tst_bench_QImageReader: add a benchmark for raw QFatoryLoader operations

This is Eirik's QtCore-only QTBUG-114253 reproducer added to the
nearest fitting existing benchmark.

Done-with: Eirik Aavitsland <eirik.aavitsland@qt.io>
Pick-to: 5.15
Task-number: QTBUG-114253
Change-Id: Iba68bedebae908af497267860e2b230db269787e
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>
(cherry picked from commit a5bcb41678ad7fe78ad3fac066094f6a614b6906)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
(cherry picked from commit 66cd58b2a569ae3423c319a65ab4f198c1c62714)
(cherry picked from commit 1c35f22097a5e6d9234a23567aa3f08992d81770)

tests/benchmarks/gui/image/qimagereader/CMakeLists.txt

@@ -8,6 +8,7 @@ qt_internal_add_benchmark(tst_bench_qimagereader
     SOURCES
         tst_bench_qimagereader.cpp
     PUBLIC_LIBRARIES
+        Qt::CorePrivate
         Qt::Gui
         Qt::Test
 )

tests/benchmarks/gui/image/qimagereader/tst_bench_qimagereader.cpp

@@ -36,6 +36,7 @@
 #include <QImageWriter>
 #include <QPixmap>
 
+#include <QtCore/private/qfactoryloader_p.h>
 #include <QSet>
 #include <QTimer>
 
@@ -55,6 +56,9 @@ public slots:
     void initTestCase();
 
 private slots:
+    void rawFactoryLoader_keyMap();
+    void rawFactoryLoader_instance();
+
     void readImage_data();
     void readImage();
 
@@ -70,6 +74,7 @@ private slots:
 private:
     QList< QPair<QString, QByteArray> > images; // filename, format
     QString prefix;
+    QFactoryLoader m_loader{QImageIOHandlerFactoryInterface_iid, "/imageformats"};
 };
 
 tst_bench_QImageReader::tst_bench_QImageReader()
@@ -106,6 +111,30 @@ void tst_bench_QImageReader::initTestCase()
         QFAIL("Can't find images directory!");
 }
 
+void tst_bench_QImageReader::rawFactoryLoader_keyMap()
+{
+    if (m_loader.keyMap().isEmpty())
+        QSKIP("No image plugins found.");
+
+    QBENCHMARK {
+        [[maybe_unused]] auto r = m_loader.keyMap();
+    }
+}
+
+void tst_bench_QImageReader::rawFactoryLoader_instance()
+{
+    if (m_loader.keyMap().isEmpty())
+        QSKIP("No image plugins found.");
+
+    const auto numInstances = m_loader.keyMap().uniqueKeys().size();
+
+    QBENCHMARK {
+        for (int i = 0; i < numInstances; ++i)
+            delete m_loader.instance(i);
+    }
+}
+
+
 void tst_bench_QImageReader::readImage_data()
 {
     QTest::addColumn<QString>("fileName");