[lldb] Fix performance regression after adding GNUstep ObjC runtime

We added support for the GNUstep ObjC runtime in 0b62647. In order to check if the target process uses
GNUstep we run an expensive symbol lookup in `CreateInstance()`. This turned out to cause a heavy performance
regression for non-GNUstep inferiors.

This patch puts a cheaper check in front, so that the vast majority of requests should return early. This
should fix the symptom for the moment. The conceptual question remains: Why does `LanguageRuntime::FindPlugin`
invoke `create_callback` for each available runtime unconditionally in every `Process::ModulesDidLoad`?

Reviewed By: jasonmolenda, jingham, bulbazord

Differential Revision: https://reviews.llvm.org/D158205

Author: weliveindetail

lldb/source/Plugins/LanguageRuntime/ObjC/GNUstepObjCRuntime/GNUstepObjCRuntime.cpp

@@ -37,6 +37,33 @@ void GNUstepObjCRuntime::Terminate() {
   PluginManager::UnregisterPlugin(CreateInstance);
 }
 
+static bool CanModuleBeGNUstepObjCLibrary(const ModuleSP &module_sp,
+                                          const llvm::Triple &TT) {
+  if (!module_sp)
+    return false;
+  const FileSpec &module_file_spec = module_sp->GetFileSpec();
+  if (!module_file_spec)
+    return false;
+  llvm::StringRef filename = module_file_spec.GetFilename().GetStringRef();
+  if (TT.isOSBinFormatELF())
+    return filename.starts_with("libobjc.so");
+  if (TT.isOSWindows())
+    return filename == "objc.dll";
+  return false;
+}
+
+static bool ScanForGNUstepObjCLibraryCandidate(const ModuleList &modules,
+                                               const llvm::Triple &TT) {
+  std::lock_guard<std::recursive_mutex> guard(modules.GetMutex());
+  size_t num_modules = modules.GetSize();
+  for (size_t i = 0; i < num_modules; i++) {
+    auto mod = modules.GetModuleAtIndex(i);
+    if (CanModuleBeGNUstepObjCLibrary(mod, TT))
+      return true;
+  }
+  return false;
+}
+
 LanguageRuntime *GNUstepObjCRuntime::CreateInstance(Process *process,
                                                     LanguageType language) {
   if (language != eLanguageTypeObjC)
@@ -50,6 +77,9 @@ LanguageRuntime *GNUstepObjCRuntime::CreateInstance(Process *process,
     return nullptr;
 
   const ModuleList &images = target.GetImages();
+  if (!ScanForGNUstepObjCLibraryCandidate(images, TT))
+    return nullptr;
+
   if (TT.isOSBinFormatELF()) {
     SymbolContextList eh_pers;
     RegularExpression regex("__gnustep_objc[x]*_personality_v[0-9]+");
@@ -176,18 +206,8 @@ void GNUstepObjCRuntime::UpdateISAToDescriptorMapIfNeeded() {
 }
 
 bool GNUstepObjCRuntime::IsModuleObjCLibrary(const ModuleSP &module_sp) {
-  if (!module_sp)
-    return false;
-  const FileSpec &module_file_spec = module_sp->GetFileSpec();
-  if (!module_file_spec)
-    return false;
-  llvm::StringRef filename = module_file_spec.GetFilename().GetStringRef();
   const llvm::Triple &TT = GetTargetRef().GetArchitecture().GetTriple();
-  if (TT.isOSBinFormatELF())
-    return filename.starts_with("libobjc.so");
-  if (TT.isOSWindows())
-    return filename == "objc.dll";
-  return false;
+  return CanModuleBeGNUstepObjCLibrary(module_sp, TT);
 }
 
 bool GNUstepObjCRuntime::ReadObjCLibrary(const ModuleSP &module_sp) {