In D9982#193510, @graesslin wrote:

Very interesting finding. Any idea why that's the case?

My guess is that dynamic_cast is the only type of cast that does runtime checking (see next link). Probably converting to const reduces the amount of checking.
http://gcc.gnu.org/ml/gcc/1999-12n/msg00544.html

Probably it should be safe to use static_cast that only does compile_time checks.

Can go into 5.12 branch, that should be absolutely safe.

Should it be done by cherry-pick?

Did you compare that to a profile with the updated cast?
This makes little sense and a simplified testcase doesn't measure significant differences in casting times on the wall clock time. And certainly not factor 22.

The screenshot with 22% is from the release with debug symbols from opensuse (you can see the first const cast there), the other one is from a debug build with the patch.

The 22% to 1.% is only in one method.

'key...?!
I'd assume the const casting avoids deep copies of the QList - at least that's the only explanation I could come up without a deeper investigation.

In D9982#193637, @luebking wrote:

'key...?!
I'd assume the const casting avoids deep copies of the QList - at least that's the only explanation I could come up without a deeper investigation.

unlikely. That was my first guess as well, but the code uses const iterator, so no deep copy.

In D9982#193606, @jtamate wrote:

In D9982#193510, @graesslin wrote:

Very interesting finding. Any idea why that's the case?

My guess is that dynamic_cast is the only type of cast that does runtime checking (see next link). Probably converting to const reduces the amount of checking.
http://gcc.gnu.org/ml/gcc/1999-12n/msg00544.html

Probably it should be safe to use static_cast that only does compile_time checks.

In this case I fear we cannot make it a static_cast. It could be that a ShellClient (Wayland) is a transient to Client (example: any child window KWin shows for the Client, such as Alt+F3, rules dialog, kill helper).

Can go into 5.12 branch, that should be absolutely safe.

Should it be done by cherry-pick?

We normally push to stable branch and then merge the stable branch into master.

Could you additional try qobject_cast? It is supposed to be faster than dynamic_cast and Client subclasses QObject.

Yes, indeed, it is faster, from 1.04% to 0,03%

I'm still new to arc.
This revision was created in a branch pointing to master, therefore when I'll do arc land, it will push into master.
How do I change the branch to land without creating a new revision?
And after landing the commit, should I do: >git checkout master, git merge <commit hash> ?

In D9982#194207, @jtamate wrote:

I'm still new to arc.
This revision was created in a branch pointing to master, therefore when I'll do arc land, it will push into master.

Just don't use arc land. It does terrible things. I always use normal git push.