Re: [Qemu-devel] [PATCH 1/9] qom/user-creatable: add a few helper macros

[Igor Mammedov <imammedo@redhat.com>]

On Thu, 1 Nov 2018 12:02:03 -0300
Eduardo Habkost <ehabkost@redhat.com> wrote:

> On Thu, Nov 01, 2018 at 01:16:37PM +0100, Igor Mammedov wrote:
> > On Tue, 30 Oct 2018 20:07:17 -0300
> > Eduardo Habkost <ehabkost@redhat.com> wrote:
> >   
> > > On Tue, Oct 30, 2018 at 03:22:43PM +0100, Igor Mammedov wrote:  
> > > > On Tue, 30 Oct 2018 13:26:40 +0400
> > > > Marc-André Lureau <marcandre.lureau@redhat.com> wrote:
> > > >     
> > > > > Hi
> > > > > 
> > > > > On Tue, Oct 30, 2018 at 5:37 AM Eduardo Habkost <ehabkost@redhat.com> wrote:    
> > > > > >
> > > > > > On Mon, Oct 29, 2018 at 10:56:57AM +0100, Igor Mammedov wrote:      
> > > > > > > On Fri, 26 Oct 2018 12:13:21 -0300
> > > > > > > Eduardo Habkost <ehabkost@redhat.com> wrote:
> > > > > > >      
> > > > > > > > On Mon, Oct 22, 2018 at 03:33:30PM +0100, Igor Mammedov wrote:      
> > > > > > > > > On Wed, 12 Sep 2018 16:55:23 +0400
> > > > > > > > > Marc-André Lureau <marcandre.lureau@redhat.com> wrote:  
> [...]
> > > > > > > > > >  void user_creatable_complete(Object *obj, Error **errp)
> > > > > > > > > >  {
> > > > > > > > > > -
> > > > > > > > > >      UserCreatableClass *ucc;
> > > > > > > > > > -    UserCreatable *uc =
> > > > > > > > > > -        (UserCreatable *)object_dynamic_cast(obj, TYPE_USER_CREATABLE);
> > > > > > > > > >
> > > > > > > > > > -    if (!uc) {
> > > > > > > > > > +    if (!IS_USER_CREATABLE(obj)) {
> > > > > > > > > >          return;
> > > > > > > > > >      }
> > > > > > > > > >
> > > > > > > > > > -    ucc = USER_CREATABLE_GET_CLASS(uc);
> > > > > > > > > > +    ucc = USER_CREATABLE_GET_CLASS(obj);
> > > > > > > > > >      if (ucc->complete) {
> > > > > > > > > > -        ucc->complete(uc, errp);
> > > > > > > > > > +        ucc->complete(USER_CREATABLE(obj), errp);      
> > > > > > > > >                          ^^^
> > > > > > > > > even though function becomes more concise,
> > > > > > > > > this will call expensive dynamic cast 2nd time (IS_USER_CREATABLE was the 1st and discarded)
> > > > > > > > > so I'm not sure is a good idea to regress startup time for readability.      
> > > > > 
> > > > > I hope it's not measurable, unless we create billions of objects. Do
> > > > > you want some figures?    
> > > > I recall penalty was big enough for QEMU, that we added qom debug option,
> > > > in case of -object, the cost could be multiplied by hundreds    
> > > 
> > > This sounds like premature optimization.  Did you measure how
> > > many nanoseconds we're saving per -object option?  
> > it's not optimization, it's about not making code worse and using
> > resources reasonably object_dynamic_cast() with IS_USER_CREATABLE().  
> 
> How exactly is the code worse?
making it slower /doesn't matter how much/ when there is better way
to do it.
Anyways the point is moot as Mark rewrote it in another way (better)
so this question won't arise.

 
> > 
> > I'd replace object_dynamic_cast() with IS_FOO() in places where
> > we just test but do not use the result for a better readability.
> > But is we need the result later I'd keep object_dynamic_cast(),
> > it's not much worse but allows us to reuse the result of cast.  
> 
> Readability is orders of magnitude more important to me.
> 
> This is not readable:
>   (UserCreatable *)object_dynamic_cast(obj, TYPE_USER_CREATABLE)
> 
> This is readable:
>   USER_CREATABLE(obj)
> 
> If the overhead of the extra object_dynamic_cast() call is too
> much, it's already possible to disable QOM cast debugging.

I'm typically on the side of simplifying code and readability
but in this case I have to disagree, out of context ^^^ it might
look fine but with full context

 uc = (UserCreatable *)object_dynamic_cast(obj, TYPE_USER_CREATABLE)
 if(uc)
    call(uc)

vs

 if(IS_USER_CREATABLE(obj))
   // discard above hidden cast result and do it again
   call(USER_CREATABLE(obj))
  
the former is not horrible enough to warrant doing senseless discard.
So lets agree to disagree :)

> 
> > [...]  
>