[Clang][AST] Move NamespaceDecl bits to DeclContext (#98567)

Currently, `NamespaceDecl` has a member `AnonOrFirstNamespaceAndFlags`
which stores a few pieces of data:
- a bit indicating whether the namespace was declared `inline`, and
- a bit indicating whether the namespace was declared as a
_nested-namespace-definition_, and
- a pointer a `NamespaceDecl` that either stores:
- a pointer to the first declaration of that namespace if the
declaration is no the first declaration, or
- a pointer to the unnamed namespace that inhabits the namespace
otherwise.

`Redeclarable` already stores a pointer to the first declaration of an
entity, so it's unnecessary to store this in `NamespaceDecl`.
`DeclContext` has 8 bytes in which various bitfields can be stored for a
declaration, so it's not necessary to store these in `NamespaceDecl`
either. We only need to store a pointer to the unnamed namespace that
inhabits the first declaration of a namespace. This patch moves the two
bits currently stored in `NamespaceDecl` to `DeclContext`, and only
stores a pointer to the unnamed namespace that inhabits a namespace in
the first declaration of that namespace. Since `getOriginalNamespace`
always returns the same `NamespaceDecl` as `getFirstDecl`, this function
is removed to avoid confusion.

14 files changed, 59 insertions, 111 deletions

diff --git a/clang-tools-extra/clang-tidy/bugprone/ForwardDeclarationNamespaceCheck.cpp b/clang-tools-extra/clang-tidy/bugprone/ForwardDeclarationNamespaceCheck.cpp
index f578f7e..0b38b18 100644
--- a/clang-tools-extra/clang-tidy/bugprone/ForwardDeclarationNamespaceCheck.cpp
+++ b/clang-tools-extra/clang-tidy/bugprone/ForwardDeclarationNamespaceCheck.cpp
@@ -95,7 +95,7 @@ static bool haveSameNamespaceOrTranslationUnit(const CXXRecordDecl *Decl1,
          "ParentDecl2 declaration must be a namespace");
   auto *Ns1 = NamespaceDecl::castFromDeclContext(ParentDecl1);
   auto *Ns2 = NamespaceDecl::castFromDeclContext(ParentDecl2);
-  return Ns1->getOriginalNamespace() == Ns2->getOriginalNamespace();
+  return Ns1->getFirstDecl() == Ns2->getFirstDecl();
 }
 
 static std::string getNameOfNamespace(const CXXRecordDecl *Decl) {
diff --git a/clang/include/clang/AST/Decl.h b/clang/include/clang/AST/Decl.h
index 5957f14..561a9d8 100644
--- a/clang/include/clang/AST/Decl.h
+++ b/clang/include/clang/AST/Decl.h
@@ -542,12 +542,9 @@ public:
 };
 
 /// Represent a C++ namespace.
-class NamespaceDecl : public NamedDecl, public DeclContext,
-                      public Redeclarable<NamespaceDecl>
-{
-
-  enum Flags : unsigned { F_Inline = 1 << 0, F_Nested = 1 << 1 };
-
+class NamespaceDecl : public NamedDecl,
+                      public DeclContext,
+                      public Redeclarable<NamespaceDecl> {
   /// The starting location of the source range, pointing
   /// to either the namespace or the inline keyword.
   SourceLocation LocStart;
@@ -555,12 +552,8 @@ class NamespaceDecl : public NamedDecl, public DeclContext,
   /// The ending location of the source range.
   SourceLocation RBraceLoc;
 
-  /// A pointer to either the anonymous namespace that lives just inside
-  /// this namespace or to the first namespace in the chain (the latter case
-  /// only when this is not the first in the chain), along with a
-  /// boolean value indicating whether this is an inline namespace.
-  llvm::PointerIntPair<NamespaceDecl *, 2, unsigned>
-      AnonOrFirstNamespaceAndFlags;
+  /// The unnamed namespace that inhabits this namespace, if any.
+  NamespaceDecl *AnonymousNamespace = nullptr;
 
   NamespaceDecl(ASTContext &C, DeclContext *DC, bool Inline,
                 SourceLocation StartLoc, SourceLocation IdLoc,
@@ -607,35 +600,19 @@ public:
   }
 
   /// Returns true if this is an inline namespace declaration.
-  bool isInline() const {
-    return AnonOrFirstNamespaceAndFlags.getInt() & F_Inline;
-  }
+  bool isInline() const { return NamespaceDeclBits.IsInline; }
 
   /// Set whether this is an inline namespace declaration.
-  void setInline(bool Inline) {
-    unsigned F = AnonOrFirstNamespaceAndFlags.getInt();
-    if (Inline)
-      AnonOrFirstNamespaceAndFlags.setInt(F | F_Inline);
-    else
-      AnonOrFirstNamespaceAndFlags.setInt(F & ~F_Inline);
-  }
+  void setInline(bool Inline) { NamespaceDeclBits.IsInline = Inline; }
 
   /// Returns true if this is a nested namespace declaration.
   /// \code
   /// namespace outer::nested { }
   /// \endcode
-  bool isNested() const {
-    return AnonOrFirstNamespaceAndFlags.getInt() & F_Nested;
-  }
+  bool isNested() const { return NamespaceDeclBits.IsNested; }
 
   /// Set whether this is a nested namespace declaration.
-  void setNested(bool Nested) {
-    unsigned F = AnonOrFirstNamespaceAndFlags.getInt();
-    if (Nested)
-      AnonOrFirstNamespaceAndFlags.setInt(F | F_Nested);
-    else
-      AnonOrFirstNamespaceAndFlags.setInt(F & ~F_Nested);
-  }
+  void setNested(bool Nested) { NamespaceDeclBits.IsNested = Nested; }
 
   /// Returns true if the inline qualifier for \c Name is redundant.
   bool isRedundantInlineQualifierFor(DeclarationName Name) const {
@@ -649,34 +626,18 @@ public:
       std::distance(Y.begin(), Y.end());
   }
 
-  /// Get the original (first) namespace declaration.
-  NamespaceDecl *getOriginalNamespace();
-
-  /// Get the original (first) namespace declaration.
-  const NamespaceDecl *getOriginalNamespace() const;
-
-  /// Return true if this declaration is an original (first) declaration
-  /// of the namespace. This is false for non-original (subsequent) namespace
-  /// declarations and anonymous namespaces.
-  bool isOriginalNamespace() const;
-
-  /// Retrieve the anonymous namespace nested inside this namespace,
-  /// if any.
+  /// Retrieve the anonymous namespace that inhabits this namespace, if any.
   NamespaceDecl *getAnonymousNamespace() const {
-    return getOriginalNamespace()->AnonOrFirstNamespaceAndFlags.getPointer();
+    return getFirstDecl()->AnonymousNamespace;
   }
 
   void setAnonymousNamespace(NamespaceDecl *D) {
-    getOriginalNamespace()->AnonOrFirstNamespaceAndFlags.setPointer(D);
+    getFirstDecl()->AnonymousNamespace = D;
   }
 
   /// Retrieves the canonical declaration of this namespace.
-  NamespaceDecl *getCanonicalDecl() override {
-    return getOriginalNamespace();
-  }
-  const NamespaceDecl *getCanonicalDecl() const {
-    return getOriginalNamespace();
-  }
+  NamespaceDecl *getCanonicalDecl() override { return getFirstDecl(); }
+  const NamespaceDecl *getCanonicalDecl() const { return getFirstDecl(); }
 
   SourceRange getSourceRange() const override LLVM_READONLY {
     return SourceRange(LocStart, RBraceLoc);
diff --git a/clang/include/clang/AST/DeclBase.h b/clang/include/clang/AST/DeclBase.h
index 6c711cf..40f01ab 100644
--- a/clang/include/clang/AST/DeclBase.h
+++ b/clang/include/clang/AST/DeclBase.h
@@ -1487,6 +1487,27 @@ class DeclContext {
   /// Number of bits in DeclContextBitfields.
   enum { NumDeclContextBits = 13 };
 
+  /// Stores the bits used by NamespaceDecl.
+  /// If modified NumNamespaceDeclBits and the accessor
+  /// methods in NamespaceDecl should be updated appropriately.
+  class NamespaceDeclBitfields {
+    friend class NamespaceDecl;
+    /// For the bits in DeclContextBitfields
+    LLVM_PREFERRED_TYPE(DeclContextBitfields)
+    uint64_t : NumDeclContextBits;
+
+    /// True if this is an inline namespace.
+    LLVM_PREFERRED_TYPE(bool)
+    uint64_t IsInline : 1;
+
+    /// True if this is a nested-namespace-definition.
+    LLVM_PREFERRED_TYPE(bool)
+    uint64_t IsNested : 1;
+  };
+
+  /// Number of inherited and non-inherited bits in NamespaceDeclBitfields.
+  enum { NumNamespaceDeclBits = NumDeclContextBits + 2 };
+
   /// Stores the bits used by TagDecl.
   /// If modified NumTagDeclBits and the accessor
   /// methods in TagDecl should be updated appropriately.
@@ -1985,6 +2006,7 @@ protected:
   /// 8 bytes with static_asserts in the ctor of DeclContext.
   union {
     DeclContextBitfields DeclContextBits;
+    NamespaceDeclBitfields NamespaceDeclBits;
     TagDeclBitfields TagDeclBits;
     EnumDeclBitfields EnumDeclBits;
     RecordDeclBitfields RecordDeclBits;
@@ -1998,6 +2020,8 @@ protected:
 
     static_assert(sizeof(DeclContextBitfields) <= 8,
                   "DeclContextBitfields is larger than 8 bytes!");
+    static_assert(sizeof(NamespaceDeclBitfields) <= 8,
+                  "NamespaceDeclBitfields is larger than 8 bytes!");
     static_assert(sizeof(TagDeclBitfields) <= 8,
                   "TagDeclBitfields is larger than 8 bytes!");
     static_assert(sizeof(EnumDeclBitfields) <= 8,
diff --git a/clang/lib/AST/ASTContext.cpp b/clang/lib/AST/ASTContext.cpp
index 497579d..6c89e38 100644
--- a/clang/lib/AST/ASTContext.cpp
+++ b/clang/lib/AST/ASTContext.cpp
@@ -7250,14 +7250,14 @@ ASTContext::getCanonicalNestedNameSpecifier(NestedNameSpecifier *NNS) const {
     // A namespace is canonical; build a nested-name-specifier with
     // this namespace and no prefix.
     return NestedNameSpecifier::Create(*this, nullptr,
-                                 NNS->getAsNamespace()->getOriginalNamespace());
+                                       NNS->getAsNamespace()->getFirstDecl());
 
   case NestedNameSpecifier::NamespaceAlias:
     // A namespace is canonical; build a nested-name-specifier with
     // this namespace and no prefix.
-    return NestedNameSpecifier::Create(*this, nullptr,
-                                    NNS->getAsNamespaceAlias()->getNamespace()
-                                                      ->getOriginalNamespace());
+    return NestedNameSpecifier::Create(
+        *this, nullptr,
+        NNS->getAsNamespaceAlias()->getNamespace()->getFirstDecl());
 
   // The difference between TypeSpec and TypeSpecWithTemplate is that the
   // latter will have the 'template' keyword when printed.
diff --git a/clang/lib/AST/DeclBase.cpp b/clang/lib/AST/DeclBase.cpp
index ef2c57e..bc5a920 100644
--- a/clang/lib/AST/DeclBase.cpp
+++ b/clang/lib/AST/DeclBase.cpp
@@ -1422,8 +1422,7 @@ DeclContext *DeclContext::getPrimaryContext() {
   case Decl::TranslationUnit:
     return static_cast<TranslationUnitDecl *>(this)->getFirstDecl();
   case Decl::Namespace:
-    // The original namespace is our primary context.
-    return static_cast<NamespaceDecl *>(this)->getOriginalNamespace();
+    return static_cast<NamespaceDecl *>(this)->getFirstDecl();
 
   case Decl::ObjCMethod:
     return this;
diff --git a/clang/lib/AST/DeclCXX.cpp b/clang/lib/AST/DeclCXX.cpp
index d5c140f..72d68f3 100644
--- a/clang/lib/AST/DeclCXX.cpp
+++ b/clang/lib/AST/DeclCXX.cpp
@@ -2941,7 +2941,7 @@ UsingDirectiveDecl *UsingDirectiveDecl::Create(ASTContext &C, DeclContext *DC,
                                                NamedDecl *Used,
                                                DeclContext *CommonAncestor) {
   if (auto *NS = dyn_cast_or_null<NamespaceDecl>(Used))
-    Used = NS->getOriginalNamespace();
+    Used = NS->getFirstDecl();
   return new (C, DC) UsingDirectiveDecl(DC, L, NamespaceLoc, QualifierLoc,
                                         IdentLoc, Used, CommonAncestor);
 }
@@ -2966,16 +2966,9 @@ NamespaceDecl::NamespaceDecl(ASTContext &C, DeclContext *DC, bool Inline,
                              bool Nested)
     : NamedDecl(Namespace, DC, IdLoc, Id), DeclContext(Namespace),
       redeclarable_base(C), LocStart(StartLoc) {
-  unsigned Flags = 0;
-  if (Inline)
-    Flags |= F_Inline;
-  if (Nested)
-    Flags |= F_Nested;
-  AnonOrFirstNamespaceAndFlags = {nullptr, Flags};
+  setInline(Inline);
+  setNested(Nested);
   setPreviousDecl(PrevDecl);
-
-  if (PrevDecl)
-    AnonOrFirstNamespaceAndFlags.setPointer(PrevDecl->getOriginalNamespace());
 }
 
 NamespaceDecl *NamespaceDecl::Create(ASTContext &C, DeclContext *DC,
@@ -2992,22 +2985,6 @@ NamespaceDecl *NamespaceDecl::CreateDeserialized(ASTContext &C,
                                    SourceLocation(), nullptr, nullptr, false);
 }
 
-NamespaceDecl *NamespaceDecl::getOriginalNamespace() {
-  if (isFirstDecl())
-    return this;
-
-  return AnonOrFirstNamespaceAndFlags.getPointer();
-}
-
-const NamespaceDecl *NamespaceDecl::getOriginalNamespace() const {
-  if (isFirstDecl())
-    return this;
-
-  return AnonOrFirstNamespaceAndFlags.getPointer();
-}
-
-bool NamespaceDecl::isOriginalNamespace() const { return isFirstDecl(); }
-
 NamespaceDecl *NamespaceDecl::getNextRedeclarationImpl() {
   return getNextRedeclaration();
 }
@@ -3043,7 +3020,7 @@ NamespaceAliasDecl *NamespaceAliasDecl::Create(ASTContext &C, DeclContext *DC,
                                                NamedDecl *Namespace) {
   // FIXME: Preserve the aliased namespace as written.
   if (auto *NS = dyn_cast_or_null<NamespaceDecl>(Namespace))
-    Namespace = NS->getOriginalNamespace();
+    Namespace = NS->getFirstDecl();
   return new (C, DC) NamespaceAliasDecl(C, DC, UsingLoc, AliasLoc, Alias,
                                         QualifierLoc, IdentLoc, Namespace);
 }
diff --git a/clang/lib/AST/ItaniumMangle.cpp b/clang/lib/AST/ItaniumMangle.cpp
index 5444dcf..40ef827 100644
--- a/clang/lib/AST/ItaniumMangle.cpp
+++ b/clang/lib/AST/ItaniumMangle.cpp
@@ -961,7 +961,7 @@ bool CXXNameMangler::isStd(const NamespaceDecl *NS) {
   if (!Context.getEffectiveParentContext(NS)->isTranslationUnit())
     return false;
 
-  const IdentifierInfo *II = NS->getOriginalNamespace()->getIdentifier();
+  const IdentifierInfo *II = NS->getFirstDecl()->getIdentifier();
   return II && II->isStr("std");
 }
 
diff --git a/clang/lib/AST/JSONNodeDumper.cpp b/clang/lib/AST/JSONNodeDumper.cpp
index 339477d..eeb314b 100644
--- a/clang/lib/AST/JSONNodeDumper.cpp
+++ b/clang/lib/AST/JSONNodeDumper.cpp
@@ -883,9 +883,8 @@ void JSONNodeDumper::VisitNamespaceDecl(const NamespaceDecl *ND) {
   VisitNamedDecl(ND);
   attributeOnlyIfTrue("isInline", ND->isInline());
   attributeOnlyIfTrue("isNested", ND->isNested());
-  if (!ND->isOriginalNamespace())
-    JOS.attribute("originalNamespace",
-                  createBareDeclRef(ND->getOriginalNamespace()));
+  if (!ND->isFirstDecl())
+    JOS.attribute("originalNamespace", createBareDeclRef(ND->getFirstDecl()));
 }
 
 void JSONNodeDumper::VisitUsingDirectiveDecl(const UsingDirectiveDecl *UDD) {
diff --git a/clang/lib/AST/TextNodeDumper.cpp b/clang/lib/AST/TextNodeDumper.cpp
index a26f50f..5ba9523 100644
--- a/clang/lib/AST/TextNodeDumper.cpp
+++ b/clang/lib/AST/TextNodeDumper.cpp
@@ -2386,8 +2386,8 @@ void TextNodeDumper::VisitNamespaceDecl(const NamespaceDecl *D) {
     OS << " inline";
   if (D->isNested())
     OS << " nested";
-  if (!D->isOriginalNamespace())
-    dumpDeclRef(D->getOriginalNamespace(), "original");
+  if (!D->isFirstDecl())
+    dumpDeclRef(D->getFirstDecl(), "original");
 }
 
 void TextNodeDumper::VisitUsingDirectiveDecl(const UsingDirectiveDecl *D) {
diff --git a/clang/lib/Sema/SemaCodeComplete.cpp b/clang/lib/Sema/SemaCodeComplete.cpp
index 8fea7b0..88d4732 100644
--- a/clang/lib/Sema/SemaCodeComplete.cpp
+++ b/clang/lib/Sema/SemaCodeComplete.cpp
@@ -6864,7 +6864,7 @@ void SemaCodeCompletion::CodeCompleteNamespaceDecl(Scope *S) {
              NS(Ctx->decls_begin()),
          NSEnd(Ctx->decls_end());
          NS != NSEnd; ++NS)
-      OrigToLatest[NS->getOriginalNamespace()] = *NS;
+      OrigToLatest[NS->getFirstDecl()] = *NS;
 
     // Add the most recent definition (or extended definition) of each
     // namespace to the list of results.
diff --git a/clang/lib/Sema/SemaLookup.cpp b/clang/lib/Sema/SemaLookup.cpp
index 7851c5d..7a6a645 100644
--- a/clang/lib/Sema/SemaLookup.cpp
+++ b/clang/lib/Sema/SemaLookup.cpp
@@ -2325,7 +2325,7 @@ static bool LookupQualifiedNameInUsingDirectives(Sema &S, LookupResult &R,
   // We have already looked into the initial namespace; seed the queue
   // with its using-children.
   for (auto *I : StartDC->using_directives()) {
-    NamespaceDecl *ND = I->getNominatedNamespace()->getOriginalNamespace();
+    NamespaceDecl *ND = I->getNominatedNamespace()->getFirstDecl();
     if (S.isVisible(I) && Visited.insert(ND).second)
       Queue.push_back(ND);
   }
diff --git a/clang/lib/Serialization/ASTReaderDecl.cpp b/clang/lib/Serialization/ASTReaderDecl.cpp
index cbaf1b0..76032aa 100644
--- a/clang/lib/Serialization/ASTReaderDecl.cpp
+++ b/clang/lib/Serialization/ASTReaderDecl.cpp
@@ -1847,13 +1847,8 @@ void ASTDeclReader::VisitNamespaceDecl(NamespaceDecl *D) {
   // same namespace, and we have an invariant that older declarations
   // get merged before newer ones try to merge.
   GlobalDeclID AnonNamespace;
-  if (Redecl.getFirstID() == ThisDeclID) {
+  if (Redecl.getFirstID() == ThisDeclID)
     AnonNamespace = readDeclID();
-  } else {
-    // Link this namespace back to the first declaration, which has already
-    // been deserialized.
-    D->AnonOrFirstNamespaceAndFlags.setPointer(D->getFirstDecl());
-  }
 
   mergeRedeclarable(D, Redecl);
 
@@ -2974,13 +2969,6 @@ void ASTDeclReader::mergeRedeclarable(Redeclarable<T> *DBase, T *Existing,
     ExistingCanon->Used |= D->Used;
     D->Used = false;
 
-    // When we merge a namespace, update its pointer to the first namespace.
-    // We cannot have loaded any redeclarations of this declaration yet, so
-    // there's nothing else that needs to be updated.
-    if (auto *Namespace = dyn_cast<NamespaceDecl>(D))
-      Namespace->AnonOrFirstNamespaceAndFlags.setPointer(
-          assert_cast<NamespaceDecl *>(ExistingCanon));
-
     // When we merge a template, merge its pattern.
     if (auto *DTemplate = dyn_cast<RedeclarableTemplateDecl>(D))
       mergeTemplatePattern(
@@ -3293,7 +3281,7 @@ ASTDeclReader::getOrFakePrimaryClassDefinition(ASTReader &Reader,
 DeclContext *ASTDeclReader::getPrimaryContextForMerging(ASTReader &Reader,
                                                         DeclContext *DC) {
   if (auto *ND = dyn_cast<NamespaceDecl>(DC))
-    return ND->getOriginalNamespace();
+    return ND->getFirstDecl();
 
   if (auto *RD = dyn_cast<CXXRecordDecl>(DC))
     return getOrFakePrimaryClassDefinition(Reader, RD);
diff --git a/clang/lib/Serialization/ASTWriterDecl.cpp b/clang/lib/Serialization/ASTWriterDecl.cpp
index b6583c5..5dff0ce 100644
--- a/clang/lib/Serialization/ASTWriterDecl.cpp
+++ b/clang/lib/Serialization/ASTWriterDecl.cpp
@@ -1383,7 +1383,7 @@ void ASTDeclWriter::VisitNamespaceDecl(NamespaceDecl *D) {
   Record.AddSourceLocation(D->getBeginLoc());
   Record.AddSourceLocation(D->getRBraceLoc());
 
-  if (D->isOriginalNamespace())
+  if (D->isFirstDecl())
     Record.AddDeclRef(D->getAnonymousNamespace());
   Code = serialization::DECL_NAMESPACE;
 
diff --git a/clang/tools/libclang/CXIndexDataConsumer.cpp b/clang/tools/libclang/CXIndexDataConsumer.cpp
index c102226..8d364ed 100644
--- a/clang/tools/libclang/CXIndexDataConsumer.cpp
+++ b/clang/tools/libclang/CXIndexDataConsumer.cpp
@@ -861,7 +861,7 @@ bool CXIndexDataConsumer::handleObjCProperty(const ObjCPropertyDecl *D) {
 }
 
 bool CXIndexDataConsumer::handleNamespace(const NamespaceDecl *D) {
-  DeclInfo DInfo(/*isRedeclaration=*/!D->isOriginalNamespace(),
+  DeclInfo DInfo(/*isRedeclaration=*/!D->isFirstDecl(),
                  /*isDefinition=*/true,
                  /*isContainer=*/true);
   return handleDecl(D, D->getLocation(), getCursor(D), DInfo);