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

[Fangrui Song via llvm-commits]

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@@ -205,33 +226,32 @@ size_t parallel::getThreadCount() {
 }
 #endif
 
-// Latch::sync() called by the dtor may cause one thread to block. If is a dead
-// lock if all threads in the default executor are blocked. To prevent the dead
-// lock, only allow the root TaskGroup to run tasks parallelly. In the scenario
-// of nested parallel_for_each(), only the outermost one runs parallelly.
+// Nested TaskGroups on worker threads use helpSync() which executes tasks from
+// the queue while waiting, preventing deadlock while enabling nested
+// parallelism.
 TaskGroup::TaskGroup()
     : Parallel(
 #if LLVM_ENABLE_THREADS
-          strategy.ThreadsRequested != 1 && threadIndex == UINT_MAX
+          strategy.ThreadsRequested != 1
 #else
           false
 #endif
       ) {
 }
 TaskGroup::~TaskGroup() {
-  // We must ensure that all the workloads have finished before decrementing the
-  // instances count.
+#if LLVM_ENABLE_THREADS
+  // In a nested TaskGroup (threadIndex != -1u), actively help drain the queue.
+  if (Parallel && threadIndex != UINT_MAX)
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MaskRay wrote:

Thanks for the suggestion. Added `isNested`

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