Index: include/clang/Basic/Diagnostic.h
===================================================================
--- include/clang/Basic/Diagnostic.h
+++ include/clang/Basic/Diagnostic.h
@@ -230,7 +230,10 @@
   };
 
   /// \brief Keeps and automatically disposes all DiagStates that we create.
-  std::list<DiagState> DiagStates;
+  ///
+  /// The first DiagState represents the state set by the command line. But it
+  /// also contains a cache for the defaults that is lazily populated.
+  mutable std::list<DiagState> DiagStates;
 
   /// \brief Represents a point in source where the diagnostic state was
   /// modified because of a pragma.
Index: include/clang/Basic/DiagnosticIDs.h
===================================================================
--- include/clang/Basic/DiagnosticIDs.h
+++ include/clang/Basic/DiagnosticIDs.h
@@ -84,6 +84,7 @@
   unsigned IsPragma : 1;
   unsigned HasNoWarningAsError : 1;
   unsigned HasNoErrorAsFatal : 1;
+  unsigned MightBeOverriden : 1;
 
 public:
   static DiagnosticMapping Make(diag::Severity Severity, bool IsUser,
@@ -94,6 +95,8 @@
     Result.IsPragma = IsPragma;
     Result.HasNoWarningAsError = 0;
     Result.HasNoErrorAsFatal = 0;
+    Result.MightBeOverriden = 0;
+
     return Result;
   }
 
@@ -108,6 +111,11 @@
 
   bool hasNoErrorAsFatal() const { return HasNoErrorAsFatal; }
   void setNoErrorAsFatal(bool Value) { HasNoErrorAsFatal = Value; }
+
+  /// Only makes sense for the base state: specify if the diagnostic might be
+  /// changed for other source location
+  bool mightBeOverriden() const { return MightBeOverriden; }
+  void setMightBeOverriden(bool Value) { MightBeOverriden = Value; }
 };
 
 /// \brief Used for handling and querying diagnostic IDs.
@@ -261,6 +269,10 @@
   getDiagnosticLevel(unsigned DiagID, SourceLocation Loc,
                      const DiagnosticsEngine &Diag) const LLVM_READONLY;
 
+  DiagnosticMapping &
+  getDiagnosticMapping(unsigned DiagID, SourceLocation Loc,
+                       const DiagnosticsEngine &Diag) const LLVM_READONLY;
+
   diag::Severity
   getDiagnosticSeverity(unsigned DiagID, SourceLocation Loc,
                         const DiagnosticsEngine &Diag) const LLVM_READONLY;
Index: lib/Basic/Diagnostic.cpp
===================================================================
--- lib/Basic/Diagnostic.cpp
+++ lib/Basic/Diagnostic.cpp
@@ -201,6 +201,11 @@
   }
   DiagnosticMapping Mapping = makeUserMapping(Map, L);
 
+  if (Loc.isValid()) {
+    // We are potentially overriding a default behavior
+    DiagStates.front().getOrAddMapping(Diag).setMightBeOverriden(true);
+  }
+
   // Common case; setting all the diagnostics of a group in one place.
   if (Loc.isInvalid() || Loc == LastStateChangePos) {
     GetCurDiagState()->setMapping(Diag, Mapping);
Index: lib/Basic/DiagnosticIDs.cpp
===================================================================
--- lib/Basic/DiagnosticIDs.cpp
+++ lib/Basic/DiagnosticIDs.cpp
@@ -396,6 +396,23 @@
   return toLevel(getDiagnosticSeverity(DiagID, Loc, Diag));
 }
 
+DiagnosticMapping &
+DiagnosticIDs::getDiagnosticMapping(unsigned DiagID, SourceLocation Loc,
+                                    const DiagnosticsEngine &Diag) const {
+  // Calling GetDiagStatePointForLoc might be costly as it needs to create
+  // FullSourceLoc and compare them to the ones in the states. So first query
+  // the base state and check if it might be overridden.
+  DiagnosticsEngine::DiagState &BaseState = Diag.DiagStates.front();
+  assert(&BaseState == Diag.DiagStatePoints.front().State);
+  DiagnosticMapping &Mapping = BaseState.getOrAddMapping((diag::kind)DiagID);
+  if (!Mapping.mightBeOverriden())
+    return Mapping;
+
+  DiagnosticsEngine::DiagStatePointsTy::iterator Pos =
+      Diag.GetDiagStatePointForLoc(Loc);
+  return Pos->State->getOrAddMapping((diag::kind)DiagID);
+}
+
 /// \brief Based on the way the client configured the Diagnostic
 /// object, classify the specified diagnostic ID into a Level, consumable by
 /// the DiagnosticClient.
@@ -406,17 +423,11 @@
 DiagnosticIDs::getDiagnosticSeverity(unsigned DiagID, SourceLocation Loc,
                                      const DiagnosticsEngine &Diag) const {
   assert(getBuiltinDiagClass(DiagID) != CLASS_NOTE);
-
   // Specific non-error diagnostics may be mapped to various levels from ignored
   // to error.  Errors can only be mapped to fatal.
   diag::Severity Result = diag::Severity::Fatal;
 
-  DiagnosticsEngine::DiagStatePointsTy::iterator
-    Pos = Diag.GetDiagStatePointForLoc(Loc);
-  DiagnosticsEngine::DiagState *State = Pos->State;
-
-  // Get the mapping information, or compute it lazily.
-  DiagnosticMapping &Mapping = State->getOrAddMapping((diag::kind)DiagID);
+  DiagnosticMapping &Mapping = getDiagnosticMapping(DiagID, Loc, Diag);
 
   // TODO: Can a null severity really get here?
   if (Mapping.getSeverity() != diag::Severity())
Index: lib/Serialization/ASTReader.cpp
===================================================================
--- lib/Serialization/ASTReader.cpp
+++ lib/Serialization/ASTReader.cpp
@@ -5328,6 +5328,10 @@
         diag::Severity Map = (diag::Severity)F.PragmaDiagMappings[Idx++];
         DiagnosticMapping Mapping = Diag.makeUserMapping(Map, Loc);
         Diag.GetCurDiagState()->setMapping(DiagID, Mapping);
+
+        // We are overriding a default behavior
+        Diag.DiagStates.front().getOrAddMapping(DiagID).setMightBeOverriden(
+            true);
       }
     }
   }