Improve performance of Qt::mightBeRichText by using QStringRef

I noted this taking around 1% of QtQuick Text creation with a few simple bindings
which is quite considerable, so hopefully improvements here will add up gradually.

Also add a benchmark measuring changes:

                  Test                 |      From      |       To       |        Details
+--------------------------------------+----------------+----------------+-----------------------+
  mightBeRichText:br-invalidspace      | 14466.00 instr | 10563.00 instr | -26.98% FASTER! :)
  mightBeRichText:br-nospace           | 18581.00 instr | 14635.00 instr | -21.24% FASTER! :)
  mightBeRichText:br-space             | 18470.00 instr | 14377.00 instr | -22.16% FASTER! :)
  mightBeRichText:documentation-header | 16336.00 instr | 12992.00 instr | -20.47% FASTER! :)
  mightBeRichText:empty                | 2618.00 instr  | 2618.00 instr  | more or less the same
  mightBeRichText:invalid closing tag  | 11102.00 instr | 7159.00 instr  | -35.52% FASTER! :)
  mightBeRichText:no tags              | 12503.00 instr | 8581.00 instr  | -31.37% FASTER! :)
  mightBeRichText:simple               | 17316.00 instr | 14074.00 instr | -18.72% FASTER! :)
  mightBeRichText:simple2              | 14394.00 instr | 10745.00 instr | -25.35% FASTER! :)
+--------------------------------------+----------------+----------------+-----------------------+
             Overall result            |                                        -201.81% :)

Change-Id: I1817a69959d176b381bcbf27b72bb751885c3e9b
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

src/gui/text/qtextdocument.cpp

@@ -94,7 +94,7 @@ bool Qt::mightBeRichText(const QString& text)
         ++start;
 
     // skip a leading <?xml ... ?> as for example with xhtml
-    if (text.mid(start, 5) == QLatin1String("<?xml")) {
+    if (text.midRef(start, 5).compare(QLatin1String("<?xml")) == 0) {
         while (start < text.length()) {
             if (text.at(start) == QLatin1Char('?')
                 && start + 2 < text.length()
@@ -109,12 +109,12 @@ bool Qt::mightBeRichText(const QString& text)
             ++start;
     }
 
-    if (text.mid(start, 5).toLower() == QLatin1String("<!doc"))
+    if (text.midRef(start, 5).compare(QLatin1String("<!doc"), Qt::CaseInsensitive) == 0)
         return true;
     int open = start;
     while (open < text.length() && text.at(open) != QLatin1Char('<')
             && text.at(open) != QLatin1Char('\n')) {
-        if (text.at(open) == QLatin1Char('&') &&  text.mid(open+1,3) == QLatin1String("lt;"))
+        if (text.at(open) == QLatin1Char('&') &&  text.midRef(open + 1, 3) == QLatin1String("lt;"))
             return true; // support desperate attempt of user to see <...>
         ++open;
     }

tests/benchmarks/gui/text/qtextdocument/main.cpp

@@ -0,0 +1,71 @@
+/****************************************************************************
+**
+** Copyright (C) 2016 Robin Burchell <robin.burchell@viroteck.net>
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of the test suite of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:GPL-EXCEPT$
+** 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 https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 3 as published by the Free Software
+** Foundation with exceptions as appearing in the file LICENSE.GPL3-EXCEPT
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-3.0.html.
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+#include <QDebug>
+#include <QTextDocument>
+#include <qtest.h>
+
+class tst_QTextDocument : public QObject
+{
+    Q_OBJECT
+private slots:
+    void mightBeRichText_data();
+    void mightBeRichText();
+};
+
+void tst_QTextDocument::mightBeRichText_data()
+{
+    QTest::addColumn<QString>("source");
+    QTest::addColumn<bool>("isMaybeRichText");
+    QTest::newRow("empty") << QString() << false;
+    QTest::newRow("simple") << QString::fromLatin1("<html><b>Foo</b></html>") << true;
+    QTest::newRow("simple2") << QString::fromLatin1("<b>Foo</b>") << true;
+    QTest::newRow("documentation-header") << QString("<?xml version=\"1.0\" encoding=\"iso-8859-1\"?>\n"
+                                                     "<!DOCTYPE html\n"
+                                                     "    PUBLIC ""-//W3C//DTD XHTML 1.0 Strict//EN\" \"DTD/xhtml1-strict.dtd\">\n"
+                                                     "<html xmlns=\"http://www.w3.org/1999/xhtml\" xml:lang=\"en\" lang=\"en\">")
+                                          << true;
+    QTest::newRow("br-nospace") << QString("Test <br/> new line") << true;
+    QTest::newRow("br-space") << QString("Test <br /> new line") << true;
+    QTest::newRow("br-invalidspace") << QString("Test <br/ > new line") << false;
+    QTest::newRow("invalid closing tag") << QString("Test <br/ line") << false;
+    QTest::newRow("no tags") << QString("Test line") << false;
+}
+
+void tst_QTextDocument::mightBeRichText()
+{
+    QFETCH(QString, source);
+    QFETCH(bool, isMaybeRichText);
+    QBENCHMARK {
+        QCOMPARE(isMaybeRichText, Qt::mightBeRichText(source));
+    }
+}
+
+QTEST_MAIN(tst_QTextDocument)
+
+#include "main.moc"

tests/benchmarks/gui/text/qtextdocument/qtextdocument.pro

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

tests/benchmarks/gui/text/text.pro

@@ -1,4 +1,5 @@
 TEMPLATE = subdirs
 SUBDIRS = \
         qfontmetrics \
-        qtext
+        qtext \
+        qtextdocument