Index: include/clang/Frontend/ASTUnit.h
===================================================================
--- include/clang/Frontend/ASTUnit.h
+++ include/clang/Frontend/ASTUnit.h
@@ -192,6 +192,9 @@
   /// some number of calls.
   unsigned PreambleRebuildCounter;
 
+  /// \brief Counter indicating how often the preamble was build in total.
+  unsigned PreambleCounter;
+
   /// \brief Cache pairs "filename - source location"
   ///
   /// Cache contains only source locations from preamble so it is
@@ -547,7 +550,11 @@
     return SourceRange(mapLocationToPreamble(R.getBegin()),
                        mapLocationToPreamble(R.getEnd()));
   }
-  
+
+  unsigned getPreambleCounter() const {
+    return PreambleCounter;
+  }
+
   // Retrieve the diagnostics associated with this AST
   typedef StoredDiagnostic *stored_diag_iterator;
   typedef const StoredDiagnostic *stored_diag_const_iterator;
Index: lib/Frontend/ASTUnit.cpp
===================================================================
--- lib/Frontend/ASTUnit.cpp
+++ lib/Frontend/ASTUnit.cpp
@@ -190,6 +190,7 @@
     OwnsRemappedFileBuffers(true),
     NumStoredDiagnosticsFromDriver(0),
     PreambleRebuildCounter(0),
+    PreambleCounter(0),
     NumWarningsInPreamble(0),
     ShouldCacheCodeCompletionResults(false),
     IncludeBriefCommentsInCodeCompletion(false), UserFilesAreVolatile(false),
@@ -1296,6 +1297,7 @@
         PCHContainerOps, /*StoreInMemory=*/false, Callbacks);
     if (NewPreamble) {
       Preamble = std::move(*NewPreamble);
+      ++PreambleCounter;
       PreambleRebuildCounter = 1;
     } else {
       switch (static_cast<BuildPreambleError>(NewPreamble.getError().value())) {
Index: lib/Frontend/PrecompiledPreamble.cpp
===================================================================
--- lib/Frontend/PrecompiledPreamble.cpp
+++ lib/Frontend/PrecompiledPreamble.cpp
@@ -434,21 +434,28 @@
         Status.getSize(), llvm::sys::toTimeT(Status.getLastModificationTime()));
   }
 
+  llvm::StringMap<PreambleFileHash> OverridenFileBuffers;
   for (const auto &RB : PreprocessorOpts.RemappedFileBuffers) {
-    vfs::Status Status;
-    if (!moveOnNoError(VFS->status(RB.first), Status))
-      return false;
-
-    OverriddenFiles[Status.getUniqueID()] =
+    OverridenFileBuffers[RB.first] =
         PreambleFileHash::createForMemoryBuffer(RB.second);
   }
 
   // Check whether anything has changed.
   for (const auto &F : FilesInPreamble) {
+    auto OverridenFileBuffer = OverridenFileBuffers.find(F.first());
+    if (OverridenFileBuffer != OverridenFileBuffers.end()) {
+      // The file's buffer was remapped; check whether it matches up
+      // with the previous mapping.
+      if (OverridenFileBuffer->second != F.second)
+        return false;
+      continue;
+    }
+
     vfs::Status Status;
     if (!moveOnNoError(VFS->status(F.first()), Status)) {
-      // If we can't stat the file, assume that something horrible happened.
-      return false;
+        // If the file's buffer is not remapped and we can't stat it,
+        // assume that something horrible happened.
+        return false;
     }
 
     std::map<llvm::sys::fs::UniqueID, PreambleFileHash>::iterator Overridden =
@@ -461,9 +468,10 @@
       continue;
     }
 
-    // The file was not remapped; check whether it has changed on disk.
+    // Neither the file's buffer nor the file itself was remapped;
+    // check whether it has changed on disk.
     if (Status.getSize() != uint64_t(F.second.Size) ||
-        llvm::sys::toTimeT(Status.getLastModificationTime()) !=
+         llvm::sys::toTimeT(Status.getLastModificationTime()) !=
             F.second.ModTime)
       return false;
   }
Index: unittests/Frontend/PCHPreambleTest.cpp
===================================================================
--- unittests/Frontend/PCHPreambleTest.cpp
+++ unittests/Frontend/PCHPreambleTest.cpp
@@ -124,6 +124,22 @@
   }
 };
 
+TEST_F(PCHPreambleTest, ReparseDoesNotInvalidatePreambleDueToNotExistingUnsavedFile) {
+  std::string Header1 = "//./header1.h";
+  std::string MainName = "//./main.cpp";
+  AddFile(MainName, "#include \"//./header1.h\"\n"
+                    "int main() { return ZERO; }");
+
+  std::unique_ptr<ASTUnit> AST(ParseAST(MainName));
+  ASSERT_TRUE(AST.get());
+  ASSERT_EQ(AST->getPreambleCounter(), 1U);
+
+  RemapFile(Header1, "#define ZERO 0\n");
+  ASSERT_TRUE(ReparseAST(AST));
+
+  ASSERT_EQ(AST->getPreambleCounter(), 1U);
+}
+
 TEST_F(PCHPreambleTest, ReparseWithOverriddenFileDoesNotInvalidatePreamble) {
   std::string Header1 = "//./header1.h";
   std::string Header2 = "//./header2.h";