qtimezoneprivate_tz: Apply a cache over the top of timezone data

Constantly re-reading the timezone information only to be told the exact
same thing is wildly expensive, which can hurt in operations that cause
a lot of QTimeZone creation, for example, V4's DateObject - which
creates them a lot (in DaylightSavingTA).

This performance problem was identified when I noticed that a
QDateTime binding updated once per frame was causing >100% CPU usage
(on a desktop!) thanks to a QtQuickControls 1 Calendar (which has a
number of bindings to the date's properties like getMonth() and so
on).

The newly added tst_QTimeZone::systemTimeZone benchmark gets a ~90%
decrease in instruction count:

--- before
+++ after
 PASS   : tst_QTimeZone::systemTimeZone()
 RESULT : tst_QTimeZone::systemTimeZone():
-     0.024 msecs per iteration (total: 51, iterations: 2048)
+     0.0036 msecs per iteration (total: 59, iterations: 16384)

Also impacted (over in QDateTime) is
tst_QDateTime::setMSecsSinceEpochTz(). The results here are - on the
surface - less impressive (~0.17% drop), however, it isn't even
creating QTimeZone on a hot path to begin with, so a large drop would
have been a surprise.

Added several further benchmarks to cover non-system zones and
traverse transitions.

Done-With: Edward Welbourne <edward.welbourne@qt.io>
Task-number: QTBUG-75585
Change-Id: I044a84fc2d3a2dc965f63cd3a3299fc509750bf7
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

1 files changed, 12 insertions, 4 deletions

diff --git a/src/corelib/time/qtimezoneprivate_p.h b/src/corelib/time/qtimezoneprivate_p.h
index 5f6491ef81..c3e9064b8c 100644
--- a/src/corelib/time/qtimezoneprivate_p.h
+++ b/src/corelib/time/qtimezoneprivate_p.h
@@ -287,6 +287,16 @@ Q_DECL_CONSTEXPR inline bool operator==(const QTzTransitionRule &lhs, const QTzT
 Q_DECL_CONSTEXPR inline bool operator!=(const QTzTransitionRule &lhs, const QTzTransitionRule &rhs) noexcept
 { return !operator==(lhs, rhs); }
 
+// These are stored separately from QTzTimeZonePrivate so that they can be
+// cached, avoiding the need to re-parse them from disk constantly.
+struct QTzTimeZoneCacheEntry
+{
+    QVector<QTzTransitionTime> m_tranTimes;
+    QVector<QTzTransitionRule> m_tranRules;
+    QList<QByteArray> m_abbreviations;
+    QByteArray m_posixRule;
+};
+
 class Q_AUTOTEST_EXPORT QTzTimeZonePrivate final : public QTimeZonePrivate
 {
     QTzTimeZonePrivate(const QTzTimeZonePrivate &) = default;
@@ -334,13 +344,11 @@ private:
     QVector<QTimeZonePrivate::Data> getPosixTransitions(qint64 msNear) const;
 
     Data dataForTzTransition(QTzTransitionTime tran) const;
-    QVector<QTzTransitionTime> m_tranTimes;
-    QVector<QTzTransitionRule> m_tranRules;
-    QList<QByteArray> m_abbreviations;
 #if QT_CONFIG(icu)
     mutable QSharedDataPointer<QTimeZonePrivate> m_icu;
 #endif
-    QByteArray m_posixRule;
+    QTzTimeZoneCacheEntry cached_data;
+    QVector<QTzTransitionTime> tranCache() const { return cached_data.m_tranTimes; }
 };
 #endif // Q_OS_UNIX