qom: allow turning cast debugging off

Cast debugging can have a substantial cost (20% or more).  Instead of adding
special-cased "fast casts" in the hot paths, we can just disable it in
releases.  The tracing facilities we just added make it easier to analyze
those problems that cast debugging would reveal.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1368188203-3407-7-git-send-email-pbonzini@redhat.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

3 files changed, 26 insertions, 9 deletions

diff --git a/configure b/configure
index 9439f1c727..cab6332991 100755
--- a/configure
+++ b/configure
@@ -220,6 +220,7 @@ blobs="yes"
 pkgversion=""
 pie=""
 zero_malloc=""
+qom_cast_debug="yes"
 trace_backend="nop"
 trace_file="trace"
 spice=""
@@ -688,6 +689,10 @@ for opt do
   ;;
   --enable-sdl) sdl="yes"
   ;;
+  --disable-qom-cast-debug) qom_cast_debug="no"
+  ;;
+  --enable-qom-cast-debug) qom_cast_debug="yes"
+  ;;
   --disable-virtfs) virtfs="no"
   ;;
   --enable-virtfs) virtfs="yes"
@@ -3575,6 +3580,7 @@ echo "gcov enabled      $gcov"
 echo "TPM support       $tpm"
 echo "libssh2 support   $libssh2"
 echo "TPM passthrough   $tpm_passthrough"
+echo "QOM debugging     $qom_cast_debug"
 
 if test "$sdl_too_old" = "yes"; then
 echo "-> Your SDL version is too old - please upgrade to have SDL support"
@@ -3909,6 +3915,9 @@ echo "CONFIG_UNAME_RELEASE=\"$uname_release\"" >> $config_host_mak
 if test "$zero_malloc" = "yes" ; then
   echo "CONFIG_ZERO_MALLOC=y" >> $config_host_mak
 fi
+if test "$qom_cast_debug" = "yes" ; then
+  echo "CONFIG_QOM_CAST_DEBUG=y" >> $config_host_mak
+fi
 if test "$rbd" = "yes" ; then
   echo "CONFIG_RBD=y" >> $config_host_mak
 fi
diff --git a/include/qom/object.h b/include/qom/object.h
index 38f674f0ca..63e2a404da 100644
--- a/include/qom/object.h
+++ b/include/qom/object.h
@@ -615,8 +615,9 @@ Object *object_dynamic_cast(Object *obj, const char *typename);
  *
  * See object_dynamic_cast() for a description of the parameters of this
  * function.  The only difference in behavior is that this function asserts
- * instead of returning #NULL on failure.  This function is not meant to be
- * called directly, but only through the wrapper macro OBJECT_CHECK.
+ * instead of returning #NULL on failure if QOM cast debugging is enabled.
+ * This function is not meant to be called directly, but only through
+ * the wrapper macro OBJECT_CHECK.
  */
 Object *object_dynamic_cast_assert(Object *obj, const char *typename,
                                    const char *file, int line, const char *func);
@@ -666,9 +667,9 @@ Type type_register(const TypeInfo *info);
  *
  * See object_class_dynamic_cast() for a description of the parameters
  * of this function.  The only difference in behavior is that this function
- * asserts instead of returning #NULL on failure.  This function is not
- * meant to be called directly, but only through the wrapper macros
- * OBJECT_CLASS_CHECK and INTERFACE_CHECK.
+ * asserts instead of returning #NULL on failure if QOM cast debugging is
+ * enabled.  This function is not meant to be called directly, but only through
+ * the wrapper macros OBJECT_CLASS_CHECK and INTERFACE_CHECK.
  */
 ObjectClass *object_class_dynamic_cast_assert(ObjectClass *klass,
                                               const char *typename,
diff --git a/qom/object.c b/qom/object.c
index 1b9c5ce4f8..f5f416bc81 100644
--- a/qom/object.c
+++ b/qom/object.c
@@ -435,12 +435,11 @@ Object *object_dynamic_cast(Object *obj, const char *typename)
 Object *object_dynamic_cast_assert(Object *obj, const char *typename,
                                    const char *file, int line, const char *func)
 {
-    Object *inst;
-
     trace_object_dynamic_cast_assert(obj ? obj->class->type->name : "(null)",
                                      typename, file, line, func);
 
-    inst = object_dynamic_cast(obj, typename);
+#ifdef CONFIG_QOM_CAST_DEBUG
+    Object *inst = object_dynamic_cast(obj, typename);
 
     if (!inst && obj) {
         fprintf(stderr, "%s:%d:%s: Object %p is not an instance of type %s\n",
@@ -448,7 +447,9 @@ Object *object_dynamic_cast_assert(Object *obj, const char *typename,
         abort();
     }
 
-    return inst;
+    assert(obj == inst);
+#endif
+    return obj;
 }
 
 ObjectClass *object_class_dynamic_cast(ObjectClass *class,
@@ -509,6 +510,12 @@ ObjectClass *object_class_dynamic_cast_assert(ObjectClass *class,
     trace_object_class_dynamic_cast_assert(class ? class->type->name : "(null)",
                                            typename, file, line, func);
 
+#ifndef CONFIG_QOM_CAST_DEBUG
+    if (!class->interfaces) {
+        return class;
+    }
+#endif
+
     ret = object_class_dynamic_cast(class, typename);
     if (!ret && class) {
         fprintf(stderr, "%s:%d:%s: Object %p is not an instance of type %s\n",