Query the StringMap only once when creating MDString (NFC)

[mehdi_amini]

Summary

Loading IR with debug info improves MDString::get() from 19ms to 10ms.

[pete]

Nice fix. This makes a lot of sense.

LGTM.

[mehdi_amini]

r261030

[ovyalov]

It seems this CL broke LLDB Windows buildbot - http://lab.llvm.org:8011/builders/lldb-windows7-android/builds/4316
Could you take a look?

[dblaikie]

It sounded like Duncan had some pretty strong feelings against this -
should it have really been committed?

[mehdi_amini]

It should have been fixed in the meantime by a subsequent commit. Let me know if it is not the case.

include/llvm/IR/Metadata.h

class MDString : public Metadata {
   friend class StringMapEntry<MDString>;
 
   MDString(const MDString &) = delete;
   MDString &operator=(MDString &&) = delete;
   MDString &operator=(const MDString &) = delete;
 
   StringMapEntry<MDString> *Entry;
   MDString() : Metadata(MDStringKind, Uniqued), Entry(nullptr) {}
-  MDString(MDString &&) : Metadata(MDStringKind, Uniqued) {}
 
 public:
+  MDString(MDString &&R) : Metadata(MDStringKind, Uniqued), Entry(R.Entry) {}
   static MDString *get(LLVMContext &Context, StringRef Str);
   static MDString *get(LLVMContext &Context, const char *Str) {
     return get(Context, Str ? StringRef(Str) : StringRef());
   }
 
   StringRef getString() const;
 
   unsigned getLength() const { return (unsigned)getString().size(); }

lib/IR/Metadata.cpp

 }
 
 //===----------------------------------------------------------------------===//
 // MDString implementation.
 //
 
 MDString *MDString::get(LLVMContext &Context, StringRef Str) {
   auto &Store = Context.pImpl->MDStringCache;
-  auto I = Store.find(Str);
-  if (I != Store.end())
-    return &I->second;
-
-  auto *Entry =
-      StringMapEntry<MDString>::Create(Str, Store.getAllocator(), MDString());
-  bool WasInserted = Store.insert(Entry);
-  (void)WasInserted;
-  assert(WasInserted && "Expected entry to be inserted");
-  Entry->second.Entry = Entry;
-  return &Entry->second;
+  auto I = Store.insert(std::make_pair(Str, MDString()));
+  auto &MapEntry = I.first->getValue();
+  if (!I.second)
+    return &MapEntry;
+  MapEntry.Entry = &*I.first;
+  return &MapEntry;
 }
 
 StringRef MDString::getString() const {
   assert(Entry && "Expected to find string map entry");
   return Entry->first();
 }
 
 //===----------------------------------------------------------------------===//