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Make QDateTime's default constructor not allocate memory on 32-bit 

Commit fb498a6519 introduced the short
datetime optimization, but it accidentally removed the optimization from
Qt 5.7 that made QDateTime's default constructor not allocate memory.
Instead of bringing back the Q_GLOBAL_STATIC that was used, let's use
the short date time optimization on 32-bit too.

The CanBeSmall constant remains false on 32-bit systems, but judicious
non-use of that constant in a few functions allows 32-bit systems to use
the optimization. We still say that any date/time value is "unsmall", as
there's no sense in calculating whether the date is between 1970-01-01
and July 14 of the same year.

Change-Id: I413d458307aa46469b80fffd145a9db0325c08cc
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
This commit is contained in:
Thiago Macieira 2016-06-22 22:00:08 -07:00
parent d16ba63f5c
commit cba3378732
2 changed files with 46 additions and 27 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

72
src/corelib/tools/qdatetime.cpp
View File

@ -2487,6 +2487,16 @@ static inline bool specCanBeSmall(Qt::TimeSpec spec)
return spec == Qt::LocalTime || spec == Qt::UTC;
}
static inline bool msecsCanBeSmall(qint64 msecs)
{
if (!QDateTimeData::CanBeSmall)
return false;
ShortData sd;
sd.msecs = qintptr(msecs);
return sd.msecs == msecs;
}
static Q_DECL_CONSTEXPR inline
QDateTimePrivate::StatusFlags mergeSpec(QDateTimePrivate::StatusFlags status, Qt::TimeSpec spec)
{
@ -2693,9 +2703,9 @@ static void setDateTime(QDateTimeData &d, const QDate &date, const QTime &time)
qint64 msecs = (days * MSECS_PER_DAY) + ds;
if (d.isShort()) {
// let's see if we can keep this short
d.data.msecs = qintptr(msecs);
if (d.data.msecs == msecs) {
if (msecsCanBeSmall(msecs)) {
// yes, we can
d.data.msecs = qintptr(msecs);
d.data.status &= ~(QDateTimePrivate::ValidityMask | QDateTimePrivate::DaylightMask);
d.data.status |= newStatus;
} else {
@ -2734,6 +2744,15 @@ static QPair<QDate, QTime> getDateTime(const QDateTimeData &d)
QDateTime::Data member functions
*****************************************************************************/
inline QDateTime::Data::Data()
{
// default-constructed data has a special exception:
// it can be small even if CanBeSmall == false
// (optimization so we don't allocate memory in the default constructor)
quintptr value = quintptr(mergeSpec(QDateTimePrivate::ShortData, Qt::LocalTime));
d = reinterpret_cast<QDateTimePrivate *>(value);
}
inline QDateTime::Data::Data(Qt::TimeSpec spec)
{
if (CanBeSmall && Q_LIKELY(specCanBeSmall(spec))) {
@ -2751,9 +2770,9 @@ inline QDateTime::Data::Data(const Data &other)
{
if (!isShort()) {
// check if we could shrink
ShortData sd;
sd.msecs = qintptr(d->m_msecs);
if (CanBeSmall && specCanBeSmall(extractSpec(d->m_status)) && sd.msecs == d->m_msecs) {
if (specCanBeSmall(extractSpec(d->m_status)) && msecsCanBeSmall(d->m_msecs)) {
ShortData sd;
sd.msecs = qintptr(d->m_msecs);
sd.status = d->m_status | QDateTimePrivate::ShortData;
data = sd;
} else {
@ -2766,15 +2785,10 @@ inline QDateTime::Data::Data(const Data &other)
inline QDateTime::Data::Data(Data &&other)
: d(other.d)
{
// reset the other to a short state, if we can
if (CanBeSmall) {
Data dummy(Qt::LocalTime);
Q_ASSERT(dummy.isShort());
other.d = dummy.d;
} else if (!isShort()) {
// can't be small, so do implicit sharing
d->ref.ref();
}
// reset the other to a short state
Data dummy;
Q_ASSERT(dummy.isShort());
other.d = dummy.d;
}
inline QDateTime::Data &QDateTime::Data::operator=(const Data &other)
@ -2786,9 +2800,9 @@ inline QDateTime::Data &QDateTime::Data::operator=(const Data &other)
d = other.d;
if (!other.isShort()) {
// check if we could shrink
ShortData sd;
sd.msecs = qintptr(other.d->m_msecs);
if (CanBeSmall && specCanBeSmall(extractSpec(other.d->m_status)) && sd.msecs == other.d->m_msecs) {
if (specCanBeSmall(extractSpec(other.d->m_status)) && msecsCanBeSmall(other.d->m_msecs)) {
ShortData sd;
sd.msecs = qintptr(other.d->m_msecs);
sd.status = other.d->m_status | QDateTimePrivate::ShortData;
data = sd;
} else {
@ -2797,7 +2811,7 @@ inline QDateTime::Data &QDateTime::Data::operator=(const Data &other)
}
}
if (!(CanBeSmall && quintptr(x) & QDateTimePrivate::ShortData) && !x->ref.deref())
if (!(quintptr(x) & QDateTimePrivate::ShortData) && !x->ref.deref())
delete x;
return *this;
}
@ -2810,7 +2824,13 @@ inline QDateTime::Data::~Data()
inline bool QDateTime::Data::isShort() const
{
return CanBeSmall && quintptr(d) & QDateTimePrivate::ShortData;
bool b = quintptr(d) & QDateTimePrivate::ShortData;
// even if CanBeSmall = false, we have short data for a default-constructed
// QDateTime object. But it's unlikely.
if (CanBeSmall)
return Q_LIKELY(b);
return Q_UNLIKELY(b);
}
inline void QDateTime::Data::detach()
@ -3030,7 +3050,6 @@ inline qint64 QDateTimePrivate::zoneMSecsToEpochMSecs(qint64 zoneMSecs, const QT
\sa isValid()
*/
QDateTime::QDateTime() Q_DECL_NOEXCEPT_EXPR(Data::CanBeSmall)
: d(Qt::LocalTime)
{
}
@ -3618,12 +3637,10 @@ void QDateTime::setMSecsSinceEpoch(qint64 msecs)
}
}
ShortData sd;
sd.msecs = msecs;
if (d.isShort() && sd.msecs == msecs) {
if (msecsCanBeSmall(msecs) && d.isShort()) {
// we can keep short
sd.status = status;
d.data = sd;
d.data.msecs = qintptr(msecs);
d.data.status = status;
} else {
d.detach();
d->m_status = status;
@ -4006,8 +4023,9 @@ QDateTime QDateTime::addMSecs(qint64 msecs) const
if (d.isShort()) {
// need to check if we need to enlarge first
msecs += dt.d.data.msecs;
dt.d.data.msecs = qintptr(msecs);
if (dt.d.data.msecs != msecs) {
if (msecsCanBeSmall(msecs)) {
dt.d.data.msecs = qintptr(msecs);
} else {
dt.d.detach();
dt.d->m_msecs = msecs;
}

1
src/corelib/tools/qdatetime.h
View File

@ -230,6 +230,7 @@ class Q_CORE_EXPORT QDateTime
CanBeSmall = sizeof(ShortData) * 8 > 50
};
Data();
Data(Qt::TimeSpec);
Data(const Data &other);
Data(Data &&other);