[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 9b807f34f8a7c1b3c569128e60c48de006346669 Mon Sep 17 00:00:00 2001
From: momo5502 <mauriceheumann at gmail.com>
Date: Sun, 2 Nov 2025 11:56:45 +0100
Subject: [PATCH 1/2] [Support] Join threads when stopping ThreadPoolExecutor

---
 llvm/lib/Support/Parallel.cpp | 20 +++++++++++++-------
 1 file changed, 13 insertions(+), 7 deletions(-)

diff --git a/llvm/lib/Support/Parallel.cpp b/llvm/lib/Support/Parallel.cpp
index 8e0c724accb36..2420231307196 100644
--- a/llvm/lib/Support/Parallel.cpp
+++ b/llvm/lib/Support/Parallel.cpp
@@ -57,6 +57,7 @@ class ThreadPoolExecutor : public Executor {
     if (S.UseJobserver)
       TheJobserver = JobserverClient::getInstance();
 
+    ThreadsCreatedFuture = ThreadsCreated.get_future();
     ThreadCount = S.compute_thread_count();
     // Spawn all but one of the threads in another thread as spawning threads
     // can take a while.
@@ -84,24 +85,28 @@ class ThreadPoolExecutor : public Executor {
   void stop() {
     {
       std::lock_guard<std::mutex> Lock(Mutex);
-      if (Stop)
-        return;
       Stop = true;
     }
+
     Cond.notify_all();
-    ThreadsCreated.get_future().wait();
-  }
+    ThreadsCreatedFuture.wait();
+
+    std::vector<std::thread> ThreadsToJoin;
+    {
+      std::lock_guard<std::mutex> Lock(Mutex);
+      ThreadsToJoin.swap(Threads);
+    }
 
-  ~ThreadPoolExecutor() override {
-    stop();
     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); }
   };
@@ -187,6 +192,7 @@ class ThreadPoolExecutor : public Executor {
   std::mutex Mutex;
   std::condition_variable Cond;
   std::promise<void> ThreadsCreated;
+  std::future<void> ThreadsCreatedFuture;
   std::vector<std::thread> Threads;
   unsigned ThreadCount;
 

>From 8cc0efc20d7d2e40f41fa917226478d2ca1da34b Mon Sep 17 00:00:00 2001
From: momo5502 <mauriceheumann at gmail.com>
Date: Wed, 3 Dec 2025 07:39:28 +0100
Subject: [PATCH 2/2] Use reduced implementation by @nga888

---
 llvm/lib/Support/Parallel.cpp | 16 +++++-----------
 1 file changed, 5 insertions(+), 11 deletions(-)

diff --git a/llvm/lib/Support/Parallel.cpp b/llvm/lib/Support/Parallel.cpp
index 2420231307196..29b28c199b45a 100644
--- a/llvm/lib/Support/Parallel.cpp
+++ b/llvm/lib/Support/Parallel.cpp
@@ -57,7 +57,6 @@ class ThreadPoolExecutor : public Executor {
     if (S.UseJobserver)
       TheJobserver = JobserverClient::getInstance();
 
-    ThreadsCreatedFuture = ThreadsCreated.get_future();
     ThreadCount = S.compute_thread_count();
     // Spawn all but one of the threads in another thread as spawning threads
     // can take a while.
@@ -85,24 +84,20 @@ class ThreadPoolExecutor : public Executor {
   void stop() {
     {
       std::lock_guard<std::mutex> Lock(Mutex);
+      if (Stop)
+        return;
       Stop = true;
     }
-
     Cond.notify_all();
-    ThreadsCreatedFuture.wait();
-
-    std::vector<std::thread> ThreadsToJoin;
-    {
-      std::lock_guard<std::mutex> Lock(Mutex);
-      ThreadsToJoin.swap(Threads);
-    }
+    ThreadsCreated.get_future().wait();
 
     std::thread::id CurrentThreadId = std::this_thread::get_id();
-    for (std::thread &T : ThreadsToJoin)
+    for (std::thread &T : Threads)
       if (T.get_id() == CurrentThreadId)
         T.detach();
       else
         T.join();
+    Threads.clear();
   }
 
   ~ThreadPoolExecutor() override { stop(); }
@@ -192,7 +187,6 @@ class ThreadPoolExecutor : public Executor {
   std::mutex Mutex;
   std::condition_variable Cond;
   std::promise<void> ThreadsCreated;
-  std::future<void> ThreadsCreatedFuture;
   std::vector<std::thread> Threads;
   unsigned ThreadCount;