[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 +755,244 @@ 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 map first for performance - if found, skip expensive full list
+    // search
+    if (FindModulesInMap(module_spec, matching_module_list))
+      return;
+    m_list.FindModules(module_spec, matching_module_list);
+    // Assert that modules were found in the list but not the map, it's
+    // because the module_spec has no filename or the found module has a
+    // different filename. For example, 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 map");
+  }
+
+  ModuleSP FindModule(const Module *module_ptr) {
+    if (!module_ptr)
+      return ModuleSP();
+
+    std::lock_guard<std::recursive_mutex> guard(GetMutex());
+    if (ModuleSP result = FindModuleInMap(module_ptr))
+      return result;
+    return m_list.FindModule(module_ptr);
+  }
+
+  // UUID searches bypass map 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);
+    AddToMap(module_sp);
+  }
+
+  size_t RemoveOrphans(bool mandatory) {
+    std::unique_lock<std::recursive_mutex> lock(GetMutex(), std::defer_lock);
+    if (mandatory) {
+      lock.lock();
+    } else {
+      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 is indexed or not.
+      run_count = RemoveOrphansFromMapAndList();
+      run_count += m_list.RemoveOrphans(mandatory);
+      total_count += run_count;
+      // Because removing orphans might make new orphans, remove from both
+      // containers until a fixed-point is reached.
+    } 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());
+    RemoveFromMap(module_sp.get());
+    return m_list.Remove(module_sp, use_notifier);
+  }
+
+  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);
+    ReplaceEquivalentInMap(module_sp);
+  }
+
+  bool RemoveIfOrphaned(const Module *module_ptr) {
+    std::lock_guard<std::recursive_mutex> guard(GetMutex());
+    RemoveFromMap(module_ptr, /*if_orphaned =*/true);
+    return m_list.RemoveIfOrphaned(module_ptr);
+  }
+
+  std::recursive_mutex &GetMutex() const { return m_list.GetMutex(); }
+
+private:
+  ModuleSP FindModuleInMap(const Module *module_ptr) {
+    if (!module_ptr->GetFileSpec().GetFilename())
+      return ModuleSP();
+    ConstString name = module_ptr->GetFileSpec().GetFilename();
+    auto it = m_name_to_modules.find(name);
+    if (it == m_name_to_modules.end())
+      return ModuleSP();
+    const llvm::SmallVectorImpl<ModuleSP> &vector = it->second;
+    for (auto &module_sp : vector) {
+      if (module_sp.get() == module_ptr)
+        return module_sp;
+    }
+    return ModuleSP();
+  }
+
+  bool FindModulesInMap(const ModuleSpec &module_spec,
+                          ModuleList &matching_module_list) const {
+    auto it = m_name_to_modules.find(module_spec.GetFileSpec().GetFilename());
+    if (it == m_name_to_modules.end())
+      return false;
+    const llvm::SmallVectorImpl<ModuleSP> &vector = it->second;
+    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 AddToMap(const ModuleSP &module_sp) {
+    ConstString name = module_sp->GetFileSpec().GetFilename();
+    if (name.IsEmpty())
+      return;
+    llvm::SmallVectorImpl<ModuleSP> &vec = m_name_to_modules[name];
+    vec.push_back(module_sp);
+  }
+
+  void RemoveFromMap(const Module *module_ptr, bool if_orphaned = false) {
+    ConstString name = module_ptr->GetFileSpec().GetFilename();
+    auto it = m_name_to_modules.find(name);
+    if (it == m_name_to_modules.end())
+      return;
+    llvm::SmallVectorImpl<ModuleSP> &vec = it->second;
+    for (auto *it = vec.begin(); it != vec.end(); ++it) {
+      if (it->get() == module_ptr) {
+        // use_count == 2 means only held by map and list (orphaned)
+        if (!if_orphaned || it->use_count() == 2)
+          vec.erase(it);
+        break;
+      }
+    }
+  }
+
+  void ReplaceEquivalentInMap(const ModuleSP &module_sp) {
+    RemoveEquivalentModulesFromMap(module_sp);
+    AddToMap(module_sp);
+  }
+
+  void RemoveEquivalentModulesFromMap(const ModuleSP &module_sp) {
+    ConstString name = module_sp->GetFileSpec().GetFilename();
+    if (name.IsEmpty())
+      return;
+
+    auto it = m_name_to_modules.find(name);
+    if (it == m_name_to_modules.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();
+
+    llvm::SmallVectorImpl<ModuleSP> &vec = it->second;
+    // 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 the vector.
+  ///
+  /// Returns the removed orphans.
+  ModuleList RemoveOrphansFromVector(llvm::SmallVectorImpl<ModuleSP> &vec) {
+    ModuleList to_remove;
+    for (int i = vec.size() - 1; i >= 0; --i) {
+      ModuleSP module = vec[i];
+      long kUseCountOrphaned = 2;
+      long kUseCountLocalVariable = 1;
+      // use_count == 3: map + list + local variable = orphaned.
+      if (module.use_count() == kUseCountOrphaned + kUseCountLocalVariable) {
+        to_remove.Append(module);
+        vec.erase(vec.begin() + i);
+      }
+    }
+    return to_remove;
+  }
+
+  /// Remove orphans that exist in both the map and list. This does not remove
+  /// any orphans that exist exclusively on the list.
+  ///
+  /// This assumes that the mutex is locked.
+  int RemoveOrphansFromMapAndList() {
+    // Modules might hold shared pointers to other modules, so removing one
+    // module might orphan other modules. 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_name_to_modules) {
+        if (vec.empty())
+          continue;
+        ModuleList to_remove = RemoveOrphansFromVector(vec);
+        remove_count += to_remove.GetSize();
+        made_progress = !to_remove.IsEmpty();
+        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_name_to_modules;
+};
+
 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 +1007,7 @@ static SharedModuleListInfo &GetSharedModuleListInfo()
   return *g_shared_module_list_info;
 }
 
-static ModuleList &GetSharedModuleList() {
+static SharedModuleList &GetSharedModuleList() {
   return GetSharedModuleListInfo().module_list;
 }
 
@@ -784,7 +1017,7 @@ 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 +1041,8 @@ 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;