[BOLT] Fix a bug related to iterators in ReorderData pass

[sinan]

Summary

If Itr is the last element then std::next(Itr) will be Range.end(), so that the statement std::next(Itr)->second is a UB.

I encountered a segmentation fault in test "reorder-data-writable-ptload.c" with my llvm-bolt linked against static libc++.

[yota9]

LGTM thanks!

bolt/lib/Passes/ReorderData.cpp

bool ReorderData::markUnmoveableSymbols(BinaryContext &BC,
   // suffix in another.
   auto isPrivate = [&](const BinaryData *BD) {
     auto Prefix = std::string("PG") + BC.AsmInfo->getPrivateGlobalPrefix();
     return BD->getName().startswith(Prefix.str());
   };
   auto Range = BC.getBinaryDataForSection(Section);
   bool FoundUnmoveable = false;
   for (auto Itr = Range.begin(); Itr != Range.end(); ++Itr) {
+    BinaryData *Next =
+        std::next(Itr) != Range.end() ? std::next(Itr)->second : nullptr;
     if (Itr->second->getName().startswith("PG.")) {
       BinaryData *Prev =
           Itr != Range.begin() ? std::prev(Itr)->second : nullptr;
-      BinaryData *Next = Itr != Range.end() ? std::next(Itr)->second : nullptr;
       bool PrevIsPrivate = Prev && isPrivate(Prev);
       bool NextIsPrivate = Next && isPrivate(Next);
       if (isPrivate(Itr->second) && (PrevIsPrivate || NextIsPrivate))
         Itr->second->setIsMoveable(false);
     } else {
       // check for overlapping symbols.
-      BinaryData *Next = Itr != Range.end() ? std::next(Itr)->second : nullptr;
       if (Next && Itr->second->getEndAddress() != Next->getAddress() &&
           Next->containsAddress(Itr->second->getEndAddress())) {
         Itr->second->setIsMoveable(false);
         Next->setIsMoveable(false);
       }
     }
     FoundUnmoveable |= !Itr->second->isMoveable();
   }