Fix dap stacktrace perf issue (llvm#104874)

We have got several customer reporting of slow stepping over the past
year in VSCode.
Profiling shows the slow stepping is caused by `stackTrace` request
which can take around 1 second for certain targets. Since VSCode sends
`stackTrace` during each stop event, the slow `stackTrace` request would
slow down stepping in VSCode. Below is the hot path:

```
               |--68.75%--lldb_dap::DAP::HandleObject(llvm::json::Object const&)
               |          |
               |          |--57.70%--(anonymous namespace)::request_stackTrace(llvm::json::Object const&)
               |          |          |
               |          |          |--54.43%--lldb::SBThread::GetCurrentExceptionBacktrace()
               |          |          |          lldb_private::Thread::GetCurrentExceptionBacktrace()
               |          |          |          lldb_private::Thread::GetCurrentException()
               |          |          |          lldb_private::ItaniumABILanguageRuntime::GetExceptionObjectForThread(std::shared_ptr<lldb_private::Thread>)
               |          |          |          |
               |          |          |          |--53.43%--lldb_private::FunctionCaller::ExecuteFunction(lldb_private::ExecutionContext&, unsigned long*, lldb_private::EvaluateExpressionOptions const&, lldb_private::DiagnosticManager&, lldb_private::Value&)
               |          |          |          |          |
               |          |          |          |          |--25.23%--lldb_private::FunctionCaller::InsertFunction(lldb_private::ExecutionContext&, unsigned long&, lldb_private::DiagnosticManager&)
               |          |          |          |          |          |
               |          |          |          |          |          |--24.56%--lldb_private::FunctionCaller::WriteFunctionWrapper(lldb_private::ExecutionContext&, lldb_private::DiagnosticManager&)
               |          |          |          |          |          |          |
               |          |          |          |          |          |          |--19.73%--lldb_private::ExpressionParser::PrepareForExecution(unsigned long&, unsigned long&, std::shared_ptr<lldb_private::IRExecutionUnit>&, lldb_private::ExecutionContext&, bool&, lldb_private::ExecutionPolicy)
               |          |          |          |          |          |          |          lldb_private::ClangExpressionParser::DoPrepareForExecution(unsigned long&, unsigned long&, std::shared_ptr<lldb_private::IRExecutionUnit>&, lldb_private::ExecutionContext&, bool&, lldb_private::ExecutionPolicy)
               |          |          |          |          |          |          |          lldb_private::IRExecutionUnit::GetRunnableInfo(lldb_private::Status&, unsigned long&, unsigned long&)
               |          |          |          |          |          |          |          |
```

The hot path is added by https://reviews.llvm.org/D156465 which should
at least be disabled for Linux. Note: I am seeing similar performance
hot path on Mac.

This PR hides the feature behind `enableDisplayExtendedBacktrace` option
which needs to be enabled on-demand.

---------

Co-authored-by: jeffreytan81 <[email protected]>
(cherry picked from commit e5140ae)

Author: 2 people

lldb/tools/lldb-dap/DAP.cpp

@@ -36,6 +36,7 @@ DAP::DAP()
       focus_tid(LLDB_INVALID_THREAD_ID), stop_at_entry(false), is_attach(false),
       enable_auto_variable_summaries(false),
       enable_synthetic_child_debugging(false),
+      enable_display_extended_backtrace(false),
       restarting_process_id(LLDB_INVALID_PROCESS_ID),
       configuration_done_sent(false), waiting_for_run_in_terminal(false),
       progress_event_reporter(

lldb/tools/lldb-dap/DAP.h

@@ -181,6 +181,7 @@ struct DAP {
   bool is_attach;
   bool enable_auto_variable_summaries;
   bool enable_synthetic_child_debugging;
+  bool enable_display_extended_backtrace;
   // The process event thread normally responds to process exited events by
   // shutting down the entire adapter. When we're restarting, we keep the id of
   // the old process here so we can detect this case and keep running.

lldb/tools/lldb-dap/lldb-dap.cpp

@@ -701,6 +701,8 @@ void request_attach(const llvm::json::Object &request) {
       GetBoolean(arguments, "enableAutoVariableSummaries", false);
   g_dap.enable_synthetic_child_debugging =
       GetBoolean(arguments, "enableSyntheticChildDebugging", false);
+  g_dap.enable_display_extended_backtrace =
+      GetBoolean(arguments, "enableDisplayExtendedBacktrace", false);
   g_dap.command_escape_prefix =
       GetString(arguments, "commandEscapePrefix", "`");
   g_dap.SetFrameFormat(GetString(arguments, "customFrameFormat"));
@@ -1925,6 +1927,8 @@ void request_launch(const llvm::json::Object &request) {
       GetBoolean(arguments, "enableAutoVariableSummaries", false);
   g_dap.enable_synthetic_child_debugging =
       GetBoolean(arguments, "enableSyntheticChildDebugging", false);
+  g_dap.enable_display_extended_backtrace =
+      GetBoolean(arguments, "enableDisplayExtendedBacktrace", false);
   g_dap.command_escape_prefix =
       GetString(arguments, "commandEscapePrefix", "`");
   g_dap.SetFrameFormat(GetString(arguments, "customFrameFormat"));
@@ -3111,17 +3115,20 @@ void request_stackTrace(const llvm::json::Object &request) {
     // This will always return an invalid thread when
     // libBacktraceRecording.dylib is not loaded or if there is no extended
     // backtrace.
-    lldb::SBThread queue_backtrace_thread =
-        thread.GetExtendedBacktraceThread("libdispatch");
+    lldb::SBThread queue_backtrace_thread;
+    if (g_dap.enable_display_extended_backtrace)
+      queue_backtrace_thread = thread.GetExtendedBacktraceThread("libdispatch");
     if (queue_backtrace_thread.IsValid()) {
       // One extra frame as a label to mark the enqueued thread.
       totalFrames += queue_backtrace_thread.GetNumFrames() + 1;
     }
 
     // This will always return an invalid thread when there is no exception in
     // the current thread.
-    lldb::SBThread exception_backtrace_thread =
-        thread.GetCurrentExceptionBacktrace();
+    lldb::SBThread exception_backtrace_thread;
+    if (g_dap.enable_display_extended_backtrace)
+      exception_backtrace_thread = thread.GetCurrentExceptionBacktrace();
+
     if (exception_backtrace_thread.IsValid()) {
       // One extra frame as a label to mark the exception thread.
       totalFrames += exception_backtrace_thread.GetNumFrames() + 1;