ScalarReplAggregates.cpp

//===- ScalarReplAggregates.cpp - Scalar Replacement of Aggregates --------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This transformation implements the well known scalar replacement of
// aggregates transformation.  This xform breaks up alloca instructions of
// aggregate type (structure or array) into individual alloca instructions for
// each member (if possible).  Then, if possible, it transforms the individual
// alloca instructions into nice clean scalar SSA form.
//
// This combines a simple SRoA algorithm with the Mem2Reg algorithm because
// often interact, especially for C++ programs.  As such, iterating between
// SRoA, then Mem2Reg until we run out of things to promote works well.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "scalarrepl"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Constants.h"
#include "llvm/DerivedTypes.h"
#include "llvm/Function.h"
#include "llvm/GlobalVariable.h"
#include "llvm/Instructions.h"
#include "llvm/IntrinsicInst.h"
#include "llvm/LLVMContext.h"
#include "llvm/Module.h"
#include "llvm/Pass.h"
#include "llvm/Analysis/DebugInfo.h"
#include "llvm/Analysis/DIBuilder.h"
#include "llvm/Analysis/Dominators.h"
#include "llvm/Analysis/Loads.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/Target/TargetData.h"
#include "llvm/Transforms/Utils/PromoteMemToReg.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Utils/SSAUpdater.h"
#include "llvm/Support/CallSite.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/GetElementPtrTypeIterator.h"
#include "llvm/Support/IRBuilder.h"
#include "llvm/Support/MathExtras.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
using namespace llvm;

STATISTIC(NumReplaced,  "Number of allocas broken up");
STATISTIC(NumPromoted,  "Number of allocas promoted");
STATISTIC(NumAdjusted,  "Number of scalar allocas adjusted to allow promotion");
STATISTIC(NumConverted, "Number of aggregates converted to scalar");
STATISTIC(NumGlobals,   "Number of allocas copied from constant global");

namespace {
  struct SROA : public FunctionPass {
    SROA(int T, bool hasDT, char &ID)
      : FunctionPass(ID), HasDomTree(hasDT) {
      if (T == -1)
        SRThreshold = 128;
      else
        SRThreshold = T;
    }

    bool runOnFunction(Function &F);

    bool performScalarRepl(Function &F);
    bool performPromotion(Function &F);

  private:
    bool HasDomTree;
    TargetData *TD;

    /// DeadInsts - Keep track of instructions we have made dead, so that
    /// we can remove them after we are done working.
    SmallVector<Value*, 32> DeadInsts;

    /// AllocaInfo - When analyzing uses of an alloca instruction, this captures
    /// information about the uses.  All these fields are initialized to false
    /// and set to true when something is learned.
    struct AllocaInfo {
      /// The alloca to promote.
      AllocaInst *AI;
      
      /// CheckedPHIs - This is a set of verified PHI nodes, to prevent infinite
      /// looping and avoid redundant work.
      SmallPtrSet<PHINode*, 8> CheckedPHIs;
      
      /// isUnsafe - This is set to true if the alloca cannot be SROA'd.
      bool isUnsafe : 1;

      /// isMemCpySrc - This is true if this aggregate is memcpy'd from.
      bool isMemCpySrc : 1;

      /// isMemCpyDst - This is true if this aggregate is memcpy'd into.
      bool isMemCpyDst : 1;

      /// hasSubelementAccess - This is true if a subelement of the alloca is
      /// ever accessed, or false if the alloca is only accessed with mem
      /// intrinsics or load/store that only access the entire alloca at once.
      bool hasSubelementAccess : 1;
      
      /// hasALoadOrStore - This is true if there are any loads or stores to it.
      /// The alloca may just be accessed with memcpy, for example, which would
      /// not set this.
      bool hasALoadOrStore : 1;
      
      explicit AllocaInfo(AllocaInst *ai)
        : AI(ai), isUnsafe(false), isMemCpySrc(false), isMemCpyDst(false),
          hasSubelementAccess(false), hasALoadOrStore(false) {}
    };

    unsigned SRThreshold;

    void MarkUnsafe(AllocaInfo &I, Instruction *User) {
      I.isUnsafe = true;
      DEBUG(dbgs() << "  Transformation preventing inst: " << *User << '\n');
    }

    bool isSafeAllocaToScalarRepl(AllocaInst *AI);

    void isSafeForScalarRepl(Instruction *I, uint64_t Offset, AllocaInfo &Info);
    void isSafePHISelectUseForScalarRepl(Instruction *User, uint64_t Offset,
                                         AllocaInfo &Info);
    void isSafeGEP(GetElementPtrInst *GEPI, uint64_t &Offset, AllocaInfo &Info);
    void isSafeMemAccess(uint64_t Offset, uint64_t MemSize,
                         Type *MemOpType, bool isStore, AllocaInfo &Info,
                         Instruction *TheAccess, bool AllowWholeAccess);
    bool TypeHasComponent(Type *T, uint64_t Offset, uint64_t Size);
    uint64_t FindElementAndOffset(Type *&T, uint64_t &Offset,
                                  Type *&IdxTy);

    void DoScalarReplacement(AllocaInst *AI,
                             std::vector<AllocaInst*> &WorkList);
    void DeleteDeadInstructions();

    void RewriteForScalarRepl(Instruction *I, AllocaInst *AI, uint64_t Offset,
                              SmallVector<AllocaInst*, 32> &NewElts);
    void RewriteBitCast(BitCastInst *BC, AllocaInst *AI, uint64_t Offset,
                        SmallVector<AllocaInst*, 32> &NewElts);
    void RewriteGEP(GetElementPtrInst *GEPI, AllocaInst *AI, uint64_t Offset,
                    SmallVector<AllocaInst*, 32> &NewElts);
    void RewriteLifetimeIntrinsic(IntrinsicInst *II, AllocaInst *AI,
                                  uint64_t Offset,
                                  SmallVector<AllocaInst*, 32> &NewElts);
    void RewriteMemIntrinUserOfAlloca(MemIntrinsic *MI, Instruction *Inst,
                                      AllocaInst *AI,
                                      SmallVector<AllocaInst*, 32> &NewElts);
    void RewriteStoreUserOfWholeAlloca(StoreInst *SI, AllocaInst *AI,
                                       SmallVector<AllocaInst*, 32> &NewElts);
    void RewriteLoadUserOfWholeAlloca(LoadInst *LI, AllocaInst *AI,
                                      SmallVector<AllocaInst*, 32> &NewElts);

    static MemTransferInst *isOnlyCopiedFromConstantGlobal(
        AllocaInst *AI, SmallVector<Instruction*, 4> &ToDelete);
  };
  
  // SROA_DT - SROA that uses DominatorTree.
  struct SROA_DT : public SROA {
    static char ID;
  public:
    SROA_DT(int T = -1) : SROA(T, true, ID) {
      initializeSROA_DTPass(*PassRegistry::getPassRegistry());
    }
    
    // getAnalysisUsage - This pass does not require any passes, but we know it
    // will not alter the CFG, so say so.
    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
      AU.addRequired<DominatorTree>();
      AU.setPreservesCFG();
    }
  };
  
  // SROA_SSAUp - SROA that uses SSAUpdater.
  struct SROA_SSAUp : public SROA {
    static char ID;
  public:
    SROA_SSAUp(int T = -1) : SROA(T, false, ID) {
      initializeSROA_SSAUpPass(*PassRegistry::getPassRegistry());
    }
    
    // getAnalysisUsage - This pass does not require any passes, but we know it
    // will not alter the CFG, so say so.
    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
      AU.setPreservesCFG();
    }
  };
  
}

char SROA_DT::ID = 0;
char SROA_SSAUp::ID = 0;

INITIALIZE_PASS_BEGIN(SROA_DT, "scalarrepl",
                "Scalar Replacement of Aggregates (DT)", false, false)
INITIALIZE_PASS_DEPENDENCY(DominatorTree)
INITIALIZE_PASS_END(SROA_DT, "scalarrepl",
                "Scalar Replacement of Aggregates (DT)", false, false)

INITIALIZE_PASS_BEGIN(SROA_SSAUp, "scalarrepl-ssa",
                      "Scalar Replacement of Aggregates (SSAUp)", false, false)
INITIALIZE_PASS_END(SROA_SSAUp, "scalarrepl-ssa",
                    "Scalar Replacement of Aggregates (SSAUp)", false, false)

// Public interface to the ScalarReplAggregates pass
FunctionPass *llvm::createScalarReplAggregatesPass(int Threshold,
                                                   bool UseDomTree) {
  if (UseDomTree)
    return new SROA_DT(Threshold);
  return new SROA_SSAUp(Threshold);
}


//===----------------------------------------------------------------------===//
// Convert To Scalar Optimization.
//===----------------------------------------------------------------------===//

namespace {
/// ConvertToScalarInfo - This class implements the "Convert To Scalar"
/// optimization, which scans the uses of an alloca and determines if it can
/// rewrite it in terms of a single new alloca that can be mem2reg'd.
class ConvertToScalarInfo {
  /// AllocaSize - The size of the alloca being considered in bytes.
  unsigned AllocaSize;
  const TargetData &TD;

  /// IsNotTrivial - This is set to true if there is some access to the object
  /// which means that mem2reg can't promote it.
  bool IsNotTrivial;

  /// ScalarKind - Tracks the kind of alloca being considered for promotion,
  /// computed based on the uses of the alloca rather than the LLVM type system.
  enum {
    Unknown,

    // Accesses via GEPs that are consistent with element access of a vector
    // type. This will not be converted into a vector unless there is a later
    // access using an actual vector type.
    ImplicitVector,

    // Accesses via vector operations and GEPs that are consistent with the
    // layout of a vector type.
    Vector,

    // An integer bag-of-bits with bitwise operations for insertion and
    // extraction. Any combination of types can be converted into this kind
    // of scalar.
    Integer
  } ScalarKind;

  /// VectorTy - This tracks the type that we should promote the vector to if
  /// it is possible to turn it into a vector.  This starts out null, and if it
  /// isn't possible to turn into a vector type, it gets set to VoidTy.
  VectorType *VectorTy;

  /// HadNonMemTransferAccess - True if there is at least one access to the 
  /// alloca that is not a MemTransferInst.  We don't want to turn structs into
  /// large integers unless there is some potential for optimization.
  bool HadNonMemTransferAccess;

public:
  explicit ConvertToScalarInfo(unsigned Size, const TargetData &td)
    : AllocaSize(Size), TD(td), IsNotTrivial(false), ScalarKind(Unknown),
      VectorTy(0), HadNonMemTransferAccess(false) { }

  AllocaInst *TryConvert(AllocaInst *AI);

private:
  bool CanConvertToScalar(Value *V, uint64_t Offset);
  void MergeInTypeForLoadOrStore(Type *In, uint64_t Offset);
  bool MergeInVectorType(VectorType *VInTy, uint64_t Offset);
  void ConvertUsesToScalar(Value *Ptr, AllocaInst *NewAI, uint64_t Offset);

  Value *ConvertScalar_ExtractValue(Value *NV, Type *ToType,
                                    uint64_t Offset, IRBuilder<> &Builder);
  Value *ConvertScalar_InsertValue(Value *StoredVal, Value *ExistingVal,
                                   uint64_t Offset, IRBuilder<> &Builder);
};
} // end anonymous namespace.


/// TryConvert - Analyze the specified alloca, and if it is safe to do so,
/// rewrite it to be a new alloca which is mem2reg'able.  This returns the new
/// alloca if possible or null if not.
AllocaInst *ConvertToScalarInfo::TryConvert(AllocaInst *AI) {
  // If we can't convert this scalar, or if mem2reg can trivially do it, bail
  // out.
  if (!CanConvertToScalar(AI, 0) || !IsNotTrivial)
    return 0;

  // If an alloca has only memset / memcpy uses, it may still have an Unknown
  // ScalarKind. Treat it as an Integer below.
  if (ScalarKind == Unknown)
    ScalarKind = Integer;

  // FIXME: It should be possible to promote the vector type up to the alloca's
  // size.
  if (ScalarKind == Vector && VectorTy->getBitWidth() != AllocaSize * 8)
    ScalarKind = Integer;

  // If we were able to find a vector type that can handle this with
  // insert/extract elements, and if there was at least one use that had
  // a vector type, promote this to a vector.  We don't want to promote
  // random stuff that doesn't use vectors (e.g. <9 x double>) because then
  // we just get a lot of insert/extracts.  If at least one vector is
  // involved, then we probably really do have a union of vector/array.
  Type *NewTy;
  if (ScalarKind == Vector) {
    assert(VectorTy && "Missing type for vector scalar.");
    DEBUG(dbgs() << "CONVERT TO VECTOR: " << *AI << "\n  TYPE = "
          << *VectorTy << '\n');
    NewTy = VectorTy;  // Use the vector type.
  } else {
    unsigned BitWidth = AllocaSize * 8;
    if ((ScalarKind == ImplicitVector || ScalarKind == Integer) &&
        !HadNonMemTransferAccess && !TD.fitsInLegalInteger(BitWidth))
      return 0;

    DEBUG(dbgs() << "CONVERT TO SCALAR INTEGER: " << *AI << "\n");
    // Create and insert the integer alloca.
    NewTy = IntegerType::get(AI->getContext(), BitWidth);
  }
  AllocaInst *NewAI = new AllocaInst(NewTy, 0, "", AI->getParent()->begin());
  ConvertUsesToScalar(AI, NewAI, 0);
  return NewAI;
}

/// MergeInTypeForLoadOrStore - Add the 'In' type to the accumulated vector type
/// (VectorTy) so far at the offset specified by Offset (which is specified in
/// bytes).
///
/// There are three cases we handle here:
///   1) A union of vector types of the same size and potentially its elements.
///      Here we turn element accesses into insert/extract element operations.
///      This promotes a <4 x float> with a store of float to the third element
///      into a <4 x float> that uses insert element.
///   2) A union of vector types with power-of-2 size differences, e.g. a float,
///      <2 x float> and <4 x float>.  Here we turn element accesses into insert
///      and extract element operations, and <2 x float> accesses into a cast to
///      <2 x double>, an extract, and a cast back to <2 x float>.
///   3) A fully general blob of memory, which we turn into some (potentially
///      large) integer type with extract and insert operations where the loads
///      and stores would mutate the memory.  We mark this by setting VectorTy
///      to VoidTy.
void ConvertToScalarInfo::MergeInTypeForLoadOrStore(Type *In,
                                                    uint64_t Offset) {
  // If we already decided to turn this into a blob of integer memory, there is
  // nothing to be done.
  if (ScalarKind == Integer)
    return;

  // If this could be contributing to a vector, analyze it.

  // If the In type is a vector that is the same size as the alloca, see if it
  // matches the existing VecTy.
  if (VectorType *VInTy = dyn_cast<VectorType>(In)) {
    if (MergeInVectorType(VInTy, Offset))
      return;
  } else if (In->isFloatTy() || In->isDoubleTy() ||
             (In->isIntegerTy() && In->getPrimitiveSizeInBits() >= 8 &&
              isPowerOf2_32(In->getPrimitiveSizeInBits()))) {
    // Full width accesses can be ignored, because they can always be turned
    // into bitcasts.
    unsigned EltSize = In->getPrimitiveSizeInBits()/8;
    if (EltSize == AllocaSize)
      return;

    // If we're accessing something that could be an element of a vector, see
    // if the implied vector agrees with what we already have and if Offset is
    // compatible with it.
    if (Offset % EltSize == 0 && AllocaSize % EltSize == 0 &&
        (!VectorTy || Offset * 8 < VectorTy->getPrimitiveSizeInBits())) {
      if (!VectorTy) {
        ScalarKind = ImplicitVector;
        VectorTy = VectorType::get(In, AllocaSize/EltSize);
        return;
      }

      unsigned CurrentEltSize = VectorTy->getElementType()
                                ->getPrimitiveSizeInBits()/8;
      if (EltSize == CurrentEltSize)
        return;

      if (In->isIntegerTy() && isPowerOf2_32(AllocaSize / EltSize))
        return;
    }
  }

  // Otherwise, we have a case that we can't handle with an optimized vector
  // form.  We can still turn this into a large integer.
  ScalarKind = Integer;
}

/// MergeInVectorType - Handles the vector case of MergeInTypeForLoadOrStore,
/// returning true if the type was successfully merged and false otherwise.
bool ConvertToScalarInfo::MergeInVectorType(VectorType *VInTy,
                                            uint64_t Offset) {
  // TODO: Support nonzero offsets?
  if (Offset != 0)
    return false;

  // Only allow vectors that are a power-of-2 away from the size of the alloca.
  if (!isPowerOf2_64(AllocaSize / (VInTy->getBitWidth() / 8)))
    return false;

  // If this the first vector we see, remember the type so that we know the
  // element size.
  if (!VectorTy) {
    ScalarKind = Vector;
    VectorTy = VInTy;
    return true;
  }

  unsigned BitWidth = VectorTy->getBitWidth();
  unsigned InBitWidth = VInTy->getBitWidth();

  // Vectors of the same size can be converted using a simple bitcast.
  if (InBitWidth == BitWidth && AllocaSize == (InBitWidth / 8)) {
    ScalarKind = Vector;
    return true;
  }

  Type *ElementTy = VectorTy->getElementType();
  Type *InElementTy = VInTy->getElementType();

  // If they're the same alloc size, we'll be attempting to convert between
  // them with a vector shuffle, which requires the element types to match.
  if (TD.getTypeAllocSize(VectorTy) == TD.getTypeAllocSize(VInTy) &&
      ElementTy != InElementTy)
    return false;

  // Do not allow mixed integer and floating-point accesses from vectors of
  // different sizes.
  if (ElementTy->isFloatingPointTy() != InElementTy->isFloatingPointTy())
    return false;

  if (ElementTy->isFloatingPointTy()) {
    // Only allow floating-point vectors of different sizes if they have the
    // same element type.
    // TODO: This could be loosened a bit, but would anything benefit?
    if (ElementTy != InElementTy)
      return false;

    // There are no arbitrary-precision floating-point types, which limits the
    // number of legal vector types with larger element types that we can form
    // to bitcast and extract a subvector.
    // TODO: We could support some more cases with mixed fp128 and double here.
    if (!(BitWidth == 64 || BitWidth == 128) ||
        !(InBitWidth == 64 || InBitWidth == 128))
      return false;
  } else {
    assert(ElementTy->isIntegerTy() && "Vector elements must be either integer "
                                       "or floating-point.");
    unsigned BitWidth = ElementTy->getPrimitiveSizeInBits();
    unsigned InBitWidth = InElementTy->getPrimitiveSizeInBits();

    // Do not allow integer types smaller than a byte or types whose widths are
    // not a multiple of a byte.
    if (BitWidth < 8 || InBitWidth < 8 ||
        BitWidth % 8 != 0 || InBitWidth % 8 != 0)
      return false;
  }

  // Pick the largest of the two vector types.
  ScalarKind = Vector;
  if (InBitWidth > BitWidth)
    VectorTy = VInTy;

  return true;
}

/// CanConvertToScalar - V is a pointer.  If we can convert the pointee and all
/// its accesses to a single vector type, return true and set VecTy to
/// the new type.  If we could convert the alloca into a single promotable
/// integer, return true but set VecTy to VoidTy.  Further, if the use is not a
/// completely trivial use that mem2reg could promote, set IsNotTrivial.  Offset
/// is the current offset from the base of the alloca being analyzed.
///
/// If we see at least one access to the value that is as a vector type, set the
/// SawVec flag.
bool ConvertToScalarInfo::CanConvertToScalar(Value *V, uint64_t Offset) {
  for (Value::use_iterator UI = V->use_begin(), E = V->use_end(); UI!=E; ++UI) {
    Instruction *User = cast<Instruction>(*UI);

    if (LoadInst *LI = dyn_cast<LoadInst>(User)) {
      // Don't break volatile loads.
      if (LI->isVolatile())
        return false;
      // Don't touch MMX operations.
      if (LI->getType()->isX86_MMXTy())
        return false;
      HadNonMemTransferAccess = true;
      MergeInTypeForLoadOrStore(LI->getType(), Offset);
      continue;
    }

    if (StoreInst *SI = dyn_cast<StoreInst>(User)) {
      // Storing the pointer, not into the value?
      if (SI->getOperand(0) == V || SI->isVolatile()) return false;
      // Don't touch MMX operations.
      if (SI->getOperand(0)->getType()->isX86_MMXTy())
        return false;
      HadNonMemTransferAccess = true;
      MergeInTypeForLoadOrStore(SI->getOperand(0)->getType(), Offset);
      continue;
    }

    if (BitCastInst *BCI = dyn_cast<BitCastInst>(User)) {
      if (!onlyUsedByLifetimeMarkers(BCI))
        IsNotTrivial = true;  // Can't be mem2reg'd.
      if (!CanConvertToScalar(BCI, Offset))
        return false;
      continue;
    }

    if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(User)) {
      // If this is a GEP with a variable indices, we can't handle it.
      if (!GEP->hasAllConstantIndices())
        return false;

      // Compute the offset that this GEP adds to the pointer.
      SmallVector<Value*, 8> Indices(GEP->op_begin()+1, GEP->op_end());
      uint64_t GEPOffset = TD.getIndexedOffset(GEP->getPointerOperandType(),
                                               Indices);
      // See if all uses can be converted.
      if (!CanConvertToScalar(GEP, Offset+GEPOffset))
        return false;
      IsNotTrivial = true;  // Can't be mem2reg'd.
      HadNonMemTransferAccess = true;
      continue;
    }

    // If this is a constant sized memset of a constant value (e.g. 0) we can
    // handle it.
    if (MemSetInst *MSI = dyn_cast<MemSetInst>(User)) {
      // Store of constant value.
      if (!isa<ConstantInt>(MSI->getValue()))
        return false;

      // Store of constant size.
      ConstantInt *Len = dyn_cast<ConstantInt>(MSI->getLength());
      if (!Len)
        return false;

      // If the size differs from the alloca, we can only convert the alloca to
      // an integer bag-of-bits.
      // FIXME: This should handle all of the cases that are currently accepted
      // as vector element insertions.
      if (Len->getZExtValue() != AllocaSize || Offset != 0)
        ScalarKind = Integer;

      IsNotTrivial = true;  // Can't be mem2reg'd.
      HadNonMemTransferAccess = true;
      continue;
    }

    // If this is a memcpy or memmove into or out of the whole allocation, we
    // can handle it like a load or store of the scalar type.
    if (MemTransferInst *MTI = dyn_cast<MemTransferInst>(User)) {
      ConstantInt *Len = dyn_cast<ConstantInt>(MTI->getLength());
      if (Len == 0 || Len->getZExtValue() != AllocaSize || Offset != 0)
        return false;

      IsNotTrivial = true;  // Can't be mem2reg'd.
      continue;
    }

    // If this is a lifetime intrinsic, we can handle it.
    if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(User)) {
      if (II->getIntrinsicID() == Intrinsic::lifetime_start ||
          II->getIntrinsicID() == Intrinsic::lifetime_end) {
        continue;
      }
    }

    // Otherwise, we cannot handle this!
    return false;
  }

  return true;
}

/// ConvertUsesToScalar - Convert all of the users of Ptr to use the new alloca
/// directly.  This happens when we are converting an "integer union" to a
/// single integer scalar, or when we are converting a "vector union" to a
/// vector with insert/extractelement instructions.
///
/// Offset is an offset from the original alloca, in bits that need to be
/// shifted to the right.  By the end of this, there should be no uses of Ptr.
void ConvertToScalarInfo::ConvertUsesToScalar(Value *Ptr, AllocaInst *NewAI,
                                              uint64_t Offset) {
  while (!Ptr->use_empty()) {
    Instruction *User = cast<Instruction>(Ptr->use_back());

    if (BitCastInst *CI = dyn_cast<BitCastInst>(User)) {
      ConvertUsesToScalar(CI, NewAI, Offset);
      CI->eraseFromParent();
      continue;
    }

    if (GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(User)) {
      // Compute the offset that this GEP adds to the pointer.
      SmallVector<Value*, 8> Indices(GEP->op_begin()+1, GEP->op_end());
      uint64_t GEPOffset = TD.getIndexedOffset(GEP->getPointerOperandType(),
                                               Indices);
      ConvertUsesToScalar(GEP, NewAI, Offset+GEPOffset*8);
      GEP->eraseFromParent();
      continue;
    }

    IRBuilder<> Builder(User);

    if (LoadInst *LI = dyn_cast<LoadInst>(User)) {
      // The load is a bit extract from NewAI shifted right by Offset bits.
      Value *LoadedVal = Builder.CreateLoad(NewAI, "tmp");
      Value *NewLoadVal
        = ConvertScalar_ExtractValue(LoadedVal, LI->getType(), Offset, Builder);
      LI->replaceAllUsesWith(NewLoadVal);
      LI->eraseFromParent();
      continue;
    }

    if (StoreInst *SI = dyn_cast<StoreInst>(User)) {
      assert(SI->getOperand(0) != Ptr && "Consistency error!");
      Instruction *Old = Builder.CreateLoad(NewAI, NewAI->getName()+".in");
      Value *New = ConvertScalar_InsertValue(SI->getOperand(0), Old, Offset,
                                             Builder);
      Builder.CreateStore(New, NewAI);
      SI->eraseFromParent();

      // If the load we just inserted is now dead, then the inserted store
      // overwrote the entire thing.
      if (Old->use_empty())
        Old->eraseFromParent();
      continue;
    }

    // If this is a constant sized memset of a constant value (e.g. 0) we can
    // transform it into a store of the expanded constant value.
    if (MemSetInst *MSI = dyn_cast<MemSetInst>(User)) {
      assert(MSI->getRawDest() == Ptr && "Consistency error!");
      unsigned NumBytes = cast<ConstantInt>(MSI->getLength())->getZExtValue();
      if (NumBytes != 0) {
        unsigned Val = cast<ConstantInt>(MSI->getValue())->getZExtValue();

        // Compute the value replicated the right number of times.
        APInt APVal(NumBytes*8, Val);

        // Splat the value if non-zero.
        if (Val)
          for (unsigned i = 1; i != NumBytes; ++i)
            APVal |= APVal << 8;

        Instruction *Old = Builder.CreateLoad(NewAI, NewAI->getName()+".in");
        Value *New = ConvertScalar_InsertValue(
                                    ConstantInt::get(User->getContext(), APVal),
                                               Old, Offset, Builder);
        Builder.CreateStore(New, NewAI);

        // If the load we just inserted is now dead, then the memset overwrote
        // the entire thing.
        if (Old->use_empty())
          Old->eraseFromParent();
      }
      MSI->eraseFromParent();
      continue;
    }

    // If this is a memcpy or memmove into or out of the whole allocation, we
    // can handle it like a load or store of the scalar type.
    if (MemTransferInst *MTI = dyn_cast<MemTransferInst>(User)) {
      assert(Offset == 0 && "must be store to start of alloca");

      // If the source and destination are both to the same alloca, then this is
      // a noop copy-to-self, just delete it.  Otherwise, emit a load and store
      // as appropriate.
      AllocaInst *OrigAI = cast<AllocaInst>(GetUnderlyingObject(Ptr, &TD, 0));

      if (GetUnderlyingObject(MTI->getSource(), &TD, 0) != OrigAI) {
        // Dest must be OrigAI, change this to be a load from the original
        // pointer (bitcasted), then a store to our new alloca.
        assert(MTI->getRawDest() == Ptr && "Neither use is of pointer?");
        Value *SrcPtr = MTI->getSource();
        PointerType* SPTy = cast<PointerType>(SrcPtr->getType());
        PointerType* AIPTy = cast<PointerType>(NewAI->getType());
        if (SPTy->getAddressSpace() != AIPTy->getAddressSpace()) {
          AIPTy = PointerType::get(AIPTy->getElementType(),
                                   SPTy->getAddressSpace());
        }
        SrcPtr = Builder.CreateBitCast(SrcPtr, AIPTy);

        LoadInst *SrcVal = Builder.CreateLoad(SrcPtr, "srcval");
        SrcVal->setAlignment(MTI->getAlignment());
        Builder.CreateStore(SrcVal, NewAI);
      } else if (GetUnderlyingObject(MTI->getDest(), &TD, 0) != OrigAI) {
        // Src must be OrigAI, change this to be a load from NewAI then a store
        // through the original dest pointer (bitcasted).
        assert(MTI->getRawSource() == Ptr && "Neither use is of pointer?");
        LoadInst *SrcVal = Builder.CreateLoad(NewAI, "srcval");

        PointerType* DPTy = cast<PointerType>(MTI->getDest()->getType());
        PointerType* AIPTy = cast<PointerType>(NewAI->getType());
        if (DPTy->getAddressSpace() != AIPTy->getAddressSpace()) {
          AIPTy = PointerType::get(AIPTy->getElementType(),
                                   DPTy->getAddressSpace());
        }
        Value *DstPtr = Builder.CreateBitCast(MTI->getDest(), AIPTy);

        StoreInst *NewStore = Builder.CreateStore(SrcVal, DstPtr);
        NewStore->setAlignment(MTI->getAlignment());
      } else {
        // Noop transfer. Src == Dst
      }

      MTI->eraseFromParent();
      continue;
    }

    if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(User)) {
      if (II->getIntrinsicID() == Intrinsic::lifetime_start ||
          II->getIntrinsicID() == Intrinsic::lifetime_end) {
        // There's no need to preserve these, as the resulting alloca will be
        // converted to a register anyways.
        II->eraseFromParent();
        continue;
      }
    }

    llvm_unreachable("Unsupported operation!");
  }
}

/// getScaledElementType - Gets a scaled element type for a partial vector
/// access of an alloca. The input types must be integer or floating-point
/// scalar or vector types, and the resulting type is an integer, float or
/// double.
static Type *getScaledElementType(Type *Ty1, Type *Ty2,
                                        unsigned NewBitWidth) {
  bool IsFP1 = Ty1->isFloatingPointTy() ||
               (Ty1->isVectorTy() &&
                cast<VectorType>(Ty1)->getElementType()->isFloatingPointTy());
  bool IsFP2 = Ty2->isFloatingPointTy() ||
               (Ty2->isVectorTy() &&
                cast<VectorType>(Ty2)->getElementType()->isFloatingPointTy());

  LLVMContext &Context = Ty1->getContext();

  // Prefer floating-point types over integer types, as integer types may have
  // been created by earlier scalar replacement.
  if (IsFP1 || IsFP2) {
    if (NewBitWidth == 32)
      return Type::getFloatTy(Context);
    if (NewBitWidth == 64)
      return Type::getDoubleTy(Context);
  }

  return Type::getIntNTy(Context, NewBitWidth);
}

/// CreateShuffleVectorCast - Creates a shuffle vector to convert one vector
/// to another vector of the same element type which has the same allocation
/// size but different primitive sizes (e.g. <3 x i32> and <4 x i32>).
static Value *CreateShuffleVectorCast(Value *FromVal, Type *ToType,
                                      IRBuilder<> &Builder) {
  Type *FromType = FromVal->getType();
  VectorType *FromVTy = cast<VectorType>(FromType);
  VectorType *ToVTy = cast<VectorType>(ToType);
  assert((ToVTy->getElementType() == FromVTy->getElementType()) &&
         "Vectors must have the same element type");
   Value *UnV = UndefValue::get(FromType);
   unsigned numEltsFrom = FromVTy->getNumElements();
   unsigned numEltsTo = ToVTy->getNumElements();

   SmallVector<Constant*, 3> Args;
   Type* Int32Ty = Builder.getInt32Ty();
   unsigned minNumElts = std::min(numEltsFrom, numEltsTo);
   unsigned i;
   for (i=0; i != minNumElts; ++i)
     Args.push_back(ConstantInt::get(Int32Ty, i));

   if (i < numEltsTo) {
     Constant* UnC = UndefValue::get(Int32Ty);
     for (; i != numEltsTo; ++i)
       Args.push_back(UnC);
   }
   Constant *Mask = ConstantVector::get(Args);
   return Builder.CreateShuffleVector(FromVal, UnV, Mask, "tmpV");
}

/// ConvertScalar_ExtractValue - Extract a value of type ToType from an integer
/// or vector value FromVal, extracting the bits from the offset specified by
/// Offset.  This returns the value, which is of type ToType.
///
/// This happens when we are converting an "integer union" to a single
/// integer scalar, or when we are converting a "vector union" to a vector with
/// insert/extractelement instructions.
///
/// Offset is an offset from the original alloca, in bits that need to be
/// shifted to the right.
Value *ConvertToScalarInfo::
ConvertScalar_ExtractValue(Value *FromVal, Type *ToType,
                           uint64_t Offset, IRBuilder<> &Builder) {
  // If the load is of the whole new alloca, no conversion is needed.
  Type *FromType = FromVal->getType();
  if (FromType == ToType && Offset == 0)
    return FromVal;

  // If the result alloca is a vector type, this is either an element
  // access or a bitcast to another vector type of the same size.
  if (VectorType *VTy = dyn_cast<VectorType>(FromType)) {
    unsigned FromTypeSize = TD.getTypeAllocSize(FromType);
    unsigned ToTypeSize = TD.getTypeAllocSize(ToType);
    if (FromTypeSize == ToTypeSize) {
      // If the two types have the same primitive size, use a bit cast.
      // Otherwise, it is two vectors with the same element type that has
      // the same allocation size but different number of elements so use
      // a shuffle vector.
      if (FromType->getPrimitiveSizeInBits() ==
          ToType->getPrimitiveSizeInBits())
        return Builder.CreateBitCast(FromVal, ToType, "tmp");
      else
        return CreateShuffleVectorCast(FromVal, ToType, Builder);
    }

    if (isPowerOf2_64(FromTypeSize / ToTypeSize)) {
      assert(!(ToType->isVectorTy() && Offset != 0) && "Can't extract a value "
             "of a smaller vector type at a nonzero offset.");

      Type *CastElementTy = getScaledElementType(FromType, ToType,
                                                       ToTypeSize * 8);
      unsigned NumCastVectorElements = FromTypeSize / ToTypeSize;

      LLVMContext &Context = FromVal->getContext();
      Type *CastTy = VectorType::get(CastElementTy,
                                           NumCastVectorElements);
      Value *Cast = Builder.CreateBitCast(FromVal, CastTy, "tmp");

      unsigned EltSize = TD.getTypeAllocSizeInBits(CastElementTy);
      unsigned Elt = Offset/EltSize;
      assert(EltSize*Elt == Offset && "Invalid modulus in validity checking");
      Value *Extract = Builder.CreateExtractElement(Cast, ConstantInt::get(
                                        Type::getInt32Ty(Context), Elt), "tmp");
      return Builder.CreateBitCast(Extract, ToType, "tmp");
    }

    // Otherwise it must be an element access.
    unsigned Elt = 0;
    if (Offset) {
      unsigned EltSize = TD.getTypeAllocSizeInBits(VTy->getElementType());
      Elt = Offset/EltSize;
      assert(EltSize*Elt == Offset && "Invalid modulus in validity checking");
    }
    // Return the element extracted out of it.
    Value *V = Builder.CreateExtractElement(FromVal, ConstantInt::get(
                    Type::getInt32Ty(FromVal->getContext()), Elt), "tmp");
    if (V->getType() != ToType)
      V = Builder.CreateBitCast(V, ToType, "tmp");
    return V;
  }

  // If ToType is a first class aggregate, extract out each of the pieces and
  // use insertvalue's to form the FCA.
  if (StructType *ST = dyn_cast<StructType>(ToType)) {
    const StructLayout &Layout = *TD.getStructLayout(ST);
    Value *Res = UndefValue::get(ST);
    for (unsigned i = 0, e = ST->getNumElements(); i != e; ++i) {
      Value *Elt = ConvertScalar_ExtractValue(FromVal, ST->getElementType(i),
                                        Offset+Layout.getElementOffsetInBits(i),
                                              Builder);
      Res = Builder.CreateInsertValue(Res, Elt, i, "tmp");
    }
    return Res;
  }

  if (ArrayType *AT = dyn_cast<ArrayType>(ToType)) {
    uint64_t EltSize = TD.getTypeAllocSizeInBits(AT->getElementType());
    Value *Res = UndefValue::get(AT);
    for (unsigned i = 0, e = AT->getNumElements(); i != e; ++i) {
      Value *Elt = ConvertScalar_ExtractValue(FromVal, AT->getElementType(),
                                              Offset+i*EltSize, Builder);
      Res = Builder.CreateInsertValue(Res, Elt, i, "tmp");
    }
    return Res;
  }

  // Otherwise, this must be a union that was converted to an integer value.
  IntegerType *NTy = cast<IntegerType>(FromVal->getType());

  // If this is a big-endian system and the load is narrower than the
  // full alloca type, we need to do a shift to get the right bits.
  int ShAmt = 0;
  if (TD.isBigEndian()) {
    // On big-endian machines, the lowest bit is stored at the bit offset
    // from the pointer given by getTypeStoreSizeInBits.  This matters for
    // integers with a bitwidth that is not a multiple of 8.
    ShAmt = TD.getTypeStoreSizeInBits(NTy) -
            TD.getTypeStoreSizeInBits(ToType) - Offset;
  } else {
    ShAmt = Offset;
  }

  // Note: we support negative bitwidths (with shl) which are not defined.
  // We do this to support (f.e.) loads off the end of a structure where
  // only some bits are used.
  if (ShAmt > 0 && (unsigned)ShAmt < NTy->getBitWidth())
    FromVal = Builder.CreateLShr(FromVal,
                                 ConstantInt::get(FromVal->getType(),
                                                           ShAmt), "tmp");
  else if (ShAmt < 0 && (unsigned)-ShAmt < NTy->getBitWidth())
    FromVal = Builder.CreateShl(FromVal,
                                ConstantInt::get(FromVal->getType(),
                                                          -ShAmt), "tmp");

  // Finally, unconditionally truncate the integer to the right width.
  unsigned LIBitWidth = TD.getTypeSizeInBits(ToType);
  if (LIBitWidth < NTy->getBitWidth())
    FromVal =
      Builder.CreateTrunc(FromVal, IntegerType::get(FromVal->getContext(),
                                                    LIBitWidth), "tmp");
  else if (LIBitWidth > NTy->getBitWidth())
    FromVal =
       Builder.CreateZExt(FromVal, IntegerType::get(FromVal->getContext(),
                                                    LIBitWidth), "tmp");

  // If the result is an integer, this is a trunc or bitcast.
  if (ToType->isIntegerTy()) {
    // Should be done.
  } else if (ToType->isFloatingPointTy() || ToType->isVectorTy()) {
    // Just do a bitcast, we know the sizes match up.
    FromVal = Builder.CreateBitCast(FromVal, ToType, "tmp");
  } else {
    // Otherwise must be a pointer.
    FromVal = Builder.CreateIntToPtr(FromVal, ToType, "tmp");
  }
  assert(FromVal->getType() == ToType && "Didn't convert right?");
  return FromVal;
}

/// ConvertScalar_InsertValue - Insert the value "SV" into the existing integer
/// or vector value "Old" at the offset specified by Offset.
///
/// This happens when we are converting an "integer union" to a
/// single integer scalar, or when we are converting a "vector union" to a
/// vector with insert/extractelement instructions.
///
/// Offset is an offset from the original alloca, in bits that need to be
/// shifted to the right.
Value *ConvertToScalarInfo::
ConvertScalar_InsertValue(Value *SV, Value *Old,
                          uint64_t Offset, IRBuilder<> &Builder) {
  // Convert the stored type to the actual type, shift it left to insert
  // then 'or' into place.
  Type *AllocaType = Old->getType();
  LLVMContext &Context = Old->getContext();

  if (VectorType *VTy = dyn_cast<VectorType>(AllocaType)) {
    uint64_t VecSize = TD.getTypeAllocSizeInBits(VTy);
    uint64_t ValSize = TD.getTypeAllocSizeInBits(SV->getType());

    // Changing the whole vector with memset or with an access of a different
    // vector type?
    if (ValSize == VecSize) {
      // If the two types have the same primitive size, use a bit cast.
      // Otherwise, it is two vectors with the same element type that has
      // the same allocation size but different number of elements so use
      // a shuffle vector.
      if (VTy->getPrimitiveSizeInBits() ==
          SV->getType()->getPrimitiveSizeInBits())
        return Builder.CreateBitCast(SV, AllocaType, "tmp");
      else
        return CreateShuffleVectorCast(SV, VTy, Builder);
    }

    if (isPowerOf2_64(VecSize / ValSize)) {
      assert(!(SV->getType()->isVectorTy() && Offset != 0) && "Can't insert a "
             "value of a smaller vector type at a nonzero offset.");

      Type *CastElementTy = getScaledElementType(VTy, SV->getType(),
                                                       ValSize);
      unsigned NumCastVectorElements = VecSize / ValSize;

      LLVMContext &Context = SV->getContext();
      Type *OldCastTy = VectorType::get(CastElementTy,
                                              NumCastVectorElements);
      Value *OldCast = Builder.CreateBitCast(Old, OldCastTy, "tmp");

      Value *SVCast = Builder.CreateBitCast(SV, CastElementTy, "tmp");

      unsigned EltSize = TD.getTypeAllocSizeInBits(CastElementTy);
      unsigned Elt = Offset/EltSize;
      assert(EltSize*Elt == Offset && "Invalid modulus in validity checking");
      Value *Insert =
        Builder.CreateInsertElement(OldCast, SVCast, ConstantInt::get(
                                        Type::getInt32Ty(Context), Elt), "tmp");
      return Builder.CreateBitCast(Insert, AllocaType, "tmp");