Linker: Avoid constructing ValueMap::MDMapT

Calling ValueMap::MD lazily constructs a ValueMap, which mallocs the
buckets.  Instead of swapping constructed maps, move around the
underlying Optional<MDMapT>.  This gets rid of some unnecessary malloc
traffic from r266579 (not that it showed up on a profile).

llvm-svn: 266761

llvm/include/llvm/IR/ValueMap.h

@@ -109,6 +109,7 @@ public:
       MDMap.emplace();
     return *MDMap;
   }
+  Optional<MDMapT> &getMDMap() { return MDMap; }
 
   bool mayMapMetadata() const { return MayMapMetadata; }
   void enableMapMetadata() { MayMapMetadata = true; }

llvm/lib/Linker/IRMover.cpp

@@ -483,11 +483,11 @@ public:
                &GValMaterializer),
         AliasMCID(Mapper.registerAlternateMappingContext(AliasValueMap,
                                                          &LValMaterializer)) {
-    ValueMap.MD().swap(SharedMDs);
+    ValueMap.getMDMap() = std::move(SharedMDs);
     for (GlobalValue *GV : ValuesToLink)
       maybeAdd(GV);
   }
-  ~IRLinker() { ValueMap.MD().swap(SharedMDs); }
+  ~IRLinker() { SharedMDs = std::move(*ValueMap.getMDMap()); }
 
   bool run();
   Value *materializeDeclFor(Value *V, bool ForAlias);