llvm/lib/Transforms/IPO/ArgumentPromotion.cpp

//===-- ArgumentPromotion.cpp - Promote by-reference arguments ------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass promotes "by reference" arguments to be "by value" arguments.  In
// practice, this means looking for internal functions that have pointer
// arguments.  If it can prove, through the use of alias analysis, that an
// argument is *only* loaded, then it can pass the value into the function
// instead of the address of the value.  This can cause recursive simplification
// of code and lead to the elimination of allocas (especially in C++ template
// code like the STL).
//
// This pass also handles aggregate arguments that are passed into a function,
// scalarizing them if the elements of the aggregate are only loaded.  Note that
// by default it refuses to scalarize aggregates which would require passing in
// more than three operands to the function, because passing thousands of
// operands for a large array or structure is unprofitable! This limit is can be
// configured or disabled, however.
//
// Note that this transformation could also be done for arguments that are only
// stored to (returning the value instead), but does not currently.  This case
// would be best handled when and if LLVM begins supporting multiple return
// values from functions.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "argpromotion"
#include "llvm/Transforms/IPO.h"
#include "llvm/Constants.h"
#include "llvm/DerivedTypes.h"
#include "llvm/Module.h"
#include "llvm/CallGraphSCCPass.h"
#include "llvm/Instructions.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/CallGraph.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Support/CallSite.h"
#include "llvm/Support/CFG.h"
#include "llvm/Support/Debug.h"
#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/StringExtras.h"
#include "llvm/Support/Compiler.h"
#include <set>
using namespace llvm;

STATISTIC(NumArgumentsPromoted , "Number of pointer arguments promoted");
STATISTIC(NumAggregatesPromoted, "Number of aggregate arguments promoted");
STATISTIC(NumByValArgsPromoted , "Number of byval arguments promoted");
STATISTIC(NumArgumentsDead     , "Number of dead pointer args eliminated");

namespace {
  /// ArgPromotion - The 'by reference' to 'by value' argument promotion pass.
  ///
  struct VISIBILITY_HIDDEN ArgPromotion : public CallGraphSCCPass {
    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
      AU.addRequired<AliasAnalysis>();
      AU.addRequired<TargetData>();
      CallGraphSCCPass::getAnalysisUsage(AU);
    }

    virtual bool runOnSCC(const std::vector<CallGraphNode *> &SCC);
    static char ID; // Pass identification, replacement for typeid
    ArgPromotion(unsigned maxElements = 3) : CallGraphSCCPass((intptr_t)&ID),
                                             maxElements(maxElements) {}
    
    /// A vector used to hold the indices of a single GEP instruction
    typedef std::vector<uint64_t> IndicesVector;

  private:
    bool PromoteArguments(CallGraphNode *CGN);
    bool isSafeToPromoteArgument(Argument *Arg, bool isByVal) const;
    Function *DoPromotion(Function *F, 
                          SmallPtrSet<Argument*, 8> &ArgsToPromote,
                          SmallPtrSet<Argument*, 8> &ByValArgsToTransform);
    /// The maximum number of elements to expand, or 0 for unlimited.
    unsigned maxElements;
  };
}

char ArgPromotion::ID = 0;
static RegisterPass<ArgPromotion>
X("argpromotion", "Promote 'by reference' arguments to scalars");

Pass *llvm::createArgumentPromotionPass(unsigned maxElements) {
  return new ArgPromotion(maxElements);
}

bool ArgPromotion::runOnSCC(const std::vector<CallGraphNode *> &SCC) {
  bool Changed = false, LocalChange;

  do {  // Iterate until we stop promoting from this SCC.
    LocalChange = false;
    // Attempt to promote arguments from all functions in this SCC.
    for (unsigned i = 0, e = SCC.size(); i != e; ++i)
      LocalChange |= PromoteArguments(SCC[i]);
    Changed |= LocalChange;               // Remember that we changed something.
  } while (LocalChange);

  return Changed;
}

/// PromoteArguments - This method checks the specified function to see if there
/// are any promotable arguments and if it is safe to promote the function (for
/// example, all callers are direct).  If safe to promote some arguments, it
/// calls the DoPromotion method.
///
bool ArgPromotion::PromoteArguments(CallGraphNode *CGN) {
  Function *F = CGN->getFunction();

  // Make sure that it is local to this module.
  if (!F || !F->hasInternalLinkage()) return false;

  // First check: see if there are any pointer arguments!  If not, quick exit.
  SmallVector<std::pair<Argument*, unsigned>, 16> PointerArgs;
  unsigned ArgNo = 0;
  for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end();
       I != E; ++I, ++ArgNo)
    if (isa<PointerType>(I->getType()))
      PointerArgs.push_back(std::pair<Argument*, unsigned>(I, ArgNo));
  if (PointerArgs.empty()) return false;

  // Second check: make sure that all callers are direct callers.  We can't
  // transform functions that have indirect callers.
  for (Value::use_iterator UI = F->use_begin(), E = F->use_end();
       UI != E; ++UI) {
    CallSite CS = CallSite::get(*UI);
    if (!CS.getInstruction())       // "Taking the address" of the function
      return false;

    // Ensure that this call site is CALLING the function, not passing it as
    // an argument.
    if (UI.getOperandNo() != 0) 
      return false;
  }

  // Check to see which arguments are promotable.  If an argument is promotable,
  // add it to ArgsToPromote.
  SmallPtrSet<Argument*, 8> ArgsToPromote;
  SmallPtrSet<Argument*, 8> ByValArgsToTransform;
  for (unsigned i = 0; i != PointerArgs.size(); ++i) {
    bool isByVal = F->paramHasAttr(PointerArgs[i].second+1, ParamAttr::ByVal);
    
    // If this is a byval argument, and if the aggregate type is small, just
    // pass the elements, which is always safe.
    Argument *PtrArg = PointerArgs[i].first;
    if (isByVal) {
      const Type *AgTy = cast<PointerType>(PtrArg->getType())->getElementType();
      if (const StructType *STy = dyn_cast<StructType>(AgTy)) {
        if (maxElements > 0 && STy->getNumElements() > maxElements) {
          DOUT << "argpromotion disable promoting argument '"
               << PtrArg->getName() << "' because it would require adding more "
               << "than " << maxElements << " arguments to the function.\n";
        } else {
          // If all the elements are single-value types, we can promote it.
          bool AllSimple = true;
          for (unsigned i = 0, e = STy->getNumElements(); i != e; ++i)
            if (!STy->getElementType(i)->isSingleValueType()) {
              AllSimple = false;
              break;
            }
          
          // Safe to transform, don't even bother trying to "promote" it.
          // Passing the elements as a scalar will allow scalarrepl to hack on
          // the new alloca we introduce.
          if (AllSimple) {
            ByValArgsToTransform.insert(PtrArg);
            continue;
          }
        }
      }
    }
    
    // Otherwise, see if we can promote the pointer to its value.
    if (isSafeToPromoteArgument(PtrArg, isByVal))
      ArgsToPromote.insert(PtrArg);
  }
  
  // No promotable pointer arguments.
  if (ArgsToPromote.empty() && ByValArgsToTransform.empty()) return false;

  Function *NewF = DoPromotion(F, ArgsToPromote, ByValArgsToTransform);

  // Update the call graph to know that the function has been transformed.
  getAnalysis<CallGraph>().changeFunction(F, NewF);
  return true;
}

/// IsAlwaysValidPointer - Return true if the specified pointer is always legal
/// to load.
static bool IsAlwaysValidPointer(Value *V) {
  if (isa<AllocaInst>(V) || isa<GlobalVariable>(V)) return true;
  if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(V))
    return IsAlwaysValidPointer(GEP->getOperand(0));
  if (ConstantExpr *CE = dyn_cast<ConstantExpr>(V))
    if (CE->getOpcode() == Instruction::GetElementPtr)
      return IsAlwaysValidPointer(CE->getOperand(0));

  return false;
}

/// AllCalleesPassInValidPointerForArgument - Return true if we can prove that
/// all callees pass in a valid pointer for the specified function argument.
static bool AllCalleesPassInValidPointerForArgument(Argument *Arg) {
  Function *Callee = Arg->getParent();

  unsigned ArgNo = std::distance(Callee->arg_begin(),
                                 Function::arg_iterator(Arg));

  // Look at all call sites of the function.  At this pointer we know we only
  // have direct callees.
  for (Value::use_iterator UI = Callee->use_begin(), E = Callee->use_end();
       UI != E; ++UI) {
    CallSite CS = CallSite::get(*UI);
    assert(CS.getInstruction() && "Should only have direct calls!");

    if (!IsAlwaysValidPointer(CS.getArgument(ArgNo)))
      return false;
  }
  return true;
}

/// Returns true if Prefix is a prefix of longer. That means, Longer has a size
/// that is greater than or equal to the size of prefix, and each of the
/// elements in Prefix is the same as the corresponding elements in Longer.
///
/// This means it also returns true when Prefix and Longer are equal!
static bool IsPrefix(const ArgPromotion::IndicesVector &Prefix,
                     const ArgPromotion::IndicesVector &Longer) {
  if (Prefix.size() > Longer.size())
    return false;
  for (unsigned i = 0, e = Prefix.size(); i != e; ++i)
    if (Prefix[i] != Longer[i])
      return false;
  return true;
}


/// Checks if Indices, or a prefix of Indices, is in Set.
static bool PrefixIn(const ArgPromotion::IndicesVector &Indices,
                     std::set<ArgPromotion::IndicesVector> &Set) {
    std::set<ArgPromotion::IndicesVector>::iterator Low;
    Low = Set.upper_bound(Indices);
    if (Low != Set.begin())
      Low--;
    // Low is now the last element smaller than or equal to Indices. This means
    // it points to a prefix of Indices (possibly Indices itself), if such
    // prefix exists.
    //
    // This load is safe if any prefix of its operands is safe to load.
    return Low != Set.end() && IsPrefix(*Low, Indices);
}

/// Mark the given indices (ToMark) as safe in the the given set of indices
/// (Safe). Marking safe usually means adding ToMark to Safe. However, if there
/// is already a prefix of Indices in Safe, Indices are implicitely marked safe
/// already. Furthermore, any indices that Indices is itself a prefix of, are
/// removed from Safe (since they are implicitely safe because of Indices now).
static void MarkIndicesSafe(const ArgPromotion::IndicesVector &ToMark,
                            std::set<ArgPromotion::IndicesVector> &Safe) {
  std::set<ArgPromotion::IndicesVector>::iterator Low;
  Low = Safe.upper_bound(ToMark);
  // Guard against the case where Safe is empty
  if (Low != Safe.begin())
    Low--;
  // Low is now the last element smaller than or equal to Indices. This
  // means it points to a prefix of Indices (possibly Indices itself), if
  // such prefix exists.
  if (Low != Safe.end()) {
    if (IsPrefix(*Low, ToMark))
      // If there is already a prefix of these indices (or exactly these
      // indices) marked a safe, don't bother adding these indices
      return;

    // Increment Low, so we can use it as a "insert before" hint
    ++Low; 
  }
  // Insert 
  Low = Safe.insert(Low, ToMark);
  ++Low;
  // If there we're a prefix of longer index list(s), remove those
  std::set<ArgPromotion::IndicesVector>::iterator End = Safe.end();
  while (Low != End && IsPrefix(ToMark, *Low)) {
    std::set<ArgPromotion::IndicesVector>::iterator Remove = Low;
    ++Low;
    Safe.erase(Remove);
  }
}

/// isSafeToPromoteArgument - As you might guess from the name of this method,
/// it checks to see if it is both safe and useful to promote the argument.
/// This method limits promotion of aggregates to only promote up to three
/// elements of the aggregate in order to avoid exploding the number of
/// arguments passed in.
bool ArgPromotion::isSafeToPromoteArgument(Argument *Arg, bool isByVal) const {
  typedef std::set<IndicesVector> GEPIndicesSet;

  // Quick exit for unused arguments
  if (Arg->use_empty())
    return true;

  // We can only promote this argument if all of the uses are loads, or are GEP
  // instructions (with constant indices) that are subsequently loaded.
  //
  // Promoting the argument causes it to be loaded in the caller
  // unconditionally. This is only safe if we can prove that either the load
  // would have happened in the callee anyway (ie, there is a load in the entry
  // block) or the pointer passed in at every call site is guaranteed to be
  // valid.
  // In the former case, invalid loads can happen, but would have happened
  // anyway, in the latter case, invalid loads won't happen. This prevents us
  // from introducing an invalid load that wouldn't have happened in the
  // original code.
  //
  // This set will contain all sets of indices that are loaded in the entry
  // block, and thus are safe to unconditionally load in the caller.
  GEPIndicesSet SafeToUnconditionallyLoad;

  // This set contains all the sets of indices that we are planning to promote.
  // This makes it possible to limit the number of arguments added.
  GEPIndicesSet ToPromote;
  
  // If the pointer is always valid, any load with first index 0 is valid.
  if(isByVal || AllCalleesPassInValidPointerForArgument(Arg))
    SafeToUnconditionallyLoad.insert(IndicesVector(1, 0));

  // First, iterate the entry block and mark loads of (geps of) arguments as
  // safe.
  BasicBlock *EntryBlock = Arg->getParent()->begin();
  // Declare this here so we can reuse it
  IndicesVector Indices;
  for (BasicBlock::iterator I = EntryBlock->begin(), E = EntryBlock->end();
       I != E; ++I)
    if (LoadInst *LI = dyn_cast<LoadInst>(I)) {
      Value *V = LI->getPointerOperand();
      if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(V)) {
        V = GEP->getPointerOperand();
        if (V == Arg) {
          // This load actually loads (part of) Arg? Check the indices then.
          Indices.reserve(GEP->getNumIndices());
          for (User::op_iterator II = GEP->idx_begin(), IE = GEP->idx_end();
               II != IE; ++II)
            if (ConstantInt *CI = dyn_cast<ConstantInt>(*II))
              Indices.push_back(CI->getSExtValue());
            else
              // We found a non-constant GEP index for this argument? Bail out
              // right away, can't promote this argument at all.
              return false;

          // Indices checked out, mark them as safe
          MarkIndicesSafe(Indices, SafeToUnconditionallyLoad);
          Indices.clear();
        }
      } else if (V == Arg) {
        // Direct loads are equivalent to a GEP with a single 0 index.
        MarkIndicesSafe(IndicesVector(1, 0), SafeToUnconditionallyLoad);
      }
    }

  // Now, iterate all uses of the argument to see if there are any uses that are
  // not (GEP+)loads, or any (GEP+)loads that are not safe to promote.
  SmallVector<LoadInst*, 16> Loads;
  IndicesVector Operands;
  for (Value::use_iterator UI = Arg->use_begin(), E = Arg->use_end();
       UI != E; ++UI) {
    Operands.clear();
    if (LoadInst *LI = dyn_cast<LoadInst>(*UI)) {
      if (LI->isVolatile()) return false;  // Don't hack volatile loads
      Loads.push_back(LI);
      // Direct loads are equivalent to a GEP with a zero index and then a load.
      Operands.push_back(0);
    } else if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(*UI)) {
      if (GEP->use_empty()) {
        // Dead GEP's cause trouble later.  Just remove them if we run into
        // them.
        getAnalysis<AliasAnalysis>().deleteValue(GEP);
        GEP->eraseFromParent();
        // TODO: This runs the above loop over and over again for dead GEPS
        // Couldn't we just do increment the UI iterator earlier and erase the
        // use?
        return isSafeToPromoteArgument(Arg, isByVal);
      }

      // Ensure that all of the indices are constants.
      for (User::op_iterator i = GEP->idx_begin(), e = GEP->idx_end();
        i != e; ++i)
        if (ConstantInt *C = dyn_cast<ConstantInt>(*i))
          Operands.push_back(C->getSExtValue());
        else
          return false;  // Not a constant operand GEP!

      // Ensure that the only users of the GEP are load instructions.
      for (Value::use_iterator UI = GEP->use_begin(), E = GEP->use_end();
           UI != E; ++UI)
        if (LoadInst *LI = dyn_cast<LoadInst>(*UI)) {
          if (LI->isVolatile()) return false;  // Don't hack volatile loads
          Loads.push_back(LI);
        } else {
          // Other uses than load?
          return false;
        }
    } else {
      return false;  // Not a load or a GEP.
    }
     
    // Now, see if it is safe to promote this load / loads of this GEP. Loading
    // is safe if Operands, or a prefix of Operands, is marked as safe.
    if (!PrefixIn(Operands, SafeToUnconditionallyLoad))
      return false;

    // See if we are already promoting a load with these indices. If not, check
    // to make sure that we aren't promoting too many elements.  If so, nothing
    // to do.
    if (ToPromote.find(Operands) == ToPromote.end()) {
      if (maxElements > 0 && ToPromote.size() == maxElements) {
        DOUT << "argpromotion not promoting argument '"
             << Arg->getName() << "' because it would require adding more "
             << "than " << maxElements << " arguments to the function.\n";
        // We limit aggregate promotion to only promoting up to a fixed number
        // of elements of the aggregate.
        return false;
      }
      ToPromote.insert(Operands);
    }
  }

  if (Loads.empty()) return true;  // No users, this is a dead argument.

  // Okay, now we know that the argument is only used by load instructions and
  // it is safe to unconditionally perform all of them. Use alias analysis to
  // check to see if the pointer is guaranteed to not be modified from entry of
  // the function to each of the load instructions.

  // Because there could be several/many load instructions, remember which
  // blocks we know to be transparent to the load.
  SmallPtrSet<BasicBlock*, 16> TranspBlocks;

  AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
  TargetData &TD = getAnalysis<TargetData>();

  for (unsigned i = 0, e = Loads.size(); i != e; ++i) {
    // Check to see if the load is invalidated from the start of the block to
    // the load itself.
    LoadInst *Load = Loads[i];
    BasicBlock *BB = Load->getParent();

    const PointerType *LoadTy =
      cast<PointerType>(Load->getPointerOperand()->getType());
    unsigned LoadSize = (unsigned)TD.getTypeStoreSize(LoadTy->getElementType());

    if (AA.canInstructionRangeModify(BB->front(), *Load, Arg, LoadSize))
      return false;  // Pointer is invalidated!

    // Now check every path from the entry block to the load for transparency.
    // To do this, we perform a depth first search on the inverse CFG from the
    // loading block.
    for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
      for (idf_ext_iterator<BasicBlock*, SmallPtrSet<BasicBlock*, 16> >
             I = idf_ext_begin(*PI, TranspBlocks),
             E = idf_ext_end(*PI, TranspBlocks); I != E; ++I)
        if (AA.canBasicBlockModify(**I, Arg, LoadSize))
          return false;
  }

  // If the path from the entry of the function to each load is free of
  // instructions that potentially invalidate the load, we can make the
  // transformation!
  return true;
}

/// DoPromotion - This method actually performs the promotion of the specified
/// arguments, and returns the new function.  At this point, we know that it's
/// safe to do so.
Function *ArgPromotion::DoPromotion(Function *F,
                                    SmallPtrSet<Argument*, 8> &ArgsToPromote,
                              SmallPtrSet<Argument*, 8> &ByValArgsToTransform) {

  // Start by computing a new prototype for the function, which is the same as
  // the old function, but has modified arguments.
  const FunctionType *FTy = F->getFunctionType();
  std::vector<const Type*> Params;

  typedef std::set<IndicesVector> ScalarizeTable;

  // ScalarizedElements - If we are promoting a pointer that has elements
  // accessed out of it, keep track of which elements are accessed so that we
  // can add one argument for each.
  //
  // Arguments that are directly loaded will have a zero element value here, to
  // handle cases where there are both a direct load and GEP accesses.
  //
  std::map<Argument*, ScalarizeTable> ScalarizedElements;

  // OriginalLoads - Keep track of a representative load instruction from the
  // original function so that we can tell the alias analysis implementation
  // what the new GEP/Load instructions we are inserting look like.
  std::map<IndicesVector, LoadInst*> OriginalLoads;

  // ParamAttrs - Keep track of the parameter attributes for the arguments
  // that we are *not* promoting. For the ones that we do promote, the parameter
  // attributes are lost
  SmallVector<ParamAttrsWithIndex, 8> ParamAttrsVec;
  const PAListPtr &PAL = F->getParamAttrs();

  // Add any return attributes.
  if (ParameterAttributes attrs = PAL.getParamAttrs(0))
    ParamAttrsVec.push_back(ParamAttrsWithIndex::get(0, attrs));
  
  // First, determine the new argument list
  unsigned ArgIndex = 1;
  for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(); I != E;
       ++I, ++ArgIndex) {
    if (ByValArgsToTransform.count(I)) {
      // Simple byval argument? Just add all the struct element types.
      const Type *AgTy = cast<PointerType>(I->getType())->getElementType();
      const StructType *STy = cast<StructType>(AgTy);
      for (unsigned i = 0, e = STy->getNumElements(); i != e; ++i)
        Params.push_back(STy->getElementType(i));
      ++NumByValArgsPromoted;
    } else if (!ArgsToPromote.count(I)) {
      // Unchanged argument
      Params.push_back(I->getType());
      if (ParameterAttributes attrs = PAL.getParamAttrs(ArgIndex))
        ParamAttrsVec.push_back(ParamAttrsWithIndex::get(Params.size(), attrs));
    } else if (I->use_empty()) {
      // Dead argument (which are always marked as promotable)
      ++NumArgumentsDead;
    } else {
      // Okay, this is being promoted. This means that the only uses are loads
      // or GEPs which are only used by loads

      // In this table, we will track which indices are loaded from the argument
      // (where direct loads are tracked as no indices).
      ScalarizeTable &ArgIndices = ScalarizedElements[I];
      for (Value::use_iterator UI = I->use_begin(), E = I->use_end(); UI != E;
           ++UI) {
        Instruction *User = cast<Instruction>(*UI);
        assert(isa<LoadInst>(User) || isa<GetElementPtrInst>(User));
        IndicesVector Indices;
        Indices.reserve(User->getNumOperands() - 1);
        // Since loads will only have a single operand, and GEPs only a single
        // non-index operand, this will record direct loads without any indices,
        // and gep+loads with the GEP indices.
        for (User::op_iterator II = User->op_begin() + 1, IE = User->op_end();
             II != IE; ++II)
          Indices.push_back(cast<ConstantInt>(*II)->getSExtValue());
        // GEPs with a single 0 index can be merged with direct loads
        if (Indices.size() == 1 && Indices.front() == 0)
          Indices.clear();
        ArgIndices.insert(Indices);
        LoadInst *OrigLoad;
        if (LoadInst *L = dyn_cast<LoadInst>(User))
          OrigLoad = L;
        else
          // Take any load, we will use it only to update Alias Analysis
          OrigLoad = cast<LoadInst>(User->use_back());
        OriginalLoads[Indices] = OrigLoad;
      }

      // Add a parameter to the function for each element passed in.
      for (ScalarizeTable::iterator SI = ArgIndices.begin(),
             E = ArgIndices.end(); SI != E; ++SI) {
        Params.push_back(GetElementPtrInst::getIndexedType(I->getType(),
                                                           &*SI->begin(),
                                                           SI->size()));
        assert(Params.back());
      }

      if (ArgIndices.size() == 1 && ArgIndices.begin()->empty())
        ++NumArgumentsPromoted;
      else
        ++NumAggregatesPromoted;
    }
  }

  const Type *RetTy = FTy->getReturnType();

  // Work around LLVM bug PR56: the CWriter cannot emit varargs functions which
  // have zero fixed arguments.
  bool ExtraArgHack = false;
  if (Params.empty() && FTy->isVarArg()) {
    ExtraArgHack = true;
    Params.push_back(Type::Int32Ty);
  }

  // Construct the new function type using the new arguments.
  FunctionType *NFTy = FunctionType::get(RetTy, Params, FTy->isVarArg());

  // Create the new function body and insert it into the module...
  Function *NF = Function::Create(NFTy, F->getLinkage(), F->getName());
  NF->copyAttributesFrom(F);

  // Recompute the parameter attributes list based on the new arguments for
  // the function.
  NF->setParamAttrs(PAListPtr::get(ParamAttrsVec.begin(), ParamAttrsVec.end()));
  ParamAttrsVec.clear();

  F->getParent()->getFunctionList().insert(F, NF);
  NF->takeName(F);

  // Get the alias analysis information that we need to update to reflect our
  // changes.
  AliasAnalysis &AA = getAnalysis<AliasAnalysis>();

  // Loop over all of the callers of the function, transforming the call sites
  // to pass in the loaded pointers.
  //
  SmallVector<Value*, 16> Args;
  while (!F->use_empty()) {
    CallSite CS = CallSite::get(F->use_back());
    Instruction *Call = CS.getInstruction();
    const PAListPtr &CallPAL = CS.getParamAttrs();
    
    // Add any return attributes.
    if (ParameterAttributes attrs = CallPAL.getParamAttrs(0))
      ParamAttrsVec.push_back(ParamAttrsWithIndex::get(0, attrs));

    // Loop over the operands, inserting GEP and loads in the caller as
    // appropriate.
    CallSite::arg_iterator AI = CS.arg_begin();
    ArgIndex = 1;
    for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end();
         I != E; ++I, ++AI, ++ArgIndex)
      if (!ArgsToPromote.count(I) && !ByValArgsToTransform.count(I)) {
        Args.push_back(*AI);          // Unmodified argument
        
        if (ParameterAttributes Attrs = CallPAL.getParamAttrs(ArgIndex))
          ParamAttrsVec.push_back(ParamAttrsWithIndex::get(Args.size(), Attrs));
        
      } else if (ByValArgsToTransform.count(I)) {
        // Emit a GEP and load for each element of the struct.
        const Type *AgTy = cast<PointerType>(I->getType())->getElementType();
        const StructType *STy = cast<StructType>(AgTy);
        Value *Idxs[2] = { ConstantInt::get(Type::Int32Ty, 0), 0 };
        for (unsigned i = 0, e = STy->getNumElements(); i != e; ++i) {
          Idxs[1] = ConstantInt::get(Type::Int32Ty, i);
          Value *Idx = GetElementPtrInst::Create(*AI, Idxs, Idxs+2,
                                                 (*AI)->getName()+"."+utostr(i),
                                                 Call);
          // TODO: Tell AA about the new values?
          Args.push_back(new LoadInst(Idx, Idx->getName()+".val", Call));
        }        
      } else if (!I->use_empty()) {
        // Non-dead argument: insert GEPs and loads as appropriate.
        ScalarizeTable &ArgIndices = ScalarizedElements[I];
        // Store the Value* version of the indices in here, but declare it now
        // for reuse
        std::vector<Value*> Ops;
        for (ScalarizeTable::iterator SI = ArgIndices.begin(),
               E = ArgIndices.end(); SI != E; ++SI) {
          Value *V = *AI;
          LoadInst *OrigLoad = OriginalLoads[*SI];
          if (!SI->empty()) {
            Ops.reserve(SI->size());
            const Type *ElTy = V->getType();
            for (IndicesVector::const_iterator II = SI->begin(), 
                 IE = SI->end(); II != IE; ++II) {
              // Use i32 to index structs, and i64 for others (pointers/arrays).
              // This satisfies GEP constraints.
              const Type *IdxTy = (isa<StructType>(ElTy) ? Type::Int32Ty : Type::Int64Ty);
              Ops.push_back(ConstantInt::get(IdxTy, *II));
              // Keep track of the type we're currently indexing
              ElTy = cast<CompositeType>(ElTy)->getTypeAtIndex(*II);
            }
            // And create a GEP to extract those indices
            V = GetElementPtrInst::Create(V, Ops.begin(), Ops.end(),
                                          V->getName()+".idx", Call);
            Ops.clear();
            AA.copyValue(OrigLoad->getOperand(0), V);
          }
          Args.push_back(new LoadInst(V, V->getName()+".val", Call));
          AA.copyValue(OrigLoad, Args.back());
        }
      }

    if (ExtraArgHack)
      Args.push_back(Constant::getNullValue(Type::Int32Ty));

    // Push any varargs arguments on the list
    for (; AI != CS.arg_end(); ++AI, ++ArgIndex) {
      Args.push_back(*AI);
      if (ParameterAttributes Attrs = CallPAL.getParamAttrs(ArgIndex))
        ParamAttrsVec.push_back(ParamAttrsWithIndex::get(Args.size(), Attrs));
    }

    Instruction *New;
    if (InvokeInst *II = dyn_cast<InvokeInst>(Call)) {
      New = InvokeInst::Create(NF, II->getNormalDest(), II->getUnwindDest(),
                               Args.begin(), Args.end(), "", Call);
      cast<InvokeInst>(New)->setCallingConv(CS.getCallingConv());
      cast<InvokeInst>(New)->setParamAttrs(PAListPtr::get(ParamAttrsVec.begin(),
                                                          ParamAttrsVec.end()));
    } else {
      New = CallInst::Create(NF, Args.begin(), Args.end(), "", Call);
      cast<CallInst>(New)->setCallingConv(CS.getCallingConv());
      cast<CallInst>(New)->setParamAttrs(PAListPtr::get(ParamAttrsVec.begin(),
                                                        ParamAttrsVec.end()));
      if (cast<CallInst>(Call)->isTailCall())
        cast<CallInst>(New)->setTailCall();
    }
    Args.clear();
    ParamAttrsVec.clear();

    // Update the alias analysis implementation to know that we are replacing
    // the old call with a new one.
    AA.replaceWithNewValue(Call, New);

    if (!Call->use_empty()) {
      Call->replaceAllUsesWith(New);
      New->takeName(Call);
    }

    // Finally, remove the old call from the program, reducing the use-count of
    // F.
    Call->eraseFromParent();
  }

  // Since we have now created the new function, splice the body of the old
  // function right into the new function, leaving the old rotting hulk of the
  // function empty.
  NF->getBasicBlockList().splice(NF->begin(), F->getBasicBlockList());

  // Loop over the argument list, transfering uses of the old arguments over to
  // the new arguments, also transfering over the names as well.
  //
  for (Function::arg_iterator I = F->arg_begin(), E = F->arg_end(),
       I2 = NF->arg_begin(); I != E; ++I) {
    if (!ArgsToPromote.count(I) && !ByValArgsToTransform.count(I)) {
      // If this is an unmodified argument, move the name and users over to the
      // new version.
      I->replaceAllUsesWith(I2);
      I2->takeName(I);
      AA.replaceWithNewValue(I, I2);
      ++I2;
      continue;
    }
    
    if (ByValArgsToTransform.count(I)) {
      // In the callee, we create an alloca, and store each of the new incoming
      // arguments into the alloca.
      Instruction *InsertPt = NF->begin()->begin();
      
      // Just add all the struct element types.
      const Type *AgTy = cast<PointerType>(I->getType())->getElementType();
      Value *TheAlloca = new AllocaInst(AgTy, 0, "", InsertPt);
      const StructType *STy = cast<StructType>(AgTy);
      Value *Idxs[2] = { ConstantInt::get(Type::Int32Ty, 0), 0 };
      
      for (unsigned i = 0, e = STy->getNumElements(); i != e; ++i) {
        Idxs[1] = ConstantInt::get(Type::Int32Ty, i);
        std::string Name = TheAlloca->getName()+"."+utostr(i);
        Value *Idx = GetElementPtrInst::Create(TheAlloca, Idxs, Idxs+2,
                                               Name, InsertPt);
        I2->setName(I->getName()+"."+utostr(i));
        new StoreInst(I2++, Idx, InsertPt);
      }
      
      // Anything that used the arg should now use the alloca.
      I->replaceAllUsesWith(TheAlloca);
      TheAlloca->takeName(I);
      AA.replaceWithNewValue(I, TheAlloca);
      continue;
    } 
    
    if (I->use_empty()) {
      AA.deleteValue(I);
      continue;
    }
    
    // Otherwise, if we promoted this argument, then all users are load
    // instructions (or GEPs with only load users), and all loads should be
    // using the new argument that we added.
    ScalarizeTable &ArgIndices = ScalarizedElements[I];

    while (!I->use_empty()) {
      if (LoadInst *LI = dyn_cast<LoadInst>(I->use_back())) {
        assert(ArgIndices.begin()->empty() &&
               "Load element should sort to front!");
        I2->setName(I->getName()+".val");
        LI->replaceAllUsesWith(I2);
        AA.replaceWithNewValue(LI, I2);
        LI->eraseFromParent();
        DOUT << "*** Promoted load of argument '" << I->getName()
             << "' in function '" << F->getName() << "'\n";
      } else {
        GetElementPtrInst *GEP = cast<GetElementPtrInst>(I->use_back());
        IndicesVector Operands;
        Operands.reserve(GEP->getNumIndices());
        for (User::op_iterator II = GEP->idx_begin(), IE = GEP->idx_end();
             II != IE; ++II)
          Operands.push_back(cast<ConstantInt>(*II)->getSExtValue());

        // GEPs with a single 0 index can be merged with direct loads
        if (Operands.size() == 1 && Operands.front() == 0)
          Operands.clear();

        Function::arg_iterator TheArg = I2;
        for (ScalarizeTable::iterator It = ArgIndices.begin();
             *It != Operands; ++It, ++TheArg) {
          assert(It != ArgIndices.end() && "GEP not handled??");
        }

        std::string NewName = I->getName();
        for (unsigned i = 0, e = Operands.size(); i != e; ++i) {
            NewName += "." + utostr(Operands[i]);
        }
        NewName += ".val";
        TheArg->setName(NewName);

        DOUT << "*** Promoted agg argument '" << TheArg->getName()
             << "' of function '" << NF->getName() << "'\n";

        // All of the uses must be load instructions.  Replace them all with
        // the argument specified by ArgNo.
        while (!GEP->use_empty()) {
          LoadInst *L = cast<LoadInst>(GEP->use_back());
          L->replaceAllUsesWith(TheArg);
          AA.replaceWithNewValue(L, TheArg);
          L->eraseFromParent();
        }
        AA.deleteValue(GEP);
        GEP->eraseFromParent();
      }
    }

    // Increment I2 past all of the arguments added for this promoted pointer.
    for (unsigned i = 0, e = ArgIndices.size(); i != e; ++i)
      ++I2;
  }

  // Notify the alias analysis implementation that we inserted a new argument.
  if (ExtraArgHack)
    AA.copyValue(Constant::getNullValue(Type::Int32Ty), NF->arg_begin());


  // Tell the alias analysis that the old function is about to disappear.
  AA.replaceWithNewValue(F, NF);

  // Now that the old function is dead, delete it.
  F->eraseFromParent();
  return NF;
}