gtype: improve get_type fast path

The -fstack-protector-strong used in many distributions by default has a
rather drastic slowdown of the fast path in generated _get_type()
functions using G_DEFINE_* macros. The amount can vary by architecture,
GCC version, and compiler flags.

To work around this, and ensure a higher probability that our fast-path
will match what we had previously, we need to break out the slow-path
(registering the type) into a secondary function that is not a candidate
for inlining.

This ensures that the common case (type registered, return the GType id)
is the hot path and handled in the prologue of the generated assembly even
when -fstack-protector-strong is enabled.

https://bugzilla.gnome.org/show_bug.cgi?id=795180

gobject/gtype.h

@@ -1948,6 +1948,7 @@ static void     type_name##_class_intern_init (gpointer klass) \
 \
 static void     type_name##_init              (TypeName        *self); \
 static void     type_name##_class_init        (TypeName##Class *klass); \
+static GType    type_name##_get_type_once     (void); \
 static gpointer type_name##_parent_class = NULL; \
 static gint     TypeName##_private_offset; \
 \
@@ -1969,6 +1970,16 @@ type_name##_get_type (void) \
 /* Added for _G_DEFINE_TYPE_EXTENDED_WITH_PRELUDE */
 #define _G_DEFINE_TYPE_EXTENDED_BEGIN_REGISTER(TypeName, type_name, TYPE_PARENT, flags) \
   if (g_once_init_enter (&g_define_type_id__volatile))  \
+    { \
+      GType g_define_type_id = type_name##_get_type_once (); \
+      g_once_init_leave (&g_define_type_id__volatile, g_define_type_id); \
+    }					\
+  return g_define_type_id__volatile;	\
+} /* closes type_name##_get_type() */ \
+\
+G_GNUC_NO_INLINE \
+static GType \
+type_name##_get_type_once (void) \
 { \
   GType g_define_type_id = \
         g_type_register_static_simple (TYPE_PARENT, \
@@ -1982,10 +1993,8 @@ type_name##_get_type (void) \
 #define _G_DEFINE_TYPE_EXTENDED_END()	\
       /* following custom code */	\
     }					\
-      g_once_init_leave (&g_define_type_id__volatile, g_define_type_id); \
+  return g_define_type_id; \
-    }					\
+} /* closes type_name##_get_type_once() */
-  return g_define_type_id__volatile;	\
-} /* closes type_name##_get_type() */
 
 /* This was defined before we had G_DEFINE_TYPE_WITH_CODE_AND_PRELUDE, it's simplest
  * to keep it.
@@ -2070,11 +2079,23 @@ type_name##_get_type (void) \
  */
 #if !defined (__cplusplus) && (__GNUC__ > 2 || (__GNUC__ == 2 && __GNUC_MINOR__ >= 7)) && !(defined (__APPLE__) && defined (__ppc64__))
 #define _G_DEFINE_BOXED_TYPE_BEGIN(TypeName, type_name, copy_func, free_func) \
+static GType type_name##_get_type_once (void); \
+\
 GType \
 type_name##_get_type (void) \
 { \
   static volatile gsize g_define_type_id__volatile = 0; \
   if (g_once_init_enter (&g_define_type_id__volatile))  \
+    { \
+      GType g_define_type_id = type_name##_get_type_once (); \
+      g_once_init_leave (&g_define_type_id__volatile, g_define_type_id); \
+    } \
+  return g_define_type_id__volatile; \
+} \
+\
+G_GNUC_NO_INLINE \
+static GType \
+type_name##_get_type_once (void) \
 { \
   GType (* _g_register_boxed) \
     (const gchar *, \
@@ -2095,11 +2116,23 @@ type_name##_get_type (void) \
   { /* custom code follows */
 #else
 #define _G_DEFINE_BOXED_TYPE_BEGIN(TypeName, type_name, copy_func, free_func) \
+static GType type_name##_get_type_once (void); \
+\
 GType \
 type_name##_get_type (void) \
 { \
   static volatile gsize g_define_type_id__volatile = 0; \
   if (g_once_init_enter (&g_define_type_id__volatile))  \
+    { \
+      GType g_define_type_id = type_name##_get_type_once (); \
+      g_once_init_leave (&g_define_type_id__volatile, g_define_type_id); \
+    } \
+  return g_define_type_id__volatile; \
+} \
+\
+G_GNUC_NO_INLINE \
+static GType \
+type_name##_get_type_once (void) \
 { \
   GType g_define_type_id = \
     g_boxed_type_register_static (g_intern_static_string (#TypeName), \
@@ -2136,11 +2169,23 @@ type_name##_get_type (void) \
 #define G_DEFINE_POINTER_TYPE_WITH_CODE(TypeName, type_name, _C_) _G_DEFINE_POINTER_TYPE_BEGIN (TypeName, type_name) {_C_;} _G_DEFINE_TYPE_EXTENDED_END()
 
 #define _G_DEFINE_POINTER_TYPE_BEGIN(TypeName, type_name) \
+static GType type_name##_get_type_once (void); \
+\
 GType \
 type_name##_get_type (void) \
 { \
   static volatile gsize g_define_type_id__volatile = 0; \
   if (g_once_init_enter (&g_define_type_id__volatile))  \
+    { \
+      GType g_define_type_id = type_name##_get_type_once (); \
+      g_once_init_leave (&g_define_type_id__volatile, g_define_type_id); \
+    } \
+  return g_define_type_id__volatile; \
+} \
+\
+G_GNUC_NO_INLINE \
+static GType \
+type_name##_get_type_once (void) \
 { \
   GType g_define_type_id = \
     g_pointer_type_register_static (g_intern_static_string (#TypeName)); \