diff --git a/clang/lib/CodeGen/CodeGenPGO.h b/clang/lib/CodeGen/CodeGenPGO.h
--- a/clang/lib/CodeGen/CodeGenPGO.h
+++ b/clang/lib/CodeGen/CodeGenPGO.h
@@ -114,7 +114,12 @@
       return 0;
     if (!haveRegionCounts())
       return 0;
-    return RegionCounts[(*RegionCounterMap)[S]];
+    // With profiles from a differing version of clang we can have mismatched
+    // decl counts. Don't crash in such a case.
+    auto Index = (*RegionCounterMap)[S];
+    if (Index >= RegionCounts.size())
+      return 0;
+    return RegionCounts[Index];
   }
 };