Improve performance of hfw in qgridlayoutengine by adding more caching.

The previous code tried to do caching of queries with constraints too,
but it's usecase was rather limited.
The caching worked for the simple case of
effectiveSizeHint(Qt::PreferredSize, QSize(10, -1));
effectiveSizeHint(Qt::PreferredSize, QSize(10, -1));    // uses cache

The problem was that if somebody called this sequence:
effectiveSizeHint(Qt::PreferredSize, QSize(10, -1));
effectiveSizeHint(Qt::PreferredSize, QSize(-1, -1));
effectiveSizeHint(Qt::PreferredSize, QSize(10, -1));

Each call would disregard the cache because the constraint was
different.

Now the pattern is used in the qgridlayoutengine itself when we
calculate hfw: (yes, height-for-width).
First, we ask for the horizontal size hints with no constraints.
Then, we'll ask for the vertical size hints with constraints.
Since horizontal and vertical ultimately comes from the same function
(effectiveSizeHint) it will invalidate the cache each time.

The solution is to add another cache for the sizeHints with
constraints.

The most notable improvement is in the hfw, nested case. Result:
RESULT : tst_QGraphicsLinearLayout::heightForWidth():"hfw, nested":
     546 msecs per iteration (total: 546, iterations: 1)

RESULT : tst_QGraphicsLinearLayout::heightForWidth():"hfw, nested":
     0.000029 msecs per iteration (total: 62, iterations: 2097152)

Improvement: 18,827,586 times faster (!!)

4 files changed, 165 insertions, 11 deletions

diff --git a/src/gui/graphicsview/qgraphicslayoutitem.cpp b/src/gui/graphicsview/qgraphicslayoutitem.cpp
index e43f7fa..016cfbf 100644
--- a/src/gui/graphicsview/qgraphicslayoutitem.cpp
+++ b/src/gui/graphicsview/qgraphicslayoutitem.cpp
@@ -137,19 +137,28 @@ void QGraphicsLayoutItemPrivate::init()
 QSizeF *QGraphicsLayoutItemPrivate::effectiveSizeHints(const QSizeF &constraint) const
 {
     Q_Q(const QGraphicsLayoutItem);
-    if (!sizeHintCacheDirty && cachedConstraint == constraint)
-        return cachedSizeHints;
+    QSizeF *sizeHintCache;
+    const bool hasConstraint = constraint.width() >= 0 || constraint.height() >= 0;
+    if (hasConstraint) {
+        if (!sizeHintWithConstraintCacheDirty && constraint == cachedConstraint)
+            return cachedSizeHintsWithConstraints;
+        sizeHintCache = cachedSizeHintsWithConstraints;
+    } else {
+        if (!sizeHintCacheDirty)
+            return cachedSizeHints;
+        sizeHintCache = cachedSizeHints;
+    }
 
     for (int i = 0; i < Qt::NSizeHints; ++i) {
-        cachedSizeHints[i] = constraint;
+        sizeHintCache[i] = constraint;
         if (userSizeHints)
-            combineSize(cachedSizeHints[i], userSizeHints[i]);
+            combineSize(sizeHintCache[i], userSizeHints[i]);
     }
 
-    QSizeF &minS = cachedSizeHints[Qt::MinimumSize];
-    QSizeF &prefS = cachedSizeHints[Qt::PreferredSize];
-    QSizeF &maxS = cachedSizeHints[Qt::MaximumSize];
-    QSizeF &descentS = cachedSizeHints[Qt::MinimumDescent];
+    QSizeF &minS = sizeHintCache[Qt::MinimumSize];
+    QSizeF &prefS = sizeHintCache[Qt::PreferredSize];
+    QSizeF &maxS = sizeHintCache[Qt::MaximumSize];
+    QSizeF &descentS = sizeHintCache[Qt::MinimumDescent];
 
     normalizeHints(minS.rwidth(), prefS.rwidth(), maxS.rwidth(), descentS.rwidth());
     normalizeHints(minS.rheight(), prefS.rheight(), maxS.rheight(), descentS.rheight());
@@ -175,9 +184,13 @@ QSizeF *QGraphicsLayoutItemPrivate::effectiveSizeHints(const QSizeF &constraint)
     // Not supported yet
     // COMBINE_SIZE(descentS, q->sizeHint(Qt::MinimumDescent, constraint));
 
-    cachedConstraint = constraint;
-    sizeHintCacheDirty = false;
-    return cachedSizeHints;
+    if (hasConstraint) {
+        cachedConstraint = constraint;
+        sizeHintWithConstraintCacheDirty = false;
+    } else {
+        sizeHintCacheDirty = false;
+    }
+    return sizeHintCache;
 }
 
 
diff --git a/src/gui/graphicsview/qgraphicslayoutitem_p.h b/src/gui/graphicsview/qgraphicslayoutitem_p.h
index b752e03..d72ee9f 100644
--- a/src/gui/graphicsview/qgraphicslayoutitem_p.h
+++ b/src/gui/graphicsview/qgraphicslayoutitem_p.h
@@ -85,8 +85,10 @@ public:
     QSizeF *userSizeHints;
     mutable QSizeF cachedSizeHints[Qt::NSizeHints];
     mutable QSizeF cachedConstraint;
+    mutable QSizeF cachedSizeHintsWithConstraints[Qt::NSizeHints];
 
     mutable quint32 sizeHintCacheDirty : 1;
+    mutable quint32 sizeHintWithConstraintCacheDirty : 1;
     quint32 isLayout : 1;
     quint32 ownedByLayout : 1;
 
diff --git a/tests/benchmarks/gui/graphicsview/qgraphicslinearlayout/qgraphicslinearlayout.pro b/tests/benchmarks/gui/graphicsview/qgraphicslinearlayout/qgraphicslinearlayout.pro
new file mode 100644
index 0000000..ff85fe8
--- /dev/null
+++ b/tests/benchmarks/gui/graphicsview/qgraphicslinearlayout/qgraphicslinearlayout.pro
@@ -0,0 +1,6 @@
+load(qttest_p4)
+TEMPLATE = app
+TARGET = tst_bench_qgraphicslinearlayout
+
+SOURCES += tst_qgraphicslinearlayout.cpp
+
diff --git a/tests/benchmarks/gui/graphicsview/qgraphicslinearlayout/tst_qgraphicslinearlayout.cpp b/tests/benchmarks/gui/graphicsview/qgraphicslinearlayout/tst_qgraphicslinearlayout.cpp
new file mode 100644
index 0000000..0dd9543
--- /dev/null
+++ b/tests/benchmarks/gui/graphicsview/qgraphicslinearlayout/tst_qgraphicslinearlayout.cpp
@@ -0,0 +1,133 @@
+/****************************************************************************
+**
+** Copyright (C) 2010 Nokia Corporation and/or its subsidiary(-ies).
+** All rights reserved.
+** Contact: Nokia Corporation (qt-info@nokia.com)
+**
+** This file is part of the test suite of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:LGPL$
+** No Commercial Usage
+** This file contains pre-release code and may not be distributed.
+** You may use this file in accordance with the terms and conditions
+** contained in the Technology Preview License Agreement accompanying
+** this package.
+**
+** 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 as published by the Free Software
+** Foundation and appearing in the file LICENSE.LGPL included in the
+** packaging of this file.  Please review the following information to
+** ensure the GNU Lesser General Public License version 2.1 requirements
+** will be met: http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html.
+**
+** In addition, as a special exception, Nokia gives you certain additional
+** rights.  These rights are described in the Nokia Qt LGPL Exception
+** version 1.1, included in the file LGPL_EXCEPTION.txt in this package.
+**
+** If you have questions regarding the use of this file, please contact
+** Nokia at qt-info@nokia.com.
+**
+**
+**
+**
+**
+**
+**
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+#include <QtTest/QtTest>
+#include <QtGui/qgraphicslinearlayout.h>
+#include <QtGui/qgraphicswidget.h>
+#include <QtGui/qgraphicsview.h>
+
+class tst_QGraphicsLinearLayout : public QObject
+{
+    Q_OBJECT
+public:
+    tst_QGraphicsLinearLayout() {}
+    ~tst_QGraphicsLinearLayout() {}
+
+private slots:
+    void heightForWidth_data();
+    void heightForWidth();
+};
+
+
+struct MySquareWidget : public QGraphicsWidget
+{
+    MySquareWidget() {}
+    virtual QSizeF  sizeHint ( Qt::SizeHint which, const QSizeF & constraint = QSizeF() ) const
+    {
+        if (which != Qt::PreferredSize)
+            return QGraphicsWidget::sizeHint(which, constraint);
+        if (constraint.width() < 0)
+            return QGraphicsWidget::sizeHint(which, constraint);
+        return QSizeF(constraint.width(), constraint.width());
+    }
+};
+
+void tst_QGraphicsLinearLayout::heightForWidth_data()
+{
+    QTest::addColumn<bool>("hfw");
+    QTest::addColumn<bool>("nested");
+
+    QTest::newRow("hfw") << true << false;
+    QTest::newRow("hfw, nested") << true << true;
+    QTest::newRow("not hfw") << false << false;
+    QTest::newRow("not hfw, nested") << false << true;
+}
+
+void tst_QGraphicsLinearLayout::heightForWidth()
+{
+    QFETCH(bool, hfw);
+    QFETCH(bool, nested);
+
+    QGraphicsScene scene;
+    QGraphicsWidget *form = new QGraphicsWidget;
+    scene.addItem(form);
+
+    QGraphicsLinearLayout *outerlayout = 0;
+    if (nested) {
+       outerlayout = new QGraphicsLinearLayout(form);
+       for(int i = 0; i < 8; i++) {
+           QGraphicsLinearLayout *layout = new QGraphicsLinearLayout(Qt::Vertical);
+           outerlayout->addItem(layout);
+           outerlayout = layout;
+       }
+    }
+
+    QGraphicsLinearLayout *qlayout = 0;
+    qlayout = new QGraphicsLinearLayout(Qt::Vertical);
+    if (nested)
+        outerlayout->addItem(qlayout);
+    else
+        form->setLayout(qlayout);
+
+    MySquareWidget *widget = new MySquareWidget;
+    for (int i = 0; i < 1; i++) {
+        widget = new MySquareWidget;
+        QSizePolicy sizepolicy = widget->sizePolicy();
+        sizepolicy.setHeightForWidth(hfw);
+        widget->setSizePolicy(sizepolicy);
+        qlayout->addItem(widget);
+    }
+    // make sure only one iteration is done.
+    // run with tst_QGraphicsLinearLayout.exe "heightForWidth" -tickcounter -iterations 6
+    // this will iterate 6 times the whole test, (not only the benchmark)
+    // which should reduce warmup time and give a realistic picture of the performance of
+    // effectiveSizeHint()
+    QSizeF constraint(hfw ? 100 : -1, -1);
+    QBENCHMARK {
+        (void)form->effectiveSizeHint(Qt::PreferredSize, constraint);
+    }
+
+}
+
+
+QTEST_MAIN(tst_QGraphicsLinearLayout)
+
+#include "tst_qgraphicslinearlayout.moc"