QVector: check d for equality before d->size for inequality

Assuming the CPU has already loaded 'this', the value of 'd' is just
an indirect load away. The value of d->size, however, is two indirect
loads away, one of which is the load of 'd'.

So it makes more sense to check for d-pointer equality first, as that
can proceed in parallel with the fetch for d->size, which the CPU
may speculatively trigger.

In addition, at least GCC in release mode after this change doesn't
set up the stack frame if the d-pointer check succeeds.

Change-Id: I61f9b245070dd1742fca6ccb8d4936a0b1aa7c07
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>

src/corelib/tools/qvector.h

@@ -711,10 +711,10 @@ typename QVector<T>::iterator QVector<T>::erase(iterator abegin, iterator aend)
 template <typename T>
 bool QVector<T>::operator==(const QVector<T> &v) const
 {
-    if (d->size != v.d->size)
-        return false;
     if (d == v.d)
         return true;
+    if (d->size != v.d->size)
+        return false;
     T* b = d->begin();
     T* i = b + d->size;
     T* j = v.d->end();