qdiriterator benchmark: test against std::filesystem

If c++17 is available and the header is available (not
experimental/filesystem) then we'll try
std::filesystem::recursive_directory_iterator as well.

Results on my PC (Windows, VS 2019):

PASS   : tst_qdiriterator::posix(C:\depot\qt\main/src/corelib)
RESULT : tst_qdiriterator::posix():"C:\depot\qt\main/src/corelib":
     23 msecs per iteration (total: 94, iterations: 4)
PASS   : tst_qdiriterator::diriterator(C:\depot\qt\main/src/corelib)
RESULT : tst_qdiriterator::diriterator():"C:\depot\qt\main/src/corelib":
     17 msecs per iteration (total: 71, iterations: 4)
PASS   : tst_qdiriterator::stdRecursiveDirectoryIterator(C:\depot\qt\main/src/corelib)
RESULT : tst_qdiriterator::stdRecursiveDirectoryIterator():"C:\depot\qt\main/src/corelib":
     6.7 msecs per iteration (total: 54, iterations: 8)
PASS   : tst_qdiriterator::fsiterator(C:\depot\qt\main/src/corelib)
RESULT : tst_qdiriterator::fsiterator():"C:\depot\qt\main/src/corelib":
     23 msecs per iteration (total: 92, iterations: 4)

And as a drive-by fix: move the 'data' function out of the private slots
so it is not invoked as a test function (it doesn't cause any problems
but is ultimately pointless).

Change-Id: Ia160ee276423ec51e35e554a4cd63d4d940c0e6a
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

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

@@ -44,9 +44,19 @@
 
 #include "qfilesystemiterator.h"
 
+#if QT_HAS_INCLUDE(<filesystem>) && defined(__cpp_lib_filesystem) && __cpp_lib_filesystem >= 201703L
+#define HAS_STD_FILESYSTEM
+#endif
+
+#ifdef HAS_STD_FILESYSTEM
+#include <filesystem>
+#endif
+
 class tst_qdiriterator : public QObject
 {
     Q_OBJECT
+
+    void data();
 private slots:
     void posix();
     void posix_data() { data(); }
@@ -54,7 +64,8 @@ private slots:
     void diriterator_data() { data(); }
     void fsiterator();
     void fsiterator_data() { data(); }
-    void data();
+    void stdRecursiveDirectoryIterator();
+    void stdRecursiveDirectoryIterator_data() { data(); }
 };
 
 
@@ -235,6 +246,28 @@ void tst_qdiriterator::fsiterator()
     qDebug() << count;
 }
 
+void tst_qdiriterator::stdRecursiveDirectoryIterator()
+{
+#ifdef HAS_STD_FILESYSTEM
+    QFETCH(QByteArray, dirpath);
+
+    int count = 0;
+
+    QBENCHMARK {
+        int c = 0;
+        for (auto obj : std::filesystem::recursive_directory_iterator(dirpath.data())) {
+            if (obj.is_directory())
+                continue;
+            c++;
+        }
+        count = c;
+    }
+    qDebug() << count;
+#else
+    QSKIP("Not supported.");
+#endif
+}
+
 QTEST_MAIN(tst_qdiriterator)
 
 #include "main.moc"

tests/benchmarks/corelib/io/qdiriterator/qdiriterator.pro

@@ -3,6 +3,8 @@ TARGET = tst_bench_qdiriterator
 QT = core testlib
 
 CONFIG += release
+# Enable c++17 support for std::filesystem
+qtConfig(c++1z): CONFIG += c++17
 
 SOURCES += main.cpp qfilesystemiterator.cpp
 HEADERS += qfilesystemiterator.h