D41416: [modules] [pch] Do not deserialize all lazy template specializations when looking for one.

[ChuanqiXu9]

This from https://reviews.llvm.org/D41416. And we plan to introduce on disk hash table based on this. See #76774.

Following off are cited from https://reviews.llvm.org/D41416:

Currently, we load all lazy template specializations when we search whether there is a suitable template specialization for a template. This is especially suboptimal with modules. If module B specializes a template from module A the ASTReader would only read the specialization DeclID. This is observed to be especially pathological when module B is stl. However, the template instantiator (almost immediately after) will call findSpecialization. In turn, findSpecialization will load all lazy specializations to give an answer.

This patch teaches findSpecialization to work with lazy specializations without having to deserialize their full content. It provides along with the DeclID an cross-TU stable ODRHash of the template arguments which is enough to decide if we have already specialization and which are the exact ones (we load all decls with the same hash to avoid potential collisions) to deserialize.

While we make finding a template specialization more memory-efficient we are far from being done. There are still a few places which trigger eager deserialization of template specializations: the places where we require completion of the redeclaration chain.

[llvmbot]

@llvm/pr-subscribers-clang-modules

@llvm/pr-subscribers-clang

Author: Chuanqi Xu (ChuanqiXu9)

Changes

This from https://reviews.llvm.org/D41416. And we plan to introduce on disk hash table based on this. See #76774.

Following off are cited from https://reviews.llvm.org/D41416:

Currently, we load all lazy template specializations when we search whether there is a suitable template specialization for a template. This is especially suboptimal with modules. If module B specializes a template from module A the ASTReader would only read the specialization DeclID. This is observed to be especially pathological when module B is stl. However, the template instantiator (almost immediately after) will call findSpecialization. In turn, findSpecialization will load all lazy specializations to give an answer.

This patch teaches findSpecialization to work with lazy specializations without having to deserialize their full content. It provides along with the DeclID an cross-TU stable ODRHash of the template arguments which is enough to decide if we have already specialization and which are the exact ones (we load all decls with the same hash to avoid potential collisions) to deserialize.

While we make finding a template specialization more memory-efficient we are far from being done. There are still a few places which trigger eager deserialization of template specializations: the places where we require completion of the redeclaration chain.

---

Patch is 26.57 KiB, truncated to 20.00 KiB below, full version: https://github.com/llvm/llvm-project/pull/83108.diff

7 Files Affected:

• (modified) clang/include/clang/AST/DeclTemplate.h (+32-4)
• (modified) clang/lib/AST/DeclTemplate.cpp (+74-22)
• (modified) clang/lib/AST/ODRHash.cpp (+15)
• (modified) clang/lib/Serialization/ASTReader.cpp (+17-8)
• (modified) clang/lib/Serialization/ASTReaderDecl.cpp (+23-13)
• (modified) clang/lib/Serialization/ASTWriter.cpp (+19-2)
• (modified) clang/lib/Serialization/ASTWriterDecl.cpp (+62-12)

diff --git a/clang/include/clang/AST/DeclTemplate.h b/clang/include/clang/AST/DeclTemplate.h
index e3b6a7efb1127a..4ed9b58d4ff609 100644
--- a/clang/include/clang/AST/DeclTemplate.h
+++ b/clang/include/clang/AST/DeclTemplate.h
@@ -256,6 +256,9 @@ class TemplateArgumentList final
   TemplateArgumentList(const TemplateArgumentList &) = delete;
   TemplateArgumentList &operator=(const TemplateArgumentList &) = delete;
 
+  /// Create hash for the given arguments.
+  static unsigned ComputeODRHash(ArrayRef<TemplateArgument> Args);
+
   /// Create a new template argument list that copies the given set of
   /// template arguments.
   static TemplateArgumentList *CreateCopy(ASTContext &Context,
@@ -730,6 +733,26 @@ class RedeclarableTemplateDecl : public TemplateDecl,
   }
 
   void anchor() override;
+  struct LazySpecializationInfo {
+    uint32_t DeclID = ~0U;
+    unsigned ODRHash = ~0U;
+    bool IsPartial = false;
+    LazySpecializationInfo(uint32_t ID, unsigned Hash = ~0U,
+                           bool Partial = false)
+      : DeclID(ID), ODRHash(Hash), IsPartial(Partial) { }
+    LazySpecializationInfo() { }
+    bool operator<(const LazySpecializationInfo &Other) const {
+      return DeclID < Other.DeclID;
+    }
+    bool operator==(const LazySpecializationInfo &Other) const {
+      assert((DeclID != Other.DeclID || ODRHash == Other.ODRHash) &&
+             "Hashes differ!");
+      assert((DeclID != Other.DeclID || IsPartial == Other.IsPartial) &&
+             "Both must be the same kinds!");
+      return DeclID == Other.DeclID;
+    }
+  };
+
 protected:
   template <typename EntryType> struct SpecEntryTraits {
     using DeclType = EntryType;
@@ -770,7 +793,12 @@ class RedeclarableTemplateDecl : public TemplateDecl,
     return SpecIterator<EntryType>(isEnd ? Specs.end() : Specs.begin());
   }
 
-  void loadLazySpecializationsImpl() const;
+  void loadLazySpecializationsImpl(bool OnlyPartial = false) const;
+
+  void loadLazySpecializationsImpl(llvm::ArrayRef<TemplateArgument> Args,
+                                   TemplateParameterList *TPL = nullptr) const;
+
+  Decl *loadLazySpecializationImpl(LazySpecializationInfo &LazySpecInfo) const;
 
   template <class EntryType, typename ...ProfileArguments>
   typename SpecEntryTraits<EntryType>::DeclType*
@@ -797,7 +825,7 @@ class RedeclarableTemplateDecl : public TemplateDecl,
     ///
     /// The first value in the array is the number of specializations/partial
     /// specializations that follow.
-    uint32_t *LazySpecializations = nullptr;
+    LazySpecializationInfo *LazySpecializations = nullptr;
 
     /// The set of "injected" template arguments used within this
     /// template.
@@ -2268,7 +2296,7 @@ class ClassTemplateDecl : public RedeclarableTemplateDecl {
   friend class TemplateDeclInstantiator;
 
   /// Load any lazily-loaded specializations from the external source.
-  void LoadLazySpecializations() const;
+  void LoadLazySpecializations(bool OnlyPartial = false) const;
 
   /// Get the underlying class declarations of the template.
   CXXRecordDecl *getTemplatedDecl() const {
@@ -3039,7 +3067,7 @@ class VarTemplateDecl : public RedeclarableTemplateDecl {
   friend class ASTDeclWriter;
 
   /// Load any lazily-loaded specializations from the external source.
-  void LoadLazySpecializations() const;
+  void LoadLazySpecializations(bool OnlyPartial = false) const;
 
   /// Get the underlying variable declarations of the template.
   VarDecl *getTemplatedDecl() const {
diff --git a/clang/lib/AST/DeclTemplate.cpp b/clang/lib/AST/DeclTemplate.cpp
index 3c217d6a6a5ae3..1babe39ee2a7e5 100644
--- a/clang/lib/AST/DeclTemplate.cpp
+++ b/clang/lib/AST/DeclTemplate.cpp
@@ -20,6 +20,8 @@
 #include "clang/AST/TemplateBase.h"
 #include "clang/AST/TemplateName.h"
 #include "clang/AST/Type.h"
+#include "clang/AST/ODRHash.h"
+#include "clang/AST/ExprCXX.h"
 #include "clang/AST/TypeLoc.h"
 #include "clang/Basic/Builtins.h"
 #include "clang/Basic/LLVM.h"
@@ -331,16 +333,43 @@ RedeclarableTemplateDecl::CommonBase *RedeclarableTemplateDecl::getCommonPtr() c
   return Common;
 }
 
-void RedeclarableTemplateDecl::loadLazySpecializationsImpl() const {
+void RedeclarableTemplateDecl::loadLazySpecializationsImpl(
+                                             bool OnlyPartial/*=false*/) const {
   // Grab the most recent declaration to ensure we've loaded any lazy
   // redeclarations of this template.
   CommonBase *CommonBasePtr = getMostRecentDecl()->getCommonPtr();
-  if (CommonBasePtr->LazySpecializations) {
-    ASTContext &Context = getASTContext();
-    uint32_t *Specs = CommonBasePtr->LazySpecializations;
-    CommonBasePtr->LazySpecializations = nullptr;
-    for (uint32_t I = 0, N = *Specs++; I != N; ++I)
-      (void)Context.getExternalSource()->GetExternalDecl(Specs[I]);
+  if (auto *Specs = CommonBasePtr->LazySpecializations) {
+    if (!OnlyPartial)
+      CommonBasePtr->LazySpecializations = nullptr;
+    for (uint32_t I = 0, N = Specs[0].DeclID; I != N; ++I) {
+      // Skip over already loaded specializations.
+      if (!Specs[I+1].ODRHash)
+        continue;
+      if (!OnlyPartial || Specs[I+1].IsPartial)
+        (void)loadLazySpecializationImpl(Specs[I+1]);
+    }
+  }
+}
+
+Decl *RedeclarableTemplateDecl::loadLazySpecializationImpl(
+                                   LazySpecializationInfo &LazySpecInfo) const {
+  uint32_t ID = LazySpecInfo.DeclID;
+  assert(ID && "Loading already loaded specialization!");
+  // Note that we loaded the specialization.
+  LazySpecInfo.DeclID = LazySpecInfo.ODRHash = LazySpecInfo.IsPartial = 0;
+  return getASTContext().getExternalSource()->GetExternalDecl(ID);
+}
+
+void
+RedeclarableTemplateDecl::loadLazySpecializationsImpl(ArrayRef<TemplateArgument>
+                                                      Args,
+                                                      TemplateParameterList *TPL) const {
+  CommonBase *CommonBasePtr = getMostRecentDecl()->getCommonPtr();
+  if (auto *Specs = CommonBasePtr->LazySpecializations) {
+    unsigned Hash = TemplateArgumentList::ComputeODRHash(Args);
+    for (uint32_t I = 0, N = Specs[0].DeclID; I != N; ++I)
+      if (Specs[I+1].ODRHash && Specs[I+1].ODRHash == Hash)
+        (void)loadLazySpecializationImpl(Specs[I+1]);
   }
 }
 
@@ -351,6 +380,8 @@ RedeclarableTemplateDecl::findSpecializationImpl(
     ProfileArguments&&... ProfileArgs) {
   using SETraits = SpecEntryTraits<EntryType>;
 
+  loadLazySpecializationsImpl(std::forward<ProfileArguments>(ProfileArgs)...);
+
   llvm::FoldingSetNodeID ID;
   EntryType::Profile(ID, std::forward<ProfileArguments>(ProfileArgs)...,
                      getASTContext());
@@ -366,10 +397,14 @@ void RedeclarableTemplateDecl::addSpecializationImpl(
 
   if (InsertPos) {
 #ifndef NDEBUG
+    auto Args = SETraits::getTemplateArgs(Entry);
+    // Due to hash collisions, it can happen that we load another template
+    // specialization with the same hash. This is fine, as long as the next
+    // call to findSpecializationImpl does not find a matching Decl for the
+    // template arguments.
+    loadLazySpecializationsImpl(Args);
     void *CorrectInsertPos;
-    assert(!findSpecializationImpl(Specializations,
-                                   CorrectInsertPos,
-                                   SETraits::getTemplateArgs(Entry)) &&
+    assert(!findSpecializationImpl(Specializations, CorrectInsertPos, Args) &&
            InsertPos == CorrectInsertPos &&
            "given incorrect InsertPos for specialization");
 #endif
@@ -443,12 +478,14 @@ FunctionTemplateDecl::getSpecializations() const {
 FunctionDecl *
 FunctionTemplateDecl::findSpecialization(ArrayRef<TemplateArgument> Args,
                                          void *&InsertPos) {
-  return findSpecializationImpl(getSpecializations(), InsertPos, Args);
+  auto *Common = getCommonPtr();
+  return findSpecializationImpl(Common->Specializations, InsertPos, Args);
 }
 
 void FunctionTemplateDecl::addSpecialization(
       FunctionTemplateSpecializationInfo *Info, void *InsertPos) {
-  addSpecializationImpl<FunctionTemplateDecl>(getSpecializations(), Info,
+  auto *Common = getCommonPtr();
+  addSpecializationImpl<FunctionTemplateDecl>(Common->Specializations, Info,
                                               InsertPos);
 }
 
@@ -508,8 +545,9 @@ ClassTemplateDecl *ClassTemplateDecl::CreateDeserialized(ASTContext &C,
                                        DeclarationName(), nullptr, nullptr);
 }
 
-void ClassTemplateDecl::LoadLazySpecializations() const {
-  loadLazySpecializationsImpl();
+void ClassTemplateDecl::LoadLazySpecializations(
+                                             bool OnlyPartial/*=false*/) const {
+  loadLazySpecializationsImpl(OnlyPartial);
 }
 
 llvm::FoldingSetVector<ClassTemplateSpecializationDecl> &
@@ -520,7 +558,7 @@ ClassTemplateDecl::getSpecializations() const {
 
 llvm::FoldingSetVector<ClassTemplatePartialSpecializationDecl> &
 ClassTemplateDecl::getPartialSpecializations() const {
-  LoadLazySpecializations();
+  LoadLazySpecializations(/*PartialOnly = */ true);
   return getCommonPtr()->PartialSpecializations;
 }
 
@@ -534,12 +572,15 @@ ClassTemplateDecl::newCommon(ASTContext &C) const {
 ClassTemplateSpecializationDecl *
 ClassTemplateDecl::findSpecialization(ArrayRef<TemplateArgument> Args,
                                       void *&InsertPos) {
-  return findSpecializationImpl(getSpecializations(), InsertPos, Args);
+  auto *Common = getCommonPtr();
+  return findSpecializationImpl(Common->Specializations, InsertPos, Args);
 }
 
 void ClassTemplateDecl::AddSpecialization(ClassTemplateSpecializationDecl *D,
                                           void *InsertPos) {
-  addSpecializationImpl<ClassTemplateDecl>(getSpecializations(), D, InsertPos);
+  auto *Common = getCommonPtr();
+  addSpecializationImpl<ClassTemplateDecl>(Common->Specializations, D,
+                                           InsertPos);
 }
 
 ClassTemplatePartialSpecializationDecl *
@@ -883,6 +924,14 @@ TemplateArgumentList::CreateCopy(ASTContext &Context,
   return new (Mem) TemplateArgumentList(Args);
 }
 
+unsigned TemplateArgumentList::ComputeODRHash(ArrayRef<TemplateArgument> Args) {
+  ODRHash Hasher;
+  for (TemplateArgument TA : Args)
+    Hasher.AddTemplateArgument(TA);
+
+  return Hasher.CalculateHash();
+}
+
 FunctionTemplateSpecializationInfo *FunctionTemplateSpecializationInfo::Create(
     ASTContext &C, FunctionDecl *FD, FunctionTemplateDecl *Template,
     TemplateSpecializationKind TSK, const TemplateArgumentList *TemplateArgs,
@@ -1225,8 +1274,9 @@ VarTemplateDecl *VarTemplateDecl::CreateDeserialized(ASTContext &C,
                                      DeclarationName(), nullptr, nullptr);
 }
 
-void VarTemplateDecl::LoadLazySpecializations() const {
-  loadLazySpecializationsImpl();
+void VarTemplateDecl::LoadLazySpecializations(
+                                             bool OnlyPartial/*=false*/) const {
+  loadLazySpecializationsImpl(OnlyPartial);
 }
 
 llvm::FoldingSetVector<VarTemplateSpecializationDecl> &
@@ -1237,7 +1287,7 @@ VarTemplateDecl::getSpecializations() const {
 
 llvm::FoldingSetVector<VarTemplatePartialSpecializationDecl> &
 VarTemplateDecl::getPartialSpecializations() const {
-  LoadLazySpecializations();
+  LoadLazySpecializations(/*PartialOnly = */ true);
   return getCommonPtr()->PartialSpecializations;
 }
 
@@ -1251,12 +1301,14 @@ VarTemplateDecl::newCommon(ASTContext &C) const {
 VarTemplateSpecializationDecl *
 VarTemplateDecl::findSpecialization(ArrayRef<TemplateArgument> Args,
                                     void *&InsertPos) {
-  return findSpecializationImpl(getSpecializations(), InsertPos, Args);
+  auto *Common = getCommonPtr();
+  return findSpecializationImpl(Common->Specializations, InsertPos, Args);
 }
 
 void VarTemplateDecl::AddSpecialization(VarTemplateSpecializationDecl *D,
                                         void *InsertPos) {
-  addSpecializationImpl<VarTemplateDecl>(getSpecializations(), D, InsertPos);
+  auto *Common = getCommonPtr();
+  addSpecializationImpl<VarTemplateDecl>(Common->Specializations, D, InsertPos);
 }
 
 VarTemplatePartialSpecializationDecl *
diff --git a/clang/lib/AST/ODRHash.cpp b/clang/lib/AST/ODRHash.cpp
index 2dbc259138a897..ffc48ee7cb95ac 100644
--- a/clang/lib/AST/ODRHash.cpp
+++ b/clang/lib/AST/ODRHash.cpp
@@ -812,6 +812,21 @@ void ODRHash::AddDecl(const Decl *D) {
     for (const TemplateArgument &TA : List.asArray())
       AddTemplateArgument(TA);
   }
+
+  // If this was a specialization we should take into account its template
+  // arguments. This helps to reduce collisions coming when visiting template
+  // specialization types (eg. when processing type template arguments).
+  ArrayRef<TemplateArgument> Args;
+  if (auto *CTSD = dyn_cast<ClassTemplateSpecializationDecl>(D))
+    Args = CTSD->getTemplateArgs().asArray();
+  else if (auto *VTSD = dyn_cast<VarTemplateSpecializationDecl>(D))
+    Args = VTSD->getTemplateArgs().asArray();
+  else if (auto *FD = dyn_cast<FunctionDecl>(D))
+    if (FD->getTemplateSpecializationArgs())
+      Args = FD->getTemplateSpecializationArgs()->asArray();
+
+  for (auto &TA : Args)
+    AddTemplateArgument(TA);
 }
 
 namespace {
diff --git a/clang/lib/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
index 683a076e6bc399..3a4fcc8c118fee 100644
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/clang/lib/Serialization/ASTReader.cpp
@@ -7549,14 +7549,23 @@ void ASTReader::CompleteRedeclChain(const Decl *D) {
     }
   }
 
-  if (auto *CTSD = dyn_cast<ClassTemplateSpecializationDecl>(D))
-    CTSD->getSpecializedTemplate()->LoadLazySpecializations();
-  if (auto *VTSD = dyn_cast<VarTemplateSpecializationDecl>(D))
-    VTSD->getSpecializedTemplate()->LoadLazySpecializations();
-  if (auto *FD = dyn_cast<FunctionDecl>(D)) {
-    if (auto *Template = FD->getPrimaryTemplate())
-      Template->LoadLazySpecializations();
-  }
+  RedeclarableTemplateDecl *Template = nullptr;
+  ArrayRef<TemplateArgument> Args;
+  if (auto *CTSD = dyn_cast<ClassTemplateSpecializationDecl>(D)) {
+    Template = CTSD->getSpecializedTemplate();
+    Args = CTSD->getTemplateArgs().asArray();
+  } else if (auto *VTSD = dyn_cast<VarTemplateSpecializationDecl>(D)) {
+    Template = VTSD->getSpecializedTemplate();
+    Args = VTSD->getTemplateArgs().asArray();
+  } else if (auto *FD = dyn_cast<FunctionDecl>(D)) {
+    if (auto *Tmplt = FD->getPrimaryTemplate()) {
+      Template = Tmplt;
+      Args = FD->getTemplateSpecializationArgs()->asArray();
+    }
+  }
+
+  if (Template)
+    Template->loadLazySpecializationsImpl(Args);
 }
 
 CXXCtorInitializer **
diff --git a/clang/lib/Serialization/ASTReaderDecl.cpp b/clang/lib/Serialization/ASTReaderDecl.cpp
index ffba04f28782ea..07d4fe81957d7c 100644
--- a/clang/lib/Serialization/ASTReaderDecl.cpp
+++ b/clang/lib/Serialization/ASTReaderDecl.cpp
@@ -88,6 +88,8 @@ namespace clang {
     const SourceLocation ThisDeclLoc;
 
     using RecordData = ASTReader::RecordData;
+    using LazySpecializationInfo
+      = RedeclarableTemplateDecl::LazySpecializationInfo;
 
     TypeID DeferredTypeID = 0;
     unsigned AnonymousDeclNumber = 0;
@@ -134,9 +136,16 @@ namespace clang {
       return Record.readString();
     }
 
-    void readDeclIDList(SmallVectorImpl<DeclID> &IDs) {
+    LazySpecializationInfo ReadLazySpecializationInfo() {
+      DeclID ID = readDeclID();
+      unsigned Hash = Record.readInt();
+      bool IsPartial = Record.readInt();
+      return LazySpecializationInfo(ID, Hash, IsPartial);
+    }
+
+    void readDeclIDList(SmallVectorImpl<LazySpecializationInfo> &IDs) {
       for (unsigned I = 0, Size = Record.readInt(); I != Size; ++I)
-        IDs.push_back(readDeclID());
+        IDs.push_back(ReadLazySpecializationInfo());
     }
 
     Decl *readDecl() {
@@ -267,7 +276,7 @@ namespace clang {
 
     template <typename T> static
     void AddLazySpecializations(T *D,
-                                SmallVectorImpl<serialization::DeclID>& IDs) {
+                                SmallVectorImpl<LazySpecializationInfo>& IDs) {
       if (IDs.empty())
         return;
 
@@ -277,12 +286,11 @@ namespace clang {
       auto *&LazySpecializations = D->getCommonPtr()->LazySpecializations;
 
       if (auto &Old = LazySpecializations) {
-        IDs.insert(IDs.end(), Old + 1, Old + 1 + Old[0]);
+        IDs.insert(IDs.end(), Old + 1, Old + 1 + Old[0].DeclID);
         llvm::sort(IDs);
         IDs.erase(std::unique(IDs.begin(), IDs.end()), IDs.end());
       }
-
-      auto *Result = new (C) serialization::DeclID[1 + IDs.size()];
+      auto *Result = new (C) LazySpecializationInfo[1 + IDs.size()];
       *Result = IDs.size();
       std::copy(IDs.begin(), IDs.end(), Result + 1);
 
@@ -320,7 +328,7 @@ namespace clang {
     void ReadFunctionDefinition(FunctionDecl *FD);
     void Visit(Decl *D);
 
-    void UpdateDecl(Decl *D, SmallVectorImpl<serialization::DeclID> &);
+    void UpdateDecl(Decl *D, llvm::SmallVectorImpl<LazySpecializationInfo>&);
 
     static void setNextObjCCategory(ObjCCategoryDecl *Cat,
                                     ObjCCategoryDecl *Next) {
@@ -2450,7 +2458,7 @@ void ASTDeclReader::VisitClassTemplateDecl(ClassTemplateDecl *D) {
   if (ThisDeclID == Redecl.getFirstID()) {
     // This ClassTemplateDecl owns a CommonPtr; read it to keep track of all of
     // the specializations.
-    SmallVector<serialization::DeclID, 32> SpecIDs;
+    SmallVector<LazySpecializationInfo, 32> SpecIDs;
     readDeclIDList(SpecIDs);
     ASTDeclReader::AddLazySpecializations(D, SpecIDs);
   }
@@ -2478,7 +2486,7 @@ void ASTDeclReader::VisitVarTemplateDecl(VarTemplateDecl *D) {
   if (ThisDeclID == Redecl.getFirstID()) {
     // This VarTemplateDecl owns a CommonPtr; read it to keep track of all of
     // the specializations.
-    SmallVector<serialization::DeclID, 32> SpecIDs;
+    SmallVector<LazySpecializationInfo, 32> SpecIDs;
     readDeclIDList(SpecIDs);
     ASTDeclReader::AddLazySpecializations(D, SpecIDs);
   }
@@ -2580,7 +2588,7 @@ void ASTDeclReader::VisitFunctionTemplateDecl(FunctionTemplateDecl *D) {
 
   if (ThisDeclID == Redecl.getFirstID()) {
     // This FunctionTemplateDecl owns a CommonPtr; read it.
-    SmallVector<serialization::DeclID, 32> SpecIDs;
+    SmallVector<LazySpecializationInfo, 32> SpecIDs;
     readDeclIDList(SpecIDs);
     ASTDeclReader::AddLazySpecializations(D, SpecIDs);
   }
@@ -4191,7 +4199,9 @@ void ASTReader::loadDeclUpdateRecords(PendingUpdateRecord &Record) {
   ProcessingUpdatesRAIIObj ProcessingUpdates(*this);
   DeclUpdateOffsetsMap::iterator UpdI = DeclUpdateOffsets.find(ID);
 
-  SmallVector<serialization::DeclID, 8> PendingLazySpecializationIDs;
+  using LazySpecializationInfo
+    = RedeclarableTemplateDecl::LazySpecializationInfo;
+  llvm::SmallVector<LazySpecializationInfo, 8> PendingLazySpecializationIDs;
 
   if (UpdI != DeclUpdateOffsets.end()) {
     auto UpdateOffsets = std::move(UpdI->second);
@@ -4461,7 +4471,7 @@ static void forAllLaterRedecls(DeclT *D, Fn F) {
 }
 
 void ASTDeclReader::UpdateDecl(Decl *D,
-   llvm::SmallVectorImpl<serialization::DeclID> &PendingLazySpecializationIDs) {
+        SmallVectorImpl<LazySpecializationInfo> &PendingLazySpecializationIDs) {
   while (Record.getIdx() < Record.size()) {
     switch ((DeclUpdateKind)Record.readInt()) {
     case UPD_CXX_ADDED_IMPLICIT_MEMBER: {
@@ -4474,7 +4484,7 @@ void ASTDeclReader::UpdateDecl(Decl *D,
 
     case UPD_CXX_ADDED_TEMPLATE_SPECIALIZATION:
       // It will be added to the template's lazy specialization set.
-      PendingLazySpecializationIDs.push_back(readDeclID());
+      PendingLazySpecializationIDs.push_back(ReadLazySpecializationInfo());
       break;
 
     case UPD_CXX_ADDED_ANONYMOUS_NAMESPACE: {
diff --git a/clang/lib/Serialization/ASTWriter.cpp b/clang/lib/Serialization/ASTWriter.cpp
index 740bec586a5e33..6cb6d3d8fae4d8 100644
--- a/clang/lib/Serialization/ASTWriter.cpp
+++ b/clang/lib/Serialization/ASTWriter.cpp
@@ -5329,12 +5329,29 @@ void ASTWriter::WriteDeclUpdatesBlocks(RecordDataImpl &OffsetsRecord) {
 
       switch (Kind) {
       case UPD_CXX_ADDED_IMPLICIT_MEMBER:
-      case UPD_CXX_ADDED_TEMPLATE_SPECIALIZATION:
       case UPD_CXX_ADDED_ANONYMOUS_NAMESPACE:
         assert(Update.getDecl() && "no decl to add?");
         Record.push_back(GetDeclRef(Update.getDecl()));
         break;
-
+  ...
[truncated]

[ChuanqiXu9]

Personally I feel this patch is good and the testing result from our workload shows it is good too. But it looks like the performance testing results from google @zygoloid @ilya-biryukov was not good. So maybe we need to wait that for landing this. (It will be great if @ilya-biryukov would like to test again)

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[vgvassilev]

Personally I feel this patch is good and the testing result from our workload shows it is good too. But it looks like the performance testing results from google @zygoloid @ilya-biryukov is not good. So maybe we need to wait for landing this. (It will be great if @ilya-biryukov would like to test again)

Thanks for putting this up. I was planning to do so, however, locally I got:

  Clang :: CXX/module/basic/basic.link/p2.cppm
  Clang :: Driver/clang_f_opts.c
  Clang :: Driver/darwin-header-search-libcxx.cpp
  Clang :: Index/crash-recovery-modules.m
  Clang :: Modules/ExtDebugInfo.cpp
  Clang :: Modules/cxx-templates.cpp
  Clang :: Modules/odr_hash.cpp
  Clang :: Modules/using-directive-redecl.cpp
  Clang :: Modules/using-directive.cpp
  Clang :: PCH/chain-late-anonymous-namespace.cpp
  Clang :: PCH/cxx-namespaces.cpp
  Clang :: PCH/namespaces.cpp

I believe some tests require just adjusting the reference files, but others may need some debugging.

[ChuanqiXu9]

Weird. I only see two failures in my local environment:

Failed Tests (2):
  Clang :: Modules/cxx-templates.cpp
  Clang :: Modules/odr_hash.cpp

And I saw both of them in my patch. It is simply order mismatches.

[vgvassilev]

Weird. I only see two failures in my local environment:

Failed Tests (2):
  Clang :: Modules/cxx-templates.cpp
  Clang :: Modules/odr_hash.cpp

And I saw both of them in my patch. It is simply order mismatches.

Ha, ok. I know that my system constantly have some tests that fail even fro the master but I thought these were real. If we don't see them in the bots then that's probably a problem only on my side...

[ChuanqiXu9]

Rebased with main.