Update aosp/master LLVM for rebase to r222494.

Change-Id: Ic787f5e0124df789bd26f3f24680f45e678eef2d
diff --git a/lib/Bitcode/Writer/BitWriter.cpp b/lib/Bitcode/Writer/BitWriter.cpp
index 3747122..7218ea0 100644
--- a/lib/Bitcode/Writer/BitWriter.cpp
+++ b/lib/Bitcode/Writer/BitWriter.cpp
@@ -18,10 +18,10 @@
 /*===-- Operations on modules ---------------------------------------------===*/
 
 int LLVMWriteBitcodeToFile(LLVMModuleRef M, const char *Path) {
-  std::string ErrorInfo;
-  raw_fd_ostream OS(Path, ErrorInfo, sys::fs::F_None);
+  std::error_code EC;
+  raw_fd_ostream OS(Path, EC, sys::fs::F_None);
 
-  if (!ErrorInfo.empty())
+  if (EC)
     return -1;
 
   WriteBitcodeToFile(unwrap(M), OS);
@@ -39,3 +39,11 @@
 int LLVMWriteBitcodeToFileHandle(LLVMModuleRef M, int FileHandle) {
   return LLVMWriteBitcodeToFD(M, FileHandle, true, false);
 }
+
+LLVMMemoryBufferRef LLVMWriteBitcodeToMemoryBuffer(LLVMModuleRef M) {
+  std::string Data;
+  raw_string_ostream OS(Data);
+
+  WriteBitcodeToFile(unwrap(M), OS);
+  return wrap(MemoryBuffer::getMemBufferCopy(OS.str()).release());
+}
diff --git a/lib/Bitcode/Writer/BitcodeWriter.cpp b/lib/Bitcode/Writer/BitcodeWriter.cpp
index dd9282a..6cfc357 100644
--- a/lib/Bitcode/Writer/BitcodeWriter.cpp
+++ b/lib/Bitcode/Writer/BitcodeWriter.cpp
@@ -22,6 +22,7 @@
 #include "llvm/IR/Instructions.h"
 #include "llvm/IR/Module.h"
 #include "llvm/IR/Operator.h"
+#include "llvm/IR/UseListOrder.h"
 #include "llvm/IR/ValueSymbolTable.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/ErrorHandling.h"
@@ -32,12 +33,6 @@
 #include <map>
 using namespace llvm;
 
-static cl::opt<bool>
-EnablePreserveUseListOrdering("enable-bc-uselist-preserve",
-                              cl::desc("Turn on experimental support for "
-                                       "use-list order preservation."),
-                              cl::init(false), cl::Hidden);
-
 /// These are manifest constants used by the bitcode writer. They do not need to
 /// be kept in sync with the reader, but need to be consistent within this file.
 enum {
@@ -201,6 +196,8 @@
     return bitc::ATTR_KIND_NON_LAZY_BIND;
   case Attribute::NonNull:
     return bitc::ATTR_KIND_NON_NULL;
+  case Attribute::Dereferenceable:
+    return bitc::ATTR_KIND_DEREFERENCEABLE;
   case Attribute::NoRedZone:
     return bitc::ATTR_KIND_NO_RED_ZONE;
   case Attribute::NoReturn:
@@ -272,7 +269,7 @@
         if (Attr.isEnumAttribute()) {
           Record.push_back(0);
           Record.push_back(getAttrKindEncoding(Attr.getKindAsEnum()));
-        } else if (Attr.isAlignAttribute()) {
+        } else if (Attr.isIntAttribute()) {
           Record.push_back(1);
           Record.push_back(getAttrKindEncoding(Attr.getKindAsEnum()));
           Record.push_back(Attr.getValueAsInt());
@@ -713,18 +710,15 @@
 static uint64_t GetOptimizationFlags(const Value *V) {
   uint64_t Flags = 0;
 
-  if (const OverflowingBinaryOperator *OBO =
-        dyn_cast<OverflowingBinaryOperator>(V)) {
+  if (const auto *OBO = dyn_cast<OverflowingBinaryOperator>(V)) {
     if (OBO->hasNoSignedWrap())
       Flags |= 1 << bitc::OBO_NO_SIGNED_WRAP;
     if (OBO->hasNoUnsignedWrap())
       Flags |= 1 << bitc::OBO_NO_UNSIGNED_WRAP;
-  } else if (const PossiblyExactOperator *PEO =
-               dyn_cast<PossiblyExactOperator>(V)) {
+  } else if (const auto *PEO = dyn_cast<PossiblyExactOperator>(V)) {
     if (PEO->isExact())
       Flags |= 1 << bitc::PEO_EXACT;
-  } else if (const FPMathOperator *FPMO =
-             dyn_cast<const FPMathOperator>(V)) {
+  } else if (const auto *FPMO = dyn_cast<FPMathOperator>(V)) {
     if (FPMO->hasUnsafeAlgebra())
       Flags |= FastMathFlags::UnsafeAlgebra;
     if (FPMO->hasNoNaNs())
@@ -762,13 +756,13 @@
 static void WriteModuleMetadata(const Module *M,
                                 const ValueEnumerator &VE,
                                 BitstreamWriter &Stream) {
-  const ValueEnumerator::ValueList &Vals = VE.getMDValues();
+  const auto &Vals = VE.getMDValues();
   bool StartedMetadataBlock = false;
   unsigned MDSAbbrev = 0;
   SmallVector<uint64_t, 64> Record;
   for (unsigned i = 0, e = Vals.size(); i != e; ++i) {
 
-    if (const MDNode *N = dyn_cast<MDNode>(Vals[i].first)) {
+    if (const MDNode *N = dyn_cast<MDNode>(Vals[i])) {
       if (!N->isFunctionLocal() || !N->getFunction()) {
         if (!StartedMetadataBlock) {
           Stream.EnterSubblock(bitc::METADATA_BLOCK_ID, 3);
@@ -776,7 +770,7 @@
         }
         WriteMDNode(N, VE, Stream, Record);
       }
-    } else if (const MDString *MDS = dyn_cast<MDString>(Vals[i].first)) {
+    } else if (const MDString *MDS = dyn_cast<MDString>(Vals[i])) {
       if (!StartedMetadataBlock)  {
         Stream.EnterSubblock(bitc::METADATA_BLOCK_ID, 3);
 
@@ -854,7 +848,7 @@
 
   // Write metadata attachments
   // METADATA_ATTACHMENT - [m x [value, [n x [id, mdnode]]]
-  SmallVector<std::pair<unsigned, MDNode*>, 4> MDs;
+  SmallVector<std::pair<unsigned, MDNode *>, 4> MDs;
 
   for (Function::const_iterator BB = F.begin(), E = F.end(); BB != E; ++BB)
     for (BasicBlock::const_iterator I = BB->begin(), E = BB->end();
@@ -1431,13 +1425,20 @@
     break;
   }
 
-  case Instruction::Alloca:
+  case Instruction::Alloca: {
     Code = bitc::FUNC_CODE_INST_ALLOCA;
     Vals.push_back(VE.getTypeID(I.getType()));
     Vals.push_back(VE.getTypeID(I.getOperand(0)->getType()));
     Vals.push_back(VE.getValueID(I.getOperand(0))); // size.
-    Vals.push_back(Log2_32(cast<AllocaInst>(I).getAlignment())+1);
+    const AllocaInst &AI = cast<AllocaInst>(I);
+    unsigned AlignRecord = Log2_32(AI.getAlignment()) + 1;
+    assert(Log2_32(Value::MaximumAlignment) + 1 < 1 << 5 &&
+           "not enough bits for maximum alignment");
+    assert(AlignRecord < 1 << 5 && "alignment greater than 1 << 64");
+    AlignRecord |= AI.isUsedWithInAlloca() << 5;
+    Vals.push_back(AlignRecord);
     break;
+  }
 
   case Instruction::Load:
     if (cast<LoadInst>(I).isAtomic()) {
@@ -1598,6 +1599,39 @@
   Stream.ExitBlock();
 }
 
+static void WriteUseList(ValueEnumerator &VE, UseListOrder &&Order,
+                         BitstreamWriter &Stream) {
+  assert(Order.Shuffle.size() >= 2 && "Shuffle too small");
+  unsigned Code;
+  if (isa<BasicBlock>(Order.V))
+    Code = bitc::USELIST_CODE_BB;
+  else
+    Code = bitc::USELIST_CODE_DEFAULT;
+
+  SmallVector<uint64_t, 64> Record;
+  for (unsigned I : Order.Shuffle)
+    Record.push_back(I);
+  Record.push_back(VE.getValueID(Order.V));
+  Stream.EmitRecord(Code, Record);
+}
+
+static void WriteUseListBlock(const Function *F, ValueEnumerator &VE,
+                              BitstreamWriter &Stream) {
+  auto hasMore = [&]() {
+    return !VE.UseListOrders.empty() && VE.UseListOrders.back().F == F;
+  };
+  if (!hasMore())
+    // Nothing to do.
+    return;
+
+  Stream.EnterSubblock(bitc::USELIST_BLOCK_ID, 3);
+  while (hasMore()) {
+    WriteUseList(VE, std::move(VE.UseListOrders.back()), Stream);
+    VE.UseListOrders.pop_back();
+  }
+  Stream.ExitBlock();
+}
+
 /// WriteFunction - Emit a function body to the module stream.
 static void WriteFunction(const Function &F, ValueEnumerator &VE,
                           BitstreamWriter &Stream) {
@@ -1666,6 +1700,8 @@
 
   if (NeedsMetadataAttachment)
     WriteMetadataAttachment(F, VE, Stream);
+  if (shouldPreserveBitcodeUseListOrder())
+    WriteUseListBlock(&F, VE, Stream);
   VE.purgeFunction();
   Stream.ExitBlock();
 }
@@ -1831,98 +1867,6 @@
   Stream.ExitBlock();
 }
 
-// Sort the Users based on the order in which the reader parses the bitcode
-// file.
-static bool bitcodereader_order(const User *lhs, const User *rhs) {
-  // TODO: Implement.
-  return true;
-}
-
-static void WriteUseList(const Value *V, const ValueEnumerator &VE,
-                         BitstreamWriter &Stream) {
-
-  // One or zero uses can't get out of order.
-  if (V->use_empty() || V->hasNUses(1))
-    return;
-
-  // Make a copy of the in-memory use-list for sorting.
-  SmallVector<const User*, 8> UserList(V->user_begin(), V->user_end());
-
-  // Sort the copy based on the order read by the BitcodeReader.
-  std::sort(UserList.begin(), UserList.end(), bitcodereader_order);
-
-  // TODO: Generate a diff between the BitcodeWriter in-memory use-list and the
-  // sorted list (i.e., the expected BitcodeReader in-memory use-list).
-
-  // TODO: Emit the USELIST_CODE_ENTRYs.
-}
-
-static void WriteFunctionUseList(const Function *F, ValueEnumerator &VE,
-                                 BitstreamWriter &Stream) {
-  VE.incorporateFunction(*F);
-
-  for (Function::const_arg_iterator AI = F->arg_begin(), AE = F->arg_end();
-       AI != AE; ++AI)
-    WriteUseList(AI, VE, Stream);
-  for (Function::const_iterator BB = F->begin(), FE = F->end(); BB != FE;
-       ++BB) {
-    WriteUseList(BB, VE, Stream);
-    for (BasicBlock::const_iterator II = BB->begin(), IE = BB->end(); II != IE;
-         ++II) {
-      WriteUseList(II, VE, Stream);
-      for (User::const_op_iterator OI = II->op_begin(), E = II->op_end();
-           OI != E; ++OI) {
-        if ((isa<Constant>(*OI) && !isa<GlobalValue>(*OI)) ||
-            isa<InlineAsm>(*OI))
-          WriteUseList(*OI, VE, Stream);
-      }
-    }
-  }
-  VE.purgeFunction();
-}
-
-// Emit use-lists.
-static void WriteModuleUseLists(const Module *M, ValueEnumerator &VE,
-                                BitstreamWriter &Stream) {
-  Stream.EnterSubblock(bitc::USELIST_BLOCK_ID, 3);
-
-  // XXX: this modifies the module, but in a way that should never change the
-  // behavior of any pass or codegen in LLVM. The problem is that GVs may
-  // contain entries in the use_list that do not exist in the Module and are
-  // not stored in the .bc file.
-  for (Module::const_global_iterator I = M->global_begin(), E = M->global_end();
-       I != E; ++I)
-    I->removeDeadConstantUsers();
-
-  // Write the global variables.
-  for (Module::const_global_iterator GI = M->global_begin(),
-         GE = M->global_end(); GI != GE; ++GI) {
-    WriteUseList(GI, VE, Stream);
-
-    // Write the global variable initializers.
-    if (GI->hasInitializer())
-      WriteUseList(GI->getInitializer(), VE, Stream);
-  }
-
-  // Write the functions.
-  for (Module::const_iterator FI = M->begin(), FE = M->end(); FI != FE; ++FI) {
-    WriteUseList(FI, VE, Stream);
-    if (!FI->isDeclaration())
-      WriteFunctionUseList(FI, VE, Stream);
-    if (FI->hasPrefixData())
-      WriteUseList(FI->getPrefixData(), VE, Stream);
-  }
-
-  // Write the aliases.
-  for (Module::const_alias_iterator AI = M->alias_begin(), AE = M->alias_end();
-       AI != AE; ++AI) {
-    WriteUseList(AI, VE, Stream);
-    WriteUseList(AI->getAliasee(), VE, Stream);
-  }
-
-  Stream.ExitBlock();
-}
-
 /// WriteModule - Emit the specified module to the bitstream.
 static void WriteModule(const Module *M, BitstreamWriter &Stream) {
   Stream.EnterSubblock(bitc::MODULE_BLOCK_ID, 3);
@@ -1933,7 +1877,7 @@
   Stream.EmitRecord(bitc::MODULE_CODE_VERSION, Vals);
 
   // Analyze the module, enumerating globals, functions, etc.
-  ValueEnumerator VE(M);
+  ValueEnumerator VE(*M);
 
   // Emit blockinfo, which defines the standard abbreviations etc.
   WriteBlockInfo(VE, Stream);
@@ -1965,9 +1909,9 @@
   // Emit names for globals/functions etc.
   WriteValueSymbolTable(M->getValueSymbolTable(), VE, Stream);
 
-  // Emit use-lists.
-  if (EnablePreserveUseListOrdering)
-    WriteModuleUseLists(M, VE, Stream);
+  // Emit module-level use-lists.
+  if (shouldPreserveBitcodeUseListOrder())
+    WriteUseListBlock(nullptr, VE, Stream);
 
   // Emit function bodies.
   for (Module::const_iterator F = M->begin(), E = M->end(); F != E; ++F)
diff --git a/lib/Bitcode/Writer/ValueEnumerator.cpp b/lib/Bitcode/Writer/ValueEnumerator.cpp
index 15f8034..f065c83 100644
--- a/lib/Bitcode/Writer/ValueEnumerator.cpp
+++ b/lib/Bitcode/Writer/ValueEnumerator.cpp
@@ -18,31 +18,280 @@
 #include "llvm/IR/DerivedTypes.h"
 #include "llvm/IR/Instructions.h"
 #include "llvm/IR/Module.h"
+#include "llvm/IR/UseListOrder.h"
 #include "llvm/IR/ValueSymbolTable.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/raw_ostream.h"
 #include <algorithm>
 using namespace llvm;
 
+namespace {
+struct OrderMap {
+  DenseMap<const Value *, std::pair<unsigned, bool>> IDs;
+  unsigned LastGlobalConstantID;
+  unsigned LastGlobalValueID;
+
+  OrderMap() : LastGlobalConstantID(0), LastGlobalValueID(0) {}
+
+  bool isGlobalConstant(unsigned ID) const {
+    return ID <= LastGlobalConstantID;
+  }
+  bool isGlobalValue(unsigned ID) const {
+    return ID <= LastGlobalValueID && !isGlobalConstant(ID);
+  }
+
+  unsigned size() const { return IDs.size(); }
+  std::pair<unsigned, bool> &operator[](const Value *V) { return IDs[V]; }
+  std::pair<unsigned, bool> lookup(const Value *V) const {
+    return IDs.lookup(V);
+  }
+  void index(const Value *V) {
+    // Explicitly sequence get-size and insert-value operations to avoid UB.
+    unsigned ID = IDs.size() + 1;
+    IDs[V].first = ID;
+  }
+};
+}
+
+static void orderValue(const Value *V, OrderMap &OM) {
+  if (OM.lookup(V).first)
+    return;
+
+  if (const Constant *C = dyn_cast<Constant>(V))
+    if (C->getNumOperands() && !isa<GlobalValue>(C))
+      for (const Value *Op : C->operands())
+        if (!isa<BasicBlock>(Op) && !isa<GlobalValue>(Op))
+          orderValue(Op, OM);
+
+  // Note: we cannot cache this lookup above, since inserting into the map
+  // changes the map's size, and thus affects the other IDs.
+  OM.index(V);
+}
+
+static OrderMap orderModule(const Module &M) {
+  // This needs to match the order used by ValueEnumerator::ValueEnumerator()
+  // and ValueEnumerator::incorporateFunction().
+  OrderMap OM;
+
+  // In the reader, initializers of GlobalValues are set *after* all the
+  // globals have been read.  Rather than awkwardly modeling this behaviour
+  // directly in predictValueUseListOrderImpl(), just assign IDs to
+  // initializers of GlobalValues before GlobalValues themselves to model this
+  // implicitly.
+  for (const GlobalVariable &G : M.globals())
+    if (G.hasInitializer())
+      if (!isa<GlobalValue>(G.getInitializer()))
+        orderValue(G.getInitializer(), OM);
+  for (const GlobalAlias &A : M.aliases())
+    if (!isa<GlobalValue>(A.getAliasee()))
+      orderValue(A.getAliasee(), OM);
+  for (const Function &F : M)
+    if (F.hasPrefixData())
+      if (!isa<GlobalValue>(F.getPrefixData()))
+        orderValue(F.getPrefixData(), OM);
+  OM.LastGlobalConstantID = OM.size();
+
+  // Initializers of GlobalValues are processed in
+  // BitcodeReader::ResolveGlobalAndAliasInits().  Match the order there rather
+  // than ValueEnumerator, and match the code in predictValueUseListOrderImpl()
+  // by giving IDs in reverse order.
+  //
+  // Since GlobalValues never reference each other directly (just through
+  // initializers), their relative IDs only matter for determining order of
+  // uses in their initializers.
+  for (const Function &F : M)
+    orderValue(&F, OM);
+  for (const GlobalAlias &A : M.aliases())
+    orderValue(&A, OM);
+  for (const GlobalVariable &G : M.globals())
+    orderValue(&G, OM);
+  OM.LastGlobalValueID = OM.size();
+
+  for (const Function &F : M) {
+    if (F.isDeclaration())
+      continue;
+    // Here we need to match the union of ValueEnumerator::incorporateFunction()
+    // and WriteFunction().  Basic blocks are implicitly declared before
+    // anything else (by declaring their size).
+    for (const BasicBlock &BB : F)
+      orderValue(&BB, OM);
+    for (const Argument &A : F.args())
+      orderValue(&A, OM);
+    for (const BasicBlock &BB : F)
+      for (const Instruction &I : BB)
+        for (const Value *Op : I.operands())
+          if ((isa<Constant>(*Op) && !isa<GlobalValue>(*Op)) ||
+              isa<InlineAsm>(*Op))
+            orderValue(Op, OM);
+    for (const BasicBlock &BB : F)
+      for (const Instruction &I : BB)
+        orderValue(&I, OM);
+  }
+  return OM;
+}
+
+static void predictValueUseListOrderImpl(const Value *V, const Function *F,
+                                         unsigned ID, const OrderMap &OM,
+                                         UseListOrderStack &Stack) {
+  // Predict use-list order for this one.
+  typedef std::pair<const Use *, unsigned> Entry;
+  SmallVector<Entry, 64> List;
+  for (const Use &U : V->uses())
+    // Check if this user will be serialized.
+    if (OM.lookup(U.getUser()).first)
+      List.push_back(std::make_pair(&U, List.size()));
+
+  if (List.size() < 2)
+    // We may have lost some users.
+    return;
+
+  bool IsGlobalValue = OM.isGlobalValue(ID);
+  std::sort(List.begin(), List.end(), [&](const Entry &L, const Entry &R) {
+    const Use *LU = L.first;
+    const Use *RU = R.first;
+    if (LU == RU)
+      return false;
+
+    auto LID = OM.lookup(LU->getUser()).first;
+    auto RID = OM.lookup(RU->getUser()).first;
+
+    // Global values are processed in reverse order.
+    //
+    // Moreover, initializers of GlobalValues are set *after* all the globals
+    // have been read (despite having earlier IDs).  Rather than awkwardly
+    // modeling this behaviour here, orderModule() has assigned IDs to
+    // initializers of GlobalValues before GlobalValues themselves.
+    if (OM.isGlobalValue(LID) && OM.isGlobalValue(RID))
+      return LID < RID;
+
+    // If ID is 4, then expect: 7 6 5 1 2 3.
+    if (LID < RID) {
+      if (RID <= ID)
+        if (!IsGlobalValue) // GlobalValue uses don't get reversed.
+          return true;
+      return false;
+    }
+    if (RID < LID) {
+      if (LID <= ID)
+        if (!IsGlobalValue) // GlobalValue uses don't get reversed.
+          return false;
+      return true;
+    }
+
+    // LID and RID are equal, so we have different operands of the same user.
+    // Assume operands are added in order for all instructions.
+    if (LID <= ID)
+      if (!IsGlobalValue) // GlobalValue uses don't get reversed.
+        return LU->getOperandNo() < RU->getOperandNo();
+    return LU->getOperandNo() > RU->getOperandNo();
+  });
+
+  if (std::is_sorted(
+          List.begin(), List.end(),
+          [](const Entry &L, const Entry &R) { return L.second < R.second; }))
+    // Order is already correct.
+    return;
+
+  // Store the shuffle.
+  Stack.emplace_back(V, F, List.size());
+  assert(List.size() == Stack.back().Shuffle.size() && "Wrong size");
+  for (size_t I = 0, E = List.size(); I != E; ++I)
+    Stack.back().Shuffle[I] = List[I].second;
+}
+
+static void predictValueUseListOrder(const Value *V, const Function *F,
+                                     OrderMap &OM, UseListOrderStack &Stack) {
+  auto &IDPair = OM[V];
+  assert(IDPair.first && "Unmapped value");
+  if (IDPair.second)
+    // Already predicted.
+    return;
+
+  // Do the actual prediction.
+  IDPair.second = true;
+  if (!V->use_empty() && std::next(V->use_begin()) != V->use_end())
+    predictValueUseListOrderImpl(V, F, IDPair.first, OM, Stack);
+
+  // Recursive descent into constants.
+  if (const Constant *C = dyn_cast<Constant>(V))
+    if (C->getNumOperands()) // Visit GlobalValues.
+      for (const Value *Op : C->operands())
+        if (isa<Constant>(Op)) // Visit GlobalValues.
+          predictValueUseListOrder(Op, F, OM, Stack);
+}
+
+static UseListOrderStack predictUseListOrder(const Module &M) {
+  OrderMap OM = orderModule(M);
+
+  // Use-list orders need to be serialized after all the users have been added
+  // to a value, or else the shuffles will be incomplete.  Store them per
+  // function in a stack.
+  //
+  // Aside from function order, the order of values doesn't matter much here.
+  UseListOrderStack Stack;
+
+  // We want to visit the functions backward now so we can list function-local
+  // constants in the last Function they're used in.  Module-level constants
+  // have already been visited above.
+  for (auto I = M.rbegin(), E = M.rend(); I != E; ++I) {
+    const Function &F = *I;
+    if (F.isDeclaration())
+      continue;
+    for (const BasicBlock &BB : F)
+      predictValueUseListOrder(&BB, &F, OM, Stack);
+    for (const Argument &A : F.args())
+      predictValueUseListOrder(&A, &F, OM, Stack);
+    for (const BasicBlock &BB : F)
+      for (const Instruction &I : BB)
+        for (const Value *Op : I.operands())
+          if (isa<Constant>(*Op) || isa<InlineAsm>(*Op)) // Visit GlobalValues.
+            predictValueUseListOrder(Op, &F, OM, Stack);
+    for (const BasicBlock &BB : F)
+      for (const Instruction &I : BB)
+        predictValueUseListOrder(&I, &F, OM, Stack);
+  }
+
+  // Visit globals last, since the module-level use-list block will be seen
+  // before the function bodies are processed.
+  for (const GlobalVariable &G : M.globals())
+    predictValueUseListOrder(&G, nullptr, OM, Stack);
+  for (const Function &F : M)
+    predictValueUseListOrder(&F, nullptr, OM, Stack);
+  for (const GlobalAlias &A : M.aliases())
+    predictValueUseListOrder(&A, nullptr, OM, Stack);
+  for (const GlobalVariable &G : M.globals())
+    if (G.hasInitializer())
+      predictValueUseListOrder(G.getInitializer(), nullptr, OM, Stack);
+  for (const GlobalAlias &A : M.aliases())
+    predictValueUseListOrder(A.getAliasee(), nullptr, OM, Stack);
+  for (const Function &F : M)
+    if (F.hasPrefixData())
+      predictValueUseListOrder(F.getPrefixData(), nullptr, OM, Stack);
+
+  return Stack;
+}
+
 static bool isIntOrIntVectorValue(const std::pair<const Value*, unsigned> &V) {
   return V.first->getType()->isIntOrIntVectorTy();
 }
 
-/// ValueEnumerator - Enumerate module-level information.
-ValueEnumerator::ValueEnumerator(const Module *M) {
+ValueEnumerator::ValueEnumerator(const Module &M) {
+  if (shouldPreserveBitcodeUseListOrder())
+    UseListOrders = predictUseListOrder(M);
+
   // Enumerate the global variables.
-  for (Module::const_global_iterator I = M->global_begin(),
-         E = M->global_end(); I != E; ++I)
+  for (Module::const_global_iterator I = M.global_begin(), E = M.global_end();
+       I != E; ++I)
     EnumerateValue(I);
 
   // Enumerate the functions.
-  for (Module::const_iterator I = M->begin(), E = M->end(); I != E; ++I) {
+  for (Module::const_iterator I = M.begin(), E = M.end(); I != E; ++I) {
     EnumerateValue(I);
     EnumerateAttributes(cast<Function>(I)->getAttributes());
   }
 
   // Enumerate the aliases.
-  for (Module::const_alias_iterator I = M->alias_begin(), E = M->alias_end();
+  for (Module::const_alias_iterator I = M.alias_begin(), E = M.alias_end();
        I != E; ++I)
     EnumerateValue(I);
 
@@ -50,30 +299,30 @@
   unsigned FirstConstant = Values.size();
 
   // Enumerate the global variable initializers.
-  for (Module::const_global_iterator I = M->global_begin(),
-         E = M->global_end(); I != E; ++I)
+  for (Module::const_global_iterator I = M.global_begin(), E = M.global_end();
+       I != E; ++I)
     if (I->hasInitializer())
       EnumerateValue(I->getInitializer());
 
   // Enumerate the aliasees.
-  for (Module::const_alias_iterator I = M->alias_begin(), E = M->alias_end();
+  for (Module::const_alias_iterator I = M.alias_begin(), E = M.alias_end();
        I != E; ++I)
     EnumerateValue(I->getAliasee());
 
   // Enumerate the prefix data constants.
-  for (Module::const_iterator I = M->begin(), E = M->end(); I != E; ++I)
+  for (Module::const_iterator I = M.begin(), E = M.end(); I != E; ++I)
     if (I->hasPrefixData())
       EnumerateValue(I->getPrefixData());
 
   // Insert constants and metadata that are named at module level into the slot
   // pool so that the module symbol table can refer to them...
-  EnumerateValueSymbolTable(M->getValueSymbolTable());
+  EnumerateValueSymbolTable(M.getValueSymbolTable());
   EnumerateNamedMetadata(M);
 
-  SmallVector<std::pair<unsigned, MDNode*>, 8> MDs;
+  SmallVector<std::pair<unsigned, MDNode *>, 8> MDs;
 
   // Enumerate types used by function bodies and argument lists.
-  for (const Function &F : *M) {
+  for (const Function &F : M) {
     for (const Argument &A : F.args())
       EnumerateType(A.getType());
 
@@ -179,6 +428,11 @@
 void ValueEnumerator::OptimizeConstants(unsigned CstStart, unsigned CstEnd) {
   if (CstStart == CstEnd || CstStart+1 == CstEnd) return;
 
+  if (shouldPreserveBitcodeUseListOrder())
+    // Optimizing constants makes the use-list order difficult to predict.
+    // Disable it for now when trying to preserve the order.
+    return;
+
   std::stable_sort(Values.begin() + CstStart, Values.begin() + CstEnd,
                    [this](const std::pair<const Value *, unsigned> &LHS,
                           const std::pair<const Value *, unsigned> &RHS) {
@@ -209,11 +463,12 @@
     EnumerateValue(VI->getValue());
 }
 
-/// EnumerateNamedMetadata - Insert all of the values referenced by
-/// named metadata in the specified module.
-void ValueEnumerator::EnumerateNamedMetadata(const Module *M) {
-  for (Module::const_named_metadata_iterator I = M->named_metadata_begin(),
-       E = M->named_metadata_end(); I != E; ++I)
+/// Insert all of the values referenced by named metadata in the specified
+/// module.
+void ValueEnumerator::EnumerateNamedMetadata(const Module &M) {
+  for (Module::const_named_metadata_iterator I = M.named_metadata_begin(),
+                                             E = M.named_metadata_end();
+       I != E; ++I)
     EnumerateNamedMDNode(I);
 }
 
@@ -239,31 +494,31 @@
 void ValueEnumerator::EnumerateMetadata(const Value *MD) {
   assert((isa<MDNode>(MD) || isa<MDString>(MD)) && "Invalid metadata kind");
 
-  // Enumerate the type of this value.
-  EnumerateType(MD->getType());
-
+  // Skip function-local nodes themselves, but walk their operands.
   const MDNode *N = dyn_cast<MDNode>(MD);
-
-  // In the module-level pass, skip function-local nodes themselves, but
-  // do walk their operands.
   if (N && N->isFunctionLocal() && N->getFunction()) {
     EnumerateMDNodeOperands(N);
     return;
   }
 
-  // Check to see if it's already in!
-  unsigned &MDValueID = MDValueMap[MD];
-  if (MDValueID) {
-    // Increment use count.
-    MDValues[MDValueID-1].second++;
+  // Insert a dummy ID to block the co-recursive call to
+  // EnumerateMDNodeOperands() from re-visiting MD in a cyclic graph.
+  //
+  // Return early if there's already an ID.
+  if (!MDValueMap.insert(std::make_pair(MD, 0)).second)
     return;
-  }
-  MDValues.push_back(std::make_pair(MD, 1U));
-  MDValueID = MDValues.size();
 
-  // Enumerate all non-function-local operands.
+  // Enumerate the type of this value.
+  EnumerateType(MD->getType());
+
+  // Visit operands first to minimize RAUW.
   if (N)
     EnumerateMDNodeOperands(N);
+
+  // Replace the dummy ID inserted above with the correct one.  MDValueMap may
+  // have changed by inserting operands, so we need a fresh lookup here.
+  MDValues.push_back(MD);
+  MDValueMap[MD] = MDValues.size();
 }
 
 /// EnumerateFunctionLocalMetadataa - Incorporate function-local metadata
@@ -277,12 +532,10 @@
 
   // Check to see if it's already in!
   unsigned &MDValueID = MDValueMap[N];
-  if (MDValueID) {
-    // Increment use count.
-    MDValues[MDValueID-1].second++;
+  if (MDValueID)
     return;
-  }
-  MDValues.push_back(std::make_pair(N, 1U));
+
+  MDValues.push_back(N);
   MDValueID = MDValues.size();
 
   // To incoroporate function-local information visit all function-local
@@ -487,7 +740,7 @@
             FnLocalMDVector.push_back(MD);
       }
 
-      SmallVector<std::pair<unsigned, MDNode*>, 8> MDs;
+      SmallVector<std::pair<unsigned, MDNode *>, 8> MDs;
       I->getAllMetadataOtherThanDebugLoc(MDs);
       for (unsigned i = 0, e = MDs.size(); i != e; ++i) {
         MDNode *N = MDs[i].second;
@@ -510,7 +763,7 @@
   for (unsigned i = NumModuleValues, e = Values.size(); i != e; ++i)
     ValueMap.erase(Values[i].first);
   for (unsigned i = NumModuleMDValues, e = MDValues.size(); i != e; ++i)
-    MDValueMap.erase(MDValues[i].first);
+    MDValueMap.erase(MDValues[i]);
   for (unsigned i = 0, e = BasicBlocks.size(); i != e; ++i)
     ValueMap.erase(BasicBlocks[i]);
 
diff --git a/lib/Bitcode/Writer/ValueEnumerator.h b/lib/Bitcode/Writer/ValueEnumerator.h
index 1c9f38e..563c214 100644
--- a/lib/Bitcode/Writer/ValueEnumerator.h
+++ b/lib/Bitcode/Writer/ValueEnumerator.h
@@ -11,13 +11,14 @@
 //
 //===----------------------------------------------------------------------===//
 
-#ifndef VALUE_ENUMERATOR_H
-#define VALUE_ENUMERATOR_H
+#ifndef LLVM_LIB_BITCODE_WRITER_VALUEENUMERATOR_H
+#define LLVM_LIB_BITCODE_WRITER_VALUEENUMERATOR_H
 
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/UniqueVector.h"
 #include "llvm/IR/Attributes.h"
+#include "llvm/IR/UseListOrder.h"
 #include <vector>
 
 namespace llvm {
@@ -42,6 +43,9 @@
 
   // For each value, we remember its Value* and occurrence frequency.
   typedef std::vector<std::pair<const Value*, unsigned> > ValueList;
+
+  UseListOrderStack UseListOrders;
+
 private:
   typedef DenseMap<Type*, unsigned> TypeMapType;
   TypeMapType TypeMap;
@@ -54,7 +58,7 @@
   typedef UniqueVector<const Comdat *> ComdatSetType;
   ComdatSetType Comdats;
 
-  ValueList MDValues;
+  std::vector<const Value *> MDValues;
   SmallVector<const MDNode *, 8> FunctionLocalMDs;
   ValueMapType MDValueMap;
 
@@ -92,7 +96,7 @@
   ValueEnumerator(const ValueEnumerator &) LLVM_DELETED_FUNCTION;
   void operator=(const ValueEnumerator &) LLVM_DELETED_FUNCTION;
 public:
-  ValueEnumerator(const Module *M);
+  ValueEnumerator(const Module &M);
 
   void dump() const;
   void print(raw_ostream &OS, const ValueMapType &Map, const char *Name) const;
@@ -130,7 +134,7 @@
   }
 
   const ValueList &getValues() const { return Values; }
-  const ValueList &getMDValues() const { return MDValues; }
+  const std::vector<const Value *> &getMDValues() const { return MDValues; }
   const SmallVectorImpl<const MDNode *> &getFunctionLocalMDValues() const {
     return FunctionLocalMDs;
   }
@@ -172,7 +176,7 @@
   void EnumerateAttributes(AttributeSet PAL);
 
   void EnumerateValueSymbolTable(const ValueSymbolTable &ST);
-  void EnumerateNamedMetadata(const Module *M);
+  void EnumerateNamedMetadata(const Module &M);
 };
 
 } // End llvm namespace