Improve performance of QColor::name, now more than 4 times faster

Before: HexRgb: 0.00230 ms per iteration, HexArgb: 0.00290 ms per iteration
After:  HexRgb: 0.00051 ms per iteration, HexArgb: 0.00061 ms per iteration

This showed up as a relevant optimization when profiling KIconLoader
which uses QColor::name() as part of the key -- thanks to Mark Gaiser for
the investigation and first suggestion of a solution. I have also seen
customer code writing a replacement for QColor::name() because it was
too slow to be used as a hash key.

Change-Id: I009ccdd712ea0d869d466e2c9894e0cea58f0e68
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>

src/gui/painting/qcolor.cpp

@@ -521,9 +521,10 @@ QString QColor::name(NameFormat format) const
 {
     switch (format) {
     case HexRgb:
-        return QString::asprintf("#%02x%02x%02x", red(), green(), blue());
+        return QLatin1Char('#') + QString::number(rgba() | 0x1000000, 16).rightRef(6);
     case HexArgb:
-        return QString::asprintf("#%02x%02x%02x%02x", alpha(), red(), green(), blue());
+        // it's called rgba() but it does return AARRGGBB
+        return QLatin1Char('#') + QString::number(rgba() | 0x100000000, 16).rightRef(8);
     }
     return QString();
 }

tests/auto/gui/painting/qcolor/tst_qcolor.cpp

@@ -284,6 +284,8 @@ void tst_QColor::name_data()
     QTest::newRow("global color darkMagenta") << QColor(Qt::darkMagenta) << "#800080" << QColor::HexRgb;
     QTest::newRow("global color darkYellow") << QColor(Qt::darkYellow) << "#808000" << QColor::HexRgb;
     QTest::newRow("transparent red") << QColor(255, 0, 0, 102) << "#66ff0000" << QColor::HexArgb;
+    QTest::newRow("fully_transparent_green_rgb") << QColor(0, 0, 255, 0) << "#0000ff" << QColor::HexRgb;
+    QTest::newRow("fully_transparent_green_argb") << QColor(0, 0, 255, 0) << "#000000ff" << QColor::HexArgb;
 }
 
 void tst_QColor::name()

tests/benchmarks/gui/painting/painting.pro

@@ -1,5 +1,6 @@
 TEMPLATE = subdirs
 SUBDIRS = \
+        qcolor \
         qpainter \
         qregion \
         qtransform \

tests/benchmarks/gui/painting/qcolor/qcolor.pro

@@ -0,0 +1,7 @@
+QT += testlib
+QT += gui-private
+
+TEMPLATE = app
+TARGET = tst_bench_qcolor
+
+SOURCES += tst_qcolor.cpp

tests/benchmarks/gui/painting/qcolor/tst_qcolor.cpp

@@ -0,0 +1,67 @@
+/****************************************************************************
+**
+** Copyright (C) 2016 Klarälvdalens Datakonsult AB, a KDAB Group company, info@kdab.com, author David Faure <david.faure@kdab.com>
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of the test suite of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:LGPL21$
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see http://www.qt.io/terms-conditions. For further
+** information use the contact form at http://www.qt.io/contact-us.
+**
+** GNU Lesser General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU Lesser
+** General Public License version 2.1 or version 3 as published by the Free
+** Software Foundation and appearing in the file LICENSE.LGPLv21 and
+** LICENSE.LGPLv3 included in the packaging of this file. Please review the
+** following information to ensure the GNU Lesser General Public License
+** requirements will be met: https://www.gnu.org/licenses/lgpl.html and
+** http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html.
+**
+** As a special exception, The Qt Company gives you certain additional
+** rights. These rights are described in The Qt Company LGPL Exception
+** version 1.1, included in the file LGPL_EXCEPTION.txt in this package.
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+#include <qtest.h>
+#include <QColor>
+
+
+class tst_QColor : public QObject
+{
+    Q_OBJECT
+
+private slots:
+    void nameRgb();
+    void nameArgb();
+};
+
+void tst_QColor::nameRgb()
+{
+    QColor color(128, 255, 10);
+    QCOMPARE(color.name(), QStringLiteral("#80ff0a"));
+    QBENCHMARK {
+        color.name();
+    }
+}
+
+void tst_QColor::nameArgb()
+{
+    QColor color(128, 255, 0, 102);
+    QCOMPARE(color.name(QColor::HexArgb), QStringLiteral("#6680ff00"));
+    QBENCHMARK {
+        color.name(QColor::HexArgb);
+    }
+}
+
+QTEST_MAIN(tst_QColor)
+
+#include "tst_qcolor.moc"