From d01bd60d43a649008e31e12a2ec42a975e3896b1 Mon Sep 17 00:00:00 2001
From: Sona Kurazyan <sona.kurazyan@qt.io>
Date: Tue, 18 Jan 2022 15:45:56 +0100
Subject: [PATCH] Optimize ContinuationWrapper used for support of move-only
 continuations
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

After QFuture continuations became non-copyable (see earlier commits),
we have to always use ContinuationWrapper to save the continuations
inside std::function, since it requires the callable to be copyable.
Optimize the wrapper, by storing the callable directly (instead of using
a ref-counted QSharedPointer) and introducing a fake copy-constructor
that makes sure that it's never called.

Change-Id: I0ed5f90ad62ede3b5c6d6e56ef58eb6377122920
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Andrei Golubev <andrei.golubev@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
(cherry picked from commit b292928e35b9864c2c3a023f79b892f40b08cd86)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
---
 src/corelib/thread/qfuture_impl.h       | 14 +++++++++++---
 src/corelib/thread/qfutureinterface.cpp |  2 +-
 2 files changed, 12 insertions(+), 4 deletions(-)

diff --git a/src/corelib/thread/qfuture_impl.h b/src/corelib/thread/qfuture_impl.h
index 69f57652ea0..b09d84a6d39 100644
--- a/src/corelib/thread/qfuture_impl.h
+++ b/src/corelib/thread/qfuture_impl.h
@@ -538,11 +538,19 @@ bool Continuation<Function, ResultType, ParentResultType>::execute()
 template<class Function>
 struct ContinuationWrapper
 {
-    ContinuationWrapper(Function &&f) : function(QSharedPointer<Function>::create(std::move(f))) { }
-    void operator()(const QFutureInterfaceBase &parentData) { (*function)(parentData); }
+    ContinuationWrapper(Function &&f) : function(std::move(f)) { }
+    ContinuationWrapper(const ContinuationWrapper &other)
+        : function(std::move(const_cast<ContinuationWrapper &>(other).function))
+    {
+        Q_ASSERT_X(false, "QFuture", "Continuation shouldn't be copied");
+    }
+    ContinuationWrapper(ContinuationWrapper &&other) = default;
+    ContinuationWrapper &operator=(ContinuationWrapper &&) = default;
+
+    void operator()(const QFutureInterfaceBase &parentData) { function(parentData); }
 
 private:
-    QSharedPointer<Function> function;
+    Function function;
 };
 
 template<typename Function, typename ResultType, typename ParentResultType>
diff --git a/src/corelib/thread/qfutureinterface.cpp b/src/corelib/thread/qfutureinterface.cpp
index dfc905e8283..cad6111ab94 100644
--- a/src/corelib/thread/qfutureinterface.cpp
+++ b/src/corelib/thread/qfutureinterface.cpp
@@ -837,7 +837,7 @@ void QFutureInterfaceBasePrivate::setState(QFutureInterfaceBase::State newState)
 
 void QFutureInterfaceBase::setContinuation(std::function<void(const QFutureInterfaceBase &)> func)
 {
-    setContinuation(func, nullptr);
+    setContinuation(std::move(func), nullptr);
 }
 
 void QFutureInterfaceBase::setContinuation(std::function<void(const QFutureInterfaceBase &)> func,