On Thu, 1 Nov 2018 12:02:03 -0300
Eduardo Habkost <ehabk...@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 <ehabk...@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.lur...@redhat.com> wrote:
> > > >
> > > > > Hi
> > > > >
> > > > > On Tue, Oct 30, 2018 at 5:37 AM Eduardo Habkost <ehabk...@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 <ehabk...@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.lur...@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 :)
>
> > [...]
>
