From 0acb56518d72c09650dd83ca34a7d5bb0763b06a Mon Sep 17 00:00:00 2001
From: Edward Welbourne <edward.welbourne@qt.io>
Date: Wed, 10 Aug 2022 16:29:39 +0200
Subject: [PATCH] Rearrange mapLocalTime() so its millis always have the right
 sign
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Doing a simple division to get seconds, before using rounding-down
division to get days and positive seconds, saves the need to check for
seconds and millis in opposite directions that might cause a needless
overflow when computing the final result.

Change-Id: Ia4f95bb0510eb4f2c1f9131a34d317bd41bbed2a
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
---
 src/corelib/time/qlocaltime.cpp | 23 ++++++++++++-----------
 1 file changed, 12 insertions(+), 11 deletions(-)

diff --git a/src/corelib/time/qlocaltime.cpp b/src/corelib/time/qlocaltime.cpp
index 2081b167534..19aaeb25408 100644
--- a/src/corelib/time/qlocaltime.cpp
+++ b/src/corelib/time/qlocaltime.cpp
@@ -348,11 +348,13 @@ QString localTimeAbbbreviationAt(qint64 local, QDateTimePrivate::DaylightStatus
 
 QDateTimePrivate::ZoneState mapLocalTime(qint64 local, QDateTimePrivate::DaylightStatus dst)
 {
-    const qint64 localDays = QRoundingDown::qDiv(local, MSECS_PER_DAY);
-    qint64 millis = local - localDays * MSECS_PER_DAY;
-    Q_ASSERT(0 <= millis && millis < MSECS_PER_DAY); // Definition of QRD::qDiv.
-    struct tm tmLocal = timeToTm(localDays, int(millis / MSECS_PER_SEC), dst);
-    millis %= MSECS_PER_SEC;
+    qint64 localSecs = local / MSECS_PER_SEC;
+    qint64 millis = local - localSecs * MSECS_PER_SEC; // 0 or with same sign as local
+    const qint64 localDays = QRoundingDown::qDiv(localSecs, SECS_PER_DAY);
+    qint64 daySecs = localSecs - localDays * SECS_PER_DAY;
+    Q_ASSERT(0 <= daySecs && daySecs < SECS_PER_DAY); // Definition of QRD::qDiv.
+
+    struct tm tmLocal = timeToTm(localDays, daySecs, dst);
     time_t utcSecs;
     if (!callMkTime(&tmLocal, &utcSecs))
         return {local};
@@ -368,22 +370,21 @@ QDateTimePrivate::ZoneState mapLocalTime(qint64 local, QDateTimePrivate::Dayligh
                        &jd))) {
         return {local, offset, dst, false};
     }
-    qint64 daySecs = tmSecsWithinDay(tmLocal);
+    daySecs = tmSecsWithinDay(tmLocal);
     Q_ASSERT(0 <= daySecs && daySecs < SECS_PER_DAY);
     if (daySecs > 0 && jd < JULIAN_DAY_FOR_EPOCH) {
         ++jd;
         daySecs -= SECS_PER_DAY;
     }
-    qint64 localSecs;
     if (Q_UNLIKELY(daysAndSecondsOverflow(jd, daySecs, &localSecs)))
         return {local, offset, dst, false};
 
     offset = localSecs - utcSecs;
 
-    if (localSecs < 0 && millis > 0) {
-        ++localSecs;
-        millis -= MSECS_PER_SEC;
-    }
+    // The only way localSecs and millis can now have opposite sign is for
+    // resolution of the local time to have kicked us across the epoch, in which
+    // case there's no danger of overflow. So if overflow is in danger of
+    // happening, we're already doing the best we can to avoid it.
     qint64 revised;
     const bool overflow = secondsAndMillisOverflow(localSecs, millis, &revised);
     return {overflow ? local : revised, offset, dst, !overflow};