xref: /external/llvm/lib/Transforms/IPO/Inliner.cpp

//===- Inliner.cpp - Code common to all inliners --------------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements the mechanics required to implement inlining without
// missing any calls and updating the call graph.  The decisions of which calls
// are profitable to inline are implemented elsewhere.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "inline"
#include "llvm/Transforms/IPO/InlinerPass.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/CallGraph.h"
#include "llvm/Analysis/InlineCost.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Module.h"
#include "llvm/Support/CallSite.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Target/TargetLibraryInfo.h"
#include "llvm/Transforms/Utils/Cloning.h"
#include "llvm/Transforms/Utils/Local.h"
using namespace llvm;

STATISTIC(NumInlined, "Number of functions inlined");
STATISTIC(NumCallsDeleted, "Number of call sites deleted, not inlined");
STATISTIC(NumDeleted, "Number of functions deleted because all callers found");
STATISTIC(NumMergedAllocas, "Number of allocas merged together");

// This weirdly named statistic tracks the number of times that, when attempting
// to inline a function A into B, we analyze the callers of B in order to see
// if those would be more profitable and blocked inline steps.
STATISTIC(NumCallerCallersAnalyzed, "Number of caller-callers analyzed");

static cl::opt<int>
InlineLimit("inline-threshold", cl::Hidden, cl::init(225), cl::ZeroOrMore,
        cl::desc("Control the amount of inlining to perform (default = 225)"));

static cl::opt<int>
HintThreshold("inlinehint-threshold", cl::Hidden, cl::init(325),
              cl::desc("Threshold for inlining functions with inline hint"));

// Threshold to use when optsize is specified (and there is no -inline-limit).
const int OptSizeThreshold = 75;

Inliner::Inliner(char &ID)
  : CallGraphSCCPass(ID), InlineThreshold(InlineLimit), InsertLifetime(true) {}

Inliner::Inliner(char &ID, int Threshold, bool InsertLifetime)
  : CallGraphSCCPass(ID), InlineThreshold(InlineLimit.getNumOccurrences() > 0 ?
                                          InlineLimit : Threshold),
    InsertLifetime(InsertLifetime) {}

/// getAnalysisUsage - For this class, we declare that we require and preserve
/// the call graph.  If the derived class implements this method, it should
/// always explicitly call the implementation here.
void Inliner::getAnalysisUsage(AnalysisUsage &AU) const {
  CallGraphSCCPass::getAnalysisUsage(AU);
}


typedef DenseMap<ArrayType*, std::vector<AllocaInst*> >
InlinedArrayAllocasTy;

/// \brief If the inlined function had a higher stack protection level than the
/// calling function, then bump up the caller's stack protection level.
static void AdjustCallerSSPLevel(Function *Caller, Function *Callee) {
  // If upgrading the SSP attribute, clear out the old SSP Attributes first.
  // Having multiple SSP attributes doesn't actually hurt, but it adds useless
  // clutter to the IR.
  AttrBuilder B;
  B.addAttribute(Attribute::StackProtect)
    .addAttribute(Attribute::StackProtectStrong);
  AttributeSet OldSSPAttr = AttributeSet::get(Caller->getContext(),
                                              AttributeSet::FunctionIndex,
                                              B);
  AttributeSet CallerAttr = Caller->getAttributes(),
               CalleeAttr = Callee->getAttributes();

  if (CalleeAttr.hasAttribute(AttributeSet::FunctionIndex,
                              Attribute::StackProtectReq)) {
    Caller->removeAttributes(AttributeSet::FunctionIndex, OldSSPAttr);
    Caller->addFnAttr(Attribute::StackProtectReq);
  } else if (CalleeAttr.hasAttribute(AttributeSet::FunctionIndex,
                                     Attribute::StackProtectStrong) &&
             !CallerAttr.hasAttribute(AttributeSet::FunctionIndex,
                                      Attribute::StackProtectReq)) {
    Caller->removeAttributes(AttributeSet::FunctionIndex, OldSSPAttr);
    Caller->addFnAttr(Attribute::StackProtectStrong);
  } else if (CalleeAttr.hasAttribute(AttributeSet::FunctionIndex,
                                     Attribute::StackProtect) &&
           !CallerAttr.hasAttribute(AttributeSet::FunctionIndex,
                                    Attribute::StackProtectReq) &&
           !CallerAttr.hasAttribute(AttributeSet::FunctionIndex,
                                    Attribute::StackProtectStrong))
    Caller->addFnAttr(Attribute::StackProtect);
}

/// InlineCallIfPossible - If it is possible to inline the specified call site,
/// do so and update the CallGraph for this operation.
///
/// This function also does some basic book-keeping to update the IR.  The
/// InlinedArrayAllocas map keeps track of any allocas that are already
/// available from other  functions inlined into the caller.  If we are able to
/// inline this call site we attempt to reuse already available allocas or add
/// any new allocas to the set if not possible.
static bool InlineCallIfPossible(CallSite CS, InlineFunctionInfo &IFI,
                                 InlinedArrayAllocasTy &InlinedArrayAllocas,
                                 int InlineHistory, bool InsertLifetime,
                                 const DataLayout *TD) {
  Function *Callee = CS.getCalledFunction();
  Function *Caller = CS.getCaller();

  // Try to inline the function.  Get the list of static allocas that were
  // inlined.
  if (!InlineFunction(CS, IFI, InsertLifetime))
    return false;

  AdjustCallerSSPLevel(Caller, Callee);

  // Look at all of the allocas that we inlined through this call site.  If we
  // have already inlined other allocas through other calls into this function,
  // then we know that they have disjoint lifetimes and that we can merge them.
  //
  // There are many heuristics possible for merging these allocas, and the
  // different options have different tradeoffs.  One thing that we *really*
  // don't want to hurt is SRoA: once inlining happens, often allocas are no
  // longer address taken and so they can be promoted.
  //
  // Our "solution" for that is to only merge allocas whose outermost type is an
  // array type.  These are usually not promoted because someone is using a
  // variable index into them.  These are also often the most important ones to
  // merge.
  //
  // A better solution would be to have real memory lifetime markers in the IR
  // and not have the inliner do any merging of allocas at all.  This would
  // allow the backend to do proper stack slot coloring of all allocas that
  // *actually make it to the backend*, which is really what we want.
  //
  // Because we don't have this information, we do this simple and useful hack.
  //
  SmallPtrSet<AllocaInst*, 16> UsedAllocas;

  // When processing our SCC, check to see if CS was inlined from some other
  // call site.  For example, if we're processing "A" in this code:
  //   A() { B() }
  //   B() { x = alloca ... C() }
  //   C() { y = alloca ... }
  // Assume that C was not inlined into B initially, and so we're processing A
  // and decide to inline B into A.  Doing this makes an alloca available for
  // reuse and makes a callsite (C) available for inlining.  When we process
  // the C call site we don't want to do any alloca merging between X and Y
  // because their scopes are not disjoint.  We could make this smarter by
  // keeping track of the inline history for each alloca in the
  // InlinedArrayAllocas but this isn't likely to be a significant win.
  if (InlineHistory != -1)  // Only do merging for top-level call sites in SCC.
    return true;

  // Loop over all the allocas we have so far and see if they can be merged with
  // a previously inlined alloca.  If not, remember that we had it.
  for (unsigned AllocaNo = 0, e = IFI.StaticAllocas.size();
       AllocaNo != e; ++AllocaNo) {
    AllocaInst *AI = IFI.StaticAllocas[AllocaNo];

    // Don't bother trying to merge array allocations (they will usually be
    // canonicalized to be an allocation *of* an array), or allocations whose
    // type is not itself an array (because we're afraid of pessimizing SRoA).
    ArrayType *ATy = dyn_cast<ArrayType>(AI->getAllocatedType());
    if (ATy == 0 || AI->isArrayAllocation())
      continue;

    // Get the list of all available allocas for this array type.
    std::vector<AllocaInst*> &AllocasForType = InlinedArrayAllocas[ATy];

    // Loop over the allocas in AllocasForType to see if we can reuse one.  Note
    // that we have to be careful not to reuse the same "available" alloca for
    // multiple different allocas that we just inlined, we use the 'UsedAllocas'
    // set to keep track of which "available" allocas are being used by this
    // function.  Also, AllocasForType can be empty of course!
    bool MergedAwayAlloca = false;
    for (unsigned i = 0, e = AllocasForType.size(); i != e; ++i) {
      AllocaInst *AvailableAlloca = AllocasForType[i];

      unsigned Align1 = AI->getAlignment(),
               Align2 = AvailableAlloca->getAlignment();
      // If we don't have data layout information, and only one alloca is using
      // the target default, then we can't safely merge them because we can't
      // pick the greater alignment.
      if (!TD && (!Align1 || !Align2) && Align1 != Align2)
        continue;

      // The available alloca has to be in the right function, not in some other
      // function in this SCC.
      if (AvailableAlloca->getParent() != AI->getParent())
        continue;

      // If the inlined function already uses this alloca then we can't reuse
      // it.
      if (!UsedAllocas.insert(AvailableAlloca))
        continue;

      // Otherwise, we *can* reuse it, RAUW AI into AvailableAlloca and declare
      // success!
      DEBUG(dbgs() << "    ***MERGED ALLOCA: " << *AI << "\n\t\tINTO: "
                   << *AvailableAlloca << '\n');

      AI->replaceAllUsesWith(AvailableAlloca);

      if (Align1 != Align2) {
        if (!Align1 || !Align2) {
          assert(TD && "DataLayout required to compare default alignments");
          unsigned TypeAlign = TD->getABITypeAlignment(AI->getAllocatedType());

          Align1 = Align1 ? Align1 : TypeAlign;
          Align2 = Align2 ? Align2 : TypeAlign;
        }

        if (Align1 > Align2)
          AvailableAlloca->setAlignment(AI->getAlignment());
      }

      AI->eraseFromParent();
      MergedAwayAlloca = true;
      ++NumMergedAllocas;
      IFI.StaticAllocas[AllocaNo] = 0;
      break;
    }

    // If we already nuked the alloca, we're done with it.
    if (MergedAwayAlloca)
      continue;

    // If we were unable to merge away the alloca either because there are no
    // allocas of the right type available or because we reused them all
    // already, remember that this alloca came from an inlined function and mark
    // it used so we don't reuse it for other allocas from this inline
    // operation.
    AllocasForType.push_back(AI);
    UsedAllocas.insert(AI);
  }

  return true;
}

unsigned Inliner::getInlineThreshold(CallSite CS) const {
  int thres = InlineThreshold; // -inline-threshold or else selected by
                               // overall opt level

  // If -inline-threshold is not given, listen to the optsize attribute when it
  // would decrease the threshold.
  Function *Caller = CS.getCaller();
  bool OptSize = Caller && !Caller->isDeclaration() &&
    Caller->getAttributes().hasAttribute(AttributeSet::FunctionIndex,
                                         Attribute::OptimizeForSize);
  if (!(InlineLimit.getNumOccurrences() > 0) && OptSize &&
      OptSizeThreshold < thres)
    thres = OptSizeThreshold;

  // Listen to the inlinehint attribute when it would increase the threshold
  // and the caller does not need to minimize its size.
  Function *Callee = CS.getCalledFunction();
  bool InlineHint = Callee && !Callee->isDeclaration() &&
    Callee->getAttributes().hasAttribute(AttributeSet::FunctionIndex,
                                         Attribute::InlineHint);
  if (InlineHint && HintThreshold > thres
      && !Caller->getAttributes().hasAttribute(AttributeSet::FunctionIndex,
                                               Attribute::MinSize))
    thres = HintThreshold;

  return thres;
}

/// shouldInline - Return true if the inliner should attempt to inline
/// at the given CallSite.
bool Inliner::shouldInline(CallSite CS) {
  InlineCost IC = getInlineCost(CS);

  if (IC.isAlways()) {
    DEBUG(dbgs() << "    Inlining: cost=always"
          << ", Call: " << *CS.getInstruction() << "\n");
    return true;
  }

  if (IC.isNever()) {
    DEBUG(dbgs() << "    NOT Inlining: cost=never"
          << ", Call: " << *CS.getInstruction() << "\n");
    return false;
  }

  Function *Caller = CS.getCaller();
  if (!IC) {
    DEBUG(dbgs() << "    NOT Inlining: cost=" << IC.getCost()
          << ", thres=" << (IC.getCostDelta() + IC.getCost())
          << ", Call: " << *CS.getInstruction() << "\n");
    return false;
  }

  // Try to detect the case where the current inlining candidate caller (call
  // it B) is a static or linkonce-ODR function and is an inlining candidate
  // elsewhere, and the current candidate callee (call it C) is large enough
  // that inlining it into B would make B too big to inline later. In these
  // circumstances it may be best not to inline C into B, but to inline B into
  // its callers.
  //
  // This only applies to static and linkonce-ODR functions because those are
  // expected to be available for inlining in the translation units where they
  // are used. Thus we will always have the opportunity to make local inlining
  // decisions. Importantly the linkonce-ODR linkage covers inline functions
  // and templates in C++.
  //
  // FIXME: All of this logic should be sunk into getInlineCost. It relies on
  // the internal implementation of the inline cost metrics rather than
  // treating them as truly abstract units etc.
  if (Caller->hasLocalLinkage() ||
      Caller->getLinkage() == GlobalValue::LinkOnceODRLinkage) {
    int TotalSecondaryCost = 0;
    // The candidate cost to be imposed upon the current function.
    int CandidateCost = IC.getCost() - (InlineConstants::CallPenalty + 1);
    // This bool tracks what happens if we do NOT inline C into B.
    bool callerWillBeRemoved = Caller->hasLocalLinkage();
    // This bool tracks what happens if we DO inline C into B.
    bool inliningPreventsSomeOuterInline = false;
    for (Value::use_iterator I = Caller->use_begin(), E =Caller->use_end();
         I != E; ++I) {
      CallSite CS2(*I);

      // If this isn't a call to Caller (it could be some other sort
      // of reference) skip it.  Such references will prevent the caller
      // from being removed.
      if (!CS2 || CS2.getCalledFunction() != Caller) {
        callerWillBeRemoved = false;
        continue;
      }

      InlineCost IC2 = getInlineCost(CS2);
      ++NumCallerCallersAnalyzed;
      if (!IC2) {
        callerWillBeRemoved = false;
        continue;
      }
      if (IC2.isAlways())
        continue;

      // See if inlining or original callsite would erase the cost delta of
      // this callsite. We subtract off the penalty for the call instruction,
      // which we would be deleting.
      if (IC2.getCostDelta() <= CandidateCost) {
        inliningPreventsSomeOuterInline = true;
        TotalSecondaryCost += IC2.getCost();
      }
    }
    // If all outer calls to Caller would get inlined, the cost for the last
    // one is set very low by getInlineCost, in anticipation that Caller will
    // be removed entirely.  We did not account for this above unless there
    // is only one caller of Caller.
    if (callerWillBeRemoved && Caller->use_begin() != Caller->use_end())
      TotalSecondaryCost += InlineConstants::LastCallToStaticBonus;

    if (inliningPreventsSomeOuterInline && TotalSecondaryCost < IC.getCost()) {
      DEBUG(dbgs() << "    NOT Inlining: " << *CS.getInstruction() <<
           " Cost = " << IC.getCost() <<
           ", outer Cost = " << TotalSecondaryCost << '\n');
      return false;
    }
  }

  DEBUG(dbgs() << "    Inlining: cost=" << IC.getCost()
        << ", thres=" << (IC.getCostDelta() + IC.getCost())
        << ", Call: " << *CS.getInstruction() << '\n');
  return true;
}

/// InlineHistoryIncludes - Return true if the specified inline history ID
/// indicates an inline history that includes the specified function.
static bool InlineHistoryIncludes(Function *F, int InlineHistoryID,
            const SmallVectorImpl<std::pair<Function*, int> > &InlineHistory) {
  while (InlineHistoryID != -1) {
    assert(unsigned(InlineHistoryID) < InlineHistory.size() &&
           "Invalid inline history ID");
    if (InlineHistory[InlineHistoryID].first == F)
      return true;
    InlineHistoryID = InlineHistory[InlineHistoryID].second;
  }
  return false;
}

bool Inliner::runOnSCC(CallGraphSCC &SCC) {
  CallGraph &CG = getAnalysis<CallGraph>();
  const DataLayout *TD = getAnalysisIfAvailable<DataLayout>();
  const TargetLibraryInfo *TLI = getAnalysisIfAvailable<TargetLibraryInfo>();

  SmallPtrSet<Function*, 8> SCCFunctions;
  DEBUG(dbgs() << "Inliner visiting SCC:");
  for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
    Function *F = (*I)->getFunction();
    if (F) SCCFunctions.insert(F);
    DEBUG(dbgs() << " " << (F ? F->getName() : "INDIRECTNODE"));
  }

  // Scan through and identify all call sites ahead of time so that we only
  // inline call sites in the original functions, not call sites that result
  // from inlining other functions.
  SmallVector<std::pair<CallSite, int>, 16> CallSites;

  // When inlining a callee produces new call sites, we want to keep track of
  // the fact that they were inlined from the callee.  This allows us to avoid
  // infinite inlining in some obscure cases.  To represent this, we use an
  // index into the InlineHistory vector.
  SmallVector<std::pair<Function*, int>, 8> InlineHistory;

  for (CallGraphSCC::iterator I = SCC.begin(), E = SCC.end(); I != E; ++I) {
    Function *F = (*I)->getFunction();
    if (!F) continue;

    for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB)
      for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
        CallSite CS(cast<Value>(I));
        // If this isn't a call, or it is a call to an intrinsic, it can
        // never be inlined.
        if (!CS || isa<IntrinsicInst>(I))
          continue;

        // If this is a direct call to an external function, we can never inline
        // it.  If it is an indirect call, inlining may resolve it to be a
        // direct call, so we keep it.
        if (CS.getCalledFunction() && CS.getCalledFunction()->isDeclaration())
          continue;

        CallSites.push_back(std::make_pair(CS, -1));
      }
  }

  DEBUG(dbgs() << ": " << CallSites.size() << " call sites.\n");

  // If there are no calls in this function, exit early.
  if (CallSites.empty())
    return false;

  // Now that we have all of the call sites, move the ones to functions in the
  // current SCC to the end of the list.
  unsigned FirstCallInSCC = CallSites.size();
  for (unsigned i = 0; i < FirstCallInSCC; ++i)
    if (Function *F = CallSites[i].first.getCalledFunction())
      if (SCCFunctions.count(F))
        std::swap(CallSites[i--], CallSites[--FirstCallInSCC]);


  InlinedArrayAllocasTy InlinedArrayAllocas;
  InlineFunctionInfo InlineInfo(&CG, TD);

  // Now that we have all of the call sites, loop over them and inline them if
  // it looks profitable to do so.
  bool Changed = false;
  bool LocalChange;
  do {
    LocalChange = false;
    // Iterate over the outer loop because inlining functions can cause indirect
    // calls to become direct calls.
    for (unsigned CSi = 0; CSi != CallSites.size(); ++CSi) {
      CallSite CS = CallSites[CSi].first;

      Function *Caller = CS.getCaller();
      Function *Callee = CS.getCalledFunction();

      // If this call site is dead and it is to a readonly function, we should
      // just delete the call instead of trying to inline it, regardless of
      // size.  This happens because IPSCCP propagates the result out of the
      // call and then we're left with the dead call.
      if (isInstructionTriviallyDead(CS.getInstruction(), TLI)) {
        DEBUG(dbgs() << "    -> Deleting dead call: "
                     << *CS.getInstruction() << "\n");
        // Update the call graph by deleting the edge from Callee to Caller.
        CG[Caller]->removeCallEdgeFor(CS);
        CS.getInstruction()->eraseFromParent();
        ++NumCallsDeleted;
      } else {
        // We can only inline direct calls to non-declarations.
        if (Callee == 0 || Callee->isDeclaration()) continue;

        // If this call site was obtained by inlining another function, verify
        // that the include path for the function did not include the callee
        // itself.  If so, we'd be recursively inlining the same function,
        // which would provide the same callsites, which would cause us to
        // infinitely inline.
        int InlineHistoryID = CallSites[CSi].second;
        if (InlineHistoryID != -1 &&
            InlineHistoryIncludes(Callee, InlineHistoryID, InlineHistory))
          continue;


        // If the policy determines that we should inline this function,
        // try to do so.
        if (!shouldInline(CS))
          continue;

        // Attempt to inline the function.
        if (!InlineCallIfPossible(CS, InlineInfo, InlinedArrayAllocas,
                                  InlineHistoryID, InsertLifetime, TD))
          continue;
        ++NumInlined;

        // If inlining this function gave us any new call sites, throw them
        // onto our worklist to process.  They are useful inline candidates.
        if (!InlineInfo.InlinedCalls.empty()) {
          // Create a new inline history entry for this, so that we remember
          // that these new callsites came about due to inlining Callee.
          int NewHistoryID = InlineHistory.size();
          InlineHistory.push_back(std::make_pair(Callee, InlineHistoryID));

          for (unsigned i = 0, e = InlineInfo.InlinedCalls.size();
               i != e; ++i) {
            Value *Ptr = InlineInfo.InlinedCalls[i];
            CallSites.push_back(std::make_pair(CallSite(Ptr), NewHistoryID));
          }
        }
      }

      // If we inlined or deleted the last possible call site to the function,
      // delete the function body now.
      if (Callee && Callee->use_empty() && Callee->hasLocalLinkage() &&
          // TODO: Can remove if in SCC now.
          !SCCFunctions.count(Callee) &&

          // The function may be apparently dead, but if there are indirect
          // callgraph references to the node, we cannot delete it yet, this
          // could invalidate the CGSCC iterator.
          CG[Callee]->getNumReferences() == 0) {
        DEBUG(dbgs() << "    -> Deleting dead function: "
              << Callee->getName() << "\n");
        CallGraphNode *CalleeNode = CG[Callee];

        // Remove any call graph edges from the callee to its callees.
        CalleeNode->removeAllCalledFunctions();

        // Removing the node for callee from the call graph and delete it.
        delete CG.removeFunctionFromModule(CalleeNode);
        ++NumDeleted;
      }

      // Remove this call site from the list.  If possible, use
      // swap/pop_back for efficiency, but do not use it if doing so would
      // move a call site to a function in this SCC before the
      // 'FirstCallInSCC' barrier.
      if (SCC.isSingular()) {
        CallSites[CSi] = CallSites.back();
        CallSites.pop_back();
      } else {
        CallSites.erase(CallSites.begin()+CSi);
      }
      --CSi;

      Changed = true;
      LocalChange = true;
    }
  } while (LocalChange);

  return Changed;
}

// doFinalization - Remove now-dead linkonce functions at the end of
// processing to avoid breaking the SCC traversal.
bool Inliner::doFinalization(CallGraph &CG) {
  return removeDeadFunctions(CG);
}

/// removeDeadFunctions - Remove dead functions that are not included in
/// DNR (Do Not Remove) list.
bool Inliner::removeDeadFunctions(CallGraph &CG, bool AlwaysInlineOnly) {
  SmallVector<CallGraphNode*, 16> FunctionsToRemove;

  // Scan for all of the functions, looking for ones that should now be removed
  // from the program.  Insert the dead ones in the FunctionsToRemove set.
  for (CallGraph::iterator I = CG.begin(), E = CG.end(); I != E; ++I) {
    CallGraphNode *CGN = I->second;
    Function *F = CGN->getFunction();
    if (!F || F->isDeclaration())
      continue;

    // Handle the case when this function is called and we only want to care
    // about always-inline functions. This is a bit of a hack to share code
    // between here and the InlineAlways pass.
    if (AlwaysInlineOnly &&
        !F->getAttributes().hasAttribute(AttributeSet::FunctionIndex,
                                         Attribute::AlwaysInline))
      continue;

    // If the only remaining users of the function are dead constants, remove
    // them.
    F->removeDeadConstantUsers();

    if (!F->isDefTriviallyDead())
      continue;

    // Remove any call graph edges from the function to its callees.
    CGN->removeAllCalledFunctions();

    // Remove any edges from the external node to the function's call graph
    // node.  These edges might have been made irrelegant due to
    // optimization of the program.
    CG.getExternalCallingNode()->removeAnyCallEdgeTo(CGN);

    // Removing the node for callee from the call graph and delete it.
    FunctionsToRemove.push_back(CGN);
  }
  if (FunctionsToRemove.empty())
    return false;

  // Now that we know which functions to delete, do so.  We didn't want to do
  // this inline, because that would invalidate our CallGraph::iterator
  // objects. :(
  //
  // Note that it doesn't matter that we are iterating over a non-stable order
  // here to do this, it doesn't matter which order the functions are deleted
  // in.
  array_pod_sort(FunctionsToRemove.begin(), FunctionsToRemove.end());
  FunctionsToRemove.erase(std::unique(FunctionsToRemove.begin(),
                                      FunctionsToRemove.end()),
                          FunctionsToRemove.end());
  for (SmallVectorImpl<CallGraphNode *>::iterator I = FunctionsToRemove.begin(),
                                                  E = FunctionsToRemove.end();
       I != E; ++I) {
    delete CG.removeFunctionFromModule(*I);
    ++NumDeleted;
  }
  return true;
}