[NFC][lldb] Speed up lookup of shared modules

By profiling LLDB debugging a Swift application without a dSYM and a
large amount of .o files, I identified that querying shared modules was
the biggest bottleneck when running "frame variable", and Clang types
need to be searched.

One of the reasons for that slowness is that the shared module list can
grow very large, and the search through it is O(n).

To solve this issue, this patch adds a new hashmap to the shared module
list whose key is the name of the module, and the value is all the
modules that share that name. This should speed up any search where the
query contains the module name.

rdar://156753350

Author: augusto2112

lldb/source/Core/ModuleList.cpp

@@ -755,11 +755,236 @@ size_t ModuleList::GetIndexForModule(const Module *module) const {
 }
 
 namespace {
+/// A wrapper around ModuleList for shared modules. Provides fast lookups for
+/// file-based ModuleSpec queries.
+class SharedModuleList {
+public:
+  /// Finds all the modules matching the module_spec, and adds them to \p
+  /// matching_module_list.
+  void FindModules(const ModuleSpec &module_spec,
+                   ModuleList &matching_module_list) const {
+    std::lock_guard<std::recursive_mutex> guard(GetMutex());
+    // Try index first for performance - if found, skip expensive full list search
+    if (FindModulesInIndex(module_spec, matching_module_list))
+      return;
+    m_list.FindModules(module_spec, matching_module_list);
+    // Assertion validates that if we found modules in the list but not the
+    // index, it's because the module_spec has no filename or the found module
+    // has a different filename (e.g., when searching by UUID and finding a
+    // module with an alias)
+    assert((matching_module_list.IsEmpty() ||
+            module_spec.GetFileSpec().GetFilename().IsEmpty() ||
+            module_spec.GetFileSpec().GetFilename() !=
+                matching_module_list.GetModuleAtIndex(0)
+                    ->GetFileSpec()
+                    .GetFilename()) &&
+           "Search by name not found in SharedModuleList's index");
+  }
+
+  ModuleSP FindModule(const Module *module_ptr) {
+    if (!module_ptr)
+      return ModuleSP();
+
+    std::lock_guard<std::recursive_mutex> guard(GetMutex());
+    // Try index first, fallback to full list search
+    if (ModuleSP result = FindModuleInIndex(module_ptr))
+      return result;
+    return m_list.FindModule(module_ptr);
+  }
+
+  // UUID searches bypass index since UUIDs aren't indexed by filename
+  ModuleSP FindModule(const UUID &uuid) const {
+    return m_list.FindModule(uuid);
+  }
+
+  void Append(const ModuleSP &module_sp, bool use_notifier) {
+    if (!module_sp)
+      return;
+    std::lock_guard<std::recursive_mutex> guard(GetMutex());
+    m_list.Append(module_sp, use_notifier);
+    AddToIndex(module_sp);
+  }
+
+  size_t RemoveOrphans(bool mandatory) {
+    std::unique_lock<std::recursive_mutex> lock(GetMutex(), std::defer_lock);
+    if (mandatory) {
+      lock.lock();
+    } else {
+      // Skip orphan removal if mutex unavailable (non-blocking)
+      if (!lock.try_lock())
+        return 0;
+    }
+    size_t total_count = 0;
+    size_t run_count;
+    do {
+      // Remove indexed orphans first, then remove non-indexed orphans. This
+      // order is important because the shared count will be different if a 
+      // module if indexed or not.
+      run_count = RemoveOrphansFromIndexAndList();
+      run_count += m_list.RemoveOrphans(mandatory);
+      total_count += run_count;
+      // Because removing orphans might make new orphans, we must continuously
+      // remove from both until both operations fail to remove new orphans.
+    } while (run_count != 0);
+
+    return total_count;
+  }
+
+  bool Remove(const ModuleSP &module_sp, bool use_notifier = true) {
+    if (!module_sp)
+      return false;
+    std::lock_guard<std::recursive_mutex> guard(GetMutex());
+    RemoveFromIndex(module_sp.get());
+    bool success = m_list.Remove(module_sp, use_notifier);
+    return success;
+  }
+
+  void ReplaceEquivalent(const ModuleSP &module_sp,
+                         llvm::SmallVectorImpl<lldb::ModuleSP> *old_modules) {
+    std::lock_guard<std::recursive_mutex> guard(GetMutex());
+    m_list.ReplaceEquivalent(module_sp, old_modules);
+    ReplaceEquivalentInIndex(module_sp);
+  }
+
+  bool RemoveIfOrphaned(const Module *module_ptr) {
+    std::lock_guard<std::recursive_mutex> guard(GetMutex());
+    RemoveFromIndex(module_ptr, /*if_orphaned =*/true);
+    bool result = m_list.RemoveIfOrphaned(module_ptr);
+    return result;
+  }
+
+  std::recursive_mutex &GetMutex() const { return m_list.GetMutex(); }
+
+private:
+  ModuleSP FindModuleInIndex(const Module *module_ptr) {
+    if (!module_ptr->GetFileSpec().GetFilename())
+      return ModuleSP();
+    ConstString name = module_ptr->GetFileSpec().GetFilename();
+    auto it = m_index.find(name);
+    if (it != m_index.end()) {
+      auto &vector = it->getSecond();
+      for (auto &module_sp : vector)
+        if (module_sp.get() == module_ptr)
+          return module_sp;
+    }
+    return ModuleSP();
+  }
+
+  bool FindModulesInIndex(const ModuleSpec &module_spec,
+                          ModuleList &matching_module_list) const {
+    auto it = m_index.find(module_spec.GetFileSpec().GetFilename());
+    if (it == m_index.end())
+      return false;
+    auto vector = it->getSecond();
+    bool found = false;
+    for (auto &module_sp : vector) {
+      if (module_sp->MatchesModuleSpec(module_spec)) {
+        matching_module_list.Append(module_sp);
+        found = true;
+      }
+    }
+    return found;
+  }
+
+  void AddToIndex(const ModuleSP &module_sp) {
+    auto name = module_sp->GetFileSpec().GetFilename();
+    if (name.IsEmpty())
+      return;
+    auto &vec = m_index[name];
+    vec.push_back(module_sp);
+  }
+
+  void RemoveFromIndex(const Module *module_ptr, bool if_orphaned = false) {
+    auto name = module_ptr->GetFileSpec().GetFilename();
+    auto it = m_index.find(name);
+    if (it == m_index.end())
+      return;
+    auto &vec = it->getSecond();
+    for (auto *it = vec.begin(); it != vec.end(); ++it) {
+      if (it->get() == module_ptr) {
+        // use_count == 2 means only held by index and list (orphaned)
+        if (!if_orphaned || it->use_count() == 2)
+          vec.erase(it);
+        break;
+      }
+    }
+  }
+
+  void ReplaceEquivalentInIndex(const ModuleSP &module_sp) {
+    RemoveEquivalentModulesFromIndex(module_sp);
+    AddToIndex(module_sp);
+  }
+
+  void RemoveEquivalentModulesFromIndex(const ModuleSP &module_sp) {
+    auto name = module_sp->GetFileSpec().GetFilename();
+    if (name.IsEmpty())
+      return;
+
+    auto it = m_index.find(name);
+    if (it == m_index.end())
+      return;
+
+    // First remove any equivalent modules. Equivalent modules are modules
+    // whose path, platform path and architecture match.
+    ModuleSpec equivalent_module_spec(module_sp->GetFileSpec(),
+                                      module_sp->GetArchitecture());
+    equivalent_module_spec.GetPlatformFileSpec() =
+        module_sp->GetPlatformFileSpec();
+
+    auto &vec = it->getSecond();
+    // Iterate backwards to minimize element shifting during removal
+    for (int i = vec.size() - 1; i >= 0; --i) {
+      auto *it = vec.begin() + i;
+      if ((*it)->MatchesModuleSpec(equivalent_module_spec))
+        vec.erase(it);
+    }
+  }
+
+  /// Remove orphans from both the index and list, if orphaned.
+  ///
+  /// This assumes that the mutex is locked.
+  int RemoveOrphansFromIndexAndList() {
+    // Modules might hold shared pointers to other modules, so removing one
+    // module might make other modules orphans. Keep removing modules until
+    // there are no further modules that can be removed.
+    bool made_progress = true;
+    int remove_count = 0;
+    while (made_progress) {
+      made_progress = false;
+      for (auto &[name, vec] : m_index) {
+        if (vec.empty())
+          continue;
+        ModuleList to_remove;
+        // Iterate backwards to minimize element shifting during removal
+        for (int i = vec.size() - 1; i >= 0; --i) {
+          ModuleSP module = vec[i];
+          // use_count == 3: index + list + local variable = orphaned
+          if (module.use_count() == 3) {
+            to_remove.Append(module);
+            vec.erase(vec.begin() + i);
+            remove_count += 1;
+            made_progress = true;
+          }
+        }
+        m_list.Remove(to_remove);
+      }
+    }
+    return remove_count;
+  }
+
+  ModuleList m_list;
+
+  /// A hash map from a module's filename to all the modules that share that
+  /// filename, for fast module lookups by name
+  llvm::DenseMap<ConstString, llvm::SmallVector<ModuleSP, 1>> m_index;
+};
+
 struct SharedModuleListInfo {
-  ModuleList module_list;
+  SharedModuleList module_list;
   ModuleListProperties module_list_properties;
 };
-}
+} // namespace
+
 static SharedModuleListInfo &GetSharedModuleListInfo()
 {
   static SharedModuleListInfo *g_shared_module_list_info = nullptr;
@@ -774,7 +999,7 @@ static SharedModuleListInfo &GetSharedModuleListInfo()
   return *g_shared_module_list_info;
 }
 
-static ModuleList &GetSharedModuleList() {
+static SharedModuleList &GetSharedModuleList() {
   return GetSharedModuleListInfo().module_list;
 }
 
@@ -784,7 +1009,7 @@ ModuleListProperties &ModuleList::GetGlobalModuleListProperties() {
 
 bool ModuleList::ModuleIsInCache(const Module *module_ptr) {
   if (module_ptr) {
-    ModuleList &shared_module_list = GetSharedModuleList();
+    SharedModuleList &shared_module_list = GetSharedModuleList();
     return shared_module_list.FindModule(module_ptr).get() != nullptr;
   }
   return false;
@@ -808,9 +1033,8 @@ ModuleList::GetSharedModule(const ModuleSpec &module_spec, ModuleSP &module_sp,
                             const FileSpecList *module_search_paths_ptr,
                             llvm::SmallVectorImpl<lldb::ModuleSP> *old_modules,
                             bool *did_create_ptr, bool always_create) {
-  ModuleList &shared_module_list = GetSharedModuleList();
-  std::lock_guard<std::recursive_mutex> guard(
-      shared_module_list.m_modules_mutex);
+  SharedModuleList &shared_module_list = GetSharedModuleList();
+  std::lock_guard<std::recursive_mutex> guard(shared_module_list.GetMutex());
   char path[PATH_MAX];
 
   Status error;