text: //===- MemCpyOptimizer.cpp - Optimize use of memcpy and friends -----------===//
  //
  // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
  // See https://llvm.org/LICENSE.txt for license information.
  // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
  //
  //===----------------------------------------------------------------------===//
  //
  // This pass performs various transformations related to eliminating memcpy
  // calls, or transforming sets of stores into memset's.
  //
  //===----------------------------------------------------------------------===//
  
  #include "llvm/Transforms/Scalar/MemCpyOptimizer.h"
  #include "llvm/ADT/DenseSet.h"
  #include "llvm/ADT/None.h"
  #include "llvm/ADT/STLExtras.h"
  #include "llvm/ADT/SmallVector.h"
  #include "llvm/ADT/Statistic.h"
  #include "llvm/ADT/iterator_range.h"
  #include "llvm/Analysis/AliasAnalysis.h"
  #include "llvm/Analysis/AssumptionCache.h"
  #include "llvm/Analysis/GlobalsModRef.h"
  #include "llvm/Analysis/MemoryDependenceAnalysis.h"
  #include "llvm/Analysis/MemoryLocation.h"
  #include "llvm/Analysis/TargetLibraryInfo.h"
  #include "llvm/Analysis/ValueTracking.h"
  #include "llvm/IR/Argument.h"
  #include "llvm/IR/BasicBlock.h"
  #include "llvm/IR/Constants.h"
  #include "llvm/IR/DataLayout.h"
  #include "llvm/IR/DerivedTypes.h"
  #include "llvm/IR/Dominators.h"
  #include "llvm/IR/Function.h"
  #include "llvm/IR/GetElementPtrTypeIterator.h"
  #include "llvm/IR/GlobalVariable.h"
  #include "llvm/IR/IRBuilder.h"
  #include "llvm/IR/InstrTypes.h"
  #include "llvm/IR/Instruction.h"
  #include "llvm/IR/Instructions.h"
  #include "llvm/IR/IntrinsicInst.h"
  #include "llvm/IR/Intrinsics.h"
  #include "llvm/IR/LLVMContext.h"
  #include "llvm/IR/Module.h"
  #include "llvm/IR/Operator.h"
  #include "llvm/IR/PassManager.h"
  #include "llvm/IR/Type.h"
  #include "llvm/IR/User.h"
  #include "llvm/IR/Value.h"
  #include "llvm/InitializePasses.h"
  #include "llvm/Pass.h"
  #include "llvm/Support/Casting.h"
  #include "llvm/Support/Debug.h"
  #include "llvm/Support/MathExtras.h"
  #include "llvm/Support/raw_ostream.h"
  #include "llvm/Transforms/Scalar.h"
  #include "llvm/Transforms/Utils/Local.h"
  #include <algorithm>
  #include <cassert>
  #include <cstdint>
  #include <utility>
  
  using namespace llvm;
  
  #define DEBUG_TYPE "memcpyopt"
  
  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");
  
  namespace {
  
  /// 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.
  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 &DL) const;
  };
  
  } // end anonymous namespace
  
  bool MemsetRange::isProfitableToUseMemset(const DataLayout &DL) 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 (Instruction *SI : TheStores)
      if (!isa<StoreInst>(SI))
        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 largest legal integer
    // size. If so, check to see whether we will end up actually reducing the
    // number of stores used.
    unsigned Bytes = unsigned(End-Start);
    unsigned MaxIntSize = DL.getLargestLegalIntTypeSizeInBits() / 8;
    if (MaxIntSize == 0)
      MaxIntSize = 1;
    unsigned NumPointerStores = Bytes / MaxIntSize;
  
    // Assume the remaining bytes if any are done a byte at a time.
    unsigned NumByteStores = Bytes % MaxIntSize;
  
    // 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 {
    using range_iterator = SmallVectorImpl<MemsetRange>::iterator;
  
    /// A sorted list of the memset ranges.
    SmallVector<MemsetRange, 8> Ranges;
  
    const DataLayout &DL;
  
  public:
    MemsetRanges(const DataLayout &DL) : DL(DL) {}
  
    using const_iterator = SmallVectorImpl<MemsetRange>::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 = DL.getTypeStoreSize(SI->getOperand(0)->getType());
  
      addRange(OffsetFromFirst, StoreSize, SI->getPointerOperand(),
               SI->getAlign().value(), SI);
    }
  
    void addMemSet(int64_t OffsetFromFirst, MemSetInst *MSI) {
      int64_t Size = cast<ConstantInt>(MSI->getLength())->getZExtValue();
      addRange(OffsetFromFirst, Size, MSI->getDest(), MSI->getDestAlignment(), MSI);
    }
  
    void addRange(int64_t Start, int64_t Size, Value *Ptr,
                  unsigned Alignment, Instruction *Inst);
  };
  
  } // end anonymous namespace
  
  /// 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.
  void MemsetRanges::addRange(int64_t Start, int64_t Size, Value *Ptr,
                              unsigned Alignment, Instruction *Inst) {
    int64_t End = Start+Size;
  
    range_iterator I = partition_point(
        Ranges, [=](const MemsetRange &O) { return O.End < Start; });
  
    // 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 == Ranges.end() || 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 != Ranges.end() && 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;
      }
    }
  }
  
  //===----------------------------------------------------------------------===//
  //                         MemCpyOptLegacyPass Pass
  //===----------------------------------------------------------------------===//
  
  namespace {
  
  class MemCpyOptLegacyPass : public FunctionPass {
    MemCpyOptPass Impl;
  
  public:
    static char ID; // Pass identification, replacement for typeid
  
    MemCpyOptLegacyPass() : FunctionPass(ID) {
      initializeMemCpyOptLegacyPassPass(*PassRegistry::getPassRegistry());
    }
  
    bool runOnFunction(Function &F) override;
  
  private:
    // This transformation requires dominator postdominator info
    void getAnalysisUsage(AnalysisUsage &AU) const override {
      AU.setPreservesCFG();
      AU.addRequired<AssumptionCacheTracker>();
      AU.addRequired<DominatorTreeWrapperPass>();
      AU.addPreserved<DominatorTreeWrapperPass>();
      AU.addPreserved<GlobalsAAWrapperPass>();
      AU.addRequired<TargetLibraryInfoWrapperPass>();
      AU.addRequired<MemoryDependenceWrapperPass>();
      AU.addPreserved<MemoryDependenceWrapperPass>();
      AU.addRequired<AAResultsWrapperPass>();
      AU.addPreserved<AAResultsWrapperPass>();
    }
  };
  
  } // end anonymous namespace
  
  char MemCpyOptLegacyPass::ID = 0;
  
  /// The public interface to this file...
  FunctionPass *llvm::createMemCpyOptPass() { return new MemCpyOptLegacyPass(); }
  
  INITIALIZE_PASS_BEGIN(MemCpyOptLegacyPass, "memcpyopt", "MemCpy Optimization",
                        false, false)
  INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
  INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
  INITIALIZE_PASS_DEPENDENCY(MemoryDependenceWrapperPass)
  INITIALIZE_PASS_DEPENDENCY(TargetLibraryInfoWrapperPass)
  INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
  INITIALIZE_PASS_DEPENDENCY(GlobalsAAWrapperPass)
  INITIALIZE_PASS_END(MemCpyOptLegacyPass, "memcpyopt", "MemCpy Optimization",
                      false, false)
  
  /// 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 *MemCpyOptPass::tryMergingIntoMemset(Instruction *StartInst,
                                                   Value *StartPtr,
                                                   Value *ByteVal) {
    const DataLayout &DL = StartInst->getModule()->getDataLayout();
  
    // 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(DL);
  
    BasicBlock::iterator BI(StartInst);
    for (++BI; !BI->isTerminator(); ++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.
        Value *StoredByte = isBytewiseValue(NextStore->getOperand(0), DL);
        if (isa<UndefValue>(ByteVal) && StoredByte)
          ByteVal = StoredByte;
        if (ByteVal != StoredByte)
          break;
  
        // Check to see if this store is to a constant offset from the start ptr.
        Optional<int64_t> Offset =
            isPointerOffset(StartPtr, NextStore->getPointerOperand(), DL);
        if (!Offset)
          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.
        Optional<int64_t> Offset = isPointerOffset(StartPtr, MSI->getDest(), DL);
        if (!Offset)
          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 nullptr;
  
    // 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 = nullptr;
    for (const MemsetRange &Range : Ranges) {
      if (Range.TheStores.size() == 1) continue;
  
      // If it is profitable to lower this range to memset, do so now.
      if (!Range.isProfitableToUseMemset(DL))
        continue;
  
      // Otherwise, we do want to transform this!  Create a new memset.
      // Get the starting pointer of the block.
      StartPtr = Range.StartPtr;
  
      AMemSet = Builder.CreateMemSet(StartPtr, ByteVal, Range.End - Range.Start,
                                     MaybeAlign(Range.Alignment));
      LLVM_DEBUG(dbgs() << "Replace stores:\n"; for (Instruction *SI
                                                     : Range.TheStores) dbgs()
                                                << *SI << '\n';
                 dbgs() << "With: " << *AMemSet << '\n');
  
      if (!Range.TheStores.empty())
        AMemSet->setDebugLoc(Range.TheStores[0]->getDebugLoc());
  
      // Zap all the stores.
      for (Instruction *SI : Range.TheStores) {
        MD->removeInstruction(SI);
        SI->eraseFromParent();
      }
      ++NumMemSetInfer;
    }
  
    return AMemSet;
  }
  
  // This method try to lift a store instruction before position P.
  // It will lift the store and its argument + that anything that
  // may alias with these.
  // The method returns true if it was successful.
  static bool moveUp(AliasAnalysis &AA, StoreInst *SI, Instruction *P,
                     const LoadInst *LI) {
    // If the store alias this position, early bail out.
    MemoryLocation StoreLoc = MemoryLocation::get(SI);
    if (isModOrRefSet(AA.getModRefInfo(P, StoreLoc)))
      return false;
  
    // Keep track of the arguments of all instruction we plan to lift
    // so we can make sure to lift them as well if appropriate.
    DenseSet<Instruction*> Args;
    if (auto *Ptr = dyn_cast<Instruction>(SI->getPointerOperand()))
      if (Ptr->getParent() == SI->getParent())
        Args.insert(Ptr);
  
    // Instruction to lift before P.
    SmallVector<Instruction*, 8> ToLift;
  
    // Memory locations of lifted instructions.
    SmallVector<MemoryLocation, 8> MemLocs{StoreLoc};
  
    // Lifted calls.
    SmallVector<const CallBase *, 8> Calls;
  
    const MemoryLocation LoadLoc = MemoryLocation::get(LI);
  
    for (auto I = --SI->getIterator(), E = P->getIterator(); I != E; --I) {
      auto *C = &*I;
  
      bool MayAlias = isModOrRefSet(AA.getModRefInfo(C, None));
  
      bool NeedLift = false;
      if (Args.erase(C))
        NeedLift = true;
      else if (MayAlias) {
        NeedLift = llvm::any_of(MemLocs, [C, &AA](const MemoryLocation &ML) {
          return isModOrRefSet(AA.getModRefInfo(C, ML));
        });
  
        if (!NeedLift)
          NeedLift = llvm::any_of(Calls, [C, &AA](const CallBase *Call) {
            return isModOrRefSet(AA.getModRefInfo(C, Call));
          });
      }
  
      if (!NeedLift)
        continue;
  
      if (MayAlias) {
        // Since LI is implicitly moved downwards past the lifted instructions,
        // none of them may modify its source.
        if (isModSet(AA.getModRefInfo(C, LoadLoc)))
          return false;
        else if (const auto *Call = dyn_cast<CallBase>(C)) {
          // If we can't lift this before P, it's game over.
          if (isModOrRefSet(AA.getModRefInfo(P, Call)))
            return false;
  
          Calls.push_back(Call);
        } else if (isa<LoadInst>(C) || isa<StoreInst>(C) || isa<VAArgInst>(C)) {
          // If we can't lift this before P, it's game over.
          auto ML = MemoryLocation::get(C);
          if (isModOrRefSet(AA.getModRefInfo(P, ML)))
            return false;
  
          MemLocs.push_back(ML);
        } else
          // We don't know how to lift this instruction.
          return false;
      }
  
      ToLift.push_back(C);
      for (unsigned k = 0, e = C->getNumOperands(); k != e; ++k)
        if (auto *A = dyn_cast<Instruction>(C->getOperand(k))) {
          if (A->getParent() == SI->getParent()) {
            // Cannot hoist user of P above P
            if(A == P) return false;
            Args.insert(A);
          }
        }
    }
  
    // We made it, we need to lift
    for (auto *I : llvm::reverse(ToLift)) {
      LLVM_DEBUG(dbgs() << "Lifting " << *I << " before " << *P << "\n");
      I->moveBefore(P);
    }
  
    return true;
  }
  
  bool MemCpyOptPass::processStore(StoreInst *SI, BasicBlock::iterator &BBI) {
    if (!SI->isSimple()) return false;
  
    // Avoid merging nontemporal stores since the resulting
    // memcpy/memset would not be able to preserve the nontemporal hint.
    // In theory we could teach how to propagate the !nontemporal metadata to
    // memset calls. However, that change would force the backend to
    // conservatively expand !nontemporal memset calls back to sequences of
    // store instructions (effectively undoing the merging).
    if (SI->getMetadata(LLVMContext::MD_nontemporal))
      return false;
  
    const DataLayout &DL = SI->getModule()->getDataLayout();
  
    // Load to store forwarding can be interpreted as memcpy.
    if (LoadInst *LI = dyn_cast<LoadInst>(SI->getOperand(0))) {
      if (LI->isSimple() && LI->hasOneUse() &&
          LI->getParent() == SI->getParent()) {
  
        auto *T = LI->getType();
        if (T->isAggregateType()) {
          MemoryLocation LoadLoc = MemoryLocation::get(LI);
  
          // We use alias analysis to check if an instruction may store to
          // the memory we load from in between the load and the store. If
          // such an instruction is found, we try to promote there instead
          // of at the store position.
          Instruction *P = SI;
          for (auto &I : make_range(++LI->getIterator(), SI->getIterator())) {
            if (isModSet(AA->getModRefInfo(&I, LoadLoc))) {
              P = &I;
              break;
            }
          }
  
          // We found an instruction that may write to the loaded memory.
          // We can try to promote at this position instead of the store
          // position if nothing alias the store memory after this and the store
          // destination is not in the range.
          if (P && P != SI) {
            if (!moveUp(*AA, SI, P, LI))
              P = nullptr;
          }
  
          // If a valid insertion position is found, then we can promote
          // the load/store pair to a memcpy.
          if (P) {
            // If we load from memory that may alias the memory we store to,
            // memmove must be used to preserve semantic. If not, memcpy can
            // be used.
            bool UseMemMove = false;
            if (!AA->isNoAlias(MemoryLocation::get(SI), LoadLoc))
              UseMemMove = true;
  
            uint64_t Size = DL.getTypeStoreSize(T);
  
            IRBuilder<> Builder(P);
            Instruction *M;
            if (UseMemMove)
              M = Builder.CreateMemMove(
                  SI->getPointerOperand(), SI->getAlign(),
                  LI->getPointerOperand(), LI->getAlign(), Size);
            else
              M = Builder.CreateMemCpy(
                  SI->getPointerOperand(), SI->getAlign(),
                  LI->getPointerOperand(), LI->getAlign(), Size);
  
            LLVM_DEBUG(dbgs() << "Promoting " << *LI << " to " << *SI << " => "
                              << *M << "\n");
  
            MD->removeInstruction(SI);
            SI->eraseFromParent();
            MD->removeInstruction(LI);
            LI->eraseFromParent();
            ++NumMemCpyInstr;
  
            // Make sure we do not invalidate the iterator.
            BBI = M->getIterator();
            return true;
          }
        }
  
        // 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.
        MemDepResult ldep = MD->getDependency(LI);
        CallInst *C = nullptr;
        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.
          Value *CpyDest = SI->getPointerOperand()->stripPointerCasts();
          bool CpyDestIsLocal = isa<AllocaInst>(CpyDest);
          MemoryLocation StoreLoc = MemoryLocation::get(SI);
          for (BasicBlock::iterator I = --SI->getIterator(), E = C->getIterator();
               I != E; --I) {
            if (isModOrRefSet(AA->getModRefInfo(&*I, StoreLoc))) {
              C = nullptr;
              break;
            }
            // The store to dest may never happen if an exception can be thrown
            // between the load and the store.
            if (I->mayThrow() && !CpyDestIsLocal) {
              C = nullptr;
              break;
            }
          }
        }
  
        if (C) {
          bool changed = performCallSlotOptzn(
              LI, SI->getPointerOperand()->stripPointerCasts(),
              LI->getPointerOperand()->stripPointerCasts(),
              DL.getTypeStoreSize(SI->getOperand(0)->getType()),
              commonAlignment(SI->getAlign(), LI->getAlign()), 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.
    auto *V = SI->getOperand(0);
    if (Value *ByteVal = isBytewiseValue(V, DL)) {
      if (Instruction *I = tryMergingIntoMemset(SI, SI->getPointerOperand(),
                                                ByteVal)) {
        BBI = I->getIterator(); // Don't invalidate iterator.
        return true;
      }
  
      // If we have an aggregate, we try to promote it to memset regardless
      // of opportunity for merging as it can expose optimization opportunities
      // in subsequent passes.
      auto *T = V->getType();
      if (T->isAggregateType()) {
        uint64_t Size = DL.getTypeStoreSize(T);
        IRBuilder<> Builder(SI);
        auto *M = Builder.CreateMemSet(SI->getPointerOperand(), ByteVal, Size,
                                       SI->getAlign());
  
        LLVM_DEBUG(dbgs() << "Promoting " << *SI << " to " << *M << "\n");
  
        MD->removeInstruction(SI);
        SI->eraseFromParent();
        NumMemSetInfer++;
  
        // Make sure we do not invalidate the iterator.
        BBI = M->getIterator();
        return true;
      }
    }
  
    return false;
  }
  
  bool MemCpyOptPass::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->getIterator(); // Don't invalidate iterator.
        return true;
      }
    return false;
  }
  
  /// 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 MemCpyOptPass::performCallSlotOptzn(Instruction *cpy, Value *cpyDest,
                                           Value *cpySrc, uint64_t cpyLen,
                                           Align 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.
  
    // Lifetime marks shouldn't be operated on.
    if (Function *F = C->getCalledFunction())
      if (F->isIntrinsic() && F->getIntrinsicID() == Intrinsic::lifetime_start)
        return false;
  
    // Require that src be an alloca.  This simplifies the reasoning considerably.
    AllocaInst *srcAlloca = dyn_cast<AllocaInst>(cpySrc);
    if (!srcAlloca)
      return false;
  
    ConstantInt *srcArraySize = dyn_cast<ConstantInt>(srcAlloca->getArraySize());
    if (!srcArraySize)
      return false;
  
    const DataLayout &DL = cpy->getModule()->getDataLayout();
    uint64_t srcSize = DL.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 = DL.getTypeAllocSize(A->getAllocatedType()) *
                          destArraySize->getZExtValue();
  
      if (destSize < srcSize)
        return false;
    } else if (Argument *A = dyn_cast<Argument>(cpyDest)) {
      // The store to dest may never happen if the call can throw.
      if (C->mayThrow())
        return false;
  
      if (A->getDereferenceableBytes() < srcSize) {
        // 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();
        if (!StructTy->isSized()) {
          // The call may never return and hence the copy-instruction may never
          // be executed, and therefore it's not safe to say "the destination
          // has at least <cpyLen> bytes, as implied by the copy-instruction",
          return false;
        }
  
        uint64_t destSize = DL.getTypeAllocSize(StructTy);
        if (destSize < srcSize)
          return false;
      }
    } else {
      return false;
    }
  
    // Check that dest points to memory that is at least as aligned as src.
    Align srcAlign = srcAlloca->getAlign();
    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->user_begin(),
                                     srcAlloca->user_end());
    while (!srcUseList.empty()) {
      User *U = srcUseList.pop_back_val();
  
      if (isa<BitCastInst>(U) || isa<AddrSpaceCastInst>(U)) {
        for (User *UU : U->users())
          srcUseList.push_back(UU);
        continue;
      }
      if (GetElementPtrInst *G = dyn_cast<GetElementPtrInst>(U)) {
        if (!G->hasAllZeroIndices())
          return false;
  
        for (User *UU : U->users())
          srcUseList.push_back(UU);
        continue;
      }
      if (const IntrinsicInst *IT = dyn_cast<IntrinsicInst>(U))
        if (IT->isLifetimeStartOrEnd())
          continue;
  
      if (U != C && U != cpy)
        return false;
    }
  
    // Check that src isn't captured by the called function since the
    // transformation can cause aliasing issues in that case.
    for (unsigned ArgI = 0, E = C->arg_size(); ArgI != E; ++ArgI)
      if (C->getArgOperand(ArgI) == cpySrc && !C->doesNotCapture(ArgI))
        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.
    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.
    ModRefInfo MR = AA->getModRefInfo(C, cpyDest, LocationSize::precise(srcSize));
    // If necessary, perform additional analysis.
    if (isModOrRefSet(MR))
      MR = AA->callCapturesBefore(C, cpyDest, LocationSize::precise(srcSize), DT);
    if (isModOrRefSet(MR))
      return false;
  
    // We can't create address space casts here because we don't know if they're
    // safe for the target.
    if (cpySrc->getType()->getPointerAddressSpace() !=
        cpyDest->getType()->getPointerAddressSpace())
      return false;
    for (unsigned ArgI = 0; ArgI < C->arg_size(); ++ArgI)
      if (C->getArgOperand(ArgI)->stripPointerCasts() == cpySrc &&
          cpySrc->getType()->getPointerAddressSpace() !=
              C->getArgOperand(ArgI)->getType()->getPointerAddressSpace())
        return false;
  
    // All the checks have passed, so do the transformation.
    bool changedArgument = false;
    for (unsigned ArgI = 0; ArgI < C->arg_size(); ++ArgI)
      if (C->getArgOperand(ArgI)->stripPointerCasts() == cpySrc) {
        Value *Dest = cpySrc->getType() == cpyDest->getType() ?  cpyDest
          : CastInst::CreatePointerCast(cpyDest, cpySrc->getType(),
                                        cpyDest->getName(), C);
        changedArgument = true;
        if (C->getArgOperand(ArgI)->getType() == Dest->getType())
          C->setArgOperand(ArgI, Dest);
        else
          C->setArgOperand(ArgI, CastInst::CreatePointerCast(
                                     Dest, C->getArgOperand(ArgI)->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);
  
    // Update AA metadata
    // FIXME: MD_tbaa_struct and MD_mem_parallel_loop_access should also be
    // handled here, but combineMetadata doesn't support them yet
    unsigned KnownIDs[] = {LLVMContext::MD_tbaa, LLVMContext::MD_alias_scope,
                           LLVMContext::MD_noalias,
                           LLVMContext::MD_invariant_group,
                           LLVMContext::MD_access_group};
    combineMetadata(C, cpy, KnownIDs, true);
  
    // Remove the memcpy.
    MD->removeInstruction(cpy);
    ++NumMemCpyInstr;
  
    return true;
  }
  
  /// 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.
  bool MemCpyOptPass::processMemCpyMemCpyDependence(MemCpyInst *M,
                                                    MemCpyInst *MDep) {
    // 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;
  
    // 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(MemoryLocation::getForSource(MDep), false,
                                     M->getIterator(), 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(MemoryLocation::getForDest(M),
                       MemoryLocation::getForSource(MDep)))
      UseMemMove = true;
  
    // If all checks passed, then we can transform M.
    LLVM_DEBUG(dbgs() << "MemCpyOptPass: Forwarding memcpy->memcpy src:\n"
                      << *MDep << '\n' << *M << '\n');
  
    // TODO: Is this worth it if we're creating a less aligned memcpy? For
    // example we could be moving from movaps -> movq on x86.
    IRBuilder<> Builder(M);
    if (UseMemMove)
      Builder.CreateMemMove(M->getRawDest(), M->getDestAlign(),
                            MDep->getRawSource(), MDep->getSourceAlign(),
                            M->getLength(), M->isVolatile());
    else
      Builder.CreateMemCpy(M->getRawDest(), M->getDestAlign(),
                           MDep->getRawSource(), MDep->getSourceAlign(),
                           M->getLength(), M->isVolatile());
  
    // Remove the instruction we're replacing.
    MD->removeInstruction(M);
    M->eraseFromParent();
    ++NumMemCpyInstr;
    return true;
  }
  
  /// We've found that the (upward scanning) memory dependence of \p MemCpy is
  /// \p MemSet.  Try to simplify \p MemSet to only set the trailing bytes that
  /// weren't copied over by \p MemCpy.
  ///
  /// In other words, transform:
  /// \code
  ///   memset(dst, c, dst_size);
  ///   memcpy(dst, src, src_size);
  /// \endcode
  /// into:
  /// \code
  ///   memcpy(dst, src, src_size);
  ///   memset(dst + src_size, c, dst_size <= src_size ? 0 : dst_size - src_size);
  /// \endcode
  bool MemCpyOptPass::processMemSetMemCpyDependence(MemCpyInst *MemCpy,
                                                    MemSetInst *MemSet) {
    // We can only transform memset/memcpy with the same destination.
    if (MemSet->getDest() != MemCpy->getDest())
      return false;
  
    // Check that there are no other dependencies on the memset destination.
    MemDepResult DstDepInfo =
        MD->getPointerDependencyFrom(MemoryLocation::getForDest(MemSet), false,
                                     MemCpy->getIterator(), MemCpy->getParent());
    if (DstDepInfo.getInst() != MemSet)
      return false;
  
    // Use the same i8* dest as the memcpy, killing the memset dest if different.
    Value *Dest = MemCpy->getRawDest();
    Value *DestSize = MemSet->getLength();
    Value *SrcSize = MemCpy->getLength();
  
    // By default, create an unaligned memset.
    unsigned Align = 1;
    // If Dest is aligned, and SrcSize is constant, use the minimum alignment
    // of the sum.
    const unsigned DestAlign =
        std::max(MemSet->getDestAlignment(), MemCpy->getDestAlignment());
    if (DestAlign > 1)
      if (ConstantInt *SrcSizeC = dyn_cast<ConstantInt>(SrcSize))
        Align = MinAlign(SrcSizeC->getZExtValue(), DestAlign);
  
    IRBuilder<> Builder(MemCpy);
  
    // If the sizes have different types, zext the smaller one.
    if (DestSize->getType() != SrcSize->getType()) {
      if (DestSize->getType()->getIntegerBitWidth() >
          SrcSize->getType()->getIntegerBitWidth())
        SrcSize = Builder.CreateZExt(SrcSize, DestSize->getType());
      else
        DestSize = Builder.CreateZExt(DestSize, SrcSize->getType());
    }
  
    Value *Ule = Builder.CreateICmpULE(DestSize, SrcSize);
    Value *SizeDiff = Builder.CreateSub(DestSize, SrcSize);
    Value *MemsetLen = Builder.CreateSelect(
        Ule, ConstantInt::getNullValue(DestSize->getType()), SizeDiff);
    Builder.CreateMemSet(
        Builder.CreateGEP(Dest->getType()->getPointerElementType(), Dest,
                          SrcSize),
        MemSet->getOperand(1), MemsetLen, MaybeAlign(Align));
  
    MD->removeInstruction(MemSet);
    MemSet->eraseFromParent();
    return true;
  }
  
  /// Determine whether the instruction has undefined content for the given Size,
  /// either because it was freshly alloca'd or started its lifetime.
  static bool hasUndefContents(Instruction *I, ConstantInt *Size) {
    if (isa<AllocaInst>(I))
      return true;
  
    if (IntrinsicInst *II = dyn_cast<IntrinsicInst>(I))
      if (II->getIntrinsicID() == Intrinsic::lifetime_start)
        if (ConstantInt *LTSize = dyn_cast<ConstantInt>(II->getArgOperand(0)))
          if (LTSize->getZExtValue() >= Size->getZExtValue())
            return true;
  
    return false;
  }
  
  /// Transform memcpy to memset when its source was just memset.
  /// In other words, turn:
  /// \code
  ///   memset(dst1, c, dst1_size);
  ///   memcpy(dst2, dst1, dst2_size);
  /// \endcode
  /// into:
  /// \code
  ///   memset(dst1, c, dst1_size);
  ///   memset(dst2, c, dst2_size);
  /// \endcode
  /// When dst2_size <= dst1_size.
  ///
  /// The \p MemCpy must have a Constant length.
  bool MemCpyOptPass::performMemCpyToMemSetOptzn(MemCpyInst *MemCpy,
                                                 MemSetInst *MemSet) {
    // Make sure that memcpy(..., memset(...), ...), that is we are memsetting and
    // memcpying from the same address. Otherwise it is hard to reason about.
    if (!AA->isMustAlias(MemSet->getRawDest(), MemCpy->getRawSource()))
      return false;
  
    // A known memset size is required.
    ConstantInt *MemSetSize = dyn_cast<ConstantInt>(MemSet->getLength());
    if (!MemSetSize)
      return false;
  
    // Make sure the memcpy doesn't read any more than what the memset wrote.
    // Don't worry about sizes larger than i64.
    ConstantInt *CopySize = cast<ConstantInt>(MemCpy->getLength());
    if (CopySize->getZExtValue() > MemSetSize->getZExtValue()) {
      // If the memcpy is larger than the memset, but the memory was undef prior
      // to the memset, we can just ignore the tail. Technically we're only
      // interested in the bytes from MemSetSize..CopySize here, but as we can't
      // easily represent this location, we use the full 0..CopySize range.
      MemoryLocation MemCpyLoc = MemoryLocation::getForSource(MemCpy);
      MemDepResult DepInfo = MD->getPointerDependencyFrom(
          MemCpyLoc, true, MemSet->getIterator(), MemSet->getParent());
      if (DepInfo.isDef() && hasUndefContents(DepInfo.getInst(), CopySize))
        CopySize = MemSetSize;
      else
        return false;
    }
  
    IRBuilder<> Builder(MemCpy);
    Builder.CreateMemSet(MemCpy->getRawDest(), MemSet->getOperand(1), CopySize,
                         MaybeAlign(MemCpy->getDestAlignment()));
    return true;
  }
  
  /// 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 MemCpyOptPass::processMemCpy(MemCpyInst *M, BasicBlock::iterator &BBI) {
    // We can only optimize non-volatile memcpy's.
    if (M->isVolatile()) return false;
  
    // If the source and destination of the memcpy are the same, then zap it.
    if (M->getSource() == M->getDest()) {
      ++BBI;
      MD->removeInstruction(M);
      M->eraseFromParent();
      return true;
    }
  
    // 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(),
                                             M->getModule()->getDataLayout())) {
          IRBuilder<> Builder(M);
          Builder.CreateMemSet(M->getRawDest(), ByteVal, M->getLength(),
                               MaybeAlign(M->getDestAlignment()), false);
          MD->removeInstruction(M);
          M->eraseFromParent();
          ++NumCpyToSet;
          return true;
        }
  
    MemDepResult DepInfo = MD->getDependency(M);
  
    // Try to turn a partially redundant memset + memcpy into
    // memcpy + smaller memset.  We don't need the memcpy size for this.
    if (DepInfo.isClobber())
      if (MemSetInst *MDep = dyn_cast<MemSetInst>(DepInfo.getInst()))
        if (processMemSetMemCpyDependence(M, MDep))
          return true;
  
    // The optimizations after this point require the memcpy size.
    ConstantInt *CopySize = dyn_cast<ConstantInt>(M->getLength());
    if (!CopySize) return false;
  
    // There are four possible optimizations we can do for memcpy:
    //   a) memcpy-memcpy xform which exposes redundance for DSE.
    //   b) call-memcpy xform for return slot optimization.
    //   c) memcpy from freshly alloca'd space or space that has just started its
    //      lifetime copies undefined data, and we can therefore eliminate the
    //      memcpy in favor of the data that was already at the destination.
    //   d) memcpy from a just-memset'd source can be turned into memset.
    if (DepInfo.isClobber()) {
      if (CallInst *C = dyn_cast<CallInst>(DepInfo.getInst())) {
        // FIXME: Can we pass in either of dest/src alignment here instead
        // of conservatively taking the minimum?
        Align Alignment = std::min(M->getDestAlign().valueOrOne(),
                                   M->getSourceAlign().valueOrOne());
        if (performCallSlotOptzn(M, M->getDest(), M->getSource(),
                                 CopySize->getZExtValue(), Alignment, C)) {
          MD->removeInstruction(M);
          M->eraseFromParent();
          return true;
        }
      }
    }
  
    MemoryLocation SrcLoc = MemoryLocation::getForSource(M);
    MemDepResult SrcDepInfo = MD->getPointerDependencyFrom(
        SrcLoc, true, M->getIterator(), M->getParent());
  
    if (SrcDepInfo.isClobber()) {
      if (MemCpyInst *MDep = dyn_cast<MemCpyInst>(SrcDepInfo.getInst()))
        return processMemCpyMemCpyDependence(M, MDep);
    } else if (SrcDepInfo.isDef()) {
      if (hasUndefContents(SrcDepInfo.getInst(), CopySize)) {
        MD->removeInstruction(M);
        M->eraseFromParent();
        ++NumMemCpyInstr;
        return true;
      }
    }
  
    if (SrcDepInfo.isClobber())
      if (MemSetInst *MDep = dyn_cast<MemSetInst>(SrcDepInfo.getInst()))
        if (performMemCpyToMemSetOptzn(M, MDep)) {
          MD->removeInstruction(M);
          M->eraseFromParent();
          ++NumCpyToSet;
          return true;
        }
  
    return false;
  }
  
  /// Transforms memmove calls to memcpy calls when the src/dst are guaranteed
  /// not to alias.
  bool MemCpyOptPass::processMemMove(MemMoveInst *M) {
    if (!TLI->has(LibFunc_memmove))
      return false;
  
    // See if the pointers alias.
    if (!AA->isNoAlias(MemoryLocation::getForDest(M),
                       MemoryLocation::getForSource(M)))
      return false;
  
    LLVM_DEBUG(dbgs() << "MemCpyOptPass: Optimizing memmove -> memcpy: " << *M
                      << "\n");
  
    // If not, then we know we can transform this.
    Type *ArgTys[3] = { M->getRawDest()->getType(),
                        M->getRawSource()->getType(),
                        M->getLength()->getType() };
    M->setCalledFunction(Intrinsic::getDeclaration(M->getModule(),
                                                   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;
  }
  
  /// This is called on every byval argument in call sites.
  bool MemCpyOptPass::processByValArgument(CallBase &CB, unsigned ArgNo) {
    const DataLayout &DL = CB.getCaller()->getParent()->getDataLayout();
    // Find out what feeds this byval argument.
    Value *ByValArg = CB.getArgOperand(ArgNo);
    Type *ByValTy = cast<PointerType>(ByValArg->getType())->getElementType();
    uint64_t ByValSize = DL.getTypeAllocSize(ByValTy);
    MemDepResult DepInfo = MD->getPointerDependencyFrom(
        MemoryLocation(ByValArg, LocationSize::precise(ByValSize)), true,
        CB.getIterator(), CB.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 || 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 || 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.
    MaybeAlign ByValAlign = CB.getParamAlign(ArgNo);
    if (!ByValAlign) 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.
    MaybeAlign MemDepAlign = MDep->getSourceAlign();
    if ((!MemDepAlign || *MemDepAlign < *ByValAlign) &&
        getOrEnforceKnownAlignment(MDep->getSource(), ByValAlign, DL, &CB, AC,
                                   DT) < *ByValAlign)
      return false;
  
    // The address space of the memcpy source must match the byval argument
    if (MDep->getSource()->getType()->getPointerAddressSpace() !=
        ByValArg->getType()->getPointerAddressSpace())
      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(
        MemoryLocation::getForSource(MDep), false,
        CB.getIterator(), MDep->getParent());
    if (!SourceDep.isClobber() || SourceDep.getInst() != MDep)
      return false;
  
    Value *TmpCast = MDep->getSource();
    if (MDep->getSource()->getType() != ByValArg->getType()) {
      BitCastInst *TmpBitCast = new BitCastInst(MDep->getSource(), ByValArg->getType(),
                                                "tmpcast", &CB);
      // Set the tmpcast's DebugLoc to MDep's
      TmpBitCast->setDebugLoc(MDep->getDebugLoc());
      TmpCast = TmpBitCast;
    }
  
    LLVM_DEBUG(dbgs() << "MemCpyOptPass: Forwarding memcpy to byval:\n"
                      << "  " << *MDep << "\n"
                      << "  " << CB << "\n");
  
    // Otherwise we're good!  Update the byval argument.
    CB.setArgOperand(ArgNo, TmpCast);
    ++NumMemCpyInstr;
    return true;
  }
  
  /// Executes one iteration of MemCpyOptPass.
  bool MemCpyOptPass::iterateOnFunction(Function &F) {
    bool MadeChange = false;
  
    // Walk all instruction in the function.
    for (BasicBlock &BB : F) {
      // Skip unreachable blocks. For example processStore assumes that an
      // instruction in a BB can't be dominated by a later instruction in the
      // same BB (which is a scenario that can happen for an unreachable BB that
      // has itself as a predecessor).
      if (!DT->isReachableFromEntry(&BB))
        continue;
  
      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, BI);
        else if (MemMoveInst *M = dyn_cast<MemMoveInst>(I))
          RepeatInstruction = processMemMove(M);
        else if (auto *CB = dyn_cast<CallBase>(I)) {
          for (unsigned i = 0, e = CB->arg_size(); i != e; ++i)
            if (CB->isByValArgument(i))
              MadeChange |= processByValArgument(*CB, i);
        }
  
        // Reprocess the instruction if desired.
        if (RepeatInstruction) {
          if (BI != BB.begin())
            --BI;
          MadeChange = true;
        }
      }
    }
  
    return MadeChange;
  }
  
  PreservedAnalyses MemCpyOptPass::run(Function &F, FunctionAnalysisManager &AM) {
    auto &MD = AM.getResult<MemoryDependenceAnalysis>(F);
    auto &TLI = AM.getResult<TargetLibraryAnalysis>(F);
    auto *AA = &AM.getResult<AAManager>(F);
    auto *AC = &AM.getResult<AssumptionAnalysis>(F);
    auto *DT = &AM.getResult<DominatorTreeAnalysis>(F);
  
    bool MadeChange = runImpl(F, &MD, &TLI, AA, AC, DT);
    if (!MadeChange)
      return PreservedAnalyses::all();
  
    PreservedAnalyses PA;
    PA.preserveSet<CFGAnalyses>();
    PA.preserve<GlobalsAA>();
    PA.preserve<MemoryDependenceAnalysis>();
    return PA;
  }
  
  bool MemCpyOptPass::runImpl(Function &F, MemoryDependenceResults *MD_,
                              TargetLibraryInfo *TLI_, AliasAnalysis *AA_,
                              AssumptionCache *AC_, DominatorTree *DT_) {
    bool MadeChange = false;
    MD = MD_;
    TLI = TLI_;
    AA = AA_;
    AC = AC_;
    DT = DT_;
    // 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 (true) {
      if (!iterateOnFunction(F))
        break;
      MadeChange = true;
    }
  
    MD = nullptr;
    return MadeChange;
  }
  
  /// This is the main transformation entry point for a function.
  bool MemCpyOptLegacyPass::runOnFunction(Function &F) {
    if (skipFunction(F))
      return false;
  
    auto *MD = &getAnalysis<MemoryDependenceWrapperPass>().getMemDep();
    auto *TLI = &getAnalysis<TargetLibraryInfoWrapperPass>().getTLI(F);
    auto *AA = &getAnalysis<AAResultsWrapperPass>().getAAResults();
    auto *AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
    auto *DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
  
    return Impl.runImpl(F, MD, TLI, AA, AC, DT);
  }
  