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;
@@ -1391,24 +1391,38 @@
           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 (llvm::StringMap<PreambleFileHash>::iterator
              F = FilesInPreamble.begin(), FEnd = FilesInPreamble.end();
            !AnyFileChanged && F != FEnd; 
            ++F) {
-        llvm::StringMap<PreambleFileHash>::iterator Overridden
-          = OverriddenFiles.find(F->first());
+        vfs::Status Status;
+        if (FileMgr->getNoncachedStatValue(F->first(), Status)) {
+          // If we can't stat the file, assume that something horrible happened.
+          AnyFileChanged = true;
+          break;
+        }
+
+        std::map<llvm::sys::fs::UniqueID, PreambleFileHash>::iterator Overridden
+          = OverriddenFiles.find(Status.getUniqueID());
         if (Overridden != OverriddenFiles.end()) {
           // This file was remapped; check whether the newly-mapped file 
           // matches up with the previous mapping.
@@ -1418,11 +1432,7 @@
         }
         
         // The file was not remapped; check whether it has changed on disk.
-        vfs::Status Status;
-        if (FileMgr->getNoncachedStatValue(F->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) ||
+        if (Status.getSize() != uint64_t(F->second.Size) ||
                    Status.getLastModificationTime().toEpochTime() !=
                        uint64_t(F->second.ModTime))
           AnyFileChanged = true;