diff --git a/clang/include/clang/Serialization/ASTReader.h b/clang/include/clang/Serialization/ASTReader.h
--- a/clang/include/clang/Serialization/ASTReader.h
+++ b/clang/include/clang/Serialization/ASTReader.h
@@ -2196,6 +2196,18 @@
     return ReadSourceLocation(ModuleFile, Record[Idx++], Seq);
   }
 
+  /// Read a FileID.
+  FileID ReadFileID(ModuleFile &F, const RecordDataImpl &Record,
+                    unsigned &Idx) const {
+    return TranslateFileID(F, FileID::get(Record[Idx++]));
+  }
+
+  /// Translate a FileID from another module file's FileID space into ours.
+  FileID TranslateFileID(ModuleFile &F, FileID FID) const {
+    assert(FID.ID >= 0 && "Reading non-local FileID.");
+    return FileID::get(F.SLocEntryBaseID + FID.ID - 1);
+  }
+
   /// Read a source range.
   SourceRange ReadSourceRange(ModuleFile &F, const RecordData &Record,
                               unsigned &Idx, LocSeq *Seq = nullptr);
diff --git a/clang/include/clang/Serialization/ASTWriter.h b/clang/include/clang/Serialization/ASTWriter.h
--- a/clang/include/clang/Serialization/ASTWriter.h
+++ b/clang/include/clang/Serialization/ASTWriter.h
@@ -582,6 +582,9 @@
   void AddAlignPackInfo(const Sema::AlignPackInfo &Info,
                         RecordDataImpl &Record);
 
+  /// Emit a FileID.
+  void AddFileID(FileID FID, RecordDataImpl &Record);
+
   /// Emit a source location.
   void AddSourceLocation(SourceLocation Loc, RecordDataImpl &Record,
                          LocSeq *Seq = nullptr);
diff --git a/clang/lib/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/clang/lib/Serialization/ASTReader.cpp
@@ -1338,10 +1338,7 @@
   // Parse the line entries
   std::vector<LineEntry> Entries;
   while (Idx < Record.size()) {
-    int FID = Record[Idx++];
-    assert(FID >= 0 && "Serialized line entries for non-local file.");
-    // Remap FileID from 1-based old view.
-    FID += F.SLocEntryBaseID - 1;
+    FileID FID = ReadFileID(F, Record, Idx);
 
     // Extract the line entries
     unsigned NumEntries = Record[Idx++];
@@ -1358,7 +1355,7 @@
       Entries.push_back(LineEntry::get(FileOffset, LineNo, FilenameID,
                                        FileKind, IncludeOffset));
     }
-    LineTable.AddEntry(FileID::get(FID), Entries);
+    LineTable.AddEntry(FID, Entries);
   }
 }
 
@@ -4294,10 +4291,8 @@
 
     // Map the original source file ID into the ID space of the current
     // compilation.
-    if (F.OriginalSourceFileID.isValid()) {
-      F.OriginalSourceFileID = FileID::get(
-          F.SLocEntryBaseID + F.OriginalSourceFileID.getOpaqueValue() - 1);
-    }
+    if (F.OriginalSourceFileID.isValid())
+      F.OriginalSourceFileID = TranslateFileID(F, F.OriginalSourceFileID);
 
     // Preload all the pending interesting identifiers by marking them out of
     // date.
diff --git a/clang/lib/Serialization/ASTWriter.cpp b/clang/lib/Serialization/ASTWriter.cpp
--- a/clang/lib/Serialization/ASTWriter.cpp
+++ b/clang/lib/Serialization/ASTWriter.cpp
@@ -1469,12 +1469,12 @@
 
     Record.clear();
     Record.push_back(ORIGINAL_FILE);
-    Record.push_back(SM.getMainFileID().getOpaqueValue());
+    AddFileID(SM.getMainFileID(), Record);
     EmitRecordWithPath(FileAbbrevCode, Record, MainFile->getName());
   }
 
   Record.clear();
-  Record.push_back(SM.getMainFileID().getOpaqueValue());
+  AddFileID(SM.getMainFileID(), Record);
   Stream.EmitRecord(ORIGINAL_FILE_ID, Record);
 
   std::set<const FileEntry *> AffectingClangModuleMaps;
@@ -2206,8 +2206,7 @@
       if (L.first.ID < 0)
         continue;
 
-      // Emit the file ID
-      Record.push_back(L.first.ID);
+      AddFileID(L.first, Record);
 
       // Emit the line entries
       Record.push_back(L.second.size());
@@ -5228,6 +5227,10 @@
   Record.push_back(Raw);
 }
 
+void ASTWriter::AddFileID(FileID FID, RecordDataImpl &Record) {
+  Record.push_back(FID.getOpaqueValue());
+}
+
 void ASTWriter::AddSourceLocation(SourceLocation Loc, RecordDataImpl &Record,
                                   SourceLocationSequence *Seq) {
   Record.push_back(SourceLocationEncoding::encode(Loc, Seq));