qapi: Adjust layout of FooList types

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.

Backports commit e65d89bf1a4484e0db0f3dc820a8b209f2fb1e8b from qemu

qemu/include/qapi/visitor-impl.h

@@ -38,7 +38,7 @@ struct Visitor
     void (*end_implicit_struct)(Visitor *v);
 
     void (*start_list)(Visitor *v, const char *name, Error **errp);
-    GenericList *(*next_list)(Visitor *v, GenericList **list);
+    GenericList *(*next_list)(Visitor *v, GenericList **list, size_t size);
     void (*end_list)(Visitor *v);
 
     /* May be NULL; only needed for input visitors. */

qemu/include/qapi/visitor.h

@@ -18,13 +18,13 @@
 #include "qapi/error.h"
 #include <stdlib.h>
 
+/* This struct is layout-compatible with all other *List structs
+ * created by the qapi generator.  It is used as a typical
+ * singly-linked list. */
 typedef struct GenericList
 {
-    union {
-        void *value;
-        uint64_t padding;
-    };
     struct GenericList *next;
+    char padding[];
 } GenericList;
 
 void visit_start_struct(Visitor *v, const char *name, void **obj,
@@ -34,7 +34,7 @@ void visit_start_implicit_struct(Visitor *v, void **obj, size_t size,
                                  Error **errp);
 void visit_end_implicit_struct(Visitor *v);
 void visit_start_list(Visitor *v, const char *name, Error **errp);
-GenericList *visit_next_list(Visitor *v, GenericList **list);
+GenericList *visit_next_list(Visitor *v, GenericList **list, size_t size);
 void visit_end_list(Visitor *v);
 
 /**

qemu/qapi/qapi-dealloc-visitor.c

@@ -101,7 +101,8 @@ static void qapi_dealloc_start_list(Visitor *v, const char *name, Error **errp)
     qapi_dealloc_push(qov, NULL);
 }
 
-static GenericList *qapi_dealloc_next_list(Visitor *v, GenericList **listp)
+static GenericList *qapi_dealloc_next_list(Visitor *v, GenericList **listp,
+                                           size_t size)
 {
     GenericList *list = *listp;
     QapiDeallocVisitor *qov = to_qov(v);

qemu/qapi/qapi-visit-core.c

@@ -52,9 +52,10 @@ void visit_start_list(Visitor *v, const char *name, Error **errp)
     v->start_list(v, name, errp);
 }
 
-GenericList *visit_next_list(Visitor *v, GenericList **list)
+GenericList *visit_next_list(Visitor *v, GenericList **list, size_t size)
 {
-    return v->next_list(v, list);
+    assert(list && size >= sizeof(GenericList));
+    return v->next_list(v, list, size);
 }
 
 void visit_end_list(Visitor *v)

qemu/qapi/qmp-input-visitor.c

@@ -173,7 +173,8 @@ static void qmp_input_start_list(Visitor *v, const char *name, Error **errp)
     qmp_input_push(qiv, qobj, errp);
 }
 
-static GenericList *qmp_input_next_list(Visitor *v, GenericList **list)
+static GenericList *qmp_input_next_list(Visitor *v, GenericList **list,
+                                        size_t size)
 {
     QmpInputVisitor *qiv = to_qiv(v);
     GenericList *entry;
@@ -192,7 +193,7 @@ static GenericList *qmp_input_next_list(Visitor *v, GenericList **list)
         return NULL;
     }
 
-    entry = g_malloc0(sizeof(*entry));
+    entry = g_malloc0(size);
     if (first) {
         *list = entry;
     } else {

qemu/qapi/qmp-output-visitor.c

@@ -127,7 +127,8 @@ static void qmp_output_start_list(Visitor *v, const char *name, Error **errp)
     qmp_output_push(qov, list);
 }
 
-static GenericList *qmp_output_next_list(Visitor *v, GenericList **listp)
+static GenericList *qmp_output_next_list(Visitor *v, GenericList **listp,
+                                         size_t size)
 {
     GenericList *list = *listp;
     QmpOutputVisitor *qov = to_qov(v);

qemu/qapi/string-input-visitor.c

@@ -140,8 +140,7 @@ start_list(Visitor *v, const char *name, Error **errp)
     }
 }
 
-static GenericList *
-next_list(Visitor *v, GenericList **list)
+static GenericList *next_list(Visitor *v, GenericList **list, size_t size)
 {
     StringInputVisitor *siv = to_siv(v);
     GenericList **link;
@@ -175,7 +174,7 @@ next_list(Visitor *v, GenericList **list)
         link = &(*list)->next;
     }
 
-    *link = g_malloc0(sizeof **link);
+    *link = g_malloc0(size);
     return *link;
 }
 

qemu/scripts/qapi-types.py

@@ -28,11 +28,8 @@ typedef struct %(c_name)s %(c_name)s;
 def gen_array(name, element_type):
     return mcgen('''
 struct %(c_name)s {
-    union {
-        %(c_type)s value;
-        uint64_t padding;
-    };
-    struct %(c_name)s *next;
+    %(c_name)s *next;
+    %(c_type)s value;
 };
 ''',
                  c_name=c_name(name), c_type=element_type.c_type())

qemu/scripts/qapi-visit.py

@@ -123,7 +123,7 @@ void visit_type_%(c_name)s(Visitor *v, const char *name, %(c_name)s **obj, Error
     }
 
     for (prev = (GenericList **)obj;
-         !err && (i = visit_next_list(v, prev)) != NULL;
+         !err && (i = visit_next_list(v, prev, sizeof(**obj))) != NULL;
          prev = &i) {
         %(c_name)s *native_i = (%(c_name)s *)i;
         visit_type_%(c_elt_type)s(v, NULL, &native_i->value, &err);