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, 43 insertions, 8 deletions

diff --git a/src/corelib/tools/qdatetime.h b/src/corelib/tools/qdatetime.h
index cd4644d76a..33dd20953a 100644
--- a/src/corelib/tools/qdatetime.h
+++ b/src/corelib/tools/qdatetime.h
@@ -1,6 +1,7 @@
 /****************************************************************************
 **
 ** Copyright (C) 2016 The Qt Company Ltd.
+** Copyright (C) 2016 Intel Corporation.
 ** Contact: https://www.qt.io/licensing/
 **
 ** This file is part of the QtCore module of the Qt Toolkit.
@@ -208,8 +209,45 @@ class QDateTimePrivate;
 
 class Q_CORE_EXPORT QDateTime
 {
+    // ### Qt 6: revisit the optimization
+    struct ShortData {
+#if Q_BYTE_ORDER == Q_LITTLE_ENDIAN
+        quintptr status : 8;
+#endif
+        // note: this is only 24 bits on 32-bit systems...
+        qintptr msecs : sizeof(void *) * 8 - 8;
+
+#if Q_BYTE_ORDER == Q_BIG_ENDIAN
+        quintptr status : 8;
+#endif
+    };
+
+    union Data {
+        enum {
+            // To be of any use, we need at least 60 years around 1970, which
+            // is 1,893,456,000,000 ms. That requires 41 bits to store, plus
+            // the sign bit. With the status byte, the minimum size is 50 bits.
+            CanBeSmall = sizeof(ShortData) * 8 > 50
+        };
+
+        Data(Qt::TimeSpec);
+        Data(const Data &other);
+        Data &operator=(const Data &other);
+        // no move semantics (would be the same as copy)
+        ~Data();
+
+        bool isShort() const;
+        void detach();
+
+        const QDateTimePrivate *operator->() const;
+        QDateTimePrivate *operator->();
+
+        QDateTimePrivate *d;
+        ShortData data;
+    };
+
 public:
-    QDateTime();
+    QDateTime() Q_DECL_NOEXCEPT_EXPR(Data::CanBeSmall);
     explicit QDateTime(const QDate &);
     QDateTime(const QDate &, const QTime &, Qt::TimeSpec spec = Qt::LocalTime);
     // ### Qt 6: Merge with above with default offsetSeconds = 0
@@ -217,15 +255,15 @@ public:
 #ifndef QT_BOOTSTRAPPED
     QDateTime(const QDate &date, const QTime &time, const QTimeZone &timeZone);
 #endif // QT_BOOTSTRAPPED
-    QDateTime(const QDateTime &other);
+    QDateTime(const QDateTime &other) Q_DECL_NOTHROW;
     ~QDateTime();
 
 #ifdef Q_COMPILER_RVALUE_REFS
     QDateTime &operator=(QDateTime &&other) Q_DECL_NOTHROW { swap(other); return *this; }
 #endif
-    QDateTime &operator=(const QDateTime &other);
+    QDateTime &operator=(const QDateTime &other) Q_DECL_NOTHROW;
 
-    void swap(QDateTime &other) Q_DECL_NOTHROW { qSwap(d, other.d); }
+    void swap(QDateTime &other) Q_DECL_NOTHROW { qSwap(d.d, other.d.d); }
 
     bool isNull() const;
     bool isValid() const;
@@ -321,10 +359,7 @@ public:
 private:
     friend class QDateTimePrivate;
 
-    // ### Qt6: Using a private here has high impact on runtime
-    // on users such as QFileInfo. In Qt 6, the data members
-    // should be inlined.
-    QSharedDataPointer<QDateTimePrivate> d;
+    Data d;
 
 #ifndef QT_NO_DATASTREAM
     friend Q_CORE_EXPORT QDataStream &operator<<(QDataStream &, const QDateTime &);