コード例 #1
0
ファイル: PtrUseVisitor.cpp プロジェクト: 32bitmicro/llvm
void detail::PtrUseVisitorBase::enqueueUsers(Instruction &I) {
  for (Value::use_iterator UI = I.use_begin(), UE = I.use_end();
       UI != UE; ++UI) {
    if (VisitedUses.insert(&UI.getUse())) {
      UseToVisit NewU = {
        UseToVisit::UseAndIsOffsetKnownPair(&UI.getUse(), IsOffsetKnown),
        Offset
      };
      Worklist.push_back(llvm_move(NewU));
    }
  }
}
コード例 #2
0
ファイル: BBCloner.cpp プロジェクト: columbia/loom
void BBCloner::UpdateSSA(Function &F) {
  DominatorTree &DT = getAnalysis<DominatorTree>();
  // The function has been greatly modified since the beginning.
  DT.runOnFunction(F);

  vector<pair<Instruction *, Use *> > ToResolve;
  for (ValueToValueMapTy::iterator I = CloneMap.begin();
       I != CloneMap.end();
       ++I) {
    Value *Key = const_cast<Value *>(I->first);
    if (Instruction *OldIns = dyn_cast<Instruction>(Key)) {
      for (Value::use_iterator UI = OldIns->use_begin();
           UI != OldIns->use_end();
           ++UI) {
        if (Instruction *User = dyn_cast<Instruction>(*UI)) {
          if (!DT.dominates(OldIns, User))
            ToResolve.push_back(make_pair(OldIns, &UI.getUse()));
        }
      }
      Instruction *NewIns = cast<Instruction>(I->second);
      for (Value::use_iterator UI = NewIns->use_begin();
           UI != NewIns->use_end();
           ++UI) {
        if (Instruction *User = dyn_cast<Instruction>(*UI)) {
          if (!DT.dominates(NewIns, User)) {
            // Use OldIns intentionally.
            ToResolve.push_back(make_pair(OldIns, &UI.getUse()));
          }
        }
      }
    }
  }

  for (size_t i = 0; i < ToResolve.size(); ) {
    Instruction *OldIns = ToResolve[i].first;
    Instruction *NewIns = cast<Instruction>(CloneMap.lookup(OldIns));
    SSAUpdater SU;
    SU.Initialize(OldIns->getType(), OldIns->getName());
    SU.AddAvailableValue(OldIns->getParent(), OldIns);
    SU.AddAvailableValue(NewIns->getParent(), NewIns);
    size_t j = i;
    while (j < ToResolve.size() && ToResolve[j].first == ToResolve[i].first) {
      SU.RewriteUse(*ToResolve[j].second);
      ++j;
    }
    i = j;
  }
}
コード例 #3
0
/// RewriteUsesOfClonedInstructions - We just cloned the instructions from the
/// old header into the preheader.  If there were uses of the values produced by
/// these instruction that were outside of the loop, we have to insert PHI nodes
/// to merge the two values.  Do this now.
static void RewriteUsesOfClonedInstructions(BasicBlock *OrigHeader,
                                            BasicBlock *OrigPreheader,
                                            ValueToValueMapTy &ValueMap) {
  // Remove PHI node entries that are no longer live.
  BasicBlock::iterator I, E = OrigHeader->end();
  for (I = OrigHeader->begin(); PHINode *PN = dyn_cast<PHINode>(I); ++I)
    PN->removeIncomingValue(PN->getBasicBlockIndex(OrigPreheader));

  // Now fix up users of the instructions in OrigHeader, inserting PHI nodes
  // as necessary.
  SSAUpdater SSA;
  for (I = OrigHeader->begin(); I != E; ++I) {
    Value *OrigHeaderVal = I;

    // If there are no uses of the value (e.g. because it returns void), there
    // is nothing to rewrite.
    if (OrigHeaderVal->use_empty())
      continue;

    Value *OrigPreHeaderVal = ValueMap[OrigHeaderVal];

    // The value now exits in two versions: the initial value in the preheader
    // and the loop "next" value in the original header.
    SSA.Initialize(OrigHeaderVal->getType(), OrigHeaderVal->getName());
    SSA.AddAvailableValue(OrigHeader, OrigHeaderVal);
    SSA.AddAvailableValue(OrigPreheader, OrigPreHeaderVal);

    // Visit each use of the OrigHeader instruction.
    for (Value::use_iterator UI = OrigHeaderVal->use_begin(),
         UE = OrigHeaderVal->use_end(); UI != UE; ) {
      // Grab the use before incrementing the iterator.
      Use &U = UI.getUse();

      // Increment the iterator before removing the use from the list.
      ++UI;

      // SSAUpdater can't handle a non-PHI use in the same block as an
      // earlier def. We can easily handle those cases manually.
      Instruction *UserInst = cast<Instruction>(U.getUser());
      if (!isa<PHINode>(UserInst)) {
        BasicBlock *UserBB = UserInst->getParent();

        // The original users in the OrigHeader are already using the
        // original definitions.
        if (UserBB == OrigHeader)
          continue;

        // Users in the OrigPreHeader need to use the value to which the
        // original definitions are mapped.
        if (UserBB == OrigPreheader) {
          U = OrigPreHeaderVal;
          continue;
        }
      }

      // Anything else can be handled by SSAUpdater.
      SSA.RewriteUse(U);
    }
  }
}
コード例 #4
0
/// WriteThunk - Replace G with a simple tail call to bitcast(F). Also replace
/// direct uses of G with bitcast(F). Deletes G.
void MergeFunctions::WriteThunk(Function *F, Function *G) const {
  if (!G->mayBeOverridden()) {
    // Redirect direct callers of G to F.
    Constant *BitcastF = ConstantExpr::getBitCast(F, G->getType());
    for (Value::use_iterator UI = G->use_begin(), UE = G->use_end();
         UI != UE;) {
      Value::use_iterator TheIter = UI;
      ++UI;
      CallSite CS(*TheIter);
      if (CS && CS.isCallee(TheIter))
        TheIter.getUse().set(BitcastF);
    }
  }

  // If G was internal then we may have replaced all uses of G with F. If so,
  // stop here and delete G. There's no need for a thunk.
  if (G->hasLocalLinkage() && G->use_empty()) {
    G->eraseFromParent();
    return;
  }

  Function *NewG = Function::Create(G->getFunctionType(), G->getLinkage(), "",
                                    G->getParent());
  BasicBlock *BB = BasicBlock::Create(F->getContext(), "", NewG);
  IRBuilder<false> Builder(BB);

  SmallVector<Value *, 16> Args;
  unsigned i = 0;
  const FunctionType *FFTy = F->getFunctionType();
  for (Function::arg_iterator AI = NewG->arg_begin(), AE = NewG->arg_end();
       AI != AE; ++AI) {
    Args.push_back(Builder.CreateBitCast(AI, FFTy->getParamType(i)));
    ++i;
  }

  CallInst *CI = Builder.CreateCall(F, Args.begin(), Args.end());
  CI->setTailCall();
  CI->setCallingConv(F->getCallingConv());
  if (NewG->getReturnType()->isVoidTy()) {
    Builder.CreateRetVoid();
  } else {
    Builder.CreateRet(Builder.CreateBitCast(CI, NewG->getReturnType()));
  }

  NewG->copyAttributesFrom(G);
  NewG->takeName(G);
  G->replaceAllUsesWith(NewG);
  G->eraseFromParent();

  DEBUG(dbgs() << "WriteThunk: " << NewG->getName() << '\n');
  ++NumThunksWritten;
}
コード例 #5
0
// Replace direct callers of Old with New.
void MergeFunctions::replaceDirectCallers(Function *Old, Function *New) {
  Constant *BitcastNew = ConstantExpr::getBitCast(New, Old->getType());
  for (Value::use_iterator UI = Old->use_begin(), UE = Old->use_end();
       UI != UE;) {
    Value::use_iterator TheIter = UI;
    ++UI;
    CallSite CS(*TheIter);
    if (CS && CS.isCallee(TheIter)) {
      remove(CS.getInstruction()->getParent()->getParent());
      TheIter.getUse().set(BitcastNew);
    }
  }
}
コード例 #6
0
ファイル: CodeGenPrepare.cpp プロジェクト: colgur/llvm
/// OptimizeCmpExpression - sink the given CmpInst into user blocks to reduce
/// the number of virtual registers that must be created and coalesced.  This is
/// a clear win except on targets with multiple condition code registers
///  (PowerPC), where it might lose; some adjustment may be wanted there.
///
/// Return true if any changes are made.
static bool OptimizeCmpExpression(CmpInst *CI) {
  BasicBlock *DefBB = CI->getParent();

  /// InsertedCmp - Only insert a cmp in each block once.
  DenseMap<BasicBlock*, CmpInst*> InsertedCmps;

  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);

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

    // Don't bother for PHI nodes.
    if (isa<PHINode>(User))
      continue;

    // Figure out which BB this cmp is used in.
    BasicBlock *UserBB = User->getParent();

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

    // If we have already inserted a cmp into this block, use it.
    CmpInst *&InsertedCmp = InsertedCmps[UserBB];

    if (!InsertedCmp) {
      BasicBlock::iterator InsertPt = UserBB->getFirstNonPHI();

      InsertedCmp =
        CmpInst::Create(CI->getOpcode(),
                        CI->getPredicate(),  CI->getOperand(0),
                        CI->getOperand(1), "", InsertPt);
      MadeChange = true;
    }

    // Replace a use of the cmp with a use of the new cmp.
    TheUse = InsertedCmp;
    ++NumCmpUses;
  }

  // If we removed all uses, nuke the cmp.
  if (CI->use_empty())
    CI->eraseFromParent();

  return MadeChange;
}
コード例 #7
0
// For each instruction used by the value, remove() the function that contains
// the instruction. This should happen right before a call to RAUW.
void MergeFunctions::removeUsers(Value *V) {
  std::vector<Value *> Worklist;
  Worklist.push_back(V);
  while (!Worklist.empty()) {
    Value *V = Worklist.back();
    Worklist.pop_back();

    for (Value::use_iterator UI = V->use_begin(), UE = V->use_end();
         UI != UE; ++UI) {
      Use &U = UI.getUse();
      if (Instruction *I = dyn_cast<Instruction>(U.getUser())) {
        remove(I->getParent()->getParent());
      } else if (isa<GlobalValue>(U.getUser())) {
        // do nothing
      } else if (Constant *C = dyn_cast<Constant>(U.getUser())) {
        for (Value::use_iterator CUI = C->use_begin(), CUE = C->use_end();
             CUI != CUE; ++CUI)
          Worklist.push_back(*CUI);
      }
    }
  }
}
コード例 #8
0
ファイル: CodeGenPrepare.cpp プロジェクト: PhongNgo/llvm
/// 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;
}
コード例 #9
0
ファイル: CodeGenPrepare.cpp プロジェクト: PhongNgo/llvm
bool CodeGenPrepare::OptimizeExtUses(Instruction *I) {
    BasicBlock *DefBB = I->getParent();

    // If the result of a {s|z}ext and its source are both live out, rewrite all
    // other uses of the source with result of extension.
    Value *Src = I->getOperand(0);
    if (Src->hasOneUse())
        return false;

    // Only do this xform if truncating is free.
    if (TLI && !TLI->isTruncateFree(I->getType(), Src->getType()))
        return false;

    // Only safe to perform the optimization if the source is also defined in
    // this block.
    if (!isa<Instruction>(Src) || DefBB != cast<Instruction>(Src)->getParent())
        return false;

    bool DefIsLiveOut = false;
    for (Value::use_iterator UI = I->use_begin(), E = I->use_end();
            UI != E; ++UI) {
        Instruction *User = cast<Instruction>(*UI);

        // Figure out which BB this ext is used in.
        BasicBlock *UserBB = User->getParent();
        if (UserBB == DefBB) continue;
        DefIsLiveOut = true;
        break;
    }
    if (!DefIsLiveOut)
        return false;

    // Make sure non of the uses are PHI nodes.
    for (Value::use_iterator UI = Src->use_begin(), E = Src->use_end();
            UI != E; ++UI) {
        Instruction *User = cast<Instruction>(*UI);
        BasicBlock *UserBB = User->getParent();
        if (UserBB == DefBB) continue;
        // Be conservative. We don't want this xform to end up introducing
        // reloads just before load / store instructions.
        if (isa<PHINode>(User) || isa<LoadInst>(User) || isa<StoreInst>(User))
            return false;
    }

    // InsertedTruncs - Only insert one trunc in each block once.
    DenseMap<BasicBlock*, Instruction*> InsertedTruncs;

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

        // Figure out which BB this ext is used in.
        BasicBlock *UserBB = User->getParent();
        if (UserBB == DefBB) continue;

        // Both src and def are live in this block. Rewrite the use.
        Instruction *&InsertedTrunc = InsertedTruncs[UserBB];

        if (!InsertedTrunc) {
            BasicBlock::iterator InsertPt = UserBB->getFirstInsertionPt();
            InsertedTrunc = new TruncInst(I, Src->getType(), "", InsertPt);
        }

        // Replace a use of the {s|z}ext source with a use of the result.
        TheUse = InsertedTrunc;
        ++NumExtUses;
        MadeChange = true;
    }

    return MadeChange;
}
コード例 #10
0
ファイル: LICM.cpp プロジェクト: nobled/llvm
/// sink - When an instruction is found to only be used outside of the loop,
/// this function moves it to the exit blocks and patches up SSA form as needed.
/// This method is guaranteed to remove the original instruction from its
/// position, and may either delete it or move it to outside of the loop.
///
void LICM::sink(Instruction &I) {
  DEBUG(dbgs() << "LICM sinking instruction: " << I << "\n");

  SmallVector<BasicBlock*, 8> ExitBlocks;
  CurLoop->getUniqueExitBlocks(ExitBlocks);

  if (isa<LoadInst>(I)) ++NumMovedLoads;
  else if (isa<CallInst>(I)) ++NumMovedCalls;
  ++NumSunk;
  Changed = true;

  // The case where there is only a single exit node of this loop is common
  // enough that we handle it as a special (more efficient) case.  It is more
  // efficient to handle because there are no PHI nodes that need to be placed.
  if (ExitBlocks.size() == 1) {
    if (!DT->dominates(I.getParent(), ExitBlocks[0])) {
      // Instruction is not used, just delete it.
      CurAST->deleteValue(&I);
      // If I has users in unreachable blocks, eliminate.
      // If I is not void type then replaceAllUsesWith undef.
      // This allows ValueHandlers and custom metadata to adjust itself.
      if (!I.use_empty())
        I.replaceAllUsesWith(UndefValue::get(I.getType()));
      I.eraseFromParent();
    } else {
      // Move the instruction to the start of the exit block, after any PHI
      // nodes in it.
      I.moveBefore(ExitBlocks[0]->getFirstInsertionPt());

      // This instruction is no longer in the AST for the current loop, because
      // we just sunk it out of the loop.  If we just sunk it into an outer
      // loop, we will rediscover the operation when we process it.
      CurAST->deleteValue(&I);
    }
    return;
  }

  if (ExitBlocks.empty()) {
    // The instruction is actually dead if there ARE NO exit blocks.
    CurAST->deleteValue(&I);
    // If I has users in unreachable blocks, eliminate.
    // If I is not void type then replaceAllUsesWith undef.
    // This allows ValueHandlers and custom metadata to adjust itself.
    if (!I.use_empty())
      I.replaceAllUsesWith(UndefValue::get(I.getType()));
    I.eraseFromParent();
    return;
  }

  // Otherwise, if we have multiple exits, use the SSAUpdater to do all of the
  // hard work of inserting PHI nodes as necessary.
  SmallVector<PHINode*, 8> NewPHIs;
  SSAUpdater SSA(&NewPHIs);

  if (!I.use_empty())
    SSA.Initialize(I.getType(), I.getName());

  // Insert a copy of the instruction in each exit block of the loop that is
  // dominated by the instruction.  Each exit block is known to only be in the
  // ExitBlocks list once.
  BasicBlock *InstOrigBB = I.getParent();
  unsigned NumInserted = 0;

  for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i) {
    BasicBlock *ExitBlock = ExitBlocks[i];

    if (!DT->dominates(InstOrigBB, ExitBlock))
      continue;

    // Insert the code after the last PHI node.
    BasicBlock::iterator InsertPt = ExitBlock->getFirstInsertionPt();

    // If this is the first exit block processed, just move the original
    // instruction, otherwise clone the original instruction and insert
    // the copy.
    Instruction *New;
    if (NumInserted++ == 0) {
      I.moveBefore(InsertPt);
      New = &I;
    } else {
      New = I.clone();
      if (!I.getName().empty())
        New->setName(I.getName()+".le");
      ExitBlock->getInstList().insert(InsertPt, New);
    }

    // Now that we have inserted the instruction, inform SSAUpdater.
    if (!I.use_empty())
      SSA.AddAvailableValue(ExitBlock, New);
  }

  // If the instruction doesn't dominate any exit blocks, it must be dead.
  if (NumInserted == 0) {
    CurAST->deleteValue(&I);
    if (!I.use_empty())
      I.replaceAllUsesWith(UndefValue::get(I.getType()));
    I.eraseFromParent();
    return;
  }

  // Next, rewrite uses of the instruction, inserting PHI nodes as needed.
  for (Value::use_iterator UI = I.use_begin(), UE = I.use_end(); UI != UE; ) {
    // Grab the use before incrementing the iterator.
    Use &U = UI.getUse();
    // Increment the iterator before removing the use from the list.
    ++UI;
    SSA.RewriteUseAfterInsertions(U);
  }

  // Update CurAST for NewPHIs if I had pointer type.
  if (I.getType()->isPointerTy())
    for (unsigned i = 0, e = NewPHIs.size(); i != e; ++i)
      CurAST->copyValue(&I, NewPHIs[i]);

  // Finally, remove the instruction from CurAST.  It is no longer in the loop.
  CurAST->deleteValue(&I);
}
コード例 #11
0
ファイル: LCSSA.cpp プロジェクト: jyasskin/llvm-mirror
/// ProcessInstruction - Given an instruction in the loop, check to see if it
/// has any uses that are outside the current loop.  If so, insert LCSSA PHI
/// nodes and rewrite the uses.
bool LCSSA::ProcessInstruction(Instruction *Inst,
                               const SmallVectorImpl<BasicBlock*> &ExitBlocks) {
  SmallVector<Use*, 16> UsesToRewrite;
  
  BasicBlock *InstBB = Inst->getParent();
  
  for (Value::use_iterator UI = Inst->use_begin(), E = Inst->use_end();
       UI != E; ++UI) {
    User *U = *UI;
    BasicBlock *UserBB = cast<Instruction>(U)->getParent();
    if (PHINode *PN = dyn_cast<PHINode>(U))
      UserBB = PN->getIncomingBlock(UI);
    
    if (InstBB != UserBB && !inLoop(UserBB))
      UsesToRewrite.push_back(&UI.getUse());
  }

  // If there are no uses outside the loop, exit with no change.
  if (UsesToRewrite.empty()) return false;
  
  ++NumLCSSA; // We are applying the transformation

  // Invoke instructions are special in that their result value is not available
  // along their unwind edge. The code below tests to see whether DomBB dominates
  // the value, so adjust DomBB to the normal destination block, which is
  // effectively where the value is first usable.
  BasicBlock *DomBB = Inst->getParent();
  if (InvokeInst *Inv = dyn_cast<InvokeInst>(Inst))
    DomBB = Inv->getNormalDest();

  DomTreeNode *DomNode = DT->getNode(DomBB);

  SSAUpdater SSAUpdate;
  SSAUpdate.Initialize(Inst->getType(), Inst->getName());
  
  // Insert the LCSSA phi's into all of the exit blocks dominated by the
  // value, and add them to the Phi's map.
  for (SmallVectorImpl<BasicBlock*>::const_iterator BBI = ExitBlocks.begin(),
      BBE = ExitBlocks.end(); BBI != BBE; ++BBI) {
    BasicBlock *ExitBB = *BBI;
    if (!DT->dominates(DomNode, DT->getNode(ExitBB))) continue;
    
    // If we already inserted something for this BB, don't reprocess it.
    if (SSAUpdate.HasValueForBlock(ExitBB)) continue;
    
    PHINode *PN = PHINode::Create(Inst->getType(), Inst->getName()+".lcssa",
                                  ExitBB->begin());
    PN->reserveOperandSpace(PredCache.GetNumPreds(ExitBB));

    // Add inputs from inside the loop for this PHI.
    for (BasicBlock **PI = PredCache.GetPreds(ExitBB); *PI; ++PI) {
      PN->addIncoming(Inst, *PI);

      // If the exit block has a predecessor not within the loop, arrange for
      // the incoming value use corresponding to that predecessor to be
      // rewritten in terms of a different LCSSA PHI.
      if (!inLoop(*PI))
        UsesToRewrite.push_back(
          &PN->getOperandUse(
            PN->getOperandNumForIncomingValue(PN->getNumIncomingValues()-1)));
    }
    
    // Remember that this phi makes the value alive in this block.
    SSAUpdate.AddAvailableValue(ExitBB, PN);
  }
  
  // Rewrite all uses outside the loop in terms of the new PHIs we just
  // inserted.
  for (unsigned i = 0, e = UsesToRewrite.size(); i != e; ++i) {
    // If this use is in an exit block, rewrite to use the newly inserted PHI.
    // This is required for correctness because SSAUpdate doesn't handle uses in
    // the same block.  It assumes the PHI we inserted is at the end of the
    // block.
    Instruction *User = cast<Instruction>(UsesToRewrite[i]->getUser());
    BasicBlock *UserBB = User->getParent();
    if (PHINode *PN = dyn_cast<PHINode>(User))
      UserBB = PN->getIncomingBlock(*UsesToRewrite[i]);

    if (isa<PHINode>(UserBB->begin()) &&
        isExitBlock(UserBB, ExitBlocks)) {
      UsesToRewrite[i]->set(UserBB->begin());
      continue;
    }
    
    // Otherwise, do full PHI insertion.
    SSAUpdate.RewriteUse(*UsesToRewrite[i]);
  }
  
  return true;
}