QDataStream: Move trasactionDepth to the main class

The q_status member has too much space dedicated to it: 32 bits. So we
can shrink it to a mere 8 bits and move it up to the 1-byte padding hole
and repurpose the space it used to use. This only works because the
q_status member was not accessed by any inline function before Qt 6.8
(see commit fc23fa459c5924bf1cc4564c7bce1fd59d7c972b).

After this, pahole says:
  class QScopedPointer<QDataStreamPrivate> d;      /*     0     8 */
  class QIODevice *          dev;                  /*     8     8 */
  bool                       owndev;               /*    16     1 */
  bool                       noswap;               /*    17     1 */
  quint8                     fpPrecision;          /*    18     1 */
  quint8                     q_status;             /*    19     1 */
  enum ByteOrder             byteorder;            /*    20     4 */
  int                        ver;                  /*    24     4 */
  quint16                    transactionDepth;     /*    28     2 */
  /* size: 32, cachelines: 1, members: 10 */

That leaves 16 bits of tail padding unused, so reduces the maximum
number of nested transactions to 65536. That is to support a Qt 7 layout
in an upcoming commit.

Change-Id: I50e2158aeade4256ad1dfffd17b29d75fd13f472
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

3 files changed, 9 insertions, 11 deletions

diff --git a/src/corelib/serialization/qdatastream.cpp b/src/corelib/serialization/qdatastream.cpp
index 5be4e734c6..f1f45895aa 100644
--- a/src/corelib/serialization/qdatastream.cpp
+++ b/src/corelib/serialization/qdatastream.cpp
@@ -258,7 +258,7 @@ constexpr quint32 QDataStream::ExtendedSize;
         return retVal;
 
 #define CHECK_STREAM_TRANSACTION_PRECOND(retVal) \
-    if (!d || d->transactionDepth == 0) { \
+    if (transactionDepth == 0) { \
         qWarning("QDataStream: No transaction in progress"); \
         return retVal; \
     }
@@ -623,10 +623,7 @@ void QDataStream::startTransaction()
 {
     CHECK_STREAM_PRECOND(Q_VOID)
 
-    if (!d)
-        d.reset(new QDataStreamPrivate());
-
-    if (++d->transactionDepth == 1) {
+    if (++transactionDepth == 1) {
         dev->startTransaction();
         resetStatus();
     }
@@ -655,7 +652,7 @@ void QDataStream::startTransaction()
 bool QDataStream::commitTransaction()
 {
     CHECK_STREAM_TRANSACTION_PRECOND(false)
-    if (--d->transactionDepth == 0) {
+    if (--transactionDepth == 0) {
         CHECK_STREAM_PRECOND(false)
 
         if (q_status == ReadPastEnd) {
@@ -695,7 +692,7 @@ void QDataStream::rollbackTransaction()
     setStatus(ReadPastEnd);
 
     CHECK_STREAM_TRANSACTION_PRECOND(Q_VOID)
-    if (--d->transactionDepth != 0)
+    if (--transactionDepth != 0)
         return;
 
     CHECK_STREAM_PRECOND(Q_VOID)
@@ -731,7 +728,7 @@ void QDataStream::abortTransaction()
     q_status = ReadCorruptData;
 
     CHECK_STREAM_TRANSACTION_PRECOND(Q_VOID)
-    if (--d->transactionDepth != 0)
+    if (--transactionDepth != 0)
         return;
 
     CHECK_STREAM_PRECOND(Q_VOID)
diff --git a/src/corelib/serialization/qdatastream.h b/src/corelib/serialization/qdatastream.h
index b4be27f474..27694aacc3 100644
--- a/src/corelib/serialization/qdatastream.h
+++ b/src/corelib/serialization/qdatastream.h
@@ -220,9 +220,11 @@ private:
     bool owndev;
     bool noswap;
     quint8 fpPrecision = QDataStream::DoublePrecision;
+    quint8 q_status;
     ByteOrder byteorder;
     int ver;
-    Status q_status;
+    quint16 transactionDepth = 0;
+
 #if QT_CORE_REMOVED_SINCE(6, 7)
     int readBlock(char *data, int len);
 #endif
@@ -426,7 +428,7 @@ inline QIODevice *QDataStream::device() const
 #if QT_CORE_INLINE_IMPL_SINCE(6, 8)
 QDataStream::Status QDataStream::status() const
 {
-    return q_status;
+    return Status(q_status);
 }
 
 QDataStream::FloatingPointPrecision QDataStream::floatingPointPrecision() const
diff --git a/src/corelib/serialization/qdatastream_p.h b/src/corelib/serialization/qdatastream_p.h
index 4600fa64d0..c4fe7c784c 100644
--- a/src/corelib/serialization/qdatastream_p.h
+++ b/src/corelib/serialization/qdatastream_p.h
@@ -24,7 +24,6 @@ QT_BEGIN_NAMESPACE
 class QDataStreamPrivate
 {
 public:
-    int transactionDepth = 0;
 };
 #endif