Auto Causes copy problems

[XinWang10]

Summary

Use variable without & cause the copy of an object

[LuoYuanke]

LGTM

[pengfei]

llvm/lib/CodeGen/BasicBlockSectionsProfileReader.cpp

100

StringRef is a small class. It's common to use copy in LLVM code. I guess it may be better to use copy than reference in small size sturctures.

[XinWang10]

llvm/lib/CodeGen/BasicBlockSectionsProfileReader.cpp

100

I tried to make an assemble file, seems it is better to use copy here, little struct can be transfered by regs.

[XinWang10]

llvm/lib/CodeGen/BasicBlockSectionsProfileReader.cpp

100

sorry, by stack not regs.

llvm/lib/CodeGen/BasicBlockSectionsProfileReader.cpp

for (; !LineIt.is_at_eof(); ++LineIt) {
     if (S.consume_front("!")) {
       if (FI == ProgramBBClusterInfo.end())
         return invalidProfileError(
             "Cluster list does not follow a function name specifier.");
       SmallVector<StringRef, 4> BBIndexes;
       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: '") +
                                      BBIndexStr + "'.");
         if (!FuncBBIDs.insert(BBIndex).second)
           return invalidProfileError(Twine("Duplicate basic block id found '") +
                                      BBIndexStr + "'.");
         if (!BBIndex && CurrentPosition)