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

[Eric Blake <eblake@redhat.com>]

On 03/21/2018 09:08 AM, Marc-André Lureau wrote:
> Hi
> 
> On Wed, Mar 21, 2018 at 3:01 PM, Paolo Bonzini <pbonzini@redhat.com> 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