Re: [Qemu-devel] [PATCH 2/3] qobject: assume base of a qobject is at offset 0

[Eric Blake]

On 03/21/2018 09:08 AM, Marc-André Lureau wrote:
> Hi
>
> On Wed, Mar 21, 2018 at 3:01 PM, Paolo Bonzini <address@hidden> wrote:
> > On 21/03/2018 14:40, Marc-André Lureau wrote:
> > > +/* A typecast, checking for the type of arguments */
> > > +/* QObject is at offset 0, for all QObject-derived types */
> > > +#define QOBJECT(x) QEMU_GENERIC(x,              \
> > > +    (QNull *, (QObject *) x),                   \
> > > +    (const QNull *, (const QObject *) x),       \
> > > +    (QNum *, (QObject *) x),                    \
> > > +    (const QNum *, (const QObject *) x),        \
> > > +    (QString *, (QObject *) x),                 \
> > > +    (const QString *, (const QObject *) x),     \
> > > +    (QDict *, (QObject *) x),                   \
> > > +    (const QDict *, (const QObject *) x),       \
> > > +    (QList *, (QObject *) x),                   \
> > > +    (const QList *, (const QObject *) x),       \
> > > +    (QBool *, (QObject *) x),                   \
> > > +    (const QBool *, (const QObject *) x),       \
> > > +    (QObject *, x),                             \
> > > +    (const QObject *, x),                       \
> > > +    qobject_unknown_type(x))
> >
> > Why not just
> >
> >          QEMU_GENERIC(x,
> >                  (QObject *, x),
> >                  (const QObject *, x),
> >                  ({ \
> >                          QEMU_BUILD_BUG_ON(offsetof(typeof(*x), base));
> >                          &(x)->base;
> >                  }))
> >
> > That is just an extension of what was being done before, and it is
> > resilient against people putting a random "QObject base" in the middle
> > of a struct.
>
> Yeah, I tried a few of those approaches. Here the problem is that
> QObject doesn't have base field. So you get a compile time error with
> a QObject * as argument.

So the compiler requires &(x)->base to resolve even when it is not on 
the branch that gets selected?

--
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org