[llvm] [Parallel] Ensure getThreadIndex()==0 for sequential tasks (PR #109084)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-support

Author: Fangrui Song (MaskRay)

<details>
<summary>Changes</summary>

https://reviews.llvm.org/D148728 introduced `bool Sequential` to unify
`execute` and the old `spawn` without argument. However, sequential
tasks might be executed by any worker thread (non-deterministic),
leading to non-determinism output for ld.lld -z nocombreloc.

In addition, the extra member variables have overhead.

This patch restores the behavior before https://reviews.llvm.org/D148728
, but temporarily sets `threadIndex` to 0 when the main thread is
executing the sequential task. In addition, use `vector` instead of
`stack` for `WorkStack`.

Fix #<!-- -->105958


---
Full diff: https://github.com/llvm/llvm-project/pull/109084.diff


1 Files Affected:

- (modified) llvm/lib/Support/Parallel.cpp (+18-36) 


``````````diff
diff --git a/llvm/lib/Support/Parallel.cpp b/llvm/lib/Support/Parallel.cpp
index a3ef3d9c621b98..4aec5b14ebb146 100644
--- a/llvm/lib/Support/Parallel.cpp
+++ b/llvm/lib/Support/Parallel.cpp
@@ -12,7 +12,6 @@
 #include "llvm/Support/Threading.h"
 
 #include <atomic>
-#include <deque>
 #include <future>
 #include <thread>
 #include <vector>
@@ -39,7 +38,7 @@ namespace {
 class Executor {
 public:
   virtual ~Executor() = default;
-  virtual void add(std::function<void()> func, bool Sequential = false) = 0;
+  virtual void add(std::function<void()> func) = 0;
   virtual size_t getThreadCount() const = 0;
 
   static Executor *getDefaultExecutor();
@@ -98,13 +97,10 @@ class ThreadPoolExecutor : public Executor {
     static void call(void *Ptr) { ((ThreadPoolExecutor *)Ptr)->stop(); }
   };
 
-  void add(std::function<void()> F, bool Sequential = false) override {
+  void add(std::function<void()> F) override {
     {
       std::lock_guard<std::mutex> Lock(Mutex);
-      if (Sequential)
-        WorkQueueSequential.emplace_front(std::move(F));
-      else
-        WorkQueue.emplace_back(std::move(F));
+      WorkStack.push_back(std::move(F));
     }
     Cond.notify_one();
   }
@@ -112,42 +108,23 @@ class ThreadPoolExecutor : public Executor {
   size_t getThreadCount() const override { return ThreadCount; }
 
 private:
-  bool hasSequentialTasks() const {
-    return !WorkQueueSequential.empty() && !SequentialQueueIsLocked;
-  }
-
-  bool hasGeneralTasks() const { return !WorkQueue.empty(); }
-
   void work(ThreadPoolStrategy S, unsigned ThreadID) {
     threadIndex = ThreadID;
     S.apply_thread_strategy(ThreadID);
     while (true) {
       std::unique_lock<std::mutex> Lock(Mutex);
-      Cond.wait(Lock, [&] {
-        return Stop || hasGeneralTasks() || hasSequentialTasks();
-      });
+      Cond.wait(Lock, [&] { return Stop || !WorkStack.empty(); });
       if (Stop)
         break;
-      bool Sequential = hasSequentialTasks();
-      if (Sequential)
-        SequentialQueueIsLocked = true;
-      else
-        assert(hasGeneralTasks());
-
-      auto &Queue = Sequential ? WorkQueueSequential : WorkQueue;
-      auto Task = std::move(Queue.back());
-      Queue.pop_back();
+      auto Task = std::move(WorkStack.back());
+      WorkStack.pop_back();
       Lock.unlock();
       Task();
-      if (Sequential)
-        SequentialQueueIsLocked = false;
     }
   }
 
   std::atomic<bool> Stop{false};
-  std::atomic<bool> SequentialQueueIsLocked{false};
-  std::deque<std::function<void()>> WorkQueue;
-  std::deque<std::function<void()>> WorkQueueSequential;
+  std::vector<std::function<void()>> WorkStack;
   std::mutex Mutex;
   std::condition_variable Cond;
   std::promise<void> ThreadsCreated;
@@ -217,13 +194,18 @@ TaskGroup::~TaskGroup() {
 void TaskGroup::spawn(std::function<void()> F, bool Sequential) {
 #if LLVM_ENABLE_THREADS
   if (Parallel) {
+    if (Sequential) {
+      // Act as worker thread 0.
+      threadIndex = 0;
+      F();
+      threadIndex = UINT_MAX;
+      return;
+    }
     L.inc();
-    detail::Executor::getDefaultExecutor()->add(
-        [&, F = std::move(F)] {
-          F();
-          L.dec();
-        },
-        Sequential);
+    detail::Executor::getDefaultExecutor()->add([&, F = std::move(F)] {
+      F();
+      L.dec();
+    });
     return;
   }
 #endif

``````````

</details>


https://github.com/llvm/llvm-project/pull/109084