QTimeZone: return a shared copy from utc()

The UTC time-zone never changes, not even under a locale change, so
memoize utc() using a Q_GLOBAL_STATIC cache. Check its isDestroyed()
and fall back to the current implementation in that case. This keeps
the function available even in late shutdown stages.

_This_ is a knife^Woptimization:

 PASS   : tst_QTimeZone::utc()
 RESULT : tst_QTimeZone::utc():
-    33.1472430 nsecs per iteration (total: 331,472,431, iterations: 10000000)
+    12.8953219 nsecs per iteration (total: 128,953,219, iterations: 10000000)
-    150.5929452 CPU cycles per iteration, 4,54 GHz (total: 1,505,929,452, iterations: 10000000)
+    60.3382987 CPU cycles per iteration, 4,68 GHz (total: 603,382,988, iterations: 10000000)
-    396.0402927 instructions per iteration, 2,630 instr/cycle (total: 3,960,402,928, iterations: 10000000)
+    84.0138436 instructions per iteration, 1,392 instr/cycle (total: 840,138,437, iterations: 10000000)
-    96.0069811 branch instructions per iteration, 2,9 G/sec (total: 960,069,811, iterations: 10000000)
+    28.0024131 branch instructions per iteration, 2,17 G/sec (total: 280,024,132, iterations: 10000000)

Amends HEAD^.

Change-Id: I4496f3a4c1f64b0615930337662bcd04af0a2ade
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Milian Wolff <milian.wolff@kdab.com>
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>

src/corelib/time/qtimezone.cpp

@@ -1443,6 +1443,7 @@ QTimeZone QTimeZone::systemTimeZone()
 }
 
 /*!
+    \fn QTimeZone QTimeZone::utc()
     \since 5.5
     Returns a QTimeZone object that describes UTC as a time zone.
 
@@ -1453,11 +1454,20 @@ QTimeZone QTimeZone::systemTimeZone()
 
     \sa systemTimeZone(), Initialization, asBackendZone()
 */
-QTimeZone QTimeZone::utc()
+QTimeZone QTimeZonePrivate::utcQTimeZone()
 {
     return QTimeZone(*new QUtcTimeZonePrivate());
 }
 
+Q_GLOBAL_STATIC(QTimeZone, utcTimeZone, QTimeZonePrivate::utcQTimeZone());
+
+QTimeZone QTimeZone::utc()
+{
+    if (Q_UNLIKELY(utcTimeZone.isDestroyed()))
+        return QTimeZonePrivate::utcQTimeZone(); // create a new, unshared one
+    return *utcTimeZone; // take a shallow copy
+}
+
 /*!
     Returns \c true if a given time zone \a ianaId is available on this system.
 

src/corelib/time/qtimezoneprivate_p.h

@@ -161,6 +161,8 @@ public:
         return QByteArrayLiteral("UTC");
     }
 
+    [[nodiscard]] static QTimeZone utcQTimeZone();
+
 protected:
     // Zones CLDR data says match a condition.
     // Use to filter what the backend has available.