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

[Eric Blake]

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?


>>  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?

>>  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.

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

signature.asc
Description: OpenPGP digital signature