Index: cfe/trunk/include/clang/Frontend/ASTUnit.h
===================================================================
--- cfe/trunk/include/clang/Frontend/ASTUnit.h
+++ cfe/trunk/include/clang/Frontend/ASTUnit.h
@@ -188,6 +188,14 @@
   /// some number of calls.
   unsigned PreambleRebuildCounter;
 
+  /// \brief Cache pairs "filename - source location"
+  ///
+  /// Cache contains only source locations from preamble so it is
+  /// guaranteed that they stay valid when the SourceManager is recreated.
+  /// This cache is used when loading preambule to increase performance
+  /// of that loading. It must be cleared when preamble is recreated.
+  llvm::StringMap<SourceLocation> PreambleSrcLocCache;
+
 public:
   class PreambleData {
     const FileEntry *File;
Index: cfe/trunk/lib/Frontend/ASTUnit.cpp
===================================================================
--- cfe/trunk/lib/Frontend/ASTUnit.cpp
+++ cfe/trunk/lib/Frontend/ASTUnit.cpp
@@ -1159,6 +1159,8 @@
   if (SavedMainFileBuffer)
     TranslateStoredDiagnostics(getFileManager(), getSourceManager(),
                                PreambleDiagnostics, StoredDiagnostics);
+  else
+    PreambleSrcLocCache.clear();
 
   if (!Act->Execute())
     goto error;
@@ -2602,11 +2604,9 @@
   // remap all the locations to the new view. This includes the diag location,
   // any associated source ranges, and the source ranges of associated fix-its.
   // FIXME: There should be a cleaner way to do this.
-
   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())
@@ -2614,11 +2614,16 @@
     const FileEntry *FE = FileMgr.getFile(SD.Filename);
     if (!FE)
       continue;
-    if (FE != PreviousFE) {
-      FID = SrcMgr.translateFile(FE);
-      PreviousFE = FE;
+    SourceLocation FileLoc;
+    auto ItFileID = PreambleSrcLocCache.find(SD.Filename);
+    if (ItFileID == PreambleSrcLocCache.end()) {
+      FileID FID = SrcMgr.translateFile(FE);
+      FileLoc = SrcMgr.getLocForStartOfFile(FID);
+      PreambleSrcLocCache[SD.Filename] = FileLoc;
+    } else {
+      FileLoc = ItFileID->getValue();
     }
-    SourceLocation FileLoc = SrcMgr.getLocForStartOfFile(FID);
+
     if (FileLoc.isInvalid())
       continue;
     SourceLocation L = FileLoc.getLocWithOffset(SD.LocOffset);