QSlotObjectBase: move the `which` parameter to the 4th position

This places the first through third parameters on the exact positions
that they will be used to perform the operations in the switch, saving
the compiler from generating a few instructions to move data around. All
ABIs Qt supports that pass any function parameters in registers at all
pass at least 4.

We keep the return type as void (instead of returning bool, for the
Compare case) so the compiler can apply tail-call optimizations for
those two typical cases.

PMF case: https://gcc.godbolt.org/z/9oP5boKfj
Function case: https://gcc.godbolt.org/z/e9vEzd5dj
Functor case: https://gcc.godbolt.org/z/s8Ejjra7P

Change-Id: I3e3bfef633af4130a03afffd175d3e3009c56323
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

1 files changed, 4 insertions, 0 deletions

diff --git a/tests/auto/corelib/kernel/qobject/tst_qobject.cpp b/tests/auto/corelib/kernel/qobject/tst_qobject.cpp
index 16f53d58da..5cb9faf01f 100644
--- a/tests/auto/corelib/kernel/qobject/tst_qobject.cpp
+++ b/tests/auto/corelib/kernel/qobject/tst_qobject.cpp
@@ -6813,7 +6813,11 @@ struct QmlReceiver : public QtPrivate::QSlotObjectBase
         , magic(0)
     {}
 
+#if QT_VERSION < QT_VERSION_CHECK(7, 0, 0)
     static void impl(int which, QSlotObjectBase *this_, QObject *, void **metaArgs, bool *ret)
+#else
+    static void impl(QSlotObjectBase *this_, QObject *, void **metaArgs, int which, bool *ret)
+#endif
     {
         switch (which) {
         case Destroy: delete static_cast<QmlReceiver*>(this_); return;