[Serialization] No transitive identifier change (#92085)

Following of https://github.com/llvm/llvm-project/pull/92083

The motivation is still cutting of the unnecessary change in the
dependency chain. See the above link (recursively) for details.

After this patch, (and the above patch), we can already do something
pretty interesting. For example,

#### Motivation example

```

//--- m-partA.cppm
export module m:partA;

export inline int getA() {
    return 43;
}

export class A {
public:
    int getMem();
};

export template <typename T>
class ATempl {
public:
    T getT();
};

//--- m-partA.v1.cppm
export module m:partA;

export inline int getA() {
    return 43;
}

// Now we add a new declaration without introducing a new type.
// The consuming module which didn't use m:partA completely is expected to be
// not changed.
export inline int getA2() {
    return 88;
}

export class A {
public:
    int getMem();
    // Now we add a new declaration without introducing a new type.
    // The consuming module which didn't use m:partA completely is expected to be
    // not changed.
    int getMem2();
};

export template <typename T>
class ATempl {
public:
    T getT();
    // Add a new declaration without introducing a new type.
    T getT2();
};

//--- 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();
}
```

In this example, module `m` exports two partitions `:partA` and
`:partB`. And a consumer `useBOnly` only consumes the entities from
`:partB`. So we don't hope the BMI of `useBOnly` changes if only
`:partA` changes. After this patch, we can make it if the change of
`:partA` doesn't introduce new types. (And we can get rid of this if we
make no-transitive-type-change).

As the example shows, when we change the implementation of `:partA` from
`m-partA.cppm` to `m-partA.v1.cppm`, we add new function declaration
`getA2()` at the global namespace, add a new member function `getMem2()`
to class `A` and add a new member function to `getT2()` to class
template `ATempl`. And since `:partA` is not used by `useBOnly`
completely, the BMI of `useBOnly` won't change after we made above
changes.

#### Design details

Method used in this patch is similar with
https://github.com/llvm/llvm-project/pull/92083 and
https://github.com/llvm/llvm-project/pull/86912. It extends the 32 bit
IdentifierID to 64 bits and use the higher 32 bits to store the module
file index. So that the encoding of the identifier won't get affected by
other modules.

#### Overhead

Similar with https://github.com/llvm/llvm-project/pull/92083 and
https://github.com/llvm/llvm-project/pull/86912. The change is only
expected to increase the size of the on-disk .pcm files and not affect
the compile-time performances. And from my experiment, the size of the
on-disk change only increase 1%+ and observe no compile-time impacts.

#### Future Plans

I'll try to do the same thing for type ids. IIRC, it won't change the
dependency graph if we add a new type in an unused units. I do think
this is a significant win. And this will be a pretty good answer to "why
modules are better than headers."

9 files changed, 210 insertions, 82 deletions

diff --git a/clang/include/clang/Lex/ExternalPreprocessorSource.h b/clang/include/clang/Lex/ExternalPreprocessorSource.h
index 6672ada..4842994 100644
--- a/clang/include/clang/Lex/ExternalPreprocessorSource.h
+++ b/clang/include/clang/Lex/ExternalPreprocessorSource.h
@@ -36,7 +36,7 @@ public:
   /// Return the identifier associated with the given ID number.
   ///
   /// The ID 0 is associated with the NULL identifier.
-  virtual IdentifierInfo *GetIdentifier(unsigned ID) = 0;
+  virtual IdentifierInfo *GetIdentifier(uint64_t ID) = 0;
 
   /// Map a module ID to a module.
   virtual Module *getModule(unsigned ModuleID) = 0;
diff --git a/clang/include/clang/Serialization/ASTBitCodes.h b/clang/include/clang/Serialization/ASTBitCodes.h
index 9f0f900..570b088 100644
--- a/clang/include/clang/Serialization/ASTBitCodes.h
+++ b/clang/include/clang/Serialization/ASTBitCodes.h
@@ -59,7 +59,7 @@ const unsigned VERSION_MINOR = 1;
 ///
 /// The ID numbers of identifiers are consecutive (in order of discovery)
 /// and start at 1. 0 is reserved for NULL.
-using IdentifierID = uint32_t;
+using IdentifierID = uint64_t;
 
 /// The number of predefined identifier IDs.
 const unsigned int NUM_PREDEF_IDENT_IDS = 1;
diff --git a/clang/include/clang/Serialization/ASTReader.h b/clang/include/clang/Serialization/ASTReader.h
index 480f852..0e95d82 100644
--- a/clang/include/clang/Serialization/ASTReader.h
+++ b/clang/include/clang/Serialization/ASTReader.h
@@ -661,14 +661,6 @@ private:
   /// been loaded.
   std::vector<IdentifierInfo *> IdentifiersLoaded;
 
-  using GlobalIdentifierMapType =
-      ContinuousRangeMap<serialization::IdentifierID, ModuleFile *, 4>;
-
-  /// Mapping from global identifier IDs to the module in which the
-  /// identifier resides along with the offset that should be added to the
-  /// global identifier ID to produce a local ID.
-  GlobalIdentifierMapType GlobalIdentifierMap;
-
   /// A vector containing macros that have already been
   /// loaded.
   ///
@@ -1547,6 +1539,11 @@ private:
   /// Translate a \param GlobalDeclID to the index of DeclsLoaded array.
   unsigned translateGlobalDeclIDToIndex(GlobalDeclID ID) const;
 
+  /// Translate an \param IdentifierID ID to the index of IdentifiersLoaded
+  /// array and the corresponding module file.
+  std::pair<ModuleFile *, unsigned>
+  translateIdentifierIDToIndex(serialization::IdentifierID ID) const;
+
 public:
   /// Load the AST file and validate its contents against the given
   /// Preprocessor.
@@ -2131,7 +2128,7 @@ public:
   /// Load a selector from disk, registering its ID if it exists.
   void LoadSelector(Selector Sel);
 
-  void SetIdentifierInfo(unsigned ID, IdentifierInfo *II);
+  void SetIdentifierInfo(serialization::IdentifierID ID, IdentifierInfo *II);
   void SetGloballyVisibleDecls(IdentifierInfo *II,
                                const SmallVectorImpl<GlobalDeclID> &DeclIDs,
                                SmallVectorImpl<Decl *> *Decls = nullptr);
@@ -2158,10 +2155,10 @@ public:
     return DecodeIdentifierInfo(ID);
   }
 
-  IdentifierInfo *getLocalIdentifier(ModuleFile &M, unsigned LocalID);
+  IdentifierInfo *getLocalIdentifier(ModuleFile &M, uint64_t LocalID);
 
   serialization::IdentifierID getGlobalIdentifierID(ModuleFile &M,
-                                                    unsigned LocalID);
+                                                    uint64_t LocalID);
 
   void resolvePendingMacro(IdentifierInfo *II, const PendingMacroInfo &PMInfo);
 
diff --git a/clang/include/clang/Serialization/ModuleFile.h b/clang/include/clang/Serialization/ModuleFile.h
index 56193d4..3787f4e 100644
--- a/clang/include/clang/Serialization/ModuleFile.h
+++ b/clang/include/clang/Serialization/ModuleFile.h
@@ -310,9 +310,6 @@ public:
   /// Base identifier ID for identifiers local to this module.
   serialization::IdentifierID BaseIdentifierID = 0;
 
-  /// Remapping table for identifier IDs in this module.
-  ContinuousRangeMap<uint32_t, int, 2> IdentifierRemap;
-
   /// Actual data for the on-disk hash table of identifiers.
   ///
   /// This pointer points into a memory buffer, where the on-disk hash
diff --git a/clang/lib/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
index 0afbc44..43013ab 100644
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/clang/lib/Serialization/ASTReader.cpp
@@ -954,7 +954,7 @@ ASTSelectorLookupTrait::ReadKey(const unsigned char* d, unsigned) {
   SelectorTable &SelTable = Reader.getContext().Selectors;
   unsigned N = endian::readNext<uint16_t, llvm::endianness::little>(d);
   const IdentifierInfo *FirstII = Reader.getLocalIdentifier(
-      F, endian::readNext<uint32_t, llvm::endianness::little>(d));
+      F, endian::readNext<IdentifierID, llvm::endianness::little>(d));
   if (N == 0)
     return SelTable.getNullarySelector(FirstII);
   else if (N == 1)
@@ -964,7 +964,7 @@ ASTSelectorLookupTrait::ReadKey(const unsigned char* d, unsigned) {
   Args.push_back(FirstII);
   for (unsigned I = 1; I != N; ++I)
     Args.push_back(Reader.getLocalIdentifier(
-        F, endian::readNext<uint32_t, llvm::endianness::little>(d)));
+        F, endian::readNext<IdentifierID, llvm::endianness::little>(d)));
 
   return SelTable.getSelector(N, Args.data());
 }
@@ -1047,7 +1047,8 @@ static bool readBit(unsigned &Bits) {
 IdentifierID ASTIdentifierLookupTrait::ReadIdentifierID(const unsigned char *d) {
   using namespace llvm::support;
 
-  unsigned RawID = endian::readNext<uint32_t, llvm::endianness::little>(d);
+  IdentifierID RawID =
+      endian::readNext<IdentifierID, llvm::endianness::little>(d);
   return Reader.getGlobalIdentifierID(F, RawID >> 1);
 }
 
@@ -1065,9 +1066,12 @@ IdentifierInfo *ASTIdentifierLookupTrait::ReadData(const internal_key_type& k,
                                                    unsigned DataLen) {
   using namespace llvm::support;
 
-  unsigned RawID = endian::readNext<uint32_t, llvm::endianness::little>(d);
+  IdentifierID RawID =
+      endian::readNext<IdentifierID, llvm::endianness::little>(d);
   bool IsInteresting = RawID & 0x01;
 
+  DataLen -= sizeof(IdentifierID);
+
   // Wipe out the "is interesting" bit.
   RawID = RawID >> 1;
 
@@ -1098,7 +1102,7 @@ IdentifierInfo *ASTIdentifierLookupTrait::ReadData(const internal_key_type& k,
   bool HadMacroDefinition = readBit(Bits);
 
   assert(Bits == 0 && "Extra bits in the identifier?");
-  DataLen -= 8;
+  DataLen -= sizeof(uint16_t) * 2;
 
   // Set or check the various bits in the IdentifierInfo structure.
   // Token IDs are read-only.
@@ -1225,7 +1229,7 @@ ASTDeclContextNameLookupTrait::ReadKey(const unsigned char *d, unsigned) {
   case DeclarationName::CXXLiteralOperatorName:
   case DeclarationName::CXXDeductionGuideName:
     Data = (uint64_t)Reader.getLocalIdentifier(
-        F, endian::readNext<uint32_t, llvm::endianness::little>(d));
+        F, endian::readNext<IdentifierID, llvm::endianness::little>(d));
     break;
   case DeclarationName::ObjCZeroArgSelector:
   case DeclarationName::ObjCOneArgSelector:
@@ -2093,7 +2097,7 @@ HeaderFileInfoTrait::ReadData(internal_key_ref key, const unsigned char *d,
   HFI.DirInfo = (Flags >> 1) & 0x07;
   HFI.IndexHeaderMapHeader = Flags & 0x01;
   HFI.LazyControllingMacro = Reader.getGlobalIdentifierID(
-      M, endian::readNext<uint32_t, llvm::endianness::little>(d));
+      M, endian::readNext<IdentifierID, llvm::endianness::little>(d));
   if (unsigned FrameworkOffset =
           endian::readNext<uint32_t, llvm::endianness::little>(d)) {
     // The framework offset is 1 greater than the actual offset,
@@ -3467,24 +3471,11 @@ llvm::Error ASTReader::ReadASTBlock(ModuleFile &F,
             "duplicate IDENTIFIER_OFFSET record in AST file");
       F.IdentifierOffsets = (const uint32_t *)Blob.data();
       F.LocalNumIdentifiers = Record[0];
-      unsigned LocalBaseIdentifierID = Record[1];
       F.BaseIdentifierID = getTotalNumIdentifiers();
 
-      if (F.LocalNumIdentifiers > 0) {
-        // Introduce the global -> local mapping for identifiers within this
-        // module.
-        GlobalIdentifierMap.insert(std::make_pair(getTotalNumIdentifiers() + 1,
-                                                  &F));
-
-        // Introduce the local -> global mapping for identifiers within this
-        // module.
-        F.IdentifierRemap.insertOrReplace(
-          std::make_pair(LocalBaseIdentifierID,
-                         F.BaseIdentifierID - LocalBaseIdentifierID));
-
+      if (F.LocalNumIdentifiers > 0)
         IdentifiersLoaded.resize(IdentifiersLoaded.size()
                                  + F.LocalNumIdentifiers);
-      }
       break;
     }
 
@@ -4089,7 +4080,6 @@ void ASTReader::ReadModuleOffsetMap(ModuleFile &F) const {
   F.ModuleOffsetMap = StringRef();
 
   using RemapBuilder = ContinuousRangeMap<uint32_t, int, 2>::Builder;
-  RemapBuilder IdentifierRemap(F.IdentifierRemap);
   RemapBuilder MacroRemap(F.MacroRemap);
   RemapBuilder PreprocessedEntityRemap(F.PreprocessedEntityRemap);
   RemapBuilder SubmoduleRemap(F.SubmoduleRemap);
@@ -4122,8 +4112,6 @@ void ASTReader::ReadModuleOffsetMap(ModuleFile &F) const {
 
     ImportedModuleVector.push_back(OM);
 
-    uint32_t IdentifierIDOffset =
-        endian::readNext<uint32_t, llvm::endianness::little>(Data);
     uint32_t MacroIDOffset =
         endian::readNext<uint32_t, llvm::endianness::little>(Data);
     uint32_t PreprocessedEntityIDOffset =
@@ -4143,7 +4131,6 @@ void ASTReader::ReadModuleOffsetMap(ModuleFile &F) const {
                                     static_cast<int>(BaseOffset - Offset)));
     };
 
-    mapOffset(IdentifierIDOffset, OM->BaseIdentifierID, IdentifierRemap);
     mapOffset(MacroIDOffset, OM->BaseMacroID, MacroRemap);
     mapOffset(PreprocessedEntityIDOffset, OM->BasePreprocessedEntityID,
               PreprocessedEntityRemap);
@@ -8238,7 +8225,6 @@ 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 identifier map", GlobalIdentifierMap);
   dumpModuleIDMap("Global macro map", GlobalMacroMap);
   dumpModuleIDMap("Global submodule map", GlobalSubmoduleMap);
   dumpModuleIDMap("Global selector map", GlobalSelectorMap);
@@ -8878,8 +8864,9 @@ void ASTReader::LoadSelector(Selector Sel) {
 
 void ASTReader::SetIdentifierInfo(IdentifierID ID, IdentifierInfo *II) {
   assert(ID && "Non-zero identifier ID required");
-  assert(ID <= IdentifiersLoaded.size() && "identifier ID out of range");
-  IdentifiersLoaded[ID - 1] = II;
+  unsigned Index = translateIdentifierIDToIndex(ID).second;
+  assert(Index < IdentifiersLoaded.size() && "identifier ID out of range");
+  IdentifiersLoaded[Index] = II;
   if (DeserializationListener)
     DeserializationListener->IdentifierRead(ID, II);
 }
@@ -8932,6 +8919,22 @@ void ASTReader::SetGloballyVisibleDecls(
   }
 }
 
+std::pair<ModuleFile *, unsigned>
+ASTReader::translateIdentifierIDToIndex(IdentifierID ID) const {
+  if (ID == 0)
+    return {nullptr, 0};
+
+  unsigned ModuleFileIndex = ID >> 32;
+  unsigned LocalID = ID & llvm::maskTrailingOnes<IdentifierID>(32);
+
+  assert(ModuleFileIndex && "not translating loaded IdentifierID?");
+  assert(getModuleManager().size() > ModuleFileIndex - 1);
+
+  ModuleFile &MF = getModuleManager()[ModuleFileIndex - 1];
+  assert(LocalID < MF.LocalNumIdentifiers);
+  return {&MF, MF.BaseIdentifierID + LocalID};
+}
+
 IdentifierInfo *ASTReader::DecodeIdentifierInfo(IdentifierID ID) {
   if (ID == 0)
     return nullptr;
@@ -8941,45 +8944,48 @@ IdentifierInfo *ASTReader::DecodeIdentifierInfo(IdentifierID ID) {
     return nullptr;
   }
 
-  ID -= 1;
-  if (!IdentifiersLoaded[ID]) {
-    GlobalIdentifierMapType::iterator I = GlobalIdentifierMap.find(ID + 1);
-    assert(I != GlobalIdentifierMap.end() && "Corrupted global identifier map");
-    ModuleFile *M = I->second;
-    unsigned Index = ID - M->BaseIdentifierID;
+  auto [M, Index] = translateIdentifierIDToIndex(ID);
+  if (!IdentifiersLoaded[Index]) {
+    assert(M != nullptr && "Untranslated Identifier ID?");
+    assert(Index >= M->BaseIdentifierID);
+    unsigned LocalIndex = Index - M->BaseIdentifierID;
     const unsigned char *Data =
-        M->IdentifierTableData + M->IdentifierOffsets[Index];
+        M->IdentifierTableData + M->IdentifierOffsets[LocalIndex];
 
     ASTIdentifierLookupTrait Trait(*this, *M);
     auto KeyDataLen = Trait.ReadKeyDataLength(Data);
     auto Key = Trait.ReadKey(Data, KeyDataLen.first);
     auto &II = PP.getIdentifierTable().get(Key);
-    IdentifiersLoaded[ID] = &II;
+    IdentifiersLoaded[Index] = &II;
     markIdentifierFromAST(*this,  II);
     if (DeserializationListener)
-      DeserializationListener->IdentifierRead(ID + 1, &II);
+      DeserializationListener->IdentifierRead(ID, &II);
   }
 
-  return IdentifiersLoaded[ID];
+  return IdentifiersLoaded[Index];
 }
 
-IdentifierInfo *ASTReader::getLocalIdentifier(ModuleFile &M, unsigned LocalID) {
+IdentifierInfo *ASTReader::getLocalIdentifier(ModuleFile &M, uint64_t LocalID) {
   return DecodeIdentifierInfo(getGlobalIdentifierID(M, LocalID));
 }
 
-IdentifierID ASTReader::getGlobalIdentifierID(ModuleFile &M, unsigned LocalID) {
+IdentifierID ASTReader::getGlobalIdentifierID(ModuleFile &M, uint64_t LocalID) {
   if (LocalID < NUM_PREDEF_IDENT_IDS)
     return LocalID;
 
   if (!M.ModuleOffsetMap.empty())
     ReadModuleOffsetMap(M);
 
-  ContinuousRangeMap<uint32_t, int, 2>::iterator I
-    = M.IdentifierRemap.find(LocalID - NUM_PREDEF_IDENT_IDS);
-  assert(I != M.IdentifierRemap.end()
-         && "Invalid index into identifier index remap");
+  unsigned ModuleFileIndex = LocalID >> 32;
+  LocalID &= llvm::maskTrailingOnes<IdentifierID>(32);
+  ModuleFile *MF =
+      ModuleFileIndex ? M.TransitiveImports[ModuleFileIndex - 1] : &M;
+  assert(MF && "malformed identifier ID encoding?");
 
-  return LocalID + I->second;
+  if (!ModuleFileIndex)
+    LocalID -= NUM_PREDEF_IDENT_IDS;
+
+  return ((IdentifierID)(MF->Index + 1) << 32) | LocalID;
 }
 
 MacroInfo *ASTReader::getMacro(MacroID ID) {
diff --git a/clang/lib/Serialization/ASTWriter.cpp b/clang/lib/Serialization/ASTWriter.cpp
index a60092a..346bab3 100644
--- a/clang/lib/Serialization/ASTWriter.cpp
+++ b/clang/lib/Serialization/ASTWriter.cpp
@@ -1993,7 +1993,7 @@ namespace {
     std::pair<unsigned, unsigned>
     EmitKeyDataLength(raw_ostream& Out, key_type_ref key, data_type_ref Data) {
       unsigned KeyLen = key.Filename.size() + 1 + 8 + 8;
-      unsigned DataLen = 1 + 4 + 4;
+      unsigned DataLen = 1 + sizeof(IdentifierID) + 4;
       for (auto ModInfo : Data.KnownHeaders)
         if (Writer.getLocalOrImportedSubmoduleID(ModInfo.getModule()))
           DataLen += 4;
@@ -3461,7 +3461,9 @@ public:
   std::pair<unsigned, unsigned>
     EmitKeyDataLength(raw_ostream& Out, Selector Sel,
                       data_type_ref Methods) {
-    unsigned KeyLen = 2 + (Sel.getNumArgs()? Sel.getNumArgs() * 4 : 4);
+    unsigned KeyLen =
+        2 + (Sel.getNumArgs() ? Sel.getNumArgs() * sizeof(IdentifierID)
+                              : sizeof(IdentifierID));
     unsigned DataLen = 4 + 2 + 2; // 2 bytes for each of the method counts
     for (const ObjCMethodList *Method = &Methods.Instance; Method;
          Method = Method->getNext())
@@ -3486,7 +3488,7 @@ public:
     if (N == 0)
       N = 1;
     for (unsigned I = 0; I != N; ++I)
-      LE.write<uint32_t>(
+      LE.write<IdentifierID>(
           Writer.getIdentifierRef(Sel.getIdentifierInfoForSlot(I)));
   }
 
@@ -3797,7 +3799,7 @@ public:
       InterestingIdentifierOffsets->push_back(Out.tell());
 
     unsigned KeyLen = II->getLength() + 1;
-    unsigned DataLen = 4; // 4 bytes for the persistent ID << 1
+    unsigned DataLen = sizeof(IdentifierID); // bytes for the persistent ID << 1
     if (isInterestingIdentifier(II, MacroOffset)) {
       DataLen += 2; // 2 bytes for builtin ID
       DataLen += 2; // 2 bytes for flags
@@ -3823,11 +3825,11 @@ public:
 
     auto MacroOffset = Writer.getMacroDirectivesOffset(II);
     if (!isInterestingIdentifier(II, MacroOffset)) {
-      LE.write<uint32_t>(ID << 1);
+      LE.write<IdentifierID>(ID << 1);
       return;
     }
 
-    LE.write<uint32_t>((ID << 1) | 0x01);
+    LE.write<IdentifierID>((ID << 1) | 0x01);
     uint32_t Bits = (uint32_t)II->getObjCOrBuiltinID();
     assert((Bits & 0xffff) == Bits && "ObjCOrBuiltinID too big for ASTReader.");
     LE.write<uint16_t>(Bits);
@@ -3860,6 +3862,10 @@ public:
 
 } // namespace
 
+/// If the \param IdentifierID ID is a local Identifier ID. If the higher
+/// bits of ID is 0, it implies that the ID doesn't come from AST files.
+static bool isLocalIdentifierID(IdentifierID ID) { return !(ID >> 32); }
+
 /// Write the identifier table into the AST file.
 ///
 /// The identifier table consists of a blob containing string data
@@ -3904,7 +3910,7 @@ void ASTWriter::WriteIdentifierTable(Preprocessor &PP,
 
       // Write out identifiers if either the ID is local or the identifier has
       // changed since it was loaded.
-      if (ID >= FirstIdentID || II->hasChangedSinceDeserialization() ||
+      if (isLocalIdentifierID(ID) || II->hasChangedSinceDeserialization() ||
           (Trait.needDecls() &&
            II->hasFETokenInfoChangedSinceDeserialization()))
         Generator.insert(II, ID, Trait);
@@ -3938,7 +3944,6 @@ void ASTWriter::WriteIdentifierTable(Preprocessor &PP,
   auto Abbrev = std::make_shared<BitCodeAbbrev>();
   Abbrev->Add(BitCodeAbbrevOp(IDENTIFIER_OFFSET));
   Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 32)); // # of identifiers
-  Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Fixed, 32)); // first ID
   Abbrev->Add(BitCodeAbbrevOp(BitCodeAbbrevOp::Blob));
   unsigned IdentifierOffsetAbbrev = Stream.EmitAbbrev(std::move(Abbrev));
 
@@ -3948,8 +3953,7 @@ void ASTWriter::WriteIdentifierTable(Preprocessor &PP,
 #endif
 
   RecordData::value_type Record[] = {IDENTIFIER_OFFSET,
-                                     IdentifierOffsets.size(),
-                                     FirstIdentID - NUM_PREDEF_IDENT_IDS};
+                                     IdentifierOffsets.size()};
   Stream.EmitRecordWithBlob(IdentifierOffsetAbbrev, Record,
                             bytes(IdentifierOffsets));
 
@@ -4043,11 +4047,13 @@ public:
     unsigned KeyLen = 1;
     switch (Name.getKind()) {
     case DeclarationName::Identifier:
+    case DeclarationName::CXXLiteralOperatorName:
+    case DeclarationName::CXXDeductionGuideName:
+      KeyLen += sizeof(IdentifierID);
+      break;
     case DeclarationName::ObjCZeroArgSelector:
     case DeclarationName::ObjCOneArgSelector:
     case DeclarationName::ObjCMultiArgSelector:
-    case DeclarationName::CXXLiteralOperatorName:
-    case DeclarationName::CXXDeductionGuideName:
       KeyLen += 4;
       break;
     case DeclarationName::CXXOperatorName:
@@ -4075,7 +4081,7 @@ public:
     case DeclarationName::Identifier:
     case DeclarationName::CXXLiteralOperatorName:
     case DeclarationName::CXXDeductionGuideName:
-      LE.write<uint32_t>(Writer.getIdentifierRef(Name.getIdentifier()));
+      LE.write<IdentifierID>(Writer.getIdentifierRef(Name.getIdentifier()));
       return;
     case DeclarationName::ObjCZeroArgSelector:
     case DeclarationName::ObjCOneArgSelector:
@@ -4793,8 +4799,15 @@ void ASTWriter::SetIdentifierOffset(const IdentifierInfo *II, uint32_t Offset) {
   IdentifierID ID = IdentifierIDs[II];
   // Only store offsets new to this AST file. Other identifier names are looked
   // up earlier in the chain and thus don't need an offset.
-  if (ID >= FirstIdentID)
-    IdentifierOffsets[ID - FirstIdentID] = Offset;
+  if (!isLocalIdentifierID(ID))
+    return;
+
+  // For local identifiers, the module file index must be 0.
+
+  assert(ID != 0);
+  ID -= NUM_PREDEF_IDENT_IDS;
+  assert(ID < IdentifierOffsets.size());
+  IdentifierOffsets[ID] = Offset;
 }
 
 /// Note that the selector Sel occurs at the given offset
@@ -5446,7 +5459,6 @@ ASTFileSignature ASTWriter::WriteASTCore(Sema &SemaRef, StringRef isysroot,
 
         // These values should be unique within a chain, since they will be read
         // as keys into ContinuousRangeMaps.
-        writeBaseIDOrNone(M.BaseIdentifierID, M.LocalNumIdentifiers);
         writeBaseIDOrNone(M.BaseMacroID, M.LocalNumMacros);
         writeBaseIDOrNone(M.BasePreprocessedEntityID,
                           M.NumPreprocessedEntities);
@@ -6647,20 +6659,26 @@ 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.
   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();
   NextTypeID = FirstTypeID;
-  NextIdentID = FirstIdentID;
   NextMacroID = FirstMacroID;
   NextSelectorID = FirstSelectorID;
   NextSubmoduleID = FirstSubmoduleID;
 }
 
 void ASTWriter::IdentifierRead(IdentifierID ID, IdentifierInfo *II) {
-  // Always keep the highest ID. See \p TypeRead() for more information.
   IdentifierID &StoredID = IdentifierIDs[II];
+  unsigned OriginalModuleFileIndex = StoredID >> 32;
+
+  // Always keep the local identifier ID. See \p TypeRead() for more
+  // information.
+  if (OriginalModuleFileIndex == 0 && StoredID)
+    return;
+
+  // Otherwise, keep the highest ID since the module file comes later has
+  // higher module file indexes.
   if (ID > StoredID)
     StoredID = ID;
 }
diff --git a/clang/lib/Serialization/GlobalModuleIndex.cpp b/clang/lib/Serialization/GlobalModuleIndex.cpp
index f09ceb8..1163943 100644
--- a/clang/lib/Serialization/GlobalModuleIndex.cpp
+++ b/clang/lib/Serialization/GlobalModuleIndex.cpp
@@ -510,7 +510,8 @@ namespace {
       // The first bit indicates whether this identifier is interesting.
       // That's all we care about.
       using namespace llvm::support;
-      unsigned RawID = endian::readNext<uint32_t, llvm::endianness::little>(d);
+      IdentifierID RawID =
+          endian::readNext<IdentifierID, llvm::endianness::little>(d);
       bool IsInteresting = RawID & 0x01;
       return std::make_pair(k, IsInteresting);
     }
diff --git a/clang/lib/Serialization/ModuleFile.cpp b/clang/lib/Serialization/ModuleFile.cpp
index f64a59b..7976c28 100644
--- a/clang/lib/Serialization/ModuleFile.cpp
+++ b/clang/lib/Serialization/ModuleFile.cpp
@@ -62,7 +62,6 @@ LLVM_DUMP_METHOD void ModuleFile::dump() {
 
   llvm::errs() << "  Base identifier ID: " << BaseIdentifierID << '\n'
                << "  Number of identifiers: " << LocalNumIdentifiers << '\n';
-  dumpLocalRemap("Identifier ID local -> global map", IdentifierRemap);
 
   llvm::errs() << "  Base macro ID: " << BaseMacroID << '\n'
                << "  Number of macros: " << LocalNumMacros << '\n';
diff --git a/clang/test/Modules/no-transitive-identifier-change.cppm b/clang/test/Modules/no-transitive-identifier-change.cppm
new file mode 100644
index 0000000..97e8ac4
--- /dev/null
+++ b/clang/test/Modules/no-transitive-identifier-change.cppm
@@ -0,0 +1,110 @@
+// Testing that adding an new identifier 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
+//
+// RUN: %clang_cc1 -std=c++20 %t/useAOnly.cppm -emit-reduced-module-interface -o %t/useAOnly.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/useAOnly.cppm -emit-reduced-module-interface -o %t/useAOnly.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
+// useAOnly should differ
+// RUN: not diff %t/useAOnly.pcm %t/useAOnly.v1.pcm &> /dev/null
+
+//--- m-partA.cppm
+export module m:partA;
+
+export inline int getA() {
+    return 43;
+}
+
+export class A {
+public:
+    int getMem();
+};
+
+export template <typename T>
+class ATempl {
+public:
+    T getT();
+};
+
+//--- m-partA.v1.cppm
+export module m:partA;
+
+export inline int getA() {
+    return 43;
+}
+
+// Now we add a new declaration without introducing a new type.
+// The consuming module which didn't use m:partA completely is expected to be
+// not changed.
+export inline int getA2() {
+    return 88;
+}
+
+export class A {
+public:
+    int getMem();
+    // Now we add a new declaration without introducing a new type.
+    // The consuming module which didn't use m:partA completely is expected to be
+    // not changed.
+    int getMem2();
+};
+
+export template <typename T>
+class ATempl {
+public:
+    T getT();
+    // Add a new declaration without introducing a new type.
+    T getT2();
+};
+
+//--- 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() {
+    return getA();
+}