[C++20] [Modules] [Reduced BMI] Remove unreachable decls GMF in redued BMI

clang/include/clang/Serialization/ASTBitCodes.h

@@ -698,6 +698,10 @@ enum ASTRecordTypes {
   /// Record code for an unterminated \#pragma clang assume_nonnull begin
   /// recorded in a preamble.
   PP_ASSUME_NONNULL_LOC = 67,
+
+  /// Record code for lexical and visible block for delayed namespace in
+  /// reduced BMI.
+  DELAYED_NAMESPACE_LEXICAL_VISIBLE_RECORD = 68,
 };
 
 /// Record types used within a source manager block.

clang/include/clang/Serialization/ASTReader.h

@@ -517,6 +517,20 @@ class ASTReader
   /// in the chain.
   DeclUpdateOffsetsMap DeclUpdateOffsets;
 
+  using DelayedNamespaceOffsetMapTy = llvm::DenseMap<
+      serialization::DeclID,
+      std::pair</*LexicalOffset*/ uint64_t, /*VisibleOffset*/ uint64_t>>;
+
+  /// Mapping from global declaration IDs to the lexical and visible block
+  /// offset for delayed namespace in reduced BMI.
+  ///
+  /// We can't use the existing DeclUpdate mechanism since the DeclUpdate
+  /// may only be applied in an outer most read. However, we need to know
+  /// whether or not a DeclContext has external storage during the recursive
+  /// reading. So we need to apply the offset immediately after we read the
+  /// namespace as if it is not delayed.
+  DelayedNamespaceOffsetMapTy DelayedNamespaceOffsetMap;
+
   struct PendingUpdateRecord {
     Decl *D;
     serialization::GlobalDeclID ID;

clang/include/clang/Serialization/ASTWriter.h

@@ -201,6 +201,16 @@ class ASTWriter : public ASTDeserializationListener,
   /// The declarations and types to emit.
   std::queue<DeclOrType> DeclTypesToEmit;
 
+  /// The delayed namespace to emit. Only meaningful for reduced BMI.
+  ///
+  /// In reduced BMI, we want to elide the unreachable declarations in
+  /// the global module fragment. However, in ASTWriterDecl, when we see
+  /// a namespace, all the declarations in the namespace would be emitted.
+  /// So the optimization become meaningless. To solve the issue, we
+  /// delay recording all the declarations until we emit all the declarations.
+  /// Then we can safely record the reached declarations only.
+  llvm::SmallVector<NamespaceDecl *, 16> DelayedNamespace;
+
   /// The first ID number we can use for our own declarations.
   serialization::DeclID FirstDeclID = serialization::NUM_PREDEF_DECL_IDS;
 
@@ -529,7 +539,8 @@ class ASTWriter : public ASTDeserializationListener,
   void WriteType(QualType T);
 
   bool isLookupResultExternal(StoredDeclsList &Result, DeclContext *DC);
-  bool isLookupResultEntirelyExternal(StoredDeclsList &Result, DeclContext *DC);
+  bool isLookupResultEntirelyExternalOrUnreachable(StoredDeclsList &Result,
+                                                   DeclContext *DC);
 
   void GenerateNameLookupTable(const DeclContext *DC,
                                llvm::SmallVectorImpl<char> &LookupTable);
@@ -704,6 +715,15 @@ class ASTWriter : public ASTDeserializationListener,
   /// declaration.
   serialization::DeclID getDeclID(const Decl *D);
 
+  /// Whether or not the declaration got emitted. If not, it wouldn't be
+  /// emitted.
+  ///
+  /// This may only be called after we've done the job to write the
+  /// declarations (marked by DoneWritingDeclsAndTypes).
+  ///
+  /// A declaration may only be omitted in reduced BMI.
+  bool wasDeclEmitted(const Decl *D) const;
+
   unsigned getAnonymousDeclarationNumber(const NamedDecl *D);
 
   /// Add a string to the given record.
@@ -798,6 +818,10 @@ class ASTWriter : public ASTDeserializationListener,
     return WritingModule && WritingModule->isNamedModule();
   }
 
+  bool isGeneratingReducedBMI() const { return GeneratingReducedBMI; }
+
+  bool getDoneWritingDeclsAndTypes() const { return DoneWritingDeclsAndTypes; }
+
 private:
   // ASTDeserializationListener implementation
   void ReaderInitialized(ASTReader *Reader) override;

clang/lib/Serialization/ASTReader.cpp

@@ -3795,6 +3795,29 @@ llvm::Error ASTReader::ReadASTBlock(ModuleFile &F,
       }
       break;
 
+    case DELAYED_NAMESPACE_LEXICAL_VISIBLE_RECORD: {
+      if (Record.size() % 3 != 0)
+        return llvm::createStringError(
+            std::errc::illegal_byte_sequence,
+            "invalid DELAYED_NAMESPACE_LEXICAL_VISIBLE_RECORD block in AST "
+            "file");
+      for (unsigned I = 0, N = Record.size(); I != N; I += 3) {
+        GlobalDeclID ID = getGlobalDeclID(F, Record[I]);
+
+        uint64_t BaseOffset = F.DeclsBlockStartOffset;
+        assert(BaseOffset && "Invalid DeclsBlockStartOffset for module file!");
+        uint64_t LexicalOffset = Record[I + 1] ? BaseOffset + Record[I + 1] : 0;
+        uint64_t VisibleOffset = Record[I + 2] ? BaseOffset + Record[I + 2] : 0;
+
+        DelayedNamespaceOffsetMap[ID] = {LexicalOffset, VisibleOffset};
+
+        assert(!GetExistingDecl(ID) &&
+               "We shouldn't load the namespace in the front of delayed "
+               "namespace lexical and visible block");
+      }
+      break;
+    }
+
     case OBJC_CATEGORIES_MAP:
       if (F.LocalNumObjCCategoriesInMap != 0)
         return llvm::createStringError(

clang/lib/Serialization/ASTReaderDecl.cpp

@@ -4124,6 +4124,15 @@ Decl *ASTReader::ReadDeclRecord(DeclID ID) {
   // offsets for its tables of lexical and visible declarations.
   if (auto *DC = dyn_cast<DeclContext>(D)) {
     std::pair<uint64_t, uint64_t> Offsets = Reader.VisitDeclContext(DC);
+
+    // Get the lexical and visible block for the delayed namespace.
+    // It is sufficient to judge if ID is in DelayedNamespaceOffsetMap.
+    // But it may be more efficient to filter the other cases.
+    if (!Offsets.first && !Offsets.second && isa<NamespaceDecl>(D))
+      if (auto Iter = DelayedNamespaceOffsetMap.find(ID);
+          Iter != DelayedNamespaceOffsetMap.end())
+        Offsets = Iter->second;
+
     if (Offsets.first &&
         ReadLexicalDeclContextStorage(*Loc.F, DeclsCursor, Offsets.first, DC))
       return nullptr;

clang/lib/Serialization/ASTWriter.cpp

@@ -870,6 +870,7 @@ void ASTWriter::WriteBlockInfoBlock() {
   RECORD(WEAK_UNDECLARED_IDENTIFIERS);
   RECORD(PENDING_IMPLICIT_INSTANTIATIONS);
   RECORD(UPDATE_VISIBLE);
+  RECORD(DELAYED_NAMESPACE_LEXICAL_VISIBLE_RECORD);
   RECORD(DECL_UPDATE_OFFSETS);
   RECORD(DECL_UPDATES);
   RECORD(CUDA_SPECIAL_DECL_REFS);
@@ -3029,10 +3030,12 @@ void ASTWriter::WriteSubmodules(Module *WritingModule) {
       Stream.EmitRecordWithBlob(ConfigMacroAbbrev, Record, CM);
     }
 
-    // Emit the initializers, if any.
+    // Emit the reachable initializers.
+    // The initializer may only be unreachable in reduced BMI.
     RecordData Inits;
     for (Decl *D : Context->getModuleInitializers(Mod))
-      Inits.push_back(GetDeclRef(D));
+      if (wasDeclEmitted(D))
+        Inits.push_back(GetDeclRef(D));
     if (!Inits.empty())
       Stream.EmitRecord(SUBMODULE_INITIALIZERS, Inits);
 
@@ -3211,6 +3214,9 @@ uint64_t ASTWriter::WriteDeclContextLexicalBlock(ASTContext &Context,
   uint64_t Offset = Stream.GetCurrentBitNo();
   SmallVector<uint32_t, 128> KindDeclPairs;
   for (const auto *D : DC->decls()) {
+    if (DoneWritingDeclsAndTypes && !wasDeclEmitted(D))
+      continue;
+
     KindDeclPairs.push_back(D->getKind());
     KindDeclPairs.push_back(GetDeclRef(D));
   }
@@ -3865,8 +3871,14 @@ class ASTDeclContextNameLookupTrait {
   data_type getData(const Coll &Decls) {
     unsigned Start = DeclIDs.size();
     for (NamedDecl *D : Decls) {
-      DeclIDs.push_back(
-          Writer.GetDeclRef(getDeclForLocalLookup(Writer.getLangOpts(), D)));
+      NamedDecl *DeclForLocalLookup =
+          getDeclForLocalLookup(Writer.getLangOpts(), D);
+
+      if (Writer.getDoneWritingDeclsAndTypes() &&
+          !Writer.wasDeclEmitted(DeclForLocalLookup))
+        continue;
+
+      DeclIDs.push_back(Writer.GetDeclRef(DeclForLocalLookup));
     }
     return std::make_pair(Start, DeclIDs.size());
   }
@@ -3975,11 +3987,20 @@ bool ASTWriter::isLookupResultExternal(StoredDeclsList &Result,
          DC->hasNeedToReconcileExternalVisibleStorage();
 }
 
-bool ASTWriter::isLookupResultEntirelyExternal(StoredDeclsList &Result,
-                                               DeclContext *DC) {
-  for (auto *D : Result.getLookupResult())
-    if (!getDeclForLocalLookup(getLangOpts(), D)->isFromASTFile())
-      return false;
+bool ASTWriter::isLookupResultEntirelyExternalOrUnreachable(
+    StoredDeclsList &Result, DeclContext *DC) {
+  for (auto *D : Result.getLookupResult()) {
+    auto *LocalD = getDeclForLocalLookup(getLangOpts(), D);
+    if (LocalD->isFromASTFile())
+      continue;
+
+    // We can only be sure whether the local declaration is reachable
+    // after we done writing the declarations and types.
+    if (DoneWritingDeclsAndTypes && !wasDeclEmitted(LocalD))
+      continue;
+
+    return false;
+  }
 
   return true;
 }
@@ -4017,8 +4038,17 @@ ASTWriter::GenerateNameLookupTable(const DeclContext *ConstDC,
     // don't need to write an entry for the name at all. If we can't
     // write out a lookup set without performing more deserialization,
     // just skip this entry.
-    if (isLookupResultExternal(Result, DC) &&
-        isLookupResultEntirelyExternal(Result, DC))
+    //
+    // Also in reduced BMI, we'd like to avoid writing unreachable
+    // declarations in GMF, so we need to avoid writing declarations
+    // that entirely external or unreachable.
+    //
+    // FIMXE: It looks sufficient to test
+    // isLookupResultEntirelyExternalOrUnreachable here. But due to bug we have
+    // to test isLookupResultExternal here. See
+    // https://github.com/llvm/llvm-project/issues/61065 for details.
+    if ((GeneratingReducedBMI || isLookupResultExternal(Result, DC)) &&
+        isLookupResultEntirelyExternalOrUnreachable(Result, DC))
       continue;
 
     // We also skip empty results. If any of the results could be external and
@@ -4209,9 +4239,15 @@ uint64_t ASTWriter::WriteDeclContextVisibleBlock(ASTContext &Context,
         continue;
       }
 
-      for (NamedDecl *ND : Result)
-        if (!ND->isFromASTFile())
-          GetDeclRef(ND);
+      for (NamedDecl *ND : Result) {
+        if (ND->isFromASTFile())
+          continue;
+
+        if (DoneWritingDeclsAndTypes && !wasDeclEmitted(ND))
+          continue;
+
+        GetDeclRef(ND);
+      }
     }
 
     return 0;
@@ -4979,9 +5015,18 @@ ASTFileSignature ASTWriter::WriteASTCore(Sema &SemaRef, StringRef isysroot,
   const TranslationUnitDecl *TU = Context.getTranslationUnitDecl();
 
   // Force all top level declarations to be emitted.
-  for (const auto *D : TU->noload_decls())
-    if (!D->isFromASTFile())
-      GetDeclRef(D);
+  //
+  // We start emitting top level declarations from the module purview to
+  // implement the eliding unreachable declaration feature.
+  for (const auto *D : TU->noload_decls()) {
+    if (D->isFromASTFile())
+      continue;
+
+    if (GeneratingReducedBMI && D->isFromExplicitGlobalModule())
+      continue;
+
+    GetDeclRef(D);
+  }
 
   // If the translation unit has an anonymous namespace, and we don't already
   // have an update block for it, write it as an update block.
@@ -5291,24 +5336,59 @@ void ASTWriter::WriteDeclAndTypes(ASTContext &Context) {
         WriteDecl(Context, DOT.getDecl());
     }
   } while (!DeclUpdates.empty());
-  Stream.ExitBlock();
 
   DoneWritingDeclsAndTypes = true;
 
+  // DelayedNamespace is only meaningful in reduced BMI.
+  // See the comments of DelayedNamespace for details.
+  assert(DelayedNamespace.empty() || GeneratingReducedBMI);
+  RecordData DelayedNamespaceRecord;
+  for (NamespaceDecl *NS : DelayedNamespace) {
+    uint64_t LexicalOffset = WriteDeclContextLexicalBlock(Context, NS);
+    uint64_t VisibleOffset = WriteDeclContextVisibleBlock(Context, NS);
+
+    // Write the offset relative to current block.
+    if (LexicalOffset)
+      LexicalOffset -= DeclTypesBlockStartOffset;
+
+    if (VisibleOffset)
+      VisibleOffset -= DeclTypesBlockStartOffset;
+
+    DelayedNamespaceRecord.push_back(getDeclID(NS));
+    DelayedNamespaceRecord.push_back(LexicalOffset);
+    DelayedNamespaceRecord.push_back(VisibleOffset);
+  }
+
+  // The process of writing lexical and visible block for delayed namespace
+  // shouldn't introduce any new decls, types or update to emit.
+  assert(DeclTypesToEmit.empty());
+  assert(DeclUpdates.empty());
+
+  Stream.ExitBlock();
+
   // These things can only be done once we've written out decls and types.
   WriteTypeDeclOffsets();
   if (!DeclUpdatesOffsetsRecord.empty())
     Stream.EmitRecord(DECL_UPDATE_OFFSETS, DeclUpdatesOffsetsRecord);
 
+  if (!DelayedNamespaceRecord.empty())
+    Stream.EmitRecord(DELAYED_NAMESPACE_LEXICAL_VISIBLE_RECORD,
+                      DelayedNamespaceRecord);
+
   const TranslationUnitDecl *TU = Context.getTranslationUnitDecl();
   // Create a lexical update block containing all of the declarations in the
   // translation unit that do not come from other AST files.
   SmallVector<uint32_t, 128> NewGlobalKindDeclPairs;
   for (const auto *D : TU->noload_decls()) {
-    if (!D->isFromASTFile()) {
-      NewGlobalKindDeclPairs.push_back(D->getKind());
-      NewGlobalKindDeclPairs.push_back(GetDeclRef(D));
-    }
+    if (D->isFromASTFile())
+      continue;
+
+    // In reduced BMI, skip unreached declarations.
+    if (!wasDeclEmitted(D))
+      continue;
+
+    NewGlobalKindDeclPairs.push_back(D->getKind());
+    NewGlobalKindDeclPairs.push_back(GetDeclRef(D));
   }
 
   auto Abv = std::make_shared<llvm::BitCodeAbbrev>();
@@ -5817,6 +5897,21 @@ DeclID ASTWriter::getDeclID(const Decl *D) {
   return DeclIDs[D];
 }
 
+bool ASTWriter::wasDeclEmitted(const Decl *D) const {
+  assert(D);
+
+  assert(DoneWritingDeclsAndTypes &&
+         "wasDeclEmitted should only be called after writing declarations");
+
+  if (D->isFromASTFile())
+    return true;
+
+  bool Emitted = DeclIDs.contains(D);
+  assert((Emitted || GeneratingReducedBMI) &&
+         "The declaration can only be omitted in reduced BMI.");
+  return Emitted;
+}
+
 void ASTWriter::associateDeclWithFile(const Decl *D, DeclID ID) {
   assert(ID);
   assert(D);

clang/lib/Serialization/ASTWriterDecl.cpp

@@ -1719,6 +1719,15 @@ void ASTDeclWriter::VisitClassTemplateDecl(ClassTemplateDecl *D) {
 
   if (D->isFirstDecl())
     AddTemplateSpecializations(D);
+
+  // Force emitting the corresponding deduction guide in reduced BMI mode.
+  // Otherwise, the deduction guide may be optimized out incorrectly.
+  if (Writer.isGeneratingReducedBMI()) {
+    auto Name = Context.DeclarationNames.getCXXDeductionGuideName(D);
+    for (auto *DG : D->getDeclContext()->noload_lookup(Name))
+      Writer.GetDeclRef(DG);
+  }
+
   Code = serialization::DECL_CLASS_TEMPLATE;
 }
 
@@ -1962,8 +1971,22 @@ void ASTDeclWriter::VisitDeclContext(DeclContext *DC) {
                 "You need to update the serializer after you change the "
                 "DeclContextBits");
 
-  Record.AddOffset(Writer.WriteDeclContextLexicalBlock(Context, DC));
-  Record.AddOffset(Writer.WriteDeclContextVisibleBlock(Context, DC));
+  uint64_t LexicalOffset = 0;
+  uint64_t VisibleOffset = 0;
+
+  if (Writer.isGeneratingReducedBMI() && isa<NamespaceDecl>(DC) &&
+      cast<NamespaceDecl>(DC)->isFromExplicitGlobalModule()) {
+    // In reduced BMI, delay writing lexical and visible block for namespace
+    // in the global module fragment. See the comments of DelayedNamespace for
+    // details.
+    Writer.DelayedNamespace.push_back(cast<NamespaceDecl>(DC));
+  } else {
+    LexicalOffset = Writer.WriteDeclContextLexicalBlock(Context, DC);
+    VisibleOffset = Writer.WriteDeclContextVisibleBlock(Context, DC);
+  }
+
+  Record.AddOffset(LexicalOffset);
+  Record.AddOffset(VisibleOffset);
 }
 
 const Decl *ASTWriter::getFirstLocalDecl(const Decl *D) {