diff --git a/llvm/lib/CodeGen/BasicBlockSectionsProfileReader.cpp b/llvm/lib/CodeGen/BasicBlockSectionsProfileReader.cpp
--- a/llvm/lib/CodeGen/BasicBlockSectionsProfileReader.cpp
+++ b/llvm/lib/CodeGen/BasicBlockSectionsProfileReader.cpp
@@ -97,7 +97,7 @@
       S.split(BBIndexes, ' ');
       // Reset current cluster position.
       CurrentPosition = 0;
-      for (auto BBIndexStr : BBIndexes) {
+      for (auto &BBIndexStr : BBIndexes) {
         unsigned long long BBIndex;
         if (getAsUnsignedInteger(BBIndexStr, 10, BBIndex))
           return invalidProfileError(Twine("Unsigned integer expected: '") +