[TableGen] Return StringRef from ValueTypeByHwMode::getMVTName

[RKSimon]

Summary

Avoid unnecessary std::string creations during TypeSetByHwMode::writeToStream.

Found during investigations into PR28222

[kparzysz]

This is good regardless of any future changes to replace getAsString.

llvm/trunk/utils/TableGen/InfoByHwMode.h

struct ValueTypeByHwMode : public InfoByHwMode<MVT> {
   bool operator< (const ValueTypeByHwMode &T) const;
 
   bool isValid() const {
     return !Map.empty();
   }
   MVT getType(unsigned Mode) const { return get(Mode); }
   MVT &getOrCreateTypeForMode(unsigned Mode, MVT Type);
 
-  static std::string getMVTName(MVT T);
+  static StringRef getMVTName(MVT T);
   std::string getAsString() const;
   void dump() const;
 };
 
 ValueTypeByHwMode getValueTypeByHwMode(Record *Rec,
                                        const CodeGenHwModes &CGH);
 
 struct RegSizeInfo {

llvm/trunk/utils/TableGen/InfoByHwMode.cpp

MVT &ValueTypeByHwMode::getOrCreateTypeForMode(unsigned Mode, MVT Type) {
   // make a copy of it for Mode and return it.
   auto D = Map.find(DefaultMode);
   if (D != Map.end())
     return Map.insert(std::make_pair(Mode, D->second)).first->second;
   // If default mode is not present either, use provided Type.
   return Map.insert(std::make_pair(Mode, Type)).first->second;
 }
 
-std::string ValueTypeByHwMode::getMVTName(MVT T) {
-  std::string N = llvm::getEnumName(T.SimpleTy);
-  if (N.substr(0,5) == "MVT::")
-    N = N.substr(5);
+StringRef ValueTypeByHwMode::getMVTName(MVT T) {
+  StringRef N = llvm::getEnumName(T.SimpleTy);
+  N.consume_front("MVT::");
   return N;
 }
 
 std::string ValueTypeByHwMode::getAsString() const {
   if (isSimple())
     return getMVTName(getSimple());
 
   std::vector<const PairType*> Pairs;
   for (const auto &P : Map)
     Pairs.push_back(&P);
   std::sort(Pairs.begin(), Pairs.end(), deref<std::less<PairType>>());
 
   std::stringstream str;
   str << '{';
   for (unsigned i = 0, e = Pairs.size(); i != e; ++i) {
     const PairType *P = Pairs[i];
     str << '(' << getModeName(P->first)
-        << ':' << getMVTName(P->second) << ')';
+        << ':' << getMVTName(P->second).str() << ')';
     if (i != e-1)
       str << ',';
   }
   str << '}';
   return str.str();
 }
 
 LLVM_DUMP_METHOD