Micro-optimize to ensure named return value optimization

Rewrite a few often used header functions to ensure they get return
value optimization.

Change-Id: I316fde21f63fedfd9cbae50855007234b0de9b70
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

Author: Allan Sandfeld Jensen

src/corelib/kernel/qproperty.h

@@ -766,9 +766,11 @@ class QUntypedBindable
         // afterwards.
         if (!(iface->getBinding && iface->setBinding))
             return QUntypedPropertyBinding {};
-        QUntypedPropertyBinding binding = iface->getBinding(data);
-        iface->setBinding(data, QUntypedPropertyBinding{});
-        return binding;
+        return [&] {
+            QUntypedPropertyBinding binding = iface->getBinding(data);
+            iface->setBinding(data, QUntypedPropertyBinding{});
+            return binding;
+        }();
     }
 
     void observe(QPropertyObserver *observer) const

src/corelib/kernel/qproperty_p.h

@@ -387,10 +387,12 @@ class Q_CORE_EXPORT QPropertyBindingPrivate : public QtPrivate::RefCounted
     {
         if (!hasCustomVTable())
             return location;
-        QPropertyBindingSourceLocation result;
-        constexpr auto msg = "Custom location";
-        result.fileName = msg;
-        return result;
+        return []() {
+            constexpr auto msg = "Custom location";
+            QPropertyBindingSourceLocation result;
+            result.fileName = msg;
+            return result;
+        }();
     }
     QPropertyBindingError bindingError() const { return m_error; }
     QMetaType valueMetaType() const { return metaType; }

src/corelib/kernel/qvariant.h

@@ -762,6 +762,15 @@ inline void swap(QVariant &value1, QVariant &value2) noexcept
 
 #ifndef QT_MOC
 
+namespace QtPrivate {
+template<typename T> inline T qvariant_cast_qmetatype_converted(const QVariant &v, QMetaType targetType)
+{
+    T t{};
+    QMetaType::convert(v.metaType(), v.constData(), targetType, &t);
+    return t;
+}
+} // namespace QtPrivate
+
 template<typename T> inline T qvariant_cast(const QVariant &v)
 {
     QMetaType targetType = QMetaType::fromType<T>();
@@ -774,9 +783,7 @@ template<typename T> inline T qvariant_cast(const QVariant &v)
             return v.d.get<nonConstT>();
     }
 
-    T t{};
-    QMetaType::convert(v.metaType(), v.constData(), targetType, &t);
-    return t;
+    return QtPrivate::qvariant_cast_qmetatype_converted<T>(v, targetType);
 }
 
 template<typename T> inline T qvariant_cast(QVariant &&v)
@@ -803,9 +810,7 @@ template<typename T> inline T qvariant_cast(QVariant &&v)
             return v.d.get<nonConstT>();
     }
 
-    T t{};
-    QMetaType::convert(v.metaType(), v.constData(), targetType, &t);
-    return t;
+    return QtPrivate::qvariant_cast_qmetatype_converted<T>(v, targetType);
 }
 
 #  ifndef QT_NO_VARIANT

src/corelib/text/qstringbuilder.h

@@ -100,8 +100,10 @@ class QStringBuilder : public QStringBuilderBase<QStringBuilder<A, B>,
             // so we're special casing this one out, QTBUG-114206
             return T();
         }
-
-        const qsizetype len = Concatenable::size(*this);
+        return convertToImpl<T>(Concatenable::size(*this));
+    }
+    template <typename T> T convertToImpl(const qsizetype len) const
+    {
         T s(len, Qt::Uninitialized);
 
         // Using data_ptr() here (private API) so we can bypass the

src/corelib/text/qstringconverter.h

@@ -70,20 +70,24 @@ class QStringEncoder : public QStringConverter
     FinalizeResult finalize() { return finalize(nullptr, 0); }
 
 private:
-    QByteArray encodeAsByteArray(QStringView in)
+    QByteArray invalidateAndReturnNull()
+    {
+        // ensure that hasError returns true
+        state.invalidChars = 1;
+        return QByteArray();
+    }
+    QByteArray encodeAsByteArrayImpl(QStringView in)
     {
-        if (!iface) {
-            // ensure that hasError returns true
-            state.invalidChars = 1;
-            return {};
-        }
         QByteArray result(iface->fromUtf16Len(in.size()), Qt::Uninitialized);
         char *out = result.data();
         out = iface->fromUtf16(out, in, &state);
         result.truncate(out - result.constData());
         return result;
     }
-
+    QByteArray encodeAsByteArray(QStringView in)
+    {
+        return iface ? encodeAsByteArrayImpl(in) : invalidateAndReturnNull();
+    }
 };
 
 class QStringDecoder : public QStringConverter
@@ -153,18 +157,23 @@ class QStringDecoder : public QStringConverter
     Q_CORE_EXPORT static QStringDecoder decoderForHtml(QByteArrayView data);
 
 private:
-    QString decodeAsString(QByteArrayView in)
+    QString invalidateAndReturnNull()
+    {
+        // ensure that hasError returns true
+        state.invalidChars = 1;
+        return QString();
+    }
+    QString decodeAsStringImpl(QByteArrayView in)
     {
-        if (!iface) {
-            // ensure that hasError returns true
-            state.invalidChars = 1;
-            return {};
-        }
         QString result(iface->toUtf16Len(in.size()), Qt::Uninitialized);
         const QChar *out = iface->toUtf16(result.data(), in, &state);
         result.truncate(out - result.constData());
         return result;
     }
+    QString decodeAsString(QByteArrayView in)
+    {
+        return iface ? decodeAsStringImpl(in) : invalidateAndReturnNull();
+    }
 };
 
 

src/corelib/tools/qhash.h

@@ -1031,9 +1031,11 @@ class QHash
 
         if (it.isUnused())
             return T();
-        T value = it.node()->takeValue();
-        d->erase(it);
-        return value;
+        return [&] {
+            T value = it.node()->takeValue();
+            d->erase(it);
+            return value;
+        }();
     }
 
 public: