[llvm] [ADT] Avoid creating iterators in DenseMap::operator[] (NFC) (PR #155862)

[Benjamin Maxwell via llvm-commits]

https://github.com/MacDue created https://github.com/llvm/llvm-project/pull/155862

An attempt to resolve the slowdown from #155204

>From 5695702094947c5c808d5e96c5ff63b3a5ac3c5e Mon Sep 17 00:00:00 2001
From: Benjamin Maxwell <benjamin.maxwell at arm.com>
Date: Thu, 28 Aug 2025 15:44:26 +0000
Subject: [PATCH] [ADT] Avoid creating iterators in DenseMap::operator[] (NFC)

An attempt to resolve the slowdown from #155204
---
 llvm/include/llvm/ADT/DenseMap.h | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/llvm/include/llvm/ADT/DenseMap.h b/llvm/include/llvm/ADT/DenseMap.h
index 2dfd1dabd07f1..be4b21da9eb32 100644
--- a/llvm/include/llvm/ADT/DenseMap.h
+++ b/llvm/include/llvm/ADT/DenseMap.h
@@ -347,11 +347,11 @@ class DenseMapBase : public DebugEpochBase {
   }
 
   ValueT &operator[](const KeyT &Key) {
-    return try_emplace_impl(Key).first->second;
+    return lookupOrInsertIntoBucket(Key).first->second;
   }
 
   ValueT &operator[](KeyT &&Key) {
-    return try_emplace_impl(std::move(Key)).first->second;
+    return lookupOrInsertIntoBucket(std::move(Key)).first->second;
   }
 
   /// isPointerIntoBucketsArray - Return true if the specified pointer points
@@ -476,15 +476,23 @@ class DenseMapBase : public DebugEpochBase {
 
 private:
   template <typename KeyArgT, typename... Ts>
-  std::pair<iterator, bool> try_emplace_impl(KeyArgT &&Key, Ts &&...Args) {
+  std::pair<BucketT *, bool> lookupOrInsertIntoBucket(KeyArgT &&Key,
+                                                      Ts &&...Args) {
     BucketT *TheBucket = nullptr;
     if (LookupBucketFor(Key, TheBucket))
-      return {makeInsertIterator(TheBucket), false}; // Already in the map.
+      return {TheBucket, false}; // Already in the map.
 
     // Otherwise, insert the new element.
     TheBucket = InsertIntoBucket(TheBucket, std::forward<KeyArgT>(Key),
                                  std::forward<Ts>(Args)...);
-    return {makeInsertIterator(TheBucket), true};
+    return {TheBucket, true};
+  }
+
+  template <typename KeyArgT, typename... Ts>
+  std::pair<iterator, bool> try_emplace_impl(KeyArgT &&Key, Ts &&...Args) {
+    auto [Bucket, Inserted] = lookupOrInsertIntoBucket(
+        std::forward<KeyArgT>(Key), std::forward<Ts>(Args)...);
+    return {makeInsertIterator(Bucket), Inserted};
   }
 
   iterator makeIterator(BucketT *P, BucketT *E, DebugEpochBase &Epoch,