[llvm-dev] Trying to use unordered_map

[Paul C. Anagnostopoulos via llvm-dev]

Okay, I'm at a loss to understand what I'm doing wrong with the unordered_map class. Thank you for any help you can give.

I have two data structures:

  SmallVector<RecordVal, 0> Values;
  std::unordered_map<const Init *, const RecordVal *> ValueMap; 

I just added the unordered_map to experiment with faster lookups for the fields in  TableGen records. The key is a StringInit and the corresponding value is a RecordVal. I made it a pointer to the RecordVal since the existing SmallVector holds the actual RecordVal instance.

Here is the code that adds a field to the structures:

  void addValue(const RecordVal &RV) {
    Values.push_back(RV);
    ValueMap[RV.getNameInit()] = &RV; 

The RecordVal is push_backed onto the vector and inserted into the map by assignment.

Here is the code that looks up an entry. First the new code and then the old code.

  const RecordVal *getValue(const Init *Name) const {
    auto It = ValueMap.find(Name);
    if (It != ValueMap.end()) {
      return It->second;
    }
    return nullptr;

    for (const RecordVal &Val : Values)
      if (Val.Name == Name) {
        return &Val;
      }
    return nullptr;
  }

As far as I can tell, 'return It->second' is not returning the RecordVal. However, I'm thoroughly confused because I threw in a ton of prints to check it. Using It->Second I can print the name and value of the RecordVal and they are correct. I even printed the addresses of the name and value data structures from both It-Second and Val and they are the same. But when I print the address It-Second and  the address &Val, they are different. 

(Indeed, I should use Visual Studio to look at this, but that is a lesson for another day.)