[PATCH] D114363: [ThreadPool] Do not return shared futures.

[Florian Hahn via Phabricator via llvm-commits]

fhahn created this revision.
fhahn added reviewers: Meinersbur, dblaikie, mehdi_amini, rriddle.
Herald added a subscriber: dexonsmith.
fhahn requested review of this revision.
Herald added a project: LLVM.

The only users of returned futures from ThreadPool is llvm-reduce after
D113857 <https://reviews.llvm.org/D113857>.

There should be no cases where multiple threads wait on the same future,
so there should be no need to return std::shared_future<>. Instead return
plain std::future<>.

If users need to share a future between multiple threads, they can share
the futures themselves.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D114363

Files:
  llvm/include/llvm/Support/ThreadPool.h
  llvm/tools/llvm-reduce/deltas/Delta.cpp


Index: llvm/tools/llvm-reduce/deltas/Delta.cpp
===================================================================
--- llvm/tools/llvm-reduce/deltas/Delta.cpp
+++ llvm/tools/llvm-reduce/deltas/Delta.cpp
@@ -267,7 +267,7 @@
       WriteBitcodeToFile(*Test.getProgram().M, BCOS);
     }
 
-    std::deque<std::shared_future<SmallString<0>>> TaskQueue;
+    std::deque<std::future<SmallString<0>>> TaskQueue;
     for (auto I = ChunksStillConsideredInteresting.rbegin(),
               E = ChunksStillConsideredInteresting.rend();
          I != E; ++I) {
Index: llvm/include/llvm/Support/ThreadPool.h
===================================================================
--- llvm/include/llvm/Support/ThreadPool.h
+++ llvm/include/llvm/Support/ThreadPool.h
@@ -56,8 +56,7 @@
 
   /// Asynchronous submission of a task to the pool. The returned future can be
   /// used to wait for the task to finish and is *non-blocking* on destruction.
-  template <typename Func>
-  auto async(Func &&F) -> std::shared_future<decltype(F())> {
+  template <typename Func> auto async(Func &&F) -> std::future<decltype(F())> {
     return asyncImpl(std::function<decltype(F())()>(std::forward<Func>(F)));
   }
 
@@ -101,7 +100,7 @@
   /// Asynchronous submission of a task to the pool. The returned future can be
   /// used to wait for the task to finish and is *non-blocking* on destruction.
   template <typename ResTy>
-  std::shared_future<ResTy> asyncImpl(std::function<ResTy()> Task) {
+  std::future<ResTy> asyncImpl(std::function<ResTy()> Task) {
 
 #if LLVM_ENABLE_THREADS
     /// Wrap the Task in a std::function<void()> that sets the result of the
@@ -117,12 +116,12 @@
       Tasks.push(std::move(R.first));
     }
     QueueCondition.notify_one();
-    return R.second.share();
+    return std::move(R.second);
 
 #else // LLVM_ENABLE_THREADS Disabled
 
     // Get a Future with launch::deferred execution using std::async
-    auto Future = std::async(std::launch::deferred, std::move(Task)).share();
+    auto Future = std::async(std::launch::deferred, std::move(Task));
     // Wrap the future so that both ThreadPool::wait() can operate and the
     // returned future can be sync'ed on.
     Tasks.push([Future]() { Future.get(); });


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