[serialization] no transitive decl changeupstream/users/ChuanqiXu9/NoTransitiveDeclChange

12 files changed, 283 insertions, 147 deletions

diff --git a/clang/include/clang/AST/DeclBase.h b/clang/include/clang/AST/DeclBase.h
index e43e812cd945..4bdf27aa9940 100644
--- a/clang/include/clang/AST/DeclBase.h
+++ b/clang/include/clang/AST/DeclBase.h
@@ -701,10 +701,7 @@ public:
 
   /// Set the owning module ID.  This may only be called for
   /// deserialized Decls.
-  void setOwningModuleID(unsigned ID) {
-    assert(isFromASTFile() && "Only works on a deserialized declaration");
-    *((unsigned*)this - 2) = ID;
-  }
+  void setOwningModuleID(unsigned ID);
 
 public:
   /// Determine the availability of the given declaration.
@@ -777,19 +774,11 @@ public:
 
   /// Retrieve the global declaration ID associated with this
   /// declaration, which specifies where this Decl was loaded from.
-  GlobalDeclID getGlobalID() const {
-    if (isFromASTFile())
-      return (*((const GlobalDeclID *)this - 1));
-    return GlobalDeclID();
-  }
+  GlobalDeclID getGlobalID() const;
 
   /// Retrieve the global ID of the module that owns this particular
   /// declaration.
-  unsigned getOwningModuleID() const {
-    if (isFromASTFile())
-      return *((const unsigned*)this - 2);
-    return 0;
-  }
+  unsigned getOwningModuleID() const;
 
 private:
   Module *getOwningModuleSlow() const;
diff --git a/clang/include/clang/AST/DeclID.h b/clang/include/clang/AST/DeclID.h
index 614ba06b6386..32d2ed41a374 100644
--- a/clang/include/clang/AST/DeclID.h
+++ b/clang/include/clang/AST/DeclID.h
@@ -19,6 +19,8 @@
 #include "llvm/ADT/DenseMapInfo.h"
 #include "llvm/ADT/iterator.h"
 
+#include <climits>
+
 namespace clang {
 
 /// Predefined declaration IDs.
@@ -107,12 +109,16 @@ public:
   ///
   /// DeclID should only be used directly in serialization. All other users
   /// should use LocalDeclID or GlobalDeclID.
-  using DeclID = uint32_t;
+  using DeclID = uint64_t;
 
 protected:
   DeclIDBase() : ID(PREDEF_DECL_NULL_ID) {}
   explicit DeclIDBase(DeclID ID) : ID(ID) {}
 
+  explicit DeclIDBase(unsigned LocalID, unsigned ModuleFileIndex) {
+    ID = (DeclID)LocalID | ((DeclID)ModuleFileIndex << 32);
+  }
+
 public:
   DeclID get() const { return ID; }
 
@@ -124,6 +130,10 @@ public:
 
   bool isInvalid() const { return ID == PREDEF_DECL_NULL_ID; }
 
+  unsigned getModuleFileIndex() const { return ID >> 32; }
+
+  unsigned getLocalDeclIndex() const;
+
   friend bool operator==(const DeclIDBase &LHS, const DeclIDBase &RHS) {
     return LHS.ID == RHS.ID;
   }
@@ -156,6 +166,9 @@ public:
   LocalDeclID(PredefinedDeclIDs ID) : Base(ID) {}
   explicit LocalDeclID(DeclID ID) : Base(ID) {}
 
+  explicit LocalDeclID(unsigned LocalID, unsigned ModuleFileIndex)
+      : Base(LocalID, ModuleFileIndex) {}
+
   LocalDeclID &operator++() {
     ++ID;
     return *this;
@@ -175,6 +188,9 @@ public:
   GlobalDeclID() : Base() {}
   explicit GlobalDeclID(DeclID ID) : Base(ID) {}
 
+  explicit GlobalDeclID(unsigned LocalID, unsigned ModuleFileIndex)
+      : Base(LocalID, ModuleFileIndex) {}
+
   // For DeclIDIterator<GlobalDeclID> to be able to convert a GlobalDeclID
   // to a LocalDeclID.
   explicit operator LocalDeclID() const { return LocalDeclID(this->ID); }
diff --git a/clang/include/clang/Serialization/ASTBitCodes.h b/clang/include/clang/Serialization/ASTBitCodes.h
index fe1bd47348be..9e4b21baa7d2 100644
--- a/clang/include/clang/Serialization/ASTBitCodes.h
+++ b/clang/include/clang/Serialization/ASTBitCodes.h
@@ -255,6 +255,12 @@ public:
   }
 };
 
+// The unaligned decl ID used in the Blobs of bistreams.
+using unaligned_decl_id_t =
+    llvm::support::detail::packed_endian_specific_integral<
+        serialization::DeclID, llvm::endianness::native,
+        llvm::support::unaligned>;
+
 /// The number of predefined preprocessed entity IDs.
 const unsigned int NUM_PREDEF_PP_ENTITY_IDS = 1;
 
diff --git a/clang/include/clang/Serialization/ASTReader.h b/clang/include/clang/Serialization/ASTReader.h
index 4ece4593f073..a2e094354d96 100644
--- a/clang/include/clang/Serialization/ASTReader.h
+++ b/clang/include/clang/Serialization/ASTReader.h
@@ -501,12 +501,6 @@ private:
   /// = I + 1 has already been loaded.
   llvm::PagedVector<Decl *> DeclsLoaded;
 
-  using GlobalDeclMapType = ContinuousRangeMap<GlobalDeclID, ModuleFile *, 4>;
-
-  /// Mapping from global declaration IDs to the module in which the
-  /// declaration resides.
-  GlobalDeclMapType GlobalDeclMap;
-
   using FileOffset = std::pair<ModuleFile *, uint64_t>;
   using FileOffsetsTy = SmallVector<FileOffset, 2>;
   using DeclUpdateOffsetsMap = llvm::DenseMap<GlobalDeclID, FileOffsetsTy>;
@@ -589,10 +583,11 @@ private:
 
   struct FileDeclsInfo {
     ModuleFile *Mod = nullptr;
-    ArrayRef<LocalDeclID> Decls;
+    ArrayRef<serialization::unaligned_decl_id_t> Decls;
 
     FileDeclsInfo() = default;
-    FileDeclsInfo(ModuleFile *Mod, ArrayRef<LocalDeclID> Decls)
+    FileDeclsInfo(ModuleFile *Mod,
+                  ArrayRef<serialization::unaligned_decl_id_t> Decls)
         : Mod(Mod), Decls(Decls) {}
   };
 
@@ -601,11 +596,7 @@ private:
 
   /// An array of lexical contents of a declaration context, as a sequence of
   /// Decl::Kind, DeclID pairs.
-  using unaligned_decl_id_t =
-      llvm::support::detail::packed_endian_specific_integral<
-          serialization::DeclID, llvm::endianness::native,
-          llvm::support::unaligned>;
-  using LexicalContents = ArrayRef<unaligned_decl_id_t>;
+  using LexicalContents = ArrayRef<serialization::unaligned_decl_id_t>;
 
   /// Map from a DeclContext to its lexical contents.
   llvm::DenseMap<const DeclContext*, std::pair<ModuleFile*, LexicalContents>>
@@ -1486,10 +1477,11 @@ private:
                                unsigned ClientLoadCapabilities);
 
 public:
-  class ModuleDeclIterator : public llvm::iterator_adaptor_base<
-                                 ModuleDeclIterator, const LocalDeclID *,
-                                 std::random_access_iterator_tag, const Decl *,
-                                 ptrdiff_t, const Decl *, const Decl *> {
+  class ModuleDeclIterator
+      : public llvm::iterator_adaptor_base<
+            ModuleDeclIterator, const serialization::unaligned_decl_id_t *,
+            std::random_access_iterator_tag, const Decl *, ptrdiff_t,
+            const Decl *, const Decl *> {
     ASTReader *Reader = nullptr;
     ModuleFile *Mod = nullptr;
 
@@ -1497,11 +1489,11 @@ public:
     ModuleDeclIterator() : iterator_adaptor_base(nullptr) {}
 
     ModuleDeclIterator(ASTReader *Reader, ModuleFile *Mod,
-                       const LocalDeclID *Pos)
+                       const serialization::unaligned_decl_id_t *Pos)
         : iterator_adaptor_base(Pos), Reader(Reader), Mod(Mod) {}
 
     value_type operator*() const {
-      return Reader->GetDecl(Reader->getGlobalDeclID(*Mod, *I));
+      return Reader->GetDecl(Reader->getGlobalDeclID(*Mod, (LocalDeclID)*I));
     }
 
     value_type operator->() const { return **this; }
@@ -1541,6 +1533,9 @@ private:
              StringRef Arg2 = StringRef(), StringRef Arg3 = StringRef()) const;
   void Error(llvm::Error &&Err) const;
 
+  /// Translate a \param GlobalDeclID to the index of DeclsLoaded array.
+  unsigned translateGlobalDeclIDToIndex(GlobalDeclID ID) const;
+
 public:
   /// Load the AST file and validate its contents against the given
   /// Preprocessor.
@@ -1912,7 +1907,8 @@ public:
 
   /// Retrieve the module file that owns the given declaration, or NULL
   /// if the declaration is not from a module file.
-  ModuleFile *getOwningModuleFile(const Decl *D);
+  ModuleFile *getOwningModuleFile(const Decl *D) const;
+  ModuleFile *getOwningModuleFile(GlobalDeclID ID) const;
 
   /// Returns the source location for the decl \p ID.
   SourceLocation getSourceLocationForDeclID(GlobalDeclID ID);
diff --git a/clang/include/clang/Serialization/ModuleFile.h b/clang/include/clang/Serialization/ModuleFile.h
index 992d26a8b88c..56193d44dd6f 100644
--- a/clang/include/clang/Serialization/ModuleFile.h
+++ b/clang/include/clang/Serialization/ModuleFile.h
@@ -454,23 +454,11 @@ public:
   /// by the declaration ID (-1).
   const DeclOffset *DeclOffsets = nullptr;
 
-  /// Base declaration ID for declarations local to this module.
-  serialization::DeclID BaseDeclID = 0;
-
-  /// Remapping table for declaration IDs in this module.
-  ContinuousRangeMap<serialization::DeclID, int, 2> DeclRemap;
-
-  /// Mapping from the module files that this module file depends on
-  /// to the base declaration ID for that module as it is understood within this
-  /// module.
-  ///
-  /// This is effectively a reverse global-to-local mapping for declaration
-  /// IDs, so that we can interpret a true global ID (for this translation unit)
-  /// as a local ID (for this module file).
-  llvm::DenseMap<ModuleFile *, serialization::DeclID> GlobalToLocalDeclIDs;
+  /// Base declaration index in ASTReader for declarations local to this module.
+  unsigned BaseDeclIndex = 0;
 
   /// Array of file-level DeclIDs sorted by file.
-  const LocalDeclID *FileSortedDecls = nullptr;
+  const serialization::unaligned_decl_id_t *FileSortedDecls = nullptr;
   unsigned NumFileSortedDecls = 0;
 
   /// Array of category list location information within this
diff --git a/clang/include/clang/Serialization/ModuleManager.h b/clang/include/clang/Serialization/ModuleManager.h
index 3bd379acf7ed..097a1c16a057 100644
--- a/clang/include/clang/Serialization/ModuleManager.h
+++ b/clang/include/clang/Serialization/ModuleManager.h
@@ -46,7 +46,7 @@ namespace serialization {
 /// Manages the set of modules loaded by an AST reader.
 class ModuleManager {
   /// The chain of AST files, in the order in which we started to load
-  /// them (this order isn't really useful for anything).
+  /// them.
   SmallVector<std::unique_ptr<ModuleFile>, 2> Chain;
 
   /// The chain of non-module PCH files. The first entry is the one named
diff --git a/clang/lib/AST/DeclBase.cpp b/clang/lib/AST/DeclBase.cpp
index 65d5eeb6354e..e74d69e05b28 100644
--- a/clang/lib/AST/DeclBase.cpp
+++ b/clang/lib/AST/DeclBase.cpp
@@ -74,18 +74,17 @@ void *Decl::operator new(std::size_t Size, const ASTContext &Context,
                          GlobalDeclID ID, std::size_t Extra) {
   // Allocate an extra 8 bytes worth of storage, which ensures that the
   // resulting pointer will still be 8-byte aligned.
-  static_assert(sizeof(unsigned) * 2 >= alignof(Decl),
-                "Decl won't be misaligned");
+  static_assert(sizeof(uint64_t) >= alignof(Decl), "Decl won't be misaligned");
   void *Start = Context.Allocate(Size + Extra + 8);
   void *Result = (char*)Start + 8;
 
-  unsigned *PrefixPtr = (unsigned *)Result - 2;
+  uint64_t *PrefixPtr = (uint64_t *)Result - 1;
 
-  // Zero out the first 4 bytes; this is used to store the owning module ID.
-  PrefixPtr[0] = 0;
+  *PrefixPtr = ID.get();
 
-  // Store the global declaration ID in the second 4 bytes.
-  PrefixPtr[1] = ID.get();
+  // We leave the upper 16 bits to store the module IDs. 48 bits should be
+  // sufficient to store a declaration ID.
+  assert(*PrefixPtr < llvm::maskTrailingOnes<uint64_t>(48));
 
   return Result;
 }
@@ -111,6 +110,29 @@ void *Decl::operator new(std::size_t Size, const ASTContext &Ctx,
   return ::operator new(Size + Extra, Ctx);
 }
 
+GlobalDeclID Decl::getGlobalID() const {
+  if (!isFromASTFile())
+    return GlobalDeclID();
+  // See the comments in `Decl::operator new` for details.
+  uint64_t ID = *((const uint64_t *)this - 1);
+  return GlobalDeclID(ID & llvm::maskTrailingOnes<uint64_t>(48));
+}
+
+unsigned Decl::getOwningModuleID() const {
+  if (!isFromASTFile())
+    return 0;
+
+  uint64_t ID = *((const uint64_t *)this - 1);
+  return ID >> 48;
+}
+
+void Decl::setOwningModuleID(unsigned ID) {
+  assert(isFromASTFile() && "Only works on a deserialized declaration");
+  uint64_t *IDAddress = (uint64_t *)this - 1;
+  assert(!((*IDAddress) >> 48) && "We should only set the module ID once");
+  *IDAddress |= (uint64_t)ID << 48;
+}
+
 Module *Decl::getOwningModuleSlow() const {
   assert(isFromASTFile() && "Not from AST file?");
   return getASTContext().getExternalSource()->getModule(getOwningModuleID());
@@ -2163,3 +2185,7 @@ DependentDiagnostic *DependentDiagnostic::Create(ASTContext &C,
 
   return DD;
 }
+
+unsigned DeclIDBase::getLocalDeclIndex() const {
+  return ID & llvm::maskTrailingOnes<DeclID>(32);
+}
diff --git a/clang/lib/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
index 4a6e1d23161b..7f17e09adc29 100644
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/clang/lib/Serialization/ASTReader.cpp
@@ -1656,7 +1656,7 @@ bool ASTReader::ReadSLocEntry(int ID) {
 
     unsigned NumFileDecls = Record[7];
     if (NumFileDecls && ContextObj) {
-      const LocalDeclID *FirstDecl = F->FileSortedDecls + Record[6];
+      const unaligned_decl_id_t *FirstDecl = F->FileSortedDecls + Record[6];
       assert(F->FileSortedDecls && "FILE_SORTED_DECLS not encountered yet ?");
       FileDeclIDs[FID] =
           FileDeclsInfo(F, llvm::ArrayRef(FirstDecl, NumFileDecls));
@@ -3375,26 +3375,11 @@ llvm::Error ASTReader::ReadASTBlock(ModuleFile &F,
             "duplicate DECL_OFFSET record in AST file");
       F.DeclOffsets = (const DeclOffset *)Blob.data();
       F.LocalNumDecls = Record[0];
-      unsigned LocalBaseDeclID = Record[1];
-      F.BaseDeclID = getTotalNumDecls();
-
-      if (F.LocalNumDecls > 0) {
-        // Introduce the global -> local mapping for declarations within this
-        // module.
-        GlobalDeclMap.insert(std::make_pair(
-            GlobalDeclID(getTotalNumDecls() + NUM_PREDEF_DECL_IDS), &F));
-
-        // Introduce the local -> global mapping for declarations within this
-        // module.
-        F.DeclRemap.insertOrReplace(
-          std::make_pair(LocalBaseDeclID, F.BaseDeclID - LocalBaseDeclID));
-
-        // Introduce the global -> local mapping for declarations within this
-        // module.
-        F.GlobalToLocalDeclIDs[&F] = LocalBaseDeclID;
+      F.BaseDeclIndex = getTotalNumDecls();
 
+      if (F.LocalNumDecls > 0)
         DeclsLoaded.resize(DeclsLoaded.size() + F.LocalNumDecls);
-      }
+
       break;
     }
 
@@ -3629,7 +3614,7 @@ llvm::Error ASTReader::ReadASTBlock(ModuleFile &F,
       break;
 
     case FILE_SORTED_DECLS:
-      F.FileSortedDecls = (const LocalDeclID *)Blob.data();
+      F.FileSortedDecls = (const unaligned_decl_id_t *)Blob.data();
       F.NumFileSortedDecls = Record[0];
       break;
 
@@ -4056,7 +4041,6 @@ void ASTReader::ReadModuleOffsetMap(ModuleFile &F) const {
   RemapBuilder PreprocessedEntityRemap(F.PreprocessedEntityRemap);
   RemapBuilder SubmoduleRemap(F.SubmoduleRemap);
   RemapBuilder SelectorRemap(F.SelectorRemap);
-  RemapBuilder DeclRemap(F.DeclRemap);
   RemapBuilder TypeRemap(F.TypeRemap);
 
   auto &ImportedModuleVector = F.TransitiveImports;
@@ -4095,8 +4079,6 @@ void ASTReader::ReadModuleOffsetMap(ModuleFile &F) const {
         endian::readNext<uint32_t, llvm::endianness::little>(Data);
     uint32_t SelectorIDOffset =
         endian::readNext<uint32_t, llvm::endianness::little>(Data);
-    uint32_t DeclIDOffset =
-        endian::readNext<uint32_t, llvm::endianness::little>(Data);
     uint32_t TypeIndexOffset =
         endian::readNext<uint32_t, llvm::endianness::little>(Data);
 
@@ -4114,11 +4096,7 @@ void ASTReader::ReadModuleOffsetMap(ModuleFile &F) const {
               PreprocessedEntityRemap);
     mapOffset(SubmoduleIDOffset, OM->BaseSubmoduleID, SubmoduleRemap);
     mapOffset(SelectorIDOffset, OM->BaseSelectorID, SelectorRemap);
-    mapOffset(DeclIDOffset, OM->BaseDeclID, DeclRemap);
     mapOffset(TypeIndexOffset, OM->BaseTypeIndex, TypeRemap);
-
-    // Global -> local mappings.
-    F.GlobalToLocalDeclIDs[OM] = DeclIDOffset;
   }
 }
 
@@ -7642,18 +7620,25 @@ CXXBaseSpecifier *ASTReader::GetExternalCXXBaseSpecifiers(uint64_t Offset) {
 
 GlobalDeclID ASTReader::getGlobalDeclID(ModuleFile &F,
                                         LocalDeclID LocalID) const {
-  DeclID ID = LocalID.get();
-  if (ID < NUM_PREDEF_DECL_IDS)
-    return GlobalDeclID(ID);
+  if (LocalID.get() < NUM_PREDEF_DECL_IDS)
+    return GlobalDeclID(LocalID.get());
+
+  unsigned OwningModuleFileIndex = LocalID.getModuleFileIndex();
+  DeclID ID = LocalID.getLocalDeclIndex();
 
   if (!F.ModuleOffsetMap.empty())
     ReadModuleOffsetMap(F);
 
-  ContinuousRangeMap<DeclID, int, 2>::iterator I =
-      F.DeclRemap.find(ID - NUM_PREDEF_DECL_IDS);
-  assert(I != F.DeclRemap.end() && "Invalid index into decl index remap");
+  ModuleFile *OwningModuleFile =
+      OwningModuleFileIndex == 0
+          ? &F
+          : F.TransitiveImports[OwningModuleFileIndex - 1];
+
+  if (OwningModuleFileIndex == 0)
+    ID -= NUM_PREDEF_DECL_IDS;
 
-  return GlobalDeclID(ID + I->second);
+  uint64_t NewModuleFileIndex = OwningModuleFile->Index + 1;
+  return GlobalDeclID(ID, NewModuleFileIndex);
 }
 
 bool ASTReader::isDeclIDFromModule(GlobalDeclID ID, ModuleFile &M) const {
@@ -7661,31 +7646,33 @@ bool ASTReader::isDeclIDFromModule(GlobalDeclID ID, ModuleFile &M) const {
   if (ID.get() < NUM_PREDEF_DECL_IDS)
     return false;
 
-  return ID.get() - NUM_PREDEF_DECL_IDS >= M.BaseDeclID &&
-         ID.get() - NUM_PREDEF_DECL_IDS < M.BaseDeclID + M.LocalNumDecls;
+  unsigned ModuleFileIndex = ID.getModuleFileIndex();
+  return M.Index == ModuleFileIndex - 1;
+}
+
+ModuleFile *ASTReader::getOwningModuleFile(GlobalDeclID ID) const {
+  // Predefined decls aren't from any module.
+  if (ID.get() < NUM_PREDEF_DECL_IDS)
+    return nullptr;
+
+  uint64_t ModuleFileIndex = ID.getModuleFileIndex();
+  assert(ModuleFileIndex && "Untranslated Local Decl?");
+
+  return &getModuleManager()[ModuleFileIndex - 1];
 }
 
-ModuleFile *ASTReader::getOwningModuleFile(const Decl *D) {
+ModuleFile *ASTReader::getOwningModuleFile(const Decl *D) const {
   if (!D->isFromASTFile())
     return nullptr;
-  GlobalDeclMapType::const_iterator I =
-      GlobalDeclMap.find(GlobalDeclID(D->getGlobalID()));
-  assert(I != GlobalDeclMap.end() && "Corrupted global declaration map");
-  return I->second;
+
+  return getOwningModuleFile(GlobalDeclID(D->getGlobalID()));
 }
 
 SourceLocation ASTReader::getSourceLocationForDeclID(GlobalDeclID ID) {
   if (ID.get() < NUM_PREDEF_DECL_IDS)
     return SourceLocation();
 
-  unsigned Index = ID.get() - NUM_PREDEF_DECL_IDS;
-
-  if (Index > DeclsLoaded.size()) {
-    Error("declaration ID out-of-range for AST file");
-    return SourceLocation();
-  }
-
-  if (Decl *D = DeclsLoaded[Index])
+  if (Decl *D = GetExistingDecl(ID))
     return D->getLocation();
 
   SourceLocation Loc;
@@ -7752,8 +7739,19 @@ static Decl *getPredefinedDecl(ASTContext &Context, PredefinedDeclIDs ID) {
   llvm_unreachable("PredefinedDeclIDs unknown enum value");
 }
 
+unsigned ASTReader::translateGlobalDeclIDToIndex(GlobalDeclID GlobalID) const {
+  ModuleFile *OwningModuleFile = getOwningModuleFile(GlobalID);
+  if (!OwningModuleFile) {
+    assert(GlobalID.get() < NUM_PREDEF_DECL_IDS && "Untransalted Global ID?");
+    return GlobalID.get();
+  }
+
+  return OwningModuleFile->BaseDeclIndex + GlobalID.getLocalDeclIndex();
+}
+
 Decl *ASTReader::GetExistingDecl(GlobalDeclID ID) {
   assert(ContextObj && "reading decl with no AST context");
+
   if (ID.get() < NUM_PREDEF_DECL_IDS) {
     Decl *D = getPredefinedDecl(*ContextObj, (PredefinedDeclIDs)ID);
     if (D) {
@@ -7766,7 +7764,7 @@ Decl *ASTReader::GetExistingDecl(GlobalDeclID ID) {
     return D;
   }
 
-  unsigned Index = ID.get() - NUM_PREDEF_DECL_IDS;
+  unsigned Index = translateGlobalDeclIDToIndex(ID);
 
   if (Index >= DeclsLoaded.size()) {
     assert(0 && "declaration ID out-of-range for AST file");
@@ -7781,7 +7779,7 @@ Decl *ASTReader::GetDecl(GlobalDeclID ID) {
   if (ID.get() < NUM_PREDEF_DECL_IDS)
     return GetExistingDecl(ID);
 
-  unsigned Index = ID.get() - NUM_PREDEF_DECL_IDS;
+  unsigned Index = translateGlobalDeclIDToIndex(ID);
 
   if (Index >= DeclsLoaded.size()) {
     assert(0 && "declaration ID out-of-range for AST file");
@@ -7800,20 +7798,31 @@ Decl *ASTReader::GetDecl(GlobalDeclID ID) {
 
 LocalDeclID ASTReader::mapGlobalIDToModuleFileGlobalID(ModuleFile &M,
                                                        GlobalDeclID GlobalID) {
-  DeclID ID = GlobalID.get();
-  if (ID < NUM_PREDEF_DECL_IDS)
+  if (GlobalID.get() < NUM_PREDEF_DECL_IDS)
+    return LocalDeclID(GlobalID.get());
+
+  if (!M.ModuleOffsetMap.empty())
+    ReadModuleOffsetMap(M);
+
+  ModuleFile *Owner = getOwningModuleFile(GlobalID);
+  DeclID ID = GlobalID.getLocalDeclIndex();
+
+  if (Owner == &M) {
+    ID += NUM_PREDEF_DECL_IDS;
     return LocalDeclID(ID);
+  }
 
-  GlobalDeclMapType::const_iterator I = GlobalDeclMap.find(GlobalID);
-  assert(I != GlobalDeclMap.end() && "Corrupted global declaration map");
-  ModuleFile *Owner = I->second;
+  uint64_t OrignalModuleFileIndex = 0;
+  for (unsigned I = 0; I < M.TransitiveImports.size(); I++)
+    if (M.TransitiveImports[I] == Owner) {
+      OrignalModuleFileIndex = I + 1;
+      break;
+    }
 
-  llvm::DenseMap<ModuleFile *, DeclID>::iterator Pos =
-      M.GlobalToLocalDeclIDs.find(Owner);
-  if (Pos == M.GlobalToLocalDeclIDs.end())
+  if (!OrignalModuleFileIndex)
     return LocalDeclID();
 
-  return LocalDeclID(ID - Owner->BaseDeclID + Pos->second);
+  return LocalDeclID(ID, OrignalModuleFileIndex);
 }
 
 GlobalDeclID ASTReader::ReadDeclID(ModuleFile &F, const RecordData &Record,
@@ -7892,32 +7901,34 @@ void ASTReader::FindExternalLexicalDecls(
 
 namespace {
 
-class DeclIDComp {
+class UnalignedDeclIDComp {
   ASTReader &Reader;
   ModuleFile &Mod;
 
 public:
-  DeclIDComp(ASTReader &Reader, ModuleFile &M) : Reader(Reader), Mod(M) {}
+  UnalignedDeclIDComp(ASTReader &Reader, ModuleFile &M)
+      : Reader(Reader), Mod(M) {}
 
-  bool operator()(LocalDeclID L, LocalDeclID R) const {
+  bool operator()(unaligned_decl_id_t L, unaligned_decl_id_t R) const {
     SourceLocation LHS = getLocation(L);
     SourceLocation RHS = getLocation(R);
     return Reader.getSourceManager().isBeforeInTranslationUnit(LHS, RHS);
   }
 
-  bool operator()(SourceLocation LHS, LocalDeclID R) const {
+  bool operator()(SourceLocation LHS, unaligned_decl_id_t R) const {
     SourceLocation RHS = getLocation(R);
     return Reader.getSourceManager().isBeforeInTranslationUnit(LHS, RHS);
   }
 
-  bool operator()(LocalDeclID L, SourceLocation RHS) const {
+  bool operator()(unaligned_decl_id_t L, SourceLocation RHS) const {
     SourceLocation LHS = getLocation(L);
     return Reader.getSourceManager().isBeforeInTranslationUnit(LHS, RHS);
   }
 
-  SourceLocation getLocation(LocalDeclID ID) const {
+  SourceLocation getLocation(unaligned_decl_id_t ID) const {
     return Reader.getSourceManager().getFileLoc(
-            Reader.getSourceLocationForDeclID(Reader.getGlobalDeclID(Mod, ID)));
+        Reader.getSourceLocationForDeclID(
+            Reader.getGlobalDeclID(Mod, (LocalDeclID)ID)));
   }
 };
 
@@ -7940,8 +7951,8 @@ void ASTReader::FindFileRegionDecls(FileID File,
     BeginLoc = SM.getLocForStartOfFile(File).getLocWithOffset(Offset);
   SourceLocation EndLoc = BeginLoc.getLocWithOffset(Length);
 
-  DeclIDComp DIDComp(*this, *DInfo.Mod);
-  ArrayRef<LocalDeclID>::iterator BeginIt =
+  UnalignedDeclIDComp DIDComp(*this, *DInfo.Mod);
+  ArrayRef<unaligned_decl_id_t>::iterator BeginIt =
       llvm::lower_bound(DInfo.Decls, BeginLoc, DIDComp);
   if (BeginIt != DInfo.Decls.begin())
     --BeginIt;
@@ -7950,17 +7961,18 @@ void ASTReader::FindFileRegionDecls(FileID File,
   // to backtrack until we find it otherwise we will fail to report that the
   // region overlaps with an objc container.
   while (BeginIt != DInfo.Decls.begin() &&
-         GetDecl(getGlobalDeclID(*DInfo.Mod, *BeginIt))
+         GetDecl(getGlobalDeclID(*DInfo.Mod, (LocalDeclID)(*BeginIt)))
              ->isTopLevelDeclInObjCContainer())
     --BeginIt;
 
-  ArrayRef<LocalDeclID>::iterator EndIt =
+  ArrayRef<unaligned_decl_id_t>::iterator EndIt =
       llvm::upper_bound(DInfo.Decls, EndLoc, DIDComp);
   if (EndIt != DInfo.Decls.end())
     ++EndIt;
 
-  for (ArrayRef<LocalDeclID>::iterator DIt = BeginIt; DIt != EndIt; ++DIt)
-    Decls.push_back(GetDecl(getGlobalDeclID(*DInfo.Mod, *DIt)));
+  for (ArrayRef<unaligned_decl_id_t>::iterator DIt = BeginIt; DIt != EndIt;
+       ++DIt)
+    Decls.push_back(GetDecl(getGlobalDeclID(*DInfo.Mod, (LocalDeclID)(*DIt))));
 }
 
 bool
@@ -8167,7 +8179,6 @@ LLVM_DUMP_METHOD void ASTReader::dump() {
   dumpModuleIDMap("Global bit offset map", GlobalBitOffsetsMap);
   dumpModuleIDMap("Global source location entry map", GlobalSLocEntryMap);
   dumpModuleIDMap("Global type map", GlobalTypeMap);
-  dumpModuleIDMap("Global declaration map", GlobalDeclMap);
   dumpModuleIDMap("Global identifier map", GlobalIdentifierMap);
   dumpModuleIDMap("Global macro map", GlobalMacroMap);
   dumpModuleIDMap("Global submodule map", GlobalSubmoduleMap);
diff --git a/clang/lib/Serialization/ASTReaderDecl.cpp b/clang/lib/Serialization/ASTReaderDecl.cpp
index 61cc99d4df68..519c7d5c20fd 100644
--- a/clang/lib/Serialization/ASTReaderDecl.cpp
+++ b/clang/lib/Serialization/ASTReaderDecl.cpp
@@ -3244,11 +3244,10 @@ bool ASTReader::isConsumerInterestedIn(Decl *D) {
 /// Get the correct cursor and offset for loading a declaration.
 ASTReader::RecordLocation ASTReader::DeclCursorForID(GlobalDeclID ID,
                                                      SourceLocation &Loc) {
-  GlobalDeclMapType::iterator I = GlobalDeclMap.find(ID);
-  assert(I != GlobalDeclMap.end() && "Corrupted global declaration map");
-  ModuleFile *M = I->second;
-  const DeclOffset &DOffs =
-      M->DeclOffsets[ID.get() - M->BaseDeclID - NUM_PREDEF_DECL_IDS];
+  ModuleFile *M = getOwningModuleFile(ID);
+  assert(M);
+  unsigned LocalDeclIndex = ID.getLocalDeclIndex();
+  const DeclOffset &DOffs = M->DeclOffsets[LocalDeclIndex];
   Loc = ReadSourceLocation(*M, DOffs.getRawLoc());
   return RecordLocation(M, DOffs.getBitOffset(M->DeclsBlockStartOffset));
 }
@@ -3791,7 +3790,6 @@ void ASTReader::markIncompleteDeclChain(Decl *D) {
 
 /// Read the declaration at the given offset from the AST file.
 Decl *ASTReader::ReadDeclRecord(GlobalDeclID ID) {
-  unsigned Index = ID.get() - NUM_PREDEF_DECL_IDS;
   SourceLocation DeclLoc;
   RecordLocation Loc = DeclCursorForID(ID, DeclLoc);
   llvm::BitstreamCursor &DeclsCursor = Loc.F->DeclsCursor;
@@ -4122,7 +4120,7 @@ Decl *ASTReader::ReadDeclRecord(GlobalDeclID ID) {
   }
 
   assert(D && "Unknown declaration reading AST file");
-  LoadedDecl(Index, D);
+  LoadedDecl(translateGlobalDeclIDToIndex(ID), D);
   // Set the DeclContext before doing any deserialization, to make sure internal
   // calls to Decl::getASTContext() by Decl's methods will find the
   // TranslationUnitDecl without crashing.
diff --git a/clang/lib/Serialization/ASTWriter.cpp b/clang/lib/Serialization/ASTWriter.cpp
index 00b0e4808321..b9f1a4cbca21 100644
--- a/clang/lib/Serialization/ASTWriter.cpp
+++ b/clang/lib/Serialization/ASTWriter.cpp
@@ -3358,12 +3358,10 @@ void ASTWriter::WriteTypeDeclOffsets() {
   Abbrev = std::make_shared<BitCodeAbbrev>();
   Abbrev->Add(BitCodeAbbrevOp(DECL_OFFSET));
   Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 32)); // # of declarations
-  Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 32)); // base decl ID
   Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Blob)); // declarations block
   unsigned DeclOffsetAbbrev = Stream.EmitAbbrev(std::move(Abbrev));
   {
-    RecordData::value_type Record[] = {DECL_OFFSET, DeclOffsets.size(),
-                                       FirstDeclID.get() - NUM_PREDEF_DECL_IDS};
+    RecordData::value_type Record[] = {DECL_OFFSET, DeclOffsets.size()};
     Stream.EmitRecordWithBlob(DeclOffsetAbbrev, Record, bytes(DeclOffsets));
   }
 }
@@ -5417,7 +5415,6 @@ ASTFileSignature ASTWriter::WriteASTCore(Sema &SemaRef, StringRef isysroot,
                           M.NumPreprocessedEntities);
         writeBaseIDOrNone(M.BaseSubmoduleID, M.LocalNumSubmodules);
         writeBaseIDOrNone(M.BaseSelectorID, M.LocalNumSelectors);
-        writeBaseIDOrNone(M.BaseDeclID, M.LocalNumDecls);
         writeBaseIDOrNone(M.BaseTypeIndex, M.LocalNumTypes);
       }
     }
@@ -6609,13 +6606,11 @@ void ASTWriter::ReaderInitialized(ASTReader *Reader) {
 
   // Note, this will get called multiple times, once one the reader starts up
   // and again each time it's done reading a PCH or module.
-  FirstDeclID = LocalDeclID(NUM_PREDEF_DECL_IDS + Chain->getTotalNumDecls());
   FirstTypeID = NUM_PREDEF_TYPE_IDS + Chain->getTotalNumTypes();
   FirstIdentID = NUM_PREDEF_IDENT_IDS + Chain->getTotalNumIdentifiers();
   FirstMacroID = NUM_PREDEF_MACRO_IDS + Chain->getTotalNumMacros();
   FirstSubmoduleID = NUM_PREDEF_SUBMODULE_IDS + Chain->getTotalNumSubmodules();
   FirstSelectorID = NUM_PREDEF_SELECTOR_IDS + Chain->getTotalNumSelectors();
-  NextDeclID = FirstDeclID;
   NextTypeID = FirstTypeID;
   NextIdentID = FirstIdentID;
   NextMacroID = FirstMacroID;
diff --git a/clang/lib/Serialization/ModuleFile.cpp b/clang/lib/Serialization/ModuleFile.cpp
index 2c42d33a8f5d..f64a59bd9489 100644
--- a/clang/lib/Serialization/ModuleFile.cpp
+++ b/clang/lib/Serialization/ModuleFile.cpp
@@ -87,7 +87,6 @@ LLVM_DUMP_METHOD void ModuleFile::dump() {
                << "  Number of types: " << LocalNumTypes << '\n';
   dumpLocalRemap("Type index local -> global map", TypeRemap);
 
-  llvm::errs() << "  Base decl ID: " << BaseDeclID << '\n'
+  llvm::errs() << "  Base decl index: " << BaseDeclIndex << '\n'
                << "  Number of decls: " << LocalNumDecls << '\n';
-  dumpLocalRemap("Decl ID local -> global map", DeclRemap);
 }
diff --git a/clang/test/Modules/no-transitive-decls-change.cppm b/clang/test/Modules/no-transitive-decls-change.cppm
new file mode 100644
index 000000000000..42ac061bc90b
--- /dev/null
+++ b/clang/test/Modules/no-transitive-decls-change.cppm
@@ -0,0 +1,112 @@
+// Testing that changing a declaration in an unused module file won't change 
+// the BMI of the current module file.
+//
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+//
+// RUN: %clang_cc1 -std=c++20 %t/m-partA.cppm -emit-reduced-module-interface -o %t/m-partA.pcm
+// RUN: %clang_cc1 -std=c++20 %t/m-partA.v1.cppm -emit-reduced-module-interface -o \
+// RUN:     %t/m-partA.v1.pcm
+// RUN: %clang_cc1 -std=c++20 %t/m-partB.cppm -emit-reduced-module-interface -o %t/m-partB.pcm
+// RUN: %clang_cc1 -std=c++20 %t/m.cppm -emit-reduced-module-interface -o %t/m.pcm \
+// RUN:     -fmodule-file=m:partA=%t/m-partA.pcm -fmodule-file=m:partB=%t/m-partB.pcm
+// RUN: %clang_cc1 -std=c++20 %t/m.cppm -emit-reduced-module-interface -o %t/m.v1.pcm \
+// RUN:     -fmodule-file=m:partA=%t/m-partA.v1.pcm -fmodule-file=m:partB=%t/m-partB.pcm
+//
+// RUN: %clang_cc1 -std=c++20 %t/useBOnly.cppm -emit-reduced-module-interface -o %t/useBOnly.pcm \
+// RUN:     -fmodule-file=m=%t/m.pcm -fmodule-file=m:partA=%t/m-partA.pcm \
+// RUN:     -fmodule-file=m:partB=%t/m-partB.pcm
+// RUN: %clang_cc1 -std=c++20 %t/useBOnly.cppm -emit-reduced-module-interface -o %t/useBOnly.v1.pcm \
+// RUN:     -fmodule-file=m=%t/m.v1.pcm -fmodule-file=m:partA=%t/m-partA.v1.pcm \
+// RUN:     -fmodule-file=m:partB=%t/m-partB.pcm
+// Since useBOnly only uses partB from module M, the change in partA shouldn't affect
+// useBOnly.
+// RUN: diff %t/useBOnly.pcm %t/useBOnly.v1.pcm &> /dev/null
+
+//--- m-partA.cppm
+export module m:partA;
+
+namespace A_Impl {
+    inline int getAImpl() {
+        return 43;
+    }
+
+    inline int getA2Impl() {
+        return 43;
+    }
+}
+
+namespace A {
+    using A_Impl::getAImpl;
+}
+
+export inline int getA() {
+    return 43;
+}
+
+export inline int getA2(int) {
+    return 88;
+}
+
+//--- m-partA.v1.cppm
+export module m:partA;
+
+namespace A_Impl {
+    inline int getAImpl() {
+        return 43;
+    }
+
+    inline int getA2Impl() {
+        return 43;
+    }
+}
+
+namespace A {
+    using A_Impl::getAImpl;
+    // Adding a new declaration without introducing a new declaration name.
+    using A_Impl::getA2Impl;
+}
+
+inline int getA() {
+    return 43;
+}
+
+inline int getA2(int) {
+    return 88;
+}
+
+// Now we add a new declaration without introducing new identifier and new types.
+// The consuming module which didn't use m:partA completely is expected to be
+// not changed.
+inline int getA(int) {
+    return 88;
+}
+
+//--- m-partB.cppm
+export module m:partB;
+
+export inline int getB() {
+    return 430;
+}
+
+//--- m.cppm
+export module m;
+export import :partA;
+export import :partB;
+
+//--- useBOnly.cppm
+export module useBOnly;
+import m;
+
+export inline int get() {
+    return getB();
+}
+
+//--- useAOnly.cppm
+export module useAOnly;
+import m;
+
+export inline int get() {
+    A<int> a;
+    return a.getValue();
+}