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.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1455778109-6278-10-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

include/qapi/visitor-impl.h

@@ -29,7 +29,7 @@ struct Visitor
 
     void (*start_list)(Visitor *v, const char *name, Error **errp);
     /* Must be set */
-    GenericList *(*next_list)(Visitor *v, GenericList **list);
+    GenericList *(*next_list)(Visitor *v, GenericList **list, size_t size);
     /* Must be set */
     void (*end_list)(Visitor *v);
 

include/qapi/visitor.h

@@ -19,13 +19,12 @@
 #include "qapi/error.h"
 #include <stdlib.h>
 
-typedef struct GenericList
-{
-    union {
-        void *value;
-        uint64_t padding;
-    };
+/* 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 {
     struct GenericList *next;
+    char padding[];
 } GenericList;
 
 void visit_start_struct(Visitor *v, const char *name, void **obj,
@@ -36,7 +35,7 @@ void visit_start_implicit_struct(Visitor *v, void **obj, size_t size,
 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);
 
 /**

qapi/opts-visitor.c

@@ -215,7 +215,7 @@ opts_start_list(Visitor *v, const char *name, Error **errp)
 
 
 static GenericList *
-opts_next_list(Visitor *v, GenericList **list)
+opts_next_list(Visitor *v, GenericList **list, size_t size)
 {
     OptsVisitor *ov = to_ov(v);
     GenericList **link;
@@ -258,7 +258,7 @@ opts_next_list(Visitor *v, GenericList **list)
         abort();
     }
 
-    *link = g_malloc0(sizeof **link);
+    *link = g_malloc0(size);
     return *link;
 }
 

qapi/qapi-dealloc-visitor.c

@@ -100,7 +100,8 @@ 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);

qapi/qapi-visit-core.c

@@ -50,9 +50,10 @@ 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)

qapi/qmp-input-visitor.c

@@ -165,7 +165,8 @@ 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;
@@ -184,7 +185,7 @@ 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 {

qapi/qmp-output-visitor.c

@@ -126,7 +126,8 @@ 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);

qapi/string-input-visitor.c

@@ -139,7 +139,7 @@ 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;
@@ -173,7 +173,7 @@ static GenericList *next_list(Visitor *v, GenericList **list)
         link = &(*list)->next;
     }
 
-    *link = g_malloc0(sizeof **link);
+    *link = g_malloc0(size);
     return *link;
 }
 

qapi/string-output-visitor.c

@@ -276,7 +276,7 @@ start_list(Visitor *v, const char *name, Error **errp)
     sov->head = true;
 }
 
-static GenericList *next_list(Visitor *v, GenericList **list)
+static GenericList *next_list(Visitor *v, GenericList **list, size_t size)
 {
     StringOutputVisitor *sov = to_sov(v);
     GenericList *ret = NULL;

scripts/qapi-types.py

@@ -26,11 +26,8 @@ def gen_array(name, element_type):
     return mcgen('''
 
 struct %(c_name)s {
-    union {
-        %(c_type)s value;
-        uint64_t padding;
-    };
     %(c_name)s *next;
+    %(c_type)s value;
 };
 ''',
                  c_name=c_name(name), c_type=element_type.c_type())

scripts/qapi-visit.py

@@ -173,7 +173,7 @@ def gen_visit_list(name, element_type):
     }
 
     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);