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

[Eduardo Habkost]

On Mon, Oct 29, 2018 at 10:56:57AM +0100, Igor Mammedov wrote:
> On Fri, 26 Oct 2018 12:13:21 -0300
> Eduardo Habkost <address@hidden> 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 <address@hidden> wrote:
> > >   
> > > > Improve a bit code readability.
> > > > 
> > > > Signed-off-by: Marc-André Lureau <address@hidden>
> > > > ---
> > > >  include/qom/object_interfaces.h | 4 ++++
> > > >  qom/object.c                    | 4 ++--
> > > >  qom/object_interfaces.c         | 9 +++------
> > > >  3 files changed, 9 insertions(+), 8 deletions(-)
> > > > 
> > > > diff --git a/include/qom/object_interfaces.h 
> > > > b/include/qom/object_interfaces.h
> > > > index 4d513fb329..46b0861457 100644
> > > > --- a/include/qom/object_interfaces.h
> > > > +++ b/include/qom/object_interfaces.h
> > > > @@ -9,9 +9,13 @@
> > > >  #define USER_CREATABLE_CLASS(klass) \
> > > >       OBJECT_CLASS_CHECK(UserCreatableClass, (klass), \
> > > >                          TYPE_USER_CREATABLE)
> > > > +#define IS_USER_CREATABLE_CLASS(klass) \
> > > > +    object_class_dynamic_cast(OBJECT_CLASS(oc), TYPE_USER_CREATABLE)
> > > >  #define USER_CREATABLE_GET_CLASS(obj) \
> > > >       OBJECT_GET_CLASS(UserCreatableClass, (obj), \
> > > >                        TYPE_USER_CREATABLE)
> > > > +#define IS_USER_CREATABLE(obj) \
> > > > +    object_dynamic_cast(OBJECT(obj), TYPE_USER_CREATABLE)
> > > >  #define USER_CREATABLE(obj) \
> > > >       INTERFACE_CHECK(UserCreatable, (obj), \
> > > >                       TYPE_USER_CREATABLE)
> > > > diff --git a/qom/object.c b/qom/object.c
> > > > index 75d1d48944..0703e8e4ff 100644
> > > > --- a/qom/object.c
> > > > +++ b/qom/object.c
> > > > @@ -424,7 +424,7 @@ void object_initialize_childv(Object *parentobj, 
> > > > const char *propname,
> > > >          goto out;
> > > >      }
> > > >  
> > > > -    if (object_dynamic_cast(obj, TYPE_USER_CREATABLE)) {
> > > > +    if (IS_USER_CREATABLE(obj)) {
> > > >          user_creatable_complete(obj, &local_err);
> > > >          if (local_err) {
> > > >              object_unparent(obj);
> > > > @@ -605,7 +605,7 @@ Object *object_new_with_propv(const char *typename,
> > > >          goto error;
> > > >      }
> > > >  
> > > > -    if (object_dynamic_cast(obj, TYPE_USER_CREATABLE)) {
> > > > +    if (IS_USER_CREATABLE(obj)) {
> > > >          user_creatable_complete(obj, &local_err);
> > > >          if (local_err) {
> > > >              object_unparent(obj);
> > > > diff --git a/qom/object_interfaces.c b/qom/object_interfaces.c
> > > > index 72b97a8bed..e3084bc04a 100644
> > > > --- a/qom/object_interfaces.c
> > > > +++ b/qom/object_interfaces.c
> > > > @@ -10,18 +10,15 @@
> > > >  
> > > >  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.   
> > 
> > (INTERFACE_CHECK is a nop if CONFIG_QOM_CAST_DEBUG is not
> > enabled, so I don't understand how it would regress startup time.
> Isn't it enabled by default though?
> Maybe we should flip default to disabled then cast should be ok and enable it
> when generic '--debug' is enabled.

This is called only once for each -object option.  Even if QOM
debugging is enabled by default I don't see why it wouldn't be OK
to call object_dynamic_cast_assert() here.

-- 
Eduardo