diff --git a/lib/Serialization/ASTReader.cpp b/lib/Serialization/ASTReader.cpp
index 0ef94aa..e6d16fe 100644
--- a/lib/Serialization/ASTReader.cpp
+++ b/lib/Serialization/ASTReader.cpp
@@ -1735,15 +1735,8 @@
   // Note that this identifier has a macro definition.
   II->setHasMacroDefinition(true);
   
-  // Adjust the offset based on our position in the chain.
-  for (unsigned I = 0, N = Chain.size(); I != N; ++I) {
-    if (Chain[I] == &F)
-      break;
-    
-    Offset += Chain[I]->SizeInBits;
-  }
-    
-  UnreadMacroRecordOffsets[II] = Offset;
+  // Adjust the offset to a global offset.
+  UnreadMacroRecordOffsets[II] = F.GlobalBitOffset + Offset;
 }
 
 void ASTReader::ReadDefinedMacros() {
@@ -1807,24 +1800,11 @@
 void ASTReader::LoadMacroDefinition(
                      llvm::DenseMap<IdentifierInfo *, uint64_t>::iterator Pos) {
   assert(Pos != UnreadMacroRecordOffsets.end() && "Unknown macro definition");
-  PerFileData *F = 0;
   uint64_t Offset = Pos->second;
   UnreadMacroRecordOffsets.erase(Pos);
   
-  for (unsigned I = 0, N = Chain.size(); I != N; ++I) {
-    if (Offset < Chain[I]->SizeInBits) {
-      F = Chain[I];
-      break;
-    }
-    
-    Offset -= Chain[I]->SizeInBits;
-  }    
-  if (!F) {
-    Error("Malformed macro record offset");
-    return;
-  }
-  
-  ReadMacroRecord(*F, Offset);
+  RecordLocation Loc = getLocalBitOffset(Offset);
+  ReadMacroRecord(*Loc.F, Loc.Offset);
 }
 
 void ASTReader::LoadMacroDefinition(IdentifierInfo *II) {
@@ -2027,7 +2007,7 @@
         // If we have to ignore the dependency, we'll have to ignore this too.
       case IgnorePCH: return IgnorePCH;
       case Success: break;
-      }
+      }     
       break;
     }
 
@@ -2420,10 +2400,6 @@
                                             -getTotalNumCXXBaseSpecifiers())));
 
       NumCXXBaseSpecifiersLoaded += F.LocalNumCXXBaseSpecifiers;
-
-      F.GlobalBitOffset = TotalModulesSizeInBits;
-      TotalModulesSizeInBits += F.SizeInBits;
-
       break;
     }
 
@@ -2705,7 +2681,7 @@
   llvm::BitstreamCursor &Stream = F.Stream;
   Stream.init(F.StreamFile);
   F.SizeInBits = F.Buffer->getBufferSize() * 8;
-
+  
   // Sniff for the signature.
   if (Stream.Read(8) != 'C' ||
       Stream.Read(8) != 'P' ||
@@ -2764,7 +2740,12 @@
       break;
     }
   }
-
+  
+  // Once read, set the PerFileData bit base offset and update the size in 
+  // bits of all files we've seen.
+  F.GlobalBitOffset = TotalModulesSizeInBits;
+  TotalModulesSizeInBits += F.SizeInBits;
+  GlobalBitOffsetsMap.insert(std::make_pair(F.GlobalBitOffset, &F));
   return Success;
 }
 
@@ -3090,26 +3071,13 @@
 }
 
 PreprocessedEntity *ASTReader::ReadPreprocessedEntityAtOffset(uint64_t Offset) {
-  PerFileData *F = 0;  
-  for (unsigned I = 0, N = Chain.size(); I != N; ++I) {
-    if (Offset < Chain[I]->SizeInBits) {
-      F = Chain[I];
-      break;
-    }
-    
-    Offset -= Chain[I]->SizeInBits;
-  }
-  
-  if (!F) {
-    Error("Malformed preprocessed entity offset");
-    return 0;
-  }
+  RecordLocation Loc = getLocalBitOffset(Offset);
 
   // Keep track of where we are in the stream, then jump back there
   // after reading this entity.
-  SavedStreamPosition SavedPosition(F->PreprocessorDetailCursor);  
-  F->PreprocessorDetailCursor.JumpToBit(Offset);
-  return LoadPreprocessedEntity(*F);
+  SavedStreamPosition SavedPosition(Loc.F->PreprocessorDetailCursor);  
+  Loc.F->PreprocessorDetailCursor.JumpToBit(Loc.Offset);
+  return LoadPreprocessedEntity(*Loc.F);
 }
 
 HeaderFileInfo ASTReader::GetHeaderFileInfo(const FileEntry *FE) {
@@ -4006,25 +3974,10 @@
 }
 
 CXXBaseSpecifier *ASTReader::GetExternalCXXBaseSpecifiers(uint64_t Offset) {
-  // Figure out which AST file contains this offset.
-  PerFileData *F = 0;
-  for (unsigned I = 0, N = Chain.size(); I != N; ++I) {
-    if (Offset < Chain[N - I - 1]->SizeInBits) {
-      F = Chain[N - I - 1];
-      break;
-    }
-    
-    Offset -= Chain[N - I - 1]->SizeInBits;
-  }
-
-  if (!F) {
-    Error("Malformed AST file: C++ base specifiers at impossible offset");
-    return 0;
-  }
-  
-  llvm::BitstreamCursor &Cursor = F->DeclsCursor;
+  RecordLocation Loc = getLocalBitOffset(Offset);
+  llvm::BitstreamCursor &Cursor = Loc.F->DeclsCursor;
   SavedStreamPosition SavedPosition(Cursor);
-  Cursor.JumpToBit(Offset);
+  Cursor.JumpToBit(Loc.Offset);
   ReadingKindTracker ReadingKind(Read_Decl, *this);
   RecordData Record;
   unsigned Code = Cursor.ReadCode();
@@ -4039,7 +3992,7 @@
   void *Mem = Context->Allocate(sizeof(CXXBaseSpecifier) * NumBases);
   CXXBaseSpecifier *Bases = new (Mem) CXXBaseSpecifier [NumBases];
   for (unsigned I = 0; I != NumBases; ++I)
-    Bases[I] = ReadCXXBaseSpecifier(*F, Record, Idx);
+    Bases[I] = ReadCXXBaseSpecifier(*Loc.F, Record, Idx);
   return Bases;
 }
 
@@ -4099,17 +4052,9 @@
   ClearSwitchCaseIDs();
 
   // Offset here is a global offset across the entire chain.
-  for (unsigned I = 0, N = Chain.size(); I != N; ++I) {
-    PerFileData &F = *Chain[N - I - 1];
-    if (Offset < F.SizeInBits) {
-      // Since we know that this statement is part of a decl, make sure to use
-      // the decl cursor to read it.
-      F.DeclsCursor.JumpToBit(Offset);
-      return ReadStmtFromStream(F);
-    }
-    Offset -= F.SizeInBits;
-  }
-  llvm_unreachable("Broken chain");
+  RecordLocation Loc = getLocalBitOffset(Offset);
+  Loc.F->DeclsCursor.JumpToBit(Loc.Offset);
+  return ReadStmtFromStream(*Loc.F);
 }
 
 ExternalLoadResult ASTReader::FindExternalLexicalDecls(const DeclContext *DC,
@@ -4342,6 +4287,7 @@
                             
 void ASTReader::dump() {
   llvm::errs() << "*** AST File Remapping:\n";
+  dumpModuleIDMap("Global bit offset map", GlobalBitOffsetsMap);
   dumpModuleIDMap("Global source location entry map", GlobalSLocEntryMap);
   dumpModuleIDOffsetMap("Global type map", GlobalTypeMap);
   dumpModuleIDOffsetMap("Global declaration map", GlobalDeclMap);
diff --git a/lib/Serialization/ASTReaderDecl.cpp b/lib/Serialization/ASTReaderDecl.cpp
index 20b72ec..841c4d7 100644
--- a/lib/Serialization/ASTReaderDecl.cpp
+++ b/lib/Serialization/ASTReaderDecl.cpp
@@ -178,16 +178,7 @@
 }
 
 uint64_t ASTDeclReader::GetCurrentCursorOffset() {
-  uint64_t Off = 0;
-  for (unsigned I = 0, N = Reader.Chain.size(); I != N; ++I) {
-    ASTReader::PerFileData &F = *Reader.Chain[N - I - 1];
-    if (&Cursor == &F.DeclsCursor) {
-      Off += F.DeclsCursor.GetCurrentBitNo();
-      break;
-    }
-    Off += F.SizeInBits;
-  }
-  return Off;
+  return F.DeclsCursor.GetCurrentBitNo() + F.GlobalBitOffset;
 }
 
 void ASTDeclReader::Visit(Decl *D) {
@@ -1419,6 +1410,14 @@
                         I->second.first->DeclOffsets[Index + I->second.second]);
 }
 
+ASTReader::RecordLocation ASTReader::getLocalBitOffset(uint64_t GlobalOffset) {
+  ContinuousRangeMap<uint64_t, PerFileData*, 4>::iterator I
+    = GlobalBitOffsetsMap.find(GlobalOffset);
+
+  assert(I != GlobalBitOffsetsMap.end() && "Corrupted global bit offsets map");
+  return RecordLocation(I->second, GlobalOffset - I->second->GlobalBitOffset);
+}
+
 void ASTDeclReader::attachPreviousDecl(Decl *D, Decl *previous) {
   assert(D && previous);
   if (TagDecl *TD = dyn_cast<TagDecl>(D)) {
diff --git a/lib/Serialization/ASTWriter.cpp b/lib/Serialization/ASTWriter.cpp
index aaf4678..e6e88f8 100644
--- a/lib/Serialization/ASTWriter.cpp
+++ b/lib/Serialization/ASTWriter.cpp
@@ -1803,6 +1803,7 @@
   
   unsigned IndexBase = Chain ? PPRec.getNumLoadedPreprocessedEntities() : 0;
   RecordData Record;
+  uint64_t BitsInChain = Chain? Chain->TotalModulesSizeInBits : 0;
   for (PreprocessingRecord::iterator E = PPRec.begin(Chain),
                                   EEnd = PPRec.end(Chain);
        E != EEnd; ++E) {
@@ -1819,7 +1820,7 @@
       // Notify the serialization listener that we're serializing this entity.
       if (SerializationListener)
         SerializationListener->SerializedPreprocessedEntity(*E, 
-                                                    Stream.GetCurrentBitNo());
+          BitsInChain + Stream.GetCurrentBitNo());
 
       unsigned Position = ID - FirstMacroID;
       if (Position != MacroDefinitionOffsets.size()) {
@@ -1843,7 +1844,7 @@
     // Notify the serialization listener that we're serializing this entity.
     if (SerializationListener)
       SerializationListener->SerializedPreprocessedEntity(*E, 
-                                                    Stream.GetCurrentBitNo());
+        BitsInChain + Stream.GetCurrentBitNo());
 
     if (MacroExpansion *ME = dyn_cast<MacroExpansion>(*E)) {
       Record.push_back(IndexBase + NumPreprocessingRecords++);