[llvm-branch-commits] [lldb] [lldb][NFC] Use DenseMap in ObjCLanguageRuntime (PR #197444)

[Felipe de Azevedo Piovezan via llvm-branch-commits]

https://github.com/felipepiovezan created https://github.com/llvm/llvm-project/pull/197444

This is a multimap of Hash -> ObjCISA (lldb::addr_t), and it shows up cpu traces of swift applications. This commit replaces `std::multimap<hash, addr_t>` with `DenseMap<hash, SmallVector<addr_t, 3>>`.

This data structure was chosen because of the following experiment. When evaluating a frame variable command for a SwiftUI variable, this object gets populated with ~186,000 entries:

* over a thousand of them had 2 hash collisions.
* 300 of them had 3 hash collisions.
* 21 of them had 4 hash collisions.
* 1 of them had 5 hash collisions.

On a release build (no asserts), this patch brought down the CPU cycles measured for that command from 487M cycles to 389M cycles.

>From 2f14d7561f9f3326a0715b5e88b07ba102b8b8f4 Mon Sep 17 00:00:00 2001
From: Felipe de Azevedo Piovezan <fpiovezan at apple.com>
Date: Wed, 13 May 2026 13:59:43 +0100
Subject: [PATCH] [lldb][NFC] Use DenseMap in ObjCLanguageRuntime

This is a multimap of Hash -> ObjCISA (lldb::addr_t), and it shows up
cpu traces of swift applications. This commit replaces
`std::multimap<hash, addr_t>` with `DenseMap<hash, SmallVector<addr_t, 3>>`.

This data structure was chosen because of the following experiment. When
evaluating a frame variable command for a SwiftUI variable, this object
gets populated with ~186,000 entries:

* over a thousand of them had 2 hash collisions.
* 300 of them had 3 hash collisions.
* 21 of them had 4 hash collisions.
* 1 of them had 5 hash collisions.

On a release build (no asserts), this patch brought down the CPU cycles
measured for that command from 487M cycles to 389M cycles.
---
 .../ObjC/ObjCLanguageRuntime.cpp               | 18 ++++++++----------
 .../LanguageRuntime/ObjC/ObjCLanguageRuntime.h |  5 ++---
 2 files changed, 10 insertions(+), 13 deletions(-)

diff --git a/lldb/source/Plugins/LanguageRuntime/ObjC/ObjCLanguageRuntime.cpp b/lldb/source/Plugins/LanguageRuntime/ObjC/ObjCLanguageRuntime.cpp
index 512426d0bd254..d0dc252199800 100644
--- a/lldb/source/Plugins/LanguageRuntime/ObjC/ObjCLanguageRuntime.cpp
+++ b/lldb/source/Plugins/LanguageRuntime/ObjC/ObjCLanguageRuntime.cpp
@@ -194,16 +194,14 @@ ObjCLanguageRuntime::GetDescriptorIterator(ConstString name) {
   // Name hashes were provided, so use them to efficiently lookup name to
   // isa/descriptor
   const uint32_t name_hash = llvm::djbHash(name.GetStringRef());
-  std::pair<HashToISAIterator, HashToISAIterator> range =
-      m_hash_to_isa_map.equal_range(name_hash);
-  for (HashToISAIterator range_pos = range.first; range_pos != range.second;
-       ++range_pos) {
-    ISAToDescriptorIterator pos = m_isa_to_descriptor.find(range_pos->second);
-    if (pos != m_isa_to_descriptor.end()) {
-      if (pos->second->GetClassName() == name)
-        return pos;
-    }
-  }
+  auto matches_it = m_hash_to_isa_map.find(name_hash);
+  if (matches_it == m_hash_to_isa_map.end())
+    return m_isa_to_descriptor.end();
+
+  for (auto isa : matches_it->second)
+    if (auto pos = m_isa_to_descriptor.find(isa);
+        pos != m_isa_to_descriptor.end() && pos->second->GetClassName() == name)
+      return pos;
 
   return m_isa_to_descriptor.end();
 }
diff --git a/lldb/source/Plugins/LanguageRuntime/ObjC/ObjCLanguageRuntime.h b/lldb/source/Plugins/LanguageRuntime/ObjC/ObjCLanguageRuntime.h
index 74ac557906d41..104c6033643aa 100644
--- a/lldb/source/Plugins/LanguageRuntime/ObjC/ObjCLanguageRuntime.h
+++ b/lldb/source/Plugins/LanguageRuntime/ObjC/ObjCLanguageRuntime.h
@@ -353,7 +353,7 @@ class ObjCLanguageRuntime : public LanguageRuntime {
                 uint32_t class_name_hash) {
     if (isa != 0) {
       m_isa_to_descriptor.insert_or_assign(isa, descriptor_sp);
-      m_hash_to_isa_map.insert(std::make_pair(class_name_hash, isa));
+      m_hash_to_isa_map[class_name_hash].push_back(isa);
       return true;
     }
     return false;
@@ -420,9 +420,8 @@ class ObjCLanguageRuntime : public LanguageRuntime {
   typedef std::map<ClassAndSel, lldb::addr_t> MsgImplMap;
   typedef std::map<ClassAndSelStr, lldb::addr_t> MsgImplStrMap;
   typedef llvm::DenseMap<ObjCISA, ClassDescriptorSP> ISAToDescriptorMap;
-  typedef std::multimap<uint32_t, ObjCISA> HashToISAMap;
+  typedef llvm::DenseMap<uint32_t, llvm::SmallVector<ObjCISA, 3>> HashToISAMap;
   typedef ISAToDescriptorMap::iterator ISAToDescriptorIterator;
-  typedef HashToISAMap::iterator HashToISAIterator;
   typedef ThreadSafeDenseMap<void *, uint64_t> TypeSizeCache;
 
   MsgImplMap m_impl_cache;