QSettings: port key processing to QAnyStringView

... in preparation for replacing the QString keys in the public API
with QAnyStringView ones.

This removes the "important optimization" that avoids a detach in the
common case where the input is the same as the output of
normalization. But that optimization is beside the point, because it
trades a memory allocation avoided in the library for O(N) allocations
inserted into user code for each call to QSettings::value(), the vast
majority of which are calls with string literals.

With the public interface ported to QAnyStringView in the follow-up
patch, we can then internally optimize memory allocations _in a central
place_ (e.g. by returning std::u16string or QVarLengthArray<QChar> from
normalizeKey() instead of QString). But first we need to get rid of all
the unwarranted allocations in user code.

Change-Id: I45fc83d972c552a220c9c29508001d3f172e1162
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Sona Kurazyan <sona.kurazyan@qt.io>

src/corelib/io/qsettings.cpp

@@ -1,6 +1,6 @@
 /****************************************************************************
 **
-** Copyright (C) 2020 The Qt Company Ltd.
+** Copyright (C) 2022 The Qt Company Ltd.
 ** Contact: https://www.qt.io/licensing/
 **
 ** This file is part of the QtCore module of the Qt Toolkit.
@@ -53,7 +53,7 @@
 #include "qtemporaryfile.h"
 #include "qstandardpaths.h"
 #include <qdatastream.h>
-#include <qstringconverter.h>
+#include "private/qstringconverter_p.h"
 
 #ifndef QT_NO_GEOM_VARIANT
 #include "qsize.h"
@@ -230,13 +230,32 @@ QSettingsPrivate::~QSettingsPrivate()
 {
 }
 
-QString QSettingsPrivate::actualKey(const QString &key) const
+QString QSettingsPrivate::actualKey(QAnyStringView key) const
 {
     auto n = normalizedKey(key);
     Q_ASSERT_X(!n.isEmpty(), "QSettings", "empty key");
     return groupPrefix + n;
 }
 
+namespace {
+    // ### this needs some public API (QStringConverter?)
+    QChar *write(QChar *out, QUtf8StringView v)
+    {
+        return QUtf8::convertToUnicode(out, QByteArrayView(v));
+    }
+    QChar *write(QChar *out, QLatin1String v)
+    {
+        for (char ch : v)
+            *out++ = QLatin1Char(ch);
+        return out;
+    }
+    QChar *write(QChar *out, QStringView v)
+    {
+        memcpy(out, v.data(), v.size() * sizeof(QChar));
+        return out + v.size();
+    }
+}
+
 /*
     Returns a string that never starts nor ends with a slash (or an
     empty string). Examples:
@@ -244,32 +263,43 @@ QString QSettingsPrivate::actualKey(const QString &key) const
             "foo"            becomes   "foo"
             "/foo//bar///"   becomes   "foo/bar"
             "///"            becomes   ""
-
-    This function is optimized to avoid a QString deep copy in the
-    common case where the key is already normalized.
 */
-QString QSettingsPrivate::normalizedKey(const QString &key)
+QString QSettingsPrivate::normalizedKey(QAnyStringView key)
 {
-    QString result = key;
+    QString result(key.size(), Qt::Uninitialized);
+    auto out = const_cast<QChar*>(result.constData()); // don't detach
 
-    int i = 0;
-    while (i < result.size()) {
-        while (result.at(i) == QLatin1Char('/')) {
-            result.remove(i, 1);
-            if (i == result.size())
-                goto after_loop;
-        }
-        while (result.at(i) != QLatin1Char('/')) {
-            ++i;
-            if (i == result.size())
-                return result;
-        }
-        ++i; // leave the slash alone
-    }
+    const bool maybeEndsInSlash = key.visit([&out](auto key) {
+        using View = decltype(key);
 
-after_loop:
-    if (!result.isEmpty())
-        result.truncate(i - 1); // remove the trailing slash
+        auto it = key.begin();
+        const auto end = key.end();
+
+        while (it != end) {
+            while (*it == u'/') {
+                ++it;
+                if (it == end)
+                    return true;
+            }
+            auto mark = it;
+            while (*it != u'/') {
+                ++it;
+                if (it == end)
+                    break;
+            }
+            out = write(out, View{mark, it});
+            if (it == end)
+                return false;
+            Q_ASSERT(*it == u'/');
+            *out++ = u'/';
+            ++it;
+        }
+        return true;
+    });
+
+    if (maybeEndsInSlash && out != result.constData())
+        --out; // remove the trailing slash
+    result.truncate(out - result.constData());
     return result;
 }
 
@@ -3290,7 +3320,7 @@ QVariant QSettings::value(const QString &key, const QVariant &defaultValue) cons
     return d->value(key, &defaultValue);
 }
 
-QVariant QSettingsPrivate::value(const QString &key, const QVariant *defaultValue) const
+QVariant QSettingsPrivate::value(QAnyStringView key, const QVariant *defaultValue) const
 {
     if (key.isEmpty()) {
         qWarning("QSettings::value: Empty key passed");

src/corelib/io/qsettings_p.h

@@ -1,6 +1,6 @@
 /****************************************************************************
 **
-** Copyright (C) 2016 The Qt Company Ltd.
+** Copyright (C) 2022 The Qt Company Ltd.
 ** Contact: https://www.qt.io/licensing/
 **
 ** This file is part of the QtCore module of the Qt Toolkit.
@@ -209,14 +209,14 @@ public:
     virtual bool isWritable() const = 0;
     virtual QString fileName() const = 0;
 
-    QVariant value(const QString &key, const QVariant *defaultValue) const;
-    QString actualKey(const QString &key) const;
+    QVariant value(QAnyStringView key, const QVariant *defaultValue) const;
+    QString actualKey(QAnyStringView key) const;
     void beginGroupOrArray(const QSettingsGroup &group);
     void setStatus(QSettings::Status status) const;
     void requestUpdate();
     void update();
 
-    static QString normalizedKey(const QString &key);
+    static QString normalizedKey(QAnyStringView key);
     static QSettingsPrivate *create(QSettings::Format format, QSettings::Scope scope,
                                         const QString &organization, const QString &application);
     static QSettingsPrivate *create(const QString &fileName, QSettings::Format format);

tests/auto/corelib/io/qsettings/tst_qsettings.cpp

@@ -1,6 +1,6 @@
 /****************************************************************************
 **
-** Copyright (C) 2016 The Qt Company Ltd.
+** Copyright (C) 2022 The Qt Company Ltd.
 ** Contact: https://www.qt.io/licensing/
 **
 ** This file is part of the test suite of the Qt Toolkit.
@@ -2160,20 +2160,17 @@ void tst_QSettings::testNormalizedKey()
 
     inKey.detach();
 
-    QString result = QSettingsPrivate::normalizedKey(inKey);
-    QCOMPARE(result, outKey);
-
-    /*
-        If the key is already normalized, we verify that outKey is
-        just a shallow copy of the input string. This is an important
-        optimization that shouldn't be removed accidentally.
-    */
-    if (inKey == outKey) {
-        QVERIFY(!result.isDetached());
-    } else {
-        if (!result.isEmpty()) {
-            QVERIFY(result.isDetached());
-        }
+    {
+        auto result = QSettingsPrivate::normalizedKey(inKey);
+        QCOMPARE(result, outKey);
+    }
+    {
+        auto result = QSettingsPrivate::normalizedKey(QUtf8StringView{inKey.toUtf8()});
+        QCOMPARE(result, outKey);
+    }
+    {
+        auto result = QSettingsPrivate::normalizedKey(QLatin1String{inKey.toLatin1()});
+        QCOMPARE(result, outKey);
     }
 }
 #endif