[llvm] [DenseMap] Introduce emplace_or_assign (PR #138886)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-adt

Author: Ramkumar Ramachandra (artagnon)

<details>
<summary>Changes</summary>

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

---
Full diff: https://github.com/llvm/llvm-project/pull/138886.diff


2 Files Affected:

- (modified) llvm/include/llvm/ADT/DenseMap.h (+16) 
- (modified) llvm/unittests/ADT/DenseMapTest.cpp (+35) 


``````````diff
diff --git a/llvm/include/llvm/ADT/DenseMap.h b/llvm/include/llvm/ADT/DenseMap.h
index bb99a41646b08..5f4442c777fad 100644
--- a/llvm/include/llvm/ADT/DenseMap.h
+++ b/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)
diff --git a/llvm/unittests/ADT/DenseMapTest.cpp b/llvm/unittests/ADT/DenseMapTest.cpp
index a4c045585fc28..c8a131b4f5f7e 100644
--- a/llvm/unittests/ADT/DenseMapTest.cpp
+++ b/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;

``````````

</details>


https://github.com/llvm/llvm-project/pull/138886