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

[Markus Armbruster]

Eric Blake <address@hidden> 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:
>
> | struct SpiceChannel {
> |-    SpiceBasicInfo *base;
> |+    /* Members inherited from SpiceBasicInfo: */
> |+    char *host;
> |+    char *port;
> |+    NetworkAddressFamily family;
> |+    /* Own members: */
> |     int64_t connection_id;
>
> 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.

>
> 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/

> |     if (err) {
>
> plus the wholesale elimination of some now-unused
> visit_type_implicit_FOO() functions.
>
> Without boxing, the corner case of one empty struct having
> another empty struct as its base type now requires inserting a
> dummy member (previously, the pointer to the base would have
> sufficed).
>
> And now that we no longer consume a 'base' member in the generated
> C struct, we can delete the former negative struct-base-clash-base
> test.
>
> Signed-off-by: Eric Blake <address@hidden>

Patch looks good.