SCCP.cpp

#if 0
  LatticeVal &ValState = getValueState(I.getOperand(0));
  LatticeVal &IdxState = getValueState(I.getOperand(1));

  if (ValState.isOverdefined() || IdxState.isOverdefined())
    markOverdefined(&I);
  else if(ValState.isConstant() && IdxState.isConstant())
    markConstant(&I, ConstantExpr::getExtractElement(ValState.getConstant(),
                                                     IdxState.getConstant()));
#endif
}

void SCCPSolver::visitInsertElementInst(InsertElementInst &I) {
  // FIXME : SCCP does not handle vectors properly.
  markOverdefined(&I);
  return;
#if 0
  LatticeVal &ValState = getValueState(I.getOperand(0));
  LatticeVal &EltState = getValueState(I.getOperand(1));
  LatticeVal &IdxState = getValueState(I.getOperand(2));

  if (ValState.isOverdefined() || EltState.isOverdefined() ||
      IdxState.isOverdefined())
    markOverdefined(&I);
  else if(ValState.isConstant() && EltState.isConstant() &&
          IdxState.isConstant())
    markConstant(&I, ConstantExpr::getInsertElement(ValState.getConstant(),
                                                    EltState.getConstant(),
                                                    IdxState.getConstant()));
  else if (ValState.isUndefined() && EltState.isConstant() &&
           IdxState.isConstant()) 
    markConstant(&I,ConstantExpr::getInsertElement(UndefValue::get(I.getType()),
                                                   EltState.getConstant(),
                                                   IdxState.getConstant()));
#endif
}

void SCCPSolver::visitShuffleVectorInst(ShuffleVectorInst &I) {
  // FIXME : SCCP does not handle vectors properly.
  markOverdefined(&I);
  return;
#if 0
  LatticeVal &V1State   = getValueState(I.getOperand(0));
  LatticeVal &V2State   = getValueState(I.getOperand(1));
  LatticeVal &MaskState = getValueState(I.getOperand(2));

  if (MaskState.isUndefined() ||
      (V1State.isUndefined() && V2State.isUndefined()))
    return;  // Undefined output if mask or both inputs undefined.
  
  if (V1State.isOverdefined() || V2State.isOverdefined() ||
      MaskState.isOverdefined()) {
    markOverdefined(&I);
  } else {
    // A mix of constant/undef inputs.
    Constant *V1 = V1State.isConstant() ? 
        V1State.getConstant() : UndefValue::get(I.getType());
    Constant *V2 = V2State.isConstant() ? 
        V2State.getConstant() : UndefValue::get(I.getType());
    Constant *Mask = MaskState.isConstant() ? 
      MaskState.getConstant() : UndefValue::get(I.getOperand(2)->getType());
    markConstant(&I, ConstantExpr::getShuffleVector(V1, V2, Mask));
  }
#endif
}

// Handle getelementptr instructions... if all operands are constants then we
// can turn this into a getelementptr ConstantExpr.
//
void SCCPSolver::visitGetElementPtrInst(GetElementPtrInst &I) {
  LatticeVal &IV = ValueState[&I];
  if (IV.isOverdefined()) return;

  SmallVector<Constant*, 8> Operands;
  Operands.reserve(I.getNumOperands());

  for (unsigned i = 0, e = I.getNumOperands(); i != e; ++i) {
    LatticeVal &State = getValueState(I.getOperand(i));
    if (State.isUndefined())
      return;  // Operands are not resolved yet...
    else if (State.isOverdefined()) {
      markOverdefined(IV, &I);
      return;
    }
    assert(State.isConstant() && "Unknown state!");
    Operands.push_back(State.getConstant());
  }

  Constant *Ptr = Operands[0];
  Operands.erase(Operands.begin());  // Erase the pointer from idx list...

  markConstant(IV, &I, ConstantExpr::getGetElementPtr(Ptr, &Operands[0],
                                                      Operands.size()));
}

void SCCPSolver::visitStoreInst(Instruction &SI) {
  if (TrackedGlobals.empty() || !isa<GlobalVariable>(SI.getOperand(1)))
    return;
  GlobalVariable *GV = cast<GlobalVariable>(SI.getOperand(1));
  DenseMap<GlobalVariable*, LatticeVal>::iterator I = TrackedGlobals.find(GV);
  if (I == TrackedGlobals.end() || I->second.isOverdefined()) return;

  // Get the value we are storing into the global.
  LatticeVal &PtrVal = getValueState(SI.getOperand(0));

  mergeInValue(I->second, GV, PtrVal);
  if (I->second.isOverdefined())
    TrackedGlobals.erase(I);      // No need to keep tracking this!
}


// Handle load instructions.  If the operand is a constant pointer to a constant
// global, we can replace the load with the loaded constant value!
void SCCPSolver::visitLoadInst(LoadInst &I) {
  LatticeVal &IV = ValueState[&I];
  if (IV.isOverdefined()) return;

  LatticeVal &PtrVal = getValueState(I.getOperand(0));
  if (PtrVal.isUndefined()) return;   // The pointer is not resolved yet!
  if (PtrVal.isConstant() && !I.isVolatile()) {
    Value *Ptr = PtrVal.getConstant();
    // TODO: Consider a target hook for valid address spaces for this xform.
    if (isa<ConstantPointerNull>(Ptr) && 
        cast<PointerType>(Ptr->getType())->getAddressSpace() == 0) {
      // load null -> null
      markConstant(IV, &I, Constant::getNullValue(I.getType()));
      return;
    }

    // Transform load (constant global) into the value loaded.
    if (GlobalVariable *GV = dyn_cast<GlobalVariable>(Ptr)) {
      if (GV->isConstant()) {
        if (!GV->isDeclaration()) {
          markConstant(IV, &I, GV->getInitializer());
          return;
        }
      } else if (!TrackedGlobals.empty()) {
        // If we are tracking this global, merge in the known value for it.
        DenseMap<GlobalVariable*, LatticeVal>::iterator It =
          TrackedGlobals.find(GV);
        if (It != TrackedGlobals.end()) {
          mergeInValue(IV, &I, It->second);
          return;
        }
      }
    }

    // Transform load (constantexpr_GEP global, 0, ...) into the value loaded.
    if (ConstantExpr *CE = dyn_cast<ConstantExpr>(Ptr))
      if (CE->getOpcode() == Instruction::GetElementPtr)
    if (GlobalVariable *GV = dyn_cast<GlobalVariable>(CE->getOperand(0)))
      if (GV->isConstant() && !GV->isDeclaration())
        if (Constant *V =
             ConstantFoldLoadThroughGEPConstantExpr(GV->getInitializer(), CE)) {
          markConstant(IV, &I, V);
          return;
        }
  }

  // Otherwise we cannot say for certain what value this load will produce.
  // Bail out.
  markOverdefined(IV, &I);
}

void SCCPSolver::visitCallSite(CallSite CS) {
  Function *F = CS.getCalledFunction();
  Instruction *I = CS.getInstruction();
  
  // The common case is that we aren't tracking the callee, either because we
  // are not doing interprocedural analysis or the callee is indirect, or is
  // external.  Handle these cases first.
  if (F == 0 || !F->hasInternalLinkage()) {
CallOverdefined:
    // Void return and not tracking callee, just bail.
    if (I->getType() == Type::VoidTy) return;
    
    // Otherwise, if we have a single return value case, and if the function is
    // a declaration, maybe we can constant fold it.
    if (!isa<StructType>(I->getType()) && F && F->isDeclaration() && 
        canConstantFoldCallTo(F)) {
      
      SmallVector<Constant*, 8> Operands;
      for (CallSite::arg_iterator AI = CS.arg_begin(), E = CS.arg_end();
           AI != E; ++AI) {
        LatticeVal &State = getValueState(*AI);
        if (State.isUndefined())
          return;  // Operands are not resolved yet.
        else if (State.isOverdefined()) {
          markOverdefined(I);
          return;
        }
        assert(State.isConstant() && "Unknown state!");
        Operands.push_back(State.getConstant());
      }
     
      // If we can constant fold this, mark the result of the call as a
      // constant.
      if (Constant *C = ConstantFoldCall(F, &Operands[0], Operands.size())) {
        markConstant(I, C);
        return;
      }
    }

    // Otherwise, we don't know anything about this call, mark it overdefined.
    markOverdefined(I);
    return;
  }

  // If this is a single/zero retval case, see if we're tracking the function.
  DenseMap<Function*, LatticeVal>::iterator TFRVI = TrackedRetVals.find(F);
  if (TFRVI != TrackedRetVals.end()) {
    // If so, propagate the return value of the callee into this call result.
    mergeInValue(I, TFRVI->second);
  } else if (isa<StructType>(I->getType())) {
    // Check to see if we're tracking this callee, if not, handle it in the
    // common path above.
    DenseMap<std::pair<Function*, unsigned>, LatticeVal>::iterator
    TMRVI = TrackedMultipleRetVals.find(std::make_pair(F, 0));
    if (TMRVI == TrackedMultipleRetVals.end())
      goto CallOverdefined;
    
    // If we are tracking this callee, propagate the return values of the call
    // into this call site.  We do this by walking all the uses. Single-index
    // ExtractValueInst uses can be tracked; anything more complicated is
    // currently handled conservatively.
    for (Value::use_iterator UI = I->use_begin(), E = I->use_end();
         UI != E; ++UI) {
      if (ExtractValueInst *EVI = dyn_cast<ExtractValueInst>(*UI)) {
        if (EVI->getNumIndices() == 1) {
          mergeInValue(EVI, 
                  TrackedMultipleRetVals[std::make_pair(F, *EVI->idx_begin())]);
          continue;
        }
      }
      // The aggregate value is used in a way not handled here. Assume nothing.
      markOverdefined(*UI);
    }
  } else {
    // Otherwise we're not tracking this callee, so handle it in the
    // common path above.
    goto CallOverdefined;
  }
   
  // Finally, if this is the first call to the function hit, mark its entry
  // block executable.
  if (!BBExecutable.count(F->begin()))
    MarkBlockExecutable(F->begin());
  
  // Propagate information from this call site into the callee.
  CallSite::arg_iterator CAI = CS.arg_begin();
  for (Function::arg_iterator AI = F->arg_begin(), E = F->arg_end();
       AI != E; ++AI, ++CAI) {
    LatticeVal &IV = ValueState[AI];
    if (!IV.isOverdefined())
      mergeInValue(IV, AI, getValueState(*CAI));
  }
}


void SCCPSolver::Solve() {
  // Process the work lists until they are empty!
  while (!BBWorkList.empty() || !InstWorkList.empty() ||
         !OverdefinedInstWorkList.empty()) {
    // Process the instruction work list...
    while (!OverdefinedInstWorkList.empty()) {
      Value *I = OverdefinedInstWorkList.back();
      OverdefinedInstWorkList.pop_back();

      DOUT << "\nPopped off OI-WL: " << *I;

      // "I" got into the work list because it either made the transition from
      // bottom to constant
      //
      // Anything on this worklist that is overdefined need not be visited
      // since all of its users will have already been marked as overdefined
      // Update all of the users of this instruction's value...
      //
      for (Value::use_iterator UI = I->use_begin(), E = I->use_end();
           UI != E; ++UI)
        OperandChangedState(*UI);
    }
    // Process the instruction work list...
    while (!InstWorkList.empty()) {
      Value *I = InstWorkList.back();
      InstWorkList.pop_back();

      DOUT << "\nPopped off I-WL: " << *I;

      // "I" got into the work list because it either made the transition from
      // bottom to constant
      //
      // Anything on this worklist that is overdefined need not be visited
      // since all of its users will have already been marked as overdefined.
      // Update all of the users of this instruction's value...
      //
      if (!getValueState(I).isOverdefined())
        for (Value::use_iterator UI = I->use_begin(), E = I->use_end();
             UI != E; ++UI)
          OperandChangedState(*UI);
    }

    // Process the basic block work list...
    while (!BBWorkList.empty()) {
      BasicBlock *BB = BBWorkList.back();
      BBWorkList.pop_back();

      DOUT << "\nPopped off BBWL: " << *BB;

      // Notify all instructions in this basic block that they are newly
      // executable.
      visit(BB);
    }
  }
}

/// ResolvedUndefsIn - While solving the dataflow for a function, we assume
/// that branches on undef values cannot reach any of their successors.
/// However, this is not a safe assumption.  After we solve dataflow, this
/// method should be use to handle this.  If this returns true, the solver
/// should be rerun.
///
/// This method handles this by finding an unresolved branch and marking it one
/// of the edges from the block as being feasible, even though the condition
/// doesn't say it would otherwise be.  This allows SCCP to find the rest of the
/// CFG and only slightly pessimizes the analysis results (by marking one,
/// potentially infeasible, edge feasible).  This cannot usefully modify the
/// constraints on the condition of the branch, as that would impact other users
/// of the value.
///
/// This scan also checks for values that use undefs, whose results are actually
/// defined.  For example, 'zext i8 undef to i32' should produce all zeros
/// conservatively, as "(zext i8 X -> i32) & 0xFF00" must always return zero,
/// even if X isn't defined.
bool SCCPSolver::ResolvedUndefsIn(Function &F) {
  for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
    if (!BBExecutable.count(BB))
      continue;
    
    for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E; ++I) {
      // Look for instructions which produce undef values.
      if (I->getType() == Type::VoidTy) continue;
      
      LatticeVal &LV = getValueState(I);
      if (!LV.isUndefined()) continue;

      // Get the lattice values of the first two operands for use below.
      LatticeVal &Op0LV = getValueState(I->getOperand(0));
      LatticeVal Op1LV;
      if (I->getNumOperands() == 2) {
        // If this is a two-operand instruction, and if both operands are
        // undefs, the result stays undef.
        Op1LV = getValueState(I->getOperand(1));
        if (Op0LV.isUndefined() && Op1LV.isUndefined())
          continue;
      }
      
      // If this is an instructions whose result is defined even if the input is
      // not fully defined, propagate the information.
      const Type *ITy = I->getType();
      switch (I->getOpcode()) {
      default: break;          // Leave the instruction as an undef.
      case Instruction::ZExt:
        // After a zero extend, we know the top part is zero.  SExt doesn't have
        // to be handled here, because we don't know whether the top part is 1's
        // or 0's.
        assert(Op0LV.isUndefined());
        markForcedConstant(LV, I, Constant::getNullValue(ITy));
        return true;
      case Instruction::Mul:
      case Instruction::And:
        // undef * X -> 0.   X could be zero.
        // undef & X -> 0.   X could be zero.
        markForcedConstant(LV, I, Constant::getNullValue(ITy));
        return true;

      case Instruction::Or:
        // undef | X -> -1.   X could be -1.
        if (const VectorType *PTy = dyn_cast<VectorType>(ITy))
          markForcedConstant(LV, I, ConstantVector::getAllOnesValue(PTy));
        else          
          markForcedConstant(LV, I, ConstantInt::getAllOnesValue(ITy));
        return true;

      case Instruction::SDiv:
      case Instruction::UDiv:
      case Instruction::SRem:
      case Instruction::URem:
        // X / undef -> undef.  No change.
        // X % undef -> undef.  No change.
        if (Op1LV.isUndefined()) break;
        
        // undef / X -> 0.   X could be maxint.
        // undef % X -> 0.   X could be 1.
        markForcedConstant(LV, I, Constant::getNullValue(ITy));
        return true;
        
      case Instruction::AShr:
        // undef >>s X -> undef.  No change.
        if (Op0LV.isUndefined()) break;
        
        // X >>s undef -> X.  X could be 0, X could have the high-bit known set.
        if (Op0LV.isConstant())
          markForcedConstant(LV, I, Op0LV.getConstant());
        else
          markOverdefined(LV, I);
        return true;
      case Instruction::LShr:
      case Instruction::Shl:
        // undef >> X -> undef.  No change.
        // undef << X -> undef.  No change.
        if (Op0LV.isUndefined()) break;
        
        // X >> undef -> 0.  X could be 0.
        // X << undef -> 0.  X could be 0.
        markForcedConstant(LV, I, Constant::getNullValue(ITy));
        return true;
      case Instruction::Select:
        // undef ? X : Y  -> X or Y.  There could be commonality between X/Y.
        if (Op0LV.isUndefined()) {
          if (!Op1LV.isConstant())  // Pick the constant one if there is any.
            Op1LV = getValueState(I->getOperand(2));
        } else if (Op1LV.isUndefined()) {
          // c ? undef : undef -> undef.  No change.
          Op1LV = getValueState(I->getOperand(2));
          if (Op1LV.isUndefined())
            break;
          // Otherwise, c ? undef : x -> x.
        } else {
          // Leave Op1LV as Operand(1)'s LatticeValue.
        }
        
        if (Op1LV.isConstant())
          markForcedConstant(LV, I, Op1LV.getConstant());
        else
          markOverdefined(LV, I);
        return true;
      case Instruction::Call:
        // If a call has an undef result, it is because it is constant foldable
        // but one of the inputs was undef.  Just force the result to
        // overdefined.
        markOverdefined(LV, I);
        return true;
      }
    }
  
    TerminatorInst *TI = BB->getTerminator();
    if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
      if (!BI->isConditional()) continue;
      if (!getValueState(BI->getCondition()).isUndefined())
        continue;
    } else if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
      if (SI->getNumSuccessors()<2)   // no cases
        continue;
      if (!getValueState(SI->getCondition()).isUndefined())
        continue;
    } else {
      continue;
    }
    
    // If the edge to the second successor isn't thought to be feasible yet,
    // mark it so now.  We pick the second one so that this goes to some
    // enumerated value in a switch instead of going to the default destination.
    if (KnownFeasibleEdges.count(Edge(BB, TI->getSuccessor(1))))
      continue;
    
    // Otherwise, it isn't already thought to be feasible.  Mark it as such now
    // and return.  This will make other blocks reachable, which will allow new
    // values to be discovered and existing ones to be moved in the lattice.
    markEdgeExecutable(BB, TI->getSuccessor(1));
    
    // This must be a conditional branch of switch on undef.  At this point,
    // force the old terminator to branch to the first successor.  This is
    // required because we are now influencing the dataflow of the function with
    // the assumption that this edge is taken.  If we leave the branch condition
    // as undef, then further analysis could think the undef went another way
    // leading to an inconsistent set of conclusions.
    if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
      BI->setCondition(ConstantInt::getFalse());
    } else {
      SwitchInst *SI = cast<SwitchInst>(TI);
      SI->setCondition(SI->getCaseValue(1));
    }
    
    return true;
  }

  return false;
}


namespace {
  //===--------------------------------------------------------------------===//
  //
  /// SCCP Class - This class uses the SCCPSolver to implement a per-function
  /// Sparse Conditional Constant Propagator.
  ///
  struct VISIBILITY_HIDDEN SCCP : public FunctionPass {
    static char ID; // Pass identification, replacement for typeid
    SCCP() : FunctionPass(&ID) {}

    // runOnFunction - Run the Sparse Conditional Constant Propagation
    // algorithm, and return true if the function was modified.
    //
    bool runOnFunction(Function &F);

    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
      AU.setPreservesCFG();
    }
  };
} // end anonymous namespace

char SCCP::ID = 0;
static RegisterPass<SCCP>
X("sccp", "Sparse Conditional Constant Propagation");

// createSCCPPass - This is the public interface to this file...
FunctionPass *llvm::createSCCPPass() {
  return new SCCP();
}


// runOnFunction() - Run the Sparse Conditional Constant Propagation algorithm,
// and return true if the function was modified.
//
bool SCCP::runOnFunction(Function &F) {
  DOUT << "SCCP on function '" << F.getNameStart() << "'\n";
  SCCPSolver Solver;

  // Mark the first block of the function as being executable.
  Solver.MarkBlockExecutable(F.begin());

  // Mark all arguments to the function as being overdefined.
  for (Function::arg_iterator AI = F.arg_begin(), E = F.arg_end(); AI != E;++AI)
    Solver.markOverdefined(AI);

  // Solve for constants.
  bool ResolvedUndefs = true;
  while (ResolvedUndefs) {
    Solver.Solve();
    DOUT << "RESOLVING UNDEFs\n";
    ResolvedUndefs = Solver.ResolvedUndefsIn(F);
  }

  bool MadeChanges = false;

  // If we decided that there are basic blocks that are dead in this function,
  // delete their contents now.  Note that we cannot actually delete the blocks,
  // as we cannot modify the CFG of the function.
  //
  SmallVector<Instruction*, 512> Insts;
  std::map<Value*, LatticeVal> &Values = Solver.getValueMapping();

  for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
    if (!Solver.isBlockExecutable(BB)) {
      DOUT << "  BasicBlock Dead:" << *BB;
      ++NumDeadBlocks;

      // Delete the instructions backwards, as it has a reduced likelihood of
      // having to update as many def-use and use-def chains.
      for (BasicBlock::iterator I = BB->begin(), E = BB->getTerminator();
           I != E; ++I)
        Insts.push_back(I);
      while (!Insts.empty()) {
        Instruction *I = Insts.back();
        Insts.pop_back();
        if (!I->use_empty())
          I->replaceAllUsesWith(UndefValue::get(I->getType()));
        BB->getInstList().erase(I);
        MadeChanges = true;
        ++NumInstRemoved;
      }
    } else {
      // Iterate over all of the instructions in a function, replacing them with
      // constants if we have found them to be of constant values.
      //
      for (BasicBlock::iterator BI = BB->begin(), E = BB->end(); BI != E; ) {
        Instruction *Inst = BI++;
        if (Inst->getType() == Type::VoidTy ||
            isa<TerminatorInst>(Inst))
          continue;
        
        LatticeVal &IV = Values[Inst];
        if (!IV.isConstant() && !IV.isUndefined())
          continue;
        
        Constant *Const = IV.isConstant()
          ? IV.getConstant() : UndefValue::get(Inst->getType());
        DOUT << "  Constant: " << *Const << " = " << *Inst;

        // Replaces all of the uses of a variable with uses of the constant.
        Inst->replaceAllUsesWith(Const);
        
        // Delete the instruction.
        Inst->eraseFromParent();
        
        // Hey, we just changed something!
        MadeChanges = true;
        ++NumInstRemoved;
      }
    }

  return MadeChanges;
}

namespace {
  //===--------------------------------------------------------------------===//
  //
  /// IPSCCP Class - This class implements interprocedural Sparse Conditional
  /// Constant Propagation.
  ///
  struct VISIBILITY_HIDDEN IPSCCP : public ModulePass {
    static char ID;
    IPSCCP() : ModulePass(&ID) {}
    bool runOnModule(Module &M);
  };
} // end anonymous namespace

char IPSCCP::ID = 0;
static RegisterPass<IPSCCP>
Y("ipsccp", "Interprocedural Sparse Conditional Constant Propagation");

// createIPSCCPPass - This is the public interface to this file...
ModulePass *llvm::createIPSCCPPass() {
  return new IPSCCP();
}


static bool AddressIsTaken(GlobalValue *GV) {
  // Delete any dead constantexpr klingons.
  GV->removeDeadConstantUsers();

  for (Value::use_iterator UI = GV->use_begin(), E = GV->use_end();
       UI != E; ++UI)
    if (StoreInst *SI = dyn_cast<StoreInst>(*UI)) {
      if (SI->getOperand(0) == GV || SI->isVolatile())
        return true;  // Storing addr of GV.
    } else if (isa<InvokeInst>(*UI) || isa<CallInst>(*UI)) {
      // Make sure we are calling the function, not passing the address.
      CallSite CS = CallSite::get(cast<Instruction>(*UI));
      if (CS.hasArgument(GV))
        return true;
    } else if (LoadInst *LI = dyn_cast<LoadInst>(*UI)) {
      if (LI->isVolatile())
        return true;
    } else {
      return true;
    }
  return false;
}

bool IPSCCP::runOnModule(Module &M) {
  SCCPSolver Solver;

  // Loop over all functions, marking arguments to those with their addresses
  // taken or that are external as overdefined.
  //
  for (Module::iterator F = M.begin(), E = M.end(); F != E; ++F)
    if (!F->hasInternalLinkage() || AddressIsTaken(F)) {
      if (!F->isDeclaration())
        Solver.MarkBlockExecutable(F->begin());
      for (Function::arg_iterator AI = F->arg_begin(), E = F->arg_end();
           AI != E; ++AI)
        Solver.markOverdefined(AI);
    } else {
      Solver.AddTrackedFunction(F);
    }

  // Loop over global variables.  We inform the solver about any internal global
  // variables that do not have their 'addresses taken'.  If they don't have
  // their addresses taken, we can propagate constants through them.
  for (Module::global_iterator G = M.global_begin(), E = M.global_end();
       G != E; ++G)
    if (!G->isConstant() && G->hasInternalLinkage() && !AddressIsTaken(G))
      Solver.TrackValueOfGlobalVariable(G);

  // Solve for constants.
  bool ResolvedUndefs = true;
  while (ResolvedUndefs) {
    Solver.Solve();

    DOUT << "RESOLVING UNDEFS\n";
    ResolvedUndefs = false;
    for (Module::iterator F = M.begin(), E = M.end(); F != E; ++F)
      ResolvedUndefs |= Solver.ResolvedUndefsIn(*F);
  }

  bool MadeChanges = false;

  // Iterate over all of the instructions in the module, replacing them with
  // constants if we have found them to be of constant values.
  //
  SmallVector<Instruction*, 512> Insts;
  SmallVector<BasicBlock*, 512> BlocksToErase;
  std::map<Value*, LatticeVal> &Values = Solver.getValueMapping();

  for (Module::iterator F = M.begin(), E = M.end(); F != E; ++F) {
    for (Function::arg_iterator AI = F->arg_begin(), E = F->arg_end();
         AI != E; ++AI)
      if (!AI->use_empty()) {
        LatticeVal &IV = Values[AI];
        if (IV.isConstant() || IV.isUndefined()) {
          Constant *CST = IV.isConstant() ?
            IV.getConstant() : UndefValue::get(AI->getType());
          DOUT << "***  Arg " << *AI << " = " << *CST <<"\n";

          // Replaces all of the uses of a variable with uses of the
          // constant.
          AI->replaceAllUsesWith(CST);
          ++IPNumArgsElimed;
        }
      }

    for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB)
      if (!Solver.isBlockExecutable(BB)) {
        DOUT << "  BasicBlock Dead:" << *BB;
        ++IPNumDeadBlocks;

        // Delete the instructions backwards, as it has a reduced likelihood of
        // having to update as many def-use and use-def chains.
        TerminatorInst *TI = BB->getTerminator();
        for (BasicBlock::iterator I = BB->begin(), E = TI; I != E; ++I)
          Insts.push_back(I);

        while (!Insts.empty()) {
          Instruction *I = Insts.back();
          Insts.pop_back();
          if (!I->use_empty())
            I->replaceAllUsesWith(UndefValue::get(I->getType()));
          BB->getInstList().erase(I);
          MadeChanges = true;
          ++IPNumInstRemoved;
        }

        for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i) {
          BasicBlock *Succ = TI->getSuccessor(i);
          if (!Succ->empty() && isa<PHINode>(Succ->begin()))
            TI->getSuccessor(i)->removePredecessor(BB);
        }
        if (!TI->use_empty())
          TI->replaceAllUsesWith(UndefValue::get(TI->getType()));
        BB->getInstList().erase(TI);

        if (&*BB != &F->front())
          BlocksToErase.push_back(BB);
        else
          new UnreachableInst(BB);

      } else {
        for (BasicBlock::iterator BI = BB->begin(), E = BB->end(); BI != E; ) {
          Instruction *Inst = BI++;
          if (Inst->getType() == Type::VoidTy ||
              isa<TerminatorInst>(Inst))
            continue;
          
          LatticeVal &IV = Values[Inst];
          if (!IV.isConstant() && !IV.isUndefined())
            continue;
          
          Constant *Const = IV.isConstant()
            ? IV.getConstant() : UndefValue::get(Inst->getType());
          DOUT << "  Constant: " << *Const << " = " << *Inst;

          // Replaces all of the uses of a variable with uses of the
          // constant.
          Inst->replaceAllUsesWith(Const);
          
          // Delete the instruction.
          if (!isa<CallInst>(Inst))
            Inst->eraseFromParent();

          // Hey, we just changed something!
          MadeChanges = true;
          ++IPNumInstRemoved;
        }
      }

    // Now that all instructions in the function are constant folded, erase dead
    // blocks, because we can now use ConstantFoldTerminator to get rid of
    // in-edges.
    for (unsigned i = 0, e = BlocksToErase.size(); i != e; ++i) {
      // If there are any PHI nodes in this successor, drop entries for BB now.
      BasicBlock *DeadBB = BlocksToErase[i];
      while (!DeadBB->use_empty()) {
        Instruction *I = cast<Instruction>(DeadBB->use_back());
        bool Folded = ConstantFoldTerminator(I->getParent());
        if (!Folded) {
          // The constant folder may not have been able to fold the terminator
          // if this is a branch or switch on undef.  Fold it manually as a
          // branch to the first successor.
          if (BranchInst *BI = dyn_cast<BranchInst>(I)) {
            assert(BI->isConditional() && isa<UndefValue>(BI->getCondition()) &&
                   "Branch should be foldable!");
          } else if (SwitchInst *SI = dyn_cast<SwitchInst>(I)) {
            assert(isa<UndefValue>(SI->getCondition()) && "Switch should fold");
          } else {
            assert(0 && "Didn't fold away reference to block!");
          }
          
          // Make this an uncond branch to the first successor.
          TerminatorInst *TI = I->getParent()->getTerminator();
          BranchInst::Create(TI->getSuccessor(0), TI);
          
          // Remove entries in successor phi nodes to remove edges.
          for (unsigned i = 1, e = TI->getNumSuccessors(); i != e; ++i)
            TI->getSuccessor(i)->removePredecessor(TI->getParent());
          
          // Remove the old terminator.
          TI->eraseFromParent();
        }
      }

      // Finally, delete the basic block.
      F->getBasicBlockList().erase(DeadBB);
    }
    BlocksToErase.clear();
  }

  // If we inferred constant or undef return values for a function, we replaced
  // all call uses with the inferred value.  This means we don't need to bother
  // actually returning anything from the function.  Replace all return
  // instructions with return undef.
  // TODO: Process multiple value ret instructions also.
  const DenseMap<Function*, LatticeVal> &RV = Solver.getTrackedRetVals();
  for (DenseMap<Function*, LatticeVal>::const_iterator I = RV.begin(),
         E = RV.end(); I != E; ++I)
    if (!I->second.isOverdefined() &&
        I->first->getReturnType() != Type::VoidTy) {
      Function *F = I->first;
      for (Function::iterator BB = F->begin(), E = F->end(); BB != E; ++BB)
        if (ReturnInst *RI = dyn_cast<ReturnInst>(BB->getTerminator()))
          if (!isa<UndefValue>(RI->getOperand(0)))
            RI->setOperand(0, UndefValue::get(F->getReturnType()));
    }

  // If we infered constant or undef values for globals variables, we can delete
  // the global and any stores that remain to it.
  const DenseMap<GlobalVariable*, LatticeVal> &TG = Solver.getTrackedGlobals();
  for (DenseMap<GlobalVariable*, LatticeVal>::const_iterator I = TG.begin(),
         E = TG.end(); I != E; ++I) {
    GlobalVariable *GV = I->first;
    assert(!I->second.isOverdefined() &&
           "Overdefined values should have been taken out of the map!");
    DOUT << "Found that GV '" << GV->getNameStart() << "' is constant!\n";
    while (!GV->use_empty()) {
      StoreInst *SI = cast<StoreInst>(GV->use_back());
      SI->eraseFromParent();
    }
    M.getGlobalList().erase(GV);
    ++IPNumGlobalConst;
  }

  return MadeChanges;
}