QTextStream: add fast path for writing char and QChar.

Avoids allocating a QString for every char being written out.
The benchmark went from 5.5 ms per iteration to 0.8 ms,
and from 40 million instructions to 6 million.

Found using Milian Wolff's heaptrack tool.

Change-Id: I1784c47b944454bc947a607a22c39d249372ed55
Reviewed-by: Adam Majer <adamm@zombino.com>
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>

src/corelib/io/qtextstream.cpp

@@ -832,6 +832,21 @@ inline void QTextStreamPrivate::write(const QString &data)
     }
 }
 
+/*!
+    \internal
+*/
+inline void QTextStreamPrivate::write(QChar ch)
+{
+    if (string) {
+        // ### What about seek()??
+        string->append(ch);
+    } else {
+        writeBuffer += ch;
+        if (writeBuffer.size() > QTEXTSTREAM_BUFFERSIZE)
+            flushWriteBuffer();
+    }
+}
+
 /*!
     \internal
 */
@@ -870,6 +885,17 @@ inline void QTextStreamPrivate::ungetChar(QChar ch)
     readBuffer[--readBufferOffset] = ch;
 }
 
+/*!
+    \internal
+*/
+inline void QTextStreamPrivate::putChar(QChar ch)
+{
+    if (params.fieldWidth > 0)
+        putString(QString(ch));
+    else
+        write(ch);
+}
+
 /*!
     \internal
 */
@@ -2240,7 +2266,7 @@ QTextStream &QTextStream::operator<<(QChar c)
 {
     Q_D(QTextStream);
     CHECK_VALID_STREAM(*this);
-    d->putString(QString(c));
+    d->putChar(c);
     return *this;
 }
 
@@ -2253,7 +2279,7 @@ QTextStream &QTextStream::operator<<(char c)
 {
     Q_D(QTextStream);
     CHECK_VALID_STREAM(*this);
-    d->putString(QString(QChar::fromLatin1(c)));
+    d->putChar(QChar::fromLatin1(c));
     return *this;
 }
 

src/corelib/io/qtextstream_p.h

@@ -174,7 +174,9 @@ public:
     bool getReal(double *f);
 
     inline void write(const QString &data);
+    inline void write(QChar ch);
     inline void putString(const QString &ch, bool number = false);
+    inline void putChar(QChar ch);
     void putNumber(qulonglong number, bool negative);
 
     // buffers

tests/benchmarks/corelib/io/io.pro

@@ -6,5 +6,6 @@ SUBDIRS = \
         qfileinfo \
         qiodevice \
         qprocess \
-        qtemporaryfile
+        qtemporaryfile \
+        qtextstream
 

tests/benchmarks/corelib/io/qtextstream/main.cpp

@@ -0,0 +1,133 @@
+/****************************************************************************
+**
+** Copyright (C) 2014 David Faure <david.faure@kdab.com>
+** Contact: http://www.qt-project.org/legal
+**
+** This file is part of the test suite of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:LGPL$
+** 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 Digia.  For licensing terms and
+** conditions see http://qt.digia.com/licensing.  For further information
+** use the contact form at http://qt.digia.com/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 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, Digia gives you certain additional
+** rights.  These rights are described in the Digia Qt LGPL Exception
+** version 1.1, included in the file LGPL_EXCEPTION.txt in this package.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 3.0 as published by the Free Software
+** Foundation and appearing in the file LICENSE.GPL included in the
+** packaging of this file.  Please review the following information to
+** ensure the GNU General Public License version 3.0 requirements will be
+** met: http://www.gnu.org/copyleft/gpl.html.
+**
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+#include <QDebug>
+#include <QIODevice>
+#include <QString>
+#include <QBuffer>
+#include <qtest.h>
+
+class tst_qtextstream : public QObject
+{
+    Q_OBJECT
+private slots:
+    void writeSingleChar_data();
+    void writeSingleChar();
+
+private:
+};
+
+enum Output { StringOutput, DeviceOutput };
+Q_DECLARE_METATYPE(Output);
+
+enum Input { CharStarInput, QStringInput, CharInput, QCharInput };
+Q_DECLARE_METATYPE(Input);
+
+void tst_qtextstream::writeSingleChar_data()
+{
+    QTest::addColumn<Output>("output");
+    QTest::addColumn<Input>("input");
+
+    QTest::newRow("string_charstar") << StringOutput << CharStarInput;
+    QTest::newRow("string_string") << StringOutput << QStringInput;
+    QTest::newRow("string_char") << StringOutput << CharInput;
+    QTest::newRow("string_qchar") << StringOutput << QCharInput;
+    QTest::newRow("device_charstar") << DeviceOutput << CharStarInput;
+    QTest::newRow("device_string") << DeviceOutput << QStringInput;
+    QTest::newRow("device_char") << DeviceOutput << CharInput;
+    QTest::newRow("device_qchar") << DeviceOutput << QCharInput;
+}
+
+void tst_qtextstream::writeSingleChar()
+{
+    QFETCH(Output, output);
+    QFETCH(Input, input);
+
+    QString str;
+    QBuffer buffer;
+    QTextStream stream;
+    if (output == StringOutput) {
+        stream.setString(&str, QIODevice::WriteOnly);
+    } else {
+        QVERIFY(buffer.open(QIODevice::WriteOnly));
+        stream.setDevice(&buffer);
+    }
+    // Test many different ways to write a single char into a QTextStream
+    QString inputString = "h";
+    const int amount = 100000;
+    switch (input) {
+    case CharStarInput:
+        QBENCHMARK {
+            for (qint64 i = 0; i < amount; ++i)
+                stream << "h";
+        }
+        break;
+    case QStringInput:
+        QBENCHMARK {
+            for (qint64 i = 0; i < amount; ++i)
+                stream << inputString;
+        }
+        break;
+    case CharInput:
+        QBENCHMARK {
+            for (qint64 i = 0; i < amount; ++i)
+                stream << 'h';
+        }
+        break;
+    case QCharInput:
+        QBENCHMARK {
+            for (qint64 i = 0; i < amount; ++i)
+                stream << QChar('h');
+        }
+        break;
+    }
+    QString result;
+    if (output == StringOutput)
+        result = str;
+    else
+        result = QString(buffer.data());
+
+    QCOMPARE(result.left(10), QString("hhhhhhhhhh"));
+}
+
+QTEST_MAIN(tst_qtextstream)
+
+#include "main.moc"

tests/benchmarks/corelib/io/qtextstream/qtextstream.pro

@@ -0,0 +1,9 @@
+TEMPLATE = app
+TARGET = tst_bench_qtemporaryfile
+
+QT = core testlib
+
+CONFIG += release
+
+SOURCES += main.cpp
+DEFINES += QT_DISABLE_DEPRECATED_BEFORE=0