTailCallPosition(SelectionDAG &DAG, SDNode *Node,
                                const TargetLowering &TLI) {
  const Function *F = DAG.getMachineFunction().getFunction();

  // Conservatively require the attributes of the call to match those of
  // the return. Ignore noalias because it doesn't affect the call sequence.
  unsigned CallerRetAttr = F->getAttributes().getRetAttributes();
  if (CallerRetAttr & ~Attribute::NoAlias)
    return false;

  // It's not safe to eliminate the sign / zero extension of the return value.
  if ((CallerRetAttr & Attribute::ZExt) || (CallerRetAttr & Attribute::SExt))
    return false;

  // Check if the only use is a function return node.
  return TLI.isUsedByReturnOnly(Node);
}

/// hasInlineAsmMemConstraint - Return true if the inline asm instruction being
/// processed uses a memory 'm' constraint.
bool
llvm::hasInlineAsmMemConstraint(InlineAsm::ConstraintInfoVector &CInfos,
                                const TargetLowering &TLI) {
  for (unsigned i = 0, e = CInfos.size(); i != e; ++i) {
    InlineAsm::ConstraintInfo &CI = CInfos[i];
    for (unsigned j = 0, ee = CI.Codes.size(); j != ee; ++j) {
      TargetLowering::ConstraintType CType = TLI.getConstraintType(CI.Codes[j]);
      if (CType == TargetLowering::C_Memory)
        return true;
    }

    // Indirect operand accesses access memory.
    if (CI.isIndirect)
      return true;
  }

  return false;
}

/// Test if the given instruction is in a position to be optimized
/// with a tail-call. This roughly means that it's in a block with
/// a return and there's nothing that needs to be scheduled
/// between it and the return.
///
/// This function only tests target-independent requirements.
bool llvm::isInTailCallPosition(ImmutableCallSite CS,
                                const TargetLowering &TLI) {
  const Instruction *I = CS.getInstruction();
  const BasicBlock *ExitBB = I->getParent();
  const TerminatorInst *Term = ExitBB->getTerminator();
  const ReturnInst *Ret = dyn_cast<ReturnInst>(Term);

  // The block must end in a return statement or unreachable.
  //
  // FIXME: Decline tailcall if it's not guaranteed and if the block ends in
  // an unreachable, for now. The way tailcall optimization is currently
  // implemented means it will add an epilogue followed by a jump. That is
  // not profitable. Also, if the callee is a special function (e.g.
  // longjmp on x86), it can end up causing miscompilation that has not
  // been fully understood.
  if (!Ret &&
      (!TLI.getTargetMachine().Options.GuaranteedTailCallOpt ||
       !isa<UnreachableInst>(Term)))
    return false;

  // If I will have a chain, make sure no other instruction that will have a
  // chain interposes between I and the return.
  if (I->mayHaveSideEffects() || I->mayReadFromMemory() ||
      !isSafeToSpeculativelyExecute(I))
    for (BasicBlock::const_iterator BBI = prior(prior(ExitBB->end())); ;
         --BBI) {
      if (&*BBI == I)
        break;
      // Debug info intrinsics do not get in the way of tail call optimization.
      if (isa<DbgInfoIntrinsic>(BBI))
        continue;
      if (BBI->mayHaveSideEffects() || BBI->mayReadFromMemory() ||
          !isSafeToSpeculativelyExecute(BBI))
        return false;
    }

  return returnTypeIsEligibleForTailCall(ExitBB->getParent(), I, Ret, TLI);
}

/// IsOperandAMemoryOperand - Check to see if all uses of OpVal by the specified
/// inline asm call are due to memory operands.  If so, return true, otherwise
/// return false.
static bool IsOperandAMemoryOperand(CallInst *CI, InlineAsm *IA, Value *OpVal,
                                    const TargetLowering &TLI) {
  std::vector<InlineAsm::ConstraintInfo>
  Constraints = IA->ParseConstraints();
  
  unsigned ArgNo = 1;   // ArgNo - The operand of the CallInst.
  for (unsigned i = 0, e = Constraints.size(); i != e; ++i) {
    TargetLowering::AsmOperandInfo OpInfo(Constraints[i]);
    
    // Compute the value type for each operand.
    switch (OpInfo.Type) {
      case InlineAsm::isOutput:
        if (OpInfo.isIndirect)
          OpInfo.CallOperandVal = CI->getOperand(ArgNo++);
        break;
      case InlineAsm::isInput:
        OpInfo.CallOperandVal = CI->getOperand(ArgNo++);
        break;
      case InlineAsm::isClobber:
        // Nothing to do.
        break;
    }
    
    // Compute the constraint code and ConstraintType to use.
    TLI.ComputeConstraintToUse(OpInfo, SDValue(),
                             OpInfo.ConstraintType == TargetLowering::C_Memory);
    
    // If this asm operand is our Value*, and if it isn't an indirect memory
    // operand, we can't fold it!
    if (OpInfo.CallOperandVal == OpVal &&
        (OpInfo.ConstraintType != TargetLowering::C_Memory ||
         !OpInfo.isIndirect))
      return false;
  }
  
  return true;
}

/// OptimizeNoopCopyExpression - If the specified cast instruction is a noop
/// copy (e.g. it's casting from one pointer type to another, i32->i8 on PPC),
/// sink it into user blocks to reduce the number of virtual
/// registers that must be created and coalesced.
///
/// Return true if any changes are made.
///
static bool OptimizeNoopCopyExpression(CastInst *CI, const TargetLowering &TLI) {
    // If this is a noop copy,
    EVT SrcVT = TLI.getValueType(CI->getOperand(0)->getType());
    EVT DstVT = TLI.getValueType(CI->getType());

    // This is an fp<->int conversion?
    if (SrcVT.isInteger() != DstVT.isInteger())
        return false;

    // If this is an extension, it will be a zero or sign extension, which
    // isn't a noop.
    if (SrcVT.bitsLT(DstVT)) return false;

    // If these values will be promoted, find out what they will be promoted
    // to.  This helps us consider truncates on PPC as noop copies when they
    // are.
    if (TLI.getTypeAction(CI->getContext(), SrcVT) ==
            TargetLowering::TypePromoteInteger)
        SrcVT = TLI.getTypeToTransformTo(CI->getContext(), SrcVT);
    if (TLI.getTypeAction(CI->getContext(), DstVT) ==
            TargetLowering::TypePromoteInteger)
        DstVT = TLI.getTypeToTransformTo(CI->getContext(), DstVT);

    // If, after promotion, these are the same types, this is a noop copy.
    if (SrcVT != DstVT)
        return false;

    BasicBlock *DefBB = CI->getParent();

    /// InsertedCasts - Only insert a cast in each block once.
    DenseMap<BasicBlock*, CastInst*> InsertedCasts;

    bool MadeChange = false;
    for (Value::use_iterator UI = CI->use_begin(), E = CI->use_end();
            UI != E; ) {
        Use &TheUse = UI.getUse();
        Instruction *User = cast<Instruction>(*UI);

        // Figure out which BB this cast is used in.  For PHI's this is the
        // appropriate predecessor block.
        BasicBlock *UserBB = User->getParent();
        if (PHINode *PN = dyn_cast<PHINode>(User)) {
            UserBB = PN->getIncomingBlock(UI);
        }

        // Preincrement use iterator so we don't invalidate it.
        ++UI;

        // If this user is in the same block as the cast, don't change the cast.
        if (UserBB == DefBB) continue;

        // If we have already inserted a cast into this block, use it.
        CastInst *&InsertedCast = InsertedCasts[UserBB];

        if (!InsertedCast) {
            BasicBlock::iterator InsertPt = UserBB->getFirstInsertionPt();
            InsertedCast =
                CastInst::Create(CI->getOpcode(), CI->getOperand(0), CI->getType(), "",
                                 InsertPt);
            MadeChange = true;
        }

        // Replace a use of the cast with a use of the new cast.
        TheUse = InsertedCast;
        ++NumCastUses;
    }

    // If we removed all uses, nuke the cast.
    if (CI->use_empty()) {
        CI->eraseFromParent();
        MadeChange = true;
    }

    return MadeChange;
}

/// Test if the given instruction is in a position to be optimized
/// with a tail-call. This roughly means that it's in a block with
/// a return and there's nothing that needs to be scheduled
/// between it and the return.
///
/// This function only tests target-independent requirements.
bool llvm::isInTailCallPosition(ImmutableCallSite CS, Attribute CalleeRetAttr,
                                const TargetLowering &TLI) {
  const Instruction *I = CS.getInstruction();
  const BasicBlock *ExitBB = I->getParent();
  const TerminatorInst *Term = ExitBB->getTerminator();
  const ReturnInst *Ret = dyn_cast<ReturnInst>(Term);

  // The block must end in a return statement or unreachable.
  //
  // FIXME: Decline tailcall if it's not guaranteed and if the block ends in
  // an unreachable, for now. The way tailcall optimization is currently
  // implemented means it will add an epilogue followed by a jump. That is
  // not profitable. Also, if the callee is a special function (e.g.
  // longjmp on x86), it can end up causing miscompilation that has not
  // been fully understood.
  if (!Ret &&
      (!TLI.getTargetMachine().Options.GuaranteedTailCallOpt ||
       !isa<UnreachableInst>(Term)))
    return false;

  // If I will have a chain, make sure no other instruction that will have a
  // chain interposes between I and the return.
  if (I->mayHaveSideEffects() || I->mayReadFromMemory() ||
      !isSafeToSpeculativelyExecute(I))
    for (BasicBlock::const_iterator BBI = prior(prior(ExitBB->end())); ;
         --BBI) {
      if (&*BBI == I)
        break;
      // Debug info intrinsics do not get in the way of tail call optimization.
      if (isa<DbgInfoIntrinsic>(BBI))
        continue;
      if (BBI->mayHaveSideEffects() || BBI->mayReadFromMemory() ||
          !isSafeToSpeculativelyExecute(BBI))
        return false;
    }

  // If the block ends with a void return or unreachable, it doesn't matter
  // what the call's return type is.
  if (!Ret || Ret->getNumOperands() == 0) return true;

  // If the return value is undef, it doesn't matter what the call's
  // return type is.
  if (isa<UndefValue>(Ret->getOperand(0))) return true;

  // Conservatively require the attributes of the call to match those of
  // the return. Ignore noalias because it doesn't affect the call sequence.
  const Function *F = ExitBB->getParent();
  Attribute CallerRetAttr = F->getAttributes().getRetAttributes();
  if (AttrBuilder(CalleeRetAttr).removeAttribute(Attribute::NoAlias) !=
      AttrBuilder(CallerRetAttr).removeAttribute(Attribute::NoAlias))
    return false;

  // It's not safe to eliminate the sign / zero extension of the return value.
  if (CallerRetAttr.hasAttribute(Attribute::ZExt) ||
      CallerRetAttr.hasAttribute(Attribute::SExt))
    return false;

  // Otherwise, make sure the unmodified return value of I is the return value.
  // We handle two cases: multiple return values + scalars.
  Value *RetVal = Ret->getOperand(0);
  if (!isa<InsertValueInst>(RetVal) || !isa<StructType>(RetVal->getType()))
    // Handle scalars first.
    return getNoopInput(Ret->getOperand(0), TLI) == I;
  
  // If this is an aggregate return, look through the insert/extract values and
  // see if each is transparent.
  for (unsigned i = 0, e =cast<StructType>(RetVal->getType())->getNumElements();
       i != e; ++i) {
    const Value *InScalar = FindInsertedValue(RetVal, i);
    if (InScalar == 0) return false;
    InScalar = getNoopInput(InScalar, TLI);
    
    // If the scalar value being inserted is an extractvalue of the right index
    // from the call, then everything is good.
    const ExtractValueInst *EVI = dyn_cast<ExtractValueInst>(InScalar);
    if (EVI == 0 || EVI->getOperand(0) != I || EVI->getNumIndices() != 1 ||
        EVI->getIndices()[0] != i)
      return false;
  }
  
  return true;
}

/// EmitJumpTableInfo - Print assembly representations of the jump tables used
/// by the current function to the current output stream.  
///
void AsmPrinter::EmitJumpTableInfo(MachineJumpTableInfo *MJTI,
                                   MachineFunction &MF) {
  const std::vector<MachineJumpTableEntry> &JT = MJTI->getJumpTables();
  if (JT.empty()) return;
  bool IsPic = TM.getRelocationModel() == Reloc::PIC_;
  
  // Use JumpTableDirective otherwise honor the entry size from the jump table
  // info.
  const char *JTEntryDirective = TAI->getJumpTableDirective();
  bool HadJTEntryDirective = JTEntryDirective != NULL;
  if (!HadJTEntryDirective) {
    JTEntryDirective = MJTI->getEntrySize() == 4 ?
      TAI->getData32bitsDirective() : TAI->getData64bitsDirective();
  }
  
  // Pick the directive to use to print the jump table entries, and switch to 
  // the appropriate section.
  TargetLowering *LoweringInfo = TM.getTargetLowering();

  const char* JumpTableDataSection = TAI->getJumpTableDataSection();  
  if ((IsPic && !(LoweringInfo && LoweringInfo->usesGlobalOffsetTable())) ||
     !JumpTableDataSection) {
    // In PIC mode, we need to emit the jump table to the same section as the
    // function body itself, otherwise the label differences won't make sense.
    // We should also do if the section name is NULL.
    const Function *F = MF.getFunction();
    SwitchToTextSection(getSectionForFunction(*F).c_str(), F);
  } else {
    SwitchToDataSection(JumpTableDataSection);
  }
  
  EmitAlignment(Log2_32(MJTI->getAlignment()));
  
  for (unsigned i = 0, e = JT.size(); i != e; ++i) {
    const std::vector<MachineBasicBlock*> &JTBBs = JT[i].MBBs;
    
    // If this jump table was deleted, ignore it. 
    if (JTBBs.empty()) continue;

    // For PIC codegen, if possible we want to use the SetDirective to reduce
    // the number of relocations the assembler will generate for the jump table.
    // Set directives are all printed before the jump table itself.
    std::set<MachineBasicBlock*> EmittedSets;
    if (TAI->getSetDirective() && IsPic)
      for (unsigned ii = 0, ee = JTBBs.size(); ii != ee; ++ii)
        if (EmittedSets.insert(JTBBs[ii]).second)
          printSetLabel(i, JTBBs[ii]);
    
    // On some targets (e.g. darwin) we want to emit two consequtive labels
    // before each jump table.  The first label is never referenced, but tells
    // the assembler and linker the extents of the jump table object.  The
    // second label is actually referenced by the code.
    if (const char *JTLabelPrefix = TAI->getJumpTableSpecialLabelPrefix())
      O << JTLabelPrefix << "JTI" << getFunctionNumber() << '_' << i << ":\n";
    
    O << TAI->getPrivateGlobalPrefix() << "JTI" << getFunctionNumber() 
      << '_' << i << ":\n";
    
    for (unsigned ii = 0, ee = JTBBs.size(); ii != ee; ++ii) {
      O << JTEntryDirective << ' ';
      // If we have emitted set directives for the jump table entries, print 
      // them rather than the entries themselves.  If we're emitting PIC, then
      // emit the table entries as differences between two text section labels.
      // If we're emitting non-PIC code, then emit the entries as direct
      // references to the target basic blocks.
      if (!EmittedSets.empty()) {
        O << TAI->getPrivateGlobalPrefix() << getFunctionNumber()
          << '_' << i << "_set_" << JTBBs[ii]->getNumber();
      } else if (IsPic) {
        printBasicBlockLabel(JTBBs[ii], false, false);
        // If the arch uses custom Jump Table directives, don't calc relative to
        // JT
        if (!HadJTEntryDirective) 
          O << '-' << TAI->getPrivateGlobalPrefix() << "JTI"
            << getFunctionNumber() << '_' << i;
      } else {
        printBasicBlockLabel(JTBBs[ii], false, false);
      }
      O << '\n';
    }
  }
}

/// Test if the given instruction is in a position to be optimized
/// with a tail-call. This roughly means that it's in a block with
/// a return and there's nothing that needs to be scheduled
/// between it and the return.
///
/// This function only tests target-independent requirements.
bool llvm::isInTailCallPosition(ImmutableCallSite CS,
                                const TargetLowering &TLI) {
  const Instruction *I = CS.getInstruction();
  const BasicBlock *ExitBB = I->getParent();
  const TerminatorInst *Term = ExitBB->getTerminator();
  const ReturnInst *Ret = dyn_cast<ReturnInst>(Term);

  // The block must end in a return statement or unreachable.
  //
  // FIXME: Decline tailcall if it's not guaranteed and if the block ends in
  // an unreachable, for now. The way tailcall optimization is currently
  // implemented means it will add an epilogue followed by a jump. That is
  // not profitable. Also, if the callee is a special function (e.g.
  // longjmp on x86), it can end up causing miscompilation that has not
  // been fully understood.
  if (!Ret &&
      (!TLI.getTargetMachine().Options.GuaranteedTailCallOpt ||
       !isa<UnreachableInst>(Term)))
    return false;

  // If I will have a chain, make sure no other instruction that will have a
  // chain interposes between I and the return.
  if (I->mayHaveSideEffects() || I->mayReadFromMemory() ||
      !isSafeToSpeculativelyExecute(I))
    for (BasicBlock::const_iterator BBI = prior(prior(ExitBB->end())); ;
         --BBI) {
      if (&*BBI == I)
        break;
      // Debug info intrinsics do not get in the way of tail call optimization.
      if (isa<DbgInfoIntrinsic>(BBI))
        continue;
      if (BBI->mayHaveSideEffects() || BBI->mayReadFromMemory() ||
          !isSafeToSpeculativelyExecute(BBI))
        return false;
    }

  // If the block ends with a void return or unreachable, it doesn't matter
  // what the call's return type is.
  if (!Ret || Ret->getNumOperands() == 0) return true;

  // If the return value is undef, it doesn't matter what the call's
  // return type is.
  if (isa<UndefValue>(Ret->getOperand(0))) return true;

  // Make sure the attributes attached to each return are compatible.
  AttrBuilder CallerAttrs(ExitBB->getParent()->getAttributes(),
                          AttributeSet::ReturnIndex);
  AttrBuilder CalleeAttrs(cast<CallInst>(I)->getAttributes(),
                          AttributeSet::ReturnIndex);

  // Noalias is completely benign as far as calling convention goes, it
  // shouldn't affect whether the call is a tail call.
  CallerAttrs = CallerAttrs.removeAttribute(Attribute::NoAlias);
  CalleeAttrs = CalleeAttrs.removeAttribute(Attribute::NoAlias);

  bool AllowDifferingSizes = true;
  if (CallerAttrs.contains(Attribute::ZExt)) {
    if (!CalleeAttrs.contains(Attribute::ZExt))
      return false;

    AllowDifferingSizes = false;
    CallerAttrs.removeAttribute(Attribute::ZExt);
    CalleeAttrs.removeAttribute(Attribute::ZExt);
  } else if (CallerAttrs.contains(Attribute::SExt)) {
    if (!CalleeAttrs.contains(Attribute::SExt))
      return false;

    AllowDifferingSizes = false;
    CallerAttrs.removeAttribute(Attribute::SExt);
    CalleeAttrs.removeAttribute(Attribute::SExt);
  }

  // If they're still different, there's some facet we don't understand
  // (currently only "inreg", but in future who knows). It may be OK but the
  // only safe option is to reject the tail call.
  if (CallerAttrs != CalleeAttrs)
    return false;

  const Value *RetVal = Ret->getOperand(0), *CallVal = I;
  SmallVector<unsigned, 4> RetPath, CallPath;
  SmallVector<CompositeType *, 4> RetSubTypes, CallSubTypes;

  bool RetEmpty = !firstRealType(RetVal->getType(), RetSubTypes, RetPath);
  bool CallEmpty = !firstRealType(CallVal->getType(), CallSubTypes, CallPath);

  // Nothing's actually returned, it doesn't matter what the callee put there
  // it's a valid tail call.
  if (RetEmpty)
    return true;

  // Iterate pairwise through each of the value types making up the tail call
  // and the corresponding return. For each one we want to know whether it's
  // essentially going directly from the tail call to the ret, via operations
  // that end up not generating any code.
//.........这里部分代码省略.........