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Implement some TODOs: make use of GLibc's tm_gmtoff and tm_zone

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Save some complicated computations using struct tm and (effectively)
time_t used in working out local-time's UTC offset and updating the
local time's representation in terms of seconds (since a nominal local
epoch). GLibc gives us the offset for free, making computation of the
seconds easy.

Use the same extension to catch an opportunity for a seldom-needed
early return in one more case.

When determining a zone's abbreviation, we can use tm_zone as a
short-cut to save some mutex-locking.

Change-Id: Id2958f75c1d49ad4aed8f9c483ec13f4fe3a86c2
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
This commit is contained in:
Edward Welbourne 2023-08-16 10:37:18 +02:00
parent b6761d098b
commit 781042efcb
1 changed files with 28 additions and 9 deletions
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37
src/corelib/time/qlocaltime.cpp
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@ -20,6 +20,11 @@
# include <qt_windows.h>
#endif
#ifdef __GLIBC__ // Extends struct tm with some extra fields:
#define HAVE_TM_GMTOFF // tm_gmtoff is the UTC offset.
#define HAVE_TM_ZONE // tm_zone is the zone abbreviation.
#endif
QT_BEGIN_NAMESPACE
using namespace QtPrivate::DateTimeConstants;
@ -80,10 +85,12 @@ public:
We can assume time-zone offsets are less than a day, so this can only arise
if the struct tm describes either the last day of 1969 or the first day of
1970. That makes for a cheap pre-test; if it holds, we can ask mktime about
the preceding second; if it gives us -2, then the -1 we originally saw is not
an error (or was an error, but needn't have been). We can then synthesize a
corrected value for local using the -2 result.
1970. When we do know the offset (a glibc extension supplies it as a member
of struct tm), we can determine whether we're on the last second of the day,
refining that check. That makes for a cheap pre-test; if it holds, we can ask
mktime() about the preceding second; if it gives us -2, then the -1 we
originally saw is not (or at least didn't need to be) an error. We can then
synthesize a corrected value for local using the -2 result.
*/
inline bool MkTimeResult::meansEnd1969()
{
@ -91,6 +98,12 @@ inline bool MkTimeResult::meansEnd1969()
return false;
#else
if (local.tm_year < 69 || local.tm_year > 70
# ifdef HAVE_TM_GMTOFF
// Africa/Monrovia had offset 00:44:30 at the epoch, so (although all
// other zones' offsets were round multiples of five minutes) we need
// the offset to determine whether the time might match:
|| (tmSecsWithinDay(local) - local.tm_gmtoff + 1) % SECS_PER_DAY
# endif
|| (local.tm_year == 69 // ... and less than a day:
? local.tm_mon < 11 || local.tm_mday < 31
: local.tm_mon > 0 || local.tm_mday > 1)) {
@ -555,7 +568,10 @@ QString localTimeAbbbreviationAt(qint64 local, QDateTimePrivate::DaylightStatus
auto use = resolveLocalTime(QRoundingDown::qDiv<MSECS_PER_SEC>(local), dst);
if (!use.good)
return {};
// TODO: for glibc, use.local.tm_zone is the zone abbreviation.
#ifdef HAVE_TM_ZONE
if (use.local.tm_zone)
return QString::fromLocal8Bit(use.local.tm_zone);
#endif
return qTzName(use.local.tm_isdst > 0 ? 1 : 0);
}
@ -572,12 +588,14 @@ QDateTimePrivate::ZoneState mapLocalTime(qint64 local, QDateTimePrivate::Dayligh
Q_ASSERT(local < 0 ? (millis <= 0 && millis > -MSECS_PER_SEC)
: (millis >= 0 && millis < MSECS_PER_SEC));
// TODO: for glibc, use.local.tm_gmtoff is the offset
// That would give us offset directly, hence localSecs = offset + use.utcSecs
// Provisional offset, until we have a revised localSecs:
int offset = localSecs - use.utcSecs;
// Revise our original hint-dst to what it resolved to:
dst = use.local.tm_isdst > 0 ? QDateTimePrivate::DaylightTime : QDateTimePrivate::StandardTime;
#ifdef HAVE_TM_GMTOFF
const int offset = use.local.tm_gmtoff;
localSecs = offset + use.utcSecs;
#else
// Provisional offset, until we have a revised localSecs:
int offset = localSecs - use.utcSecs;
auto jd = tmToJd(use.local);
if (Q_UNLIKELY(!jd))
return {local, offset, dst, false};
@ -593,6 +611,7 @@ QDateTimePrivate::ZoneState mapLocalTime(qint64 local, QDateTimePrivate::Dayligh
// Use revised localSecs to refine offset:
offset = localSecs - use.utcSecs;
#endif // HAVE_TM_GMTOFF
// 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