QMimeDatabasePrivate: deduplicate QStringLiteral

defaultMimeType() cannot be moved out of the class and defined in
qmimedatabase.cpp, because it is called from QMimeType::isDefault() in
qmimetype.cpp.

Return m_defaultMimeType constant by reference rather than by value from
the inline helper function. This eliminates useless copying of QString
when calling code needs only a reference, and so improves performance.
Even though m_defaultMimeType is initialized with a QStringLiteral and
its QArrayDataPointer has d == nullptr, QArrayDataPointer's ref() and
deref() still have to compare d with nullptr to decide that no reference
count manipulation is needed. Compilers might be unable to reorder these
checks with surrounding code because they guard reference count
manipulation. Branch misprediction could further harm performance.
Optimizing these checks out can be difficult for compilers.

Change-Id: I3ffa0fdc5ad948452301dd9688073b5323a7983b
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/mimetypes/qmimedatabase.cpp

@@ -61,6 +61,15 @@
 
 QT_BEGIN_NAMESPACE
 
+static QString directoryMimeType()
+{
+    return QStringLiteral("inode/directory");
+}
+static QString plainTextMimeType()
+{
+    return QStringLiteral("text/plain");
+}
+
 Q_GLOBAL_STATIC(QMimeDatabasePrivate, staticQMimeDatabase)
 
 QMimeDatabasePrivate *QMimeDatabasePrivate::instance()
@@ -205,7 +214,7 @@ QMimeType QMimeDatabasePrivate::mimeTypeForName(const QString &nameOrAlias)
 QStringList QMimeDatabasePrivate::mimeTypeForFileName(const QString &fileName)
 {
     if (fileName.endsWith(QLatin1Char('/')))
-        return QStringList() << QStringLiteral("inode/directory");
+        return { directoryMimeType() };
 
     const QMimeGlobMatchResult result = findByFileName(fileName);
     QStringList matchingMimeTypes = result.m_matchingMimeTypes;
@@ -272,18 +281,18 @@ void QMimeDatabasePrivate::loadIcon(QMimeTypePrivate &mimePrivate)
     }
 }
 
-static QString fallbackParent(const QString &mimeTypeName)
+QString QMimeDatabasePrivate::fallbackParent(const QString &mimeTypeName) const
 {
     const QStringView myGroup = QStringView{mimeTypeName}.left(mimeTypeName.indexOf(QLatin1Char('/')));
     // All text/* types are subclasses of text/plain.
-    if (myGroup == QLatin1String("text") && mimeTypeName != QLatin1String("text/plain"))
-        return QStringLiteral("text/plain");
+    if (myGroup == QLatin1String("text") && mimeTypeName != plainTextMimeType())
+        return plainTextMimeType();
     // All real-file mimetypes implicitly derive from application/octet-stream
     if (myGroup != QLatin1String("inode") &&
         // ignore non-file extensions
         myGroup != QLatin1String("all") && myGroup != QLatin1String("fonts") && myGroup != QLatin1String("print") && myGroup != QLatin1String("uri")
-        && mimeTypeName != QLatin1String("application/octet-stream")) {
-        return QStringLiteral("application/octet-stream");
+        && mimeTypeName != defaultMimeType()) {
+        return defaultMimeType();
     }
     return QString();
 }
@@ -359,7 +368,7 @@ QMimeType QMimeDatabasePrivate::findByData(const QByteArray &data, int *accuracy
 
     if (isTextFile(data)) {
         *accuracyPtr = 5;
-        return mimeTypeForName(QStringLiteral("text/plain"));
+        return mimeTypeForName(plainTextMimeType());
     }
 
     return mimeTypeForName(defaultMimeType());
@@ -469,7 +478,7 @@ QMimeType QMimeDatabasePrivate::mimeTypeForFile(const QString &fileName,
     QT_STATBUF statBuffer;
     if (QT_STAT(nativeFilePath.constData(), &statBuffer) == 0) {
         if (S_ISDIR(statBuffer.st_mode))
-            return mimeTypeForName(QStringLiteral("inode/directory"));
+            return mimeTypeForName(directoryMimeType());
         if (S_ISCHR(statBuffer.st_mode))
             return mimeTypeForName(QStringLiteral("inode/chardevice"));
         if (S_ISBLK(statBuffer.st_mode))
@@ -482,7 +491,7 @@ QMimeType QMimeDatabasePrivate::mimeTypeForFile(const QString &fileName,
 #else
     const bool isDirectory = fileInfo ? fileInfo->isDir() : QFileInfo(fileName).isDir();
     if (isDirectory)
-        return mimeTypeForName(QStringLiteral("inode/directory"));
+        return mimeTypeForName(directoryMimeType());
 #endif
 
     switch (mode) {
@@ -764,7 +773,7 @@ QMimeType QMimeDatabase::mimeTypeForFileNameAndData(const QString &fileName, QIO
     QMutexLocker locker(&d->mutex);
 
     if (fileName.endsWith(QLatin1Char('/')))
-        return d->mimeTypeForName(QStringLiteral("inode/directory"));
+        return d->mimeTypeForName(directoryMimeType());
 
     const bool openedByUs = !device->isOpen() && device->open(QIODevice::ReadOnly);
     const QMimeType result = d->mimeTypeForFileNameAndData(fileName, device);
@@ -794,7 +803,7 @@ QMimeType QMimeDatabase::mimeTypeForFileNameAndData(const QString &fileName, con
     QMutexLocker locker(&d->mutex);
 
     if (fileName.endsWith(QLatin1Char('/')))
-        return d->mimeTypeForName(QStringLiteral("inode/directory"));
+        return d->mimeTypeForName(directoryMimeType());
 
     QBuffer buffer(const_cast<QByteArray *>(&data));
     buffer.open(QIODevice::ReadOnly);

src/corelib/mimetypes/qmimedatabase_p.h

@@ -84,7 +84,7 @@ public:
 
     static QMimeDatabasePrivate *instance();
 
-    inline QString defaultMimeType() const { return m_defaultMimeType; }
+    const QString &defaultMimeType() const { return m_defaultMimeType; }
 
     bool inherits(const QString &mime, const QString &parent);
 
@@ -114,12 +114,13 @@ private:
     const Providers &providers();
     bool shouldCheck();
     void loadProviders();
+    QString fallbackParent(const QString &mimeTypeName) const;
 
+    const QString m_defaultMimeType;
     mutable Providers m_providers;
     QElapsedTimer m_lastCheck;
 
 public:
-    const QString m_defaultMimeType;
     QMutex mutex;
 };