Index: include/clang/Frontend/ASTUnit.h
===================================================================
--- include/clang/Frontend/ASTUnit.h
+++ include/clang/Frontend/ASTUnit.h
@@ -190,7 +190,10 @@
   /// we'll attempt to rebuild the precompiled header. This way, if
   /// building the precompiled preamble fails, we won't try again for
   /// some number of calls.
-  unsigned PreambleRebuildCounter;
+  unsigned PreambleRebuildCountdownCounter;
+
+  /// \brief Counter indicating how often the preamble was build in total.
+  unsigned PreambleCounter;
 
   /// \brief Cache pairs "filename - source location"
   ///
@@ -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
@@ -189,7 +189,8 @@
     TUKind(TU_Complete), WantTiming(getenv("LIBCLANG_TIMING")),
     OwnsRemappedFileBuffers(true),
     NumStoredDiagnosticsFromDriver(0),
-    PreambleRebuildCounter(0),
+    PreambleRebuildCountdownCounter(0),
+    PreambleCounter(0),
     NumWarningsInPreamble(0),
     ShouldCacheCodeCompletionResults(false),
     IncludeBriefCommentsInCodeCompletion(false), UserFilesAreVolatile(false),
@@ -1253,21 +1254,21 @@
                             PreambleInvocationIn.getDiagnosticOpts());
       getDiagnostics().setNumWarnings(NumWarningsInPreamble);
 
-      PreambleRebuildCounter = 1;
+      PreambleRebuildCountdownCounter = 1;
       return MainFileBuffer;
     } else {
       Preamble.reset();
       PreambleDiagnostics.clear();
       TopLevelDeclsInPreamble.clear();
-      PreambleRebuildCounter = 1;
+      PreambleRebuildCountdownCounter = 1;
     }
   }
 
   // If the preamble rebuild counter > 1, it's because we previously
   // failed to build a preamble and we're not yet ready to try
   // again. Decrement the counter and return a failure.
-  if (PreambleRebuildCounter > 1) {
-    --PreambleRebuildCounter;
+  if (PreambleRebuildCountdownCounter > 1) {
+    --PreambleRebuildCountdownCounter;
     return nullptr;
   }
 
@@ -1295,20 +1296,21 @@
         PCHContainerOps, /*StoreInMemory=*/false, Callbacks);
     if (NewPreamble) {
       Preamble = std::move(*NewPreamble);
-      PreambleRebuildCounter = 1;
+      ++PreambleCounter;
+      PreambleRebuildCountdownCounter = 1;
     } else {
       switch (static_cast<BuildPreambleError>(NewPreamble.getError().value())) {
       case BuildPreambleError::CouldntCreateTempFile:
       case BuildPreambleError::PreambleIsEmpty:
         // Try again next time.
-        PreambleRebuildCounter = 1;
+        PreambleRebuildCountdownCounter = 1;
         return nullptr;
       case BuildPreambleError::CouldntCreateTargetInfo:
       case BuildPreambleError::BeginSourceFileFailed:
       case BuildPreambleError::CouldntEmitPCH:
       case BuildPreambleError::CouldntCreateVFSOverlay:
         // These erros are more likely to repeat, retry after some period.
-        PreambleRebuildCounter = DefaultPreambleRebuildInterval;
+        PreambleRebuildCountdownCounter = DefaultPreambleRebuildInterval;
         return nullptr;
       }
       llvm_unreachable("unexpected BuildPreambleError");
@@ -1450,7 +1452,7 @@
   AST->OnlyLocalDecls = OnlyLocalDecls;
   AST->CaptureDiagnostics = CaptureDiagnostics;
   if (PrecompilePreambleAfterNParses > 0)
-    AST->PreambleRebuildCounter = PrecompilePreambleAfterNParses;
+    AST->PreambleRebuildCountdownCounter = PrecompilePreambleAfterNParses;
   AST->TUKind = Action ? Action->getTranslationUnitKind() : TU_Complete;
   AST->ShouldCacheCodeCompletionResults = CacheCodeCompletionResults;
   AST->IncludeBriefCommentsInCodeCompletion
@@ -1584,7 +1586,7 @@
 
   std::unique_ptr<llvm::MemoryBuffer> OverrideMainBuffer;
   if (PrecompilePreambleAfterNParses > 0) {
-    PreambleRebuildCounter = PrecompilePreambleAfterNParses;
+    PreambleRebuildCountdownCounter = PrecompilePreambleAfterNParses;
     OverrideMainBuffer =
         getMainBufferWithPrecompiledPreamble(PCHContainerOps, *Invocation, VFS);
     getDiagnostics().Reset();
@@ -1763,7 +1765,7 @@
   // If we have a preamble file lying around, or if we might try to
   // build a precompiled preamble, do so now.
   std::unique_ptr<llvm::MemoryBuffer> OverrideMainBuffer;
-  if (Preamble || PreambleRebuildCounter > 0)
+  if (Preamble || PreambleRebuildCountdownCounter > 0)
     OverrideMainBuffer =
         getMainBufferWithPrecompiledPreamble(PCHContainerOps, *Invocation, VFS);
 
Index: lib/Frontend/PrecompiledPreamble.cpp
===================================================================
--- lib/Frontend/PrecompiledPreamble.cpp
+++ lib/Frontend/PrecompiledPreamble.cpp
@@ -446,21 +446,28 @@
         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 =
@@ -473,9 +480,10 @@
       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";