Long live the short QDateTime optimization

Well, maybe not for so long: we may want to revisit it for Qt 6. At the
very least, we should enlarge the size of QDateTime on 32-bit system so
that they too can benefit from the optimization.

With this optimization, on 64-bit systems, the most common uses of
QDateTime now no longer allocate memory at all. The range is slightly
reduced from 584,554,049 years to 2,283,414 years around 1970. The other
drawback is that calling QDateTime::offsetFromUtc() on a localtime now
needs to recalculate the offset, instead of using the cached offset.
(QDateTime::toMSecsSinceEpoch() didn't use the cache).

Change-Id: Id5480807d25e49e78b79ffff144a8b2c9af91814
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

1 files changed, 7 insertions, 3 deletions

diff --git a/src/corelib/tools/qdatetime_p.h b/src/corelib/tools/qdatetime_p.h
index 6a5173f812..c4d00c99f9 100644
--- a/src/corelib/tools/qdatetime_p.h
+++ b/src/corelib/tools/qdatetime_p.h
@@ -64,6 +64,10 @@ QT_BEGIN_NAMESPACE
 class QDateTimePrivate
 {
 public:
+    // forward the declarations from QDateTime (this makes them public)
+    typedef QDateTime::ShortData QDateTimeShortData;
+    typedef QDateTime::Data QDateTimeData;
+
     // Never change or delete this enum, it is required for backwards compatible
     // serialization of QDateTime before 5.2, so is essentially public API
     enum Spec {
@@ -110,11 +114,11 @@ public:
     {
     }
 
-    QDateTimePrivate(const QDate &toDate, const QTime &toTime, Qt::TimeSpec toSpec,
-                     int offsetSeconds);
+    static QDateTime::Data create(const QDate &toDate, const QTime &toTime, Qt::TimeSpec toSpec,
+                                  int offsetSeconds);
 
 #ifndef QT_BOOTSTRAPPED
-    QDateTimePrivate(const QDate &toDate, const QTime &toTime, const QTimeZone & timeZone);
+    static QDateTime::Data create(const QDate &toDate, const QTime &toTime, const QTimeZone & timeZone);
 #endif // QT_BOOTSTRAPPED
 
     qint64 m_msecs;