QPen/QBrush: add optimized assignment operators for color and style

Assigning a color or style to an existing pen or brush (which we do
frequently in QPainter and elsewhere in Qt) used to implicitly create a
new QPen/QBrush instance and then move-assign that. This involves an
allocation of the new private data, and often also a deallocation of the
old pen or brush private if that wasn't shared (which it often isn't, as
we usually set a temporary object with no further copies).

By implementing specific assignment operators from color and style we
can shortcut this and avoid the allocations.

With this set of changes, the setPen/setBrush benchmarks now compare
to Qt 6.8 as follows:

setPen
6.8: 0.00016 msecs per iteration (total: 89, iterations: 524288)
6.9: 0.000032 msecs per iteration (total: 69, iterations: 2097152)

setBrush
6.8: 0.000078 msecs per iteration (total: 82, iterations: 1048576)
6.9: 0.000017 msecs per iteration (total: 73, iterations: 4194304)

i.e. improve by a factor of >4.5.

Change-Id: Ia9c0229b0f540ecf9afba2a598973c6931712f64
Reviewed-by: Christian Ehrlicher <ch.ehrlicher@gmx.de>

src/gui/painting/qbrush.cpp

@@ -598,19 +598,49 @@ void QBrush::detach(Qt::BrushStyle newStyle)
 
 
 /*!
-    \fn QBrush &QBrush::operator=(const QBrush &brush)
-
     Assigns the given \a brush to \e this brush and returns a
     reference to \e this brush.
 */
 
-QBrush &QBrush::operator=(const QBrush &b)
+QBrush &QBrush::operator=(const QBrush &brush)
 {
-    if (d == b.d)
+    if (d == brush.d)
         return *this;
 
-    b.d->ref.ref();
-    d.reset(b.d.get());
+    brush.d->ref.ref();
+    d.reset(brush.d.get());
+    return *this;
+}
+
+/*!
+    \fn QBrush &QBrush::operator=(QColor color)
+    \fn QBrush &QBrush::operator=(Qt::GlobalColor color)
+    \overload
+    \since 6.9
+
+    Makes this brush a solid pattern brush of the given \a color,
+    and returns a reference to \e this brush.
+*/
+QBrush &QBrush::operator=(QColor color)
+{
+    detach(Qt::SolidPattern);
+    d->color = color;
+    d->transform = {};
+    return *this;
+}
+
+/*!
+    \overload
+    \since 6.9
+
+    Makes this brush a black brush of the given \a style,
+    and returns a reference to \e this brush.
+*/
+QBrush &QBrush::operator=(Qt::BrushStyle style)
+{
+    detach(style);
+    d->color = Qt::black;
+    d->transform = {};
     return *this;
 }
 

src/gui/painting/qbrush.h

@@ -49,6 +49,10 @@ public:
     inline void swap(QBrush &other) noexcept
     { d.swap(other.d); }
 
+    QBrush &operator=(Qt::BrushStyle style);
+    QBrush &operator=(QColor color);
+    QBrush &operator=(Qt::GlobalColor color) { return operator=(QColor(color)); }
+
     operator QVariant() const;
 
     inline Qt::BrushStyle style() const;

src/gui/painting/qpen.cpp

@@ -344,6 +344,47 @@ QPen &QPen::operator=(const QPen &p) noexcept
     \memberswap{pen}
 */
 
+/*!
+    \overload
+    \since 6.9
+
+    Makes this pen a solid pen with the given color, and default
+    cap and join styles, and returns a reference to \e this pen.
+*/
+QPen &QPen::operator=(QColor color)
+{
+    detach();
+    d->brush = color;
+    d->width = 1;
+    d->style = Qt::SolidLine;
+    d->capStyle = qpen_default_cap;
+    d->joinStyle = qpen_default_join;
+
+    return *this;
+}
+
+/*!
+    \overload
+    \since 6.9
+
+    Makes this pen a solid, black pen with default cap and join styles,
+    and returns a reference to \e this pen.
+*/
+QPen &QPen::operator=(Qt::PenStyle style)
+{
+    detach();
+    if (style == Qt::NoPen) {
+        d = nullPenInstance()->pen;
+    } else {
+        d->brush = Qt::black;
+        d->width = 1;
+        d->style = style;
+        d->capStyle = qpen_default_cap;
+        d->joinStyle = qpen_default_join;
+    }
+    return *this;
+}
+
 /*!
    Returns the pen as a QVariant.
 */

src/gui/painting/qpen.h

@@ -41,6 +41,9 @@ public:
     QT_MOVE_ASSIGNMENT_OPERATOR_IMPL_VIA_PURE_SWAP(QPen)
     void swap(QPen &other) noexcept { d.swap(other.d); }
 
+    QPen &operator=(QColor color);
+    QPen &operator=(Qt::PenStyle style);
+
     Qt::PenStyle style() const;
     void setStyle(Qt::PenStyle);