[lld] [llvm] [Support] Support nested parallel TaskGroup via work-stealing (PR #189293)

[Fangrui Song via llvm-commits]

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@@ -96,14 +96,30 @@ class ThreadPoolExecutor {
     static void call(void *Ptr) { ((ThreadPoolExecutor *)Ptr)->stop(); }
   };
 
-  void add(std::function<void()> F) {
+  void add(std::function<void()> F, parallel::detail::Latch &L) {
     {
       std::lock_guard<std::mutex> Lock(Mutex);
-      WorkStack.push_back(std::move(F));
+      WorkStack.push_back({std::move(F), std::ref(L)});
     }
     Cond.notify_one();
   }
 
+  // Execute tasks from the work queue until the latch reaches zero.
+  // Used by nested TaskGroups (on worker threads) to prevent deadlock:
+  // instead of blocking in sync(), actively help drain the queue.
+  void helpSync(const parallel::detail::Latch &L) {
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MaskRay wrote:

Done `Item()`

https://github.com/llvm/llvm-project/pull/189293