QmlDebug: Reuse packets instead of deleting and recreating them

This reduces memory churn as we don't have to reallocate the various
QPacket, QBuffer, QByteArray objects and their private classes all the
time. Also, subsequent packets often have similar sizes, which makes it
beneficial to preallocate the underlying byte array to the size of the
previous packet on clear(). In order not to carry the extra reserved
space in the lists passed to the debug server, we squeeze the byte
arrays before inserting them into the lists. That detaches and copies
them, which would have been done anyway when reusing the same packet
for the next message.

The result is a reduction in the number of temporary memory allocations
from QQmlProfilerServiceImpl::sendMessages() by about 75%.

Change-Id: Ief98cd7a93a2e8e840e111dee6b346cae00e06bf
Reviewed-by: Simon Hausmann <simon.hausmann@theqtcompany.com>

2 files changed, 28 insertions, 3 deletions

diff --git a/src/plugins/qmltooling/packetprotocol/qpacket.cpp b/src/plugins/qmltooling/packetprotocol/qpacket.cpp
index c42288e920..fab0a5b189 100644
--- a/src/plugins/qmltooling/packetprotocol/qpacket.cpp
+++ b/src/plugins/qmltooling/packetprotocol/qpacket.cpp
@@ -108,11 +108,34 @@ QPacket::QPacket(int version, const QByteArray &data)
 }
 
 /*!
-  Returns raw packet data.
+  Returns a reference to the raw packet data.
   */
-QByteArray QPacket::data() const
+const QByteArray &QPacket::data() const
 {
     return buf.data();
 }
 
+/*!
+  Returns a copy of the raw packet data, with extra reserved space removed.
+  Mind that this triggers a deep copy. Use it if you anticipate the data to be detached soon anyway.
+  */
+QByteArray QPacket::squeezedData() const
+{
+    QByteArray ret = buf.data();
+    ret.squeeze();
+    return ret;
+}
+
+/*!
+  Clears the packet, discarding any data.
+ */
+void QPacket::clear()
+{
+    buf.reset();
+    QByteArray &buffer = buf.buffer();
+    // Keep the old size to prevent unnecessary allocations
+    buffer.reserve(buffer.capacity());
+    buffer.truncate(0);
+}
+
 QT_END_NAMESPACE
diff --git a/src/plugins/qmltooling/packetprotocol/qpacket_p.h b/src/plugins/qmltooling/packetprotocol/qpacket_p.h
index a079b244e8..b6fda2411d 100644
--- a/src/plugins/qmltooling/packetprotocol/qpacket_p.h
+++ b/src/plugins/qmltooling/packetprotocol/qpacket_p.h
@@ -61,7 +61,9 @@ class QPacket : public QDataStream
 public:
     QPacket(int version);
     explicit QPacket(int version, const QByteArray &ba);
-    QByteArray data() const;
+    const QByteArray &data() const;
+    QByteArray squeezedData() const;
+    void clear();
 
 private:
     void init(QIODevice::OpenMode mode);