lib/Transforms/Scalar/GVN.cpp

//===- GVN.cpp - Eliminate redundant values and loads ------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file was developed by the Owen Anderson and is distributed under
// the University of Illinois Open Source License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass performs global value numbering to eliminate fully redundant
// instructions.  It also performs simple dead load elimination.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "gvn"
#include "llvm/Value.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/BasicBlock.h"
#include "llvm/Instructions.h"
#include "llvm/Function.h"
#include "llvm/DerivedTypes.h"
#include "llvm/Analysis/Dominators.h"
#include "llvm/ADT/BitVector.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/MemoryDependenceAnalysis.h"
#include "llvm/Support/CFG.h"
#include "llvm/Support/Compiler.h"
using namespace llvm;

//===----------------------------------------------------------------------===//
//                         ValueTable Class
//===----------------------------------------------------------------------===//

/// This class holds the mapping between values and value numbers.  It is used
/// as an efficient mechanism to determine the expression-wise equivalence of
/// two values.
namespace {
  struct VISIBILITY_HIDDEN Expression {
    enum ExpressionOpcode { ADD, SUB, MUL, UDIV, SDIV, FDIV, UREM, SREM, 
                            FREM, SHL, LSHR, ASHR, AND, OR, XOR, ICMPEQ, 
                            ICMPNE, ICMPUGT, ICMPUGE, ICMPULT, ICMPULE, 
                            ICMPSGT, ICMPSGE, ICMPSLT, ICMPSLE, FCMPOEQ, 
                            FCMPOGT, FCMPOGE, FCMPOLT, FCMPOLE, FCMPONE, 
                            FCMPORD, FCMPUNO, FCMPUEQ, FCMPUGT, FCMPUGE, 
                            FCMPULT, FCMPULE, FCMPUNE, EXTRACT, INSERT,
                            SHUFFLE, SELECT, TRUNC, ZEXT, SEXT, FPTOUI,
                            FPTOSI, UITOFP, SITOFP, FPTRUNC, FPEXT, 
                            PTRTOINT, INTTOPTR, BITCAST, GEP, EMPTY,
                            TOMBSTONE };

    ExpressionOpcode opcode;
    const Type* type;
    uint32_t firstVN;
    uint32_t secondVN;
    uint32_t thirdVN;
    SmallVector<uint32_t, 4> varargs;
  
    Expression() { }
    Expression(ExpressionOpcode o) : opcode(o) { }
  
    bool operator==(const Expression &other) const {
      if (opcode != other.opcode)
        return false;
      else if (opcode == EMPTY || opcode == TOMBSTONE)
        return true;
      else if (type != other.type)
        return false;
      else if (firstVN != other.firstVN)
        return false;
      else if (secondVN != other.secondVN)
        return false;
      else if (thirdVN != other.thirdVN)
        return false;
      else {
        if (varargs.size() != other.varargs.size())
          return false;
      
        for (size_t i = 0; i < varargs.size(); ++i)
          if (varargs[i] != other.varargs[i])
            return false;
    
        return true;
      }
    }
  
    bool operator!=(const Expression &other) const {
      if (opcode != other.opcode)
        return true;
      else if (opcode == EMPTY || opcode == TOMBSTONE)
        return false;
      else if (type != other.type)
        return true;
      else if (firstVN != other.firstVN)
        return true;
      else if (secondVN != other.secondVN)
        return true;
      else if (thirdVN != other.thirdVN)
        return true;
      else {
        if (varargs.size() != other.varargs.size())
          return true;
      
        for (size_t i = 0; i < varargs.size(); ++i)
          if (varargs[i] != other.varargs[i])
            return true;
    
          return false;
      }
    }
  };
  
  class VISIBILITY_HIDDEN ValueTable {
    private:
      DenseMap<Value*, uint32_t> valueNumbering;
      DenseMap<Expression, uint32_t> expressionNumbering;
  
      uint32_t nextValueNumber;
    
      Expression::ExpressionOpcode getOpcode(BinaryOperator* BO);
      Expression::ExpressionOpcode getOpcode(CmpInst* C);
      Expression::ExpressionOpcode getOpcode(CastInst* C);
      Expression create_expression(BinaryOperator* BO);
      Expression create_expression(CmpInst* C);
      Expression create_expression(ShuffleVectorInst* V);
      Expression create_expression(ExtractElementInst* C);
      Expression create_expression(InsertElementInst* V);
      Expression create_expression(SelectInst* V);
      Expression create_expression(CastInst* C);
      Expression create_expression(GetElementPtrInst* G);
    public:
      ValueTable() { nextValueNumber = 1; }
      uint32_t lookup_or_add(Value* V);
      uint32_t lookup(Value* V) const;
      void add(Value* V, uint32_t num);
      void clear();
      void erase(Value* v);
      unsigned size();
  };
}

namespace llvm {
template <> struct DenseMapKeyInfo<Expression> {
  static inline Expression getEmptyKey() { return Expression(Expression::EMPTY); }
  static inline Expression getTombstoneKey() { return Expression(Expression::TOMBSTONE); }
  
  static unsigned getHashValue(const Expression e) {
    unsigned hash = e.opcode;
    
    hash = e.firstVN + hash * 37;
    hash = e.secondVN + hash * 37;
    hash = e.thirdVN + hash * 37;
    
    hash = (unsigned)((uintptr_t)e.type >> 4) ^
            (unsigned)((uintptr_t)e.type >> 9) +
            hash * 37;
    
    for (SmallVector<uint32_t, 4>::const_iterator I = e.varargs.begin(), E = e.varargs.end();
         I != E; ++I)
      hash = *I + hash * 37;
    
    return hash;
  }
  static bool isPod() { return true; }
};
}

//===----------------------------------------------------------------------===//
//                     ValueTable Internal Functions
//===----------------------------------------------------------------------===//
Expression::ExpressionOpcode 
                             ValueTable::getOpcode(BinaryOperator* BO) {
  switch(BO->getOpcode()) {
    case Instruction::Add:
      return Expression::ADD;
    case Instruction::Sub:
      return Expression::SUB;
    case Instruction::Mul:
      return Expression::MUL;
    case Instruction::UDiv:
      return Expression::UDIV;
    case Instruction::SDiv:
      return Expression::SDIV;
    case Instruction::FDiv:
      return Expression::FDIV;
    case Instruction::URem:
      return Expression::UREM;
    case Instruction::SRem:
      return Expression::SREM;
    case Instruction::FRem:
      return Expression::FREM;
    case Instruction::Shl:
      return Expression::SHL;
    case Instruction::LShr:
      return Expression::LSHR;
    case Instruction::AShr:
      return Expression::ASHR;
    case Instruction::And:
      return Expression::AND;
    case Instruction::Or:
      return Expression::OR;
    case Instruction::Xor:
      return Expression::XOR;
    
    // THIS SHOULD NEVER HAPPEN
    default:
      assert(0 && "Binary operator with unknown opcode?");
      return Expression::ADD;
  }
}

Expression::ExpressionOpcode ValueTable::getOpcode(CmpInst* C) {
  if (C->getOpcode() == Instruction::ICmp) {
    switch (C->getPredicate()) {
      case ICmpInst::ICMP_EQ:
        return Expression::ICMPEQ;
      case ICmpInst::ICMP_NE:
        return Expression::ICMPNE;
      case ICmpInst::ICMP_UGT:
        return Expression::ICMPUGT;
      case ICmpInst::ICMP_UGE:
        return Expression::ICMPUGE;
      case ICmpInst::ICMP_ULT:
        return Expression::ICMPULT;
      case ICmpInst::ICMP_ULE:
        return Expression::ICMPULE;
      case ICmpInst::ICMP_SGT:
        return Expression::ICMPSGT;
      case ICmpInst::ICMP_SGE:
        return Expression::ICMPSGE;
      case ICmpInst::ICMP_SLT:
        return Expression::ICMPSLT;
      case ICmpInst::ICMP_SLE:
        return Expression::ICMPSLE;
      
      // THIS SHOULD NEVER HAPPEN
      default:
        assert(0 && "Comparison with unknown predicate?");
        return Expression::ICMPEQ;
    }
  } else {
    switch (C->getPredicate()) {
      case FCmpInst::FCMP_OEQ:
        return Expression::FCMPOEQ;
      case FCmpInst::FCMP_OGT:
        return Expression::FCMPOGT;
      case FCmpInst::FCMP_OGE:
        return Expression::FCMPOGE;
      case FCmpInst::FCMP_OLT:
        return Expression::FCMPOLT;
      case FCmpInst::FCMP_OLE:
        return Expression::FCMPOLE;
      case FCmpInst::FCMP_ONE:
        return Expression::FCMPONE;
      case FCmpInst::FCMP_ORD:
        return Expression::FCMPORD;
      case FCmpInst::FCMP_UNO:
        return Expression::FCMPUNO;
      case FCmpInst::FCMP_UEQ:
        return Expression::FCMPUEQ;
      case FCmpInst::FCMP_UGT:
        return Expression::FCMPUGT;
      case FCmpInst::FCMP_UGE:
        return Expression::FCMPUGE;
      case FCmpInst::FCMP_ULT:
        return Expression::FCMPULT;
      case FCmpInst::FCMP_ULE:
        return Expression::FCMPULE;
      case FCmpInst::FCMP_UNE:
        return Expression::FCMPUNE;
      
      // THIS SHOULD NEVER HAPPEN
      default:
        assert(0 && "Comparison with unknown predicate?");
        return Expression::FCMPOEQ;
    }
  }
}

Expression::ExpressionOpcode 
                             ValueTable::getOpcode(CastInst* C) {
  switch(C->getOpcode()) {
    case Instruction::Trunc:
      return Expression::TRUNC;
    case Instruction::ZExt:
      return Expression::ZEXT;
    case Instruction::SExt:
      return Expression::SEXT;
    case Instruction::FPToUI:
      return Expression::FPTOUI;
    case Instruction::FPToSI:
      return Expression::FPTOSI;
    case Instruction::UIToFP:
      return Expression::UITOFP;
    case Instruction::SIToFP:
      return Expression::SITOFP;
    case Instruction::FPTrunc:
      return Expression::FPTRUNC;
    case Instruction::FPExt:
      return Expression::FPEXT;
    case Instruction::PtrToInt:
      return Expression::PTRTOINT;
    case Instruction::IntToPtr:
      return Expression::INTTOPTR;
    case Instruction::BitCast:
      return Expression::BITCAST;
    
    // THIS SHOULD NEVER HAPPEN
    default:
      assert(0 && "Cast operator with unknown opcode?");
      return Expression::BITCAST;
  }
}

Expression ValueTable::create_expression(BinaryOperator* BO) {
  Expression e;
    
  e.firstVN = lookup_or_add(BO->getOperand(0));
  e.secondVN = lookup_or_add(BO->getOperand(1));
  e.thirdVN = 0;
  e.type = BO->getType();
  e.opcode = getOpcode(BO);
  
  return e;
}

Expression ValueTable::create_expression(CmpInst* C) {
  Expression e;
    
  e.firstVN = lookup_or_add(C->getOperand(0));
  e.secondVN = lookup_or_add(C->getOperand(1));
  e.thirdVN = 0;
  e.type = C->getType();
  e.opcode = getOpcode(C);
  
  return e;
}

Expression ValueTable::create_expression(CastInst* C) {
  Expression e;
    
  e.firstVN = lookup_or_add(C->getOperand(0));
  e.secondVN = 0;
  e.thirdVN = 0;
  e.type = C->getType();
  e.opcode = getOpcode(C);
  
  return e;
}

Expression ValueTable::create_expression(ShuffleVectorInst* S) {
  Expression e;
    
  e.firstVN = lookup_or_add(S->getOperand(0));
  e.secondVN = lookup_or_add(S->getOperand(1));
  e.thirdVN = lookup_or_add(S->getOperand(2));
  e.type = S->getType();
  e.opcode = Expression::SHUFFLE;
  
  return e;
}

Expression ValueTable::create_expression(ExtractElementInst* E) {
  Expression e;
    
  e.firstVN = lookup_or_add(E->getOperand(0));
  e.secondVN = lookup_or_add(E->getOperand(1));
  e.thirdVN = 0;
  e.type = E->getType();
  e.opcode = Expression::EXTRACT;
  
  return e;
}

Expression ValueTable::create_expression(InsertElementInst* I) {
  Expression e;
    
  e.firstVN = lookup_or_add(I->getOperand(0));
  e.secondVN = lookup_or_add(I->getOperand(1));
  e.thirdVN = lookup_or_add(I->getOperand(2));
  e.type = I->getType();
  e.opcode = Expression::INSERT;
  
  return e;
}

Expression ValueTable::create_expression(SelectInst* I) {
  Expression e;
    
  e.firstVN = lookup_or_add(I->getCondition());
  e.secondVN = lookup_or_add(I->getTrueValue());
  e.thirdVN = lookup_or_add(I->getFalseValue());
  e.type = I->getType();
  e.opcode = Expression::SELECT;
  
  return e;
}

Expression ValueTable::create_expression(GetElementPtrInst* G) {
  Expression e;
    
  e.firstVN = lookup_or_add(G->getPointerOperand());
  e.secondVN = 0;
  e.thirdVN = 0;
  e.type = G->getType();
  e.opcode = Expression::GEP;
  
  for (GetElementPtrInst::op_iterator I = G->idx_begin(), E = G->idx_end();
       I != E; ++I)
    e.varargs.push_back(lookup_or_add(*I));
  
  return e;
}

//===----------------------------------------------------------------------===//
//                     ValueTable External Functions
//===----------------------------------------------------------------------===//

/// lookup_or_add - Returns the value number for the specified value, assigning
/// it a new number if it did not have one before.
uint32_t ValueTable::lookup_or_add(Value* V) {
  DenseMap<Value*, uint32_t>::iterator VI = valueNumbering.find(V);
  if (VI != valueNumbering.end())
    return VI->second;
  
  
  if (BinaryOperator* BO = dyn_cast<BinaryOperator>(V)) {
    Expression e = create_expression(BO);
    
    DenseMap<Expression, uint32_t>::iterator EI = expressionNumbering.find(e);
    if (EI != expressionNumbering.end()) {
      valueNumbering.insert(std::make_pair(V, EI->second));
      return EI->second;
    } else {
      expressionNumbering.insert(std::make_pair(e, nextValueNumber));
      valueNumbering.insert(std::make_pair(V, nextValueNumber));
      
      return nextValueNumber++;
    }
  } else if (CmpInst* C = dyn_cast<CmpInst>(V)) {
    Expression e = create_expression(C);
    
    DenseMap<Expression, uint32_t>::iterator EI = expressionNumbering.find(e);
    if (EI != expressionNumbering.end()) {
      valueNumbering.insert(std::make_pair(V, EI->second));
      return EI->second;
    } else {
      expressionNumbering.insert(std::make_pair(e, nextValueNumber));
      valueNumbering.insert(std::make_pair(V, nextValueNumber));
      
      return nextValueNumber++;
    }
  } else if (ShuffleVectorInst* U = dyn_cast<ShuffleVectorInst>(V)) {
    Expression e = create_expression(U);
    
    DenseMap<Expression, uint32_t>::iterator EI = expressionNumbering.find(e);
    if (EI != expressionNumbering.end()) {
      valueNumbering.insert(std::make_pair(V, EI->second));
      return EI->second;
    } else {
      expressionNumbering.insert(std::make_pair(e, nextValueNumber));
      valueNumbering.insert(std::make_pair(V, nextValueNumber));
      
      return nextValueNumber++;
    }
  } else if (ExtractElementInst* U = dyn_cast<ExtractElementInst>(V)) {
    Expression e = create_expression(U);
    
    DenseMap<Expression, uint32_t>::iterator EI = expressionNumbering.find(e);
    if (EI != expressionNumbering.end()) {
      valueNumbering.insert(std::make_pair(V, EI->second));
      return EI->second;
    } else {
      expressionNumbering.insert(std::make_pair(e, nextValueNumber));
      valueNumbering.insert(std::make_pair(V, nextValueNumber));
      
      return nextValueNumber++;
    }
  } else if (InsertElementInst* U = dyn_cast<InsertElementInst>(V)) {
    Expression e = create_expression(U);
    
    DenseMap<Expression, uint32_t>::iterator EI = expressionNumbering.find(e);
    if (EI != expressionNumbering.end()) {
      valueNumbering.insert(std::make_pair(V, EI->second));
      return EI->second;
    } else {
      expressionNumbering.insert(std::make_pair(e, nextValueNumber));
      valueNumbering.insert(std::make_pair(V, nextValueNumber));
      
      return nextValueNumber++;
    }
  } else if (SelectInst* U = dyn_cast<SelectInst>(V)) {
    Expression e = create_expression(U);
    
    DenseMap<Expression, uint32_t>::iterator EI = expressionNumbering.find(e);
    if (EI != expressionNumbering.end()) {
      valueNumbering.insert(std::make_pair(V, EI->second));
      return EI->second;
    } else {
      expressionNumbering.insert(std::make_pair(e, nextValueNumber));
      valueNumbering.insert(std::make_pair(V, nextValueNumber));
      
      return nextValueNumber++;
    }
  } else if (CastInst* U = dyn_cast<CastInst>(V)) {
    Expression e = create_expression(U);
    
    DenseMap<Expression, uint32_t>::iterator EI = expressionNumbering.find(e);
    if (EI != expressionNumbering.end()) {
      valueNumbering.insert(std::make_pair(V, EI->second));
      return EI->second;
    } else {
      expressionNumbering.insert(std::make_pair(e, nextValueNumber));
      valueNumbering.insert(std::make_pair(V, nextValueNumber));
      
      return nextValueNumber++;
    }
  } else if (GetElementPtrInst* U = dyn_cast<GetElementPtrInst>(V)) {
    Expression e = create_expression(U);
    
    DenseMap<Expression, uint32_t>::iterator EI = expressionNumbering.find(e);
    if (EI != expressionNumbering.end()) {
      valueNumbering.insert(std::make_pair(V, EI->second));
      return EI->second;
    } else {
      expressionNumbering.insert(std::make_pair(e, nextValueNumber));
      valueNumbering.insert(std::make_pair(V, nextValueNumber));
      
      return nextValueNumber++;
    }
  } else {
    valueNumbering.insert(std::make_pair(V, nextValueNumber));
    return nextValueNumber++;
  }
}

/// lookup - Returns the value number of the specified value. Fails if
/// the value has not yet been numbered.
uint32_t ValueTable::lookup(Value* V) const {
  DenseMap<Value*, uint32_t>::iterator VI = valueNumbering.find(V);
  if (VI != valueNumbering.end())
    return VI->second;
  else
    assert(0 && "Value not numbered?");
  
  return 0;
}

/// clear - Remove all entries from the ValueTable
void ValueTable::clear() {
  valueNumbering.clear();
  expressionNumbering.clear();
  nextValueNumber = 1;
}

//===----------------------------------------------------------------------===//
//                       ValueNumberedSet Class
//===----------------------------------------------------------------------===//
namespace {
class ValueNumberedSet {
  private:
    SmallPtrSet<Value*, 8> contents;
    BitVector numbers;
  public:
    ValueNumberedSet() { numbers.resize(1); }
    ValueNumberedSet(const ValueNumberedSet& other) {
      numbers = other.numbers;
      contents = other.contents;
    }
    
    typedef SmallPtrSet<Value*, 8>::iterator iterator;
    
    iterator begin() { return contents.begin(); }
    iterator end() { return contents.end(); }
    
    bool insert(Value* v) { return contents.insert(v); }
    void insert(iterator I, iterator E) { contents.insert(I, E); }
    void erase(Value* v) { contents.erase(v); }
    unsigned count(Value* v) { return contents.count(v); }
    size_t size() { return contents.size(); }
    
    void set(unsigned i)  {
      if (i >= numbers.size())
        numbers.resize(i+1);
      
      numbers.set(i);
    }
    
    void operator=(const ValueNumberedSet& other) {
      contents = other.contents;
      numbers = other.numbers;
    }
    
    void reset(unsigned i)  {
      if (i < numbers.size())
        numbers.reset(i);
    }
    
    bool test(unsigned i)  {
      if (i >= numbers.size())
        return false;
      
      return numbers.test(i);
    }
    
    void clear() {
      contents.clear();
      numbers.clear();
    }
};
}

//===----------------------------------------------------------------------===//
//                         GVN Pass
//===----------------------------------------------------------------------===//

namespace {

  class VISIBILITY_HIDDEN GVN : public FunctionPass {
    bool runOnFunction(Function &F);
  public:
    static char ID; // Pass identification, replacement for typeid
    GVN() : FunctionPass((intptr_t)&ID) { }

  private:
    ValueTable VN;
    
    DenseMap<BasicBlock*, ValueNumberedSet> availableOut;
    
    // This transformation requires dominator postdominator info
    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
      AU.setPreservesCFG();
      AU.addRequired<DominatorTree>();
      AU.addRequired<MemoryDependenceAnalysis>();
      AU.addPreserved<MemoryDependenceAnalysis>();
    }
  
    // Helper fuctions
    // FIXME: eliminate or document these better
    Value* find_leader(ValueNumberedSet& vals, uint32_t v) ;
    void val_insert(ValueNumberedSet& s, Value* v);
    bool processLoad(LoadInst* L,
                     DenseMap<Value*, LoadInst*>& lastLoad,
                     SmallVector<Instruction*, 4>& toErase);
    bool processInstruction(Instruction* I,
                            ValueNumberedSet& currAvail,
                            DenseMap<Value*, LoadInst*>& lastSeenLoad,
                            SmallVector<Instruction*, 4>& toErase);
    bool processNonLocalLoad(LoadInst* L, SmallVector<Instruction*, 4>& toErase);
    Value *performPHIConstruction(BasicBlock *BB, LoadInst* orig,
                                  DenseMap<BasicBlock*, Value*> &Phis);
    void dump(DenseMap<BasicBlock*, Value*>& d);
  };
  
  char GVN::ID = 0;
  
}

// createGVNPass - The public interface to this file...
FunctionPass *llvm::createGVNPass() { return new GVN(); }

static RegisterPass<GVN> X("gvn",
                           "Global Value Numbering");

STATISTIC(NumGVNInstr, "Number of instructions deleted");
STATISTIC(NumGVNLoad, "Number of loads deleted");

/// find_leader - Given a set and a value number, return the first
/// element of the set with that value number, or 0 if no such element
/// is present
Value* GVN::find_leader(ValueNumberedSet& vals, uint32_t v) {
  if (!vals.test(v))
    return 0;
  
  for (ValueNumberedSet::iterator I = vals.begin(), E = vals.end();
       I != E; ++I)
    if (v == VN.lookup(*I))
      return *I;
  
  assert(0 && "No leader found, but present bit is set?");
  return 0;
}

/// val_insert - Insert a value into a set only if there is not a value
/// with the same value number already in the set
void GVN::val_insert(ValueNumberedSet& s, Value* v) {
  uint32_t num = VN.lookup(v);
  if (!s.test(num))
    s.insert(v);
}

void GVN::dump(DenseMap<BasicBlock*, Value*>& d) {
  printf("{\n");
  for (DenseMap<BasicBlock*, Value*>::iterator I = d.begin(),
       E = d.end(); I != E; ++I) {
    if (I->second == MemoryDependenceAnalysis::None)
      printf("None\n");
    else
      I->second->dump();
  }
  printf("}\n");
}


Value *GVN::performPHIConstruction(BasicBlock *BB, LoadInst* orig,
                                   DenseMap<BasicBlock*, Value*> &Phis) {
  DenseMap<BasicBlock*, Value*>::iterator DI = Phis.find(BB);
  if (DI != Phis.end())
    return DI->second;
  
  unsigned numPreds = std::distance(pred_begin(BB), pred_end(BB));
  
  if (numPreds == 1) {
    Phis[BB] = Phis[*pred_begin(BB)];
    return Phis[BB];
  } else {
    PHINode *PN = new PHINode(orig->getType(), orig->getName()+".rle", BB->begin());
    PN->reserveOperandSpace(numPreds);
                                 
    // Fill in the incoming values for the block.
    for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
      PN->addIncoming(performPHIConstruction(*PI, orig, Phis), *PI);
    
    bool all_same = PN->getNumIncomingValues() != 1;
    Value* first = PN->getIncomingValue(0);
    for (unsigned i = 1; i < PN->getNumIncomingValues(); ++i)
      all_same &= (PN->getIncomingValue(i) == first);
    
    if (all_same) {
      PN->eraseFromParent();
      return first;
    } else {
      return PN;
    }
  }
}

bool GVN::processNonLocalLoad(LoadInst* L, SmallVector<Instruction*, 4>& toErase) {
  MemoryDependenceAnalysis& MD = getAnalysis<MemoryDependenceAnalysis>();
  
  DenseMap<BasicBlock*, Value*> deps;
  bool ret = MD.getNonLocalDependency(L, deps);
  if (!ret)
    return false;
  
  DenseMap<BasicBlock*, Value*> repl;
  for (DenseMap<BasicBlock*, Value*>::iterator I = deps.begin(), E = deps.end();
       I != E; ++I)
    if (I->second == MemoryDependenceAnalysis::None) {
      return false;
    } else if (StoreInst* S = dyn_cast<StoreInst>(I->second)) {
      if (S->getPointerOperand() == L->getPointerOperand())
        repl.insert(std::make_pair(I->first, S->getOperand(0)));
      else
        return false;
    } else if (LoadInst* LD = dyn_cast<LoadInst>(I->second)) {
      if (LD->getPointerOperand() == L->getPointerOperand())
        repl.insert(std::make_pair(I->first, LD));
      else
        return false;
    } else {
      return false;
    }
  
  Value* v = performPHIConstruction(L->getParent(), L, repl);
  
  MD.removeInstruction(L);
  L->replaceAllUsesWith(v);
  toErase.push_back(L);

  return true;
}

bool GVN::processLoad(LoadInst* L,
                         DenseMap<Value*, LoadInst*>& lastLoad,
                         SmallVector<Instruction*, 4>& toErase) {
  if (L->isVolatile()) {
    lastLoad[L->getPointerOperand()] = L;
    return false;
  }
  
  Value* pointer = L->getPointerOperand();
  LoadInst*& last = lastLoad[pointer];
  
  // ... to a pointer that has been loaded from before...
  MemoryDependenceAnalysis& MD = getAnalysis<MemoryDependenceAnalysis>();
  Instruction* dep = MD.getDependency(L);
  if (dep == MemoryDependenceAnalysis::NonLocal &&
      L->getParent() != &L->getParent()->getParent()->getEntryBlock())
    processNonLocalLoad(L, toErase);
  bool deletedLoad = false;
  
  while (dep != MemoryDependenceAnalysis::None &&
         dep != MemoryDependenceAnalysis::NonLocal &&
         (isa<LoadInst>(dep) || isa<StoreInst>(dep))) {
    // ... that depends on a store ...
    if (StoreInst* S = dyn_cast<StoreInst>(dep)) {
      if (S->getPointerOperand() == pointer) {
        // Remove it!
        MD.removeInstruction(L);
        
        L->replaceAllUsesWith(S->getOperand(0));
        toErase.push_back(L);
        deletedLoad = true;
        NumGVNLoad++;
      }
      
      // Whether we removed it or not, we can't
      // go any further
      break;
    } else if (!last) {
      // If we don't depend on a store, and we haven't
      // been loaded before, bail.
      break;
    } else if (dep == last) {
      // Remove it!
      MD.removeInstruction(L);
      
      L->replaceAllUsesWith(last);
      toErase.push_back(L);
      deletedLoad = true;
      NumGVNLoad++;
        
      break;
    } else {
      dep = MD.getDependency(L, dep);
    }
  }
  
  if (!deletedLoad)
    last = L;
  
  return deletedLoad;
}

/// buildsets_availout - When calculating availability, handle an instruction
/// by inserting it into the appropriate sets
bool GVN::processInstruction(Instruction* I,
                                ValueNumberedSet& currAvail,
                                DenseMap<Value*, LoadInst*>& lastSeenLoad,
                                SmallVector<Instruction*, 4>& toErase) {
  if (LoadInst* L = dyn_cast<LoadInst>(I)) {
    return processLoad(L, lastSeenLoad, toErase);
  }
  
  unsigned num = VN.lookup_or_add(I);
  
  if (currAvail.test(num)) {
    Value* repl = find_leader(currAvail, num);
    
    I->replaceAllUsesWith(repl);
    toErase.push_back(I);
    return true;
  } else if (!I->isTerminator()) {
    currAvail.set(num);
    currAvail.insert(I);
  }
  
  return false;
}

// GVN::runOnFunction - This is the main transformation entry point for a
// function.
//
bool GVN::runOnFunction(Function &F) {
  // Clean out global sets from any previous functions
  VN.clear();
  availableOut.clear();
 
  bool changed_function = false;
  
  DominatorTree &DT = getAnalysis<DominatorTree>();   
  
  SmallVector<Instruction*, 4> toErase;
  
  // Top-down walk of the dominator tree
  for (df_iterator<DomTreeNode*> DI = df_begin(DT.getRootNode()),
         E = df_end(DT.getRootNode()); DI != E; ++DI) {
    
    // Get the set to update for this block
    ValueNumberedSet& currAvail = availableOut[DI->getBlock()];     
    DenseMap<Value*, LoadInst*> lastSeenLoad;
    
    BasicBlock* BB = DI->getBlock();
  
    // A block inherits AVAIL_OUT from its dominator
    if (DI->getIDom() != 0)
      currAvail = availableOut[DI->getIDom()->getBlock()];

    for (BasicBlock::iterator BI = BB->begin(), BE = BB->end();
         BI != BE; ++BI) {
      changed_function |= processInstruction(BI, currAvail, lastSeenLoad, toErase);
      
      NumGVNInstr += toErase.size();
      
      for (SmallVector<Instruction*, 4>::iterator I = toErase.begin(),
           E = toErase.end(); I != E; ++I)
        (*I)->eraseFromParent();
      
      toErase.clear();
    }
  }
  
  return changed_function;
}