Add a static QFileInfo::exists(fileName) function

This avoids dynamic construction of the private class. According to
the benchmark we go from 4,550 to 3,900 instruction reads per iteration.
(without change 32629676 the baseline is 5,600)

Change-Id: I5df925e30dbd49bdde87173e481820574ce5abe1
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/io/qfile.cpp

@@ -429,7 +429,7 @@ QFile::exists() const
 bool
 QFile::exists(const QString &fileName)
 {
-    return QFileInfo(fileName).exists();
+    return QFileInfo::exists(fileName);
 }
 
 /*!

src/corelib/io/qfileinfo.cpp

@@ -666,6 +666,28 @@ bool QFileInfo::exists() const
     return d->getFileFlags(QAbstractFileEngine::ExistsFlag);
 }
 
+/*!
+    Returns true if the \a file exists; otherwise returns false.
+
+    \note If \a file is a symlink that points to a non-existing
+    file, false is returned.
+
+    \note Using this function is faster for than using
+    \c QFileInfo(file).exists() for file system access.
+*/
+bool QFileInfo::exists(const QString &file)
+{
+    QFileSystemEntry entry(file);
+    QFileSystemMetaData data;
+    QAbstractFileEngine *engine =
+        QFileSystemEngine::resolveEntryAndCreateLegacyEngine(entry, data);
+    // Expensive fallback to non-QFileSystemEngine implementation
+    if (engine)
+        return QFileInfo(file).exists();
+    QFileSystemEngine::fillMetaData(entry, data, QFileSystemMetaData::ExistsAttribute);
+    return data.exists();
+}
+
 /*!
     Refreshes the information about the file, i.e. reads in information
     from the file system the next time a cached property is fetched.

src/corelib/io/qfileinfo.h

@@ -84,6 +84,7 @@ public:
     void setFile(const QFile &file);
     void setFile(const QDir &dir, const QString &file);
     bool exists() const;
+    static bool exists(const QString &file);
     void refresh();
 
     QString filePath() const;

tests/auto/corelib/io/qfileinfo/tst_qfileinfo.cpp

@@ -438,6 +438,7 @@ void tst_QFileInfo::exists()
 
     QFileInfo fi(path);
     QCOMPARE(fi.exists(), expected);
+    QCOMPARE(QFileInfo::exists(path), expected);
 }
 
 void tst_QFileInfo::absolutePath_data()

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

@@ -52,6 +52,8 @@ class qfileinfo : public QObject
 {
     Q_OBJECT
 private slots:
+    void existsTemporary();
+    void existsStatic();
 #if defined(Q_OS_WIN) && !defined(Q_OS_WINCE)
     void symLinkTargetPerformanceLNK();
     void symLinkTargetPerformanceMounpoint();
@@ -70,6 +72,18 @@ void qfileinfo::cleanupTestCase()
 {
 }
 
+void qfileinfo::existsTemporary()
+{
+    QString appPath = QCoreApplication::applicationFilePath();
+    QBENCHMARK { QFileInfo(appPath).exists(); }
+}
+
+void qfileinfo::existsStatic()
+{
+    QString appPath = QCoreApplication::applicationFilePath();
+    QBENCHMARK { QFileInfo::exists(appPath); }
+}
+
 #if defined(Q_OS_WIN) && !defined(Q_OS_WINCE)
 void qfileinfo::symLinkTargetPerformanceLNK()
 {