Add QList::emplaceFront() that fixes huge performance issue in prepend

Prepend in QList was using insert() logic that always uses append-aware
functions. This results in the fact that freeSpaceAtBegin() is always 0
and forces us to actually allocate on *every* call (with the same
capacity!). Vicious cycle is hot-fixable with introduction of
emplaceFront (or anything prepend-aware, really)

This brings me from 632ms to 0.65ms for 100k iterations of
list.prepend(int(0)). Still ~3x worse than QList in 5.15 but much
faster than QVector, which takes 382ms in the same workload

Not addressed:
- QString/QBA
- Other prepend functions in QList e.g. prepend(it1, it2)
- Lower-level array operations that should just be extended

Task-number: QTBUG-86583
Change-Id: Ie82b07d81a67605cd308d9fabf9532d57935647f
Reviewed-by: Lars Knoll <lars.knoll@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qlist.h

@@ -316,12 +316,15 @@ public:
     void append(rvalue_ref t) { emplaceBack(std::move(t)); }
     void append(const QList<T> &l) { append(l.constBegin(), l.constEnd()); }
     void append(QList<T> &&l);
-    void prepend(rvalue_ref t);
-    void prepend(parameter_type t);
+    void prepend(rvalue_ref t) { emplaceFront(std::move(t)); }
+    void prepend(parameter_type t) { emplaceFront(t); }
 
     template <typename ...Args>
     reference emplaceBack(Args&&... args) { return *emplace(count(), std::forward<Args>(args)...); }
 
+    template <typename ...Args>
+    inline reference emplaceFront(Args&&... args);
+
     iterator insert(qsizetype i, parameter_type t)
     { return insert(i, 1, t); }
     iterator insert(qsizetype i, qsizetype n, parameter_type t);
@@ -651,13 +654,6 @@ inline void QList<T>::remove(qsizetype i, qsizetype n)
     }
 }
 
-template <typename T>
-inline void QList<T>::prepend(parameter_type t)
-{ insert(0, 1, t); }
-template <typename T>
-void QList<T>::prepend(rvalue_ref t)
-{ insert(0, std::move(t)); }
-
 template<typename T>
 inline T QList<T>::value(qsizetype i, parameter_type defaultValue) const
 {
@@ -711,6 +707,29 @@ inline void QList<T>::append(QList<T> &&other)
     }
 }
 
+template<typename T>
+template<typename... Args>
+inline typename QList<T>::reference QList<T>::emplaceFront(Args &&... args)
+{
+    const bool shouldGrow = d->shouldGrowBeforeInsert(d.begin(), 1);
+    const auto newSize = size() + 1;
+    if (d->needsDetach() || newSize > d->constAllocatedCapacity() || shouldGrow) {
+        const auto flags = d->detachFlags() | Data::GrowsBackwards;
+        DataPointer detached(DataPointer::allocateGrow(d, newSize, flags));
+
+        T tmp(std::forward<Args>(args)...);
+        detached->copyAppend(constBegin(), constEnd());
+        // insert here makes sure we have extra free space at beginning. we
+        // actually need a proper copyPrepend here instead.
+        detached->insert(detached.begin(), 1, std::move(tmp));
+        d.swap(detached);
+    } else {
+        // ### replace with emplaceFront
+        d->emplace(d.begin(), std::forward<Args>(args)...);
+    }
+    return *d.begin();
+}
+
 
 template <typename T>
 inline typename QList<T>::iterator