[DenseMap] Introduce emplace_or_assign

Introduce emplace_or_assign, a variant of insert_or_assign that has
slightly better characteristics.

Author: artagnon

llvm/include/llvm/ADT/DenseMap.h

@@ -324,6 +324,22 @@ class DenseMapBase : public DebugEpochBase {
     return Ret;
   }
 
+  template <typename... Ts>
+  std::pair<iterator, bool> emplace_or_assign(const KeyT &Key, Ts &&...Args) {
+    auto Ret = try_emplace(Key, std::forward<Ts>(Args)...);
+    if (!Ret.second)
+      Ret.first->second = ValueT(std::forward<Ts>(Args)...);
+    return Ret;
+  }
+
+  template <typename... Ts>
+  std::pair<iterator, bool> emplace_or_assign(KeyT &&Key, Ts &&...Args) {
+    auto Ret = try_emplace(std::move(Key), std::forward<Ts>(Args)...);
+    if (!Ret.second)
+      Ret.first->second = ValueT(std::forward<Ts>(Args)...);
+    return Ret;
+  }
+
   bool erase(const KeyT &Val) {
     BucketT *TheBucket = doFind(Val);
     if (!TheBucket)

llvm/unittests/ADT/DenseMapTest.cpp

@@ -545,6 +545,41 @@ TEST(DenseMapCustomTest, InsertOrAssignTest) {
   EXPECT_EQ(1, CountCopyAndMove::MoveAssignments);
 }
 
+TEST(DenseMapCustomTest, EmplaceOrAssign) {
+  DenseMap<int, CountCopyAndMove> Map;
+
+  CountCopyAndMove::ResetCounts();
+  auto Try0 = Map.emplace_or_assign(3, 3);
+  EXPECT_TRUE(Try0.second);
+  EXPECT_EQ(0, CountCopyAndMove::TotalCopies());
+  EXPECT_EQ(0, CountCopyAndMove::TotalMoves());
+  EXPECT_EQ(1, CountCopyAndMove::ValueConstructions);
+
+  CountCopyAndMove::ResetCounts();
+  auto Try1 = Map.emplace_or_assign(3, 4);
+  EXPECT_FALSE(Try1.second);
+  EXPECT_EQ(0, CountCopyAndMove::TotalCopies());
+  EXPECT_EQ(1, CountCopyAndMove::ValueConstructions);
+  EXPECT_EQ(0, CountCopyAndMove::MoveConstructions);
+  EXPECT_EQ(1, CountCopyAndMove::MoveAssignments);
+
+  int Key = 5;
+  CountCopyAndMove::ResetCounts();
+  auto Try2 = Map.emplace_or_assign(Key, 3);
+  EXPECT_TRUE(Try2.second);
+  EXPECT_EQ(0, CountCopyAndMove::TotalCopies());
+  EXPECT_EQ(0, CountCopyAndMove::TotalMoves());
+  EXPECT_EQ(1, CountCopyAndMove::ValueConstructions);
+
+  CountCopyAndMove::ResetCounts();
+  auto Try3 = Map.emplace_or_assign(Key, 4);
+  EXPECT_FALSE(Try3.second);
+  EXPECT_EQ(0, CountCopyAndMove::TotalCopies());
+  EXPECT_EQ(1, CountCopyAndMove::ValueConstructions);
+  EXPECT_EQ(0, CountCopyAndMove::MoveConstructions);
+  EXPECT_EQ(1, CountCopyAndMove::MoveAssignments);
+}
+
 // Make sure DenseMap works with StringRef keys.
 TEST(DenseMapCustomTest, StringRefTest) {
   DenseMap<StringRef, int> M;