qapi: Unbox base members

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 allows us to turn on
automatic type-safe wrappers for upcasting 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 additional upcast functions like qapi_SpiceChannel_base().
Meanwhile, changes to qapi-visit.c look like:

| 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_SpiceBasicInfo_fields(v, (SpiceBasicInfo **)obj, &err);
| if (err) {

(the cast is necessary, since our upcast wrappers only deal with a
single pointer, not pointer-to-pointer); 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 'Base *base' member 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.

Backports commit ddf21908961073199f3d186204da4810f2ea150b from qemu

qemu/scripts/qapi-types.py

@@ -73,16 +73,13 @@ struct %(c_name)s {
 ''',
                 c_name=c_name(name))
 
-    if base:
-        ret += gen_struct_field('base', base, False)
-
-    ret += gen_struct_fields(members)
+    ret += gen_struct_fields(members, base)
 
     # Make sure that all structs have at least one field; this avoids
     # potential issues with attempting to malloc space for zero-length
     # structs in C, and also incompatibility with C++ (where an empty
     # struct is size 1).
-    if not base and not members:
+    if not (base and base.members) and not members:
         ret += mcgen('''
     char qapi_dummy_field_for_empty_struct;
 ''')
@@ -275,11 +272,10 @@ class QAPISchemaGenTypeVisitor(QAPISchemaVisitor):
         if variants:
             assert not members      # not implemented
             self.decl += gen_union(name, base, variants)
-            # TODO Use gen_upcast on structs too, once they have sane layout
-            if base:
-                self.decl += gen_upcast(name, base)
         else:
             self.decl += gen_struct(name, base, members)
+        if base:
+            self.decl += gen_upcast(name, base)
         self._gen_type_cleanup(name)
 
     def visit_alternate_type(self, name, info, variants):

qemu/scripts/qapi-visit.py

@@ -69,13 +69,12 @@ static void visit_type_implicit_%(c_type)s(Visitor *v, %(c_type)s **obj, Error *
     return ret
 
 def gen_visit_struct_fields(name, base, members):
-    struct_fields_seen.add(name)
-
     ret = ''
 
     if base:
-        ret += gen_visit_implicit_struct(base)
+        ret += gen_visit_fields_decl(base)
 
+    struct_fields_seen.add(name)
     ret += mcgen('''
 
 static void visit_type_%(c_name)s_fields(Visitor *v, %(c_name)s **obj, Error **errp)
@@ -87,9 +86,9 @@ static void visit_type_%(c_name)s_fields(Visitor *v, %(c_name)s **obj, Error **e
 
     if base:
         ret += mcgen('''
-    visit_type_implicit_%(c_type)s(v, &(*obj)->%(c_name)s, &err);
+    visit_type_%(c_type)s_fields(v, (%(c_type)s **)obj, &err);
 ''',
-                     c_type=base.c_name(), c_name=c_name('base'))
+                     c_type=base.c_name())
         ret += gen_err_check()
 
     ret += gen_visit_fields(members, prefix='(*obj)->')