diff --git a/clang/include/clang/Frontend/CompilerInstance.h b/clang/include/clang/Frontend/CompilerInstance.h
index fd6224bd3c3f..430cc603c78f 100644
--- a/clang/include/clang/Frontend/CompilerInstance.h
+++ b/clang/include/clang/Frontend/CompilerInstance.h
@@ -537,6 +537,7 @@ public:
   /// context.
   void createPCHExternalASTSource(llvm::StringRef Path,
                                   bool DisablePCHValidation,
+                                  bool DisableStatCache,
                                   void *DeserializationListener);
 
   /// Create an external AST source to read a PCH file.
@@ -545,6 +546,7 @@ public:
   static ExternalASTSource *
   createPCHExternalASTSource(llvm::StringRef Path, const std::string &Sysroot,
                              bool DisablePCHValidation,
+                             bool DisableStatCache,
                              Preprocessor &PP, ASTContext &Context,
                              void *DeserializationListener, bool Preamble);
 
diff --git a/clang/include/clang/Frontend/PreprocessorOptions.h b/clang/include/clang/Frontend/PreprocessorOptions.h
index 2a540b61df7f..0d52e53ea16a 100644
--- a/clang/include/clang/Frontend/PreprocessorOptions.h
+++ b/clang/include/clang/Frontend/PreprocessorOptions.h
@@ -48,6 +48,10 @@ public:
   /// precompiled headers.
   bool DisablePCHValidation;
 
+  /// \brief When true, disables the use of the stat cache within a
+  /// precompiled header or AST file.
+  bool DisableStatCache;
+
   /// \brief Dump declarations that are deserialized from PCH, for testing.
   bool DumpDeserializedPCHDecls;
 
@@ -125,7 +129,7 @@ public:
   
 public:
   PreprocessorOptions() : UsePredefines(true), DetailedRecord(false),
-                          DisablePCHValidation(false),
+                          DisablePCHValidation(false), DisableStatCache(false),
                           DumpDeserializedPCHDecls(false),
                           PrecompiledPreambleBytes(0, true),
                           RetainRemappedFileBuffers(false) { }
diff --git a/clang/include/clang/Serialization/ASTReader.h b/clang/include/clang/Serialization/ASTReader.h
index 38b0c57d221c..b9bf5f60b69e 100644
--- a/clang/include/clang/Serialization/ASTReader.h
+++ b/clang/include/clang/Serialization/ASTReader.h
@@ -590,6 +590,9 @@ private:
   /// headers when they are loaded.
   bool DisableValidation;
       
+  /// \brief Whether to disable the use of stat caches in AST files.
+  bool DisableStatCache;
+
   /// \brief Mapping from switch-case IDs in the chain to switch-case statements
   ///
   /// Statements usually don't have IDs, but switch cases need them, so that the
@@ -783,8 +786,14 @@ public:
   /// \param DisableValidation If true, the AST reader will suppress most
   /// of its regular consistency checking, allowing the use of precompiled
   /// headers that cannot be determined to be compatible.
+  ///
+  /// \param DisableStatCache If true, the AST reader will ignore the
+  /// stat cache in the AST files. This performance pessimization can
+  /// help when an AST file is being used in cases where the
+  /// underlying files in the file system may have changed, but
+  /// parsing should still continue.
   ASTReader(Preprocessor &PP, ASTContext *Context, const char *isysroot = 0,
-            bool DisableValidation = false);
+            bool DisableValidation = false, bool DisableStatCache = false);
 
   /// \brief Load the AST file without using any pre-initialized Preprocessor.
   ///
@@ -805,9 +814,15 @@ public:
   /// \param DisableValidation If true, the AST reader will suppress most
   /// of its regular consistency checking, allowing the use of precompiled
   /// headers that cannot be determined to be compatible.
+  ///
+  /// \param DisableStatCache If true, the AST reader will ignore the
+  /// stat cache in the AST files. This performance pessimization can
+  /// help when an AST file is being used in cases where the
+  /// underlying files in the file system may have changed, but
+  /// parsing should still continue.
   ASTReader(SourceManager &SourceMgr, FileManager &FileMgr,
             Diagnostic &Diags, const char *isysroot = 0,
-            bool DisableValidation = false);
+            bool DisableValidation = false, bool DisableStatCache = false);
   ~ASTReader();
 
   /// \brief Load the precompiled header designated by the given file
diff --git a/clang/lib/Basic/FileManager.cpp b/clang/lib/Basic/FileManager.cpp
index cbe90bfdc167..138b54cc0865 100644
--- a/clang/lib/Basic/FileManager.cpp
+++ b/clang/lib/Basic/FileManager.cpp
@@ -359,12 +359,43 @@ FileManager::getVirtualFile(llvm::StringRef Filename, off_t Size,
   NamedFileEnt.setValue(NON_EXISTENT_FILE);
 
   // We allow the directory to not exist. If it does exist we store it.
-  // 
+  FileEntry *UFE = 0;
   const DirectoryEntry *DirInfo = getDirectoryFromFile(*this, Filename);
+  if (DirInfo) {
+    // Check to see if the file exists. If so, drop the virtual file
+    int FileDescriptor = -1;
+    struct stat StatBuf;
+    const char *InterndFileName = NamedFileEnt.getKeyData();
+    if (getStatValue(InterndFileName, StatBuf, &FileDescriptor) == 0) {
+      // If the stat process opened the file, close it to avoid a FD leak.
+      if (FileDescriptor != -1)
+        close(FileDescriptor);
 
-  FileEntry *UFE = new FileEntry();
-  VirtualFileEntries.push_back(UFE);
-  NamedFileEnt.setValue(UFE);
+      StatBuf.st_size = Size;
+      StatBuf.st_mtime = ModificationTime;
+      UFE = &UniqueFiles.getFile(InterndFileName, StatBuf);
+
+      NamedFileEnt.setValue(UFE);
+
+      // If we had already opened this file, close it now so we don't
+      // leak the descriptor. We're not going to use the file
+      // descriptor anyway, since this is a virtual file.
+      if (UFE->FD != -1) {
+        close(UFE->FD); 
+        UFE->FD = -1;
+      }
+
+      // If we already have an entry with this inode, return it.
+      if (UFE->getName()) 
+        return UFE;
+    }
+  }
+
+  if (!UFE) {
+    UFE = new FileEntry();
+    VirtualFileEntries.push_back(UFE);
+    NamedFileEnt.setValue(UFE);
+  }
 
   // Get the null-terminated file name as stored as the key of the
   // FileEntries map.
@@ -375,23 +406,7 @@ FileManager::getVirtualFile(llvm::StringRef Filename, off_t Size,
   UFE->ModTime = ModificationTime;
   UFE->Dir     = DirInfo;
   UFE->UID     = NextFileUID++;
-  
-  // If this virtual file resolves to a file, also map that file to the 
-  // newly-created file entry.
-  int FileDescriptor = -1;
-  struct stat StatBuf;
-  if (getStatValue(InterndFileName, StatBuf, &FileDescriptor)) {
-    // If the stat process opened the file, close it to avoid a FD leak.
-    if (FileDescriptor != -1)
-      close(FileDescriptor);
-
-    return UFE;
-  }
-  
-  UFE->FD = FileDescriptor;
-  llvm::SmallString<128> FilePath(UFE->Name);
-  llvm::sys::fs::make_absolute(FilePath);
-  FileEntries[FilePath] = UFE;
+  UFE->FD      = -1;
   return UFE;
 }
 
diff --git a/clang/lib/Frontend/ASTUnit.cpp b/clang/lib/Frontend/ASTUnit.cpp
index 1db81cb13e8a..1138cd7db0dd 100644
--- a/clang/lib/Frontend/ASTUnit.cpp
+++ b/clang/lib/Frontend/ASTUnit.cpp
@@ -1580,6 +1580,7 @@ bool ASTUnit::Reparse(RemappedFile *RemappedFiles, unsigned NumRemappedFiles) {
 
   // Remap files.
   PreprocessorOptions &PPOpts = Invocation->getPreprocessorOpts();
+  PPOpts.DisableStatCache = true;
   for (PreprocessorOptions::remapped_file_buffer_iterator 
          R = PPOpts.remapped_file_buffer_begin(),
          REnd = PPOpts.remapped_file_buffer_end();
@@ -1948,6 +1949,7 @@ void ASTUnit::CodeComplete(llvm::StringRef File, unsigned Line, unsigned Column,
 
   // If the main file has been overridden due to the use of a preamble,
   // make that override happen and introduce the preamble.
+  PreprocessorOpts.DisableStatCache = true;
   StoredDiagnostics.insert(StoredDiagnostics.end(),
                            this->StoredDiagnostics.begin(),
              this->StoredDiagnostics.begin() + NumStoredDiagnosticsFromDriver);
diff --git a/clang/lib/Frontend/CompilerInstance.cpp b/clang/lib/Frontend/CompilerInstance.cpp
index 92aed39b5c34..412e7111e480 100644
--- a/clang/lib/Frontend/CompilerInstance.cpp
+++ b/clang/lib/Frontend/CompilerInstance.cpp
@@ -234,11 +234,13 @@ void CompilerInstance::createASTContext() {
 
 void CompilerInstance::createPCHExternalASTSource(llvm::StringRef Path,
                                                   bool DisablePCHValidation,
+                                                  bool DisableStatCache,
                                                  void *DeserializationListener){
   llvm::OwningPtr<ExternalASTSource> Source;
   bool Preamble = getPreprocessorOpts().PrecompiledPreambleBytes.first != 0;
   Source.reset(createPCHExternalASTSource(Path, getHeaderSearchOpts().Sysroot,
-                                          DisablePCHValidation,
+                                          DisablePCHValidation, 
+                                          DisableStatCache,
                                           getPreprocessor(), getASTContext(),
                                           DeserializationListener,
                                           Preamble));
@@ -249,6 +251,7 @@ ExternalASTSource *
 CompilerInstance::createPCHExternalASTSource(llvm::StringRef Path,
                                              const std::string &Sysroot,
                                              bool DisablePCHValidation,
+                                             bool DisableStatCache,
                                              Preprocessor &PP,
                                              ASTContext &Context,
                                              void *DeserializationListener,
@@ -256,7 +259,7 @@ CompilerInstance::createPCHExternalASTSource(llvm::StringRef Path,
   llvm::OwningPtr<ASTReader> Reader;
   Reader.reset(new ASTReader(PP, &Context,
                              Sysroot.empty() ? 0 : Sysroot.c_str(),
-                             DisablePCHValidation));
+                             DisablePCHValidation, DisableStatCache));
 
   Reader->setDeserializationListener(
             static_cast<ASTDeserializationListener *>(DeserializationListener));
diff --git a/clang/lib/Frontend/FrontendAction.cpp b/clang/lib/Frontend/FrontendAction.cpp
index 5f78fb17723d..af23923002dc 100644
--- a/clang/lib/Frontend/FrontendAction.cpp
+++ b/clang/lib/Frontend/FrontendAction.cpp
@@ -224,6 +224,7 @@ bool FrontendAction::BeginSourceFile(CompilerInstance &CI,
       CI.createPCHExternalASTSource(
                                 CI.getPreprocessorOpts().ImplicitPCHInclude,
                                 CI.getPreprocessorOpts().DisablePCHValidation,
+                                CI.getPreprocessorOpts().DisableStatCache,
                                 DeserialListener);
       if (!CI.getASTContext().getExternalSource())
         goto failure;
diff --git a/clang/lib/Sema/SemaType.cpp b/clang/lib/Sema/SemaType.cpp
index a1908126efcb..9206b1bed6ea 100644
--- a/clang/lib/Sema/SemaType.cpp
+++ b/clang/lib/Sema/SemaType.cpp
@@ -505,8 +505,6 @@ static void maybeSynthesizeBlockSignature(TypeProcessingState &state,
         break;
       }
     }
-    assert(!isOnlyParens &&
-           "non-empty abstract-declarator contained only parens!");
   }
 #endif
 
diff --git a/clang/lib/Serialization/ASTReader.cpp b/clang/lib/Serialization/ASTReader.cpp
index 68e2e79dde4a..d2d20bfdbdf8 100644
--- a/clang/lib/Serialization/ASTReader.cpp
+++ b/clang/lib/Serialization/ASTReader.cpp
@@ -2034,12 +2034,14 @@ ASTReader::ReadASTBlock(PerFileData &F) {
       break;
 
     case STAT_CACHE: {
-      ASTStatCache *MyStatCache =
-        new ASTStatCache((const unsigned char *)BlobStart + Record[0],
-                         (const unsigned char *)BlobStart,
-                         NumStatHits, NumStatMisses);
-      FileMgr.addStatCache(MyStatCache);
-      F.StatCache = MyStatCache;
+      if (!DisableStatCache) {
+        ASTStatCache *MyStatCache =
+          new ASTStatCache((const unsigned char *)BlobStart + Record[0],
+                           (const unsigned char *)BlobStart,
+                           NumStatHits, NumStatMisses);
+        FileMgr.addStatCache(MyStatCache);
+        F.StatCache = MyStatCache;
+      }
       break;
     }
 
@@ -4668,28 +4670,32 @@ void ASTReader::FinishedDeserializing() {
 }
 
 ASTReader::ASTReader(Preprocessor &PP, ASTContext *Context,
-                     const char *isysroot, bool DisableValidation)
+                     const char *isysroot, bool DisableValidation,
+                     bool DisableStatCache)
   : Listener(new PCHValidator(PP, *this)), DeserializationListener(0),
     SourceMgr(PP.getSourceManager()), FileMgr(PP.getFileManager()),
     Diags(PP.getDiagnostics()), SemaObj(0), PP(&PP), Context(Context),
     Consumer(0), isysroot(isysroot), DisableValidation(DisableValidation),
-    NumStatHits(0), NumStatMisses(0), NumSLocEntriesRead(0),
-    TotalNumSLocEntries(0), NextSLocOffset(0), NumStatementsRead(0),
-    TotalNumStatements(0), NumMacrosRead(0), TotalNumMacros(0),
-    NumSelectorsRead(0), NumMethodPoolEntriesRead(0), NumMethodPoolMisses(0),
-    TotalNumMethodPoolEntries(0), NumLexicalDeclContextsRead(0),
-    TotalLexicalDeclContexts(0), NumVisibleDeclContextsRead(0),
-    TotalVisibleDeclContexts(0), NumCurrentElementsDeserializing(0) {
+    DisableStatCache(DisableStatCache), NumStatHits(0), NumStatMisses(0), 
+    NumSLocEntriesRead(0), TotalNumSLocEntries(0), NextSLocOffset(0), 
+    NumStatementsRead(0), TotalNumStatements(0), NumMacrosRead(0), 
+    TotalNumMacros(0), NumSelectorsRead(0), NumMethodPoolEntriesRead(0), 
+    NumMethodPoolMisses(0), TotalNumMethodPoolEntries(0), 
+    NumLexicalDeclContextsRead(0), TotalLexicalDeclContexts(0), 
+    NumVisibleDeclContextsRead(0), TotalVisibleDeclContexts(0), 
+    NumCurrentElementsDeserializing(0) 
+{
   RelocatablePCH = false;
 }
 
 ASTReader::ASTReader(SourceManager &SourceMgr, FileManager &FileMgr,
                      Diagnostic &Diags, const char *isysroot,
-                     bool DisableValidation)
+                     bool DisableValidation, bool DisableStatCache)
   : DeserializationListener(0), SourceMgr(SourceMgr), FileMgr(FileMgr),
     Diags(Diags), SemaObj(0), PP(0), Context(0), Consumer(0),
-    isysroot(isysroot), DisableValidation(DisableValidation), NumStatHits(0),
-    NumStatMisses(0), NumSLocEntriesRead(0), TotalNumSLocEntries(0),
+    isysroot(isysroot), DisableValidation(DisableValidation), 
+    DisableStatCache(DisableStatCache), NumStatHits(0), NumStatMisses(0), 
+    NumSLocEntriesRead(0), TotalNumSLocEntries(0),
     NextSLocOffset(0), NumStatementsRead(0), TotalNumStatements(0),
     NumMacrosRead(0), TotalNumMacros(0), NumSelectorsRead(0),
     NumMethodPoolEntriesRead(0), NumMethodPoolMisses(0),