Query the StringMap only once when creating MDString (NFC)

Summary:
Loading IR with debug info improves MDString::get() from 19ms to 10ms.
This is a rework of D16597 with adding an "emplace" method on the StringMap
to avoid requiring the MDString move ctor to be public.

Reviewers: dexonsmith

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D17920

From: Mehdi Amini <mehdi.amini@apple.com>
llvm-svn: 264386

1 files changed, 6 insertions, 11 deletions

diff --git a/llvm/lib/IR/Metadata.cpp b/llvm/lib/IR/Metadata.cpp
index da7442281ee..bcbda13f918 100644
--- a/llvm/lib/IR/Metadata.cpp
+++ b/llvm/lib/IR/Metadata.cpp
@@ -397,17 +397,12 @@ void ValueAsMetadata::handleRAUW(Value *From, Value *To) {
 
 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.emplace_second(Str);
+  auto &MapEntry = I.first->getValue();
+  if (!I.second)
+    return &MapEntry;
+  MapEntry.Entry = &*I.first;
+  return &MapEntry;
 }
 
 StringRef MDString::getString() const {