lib/Transforms/Scalar/GVN.cpp

//===- GVN.cpp - Eliminate redundant values and loads ------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file 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.
//
// Note that this pass does the value numbering itself, it does not use the
// ValueNumbering analysis passes.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "gvn"
#include "llvm/Transforms/Scalar.h"
#include "llvm/BasicBlock.h"
#include "llvm/Constants.h"
#include "llvm/DerivedTypes.h"
#include "llvm/Function.h"
#include "llvm/Instructions.h"
#include "llvm/Value.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/Dominators.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/MemoryDependenceAnalysis.h"
#include "llvm/Support/CFG.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Compiler.h"
#include "llvm/Support/Debug.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
using namespace llvm;

STATISTIC(NumGVNInstr, "Number of instructions deleted");
STATISTIC(NumGVNLoad, "Number of loads deleted");
STATISTIC(NumGVNPRE, "Number of instructions PRE'd");
STATISTIC(NumGVNBlocks, "Number of blocks merged");

static cl::opt<bool> EnablePRE("enable-pre",
                               cl::init(false), cl::Hidden);

//===----------------------------------------------------------------------===//
//                         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, CALL, CONSTANT,
                            EMPTY, TOMBSTONE };

    ExpressionOpcode opcode;
    const Type* type;
    uint32_t firstVN;
    uint32_t secondVN;
    uint32_t thirdVN;
    SmallVector<uint32_t, 4> varargs;
    Value* function;
  
    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 (function != other.function)
        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 (function != other.function)
        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;
      AliasAnalysis* AA;
      MemoryDependenceAnalysis* MD;
      DominatorTree* DT;
  
      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);
      Expression create_expression(CallInst* C);
      Expression create_expression(Constant* C);
    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();
      void setAliasAnalysis(AliasAnalysis* A) { AA = A; }
      void setMemDep(MemoryDependenceAnalysis* M) { MD = M; }
      void setDomTree(DominatorTree* D) { DT = D; }
      uint32_t getNextUnusedValueNumber() { return nextValueNumber; }
  };
}

namespace llvm {
template <> struct DenseMapInfo<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;
    
    hash = ((unsigned)((uintptr_t)e.function >> 4) ^
            (unsigned)((uintptr_t)e.function >> 9)) +
           hash * 37;
    
    return hash;
  }
  static bool isEqual(const Expression &LHS, const Expression &RHS) {
    return LHS == RHS;
  }
  static bool isPod() { return true; }
};
}

//===----------------------------------------------------------------------===//
//                     ValueTable Internal Functions
//===----------------------------------------------------------------------===//
Expression::ExpressionOpcode ValueTable::getOpcode(BinaryOperator* BO) {
  switch(BO->getOpcode()) {
  default: // THIS SHOULD NEVER HAPPEN
    assert(0 && "Binary operator with unknown opcode?");
  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;
  }
}

Expression::ExpressionOpcode ValueTable::getOpcode(CmpInst* C) {
  if (isa<ICmpInst>(C) || isa<VICmpInst>(C)) {
    switch (C->getPredicate()) {
    default:  // THIS SHOULD NEVER HAPPEN
      assert(0 && "Comparison with unknown predicate?");
    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;
    }
  }
  assert((isa<FCmpInst>(C) || isa<VFCmpInst>(C)) && "Unknown compare");
  switch (C->getPredicate()) {
  default: // THIS SHOULD NEVER HAPPEN
    assert(0 && "Comparison with unknown predicate?");
  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;
  }
}

Expression::ExpressionOpcode ValueTable::getOpcode(CastInst* C) {
  switch(C->getOpcode()) {
  default: // THIS SHOULD NEVER HAPPEN
    assert(0 && "Cast operator with unknown opcode?");
  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;
  }
}

Expression ValueTable::create_expression(CallInst* C) {
  Expression e;
  
  e.type = C->getType();
  e.firstVN = 0;
  e.secondVN = 0;
  e.thirdVN = 0;
  e.function = C->getCalledFunction();
  e.opcode = Expression::CALL;
  
  for (CallInst::op_iterator I = C->op_begin()+1, E = C->op_end();
       I != E; ++I)
    e.varargs.push_back(lookup_or_add(*I));
  
  return e;
}

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.function = 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.function = 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.function = 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.function = 0;
  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.function = 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.function = 0;
  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.function = 0;
  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.function = 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
//===----------------------------------------------------------------------===//

/// add - Insert a value into the table with a specified value number.
void ValueTable::add(Value* V, uint32_t num) {
  valueNumbering.insert(std::make_pair(V, num));
}

/// 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 (CallInst* C = dyn_cast<CallInst>(V)) {
    if (AA->doesNotAccessMemory(C)) {
      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 (AA->onlyReadsMemory(C)) {
      Expression e = create_expression(C);
      
      if (expressionNumbering.find(e) == expressionNumbering.end()) {
        expressionNumbering.insert(std::make_pair(e, nextValueNumber));
        valueNumbering.insert(std::make_pair(V, nextValueNumber));
        return nextValueNumber++;
      }
      
      Instruction* local_dep = MD->getDependency(C);
      
      if (local_dep == MemoryDependenceAnalysis::None) {
        valueNumbering.insert(std::make_pair(V, nextValueNumber));
        return nextValueNumber++;
      } else if (local_dep != MemoryDependenceAnalysis::NonLocal) {
        if (!isa<CallInst>(local_dep)) {
          valueNumbering.insert(std::make_pair(V, nextValueNumber));
          return nextValueNumber++;
        }
        
        CallInst* local_cdep = cast<CallInst>(local_dep);
        
        if (local_cdep->getCalledFunction() != C->getCalledFunction() ||
            local_cdep->getNumOperands() != C->getNumOperands()) {
          valueNumbering.insert(std::make_pair(V, nextValueNumber));
          return nextValueNumber++;
        } else if (!C->getCalledFunction()) { 
          valueNumbering.insert(std::make_pair(V, nextValueNumber));
          return nextValueNumber++;
        } else {
          for (unsigned i = 1; i < C->getNumOperands(); ++i) {
            uint32_t c_vn = lookup_or_add(C->getOperand(i));
            uint32_t cd_vn = lookup_or_add(local_cdep->getOperand(i));
            if (c_vn != cd_vn) {
              valueNumbering.insert(std::make_pair(V, nextValueNumber));
              return nextValueNumber++;
            }
          }
        
          uint32_t v = lookup_or_add(local_cdep);
          valueNumbering.insert(std::make_pair(V, v));
          return v;
        }
      }
      
      
      DenseMap<BasicBlock*, Value*> deps;
      MD->getNonLocalDependency(C, deps);
      CallInst* cdep = 0;
      
      for (DenseMap<BasicBlock*, Value*>::iterator I = deps.begin(),
           E = deps.end(); I != E; ++I) {
        if (I->second == MemoryDependenceAnalysis::None) {
          valueNumbering.insert(std::make_pair(V, nextValueNumber));

          return nextValueNumber++;
        } else if (I->second != MemoryDependenceAnalysis::NonLocal) {
          if (DT->properlyDominates(I->first, C->getParent())) {
            if (CallInst* CD = dyn_cast<CallInst>(I->second))
              cdep = CD;
            else {
              valueNumbering.insert(std::make_pair(V, nextValueNumber));
              return nextValueNumber++;
            }
          } else {
            valueNumbering.insert(std::make_pair(V, nextValueNumber));
            return nextValueNumber++;
          }
        }
      }
      
      if (!cdep) {
        valueNumbering.insert(std::make_pair(V, nextValueNumber));
        return nextValueNumber++;
      }
      
      if (cdep->getCalledFunction() != C->getCalledFunction() ||
          cdep->getNumOperands() != C->getNumOperands()) {
        valueNumbering.insert(std::make_pair(V, nextValueNumber));
        return nextValueNumber++;
      } else if (!C->getCalledFunction()) { 
        valueNumbering.insert(std::make_pair(V, nextValueNumber));
        return nextValueNumber++;
      } else {
        for (unsigned i = 1; i < C->getNumOperands(); ++i) {
          uint32_t c_vn = lookup_or_add(C->getOperand(i));
          uint32_t cd_vn = lookup_or_add(cdep->getOperand(i));
          if (c_vn != cd_vn) {
            valueNumbering.insert(std::make_pair(V, nextValueNumber));
            return nextValueNumber++;
          }
        }
        
        uint32_t v = lookup_or_add(cdep);
        valueNumbering.insert(std::make_pair(V, v));
        return v;
      }
      
    } else {
      valueNumbering.insert(std::make_pair(V, nextValueNumber));
      return nextValueNumber++;
    }
  } else 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);
  assert(VI != valueNumbering.end() && "Value not numbered?");
  return VI->second;
}

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

/// erase - Remove a value from the value numbering
void ValueTable::erase(Value* V) {
  valueNumbering.erase(V);
}

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

namespace llvm {
  template<> struct DenseMapInfo<uint32_t> {
    static inline uint32_t getEmptyKey() { return ~0; }
    static inline uint32_t getTombstoneKey() { return ~0 - 1; }
    static unsigned getHashValue(const uint32_t& Val) { return Val * 37; }
    static bool isPod() { return true; }
    static bool isEqual(const uint32_t& LHS, const uint32_t& RHS) {
      return LHS == RHS;
    }
  };
}

namespace {
  struct VISIBILITY_HIDDEN ValueNumberScope {
    ValueNumberScope* parent;
    DenseMap<uint32_t, Value*> table;
    
    ValueNumberScope(ValueNumberScope* p) : parent(p) { }
  };
}

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*, ValueNumberScope*> localAvail;
    
    typedef DenseMap<Value*, SmallPtrSet<Instruction*, 4> > PhiMapType;
    PhiMapType phiMap;
    
    
    // This transformation requires dominator postdominator info
    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
      AU.addRequired<DominatorTree>();
      AU.addRequired<MemoryDependenceAnalysis>();
      AU.addRequired<AliasAnalysis>();
      
      AU.addPreserved<DominatorTree>();
      AU.addPreserved<AliasAnalysis>();
      AU.addPreserved<MemoryDependenceAnalysis>();
    }
  
    // Helper fuctions
    // FIXME: eliminate or document these better
    bool processLoad(LoadInst* L,
                     DenseMap<Value*, LoadInst*> &lastLoad,
                     SmallVectorImpl<Instruction*> &toErase);
    bool processInstruction(Instruction* I,
                            DenseMap<Value*, LoadInst*>& lastSeenLoad,
                            SmallVectorImpl<Instruction*> &toErase);
    bool processNonLocalLoad(LoadInst* L,
                             SmallVectorImpl<Instruction*> &toErase);
    bool processBlock(DomTreeNode* DTN);
    Value *GetValueForBlock(BasicBlock *BB, LoadInst* orig,
                            DenseMap<BasicBlock*, Value*> &Phis,
                            bool top_level = false);
    void dump(DenseMap<uint32_t, Value*>& d);
    bool iterateOnFunction(Function &F);
    Value* CollapsePhi(PHINode* p);
    bool isSafeReplacement(PHINode* p, Instruction* inst);
    bool performPRE(Function& F);
    Value* lookupNumber(BasicBlock* BB, uint32_t num);
    bool mergeBlockIntoPredecessor(BasicBlock* BB);
  };
  
  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");

void GVN::dump(DenseMap<uint32_t, Value*>& d) {
  printf("{\n");
  for (DenseMap<uint32_t, Value*>::iterator I = d.begin(),
       E = d.end(); I != E; ++I) {
      printf("%d\n", I->first);
      I->second->dump();
  }
  printf("}\n");
}

Value* GVN::CollapsePhi(PHINode* p) {
  DominatorTree &DT = getAnalysis<DominatorTree>();
  Value* constVal = p->hasConstantValue();
  
  if (!constVal) return 0;
  
  Instruction* inst = dyn_cast<Instruction>(constVal);
  if (!inst)
    return constVal;
    
  if (DT.dominates(inst, p))
    if (isSafeReplacement(p, inst))
      return inst;
  return 0;
}

bool GVN::isSafeReplacement(PHINode* p, Instruction* inst) {
  if (!isa<PHINode>(inst))
    return true;
  
  for (Instruction::use_iterator UI = p->use_begin(), E = p->use_end();
       UI != E; ++UI)
    if (PHINode* use_phi = dyn_cast<PHINode>(UI))
      if (use_phi->getParent() == inst->getParent())
        return false;
  
  return true;
}

/// GetValueForBlock - Get the value to use within the specified basic block.
/// available values are in Phis.
Value *GVN::GetValueForBlock(BasicBlock *BB, LoadInst* orig,
                             DenseMap<BasicBlock*, Value*> &Phis,
                             bool top_level) { 
                                 
  // If we have already computed this value, return the previously computed val.
  DenseMap<BasicBlock*, Value*>::iterator V = Phis.find(BB);
  if (V != Phis.end() && !top_level) return V->second;
  
  // If the block is unreachable, just return undef, since this path
  // can't actually occur at runtime.
  if (!getAnalysis<DominatorTree>().isReachableFromEntry(BB))
    return Phis[BB] = UndefValue::get(orig->getType());
  
  BasicBlock* singlePred = BB->getSinglePredecessor();
  if (singlePred) {
    Value *ret = GetValueForBlock(singlePred, orig, Phis);
    Phis[BB] = ret;
    return ret;
  }
  
  // Otherwise, the idom is the loop, so we need to insert a PHI node.  Do so
  // now, then get values to fill in the incoming values for the PHI.
  PHINode *PN = PHINode::Create(orig->getType(), orig->getName()+".rle",
                                BB->begin());
  PN->reserveOperandSpace(std::distance(pred_begin(BB), pred_end(BB)));
  
  if (Phis.count(BB) == 0)
    Phis.insert(std::make_pair(BB, PN));
  
  // Fill in the incoming values for the block.
  for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
    Value* val = GetValueForBlock(*PI, orig, Phis);
    PN->addIncoming(val, *PI);
  }
  
  AliasAnalysis& AA = getAnalysis<AliasAnalysis>();
  AA.copyValue(orig, PN);
  
  // Attempt to collapse PHI nodes that are trivially redundant
  Value* v = CollapsePhi(PN);
  if (!v) {
    // Cache our phi construction results
    phiMap[orig->getPointerOperand()].insert(PN);
    return PN;
  }
    
  MemoryDependenceAnalysis& MD = getAnalysis<MemoryDependenceAnalysis>();

  MD.removeInstruction(PN);
  PN->replaceAllUsesWith(v);

  for (DenseMap<BasicBlock*, Value*>::iterator I = Phis.begin(),
       E = Phis.end(); I != E; ++I)
    if (I->second == PN)
      I->second = v;

  PN->eraseFromParent();

  Phis[BB] = v;
  return v;
}

/// processNonLocalLoad - Attempt to eliminate a load whose dependencies are
/// non-local by performing PHI construction.
bool GVN::processNonLocalLoad(LoadInst* L,
                              SmallVectorImpl<Instruction*> &toErase) {
  MemoryDependenceAnalysis& MD = getAnalysis<MemoryDependenceAnalysis>();
  
  // Find the non-local dependencies of the load
  DenseMap<BasicBlock*, Value*> deps;
  MD.getNonLocalDependency(L, deps);
  
  DenseMap<BasicBlock*, Value*> repl;
  
  // Filter out useless results (non-locals, etc)
  for (DenseMap<BasicBlock*, Value*>::iterator I = deps.begin(), E = deps.end();
       I != E; ++I) {
    if (I->second == MemoryDependenceAnalysis::None)
      return false;
  
    if (I->second == MemoryDependenceAnalysis::NonLocal)
      continue;
  
    if (StoreInst* S = dyn_cast<StoreInst>(I->second)) {
      if (S->getPointerOperand() != L->getPointerOperand())
        return false;
      repl[I->first] = S->getOperand(0);
    } else if (LoadInst* LD = dyn_cast<LoadInst>(I->second)) {
      if (LD->getPointerOperand() != L->getPointerOperand())
        return false;
      repl[I->first] = LD;
    } else {
      return false;
    }
  }
  
  // Use cached PHI construction information from previous runs
  SmallPtrSet<Instruction*, 4>& p = phiMap[L->getPointerOperand()];
  for (SmallPtrSet<Instruction*, 4>::iterator I = p.begin(), E = p.end();
       I != E; ++I) {
    if ((*I)->getParent() == L->getParent()) {
      MD.removeInstruction(L);
      L->replaceAllUsesWith(*I);
      toErase.push_back(L);
      NumGVNLoad++;
      return true;
    }
    
    repl.insert(std::make_pair((*I)->getParent(), *I));
  }
  
  // Perform PHI construction
  SmallPtrSet<BasicBlock*, 4> visited;
  Value* v = GetValueForBlock(L->getParent(), L, repl, true);
  
  MD.removeInstruction(L);
  L->replaceAllUsesWith(v);
  toErase.push_back(L);
  NumGVNLoad++;

  return true;
}

/// processLoad - Attempt to eliminate a load, first by eliminating it
/// locally, and then attempting non-local elimination if that fails.
bool GVN::processLoad(LoadInst *L, DenseMap<Value*, LoadInst*> &lastLoad,
                      SmallVectorImpl<Instruction*> &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>();
  bool removedNonLocal = false;
  Instruction* dep = MD.getDependency(L);
  if (dep == MemoryDependenceAnalysis::NonLocal &&
      L->getParent() != &L->getParent()->getParent()->getEntryBlock()) {
    removedNonLocal = processNonLocalLoad(L, toErase);
    
    if (!removedNonLocal)
      last = L;
    
    return removedNonLocal;
  }
  
  
  bool deletedLoad = false;
  
  // Walk up the dependency chain until we either find
  // a dependency we can use, or we can't walk any further
  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 (dep != MemoryDependenceAnalysis::None &&
      dep != MemoryDependenceAnalysis::NonLocal &&
      isa<AllocationInst>(dep)) {
    // Check that this load is actually from the
    // allocation we found
    Value* v = L->getOperand(0);
    while (true) {
      if (BitCastInst *BC = dyn_cast<BitCastInst>(v))
        v = BC->getOperand(0);
      else if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(v))
        v = GEP->getOperand(0);
      else
        break;
    }
    if (v == dep) {
      // If this load depends directly on an allocation, there isn't
      // anything stored there; therefore, we can optimize this load
      // to undef.
      MD.removeInstruction(L);

      L->replaceAllUsesWith(UndefValue::get(L->getType()));
      toErase.push_back(L);
      deletedLoad = true;
      NumGVNLoad++;
    }
  }

  if (!deletedLoad)
    last = L;
  
  return deletedLoad;
}

Value* GVN::lookupNumber(BasicBlock* BB, uint32_t num) {
  DenseMap<BasicBlock*, ValueNumberScope*>::iterator I = localAvail.find(BB);
  if (I == localAvail.end())
    return 0;
  
  ValueNumberScope* locals = I->second;
  
  while (locals) {
    DenseMap<uint32_t, Value*>::iterator I = locals->table.find(num);
    if (I != locals->table.end())
      return I->second;
    else
      locals = locals->parent;
  }
  
  return 0;
}

/// processInstruction - When calculating availability, handle an instruction
/// by inserting it into the appropriate sets
bool GVN::processInstruction(Instruction *I,
                             DenseMap<Value*, LoadInst*> &lastSeenLoad,
                             SmallVectorImpl<Instruction*> &toErase) {
  if (LoadInst* L = dyn_cast<LoadInst>(I)) {
    bool changed = processLoad(L, lastSeenLoad, toErase);
    
    if (!changed) {
      unsigned num = VN.lookup_or_add(L);
      localAvail[I->getParent()]->table.insert(std::make_pair(num, L));
    }
    
    return changed;
  }
  
  uint32_t nextNum = VN.getNextUnusedValueNumber();
  unsigned num = VN.lookup_or_add(I);
  
  // Allocations are always uniquely numbered, so we can save time and memory
  // by fast failing them.
  if (isa<AllocationInst>(I) || isa<TerminatorInst>(I)) {
    localAvail[I->getParent()]->table.insert(std::make_pair(num, I));
    return false;
  }
  
  // Collapse PHI nodes
  if (PHINode* p = dyn_cast<PHINode>(I)) {
    Value* constVal = CollapsePhi(p);
    
    if (constVal) {
      for (PhiMapType::iterator PI = phiMap.begin(), PE = phiMap.end();
           PI != PE; ++PI)
        if (PI->second.count(p))
          PI->second.erase(p);
        
      p->replaceAllUsesWith(constVal);
      toErase.push_back(p);
    } else {
      localAvail[I->getParent()]->table.insert(std::make_pair(num, I));
    }
  
  // If the number we were assigned was a brand new VN, then we don't
  // need to do a lookup to see if the number already exists
  // somewhere in the domtree: it can't!
  } else if (num == nextNum) {
    localAvail[I->getParent()]->table.insert(std::make_pair(num, I));
    
  // Perform value-number based elimination
  } else if (Value* repl = lookupNumber(I->getParent(), num)) {
    // Remove it!
    MemoryDependenceAnalysis& MD = getAnalysis<MemoryDependenceAnalysis>();
    MD.removeInstruction(I);
    
    VN.erase(I);
    I->replaceAllUsesWith(repl);
    toErase.push_back(I);
    return true;
  } else {
    localAvail[I->getParent()]->table.insert(std::make_pair(num, I));
  }
  
  return false;
}

// GVN::runOnFunction - This is the main transformation entry point for a
// function.
//
bool GVN::runOnFunction(Function& F) {
  VN.setAliasAnalysis(&getAnalysis<AliasAnalysis>());
  VN.setMemDep(&getAnalysis<MemoryDependenceAnalysis>());
  VN.setDomTree(&getAnalysis<DominatorTree>());
  
  bool changed = false;
  bool shouldContinue = true;
  
  while (shouldContinue) {
    shouldContinue = iterateOnFunction(F);
    changed |= shouldContinue;
  }
  
  return changed;
}


bool GVN::processBlock(DomTreeNode* DTN) {
  BasicBlock* BB = DTN->getBlock();

  SmallVector<Instruction*, 8> toErase;
  DenseMap<Value*, LoadInst*> lastSeenLoad;
  bool changed_function = false;
  
  if (DTN->getIDom())
    localAvail[BB] =
                  new ValueNumberScope(localAvail[DTN->getIDom()->getBlock()]);
  else
    localAvail[BB] = new ValueNumberScope(0);
  
  for (BasicBlock::iterator BI = BB->begin(), BE = BB->end();
       BI != BE;) {
    changed_function |= processInstruction(BI, lastSeenLoad, toErase);
    if (toErase.empty()) {
      ++BI;
      continue;
    }
    
    // If we need some instructions deleted, do it now.
    NumGVNInstr += toErase.size();
    
    // Avoid iterator invalidation.
    bool AtStart = BI == BB->begin();
    if (!AtStart)
      --BI;

    for (SmallVector<Instruction*, 4>::iterator I = toErase.begin(),
         E = toErase.end(); I != E; ++I)
      (*I)->eraseFromParent();

    if (AtStart)
      BI = BB->begin();
    else
      ++BI;
    
    toErase.clear();
  }
  
  return changed_function;
}

/// performPRE - Perform a purely local form of PRE that looks for diamond
/// control flow patterns and attempts to perform simple PRE at the join point.
bool GVN::performPRE(Function& F) {
  bool changed = false;
  SmallVector<std::pair<TerminatorInst*, unsigned>, 4> toSplit;
  for (df_iterator<BasicBlock*> DI = df_begin(&F.getEntryBlock()),
       DE = df_end(&F.getEntryBlock()); DI != DE; ++DI) {
    BasicBlock* CurrentBlock = *DI;
    
    // Nothing to PRE in the entry block.
    if (CurrentBlock == &F.getEntryBlock()) continue;
    
    for (BasicBlock::iterator BI = CurrentBlock->begin(),
         BE = CurrentBlock->end(); BI != BE; ) {
      if (isa<AllocationInst>(BI) || isa<TerminatorInst>(BI) ||
          isa<PHINode>(BI) || BI->mayReadFromMemory() ||
          BI->mayWriteToMemory()) {
        BI++;
        continue;
      }
      
      uint32_t valno = VN.lookup(BI);
      
      // Look for the predecessors for PRE opportunities.  We're
      // only trying to solve the basic diamond case, where
      // a value is computed in the successor and one predecessor,
      // but not the other.  We also explicitly disallow cases
      // where the successor is its own predecessor, because they're
      // more complicated to get right.
      unsigned numWith = 0;
      unsigned numWithout = 0;
      BasicBlock* PREPred = 0;
      DenseMap<BasicBlock*, Value*> predMap;
      for (pred_iterator PI = pred_begin(CurrentBlock),
           PE = pred_end(CurrentBlock); PI != PE; ++PI) {
        // We're not interested in PRE where the block is its
        // own predecessor, on in blocks with predecessors
        // that are not reachable.
        if (*PI == CurrentBlock) {
          numWithout = 2;
          break;
        } else if (!localAvail.count(*PI))  {
          numWithout = 2;
          break;
        }
        
        DenseMap<uint32_t, Value*>::iterator predV = 
                                            localAvail[*PI]->table.find(valno);
        if (predV == localAvail[*PI]->table.end()) {
          PREPred = *PI;
          numWithout++;
        } else if (predV->second == BI) {
          numWithout = 2;
        } else {
          predMap[*PI] = predV->second;
          numWith++;
        }
      }
      
      // Don't do PRE when it might increase code size, i.e. when
      // we would need to insert instructions in more than one pred.
      if (numWithout != 1 || numWith == 0) {
        BI++;
        continue;
      }
      
      // We can't do PRE safely on a critical edge, so instead we schedule
      // the edge to be split and perform the PRE the next time we iterate
      // on the function.
      unsigned succNum = 0;
      for (unsigned i = 0, e = PREPred->getTerminator()->getNumSuccessors();
           i != e; ++i)
        if (PREPred->getTerminator()->getSuccessor(i) == PREPred) {
          succNum = i;
          break;
        }
        
      if (isCriticalEdge(PREPred->getTerminator(), succNum)) {
        toSplit.push_back(std::make_pair(PREPred->getTerminator(), succNum));
        changed = true;
        BI++;
        continue;
      }
      
      // Instantiate the expression the in predecessor that lacked it.
      // Because we are going top-down through the block, all value numbers
      // will be available in the predecessor by the time we need them.  Any
      // that weren't original present will have been instantiated earlier
      // in this loop.
      Instruction* PREInstr = BI->clone();
      bool success = true;
      for (unsigned i = 0; i < BI->getNumOperands(); ++i) {
        Value* op = BI->getOperand(i);
        if (isa<Argument>(op) || isa<Constant>(op) || isa<GlobalValue>(op))
          PREInstr->setOperand(i, op);
        else {
          Value* V = lookupNumber(PREPred, VN.lookup(op));
          if (!V) {
            success = false;
            break;
          } else
            PREInstr->setOperand(i, V);
        }
      }
      
      // Fail out if we encounter an operand that is not available in
      // the PRE predecessor.  This is typically because of loads which 
      // are not value numbered precisely.
      if (!success) {
        delete PREInstr;
        BI++;
        continue;
      }
      
      PREInstr->insertBefore(PREPred->getTerminator());
      PREInstr->setName(BI->getName() + ".pre");
      predMap[PREPred] = PREInstr;
      VN.add(PREInstr, valno);
      NumGVNPRE++;
      
      // Update the availability map to include the new instruction.
      localAvail[PREPred]->table.insert(std::make_pair(valno, PREInstr));
      
      // Create a PHI to make the value available in this block.
      PHINode* Phi = PHINode::Create(BI->getType(),
                                     BI->getName() + ".pre-phi",
                                     CurrentBlock->begin());
      for (pred_iterator PI = pred_begin(CurrentBlock),
           PE = pred_end(CurrentBlock); PI != PE; ++PI)
        Phi->addIncoming(predMap[*PI], *PI);
      
      VN.add(Phi, valno);
      localAvail[CurrentBlock]->table[valno] = Phi;
      
      BI->replaceAllUsesWith(Phi);
      VN.erase(BI);
      
      Instruction* erase = BI;
      BI++;
      erase->eraseFromParent();
      
      changed = true;
    }
  }
  
  for (SmallVector<std::pair<TerminatorInst*, unsigned>, 4>::iterator
       I = toSplit.begin(), E = toSplit.end(); I != E; ++I)
    SplitCriticalEdge(I->first, I->second, this);
  
  return changed;
}

// mergeBlockIntoPredecessor - If this block is the only successor
// of its predecessor, and the edge is non-critical, 
// fold it into that predecessor.
bool GVN::mergeBlockIntoPredecessor(BasicBlock* BB) {
  // Can't merge the entry block.
  if (pred_begin(BB) == pred_end(BB)) return false;
  // Can't merge if there are multiple preds.
  if (++pred_begin(BB) != pred_end(BB)) return false;
  
  BasicBlock* PredBB = *pred_begin(BB);
  
  // Can't merge if the edge is critical.
  if (PredBB->getTerminator()->getNumSuccessors() != 1) return false;
  
  // Begin by getting rid of unneeded PHIs.
  while (PHINode *PN = dyn_cast<PHINode>(&BB->front())) {
    PN->replaceAllUsesWith(PN->getIncomingValue(0));
    BB->getInstList().pop_front();  // Delete the phi node...
  }
  
  // Delete the unconditional branch from the predecessor...
  PredBB->getInstList().pop_back();
  
  // Move all definitions in the successor to the predecessor...
  PredBB->getInstList().splice(PredBB->end(), BB->getInstList());
  
  // Make all PHI nodes that referred to BB now refer to Pred as their
  // source...
  BB->replaceAllUsesWith(PredBB);
  
  // Finally, erase the old block and update dominator info.
  DominatorTree& DT = getAnalysis<DominatorTree>();
  DomTreeNode* DTN = DT[BB];
  DomTreeNode* PredDTN = DT[PredBB];
  
  if (DTN) {
    SmallPtrSet<DomTreeNode*, 8> Children(DTN->begin(), DTN->end());
    for (SmallPtrSet<DomTreeNode*, 8>::iterator DI = Children.begin(),
         DE = Children.end(); DI != DE; ++DI)
      DT.changeImmediateDominator(*DI, PredDTN);

    DT.eraseNode(BB);
  }
  
  BB->eraseFromParent();
  
  NumGVNBlocks++;
  return true;
}

// iterateOnFunction - Executes one iteration of GVN
bool GVN::iterateOnFunction(Function &F) {
  // Clean out global sets from any previous functions
  VN.clear();
  phiMap.clear();
  
  // Merge unconditional branches, allowing PRE to catch more
  // optimization opportunities.
  bool mergedBlocks = false;
  for (Function::iterator FI = F.begin(), FE = F.end(); FI != FE; ) {
    BasicBlock* BB = FI;
    ++FI;
    mergedBlocks |= mergeBlockIntoPredecessor(BB);
  }
  
  for (DenseMap<BasicBlock*, ValueNumberScope*>::iterator
       I = localAvail.begin(), E = localAvail.end(); I != E; ++I)
    delete I->second;
  localAvail.clear();
  
  DominatorTree &DT = getAnalysis<DominatorTree>();   

  // Top-down walk of the dominator tree
  bool changed = false;
  for (df_iterator<DomTreeNode*> DI = df_begin(DT.getRootNode()),
       DE = df_end(DT.getRootNode()); DI != DE; ++DI)
    changed |= processBlock(*DI);
  
  if (EnablePRE)
    changed |= performPRE(F);
  
  return changed || mergedBlocks;
}