Index: include/clang/Frontend/ASTUnit.h
===================================================================
--- include/clang/Frontend/ASTUnit.h
+++ include/clang/Frontend/ASTUnit.h
@@ -31,6 +31,7 @@
 #include "llvm/ADT/StringMap.h"
 #include "llvm/Support/MD5.h"
 #include "llvm/Support/Path.h"
+#include "llvm/Support/FileSystem.h"
 #include <cassert>
 #include <map>
 #include <memory>
@@ -264,12 +265,15 @@
   /// a line after skipping the preamble.
   bool PreambleEndsAtStartOfLine;
 
+  typedef std::map<llvm::sys::fs::UniqueID,
+      std::pair<std::string, PreambleFileHash>> FilesInPreambleMap;
+
   /// \brief Keeps track of the files that were used when computing the 
   /// preamble, with both their buffer size and their modification time.
   ///
   /// If any of the files have changed from one compile to the next,
   /// the preamble must be thrown away.
-  llvm::StringMap<PreambleFileHash> FilesInPreamble;
+  FilesInPreambleMap FilesInPreamble;
 
   /// \brief When non-NULL, this is the buffer used to store the contents of
   /// the main file when it has been padded for use with the precompiled
Index: lib/Frontend/ASTUnit.cpp
===================================================================
--- lib/Frontend/ASTUnit.cpp
+++ lib/Frontend/ASTUnit.cpp
@@ -1378,7 +1378,7 @@
           
       // First, make a record of those files that have been overridden via
       // remapping or unsaved_files.
-      llvm::StringMap<PreambleFileHash> OverriddenFiles;
+      std::map<llvm::sys::fs::UniqueID, PreambleFileHash> OverriddenFiles;
       for (const auto &R : PreprocessorOpts.RemappedFiles) {
         if (AnyFileChanged)
           break;
@@ -1391,40 +1391,47 @@
           break;
         }
 
-        OverriddenFiles[R.first] = PreambleFileHash::createForFile(
+        OverriddenFiles[Status.getUniqueID()] = PreambleFileHash::createForFile(
             Status.getSize(), Status.getLastModificationTime().toEpochTime());
       }
 
       for (const auto &RB : PreprocessorOpts.RemappedFileBuffers) {
         if (AnyFileChanged)
           break;
-        OverriddenFiles[RB.first] =
+
+        vfs::Status Status;
+        if (FileMgr->getNoncachedStatValue(RB.first, Status)) {
+          AnyFileChanged = true;
+          break;
+        }
+
+        OverriddenFiles[Status.getUniqueID()] =
             PreambleFileHash::createForMemoryBuffer(RB.second);
       }
        
       // Check whether anything has changed.
-      for (llvm::StringMap<PreambleFileHash>::iterator 
+      for (FilesInPreambleMap::iterator
              F = FilesInPreamble.begin(), FEnd = FilesInPreamble.end();
            !AnyFileChanged && F != FEnd; 
            ++F) {
-        llvm::StringMap<PreambleFileHash>::iterator Overridden
-          = OverriddenFiles.find(F->first());
+        std::map<llvm::sys::fs::UniqueID, PreambleFileHash>::iterator Overridden
+          = OverriddenFiles.find(F->first);
         if (Overridden != OverriddenFiles.end()) {
           // This file was remapped; check whether the newly-mapped file 
           // matches up with the previous mapping.
-          if (Overridden->second != F->second)
+          if (Overridden->second != F->second.second)
             AnyFileChanged = true;
           continue;
         }
         
         // The file was not remapped; check whether it has changed on disk.
         vfs::Status Status;
-        if (FileMgr->getNoncachedStatValue(F->first(), Status)) {
+        if (FileMgr->getNoncachedStatValue(F->second.first, Status)) {
           // If we can't stat the file, assume that something horrible happened.
           AnyFileChanged = true;
-        } else if (Status.getSize() != uint64_t(F->second.Size) ||
+        } else if (Status.getSize() != uint64_t(F->second.second.Size) ||
                    Status.getLastModificationTime().toEpochTime() !=
-                       uint64_t(F->second.ModTime))
+                       uint64_t(F->second.second.ModTime))
           AnyFileChanged = true;
       }
           
@@ -1612,12 +1619,14 @@
     if (!File || File == SourceMgr.getFileEntryForID(SourceMgr.getMainFileID()))
       continue;
     if (time_t ModTime = File->getModificationTime()) {
-      FilesInPreamble[File->getName()] = PreambleFileHash::createForFile(
-          File->getSize(), ModTime);
+      FilesInPreamble[File->getUniqueID()] = std::make_pair(
+        File->getName(),
+        PreambleFileHash::createForFile(File->getSize(), ModTime));
     } else {
       llvm::MemoryBuffer *Buffer = SourceMgr.getMemoryBufferForFile(File);
-      FilesInPreamble[File->getName()] =
-          PreambleFileHash::createForMemoryBuffer(Buffer);
+      FilesInPreamble[File->getUniqueID()] = std::make_pair(
+        File->getName(),
+        PreambleFileHash::createForMemoryBuffer(Buffer));
     }
   }