Re: [Qemu-devel] [PATCH v10 10/25] qapi: Unbox base members

[Eric Blake <eblake@redhat.com>]

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On 10/23/2015 01:14 PM, Markus Armbruster wrote:
> Eric Blake <eblake@redhat.com> writes:
> 
>> Rather than storing a base class as a pointer to a box, just
>> store the fields of that base class in the same order, so that
>> a child struct can be directly cast to its parent.  This gives
>> less malloc overhead, less pointer dereferencing, and even less
>> generated code.  Compare to the earlier commit 1e6c1616a "qapi:
>> Generate a nicer struct for flat unions" (although that patch
>> had fewer places to change, as less of qemu was directly using
>> qapi structs for flat unions).  It also drops a hack that was
>> needed for type-safe casting to the base class of a struct.
>>
>> Changes to the generated code look like this in qapi-types.h:
>>

>>
>> as well as:
>>
>> | static inline SpiceBasicInfo *qapi_SpiceChannel_base(const SpiceChannel *obj)
>> | {
>> |-    return (SpiceBasicInfo *)obj->base;
>> |+    return (SpiceBasicInfo *)obj;
>> | }
> 
> No more, PATCH 09 hides this change.  If you limit that one to just
> unions, it'll be visible again here, but it'll look different.

No, this change is new to v10, because of patch 09.  In v9, we did not
have gen_upcast(); in v10, patch 9's version of gen_upcast() had to
special-case struct to output 'obj->base', and patch 10 gets rid of the
special case to output 'obj' always.

If I limit patch 9 to just unions, then there would be no clients using
the upcast wrappers in patch 9.  The only users of upcasts that benefit
from the result of gen_upcast() are the struct users in ui/*.c, that
were touched in patch 9, and then where _this_ change in the generated
code in patch 10 lets ui/ code remain unchanged (the way the upcast
happens changed, but it was nicely hidden behind the nice wrapper
interface).

> 
>>
>> and this in qapi-visit.c:
>>
>> | static void visit_type_SpiceChannel_fields(Visitor *v, SpiceChannel **obj, Error **errp)
>> | {
>> |     Error *err = NULL;
>> |
>> |-    visit_type_implicit_SpiceBasicInfo(v, &(*obj)->base, &err);
>> |+    visit_type_SpiceBasiInfo_fields(v, (SpiceBasicInfo **)obj, &err);
> 
> s/BasiI/BasicI/

I should have used the mouse to copy-paste instead of typing late at
night :)

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


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