Re: [Qemu-ppc] [Qemu-devel] [PATCH v9 36/37] RFC: qapi: Adjust layout of FooList types

[Markus Armbruster]

Eric Blake <address@hidden> writes:

> On 01/28/2016 08:34 AM, Markus Armbruster wrote:
>> Eric Blake <address@hidden> writes:
>> 
>>> By sticking the next pointer first, we don't need a union with
>>> 64-bit padding for smaller types.  On 32-bit platforms, this
>>> can reduce the size of uint8List from 16 bytes (or 12, depending
>>> on whether 64-bit ints can tolerate 4-byte alignment) down to 8.
>>> It has no effect on 64-bit platforms (where alignment still
>>> dictates a 16-byte struct); but fewer anonymous unions is still
>>> a win in my book.
>>>
>>> However, this requires visit_start_list() and visit_next_list()
>>> to gain a size parameter, to know what size element to allocate.
>>>
>>> I debated about going one step further, to allow for fewer casts,
>>> by doing:
>>>     typedef GenericList GenericList;
>>>     struct GenericList {
>>>         GenericList *next;
>>>     };
>>>     struct FooList {
>>>         GenericList base;
>>>         Foo value;
>>>     };
>>> so that you convert to 'GenericList *' by '&foolist->base', and
>>> back by 'container_of(generic, GenericList, base)' (as opposed to
>>> the existing '(GenericList *)foolist' and '(FooList *)generic').
>>> But doing that would require hoisting the declaration of
>>> GenericList prior to inclusion of qapi-types.h, rather than its
>>> current spot in visitor.h; it also makes iteration a bit more
>>> verbose through 'foolist->base.next' instead of 'foolist->next'.
>
> Should I attempt this?

A quick grep for '(GenericList' finds two in qapi-code-gen.txt, and two
in qapi-visit-py.  I doubt avoiding them is worth much of your time or
mine :)

>>>  typedef struct GenericList
>>>  {
>>> -    union {
>>> -        void *value;
>>> -        uint64_t padding;
>>> -    };
>>>      struct GenericList *next;
>>> +    char padding[];
>>>  } GenericList;
>> 
>> Less trickery, I like it.
>> 
>> Member padding appears to be unused.
>
> or just leave it at this?

I'd say good enough.

>>>  bool visit_start_list(Visitor *v, const char *name, GenericList **list,
>>> -                      Error **errp)
>>> +                      size_t size, Error **errp)
>>>  {
>>> -    bool result = v->start_list(v, name, list, errp);
>>> +    bool result;
>>> +
>>> +    assert(list ? size : !size);
>> 
>> Tighter than size != 0 would be size >= GenericList.  Same elsewhere.
>
> Makes sense.
>
>> 
>> Rest looks good.  Didn't look as closely as for the previous patches
>> (getting tired), but so far I like the idea.
>
> Okay, I'll keep it and drop the RFC.

Thanks!