Index: include/clang/Frontend/ASTUnit.h
===================================================================
--- include/clang/Frontend/ASTUnit.h
+++ include/clang/Frontend/ASTUnit.h
@@ -185,6 +185,12 @@
   /// some number of calls.
   unsigned PreambleRebuildCounter;
 
+  /// \brief Cache pairs "filename - source location"
+  ///
+  /// This cache is used when loading preambule to increase performance
+  /// of that loading
+  std::map<const std::string, SourceLocation> SrcLocCache;
+
 public:
   class PreambleData {
     const FileEntry *File;
Index: lib/Frontend/ASTUnit.cpp
===================================================================
--- lib/Frontend/ASTUnit.cpp
+++ lib/Frontend/ASTUnit.cpp
@@ -470,7 +470,7 @@
   }
   
   // Save the current top-level hash value.
-  CompletionCacheTopLevelHashValue = CurrentTopLevelHashValue;
+  CompletionCacheTopLevelHashValue = PreambleTopLevelHashValue;
 }
 
 void ASTUnit::ClearCachedCompletionResults() {
@@ -1148,6 +1148,8 @@
   if (SavedMainFileBuffer)
     TranslateStoredDiagnostics(getFileManager(), getSourceManager(),
                                PreambleDiagnostics, StoredDiagnostics);
+  else
+    SrcLocCache.clear();
 
   if (!Act->Execute())
     goto error;
@@ -1506,6 +1508,8 @@
   SimpleTimer PreambleTimer(WantTiming);
   PreambleTimer.setOutput("Precompiling preamble");
 
+  CompletionCacheTopLevelHashValue = 0;
+
   // Save the preamble text for later; we'll need to compare against it for
   // subsequent reparses.
   StringRef MainFilename = FrontendOpts.Inputs[0].getFile();
@@ -1656,7 +1660,6 @@
   // entities the last time we rebuilt the preamble, clear out the completion
   // cache.
   if (CurrentTopLevelHashValue != PreambleTopLevelHashValue) {
-    CompletionCacheTopLevelHashValue = 0;
     PreambleTopLevelHashValue = CurrentTopLevelHashValue;
   }
 
@@ -2105,7 +2108,7 @@
   // If we're caching global code-completion results, and the top-level 
   // declarations have changed, clear out the code-completion cache.
   if (!Result && ShouldCacheCodeCompletionResults &&
-      CurrentTopLevelHashValue != CompletionCacheTopLevelHashValue)
+      PreambleTopLevelHashValue != CompletionCacheTopLevelHashValue)
     CacheCodeCompletionResults();
 
   // We now need to clear out the completion info related to this translation
@@ -2579,8 +2582,7 @@
 
   SmallVector<StoredDiagnostic, 4> Result;
   Result.reserve(Diags.size());
-  const FileEntry *PreviousFE = nullptr;
-  FileID FID;
+
   for (const StandaloneDiagnostic &SD : Diags) {
     // Rebuild the StoredDiagnostic.
     if (SD.Filename.empty())
@@ -2588,11 +2590,17 @@
     const FileEntry *FE = FileMgr.getFile(SD.Filename);
     if (!FE)
       continue;
-    if (FE != PreviousFE) {
+    FileID FID;
+    SourceLocation FileLoc;
+    auto ItFileID = SrcLocCache.find(SD.Filename);
+    if (ItFileID == SrcLocCache.end()) {
       FID = SrcMgr.translateFile(FE);
-      PreviousFE = FE;
+      FileLoc = SrcMgr.getLocForStartOfFile(FID);
+      SrcLocCache.insert(std::make_pair(SD.Filename, FileLoc));
+    } else {
+      FileLoc = ItFileID->second;
     }
-    SourceLocation FileLoc = SrcMgr.getLocForStartOfFile(FID);
+
     if (FileLoc.isInvalid())
       continue;
     SourceLocation L = FileLoc.getLocWithOffset(SD.LocOffset);