text: //===- MemCpyOptimizer.cpp - Optimize use of memcpy and friends -----------===//
  //
  //                     The LLVM Compiler Infrastructure
  //
  // This file is distributed under the University of Illinois Open Source
  // License. See LICENSE.TXT for details.
  //
  //===----------------------------------------------------------------------===//
  //
  // This pass performs various transformations related to eliminating memcpy
  // calls, or transforming sets of stores into memset's.
  //
  //===----------------------------------------------------------------------===//
  
  #define DEBUG_TYPE "memcpyopt"
  #include "llvm/Transforms/Scalar.h"
  #include "llvm/ADT/SmallVector.h"
  #include "llvm/ADT/Statistic.h"
  #include "llvm/Analysis/AliasAnalysis.h"
  #include "llvm/Analysis/Dominators.h"
  #include "llvm/Analysis/MemoryDependenceAnalysis.h"
  #include "llvm/Analysis/ValueTracking.h"
  #include "llvm/IR/DataLayout.h"
  #include "llvm/IR/GlobalVariable.h"
  #include "llvm/IR/IRBuilder.h"
  #include "llvm/IR/Instructions.h"
  #include "llvm/IR/IntrinsicInst.h"
  #include "llvm/Support/Debug.h"
  #include "llvm/Support/GetElementPtrTypeIterator.h"
  #include "llvm/Support/raw_ostream.h"
  #include "llvm/Target/TargetLibraryInfo.h"
  #include "llvm/Transforms/Utils/Local.h"
  #include <list>
  using namespace llvm;
  
  STATISTIC(NumMemCpyInstr, "Number of memcpy instructions deleted");
  STATISTIC(NumMemSetInfer, "Number of memsets inferred");
  STATISTIC(NumMoveToCpy,   "Number of memmoves converted to memcpy");
  STATISTIC(NumCpyToSet,    "Number of memcpys converted to memset");
  
  static int64_t GetOffsetFromIndex(const GEPOperator *GEP, unsigned Idx,
                                    bool &VariableIdxFound, const DataLayout &TD){
    // Skip over the first indices.
    gep_type_iterator GTI = gep_type_begin(GEP);
    for (unsigned i = 1; i != Idx; ++i, ++GTI)
      /*skip along*/;
  
    // Compute the offset implied by the rest of the indices.
    int64_t Offset = 0;
    for (unsigned i = Idx, e = GEP->getNumOperands(); i != e; ++i, ++GTI) {
      ConstantInt *OpC = dyn_cast<ConstantInt>(GEP->getOperand(i));
      if (OpC == 0)
        return VariableIdxFound = true;
      if (OpC->isZero()) continue;  // No offset.
  
      // Handle struct indices, which add their field offset to the pointer.
      if (StructType *STy = dyn_cast<StructType>(*GTI)) {
        Offset += TD.getStructLayout(STy)->getElementOffset(OpC->getZExtValue());
        continue;
      }
  
      // Otherwise, we have a sequential type like an array or vector.  Multiply
      // the index by the ElementSize.
      uint64_t Size = TD.getTypeAllocSize(GTI.getIndexedType());
      Offset += Size*OpC->getSExtValue();
    }
  
    return Offset;
  }
  
  /// IsPointerOffset - Return true if Ptr1 is provably equal to Ptr2 plus a
  /// constant offset, and return that constant offset.  For example, Ptr1 might
  /// be &A[42], and Ptr2 might be &A[40].  In this case offset would be -8.
  static bool IsPointerOffset(Value *Ptr1, Value *Ptr2, int64_t &Offset,
                              const DataLayout &TD) {
    Ptr1 = Ptr1->stripPointerCasts();
    Ptr2 = Ptr2->stripPointerCasts();
    GEPOperator *GEP1 = dyn_cast<GEPOperator>(Ptr1);
    GEPOperator *GEP2 = dyn_cast<GEPOperator>(Ptr2);
  
    bool VariableIdxFound = false;
  
    // If one pointer is a GEP and the other isn't, then see if the GEP is a
    // constant offset from the base, as in "P" and "gep P, 1".
    if (GEP1 && GEP2 == 0 && GEP1->getOperand(0)->stripPointerCasts() == Ptr2) {
      Offset = -GetOffsetFromIndex(GEP1, 1, VariableIdxFound, TD);
      return !VariableIdxFound;
    }
  
    if (GEP2 && GEP1 == 0 && GEP2->getOperand(0)->stripPointerCasts() == Ptr1) {
      Offset = GetOffsetFromIndex(GEP2, 1, VariableIdxFound, TD);
      return !VariableIdxFound;
    }
  
    // Right now we handle the case when Ptr1/Ptr2 are both GEPs with an identical
    // base.  After that base, they may have some number of common (and
    // potentially variable) indices.  After that they handle some constant
    // offset, which determines their offset from each other.  At this point, we
    // handle no other case.
    if (!GEP1 || !GEP2 || GEP1->getOperand(0) != GEP2->getOperand(0))
      return false;
  
    // Skip any common indices and track the GEP types.
    unsigned Idx = 1;
    for (; Idx != GEP1->getNumOperands() && Idx != GEP2->getNumOperands(); ++Idx)
      if (GEP1->getOperand(Idx) != GEP2->getOperand(Idx))
        break;
  
    int64_t Offset1 = GetOffsetFromIndex(GEP1, Idx, VariableIdxFound, TD);
    int64_t Offset2 = GetOffsetFromIndex(GEP2, Idx, VariableIdxFound, TD);
    if (VariableIdxFound) return false;
  
    Offset = Offset2-Offset1;
    return true;
  }
  
  
  /// MemsetRange - Represents a range of memset'd bytes with the ByteVal value.
  /// This allows us to analyze stores like:
  ///   store 0 -> P+1
  ///   store 0 -> P+0
  ///   store 0 -> P+3
  ///   store 0 -> P+2
  /// which sometimes happens with stores to arrays of structs etc.  When we see
  /// the first store, we make a range [1, 2).  The second store extends the range
  /// to [0, 2).  The third makes a new range [2, 3).  The fourth store joins the
  /// two ranges into [0, 3) which is memset'able.
  namespace {
  struct MemsetRange {
    // Start/End - A semi range that describes the span that this range covers.
    // The range is closed at the start and open at the end: [Start, End).
    int64_t Start, End;
  
    /// StartPtr - The getelementptr instruction that points to the start of the
    /// range.
    Value *StartPtr;
  
    /// Alignment - The known alignment of the first store.
    unsigned Alignment;
  
    /// TheStores - The actual stores that make up this range.
    SmallVector<Instruction*, 16> TheStores;
  
    bool isProfitableToUseMemset(const DataLayout &TD) const;
  
  };
  } // end anon namespace
  
  bool MemsetRange::isProfitableToUseMemset(const DataLayout &TD) const {
    // If we found more than 4 stores to merge or 16 bytes, use memset.
    if (TheStores.size() >= 4 || End-Start >= 16) return true;
  
    // If there is nothing to merge, don't do anything.
    if (TheStores.size() < 2) return false;
  
    // If any of the stores are a memset, then it is always good to extend the
    // memset.
    for (unsigned i = 0, e = TheStores.size(); i != e; ++i)
      if (!isa<StoreInst>(TheStores[i]))
        return true;
  
    // Assume that the code generator is capable of merging pairs of stores
    // together if it wants to.
    if (TheStores.size() == 2) return false;
  
    // If we have fewer than 8 stores, it can still be worthwhile to do this.
    // For example, merging 4 i8 stores into an i32 store is useful almost always.
    // However, merging 2 32-bit stores isn't useful on a 32-bit architecture (the
    // memset will be split into 2 32-bit stores anyway) and doing so can
    // pessimize the llvm optimizer.
    //
    // Since we don't have perfect knowledge here, make some assumptions: assume
    // the maximum GPR width is the same size as the pointer size and assume that
    // this width can be stored.  If so, check to see whether we will end up
    // actually reducing the number of stores used.
    unsigned Bytes = unsigned(End-Start);
    unsigned NumPointerStores = Bytes/TD.getPointerSize();
  
    // Assume the remaining bytes if any are done a byte at a time.
    unsigned NumByteStores = Bytes - NumPointerStores*TD.getPointerSize();
  
    // If we will reduce the # stores (according to this heuristic), do the
    // transformation.  This encourages merging 4 x i8 -> i32 and 2 x i16 -> i32
    // etc.
    return TheStores.size() > NumPointerStores+NumByteStores;
  }
  
  
  namespace {
  class MemsetRanges {
    /// Ranges - A sorted list of the memset ranges.  We use std::list here
    /// because each element is relatively large and expensive to copy.
    std::list<MemsetRange> Ranges;
    typedef std::list<MemsetRange>::iterator range_iterator;
    const DataLayout &TD;
  public:
    MemsetRanges(const DataLayout &td) : TD(td) {}
  
    typedef std::list<MemsetRange>::const_iterator const_iterator;
    const_iterator begin() const { return Ranges.begin(); }
    const_iterator end() const { return Ranges.end(); }
    bool empty() const { return Ranges.empty(); }
  
    void addInst(int64_t OffsetFromFirst, Instruction *Inst) {
      if (StoreInst *SI = dyn_cast<StoreInst>(Inst))
        addStore(OffsetFromFirst, SI);
      else
        addMemSet(OffsetFromFirst, cast<MemSetInst>(Inst));
    }
  
    void addStore(int64_t OffsetFromFirst, StoreInst *SI) {
      int64_t StoreSize = TD.getTypeStoreSize(SI->getOperand(0)->getType());
  
      addRange(OffsetFromFirst, StoreSize,
               SI->getPointerOperand(), SI->getAlignment(), SI);
    }
  
    void addMemSet(int64_t OffsetFromFirst, MemSetInst *MSI) {
      int64_t Size = cast<ConstantInt>(MSI->getLength())->getZExtValue();
      addRange(OffsetFromFirst, Size, MSI->getDest(), MSI->getAlignment(), MSI);
    }
  
    void addRange(int64_t Start, int64_t Size, Value *Ptr,
                  unsigned Alignment, Instruction *Inst);
  
  };
  
  } // end anon namespace
  
  
  /// addRange - Add a new store to the MemsetRanges data structure.  This adds a
  /// new range for the specified store at the specified offset, merging into
  /// existing ranges as appropriate.
  ///
  /// Do a linear search of the ranges to see if this can be joined and/or to
  /// find the insertion point in the list.  We keep the ranges sorted for
  /// simplicity here.  This is a linear search of a linked list, which is ugly,
  /// however the number of ranges is limited, so this won't get crazy slow.
  void MemsetRanges::addRange(int64_t Start, int64_t Size, Value *Ptr,
                              unsigned Alignment, Instruction *Inst) {
    int64_t End = Start+Size;
    range_iterator I = Ranges.begin(), E = Ranges.end();
  
    while (I != E && Start > I->End)
      ++I;
  
    // We now know that I == E, in which case we didn't find anything to merge
    // with, or that Start <= I->End.  If End < I->Start or I == E, then we need
    // to insert a new range.  Handle this now.
    if (I == E || End < I->Start) {
      MemsetRange &R = *Ranges.insert(I, MemsetRange());
      R.Start        = Start;
      R.End          = End;
      R.StartPtr     = Ptr;
      R.Alignment    = Alignment;
      R.TheStores.push_back(Inst);
      return;
    }
  
    // This store overlaps with I, add it.
    I->TheStores.push_back(Inst);
  
    // At this point, we may have an interval that completely contains our store.
    // If so, just add it to the interval and return.
    if (I->Start <= Start && I->End >= End)
      return;
  
    // Now we know that Start <= I->End and End >= I->Start so the range overlaps
    // but is not entirely contained within the range.
  
    // See if the range extends the start of the range.  In this case, it couldn't
    // possibly cause it to join the prior range, because otherwise we would have
    // stopped on *it*.
    if (Start < I->Start) {
      I->Start = Start;
      I->StartPtr = Ptr;
      I->Alignment = Alignment;
    }
  
    // Now we know that Start <= I->End and Start >= I->Start (so the startpoint
    // is in or right at the end of I), and that End >= I->Start.  Extend I out to
    // End.
    if (End > I->End) {
      I->End = End;
      range_iterator NextI = I;
      while (++NextI != E && End >= NextI->Start) {
        // Merge the range in.
        I->TheStores.append(NextI->TheStores.begin(), NextI->TheStores.end());
        if (NextI->End > I->End)
          I->End = NextI->End;
        Ranges.erase(NextI);
        NextI = I;
      }
    }
  }
  
  //===----------------------------------------------------------------------===//
  //                         MemCpyOpt Pass
  //===----------------------------------------------------------------------===//
  
  namespace {
    class MemCpyOpt : public FunctionPass {
      MemoryDependenceAnalysis *MD;
      TargetLibraryInfo *TLI;
      const DataLayout *TD;
    public:
      static char ID; // Pass identification, replacement for typeid
      MemCpyOpt() : FunctionPass(ID) {
        initializeMemCpyOptPass(*PassRegistry::getPassRegistry());
        MD = 0;
        TLI = 0;
        TD = 0;
      }
  
      bool runOnFunction(Function &F);
  
    private:
      // This transformation requires dominator postdominator info
      virtual void getAnalysisUsage(AnalysisUsage &AU) const {
        AU.setPreservesCFG();
        AU.addRequired<DominatorTree>();
        AU.addRequired<MemoryDependenceAnalysis>();
        AU.addRequired<AliasAnalysis>();
        AU.addRequired<TargetLibraryInfo>();
        AU.addPreserved<AliasAnalysis>();
        AU.addPreserved<MemoryDependenceAnalysis>();
      }
  
      // Helper fuctions
      bool processStore(StoreInst *SI, BasicBlock::iterator &BBI);
      bool processMemSet(MemSetInst *SI, BasicBlock::iterator &BBI);
      bool processMemCpy(MemCpyInst *M);
      bool processMemMove(MemMoveInst *M);
      bool performCallSlotOptzn(Instruction *cpy, Value *cpyDst, Value *cpySrc,
                                uint64_t cpyLen, unsigned cpyAlign, CallInst *C);
      bool processMemCpyMemCpyDependence(MemCpyInst *M, MemCpyInst *MDep,
                                         uint64_t MSize);
      bool processByValArgument(CallSite CS, unsigned ArgNo);
      Instruction *tryMergingIntoMemset(Instruction *I, Value *StartPtr,
                                        Value *ByteVal);
  
      bool iterateOnFunction(Function &F);
    };
  
    char MemCpyOpt::ID = 0;
  }
  
  // createMemCpyOptPass - The public interface to this file...
  FunctionPass *llvm::createMemCpyOptPass() { return new MemCpyOpt(); }
  
  INITIALIZE_PASS_BEGIN(MemCpyOpt, "memcpyopt", "MemCpy Optimization",
                        false, false)
  INITIALIZE_PASS_DEPENDENCY(DominatorTree)
  INITIALIZE_PASS_DEPENDENCY(MemoryDependenceAnalysis)
  INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfo)
  INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
  INITIALIZE_PASS_END(MemCpyOpt, "memcpyopt", "MemCpy Optimization",
                      false, false)
  
  /// tryMergingIntoMemset - When scanning forward over instructions, we look for
  /// some other patterns to fold away.  In particular, this looks for stores to
  /// neighboring locations of memory.  If it sees enough consecutive ones, it
  /// attempts to merge them together into a memcpy/memset.
  Instruction *MemCpyOpt::tryMergingIntoMemset(Instruction *StartInst,
                                               Value *StartPtr, Value *ByteVal) {
    if (TD == 0) return 0;
  
    // Okay, so we now have a single store that can be splatable.  Scan to find
    // all subsequent stores of the same value to offset from the same pointer.
    // Join these together into ranges, so we can decide whether contiguous blocks
    // are stored.
    MemsetRanges Ranges(*TD);
  
    BasicBlock::iterator BI = StartInst;
    for (++BI; !isa<TerminatorInst>(BI); ++BI) {
      if (!isa<StoreInst>(BI) && !isa<MemSetInst>(BI)) {
        // If the instruction is readnone, ignore it, otherwise bail out.  We
        // don't even allow readonly here because we don't want something like:
        // A[1] = 2; strlen(A); A[2] = 2; -> memcpy(A, ...); strlen(A).
        if (BI->mayWriteToMemory() || BI->mayReadFromMemory())
          break;
        continue;
      }
  
      if (StoreInst *NextStore = dyn_cast<StoreInst>(BI)) {
        // If this is a store, see if we can merge it in.
        if (!NextStore->isSimple()) break;
  
        // Check to see if this stored value is of the same byte-splattable value.
        if (ByteVal != isBytewiseValue(NextStore->getOperand(0)))
          break;
  
        // Check to see if this store is to a constant offset from the start ptr.
        int64_t Offset;
        if (!IsPointerOffset(StartPtr, NextStore->getPointerOperand(),
                             Offset, *TD))
          break;
  
        Ranges.addStore(Offset, NextStore);
      } else {
        MemSetInst *MSI = cast<MemSetInst>(BI);
  
        if (MSI->isVolatile() || ByteVal != MSI->getValue() ||
            !isa<ConstantInt>(MSI->getLength()))
          break;
  
        // Check to see if this store is to a constant offset from the start ptr.
        int64_t Offset;
        if (!IsPointerOffset(StartPtr, MSI->getDest(), Offset, *TD))
          break;
  
        Ranges.addMemSet(Offset, MSI);
      }
    }
  
    // If we have no ranges, then we just had a single store with nothing that
    // could be merged in.  This is a very common case of course.
    if (Ranges.empty())
      return 0;
  
    // If we had at least one store that could be merged in, add the starting
    // store as well.  We try to avoid this unless there is at least something
    // interesting as a small compile-time optimization.
    Ranges.addInst(0, StartInst);
  
    // If we create any memsets, we put it right before the first instruction that
    // isn't part of the memset block.  This ensure that the memset is dominated
    // by any addressing instruction needed by the start of the block.
    IRBuilder<> Builder(BI);
  
    // Now that we have full information about ranges, loop over the ranges and
    // emit memset's for anything big enough to be worthwhile.
    Instruction *AMemSet = 0;
    for (MemsetRanges::const_iterator I = Ranges.begin(), E = Ranges.end();
         I != E; ++I) {
      const MemsetRange &Range = *I;
  
      if (Range.TheStores.size() == 1) continue;
  
      // If it is profitable to lower this range to memset, do so now.
      if (!Range.isProfitableToUseMemset(*TD))
        continue;
  
      // Otherwise, we do want to transform this!  Create a new memset.
      // Get the starting pointer of the block.
      StartPtr = Range.StartPtr;
  
      // Determine alignment
      unsigned Alignment = Range.Alignment;
      if (Alignment == 0) {
        Type *EltType =
          cast<PointerType>(StartPtr->getType())->getElementType();
        Alignment = TD->getABITypeAlignment(EltType);
      }
  
      AMemSet =
        Builder.CreateMemSet(StartPtr, ByteVal, Range.End-Range.Start, Alignment);
  
      DEBUG(dbgs() << "Replace stores:\n";
            for (unsigned i = 0, e = Range.TheStores.size(); i != e; ++i)
              dbgs() << *Range.TheStores[i] << '\n';
            dbgs() << "With: " << *AMemSet << '\n');
  
      if (!Range.TheStores.empty())
        AMemSet->setDebugLoc(Range.TheStores[0]->getDebugLoc());
  
      // Zap all the stores.
      for (SmallVector<Instruction*, 16>::const_iterator
           SI = Range.TheStores.begin(),
           SE = Range.TheStores.end(); SI != SE; ++SI) {
        MD->removeInstruction(*SI);
        (*SI)->eraseFromParent();
      }
      ++NumMemSetInfer;
    }
  
    return AMemSet;
  }
  
  
  bool MemCpyOpt::processStore(StoreInst *SI, BasicBlock::iterator &BBI) {
    if (!SI->isSimple()) return false;
  
    if (TD == 0) return false;
  
    // Detect cases where we're performing call slot forwarding, but
    // happen to be using a load-store pair to implement it, rather than
    // a memcpy.
    if (LoadInst *LI = dyn_cast<LoadInst>(SI->getOperand(0))) {
      if (LI->isSimple() && LI->hasOneUse() &&
          LI->getParent() == SI->getParent()) {
        MemDepResult ldep = MD->getDependency(LI);
        CallInst *C = 0;
        if (ldep.isClobber() && !isa<MemCpyInst>(ldep.getInst()))
          C = dyn_cast<CallInst>(ldep.getInst());
  
        if (C) {
          // Check that nothing touches the dest of the "copy" between
          // the call and the store.
          AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
          AliasAnalysis::Location StoreLoc = AA.getLocation(SI);
          for (BasicBlock::iterator I = --BasicBlock::iterator(SI),
                                    E = C; I != E; --I) {
            if (AA.getModRefInfo(&*I, StoreLoc) != AliasAnalysis::NoModRef) {
              C = 0;
              break;
            }
          }
        }
  
        if (C) {
          unsigned storeAlign = SI->getAlignment();
          if (!storeAlign)
            storeAlign = TD->getABITypeAlignment(SI->getOperand(0)->getType());
          unsigned loadAlign = LI->getAlignment();
          if (!loadAlign)
            loadAlign = TD->getABITypeAlignment(LI->getType());
  
          bool changed = performCallSlotOptzn(LI,
                          SI->getPointerOperand()->stripPointerCasts(),
                          LI->getPointerOperand()->stripPointerCasts(),
                          TD->getTypeStoreSize(SI->getOperand(0)->getType()),
                          std::min(storeAlign, loadAlign), C);
          if (changed) {
            MD->removeInstruction(SI);
            SI->eraseFromParent();
            MD->removeInstruction(LI);
            LI->eraseFromParent();
            ++NumMemCpyInstr;
            return true;
          }
        }
      }
    }
  
    // There are two cases that are interesting for this code to handle: memcpy
    // and memset.  Right now we only handle memset.
  
    // Ensure that the value being stored is something that can be memset'able a
    // byte at a time like "0" or "-1" or any width, as well as things like
    // 0xA0A0A0A0 and 0.0.
    if (Value *ByteVal = isBytewiseValue(SI->getOperand(0)))
      if (Instruction *I = tryMergingIntoMemset(SI, SI->getPointerOperand(),
                                                ByteVal)) {
        BBI = I;  // Don't invalidate iterator.
        return true;
      }
  
    return false;
  }
  
  bool MemCpyOpt::processMemSet(MemSetInst *MSI, BasicBlock::iterator &BBI) {
    // See if there is another memset or store neighboring this memset which
    // allows us to widen out the memset to do a single larger store.
    if (isa<ConstantInt>(MSI->getLength()) && !MSI->isVolatile())
      if (Instruction *I = tryMergingIntoMemset(MSI, MSI->getDest(),
                                                MSI->getValue())) {
        BBI = I;  // Don't invalidate iterator.
        return true;
      }
    return false;
  }
  
  
  /// performCallSlotOptzn - takes a memcpy and a call that it depends on,
  /// and checks for the possibility of a call slot optimization by having
  /// the call write its result directly into the destination of the memcpy.
  bool MemCpyOpt::performCallSlotOptzn(Instruction *cpy,
                                       Value *cpyDest, Value *cpySrc,
                                       uint64_t cpyLen, unsigned cpyAlign,
                                       CallInst *C) {
    // The general transformation to keep in mind is
    //
    //   call @func(..., src, ...)
    //   memcpy(dest, src, ...)
    //
    // ->
    //
    //   memcpy(dest, src, ...)
    //   call @func(..., dest, ...)
    //
    // Since moving the memcpy is technically awkward, we additionally check that
    // src only holds uninitialized values at the moment of the call, meaning that
    // the memcpy can be discarded rather than moved.
  
    // Deliberately get the source and destination with bitcasts stripped away,
    // because we'll need to do type comparisons based on the underlying type.
    CallSite CS(C);
  
    // Require that src be an alloca.  This simplifies the reasoning considerably.
    AllocaInst *srcAlloca = dyn_cast<AllocaInst>(cpySrc);
    if (!srcAlloca)
      return false;
  
    // Check that all of src is copied to dest.
    if (TD == 0) return false;
  
    ConstantInt *srcArraySize = dyn_cast<ConstantInt>(srcAlloca->getArraySize());
    if (!srcArraySize)
      return false;
  
    uint64_t srcSize = TD->getTypeAllocSize(srcAlloca->getAllocatedType()) *
      srcArraySize->getZExtValue();
  
    if (cpyLen < srcSize)
      return false;
  
    // Check that accessing the first srcSize bytes of dest will not cause a
    // trap.  Otherwise the transform is invalid since it might cause a trap
    // to occur earlier than it otherwise would.
    if (AllocaInst *A = dyn_cast<AllocaInst>(cpyDest)) {
      // The destination is an alloca.  Check it is larger than srcSize.
      ConstantInt *destArraySize = dyn_cast<ConstantInt>(A->getArraySize());
      if (!destArraySize)
        return false;
  
      uint64_t destSize = TD->getTypeAllocSize(A->getAllocatedType()) *
        destArraySize->getZExtValue();
  
      if (destSize < srcSize)
        return false;
    } else if (Argument *A = dyn_cast<Argument>(cpyDest)) {
      // If the destination is an sret parameter then only accesses that are
      // outside of the returned struct type can trap.
      if (!A->hasStructRetAttr())
        return false;
  
      Type *StructTy = cast<PointerType>(A->getType())->getElementType();
      uint64_t destSize = TD->getTypeAllocSize(StructTy);
  
      if (destSize < srcSize)
        return false;
    } else {
      return false;
    }
  
    // Check that dest points to memory that is at least as aligned as src.
    unsigned srcAlign = srcAlloca->getAlignment();
    if (!srcAlign)
      srcAlign = TD->getABITypeAlignment(srcAlloca->getAllocatedType());
    bool isDestSufficientlyAligned = srcAlign <= cpyAlign;
    // If dest is not aligned enough and we can't increase its alignment then
    // bail out.
    if (!isDestSufficientlyAligned && !isa<AllocaInst>(cpyDest))
      return false;
  
    // Check that src is not accessed except via the call and the memcpy.  This
    // guarantees that it holds only undefined values when passed in (so the final
    // memcpy can be dropped), that it is not read or written between the call and
    // the memcpy, and that writing beyond the end of it is undefined.
    SmallVector<User*, 8> srcUseList(srcAlloca->use_begin(),
                                     srcAlloca->use_end());
    while (!srcUseList.empty()) {
      User *UI = srcUseList.pop_back_val();
  
      if (isa<BitCastInst>(UI)) {
        for (User::use_iterator I = UI->use_begin(), E = UI->use_end();
             I != E; ++I)
          srcUseList.push_back(*I);
      } else if (GetElementPtrInst *G = dyn_cast<GetElementPtrInst>(UI)) {
        if (G->hasAllZeroIndices())
          for (User::use_iterator I = UI->use_begin(), E = UI->use_end();
               I != E; ++I)
            srcUseList.push_back(*I);
        else
          return false;
      } else if (UI != C && UI != cpy) {
        return false;
      }
    }
  
    // Since we're changing the parameter to the callsite, we need to make sure
    // that what would be the new parameter dominates the callsite.
    DominatorTree &DT = getAnalysis<DominatorTree>();
    if (Instruction *cpyDestInst = dyn_cast<Instruction>(cpyDest))
      if (!DT.dominates(cpyDestInst, C))
        return false;
  
    // In addition to knowing that the call does not access src in some
    // unexpected manner, for example via a global, which we deduce from
    // the use analysis, we also need to know that it does not sneakily
    // access dest.  We rely on AA to figure this out for us.
    AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
    AliasAnalysis::ModRefResult MR = AA.getModRefInfo(C, cpyDest, srcSize);
    // If necessary, perform additional analysis.
    if (MR != AliasAnalysis::NoModRef)
      MR = AA.callCapturesBefore(C, cpyDest, srcSize, &DT);
    if (MR != AliasAnalysis::NoModRef)
      return false;
  
    // All the checks have passed, so do the transformation.
    bool changedArgument = false;
    for (unsigned i = 0; i < CS.arg_size(); ++i)
      if (CS.getArgument(i)->stripPointerCasts() == cpySrc) {
        Value *Dest = cpySrc->getType() == cpyDest->getType() ?  cpyDest
          : CastInst::CreatePointerCast(cpyDest, cpySrc->getType(),
                                        cpyDest->getName(), C);
        changedArgument = true;
        if (CS.getArgument(i)->getType() == Dest->getType())
          CS.setArgument(i, Dest);
        else
          CS.setArgument(i, CastInst::CreatePointerCast(Dest,
                            CS.getArgument(i)->getType(), Dest->getName(), C));
      }
  
    if (!changedArgument)
      return false;
  
    // If the destination wasn't sufficiently aligned then increase its alignment.
    if (!isDestSufficientlyAligned) {
      assert(isa<AllocaInst>(cpyDest) && "Can only increase alloca alignment!");
      cast<AllocaInst>(cpyDest)->setAlignment(srcAlign);
    }
  
    // Drop any cached information about the call, because we may have changed
    // its dependence information by changing its parameter.
    MD->removeInstruction(C);
  
    // Remove the memcpy.
    MD->removeInstruction(cpy);
    ++NumMemCpyInstr;
  
    return true;
  }
  
  /// processMemCpyMemCpyDependence - We've found that the (upward scanning)
  /// memory dependence of memcpy 'M' is the memcpy 'MDep'.  Try to simplify M to
  /// copy from MDep's input if we can.  MSize is the size of M's copy.
  ///
  bool MemCpyOpt::processMemCpyMemCpyDependence(MemCpyInst *M, MemCpyInst *MDep,
                                                uint64_t MSize) {
    // We can only transforms memcpy's where the dest of one is the source of the
    // other.
    if (M->getSource() != MDep->getDest() || MDep->isVolatile())
      return false;
  
    // If dep instruction is reading from our current input, then it is a noop
    // transfer and substituting the input won't change this instruction.  Just
    // ignore the input and let someone else zap MDep.  This handles cases like:
    //    memcpy(a <- a)
    //    memcpy(b <- a)
    if (M->getSource() == MDep->getSource())
      return false;
  
    // Second, the length of the memcpy's must be the same, or the preceding one
    // must be larger than the following one.
    ConstantInt *MDepLen = dyn_cast<ConstantInt>(MDep->getLength());
    ConstantInt *MLen = dyn_cast<ConstantInt>(M->getLength());
    if (!MDepLen || !MLen || MDepLen->getZExtValue() < MLen->getZExtValue())
      return false;
  
    AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
  
    // Verify that the copied-from memory doesn't change in between the two
    // transfers.  For example, in:
    //    memcpy(a <- b)
    //    *b = 42;
    //    memcpy(c <- a)
    // It would be invalid to transform the second memcpy into memcpy(c <- b).
    //
    // TODO: If the code between M and MDep is transparent to the destination "c",
    // then we could still perform the xform by moving M up to the first memcpy.
    //
    // NOTE: This is conservative, it will stop on any read from the source loc,
    // not just the defining memcpy.
    MemDepResult SourceDep =
      MD->getPointerDependencyFrom(AA.getLocationForSource(MDep),
                                   false, M, M->getParent());
    if (!SourceDep.isClobber() || SourceDep.getInst() != MDep)
      return false;
  
    // If the dest of the second might alias the source of the first, then the
    // source and dest might overlap.  We still want to eliminate the intermediate
    // value, but we have to generate a memmove instead of memcpy.
    bool UseMemMove = false;
    if (!AA.isNoAlias(AA.getLocationForDest(M), AA.getLocationForSource(MDep)))
      UseMemMove = true;
  
    // If all checks passed, then we can transform M.
  
    // Make sure to use the lesser of the alignment of the source and the dest
    // since we're changing where we're reading from, but don't want to increase
    // the alignment past what can be read from or written to.
    // TODO: Is this worth it if we're creating a less aligned memcpy? For
    // example we could be moving from movaps -> movq on x86.
    unsigned Align = std::min(MDep->getAlignment(), M->getAlignment());
  
    IRBuilder<> Builder(M);
    if (UseMemMove)
      Builder.CreateMemMove(M->getRawDest(), MDep->getRawSource(), M->getLength(),
                            Align, M->isVolatile());
    else
      Builder.CreateMemCpy(M->getRawDest(), MDep->getRawSource(), M->getLength(),
                           Align, M->isVolatile());
  
    // Remove the instruction we're replacing.
    MD->removeInstruction(M);
    M->eraseFromParent();
    ++NumMemCpyInstr;
    return true;
  }
  
  
  /// processMemCpy - perform simplification of memcpy's.  If we have memcpy A
  /// which copies X to Y, and memcpy B which copies Y to Z, then we can rewrite
  /// B to be a memcpy from X to Z (or potentially a memmove, depending on
  /// circumstances). This allows later passes to remove the first memcpy
  /// altogether.
  bool MemCpyOpt::processMemCpy(MemCpyInst *M) {
    // We can only optimize statically-sized memcpy's that are non-volatile.
    ConstantInt *CopySize = dyn_cast<ConstantInt>(M->getLength());
    if (CopySize == 0 || M->isVolatile()) return false;
  
    // If the source and destination of the memcpy are the same, then zap it.
    if (M->getSource() == M->getDest()) {
      MD->removeInstruction(M);
      M->eraseFromParent();
      return false;
    }
  
    // If copying from a constant, try to turn the memcpy into a memset.
    if (GlobalVariable *GV = dyn_cast<GlobalVariable>(M->getSource()))
      if (GV->isConstant() && GV->hasDefinitiveInitializer())
        if (Value *ByteVal = isBytewiseValue(GV->getInitializer())) {
          IRBuilder<> Builder(M);
          Builder.CreateMemSet(M->getRawDest(), ByteVal, CopySize,
                               M->getAlignment(), false);
          MD->removeInstruction(M);
          M->eraseFromParent();
          ++NumCpyToSet;
          return true;
        }
  
    // The are two possible optimizations we can do for memcpy:
    //   a) memcpy-memcpy xform which exposes redundance for DSE.
    //   b) call-memcpy xform for return slot optimization.
    MemDepResult DepInfo = MD->getDependency(M);
    if (DepInfo.isClobber()) {
      if (CallInst *C = dyn_cast<CallInst>(DepInfo.getInst())) {
        if (performCallSlotOptzn(M, M->getDest(), M->getSource(),
                                 CopySize->getZExtValue(), M->getAlignment(),
                                 C)) {
          MD->removeInstruction(M);
          M->eraseFromParent();
          return true;
        }
      }
    }
  
    AliasAnalysis::Location SrcLoc = AliasAnalysis::getLocationForSource(M);
    MemDepResult SrcDepInfo = MD->getPointerDependencyFrom(SrcLoc, true,
                                                           M, M->getParent());
    if (SrcDepInfo.isClobber()) {
      if (MemCpyInst *MDep = dyn_cast<MemCpyInst>(SrcDepInfo.getInst()))
        return processMemCpyMemCpyDependence(M, MDep, CopySize->getZExtValue());
    }
  
    return false;
  }
  
  /// processMemMove - Transforms memmove calls to memcpy calls when the src/dst
  /// are guaranteed not to alias.
  bool MemCpyOpt::processMemMove(MemMoveInst *M) {
    AliasAnalysis &AA = getAnalysis<AliasAnalysis>();
  
    if (!TLI->has(LibFunc::memmove))
      return false;
  
    // See if the pointers alias.
    if (!AA.isNoAlias(AA.getLocationForDest(M), AA.getLocationForSource(M)))
      return false;
  
    DEBUG(dbgs() << "MemCpyOpt: Optimizing memmove -> memcpy: " << *M << "\n");
  
    // If not, then we know we can transform this.
    Module *Mod = M->getParent()->getParent()->getParent();
    Type *ArgTys[3] = { M->getRawDest()->getType(),
                        M->getRawSource()->getType(),
                        M->getLength()->getType() };
    M->setCalledFunction(Intrinsic::getDeclaration(Mod, Intrinsic::memcpy,
                                                   ArgTys));
  
    // MemDep may have over conservative information about this instruction, just
    // conservatively flush it from the cache.
    MD->removeInstruction(M);
  
    ++NumMoveToCpy;
    return true;
  }
  
  /// processByValArgument - This is called on every byval argument in call sites.
  bool MemCpyOpt::processByValArgument(CallSite CS, unsigned ArgNo) {
    if (TD == 0) return false;
  
    // Find out what feeds this byval argument.
    Value *ByValArg = CS.getArgument(ArgNo);
    Type *ByValTy = cast<PointerType>(ByValArg->getType())->getElementType();
    uint64_t ByValSize = TD->getTypeAllocSize(ByValTy);
    MemDepResult DepInfo =
      MD->getPointerDependencyFrom(AliasAnalysis::Location(ByValArg, ByValSize),
                                   true, CS.getInstruction(),
                                   CS.getInstruction()->getParent());
    if (!DepInfo.isClobber())
      return false;
  
    // If the byval argument isn't fed by a memcpy, ignore it.  If it is fed by
    // a memcpy, see if we can byval from the source of the memcpy instead of the
    // result.
    MemCpyInst *MDep = dyn_cast<MemCpyInst>(DepInfo.getInst());
    if (MDep == 0 || MDep->isVolatile() ||
        ByValArg->stripPointerCasts() != MDep->getDest())
      return false;
  
    // The length of the memcpy must be larger or equal to the size of the byval.
    ConstantInt *C1 = dyn_cast<ConstantInt>(MDep->getLength());
    if (C1 == 0 || C1->getValue().getZExtValue() < ByValSize)
      return false;
  
    // Get the alignment of the byval.  If the call doesn't specify the alignment,
    // then it is some target specific value that we can't know.
    unsigned ByValAlign = CS.getParamAlignment(ArgNo+1);
    if (ByValAlign == 0) return false;
  
    // If it is greater than the memcpy, then we check to see if we can force the
    // source of the memcpy to the alignment we need.  If we fail, we bail out.
    if (MDep->getAlignment() < ByValAlign &&
        getOrEnforceKnownAlignment(MDep->getSource(),ByValAlign, TD) < ByValAlign)
      return false;
  
    // Verify that the copied-from memory doesn't change in between the memcpy and
    // the byval call.
    //    memcpy(a <- b)
    //    *b = 42;
    //    foo(*a)
    // It would be invalid to transform the second memcpy into foo(*b).
    //
    // NOTE: This is conservative, it will stop on any read from the source loc,
    // not just the defining memcpy.
    MemDepResult SourceDep =
      MD->getPointerDependencyFrom(AliasAnalysis::getLocationForSource(MDep),
                                   false, CS.getInstruction(), MDep->getParent());
    if (!SourceDep.isClobber() || SourceDep.getInst() != MDep)
      return false;
  
    Value *TmpCast = MDep->getSource();
    if (MDep->getSource()->getType() != ByValArg->getType())
      TmpCast = new BitCastInst(MDep->getSource(), ByValArg->getType(),
                                "tmpcast", CS.getInstruction());
  
    DEBUG(dbgs() << "MemCpyOpt: Forwarding memcpy to byval:\n"
                 << "  " << *MDep << "\n"
                 << "  " << *CS.getInstruction() << "\n");
  
    // Otherwise we're good!  Update the byval argument.
    CS.setArgument(ArgNo, TmpCast);
    ++NumMemCpyInstr;
    return true;
  }
  
  /// iterateOnFunction - Executes one iteration of MemCpyOpt.
  bool MemCpyOpt::iterateOnFunction(Function &F) {
    bool MadeChange = false;
  
    // Walk all instruction in the function.
    for (Function::iterator BB = F.begin(), BBE = F.end(); BB != BBE; ++BB) {
      for (BasicBlock::iterator BI = BB->begin(), BE = BB->end(); BI != BE;) {
        // Avoid invalidating the iterator.
        Instruction *I = BI++;
  
        bool RepeatInstruction = false;
  
        if (StoreInst *SI = dyn_cast<StoreInst>(I))
          MadeChange |= processStore(SI, BI);
        else if (MemSetInst *M = dyn_cast<MemSetInst>(I))
          RepeatInstruction = processMemSet(M, BI);
        else if (MemCpyInst *M = dyn_cast<MemCpyInst>(I))
          RepeatInstruction = processMemCpy(M);
        else if (MemMoveInst *M = dyn_cast<MemMoveInst>(I))
          RepeatInstruction = processMemMove(M);
        else if (CallSite CS = (Value*)I) {
          for (unsigned i = 0, e = CS.arg_size(); i != e; ++i)
            if (CS.isByValArgument(i))
              MadeChange |= processByValArgument(CS, i);
        }
  
        // Reprocess the instruction if desired.
        if (RepeatInstruction) {
          if (BI != BB->begin()) --BI;
          MadeChange = true;
        }
      }
    }
  
    return MadeChange;
  }
  
  // MemCpyOpt::runOnFunction - This is the main transformation entry point for a
  // function.
  //
  bool MemCpyOpt::runOnFunction(Function &F) {
    bool MadeChange = false;
    MD = &getAnalysis<MemoryDependenceAnalysis>();
    TD = getAnalysisIfAvailable<DataLayout>();
    TLI = &getAnalysis<TargetLibraryInfo>();
  
    // If we don't have at least memset and memcpy, there is little point of doing
    // anything here.  These are required by a freestanding implementation, so if
    // even they are disabled, there is no point in trying hard.
    if (!TLI->has(LibFunc::memset) || !TLI->has(LibFunc::memcpy))
      return false;
  
    while (1) {
      if (!iterateOnFunction(F))
        break;
      MadeChange = true;
    }
  
    MD = 0;
    return MadeChange;
  }
  