Introduce a new class QStringBuilder to speed up the creation of

QString objects from smaller chunks.

The QStringBuilder class:

QStringBuilder uses expression templates (using the '%' operator)
to postpone any actual concatenation until it is assigned to an
actual QString. At that time it knows the exact sizes of all chunks,
can compute the required space, allocates once a QString of
appriopriate size and then copies over the chunk data one-by-one.

In addition, QLatin1Literal is a drop-in replacement for QLatin1String
(which we can't change for compatibility reasons) that knows its
size, therefore saving a few cycles when computing the size of the
resulting string.

Some further saved cycles stem from inlining and reduced reference
counting logic (the QString created from a QStringBuilder has typically
ref count equal to 1, while QString::append() needs an extra test)

Minor changes to the existing QString class:

- Introduce QString constructor to create an uninitialized QString of a given size.
This particular constructor is used by QStringBuilder class.

- Introduce a QT_USE_FAST_CONCATENATION macro to disable the existing
overloads of operator+() and helps finding the places where they are used in code.

- Introduce QT_USE_FAST_OPERATOR_PLUS.  This also disables the existing
overloads of operator+() and creates a new templated operator+() with
identical implementation of operator%().  This allows code that is compilable
QT_CAST_{TO,FROM}_ASCII to use QStringBuilder almost transparently. The only
case that is not covered is creating objects like QUrl that are implicitly
constructible from a QString from a QStringBuilder result.  This needs to be
converted explicitly to a QString first, e.g. by using QUrl
url(QString(QLatin1String("http://") + hostName));

Reviewed-by: MariusSO

2 files changed, 316 insertions, 0 deletions

diff --git a/tests/benchmarks/qstringbuilder/main.cpp b/tests/benchmarks/qstringbuilder/main.cpp
new file mode 100644
index 0000000..cb76925
--- /dev/null
+++ b/tests/benchmarks/qstringbuilder/main.cpp
@@ -0,0 +1,304 @@
+
+#include "qstringbuilder.h"
+
+#include <QDebug>
+#include <QString>
+
+#include <qtest.h>
+
+
+#define COMPARE(a, b) QCOMPARE(a, b)
+//#define COMPARE(a, b)
+
+#define SEP(s) qDebug() << "\n\n-------- " s "  ---------";
+#define L(s) QLatin1String(s)
+
+class tst_qstringbuilder : public QObject
+{
+    Q_OBJECT
+
+public:
+    tst_qstringbuilder()
+      : l1literal("some string literal"),
+        l1string("some string literal"),
+        ba("some string literal"),
+        string(l1string),
+        stringref(&string, 2, 10),
+        achar('c')
+    {}
+
+
+public:
+    enum { N = 10000 };
+
+    int run_traditional()
+    {
+        int s = 0;
+        for (int i = 0; i < N; ++i) {
+#if 0
+            s += QString(l1string + l1string).size();
+            s += QString(l1string + l1string + l1string).size();
+            s += QString(l1string + l1string + l1string + l1string).size();
+            s += QString(l1string + l1string + l1string + l1string + l1string).size();
+#endif
+            s += QString(achar + l1string + achar).size();
+        }
+        return s;
+    }
+
+    int run_builder()
+    {
+        int s = 0;
+        for (int i = 0; i < N; ++i) {
+#if 0
+            s += QString(l1literal % l1literal).size();
+            s += QString(l1literal % l1literal % l1literal).size();
+            s += QString(l1literal % l1literal % l1literal % l1literal).size();
+            s += QString(l1literal % l1literal % l1literal % l1literal % l1literal).size();
+#endif
+            s += QString(achar % l1literal % achar).size();
+        }
+        return s;
+    }
+
+private slots:
+
+    void separator_0() {
+        qDebug() << "\nIn each block the QStringBuilder based result appear first, "
+            "QStringBased second.\n";
+    }
+
+    void separator_1() { SEP("literal + literal  (builder first)"); }
+
+    void b_2_l1literal() {
+        QBENCHMARK { r = l1literal % l1literal; }
+        COMPARE(r, l1string + l1string);
+    }
+    void s_2_l1string() {
+        QBENCHMARK { r = l1string + l1string; }
+        COMPARE(r, QString(l1literal % l1literal));
+    }
+
+
+    void separator_2() { SEP("2 strings"); }
+
+    void b_2_string() {
+        QBENCHMARK { r = string % string; }
+        COMPARE(r, string + string);
+    }
+    void s_2_string() {
+        QBENCHMARK { r = string + string; }
+        COMPARE(r, QString(string % string));
+    }
+
+
+    void separator_2c() { SEP("2 string refs"); }
+
+    void b_2_stringref() {
+        QBENCHMARK { r = stringref % stringref; }
+        COMPARE(r, stringref.toString() + stringref.toString());
+    }
+    void s_2_stringref() {
+        QBENCHMARK { r = stringref.toString() + stringref.toString(); }
+        COMPARE(r, QString(stringref % stringref));
+    }
+
+
+    void separator_2b() { SEP("3 strings"); }
+
+    void b_3_string() {
+        QBENCHMARK { r = string % string % string; }
+        COMPARE(r, string + string + string);
+    }
+    void s_3_string() {
+        QBENCHMARK { r = string + string + string; }
+        COMPARE(r, QString(string % string % string));
+    }
+
+
+    void separator_2a() { SEP("string + literal  (builder first)"); }
+
+    void b_string_l1literal() {
+        QBENCHMARK { r = string % l1literal; }
+        COMPARE(r, string + l1string);
+    }
+    void b_string_l1string() {
+        QBENCHMARK { r = string % l1string; }
+        COMPARE(r, string + l1string);
+    }
+    void s_string_l1literal() {
+        QBENCHMARK { r = string + l1string; }
+        COMPARE(r, QString(string % l1literal));
+    }
+    void s_string_l1string() {
+        QBENCHMARK { r = string + l1string; }
+        COMPARE(r, QString(string % l1literal));
+    }
+
+
+    void separator_3() { SEP("3 literals"); }
+
+    void b_3_l1literal() {
+        QBENCHMARK { r = l1literal % l1literal % l1literal; }
+        COMPARE(r, l1string + l1string + l1string);
+    }
+    void s_3_l1string() {
+        QBENCHMARK { r = l1string + l1string + l1string; }
+        COMPARE(r, QString(l1literal % l1literal % l1literal));
+    }
+
+
+    void separator_4() { SEP("4 literals"); }
+
+    void b_4_l1literal() {
+        QBENCHMARK { r = l1literal % l1literal % l1literal % l1literal; }
+        COMPARE(r, l1string + l1string + l1string + l1string);
+    }
+    void s_4_l1string() {
+        QBENCHMARK { r = l1string + l1string + l1string + l1string; }
+        COMPARE(r, QString(l1literal % l1literal % l1literal % l1literal));
+    }
+
+
+    void separator_5() { SEP("5 literals"); }
+
+    void b_5_l1literal() {
+        QBENCHMARK { r = l1literal % l1literal % l1literal % l1literal %l1literal; }
+        COMPARE(r, l1string + l1string + l1string + l1string + l1string);
+    }
+
+    void s_5_l1string() {
+        QBENCHMARK { r = l1string + l1string + l1string + l1string + l1string; }
+        COMPARE(r, QString(l1literal % l1literal % l1literal % l1literal % l1literal));
+    }
+
+
+    void separator_6() { SEP("4 chars"); }
+
+    void b_string_4_char() {
+        QBENCHMARK { r = string + achar + achar + achar + achar; }
+        COMPARE(r, QString(string % achar % achar % achar % achar));
+    }
+
+    void s_string_4_char() {
+        QBENCHMARK { r = string + achar + achar + achar + achar; }
+        COMPARE(r, QString(string % achar % achar % achar % achar));
+    }
+
+
+    void separator_7() { SEP("char + string + char"); }
+
+    void b_char_string_char() {
+        QBENCHMARK { r = achar + string + achar; }
+        COMPARE(r, QString(achar % string % achar));
+    }
+
+    void s_char_string_char() {
+        QBENCHMARK { r = achar + string + achar; }
+        COMPARE(r, QString(achar % string % achar));
+    }
+
+    void separator_8() { SEP("string.arg"); }
+
+    void b_string_arg() {
+        const QString pattern = l1string + "%1" + l1string;
+        QBENCHMARK { r = l1literal % string % l1literal; }
+        COMPARE(r, l1string + string + l1string);
+    }
+
+    void s_string_arg() {
+        const QString pattern = l1string + "%1" + l1string;
+        QBENCHMARK { r = pattern.arg(string); }
+        COMPARE(r, l1string + string + l1string);
+    }
+
+    void s_bytearray_arg() {
+        QByteArray result;
+        QBENCHMARK { result = ba + ba + ba; }
+    }
+
+
+    void separator_9() { SEP("QString::reserve()"); }
+
+    void b_reserve() {
+        QBENCHMARK {
+            r.clear();
+            r = string % string % string % string;
+        }
+        COMPARE(r, string + string + string + string);
+    }
+    void b_reserve_lit() {
+        QBENCHMARK {
+            r.clear();
+            r = string % l1literal % string % string;
+        }
+        COMPARE(r, string + string + string + string);
+    }
+    void s_reserve() {
+        QBENCHMARK {
+            r.clear();
+            r.reserve(string.size() + string.size() + string.size() + string.size());
+            r += string;
+            r += string;
+            r += string;
+            r += string;
+        }
+        COMPARE(r, string + string + string + string);
+    }
+    void s_reserve_lit() {
+        QBENCHMARK {
+            r.clear();
+            //r.reserve(string.size() + qstrlen(l1string.latin1())
+            //    + string.size() + string.size());
+            r.reserve(1024);
+            r += string;
+            r += l1string;
+            r += string;
+            r += string;
+        }
+        COMPARE(r, string + string + string + string);
+    }
+
+private:
+    const QLatin1Literal l1literal;
+    const QLatin1String l1string;
+    const QByteArray ba;
+    const QString string;
+    const QStringRef stringref;
+    const QLatin1Char achar;
+
+    QString r;
+};
+
+
+//void operator%(QString, int) {}
+
+int main(int argc, char *argv[])
+{
+    //qDebug() << (QString("xx") * QLatin1String("y")).toString();
+    //42 % 3; // Sanity test, should always work.
+    //QString("x") % 2; // Sanity test, should only compile when the 
+    // operator%(QString, int) is visible.
+
+    if (argc == 2 && (argv[1] == L("--run-builder") || argv[1] == L("-b"))) {
+        tst_qstringbuilder test;
+        return test.run_builder();
+    }
+
+    if (argc == 2 && (argv[1] == L("--run-traditional") || argv[1] == L("-t"))) {
+        tst_qstringbuilder test;
+        return test.run_traditional();
+    }
+
+    if (argc == 1) {
+        QCoreApplication app(argc, argv);
+        QStringList args = app.arguments();
+        tst_qstringbuilder test;
+        return QTest::qExec(&test, argc, argv);
+    }
+
+    qDebug() << "Usage: " << argv[0] << " [--run-builder|-r|--run-traditional|-t]";
+}
+
+
+#include "main.moc"
diff --git a/tests/benchmarks/qstringbuilder/qstringbuilder.pro b/tests/benchmarks/qstringbuilder/qstringbuilder.pro
new file mode 100644
index 0000000..79171b4
--- /dev/null
+++ b/tests/benchmarks/qstringbuilder/qstringbuilder.pro
@@ -0,0 +1,12 @@
+load(qttest_p4)
+TEMPLATE = app
+TARGET = tst_qstringbuilder
+
+QMAKE_CXXFLAGS += -g
+QMAKE_CFLAGS += -g
+
+QT -= gui
+
+CONFIG += release
+
+SOURCES += main.cpp