SCCP.cpp

      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) {
  if (ValueState[&I].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.
    
    if (State.isOverdefined())
      return markOverdefined(&I);

    assert(State.isConstant() && "Unknown state!");
    Operands.push_back(State.getConstant());
  }

  Constant *Ptr = Operands[0];
  ArrayRef<Constant *> Indices(Operands.begin() + 1, Operands.end());
  markConstant(&I, ConstantExpr::getGetElementPtr(Ptr, Indices));
}

void SCCPSolver::visitStoreInst(StoreInst &SI) {
  // If this store is of a struct, ignore it.
  if (SI.getOperand(0)->getType()->isStructTy())
    return;
  
  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, then merge it.
  mergeInValue(I->second, GV, getValueState(SI.getOperand(0)));
  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) {
  // If this load is of a struct, just mark the result overdefined.
  if (I.getType()->isStructTy())
    return markAnythingOverdefined(&I);
  
  LatticeVal PtrVal = getValueState(I.getOperand(0));
  if (PtrVal.isUndefined()) return;   // The pointer is not resolved yet!
  
  LatticeVal &IV = ValueState[&I];
  if (IV.isOverdefined()) return;

  if (!PtrVal.isConstant() || I.isVolatile())
    return markOverdefined(IV, &I);
    
  Constant *Ptr = PtrVal.getConstant();

  // load null -> null
  if (isa<ConstantPointerNull>(Ptr) && I.getPointerAddressSpace() == 0)
    return markConstant(IV, &I, Constant::getNullValue(I.getType()));
  
  // Transform load (constant global) into the value loaded.
  if (GlobalVariable *GV = dyn_cast<GlobalVariable>(Ptr)) {
    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 from a constant into a constant if possible.
  if (Constant *C = ConstantFoldLoadFromConstPtr(Ptr, TD))
    return markConstant(IV, &I, C);

  // 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->isDeclaration()) {
CallOverdefined:
    // Void return and not tracking callee, just bail.
    if (I->getType()->isVoidTy()) return;
    
    // Otherwise, if we have a single return value case, and if the function is
    // a declaration, maybe we can constant fold it.
    if (F && F->isDeclaration() && !I->getType()->isStructTy() &&
        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.
        if (State.isOverdefined())
          return markOverdefined(I);
        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))
        return markConstant(I, C);
    }

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

  // If this is a local function that doesn't have its address taken, mark its
  // entry block executable and merge in the actual arguments to the call into
  // the formal arguments of the function.
  if (!TrackingIncomingArguments.empty() && TrackingIncomingArguments.count(F)){
    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) {
      // If this argument is byval, and if the function is not readonly, there
      // will be an implicit copy formed of the input aggregate.
      if (AI->hasByValAttr() && !F->onlyReadsMemory()) {
        markOverdefined(AI);
        continue;
      }
      
      if (StructType *STy = dyn_cast<StructType>(AI->getType())) {
        for (unsigned i = 0, e = STy->getNumElements(); i != e; ++i) {
          LatticeVal CallArg = getStructValueState(*CAI, i);
          mergeInValue(getStructValueState(AI, i), AI, CallArg);
        }
      } else {
        mergeInValue(AI, getValueState(*CAI));
      }
    }
  }
  
  // If this is a single/zero retval case, see if we're tracking the function.
  if (StructType *STy = dyn_cast<StructType>(F->getReturnType())) {
    if (!MRVFunctionsTracked.count(F))
      goto CallOverdefined;  // Not tracking this callee.
    
    // If we are tracking this callee, propagate the result of the function
    // into this call site.
    for (unsigned i = 0, e = STy->getNumElements(); i != e; ++i)
      mergeInValue(getStructValueState(I, i), I, 
                   TrackedMultipleRetVals[std::make_pair(F, i)]);
  } else {
    DenseMap<Function*, LatticeVal>::iterator TFRVI = TrackedRetVals.find(F);
    if (TFRVI == TrackedRetVals.end())
      goto CallOverdefined;  // Not tracking this callee.
      
    // If so, propagate the return value of the callee into this call result.
    mergeInValue(I, TFRVI->second);
  }
}

void SCCPSolver::Solve() {
  // Process the work lists until they are empty!
  while (!BBWorkList.empty() || !InstWorkList.empty() ||
         !OverdefinedInstWorkList.empty()) {
    // Process the overdefined instruction's work list first, which drives other
    // things to overdefined more quickly.
    while (!OverdefinedInstWorkList.empty()) {
      Value *I = OverdefinedInstWorkList.pop_back_val();

      DEBUG(dbgs() << "\nPopped off OI-WL: " << *I << '\n');

      // "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)
        if (Instruction *I = dyn_cast<Instruction>(*UI))
          OperandChangedState(I);
    }
    
    // Process the instruction work list.
    while (!InstWorkList.empty()) {
      Value *I = InstWorkList.pop_back_val();

      DEBUG(dbgs() << "\nPopped off I-WL: " << *I << '\n');

      // "I" got into the work list because it made the transition from undef 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 (I->getType()->isStructTy() || !getValueState(I).isOverdefined())
        for (Value::use_iterator UI = I->use_begin(), E = I->use_end();
             UI != E; ++UI)
          if (Instruction *I = dyn_cast<Instruction>(*UI))
            OperandChangedState(I);
    }

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

      DEBUG(dbgs() << "\nPopped off BBWL: " << *BB << '\n');

      // 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()->isVoidTy()) continue;
      
      if (StructType *STy = dyn_cast<StructType>(I->getType())) {
        // Only a few things that can be structs matter for undef.

        // Tracked calls must never be marked overdefined in ResolvedUndefsIn.
        if (CallSite CS = CallSite(I))
          if (Function *F = CS.getCalledFunction())
            if (MRVFunctionsTracked.count(F))
              continue;

        // extractvalue and insertvalue don't need to be marked; they are
        // tracked as precisely as their operands. 
        if (isa<ExtractValueInst>(I) || isa<InsertValueInst>(I))
          continue;

        // Send the results of everything else to overdefined.  We could be
        // more precise than this but it isn't worth bothering.
        for (unsigned i = 0, e = STy->getNumElements(); i != e; ++i) {
          LatticeVal &LV = getStructValueState(I, i);
          if (LV.isUndefined())
            markOverdefined(LV, I);
        }
        continue;
      }

      LatticeVal &LV = getValueState(I);
      if (!LV.isUndefined()) continue;

      // extractvalue is safe; check here because the argument is a struct.
      if (isa<ExtractValueInst>(I))
        continue;

      // Compute the operand LatticeVals, for convenience below.
      // Anything taking a struct is conservatively assumed to require
      // overdefined markings.
      if (I->getOperand(0)->getType()->isStructTy()) {
        markOverdefined(I);
        return true;
      }
      LatticeVal Op0LV = getValueState(I->getOperand(0));
      LatticeVal Op1LV;
      if (I->getNumOperands() == 2) {
        if (I->getOperand(1)->getType()->isStructTy()) {
          markOverdefined(I);
          return true;
        }

        Op1LV = getValueState(I->getOperand(1));
      }
      // If this is an instructions whose result is defined even if the input is
      // not fully defined, propagate the information.
      Type *ITy = I->getType();
      switch (I->getOpcode()) {
      case Instruction::Add:
      case Instruction::Sub:
      case Instruction::Trunc:
      case Instruction::FPTrunc:
      case Instruction::BitCast:
        break; // Any undef -> undef
      case Instruction::FSub:
      case Instruction::FAdd:
      case Instruction::FMul:
      case Instruction::FDiv:
      case Instruction::FRem:
        // Floating-point binary operation: be conservative.
        if (Op0LV.isUndefined() && Op1LV.isUndefined())
          markForcedConstant(I, Constant::getNullValue(ITy));
        else
          markOverdefined(I);
        return true;
      case Instruction::ZExt:
      case Instruction::SExt:
      case Instruction::FPToUI:
      case Instruction::FPToSI:
      case Instruction::FPExt:
      case Instruction::PtrToInt:
      case Instruction::IntToPtr:
      case Instruction::SIToFP:
      case Instruction::UIToFP:
        // undef -> 0; some outputs are impossible
        markForcedConstant(I, Constant::getNullValue(ITy));
        return true;
      case Instruction::Mul:
      case Instruction::And:
        // Both operands undef -> undef
        if (Op0LV.isUndefined() && Op1LV.isUndefined())
          break;
        // undef * X -> 0.   X could be zero.
        // undef & X -> 0.   X could be zero.
        markForcedConstant(I, Constant::getNullValue(ITy));
        return true;

      case Instruction::Or:
        // Both operands undef -> undef
        if (Op0LV.isUndefined() && Op1LV.isUndefined())
          break;
        // undef | X -> -1.   X could be -1.
        markForcedConstant(I, Constant::getAllOnesValue(ITy));
        return true;

      case Instruction::Xor:
        // undef ^ undef -> 0; strictly speaking, this is not strictly
        // necessary, but we try to be nice to people who expect this
        // behavior in simple cases
        if (Op0LV.isUndefined() && Op1LV.isUndefined()) {
          markForcedConstant(I, Constant::getNullValue(ITy));
          return true;
        }
        // undef ^ X -> undef
        break;

      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(I, Constant::getNullValue(ITy));
        return true;
        
      case Instruction::AShr:
        // X >>a undef -> undef.
        if (Op1LV.isUndefined()) break;

        // undef >>a X -> all ones
        markForcedConstant(I, Constant::getAllOnesValue(ITy));
        return true;
      case Instruction::LShr:
      case Instruction::Shl:
        // X << undef -> undef.
        // X >> undef -> undef.
        if (Op1LV.isUndefined()) break;

        // undef << X -> 0
        // undef >> X -> 0
        markForcedConstant(I, Constant::getNullValue(ITy));
        return true;
      case Instruction::Select:
        Op1LV = getValueState(I->getOperand(1));
        // 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(I, Op1LV.getConstant());
        else
          markOverdefined(I);
        return true;
      case Instruction::Load:
        // A load here means one of two things: a load of undef from a global,
        // a load from an unknown pointer.  Either way, having it return undef
        // is okay.
        break;
      case Instruction::ICmp:
        // X == undef -> undef.  Other comparisons get more complicated.
        if (cast<ICmpInst>(I)->isEquality())
          break;
        markOverdefined(I);
        return true;
      case Instruction::Call:
      case Instruction::Invoke: {
        // There are two reasons a call can have an undef result
        // 1. It could be tracked.
        // 2. It could be constant-foldable.
        // Because of the way we solve return values, tracked calls must
        // never be marked overdefined in ResolvedUndefsIn.
        if (Function *F = CallSite(I).getCalledFunction())
          if (TrackedRetVals.count(F))
            break;

        // If the call is constant-foldable, we mark it overdefined because
        // we do not know what return values are valid.
        markOverdefined(I);
        return true;
      }
      default:
        // If we don't know what should happen here, conservatively mark it
        // overdefined.
        markOverdefined(I);
        return true;
      }
    }
  
    // Check to see if we have a branch or switch on an undefined value.  If so
    // we force the branch to go one way or the other to make the successor
    // values live.  It doesn't really matter which way we force it.
    TerminatorInst *TI = BB->getTerminator();
    if (BranchInst *BI = dyn_cast<BranchInst>(TI)) {
      if (!BI->isConditional()) continue;
      if (!getValueState(BI->getCondition()).isUndefined())
        continue;
    
      // If the input to SCCP is actually branch on undef, fix the undef to
      // false.
      if (isa<UndefValue>(BI->getCondition())) {
        BI->setCondition(ConstantInt::getFalse(BI->getContext()));
        markEdgeExecutable(BB, TI->getSuccessor(1));
        return true;
      }
      
      // Otherwise, it is a branch on a symbolic value which is currently
      // considered to be undef.  Handle this by forcing the input value to the
      // branch to false.
      markForcedConstant(BI->getCondition(),
                         ConstantInt::getFalse(TI->getContext()));
      return true;
    }
    
    if (SwitchInst *SI = dyn_cast<SwitchInst>(TI)) {
      if (SI->getNumSuccessors() < 2)   // no cases
        continue;
      if (!getValueState(SI->getCondition()).isUndefined())
        continue;
      
      // If the input to SCCP is actually switch on undef, fix the undef to
      // the first constant.
      if (isa<UndefValue>(SI->getCondition())) {
        SI->setCondition(SI->getCaseValue(1));
        markEdgeExecutable(BB, TI->getSuccessor(1));
        return true;
      }
      
      markForcedConstant(SI->getCondition(), 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 SCCP : public FunctionPass {
    static char ID; // Pass identification, replacement for typeid
    SCCP() : FunctionPass(ID) {
      initializeSCCPPass(*PassRegistry::getPassRegistry());
    }

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

char SCCP::ID = 0;
INITIALIZE_PASS(SCCP, "sccp",
                "Sparse Conditional Constant Propagation", false, false)

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

static void DeleteInstructionInBlock(BasicBlock *BB) {
  DEBUG(dbgs() << "  BasicBlock Dead:" << *BB);
  ++NumDeadBlocks;

  // Check to see if there are non-terminating instructions to delete.
  if (isa<TerminatorInst>(BB->begin()))
    return;

  // Delete the instructions backwards, as it has a reduced likelihood of having
  // to update as many def-use and use-def chains.
  Instruction *EndInst = BB->getTerminator(); // Last not to be deleted.
  while (EndInst != BB->begin()) {
    // Delete the next to last instruction.
    BasicBlock::iterator I = EndInst;
    Instruction *Inst = --I;
    if (!Inst->use_empty())
      Inst->replaceAllUsesWith(UndefValue::get(Inst->getType()));
    if (isa<LandingPadInst>(Inst)) {
      EndInst = Inst;
      continue;
    }
    BB->getInstList().erase(Inst);
    ++NumInstRemoved;
  }
}

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

  // 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.markAnythingOverdefined(AI);

  // Solve for constants.
  bool ResolvedUndefs = true;
  while (ResolvedUndefs) {
    Solver.Solve();
    DEBUG(dbgs() << "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.

  for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
    if (!Solver.isBlockExecutable(BB)) {
      DeleteInstructionInBlock(BB);
      MadeChanges = true;
      continue;
    }
  
    // 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()->isVoidTy() || isa<TerminatorInst>(Inst))
        continue;
      
      // TODO: Reconstruct structs from their elements.
      if (Inst->getType()->isStructTy())
        continue;
      
      LatticeVal IV = Solver.getLatticeValueFor(Inst);
      if (IV.isOverdefined())
        continue;
      
      Constant *Const = IV.isConstant()
        ? IV.getConstant() : UndefValue::get(Inst->getType());
      DEBUG(dbgs() << "  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 IPSCCP : public ModulePass {
    static char ID;
    IPSCCP() : ModulePass(ID) {
      initializeIPSCCPPass(*PassRegistry::getPassRegistry());
    }
    bool runOnModule(Module &M);
  };
} // end anonymous namespace

char IPSCCP::ID = 0;
INITIALIZE_PASS(IPSCCP, "ipsccp",
                "Interprocedural Sparse Conditional Constant Propagation",
                false, false)

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


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

  for (Value::const_use_iterator UI = GV->use_begin(), E = GV->use_end();
       UI != E; ++UI) {
    const User *U = *UI;
    if (const StoreInst *SI = dyn_cast<StoreInst>(U)) {
      if (SI->getOperand(0) == GV || SI->isVolatile())
        return true;  // Storing addr of GV.
    } else if (isa<InvokeInst>(U) || isa<CallInst>(U)) {
      // Make sure we are calling the function, not passing the address.
      ImmutableCallSite CS(cast<Instruction>(U));
      if (!CS.isCallee(UI))
        return true;
    } else if (const LoadInst *LI = dyn_cast<LoadInst>(U)) {
      if (LI->isVolatile())
        return true;
    } else if (isa<BlockAddress>(U)) {
      // blockaddress doesn't take the address of the function, it takes addr
      // of label.
    } else {
      return true;
    }
  }
  return false;
}

bool IPSCCP::runOnModule(Module &M) {
  SCCPSolver Solver(getAnalysisIfAvailable<TargetData>());

  // AddressTakenFunctions - This set keeps track of the address-taken functions
  // that are in the input.  As IPSCCP runs through and simplifies code,
  // functions that were address taken can end up losing their
  // address-taken-ness.  Because of this, we keep track of their addresses from
  // the first pass so we can use them for the later simplification pass.
  SmallPtrSet<Function*, 32> AddressTakenFunctions;
  
  // 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->isDeclaration())
      continue;
    
    // If this is a strong or ODR definition of this function, then we can
    // propagate information about its result into callsites of it.
    if (!F->mayBeOverridden())
      Solver.AddTrackedFunction(F);
    
    // If this function only has direct calls that we can see, we can track its
    // arguments and return value aggressively, and can assume it is not called
    // unless we see evidence to the contrary.
    if (F->hasLocalLinkage()) {
      if (AddressIsTaken(F))
        AddressTakenFunctions.insert(F);
      else {
        Solver.AddArgumentTrackedFunction(F);
        continue;
      }
    }

    // Assume the function is called.
    Solver.MarkBlockExecutable(F->begin());
    
    // Assume nothing about the incoming arguments.
    for (Function::arg_iterator AI = F->arg_begin(), E = F->arg_end();
         AI != E; ++AI)
      Solver.markAnythingOverdefined(AI);
  }

  // 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->hasLocalLinkage() && !AddressIsTaken(G))
      Solver.TrackValueOfGlobalVariable(G);

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

    DEBUG(dbgs() << "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<BasicBlock*, 512> BlocksToErase;

  for (Module::iterator F = M.begin(), E = M.end(); F != E; ++F) {
    if (Solver.isBlockExecutable(F->begin())) {
      for (Function::arg_iterator AI = F->arg_begin(), E = F->arg_end();
           AI != E; ++AI) {
        if (AI->use_empty() || AI->getType()->isStructTy()) continue;
        
        // TODO: Could use getStructLatticeValueFor to find out if the entire
        // result is a constant and replace it entirely if so.

        LatticeVal IV = Solver.getLatticeValueFor(AI);
        if (IV.isOverdefined()) continue;
        
        Constant *CST = IV.isConstant() ?
        IV.getConstant() : UndefValue::get(AI->getType());
        DEBUG(dbgs() << "***  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)) {
        DeleteInstructionInBlock(BB);
        MadeChanges = true;

        TerminatorInst *TI = BB->getTerminator();
        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()));
        TI->eraseFromParent();

        if (&*BB != &F->front())
          BlocksToErase.push_back(BB);
        else
          new UnreachableInst(M.getContext(), BB);
        continue;
      }
      
      for (BasicBlock::iterator BI = BB->begin(), E = BB->end(); BI != E; ) {
        Instruction *Inst = BI++;
        if (Inst->getType()->isVoidTy() || Inst->getType()->isStructTy())
          continue;
        
        // TODO: Could use getStructLatticeValueFor to find out if the entire
        // result is a constant and replace it entirely if so.
        
        LatticeVal IV = Solver.getLatticeValueFor(Inst);
        if (IV.isOverdefined())
          continue;
        
        Constant *Const = IV.isConstant()
          ? IV.getConstant() : UndefValue::get(Inst->getType());
        DEBUG(dbgs() << "  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) && !isa<TerminatorInst>(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];
      for (Value::use_iterator UI = DeadBB->use_begin(), UE = DeadBB->use_end();
           UI != UE; ) {
        // Grab the user and then increment the iterator early, as the user
        // will be deleted. Step past all adjacent uses from the same user.
        Instruction *I = dyn_cast<Instruction>(*UI);
        do { ++UI; } while (UI != UE && *UI == I);

        // Ignore blockaddress users; BasicBlock's dtor will handle them.
        if (!I) continue;

        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.
#ifndef NDEBUG
          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 {
            llvm_unreachable("Didn't fold away reference to block!");
          }
#endif
          
          // 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.
  //
  // Do this in two stages: first identify the functions we should process, then
  // actually zap their returns.  This is important because we can only do this
  // if the address of the function isn't taken.  In cases where a return is the
  // last use of a function, the order of processing functions would affect
  // whether other functions are optimizable.
  SmallVector<ReturnInst*, 8> ReturnsToZap;
  
  // 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) {
    Function *F = I->first;
    if (I->second.isOverdefined() || F->getReturnType()->isVoidTy())
      continue;
  
    // We can only do this if we know that nothing else can call the function.
    if (!F->hasLocalLinkage() || AddressTakenFunctions.count(F))
      continue;
    
    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)))
          ReturnsToZap.push_back(RI);
  }

  // Zap all returns which we've identified as zap to change.
  for (unsigned i = 0, e = ReturnsToZap.size(); i != e; ++i) {
    Function *F = ReturnsToZap[i]->getParent()->getParent();
    ReturnsToZap[i]->setOperand(0, UndefValue::get(F->getReturnType()));
  }
    
  // If we inferred 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!");
    DEBUG(dbgs() << "Found that GV '" << GV->getName() << "' is constant!\n");
    while (!GV->use_empty()) {
      StoreInst *SI = cast<StoreInst>(GV->use_back());
      SI->eraseFromParent();
    }
    M.getGlobalList().erase(GV);
    ++IPNumGlobalConst;
  }

  return MadeChanges;
}