gtype: avoid "-Wcast-align" warning with optimized G_TYPE_CHECK_INSTANCE_CAST()

We can get a "-Wcast-align", if the target type that we cast to ("ct") has a
larger alignment than GTypeInstance.

That can happen on i686 architecture, if the GObject type has larger
alignment than the parent struct (or GObject). Since on i686, embeding
a "long long" or a "long double" in a struct still does not increase
the alignment beyond 4 bytes, this usually only happens when using the
__attribute__() to increase the alignment (or to have a field that has
the alignment increased).

It can happen on x86_64 when having a "long double" field.

The compiler warning is hard to avoid but not very useful, because it purely
operates on the pointer types at compile time. G_TYPE_CHECK_INSTANCE_CAST()
instead asserts (in non-optimized mode) that the pointer really points
to the expected GTypeInstance (and if that's the case, then the alignment
should be suitable already).

This is like in commit ed553e8e309d ('gtype: Eliminate -Wcast-align warnings
with G_TYPE_CHECK_INSTANCE_CAST'). But also fix the optimized code path.

With the unpatched G_TYPE_CHECK_INSTANCE_CAST() macro, the unit test would
now show the problem (with gcc-9.3.1-2.fc30.i686 or
gcc-12.2.1-4.fc37.x86_64):

  $ export G_DISABLE_CAST_CHECKS=1
  $ export CFLAGS='-Wcast-align=strict'
  $ meson build
  $ ninja -C build
  ...
  In file included from ../gobject/gobject.h:26,
                   from ../gobject/gbinding.h:31,
                   from ../glib/glib-object.h:24,
                   from ../gobject/tests/objects-refcount1.c:2:
  ../gobject/tests/objects-refcount1.c: In function ‘my_test_dispose’:
  ../gobject/gtype.h:2523:42: warning: cast increases required alignment of target type [-Wcast-align]
   2523 | #  define _G_TYPE_CIC(ip, gt, ct)       ((ct*) ip)
        |                                          ^
  ../gobject/gtype.h:517:66: note: in expansion of macro ‘_G_TYPE_CIC’
    517 | #define G_TYPE_CHECK_INSTANCE_CAST(instance, g_type, c_type)    (_G_TYPE_CIC ((instance), (g_type), c_type))
        |                                                                  ^~~~~~~~~~~
  ../gobject/tests/objects-refcount1.c:9:37: note: in expansion of macro ‘G_TYPE_CHECK_INSTANCE_CAST’
      9 | #define MY_TEST(test)              (G_TYPE_CHECK_INSTANCE_CAST ((test), G_TYPE_TEST, GTest))
        |                                     ^~~~~~~~~~~~~~~~~~~~~~~~~~
  ../gobject/tests/objects-refcount1.c:96:10: note: in expansion of macro ‘MY_TEST’
     96 |   test = MY_TEST (object);
        |          ^~~~~~~

2 files changed, 20 insertions, 3 deletions

diff --git a/gobject/gtype.h b/gobject/gtype.h
index f77a1a37b..04e373385 100644
--- a/gobject/gtype.h
+++ b/gobject/gtype.h
@@ -2520,8 +2520,8 @@ const gchar *    g_type_name_from_class         (GTypeClass	*g_class);
 
 /* --- implementation bits --- */
 #if defined(G_DISABLE_CAST_CHECKS) || defined(__OPTIMIZE__)
-#  define _G_TYPE_CIC(ip, gt, ct)       ((ct*) ip)
-#  define _G_TYPE_CCC(cp, gt, ct)       ((ct*) cp)
+#  define _G_TYPE_CIC(ip, gt, ct)       ((ct*) (void *) ip)
+#  define _G_TYPE_CCC(cp, gt, ct)       ((ct*) (void *) cp)
 #else
 #  define _G_TYPE_CIC(ip, gt, ct) \
     ((ct*) (void *) g_type_check_instance_cast ((GTypeInstance*) ip, gt))
diff --git a/gobject/tests/objects-refcount1.c b/gobject/tests/objects-refcount1.c
index 4ecf877a9..5ba55890a 100644
--- a/gobject/tests/objects-refcount1.c
+++ b/gobject/tests/objects-refcount1.c
@@ -15,10 +15,27 @@
 typedef struct _GTest GTest;
 typedef struct _GTestClass GTestClass;
 
+#if G_GNUC_CHECK_VERSION (4, 0)
+/* Increase the alignment of GTest to check whether
+ * G_TYPE_CHECK_INSTANCE_CAST() would trigger a "-Wcast-align=strict" warning.
+ * That would happen, when trying to cast a "GObject*" to "GTest*", if latter
+ * has larger alignment.
+ *
+ * Note that merely adding a int64 field to GTest does not increase the
+ * alignment above 4 bytes on i386, hence use the __attribute__((__aligned__())).
+ */
+#define _GTest_increase_alignment __attribute__((__aligned__(__alignof(gint64))))
+#else
+#define _GTest_increase_alignment
+#endif
+
 struct _GTest
 {
   GObject object;
-};
+
+  /* See _GTest_increase_alignment. */
+  long double increase_alignment2;
+} _GTest_increase_alignment;
 
 struct _GTestClass
 {