QDateTime::fromString(): improve performance by 33%

getMaximum() and getMinimum(), called during parsing, create new
QDateTime instances, which on Linux end up calling mktime().
Making these static (for the common case of LocalTime spec)
improves performance dramatically, when parsing several date/times.

tests/benchmarks/corelib/tools/qdatetime/ (after fixing it to
actually parse a valid date/time) says:
 RESULT : tst_QDateTime::fromString():
-     36,742,060 instruction reads per iteration (total: 36,742,060, iterations: 1)
+     24,230,060 instruction reads per iteration (total: 24,230,060, iterations: 1)

Change-Id: I0c3931285475bf19a5be8cba1486ed07cbf5e134
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 15 insertions, 5 deletions

diff --git a/src/corelib/tools/qdatetimeparser.cpp b/src/corelib/tools/qdatetimeparser.cpp
index 621c877174..ae429950c8 100644
--- a/src/corelib/tools/qdatetimeparser.cpp
+++ b/src/corelib/tools/qdatetimeparser.cpp
@@ -1216,15 +1216,15 @@ end:
         } else {
             if (context == FromString) {
                 // optimization
-                Q_ASSERT(getMaximum().date().toJulianDay() == 4642999);
+                Q_ASSERT(maximum.date().toJulianDay() == 4642999);
                 if (newCurrentValue.date().toJulianDay() > 4642999)
                     state = Invalid;
             } else {
-                if (newCurrentValue > getMaximum())
+                if (newCurrentValue > maximum)
                     state = Invalid;
             }
 
-            QDTPDEBUG << "not checking intermediate because newCurrentValue is" << newCurrentValue << getMinimum() << getMaximum();
+            QDTPDEBUG << "not checking intermediate because newCurrentValue is" << newCurrentValue << minimum << maximum;
         }
     }
     StateNode node;
@@ -1607,13 +1607,13 @@ bool QDateTimeParser::potentialValue(const QStringRef &str, int min, int max, in
 
 bool QDateTimeParser::skipToNextSection(int index, const QDateTime &current, const QStringRef &text) const
 {
-    Q_ASSERT(current >= getMinimum() && current <= getMaximum());
-
     const SectionNode &node = sectionNode(index);
     Q_ASSERT(text.size() < sectionMaxSize(index));
 
     const QDateTime maximum = getMaximum();
     const QDateTime minimum = getMinimum();
+    Q_ASSERT(current >= minimum && current <= maximum);
+
     QDateTime tmp = current;
     int min = absoluteMin(index);
     setDigit(tmp, index, min);
@@ -1713,11 +1713,21 @@ bool QDateTimeParser::fromString(const QString &t, QDate *date, QTime *time) con
 
 QDateTime QDateTimeParser::getMinimum() const
 {
+    // Cache the most common case
+    if (spec == Qt::LocalTime) {
+        static const QDateTime localTimeMin(QDATETIMEEDIT_DATE_MIN, QDATETIMEEDIT_TIME_MIN, Qt::LocalTime);
+        return localTimeMin;
+    }
     return QDateTime(QDATETIMEEDIT_DATE_MIN, QDATETIMEEDIT_TIME_MIN, spec);
 }
 
 QDateTime QDateTimeParser::getMaximum() const
 {
+    // Cache the most common case
+    if (spec == Qt::LocalTime) {
+        static const QDateTime localTimeMax(QDATETIMEEDIT_DATE_MAX, QDATETIMEEDIT_TIME_MAX, Qt::LocalTime);
+        return localTimeMax;
+    }
     return QDateTime(QDATETIMEEDIT_DATE_MAX, QDATETIMEEDIT_TIME_MAX, spec);
 }