From 6c624c80fd9f2b5d43dabf955ef42657c8d029bd Mon Sep 17 00:00:00 2001
From: Edward Welbourne <edward.welbourne@qt.io>
Date: Thu, 1 Sep 2022 18:24:30 +0200
Subject: [PATCH] Avoid recomputing DST-ness for time-zone when already known
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When a QDateTime is constructed it determines whether daylight-saving
time is in effect and records it in its status field. So consult that
cached answer first, before asking the time-zone backend to work out
the answer the slow way.

Change-Id: I1d380d41f28a9b866216ac0f65425c30a8988cac
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
---
 src/corelib/time/qdatetime.cpp | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/src/corelib/time/qdatetime.cpp b/src/corelib/time/qdatetime.cpp
index 865e9befdf1..418466033de 100644
--- a/src/corelib/time/qdatetime.cpp
+++ b/src/corelib/time/qdatetime.cpp
@@ -3613,6 +3613,10 @@ bool QDateTime::isDaylightTime() const
         break;
 #else
         Q_ASSERT(d->m_timeZone.isValid());
+        if (auto dst = extractDaylightStatus(getStatus(d));
+            dst != QDateTimePrivate::UnknownDaylightTime) {
+            return dst == QDateTimePrivate::DaylightTime;
+        }
         return d->m_timeZone.d->isDaylightTime(toMSecsSinceEpoch());
 #endif // timezone
     case Qt::LocalTime: {