build: disable type checks in G_TYPE_CHECK_INSTANCE_CAST() macros (bgo#771120)

Avoid the pointless overhead. Even glib disables them unless you build
with --enable-debug.

https://bugzilla.gnome.org/show_bug.cgi?id=771120

1 files changed, 57 insertions, 4 deletions

diff --git a/shared/nm-default.h b/shared/nm-default.h
index e3e3ea4414..9e2377cf80 100644
--- a/shared/nm-default.h
+++ b/shared/nm-default.h
@@ -52,15 +52,68 @@
 #define NM_VERSION_MAX_ALLOWED   NM_VERSION_NEXT_STABLE
 #define NM_VERSION_MIN_REQUIRED  NM_VERSION_0_9_8
 
+#ifndef NM_MORE_ASSERTS
+#define NM_MORE_ASSERTS 0
+#endif
+
+#if NM_MORE_ASSERTS == 0
+/* The cast macros like NM_TYPE() are implemented via G_TYPE_CHECK_INSTANCE_CAST()
+ * and _G_TYPE_CIC(). The latter, by default performs runtime checks of the type
+ * by calling g_type_check_instance_cast().
+ * This check has a certain overhead without being helpful.
+ *
+ * Example 1:
+ *     static void foo (NMType *obj)
+ *     {
+ *         access_obj_without_check (obj);
+ *     }
+ *     foo ((NMType *) obj);
+ *     // There is no runtime check and passing an invalid pointer
+ *     // leads to a crash.
+ *
+ * Example 2:
+ *     static void foo (NMType *obj)
+ *     {
+ *         access_obj_without_check (obj);
+ *     }
+ *     foo (NM_TYPE (obj));
+ *     // There is a runtime check which prints a g_warning(), but that doesn't
+ *     // avoid the crash as NM_TYPE() cannot do anything then passing on the
+ *     // invalid pointer.
+ *
+ * Example 3:
+ *     static void foo (NMType *obj)
+ *     {
+ *         g_return_if_fail (NM_IS_TYPE (obj));
+ *         access_obj_without_check (obj);
+ *     }
+ *     foo ((NMType *) obj);
+ *     // There is a runtime check which prints a g_critical() which also avoids
+ *     // the crash. That is actually helpful to catch bugs and avoid crashes.
+ *
+ * Example 4:
+ *     static void foo (NMType *obj)
+ *     {
+ *         g_return_if_fail (NM_IS_TYPE (obj));
+ *         access_obj_without_check (obj);
+ *     }
+ *     foo (NM_TYPE (obj));
+ *     // The runtime check is performed twice, with printing a g_warning() and
+ *     // a g_critical() and avoiding the crash.
+ *
+ * Example 3 is how it should be done. Type checks in NM_TYPE() are pointless.
+ * Disable them for our production builds.
+ */
+#ifndef G_DISABLE_CAST_CHECKS
+#define G_DISABLE_CAST_CHECKS
+#endif
+#endif
+
 #include <stdlib.h>
 #include <glib.h>
 
 /*****************************************************************************/
 
-#ifndef NM_MORE_ASSERTS
-#define NM_MORE_ASSERTS 0
-#endif
-
 #if NM_MORE_ASSERTS == 0
 
 /* glib assertions (g_return_*(), g_assert*()) contain a textual representation