Re: [Qemu-devel] [PATCH v10 10/13] qapi: Don't box struct branch of alternate

[Eric Blake <eblake@redhat.com>]

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On 02/17/2016 07:40 AM, Markus Armbruster wrote:

>>>> There's no reason to do two malloc's for an alternate type visiting
>>>> a QAPI struct; let's just inline the struct directly as the C union
>>>> branch of the struct.
>>>>

>> Also, here we pass 'obj'; visit_type_FOO() had to pass '*obj' (again,
>> because we have one less level of indirection, and 7/13 reduced the
>> indirection required in visit_type_FOO_fields()).
>>
>>>         // visit_start_union() + switch dropped
>>>         error_propagate(errp, err);
>>>         err = NULL;
>>>         visit_end_struct(v, &err);
>>>     out:
>>>         error_propagate(errp, err);
>>>     }
>>>
>>> Why can we drop visit_start_union() + switch?
>>
>> visit_start_union() is dropped because its only purpose was to determine
>> if the dealloc visitor needs to visit the default branch. When we had a
>> separate allocation, we did not want to visit the branch if the
>> discriminator was not successfully parsed, because otherwise we would
>> dereference NULL.  But now that we don't have a second pointer
>> allocation, we no longer have anything to dealloc, and we can no longer
>> dereference NULL. Explained better in 12/13, where I delete
>> visit_start_union() altogether.  But maybe I could keep it in this patch
>> in the meantime, to minimize confusion.
> 
> Perhaps squashing the two patches together could be less confusing.

Yes, I'm closer to posting v11, and in that version, visit_start_union
is only dropped in a single patch; and this patch just inlines the
visit_start_struct() allocation directly into the visit_type_ALT()
rather than creating a new visit_type_alternate_ALT().

> 
>> Dropped switch, on the other hand, looks to be a genuine bug.  Eeek.
>> That should absolutely be present, and it proves that our testsuite is
>> not strong enough for not catching me on it.
>>
>> And now that you've made me think about it, maybe I have yet another
>> idea.  Right now, we've split the visit of local members into
>> visit_type_FOO_fields(), while leaving the variant members to be visited
>> in visit_type_FOO()
> 
> Yes.  I guess that's to support visiting "implicit" structs.

Up to now, we've forbidden the use of one union as a branch of another
(but allowed a union as a branch of an alternate), so the types passed
to visit_struct_FOO_fields() never had variants.  As part of our generic
"object" work, I _want_ to support one union as a branch of another (as
long as we can prove there will be no QMP name collisions); and that's
another reason why visit_struct_FOO_fields() would need to always visit
variants if present.


> Let me get to this result from a different angle.  A "boxed" visit is
> 
>     allocate hook
>     visit the members (common and variant)
>     cleanup hook

Correct, and we have two choices of allocate hook: visit_start_struct()
[allocate and consume {}, for visit_type_FOO() in general], and
visit_start_implicit_struct() [allocate, but don't consume {}, for flat
union branches prior to this series].

> 
> An "unboxed" visit is basically the same without the two hooks.

Done anywhere we don't have a separate C struct [base classes prior to
this series; and then this series is adding unboxed variant visits
within flat unions and alternates].

> 
> "Implicit" is like unboxed, except the members are inlined rather than
> wrapped in a JSON object.
> 
> So the common code to factor out is "visit the members".

Yep, we're on the same wavelength, and it is looking fairly nice for
what I'm about to post as v11.  And I like 'unboxed' better than
'is_member':

>>>> -                     c_type=typ.c_type(),
>>>> +                     c_type=typ.c_type(is_member=inline),
> 
> I don't like the name is_member.  The thing we're dealing with here is a
> member, regardless of the value of inline.  I think it's boxed
> vs. unboxed.

Hmm. I have later patches that add a 'box':true QAPI designation to
commands and events, to let us do qmp_command(Foo *arg, Error **errp)
instead of qmp_command(type Foo_member1, type Foo_member2 ..., Error
**errp) (that is, instead of breaking out each parameter, we pass them
all boxed behind a single pointer).  What we are doing here is in the
opposite direction - we are taking code that has boxed all the sub-type
fields behind a single pointer, and unboxing it so that they occur
inline in the parent type's storage.  Works for me; I'm switching to
'is_unboxed' as the case for when we want to omit the pointer
designation during the member declaration.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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