diff --git a/include/clang/Basic/DiagnosticIDs.h b/include/clang/Basic/DiagnosticIDs.h
index 5d36285..25e5dfd 100644
--- a/include/clang/Basic/DiagnosticIDs.h
+++ b/include/clang/Basic/DiagnosticIDs.h
@@ -28,7 +28,8 @@
   namespace diag {
     // Start position for diagnostics.
     enum {
-      DIAG_START_DRIVER        =                               300,
+      DIAG_START_COMMON        =                                 0,
+      DIAG_START_DRIVER        = DIAG_START_COMMON          +  300,
       DIAG_START_FRONTEND      = DIAG_START_DRIVER          +  100,
       DIAG_START_SERIALIZATION = DIAG_START_FRONTEND        +  100,
       DIAG_START_LEX           = DIAG_START_SERIALIZATION   +  120,
@@ -49,6 +50,7 @@
     enum {
 #define DIAG(ENUM,FLAGS,DEFAULT_MAPPING,DESC,GROUP,\
              SFINAE,ACCESS,CATEGORY,NOWERROR,SHOWINSYSHEADER) ENUM,
+#define COMMONSTART
 #include "clang/Basic/DiagnosticCommonKinds.inc"
       NUM_BUILTIN_COMMON_DIAGNOSTICS
 #undef DIAG
diff --git a/lib/Basic/DiagnosticIDs.cpp b/lib/Basic/DiagnosticIDs.cpp
index a703876..e96d509 100644
--- a/lib/Basic/DiagnosticIDs.cpp
+++ b/lib/Basic/DiagnosticIDs.cpp
@@ -15,8 +15,8 @@
 #include "clang/Basic/AllDiagnostics.h"
 #include "clang/Basic/DiagnosticCategories.h"
 #include "clang/Basic/SourceManager.h"
-#include "llvm/ADT/STLExtras.h"
 #include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/STLExtras.h"
 #include "llvm/Support/ErrorHandling.h"
 #include <map>
 using namespace clang;
@@ -83,11 +83,9 @@
 #include "clang/Basic/DiagnosticSemaKinds.inc"
 #include "clang/Basic/DiagnosticAnalysisKinds.inc"
 #undef DIAG
-  { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}
 };
 
-static const unsigned StaticDiagInfoSize =
-  llvm::array_lengthof(StaticDiagInfo)-1;
+static const unsigned StaticDiagInfoSize = llvm::array_lengthof(StaticDiagInfo);
 
 /// GetDiagInfo - Return the StaticDiagInfoRec entry for the specified DiagID,
 /// or null if the ID is invalid.
@@ -106,7 +104,7 @@
 
   // Out of bounds diag. Can't be in the table.
   using namespace diag;
-  if (DiagID >= DIAG_UPPER_LIMIT)
+  if (DiagID >= DIAG_UPPER_LIMIT || DiagID <= DIAG_START_COMMON)
     return 0;
 
   // Compute the index of the requested diagnostic in the static table.
@@ -118,8 +116,7 @@
   // This is cheaper than a binary search on the table as it doesn't touch
   // memory at all.
   unsigned Offset = 0;
-  unsigned ID = DiagID;
-#define DIAG_START_COMMON 0 // Sentinel value.
+  unsigned ID = DiagID - DIAG_START_COMMON - 1;
 #define CATEGORY(NAME, PREV) \
   if (DiagID > DIAG_START_##NAME) { \
     Offset += NUM_BUILTIN_##PREV##_DIAGNOSTICS - DIAG_START_##PREV - 1; \
@@ -135,7 +132,6 @@
 CATEGORY(SEMA, COMMENT)
 CATEGORY(ANALYSIS, SEMA)
 #undef CATEGORY
-#undef DIAG_START_COMMON
 
   // Avoid out of bounds reads.
   if (ID + Offset >= StaticDiagInfoSize)