Do not shrink on remove()

This is in line with Qt 5 behavior, how std::vector works and also
how QString and QByteArray behave. If you need the shrink allocated
storage, call squeeze() manually.

Change-Id: I16cadd4f2a89bb2ec5de02fe69186f5da321cd06
Reviewed-by: Paul Wicking <paul.wicking@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/doc/src/qt6-changes.qdoc

@@ -161,10 +161,9 @@
     \section3 Stability of References
 
     There are several changes made to the QVector/QList implementation. The
-    QVector related ones are: insertion at the beginning is optimized (similarly
-    to QList in Qt 5) and element removal can reallocate in order to remove the
-    unused capacity. The QList related one is: memory layout for the elements is
-    simplified.
+    QVector related one is: insertion at the beginning is optimized (similarly
+    to QList in Qt 5). The QList related one is: memory layout for the elements
+    is simplified.
 
     \important These changes impact the stability of references. In Qt 6, you
     should consider any size or capacity modifying method to invalidate all

src/corelib/tools/qlist.h

@@ -639,22 +639,11 @@ inline void QList<T>::remove(qsizetype i, qsizetype n)
     if (n == 0)
         return;
 
-    const auto newSize = size() - n;
-    if (d->needsDetach() ||
-            ((d->flags() & Data::CapacityReserved) == 0
-             && newSize < d->allocatedCapacity()/2)) {
-        // allocate memory
-        DataPointer detached(Data::allocate(d->detachCapacity(newSize)));
-        const_iterator where = constBegin() + i;
-        if (newSize) {
-            detached->copyAppend(constBegin(), where);
-            detached->copyAppend(where + n, constEnd());
-        }
-        d.swap(detached);
-    } else {
-        // we're detached and we can just move data around
-        d->erase(d->begin() + i, d->begin() + i + n);
-    }
+    if (d->needsDetach())
+        d.detach();
+
+    d->erase(d->begin() + i, d->begin() + i + n);
+}
 
 template <typename T>
 inline void QList<T>::removeFirst()

src/corelib/tools/qlist.qdoc

@@ -501,11 +501,6 @@
     may have used more memory than the normal QList growth strategy
     would have allocated—or you may have used less.
 
-    \note Calling reserve() changes the growth strategy of QList to the one that
-    avoids unnecessary reallocations. This may give worse performance when \a
-    size is underestimated. To restore automatic growth strategy, call
-    squeeze().
-
     \warning reserve() reserves memory but does not change the size of the list.
     Accessing data beyond the end of the list is undefined behavior.
 
@@ -799,19 +794,14 @@
     Removes \a n elements from the list, starting at index position \a i.
 
 //! [shrinking-erase]
-    Element removal may cause the list to shrink the capacity to reduce the
-    allocated memory size. The automatic shrinking only happens when the new
-    size is below some threshold (e.g. half of the capacity). This means that,
-    for large lists, removing a couple of elements often won't cause any side
-    effects. To make sure shrinking does not happen at all, use reserve()
-    beforehand to give a hint to the QList that the capacity should be
-    preserved.
+    Element removal will preserve the list's capacity and not reduce the amount of
+    allocated memory. To shed extra capacity and free as much memory as possible,
+    call squeeze().
 //! [shrinking-erase]
 
 //! [iterator-invalidation-erase]
-    \note When QList is not \l{implicitly shared} and the capacity is reserved
-    with a call to reserve(), this function only invalidates iterators at or
-    after the specified position.
+    \note When QList is not \l{implicitly shared}, this function only
+    invalidates iterators at or after the specified position.
 //! [iterator-invalidation-erase]
 
     \sa insert(), replace(), fill()