llvm-mirror/lib/Transforms/Utils/PromoteMemoryToRegister.cpp

//===- PromoteMemoryToRegister.cpp - Convert allocas to registers ---------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file promotes memory references to be register references.  It promotes
// alloca instructions which only have loads and stores as uses.  An alloca is
// transformed by using iterated dominator frontiers to place PHI nodes, then
// traversing the function in depth-first order to rewrite loads and stores as
// appropriate.
//
// The algorithm used here is based on:
//
//   Sreedhar and Gao. A linear time algorithm for placing phi-nodes.
//   In Proceedings of the 22nd ACM SIGPLAN-SIGACT Symposium on Principles of
//   Programming Languages
//   POPL '95. ACM, New York, NY, 62-73.
//
// It has been modified to not explicitly use the DJ graph data structure and to
// directly compute pruned SSA using per-variable liveness information.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "mem2reg"
#include "llvm/Transforms/Utils/PromoteMemToReg.h"
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AliasSetTracker.h"
#include "llvm/Analysis/Dominators.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/DIBuilder.h"
#include "llvm/DebugInfo.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/Metadata.h"
#include "llvm/InstVisitor.h"
#include "llvm/Support/CFG.h"
#include "llvm/Transforms/Utils/Local.h"
#include <algorithm>
#include <queue>
using namespace llvm;

STATISTIC(NumLocalPromoted, "Number of alloca's promoted within one block");
STATISTIC(NumSingleStore,   "Number of alloca's promoted with a single store");
STATISTIC(NumDeadAlloca,    "Number of dead alloca's removed");
STATISTIC(NumPHIInsert,     "Number of PHI nodes inserted");

namespace {

struct AllocaInfo : private InstVisitor<AllocaInfo, bool> {
  SmallVector<BasicBlock *, 32> DefiningBlocks;
  SmallVector<BasicBlock *, 32> UsingBlocks;
  SmallVector<Instruction *, 8> DeadInsts;

  Type *AllocaTy;
  StoreInst *OnlyStore;
  BasicBlock *OnlyBlock;
  bool OnlyUsedInOneBlock;

  Value *AllocaPointerVal;
  DbgDeclareInst *DbgDeclare;

  void clear() {
    DefiningBlocks.clear();
    UsingBlocks.clear();
    DeadInsts.clear();
    AllocaTy = 0;
    OnlyStore = 0;
    OnlyBlock = 0;
    OnlyUsedInOneBlock = true;
    AllocaPointerVal = 0;
    DbgDeclare = 0;
  }

  /// Scan the uses of the specified alloca, filling in the AllocaInfo used
  /// by the rest of the pass to reason about the uses of this alloca.
  bool analyzeAlloca(AllocaInst &AI) {
    clear();

    AllocaTy = AI.getAllocatedType();
    enqueueUsers(AI);

    // Walk queued up uses in the worklist to handle nested uses.
    while (!UseWorklist.empty()) {
      U = UseWorklist.pop_back_val();
      Instruction &I = *cast<Instruction>(U->getUser());
      if (!visit(I))
        return false; // Propagate failure to promote up.

      if (OnlyUsedInOneBlock) {
        if (OnlyBlock == 0)
          OnlyBlock = I.getParent();
        else if (OnlyBlock != I.getParent())
          OnlyUsedInOneBlock = false;
      }
    }

    DbgDeclare = FindAllocaDbgDeclare(&AI);
    return true;
  }

private:
  // Befriend the base class so it can call through private visitor methods.
  friend class InstVisitor<AllocaInfo, bool>;

  /// \brief A use pointer that is non-null when visiting uses.
  Use *U;

  /// \brief A worklist for recursively visiting all uses of an alloca.
  SmallVector<Use *, 8> UseWorklist;

  /// \brief A set for preventing cyclic visitation.
  SmallPtrSet<Use *, 8> VisitedUses;

  void enqueueUsers(Instruction &I) {
    for (Value::use_iterator UI = I.use_begin(), UE = I.use_end(); UI != UE;
         ++UI)
      if (VisitedUses.insert(&UI.getUse()))
        UseWorklist.push_back(&UI.getUse());
  }

  bool visitLoadInst(LoadInst &LI) {
    if (LI.isVolatile() || LI.getType() != AllocaTy)
      return false;

    // Keep track of variable reads.
    UsingBlocks.push_back(LI.getParent());
    AllocaPointerVal = &LI;
    return true;
  }

  bool visitStoreInst(StoreInst &SI) {
    if (SI.isVolatile() || SI.getValueOperand() == U->get() ||
        SI.getValueOperand()->getType() != AllocaTy)
      return false;

    // Remember the basic blocks which define new values for the alloca
    DefiningBlocks.push_back(SI.getParent());
    AllocaPointerVal = SI.getOperand(0);
    OnlyStore = &SI;
    return true;
  }

  bool visitBitCastInst(BitCastInst &BC) {
    if (BC.use_empty())
      DeadInsts.push_back(&BC);
    else
      enqueueUsers(BC);
    return true;
  }

  bool visitGetElementPtrInst(GetElementPtrInst &GEPI) {
    if (GEPI.use_empty()) {
      DeadInsts.push_back(&GEPI);
      return true;
    }

    enqueueUsers(GEPI);

    return GEPI.hasAllZeroIndices();
  }

  // We can promote through debug info intrinsics as they don't alter the
  // value stored in memory.
  bool visitDbgInfoIntrinsic(DbgInfoIntrinsic &I) {
    DeadInsts.push_back(&I);
    return true;
  }

  bool visitIntrinsicInst(IntrinsicInst &II) {
    switch (II.getIntrinsicID()) {
    default:
      return false;

      // Lifetime intrinsics don't preclude promoting the memory to a register.
      // FIXME: We should use these to promote to undef when outside of a valid
      // lifetime.
    case Intrinsic::lifetime_start:
    case Intrinsic::lifetime_end:
      DeadInsts.push_back(&II);
      return true;
    }
  }

  // The fallback is that the alloca cannot be promoted.
  bool visitInstruction(Instruction &I) { return false; }
};

// Data package used by RenamePass()
class RenamePassData {
public:
  typedef std::vector<Value *> ValVector;

  RenamePassData() : BB(NULL), Pred(NULL), Values() {}
  RenamePassData(BasicBlock *B, BasicBlock *P, const ValVector &V)
      : BB(B), Pred(P), Values(V) {}
  BasicBlock *BB;
  BasicBlock *Pred;
  ValVector Values;

  void swap(RenamePassData &RHS) {
    std::swap(BB, RHS.BB);
    std::swap(Pred, RHS.Pred);
    Values.swap(RHS.Values);
  }
};

/// \brief This assigns and keeps a per-bb relative ordering of load/store
/// instructions in the block that directly load or store an alloca.
///
/// This functionality is important because it avoids scanning large basic
/// blocks multiple times when promoting many allocas in the same block.
class LargeBlockInfo {
  /// \brief For each instruction that we track, keep the index of the
  /// instruction.
  ///
  /// The index starts out as the number of the instruction from the start of
  /// the block.
  DenseMap<const Instruction *, unsigned> InstNumbers;

public:

  /// This code only looks at accesses to allocas.
  static bool isInterestingInstruction(const Instruction *I) {
    return (isa<LoadInst>(I) && isa<AllocaInst>(I->getOperand(0))) ||
           (isa<StoreInst>(I) && isa<AllocaInst>(I->getOperand(1)));
  }

  /// Get or calculate the index of the specified instruction.
  unsigned getInstructionIndex(const Instruction *I) {
    assert(isInterestingInstruction(I) &&
           "Not a load/store to/from an alloca?");

    // If we already have this instruction number, return it.
    DenseMap<const Instruction *, unsigned>::iterator It = InstNumbers.find(I);
    if (It != InstNumbers.end())
      return It->second;

    // Scan the whole block to get the instruction.  This accumulates
    // information for every interesting instruction in the block, in order to
    // avoid gratuitus rescans.
    const BasicBlock *BB = I->getParent();
    unsigned InstNo = 0;
    for (BasicBlock::const_iterator BBI = BB->begin(), E = BB->end(); BBI != E;
         ++BBI)
      if (isInterestingInstruction(BBI))
        InstNumbers[BBI] = InstNo++;
    It = InstNumbers.find(I);

    assert(It != InstNumbers.end() && "Didn't insert instruction?");
    return It->second;
  }

  void deleteValue(const Instruction *I) { InstNumbers.erase(I); }

  void clear() { InstNumbers.clear(); }
};

struct PromoteMem2Reg {
  /// The alloca instructions being promoted.
  std::vector<AllocaInst *> Allocas;
  DominatorTree &DT;
  DIBuilder DIB;

  /// An AliasSetTracker object to update.  If null, don't update it.
  AliasSetTracker *AST;

  /// Reverse mapping of Allocas.
  DenseMap<AllocaInst *, unsigned> AllocaLookup;

  /// \brief The PhiNodes we're adding.
  ///
  /// That map is used to simplify some Phi nodes as we iterate over it, so
  /// it should have deterministic iterators.  We could use a MapVector, but
  /// since we already maintain a map from BasicBlock* to a stable numbering
  /// (BBNumbers), the DenseMap is more efficient (also supports removal).
  DenseMap<std::pair<unsigned, unsigned>, PHINode *> NewPhiNodes;

  /// For each PHI node, keep track of which entry in Allocas it corresponds
  /// to.
  DenseMap<PHINode *, unsigned> PhiToAllocaMap;

  /// If we are updating an AliasSetTracker, then for each alloca that is of
  /// pointer type, we keep track of what to copyValue to the inserted PHI
  /// nodes here.
  std::vector<Value *> PointerAllocaValues;

  /// For each alloca, we keep track of the dbg.declare intrinsic that
  /// describes it, if any, so that we can convert it to a dbg.value
  /// intrinsic if the alloca gets promoted.
  SmallVector<DbgDeclareInst *, 8> AllocaDbgDeclares;

  /// The set of basic blocks the renamer has already visited.
  ///
  SmallPtrSet<BasicBlock *, 16> Visited;

  /// Contains a stable numbering of basic blocks to avoid non-determinstic
  /// behavior.
  DenseMap<BasicBlock *, unsigned> BBNumbers;

  /// Maps DomTreeNodes to their level in the dominator tree.
  DenseMap<DomTreeNode *, unsigned> DomLevels;

  /// Lazily compute the number of predecessors a block has.
  DenseMap<const BasicBlock *, unsigned> BBNumPreds;

public:
  PromoteMem2Reg(ArrayRef<AllocaInst *> Allocas, DominatorTree &DT,
                 AliasSetTracker *AST)
      : Allocas(Allocas.begin(), Allocas.end()), DT(DT),
        DIB(*DT.getRoot()->getParent()->getParent()), AST(AST) {}

  void run();

private:
  void RemoveFromAllocasList(unsigned &AllocaIdx) {
    Allocas[AllocaIdx] = Allocas.back();
    Allocas.pop_back();
    --AllocaIdx;
  }

  unsigned getNumPreds(const BasicBlock *BB) {
    unsigned &NP = BBNumPreds[BB];
    if (NP == 0)
      NP = std::distance(pred_begin(BB), pred_end(BB)) + 1;
    return NP - 1;
  }

  void DetermineInsertionPoint(AllocaInst *AI, unsigned AllocaNum,
                               AllocaInfo &Info);
  void ComputeLiveInBlocks(AllocaInst *AI, AllocaInfo &Info,
                           const SmallPtrSet<BasicBlock *, 32> &DefBlocks,
                           SmallPtrSet<BasicBlock *, 32> &LiveInBlocks);
  void RenamePass(BasicBlock *BB, BasicBlock *Pred,
                  RenamePassData::ValVector &IncVals,
                  std::vector<RenamePassData> &Worklist);
  bool QueuePhiNode(BasicBlock *BB, unsigned AllocaIdx, unsigned &Version);
};

} // end of anonymous namespace

/// \brief Walk a small vector of dead instructions and recursively remove them
/// and subsequently dead instructions.
///
/// This is only valid to call on dead instructions using an alloca which is
/// promotable, as we leverage that assumption to delete them faster.
static void removeDeadInstructions(AllocaInst *AI,
                                   SmallVectorImpl<Instruction *> &DeadInsts) {
  while (!DeadInsts.empty()) {
    Instruction *I = DeadInsts.pop_back_val();

    // Don't delete the alloca itself.
    if (I == AI)
      continue;

    // Note that we open code the deletion algorithm here because we know
    // apriori that all of the instructions using an alloca that reaches here
    // are trivially dead when their use list becomes empty (The only risk are
    // lifetime markers which we specifically want to nuke). By coding it here
    // we can skip the triviality test and be more efficient.
    //
    // Null out all of the instruction's operands to see if any operand becomes
    // dead as we go.
    for (User::op_iterator OI = I->op_begin(), OE = I->op_end(); OI != OE;
         ++OI) {
      Instruction *Op = dyn_cast<Instruction>(*OI);
      if (!Op)
        continue;

      OI->set(0);
      if (!Op->use_empty())
        continue;

      DeadInsts.push_back(Op);
    }
    I->eraseFromParent();
  }
}

/// \brief Rewrite as many loads as possible given a single store.
///
/// When there is only a single store, we can use the domtree to trivially
/// replace all of the dominated loads with the stored value. Do so, and return
/// true if this has successfully promoted the alloca entirely. If this returns
/// false there were some loads which were not dominated by the single store
/// and thus must be phi-ed with undef. We fall back to the standard alloca
/// promotion algorithm in that case.
static bool rewriteSingleStoreAlloca(AllocaInst *AI, AllocaInfo &Info,
                                     LargeBlockInfo &LBI,
                                     DominatorTree &DT,
                                     AliasSetTracker *AST) {
  StoreInst *OnlyStore = Info.OnlyStore;
  bool StoringGlobalVal = !isa<Instruction>(OnlyStore->getOperand(0));
  BasicBlock *StoreBB = OnlyStore->getParent();
  int StoreIndex = -1;

  // Clear out UsingBlocks.  We will reconstruct it here if needed.
  Info.UsingBlocks.clear();

  for (Value::use_iterator UI = AI->use_begin(), E = AI->use_end(); UI != E;) {
    Instruction *UserInst = cast<Instruction>(*UI++);
    if (!isa<LoadInst>(UserInst)) {
      assert(UserInst == OnlyStore && "Should only have load/stores");
      continue;
    }
    LoadInst *LI = cast<LoadInst>(UserInst);

    // Okay, if we have a load from the alloca, we want to replace it with the
    // only value stored to the alloca.  We can do this if the value is
    // dominated by the store.  If not, we use the rest of the mem2reg machinery
    // to insert the phi nodes as needed.
    if (!StoringGlobalVal) { // Non-instructions are always dominated.
      if (LI->getParent() == StoreBB) {
        // If we have a use that is in the same block as the store, compare the
        // indices of the two instructions to see which one came first.  If the
        // load came before the store, we can't handle it.
        if (StoreIndex == -1)
          StoreIndex = LBI.getInstructionIndex(OnlyStore);

        if (unsigned(StoreIndex) > LBI.getInstructionIndex(LI)) {
          // Can't handle this load, bail out.
          Info.UsingBlocks.push_back(StoreBB);
          continue;
        }

      } else if (LI->getParent() != StoreBB &&
                 !DT.dominates(StoreBB, LI->getParent())) {
        // If the load and store are in different blocks, use BB dominance to
        // check their relationships.  If the store doesn't dom the use, bail
        // out.
        Info.UsingBlocks.push_back(LI->getParent());
        continue;
      }
    }

    // Otherwise, we *can* safely rewrite this load.
    Value *ReplVal = OnlyStore->getOperand(0);
    // If the replacement value is the load, this must occur in unreachable
    // code.
    if (ReplVal == LI)
      ReplVal = UndefValue::get(LI->getType());
    LI->replaceAllUsesWith(ReplVal);
    if (AST && LI->getType()->isPointerTy())
      AST->deleteValue(LI);
    LI->eraseFromParent();
    LBI.deleteValue(LI);
  }

  // Finally, after the scan, check to see if the store is all that is left.
  if (!Info.UsingBlocks.empty())
    return false; // If not, we'll have to fall back for the remainder.

  // Record debuginfo for the store and remove the declaration's
  // debuginfo.
  if (DbgDeclareInst *DDI = Info.DbgDeclare) {
    DIBuilder DIB(*AI->getParent()->getParent()->getParent());
    ConvertDebugDeclareToDebugValue(DDI, Info.OnlyStore, DIB);
    DDI->eraseFromParent();
  }
  // Remove the (now dead) store and alloca.
  Info.OnlyStore->eraseFromParent();
  LBI.deleteValue(Info.OnlyStore);

  if (AST)
    AST->deleteValue(AI);
  AI->eraseFromParent();
  LBI.deleteValue(AI);
  return true;
}

namespace {
/// This is a helper predicate used to search by the first element of a pair.
struct StoreIndexSearchPredicate {
  bool operator()(const std::pair<unsigned, StoreInst *> &LHS,
                  const std::pair<unsigned, StoreInst *> &RHS) {
    return LHS.first < RHS.first;
  }
};
}

/// Many allocas are only used within a single basic block.  If this is the
/// case, avoid traversing the CFG and inserting a lot of potentially useless
/// PHI nodes by just performing a single linear pass over the basic block
/// using the Alloca.
///
/// If we cannot promote this alloca (because it is read before it is written),
/// return true.  This is necessary in cases where, due to control flow, the
/// alloca is potentially undefined on some control flow paths.  e.g. code like
/// this is potentially correct:
///
///   for (...) { if (c) { A = undef; undef = B; } }
///
/// ... so long as A is not used before undef is set.
static void promoteSingleBlockAlloca(AllocaInst *AI, const AllocaInfo &Info,
                                     LargeBlockInfo &LBI,
                                     AliasSetTracker *AST) {
  // The trickiest case to handle is when we have large blocks. Because of this,
  // this code is optimized assuming that large blocks happen.  This does not
  // significantly pessimize the small block case.  This uses LargeBlockInfo to
  // make it efficient to get the index of various operations in the block.

  // Walk the use-def list of the alloca, getting the locations of all stores.
  typedef SmallVector<std::pair<unsigned, StoreInst *>, 64> StoresByIndexTy;
  StoresByIndexTy StoresByIndex;

  for (Value::use_iterator UI = AI->use_begin(), E = AI->use_end(); UI != E;
       ++UI)
    if (StoreInst *SI = dyn_cast<StoreInst>(*UI))
      StoresByIndex.push_back(std::make_pair(LBI.getInstructionIndex(SI), SI));

  // Sort the stores by their index, making it efficient to do a lookup with a
  // binary search.
  std::sort(StoresByIndex.begin(), StoresByIndex.end(),
            StoreIndexSearchPredicate());

  // Walk all of the loads from this alloca, replacing them with the nearest
  // store above them, if any.
  for (Value::use_iterator UI = AI->use_begin(), E = AI->use_end(); UI != E;) {
    LoadInst *LI = dyn_cast<LoadInst>(*UI++);
    if (!LI)
      continue;

    unsigned LoadIdx = LBI.getInstructionIndex(LI);

    // Find the nearest store that has a lower index than this load.
    StoresByIndexTy::iterator I =
        std::lower_bound(StoresByIndex.begin(), StoresByIndex.end(),
                         std::make_pair(LoadIdx, static_cast<StoreInst *>(0)),
                         StoreIndexSearchPredicate());

    if (I == StoresByIndex.begin())
      // If there is no store before this load, the load takes the undef value.
      LI->replaceAllUsesWith(UndefValue::get(LI->getType()));
    else
      // Otherwise, there was a store before this load, the load takes its value.
      LI->replaceAllUsesWith(llvm::prior(I)->second->getOperand(0));

    if (AST && LI->getType()->isPointerTy())
      AST->deleteValue(LI);
    LI->eraseFromParent();
    LBI.deleteValue(LI);
  }

  // Remove the (now dead) stores and alloca.
  while (!AI->use_empty()) {
    StoreInst *SI = cast<StoreInst>(AI->use_back());
    // Record debuginfo for the store before removing it.
    if (DbgDeclareInst *DDI = Info.DbgDeclare) {
      DIBuilder DIB(*AI->getParent()->getParent()->getParent());
      ConvertDebugDeclareToDebugValue(DDI, SI, DIB);
    }
    SI->eraseFromParent();
    LBI.deleteValue(SI);
  }

  if (AST)
    AST->deleteValue(AI);
  AI->eraseFromParent();
  LBI.deleteValue(AI);

  // The alloca's debuginfo can be removed as well.
  if (DbgDeclareInst *DDI = Info.DbgDeclare)
    DDI->eraseFromParent();

  ++NumLocalPromoted;
}

void PromoteMem2Reg::run() {
  Function &F = *DT.getRoot()->getParent();

  if (AST)
    PointerAllocaValues.resize(Allocas.size());
  AllocaDbgDeclares.resize(Allocas.size());

  AllocaInfo Info;
  LargeBlockInfo LBI;

  for (unsigned AllocaNum = 0; AllocaNum != Allocas.size(); ++AllocaNum) {
    AllocaInst *AI = Allocas[AllocaNum];

    assert(AI->getParent()->getParent() == &F &&
           "All allocas should be in the same function, which is same as DF!");

    // Calculate the set of read and write-locations for each alloca.  This is
    // analogous to finding the 'uses' and 'definitions' of each variable.
    bool Good = Info.analyzeAlloca(*AI);
    (void)Good;
    assert(Good && "Cannot promote non-promotable alloca!");

    // Nuke all of the dead instructions.
    removeDeadInstructions(AI, Info.DeadInsts);

    if (AI->use_empty()) {
      // If there are no uses of the alloca, just delete it now.
      if (AST)
        AST->deleteValue(AI);
      AI->eraseFromParent();

      // Remove the alloca from the Allocas list, since it has been processed
      RemoveFromAllocasList(AllocaNum);
      ++NumDeadAlloca;
      continue;
    }

    // If there is only a single store to this value, replace any loads of
    // it that are directly dominated by the definition with the value stored.
    if (Info.DefiningBlocks.size() == 1) {
      if (rewriteSingleStoreAlloca(AI, Info, LBI, DT, AST)) {
        // The alloca has been processed, move on.
        RemoveFromAllocasList(AllocaNum);
        ++NumSingleStore;
        continue;
      }
    }

    // If the alloca is only read and written in one basic block, just perform a
    // linear sweep over the block to eliminate it.
    if (Info.OnlyUsedInOneBlock) {
      promoteSingleBlockAlloca(AI, Info, LBI, AST);

      // The alloca has been processed, move on.
      RemoveFromAllocasList(AllocaNum);
      continue;
    }

    // If we haven't computed dominator tree levels, do so now.
    if (DomLevels.empty()) {
      SmallVector<DomTreeNode *, 32> Worklist;

      DomTreeNode *Root = DT.getRootNode();
      DomLevels[Root] = 0;
      Worklist.push_back(Root);

      while (!Worklist.empty()) {
        DomTreeNode *Node = Worklist.pop_back_val();
        unsigned ChildLevel = DomLevels[Node] + 1;
        for (DomTreeNode::iterator CI = Node->begin(), CE = Node->end();
             CI != CE; ++CI) {
          DomLevels[*CI] = ChildLevel;
          Worklist.push_back(*CI);
        }
      }
    }

    // If we haven't computed a numbering for the BB's in the function, do so
    // now.
    if (BBNumbers.empty()) {
      unsigned ID = 0;
      for (Function::iterator I = F.begin(), E = F.end(); I != E; ++I)
        BBNumbers[I] = ID++;
    }

    // If we have an AST to keep updated, remember some pointer value that is
    // stored into the alloca.
    if (AST)
      PointerAllocaValues[AllocaNum] = Info.AllocaPointerVal;

    // Remember the dbg.declare intrinsic describing this alloca, if any.
    if (Info.DbgDeclare)
      AllocaDbgDeclares[AllocaNum] = Info.DbgDeclare;

    // Keep the reverse mapping of the 'Allocas' array for the rename pass.
    AllocaLookup[Allocas[AllocaNum]] = AllocaNum;

    // At this point, we're committed to promoting the alloca using IDF's, and
    // the standard SSA construction algorithm.  Determine which blocks need PHI
    // nodes and see if we can optimize out some work by avoiding insertion of
    // dead phi nodes.
    DetermineInsertionPoint(AI, AllocaNum, Info);
  }

  if (Allocas.empty())
    return; // All of the allocas must have been trivial!

  LBI.clear();

  // Set the incoming values for the basic block to be null values for all of
  // the alloca's.  We do this in case there is a load of a value that has not
  // been stored yet.  In this case, it will get this null value.
  //
  RenamePassData::ValVector Values(Allocas.size());
  for (unsigned i = 0, e = Allocas.size(); i != e; ++i)
    Values[i] = UndefValue::get(Allocas[i]->getAllocatedType());

  // Walks all basic blocks in the function performing the SSA rename algorithm
  // and inserting the phi nodes we marked as necessary
  //
  std::vector<RenamePassData> RenamePassWorkList;
  RenamePassWorkList.push_back(RenamePassData(F.begin(), 0, Values));
  do {
    RenamePassData RPD;
    RPD.swap(RenamePassWorkList.back());
    RenamePassWorkList.pop_back();
    // RenamePass may add new worklist entries.
    RenamePass(RPD.BB, RPD.Pred, RPD.Values, RenamePassWorkList);
  } while (!RenamePassWorkList.empty());

  // The renamer uses the Visited set to avoid infinite loops.  Clear it now.
  Visited.clear();

  // Remove the allocas themselves from the function.
  for (unsigned i = 0, e = Allocas.size(); i != e; ++i) {
    Instruction *A = Allocas[i];

    // If there are any uses of the alloca instructions left, they must be in
    // unreachable basic blocks that were not processed by walking the dominator
    // tree. Just delete the users now.
    if (!A->use_empty())
      A->replaceAllUsesWith(UndefValue::get(A->getType()));
    if (AST)
      AST->deleteValue(A);
    A->eraseFromParent();
  }

  // Remove alloca's dbg.declare instrinsics from the function.
  for (unsigned i = 0, e = AllocaDbgDeclares.size(); i != e; ++i)
    if (DbgDeclareInst *DDI = AllocaDbgDeclares[i])
      DDI->eraseFromParent();

  // Loop over all of the PHI nodes and see if there are any that we can get
  // rid of because they merge all of the same incoming values.  This can
  // happen due to undef values coming into the PHI nodes.  This process is
  // iterative, because eliminating one PHI node can cause others to be removed.
  bool EliminatedAPHI = true;
  while (EliminatedAPHI) {
    EliminatedAPHI = false;

    // Iterating over NewPhiNodes is deterministic, so it is safe to try to
    // simplify and RAUW them as we go.  If it was not, we could add uses to
    // the values we replace with in a non deterministic order, thus creating
    // non deterministic def->use chains.
    for (DenseMap<std::pair<unsigned, unsigned>, PHINode *>::iterator
             I = NewPhiNodes.begin(),
             E = NewPhiNodes.end();
         I != E;) {
      PHINode *PN = I->second;

      // If this PHI node merges one value and/or undefs, get the value.
      if (Value *V = SimplifyInstruction(PN, 0, 0, &DT)) {
        if (AST && PN->getType()->isPointerTy())
          AST->deleteValue(PN);
        PN->replaceAllUsesWith(V);
        PN->eraseFromParent();
        NewPhiNodes.erase(I++);
        EliminatedAPHI = true;
        continue;
      }
      ++I;
    }
  }

  // At this point, the renamer has added entries to PHI nodes for all reachable
  // code.  Unfortunately, there may be unreachable blocks which the renamer
  // hasn't traversed.  If this is the case, the PHI nodes may not
  // have incoming values for all predecessors.  Loop over all PHI nodes we have
  // created, inserting undef values if they are missing any incoming values.
  //
  for (DenseMap<std::pair<unsigned, unsigned>, PHINode *>::iterator
           I = NewPhiNodes.begin(),
           E = NewPhiNodes.end();
       I != E; ++I) {
    // We want to do this once per basic block.  As such, only process a block
    // when we find the PHI that is the first entry in the block.
    PHINode *SomePHI = I->second;
    BasicBlock *BB = SomePHI->getParent();
    if (&BB->front() != SomePHI)
      continue;

    // Only do work here if there the PHI nodes are missing incoming values.  We
    // know that all PHI nodes that were inserted in a block will have the same
    // number of incoming values, so we can just check any of them.
    if (SomePHI->getNumIncomingValues() == getNumPreds(BB))
      continue;

    // Get the preds for BB.
    SmallVector<BasicBlock *, 16> Preds(pred_begin(BB), pred_end(BB));

    // Ok, now we know that all of the PHI nodes are missing entries for some
    // basic blocks.  Start by sorting the incoming predecessors for efficient
    // access.
    std::sort(Preds.begin(), Preds.end());

    // Now we loop through all BB's which have entries in SomePHI and remove
    // them from the Preds list.
    for (unsigned i = 0, e = SomePHI->getNumIncomingValues(); i != e; ++i) {
      // Do a log(n) search of the Preds list for the entry we want.
      SmallVectorImpl<BasicBlock *>::iterator EntIt = std::lower_bound(
          Preds.begin(), Preds.end(), SomePHI->getIncomingBlock(i));
      assert(EntIt != Preds.end() && *EntIt == SomePHI->getIncomingBlock(i) &&
             "PHI node has entry for a block which is not a predecessor!");

      // Remove the entry
      Preds.erase(EntIt);
    }

    // At this point, the blocks left in the preds list must have dummy
    // entries inserted into every PHI nodes for the block.  Update all the phi
    // nodes in this block that we are inserting (there could be phis before
    // mem2reg runs).
    unsigned NumBadPreds = SomePHI->getNumIncomingValues();
    BasicBlock::iterator BBI = BB->begin();
    while ((SomePHI = dyn_cast<PHINode>(BBI++)) &&
           SomePHI->getNumIncomingValues() == NumBadPreds) {
      Value *UndefVal = UndefValue::get(SomePHI->getType());
      for (unsigned pred = 0, e = Preds.size(); pred != e; ++pred)
        SomePHI->addIncoming(UndefVal, Preds[pred]);
    }
  }

  NewPhiNodes.clear();
}

/// \brief Determine which blocks the value is live in.
///
/// These are blocks which lead to uses.  Knowing this allows us to avoid
/// inserting PHI nodes into blocks which don't lead to uses (thus, the
/// inserted phi nodes would be dead).
void PromoteMem2Reg::ComputeLiveInBlocks(
    AllocaInst *AI, AllocaInfo &Info,
    const SmallPtrSet<BasicBlock *, 32> &DefBlocks,
    SmallPtrSet<BasicBlock *, 32> &LiveInBlocks) {

  // To determine liveness, we must iterate through the predecessors of blocks
  // where the def is live.  Blocks are added to the worklist if we need to
  // check their predecessors.  Start with all the using blocks.
  SmallVector<BasicBlock *, 64> LiveInBlockWorklist(Info.UsingBlocks.begin(),
                                                    Info.UsingBlocks.end());

  // If any of the using blocks is also a definition block, check to see if the
  // definition occurs before or after the use.  If it happens before the use,
  // the value isn't really live-in.
  for (unsigned i = 0, e = LiveInBlockWorklist.size(); i != e; ++i) {
    BasicBlock *BB = LiveInBlockWorklist[i];
    if (!DefBlocks.count(BB))
      continue;

    // Okay, this is a block that both uses and defines the value.  If the first
    // reference to the alloca is a def (store), then we know it isn't live-in.
    for (BasicBlock::iterator I = BB->begin();; ++I) {
      if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
        if (SI->getOperand(1) != AI)
          continue;

        // We found a store to the alloca before a load.  The alloca is not
        // actually live-in here.
        LiveInBlockWorklist[i] = LiveInBlockWorklist.back();
        LiveInBlockWorklist.pop_back();
        --i, --e;
        break;
      }

      if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
        if (LI->getOperand(0) != AI)
          continue;

        // Okay, we found a load before a store to the alloca.  It is actually
        // live into this block.
        break;
      }
    }
  }

  // Now that we have a set of blocks where the phi is live-in, recursively add
  // their predecessors until we find the full region the value is live.
  while (!LiveInBlockWorklist.empty()) {
    BasicBlock *BB = LiveInBlockWorklist.pop_back_val();

    // The block really is live in here, insert it into the set.  If already in
    // the set, then it has already been processed.
    if (!LiveInBlocks.insert(BB))
      continue;

    // Since the value is live into BB, it is either defined in a predecessor or
    // live into it to.  Add the preds to the worklist unless they are a
    // defining block.
    for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
      BasicBlock *P = *PI;

      // The value is not live into a predecessor if it defines the value.
      if (DefBlocks.count(P))
        continue;

      // Otherwise it is, add to the worklist.
      LiveInBlockWorklist.push_back(P);
    }
  }
}

namespace {
typedef std::pair<DomTreeNode *, unsigned> DomTreeNodePair;

struct DomTreeNodeCompare {
  bool operator()(const DomTreeNodePair &LHS, const DomTreeNodePair &RHS) {
    return LHS.second < RHS.second;
  }
};
} // end anonymous namespace

/// At this point, we're committed to promoting the alloca using IDF's, and the
/// standard SSA construction algorithm.  Determine which blocks need phi nodes
/// and see if we can optimize out some work by avoiding insertion of dead phi
/// nodes.
void PromoteMem2Reg::DetermineInsertionPoint(AllocaInst *AI, unsigned AllocaNum,
                                             AllocaInfo &Info) {
  // Unique the set of defining blocks for efficient lookup.
  SmallPtrSet<BasicBlock *, 32> DefBlocks;
  DefBlocks.insert(Info.DefiningBlocks.begin(), Info.DefiningBlocks.end());

  // Determine which blocks the value is live in.  These are blocks which lead
  // to uses.
  SmallPtrSet<BasicBlock *, 32> LiveInBlocks;
  ComputeLiveInBlocks(AI, Info, DefBlocks, LiveInBlocks);

  // Use a priority queue keyed on dominator tree level so that inserted nodes
  // are handled from the bottom of the dominator tree upwards.
  typedef std::priority_queue<DomTreeNodePair,
                              SmallVector<DomTreeNodePair, 32>,
                              DomTreeNodeCompare> IDFPriorityQueue;
  IDFPriorityQueue PQ;

  for (SmallPtrSet<BasicBlock *, 32>::const_iterator I = DefBlocks.begin(),
                                                     E = DefBlocks.end();
       I != E; ++I) {
    if (DomTreeNode *Node = DT.getNode(*I))
      PQ.push(std::make_pair(Node, DomLevels[Node]));
  }

  SmallVector<std::pair<unsigned, BasicBlock *>, 32> DFBlocks;
  SmallPtrSet<DomTreeNode *, 32> Visited;
  SmallVector<DomTreeNode *, 32> Worklist;
  while (!PQ.empty()) {
    DomTreeNodePair RootPair = PQ.top();
    PQ.pop();
    DomTreeNode *Root = RootPair.first;
    unsigned RootLevel = RootPair.second;

    // Walk all dominator tree children of Root, inspecting their CFG edges with
    // targets elsewhere on the dominator tree. Only targets whose level is at
    // most Root's level are added to the iterated dominance frontier of the
    // definition set.

    Worklist.clear();
    Worklist.push_back(Root);

    while (!Worklist.empty()) {
      DomTreeNode *Node = Worklist.pop_back_val();
      BasicBlock *BB = Node->getBlock();

      for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB); SI != SE;
           ++SI) {
        DomTreeNode *SuccNode = DT.getNode(*SI);

        // Quickly skip all CFG edges that are also dominator tree edges instead
        // of catching them below.
        if (SuccNode->getIDom() == Node)
          continue;

        unsigned SuccLevel = DomLevels[SuccNode];
        if (SuccLevel > RootLevel)
          continue;

        if (!Visited.insert(SuccNode))
          continue;

        BasicBlock *SuccBB = SuccNode->getBlock();
        if (!LiveInBlocks.count(SuccBB))
          continue;

        DFBlocks.push_back(std::make_pair(BBNumbers[SuccBB], SuccBB));
        if (!DefBlocks.count(SuccBB))
          PQ.push(std::make_pair(SuccNode, SuccLevel));
      }

      for (DomTreeNode::iterator CI = Node->begin(), CE = Node->end(); CI != CE;
           ++CI) {
        if (!Visited.count(*CI))
          Worklist.push_back(*CI);
      }
    }
  }

  if (DFBlocks.size() > 1)
    std::sort(DFBlocks.begin(), DFBlocks.end());

  unsigned CurrentVersion = 0;
  for (unsigned i = 0, e = DFBlocks.size(); i != e; ++i)
    QueuePhiNode(DFBlocks[i].second, AllocaNum, CurrentVersion);
}

/// \brief Queue a phi-node to be added to a basic-block for a specific Alloca.
///
/// Returns true if there wasn't already a phi-node for that variable
bool PromoteMem2Reg::QueuePhiNode(BasicBlock *BB, unsigned AllocaNo,
                                  unsigned &Version) {
  // Look up the basic-block in question.
  PHINode *&PN = NewPhiNodes[std::make_pair(BBNumbers[BB], AllocaNo)];

  // If the BB already has a phi node added for the i'th alloca then we're done!
  if (PN)
    return false;

  // Create a PhiNode using the dereferenced type... and add the phi-node to the
  // BasicBlock.
  PN = PHINode::Create(Allocas[AllocaNo]->getAllocatedType(), getNumPreds(BB),
                       Allocas[AllocaNo]->getName() + "." + Twine(Version++),
                       BB->begin());
  ++NumPHIInsert;
  PhiToAllocaMap[PN] = AllocaNo;

  if (AST && PN->getType()->isPointerTy())
    AST->copyValue(PointerAllocaValues[AllocaNo], PN);

  return true;
}

/// \brief Recursively traverse the CFG of the function, renaming loads and
/// stores to the allocas which we are promoting.
///
/// IncomingVals indicates what value each Alloca contains on exit from the
/// predecessor block Pred.
void PromoteMem2Reg::RenamePass(BasicBlock *BB, BasicBlock *Pred,
                                RenamePassData::ValVector &IncomingVals,
                                std::vector<RenamePassData> &Worklist) {
NextIteration:
  // If we are inserting any phi nodes into this BB, they will already be in the
  // block.
  if (PHINode *APN = dyn_cast<PHINode>(BB->begin())) {
    // If we have PHI nodes to update, compute the number of edges from Pred to
    // BB.
    if (PhiToAllocaMap.count(APN)) {
      // We want to be able to distinguish between PHI nodes being inserted by
      // this invocation of mem2reg from those phi nodes that already existed in
      // the IR before mem2reg was run.  We determine that APN is being inserted
      // because it is missing incoming edges.  All other PHI nodes being
      // inserted by this pass of mem2reg will have the same number of incoming
      // operands so far.  Remember this count.
      unsigned NewPHINumOperands = APN->getNumOperands();

      unsigned NumEdges = std::count(succ_begin(Pred), succ_end(Pred), BB);
      assert(NumEdges && "Must be at least one edge from Pred to BB!");

      // Add entries for all the phis.
      BasicBlock::iterator PNI = BB->begin();
      do {
        unsigned AllocaNo = PhiToAllocaMap[APN];

        // Add N incoming values to the PHI node.
        for (unsigned i = 0; i != NumEdges; ++i)
          APN->addIncoming(IncomingVals[AllocaNo], Pred);

        // The currently active variable for this block is now the PHI.
        IncomingVals[AllocaNo] = APN;

        // Get the next phi node.
        ++PNI;
        APN = dyn_cast<PHINode>(PNI);
        if (APN == 0)
          break;

        // Verify that it is missing entries.  If not, it is not being inserted
        // by this mem2reg invocation so we want to ignore it.
      } while (APN->getNumOperands() == NewPHINumOperands);
    }
  }

  // Don't revisit blocks.
  if (!Visited.insert(BB))
    return;

  for (BasicBlock::iterator II = BB->begin(); !isa<TerminatorInst>(II);) {
    Instruction *I = II++; // get the instruction, increment iterator

    if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
      AllocaInst *Src = dyn_cast<AllocaInst>(LI->getPointerOperand());
      if (!Src)
        continue;

      DenseMap<AllocaInst *, unsigned>::iterator AI = AllocaLookup.find(Src);
      if (AI == AllocaLookup.end())
        continue;

      Value *V = IncomingVals[AI->second];

      // Anything using the load now uses the current value.
      LI->replaceAllUsesWith(V);
      if (AST && LI->getType()->isPointerTy())
        AST->deleteValue(LI);
      BB->getInstList().erase(LI);
    } else if (StoreInst *SI = dyn_cast<StoreInst>(I)) {
      // Delete this instruction and mark the name as the current holder of the
      // value
      AllocaInst *Dest = dyn_cast<AllocaInst>(SI->getPointerOperand());
      if (!Dest)
        continue;

      DenseMap<AllocaInst *, unsigned>::iterator ai = AllocaLookup.find(Dest);
      if (ai == AllocaLookup.end())
        continue;

      // what value were we writing?
      IncomingVals[ai->second] = SI->getOperand(0);
      // Record debuginfo for the store before removing it.
      if (DbgDeclareInst *DDI = AllocaDbgDeclares[ai->second])
        ConvertDebugDeclareToDebugValue(DDI, SI, DIB);
      BB->getInstList().erase(SI);
    }
  }

  // 'Recurse' to our successors.
  succ_iterator I = succ_begin(BB), E = succ_end(BB);
  if (I == E)
    return;

  // Keep track of the successors so we don't visit the same successor twice
  SmallPtrSet<BasicBlock *, 8> VisitedSuccs;

  // Handle the first successor without using the worklist.
  VisitedSuccs.insert(*I);
  Pred = BB;
  BB = *I;
  ++I;

  for (; I != E; ++I)
    if (VisitedSuccs.insert(*I))
      Worklist.push_back(RenamePassData(*I, Pred, IncomingVals));

  goto NextIteration;
}

bool llvm::isAllocaPromotable(const AllocaInst *AI) {
  // We cast away constness because we re-use the non-const analysis that the
  // actual promotion routine uses. While it is non-const, it doesn't actually
  // mutate anything at this phase, and we discard the non-const results that
  // promotion uses to mutate the alloca.
  return AllocaInfo().analyzeAlloca(*const_cast<AllocaInst *>(AI));
}

void llvm::PromoteMemToReg(ArrayRef<AllocaInst *> Allocas,
                           DominatorTree &DT, AliasSetTracker *AST) {
  // If there is nothing to do, bail out...
  if (Allocas.empty())
    return;

  PromoteMem2Reg(Allocas, DT, AST).run();
}