qobject: Simplify QObject

The QObject hierarchy is small enough, and unlikely to grow further
(since we only use it to map to JSON and already cover all JSON
types), that we can simplify things by not tracking a separate
vtable, but just inline the code element of the vtable QType
directly into QObject (renamed to type), and track a separate array
of destroy functions.  We can drop qnull_destroy_obj() in the
process.

The remaining QObject subclasses must export their destructor.

This also has the nice benefit of moving the typename 'QType'
out of the way, so that the next patch can repurpose it for a
nicer name for 'qtype_code'.

The various objects are still the same size (so no change in cache
line pressure), but now have less indirection (although I didn't
bother benchmarking to see if there is a noticeable speedup, as
we don't have hard evidence that this was in a performance hotspot
in the first place).

A future patch could drop the refcnt size to 32 bits for a smaller
struct on 64-bit architectures, if desired (we have limits on the
largest JSON that we are willing to parse, and will probably never
need to take full advantage of a 64-bit refcnt).

Suggested-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1449033659-25497-2-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>

7 files changed, 21 insertions, 16 deletions

diff --git a/include/qapi/qmp/qbool.h b/include/qapi/qmp/qbool.h
index d9256e4268..836d078866 100644
--- a/include/qapi/qmp/qbool.h
+++ b/include/qapi/qmp/qbool.h
@@ -25,5 +25,6 @@ typedef struct QBool {
 QBool *qbool_from_bool(bool value);
 bool qbool_get_bool(const QBool *qb);
 QBool *qobject_to_qbool(const QObject *obj);
+void qbool_destroy_obj(QObject *obj);
 
 #endif /* QBOOL_H */
diff --git a/include/qapi/qmp/qdict.h b/include/qapi/qmp/qdict.h
index 787c658967..6c2a0e501e 100644
--- a/include/qapi/qmp/qdict.h
+++ b/include/qapi/qmp/qdict.h
@@ -48,6 +48,7 @@ void qdict_iter(const QDict *qdict,
                 void *opaque);
 const QDictEntry *qdict_first(const QDict *qdict);
 const QDictEntry *qdict_next(const QDict *qdict, const QDictEntry *entry);
+void qdict_destroy_obj(QObject *obj);
 
 /* Helper to qdict_put_obj(), accepts any object */
 #define qdict_put(qdict, key, obj) \
diff --git a/include/qapi/qmp/qfloat.h b/include/qapi/qmp/qfloat.h
index 46745e50d1..a8af2a89b2 100644
--- a/include/qapi/qmp/qfloat.h
+++ b/include/qapi/qmp/qfloat.h
@@ -25,5 +25,6 @@ typedef struct QFloat {
 QFloat *qfloat_from_double(double value);
 double qfloat_get_double(const QFloat *qi);
 QFloat *qobject_to_qfloat(const QObject *obj);
+void qfloat_destroy_obj(QObject *obj);
 
 #endif /* QFLOAT_H */
diff --git a/include/qapi/qmp/qint.h b/include/qapi/qmp/qint.h
index 339a9abb8f..049e528079 100644
--- a/include/qapi/qmp/qint.h
+++ b/include/qapi/qmp/qint.h
@@ -24,5 +24,6 @@ typedef struct QInt {
 QInt *qint_from_int(int64_t value);
 int64_t qint_get_int(const QInt *qi);
 QInt *qobject_to_qint(const QObject *obj);
+void qint_destroy_obj(QObject *obj);
 
 #endif /* QINT_H */
diff --git a/include/qapi/qmp/qlist.h b/include/qapi/qmp/qlist.h
index b1bf7852c5..a84117ecb1 100644
--- a/include/qapi/qmp/qlist.h
+++ b/include/qapi/qmp/qlist.h
@@ -49,6 +49,7 @@ QObject *qlist_peek(QList *qlist);
 int qlist_empty(const QList *qlist);
 size_t qlist_size(const QList *qlist);
 QList *qobject_to_qlist(const QObject *obj);
+void qlist_destroy_obj(QObject *obj);
 
 static inline const QListEntry *qlist_first(const QList *qlist)
 {
diff --git a/include/qapi/qmp/qobject.h b/include/qapi/qmp/qobject.h
index 4b96ed5837..550ba40dde 100644
--- a/include/qapi/qmp/qobject.h
+++ b/include/qapi/qmp/qobject.h
@@ -47,15 +47,8 @@ typedef enum {
     QTYPE_MAX,
 } qtype_code;
 
-struct QObject;
-
-typedef struct QType {
-    qtype_code code;
-    void (*destroy)(struct QObject *);
-} QType;
-
 typedef struct QObject {
-    const QType *type;
+    qtype_code type;
     size_t refcnt;
 } QObject;
 
@@ -71,9 +64,12 @@ typedef struct QObject {
     qobject_decref(obj ? QOBJECT(obj) : NULL)
 
 /* Initialize an object to default values */
-#define QOBJECT_INIT(obj, qtype_type)   \
-    obj->base.refcnt = 1;               \
-    obj->base.type   = qtype_type
+static inline void qobject_init(QObject *obj, qtype_code type)
+{
+    assert(QTYPE_NONE < type && type < QTYPE_MAX);
+    obj->refcnt = 1;
+    obj->type = type;
+}
 
 /**
  * qobject_incref(): Increment QObject's reference count
@@ -85,6 +81,11 @@ static inline void qobject_incref(QObject *obj)
 }
 
 /**
+ * qobject_destroy(): Free resources used by the object
+ */
+void qobject_destroy(QObject *obj);
+
+/**
  * qobject_decref(): Decrement QObject's reference count, deallocate
  * when it reaches zero
  */
@@ -92,9 +93,7 @@ static inline void qobject_decref(QObject *obj)
 {
     assert(!obj || obj->refcnt);
     if (obj && --obj->refcnt == 0) {
-        assert(obj->type != NULL);
-        assert(obj->type->destroy != NULL);
-        obj->type->destroy(obj);
+        qobject_destroy(obj);
     }
 }
 
@@ -103,8 +102,8 @@ static inline void qobject_decref(QObject *obj)
  */
 static inline qtype_code qobject_type(const QObject *obj)
 {
-    assert(obj->type != NULL);
-    return obj->type->code;
+    assert(QTYPE_NONE < obj->type && obj->type < QTYPE_MAX);
+    return obj->type;
 }
 
 extern QObject qnull_;
diff --git a/include/qapi/qmp/qstring.h b/include/qapi/qmp/qstring.h
index 34675a7fc0..df7df558b2 100644
--- a/include/qapi/qmp/qstring.h
+++ b/include/qapi/qmp/qstring.h
@@ -32,5 +32,6 @@ void qstring_append_int(QString *qstring, int64_t value);
 void qstring_append(QString *qstring, const char *str);
 void qstring_append_chr(QString *qstring, int c);
 QString *qobject_to_qstring(const QObject *obj);
+void qstring_destroy_obj(QObject *obj);
 
 #endif /* QSTRING_H */