Improve QIODevice::peek() performance on buffered devices

Since 5.7, QIODevice::peek() implementation is based on transaction
mechanism. While technically it's correct, seeking backward on a
buffered random-access device clears the internal buffer that affects
the performance of reading.

To solve the problem, this patch implements peek mode directly inside
the reading procedure.

Task-number: QTBUG-56032
Change-Id: Ic5269f76e44c491a0309e13aba87fa7cf7b9259f
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

1 files changed, 32 insertions, 0 deletions

diff --git a/tests/benchmarks/corelib/io/qiodevice/main.cpp b/tests/benchmarks/corelib/io/qiodevice/main.cpp
index b106a9fd3d..de4660a253 100644
--- a/tests/benchmarks/corelib/io/qiodevice/main.cpp
+++ b/tests/benchmarks/corelib/io/qiodevice/main.cpp
@@ -39,6 +39,8 @@ class tst_qiodevice : public QObject
 private slots:
     void read_old();
     void read_old_data() { read_data(); }
+    void peekAndRead();
+    void peekAndRead_data() { read_data(); }
     //void read_new();
     //void read_new_data() { read_data(); }
 private:
@@ -86,6 +88,36 @@ void tst_qiodevice::read_old()
     }
 }
 
+void tst_qiodevice::peekAndRead()
+{
+    QFETCH(qint64, size);
+
+    QString name = "tmp" + QString::number(size);
+
+    {
+        QFile file(name);
+        file.open(QIODevice::WriteOnly);
+        file.seek(size);
+        file.write("x", 1);
+        file.close();
+    }
+
+    QBENCHMARK {
+        QFile file(name);
+        file.open(QIODevice::ReadOnly);
+
+        QByteArray ba(size / 1024, Qt::Uninitialized);
+        while (!file.atEnd()) {
+            file.peek(ba.data(), ba.size());
+            file.read(ba.data(), ba.size());
+        }
+    }
+
+    {
+        QFile file(name);
+        file.remove();
+    }
+}
 
 QTEST_MAIN(tst_qiodevice)