[ThinLTO] Optimize write-only globals out

[evgeny777]

Summary

This patch uses approach introduced in D49362 to internalize variables with write-only access. Optimizer in the backend eliminates them together with corresponding stores.

Here is test result using LLVM test suite:

[steven_wu]

Sounds like an interesting optimization. Is the global storage (from the other module) also eliminated in this case? If so, can you add a testcase for that?

include/llvm/IR/ModuleSummaryIndex.h

165–166

Would it be cleaner if you create a new enum bitfield for access specifier?

enum Access { ReadWrite = 0, ReadOnly = 1, WriteOnly =2, Invalid =3 };

This should make all the method below a bit cleaner.

[evgeny777]

Is the global storage (from the other module) also eliminated in this case? If so, can you add a testcase for that?

Yes, optimized variable is internalized in both source and destination module. I'll add test case for that

include/llvm/IR/ModuleSummaryIndex.h

165–166

I'm not sure if this would make it cleaner. There is also a HaveGV flag in PointerIntPair, so to manipulate access bits as suggested you'll have to do something like this:

bool isValidAccessSpecifier() const {
    return (RefAndFlags.getInt() &  (Invalid << 1)) !=  (Invalid << 1);
}
void setWriteOnly() {
    assert(isValidAccessSpecifier());
    RefAndFlags.setInt(RefAndFlags.getInt() | (WriteOnly << 1));
 }

so technically almost the same as before

[evgeny777]

Added test case for internalization in the source module (see index-const-prop2.ll)

[steven_wu]

LGTM.

include/llvm/IR/ModuleSummaryIndex.h

165–166

My suggestion was something like (simplified):

Access getAccess() const { return RefAndFlags.getInt() >> 1; }
void setAccess(Access status) { RefAndFlags.setInt(status <<1) }
bool isReadOnly() const { getAccess() == ReadOnly; }

This is has slightly different semantics than the one you have (it overwrites status and cannot get into invalid state). I am fine with current implementation as well.

[evgeny777]

@tejohnson Teresa, do you have any comments/objections?

[tejohnson]

I didn't get very far today, will finish the review tomorrow morning.

include/llvm/IR/ModuleSummaryIndex.h

210

Is this checked anywhere? Should the assert be checking that the bit is not yet set?

212

Probably better to check this after it is set below, to catch case of both being set more immediately.

216

ditto

745

s/think/thin/

[tejohnson]

Thanks! A bunch of misc comments/suggestions below.

include/llvm/IR/ModuleSummaryIndex.h

743

This is a little confusing. Maybe change the names to "PossibleReadOnly" and "PossibleWriteOnly" or something like that?

lib/Analysis/ModuleSummaryAnalysis.cpp

280

Clarify that this first operand is the stored value. Also clarify why can't we just let these get added naturally when we do the deferred invocations of findRefEdges - I guess it is because we want to capture any references to pointers being considered (e.g. that they don't escape)?

281

s/refernces/references/. Also, the references you are referring to on this line is (I think) different than the references you mention on the prior line - can you adjust the comment to reflect (or maybe I am confused??).

287

is there a fall through else case that we aren't handling?

426–427

adjust comment about writeonly as well. Actually, would it be better to simply skip the special case handling of non-volatile loads and stores in the above loop if !IsThinLTO in the first place? Then skip all of the handling for those here as well.

lib/AsmParser/LLParser.cpp

7779

should be calling setWriteOnly. Can you add a test that would have caught this?

8618

This code won't handle the case where a GVar is writeonly but not readonly. Suggest turning into a:

do {

...

} while (EatIfPresent(lltok::comma));

loop (there are some examples in other summary parsing functions). As a bonus, that allows specifying the flags in either order.

lib/Bitcode/Reader/BitcodeReader.cpp

5380–5381

Update comment

5589–5590

Can you add comments for the const parameters? E.g. /*ReadOnly=*/

5702–5703

Ditto on const params

lib/IR/ModuleSummaryIndex.cpp

151–152

this last one doesn't go with the writeonly case, probably needs updating

154

s/is reference/if reference/

155

"and clear readonly attribute"... sounds clearer to mean

163–164

comment needs update for write only case in a few places

lib/LTO/LTO.cpp

196

Probably they should each be added in individually or we'd end up with the same hash if in one version summary was read only and in another it is write only.

test/ThinLTO/X86/index-const-prop.ll

16 ↗

(On Diff #205262)

Clearer to pull these new test cases into different files, since they aren't about constant prop

test/ThinLTO/X86/index-const-prop2.ll

54

I see the check below that the stores were eliminated from the source module, but not where we are checking that the variables were eliminated altogether?

[evgeny777]

Addressed review comments

lib/Analysis/ModuleSummaryAnalysis.cpp

280

The idea of deferred processing is that at some moment we know that all further added references are either readonly or writeonly, so we can set correct bits in ValueInfo. We can't say anything about objects referenced by first operand of store, for instance this instruction:

store i32* @value, i32** @value_ptr

is identical to C construct:

int *value_ptr = &value;

And while value_ptr can be considered write-only we can't say anything about value

287

findRefEdges can't be called with anything except llvm::User object. This means we can't call it wth, say, integer constant.

lib/AsmParser/LLParser.cpp

7779

Nice catch. I've modified tests/Assembler/thinlto-summary.ll to address this

8618

Makes sense. See thinlto-summary.ll for test case

[tejohnson]

include/llvm/IR/ModuleSummaryIndex.h

210

As noted in my last comments - we aren't validating this assumption anywhere. Should this check that getAccessSpecifier is 0 before we set? Ditto below for setWriteOnly.

lib/Analysis/ModuleSummaryAnalysis.cpp

280

Ok sure, just clarify that "second operand" means the pointer address being stored, and "first operand" means the value we are storing.

290

Remove "with" at end of above line? I.e. I think it should be "to RefEdges when processing"

426–427

I see you moved the IsThinLTO check up as suggested, but can we also skip this whole block of code (that handles NonVolatileLoads/Stores and then processes the resulting Load/StoreRefEdges sets) if !IsThinLTO? I realize this code won't do anything since those sets will be empty, but I think it would be clearer.

lib/Transforms/Utils/FunctionImportUtils.cpp

244

s/definitly/definitely/

[evgeny777]

Addressed

[tejohnson]

LGTM with one minor comment nit below. Thanks!

lib/Analysis/ModuleSummaryAnalysis.cpp

292

Avoid double negative by changing to "can't be treated either as read- or as write-only..."

[rnk]

I suspect this caused https://crbug.com/981168, so I reverted this in rL365097. The steps to repro are to build LLD with thinlto and run check-lld on Linux. Some of the wasm tests fail because the WasmSym::MemoryBase global variable is wrongly internalized. I'll try to confirm that this is the culprit and provide more concrete repro steps momentarily, but I want to get things back to green before the US holidays.

[rnk]

Just wanted to confirm, after reverting this, I was able to build and test LLD with ThinLTO enabled again, so this did cause the issue.

Unfortunately, I have not been able to come up with a reduced test case for the issue, so I will have to leave the instructions at "run check-lld with thinlto enabled".

include/llvm/IR/ModuleSummaryIndex.h

 /// of the map is unknown, resulting in inefficiencies due to repeated
 /// insertions and resizing.
 using GlobalValueSummaryMapTy =
     std::map<GlobalValue::GUID, GlobalValueSummaryInfo>;
 
 /// Struct that holds a reference to a particular GUID in a global value
 /// summary.
 struct ValueInfo {
-  PointerIntPair<const GlobalValueSummaryMapTy::value_type *, 2, int>
+  enum Flags { HaveGV = 1, ReadOnly = 2, WriteOnly = 4 };
+  PointerIntPair<const GlobalValueSummaryMapTy::value_type *, 3, int>
       RefAndFlags;
 
   ValueInfo() = default;
   ValueInfo(bool HaveGVs, const GlobalValueSummaryMapTy::value_type *R) {
     RefAndFlags.setPointer(R);
     RefAndFlags.setInt(HaveGVs);
   }
 
ArrayRef<std::unique_ptr<GlobalValueSummary>> getSummaryList() const {
     return getRef()->second.SummaryList;
   }
 
   StringRef name() const {
     return haveGVs() ? getRef()->second.U.GV->getName()
                      : getRef()->second.U.Name;
   }
 
-  bool haveGVs() const { return RefAndFlags.getInt() & 0x1; }
-  bool isReadOnly() const { return RefAndFlags.getInt() & 0x2; }
-  void setReadOnly() { RefAndFlags.setInt(RefAndFlags.getInt() | 0x2); }
+  bool haveGVs() const { return RefAndFlags.getInt() & HaveGV; }
+  bool isReadOnly() const {
+    assert(isValidAccessSpecifier());
+    return RefAndFlags.getInt() & ReadOnly;
+  }
+  bool isWriteOnly() const {
+    assert(isValidAccessSpecifier());
+    return RefAndFlags.getInt() & WriteOnly;
+  }
+  unsigned getAccessSpecifier() const {
+    assert(isValidAccessSpecifier());
+    return RefAndFlags.getInt() & (ReadOnly | WriteOnly);
+  }
+  bool isValidAccessSpecifier() const {
+    unsigned BadAccessMask = ReadOnly | WriteOnly;
+    return (RefAndFlags.getInt() & BadAccessMask) != BadAccessMask;
+  }
+  void setReadOnly() {
+    RefAndFlags.setInt(RefAndFlags.getInt() | ReadOnly);
+    // We expect ro/wo attribute to set only once during
+    // ValueInfo lifetime.
+    assert(isValidAccessSpecifier());
+  }
+  void setWriteOnly() {
+    RefAndFlags.setInt(RefAndFlags.getInt() | WriteOnly);
+    assert(isValidAccessSpecifier());
+  }
 
   const GlobalValueSummaryMapTy::value_type *getRef() const {
     return RefAndFlags.getPointer();
   }
 
   bool isDSOLocal() const;
 
   /// Checks if all copies are eligible for auto-hiding (have flag set).
if (!TypeTests.empty() || !TypeTestAssumeVCalls.empty() ||
         !TypeCheckedLoadVCalls.empty() || !TypeTestAssumeConstVCalls.empty() ||
         !TypeCheckedLoadConstVCalls.empty())
       TIdInfo = llvm::make_unique<TypeIdInfo>(TypeIdInfo{
           std::move(TypeTests), std::move(TypeTestAssumeVCalls),
           std::move(TypeCheckedLoadVCalls),
           std::move(TypeTestAssumeConstVCalls),
           std::move(TypeCheckedLoadConstVCalls)});
   }
-  // Gets the number of immutable refs in RefEdgeList
-  unsigned immutableRefCount() const;
+  // Gets the number of readonly and writeonly refs in RefEdgeList
+  std::pair<unsigned, unsigned> specialRefCounts() const;
 
   /// Check if this is a function summary.
   static bool classof(const GlobalValueSummary *GVS) {
     return GVS->getSummaryKind() == FunctionKind;
   }
 
   /// Get function summary flags.
   FFlags fflags() const { return FunFlags; }
template <> struct DenseMapInfo<FunctionSummary::ConstVCall> {
   static unsigned getHashValue(FunctionSummary::ConstVCall I) {
     return I.VFunc.GUID;
   }
 };
 
 /// Global variable summary information to aid decisions and
 /// implementation of importing.
 ///
-/// Global variable summary has extra flag, telling if it is
-/// modified during the program run or not. This affects ThinLTO
-/// internalization
+/// Global variable summary has two extra flag, telling if it is
+/// readonly or writeonly. Both readonly and writeonly variables
+/// can be optimized in the backed: readonly variables can be
+/// const-folded, while writeonly vars can be completely eliminated
+/// together with corresponding stores. We let both things happen
+/// by means of internalizing such variables after ThinLTO import.
 class GlobalVarSummary : public GlobalValueSummary {
 public:
   struct GVarFlags {
-    GVarFlags(bool ReadOnly = false) : ReadOnly(ReadOnly) {}
+    GVarFlags(bool ReadOnly, bool WriteOnly)
+        : MaybeReadOnly(ReadOnly), MaybeWriteOnly(WriteOnly) {}
 
-    unsigned ReadOnly : 1;
+    // In permodule summaries both MaybeReadOnly and MaybeWriteOnly
+    // bits are set, because attribute propagation occurs later on
+    // thin link phase.
+    unsigned MaybeReadOnly : 1;
+    unsigned MaybeWriteOnly : 1;
   } VarFlags;
 
   GlobalVarSummary(GVFlags Flags, GVarFlags VarFlags,
                    std::vector<ValueInfo> Refs)
       : GlobalValueSummary(GlobalVarKind, Flags, std::move(Refs)),
         VarFlags(VarFlags) {}
 
   /// Check if this is a global variable summary.
   static bool classof(const GlobalValueSummary *GVS) {
     return GVS->getSummaryKind() == GlobalVarKind;
   }
 
   GVarFlags varflags() const { return VarFlags; }
-  void setReadOnly(bool RO) { VarFlags.ReadOnly = RO; }
-  bool isReadOnly() const { return VarFlags.ReadOnly; }
+  void setReadOnly(bool RO) { VarFlags.MaybeReadOnly = RO; }
+  void setWriteOnly(bool WO) { VarFlags.MaybeWriteOnly = WO; }
+  bool maybeReadOnly() const { return VarFlags.MaybeReadOnly; }
+  bool maybeWriteOnly() const { return VarFlags.MaybeWriteOnly; }
 };
 
 struct TypeTestResolution {
   /// Specifies which kind of type check we should emit for this byte array.
   /// See http://clang.llvm.org/docs/ControlFlowIntegrityDesign.html for full
   /// details on each kind of check; the enumerators are described with
   /// reference to that document.
   enum Kind {
public:
 
   /// Export summary to dot file for GraphViz.
   void exportToDot(raw_ostream& OS) const;
 
   /// Print out strongly connected components for debugging.
   void dumpSCCs(raw_ostream &OS);
 
   /// Analyze index and detect unmodified globals
-  void propagateConstants(const DenseSet<GlobalValue::GUID> &PreservedSymbols);
+  void propagateAttributes(const DenseSet<GlobalValue::GUID> &PreservedSymbols);
 };
 
 /// GraphTraits definition to build SCC for the index
 template <> struct GraphTraits<ValueInfo> {
   typedef ValueInfo NodeRef;
   using EdgeRef = FunctionSummary::EdgeTy &;
 
   static NodeRef valueInfoFromEdge(FunctionSummary::EdgeTy &P) {

lib/Analysis/ModuleSummaryAnalysis.cpp

 
 static bool isNonVolatileLoad(const Instruction *I) {
   if (const auto *LI = dyn_cast<LoadInst>(I))
     return !LI->isVolatile();
 
   return false;
 }
 
+static bool isNonVolatileStore(const Instruction *I) {
+  if (const auto *SI = dyn_cast<StoreInst>(I))
+    return !SI->isVolatile();
+
+  return false;
+}
+
 static void computeFunctionSummary(ModuleSummaryIndex &Index, const Module &M,
                                    const Function &F, BlockFrequencyInfo *BFI,
                                    ProfileSummaryInfo *PSI, DominatorTree &DT,
                                    bool HasLocalsInUsedOrAsm,
                                    DenseSet<GlobalValue::GUID> &CantBePromoted,
                                    bool IsThinLTO) {
   // Summary not currently supported for anonymous functions, they should
   // have been named.
   assert(F.hasName());
 
   unsigned NumInsts = 0;
   // Map from callee ValueId to profile count. Used to accumulate profile
   // counts for all static calls to a given callee.
   MapVector<ValueInfo, CalleeInfo> CallGraphEdges;
-  SetVector<ValueInfo> RefEdges;
+  SetVector<ValueInfo> RefEdges, LoadRefEdges, StoreRefEdges;
   SetVector<GlobalValue::GUID> TypeTests;
   SetVector<FunctionSummary::VFuncId> TypeTestAssumeVCalls,
       TypeCheckedLoadVCalls;
   SetVector<FunctionSummary::ConstVCall> TypeTestAssumeConstVCalls,
       TypeCheckedLoadConstVCalls;
   ICallPromotionAnalysis ICallAnalysis;
   SmallPtrSet<const User *, 8> Visited;
 
   // Add personality function, prefix data and prologue data to function's ref
   // list.
   findRefEdges(Index, &F, RefEdges, Visited);
   std::vector<const Instruction *> NonVolatileLoads;
+  std::vector<const Instruction *> NonVolatileStores;
 
   bool HasInlineAsmMaybeReferencingInternal = false;
   for (const BasicBlock &BB : F)
     for (const Instruction &I : BB) {
       if (isa<DbgInfoIntrinsic>(I))
         continue;
       ++NumInsts;
+      // Regular LTO module doesn't participate in ThinLTO import,
+      // so no reference from it can be read/writeonly, since this
+      // would require importing variable as local copy
+      if (IsThinLTO) {
         if (isNonVolatileLoad(&I)) {
           // Postpone processing of non-volatile load instructions
           // See comments below
           Visited.insert(&I);
           NonVolatileLoads.push_back(&I);
           continue;
+        } else if (isNonVolatileStore(&I)) {
+          Visited.insert(&I);
+          NonVolatileStores.push_back(&I);
+          // All references from second operand of store can be considered
+          // write-only if we haven't added them previously to RefEdges with
+          // when processing some other instruction. References from first
+          // can't be considered as read- or write-only so we add them to
+          // RefEdges as we do from all other instructions except non-volatile
+          // load.
+          Value *Stored = I.getOperand(0);
+          if (auto *GV = dyn_cast<GlobalValue>(Stored))
+            // findRefEdges will try to examine GV operands, so instead
+            // of calling it we should add GV to RefEdges directly.
+            RefEdges.insert(Index.getOrInsertValueInfo(GV));
+          else if (auto *U = dyn_cast<User>(Stored))
+            findRefEdges(Index, U, RefEdges, Visited);
+          continue;
+        }
       }
       findRefEdges(Index, &I, RefEdges, Visited);
       auto CS = ImmutableCallSite(&I);
       if (!CS)
         continue;
 
       const auto *CI = dyn_cast<CallInst>(&I);
       // Since we don't know exactly which local values are referenced in inline
for (const Instruction &I : BB) {
             ICallAnalysis.getPromotionCandidatesForInstruction(
                 &I, NumVals, TotalCount, NumCandidates);
         for (auto &Candidate : CandidateProfileData)
           CallGraphEdges[Index.getOrInsertValueInfo(Candidate.Value)]
               .updateHotness(getHotness(Candidate.Count, PSI));
       }
     }
 
-  // By now we processed all instructions in a function, except
-  // non-volatile loads. All new refs we add in a loop below
-  // are obviously constant. All constant refs are grouped in the
-  // end of RefEdges vector, so we can use a single integer value
-  // to identify them.
-  unsigned RefCnt = RefEdges.size();
-  for (const Instruction *I : NonVolatileLoads) {
+  auto AddRefEdges = [&](const std::vector<const Instruction *> &Instrs,
+                         SetVector<ValueInfo> &Edges) {
+    for (const auto *I : Instrs) {
       Visited.erase(I);
-    findRefEdges(Index, I, RefEdges, Visited);
+      findRefEdges(Index, I, Edges, Visited);
     }
+  };
+
+  // By now we processed all instructions in a function, except
+  // non-volatile loads and non-volatile value stores. Let's find
+  // ref edges for both of instruction sets
+  AddRefEdges(NonVolatileLoads, LoadRefEdges);
+  AddRefEdges(NonVolatileStores, StoreRefEdges);
+
+  // If both load and store instruction reference the same variable
+  // we won't be able to optimize it. Add all such reference edges
+  // to RefEdges set.
+  for (auto &VI : StoreRefEdges)
+    if (LoadRefEdges.remove(VI))
+      RefEdges.insert(VI);
+
+  unsigned RefCnt = RefEdges.size();
+  // All new reference edges inserted in two loops below are either
+  // read or write only. They will be grouped in the end of RefEdges
+  // vector, so we can use a single integer value to identify them.
+  for (auto &VI : LoadRefEdges)
+    RefEdges.insert(VI);
+
+  unsigned FirstWORef = RefEdges.size();
+  for (auto &VI : StoreRefEdges)
+    RefEdges.insert(VI);
+
   std::vector<ValueInfo> Refs = RefEdges.takeVector();
-  // Regular LTO module doesn't participate in ThinLTO import,
-  // so no reference from it can be readonly, since this would
-  // require importing variable as local copy
-  if (IsThinLTO)
-    for (; RefCnt < Refs.size(); ++RefCnt)
-      Refs[RefCnt].setReadOnly();
+  for (; RefCnt < FirstWORef; ++RefCnt)
 
+  for (; RefCnt < Refs.size(); ++RefCnt)
+    Refs[RefCnt].setWriteOnly();
+
   // Explicit add hot edges to enforce importing for designated GUIDs for
   // sample PGO, to enable the same inlines as the profiled optimized binary.
   for (auto &I : F.getImportGUIDs())
     CallGraphEdges[Index.getOrInsertValueInfo(I)].updateHotness(
         ForceSummaryEdgesCold == FunctionSummary::FSHT_All
             ? CalleeInfo::HotnessType::Cold
             : CalleeInfo::HotnessType::Critical);
 
computeVariableSummary(ModuleSummaryIndex &Index, const GlobalVariable &V,
   SetVector<ValueInfo> RefEdges;
   SmallPtrSet<const User *, 8> Visited;
   bool HasBlockAddress = findRefEdges(Index, &V, RefEdges, Visited);
   bool NonRenamableLocal = isNonRenamableLocal(V);
   GlobalValueSummary::GVFlags Flags(V.getLinkage(), NonRenamableLocal,
                                     /* Live = */ false, V.isDSOLocal(),
                                     V.hasLinkOnceODRLinkage() && V.hasGlobalUnnamedAddr());
 
-  // Don't mark variables we won't be able to internalize as read-only.
-  GlobalVarSummary::GVarFlags VarFlags(
+  // Don't mark variables we won't be able to internalize as read/write-only.
+  bool CanBeInternalized =
       !V.hasComdat() && !V.hasAppendingLinkage() && !V.isInterposable() &&
-      !V.hasAvailableExternallyLinkage() && !V.hasDLLExportStorageClass());
+      !V.hasAvailableExternallyLinkage() && !V.hasDLLExportStorageClass();
+  GlobalVarSummary::GVarFlags VarFlags(CanBeInternalized, CanBeInternalized);
   auto GVarSummary = llvm::make_unique<GlobalVarSummary>(Flags, VarFlags,
                                                          RefEdges.takeVector());
   if (NonRenamableLocal)
     CantBePromoted.insert(V.getGUID());
   if (HasBlockAddress)
     GVarSummary->setNotEligibleToImport();
   Index.addGlobalValueSummary(V, std::move(GVarSummary));
 }
ModuleSymbolTable::CollectAsmSymbols(
                     ArrayRef<FunctionSummary::VFuncId>{},
                     ArrayRef<FunctionSummary::VFuncId>{},
                     ArrayRef<FunctionSummary::ConstVCall>{},
                     ArrayRef<FunctionSummary::ConstVCall>{});
             Index.addGlobalValueSummary(*GV, std::move(Summary));
           } else {
             std::unique_ptr<GlobalVarSummary> Summary =
                 llvm::make_unique<GlobalVarSummary>(
-                    GVFlags, GlobalVarSummary::GVarFlags(),
+                    GVFlags, GlobalVarSummary::GVarFlags(false, false),
                     ArrayRef<ValueInfo>{});
             Index.addGlobalValueSummary(*GV, std::move(Summary));
           }
         });
   }
 
   bool IsThinLTO = true;
   if (auto *MD =

lib/AsmParser/LLParser.cpp

bool LLParser::ParseArgs(std::vector<uint64_t> &Args) {
 
   return false;
 }
 
 static const auto FwdVIRef = (GlobalValueSummaryMapTy::value_type *)-8;
 
 static void resolveFwdRef(ValueInfo *Fwd, ValueInfo &Resolved) {
   bool ReadOnly = Fwd->isReadOnly();
+  bool WriteOnly = Fwd->isWriteOnly();
+  assert(!(ReadOnly && WriteOnly));
   *Fwd = Resolved;
   if (ReadOnly)
     Fwd->setReadOnly();
+  if (WriteOnly)
+    Fwd->setWriteOnly();
 }
 
 /// Stores the given Name/GUID and associated summary into the Index.
 /// Also updates any forward references to the associated entry ID.
 void LLParser::AddGlobalValueToIndex(
     std::string Name, GlobalValue::GUID GUID, GlobalValue::LinkageTypes Linkage,
     unsigned ID, std::unique_ptr<GlobalValueSummary> Summary) {
   // First create the ValueInfo utilizing the Name or GUID.
bool LLParser::ParseVariableSummary(std::string Name, GlobalValue::GUID GUID,
                                     unsigned ID) {
   assert(Lex.getKind() == lltok::kw_variable);
   Lex.Lex();
 
   StringRef ModulePath;
   GlobalValueSummary::GVFlags GVFlags = GlobalValueSummary::GVFlags(
       /*Linkage=*/GlobalValue::ExternalLinkage, /*NotEligibleToImport=*/false,
       /*Live=*/false, /*IsLocal=*/false, /*CanAutoHide=*/false);
-  GlobalVarSummary::GVarFlags GVarFlags(/*ReadOnly*/ false);
+  GlobalVarSummary::GVarFlags GVarFlags(/*ReadOnly*/ false,
+                                        /* WriteOnly */ false);
   std::vector<ValueInfo> Refs;
   if (ParseToken(lltok::colon, "expected ':' here") ||
       ParseToken(lltok::lparen, "expected '(' here") ||
       ParseModuleReference(ModulePath) ||
       ParseToken(lltok::comma, "expected ',' here") || ParseGVFlags(GVFlags) ||
       ParseToken(lltok::comma, "expected ',' here") ||
       ParseGVarFlags(GVarFlags))
     return true;
bool LLParser::ParseOptionalRefs(std::vector<ValueInfo> &Refs) {
   do {
     ValueContext VC;
     VC.Loc = Lex.getLoc();
     if (ParseGVReference(VC.VI, VC.GVId))
       return true;
     VContexts.push_back(VC);
   } while (EatIfPresent(lltok::comma));
 
-  // Sort value contexts so that ones with readonly ValueInfo are at the end
-  // of VContexts vector. This is needed to match immutableRefCount() behavior.
+  // Sort value contexts so that ones with writeonly
+  // and readonly ValueInfo  are at the end of VContexts vector.
+  // See FunctionSummary::specialRefCounts()
   llvm::sort(VContexts, [](const ValueContext &VC1, const ValueContext &VC2) {
-    return VC1.VI.isReadOnly() < VC2.VI.isReadOnly();
+    return VC1.VI.getAccessSpecifier() < VC2.VI.getAccessSpecifier();
   });
 
   IdToIndexMapType IdToIndexMap;
   for (auto &VC : VContexts) {
     // Keep track of the Refs array index needing a forward reference.
     // We will save the location of the ValueInfo needing an update, but
     // can only do so once the std::vector is finalized.
     if (VC.VI.getRef() == FwdVIRef)
bool LLParser::ParseGVFlags(GlobalValueSummary::GVFlags &GVFlags) {
 
   if (ParseToken(lltok::rparen, "expected ')' here"))
     return true;
 
   return false;
 }
 
 /// GVarFlags
-///   ::= 'varFlags' ':' '(' 'readonly' ':' Flag ')'
+///   ::= 'varFlags' ':' '(' 'readonly' ':' Flag
+///                      ',' 'writeonly' ':' Flag ')'
 bool LLParser::ParseGVarFlags(GlobalVarSummary::GVarFlags &GVarFlags) {
   assert(Lex.getKind() == lltok::kw_varFlags);
   Lex.Lex();
 
-  unsigned Flag = 0;
   if (ParseToken(lltok::colon, "expected ':' here") ||
-      ParseToken(lltok::lparen, "expected '(' here") ||
-      ParseToken(lltok::kw_readonly, "expected 'readonly' here") ||
-      ParseToken(lltok::colon, "expected ':' here"))
+      ParseToken(lltok::lparen, "expected '(' here"))
     return true;
 
-  ParseFlag(Flag);
-  GVarFlags.ReadOnly = Flag;
+  auto ParseRest = [this](unsigned int &Val) {
+    Lex.Lex();
+    if (ParseToken(lltok::colon, "expected ':'"))
+      return true;
+    return ParseFlag(Val);
+  };
 
-  if (ParseToken(lltok::rparen, "expected ')' here"))
+  do {
+    unsigned Flag = 0;
+    switch (Lex.getKind()) {
+    case lltok::kw_readonly:
+      if (ParseRest(Flag))
         return true;
-  return false;
+      GVarFlags.MaybeReadOnly = Flag;
+      break;
+    case lltok::kw_writeonly:
+      if (ParseRest(Flag))
+        return true;
+      GVarFlags.MaybeWriteOnly = Flag;
+      break;
+    default:
+      return Error(Lex.getLoc(), "expected gvar flag type");
+    }
+  } while (EatIfPresent(lltok::comma));
+  return ParseToken(lltok::rparen, "expected ')' here");
 }
 
 /// ModuleReference
 ///   ::= 'module' ':' UInt
 bool LLParser::ParseModuleReference(StringRef &ModulePath) {
   // Parse module id.
   if (ParseToken(lltok::kw_module, "expected 'module' here") ||
       ParseToken(lltok::colon, "expected ':' here") ||
       ParseToken(lltok::SummaryID, "expected module ID"))
     return true;
 
   unsigned ModuleID = Lex.getUIntVal();
   auto I = ModuleIdMap.find(ModuleID);
   // We should have already parsed all module IDs
   assert(I != ModuleIdMap.end());
   ModulePath = I->second;
   return false;
 }
 
 /// GVReference
 ///   ::= SummaryID
 bool LLParser::ParseGVReference(ValueInfo &VI, unsigned &GVId) {
-  bool ReadOnly = EatIfPresent(lltok::kw_readonly);
+  bool WriteOnly = false, ReadOnly = EatIfPresent(lltok::kw_readonly);
+  if (!ReadOnly)
+    WriteOnly = EatIfPresent(lltok::kw_writeonly);
   if (ParseToken(lltok::SummaryID, "expected GV ID"))
     return true;
 
   GVId = Lex.getUIntVal();
   // Check if we already have a VI for this GV
   if (GVId < NumberedValueInfos.size()) {
     assert(NumberedValueInfos[GVId].getRef() != FwdVIRef);
     VI = NumberedValueInfos[GVId];
   } else
     // We will create a forward reference to the stored location.
     VI = ValueInfo(false, FwdVIRef);
 
   if (ReadOnly)
     VI.setReadOnly();
+  if (WriteOnly)
+    VI.setWriteOnly();
   return false;
 }

lib/Bitcode/Reader/BitcodeReader.cpp

static GlobalValueSummary::GVFlags getDecodedGVSummaryFlags(uint64_t RawFlags,
   bool Local = (RawFlags & 0x4);
   bool AutoHide = (RawFlags & 0x8);
 
   return GlobalValueSummary::GVFlags(Linkage, NotEligibleToImport, Live, Local, AutoHide);
 }
 
 // Decode the flags for GlobalVariable in the summary
 static GlobalVarSummary::GVarFlags getDecodedGVarFlags(uint64_t RawFlags) {
-  return GlobalVarSummary::GVarFlags((RawFlags & 0x1) ? true : false);
+  return GlobalVarSummary::GVarFlags((RawFlags & 0x1) ? true : false,
+                                     (RawFlags & 0x2) ? true : false);
 }
 
 static GlobalValue::VisibilityTypes getDecodedVisibility(unsigned Val) {
   switch (Val) {
   default: // Map unknown visibilities to default.
   case 0: return GlobalValue::DefaultVisibility;
   case 1: return GlobalValue::HiddenVisibility;
   case 2: return GlobalValue::ProtectedVisibility;
static void parseTypeIdSummaryRecord(ArrayRef<uint64_t> Record,
   TypeId.TTRes.AlignLog2 = Record[Slot++];
   TypeId.TTRes.SizeM1 = Record[Slot++];
   TypeId.TTRes.BitMask = Record[Slot++];
   TypeId.TTRes.InlineBits = Record[Slot++];
 
   while (Slot < Record.size())
     parseWholeProgramDevirtResolution(Record, Strtab, Slot, TypeId);
 }
 
-static void setImmutableRefs(std::vector<ValueInfo> &Refs, unsigned Count) {
-  // Read-only refs are in the end of the refs list.
-  for (unsigned RefNo = Refs.size() - Count; RefNo < Refs.size(); ++RefNo)
+static void setSpecialRefs(std::vector<ValueInfo> &Refs, unsigned ROCnt,
+                           unsigned WOCnt) {
+  // Readonly and writeonly refs are in the end of the refs list.
+  assert(ROCnt + WOCnt <= Refs.size());
+  unsigned FirstWORef = Refs.size() - WOCnt;
+  unsigned RefNo = FirstWORef - ROCnt;
+  for (; RefNo < FirstWORef; ++RefNo)
     Refs[RefNo].setReadOnly();
+  for (; RefNo < Refs.size(); ++RefNo)
+    Refs[RefNo].setWriteOnly();
 }
 
 // Eagerly parse the entire summary block. This populates the GlobalValueSummary
 // objects in the index.
 Error ModuleSummaryIndexBitcodeReader::parseEntireSummary(unsigned ID) {
   if (Stream.EnterSubBlock(ID))
     return error("Invalid record");
   SmallVector<uint64_t, 64> Record;
 
   // Parse version
   {
     BitstreamEntry Entry = Stream.advanceSkippingSubblocks();
     if (Entry.Kind != BitstreamEntry::Record)
       return error("Invalid Summary Block: record for version expected");
     if (Stream.readRecord(Entry.ID, Record) != bitc::FS_VERSION)
       return error("Invalid Summary Block: version expected");
   }
   const uint64_t Version = Record[0];
   const bool IsOldProfileFormat = Version == 1;
-  if (Version < 1 || Version > 6)
+  if (Version < 1 || Version > 7)
     return error("Invalid summary version " + Twine(Version) +
-                 ". Version should be in the range [1-6].");
+                 ". Version should be in the range [1-7].");
   Record.clear();
 
   // Keep around the last seen summary to be used when we see an optional
   // "OriginalName" attachement.
   GlobalValueSummary *LastSeenSummary = nullptr;
   GlobalValue::GUID LastSeenGUID = 0;
 
   // We can expect to see any number of type ID information records before
while (true) {
     case bitc::FS_PERMODULE:
     case bitc::FS_PERMODULE_RELBF:
     case bitc::FS_PERMODULE_PROFILE: {
       unsigned ValueID = Record[0];
       uint64_t RawFlags = Record[1];
       unsigned InstCount = Record[2];
       uint64_t RawFunFlags = 0;
       unsigned NumRefs = Record[3];
-      unsigned NumImmutableRefs = 0;
+      unsigned NumRORefs = 0, NumWORefs = 0;
       int RefListStartIndex = 4;
       if (Version >= 4) {
         RawFunFlags = Record[3];
         NumRefs = Record[4];
         RefListStartIndex = 5;
         if (Version >= 5) {
-          NumImmutableRefs = Record[5];
+          NumRORefs = Record[5];
           RefListStartIndex = 6;
+          if (Version >= 7) {
+            NumWORefs = Record[6];
+            RefListStartIndex = 7;
+          }
         }
       }
 
       auto Flags = getDecodedGVSummaryFlags(RawFlags, Version);
       // The module path string ref set in the summary must be owned by the
       // index's module string table. Since we don't have a module path
       // string table section in the per-module index, we create a single
       // module path string table entry with an empty (0) ID to take
       // ownership.
       int CallGraphEdgeStartIndex = RefListStartIndex + NumRefs;
       assert(Record.size() >= RefListStartIndex + NumRefs &&
              "Record size inconsistent with number of references");
       std::vector<ValueInfo> Refs = makeRefList(
           ArrayRef<uint64_t>(Record).slice(RefListStartIndex, NumRefs));
       bool HasProfile = (BitCode == bitc::FS_PERMODULE_PROFILE);
       bool HasRelBF = (BitCode == bitc::FS_PERMODULE_RELBF);
       std::vector<FunctionSummary::EdgeTy> Calls = makeCallList(
           ArrayRef<uint64_t>(Record).slice(CallGraphEdgeStartIndex),
           IsOldProfileFormat, HasProfile, HasRelBF);
-      setImmutableRefs(Refs, NumImmutableRefs);
+      setSpecialRefs(Refs, NumRORefs, NumWORefs);
       auto FS = llvm::make_unique<FunctionSummary>(
           Flags, InstCount, getDecodedFFlags(RawFunFlags), /*EntryCount=*/0,
           std::move(Refs), std::move(Calls), std::move(PendingTypeTests),
           std::move(PendingTypeTestAssumeVCalls),
           std::move(PendingTypeCheckedLoadVCalls),
           std::move(PendingTypeTestAssumeConstVCalls),
           std::move(PendingTypeCheckedLoadConstVCalls));
       PendingTypeTests.clear();
case bitc::FS_ALIAS: {
       TheIndex.addGlobalValueSummary(GUID.first, std::move(AS));
       break;
     }
     // FS_PERMODULE_GLOBALVAR_INIT_REFS: [valueid, flags, varflags, n x valueid]
     case bitc::FS_PERMODULE_GLOBALVAR_INIT_REFS: {
       unsigned ValueID = Record[0];
       uint64_t RawFlags = Record[1];
       unsigned RefArrayStart = 2;
-      GlobalVarSummary::GVarFlags GVF;
+      GlobalVarSummary::GVarFlags GVF(/* ReadOnly */ false,
+                                      /* WriteOnly */ false);
       auto Flags = getDecodedGVSummaryFlags(RawFlags, Version);
       if (Version >= 5) {
         GVF = getDecodedGVarFlags(Record[2]);
         RefArrayStart = 3;
       }
       std::vector<ValueInfo> Refs =
           makeRefList(ArrayRef<uint64_t>(Record).slice(RefArrayStart));
       auto FS =
while (true) {
     case bitc::FS_COMBINED_PROFILE: {
       unsigned ValueID = Record[0];
       uint64_t ModuleId = Record[1];
       uint64_t RawFlags = Record[2];
       unsigned InstCount = Record[3];
       uint64_t RawFunFlags = 0;
       uint64_t EntryCount = 0;
       unsigned NumRefs = Record[4];
-      unsigned NumImmutableRefs = 0;
+      unsigned NumRORefs = 0, NumWORefs = 0;
       int RefListStartIndex = 5;
 
       if (Version >= 4) {
         RawFunFlags = Record[4];
         RefListStartIndex = 6;
         size_t NumRefsIndex = 5;
         if (Version >= 5) {
+          unsigned NumRORefsOffset = 1;
           RefListStartIndex = 7;
           if (Version >= 6) {
             NumRefsIndex = 6;
             EntryCount = Record[5];
             RefListStartIndex = 8;
+            if (Version >= 7) {
+              RefListStartIndex = 9;
+              NumWORefs = Record[8];
+              NumRORefsOffset = 2;
+            }
           }
-          NumImmutableRefs = Record[RefListStartIndex - 1];
+          NumRORefs = Record[RefListStartIndex - NumRORefsOffset];
         }
         NumRefs = Record[NumRefsIndex];
       }
 
       auto Flags = getDecodedGVSummaryFlags(RawFlags, Version);
       int CallGraphEdgeStartIndex = RefListStartIndex + NumRefs;
       assert(Record.size() >= RefListStartIndex + NumRefs &&
              "Record size inconsistent with number of references");
       std::vector<ValueInfo> Refs = makeRefList(
           ArrayRef<uint64_t>(Record).slice(RefListStartIndex, NumRefs));
       bool HasProfile = (BitCode == bitc::FS_COMBINED_PROFILE);
       std::vector<FunctionSummary::EdgeTy> Edges = makeCallList(
           ArrayRef<uint64_t>(Record).slice(CallGraphEdgeStartIndex),
           IsOldProfileFormat, HasProfile, false);
       ValueInfo VI = getValueInfoFromValueId(ValueID).first;
-      setImmutableRefs(Refs, NumImmutableRefs);
+      setSpecialRefs(Refs, NumRORefs, NumWORefs);
       auto FS = llvm::make_unique<FunctionSummary>(
           Flags, InstCount, getDecodedFFlags(RawFunFlags), EntryCount,
           std::move(Refs), std::move(Edges), std::move(PendingTypeTests),
           std::move(PendingTypeTestAssumeVCalls),
           std::move(PendingTypeCheckedLoadVCalls),
           std::move(PendingTypeTestAssumeConstVCalls),
           std::move(PendingTypeCheckedLoadConstVCalls));
       PendingTypeTests.clear();
case bitc::FS_COMBINED_ALIAS: {
       break;
     }
     // FS_COMBINED_GLOBALVAR_INIT_REFS: [valueid, modid, flags, n x valueid]
     case bitc::FS_COMBINED_GLOBALVAR_INIT_REFS: {
       unsigned ValueID = Record[0];
       uint64_t ModuleId = Record[1];
       uint64_t RawFlags = Record[2];
       unsigned RefArrayStart = 3;
-      GlobalVarSummary::GVarFlags GVF;
+      GlobalVarSummary::GVarFlags GVF(/* ReadOnly */ false,
+                                      /* WriteOnly */ false);
       auto Flags = getDecodedGVSummaryFlags(RawFlags, Version);
       if (Version >= 5) {
         GVF = getDecodedGVarFlags(Record[3]);
         RefArrayStart = 4;
       }
       std::vector<ValueInfo> Refs =
           makeRefList(ArrayRef<uint64_t>(Record).slice(RefArrayStart));
       auto FS =

lib/Bitcode/Writer/BitcodeWriter.cpp

static uint64_t getEncodedGVSummaryFlags(GlobalValueSummary::GVFlags Flags) {
   // change to the getEncodedLinkage() function will need to be taken into
   // account here as well.
   RawFlags = (RawFlags << 4) | Flags.Linkage; // 4 bits
 
   return RawFlags;
 }
 
 static uint64_t getEncodedGVarFlags(GlobalVarSummary::GVarFlags Flags) {
-  uint64_t RawFlags = Flags.ReadOnly;
+  uint64_t RawFlags = Flags.MaybeReadOnly | (Flags.MaybeWriteOnly << 1);
   return RawFlags;
 }
 
 static unsigned getEncodedVisibility(const GlobalValue &GV) {
   switch (GV.getVisibility()) {
   case GlobalValue::DefaultVisibility:   return 0;
   case GlobalValue::HiddenVisibility:    return 1;
   case GlobalValue::ProtectedVisibility: return 2;
void ModuleBitcodeWriterBase::writePerModuleFunctionSummaryRecord(
     SmallVector<uint64_t, 64> &NameVals, GlobalValueSummary *Summary,
     unsigned ValueID, unsigned FSCallsAbbrev, unsigned FSCallsProfileAbbrev,
     const Function &F) {
   NameVals.push_back(ValueID);
 
   FunctionSummary *FS = cast<FunctionSummary>(Summary);
   writeFunctionTypeMetadataRecords(Stream, FS);
 
+  auto SpecialRefCnts = FS->specialRefCounts();
   NameVals.push_back(getEncodedGVSummaryFlags(FS->flags()));
   NameVals.push_back(FS->instCount());
   NameVals.push_back(getEncodedFFlags(FS->fflags()));
   NameVals.push_back(FS->refs().size());
-  NameVals.push_back(FS->immutableRefCount());
+  NameVals.push_back(SpecialRefCnts.first);  // rorefcnt
+  NameVals.push_back(SpecialRefCnts.second); // worefcnt
 
   for (auto &RI : FS->refs())
     NameVals.push_back(VE.getValueID(RI.getValue()));
 
   bool HasProfileData =
       F.hasProfileData() || ForceSummaryEdgesCold != FunctionSummary::FSHT_None;
   for (auto &ECI : FS->calls()) {
     NameVals.push_back(getValueId(ECI.first));
void ModuleBitcodeWriterBase::writeModuleLevelReferences(
   Stream.EmitRecord(bitc::FS_PERMODULE_GLOBALVAR_INIT_REFS, NameVals,
                     FSModRefsAbbrev);
   NameVals.clear();
 }
 
 // Current version for the summary.
 // This is bumped whenever we introduce changes in the way some record are
 // interpreted, like flags for instance.
-static const uint64_t INDEX_VERSION = 6;
+static const uint64_t INDEX_VERSION = 7;
 
 /// Emit the per-module summary section alongside the rest of
 /// the module's bitcode.
 void ModuleBitcodeWriterBase::writePerModuleGlobalValueSummary() {
   // By default we compile with ThinLTO if the module has a summary, but the
   // client can request full LTO with a module flag.
   bool IsThinLTO = true;
   if (auto *MD =
void ModuleBitcodeWriterBase::writePerModuleGlobalValueSummary() {
   // Abbrev for FS_PERMODULE_PROFILE.
   auto Abbv = std::make_shared<BitCodeAbbrev>();
   Abbv->Add(BitCodeAbbrevOp(bitc::FS_PERMODULE_PROFILE));
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 8));   // valueid
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 6));   // flags
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 8));   // instcount
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 4));   // fflags
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 4));   // numrefs
-  Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 4));   // immutablerefcnt
+  Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 4));   // rorefcnt
+  Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 4));   // worefcnt
   // numrefs x valueid, n x (valueid, hotness)
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Array));
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 8));
   unsigned FSCallsProfileAbbrev = Stream.EmitAbbrev(std::move(Abbv));
 
   // Abbrev for FS_PERMODULE or FS_PERMODULE_RELBF.
   Abbv = std::make_shared<BitCodeAbbrev>();
   if (WriteRelBFToSummary)
     Abbv->Add(BitCodeAbbrevOp(bitc::FS_PERMODULE_RELBF));
   else
     Abbv->Add(BitCodeAbbrevOp(bitc::FS_PERMODULE));
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 8));   // valueid
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 6));   // flags
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 8));   // instcount
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 4));   // fflags
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 4));   // numrefs
-  Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 4));   // immutablerefcnt
+  Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 4));   // rorefcnt
+  Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 4));   // worefcnt
   // numrefs x valueid, n x (valueid [, rel_block_freq])
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Array));
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 8));
   unsigned FSCallsAbbrev = Stream.EmitAbbrev(std::move(Abbv));
 
   // Abbrev for FS_PERMODULE_GLOBALVAR_INIT_REFS.
   Abbv = std::make_shared<BitCodeAbbrev>();
   Abbv->Add(BitCodeAbbrevOp(bitc::FS_PERMODULE_GLOBALVAR_INIT_REFS));
void IndexBitcodeWriter::writeCombinedGlobalValueSummary() {
   Abbv->Add(BitCodeAbbrevOp(bitc::FS_COMBINED));
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 8));   // valueid
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 8));   // modid
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 6));   // flags
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 8));   // instcount
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 4));   // fflags
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 8));   // entrycount
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 4));   // numrefs
-  Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 4));   // immutablerefcnt
+  Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 4));   // rorefcnt
+  Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 4));   // worefcnt
   // numrefs x valueid, n x (valueid)
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Array));
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 8));
   unsigned FSCallsAbbrev = Stream.EmitAbbrev(std::move(Abbv));
 
   // Abbrev for FS_COMBINED_PROFILE.
   Abbv = std::make_shared<BitCodeAbbrev>();
   Abbv->Add(BitCodeAbbrevOp(bitc::FS_COMBINED_PROFILE));
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 8));   // valueid
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 8));   // modid
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 6));   // flags
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 8));   // instcount
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 4));   // fflags
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 8));   // entrycount
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 4));   // numrefs
-  Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 4));   // immutablerefcnt
+  Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 4));   // rorefcnt
+  Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 4));   // worefcnt
   // numrefs x valueid, n x (valueid, hotness)
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Array));
   Abbv->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::VBR, 8));
   unsigned FSCallsProfileAbbrev = Stream.EmitAbbrev(std::move(Abbv));
 
   // Abbrev for FS_COMBINED_GLOBALVAR_INIT_REFS.
   Abbv = std::make_shared<BitCodeAbbrev>();
   Abbv->Add(BitCodeAbbrevOp(bitc::FS_COMBINED_GLOBALVAR_INIT_REFS));
forEachSummary([&](GVInfo I, bool IsAliasee) {
     NameVals.push_back(Index.getModuleId(FS->modulePath()));
     NameVals.push_back(getEncodedGVSummaryFlags(FS->flags()));
     NameVals.push_back(FS->instCount());
     NameVals.push_back(getEncodedFFlags(FS->fflags()));
     NameVals.push_back(FS->entryCount());
 
     // Fill in below
     NameVals.push_back(0); // numrefs
-    NameVals.push_back(0); // immutablerefcnt
+    NameVals.push_back(0); // rorefcnt
+    NameVals.push_back(0); // worefcnt
 
-    unsigned Count = 0, ImmutableRefCnt = 0;
+    unsigned Count = 0, RORefCnt = 0, WORefCnt = 0;
     for (auto &RI : FS->refs()) {
       auto RefValueId = getValueId(RI.getGUID());
       if (!RefValueId)
         continue;
       NameVals.push_back(*RefValueId);
       if (RI.isReadOnly())
-        ImmutableRefCnt++;
+        RORefCnt++;
+      else if (RI.isWriteOnly())
+        WORefCnt++;
       Count++;
     }
     NameVals[6] = Count;
-    NameVals[7] = ImmutableRefCnt;
+    NameVals[7] = RORefCnt;
+    NameVals[8] = WORefCnt;
 
     bool HasProfileData = false;
     for (auto &EI : FS->calls()) {
       HasProfileData |=
           EI.second.getHotness() != CalleeInfo::HotnessType::Unknown;
       if (HasProfileData)
         break;
     }

lib/IR/AsmWriter.cpp

void AssemblyWriter::printAliasSummary(const AliasSummary *AS) {
   // that case by just emitting "null" as the aliasee.
   if (AS->hasAliasee())
     Out << "^" << Machine.getGUIDSlot(SummaryToGUIDMap[&AS->getAliasee()]);
   else
     Out << "null";
 }
 
 void AssemblyWriter::printGlobalVarSummary(const GlobalVarSummary *GS) {
-  Out << ", varFlags: (readonly: " << GS->VarFlags.ReadOnly << ")";
+  Out << ", varFlags: (readonly: " << GS->VarFlags.MaybeReadOnly << ", "
+      << "writeonly: " << GS->VarFlags.MaybeWriteOnly << ")";
 }
 
 static std::string getLinkageName(GlobalValue::LinkageTypes LT) {
   switch (LT) {
   case GlobalValue::ExternalLinkage:
     return "external";
   case GlobalValue::PrivateLinkage:
     return "private";
void AssemblyWriter::printSummary(const GlobalValueSummary &Summary) {
   auto RefList = Summary.refs();
   if (!RefList.empty()) {
     Out << ", refs: (";
     FieldSeparator FS;
     for (auto &Ref : RefList) {
       Out << FS;
       if (Ref.isReadOnly())
         Out << "readonly ";
+      else if (Ref.isWriteOnly())
+        Out << "writeonly ";
       Out << "^" << Machine.getGUIDSlot(Ref.getGUID());
     }
     Out << ")";
   }
 
   Out << ")";
 }
 

lib/IR/ModuleSummaryIndex.cpp

 #include "llvm/Support/Path.h"
 #include "llvm/Support/raw_ostream.h"
 using namespace llvm;
 
 #define DEBUG_TYPE "module-summary-index"
 
 STATISTIC(ReadOnlyLiveGVars,
           "Number of live global variables marked read only");
+STATISTIC(WriteOnlyLiveGVars,
+          "Number of live global variables marked write only");
 
 FunctionSummary FunctionSummary::ExternalNode =
     FunctionSummary::makeDummyFunctionSummary({});
 
 bool ValueInfo::isDSOLocal() const {
   // Need to check all summaries are local in case of hash collisions.
   return getSummaryList().size() &&
          llvm::all_of(getSummaryList(),
                       [](const std::unique_ptr<GlobalValueSummary> &Summary) {
                         return Summary->isDSOLocal();
                       });
 }
 
 bool ValueInfo::canAutoHide() const {
   // Can only auto hide if all copies are eligible to auto hide.
   return getSummaryList().size() &&
          llvm::all_of(getSummaryList(),
                       [](const std::unique_ptr<GlobalValueSummary> &Summary) {
                         return Summary->canAutoHide();
                       });
 }
 
-// Gets the number of immutable refs in RefEdgeList
-unsigned FunctionSummary::immutableRefCount() const {
-  // Here we take advantage of having all readonly references
+// Gets the number of readonly and writeonly refs in RefEdgeList
+std::pair<unsigned, unsigned> FunctionSummary::specialRefCounts() const {
+  // Here we take advantage of having all readonly and writeonly references
   // located in the end of the RefEdgeList.
   auto Refs = refs();
-  unsigned ImmutableRefCnt = 0;
-  for (int I = Refs.size() - 1; I >= 0 && Refs[I].isReadOnly(); --I)
-    ImmutableRefCnt++;
-  return ImmutableRefCnt;
+  unsigned RORefCnt = 0, WORefCnt = 0;
+  int I;
+  for (I = Refs.size() - 1; I >= 0 && Refs[I].isWriteOnly(); --I)
+    WORefCnt++;
+  for (; I >= 0 && Refs[I].isReadOnly(); --I)
+    RORefCnt++;
+  return {RORefCnt, WORefCnt};
 }
 
 // Collect for the given module the list of function it defines
 // (GUID -> Summary).
 void ModuleSummaryIndex::collectDefinedFunctionsForModule(
     StringRef ModulePath, GVSummaryMapTy &GVSummaryMap) const {
   for (auto &GlobalList : *this) {
     auto GUID = GlobalList.first;
bool ModuleSummaryIndex::isGUIDLive(GlobalValue::GUID GUID) const {
   if (SummaryList.empty())
     return true;
   for (auto &I : SummaryList)
     if (isGlobalValueLive(I.get()))
       return true;
   return false;
 }
 
-static void propagateConstantsToRefs(GlobalValueSummary *S) {
-  // If reference is not readonly then referenced summary is not
-  // readonly either. Note that:
+static void propagateAttributesToRefs(GlobalValueSummary *S) {
+  // If reference is not readonly or writeonly then referenced summary is not
+  // read/writeonly either. Note that:
   // - All references from GlobalVarSummary are conservatively considered as
-  //   not readonly. Tracking them properly requires more complex analysis
-  //   then we have now.
+  //   not readonly or writeonly. Tracking them properly requires more complex
+  //   analysis then we have now.
   //
   // - AliasSummary objects have no refs at all so this function is a no-op
   //   for them.
   for (auto &VI : S->refs()) {
-    if (VI.isReadOnly()) {
-      // We only mark refs as readonly when computing function summaries on
-      // analysis phase.
-      assert(isa<FunctionSummary>(S));
-      continue;
-    }
+    assert(VI.getAccessSpecifier() == 0 || isa<FunctionSummary>(S));
     for (auto &Ref : VI.getSummaryList())
-      // If references to alias is not readonly then aliasee is not readonly
-      if (auto *GVS = dyn_cast<GlobalVarSummary>(Ref->getBaseObject()))
+      // If references to alias is not read/writeonly then aliasee
+      // is not read/writeonly
+      if (auto *GVS = dyn_cast<GlobalVarSummary>(Ref->getBaseObject())) {
+        if (!VI.isReadOnly())
           GVS->setReadOnly(false);
+        if (!VI.isWriteOnly())
+          GVS->setWriteOnly(false);
+      }
   }
 }
 
-// Do the constant propagation in combined index.
-// The goal of constant propagation is internalization of readonly
-// variables. To determine which variables are readonly and which
-// are not we take following steps:
-// - During analysis we speculatively assign readonly attribute to
-//   all variables which can be internalized. When computing function
-//   summary we also assign readonly attribute to a reference if
-//   function doesn't modify referenced variable.
+// Do the access attribute propagation in combined index.
+// The goal of attribute propagation is internalization of readonly (RO)
+// or writeonly (WO) variables. To determine which variables are RO or WO
+// and which are not we take following steps:
+// - During analysis we speculatively assign readonly and writeonly
+//   attribute to all variables which can be internalized. When computing
+//   function summary we also assign readonly or writeonly attribute to a
+//   reference if function doesn't modify referenced variable (readonly)
+//   or doesn't read it (writeonly).
 //
-// - After computing dead symbols in combined index we do the constant
-//   propagation. During this step we clear readonly attribute from
-//   all variables which:
-//   a. are preserved or can't be imported
-//   b. referenced by any global variable initializer
-//   c. referenced by a function and reference is not readonly
+// - After computing dead symbols in combined index we do the attribute
+//   propagation. During this step we:
+//   a. clear RO and WO attributes from variables which are preserved or
+//      can't be imported
+//   b. clear RO and WO attributes from variables referenced by any global
+//      variable initializer
+//   c. clear RO attribute from variable referenced by a function when
+//      reference is not readonly
+//   d. clear WO attribute from variable referenced by a function when
+//      reference is not writeonly
+//
+//   Because of (c, d) we don't internalize variables read by function A
+//   and modified by function B.
 //
 // Internalization itself happens in the backend after import is finished
-// See internalizeImmutableGVs.
-void ModuleSummaryIndex::propagateConstants(
+// See internalizeGVsAfterImport.
+void ModuleSummaryIndex::propagateAttributes(
     const DenseSet<GlobalValue::GUID> &GUIDPreservedSymbols) {
   for (auto &P : *this)
     for (auto &S : P.second.SummaryList) {
       if (!isGlobalValueLive(S.get()))
         // We don't examine references from dead objects
         continue;
 
-      // Global variable can't be marked read only if it is not eligible
-      // to import since we need to ensure that all external references
-      // get a local (imported) copy. It also can't be marked read only
-      // if it or any alias (since alias points to the same memory) are
-      // preserved or notEligibleToImport, since either of those means
-      // there could be writes that are not visible (because preserved
-      // means it could have external to DSO writes, and notEligibleToImport
-      // means it could have writes via inline assembly leading it to be
-      // in the @llvm.*used).
+      // Global variable can't be marked read/writeonly if it is not eligible
+      // to import since we need to ensure that all external references get
+      // a local (imported) copy. It also can't be marked read/writeonly if
+      // it or any alias (since alias points to the same memory) are preserved
+      // or notEligibleToImport, since either of those means there could be
+      // writes (or reads in case of writeonly) that are not visible (because
+      // preserved means it could have external to DSO writes or reads, and
+      // notEligibleToImport means it could have writes or reads via inline
+      // assembly leading it to be in the @llvm.*used).
       if (auto *GVS = dyn_cast<GlobalVarSummary>(S->getBaseObject()))
         // Here we intentionally pass S.get() not GVS, because S could be
         // an alias.
-        if (!canImportGlobalVar(S.get()) || GUIDPreservedSymbols.count(P.first))
+        if (!canImportGlobalVar(S.get()) ||
+            GUIDPreservedSymbols.count(P.first)) {
           GVS->setReadOnly(false);
-      propagateConstantsToRefs(S.get());
+          GVS->setWriteOnly(false);
+        }
+      propagateAttributesToRefs(S.get());
     }
   if (llvm::AreStatisticsEnabled())
     for (auto &P : *this)
       if (P.second.SummaryList.size())
         if (auto *GVS = dyn_cast<GlobalVarSummary>(
                 P.second.SummaryList[0]->getBaseObject()))
-          if (isGlobalValueLive(GVS) && GVS->isReadOnly())
+          if (isGlobalValueLive(GVS)) {
+            if (GVS->maybeReadOnly())
               ReadOnlyLiveGVars++;
+            if (GVS->maybeWriteOnly())
+              WriteOnlyLiveGVars++;
+          }
 }
 
 // TODO: write a graphviz dumper for SCCs (see ModuleSummaryIndex::exportToDot)
 // then delete this function and update its tests
 LLVM_DUMP_METHOD
 void ModuleSummaryIndex::dumpSCCs(raw_ostream &O) {
   for (scc_iterator<ModuleSummaryIndex *> I =
            scc_begin<ModuleSummaryIndex *>(this);
static void defineExternalNode(raw_ostream &OS, const char *Pfx,
   } else {
     OS << getNodeVisualName(Id);
   }
   OS << "\"]; // defined externally\n";
 }
 
 static bool hasReadOnlyFlag(const GlobalValueSummary *S) {
   if (auto *GVS = dyn_cast<GlobalVarSummary>(S))
-    return GVS->isReadOnly();
+    return GVS->maybeReadOnly();
+  return false;
+}
+
+static bool hasWriteOnlyFlag(const GlobalValueSummary *S) {
+  if (auto *GVS = dyn_cast<GlobalVarSummary>(S))
+    return GVS->maybeWriteOnly();
   return false;
 }
 
 void ModuleSummaryIndex::exportToDot(raw_ostream &OS) const {
   std::vector<Edge> CrossModuleEdges;
   DenseMap<GlobalValue::GUID, std::vector<uint64_t>> NodeMap;
   using GVSOrderedMapTy = std::map<GlobalValue::GUID, GlobalValueSummary *>;
   std::map<StringRef, GVSOrderedMapTy> ModuleToDefinedGVS;
   collectDefinedGVSummariesPerModule(ModuleToDefinedGVS);
 
   // Get node identifier in form MXXX_<GUID>. The MXXX prefix is required,
   // because we may have multiple linkonce functions summaries.
   auto NodeId = [](uint64_t ModId, GlobalValue::GUID Id) {
     return ModId == (uint64_t)-1 ? std::to_string(Id)
                                  : std::string("M") + std::to_string(ModId) +
                                        "_" + std::to_string(Id);
   };
 
   auto DrawEdge = [&](const char *Pfx, uint64_t SrcMod, GlobalValue::GUID SrcId,
                       uint64_t DstMod, GlobalValue::GUID DstId,
                       int TypeOrHotness) {
     // 0 - alias
     // 1 - reference
     // 2 - constant reference
-    // Other value: (hotness - 3).
-    TypeOrHotness += 3;
+    // 3 - writeonly reference
+    // Other value: (hotness - 4).
+    TypeOrHotness += 4;
     static const char *EdgeAttrs[] = {
         " [style=dotted]; // alias",
         " [style=dashed]; // ref",
         " [style=dashed,color=forestgreen]; // const-ref",
+        " [style=dashed,color=violetred]; // writeOnly-ref",
         " // call (hotness : Unknown)",
         " [color=blue]; // call (hotness : Cold)",
         " // call (hotness : None)",
         " [color=brown]; // call (hotness : Hot)",
         " [style=bold,color=red]; // call (hotness : Critical)"};
 
     assert(static_cast<size_t>(TypeOrHotness) <
            sizeof(EdgeAttrs) / sizeof(EdgeAttrs[0]));
for (auto &SummaryIt : GVSMap) {
         A.add("shape", "record", "function");
       } else if (isa<AliasSummary>(SummaryIt.second)) {
         A.add("style", "dotted,filled", "alias");
         A.add("shape", "box");
       } else {
         A.add("shape", "Mrecord", "variable");
         if (Flags.Live && hasReadOnlyFlag(SummaryIt.second))
           A.addComment("immutable");
+        if (Flags.Live && hasWriteOnlyFlag(SummaryIt.second))
+          A.addComment("writeOnly");
       }
       if (Flags.DSOLocal)
         A.addComment("dsoLocal");
       if (Flags.CanAutoHide)
         A.addComment("canAutoHide");
 
       auto VI = getValueInfo(SummaryIt.first);
       A.add("label", getNodeLabel(VI, SummaryIt.second));
       if (!Flags.Live)
         A.add("fillcolor", "red", "dead");
       else if (Flags.NotEligibleToImport)
         A.add("fillcolor", "yellow", "not eligible to import");
 
       OS << "    " << NodeId(ModId, SummaryIt.first) << " " << A.getAsString()
          << "\n";
     }
     OS << "    // Edges:\n";
 
     for (auto &SummaryIt : GVSMap) {
       auto *GVS = SummaryIt.second;
       for (auto &R : GVS->refs())
-        Draw(SummaryIt.first, R.getGUID(), R.isReadOnly() ? -1 : -2);
+        Draw(SummaryIt.first, R.getGUID(),
+             R.isWriteOnly() ? -1 : (R.isReadOnly() ? -2 : -3));
 
       if (auto *AS = dyn_cast_or_null<AliasSummary>(SummaryIt.second)) {
-        Draw(SummaryIt.first, AS->getAliaseeGUID(), -3);
+        Draw(SummaryIt.first, AS->getAliaseeGUID(), -4);
         continue;
       }
 
       if (auto *FS = dyn_cast_or_null<FunctionSummary>(SummaryIt.second))
         for (auto &CGEdge : FS->calls())
           Draw(SummaryIt.first, CGEdge.first.getGUID(),
                static_cast<int>(CGEdge.second.Hotness));
     }

lib/LTO/LTO.cpp

#endif
   auto AddUsedThings = [&](GlobalValueSummary *GS) {
     if (!GS) return;
     AddUnsigned(GS->isLive());
     AddUnsigned(GS->canAutoHide());
     for (const ValueInfo &VI : GS->refs()) {
       AddUnsigned(VI.isDSOLocal());
       AddUsedCfiGlobal(VI.getGUID());
     }
-    if (auto *GVS = dyn_cast<GlobalVarSummary>(GS))
-      AddUnsigned(GVS->isReadOnly());
+    if (auto *GVS = dyn_cast<GlobalVarSummary>(GS)) {
+      AddUnsigned(GVS->maybeReadOnly());
+      AddUnsigned(GVS->maybeWriteOnly());
+    }
     if (auto *FS = dyn_cast<FunctionSummary>(GS)) {
       for (auto &TT : FS->type_tests())
         UsedTypeIds.insert(TT);
       for (auto &TT : FS->type_test_assume_vcalls())
         UsedTypeIds.insert(TT.GUID);
       for (auto &TT : FS->type_checked_load_vcalls())
         UsedTypeIds.insert(TT.GUID);
       for (auto &TT : FS->type_test_assume_const_vcalls())
for (auto &S : I.second.SummaryList)
         GlobalInvolvedWithAlias.insert(&AS->getAliasee());
 
   for (auto &I : Index)
     thinLTOResolvePrevailingGUID(Index.getValueInfo(I), GlobalInvolvedWithAlias,
                                  isPrevailing, recordNewLinkage,
                                  GUIDPreservedSymbols);
 }
 
-static bool isWeakWriteableObject(GlobalValueSummary *GVS) {
+static bool isWeakObjectWithRWAccess(GlobalValueSummary *GVS) {
   if (auto *VarSummary = dyn_cast<GlobalVarSummary>(GVS->getBaseObject()))
-    return !VarSummary->isReadOnly() &&
+    return !VarSummary->maybeReadOnly() && !VarSummary->maybeWriteOnly() &&
            (VarSummary->linkage() == GlobalValue::WeakODRLinkage ||
             VarSummary->linkage() == GlobalValue::LinkOnceODRLinkage);
   return false;
 }
 
 static void thinLTOInternalizeAndPromoteGUID(
     GlobalValueSummaryList &GVSummaryList, GlobalValue::GUID GUID,
     function_ref<bool(StringRef, GlobalValue::GUID)> isExported) {
   for (auto &S : GVSummaryList) {
     if (isExported(S->modulePath(), GUID)) {
       if (GlobalValue::isLocalLinkage(S->linkage()))
         S->setLinkage(GlobalValue::ExternalLinkage);
     } else if (EnableLTOInternalization &&
                // Ignore local and appending linkage values since the linker
                // doesn't resolve them.
                !GlobalValue::isLocalLinkage(S->linkage()) &&
                S->linkage() != GlobalValue::AppendingLinkage &&
                // We can't internalize available_externally globals because this
                // can break function pointer equality.
                S->linkage() != GlobalValue::AvailableExternallyLinkage &&
-               // Functions and read-only variables with linkonce_odr and weak_odr 
-               // linkage can be internalized. We can't internalize linkonce_odr
-               // and weak_odr variables which are modified somewhere in the
-               // program because reads and writes will become inconsistent.
-               !isWeakWriteableObject(S.get()))
+               // Functions and read-only variables with linkonce_odr and
+               // weak_odr linkage can be internalized. We can't internalize
+               // linkonce_odr and weak_odr variables which are both modified
+               // and read somewhere in the program because reads and writes
+               // will become inconsistent.
+               !isWeakObjectWithRWAccess(S.get()))
       S->setLinkage(GlobalValue::InternalLinkage);
   }
 }
 
 // Update the linkages in the given \p Index to mark exported values
 // as external and non-exported values as internal.
 void llvm::thinLTOInternalizeAndPromoteInIndex(
     ModuleSummaryIndex &Index,

lib/Transforms/IPO/FunctionImport.cpp

 // Compute dead symbols and propagate constants in combined index.
 void llvm::computeDeadSymbolsWithConstProp(
     ModuleSummaryIndex &Index,
     const DenseSet<GlobalValue::GUID> &GUIDPreservedSymbols,
     function_ref<PrevailingType(GlobalValue::GUID)> isPrevailing,
     bool ImportEnabled) {
   computeDeadSymbols(Index, GUIDPreservedSymbols, isPrevailing);
   if (ImportEnabled) {
-    Index.propagateConstants(GUIDPreservedSymbols);
+    Index.propagateAttributes(GUIDPreservedSymbols);
   } else {
-    // If import is disabled we should drop read-only attribute
+    // If import is disabled we should drop read/write-only attribute
     // from all summaries to prevent internalization.
     for (auto &P : Index)
       for (auto &S : P.second.SummaryList)
-        if (auto *GVS = dyn_cast<GlobalVarSummary>(S.get()))
+        if (auto *GVS = dyn_cast<GlobalVarSummary>(S.get())) {
           GVS->setReadOnly(false);
+          GVS->setWriteOnly(false);
+        }
   }
 }
 
 /// Compute the set of summaries needed for a ThinLTO backend compilation of
 /// \p ModulePath.
 void llvm::gatherImportedSummariesForModule(
     StringRef ModulePath,
     const StringMap<GVSummaryMapTy> &ModuleToDefinedGVSummaries,
static Function *replaceAliasWithAliasee(Module *SrcModule, GlobalAlias *GA) {
   NewFn->setVisibility(GA->getVisibility());
   GA->replaceAllUsesWith(ConstantExpr::getBitCast(NewFn, GA->getType()));
   NewFn->takeName(GA);
   return NewFn;
 }
 
 // Internalize values that we marked with specific attribute
 // in processGlobalForThinLTO.
-static void internalizeImmutableGVs(Module &M) {
+static void internalizeGVsAfterImport(Module &M) {
   for (auto &GV : M.globals())
     // Skip GVs which have been converted to declarations
     // by dropDeadSymbols.
     if (!GV.isDeclaration() && GV.hasAttribute("thinlto-internalize")) {
       GV.setLinkage(GlobalValue::InternalLinkage);
       GV.setVisibility(GlobalValue::DefaultVisibility);
     }
 }
if (Mover.move(std::move(SrcModule), GlobalsToImport.getArrayRef(),
                    [](GlobalValue &, IRMover::ValueAdder) {},
                    /*IsPerformingImport=*/true))
       report_fatal_error("Function Import: link error");
 
     ImportedCount += GlobalsToImport.size();
     NumImportedModules++;
   }
 
-  internalizeImmutableGVs(DestModule);
+  internalizeGVsAfterImport(DestModule);
 
   NumImportedFunctions += (ImportedCount - ImportedGVCount);
   NumImportedGlobalVars += ImportedGVCount;
 
   LLVM_DEBUG(dbgs() << "Imported " << ImportedCount - ImportedGVCount
                     << " functions for Module "
                     << DestModule.getModuleIdentifier() << "\n");
   LLVM_DEBUG(dbgs() << "Imported " << ImportedGVCount

lib/Transforms/Utils/FunctionImportUtils.cpp

if (GV.hasName()) {
     // definition.
     if (VI && VI.isDSOLocal()) {
       GV.setDSOLocal(true);
       if (GV.hasDLLImportStorageClass())
         GV.setDLLStorageClass(GlobalValue::DefaultStorageClass);
     }
   }
 
-  // Mark read-only variables which can be imported with specific attribute.
-  // We can't internalize them now because IRMover will fail to link variable
-  // definitions to their external declarations during ThinLTO import. We'll
-  // internalize read-only variables later, after import is finished.
-  // See internalizeImmutableGVs.
+  // Mark read/write-only variables which can be imported with specific
+  // attribute. We can't internalize them now because IRMover will fail
+  // to link variable definitions to their external declarations during
+  // ThinLTO import. We'll internalize read-only variables later, after
+  // import is finished. See internalizeGVsAfterImport.
   //
   // If global value dead stripping is not enabled in summary then
   // propagateConstants hasn't been run. We can't internalize GV
   // in such case.
   if (!GV.isDeclaration() && VI && ImportIndex.withGlobalValueDeadStripping()) {
     const auto &SL = VI.getSummaryList();
     auto *GVS = SL.empty() ? nullptr : dyn_cast<GlobalVarSummary>(SL[0].get());
-    if (GVS && GVS->isReadOnly())
+    // At this stage "maybe" is "definitly"
+    if (GVS && (GVS->maybeReadOnly() || GVS->maybeWriteOnly()))
       cast<GlobalVariable>(&GV)->addAttribute("thinlto-internalize");
   }
 
   bool DoPromote = false;
   if (GV.hasLocalLinkage() &&
       ((DoPromote = shouldPromoteLocalToGlobal(&GV)) || isPerformingImport())) {
     // Save the original name string before we rename GV below.
     auto Name = GV.getName().str();

test/Assembler/thinlto-summary.ll

 ; Test summary parsing (tests all types/fields in various combinations).
 ; RUN: llvm-as %s -o - | llvm-dis -o - | FileCheck %s
 
 ; ModuleID = 'thinlto-summary.thinlto.bc'
 
 ^0 = module: (path: "thinlto-summary1.o", hash: (1369602428, 2747878711, 259090915, 2507395659, 1141468049))
 ^1 = module: (path: "thinlto-summary2.o", hash: (2998369023, 4283347029, 1195487472, 2757298015, 1852134156))
 
 ; Check a function that makes several calls with various profile hotness, and a
 ; reference (also tests forward references to function and variables in calls
 ; and refs).
-^2 = gv: (guid: 1, summaries: (function: (module: ^0, flags: (linkage: external, notEligibleToImport: 0, live: 0, dsoLocal: 0), insts: 10, calls: ((callee: ^15, hotness: hot), (callee: ^17, hotness: cold), (callee: ^16, hotness: none)), refs: (readonly ^13, ^14))))
+^2 = gv: (guid: 1, summaries: (function: (module: ^0, flags: (linkage: external, notEligibleToImport: 0, live: 0, dsoLocal: 0), insts: 10, calls: ((callee: ^15, hotness: hot), (callee: ^17, hotness: cold), (callee: ^16, hotness: none)), refs: (writeonly ^14, readonly ^13, ^11))))
 
 ; Function with a call that has relative block frequency instead of profile
 ; hotness.
 ^3 = gv: (guid: 2, summaries: (function: (module: ^1, flags: (linkage: external, notEligibleToImport: 0, live: 0, dsoLocal: 0), insts: 10, calls: ((callee: ^15, relbf: 256)))))
 
 ; Summaries with different linkage types.
 ^4 = gv: (guid: 3, summaries: (function: (module: ^0, flags: (linkage: internal, notEligibleToImport: 0, live: 0, dsoLocal: 1), insts: 1)))
 ; Make this one an alias with a forward reference to aliasee.
 ^5 = gv: (guid: 4, summaries: (alias: (module: ^0, flags: (linkage: private, notEligibleToImport: 0, live: 0, dsoLocal: 1), aliasee: ^14)))
 ^6 = gv: (guid: 5, summaries: (function: (module: ^0, flags: (linkage: available_externally, notEligibleToImport: 0, live: 0, dsoLocal: 0), insts: 1)))
 ^7 = gv: (guid: 6, summaries: (function: (module: ^0, flags: (linkage: linkonce, notEligibleToImport: 0, live: 0, dsoLocal: 0), insts: 1)))
 ^8 = gv: (guid: 7, summaries: (function: (module: ^0, flags: (linkage: linkonce_odr, notEligibleToImport: 0, live: 0, dsoLocal: 0), insts: 1)))
 ^9 = gv: (guid: 8, summaries: (function: (module: ^0, flags: (linkage: weak_odr, notEligibleToImport: 0, live: 0, dsoLocal: 0, canAutoHide: 1), insts: 1)))
 ^10 = gv: (guid: 9, summaries: (function: (module: ^0, flags: (linkage: weak, notEligibleToImport: 0, live: 0, dsoLocal: 0), insts: 1)))
 ^11 = gv: (guid: 10, summaries: (variable: (module: ^0, flags: (linkage: common, notEligibleToImport: 0, live: 0, dsoLocal: 0), varFlags: (readonly: 0))))
 ; Test appending globel variable with reference (tests backward reference on
 ; refs).
 ^12 = gv: (guid: 11, summaries: (variable: (module: ^0, flags: (linkage: appending, notEligibleToImport: 0, live: 0, dsoLocal: 0), varFlags: (readonly: 0), refs: (^4))))
 
 ; Test a referenced global variable.
 ^13 = gv: (guid: 12, summaries: (variable: (module: ^0, flags: (linkage: external, notEligibleToImport: 0, live: 0, dsoLocal: 0), varFlags: (readonly: 1))))
 
 ; Test a dsoLocal variable.
-^14 = gv: (guid: 13, summaries: (variable: (module: ^0, flags: (linkage: external, notEligibleToImport: 0, live: 0, dsoLocal: 1), varFlags: (readonly: 0))))
+^14 = gv: (guid: 13, summaries: (variable: (module: ^0, flags: (linkage: external, notEligibleToImport: 0, live: 0, dsoLocal: 1), varFlags: (writeonly: 0))))
 
 ; Functions with various flag combinations (notEligibleToImport, Live,
 ; combinations of optional function flags).
 ^15 = gv: (guid: 14, summaries: (function: (module: ^1, flags: (linkage: external, notEligibleToImport: 1, live: 1, dsoLocal: 0), insts: 1)))
 ^16 = gv: (guid: 15, summaries: (function: (module: ^1, flags: (linkage: external, notEligibleToImport: 0, live: 0, dsoLocal: 0), insts: 1, funcFlags: (readNone: 1, noRecurse: 1))))
 ; This one also tests backwards reference in calls.
 ^17 = gv: (guid: 16, summaries: (function: (module: ^1, flags: (linkage: external, notEligibleToImport: 0, live: 0, dsoLocal: 0), insts: 1, funcFlags: (readOnly: 1, returnDoesNotAlias: 1), calls: ((callee: ^15)))))
 

 ^26 = typeid: (name: "_ZTS1A", summary: (typeTestRes: (kind: allOnes, sizeM1BitWidth: 7), wpdResolutions: ((offset: 0, wpdRes: (kind: branchFunnel)), (offset: 8, wpdRes: (kind: singleImpl, singleImplName: "_ZN1A1nEi")), (offset: 16, wpdRes: (kind: indir, resByArg: (args: (1, 2), byArg: (kind: indir, byte: 2, bit: 3), args: (3), byArg: (kind: uniformRetVal, info: 1), args: (4), byArg: (kind: uniqueRetVal, info: 1), args: (5), byArg: (kind: virtualConstProp)))))))
 ; Test the other kinds of type test resoultions
 ^27 = typeid: (name: "_ZTS1D", summary: (typeTestRes: (kind: byteArray, sizeM1BitWidth: 0)))
 ^28 = typeid: (name: "_ZTS1E", summary: (typeTestRes: (kind: unsat, sizeM1BitWidth: 0)))
 
 ; Make sure we get back from llvm-dis essentially what we put in via llvm-as.
 ; CHECK: ^0 = module: (path: "thinlto-summary1.o", hash: (1369602428, 2747878711, 259090915, 2507395659, 1141468049))
 ; CHECK: ^1 = module: (path: "thinlto-summary2.o", hash: (2998369023, 4283347029, 1195487472, 2757298015, 1852134156))
-; CHECK: ^2 = gv: (guid: 1, summaries: (function: (module: ^0, flags: (linkage: external, notEligibleToImport: 0, live: 0, dsoLocal: 0, canAutoHide: 0), insts: 10, calls: ((callee: ^15, hotness: hot), (callee: ^17, hotness: cold), (callee: ^16, hotness: none)), refs: (^14, readonly ^13))))
+; CHECK: ^2 = gv: (guid: 1, summaries: (function: (module: ^0, flags: (linkage: external, notEligibleToImport: 0, live: 0, dsoLocal: 0, canAutoHide: 0), insts: 10, calls: ((callee: ^15, hotness: hot), (callee: ^17, hotness: cold), (callee: ^16, hotness: none)), refs: (^11, readonly ^13, writeonly ^14))))
 ; CHECK: ^3 = gv: (guid: 2, summaries: (function: (module: ^1, flags: (linkage: external, notEligibleToImport: 0, live: 0, dsoLocal: 0, canAutoHide: 0), insts: 10, calls: ((callee: ^15)))))
 ; CHECK: ^4 = gv: (guid: 3, summaries: (function: (module: ^0, flags: (linkage: internal, notEligibleToImport: 0, live: 0, dsoLocal: 1, canAutoHide: 0), insts: 1)))
 ; CHECK: ^5 = gv: (guid: 4, summaries: (alias: (module: ^0, flags: (linkage: private, notEligibleToImport: 0, live: 0, dsoLocal: 1, canAutoHide: 0), aliasee: ^14)))
 ; CHECK: ^6 = gv: (guid: 5, summaries: (function: (module: ^0, flags: (linkage: available_externally, notEligibleToImport: 0, live: 0, dsoLocal: 0, canAutoHide: 0), insts: 1)))
 ; CHECK: ^7 = gv: (guid: 6, summaries: (function: (module: ^0, flags: (linkage: linkonce, notEligibleToImport: 0, live: 0, dsoLocal: 0, canAutoHide: 0), insts: 1)))
 ; CHECK: ^8 = gv: (guid: 7, summaries: (function: (module: ^0, flags: (linkage: linkonce_odr, notEligibleToImport: 0, live: 0, dsoLocal: 0, canAutoHide: 0), insts: 1)))
 ; CHECK: ^9 = gv: (guid: 8, summaries: (function: (module: ^0, flags: (linkage: weak_odr, notEligibleToImport: 0, live: 0, dsoLocal: 0, canAutoHide: 1), insts: 1)))
 ; CHECK: ^10 = gv: (guid: 9, summaries: (function: (module: ^0, flags: (linkage: weak, notEligibleToImport: 0, live: 0, dsoLocal: 0, canAutoHide: 0), insts: 1)))
-; CHECK: ^11 = gv: (guid: 10, summaries: (variable: (module: ^0, flags: (linkage: common, notEligibleToImport: 0, live: 0, dsoLocal: 0, canAutoHide: 0), varFlags: (readonly: 0))))
-; CHECK: ^12 = gv: (guid: 11, summaries: (variable: (module: ^0, flags: (linkage: appending, notEligibleToImport: 0, live: 0, dsoLocal: 0, canAutoHide: 0), varFlags: (readonly: 0), refs: (^4))))
-; CHECK: ^13 = gv: (guid: 12, summaries: (variable: (module: ^0, flags: (linkage: external, notEligibleToImport: 0, live: 0, dsoLocal: 0, canAutoHide: 0), varFlags: (readonly: 1))))
-; CHECK: ^14 = gv: (guid: 13, summaries: (variable: (module: ^0, flags: (linkage: external, notEligibleToImport: 0, live: 0, dsoLocal: 1, canAutoHide: 0), varFlags: (readonly: 0))))
+; CHECK: ^11 = gv: (guid: 10, summaries: (variable: (module: ^0, flags: (linkage: common, notEligibleToImport: 0, live: 0, dsoLocal: 0, canAutoHide: 0), varFlags: (readonly: 0, writeonly: 0))))
+; CHECK: ^12 = gv: (guid: 11, summaries: (variable: (module: ^0, flags: (linkage: appending, notEligibleToImport: 0, live: 0, dsoLocal: 0, canAutoHide: 0), varFlags: (readonly: 0, writeonly: 0), refs: (^4))))
+; CHECK: ^13 = gv: (guid: 12, summaries: (variable: (module: ^0, flags: (linkage: external, notEligibleToImport: 0, live: 0, dsoLocal: 0, canAutoHide: 0), varFlags: (readonly: 1, writeonly: 0))))
+; CHECK: ^14 = gv: (guid: 13, summaries: (variable: (module: ^0, flags: (linkage: external, notEligibleToImport: 0, live: 0, dsoLocal: 1, canAutoHide: 0), varFlags: (readonly: 0, writeonly: 0))))
 ; CHECK: ^15 = gv: (guid: 14, summaries: (function: (module: ^1, flags: (linkage: external, notEligibleToImport: 1, live: 1, dsoLocal: 0, canAutoHide: 0), insts: 1)))
 ; CHECK: ^16 = gv: (guid: 15, summaries: (function: (module: ^1, flags: (linkage: external, notEligibleToImport: 0, live: 0, dsoLocal: 0, canAutoHide: 0), insts: 1, funcFlags: (readNone: 1, readOnly: 0, noRecurse: 1, returnDoesNotAlias: 0, noInline: 0))))
 ; CHECK: ^17 = gv: (guid: 16, summaries: (function: (module: ^1, flags: (linkage: external, notEligibleToImport: 0, live: 0, dsoLocal: 0, canAutoHide: 0), insts: 1, funcFlags: (readNone: 0, readOnly: 1, noRecurse: 0, returnDoesNotAlias: 1, noInline: 0), calls: ((callee: ^15)))))
 ; CHECK: ^18 = gv: (guid: 17, summaries: (alias: (module: ^0, flags: (linkage: external, notEligibleToImport: 0, live: 0, dsoLocal: 1, canAutoHide: 0), aliasee: ^14)))
 ; CHECK: ^19 = gv: (guid: 18, summaries: (function: (module: ^0, flags: (linkage: external, notEligibleToImport: 0, live: 0, dsoLocal: 0, canAutoHide: 0), insts: 4, typeIdInfo: (typeTests: (^24, ^26)))))
 ; CHECK: ^20 = gv: (guid: 19, summaries: (function: (module: ^0, flags: (linkage: external, notEligibleToImport: 0, live: 0, dsoLocal: 0, canAutoHide: 0), insts: 8, typeIdInfo: (typeTestAssumeVCalls: (vFuncId: (^27, offset: 16))))))
 ; CHECK: ^21 = gv: (guid: 20, summaries: (function: (module: ^0, flags: (linkage: external, notEligibleToImport: 0, live: 0, dsoLocal: 0, canAutoHide: 0), insts: 5, typeIdInfo: (typeCheckedLoadVCalls: (vFuncId: (^25, offset: 16))))))
 ; CHECK: ^22 = gv: (guid: 21, summaries: (function: (module: ^0, flags: (linkage: external, notEligibleToImport: 0, live: 0, dsoLocal: 0, canAutoHide: 0), insts: 15, typeIdInfo: (typeTestAssumeConstVCalls: ((vFuncId: (^27, offset: 16), args: (42)), (vFuncId: (^27, offset: 24)))))))

test/Bitcode/summary_version.ll

 ; Check summary versioning
 ; RUN: opt  -module-summary  %s -o - | llvm-bcanalyzer -dump | FileCheck %s
 
 ; CHECK: <GLOBALVAL_SUMMARY_BLOCK
-; CHECK: <VERSION op0=6/>
+; CHECK: <VERSION op0=7/>
 
 
 
 ; Need a function for the summary to be populated.
 define void @foo() {
     ret void
 }

test/Bitcode/thinlto-alias.ll

 ; CHECK-NEXT: <FUNCTION op0=0 op1=4
 ; "analias"
 ; CHECK-NEXT: <FUNCTION op0=4 op1=7
 ; CHECK:       <GLOBALVAL_SUMMARY_BLOCK
 ; CHECK-NEXT:    <VERSION
 ; CHECK-NEXT:    <FLAGS
 ; See if the call to func is registered.
 ; The value id 1 matches the second FUNCTION record above.
-; CHECK-NEXT:    <PERMODULE {{.*}} op6=1/>
+; CHECK-NEXT:    <PERMODULE {{.*}} op7=1/>
 ; CHECK-NEXT:  </GLOBALVAL_SUMMARY_BLOCK>
 
 ; CHECK: <STRTAB_BLOCK
 ; CHECK-NEXT: blob data = 'mainanalias{{.*}}'
 
 ; COMBINED:       <GLOBALVAL_SUMMARY_BLOCK
 ; COMBINED-NEXT:    <VERSION
 ; COMBINED-NEXT:    <FLAGS
 ; See if the call to analias is registered, using the expected value id.
 ; COMBINED-NEXT:    <VALUE_GUID op0=[[ALIASID:[0-9]+]] op1=-5751648690987223394/>
 ; COMBINED-NEXT:    <VALUE_GUID
 ; COMBINED-NEXT:    <VALUE_GUID op0=[[ALIASEEID:[0-9]+]] op1=-1039159065113703048/>
-; COMBINED-NEXT:    <COMBINED {{.*}} op8=[[ALIASID]]/>
+; COMBINED-NEXT:    <COMBINED {{.*}} op9=[[ALIASID]]/>
 ; COMBINED-NEXT:    <COMBINED {{.*}}
 ; COMBINED-NEXT:    <COMBINED_ALIAS  {{.*}} op3=[[ALIASEEID]]
 ; COMBINED-NEXT:  </GLOBALVAL_SUMMARY_BLOCK
 
 ; ModuleID = 'thinlto-function-summary-callgraph.ll'
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
 

test/Bitcode/thinlto-alias2.ll

 ; Test to check the callgraph for call to alias in module.
 ; RUN: opt -module-summary %s -o %t.o
 ; RUN: llvm-bcanalyzer -dump %t.o | FileCheck %s
 
 ; CHECK:       <GLOBALVAL_SUMMARY_BLOCK
 ; CHECK-NEXT:    <VERSION
 ; CHECK-NEXT:    <FLAGS
-; CHECK-NEXT:    <PERMODULE {{.*}} op4=0 op5=0 op6=[[ALIASID:[0-9]+]]/>
+; CHECK-NEXT:    <PERMODULE {{.*}} op4=0 op5=0 op6=0 op7=[[ALIASID:[0-9]+]]/>
 ; CHECK-NEXT:    <PERMODULE {{.*}} op0=[[ALIASEEID:[0-9]+]]
 ; CHECK-NEXT:    <ALIAS {{.*}} op0=[[ALIASID]] {{.*}} op2=[[ALIASEEID]]/>
 ; CHECK-NEXT:  </GLOBALVAL_SUMMARY_BLOCK>
 
 ; ModuleID = 'thinlto-alias2.ll'
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
 

test/Bitcode/thinlto-function-summary-callgraph-cast.ll

 ; Test to check the callgraph for calls to casts.
 ; RUN: opt -module-summary %s -o %t.o
 ; RUN: llvm-bcanalyzer -dump %t.o | FileCheck %s
 ; PR34966
 
 ; CHECK:       <GLOBALVAL_SUMMARY_BLOCK
 ; CHECK-NEXT:    <VERSION
 ; CHECK-NEXT:    <FLAGS
 ; "op7" is a call to "callee" function.
-; CHECK-NEXT:    <PERMODULE {{.*}} op8=3 op9=[[ALIASID:[0-9]+]]/>
+; CHECK-NEXT:    <PERMODULE {{.*}} op9=3 op10=[[ALIASID:[0-9]+]]/>
 ; "another_caller" has only references but no calls.
-; CHECK-NEXT:    <PERMODULE {{.*}} op4=3 {{.*}} op8={{[0-9]+}}/>
+; CHECK-NEXT:    <PERMODULE {{.*}} op4=3 {{.*}} op9={{[0-9]+}}/>
 ; CHECK-NEXT:    <PERMODULE {{.*}} op0=[[ALIASEEID:[0-9]+]]
 ; CHECK-NEXT:    <ALIAS {{.*}} op0=[[ALIASID]] {{.*}} op2=[[ALIASEEID]]/>
 ; CHECK-NEXT:  </GLOBALVAL_SUMMARY_BLOCK>
 
 ; ModuleID = 'thinlto-function-summary-callgraph-cast.ll'
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
 

test/Bitcode/thinlto-function-summary-callgraph-pgo.ll

 ; CHECK: <SOURCE_FILENAME
 ; CHECK-NEXT: <FUNCTION
 ; "func"
 ; CHECK-NEXT: <FUNCTION op0=4 op1=4
 ; CHECK:       <GLOBALVAL_SUMMARY_BLOCK
 ; CHECK-NEXT:    <VERSION
 ; CHECK-NEXT:    <FLAGS
 ; See if the call to func is registered, using the expected hotness type.
-; CHECK-NEXT:    <PERMODULE_PROFILE {{.*}} op6=1 op7=2/>
+; CHECK-NEXT:    <PERMODULE_PROFILE {{.*}} op7=1 op8=2/>
 ; CHECK-NEXT:  </GLOBALVAL_SUMMARY_BLOCK>
 ; CHECK: <STRTAB_BLOCK
 ; CHECK-NEXT: blob data = 'mainfunc{{.*}}'
 
 ; COMBINED:       <GLOBALVAL_SUMMARY_BLOCK
 ; COMBINED-NEXT:    <VERSION
 ; COMBINED-NEXT:    <FLAGS
 ; COMBINED-NEXT:    <VALUE_GUID op0=[[FUNCID:[0-9]+]] op1=7289175272376759421/>
 ; COMBINED-NEXT:    <VALUE_GUID
 ; COMBINED-NEXT:    <COMBINED
 ; See if the call to func is registered, using the expected hotness type.
 ; op6=2 which is hotnessType::None.
-; COMBINED-NEXT:    <COMBINED_PROFILE {{.*}} op8=[[FUNCID]] op9=2/>
+; COMBINED-NEXT:    <COMBINED_PROFILE {{.*}} op9=[[FUNCID]] op10=2/>
 ; COMBINED-NEXT:  </GLOBALVAL_SUMMARY_BLOCK>
 
 ; ModuleID = 'thinlto-function-summary-callgraph.ll'
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
 
 ; Function Attrs: nounwind uwtable
 define i32 @main() #0 !prof !2 {

test/Bitcode/thinlto-function-summary-callgraph-profile-summary.ll

 ; CHECK-NEXT: <FUNCTION op0=37 op1=5
 ; "none3"
 ; CHECK-NEXT: <FUNCTION op0=42 op1=5
 ; CHECK-LABEL:       <GLOBALVAL_SUMMARY_BLOCK
 ; CHECK-NEXT:    <VERSION
 ; CHECK-NEXT:    <FLAGS
 ; CHECK-NEXT:    <VALUE_GUID op0=25 op1=123/>
 ; op4=hot1 op6=cold op8=hot2 op10=hot4 op12=none1 op14=hot3 op16=none2 op18=none3 op20=123
-; CHECK-NEXT:    <PERMODULE_PROFILE {{.*}} op6=1 op7=3 op8=5 op9=1 op10=2 op11=3 op12=4 op13=1 op14=6 op15=2 op16=3 op17=3 op18=7 op19=2 op20=8 op21=2 op22=25 op23=4/>
+; CHECK-NEXT:    <PERMODULE_PROFILE {{.*}} op7=1 op8=3 op9=5 op10=1 op11=2 op12=3 op13=4 op14=1 op15=6 op16=2 op17=3 op18=3 op19=7 op20=2 op21=8 op22=2 op23=25 op24=4/>
 ; CHECK-NEXT:  </GLOBALVAL_SUMMARY_BLOCK>
 
 ; CHECK: <STRTAB_BLOCK
 ; CHECK-NEXT: blob data = 'hot_functionhot1hot2hot3hot4coldnone1none2none3{{.*}}'
 
 ; COMBINED:       <GLOBALVAL_SUMMARY_BLOCK
 ; COMBINED-NEXT:    <VERSION
 ; COMBINED-NEXT:    <FLAGS
 ; COMBINED-NEXT:    <VALUE_GUID
 ; COMBINED-NEXT:    <VALUE_GUID
 ; COMBINED-NEXT:    <VALUE_GUID
 ; COMBINED-NEXT:    <VALUE_GUID
 ; COMBINED-NEXT:    <VALUE_GUID
 ; COMBINED-NEXT:    <VALUE_GUID
 ; COMBINED-NEXT:    <VALUE_GUID
 ; COMBINED-NEXT:    <VALUE_GUID
 ; COMBINED-NEXT:    <COMBINED abbrevid=
 ; COMBINED-NEXT:    <COMBINED abbrevid=
 ; COMBINED-NEXT:    <COMBINED abbrevid=
 ; COMBINED-NEXT:    <COMBINED abbrevid=
 ; COMBINED-NEXT:    <COMBINED abbrevid=
 ; COMBINED-NEXT:    <COMBINED abbrevid=
-; COMBINED-NEXT:    <COMBINED_PROFILE {{.*}} op8=[[HOT1:.*]] op9=3 op10=[[COLD:.*]] op11=1 op12=[[HOT2:.*]] op13=3 op14=[[NONE1:.*]] op15=2 op16=[[HOT3:.*]] op17=3 op18=[[NONE2:.*]] op19=2 op20=[[NONE3:.*]] op21=2/>
+; COMBINED-NEXT:    <COMBINED_PROFILE {{.*}} op9=[[HOT1:.*]] op10=3 op11=[[COLD:.*]] op12=1 op13=[[HOT2:.*]] op14=3 op15=[[NONE1:.*]] op16=2 op17=[[HOT3:.*]] op18=3 op19=[[NONE2:.*]] op20=2 op21=[[NONE3:.*]] op22=2/>
 ; COMBINED_NEXT:    <COMBINED abbrevid=
 ; COMBINED_NEXT:  </GLOBALVAL_SUMMARY_BLOCK>
 
 
 ; ModuleID = 'thinlto-function-summary-callgraph.ll'
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
 

test/Bitcode/thinlto-function-summary-callgraph-relbf.ll

 ; Test to check the callgraph in summary
 ; RUN: opt -write-relbf-to-summary -module-summary %s -o %t.o
 ; RUN: llvm-bcanalyzer -dump %t.o | FileCheck %s
 ; RUN: llvm-dis -o - %t.o | FileCheck %s --check-prefix=DIS
 ; Round trip it through llvm-as
 ; RUN: llvm-dis -o - %t.o | llvm-as -write-relbf-to-summary -o - | llvm-dis -o - | FileCheck %s --check-prefix=DIS
 
 ; CHECK: <SOURCE_FILENAME
 ; CHECK-NEXT: <GLOBALVAR
 ; CHECK-NEXT: <FUNCTION
 ; "func"
 ; CHECK-NEXT: <FUNCTION op0=17 op1=4
 ; CHECK:       <GLOBALVAL_SUMMARY_BLOCK
 ; CHECK-NEXT:    <VERSION
 ; CHECK-NEXT:    <FLAGS
 ; See if the call to func is registered.
-; CHECK-NEXT:    <PERMODULE_RELBF {{.*}} op4=1 {{.*}} op8=256
+; CHECK-NEXT:    <PERMODULE_RELBF {{.*}} op4=1 {{.*}} op9=256
 ; CHECK-NEXT:  </GLOBALVAL_SUMMARY_BLOCK>
 ; CHECK: <STRTAB_BLOCK
 ; CHECK-NEXT: blob data = 'undefinedglobmainfunc{{.*}}'
 
 
 ; ModuleID = 'thinlto-function-summary-callgraph.ll'
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"

test/Bitcode/thinlto-function-summary-callgraph-sample-profile-summary.ll

 ; CHECK-NEXT: <FUNCTION op0=44 op1=5
 ; CHECK-NEXT: <FUNCTION op0=49 op1=5
 
 ; CHECK-LABEL:       <GLOBALVAL_SUMMARY_BLOCK
 ; CHECK-NEXT:    <VERSION
 ; CHECK-NEXT:    <FLAGS
 ; CHECK-NEXT:    <VALUE_GUID op0=26 op1=123/>
 ; op4=none1 op6=hot1 op8=cold1 op10=none2 op12=hot2 op14=cold2 op16=none3 op18=hot3 op20=cold3 op22=123
-; CHECK-NEXT:    <PERMODULE_PROFILE {{.*}} op6=7 op7=0 op8=1 op9=3 op10=4 op11=1 op12=8 op13=0 op14=2 op15=3 op16=5 op17=1 op18=9 op19=0 op20=3 op21=3 op22=6 op23=1 op24=26 op25=4/>
+; CHECK-NEXT:    <PERMODULE_PROFILE {{.*}} op7=7 op8=0 op9=1 op10=3 op11=4 op12=1 op13=8 op14=0 op15=2 op16=3 op17=5 op18=1 op19=9 op20=0 op21=3 op22=3 op23=6 op24=1 op25=26 op26=4/>
 ; CHECK-NEXT:  </GLOBALVAL_SUMMARY_BLOCK>
 
 ; CHECK: <STRTAB_BLOCK
 ; CHECK-NEXT: blob data = 'hot_functionhot1hot2hot3cold1cold2cold3none1none2none3{{.*}}'
 
 ; COMBINED:       <GLOBALVAL_SUMMARY_BLOCK
 ; COMBINED-NEXT:    <VERSION
 ; COMBINED-NEXT:    <FLAGS

 ; COMBINED-NEXT:    <COMBINED abbrevid=
 ; COMBINED-NEXT:    <COMBINED abbrevid=
 ; COMBINED-NEXT:    <COMBINED abbrevid=
 ; COMBINED-NEXT:    <COMBINED abbrevid=
 ; COMBINED-NEXT:    <COMBINED abbrevid=
 ; COMBINED-NEXT:    <COMBINED abbrevid=
 ; COMBINED-NEXT:    <COMBINED abbrevid=
 ; COMBINED-NEXT:    <COMBINED abbrevid=
-; COMBINED-NEXT:    <COMBINED_PROFILE {{.*}} op8=[[NONE1:.*]] op9=0 op10=[[HOT1:.*]] op11=3 op12=[[COLD1:.*]] op13=1 op14=[[NONE2:.*]] op15=0 op16=[[HOT2:.*]] op17=3 op18=[[COLD2:.*]] op19=1 op20=[[NONE3:.*]] op21=0 op22=[[HOT3:.*]] op23=3 op24=[[COLD3:.*]] op25=1/>
+; COMBINED-NEXT:    <COMBINED_PROFILE {{.*}} op9=[[NONE1:.*]] op10=0 op11=[[HOT1:.*]] op12=3 op13=[[COLD1:.*]] op14=1 op15=[[NONE2:.*]] op16=0 op17=[[HOT2:.*]] op18=3 op19=[[COLD2:.*]] op20=1 op21=[[NONE3:.*]] op22=0 op23=[[HOT3:.*]] op24=3 op25=[[COLD3:.*]] op26=1/>
 ; COMBINED_NEXT:    <COMBINED abbrevid=
 ; COMBINED_NEXT:  </GLOBALVAL_SUMMARY_BLOCK>
 
 
 ; ModuleID = 'thinlto-function-summary-callgraph.ll'
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
 

test/Bitcode/thinlto-function-summary-callgraph.ll

 ; COMBINED-NEXT:    <VERSION
 ; COMBINED-NEXT:    <FLAGS
 ; Only 2 VALUE_GUID since reference to undefinedglob should not be included in
 ; combined index.
 ; COMBINED-NEXT:    <VALUE_GUID op0=[[FUNCID:[0-9]+]] op1=7289175272376759421/>
 ; COMBINED-NEXT:    <VALUE_GUID
 ; COMBINED-NEXT:    <COMBINED
 ; See if the call to func is registered.
-; COMBINED-NEXT:    <COMBINED {{.*}} op8=[[FUNCID]]/>
+; COMBINED-NEXT:    <COMBINED {{.*}} op9=[[FUNCID]]/>
 ; COMBINED-NEXT:  </GLOBALVAL_SUMMARY_BLOCK>
 
 ; ModuleID = 'thinlto-function-summary-callgraph.ll'
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
 
 ; Function Attrs: nounwind uwtable
 define i32 @main() #0 {

test/Bitcode/thinlto-function-summary-refgraph.ll

 ; for calls). Use different linkage types for the various test cases to
 ; distinguish the test cases here (op1 contains the linkage type).
 ; Note that op3 contains the # non-call references.
 ; This also ensures that we didn't include a call or reference to intrinsic
 ; llvm.ctpop.i8.
 ; CHECK:       <GLOBALVAL_SUMMARY_BLOCK
 ; Function main contains call to func, as well as address reference to func:
 ; op0=main op4=func op5=func
-; CHECK-DAG:    <PERMODULE {{.*}} op0=11 op1=0 {{.*}} op4=1 op5=0 op6=2 op7=2/>
+; CHECK-DAG:    <PERMODULE {{.*}} op0=11 op1=0 {{.*}} op4=1 op5=0 op6=0 op7=2 op8=2/>
 ; Function W contains a call to func3 as well as a reference to globalvar:
 ; op0=W op4=globalvar op5=func3
-; CHECK-DAG:    <PERMODULE {{.*}} op0=6 op1=5 {{.*}} op4=1 op5=0 op6=1 op7=5/>
+; CHECK-DAG:    <PERMODULE {{.*}} op0=6 op1=5 {{.*}} op4=1 op5=0 op6=0 op7=1 op8=5/>
 ; Function X contains call to foo, as well as address reference to foo
 ; which is in the same instruction as the call:
 ; op0=X op4=foo op5=foo
-; CHECK-DAG:    <PERMODULE {{.*}} op0=7 op1=1 {{.*}} op4=1 op5=0 op6=4 op7=4/>
+; CHECK-DAG:    <PERMODULE {{.*}} op0=7 op1=1 {{.*}} op4=1 op5=0 op6=0 op7=4 op8=4/>
 ; Function Y contains call to func2, and ensures we don't incorrectly add
 ; a reference to it when reached while earlier analyzing the phi using its
 ; return value:
 ; op0=Y op4=func2
-; CHECK-DAG:    <PERMODULE {{.*}} op0=8 op1=72 {{.*}} op4=0 op5=0 op6=3/>
+; CHECK-DAG:    <PERMODULE {{.*}} op0=8 op1=72 {{.*}} op4=0 op5=0 op6=0 op7=3/>
 ; Function Z contains call to func2, and ensures we don't incorrectly add
 ; a reference to it when reached while analyzing subsequent use of its return
 ; value:
 ; op0=Z op4=func2
-; CHECK-DAG:    <PERMODULE {{.*}} op0=9 op1=3 {{.*}} op4=0 op5=0 op6=3/>
+; CHECK-DAG:    <PERMODULE {{.*}} op0=9 op1=3 {{.*}} op4=0 op5=0 op6=0 op7=3/>
 ; Variable bar initialization contains address reference to func:
 ; op0=bar op2=func
-; CHECK-DAG:    <PERMODULE_GLOBALVAR_INIT_REFS {{.*}} op0=0 op1=0 op2=1 op3=2/>
+; CHECK-DAG:    <PERMODULE_GLOBALVAR_INIT_REFS {{.*}} op0=0 op1=0 op2=3 op3=2/>
 ; CHECK:  </GLOBALVAL_SUMMARY_BLOCK>
 
 ; CHECK: <STRTAB_BLOCK
 ; CHECK-NEXT: blob data = 'barglobalvarfuncfunc2foofunc3WXYZllvm.ctpop.i8main{{.*}}'
 
 ; ModuleID = 'thinlto-function-summary-refgraph.ll'
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"

 ; on the path to the test.
 ; DIS: ^0 = module: (path: "{{.*}}", hash: (0, 0, 0, 0, 0))
 ; DIS-DAG: = gv: (name: "Z", summaries: (function: (module: ^0, flags: (linkage: linkonce_odr, notEligibleToImport: 0, live: 0, dsoLocal: 0, canAutoHide: 0), insts: 2, calls: ((callee: ^{{.*}}))))) ; guid = 104084381700047393
 ; DIS-DAG: = gv: (name: "X", summaries: (function: (module: ^0, flags: (linkage: available_externally, notEligibleToImport: 0, live: 0, dsoLocal: 0, canAutoHide: 0), insts: 2, calls: ((callee: ^{{.*}})), refs: (^{{.*}})))) ; guid = 1881667236089500162
 ; DIS-DAG: = gv: (name: "W", summaries: (function: (module: ^0, flags: (linkage: weak_odr, notEligibleToImport: 0, live: 0, dsoLocal: 0, canAutoHide: 0), insts: 2, calls: ((callee: ^{{.*}})), refs: (^{{.*}})))) ; guid = 5790125716599269729
 ; DIS-DAG: = gv: (name: "foo") ; guid = 6699318081062747564
 ; DIS-DAG: = gv: (name: "func") ; guid = 7289175272376759421
 ; DIS-DAG: = gv: (name: "func3") ; guid = 11517462787082255043
-; DIS-DAG: = gv: (name: "globalvar", summaries: (variable: (module: ^0, flags: (linkage: external, notEligibleToImport: 0, live: 0, dsoLocal: 0, canAutoHide: 0), varFlags: (readonly: 1)))) ; guid = 12887606300320728018
+; DIS-DAG: = gv: (name: "globalvar", summaries: (variable: (module: ^0, flags: (linkage: external, notEligibleToImport: 0, live: 0, dsoLocal: 0, canAutoHide: 0), varFlags: (readonly: 1, writeonly: 1)))) ; guid = 12887606300320728018
 ; DIS-DAG: = gv: (name: "func2") ; guid = 14069196320850861797
 ; DIS-DAG: = gv: (name: "llvm.ctpop.i8") ; guid = 15254915475081819833
 ; DIS-DAG: = gv: (name: "main", summaries: (function: (module: ^0, flags: (linkage: external, notEligibleToImport: 0, live: 0, dsoLocal: 0, canAutoHide: 0), insts: 9, calls: ((callee: ^{{.*}})), refs: (^{{.*}})))) ; guid = 15822663052811949562
-; DIS-DAG: = gv: (name: "bar", summaries: (variable: (module: ^0, flags: (linkage: external, notEligibleToImport: 0, live: 0, dsoLocal: 0, canAutoHide: 0), varFlags: (readonly: 1),  refs: (^{{.*}})))) ; guid = 16434608426314478903
+; DIS-DAG: = gv: (name: "bar", summaries: (variable: (module: ^0, flags: (linkage: external, notEligibleToImport: 0, live: 0, dsoLocal: 0, canAutoHide: 0), varFlags: (readonly: 1, writeonly: 1),  refs: (^{{.*}})))) ; guid = 16434608426314478903
 ; Don't try to match the exact GUID. Since it is private, the file path
 ; will get hashed, and that will be test dependent.
 ; DIS-DAG: = gv: (name: "Y", summaries: (function: (module: ^0, flags: (linkage: private, notEligibleToImport: 0, live: 0, dsoLocal: 1, canAutoHide: 0), insts: 14, calls: ((callee: ^{{.*}}))))) ; guid =

test/ThinLTO/X86/Inputs/dot-dumper2.ll

+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+@A = local_unnamed_addr global i32 10, align 4

test/ThinLTO/X86/dot-dumper2.ll

+; Test writeOnly attribute
+; RUN: opt -module-summary %s -o %t1.bc
+; RUN: opt -module-summary %p/Inputs/dot-dumper2.ll -o %t2.bc
+; RUN: llvm-lto2 run -save-temps %t1.bc %t2.bc -o %t3 \
+; RUN:  -r=%t1.bc,main,px \
+; RUN:  -r=%t1.bc,A, \
+; RUN:  -r=%t2.bc,A,p
+
+; RUN: cat %t3.index.dot | FileCheck --check-prefix=COMBINED %s
+
+; COMBINED: digraph Summary {
+; COMBINED-NEXT:  // Module:
+; COMBINED-NEXT:  subgraph cluster_0 {
+; COMBINED-NEXT:    style = filled;
+; COMBINED-NEXT:    color = lightgrey;
+; COMBINED-NEXT:    label =
+; COMBINED-NEXT:    node [style=filled,fillcolor=lightblue];
+; COMBINED-NEXT:    M0_[[MAIN:[0-9]+]] [shape="record",label="main|extern (inst: 2, ffl: 00000)}"]; // function
+; COMBINED-NEXT:    // Edges:
+; COMBINED-NEXT:  }
+; COMBINED-NEXT:  // Module:
+; COMBINED-NEXT:  subgraph cluster_1 {
+; COMBINED-NEXT:    style = filled;
+; COMBINED-NEXT:    color = lightgrey;
+; COMBINED-NEXT:    label =
+; COMBINED-NEXT:    node [style=filled,fillcolor=lightblue];
+; COMBINED-NEXT:    M1_[[A:[0-9]+]] [shape="Mrecord",label="A|extern}"]; // variable, writeOnly
+; COMBINED-NEXT:    // Edges:
+; COMBINED-NEXT:  }
+; COMBINED-NEXT:  // Cross-module edges:
+; COMBINED-NEXT:  M0_[[MAIN]] -> M1_[[A]] [style=dashed,color=violetred]; // writeOnly-ref
+; COMBINED-NEXT: }
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+@A = external local_unnamed_addr global i32, align 4
+
+; Function Attrs: nounwind uwtable
+define i32 @main() local_unnamed_addr {
+  store i32 42, i32* @A, align 4
+  ret i32 0
+}

test/ThinLTO/X86/index-const-prop2.ll

 ; Check constant propagation in thinlto combined summary. This allows us to do 2 things:
 ;  1. Internalize global definition which is not used externally if all accesses to it are read-only
 ;  2. Make a local copy of internal definition if all accesses to it are readonly. This allows constant
 ;     folding it during optimziation phase.
 ; RUN: opt -module-summary %s -o %t1.bc
 ; RUN: opt -module-summary %p/Inputs/index-const-prop.ll -o %t2.bc
 ; RUN: llvm-lto2 run %t1.bc %t2.bc -save-temps \
 ; RUN:  -r=%t2.bc,foo,pl \
 ; RUN:  -r=%t2.bc,bar,pl \
 ; RUN:  -r=%t2.bc,baz,pl \
 ; RUN:  -r=%t2.bc,rand, \
 ; RUN:  -r=%t2.bc,gBar,pl \
 ; RUN:  -r=%t1.bc,main,plx \
+; RUN:  -r=%t1.bc,main2,pl \
 ; RUN:  -r=%t1.bc,foo, \
 ; RUN:  -r=%t1.bc,bar, \
+; RUN:  -r=%t1.bc,baz, \
 ; RUN:  -r=%t1.bc,gBar, \
 ; RUN:  -o %t3
 ; RUN: llvm-dis %t3.1.3.import.bc -o - | FileCheck %s --check-prefix=IMPORT
 ; RUN: llvm-dis %t3.1.5.precodegen.bc -o - | FileCheck %s --check-prefix=CODEGEN
 
 ; Now check that we won't internalize global (gBar) if it's externally referenced
 ; RUN: llvm-lto2 run %t1.bc %t2.bc -save-temps \
 ; RUN:  -r=%t2.bc,foo,pl \
 ; RUN:  -r=%t2.bc,bar,pl \
 ; RUN:  -r=%t2.bc,baz,pl \
 ; RUN:  -r=%t2.bc,rand, \
 ; RUN:  -r=%t2.bc,gBar,plx \
 ; RUN:  -r=%t1.bc,main,plx \
+; RUN:  -r=%t1.bc,main2,pl \
 ; RUN:  -r=%t1.bc,foo, \
 ; RUN:  -r=%t1.bc,bar, \
+; RUN:  -r=%t1.bc,baz, \
 ; RUN:  -r=%t1.bc,gBar, \
-; RUN:  -o %t3
-; RUN: llvm-dis %t3.1.3.import.bc -o - | FileCheck %s --check-prefix=IMPORT2
+; RUN:  -o %t4
+; RUN: llvm-dis %t4.1.3.import.bc -o - | FileCheck %s --check-prefix=IMPORT2
+
+; RUN: llvm-lto2 run %t1.bc %t2.bc -save-temps \
+; RUN:  -r=%t2.bc,foo,pl \
+; RUN:  -r=%t2.bc,bar,pl \
+; RUN:  -r=%t2.bc,baz,pl \
+; RUN:  -r=%t2.bc,rand, \
+; RUN:  -r=%t2.bc,gBar,pl \
+; RUN:  -r=%t1.bc,main,pl \
+; RUN:  -r=%t1.bc,main2,plx \
+; RUN:  -r=%t1.bc,foo, \
+; RUN:  -r=%t1.bc,bar, \
+; RUN:  -r=%t1.bc,baz, \
+; RUN:  -r=%t1.bc,gBar, \
+; RUN:  -o %t5
+; RUN: llvm-dis %t5.1.3.import.bc -o - | FileCheck %s --check-prefix=IMPORT
+; RUN: llvm-dis %t5.1.5.precodegen.bc -o - | FileCheck %s --check-prefix=CODEGEN2
+; Check that gFoo and gBar were eliminated from source module together
+; with corresponsing stores
+; RUN: llvm-dis %t5.2.5.precodegen.bc -o - | FileCheck %s --check-prefix=CODEGEN2-SRC
 
 ; IMPORT:       @gFoo.llvm.0 = internal unnamed_addr global i32 1, align 4
 ; IMPORT-NEXT:  @gBar = internal local_unnamed_addr global i32 2, align 4
 ; IMPORT:       !DICompileUnit({{.*}})
 
 ; CODEGEN:        i32 @main()
 ; CODEGEN-NEXT:     ret i32 3
 
 ; IMPORT2: @gBar = available_externally dso_local local_unnamed_addr global i32 2, align 4
 
+; CODEGEN2:      i32 @main2
+; CODEGEN2-NEXT:   %1 = tail call i32 @rand()
+; CODEGEN2-NEXT:   %2 = tail call i32 @rand()
+; CODEGEN2-NEXT:   ret i32 0
+
+; CODEGEN2-SRC:       void @baz()
+; CODEGEN2-SRC-NEXT:    %1 = tail call i32 @rand()
+; CODEGEN2-SRC-NEXT:    %2 = tail call i32 @rand()
+; CODEGEN2-SRC-NEXT:    ret void
+
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-pc-linux-gnu"
 
 ; We should be able to link external definition of gBar to its declaration
 @gBar = external global i32
 
 define i32 @main() local_unnamed_addr {
   %call = tail call i32 bitcast (i32 (...)* @foo to i32 ()*)()
   %call1 = tail call i32 bitcast (i32 (...)* @bar to i32 ()*)()
   %add = add nsw i32 %call1, %call
   ret i32 %add
 }
 
+define i32 @main2() local_unnamed_addr {
+  tail call void @baz()
+  ret i32 0
+}
+
 declare i32 @foo(...) local_unnamed_addr
 
 declare i32 @bar(...) local_unnamed_addr
+
+declare void @baz() local_unnamed_addr

test/ThinLTO/X86/writeonly.ll

+; Checks that we optimize writeonly variables and corresponding stores using llvm-lto
+; -stats requires asserts
+; REQUIRES: asserts
+
+; RUN: opt -module-summary %s -o %t1.bc
+; RUN: opt -module-summary %p/Inputs/index-const-prop.ll -o %t2.bc
+; RUN: llvm-lto -thinlto-action=thinlink -o %t3.index.bc %t1.bc %t2.bc
+
+; Check that we optimize write-only variables
+; RUN: llvm-lto -thinlto-action=import -exported-symbol=main  %t1.bc -thinlto-index=%t3.index.bc -o %t1.imported.bc -stats 2>&1 | FileCheck %s --check-prefix=STATS
+; RUN: llvm-dis %t1.imported.bc -o - | FileCheck %s --check-prefix=IMPORT
+; RUN: llvm-lto -thinlto-action=optimize %t1.imported.bc -o - | llvm-dis - -o - | FileCheck %s --check-prefix=OPTIMIZE
+
+; IMPORT: @gFoo.llvm.0 = internal unnamed_addr global i32 1, align 4, !dbg !0
+; IMPORT-NEXT: @gBar = internal local_unnamed_addr global i32 2, align 4, !dbg !5
+; IMPORT: !DICompileUnit({{.*}})
+
+; STATS:  2 module-summary-index - Number of live global variables marked write only 
+
+; Check that we've optimized out variables and corresponding stores
+; OPTIMIZE-NOT:  gFoo
+; OPTIMIZE-NOT:  gBar
+; OPTIMIZE:      i32 @main
+; OPTIMIZE-NEXT:   %1 = tail call i32 @rand()
+; OPTIMIZE-NEXT:   %2 = tail call i32 @rand()
+; OPTIMIZE-NEXT:   ret i32 0
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-pc-linux-gnu"
+
+@gBar = external global i32
+
+; Should not be counted in the stats of live write only variables since it is
+; dead and will be dropped anyway.
+@gDead = internal unnamed_addr global i32 1, align 4
+
+define i32 @main() local_unnamed_addr {
+  tail call void @baz()
+  ret i32 0
+}
+declare void @baz() local_unnamed_addr

test/ThinLTO/X86/writeonly2.ll

+; Check that we optimize out writeonly variables and corresponding stores.
+; This test uses llvm-lto2
+
+; RUN: opt -module-summary %s -o %t1.bc
+; RUN: opt -module-summary %p/Inputs/index-const-prop.ll -o %t2.bc
+; RUN: llvm-lto2 run %t1.bc %t2.bc -save-temps \
+; RUN:  -r=%t2.bc,foo,pl \
+; RUN:  -r=%t2.bc,bar,pl \
+; RUN:  -r=%t2.bc,baz,pl \
+; RUN:  -r=%t2.bc,rand, \
+; RUN:  -r=%t2.bc,gBar,pl \
+; RUN:  -r=%t1.bc,main,plx \
+; RUN:  -r=%t1.bc,baz, \
+; RUN:  -r=%t1.bc,gBar, \
+; RUN:  -o %t3
+; RUN: llvm-dis %t3.1.3.import.bc -o - | FileCheck %s --check-prefix=IMPORT
+; RUN: llvm-dis %t3.1.5.precodegen.bc -o - | FileCheck %s --check-prefix=CODEGEN
+; Check that gFoo and gBar were eliminated from source module together
+; with corresponsing stores
+; RUN: llvm-dis %t3.2.5.precodegen.bc -o - | FileCheck %s --check-prefix=CODEGEN-SRC
+
+; IMPORT:       @gFoo.llvm.0 = internal unnamed_addr global i32 1, align 4
+; IMPORT-NEXT:  @gBar = internal local_unnamed_addr global i32 2, align 4
+; IMPORT:       !DICompileUnit({{.*}})
+
+; CODEGEN-NOT:  gFoo
+; CODEGEN-NOT:  gBar
+; CODEGEN:      i32 @main
+; CODEGEN-NEXT:   %1 = tail call i32 @rand()
+; CODEGEN-NEXT:   %2 = tail call i32 @rand()
+; CODEGEN-NEXT:   ret i32 0
+
+; CODEGEN-SRC-NOT:   gFoo
+; CODEGEN-SRC-NOT:   gBar
+; CODEGEN-SRC:       void @baz()
+; CODEGEN-SRC-NEXT:    %1 = tail call i32 @rand()
+; CODEGEN-SRC-NEXT:    %2 = tail call i32 @rand()
+; CODEGEN-SRC-NEXT:    ret void
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-pc-linux-gnu"
+
+; We should be able to link external definition of gBar to its declaration
+@gBar = external global i32
+
+define i32 @main() local_unnamed_addr {
+  tail call void @baz()
+  ret i32 0
+}
+declare void @baz() local_unnamed_addr