[llvm] [Support] Join threads when stopping the ThreadPoolExecutor (PR #166054)

[Maurice Heumann via llvm-commits]

https://github.com/momo5502 updated https://github.com/llvm/llvm-project/pull/166054

>From f2907665dd3e0a0c839bbf9301aa7532548ee593 Mon Sep 17 00:00:00 2001
From: momo5502 <mauriceheumann at gmail.com>
Date: Sun, 2 Nov 2025 11:56:45 +0100
Subject: [PATCH] [Support] Join threads when stopping ThreadPoolExecutor

---
 llvm/lib/Support/Parallel.cpp | 18 ++++++++++++------
 1 file changed, 12 insertions(+), 6 deletions(-)

diff --git a/llvm/lib/Support/Parallel.cpp b/llvm/lib/Support/Parallel.cpp
index 8e0c724accb36..e20e6af262fc6 100644
--- a/llvm/lib/Support/Parallel.cpp
+++ b/llvm/lib/Support/Parallel.cpp
@@ -83,25 +83,31 @@ class ThreadPoolExecutor : public Executor {
 
   void stop() {
     {
-      std::lock_guard<std::mutex> Lock(Mutex);
-      if (Stop)
+      std::unique_lock<std::mutex> Lock(Mutex);
+      if (Stop && Threads.empty())
         return;
       Stop = true;
     }
+
     Cond.notify_all();
     ThreadsCreated.get_future().wait();
-  }
 
-  ~ThreadPoolExecutor() override {
-    stop();
+    std::vector<std::thread> ThreadsToJoin;
+    {
+      std::unique_lock<std::mutex> Lock(Mutex);
+      ThreadsToJoin.swap(Threads);
+    }
+
     std::thread::id CurrentThreadId = std::this_thread::get_id();
-    for (std::thread &T : Threads)
+    for (std::thread &T : ThreadsToJoin)
       if (T.get_id() == CurrentThreadId)
         T.detach();
       else
         T.join();
   }
 
+  ~ThreadPoolExecutor() override { stop(); }
+
   struct Creator {
     static void *call() { return new ThreadPoolExecutor(strategy); }
   };