Re: [Qemu-devel] [PATCH v11 09/15] qapi: Adjust layout of FooList types

[Markus Armbruster]

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.
>
> It requires visit_next_list() to gain a size parameter, to know
> what size element to allocate; comparable to the size parameter
> of visit_start_struct().
>
> 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'.
>
> Note that for lists of objects, the 'value' payload is still
> hidden behind a boxed pointer.  Someday, it would be nice to do:
>
> struct FooList {
>     FooList *next;
>     Foo value;
> };
>
> for one less level of malloc for each list element.  This patch
> is a step in that direction (now that 'next' is no longer at a
> fixed non-zero offset within the struct, we can store more than
> just a pointer's-worth of data as the value payload), but the
> actual conversion would be a task for another series, as it will
> touch a lot of code.
>
> Signed-off-by: Eric Blake <address@hidden>

Bonus: qapi-types.h shrinks by 5%.

 qapi-types.h |  350 +++++++++++------------------------------------------------
 qapi-visit.c |  140 +++++++++++------------
 2 files changed, 140 insertions(+), 350 deletions(-)