Import LLVM, at r72732.

1 files changed, 1003 insertions, 0 deletions

diff --git a/lib/Transforms/Utils/PromoteMemoryToRegister.cpp b/lib/Transforms/Utils/PromoteMemoryToRegister.cpp
new file mode 100644
index 0000000..b717699
--- /dev/null
+++ b/lib/Transforms/Utils/PromoteMemoryToRegister.cpp
@@ -0,0 +1,1003 @@
+//===- 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 dominator frontiers to place PHI nodes, then traversing
+// the function in depth-first order to rewrite loads and stores as appropriate.
+// This is just the standard SSA construction algorithm to construct "pruned"
+// SSA form.
+//
+//===----------------------------------------------------------------------===//
+
+#define DEBUG_TYPE "mem2reg"
+#include "llvm/Transforms/Utils/PromoteMemToReg.h"
+#include "llvm/Constants.h"
+#include "llvm/DerivedTypes.h"
+#include "llvm/Function.h"
+#include "llvm/Instructions.h"
+#include "llvm/IntrinsicInst.h"
+#include "llvm/Analysis/Dominators.h"
+#include "llvm/Analysis/AliasSetTracker.h"
+#include "llvm/ADT/DenseMap.h"
+#include "llvm/ADT/SmallPtrSet.h"
+#include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/ADT/StringExtras.h"
+#include "llvm/ADT/STLExtras.h"
+#include "llvm/Support/CFG.h"
+#include "llvm/Support/Compiler.h"
+#include <algorithm>
+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");
+
+// Provide DenseMapInfo for all pointers.
+namespace llvm {
+template<>
+struct DenseMapInfo<std::pair<BasicBlock*, unsigned> > {
+  typedef std::pair<BasicBlock*, unsigned> EltTy;
+  static inline EltTy getEmptyKey() {
+    return EltTy(reinterpret_cast<BasicBlock*>(-1), ~0U);
+  }
+  static inline EltTy getTombstoneKey() {
+    return EltTy(reinterpret_cast<BasicBlock*>(-2), 0U);
+  }
+  static unsigned getHashValue(const std::pair<BasicBlock*, unsigned> &Val) {
+    return DenseMapInfo<void*>::getHashValue(Val.first) + Val.second*2;
+  }
+  static bool isEqual(const EltTy &LHS, const EltTy &RHS) {
+    return LHS == RHS;
+  }
+  static bool isPod() { return true; }
+};
+}
+
+/// isAllocaPromotable - Return true if this alloca is legal for promotion.
+/// This is true if there are only loads and stores to the alloca.
+///
+bool llvm::isAllocaPromotable(const AllocaInst *AI) {
+  // FIXME: If the memory unit is of pointer or integer type, we can permit
+  // assignments to subsections of the memory unit.
+
+  // Only allow direct and non-volatile loads and stores...
+  for (Value::use_const_iterator UI = AI->use_begin(), UE = AI->use_end();
+       UI != UE; ++UI)     // Loop over all of the uses of the alloca
+    if (const LoadInst *LI = dyn_cast<LoadInst>(*UI)) {
+      if (LI->isVolatile())
+        return false;
+    } else if (const StoreInst *SI = dyn_cast<StoreInst>(*UI)) {
+      if (SI->getOperand(0) == AI)
+        return false;   // Don't allow a store OF the AI, only INTO the AI.
+      if (SI->isVolatile())
+        return false;
+    } else if (const BitCastInst *BC = dyn_cast<BitCastInst>(*UI)) {
+      // A bitcast that does not feed into debug info inhibits promotion.
+      if (!BC->hasOneUse() || !isa<DbgInfoIntrinsic>(*BC->use_begin()))
+        return false;
+      // If the only use is by debug info, this alloca will not exist in
+      // non-debug code, so don't try to promote; this ensures the same
+      // codegen with debug info.  Otherwise, debug info should not
+      // inhibit promotion (but we must examine other uses).
+      if (AI->hasOneUse())
+        return false;
+    } else {
+      return false;
+    }
+
+  return true;
+}
+
+namespace {
+  struct AllocaInfo;
+
+  // Data package used by RenamePass()
+  class VISIBILITY_HIDDEN RenamePassData {
+  public:
+    typedef std::vector<Value *> ValVector;
+    
+    RenamePassData() {}
+    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);
+    }
+  };
+  
+  /// LargeBlockInfo - 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 VISIBILITY_HIDDEN LargeBlockInfo {
+    /// InstNumbers - 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:
+    
+    /// isInterestingInstruction - 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)));
+    }
+    
+    /// getInstructionIndex - 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 VISIBILITY_HIDDEN PromoteMem2Reg {
+    /// Allocas - The alloca instructions being promoted.
+    ///
+    std::vector<AllocaInst*> Allocas;
+    DominatorTree &DT;
+    DominanceFrontier &DF;
+
+    /// AST - An AliasSetTracker object to update.  If null, don't update it.
+    ///
+    AliasSetTracker *AST;
+
+    /// AllocaLookup - Reverse mapping of Allocas.
+    ///
+    std::map<AllocaInst*, unsigned>  AllocaLookup;
+
+    /// NewPhiNodes - The PhiNodes we're adding.
+    ///
+    DenseMap<std::pair<BasicBlock*, unsigned>, PHINode*> NewPhiNodes;
+    
+    /// PhiToAllocaMap - For each PHI node, keep track of which entry in Allocas
+    /// it corresponds to.
+    DenseMap<PHINode*, unsigned> PhiToAllocaMap;
+    
+    /// PointerAllocaValues - 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;
+
+    /// Visited - The set of basic blocks the renamer has already visited.
+    ///
+    SmallPtrSet<BasicBlock*, 16> Visited;
+
+    /// BBNumbers - Contains a stable numbering of basic blocks to avoid
+    /// non-determinstic behavior.
+    DenseMap<BasicBlock*, unsigned> BBNumbers;
+
+    /// BBNumPreds - Lazily compute the number of predecessors a block has.
+    DenseMap<const BasicBlock*, unsigned> BBNumPreds;
+  public:
+    PromoteMem2Reg(const std::vector<AllocaInst*> &A, DominatorTree &dt,
+                   DominanceFrontier &df, AliasSetTracker *ast)
+      : Allocas(A), DT(dt), DF(df), AST(ast) {}
+
+    void run();
+
+    /// properlyDominates - Return true if I1 properly dominates I2.
+    ///
+    bool properlyDominates(Instruction *I1, Instruction *I2) const {
+      if (InvokeInst *II = dyn_cast<InvokeInst>(I1))
+        I1 = II->getNormalDest()->begin();
+      return DT.properlyDominates(I1->getParent(), I2->getParent());
+    }
+    
+    /// dominates - Return true if BB1 dominates BB2 using the DominatorTree.
+    ///
+    bool dominates(BasicBlock *BB1, BasicBlock *BB2) const {
+      return DT.dominates(BB1, BB2);
+    }
+
+  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 RewriteSingleStoreAlloca(AllocaInst *AI, AllocaInfo &Info,
+                                  LargeBlockInfo &LBI);
+    void PromoteSingleBlockAlloca(AllocaInst *AI, AllocaInfo &Info,
+                                  LargeBlockInfo &LBI);
+
+    
+    void RenamePass(BasicBlock *BB, BasicBlock *Pred,
+                    RenamePassData::ValVector &IncVals,
+                    std::vector<RenamePassData> &Worklist);
+    bool QueuePhiNode(BasicBlock *BB, unsigned AllocaIdx, unsigned &Version,
+                      SmallPtrSet<PHINode*, 16> &InsertedPHINodes);
+  };
+  
+  struct AllocaInfo {
+    std::vector<BasicBlock*> DefiningBlocks;
+    std::vector<BasicBlock*> UsingBlocks;
+    
+    StoreInst  *OnlyStore;
+    BasicBlock *OnlyBlock;
+    bool OnlyUsedInOneBlock;
+    
+    Value *AllocaPointerVal;
+    
+    void clear() {
+      DefiningBlocks.clear();
+      UsingBlocks.clear();
+      OnlyStore = 0;
+      OnlyBlock = 0;
+      OnlyUsedInOneBlock = true;
+      AllocaPointerVal = 0;
+    }
+    
+    /// AnalyzeAlloca - Scan the uses of the specified alloca, filling in our
+    /// ivars.
+    void AnalyzeAlloca(AllocaInst *AI) {
+      clear();
+
+      // As we scan the uses of the alloca instruction, keep track of stores,
+      // and decide whether all of the loads and stores to the alloca are within
+      // the same basic block.
+      for (Value::use_iterator U = AI->use_begin(), E = AI->use_end();
+           U != E;)  {
+        Instruction *User = cast<Instruction>(*U);
+        ++U;
+        if (BitCastInst *BC = dyn_cast<BitCastInst>(User)) {
+          // Remove any uses of this alloca in DbgInfoInstrinsics.
+          assert(BC->hasOneUse() && "Unexpected alloca uses!");
+          DbgInfoIntrinsic *DI = cast<DbgInfoIntrinsic>(*BC->use_begin());
+          DI->eraseFromParent();
+          BC->eraseFromParent();
+          continue;
+        } 
+        else if (StoreInst *SI = dyn_cast<StoreInst>(User)) {
+          // Remember the basic blocks which define new values for the alloca
+          DefiningBlocks.push_back(SI->getParent());
+          AllocaPointerVal = SI->getOperand(0);
+          OnlyStore = SI;
+        } else {
+          LoadInst *LI = cast<LoadInst>(User);
+          // Otherwise it must be a load instruction, keep track of variable
+          // reads.
+          UsingBlocks.push_back(LI->getParent());
+          AllocaPointerVal = LI;
+        }
+        
+        if (OnlyUsedInOneBlock) {
+          if (OnlyBlock == 0)
+            OnlyBlock = User->getParent();
+          else if (OnlyBlock != User->getParent())
+            OnlyUsedInOneBlock = false;
+        }
+      }
+    }
+  };
+}  // end of anonymous namespace
+
+
+void PromoteMem2Reg::run() {
+  Function &F = *DF.getRoot()->getParent();
+
+  if (AST) PointerAllocaValues.resize(Allocas.size());
+
+  AllocaInfo Info;
+  LargeBlockInfo LBI;
+
+  for (unsigned AllocaNum = 0; AllocaNum != Allocas.size(); ++AllocaNum) {
+    AllocaInst *AI = Allocas[AllocaNum];
+
+    assert(isAllocaPromotable(AI) &&
+           "Cannot promote non-promotable alloca!");
+    assert(AI->getParent()->getParent() == &F &&
+           "All allocas should be in the same function, which is same as DF!");
+
+    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;
+    }
+    
+    // Calculate the set of read and write-locations for each alloca.  This is
+    // analogous to finding the 'uses' and 'definitions' of each variable.
+    Info.AnalyzeAlloca(AI);
+
+    // 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) {
+      RewriteSingleStoreAlloca(AI, Info, LBI);
+
+      // Finally, after the scan, check to see if the store is all that is left.
+      if (Info.UsingBlocks.empty()) {
+        // Remove the (now dead) store and alloca.
+        Info.OnlyStore->eraseFromParent();
+        LBI.deleteValue(Info.OnlyStore);
+
+        if (AST) AST->deleteValue(AI);
+        AI->eraseFromParent();
+        LBI.deleteValue(AI);
+        
+        // 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);
+      
+      // Finally, after the scan, check to see if the stores are all that is
+      // left.
+      if (Info.UsingBlocks.empty()) {
+        
+        // Remove the (now dead) stores and alloca.
+        while (!AI->use_empty()) {
+          StoreInst *SI = cast<StoreInst>(AI->use_back());
+          SI->eraseFromParent();
+          LBI.deleteValue(SI);
+        }
+        
+        if (AST) AST->deleteValue(AI);
+        AI->eraseFromParent();
+        LBI.deleteValue(AI);
+        
+        // The alloca has been processed, move on.
+        RemoveFromAllocasList(AllocaNum);
+        
+        ++NumLocalPromoted;
+        continue;
+      }
+    }
+    
+    // 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;
+    
+    // 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));
+  while (!RenamePassWorkList.empty()) {
+    RenamePassData RPD;
+    RPD.swap(RenamePassWorkList.back());
+    RenamePassWorkList.pop_back();
+    // RenamePass may add new worklist entries.
+    RenamePass(RPD.BB, RPD.Pred, RPD.Values, RenamePassWorkList);
+  }
+  
+  // 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
+    // sections of dead code that were not processed on the dominance frontier.
+    // Just delete the users now.
+    //
+    if (!A->use_empty())
+      A->replaceAllUsesWith(UndefValue::get(A->getType()));
+    if (AST) AST->deleteValue(A);
+    A->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;
+    
+    for (DenseMap<std::pair<BasicBlock*, 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 = PN->hasConstantValue(true)) {
+        if (!isa<Instruction>(V) ||
+            properlyDominates(cast<Instruction>(V), PN)) {
+          if (AST && isa<PointerType>(PN->getType()))
+            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<BasicBlock*, 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.
+      SmallVector<BasicBlock*, 16>::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();
+}
+
+
+/// ComputeLiveInBlocks - 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;
+  LiveInBlockWorklist.insert(LiveInBlockWorklist.end(), 
+                             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;
+      } else 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);
+    }
+  }
+}
+
+/// DetermineInsertionPoint - 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);
+
+  // Compute the locations where PhiNodes need to be inserted.  Look at the
+  // dominance frontier of EACH basic-block we have a write in.
+  unsigned CurrentVersion = 0;
+  SmallPtrSet<PHINode*, 16> InsertedPHINodes;
+  std::vector<std::pair<unsigned, BasicBlock*> > DFBlocks;
+  while (!Info.DefiningBlocks.empty()) {
+    BasicBlock *BB = Info.DefiningBlocks.back();
+    Info.DefiningBlocks.pop_back();
+    
+    // Look up the DF for this write, add it to defining blocks.
+    DominanceFrontier::const_iterator it = DF.find(BB);
+    if (it == DF.end()) continue;
+    
+    const DominanceFrontier::DomSetType &S = it->second;
+    
+    // In theory we don't need the indirection through the DFBlocks vector.
+    // In practice, the order of calling QueuePhiNode would depend on the
+    // (unspecified) ordering of basic blocks in the dominance frontier,
+    // which would give PHI nodes non-determinstic subscripts.  Fix this by
+    // processing blocks in order of the occurance in the function.
+    for (DominanceFrontier::DomSetType::const_iterator P = S.begin(),
+         PE = S.end(); P != PE; ++P) {
+      // If the frontier block is not in the live-in set for the alloca, don't
+      // bother processing it.
+      if (!LiveInBlocks.count(*P))
+        continue;
+      
+      DFBlocks.push_back(std::make_pair(BBNumbers[*P], *P));
+    }
+    
+    // Sort by which the block ordering in the function.
+    if (DFBlocks.size() > 1)
+      std::sort(DFBlocks.begin(), DFBlocks.end());
+    
+    for (unsigned i = 0, e = DFBlocks.size(); i != e; ++i) {
+      BasicBlock *BB = DFBlocks[i].second;
+      if (QueuePhiNode(BB, AllocaNum, CurrentVersion, InsertedPHINodes))
+        Info.DefiningBlocks.push_back(BB);
+    }
+    DFBlocks.clear();
+  }
+}
+
+/// RewriteSingleStoreAlloca - 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.
+void PromoteMem2Reg::RewriteSingleStoreAlloca(AllocaInst *AI,
+                                              AllocaInfo &Info,
+                                              LargeBlockInfo &LBI) {
+  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 &&
+                 !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.
+    LI->replaceAllUsesWith(OnlyStore->getOperand(0));
+    if (AST && isa<PointerType>(LI->getType()))
+      AST->deleteValue(LI);
+    LI->eraseFromParent();
+    LBI.deleteValue(LI);
+  }
+}
+
+
+/// StoreIndexSearchPredicate - 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;
+  }
+};
+
+/// PromoteSingleBlockAlloca - 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.
+///
+void PromoteMem2Reg::PromoteSingleBlockAlloca(AllocaInst *AI, AllocaInfo &Info,
+                                              LargeBlockInfo &LBI) {
+  // 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.
+  
+  // Clear out UsingBlocks.  We will reconstruct it here if needed.
+  Info.UsingBlocks.clear();
+  
+  // 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));
+
+  // If there are no stores to the alloca, just replace any loads with undef.
+  if (StoresByIndex.empty()) {
+    for (Value::use_iterator UI = AI->use_begin(), E = AI->use_end(); UI != E;) 
+      if (LoadInst *LI = dyn_cast<LoadInst>(*UI++)) {
+        LI->replaceAllUsesWith(UndefValue::get(LI->getType()));
+        if (AST && isa<PointerType>(LI->getType()))
+          AST->deleteValue(LI);
+        LBI.deleteValue(LI);
+        LI->eraseFromParent();
+      }
+    return;
+  }
+  
+  // Sort the stores by their index, making it efficient to do a lookup with a
+  // binary search.
+  std::sort(StoresByIndex.begin(), StoresByIndex.end());
+  
+  // 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 than this load. 
+    StoresByIndexTy::iterator I = 
+      std::lower_bound(StoresByIndex.begin(), StoresByIndex.end(),
+                       std::pair<unsigned, StoreInst*>(LoadIdx, 0),
+                       StoreIndexSearchPredicate());
+    
+    // If there is no store before this load, then we can't promote this load.
+    if (I == StoresByIndex.begin()) {
+      // Can't handle this load, bail out.
+      Info.UsingBlocks.push_back(LI->getParent());
+      continue;
+    }
+      
+    // Otherwise, there was a store before this load, the load takes its value.
+    --I;
+    LI->replaceAllUsesWith(I->second->getOperand(0));
+    if (AST && isa<PointerType>(LI->getType()))
+      AST->deleteValue(LI);
+    LI->eraseFromParent();
+    LBI.deleteValue(LI);
+  }
+}
+
+
+// QueuePhiNode - queues 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,
+                                  SmallPtrSet<PHINode*, 16> &InsertedPHINodes) {
+  // Look up the basic-block in question.
+  PHINode *&PN = NewPhiNodes[std::make_pair(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(),
+                       Allocas[AllocaNo]->getName() + "." +
+                       utostr(Version++), BB->begin());
+  ++NumPHIInsert;
+  PhiToAllocaMap[PN] = AllocaNo;
+  PN->reserveOperandSpace(getNumPreds(BB));
+  
+  InsertedPHINodes.insert(PN);
+
+  if (AST && isa<PointerType>(PN->getType()))
+    AST->copyValue(PointerAllocaValues[AllocaNo], PN);
+
+  return true;
+}
+
+// RenamePass - 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 = 0;
+      for (succ_iterator I = succ_begin(Pred), E = succ_end(Pred); I != E; ++I)
+        if (*I == BB)
+          ++NumEdges;
+      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;
+  
+      std::map<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 && isa<PointerType>(LI->getType()))
+        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;
+      
+      std::map<AllocaInst *, unsigned>::iterator ai = AllocaLookup.find(Dest);
+      if (ai == AllocaLookup.end())
+        continue;
+      
+      // what value were we writing?
+      IncomingVals[ai->second] = SI->getOperand(0);
+      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;
+}
+
+/// PromoteMemToReg - Promote the specified list of alloca instructions into
+/// scalar registers, inserting PHI nodes as appropriate.  This function makes
+/// use of DominanceFrontier information.  This function does not modify the CFG
+/// of the function at all.  All allocas must be from the same function.
+///
+/// If AST is specified, the specified tracker is updated to reflect changes
+/// made to the IR.
+///
+void llvm::PromoteMemToReg(const std::vector<AllocaInst*> &Allocas,
+                           DominatorTree &DT, DominanceFrontier &DF,
+                           AliasSetTracker *AST) {
+  // If there is nothing to do, bail out...
+  if (Allocas.empty()) return;
+
+  PromoteMem2Reg(Allocas, DT, DF, AST).run();
+}